Vinyl ester resins curing

Vinyl ester resins generally offer mechanical properties superior to those of polyester matrices but at an increased cost. Vinyl esters are chemically similar to epoxy resins but are manufactured via a cold-curing process similar to that used in the manufacture of polyester resins. Vinyl esters offer superior resistance to water and chemical attack and are used in such appHcations as underground pipes, tank liners, and storage tanks (see Vinyl polymers). 

Epoxy system curing profile, 10 17-18 Epoxy systems, post-cure for, 10 423 Epoxy vinyl ester composites, 10 452 Epoxy vinyl ester resins, 10 349 Epoxy vinyl esters, 10 383 Eprosartan, 5 187... 

In most applications, polyester and vinyl ester resins are used as the matrix materials. Epoxies are also used, although they require longer cure times and do not release easily from the pultrusion dies. Hence, thermosetting resins are most commonly used with pultrusion, although some high-performance thermoplastics such as PEEK and polysulfone can also be accommodated. In addition to the resin, the resin bath may contain a curing agent (initiator, cf. Section 3.3.1.2), colorants, ultraviolet stabilizer, and fire retardant. 

Because of all these minor components (e.g., catalysts and inhibitors, added to major ones) the cure of vinyl ester resins is very complex, involving many competitive reactions. There are some new variables to account for, such as the inhibitor and initiator concentrations and induction time. Several papers [81,96,200,201] use the mechanistic approach, claiming that the phenomenological models do not explicitly include these facts, resulting in a new parameter characterization after each change in resin formulation [96]. Despite these arguments, the phenomenological approach is the most widely used and is based on an autocatalytic model which has been successfully applied to epoxy resins. Many authors [30,34,74,199,202,203] proposed the Equation 2.30 to describe the cure kinetic of unsaturated polyesters ... 

Sheet molding compound (SMC) is a composite material that has been used as an alternative to steel for more than 30 years. SMC consists of a glass-fiber-filled unsaturated polyester and vinyl ester resin that has been compacted into a sheet. These sheets are then placed into compression molds and formed into body panels, and these panels are then coated mainly by dual-cure coatings. ... 

VINYL ESTER RESINS. The vinyl ester resins are a relatively recent addition1 to thermosetting-polymer-chemistry. Superficially, they are similar to unsaturated polyester resins insofar as they contain ethylmic lmsaturation and are cured throngh a free-radical mechanism, usually in the presence of a vinyl monomer, such as styrene. However, close examination of the chemistry and structure of the vinyl ester resins demonstrates several basic differences which lead to their unique characteristics. 

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... 

Two resins were used to do the first study on laminate construction. The first was a brominated epoxy vinyl ester resin with antimony pentoxide and the second was a brominated unsaturated polyester resin. They were both promoted to cure at room temperature with methyl ethyl ketone peroxide catalyst. The panels were then postcured at 250°F (121°C) for 8 h. Panels were prepared that varied in glass content from 25% to 70% and panel thickness varying from 0.05 in. to 0.25 in. and were tested at the same testing facility. A summary of the FSI test data for the first set of panels tested are shown in Figures 23.1 and 23.2. This graph in Figure 23.1 plots the FSI value versus the panel thickness. This data would indicate that the thickness of the test panel has no effect on the measured... 

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. 

These resins act more as polyester resins than they do as epoxy resins. They are easily processed, have fast cure rates at room temperature, and can be cured with peroxides. They effectively wet out glass fiber reinforcement and cure quickly. Therefore, vinyl ester resins are often used in the manufacture of composites such as filament- wound and pultruded structures. 

Curing Kinetics of Unsaturated Polyester and Vinyl Ester Resins... 

To overcome these drawbacks, in recent years much attention has been paid to the development of resins which cem be f2d ricated with the same processes as those for conventional polyester resins, but having superior properties. Vinyl ester resins are the result of such development efforts (4-6). Vinyl ester resins are addition products of Vcurious epoxide resins and ethylenically unsaturated mono-carboxylic acids ( ). It condsines the excellent mechcuiical, chemical cuid solvent resistemce of epoxy resins with the properties found in the unsaturated polyester resins. In general, the cured vinyl ester resin has physical properties superior to the cured conventional ester resin, particulcurly corrosion resistcuice. This arises from the differences in the number and arremgement of polar groups such as ester and hydroxyl groups eind ccurbon-to-ccirbon double bonds present in the polymer chains. 

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. 

Figure 3 gives plots of the rate of heat generated versus ctire time under isothermal DSC conditions, at various ten ratures, for both the Ashland polyester and the Dow vinyl ester resins. It is seen that the eunount of heat generated (i.e., the area under the dQ/dt-cure time curve) increases as the cure temperat ire increases, as would be expected for exothermic chemical reactions. 

Heat generated versus isothermal cure temperature for (a) the Ashlemd polyester resin, emd (b) the Dow vinyl ester resin. 

Figure 5 gives plots of 0 . and versus cure time for the Ashland polyester resin, and Figure 6 gives similar plots for the Dow vinyl ester resin. 

It is seen that in both cases the use of gives rise to very high values for the degree of cure (approaching 1.0) at the end of each isothermal cure, compared to that obtained when was used. We know that the curing reaction cannot approach its completion at such low temperatures, say at 50 C, because the ratio of the residual heat (Q ) to the ultimate heat (Qxj ) is fairly large 0.376 for the Ashland polyester resin and 0.557 for the Dow vinyl ester resin. 

First, it is seen in Figure 3 that the meiximum value of dQ/dt (i.e., (dQ/dt)p) increases, and the time (tp) at which the mcucimum of dQ/dt occurs decreases, as the cure temperature (hence the rate of cure) increases. Over the range of temperatures investigated, the Ashlemd polyester resin exhibits higher values of (dQ/dt)p emd lower values of tp than the Dow vinyl ester resin, indicating that the former is more reactive than the latter. Second, although... 

As may be seen in Table II, in both resins the reaction rate constants kj emd k2 appearing in eq. (5) increase with the isothermal curing temperature. In most cases, the magnitudes of kj amd k2 (only at low temperatures) are greater for the Ashland polyester resin them for the Dow vinyl ester resin. 

Figure 2. TEM image of a cured sample of an elastomer-modified vinyl ester resin (Table I, recipe 1). The sample shown is an ultrathin, OsO stained section. Magnification 60,000x.

Figure 3. Fatigue-fracture surface of an elastomer-modified vinyl ester resin to which ETBN has been added (Table I, recipe 2). The magnifications of these SEM images of cured samples are lOOOx (top) and 5000x (bottom).

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)). 

The chemical reactivities of the unsaturated groups at the ends of vinyl ester resin and vinyl urethane resin chains are different in several respects from those of the same groups when situated in mid-chain positions, as they are in polyester resins. As a consequence of the different reactivity ratios, the two kinds of cured resin behave differently from moisture and chemical resistance points of view. The structural differences are also reflected in the mechanical properties, such as fracture toughness. 

Conventional vinyl ester resins are increasingly used in anticorrosion applications and in marine applications. They are essentially epoxy resins, but chemically modified to enable them to cure in just the same way, and with virtually the same chemicals and fabrication procedures as polyesters. They have excellent water resistance and chemical resistance, but poor resistance to UV light, although modifications can be made to make them light stable and suitable for use in weatherable gelcoats [13]. 

Chem. Descrip. MEK peroxide in dimethyl phthalate Uses Curing agent for reduced gas generation in vinyl ester resin-based gel coals, bamer coatings, and corrosion resist, constniction Properties Clear liq. act. oxygen (9%)... 

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