GB2169907A - Moulding materials comprising polyamides - Google Patents

Abstract

Moulding materials comprising polyamides, i.e. homopolyamides or polyamide mixtures, with greatly reduced adhesiveness, i.e. without particles of the moulding material bonding together or tending to aggregate at elevated temperature, contain a hydrophilic silicone compounds.

Description

SPECIFICATION Moulding materials comprising polyamides The present invention relates to moulding materials comprising polyamides.

It is known that polymers, at above their glass transition point, have a more or less pronounced tendency towards adhesiveness. For example, there is the property of particles of moulding materials, in particular as granulate or powder, to bond to one another. This property grows with increasing temperature, and can lead to aggregation unless suitable measures are taken to prevent it.

The adhesiveness is directly dependent on the softening properties ofthe polymers. If a polymer softens, "tack", i.e. a property of adhesiveness, results, which grows with increasing temperature.

The greaterthe difference between the glasstransition point and the temperature ofthe product, the more marked is the adhesiveness and also the tendency to aggregation of, for example, the granulate or powder.

The extent ofthe adhesiveness is very greatly influenced by the degree ofcrystallinity ofthe polymer. While amorphous polymers are already tacky at temperatures above the glass transition point, the corresponding temperature for crystalline polymers is distinctly higher. Adhesiveness is only observed at the start of the melt range, for polymers with a very high degreeofcrystallinity.

In the preparation and processing of polymeric moulding materials, it is often advantageous, and sometimes unavoidable, to operate at temperatures above their glass transition point. As a consequence of adhesiveness, aggregation or balling ofthe particles ofthe moulding material leads to a disadvantage in operation. Forexample,thedrying of polymers is often conducted at temperatures above the glass transition point, in orderto reach the desired moisture content in drying times which are commercially acceptable. For moulding materials whose glass transition point is in the range of room temperature or below, storage can lead to undesirable aggregation, particularly in countries with hot climates where storage at temperatures of 40 Cand above is customary.

In orderto reducethetendencyto adhesiveness and thus to aggregation, and also the resulting problems in working the moulding materials, various measures are known. For example, for moulding materials which can be crystallised, the property already described, that the adhesiveness decreases with increasing deg ree of crystallinity, is used. The degree of crystallinity ofthe moulding materials is increased by tempering conducted, according to a given temperature programme, by raising the temperature in dependence on the time in the mass, so allowing decreasing adhesiveness by the increasing degree of crystallinity.This tempering can be very time-consuming for polymer mixtures, becausetheyhavea much lower rate of crystallisation as well as a lower maximum degree of crystallisation than the corresponding homopolymers, and the temperature during tempering can only be raised very slowly, in orderto avoid aggregation.

Another way in which adhesiveness can be reduced is to add to the moulding material a substance which acts as a separating agent. For this purpose, the separating agent is applied superficially on to the moulding material (in the form of a granulate, powder or similar state), in a separate process step. However, such separating agents have the disadvantage that their effect is lost when the moulding materials are melted and the separating agents are thereby mixed into the moulding material. If the separating agents cannot be mixed homogeneously with the polymeric moulding material,the properties ofthe moulding material can be influenced in an undesirable manner.

The properties described herein for polymeric moulding materials in general are alsofullyapplicable to moulding materials comprising polyamides in particular.

According to the present invention, a moulding material comprises a polyamide and a hydrophilic silicone compound.

Polyamides in the context of the present invention are all homopolyamidesand polyamidemixtures which can be prepared in known mannerfrom aminocarboxylic acids or, asfar is possible,the corresponding lactamsand/ordiaminesanddicar- boxylic acids ortheir salts. The moulding materials of the invention can contain additives such asstabilisers, plasticisers, pigments, optical brighteners, reinforcing agents and fillers.

Advantageously, silicone-containing surfactants are used asthe hydrophilicsiliconecompoundsfor the present invention. Such compounds are, primarily, silicon polyalkylene oxide copolymerswhich can basically be distinguished as the hydrolysable silicone surfactantsoftheformula R' (4.m)Si[[(-o-Si(CH3)2-]x-o-Zz-R]m and the non-hydrolysable silicone surfactants of the formnula (CH3)3SI-O-[-Si (CH3)2 - 0 -]x - [ - SiCH3(C3H6 - O - Zz - R) - O -]y Si (CH3)3 wherein m, x, y and z are integers, R is H oran alkyl group, R' is an alkyl group and Zz represents one or more polyalkylene oxide groups -(CnH2nO)z-which can be derived from polyethylene oxide or a block polymer of polyethylene oxide and polypropylene oxide blocks.

Products of this type are commercially available, typical examples including Silwet L-720 and Silwet L-7550 (from Union Carbide Corp., Danbury), Baysilon OF/OH-OR 603 (from BayerAG, Leverkusen) and Silicone Oil L 050 (from Wacker Chemie GmbH, Munich).

The hydrophilic silicone compounds can be added to the polyamide during or after its preparation. They can thus be mixed either by addition before, during or after the polymerisation ofthe polyamide in meltflowableform, orthey can be introduced superficially on to the solid polyamide, preferably in granulate or powder form.

The hydrophilic silicone compounds are added in amounts between 0.005 and 5% by weight, preferably between 0.05 and 2% by weight, based on the polymer. The optimum amounts depend on the manner of addition. If the hydrophilic silicone compound is mixed with the polyamide in melt-flowable form, the best results are obtained with amounts between 0.05 and 2% by weight, whereas amounts between 0.05 and 0.5% by weight are advantageous forsuperficial application on to the solid polyamide.

Thefollowing Examples 1 and 2 illustrate the present invention, while ExamplesAand B are comparative.

Example 1 40 parts by weight laurolactam, 60 parts by weight caprolactam, 2 parts by weight water and 0.5 part by weight adipic acid were introduced into an autoclave with stirrer and, after displacement ofthe air by nitrogen, heated in the closed autoclave for 5 hours at 290 C. The pressure was maintained at a maximum of 20 bar by release as necessary. Then, within 1 hour, the pressure was reduced to atmospheric by release of the autoclave, and the temperature of the reaction mixture fell to 260 C at the same time. 0.8 part by weight of the silicone surfactant Silwet L-7500 from Union Carbide Corp. was added and the polymerisation was continued underthe passage of nitrogen.

After 8 hours, the desired degree of polymerisation, corresponding to a solution viscosity of 1.83 (measured as a 0.5% solution in M-cresol at 20 C) was reached.

The polymer was removed from the autoclave in cord form, solidified by passage through a water bath, and granulated. The granulate was introduced into a silo and stored there for 24 hours. The granulate temperature fell during storage from 35 C initially to 30 C. The granulate was then introduced into a tumble drier whose heating medium was cooled to room temperature, so that no problems associated with aggregation ofthe granulate were encountered. After the application of a vacuum of 20 torr, the tumble drier was heated. After about 1 hour, the heating medium reached a temperature of 90 C, and the product temperature reached about 86 C after2 hours. After a drying period of a further6 hours, the desired moisture content ofthe granulate, 0.02% H20, was reached.On removal ofthe granulate from the tumble drier, at the end of drying, there were no aggregates of the granulate orany granulates sticking to the walls of the drier.

Example A A polyamide granulate was prepared in the same way as in Example 1, with the difference that no silicone compound was added. The granulate was introduced into a silo as in Example 1, butthen had to be cooled by the passage of air at about 15 C, in order to avoid aggregation. After 24 hours storage, the granulate was introduced into a tumble drier and dried, as in example 1. In orderto avoid sticking ofthe granulate with itself or to the walls of the drier, the temperature of the heating medium was raised step-wise,flrst to 45 C,to 65 C after 1 hourand then to 85 C after a further hour. The producttemperature was 82 C after a further hour. In order to reach the desired moisture content ofthe granulate of 0.02% H20, a drying period of an additional 8 hours is necessary.

Acomparison ofthe procedures in Examples 1 and A clearly shows that, by the addition of silicone compounds in accordance with the invention, and the resultant reduced adhesiveness of polyamides, problem-free storage, especially of as yet unternpered and thereby crystallised granulate, is possible, as well as a quickandtherebyfunctional procedure in drying of this granulate.

Example 2 40 parts by weight caprolactam, 20 parts by weight AH-salt, 16 parts by weight hexamethylenediamine, 14 parts by weight azelaic acid, 15 parts by weight dodecanedicarboxylic acid and 3 parts by weight water wee introduced into an autoclave with stirrer and, after driving outthe air, heated to 260 Cforl hour.

The pressure in the autoclave was thereby held at a maximum of 20 bar. Subsequently, within 1 hour, the pressure in the autoclave was reduced to atmospheric pressure, and the polymerisation was continued, underthe introduction of nitrogen, to a degree of polymerisation corresponding to a solution viscosity of 1.55. The polymer was removed from the autoclave, granulated and dried, as described in Example 1.

100 parts by weight ofthe resultantgranulatewere mixed with 0.1 part by weight of Silicone Oil L 050 from WackerChemie, and then milled in a peg mill with simultaneous cooling with liquid nitrogen. The fraction of raw milled material having a particle size between 80 and 200 m was sieved off and mixed with 0.1 % by weight magnesium stearate. A sample ofthe powderthus obtained was stored for 3 days at 40 C. Its tendency to aggregation was then estimated as zero on the basis of a scale in which 0 = no aggregation; 1 = some aggregation, ciumps can be crushed manually; 2 = average aggregation, ciumps can only be crushed understrong manual pressure; and 3 = strong aggregation, clumps cannot be crushed manually.

Example B The procedure of Example 2 was conducted, with the difference that no hydrophilic silicone compound was added to the granulate before milling. On the same scale as before, the tendency to aggregation was estimated as 2.

Claims (6)

1. A moulding material which comprises a polyamide and a hydrophilic silicone compound.

2. Amoulding material according to claim 1,which comprises 0.005 to 5% w/w ofthe hydrophilic silicone compound, based on the polyamide.

3. A moulding material according to claim 1 or claim 2, produced by adding the hydrophilic silicone compound to the polyamide before, during or after polymerisation.

4. Amoulding material according to claim 1 or claim 2, produced by applying the hydrophilic silicone compound on to the polyamide, the polyamide being in granulate or powderform.

5. A moulding material according to any preceding claim, in which the hydrophilicsilicone compound is a silicone-based surfactant.

6. A moulding material according to claim 1, substantially as described in Example 1 or Example 2.