1. Field of the Invention
The present invention relates to a semiconductive polymer composition, to a method for preparing said semiconductive polymer composition, to its use for the production of a semiconductive layer of an electric power cable, and to an electric power cable comprising at least one semiconductive layer, which layer comprises the above mentioned semiconductive polymer composition.
2. Description of Related Art
In wire and cable applications a typical cable comprises at least one conductor surrounded by one or more layers of polymeric materials. In power cables, including medium voltage (MV), high voltage (HV) and extra high voltage (EHV), said conductor is surrounded by several layers including an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order. The cables are commonly produced by extruding the layers on a conductor. Such polymeric semiconductive layers are well known and widely used in dielectric power cables rated for voltages greater than 1 kilo Volt. These layers are used to provide layers of intermediate resistivity between the conductor and the insulation, and between the insulation and the ground or neutral potential.
These compositions are usually prepared in granular or pellet form. Polyolefin formulations such as these are disclosed in U.S. Pat. Nos. 4,286,023; 4,612,139; and 5,556,697; and European Patent 420 271.
One or more of said layers of the power cable are typically crosslinked to achieve desired properties to the end product cable. Crosslinking of polymers, i.e. forming primarily interpolymer crosslinks (bridges), is one well known modification method in many end applications of polymers. Crosslinking of polymers, such as polyolefins, substantially contributes i.a. to heat and deformation resistance, creep properties, mechanical strength, as well as to chemical and abrasion resistance of a polymer. In wire and cable applications crosslinked polymers, such as crosslinked polyethylenes, are commonly used as a layer material, e.g. in insulating, semi-conducting and/or jacketing layers.
Crosslinking can be effected i.a. by radical reaction using radiation or free radical generating agents, also called crosslinking agents. Examples of such free radical generating agents are peroxides including inorganic and organic peroxides. Crosslinking using peroxide is known as peroxide technology. A further well known crosslinking method is crosslinking functional groups, e.g. by hydrolysing hydrolysable silane groups, which are linked to polymer, and subsequently condensing the formed silanol groups using a silanol condensation catalyst, for instance carboxylates of metals, such as tin, zinc, iron, lead and cobalt; organic bases; inorganic acids; and organic acids. The crosslinking of polymers via silane groups thereof is known as silane-crosslinking technology, and for hydrolysable silane groups also called as moisture curing technology. Such silane-crosslinking techniques are known e.g. from U.S. Pat. No. 4,413,066, U.S. Pat. No. 4,297,310, U.S. Pat. No. 4,351,876, U.S. Pat. No. 4,397,981, U.S. Pat. No. 4,446,283 and U.S. Pat. No. 4,456,704. These two type of crosslinking methods are referred herein below shortly as “crosslinking via radical reaction” and, respectively, “crosslinking via silane groups”. In case of crosslinkable semiconductive layer materials using crosslinking via radical reaction, said layer composition may also comprise a crosslinking agent, such as peroxide, which is preferably added onto the pellets after producing the polymer pellets as described e.g in WO00038895 of Pirelli.
The purpose of a semiconductive layer is to prolong the service life, i.e. long term viability, of a power cable i.a. by preventing partial discharge at the interface of conductive and dielectric layers. Surface smoothness of the extruded semiconductive layer is also a property that plays also an important role in prolonging the surface life of the cable. The smoothness is influenced i.a. by the used carbon black (CB). I.a. an uneven distribution of the particle size of carbon black particles can adverse said surface smoothness and cause localised electrical stress concentration which is a defect that can initiate a phenomenon well known as vented trees. Moreover, i.a. the surface properties and particle size as such of the CB may affect the surface smoothness of the semiconductive layer of a power cable. E.g. it is known that the larger the CB particles, the smoother the surface of the semiconductive layer. However, increasing the particle size of a CB for improving smoothness in turn deteriorates, i.e increases, the resistivity of the semiconductive layer material, whereby these properties need often be balanced, especially in case of so called furnace carbon black.
Furnace carbon black is generally acknowledged term for the well known CB type that is produced in a furnace-type reactor by pyroliyzing a hydrocarbon feedstock with hot combustion gases. A variety of preparation methods thereof are known and such furnace carbon blacks are described i.a. in EP629222 of Cabot, U.S. Pat. No. 4,391,789, U.S. Pat. No. 3,922,335 and U.S. Pat. No. 3,401,020. Furnace carbon black is distinguished herein from acetylene carbon black which is also generally acknowledged term for the well known type of CB produced by reaction of acetylene and unsaturated hydrocarbons, e.g. as described in U.S. Pat. No. 4,340,577.
As an example of commercial furnace carbon black grades described in ASTM D 1765-98b are i.a. N351, N293 and N550.
Moreover, many carbon blacks, e.g. above mentioned furnace carbon blacks, are commercially available in a form of “pellet” agglomerates formed from primary CB particles thereof. These agglomerates are broken during the processing, i.e. compounding, step of the preparation of said semiconductive polymer composition. The break down of said agglomerates thus may also have an effect on said surface smoothness property. Without binding to any theory it appears that an extensive mixing of semiconductive polymer mixture in order to get an even particle size distribution amongst the CB particles may adverse the resistivity of the composition. Accordingly there seems to be limitations in particle size window for the CB particles to enable sufficient smoothness and resistivity of the final product.
Thus there is a continuous need in prior art to provide new semiconductive polymer compositions with improved smoothness and at the same time maintain feasible balance with other properties.