Читать книгу Concise Handbook of Fluorocarbon Gases - Sina Ebnesajjad - Страница 28

The inception of fluoropolymers as a group dates back to the serendipitous discovery of polytetrafluoroethylene (PTFE) in a DuPont laboratory by Roy Plunket. His research program was aimed at the discovery and development of new refrigerants. The initial testing of the waxy white powder found in a tetrafluoroethylene gas cylinder revealed something unusual. The powder had a high melting point, was insoluble and was unaffected by any chemical and intractable as a thermoplastic. Manhattan Project’s needs for chemical resistance materials led to the first application of PTFE.

Following the successful introduction of PTFE, produced in a pilot plant, into the processing equipment for uranium manufacturing and purification, interest in this plastic began gathering. The DuPont Company in West Virginia, USA in 1946 built the first commercial plant. The story of the development fluoropolymers has been covered, in detail, in other publications [12].

The basic properties of PTFE may be justifiably called extreme (Table 2.8). The properties and characteristics of PTFE include high chemical resistance, low and high temperature capability, resistance to weathering, low friction, electrical and thermal insulation, and low friction or slipperiness. One drawback of this polymer is its relative softness, compared to engineering polymers, resulting in cold flow and ease of abrasion both of which are undesirable for many applications. Fillers are incorporated in PTFE to enhance its hardness.

The first fluoropolymer PTFE was quite peculiar in that it would not flow upon melting. It could not be processed by typical melt processing techniques such as extrusion. Metal powder processing techniques were modified and adopted to fabricate parts from PTFE examples of which include modified compression molding, ram extrusion and paste extrusion. Afterward a number of melt processible fluoropolymers were developed in the decades since the Second World War [13, 14].

Table 2.8 Fundamental properties of polytetrafluoroethylene.

High melting point, 342°C

Exceptional thermal stability

Useful mechanical properties at extremely low and high temperatures (-260 to 260°C)

Insolubility

Chemical inertness

Low coefficient of friction

Low dielectric constant/dissipation factor

Low water ab/adsorptivity

Excellent outdoor weatherability

Flame resistance (limiting oxygen index = 95)

Exceptional Purity

A whole family of thermoplastic polymers has been developed based on homopolymers and copolymers of tetrafluoroethylene (TFE). After PTFE the largest volume member is perfluorinated ethylene propylene copolymer (FEP). The second monomer in the FEP is hexafluoropropylene (CF₃-CF=CF₂), which is conveniently co-produced along with TFE in the monomer manufacturing process. Other important products included a copolymer of TFE with ethylene at a molecular ratio of 1:1 and TFE (ETFE) and perfluoroalky vinyl ethers (PFA and MFA).

The third largest volume family of fluoropolymers is based on vinylidene fluoride (VDF) hompolymers and copolymers. These thermoplastics are processed by normal melt processing techniques such as extrusion, film blowing and various molding methods. VDF is copolymerized with other monomers such as chlorotrifluoroethylene (CTFE). Both thermoplastics and elastomers can be prepared depending on the concentration of the CTFE in the copolymer. A new example is Arkema Corporation’s VDF copolymers containing a reactive group that allows its ease of bondability [15].

The important polymers based on CTFE are its homopolymer polychlorotrifluoroethylene (PCTFE) and its copolymer with ethylene. The latter chlorotrifluoroethylene ethylene copolymer (ECTFE) is the analog of ETFE. PCTFE is borderline melt processible and is considered somewhat difficult to process. This CTFE homopolymer degrades when it is heated in the air lead to rapid decrease in its molecular weight.

Chemours Corp is the only commercial manufacturer of vinyl fluoride and its homopolymer polyvinyl fluoride (PVF). PVF cannot be processed by the standard melt processing method because it undergoes rapid thermal decomposition when heated above its melting point (195°C). PVF is fabricated into films and coatings using a somewhat unusual process. PVF is first dispersed in a latent (polar) solvent that suppresses its melting point thus permitting its melt processing without decomposition [16].

Fluoropolymer applications traverse across virtually every industrial segment and geographical regions. Examples of industries include automotive, aerospace, chemical and petrochemical processing, electrical, microelectronic, semi-conductor, pharmaceutical, biopharmaceutical, consumer, sports and recreation, food, beverage, laboratory applications, construction and architectural, military and others.

PTFE is the best material for manufacturing parts including gaskets, vessel and pipe linings, seals, spacers, dip tubes and other applications where corrosion resistance against most chemically challenging agents are required. Copolymers of TFE may also be used to manufacture some of those parts and wire and cable insulation. Most acids, bases and organic solvents do not affect these polymers even at elevated temperatures. PVDF applications include wire insulation, tubing and pipes for high purity water transportation, membrane distillation, gas separation, separator for lithium ion battery. PCTFE films are part of the composites for manufacturing bubble packs for pharmaceuticals because of their resistance to moisture vapor permeation.