GB2297092A

GB2297092A - Low compression set thermoplastic elastomers

Info

Publication number: GB2297092A
Application number: GB9500439A
Authority: GB
Inventors: Antonio Fernando Pileci
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-01-10
Filing date: 1995-01-10
Publication date: 1996-07-24
Also published as: GB9500439D0

Abstract

Thermoplastic elastomers with a compression set of less than 25% were made by melt-blending a mixture comprising 100 parts by weight of PVC, with from 5 to 95 parts by weight of a nitrile rubber mixture of a particulate cross-linked nitrile rubber and an unvulcanised nitrile rubber, with from 20 to 200 parts be weight of a PVC plasticizer and a rubber curing agent.

Description

LOW COMPRESSION SET THERMOPLASTIC ELASTOMHZS A composition comprising a fully cured rubber in fine particulate form dispersed uniformly throughout a chemically compatible thermoplastic is known as a the plastic elastomer (TPE).

Thermoplastic elastomers combine the desirable properties of a vulcanized rubber, including, resilience, heat-resistance, inpact strength and low compression set, with the melt-processing ease of thermoplastics. These products may be typified by Santoprene (manufactured by Monsanto Ltd.), a material which is constituted of a blend of polypropylene and ethylene-propylene-diene monomer rubber (EPDM), the latter being cured and dispersed as fine particles throughout the thermoplastic phase (polypropylene) during the blending operation. Further details are given by N.R.Legge, et al.

in "Thermoplastic Elastorners" Hanser Publishers, 1987.

In the case of thenrroplastic elastomers based on PVC and nitrile rubber, the cured rubber particles are pre-cross linked and these are subsequently dispersed in plasticized PVC. This family of TPE's has particularly good resistance to ozone and weathering and to oil and chemicals. For these reasons and because of their comparatively low cost they find use as alternatives to EP3M and nitrile rubbers in automobile applications and in the glazing seal industry.

However, these uses have been limited to areas where values for the compression set of the extruded artefacts are not required to be less than 50%, as measured after 22 hours compression at 70 C, according to ASTM D 395B. The compression set can be lowered to a value of 40-45% by increasing the nitrile rubber content of the TPE.

This technique has drawbacks, however, in that apart from increasing the cost it leads to a deterioration in important mechanical properties, notably the tensile strength and tear resistance.

A procedure has recently been described in EP 0353525A1 for reducing the compression set of PVC/nitrile rubber TPE's to a value claimed to be about 40% without adversely affecting the mechanical strength and maintaining the thermoplastic melt-extrusion characteristics. These improved properties were claimed to be achieved by adding a conventional rubber vulcanizing agent, including a sulphur based system, to a composition comprising PVC, partially vulcanized (or unvulcanized) nitrile rubber and a PVC plasticizer, and allowing the mixture to react during the melt mixing and forming process.However, for PVC/nitrile rubber TPE's to be suitable alternatives to vulcanized rubbers for more general use than has hitherto been the case, it is necessary for the compression set not to exceed about 25% - a value substantially lower than the 40% figure achieved ,cloying the methods described in EP 0353525A1.

The present invention entails a procedure whereby PVC/nitrile rubber TPE's can be prepared which have a compression of less than 25% and a tensile strength of at least 9N/rmf and they are readily melt extruded. The improvement in compression set (achieved without detriment to the mechanical or melt-rheological properties) is achieved by employing, not a single class of nitrile rubber, but rather a mixture of an unvulcanized nitrile rubber and cross-linked nitrile rubber for blending with the PVC, a PVC plasticizer and a rubber cross-linking agent. During melt-mixing of these components (and other optional additives in the fomulation) certain chemical reactions occur which lead to the low compression set PVC/nitrile rubber TPE's of the present invention.

In this invention the amount of nitrile rubber dispersed in the PVC is in the range 5 parts to 95 parts by weight per 100 parts by weight of PVC and is preferably in the range 10 parts to 60 parts by weight per 100 parts by weight of PVC. The nitrile rubber is a mixture of a pelletized unvulcanized nitrile rubber and a particulate cross-linked nitrile rubber, the former is present preferably in the range 5 parts to 90 parts by weight per 100 parts by weight of the cross-linked nitrile rubber and more preferably in the range 10 parts to 50 parts by weight per 100 parts by weight of the cross-linked nitrile rubber. The nitrile rubber, both cross-linked and unvulcanized, has an acrylonitrile content which is preferably in the range 27 parts to 45 parts by weight per 100 parts by weight of the rubber.The Mooney viscosity (of the unvulcanized rubber) is preferably in the range 40 to 100 and the cross-linked rubber has a particle size which is preferably in the range 0.2 to 0.7mum. The unvulcanized rubber is preferably in pellet form of size 3mn to 7mn.

The PVC suitable for the purpose of the present invention is either a suspension grade, or an emulsion-prepared grade or a mass-polymerized grade or a mixture of any of these grades. The K-value is preferably in the range 45 to 90 (DIN 53726).

The PVC plasticizer is preferably but not exclusively a monomeric ester, such as dioctyl phthalate or a mixture of such esters and is employed in the range 20 parts to 200 parts by weight per 100 parts by weight of PVC, but preferably in the range 30 parts to 100 parts by weight per 100 parts by weight of PVC.

Suitable rubber curing agents for the purpose of the present invention include organic peroxides and these include: 2,2-ditertiarybutyl peroxybutane; 1,1-ditertiarybutyl l,l-ditertiarybutylperoxy- 3,5,5-trimethylcyclohexane and and 1,1, l-ditertiaryperoxycyc lohexane.

The amount of peroxide used is preferably in the range 0.01 to 5.0 parts by weight per 100 parts by crnbined weight of the PVC and nitrile rubber in the formulation and, more preferably, it is in the range 0.05 parts to 3.5 parts by weight per 100 parts by combined weight of the PVC and nitrile rubber in the formulation, and is such that the PVC/nitrile rubber composition remains thermoplastic during, and after, melt-mixing, and it is essentially all decomposed by the time that melt-mixing is complete.

Although an organic peroxide is the preferred curing agent for the nitrile rubber in the practice of the present invention, a conventional sulphur-accelerator diene rubber curing system such as,for example, those described in U.S.Patent 4350796 may also be used.

In addition to PVC, the PVC plasticizer, nitrile rubber and organic peroxide, the composition may, optionally, contain one or more of the following: powdered cork; filler; a heat stabilizer; a lubricant; a pigment; zinc oxide; stearic acid; epoxidized soya bean oil and a blowing agent.

The powdered cork may be included in the formulation where further reduction in the compression set of the extruded or moulded artefacts is required.

The filler may be carbon black, alumina, chalk, magnesia, silica, clay, titania, etc. The amount of filler should not exceed 170 parts by weight per 100 parts by weight of PVC, otherwise the mechanical properties of the extruded or moulded artefacts will start to deteriorate.

The e amount of PVC heat-stabilizer in the formulation will be determined largely by the end-use requirement of the PVC/nitrile rubber TPE. Suitable stabilizers include those based on lead, such as lead fumarate and tribasic lead sulphate; an organic tin stabilizer such as dibutyl tin dilaurate; a mixture of barium and cadmium compounds such as barium/cadmium phenates.

The lubricant, added to facilitate extrusion and injection moulding of PVC/nitrile rubber TPE's, may be either zinc stearate, stearic acid, polyethylene wax, etc. The amount used will depend upon the composition of the compound being extruded or moulded.

The amount of pigment taken in the formulation will depend on the particular colour requirements. Suitable pigments include titanium dioxide, carbon black and chrome yellow.

The blowing agent may, for example, be azocarbonamide and the amount used will depend on the required bulk density of the extruded or moulded articles.

By way of illustration of the practice of the present invention, in the case where peroxide curing of the nitrile rubber is carried out, the PVC powder, the PVC plasticizer, the cross-linked nitrile rubber powder and the peroxide are thoroughly mixed together in a powder blender for preferably 5 to 15 minutes at a temperature preferably in the range 55 to 105'C. Thereafter, the unvulcanized nitrile rubber pellets are added and, optionally, one or more of the following: a mineral filler; carbon black or other pigment; a lubricant and a PVC heat-stabilizer. Powder mixing is continued for a further 5-10 minutes.The powder is thereafter thoroughly melt-mixed in a 2-roll mill, or other suitable melt-mixing equipment, such as a Banbury mixer or a Buss Ko-Kneader, at a temperature preferably in the range 125-185 C. The period of mixing is numerically equal to at least 7-8 half-lives of the peroxide at the prevailing melt temperature and will generally fall in the range 2-10 minutes. For all intents and purposes the peroxide is then completely decomposed. The thoroughly mixed blend is removed from the 2-roll mill, or other mixer, and cooled and granulated. The granules are subsequently fed into an extruder or injection moulding machine from which emerges PVC/nitrile rubber TPE artefacts of the present invention.

These thermoplastic elastomer artefacts of the present invention have rubber-like elasticity and feel like rubber. They have a brittle temperature of about -60'C, high mechanical strength and elongation at break, and, in particular, a compression set of less than 25% measured after 25% compression for 22 hours at 70'C as described in ASTM D 395B. Thus, these PVC/nitrile rubber TPE's are suitable alternatives to vulcanized rubbers, particularly, but not exclusively, nitrile rubber and EPIM for most if not all applications in the automobile and glazing seal industries.

During the melt-mixing of the polymer composition on the 2-roll mill, or other mixing equipment, the organic peroxide, when employed as the curing agent, decomposes with the generation of free-radicals. It is speculated that these free radicals interact with the nitrile rubber to effect chemical attachment of the unvulcanized rubber to the discrete particles of the cross-linked rubber, thereby forming a network of elastomeric particles which is responsible, in part at least, for the low compression set of the PVC/nitrile rubber thermoplastic elastomers of the present invention. It is conceivable that comparable free radical reactions occur when an accelerated sulphur curing system is used instead of a peroxide.These interpretations and suggested mechanisms are of a speculative nature only and are not intended to constitute any part of the invention.

ExamPle 1 One hundred parts by weight of PVC powder (Vinnol H70DF, K-value 70, manufactured by Wacker-Chemie GmtH), 30 parts of vulcanised nitrile rubber powder (Chernigtin P83, manufactured by Goodyear) and 70 parts of dioctyl phthalate were mixed in a ribbon blender for 8 minutes at 75 C. Next, 10 parts of unvulcanized nitrile rubber pellets (Hycar 1853HM, manufactured by B.F.Goodrich), 0.6 parts of calcium stearate, 1.5 parts of tri -sic lead sulphate, 5 parts of epoxidized soya bean oil and 2 parts of carbon black (Printex P, manufactured by Degussa) were added to the blender and powder blending continued for a further 5 minutes.The resultant powder blend was then thoroughly melt-mixed for about 5 minutes on a 150mm diameter 2-roll mill at 150-C. At this stage 0.2 parts of 1,1ditertiarybutyl peroxy-3,5,5-trimethyl cyclohexane (trigonox 29, manufactured by Akzo Chemie) were added unifonnly to the melt on the mill and mixing continued for a further 8 minutes. The hide was then removed from the mill, cooled to ambient temperature and granulated. The granules were fed into an extruder from which emerged a strip of cross-section 3.Orrrn x 25mm. The mechanical properties of the strip were determined and the results obtained are given in Table 1.

Exartvle 2 One hundred parts by weight of PVC powder (Vinnol H70DF), 40 parts of vulcanized nitrile rubber powder (Chemigum P83), 70 parts of dioctyl phthalate, 0.6 parts of calcium stearate, 1.5 parts of tribasic lead sulphate, 5 parts of epoxidised soya bean oil, 2 parts of carbon black (Printex P) and 0.2 parts of 1,1-ditertiarybutyl peroxy-3,5,5-trimethylcyclohexane (Trigonox 29) were mixed in a ribbon blender for 10 minutes at 75 c. The thoroughly mixed powder blend was then melt mixed on a 15Orrrn diameter 2-roll mill for 8 minutes at 150'C. The hide was removed fran the mill, cooled to ambient temperature and granulated. The granules were fed to an extruder from which emerged a strip of cross-section 3.0mm x 25mm.

The mechanical properties of the strip were determined and the results obtained are given in Table 1.

Example 3 The procedure described in Example 2 was repeated except that the nitrile rubber used was the unvulcanized grade, Hycar 1853HM, and the curing agent was omitted fran the formulation.

Table 1. Mechanical Properties of PVC/nitrile rubber TPE artefacts described in Examples 1, 2 and 3.

Example 1 Example 2 Example 3 Compression set after 25% compression for 22 hours &commat; 23.7 38 74 70 C - ASIM D 395B (%) Tear strength - ASTM D 624 30 28 26 (N/mm) Kardness - ASTM D 2240 63 65 64 (Shore A degrees) Elongation at break - ASTM 485 476 375 D 412A (%) Ultimate tensile strength 10.9 10.7 9.0 - ASTM D 412A (N/rmf)

Claims

What we claim is:1 A method of producing plastic artefacts having a permanent compression set of less than 40% (ASTM D395-89, Method B), a hardness of less than 75 Shore A and meeting the requirements for glazing seals (British Standard 7412, 1991), comprising steps of: 1(a) powder mixing a nitrile rubber blend with PVC and an ester for plasticizing the PVC, together with a reagent which generates free radicals on heating; 1(b) one or more of the following optionally incorporated in the powder mix according to claim 1(a): an ethylenically unsaturated compound, carbon black, an external lubricant, a rubber antioxidant, a PVC heat stabilizer, clay, silica, chalk, titania and pastel coloured pigments.

1(c) melt-mixing the blend according to claims 1(a) and 1(b) for sufficient time to achieve thorough mixing of the constituents and to decompose all or most of the reagent which generates free radicals on heating; 1(d) extruding or moulding the melt-mixed blend according to claims 1(a) to 1(c) to give artefacts which may be used as glazing seals.

2 A composition as claimed in claim 1(a) wherein the nitrile rubber blend is a mixture of a pre-cross linked nitrile rubber and an unvulcanized nitrile rubber having a Mooney viscosity in the range 20 to 120, both rubbers being co polymers of butadiene and acrylonitrile and having an acrylonitrile content of 10 parts to 60 parts by weight per

100 parts of nitrile rubber.

3 A composition wherein there are 5 parts to 90 parts by weight of the nitrile rubber blend per 100 parts of PVC in the composition as claimed in Claims 1(a) to 1(d) and 2.

4 A composition as claimed in claims 1(a) to 1(d), 2 and 3 wherein the PVC has a K-value in the range 55 to 90 (DIN 53726).

5 A composition according to claims 1(a) to 1(d) wherein the PVC plasticizer is an organic ester having a molecular weight of more than 275 and which does not contain any ethylenically unsaturated groups in the molecule.

6 A composition according to claims 1(a) to 1(d) wherein the PVC plasticizer is a mixture of organic esters, each having a molecular weight of more than 275 and none of which contain ethylenically unsaturated groups in the molecules.

7 A composition according to claims 1(b) to 1(d) wherein the ethylenically unsaturated compound is an organic compound which contains two or more ethylenically unsaturated groups per molecule.

8 A composition according to claims 1(b) to 1(d) wherein the ethylenically unsaturated compound has a molecular weight in the range 120 to 450 and a boiling point greater than 150"C at 76one pressure.

9 A composition according to claims 1(b) to 1(d) wherein there are 0.5 parts to 50 parts by weight of the ethylenically unsaturated compound per 100 parts by weight of PVC in the formulation.

10 A composition according to claims 1(a) to 1(d) wherein the reagent which generates free radicals on heating is an organic peroxide having a half-life, measured in chlorobenzene, of more than 7 minutes at 80"C and less than

2 seconds at 240 C.

11 A composition according to claims 1(a) to 1(d) wherein the reagent which generates free radicals on heating is an azo compound having a half-life, measured in chlorobenzene, of

60 seconds at a temperature in the range 125it to 145-C.

12 A composition according to claims 1(a) to 1(d) wherein the reagent which generates free radicals on heating is an inorganic persulphate.

13 A composition according to claims 1(a) to 1(d), 10, 11 and

12 wherein there are 0.02 parts to 10 parts of the reagent which generates free radicals on heating per 100 parts of PVC in the formulation.

14 A procedure whereby a composition according to the previous claims is first powder mixed and thereafter melt-mixed at a temperature in the range 95 C to 205"C so as to thoroughly mix and at the same time to generate free radicals in situ for chemical reaction with and between constituents of the formulation.

15 A procedure whereby compositions according to claims 1 to 13 which have been melt blended, and then chemically reacted with free radicals 'in situ' according to claim 14 are converted to thermoplastic elastomers having a compression set of less than 40% (ASm D395-89, Method B), a hardness of less than 75" Shore A and at the same time meeting the requirements for the United Kingdam glazing seals industry (British Standard 7412, 1991).

16 Hollow profiles produced by extrusion from the thermoplastic elastomers according to claim 15.

17 Solid profiles produced by extrusion from the thermoplastic elastomers according to claim 15.

18 Artefacts produced by injection moulding or compression moulding from the thermoplastic elastomers according to claim 15.