Polyurethane Elastic limit

Keywords Adhesive modulus Adhesys expert system Co-axial joints Compression Concealed joints Creep Elastic limit Epoxy Epoxy composite Einite element analysis Glue line thickness Goland and Reissner Hart-Smith Heat exchanger Hooke s Law Joint designs Joint thickness Lap shear strength Peel Plastic behaviour Polyurethane Pipe bonding Shear stresses Shear modulus Stress distribution Thick adherend shear test Tubular joints Volkersen equation Young s modulus... 

This appears at first sight to be a good option. However, when the modulus of the adhesive is reduced, the elastic limit is also reduced and the net result is usually negative. Fig. 27 shows that the load on the polyurethane product takes the whole of the joint into the plastic region. The challenge to the adhesive manufacturer is... 

Figure 27 The low elastic limit of polyurethane adhesives counteracts the benefit of...

Elasticity If the product requires flexibility, examples of the choices includes polyethylene, vinyl, polypropylene, EVA, ionomer, urethane-polyester, fluorocarbon, silicone, polyurethane, plastisols, acetal, nylon, or some of the rigid plastics that have limited flexibility in thin sections. 

Coal tar-modified polyurethane is a cold-applied liquid waterproofing system. The system by Sonneborn is an example of this approach to waterproofing. It is applied as a liquid at the rate of 10-15 mils/coat. The coating dries hard, but has some elasticity. This material may be attacked by acids in groundwater but can be defended by a protection board. The performance of any liquid-applied waterproofing systems is limited by the capabilities of the applicator (it is difficult to achieve even coats on vertical surfaces). 

The factors which influence pre-gel intramolecular reaction in random polymerisations are shown to influence strongly the moduli of the networks formed at complete reaction. For the polyurethane and polyester networks studied, the moduli are always lower than those expected for no pre-gel intramolecular reaction, indicating the importance of such reaction in determining the number of elastically ineffective loops in the networks. In the limit of the ideal gel point, perfect networks are predicted to be formed. Perfect networks are not realised with bulk reaction systems. At a given extent of pre-gel intramolecular... 

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). 

Neither the uniform strain model nor the uniform stress model is appropriate for this microstructure. Consequently, the elastic moduli of polyurethanes lie between the limits set by Eqs (4.11) and (4.12). For a network chain of Me = 6000, the rubber elasticity theory of Eq. (3.20) predicts a shear modulus of about 0.4 MPa. The hard blocks will have the typical 3GPa Young s modulus of glassy polymers. Increases in the hard block content cause the Young s modulus to increase from 30 to 500 MPa (Fig. 7.13). For automobile panel applications it is usual to have a high per cent of hard blocks so that the room temperature flexural modulus is 500 MPa. 

Polyurethanes are another class of TPEs. They are a large family of chemical compounds that can consist of ether-based, ester-based, polycarbonate-based or polypropylene-based varieties. A number of copolymers are also included , polyurethanes are combinations of macroglycols and diisocyanates that have been polymerized into tough and elastic materials. TPE polyurethanes have been used for peristaltic pump tubing, parenteral solution tubing and catheters. The tables list the majority of those that are commercially available. Among others are those either of limited supply, available for proprietary use only or that have been successful, but recently discontinued such as ... 

Polyurethane rubbers have now been used as seal materials for some time on account of their unique ability to combine resistance to swelling in oil with high strengths and high stiffnesses. Their ability, in some classes, to be processed as thermoplastics, is also considered useful as manufacture of the seal can then be automated and hence quality is more reproducible. A limitation in their use has been that they depend upon physical types of crosslinking for their elastic and strength properties and when certain specific temperatures are reached these crosslinks rapidly weaken and the polyurethane elastomer melts and fails. At present most rubber seals are made from vulcanized covalently crosslinked rubbers where crosslinks are based on sulphur or carbon, and these do not melt at elevated temperatures, but instead decompose. 

One-part urethane adhesives have been used for many years as high performance sealants. In this capacity they provide a useful combination of strength, flexibility, and elastic recovery. As adhesives, these systems have limited use im-less formulated to overcome their inherent disadvantages. One-part polyurethane adhesives are typically moisture-cured and rely on a multistep reaction sequence as follows isocyanate reacts with water to form carbamic acid, the unstable car-bamic acid loses carbon dioxide and generates an amine, the amine reacts with additional isocyanate to form a urea, and the urea reacts with additional isocyanate to form a biuret, which includes a cross-link. Unless it diffuses out of the system, the CO2 can cause foaming. Formulators learn to minimize the isocyanate content (%NCO) of a system in order to balance cure speed with foam control. Cure speeds—and foaming rates—of these systems decrease from the outside in and vary with the amount of atmospheric moisture in the air, which changes hourly and seasonally. 

To illustrate this relationship, consider a random ethylene copolymer of a density 0.856gcm" containing 19mol.% 1-octene comonomer. At room temperature, the material is elastic, or capable of recovering its size and shape after deformation. However, the copolymer loses the desirable properties of an elastomer at higher temperatures for example, the material has a compression set, a measure of a material s ability to recover its size after compression, of 100% at 70 °C (Figure 2). The ability to make PE with properties that fall outside these limitations would lead to a tremendous expansion of uses for this polymer, for example, replacing flexible polyvinylchloride (f-PVC), which cannot be incinerated or recycled, or more expensive thermoplastic polyurethanes (TPUs) or thermoplastic vulcanizates (TPVs). 

Spandex" Fibers. It is the name of polyurethane-based elastic fibers these are materials experiencing a very significant development in the textile industry. These spandex fibers have an initial molecular structure close to that of previously described TPU, but their preparation is performed out of stoichiometry in order to limit their molar mass and to produce two-ended chains carrying isocyanate functional groups. 

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