Patent GB-1087519-A (GB)
Title: Process and apparatus for dimensional stabilization of thermoplastic film
IPC: B29D7/00
Filed: 1966-09-07
Abstract: 1,087,519. Stabilization of films. E. I. DU PONT DE NEMOURS & CO. Sept. 7, 1966 [Sept. 8, 1965], No. 40007/66. Heading B5B. In a process for the dimensional stabilization of a continuous web of biaxially oriented polypropylene film wherein the film maintained at a temperature within the range of from 120‹ to 150‹ C. is allowed to retract between 5 and 40% in its transverse direction and between 1 and 15% in its longitudinal direction as it is passed successively over a series of driven rolls, the web is passed over and in contact with at least 90 degrees of the circumference of each of a series of idler rolls located between the first and second driven rolls of said series of driven rolls, whereby the major portion of transverse retraction of the film takes place between the first and second driven rolls, guiding the film into smooth contact with at least one of the driven rolls following the last of said idler rolls by passing the film over a guide member immediately adjacent the driven roll which presents a film-spreading surface operative to bow the film of transverse to the direction of travel of the web and contacting the entire width of the web, and thereafter passing said film over the driven roll adjacent thereto. In the operation of the apparatus shown, heated air is circulated through the container E to maintain the atmosphere at from 120‹ to 150‹ C., preferably from 125‹ to 140‹ C. Bi-axially oriented polypropylene film F in the form of a continuous web is guided into the chamber over idler guide roll G and passes between a first driven feed roll 1 and associated nip roll 1A, between driven roll 11 and nip roll 11A, and thence over a series of closely spaced idler rolls 2A, 2B, 2C, 2D, 2E and 2F disposed in static relationship such that the film contacts at least 90‹, and preferably about 180‹ of the circumference of each roll. From the last idler roll the web of film is passed over each of a series of relatively widely spaced driven rolls comprising second driven roll 3, and rolls 4, 5, 6, 7, 8 and 9 and finally over driven take-off roll 10 and its associated nip roll 10A. In the preferred embodiment the film is smoothed out, i.e. longitudinal wrinkles are removed before passing over at least one of the series of driven rolls in the path of the film following the idler rolls by passing the film over a spreading means in the form of a moderately arched or curved spreader rod or roll S the arched surface of which extends transversely of the web and for the full width thereof, and thence over the immediately adjacent driven roll. The radius of curvature of rod S may be from 60 to 40 feet or more. To ensure uniform and efficient heating of the film to the conditioning temperature all but the last of the series of driven rolls are controllably heated by suitable means not shown and the take-off roll 10 is cooled to bring the temperature of the film to near the normal temperature as it leaves the conditioner. To ensure positive traction and uniform contact of the film with the driven intermediate rolls co-operating nip rolls (3A, 4A &c.) each surfaced with an elastomeric material are also preferably associated with each driven intermediate roll. With incoming film which is especially baggy the speed of roll 11 with its associated nip roll 11 A is set slightly higher than the speed of roll 1 to provide a slight pull up of this point; otherwise rolls 1 and 11 are driven at the same speed. When roll 11 is used, relaxation of the film occurs first between this roll and roll 3, referred to above as the second driven roll, in which event roll 11 will become the first driven roll for purposes of defining the position of the idler rolls. The tension on the film and hence the extent of retraction of the film as it passes through the conditioner is controlled by control of the relative speeds of the feed and take-off rolls 1 (or 11) and 10 respectively, by conventional speed control means not shown. The idler rolls are desirably closely spaced, e.g. 4-inch diameter rolls may be used with their centres spaced apart about 6 to 18 inches, and so positioned relatively that the film contacts at least 90 degrees and preferably about 180 degrees of the circumference of each roll. An example illustrates the treatment of polypropylene film.