Polyvinylidenefluoride (PVDF):
General Description
- Introduced by Pennwalt Corp. in 1961, PVF2 or PVDF is a high molecular weight homopolymer.
- The monomer an be obtained from trichloroethylene by reacting with hydrofluoric acid and the action of metallic zinc on the difluorodichloroethane formed.
- The partially crystalline polymer contains over 57% w/w fluorine.
- high mechanical strength, stiffness and toughness,
- relatively resistant to high and low temperatures
- tough even at low temperatures,
- good chemical resistance.
Structure and Properties
- The degree of crystallinity depends on the history of the molding.
- Rapid cooling of thin walled moldings and film results in transparent products; slow cooling or annealing at 135°C lead to highly crystalline products with high stiffness and compressive creep strength.
Availability
PVDF is supplied in various grades as:
- granules for injection molding and extrusion,
- as powder and
- as 45 to 50% dispersions in organic solvents (dimethyl phthalate & diisobutylketone).
Mechanical Properties
- Transition Temperatures - PVDF has a glass transition temperature Tg of 40 °C, a crystallization temperature of 140 °C and a melting point of 171 °C.
Thermal Properties
PVDF can be used over the range -60 to +150°C.
It withstands temperatures as follows without any noticeable degradation:
- 150°C over 1 year,
- 260°C up to 12 hours,
- 240 °C up to 30 minutes,
- at 480 °C it decomposes within a few minutes.
Copper, aluminium and iron catalyze decomposition.
Electrical Properties
-Like PVF, PVC and PVDC, the polar nature of PVDF precludes its use for high frequency applications.
- The volume resistivity is greater than 1015 Ω at 30 °C making it highly suitable for use in the mains frequency range.
- The dielectric strength (1 mm film) is 22 kV/mm, the surface resistance exceeds 1013 Ω.
Resistance to Chemicals
- PVDF is resistant to acids (apart from fuming nitric acid), alkaline solutions, solvents, aromatic, aliphatic or chlorinated hydrocarbons, oils, fats, fluoro-refrigerants.
- It is not resistant to primary amines at elevated temperatures, hot acetone (thin films), strongly polar organic compounds such as dimethylformamide, diethyl acetamide, cyclohexanone, ketones, esters.
Weathering Resistance
- PVDF deteriorates slightly outdoors.
Resistance to Radiation
- PVDF is resistant to UV; there is slight degradation in the range 200 to 400 nm.
- Its resistance to high energy radiation is superior to that of other fluoroplastics.
Flammability
- PVDF is classified as V-0 to UL94. The oxygen index is 44%.
Toxicological Assessment
- There are no restrictions on the use of PVDF with foodstuffs.
- It is non-toxic.
- Gases liberated during processes must be
ventilated.
Processing - 1
- Injection molding, extrusion and coating are the most important methods of processing PVDF.
- The storing temperature of coatings produced by spraying, dipping or casting is 190 to 215°C.
Processing-2
- It is absolutely essential that the surfaces of plasticizing screws in injection molders or extruders do not contain any boron.
- Boron-containing products such as various glass fiber materials or MoS2 are incompatible with PVDF and cause spontaneous decomposition of the product during processing (fire, vaporization).
- This can also occur with colorants, fillers and reinforcements not certified by the manufacturer. Suitable pigment concentrates are available.
Joining -1
Welding
- Welding with round nozzle 345 + 10°C
- Welding with high speed nozzle 350 + 10°C
- Heated tool welding 230 °C
- Melt pressure 0.5 bar
- Welding pressure 2 bar
HF welding is suitable for wall thicknesses up to 2 mm.
Ultrasonic welding: near and remote field methods.
Joining-2
Bonding
- PVDF with PVDF: solvent adhesives as for PVC-U
- PVDF with PVDF or other materials: two-pack adhesives based on Epoxide resin.
- Surfaces to be bonded must be roughened.
Typical Applications
- Seals, pump and valve components, membranes, transparent rigid pipes (for HF and HC!), fittings, linings for pipes, vessels and autoclaves. films for packaging pharmaceuticals, medical instruments, shrink tubing, corrosion- and weather-resistant laminates for wood and metal, cable sheathing, monofilaments for manufacturing filter cloth and piezo and pyroelectrical films for hydrophones, infrared detectors, respiration monitors, muscle sensors and security equipment.
- Modification of the relatively stiff homopolymer has resulted in more flexible grades thus opening up new applications. The melting range of the modified material is 162 to l65°C.
Typical Applications-1
- Seals, pump and valve components, membranes, transparent rigid pipes (for HF and HC!), fittings, linings for pipes, vessels and autoclaves. films for packaging pharmaceuticals, medical instruments, shrink tubing, corrosion- and weather-resistant laminates for wood and metal, cable sheathing, monofilaments for manufacturing filter cloth.
Applications-2
- Piezo and pyroelectric films for hydrophones, infrared detectors, respiration monitors, muscle sensors and security equipment.
- Modification of the relatively stiff homopolymer has resulted in more flexible grades thus opening up new applications. The melting range of the modified material is 162 to l65°C.
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