A material consisting of gas and polymer in a closed cell structure is called a foam. An insulation material that is foamed will have a lower dielectric constant and dissipation factor than the solid material. A polymer with a low dielectric constant, when foamed, will yield cable designs that are smaller and lighter than electrically equivalent designs using solid insulation materials. This paper describes the basic properties of such a polymer. Data is presented on the mechanical properties of wires insulated with this foamed material. Theoretical electrical properties and calculated polymer weights for MIL-STD-1553B data bus constructions are also presented.
THE COMMERCIAL COMPUTER and telecommunications industries are driven by the demand for more information and faster communications. The cable designer has the task of providing voice and data communications at higher speeds, over longer distances, and with less signal distortion or loss. In addition, as the number and complexity of electronics devices increase, more and more wiring is being squeezed into less and less space. These problems are not unlike those facing the electronic systems integrator on military aircraft programs.
Making wires smaller and using less insulation has the unsatisfactory result of decreasing electrical performance. These problems can be solved by using insulating materials with a lower dielectric constant. A new fluoroplymer foam specifically designed as primary insulation for twisted pair data transmission cables is now available from Allied Engineered Plastics. This foam is based on ethylenechlorotrifluoroethylene (ECTFE) resin and is trade named HALAR Datashield. Although this polymer was developed to meet the demands of the commercial computer market, it has the potential to provide significant weight savings in MIL-STD-1553B cable constructions.
ECTFE is a fluoropolymer. As a family of materials, fluoropolymers have the combination of properties required to meet the demands of the military aerospace market. No other materials have the ability to resist attack by most chemicals, even at elevated temperatures. Most fluoroplymers retain much of their strength at temperatures where other polymers are no longer solid, and remain ductile at lower temperatures when other polymers become brittle. Fluoropolymers have been used as wire and cable insulation on military aircraft for over 20 years. Current specifications include MIL-W-16878 (FEP, TFE) MIL-W-81044 (PVF2) and MIL-W-22759 (ETFE, TFE, FEP). Typical properties of the most common fluoropolymers are shown in Table I.