ACRYLONITRILE POLYMER FILM AND PROCESS FOR PREPARING SAME

Abstract: An acrylonitrile polymer film comprising at least 85 wt.% of acrylonitrile, wherein the difference in the value of the sec­ ond moment of the broad-line NMR spectrum at 20°C between the film and the starting polymer powder thereof, or the poly­ mer powder formed by dissolving the film and then precipitat­ ing the polymer from the solution, is at least 0.3 Gauss² and the infrared dichroic ratio of the nitrile groups satisfies the require­ ments that 0.4 < Ax/Az < 0.8, 0.4 < Ay/Az < 0.8, and 0.8 < Ax/Ay < 1.25, where Ax and Ay are the absorption intensities of polarized infrared rays oscillating in respective directions parallel to the surface of the film and perpendicular to each other, and Az is the absorption intensity of polarized infrared rays oscillating in the direction perpendicular to the film sur­ face. This film is prepared by coagulating an acrylonitrile po­ lymer solution in an aqueous coagulating bath, adjusting the water content of the obtained water-containing coagulated film to from 20 to 100 wt.% based on the weight of the dry polymer, longitudinally stretching the film using rolls having a surface temperature of 65° to 95°C, then transversely stretching the film at a water content of from 10 to 40 wt.% based on the weight of the dry polymer, which is lower than the water con­ tent at the longitudinally stretching step, in an atmosphere maintained at from 100° to 180°C by using a tenter. The film has improved brittleness characteristics such as tensile elon­ gation, folding endurance, and impact strength.

Description: This invention relates to an acrylonitrile polymer film and a process for the preparation thereof. More particularly, it relates to an acrylonitrile polymer film with highly reduced brittleness characteristics, namely, highly improved tensile elongation, folding endurance, and impact strength, and a process for the preparation of this film. (2) Description of the Prior Art Films made of acrylonitrile polymers, especially polymers with a high nitrile content, have excellent gas barrier properties, weatherability, and stain resistance, but have inferior characteristics with regard to brittleness, such as tensile elongation, folding endurance, and impact strength. Moreover, since such polymers have poor thermoplasticity, the film should be prepared by a solvent shaping process using a solvent or a semi-melt shaping process using a plasticizer. However, the films obtained by removing the solvent or plasticizer from the as-formed film have poor mechanical strengths and require orientation, and especially biaxial orientation. Various methods for obtaining acrylonitrile polymer films having good mechanical strengths by biaxially stretching unoriented films of acrylonitrile polymer have already been proposed. However, when an acrylonitrile polymer film is oriented by stretching, fibrillation is readily caused, making biaxial stretching, and especially sequential biaxial stretching, difficult. As a typical example of the methods which enable biaxial stretching of an acrylonitrile polymer film, there can be mentioned a method in which an unoriented film of an acrylonitrile polymer still containing a solvent, a plasticizer, or water is biaxially stretched. However, this method has drawbacks in connection with orientation and densification, making it impossible to obtain satisfactory improvements in the mechanical strength characteristics, and especially a satisfactory reduction in the brittleness characteristics, of the film. U.S. Patent No. 3,437,717 discloses another method for biaxially stretching an unoriented acrylonitrile polymer film in the plasticized state, in which a solution of an acrylonitrile polymer in a polar organic solvent is extruded in the form of a sheet and part of the solvent removed, after which the sheet is biaxially stretched and the remainder of the solvent is removed by drying with hot air. In this method, however, fluidization of molecules occurs, as a result of which sufficient orientation and densification are not attained. U.S. Patent No. 2,706,674 discloses a method in which an acrylonitrile polymer is brought into the semi-molten state by the hydration of the nitrile groups in the polymer at high temperature and pressure. This semi-molten acrylonitrile polymer is then extruded as a film through a slot die into an inert gas atmosphere at room temperature. However, the film thus obtained has a large void ratio and an insufficient mechanical strength. Japanese Examined Patent Publication No. 53-15,069 and U.S. Patent No. 4,144,299 provide a method in which a solution of an acrylonitrile polymer is coagulated in an aqueous coagulating bath, the film formed is washed with water to remove the solvent, and the obtained undrawn water-containing film is biaxially stretched in the presence of water at a temperature of at least 100°C by a tenter method or an inflation method. The film thus obtained is highly oriented, but still has poor tensile elongation, and other brittleness characteristics. In addition, U.S. Patent No. 4,066,731 discloses a method in which an undrawn film of acrylonitrile polymer is biaxially stretched in the presence of water in a humid atmosphere at a temperature not higher than 90°C (200°F), and preferably at a temperature not higher than 70°C (160°F), whereafter it is densified by drying. Even by this method, however, sufficient densification is difficult to attain, and the brittleness characteristics are not sufficiently reduced. It is therefore a primary object of the present invention to provide a film of an acrylonitrile polymer with reduced brittleness characteristics, namely, improved tensile elongation, folding endurance, and impact strength. Another object of the present invention is to provide an industrially advantageous process for preparing a film of an acrylonitrile polymer with reduced brittleness characteristics. More specifically, in accordance with one aspect of the present invention, there is provided an acrylonitrile polymer film comprising at least 85% by weight of units of acrylonitrile, wherein the difference in the value of the second moment of the broad-line NMR spectrum at 20°C between the film and the starting polymer powder used for the preparation thereof or the polymer powder formed by dissolving the film and then precipitating the polymer from the solution is at least 0.3 Gauss² and the infrared dichroic ratio of the nitrile groups satisfies the requirements that 0.4 < Ax/Az < 0.8, 0.4 < Ay/Az < 0.8 and 0.8 < Ax/Ay < 1.25, in which Ax and Ay stand for the absorption intensities of polarized infrared rays oscillating in respective directions parallel to the surface of the film and perpendicular to each other, and Az stands for the absorption intensity of polarized infrared rays oscillating in the direction perpendicular to the surface of the film. In accordance with another aspect of the present invention, there is provided a process for the preparation of acrylonitrile polymer films, which comprises coagulating a solution of an acrylonitrile polymer in an aqueous coagulating bath, adjusting the water content of the coagulated water-containing film obtained to from 20 to 100% by weight, based on the weight of the dry polymer, stretching the film in the longitudinal direction by using rolls having a surface temperature of 65 to 95°C, then stretching the film in the transverse direction at a water content of from 10 to 40% by weight, based on the weight of the dry polymer, which is lower than the water content in the longitudinal stretching step, in an atmosphere maintained at from 100° to 180°C by using a tenter. The invention will be described in more detail by way of the preferred embodiments. The acrylonitrile polymer used in the present invention is a homopolymer of acrylonitrile or an acrylonitrile copolymer containing at lease 85% by weight of acrylonitrile units. As the comonomer to be copolymerized with acrylonitrile, there can be mentioned acrylic acid esters such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, amyl acrylate, hexyl acrylate, phenyl acrylate and octyl acrylate; methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, amyl methacrylate, hexyl methacrylate, phenyl methacrylate and octyl methacrylate; vinyl halides and vinylidene halides such as vinyl chloride, vinyl bromide, vinyl fluoride, vinylidene chloride and vinylidene flouride; vinyl amides such as acrylamide, methacrylamide, N-methylacrylamide and N-methylacrylamide and N=vinylpyrrolidone; vinyl esters such as vinyl acetate, vinyl propionate and vinyl butyrate; vinyl aromatic compounds such as styrene, vinylnaphthalene and vinylpyridine; vinyl carboxylic acids such as acrylic acid and methacrylic acid; unsaturated dicarboxylic acids such as itaconic acid, maleic acid and fumaric acid; unsaturated dicarboxylic anhydrides such as itaconic anhydride and maleic anhydride; allylsulfonic acid, methallylsulfonic acid, and their salts; methacrylonitrile; and olefins such as ethylene and propylene. Copolymers comprising acrylonitrile and at least two of the foregoing comonomers may be used. Blends of two or more of the foregoing polymers may also be used. If the acrylonitrile content of the acrylonitrile copolymer is lower than 85% by weight, the plasticizing effect of water is reduced, making the stretching operation difficult, although the extent of the plasticizing effect reduction varies depending on the particular comonomer. In addition, the properties required for acrylonitrile polymer films, such as the gas barrier properties and weatherability, are insufficient. The second moment of the broad-line NMR spectrum is determined by using a JNM-PW60 (60 MHz) NMR spectrometer supplied by Nippon Denshi K.K. The second moment is a parameter useful for estimating the structure of the film, especially the packing state of the film molecules. In the case of acrylonitrile polymer films, the second moment is thought to vary with the crystallinity of the polymer and the void ratio in amorphous parts of the film. A larger value of the second moment of the film indicates that the mechanical strength of the film is higher. In order to attain the intended reduction in brittleness, it is essential that the difference in the value of the second moment between the film and the starting polymer powder used for the preparation thereof or the polymer powder formed by dissolving the film and then precipitating the polymer from the solution be at least 0.3 Gauss². A range in this difference of from 0.3 to 2.0 Gauss² is especially convenient. The infrared dichroic spectrum is determined by using a JIR 40X spectrometer supplied by Nippon Denshi K.K. according to the method disclosed in "Die Makromolekulare Chemie", Vol. 176, 2451-2458 (1975). The acrylonitrile polymer is oriented by stretching, and the infrared dichroic ratio (Ax/Az or Ay/Az) of the nitrile group µCN absorption at 2245 cm -1 and the methylene group -CH₂-absorption at 1455 cm -1 decreases with an increase in the degree of stretching, i.e. orientation. The infrared dichroic ratio of the absorption of the nitrile group at 2245 cm -1 varies greatly depending on the degree of orientation; by stretching, the nitrile groups are oriented on the surface of the film in a direction perpendicular to the stretching direction. Ordinarily, the mechanical strength of the acrylonitrile polymer film increases with an increase in the degree of orientation, but the brittleness characteristics such as tensile elongation, impact strength, and folding endurance are insufficient either when orientation in both the longitudinal and transverse directions is too strong or when orientation in the longitudinal and transverse directions is unbalanced. Orientation in both the longitudinal and transverse directions should therefore lie within specific ranges. More precisely, in the present invention, the infrared dichroic ratio of the nitrile groups must satisfy the conditions that 0.4 < Ax/Az < 0.8, 0.4 < Ay/Az < 0.8, and 0.8 < Ax/Ay < 1.25, where Ax and Ay stand for the absorption intensities of polarized infrared rays oscillating in respective directions parallel to the surface of the film and perpendicular to each other, and Az stands for the absorption intensity of polarized infrared rays oscillating in the direction perpendicular to the surface of the film. As is apparent from the foregoing illustration, the mechanical strength , and especially brittleness, of the acrylonitrile polymer film is closely related to the second moment of the broad-line NMR spectrum and the infrared dichroic ratio. Acrylonitrile polymer films satisfying the above-mentioned requirements for both factors exhibit improved brittleness characteristics. The above-mentioned structure is thermally stable, and the brittleness characteristics are especially highly reduced in acrylonitrile polymer films in which the change in the second moment of the broad-line NMR spectrum caused by carrying out hot air heat treatment at 140°C for 1 minute is less than 0.5 Gauss². The acrylonitrile polymer film of the present invention not only has highly improved mechanical strength and reduced brittleness, but also the good gas barrier properties, weatherability, and stain resistance inherent in ordinary acrylonitrile polymer film. The film of the present invention has excellent gas-barrier properties even in the wet state. Therefore, it is very useful as a packaging material for water-containing substances or foods to be boiled or retorted. Furthermore, because of its excellent weatherability and stain resistance, the film of the present invention is very valuable as a laminate on steel plates, plywood, or resin plates. The film of the present invention is prepared, for example, by the process described below. A solution of an acrylonitrile polymer is coagulated in an aqueous coagulating bath. The water content of the coagulated water-containing undrawn film obtained (hereinafter referred to as "water-containing starting film") is adjusted to from 20 to 100% by weight based on the weight of the dry polymer, preferably by hot air drying and/or hot water immersion. Next, the film is stretched in the longitudinal direction using rolls having a surface temperature of from 65 to 95°C, following which it is stretched in the transverse direction at a water content of from 10 to 40% by weight, based on the weight of the dry polymer, which is lower than the water content in the longitudinal stretching step, in an atmosphere maintained at from 100 to 180°C by using a tenter. The water-containing starting film of the acrylonitrile polymer is ordinarily prepared by a wet coagulation method using an aqueous coagulating bath. More specifically, a solution of an acrylonitrile polymer is cast on the surface of a belt or drum, or extruded in the form of a sheet or tube through a slot die, and the extrudate is coagulated in an aqueous coagulating bath to obtain a film. A conventional process for casting or extruding the acrylonitrile polymer solution may be used. The solvent used to form the polymer solution may be, for example, dimethylformamide, dimethylsulfoxide, N-dimethylformamide, N-methyl-β-cyanoethylformamide, α-cyanoacetamide, acetonitrile, γ-butyrolactone, ethylene carbonate, propylene carbonate, or an aqueous solution of an inorganic acid and its salts, such as nitric acid, sulfuric acid, zinc chloride or sodium thiocyanate. The polymer concentration in the polymer solution is ordinarily higher than 10% by weight but lower than 50% by weight, and preferably in the range of from 10 to 35% by weight. As the polymer concentration becomes higher, the density of the stretched film increases, but at excessively high polymer concentrations, the viscosity of the solution becomes very large, making the solution difficult to deaerate and filtrate. Water or an aqueous solution of the solvent used for forming the polymer solution (for example, dimethylformamide or nitric acid) may be advantageously used as the aqueous coagulating bath for effecting the coagulation. In order to obtain a high-density stretched film, the solvent concentration in the aqueous solution should be made as high as possible, yet not so high as to prevent coagultion from occurring. Ordinarily, this concentration may be higher when organic solvents are used compared with the case in which inorganic solvents are used. For example, it is preferred that the concentration be from 20 to 40% by weight in the case of nitric acid, and from 30 to 60% by weight in the case of dimethylformamide. When the polymer concentration of the polymer solution is high, a dense film can be obtained even if the solvent concentration in the coagulating solution is low. The solvent contained in the coagulated film may be removed by washing with cold or warm water. A low degree of solvent removal is desirable in view of the plasticizing effect of the residual solvent, but sufficient solvent should be removed to obtain the desired properties in the film. The water-containing starting film thus obtained ordinarily has a water content of from 50 to 400% by weight based on the weight of the dry polymer. Furthermore, the water-containing acrylonitrile polymer starting film of the present invention may be prepared by one of the processes described below. As described in U.S. Patent No. 3,437,717, an acrylonitrile polymer is cast into a film using a volatile organic solvent, following which the solvent is volatilized to some extent by hot air, and the residual solvent removed by replacement with water or hot water. Moreover, a process may be adopted in which, as described in U.S. Patent 2,706,674, the nitrile groups of an acrylonitrile polymer are hydrated with water to melt the polymer, and the polymer in the semi-molten state extruded into a water vapor atmosphere at high temperature and pressure. When this process is used, a small amount of a plasticizer should preferably be added. The water content of the water-containing starting film thus prepared is adjusted to within the predetermined range, and the film then stretched. If the water content of the film and the temperature are adjusted to within the respective predetermined ranges, this ensures the stable preparation of a dense and transparent film with excellent uniformity of thickness and orientation. The water content of the water-containing coagulated acrylonitrile polymer film thus formed is adjusted to from 20 to 100% by weight based on the weight of the dry polymer. Adjustment of the water content in the water-containing starting film can be accomplished by a drying method using hot air, infrared rays, far infrared rays, steam, a hot plate, or hot rolls. However, rapid drying tends to result in devitrification or the formation of pores in the starting film, and hence, rapid drying should be avoided. Hot air drying is especially preferred because a stretched film with excellent transparency and mechanical strength can be obtained, although the drying efficiency is low. Immersing the water-containing starting film in hot water maintained at 50° to 90°C is very effective for performing dehydration and drying efficiently and uniformly. This method is especially effective for dehydration of starting films having a high water content. For example, when a starting film having a water content of about 250% by weight is immersed in hot water maintained at 80°C for about 30 seconds, the film is uniformly dehydrated and dried to a water content of about 80% by weight. If the temperature of the hot water is higher than 90°C, considerable devitrification (the formation of pores) of the film by dehydration occurs, and it becames difficult to restore sufficient density to the film by subsequent heating. As a result, the physical properties, and especially the mechanical strength, of the film are reduced. If the temperature of the hot water is lower than 50°C, the drying effect by dehydration is lowered. A standard roll-tenter sequential biaxial drawing apparatus is usually provided with a preheating zone formed upstream to a drawing zone of each the rolls and tenter. The adjustment of the water content and preliminary heating can be accomplished by utilizing this preheating zone. The film having the water content adjusted in the above-mentioned manner is then subjected to a sequential stretching operation using rolls and a tenter. More specifically, the starting film with a water content adjusted to from 20 to 100% by weight based on the weight of the dry polymer is longitudinally stretched using rolls having a surface temperature of 65° to 95°C, following which the film is transversely stretched at a water content of from 10 to 40% by weight based on the weight of the dry polymer, which is lower than the water content in the longitudinally stretching step, in an atmosphere maintained at 100° to 180°C by using a tenter. If in the roll-stretching step, the water content of the starting film is lower than 20% by weight, it is difficult to maintain a water content at a level of at least 10% by weight in the subsequent tenter-stretching step. If the water content exceeds 80% by weight in the roll-stretching step, the formation of some pores is observed, but at a water content of up to 100% by weight, the film is densified and rendered transparent by the subsequent tenter-stretching operation. However, if the water content exceeds 100% by weight in the roll-stretching step, pores (devitrification) that cannot be removed in the subsequent tenter-stretching step are formed. Ordinarily, the water content of the starting film in the roll-stretching step is preferably from 15 to 70% by weight. If the surface temperature of the rolls is lower than 65°C, the stretching stress is increased and the film readily broken during roll-stretching. If the surface temperature of the rolls is higher than 95°C, pores are formed (devitrification) by abrupt dehydration, and the film cannot be densified or rendered transparent during subsequent tenter-stretching. It is ordinarily preferred that the surface temperature of the rolls lie in a range of from 70° to 90°C. If the water content is lower than 10% by weight or the atmospheric temperature is lower than 100°C in the tenter-stretching step, the stretching stress is increased and the film readily broken by stretching. If the water content exceeds 40% by weight in the tenter -stretching step, the stretching stress is increased and uniform stretching becomes difficult; and devitrification is casued to some extent in the stretched film and the mechanical strength is reduced. If the atmosphere temperature exceeds 180°C, uniform adjustment of the water content becomes difficult and necking readily caused. Ordinarily, it is preferred that the tenter-stretching operation be carried out at a water content of from 15 to 30% by weight and an atmospheric temperature of from 105° to 150°C. If the film contains a small amount (usually not more than 5%) of a plasticizer in addition to water, the tenterstretching operation can be carried out at a water content and an atmospheric temperature, which are slightly lower than the above-mentioned water content and atmospheric temperature. If the acrylonitrile content of the polymer is close to 100%, i.e., if the copolymer has a composition close to that of an acrylonitrile homopolymer, the stretching stress is increased and uniform stretching becomes difficult. However, if a certain amount of plasticizer or solvent is incorporated in the undrawn film, the plasticizing effect of the water is enhanced and unifrom stretching becomes possible. The upper limit of the acrylonitrile content of the polymer that allows uniform stretching without such auxiliary means is 98% by weight. In order to obtain a film having high mechanical strength, it is preferred that the stretching ratio lie in a range of from 1.5 to 4, and more preferably 2.0 to 3.0, in either the longitudinal direction or the transverse direction. In order to obtain a film satisfying the requirements relating to the second moment of the broad-line NMR spectrum and the infrared dichroic ratio and having reduced brittleness characteristics according to present invention, it is preferred that the stretch ratio lie in a range of from 2.0 to 3.0, and the ratio of the stretching ratio in the longitudinal direction to the stretching ratio in the transverse direction (MD/TD; MD stands for the longitudinal direction and TD stands for the transverse direction) lie in a range of from 0.8 to 1.5. In order to improve the thermal dimensional stability and the mechanical strength characteristics, heat setting treatment may be performed, if desired, after the stretching operation. Moreover, surface treatment may also be carried out to prevent the accumulation of static charges or improve the adhesiveness. The present invention will now be described in detail with reference to the following examples that by no means limit the scope of the invention. Incidentally, in these examples, all percentages and parts are given by weight. Example 1 In 80 parts of an aqueous 70% nitric acid solution were dissolved 20 parts of a powdery acrylonitrile copolymer comprising 94% acrylonitrile and 6% methyl acrylate, and having a reduced viscosity of 1.4 as measured at 35°C in dimethylformamide as the solvent at a polymer concentration of 0.2% and a second moment of 19.6 Gauss² in the broad-line NMR spectrum. The polymer solution was deaerated in vacuo and then extruded in to an aqueous 30% nitric acid solution maintained at 0°C through a T-die having a slot width of 300 mm and a slot clearance of 2.0 mm while the resulting film was taken up at a rate of 2 m/min, to give a transparent coagulated film having a thickness of 350 µm. The film was washed in a coutercurrent-type water washing bath to remove the residual solvent (nitric acid), giving a water-containing starting film with a water content of 170%. The film was dried in a hot air oven maintained at 80°C to adjust the water content. By using a roll-tenter biaxial drawing apparatus, the film was stretched in the longitudinal direction using metal rolls, then stretched in the transverse direction using a tenter to effect sequential biaxial stretching. The stretching ratio was 3.0 in the longitudinal direction and 2.5 in the transverse direction based on the original length of the starting film. The stretched film was heat-set by hot air maintained at 180°C while maintaining the film at a constant length. The stretching conditions, stretchability, NMR second moment, infrared dichroic ratio, and physical properties of the stretched film are shown in Table 1. Note 1) Nos. 1 through 3 are runs according to the present invention, and Nos. 4 through 7 are comparative runs. 2) For the tensile strength, tensile elongation and folding endurance, the upper value is the value as measured in the longitudinal direction (MD) and the lower value is the value as measured in the transverse direction (TD). 3) The values or the tensile strength, tensile elongation and impact strength were determined according to the method in JIS Z-1707 (the falling ball test). 4) The folding endurance values were determined according to the mehtod in JIS P-8115. Example 2 In 70 parts or dimethylformamide were dissolved 30 parts or the same acrylonitrile copolymer as used in Example 1. The polymer solution was deaerated and then extruded in an aqueous 50% dimethylformamide solution through the same T-die as used in Example 1 to obtain a coagulated film having a thickness of 280 µm. The solvent was removed first by water washing in a coutercurrent-type water washing bath, then by using warm water maintained at 60°C to obtain a water-containing starting film having a residual water content of 3%. In the same manner as described in Example 1, the starting film was dried by hot air maintained at 120°C to adjust the water content, and the film subjected first to roll-stretching then to tenter-stretching to effect sequential biaxial stretching. In the roll-stretching step, the water content of the starting film was 20% [sample (a)] or 40% [sample (b) ] and the roll surface temperature was 80°C, while in the tenter-stretching step, the water content was 12% [sample (a)] or 30% [sample (b)] and the atmospheric temperature was 120°C. the stretching ratio was 3.0 in either the longitudinal direction or the transverse direction. A substantially uniform stretched film having a good transparency was obtained. The results data obtained for this film are as follows. Example 3 The water-containing starting film obtained in Example 1 was immersed for 60 seconds in hot water maintained at 40° to 95°C, dried by hot air maintained at 120°C and then roll-stretched in the longitudinal direction at a roll surface temperature of 80°C, a water content of 40% in the starting film and a stretching ratio of 3.0. Then the film was stretched in the transverse direction at a water content of 15%, an atmospheric temperature of 130°C, and a stretching ratio of 2.5. The hot water treatment conditions and physical properties of the stretched film are shown in Table 2. The physical properties of the stretched film were determined after the film had been heat-set by hot air maintained at 180°C while maintaining the film at a constant-length. In each of the stretched films shown in Table 2, the NMR second moment was 20.2 to 20.6 Gauss², and the infrared dichroic ratios Ax/Az and Ay/Az were 0.6 to 0.8. Example 4 Using a Henschel mixer, 60 parts of a powdery acrylonitrile polymer comprising 88% of acrylonitrile and 12% of vinyl acetate and having a broad-line NMR second moment of 18.3 Gauss² were homogeneously kneaded at 30°C with 40 parts of ethylene carbonate to obtain flakes. The flakes were extruded in the form of a strand by a monoaxial screw extruder maintained at 150°C, then pelletized using a pelletizer. The pellets were extruded at 155°C in the form of a sheet through a T-die by using a biaxial screw extruder to obtain a starting film having a thickness of 140 µm. The starting film was immersed in hot water maintained at 80°C and the solvent removed to an ethylene carbonate content of 3%, giving an opaque film having a water content of 48%. The starting film was stretched in the longitudinal direction at a roll surface temperature of 80°C, a water content of 30% in the starting film, and a stretching ratio of 3.0 by using a roll-drawing apparatus provided with a preheating roll maintained at 60°C, following which the film was stretched in the transverse direction at an atmospheric temperature of 110°C, a water content of 16%, and a stretching ratio of 2.7 using a tenter-drawing apparatus. The stretched film was heat-set by hot air maintained at 180°C while maintaining the film at a constant length to obtain a uniform and transparent film having the following physical properties.

Claims: An acrylonitrile polymer film comprising at least 85% by weight of acrylonitrile, wherein the difference in the value of the second moment of the broad-line NMR spectrum at 20°C between the film and the starting polymer powder used for the preparation thereof, or the polymer powder formed by dissolving the film and then precipitating the polymer from the solution is at least 0.3 Gauss² and the infrared dichroic ratio of the nitrile groups satisfies the requirements that 0.4 < Ax/Az < 0.8, 0.4 < Ay/Az < 0.8, and 0.8 < Ax/Ay < 1.25, wherein Ax and Ay stand for the absorption intensities of polarized infrared rays oscillating in respective directions parallel to the surface of the film and perpendicular to each other, and Az stands for the absorption intensity of polarized infrared rays oscillating in the direction perpendicular to the surface of the film. An acrylonitrile polymer film as set forth in claim 1, wherein the change in the second moment of the broad-line NMR spectrum, which is caused by hot air heat treatment conducted at 140°C for 1 minute, is less than 0.5 Gauss². A process for the preparation of an acrylonitrile polymer film as set forth in claim 1 or 2, which comprises coagulating a solution of an acrylonitrile polymer in an aqueous coagulating-bath, adjusting the water content of the obtained water-containing coagulated film to from 20 to 100% by weight based on the weight of the dry polymer, stretching the film in the longitudinal direction using rolls having a surface temperature of 65° to 95°C, then stretching the film in the transverse direction at a water content of from 10 to 40% by weight based on the weight of the dry polymer, which is lower than the water content in the longitudinal stretching step, in an atmosphere maintained at 100 to 180°C by using a tenter. A process for the preparation of an acrylonitrile polymer film according to claim 3, wherein the adjustment of the water content of the water-containing coagulated film is carried out by hot air drying. A process for the preparation of an acrylonitrile polymer film according to claim 3, wherein the adjustment of the water content of the water-containing coagulated film is carried out by dehydration by immersion in hot water maintained at 50° to 90°C, or by a combination of immersion in hot water maintained at 50° to 90°C, with hot air drying.

Inventors: INOUE, AKIO, TANABE, TSUNEAKI
Applicants: ASAHI KASEI KOGYO KABUSHIKI KAISHA
Application year: 1983
Main classification: C08J5
IPC codes: C08J5, B29D7