Incorporation into cured resin

Besides resin and reactive diluent, additives are commonly incorporated into polyester resins. These include not only curing agents and fillers (see Section 25.2.3) but also ultraviolet stabilisers. The latter are particularly important for outdoor applications such as roof lighting, benzotriazoles being particularly effective. 

The methylated maleic acid adduct of phthalic anhydride, known as methyl nadic anhydride VI, is somewhat more useful. Heat distortion temperatures as high as 202°C have been quoted whilst cured systems, with bis-phenol epoxides, have very good heat stability as measured by weight loss over a period of time at elevated temperatures. The other advantage of this hardener is that it is a liquid easily incorporated into the resin. About 80 phr are used but curing cycles are rather long. A typical schedule is 16 hours at 120°C and 1 hour at 180°C. 

The pigments are incorporated into the resins before crosslinking to ensure homogeneous coloration. This can be done in the molten resin, for example in a kneader at about 90°C, or in dissolved or liquid resins by the liquid resin method. Ball mills are normally used for coloring prewetted powder molding compounds that have not yet been cured. 

Commercial epoxy adhesives are composed primarily of an epoxy resin and a curing agent. Various additives and modifiers are added to the formulation to provide specific properties. Example trade names and suppliers of these ingredients are included in App. A. The curing agent may be incorporated into the resin to provide a single-component adhesive, or else it may be provided in a separate container to be mixed into the resin immediately prior to application. 

In this study, up to about 50% of the phenol-formaldehyde was replaced with carbohydrates and the modified resins used to bond wood veneer panels. The carbohydrate modified resins were formulated and cured under neutral conditions. The resins bond wood with acceptable dry- and wet-shear strengths, and wood failures. Reducing as well as nonreducing carbohydrates can be used as modifiers. The carbohydrate modifiers are being incorporated into the resin via ether linkages between the hydroxyls of the carbohydrate and methylol groups in the phenol-formaldehyde resin. The resins formulated under neutral conditions are very light in color. 

Water extraction studies of the carbohydrate modified resins in the earlier study indicate that a portion of the carbohydrate is apparently incorporated into the final cured resin. The absolute amount of modifier incorporated into the cured resin increases with the amount initially added during resin formulation. IR studies indicated that the carbohydrate derivative is probably incorporated into the resin via ether linkages. 

Incorporation of Carbohydrate into Cured Resin. Several methods were used to determine whether the carbohydrate component is chemically incorporated into the final cured resin. These methods were water extraction of the modifier from the cured resin, IR spectroscopy, and isolation of reaction products formed between a carbohydrate and a model compound that contained a phenolic methylol group. 

Incorporation of monofunctional epoxy POSS into an amine-cured epoxy network increased and broadened the Tg without changing the crosslink density and enhanced the thermal properties. Additionally, it was found that the thermal and thermal-mechanical properties of resultant styrene-POSS vinylester resin nanocomposites were dependent on the percentage of POSS incorporated into the resin [171]. Over a range of POSS incorporations, the Tg of the copolymers changed very little, but the flexural modulus increased with increasing POSS content. 

In fact, these cyclic trimers may be incorporated into silicone resins of the type employed in surface coatings. In such products, a silanetriol is incorporated to cross-link the resin (this cross-linking reaction has been previously described) and to allow the presence of excess hydroxyl groups after resin preparation. These materials containing excess hydroxyl groups are then applied to a substrate as a surface coating and subsequently cured via a thermal cycle of up to 60 min at 500 °F. When catalyzed, their cure cycle may be reduced to 30-60 min at 400 °F. Some typical catalysts and the levels at which they are usually employed are listed in Table II. 

Non-reaotive stains diffuse into cured resins uniformly with no sign of local concentration variation. When incorporated into the epoxy resin before curing these stains did tend to form streaks and to concentrate in haloes around bubbles which form during curing. 

Phthalic anhydride is the cheapest anhydride curing agent, but it has the disadvantage of being rather difhcult to mix with the resin. Liquid anhydrides (e.g., dodecenylsuccinic anhydride and nadic methyl anhydride), low-melting anhydrides (e.g., hexahydrophthalic anhydride), and eutectic mixtures are more easily incorporated into the resin. Since maleic anhydride produces brittle products, it is seldom used by itself and is used as a secondary hardener in admixture with other anhydrides. Dodecenylsuccinic anhydride imparts flexibility into the casting, whereas chlorendic anhydride confers flame resistance. 

The cure reaction in polyester gel coat systems makes them particularly useful model coatings materials since the cure kinetics can be studied by several analytical techniques. The initiation of cure in polyester gel coat resins begins with the decomposition of hydroperoxide (MEKP is predominately hydroperoxide) and the subsequent copolymerization of styrene with the unsaturated groups, either fumarate or maleate, incorporated into the resin backbone. ... 

Improved fire retardancy can be conferred to most resins by the incorporation of other materials. These materials may be specifically formulated for chemical incorporation into the resin during cure or exist as particulates in physical contact with the resin. Most organic phosphate esters and halogenated hydrocarbons, as well as antimony trioxide, are physically combined with the resin, and some halogen compounds are combined chemically. High concentrations of aluminium tri-hydrate (ATH) act as both filler and an effective low smoke/toxicity additive by releasing water of hydration at the flame temperature. 

Oligomeric phosphoric/phosphonic anhydrides can be incorporated into epoxy resins to make prepolymers or used as curing agents (184-186). 

Additives and fillers can be incorporated into the resins prior to curing, to make them easier to process and to achieve specific end-product properties. By choosing the right combination of resins, reinforcing fibres, fillers and additives, a composite material can be formulated to suit practically any type of application. 

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... 

The curing reactions of some of the functional groups incorporated into Acrylic Resins for use in surface coating applications are illustrated below. 

Tertiary amines with N-methacryloxyethyl groups have been synthesized (26). These amines copol)nnerize with acrylic monomers. They are incorporated into the resin which should have lower toxicity than polymers cured with non-pol3nnerizable accelerators. Color stability of these resins is similar to those cured with DMPT. 

Two-Stage Resins. The ratio of formaldehyde to phenol is low enough to prevent the thermosetting reaction from occurring during manufacture of the resin. At this point the resin is termed novolac resin. Subsequently, hexamethylenetetramine is incorporated into the material to act as a source of chemical cross-links during the molding operation (and conversion to the thermoset or cured state). 

Heat distortion temperatures of resins cured with pyromellitic dianhydride are often quoted at above 200°C. The high heat distortion is no doubt also associated with the rigid linkages formed between epoxy molecules because of the nature of the anhydride. The use of these two anhydrides has, however, been restricted because of difficulties in incorporating them into the resin. 

Some rubber base adhesives need vulcanization to produce adequate ultimate strength. The adhesion is mainly due to chemical interactions at the interface. Other rubber base adhesives (contact adhesives) do not necessarily need vulcanization but rather adequate formulation to produce adhesive joints, mainly with porous substrates. In this case, the mechanism of diffusion dominates their adhesion properties. Consequently, the properties of the elastomeric adhesives depend on both the variety of intrinsic properties in natural and synthetic elastomers, and the modifying additives which may be incorporated into the adhesive formulation (tackifiers, reinforcing resins, fillers, plasticizers, curing agents, etc.). 

An obvious means of increasing conductivity is to incorporate metals into the fabric. Thus fabric can be sprayed with a liquid resembling metallic paint, containing micron-sized metallic particles such as copper incorporated into a binder such as a polyester, epoxy or acrylic resin. During curing the metallic particles come into contact with one another, thus... 

Antimony trioxide, 8-stannic acid (hydrous tin(IV) oxide), zinc hydroxystannate, and zinc stannate were incorporated into the commercial brominated polyester resin at levels of 1,2,5 and 10% by weight. No processing problems were encountered and the samples cured satisfactorily to give rigid, opaque strips. 

A further improvement in flame-retardant efficiency is observed when a colloidal dispersion of tin(IV) oxide is incorporated into the polyester. At a 1.5% addition level, colloidal SnO gives an 01 value (49.0) which is markedly higher than that obtained with 5% loadings of either anhydrous SnO, (47.7) or 8-stannic acid (47.9), as powdered additives (Figure 2). In addition to its increased flame-retardant ability, colloidal Sn02 offers the further advantages of translucency in the cured plastic, ease of incorporation and nonsettling in the resin prior to cure. 

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