Biaxially Oriented Polyamide Film

This invention relates to polymeric film. It further relates to articles prepared from amorphous polyamides.

Polymeric films have a wide range of commercial uses, from beverage bottles and transparent packaging for food to strong films for industrial uses such as magnetic tape and insulation material. It is known that crystalline polymeric materials such as polyethylene, polyester, poly(vinyl chloride), polystyrene, and certain polyamides can be biaxially stretched for "orientation" of the polymer molecules. The stretched film is then annealed to heat-set the oriented molecules. The result is generally, for such crystalline linear polymers, a stronger and denser material.

It would be desirable to impart to the sheet products of non-crystalline polymers such as polyamides certain of the advantages of biaxially-oriented crystalline polymeric films.

It is therefore an object of the invention to produce an improved amorphous polyamide film.

According to the invention, there is provided a biaxially stretched amorphous polyamide film or sheet. The amorphous polyamide can contain alicyclic units and is preferably the reaction product of a condensation polymerization of at least one dicarboxylic acid and at least one alicyclic diamine. The film can be produced by extruding a sheet of an amorphous polyamide material, heating the extruded sheet at a temperature above the glass transition temperature, stretching the sheet at a draw ratio of at least about 1.5, and, optionally, heat-setting the stretched sheet. The biaxially stretched amorphous polyamide film is generally characterized by a thickness of about 0.01 mm to about 0.50 mm and a density of about 1.0 to about 1.2 g/cc, and has good optical properties for uses requiring a transparent film.

The invention film can be produced from an essentially amorphous polyamide resin. By "amorphous" is meant that the polymer does not exhibit a distinct melting temperature and crystallization temperature as are characteristic of crystalline polymeric materials. By "polyamide" is meant a solid material having repeating structural units which contain at least one --C(O)NH-- group.

The polyamides which are suitable for the film of the invention include alicyclic polyamides containing repeating structural units which contain at least one alicyclic moiety. Such polymers can be produced by copolymerizing monomers of at least one dicarboxylic acid and at least one alicyclic diamine.

A particularly suitable amorphous polyamide can be produced by the condensation polymerization of at least one C.sub.6 -C.sub.12 straight-chain alkane dicarboxylic acid with an alicyclic diamine. Such dicarboxylic acids include adipic, suberic, azelaic, sebacic, and dodecanedioic. Suitable alicyclic diamines include 2,2-bis(4-aminocyclohexyl)propane (PACP) or methyl derivatives thereof, which can be represented by the formula ##STR1## wherein each R is selected from hydrogen and methyl radicals. Geometric isomers and mixtures of the diamines can be used. An amorphous terpolymer can be prepared by contacting, under polymerization conditions, the diamines with the diacids. In a preferred polymerization, 2,2-bis(4-aminocyclohexyl)propane or methyl derivatives thereof are contacted with a mixture of adipic acid and a higher molecular weight straight-chain dicarboxylic acid. Preferably, the mixture of acids contains about 20 to about 65 mole percent adipic acid and about 35 to about 80 mole percent of at least one higher, straight-chain dicarboxylic acid selected from suberic, azelaic, sebacic and dodecanedioic acids. The PACP or methyl derivative thereof is preferably employed in a molar ratio of about 1 mole per mole of the acid mixture, but the reaction can be carried out in systems having a slight excess up to about 5 mole percent of the acids or diamine.

The polyamides used in the invention film can be prepared by a melt polymerization process such as that described in U.S. Pat. No. 3,840,501. They can also be prepared by a two step process in which a prepolymer is prepared, the prepolymer is extruded, and the extruded prepolymer is again subjected to polymerization conditions to produce the desired polyamide, as described in U.S. Pat. No. 4,097,469.

Suitable PACP 9/6 resin for producing an amorphous biaxially-oriented film has an inherent viscosity in the range of about 0.8 to about 2.0 dl/g in m-cresol at 30.degree. C. It has a glass transition temperature in the range of about 150.degree. C. to about 200.degree. C. and has essentially no DSC (differential scanning calorimetry; 20.degree. C./min in nitrogen) melting point. The density of the starting resin will be in the range of about 1.0 g/cc to about 1.2 g/cc.

For production of biaxially-oriented film, the chosen polymer resin is first extruded into film or sheet at a temperature of about 250.degree. C. to about 350.degree. C. under an inert atmosphere such as nitrogen gas to minimize oxidation and discoloration of the polymer. For optimum clarity and light transmission, impurities, polymer gel and die streaks should be minimized.

Biaxial orientation of the polyamide can be carried out by any biaxial orientation process. Generally, orientation will be carried out above the glass transition temperature (Tg) of the polymer, preferably from about 5.degree. C. to about 20.degree. C. above the Tg. For example, for PACP 9/6 (60/40) copolyamide having a sheet Tg of about 174.degree. C., the preferred biaxial orientation temperature is from about 184.degree. C. to about 190.degree. C. to obtain film having the most desired physical properties and appearance.

The extruded sheet or film is then subjected to biaxial stretching, which may be carried out on the extruded sheet or film prior to cooling or after cooling and reheating to the stretching temperature.

Methods of biaxial orientation include tentering and bubble-blowing. Tentering, which has been used extensively in the textile industry, consists of stretching a formed flat sheet first in the machine direction under heat and pressure, such as over heated rolls or through an oven between rolls, and then in the transverse direction with a tenter-frame at an elevated temperature.

In bubble-blowing orientation, the film can be stretched in the machine and transverse directions simultaneously.

The biaxially-oriented film can optionally be subjected to a post-orienting heat treatment for annealing (heat-setting) the polymer; however, an advantage of biaxial orientation of the amorphous polymers is that annealing is not necessary to preserve the properties imparted by orientation. Annealing is generally carried out by exposing the biaxially-oriented sheet or film to a temperature within the range of about 100.degree. C. to about 225.degree. C., preferably about 125.degree. C. to about 200.degree. C., generally for about 10 seconds to several minutes.

In a typical process, PACP 9/6 pellets, from which the moisture has been removed by pre-drying or by using a vented extruder, are heated in an extruder to a temperature sufficiently high to produce a suitable extrusion viscosity but not so high as to degrade the polymer, preferably about 250.degree. C. to about 350.degree. C. The polymer is extruded into a film or sheet, usually a sheet at least about 10 mils thick. The hot sheet is passed over cooling rolls and, in a continuous process, cooled slightly to the biaxial orientation temperature or, in a noncontinuous process, cooled to ambient and rolled for later orientation. The sheet is then stretched, generally in one direction at a time, at a temperature above the glass transition temperature, generally about 5.degree. C. to about 20.degree. C. above the glass transition temperature of the extruded sheet or the polymer. The stretched sheet is then stretched at roughly a 90.degree. angle to the first stretching direction, at a temperature as described above. The stretching is generally carried out at a draw ratio of about 1.5 to about 3.0 times, based on the extruded dimensions, preferably about 2.2 to about 2.75. The biaxial stretching will generally have the effect of producing a film having a thickness of less than about 12 mils, generally about 1 mil to about 10 mils thick. Suitable biaxial stretching can be accomplished using a combination of rolls for the initial stretching and a tenter frame for the second stretching.

The physical properties of the invention biaxially-oriented polyamide film will vary depending upon its starting monomers and preparation method. A PACP 9/6 (60/40) polymeric film will generally have the following properties: The film will range in thickness from about 0.02 mm (0.8 mil) to about 0.3 mm (12 mil). The tensile strength will most suitably be greater than about 100 MPa, and will generally range from about 75 MPa to about 200 MPa. Its elongation values will generally range from about 10 percent to about 60 percent. Haze of a sample will generally fall within the range of about 1.0 to about 3.0. An optical brightener can be added to the polymer if an essentially clear, non-tinted film is desired. Light transmittance will generally be about 88 to about 93 percent. The glass transition temperature (Tg) determined by DSC is about 174.degree. C. to about 185.degree. C. Density will generally be at least about 1.0 g/cc, and will usually fall within the range of about 1.04 to about 1.08 g/cc.

The polyamide films of the invention are formed from essentially amorphous polymeric material. By "amorphous" is meant that the polymers have no melting point (Tm) as determined by DTA (differential thermal analysis) and DSC. It is noted that the polymers used in the invention film are inherently amorphous, and are not amorphous merely under certain imposed conditions, such as low temperature. They are thus distinguishable from inherently crystalline polymers, including some polyamides, in which crystalline character can be induced under, for example, elevated temperature. When samples of PACP 9/6 film are annealed following orientation, very little density change occurs, indicating that crystallization does not take place.

The amorphous polyamide film of the invention offers the good combination of film properties of clarity (low haze), high glass transition temperature and good chemical resistance. The glass transition temperature for PACP 9/6 of about 170.degree. C.-190.degree. C. is relatively high compared with that for many conventional film materials. This permits the retention of properties at temperatures approaching 185.degree. C., making it suitable for high-temperature film applications such as adhesive tape for electrical applications. The haze of a sample of PACP 9/6 film about 2 mil in thickness is about 1.3 percent, making it highly suitable for imaging applications requiring a transparent, non-distortive film, such as printing plates.

As can be seen from the data in the examples, the biaxial orientation of the amorphous polyamide results in better clarity (lower haze), greater strength and better elongation properties. Contrary to what is generally seen for crystalline materials, the density of the PACP 9/6 does not change appreciably with annealing.

As used herein, the term "biaxial orientation" refers to stretching of a sheet of the polyamide in at least two different directions. The draw ratio of the biaxial orientation can be any draw ratio effective to achieve the desired properties or appearance of the film. Generally, the draw ratio (extended length divided by original length) will be within the range of from about 2 to about 3 in each direction.

The process of biaxial orientation of the amorphous polyamide has the effect generally of increasing tensile strength and decreasing elongation and Elmendorf tear. A biaxially-oriented PACP 9/6 film is essentially colorless and has low haze and good surface gloss.

This example describes the procedure used to make the polyamide polymer and the film described herein. The polyamide polymer was produced by a 2-step process similar to that described in U.S. Pat. No. 3,840,501 and prepared from a 50:50 mole ratio of 2,2-bis(4-aminocyclohexyl)propane (PACP) and a 60/40 mole ratio mixture of azelaic acid and adipic acid (9/6).

The preparation of PACP 9/6 was as follows: To a 30-gallon 316 stainless steel autoclave fitted with a variable speed drive anchor stirrer, heat exchanger, thermocouple and a hot oil heat source through a spiral wound jacket fitted to the outside of the vessel was charged 33.1 pounds (0.14 pound moles) of PACP, 15.7 pounds (0.08 pound moles) of azelaic acid, 8.1 pounds (0.06 pound moles) of adipic acid, 14.2 pounds of distilled water, 1.67 grams of an optical brightener (Estorite OB-1, available from Eastman Chem. Co.), 2.57 grams of manganous hypophosphite (from ICN-K and K Laboratories, Inc.) and 15 grams of acetic acid. The reactor was closed and pressurized to 20 psi with nitrogen.

The reactor contents were stirred and heated to 290.degree. C. As the temperature increased, the reactor pressure increased to 150 psi. Water was vented from the reactor to maintain 150 psi until a temperature of 290.degree. C. was reached. This took about 4.5 hours. The reactor was then vented to 0 psi in 30 minutes. With the stirrer, off the reactor was pressured to 125 psi with nitrogen. The product was then extruded through a Strahman valve, stranded into a water bath for cooling, and pelletized. The pelletized prepolymer was dried for 8 hours in an oven at 100.degree. C. The prepolymer was finished by passage through a 1 inch diameter NRM bench model extruder heated to 325.degree. C. (die plate heated to 310.degree. C.) and metered into a Baker Perkins twin screw single stage Poly-Con 100 fitted with a vacuum chamber, heating jacket, variable speed drive and die plate. The temperature of the Poly-Con was about 310.degree. C. and the screw speed was about 32 rpm. The polyamide was cooled in a water trough, passed through a chopper and pelletized. The product had an inherent viscosity of 1.1 measured in m-cresol solvent at a concentration of 0.50 grams/100 milliliters at 30.degree. C. The pellets were dried in a vacuum oven at about 100.degree. C. for 1-2 days.

Film extrusion (16 mil thick X 6 inches wide) was carried out on a 1.25 inch diameter extruder equipped with a 6-inch wide flex-lip die and a sheet line. The conditions for making 16-mil sheet were extruder zone 1, 540.degree. F.; zone 2, 599.degree. F.; zone 3, 599.degree. F.; adaptor, 599.degree. F.; die, 599.degree. F.; rolls, 210.degree.-220.degree. F.; screw 55 rpm. Lighter films (3 mil and 10 mil) were also made. The film thus prepared had the properties set out in Table I.

This example describes the process by which polyamide copolymer films prepared in accordance with the procedure described in Example I were biaxially oriented and compares the properties of non-oriented and biaxially-oriented films of similar thickness. The sheet (about 16 mils thick) was placed in a T. M. Long Film Stretcher Unit (T. M. Long Co.) at 365.degree.-370.degree. F. (185.degree.-188.degree. C.) and was biaxially oriented simultaneously in both directions at a rate of 1000 percent per minute after preheating for 10 to 150 seconds. The PACP 9/6 film was biaxially oriented at draw ratios between 2X and 2.75X in both directions, draw ratio being defined as the extended length divided by the original length. The drawn film was removed from the film stretcher and was evaluated for various properties. In this manner, the aforementioned polyamide copolymers were evaluated. These results are listed in Table II along with those of a non-oriented extruded PACP 9/6 film of a similar thickness. Biaxial orientation causes significant increases (relative to non-oriented PACP 9/6 film) in the tensile strength and decreases in elongation and Elmendorf tear. The biaxially oriented PACP 9/6 film is colorless and has low (below about 1.5 percent) haze and good surface gloss.

Samples of the biaxially oriented PACP 9/6 film were annealed at temperatures between 150.degree. and 225.degree. C. for 90 seconds in a forced air oven with each sample held at constant length in a frame. The densities of the annealed films ranged from 1.060 (150.degree. C. sample) to 1.064 (225.degree. C. sample). The lack of increase in density from annealing indicates that the film does not crystallize.