Closed cell polymer foam

Details Closed cell polymer foam Closed cell polymer foam

1999-10-01

2002-05-07

Foelak, Morton (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S079000, C521S097000, C521S098000

Reexamination Certificate

active

06384095

ABSTRACT:

The present invention relates to a closed-cell polymer foam in which the polymer comprises a blend of an olefinic, particularly a styrenic polymer and a polar polymer having a lower molecular weight than the olefinic polymer. The present invention also relates to a process for preparing such a polymer foam and to foamed articles made from the foam.
Closed-cell polystyrene foams are widely used and, in particular, find utility as an insulating material. The good insulation performance of such foams is largely associated with their fine cellular structure and the gaseous material, typically a fully halogenated chlorofluorocarbon or hydrochlorofluorocarbon, which fills the cells or bubbles within the foam.
It is well known that polystyrene foam may be readily formed by an extrusion process. Such a process is described, for example, in GB-2146941 and in EP-0411923 which is concerned with the preparation of polystyrene foam having a density of from 32 to 160 kg.m
−3
using an extrusion process in which liquid carbon dioxide is employed as a blowing agent.
In a typical extrusion process, a blowing agent, which may be in the liquid or gaseous state, is injected under pressure into a stream of heat plasticised/heat softened polymer flowing through an extruder. Significant quantities of the blowing agent dissolve in the heat plasticised polymer resulting in the formation of a mobile, low viscosity mixture which is then pumped to an extrusion die under high pressure. As the mixture approaches the die, the pressure acting on the mixture drops causing dissolved blowing agent to come out of solution to form bubbles. The mixture then passes through the die and emerges from the extruder on the other side of the die where the pressure acting on the mixture drops still further to atmospheric pressure resulting in the further release of dissolved blowing agent and an increase in the volume of the bubbles that have formed. The viscosity of the polymer and the surface tension at the polymer/bubble interface influence the size of the bubbles that form from the evolution of the dissolved blowing agent. A low viscosity polymer provides for rapid bubble growth, but if too low allows the blowing agent to escape into the atmosphere before the polymeric foam has had time to set. During the extrusion process the temperature/pressure profile along the extruder is adjusted so as to control the release of the dissolved blowing agent from the polymer and the rate at which the polymer cools. In this way, the volume of the bubbles that form during the extrusion process can be controlled so as to obtain foam of the required density. A nucleating agent can also be added to the polymer in order to control the number and distribution of the bubbles that form in the blowing process.
Hitherto, polymer foams have typically been prepared using fully halogenated chlorofluorocarbon and hydrochlorofluorocarbon blowing agents. However, such materials and particularly fully halogenated chlorofluorocarbons have been implicated in the destruction of the earth's protective ozone layer so that a need has arisen to find alternative blowing agents. Fully halogenated chlorofluorocarbons and hydrochlorofluorocarbons have also been widely used as refrigerants and in respect of this particular application replacement refrigerants comprising one or more ozone benign hydrofluorocarbons have been developed. Unfortunately, however, the different solubility characteristics of hydrofluorocarbons mean that the heat plasticised/heat softened polystyrene formed in the extrusion process is unable to dissolve sufficient of a hydrofluorocarbon blowing agent to allow for the preparation of a suitable polystyrene foam. More specifically, the relatively low solubility of the hydrofluorocarbon in the heat plasticised/heat softened polystyrene phase will result in the formation of a hydrofluorocarbon/polystyrene mixture having a higher viscosity than is normal for blowing agent/polystyrene mixtures in extrusion processes and this will act to increase the extruder power requirement. Furthermore, the low solubility of the hydrofluorocarbon limits the availability of blowing agent in the mixture and this can tend to result in an unacceptable increase in foam density.
Therefore, there is a need to prepare polystyrene and other polyolefin foams of acceptable quality and performance using blowing agents which have low and preferably zero ozone depletion potentials.
The present invention provides for the use of hydrofluorocarbon containing blowing agents which have no deleterious effect on the ozone layer in the preparation of polystyrene based foams. This is achieved by incorporating a minor amount of a polar auxiliary polymer in the polystyrene which is able to dissolve significant quantities of a hydrofluorocarbon containing blowing agent when in the heat plasticised/heat softened state. The resulting polymer blend is able to dissolve greater quantities of a hydrofluorocarbon containing blowing agent when in the heat plasticised/heat softened state than polystyrene alone. Additional benefits may also include the reduced emission of gaseous blowing agents to the environment and the ability to prepare foams without the need for the inclusion of fire retardants or plasticisers within the foam. The present invention may also find utility in the preparation of other polyolefin foams.
According to a first aspect of the present invention there is provided a closed-cell polymer foam having a density of less than 100 kg.m
−3
, preferably in the range of from 20 to 80 kg.m
−3
, in which the cells contain a foam blowing agent comprising:
(a) a blend of two or more hydrofluorocarbons,
(b) a blend comprising carbon dioxide and one or more hydrofluorocarbons, or
(c) a blend comprising dimethyl ether and one or more hydrofluorocarbons, and the polymer is a blend comprising at least 50% by weight of an olefinic polymer(s) and up to 50% by weight of a polar polymer(s) having a lower molecular weight than the olefinic polymer which when in the heat plasticised state is able to dissolve the foam blowing agent.
In a preferred embodiment, the closed-cell polymer foam is a closed-cell thermoplastic polymer foam.
The closed-cell polymer foam of the invention is prepared from a foam blowing agent comprising (a) a blend of two or more hydrofluorocarbons, (b) a blend comprising carbon dioxide and one or more hydrofluorocarbons or (c) a blend comprising dimethyl ether and one or more hydrofluorocarbons and a polymer composition comprising at least 50%, e.g. from 50 to 99%, by weight of an olefinic polymer(s) and up to 50%, e.g. from 1 to 50%, by weight of a polar polymer(s) having a lower molecular weight than the olefinic polymer which when in the heat plasticised state is able to dissolve the foam blowing agent. A proportion of the foam blowing agent which is used in the preparation of the foam is encapsulated by the cells or bubbles which form during the blowing process so that the closed-cell polymer foam which is finally prepared retains a proportion of the gaseous blowing agent used in its preparation. The closed-cell polymer foam of the invention is conveniently prepared using conventional extrusion techniques of the type which are routinely employed in the preparation of polystyrene foams.
According to a second aspect of the present invention there is provided a process for preparing a closed-cell polymer foam as defined in the first aspect which comprises blowing a heat plasticised polymer composition with a foam blowing agent comprising:
(a) a blend of two or more hydrofluorocarbons,
(b) a blend comprising carbon dioxide and one or more hydrofluorocarbons, or
(c) a blend comprising dimethyl ether and one or more hydrofluorocarbons, the said polymer composition comprising at least 50% by weight of an olefinic polymer(s) and up to 50% by weight of a polar polymer(s) having a lower molecular weight than the olefinic polymer which when in the heat plasticised state is able to dissolve the foam blowing agent.
In the process of the present invention th

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