EP0799922A1

EP0799922A1 - Method of regulating the thermal bonding process of synthetic fibre webs

Info

Publication number: EP0799922A1
Application number: EP97660030A
Authority: EP
Inventors: Simo Mäkipirtti; Erkki Lampila; Heikki Bergholm
Original assignee: Jw Suominen Oy; Suominen J W Oy
Current assignee: Jw Suominen Oy; Suominen J W Oy
Priority date: 1996-03-18
Filing date: 1997-03-14
Publication date: 1997-10-08
Anticipated expiration: 2017-03-14
Also published as: FI961252L; FI961252A0; EP0799922B1; DE69700715T2; FI101087B; DE69700715D1; ATE186341T1

Abstract

A regulation method for regulating the thermobonding processes of synthetic fibre webs and the production processes of synthetic fibres for the preparation of nonwoven fibre fabrics having the desired strength properties. In the method according to the invention, fibres are made from a polymer, especially a polyolefin polymer, using melt spinning methods, and from these fibres, fibre fabric is made using thermal bonding methods. The central processing conditions of both the fibre and the fabric are adapted using the regulation method according to the invention and associated regulation model so that as a result of thermobonding, the desired nonwoven fabric is obtained having regulated strength properties.

Description

In the method according to the new invention, fibres are made from a polymer, especially polyolefin polymers, using melt spinning methods, and from such fibres, fibre fabric is made using thermal bonding methods. By using the regulation method according to the invention and associated regulation model the central processing conditions for both the fibre and the fabric production are adapted so that the thermal bonding results in the desired nonwoven fabric with regulated and desired strength characteristics. The regulation method according to the invention simulates both pilot and production scale test results, and constitutes an empirical method based on these results as such and the observations made, for which method also natural scientific basis can be found.
In the regulation method according to the invention, characteristic features of the regulation principles, and of the fibre production and thermobonding processes derived therefrom, are i.a.:

The regulation method is developed for direct control of production processes, but it can be used quite beneficially also in product development and in corresponding test runs.
The core of the regulation method is a dynamic regulation equation of the form
- φ is a strength characteristic of the fabric: longitudinal or transverse tensile strength, elongation or energy to break
- v is the speed of the web related to the rate of heating of the web
- λ is the mechanical draw ratio in the fibre production fav is the average chain orientation of the fibre structure
- T is the temperature (usually the surface temperature of the smooth bonding roll)
- E is the activation energy of the bonding process a and c are constants.
- The regulation function φ is a growing function as a function of temperature, but at a high temperature it is limited by a regulation function of similar form simulating ageing and which decreases as a function of temperature. The intersecting point of these functions corresponds to the maximum regulation technical strength value, and the corresponding temperature. It can be mentioned that this maximum strength value usually corresponds to the measurement result values.
The variable and the parameters of the regulation equations are determined in roll bonding using fib-respecific pilot tests, and in air and oven bonding using either thermomechanical loop tests or also pilot tests.
The values for the maximum strength and corresponding temperature according to the requlation equations are both functions of the crystalline lamellar thickness of the polymer structure of a spinning fibre, of the form (for example as regards the tensile strength): σ_max= C₃ D₂ ^-n1 and T_max = T - C₄ D_Z ^-1, wherein D_Z is the (Lorenz-corrected) crystalline lamellar thickness. In the regulation method, the crystalline lamellar thickness is determined as a product of the long period ob-tainable from X-ray small angle scattering (Lorenz-corrected) and the crystallinity degree obtainable from wide angle scattering. It can be mentioned that the melting point of the polymeric crystal regions (partial melting) is in a linear relationship to the inverse value of the crystalline lamellar thickness, and consequently the relationship between the afore mentioned maximum value for the bonding temperature and the lamellar thickness can be determined on a natural scientific base. It is to be noted that according to the equations, the increase in lamellar thickness has an increasing effect on the temperature values and a decreasing effect on the maximum strength values.
In the regulation method, the regulation of fibre production is based on regulating the crystalline lamellar thickness of the spinning fibre, which is carried out with respect to the degree of crystallinity by regulating the quenching rate and temperature of the spinning fibre, and with respect to the long period by regulating the quenching rate and temperature but also the chain length of the polymer and, for example, the degree of isotacticity of a propylene polymer.
The orientation of the spinning fibre and correspondingly the strength characteristics of thermobon-ding are regulated also by means of regulating the fibre spin line tension.
By regulating the temperature difference bet-ween the bonding rolls the strength characteristics of the product fabrics can be fine-regulated also in a direction opposite to that of the bonding equation. The manner of regulating is based on a change of both fibre deformation and heat transfer.
The temperature dependencies of the bonding equations corresponding to the longitudinal and transverse tensile strength values of the fabrics are usually of the same magnitude. The elongation values of the fabrics are quite sensitive to external influences (temperature, radiation forms etc.), wherefore the corresponding activation energies of the equations often deviate from the energy values of the tensile strength equations. A substantial scattering of fibre specific activation energies can be observed especially at high temperatures, above the temperatures corresponding to the maximum strength values. This usually requires that the energy values are determined fibre-specifically over the whole temperature range. At high temperatures the decrease in the strength values can be derived from fibre deformation, chain degradation as a result of autogenic oxidation and fibre shrinkage. Variation in the partial melt quantities also leads to scattering in the energy values.
The product strength values in thermobonding can be regulated within broad limits under the control of the regulation equations as a function of web speed, bonding temperature and fibre draw ratio. The temperature dependencies are not often of the same magnitude for the strength values of the product fabrics. This has to be taken into account especially when demands are put on the end product strengths as regards the mutual relationships between the longitudinal and transverse strength values. In such a case there are several paths leading to the desired end result and the path "of least damages" has to be looked for using the system control computers, or determined fibre-specifically in advance by means of an atlas made for this purpose utilizing the regulation equations.
The control and regulating equation for fabric production can be constructed also synthetically based on observations made on the dependency of the maximum strength and the corresponding temperature with respect to the crystalline lamellar thickness of the fibre polymer. The web speed exponent and the draw ratio constant in the regulation equation have narrow variation limits and they are to their value close to one, wherefore empirical coefficients can be used. The values for the activation energy of bonding are practically of the same magnitude (E ^∼ 40 kcal/mol) for stable fibre fabrics. By introducing these values into the regulation equation, an estimate for the equation is obtained, which can be used in product development and in the control of parameter measurement runs.
The said regulation equation is well suited for regulating the strength properties of thermobonded product fabrics made from webs of skin-core two-component fibres, the components being of different or similar polymer quality. In this case the crystalline lamellar thickness of the fibre polymer corresponds to the structure of the skin layer. The manufacture of skin-core fibres (concentric die spinning, peripheral oxidation, etc) is affected by the chain distribution values of the polymer, the melt viscosity values, the spin melt temperatures and the spinning draw ratio. These affect either as parameters or as variables in the regulation of the ther-mobonding. The magnitude of the apparent activation energy values for the thermobonding of skin-core fibres is a function of the spinning treatment, and susceptible to scattering due to the (large) proportion of total or partial melts. It is not easy to determine the coefficients of the regulating equation for the thermobonding of two-component fibres without carrying out accurate pilot-measurements.

When implementing the regulation method according to the invention it is to be taken into account that in fibre manufacture and in thermobonding on production scale, the quality of the fibre polymers, the spinning and bonding capacities, the fabric weights, the distribution of the "bonding points", the pattern design and the bonding surface of the bonding rolls are unchanged or subject to only small changes during long periods of time. In the development of the thermobonding regulating method, these variables have been treated as parameters, the effect of which on the regulation equation always has to be experimentally determined after changes have taken place.
The new regulation method according to the invention is defined in the first patent claim, the contents of which is included in the present disclosure.
There are many points in common between the disclosed regulation method and the rather extensive patent and technical scientific literature relating to thermobon-ding.
In this context only a very limited part of the publications can be discussed. These generally include a very broad number of patent and other reference publications. Separate aspects relating to the regulation method according to the invention are to be found in almost all ther-mobonding and fibre production methods using synthetic and natural fibres.
In the following, technical science publications and bonding models relating to the thermobonding technique, as well as some patent publications, are discussed briefly.
Alfred Watzl /1/ gives, in his publication series (1994-95), a very good and detailed description of the theoretical foundations for the present day nonwoven techniques, of practical experience and of market development.
S.B. Warner /2/ indicates the various thermomechanical processes in thermobonding and evaluates the effect of each process on thermobonding. In the publication, the limitations of conductive heat transfer in thermobonding is discussed, as well as the importance of nip pressure facilitated flow in heat transfer. In bonding, the contribution of roll bonding induced deformation heating (30-35 °C) in the web, is small but important. A consideration of the Clapeyron effect indicates only a limited amount of melting (effect appr. 10°C) taking place at high pressures. The effect of diffusion in the material transfer in bonding is not important: a diffusion distance (with associated effects) of the order of magnitude of the gyration radius (~ 500 Å) requires a delay time of 1-2 seconds, when the delay time of the "web bond" in the roll nip is of the order of only 10 ms. In the bond periphery, also the thermomechanical and the still uncontrolled thermomechanical history of the polymer restricts product strength.
D. Müller /3/ has studied, in the thermobonding process, especially the significance of roll pressure in the conduction of heat, the effect of production speed, temperature, pressure and bonding pattern on the bonding process. D. Müller /3/ and S. Klöcker /3, 4/ have developed a process model which is applicable to nonwoven thermo-bonding. The model enables a description of the nonwoven deformation characteristics in the processing conditions between the bonding rolls. The model allows a correlation between the fibre material characteristics and the process parameters. As a material model for the model, the Zerner 3 p solid material model is used, which, in addition to the temperature, time and pressure contains the elasticity and damping characteristics of the material.
The fibre material characteristics are easily obtained from elasticity and relaxation measurements. In the region of partial polymer melting, the elasticity characteristic is significantly reduced, because in the roll nip part of the volume looses its elastical properties. In short, in the model, partial melting is an axiom and it is accepted as such (parameter), the bonding strength is a function on the roll nip pressure, and the conductive heat transfer is substantially improved due to the effect of the nip pressure.
The measuring results show that for a given fibre material and unit weight, the production speed, required roll pressure and finally roll geometry, can be predicted more accurately than before. Furthermore, the mechanical nonwoven deformation characteristics described by the model can be combined with the calculations and control system for the roll deflection compensation.
T. F. Gilmore et al. /5, 6/ study the strength properties of point-bonded products by means of research and thermo-bonding models of their own and from literature. According to the study, unquantified factors relating to ther-mobonding were (1993) i.a.: the effect of fibre elongation, the effect of the thermal history of the fibres, and the effect of the basic properties of the fibres on the bonding strength; the effect of temperature on the bonding properties and the bulk polymer morphology in the bond are unknown; the effects of the Clapeyron-effect, the effects of the bonding point form and battery distribution have not been elucidated; the effect of the temperature difference between the bonding rolls is not known; the effect of the cooling rate on the product is unknown; the reasons for the differences between the longitudinal and transverse fabric properties have not been elucidated etc. T. G. Gilmore uses /6, 1994/ in a computer model based on the Box-Hunter model, in addition to regression coefficients, four process variables (roll temperature, - load, line speed and web weight) for evaluating the physical properties of a thermobonded product. In the evaluation of the results, it is acknowledged that the method is usable for optimizing thermobonding processes and analyzing the mechanisms thereof. The essential result is that almost all product properties correlate strongly to the bonding temperature, other factors being of less importance.
In the following, two computer models used for spunbon-ding process research are studied briefly, both of which relate to the fibre spinning process preceding bonding. S. Misra et al /7/ apply a mathematical model of the spunbonding process in order to study the effect of various process and polymer variables on the structure and properties of product filaments. The material parameters studied were elongational viscosity and pressure induced crystallization kinetics. Variable parameters were the extrusion temperature, spinline length (SL), temperature, flow rate of the cooling air, the design of the draw-down-venturi and die capacity. The model shows that the elongational viscosity is a key factor for controlling the draw-down rate, crystallization temperature and orientation development in the fibre. Among the process parameters, the temperature and flow rate of the cooling air has the greatest effect. A. C. Smith et al. /8/ have made a model for fibre formation in a spunbonding process. The model equations comprise the Ziabick crystallization rate equation and an empirical parameter for the pressure effect of the crystallization rate. The models make it possible to estimate i.a. the starting point and position of crystallization as it advances along the spinning line. The computer results and the experimental results are in agreement.
As regards the art of polypropylene, polyester and bicomponent fibre thermobonding, some publications are mentioned without an analysis/9/.
It can also be mentioned that R. J. Kerekes /10/ has developed a basic model for calender heat transfer, to which many scientists refer.
Finally, the art of thermobonding technique will be examined briefly by means of some patent publications. The patent publications deal mainly with spunbonding as this includes the essential factors affecting thermobonding both as regards fibre making and bonding.
R. V. Schwartz (/11/: US 4,100,319/1978) describes the manufacture of a web from fast-spun fibres (as in the process US 3,855,046) and the immediate thermobonding of the web (contrary to the process US 3,692,618) without using pre-heating. It is believed that in the process the web filament may not exceed a specified low degree of crystallinity before the formation of the bond, thus a rapid heating of the web thus being necessary. 12 patented processes are mentioned in the publication /11/ in order to illustrate the prior art corresponding to the method.
A patent publication (/12/: US 5,282,378/1994) relating to the preparation and bonding of a peripherically oxidized fibre, describes the oxidation of a polymer for high melt temperature spinning, which polymer is spundelayed and exhibits a "high" MWD dispersion, to produce a fibre having a skin-core structure with good thermobonding properties. In the prior art description of the patent publication, reference is made to 76 publications (i.a. 50 US patents). Also the 'Finnish' patent applications FIA 943072/23.06.94 and FIA 942889/16.06.94 refer to the same peripheral oxidation.
The brief study made of publications relating to thermo-bonding techniques did not reveal any general, basically "temperature controlled" method or control model comparable to the new thermobonding regulation method. In many publications the temperature, in addition to the pressure, was held to be relevant in the formation of the bonding strength.
The following examples illustrate in detail the operation principles of the new thermobonding regulation method.

Example 1

In example 1., the basic principles for the new invention and the essential observations relating thereto are disclosed by means of detailed subexamples.

Subexample 1.1

The subexample 1.1 describes the fibre and fabric manufacturing apparatuses and the function thereof for use in the process development according to the examples of the description of the invention.
In the production of the fibre sample series referred to in the description of the method according to the new invention, both so called short-spinning and high-speed spinning methods and corresponding apparatuses were used. In this study, the terms 'short- and high speed spinning method' are used.
The spinning and drawing test series were predominantly carried out in pilot scale apparatuses, which differed from production scale apparatuses only as regards the number of spinning units, but not as regards size. Compared to production apparatuses the pilot apparatuses were better equipped with regard to measuring technique.
The number of nozzles in the nozzle plate commonly used in a short-spinning apparatus was 30500 and the diameter of the nozzles was 0.25 mm. Thus the capacity of the apparatus (P, kgh^-1) was, as a function of the rotational speed of the pump (n_r, min^-1), of the form ${P = 2.0437 x n}_{r} .$
The velocity of the polymer in the said nozzle (v_s, m min^- ¹⁾) was correspondingly of the form $v_{s} {= 2.5111 x 10}^{-2} {x n}_{r}$
In the rotational speed range of the polymer pump n_r = 15-20, the following range for the capacity of the apparatus and the nozzle speeds were obtained:
P = 31-41 kgh^-1 and v_s = 0.38-0.50 m min^-1. The number and especially the diameter of the nozzles can be changed, if necessary, by changing the nozzle plate.
The high speed-spinning apparatus contained two parallell nozzle plates, the number of nozzles were altogether 1200 and the exit diameter of the flow designed nozzle was 0.4 mm. The high speed- and short-spinning apparatuses were similar as to their capacities, but there was an about 10-fold difference between the speeds of their 1-godets (appr. 1000 and 100 m min^-1). There was a substantial difference in the cooling systems for the spinning fibres of the spinning methods: the length of the cooling zone (chute) of the high speed-spinning method was appr. 2 m and that of the short-spinning method correspondingly appr. 50 mm. In the high speed-spinning methods there is an effective cooling apparatus for the cooling air for the fibres, the cooling being easily regulated, is even and fibre-specific, which is difficult to implement in short-spinning methods.
The sub-parts of the short-spinning-drawing apparatus (Figure 1) are: 1. extruder, 2. melt spinning device, 3. 1-godet, 4. drawing oven, 5. 2-godet, 6. 2-finishing (avivage), 7. crimping device, 8. stabilizing and drying oven and 9. staple fibre cutter. The apparatus diagram for the high speed-spinning method is substantially analogous to the diagram of Fig. 1.
Part of the fibre series in the examples were prepared with a production scale short-spinning drawing apparatus. In part of the fabric production tests, fibres made with high speed-spinning methods were used. Part of the samples used for analyzing details of the invention were pre-pared under controlled conditions on a Haake-Rheocord-9-apparatus, which was provided also with a mechanical drawing apparatus and oven.
A diagram for a conventional apparatus for use in the preparation and bonding of fibre webs is shown in the Figure 2. The parts of the apparatus are: 1. opener, 2. fine opener, 3. storage silo, 4. feeder, 5. carder, 6. bonding rolls and 7. winder.
The devices in the diagram are all of production scale. The production and bonding speed of the web in the bonding line can be regulated in the range of 25-100 m min^-1. In the bonding apparatus the temperature and the pressure of the upper roll provided with a diamond- or circular-shaped raised pattern and of the smooth lower roll could be regulated. The diameter of the bonding rolls was of production scale, but their width was only 2.2 m.
In the studies relating to the fibre bonding mechanism, so called fibre-loop bonding and direct bonding tests were carried out with a mechanical and dynamic-mechanical thermoanalyzer (Mettler 3000/TMA 40 and Seiko 5600/DMS-200 analyzer).

Subexample 1.2

In the subexample 1.2, the changes of the long identity period and the degree of crystallinity of a fibre polymer (here polypropylene) as a function of temperature and time are studied. From the (statistical) value of the long identity period, using the value for the degree of crystallinity, i.a. the statistical value of the crystalline (as also the amorphous) layer thickness of the polymer as a function of time and temperature, can be calculated.
For all the tested polymers, similar values for the long period as a function of temperature and time were obtained in the range measured within the limits of accuracy of measurement (irrespective of differences in the polymer starting values), which for its own part proves that the period value is independent of the degree of crystallinity.
In the structural system of the propylene polymer being studied, the value of the Lorenz-corrected long period (L(_z), Å) as a function of temperature (T, K) and time (t, min), is of the form $L_{z} = 92.64 + 1864.4/(435.44 - T) + [(465.0/(444.01 - T)) - 3.7326] ln t$
For the differential changes of the period as a function of temperature and time, the following equations are obtained $L_{z} {/dT = 1864.4/(435.44-T)}^{2} {+[465.0/444.01-T)}^{2}] ln t$
$L_{z} {/dt = [(465.0/(444.01-T)) - 3.7326] t}^{-1}$
It can be seen from the equations that the time dependency of the long period is also a function of the temperature. The value for the long period grows rapidly as a function of temperature, especially when approaching the melting point of the polymer.
The long identity period (L∗, Å) being the sum of the amorphous and crystalline lamellar thickness of the polymer, the crystalline lamellar thickness (D∗, Å) is obtained from the product f_c x L∗, where f_c is the crystallinity degree fraction. In this study the Lorenz-corrected measurement value (L_z, Å) for the period is used. A substantially linear relationship prevails between the period values, which in the test series were of the form ${L* = 1.064 L}_{z} + 30.59$
When implementing the regulation method, the absolute value of the long identity period is not necessary, especially when the difference between the values is small.
The changes in the crystallinity degree of the fibre polymer in bonding are quite important in the regulation method, but difficult to determine and estimate, as the delay time in bonding is of the order of magnitude of only a millisecond per bonding point.
The change in crystallinity degree (f_c = χ/100) of the fibre B-1 used in the bonding studies (Tables 1. and 2.) as a function of time and temperature was monitored by WAXS ans SAXS analyses (apparatuses: Philips PW1730/PW1710, Kratky) after isothermal heat treatment (ϑ = 80°-140°C, t = 1-1800 min). By using the ratio a = χ/χ_max for the crystallized fraction of the polymer (B-1: after prolonged heating χ_max = 55 %), it could be established that the increasing crystallinity fraction comp-lies with the kinetic equation of Avram. As a result, the following function was obtained for the crystallizing fraction α: ${α = 1-exp[-1.8304x10}^{3} {x exp(-5984.34/RT) x t}^{0.075}]$
where R, cal/degree mole is the general gas constant; T, K is the absolute temperature and t, min is the time of isothermal crystallization. The equation obtained simulates well the measurement results obtained. At the temperature 140 °C, slight scattering could be observed due to lamellar surface melting.
In the equation /7/ the low value of the time exponent (n=0.075) is extraordinary compared to the time exponent of a sample taken directly from the molten condition to isothermal crystallization. Also the value of the activation energy (E= - 5480 cal/mol) in the equation is lower than the usual value (the value obtained without correcting the crystallization temperature). These differences are apparently due to the fact that the primary crystallization (which takes place during the spinning cooling of the sample) with its nuclei formation and growth, advances so far that during the secondary crystallization the said values decrease, the Avram-equation type still being preserved.
Also the following equation form is suitable for simulating the measurement results for the change in crystallinity degree of the fibre polymer B-1: ${χ/100 = f}_{c} = 50.690 exp(-2002.16/T) + 0.03325 log t$
The time exponent in this equation did not, however, remain constant in all the studied samples, wherefore an equation of Avram type was taken into use.
The fibre polymer A-1 (Table 2.) was monoclinic to its initial structure and had a high primary crystallinity degree, that is χ = 43.6 % (the B-1 sample was smectic to its initial structure).
The secondary crystallization of the fibre polymer A-1 is well simulated by an Avram type equation, wherein the time exponent corresponded to that of the previous sample (n=0.075) and the rate constant was of the form ${k = 6.3088 x 10}^{3} exp(-6275.49/RT)$
Some constant values of the crystallization equations for the fibres of the bonding tests have been included as examples in the Table 3.
From the equations /3-5/ it can be concluded that the increase of the crystalline lamellar thickness is monotonic as a function of the temperature. Based on the equations it can be further concluded that, at the temperature range studied, the amorphous lamellar thickness, (1-f_c)L_z, is characterized by a deep minimum. Thus the amorphous lamellar thickness first decreases when the temperature and the crystallinity degree increase (that is, the amorphous fraction decreases), but starts thereafter to increase sharply after the minimum range in the temperature range of 100-130°C. Thus, in practice, the crystalline lamella thus increases in length and becomes narrower, whereby the amorphous lamella can increase in thickness even though the amorphous fraction of the matrix decreases.
The crystalline, as well as the amorphous lamellar thickness of the polymer and its changes can, based on the study, be quite precisely regulated by regulating the crystallinity degree of the polymer matrix and its long identity period. The regulation of the crystallinity degree takes place especially by regulating the temperature of the polymer matrix. The regulation of the long period takes place by regulating both the matrix temperature and time. The primary value of the long period can be affected upon i.a. by regulating the cooling rate of the spinning polymer and the quenching temperature. The choice of molecular weight and the weight distribution of the polymer is of importance in this connection.
The cooling temperature range relevant for the formation i of a spinning structure in the fibre polymer falls in the maximum temperature range for the crystallization rate (the crystallization temperature at which the reciprocal value of the crystallization half-time has a maximum), which area is determined experimentally (it is also an orientation function). It can be shown /13/ that when the maximum temperature for the crystallization rate is ϑ_m = 90°, in addition to an amorphous phase only a smectic phase (α-crystallinity degree: χ < 20%) is obtained in the structure at a cooling rate value of Ṫ₉₀ > 80°C/s. In the cooling rate range of 20 < Ṫ₉₀ < 80 the structure contains increasingly monoclinic phase besides the smectic and amorphous phases (χ: 20-40%) as the cooling temperature decreases. When the cooling rate decreases, Ṫ₉₀ < 20°C, the smectic phase has disappeared completely (χ: 40-60%).

Subexample 1.3

In this subexample, the development of a bonding equation simulating the changes in the properties of the product fabrics from a fibre thermobonding process is considered. By means of the equation, primarily the changes in the mechanical strength properties of the fabrics as a function of bonding temperature and time are studied.
The thermobonding of synthetic fibres is a rather complicated process, which is governed by different mechanisms at varying temperature and speed ranges. In this study, an attempt is made at studying the thermobonding results by means of two bonding equations simulating these, while not considering precisely the details of various material transfer mechanisms. In this study, the bonding mechanism is divided into three stages to form a work hypothesis:
In the first bonding stage contact points bewteen the web fibres are formed, apparently as a dislocation-mediated translational slipping process. In this case the effective compression pressure (from the bonding rolls) in the bonding point is above the yield limit of the polymer material. In practice during bonding, this bonding stage takes place almost momentarily at an increasing temperature (the material constants are a function of temperature; the bonding rolls are at a high temperature but the web is cold) and stops when the displacement movement is prevented as the compression pressure decreases below the yield limit.
It is believed that the second bonding stage takes place as a thermally activated, time dependent slip flow, which takes place below the yield limit of the material. Thereby the displacements circumvent the flow bars for example by means of dislocation-mediated, diffusion-controlled 'climbing'.
In the third bonding stage, there is formed, due to the partly melting of the polymer, so much molten phase that the contact formation takes place based on a viscotic flow mechanism. In addition, other significant phenomena take place in the system: i.a. diffusional material flow, melting of the finely divided crystalline portion of the polymer and re-crystallization etc. When the temperature increases further the bonding strength starts to decrease due to the effect of shrinkage of the bonding fibres, thermal degradation (a function of the anti-oxidant concentration and type), excessive melt formation and other factors.
A variable property of the fabric resulting from bonding (longitudinal and transverse strengths, -elongations, - toughness etc.) is marked with the letter φ. As a result of the effect of the increase of the contact surface, the following temperature activated, dynamic general velocity equation is obtained for the variable property ${dφ/dT = C" x v}^{-1} {x φ}^{n} x exp[-E/RT], wherein$
C" is a constant, E is the activation energy, R is the general gas constant, T is the absolute temperature, v is the heating rate of the fibre web.
The integrated form of the velocity equation /10/ is (provided that 2RT/E << 1)
wherein n is an exponent dependent on the material transfer mechanism.
In the third bonding stage, for the equation illustrating the decrease in the strength values, i.a. for analogy reasons, also the afore mentioned equation /10-12/ has been taken, which can also be used for simulating the changes in the properties corresponding to the ageing of the polymer.
The compatability of the strength values of the test fabrics manufactured both on pilot and production scale with the bonding equations (on both sides of the maximum values) is quite good (tensile strength and elongation measurements: test strip 5 x 20 cm, drawing speed 30 cm/min; equation: n = 0, φ_o = 0). It is presumed that the heating rate of the fibre web is constant and in a simple, linear relationship to the web line speed.
The Table 1. contains a compilation of measurement results in equation form obtained from the bonding of various types of test fibres. The appended Table 2. contains a compilation of values for the properties of web test fibres used in the bonding tests.

Subexample 1.4

In the subexample 1.4, the development of the maximum values for the tensile strength (and elongation) of fabrics obtained from thermobonding of test fibres, is studied.
In this study, the maximum value of the tensile strength of a thermobonded fibre fabric is taken to be the intersection point of the bonding equations according to the subexample 1.3 for the low and high bonding temperature ranges. The measurement values of the test fabric tensile strengths require the use of two separate equations for illustrating the bonding process, although there is a transitional zone in the near vicinity of the intersection points, where the measurement results remain slightly below the calculated values. This transitional area is, however, narrow, especially for conventional fabric strengths, whereby the change in strength as a function of temperature is lower than for high strength fabrics. The maximum tensile strengths corresponding to the intersection point of the bonding equations have been included in the Table 1., in addition to the bonding equations.
The crystallinity degree, the Lorenz-corrected long period and the crystalline lamellar thickness have been determined in the manner taught in the subexample 1.2 for fibres corresponding to the bonding equations. The value for the crystalline lamellar thickness for each fibre has been indicated in the Table 1.
In the Fig. 3, the said values for the maximum strength as a function of crystalline lamellar thickness have been given. The following equation is obtained from the logarithmic dependency of the figure $σ_{max} {= 1192.7 x D}_{z} ^{-0.854}$
Many factors affect the spread of the observation points in the Figure, of which the following can be mentioned as an example:

The different draw ratios of the test fibres in the mechanical drawing following spinning, which draw ratios are close to the draw ratio value of λ = 1.0 for structurally homogenous fibres.
The variation in carding and weight of the test webs.
The area, shape and distribution of the raised pattern on the roll surface of the bonding roll. In the Figure, the displacement resulting from changing one patterned roll to another has been indicated with a dotted line (line H).
The rate of change of the crystallinity and period values as a function of temperature.
Part of the tested fibres were imported fibres, the manufacturing conditions of which were not precisely known (fibres with unhomogenous cross-section, chain modified, skin-core-, spinning etc fibres).

The temperature corresponding to the maximum tensile strength defined by the intersection point of the bonding equations is a function of the reciprocal value of the crystalline lamellar thickness of the fibre polymer structure, of the form $T_{m} {= 455.79 - 9.777x10}^{2} {x D}_{z} ^{-1}$
A correlation according to the equation /132/ has been found to exist also between the melting point of the crystal regions of the polymer matrix and the crystalline lamellar thickness /14/, wherefore there is also a scientific base for the equation.
In the subexample 1.5 the effect of the mechanical draw ratio on the thermal bonding has been studied. From the intersection point of the bonding equations (Table 3), the following equation is obtained for the correlation between the maximum strength and the draw ratio (λ = 1.0-2.5) $σ_{max} {= 45.807 x λ}^{-0.8226}$
Thus the maximum strength of the fabric corresponds to the draw ratio of λ = 1, that is, when operating with the same polymer and the same manufacturing conditions, the highest fabric strength is obtained with spinning fibres, which observation is quite important.
In the said test run (1.5), the average chain orientation (fav) of the fibre matrix as a function of the draw ratio was of the form $ln λ = 6.6015 fav - 3.6308$
that is when substituted $σ_{max} {= 9.078 x 10}^{2} exp [-5.4302 fav]$
The equation /16/ combines the maximum strength in thermobonding with the fibre chain orientation values from corresponding manufacturing conditions and thus also with the spinning orientation.
The maximum strength values for the fibre fabrics obtainable by fibre web thermobonding, can thus be regulated by regulating the crystalline lamellar thickness of the spinning fibre. This regulation, in turn, can take place by regulating the long period, the crystallinity degree or both of the spinning fibre, in accordance with the subexample 1.2.

Subexample 1.5

In the subexample 1.5, the effect of the change in draw ratio in the mechanical drawing process of a spinning fibre, that is also the effect of the change in chain orientation of the fibre matrix, on the tensile strength and elongation values of a thermobonded product, is shown.
Fibres were made in a conventional fibre line (Fig. 1) from melt spun propylene fibres using mechanical drawing at varying draw ratios. The properties of the fibre polymer, the strength values of the product fibres as a function of draw ratio, as well as the drawing conditions, are indicated in the Table 2 (fibres D-10-14). Test fabrics were made in a fabric line from the test fibres obtained using conventional techniques (Fig. 2) as a function of temperature. The bonding equations for the longitudinal tensile strength and the elongation for the test fabrics with intersection points are indicated in the Table 4.
From the equations given in the Table 4. it can be observed that the activation energy values for both the elongation and the tensile strength of the drawing process remain constant, and are independent of the fibre draw ratio within the limits of measuring accuracy. The pre-exponential factor is a function of the used fibre draw ratio. In the system under study, the following equations are obtained from the lnc-values in Table 4. for the tensile strength and elongation in both the low and high temperature bonding areas (that is at temperatures both below and above the maximum value): ${σ =T}^{2} {x3.4691x10}^{21} x exp[-0.8698λ] x exp[-48818.7/RT]$
${∈ =T}^{2} {x6.5052x10}^{13} x exp[-1.0219λ] x exp[-33497.6/RT]$
${σ =T}^{2} {x1.2090x10}^{-7} x exp[-0.4895λ] x exp[6912.3/RT]$
${∈=T}^{2} {x2.2027x10}^{-14} x exp[-1.0999λ] x exp[21000.6/RT]$
In the test series under study, the relationship between the fibre draw ratio and the average orientation factor of the fibre polymer is indicated as equation /15/ in subexample 1.4. By inserting this equation into the equations /17-20/, the effect of orientation on the tensile strength and elongation values of the test fabrics is obtained over the whole studied range.
In the Figure 4, the maximum tensile strength and elongation values corresponding to the draw ratios, as well as the path of the temperature functions thereof, have been calculated from the equations of the Table 4., for clarity's sake only corresponding to the draw ratios of λ = 1 and 2. Some measurement values have been included in the function graphs, which show that the equations fairly well simulate the path of the measurement points.
Based on the Figure 4 it can be observed that the path of the maximal values for tensile strength and elongation of the test fabrics as a function of temperature differ from each other. In the case under study, at the conventional operational range of thermobonding, the fibre tensile strength reaches its maximal value as a function of temperature prior to elongation.
In order to compare the tensile strength-elongation relationships some elongation equations have been indicated in the Table 1., in addition to the tensile strength equations. The maximal tensile strength and elongation va-lues for a fabric bonded from fibre B-2 (under the processing conditions used) are σ_m = 65.8 N (159.4°C) and ∈_m = 24.5 % (153.0°C). Thus, at the temperature of maximal elongation, the tensile strength (45.3 N) has reached only 69 % of its maximal value. In the fabric made from the fibre B-13, the maximal tensile strength and elongation values are almost of the same magnitude and quite close to each other with respect to the temperature (42.1 N/161.2 °C and 42.2 %/161.0 °C). Both the tensile strength and the elongation values for the fabrics made from the fibres B-1 and B-9 are quite close to each other with respect to the temperature, but the tensile strength values are high (81.5 and 79.8 N) and the elongation values are low (38.2 and 37.7 %). The strength values of the fabrics made from the fibres A-1 and A-8 differ from each other both as to the maximal elongations and the corresponding temperatures, the maximal strength values being quite low (Table 1.).
The elongation of a fabric (fibre) generally reacts more strongly to exterior effects (temperature, high energy and light radiation, etc) than the tensile strength. These characteristics, which vary for each polymer spunbonding system, form the basis for the regulation of the strength properties for thermobonded fabrics.
In order to show the effect of high-energy radiation, fibres from the test series were irradiated (γ-radiation: 3.06 Mrad, 10 days of storage) before bonding. The equations simulating the test fabric tensile strength and elongation are of the form
(T < Tm and T > Tm): ${σ = T}^{2} {x 1.0208x10}^{6} x exp[-0.9427λ]exp[-18766.6/RT]$
${∈ = T}^{2} {x 2.9380x10}^{-2} x exp[-0.5171λ]exp[-4112.9/RT]$
${σ = T}^{2} {x 4.7812x10}^{-19} x exp[-0.8622λ]exp[28769.3/RT]$
${∈ = T}^{2} {x 5.2692x10}^{-19} x exp[-0.7195λ]exp[28753.0/RT]$
From the equations /21-24/ and the Figure 4. it can be seen that the position of both the elongation and the tensile strength of the test fabrics corresponding to the radiated fibres have changed substantially, both with respect to temperature and to each other. It can also be seen that the values for the activation energy of the equations corresponding to the tensile strength and elongation of the test fabrics have decreased at temperatures below the intersection maximum, and increased above such temperatures. Similarly, it can be noted that the speed term having a draw ratio effect still is concentrated in the temperature independent speed term.
When a fibre web is roll bonded using temperature and pressure, strong and rapid shear is applied to the bonding point. Test bonding was also studied under low mechanical load by means of so-called loop bonding tests. Loop bonding is carried out using a thermomechanical fibre analyzing device (Metler 3000). To each socket of the analyzing device a fibre sample (50 x 2.2 dtex) is fastened at both ends and so that the fibre portions of the formed loops form a cross loop with each other. This fibre loop system is loaded with a constant load (1 mN/tex) and heated at a constant rate (10-50 °C/min) while simul-taneously registering the elongation and temperature, to the desired "bonding temperature" and is cooled. A bond is formed at the cross of the loops, the strength of which is measured with a drawing apparatus (Zwick-1435) after one leg has been cut from each loop. The strength value for this loop bond (σ_e, mN) together with the said bonding temperature, satisfy the conventional bonding equation /10-12/.
The strength values corresponding to different draw ra-tios for the fibres (E: 1-5) used in the loop bonding test series, as well as the polymer molecular weight distribution have been included in the Table 2. The following equation simulating the loop strength values in loop bonding were obtained:
The activation energy values in bonding fibres of diffe-rent draw ratios are independent of the draw ratios and thus correspond to the bonding result of the afore mentioned conventional webs. Fibres of the test series (E 2-4) were radiated (γ-radiation: 3.40 Mrad, storage 70 days) and loop bonding tests were carried out on the fib-res obtained. The loop strengths obtained for the radiated samples were substantially of similar magnitude, that is the draw ratio (orientation) effect on the bonding strength of the bond had disappeared (within the limits of measuring accuracy). As the equation for the loop bonding strength for radiated samples, the following was obtained $σ_{e,s} {= 7.7752 x 10}^{36} {x T}^{2} x exp[-76679/RT]$
The bonding result from the loop test series can be seen in the Figure 5. The bonding results for individual fibres (E:1-5) have been displaced by using a factor obtainable from the equation /25/ to correspond to the result for the fibre E-4, which has been taken to be the reference state. From the Figure it can be seen that the re-sulting equations fit the measurement values. In the loop bonding tests, a fairly light mechanical load is applied to the bond, and thus the fibre to be bonded is not deformed in the bonding point as is the case in conventional thermobonding with rolls. The partial melt formed in the loop bonding initially primarily wets the fibre bundle and when the mass of partial melt increases, it glues the wetted fibre bundles together and finally surrounds the whole cross loop. This type of bonding is not sensitive to an excess of partial melt, as this migrates through sorption outside the bonding point. Also changes in fibre draw ratio do not affect as readily in loop bonding as in roll bonding, although the draw ratio affects in a similar manner in both. Loop bonding thus essentially corresponds to hot air bonding under low mechanical load.
When using high-energy radiated fibres in loop bonding, the bonding strength increased in each sample irrespective of the mechanical draw ratio used in the manufacture of the fibre, that is contrary to web bonding. This difference is due to the low fibre deformation in loop bonding and also to the disappearance of the damaging irradiation effect on the polymer structure (bonding chains) from the polymer portion involved in bonding during the formation of the partial melt. The decrease in bonding temperature due to irradiation was substantially the same for both bonding methods. When evaluating the loop bonding tests carried out, it is to be borne in mind that small amounts of hindered amines had been mixed in the polymer in addition to the base antioxidant mixture (in the web bonding tests only one fibre had a low antioxidant addition). The effect of antioxidants as well as of some wetting agents becames evident, especially in air bonding.
Based on the effects of thermobonding as shown in the subexample 1.5 on the tensile strength and elongation values of a fabric product obtained from fibres made with mechanical draw ratios of differing magnitude, as well as on their relationship, i.a. the following observations can be made as regards the regulation method:
As regards different fibre qualities and thermobonding methods, a relationship according to the equation /14/ can be seen as a common characteristic, wherein the maximal bonding strength (when operating under the same manufacturing conditions) is obtained with mechanically undrawn fibres, that is with spinning fibres. The maximal strength values in thermobonding can be regulated primarily by regulating the crystalline lamellar thickness of the spinning fibre, that is thus by regulating the polymer chain length and -distribution, the spinnning draw ratio in the spinning process, the cooling rate and temperature of the spinning fibre.
As regards production fabrics, it is important to impart sufficient tensile strength and elongation and a specific elongation/tensile strength ratio to the fabrics. The nature of the regulation method required is determined by both the quality of the used fibre (a function of the spinning process) as well as the manner of carrying out the thermobonding.
A special problem constitutes the regulation of the strength properties of fabrics to be thermobonded using a bonding apparatus (i.a. roll bonding) with high compression and simultaneously rapid mechanical shear. As a result of the shear, the fibres in the bonding region are deformed, which leads to fibre breaks especially in the periphery of the bonding region. In such a case the regulation of the thermobonding has to be carried out individually for the polymer and the apparatus, in which case the regulation of the mechanical draw ratio in the fibre manufacture as well as the regulation of the bonding temperatures for the web are of substantial importance. Regulating the elongation-draw ratio in product fabrics always means operating below the maximal fabric strength values as regards elongation, draw ratio or both.
The sensitivity to shear in roll-thermobonding can be decreased to some degree by using fibres with a skin-core structure, especially if the melting range of the skin layer is lowered by blending into the fibre (or otherwise produced) of a short chained polymer fraction of a similar quality (spinning distribution) or by using a fibre coated with a low melting polymer of a completely different quality.
By using thermobonding methods with light bonding shear (i.a. hot air and oven bonding methods) some of the afore mentioned bonding problems can be avoided. In such a case the regulation of the bonding can be simpler than for methods based on shear. These methods are, however, poorly suited for the large scale production of many thin fabric qualities (from fibre web).

Subexample 1.6

In the subexample 1.6, the effect of the heating rate of the fibre web on the thermobonding temperatures and the tensile strength and elongation values of the thermobon-ded product, is shown.
It was presumed (subexample 1.3) that the thermobonding of the fibre web, as regards a variable characteristic of the product fabric, complies with the general dynamic kinetic law /10/, wherein the heating rate of the fibre web was taken to be constant. The heating of the fibre web cannot take place exactly in this manner in a production apparatus. In order to elucidate any deviation, a test series was carried out, wherein the transport speed of the web and simultaneously the hot rolling speed was increased. Thereby the function for the heating rate of the web was presumed to remain of the same form. As variable bonding speed values, the following speed series for the web was used: v, m min^-1 = 35, 45, 60 and 75. The characteristics of the used test fibre and polymer are indicated (F-1, F-2) in the Table 2. The constant terms in the tensile strength and elongation equations for the product fabrics corresponding to different web speeds are indicated in the Table 5.
From the speed equations it can be seen that within the limits of applied measuring accuracy the activation energy for bonding is independent of the web speed (bonding speed) in the equations corresponding both to the tensile strength and the elongation of the product fabric. The magnitude of the effect of the speed of the web can thus be calculated from the pre-exponential factors of the bonding equations. The following equations are obtained for the tensile strength and elongation values of the product fabrics: ${σ = v}^{-1.294} {x 5.7144 x 10}^{17} {x T}^{2} x exp[-38225.8/RT]$
${∈ = v}^{-1.223} {x 6.7918 x 10}^{17} {x T}^{2} x exp[-38225.8/RT]$
The change in tensile strength of the product fabric above the temperature corresponding to the maximal value is in this case (within the limits of applied measuring accuracy) independent of the web speed. In accordance with the measuring results, however, a reduction in tensile strength can be established for each studied speed at high bonding temperatures (after the maximal value) (in this case: ϑ_si > 165°C, ϑ < 155°C). In the case under study, the value for the maximal strength is appr. 40 (± 2) N. The intersection temperatures for the equations corresponding to this maximal strength as well as the equation parameters are indicated in the Table 5. The change in product fabric elongation (above the temperature corresponding to the maximal value), is characterized by substantial scattering. In the case under study, the measurement values for the change in elongation, can, in the vicinity of the maximal values, be simulated by means of one equation. This equation, and the maximal values for elongation with temperatures at corresponding speeds, are indicated in the Table 5. When the bonding temperature increases further, the elongation starts to decrease rapidly (too much molten phase is formed with respect to that required for bonding, machining deformation increases, oxidation-induced ageing starts, etc). This rapid decrease in elongation seems to start at the higher a temperature range, the higher the web speed is. Into the Table 5., the equations for the product fabric elongation (∈₂) at high temperatures have been introduced.
The decrease in fabric elongations in the area of these maximal values has (independently of the web speed) the form of the equation /30/, and at higher temperatures than the afore mentioned, as a function of bonding speed, the form /31/: ${∈ = 8.8887 x 10}^{-3} {x T}^{2} x exp[16888.3/RT]$
${∈ = 2.0661 x 10}^{-49} {x v}^{0.8774} {x T}^{2} x exp[87590.7/RT]$
The graphs of the equations simulating the bonding strength values are shown in Figure 6. Some measuring points are also seen in the Figure.
For comparison, in the Table 5. and the Figure 6., the corresponding bonding strength values (v = 35 m min^-1) for the import fibre F-2 (Table 2.) have been indicated. It is apparent from the Figure that the behaviour of tensile strength and elongation of the product fabric for the fibre F-2 is similar to that of the fibre F-1, irrespective of different thermal activation values.
In the thermobonding line used in the bonding speed tests, the differences in speed between the carding rolls and between the carding rolls and bonding rolls were so adjusted that the final compression values were constant for the different web speeds used in the test series. The uniformity of the compression values was monitored by analyzing the ratio between the longitudinal and transverse tensile strength. According to the calculations, only when using the highest web speed, only a 4.7 % deviation was observed in the compression values.
The regulation of the strength properties of the the product fabrics in thermobonding is based i.a. on the following test observations

by means of regulating both web speed and bonding temperature, the strength values of the fabrics can be regulated within broad limits, especially when operating below the maximal strength values and corresponding temperatures. From the results also the great effect of polymer-blended antioxidant mixtures, both on the tensile strength and especially elongation values when operating at temperatures above the maximal strength values, is evident
important from the point of view of production and regulation is also the fact that the thermal resistance of elongation is all the better the higher the bonding speed, that is the shorter the bonding time.

Subexample 1.7

In the subexample 1.7, the effect of fibre line tension in melt spinning on the thermobonding of the product fibres is studied.
Chain orientation developing in melt spinning is largely controlled by the melt flow rate field and, opposite thereto, the structural relaxation resulting from the molecular thermal movement. The orientation developed in in the range of capillary flow (in the dies) is of low stability and of little importance due to the low speed gradient and the short relaxation times resulting from the high die temperature. A kinetically stable fibre chain orientation is developed only during the melt elongation flow, the velocity of the orientation-controlling flow parallel to the fibre axis being high and the melt viscosity, which increases due to the decreasing temperature, increasing the molecular relaxation time (preventing disorientation) and finally the melt solidification free-zing the formed orientation. It can be shown that the tension affecting the fibre filament at the solidification point and the birefringence of the fibre polymer both relate in the same way to the melt draw ratio at spinning, and thus also birefringence and tensile strength at the solidification point are directly proportional to each other. The tensile strength at the solidification point is again substantially the same as the tensile strength at the 1-godet (Figure 1.). Thus the factors affecting the increase in spinning tension also increase the chain orientation of the fibre filament. I.a. the following factors facilitate the increase in spinning tension:
increase in polymer viscosity, decrease in spinning temperature, increase in spinning capacity (at constant gauge), decrease in fibre gauge, increase in quenching speed etc.
In the following, the effect of fibre line tension in the melt spinning process on fibre thermobonding is studied by means of some examples. In the Table 6., the constant terms for the bonding equations of the strength values for the fabrics corresponding to the thermobonding result for the fibre series under study, the fibre line tensions corresponding to the bonding equations (SLT: N/spinline-fibre number 30500-), the crystallinity degrees and the polymer MWD values. The spinning draw ratio of the test fibres was λ_k = 206 and the mechanical draw ratio, λ = 1.15 (for samples H-7 and H-8: λ_k = 189, λ = 1.25), and the staple fibre gauge was 2.2 dtex. The product fabric weight was on an average, w = 23 g/m². The tensile strength-elongation graphs for the test fabrics corresponding to the bonding equations have been indicated in the Figure 7.
The Figure 7 shows the results of thermobonding corresponding to the test fibres H-1 and H-2 (SLT: 41 and 57N) as tensile strength-elongation function values, the bonding temperatures and line tension being the parameters. According to the Figure, it can be seen that the fabric strength values, in a manner corresponding to the bonding equations, pass through a maximal value as a function of temperature. Further it can be seen that when the SLT value decreases, the strength value increases on all isotherms before the maximal value and decreases thereafter. Prior to reaching the maximal values, the changes in the strength values as a function of the SLT-values are quite substantial, but on the subsequent isotherms, low.
The fibre samples H-3 and H-4 have been prepared from a polymer having a melt index of the same magnitude, but a substantially narrower chain distribution as compared to the polymer of the samples H-1 and H-2. From the Table 7. it can be seen that the fabrics obtained from the test fibres (H-3 and H-4) are of low strength over the whole bonding range as compared to the test series studied earlier. From the graphs of the bonding equations it can be seen that the effect of SLT on the elongation values below the elongation maximum is low, but substantial on the isotherms above. By adding to the polymer in question a more effective anti-oxidant mixture, the effect of SLT on the elongation isotherms can be removed completely over the whole bonding temperature range [Fibres H-31 and H-41, SLT: 24 and 41N:high/temperatures:
E(σ) = 34684.4, ln c - -48.7365, σm = 39.2N/433.7°K;
E(∈) = 34684.4, ln c = -49.1619, ∈m = 31.4%/43.,4°K/H-31 and ln c = -49.2113, ∈m = 30.0%/431.4°K/H-41].
In the thermobonding of the fibre samples H-3 (-31) and H-4 (-41), regulation methods based on both the bonding temperature and the spin line tension are of very little importance.
A third object under study is melt spinning of a polymer with a melt index of 25 and a broad chain distribution is taken. The equation constants corresponding to the ther-mobonding results for the fibres H-5, H-6 and H-7, H-8 made using mechanical draw ratios of λ = 1.15 and 1.25 are given in the Table 6. The tensile strength-elongation graphs for the test fabrics corresponding to the fibres H-5 and H-6, are given in the Figure 7.
From the Figure it can be seen that the effect of the spin line tension on the tensile strength of the fabrics is fairly low on the various isotherms. As regards the fabric elongations, the SLT-effect is substantial only at temperatures after the elongation maximum, where the elongation values decrease isothermally when the SLT-value decreases.
Based on the bonding equations corresponding to the test fibres H-7 and H-8 (λ = 1.25), it can be seen that the SLT-values affect the thermobonding result over the whole temperature range. Before the maximum for the strength values, lowering of the SLT-value increases the tensile strength and elongation values for the fabrics, and after the maximum, increases the tensile strength and reduces the elongation values.
Finally, melt spinning is studied using a fibre polymer with a melt index of MI = 18 and a chain distribution value MWD: 221000-6.4. The chain dispersion of the polymer is thus the highest of the tested polymers. At quenching conditions corresponding to those of the test series, also the highest line tension values at melt spinning are obtained for this polymer. According to the equation constants given in the Table 6. and the functional graphs of Figure 7., a decrease in the spin-ning line tension increases both the tensile strength and the elongation values on each isotherm in the whole temperature range. The changes in strength are, however, the biggest on the isotherms prior to the strength maximum.
In the beginning of the subexample, some factors affecting the tension in spinning have been listed. When manufacturing high-strength, high-oriented monofilaments, tension at spinning is of importance as regards the orientation development of the fibres. In the manufacture of fibres with a fairly low orientation, the effect of easily implemented changes in the spinning tension is small. In the analysis of the thermobonding strength of multifunctional fibres, the effects of any changes in spinning tension during fibre manufacture are often difficult to distinguish from a variety of other affecting factors. In the regulation of the thermobonding strength values, the regulation of the spinning tension is, however, of importance especially in the manufacture of disposable fabrics when regulating the required low limit strengths thereof.

Example 2

In the example 2, factors affecting thermobonding of some fibres with skin-core structure are studied. The subexample is directed at studying the behaviour in thermobon-ding of 'concentric layer fibres' containing either two different or two similar polymers, and one similar polymer, but modified after spinning.

Subexample 2.1

In the subexample 2.1, bi-component fibres are studied which have been prepared from two polymers of a different quality (mutually dissolving to a limited degree) by using concentric dies in spinning. The polymer pairs forming the skin-core-structure of the fibres to be thermo-bonded were polyethylene/polypropylene (PE/PP-fibres I-1 and -2, Table 2.), polyethylene/polyethylene terephtalate (PE/PET-fibre I-3) and polypropylene/polyethylene terephtalate (PP/PET-fibre I-4).
The constant values of the bonding equations /12/ corresponding to the strength values (σ ja ∈) of the fabrics obtained in thermobonding of the polyethylene/polypropylene-test fibres I-1 and -2 are indicated in the Table 7. The graphs for the equations and some measuring points are indicated in the Figure 8. From the Figure it can be seen that the measuring results for both test fibres are similar within the accuracy of measurment used. There is a transitional region between the measuring values corresponding to the low and the high temperature ranges of the equations, in which range the measuring values satisfy neither equation. It is also to be noted that the activation energy terms for the bonding equations corresponding to the tensile strength of both measuring ranges are of the same sign, wherefore the tensile strength increases in both ranges when the temperature increases.
In order to clarify the operability of the bonding regions for the two-component-test fibres, a number of TMA-loop test were carried out with these fibres (subexample 1.5). The constants for the bonding equations corresponding to the loop tests are indicated in the Table 8. Also the DSC-analyses of the test fibres have been indicated in the same table, from which the melting and solidification points of the polymers (enthalpy peaks) and the degrees of crystallinity can be seen.
In order to understand the behaviour of the melt phase which facilitates thermobonding, the Figure 9 includes a calculation of the temperature distributions of a cross-section of a bonding point of a pure polypropylene fibre fabric (normal to the fabric surface) as a function of the transitional speed of the fibre web. In the calculation, the known Binder-Smith differential method has been used, and on the starting values, the Gröber distribution function. It has been assumed in the calculations that during each delay time determined by the web speed, the quantity of heat needed for bonding has been transferred to the bond. In the calculations, the temperature difference between the bonding rolls has been defined as Δϑ = 15°C. The strong anomaly of the heat transfer rates and specific heat values in the range studied, results in a qualification of the calculated values, but the necessary essential data is obtained from the distribution. The temperature profile of the bonding point resembles, according to Figure 9, to its shape rather a parabola. When the bonding velocity increases, the temperature decreases rapidly in the interior parts of the bond close to room temperature and increases substantially in the surface areas of the bond and on the surface (with the amount of heat supplied for reaching heat equilibrium). In the system, the temperature difference between the roll surfaces does not seem to have a big effect on the position of the temperature minimum in the cross-section. It is evident, based on the calculated temperature profile, that in the interior parts of the cross-section of the bonding area, the temperature does not increase at technical bonding velocities to such a high value that partial melting of the polymer (PP) would produce the amount of melt needed for bonding. It is thus necessary to provide molten polymer (from either fully or partly molten fibres) from the surface areas of the bond which are at a sufficiently high temperature, which molten polymer under the effect of the compression and shear of the bonding rolls is transferred to the interior part of the fibre aggregate of the bonding area, to cause cold and hot bonding between the fibres. This "thermobonding mechanism" which is conventional for one-component fibres is applicable also to the two-component fibres being studied.
From the TMA-results from the loop tests for the PE/PP-fibre system it can be seen that the elongation values of the system reach a minimum at the temperature range ϑ = 90°-100°C. In the usual thermal contraction graph following the elongation minimum (ϑ = 95°-161°C) there is an anomalous temperature range between ϑ - 118°-125°C. The anomaly begins as a contraction inversion point at the temperature ϑ = 118.7°C, continues to the contraction peak at the temperature ϑ = 122°C and ends in a small elongation minimum at the temperature ϑ = 125°C. This region results from the melting of the shell layer of the fibre system.
In the loop tests the bonding of the fibres starts at the temperature appr. ϑ = 112°C and the bonding strength increases in relation to the partial melting of the polyethylene layer in accordance with the bonding equation I-1 (Table 7, I-1:3). After the ethylene melting range, the increase in bonding strength as a function of temperature is divided in two areas both with respect to the strength values and the activation energies (Table 7, I-1: 1 and 2). The loop bond strengths can be approximately described with the equations (ϑ - 127°-160°C): $σ_{1} = 18.18 ϑ - 1581.6$
and $σ_{2} = 21.95 ϑ - 2443.7$
In the subexample 1.5 it was established that the loop bonding primarily corresponds to hot air and oven bonding, which differs from roll bonding primarily in the absence of compression and shear stress. No transitional ranges for tensile strength and elongation, as in fabric tests, exist as such in the loop bonding of the 2-component fibres studied. After melting of the polyethylene phase, there is a strengthening of the loop bonds following an increase in temperature (negative E-values) in a manner corresponding to fabric bonding. Thus the ranges immediately following melting could be considered to be transitional ranges, where the values for the loop strengths fluctuate and the elongations are low. The observed scattering of the strength values in this range is a consequence of both the increase of the PP-solubility in the PE-phase as a consequence of the long melting times in the loop tests and the solidification of the PE+PP-melt phase as a separate or as the same crystallization as a result of the slow cooling of the molten phase (separate crystallization: PE solidifies in the area of 115-119 °C and PP in the area 136-140 °C, aggregate crystallization: PE+PP solidifies in the area of 115-119 °C). A corresponding phenomenon did not exist in the thermobonding of some production fibre batches. It is also to be noted that the strong increase in the melt portion (mutually soluble PE+PP-partial melt) cannot be advantageous in thermobonding. In the loop tests, excess melt can flow into the fibre bundle above and below the bonding point. From the high temperature range TMA results of the loop bonding tests, a partial melting of polypropylene is evident as well as an associated substantial increase in thermal contraction and also loop bonding strength of the system.
The bonding tests for webs corresponding to the test fibres I:1-6 have been carried out using a temperature difference between the smooth and the patterned roll of Δϑ = 7°C. When evaluating the significance of the magnitude of this gradient in thermobonding, it could be seen that when decreasing the temperature gradient by increasing the temperature of the patterned roll, both the tensile strengths and the elongations increased strongly. This is an important observation especially as regards fabric elongations, as these usually decrease when the temperature increases. According to the measurement results, the following equations are obtained for illustrating the effect of the gradient Δ(=ϑsi-ϑpi) in the thermobonding of the test fibres I-1 and -2 (T=Tsi): ${σ = 8.8289 x 10}^{7} {x T}^{2} {x exp[-8.1018 x 10}^{-2} x Δ] x exp[-23054.5/RT]$
${∈ = 1.7087 x 10}^{-5} {x T}^{2} {x exp[-3.6862 x 10}^{-2} x Δ] x exp[+1720.7/RT]$
The effect of the web speed on the thermobonding of a PE/PP-bi-component fibre was studied in a test run on production scale. The increase of the web speed in the interval v = 36-60 m/min weakened both the tensile strength and also (slightly) elongation values of the fabric under all conditions.
In this connection it can also be observed that the structural factors of the polymer affecting solidification and partial melt temperature, and thermobonding tensile strengths, are, in addition to the quenching temperature and rate and crystallinity degree, the values for the polymer chain length and distribution. Especially a high value for the chain distribution is a prerequisite for the good strength properties of the fabrics. For bi-component fibres this applies especially to the polyethylene of the shell layer, the dispersion value of which is low for the common qualities. As examples, some chain size values (Mw/D) for polyethylene can be given: 33110/2.56, 42650/2.94, 55500/3.35, 48960/4.36, 56000/5.17 etc.
In the evaluation of the thermobonding of the fibre webs the temperatures of the bonding roll have been used (ϑsi). In the Table 7., the equation constants for the bonding equation of the fibre I:1-2 have been calculated by subtracting 15 °C from the temperature of the smooth roll, that is T = Tsi-15, in order to approximate the real temperature in the bond. From the Table it can be seen that the effect of the temperature change on the activation energy values is quite small. The differences in the activation energies for loop and web bonding cannot be explained as resulting from the temperature differences in the system.
In the Table 7, the equation constants corresponding to the bonding test series of some PE/PP-bi-component fibre batches (I:6-14) are given. From the Table it can be seen that the activation energy values for the bonding equations for the fibre batches differ substantially from each other, especially at the low temperature range. These differences are due to the already observed differences in melt quantity and solidification, as well as the presence of different surface active (finishing agents, antioxidants, etc).
Bi-component fibres having a fibre core of polyethylene terephtalate and a shell layer of either polyethylene or polypropylene, behave, according to the TMA results, in thermobonding in an analogous manner to the fibre systems studied earlier. The polyester fibre did not, however, participate in the bonding process in the temperature range studied. When the temperature increases, melt bonding of the PE- and PP-phases of fibres coated with polyethylene and polypropylene (I:3-5) is followed by a decrease in the bonding strength both as a result of ageing and for other reasons (Table 8.). In the fibre web bonding tests corresponding to both fibre qualities, the bonding of the shell layer at its melting range corresponds to the PE/PP-fibre system (for example I-3 and I-6). Contrary to the loop tests, however, for PE/PET-fib-res, also after melt bonding of the shell layer, the tensile strength and elongation values of the fabrics show a slight increase, which could be due to differences in processing times (ageing and other phenomena). It can also be said that decreasing the temperature difference between the bonding rolls did not significantly affect the fabric strength values in either PET-core fibre (contrary to the PE/PP-system).

Subexample 2.2

In the subexample 2.2, the thermobonding of two skin-core fibre series are studied, which have been made in a different manner from polypropylene.
In the first fibre series (Table 2., D: 7-12) the skin core structure is obtained as a result of polymer chain degradation resulting from peripheral oxidation and simultaneous retarded quenching of a superheated spinning fibre. Thus, onto the fibre surface, a layer of a low melting, short-chain polymer is formed, which is coherent with the long-chained core polymer. The product fibre has a high crystallinity degree. The manufacturing method of the peripherically oxidized layered fibre has been disclosed i.a. in the description and the prior art section of US patent 5,281,378/1994.
In the second fibre series (Table 2., B: 1,4,6,8,12) the skin-core structure is obtained in a simultaneous high-speed spinning process of a very short-chained and a long-chained polymer. The product fibre has, in this case, a low monoclinic crystallinity degree and a 'para-crystalline' superstructure. The values for the constants of the bonding equations corresponding to the tensile strengths of the fibres produced by the peripheral oxidation method are given in the Table 9., wherefrom also the DSC-analysis results of the corresponding fibres can be seen.
The following observations can be made based on the results of spinning and thermobonding of the fibres from the oxidation test series, which also affect the regulation method:

The spinning of the sample D121 from Table 9. has been carried out at the temperature ϑ = 292°C, at a spinning draw ratio of, λk = 210 and nozzle speed of, v = 0.389 m/min, without decreasing the cooling rate. The spinning of the sample D100 has been carried out at the same melt temperature as that of the sample D121, but the quenching of the sample has been retarded by lowering the nozzle speed to the value v = 0.25 m/min and by reducing the quantity of cooling air (21°C) by about 50 % from its starting value (V = 735 m³/h at the capacity, P = 31.8 kg/h, d = 2.2 dtex, λ = 1.15), whereby the spinning draw ratio has increased to the value λk = 1630.

From the Table 9 it can be seen that due to the changes, the value for the activation energy (E, kcal/mol) for the bonding equation corresponding to the fabric tensile strength has decreased from the value |E| = 48.7 to the value |E| = 20.8. Correspondingly, the maximal values for the tensile strength and elongation have changed with the value intervals σ: 39.5-56.5 N and ∈: 72-89 %. Especially to be observed is the decrease of the temperature corresponding to the tensile strength maximum as a result of retarded quenching with the value interval ϑ = 157.9°-153.0°C.
In the preparation of the fibre sample D12, retarded quenching and decreased nozzle speed have been used, but simultaneously the temperature of the spinning melt has been decreased by 7 °C. The maximal values for the tensile strength and elongation of the bonding equation then decreased substantially, that is to the values σ = 27.7 N and ∈ = 34.0%. The sample D12 is not directly comparable to the sample D100, as their draw ratio values in manufacture differed from each other.

In the Table 9 for the fibre samples D: 7-11, the spinning conditions in the oven are mutually similar, but the quenching of the fibre melt is retarded primarily by decreasing the quantity of quenching gas (pressure p = 300-50 Pa, line tension, SLT=18-11 N). In some of the samples, also the mechanical draw ratio has been lowered. According to the Table, the draw ratio and elongation maximum values change only slightly when lowering the quantity of quenching gas. The temperatures corresponding to the maximum values do, however, change considerably. As a result of chain degradation following peripheral oxidation, the melt indeces of the fibre samples (MI, ASTM D1238) increase rapidly as a function of oxidation. The following equation is obtained for the activation energies of the bonding equations and the melt index ratio S_MI (fibre melt index/melt index of original polymer) of the corresponding fibre samples within the limits of the applied measuring accuracy: ${E = -77906 + 15519 x S}_{MI}$
A similar equation can be obtained also between the melt index ratio and the bonding temperatures corresponding to the maximum values for the strength values of the bonding equations.
The correlation between the maximum values for the fabric tensile strength and the crystalline lamellar thickness as observed in the subexample 1.4 applies and can be verified in those cases where it can be determined by SAXS-analysis in accordance with the scattering layer (mostly with a combined SAXS- and Raman spectrum analysis). Also the correlation between the mechanical draw ratio and the tensile strength maximum values seems to apply.

The behaviour of skin-core fibres made from polypropylene by means of a spinning and quenching process in the ther-mobonding of a fibre web and fibre loop is studied briefly. The WAXS analysis of the test fibres usually shows a smectic structure and thus very low monoclinic crystallinity. The SAXS analysis, on the other hand, shows structure dependant anomaly. The SAXS peak of the first order is difficult to establish from the intensity and angle values. From the Lorenz-corrected intensity values, besides the first order peak, usually a strong peak is obtained, which primarily is zero angle scattering. The SAXS-anomaly is a result of the differences in the lamellar structural systems between the shell and the core layers, of which a scattering sum is obtained in the analysis.
Some of the properties of fibres belonging to this structural group and the standard constants of the bonding equations are given in the Tables 2. and 10.
(B:1,2,4,6,8,12).
The test fibres were made from the same polymer quality (M_W = 225000, M_n = 37600, D = 5.93). The strength values of the test fibres essentially satisfy the equation σ = 771.7 x ∈^-0.6117, (amN/dtex; ∈ %). A clear yield limit characterizes the tensile strength-elongation ratios for the fibres. The test fibres comply with the correlations between the fabric strength and fibre structure as disclosed in the subexample 1.4, that is σmax = F(D_(z), λ, fav). From the point of view of the regulation method, of crucial importance is thus the regulation of the crystalline la-mellar thickness, in addition to the regulation of the spinning and mechanical draw ratio.
Based on the results of the thermomechanical loop tests of the fibre series B under study, it can be seen that the elongation maximum values of the various fibre qualities differ from each other both as regards position and magnitude. Also in the fibre-specific elongation results there seems to be scatter apparently due to the nature of the loop bonding (low initial load: 1 mN/tex). The position of the thermomechanical contraction with respect to temperature is (for the same fibre quality) the same for different fibre samples, and there is no scattering. The interbonding of loop fibres always starts at temperatures of the base part of the TM-contraction peak and reaches its maximum strength usually at the temperatures of the TM-peak. When the temperature increases above the temperature corresponding to the TM-contraction peak, the bonding strength decreases instantly.
According to the Table 10., the values for the activation energies for the loop and web bonding (Tables L and Y) of the fibre qualities B-4, -6 and -8 are for each fibre close to each other. In order to allow for a comparison of bonding, the bonding temperatures corresponding to loop strengths of 100 mN and 1000 mN have been calculated into the Table from the bonding equations. These strength values are close to both initial and terminating strength development. According to the Table 10., bonding with the said fibres starting at the temperature 131.4 ± 0.4°C reaches the limit 1000 mN depending on the activation energy values at the temperature range of 148-151 °C. The loop strengths of the fibre quality B-1 having the best bonding strength in fabric bonding appear at temperatures higher than the said loop strengths (Δϑ = 143.1-159.4°C). The fibre quality A-1 which bonds as a web at a very high temperature and low strength (Tables 1 and 2, loop: E =-38766, ln c = 40.1979, Δϑ = 136.6-156.5°C) is positioned at lower temperatures in loop bonding as compared to the fibre quality B-1. The favourable positioning of the fibre quaility A-1 in loop bonding is primarily due to the fact that in this manner of bonding, the finishing agent for the fibre surface does not disturb the strengthening of the bond as is the case in bonding under stress.
In loop bonding the fibre quality B-12 positions in a manner corresponding to the values of the fibre quality B-1. For this fibre quality, which is close to a conventional homogenous polypropylene fibre, also a high temperature 'region of ageing' can be observed.
In order to allow a comparison of the fibre series B and the test series already studied, some numbers for the bonding equations of the series and derivatives thereof are indicated in the Table 11. The bonding equation corresponding to the fibre denomination A-15 is an average of the bonding equations for 15 fibre batches of the A-series. The fibre C-2 is a bi-component fibre wherein both the core and the shell layer are of propylene polymers of two different melting ranges. In the Table, the derivative of the tensile strength of the bonding equation as a function of temperature (ϑ₃₀, °C) has been indicated, where the bonding strength corresponds to the minimum value, σ = 30 N, required (in this connection) for the fabric. Also the fraction of the maximum strength (α) corresponding to minimum strength has been given in the Table, as well as the derivative dα/dT, at the fraction corresponding to minimum strength.
From the Table 11. it can be seen that the temperatures corresponding to the maximum strength values are close to each other, i.a. for the fibre series B and the average value series A-15.
At the minimum level of fabric strength requirement (σ = 30N) the bonding strength in the B-1 series as compared to the maximum value is, however, only 37 % and in the A-15-series already 78 % (100α), the derivatives, dσ/dT being close to each other at temperatures corresponding to the minimum strengths. Even though the temperatures corresponding the maximum bonding strength are close to each other, a bonding strength corresponding to minimum level is reached in the B-1-series at a temperature which is 11.9 °C lower than in the A-15-series. With the A-1-fibre quality, which bonds only at high temperatures and has poor strength properties, the minimum strength is reached only at a temperature 18 °C higher than that of the series B-1.
The fibre quality B-2 in the Table shows the effect of the unusually low activation energy in bonding on the ϑ₃₀-temperature. In the fibre samples of the fibre series D, when the σm-values remain substantially of the same magnitude, a lowering of the ϑm-temperatures takes place. In the fibres of the D-series, when the thickness of the shell layer made from a short-chained polymer increases, the activation energy values decrease and a resulting increasing lowering of the ϑ₃₀-temperature as compared to the ϑm-temperature.
The essential observations relating to regulation method can be listed as follows:
Most of the variables and parameters affecting thermo-bonding strength are strongly correlated to the bonding temperature, which for this and for some kinetic reasons has been taken as the main variable in the regulation method. According to the measuring results, thermobonding can be considered a thermally activated process, which, due to the narrowness of the operational temperature range, complies with an Arrhenius-type temperature dependency.
In the dynamic regulation equation simulating the measuring results, the speed of the web has been used in place of the value for the linear heating rate of the bond, assuming a simple dependency existing between these speed values. According to measurements, this dependency can be described within the range studied, using in addition to a constant factor, a speed exponent having a value close to one.
As a result of mechanical drawing of the fibre subsequent to spinning, the bonding strength of the fibre web decreases exponentially as the draw ratio increases, which also shows that the best strength properties in nonwoven-fabrics are obtained with spinning fibres. Naturally, the draw ratio effect does not lower the bonding strength values when bonding, for example, set multi-component fibres with a skin core structure, wherein the bonding takes place by means of the skin layer phase having a lower melting point than the core layer phase, and at a temperature range exceeding the skin layer phase melting temperature.
The increase in strength properties as a function of temperature according to the bonding equation, is followed by a decrease in the strength values at high temperatures as a result of thermal ageing of the polymer, autogenic oxidation, mechanical deformation, increase in melt portion and other factors. For monitoring the strength properties of thermobonding in this temperature range, a regulation equation of the same form is used (for analogy and other reasons) as that used at lower temperatures. The intersecting point of these regulation equations is used in the method as the so-called regulation technical maximum strength, the value of which is usually close to the measured maximal strength value (and corresponding temperatures).
According to the experimental measuring results, the regulation technical strength values of the nonwoven fabrics and corresponding temperatures, are fibre-specific functions of the reciprocal value of the crystalline lamellar thickness of the polymer structure of the spinning fibre. According to the measurements (TMA, DMS), thermal bonding starts at a temperature range where the thermomechanical contraction and the disorientation of the molecular chains of the fibres start. This temperature also indicating the onset of partial melting of the polymeric structure, is a function of the reciprocal value of the crystalline lamellar thickness of the polymer structure. The crystalline lamellar thickness of the polymer structure is thus a very important regulation parameter in the regulation method.
The crystalline lamellar thickness of the polymer structure for use in the regulation method is expressed as the product of the Lorenz-corrected long indentity period and the crystallinity degree fraction of the polymer. Each product factor is determined directly from the fibre with x-ray diffraction analysis (SAXS and WAXS). For the regulation method, from the SAXS-measurement results obtained, an equation was developed for the determination of the long identity period. For regulation estimation purposes, sufficiently exact period values are obtained as a function of temperature and time from the equation in the thermobonding temperature range.
In order to determine the increase in crystallinity degree as a function of time and temperature, measurements were carried out on some fibre polymers. From the measurement results it was discovered that an Avram-type kinetic equation was applicable for monitoring the crystallinity degree of the secondary crystallization. The fibre polymers have, however, very little in common in their crystallization behaviour, wherefore, when using different fibre polymer qualities, the kinetics of crystallization after spinning has to be always determined.
The regulation of the crystalline lamellar thickness of the fibre polymer is based on the regulation of the values of its long period and crystallinity degree, both during manfucature of the spinning fibre and, if necessary, by heating the fibres in connection with processing under the control of the said period and crystallinity degree equations. In the regulation method it is possible to use in parallel the said period equation and the spinning fibre period values, while still maintaining a good regulation accuracy, irrespective of their different manners of derivation (and also of a slight magnitude difference from the point of view of regulation).
The long identity period of the spinning fibre polymer structure is almost solely a function of the degree of supercooling of the polymer melt and it can be regulated (especially with regard to the minimum value) by means of conventional regulation of the quenching temperature of the melt spinning fibre.
The crystallinity degree of the spinning fibre polymer is regulated by means of the quenching rate of the melt. It has been observed that the temperature of the spinning melt corresponding to the quenching rate and relevant for the future polymer structure corresponds to the temperature of maximum crystallization rate of the polymer. The desired spinning fibre structures and crystallinity degrees are obtained as an almost linear function of the logarithm of the quenching rate corresponding to this temperature.
The values for the activation energy of the regulation equation for thermobonding are dependent on the fibre structure and its defective states, on the bonding method (i.a. the applied bonding pressure), melt formation and - quantity and many other factors. For this reason it is advisable to determine the activation energy value for bonding of each fibre quality either with pilot or TM-loop measurements, or also from the temperature dependency of the unloaded thermal contraction of the fibre. Below the temperature corresponding to the maximum values for the strength properties in thermobonding, activation energy values for the longitudinal and transverse tensile strength are mutually of equal magnitude. The same applies for the product elongations. The activation energy values corresponding to the tensile strength and elongation values for the product fabrics can differ from each other substantially depending on the fibre quality (for example fibres with a plastic component and skin-core structure). At a temperature range exceeding the temperature corresponding to the maximum values for the strength properties, the activation energy values are usually below the values corresponding to low temperatures, and they are susceptible to pressure variations and melt quantities.
When estimating the strength values and the regulation equation for thermobonding for parameter measurements or product design, an activation energy value of E = 40 ± 3 kcal/mole can be used at the low temperature range, especially when the values for the maximum strength and corresponding temperatures are determined using the crystalline lamellar thickness of the fibre structure. In the high temperature range, the viscous flow values for the polymer quality can be used in the estimation, as well as the common dependency function, according to which the pre-exponential factor, as a logarithm, is the same linear function of the activation energy in both temperature ranges.
When measuring the strength properties in thermobonding it was observed that the nip pressure between the bonding rolls has a very little effect on the bonding strength values after exceeding a defined 'critical' pressure limit. In the measurements performed, this pressure limit was 50 ± 5 N/mm. In most of the test runs performed in this study, a nip pressure value of 60 N/mm has been used as a constant parameter. When operating above the temperature corresponding to the maximum for the strength properties and especially when the polymer melt quantity is big (skin-core structural and similar fibres), the bonding pressure has to be regulated (in a direction below the limit value) for optimum results.
In the method for regulating the strength values in ther-mobonding, the line tension from spinning fibre manufacture (SLT) and the roll temperature difference have been used as parametric variables, which both affect the tensile strength and elongation values of the nonwoven fabric. As parametric variables these are included in the pre-exponential term (c) of the temperature function of the regulation equation.
The spinning fibre line tension is a function of the molecular size and size distribution of the polymer chains and the cooling technique of the spinning melt. The line tension affects i.a. the molecular chain orientation of the spinning polymer and the spinning fibre strength via the lamellar crystal thickness formed in spinning. Depending on the spinning system used, the SLT-effect can be quite important in the regulation and standardization of fabric strength properties. Due to their multifunc-tionality, the SLT-values applied in spinning should preferably be used in fibre-specific parametric form in processing, but under continuous strict control.
The temperature difference between the web bonding rolls in processing is polyfunctional and determined by the nonwoven web speed and surface weight (i.e. temperature load), the required melt quantities, the assymetric temperature profile of the bond cross-section, the magnitude (and position in the bond) of fibre deformation, the processing apparatus (regulation accuracy) and other factors. In the method description, only one example of the use of this complicated temperature difference effect is given in the regulation of the fabric strength properties. The optimization of the necessary temperature difference is, however, always to be carried out specifically for each fibre and for this reason it is used in the method under study in parameter form in the overall regulation of the production process.
The regulation method for the strength characteristics of thermobonded fabrics has, in connection with this study, been primarily applied to web bonding by means of bonding rolls. In addition to roll bonding, thermomechanical test runs correlating directly with air and oven bonding have, however, been carried out as well as comparative measurements for the different bonding methods using gamma-irradiated fibres.
In the cases corresponding to air and oven bonding, the bonding strengths follow the conventional exponential dependency in roll bonding of the mechanical draw ratio from fibre manufacture (or correspondingly, from the average molecular chain orientation of the fibre structure) and decrease markedly when the draw ratio (chain orientation) increases. This draw ratio effect does not, however, apply to fibres with a skin-core structure, wherein the polymer of the skin layer exhibits a substantially lower melting range as compared to the core fibre polymer and a high proportion of molten phase at the bonding temperature (for example a polyethylene-polypropylene skin-core fibre). In this case thermobonding corresponds to bonding above the temperature of maximum strength values. These fibres can, if necessary, be drawn to a high chain orientation degree already in the spinning process, or also mechanically after spinning. The said exponential draw ratio effect disappears from the bonding strengths of irradiated fibres and the bonding strengths correspond to strengths of spinning fibres, that is the bonding strengths in air and oven bonding improved substantially as a result of irradiating the fibres. In addition to the improvement in bonding strength, also the bonding temperatures decreased, the decrease being a power function of the fibre draw ratio. In roll bonding, the strength properties of the irradiated fibres decreased substantially, but the bonding temperatures decreased in an advantageous manner as a function of the draw ratio in substantially the same degree as in air bonding.
The regulation of the strength properties of thermobonded nonwoven fabrics takes place under the control of the regulation equation, the main variables being the bonding temperature, the speed of the web and the mechanical draw ratio of the fibres to be bonded. Variable parameters which are more seldom subject to change in production processing, but none the less not less important than the said practical variables, are especially the crystalline lamellar thickness of the spinning fibre structure, the spinning line tension, the temperature difference between the bonding rolls and especially the pressure between the rolls when operating at a temperature above the temperature corresponding to the maximum strength values.
The variable parameters can, if desired, naturally be included in the regulation equation in a manner corresponding to the web speed and the fibre draw ratio, and this is also advantageous in product development when combining desired fabric properties in pilot and production tests under the control of the regulation equation.
In this connection it should also be observed that when manufacturing homogenous or layered fibres according to the present state of the art and using them as such or as fibre mixtures for thermobonded nonwoven fabrics, fabrics possessing the required strength properties and ratios thereof are obtainable by mutual adaptation of both the variables and the parameters of the regulation equation, using a number of different paths, whereby also various cost factors govern the processes chosen and applicable technique.
In the description of the method, the applicability of the new regulation method has been shown, both as regards the fibre and the fabric. It is to be observed that the very complicated regulation method of the invention for the manufacturing process of fibres and nonwoven fabrics to be made therefrom, can be varied in very many different ways, however, by remaining within the operational range disclosed in the examples and the claims.
An attempt has been made at providing, in the form of examples, a theory for the phenomena relating to the regulation method in order to provide a clear natural scientific picture of the method according to the new invention. It is, however, self-evident that there is no attempt at explaining all the phenomena relating to the method, either because of their unknown nature or lack of sufficient technical scientific measuring results, wherefore it is not possible to rely solely on the principles given in the description of the new method.

The following claims are presented based on the description of the invention and the presented examples.

TABLE 3

Symbols: α = 1 - exp [-Ktⁿ] K = K_o exp [-E/RT]
Cristallinity degree fraction: α; WAXS-crystallinity degree: χ, %; α = χ/χ_oo; speed constant: K; activation energy: E, cal/mol; time exponent: n = 0.075

The change of crystallinity degree of some fibre qualities as a function of time and temperature
Fibre quality	E cal/mol	K _o -	χ _o %	χ _oo %
Planar film	9597.5	2.831 x 10⁵	16.0
B - 1	5984.3	1.830 x 10³	18.48	55
B - 4	7671.7	2.557 x 10⁴	27.19	55
B - 5	7998.9	4.592 x 10⁴	30.86
B - 13	8132.0	4.475 x 10⁵	35.38
A - 1	6275.4	6.309 x 10³	43.64	57
A - 9	9666.3	2.616 x 10⁵	29.66

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Melliand Textilberichte, 10, 1994, 840-850; 11, 1994, 933-940; 12, 1994, 1015-1020; 1-2, 1995, 76-78; 3, 1995, 170-173; 4, 1995, 265-269
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Claims

Method for regulating the strength properties of a nonwoven product obtained in a process of thermobonding a fibre web of syntethic fibres, especially polypropylene fibres, in association with a method for regulating the structure of the fibre in connection with the fibre melt spinning and drawing processes, characterized in that
a. the strength characteristics of the nonwoven product obtained as a thermobonding product of a fibre web of constant weight, are regulated as a function of bonding temperature, web speed and fibre structure, in accordance with the following regulation equation
wherein φ means a strength property of the product fabric, here especially tensile strength (σ, N/50 mm) and elongation (∈, %); λ is the mechanical draw ratio in fibre manufacture; fav is the average chain orientation of the fibre polymer structure; v (m/min) is the speed of the web; T(K) is the absolute temperature; R(cal-jou-le/°K·mol) is the general gas constant; E(cal-joule/mol) is the experimentally determined activation energy; c, n a and a₁ are experimentally determined constant terms and wherein E contains, as a parameter, the bonding pressure and c contains, as a parameter, besides the pressure, the activation energy, the temperature difference between the rolls in roll bonding, the crystalline lamellar thickness of the fibre polymer structure, the spin line tension in spinning, web weight,

b. which regulation equation is increasing as a function of temperature, but is limited at high temperatures by a regulation equation of similar form which is decreasing as a function of temperature and simulates ageing, namely of the form φ' = c' x T² x exp [-E'/RT], wherein c', T, E' and R correspond to the terms c, T, E and R above, the intersecting point of the regulation equations representing the maximum value for the regulation technical strength characteristic of the nonwoven fibre and corresponding temperature,

c. the maximum value for the regulation technical strength characteristic is a function of the crystalline lamellar thickness (D_z, Å) of the fibre polymer structure, of the form σ_max (∈_max) = c₃ x D_z ^-n1, that is, the maximum value for the bonding strength is regulated by regulating the crystalline lamellar thickness of the fibre, in an operational range where the Lorenz-corrected crystalline lamellar thickness varies in the range, D_z = 20-70 Å and the exponent value n₁ = 0.85 ± 0.15,

d. the temperature corresponding to the maximum value for the regulation technical strength characteristic is a function of the crystalline lamellar thickness of the fibre polymer structure, of the form T_max = T - c₄ D_z ^-1, that is the bonding temperature corresponding to the maximum value for the bonding strength is regulated by regulating the crystalline lamellar thickness of the fibre, in an operational range, where the Lorenz-corrected crystalline lamellar thickness varies in the range, D_z = 20-70 Å,

e. the thermobonding being carried out in a fibre-specific manner starting in the experimentally determined temperature range, wherein the thermomechanical contraction of the unloaded fibre and simultaneously the disorientation of the fibre polymer molecular chains start, and wherein the melting point of the partial regions of the fibre matrix decreases linearly as a function of the reciprocal value of the crystalline lamellar thickness passing through the polymer melting point,

f. and the activation energy values (E) and the pre-exponential factors (c) of the regulation equations corresponding to the temperature ranges below and above the maximum point for the strength values, satisfy in a polymer-specific manner the same equation ln c = -c₅ E - c₆, wherein c₅ and c₆ are experimentally determined constant terms.
The regulation method according to claim 1, characterized in that the maximum value for the thermobonding strength is regulated as a function of the mechanical draw ratio λ in fibre manufacture, and thus also as a function having the same form of the chain orientation of the fibre structure.
The method according to claim 1, characterized in that when bonding one-component fibres, desired fibre-specific thermobonding strengths which are lower than those of spinning fibres, are obtained by regulating the mechanical fibre draw ratio in the range λ > 1.0.
The regulation method according to claim 1, paragraphs a. and b., wherein the strength properties of a nonwoven fabric obtained in thermobonding is regulated in accordance with a regulation equation having the bonding temperature, the speed of the web and the mechanical draw ratio of the fibre as primary variables, characterized in that
a. the variation ranges of the parametric variables in the regulation equation are:
web weight (w): w = 15-30 g/m³
the nip pressure between the bonding rolls (p): p = 50-75 N/mm
the temperature difference (ϑ) between the bonding rolls: $Δϑ = 2-15 °C$
spin line tension (SLT): SLT = 300-3000 µN/filament

b. the speed of the web (v) varies in the range:
v = 20-150 m/min., the corresponding speed exponent (n) varying for an one-component fibre in the range: n = 1.0-1.6

c. the mechanical draw ratio in fibre drawing (λ) varies in the range λ = 1-5, the corresponding constant coefficient (a) varying for different polymer qualities prior to the maximum for the strength values in the range a = 0.75-1.25 and after the maximum in the range: $a = 0.40-1.25$

d. the bonding temperature (T) of the fibre web varies in the range: T = 375-450 K

e. for a stabilized fibre structure, at a nip pressure of p = 60 N/mm, prior to the maximum for the strength values, the value for the activation energy (E) in thermo-bonding is: E = 39 ± 4 kcal/mol; this value decreases in the range (1-0.25) x E, the deformative states of the fibre structure increasing; after the strength value maximum, when the partial melt quantity increases, the activation energy value varies in the range: (0.2-0.5) x E, where the lowest value corresponds to the activation energy of the viscous flow of the polymer

f. the values for the constant factors in the dependency function between the activation energy values and the pre-exponential terms in the regulation equation are: $c_{5} = 1.160 ± 0.0$
$c_{6} = 8.400 ± 0.200.$
The regulation method according to claim 1, paragraphs c., d. and e., where the regulation of the strength properties of the nonwoven fabric obtainable in thermobon-ding takes place by regulating the crystalline lamellar thickness of the polymer structure of the spinning fibre, that is the product of the long identity period and the crystallinity degree fraction of the polymer structure in the operational range, D_z = 20-70 Å, characterized in that
a. in order to obtain the desired crystallinity degree (f_c), a conventional spinning fibre is air-quenched so that the quenching rate (Ṫ) of the polymer is maintained in the vicinity of the temperature (T_c, max, K) corresponding to its maximal crystallization rate (T_c, max = 0.8 x T_MP), whereby the desired crystallinity degrees are obtained as a linear function of the logarithm of the quenching rate
f_c: < 0.2; Ṫ₉₀ > 100°C/s;
phases: smectic + amorphous
f_c: 0.2-0.4, 10°C/s < Ṫ₉₀ < 100°C/s;
phases: smectic + monoclinic + amorphous
f_c: 0.4-0.6; Ṫ₉₀ < 10°C/s;
phases: monoclinic + amorphous

b. in order to obtain the desired crystallinity degree (f_c) the spinning fibre is air-quenched in a conventional manner so that the spinning fibre is quenched in the velocity range, 10°C/s < Ṫ₉₀ < 100°C/s, whereby there is formed a three-phase equilibrium and a corresponding degree of crystallinity, and the final desired degree of crystallinity is obtained by heating the fibre at a temperature indicated by the Avram growth equation to the range 60-110°C in a delay time determined by the line speed,

c. in order to obtain the desired long identity period for the polymer matrix of the spinning fibre in the range, L_z = 90 Å - 160 Å, the spinning fibre is air-quenched in a known manner, using subcooling of the spin melt as the only parameter for the period length, in the temperature range, ϑ = 175-110°C.

d. in order to obtain the desired long identity period for the polymer matrix of the spinning fibre in the range, L_z = 90 Å - 160 Å, the spin melt is quenched in the two-phase range: smectic + amorphous, and the fibre is heated, in accordance with the temperature-time-dependency of the long period in the temperature range ϑ = 25-135°C, in order to obtain the desired final period length.
The method according to the claim 1 for regulating the thermobonding strength of a fibre web made from fibres with a skin-core structure, characterized in that
a. the bonding is carried out under the control of the regulation equation in a temperature range which exceeds the melting range of the skin layer,

b. the fibre is drawn in spinning and/or thereafter mechanically, using a draw ratio corresponding to the desired average chain orientation and is stabilized thermally at temperatures below the melting temperature of the skin layer.

7. The method according to claim 1 for regulating the thermobonding strength of a fibre web made from polyethylene/polypropylene or-/polyester fibres with a skin-core structure, characterized in that
a. the bonding is carried out under the control of the regulation equation in a temperature range which is below the maximum melting point of the polyethylene,

b. in the bonding, polyethylene qualities are used the weight average molecular weight and dispersion of which vary in the range M_W = 25000-60000 and D = 2.5 - 6.0.

8. The method according to claim 1 for regulating the thermobonding strength of a fibre web, characterized in that
a. the bonding temperature of fibres for use in hot air and/or oven bonding of a fibre web is lowered by irra-diating the fibres with gamma-rays prior to bonding, at a rate of 2-4 Mrad,

b. the fibres are drawn prior to irradiation during spinning (λ = 90-1600) and/or thereafter mechanically so that the average chain orientation of the product fibre is in the range fav = 0.4-0.8.