WO1986007070A1

WO1986007070A1 - Using lignin as a reaction component for producing polyurethanes

Info

Publication number: WO1986007070A1
Application number: PCT/SE1986/000237
Authority: WO
Inventors: Hirohiza Yoshida; Knut Kringstad; Roland MÖRCK
Original assignee: Svenska Traforskningsinstitutet
Current assignee: Svenska Traforskningsinstitutet
Priority date: 1985-05-29
Filing date: 1986-05-21
Publication date: 1986-12-04
Anticipated expiration: 1987-11-29
Also published as: SE8502651D0; SE8502651L

Abstract

Producing polyurethanes by polymerizing polymeric isocyanate, polyol and kraft lignin or some other type of technical lignin, in which the lignin used has been freed substantially of its high molecular weight fractions by means of some suitable method herefor. The polyurethane produced exhibits good mechanical properties and high thermal stability.

Description

DESCRIPTION

Using lignin as a reaction component for producing polyurethanes

The present invention relates to the production of polyurethanes, and more particularly to the production of polyurethanes in which lignin is used as a reaction component.

In the past 50 - 60 years many attempts have been made to incorporate lignin in polymeric materials. These materials can be divided into two categories of use:

1 ) materials in which lignin is used as an additive or a filling substance; and

2) materials in which lignin has been reacted with mono¬ mers or polymers to form a polymeric network. One example of the former category is the use of lignin as a filler in rubber used in the manufacture of motor-vehicle tires. An example of the latter category is the use of lignin as a substitute for phenol in phenolic resins, or as a polyol component in polyurethanes. In this latter case, however, the lignin has a low reactivity and the resultant material is brittle, this brittleness increas¬ ing with increasing lignin contents. Attempts have been made to overcome these drawbacks, by chemically modify¬ ing the lignin. These attempts have met with no success, however. This low reactivity of lignin is not solely due to the steric hindrance of the methoxy group in the proximity of the reactive phenolic hydroxy group, but also to the configuration of the lignin in the solid phase of the reaction system in the' production of phenolic resins and polyurethanes.

Polyurethanes are polymers which contain the group

H 0 I It

-N-C-0- for ed, for example, by reaction between a diisocyanate and a diol: n OCN-R-CNO + N HO-R ' -OH ■> {oCONHRNHCOOR ' j- n where R and R' are organic groups.

When the reaction between the diisocyanate and the diol is performed under equivalent mole-ratios

(NCO/OH = 1.0) a linear polyurethane is obtained. The polyurethane, however, is normally cross-linked to vary¬ ing degrees of density and is therefore a thermoset.

A cross-linked polyurethane is obtained by carry- ing out the reaction in the presence of an exessive quantity of isocyanate (NCO/OH > 1.0).. When isocyanate is introduced into the system in excessive quantities, the reaction takes a complex course. This is due to subsidiary reactions which, inter alia, may result in the formation of allophanate and biuret structures. Allophanate-formation:

0 If

/ NHC-O -->+ OCNΛ>- - ~ N- ?C-0 ~*>

C-NH ' II

0

Biuret-formation:

In commercially available polyurethanes the tri- functional allophanate and biuret-bonds are able to contribute to the genesis of the network structure. The degree of cross-linking, or cross-linking density, increases generally with an increasing NCO/OH-ratio. The mechanical properties of the polymers are highly dependent on the cross-linking density. An increase in cross-link¬ ing density results in a higher Young's modulus and increased maximum strength. The polymers also become brittle, however, as a result hereof.

In recent years diisocyanates have been replaced to a large extent with polymeric isocyanate. This change from diisocyanates to polymeric isocyanate was made for reasons of an economic and environmentally technical nature. When the polymeric isocyanate incorporates aromatic rings in its principal chain, there is normally obtained a rigid polyurethane.

In the presence of excessive polymeric isocyanate solely rigid polyurethanes are -formed. A soft polyurethane can be obtained by using a linear polyol or a polyester polyol. Another method of obtaining soft polyurethanes is to reduce the NCO/OH-ratio. However, it is difficult to produce strong polyurethanes under such conditions, because the mechanical properties of the polyurethanes depend to a large extent on the NCO/OH-ratio.

When carrying out the investigations which resulted in the present invention described here, it was found that when:

1 ) the high molecular fraction of the lignin is separated by fractionation or in some other way, and

2) the reaction between isocyanate and lignin is, preferably, effected in solution or in a swollen state, the reactivity and behaviour of the lignin is improved considerably when used as a component in polyurethane materials.

When undissolved lignin particles are present in the reaction system, as is the case when using high- molecular weight kraft lignin fractions or non-fractionate kraft lignin (isolated by acidification of kraft black liquor) , the resultant polyurethanes are heterogenec-us and show a tendency towards -phase separation. When examining such heterogeneous polyurethanes under a scanning electronmicroscope and by measurement with an "Instron" instrument, it was observed that fracture of the polyurethane occurs at the point at which phase separation takes place, and will commence at low load levels.

Accordingly, the present invention relates to a method for producing polyurethanes by polymerizing polymeric isocyanate, polyol and lignin, ^"characterized by using as a lignin reactant lignin which has been substantially freed from fractions of high molecular _^ weight. By "high molecular weight" is meant in accordance with the invention primarily molecular weights above 10,000, determined in a manner herein¬ after described.

Molecular weight fractionation of lignins can be effected in differing ways. Examples of such methods are solvent fractionation, ultra-filtration, fractionated precipitates of spent kraft liquor (black liquor) , and preparative GPC. In principle, solvent fractionation can be effected in two different ways, firstly by successive extraction with differing organic solvents or combinations thereof, and secondly by successive precipitation from a solution. Fractionation by successive extraction can be carried out with solvents or solvent combinations other than those used in the working example of this specification. Successive extraction can either be carried out in a Soxhlet apparatus or by applying the type of technique described in the following example. The method described in the working example of this application has been preferred to the Soxhlet-extraction process, since the preferred method has been found more gentle (in the Soxhlet- extraction process the extracted material is dissolved in a boiling solvent for several hours) , although it will be understood that the invention is not restricted to this method.

The molecular weight fractionation of kraft lignin by successive extraction with an organic solvent can be carried out, for example, in the follow¬ ing manner.

In this example fractionation is effected in four separate extraction stages: The lignin is first extracted with dichloromethane (fraction 1), then with n-propanol (fraction 2) , methanol (fraction 3) and dichloromethane/methanol 3/7 (fraction 4) . Fraction 5 consisted of the undissolved residue obtained from these four extraction stages. The yields obtained in the various extraction stages and the molecular-weight data relating to the fractions obtained are set forth in Table 1. Table 1

Fraction Solvent Yield Molecular weight No . % M data* n M w . M w /' M n

1 Dichloromethane 9 422 580 1 .38

2 n-Propanol 22 789 1085 1 .38

3 Methanol 26 1265 2139 1 .69

4 Dichloromethane/ methanol 28 3767 81919 21 .7

5 14 5796 183958 31 .7

Starting material - 1403 39132 27.9

* Molecular weight data for fractions 4 and 5 and for non- fractionated starting material has been measured on the acetyl derivative of its lignin.

It will be seen from Table 1 that the fractions 1-3 have low molecular weights with relatively narrow molecular weight distributions (^M _w/^M _n 2). The fractions 4 and 5 of higher molecular weights have considerably broader molecular weight distributions, encompassing molecular weights from some thousands to several millions. In fraction 5 about 15 - 20 % of the material has molecular weights above 100,000.

The molecular weight data recited in Table 1 was obtained with the aid of GPC (Gel Permeation Chromotography) . The GPC-columns were calibrated with p ^roly - styrene standards . Mn and Mw were calculated with the aid of a computer program.

With regard to the yields obtained in the various extraction stages, it will be seen from the Table that dichloromethane extracted 9 % of the starting material, n-propanol 22 %, methanol 26 %, and dichloro- methane/methanol (3/7) 28 %. The undissolved residue

(fraction 5) comprised 14 % of the starting material. It should be pointed out_.that different amounts of kraft lignin and also different types of lignin can provide mutually different yields in the various extraction stages depending on differences in molecular weight distribution and chemical structure.

According to the invention polyurethane (PU) are produced in solution by polymerizing polymeric isocyanate, polyol and fractions of kraft lignin. The solvent used may be, for example, tetrahydrofuran, acetone, dimethylfofmamide, or dimethylacetamide. The amount of lignin charged to the system varied from zero to about 40 % by weight, and the NCO/OH-ratio varied from about 0.5 to about 2.5 (in moles). Upon termination of the reaction, films were produced by evaporating the solvent ("solvent casting") and curing at elevated temperature. The polyurethane films which contain lignin from the fractions 1 to 3 are optically homo¬ geneous, whereas the films containing lignin from the fractions 4 and 5 and non-fractionated lignin are hetero¬ geneous. Films were also produced for the purpose of testing the mechanical and thermal properties of the polyurethanes. In the following Examples 1 to 3, rigid, flexible and soft polyurethane films, respectively., have been produced.

Table 2 illustrates the mechanical properties ^• of these polyurethanes. The samples listed within each section of the table have mutually different NC0/0H- ratios.

Cross linking NCO/OH Lignin density Youngs^' ^fex±πum Breaking ratio content Density (x 10 ~ 3 modulus strength energy

No. ( in moles ) (%fcy weight) fø-/αn ) πoles/cm ) (MPa) (MPa) ( 'J^"■")

1-1 1.524 0 1.179 1.17 700 33.4 0.635

1-2 1.499 8.1 1.194 1.32 1068 47.1 0.812

1-3 1.597 18.2 1.209 1.39 1145 36.3 0.367

2-1 1.201 0 1.171 0.75 202 20.1 0.402

2-2 1.187 9.0 1.185 0.89 435 35.9 1.880

2-3 1.187 15.0 1.202 0.95 867 44.5 1.310

2-4 1.149 17.4 1.208 1.01 900 40.2 0.933

3-1 0.548 0 1.121 0.16 - not measurable

3-2 0.559 8.4 1.145 0.41 2.6 1.6 0.101

3-3 0.518 17.2 1.173 0.49 30.1 8.1 0.804

3-4 0.534 26.6 1.197 0.63 295 16.8 0.686

3-5 0.483 33.9 1.218 0.65 595 23.2 0.180

Example 1 refers to rigid polyurethane. Example 2 to flexible polyurethane and Exaπple 3 to soft polyurethane, respectively.

SUBSTITUTE SHEET A rigid polyurethane requires a high Young's modulus without an increase in maximum strength. Flexible and soft polyurethanesrequire high maximum strengths and breaking energies. At an NCO/OH-ratio of 0.548 a lignin- free polyurethane is too weak to allow its mechanical properties to be measured. As illustrated in Table 2, the addition to this composition of up to 27 percent by weight lignin results in a polyurethane possessing irtproved mechanical properties. The NCO/OH-ratio of the polyurethanes 3 - 4 recited in the table is only half the NCO/OH-ratio of the lignin-free polyurethanes 2 - 1. Despite this, however, the mechanical properties of the two samples are the same. This shows that the addition of lignin is a useful way of obtaining soft polyurethanes from polymeric isocyanate.

Further studies of the physical properties of the polyurethanes produced show that:

1. The thermal decomposition temperature of the lignin- containing polyurethane is approximately 10 - 15 C higher than the corresponding temperature of the lignin-free polyurethane. In addition, the rate of pyrolysis of the lignin-containing polyurethane is markedly lower than that of the lignin-free polyurethane. The increased thermal stability of the lignin-containing polyurethane may be due to the anti-oxidation properties of the lignin. The lignin-containing polyurethane exhibits a broad temperature band with respect to glass transition temperatures, as is normally also the case for inter¬ penetrating polymer networks. Such material is able, among other things, to absorb vibrations over a wide frequency band, and can be used effectively as a material for damping sound and vibrations. The physical properties of these materials are also stable over a wide range of temperatures. 2. The cross-linking density of lignin-containing poly¬ urethanes is much higher than that of lignin-free poly¬ urethanes for a given isocyanate/hydroxy (NCO/OH) ratio. This indicates that the lignin has reacted with isocyanate and contributed to the formation of the network structure. 3. An increase in cross-linking density results in an increase in the Young's modulus and maximum strength of the lignin-containing polyurethane. The maximum strength obtains a constant level independent of the NCO/OH-ratio and the kraft-lignin content.

Consequently, these results show that it is possible to produce lignin-containing polyurethanes having mechanical properties which are equivalent or superior to the mechanical properties of lignin-free polyurethanes. The use in polyurethanes of lignin which has been freed from its high molecular weight portion enables both the aliphatic polyol and the expensive polymeric isocyanate to be replaced in part with the lignin, without impairing the physical properties of the polyurethane material. The possibility of substituting lignin for the more expensive isocyanate is particularly beneficial from the aspect of economy. The advantages afforded by producing lignin- containing polyurethanes in accordance with the invention, where the lignin has been substantially freed from fractions having high molecular weight, can be summarized as follows:

1. It is possible to save not only polyol but also expensive isocyanate and still obtain polyurethanes which possess strength properties comparable with the properties of those polyurethanes which can be purchased commercially today.

2. It is possible to produce a soft and stretchable polyurethane, by using polymeric isocyanate in combina¬ tion with lignin of low molecular weight.

3. It is possible to produce a broad spectrum of rigid and soft lignin-containing polyurethanes.

4. It is possible to produce lignin-containing poly¬ urethanes having a wide range of glass transisiton temperatures. 5. It is possible to raise the thermal stability of the polyurethane, by incorporating therein lignin which has been freed from high molecular weight material in the polyurethane system.

5. 6. The lignin-containing polyurethane can be used within all areas in which polyurethane is normally used, e.g. polyurethane foam, binders, etc.

New usages of interest with regard to the properties of the material may also include

10 I) use of the material as a vibration damping material, or II) as a sound-proofing material.

Example of solventfractionation of kraft lignin

500 g of kraft lignin were slurried in 2.5 liters

15 of dichloromethane. The suspension was stirred for 30 minutes at room temperature.

The undissolved material was filtered off and re-slurried in 2.5 liters of dichloromethane. This suspension was also stirred for 30 minutes at room

20 temperature. The undissolved material was filtered off and the resultant filter cake washed with approximately 1 liter of dichloromethane.

The same procedure was repeated in the remaining extraction stages with the use of the other solvents

25 (cf Table 1) . The undissolved material was dried thoroughly and pulverized between each extraction stage.

Material which had passed to solution in the respective extraction stages was cleansed from solvent in three stages:

30 1. Vacuum evaporation with the aid of water suction.

2. Subsequent to vacuum evaporation the flasks contain¬ ing extracted material were connected to a high-vacuum pump for 14 - 16 hours at a temperature of 50 C. 3. The extracted material was removed from the evaporation flasks, pulverized and dried in vacuum over P₂ at room temperature for 20 hours.

Example 1 Manufacture of rigid polyurethane.

0.998 g of fractionated kraft lignin having

Mn = 845, Mw = 1700 and Mw/Mn = 2.0 (a combination of fractions 2 and 3) was dissolved in 15 ml tetrahydro- furan by stirring at room temperature. 5.037 g of propylene oxide-based polyol and 6.288 g polymeric isocyanate were added to the solution. The reaction was carried out by stirring the solution for 8 hours at room temperature. Subsequent to termination of the reaction, films were produced by evaporating the-solvent ("solventcasting") . The dry films were cured at 96°C for 8 hours.

The resultant film had a density and cross- linking density of 1.194 g/cm 3 and 1.32 x 10—3 moles/cm3, respectively. The Young's modulus, maximum strength and breaking energy were 1.07 GPa, 47 MPa and 0.812 J, respectively.

Example 2

Manufacture of flexible polyurethane.

0.5 g of fractionated kraft lignin having M = 845, M_τw. = 1700 and Mw/Mn = 2.0 (a combination of fractions 2 and 3) was dissolved in 15 ml of tetrahydrofuran while stirring at room temperature. 2.529 g of propylene oxide- based polyol and 2.5 g of polymeric isocyanate were added to the solution. The reaction was effected while stirring for 8 hours at room temperature. Subsequent to termination of the reaction, films were produced by evaporating the solvent ("solvent casting"). The dry films were cured at 96°C for 8 hours.

The resultant film had a density and a cross- linking density of 1.185 g/cm 3 and 0.89 x 10—3 moles/cm3 respectively. The Young's modulus, maximum strength and breaking energy were 0.44 GPa, 35.9 MPa and 1.88 J, respectively.

Example 3

The production of soft polyurethane. 1.0 g of fractionated lignin having M = 845,

M„w = 1700 and Mw Mn = 2.0 (combination of fractions 2 and 3) was dissolved in 15 ml of tetrahydrofuran, by stirring at room temperature. 3.238 g of propylene oxide-based polyol and 1.563 g of polymeric isocyanate were added to the solution. The reaction was carried out while stirring for 8 hours at room temperature. Subsequent to termination of the reaction, films were prepared by evaporating the solvents ("solvent casting"). The dry films were cured at 96 C for 8 hours. The density and cross-linking density of the produced film were 1.173 g/cm 3 and 0.49 x 10-3 mole/cm3, respectively. The Young's modulus, maximum strength and breaking energy were 0.03 GPa, 8.1 MPa and 0.804 j, respectively.

Claims

1. A method for producing polyurethanes by polymerizing polymeric isocyanate, polyol and kraft lignin or some other type of technical lignin, characterized in that the lignin used has beeri substantially freed of fractions of high molecular weight by some suitable process herefor.

2. A method for producing polyurethanes in accordance with claim 1 , characterized in that the lignin used has a molecular weight between 500 and 10,000.

3. A method for producing polyurethanes in accordance with Claim 2, characterized in that the lignin used has a molecular weight between 500 and 3000, preferably between 500 and 2000.

4. A method for producing polyurethanes in accordance with any of Claim 1 - 3, characterized in that the lignin used is kraft lignin.

5. A method for producing polyurethanes in accordance with any of Claim 1 - 4, characterized in that the molecular weight fractionation is effected by solvent fractionation.

6. A method for producing polyurethanes in accordance with any of Claims 1 - 5, characterized in that polymerization is effected in solution or in a swollen state.