The present invention relates to a wire coating composition. The term
"wire" used herein embraces both wires and cables.
It is conventional to provide a polymeric insulation coating around
wires and cables. Such coatings are often made from halogenated polymers, such as
polyvinyl chloride (PVC). The main problem of PVC and other halogenated polymers
is that upon combustion a large volume of toxic, acidic and highly corrosive hydrogen
halide smoke is liberated. As a consequence halogen-free polymer compositions for
coating cables or wires have been developed. Examples of such compositions are included
in EP 082407A, EP 488381A and US 5032321. These and other prior art halogen-free
coating compositions, which are based upon thermoplastic polymer/mineral filler
mixtures, possess inadequate ageing properties, electrical insulation properties
and temperature stability. These coating compositions have a maximum temperature
range of only 70-80°C. These compositions also have poor ageing properties. Furthermore,
the extrusion speed for compositions of the type disclosed in EP 082407A, EP 488381A
and US 5032321 may only be one fifth of that for PVC. Similar compositions containing
intumescent system flame retardants, such as ammonium polyphosphate, have also been
shown to exhibit inadequate electrical resistance due to the fact that the flame
retardant attracts moisture and therefore increases the electrical conductivity
of the material. It is noted that conventional phosphorous-based flame retardants
are present in relatively large amounts, usually up to 40 php for ammonium polyphosphate.
A disadvantage with red phosphorous-based systems is that the cable
coatings have a strong red colouring, which is disadvantageous for electrical cable
coatings in general.
The aim of the invention is to provide a recyclable and/or reprocessable
halogen-free coating composition with improved ageing resistance, electrical insulation
and temperature resistance properties without a deterioration in other physical
properties, such as tensile strength or flexibility. Such a cable coating would
be ideal for use in lighting cable coatings for ships.
According to a first aspect of the present invention there is provided
a wire as coated with a coating composition, said coating composition comprising
polypropylene polymer or copolymer and magnesium hydroxide provided with a hydrophobic
coating, wherein the coating composition comprises from 100 to 200 parts per hundred
parts of polypropylene of magnesium hydroxide and wherein the wire coating composition
does not contain any halogen and is essentially free of phosphorous and phosphorous
compounds and phosphorous salts, and is furthermore free of polyethylene wax.
According to a second aspect of the present invention there is provided
the use of a coating composition in coating a wire, wherein the coating composition
comprises polypropylene polymer or copolymer and magnesium hydroxide provided with
a hydrophobic coating, wherein the coating composition comprises from 100 to 200
parts per hundred parts of polypropylene of magnesium hydroxide, and wherein the
wire coating composition does not contain any halogen and is essentially free of
phosphorous and phosphorous compounds and phosphorous salts, and is furthermore
free of polyethylene wax.
The term "essentially" is used herein to mean that the composition
is free from "phosphorous" or compounds or salts thereof or has a phosphorous content
of less than 800 ppm.
The wire coating composition of the present invention exhibits the
required coating charactaristics, but unlike some PVC compounds is readily recyclable.
The coating composition of the invention further provides the required electrical
insulation, while being flexible, flame resistant, heat stable to greater than 125°C,
abrasion resistant, readily extrudable and recyclable.
The components of the composition are ideally present in the following
100 php (parts per hundred polypropylene); and
magnesium hydroxide (coated)
substantially 140 php.
The coating compositions are particularly useful for cables in ships,
such wires being subjected to high temperatures and contact with sea water and fluids
such as fuel oil typically found in ships. It is necessary for wire coating materials
to have a temperature rating of greater than 100°C, at which temperature the coating
should remain stable and retain electrical insulation properties. The coating compositions
according to the invention possess a temperature rating of greater than 125°C. The
temperature rating is the maximum temperature at which a given insulation or jacket
may be safely maintained during continuous use without incurring any thermally-induced
In order to satisfy the ageing requirements of cables for use in ships
the cable coating must meet the demands of IEC 92-351, i.e. they must have a retention
of at least 70% in tensile strength and elongation at break following 7 days storage
at 135 ± 2°C.
The magnesium hydroxide acts as a filler and flame retardant. In order
to overcome the problems of moisture attraction associated with intumescent flame
retardant systems, the magnesium hydroxide particles are coated with a hydrophobic
material such as an alkyl silane, for example a trimethoxysiiane or triethoxysilane.
The coating enhances adhesion between the filler particles and the polymer matrix,
improves the abrasion resistance of the coating and most importantly, due to its
hydrophobic nature, increases the resistance of the coating to moisture entrapment
thereby maintaining electrical insulation properties. Magnesium hydroxide is also
stable at high temperature, whilst its low surface area gives it a low viscosity.
It is therefore easy to process.
The preferred amount of 140 php magnesium hydroxide content gives
the ideal balance between flame retardance and flexibility. A high filler content
gives excellent flame retardance. However, at the same time the coating will become
much less flexible such that the coated wire will fail a standard conductance test
whereby a wire is tightly coiled (typically around lmm diameter mandrel) and stored
in hot water (normally 80°C) whilst under high voltage. If the coating is not flexible
enough then it will crack at the coil heads.
A further advantage of the compositions according to the invention
is that they can be extruded onto wire or cable using existing extrusion equipment
at speeds similar to that of PVC coatings.
A cost saving may be made by using the coating compositions of the
invention using the high temperature rating of the coatings. Normally a copper wire
becomes hot due to the conduction of the electrical current. The high temperature
rating of the coating means that the thin layers of coating can be used on thin
wires, representing a saving of expensive copper.
A preferred embodiment of the invention involves the incorporation
into the composition of a synergistic blend of an anti-oxidant derived from phenol
and anti-oxidant based upon a phosphite. Suitable anti-oxidants are those marketed
by Ciba-Geigy under the trade marks IRGANOX and IRGAFOS. Such anti-oxidants are
present in an amount of only 0.1 to 5 php. Such phosphite contents are sufficiently
low to have a negligible moisture attraction capability. In such compositions good
processability is retained without adversely effecting the other physical properties
of the coating. This aids further reprocessing of the coatings and therefore improves
the recyclability of the compositions.
The polymeric composition of the invention may optionally further
comprise additives such as stabilisers, plasticisers, lubricants and further flame
In order that the invention may be more readily understood a specific
embodiment of the invention is now described in detail.
A composition was made from the following formulation
Araldite GT 7072
(php = parts per hundred parts of polymer)
- 1. Moplen D50G is a trade name of Himont for a propylene copolymer.
- 2. Magnefin SE630 is a trade name of Martinswerk for magnesium hydroxide.
- 3. Araldite GT7072 is a tradename of Ciba-Geigy for an adhesion promoter.
- 4. Ultranox 815A is a GE Speciality Chemicals tradename for a 50/50 weight ratio
free flowing blend of Ultranox 626 (tradename for bis-(2,4-di-t-butylphenyl) pentaerythritol
diphosphite, an anti-oxidant) and Ultranox 210 (tradename for tetrakis-[methylene-(3,5-di-t-butyl-4-hydroxyhydrocinnamate)]-methane,
A cable coating prepared from this compound gave the following properties:
Oil Resistance Properties
Elongation at break
After Ageing 7 days at 135°C
Retention of tensile strength
Retention of elongation
Hot pressure at 135°C
Cold impact at -40°C
Cold blend at -40°C
Fire and Smoke Test Properties
Halogen acid gas evolution
Corrosivity of gases
Smoke density - Flaming
Time to maximum
Smoke density - Non
Time to maximum
Retention of tensile strength %
Retention of elongation at break %
Dielectric constant at 1 MHz
Dissipation factor at 1 MHz
Insulation resistance at 20°C Initial value
After 21 days immersion in water
Insulation resistance after 2 hours
3 x 1013
1 x 1013
6 x 1012
Melt flow index
It is to be understood that the embodiment described above is by way
of illustration only. Many modifications and variations are possible.