Browse Prior Art Database

Novel Foamable Compositions and Rotational MOlded ARticles Made THerefrom

IP.com Disclosure Number: IPCOM000008109D
Publication Date: 2002-May-17
Document File: 10 page(s) / 64K

Publishing Venue

The IP.com Prior Art Database

Abstract

This invention describes foamable thermoplastic compositions and rotational molded articles made therefrom in which the blowing agents are dispersed uniformly and activate relatively slowly, preferably after fusion has been substantially completed, resulting in foamed articles with uniform cell structure and properties. Conventional rotomolded foams are made by dry-blending thermoplastic powder with chemical blowing agents (CBA) followed by biaxial rotational molding at zero shear rate to melt and sinter the powder particles. The blowing agents generate a lot of energy and decompose very fast at peak temperatures of about 190°C or less, before fusion of the thermoplastic powder has been completed. Hence, the foamed rotomolded articles made with conventional chemical blowing agents typically contain a lot of irregular sized voids.

This text was extracted from a Microsoft Word document.
This is the abbreviated version, containing approximately 10% of the total text.

                       This invention describes foamable thermoplastic compositions and rotational molded articles made therefrom in which the blowing agents are dispersed uniformly and activate relatively slowly, preferably after fusion has been substantially completed, resulting in foamed articles with uniform cell structure and properties. Conventional rotomolded foams are made by dry-blending thermoplastic powder with chemical blowing agents (CBA) followed by biaxial rotational molding at zero shear rate to melt and sinter the powder particles. The blowing agents generate a lot of energy and decompose very fast at peak temperatures of about 190°C or less, before fusion of the thermoplastic powder has been completed. Hence, the foamed rotomolded articles made with conventional chemical blowing agents typically contain a lot of irregular sized voids.

                       Rotational molding, also referred to as rotomolding, is widely used to produce hollow articles such as toys, sporting equipment, containers, water tanks, etc. For the process, a thermoplastic resin is placed in a mold which is then closed, heated and rotated on two axes, i.e., biaxially, to allow the resin to melt and uniformly coat the interior of the mold. The mold is then cooled and the molded article is removed.

In many instances, it is highly desirable to have a foam layer or core in the interior of the molded article to provide insulation, impart structural integrity or stiffness to the article, reduce weight, or the like. This is accomplished by including a foaming or blowing agent with the resin which decomposes at the molding temperature to release a gas, such as CO2 or N2.

The use of foaming agents presents a problem where formed articles having a smooth exterior surface are desired and various techniques have been employed to produce foamed rotomolded goods having a smooth skin layer. In one approach, referred to as the "two-step" method, a non-foamable resin is first introduced into the mold and molded to produce a non-foamed exterior layer of the desired thickness. A foamable resin is then introduced into the mold and the molding operation resumed so that a foamed layer is formed on the inside of the non-foamed layer. Such a method is disclosed in U.S. Pat. No. 3,976,821. While it is possible to produce acceptable molded goods in this manner by proper selection of resin(s) and operating conditions, the procedure is labor intensive and time-consuming. Also, it requires use of a mold with an opening to permit introduction of the foamable resin.

Another approach has been to utilize specially designed equipment, such as disclosed in U.S. Pat. No. 4,952,350, which permits both the non-foamable and foamable resin to be introduced at the beginning of the operation but maintained separately. In this way, the foamable resin can be released, e.g., from a dump box, at some point in the operation after the non-foamable resin has melted and uniformly coated the interior surface of the mold.

In other cases, such as in...