Kynar - The universal polymer
Among fluoropolymers, there are few with the processing versatility as Kynar (PVDF). Kynar – or polyvinylidene fluoride – is particularly useful because it lends itself to numerous applications while also allowing itself to be processed in a number of different methods.
While PTFE shares – and possibly exceeds – the range of Kynar when it comes to multiple applications, the fact that PTFE cannot be melt processed means there are limitations in part shape and design. It is here that Kynar comes out ahead.
Kynar (PVDF) offers the user the option to combine rigid and flexible materials when processing. As a material of construction for pumps and pipes, it exhibits excellent resistance to abrasion. Kynar (PVDF) can also be manufactured in thin, flexible and transparent sections such as films, filament, and tubing. Unlike many polymers (including PTFE) the material is unaffected by sunlight and can therefore be used in an exposed condition outdoors without the risk of degradation.
Strength and toughness
Kynar (PVDF) is inherently strong and tough as reflected by its tensile properties and impact strength. An ambient temperature tensile strength at yield of 35-55 MPa and an un-notched impact strength of 800-4270 kJ/m offered by select resins emphasize this. These characteristics are retained over a wide range of temperatures.
Compared to many thermoplastics, Kynar (PVDF) has excellent resistance to tensile creep and fatigue. The long-term resistance of Kynar (PVDF) to flexural creep at elevated temperatures is significant. Kynar (PVDF) is suitable for many applications in which load bearing characteristics are important. Likewise, the short-term flexural creep resistance of the material reflects superior load bearing performance.
Kynar (PVDF) is rigid and resistant to creep under mechanical stress and load.
It is able to maintain a low tensile creep when subjected to constant stress. For example, when Kynar (PVDF) is subjected to a stress of 0.69 MPa (100 psi), the resin is able to maintain outstanding resistance even at temperatures as high as 140°C.
Kynar (PVDF) exhibits high thermal stability. Prolonged exposure at 250°C in air does not lead to weight loss. No oxidative or thermal degradation has been detected during continuous exposure to 150°C for a period of ten years.
In general, Kynar (PVDF) is one of the easiest fluoropolymers to process. The resins can be recycled up to three times without detriment to their mechanical properties because Kynar (PVDF) is inherently thermally stable and does not contain additives. Similar to most thermoplastics, Kynar (PVDF) resins discolour and degrade during processing if the processing temperature is too high, the residence time is too long, or the shear rate is too high.
Kynar (PVDF) exhibits a combination of high dielectric strength and excellent mechanical properties over a broad temperature range. This has led Kynar (PVDF) to be used for thin-wall primary insulation and as a jacket for industrial control wiring. Kynar (PVDF) has a high dissipation factor that lends an advantage as a material for parts requiring dielectric high heating strengths such as impedance welding. With proper shielding, Kynar (PVDF) can be used as jacketing for high frequency data cables because of its excellent flame and smoke performance.
Kynar (PVDF) is chemically resistant to a wide range of chemicals. Most acids and acid mixtures, weak bases, halogens, halogenated solvents, hydrocarbons, alcohols, salts and oxidants pose little problem for Kynar (PVDF).
Many factors can affect a material’s chemical resistance. These include, but are not limited to, exposure time, chemical concentration, extreme temperature and pressure, frequency of temperature and pressure cycling, attrition due to abrasive particles, and the type of mechanical stress imposed. The fact that certain combinations of chemical exposure and mechanical load can induce stress cracking in many otherwise chemically resistant materials, both metallic and non-metallic, is of particular significance. In general, the broad molecular weight distribution of Kynar (PVDF) results in greater resistance to stress cracking.
Factors such as permeability and adhesion affect the chemical resistance of Kynar (PVDF) coatings. Consequently, coatings may not exhibit exactly the same properties as melt-processed resins. Maximum use temperature for dispersion-applied or powder coatings should not exceed 100°C (212°F).
However, assuming chemical resistance is still adequate, laminated systems can be used from 120°- 135°C (248°- 275°F).
Operating parameters are dependent on the particular application of Kynar (PVDF) and differ from those experienced in either laboratory testing or apparently similar field service. Because corrosive fluids or vapours are often mixtures of various individual chemicals, it is strongly recommended that trial installations be evaluated under actual service conditions. For example, immersion testing of Kynar (PVDF) in individual chemicals at a specific operating temperature, will not necessarily predict the performance of fabricated components when they are exposed to an exothermic reaction between the individual chemicals.
The chemical resistance of Kynar (PVDF) is indicated in the chart below. In this chart, the behaviour of Kynar (PVDF) at 93°C (200°F) in contact with nine general chemical species is compared with that of other well-known plastics. The rating system ranges from unacceptable severe attack in the outer segment of the circle to excellent (inert) in the bull’s-eye.
Kynar (PVDF) films up to 0.125 mm thick are translucent to transparent.
The material shows excellent resistance to UV and film thicknesses above 0.5mm have been shown to completely block UV rays of wavelengths less than 250 Nm.
Many years of outdoor exposure in direct sunlight have little effect on the physical properties of Kynar (PVDF). However, some increases in tensile strength and reduction in elongation do occur over time.
Ozone is a powerful oxidizing agent characterized by a high degree of chemical instability. Kynar (PVDF) offers excellent chemical resistance to ozone exposure.
Kynar (PVDF) is also highly resistant to fungi and does not support the growth of the same.
Resistance to nuclear radiation
The resistance of Kynar (PVDF) to nuclear radiation is excellent. The original tensile strength of the resin is essentially unchanged after exposure to 100 megarads (Mrads) of gamma radiation from a Cobalt-60 source at 50°C (122°F) and in high vacuum (10 -6 torr). The impact strength and elongation are slightly reduced due to cross-linking.
This stability to effects of radiation, combined with chemical resistance, has resulted in the successful use of Kynar (PVDF) components in nuclear reclamation plants.
Smooth Kynar (PVDF) products of all types can be extruded at high rates without extrusion aids, lubricants or heat stabilisers. Resins can be processed on standard equipment with materials of construction similar to those used to process PVC or polypropylene. Drying of Kynar (PVDF) is usually not required; however, it has been shown to reduce some surface blemishes in film, sheet and pipe extrusion.
The extrusion process lends itself to the production of rods, tubes, pipes and profiles.
Kynar (PVDF) can be injection moulded to produce more intricate parts than can be achieved via machining.
Standard injection moulding equipment and tooling can be used to process Kynar (PVDF) resin. No specialty materials of construction are required, but chrome or nickel plating of polymer contact surfaces is recommended to prevent pitting.
Kynar (PVDF) components are used extensively in:
- high purity semiconductor market
- pulp and paper industry
- nuclear waste processing
- the general chemical processing industry
- water treatment membranes
Kynar (PVDF) finds preference as a pipe lining and tank lining material in plants handling corrosive chemicals.
Kynar (PVDF) also meets specifications for food and pharmaceutical processing industries.
Kynar (PVDF) film can be used for applications requiring long-term protection. The film is produced by monolayer or multilayer technology as thin, thick, wide or narrow (from 10 to 175 μm), allowing great freedom of design. The commercial range includes both mass-tinted and transparent films, which can be printed with a variety of designs. Film can be laminated onto thermoplastic, thermoset and coated metal supports
Kynar (PVDF) has gained success in the battery industry as binders for cathodes and anodes in lithium-ion technology, and as battery separators in lithium-ion polymer technology.
Kynar (PVDF) resin is a respected membrane material for applications ranging from bioprocess separations to water purification because it is extremely chemically resistant and thus well suited to aggressive chemical environments.
Kynar (PVDF) has a high temperature resistance, which makes it appropriate for applications that require high temperature cleaning. It tolerates ozone (an oxidant increasingly used for water purification) very well, compared to less robust polymer materials.
It is also a high purity resin with FDA and NSF listings, making it compatible with direct food/beverage contact applications.
Select grades of Kynar (PVDF) resin easily achieve the flame spread/smoke developed rating of 25/50 when tested in accordance with ASTM E 84. This enables Kynar (PVDF) pipe to be used in the plenum for applications such as corrosive waste drainage and laboratory chemical systems.