CA2778513A1

CA2778513A1 - Industrial textiles comprised of bi-axially oriented, hydrolytically stabilized polymer film

Info

Publication number: CA2778513A1
Application number: CA2778513A
Authority: CA
Inventors: Allan Manninen
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-05-28
Filing date: 2012-05-28
Publication date: 2013-11-28
Also published as: WO2013177670A1

Abstract

A biaxially oriented multilayer thermoplastic film, and seaming components comprised of the film. Each layer comprises a polyester having an intrinsic viscosity (IV) of at least 0.5; and a hydrolytic stabilizer comprising a carbodiimide, and the film has a thickness of at least 100µm. The carbodiimide provides resistance to depolymerization resulting from prolonged exposure to heat and humidity. The film is preferably produced as a multilayer co-extrusion which is subsequently oriented in each of the machine and transverse directions to maximize its mechanical properties. The final film is processed to render it suitable for use as components in nonwoven film type industrial textiles where durability and longevity are desirable features.

Description

INDUSTRIAL TEXTILES COMPRISED OF BI-AXIALLY ORIENTED, HYDROLYTICALLY STABILIZED POLYMER FILM
FIELD OF THE INVENTION
The invention generally concerns hydrolytically stabilized, bi-axially oriented polymeric films. It is particularly concerned with the use of such films in the manufacture and production of components for industrial textiles and other applications where durability and stability in adverse environmental conditions are important.
BACKGROUND OF THE INVENTION
Industrial fabrics used in filtration, conveyance and similar processes have been and are typically manufactured by weaving, whereby synthetic yarns are interwoven to provide either the entire fabric, or only a base portion which may subsequently be either encapsulated (e.g. with polyurethane or other similar rugged material) or needled to attach a nonwoven ban. material. Such fabrics have been satisfactory for these uses, but the cost of their production is high, particularly when the fabrics must be finely and precisely woven using relatively small yarns. Further, these fabrics must be rendered endless in some manner, either by installing a seaming element at their opposed longitudinal ends, or by re-weaving the longitudinal yarns back into the fabric structure to form seaming loops or similar joining means, for secure connection by a pintle, coil or similar securing means. It is also known to weave such fabrics in an endless manner, so that there is no seam, or to interweave the yarns from one longitudinal end into the yarns of the opposed end to forni a woven seam. These fabrics are expensive to produce and require a high capital investment in wide industrial looms and similar related equipment for subsequent processing, as well as a skilled workforce to operate the equipment and produce an acceptable finished product.
It has recently been proposed by Manninen (WO 2011/069259) to construct nonwoven fabrics suitable for the same or similar processes as those which currently use woven fabrics by using selectively slit and embossed polymeric films. It is also proposed to form components from suitably shaped and processed film for use in seaming both these nonwoven and woven textiles (see WO 2010/121360, WO 2011/069258, or CA
2749477).
The '259 document proposes that thermoplastic polymeric films used in such fabrics described by Manninen in WO 2011/069259, WO 2010/121360, WO 2011/069258, or CA 2749477 and which are comprised of bi-axially oriented hydrolytically stabilized polymer films which are resistant to heat, humidity and abrasive wear.
Bi-axially oriented hydrolytically stabilized polyester films are known. For example, US
6,855,758 to Murschall et al. discloses a hydrolysis resistant bi-axially oriented film made from a crystallizable thermoplastic polyester. This patent teaches that phenolic stabilizers, in particular the 3,5-di-tert-butyl-4-hydroxyphenyl propionates of

2 monomeric carbodiimides such as dicyclohexylcarbodiimide, or aromatic polymeric carbodiimides having a molecular weight of from 2,000 to 5,000 obtainable as Stabaxol0 P from Rhein Chemie GmbH of Mannheim, Geimany may also be used. In a preferred embodiment, the film includes from 0.1% to 5% pbw of aromatic polymeric carbodiimides and from 0.1% to 5% pbw of a blend made from 30% to 90% pbw organic phosphite (in particular a triaryl phosphite) and from 70% to 10% pbw of a hydroxyphenyl propionate. The film as claimed includes a crystallizable polyester or copolyester and a hydrolysis stabilizer consisting essentially of (1) either a monomeric carbodiimide, an aromatic polymeric carbodiimide or oxazolines, and (2) optionally at least one of either a phenolic compound or an organic phosphite. The examples noted in the reference do not include any film of a thickness exceeding 100 m, in particular in a range suitable for use in industrial textiles and components thereof US 6,020,056 to Walker et al. discloses a bi-axially oriented hydrolytically stabilized PET film which does not employ end-capping agents such as carbodiimides. The PET has an initial IV of from 0.95 to 1.1 and, when cast, has an IV of from about 0.8 to 1.0; the resulting film is stretched and oriented at least 2 times in the MD and CD, and its intended end use is as a motor insulation.
US 7,229,697 to Kliesch et al. discloses a hydrolysis stabilized PET film whose thickness is from 0.4 to 500 pan, using alternatives to carbodiimides as hydrolysis stabilizers for PET to address health issues relating to the off-gassing of isocyanates during extrusion and increases in the molecular weight of the PET and extrusion problems associated with it.
WO 2011/030098 to Brennan et al. discloses a bi-axially oriented PET film further including a hydrolysis stabilizer which is a glycidyl ester of a branched monocarboxylic acid having from 5 to 50 carbon atoms and which is present in the film as its reaction product along with some of the polyester end-groups. Use of the film as a layer in a photovoltaic cell is disclosed.
It is known from US 5,885,709 to Wick et al. to provide hydrolysis resistant polyester fibers and filaments which have capped carboxyl end groups following reaction with

3 carbodiimides. The polyesters can include any aliphatic or aromatic filament forming polyester (e.g. PET) with preference given to those having a molecular weight corresponding to an intrinsic viscosity (IV) of at least 0.64 and preferably at least 0.7 dL/g as measured in dichloroacetic acid at 25 C.
None of the aforementioned prior art proposes a bi-axially oriented, hydrolytically stabilized polymeric film with physical characteristics making it suitable for use as components of nonwoven industrial textiles. In order to be useful and have physical properties sufficient to allow such textiles to survive the various rigors of the environment for which they are intended, a suitable film should be formed from a medium to high IV polyester; the IV should be between about 0.55 and 1Ø The PET
must also be hydrolytically stabilized to prevent premature depolymerization in hot and moist environments due to hydrolytic degradation; carbodiimides are preferred for this application. The film must be stretched and oriented as it is produced so as to increase and maximize its elastic modulus and other physical properties. The film should have a thickness of from about 100 up to 500 um, but ideally in the range of about 250 to 350 um and this caliper should be uniform throughout. None of the prior art describes such a film.
SUMMARY OF THE INVENTION
In a first broad embodiment, the present invention seeks to provide a biaxially oriented multilayer thermoplastic film, wherein (i) each layer comprises a polyester having an intrinsic viscosity (IV) of at least 0.5;
(ii) at least one layer comprises a hydrolytic stabilizer comprising a carbodiimide;
and (iii) the film has a thickness of at least 100um.
Preferably, the polyester for each layer is selected from one of polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN) and

4 poly(cyclohexylene dimethylene terephthalate) acid (PCTA). Preferably, the polyester for each layer is PET.
Preferably, the PET has an intrinsic viscosity (IV) of from about 0.55 to at least 1.0; more preferably the PET has an IV in the range of about 0.5 to 0.8 when measured at 30 C in a solution of trifluoroacetic acid and dichloromethane.
Preferably, the film consists of at least two layers of miscible polymeric extrudate. When the film is comprised of two layers, one layer will have a thickness of from

5% to 15% of the overall film thickness, and the second layer will have a thickness of from 85% to 95%.
More preferably, the thickness of the first film layer is 10% of the overall film thickness and the thickness of the second layer is 90% of the film thickness.
Preferably, the film is comprised of at least three coextruded layers. Where the film is comprised of three coextruded layers, each of the outer layers preferably comprises from 5% to 20% of the overall film thickness and the inner layer comprises from 60%
to 90%
of the film thickness. More preferably, each of the outer layers comprises from 10% to 15% of the final film thickness.
Preferably, at least one outer film layer includes an antiblock agent.
Preferably, at least one film layer includes a colorant or other additive, such as titanium dioxide, carbon black, or a dye. Alternatively, at least one outer layer includes a radiant energy absorbing material.
Preferably, the overall thickness of the finished film is at least 100 pm;
more preferably, the film thickness is between 100 ¨ 500 p.m. Most preferably, the film thickness is between about 175 and about 350 m.
Preferably, the hydrolysis stabilizer is a carbodiimide and is added to and blended with the polyester in masterbatch form sufficient to comprise from about 0.1% to 5%
pbw (parts by weight), preferably about 0.5% to 3% pbw and more preferably about 1.5% to 3%
pbw based on the weight of the material in each film layer. Preference is presently given to aromatic polymeric carbodiimides; alternatively the carbodiimide is monomeric.
Preferably, the carbodiimide is incorporated as a masterbatch in the polymer melt.
Preferably, the film is oriented and stretched in each of the machine direction (MD) and transverse direction (TD) by a factor of from about 2 to about 4 times its original dimension; preferably it is oriented by a factor of at least 3. The resulting film is subsequently annealed, cooled and formed into rolls for later use.
When intended for use as components in nonwoven industrial textiles, the film can be processed in one of several ways. It may be first cut to a desired size and then a desired topography imparted by means of a thermoforming process whereby heat and pressure are used to deform the film out of plane in a desired shape. The film is then slit by either mechanical means or by means of radiant energy such as from tuned laser.
Alternatively, the film is first slit or perforated by chosen means, and then embossed with a suitable pattern. In either case, the slit and profiled film sections are then assembled into nonwoven industrial textiles and associated components using known means. As a further alternative, the film may first be embossed according to a desired pattern, and then assembled in two or more layers, and finally perforated as desired.
DETAILED DESCRIPTION OF THE INVENTION
The fabrics and seaming components of the invention are formed from an extruded and bi-axially oriented film which is comprised of at least one, and preferably three coextruded polymeric layers that are oriented and heatset together, and which optionally include a hydrolysis stabilizer in the form of a monomeric or polymeric carbodiimide.
As examples only of polyesters suitable for use in the films of the invention, it has been found that Invista Type 4027 PET (available from Invista S.a.r.l. of Wichita, Kansas) having an IV of about 0.60, Invista Type 8326 PET at an IV = 0.55, and DAK
Americas LLC of Charlotte, NC Type 80 PET which has an IV of 0.80 have proven suitable.
These polyesters are commercially available in dry pelletized form from the supplier with the specified IV. If necessary or desired, the IV of these or any other PET resins can be

6 increased by means of known solid state polymerization processes whereby the polyester is exposed to high temperatures and vacuum (or inert gas, to prevent oxidative degradation); the result is a relatively higher molecular weight polyester in comparison to that of the starting material. In general, higher IV PET resins will allow for the production of a film with improved physical properties, in particular resistance to abrasion and hydrolysis, when compared to films produced from resins of lower IV. High intrinsic viscosity polymer resins will allow the resulting films to better withstand the rigorous demands of certain of the industrial environments to which it may be exposed, such as in the hot and moist dryer section of a papermaking machine, or continuous exposure to sunlight on a solar panel.
Depending on the intended end use of the film, it may be either desirable or necessary to increase its resistance to hydrolysis. Hydrolysis is a chemical process by which a water molecule is added to a substance resulting in that substance splitting into two parts. It is the type of reaction that will break down certain polymers, especially those such as PET
which are made by condensation polymerization. Hydrolysis stabilizers are often added to PET resins when the intended end product will be used in hot and moist environments.
Hydrolysis stabilization additives are well known and function by reacting with free polymeric carboxyl end groups in the polymer melt prior to extrusion. One such additive, which has proven successful when incorporated into polyester monofilaments, is Stabaxol KE7646. This additive is commercially available from Rhein Chemie Corp. of Chardon, OH and is comprised of from about 10% to 30% pbw of a polymeric carbodiimide in 70 to 90% pbw of a high IV PET (IV approx. 0.80). A monomeric form of the Stabaxol additive is also available and is anticipated to be equally as successful in imparting hydrolysis resistance as the polymeric form; either form is suitable for use in the polymeric films of the invention.
The Stabaxol KE7646 is the masterbatch form of a polycarbodiimide; according to the manufacturer, it contains 15% Stabaxol P100 (the active ingredient) uniformly blended in the PET. Addition of 10-20 pbw Stabaxol KE7646 per 100 pbw PET should provide an active ingredient content of Stabaxol P100 of 1.5% to 3%, which is the preferred

7 range of carbodiimide hydrolysis stabilizer in the films and components of the present invention. It has been found that compositions including relatively high IV
PET (e.g. 0.8, such as is found in the DAK Type 80 resin) and about 5% pbw carbodiimide, or more, may become difficult to reliably extrude; thickness variation, shrinkage and film orientation are problematic because the stabilizer appears to increase crosslinking and extensional viscosity of melt.
The films of the invention are made as follows. A desired PET or other polyester resin is first obtained and an appropriate amount of hydrolysis stabilizer is added according to normal blending processes as are known in the art. As previously mentioned, if the end use of the film is as a belt component in a hot and/or humid environment, the polymer should be hydrolysis stabilized and the IV selected as appropriate. The polymer is preferably obtained as resin pellets which are then loaded into the hoppers of the film extruder(s). Once heated to the melt point, the polymer melt is then extruded through a slot die according to techniques and equipment common in the industry. The amorphous prefilm is subsequently quenched on a chill roll and then reheated and oriented in both the MD (machine direction) and TD (transverse direction) so as to impart stretch-induced structure through biaxial orientation. This step is important in order to provide a mechanically stable film as the stretching process will straighten out the polymer chains in the film and provide crystals with the desired morphology. Stretching temperatures are normally above the glass transition temperature T, by at least 10 C. The stretching ratio in each of the MD and TD will be about 3, but may range from 2 to about 4 as required.
There are essentially two film stretching processes in use at present:
simultaneous stretching, in which the film is exposed to both MD and TD force simultaneously; and sequential stretching, in which the film is exposed first to MD and then TD
stretch forces.
The films of the invention can be made using the simultaneous stretch process, or sequential stretching. Depending on the end use of the film, it may be desired to preferentially stretch the film in one of these directions over the other. A
second subsequent stretch in either or both the MD and TD may be employed as needed.
The MD and TD shrinkage can be adjusted as appropriate by temperature settings and frame geometry. It should be noted that, as film thickness and carbodiimide content increases, it

8 becomes increasingly difficult to reliably and uniformly control properties, particularly thickness. Heat setting or annealing of the film at oven temperatures of about 180 C to 260 C follows stretch and orientation; the film is then cooled and wound. The oriented film has a final thickness of from about 175 um to 350um; depending on the intended end use, the film thickness may be increased or decreased around these limits as necessary.
The polyester films of the invention may be monolayer films, but are preferably multilayer and more preferably are comprised of three layers. In experimental trials, the film was extruded using a three layer die with a feedblock designed to feed both outside skins from one extruder and the core layer from another. The resultant film thus included three polymer layers arranged according to an A-B-A configuration in which each of A
and B are essentially the same polymer but are of two differing thicknesses.
It was found that multilayer films of the invention in which the layers A each accounted for 15% of the overall film thickness and the layer B provided the remaining 70% were suitable for use as components in industrial textiles, however, other film thickness ratios such as 10-80-10 may also prove suitable. Preference is given to A-B-A or A-B-C three layer structures.
In such structures, it is possible for at least one and preferably both of the outer layers, and an intermediate layer to include the hydrolysis stabilizer. The concentration of the stabilizer may vary from one layer to the next. However, in such structures, it is important that the adjacent layers be compatible, or miscible; alternatively a so-called "tie layer" may be located in between the adjacent layers to prevent layer separation and provide a unified film structure.
The hydrolytically stabilized film of the present invention bears some similarities to the coextruded laser weld enabled film described in CA 2758622 (Manninen). As described in that document, one layer of the film is different from the others in that it includes a laser weld enabling material. In the present invention, the film is comprised entirely of essentially the same polymer (although a dye or other common additive may be included in one or more of the layers). For example, it may be necessary to provide an antiblock agent, such as Invista V388 (available from Invista S.a.r.l. of Wichita, Kansas) at a 5%

9 pbw concentration, to the outer "A" layers of the film to prevent them from sticking to a roll or other component of the extruder and/or stretching arrangement.
The polymer films of the present invention are of particular importance to the industrial textile industry for several reasons. First, they are formed from a higher IV
PET resin than others that have been used previously and which are commercially available PET
films. It has been found that the high IV PET retards brittle crystal formation during heatsetting/thermoforming steps. Commercially available PET films are farmed from polyester resins whose IV is less than 0.5; if exposed to heat in the range of about 200 C
or more, or prolonged exposure to sunlight, such films will become very brittle and fail in various ways (their tensile strength will diminish, they will become prone to stress cracking, etc.) whereas the films of the present invention will not degrade in this manner.
Also, the grade of the PET resin is important when the end use application involves hydrolysis; generally the resin should have a relatively low carboxyl end group concentration and contain low residual diethylene glycol. Further, the hydrolysis stabilizer is reactive and affects the extensional viscosity of melt of the blend ¨ therefore the stabilizer loading affects the processability of the film. It has been found that the quality of the film is significantly affected by the line process parameters, i.e.
temperatures, stretching ratio, etc.
The final film is now available for use in the manufacture of industrial textiles which are uniquely suitable for conveyance, filtration and separation processes. For example, hydrolytically stabilized PET film having a thickness of from about 250 m to about 350 pm is suitable for use in the production of selectively slit and embossed films intended for subsequent assembly as nonwoven papermaking fabrics. Similar films of greater or lesser thickness will be appropriate for the manufacture of seaming components which will be used to join the opposing ends of these fabrics on the machines for which they are intended. The films will also be suitable for use in other applications, such as in solar panels, where resistance to degradation and embrittlement are important.

Claims

I CLAIM:

1. A biaxially oriented multilayer thermoplastic film, wherein (i) each layer comprises a polyester having an intrinsic viscosity (IV) of at least 0.5;
(ii) at least one layer comprises a hydrolytic stabilizer comprising a carbodiimide;
and (iii) the film has a thickness of at least 100µm.

2. A film according to Claim 1, wherein the polyester for each layer is selected from one of PET, PBT, PEN, PCTA.

3. A film according to Claim 2, wherein the polyester for each layer is PET.

4. A film according to any one of Claims 1 to 3, wherein the IV is in the range of 0.5 to 1Ø

5. A film according to any one of Claims 1 to 4, wherein the film thickness is in the range of 100µm to 500µm.

6. A film according to Claim 5 comprising two layers, wherein a first layer comprises from 5% to 15% of the film thickness and the second layer comprises from 85%
to 95% of the film thickness.

7. A film according to Claim 6, wherein the first layer comprises substantially 10%
of the film thickness and the second layer comprises substantially 90% of the film thickness.

8. A film according to Claim 5, comprising three layers, wherein each outer layer comprises from 5% to 20% of the film thickness and an inner layer comprises from 60%
to 90% of the film thickness.

9. A film according to Claim 8, wherein each outer layer comprises from 10%
to 15%
of the film thickness and the inner layer comprises from 70% to 80% of the film thickness.

10. A film according to any one of Claims 1 to 9 wherein for each layer comprising a hydrolytic stabilizer, the carbodiimide comprises between 0.5%pbw and 5%pbw of the material of that layer.

11. A film according to any one of Claims 1 to 10, wherein the carbodiimide is selected from a monomeric form and a polymeric form.

12. A film according to Claim 11, wherein the carbodiimide is polymeric.

13. A film according to any one of Claims 1 to 12, wherein the film is stretched in each of a longitudinal and a transverse direction by a factor of from two to at least four.

14. A film according to Claim 13, wherein the film is stretched by a factor of at least three.

15. A film according to any one of Claims 1 to 14, wherein at least one layer further comprises an additive.

16. A film according to Claim 15, wherein the additive is selected from carbon black, titanium dioxide, and at least one dye.

17. A film according to any one of Claims 1 to 15, wherein at least one layer further comprises an antiblock agent.

18. A film according to any one of Claims 1 to 15, wherein at least one layer further comprises a radiant energy absorbent material.

19. A component for use with an industrial textile, comprising a film strip prepared from a film according to any one of Claims 1 to 18.

20. A component according to Claim 19, comprising a seaming component constructed and arranged to be secured to a seamable edge of the industrial textile.