NL8400417A - Polyoxymethylene moulding compsn. having improved sliding properties - contg. hydroxy-carboxylic acid ester of higher alkanol as lubricant esp. tri:stearyl citrate - Google Patents

Abstract

The compsns. contain by wt., 0.3-5% of a hydroxycarboxy-lic acid ester, (A), of: (a) a 3-30C hydroxycarboxylic acid contg. 1-10 COOH and 1-5 OH gps. and (b) one or more 10-25C alkanols. Pref. the lubricant (A) is esp. tristearyl citrate.

Description

^ 1 N.0. 32288

Polyoxymethylene molding materials * with improved sliding properties.

The invention relates to polyoxymethylene molding compositions with improved sliding properties, which contain as a lubricant a hydroxycarboxylic acid ester.

Polyoxymethylene moldings, for example bearing elements, which are exposed to dragging friction with other materials, in particular metal parts, must exhibit favorable sliding and wear behavior. This can be achieved by the addition of lubricants. However, the known lubricants have considerable drawbacks.

Molybdenum sulfide does not sufficiently improve the wear behavior, so that, especially at high roughnesses of the metal parts, a strong wear of the polyoxymethylene moldings occurs.

Polytetrafluoroethylene is a relatively expensive additive which additionally degrades the mechanical properties, in particular the tensile strength and elongation of the polyoxymethylene molding compositions.

Other additives, such as stearyl alcohol or stearylal, tend to migrate and sweat out of the polymers, so that the lubricant action decreases over time and an unsightly deposit is formed on the molding.

German Qffenlegungsschrift 2,262,788 describes lubricants based on fatty acid esters of polyhydric alcohols. These additives also still tend to form deposits. In particular, however, the frictional wear caused by sliding on materials with low roughnesses, which are important in particular in the fine work technique, is still too high.

The object of the invention, therefore, was to develop polyoxyethylene molding materials with improved sliding properties both with respect to materials with a high and also with those with a low roughness. The molding materials must be thermoplastic easy to process and have no tendency to deposit.

This problem is solved by the addition of 0.3 to 5, preferably 0.5 to 3, weight percent of the long chain hydroxycarboxylic acid esters of the invention.

Polyoxymethylenes for the purposes of the present invention are homopolymers of formaldehyde and copolymers of formaldehyde or of trioxane with cyclic and linear formals, such as, for example, 1,3-dioxolane, dioxepane or epoxides, such as ethylene oxide or propylene oxide.

The homopolymers should have thermally stable terminal groups, such as, for example, ester or ethyl groups. The copolymers preferably contain 14 U 4 4 1 7 / ... 2 more than 75% oxymethylene groups and at least 0.1% groups of the comonomer, with which 2 adjacent carbon atoms are introduced into the chain. Such copolymers can be known in a known manner by cationic copolymerization in trioxane with suitable comonomers, for example cyclic ethers or acetals, such as ethylene oxide, propylene oxide, 1,3-dioxane, 1,3-dioxolane, 1,3-dioxepane or with linear oil. -go- or polyformals or acetals, such as polydioxolan, polydioxepan.

The hydroxycarboxylic acid esters according to the invention consist of a hydroxycarboxylic acid a) and one or more alkanols b).

The hydroxycarboxylic acids contain 3 to 30 carbon atoms and carry 1 to 10, preferably 1 to 3, carboxyl groups and 1 to 5, preferably 1 or 2, hydroxyl groups. Preferred hydroxycarboxylic acids are citric, malic, tartaric and lactic acids.

The alkanols contain 10 to 25 carbon atoms. In order to increase the compatibility with the polyoxymethylene, they may also contain ether groups in addition to the hydroxy group, which are obtained, for example, by reacting alkanols with epoxides, such as ethylene oxide or propylene oxide. The number of added epoxide units should preferably not exceed 5. Preferred alcohols are stearyl alcohol or alfols which are prepared by oligomerization of ethylene to Ziegler catalysts and subsequent oxidation with air.

The esters can be prepared by azeotropic esterification, for example with cyclohexane or toluene as water entraining agent in the presence of p-toluenesulfonic acid and is known in the art.

The polymers may contain conventional heat and light stabilizers as well as other additives such as, for example, dyes, fillers and reinforcing agents. The lubricant has been incorporated into it expediently together with the other additives by known methods, eg by mixing on a suitable extruder or kneader, preferably at temperatures between 180 and 260 ° C. In addition to the lubricants according to the invention, they can also contain customary additives such as molybdenum sulfide, silicic acid, alkaline earth metal salts or talc.

The trailing friction number and the wear due to sliding were measured according to the following method:

A cylindrical polyoxymethylene test rod with a length of 10 + 0.1 mm, an upper diameter of 6 mm, which is 1 mm high at the load surface of the cylinder and a diameter of 3 mm. 8400417 3 w.

is pressed on a metal disc rotating at a speed of 0.5 m / s (depth of the surface roughness 0.1 to 0.2 µm) with a load of 1 N / mm 3. To determine the wear by sliding, the decrease in the length and the weight loss of the test rod 5 after a run of 20 km is measured. To determine the trailing frictional number, the frictional force is determined by a touch-free path length determination with inductive automatic automats, the deflection of the bending rod to which the test body is attached being measured during movement of the disc in relation to the rest state. The spring constant of the bending rod then provides a direct conversion from road to force. The ratio of the frictional force to the load on the test body gives the trailing friction value μ.

The parts and percentages mentioned in the examples are by weight.

15

Example I

1% Citric acid tristearyl ester is prepared in powdered polyoxymethylene, prepared from 96.9 parts of trioxane and 3 parts of dioxolan (melt index 9), together with 0.1% polyamide and 0.5% 2,2'-methylene-bis-4-methyl -6-tert-20 butylphenol mixed and melted on a twin screw extruder, extruded and granulated.

The granulate is processed into a semi-finished product, from which the test bodies are manufactured. These test bodies are then examined for their sliding rubbing behavior. The results are summarized in Table A along with 25 other examples and the prior art.

Example II

As in Example 1, however, using a stabilized, granulated trioxane homopolymer with 0.75% polyamide and 0.5% 2,2'-30 methylene-bis-4-methyl-6-tert-butylphenol.

Example III

As in Example 1, but with the addition of 1% basic magnesium carbonate.

35

Example IV

Like Example III, but with 2.5-fold ethoxylated Alfol 20 as an alcohol component in the ester.

40 8400417 r; SN - 4

Example V

Like Example III, but with malic acid as hydroxycarboxylic acid.

5 Comparative example

As in Example 1, but with 1% ethylene glycol distearate according to German Offenlegungsschrift 2,262,788.

Table A

10 Sliding wear test on the rod-disk system with a surface pressure of 1 W / inm ^, a speed of 0.5 m / s and a surface temperature u 40 ° C as well as a sliding range of 20 km roughness of the _________ sliding rubbing partner 0 , 1 to 0.2 ym ._

Example Sliding friction μ Sliding wear s 15 ___ 0.1 to 0.2 ym__ I 0.35 0.70 II 0.40 0.68 III 0.38 0.60 IV 0.34 0.63 20 V 0.31 0 , 70

Comparative example_0.60_0.70_ 8400417

Claims (2)

1. Polyoxymethylene molding compositions with improved sliding properties, characterized by a content of 0.3 to 5% by weight of a hydroxycarboxylic acid ester of a) a hydroxycarboxylic acid with 3 to 30 carbon atoms, which carries 1 to 30 carboxyl groups and 1 to 5 hydroxyl groups and b) one or more alkanols with 10 to 25 carbon atoms. Polyoxymethylene molding compositions according to claim 1, characterized in that the lubricant is citric acid tristearyl ester. ****** 8400417