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The
following characteristics contribute to the unique properties of
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
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the
forces within one chain. PTFE linear polymer chains are otherwise
restrained. However, in FEP and PFA, interpolymer chain
entanglement of the pendant structure precludes the shifting of polymer
chains to relieve the implied load. The ìcreepî normally associated
with PTFE is mostly avoided with 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.
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• 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 PTFE and entanglement in 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 (327ƒC [621ƒF] for PTFE; 260ƒC [500ƒF]
for FEP, and 305ƒC [582ƒF] for PFA). The melting point
of PTFE is one of the highest in organic polymer chemistry.
Other materials can attain higher temperatures, but they degrade
rather than melt. Compared to PTFE, the lower melting temperature
of FEP results from lower degrees of polymerization and crystallinity.
In 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 PTFE.
• High thermal stability–Due to the strength of the carbon-fluorine
and carbon-carbon single bonds, appreciable thermal energy must
be sorbed by the polymers before thermal degradation. The rate of
decomposition of a part of 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 400ƒC,
FEP is measured at 4/100,000 of 1 percent, and PTFE
at 1/100,000 of 1 percent. At high processing temperatures, adequate
ventilation is recommended.
• High upper service temperature (260ƒC [500ƒF] for PTFE,
204ƒC [400ƒF] for FEP and 260ƒC [500ƒF] for PFA).
The polymersí high melting points and morphological features allow
components made from the resin to be used continuously at the stated
temperatures. Above this temperature, the componentís physical properties
may begin to decrease. The polymer itself, however, will be unaffected
if the temperature is insufficient for thermal degradation.
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• Insolubility–There is no known solvent for 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.
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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
results from low interfacial forces between its surface and another
material and the comparatively low force to deform.
• Low dielectric constant and dissipation factor–provides low,
if not the lowest, values for these parameters. These low values arise
from the polymerís nonpolarity as well as the tight electron hold
in the ultrapolymer bonds.
• Low water absorptivity–For 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 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
makes it inert to these influences.
• Flame resistant–will burn when exposed to flame, but will
not continue to burn when the flame is removed.
• Excellent toughness–Some mechanical properties of resins
are shown in Table 1. Toughness characteristics are high and differ
somewhat between resin types. |
