Resins thermosetting

The lUPAC defines thermosetting polymers, or thermosets, as prepolymers in a soft sohd or viscous state that changes irreversibly into an infusible, insoluble 

Edited by SabuThomas, Kuruvilla Joseph, S. K. Malhotra, KoichiGoda and M. S. Sreekala. 

Thermosetting acrylic resins can be formulated to achieve the balance of properties required for sheet fed and DWI application. 

The base acrylic resins contain high levels of plasticising monomer such as ethyl or butyl acrylates combined with a hard comonomer, such as methyl methacrylate or styrene. Levels of functional crosslinking monomers are kept low in order to maximise film flexilnlity. 

Three types of thermosetting acrylics are used in metal decorating applications. 

These products have acid values in the region of 60-80mg KOH/g and can be crosslinked with either- 

Solvent based carboxyl acrylics in Europe continue to be used as base coats in some 3 piece can applications, where they are applied at film weights of lOmg/m and cured for 10 minutes 190-200°C. 

Oligomers end-capped with maleimide rings, which are known as bismaleimide (BMI) resins, exhibit thermal stability intermediate between epoxies and poly-imides. BMI systems are mainly used to fabricate structural composites capable of sustaining temperatures up to 230°C. Specific versions, such as American Cyanamid FM 32 and Ciba-Geigy Kerimid 601 have been developed to prepare adhesive compositions. Fig. 17 displays a typical constitutive unit of various commercial BMI resins based on bismaleimide 38, prepared from maleic anhydride 37 and 4,4 -methylenebisbenzeneamine (MDA) 34. 

However, a number of other aromatic diamines, in particular the alkyl-substituted MDA derivatives, have been converted into the corresponding bismaleimides to study the effect of the chemical structure on both the melting and 

Thermal polymerisation of short chain BMI resins occurs through radical polymerisation of the maleimide double bond producing highly cross-linked and brittle networks. A number of experimental results have been published that allow one to correlate the chemical structure of BMIs with the processability windows Tp - Tin , where Tp is the onset of polymerisation exotherm and is the maximum 

The thermal stability of cured bismaleimide formulations during long-term ageing experiments was found to be intermediate between the heat resistance of epoxy-novolacs and that of norbomene-terminated polyimides or nadimide resins discussed in the following paragraph. Composite materials made with BMI resins can be safely used for about 10,000 h at 200°C and approximately 100 to 200 h at 240°C. For long-term applications at 300°C, that is, 2000-3000 h, other resins are more effective. 

The concept of norbornene-terminated oligoimides was further studied at the NASA Lewis Research Centre by researchers who developed the polymerisation of monomeric reactants (PMR) approach [38]. The method, represented in Fig. 18, involves a blend of both the dimethyl ester of 3,3 -benzophenonetetra-carboxylic acid 42 and the monomethylester of 5-norbornene-2,3-dicarboxylic acid 41 with 4,4 -methylenebisbenzeneamine 34 in solution in methanol. 

One of the oldest group of polymers, still widely used in the electrical industry as Bakelite , is the phenol-formaldehyde (PF) resins formed in situ by reaction of phenol with formaldehyde. Owing to the polyfunc-tionahty of phenol which can react with three difunctional formaldehyde molecules, the result is a highly cross-linked structure which can be formed in one step from the low precursors. 

Similar cross-linked macromolecules can be prepared by reaction of formaldehyde with urea (UF), and by melamine (MF) with formaldehyde. These are hard but tough materials which are much less coloured than PF. They are the materials used in white electrical plugs and in pigmented form as Formica for working surfaces. 

The main environmental limitation of the cross-linked resins is that they cannot be readily mechanically recycled. They can of course be burned and may be ground to powder and used as inert fillers for the thermoplastic polymers. However, since they are intended to be durable materials, they represent no real threat to the environment. 

However, a number of other aromatic diamines, in particular the alkyl-substituted MDA derivatives, have been converted into the corresponding BMIs to study the effect of the chemical structure on both the melting and curing temperatures. Most of these BMIs are crystalline materials with high melting points in the range of 150—250 °C. The chemical reactivity of these resins is a consequence of the eleetron-deficient earbon—earbon double bond that ean be subjected to different reaetions sueh as nucleophilic attack of amines or thiols, radieal polymerization, and ene-, diene-, and cycloadditions. Polymers known as poly (amino-BMIs) or polyaspartimides are prepared by 

Fibers or fabrics are impregnated with the low viscosity solution, and the solvent is removed at a relatively low temperature to provide tacky prepregs containing the three monomer reactants and 5—10% by weight of residual methanol. To prevent any reaction of the monomers, prepreg rolls protected with polyethylene films are stored at low temperature (-18 °C). 

At temperatures between 120 and 230 °C, polycondensation and imidization occur in situ to form nadimide end-capped oligoimides 43 with significant evaporation of methanol and water that must be eliminated before final curing. This obviously implies that a complicated curing schedule, alternating partial vacuum and applied pressure, has to be implemented to prepare void-free composites. Up to now, the most widely known PMR resin is PMR-15, optimized to exhibit the best overall performance. An impressive number of experiments were conducted to achieve a chemical composition containing diester 42, monoester 41, and diamine 34 in a molar ratio of 

Epoxy (EP) resin contains epoxy groups in its molecular structure. There are many types of EP resin, but more than 90% of EP is bisphenol A-type epoxy resin, which is formed via the polymerization of bisphenol A and epichlorohydrin. The molecular structure is 

Unsaturated polyester (UP) resin is an unsaturated linear polyester resin polycondensed by unsaturated dibasic acid or anhydride (mainly maleic anhydride and fumaric acid), a certain amount of saturated dibasic acids (such as phthalic acid and terephthalic acid), and dihydric alcohol or polyhydric alcohols (such as ethylene glycol, propylene glycol, and glycerin). 

Polyether ether ketones (PEEKs) are a polycondensation product of 4, 4 -difluorophenyl ketone, hydroquinone, and anhydrous sodium carbonate (or potassium carbonate). The molecular structure is as follows  

The benzene and benzophenone structure makes the acromolecular chains rigid, and ether linkages make the macromolecular chains flexible. Thus, PEEK is an engineering thermoplastic with both rigidity and flexibility. 

Under normal conditions, some resins such as PE, PS, and PA can be directly molded however, to meet the requirements of a particular application performance, treatments such as 

The majority of COCs use norbornene and ethene as comonomers. However, heat curable compositions have been described (41). These compositions find use in glass reinforced materials. 

They can be handled analogous to thermosetting resins, and thus the use of highly volatile comonomers, such as ethene or prop-ene is prohibitive. Instead, other vinyl monomers are used. A heat curable formulation uses a mixture of tetracyclododecene, 2-norbomene, 5-vinyl-2-norbomene, and divinylbenzene as reactive components (41). The mixture further contains 3,5-di-ferf-butylhy-droxyanisole as antioxidant and a hybrid catalyst system containing a zirconium based metathesis catalyst and a radical catalyst. The metathesis catalyst is benzylidene (l,3-dimesitylimidazolidin-2-yl-idene)(tricyclohexylphosphine)ruthenium dichloride and the radical catalyst is di-ferf-butyl peroxide. 

In order to fabricate a resin laminated copper foil, the composition is poured over a glass cloth placed on a glass fiber reinforced poly-(tetrafluoroethylene) (PTFE) film. Another sheet PTFE film is placed 

The laminate in which the impregnated glass cloths are sandwiched between the PTFE films is then polymerized by adhering onto a hot plate heated at 145°C for 1 min. Thereafter, the PTFE films are peeled off to get a prepreg. Eventually the prepreg is laminated on both sides with a copper foil and a PTFE film having a thickness of 0.05 mm. The laminate is then put into a mold and heat pressed in the mold frame under a press pressure of 4.1 MPa at 200°C for 15 min. 

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It is manufactured by heating dicyandiamide, H2N C(NH) NH CN, either alone or in the presence of ammonia or other alkalis, in various organic solvents. Melamine is an important material in the plastics industry. Condensed with melhanal and other substances it gives thermosetting resins that are remarkably stable to heat and light. U.S. production 1980 80 000 tonnes. 

Mineral hydrates, such as alumina trihydrate and magnesium sulfate heptahydrate, are used in highly filled thermoset resins. 

Useful thermosetting resins are obtained by interaction of furfural with phenol. The reaction occurs under both acidic and basic catalysis. Other large uses of furfural together with phenol are in the manufacture of resin-bonded grinding wheels and coated abrasives (5). 

Usage of phosphoms-based flame retardants for 1994 in the United States has been projected to be 150 million (168). The largest volume use maybe in plasticized vinyl. Other use areas for phosphoms flame retardants are flexible urethane foams, polyester resins and other thermoset resins, adhesives, textiles, polycarbonate—ABS blends, and some other thermoplastics. Development efforts are well advanced to find appHcations for phosphoms flame retardants, especially ammonium polyphosphate combinations, in polyolefins, and red phosphoms in nylons. Interest is strong in finding phosphoms-based alternatives to those halogen-containing systems which have encountered environmental opposition, especially in Europe. 

Vinyl organosol coatings, which incorporate a high molecular weight thermoplastic PVC organosol dispersion resin, are extremely flexible. Soluble thermosetting resins, including epoxy, phenoHc, and polyesters, are added to enhance the film s product resistance and adhesion. 

In the final product, the formaldehyde has completely reacted to form a very inert thermoset resin. Spontaneous emission of formaldehyde from high pressure laminates is measured at approximately the accepted background level of 0.035 ppm (15). Melamine surfaced laminates are tested and approved for food service equipment by the National Sanitation Foundation (16). 

Although it would be desirable to recycle laminate scrap, this has been difficult because of its thermoset nature. However, a 1993 patent (18) suggested a means whereby scrap consisting of cellulose, thermoset resins, and partially reacted resins can be ground to a powder which is used as a filler in a thermoplastic resin. The filled thermoplastic resin is then used for mol ding of various articles. 

For many moderate-duty films for operating temperatures below 80 to 120°C, M0S2 is used in combination with acryflcs, alkyds, vinyls, and acetate room temperature curing resins. For improved wear life and temperatures up to 150—300°C, baked coatings are commonly used with thermosetting resins, eg, phenohcs, epoxies, alkyds, siUcones, polyimides, and urethanes. Of these, the MlL-L-8937 phenoHc type is being appHed most extensively. 

Another significant use of 3-methylphenol is in the production of herbicides and insecticides. 2-/ f2 -Butyl-5-methylphenol is converted to the dinitro acetate derivative, 2-/ f2 -butyl-5-methyl-4,6-dinitrophenyl acetate [2487-01 -6] which is used as both a pre- and postemergent herbicide to control broad leaf weeds (42). Carbamate derivatives of 3-methylphenol based compounds are used as insecticides. The condensation of 3-methylphenol with formaldehyde yields a curable phenoHc resin. Since 3-methylphenol is trifunctional with respect to its reaction with formaldehyde, it is possible to form a thermosetting resin by the reaction of a prepolymer with paraformaldehyde or other suitable formaldehyde sources. 3-Methylphenol is also used in the production of fragrances and flavors. It is reduced with hydrogen under nickel catalysis and the corresponding esters are used as synthetic musk (see Table 3). 

Ammonia is used in the fibers and plastic industry as the source of nitrogen for the production of caprolactam, the monomer for nylon 6. Oxidation of propylene with ammonia gives acrylonitrile (qv), used for the manufacture of acryHc fibers, resins, and elastomers. Hexamethylenetetramine (HMTA), produced from ammonia and formaldehyde, is used in the manufacture of phenoHc thermosetting resins (see Phenolic resins). Toluene 2,4-cHisocyanate (TDI), employed in the production of polyurethane foam, indirectly consumes ammonia because nitric acid is a raw material in the TDI manufacturing process (see Amines Isocyanates). Urea, which is produced from ammonia, is used in the manufacture of urea—formaldehyde synthetic resins (see Amino resins). Melamine is produced by polymerization of dicyanodiamine and high pressure, high temperature pyrolysis of urea, both in the presence of ammonia (see Cyanamides). 

A variety of thermosetting resins are used in SMC. Polyesters represent the most volume and are available in systems that provide low shrinkage and low surface profile by means of special additives. Class A automotive surface requirements have resulted in the development of sophisticated systems that commercially produce auto body panels that can be taken direcdy from the mold and processed through standard automotive painting systems, without additional surface finishing. Vinyl ester and epoxy resins (qv) are also used in SMC for more stmcturaHy demanding appHcations. 

Reinforced Thermoplastic Sheet. This process uses precombined sheets of thermoplastic resin and glass fiber reinforcement, cut into blanks to fit the weight and size requirements of the part to be molded. The blanks, preheated to a specified temperature, are loaded into the metal mold and the material flows under mol ding pressure to fiU the mold. The mold is kept closed under pressure until the temperature of the part has been reduced, the resin solidified, and demolding is possible. Cycle time, as with thermosetting resins, depends on the thickness of the part and the heat distortion temperature of the resin. Mol ding pressures are similar to SMC, 10—21 MPa (1500—3000 psi), depending on the size and complexity of the part. 

Centrifugal casting is used to produce water softener tanks and pipe by saturating a reinforcement with thermosetting resin within a mold that is then rotated at high speed to consoHdate the laminate before curing. 

In addition to fiber and fabric influences on abrasion resistance, chemical finishes must also be considered. Many thermosetting resins used to impart durable press characteristics to ceUulosic fabrics reduce their resistance to abrasion as a result of fiber embrittlement. 

Poly(vinyl alcohol) is employed as a modifier of thermosetting resins used as adhesives in plywood and particle board manufacture (314,315). The polymer is added to urea-formaldehyde or urea—melamine—formaldehyde resins to improve initial grab, to increase viscosity, and, in general, to improve the characteristics of the board. 

Plastics. Almost all commercial plastics find some use both dry and lubricated for sliding at low speeds and light loads the most commonly used thermoplastics are nylon, acetal resins, and polytetrafluoroethylene (PTFE). Typical thermosetting resins for bearing appHcations are phenoHcs, polyesters, and polyimides. Table 8 compares the characteristics of plastic bearing materials with those of graphite, wood, and mbber which find use in somewhat similar appHcations. 

Glassy, or vitreous, carbon is a black, shiny, dense, brittle material with a vitreous or glasslike appearance (10,11). It is produced by the controUed pyrolysis of thermosetting resins phenol—formaldehyde and polyurethanes are among the most common precursors. Unlike conventional artificial graphites, glassy carbon has no filler material. The Hquid resin itself becomes the binder. 

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