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Application Of Nanotechnology In Fluoropolymers

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Nano-fillers differ from conventional reinforcement in terms of the dimensions of the reinforcing phase being in the nanometres scale, i.e. three to four orders of magnitude smaller. In many cases, the geometric distribution of these discrete phases may also be unique, such as the formation of co-continuous morphology or interpenetrating network.

Conceptually, nano-reinforcement of amorphous fluoropolymers presents a new opportunity in overcoming the detrimental effects associated conventional compounding technology, such as contamination and reduced heat and chemical resistances
The addition of filler materials to elastomers is widely accepted, and the ingredients now widely varied. However, new advances in the use of nano-technology offers some exciting opportunities to improve physical properties in some important areas.

Common compounding techniques rely upon particulate fillers to reinforce the elastomer. Without reinforcement, the elastomers strength would come from a combination of chain entanglements, crosslinking and strain-induced crystallization. Enhancement of physical properties by utilizing fillers is widely reported, however some of the mechanisms behind the effects are not yet fully understood. This is especially the case for carbon black reinforcement: Hamed(1) discusses a number of different mechanisms for carbon black reinforcement within rubbers which include “filler induced networks” and “crack deflection”. Medalia and Kraus(2) also discuss the possibility of chemical bonding at the carbon-polymer interface - this effect was further analysed by Bertrand, Weng and Lauer(3) using Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS).

Although the use of carbon black as a reinforcing filler is common, it is not appropriate for all applications. For some uses, just the colour alone is not acceptable, for others, associated impurities can be problematic.

There are a number of other filler types that can be considered in place of carbon black, for example clay fillers. However, these materials tend not to offer the same degree of reinforcement as the carbon blacks, and have complications related to chemical resistance. Each filler type, even though solid, will have an associated acidity or alkalinity (‘pH') due to the presence of various chemical groups on the particle surface. An excellent investigation of the surface chemistry of carbon blacks was performed by Darmstadt et al(4).

For some silicas though, the pH can be as low as 4.0 - and, depending on the elastomer cure system, if these are not compounded properly the crosslinking reactions can be suppressed completely. A comprehensive review of these effects are discussed by Fowkes(5).

Surface activity can also lead to significant complications related to filler selection, where the chemical compatibility can be greatly affected. As discussed above, each of the fillers have an associated ‘pH' - this is essentially a function of the molecular groups on the surface of the particle and the media into which it is immersed. Hertz discusses this effect (6), and uses elastomers in sour gas environments as his example.

Briggs, Edwards and Storey(7) also offer evidence to show the mechanism of absorption of chemicals, causing swelling in rubber compounds. Here, the authors examine the effect of filler solubilities as a mechanism for the swelling of elastomers in water.

Within the sealing industry, one of the most challenging environments are found within the semi-conductor processing sector. Here, seals are exposed to hot, silane-based materials or halogenated plasmas, and parts are required to be ultra-clean. On degradation of the polymer, these fillers can fall onto micro-chip wafers, thus destroying the product - this is known as ‘particulation'. An example of the effects of plasma attack is shown in Figure 1.

Nanotechnology

Utilising nano-technology could aid in the reduction of some of the negative aspects discussed above. The ultimate aim is to have a filler system which imparts good reinforcement and is essentially chemically inert.

Numerous routes for nano-reinforcement have been suggested, with one of the most common method being silica reinforcement. This work is reported widely, especially in materials involving polydimethylsiloxane (PDMS) (8) (9). The physical properties are also reported (10) (11). In this work, the polymer is allowed to swell in alkoxysilane until equilibrium is reached. The alkoxide can then be precipitated into nano-size silica particles. With proper compatibilisation and controlled phase separation, the resultant silica can be intimately dispersed within the polymer, forming a nano-composite. This process offers the potential of giving not only an extremely pure final product, but a product where the surface of the silica particles and it's geometric distribution can be controlled (12) (13).

However, even with careful control of the silica surface properties, the silica still has the associated problems of it's pH, potentially leading to excessive swelling in some environments.

Another potential route of nano-reinforcement is with crystalline domains of a perfluoropolymer, which are introduced during the polymerization step of an elastomer. The resulting material would have the crystalline perfluoropolymer domains intimately mixed into the elastomer matrix. This method of nano-reinforcement is a novel approach, with little literature available on the subject. This form of reinforcement offers a number of significant advantages:

1) As the filler is inert, it will not interfere with any cure reactions.
2) Offer no disadvantages in terms of chemical compatibility.
3) Purity: as the ingredients are all synthetic, sources of impurities can be reduced or eliminated.

In the evaluation of nano-technology, Precision Polymer Engineering Ltd selected the crystalline perfluoropolymer nano-filler route for further analysis. Perfluoroelastomer (FFKM) material was used as the polymer in which to test the effectiveness of the novel filler system.

Nano-filled Perfluoroelastomer

The initial premise of utilizing perfluoropolymer nano-fillers offers some significant advantages in terms of chemical resistance, but tests were required to determine whether the system offered any physical reinforcement. To address this, a series of stress-strain tests were performed, the results are shown below in Table 1.

Table 1: Reinforcement effect of perfluoropolymer nano-filler
Physical Property FFKM (no filler) FFKM - perfluorinated Nano-filler
Tensile Strength (MPa) 8.6 18.3
Stress at 100% Strain (MPa) 1.5 3.5
Elongation at Break (%) 280 331
Hardness (Shore A) 52 64


When compared to an unfilled grade, the nano-filled material does indeed give some reinforcement. It can, therefore, be concluded that the perfluorinated nano-filler system would fulfill the main function of a filler - to offer some reinforcement.

With both the elastomer and filler being organic in nature, it is also possible that the material could be susceptible to degradation at elevated temperatures. By accurately measuring weight loss with increasing temperature, any degradation in the polymer or filler structure can be identified. Thermogravimetric analysis (TGA) was used to gain an understanding of how the material would perform at elevated temperatures. A Netzsch F1 Phoenix TGA was used in the analysis, with a 10°/min ramp to 700°C. A nitrogen atmosphere was used to 550°C, at which point it was switched to an air/nitrogen mix. The resulting chart is shown in Figure 2.

Figure 2: TGA Analysis of Nano-particle Filled Elastomer



















Figure 2 shows the benefits of using the perfluoro nano-filler: on degradation, there is no residue generated. The typical inorganic-filled system showed 17% of residual material. The new crystalline perfluoropolymer filler system degrades totally into gaseous products, reducing, or possibly even eliminating the potential for particulation.

Also in Figure 2, it can be seen that a polymer system utilising inorganic fillers does start to degrade at a slightly higher temperature (an onset temperature approaching 425°C), with the new nano-filled material starting degradation in the region of 400°C. These temperatures however are still greatly in excess of any that would be seen in an application environment.

Based on the above results, further functional tests were performed to determine the service performance of the nano-filled material. A number of tests were performed in different plasma environments, the results are shown in Figures 3 and 4. The figures chart the etch rate of comparable elastomers.

Figure 3: C4F8 Plasma Testing at 150°C
Etch time: 60 minutes, Etch Pressure: 8 0mT,
Distance from source: 10 cm, Plasma density: 1012 cm3



Figure 4: SF6 Plasma Testing
Etch time: 60 minutes, Etch Pressure: 80 mT,
Distance from source: 10 cm, Plasma density: 1012 cm3

The results show that the nano-filled grades performed exceptionally well in the plasma testing, with one result out-performing the industry benchmark of polytetrafluoroethylene (PTFE).

To assess chemical compatibility, a series of chemical immersions were performed, the results are shown below in Table 2.

Table 2: Chemical Immersion Testing
Chemical Conditions Volume Swell
[%] Hardness
Change
[IRHD]
Acetaldehyde 70 hours at 40°C 5% -4
Ethylene diamine 70 hours at 23°C 3% 0
Glacial acetic acid 72 hours at 100°C 7% -5
Glacial acetic acid 336 hours at 100°C 6% -5
Hydrochloric Acid (37%) 70 hours at 80°C 6% -2

Results show that the nano-filled grade has no significant weakness in any of the fluids tested, with all showing volume swells of less than 10% in each of the tests.

The final test for the product was to address concerns over purity. Even when manufactured in a clean room environment, by adding reinforcing fillers, there is a potential for contamination, as this is very dependent upon the purity of the reinforcing agents.

To test the effect of the new nano-filler systems with respect to purity, a hydrogen fluoride extraction test was performed. In this test, a seal is immersed in hydrofluoric acid for a specific period of time, once complete, the acid is analysed by to determine the presence of any metallic ions. The results are shown in Figure 5.

Figure 5: Extraction Test Results


The results, measured in parts-per-billion (ppb) show excellent results, with very little in terms of leachable components.

To further confirm the purity of the new nano-filled grades, scanning electron microscopy (SEM) analysis combined with electrodispersive x-ray (EDX) analysis was used to determine if any foreign matter was present within an actual part. The results are shown below in Figure 6.

Figure 6: EDX analysis of nano-filled FFKM


These results show that the only detectable elements were carbon and fluorine, again confirming that the new nano-filled material has particularly high purity.



Conclusions

It has been shown that in the most aggressive environments, regular filler reinforcement can have some weaknesses. Literature reviews have shown that the use of silica nano-fillers formed in-situ is becoming more common, but would have some of the same associated weaknesses in terms of chemical resistance.

Testing of the new crystalline perfluoropolymer nano-fillers has shown that the use of nano-technology can have a significant role in elastomeric seals, with excellent results in terms of plasma resistance and good results in chemical immersions.








References


(1) Gary R. Hamed, “Rubber Reinforcement”, American Chemical Society - Rubber Division, Rubber Chemistry and Technology, Ref. V73-I3-0524.

(2) A.I. Medalia, G.Kraus, “Reinforcement of Elastomers by Particulate Fillers”, Science and Technology of Rubber, Second Edition 1994, pp394-5.

(3) P. Bertrand, L.T. Weng, Lauer, “Interaction Between Rubber and Carbon Black Studied by TOF-SIMS', American Chemical Society - Rubber Division, Rubber Chemistry and Technology, Ref. V75-I4-0627, 2004

(4) H. Darmstadt et al, “Surface Activity And Chemistry Of Thermal Carbon Blacks “, American Chemical Society - Rubber Division, Rubber Chemistry and Technology, Ref. V73-I2-0293, 2000

(5) F.M. Fowkes, “Acid-Base Contributions to Polymer-Filler Interactions”, American Chemical Society - Rubber Division, Rubber Chemistry and Technology, Ref. V57-I2-0328, 1984

(6) D. Hertz, “Theory of Rubber Compounding”, Energy Rubber Group Educational Symposium, September 24-25 1991, p7.

(7) G.J. Briggs, D.C. Edwards, E.B. Storey, “Water Absorption of Elastomers”, American Chemical Society - Rubber Division, Rubber Chemistry and Technology, Ref. V36-I3-0621, 1963.

(8) Y.Ikeda et al, “Reinforcement of styrene-butadiene rubber vulcanizate by in-situ prepared by the sol-gel reaction of tetraethoxysilane”, J. Material Chem., 1997, 7(8), 1497-1503.

(9) Jianye Wen and James E. Mark, “Precipitation of Silica-Titania Mixed Oxide Fillers into Poly(dimethylsiloxane) Networks”, American Chemical Society - Rubber Division, Rubber Chemistry and Technology, Ref. V67-I5-0806, 1994.

(10) Kanji Kajiwara, Yoshihiro Kameda, Yuko Ikeda, Hiroshi Urakawa, “Biaxial Tensile Behavior Of Rubber Vulcanizates: I. Silica And Gum Stocks”, American Chemical Society - Rubber Division, Rubber Chemistry and Technology, Ref. V77-I4-0611, 2004.

(11) McCarthy, D. W., J. E. Mark, et al. (1998). "Synthesis, Structure, and Properties of Hybrid Organic-Inorganic Composites Based on Polysiloxanes 1: Silica in Poly(Dimethylsiloxane)." Journal of Polymer Science: Part B: Polymer Physics 36: 1167-1189.

(12) Vu, B. T., J. E. Mark, et al. (2000). "Surface Modification of Silica Fillers Formed in-situ for the Reinforcement of Polydimethylsiloxane Networks." Proceedings of the American Chemical Society Division of Polymer Materials: Science and Engineering 83: 411-412

(13) Vu, B. T. N., J. E. Mark and D. W. Schaefer (2003). "Interfacial modification for controlling silica-polysiloxane interactions and bonding in some


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