Film made of an aromatic polyamide and/or aromatic polyimide and a magnetic recording medium using it

#The present invention relates to a film made of an aromatic polyamide and/or aromatic polyimide. In more detail, it relates to a film made of an aromatic polyamide and/or aromatic polyimide free from curling and excellent in flatness.

#It is conventionally proposed to use films made of an aromatic polyamide and/or aromatic polyimide as base films for magnetic recording media, flexible printed boards, heat sensitive transfer films, etc. since they are excellent in mechanical properties and heat resistance. The base films used in these applications are required to have a flat surface free from curling and wrinkles since the yield in processing and the functions as products are adversely affected.

#It is proposed to use them as base films for magnetic recording media such as video tapes and data memory tapes which are densified especially in recent years. In these applications, the films are exposed to high temperatures when the magnetic layer is dried after coating or vapor-deposited, and they are curled or wrinkled to be poor in flatness. So, the tapes produced from them are wound in awkward styles or touch the head imperfectly, not allowing sufficient electromagnetic conversion characteristics to be obtained disadvantageously.

#As aromatic diamine components, 2-chloroparaphenylenediamine corresponding to 85 mol % and 4,4'-diaminodiphenyl ether corresponding to 15 mol % were dissolved into N-methyl-2-pyrrolidone (NMP), and 2-chloroterephthalic acid chloride corresponding to 98.5 mol % was added. The mixture was stirred for 2hrs to complete polymerization. It was neutralized by lithium hydroxide, to obtain an aromatic polyamide solution with a polymer concentration of 10.5 wt %. To the solution, colloidal silica particles with a primary particle size of 45 nm were added by 0.3 wt % based on the weight of the polymer.

#The polymer solution was filtered, introduced into the dry process, cast onto an endless belt with a belt temperature of 150.degree. C., heated by hot air of 160.degree. C. at a solvent extracting rate of 0.2 wt %/sec, to evaporate the solvent, and the self-sustainable film obtained was continuously separated from the belt. Then a 50.degree. C. solution of NMP/water=40/60 was blown from the belt side of the film using a nozzle, and the film was introduced into a wet bath of the same composition, for extracting the remaining solvent, the inorganic salt produced by neutralization and impurities for 5 minutes, and furthermore introduced into a 50.degree. C. water bath for 5 minutes, for extraction. In this duration, the film was drawn to 1.2 times in the machine direction (MD). Then, in a tenter, it was drawn to 1.3 times in the transverse direction (TD), while being dried and heat-treated on both sides of the film at 280.degree. C., and gradually cooled, to obtain a 4.4 .mu.m thick film.

#The values of orientation degree P obtained by measuring the film at 4 points in the thickness direction by the laser Raman microprobe method were 6.49, 12.53, 9.06 and 7.91 in this order from one side of the film. The (Pmax-Pmin)/Pavr was 0.67, Pa/Pb was 1.1. Pavr was 9.0. The obtained film was evaluated in flatness and found to be as good as .largecircle. without being curled. The heat shrinkage of the film in the machine and transverse directions were respectively 2.4 and 2.3%. The elastic modulus in the machine and transverse directions were respectively 13 and 13 GPa. The elongations in the machine and transverse directions were 45 and 45% respectively. The moisture absorption was 1.5%. The magnetic tape prepared by using the film was as good as .largecircle. in flatness and also as good as .largecircle. in electromagnetic conversion characteristics. The magnetic film could sufficiently withstand recording and reproduction at a recording density 8 kilobytes/mm.sup.2.