Thermosetting Acrylic Emulsion

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. 

In this book I have confined discussion to those polymeric materials which are cured by chemical reaction and which have found widespread application in the construction industry. As such, the book covers materials based on epoxies, polyurethanes, silicones, polysulphides, alkyds and polyesters. In addition, there is a chapter on hybrid polymer systems and one on acrylics. It is true that acrylic emulsions are not strictly thermosetting polymer systems, but their widespread use and importance made their exclusion difficult. These materials find use as coatings, sealants, adhesives, grouts, flooring compounds, repair compounds and waterproofing agents. 

In textile industry it is used in laminating and as adhesive in the flocking of fabrics and in the finishing product of the fabrics as a mixture with formaldehyde containing reactants and resins. Nonwoven textile industry uses it as binder and thermosetting of acrylic emulsion. 

Example 5 of Reference 36 describes the preparation of two latexes, one reactive with the other. The graded rubber latex has a core of crosslinked poly(butyl acrylate) and a shell of linear poly(methyl methacrylate) containing hydroxypropylmethacrylate as the reactive species. The thermoset prepolymer emulsion is prepared from methyl methacrylate, containing glycidyl methacrylate as its reactive species. After polymerization, the two latexes are blended to yield a molding compound containing about 25% rubber, based on the core portions of the graded-... 

Melamine formaldehyde can be crosslinked at elevated temperatures with both hydroxyl and carboxyl functional groups. (See thermosetting acrylics chapter for reaction mechanisms.) The temperature required is at least 120°C, at which point the hydroxyl group will react, but the carboxyl group needs a slightly higher heat input, approximately 150°C. Systems are unlikely to require an acid catalyst because of the catalytic effects of the polymerisation catalysts and surfactants in the acrylic latices. If required, p-toluene sulphonic acid is the most suitable (typically at levels of 0.2 - 0.4%). Alternatively, the melamine resin could be incorporated in an unneutralised, acidic emulsion, which reduces the cure temperature, but will sacrifice stability. 

Core-shell rubber (CSR) particles are prepared by emulsion polymerization, and typically exhibit two or more alternating rubbery and glassy spherical layers (Lovell 1996 Chapter 8). These core-shell particles are widely used in thermoplastics, especially in acrylic materials (Lovell, 1996), and have also been used to modify thermosets, such as epoxies, cyanates, vinyl ester resins, etc. (Becu et al., 1995). 

It is not possible to produce a dispersion of rubber particles in the thermoset precursors due to their agglomeration. It is possible, however, to synthesize a stable emulsion or suspension of rubber particles in one of the monomers. These particles, stabilized by copolymers and surfactants, may be considered as a limiting case of CSR particles when the shell thickness tends to zero. The use of dispersed acrylic rubbers (Sue et al., 1996a and Ashida et al., 1999) and poly(dimethyl-siloxane) (PDMS) emulsions (Rey et al., 1999), have been reported. 

Hand building finishes that retain their stiffening and fullness effects after repeated launderings are considered to be durable. These products are usually aqueous emulsions of polymers that form water-insoluble films on the fibre surface when dried. The three main types of products are vinyl acetate-containing polymers, acrylic copolymers and thermosetting polymers. 

Rhoplex [Rohm Haas], TM for aqueous dispersions of acrylic copolymers. White, opaque emulsions various grades differing in hardness, flexibility, adhesion, and tack of film some thermosetting. Produce colorless, transparent films with outstanding permanence, durability, adhesion, and pigment-binding capacity. 

Acrylics are formulated as water-emulsion latex, thermoplastic, or thermosetting resin. 

Uses Self-crosslinking emulsions, heat-curing coatings, silk grafting, crosslinked acrylic sheets, thermosetting paints Properties Pt-Co 100 max. colorless clear to yish. liq. faintly ester-like odor misc. with water m.w. 115.1 sp.gr. 1.1 b.p. 100 C solid, pt. -37 C flash pt. none pH 7 1 40 1.5% water Toxicology LD50 (oral, rat) 1815 mg/kg avoid contact with skin, eyes, mucous membranes TSCA listed... 

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... 

The acrylics can be formulated as thermoplastic resins, thermosetting resins, and as a water emulsion latex. The resins are formed from polymers of acrylate esters, primarily polymethyl methacrylate and polyethyl acrylate. Since the acrylate resins do not contain tertiary hydrogens attached directly to the polymer backbone chain, they are esceptionally stable to oxygen and UV light. The repeating units for the methacrylate and acrylate are as follows ... 

The acrylics can be formulated as thermoplastic resins, thermosetting resins, and as a water emulsion latex. 

Another way to classify polymers results from the consideration of their typical applications. Typical classes are Compression molding compounds, injection molding compounds, semi-finished products, films, fibers, foams (urethane foam, styrofoam), adhesives (synthetic adhesives are based on elastomers, thermoplastics, emulsions, and thermosets. Examples of thermosetting adhesives are Epoxy, polyurethane, cyanoacrylate, acrylic polymers), coatings, membranes, ion exchangers, resins (polyester resin, epoxy resin, vinylether resin), thermosets (polymer material that irreversibly cures), elastomers (BR, silicon rubber). 

The acrylics are exceptionally resistant to oxygen and UV light deterioration and exhibit good gloss or color retention. They can be formulated as thermoplastic resins, thermosetting resins, or a water emulsion latex. 

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