**Statements regarding behavior in a flame situation are not intended to reflect hazards presented by this or any other material when under actual fire conditions. The following characteristics contribute to the unique properties of PFA fluorocarbon resins:
- Nonpolarity: The carbon backbone of the linear polymer is completely sheathed by the electron cloud of fluorine atoms, much like a wire core is protected by insulation coating. This ensheathment, and the angles at which the carbon-fluorine bonds are disposed, causes the centers of electronegativity and electropositivity to be perfectly balanced across the polymer chain cross section. As a result, no net charge difference prevails. This nonpolarity of the polymer is partly responsible for its lack of chemical reactivity.
Low interchain forces: The bond forces between two adjacent polymer chains are significantly lower than the forces within one chain. PFA PTFE linear polymer chains are otherwise restrained. However, in PFA FEP and PFA, interpolymer chain entanglement of the pendant structure precludes the shifting of polymer chains to relieve the implied load. The "creep" normally associated with PFA PTFE is mostly avoided with PFA FEP and even more so with PFA.
High C-F and C-C bond strengths are among the strongest in single bond organic chemistry. The polymer must absorb considerable energy to disrupt these bonds. Chemical reactions represent a kinetic and thermodynamic resolution of bond-making and bond-breaking in favor of the most stable system. These bond strengths are hard to overcome.
- Crystallinity: The high degree of crystallinity in these semicrystalline polymers results in high melting points, mechanical properties, and an integral barrier to migrating, small, nonpolar molecules. Under certain conditions, these molecules penetrate the plastics.
High degree of polymerization: The unbranched nature of the polymers and their low interpolymer chain attraction requires very long chain lengths in PFA PTFE and entanglement in PFA FEP and PFA to provide load-bearing mechanical properties. The chain length also has an impact on flow and crystallinity of the polymers. These unique properties lead to the following benefits:
High melting points (327Â°C [621Â°F] for PFA PTFE; 260Â°C [500Â°F] for PFA FEP, and 305Â°C [582Â°F] for PFA PFA). The melting point of PFA PTFE is one of the highest in organic polymer chemistry. Other materials can attain higher temperatures, but they degrade rather than melt. Compared to PFA PTFE, the lower melting temperature of PFA FEP results from lowerÂ°of polymerization and crystallinity. In PFA PFA, a higher degree of polymerization, enhanced entanglement of the pendant structure, and lower comonomer content combine to provide a melting point closer to that of PFA PTFE.
High thermal stability: Due to the strength of the carbon-fluorine and carbon-carbon single bonds, appreciable thermal energy must be absorbed by the polymers before thermal degradation. The rate of decomposition of a part of PFA depends on the particular resin, temperature, and heat exposure time; and to a lesser extent, pressure and nature of the environment. At maximum continuous service temperatures, thermal degradation of the resins is minimal. For example, at 400Â°C, PFA FEP is measured at 4/100,000 of 1 percent, and PFA PTFE at 1/100,000 of 1 percent. At high processing temperatures, adequate ventilation is recommended.
High upper service temperature (260Â°C [500Â°F] for PFA PTFE, 204Â°C [400Â°F] for PFA FEP and 260Â°C [500Â°F] for PFA). The polymers' high melting points and morphological features allow components made from the resin to be used continuously at the stated temperatures. Above this temperature, the component's physical properties may begin to decrease. The polymer itself, however, will be unaffected if the temperature is insufficient for thermal degradation.
- Insolubility: There is no known solvent for PFA fluorocarbon resins under ordinary conditions.
Inertness to chemical attack: The intrapolymer-chain bond strengths preclude reaction with most chemicals. Under relatively unusual circumstances the polymer can be made to react. Examples of unusual reagents include:
° Sodium, in a suitable media, etches the fluorocarbon polymer.
° Finely divided metals often interact with the polymer.
° Interhalogen compounds often induce halogen interchange with the fluorine.
° Ionized oxygen in oxygen plasma is often sufficiently energetic to react with the polymer chain.
° Electron bombardment at the megarad level can sever the polymer chain.
- Low coefficient of friction: The low coefficient of friction of PFA results from low interfacial forces between its surface and another material and the comparatively low force to deform.
- Low dielectric constant and dissipation factor: PFA provides low, if not the lowest, values for these parameters. These low values arise from the polymer's nonpolarity as well as the tight electron hold in the ultrapolymer bonds.
- Low water absorptivity: For PFA to absorb water, the surface must remain wet for a long enough time for water to become physico-chemically associated with the polymer chains, and then it must become included in the polymer bulk structure. Water is a very high energy material and PFA has a very low surface energy. Therefore, these events are energetically incompatible and only occur under special circumstances and to a small extent.
- Excellent weatherability: Weather includes light of various wavelengths (IR, visible, UV), water (liquid or gas), other gases, and normal temperatures and pressure. The physical and chemical makeup of PFA makes it inert to these influences.
- Flame resistant: PFA will burn when exposed to flame, but will not continue to burn when the flame is removed.
- Excellent toughness: Some mechanical properties of PFA resins are shown in Table 1. Toughness characteristics are high and differ somewhat between resin types.