WO2005105903A2

WO2005105903A2 - Low density white polyester film

Info

Publication number: WO2005105903A2
Application number: PCT/IN2005/000111
Authority: WO
Inventors: Nagnath Trimbakrao Lahurikar
Original assignee: Garware Polyester Ltd
Current assignee: Garware Hi Tech Films Ltd
Priority date: 2004-05-05
Filing date: 2005-04-11
Publication date: 2005-11-10
Anticipated expiration: 2006-11-05
Also published as: CN1693050A; TW200600321A; WO2005105903A3; KR20060045911A

Abstract

The present invention relates to a process of preparing non-laminated blended biaxially orientation extruded polyester film without any co-extrusion layers with the voids being dispersed in the monolayered film for applications as base material for label, poster, recording paper, wrapping paper including reflector of a surface light source for illuminating a scope of liquid crystal displays or electronic decorative boards. Such films exhibit % transmission of 10 % max, reflection at 450 nm, 550 nm and 650 nm of > 90 %. An appropriately blended composition comprising a mix polyester, a polycondensed resin of a dicarboxylic ester and polyhydric alcohols, density reducing polymers, optical brightners and inorganic particles is extruded and processed to produce the non-laminated biaxially oriented polyester film.

Description

LOW DENSITY WHITE POLYESTER FILM

Field of the Invention

The present invention relates to a non-laminated blended biaxially orientation extruded polyester film for applications as base material for label, poster, recording paper, wrapping paper including reflector of a surface light source for illuminating a scope of liquid crystal displays or electronic decorative boards.

Background and prior art

Several attempts have been made to prepare appropriate polyester films for such purposes.

JP2002098811 provides a white laminated polyester film for the reflecting member of a surface light source excellent in reflecting characteristics and hiding performance. This patent discloses a process wherein the white laminated polyester film is obtained by disposing a white polyester layer (A) containing at least inorganic fine particles (a) on one face of a white polyester layer (B) containing fine bubbles and disposing a white polyester layer (C) containing at least inorganic fine particles (c) on the other face of the layer (B). The refractive indexes of the inorganic fine particles (a), the inorganic fine particles (c) and the polyester represented by na, nc and np, respectively, satisfy the relational expressions nc-na>0.2 and nc-np>0.2

JP2002098808 provides a white laminated polyester film for the reflecting member of a surface light source excellent in reflecting characteristics and light resistance. In this patent the white laminated polyester film comprises of two or more layers of white polyester layer (A) on at least one face of a white polyester layer (B) containing fine bubbles. The white polyester layer (B) contains a light resistance-imparting agent

JP2001228313 discloses a white laminated polyester film for a reflection member of a surface light source excellent in reflection characteristics and light resistance. The white laminated polyester film for the reflection member of the surface light source comprises two or more layers including a white polyester layer A at least on one surface of a white polyester B containing fine air bubbles and is characterized by including a light resistant agent in the white polyester layer A.

JP2002040214 describes a member for surface light source reflection excellent in initial reflective characteristics, small in reduction with lapse of time of reflectance and luminance in long time usage and capable of maintaining high quality image for a long term. A white film for surface light source reflection plate is characterized in that a coating layer containing a substance having UV absorbing ability and a fluorescent whitening agent is provided on at least one surface of a white film having bubbles in its internal part

JP9001648 discloses a method of enhancing reflectivity and abrasion resistance such that particles do not fall off even by surface rubbing by specifying the surface glossiness of at least the single surface of a polyester film, the concentration of particles in the surface layer thereof and the refractive index in the thickness direction of the polyester film. The surface glossiness of at least the single surface of a polyester film is set to 5- 80% and the concentration of particles in the surface layer thereof is set to 0.5wt% or less and the refractive index in the thickness direction of the polyester film is set to 1.46- 1.54. Therefore, pellets of high speed crystallizing polyester A are dried to be supplied to a known melt extruder. Then, polyester A is laminated to at least the single surface of polyester B by using a two-or three-layered manifold or confluent block and a two-or three-layered sheet is extruded from a slit like cap and the extrudate is cooled by a casting roll to form a non-stretched film which is, in turn, heat-treated to be stretched

JP59195624 discloses a means of obtaining a liquid crystal display cell superior in sharpness and brightness and suitable for size enlargement and formation of a curved face by using two oriented plastic films with their principal orientation directions in parallel to each other, for the protective cover of polarizing films. An extruder and a die is used to form an unstretched plastic film and the film is stretched uni- or bi-axially to obtain two oriented films. One of the two may be raised in birefringency by raising stretching ratio a little or the like, or changed in film thickness by changing an extrusion amount of the extruder. Each of two polarizing films are applied to each of said plastic films to form two pairs of polarizing plates, with each oriented plastic films inside and the principal orientation directions in parallel to each other to form a liquid crystal display cell by joining the two pairs with adhesive or the like. High density PE, low density PE, PVC, and polyphenylene sulfide films, etc. are usable for said plastic film, and above all, a polyester film having polyethylene terephthalate component is preferable.

JP4239540 provides means to obtain a white polyester film capable of obtaining brighter screen having higher percentage of reflectance when used as a substrate for liquid crystal display reflecting sheet and optimum as the substrate for liquid crystal display reflecting sheet. The objective white polyester film in which average percentage of reflectance in wavelength area of light of 400-700nm on the surface is >90% and (maximum value - minimum value) of percentage of reflectance in the wavelength area is <10% and the objective white polyester film in which average percentage of reflectance in wavelength area of light of 330-380nm on the surface is >90%. The equivalent US Patent is US 5,672.409 describes a reflector used in a surface light source comprising a white polyester film in which fine voids are formed and whose apparent specific gravity is in the range of 0.5 to 1.2. The reflectance of the polyester film can be increased by the fine voids. When the reflector having a high reflectance is used in a surface light source having a side light system, a bright scope of, for example, a liquid crystal display easy to see can be obtained.

Most of the prior art teach that a laminated polyester film for the purpose necessarily needs to have a white polyester layer (A) containing at least inorganic fine particles on one face of a white polyester layer (B) containing fine bubbles and disposing a white polyester layer (C) containing at least inorganic fine particles on the other face of the layer (B). The lamination may be A/B or A/B/A. The system may be coated with a fluorescent whitening agent if required. The inorganic particles and the whitening agents must meet certain specific criteria to be used as the substrate for liquid crystal display reflecting sheet.

Such multi-step lamination processes as described in the prior art need several controls and an array of equipment making the process slow, cumbersome and expensive. It is a long-standing need of the industry to provide cost effective and simpler process for preparation of non-laminated blended biaxially orientation polyester film for diverse applications including liquid crystal displays or electronic decorative boards.

Summary of the invention The main object of the invention is to provide a cost effective commercially viable process for the preparation of non-laminated blended biaxially orientation polyester film for applications as base material for label, poster, recording paper, wrapping paper including reflector of a surface light source for illuminating a scope. of liquid crystal displays or electronic decorative boards. It is another object of the invention to provide a process that eliminates the multi-step process and to produce the polyester films though the easier route of extrusion of blended formulations without compromising on the functionality for the films.

It is yet another of the invention to provide a process in which the range of raw materials to be used can be widened in contract to the limited number of raw materials with very narrow specifications as is taught in the prior art.

Thus in accordance of this invention in contrast to the prior art an appropriately blended composition comprising a mix polyester, a polycondensed resin of a dicarboxylic ester and polyhydric alcohols, density reducing polymers, optical brightners and inorganic particles is extruded and processed to produce non-laminated biaxially oriented polyester film.

Detailed description of the Invention

In a preferred embodiment of this invention the process involves transesterification of dimethyl terephthalate (DMT) and monoethylene glycol (MEG) to produce granules of high moleculer polymer (PET)

The granules of PET and the master batches of inorganic particles, optical brightners and incompatible polymers such as polyolefins such as polypropylene are mixed, dried at 100-160°C and extruded through extruder. The cast film is quenched on chill roll using electrostatic pinning, then oriented in the longitudinal direction using a group of rollers heated at temperature of 80-120°C, to achieve a draw ratio of 2-5 times. Thereafter, the stretched film is cooled on a group of rollers at temperature 30-60°C and then subjected to transverse orientation in a tenterat at 90-140°C where the film is stretched 2-5 times in transverse direction. The film is then heat set at 150-230°C and finally wound on a winder roll.

The film is drawn in longitudinal and transverse direction 2 to 4 times in each direction so that the area ratio of non stretched film and biaxially stretched film is maintained at least 6 times and to a maximum of 25 times.

In various embodiments of the invention polypropylene may be added in different ways. The polypropylene (PP) 6-12% by weight of the feeding composition is added in the form of granules in the mixture subjected for extrusion or it may be added in the master batch form with PET before extrusion. A side feeding arrangement in extrusion may also be used to add the PP separately.

The polypropylene (PP) 6-12% by weight of the feeding composition is added in the form of granules in the mixture subjected for extrusion or it may be added in the master batch form with PET before extrusion. A side feeding arrangement in extrusion may also be used to add the PP separately.

The OB can be added in the form of powder in the extrusion composition before extrusion. Alternatively the OB may be added by making a master batch with PET or PP. The master batch is then mixed with the extrusion composition.

In the present invention, "polyester" means a polymer obtained by condensation polymerization of a diol and a dicarboxylic acids such as Terephthalic acid, isophthalic acid, phthalic acid, naphthalene dicarboxylic acid, adipic acid, sebacic acid may be the preferred dicarboxylic acids.

The Polyester employed in the present invention is a polycondensed resin of a dicarboxylic ester and polyhydric alcohols. Dicarboxylic esters are selected from dimethyl terephthalate, dimethyl isophthalate, dimethyl sebacate, dimethyl adipate, naphthalene dicarboxylate and the like. Dimethylterephthalate is prefered.

Alkylene glycois include: ethylene glycol, diethylene glycol, trimetylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol and hexylene glycol. Among these, ethylene glycol is most preferred. The catalyst system used in this invention to prepare polyester is acetates of alkylene earth metals such as Manganese acetate, sodium acetate, calcium acetate and the like as a transesterification catalysts and compounds of antimony as antimony trioxide used as a polycondensation catalyst. Thermal stabilizer triphenyl phosphate used alongwith polycondensation catalyst to impart thermal stability to polymer.

Incompatible polyolefins used in the process are: poly-3-methyl butane, poly-4- methylpentene-1 , polypropylene, polyvinyl-t-butane, 1,4 transpoly-2,3- dimethylbutadiene, polyvinylcyclohexane, polystyrene, polyfluorostyrene. Additionally cellulose acetate, cellulose propionate or polychlorotrifluoroethylene may be used. Among these polymers, polyolefin, particularly polymethyl pentene or polypropylene are most most preferred.

The inorganic material is selected from titanium dioxide, calcium carbonate, barium sulphate, aluminium trioxide, silica, kaolin or aluminium silicate and their mixtures. The average particle size of the inorganic filler used in the range of 0.5-1 Oμ more preferably 1-5 μ. The concentrations vary from 5-15% preferably from 6-12%.

The density lowering agent may be added directly in required amounts preferably 2 - 25wt%, more preferably 0.5 to 5.0 wt% to the matrix polymer of the film to make a master polymer and thereafter blended.

Optical brighting agents such as Uvitex (Ciba Geigy), Eastobrite OB-1 (Eastmab) and the like is added in the polyester master-batch prior to extrusion in 0.01 -1.0% range preferably in 0.05-1.0 %.

In the present invention, a density-lowering agent is such as polyalkylene glycol such as polyethylene glycol, methoxypolyalkylene glycol, polytetramethylene glycol or polypropylene glycol or derivative thereof, or copolymer thereof such as polyalkylene glycol and a polyester is added to facilitate uniform dispersion of the non-compatible polymer. In the present invention, the polyalkylene glycol used is polyethylene glycol of molecular weight 2000-4000 dalton. This glycol is added in the range of 1-10% preferably 1-5 % and more preferably 1-3%.

Measurement of Film Characteristics

a) % Reflectance and colour values:

Percentage Reflectance is the % of incident light on film surface reflected back.

% reflection of light is measured in 400 to 700 nm wavelength region. Colour values are the measurement of yellowness, whiteness of the film. The L,a,b values indicate the color of the film. Reflectance is determined in the wavelength range of 400 to 700 nm in a UV-2401 PC of Shimadzu spectrometer. A BaSO4 coated plate is used as standard. The reflectance is measured from graph and points table at an interval of 5 nm and also at 450, 550 & 650 nm respectively. Also colour values are also measured on Hunter scale provided with this instrument. Difference between the maximum and minimum reflectance in the range of 400 - 700 nm range is measured by taking the difference between maximum and minimum reflectance is calculated.

The graph showing reflectance % v/s wavelength in nm in fig.1

b) Apparent specific gravity: " It is the specific gravity of film containing voids."

Mixing n-hexane and carbon tetrachloride results in solutions of different densities.

The film samples are cut from different portions of the film and added to the known density mixtures kept in Nesceller's cylinder (100 ml) filled upto 100 ml mark. The samples, which remain steady at centre of the cylinder, indicate the apparent density of that sample.

c) Surface roughness (Ra):

"Ra": "It is an arithmetic mean of absolute value of the profile departure within the measuring length"

Stylus type surface roughness tester such as "Surfcorder Analyser AY-41 of Kosaka Lab Ltd" is used to measure the surface roughness. The samples are mounted on the sample port and the surface is scanned by mounting stylus and Ra is measured as the surface roughness.

d) SEM: (Scanning Electron Microscopic analysis)

The polyester film is coated with a gold layer and mounted in sample port maintained under vacuum. The sample is scanned for transverse section by adjusting the microscope to obtain the microscopic images showing voids etc. e) Mean particle size of the inorganic particles is measured by dispersing the particles in glycol and the average particle size and distribution is determined by the particle size analyser .

The process of this invention is now illustrated with a few non-limiting examples.

Examplel

Polyethylene terephthalate (PET) pellets are extruded without any additive or brightner agent. Extruded the film oriented biaxially i.e. in longitudinal and transverse direction, heat set at 230°c and wound. This is taken as a reference reading to compare with samples prepared in examples 2-7.

Example 2

Pellets PET and master pellets prepared of an inorganic additive e.g. Barium sulphate @ 50 % loading in PET. Also PEG (polyethylene-glycol) molecular wt. 4000 was added 1-2% during polymerisation of polyethylene terephthalate is air dried at 180^CC for 4 hours. Also the pellets of polyolefin e.g. polypropylene dried at 130°C in dehumidified airflow. After sufficient drying the pellets of PET, master batch are to be cooled upto 130°C and the polypropylene pellets are mixed thoroughly to have a uniformity in final film composition. This mixture is subjected to an extrusion also master pellets of brightening agent are also mixed with this mixture before extrusion. The extruder temperature is maintained at 270 - 290°C, the mixture is melt extruded, the melt is delivered out from a hanger die in the form of a sheet cast on a chill drum whose surface temperature is maintained at 25°C. An electrostatic force is also applied for proper cooling of the cast sheet. Further this cooled sheet was forwarded to the group of rollers maintained at 85 to 98 °C. The sheet was stretched in longitudinal direction at a draw ratio of 3.2 to 3.4 times and thereafter this uniaxially film is cooled by a group of rollers maintained at 25°C. Further to this, it is subjected for stretching in tenter by cramping the edges of the film in clips and the film is stretched in a transverse direction at a draw ratio of 3.6 times in an atmosphere of 130°C maintained in tenter. After that, the film is thermoset in the tenter at 230°C, the film is uniformly and gradually cooled to room temperature, and thereafter, the film is wound. The thickness of the film obtained is 188μ. The characteristics of the film obtained are given in table 1 and the reflectance % vs wavelength as shown in fig.1 Example 3:

Pellets of PET and master pellets of an inorganic additive e.g. calcium carbonate (@40% cone.) also PEG of mol.wt.4000 1-2% added in polymerization of Polyethylene terephthalate. The pellets are dried at 180°C for 4 hours in dehumidified air and pellets of polyolefin e.g. polypropylene dried at 130°C are mixed with this after colling PET chips and Master PET chips to 130°C. Optical brightner master batch is also added (Uvitex OB) giving 400-500 ppm concentration in final film. This is extruded and drawn to obtain a biaxially oriented film. The resultant film has shown value just 90%, which is less than the value, obtained in example 2.

Example 4:

10kg PET granules are pulverised in a stainless steel blender and mixed with 40% TiO2 powder loading. The mixture is extruded and made into a master batch. A master batch of 5% Optical Brightner (OB) Uvitex-OB of Ciba Geigy is made. The two master batches are mixed to get a composition as PET pellets 10 kg+ TiO2 master batch 4.7 kg + 2.2 kg PP granules + OB master batch 0.52 kg which is mixed well and extruded at 270- 290°C. The cast film is oriented in longitudinal as well as transverse direction with a draw ratio 3-4 times in each direction. The film is heat set at 225 to 230 °C.

Example 5:

A master batch of 10 kg PET pulverised granules is mixed with a master batch of 40% BaSO4 and extruded and pelletised.. A master batch of 5% optical brightener in PET is prepared in a similar manner. The two master batches are well mixed to give a composition containing 10 kg PET + 4.7 kg BaSO4 + 2.2 kg polypropylene + 0.52 kg and extruded at 270-290°C. The cast film is oriented in longitudinal direction at a draw ratio of 3-4 times and in transverse direction at a draw ratio of 3-4 times. The film is further heat set at 230°C.

Example 6: In this case a master batch containing 50% TiO2 is made in polypropylene following the process given in examples 4 and 5. A master batch containing 5% Optical Brightener is made in PET. These are mixed well to give a composition PET pellets 10 kg + TiO2 2.13 kg + OB 267 kg + Polypropylene granules 0.933 kg The mixed composition is extruded at 270-290°C . The cast film is quenched on chill drum by pinning wire and then oriented in the longitudinal direction and transverse direction at a draw ratio of 3-4 times in each direction. The film is then heat set at 230°C.

Example 7: In this example, the PET pellets, inorganic particle master batch, optical brightner master batch are mixed well and added in dryer. Polypropylene is fed through a side feeding arrangement in the extruder. The master batch of inorganic filler TiO₂ (50% in PP), optical brightner master batch (5% in PET) and PET granules are mixed well to give a batch composition of PET 10 kg + TiO₂ 2.46 kg + OB master batch 0.54 kg and 0.68 kg polypropylene. The mixed composition is extruded at 270-290°C. The cast film is quenched on chill drum by pinning wire and then oriented in the longitudinal direction and transverse direction at a draw ratio of 3-4 times in each direction. The film is then heat set at 230°C.

Scanning Electron Micrographs of the film prepared in example 7.... is given in figure 2.

The cross section of the film does not show any co-extrusion layer as the film is a monolayer in contrast to the multi layered laminated films prepared by the method of JP4239540 (figure 3). The arrows in the photographs indicate the dispersed voids in the mono layered film of the present invention. Figure 4 shows the multilayered lamination A/B structure of the film prepared by the process of JP4239540. Out of the total thickness of 190μ, the lamination layer is 40μ and contains no voids. The layer of 150μ contains the voids and has a structure different from the co-extrusion layer. Similarly the A/B/A laminated structure prepared by the process of JP 4239540 shows no voids in the co-extruded lamination layer. The central portion i.e. "B" contains all the voids.

The structure of the films prepared by the process of the present invention are distinctly different from those prepared by methods described in the prior art.

Claims

1. A process for the preparation of non-laminated blended biaxially orientation polyester film for applications as base material for label, poster, recording paper, wrapping paper including reflector of a surface light source for illuminating a scope of liquid crystal displays or electronic decorative boards where in i) Polyester obtained by polymerization of a diol and a dicarboxylic acid inorganic particles, incompatible polyolefin polymers that do not homogenously mix in the polyester matrix, optical brighter pigment is blended, dried and fed to an extruder at temperature in the range of about 270°C to about 290°C ii) The melt form is cast on a cool drum and quenched by electrostatic pinning/quenching iii) The sheet is introduced to a group of rollers heated to temperature of about 80°C to about 120°C and stretched in a longitudinal direction at a draw ratio of 2-5 times while passing through the rollers iv) The uniaxially stretched film is cooled by a group of rollers at temperature of about 20°C - about 50°C v) The film is introduced into a tenter and clips are used to clamp the edges of the film wherein the film is stretched in a transverse direction in hot atmosphere of temperature of about 90°C - about 140°C vi) The film is drawn in longitudinal and transverse direction several times in each direction so that the area ratio of non stretched film and biaxially stretched film is maintained at least 6 times and to a maximum of 10 times vii) The film is thermoset at about 150^CC to about 230 °C followed by cooling to room temperature and winding.

2. A process as claimed in claim 1 wherein the polyolefin in step (i) of the reaction is optionally added to a master blended batch of Polyester, inorganic particles, incompatible polyolefin polymers and optical brighter pigment prior to extrusion.

3. A process as claimed in claim 1 wherein the optical brightener is optionally added in step (i) to the master blended batch of Polyester, inorganic particles, incompatible polyolefin polymers prior to the extrusion.

4. A process claimed in claim 1 wherein the polyester is a polycondensed resin of a dicarboxylic ester and polyhydric alcohols the dicarboxylic ester being selected from dimethyl terephthalate, dimethyl isophthalate, dimethyl sebacate, dimethyl adipate, naphthalene dicarboxylate and the like preferably Dimethylterephthalate.

5. A process as claimed in claim 1 wherein the alkylene glycols include ethylene glycol, diethylene glycol, trimetylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol and hexylene glycol preferably ethylene glycol.

6. A process as claimed in claim 1 wherein the incompatible polymers are selected from poly-3-methyl butane, poly-4-methylpentene-1 , polypropylene, polyvinyl-t-butane, 1,4 transpoly-2,3-dimethylbutadiene, polyvinyl cyclohexane, polystyrene, polyfluorostyrene, cellulose acetate, cellulose propionate or polychlorotrifluoroethylene preferably polymethyl pentene or polypropylene.

7. A process as claimed in claim 1 wherein the inorganic material is selected from titanium dioxide such as rutile, calcium carbonate, barium sulphate, aluminium trioxide, silica, kaolin or their mixtures of average particle size in the range of 0.5-5μ, preferably 1-3 μ in concentrations of 5-15% preferably from 8-12%.

8. A process as claimed in claim 1 wherein the density-lowering agent is selected from polyalkylene glycol such as polyethylene glycol, methoxypolyalkylene glycol, polytetramethylene glycol or polypropylene glycol or derivative thereof, or copolymer thereof such as polyalkylene glycol and a polyester preferably a copolymer of polyalkylene glycol-polyethylene terephthalate in the molecular weight distribution of 200-4000 Daltons preferably from 2000-4000 Daltons in concentrations of 1-10% preferably 1- 5% and more preferably 1 -3%.

9. A process as claimed in claims 1 and 7 wherein 2 - 25wt%, more preferably 0.5 to 5.0wt% of the density-lowering agent is added directly to the matrix polymer.

10. A process as claimed in claim 1 wherein the optical brightening agents such as Uvitex (Ciba Geigy), Eastobrite OB-1 (Eastmab Make) and the like are added in 0.01 -1.0% preferably in 0.05-1.0 % in the polyester master-batch prior to extrusion.

11. A process as claimed in claims 1-10 for the preparation of non-laminated blended biaxially orientation co-extruded polyester film without any co- extrusion layers and the voids being dispersed in the monolayered film exhibiting % transmission of 10%max, reflection at 450 nm, 550nm and 650nm of >90% where in i) Polyester obtained by polymerization of a diol and a dicarboxylic acid inorganic particles, incompatible polyolefin polymers that do not homogenously mix in the polyester matrix, optical brighter pigment is blended, dried and fed to an extruder at temperature in the range of about 270°C to about 290°C ii) The melt form is cast on a cool drum and quenched by electrostatic pinning/quenching iii) The sheet is introduced to a group of rollers heated to temperature of about 80°C to about 120°C and stretched in a longitudinal direction at a draw ratio of 2-5 times while passing through the rollers iv) The uniaxially stretched film is cooled by a group of rollers at temperature of about 20°C - about 50°C v) The film is introduced into a tenter and clips are used to clamp the edges of the film wherein the film is stretched in a transverse direction in hot atmosphere of temperature of about 90°C - about 140°C vi) The film is drawn in longitudinal and transverse direction several times in each direction so that the area ratio of non stretched film and biaxially stretched film is maintained at least 6 times and to a maximum of 10 times vii) The film is thermoset at about 150°C to about 230 °C followed by cooling to room temperature and winding.

Table:1 Experimental Results