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.Thames S.F. "Conversion to Water-Soluble/Water-borne polymers" February 1996 Lecture notes from the University of Southern Mississippi short course
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.Darwen S. "Developments in high performance water-borne epoxy coatings" Polymers Paints and Colours Journal February. 23 1994 pages 65–67 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.Richardson F.B "Water-borne Epoxy Coatings: Past, Present and Future" Modern Paints and Coatings April 1988 pages 84-88 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.Howarth G.A "Synthesis of a legislation compliant corrosion protection coating system based on urethane, oxazolidine and waterborne epoxy technology" Master of Science Thesis April 1997 Imperial College London 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
Water reducible alkyds are basically conventional alkyd resins (i.e., polyesters based on saturated or unsaturated oils or fatty acids, polybasic acids and alcohols) modified to confer water miscibility. Typical components are vegetable oils or fatty acids such as linseed, soybean, castor, dehydrated castor, safflower, tung, coconut and tall oil. Acids include isophthalic, terephthalic, adipic, benzoic, succinic acids and phthalic, maleic and trimellitic anhydride. Polyols include glycerol, pentaerythritol, trimethylolpropane, ethylene glycol, propylene glycol, diethylene glycol, neopentyl glycol, 1,6-hexanediol and 1,4-butanediol. Typical methods for introducing varying degrees of water miscibility are similar to other resin systems. Methods basically involve introducing hydrophilic centres such as acid groups that can then be neutralised to form a salt. Introducing polar groups onto the backbone is another method. With alkyds typical methods include maleinazation of unsaturated fatty acids with maleic anhydride. This involves making a Diels-Alder adduct near the double bond sites. The acid groups introduced can then be further reacted with polyols. A Diels-Alder reaction only occurs where there is a conjugated double bond system. Simple addition occurs if not conjugated. Other techniques include synthesizing the resin with hydroxyl functional oligomers e.g. containing ethylene glycol then adding specific acid or hydroxyl containing substances towards the end of the reaction. Another technique is making an acrylic functional alkyd with an acrylic monomer blend rich in carboxylic acid groups. Synthesis techniques have been studied and published for acrylic modified water-reducible alkyds.
Alkyd emulsions Late twentieth century technology allowed the production of alkyd emulsions. The technology continues to evolve including production of direct-to-metal (DTM) finishes. The biggest issue has been getting VOC content below 250g/L. Poor corrosion resistance has also been an issue. Alkyd emulsion technology uses a reactive surfactant that has double bonds and thus oxidative drying properties like a conventional alkyd. The material is then put under shear and water added slowly. Initially a water in oil emulsion is formed but continued water addition and shear results in inversion and a stable oil in water emulsion is formed. Sustainability and other market factors mean a number of companies are entering the market. As well as patents, doctoral theses are being done at universities on the subject.
Waterborne polyester resins
Saturated polyester resins contain many of the materials used in conventional alkyd resins but without the oil or fatty acid components. Typical components for these resins are poly carboxylic and polyhydroxyl components. The more commonly used polyacids are phthalic, isophthalic, terephthalic and adipic acid. Phthalic and trimellitic anhydrides may also be used. Polyols tend to be neopentyl glycol, 1,6-hexanediol and trimethylolpropane. To make them waterborne organic acids or anhydrides are added in a two-stage process but there are other methods too.
Waterborne polyurethane resins
Polyurethane resins are available waterborne. The single component versions are usually referred to as polyurethane dispersions (PUD). They are available in anionic, cationic and nonionic versions though anionic moieties are the most readily available commercially. The use of an anionic or cationic center or indeed a hydrophilic non-ionic manufacturing technique tends to result in a permanent inbuilt water resistance weakness. Research is being conducted and techniques developed to combat this weakness. Cationic PUD also introduce hydrophilic components when synthesized, but techniques have and are being researched to improve the performance and water resistance properties by various techniques. This includes introducing star-branched polydimethylsiloxane.
Waterborne polyurethanes are also available in two-component versions. As a two-component polyurethane consists of polyol(s) and an isocyanate and isocyanates react with water this requires special formulating and production techniques. The polyisocyanate that is water-dispersible maybe modified with sulfonate for example. PUDs are not usually synthesised with plant based polyols because they don't have other performance enhancing functional groups. Recent work (2021) reports modification to achieve this and enable even greener versions. Work is also ongoing to get the performance of one-component waterborne polyurethanes to match that of two-component versions. Self-healing versions of two-component waterborne polyurethanes are being researched. Research has shown that modification of these resin systems with polyaniline improves a number of properties including corrosion resistance. Higher solid content is also desired for economics of not transporting water.
Ionic centers are usually introduced with waterborne PUDs, and so the water resistance in the resultant film has been studied. The nature of the polyol and the level of COOH groups and hydrophobic modification with other moieties can improve the hydrophilicity. Polyester polyols give the biggest improvements. Polycarbonate polyols also enhance properties, especially if the polycarbonate is also fluorinated.
Silicone modification of the resin makes the species much more hydrophobic and water resistant.
As the world attempts to move towards a low-carbon economy, carbon capture by using carbon dioxide from the atmosphere is gaining attention and research being done. Using carbon dioxide in PUD production is being researched.
Waterborne Latexes