Acrylic adhesives polymerisation

The reluctance of acrylic monomers to polymerise in the presence of air has been made a virtue with the anaerobic acrylic adhesives. These are usually dimethacrylates such as tetramethylene glycol dimethacrylate. The monomers are supplied with a curing system comprising a peroxide and an amine as part of a one-part pack. When the adhesive is placed between mild steel surfaces air is excluded, which prevents air inhibition, and the iron present acts as a polymerisation promoter. The effectiveness as a promoter varies from one metal to another and it may be necessary to use a primer such as cobalt naphthenate. The anaerobic adhesives have been widely used for sealing nuts and bolts and for a variety of engineering purposes. Small tube containers are also available for domestic use. 

The majority of adhesive laminations in multilayer flexible packaging are manufactured using the dry-bond process. In this technique, a liquid adhesive is applied to one substrate. The adhesive is then dried using hot air. This dried surface can be adhered to a second substrate using heat and pressure at a nip point. The adhesive formulations themselves represent a reactive chemistry (typically urethanes or acrylics) that is chosen to withstand the processing and storage/distribution environment of the filled product. The adhesives polymerise and/or cross-link during production of the laminated product. 

Acrylic adhesives (or structural adhesives) are essentially a two-part product radical polymerisation is initiated by use of a second component which is supplied as a primer. The curative of these products is divided between the two parts with one half containing... 

Acrylic adhesives cure by addition polymerisation reactions, which provide a rapid setting of the adhesive once the cure has initiated. 

Two-part acrylic adhesive An initiator, such as peroxides, polymerises acrylate groups of the main component... 

Engineering acrylic adhesives are two-part systems which, when mixed or activated, polymerise (cure) to form an impact-resistant plastic layer which is well adhered to the surfaces of the adherends. The activator for the curing process may be a chemical (either mixed with the adhesive or applied to the adherend surfaces prior to the adhesive), or, for some varieties, UV or electron-beam radiation. The toughened or modified engineering adhesives have been widely used since the 1980 s in vehicle construction, wood to metal bonding, aerospace applications, panels and computer equipment. 

Reactive acrylic adhesives (sometimes called tough acrylics, reactive fluids, second-generation acrylics or methyl methacrylate adhesives are based on acrylate and methacrylate monomers, and have been used commercially for >50 years. Reactive acrylic adhesives are based on acrylic and methacrylic monomers, and polymerise via a free-radical route similar to catalysed anaerobic adhesives. However, very significant differences exist. Whereas the monomers in anaerobics are predominantly difunctional or trifunctional to achieve highly crosslinked thermoset systems, reactive acrylics are based mainly on monofunctional monomers e.g., methyl methacrylate, or cyclo hexyl methacrylate ... 

Transparency is often required. This is achieved by arranging that the particle size of the modifier to be below that of the wavelength of visible light (0.4-0.8 pm). This can normally be achieved by emulsion polymerisation, e.g., polybutadiene, polystyrene. Adhesion and surface compatibility between the polymer and modifier can be achieved by surface grafting of polar groups, e.g., acrylonitrile, various acrylates, onto the impact modifier surface before blending. 

Both acrylic acid and methacrylic acid polymerise to give water soluble hard resins. The viscous solutions so formed have been used as emulsifying agents, adhesives and as thickening agents for inks and dyes. Polymers of esters of these acids are of greater commercial importance. Esters can be prepared from cyanhydrins by reaction with an alcohol ... 

Copolymerisation reactions do not always succeed in the presence of phenols. Cardanol, converted to the acrylate (R = H) by reaction with acryloyl chloride, has been co-polymerised in the presence of benzoyl peroxide with methyl methacrylate leading to a product vrith improved thermal stability compared with polymethyl methacrylate alone (ref. 265). In a similar way an acrylate and a methacrylate (R = Me) have been synthesised from 3-pentadecylphenol. Polymerisation yielded moderately high molecular weight compounds of potential interest as pressure-sensitive adhesives (ref. 266). 

The liquid acrylics form a further group of unsaturated reactive resins and these are now available as two-part mixed or unmixed products. Compared with polyesters they are a relatively recent addition to the range of adhesives potentially suitable for structural joints. Many are based on the monomer methylmethacrylate which is polymerised by the addition of a small quantity of initiator or hardener. 

A broad range of monomers with relatively low water solubility have been polymerised by conventional emulsion polymerisation. Acrylics, methacrylics, styrene and vinyl acetate are the most common monomers used in preparing latexes for paints, textile binders, and adhesives. Acrylic, polyester, epoxy and urethane dispersions are used in industrial coatings, where higher strength is required. Butadiene is often copolymerised with styrene in producing synthetic rubber for tyre manufacture. 

Carboxylic monomers, such as acrylic or methacrylic acid, are included in emulsion polymerisation formulations for several reasons to increase the stability of latex particles, to improve the adhesion of resultant films to various substrates, to provide functional groups for interparticle crosslinking reactions and to control the viscosity of latex via neutralisation. Acrylic latices with and without incorporation of carboxylic groups, together with addition of various amounts of anionic surfactant, are used to investigate rheological and drying behaviours of the latices. 9 refs. 

This paper reports on the synthesis, characterisation, and applications of novel flame retardant dibromostyrene-based latexes. They are copolymers of dibromostyrene with butadiene, alkyl acrylates and methacrylates, vinyl acetate, styrene and unsaturated carboxylic acids, which form a wide variety of flame retardant latexes via an emulsion polymerisation technique. Choice of monomer or monomer blend is based upon the final glass transition temperature of the copolymer desired. Other criteria include desired physical properties and chemical resistance. Dibromostyrene-based butadiene and acryUc latexes are shown to possess the desired physical properties for use in coatings, adhesives and sealants, and the bromine content of the latexes has enabled the material to pass six different flammability requirements for the end uses such as textile backcoating, latex-based paint, contact adhesive, latex sealant, nonwoven binder, and carpet backing. 18 refs. 

For the whole family, 100 per cent liquid-to-solid conversion occurs by radical polymerisation of the vinyl group of the acrylic ester, a reaction catalysed by metal and inhibited by atmospheric oxygen. Typically, therefore, the adhesives cure only when the treated parts are assembled and air is excluded from the mating surfaces. The cure rate on non-metallic surfaces is generally too low for normal commercial use and here a secondary catalyst in the form of a surface primer is beneficial. With closely fitting parts, one metal surface is usually sufficient catalyst and when both surfaces are metallic, polymerisation is rapid at room temperature. Normally, components may be handled between five and twenty minutes after assembly with full strength after at least an hour - possibly very much longer with some materials. 

The curing process depends on the radical polymerisation of the acrylic vinyl group giving total liquid-to-solid conversion. In anaerobic variants, this may be by the same mechanism described in Section 5.1.2 but all respond to a catalytic primer - either a surface initiator or mixed directly into the adhesive. 

Keywords Acrylates Crosslinking Dual cure Epoxides Glass laminates Interpenetrating polymer networks Isocyanates Photo initiated cationic polymerisation Photoinitiators Photopolymerisation Pressure sensitive adhesives Release coatings Structural adhesives Thiol/polyene UV radiation curing Vinyl ethers. 

To cure thin coatings in contact with air, it is therefore necessary to work under intense illumination in order to consume rapidly the O2 dissolved in the sample and shorten the exposure time during which atmospheric oxygen diffuses into the coating. This is not a problem anymore in adhesive applications which are performed under oxygen diffusion-free conditions (laminates). After a short induction period during which the dissolved O2 is consumed by the initiator radicals, the polymerisation of the acrylate double bond proceeds as fast in the laminated sample as in an inert atmosphere and much faster than for a coating, as shown in Fig. 2 for a polyurethane-acrylate resin. 

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