Closing the Fluoro Elastomers High Temperature Gap: New Developments in Fluorosilicone Rubber

Transcription

1 Automotive and Transportation Consumer Solutions Closing the Fluoro Elastomers High Temperature Gap: New Developments in Fluorosilicone Rubber By Phillip Griffith, Craig Gross, Robert Drake, Scott Miller

2 Abstract As rubber applications continue to evolve and require highertemperature performance and higher resistance against fuels and oils, many industries including automotive, aviation, and oil and gas are looking beyond traditional materials for solutions. Applications include everything from turbocharger hose lining to blowback preventer seals to gaskets for airplane hydraulic systems. FKM (fluorocarbon rubber) and FVMQ (fluorosilicone rubber) have historically been used in these applications, with FKM being used for many of the applications requiring the highest-temperature resistance and FVMQ being used for temperatures up to 220 C. Recent advancements in FVMQ technology have extended the upper service-temperature performance up to 240 C with peak temperatures to 250 C. This advancement, in addition to greater property stability over a wide temperature range compared to FKM, now allows FVMQ to be used in higher-temperature applications. Additionally, FVMQ provides 25% lower density, allowing for better weight savings and lower material usage. This paper discusses the technology advancements in FVMQ, showing comparisons to traditional FVMQ and FKM materials. Comparisons will be made to newly developed FVMQ materials with higher heat stability, showing test results for physical properties measured at elevated temperature (up to 200 C), aging performance in automotive fluids (fuels and oils), adhesion performance to VMQ (as it relates to co-extrusion application), and acid-gas resistance. This paper will demonstrate how FVMQ elastomers can be designed to precisely meet specific application requirements and highlight performance attributes in which FVMQ provides advantages over FKM. Introduction To keep pace with change in the automotive industry, manufacturers are facing a number of pressures. There are regulatory and governmental mandates and continuous costreduction initiatives. In addition, new customer preferences are shaping design trends for smarter, safer and more energyefficient vehicle technologies, along with steadily reducing environmental impact. Intending to meet these needs, many initiatives such as weight reduction, use of smaller yet more powerful engines, and increased electronics content have created additional challenges for suppliers and manufacturers. Megatrends in vehicle design requirements present many significant technical challenges for fuel economy improvements, emissions reduction, vehicle control and occupant comfort. To match the advancements in vehicle design and systems technology, traditional materials are being pushed to deliver performance aspects that extend beyond the requirements they were originally developed to meet. Fluorosilicone elastomers are one class of traditional materials being expected to deliver innovative solutions to enable these new technology advancements. The use of alternative fuels, synthetic oils and aggressive fluids; increased operating temperatures with less cooling airflow; lighterweight vehicles with reduced material consumption; and global vehicle platforms all pose unique challenges related to material performance. First developed in the 1950s to provide high resistance to fuels, oils and solvents in applications across a broad temperature range, fluorosilicone rubber (FVMQ) development has continued to evolve and, as a result, has found wide use in automotive, aviation and industrial applications for many decades now. Close collaboration with these industries and/or markets is necessary to ensure technology development of fluorosilicone rubber results in innovations that can help customers meet design trends for performance improvement and cost reduction. This paper highlights some key advancements in fluorosilicone rubber, developed in response to industry expectations for improved materials to meet challenging automotive-design trends. Basic Fluorosilicone Technology Fluorosilicone rubber is formulated from fluorosilicone polymers that contain a (-Si-O-) repeating group on the polymer backbone. One unique difference, compared to their dimethylsiloxane counterparts, is the incorporation of a fluorine component attached to the polymer backbone. pg 2

3 Fluorosilicone polymers replace one (methyl) side group on each silicon with a (trifluoropropyl) side group (Figure 1). The resulting polymer retains dimethylsilicone s durability in very hot and very cold temperatures while adding high resistance to nonpolar solvents and aggressive fluids. A vinyl side group also can be added to allow the polymer to be crosslinked. These polymers are then utilized to produce fluoro-silicone rubber, which is commonly referred to as FVMQ (fluorovinylmethylsiloxane) per ASTM Depending on the application, FVMQ technologies compete directly and indirectly with carbon-based fluoroelastomers such as fluorocarbon rubber (FKM). Figure 1. Structure of silicone and fluorosilicone rubber VMQ FVMQ CH=CH 2 Si CH 3 O n CH 3 Si CH 3 O m CH 2 Early applications for fluorosilicone rubber technology involved various aerospace vehicles and equipment as well as a number of military aircraft programs, most commonly for seals exposed to fuels and hydraulic fluids. Automotive applications include a wide range of fuel-resistant sealing and gasketing applications, as well as membranes, diaphragms and flexible valves for fuel and vapor recovery systems. Fluorosilicone rubber is especially beneficial for chemical-resistant liners on turbocharger hoses exposed to temperatures exceeding 180 C and is commonly used in combination with an outer layer of reinforced HCR (high-consistency rubber) dimethylsilicone elastomer. FVMQ- HCR hoses have broad application in light-duty and heavy-duty trucks. In addition, fluorosilicone rubber has found wide use in applications requiring high chemical resistance when exposed to low temperatures or when a soft material with high chemical resistance is required for sealing applications. Unlike many fluorocarbon elastomers, fluorosilicone rubber can be used in applications requiring temperature exposure down to -65 C in addition to applications requiring low modulus and hardness values below 40 Shore A durometer. Si CH 3 CH 2 O m CF 3 CH=CH 2 Si CH 3 O n Silicones are known as an enabling technology with unique properties not offered by other materials. The versatility of silicone chemistry helps to create numerous opportunities for developing specialized fluorosilicone rubber bases and custom compounds to meet specific application needs exactly what today s design trends demand. Innovation in FVMQ A multigenerational approach to improvement of FVMQ technology has been used that is focused on market needs for higher-temperature resistance and improved thermal stability, increased fuel resistance, and higher adhesion to VMQ. These market needs, in conjunction with close collaboration with end users, has enabled the development of specific, fully formulated compounds designed to meet critical customer requirements and provides an expanded FVMQ toolbox of new technologies that can be applied to many other industries and applications. Use of a platform project that explores all aspects of FVMQ technology including new additives and intermediates; improvements in thermal stability; higher adhesion to VMQ; and improvements in compression set, handling, and fuel and oil stability expands the use of FVMQ in applications where the material may not have been considered in the past. Activities in this multigenerational project plan are continually evolving to find and resolve gaps between current technology and new market and customer needs requiring improved performance of FVMQ solutions. This has led directly to the commercialization of new compounds available to the market that address many of these performance gaps, such as higher-temperature performance for turbocharger hoses (TCH) and other applications, low compression set for better sealing, and higher fuel resistance and retention of properties after immersion. High Resilience and Low Compression Set Developments New FVMQ bases (shown in Table I on page 10) have been developed to meet needs for high resilience/rebound, low compression set, low volume swell in fuel, reduced stickiness when unwrapping and mill handling, and improved ply adhesion when cured with VMQ. pg 3

4 Specially formulated to meet fabricator, compounder and specifier needs, two uncatalyzed FVMQ materials were developed with a choice of either 40 or 70 Shore A durometer cured hardness. These fluorosilicone rubbers are designed to help optimize sealing performance and to provide both easy processing and process versatility. Both FVMQs offer extremely low compression set, which is a critical property required to improve long-term durability for components like O-rings and seals. These materials achieve compression set values below 10% when post-cured and tested at 175 C for 22 hours, as well as comparatively low compression set at the harsher conditions of 70 hours at 200 C. These unique FVMQ products also offer high resilience for dynamic sealing applications and stable performance properties, even with no post-curing. These new FVMQ materials provide typical fluorosilicone rubber properties including excellent resistance to fuels, oils, solvents and aggressive fluids with low volume swell. In testing, the fluorosilcone rubber materials each exhibit excellent fluid resistance with low volume swell when exposed to a variety of different fuels and oils. In addition, good property retention and aging resistance are seen over a wide service-temperature range. These performance attributes can be further enhanced for specific needs. In terms of better handling characteristics, these specialty FVMQ products can be custom-compounded, pigmented, blended, modified with additives and easily processed. With custom compounding, specific performance attributes can be especially tailored for particular application requirements, including increased temperature resistance. Pigment masterbatches provide easy coloring, while performance additives are available for addressing a variety of other needs. In addition, the new products can be blended with each other to achieve intermediate hardness levels, or they can be blended with other fluorosilicone or silicone bases for further tailoring of uncured and cured properties. These materials are designed for molding, extrusion and calendering processes and for applications such as O-rings, diaphragms and other fuel contact applications. The improved adhesion of these products to VMQ allows this technology to be used for TCH applications where adhesion to VMQ is necessary for reliable performance. FVMQ for Higher Operating Temperatures Development efforts focused on a more heat-resistant FVMQ technology for extreme high-temperature applications aim to meet more stringent component-design and reliability requirements demanded for under-the-hood applications. These performance and reliability requirements are necessary due to much higher operating temperatures seen with smaller and more powerful engines, increased exhaust gas recirculation, advanced emission controls, and more underhood content with less airflow. In close collaboration with specific customers and end users, a diverse, global team of scientists and engineers responded to specifications calling for increased heat-resistance properties in fluorosilicone rubber. With specific milestones and targeted improvements in the development process, the team worked closely with fabricators and specifiers to develop the custom FVMQ technology and to validate its performance with hightemperature aging tests (Figure 2). Figure 2. Heat aging 14 days at 250 C % Change 0% -20% -40% -60% -80% -100% Tensile 40 Durometer Current Technology Elongation 40 Durometer New Technology Previously limited to applications with long-term exposure to application temperatures of 200 C or less, fluorosilicone rubber can now be custom-compounded for automotive components that must withstand continuous temperatures up to 220 C and limited-time peak temperatures up to 240 C or beyond. Rigorous heat-aging tests show considerable improvements in retention of tensile strength and elongation, as well as improved performance when exposed to aggressive fluids in extreme heat (Figure 3). pg 4

5 Figure 3. Property change after immersion in automatic transmission fluid for 168 hours at 150 C % Change 0% -10% -20% -30% -40% -50% -60% Standard FSR Shore MPa TS, 425% EB Tensile Strength Advanced FSR Shore MPa TS, 340% EB Standard FSR Shore MPa TS, 400% EB Elongation Advanced FSR Shore MPa TS, 225% EB Acid Receptors Acids form from the presence of the fluorine ion and vinyl groups in the FVMQ polymer chain when heat-aged. These acids can cause the polymer to chain cleave, which reduces mechanical strength with the smaller polymer fragments. The addition of an acid acceptor technology has been shown to minimize this degradation. Addition of the correct type and amount of an acid acceptor can significantly improve thermal performance but can have conflicting impacts on stability in fluids and can cause interactions with other components that need to be managed (Figures 4 and 5). Figure 4. Thermal degradation of FVMQ with different types of acid acceptors FVMQ Stability Several degradation mechanisms were researched and investigated to better understand FVMQ thermal stability. These degradation mechanisms occur at different temperatures and rates and under different environmental conditions. Therefore, different technology solutions are required to address all of these failure modes with minimum negative impact on other properties. Degradation mechanisms of FVMQ include chain cleavage and depolymerization, oxidative cleavage of crosslinks and side groups, and changes to filler/polymer interactions. These materials were tested in a range of environments, including: Dry heat - 70 hours to 14 days at 250 to 275 C hours at 220 C plus 70 hours at 240 C Oil Dexron VI used as an aggressive test oil, 7 days at 150 C Compression set 3 and 7 days at 200 C Fuel Acid gas condensate Tensile strength, MPa Days at 250 C AA 1 AA 2 AA 3 AA 4 pg 5

6 Figure 5. Thermal degradation of FVMQ with different levels of acid acceptor Tensile strength, MPa Days at 250 C No AA 4 Level 1 AA 4 Level 2 AA 4 Oxygen diffusion and sample thickness are important parameters in thermal stability. An absence of added redox stabilizer will result in rapid decomposition at elevated temperatures. Design of experiment methodology was used to optimize the type and quantity of redox stabilizers for higher-temperature FVMQ to maximize performance (Figure 6). A wide range of properties was tested and evaluated to ensure a balanced solution to meet customer needs. Efforts are continuing to explore new additives for improved performance at temperature and then optimize for use in new compounds. Figure 6. Designed experiment interaction of redox stabilizer Design-Expert Software Factor Coding: Coded EB, 14 d at 250ºC Design Points EB, 14 d at 250ºC 2 Metal Oxide Redox Stabilizers In addition to acid acceptors, the use of the metal oxide redox stabilizer is required to help address the oxidative cleavage of the polymer. These have been widely used in VMQ applications requiring higher-temperature stability but not commonly used in FVMQ until recently, as there has been minimal need for > 200 C applications. X1 = B: Redox 1 X2 = A: Redox 3 A: Redox A wide range of redox-active oxides is available, with variable interactions between combinations of redox stabilizers. The initial oxidation state, particle size and shape all play a role in obtaining good redox stabilization. Table II on page 11 shows variable performance for stabilization of FVMQ. Redox stabilizers Optimized redox stabilizers for FVMQ have been identified that improve thermal stability with minimum negative effects on other performance properties. FVMQ typically is stabilized with cerium complexes in addition to other redox-active metals, which are known for stabilization of siloxanes against thermal decomposition. Although there is much debate in literature, redox stabilizers are thought to work by decomposition of peroxides formed from O 2 oxidation, thus preventing branching chain reactions: R + M x+1+ R + + M x+ (1) 4 M x+ + O 2 4 M x O 2- (2) Analysis of Heat Age Conditions B: Redox 1 A comprehensive approach was executed to understand the impact of various exposure times and temperatures on FVMQ degradation. The data generated from this comprehensive evaluation identified accelerated higher-temperature aging conditions comparable to longer-term lower-temperature conditions typically required for specifications (Figure 7). Identifying short-term testing conditions was critical for accelerating development activities and providing quicker results while still allowing long-term aging performance to be predictable. In addition, performance at a given condition can vary widely by the FVMQ sample, which allows a better understanding of technology changes and their impact on performance at various conditions. pg 6

7 Peroxide Choice Peroxide choice also affects thermal stability, which allows some flexibility in improving performance depending on how the material is processed and cured. Choice of peroxide influences the cured network and thus the thermal stability, and it also has an effect on oil stability and compression set. Dialkyl or diacyl peroxides are preferred for FVMQ cure and thermal stability (Figures 8 and 9). Figure 7. Interval plot of heat aging Figure 8. Effect of peroxide choice on tensile strength 12.0 Tensile strength, MPa Days at 250 C Interval Plot of Durometer, S, 100% Modulus, Tensile, MPa, EB, % 95% CI for the Mean Peroxide 1 Peroxide 2 Peroxide 3 Peroxide 4 Figure 9. Effect of peroxide type on elongation Durometer, Shore A 100% Modulus, MPa Tensile, MPa EB, % 400 Elongation at break, % Days at 250 C Peroxide 1 Peroxide 2 Peroxide 3 Peroxide HA Hardness HA 1. PC 2. 3 d 250 C 3. 7 d 250 C d 250 C h 220 C h 240 C pg 7

8 Properties at Temperature Though FVMQ has lower tensile strength than FKM at room temperature, property performance at elevated temperatures is similar between FKM and FVMQ when evaluating tensile strength and is higher for FVMQ when evaluating elongation at break. FVMQ materials are stable over a wider range of temperatures than FKM and can have higher performance at elevated temperatures. Material properties at elevated temperatures often are not considered even though the application may be exposed to these elevated temperatures. (Figures 10, 11 and 12). Figure 10. Tensile strength of FVMQ and FKM at various temperatures Dumbbells placed in grips and measured ~15 when oven is at temp Tensile MPa at temperature Dow High-temp FVMQ FKM-1 Hose Grade FKM-2 O-ring Grade Temperature, C FKM Bisphenol, Lower F FKM Peroxide-cured, Higher F Figure 11. Elongation of FVMQ and FKM at various temperatures Dumbbells placed in grips and measured ~15 min after oven is at temp Elongation % at temperature 100% modulus MPa at temperature Dow High-temp FVMQ FKM-1 Hose Grade FKM-2 O-ring Grade Temperature, C FKM Bisphenol, Lower F FKM Peroxide-cured, Higher F Figure 12. Modulus of FVMQ and FKM at various temperatures Dumbbells placed in grips and measured ~15 min after oven is at temp Temperature, C Dow High-temp FVMQ FKM-1 Hose Grade FKM-2 O-ring Grade FKM Bisphenol, Lower F FKM Peroxide-cured, Higher F pg 8

9 FVMQ to VMQ Adhesion In TCH applications, the design of the hose requires good adhesion of the fluoro elastomer to the lower-cost VMQ layer. Improved adhesion strength of FVMQ for higher initial adhesion as well as higher adhesion after aging has been accomplished. Measurements were taken after 225 C aging on both baseline and improved FVMQ, as well as on competitive FKM samples (Figure 13). Specifically developed to meet increased heat-resistance requirements for turbocharger hoses, the high-temperature fluorosilicone rubber can be designed to meet various OEM specifications for performance at increased temperatures. Combined with a reinforced HCR silicone rubber outer layer and utilizing the correct adhesion technology, the more heatresistant fluorosilicone rubber can provide simplified design, stronger adhesion, easier processing via co-extrusion or calendering, thinner wall sections, lighter weight, and potentially lower cost-in-use than some fluorocarbon options. Similarities between FVMQ as the liner and VMQ as the outerlayer including the chemical nature of siloxane backbones, cure chemistries and cure speeds make an ideal combination. In addition, the high-temperature FVMQ technology also can provide added durability for fuel, oil and transmission seals; O-rings and gaskets; diaphragms; membranes; and flexible valves. Acid Gas and Heat Aging FKM mechanical properties after elevated-temperature aging can show good thermal stability and property retention (Table III on page 11). However, many of these thermally stable FKMs are not as resistant to exhaust gas condensate such as aqueous acidic solutions, with very large volume swells of > 300% possible, plus > 60 to 70% change in tensile and elongation. FKM materials can be improved for better stability in acidic solutions with minimum volume swell and change in properties, but this usually results in decreased thermal stability. In TCH applications, both thermal stability and acid gas performance are required. Figures 14 and 15 show four FKM grades. The three that have reasonable thermal stability have poor acid-gas resistance, as measured after 21 days in 1 M acetic acid at 95 C, with the yellow reference sample showing the original size before aging. The higher-fluorine-content FKM has good acid-gas resistance but poor thermal stability. Figure 13. FKM and FVMQ adhesion to a 40 Shore VMQ peel adhesion, kn/m FVMQ Breaks Figure 14. Tensile strength of FKM after heat aging Tensile strength, MPa Days at 225 C Hose-grade FKM/VMQ Gen I FVMQ/VMQ Higher-fluorine FKM/VMQ Development FVMQ/VMQ Days at 250 C Hose-grade FKM Bisphenol FKM Higher-fluorine FKM O-ring-grade FKM pg 9

10 Figure 15. FKM samples after 21 days in 1 M acetic acid at 95 C New FVMQ materials were developed for these conditions that exhibit a balance of thermal stability with improved acid-gas performance, optimized by carefully selecting the correct FVMQ base and thermal-stability additives (Table IV on page 12). Conclusion With in-depth industry experience, materials creativity and problem-solving collaboration, teams of leading scientists and application engineers are succeeding in generating significant upgrades in fluorosilicone rubber capabilities. This proven, effective solution for durable automotive components exposed to aggressive fluids and very hot or cold conditions can now be custom-compounded for better handling characteristics, as well as for extremely high continuous and peak temperatures. In addition, improvements in compression set, resilience and acid-gas resistance also have been achieved, which allows OEMs, specifiers and end users another material option to consider for challenging applications requiring these improvements. These technology upgrades can be built into the full range of fluorosilicone rubber choices and tailored to meet customers precise application requirements. Table I. Properties of high-resilience/low-compression-set FVMQ bases SILASTIC LS-2940 U Fluorosilicone Rubber SILASTIC LS-2970 U Fluorosilicone Rubber Test a Property b ASTM D2240 Hardness, Shore A durometer ASTM D412, Die C Tensile strength, MPa ASTM D412, Die C Elongation at break, % ASTM D412, Die C Modulus 100%, MPa ASTM D2632 Resilience, % rebound ASTM D624B Tear strength, kn/m ASTM D395 Compression set 22 hr at 177 C c, % set ASTM D395 Compression set 70 hr at 200 C c, % set ASTM D395 Compression set 70 hr at 23 C c, % set ASTM D1329 Temperature retraction TR10, C ASTM D471 Reference fuel B, 23 C for 24 hr, % vol swell ASTM D471 Reference fuel C, 23 C for 70 hr, % vol swell ASTM D471 Reference fuel C, 60 C for 168 hr, % vol swell ASTM D471 FAM B fuel, 60 C for 168 hr, % vol swell a ASTM: American Society for Testing and Materials b Properties obtained using 1.0 phr DBPH-50 (DHBP) (2,5-bis [tert-butylperoxy] 2,5 dimethyl hexane) on 1.91 mm (0.075 inch thick) slabs, molded 10 minutes at 171 C (340 F) and post-cured four hours at 200 C (392 F) c Tested according to method B, type II (12 mm), plied disks pg 10

11 Table II. Variable performance for stabilization of FVMQ Stabilizer Redox 1 Redox 2 Significant difference Number of samples Student's t-test 14 d at 250 C Hardness Yes 50% modulus, MPa No 100% modulus, MPa No Tensile strength, MPa Yes Elongation, % Yes Δ Hardness, Shore A Yes Δ Weight, % No Δ 50% modulus, % No Δ 100% modulus, % No Δ Tensile strength, % Yes Δ Elongation, % Yes 700 hr at 220 C plus 70 hr at 240 C Hardness No 50% modulus, MPa Yes 100% modulus, MPa Yes Tensile strength, MPa Yes Elongation, % Yes Δ Hardness, Shore A Yes Δ Weight, % No Δ 50% modulus, % No Δ 100% modulus, % Yes Δ Tensile strength, % Yes Δ Elongation, % No Table III. Performance of FKM after acid-gas exposure 1M Acetic Acid (Wet) Hose-grade FKM O-ring-grade FKM Bisphenol FKM Higher-fluorine FKM Durometer, Shore A % modulus, MPa 2.65 Tensile, MPa Elongation, % Changes Hardness change Volume swell, % Δ 100% modulus, % 9 Δ Tensile strength, % Δ Elongation, % pg 11

12 Table IV. Properties of improved FVMQ materials in acid-gas and heat-aging conditions Properties and changes Intermediate Renault PSA, 1 solution in liquid, test wet 700 hr at 220 C + 70 hr at 240 C heat aging 60 Shore New 60 New Shore New 60 New 60 Max Min FVMQ Shore 1 Shore 2 Max Min FVMQ Shore 1 Shore 2 Durometer, Shore A % modulus, MPa % modulus, MPa Tensile, MPa Elongation, % Not tested; expect very poor results Changes Hardness change, Shore A Volume swell, % Δ 100% modulus, % Δ Tensile strength, % Δ Elongation, % Brands from Dow Serve You Whether you seek industry-leading innovation or greater cost efficiency, Dow can help. DOWSIL and SILASTIC solutions deliver specialty materials, collaborative problem-solving and innovation support tailored to your needs. See details at consumer.dow.com/auto. Contact Information consumer.dow.com/contactus HANDLING PRECAUTIONS PRODUCT SAFETY INFORMATION REQUIRED FOR SAFE USE IS NOT INCLUDED IN THIS DOCUMENT. BEFORE HANDLING, READ PRODUCT AND SAFETY DATA SHEETS AND CONTAINER LABELS FOR SAFE USE, PHYSICAL AND HEALTH HAZARD INFORMATION. THE SAFETY DATA SHEET IS AVAILABLE ON THE DOW WEBSITE AT OR FROM YOUR DOW SALES APPLICATION ENGINEER, OR DISTRIBUTOR, OR BY CALLING DOW CUSTOMER SERVICE. 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 our 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 s sole warranty is that our products will meet the 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. TO THE FULLEST EXTENT PERMITTED BY APPLICABLE LAW, DOW SPECIFICALLY DISCLAIMS ANY OTHER EXPRESS OR IMPLIED WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY. DOW DISCLAIMS LIABILITY FOR ANY INCIDENTAL OR CONSEQUENTIAL DAMAGES. Trademark of The Dow Chemical Company ( Dow ) or an affiliated company of Dow 2018 The Dow Chemical Company. All rights reserved Form No A