Лако-красочные материалы — производство

Although not strictly used as prime structural adhesives, polymer latices or dispersions have been included here because of their increasing use as aids to bonding in the patch repair of spalled concrete. They usually take the form of a polymer-cement slurry which is applied to the moistened and prepared concrete surface. In general, the repair mortar must then be applied before a film

forms on the surface of the bonding aid. Many cementitious repair mortars themselves are today modified by polymers of similar generic type to those used in bonding aids.

Before discussing the various forms of polymers which may be encountered it is pertinent to describe briefly the basic structure of water-based polymer dispersions(ll). The starting point is a monomer which forms droplets in water. Aqueous surfactants are adsorbed at the droplet surface to stabilise the emulsion before an initiator is added to cause polymerisation under controlled conditions of pressure, temperature and stirring rate. The latex is thus an aqueous dispersion of small discrete polymer particles to which is added a range of additives, such as coalescents, anti-foaming agents, bacteriocide and anti-oxidants, to improve shelf life and properties related to its end use.

Most bonding aids are supplied for civil engineering use at a total solids content of about 50% by weight. The materials must be formulated to meet the particular requirements for concrete repair, that is to be stable in a relatively high alkaline environment and to have a minimum film-forming temperature of 5 °С. Film-forming properties are particularly important but the process is complex in polymer dispersions. Essentially it occurs as a result of water evaporation which eventually leads to particle contact. However, the stabilising surfactant system employed and other properties such as particle size, surface tension and viscosity will also influence film formation. As with all the previous adhesive systems which have been discussed the various additives incorporated into such bonding aids can have as much influence on the final properties as can the nature of the polymer itself.

Thermoplastic polymers supplied for use as concrete bonding aids include polyvinyl acetate (PVA), acrylics, styrene acrylates and styrene butadiene rubber (SBR). The PVAs tend to be used for non-structural applications, for example bonding render to concrete, as they tend to be moisture susceptible. Acrylics and styrene acrylates exhibit superior resistance to water and can be nearly as good as SBRs in this respect. There is also some evidence to suggest that the acrylics are less sensitive to premature film-forming in drying conditions than are the SBRs. Indeed, the Department of Transport in their specification on concrete bridge repair(12,13), restrict the use of polymer bonding aids to acrylic systems. Counter to this is the much longer and generally successful track record of SBR cement bonding agents.

The neat or diluted latex can be used without cement but can only be applied in relatively thin films, resulting in a lack of tack and lower tensile strengths than systems containing ordinary portland cement. Latices combined with cement, typically in ratios of between 1 : 1 and 1 : 2 by volume, result in reasonably resilient yet strong bond coats. The latex/cement mixture achieves its desirable properties of substrate wetting, substrate adhesion, strength and resilience from the way in which the polymer latex particles coalesce on drying to form continuous strands which penetrate the cement matrix and bridge voids between cement particles and the substrate. In addition the latex improves workability and enables a reduction in the water : cement ratio to be made.