GB740393A - Improvements relating to the stretching of film of synthetic linear organic polymeric material - Google Patents

Abstract

<PICT:0740393/IV(a)/1> A travelling continuous web of film of synthetic linear organic polymeric material is continuously stretched longitudinally by opposed tensional forces acting lengthwise of the web over a substantial distance, the web being maintained between the forces in contact with a number of closely-spaced, parallel idler rolls, each freely rotatable on an axis at right-angles to the length of the web, and the web following a sinuous path over and under alternate idler rolls. The opposed tensional forces are preferably provided by means of two groups of positively driven rolls, the number of idler rolls between them being such that the last idler roll is rotating at substantially the same speed as the fast rolls. Preferably also the clearances between the last positively-driven slow roll and the first idler roll, between adjacent idler rolls, and between the last idler roll and the first fast roll do not exceed 1.5-5 times the thickness of the film before stretching. As shown in Fig. 1, the first group of five rolls A-E are the positively driven "slow" rolls, the second group of five rolls are idler rolls over which actual stretching is effected, and the third group of five rolls F-J are positively driven "fast" rolls. All the rolls are 30 inches wide and 4.5 inches o.d. The clearance between adjacent slow rolls and adjacent fast rolls is 4 inches, and that between the last slow roll and the first idler roll, between adjacent idler rolls and between the last idler roll and the first fast roll is 0.012 inch. In an example polymeric polyethylene glycol terephthalate is extruded to form an amorphous film 20 inches wide and 0.004 inch thick which is stretched as described above, the slow rolls being driven at a peripheral speed of about 9.7 yards/minute and the idler rolls and the fast rolls at a peripheral speed of about 26.7 yards/minute, giving a stretch ratio of about 2.75. All the stretch occurs by the time the seventh idler roll is reached. The stretched film is 18 inches wide and 0.0014 inch thick. All the rolls are internally heated to give a surface temperature of 85-90 DEG C. The film material may be a polyamide such as polyhexamethylene adipamide or sebacamide or polycaproamide, vinylidene chloride, rubber hydrochloride, polystyrene, polyvinyl alcohol, polyvinyl acetals or vinyl acetate-chloride copolymer. U.S.A. Specifications 2,071,250, 2,071,253 and 2,465,319 are referred to.