Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By gas forming or expanding
Patent
1992-01-17
1994-01-11
Kuhns, Allan R.
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By gas forming or expanding
264 54, 264338, 425 4C, 425325, B29C 6722
Patent
active
052778525
DESCRIPTION:
BRIEF SUMMARY
THE PRIOR ART
The present invention relates to a process for the continuous or semi-continuous production of cellular plastic from a plastic mass containing a blowing agent and, optionally, other process-regulative substances and additives.
Production of cellular plastic (foamed plastic) from thermoplastic materials has increased sharply. The key reasons for this are low consumption of raw materials relative to volume and high thermal insulation value.
There are several known processes for production of plastic foam from thermoplastic resins, including, for example "bead foam" of polystyrene (Styropor) and the static high-pressure process for the production of PVC foam having low density and closed cells. The "bead foam" process, which is a multi-stage process, produces a polystyrene foam having low density but with relatively low physical values. The static high-pressure process produces PVC foam having low density, closed cells and high quality, but is a highly manual process associated with high production costs, and it suffers considerable limitations with respect to the product's shape.
Of fully continuous processes, extrusion is the one used most frequently. It is usually relatively easy to control and is normally associated with low production costs.
Today, the use of plastic processing machines for continuous production of foamed plastic, for example extruders, generally involves the employment of physical blowing agents for foaming of the cellular plastic with low density. For higher densities, chemical blowing agents are usually the basis for the foaming process.
The thermoplastics resins best suited for foaming are: Polystyrenes (PS), including polymerisates (HIPS, SB, SAN, ABS), polyethylene (LDPE) and the copolymerisate (EVA), polyvinylchlorides (PVC) including polyblends, and polypropylenes (PP).
Favorable conditions for foaming are obtained when the melt viscosity of the raw material falls slowly and evenly in the temperature ranges for softening. Therefore, the amorphous thermoplastic resins are easier to foam than the crystalline ones. The partly crystalline thermoplastic materials are usually cross-linked chemically or cross-linked by radiation prior to or in connection with the foaming. In this way there is obtained a more favorable viscosity curve as function of the temperature.
In the extrusion of plastic foam, the foaming process itself is dependent on the plastic's viscosity and the melt's tensile capacity, the gas pressure of the blowing agent and the exterior pressure and interaction between the melt and the blowing agent.
Because plastic foam has greater friction against the wall of a tool connected with a plastic processing machine such as an extruder, than does an unexpanded plastic material, it is generally desirable that the foaming should take place only after the plastic compound has left the tool. One condition for this is that the exterior pressure on the melt containing active blowing agent be high enough to prevent expansion. In presently known production processes, after the plastic melt leaves the processing machine's tool, the external pressure on the melt decreases and the plastic material expands as a result of oversaturation of the gas in the plastic material.
One process parameter that has a heavy influence on the cellular structure and thus the quality of the extrudate is the temperature of the material. In the case of too low a material temperature, the plastic mixture would not be capable of expanding completely due to a relatively high melt viscosity, resulting in a relatively high density. If the temperature of the material is too high, the melt viscosity will be low, which easily causes the cells to be ruptured with a high density as a result.
In order to produce plastic foam having low, uniform density and closed cells, it is necessary to produce an extrudate where the gas cells are not ruptured to pieces due to unequal velocity in the direction of low at the outlet of the forming tool where the pressure is reduced and the gas cells are formed. At the same
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Kuhns Allan R.
Scandinor A/S
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