has developed patented technology for the compounding and processing of fluoropolymer foam resins. The foam process involves the continuous injection of a gas such as nitrogen directly into an extruder filled with molten resin. A specially designed extruder screw is typically utilized to create the polymer gas mixture with an inert nucleating package contained in the resin to help promote cell growth. Foams of up to 60% voids have been demonstrated through the use of this process.
APPLICATIONS FOR MELT PROCESSABLE PERFLUORINATED RESINS
SukoPTFE Teflon® FEP
and PFA perfluorinated resins possess melt viscosities low enough for processing in melt extruders. Additionally, FEP
and PFA are well suited for extrusion foaming while maintaining their low dielectric constant and low dissipation factors, both necessary for twisted pair and coaxial cable insulations.
Data cables are used in the transmission of electronic signals in a variety of settings and applications. The key electrical properties required for these cables include a low dielectric constant and a low dissipation factor. These properties can be enhanced by foaming the insulation. Depending on the resin used, cables having a foamed insulation allow cable miniaturization, weight reduction of the end product,and the transmission of clear high-quality electrical signals at high signal speeds.
Foamed cables fabricated from sukoptfe fluoropolymer
resins have become extremely popular in computer manufacturing and installation of network systems. Cables made with sukoptfe Teflon® FEP
or PFA have low-flame and low-smoke attributes. These cables can be routed through plenums and other air
handling ducts, reducing the need for the costly installation of electrical conduit.
STRUCTURE AND PROPERTIES OF FLUOROPOLYMERS
The ability to melt extrude these polymers, combined with such characteristics as low dielectric constant and low flammability made FEP
and PFA well suited for wire
and cable applications
GENERAL ASPECTS OF FOAMED INSULATION
The dielectric properties of polymeric insulation on wire
can be improved by the inclusion of gaseous bubbles. However, this must be done in a way that results in small and evenly distributed voids throughout the insulation. Large bubbles can lead to failures within the insulation and uneven void distribution can cause mismatched performance around a conductor. DuPont has developed technology for the controlled nucleation of nitrogen gas in the fluoropolymer melt as it is extruded and drawn onto wire
Foaming provides a collection of distinct advantages that are not achievable with conventional melt extrusion techniques. For FEP
and PFA, when foaming technology is utilized, the dielectric constant drops
as the void content increases. The reduced dielectric constant achieved through foaming increases the relative velocity of propagation while lowering capacitance. Because of these improved attributes over conventional melt extrusion, foaming also allows for reduced wall thickness, leading to a corresponding drop
in overall weight of the required insulation.
EQUIPMENT REQUIREMENTS FOR FOAMING
Conventional extruder screws for melt extrusion have two key zones, a feed and a shallow metering zone. In foam extrusion applications, a 3 or 4 stage screw is used with feed, compression, metering, and mixing stages. A reduced diameter ring is machined at the middle of the first mixing section to reduce the pressure in that region, facilitating the gas injection.
Proper sizing of the extruder and appropriate screw design are key factors in the foaming process.
As the polymer is melted and fed
through the extruder barrel, pressurized gas is injected at or about sonic velocity by a high pressure
pump. Proper sizing of the injector via the metering orifice is required to deliver a controlled gas injection into the polymer at the desired levels.
The major safety consideration for the extrusion of fluoropolymers and other organic polymers is the removal of off-gases released from hot polymers into work areas. This can be accomplished by the installation of exhaust hoods at the die and the hopper heaters, if utilized. Extruding into water – either a quench tank or a partially filled container – for purging is also recommended.