Back to Index Soft, Processable SEBS Polymers for Compounds Dale Handlin Ph.D., Ching Ting Ph.D., Ziv Cheng, and Parashar Davé TSRC-Dexco Polymers, Plaquemine, LA, USA Abstract Two of the important trends in elastomeric materials today are lower hardness and easier processability. Better processability allows molded parts such as cell phone cases and wearable devices to have complex surface grains, while softer compounds begin to blur the line between solids and gels. TSRC has developed three different polymers that enable compounders to meet these needs in different ways. DP-029 is a new high molecular weight SEBS designed for better oil holding capability to allow softer compounds through traditional SEBS compounding. DP- 6014 is a lower styrene SEBS that is not only softer but can be processed without oil like a polyolefin. This allows it to be combined with typical polyolefins and olefin elastomers in soft, elastic compounds, films and fibers. Finally DP-027 is the softest SEBS available. It is similar in hardness to a low styrene SIS used in adhesives. To reach this low hardness we combined low styrene content with a high vinyl backbone. The high vinyl backbone of DP-027 also provides thermodynamic compatibility with polypropylene for soft, transparent parts for packaging and medical applications. Both DP-014 and 027 can be processed without oil for use in personal electronics such as ear bud insulation, cellphone cases and wearable fitness trackers. Introduction SBCs (styrene block copolymers) produced by anionic polymerization are based on simple A-B- A molecular architecture where A, polystyrene is the hard phase, and B, the elastomeric segment is the soft phase. In most common SBC structures the elastomeric segment is butadiene or isoprene, forming SBS (styrene-butadiene-styrene) or SIS (styrene-isoprene-styrene) block copolymers [1]. These unique block-like structures are known to form distinct phase separated systems. The two phases are known to retain many properties of their respective polymers including two separate glass transition temperatures. The consequence of this structure is that rigid polystyrene is stiff at room temperature and provides the strength characteristic, and the soft phase provides the elastic property. These can easily be compounded and formed into different shapes and sizes using conventional processing equipment. Poly(styrene-b-(ethylene-butylene)-b-styrene) block copolymer (SEBS) are synthesized by selective hydrogenation of the butadiene mid-block of (SBS) block copolymer [2]. Since its structure contains very few double bonds, it exhibits excellent heat, UV and aging resistance. They are also stable up to 250 C processing temperature unlike SBS and SIS making them useful in high temperature compounding applications. They also offer excellent oil absorption, ease of
processability, compatibility with polyolefins, and balanced strength-toughness and compression set properties. These hydrogenated polymers are recyclable making them useful in automotive applications, unlike vulcanized rubber. Currently SEBS is widely used in wire and cable, medical, film and sheet, footwear, gels and soft-touch, consumer goods, appliance, and electronics applications. It is important to understanding the structure-property relationship of these block copolymers when designing formulations to meet specific customer requirements. By fine tuning the hard phase and soft phase segment during polymerization one can significantly alter their physical property characteristics [4]. With same molecular weight, lowering the polystyrene (PS) content will reduce the hardness, making them more rubber-like. They are also going to be more compatible with polyolefins such as polyethylene (PE) and polypropylene (PP). Keeping the PS content and molecular microstructure similar, lowering the molecular weight will improve its processability and transparency. By increasing the molecular weight these trends will be reversed. Finally, vinyl content can be raised by increasing the of amount of 1,2 butadiene addition. High vinyl SEBS polymers are more compatible with PP for transparent films and can be used in energy damping applications. Product Features and Characteristics DP-6014, DP-027 and DP-029 are a family of low styrene SEBS grades developed by TSRC technology center in Kaohsiung, Taiwan. Low styrene allows these polymers ease of processability and greater compatibility to polyolefins. Lower melt elasticity provides formulation flexibility in film and thin wall extrusion and injection molding applications. Table 1 Taipol 6152 DP-6014 DP-027 DP-029 Polymer Structure Linear, Linear, Linear, Linear, triblock triblock triblock triblock Molecular weight Low Low-Medium Medium High Vinyl Content, % Normal Normal High Normal Polystyrene, % 29% 18% 13% ~23% Hardness, A Scale 80 A 70 A 35 ---- Melt Flow, 230 C/2.16 kg 1 5.50 3.0 ---- DP-6014: A new hydrogenated styrenic block copolymer has been designed to process as a single phase melt but retain its two phase nature at use temperature to provide strength and creep resistance
[3]. This development allows the production of compounds which can be oil-free with acceptable rheology for applications sensitive to orientation and applications requiring low odor or taste. These polymers can be particularly useful in thin wall molding and extrusion applications. Its easy processing allows it to be highly filled for reduced cost or improved flammability performance. Since DP-6014 flows like a polyolefin it can be melt blended with a wide range of propylene and ethylene based polyolefin plastics and plastomers with or without processing oil. It also improves strength and elasticity in soft polyolefins, including hysteresis and permanent set performance of both soft polyethylene and polypropylenes. S EB S DP-027: DP-027 is a high vinyl SEBS polymer designed specifically for compatibility and processability with polypropylene and polypropylene copolymers. The high vinyl content provides excellent compatibility with polypropylenes. Combined with its low modulus, the compatibility of DP-027 with polypropylene allows the modification of polypropylenes to create a range of hardnesses without oil or increased haze. Clarified polypropylene copolymers can be used with DP-027 to produce clear, tough packaging and medical devices that have many of the properties of plasticized PVC, but without extractable plasticizers. The molecular design of DP-027 creates a polymer that flows just like polypropylene at polypropylene processing temperatures of around 230C for easy compounding even in a single screw extruder. The combination of easy compounding, compatibility and refractive index matching makes DP-027/polypropylene compounds ideally suitable for applications requiring high clarity, softness, low modulus and high elongation. For example, blends of DP-027 with random copolymer PP are well suited for high transparency films and medical tubing applications. Films of DP-27/random copolymer PP also show excellent resistance to steam sterilization. Blends with polypropylene can be used in clear rigid and flexible packaging applications. S EB S
DP-029 DP-029 takes a more traditional approach to creating soft compounds using oil and polypropylene. It combines low styrene content and high molecular weight, similar to Taipol 6159. Hence, it combines softness with excellent oil retention, compression set and high temperature performance compared to Taipol 6151, and similar to Taipol 6159. Ultra-high molecular weight offers excellent high temperature performance and resistance to high temperature compression deformation [5]. It also provides excellent a matte surface finish with excellent oil holding even at high oil levels for very soft compounds. By utilizing DP-029 formulators can obtain a lower hardness compound with less oil usage. This can be particularly useful when designing products for soft-touch applications such as toys and sporting goods. They can also perform well in flexible components, seals and gaskets for construction and automotive. Taipol 6151 Table 2 SEBS polymer MW ratio Styrene, % Taipol 6151 1.0 32 Taipol 6159 1.50 29 DP-029 1.50 ~23 DP-029 Experimental Details Dryblend samples were prepared using a Henschel type high speed mixer. SEBS was first added followed by oil, PP, and other raw materials. Batches were allowed to mix for a few minutes to
get good dispersion. Compounding was done on a 30-mm Lab size, co-rotating twin screw extrusion line using typical processing temperatures. Molding was carried out on a 110 ton injection molding machine. Plaques were allowed to condition at 23 C and 50% RH prior to testing. DP-6014: Results and Discussion One area of potential interest where DP-6014 can be used is wire and cable. Typically, these are in the Shore A 80-90 hardness range. This range provides the necessary abrasion resistance seen during the installation process. Additives such as naphthenic/paraffinic oil are used to increase flow to the desired range whilst polymers such as polypropylene and polyethylene are used to adjust other properties such as tensile strength or heat distortion. While this approach works well, the addition of oil creates issues such as flammability, color/odor and concern about migration. Another area of concern is smoke generation. Migration and smoke are not desired in wire and cable formulations. There is a concerted effort by formulators to shift toward more thermally stable polymeric solutions. We wanted to understand the effect of substituting Taipol 6151 with 6014 and compare results with only oil addition. The results would show what the net effect of oil replacement with 6014 is. Up to 30 phr was replaced in a formulation containing 75 phr 4 MF PP homopolymer and 100. All formulations contained a standard AO package. Refer to Table 3 and 4 for formulation details. Table 3 Raw Material Control Substitute 6151 with 6014 (phr) Taipol 6151 100 90 80 70 Taipol 6014 0 10 20 30 Oil and PP 100 / 75 100 / 75 100 / 75 100 / 75 Table 4 Raw Material Control Substitute 6151 with Oil (phr) Taipol 6151 100 90 80 70 Oil 100 110 120 130 PP 75 75 75 75 Table 5 and 6 summarize their physical properties.
6014 substitution. Table 5 Control Substitute 6151 with 6014 (phr) Hardness, A Scale 85 81 81 82 MF, 200 C, 5 Kg 12.4 12.8 14.1 18.0 Tensile Strength, psi 3,210 2,930 2,278 1,858 Oil addition. Table 6 Control Substitute 6151 with Oil (phr) Hardness, A Scale 85 85 82 80 MF, 200 C, 5 Kg 12.4 21.3 45.6 97.0 Tensile Strength, psi 3,210 2,714 1,964 1,468 DP-6014 maintains the melt flow rate and hardness of these formulations, unlike oil. Addition of extra 10 increases the melt flow by 9 points. There is also a 10% loss in tensile strength in formulations containing additional oil. 6014 can be effectively used as a polymer to address oil bleeding issues. By replacing oil with 6014 one can formulate products with no loss in hardness while maintaining its physical properties. This feature should be useful in wire and cable and gel applications. 6014 blends also retain high loading of mineral fillers such as calcium carbonate. DP-027: DP-027 was primarily developed for medical applications. The first approach was to soften PP to feel like plasticized PVC and provide the necessary yield strength and elongation properties. The second was to produce transparent blends with clarified random polypropylene to provide excellent clarity. Table 7 shows physical properties of DP-027 produced in the semi-commercial plant and random copolymer (5060T, Formosa). Property Table 7 DP-027 (HP-06) Random copolymer PP Hardness, Shore A 40 97 (D 76) Melt flow (230 C, 2.16 kg) 3.3 8.8 Transparency (2.3 mm, UV550 nm) 65.9 % 27% Haze, % 17* %
Transverse Direction Tensile Strength, MPa 9.5 146 Elongation at break, % 970 180 Machine Direction Tensile Strength, MPa 6.7 179 Elongation at break, % 870 91 Rebound (45 ), % 74.3 -- *- presence of anti-block Table 8 shows key properties for 20 and 40% DP-027 bends with random copolymer. Table 8 Property 20% DP-27 40% DP-027 Hardness, Shore D 65 (A 97) 50 (A 95) Melt flow (230 C, 2.16 kg) 7.8 7.2 Transparency (2.3 mm, UV550 nm) 28.3 34.4 Transverse Direction Tensile Strength, MPa 160 166 Elongation at break, % 920 880 Machine Direction Tensile Strength, MPa 161 170 Elongation at break, % 800 720 By adding 20 or 40% SEBS DP-027 to the random copolymer there was a significant increase in elongation at break values (+700%) compared to unmodified random PP. The stress-strain curves (Figure 1) indicate that although these blends show plastic behavior with a distinct yield point, they have excellent rubber-like elongation. This shows the compatibility of these blends in a cocontinuous phase morphology. There is also an increase in tensile strength at break at 20 and 40% random copolymer addition. This may indicate that SEBS rubber is well dispersed in PP matrix. Only after adding more than 50% SEBS do the hardness values begin to drop significantly.
Figure 1 Tensile Stress (MPa) DP: 80 DP: 60 DP: 50 DP: 40 DP: 20 In summary DP-027/random copolymer blends provide balance in tensile strength/elongation break values while providing excellent transparency. These blends are well suited to replace PVC in medical applications. DP-029: Tensile Strain (%) DP-029 is a linear sequential triblock high molecular weight copolymer with approximately 23% styrene content. The uniqueness of DP-029 is its low styrene content and high molecular weight that allows extra oil holding capacity. It also offers good processability and softness combined with low compression set values. Table 9 shows formulations of DP-029, 6151 and 6159 with 130, 200 and 300 phr of oil and 37 parts of polypropylene. Table 9 Formulation DP-029 + 130 DP-029 + 200 DP-029 + 300 Hardness, A Scale 45 29 18 Compression set, 100 C 62 65 76 Rebound Resilience, % 55 44 46 Formulation 6151 + 130 6151 + 200 phr 6151 + 300 phr oil oil Hardness, A Scale 48 34 22
Compression set, 100 C 68 73 79 Rebound Resilience, % 52 47 43 Formulation 6159 + 130 6159 + 200 phr oil 6159 + 300 phr oil Hardness, A Scale 61 39 25 Compression set, 100 C 56 63 74 Rebound Resilience, % 49 44 40 As expected, lower styrenic content in DP-029 provides lower hardness and better resilience in flexible compounds with increased oil content. On the other hand higher molecular weight provides lower compression set than 6151. These formulations should also have better heat resistance properties. Conclusions These novel low styrene SEBS polymers provide a balance in property, processability and formulation flexibility to fit a variety of applications. DP-6014 provides ease of processabilitysoftness and strength-elasticity balance in formulations. The molecular design of DP-027 provides the softness and excellent miscibility with random copolymer propylene to make highly transparent blends for films and molded articles. DP-029 provides low hardness grades with excellent compression set and rebound. Acknowledgment The authors would like to acknowledge coworkers in Plaquemine, LA, and Kaohsiung, Taiwan for their help and support with this project. References [1] Jiri George Drobny, Handbook of Thermoplastic Elastomers, William Andrew Publishing (2007) [2] N.R. Legge, G. Holden, and H.E. Schroeder, Eds., Thermoplastic Elastomers, Hanser, New York (1987) [3] D. Handlin, Z. Cheng, M. Berard, A New Styrenic Block Copolymer Designed for Polyolefin-like Processing for Compounding, Films and Fibers, SPE ANTEC Conference (2015) [4] H.L. Hsieh and R.P. Quirk, Anionic Polymerization, CRC Press, New York (1996) [5] M. Berard, J. Nelson, C. Su and C.H. Wang, Ultra-high MW SEBS for High Performance Applications, SPE TOPCON Conference 2012