Crosslinking unsaturated polyester

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. 

Figure 14.7 Rubbery modulus against dangling chains concentration in styrene crosslinked unsaturated polyester. (After Mortaigne et a/., 1992.)...

Table 14.3 Characteristics of the hydrolysis kinetics at 100°C, 100% HR for styrene crosslinked unsaturated polyester and vinyl esters. (After Ganem et a ., 1994.

Figure 1.9 Apparent critical stress intensity factor, K of an unreinforced, crosslinked, unsaturated polyester resin containing notches of various degrees of sharpness [19]...

The materials in this group are linear copolyesters. One of the dicarboxylic acids is an aliphatic unsaturated diacid. The unsaturation is introduced into the polymer backbone for the purpose of subsequent crosslinking. Unsaturated polyester technology was developed for use in glass fiber laminates, thermosetting molding compositions, casting resins, and solventless lacquers. 

Styrene is the most common monomer used in crosslinking unsaturated polyesters. When special properties are required, other monomers like methyl methacrylate may be employed. Sometimes this is done in combination with styrene. Diallyl phthalate and triallyl cyanurate form better heat-resistant products. 

Zhang and co-workers developed unique crosslinkable unsaturated polyesters by a one-pot lipase-catalysed oxidation followed by copolymerisation of crude linoleic acid, glycerol and l,18-ds-9-octanedecenedioic acid [81]. In a similar approach, Roumanet and co-workers prepared aliphatic unsaturated polyesters derived from l,18-(Z)-octadec-9-enedioic acid (a derivative of fatty acids from sunflower oil) and aliphatic diols of different molecular weight [82]. The double bonds of the ensuing polyester backbones were then epoxidised and photocrosslinked, yielding transparent and homogeneous crosslinked films with high hydrophobicity. 

Peroxyketals Polyethylene crosslinking Unsaturated polyester curing... 

The thermoplastic and crosslinked unsaturated polyester materials must form separate phases. 

Triallyl Cyanurate n (TAG, 2,4,6-triallyloxyl-l,3,5-tri-azine) (CH2=CHCH20C)3N3. This heterocyclic compound, a solid below 110°C, is highly reactive and is used in copolymerizations with vinyl-type monomers to form AUyl Resins. It is also used to crosslink unsaturated polyesters and raise their softening temperatures. 

Case L C and Case L K (1968) Vinyl-crosslinked unsaturated polyester-polyether copolymers, US Patent 3,375,301. 

This is also known as Bulk Moulding Compound (BMC). It is blended through a mix of unsaturated polyester resin, crosslinking monomer, catalyst, mineral fillers and short-length fibrous reinforcement materials such as chopped glass fibre, usually in lengths of 6-25 mm. They are all mixed in different proportions to obtain the required electromechanical properties. The mix is processed and cured for a specific time, under a prescribed pressure and temperature, to obtain the DMC. 

Unsaturated polyester finishes of this type do not need to be stoved to effect crosslinking, but will cure at room temperature once a suitable peroxide initiator cobalt salt activator are added. The system then has a finite pot life and needs to be applied soon after mixing. Such a system is an example of a two-pack system. That is the finish is supplied in two packages to be mixed shortly before use, with obvious limitations. However, polymerisation can also be induced by ultra violet radiation or electron beam exposure when polymerisation occurs almost instantaneously. These techniques are used widely in packaging, particularly cans, for which many other unsaturated polymers, such as unsaturated acrylic resins have been devised. 

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. 

Polyester-based networks are typically prepared from polyester prepolymers bearing unsaturations which can be crosslinked. The crosslinking process is either an autoxidation in the presence of air oxygen (alkyd resins) or a copolymerization with unsaturated comonomers in the presence of radical initiators (unsaturated polyester resins). It should also be mentioned that hydroxy-terminated saturated polyesters are one of the basis prepolymers used in polyurethane network preparation (see Chapter 5). 

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. 

Unsaturated polyesters that are terminated by carboxylic acid groups at both ends of the chain after neutralization are efficient emulsifiers for lipophilic monomers [110] and thus act as self-emulsifying crosslinking agents in the ECP of these systems. Normal emulsions of EUP and comonomers have a white, milky appearance. With an appropriate structure and molar mass of the EUP and within a certain range of EUP/comonomer ratios, however, microemulsions are... 

The equivalent weight increased from 210 for UPE A to 316 for UPE E with one third of the maleic anhydride was replaced with succinic anhydride and 634 for UPE F with two thirds of the maleic anhydride is replaced with succinic anhydride. This change will effectively decrease the crosslink density of the final cured film. The Brookfield viscosities of the two new polyesters (E and F) are less than those from the first series. Since the molecular weights should be about the same (1760), it is not surprising that the Brookfield viscosities were relatively constant for all three unsaturated polyesters. The slight decreasing trend in viscosity with increasing succinic anhydride in the unsaturated polyester may be due to the increased free volume obtained when succinic anhydride is substituted for maleic anhydride. 

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). 

Simple polyesters of the type described by Eq. 2-170 are too limited to be of commercial interest. Almost all crosslinked polyesters are either unsaturated polyesters or alkyd polyesters. These offer a greater ability to vary the final product properties to suit a targeted market. Also, they offer greater process control since different chemical reactions are involved in the polymerization and crosslinking reactions. A typical unsaturated polyester... 

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