SPECIALTY PLASTICS Polyvinylidenefluoride (PVDF)

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.

Particular advantages of PVDF are: - 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 W 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 W,

Resistance to Chemicals Chemical Properties 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 Resistance to Radiation 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.

Toxicological Assessment 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.

Trade Names Kynar (Arkema Inc.) Dycon (Dyncon, US) Kureha (Kureha Chern. Ind., IP) Solef (Solvay Solexis, US) Hylar (Solvay Solexis, US) Dykor (Whitford Corp. US)

Further Reading BRYDSON, J.A, Plastics Material, Butterworth Heinemann, oxford, New Delhi (2005) DOMININGHAUS.H, Plastics for Engineering, Hanser publishers, Munich, New York (1998) CHARLESS A . HARPER, Modern Plastics Hand Book McGraw –Hill, New York (1999)

Table 1 Guide values of properties of Fluorine containing polymers Units PTFE FEP PCTFE PVF PVDF Density g/cm3 2.15-2.20 2.12-2.17 2.10-2.12 1.38-1.57 1.7 Water absorption (23°C, 24h) % 0.04 Mechanical Tensile strength N/mm2 25-36 22-28 32-40 49-127 46 Elongation at break 350-550 250-330 128-175 115-250 100-400 Tensile modulus of elasticity 408 350 1050-2110 1800 840 Thermal Melting Point °C 327 290 216 198 171 Service temperaturein air without mechanical loading short-term C 300 250 180 long-term 205 150 100-120 Glass transition temperature 127 45 -20 40 Heat deflection temperature 121 70 126 Coefficient of linear expansion between 0- 50°C K-1.10.-6 100 80 60 20 85 Specific heat capacity kJ/kg K 1.0 1.2 0.9

Transparent - transluscent Table 1 Guide values of properties of Fluorine containing polymers Properties Units PTFE FEP PCTFE PVF PVDF Electrical Volume resistivity Wcm >1018 >2x1018 >1.20x1018 3x1013 2x1014 Surface resistance W 1017 1016 Dielectric constant <2.1 2.1 2.3-2.7 8.5 7.72 Dissipation factor, tan d (103Hz) 0.0002 0.023-0.027 0.015 0.018 Dielectric strength kV/mm 600-800 800 450-550 Flammability (ASTM D65) Non-flammable Clarity opaque Transparent - transluscent