Thermoset resins Polymerisation

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

The metathesis polymerisation of dicyclopentadiene, an inexpensive monomer (commercially available cyclopentadiene dimer produced by a Diels-Alder addition reaction containing ca 95 % endo and ca 5 % exo form), leads to a polymer that may be transformed into a technically useful elastomer [144-146, 179] and thermosetting resin [180,181]. The polymerisation has characteristics that make it readily adaptable to the reaction injection moulding ( rim ) process [182], The main feature of this process comes from the fact that the polymerisation is carried out directly in the mould of the desired final product. The active metathesis catalyst is formed when two separate reactants, a precatalyst (tungsten-based) component and an activator (aluminium-based) component, are combined. Monomer streams containing one respective component are mixed directly just before entering the mould, and the polymerisation into a partly crosslinked material takes place directly in this mould (Figure 6.5) [147,168,183-186],... 

In the modification with liquid thermosetting resins, considerable amounts of polymerisable low-molecular weight polymers or prepolymers are added in a liquid form to cement mortar and concrete during mixing. The... 

Norbornenyl functionalised linseed oil and cyclopentadiene oligomer-based thermosetting resins have been successfully synthesised using dicy-clopentadiene or other polycyclic norbornene-based comonomers as cross-linkers. New biorenewable-based thermosets have been successfully synthesised by the ring opening metathesis polymerisation of norbornenyl functionalised fatty alcohols of soybean and castor oils. The castor oil-based thermoset exhibits lower soluble fractions and higher thermal stability and mechanical properties compared with the soybean oil-based thermoset. This is due to more effective cross-linking. ... 

Polyester Resin—Thermosetting resins produced by dissolving unsaturated, generally linear, alkyd resins in a vinyl type monomer such as styrene and capable of being crosslinked by vinyl polymerisation using initiators and promoters. 

The report does not cover polymerisation additives, compounding machinery or masterbatches ( concentrates ). Nor does it cover some of the more specialised additives used in active packaging, such as oxygen scavengers, or those that are specific to the thermosetting resin, rubber and coatings industries. 

Epoxy adhesives are thermosetting resins which solidify by polymerisations and, once set, will soften but not melt on heating. Two-part resin/hardener systems solidify on mixing (sometimes accelerated by heat), while one-part materials... 

Presently, the main limitation of FRP advanced composites, in terms of sustainability, arises at the end of the life cycle and has to do with the limited waste management possibilities, since thermosetting resins (by far the most commonly used for infrastracture applications), once polymerised, can no longer be reprocessed. This means that the most common option at the end of the service life is to process the FRP materials to granulates, which are then used as landhll material. 

Although there are some exceptions, most of the thermoset resins listed and described within this Databook are received by the purchaser as viscous liquids, typically but not necessarily in solvent monomers. During the process of first forming and then moulding the composite of matrix resin and fibre reinforcement into the desired component, that liquid state is converted into an irreversible solid by the chemically activated, time-temperature sensitive process known as polymerisation. 

A low percentage chemical addition, frequently a peroxide, and also called a harderter, which promotes the cure of a thermoset resin. Typically it oxidises the accelerator to release the necessary free radicals which cause the molecular change associated with cure or polymerisation. 

The layman s term for the time/lemperature related molecular cross-linking process - known more correctly as polymerisation - which occurs when a thermoset resin changes from the typically viscous liquid to the solid state. This cure process is normally also promoted following chemical activation by a catalyst - or often called, hardener -addition. Other chemical accelerators may be added by the resin manufacturer, or alternatively added to the resin immediately prior to the catalyst addition. Cure can also be activated in other ways, such as by the influence of ultra-violet radiation. No by-products are formed during the formation of these long molecular cross-linked chain links. 

With most thermosetting resin systems additives such as catalysts and hardeners have to be added to the resins to cause polymerisation to occur. Many other additives are also used with thermosetting systems and these include inorganic fillers, internal release agents, pigments, etc. These were dealt with in detail in Chapter 2. 

The polymers can be categorised as formaldehyde containing and formaldehyde free and as thermoset or thermoplastic resins. Typical formaldehyde containing resins are melamine formaldehyde sulfonamide resins, where the sulfonamide is ortho and para toluenesulfonamide. The sulfonamide, which is a solvent for the dye, undergoes a condensation polymerisation with formaldehyde and melamine, the latter acting as a cross-linking agent. Non-formaldehyde, thermoplastic resins are usually polyamides and polyesters. 

In some cases the raw material is already in a fluid state with thermosets as a resin for casting or impregnating, with thermoplasts, as an exception, sometimes as a monomer which is able to polymerise in the mould. In these cases the forming processes are relatively simple, since they can be carried out on low-viscous fluids which do not require high pressures to be transported. 

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