Styrene unsaturated polyester diluent

The second largest use at 21% is for unsaturated polyester resins, which are the products of polycondensation reactions between molar equivalents of certain dicarboxyhc acids or thek anhydrides and glycols. One component, usually the diacid or anhydride, must be unsaturated. A vinyl monomer, usually styrene, is a diluent which later serves to fully cross-link the unsaturated portion of the polycondensate when a catalyst, usually a peroxide, is added. The diacids or anhydrides are usually phthahc anhydride, isophthahc acid, and maleic anhydride. Maleic anhydride provides the unsaturated bonds. The exact composition is adjusted to obtain the requked performance. Resins based on phthahc anhydride are used in boat hulls, tubs and spas, constmction, and synthetic marble surfaces. In most cases, the resins contain mineral or glass fibers that provide the requked stmctural strength. The market for the resins tends to be cychcal because products made from them sell far better in good economic times (see Polyesters,unsaturated). 

Because of its low price, compatibility, low viscosity and ease of use styrene is the preferred reactive diluent in general purpose resins. Methyl methacrylate is sometimes used, but as it does not copolymerise alone with most unsaturated polyesters, usually in conjunction with styrene in resins for translucent sheeting. Vinyl toluene and diallyl phthalate are also occasionally employed. The use of many other monomers is described in the literature. 

Similar to unsaturated polyesters, they are copolymerized with diluents such as styrene using similar free-radical initiators. They differ from polyesters in that the unsaturation is at the end of the molecule and not along the polymer chains. Their burning behavior falls between that of polyester and epoxy resins (LOI = 20-23 vol%, Table 2.4). 

There are two basic VPI resin formulations. One is based on epoxy resins, and the other is based on unsaturated polyesters. Both are typically one-part solventless compositions that are completely polymerizable. Volatile, nonpolymerizable solvents cannot be tolerated because these will lead to bubbling in the vacuum impregnation procedure. To control viscosity of the resin formulations, the epoxies contain low-viscosity, reactive diluents of the monoglycidyl ether type the unsaturated polyester formulations contain styrene or vinyltoluene as reactive diluents. Both resin types contain catalysts that become active only at elevated temperatures to insure long-term stability at room temperature. The epoxy resins are frequently catalyzed with metal organic compounds such as titanium complexes, and peroxides are usually the main catalyst in the unsaturated polyester formulations. In addition to the ingredients mentioned here, the compositions may contain additives such as cocatalysts, activators, and accelerators. However, there are no particular fillers used in VPI resins. 

Unsaturated polyester resins and vinyl ester resins are styrene-based matrices cured by free radical copolymerization of the reactive diluent (solvent) styrene and the unsaturated groups in the dissolved polymeric ester. In the case of the polyester resins the unsaturated groups are within the molecular backbone of the polyester (Structure (V)). 

Unsaturated polyester Dicarboxylic acids, diols and reactive diluents e.g., styrene Surface coating, fibre composites for mechanical equipment and building construction, electrical and lighting industries... 

The composite resin industry is the main exception with respect to the selective choice of reactive diluents. Within this industry, styrene is used as well as reactive diluent (known in combination with unsaturated polyesters). Methacrylates are then often referred to as - chemically erroneously - vinyl esters. 

Unsaturated polyester resins are generally linear low MW polyesters having unsaturation (reactive double bonds) within their backbone. They are supplied as a solution of polymerizable monomers (reactive diluents), most commonly in styrene. 

Reactive diluent is a very important component of unsaturated polyester resins. It should act as a solvent for unsaturated polyester resins as well as copolymerize with unsaturated bonds in the polymer backbone. Styrene is the most commonly used diluent due to good solvency, low cost, low viscosity and good reactivity. Apart from styrene, other monomers such as vinyl toluene, methyl methacrylate and a-methyl styrene as well as some polyfunctional diluents such as 1,6-hexanediol diacrylate, diallyl phthalate, divinyl benzene, and trimethylolpropane triacrylate are used. 

Unsaturated polyester resins are low MW condensation polymers which are transformed to a cross-linked network via radical initiated polymerization. The double bonds in the backbone copolymerize with unsaturated monomer (reactive diluent) present in the system. During polymerization, relatively short low MW polyester chains are cross-linked by short bridges consisting of, on average, around two to three styrene units, forming a densely cross-linked polymer network (see Figure 2.17). 

CIO aromatics. Styrene and vinyl toluene are also aromatic hydrocarbons that act simultaneously as solvents and reactive diluents for chemical crosslinking with unsaturated polyester resins and in UV-cured coatings. 

As pointed out in the introduction, unsaturated polyester resins are commonly combined with reactive diluent monomers (Table 12.3). Styrene is most frequently used for this purpose.However, monomers such as methyl methacrylate, diallyl phthalate, " triallyl cyanurate, " vinyl toluene,a -methylstyrene, p-t rt-butylstyrene, and chlorostyrene have also been studied (Table 12.3), showing varying degrees of utility. 

Unsaturated polyesters are fast-curing, two-part systems that harden by the addition of catalysts, usually peroxides. Styrene monomer is generally used as a reactive diluent for polyester resins. Cure can occur at room or elevated temperature depending on the type of catalyst Accelerators such as cobalt naphthalene are sometimes incorporated into the resin to speed cure. Unsaturated polyester adhesives exhibit greater shrinkage during cure and poorer chemical resistance than epoxy adhesives. Certain types of polyesters are inhibited from curing by the presence of air, but they cure fully when enclosed between two... 

There are, however, important solution polymerization processes. Examples include processes for making poly (vinyl acetate) and copolymers of vinyl chloride and of styrene. Another is the manufacture of unsaturated polyester resins, discussed above. In this case, the solvent for the resin, usually styrene, is a reactive diluent for subsequent polymerization. The styrene copolymerizes with the resin during the curing process, making it unnecessary to devolatilize the polymer or recycle solvent. 

The polymer is dissolved in styrene or a monomer containing vinyl unsaturated. With heat and a chemically activated free radical initiation, the polyester and the reactive diluent crosslink to form a 3-D nonmelting network. The reaction occurs at 170-200C (338-390F), with components in roughly equal molecular ratio and water eliminated. The polyester is then dissolved in stabilized comonomers. Protected from light, the resin has a shelf life of about six months. Thirty percent styrene usually serves as comonomer solvent. In response to growing unease about the presence of styrene vapor at the workplace, low-styrene emission types have been introduced in recent years. 

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