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.

Product Properties

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.

Creep properties

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.

Temperature resistance

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.

Electrical properties

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.

Chemical resistance

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.

Environmental properties

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.

Processing methods

Extrusion

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.

Injection moulding

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.

Applications

Industrial

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.

Foams

Kynar (PVDF) closed cell foams are available in sheets or rolls. Kynar (PVDF) foams are of very high purity, very low flammability and are UV and corrosion resistant.

Films

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

Batteries

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.

Membranes

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.

Cables

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.

PEEK Seals – Numerous Applications, Many Choices

As a polymer, PEEK is most often compared with PTFE. The two have multiple similarities including good temperature resistance, chemical inertness and dielectric strength. When it comes to pure physical strength however, PEEK moves ahead on two counts.

First – the absolute strength of the material is much higher. With a higher tensile strength and hardness, PEEK is preferred to PTFE in applications where dimensional stability over prolonged physical strain is required. Although PTFE does have fillers, such as glass and carbon, which allow for increased stiffness, it still does not compare with PEEK on this metric.

Second – PEEK has a lower specific gravity (1.35 against 2.25 for PTFE). As a result, in applications where the overall weight of the assembly needs to be minimized, PEEK emerges a winner.

One such application where PEEK is highly sought after is in the seals industry. Seals themselves include a huge range of polymers, elastomers and metals, each of which rely on the specific characteristics of the material being used to achieve effectiveness in its application.

Types of PEEK seals

Piston Ring Seals

Piston rings are used primarily to aid wear absorption on the outer diameter of the piston shaft. PEEK is hard enough to withstand the extensive wear induced within the piston, but not hard enough to damage the metal components themselves. The rings are usually machined from a PEEK bush and have different types of cuts, which aid in installation and performance.

Ball Valve Seats

Ball valve seats show a predominant preference for PTFE, as they require a soft material that yields easily to the shape of the ball valve. However, there are a significant number of PEEK seats being used in high-performance valves, where both the PTFE and the metal are machined to ensure a proper fit. Typically, we see these being used in valves employed on oil-rigs or power plants, where the high temperatures indicate a requirement for a polymer slightly tougher than PTFE.

Rotary Shaft Seals

We have developed compounded grades of PTFE with PEEK to cater to the rotary shaft seals market. The combination of PTFE and PEEK is a powerful one. The PTFE provides a boost to the self-lubrication properties, while the PEEK adds strength. Although they work well together, specific applications do call for pure PEEK. The purpose is similar to that of the piston ring, except here the shaft moves radially. PEEK again serves the purpose of being able to withstand wear at high RPMs, while being soft enough not to damage the metal in the event of misalignment or seal failure.

Ball and Butterfly Valve Seats

A number of different materials are used in this application, including PTFE, Delrin and UHMWPE. PEEK finds acceptance specifically in applications with high pressures and temperatures. Butterfly valves are an integral part of any fluid regulatory system, including hydroelectric power plants, oil and gas refineries and shipping.

Manufacturing process

PEEK seals and seats are made primarily via machining. It is possible to injection mould the components directly, but this involves extensive tooling. Furthermore, the precision needed on the part’s dimensions would dictate the need for further machining. Hence, unless the volumes are vast, it is most likely machined from a bush.

The bush itself may be either extruded or compression moulded. Extrusion offers higher productivity and longer length parts, but is again dependent on the correct type of tooling being available. Compression moulding is cost effective and allows the dies to be modified easily, so that the moulded part is made with minimal excess material (a very key criterion when dealing with an expensive material like PEEK). The issue with compression moulding is that it is a slow process with very limited productivity.

So looking at the trade-off between productivity and tooling cost, an OEM can accordingly decide which method to adopt, depending on the volumes.

Variants in PEEK

While most specifications call for pure, virgin (unfilled) PEEK, there are requirements for filled variants also. Most commonly, PEEK is used with a 30% Glass or Carbon filler to aid properties such as creep, dimensional stability and flexural strength.

As mentioned above, PEEK also does well with PTFE. More specifically, compression moulding best-practices sometimes recommend the addition of 5% PTFE into the PEEK mould, as this allows for better self lubrication of the material, while letting it maintain its superior strength.

Another polymer well suited to blending with PEEK is Polyimide. Although the blend is not nearly as proven as the regular filled variants, initial studies show that the addition of Polyimide allows PEEK to maintain its flexural modulus over a much high temperature range as against unfilled PEEK.

It is difficult to combine too many other polymers with PEEK, simply because the temperatures needed to process PEEK far exceed the melting points of most of these polymers.

A word on PEK

PEK or PAEK has recently emerged as a competitor to PEEK. Industry experts have observed that while PEK does match PEEK on most metrics, it’s long-term effectiveness in maintaining its properties is still being tested.

We recently received a failed seal from an OEM, asking us to analyse whether it was PEEK. After testing it in a lab, it was found that the part was made using PEK. The end-user claimed that the part had only survived a few months in his valve assembly, before failing. This may have been a one-off incident, or could also point to the improper processing of the PEK part. However, it is useful to keep in mind.

Conclusion

PEEK is well known as a versatile polymer. Seals and seats are one more application where this material finds application. The product, however, requires precise dimensional tolerances that not all processors are able to offer. In addition to this, the availability of variants both within PEEK and amongst competing polymers makes the choice of material an exercise that the OEM must take very seriously, before committing one way or another.

Charting the standards used in defining PTFE properties

It was recently brought to our notice by an astute client that the data quoted in many of the generic online sources did not give a complete picture of the values and correct test methods needed in checking the properties of PTFE.

A quick online search of a given property of PTFE churns out a number of data sheets from various supplier websites. And although the values and standards more or less match across these sources, our own study has revealed the following:

1. Some of the standards quoted are incorrect
2. The values quoted do not have any reference as many of the standards only specify the test method and not the value reference

As a result, with an obscure polymer like PTFE, we find that information has been carried forward from older data sheets and passed on until no one is very sure what the “correct” value is anymore. We ourselves have reached a dead-end on a number of metrics, but we have done our best to fill the gaps using verifiable data.

Let’s look at point (1) above. The most commonly quoted standard for PTFE is ASTM D 1457. We see this standard in a number of places and only after trying to buy a copy online were we informed that ASTM D 4894 had replaced the ASTM D 1457 in 2001.

Clients who – due to the effect of legacy – still refer to the ASTM D 1457 sometimes get upset when we send them test reports quoting ASTM D 4894 and it requires some discussion with their QA team before the new standard is accepted.

However, even the ASTM D 4894 only applies to virgin PTFE. For filled grades of PTFE, we refer to the ASTM D 4745. This again requires a discussion with the client as is especially problematic when the client orders a very specialized grade. Since the ASTM D 4745 only covers the more general filled grades of PTFE, clients who order an irregular grade feel frustrated that there is no standard pertaining to their requirement.

Both the standards, however, do provide some basic values of tensile strength, elongation and specific gravity, which help in checking whether the properties attained after testing are in line with the requirements.

However, as the table below shows, very few of the standards actually give any values. For a whole list of properties, the standards only tell you how to check the value, but do not make any recommendations on what those values should be. Furthermore, due to PTFEbeing a niche polymer, some of the standards – such as ASTM D 2240 – actually pertain to other materials and the test method is simply employed for PTFE.

	Virgin		Filled grades
	Standard	Value in standard	Standard	Value in standard
Density	ASTM D 4894	Yes	ASTM D 4745	Yes
Avg. Particle Size	ASTM D 4894	Yes	ASTM D 4894	Yes
Tensile Strength	ASTM D 4894	Yes	ASTM D 4745	Yes
Elongation at break	ASTM D 4894	Yes	ASTM D 4745	Yes
Shore D Hardness	ASTM D 2240	No	ASTM D 2240	No
Linear Expansion Coefficient (-50°C to +15°C)	ASTM E 831 / ASTM D 696	Yes (696)	ASTM E 831 / ASTM D 696	No
E-modulus (tensile)	ISO R 527	No	ISO R 527	No
Wear resistance (with Taber abraser method)	ASTM G 195-08	No	ASTM G 195-08	No
Deformation Under Load – Total deformation after 24H	ASTM D 621 A	No	ASTM D 621 A	No
Static Coefficient of friction	ASTM D 1894	No	ASTM D 1894	No
Dielectric Strength	ASTM D 149	No	ASTM D 149	No
Dieelectric Constant	ASTM D 150	No	ASTM D 150	No

As mentioned earlier, my client was curious to know what benchmarks were being followed when we quoted the values expected for each metric. However, we were unable to find any organisation that actively published data on PTFE and its filled grades.

In trying to trace back the values seen across so many data sheets (they are all in the same range, so we assumed they have some common source), we were able to find references old manuals released by DuPont, Dyneon and Daikin from where these values were obtained. Obviously, once we referenced DuPont, the client was satisfied and we were able to neatly define both the correct standard and the value with the proper reference.

It is however interesting to note that as widely used and accepted as PTFE is, there still does not exist any up-to-date properties standard that can be used by manufacturers as reference. The DuPont website does have values of virgin PTFE – but they reference the ASTM D 1457 – which suggests that maybe the information is dated. Not to say that the values would have changed significantly, but QA is a continuous process and something published within the last decade might offer a lot of support to both manufacturers and OEMs alike.