Detailed Product Description

Petrolatums have been used for some years to fill up voids between the conductors in a cable, thus giving it a high resistance to water penetration in axial direction. The high relative permittivity of petrolatums (approx 2.2) decreases the capacity of the cable. Petrolatums also have fairly low drop melting point, high volume contraction at cooling and a tendency to smear. A much better capacity will be obtained with a mixture of petrolatum and EXPANCEL microspheres. At the same time a lot of other properties will be improved.

THE PRODUCT

EXPANCEL microspheres are white, spherically formed particles, consisting of a polymeric shell with an enclosed blowing agent. EXPANCEL can be delivered unexpanded or expanded.  For most cable applications we recommend our pre-expanced and dried grade, EXPANCEL 551 DE 40 d42 which has the following specifications:

Particle size, weight average diameter 30-50 pm

True density, kg/m³                             42 ± 4

Dry content %                                        >99

For tropical countries when a higher melting point for the petrolatum is used, our high temperature grades like EXPANCEL 091 DE are recommended.

EXPANCEL DE is a free flowing powder with very low density and high compressibility.

EFFECTS OF ADDING MICROSPHERES

EXPANCEL DE is mixed with liquid petrolatum prior to filling the cable. Usual amounts of EXPANCEL DE are 3 - 6% by weight. The mixture can be filled into the cable in the liquid or semi-liquid state. Mixing is done either in direct connection with the filling procedure or as a separate step before filling the cable.

The use of EXPANCEL DE will effect a number of properties of the cable filling compound:

Reduced weight:. Due to the extremely low density of EXPANCEL DE the weight of the cable can be reduced. Addition of 4% microspheres to a petrolatum will decrease the density from about 900 to about 450 kg/m³.

Decreased relative permittivity (dielectric constant). For a petrolatum with a relative permittivity of 2.2, addition of 4% EXPANCEL DE will cause a reduction to about 1.6. This effect can be used to increase the capacity of the cable, to use a petrolatum with high relative permittivity or to decrease the insulation thickness on each conductor.

Increased drop melting point. The increase is about 5^oC with an addition of 4% EXPANCEL DE and even more at higher loadings of EXPANCEL.

Low volume contraction at cooling. Most petrolatums have a high volume contraction at cooling and expansion at heating. A volume expansion of 6% between 10 and 65^oC is quite normal. After addition of 4% EXPANCEL DE, the volume expansion is only 2% in the same temperature range. With such a filling compound there will be limited problems with high internal pressure in the cable at elevated temperatures and no problems with water penetration because of high shrinkage between filling and end-use temperatures.

High compressibility. The microspheres can resist high filling pressures. Under load they act as compressible rubber balls that are deformed, but regain their original form when unloaded. Due to the high compressibility, the microspheres can be forced into and stay in small interspaces between the conductors, which will further decrease the tendency of water pnetration in a damaged cable.

Increased viscosity. Adding EXPANCEL DE to a petrolatum will result in a substantially increased viscosity, even at elevated temperatures, and the mode of filling the cable has to be altered to compensate for this. On the other hand the filling compound has a much lower tendency to smear and it is easier to remove from the conductors if necessary.

MIXING PETROLATUM AND EXPANCEL DE

For the mixing operation an ordinary batch mixer can be used. The mixer should be equipped with heating and cooling. Due to the great differences in density between EXPANCEL DE and the petrolatum an impeller that forces the microspheres downwards is recommended. If the filling compound has to be free from entrapped air, the mixer must be connected to vacuum. It may also be advantageous. If the mixer can be pressurized and if the discharge pipe is heated because it facilitates a complete discharge.

The petrolatum should be completely melted and the temperature adjusted to 65 75^oC (depending on type) before addition of microspheres.

EXPANCEL DE is added to the melted petrolatum. Due to the very low density, handling may cause dusting problems. A membrane pump can be used for dosing or the microspheres can be sucked into the mixer by vacuum.

Petrolatum and EXPANCEL should be thoroughly mixed. It may also be advantageous to deaerate the filling compound (at about 0.1 atm absolute pressure). Before discharge, the temperature should be decreased to ensure that the microspheres will not tend to separate from the petrolatum. The appropriate discharge temperature depends on the melting of the petrolatum, but is usually 50 55 ^oC.

OTHER APPLICATIONS

EXPANCEL microspheres can also be used to ensure a water tight seal between the inner conductors and the outer sheath of a cable. In

this case EXPANCEL DU can be used in admixture with petrolatum, grease, etc. EXPANCEL DU is dry, unexpanded microspheres with a true density of 1000 1100 kg/m³. The mixture is applied on the inner sheath of the cable. The microspheres are expanded when the outer polyethylene sheath is applied at high temperatures, resulting in a water tight seal.

MISCELLANEOUS

The results in this bulletin are based on laboratory experiments only. A certain type of petrolatum has been used. Prospective users of EXPANCEL DE in cable filling compound should carry out tests to find the optimum mixture of EXPANCEL DE and the petrolatum type to be used.

COMMENTS

The information contained in this leaflet is the result of our research and experience. It is given in good faith, but under no circumstances does it constitute a guarantee on our part, nor does it hold us responsible, particularly in the case of legal action by a third party.