Gelation polyester resin

Polyester Resins. Reinforced polyester resins are thermosets based on unsaturated polyesters from glycols and dibasic acids, either or both of which contain reactive double bonds. The ratio of saturated to unsaturated components controls the degree of cross-linking and thus the rigidity of the product (see Polyesters, unsaturated). Typically, the glycols and acids are esterified until a viscous Hquid results, to which an inhibitor is added to prevent premature gelation. Addition of the monomer, usually styrene, reduces the viscosity to an easily workable level. 

An interest has been developed in the use of vanadium naphthenates as accelerators. In 1956 the author found that if MEKP was added to a polyester resin containing vanadium naphthenate the resin set almost immediately, that is, while the peroxide was still being stirred in. Whereas this effect was quite reproducible with the sample of naphthenate used, subsequent workers have not always obtained the same result. It would thus appear that the curing characteristics are very dependent on the particular grade of resin and of vanadium naphthenate used. It was also observed by the author that the gelation rate did not always increase with increased temperature or accelerator concentration and in some instances there was a retardation. Subsequent workers have found that whilst the behaviour of the naphthenate varies according to such factors as the resin and catalyst used, certain vanadium systems are of value where a high productivity in hand lay-up techniques is desired. 

Alternatively, Ramis and Salla (1997) define gelation of a polyester resin via Figure 3.56 as the point at which a submerged TMA probe becomes embedded ( stuck ) in the resin. 

The procedure to obtain nanocomposites based on unsaturated polyester resins leads to improvements in the order of 120% in the flexural modulus, 14% in flexural strength and 57% increase in tensile modulus with 4.7% of clay slurry content. Thermal stability augments and the gelation temperature increases to 45 °C, as compared to that of the resin (Fig. 31.6). It seems that adding water to the MMT allows better intercalation of polymer chains into the interlamellar space. Because clay is first suspended in water, this improves dispersion and distribution of the particles in the resin matrix. Longer gelation times lead to more uniform and mechanically stronger structures and to yield stresses (Fig. 31.7). Enhanced polymer-clay interactions are revealed by XPS in this case (Fig. 31.8). 

Figure 31.6 Shear viscosity versus temperature for an unsaturated polyester resin (UP)-clay slurry indicating the temperature for the onset of the gelation reaction. Source Reproduced with permission from Rivera-Gonzaga JA, Sanchez-Solis A, Manero O. J Polym Eng 2012 32,1 [91]. Copyright 2012 De Gruyter.

ISO 2535-1974(E), Plastics—Unsaturated polyester resins--Measurement of gel time at 25 C. BS 2782, Part 8, Method 835B, Determination of gelation time of polyester resins (manual method), 1980. 

The control of gelation and cure is the most critical part of polyester resin technology. Gelation is followed immediately by a self-propagated cure with considerable evolution of heat, which in thick sections and with a highly reactive resin can cause a temperature rise to over 204°C. Molded or laminated structures can be made with low or zero pressure. In closed systems, there is no loss of volatiles during cure. 

Safe preparation of polyester resins from polyfunctional monomers (Fjj > 2) without gelation requires careful optimization of average functionality and calculation of the degree of polymerization at the gel point (Pg j). Wallace Carother and PJ. Floryhave made significant contributions in this area however, a detailed discussion is outside the scope of this book. 

Known also as Diphenyl Ketone. p-Benzoquinone -kwi- non (1,4-benzoquinone, chinone) A yellow crystalline compound used, along with many of its derivatives, as an inhibitor in unsaturated polyester resins to prevent premature gelation during storage. 

It was found that the obtained adducts accelerated the cure of UPRs. The efficiency of the tested accelerators was characterized by the gelation time and peak exotherm temperature. The shortest gelation time was obtained for the adduct of p-toluidine and bisphenol A or p,p -dihydroxydiphenylmethane based low-molecular weight epoxy resins. Adducts remained in the polyester resin as permanently bonded fragments and could not be extracted with methylene chloride from the cured UPR. Moreover, products extracted from the crossHnked resins were determined by the chromatographic and gravimetric methods. 

Usually polyester resin shrinkage characteristics provide the best condition for gelation to occur under pressure. Polyester resin gels and continues to exhibit expansion, and suddenly shrinks at a high initial rate providing a last pressure reduction which reduces the frictional forces proportionally. 

When the resin temperature drops below the boiling point of the reactive diluent (usually styrene) the resin is pumped into a blending tank containing suitability inhibited diluent. It is common practice to employ a mixture of inhibitors in order to obtain a balance of properties in respect of colour, storage stability and gelation rate of catalysed resin. A typical system based on the above polyester fomulation would be ... 

Several applications of hyperbranched polymers as precursors for synthesis of crosslinked materials have been reported [91-97] but systematic studies of crosslinking kinetics, gelation, network formation and network properties are still missing. These studies include application of hyperbranched aliphatic polyesters as hydroxy group containing precursors in alkyd resins by which the hardness of alkyd films was improved [94], Several studies involved the modification of hyperbranched polyesters to introduce polymerizable unsaturated C=C double bonds (maleate or acrylic groups). A crosslinked network was formed by free-radical homopolymerization or copolymerization. 

One major use of polyesters, the alkyd coating resins, is based on random prepolymers reacted with unsaturated fatty acids. Gelation phenomena have been studied extensively and the gelation theories of Carothers, Flory, and Stock-mayer were developed with polyester random prepolymers. Only recently, Gordon (iJ, 12) has extended the gelation theory to the structopendant polyester system cross-linked by vinyl monomers. 

---