PTFE, FEP, PFA Recycling
0

PFA Recycling | Recycle PFA

1. Introduction

PFA is a perfluoroalkoxy copolymer resin available in pellet or powder. PFA combines the processing ease of conventional thermoplastic resins with the excellent properties of PTFE (polytetrafluoroethylene).

It differs from the PTFE resins in that it is melt-processable using conventional injection molding and screw extrusion techniques. PFA was invented by DuPont. Other brandnames for granules are Neoflon PFA from Daikin or Hyflon PFA from Solvay Solexis.
Products manufactured from PFA can offer continuous service temperatures up to 260°C (500°F). What’s more, PFA provides superior creep resistance at high temperatures, excellent low-temperature toughness, and exceptional flame resistance.

PFA melts at 280°C minimum and is melt processable. Some grades are chemically stabilised. It is available in the form of translucent pellets, powder, and as an aqueous dispersion.
The PFA fluoropolymer resins are processed by conventional melt-extrusion techniques and by injection, compression, rotational, transfer, and blow-molding processes. The high melt strength and heat stability of these resins permit the use of relatively large die openings and high-temperature draw-down techniques, which increase processing rates. Reciprocating screw injection molding machines are recommended. Corrosion-resistant metals should be used in contact with the molten resin. Long extruder barrels, relative to diameter, are used to provide residence time for heating the resin to 316° to 427°C (600° to 800°F).

2. Chemistry

PFA fluorocarbon resin is a copolymer of tetrafluoroethylene and a perfluorinated vinyl ether having the formula [(CF(ORf)-CF2)m(CF2 -CF2 )n ]x, where ORf represents a side chain perfluoralkoxy group

3. Properties

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 the flow and crystallinity of the polymers. These unique properties lead to the following benefits:

• High melting points (327°C [621°F] for PFA PTFE; and 305°C [582°F] for 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. 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 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.. At high processing temperatures, adequate ventilation is recommended.

• High upper service temperature (260°C [500°F] for PFA PTFE and 260°C [500°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.

4. Recycling

4.1. PFA Scraps Recycling

Eco USA has various options for recycling PFA polymer. These options depend upon the nature and amount of the scrap destined for recycle. There are three options for recycling this fluoropolymer: reprocessing, recovery or disposal.
Eco USA collects the scrap for recycling and then processes it into a form suitable for re-use. The PFA scraps can be reprocessed as thermoplastics provided following analyses are performed:

– Checking melt index
– Contamination quantified
– Thermal stability verified
– DSC analysis and other testing

Thermoplastics are easily recyclable, compared to thermosets, because the polymer chain does not degrade when melted down. This is because the weaker interactions between polymer chains break down at much lower temperatures than the chemical bonds between monomers. This allows thermoplastics to be recycled indefinitely until the polymers are broken down to the point that the material loses structural integrity.

Thermoplastics have a limit recyclable lifespan due to degradation of the polymers and contamination during the recycling process. Contaminants can be inert materials, which act as fillers, or they can be other plastics, which alters the physical properties of the resulting material.

4.2. Recovery and Disposal of Fluoropolymer Waste (i.e. PFA)

Recovery

Fluoropolymers are usually employed in small components in specific complex applications such as electronic equipment, transport (cars, trains and airplanes) or as very thin layer coatings on fabrics and metals. Where sufficient quantities of fluoropolymers can be recovered and may be sufficient to warrant recycling then they should be shipped to specialist recyclers.

A very substantial market exists for recovered fluoropolymers as low friction additives to other materials. For example PTFE and/or PFA are typically ground into fine powders and used in such products as inks and paints. In this scenario Eco USA is finding the new user for a still usable product.

Disposal

Fluoropolymer waste should be incinerated in authorized incinerators. Preferably, non-recyclable fluoropolymers should be sent to incinerators with energy recovery. Disposal in authorized landfills is also acceptable.

Decomposition products:

According to the fire conditions toxic gasses develop, predominantly carbon dioxide, carbon monoxide, hydrofluoric acid, tetrafluoroethylene, hexafluoropropylene, perfluorisobutylene, carbonyl fluoride and other low-molecular fluorohydrocarbons.

Article Sources:
1) Fluoropolymers,Vol.2, 1999
2) Du Pont notes about fluoropolymers

Post comment

XHTML: You can use these tags: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>

Random Testimonial

  • ~ Silcotech North America

    "With one phone call to Eco USA, we made a significant dent in our waste management budget, improved our bottom line and reduced our company's carbon footprint. The program has been a win-win all around. Douglas R. Snider - QMS Administrator Silcotech North"

  • Read more testimonials »