GB2177704A - Melt processable rubber/polyethylene compositions - Google Patents

Abstract

A melt processable composition comprises 30 to 90% by weight of linear low density polyethylene and 10 to 70% by weight of ground vulcanized rubber in particle form. The rubber has a particle size of less than about 1.5 mm and such particles are coated with an organic peroxide cross-linking agent. The polyethylene has a density of 0.915 to 0.925 g/cm<3>. The composition may be formed into articles using melt processing techniques e.g. injection moulding. The articles may, for example, be mud-flaps for vehicles.

Description

SPECIFICATION Melt processable rubber/polyethylene compositions The present invention relates to melt-processable compositions of rubber and polyethylene and especially to such compositions in which the rubber is ground scrap rubber obtained from, for instance, used automobile tires.

Some motor vehicles, especially trucks and other large vehicles, have rubber mudflaps near the rear of the wheels of the vehicle. These mudflaps are intended to deflect downwards water, mud, sand, stones and the like that are thrown up by the wheels of the vehicle as the vehicle travels along a road, thereby reducing the safety hazards of flying objects and the annoyance to motorists of a spray and water, mud and the like.

Mudflaps are normally made from rubber compositions using compression vulcanization processes.

While such processes have produced commercially-acceptable products, it is believed that other fabrication processes offer the potential of greater versatility, faster cycle times, better economics and/or improved properties.

Mudflaps and similar products may be manufactured from thermoplastic polymers such as polyethylene, polypropylene and polyvinyl chloride by melt-forming processes e.g. injection moulding and sheet extrusion. However, such products tend to lack the appearance and flexibility of rubber that is often expected by consumers.

Mudflaps and similar products may also be manufactured from blends of powdered rubber and polyethylene. The resultant products have the appearance and feel of rubber but have lower resistance to flexural cracking than products made from polyethylene or rubber only.

It has now been found that rubber-like articles of improved physical properties may be manufactured by melt processing compositions of rubber and polyethylene in which the rubber has been coated with an organic peroxide.

Accordingly, the present invention proviedes a composition comprising: (a) 30 to 90% by weight of a linear low density polyethylene, said polyethylene having a density in the range of 0.915 to 0.925 g/cm3, and (b) 10-70% by weight of ground vulcanized rubber, said ground rubber having a particle size of less than about 1.5 mm and being coated with 2000 to 10 000 ppm, based on the weight of rubber, of an organic cross-linking agent.

The present invention also provides a process for forming an article having the appearnace of rubber, said process comprising (i) feeding to melt processing apparatus a composition comprising (a) 30 to 90% by weight of a linear low density polyethylene, said polyethylene having a density of 0.915 to 0.925 g/cm3, and; (b) 10 to 70% by weight of ground vulcanized rubber, said ground rubber having a particle size of less than about 1.5 mm and being coated with 2000 to 10 000 ppm, based on the weight of rubber, of an organic cross-linking agent, (ii) admixing said composition within said apparatus under melt conditions, and (iii) forming the resultant admixed molten composition into an article.

In preferred embodiments of the composition and process of the present invention, the rubber is ground scrap rubber, especially ground scrap rubber obtained from automobile tires.

The composition of the present invention is comprised of polyethylene, rubber and a cross-linking agent. The polyethylene is a linear low density polyethylene having a density in the range of 0.915 to 0.925 g/cm3. Such polyethylene may be a copolymer of ethylene with at least one alpha-olefin homologue of ethylene, especially a C4 - C10 alpha-olefin. Examples of such alpha-olefins are butene-1, hexene1 and octene-1. The molecular weight of the polyethylene may be varied over a wide range, depending in particular on the intended end-use of articles fabricated from the composition and the proportions of polyethylene, rubber and cross-linking agent in the composition.

The rubber of the composition is a ground vulcanized rubber. The rubber should be ground to a particle size that will facilitate adequate mixing of the polyethylene and rubber during procesing of the composition. Thus the particle sizes that may be used will depend for instance on the mixing capabilities of the melt processing apparatus, e.g. injection moulding apparatus or extrusion apparatus. The intended end-use of articles formed from the composition may also be a factor because the homogeneity of the composition as formed into an article may affect the properties of that article. The particle size of the rubber should be less than 1.5 mm, especially less than 1.0 mm and in particular less than 0.5 mm.

In a preferred embodiment, the ground vulcanized rubber is obtained from automobile tires or the like, especially scrap automobile tires. Such tires may be ground to a suitable particle size for the compositions of the present invention. Techniques for grinding rubber are known in the art.

The compositions of the present invention also contain an organic cross-linking agent, especially an organic peroxide cross-linking agent. The cross-linking agent is coated onto the particles of the ground rubber prior to the admixing of rubber and polyethylene. It is preferred that the coating of the particles be carried out in a uniform manner, to improve the uniformity of the resultant product. For instance, the coating may be applied by using a solution of cross-linking agent and inert solvent by admixing the solution and rubber particles and subsequently removing the solvent. In a preferred method, the coating is applied using a Henschel* mixer.

The amount of the cross-linking agent may be about 2000 to 10 000 ppm, especially 2500 to 6000 ppm.

The preferred cross-linking agents are organic peroxide cross-linking agents, especially a bis(tert. alkyl peroxy alkyl) benzene, dicumyl peroxide or an acetylenic diperoxy compound. For instance, the crosslinking agent may be 2,5-dimethyl-2,5-di(t-butylperoxy) hexyne-3 which is available commercially under the trade mark Lupersol 130 from Pennwalt Corp. of Buffalo, New York, U.S.A. Alternatively, the crosslinking agent may be 2,5-dimethyl-2,5-bis (tert-butyl peroxyisopropyl) benzene which is available commercially under the trade mark Vulcup from Hercules Incorporated. A co-curing agent may also be incorporated into the composition e.g. in association with the cross-linking agent. Examples of co-curing agents include triallyl cyanurate, triallyl isocyanurate and 1,2-polybutadiene.

The compositions of the present invention may also contain stabilizers e.g. antioxidants and/or ultra violet stabilizers, pigments, fillers and the like, as is known for rubber compositions.

The compositions contain 30 to 90% by weight of the linear low density polyethylene and 10 to 70% by weight of the rubber. In preferred embodiments the compositions contain 40 to 60% by weight of polyethylene and 40 to 60% by weight of rubber. However, it is to be understood that the relative amounts of polyethylene and rubber, and the type of polyethylene, will depend in particular on the properties required in the articles fabricated from the compositions.

The compositions of the present invention are intended to be fabricated into articles using an injection moulding process, especially for the manufacture of articles that have the appearance of being fabricated from rubber.

* denotes trade mark Such articles include mudflaps and other protective devices for use on motor vehicles, especially trucks and other large vehicles.

In an injection moulding process, the compositions are admixed under melt conditions. Although the compositions could be so admixed prior to being fed to the injection moulding apparatus, it is preferred that the admixing occur in the injection mounding apparatus immediately prior to injection of the admixed composition into the mould of the apparatus. The amount of admixing should be sufficient to provide a degree of homogeneity in the moulded article subsequently obtained that is commensurate with the intended end-use of the article; homogeneity is one factor that is pertinent to the properties of the moulded article.Thus, apart from the mixing characteristics of the injection moulding apparatus, matters such as the particle size of the components of the composition, the relative particle sizes between different components and the uniformity of the particle sizes e.g. particle size distribution, of the components may be important with respect to the properties of the articles that are obtained. Such factors will be understood by those skilled in the art.

The compositions of the present invention are also intended for use in melt processes other than injection moulding. For instance, the compositions may be fed to extrusion apparatus, especially extrusion apparatus for the manufacture of sheet products. Mixing characteristics of the extrusion apparatus, particle sizes of the components of the composition, relative particle sizes between different components and the uniformity of the particle sizes, e.g. particle size distribution, may be important with respect to the properties of the articles that are obtained.

Articles fabricated from the compositions of the present invention may be used in a variety of end uses. In particular, the articles may be in the form of mudflaps for vehicles.

The present invention is illustrated by the following examples.

Example I A number of compositions of the present invention and comparative compositions were prepared. The polyethylene was in the form of pellets. The rubber was obtained from scrap automobile tires and had been ground to a particle size of 1.0 mm. Organic peroxide, if present, had been coated onto the rubber particles, prior to admixing of rubber and polyethylene, in a Henschel mixer.

To test the properties of the compositions, so-called "tensile bars" were prepared. These tensile bars had a length of 130 mm, a width of 13 mm and a thickness of 3mm, and were moulded on an Engel* injection moulding machine from a dry blend of polyethylene pellets and rubber powder. In preparing the sample tensile bars, the melt temperature used was 220"C, which was selected so as to activate any organic peroxide present in the composition being moulded while maintaining decomposition of the rubber at an acceptable level. The screw speed and back pressure on the injection moulding apparatus were both set at the maximum for the apparatus to maximise the degree of polymer/rubber homogeneity.

Mould closure time was kept to a minimum to reduce any tendency for the rubber to decompose.

The injection moulded tensile bars were subjected to the following tests: (a) Flex test ... The bars were flexed to 90 degrees on each side of the axis of the bars, at ambient temperature and at a rate of 42 cycles/minute. The bars were considered to have failed when a crack of more than 3mm in length had appeared at the point of flexure of the bars.

(b) Notched Flex test ... The procedure described above for the flex test was repeated except that a slit having a depth of 3 mm was cut into one side of the bars at the point of flexure. The bars were considered to have failed when this cut had grown by 3 mm in length.

(c) Tensile Impact test ... Type L tensile impact bars were cut from the injection moulded bars, according to the procedure of ASTM D-1822-68. The tensile impact test was carried out at -40 C.

The results obtained are given in Table I.

* denotes trade mark Table I Run No. 1 2 3 4 5 6 7 Polyethylene* 100 50 50 50 50 50 50 (parts by weight) Rubber - 50 50 50 50 50 50 (parts by weight) Organic Peroxide** - - 500 800 6000 3000 3000 (ppm) Flex Test 3200 1000 750 1450 3400 3175 200 (cycles to failure) Notched Flex Test 900 300 - - 900 900 (cycles to failure) Tensile Impact test 14 20 NA NA 17 25 (Joules/cm2) *SCLAIR 2107 ethylene/butene copolymer, density 0.924 g/cm3, melt index 5.1 dg/min.

** Lupersol 130 organic peroxide.

NA = not available Note: Runs 1-4 are comparative examples.

In Run 7, the polyethylene and rubber were poorly mixed.

Claims (7)

1. A composition comprising: (a) 30 to 90% by weight of a linear low density polyethylene, said polyethylene having a density in the range of 0.915 to 0.925 g/cm3, and (b) 10 to 70% by weight of ground vulcanized rubber, said ground rubber having a particle size of less than about 1.5 mm and being coated with 2000 to 10 000 ppm, based on the weight of rubber, of an organic cross-linking agent.

2. The composition of Claim 1 in which the cross-linking agent is an organic peroxide.

3. The composition of Claim 2 in which the rubber is coated with 2500 to 6000 ppm of an organic cross-linking agent.

4. The composition of Claim 3 in which the particle size of the ground rubber is less than 1.0 mm.

5. The composition of Claim 3 in which the organic peroxide cross-linking agent is selected from the group consisting of a bis(tert. alkyl peroxyalkyl) benzene, dicumyl peroxide and an acetylenic diperoxy compound.

6. The composition of Claim 5 in which the organic peroxide is 2,5-dimethyl-2,5-di-(tert-butyl peroxy) hexyne-3 or 2,5-dimethyl-2,5 bis (tert-butyl peroxyisopropyl) benzene.

7. A process for forming an article having the appearance of rubber, said process comprising: (i) feeding to melt processing apparatus a composition comprising (a) 30 to 90% by weight of a linear low density polyethylene, said polyethylene having a density of 0.915 to 0.925 g/cm3, and (b) 10 to 70% by weight of ground vulcanized rubber, said ground rubber being in the form of particles having a size of less than about 1.5 mm and coated with 2000 to 10,000 ppm, based on the weight of rubber, of an organic cross-linking agent, (ii) admixing said composition within said apparatus under melt conditions, and (iii) forming the resultant admixed composition into an article.

7. A process for forming an article having the appearance of rubber, said process comprising: (i) feeding to melt processing apparatus a composition comprising (a) 30 to 90% by weight of a linear low density polyethylene, said polyethylene having a density of less than 0.915 g/cm3, and (b) 10 to 70% by weight of ground vulcanized rubber, said ground rubber having a particle size of less than about 1.5 mm and being coated with 2000 to 10 000 ppm, based on the weight of rubber, of an organic cross-linking agent, (ii) admixing said composition within said apparatus under melt conditions, and (iii) forming the resultant admixed composition into an article.

8. The process of Claim 7 in which the cross-linking agent is an organic peroxide.

9. The composition of Claim 8 in which the rubber is coated with 2500 to 6000 ppm of an organic cross-linking agent.

10. The process of Claim 9 in which the particle size of the ground rubber is less than 1.0 mm.

11. The process of Claim 9 in which the organic peroxide cross-linking agent is selected from the group consisting of a bis(tert. alkyl peroxyalkyl) benzene, dicumyl peroxide and an acetylenic diperoxy compound.

12. The process of Claim 11 in which the organic peroxide is 2,5-dimethyl-2,5-di-(tert-butyl peroxy) hexyne-3 or 2,5-dimethyl-2,5-bis(tert-butyl peroxyisopropyl) benzene.

13. The process of Claim 7 in the form of an injection moulding process.

14. The process of Claim 7 in the form of a sheet extrusion process.

Amendments to the claims have been filed, and have the following effect: (a) Claims 1 and 7 above have been deleted or textually amended.

(b) New or textually amended claims have been filed as follows: (c) Claims 2, 3, 4, 5, 8, 9 & 10 above have been re-numbered as 4, 2, 3, 5, 10, 8, 9 and their appendancies corrected.

1. A composition comprising: (a) 30 to 90% by weight of a linear low density polyethylene, said polyethylene having a density in the range of 0.915 to 0.925 g/cm3, and (b) 10 to 70% by weight of ground vulcanized rubber, said ground rubber being in the form of particles having a size of less than about 1.5 mm and coated with 2000 to 10,000 ppm, based on the weight of rubber, of an organic cross-linking agent.