Slip-modified, electrically conductive polyoxymethylene

Details Slip-modified, electrically conductive polyoxymethylene Slip-modified, electrically conductive polyoxymethylene

2002-10-31

2004-09-14

Kopec, Mark (Department: 1751)

Compositions

Electrically conductive or emissive compositions

Elemental carbon containing

C106S231000, C524S496000

Reexamination Certificate

active

06790385

ABSTRACT:

The invention relates to an electrically conductive polyoxymethylene molding composition whose electrical conductivity is maintained via addition of a lubricant mixture composed of a lubricant with predominantly external lubricant action and of a lubricant with predominantly internal lubricant action in order to improve abrasion performance, and to its use.
Traditional materials, such as metals, are increasingly undergoing successful replacement by plastics. Because the electrical resistances within plastics are usually very high, there is a risk of electrostatic charging, and this can be disruptive in certain application sectors, or can even be dangerous. Attempts are therefore being made to improve the electrical conductivity of plastics. A suitable method of reducing internal electrical resistance is the addition of metal powders and metal fibers, carbon fibers, graphite, or carbon black. The last-mentioned is in particular capable of universal use and processing to provide polymers with conductivity. The use of highly structured carbon blacks means that the amounts required are markedly lower than for graphite. A disadvantage with the use of highly structured carbon blacks is their sensitivity to processing effects. On the one hand, the dispersion of the carbon black has to be sufficiently good, and on the other hand excessive shear must not be allowed to break down the carbon black agglomerates. For this reason, use is often made of lubricant additives intended to contribute to low shear within the melt.
Many of the moldings produced from electrically conductive polyoxymethylene are also subject to tribological stresses. It is well known that additions of lubricants can improve the tribological, i.e. sliding and friction, properties of thermoplastics. However, the problem is to find the correct selection and combination of lubricants for the particular plastic. The processing aids used in POM are not suitable for reducing abrasion on moldings subject to wear. Only a few lubricants are capable of reducing abrasion when used with conductivity black incorporated into POM. It is likely that they disrupt the bonding between matrix and carbon black. This may well also be the reason for the reduction in mechanical properties. Careful use of the lubricants and optimized selection of the same are therefore required.
Another problem with the conductivity blacks is the fall-off in toughness. This can be compensated by addition of elastomers.
U.S. Pat. No. 4,828,755 describes a mixture where polyethylene glycol and non-polar polyethylene wax are proposed for incorporation of conductivity black into polyoxymethylene. As is shown by the low electrical resistance values, the lubricants achieve incorporation of the carbon black into the matrix under gentle conditions, but abrasion from the resultant moldings is high, and mechanical properties, and also heat resistance, are severely reduced over the base material.
The object of the present invention was to improve abrasion performance and reduce the fall-off in mechanical properties while retaining the good electrical conductivity of a polyoxymethylene modified with conductivity black.
The object of the invention was achieved by using a lubricant mixture composed of a lubricant with predominantly external lubricant action, i.e. surface-active lubricants, and a lubricant with predominantly internal lubricant action, i.e. viscosity-reducing lubricants, that is to say lubricants whose lubricant effect acts predominantly within the melt.
The invention provides a molding composition composed of 30 to 89 parts by weight of a polyoxymethylene (A) and from 2 to 10 parts by weight, preferably from 3 to 5 parts by weight, of a conductivity black (B), and also from 0.5 to 6 parts by weight, preferably from 3 to 5 parts by weight, of a lubricant mixture (C) composed of a lubricant with predominantly internal lubricant action and of a lubricant with predominantly external lubricant action, and from 1 to 12 parts by weight, preferably from 5 to 10 parts by weight, of an impact-modifier component (D). The mixing ratio of lubricant with predominantly internal lubricant action to lubricant with predominantly external lubricant action may be from 0:100 to 100:0 parts by weight. The molding composition may comprise additives and processing aids (E) in amounts of from 0.005 to 50 parts by weight. Components (A) to (E) here always give a total of 100 parts by weight.
As stated at the outset, polyoxymethylene is a suitable component (A).
The polyoxymethylenes (POMs), for example as described in DE-A 29 47 490, are generally unbranched linear polymers, generally containing at least 80%, preferably at least 90%, of oxymethylene units (—CH
2
O—). The term polyoxymethylenes here encompasses homopolymers of formaldehyde or of its cyclic oligomers, such as trioxane or tetroxane, and also corresponding copolymers.
Homopolymers of formaldehyde or of trioxane are polymers whose hydroxy end groups have been stabilized chemically in a known manner to prevent degradation, e.g. by esterification or etherification. Copolymers are polymers of formaldehyde or of its cyclic oligomers, in particular trioxane, with cyclic ethers, with cyclic acetals, and/or with linear polyacetals.
These POM homo- or copolymers are known per se to the skilled worker and are described in the literature. Very generally, these polymers have at least 50 mol % of —CH
2
O— repeat units in the main polymer chain. The homopolymers are generally prepared by polymerization of formaldehyde or trioxane, preferably in the presence of suitable catalysts.
For the purposes of the invention, POM copolymers are preferred as component (A), in particular those which, besides the —CH
2
O— repeat units, also contain up to 50 mol %, preferably from 0.1 to 20 mol %, and in particular from 0.5 to 10 mol %, of
repeat units, where R
1
to R
4
, independently of one another, are a hydrogen atom, a C
1
-C
4
-alkyl group, or a halo-substituted alkyl group having from 1 to 4 carbon atoms, and R
5
is —CH
2
—, —CH
2
O—, a C
1
-C
4
-alkyl-substituted or C
1
-C
4
-haloalkyl-substituted methylene group, or a corresponding oxymethylene group, and n has a value in the range from 0 to 3. These groups may advantageously be introduced into the copolymers via ring-opening of cyclic ethers. Preferred cyclic ethers are those of the formula
where R
1
to R
5
and n are as defined above. Merely by way of example, mention may be made of ethylene oxide, propylene 1,2-oxide, butylene 1,2-oxide, butylene 1,3-oxide, 1,3-dioxane, 1,3-dioxolane, and 1,3-dioxepan as cyclic ethers, and also linear oligo- or polyformals, such as polydioxolane or polydioxepan as comonomers.
Particularly advantageous copolymers are those of from 99.5 to 95 mol % of trioxane and from 0.5 to 5 mol % of one of the above-mentioned comonomers.
Other suitable components (A) are oxymethyleneterpolymers, prepared, for example, by reacting trioxane, one of the cyclic ethers described above, and a third monomer, preferably a bifunctional compound of the formula
where Z is a chemical bond, —O—, or —ORO— (R=C
1
-C
8
-alkylene or C
2
-C
8
-cycloalkylene).
Preferred monomers of this type are ethylene diglycide, diglycidyl ether, and diethers made from glycidyl compounds and formaldehyde, dioxane, or trioxane in a molar ratio of 2:1, and also diethers made from 2 mol of glycidyl compound and 1 mol of an aliphatic diol having from 2 to 8 carbon atoms, for example the diglycidyl ethers of ethylene glycol, 1,4-butanediol, 1,3-butanediol, 1,3-cyclobutanediol, 1,2-propanediol, or 1,4-cyclohexanediol, to mention merely a few examples.
Processes for preparing the POM homo- and copolymers described above are known to the skilled worker and are described in the literature.
The preferred POM copolymers have melting points of at least 150° C. and molecular weights (weight-average) M
w
in the range from 5000 to 200,000, preferably from 7000 to 150,000. Particular preference is given to end-group-stabilized POM polymers which have carbon-carbon bonds at the ends of the chains.
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