EP1951496A1

EP1951496A1 - Process for extruding a thin film from an aromatic polyamide-imide composition

Info

Publication number: EP1951496A1
Application number: EP06819376A
Authority: EP
Inventors: Hong Chen; Gregory Warkoski
Original assignee: Solvay Advanced Polymer LLC
Current assignee: Solvay Specialty Polymers USA LLC
Priority date: 2005-11-09
Filing date: 2006-11-09
Publication date: 2008-08-06
Also published as: JP2009515020A; WO2007054543A1

Abstract

Process for extruding a film (F) having a thickness of below 1000 µm from a polymer composition (C), wherein the polymer composition (C) comprises at least 40 wt. %, based on the total weight of the polymer composition (C), of at least one aromatic polyamide-imide manufactured by a process including the polycondensation reaction of : (i) trimellitic anhydride monoacid halide; (ii) at least one comonomer chosen from diamines and diisocyanates, and (iii) at least one substance (S) comprising on e and only one functional group capable of reacting with either an anhydride group and an acid halide group on one hand, or an amine group and an isocyanate group on the other hand, or one and only one precursor of said functional group, wherein the amount of substance (S) is of above 1 mol. %, based on the total number of moles of trimellitic anhydride monoac id halide and substance (S).

Description

Process for extruding a thin film from an aromatic polyamide-imide composition

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. application serial

No. 60/734752 filed November 09, 2005, the whole content of which is incorporated herein by reference. FIELD OF THE INVENTION

This invention relates to a process for extruding a thin film from an aromatic polyamide-imide composition.

BACKGROUND

High temperature thin films, and especially high temperature very thin films, are currently dominated by polyimides such as KAPTON^® grades. They are produced by a solution cast process and need the use of organic solvents. It is well known that the skilled in the art dislikes the use of organic solvents. A solvent-free alternative was proposed by Emery et al. (US 4,581,264), which consisted in extruded polyamide-imide films. While successful to a certain extent, it proved unsuccessful in certain other cases, notably when very thin films had to be made from polymer compositions that have a very high polyamide-imide content, i.e. those which in practice offer the most attractive properties, as detailed in U.S. 4,581,264.

US 4,581,264 describes a process for extruding various articles from an aromatic polyamide-imide composition through melt channels and extrusion dies at continuously increasing shear rates and at temperatures of about 575⁰F to about 680⁰F. Among them are thin or thick sheets, thick tubes and thin films.

The thin films of US 4,581,264 are reported to have possibly a thickness as different as 1 mil or 15 mils. Example 1 describes the manufacture of a film having a thickness of 6.5 mils by extruding a neat polyamide-imide polymer. In contrast, the film of example 2, which has a lower thickness (namely, 3 mils), was not extruded from a neat polyamide-imide polymer, but from a 80:20 blend of a polyamide-imide polymer and of a ULTEM^®polyetherimide.

ULTEM^® polyetherimides exhibit usually a much lower level of properties than polyamide-imides but act as processing aids; thus, that the use of a ULTEM^® polyetherimide in example 2, although being detrimental to the properties, was in fact required to extrude successfully a film as thin as 0.003 inches based on the extrusion process taught in US 4,581,284.

In fact, the extrusion process described in US 4,581,264 is not well adapted for extruding very thin films (typically, below 4 mils) from neat polyamide- imide polymers and, more generally, from polymer compositions that comprise well above 80 wt. % of polyamide-imide.

There remains a strong need for an improved process for extruding a film from a polyamide-imide composition, in particular for a process that would be especially well suited for extruding a good looking very thin film from a polymer composition comprising well above 80 wt. % of polyamide-imide.

This need and still other ones are met by the instant invention. BRIEF DESCRIPTION

A certain aspect of the present invention is directed to a process for extruding a film (F) having a thickness of below 1000 μm from a polymer composition (C), wherein the polymer composition (C) comprises at least

40 wt. %, based on the total weight of the polymer composition (C), of at least one aromatic polyamide-imide manufactured by a process including the polycondensation reaction of : (i) trimellitic anhydride monoacid halide; (ii) at least one comonomer chosen from diamines and diisocyanates, and (iii) at least one substance (S) comprising one and only one functional group capable of reacting with either an anhydride group and an acid halide group on one hand, or an amine group and an isocyanate group on the other hand, or one and only one precursor of said functional group, wherein the amount of substance (S) is above 1 mol. %, based on the total number of moles of trimellitic anhydride monoacid halide and substance (S).

Such process enables to obtain polyamide-imide films of improved quality, in particular good looking polyamide-imide films as thin as 25 μm which have a very high polyamide-imide content (typically above 85 wt. %), while not requiring the use of solvents.

Another aspect of the present invention concerns a film obtainable by the process as above described.

An aspect of the present invention of particular interest is directed to a process for extruding a film (F) having a thickness of below 1000 μm from a polymer composition (C), wherein the polymer composition (C) comprises at least 40 wt. %, based on the total weight of the polymer composition (C), of at least one aromatic polyamide-imide manufactured by a process including the polycondensation reaction between (i) an acid monomer combination of a trimellitic anhydride monoacid halide with trimellitic acid, and (ii) at least one comonomer chosen from diamines and diisocyanates, wherein the trimellitic acid amount is above 2 mol. %, based on the total number of moles of acid monomer.

DETAILED DESCRIPTION

The film (F)

The thickness (t) of the film (F) is advantageously defined as : t = Iv τ(x,y,z) dx dy dz / V, wherein x, y and z are the coordinates in a three-dimensional space of an elementary volume dV (dV being equal to dx times dy times dz) of the film (F) of overall plain volume V, and τ is the local thickness.

The local thickness τ, associated to a material point of coordinates (x,y,z), is defined as the length of the shortest straight line D including the material point of concern, which goes right through the film (F) (i.e. which goes from the material point where D enters the film (F) to the material point where D exits the film (F)).

The thickness of the film (F) is preferably of below 100 μm, more preferably less than 75 μm, still more preferably less than 55 μm, even still more preferably less than 40 μm and the most preferably less than 30 μm. Besides, the thickness of the film (F) is advantageously greater than 10 μm, preferably greater than 15 μm and more preferably greater than 20 μm.

The polymer composition (C)

The aromatic polyamide-imide To the purpose of the present invention, an "aromatic polyamide-imide" is intended to denote any polymer comprising more than 50 wt. % of recurring units comprising at least one aromatic ring, at least one imide group, as such and/or in its amic acid form, and at least one amide group which is not included in the amic acid form of an imide group [recurring units (Rl)]. Preferably, the aromatic polyamide-imide comprises more than 90 wt. % of recurring units (Rl). More preferably, it contains no recurring unit other than recurring units (Rl). The recurring units (Rl) are advantageously :

Rl -a and/or Rl -b

(imide form) (amic acid form) where : - Ar is typically :

with X = , , with n= 1,2,3,4 or 5;

- R is typically :

with n= 0,1,2,3,4 or 5. Recurring units (Rl) are preferably chosen from :

and/or the corresponding amide-amic acid containing recurring unit :

wherein the attachment of the two amide groups to the aromatic ring as shown in (i-b) will be understood to represent the 1 ,3 and the 1 ,4 polyamide-amic acid configurations;

(ϋ)

and/or the corresponding amide-amic acid containing recurring unit :

wherein the attachment of the two amide groups to the aromatic ring as shown in (ii-b) will be understood to represent the 1,3 and the 1,4 polyamide-amic acid configurations; and

(iii)

(iii-a) and/or the corresponding amide-amic acid containing recurring unit :

wherein the attachment of the two amide groups to the aromatic ring as shown in (iii-b) will be understood to represent the 1 ,3 and the 1 ,4 polyamide-amic acid configurations.

Recurring units (Rl) are very preferably a mix of recurring units (ii) and (iii).

Excellent results were obtained with aromatic polyamide-imides consisting of a mix of recurring units (ii) and (iii). Polymers commercialized by Solvay Advanced Polymers as TORLON^® polyamide-imides comply with this criterion.

The aromatic polyamide-imide used in the process of the present invention is manufactured by a process including the polycondensation reaction of : (i) trimellitic anhydride monoacid halide; (ii) at least one comonomer chosen from diamines and diisocyanates, and (iii) at least one substance (S) comprising one and only one functional group capable of reacting with either an anhydride group and an acid halide group on one hand, or an amine group and an isocyanate group on the other hand, or one and only one precursor of said functional group, wherein the amount of substance (S) is above 1 mol. %, based on the total number of moles of trimellitic anhydride monoacid halide and substance (S).

As trimellitic anhydride monoacid halide, 4-trimellitoyl anhydride chloride is preferred.

The comonomer comprises preferably at least one aromatic ring. Besides, it comprises preferably at most two aromatic rings.

The comonomer is preferably a diamine. As examples of diamines, it can be cited : paraphenylene diamine, benzidine, 4-[(4-aminophenyl)methyl]aniline. More preferably, the diamine is chosen from the group consisting of 4,4'- diaminodiphenylmethane, 4,4'-diaminodiphenylether, m-phenylenediamine and mixtures thereof.

Substance (S) comprises one and only one functional group capable of reacting with either an anhydride group and an acid halide group on one hand, or an amine group and an isocyanate group on the other hand, or a precursor of said functional group.

As substances (S) comprising one and only one functional group capable of reacting with an anhydride group and an acid halide group, it can be cited aniline, napthylamine, anisidine and phenyl isocyanate.

As substances (S) comprising one and only one functional group capable of reacting with an amine group and an isocyanate group, it can be cited phthalic anhydride, 1,8-naphthalic anhydride, 1,2-cyclohexenedicarboxylic anhydride (cis or trans), succinic anhydride, maleic anhydride, benzoic anhydride, benzoyl chloride and naphthoyl chloride.

As substances (S) comprising one and only one precursor of a functional group capable of reacting with an amine group and an isocyanate group, it can be cited any substance comprising two vicinal carboxylic acids, in particular trimellitic acid and maleic acid. Substance (S) comprises preferably one and only one functional group capable of reacting with an amine group and an isocyanate group, or one and only one precursor of said functional group.

Substance (S) comprises more preferably one and only one precursor of a functional group capable of reacting with an amine group and an isocyanate group.

Substance (S) may be aliphatic or aromatic. It is preferably aromatic.

The most prefered substance (S) is trimellitic acid. Excellent results were obtained when trimellitic acid was used as the sole substance (S).

The amount of substance (S), in particular the amount of trimellitic acid, is preferably above 1 ,5 %, more preferably above 2 %, and still more preferably above 2.5 % by mole, based on the total number of moles of trimellitic anhydride monoacid halide and substance (S). Besides, the amount of substance (S) is advantageously below 25 %, preferably below 15 %, more preferably below 10 %, still more preferably 8 % by mole, based on the total number of moles of trimellitic anhydride monoacid halide and substance (S).

The aromatic polyamide-imide has an intrinsic viscosity (IV) which is advantageously below 0.60, preferably below 0.55 and more preferably below 0.50. Besides, it is advantageously above 0.20, preferably above 0.25 and more preferably above 0.30. Intrinsic viscosities measurements are operated on a Schott Gerate Viscometer Instrument (AVS 440) using 1 -methyl-2-pyrrolidinone (NMP) at 25°C following ASTM D2857, ASTM D5336 and ASTM D52251. The aromatic polyamide-imide has an average molecular weight (Mw) which is advantageously below 40,000, preferably below 35,000 and more preferably below 30,000 g/mol. Besides, it is advantageously above 10,000, preferably above 12,000 and more preferably above 15,000 g/mol, as determined by GPC at room temperature in a solution of DMF containing 0.1M LiBr

(dissolution at 100⁰C) with a polystyrene calibration, corrected by NMR data to give absolute values.

The polymer composition (C) comprises at least 40 wt. % based on the total weight of the polymer composition (C), of at least one aromatic polyamide- imide. The aromatic polyamide-imide is contained in polymer composition (C) in an amount of preferably above 85 wt. %, more preferably above 90 wt. %, still more preferably above 95 wt. % and the most preferably above 99 wt. %, based on the total weight of polymer composition (C). The external lubricant. The polymer composition (C) comprises preferably an external lubricant.

Any external lubricant is in principle suitable as long as it has no detrimental effect on the process and the extruded film there from.

The external lubricant is preferably a fluorocarbon polymer. For the purpose of the present invention, a fluorocarbon polymer is intended to denote any polymer of which more than 50 mol. % of the recurring units are derived from at least one ethylenically unsaturated monomer comprising at least one fluorine atom (hereafter, "the fluorinated monomer"), based on the total number of moles of recurring units.

The fluorocarbon polymer comprises preferably more than 75 wt. %, more preferably more than 90 wt. % of recurring units derived from fluorinated monomers, and still more preferably more than 97 wt. % of recurring units derived from fluorinated monomers, based on the total number of moles of recurring units.

The fluorocarbon polymer advantageously comprises recurring units derived from vinylidene fluoride (VF₂) or from tetrafluoroethylene (TFE).

Preferably, the fluorocarbon polymer consists of recurring units derived from vinylidene fluoride (VF₂) or from tetrafluoroethylene (TFE), and at least one other fluorinated monomer other than VF₂ or TFE, depending on whether VF₂ or TFE is used as the base monomer. The other fluorinated monomer can be notably vinylidene fluoride (or VF₂, when TFE is used as the base monomer); trifluoroethylene; chlorotrifluoroethylene (CTFE); 1 ,2-difluoroethylene; tetrafluoroethylene (or TFE, when VF₂ is used as the base monomer); hexafluoropropylene (HFP); octafluorobutene; perfluoro(alkyl vinyl) ethers, such as perfluoro(methyl vinyl) ether (PMVE), perfluoro(ethyl vinyl) ether (PEVE) and perfluoro(propyl vinyl) ether (PPVE); perfluoro(l,3-dioxole); perfluoro(2,2-dimethyl- 1 ,3-dioxole) (PDD); the product of formula

CF₂=CFOCF₂CF(CF₃)OCF₂CF₂X in which X is -SO₂F, -CO₂H, -CH₂OH, - CH₂OCN or -CH₂OPO₃H; the product of formula CF₂=CFOCF₂CF₂SO₂F; the product of formula F(CF₂)_nCH₂OCF=CF₂ in which n is 1, 2, 3, 4 or 5; the product of formula RiCH₂OCF=CF₂ in which Ri is hydrogen or F(CF₂)_Z, and z is 1 , 2, 3 or 4; the product of formula R₃OCF=CH₂ in which R₃ is F(CF₂)_Z and z is 1 , 2, 3 or 4; perfluorobutylethylene (PFBE); 3,3,3-trifluoropropene and 2-trifiuoromethyl-3 ,3 ,3-trifluoro- 1 -propene.

More preferably, the external lubricant is a homopolymer of tetrafluoroethylene or a copolymer of which the recurring units are derived from vinylidene fluoride (VF₂) and from hexafluoropropylene (HFP). In this second case, the VF₂/HFP copolymer consists of preferably at least 50 %, more preferably at least 60 % by moles of VF₂, and preferably at most 30 %, more preferably at most 40 % by moles of HFP based on the total number of moles of recurring units. Besides, the VF₂/HFP copolymer consists of preferably at most 95 %, more preferably at most 85 % by moles of VF₂ and preferably at least 5 % and more preferably at least 15 % by moles of HFP based on the total number of moles of recurring units.

Excellent results were obtained with the PTFE Dyneon™ PA5956, commercially available from 3M, or the vinylidene fluoride (VF₂)- hexafluoropropylene (HFP) copolymer consisting of from 60 to 85 % by moles ofVF₂ and from 40 to 15 % by moles of HFP, commercially available from Solvay Solexis under the name Tecnoflon^® NM fiuoroelastomer, or mixtures thereof.

It was surprisingly found that using an external lubricant did contribute not only to reduce adherence between the polymer and the extruder barrel and die surface, as taught in US 4,581,264, but also to reduce the variation of polymer flow and improve the consistency of the film thickness.

The external lubricant, in particular the fiuorocarbon polymer, is contained in polymer composition (C) in an amount of preferably at least 0.1 wt. %, and more preferably at least 0.3 wt. %, based on the total weight of the polymer composition. Besides, it is preferably contained in polymer composition (C) in an amount of preferably at most 2 wt. %, and more preferably at most 1 wt. %, based on the total weight of the polymer composition.

The external lubricant has an average molecular weight in number of preferably below 700, 000 (as determined by conventional GPC technique). The external lubricant, in particular the fiuorocarbon polymer is preferably homogeneously distributed in the matrix formed by polymer composition (C).

The polymer composition (C) may comprise other usual additives of aromatic polyamide-imide compositions, insofar as their nature and their amount does not impair the desired properties. Nonlimiting examples of such additives comprise adhesion promoters, antioxidants, antistatic agents, carbon black, carbon fibers, compatibilizers, curing agents, dyes, extending fillers, fire retardants, glass fibers, metal particles, mold release agents, pigments, plasticizers, reinforcing fillers, rubbers, silica, smoke retardants, tougheners, UV absorbers, and the like, and mixtures thereof. The polymer composition (C) comprises generally from 0 to 5 wt. % of at least one solvent of the aromatic polyamide-imide. The terms "solvent of the aromatic polyamide-imide" intend to denote a substance, usually a liquid, capable of dissolving the aromatic polyamide-imide. Typical solvents of the aromatic polyamide-imides according to the present invention are l-methyl-2- pyrrolidinone (NMP), dimethylacetamide (DMAc), dimethylformamide (DMF), dimethylsulfoxide (DMSO) or other dipolar aprotic solvents. For instance, NMP has to be heated around 100⁰C to dissolve aromatic polyamide-imides, but once dissolved, the solution can be cooled without any effect on the solubility of the polymer. Preferably, the polymer composition (C) is essentially free of solvent. Very preferably, it is free of solvent. The extrusion process.

The film (F) is usually extruded with an extruder, said extruder comprising a barrel and an extrusion die. The extruder may comprise a single barrel, or, more often, a plurality of barrels. The process according to the present invention comprises preferably the steps of : (i) feeding the polymer composition (C) into a the feed end of the barrel of the extruder.

Both single- and twin-screw extruders can give good results. Usually, powdered or pelletized polymer and possibly other materials such as fillers, additives and external lubricants are delivered to a hopper and from there fed into the feed end of a barrel in which the material is conveyed forward as it is mixed. The polymer composition (C) is preferably fed into the extruder at a feed rate of above 7 lb/hr (3.17 kg/hr); besides the feed rate does not preferably exceed 12 lb/hr (5.44 kg/hr). Screws with compression ratio of 1:1 to 3:1 can be used successfully for film extrusion process without significant differences. Both starve feeding and flood feeding can be used advantageously to make films as thin as 1 mil (25.4 μm). It is also advantageous to run the screw at low rate, typically below 100 rpm and preferably below 60 rpm. Notwithstanding, the film extruder screw runs preferably above 10 rpm, and more preferably above 14 rpm. (ii) melting the polymer composition (C) between the flights of the screw(s) of the extruder to form a melt (M) of the polymer composition (C). Usually, the materials are plasticized by the frictional heat generated by the rotating screw(s) of the extruder and external heat applied through the barrel. The barrel temperature of the extruder is advantageously above 250⁰C, preferably above 300⁰C and more preferably above 325°C. Besides, it is generally less than 730⁰F (387.8°C) and preferably less than 705⁰F (373.9°C). However, when the extruder is a corotating, intermeshing twin-screw extruder, good results might also be obtained with a barrel temperature of about 450⁰F (232°C) to about 600⁰F (316°C). Generally, the motion of the screw(s) advances the materials through the machine while applying shear to effect the melting and blending of polymers and additives, if any.

(iii) passing the melt (C) into a melt channel of the extrusion die, said extrusion die being attached to the exit of the extruder. The melted material leaves generally the extruder as a consequence of the screw action through an oval or circular shaped melt channel.

(iv) heating the extrusion die to maintain the polymer composition (C) as a melt. The die temperature is in general of at least 680 ⁰F (360.0⁰C), but when the die temperature is above 730⁰F (387.8°C), the material often starts to decompose. Thus, the die temperature of the extrusion die is advantageously less than 730⁰F (387.8°C) and preferably less than 705⁰F (373.9°C). Besides, the temperature of the extrusion die is advantageously above 645°F (340.5⁰C) and preferably above 670⁰F (354.4°C). The pressure of the extrusion die is generally comprised between 700 and 2500 psi (48.3 and 172.4 bars). (v) passing the melt (M) through the melt channel, said melt channel having a geometry which imparts a constantly increasing shear rate in the melt (M). (vi) extruding the melt (M) out of the exit of the extrusion die, said exit having a shape to form a film in the molten form.

The melted material enters the final extrusion die which gives generally the film its form. It is also preferred to adjust the die adaptor, so as to reduce residence time between screw tip and die.

(vii) solidifying the molten form after it has been extruded from the extrusion die to provide the film.

Solidifying is usually caused by cooling. The film can be cooled by many ways. It is preferably air-cooled. The process advantageously further comprises a step prior to the step (i) of feeding the polymer composition (C) into the extruder, consisting in pelletizing the polymer composition (C). During this step, it is advantageous to run the extruder at low rate, preferably below 60 rpm.

In a second aspect, the present invention concerns a film obtainable by the process as above described. Such film can notably be obtained by extrusion, using different types of extruder, notably a twin-screw or a single-screw extruder. The film obtainable by the process according to the present invention is preferably identical to film (F) and complies very preferably with all the preferred characteristics of film (F), whatever the level of preference, notably as concerns its thickness. Such a film may also be obtained by other processes while maintaining the same or substantially the same characteristics featured by the film (F)

The films (F) manufactured by the invented process or films obtainable by such process are useful in a huge number of applications, including electrical and electronic insulation applications, such as wire and cable tapes, substrates for flexible printed circuits, motor slot liners, magnet wire insulation, transformer and capacitor insulation, etc. They can also be used for friction and wear applications, such as metal-polymer bushings and bearings.

An outstanding advantage of the process according to the present invention is that it makes it possible to high quality extruded very thin films (as thin as 25 μm) from a polymer composition comprising well above 80 wt. % of aromatic polyamide-imide which is essentially free of solvent. Such beneficial merits were not achievable by any prior art extrusion process.

The following examples are included to provide additional guidance to those skilled in the art in practicing the claimed invention. The examples provided are merely representative of the work that contributes to the teaching of the application. Accordingly, these examples are not intended to limit the invention, as defined in the appended claims, in any manner.

Excellent results were obtained when using the process substantially as herein exemplified. EXAMPLES

The following raw materials were :

^■ Aromatic polyamide-imide (PAI) powders manufactured by a process including the polycondensation reaction of trimellitic acid chloride, 4,4'-diaminodiphenylether and m-phenylenediamine, and trimellitic acid. Three grades (PAI 1, PAI 2 and PAI 3) were prepared, differing only by the amount of trimellitic acid ;

^■ PTFE Dyneon™ PA5956, a homopolymer of tetrafluoroethylene, commercially available from 3M ;

^■ Tecnoflon^® NM, a fluoroelastomer made OfVF₂ZHFP copolymer, commercially available from Solvay Solexis ;

^■ Magnesium oxide, commercially available from Kyowa as KM-3150.

Six polymer compositions (samples) were prepared in the form of pellets on a Berstorff B extruder, using the same pelletizing compounding conditions as follows :

Table 1 : pelletizing compounding conditions

The components of each formulation were mixed together during the pelletization step. The screw rate was set at 60 rpm. The three grades of PAI featuring different amounts of trimellitic acid used are summarized in the first lines of table 2. They had intrinsic viscosities of 0.486 dl/g for PAI 1, 0.699 dl/g for PAI 2 and of 0.468 dl/g for PAI 3 (measured as defined in the core of the description of the present invention). Table 2 : Nature and amount of the ingredients of the polymer compositions

(samples)

Films were then prepared from the above six polymer compositions (samples). All film extrusion were carried out on an Egan single screw extruder, fitted with a 6 inch film die, using starve feeding mode. Used conditions are summarized in Table 3. Screw rate was set at 60 rpm. All films were cured for 4 hours at 500⁰F (260⁰C).

Table 3 : Film extrusion conditions

Films from samples 1, 2 and 3 were extruded with a 2.3:1 screw profile. Also, a film from sample 1 was extruded using a 1 : 1 screw profile (hereafter "Ia"). Experiments with sample 1 (1 and Ia) on table 4 using PAI 1, according to the present invention (with TMA = 3.0 % based on the total number of moles of trimellitic anhydride monoacid halide and trimellitic acid), gave very good looking thin films, while the experiment with sample 2 with PAI 2 (only 0.3 % TMA) was aborted due to high die pressure (>4300 psi). This showed that the screw profile did not have any influence on the quality of the film obtained. Raising the TMA amount from 3.0 to 5.5 % (PAI 3), led to the extrusion of good looking films as thin as 25 μm (sample 3).

Surprisingly, films obtained according to the invention featured some anisotropic properties (see table 4). Properties such as tensile strength, tensile elongation and tensile modulus along the machine direction (MD) were better than those in the transverse direction (TD).

Table 4 : Film properties

Some other films were extruded from polymer compositions containing Tecnoflon^®, magnesium oxide and eventually PTFE (samples 4, 5 and 6 on table 5). Film thicknesses of below 50 μm were obtained featuring also some anisotropic properties. Table 5 : Film properties

Once again, properties such as tensile strength, tensile elongation and tensile modulus along the machine direction (MD) were better than those in the transverse direction (TD). The coefficient of linear thermal expansion (CLTE) followed also the same trend. It was surprisingly been found that the properties of sample 6 were more consistent than the others.

The invention has been described with reference to preferred and exemplary embodiments but is not limited thereto. Those skilled in the art will appreciate that various modifications can be made without departing from the scope of the invention, which is defined by the following claims.

Claims

C L A I M S

1. Process for extruding a film (F) having a thickness of below 1000 μm from a polymer composition (C), wherein the polymer composition (C) comprises at least 40 wt. %, based on the total weight of the polymer composition (C), of at least one aromatic polyamide-imide manufactured by a process including the polycondensation reaction of :

(i) trimellitic anhydride monoacid halide;

(ii) at least one comonomer chosen from diamines and diisocyanates, and

(iii) at least one substance (S) comprising one and only one functional group capable of reacting with either an anhydride group and an acid halide group on one hand, or an amine group and an isocyanate group on the other hand, or one and only one precursor of said functional group,

wherein the amount of substance (S) is of above 1 mol. %, based on the total number of moles of trimellitic anhydride monoacid halide and substance (S).

2. Process for extruding a film (F) having a thickness of below 1000 μm from a polymer composition (C), wherein the polymer composition (C) comprises at least 40 wt. %, based on the total weight of the polymer composition (C), of at least one aromatic polyamide-imide manufactured by a process including the polycondensation reaction between (i) an acid monomer combination of a trimellitic anhydride monoacid halide with trimellitic acid, and (ii) at least one comonomer chosen from diamines and diisocyanates, wherein the trimellitic acid amount is above 2 mol. %, based on the total number of moles of acid monomer.

3. Process according to Claim 1 or 2, characterized in that the film (F) is extruded with an extruder, said extruder comprising a barrel and an extrusion die.

4. Process according to Claim 3, characterized in that the process comprises the steps of :

(i) feeding the polymer composition (C) into the feed end of the barrel of the extruder; (ii) melting the polymer composition (C) between the flights of the screw(s) of the extruder to form a melt (M) of the polymer composition (C);

(iii) passing the melt (C) into a melt channel of the extrusion die, said extrusion die being attached to the exit of the extruder;

(iv) heating the extrusion die to maintain the polymer composition (C) as a melt;

(v) passing the melt (M) through the melt channel, said melt channel having a geometry which imparts a constantly increasing shear rate in the melt (M);

(vi) extruding the melt (M) out of the exit of the extrusion die, said exit having a shape to form a film in the molten form;

5. Process according to Claim 4, further comprising the step of pelletizing the polymer composition (C), prior to the step of feeding it into the extruder.

6. Process according to any one of Claims 3 to 5, characterized in that the extruder is a corotating, intermeshing twin-screw extruder, having a barrel temperature of about 450⁰F (232°C) to about 600⁰F (316°C).

7. Process according to any one of Claims 3 to 5, characterized in that the extruder is a corotating, intermeshing single-screw extruder.

8. Process according to any one of Claims 3 to 7, characterized in that the temperature of the extrusion die is above 645°F (340.5⁰C).

9. Process according to Claim 8, characterized in that the temperature of the extrusion die is above 680⁰F (360.0⁰C).

10. Process according to any one of Claims 3 to 9, characterized in that the temperature of the extrusion die is less than 730⁰F (387.8°C).

11. Process according to Claim 10, characterized in that the temperature of the extrusion die is less than 710⁰F (376.7°C).

12. Process according to any one of Claims 3 to 11, characterized in that the polymer composition (C) is fed into the extruder at a feed rate of above 7 1b/hr (3.17 kg/hr).

13. Process according to any one of Claims 3 to 12, characterized in that the polymer composition (C) is fed into the extruder at a feed rate of less than

12 1b/hr (5.44 kg/hr).

14. Process according to any one of the preceding Claims, characterized in that the aromatic polyamide-imide has a weight average molecular weight of below 40,000 g/mol.

15. Process according to any one of the preceding Claims but 2, characterized in that the substance (S) amount is below 15 mol. %, based on the total number of moles of trimellitic anhydride monoacid halide and substance (S).

16. Process according to any one of the preceding Claims, characterized in that the polymer composition (C) comprises at least 85 wt. %, based on the total weight of polymer composition (C), of the aromatic polyamide-imide.

17. Process according to any one of the preceding Claims, characterized in that the thickness of the film (F) is of below 100 μm.

18. Process according to Claim 17, characterized in that the thickness of the film (F) is of below 55 μm.

19. Process according to any one of the preceding Claims, characterized in that the polymer composition (C) further comprises an external lubricant.

20. Process according to Claim 19, characterized in that the external lubricant is a fluorocarbon polymer.

21. Process according to Claim 20, characterized in that the fluorocarbon polymer is a homopolymer of tetrafiuoroethylene.

22. Process according to Claim 20, characterized in that the fluorocarbon polymer is a vinylidene fluoride (VF₂) - hexafluoropropylene (HFP) copolymer consisting of from 60 to 85 % by moles OfVF₂ and from 40 to 15 % by moles of HFP.

23. Process according to Claim 20, characterized in that the fluorocarbon polymer is a mix of a homopolymer of tetrafluoroethylene and a vinylidene fluoride (VF₂) -hexafluoropropylene (HFP) copolymer consisting of from 60 to 85 % by moles of VF₂ and from 40 to 15 % by moles of HFP.

24. Process according to any one of Claims 19 to 23, characterized in that the external lubricant is comprised in the polymer composition (C) in an amount of at least 0.1 wt. %, based on the total weight of the polymer composition (C).

25. Process according to any one of Claims 19 to 24, characterized in that the external lubricant is comprised in the polymer composition (C) in an amount of at most 1 wt. %, based on the total weight of the polymer composition (C).

26. Process according to any one of the preceding Claims, characterized in that the polymer composition (C) comprises from 0 to 5 wt. % of at least one solvent of the aromatic polyamide-imide.

27. Film obtainable by the process according to any one of the preceding Claims.

28. Use of the film according to Claim 27 in applications chosen from electrical and electronic insulation applications and friction and wear applications.