PU ELASTOMER

PU elastomers contain alternative soft and hard segments, which separate into different phases. Hard domains play a role of cross-links, whereas soft blocks provide extensibility. Therefore, morphology and properties of PU are defined by relative amount of soft and hard segments. For example, at 70% concentration of soft segments, the material is described as a mbbery matrix with... 

In an NMR analysis of the effects of /-irradiation induced degradation on a specific polyurethane (PU) elastomer system, Maxwell and co-workers [87] used a combination of both H and 13C NMR techniques, and correlated these with mechanical properties derived from dynamic mechanical analysis (DMA). 1H NMR was used to determine spin-echo decay curves for three samples, which consisted of a control and two samples exposed to different levels of /-irradiation in air. These results were deconvoluted into three T2 components that represented T2 values which could be attributed to an interfacial domain between hard and soft segments of the PU, the PU soft segment, and the sol... 

Thermal and physical properties of plastics (typical values) Cast PU Elastomer PU... 

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. 

Chain extenders Diols and diamines are generally used as chain extenders in PU industry and choice of chain extender influences elastomer properties considerably. The standard diol chain extender used for the synthesis of PU elastomer is 1,4-butane diol (BDO). Compared with a diol, better physical properties usually result when a diamine is employed as an extender. This is probably due to the introduction of urea linkages which enter into strong hydrogen bonded interactions. A diamine is usually chosen as the chain extender when a relatively unsymmetrical diisocyanate is employed. Diamines also function as efficient catalysts in addition to chain extenders. 

Its average molecular weight is 2000 400 with functionality 2.0. The cost of these fluorinated polyols is high. However, PU elastomers with unique chemical resistance and other exceptional properties are produced. 

Flexible PU foams are prepared from basically the same raw materials as PU elastomers. Instead of a short-chain extender, water is used. Water reacts with isocyanates to form an amine and C02 (Eq. (2.22)), which results in foaming. The diamine thus created can then react with isocyanate groups giving polyurea short segments. A surfactant is also introduced as a cell control agent. 

PU elastomers based on a polyester diol as the soft block and isocyanate groups coupled via a chain extender as the rigid block, have been investigated. The role of the chain extender is to induce some mobility and disorder within the hard microdomains. The elastomers are selectively deuterated in the rigid block. 2H NMR spectra show the coexistence of a solid and a mobile fraction [89]. The solid fraction shows no onset of fast molecular motions at all, while the mobile fraction increases with temperature. These results suggest that some of the hard microdomains (perhaps the most disordered ones) melt successively when temperature is raised. 

Originally PU elastomers were synthesized using one of the aromatic isocyanates (MDI, TDI, etc.). Recently, however, more focus has been placed on the use of distilled MDI and modified MDI products. HMDI and IPDI are used to make coatings for applications when high resistance to weathering and discoloration by light is important. ... 

Thermoplastic PU elastomers (TPU) are a special group of PU elastomers that can be processed using the traditional economical techniques available for thermoplastic polymers. These materials are designed to meet a wide variety of industrial needs, including... 

Medical PUs are another subset of PU elastomers. Segmented PUs were first suggested for use in a biomedical application in 1967. ° Early work with PU elastomers showed that these materials could be used for implants without causing a large, unwanted inflammatory response. The first medical devices made of PUs, however, were found to be susceptible to hydrolysis and degraded faster than desired. ° From that time, new biostable materials have been developed for use as pacemaker leads, catheters, vascular grafts. 

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. 31. Stress-strain curves for (A) a pristine PU elastomer and (B) a PU-clay nanocomposite...

Polyurethanes (PUs) are among the most demanded polymers, are required by modern technologies, and the worldwide consumption of PU has steadily increased. Of PU production 80% now consists of resilient and rigid foams while 20% remains solid PU (elastomers, coatings, adhesives, etc.) [1],... 

The polyether diols are used especially for PU elastomers, coatings adhesives and sealants. 

Application PU elastomers, microcellular elastomers (shoe soles) ... 

The PU elastomers obtained from polyether diols with DMC catalysts (Acclaim Polyols of Bayer) have a spectacular improvement in the majority of physico-mechanical properties when compared with PU elastomers made from the polyether diols, obtained by anionic catalysis. 

These kind of polyether polyols containing polymeric fillers are called polymer polyols and are produced on a large industrial scale, because they are one of the most important group of polyols used for high performance flexible polyurethane (PU) foams and PU elastomers [8-12]. 

The polyether polyols based exclusively on ACN are commercialised because of their high glass transition temperature (T ) of the polyacrylonitrile solid fraction. However, they are not used for production of slabstock foams, but for PU elastomers (microcellular elastomers for shoe soles) and integral skin foams. 

The polyether diols, THF-alkylene oxide copolymers are used especially for preparing PU elastomers with high elastic properties (elongation, modulus, tear and tensile strength), superior to those of PU elastomers derived from polyalkylene oxide polyether diols (PO homopolymers or PO-EO copolymers). 

The most important polyols obtained by cationic catalysis are PTHF homopolymers with a MW of 600-3000 daltons used for PU elastomers and spandex fibres. The THF-... 

The most important area for PTHF and THF-alkylene oxides copolymers remains the PU elastomers area (including elastomeric fibres), the PTHF and the high THF content copolymers confer on the resulting PU elastomers specific properties, such as excellent hydrolytic stability, resiliency, low temperature stability, elasticity at lower temperatures, and resistance to fungus attack [7, 35, 36, 37]. 

Chapter 12.5). Polyester polyols in reality, have the theoretical functionality, which is a great advantage for many polyurethane applications. For polyester diols the functionality is 2 OH groups/mol. This structural aspect results in the polyester diols giving PU elastomers with excellent physico-mechanical properties, superior to all polypropylene glycols obtained by anionic PO polymerisation. 

Representative PCL are the diols of MW of 2000-4000 daltons, used in hydrolytically stable PU elastomers. The diols used as starters are DEG, 1,4 butanediol and NPG. The melting point of PCL, of MW of 2000 daltons, is in the range of 40-60 °C and of MW of 1000 daltons in the range of 30-40 °C. If a polyfunctional polyol is used as a starter, polyfunctional PCL polyols are obtained. Thus, by polymerisation of CPL initiated by trimethylolpropane (reaction 8.32) a polyester triol is obtained and initiated by pentaerythritol, a polyester tetraol is formed. It is interesting that some low MW PCL triols with a MW of 300-900 daltons are liquid at room temperature (melting points in the range of 6-16 °C). The viscosities of PCL polyols, at 60 °C, are 40-1600 mPa-s, depending on the polyol structure. 

PCL polyols are used to produce hydrolysis and solvent resistant PU elastomers which are flexible at lower temperatures. A characteristic of these special polyester polyols is their uniform and fast reactivity due to the 100% primary terminal hydroxyl groups. A characteristic of elastic polyurethanes, based on PCL polyols, is the clickability and superior tear resistance. 

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... 

Polybutadiene diols and hydrogenated polybutadiene diols are used for producing thermoplastic polyurethane elastomers, hydrolytically stable PU elastomers, adhesives, sealants, coatings, encapsulation and electrical insulation polyurethanic products. 

The very low glass transition temperature (Tg) of polysiloxane chains (Tg = -123 °C) is a very attractive property for using these kinds of polymeric chains to build an oligo-polyol structure with terminal hydroxyl groups [1]. The resulting structure called a polysiloxane polyol gives, after reaction with diisocyanates, polyurethane (PU) elastomers which conserve their high elasticity at very low temperatures [1]. 

Reaction 15.14 is an equilibrium reaction and this equilibrium is pushed to the oxazolidine formation by the water elimination from the reaction system under vacuum distillation. This reaction is used in practice to trap the traces of water in some PU formulations which need a perfect anhydrous media, for example in PU elastomers [26]. By the addition of oxazolidines to these formulations reaction of these compounds with water takes place and the oxazolidine is transformed in an aminoalcohol which is in fact a chain extender generated in situ and an aldehyde or a ketone practically inert in the reactions involved in PU chemistry (reaction 15.15). 

Bisphenol A propoxylated or ethoxylated with 2-10 mols of alkylene oxides/mol of bisphenol A are used as chain extenders for PU elastomers and as aromatic diols for isocyanuric and urethane isocyanuric foams [30] (structure 15.30). 

The resorcinol diols are used successfully in high durability and thermal stability PU adhesives, PU elastomers (cast and thermoplastic PU elastomers), sealants and coatings. 

The alkoxylation process is easy to apply to PU foams having a low concentration of urethane and urea groups such as flexible and semiflexible foams, integral skin foams, PU elastomers and so on. Urea groups react in a similar way with urethane groups, with the formation of oxazolidones and amines by an intramolecular alcoholysis of urea groups (reaction 20.15). 

As a general rule, for linear polymers all the properties, such as tensile strength, elongation, elasticity, melting points, glass transition temperature (Tg), modulus and increase of the MW, increase up to a limited value, where all the properties remain practically constant. This behaviour is valuable for linear polymers, in our particular case in linear polyurethanes (PU elastomers, spandex fibres, etc). 

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