AU687467B2 - Polyethylene molding materials and process for the production of moldings from these molding materials - Google Patents

Description

P/UU'Ul 2&51I Regulation 3,2(2)

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Application Number: Lodged: Invention Title: POLYETHYLENE MOLDING MATERIALS AND PROCESS FOR THE PRODUCTION OF MOLDINGS FROM THESE MOLDING AMTERIALS The following statement is a full description of this invention, including the best method of performing it known to us a a. a

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Polyethylene molding materials and process for the production of moldings from these molding materials The invention relates to novel molding materials comprising ultrahigh molecular weight polyethylene (PE-UHMW) and a process for the production of moldings from these molding materials by ram extrusion. The novel molding materials avoid the formation of cracks in moldings, which is to be observed in particular in the production of thick-walled profiles by ram extrusion (PE-UHMW).

Among the polyethylenes, PE-UHMW grades occupy a special place. This term is understood as meaning linear polyethylenes obtained by the low pressure method and having an extremely high melt viscosity. Their limiting viscosity number is at least about 1,000 ml/g, corresponding to average molecular weights (weight average) of about 106 g/mol or more. The conversion of the limiting viscosity number into molecular weights is carried out according to the Margolis equation, and the method used •for the determination is described, for example, in CZ- 20 Chemische Technik 4 (1974), page 129 et seq.

PE-UHMW is distinguished by a number of physical charac- "teristics which open up a -wide range of potential applications for it. Its high resistance to wear, its low coefficient of friction with respect to other materials, its excellent toughnesL and its high heat distortion resistance are noteworthy. In addition, it is remarkably "resistant to a large number of chemicals.

Owing to these particular mechanical, thermal and chemical properties, ultrahigh molecular weight polyethylene is used as a high-quality engineering material in a very wide range of applications. Examples are the textile, food and packaging industries, the chemical industry and mechanical engineering.

Owing to the thermal behavior of PE-UHMW, only a few

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methods are suitable for processing to moldings. Sinter processes and ram extrusion have become established in industry. Processing by injection molding requires either a special design of the injection molding machines or the use of ultrahigh molecular weight polyethylene grades specially prepared for this purpose.

Ram extrusion is understood as meaning a pressure sintering method which is used in particular for the continuous production of continuous profiles of plastics having a high melt viscosity, such as PE-UHMW and polytetrafluoroethylene. In this procedure, powders are sintered in a heated die under pressure in a ram extruder to give extrudates, such as rods and pipes. For this purpose, pulverulent polymer is introduced batchwise by a metering channel into an extrusion barrel and is pressed by means of a ram, which is moved back and forth by a hydraulic, controllable drive apparatus, to give an extrudate. In addition to compaction of the powder, the object of pressing is to convey the successive moldings into the heating zone of the extrusion barrel, where they are sintered at temperatures above the crystallite melting point (about 135°C) to give the extrudate. The pressure resistance required for compaction is generated by the friction between polymeric material and die wall.

The production of moldings from PE-UHMW by ram extrusion is described in DE-C-1 778 258. According to this publication, thermally plastified, high molecular weight high density polyethylene is molded using a ram extruder having an extrusion die. Here, the polyethylene is 30 cooled in the die from about 240 to 100 0 C under the action of a compaction pressure from at least 200 to not more than 1500 kp/cm 2 (corresponding to 19.6 to not more than 147.1 MPa) for a residence time of several minutes, preferably 3 to 8 minutes.

DE-C-2 829 232 relates to a process for the optimum processing of high molecular weight polyethylene by means of ram extrusion, by providing the data required for I I I 3 designing the ram extruder used for this purpose. The process comprises exactly setting the residence time of the polyethylene in the die as a function of the equivalent diameter of the extrusion die. The equivalent diameter is understood as being the quotient of the cross-sectional area and the circumference of the crosssectional area of the extrusion die.

This procedure has proven suitable in particular for the production of profiles having a small diameter or having a small wall thickness. However, problems may arise in the production of thick-walled and larger profiles.

Their surface frequently exhibits transverse cracks, which are a consequence of excessively high wall shear stresses during the extrusion process. The occurrence of these defects is not due only to the design of the extruder, for example the geometry of the die, but may also be caused by the properties of the pulverulent polymer to be processed, for example its molecular .weight, its molecular weight distribution and its flow.

e 20 It is not always possible to take into account these and other parameters when designing the extruders. Apart from the fact that the possibility of using the extruders is greatly limited as a result of this, the parameters must be adjusted very exactly since deviations from the required values often cannot be tolerated because they lead to finished products of insufficient quality.

o• It was therefore the object to develop molding materials which are suitable for processing by ram extrusion, are based on ultrahigh molecular weight polyethylene, can be used generally and in particular do not require the use of specially designed dies. Furthermore, the excellent physical properties of PE-UHMW should not be adversely affected by the processing method. Finally, products obtained from these molding materials must also meet high quality requirements.

The invention relates to polyethylene molding materials I I 4 having a viscometrically measured average molecular weight of at least 106 g/mol. They contain 0.05 to by weight, based on the molding material, of a salt of a higher monocarboxylic acid or of a mixture of such salts, the salt or the salt mixture melts at between 80 and 220°C and the melt viscosity of the salt or of the salt mixture is not more than 50 mPa.s at the processing temperature of the molding materials.

According to the invention, all types of ultrahigh molecular weight polyethylenes may be used as a basis of the novel molding materials, regardless of their method of preparation. Accordingly, it. is possible to use polymers which were obtained by the Ziegler process by polymerization of ethylene in the presence of metals of group IV B VI B of the Periodic Table of Elements (IUPAC version) together with organometallic compounds of the elements of groups I A III A of the Periodic Table of Elements (IUPAC version). However, it is also possible to use ultrahigh molecular weight polyethylenes 20 which have been prepared from anhydrous and oxygen-free ethylene in the gas phase in the presence of supported catalysts containing metal alkyl and chromium oxide.

In addition to a large number of other substances, such as paraffins, fatty alcohols, fatty acids, fatty esters 25 and amides, salts of higher monocarboxylic acids (also referred to as metal soaps) are used as lubricants, inter alia in the processing of thermoplastics. Their purpose in this case is to promote the melting process, reduce the internal friction in the thermoplastic melt and thus facilitate the flow of the melt. In addition, they increase the output during extrusion and injection molding and prevent adhesion of the melt to the hot surfaces of the machine parts.

Surprisingly, the addition of very particular lubricants selected from a large number of different compounds prevents the formation of transverse cracks in the ram

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5 extrusion of PE-UHMW. It should be taken into account here that, in contrast to other thermoplastics, PE-UHMW does not form a melt but is converted into a viscoelastic state without thermal degradation on heating. Even in such a low concentration, the metal salts ensure that they do not adversely affect the excellent mechanical and thermal properties of the material and of the moldings produced from it.

The salts of higher monocarboxylic acids, which salts are contained in the molding materials according to the invention, are derived from straight-chain or branched aliphatic or aromatic monocarboxylic acids having 10 to 24, in particular 12 to 20, carbon atoms in the molecule.

They contain, as cations, metallic elements of groups I A to III A and I B to III B of the Periodic Table of Elements (IUPAC version) in the form of monovalent or polyvalent ions. Soaps of the metals calcium, magnesium and zinc are preferred. The soaps may also contain two or more different metals or be basic salts. Mixtures of 20 different metal soaps may also be used.

The melt behavior of the salts of the carboxylic acids is critical with regard to their suitability for improving the processability of ultrahigh molecular weight polyethylene by ram extrusion. The salts contained in the S 25 molding materials according to the invention melt at between 80 and 220°C, preferably between 100 and 200 0

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in particular between 130 and 190°C. Their melt viscosity at the processing temperature is not more than mPa.s, and melt viscosities of not more than 10 mPa.s, in particular not more than 1 mPa.s, are preferred. The processing temperature is understood as meaning the temperature at which the molding material is extruded in the ram extruder. The melt viscosity is determined in a known manner by viscometry in a rotational viscometer (cf. Kulicke, Flieiverhalten von Stoffen und Stoffgemischen [Flow behavior of substances and mixtures of substances], Hitig Wetz, Basel, Heidelberg, New York

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Bp- Al-ru I- 6 1986).

For the preparation of the molding materials of the invention, the metal soaps are added to the polymer in concentrations of 0.005 to 5.0, in particular 0.01 to 2.0, by weight, based on the molding materials. In order to ensure a uniform distribution of the salts in the polyethylene, the compc-oents are expediently mixed as a powder in conventional :mixers or homogenizers. If necessary, further additives, such as light stabilizers, antioxidants or dyes, may also be added. The homogeneous mixture may be further used directly or after agglomeration, for example to give finely divided particles.

The examples which follow describe the invention in more detail, but it is not restricted to the embodiments illustrated.

S• Examples: Ultrahigh molecular weight polyethylene with an average molecular mass of 7.3 x 106 to 9.2'106 glmol which contains in each case 0.5% by weight (based on the molding material) of the metal soaps stated in the table below is processed 20 on a horizontal ram extruder having a die length of 1.2 m under a pressure of about 12 MPa at temperatures (starting at the feed hopper) of 305/300/230/195 0 C to give solid rods of 50 mm diameter. The cracking is evaluated visually in the moldings obtained.

Metal soap Melting range Melt viscosity 3) Cracking 1) [oC] 2) [Pa.s] Ca stearate 120-160 >103 very pronounced Mg stearate 115-150 31 pronounced Zn stearate 118-122 0.07 none i I r 1) Product of Fa. Peter Greven, Fettchemie GmbH Co. KG, Bad Munstereifel based on tallow fatty acid of the composition C1/C 18 monocarboxyclic acid in the ratio of 1:2 to 1 3, purity: 94 to 98%.

2) Measured with the Kofler-bank 3) Measured at 200 0 C with the viscosimeter RV 20 of the Fa. Haake o I I I C

Claims (7)

1. A process for the production of moldings from polyethylene by ram extrusion, wherein the polyethylene used is a molding material having a viscometrically measured average molecular weight of at least 106 g/mol, wherein said molding material contains 0.005 to 5.0% by weight, based on the moulding material, of a salt of a higher monocarboxylic acid, said salt being derived from straight-chain or branched aliphatic or aromatic monocarboxylic acids having 10 to 24 carbon atoms in the molecule, or of a mixture of such salts, the salt or the salt mixture melts at between 80 and 2200C, the melt viscosity of the salt mixture at the processing temperature of the molding material is not more than 50 Pa.s and the cations are metallic elements of groups I A to III A and I B to III B of the Periodic Table of Elements in the form of monovalent or polyvalent ions. ••oe• o: o S"2. A process as claimed in claim 1, wherein the higher monocarboxylic acid has 12 to 20 carbon atoms in the molecule.

3. A process as claimed in claim 1 or claim 2, wherein the metallic element is magnesium, calcium or zinc.

4. A process as claimed in any one of claims 1 to 3, wherein the salt or the salt mixture melts at between 100 and 2000C. A process as claimed in any one of claims 1 to 4, wherein the salt or salt mixture melts at between 110 and 1800C.

6. A process as claimed in any one of claims 1 to 5, wherein the viscosity of the salt or of the salt mixture at the processing temperature of the molding material is not more than 10 Pa.s. ri" -LIa 0U -Nc -1 IP1~BI lmr 9

7. A process as claimed in any one of claims 1 to 6 wherein the viscosity of the salt or of the salt mixture at the processing temperature of the molding material is not more than 1 Pa.s.

8. A process as claimed in one or more of claims 1 to 7 wherein thp molding material, contains 0.01 to 2.0% by weight, based on the molding material, of the salt or salt mixture.

9. A polyethylene molding prepared according to the process of any one of claims 1 to 8. DATED this 25th day of November, 1997. HOECHST AKTIENGESELLSCHAFT o r o o o o o WATERMARK PATENT TRADEMARK ATTORNEYS 290 BURWOOD ROAD HAWTHORN VICTORIA 3122 AUSTRALIA KJS:PJM:RM DOC 14 AU8163794.WPC 3 3 j Q M~A i-- -O ~C~nT Frankfurt, 21.12.1993 PAT/rcht-sei Hoe93/Y021 Hoechst Aktiengesellschaft, 65926 Frankfurt am Main Abstract of the Disclosur- Polyethylene molding materials which are particularly suitable for processing by ram extrusion and have an average molecular weight of at least 106 g/mol contain 0.05 to 5.0% by weight, based on the molding material, of a salt of a higher monocarboxylic acid or of a mixture of such salts. The salts or the salt mixtures melt at between 80 and 220 C, and their melt viscosity is not more than 50 mPa.s at the processing temperature of the molding material. ©•ee• V,7. 7 tO r M