3.2.2 Olefenics and TPO elastomers
Thermoplastic polyolefin (TPO) elastomers are typically composed of ethylene propylene rubber (EPR) or ethylene propylene diene “M” (EPDM) as the elastomeric segment and polypropylene thermoplastic segment.18 LDPE, HDPE, and LLDPE; copolymers ethylene vinyl acetate (EVA), ethylene ethylacrylate (EEA), ethylene, methyl-acry- late (EMA); and polybutene-1 can be used in TPOs.18 Hydrogenation of polyisoprene can yield ethylene propylene copolymers, and hydrogena- tion of 1,4- and 1,2-stereoisomers of S-B-S yields ethylene butylene copolymers.1
TPO elastomers are the second most used TPEs on a tonnage basis, accounting for about 25% of total world consumption at the close of the twentieth century (according to what TPOs are included as thermo- plastic elastomers).
EPR and polypropylene can be polymerized in a single reactor or in two reactors. With two reactors, one polymerizes propylene monomer to polypropylene, the second copolymerizes polypropylene with ethyl- ene propylene rubber (EPR) or EPDM. Reactor grades are (co)poly- merized in a single reactor. Compounding can be done in the single reactor.
Montell’s in-reactor Catalloy®* (“catalytic alloy”) polymerization
process alloys propylene with comonomers, such as EPR and EPDM, yielding very soft, very hard, and rigid plastics, impact grades or elastomeric TPOs, depending on the EPR or EPDM % content. The term olefinic for thermoplastic olefinic elastomers is arguable, because of the generic definition of olefinic. TPVs are composed of a continuous thermoplastic polypropylene phase and a discontinuous vulcanized rubber phase, usually EPDM, EPR, nitrile rubber, or butyl rubber.
Montell describes TPOs as flexible plastics, stating “TPOs are not TPEs.”32a The company’s Catalloy catalytic polymerization is a cost- effective process, used with propylene monomers which are alloyed with comonomers, including the same comonomers with different mol- ecular architecture.
*Catalloy is a registered trademark of Montell North America Inc., wholly owned by the Royal Dutch/Shell Group.
tors that allow the separate polymerization of a variety of monomer streams.32 Alloyed or blended polymers are produced directly from a series of reactors which can be operated independently from each other to a degree.32
Typical applications are: flexible products such as boots, bellows, drive belts, conveyor belts, diaphragms, keypads; connectors, gaskets, grommets, lip seals, O-rings, plugs; bumper components, bushings, dunnage, motor mounts, sound deadening; and casters, handle grips, rollers, and step pads.9
Insite®* technology is used to produce Affinity®† polyolefin plas-
tomers (POP), which contain up to 20 wt % octene comonomer.14 Dow Chemical’s 8-carbon octene polyethylene technology produces the com- pany’s ULDPE “Attane” ethylene-octene-1 copolymer for cast and blown films. Alternative copolymers are 6-carbon hexene and 4-carbon butene, for heat-sealing packaging films. Octene copolymer POP has lower heat-sealing temperatures for high-speed form-fill-seal lines and high hot-tack strength over a wide temperature range.14 Other bene- fits cited by Dow Chemical are: toughness, clarity, and low taste/odor transmission.14
Insite technology is used for homogeneous single-site catalysts which produce virtually identical molecular structure such as branch- ing, comonomer distribution, and narrow molecular weight distribu- tion (MWD).14 Solution polymerization yields Affinity polymers with uniform, consistent structures, resulting in controllable, predictable performance properties.14
Improved performance properties are obtained without diminishing processability because the Insite process adds long-chain branching onto a linear short-chain, branched polymer.14 The addition of long- chain branches improves melt strength and flow.14 Long-chain branch- ing results in polyolefin plastomers processing at least as smooth as LLDPE and ultra-low-density polyethylene (ULDPE) film extrusion.14
Polymer design contributes to extrusion advantages such as enhanced shear flow, drawdown, and thermoformability.
For extrusion temperature and machine design, the melt temperature is 450 to 550°F (232 to 288°C), the feed zone temperature setting is 300 to 325°F (149 to 163°C), 24/1 to 32/1 L/D; for the sizing gear box, use 5 lb/h/hp (1.38 kg/h/kW) to estimate power required to extrude POP at a given rate; for single-flight screws, line draw over the length of the line,
10 to 15 ft/min (3 to 4.5 m/min) maximum. Processing conditions and equipment design vary according to the resin selection and finished product. For example, a melt temperature of 450 to 550°F (232 to 288°C)
*Insite is a registered trademark of DuPont Dow Elastomers LLC.
†Affinity is a registered trademark of The Dow Chemical Company.
applies to cast, nip-roll fabrication using an ethylene alpha-olefin POP,14a while 350 to 450°F (177 to 232°C) is recommended for extru- sion/blown film for packaging, using an ethylene alpha-olefin POP.14b
POP applications are: sealants for multilayer bags and pouches to package cake mixes, coffee, processed meats and cheese, and liquids; overwraps; shrink films; skin packaging; heavy-duty bags and sacks; and molded storage containers and lids.14
Engage®*polyolefin elastomers (POE), ethylene octene copolymer
elastomers, produced by DuPont Dow Elastomers, use Insite catalytic technology.5 Table 3.2 shows their low density and wide range of phys- ical/mechanical properties (using ASTM test methods).5
The copolymer retains toughness and flexibility down to —40°F
(—40°C).5 When cross-linked with peroxide, silane, or by radiation, heat resistance and thermal aging increase to >302°F (>150°C).5 Cross- linked copolymer is extruded into covering for low- and medium-voltage cables. POE elastomers have a saturated chain, providing inherent UV stability.5 Ethylene octene copolymers are used as impact modifiers, for example, in polypropylene. Typical products are: foams and cushioning components, sandal and slipper bottoms, sockliners and midsoles, swim fins, and winter and work boots; TPO bumpers, interior trim and rub strips, automotive interior air ducts, mats and liners, extruded hose and tube, interior trim, NVH applications, primary covering for wire and
*Engage is a registered trademark of DuPont Dow Elastomers LLC.
3.2.3 Polyurethane thermoplastic elastomers (TPUs)
TPUs are the third most used TPEs, accounting for about 15% of TPE
Linear polyurethane thermoplastic elastomers can be produced by reacting a diisocyanate [methane diisocyanate (MDI) or toluene diiso- cyanate (TDI)] with long-chain diols such as liquid polyester or poly- ether polyols, and adding a chain extender such as 1,4-butanediol.17,18c The diisocyanate and chain extender form the hard segment, and the long-chain diol forms the soft segment.18c For sulfur curing, unsatura- tion is introduced, usually with an allyl ether group.17 Peroxide curing agents can be used for cross-linking.
The two principal types of TPUs are polyether and polyester. Polyethers have good low-temperature properties and resistance to fungi; polyesters have good resistance to fuel, oil, and hydrocarbon solvents.
BASF Elastollan®* TPU elastomer property profiles show typical
properties of polyurethane thermoplastic elastomers (see Table 3.3).
Shore hardness can be as soft as 70 A and as hard as 74 D, depend- ing on the hard/soft segment ratio. Specific gravity, modulus, com- pressive stress, load-bearing strength, and tear strength are also hard/soft ratio dependent.18c TPU thermoplastic elastomers are tough, tear resistant, abrasion resistant, and exhibit low-tempera- ture properties.4
Dow Plastics Pellethane®† TPU elastomers are based on both poly-
ester and polyether soft segments.3
Five series indicate typical applications:
1. Polyester polycaprolactones for injection-molded automotive pan- els, painted (without primer) with urethane and acrylic enamels, or water-based elastomeric coating
2. Polyester polycaprolactones for seals, gaskets, and belting
3. Polyester polyadipates extruded into film, sheet, and tubing
*Elastollan is a registered trademark of BASF Corporation.
†Pellethane is a registered trademark of The Dow Chemical Company.
4. Polytetramethylene glycol ethers with excellent dielectrics for extruded wire and cable covering, and also for films, tubing, belting, and caster wheels
5. Polytetramethylene glycol ethers for healthcare applications3
Polyether-polyester hybrid specialty compounds are the softest nonplas- ticized TPU (Shore hardness 70 A), and are used as impact modifiers.
Polycaprolactones possess good low-temperature impact strength for paintable body panels, good fuel and oil resistance, and hydrolytic sta- bility for seals, gaskets, and belting.3 Polycaprolactones have fast crys- tallization rates, high crystallinity, and are generally easily processed into complex parts.
Polyester polyadipates have improved oil and chemical resistance, but slightly lower hydrolytic stability than polycaprolactones which are used for seals, gaskets, and beltings.
Polytetramethylene glycol ether resins for wire/cable covering have excellent resistance to hydrolysis and microorganisms, compared with polyester polyurethanes. Healthcare grade polyether TPUs are resis- tant to fungi, have low levels of extractable ingredients, excellent hydrolysis resistance, and can be sterilized for reuse by gamma irra- diation, ethylene oxide, and dry heat but not with pressurized steam (autoclave).3 Polyether TPUs are an option for sneakers and athletic footwear components such as outer soles.
Bayer Bayflex®* elastomeric polyurethane reaction injection mold-
ing (RIM) is a two-component diphenylmethane diisocyanate- (MDI)- based liquid system produced in unreinforced, glass-reinforced, and mineral-/microsphere-reinforced grades.15 They possess a wide stiff-
*Bayflex is a registered trademark of Bayer Corporation.
ness range, relatively high impact strength, quality molded product surface, and can be in-mold coated. Room temperature properties are: specific gravity, 0.95 to 1.18; ultimate tensile strength, 2300 to 4200 lb/in2 (16 to 29 MPa); flexural modulus, 5000 to 210,000 lb/in2 (34 to
1443 MPa); and tear strength, Die C, 230 to 700 lb/in (40 to 123 kN/m).15 Related Bayer U.S. patents are: TPU-Urea Elastomers, U.S. Patent 5,739,250 assigned to Bayer AG, April 14, 1998, and RIM Elastomers Based on Prepolymers of Cycloaliphatic Diisocyanates, U.S. Patent 5,738,253 assigned to Bayer Corp., April 14, 1998.
Representative applications are: tractor body panels and doors, automotive fascia, body panels, window encapsulation, heavy-duty truck bumpers, and recreation vehicle (RV) panels.
Bayer’s Texin®* polyester and polyether TPU and TPU/polycarbon-
ate (PC) elastomers were pioneer TPEs in early development of pas- senger car fronts and rear bumpers. PC imparts Izod impact strength toughness required for automotive exterior body panels. Extrusion applications include film/sheet, hose, tubing, profiles, and wire/cable covering. Hardness ranges from Shore A 70 to Shore D 75. Texin can be painted without a primer.
Morton International Morthane®† TPU elastomers are classified
into four groups: polyesters, polyethers, polycaprolactones, and poly- blends. Polyester polyurethanes have good tear and abrasion resis- tance, toughness, and low-temperature flexibility, and Shore hardness ranges from 75 A to 65 D. They are extruded into clear film and tubing and fuel line hose. Certain grades are blended with acrylonitrile buta- diene styrene (ABS), styrene acrylonitrile (SAN), nylon, PC, polyvinyl chloride (PVC), and other thermoplastic resins. Polyethers possess hydrolytic stability, resilience, toughness, good low-temperature flexi- bility, easy processability, and fast cycles. They also have tensile strength up to 7500 lb/in2 (52 MPa) and melt flow ranges from about 5 to 60 g/10 min. Hardnesses are in the Shore A range up to 90. Certain grades can be used in medical applications. Aliphatic polyester and polyether grades provide UV resistance for pipes, tubing, films, and liners. They can be formulated for high clarity.
The polyblends are polyester TPUs blended with ABS, SAN, PC, nylon, PVC, and other thermoplastics for injection molding and extru- sion. A 10 to 20% loading into PVC compositions can increase mechani- cal properties 30 to 40%.1 Although the elastomeric TPUs are inherently flexible, plasticizers may be recommended, for example, in films.
TPU elastomers are processed on rubber equipment, injection mold- ed, extruded, compression molded, transfer molded, and calendered. In
*Texin is a registered trademark of Bayer Corporation.
†Morthane is a registered trademark of Morton International Inc.
order to be fabricated into products, such as athletic shoe outer soles, the elastomer and ingredients are mixed in conventional rubber equip- ment: two-roll mills, internal mixers, and compounded.17 Subsequently, the compound is processed, for example, injection molded.17
Typical melt processing practices are described with Pellethane TPU (see Table 3.4). The moisture content is brought to <0.02% before molding or extruding.3 Dessicant, dehumidifying hopper dryers that can produce a —40°F (—40°C) dew point at the air inlet are suggested. A dew point of —20°F (—29°C) or lower is suggested for TPU elas- tomers.3 The suggested air-inlet temperature range is 180 to 230°F (82 to 110°C)3: 180 to 200°F (82 to 93°C) for the softer Shore A elastomers, to 210 to 230°F (99 to 110°C) for harder Shore D elastomers.
Drying time to achieve a given moisture content for resin used directly from sealed bags is shown in Fig. 3.3: about 4 h to achieve
<0.02% moisture content @ 210°F (99°C) air-inlet temperature and
—20°F (—29°C) dew point.3 When TPU elastomers are exposed to air just prior to processing, the pellets are maintained at 150 to 200°F (65 to 93°C), a warmer temperature than the ambient air.3 A polymer tem- perature that is warmer than the ambient air reduces the ambient moisture absorption.3
Melt temperature is determined by Tm of resin and processing and equipment specifications, including machine capacity, rated shot size, screw configuration (L/D, flight number, and design), part design, mold design (gate type and runner geometry), and cycle time.3 Shear energy created by the reciprocating screw contributes heat to the melt, causing the actual melt temperature to be 10 to 20°F (6 to 10°C) higher than the barrel temperature settings.3 Temperature settings should take shear energy into account. In order to ensure maximum product quality, the processor should discuss processing parameters, specific machinery, equipment, and tool and product data with the resin supplier. For exam- ple, if it is suspected that an improper screw design will be used, melt
TABLE 3.4 Typical Injection-Molding Settings* for Pellethane TPU
Temperature, °F (°C) Shore A 80 Shore D 55
Melt temperature max 415 (213) 435 (224)
Rear (feed) 350–370 (177–188) 360–380 (183–193)
Middle (transition) 360–380 (183–193) 370–390 (188–198)
Front (metering) 370–390 (188–199) 390–410 (199–210)
Nozzle 390–410 (199–210) 400–410 (204–210)
Mold temperature 80–140 (27–60)
*Typical temperature and pressure settings are based on Ref. 3. Settings are based on studies using a reciprocating screw, general-purpose screw, clamp capacity of 175 tons, and rated shot capacity of 10 oz (280 g). Molded specimen thicknesses ranged from 0.065 to 0.125-in (1.7 to 3.2-mm) thickness.
A higher mold temperature favors a uniform melt cooling rate, minimizing residual stresses, and improves the surface finish, mold release, and product quality. The mold cooling rate affects finished product quality. Polyether type TPU can set up better and release better.
High pressures and temperatures fill a high surface-to-volume ratio mold cavity more easily, but TPU melts can flash fairly easily at high pressures (Table 3.5). Pressure can be carefully controlled to achieve a quality product by using higher pressure during quick-fill, followed by lower pressure.3 The initial higher pressure may reduce mold shrink- age by compressing the elastomeric TPU.3
The back pressure ranges from 0 to 100 lb/in2 (0 to 0.69 MPa). TPU elastomers usually require very little or no back pressure.3 When addi- tives are introduced by the processor prior to molding, back pressure will enhance mixing, and when the plastication rate of the machine is insufficient for shot size or cycle time, a back pressure up to 200 lb/in2 (1.4 MPa) can be used.3
Product quality is not as sensitive to screw speed as it is to process temperatures and pressures. The rotating speed of the screw, together with flight design, affects mixing (when additives have been intro- duced) and shear energy. Higher speeds generate more shear energy (heat). A speed above 90 r/min can generate excessive shear energy, cre- ating voids and bubbles in the melt, which remain in the molded part.3
*Typical temperature and pressure settings are based on Ref. 3. Settings are based on stud- ies using a reciprocating screw, general-purpose screw, clamp capacity of 175 tons, and rated shot capacity of 10 oz (280 g). Molded specimen thicknesses ranged from 0.065- to 0.125-in (1.7- to 3.2-mm) thickness.
†U.S. units refer to line pressures; metric units are based on the pressure on the (average)
cross-sectional area of the screw.
Cycle times are related to TPU hardness, part design, temperatures, and wall thickness. Higher temperature melt and a hot mold require longer cycles, when the cooling gradient is not too steep. The cycle time for thin-wall parts, <0.125 in (<3.2 mm), is typically about 20 s.3 The wall thickness for most parts is less than 0.125 in (3.2 mm), and a wall thickness as small as 0.062 in (1.6 mm) is not uncommon. When the wall thickness is 0.250 in (6.4 mm), the cycle time can increase to about 90 s.3
Mold shrinkage is related to TPU hardness and wall thickness, part and mold designs, and processing parameters (temperatures and pres- sures). For a wall thickness of 0.062 in (1.6 mm) for durometer hardness Shore A 70, the mold shrinkage is 0.35%. Using the same wall thickness for durometer hardness Shore A 90, the mold shrinkage is 0.83%.3
Purging when advisable is accomplished with conventional purg- ing materials, polyethylene or polystyrene. Good machine mainte- nance includes removing and cleaning the screw and barrel mechanically with a salt bath or with a high-temperature fluidized- sand bath.3
Reciprocating screw injection machines are usually used to injection mold TPU, and these are the preferred machines, but ram types can be successfully used. Ram machines are slightly oversized in order to avoid: (1) incomplete melting and (2) steep temperature gradients dur- ing resin melting and freezing. Oversizing applies especially to TPU durometers harder than Shore D 55.3
Molded and extruded TPU have a wide range of applications, including:
Automotive: body panels (tractors) and RVs, doors, bumpers (heavy- duty trucks), fascia, and window encapsulations
Covering for wire and cable
Footwear and outer soles
Seals and gaskets