Crosslinked plastics

The crosslinking reaction is an extremely important one from the commercial standpoint. Crosslinked plastics are increasingly used as engineering materials because of their excellent stability toward elevated temperatures and physical stress. They are dimensionally stable under a wide variety of conditions due to their rigid network structure. Such polymers will not flow when heated and are termed thermosetting polymers or simply thermosets. More than 10 billion pounds of thermosets are produced annually in the United States. Plastics that soften and flow when heated, that is, uncrosslinked plastics, are called thermoplastics. Most of the polymers produced by chain polymerization are thermoplastics. Elastomers are a category of polymers produced by chain polymerization that are crosslinked (Sec. 1-3), but the crosslinking reactions are different from those described here (Sec. 9-2). 

An important development was the use of plasticizers which crosslink upon application of heat and thus produce a more rigid end product. This extends the range of products obtainable by plastisol techniques into rigids. Ry varying the amount of crosslinking plasticizer incorporated, various levels of flexibility are obtained. 

The interesting molecular mass range for technical applications lies between a Mr value of 200,000 to 400,000 for the thermoplastic polyvinyl plastics and between 15,000 and 25,000 for the polyamides. For polyesters which are used as precursors for hardened crosslinked plastics, the optimum molecular mass lies around 2000 to 4000. 

Because the chemical composition of crosslinked plastics cannot be given with any accuracy, this table lists starting materials and reactive groups but does not describe the products or give the trade names of the many available thermosets that differ in composition as well as content of additives, e.g., fillers. 

Crosslinked plastics cannot be separated from fillers in this way due to their insolubility. Inorganic fillers (glass fibers or chalk) can sometimes be isolated by burning the sample in a porcelain cup, although this is not always the case. Carbon black may also burn off. However, it is frequently necessary to try special methods, which vary from case to case. 

Extensive studies on the ultrasonic devulcanization of rubbers and some preliminary studies on ultrasonic decrosslinking of crosslinked plastics were... 

Sheet with improved heat resistance and mechanical performance by use of crosslinking plasticizer. [Data from Kyriyama, K. Ichihara, M. Sugawara, R. S. Hirakala, K. H., US Patent 4,402,887, Sep. 6,1983.]... 

The rate dependence of fatigue strength demands carefiil consideration of the potential for heat buildup in both the fatigue test and in service. Generally, since the buildup is a function of the viscous component of the material, the materials that tend toward viscous behavior will also display sensitivity to cyclic load frequency. Thus, TPs, particularly the crystalline polymers like polyethylene that are above their glass-transition temperatures, are expected to be more sensitive to the cyclic load rate, and highly crosslinked plastics or glass fiber reinforced TS plastics are much less sensitive to the frequency of load. 

Thermoelastics are chemically or physically wide-meshed crosslinked plastics that become elastic above the softening temperature (glass transition temperature) or fusing temperature, but do not flow viscously up to the decomposition temperature, making them nonprocessable as thermoplastics. Below the softening temperature, their characteristics are similar to thermoplastics. 

Elastomers are chemically wide-meshed crosslinked plastics that are elastic from lower temperatures (below 0 °C) up to the decomposition temperature (Fig. 2). As a result of the wide-meshed crosslinking, macrobrownian movements (molecular... 

The processing of uncrosslinked plastics (thermoplastics, thermoplastic elastomers (TPE)) and crosslinked plastics (elastomers, duroplastics) differs in two main aspects ... 

Depending upon diacetylene content, the linear polymers can be transformed (via thermolysis) to either highly crosslinked plastics or slightly crosslinked elastomers. The crosshnked polymers degrade thermally above 425 °C under inert conditions. 

Extensive studies on the ultrasonic devulcanization of rubbers and some preliminary studies on ultrasonic decrosslinking of crosslinked plastics were carried out [83-122]. It was shown that this continuous process allows one to recycle various types of rubbers and thermosets. As a most desirable consequence, ultrasonically devulcanized rubber becomes soft, therefore making it possible for this material to be reprocessed, shaped, and revulcanized in very much the same way as the virgin rubber. This new technology has been used successfully in the laboratory to devulcanize a ground tire rubber (GRT)... 

Ultrasound technology is now being implemented in recycling used tyres and crosslinked plastics. The ultrasound energy is used to break up the three-dimensional network in the crosslinked rubbers and plastics [58]. The process consists essentially of screw extruding the heated crosslinked material onto a powerful sonicator. It was shown that high power ultrasonic waves are capable of rapidly breaking down the three-dimensional network present in crosslinked elastomers and plastics. 

Highly crosslinked plastics are usually not affected by solvents. This type of plastic cannot be dissolved unless the chemical crosslinks are broken. If this should occur by a suitably aggressive solvent for the particular polymer, strength will be severely degraded. 

These materials, of course, are not strictly IPNs. However, the elastomer portion is partly cured and/or highly branched at the time of forming (somewhat similar to the urethane products discussed in Section 8.2). In addition, the polypropylene, being highly crystalline, contributes physical crosslinks to the system. The final material is composed of a partly gelled elastomer and a physically crosslinked plastic. 

Energetic radiation causes gas liberation both in crosslinking plastics and in those that tend to degrade. Table 5.21 and Table 5.28. The amount of liberated gas depends on the radiation conditions (s. Section 5.3.3). 

In the temperature range from 230 to 280 °C, which is relevant for lead-free soldering, the crystalline regions in thermoplastics will melt. In non-crosslinked plastic films, there are no sufficient bonds left to hold the material together. The crystallites in crosslinked thermoplastics also melt, but the chemical crosslinking bonds hold the macromolecule together in its amorphous zones so that they can keep their shape - even at low stiffness levels. The crosslinked film remains sufficiently strong and stable short-term up to approx. 300 °C [720]. 

It should also be noted that non-crosslinked and crosslinked plastics exhibit quite different solubility behavior. Only limited swelling is observed in crosslinked plas-... 

Crosslinking (plastics) chemical bonding of thermosetting molecules when they are cured. 

Two component silicone sealants have as one component the silanol terminated silicone polymer, plus tetraethylorthosilicate as the crosslinker, plasticizers, reinforcing fillers, and other additives. The minor component contains the activator, an organometallic catalyst such as dibutyl tin dilaurate, in a paste of silicone fluid, and filler. 

This law describes an ideal case of steady flow which is strongly influenced by the nature of polymer, crosslinks, plasticizers, fillers, nature of penetrant, and temperature. Figure 7.1.10 shows the effect of polymer crystallinity and temperature on diffusion coefficients of 1-butanol and 1-octanol in poly(ethylene terephthalate). Increase of temperature facilitates diffusion (see also Figure 7.1.1). There is also difference in diffusion between two penetrants studied. Butanol has higher diffusion rate than octanol, which is easy to understand since butanol has smaller molecular volume (see also Figirre 7.1.3). Increase in crystallinity causes decrease of diffusion rate of both solvents. ... 

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