Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2000-05-10
2001-09-18
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, C521S143000, C521S182000
Reexamination Certificate
active
06291539
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to thermoplastic foams and the processing of thermoplastic foams, and more particularly to a method for lowering the density and increasing the flexibility of thermoplastic foams.
BACKGROUND OF THE INVENTION
Thermoplastic materials are those that soften and flow upon application of pressure and heat. Thermoplastic foams are defined, generally, as foams made from thermoplastic resins. Because thermoplastic materials regain their original properties upon cooling, most thermoplastic materials can be remolded many times. Examples of thermoplastic resins include poly(vinyl chloride), polyethylene polystyrene, acrylate resins, and poly(ethylene terephthalate).
It is known to produce thermoplastic resin foam materials having substantially closed-cell structures by intimately incorporating within the resin material a volatile organic liquid which vaporizes upon heating to form a gas (the liquid is known as the blowing agent, its resultant vapor the blowing gas). It is also known to use a solid substance as the blowing agent where the solid substance decomposes to form the blowing gas. The vapor created from the blowing agent is the blowing gas (often also referred to as simply the blowing agent) and causes the thermoplastic to expand and form a cellular mass.
Thermoplastic resin materials that have been foamed by the action of a volatile organic blowing agent producing a primary foaming gas may thereafter be induced to further expand. This secondary expansion is achieved by exposing the foamed material to another gas (a secondary gas), such as steam or air, which has a permeability rate greater than the permeability rate of the primary foaming gas through the cell walls of the foamed mass. During the exposure to this secondary gas, the material is reheated to a heat softening temperature. The secondary gas, which has a permeability rate through the cell wall greater than that of the primary gas already in the cell, permeates the cell wall and joins the primary gas inside the cell. At the heat softening temperature, the combined effect of the primary gas and the secondary gas causes further expansion of the initially foamed material. The result is a lower density foam product.
It is further known that thermoplastic resin materials that have been foamed by the gas emitted upon decomposition of a solid substance may thereafter be induced to further expand. This further expansion is achieved by heating the foamed material to a temperature near the melting point of the resin while subjecting it to a secondary gas at superatmospheric pressure. After this step is performed, the foamed material is reheated to a heat softening temperature at a lower pressure (i.e. atmospheric pressure). This causes the gas to expand inside the cells. The combined expansion of the primary gas and the secondary gas (which has entered the cells of the foamed material primarily because of the internal/external pressure differential during the application of the superatmospheric pressure) produces a lower density foam product.
SUMMARY OF THE INVENTION
The present invention is an improved, low density thermoplastic foam and an improved method for treating thermoplastic resins to achieve foams of lower density and increased flexibility. Foams having densities as low as 0.008 grams/cubic centimeter (g/cc) are obtainable.
The invention involves a multi-step process. The first step calls for decreasing the pressure on a primarily foamed thermoplastic resin and, while the foam is subject to this decreased pressure (under at least a partial vacuum), increasing the temperature of the foam. The foam temperature is increased to a point between the glass transition temperature and about the melting point of the foam, if the resin is made from a semicrystalline resin. If the resin is amorphous, the temperature is raised to a point between the softening temperature and the melting point of the amorphous foam. While these temperature and pressure conditions are sustained, the foam expands. The foam expansion is the result of the cells in the foam undergoing an increase in volume due to the temperature increase and pressure decrease. The order in which the temperature and pressure are changed is irrelevant—they may even be adjusted simultaneously. Furthermore, if the foam is taken directly from a foam extrusion process, the foam may already be at the proper temperature. In fact, from the foam extrusion process, the temperature may even have to be lowered before allowing the first step to occur. The conditions reached in the first step are held for a predetermined time, to allow adequate foam expansion, before moving to the second step.
The second step involves exposing the primarily foamed (and expanded) thermoplastic resin foam to a secondary expansion gas for a sufficient amount of time to cause secondary expansion. Secondary expansion occurs when the secondary gas permeates the cells of the thermoplastic resin and joins the primary blowing agent inside each cell. The pressure under which the foam is subject during this second step is preferably at least about 1 pascal (Pa) above atmospheric pressure. For a faster permeation rate and subsequent expansion, the pressure should be at least about 500 kilopascals (kPa) above atmospheric pressure. When the pressure is released, the foam will expand again, thereby lowering its density.
In this second step, it is preferred that the temperature is in the same range as that used in the first step. Although it is possible to perform the second step at temperatures up to the melting point and down to ambient temperatures, the process would, in the later case, be unsuitably long. The temperature can be maintained from the first step, or the material can be cooled and later reheated. In addition, the temperature can be increased before, during, or after the pressure is adjusted. After the gas permeates the foam cells during the second step, the pressure is released and expansion occurs. The foam can be cooled before, during, or after the pressure is released. Preferably, the foam will be cooled after pressure is released and expansion is allowed to occur.
The density reduction achieved by this process can be up to 96%; controlled density reductions can be in the range from about 15% to about 96%. More preferred density reductions will be in the range of from about 50% to about 96%.
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Dinkel Allan L.
Kreiser David W.
Weibel Juerg Patrik
Armacell Enterprise GmbH
Foelak Morton
Womble Carlyle Sandridge & Rice PLLC
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