Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By mechanically introducing gas into material
Patent
1997-04-21
1999-09-21
Kuhns, Allan R.
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By mechanically introducing gas into material
264102, 264321, B29C 4402
Patent
active
059550147
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a procedure for the manufacture of a foamed plastic product.
2. Description of the Related Art
Foamed plastic films and blanks can be manufactured primarily by using foaming agents added directly into the film production process or by orientation stretching at a suitable temperature of a film structure containing special additives. With normal extrusion foaming methods, foaming degrees exceeding 50% are seldom achieved.
SUMMARY
E.g. U.S. Pat. No. 4,473,665 presents methods for pressurizing solid plastic with gas to achieve foaming. However, these methods require pressures of several tens, even hundreds of MPa.
The object of the present invention is to produce a procedure for pressure inflation of a prefoamed plastic film that makes it possible to manufacture strongly foamed film products, involving a high foaming degree and allowing the thickness of the product to be increased without increasing the amount of plastic material.
In the procedure of the invention, a polymer to be inflated contains a structural boundary layer or micropore in which a bubble is nucleated and which is inflated during a pressurizing processs. The details of the features characteristic of the procedure of the invention are presented in the attached claims.
Films inflated by the pressurizing method are visually more dull and have an increased opacity and untransparency as compared with uninflated products. In practice, the change in the visual characteristics of the film improves its properties relating to printability. The films have more paper-like rigidity characteristics, and the product is more elastic and has a better (thermal, optical) insulating capability than an uninflated film.
By the pressurizing method, using biaxially oriented polypropylene films prefoamed to 30%, it is possible to produce homogeneous foamed films and sheets with an 80-% foaming degree. Since the inflation of the foam bubbles occurs at a temperature lower than the melting point of the polymer, the bubble walls of the structure are oriented simultaneously. The orientation increases the structural strength of the final product.
The pressure required in the procedure of the invention is of the order of only 1 MPa and accordingly the pressurization chambers needed are structurally light, thus making the procedure economical.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following, the invention is described in detail by the aid of an example by referring to the attached drawing, in which
FIG. 1 presents an uninflated film (prefabricated product),
FIGS. 2a-2c illustrate the manufacture of foamed plastic films by the method of the invention and
FIG. 3 presents an inflated film.
DETAILED DESCRIPTION
The basic film/blank 1 presented by FIG. 1 contains discontinuities 2 which may consist of prefoamed bubbles or boundary layers/spaces (boundary layer in a lamellar structure) formed by solid particles where a gas can be diffused and stored.
The basic film/blank 1 may have a thickness of the order of D=5 .mu.m-1000 .mu.m. The basic film/blank 1 may be in an unoriented state or in a biaxially oriented state. In this context, orientation refers to a method of processing of the plastic film/blank whereby the product is melted and then precooled to a temperature below the crystallization point and heated again to the orientation temperature for the time required by the stretching. The orientation temperature is lower than the melting point of the polymer.
The foamed structure in the basic film/blank 1 may be a lenslike structure produced by adding foaming agents into the process. It may also be a "shredded" or cavitated foamed structure resulting from internal shredding of the structure during orientation stretching, caused by solid additives or particles added into the polymer.
FIG. 2 presents an apparatus comprising an unwinding roller 3 and an end roller 4 for a film web 1, placed in a chamber 5. On the unwinding roller's side of the chamber 5 (on the right in FIG. 2a) there
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Kirjavainen Kari
Minkkinen Hannu
Raukola Jaako
Kuhns Allan R.
VTT Technology OY
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