Elastomeric Materials and Processes
Another important group of polymers is that group which is elastic or rubberlike, known as elastomers. This chapter discusses this group of materials, including TPEs, MPRs, TPVs, synthetic rubbers, and natural rubber.
3.2 Thermoplastic Elastomers (TPEs)
Worldwide consumption of TPE for the year 2000 is estimated to be about 2.5 billion pounds, primarily due to new polymer and processing technologies, with an annual average growth rate of about 6% between
1996 and 2000.4 About 40% of this total is consumed in North America.4
TPE grades are often characterized by their hardness, resistance to abrasion, cutting, scratching, local strain (deformation), and wear. The chapter author, editors, publisher, and companies referred to are not responsible for the use or accuracy of information in this chapter, such as property data, processing parameters, aplications.
conventional measure of hardness is Shore A and Shore D shown in Fig. 3.1. Shore A is a softer and Shore D is a harder TPE, with ranges from as soft as Shore A 40 to as hard as Shore D 82. Durometer hard- ness (ASTM D 2240) is an industry standard test method for rubbery materials, covering two types of durometers, A and D. The durometer is the hardness measuring apparatus; and the term durometer hard- ness is often used with Shore hardness values. There are other hard- ness test methods such as Rockwell hardness for plastics and electrical insulating materials (ASTM D 785 and ISO 2039), and Barcol hard- ness (ASTM D 2583) for rigid plastics. While hardness is often a quan- tifying distinction between grades, it does not indicate comparisons between physical/mechanical, chemical, and electrical properties.
Drying times depend on moisture absorption of a given resin. TPE
producers suggest typical drying times and processing parameters. Actual processing temperature and pressure settings are determined by resin melt temperatures and rheological properties, mold cavity design, and equipment design such as screw configuration.
Performance property tables provided by suppliers usually refer to compounded grades containing property enhancers (additives) such as stabilizers, modifiers, and flame retardants. Sometimes the suppliers’ property tables refer to a polymer, rather than a formulated compound.
Styrene block copolymers are the most widely used TPEs, accounting for close to 45% of total TPE consumption worldwide at the close of the twen- tieth century.1 They are characterized by their molecular architecture which has a “hard” thermoplastic segment (block) and a “soft” elas- tomeric segment (block) (see Fig. 3.2). Styrenic TPEs are usually styrene butadiene styrene (SBS), styrene ethylene/butylene styrene (SEBS), and styrene isoprene styrene (SIS). Styrenic TPEs usually have about 30 to
40% (wt) bound styrene; certain grades have a higher bound styrene con- tent. The polystyrene endblocks create a network of reversible physical.
Figure 3.1 TPEs bridge the hardness ranges of rubbers and plastics. (Source: Ref. 10, p. 5.2.)
Figure 3.2 Structures of three common styrenic block copolymer TPEs: a and c = 50 to 80; b = 20 to 100. (Source: Ref. 10, p. 5.12.)
cross-links which allow thermoplasticity for melt processing or solvation. With cooling or solvent evaporation, the polystyrene domains reform and harden, and the rubber network is fixed in position.2
Principal styrenic TPE markets are: molded shoe soles and other footwear; extruded film/sheet and wire/cable covering; and pressure- sensitive adhesives (PSA) and hot-melt adhesives, viscosity index (VI) improver additives in lube oils, resin modifiers, and asphalt modifiers. They are also popular as grips (bike handles), kitchen utensils, clear medical products, and personal care products.1,4 Adhesives and sealants are the largest single market.1 Styrenic TPEs are useful in adhesive compositions in web coatings.1
Styrenic block copolymer (SBC) thermoplastic elastomers are pro- duced by Shell Chemical (Kraton®*), Firestone Synthetic Rubber and Latex, Division of Bridgestone/Firestone (Stereon®†), Dexco Polymers (Vector®‡), EniChem Elastomers (Europrene®§), and other compa-
nies. SBC properties and processes are described for these four pro- ducers’ TPEs.
Kraton TPEs are usually SBS, SEBS, and SIS, as are SEP (styrene ethylene/propylene) and SEB (styrene ethylene/butylene).2 The poly- mers can be precisely controlled during polymerization to meet prop- erty requirements for a given application.2
Two Kraton types are chemically distinguished: Kraton G and Kraton
D. A third type, Kraton Liquid®,¶ poly(ethylene/butylene), is described
*Kraton is a registered trademark of Shell Chemical Company.
†Stereon is a registered trademark of Firestone Synthetic Rubber and Latex
Company, Division of Bridgestone/Firestone.
‡Vector is a registered trademark of Dexco, A Dow/Exxon Partnership.
§Europrene is a registered trademark of EniChem Elastomers.
¶Kraton Liquid is a registered trademark of Shell Chemical Company.
on page 3.5. Kraton G and D have different performance and processing properties. Kraton G polymers have saturated midblocks with better resistance to oxygen, ozone, ultraviolet (UV) radiation, and higher ser- vice temperatures, depending on load, up to 350°F (177°C) for certain grades.2 They can be steam sterilized for reusable hospital products. Kraton D polymers have unsaturated midblocks with service tempera- tures up to 150°F (66°C).2 SBC upper service temperature limits depend on the type and wt % thermoplastic and type and wt % elastomer, and the addition of heat stabilizers. A number of Kraton G polymers are lin- ear SEBS, while several Kraton D polymers are linear SIS.2 Kraton G polymer compounds’ melt process is similar to polypropylene; Kraton D polymer compounds’ process is comparable to polystyrene (PS).2
Styrenic TPEs have strength properties equal to vulcanized rubber, but they do not require vulcanization.2 Properties are determined by polymer type and formulation. There is a wide latitude in compound- ing to meet a wide variety of application properties.2 According to application-driven formulations, Kratons are compounded with a hardness range from Shore A 28 to 95 (Shore A 95 is approximately equal to Shore D 40), sp gr from 0.90 to 1.18, tensile strengths from
150 to 5000 lb/in2 (1.03 to 34.4 MPa), and flexibility down to —112°F
(—80°C) (see Table 3.1).2
Kratons are resistant to acids, alkalis, and water, but long soaking in hydrocarbon solvents and oils deteriorate the polymers.2
Automotive applications range from window seals and gasketing to enhanced noise/vibration attenuation.1 The polymers are candidates for automotive seating, interior padded trim and insulation, hospital padding, and topper pads.1 SEBS is extruded/blown into 1-mil films for disposable gloves for surgical/hospital/dental, food/pharmaceutical, and household markets.1
Kratons are used in PSAs, hot-melt adhesives, sealants, solution- applied coatings, flexible oil gels, modifiers in asphalt, thermoplastics, and thermosetting resins.2 When Kratons are used as an impact mod- ifier in nylon 66, notched Izod impact strength can be increased from
0.8 ft-lb/in for unmodified nylon 66 to 19 ft-lb/in. Flexural modulus may decrease from 44,000 lb/in2 (302 MPa) for unmodified nylon 66 to about 27,000 lb/in2 (186 MPa) for impact-modified nylon 66.
SBCs are injection molded, extruded, blow molded, and compression molded.2
Kraton Liquid polymers are polymeric diols with an aliphatic, pri- mary OH— group on each terminal end of the poly(ethylene/butylene) elastomer. They are used in formulations for adhesives, sealants, coat- ings, inks, foams, fibers, surfactants, and polymer modifiers.13
Two large markets for Firestone’s styrenic block copolymer SBS Stereon TPEs are: (1) impact modifiers (enhancers) for flame-retar- dant polystyrene and polyolefin resins and (2) PSA and hot-melt adhe- sives. Moldable SBS block copolymers possess high clarity, gloss, good flex cycle stability for “living hinge” applications, FDA-compliant grades for food containers, and medical/hospital products.1 Typical mechanical properties are: 4600-lb/in2 (31.7-MPa) tensile strength,
6000-lb/in2 (41.4-MPa) flexural strength, and 200,000-lb/in2 (1.4-GPa)
Stereon stereospecific butadiene styrene block copolymer is used as an impact modifier in PS, high-impact polystyrene (HIPS), polyolefin sheet and films, such as blown film grade linear low-density polyethyl- ene (LLDPE), to achieve downgauging and improve tear resistance and heat sealing.1 Blown LLDPE film modified with 7.5% stereospecific styrene block copolymers has a Dart impact strength of 185°F per 50 g, compared with 135°F per 50 g for unmodified LLDPE film. These copolymers also improve environmental stress crack resistance (ESCR), especially to fats and oils for meat/poultry packaging trays, increase melt flow rates, increase gloss, and meet U.S. FDA 21 CFR
177.1640 (PS and rubber modified PS) with at least 60% PS for food contact packaging.1 When used with thermoformable foam PS, flexibil- ity is improved without sacrificing stiffness, allowing deeper draws.1
The stereospecific butadiene block copolymer TPEs are easily dispersed and improve blendability of primary polymer with scrap for recycling.
Vector SBS, SIS, and SB styrenic block copolymers are produced as diblock-free and diblock copolymers.29 The company’s process to make linear SBCs yields virtually no diblock residuals. Residual styrene butadiene and styrene isoprene require endblocks at both ends of the polymer in order to have a load-bearing segment in the elastomeric network.29 However, diblocks are blended into the copolymer for cer- tain applications.29 Vector SBCs are injection-molded, extruded, and formulated into pressure-sensitive adhesives for tapes and labels, hot- melt product-assembly adhesives, construction adhesives, mastics, sealants, and asphalt modifiers.29 The asphalts are used to make mem- branes for single-ply roofing and waterproofing systems, binders for pavement construction and repair, and sealants for joints and cracks.29
Vector SBCs are used as property enhancers (additives) to improve the toughness and impact strength at ambient and low temperatures of engineering thermoplastics, olefinic and styrenic thermoplastics, and thermosetting resins.29 The copolymers meet applicable U.S. FDA food additive 21 CFR 177.1810 regulations and United States Pharmacopoeia (USP) (Class VI medical devices) standards for health- care applications.29
The company’s patented hydrogenation techniques are developed to improve SBC heat resistance as well as ultraviolet resistance.29
EniChem Europrene SOL T are styrene butadiene and styrene iso- prene linear and radial block copolymers.1 They are solution-polymer- ized using anionic type catalysts.33 The molecules have polystyrene endblocks with central elastomeric polydiene (butadiene or isoprene) blocks.33 The copolymers are (S-B)n X type where S = polystyrene, B = polybutadiene and polyisoprene, and X = a coupling agent. Both con- figurations have polystyrene (PS) endblocks, with bound styrene con- tent ranging from 25 to 70% (wt).1 Polystyrene contributes styrene hardness, tensile strength, and modulus; polybutadiene and polyiso- prene contribute high resilience and flexibility, even at low tempera- tures.1 Higher molecular weight (MW) contributes a little to mechanical properties, but decreases melt flow characteristics and processability.
The polystyrene and polydiene blocks are mutually insoluble, and this shows with two Tg peaks on a cartesian graph with tan 6 (y axis) versus temperature (x axis): one Tg for the polydiene phase and a second Tg for the polystyrene phase. A synthetic rubber, such as SBR, shows one Tg.33
The two phases of a styrenic TPE are chemically bound, forming a net-
work with the PS domains dispersed in the polydiene phase. This struc- ture accounts for mechanical/elastic properties and thermoplastic processing properties.33 At temperatures up to about 167°F (80°C), which is below PS Tg of 203 to 212°F (95 to 100°C) the PS phase is rigid.33 Consequently, the PS domains behave as cross-linking sites in the polydiene phase, similar to sulfur links in vulcanized rubber.33 The rigid PS phase also acts as a reinforcement, as noted here.33 Crystal PS, HIPS, poly-alpha-methylstyrene, ethylene vinyl acetate (EVA) copoly- mers, low-density polyethylene (LDPE), and high-density polyethylene (HDPE) can be used as organic reinforcements. CaCO3, clay, silica, and silicates act as inorganic fillers, with little reinforcement, and they can adversely affect melt flow if used in excessive amounts.33
The type of PS, as well as its % content, affect properties. Crystal PS, which is the most commonly used, and HIPS increase hardness, stiffness, and tear resistance without reducing melt rheology.1 High styrene copolymers, especially Europrene SOL S types produced by solution polymerization, significantly improve tensile strength, hard- ness, and plasticity, and they enhance adhesive properties.33 High styrene content does not decrease the translucency of the compounds.33
Poly-alpha-methylstyrene provides higher hardness and modulus, but abrasion resistance decreases.33 EVA improves resistance to weather, ozone, aging, and solvents, retaining melt rheology and finished prod- uct elasticity. The highest Shore hardness is 90 A, the highest melt flow is 16 g/10 min, and specific gravity is 0.92–0.96.1
Europrene compounds can be extended with plasticizers which are basically a paraffinic oil containing specified amounts of naphthenic and aromatic fractions.33 Europrenes are produced in both oil-extended and dry forms.1 Oils were specially developed for optimum mechanical, aging, processing, and color properties.33 Increasing oil content signifi- cantly increases melt flow properties, but it reduces mechanical prop- erties. Oil extenders must be incompatible with PS in order to avoid PS swelling, which would decrease mechanical properties even more.33
The elastomers are compounded with antioxidants to prevent ther- mal and photooxidation which can be initiated through the unsaturated zones in the copolymers.33 Oxidation can take place during melt pro- cessing and during the life of the fabricated product.33 Phenolic, or phosphitic antioxidants, and dilauryldithiopropionate as a stabilizer during melt processing are recommended.33 Conventional UV stabiliz- ers are used such as benzophenone and benzotriazine.33 Depending on the application, the elastomer is compounded with flow enhancers such as low MW polyethylene (PE), microcrystalline waxes or zinc stearate, pigments, and blowing agents.33
Europrene compounds, especially oil-extended grades, are used in shoe soles and other footwear.1 Principal applications are: impact mod- ifiers in PS, HDPE, LDPE, polypropylene (PP), other thermoplastic resins and asphalt; extruded hose, tubing, O-rings, gaskets, mats, swimming equipment (eye masks, snorkels, fins, “rubberized” suits) and rafts; and pressure-sensitive adhesives (PSA) and hot melts.1 SIS types are used in PSA and hot melts; SBS types are used in footwear.1
The copolymer is supplied in crumb form, and mixing is done by con- ventional industry practices, with an internal mixer or low-speed room temperature premixing and compounding with either a single- or twin- screw extruder.33 Low-speed premixing/extrusion compounding is the process of choice.
Europrenes have thermoplastic polymer melt processing properties and characteristics of TPEs. At melt processing temperatures, they behave as thermoplastics, and below the PS Tg of 203 to 212°F (95 to
100°C) the copolymers act as cross-linked elastomers, as noted earlier.
Injection-molding barrel temperature settings are from 284 to 374°F (140 to 190°C). Extrusion temperature at the head of the extruder is maintained between 212 and 356°F (100 and 180°C).