It is known to produce thermoplastic resin foam in sheet form by compounding such a resin, e.g., polyethylene, with a thermally-decomposable blowing agent, e.g., azodicarbonamide, heat-plastifying the composition under conditions not causing decomposition of the blowing agent and fabricating therefrom a non-cellular sheet, crosslinking that sheet, e.g., by high energy electron irradiation, to obtain a crosslinked foamable sheet, and heating that crosslinked sheet to effect decomposition of the blowing agent and formation of gas-filled predominately closed cells in a resulting foamed resin sheet product. However, it is difficult to heat the crosslinked sheet product uniformly throughout to obtain uniform foaming, particularly in a continuous manner. Moreover, the foaming sheet is expanding in all directions while the resin is in heat-softened condition; in that stage the sheet would sag and distort under its own weight and must be supported in some way even though it is sticky and is expanding.
It has been suggested to foam foamable resin, e.g. sheets thereof, by immersion in hot liquid, but suitable means have not been described to keep the foaming sheet submerged against the buoyancy forces of the liquid without causing sticking or distortion of the foam. For instance, rollers, including racks of multiple rollers, are not satisfactory whether free-turning or driven or differentially driven because they provide only discontinuous support which is either highly localized (as with small diameter rollers) or is widely spaced (as is necessary with larger diameter rollers) and allows bulging of the soft foam material between such rollers. Belt conveyors in contact with the foam sheet are disadvantageous because the soft resin sticks thereto and the free expansion of the foam is impeded.
In U.S. Pat. No. 3,562,367 to Shinohara et al., there is described a float process of foaming foamable crosslinked olefinic resin sheet. In that process, such a sheet is heated in part and supported by floating it on the surface of a heavier liquid that is maintained at a temperature suitable for effecting the foaming, and the sheet is concurrently also heated from above with heat rays. A suitable such heat transfer liquid is a molten mixture of low melting salts, such as a mixture comprising potassium nitrate and sodium nitrite, which melts at a temperature below the foaming temperature of the foamable sheet and is used at a temperature that softens the sheet and decomposes the blowing agent. In such foaming step, the sheet is supported on the flat upper surface of the heat transfer liquid and is free to expand in all directions as the sheet foams while so supported. However, the heating of the foamable sheet to effect foaming thereof by heat transferred from the liquid beneath the sheet to the underneath side thereof must be exactly matched by heat transferred to the upper side of the sheet from heat ray sources above the sheet, e.g. infra-red lamps or like devices. Attaining such matching is particularly difficult in continuous operation where foamable sheet continuously moves across the surface of a heat transfer liquid bath, foams, and is continuously taken off and away as foamed sheet, wherein the temperature of the sheet changes during such transit and the temperature profiles of the sheet may also change during the operation. While these observations might suggest immersing the foamable sheet to allow the liquid heat transfer medium to uniformly heat the sheets by identical means from each side of the sheet simultaneously, suitable means to keep the sheet submerged have not heretofore been known, and the feature of supporting the sheet in a flat and unconfined condition on the liquid surface would thereby be lost. At column 3, lines 50 et seq. of printed copies of U.S. Pat. No. 3,562,367 it is stated that heating the foamable sheet in a heated liquid cannot be employed when producing foamed sheet, particularly continuously. Nevertheless, the non-uniform heating of floating sheet is at least troublesome. Moreover, sheets of some resins, such as radiation cross-linked linear polyethylene, which are susceptible to degradation and other deleterious effects of exposure to air at high temperatures, cannot consistently be foamed by the liquid bath surface float method without elaborate and inconvenient measures to maintain appropriately inert atmosphere over the liquid bath.
As to the handling problems caused by the stickiness and low tensile strength of the hot, foaming resin, similar problems are encountered in many other process operations where a soft solid is submerged in a treating liquid under conditions that the solid is sticky and/or has little tensile strength.
Accordingly, it would be desirable to provide improved method and means for carrying out such unit process with sticky and/or low tensile strength solid materials. Particularly, it would be desirable to provide method and means to effect foaming of a heat foamable material preform especially in the form of sheet comprising crosslinked thermoplastic resin and foaming agent such as thermally decomposable chemical blowing agent, which method and means would supply the necessary heat uniformly and simultaneously to all sides of the foamable and foaming material while protecting it from exposure to deleterious atmosphere, and would allow the foaming material to expand freely in all directions without distortions while carrying out the operations in continuous manner.
A general object of this invention is to provide improved method and means to fulfill the above desires.
A more specific object is to provide method and means to make thermoplastic resin foam, especially in sheet form, from foamable thermoplastic resin composition containing latent foaming agent such as thermally decomposable chemical foaming agent in continuous manner by continuously submerging the foamable material in a body of heat transfer liquid and conducting the material and resulting foaming product through such body of liquid and the resulting foamed product through and from such body of liquid.
Other objects and advantages of the invention will become evident in the description that follows.