Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-10-25
2002-04-23
Sanders, Kriellion A. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
active
06376595
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a master batch for the production of crosslinked polymer pipes, a crosslinkable polymer composition for the production of crosslinked polymer pipes, and a crosslinked polymer pipe.
TECHNICAL BACKGROUND
It is well known to make pipes of polymer materials such as polyethylene plastic or polypropylene plastic. It is also known to enhance the properties of polymer pipes by utilizing crosslinked polymer material such as e.g. crosslinked polyethylene plastic. Several properties such as e.g. the crack resistance are improved by using a crosslinked polymer material. Crosslinked polymer pipes are used inter alia for the distribution of water such as drinking water; for the distribution of gas, e.g. in connection with off-shore applications; in district heating systems; etc. The design stress of a crosslinked polymer pipe which is the circumferential stress the pipe is designed to withstand for 50 years without failure is determined for different temperatures in terms of the Minimum Required Strength (MRS) according to ISO/TR 9080. Thus, today crosslinked polyethylene pipes achieve an MRS 80 or 8.0 MPa at 20° C.
It would be a substantial technical progress if the design stress of crosslinked polymer pipes, more particularly crosslinked polyethylene pipes could be improved. Practically this would mean that for a given stress the pipe could be made thinner, i.e. less material could be used for making the pipe, or alternatively for a given thickness of the pipe it could withstand higher stress. As an example, pipes for transporting gas in the off-shore industry may be made of crosslinked polyethylene clad on the outside with steel. If the design stress of the crosslinked polyethylene used for such pipes could be increased, the outer steel cladding could be made thinner and as a result the pipe would be less expensive.
THE INVENTION
According to the present invention it has now been discovered that a substantial improvement in design stress can be achieved for crosslinked black polymer pipes, i.e. pipes having carbon black as the pigment, if the master batch for introducing the pigment into the base polymer is a Ziegler-Natta catalyzed high density polyethylene with a density of at least 0.940 g/cm
3
.
The present invention thus provides a master batch for the production of crosslinked polymer pipes which master batch comprises carbon black and a polymer carrier, characterised in that the polymer carrier is a Ziegler-Natta catalysed polyethylene with a density of at least 0.940 g/cm
3
.
Further, the present invention provides a crosslinkable polymer composition for the production of crosslinked polymer pipes which comprises a polyethylene base polymer compounded with a master batch which comprises carbon black and a polymer carrier, characterised in that the polyethylene base polymer is a chromium catalysed high density polyethylene, and that the polymer carrier of the master batch is a Ziegler-Natta catalysed polyethylene with a density of at least 0.940 g/cm
3
.
Still further, the present invention provides a crosslinked polyethylene pipe, characterised in that it has been made by crosslinking a pipe of the crosslinkable polymer composition defined above with a peroxide crosslinking agent.
These and other advantages and characterising features of the present invention will appear from the following specification and the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
By the expression “polyethylene” is meant, generally and in connection with the present invention, a homopolymer of ethylene or a copolymer of ethylene, wherein the ethylene monomer constitutes the major part of the mass. Thus, the polyethylene may include from 0 to about 25% by weight, preferably about 1-15% by weight of one or more comonomers which are copolymerisable with ethylene. Such comonomers are well known to those skilled in the art and no extensive enumeration will be required, but as examples, mention may can made of vinylically unsaturated monomers, such as C
3
-C
8
alpha olefins, for instance propene and butene.
The polymer pipe of the present invention is made of a crosslinkable polymer composition which comprises a polyethylene base polymer and a master batch including carbon black in a specific polymer carrier.
Master batches are used for incorporating additives such as pigments into polymer compositions. The pigment should be dispersed homogenously in the master batch in order to achieve a homogenous dispersion of the pigment in the base polymer when compounding the base polymer with the master batch. Since pigments are more easily dispersed in low density polyethylene, i.e. polyethylene produced by radical initiated high pressure polymerisation, low density polyethylene is normally used as the polymer carrier of the master batch.
In the present invention it has been discovered, however, that a polymer having surprisingly superior characteristics with regard to the design stress value may be obtained by using a pigment master batch which in addition to carbon black as pigment has a Ziegler-Natta catalysed high density polyethylene as the polymer carrier of the master batch instead of low density polyethylene. The density of this Ziegler-Natta catalysed hig density polyethylene is at least 0.940 g/cm
3
, preferably 0.940-0.970 g/cm
3
, and more preferably 0.955-0.965 g/cm
3
.
Ziegler-Natta catalysed high density polyethylene, i.e. polyethylene produced by the use of a coordination catalyst of the Ziegler-Natta type as disclosed in Encyclopedia of Polymer Science and Engineering, 2nd Ed., Vol. 17 (1989), p. 1027-1028, is well known. Apart from the polymer carrier being a Ziegler-Natta catalysed polyethylene having a density as defined above, there are no other particularly critical features regarding the polymer carrier of the master batch of the present invention.
Besides the polymer carrier the master batch of the present invention comprises carbon black as pigment for the polymer composition. The type of carbon black is not particularly critical at the present invention and different types of carbon black such as furnace black, channel black, etc. may be used. The amount of carbon black included in the master batch depends on the amount of carbon black which it is desired to incorporate into the polymer composition and also on the amount of carbon black possible to include in the master batch. Generally, the amount of carbon black in the master batch lies in the range 30 to 60% by weight, preferably 40 to 50% by weight of the master batch in order to obtain 1 to 5% by weight, preferably 2 to 2.5% by weight of carbon black in the polymer composition.
In addition to the specific master batch described above, the crosslinkable polymer composition of the present invention comprise a polyethylene base polymer.
Besides polyethylene the base polymer includes up to about 5% by weight, preferably about 0.3-5% by weight, and more preferably about 0.3-4% by weight of additives. These additives include stabilising additives such as antioxidants to counteract decomposition due to oxidation, radiation, etc.
As mentioned earlier, the base polymer of the present invention is a chromium catalysed high density polyethylene, i.e. a chromium-containing catalyst is used at the polymerisation of the base polymer. Such cromium catalysts are previously known and therefore no detailed description is called for here. For more detailed information, reference is made to M. P. McDaniel, Advances in Catalysis, Vol. 33 (1985), pp 47-98 and M. P. McDaniel, Ind. Eng. Chem. Res., Vol. 27 (1988), pp 1559-1569. Normally, the catalyst is supported by a carrier, preferably silica. The so-called Phillips catalyst, which is based on chromium trioxide on a silica carrier, is a chromium. catalyst suitably used in the invention. The Phillips catalyst is generally produced by activating silica together with a so-called masterbatch of chromium trioxide or chromic acetate. When chromic acetate is used, it is oxidised to chromium trioxide, so that the end product is the same no matter whether
Borealis Technology OY
Merchant & Gould P.C.
Sanders Kriellion A.
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