BACKGROUND

The present disclosure relates to polyethylene foams. In one aspect, the present disclosure relates to polyethylene foams useful as telecom insulation while in another aspect, the present disclosure relates to coaxial and radio frequency cables comprising polyethylene foams.

Coaxial/radio frequency cables made of highly foamed polyethylene are widely used as antenna feeders, cabling of antenna arrays, equipment interconnections, mobile telecommunication systems, microwave transmission systems, broadcast transmission systems and other communication systems. Rapid development of mobile telecommunication systems has increased consumer demand for high quality radio frequency cables with minimum signal attenuation and broader bandwidth in the high frequency range (e.g., >2.47 GHz, or >5 GHz, or >20 GHz). As demand for high bandwidth increases, cables require the use of a highly foamed dielectric made with polymer resins, e.g., a polyolefin, with minimum polar groups or polar additives and which are cost effective and have good electrical properties.

Usually, a high frequency cable is made of an inner conductor surrounded by a foamed insulation. The base resin for insulation is generally a mixture of high density polyethylene (HDPE), high pressure low density polyethylene (HPLDPE, or simply, LDPE), and a nucleating master batch. In general, the ratio of HDPE to LDPE is 70-80% HDPE/30-20% LDPE. The nucleating master batch is typically added at about 1-3% and is generally also based on a LDPE resin. Due to less branching in the molecular structure of HDPE, the dissipation factor (Df) of HDPE is lower than LDPE and as such, the majority of base resin for cable insulation is typically HDPE. Moreover, this provides desirable mechanical properties to the foam such as high crush resistance. LDPE, in contrast, enhances the overall melt strength of the base resin due to its branched structure. However, a need exists for a foamed composition with (i) a higher expansion ratio, (ii) a finer cell size, and/or (iii) a more uniformly distributed cell structure than conventional foamed HDPE/LDPE insulation.

A need also exists for an insulation with a thin wall (e.g., <0.4 mm) with improved crush resistance to withstand the cabling process, including twisting and shielding.

A need also exists for an insulation that can be foamed with a physical foaming agent on typical extrusion foaming equipment.

SUMMARY OF THE INVENTION

In one embodiment, the present disclosure provides a foamable composition comprising (A) a high density polyethylene (HDPE); (B) a low density polyethylene (LDPE); (C) a peroxide-modified HDPE; (D) a nucleator; (E) optionally, an additive; and (F) optionally, a foaming agent.

In one embodiment, the present disclosure provides a process for making a foam composition, the process comprising the steps of:

(i) forming a composition comprising:(A) a high density polyethylene (HDPE);(B) a low density polyethylene (LDPE);(C) a peroxide-modified HDPE,(D) a nucleator; and(E) optional additive; and

(ii) contacting the composition with a foaming agent at a pressure from 1 MPa to 40 MPa under typical extrusion conditions.

In one embodiment, the present disclosure provides a foam formed from the foamable composition. In another embodiment, the present disclosure provides a cable comprising an insulation layer comprising the foam.

