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An introduction to perfluoroelastomers.

Perfluoroelastomers are essential engineering materials that come into play where other elastomers prove insufficient in handling the harshest of chemical environments and/or extreme temperatures. These materials have the highest level of chemical and thermal resistance of any elastomer currently available. Perfluorinated elastomers (PFEs) are employed more often in many industry applications as operating conditions become increasingly intense. Additional reasons for use of PFE include reducing equipment downtime, handling more stringent environmental regulations (e.g., reduce emissions, leak prevention) and/or increasing lifetime performance of the finished article, such as a seal or progressive cavity pump.

The intention of this article is to provide technical insight into PFE polymers, their crosslink chemistry, capabilities for chemical and thermal resistance, and application considerations, along with detailed information on how to handle and work with PFE polymers. These materials are known to be very expensive, orders of magnitude more expensive than a typical fluorocarbon (FKM) or hydrocarbon elastomer, so proper handling and use are important.

However, a common misconception is that special equipment and/or environments are necessary to work with current PFE polymers. This is only partially true and only for semi-conductor processing and plasma applications where purity and cleanliness are critical considerations. Although there are some minor handling and mixing considerations for PFE polymers, special capital equipment investments on the user's part are not required. Typical FKM and hydrocarbon elastomer mixing and processing equipment is more than sufficient to work with PFE materials.

What are PFEs?

Perfluorinated elastomers, often referred to as FFKM or PFE, are copolymers of (primarily) tetrafluoroethylene (TFE) and perfluorinated vinyl ether (PFVE). In light of their inherent chemical inertness, they utilize a cure site monomer (CSM) to enable crosslinking. These CSMs typically contain a free-radically reactive bromine and/or iodine atom, or they carry a perfluoroalkyl nitrile group that can be induced to trimerize into a perfluorinated triazine network (see section on PFE crosslink chemistries).

Originally developed in the late 1960s, PFE polymers contain approximately 72.5% fluorine by weight and do not have any significant hydrocarbon segments in the polymer backbone.

Typical fluoroelastomers have lower fluorine content, approximately 65.9-70.5% by weight, and some available hydrocarbon character through the incorporation of significant amounts of vinylidene fluoride monomer. Hydrocarbons can provide a thermodynamic weak point in standard fluoropolymers. The bond dissociation energy (BDE) of a fluorocarbon bond is approximately 25-30% higher (depending on the reference used) than a hydrocarbon. The fluorocarbon BDE is 514 kJ/mol versus hydrocarbon bonds having a BDE of 338 kJ/mol.

Available PFE grades

Two primary grades of PFE are offered commercially. The first, referred to as a chemical grade, is crosslinked using suitable peroxide and a coagent. The chemical grades are good for applications where chemical resistance is the primary concern and temperatures will not likely be excessive. These grades tend to be less expensive compared to other PFE grades and are good for chemical handling and processing, cleaning and chemical etching processes in a variety of industries.

The other major PFE grade is referred to as a high temperature (HT) grade requiring some proprietary catalyst chemistry to form a triazine crosslink network or employing diaminobisphenol AF to create a benzoxazole-based cure. These materials are used when performance temperatures are in excess of 230[degrees]C. The perfluorinated triazine networks are capable of performing at temperatures continuously up to 315[degrees]C with minimal loss of properties and outstanding compression set resistance. The benzoxazole cures have an upper temperature use to about 275-280[degrees]C. The HT PFE grades are excellent for many thermally extreme processes with exposure to aggressive chemicals in aerospace, oil and gas, and chemical process industry applications.

Some specialty grades are offered as well, specific to the semiconductor market, having improved plasma resistance, varying degrees of clarity and "cleanliness." To get the maximum benefits of these materials, they should be handled, compounded and converted to finished articles in a clean room environment.

PFE crosslink chemistries

There are a few different crosslink chemistries used for PFE polymers. Some of the more common are free-radical/coagent cure, catalyst-induced triazine cure and formation of benzoxazole crosslinks. These methods will sufficiently crosslink their respective PFE polymers; however, they each have their individual benefits and deficiencies. Great care should be taken by the reader to choose the right cure chemistry and polymer combination for the end-use application being considered. Generally, a post-cure cycle is required in order to maximize the performance properties of either crosslink system.

The free-radical/coagent cure is most often related to crosslinking chemical PFE grades. Using a bromine or iodine containing cure site monomer (X-CSM; X = Br or I) and a suitable coagent to trap the peroxide generated free radicals, this type of crosslink is excellent for application temperatures up to about 230[degrees]C, and can be used in steam, acid and hot water applications. Polymers using an X-CSM and peroxide cure do not have as good thermal compression set performance as the triazine type cure.

To crosslink the chemical grade PFE polymers, triallyl isocyanurate (TAIC) has been shown to give the best overall performance as a coagent in regard to physical properties and heat resistance. TAIC can migrate away from the mixed polymer and homopolymerize readily during the cure cycle. As such, it can affect processability (e.g., mold fouling), and sometimes trimethallyl isocyanurate (TMAIC) is used instead. Some improvements to compression set have been obtained by using or blending TAIC with TMAIC, which will slightly retard the cure rate, allowing for a more ordered crosslink network to develop during cure. Chemical structures for TAIC and TMAIC are given in figure 1.

Because the reaction is peroxide initiated, these materials can become scorchy. Good temperature control and care during mixing and processing of the chemical PFE polymers is therefore vital. Typically, peroxides such as 2,5-dimethyl-2,5-di(t-butylperoxy) hexane (DBPH), shown in figure 2, are used to help impede scorching.

Scorch-protected peroxides, such as Luperox HP101XLP, can be used, if necessary. However, use of these materials in PFE can allow for the coagent to readily migrate to the surface during press cure, allowing for mold fouling and/or adhesion to the mold. These scorch-protected grades can be used in blends with other peroxides (like DBPH) to "fine tune" the cure rate.

To form the triazine crosslink network in the HT PFE grades, a proprietary catalyst is employed, reacting at a high temperature with nitrile containing cure sites (CN-CSM) to form a triazine ring structure, presented in figure 3.

This aromatic heterocyclic structure is very thermally stable and has shown good performance for continuous use to 315[degrees]C. Overall chemical resistance is exceptional. The CN-CSM can be incorporated into the polymer at various concentrations in order to obtain different crosslink densities. However, this crosslink chemistry is susceptible to hydrolysis by steam and/or hot water. Upon exposure, the ring will rapidly open through aromatic nucleophilic substitution pathways and the modulus will quickly decrease, affecting sealing and physical performance.

This weakness can be partially overcome by using a dual cure system with a peroxide/coagent and triazine forming catalyst, or using the peroxide/coagent system alone with the CN-CSM containing polymers. Again, both have some advantages and disadvantages. The benefit of using a dual cure system is that resistance to hydrolysis is significantly improved and good compression set performance is maintained. Using the peroxide/coagent system alone allows for much better performance in steam and hot water with fair-to-good compression set performance. Note that neat reagents for the peroxide and coagent should be used in both cases; carriers for these materials can contribute to moisture absorption. The detriment to both systems in upper temperature performance is reduced from 315[degrees]C to closer to 250-260[degrees]C. It is recommended that if either of these cure systems is used with the CN-CSM type polymers, a small amount of acid acceptor should be used.

Benzoxazole crosslinking (displayed in figure 4) of CN-CSM containing polymers exhibits good thermal stability as well to approximately 275-280[degrees]C.

Basic PFE polymer architecture

The basic polymer architecture for the PFE polymer (provided in figure 5) is made from tetrafluoroethylene and a perfluorovinylether.

This structure is essentially the same for both the peroxide and HT catalyst grades. The CSM can be an iodine-/bromine-containing monomer (peroxide cure) or a nitrile-containing monomer (catalyst cure). The [R.sub.f] group consists of a short chain fluorocarbon species.

Typical properties of PFE

General properties for typical quality control recipes for some various PFE polymer grades are presented in table 1. These recipes consist only of carbon black filler (15 phr N550), curatives and acid acceptors (peroxide cure only). The HT catalyst cure polymers are offered with varying concentrations of CSM. (Data for the HT polymers using benzoxazole, blended or peroxide cures are not included.) Please note that for compression sets up to 230[degrees]C, a 25% deflection was used, and for 300[degrees]C, only 18% deflection was imposed on the test specimens due to the much higher coefficient of thermal expansion of PFE polymers as compared to FKM or other hydrocarbon elastomers.

While the crosslink density for the peroxide/coagent cured grades is approximately the same, a difference in tensile and compression set performance can be observed from the difference in molecular weight of the two polymers. The differences in the CN-CSM concentration of the HT grades can be seen in the physical properties and compression set performance.

PFE chemical and thermal resistance

For all PFE polymers and cure types, resistance to a wide variety of chemicals, at varying concentrations and conditions, is exceptional. Chemistries, such as polar solvents, that would normally affect other fluoroelastomers negatively, have little to no effect upon the fully fluorinated elastomers. Examples of relative polymer performance are provided in table 2.

Thermal resistance of these materials is exceptional, as well, even with exposure to otherwise harmful chemistries. For the peroxide/coagent PFE grades, application use to approximately 230[degrees]C is feasible. The upper temperature range is significantly increased for the HT grades to 315[degrees]C because of the exceptional thermal stability of the triazine crosslink. Upper use temperatures are usually determined from using thermal analysis techniques, such as thermogravimetric analysis (TGA), to calculate expected performance and/or predetermined weight loss out to a specified time (e.g., 1,000 hours).

Readers are encouraged to perform their own analyses with PFE materials, given the wide range of compound formulations, chemistries, substance concentrations and application temperatures prior to using any PFE compound for a specified end-use. Please consult your respective PFE supplier's technical service representative for further information.

Application considerations for PFE

When considering a PFE material for use in an application, there are several questions that should be addressed and considerations to be had to make sure the right PFE polymer and/or compound is chosen. Questions to be asked include (as many answers/details should be provided as possible):

1. What is the application temperature and environment?

a. What chemicals will the finished article be exposed to?

b. Will there be exposure to steam or hot water?

2. What are the physical property specifications?

a. Tensile, elongation, modulus, durometer, tear resistance

b. Compression set, heat and fluid resistance, finished article color

3. What is the upper continuous use temperature?

a. Are there possible excursions to higher temperatures? What temperature and for how long?

4. Does the PFE need to be adhered to a substrate?

5. Will the article go into a static or dynamic application?

6. How will the article be manufactured?

Important considerations for engineering with PFE include:

1. Continuous use temperature for the application

a. For temperatures up to 230[degrees]C, use chemical PFE grades

b. For temperatures >230[degrees]C, use HT PFE grades

2. For steam and hot water applications, use peroxide/coagent cure alone or with catalyst cure

3. Use specialty PFE grades for exposure to plasma

4. Compression of PFE seals in application (related to high CTE)

a. Recommend <20% deflection for temperatures >270[degrees]C

b. Recommend <30% deflection for all other temperatures

For additional assistance related to PFE applications, please contact your technical service representative.

PFE coefficient of thermal expansion

An important consideration for PFE materials is their high coefficient of thermal expansion (CTE) compared to other materials, including standard FKM, as most PFE applications are seals. For a relative idea of the difference in CTE, a PFE was evaluated against other materials, including FKM and HNBR. All samples tested were peroxide cured academic formulations containing the same amount and type of filler (N990 carbon black). An additional sample of the HNBR was run to compare relative hardness compounds, containing 2.5 times the N990 of the other batches. An overlay of the TMA data is provided in figure 6.

Samples were run on a TA Instruments Q400 thermomechanical analyzer (TMA) heating and cooling between -50[degrees]C and 225[degrees]C at a rate of 5[degrees]C/minute using a macro-expansion probe for testing. A summary of the CTE values determined from analyzing the TMA data is available in table 3.

Comparing the HNBR and FKM samples containing 30 phr of N990 each, the relative CTE values are nearly equivalent and almost overlap in figure 6. With the increase of N990 in HNBR to reach a comparable durometer to the FKM, the average CTE decreases by ~26%. This would be expected with additional filler having significantly lower CTE values (e.g., carbon black = 0.20 [micro]m/m*[degrees]C); the actual CTEs of any finished compound will be formulation dependent. The PFE sample has an average CTE increase of ~30% compared to the FKM and HNBR with the same filler loading, and ~86% higher (on average) than the comparable hardness HNBR (75 phr N990).

PFE can continue to expand once at an elevated temperature and held there for a period of time. It may take a PFE compound up to 30-45 minutes to reach a fully expanded equilibrium at temperature.

Compounding and mixing PFE

Select the respective cure chemistry (noted above) appropriate for the grade of PFE being utilized and/or application conditions when formulating PFE compounds. Neat curatives and additives should be used whenever possible to achieve the best performance. This is especially true for steam and acid exposure applications. Keep in mind non-fluorinated additives will have a tendency to migrate out of the perfluoroelastomer due to its inherent oleophobic and hydrophobic nature. This migration may be so fast as to already occur during the mixing process. Any processing step involving shear conditions further contributes to this phase separation behavior.

Other components should be used as needed. For fillers, both carbon black and mineral fillers can be used. Typical carbon blacks used are N990, N550 and N330. Common mineral fillers used for FKM can be employed, e.g., barium sulfate, Min-U-Sil 5 [micro]m. Process aids that are common for FKM can be used with PFE and only as necessary. Peroxide PFE grades benefit from the addition of zinc oxide, acting as an acid acceptor and improving compression set performance. Note, however, that resistance to certain chemicals (e.g., acids) is further affected by the compound ingredients. For example, if acid acceptors, such as zinc oxide, are used, the acid resistance of the finished part is negatively affected.

Perfluoroelastomers can be mixed using standard elastomer mixing equipment. For best results, mill mixing is recommended. As with any fluoroelastomer mix, the mill or internal mixer should be very clean and free of contaminants, especially moisture (HT PFE grades are moisture sensitive). Also, small amounts of standard FKM (even as little as 500 ppm) can have an effect on cure rate. It is recommended that all other raw ingredients be pre-blended to better facilitate incorporation into and dispersion throughout the polymer. For highly filled compounds using a peroxide cure, performing a two-pass mix would be in order.

When starting the mix, the mill gap should be wider than usual. After passing the PFE polymer through once or twice, slowly close the nip until a normal mix gap is reached. A big key to mixing PFE is to keep the compound very warm to the touch. Add the pre-blended ingredients in several additions. Adding too quickly can cause the polymer to cool rapidly and crumble apart. If this happens, the material can be patiently worked back together.

Stay at, or below, the recommended safe temperature for peroxide mixing. Do not use release dips on PFE compounds.

Processing and adhesion of PFE compounds

When processing PFE compounds, it is recommended that the compounds be refreshed and/or warmed (~60-70[degrees]C) prior to use. This will help with flow, reincorporation of any additive(s) that may have begun to migrate out of the mixed PFE polymer, and further improve filler dispersion. As with the mixing, make sure to start with a wide nip and slowly close as the material warms. Cold PFE compounds and a narrow mill gap can damage equipment.

Compression molding is recommended for manufacturing PFE articles. They can also be extruded easily Please note that a typical PFE compound will have shrinkage of ~3.5-5.5% after post-cure, and this factor should be considered for mold design. The actual value will be formulation dependent. Therefore, pre-forms should be as close as possible to the size of the finished part to decrease the distance the material has to flow in the mold, as well as potentially limit very expensive flash. Mold release is recommended for all peroxide cured compounds for reasons mentioned in the crosslink chemistry section. Stoner A373 fluoroelastomer release agent has been found to work very well for PFE compound molding. Please be sure to closely follow the supplier's application instructions.

Cure conditions will vary with the crosslink system used. For a peroxide cure, 10-15 minutes at 177[degrees]C is a recommended starting cure cycle. The catalyst-induced triazine cures require a slightly higher cure temperature. Typical cure times are 10-15 minutes at 188[degrees]C in order to attain sufficient green strength out of the mold. Typical mold processing should be employed, making sure to bump molds 3-4 times to release any trapped gases. For thicker parts, reduce cure temperatures slightly and lengthen the cure cycle.

As with the press cure conditions previously discussed, the post-cure conditions are curative dependent, as well. Recommended starting settings for peroxide initiated systems are 4-16 hours at 232[degrees]C. Actual time will depend on final performance requirements. The HT PFE grades are typically post-cured in air for 24 hours at 250[degrees]C. Sometimes, a post-cure under nitrogen atmosphere is applied in order to achieve ultimate high temperature compression set resistance.

When bonding to PFE materials, only use an uncrosslinked compound. It is extraordinarily difficult to bond to cured perfluoroelastomers. The use of process aids should be minimized or completely eliminated, if possible, when attempting to bond PFE. While no special adhesives currently exist for perfluorinated materials, experience has shown that good bondability can be achieved using Lord Corporation's Chemlok 5150 or Dow Chemical Corporation's Megum 3290-1 and Thixon 300/301 adhesives.

As with any adhesive application process, closely follow the supplier's instructions for surface preparation and adhesive application. Sometimes a pre-bake of the adhesive layer at a low temperature (150-160[degrees]C) for ~5 minutes can be useful in adhesion promotion to metal substrates prior to molding. After molding the bonded article, a post-cure will likely be necessary. Because the adhesives cannot handle extreme temperatures for long periods of time, it is recommended that a shorter post-cure time be used at typical post-cure temperatures (~232[degrees]C), or post-cure for a longer time using a lower temperature (200[degrees]C) when bonding PFE.

PFE markets and applications

Historically, PFE has been used primarily in exotic and/or highly technically sensitive applications, such as the semiconductor industry. More recently, the material is gaining a wider acceptance and necessity in the transportation (not just aerospace), chemical processing industry (CPI), and oil and gas markets due to increasingly demanding application environments. The high cost of these materials may be a barrier for many applications. PFE is now positioned in many markets where FKM was almost 35 years ago. While very expensive compared to other materials, it is often chosen when other material choices were unsuccessful. The costs associated with the utilization of PFE in desired applications are in line with the expected benefits gained from the efficacy of the final material solution.

Much of the initial use for PFE was in aerospace, for use with some of the very corrosive lubricants and fuel additives for airliners, not to mention the high temperature exposure for many seals and other elastomeric components. In the semiconductor market, the need for plasma and thermal resistance made PFE an absolute requirement in semiconductor processing and in finished electronic devices.

As automobile and heavy equipment engines continue to see more aggressive fluids and higher operating temperatures, the use of PFE in respective components is expected to increase, as well, in these industries as they attempt to squeeze more performance and mileage out of less fuel.

As the price of oil is projected to rise, the oil and gas industry has to work harder and spend more to achieve economic yields. The days of simply drilling into the ground and having oil come out are gone, according to most in the industry. Deeper wells are being drilled, at higher and higher temperatures in more chemically aggressive environments. Horizontal drilling requires special engineering feats to extract the "black gold" from the ground. Events such as the recent Macondo incident in the Gulf of Mexico have made reliability of highly engineered components a priority. The need for materials that will not fail under extreme conditions is more critical than before. PFE can fill that gap for many applications.

There is a variety of applications for PFE in the CPI market, as well. From taking oil and refining it into other useful chemistries, to fluid transport and machinery/material handling equipment, PFE is finding a new home in many areas where improved chemical resistance is necessary. Applications include stators/progressive cavity pumps for moving chemicals, line stops for use in maintenance of high temperature and chemically aggressive continuous processes at CPI plants, and seals/diaphragms for valves and pumps having exposure to harsh materials, extending their useful life and providing increased reliability and safety.

Summary

Perfluoroelastomers can be used for the most extreme of chemical and thermal conditions in a wide range of industries. Where other materials fail, PFE can perform successfully. Specialized equipment is superfluous for processing PFE polymers. Some simple training and know-how are all that is required. Actual performance is dictated by many factors, including but not limited to compound formulation, crosslink chemistry and density, polymer molecular weight, manufacturing conditions and application conditions. All factors should be weighed thoroughly prior to choosing a PFE grade.

This article is based on a paper presented at the 184th Technical Meeting of the Rubber Division, ACS, October 2013.

References

(1.) CRC Handbook of Chemistry and Physics, 91st Edition, 2010-2011.

(2.) A.L. Moore, Fluoroelastomers Handbook: The Definitive User's Guide and Databook, 2005.

(3.) PC. Painter and M.M. Coleman, Fundamentals of Polymer Science, 2nd Edition, CRC Press, 1997.

by Ed Cole, 3M Advanced Materials Division

Table 1--general properties for various PFE grades and cure
systems

Property                               Peroxide/coagent cure

3M Dyneon                                PFE 60Z         PFE 90Z
  PFE grade
Mooney (ML 1+10)                              60              98
  @ 121[degrees]C
Tensile (psi)                              2,600           3,035
Elongation (%)                               165             155
100% modulus (psi)                         1,600           1,545
Durometer, A                                  75              75
Compression set
  -70 hrs. @ 200[degrees]C                    49              29
  -70 hrs. @ 230[degrees]C                    56              37
  -70 hrs. @ 300[degrees]C                    --              --
TR-10 ([degrees]C)                  -2[degrees]C    -2[degrees]C
Brittleness pt. ([degrees]C)       -35[degrees]C   -35[degrees]C

Property                             HT catalyst/triazine cure

3M Dyneon                               PFE 81TZ        PFE 131TZ
  PFE grade                        Low crosslink   Med. crosslink
Mooney (ML 1+10)                              80               80
  @ 121[degrees]C
Tensile (psi)                              1,720           2,300
Elongation (%)                               230             165
100% modulus (psi)                           685           1,310
Durometer, A                                  71              77
Compression set
  -70 hrs. @ 200[degrees]C                    --              --
  -70 hrs. @ 230[degrees]C                    27              20
  -70 hrs. @ 300[degrees]C                    50              43
TR-10 ([degrees]C)                  -2[degrees]C    -2[degrees]C
Brittleness pt. ([degrees]C)       -35[degrees]C   -35[degrees]C

Property                        HT catalyst/triazine cure

3M Dyneon                             PFE 191TZ
  PFE grade                       High crosslink
Mooney (ML 1+10)                              80
  @ 121[degrees]C
Tensile (psi)                              2,300
Elongation (%)                               110
100% modulus (psi)                         2,200
Durometer, A                                  80
Compression set
  -70 hrs. @ 200[degrees]C                    --
  -70 hrs. @ 230[degrees]C                    15
  -70 hrs. @ 300[degrees]C                    33
TR-10 ([degrees]C)                  -2[degrees]C
Brittleness pt. ([degrees]C)       -35[degrees]C

Table 2--generic chemical resistance
comparison of fluoropolymers

Base chemistry              Example    PFE     FKM    TFE/P

Aliphatic                   Propane    ++++   ++++       ++
  hydrocarbon
Aromatic                    Benzene    ++++   ++++       ++
  hydrocarbon
Organic acid            Acetic acid    ++++      +      +++
Inorganic acid        Sulfuric acid    ++++   ++++   Varies
Inorganic base     Sodium hydroxide    ++++     ++     ++++
Ethers                         MtBE    ++++    +++        +
Ketones                     Acetone    ++++      +       ++
Nitrous oxidizers          Ammonium    ++++    +++     ++++
                            nitrate

Table 3--numeric value comparison for CTE and [T.sub.g] by TMA of
various polymers

Sample ID           Durometer     Heat up CTE   Cool down CTE
                          (A)     ([micro]m/m     ([micro]m/m
                                x [degrees]C)   x [degrees]C)

HNBR 30 phr N990          56            238.0           284.2
HNBR 75 phr N990          68            175.3           213.1
FKM 30 phr N990           69            260.5           295.1
PFE 30 phr N990           75            340.3           384.2

Sample ID            Heat up        Cool down
                   [T.sub.g]        [T.sub.g]
                                 ([degrees]C)

HNBR 30 phr N990      -26.10           -25.16
HNBR 75 phr N990      -24.43           -25.28
FKM 30 phr N990       -24.35           -25.54
PFE 30 phr N990        -1.61            -2.95
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Date:Dec 1, 2013
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