Commercial block polymers thermoplastic rubber

S-B-S and S-I-S. Much of our discussion will refer directly to data for S-B-S and S-I-S block polymers. We justify this on several counts. These block polymers can be clearly defined as to structures, molecular weights, and compositions. They have served as model systems for much of the recent work in block polymers. They also comprise the largest volume of commercial block polymers. Finally, we believe that the discovery (20) of the S-B-S and S-I-S thermoplastic rubbers which are strong, resilient rubbers without vulcanization, and the concomitant, readily understood theory (21). provided a paradigm (terminology of T. S. Kuhn (2 2)) that significantly accelerated the scientific work on these polymers in recent years. 

Although the dynamic mechanical properties and the stress-strain behavior iV of block copolymers have been studied extensively, very little creep data are available on these materials (1-17). A number of block copolymers are now commercially available as thermoplastic elastomers to replace crosslinked rubber formulations and other plastics (16). For applications in which the finished object must bear loads for extended periods of time, it is important to know how these new materials compare with conventional crosslinked rubbers and more rigid plastics in dimensional stability or creep behavior. The creep of five commercial block polymers was measured as a function of temperature and molding conditions. Four of the polymers had crystalline hard blocks, and one had a glassy polystyrene hard block. The soft blocks were various kinds of elastomeric materials. The creep of the block polymers was also compared with that of a normal, crosslinked natural rubber and crystalline poly(tetra-methylene terephthalate) (PTMT). 

Global consumption of thermoplastic rubbers of all types is estimated at about 600,000 t/yr (51). Of this, 42% was estimated to be consumed in the United States, 39% in Western Europe, and 19% in Japan. At present, the worldwide market is estimated to be divided as follows styrenic block copolymers, 48% hard polymer/elastomer combinations, 26% thermoplastic polyurethanes, 12% thermoplastic polyesters, 4% and others, 9%. The three largest end uses were transportation, 23% footwear, 18% and adhesives, coatings, etc, 16%. The ranges of the hardness values, prices, and specific gravities of commercially available materials are given in Table 4. 

We suggest that the S-B-S thermoplastic rubbers and the domain theory have produced a paradigm upon which the block polymer field advanced significantly. In reviewing the technological discovery of the S-B-S thermoplastic rubbers and the virtually simultaneous formation of a theory that enabled us to move rapidly toward commercialization, it was of interest to us to trace a pathway to discovery for the relatively small group concerned with it. We have set forth in Table II some selected events that we believe enabled us to very rapidly understand the physical phenomena and proceed. Table II is, then, not intended in any way to be a comprehensive illustration of the history of block polymers but is rather a discrete list of events leading to the discovery of S-B-S and S-I-S thermoplastic rubbers in our laboratory. 

This discovery culminated in the commercial production and the announcement (41) in 1965 of thermoplastic elastomers from block polymers of styrene and butadiene (S-B-S) and of styrene and isoprene (S-I-S). To rubber scientists and technologists the most outstanding property of S-B-S and S-I-S was the unvulcanized tensile strength compared to that of vulcanized NR and vulcanized SBR carbon black stocks. Stress-strain curves, to break, of these latter materials are compared to that of S-B-S in Figure 2. It was pointed out that the high strength of S-B-S must be due to physical crosslinks. 

In the last years of the history shown in Table II we see the announcements of new commercial thermoplastic rubbers. Uniroyal TPR appeared in 1971. (This thermoplastic rubber many not be a block polymer. Presumably it is a blend that achieves its properties by virtue of interpenetrating networks between the plastic and rubber constituents. The exact structure has not been disclosed.) Du Pont s Hytrel, an (A-B) polyetherpolyester thermoplastic rubber, came out in 1972. Also in 1972, Shell announced a second generation block polymer, Kraton-G, which is a three-block S-EB-S thermoplastic rubber (EB represents an ethylene-butylene rubbery midblock). 

The unusual and attractive properties of the block polymers already identified, and the almost limitless combinations of possible block polymer structures, argue for an unbounded future. The rapidly growing applications for the commercial thermoplastic rubber block polymers of Table III have confirmed the trend. To lend some credibility to our look at the future, however, we have restricted it to the area of A-B-A block polymers in which we have the most experience. Some of the future trends we suggest are higher service temperature, oxidative stability, better processability, solvent resistance, flame retardance, electrical conductivity. 

Thermoplastic rubber is a relatively new class of polymer. It has the solubility and thermoplasticity of polystyrene, while at ambient temperatures it has the toughness and resilience of vulcanized natural rubber or polybutadiene. These rubbers are actually block copolymers. The simplest form consists of a rubbery mid-block with two plastic end blocks (A-B-A), as shown in Figure 5.7. Examples of commercial products are Kraton and Solprene . These materials are often compounded with plasticizers to decrease hardness and modulus, eliminate drawing, enhance pressure-sensitive tack, improve low-temperature flexibility, reduce melt and solution viscosity, decrease cohesive strength or increase plasticity if desired, and substantially lower material costs. Low levels of thermoplastic rubbers are sometimes added to other rubber adhesives. These materials are used as components in the following applications PSAs, hot-melt adhesives, heat-activated-assembly adhesives, contact adhesives, reactive contact adhesives, building construction adhesives, sealants, and binders. Two common varieties of thermoplastic rubber adhesives are styrene-butadiene-styrene (S-B-S) and styrene-isoprene-styrene (S-I-S). ... 

As indicated in Section 18.5.1, the styrene-butadiene block copolymers which are of commercial interest are those which have the characteristics of thermoplastic rubbers . These polymers have the structure S—B—S, where S represents a block of about 150 styrene units and B represents a block of about 1000 butadiene units. (This composition corresponds to a styrene content of approximately 35% and a molecular weight of 85 000.)... 

SBS and SIS Thermoplastic Rubbers (Harlan, 1977 Chu, 1986) - Styrene-butadiene s rene and styrene-isoprene-s rene are thermoplastic rubber block copolymers. They were larst marketed commercially in 1965. The polymers have rubbery midblocks of butadiene or isoprene molecules and two plastic end blocks of styrene molecules. The polymers have the modulus and resilience of vulcanized butadiene and isoprene at room temperature and act as thermoplastics at higher temperatures. When SBS or SIS molecules are combined in the solid phase, a two-phase structure is formed by the clustering of the styrene endblocks. The plastic endblock regions are called domains which act as crosslinks between the ends of the rubber chains (butadiene or isoprene) locking them in place. The block copolymers act like a typical vulcanized rubber that is filled with dispersed reactive filler particles. 

Thermoplastic elastomers (TPE), 9 565-566, 24 695-720 applications for, 24 709-717 based on block copolymers, 24 697t based on graft copolymers, ionomers, and structures with core-shell morphologies, 24 699 based on hard polymer/elastomer combinations, 24 699t based on silicone rubber blends, 24 700 commercial production of, 24 705-708 economic aspects of, 24 708-709 elastomer phase in, 24 703 glass-transition and crystal melting temperatures of, 24 702t hard phase in, 24 703-704 health and safety factors related to, 24 717-718... 

Polyurethane multiblock copolymers of the type described by Eqs. 2-197 and 2-198 constitute an important segment of the commercial polyurethane market. The annual global production is about 250 million pounds. These polyurethanes are referred to as thermoplastic polyurethanes (TPUs) (trade names Estane, Texin). They are among a broader group of elastomeric block copolymers referred to as thermoplastic elastomers (TPEs). Crosslinking is a requirement to obtain the resilience associated with a rubber. The presence of a crosslinked network prevents polymer chains from irreversibly slipping past one another on deformation and allows for rapid and complete recovery from deformation. 

The hydrogenation of the centre block of SBS copolymer produced oxidation stable thermoplastic elastomer. This product was commercialized by the Shell Development Company under the trade name of Kraton G. The field of thermoplastic elastomers based on styrene, 1-3-butadiene or isoprene has expanded so much in the last 10 years that the synthetic rubber chemist produced more of these polymers than the market could handle. However, the anionically prepared thermoplastic system is still the leader in this field, since it produced the best TPR s with the best physical properties. These TPR s can accommodate more filler, which reduces the cost. For example, the SBS Kraton type copolymer varies the monomer of the middle block to produce polyisoprene at various combinations, then, followed... 

Polystyrene is one of the most widely used thermoplastic materials ranking behind polyolefins and PVC. Owing to their special property profile, styrene polymers are placed between commodity and speciality polymers. Since its commercial introduction in the 1930s until the present day, polystyrene has been subjected to numerous improvements. The main development directions were aimed at copolymerization of styrene with polar comonomers such as acrylonitrile, (meth)acrylates or maleic anhydride, at impact modification with different rubbers or styrene-butadiene block copolymers and at blending with other polymers such as polyphenylene ether (PPE) or polyolefins. 

Multiphase polymer blends are of major economic importance in the polymer industry. The most common examples involve the impact modification of a thermoplastic by the microdispersion of a rubber into a brittle polymer matrix. Most commercial blends consist of two polymers combined with small amounts of a third, compatibilizing polymer, typically a block or graft copolymer. 

Styrene-butadiene block copolymers (SBC) with a high (70-85 %) styrene content are commercially produced and marketed as transparent, stiff, and tough thermoplastic resins under the trade names of Styrolux (Styrolution), K-Resin (Chevron-Phillips), Finaclear (Total petrochemical), and Clearene (Denka-Kaguku). Unlike other more elastomeric types of styrene-butadiene block copolymers, the rigid SBC resins contain only <25 % polybutadiene rubber content. Structurally, these SBC polymers are composed of polystyrene (S) and polybutadiene (B) blocks, linked together in an unsymmetrical star-block [(S-B)x] structure. 

Multiphase or multicomponent polymers can clearly be more complex structurally than single phase materials, for there is the distribution of the various phases to describe as well as their internal structure. Most polymer blends, block and graft copolymers and interpenetrating networks are multiphase systems. A major commercial set of multiphase polymer systems are the toughened, high impact or impact modified polymers. These are combinations of polymers with dispersed elastomer (rubber) particles in a continuous matrix. Most commonly the matrix is a glassy amorphous thermoplastic, but it can also be crystalline or a thermoset. The impact modified materials may be blends, block or graft copolymers or even all of these at once. 

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