Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
49
MELT PROCESSABLE
RUBBER,/POLYETHYLENE COMPOSITIONS
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The present invention relates ~o 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
defl~ct 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 of
water, mud and the like.
Mud1aps 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, bet~er
economics and/or improved properties.
Mudflaps and similar products may be manufactured
from thermoplastic polymers such as polyethylene, poly-
propylene 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
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processing compositions of rubber and polyethylene in which
the rubber has been coated with an organic peroxide.
Accordingly, the present invention provides 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 o ground vulcanized rubber,
said ground rukber 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 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 o~
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 and is a
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copolymer of ethylene with at least one alpha-olefin
homologue of ethylene, especially a C4 - Clo alpha-olefin.
Examples of such alpha-olefins are butene-l, hexene-l and
octene-l. 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 processing 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. The rubber should be
classified so that all, or at least essentially all, of the
rubber has a particle size of less than 1.5 mm; larger
particles tend to have detrimental effects on properties of
the resultant products.
In a preferred embodiment, the ground vulcanized
rubber is obtained from autombile 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
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admixing of rubber and polyethylene; addition of peroxide
into the composition in the form of a concentrate or directly
compounding peroxide into the polyethylene does not result in
significant improvements in flex life of the product and/or
results in processing difficulties e.g. due to cross-linking
of the polymer. 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 t the
coating may be applied 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 o the cross-linking agent may be about
2000 to lO 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 and/or an acetylenic
diperoxy compound. For instance, the cross-linking 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 cross-linking 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
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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.
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 injec~ion moulding 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 ~he
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 apparatusl especially extrusion apparatus for the
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manufacture of sheet products. Mixing characteristics o~ 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 mudElaps 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 lo O 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 ~ngel* injection moulding
machine from a dry blend of polyethylene pellets and rubber
powder. In preparing the sample tensile bars, the melt
temperature used was 220C, 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:
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(a) Flex test O~ 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 Impac~ test ... Type L tensile impa_t
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 -40C.
The results obtained are given in Table I.
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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 60003300 3000
(ppm)
Flex Test 3200 1000 750 1450 34003175 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/cm3l melt index 5.1 dg/min.
** Lupersol 130 organic peroxide.
N~ = not available
Note: Runs 1-4 are comparative examples.
In Run 7, the polyethylene and rubber were poorly
mixed.
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