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Tetrafluoroethylene/ hexafluoropropylene Copolymer (FEP):
General Description:
The high molecular weight of PTFE and associated high melt viscosity and specialist processing suggested the need for fluoro-plastics which could be melt processed like conventional thermoplastics by injection molding or extrusion. These copolymers include FEP.
Despite the close relationship it has properties somewhat different from PTFE:
- Can be molded, extruded and thermoformed like thermoplastics,
- higher impact strength,
- lower service temperature,
- comparable electrical properties, chemical resistance and weathering resistance.
Additives:
1. As this copolymer is composed of per fluorinated propylene and ethylene, fillers and reinforcements are the most important additives.
2. Besides colorants, the most commonly used functional additives are non-ionic wetting agents.
3. The main filler and reinforcement are graphite and milled glass fiber, respectively. The type and amount of filler are limited primarily by the processability.
Availability:
The current FEP range comprises 3 grades suitable for:
1. Injection molding
2. Extrusion and
3. A water-based dispersion for applying protective coatings with lubricating or non-stick properties for impregnating yarn & felt as well as a hot melt adhesive.
FEP is supplied as granules, as powder for fluidized bed coating and electrostatic coating as well as dispersion.
Film, tubes, profiles and foil are the semi-finished products available as stock shapes.
Mechanical Properties-1
Behavior at High Rate of Deformation:
Molded FEP material shows tough behavior down to - 200 °C in the flexural impact test and impact strength test.
Mechanical Properties -2
Friction and Wear Characteristics:
1. FEP exhibits good slip properties.
2. Because the coefficients of static friction are smaller than the coefficients of sliding friction, the stick-slip effect & consequent vibration do not occur.
3. The coefficient of sliding friction is only slightly dependent on the slip velocity above 20 m/min.
4. FEP is suitable for sliding surfaces at specific loads and low slip velocities.
Thermal Properties
FEP does not exhibit the sudden change in volume as with PTFE around 19°C.
Electrical Properties
1. The electrical properties of FEP are almost identical with those of PTFE which has the lowest dielectric constant and dissipation factor of all plastics as well as a high volume resistivity and surface resistance. These properties are largely independent of temperature and frequency.
2. Because FEP absorbs almost no water, the resistivity is unchanged even after extended immersion.
3. The dielectric constant of FEP is low and practically constant over a wide range of frequency from 102 to 109 Hz. It falls slightly with increasing temperature.
Optical Properties
1. The refractive index n20D is 1.338.
2. Thin-walled moldings of unfilled, natural FEP are translucent with a bluish lustre.
3. FEP films (0.05 mm) are almost opaque in the infrared region (8 to 9 mm), i.e. they absorb incident energy.
4. Light transmission increases rapidly from 0 to 90% over the UV region of 200 to 400 nm.
Performance in Vacuum
On evacuation down to 10-5 bar, FEP does not depolymerize or outgas. The weight loss at 10-5 bar and 100 °C is 0.08%.
Chemical Properties
Resistance to Chemicals
FEP is resistant to acids, alkaline solutions, all solvents, detergents, grease, oil, water.
Resistance to Stress Cracking
Resistance to stress cracking is basically high and is best for high molecular weight molding compounds that can be used for linings, expansion collars and valves in chemical plant.
Weathering Resistance
FEP has good resistance to weathering and light. Weathering tests indicate a life of over 20 years for FEP without measurable changes in properties.
Resistance to High Energy Radiation
The resistance of FEP to ionizing radiation in the presence of oxygen is low. The limiting value for degradation is 4x103 to 6.5x103 J/kg.
The change in dielectric constant and dissipation factor up to 103 J/ kg is negligible. Radiation resistance in vacuum and inert atmospheres is somewhat better.
Flammability
FEP is non-flammable. According to UL94 it belongs in Class V-0. The oxygen index of the unfilled material is 95%.
Toxicological Assessment
All grades of FEP meet the requirements of FDA regulation
Processing-1
Injection Molding
Melt temperature 320 to 360°C
Injection pressure 350 to 1000 bar
Mold temperature 200 to 230°C
Shrinkage 3.5 to 6.0%.
Processing -2
Extrusion
1. Melt temperature 315 to 410°C (also for cable manufacture), Barrel material non-ferrous alloys (Hastelloy, Xaloy, Reiloy, Monel).
2. FEP can be extrusion blow molded. Forming of semi-finished products is carried out at temperatures above 280 °C. Parts formed in the thermoelastic region take up their original shape on heating.
3. Semi-finished products and moldings can be easily machined. Bonding is possible with Epoxide and Silicone resin adhesives and requires pretreatment of the surfaces with etching reagents. Friction welding is possible but not ultrasonic welding is not possible.
Typical Applications
1. Cable insulation, (under floor electric heating),
2. coatings,
3. lining of vessels,
4. flexible printed circuits,
5. injection moldings for electrical, electronics and chemical app1ications,
6. packaging film,
7. impregnation of cloth, yam, felt,
hot melt adhesives.
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