Fluorosilicone Liquid Silicone Rubber

Transcription

1 Fluorosilicone Liquid Silicone Rubber Ronald Romanowski, Senior Industry Specialist Dow Corning Corporation, Kendallville, Indiana Paul T. Irish, Development Chemist Charles W. Olsen Jr., Global Marketing Specialist Lauren M. Tonge, Senior Development Specialist Dow Corning Corporation, Midland, Michigan International Silicone Conference 23 Akron, Ohio - USA

2 ABSTRACT Liquid silicone rubber is designed to produce rubber parts more efficiently and economically because LSR offers: 1. Fast cycle times 2. Clean operation resulting from enclosed systems 3. Automated processing Fluorosilicone rubber is a high-performance elastomer particularly suited to environments where fuel, oil or solvent is present. Advances have been made in silicone rubber technologies that allow the process efficiencies of LSR to be coupled with much of the fluid resistance capabilities of fluorosilicone rubber. Further advances in the processing of fluorosilicone liquid rubber have been made which allow a wide variety of features to be added to the product, which can bring significant benefit. Among the innovations which will be discussed in this paper are the inclusion of mold release to improve automated processing, the use of self-bleeding lubricants to ease assembly, the ability to pigment and the range of durometer which can be achieved. This paper will highlight these and other innovative possibilities and benefits of fluorosilicone liquid silicone rubber. INTRODUCTION Liquid rubber molding offers many advantages to the rubber fabricator particularly for highvolume parts. Even though the difference in price between the high consistency fluorosilicone, FSR, and the liquid fluorosilicone rubbers, FLSR, are greater than the case with dimethyl silicone rubber, the advantages of the liquid silicone product form justify its use. Fluorosilicone liquid rubber molding provides the ability to perform long runs with minimal manual intervention. Nearly unattended automatic operation is possible; in regions of the world with high labor costs, a single operator can maintain an entire bank of liquid injection molding machines. Europe has seen tremendous growth in the use of LSR in a wide variety of products, from automotive to baby care items. This trend is now moving the United States as fabricators realize the business advantages of adding LSR at the next facility expansion. Although LSR is more expensive to produce as a raw material it offers savings to the fabricator through process efficiency. As with LSRs, FLSRs can be formulated to meet various solvent or oil resistance needs and addition of ingredient packages to enhance release or lubrication. Page 2 of 8

3 TECHNOLOGY DEVELOPMENT Dow Corning scientists have prepared a full range of liquid fluorosilicone and fluorosiliconedimethyl silicone copolymers in the laboratory to evaluate their mechanical and chemical resistance properties. The benefits of a liquid silicone rubber are well recognized; the challenge was to develop liquid silicone rubber with exactly the right oil and solvent resistance to meet customer needs. There are two approaches to tailoring the oil resistance properties: blends and copolymers. Blends of fluorosilicone and dimethyl silicone polymers exhibit a range of swelling characteristics between that of dimethyl and fluorosilicone. However, because of the difference in performance characteristics of the two polymers and their poor miscibility in one another, blends of dimethyl and fluorosilicone polymers exhibit lower properties than copolymer. The following copolymer and blends were compared in a standard formulation without individual optimization. Copolymer Data Mol % Trifluoropropyl* Durometer (Shore A) Tensile (psi) Elongation (%) Compression Set (%) 13 NA NA NA NA Tear B (ppi) Resilience (Bashore) Extrusion Rate (gm/min.) *e.g. 4 mol % trifluoropropyl = 4 mol % trifluoropropyl methyl siloxane and 6 mol % dimethyl siloxane Table 1. Rubber Properties of Lab Examples of FLSR prepared from Fluoro/Dimethyl Copolymers or In the blend until high concentrations of fluorosilicone polymer are reached, the dimethyl phase remains continuous with isolated pockets of fluorosilicone rubber. Because of this effect even at blends of 6% fluorosilicone, the dimethyl rubber remains continuous and the material exhibits typical swelling properties of dimethyl phase but with reduced mechanical strength properties due to the fluorosilicone rubber component. The solution to this problem is to create a new polymer consisting of a copolymer of both dimethyl and trifluoropropylmethyl. This copolymer is produced from the polymerization of the respective components by controlling the ratio of the monomer feedstocks. One is able to tailor the level of dimethyl to the level of trifluoropropylmethyl in order to dial-in exact swelling characteristics while retaining the best properties possible. Because this mixing takes place on a molecular level there is a high degree of interaction between the dimethyl and trifluoropropylmethyl. Excellent properties are retained to 8% substitution. This gives a broad range of possible formulations. The swelling characteristics for these liquid silicone rubbers can be tailored to the level needed for the fluid of choice. The comparison of the fluid resistance and various levels of fluorine substitution are shown in the following graphs. At equivalent fluorosilicone levels, copolymers exhibit reduced swell. As a result of this, swell characteristics can be balanced against the cost of increased fluorosilicone content in order to meet a cost/performance profile that exactly meets the need. Page 3 of 8

4 Ref Fuel 23C. JP 23C Copolymer Copolymer % Volume Swell % Volume Swell Mole % Fluoro Mole % Fluoro Mil 7C. ASTM #3 15C. % Volume Swell Copolymer Mole % Fluoro % Volume Swell 4 Coploymer Mole % Fluoro -4.3 Figure 1. Volume Swell Comparison of Copolymers and Blended Polymers in Oil, Fuels, and Fluids. Page 4 of 8

5 SILASTIC COMPARISONS Silastic Fluorosilicone LSR was developed at 4 mole percent trifluoropropylmethyl silicone to achieve the desired oil resistance and fuel splash properties. Volume swell and permeation are directly related to solubility of the test fluid in the rubber. In sealing applications, oil and fuel permeation is an important performance property. Low oil permeation results in low weep, and low fuel permeation assists in meeting hydrocarbon emission standards. The effect of fluorine substitution on permeability can be measured in several different ways. The most common method is to prepare a rubber sheet, clamp it on top of the permeation cup containing the fuel to be evaluated, and measure weight loss. The permeation was done in CE1 fuel at 6 C to highlight the differences in fluorosilicones and dimethyl silicone rubbers. In this method, the performance of the 4 mole % Silastic is about equal to the performance of the LoPerm VMQ only available as an high consistency rubber,, while the LoPerm VFSR, FSR, and LoPerm FSR s have dramatically lower permeation rates. 45, 4, 35, Permeation Rate (gm*mm/m2*day) 3, 25, 2, 15, 1, 5, VM Q LoPerm VMQ 5:5 FLSR/LSR S ilastic LoPerm V FVM Q FVM Q LoPerm FV M Q Figure 2. Uncompressed Permeation Results from Rubber Sheets CE1 at 6 C Page 5 of 8

6 Figure 3. O-ring Permeation Testing Fixture for Variable Compression Sheet testing does not take into account the actual gasket or o-ring configuration. The O-ring test fixture allows the evaluation of the effect of compression. The results in Figure 4 clearly demonstrate that the effect of compression varies from material to material. For example, fluorosilicone rubber shows less permeation reduction from compression of the gasket than does the dimethyl material. By comparing the flat plate permeation with the o-ring permeation, it is evident that compression has a very dramatic effect of lowering permeation rates of VMQ based materials, while compression has a less dramatic rate of lowering permeation in the FSR materials i. 6 Permeation Rate (mm*gm/m2*day) Silastic LoPerm VMQ FVMQ LoPerm FVMQ VMQ 5: /LSR LoPerm V FVMQ -1% % 1% 2% 3% 4% 5% Nominal Compression Figure 4. Effect of Compression on Permeation in CE1 at 6 C In some instances only slight modification of the volume swell resistance properties are required, and in these cases the blend of and a dimethyl LSR can provide an effective solution. Page 6 of 8

7 SILASTIC ENHANCEMENT ADDITIVES Good mold release and ease of assembly can allow the manufacturer to take full advantage of the higher-speed processing, which is possible through the use of the FLSR. Table 2 lists the properties available through the use of two different additives. The first additive provides good mold release for automated operations; the second additive provides an additional self-lubricating effect. Ultimately, these additives make assembly of parts much easier. Automated operations require consistent and uniform release from mold cavities in order to operate unattended. Silastic Fluorosilicone LSR on its own provides acceptable release. However, with the addition of a proprietary mold release, parts easily release from the mold. Up to five parts per hundred of the mold release have been evaluated with no significant change in properties observed. Similarly, at relatively modest levels the self-lube additive allows for ease of assembly. This addition helps to avoid nicked o-rings and other parts damage during assembly operations by reducing the friction associated with sliding the rubber part over metal or plastic surfaces. Silastic Q Additive A Additive B 1 pph 2 pph 5 pph 1 pph 2 pph 5 pph Release Characteristics OK GOOD GOOD GOOD GOOD GOOD GOOD+ Lubricity NONE NONE NONE YES NONE SLIGHT HEAVY Tensile (psi) Elongation (%) % Modulus Tear B (ppi) Durometer Shore A Resiliency Volume Swell 15C/24hr/IRM % 14.6% 15.6% 15.% 15.4% 15.7% 14.7% Table 2. Effects of Self-Lubricating and Mold Release Additives after Post Cure PIGMENTABLE VERSIONS OF SILASTIC Silastic is stabilized with red iron oxide, which limits the color choices available in the product. Other equally effective heat stabilzers are available, which permit the color to be customized. A pigmentable version of Silastic was prepared utilizing an equally effective heat stability additive. Initial mechanical properties of Silastic and the pigmentable version were equivalent. The heat stability of these two materials was compared at 225 C. and is shown in Table 3. The use of different heat stability additives allows greater variability in custom compounding without reducing the good high temperature properties. Silastic Hours 72 Hours 168 Hours Durometer, point change Tensile, % change Elongation, % change % Modulus, % change Pigmentable Version 24 Hours 72 Hours 168 Hours Durometer, point change Tensile, % change Elongation, % change % Modulus, % change Table 3. Heat Stability Data at 225 C. Page 7 of 8

8 SUMMARY The desire for enhanced swell characteristics was balanced against the cost of increasing fluorosilicone content in order to meet a cost/performance profile that exactly meets the needs of the customer with Silastic As part of the family of Silastic LSR from Dow Corning, this FLSR offers advantages to the rubber fabricator particularly for applications in harsh conditions. Enhanced flexibility is available to the fabricator through FLSR products formulated to meet various solvent or oil resistance needs and through the addition of ingredient packages to enhance release or lubrication. Excellent heat stability can be obtained with color. Compressed fuel permeation rates of FLSR have been shown to be comparable to LoPerm VMQ and significantly better than uncompressed rates. The addition of mold release and ease of assembly additives allows the manufacturer to take full advantage of the higher-speed processing of FLSR. i Brumels, Mark D.; Olsen, Jr., Charles W.; Irish, Paul T.; and Altum, Stephen C., Dow Corning Corporation, Midland, Michigan, SAE Paper , The Effect of Compression on Permeation of Hydrocarbons through Dimethyl and Fluorosilicone Rubber Rubber Fabrication Solutions LIMITED WARRANTY INFORMATION PLEASE READ CAREFULLY The information contained herein is offered in good faith and is believed to be accurate. However, because conditions and methods of use of our products are beyond our control, this information should not be used in substitution for customer s tests to ensure that Dow Corning s products are safe, effective, and fully satisfactory for the intended end use. Suggestions of use shall not be taken as inducements to infringe any patent. Dow Corning s sole warranty is that the product will meet the Dow Corning sales specifications in effect at the time of shipment. Your exclusive remedy for breach of such warranty is limited to refund of purchase price or replacement of any product shown to be other than as warranted. DOW CORNING SPECIFICALLY DISCLAIMS ANY OTHER EXPRESS OR IMPLIED WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY. DOW CORNING DISCLAIMS LIABILITY FOR ANY INCIDENTAL OR CONSEQUENTIAL DAMAGES. Dow Corning and Silastic are registered trademarks of Dow Corning Corporation. We help you invent the future is a trademark of Dow Corning Corporation. 23 Dow Corning Corporation. All rights reserved. Ref:

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