Types of PU Elastomer

Polyurethane rubbers are a special type of PU elastomer. The PU rubbers are highly resistant to oil and fuel and have high abrasion resistance and tensile strength similar to PU elastomers. Polyurethane rubbers maintain their flexibility even at a temperature as low as -40° C and are thermally stable up to 125°C. Polyurethane rubbers have better mechanical strength and resist abrasion, ozone, and oil better than other specialty rubbers such as acrylates and nitrile butadiene. ... 

Fig. 12.1. Hydroxy-terminated polybutadiene (BD). (Typical trade name examples are Poly-BD ofARCO (London) and Liquiflex of Retroflex (London). Used for special types of PU elastomer where products of low water absorption and sensitivity are required. Generally their mechanical strength is lower than that of their polyester and polyether analogues with the advantage of giving PUs of very low and hence temperature flexibility.

A new type of PU elastomer has been synthesized which relies on the chemical structure of the molecule for water-vapour transmission as shown in Fig. 13.14. 

The hydroxyl terminated polybutadienes lead to PU with physico-mechanical properties significantly lower than those of PU based on polyether or polyester polyols. The nonpolar polymeric chain and the extraordinary hydrophobicity mean that hydroxy terminated polybutadienes are used for special applications, due to their excellent electrical insulation properties which are equal or superior to epoxies or silicone elastomer systems. The hydrolytic stability of PU elastomers derived from hydroxyl terminated polybutadiene is superior to the majority of other types of PU. Thus, some specific applications of... 

The standard diol chain extender used for the synthesis of PU elastomers was 1,4-butane diol (BDO). Products from this type of material, as a generalization, have inferior properties to aromatic diamine curatives and various alternatives have been introduced over the last few years to attempt to simulate the properties of the urethane diamine cure or to produce special effects. Examples of these are shown below ... 

MDI- or NDI-based PUs usually have better mechanical properties than PUs obtained from aliphatic polyisocyanates. The effect of various diisocyanates on the thermal stability of their PU elastomers is given below for a polycap-rolactone (mol. wt. 2000)/diisocyanate/BDO type elastomer of molar ratio 1 2.6 1 and synthesized using the prepolymer technique. 

The major advantage in preparation of PUs is that they can be synthesized from various types of polymers that contain different mechanical and physical properties. In particular, poly(ester-urethane) elastomers are interesting because they can be designed synthetically to have hydroxyl functional groups at their chain terminals. These react readily with diisocyanate groups to form urethane linkages. In the same sense, chemical modification of the involved monomers allows incorporation of some functionality into a PU matrix, thus Improving a specific property. An example of chemical modification of monomers was developed with chitosan. This polysaccharide is product of the alkaline deacetylation of... 

Fig. 2.9 SANS patterns from two bulk PU elastomers they differ only in the type of dusocyanate, the flexible DBDl and rigid MDl (a) DBDl BDO PTHF (b) MDl BDO PTHF by courtesy of Dr. D. BucknaU, Wginia Tech University.

Another problem known to exist with PU elastomers is that of their limited resistance to hydrolysis. This has been tackled in various ways initially by the use of linear polyether polyols of which the two most widely used types are shown in Table 1.4. 

Fig. 3.12. DSC scans of different chain extender-based PU elastomers (arrow designates the position of transition). PU type Capa 225/CHDI/chain extender molar ratio 1 26 1 (from Barikani, 1986).

The reaction sequences used in this preparation followed the conventional polyurethane prepolymer sequence with the following provision that steps were made to obtain in the final PU elastomer controlled quantities of covalent-type crosslinks by leaving calculated excesses of isocyanate groups in the initially cast and cured PU. 

Rubbers and elastomeric products for practical applications are usually blends of different elastomer types that develop specific domain morphologies at the microscale, and, therefore, they are a part of this chapter. The most common representatives of the ruhher family are natural ruhher (NR) and the synthetic polyhutadiene ruhher (PB). There are various copolymers of butadiene with styrene (styrene butadiene rubber, SBR) or acrylonitrile (acrylonitrile-butadiene rubber, NBR). Several elastomers have been developed for special purposes, such as EVA (ethylene vinyl acetate copolymer), PU (polyurethane), EPDM (ethylene propylene terpolymer), and siUcone rubber. 

Nowadays, the trend of research in this field is toward new reaction routes to establish the linking process among polyol and isocyanates obtained from biopolymers to synthesize hybrid PUs and, thus, reach to overcome thermal and mechanical properties of these materials. Also other important parameters such as functionality of diols, isocyanate type, and chain extenders are relevant in PU synthesis [13]. In these materials, the molecular structure varies from rigid cross-linked to linear or highly extensible elastomers hence, their applications are versatile, such as coating or membranes in medical devices, materials in construction engineering, automotive parts, foams and adhesives. 

Figure 3.18 A large full-automated production plant for truck floor-covering mats using the Foam Film technology. The parts are made of thermoformed PVC (or thermoplastic elastomer) foam-backed with PU, which is kept separate from the mould with a film (adhesive type).

Use of internal mixers offers a direct and environment friendly technique for preparation of nanocomposites. A two-step mixing in an internal mixer followed by addition of curative on a two roll mill for preparation of elastomer-layered silicate nanocomposites [51]. In the study on the effect of processing conditions (mixer type, mixing temperature) and formulations on the properties of EPDM nanocomposites, it has been shown that open two roll mill mixing results in inadequate dispersion of the nanofiller in the elastomer matrix compared to compounding in an internal mixer. Melt compounding with layered silicates has been reported for NR [52], ENR [46], SBR [53], NBR [54], EPDM [55] and PU [56] systems. 

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