Thermosetting materials polyurethanes

Thermoplastic foams - mainly polystyrene, PVC, polyethylene and polypropylene -account for roughly 50% of all plastic foams, the other 50% being polyurethane, a thermoset material. For all uses added together, EPS accounts for more than 90% of all thermoplastic foams, followed by PVC with a few percent, and polyethylene and polypropylene. 

Reactions of alkoxylated lignin with diisocyanates produce thermoset materials because the lignin polyol is always polyfunctional with a functionality greater than 2. The isocyanate-alcohol reaction produces a urethane linkage that when repeated creates a crosslinked, nonreformable polyurethane. This is shown in Fig. 6. A broad spectrum of lignin-based urethanes have been made and tested. The data show that these materials match if not exceed the properties of synthetic polyurethanes made without lignin [60]. 

Since the introduction of the first commercial thermoset, Bakelite, based on phenol formaldehyde condensation, a wide range of thermoset materials have been introduced. These are typically designed for specific properties related to their chemistry and processability. Some commercially important thermosets include phenolics, ureas, melamines, epoxy resins, unsaturated polyesters, silicones, rubbers, polyurethanes, acrylics, cyanates, polyimides, and benzocyclobutenes. ... 

Thermoset materials Epoxy resins, phenolic resins, polyurethanes (highly cross-linked), anaerobic adhesives. 

Solubility in organic solvents Tetrahydrofuran (THF is regarded as a solvent with very universal properties, however it is combustible and its vapors must not be inhaled (see Section 7.5.2). In addition, suitable solvents are acetone and methyl-ethyl ketone. Thermoset materials are generally insoluble, polyvinyl chloride, Plexiglas, polystyrol, rubbers and poorly crosslinked polyurethane are swellable thermoplastics. 

Polyurethanes are produced by the chemical action of di-isocyanate and polyol. The properties can be varied by the type of isocyanate used and the proportion of the two monomers. There are four main groups of classification for the thermoplastic groups of polyurethane, i.e. rigid foam, flexible foam, non-cellular and cellular polymers. Two main isocyanates used are toluene di-isocyanate (TDI) and diphenylmethane diisocyanate (MDI). Polyurethanes have limited application in the pharmaceutical or medical industries. Polyurethane is used as an adhesive for laminations (thermosetting material). Like thermosetting polyurethane, thermoplastic polyurethanes can be found as esters and ethers. 

Among those plastics which are commercially produced in cellular form are polyurethane, PVC, polystyrene, polyethylene, polypropylene, epoxy, phenol-formaldehyde, urea-formaldehyde, ABS, cellulose acetate, styrene-acrylonitrile, silicone, and ionomers. However, note that it is possible today to produce virtually every thermoplastic and thermoset material in cellular form. In general, the basic properties of the respective polymers are present in the cellular products except, of course, those changed by conversion to the cellular form. 

Polyurethane materials exist in a variety of forms including flexible or rigid foams, chemical resistant coatings, specialty adhesives and sealants, and elastomers. Most polyurethanes are thermoset materials they cannot be melted and reshaped as thermoplastic materials. Once the reactions have ceased the thermoset polyurethanes are cured and cannot be heat shaped without degradation. The thermal stability results from the croslinking degree of polymer chains (the crosslink density) and from the nature and frequency of repeating units within the polymer chains. 

Thermoset materials are produced by the direct formation of network polymers from monomers, or by crosslinking linear prepolymers. Important thermosets include alkyds, amino and phenolic resins, epoxies, unsaturated polyesters and polyurethanes. Thermosetting polymers consist of two liquid components, one containing a resin and the other a hardener [1]. 

An important group of polymers are classified as being thermosetting materials (165) (see Thermosets). The chemistry associated with their formation is often the same as that used for thermoplastic materials, except that in the case of these polymers the monomers used have more than two functions per monomer unit. It is therefore possible to have two materials that ostensibly look chemically similar, but one is a thermoplastic and the other a thermoset. This situation is to be found in Polyurethanes (qv) where, depending on the functionality of the isocyanate or soft block used, the resiJting material may be a thermoplastic or a thermoset. In most cases it is desirable to monitor the cure process to avoid either too slow a formation of the three-dimensional network, and hence poor production efficiency, or too quick a cure with the possibility of excessive exotherms and possible degradation of the material. The formation of a network for a material that has a Tg above... 

Unsaturated low molecular weight (M between 1000 and 10000), often branched, used as macromonomers for synthesis of thermosets (polyester resins), or thermosetting materials by themselves (alkyd resins). They are prepared from several monomers, namely phthalic and maleic anhydrides, adipic acid, iso-phthalic acid, natural fatty acids or triglycerides, and a great variety of multifunctional alcohols. In a few special cases, they may be saturated and/or linear for use as macromonomers in the production of polyurethanes or other polymers. 

Other thermosetting materials are used in relatively small quantities in specialist applications, including alkyd and silicon resins polyurethane resins are discussed in the next chapter. 

PUs can be both thermoplastic and thermosetting materials composed by HS and SS. Thermoplastic polyurethanes (TPU) are formed by linear polymer chains with functionality equal to two. This means that the SS, the polyol, is a diol and the HS, the isocyanate, is a diisocyanate. The thermodynamic incompatibility between the hard and the SSs as well as the NCO OH ratio, the synthesis procedure, etc. can generate a phase separation into the TPU. On the other hand, the thermosetting PUs are formed by polyols and/or isocyanates with functionalities higher than 2, with a NCO OH ratio higher than 1 or are obtained by using cross-linker agents. 

Polyurethanes can have the physical structure of a solid casting, a flexible elastomer or soft or rigid foams. It can be either a thermoplastic or more usually it is a thermoset material. The isocyanates in polyurethanes often result in health and safety implications during cure or processing and so must be carefully controlled. 

Adhesives come in several forms and include pressure-sensitive adhesives (which must have characteristics of both a liquid and a solid), mbber-based adhesives (essentially all mbbers can be used with solvent natural mbber based, neoprene based, styrene butadiene rubber (SBR) based), hot melt adhesives, and natural produce adhesives. Structural adhesives require high-strength materials and are often used instead of welding or other methods of joining. For example, structural adhesives are used to attach bodywork to automobile chassis in some makes of car. Epoxy-based thermosets, phenolics, polyurethanes, acrylics, cyanoacrylates. 

Thermosetting-encapsulation compounds, based on epoxy resins (qv) or, in some niche appHcations, organosiHcon polymers, are widely used to encase electronic devices. Polyurethanes, polyimides, and polyesters are used to encase modules and hybrids intended for use under low temperature, low humidity conditions. Modified polyimides have the advantages of thermal and moisture stabiHty, low coefficients of thermal expansion, and high material purity. Thermoplastics are rarely used for PEMs, because they are low in purity, requHe unacceptably high temperature and pressure processing conditions. 

Ammonia is used in the fibers and plastic industry as the source of nitrogen for the production of caprolactam, the monomer for nylon 6. Oxidation of propylene with ammonia gives acrylonitrile (qv), used for the manufacture of acryHc fibers, resins, and elastomers. Hexamethylenetetramine (HMTA), produced from ammonia and formaldehyde, is used in the manufacture of phenoHc thermosetting resins (see Phenolic resins). Toluene 2,4-cHisocyanate (TDI), employed in the production of polyurethane foam, indirectly consumes ammonia because nitric acid is a raw material in the TDI manufacturing process (see Amines Isocyanates). Urea, which is produced from ammonia, is used in the manufacture of urea—formaldehyde synthetic resins (see Amino resins). Melamine is produced by polymerization of dicyanodiamine and high pressure, high temperature pyrolysis of urea, both in the presence of ammonia (see Cyanamides). 

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