Applications of water in plastics are broadly known.
One application of water in plastic compositions includes its use with a thermosetting plastic as a shattering agent. U.S. Pat. No. 3,639,549, for example, discloses the use of water with a polyester thermoset in which the temperature at which the resin sets is sufficient to convert the water contained in the plastic to steam. The expansion caused by generating steam from the water works at odds with the hardening of the resin and as a result the resin is cracked to produce a decorative effect.
Water has also been reported broadly as a foaming agent for plastics. However, water as a foaminig agent for plastics is a term which heretofore has been more a misnomer than a fact. For example, water has been named as a foaming agent in the foaming of polyurethanes. Water in this case is not the foaming agent, however, but acts instead as an initiator to generate the foaming agent. Water in this capacity reacts with polyisocyanates, the precursors of polyurethanes, to generate carbamic acid which under the conditions of the reaction instantaneously decomposes to produce carbon dioxide, the true foaming agent. Water present in urethane foam production is totally consumed by the carbamic acid production and decomposition reaction. It is the carbon dioxide that functions as the foaming agent in the foaming of polyurethane and this reaction, i.e. utilizing carbon dioxide as a foaming agent, is the leading method employed for the production of foamed polyurethanes. U.S. Pat. No. 3,694,530 to Wolfe, U.S. Pat. No. 3,658,972 to Ready et al., U.S. Pat. No. 3,590,012 to Hauptman, and U.S. Pat. No. 3,706,687 to Rudzki are examples of the disclosure of the function of water as an initiator in the production of polyurethanes.
Foaming of the resin in the production of thermoplastic articles other than polyurethanes is also known. Thermoplastics having good structural integrity can be prepared from a large category of foamed plastics which includes polystyrene, ABS (acrylonitrile-butadiene-styrene) resins, polyethylene, polypropylene, acrylics and other thermoplastics characterized by rigidity of structure after forming. These resins are often molded into articles which are not only rigid, but load-bearing and thus useful in building structures. Further, structural thermoplastics may also be foam molded, and in this capacity gaseous foaming agents are utilized.
The gaseous foaming agent for foamed structural thermoplastics may be a low boiling liquid, such as pentane. However, satisfactory incorportion and homogenization of a liquid foaming agent with solid resin pellets is one of the major problems in foam molding of this type. Consequently, solids such as azodicarbonamide which liberate gases when heated above their decomposition temperature are conventionally used with resin as a dry mix. However, residues remaining within the resin from such mixes can cause considerable discoloration and surface extrudates on the plastic.
Water as a foaming agent broadly falls within the category of volatile liquids such as pentane which might be considered as possible foaming agents for foamable structural thermoplastics. For example, see U.S. Pat. No. 3,268,636 and U.S. Pat. No. 3,436,446 to Angell. The use of water in foam molding to serve as a heat sink is disclosed in U.S. Pat. No. 3,475,354 to Needham and as a source of heat in U.S. Pat. No. 3,309,439 to Norweiler. The use of water as a foaming agent has been handicapped because, in foam molding, water is known to cause indent in the final article. Indent is a phenomenon in which the surface of the finished molded article does not completely conform to the mold pattern but is instead characterized by indents or depression where the foamed thermoplastic resin retracted from the mold surfaces during the cooling and settling period. The cause of indent is not known, but may be due to the premature condensation of steam resulting in the collapse of the steam pockets in the thermoplastic resin into smaller volume water pockets thereby causing a partial vacuum in the pockets and permitting the hot thermoplastic, which has not congealed, to partially collapse and recede from the surface of the mold.
One of the most common limitations encountered in foam molding, regardless of the foaming agent used, is "swirl". Swirl is, as the name indicates, a visible imperfection in the surface of the finished article. In the latter it may be characterized generally as an erosive pitting of the surface. Swirl may be characterized as (a) optical and (b) physical or surface swirl. Optical swirl is a faintly detectable random turbulence pattern visible on the surface of the foamed thermoplastic structural article. Optical swirl is believed to be due to alignment of the gas bubbles in the foamed thermoplastic article along the flow patterns, including any turbulence patterns generated in the melted thermoplastic resin, as the thermoplastic resin fills and is expanded in the cavity in the mold. Optical swirl is thus a congealed flow pattern in the congealed thermoplastic article made visible by partial alignment of the gas bubbles in the interior of the thermoplastic article along the flow patterns of the thermoplastic resin in the cavity of the mold. Optical swirl is not a prominent phenomenon and causes little problem as it can be easily hidden, where undesirable, by surface painting.
Physical swirl, on the other hand, appears to be the result of escape of some of the gas bubbles from the interior to the surface of the expanded thermoplastic resin before the thermoplastic resin congeals to form the finished article. These surface bubbles tend to migrate across the surface of the thermoplastic article between the still pliable thermoplastic resin and the surface of the cavity in the mold, often along the flow lines of the thermoplastic resin as it fills and expands in the cavity of the mold. Each bubble leaves behind it a bubble track or small groove across the surface of the foamed thermoplastic article. These grooves, which remain in the surface of the foamed thermoplastic article, are highly visible as a swirled pattern even after painting. For applications requiring a smooth or unpatterned surface, the swirl pattern is highly objectionable. The use of higher molding temperature is sometimes effective to remove the swirl pattern. The higher temperature allows the thermoplastic resin to stay soft longer so that the bubble tracks or grooves in the surface of the thermoplastic resin fill in before the thermoplastic resin congeals. Higher temperature, however, also means a longer cooling period in the cavity of the mold. Prolonged cooling time increases the production time cycle, making the use of a higher molding temperature uneconomical.
No practical process has heretofore been known which produced swirl-free or swirl-free, indent-free foamed thermoplastic structural articles. Therefore, there is a need for an economical process for producing swirl-free and swirl-free, indent-free foamed thermoplastic structural articles.