Thermoplastic Elastomers

8 Thermoplastic Elastomers 8.1 Background Thermoplastic elastomers (TPEs) have two big advantages over the conventional thermoset (vulcanized) elastomers, namely ease and speed of processing. Other advantages of TPEs are recyclability of scrap, lower energy costs for processing, and the availability of standard, uniform grades (not generally available in thermosets). TPEs are molded or extruded on standard plasticsprocessing equipment in considerably shorter cycle times than those required for compression or transfer molding of conventional rubbers. They are made by copolymerizing two or more monomers, using either block or graft polymerization techniques. One of the monomers provides the hard, or crystalline, polymer segment that functions as a thermally stable component; the other monomer develops the soft or amorphous segment, which contributes the elastomeric or rubbery characteristic. Physical and chemical properties can be controlled by varying the ratio of the monomers and the length of the hard and soft segments. Block techniques create long-chain molecules that have various or alternating hard and soft segments. Graft polymerization methods involve attaching one polymer chain to another as a branch. The properties that are affected by each phase can be generalized as follows. "Hard phase"-plastic properties: (1) Processing temperatures

(2) Continuous use temperature (3) Tensile strength (4) Tear strength (5) Chemical and fluid resistance (6) Adhesion to inks, adhesives, and overmolding substrates "Soft phase"-elastomeric properties:

(3) Flexibility (4) Compression set and tensile set Three high performance types of TPEs make up this chapter.

8. 1.1 Thermoplastic Polyurethane Elastomers (TPUs) Urethanes are the reaction product of a diisocyanate and a long- or short-chain polyether, polyester, or caprolactone glycol. The polyols and the short-chain diols react with the diisocyanates to form linear polyurethane molecules. This combination of diisocyanate and short-chain diol produces the rigid or hard segment. The polyols form the flexible or soft segment of the final molecule. Figure 8.1 shows the molecular structure in schematic form. The properties of the resin depend on the nature of the raw materials, the reaction conditions, and the ratio of the starting raw materials. The polyols used have a significant influence on certain properties of the thermoplastic polyurethane. Polyether and polyester polyols are both used to produce many products. The polyester-based TPUs have the following characteristic features: • Good oil/solvent resistance • Good UV resistance • Abrasion resistance • Good heat resistance • Good mechanical properties The polyether-based TPUs have the following characteristic features: • Fungus resistance • Low temperature flexibility

(1) Lower service temperature limits

• Excellent hydrolytic stability

(2) Hardness

• Acidlbase resistance

297

298

THE EFFECT OF CREEP AND OTHER TIME RELATED FACTORS ON PLASTICS AND ELASTOMERS

Flexible Segment

Rigid Segment

Flexible Segment

Flexible Segment

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Figure 8.1. Molecular structure of a thermoplastic polyurethane elastomer.

In addition to the basic components described above, most resin formulations contain additives to facilitate production and processability. Other additives can also be included such as: • Demolding agents • Flame retardants • Heat/UV stabilizers • Plasticizers The polyether types are slightly more expensive and have better hydrolytic stability and low-temperature flexibility than the polyester types.

8. 1.2 Thermoplastic Copolyester Elastomers (TPE-Es or COPEs) These TPEs are generally tougher over a broader temperature range than the urethanes described in Section 8.1.1. Also, they are easier to process. • Excellent abrasion resistance • High tensile, compressive, and tear strength • Good flexibility over a wide range of temperatures • Good hydrolytic stability • Resistance to solvents and fungus attack • Selection of a wide range of hardness In these polyester TPEs, the hard polyester segments can crystallize, giving the polymer some of the attributes of semi-crystalline thermoplastics, most particularly better solvent resistance than ordinary rubbers, and also better heat resistance. Above the

melting temperature of the crystalline regions, these TPEs can have low viscosity and can be molded easily in thin sections and complex structures. The properties of thermoplastic polyester elastomers can be fine-tuned over a range by altering the ratio of hard to soft segments. In DuPont Hytrel® polyester TPEs, the resin is a block copolymer. The hard phase is polybutylene terephthalate (PBT). The soft segments are long-chain polyether glycols.

8. 1.3 Thermoplastic Polyether Block Amide (PEBA) Elastomers Polyether block amides are plasticizer-free TPEs. The soft segment is the polyether and the hard segment is the polyamide (nylon). They are easy to process by injection molding and profile or film extrusion. Often they can be easily melt-blended with other polymers, and many compounders will provide custom products by doing this. Their chemistry allows them to achieve a wide range of physical and mechanical properties by varying the monomeric block types and ratios. (I) Light weight (2) Great flexibility (extensive range) (3) Resiliency (4) Very good dynamic properties (5) High strength (6) Outstanding impact resistance properties at low temperature (7) Easy processing

(8) Good resistance to most chemicals

299

8: THERMOPLASTIC ELASTOMERS

8.2 Thermoplastic Polyurethane Elastomers (TPUs)

8.2.1 BASFElastollan@ 1164 D-Polyether Resin, Shore Hardness 64DTPUResin

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Figure 8.2. Isochronous stress-strain of Elastollan® 1164 D at 23°C.

8.2.2 BASFElastollan@ 1185 A-Polyether Resin, Shore Hardness 85A TPUResin

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Figure 8.3. Isochronous stress-strain of Elastollarr" 1185 A at 23°C.

20

300

THE EFFECT OF CREEP AND OTHER TIME RELATED FACTORS ON PLASTICS AND ELASTOMERS

8.2.3 BASF Elastollan@ C 64 D-Polyester Resin, High Crystallinity, Shore Hardness 64D TPU Resin

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Figure 8.4. Isochronous stress-strain of Elastcllanf C 64 D at 23°C.

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301

8: THERMOPLASTIC ELASTOMERS

8.2.5 BASF Elastollan@ R 3000-20% Glass Fiber, Shore Hardness 73DTPUResin

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8.3 Thermoplastic Copolyester Elastomers (TPE-Es or COPEs) 8.3.1 DuPont Engineering Polymers Hytrel@4056-HighPerformance, Low Modulus, Shore D40 TPEE

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302

THE EFFECT OF CREEP AND OTHER TIME RELATED FACTORS ON PLASTICS AND ELASTOMERS

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Figure 8.8. Isochronous stress-strain of Hytrel® 4056 at 40°C.

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MPa 5.9 MPa 5.5

303

8: THERMOPLASTIC ELASTOMERS

8.3.2 DuPont Engineering Polymers Hytrel@ 5526-High Performance, Medium-High Modulus, Shore 055 TPEE

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304

THE EFFECT OF CREEP AND OTHER TIME RELATED FACTORS ON PLASTICS AND ELASTOMERS

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8.3.3 DuPont Engineering Polymers Hytrel@ 6356-High Performance, Medium-High Modulus, Shore D6 TPEE 12

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8: THERMOPLASTIC ELASTOMERS

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Figure 8.18. Tensile creep modulus versus time of Hytrel® 6356 at 23°C and various stress levels.

306

THE EFFECT OF CREEP AND OTHER TIME RELATED FACTORS ON PLASTICS AND ELASTOMERS

8.3.4 DuPont Engineering Polymers Hytrel@ 7246-High Performance, High Modulus, Shore D72 TPEE

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Figure 8.20. Isochronous stress-strain of Hytrel®

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307

8: THERMOPLASTIC ELASTOMERS

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