Waterborne resins, or water-based resins, are resins, including or polymeric resins, that use water as the carrying medium, as opposed to those used with other solvents or with none. Resins are used in the production of coatings, adhesives, sealants, elastomers and composite materials. Waterborne resin usually describes all resins for which water is the main carrier. They could be water-soluble, water reducible or water dispersed.
History Most coatings have four basic components. These are the resin, solvent, pigment and additive systems but the resin, or binder, is the key ingredient. Continuing environmental legislation in many countries along with geopolitics such as oil production are ensuring that chemists are increasingly turning to waterborne technology for paint/coatings and since resins or binders are the most important part of a coating, more of them are being developed and designed waterborne and there is a constantly increasing use by coating formulators. The use of waterborne coatings and hence waterborne resins really started to grow in the 1960s led by the United States and was driven by: a) the need to reduce flammability; b) environmental legislation aimed at reducing the amount of solvent vapor (volatile organic compounds, VOCs) discharged into the atmosphere; c) cost; d) political factors i.e. security of supply. All these factors helped the desire to reduce the reliance on oil derived solvents. The use of water as the carrying solvent for coatings and hence resins has been increasing ever since. The same holds true for adhesives. Water is generally a low cost (but not free) commodity in plentiful supply with no toxicity problems so there has always been a desire to produce paints, inks, adhesives and textile sizes etc. with water as the carrying solvent. This has required the production of waterborne resins designed for these systems. In recent years legislative pressure has ensured that waterborne systems and hence waterborne resins are coming increasingly to the fore.
Types of waterborne resins
Waterborne epoxy resins
An epoxy resin system generally consists of a curing agent and an epoxy resin. Both the curing agent and the epoxy resin can be made waterborne. Solid epoxy resin (molecular weight >1000) dispersions are available and consist of an epoxy resin dispersed in water sometimes with the aid of co-solvents and surfactants. The resin backbone is often modified to ensure water dispersibility. These resins dry in their own right by water/co-solvent evaporation and the particles coalescence. To cure the resin and crosslink it, an amine-based curing agent is usually added. This produces a two-component system. An alternative is to use standard medium viscosity liquid epoxy resins and emulsify them in a water-soluble polyamine or polyaminoamide hardener resin which also gives a two-component system. Polyaminoamides (or polyamidoamines) are made by reacting ethylene amines with dimerized fatty acids to give a species with amide links but still having amine functionality. Water is liberated during the condensation reaction. These resins can then be made water-soluble by reacting further with glacial organic acids or formaldehyde. Resins like these are usually left with yet further amine functionality on the polymer backbone to enable them to cure and crosslink an epoxy resin. Paints may then be made from them by pigmenting either the epoxy or the amine hardener portion or even both. Polyamine curing resins as opposed to polyaminoamide resins are generally made by partially adducting polyfunctional amines with an epoxy resin and/or epoxy diluent and leaving the species with residual amine functionality. This adduct can then be dissolved in water and used to emulsify more epoxy resin and again either portion or both may be pigmented. The advantage with these systems is that they do not need glacial organic acids to solubilize them. This is an advantage if the coating is to be used over a highly alkaline substrate such as fresh concrete, as the alkali from the cement will neutralise the acid and cause instability on repeated dipping of a brush into the can. Even though water is present and is a fuel for corrosion, water-based metal coatings based on waterborne epoxy can also be formulated. Other research is investigating the benefits of combining graphene technology with waterborne epoxy. Research continues and many patents and journal papers continue to be published with novel ways of converting epoxy systems to their waterborne counterparts. One such method is to take a molecule that already is intrinsically partially hydrophilic such as a diol with a polypropylene oxide backbone, and then reacting it with epichlorohydrin and then dehydrochlorinated with sodium hydroxide. This produces a diepoxy terminated polypropylene glycol molecule. This can now be reacted with an ethyleneamine such as triethylenetetramine (TETA) to produce an amine terminated moiety that is intrinsically hydrophilic and able to cure an epoxy resin. These waterbased epoxy coatings when used with the right choice of pigments, can be used to coat the inside of oil tanks.
Waterborne alkyd resins
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