A New Styrenic Block Copolymer Designed for Polyolefin-like Processing for Compounding, Films and Fibers Dale Handlin, TSRC, Shelby North Carolina, Ziv Cheng, The TSRC, Kaohsiung, Taiwan, and Mark Berard, TSRC Dexco Company, Plaquemine, Louisiana, Abstract Block copolymers are highly valued for their ability to be extruded and injection molded in combination with other materials. However, they are typically processed in the phase separated state. The high melt elasticity of the phase separated block copolymer leads to a variety of problems with mixing and small dimension articles such as films, fibers and thin walled parts. A new hydrogenated styrenic block copolymer has been designed to process as a single phase melt but retain its two phase nature at use temperatures to provide strength and creep resistance. The processing and resulting properties of the pure polymer and combinations with a variety of polyolefins will be explored. Introduction Commercially useful styrenic block copolymers (SBCs) were first invented by Shell Chemical Company in the 1960 s. Their key innovation and the reason for their growth into a wide range of products with over >3,000 KT annual production is their ability to bind two incompatible polymers, a styrenic polymer and a polyolefin elastomer, into a form that maintains their individual characteristics of the soft polyolefin elastomer and the strength of a styrenic polymer. This combination is necessarily a balance between mechanical properties and useful rheology. If the phase separation is very strong, the polymer will be strong and resist creep and stress relaxation, essential features of a good elastomer, but thermoplastic flow in common processing equipment will be problematic if not impossible without process aids. Most styrenic block copolymers today are produced with a high or moderately high degree of phase separation. Additives such as naphthenic /paraffinic oils, are used to increase the flow to the desired range while other polymers such as polypropylenes and polyethylenes are added to adjust other properties such as heat distortion temperature or surface appearance. In other applications such as adhesives, tackifying resins and oils can be added to produce a wide range of adhesives from very tacky hot melt pressure sensitive adhesives, to cling films that can be removed with a tuned amount of force leaving no residue. While this approach has produced outstanding products, the addition of oil adds processing issues such as color, odor, taste and a concern about migration. A different approach is considered in this paper: the development of SBCs with a controlled degree of phase separation designed to match the processing temperature of polyolefins. This development allows the production of compounds which can be oil free with acceptable rheology for applications which are sensitive to orientation and applications requiring low odor or taste, or the production of oiled compounds with very high flow. These are particularly useful in thin molding and extrusion applications which require uniform flow at the die or mold for reliable, high speed processing. This polymer, which can be processed in a single phase melt like a polyolefin, yet phase separates into a two phase material at application temperatures, can easily be compounded with a wide variety of new polyolefin copolymers such as olefin statistical block copolymers to improve elasticity and reduce permanent set while maintaining easy processing. Materials A series of sequential hydrogenated styreneethylenebutylene-styrene triblock copolymers (hydrogenated styrene-butadiene-styrene) were synthesized with a controlled degree of phase separation by controlling χn near the weak phase segregation boundary [1]. The temperature below which the styrene endblocks phase separate in the melt is known as the order-disorder transition temperature (ODT). For blending components the materials can be divided into three groups: 1. Materials which increase flow such as tackifying resins and oils. Based on its high temperature stability and compatibility we have chosen a fully hydrogenated tackifying resin. Similarly we have chosen a low volatility, stable white oil. 2. Materials which decrease flow such as polystyrene. In this study we have chosen standard 7 MF crystal polystyrene. 3. Polymers with no fundamental effect on flow but with other properties of interest for compounding. In this study we have chosen three polypropylenes: a homopolymer, a polypropylene/ethylene copolymer, and a metallocene polypropylene plastomer, Vistamaxx 6103. We have also included two soft metallocene polyethylene copolymers, Tafmer 640 and Infuse 9500, and a LDPE.
Discussion The order-disorder transition temperature, as shown in Figure 1, was controlled to approximately 220 C. As Figure 1 shows, at temperatures below the ODT the elastic modulus is extremely high and the behavior is one of an elastomer, while above the ODT the loss modulus is higher and the flow is dominated by viscous flow, just as it is in a polyolefin melt. Because this SEBS is above its ODT at most processing temperatures, its rheology is more typical of normal polyolefins with a Newtonian plateau at very low shear rates followed by power law dependence at higher shear rates. The effect on viscosity is shown in Figure 2, which compares the capillary viscosity of the new material, DP014, to two typical commercial SEBS grades which have ODTs well above 300 C. The higher ODT polymers have much higher viscosities and show power law behavior throughout the shear rate range. Note that Figure 2 is on a log-log scale. Figure 3 shows the low ODT SEBS, DP014, compared to various commercial polyolefins, which also show a tendency toward a Newtonian plateau at low shear rates. Additionally, the viscosity is in the same range as the polyolefins, which will help mixing in processing equipment. Figure 3. Capillary viscosity at 230 C of the new experimental SEBS, DP014, & several commercial polypropylenes, polyethylenes, and a polystyrene. The influence of various additives on ODT can be explored by plotting the ODT against the % SEBS polymer in the blend as shown in Figure 4. As expected the ODT is reduced in the order of the flow rate of the additives: tackifying resin>oil> LDPE (in this case a 1000 MF LDPE). Figure 1. Dynamic rheology sweep through the Order- Disorder transition temperature of an SEBS with an ODT between 215 and 220 C. Figure 4. The change in ODT caused by the addition of blending materials of the three classes described above. The effect on flow of the blends caused by the addition of these lower molecular weight additives and their effect on ODT is shown in Figure 5. As expected, the oil, tackifying resin and LDPE greatly increase flow, largely because of their much higher flow. In these blends a 1000MF LDPE was used. Figure 2. Capillary viscosity at 230 C of the new experimental SEBS, DP014, & two commercial materials of higher ODT.
Figure 5. The increase in flow at 230 C caused by the addition of compatible, low molecular weight additives. Thin Molding and Extrusion SBCs have long been used in thin molded, extruded parts and films [2, 3]. Extrusion of continuous or semicontinuous thin sections is very sensitive to orientation which may lead to warping or at a minimum, property differences between the extrusion and transverse direction. Being able to extrude above ODT is essential for well-developed flow in a close tolerance die. To demonstrate this, the blends described in the previous figures were used to make transparent, thin extrusions on a laboratory scale line with a 25 cm wide die. The sheets were tested using standard tensile and hysteresis methods in both the machine and transverse direction. One of the most significant features of these profiles is that their machine direction and transverse direction properties are similar. This is shown in Figures 9 & 10 for permanent set, and was also found for the other properties. Despite the easy processing of these polymers and their blends, ultimate tensile strengths of the sheets can be quite high. Figure 6 shows the tensile strengths of the blends shown above in the machine direction. Materials which lower the ODT tend to lower tensile strengths. Although not shown, transverse tensile strength generally follows the same trend and is about 10% lower. Figure 6. Ultimate tensile strengths in the machine direction for thin extrusions of DP014 blended with modifiers. The effect on the machine direction modulus at 200% elongation, shown in Figure 7, is quite similar to the effect on the ODT as shown in Figure 4. Figure 7. The effect on blending additives on the machine direction 200% modulus. Hysteresis performance can be a very effective way to judge the performance of a thermoplastic elastomer. The figure below shows a common 200%, 3 cycle hysteresis test. Permanent set on the first cycle is one of the most sensitive measures of elasticity.
Figure 8. Example of 200% strain, three cycle hysteresis test. %Permanent set is the percent unrecovered strain at the bottom of the chart, and is shown in Figs 9&10. Tackifying resin, which can be viewed as a high molecular weight oil which is compatible with the midblock, reduces permanent set on the first cycle, up to a point, by allowing a more equilibrium chain configuration in spite of the orientation effect of the die. As the ODT is lowered too much by the tackifying resin, the polymer begins to creep, increasing set. Oil increases permanent set by inducing creep even at low levels, overcoming the effect of reduced orientation. LDPE increases permanent set uniformly beyond low levels because of the orientation induced crystallization that occurs in all readily crystallizable polyethylenes and polypropylenes. Once the crystals have been formed in the stretching process they increase set both in the machine and transverse direction. Figure 9. Machine direction permanent set for each of the blends shown above. The transverse direction permanent set is only slightly lower than the machine direction permanent set demonstrating that the extrusion results in a relatively isotropic part. The effects of additives are similar in the machine and transverse directions. Figure 10. Transverse Direction permanent set for comparison to Figure 9. Injection Molded Blends Because of its simple rheology, DP014 can be compounded with polyolefins just as any other polyolefin can. A series of polypropylenes and polyethylenes were compounded in a twin screw extruder and injection molded to measure the effect of blending on properties. Rigid polypropylenes DP014 was blended with a polypropylene copolymer and homopolymer in comparison with a standard grade, Taipol 6152. With the exception of improved processing with the higher flow DP014, both polymers improved toughness and elongation to about the same extent with the softer DP014 lowered hardness a few point more than the standard SEBS. Softer Polyolefins A range of soft polyolefins are currently available which blur the line between traditional polyolefins and elastomers and are often called plastomers or olefin block copolymers. These materials are much softer than the corresponding homopolymers and exhibit some recovery on stretching that is associated with elastomers. DP014 was blended with three different plastomers, a Polypropylene based (Vistamaxx 6012), a polyethylene based (Tafmer 640), and an olefin block copolymer (Infuse 9500). The tensile properties of the polypropylene elastomer were reduced somewhat by the addition of the DP014. However, the properties of the two polyethylene based elastomers were increased significantly as shown in Figure 11.
Figure 11. 300% Modulus and tensile strength of polyethylene elastomers blended with the high flow SEBS, DP014 comparing the neat polyolefin to a 50/50 blend. The same blends were subject to three cycle 200% hysteresis testing. In all cases the addition of DP014 significantly increased the stress at 200% in each cycle while reducing permanent set and hysteresis loss. This combination of properties is unusual and reflects the ability of the more elastic SEBS to dramatically improve elasticity in these blends. Figure 14. % Hysteresis Loss on each of three cycles of the pure polyolefin which is reduced by blending 50/50 with DP014 showing similar reductions in hysteretic loss when blending with the SEBS in spite of increased loads. Conclusions A new hydrogenated styrenic block copolymer has been designed to provide an excellent balance of processability, strength and elasticity. This polymer facilitates the formation of isotropic parts in challenging thin walled extrusion and molding conditions without the need for process oils. Blends with common additives and with polyolefin elastomers show promise for high performance thermoplastic elastomers in a wide range of applications. References Figure 12. Stress at 200% elongation on each cycle demonstrating the increase produced by blending 50/50 with DP014. 1. L. Leibler, Macromolecules, 13, 1602 (1980); 15, 1283 (1982). 2. A.J. Uzee, B.G. Witt, W.J. Grigar, S14th Internat. Conf. on TPEs, Brussels, Belgium, ismithers, (2011). 3. A. Uzee, B. Marino, F. Brown, TPE Magazine, 4, 230 (2014). Vistamaxx is a trademark of ExxonMobil Chemical. TAFMER is a trademark of Mitsui Chemicals. INFUSE is a trademark of the Dow Chemical Company. Taipol is a trademark of TSRC. Figure 13. Permanent Set on each cycle comparing the pure polyolefins with 50/50 blends with DP014. In spite of the increased stress at load, the permanent set is significantly reduced by blending with the SEBS.