Solution thermosetting acrylics

The paint plants of large automotive manufacturers have been a cause of solvent pollution in the major industrialised countries. Control regulations governing the level of emissions from industrial premises which are coming into force in many countries have given direction to the development in automotive finishing of low V.O.C. coatings. 

The move from thermoplastic topcoats (NC or TPA s) was a big step forward in reducing solvent levels. Typically, the volume solids content at spray qyplication viscosity for a thermoplastic acrylic is less than 15%. This means that if one used 10 kgs of a thermoplastic topcoat lacquer at spraying viscosity to coat a car 8.5 kgs of solvent will be released to the atmosphere. In the U.S., one objective has been to reduce solvent emissions resulting from the application of a topcoat down to 1.6 kg per vehicle. With standard thermoset acrylics it is possible to more than double the volume solids at application, thus halving the solvent emission. 

A further increase in the solids content of automotive paints came with the advent of acrylic non aqueous dispersions (N.A.D. s). These were originally developed by I.C.I. in the late 60 s and early With this development, akin to water based emulsion 

The difference in solids contents of the systems already discussed are summarised in the table below. 

TABLE 4-2 TYPICAL APPLICATION SOLIDS OF ACRYLIC AUTOMOTIVE 

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The thermoset acrylics (20) of major importance in the coating industry, in recent years, have been developed primarily by Canadian Industry Ltd. and by Pittsburgh Plate Glass Co. in this country (4). Raw materials are acrylamide, acrylic acid, acrylates, and styrene. Cross-linking agents are amino and epoxy resins. The materials are also self-cross-linking. They are usually sold as solutions in paint solvents. 

Thermoplastics in the form of a lacquer have poorer mechanical strength than the thermosetting resins but are more easily removed, should this prove necessary. Polyvinyl acetate and polyurethane are good examples of this class of consolidants, particularly on wrought iron artefacts. Many of the copper and copper-based alloys, such as bronzes and brasses recovered from the Mary Rose, were consolidated with a solution of acrylic resin dissolved in toluene (Incralac). 

Solution monomer dispersed in solvent with soluble catalyst free radical, ionic, Zeigler Nichols thermosets, acrylics, PVAlc, PVC, polybutadiene, polypropylene, melamine phenolic resins, polyisoprene, polycarbonate, chlorinated polyesters glass or s/s STR (2-8 kW/m mixen heat transfer area = 1-4 m /m depending on the volume of the reactor with small area associated with large volumes). 

Thermoset acrylic systems - where restrictions in molecular weight during their solution polymerisation phase, are overcome when they crosslink. 

Whilst there are some thermoset acrylic emulsions cormnerdally available, the bulk of the thermoset resins, used as the main binder system, are produced in solution. Some may then be made waterborne by neutralisation and inversion (dispersion) into a water phase. Lower molecular weights favour this qrproach. The rate of conversion from solvent based to waterborne industrial thermoset coating systems has been, and is, much slower than the conversion from architectural alkyd paints to emulsion altonatives. There are two principle reasons for this. Firstly there are problems of application and substrate wetting of many waterborne systems. Secondly, the modifications frequently required to induce water dispersibility reduce one or more of the essential performance properties required from the cured film, compared to a solvent based system. Water resistance, with many films having an increased tendency for blushing is one example. However, for some applications, such as electrodeposition, only waterborne systems will work. 

Acrylic homopolymers [(—CH2CH(COOR)—) ] and copolymers are synthesized from acrylates and methacrylates. Through copolymerization, the polymer properties are widely varied from soft, flexible elastomers to hard, stiff thermoplastics and thermosets. Acrylic polymers are produced in many different forms including sheet, rod, tube, pellets, beads, film, solutions, lattices, and reactive syrups. 

Base materials for circuit boards based on phenolic resin/paper laminates are used primarily for low-performance materials, which are needed, for example, in consumer electronics. The actual properties standard of these materials meets the requirements of the market. The laminating adhesives used for the manufacture of base materials are organic solutions or aqueous dispersions of thermosetting resins such as poly(vinyl butyral)-phenolic resin or acrylate-phenolic resin. 

Bonding The use of solvents (mostly with a cement made of a viscous solution of the same polymer) is common for easily soluble amorphous polymers like acrylics, polystyrene, cellulose, and PVC. However, this method does not apply to most crystalline polymers. The proper choice of solvent (or mixture of solvents) also takes into account the desired rate of diffusion. In this matter, the concept of solubility parameters may be exploited when there is a similarity between polymer and solvent. Various glues may also be used (contact glue, epoxy) in order to bind plastics to each other, including crystalline polymers or thermosets. 

Acrylics Solutions and aqueous emulsions Both thermoplastic and thermoset formulations available Very wide adhesion range Excellent resistance to discolouration, hght, and oxidation Curing types are available that have wash and dry-cleaning resistance Pressure-sensitive adhesives Laminating adhesives... 

As mentioned above, both thermoplastic and thermosetting type acrylic resins are commercially available for coatings, elastomers, sealants and adhesive applications. These resins are supplied in many different delivery forms such as powders or pellets, solutions in organic solvents and aqueous dispersions. These resin types are briefly described below. 

The term solution acrylics refers to acrylic resins prepared by chain-growth polymerization using a solutionbased polymerization process. Here, acrylic monomers and initiators are slowly added to an organic solvent and polymerization is carried out at a predetermined temperature and inert atmosphere with efficient stirring. Both monomers and the polymer formed are miscible in the selected solvent. With the progress of polymerization, the solution viscosity will Increase and heat transfer becomes difficult, limiting the solid content of the final solution. Both thermoplastic and thermosetting solution acrylics can be prepared by this technique. 

The viscosity of solution polymers increases directly with molecular weight. Performance properties begin to plateau as molecular weight increases, but viscosity continues to increase. Thermosetting solution acrylic resins are polymerized to lower molecular weight than thermoplastic acrylics, since they attain their property performance by conversion to infinite three-dimensional networks by chemical reaction (forming primary chemical bonds). 

The majority of water reducible resins prepared by the solution route are for thermoset applications if they are to be used as the principle film forming resin. As such hydroxyl functionality is normally incorporated if the resin contains acid functionality for neutralisation, which is normal. Typically, the hydroxyl value would be of a similar value to the acid value, i.e. 50 - 100 mg KOH/g. Note that there are two methods of quoting acid and hydroxyl values for vinyl and acrylic solution resins. Values can be quoted... 

Lacquer (topcoats) A solution of organics in a solvent that crosslink and form a film when the solvent is evaporated. In early 1900s terminology, the solid material was nitrocellulose-based and the solvent was non-aqueous. In newer uses of the term, the solid can also be a thermoplastic material, such as a vinyl or an acrylic, or thermosetting materials, such as epoxies and phenolics, and the solvent can be water. 

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