Use of thermoplastic polymethylmethacrylate molding...

Details Use of thermoplastic polymethylmethacrylate molding... Use of thermoplastic polymethylmethacrylate molding...

1999-02-17

2001-05-15

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Mechanical shaping or molding to form or reform shaped article

Shaping against forming surface

C264S328100, C264S331180, C264S334000

Reexamination Certificate

active

06231799

ABSTRACT:

FIELD OF THE INVENTION
The use of thermoplastic polymethylmethacrylate (hereinafter PMMA) molding compositions or batches for the formation of molded objects, in particular, by compression molding or injection molding. The production of objects of synthetic material which are required in large numbers are economically produced by injection molding procedures. Where such objects have a complex geometry which must be molded or cast with the highest precision galvanically constructed inserts, the so called electrotypes, are often integrates into the injection molding tool. The advantage of this technology, is that many galvano inserts can be taken off from a “master” and thus, the expense of the tools can be lowered.
DISCUSSION OF THE PRIOR ART
In the workup of PMMA molding compositions or batches on non-ferrous metal tools for example, in the above described electrotypes such as machine parts, more or less strong wear appearances have been noted according to the insertion area and work-up parameters. Because of this, the tools, as well as the machine parts become unusable after a particular time, since the produced molded parts no longer have the desired quality. The main problem herein is damage to the surface (polish). Because of flaking off of black covering of the tool or machine parts there can also result impurities in the formed objects themselves.
Heretofore, it had been assumed that these problems resulted from high temperatures which occurred as the result of compression of air in the casting tools. In particular, in narrow spaces in the molds small air bubbles remained which were strongly compressed by the injection pressure. The manufacturers of PMMA molding compositions therefore recommended operation at the lowest temperatures possible and provision of good ventilation in order to avoid the above mentioned problems.
SUMMARY OF THE INVENTION
It is the purpose of the present invention to reduce the wear on non-ferrous metal work tools and machine parts in an effective manner. It is further the task of the present invention to improve the quality of PMMA molded parts, in particular with respect to their purity and surface quality. These tasks are solved including not explicitly named tasks, which can be deduced from the context set forth herein, by the use of substantially sulfur free PMMA molding compositions or batches.
The use of substantially sulfur free thermoplastic polymethylmethacrylate molding compositions or batches for the formation of molded objects with non-ferrous metal containing tools results in the effective reduction of wear on the non-ferrous metal tools and machine parts. In particular, among other things, the chemical corrosion of the metal to metal sulfides is prevented. Such a situation is most surprising since the sulfur content of commercially available molding compositions is only 500 to 800 ppm. Furthermore, the bulk of the sulfur content is due to so called molecular weight modifiers. These are, for the most part, built into the polymer via covalent bonding. The concept of essentially “sulfur free” as used herein is that no sulfur containing modifiers are present and that the sulfur content of the PMMA molding compositions is less than 50 ppm, preferably less than 30 ppm, most preferably less than 10 ppm and most desirably less than 5 ppm.
Thermoplastic PMMA molding compositions are homo or co-polymers formed through the polymerization of methylmethacrylate alone, or in combination with one or more ethylenically unsaturated monomers and which can be thermoplastically processed.
Non-ferrous metal tools are tools in particular inserts, nozzles or channels which contain non-ferrous metals. Under non-ferrous metals one understands those metals which form substantially insoluble sulfides. Belonging to this group, but not restricted thereto, are nickel, copper, alloys of these two metals with another, as well as of both of these metals such as with beryllium, zinc or tin. The mode of production of the molded objects from the above-identified PMMA compositions can take place by means well known to those skilled in the art. Included in these methods are, in particular, extrusion, injection molding and compression molding. Preferred therein being injection molding and compression molding.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The above-identified PMMA molding compositions are molding compositions which involve PMMA, homo and/or copolymers which may be obtained by the polymerization of
I. 5 to 100 wt % preferably 50 to 100 wt % and particularly preferred, being 80 to 100 wt % of methylmethacrylate.
II. 0 to 95 wt %, suitably 0 to 50 wt % and particularly preferred, being 0 to 20 wt % of one or several ethylenically unsaturated monomers copolymerizable with (I) whereby, (I) and (II) together yield 100 wt %.
III. 0.05 to 5 wt % relative to 100 wt % of the sum of (I) and (II) of a substantially sulfur free molecular weight modifier and,
IV. up to 100 wt % relative to the sum of (I) and (II) of conventional additives to thermoplastic mold compositions,
and which have an weight average molecular weight of from 2×10
4
to 3×10
5
preferably, between 5×10
4
to 2.5×10
5
particularly being preferred, 8×10
4
to 2×10
5
.
The component (I) is commercially obtainable. Its weight proportion amounts to 5 to 100 wt %, preferably 50 to 100 wt %, most preferably 80 to 100 wt %. If the weight proportion is less than 5 wt %, then the qualities of the methylmethacrylate components do not appear.
The members of the group which comprise the ethylenically unsaturated monomers which form component (II) in the PMMA molding mass other than the methylmethacrylate, include among others, vinyl esters, esters of acrylic acid such as methyl and ethyl acrylate, esters of methacrylic acid which differ from methylmethacrylate for example, tertbutyl methacrylate, n-butyl methacrylate, ethylhexyl methacrylate, as well as cycloalkyl acrylates and cycloalkyl methacrylates, vinyl chloride, vinylidinechloride, vinyl acetate, styrene, substituted styrene with alkyl substituents in the side chain such as, for example, &agr;-methylstyrene and &agr;-ethylstyrene, substituted styrenes having alkyl substituents in the ring such as, for example, vinyltoulene and p-methylstyrene, halogenated styrene, such as monochlorostyrene, dichlorostyrene and tribromostyrene vinyl and isopropenylethers, maleic acid derivatives for example, maleic anhydride, methylmaleic anhydride, maleimide, methylmaleimide, phenylmaleimide, and cyclohexylmaleimide and dienes, such as, for example, 1,3-butyidiene, and divinylbenzene. Particularly preferred are acrylic esters and methacrylic acid esters which are different from those in (I), vinyl acetate, vinyl chloride, vinylidene chloride, styrene, &agr;-methylstyrene, halosubstituted styrene, vinylethers, isopropenylether and dienes.
All of the above-identified monomers commercially available. They are utilized in amounts of 0 and 95 wt % suitably 0 to 50 wt % of particularly preferred in amounts of 0 to 20 wt % relative to the total amounts of monomeric components and are co-polymerized with methyl methacrylate. If their weight proportion is greater than 95% then the properties of methylacrylic acid components (I) do not appear.
As components (III) in accordance with the present invention, for the control of molecular weight, sulfur free controllers are utilized. Without being limited thereto in accordance with the present invention as examples of sulfur free molecular weight modifiers there may be mentioned dimeric-&agr;-methylstyrene (2,4 diphenyl 4-methyl-1-pentene), enolethers of aliphatic and/or cycloaliphatic aldehydes, terpenes, 62-terpinene, terpinolene, 1,4-cyclohexadiene, 1,4-dihydronaphthalene, 1,4,5,8-tetrahydronaphthalene, 2,5-dihydrofuran, 2,5-dimethylfuran and/or 3,6-dihydro-2H-pyran, of which preferred is dimeric a-methylstyrene.
These sulfur free molecular weight modifiers are utilized in quantities of between 0.05 to 5 wt %, preferably 0.1 to 2 wt % relative to 100 wt % of all of the monomeric components utilized.

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