Styrenic thermoplastic elastomers.

Styrenic block copolymers were first commercialized in 1965 by Shell Chemical (ref. 1). The first commercial market was footwear, but in the last 30+ years styrenic block copolymers have branched out into several different market areas. Today, over a billion pounds per year of styrenic block copolymers are manufactured worldwide by more than 15 different companies. Some of the application areas include modified asphalts (roads and roofing), adhesives/sealants, thermoplastic rubber compounds and impact modification.

Block copolymer synthesis

Styrenic block copolymers owe their success to a unique block structure. The key to the structure is the "living" anionic synthesis chemistry used in the manufacturing process. Figure 1 shows the steps used to make a block copolymer (ref. 2). Typically, the polymerization begins by adding a very pure styrene monomer to an alkyl lithium catalyst in an appropriate solvent. This catalyst system has the property of polymerizing monomer units at a very uniform rate in each of the polymer chains. In addition, it does not die or undergo any side reactions. The catalyst site remains "living" at the end of the polymer chain as long as water and other chain terminating agents are excluded from the reactor. The reaction continues until all of the styrene monomer is consumed. Thus linear polymer chains of polystyrene (S) are formed which have the same molecular weight. Secondly, all of the chains have "living reactive chain ends." The next sequence of the reaction starts by adding a new monomer such as isoprene (I) or butadiene (B). The diene monomer units are polymerized onto the ends of the chain in a uniform manner, thus forming identical styrene-diene diblock, copolymer molecules. The styrene block typically will have a molecular weight of 4,000-40,000 and the diene block will have a molecular weight of 20,000 to 150,000. At this point, one of three things can be done. The reaction can be terminated to form a diblock. A second option is to add more styrene monomer to make a third block of polystyrene. The third option is to add a coupling agent. If a difunctional coupling agent is added to the reactive diblock, a triblock copolymer is formed (SI-x-IS or SIS). With multifunctional coupling agents, multiarm polymers can be formed with 3 to 20+ arms.

[Figure 1 ILLUSTRATION OMITTED]

The diene blocks, such as polyisoprene and polybutadiene are elastomeric and still contain unsaturated double bonds. The SIS and SBS polymers can then be subjected to a further chemical reaction that selectively hydrogenates the double bonds in the diene elastomer and leaves the polystyrene endblocks unchanged. Thus, SEBS (styrene-ethylene/butylenestyrene) and SEPS (styrene-ethylene/propylene-styrene) polymers can be formed which have outstanding UV and thermal stability. The SEBS or SEPS polymers can be reacted further by grafting monomers such as maleic anhydride onto the rubber block of the molecule.

Physical properties

The block structure is what creates the unique properties of these polymers. The polystyrene blocks are incompatible with the elastomeric diene segments and, therefore, they phase separate on a microscopic scale, like oil and water. The resulting structure looks like a meatballs and spaghetti structure with the styrene segments forming hard spherical domains and the elastomeric segments linking the styrene domains into a three-dimensional structure (ref. 3). The styrene domains are hard and glassy and form physical crosslinks for binding the molecules together to form a strong, elastic network. The tensile strength of the block copolymers can be as high as 20-35 MPa. This is greater than most chemically crosslinked elastomers. For many applications, this means the block copolymers can be very highly extended with oils, tackifying resins or other polymers and still have excellent strength.

The beauty of the styrene blocks is that they retain their strength up to temperatures near the glass transition of polystyrene (about 95 [degrees] C). At this point, the domain softens and the copolymer will flow like any thermoplastic polymer. When the block copolymer is cooled, the hard polystyrene domains reform and the strength returns. The reforming process is so quick that hot melt adhesive coating lines can be run in excess of 5 m/sec. Processing can also be achieved by dissolving the block copolymer in a solvent that will solvate both phases. Since the block copolymers are only about 100,000 in molecular weight versus other conventional elastomers like natural rubber (1,000,000 molecular weight), the solution viscosity of the block copolymer is relatively low. Adhesive formulations with high solids levels (50%+) can easily be achieved. By comparison, natural rubber adhesive solutions are typically limited to 25-35% solids. In addition, the block copolymers will regain their strength once the solvent is evaporated. No time consuming and complicated chemical crosslinking steps are required as is commonly practiced with natural rubber or other elastomers.

Formulating

The uniqueness of the block copolymer structure extends beyond processing. The two-phase structure gives the formulator two phases to modify to achieve an appropriate balance of elastic properties. For instance, oils and resins can be added to the elastomeric midblock phase to soften or even tackify the polymer. Aromatic endblock resins can be added to the polystyrene endblock regions to harden and strengthen compounds. Other polymeric materials can also be blended with the block copolymers to form interpenetrating networks and technologically useful mixtures.

Features of the block copolymers

Besides being strong, highly extendible, cost effective, processable and easily formulated into a variety of compounds or adhesives, the styrenic block copolymers have other attributes that are very beneficial (table 1). The polymerization chemistry requires that the systems and chemicals are very clean and pure. This results in FDA approval for many of the products. The polymers are clear (the styrene domains are too small to scatter light) and can be formulated into transparent elastomeric products. The specific gravity of the block copolymers is around 0.94 and is even less when formulated with appropriate ingredients. This can be important for paper recycling processes that separate ingredients based on density (ref. 4). The polymers have good resistance to acids and bases and can even be used in dispensing systems for battery acid. For applications requiring good UV and oxidative resistance, the hydrogenated styrenic polymers are particularly useful. For increased adhesion or compatibility with polar materials such as polyamides or polyesters, maleated block copolymers such as SEBS-MA 1901 (table 2) can be employed (ref. 5).

Table 1 - features and benefits

General styrenic block copolymers
No vulcanization required - high processing speeds
Precise molecules - wide range of structure
Clean - FDA approved
High strength - highly extendable
Low viscosity - high solids
Hot melt or solvent applied
Wide formulating latitude - two phases to modify
Clear
Aggressive adhesion to polyolefins
Low density - aids recycling
Cohesive strength adjustable with diblock content
Electrically insulating
Resistant to acids and bases
Free flowing pellets/powder - no mastication required
Table 2 - characteristics of various styrenic block copolymers

Polymer                       Commercial
                              designation

SBS 1101                      Kraton D 1101(*)
SBS 1102                      Kraton D 1102
SEBS 1650                     Kraton G 1650
SEBS 1651                     Kraton G 1651
SEBS 1652                     Kraton G 1652
SEP 1702                      Kraton G 1702
SEBS-MA 1901                  Kraton FG 1901X
[(SB).sub.2]-x-[(B).sub.2]    KX-222(*)
[(SEB).sub.2]-x-[(I).sub.2]   RP-6919(*)
el-EB-OH                      L-207(*)

Polymer                       Description

SBS 1101                      31% styrene, SBS
SBS 1102                      28% styrene, SBS
SEBS 1650                     30% styrene, SEBS
SEBS 1651                     33% styrene, SEBS
SEBS 1652                     30% styrene, SEBS
SEP 1702                      28% styrene, SEP diblock
SEBS-MA 1901                  30% styrene, SEBS with
                              1.7 wt% maleic anhydride
[(SB).sub.2]-x-[(B).sub.2]    18% styrene, dissimilar
                              arm styrene-butadiene
                              block copolymer where
                              the polybutadiene has
                              high levels of 1,2 polybutadiene
[(SEB).sub.2]-x-[(I).sub.2]   18% styrene, 36% isoprene,
                              dissimilar arm
                              styrene, ethylene/butylene,
                              isoprene block
                              copolymer
el-EB-OH                      Poly(ethylene/butylene)
                              rubber with 11 epoxy units
                              and a primary hydroxyl

(*) Shell Chemical

Application areas

Modified asphalts

Asphalt materials are typically used in the roads and roofing industry. Asphalts are conventionally "oxidized" or "blown" to improve their high temperature properties so they can be used on sloped roofs or roads that will not rut in hot climates. This process, however, compromises or severely limits the low temperature properties. The addition of 3-20% styrenic block copolymer to an unblown asphalt will significantly improve the high temperature flow properties and elasticity of the asphalt, while at the same time improving the low temperature properties (figure 2). Thus, the service temperature range of asphalts can be expanded. Rutting and low temperature cracking can be improved in roads, fatigue and low temperature cracking can be improved in roofing, and road crack sealants will have improved thermal cycling and seasonal durability. In addition, the modified asphalt can be processed like a conventional asphalt at 125-200 [degrees] C in roll roofing lines, road paving, mopping asphalts, road crack sealants or shingle manufacturing lines.

[Figure 2 ILLUSTRATION OMITTED]

Adhesives

The high tensile strength and two phase structure of the styrenic block copolymers allow them to be formulated with tackifying resins, oils, aromatic endblock resins and fillers to make a wide spectrum of adhesive products. These products can vary from pressure sensitive tape or labels, to assembly adhesives, to construction adhesives for plywood paneling. The FDA clearances and high-speed thermoplastic processing makes them ideally suited for diaper assembly and adhesives for personal care products. The key to many adhesive applications is to carefully formulate the rheological properties of the adhesive or sealant to achieve the correct balance of properties. Label and sealant applications can benefit from using diblock copolymers in combination with triblock copolymers to achieve higher peel values, better die cuttability and lower cohesive strength. Thus, sealants can be made to fail cohesively instead of adhesively. Oils can be modified with as little as 5-15% of hydrogenated block copolymers to make elastomeric water proofing sealants or cable filling oil gels.

Compounds

As mentioned previously, the styrenic block copolymers can be combined with oils, fillers, polyolefins and other polymeric materials to achieve a wide spectrum of elastomeric properties, while still maintaining good processability. Special FDA clearances allow it to be used in many food applications, such as milk tubing. The good electrical insulating properties and aging properties of the hydrogenated block copolymers allow them to be formulated into many different hardness grades, which can meet stringent UL wire and cable ratings. The low density, low temperature flexibility, high strength and the economical thermoplastic processing make SBS compounds ideal for thick soled shoes and casual sneakers. Because of the high strength of the styrenic block copolymers, highly extended and very soft elastomeric compounds are also possible.

Polymer modification

Styrenic block copolymers are ideally suited for impact modifying styrenics and polyphenylene ether polymers (figure 3). The hydrogenated block copolymers, because of their olefinic character (EB or EP rubber midblock), work well for modifying polyolefins (table 3), such as polyethylene, polypropylene or polybutylene, while maintaining good clarity and flow. Submicron interpenetrating network structures are possible with polyolefins and the hydrogenated block copolymers. Polycarbonate even benefits from the addition of small amounts of block copolymer to improve thick section impact and annealing properties (figure 4). With the advent of maleated block copolymers, polyamide and polyester engineering thermoplastics can now be modified to improve impact strength (figure 5) (ref. 6).

[Figure 3-5 ILLUSTRATION OMITTED]

Table 3 - polypropylene/block copolymer blends (80/20)

Polymer      Room temperature     Flexural   Spiral flow
            notched Izod impact   modulus        (m)
                   (J/m)           (GPa)

SEBS 1652           560             889         0.62
SEBS 1650           678             945         0.57
SEBS 1651           224             855          --
SEP 1702            577             758

New developments

The microstructure of the midblock of SBS and SIS polymers can be adjusted to yield polymers which have lower viscosities, higher reactivity and improved sound dampening properties (higher Tgs). A new [(SB).sub.2]-x-[(B).sub.2] polymer can be formulated into a hot melt pressure sensitive adhesive that can be cured with UV light and a photoinitiator even in the presence of air (ref. 7). A new partially hydrogenated block copolymer [(SEB).sub.2]-x-[(I).sub.2] has been developed which manifests a unique balance of aging properties, crosslinkability and softness that is useful for PSA or compound applications (ref. 8). The newest generation of anionically synthesized block copolymers is opening opportunities in liquid reactive systems (figure 6). The polymer shown in figure 6 is less than 10,000 molecular weight and is an epoxidized polyisoprene-poly(ethylene/butylene) block copolymer with a terminal hydroxyl group (eI-EB-OH). The functional groups offer a host of new reactive possibilities and cationic curing mechanisms, which are useful in adhesive and polymer modification applications (refs. 9 and 10).

[Figure 6 ILLUSTRATION OMITTED]

Summary

Styrenic block copolymers are an important segment of the elastomer industry. Their unique and precisely controlled molecular structure allows these elastomers to be tailored to a variety of applications. No other elastomer services such a wide diversity of application areas. From formulating, to processing, to choosing the appropriate molecular structure, styrenic block copolymers offer a wide vista of new properties and processing possibilities.

References

(1.) U.S. Patent 3,265,765.

(2.) N.R. Legge, S. Davison, H.E. DeLaMare, G. Holden, M.K. Martin, Applied Polymer Science, 2nd Ed. R.W. Tess and G.W. Poehlein, Eds. ACS Symposium Series No. 285, ch. 9, American Chemical Society, Washington, D. C., 1985.

(3.) J.T. Harlan, L.A. Petershagen, E.E. Ewins, G.A. Davies, Handbook of Adhesives, Irving Skeist, Ed., p. 239, Van Nostrand Reinhold, New York, 1990.

(4.) E.M. Wise, J.M. Arnold, Tappi Hot Melt Symposium, Tappi Press, 131 (1992).

(5.) S.H. Dillman, A. Sanders, Tappi Hot Melt Symposium, Tappi Press, 39 (1989).

(6.) R. Gelles, M. Modic, J. Kirkpatrick, Conference Proceedings for Society of Plastic Engineers 46th Annual Technical Conference, 513 (1988).

(7.) M. DuPont, The Journal of the Adhesives and Sealants Council (Spring Symposium), 229 (1997).

(8.) G.R. Himes, E.M. Loveday, Tappi Hot Melt Symposium, Tappi Press, 1 (1994).

(9.) J.R. Erickson, E.M. Zimmermann, J.G. Southwick, K.S. Kiibler, Adhesives Age, November, 18 (1995).

(10.) J.R. Erickson, P.A. Mancinelli, Proceedings of Pressure Sensitive Tape Council, 25 (1998).3