Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By gas forming or expanding
Patent
1996-03-04
1997-09-16
Kuhns, Allan R.
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By gas forming or expanding
264 455, 264 51, B29C 4402
Patent
active
056677405
DESCRIPTION:
BRIEF SUMMARY
PRIOR ART
Cellular plastics/foamed plastics can be divided into two main groups:
Closed cells in the plastic foam (i.e., cells which are not joined together) are a necessary condition if the foam is not to absorb moisture. It is also very important that the cells be closed if it is required that the cellular plastic have good, long-lasting thermal insulation properties. Also, the mechanical strength properties of the cellular plastic will generally be far better if the gas cells are closed, compared with cellular plastic with open cells, i.e., where the gas cells are joined together (e.g., as in a sponge).
There are several principally different technologies for producing foamed plastic with closed gas cells. It is conditional for all processes that the plastic not be sheared, i.e., that the layers of the plastic compound do not move in relation to one another after the blowing agent has expanded in the plastic compound and before the cell walls in the plastic compound have become sufficiently strong, either by chemical reation or by cooling, to prevent the cells from being torn to pieces by shearing. When a physical blowing agent is used, it is a condition that the gas be mixed into the plastic compound at a pressure above the expansion pressure of the gas, i.e., that the gas be mixed into the plastic compound in liquid form. If the gas has been permitted to expand before, or in connection with, being blended into the plastic compound, the result will be a greater or lesser portion of the plastic foam having open, continuous cells. When a chemical blowing agent is used, it is a condition for attaining foam with closed cells that the decomposition of the blowing agent should take place under a pressure great enough to prevent expansion of the gas until the entire decomposition phase has been completed. If the decomposition takes place under too low a pressure, the result will be foam with open gas cells.
One frequently used process is reaction injection moulding (RIM). In this process, two or more liquid components are mixed together and injected into a closed mold cavity. Polyurethane (PUR) is used for the most part in this process, but other plastic raw materials such as, e.g., thermoplastic polyamide, polyester and epoxy are also used. The method has the major advantage, compared with most other processes, of enabling the production of very large products, and allowing the plastic to be foamed at the place of usage (in situ). PUR foam can be produced with low density and has a very good thermal insulating capability as long as moisture has not penetrated into the foam. However, the foam is relatively expensive, it is brittle, and its compression strength is limited. Moreover, the gas cells collapse at very low fluid pressure, severely limiting the insulating property of the foam under water. The foam also has enviromentally negative sides. (One of the components of the foam is isocyanate. Currently, there is also an extensive use of CFC gases as the blowing agent in the foam.)
Of other known processes used especially for foaming of thermoplastics into light cellular plastic with closed cells, we can mention, in particular: static processes, extrusion processes, and injection molding processes.
Among static processes, the "Styropor process" is the most well known. It was developed especially for polystyrene and utilizes a volatile carbon-hydrogen compound as the foaming agent. The foam may be produced with very low densities and may be foamed at the place of use. Its fire and chemical properties, however, are in many cases inadequate. Moreover, the mechanical properties are relatively limited. The process can be used for production of large foam products.
Another type of static process frequently used is the so-called "static high pressure method". This process is used primarily for production of light PVC foam with a chemical blowing agent. The foam can be produced with low density (about 30 g/cm.sup.3), has a closed, uniform cell structure and has very good mechanical and chemical properties. In terms
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Kuhns Allan R.
Polynor Partners A/S
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