SPECIALTY PLASTICS Polyvinylfluoride (PVF)

Polyvinylfluoride (PVF) Polyvinylfluoride was commercialized by Du Pont in the early sixties. The monomer is manufactured by adding hydrogen fluoride to acetylene.

Structure and General Properties The crystalline melting point is 198°C and the molecular weight is 5x104 to 2x105. Many properties of PVF resemble those of PVC (low water absorption, resistance to hydrolysis, separation of HF at elevated temperature). The smaller F atoms result in a higher degree of crystallinity. The flame resistance is poorer than that of PVC; PVF continues to burn slowly after ignition.

Transition Temperatures Availability PVF is supplied only as film, weakly oriented or biaxially stretched film is available. Transition Temperatures The glass transition temperature is - 20 °C, the crystalline melting temperature 198 °C.

Optical Properties PVF films are transparent to visible and UV light. In the infrared region radiation with wavelengths between 7 and l2 mm is absorbed. The refractive index n20D is 1.45. Three optical axes must be differentiated for oriented films.

Resistance to Chemicals Chemical Properties Resistance to Chemicals PVF is resistant to acids, alkaline solutions and many solvents at room temperature. It is unaffected by boiling in carbon tetrachloride, benzene, acetone and methyl ethyl ketone. The resistance lies between that of PTFE and PCTFE.

Toxicological Assessment Flammability PVF burns slowly, i.e. its fire performance is somewhat poorer than that of PVC-U. Toxicological Assessment PVF is suitable for use with foodstuffs and is physiologically inert.

Processing-1 PVF is supplied only as film. It can be compression molded. Cast film is made by pouring a mixture of 25 to 40% PVF and 75 to 60% dimethyl sulfoxide on to a platen and heating to 130 °C for a short time. The paste-like mixture can also be extruded in hot oil at 160°C. PVF films can be welded by heat impulse or high frequency.

Organosols (g-butyrolactone) can also be extruded in a water bath. Processing-2 Organosols (g-butyrolactone) can also be extruded in a water bath. High transparency films are obtained by briefly heating the film to 250 °C and quenching rapidly. Biaxial stretching is carried out in warm solvents (g-butyrolactone, dimethyl acetamide). PVF films can be welded by heat impulse or high frequency.

Typical Applications The high weathering resistance of PVF makes it suitable for outdoor use as roof covering, cladding, pipe insulation, green houses, solar collectors (transparent to UV). Corrosion protection for metal panels, plywood and insulating board, road signs, packaging film, shrink tube.

Trade Names Kel-F (3M Co., US) KF Piezo Film (Kureha Chem. Ind., IP) Tedlar (Du Pont de Nemours, 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 Notched impact strength Ft lb/in 3 No break 2.5-2.7 Thermal Melting Point °C 327 290 216 198 171 Service temperature in air without mechanical loading short-term 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 and 50°C K-1.10.-6 100 80 60 20 85 Specific heat capacity kJ/kg K 1.0 1.2 0.9 Thermal conductivity W/mK 0.25 0.22

Table 1 Guide values of properties of Fluorine containing polymers Units PTFE FEP PCTFE PVF PVDF Electrical Volume resistivity Wcm >1018 >2.1018 >1.20.1018 3.1013 2.1014 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 Tracking resistance class C 600 Flammability (ASTM D65) Non flammable Clarity opaque Transparent -