Waterborne resins
Waterborne resins are sometimes called water-based resins. They are resins or polymeric resins that use water as the carrying medium as opposed to solvent or solvent-less. Resins are used in the production of coatings, adhesives, sealants, elastomers and composite materials..When the phrase waterborne resin is used it usually describes all resins which have water as the main carrying solvent. The resin 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 1960’s 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 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 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
see also EpoxyAn 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 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 he 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 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.
Waterborne alkyd resins
see also article AlkydWater 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, soyabean, 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 neutrazised to form a salt. Introducing polar groups onto the backbone is another method. With alkyds typical methods include maleinization 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.
Waterborne polyester resins
see also Polyester resinSaturated 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
see article Polyurethane dispersionPolyurethanes resins are available waterborne. The single component versions are usually referred to as Polyurethane dispersions.
They are available in anionic, cationic and nonionic versions though anionic moieties are the most readily available commercially. Waterborne polyurethanes are also available in 2 component versions. As a 2 component polyurethane consists of polyol 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.
Waterborne lattices
see main article LatexA latex is a stable dispersion of polymer in water. Synthetic lattices are usuaully made by polymerizing a monomer such as vinyl acetate that has been emulsified with surfactants dispersed in water.
Waterborne electrophoretic deposition resins
see article Electrophoretic depositionThe resins used for electrodeposition are usually epoxy, acrylic or phenolic resin types. They are formulated with functional groups which when neutralised form ionic groups on the polymer backbone. These confer water solubility on the polymer. They are available as anodic versions which deposit on the cathode of an electrochemical cell or cathodic which deposit on the cathode. Cathodic electrodeposition resins dominate and they have revolutionised corrosion protection in the automotive industry. They are applied as OEM rather than as a refinishing system. Cathodic resins contain amines on the polymer backbone which are neutralised by acids groups such as acetic acid to give a stable aqueous dispersion. When an electric current is passed through a car body that is dipped in a bath containing a paint based on a cathodic electrodeposition resin, the hydroxyl ions formed near the cathode deposit the paint on the car body. The electric current needed for this is determined by the number of ionic centres.
Dispersions of waterborne resins for electrocoating usually contain some co-solvents such as butyl glycol and isopropanol and are usually very low in solids content i.e. 15%. They usually have molecular weights in the region of 3–4000. Paints based on them tend to have PVCs of less than 10 i.e. a very high binder to pigment ratio.
Waterborne hybrid resins
Many resins are available waterborne but can be hybrids or blends. An example would be polyurethane dispersions blended or hybridized with acrylic resins. Waterborne epoxy resins may be modified with acrylate and then further modified with side chains having many fluorine atoms on them. Waterborne resins are also available that use both water and renewable raw materials.Water
see main article WaterWater is in some ways an unusual chemical. It is a very powerful and universal solvent. Most liquids reduce in volume on freezing, but water expands. It occurs naturally on earth in all three states of solid, liquid and gas. At 273.16 K or 0.16 °C it can coexist in all three states simultaneously. It has a very low molecular weight of 18 and yet a relatively high boiling point of 100 0 C. This is due to inter molecular forces and in particular hydrogen bonding. The surface tension is also high at 72 dynes/cm which affects its ability to wet certain surfaces. It evaporates very slowly in comparison to some solvents and hardly at all when the relative humidity is very high. It has a very high specific heat capacity and that is why it is used in central heating systems in the United Kingdom and Europe. These factors have to be borne in mind when formulating waterborne resins and other water based systems such as adhesives and coatings.