**5.1 One-step methods**

One-shot method is a commonly used industrial technique to prepare polyurethanes. The urethane reaction involves a diisocyanate (hard segment) and a diol (soft segment) (see Figure 2). Generally two diols are required, a chain extender or short chain diol and an

physical crosslinks. If the dispersed phase is elastic then the polymer is a toughened thermoplastic, not an elastomer. Elastomer reversibility must have physical cross-links, therefore these crosslinks must be reversible. Physical crosslinks do not exist permanently

Generally, thermoplastic elastomers can be categorized into two groups: multi-block copolymers and blends. The first group is copolymers consist of soft elastomers and hard thermoplastic blocks, such as styrenic block copolymers (SBCs), polyamide/elastomer block copolymers (COPAs), polyether ester/elastomer block copolymers (COPEs) and polyurethane/elastomer block copolymers (TPUs). TPE blends can be divided into

Thermoplastic elastomers are known as two-phase system consisting of rubbery elastomeric (soft) component and rigid (hard) component. The soft phase can be polybutadiene, poly(ethylene-*co*-alkene), polyisobutylene, poly(oxyethylene), poly(ester), polysiloxane or any of the typical elastomers while the hard phase are polystyrene, poly(methyl methacrylate), urethane, ionomer – poly(ethylene-*co*-acrylic acid) (sodium, Mg, Zn salt), ethylene propylene diene monomer, and fluropolymers. The structure representing a

Polystyrene domain

Polydiene rubber matrix

Polybutadiene rubber

matrix

and may disappear with the increase of temperature.

styrenic TPE is shown schematically in Figure 1.

polyolefin blends (TPOs) and dynamically vulcanized blends (TPVs).

Fig. 1. Schematic of a styrene-butadiene-styrene block copolymer

TPEs are two phase polymers, however they can be synthesised in one reaction step or in

One-shot method is a commonly used industrial technique to prepare polyurethanes. The urethane reaction involves a diisocyanate (hard segment) and a diol (soft segment) (see Figure 2). Generally two diols are required, a chain extender or short chain diol and an

**5. Synthesis of thermoplastic elastomers** 

two or three steps to create each phase separately.

**5.1 One-step methods** 

elastomeric hydroxyl terminated polymer. Examples of a diisocyanate are methane 4,4' diphenyl diisocyanate (MDI), 2,4- and 2,6-toluene diisocyanate (TDI) and 1,6-hexane diisocyanate (HDI). A chain extender may be 1,4-butanediol. When MDI and butanediol react they form a polyurethane with alternating monomer units connected by urethane groups, though other functional groups also form as by products of the reaction. This polyurethane is not elastomeric and it constitutes the hard phase of a typical TPU. Hydroxyl terminated elastomers include polyethers: poly(oxyethylene), poly(oxybutylene), polyesters: poly(ethylene succinate), poly(butylene succinate), poly(ethylene adipate), poly(butylene), hydroxyl terminated polybutadiene and hydroxyl terminated poly(butadiene-*co*acrylonitrile).

Fig. 2. Schematic representation of TPUs composed of alternating hard segment and soft segment structures

These polymeric diols react with isocyanate and are linked into the TPU as a complete elastomer block. The hard and soft (elastomeric) chain segments phase separate with the hard segments as a dispersed minor phase, since the soft segments must form a continuous phase if elastomeric properties are to be displayed. The reactivity difference between the –OH groups of the polyol and the chain extender with different isocyanate groups affect the sequence of hard segments in the polymer chain. Thus, polyurethanes obtained by using this method have a more random sequence. However, the polymer is highly crystalline due to the favoured reaction between polyol and diisocyanate before extended polymer growth has occurred.

A chain growth polymerisation can be used to form a TPE in one step. An example is a poly(ethylene-*co*-butene) with a high butane content, polymerised using a single-site metallocene initiator. This polyethylene can undergo phase separation due to crystallisation, crystals are the physical crosslinks, and the highly branched structure will exhibit elastomeric properties. It may need to be blended with a less branched polyethylene to increase the physical cross-links. Alternatively it can be partially cross-linked by dynamically vulcanising by extrusion with a peroxide initiator. While chemical cross-links are formed this type of polyethylene can still be processed as a thermoplastic. Dynamic Vulcanization can be applied to poly(ethylene-*co*-propylene) rubber (EPR) that may be blended with a thermoplastic polyethylene to provide a binding crystalline phase.

#### **5.2 Two-step methods**

TPU can be synthesised by a two-step method, which is known as prepolymer method. The reaction may be carried out in two steps where excess diisocyanate is added to the polymeric diol to form an isocycanate terminated pre-polymer with excess diisocyanate

Dynamic vulcanization is a widely used method to prepare thermoplastic elastomers comprising partially or fully cross-linked elastomer particles in melt-processable thermoplastic matrix. Thermoplastic vulcanizates are prepared by melt-mixing the elastomer and thermoplastic in an internal mixer or in a twin-screw extruder. After a wellmixed blend has formed, in the second step, vulcanizing agents such as cross-linkers or curatives are added. Vulcanization of the rubber polymer takes place during the continuation of the mixing process under conditions of high temperature and high shear. According to the earlier investigation (Aubert et al., 2004) the best combinations of elastomer and thermoplastic are those in which the surface energies of the two components are matched, the entanglement molecular weight of the elastomer is low and the thermoplastic is at least 15% crystalline. The most common used compositions are based on dynamically vulcanized ethylene propylene diene monomer (EPDM) and polyolefins. Others blends include butyl and halobutyl rubbers and polyolefin resins, polyacrylate rubber and polyolefins (Soares et al., 2008) and butadiene-acrylonitrile rubber and

Anionic polymerization remains as an important technique to for the preparation of welldefined styrene butadiene triblock copolymer. Poly(styrene-b-butadiene-b-styrene) (SBS) is an example of a tri-block copolymer, though di-block copolymers are also formed from the same monomers. Styrene is first initiated with butyllithium and polymerised until all of the styrene has reacted. The polystyrene has an anionic end group with lithium counter-ion. Butadiene is added and the polymerisation continues forming a butadiene block. After all of the butadiene has reacted, more styrene is added and the polymerisation continues until all styrene has reacted. Then the polymerisation is terminated by addition of a protic substance such as methanol or water. Termination may be carried out after the second polymerisation step to give a di-block copolymer. The size of blocks is determined by the concentration of initiator and the amounts of monomers added at each step. Molar mass distribution is characteristically low for anionic polymerisation so the macromolecular architecture is

Carbocationic polymerization has a more complex system than the anionic polymerization described above. It has been used to produce block copolymers with polyisobutylene midsegments, or poly(styrene-b-isobutylene-b-styrene) (SIBS). This polymerization involves a three-step progression: (i) controlled initiation, (ii) reversible termination (quasi-living systems), and (iii) controlled transfer. The initiators have two or more functionalities. The polymer segments are produced sequentially from monomers as in anionic polymerization. The initiator is reacted with isobutylene at the first stage. The product is a difunctional living polymer. It can initiate further polymerization when more styrene monomers are added. After termination, this gives the block polymer SIBS. Polyisobutylene is the only mid segment that can be produced by this method while there are many aromatic polymers that

Thermoplastic elastomers are technologically very attractive because they can be processed as thermoplastics, this is their main advantage compared with cross-linkable elastomers. They can be re-melted or devitrified and shaped again. Hence, they are generally processed by extrusion and injection moulding, which are the most common processing methods used

poly(vinyl chloride) (Passador et al., 2008).

accurately controlled.

can form the end segments.

**6. Processing methods** 

monomer that is then reacted or chain extended with the monomeric diol to form the segmented TPU structure. The polymeric diol can be a biopolymer or biodegradable polymer such as castor oil. Comparison has been made between poly(butylene succinate) and poly(butylene adipate) as the soft phase in TPU (Sonnenschein et al., 2010). The succinate derived TPU exhibited higher soft phase glass transitions and more hard phase to soft phase interactions than the adipate derived TPU, due to higher carbonyl content and hence polar interactions in the succinates. Abrasion resistance was a function of overall hard phase volume fraction.

Sequences are found to be more regular in the polymer obtained via prepolymer method compared to the one shot method. The structural regularity leads to a better packing of hard segments where physical cross-linking points are easier to form. Hence, a two-step method gives a product of better mechanical properties than a one-step method does (Table 2). Again, the solubility of these two products is different. The polyurethanes obtained from one-method are soluble in some of the common solvents, but the polyurethanes from the prepolymer process could not be dissolved in any common solvents (Figure 3).


Table 2. Comparison of properties of TPUs made from one-shot and prepolymer methods

Fig. 3. Solubility results of TPUs made from one-shot and prepolymer methods in various solvents at room temperature (1: not soluble; 2: soluble after a few days; 3: soluble)

Source: Herbert & Nan, 2010

monomer that is then reacted or chain extended with the monomeric diol to form the segmented TPU structure. The polymeric diol can be a biopolymer or biodegradable polymer such as castor oil. Comparison has been made between poly(butylene succinate) and poly(butylene adipate) as the soft phase in TPU (Sonnenschein et al., 2010). The succinate derived TPU exhibited higher soft phase glass transitions and more hard phase to soft phase interactions than the adipate derived TPU, due to higher carbonyl content and hence polar interactions in the succinates. Abrasion resistance was a function of overall hard

Sequences are found to be more regular in the polymer obtained via prepolymer method compared to the one shot method. The structural regularity leads to a better packing of hard segments where physical cross-linking points are easier to form. Hence, a two-step method gives a product of better mechanical properties than a one-step method does (Table 2). Again, the solubility of these two products is different. The polyurethanes obtained from one-method are soluble in some of the common solvents, but the polyurethanes from the

prepolymer process could not be dissolved in any common solvents (Figure 3).

**Properties One-shot Prepolymer**  Elongationat break (%) 550 559 Hardness Shore A 80 82 Modulus at 100% (MPa) 5.6 6.6 Tensile strength at break (MPa) 14.0 11.9 Tg (°C) -35 -34

Table 2. Comparison of properties of TPUs made from one-shot and prepolymer methods

Fig. 3. Solubility results of TPUs made from one-shot and prepolymer methods in various solvents at room temperature (1: not soluble; 2: soluble after a few days; 3: soluble)

phase volume fraction.

Source: Herbert & Nan, 2010

Dynamic vulcanization is a widely used method to prepare thermoplastic elastomers comprising partially or fully cross-linked elastomer particles in melt-processable thermoplastic matrix. Thermoplastic vulcanizates are prepared by melt-mixing the elastomer and thermoplastic in an internal mixer or in a twin-screw extruder. After a wellmixed blend has formed, in the second step, vulcanizing agents such as cross-linkers or curatives are added. Vulcanization of the rubber polymer takes place during the continuation of the mixing process under conditions of high temperature and high shear. According to the earlier investigation (Aubert et al., 2004) the best combinations of elastomer and thermoplastic are those in which the surface energies of the two components are matched, the entanglement molecular weight of the elastomer is low and the thermoplastic is at least 15% crystalline. The most common used compositions are based on dynamically vulcanized ethylene propylene diene monomer (EPDM) and polyolefins. Others blends include butyl and halobutyl rubbers and polyolefin resins, polyacrylate rubber and polyolefins (Soares et al., 2008) and butadiene-acrylonitrile rubber and poly(vinyl chloride) (Passador et al., 2008).

Anionic polymerization remains as an important technique to for the preparation of welldefined styrene butadiene triblock copolymer. Poly(styrene-b-butadiene-b-styrene) (SBS) is an example of a tri-block copolymer, though di-block copolymers are also formed from the same monomers. Styrene is first initiated with butyllithium and polymerised until all of the styrene has reacted. The polystyrene has an anionic end group with lithium counter-ion. Butadiene is added and the polymerisation continues forming a butadiene block. After all of the butadiene has reacted, more styrene is added and the polymerisation continues until all styrene has reacted. Then the polymerisation is terminated by addition of a protic substance such as methanol or water. Termination may be carried out after the second polymerisation step to give a di-block copolymer. The size of blocks is determined by the concentration of initiator and the amounts of monomers added at each step. Molar mass distribution is characteristically low for anionic polymerisation so the macromolecular architecture is accurately controlled.

Carbocationic polymerization has a more complex system than the anionic polymerization described above. It has been used to produce block copolymers with polyisobutylene midsegments, or poly(styrene-b-isobutylene-b-styrene) (SIBS). This polymerization involves a three-step progression: (i) controlled initiation, (ii) reversible termination (quasi-living systems), and (iii) controlled transfer. The initiators have two or more functionalities. The polymer segments are produced sequentially from monomers as in anionic polymerization. The initiator is reacted with isobutylene at the first stage. The product is a difunctional living polymer. It can initiate further polymerization when more styrene monomers are added. After termination, this gives the block polymer SIBS. Polyisobutylene is the only mid segment that can be produced by this method while there are many aromatic polymers that can form the end segments.

### **6. Processing methods**

Thermoplastic elastomers are technologically very attractive because they can be processed as thermoplastics, this is their main advantage compared with cross-linkable elastomers. They can be re-melted or devitrified and shaped again. Hence, they are generally processed by extrusion and injection moulding, which are the most common processing methods used

obtained from injection molding include threads, springs, storage containers, mechanical

Compression molding was among the first method of molding to be used to produce plastic parts. However, it is by far less used than injection molding. Generally, this method involves four steps. First of all, the raw polymer materials in pellets or powder form are placed in a heated and open mold cavity. The mold is closed with another half of the mold and at the same time, pressure is applied to force the materials into contact with all mold areas. The materials soften under high pressure and temperature, flowing to fill the mold. The part is hardened under pressure by cooling the mold before removal so the part maintains its shape. There are six important considerations that should be bear in mind, they are the proper amount of material, the minimum energy required to heat the material, the minimum time required to heat the material, the proper heating technique, the force needed to ensure that shots attains the proper shape, the design of the mold for rapid cooling. Compression molding of TPEs usually requires longer heating and cooling time due to their high melting points. Separate platens can be used to solve this problem where one is hot press that is electrically heated and another one is cold press that is water cooled. The part is hot pressed under pressure and then transferred immediately to the cold press to chill it under pressure.

Transfer molding is a process which the polymer is melted in a separate chamber known as pot then forced into a preheated mold through a sprue, taking a shape of the mold cavity. The mold is cooled down before opening. Thermoplastic elastomers usually have high viscosity and longer transfer time is needed. The temperature of the mold should be maintained at above melting temperature of the polymer to avoid premature cooling or freezing before the completion of transfer. The important variables during the process of transfer molding are the type of polymer, melting point of the polymer, pot hold time,

Blow molding is a manufacturing process that is used to produce hollow plastic parts. There is a wide variety of materials can be used in this process, including but not limited to high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP), Poly(vinyl chloride) (PVC) and Poly(ethylene terephtalate) (PET). The basic process begins with the melting of thermoplastic and extruding it through a die head to form a hollow tube called a parison. The parison is then clamped between two mold halves, which close around it and the parsion is inflated by pressurized air until it conforms to the inner shape of the

Basically, there are three types of blow molding used to form the parison. In extrusion blow molding, plastic is melted and extruded using a rotating screw to force the molten through a die head that forms the parsion. Injection blow molding is part injection molding and part

The hot press is usually preheated to reduce the total cycle time.

transfer pressure, transfer rate and the mold cooling time.

mold cavity. Lastly, the molds open and the finished part is removed.

parts and automotive dashboards.

**6.3 Compression molding** 

**6.4 Transfer molding** 

**6.5 Blow molding** 

by thermoplastics. A disadvantage is that TPE have an operating temperature below that at which the hard phase becomes dimensionally unstable.

Several factors need to be taken into account during the processing of TPEs, including viscosity or rheology of the two-phase polymer, temperature at which the hard phase can be processed, thermal stability since the complex structures will potential have several weak chemical links, thermal conductivity since the hard phase is surrounded by soft phase, crystallinity in the hard phase that must be melted with excess enthalpy, and moisture that may cause hydrolysis at processing temperatures.
