Polyether TPUs

The properties of TPUs based on polytetramethylene glycol with PPDI and CHDI are given in Table 9.13. Single chain extenders only were used. Use of the aromatic diol Dianol 22 alone resulted in poor-quality TPUs with this polyether polyol, whereas the aliphatic diol BDO provided good high-strength elastomers. 

In this series the chain extender HQEE was investigated with PPDI and an exceptionally high strength and low compression set TPU resulted in compound 21, which met the target specification set for the hydraulic seal in Table 9.10. 

Both PPDI and CHDI TPUs have good properties when chain-extended with BDO. However, when the use of the more bulky aromatic diol Dianol 2 was explored, both these diisocyanates produced low-strength TPUs as shown by compounds 20 and 24. 

It is also useful, at this juncture, to consider Fig. 9.7 which is based on PPDI and observe that the maximum temperature for thermal stability appears to be 140°C when the chain extension system BDO + 1,4-CHDM is used this corresponds to the analogous system used with CHDI. 

A most interesting and unusual feature of the CHDI-based TPUs is the apparently constant dynamic modulus over the very large temperature range 50-140°C for TPUs containing linear chain extenders and 50-170°C when an all-cyclic chain-extender system is used. 

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TPUs are polar materials and are therefore resistant to nonpolar organic fluids such as oils, fuels, and greases, but they are readily attacked and even dissolved by polar organic fluids such as dimethylformamide and dimethylsulfoxide. TPUs behave like copolyester TPEs toward water and aqueous solutions, being resistant to these media except at very high or low pH. Polyether TPUs are more resistant to such hydrolytic degradation than are the polyester TPUs. 

Polyether TPUs are slightly lower in specific gravity than polyester and polycaprolactone grades. They offer low-temperature flexibility and good abrasion and tear resilience. They are also durable against microbial attack and provide excellent hydrolysis resistance— making them suitable for applications where water is a consideration. 

In addition, polyester polyols are made by the reaction of caprolactone with diols. Poly(caprolactone diols) are used in the manufacture of thermoplastic polyurethane elastomers with improved hydrolytic stabiHty (22). The hydrolytic stabiHty of the poly(caprolactone diol)-derived TPUs is comparable to TPUs based on the more expensive long-chain diol adipates (23). Polyether/polyester polyol hybrids are synthesized from low molecular weight polyester diols, which are extended with propylene oxide. 

TPU is usually made from hydroxyl-terminated polyether or polyester diols, diisocyanates, and bifunctional chain extenders. Since the composition, the synthetic method, molecular weight, and its distribution are all changeable, there are numerous types of TPUs available, and their prices and properties vary significantly. 

Both polyester- and polyether-based TPU could be used to blend with PVC, although the former constitutes the majority of the commercial products. All of the blends should meet the following requirements (I) they must have good or relatively good compatibility with PVC, (2) their processing temperature should be close to or lower than that of PVC, (3) they have to meet the specific requirements of the products, for example, TPUs used for medical purposes should be colorless (if possible), transparent, nontoxic, and able to be sterilized, and (4) they should not be expensive. 

TPUs are handicapped by a lower elasticity than conventional rubbers, the more so the higher the hardness certain risks of creep, relaxation and permanent set, the more so the higher the temperature higher cost than TPOs risks of hydrolysis especially for the polyester types UV exposure yellowing incompatibility between certain polyester and polyether grades aromatic and chlorinated hydrocarbon behaviour limited thermal behaviour density inherent flammability, but FR grades are marketed risks of fume toxicity in the event of fire. 

As with other block copolymers, the nature of the soft segments determines the elastic behavior and low-temperature performance. TPUs based on polyester soft blocks have excellent resistance to nonpolar fluids and high tear strength and abrasion resistance. Those based on polyether soft blocks have excellent resistance (low heat buildup, or hysteresis), thermal stability, and hydrolytic stability. 

There are three main chemical classes of TPUs polyester, polyether, and a smaller class known as polycaprolactone [3]. 

Thermoplastic Polyurethanes (TPUs). The first commercial TPEs were the TPUs, which have the same block copolymer morphology as do the COPs. Their general structure is -A-B-A-B-, where A represents a hard crystalline block derived by chain extension of a diisocyanate with a glycol. The soft block, represented by B, can be derived from either a polyester or a polyether. Typical TPU structures, both polyester and polyether types, are represented here ... 

The polyester-type thermoset polyurethanes were commercialized in 1942, and the linear thermoplastic ones (TPU) 10 years later. Polyester-type TPUs, Texiit resins for extrusion and injection molding, were introduced in 1961, whereas polyether-type, Roylar , in 1971. Owing to great diversity of the ingredients, the TPU performance can be readily modified. For this reason, as well as because of the cost, TPUs are seldom blended. Their use can be divided into three groups (i) blends with POM, (ii) blends in which TPU is used as a compatibilizer and impact modifier, and (iii) others. 

The two principal types of TPUs are poly ether and polyester. Polyethers have good low-temperature properties and resistance to fungi polyesters have good resistance to fuel, oil, and hydrocarbon solvents. 

Thermoplastic elastomers (TPEs), which combine the characteristics of both thermoplastics and elastomers, can be classified into two major groups. The first is made up of block copolymers formed by polymerizing a thermoplastic monomer with an elastomer comonomer, as in the case of styrene block copolymers such as styrene-ethylene-butadiene-styrene (SEBS). Engineering TPEs such as thermoplastic urethanes (TPUs), copolyesters (COPEs) and polyether block amides (PEBAs) can be included in this first group. Other types of copolymer are from the polyolefin family, consisting of the polyolefin... 

The statements made regarding polyurethane (Section 5.5.9.14) also apply to hydrolysis resistance of TPUs. Polyether-based TPU is considerably more resistant to hydrolysis than polyester-based TPU because of its chemical structure. Rigid TPUs are also typically more resistant than soft ones, because the hard segments in the polymer molecule do not hydrolyze as quickly. 

Table A.40 Swelling behavior of TPU in solvent (3 weeks at 20 °C) results reflect tests with two grades of Elastollan two TPUs based on polyester, one TPU based on polyether [867]...

Both types of TPUs dissolve in cyclohexanone, tetrahydrofuran, and pyridine. The polyether type burns with yellow flame with strong acrid odor and gray smoke. The material burns fast and completely and eventually chars. The polyester type burns slowly with yellow-orange flame. For positive identification, FTIR is most commonly used. FTIR as well as differential scanning calorimetry can differentiate between the two types of TPUs. NMR has been proven to be useful in determining types of polyols. 

The polyether-based TPUs have the following characteristic features ... 

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