Unsaturated polyesters crosslinked with styrene

Other common binders include nitrocellulose (acetone as the solvent), polyvinyl alcohol (used with water), and Laminae (an unsaturated polyester crosslinked with styrene — the material is a liquid until cured by catalyst, heat, or both, and no solvent is required). Epoxy binders can also be used in hquid form during the mixing process and then allowed to cure to leave a final, rigid product. 

IR-ATR and ESCA spectroscopy have been applied to the study of photo-oxidation and photostabilization of unsaturated polyesters crosslinked with styrene. Results obtained show that both styrene and polyester units of the cured resins are vulnerable to UV oxidative degradation and are easily photo-oxidized. 

As in most chainwise polymerizations, q -> 1, the first observation arising from Eq. (3.180) is the very low value of the gel conversion. In actual systems, intramolecular cyclization and microgel formation produce an increase in the gel conversion. But reported values of xgei for the free-radical polymerization of systems containing multifunctional monomers are usually below 0.10. This is the case for the crosslinking of unsaturated polyesters (Af) with styrene (A2 ). 

Chain-Growth Polymerisation with Termination A major exponent of this class of polymerisations are free radical polymerisations in the presence of a radical initiator. A classic example is the crosslinking (co)polymerisation of unsaturated polyester resins with styrene, initiated by the decomposition of a peroxide initiator. Some important reaction steps involved in free radical polymerisations are sketched in Reaction scheme 4. 

The structure of unsaturated polyester crosslinking with vinyl monomers, such as styrene, may be described in the simplest form as short polystyrene units linked by double bonds at different polyester molecules, as follows ... 

Unsaturated polyesters (UPs) crosslinked with styrene are often used as a matrix of fiber reinforced plastics. Several reports treated the degradation of the crosslinked UPs with high temperature treatment in water (1,2), acetic acid (5), alcohols including glycols (4,5), and amines (6), often in the presence of catalysts. In these literatures, recovery of polymeric materials from the crosslinked UPs was not a main objective. However, in case we can hydrolyze polyester chains selectively, linear polystyrene derivatives can be obtained as recycled materials. 

In general, polyester resins are synthesized by the reaction between carboxylic acids and alcohols, with three or more reactive groups. Recently, unsaturated polyesters were incorporated in various ways to produce terminal, pendant, and internal double bonds [57-59]. In the case of unsaturated polyesters, maleic anhydride is most commonly used to produce internal unsaturation. The double bond present on unsaturated polyester reacts with a vinyl monomer, mainly styrene, resulting in a 3D crosslinked structure. This structure acts as a thermoset. The crosslinking is initiated through an exothermic reaction involving an organic peroxide, such as methyl ethyl ketone peroxide or benzoyl peroxide (Fig. 3.18). 

Myrcene-maleic anhydride Diels-Alder adduct and the corresponding diacid have been used as mmiomers for the polycondensation with diethylene glycol, resulting in unsaturated polyesters. The polyesters exhibited air-drying properties on crosslinking with styrene [111]. 

For example, the molecular weight of unsaturated polyesters is controlled to less than 5000 g/mol. The low molecular weight of the unsaturated polyester allows solvation in vinyl monomers such as styrene to produce a low-viscosity resin. Unsaturated polyesters are made with monomers containing carbon-carbon double bonds able to undergo free-radical crosslinking reactions with styrene and other vinyl monomers. Crosslinking the resin by free-radical polymerization produces the mechanical properties needed in various applications. 

Certain commercially important crosslinking reactions are carried out with unsaturated polymers. For example, as will be described later in this chapter, polyesters can be made using bifunctional acids which contain a double bond. The resulting polymers have such double bonds at regular intervals along the backbone. These sites of unsaturation are then crosslinked by reaction with styrene monomer in a free-radical chain (addition) process to give a material consisting of polymer backbones and poly(styrene) copolymer crosslinks. 

IPNs are found in many applications though this is not always recognised. For example conventional crosslinked polyester resins, where the polyester is unsaturated and crosslinks are formed by copolymerisation with styrene, is a material which falls within the definition of an interpenetrating polymer network. Experimental polymers for use as surface coatings have also been prepared from IPNs, such as epoxy-urethane-acrylic networks, and have been found to have promising properties. 

The largest single use of maleic anhydride is in the preparation of unsaturated polyester resins. It is first esterihed with a polyalcohol (two or more hydroxyls) and then the double bond is copolymerized (crosslinked) with a vinyl monomer such as styrene to form a rigid structure. Such resins are usually reinforced with hberglass (FRP). Maleic anhydride is also used to make oil additives and agricultural chemicals. 

Resins used were two types of epoxy resins (EP) and an unsaturated polyester resin (UP) as shown in Figure 1. EP is the bis-phenol-A type resin cured with methyl-tetrahydrophthalic anhydride (MTHPA) or 1,8-p-menthandiamine (MDA). UP is the iso-phthalic type resin which has ester bonds in the main chain and is crosslinked by styrene (10). 

The crosslinking of unsaturated polyesters (Sec. 2-12a) is carried out by copolymerization [Selley, 1988]. Low-molecular-weight unsaturated polyester (prepolymer) and radical initiator are dissolved in a monomer, the mixture poured, sprayed, or otherwise shaped into the form of the desired final product, and then transformed into a thermoset by heating. Styrene is the most commonly used monomer. Vinyltoluene, methyl methacrylate, diallyl phthalate, a-methylstyrene, and triallyl cyanurate are also used, often together with styrene. 

The crosslinking copolymerization of unsaturated polyesters with styrene is utilized industrially for the production of surface coatings. More extensive are applications in fiber-reinforced plastics where the unsaturated polyesters are used as a matrix for inorganic and organic fibers (see Example 5-20). Areas of application are (large) parts in boats, vehicles, and sport equipment construction. 

The distribution of styrene crosslinks in cured unsaturated polyesters based on maleic anhydride, phthalic anhydride, propylene glycol and dipropylene glycol is found to be dependent on the molar ratio of styrene vs. maleic/fumaric unit. If the molar ratio is higher than 1, the dyad distribution is predominant lower molar ratios yield mostly styrene microdomains, i.e. n-ads with more than 2 styrene repeating units 232>. 

Maleic anhydride [R1 equals -CH=CH- in Eq. (2.7) cis isomer] is reacted with aliphatic diols to form low molar mass unsaturated polyesters, UP. For molar masses higher than 1000 g/mol, products are diluted with a liquid vinyl monomer, most often styrene. This reactive mixture, generally called unsaturated polyester, UP resin, can be transformed into crosslinked polymers through a free-radical chain polymerization (see Sec. 2.3). 

For styrene-crosslinked unsaturated polyesters and vinyl esters, v is considerably higher v (20 5)10 4 KT1 and v/G —3. It is noteworthy that in such networks (Tg 100°C) the temperature interval between ambient temperature and Tg is a diffuse transition region in which tan 8 increases almost continuously with temperature. 

As a preliminary step in the manufacture of unsaturated polyester thermoset plastic one uses low molecular weight linear polyester (Mr 10,000) obtained by a polycondensation of polyglycols with saturated and unsaturated dicarboxylic acids. The precondensate can then be dissolved and stored in the stabilized comonomer, e.g. styrene, with which it will be crosslinked later. The crosslinking polymerization reaction between the polyester chains and the styrene bridges is initiated with the help of organic peroxides which are added dispersed in plasticizers. The reaction begins at 60-90 °C and then proceeds exothermally. In addition to this a cold hardening reaction can also be carried out. For this reaction cold accelerators are necessary, e.g. tertiary amines or cobalt naphthenate. 

It comes as a liquid that typically contains low molar-mass, unsaturated polyester and styrene. The unsaturated polyester contains carbon-carbon double bonds. The polymerization is catalyzed with a hardener, an organic peroxide dissolved in an organic solvent. The peroxide forms free radicals that initiate polymerization of the styrene and crosslinking of the polyester. 

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