Stabilized polyoxymethylene moulding materials

Details Stabilized polyoxymethylene moulding materials Stabilized polyoxymethylene moulding materials

2000-12-18

2003-07-08

Szekely, Peter (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S210000, C524S230000, C524S315000, C524S318000, C524S349000, C524S351000, C524S352000, C524S456000, C524S542000, C524S593000, C524S145000

Reexamination Certificate

active

06590020

ABSTRACT:

The present invention relates to thermoplastic polyoxymethylene molding materials containing, as essential components,
A) from 10 to 99.98% by weight of a polyoxymethylene homo- or copolymer
B) from 0.005 to 2% by weight of a sterically hindered phenol
C) from 0.001 to 2% by weight of a polyamide
D) from 0.002 to 2% by weight of an alkaline earth metal silicate or of an alkaline earth metal glycerophosphate
E) from 0.01 to 5% by weight of at least one ester or amide of saturated or unsaturated aliphatic carboxylic acids of 10 to 40 carbon atoms with polyols or aliphatic saturated alcohols or amines of 2 to 40 carbon atoms or an ether which is derived from alcohols and ethylene oxide
F) from 0 to 5% by weight of a melamine/formaldehyde condensate
G) from 0 to 74% by weight of further additives, the sum of the percentages by weight of the components A) to G) being 100% in each case.
The present invention furthermore relates to the use of such molding materials for the production of moldings of any type and to the moldings obtainable thereby.
Polyoxymethylene homo- and/or copolymers have long been known. The polymers have a number of excellent properties, making them suitable for a very wide range of industrial applications. Nevertheless, there has been no lack of attempts to find compositions for improving the processability, for example the flowability, solidification time, etc., and/or for improving the mechanical properties and the dimensional stability of moldings produced from such polymers.
According to DE-A-23 19 359, molding materials which consist of from 98 to 25% by weight of oxymethylene copolymers and from 2 to 75% by weight of acicular calcium metasilicate, the percentages by weight being based on the total material, have improved processability, dimensional stability and heat aging properties.
The prior art furthermore describes the stabilization of polyoxymethylene molding materials by suitable additives. For this purpose, antioxidants, for example sterically hindered phenol derivatives, are added to polyoxymethylene molding materials. Such phenol derivatives are listed, for example, in DE-A-27 02 661. EP-A-19 761 discloses glass fiber-reinforced polyoxymethylene molding materials which contain alkoxymethylmelamines for improving the impact strength. Polyoxymethylene molding materials which have been stabilized to the action of heat in the temperature range from 100 to 150° C. over a relatively long period are disclosed in EP-A-52 740, which describes the addition of a partially etherified special melamine/formaldehyde condensate. DE-A-3 011 280 describes stabilized oxymethylene copolymer materials which contain, as a stabilizer, a mixture of at least one amino-substituted triazine, at least one sterically hindered phenol and at least one metal-containing compound. The metal-containing compounds preferably comprise potassium hydroxide, calcium hydroxide, magnesium hydroxide or magnesium carbonate.
In spite of these measures, known polyoxymethylene molding materials have, for some applications, unsatisfactory thermal stability which adversely affects the processing to give moldings and, for example, contribute to deposits on the mold or to a deterioration in the mold release properties and/or, during subsequent use of the moldings, lead to discolorations and a decline in the mechanical properties. Another disadvantage is that the mixtures may also contain formaldehyde adducts which, on processing at elevated temperatures, may lead to an annoying odor owing to the elimination of formaldehyde.
DE-A 36 28 560, DE A-36 28 561 and DE A-36 28 562 disclose polyoxymethylene molding materials which contain, as stabilizers, a mixture of sterically hindered phenols and alkaline earth metal silicates and alkaline earth metal glycerophosphates. According to the description, polyamides may also be used as further co-stabilizers. Although these materials have relatively good heat stability, they are still in need of improvement with regard to the color quality.
It is an object of the present invention to provide polyoxymethylene molding materials which have better thermal stability during processing and at elevated temperatures of use. At the same time, it is intended to achieve better color constancy and permanent brightness of the color. Coating of the mold and formaldehyde emission are to be very substantially minimized during processing.
We have found that the object is achieved by the molding materials defined at the outset.
The novel molding materials contain, as component A), from 10 to 99.98, preferably from 30 to 99, in particular from 40 to 98, % by weight of a polyoxymethylene homo- or copolymer.
Such polymers are known per se to the person skilled in the art and are described in the literature.
Very generally, these polymers have at least 50 mol % of repeated units —CH
2
O— in the polymer main chain.
Homopolymers are generally prepared by polymerizing formaldehyde or trioxane, preferably in the presence of suitable catalysts.
For the purposes of the present invention, polyoxymethylene copolymers are preferred as component A, in particular those which, in addition to the repeating units —CH
2
O—, also contain up to 50, preferably from 0.1 to 20, in particular from 0.3 to 10, very particularly preferably from 2 to 6, mol % of repeating units
where R
1
to R
4
, independently of one another, are each hydrogen, C
1
- to C
4
-alkyl or halogen-substituted alkyl of 1 to 4 carbon atoms and R
5
is —CH
2
—, —CH
2
O— or C
1
- to C
4
-alkyl- or C
1
- to C
4
-haloalkyl-substituted methylene or a corresponding oxymethylene group and n is from 0 to 3. Advantageously, these groups can be introduced into the copolymers by ring cleavage of cyclic ethers. Preferred cyclic ethers are those of the formula
where R
1
to R
5
and n have the abovementioned meanings. Merely by way of example, ethylene oxide, 1,2-propylene oxide, 1,2-butylene oxide, 1,3-butylene oxide, 1,3-dioxane, 1,3-dioxolane and 1,3-dioxepan may be mentioned as cyclic ethers, and linear oligo- or polyformals, such as polydioxolane or polydioxepan, as comonomers.
Other suitable components A) are oxymethylene terpolymers which are prepared, for example, by reacting trioxane and one of the cyclic ethers described above with a third monomer, preferably bifunctional compounds of the formula
where Z is a chemical bond, —O—, —ORO—(R=C
1
- to C
8
-alkylene or C
2
- to C
8
-cycloalkylene).
Preferred monomers of this type are ethylene diglycide, diglycidyl ether and diethers of glycidylene compounds and formaldehyde, dioxane or trioxane in the molar ratio 2:1 and diethers of 2 mol of glycidyl compound and 1 mol of an aliphatic diol of 2 to 8 carbon atoms, for example the diglycidyl ethers of ethylene glycol, 1,4-butanediol, 1,3-butanediol, cyclobutane-1,3-diol, 1,2-propanediol and cyclohexane-1,4-diol, to mention but a few examples.
Processes for the preparation of the homo- and copolymers described above are known to a person skilled in the art and are described in the literature, so that further information is superfluous here.
The preferred polyoxymethylene copolymers have melting points of at least 150° C. and weight average molecular weights M
w
of from 5000 to 200,000, preferably from 7000 to 150,000.
Polyoxymethylene polymers which are stabilized at the terminal groups and have C—C bonds at the chain ends are particularly preferred.
Suitable sterically hindered phenols B) are in principle all compounds which have a phenolic structure and possess at least one bulky group on the phenol ring.
For example, compounds of the formula
where R
1
and R
2
may be identical or different and are each alkyl, substituted alkyl or substituted triazolyl and R
3
is alkyl, substituted alkyl, alkoxy or substituted amino, are preferred.
Antioxidants of the stated type are described, for example, in DE-A-27 02 661 (U.S. Pat. No. 4,360,617).
A further group of preferred sterically hindered phenols are derived from substituted benzenecarboxylic acids, in particular from substituted benzenepropionic acids.
Particularly preferred comp

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