Gasket for and method of casting polymer plates

Details Gasket for and method of casting polymer plates Gasket for and method of casting polymer plates

1988-11-25

1991-09-10

Lowe, James

Plastic and nonmetallic article shaping or treating: processes

Mechanical shaping or molding to form or reform shaped article

Shaping against forming surface

249 82, 2491141, 249134, 2491871, 264 22, 264216, 26433118, 264337, B29C 3300, B29C 3940

Patent

active

050471996

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a hank (which can also be referred to as a gasket) that can be used for the manufacture of polymer plates or sheets, such as, for example, acrylic polymers, by casting. More particularly, the present invention relates to a hank making up the component that provides a tight seal between the two plates between which the methylmethacrylate prepolymer "syrup" or the pure monomer is poured in order to complete or carry out the polymerization by raising the temperature, resulting in the production of cast sheets of poly(methyl methacrylate). These cast sheets form, for example, colored transparent plates that have a wide practical application, notably for making advertising signs.
To manufacture sheets of poly(methyl methacrylate), one usually starts by preparing a "syrup" of methylmethacrylate prepolymer in a reactor under agitation and with a controlled temperature until a conversion of 5-10% is achieved. After degassing the "syrup" to prevent the formation of air bubbles in the cast sheets thus formed, this "syrup" is poured in molds comprised of two flat glass plates separated by a peripheral hank, the plates being held in place by clamps for closing the mold. Once the hank has been positioned mechanically between the plates, the clamps are placed on three sides; the "syrup" is introduced through a pipe on the fourth side of the mold; after filling, the missing clamps are set in place, the air is expelled from the mold, and the latter is placed in an oven to complete polymerization.
During the methylmethacrylate polymerization cycle, the mold undergoes changes in volume due to variations in the density of the reaction mixture with the temperature and the degree of polymerization. Such changes in volume are partially borne by the hank.
The table below shows the density of the mixture to be polymerized at the different stages of polymerization and, based on certain working hypotheses, the resulting thickness of the hank, its thickness at the start of polymerization being rated as 1.
In order for the hank to be able to provide a tight seal for the mold during the rise in temperature to 60.degree. C., it must first be placed under stress and the elastic crushing to which it is subjected must be sufficient for it to be able to relax from 1 to 1.046.
Therefore, the nominal diameter of the hank, for a final plate thickness of 0.85, must be 1.046. Actually, such a diameter must be greater than this size in order to account for the following parameters: change in thickness of the hank; partial creep of the hank; reduction in the elastic forces that provide tightness for the mold during the temperature rise to 60.degree. C.; change in the filling volume and change in the positioning of the hank. To take these parameters into account, it has been calculated that the diameter of the hank must be 1.09 or more, which gives a "shimming coefficient" of 1.09/0.85, or 1.28.
In practice, "shimming coefficients" are determined experimentally for each thickness of sheet, the values chosen being those that will produce sheets that are by nature free of flaws, but above all that will give the most uniform thickness profile; the closing force of the clamps is also considered when calculating this coefficient. This closing force must also be taken into account when choosing the hardness of the material from which the hank will be made.
Plasticized poly(vinyl chloride) hanks meet all these requirements: providing a tight seal for the mold, having an adjustable diameter, an adjustable hardness, flexibility, weldability, as well as being easy to work with.
However, such hanks have the disadvantage of allowing themselves to be swollen by methyl methacrylate. The result is that after the mold has been stripped, the hank will remain integral with the sheet of poly(methyl methacrylate) and the cutting (or trimming) of the edges of the resulting sheet will lead to trimming scraps that cannot be recovered because of the physiochemical adherence of the hank to the poly(methacrylate) and because the thermal cra

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