Thermosets examples

The broadest classification for plastics is the old thermoplastic and thermosetting . Examples of the former group are polyethylene, polystyrene, and poly-(methyl methacrylate) examples of the latter are urea-formaldehyde condensation polymers, powder coatings based on polyesters, epoxy resins, and vulcanized synthetic elastomers. 

Another way to classify polymers results from the consideration of their typical applications. Typical classes are Compression molding compounds, injection molding compounds, semi-finished products, films, fibers, foams (urethane foam, styrofoam), adhesives (synthetic adhesives are based on elastomers, thermoplastics, emulsions, and thermosets. Examples of thermosetting adhesives are Epoxy, polyurethane, cyanoacrylate, acrylic polymers), coatings, membranes, ion exchangers, resins (polyester resin, epoxy resin, vinylether resin), thermosets (polymer material that irreversibly cures), elastomers (BR, silicon rubber). 

Translucency, as distinct from transparency, can be achieved in a great variety of plastics if the specimen is thin enough even in some reinforced thermosets. Examples of the application of this characteristic are PP fluid reservoirs, in which the level can be read, and interior lamp lenses in nylon, providing diffuse illumination. 

DSC can be used to monitor the curing of thermosets. Examples are the polymerisation of unsatured polyester resins, poly-addition of epoxy resins with curing agents and isocyanates with polyols. 

In practice, synthetic polymers are sometimes divided into two classes, thermosetting and thermo-plMtic. Those polymers which in their original condition will fiow and can be moulded by heat and pressime, but which in their finished or cured state cannot be re softened or moulded are known as thermo setting (examples phenol formaldehyde or urea formaldehyde polymer). Thermoplastic polymers can be resoftened and remoulded by heat (examples ethylene polymers and polymers of acrylic esters). 

In this section we examine some examples of cross-linked step-growth polymers. The systems we shall describe are thermosetting polymers of considerable industrial importance. The chemistry of these polymerization reactions is more complex than the hypothetical AB reactions of our models. We choose to describe these commercial polymers rather than model systems which might conform better to the theoretical developments of the last section both because of the importance of these materials and because the theoretical concepts provide a framework for understanding more complex systems, even if they are not quantitatively successful. 

MixedPhosphona.te Esters. Unsaturated, mixed phosphonate esters have been prepared from monoesters of 1,4-cyclohexanedimethanol and unsaturated dicarboxyhc acids. Eor example, maleic anhydride reacts with this diol to form the maleate, which is treated with benzenephosphonic acid to yield an unsaturated product. These esters have been used as flame-retardant additives for thermoplastic and thermosetting resias (97). 

Examples of thermoplastic coatings an fluoropolymers, eg. Teflon or polyamides, eg, nylon. Thermosetting coatings are more resistant to ... 

Both thermoplastics and thermosets can be formed by compression moulding (Fig. 24.5). The polymer, or mixture of resin and hardener, is heated and compressed between dies. The method is well suited to the forming of thermosets (casings for appliances, for instance) and of composites with a thermosetting matrix (car bumpers, for example). Since a thermoset can be removed while it is still hot, the cycle time is as short as 10 seconds for small components, 10 minutes for large tliick-walled mouldings. Pressures are lower than for injection mouldings, so the capital cost of the equipment is much less. 

Where plastics are to be used for electrical applications, then electrical properties as well as mechanical and other properties need to be considered. Whilst properties such as resistivity, power factor and dielectric constant are important, they may not be all-important. For example, although polyamides and many thermosetting plastics may show only moderate values for the above properties, they have frequently been used successfully in low-frequency applications. Perhaps more important for many purposes are the tracking and arcing resistance, which are frequently poor with aromatic polymers. 

Solvent Example Thermosetting resins Thermoplastics Elastomers... 

There is considerable evidence in the thermoset literature that the fracture energy decreases with increasing crosslink density, consistent with the intuitive result that crosslinking inhibits flow. In the limit of very high crosslink density, where for example we approach the structure of diamond, fracture can occur on a single crystal plane such that... 

Bis-maleimide resins composed of BMI and diamines have been reported in the early 1960s in the patent literature. Since that time, a number of patents have appeared describing improvements in their properties and uses [3]. Although many bis-maleimide resins are commercially developed, relatively few reports of their use as adhesives are to be found in scientific journals [4-10]. Improvements of maleimide resins are mirrored in the improvements of thermosetting polyimides. For example, the method of in situ polymerization of monomer reactants (PMR method) was developed [6]. 

Thermosetting resins are available as powders and liquids. The powders can be molded in a similar manner to thermoplastics and are used in process plants where large numbers of small items are required (for example, packing rings). 

Compression moulding is one of the most common methods used to produce articles from thermosetting plastics. The process can also be used for thermoplastics but this is less conunon - the most familiar example is the production of LP records. The moulding operation as used for thermosets is illustrated in Fig. 4.62. A pre-weighed charge of partially polymerised thermoset is placed in the lower half of a heated mould and the upper half is then forced down. This causes the material to be squeezed out to take the shape of the mould. The application of the heat and pressure accelerates the polymerisation of the... 

Polymer-matrix materials include a wide range of specific materials. Perhaps the most commonly used polymer is epoxy. Other polymers include vinyl ester and polyester. Polymers can be either of the thermoset type, where cross-linking of polymer chains is irreversible, or of the thermoplastic type, where cross-linking does not take place but the matrix only hardens and can be softened and hardened repeatedly. For example, thermoplastics can be heated and reheated, as is essential to any injection-molding process. In contrast, thermosets do not melt upon reheating, so they cannot be injection molded. Polyimides have a higher temperature limit than epoxies (650°F versus 250°F or 350°F) (343°C versus 121°C or 177°C), but are much more brittle and considerably harder to process. 

In these reactions, the monomers have two functional groups (whether one or two monomers are used), and a linear polymer results. With more than two functional groups present, crosslinking occurs and a thermosetting polymer results. Example of this type are polyurethanes and urea formaldehyde resins (Chapter 12). 

Acid catalysts, such as metal oxides and sulfonic acids, generally catalyze condensation polymerizations. However, some condensation polymers form under alkaline conditions. For example, the reaction of formaldehyde with phenol under alkaline conditions produces methy-lolphenols, which further condense to a thermosetting polymer. 

---