Polyester resins curing systems

Uses Unsat. polyester resin comonomer chemical intermediate for paints, light-colored alkyds, plasticizers, adhesives, pesticides hardener for resins curing system additive for bisphenol A-epichlorohydrin epoxies for food contact... 

Consider now the effect of surfactant on the properties of the initial and the cured polyester resins. Such systems show a ninnber of particular features. The structme and properties of the cured polyester resins are determined to a great extent by the compatibility of the copolymerizing components of the system. There is almost no information on the possibility of controlling this compatibility by means of surfactant, which is why it is of great interest to study the influence of surfactant upon the thermodynamics of compatibility of the components of the cured polyester resin, as well as the effect of this compatibility on the molecular mobility of the polymeric chains and on the physical-mechanical properties of the polymer. 

Similar to the vegetable oil-based polyesters, the curing system must be selected according to the nature of the oil. As an example, for non-drying oil-based poly(ester amide), the following system can be used. A homogeneous mixture of resin with 30 parts of styrene or methyl methylacrylate as the reactive diluent, four parts of methyl-ethyl ketone peroxide (MEKP) as an initiator and two parts of cobalt octate/naphthenate as an activator, are prepared in a container at room temperature by mechanical stirring. The thin films of the resin system can then be cured at the desired temperature for a predetermined period of time. 

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... 

Catalyst Selection. The low resin viscosity and ambient temperature cure systems developed from peroxides have faciUtated the expansion of polyester resins on a commercial scale, using relatively simple fabrication techniques in open molds at ambient temperatures. The dominant catalyst systems used for ambient fabrication processes are based on metal (redox) promoters used in combination with hydroperoxides and peroxides commonly found in commercial MEKP and related perketones (13). Promoters such as styrene-soluble cobalt octoate undergo controlled reduction—oxidation (redox) reactions with MEKP that generate peroxy free radicals to initiate a controlled cross-linking reaction. 

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. 

The applications of the unsaturated polyester resins were increased in the late 1960s by the introduction of water-extended polyesters. In these materials water is dispersed into the resin in very tiny droplets (ca 2-5 p.m diameter). Up to 90% of the system can consist of water but more commonly about equal parts of resin and water are used. The water component has two basic virtues in this system it is very cheap and because of its high specific heat it is a good heat sink for moderating cure exotherms and also giving good heat shielding properties of interest in ablation studies. 

Consist of a range of chemicals which promote cross-linking can initiate cure by catalysing ( catalysts , hardeners, initiators), speed up and control cure (activators, promoters) or perform the opposite function (inhibitors) producing thermosetting compounds and specialised thermoplastics (e.g. peroxides in polyesters, or amines in epoxy formulations). The right choice of a cure system is dependent on process, process temperature, application and type of resin. 

The following period between 1975 and 1985 was characterized by activities related to the development of prepreg systems for low pressure autoclave moulding, i.e. modification of the resin chemistries to achieve flow, tack and non-volatile cure. These requirements were dictated by the industry to meet the processing techniques already in place for epoxy and polyester resins. 

The free-radical cure mechanism of the vinyl ester resins is well understood. In most respects, it is similar to that of the unsaturated polyester resins. To initiate the curing process, it is necessary to generate free radicals within Ike resm mass. Organic peroxides are tlie most common source of free radicals. These peroxides will decompose under the influence of elevated temperatures or chemical promoters, e.g., organometallics or tertiary amines, to form free radicals. Generation of free radicals also can be effected by ultraviolet or high-energy radiation applied directly to the resin system. The free radicals thus formed react to open the double bond... 

Examples used in practice are polyester and epoxy casting resins, both as two-component systems. The epoxies are more expensive than the polyester resins, but they show less shrinkage the shrinkage, expressed as the volume difference before and after curing, amounts to 1 to 6 % for epoxies, and to 5 to 10 % for polyesters. 

Thermosetting adhesives are, in general, two-component systems, and may be cured either at ambient or at elevated temperatures. After the components have been mixed, the glue has a limited time of application. Phenol formaldehyde, polyester resins and epoxies are being used the latter show a very strong adhesion to practically all materials. 

There are basically two types of epoxy acrylate resins used in formulating adhesive systems. One is a vinyl ester resin that is used in two-component adhesive formulations much as a DGEB A epoxy or a polyester resin is. The other is a special type of resin that is used in radiation cure processes. This latter type of epoxy acrylate does not have any free epoxy groups, but reacts through its unsaturation. 

Although epoxies dominate the thermoset fracture literature, work has been reported on other systems, e.g., polyester resins, phenol-formaldehyde compounds, peroxide cured polystyrene, and highly crosslinked polyurethanes. In general, these materials exhibit fracture behaviors similar to epoxies, and suggest that thermosets, as a class of materials, display characteristic crack growth properties. 

In spite of the fact that the rheological behavior during c ire of the Ashlemd polyester resin looks very similar to that of the Dow vinyl ester resin, the Ashlcuid polyester resin is found to be more reactive them the Dow vinyl ester resin, with the same formulation emd initiator system used in this study. Some iii Kjrtemt differences in the rheological responses are reflected on (1) the values of (2) the time at which shrinkage begins to occur when the fluid is at rest emd (3) the slope of the -( ii- 22 versus cure time curve. 

It is interesting to note that Eq. (28) is sufficiently general and describes well the kinetics of curing of fundamentally different systems with phase segregation like, for example, unsaturated polyester resin. This is shown in Fig. 32 where there are compared experimental data of Ref. [129] and curves calculated by Eq. (28). 

Fig. 32. Experimental (dots) and calculated (Eq. (28), solid lines) time dependences of conversion in semilogarithmic coordinates for the system polyester resin-benzol peroxide. Curing temperature (°C) 90 (1) 85 (2) 80 (3) [125]...

---