Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1998-08-07
2001-02-20
Short, Patricia A. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C524S512000
Reexamination Certificate
active
06191222
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to reinforced or non-reinforced polyoxymethylene moulding compositions with improved heat stability and stability against discoloration, shaped articles therefrom, and a process for the preparation of reinforced or non-reinforced polyoxymethylene moulding compositions with improved heat stability and stability against discoloration using certain additives. The invention particularly relates to reinforced or non-reinforced polyoxymethylene moulding compositions of
(i) at least one polyoxymethylene homo- and/or copolymer,
(ii) at least one conventional additive and
(iii) at least one polymeric plastics material for improving the heat stability and the stability against discoloration.
Polyoxymethylene (polyacetal) is an excellent material from which the most diverse commodity objects can be produced, in particular, by injection moulding. The chemical resistance to many organic solvents and bases is of particular advantage here. Since the introduction of polyacetals on to the market, various attempts have been made to improve the properties of polyoxymethylene (POM) in a controlled manner, so that its spectrum of industrial use could be extended further.
In particular, polyoxymethylene homo- and/or copolymers are suitable for the preparation of reinforced or non-reinforced moulding compositions. To improve their processability and heat stability, polyoxymethylene homo- and/or copolymers are mixed with conventional additives and at least one additive for improving the heat stability.
BACKGROUND OF THE INVENTION
With regard to the closer prior art, the following publications are mentioned, in which polymeric plastics materials are proposed as an addition to POM in order to optimise the heat stability thereof in this way:
U.S. Pat. No. 2,993,025 (synthetic polyamides),
U.S. Pat. No. 3,204,014 (N-vinylpyrrolidone/acrylamide copolymers),
U.S. Pat. No. 3,210,322 (carboxyl group-containing polyamides, polyurethanes, substituted polyacrylamides, polyvinylpyrrolidones and hydrazines),
U.S. Pat. No. 3,215,671 ((meth)acrylates and (meth)acrylamides as crosslinking agents),
U.S. Pat. No. 4,464,435 (copolymers of polyesters, (meth)acrylates, (meth)acrylamides, triallyl cyanurate, diallyl phthalate, vinyl acetate and divinylbenzene),
JP-B-14329/68 (copolymers of acrylamide or acrylamide derivatives with alkyl acrylates, vinyl ethers or vinyl ketones or copolymers of acrylamide derivatives and styrene),
U.S. Pat. No. 3,960,984 (amide oligomer),
U.S. Pat. No. 4,098,843 (dispersion of a polyamide in a carrier resin),
JP-B-22669/68 (ethylene/vinyl acetate copolymers),
EP-A-0 245 962 (polymers and/or oligomers which contain hydroxyl groups and at least one further functional group with a stronger Lewis basicity than the hydroxyl group),
BE-A-722 268 (polyamides, polyurethanes, polyureas, polyvinylpyrrolidone, poly(meth)acrylamide, urea derivatives, amides, hydrazones, semicarbazones and alkylene-bis-phenols),
JP-B-17107/67 (copolymer of acrylamide with styrene or vinylnaphthalene),
EP-A-0 270 278 (mixture of a superpolyamide and a small amount of a transition metal salt),
EP-A-0 270 279 (mixture of a superpolyamide and a small amount of a cyclic amidine compound), and
DE-B-25 40 207 (precipitation polycondensate of formaldehyde and melamine in a molar ratio of between 1.2:1 and 10:1).
It is furthermore known to use vinyl polymers with amide or lactam groups to improve the heat stability.
EP-A-0 381 943 discloses polyoxymethylene moulding compositions which comprise, as an additive, either
a) a polymer of
a1) methyl, ethyl and/or n-propyl methacrylate
a2) n-propyl, n-butyl and/or pentyl acrylate and
a3) acrylamide and/or methacrylamide
or
b) a mixture of
b1) polymethyl methacrylate
b2) poly(n-butyl acrylate) and
b3) polymethacrylamide.
The polymeric additives according to EP-A-0 381 943 are therefore copolymers which are obtained according to the instructions in EP-A-0 381 943 either by the route of the emulsion polymerization method or by the suspension bead polymerization method.
Just the large number of additives proposed to date reveals that it has not yet been possible to solve the problem of improving the heat stability of reinforced or non-reinforced polyoxymethylene moulding compositions to the complete satisfaction of those in practice. For some applications, known polyoxymethylene moulding compositions still have an inadequate heat stability, which adversely influences processing to shaped articles and, for example, contributes towards deposits on the mould or impairment of the ease of release from the mould and/or leads to discolorations and to a reduction in the mechanical properties during later use of the shaped articles. It is furthermore a disadvantage that the polyoxymethylene moulding compositions known to date may still contain formaldehyde adducts, which lead to odour nuisances during processing at elevated temperatures due to formaldehyde being split off.
Even the more recent additives proposed, for example, in EP-A 0 381 943, which are based on a copolymer or blend of (meth)acrylate/(meth)acrylamide, have so far not found acceptance in practice, since the emulsion polymers incorporated into the polyoxymethylene moulding compositions probably showed no reproducible results in respect of the heat stabilization. Furthermore, the auxiliaries remaining in the emulsion polymer which is to be used to stabilize the POM lead to discolorations or even to decomposition of the polyoxymethylene moulding composition at the usual polyoxymethylene moulding composition processing temperatures of greater than 200° C.
SUMMARY OF THE INVENTION
In view of the prior art mentioned and described herein, it was consequently an object of the invention to provide reinforced or non-reinforced polyoxymethylene moulding compositions for industrial production which are superior to the POM moulding compositions used hitherto in practice. In particular, the new POM moulding compositions should have
a higher heat stability,
a lower tendency towards discoloration,
a lower residual formaldehyde content
than the POM moulding compositions known to date. Finally, attempts are also to be made to achieve this “basic stabilization”, if possible, with a single additive, which furthermore should be simple and inexpensive to prepare.
This and other objects which are not described in more detail are achieved by a polyoxymethylene moulding composition of the type described above.
By employing as additive (iii), to improve the heat stability, a copolymer which is obtainable by polymerization in bulk of a mixture of
A) 60-90 parts of one or more (meth)acrylates,
B) 10-40 parts of one or more (meth)acrylamides,
C) per 100 parts of A)+B), >0.2 to 5 parts of molecular weight regulator and
D) per 100 parts of A)+B), up to 2 parts of lipophilic free radical polymerization initiators,
and by the fact that
the moulding composition comprises (iii) in an amount of 0.01 to 2 parts, based on the sum of (i)+(ii),
where all the amounts relate to parts by weight (wt/wt) and A) and B) together must give 100 parts,
the problems on which the invention is based are solved in a manner which is not readily foreseeable.
The reinforced or non-reinforced moulding composition according to the invention based on polyoxymethylene thus offers, inter alia, the following advantages over the POM moulding compositions known from the prior art, in particular over the compositions known from EP-A-0 381 943:
Uncomplicated preparation of the additive by bulk polymerization and subsequent grinding to a powder.
No use of solvents, which means that drying of the additive is omitted. The polymer therefore cannot contain residual solvents.
Bulk polymers comprise no auxiliaries necessary for emulsion polymerization, such as, for example, emulsifiers, protective colloids, or other additives which lead to discolorations at the usual processing temperatures of greater than 200° C. when used in polyoxymethylene moulding compositions.
Highly regulated copolymers do not tend to form dirt points (speck formatio
Alt Hartmut
Dorn Klaus
Keller Bruno
Michelchen Dietrich
Schauhoff Stephanie
Degussa-Huls AG
Pillsbury Madison & Sutro LLP
Short Patricia A.
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