Waterborne resins

Summary

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.[1][2] 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.[3]

History

Most coatings have four basic components. These are the resin, solvent, pigment and additive systems[4] 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 (VOC - Volatile organic compound) discharged into the atmosphere; c) cost; d) political factors i.e. security of supply.[5] 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.[6][7]

Types of waterborne resins

Waterborne epoxy resins

see also Epoxy

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.[8] 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.[9] Paints may then be made from them by pigmenting  either the epoxy or the amine hardener portion or even both.[10] 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.[11] Even though water is present and is a fuel for corrosion, water-based metal coatings based on waterborne epoxy can also be formulated.[12]

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.[13][14]

Waterborne alkyd resins

see also article Alkyd

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, 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.[15] 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.[16] 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.

Alkyd emulsions

Late twentieth century technology allowed the production of alkyd emulsions.[17] The technology continues to evolve.[18] 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.[19][20] Sustainability and other market factors mean a number of companies are entering the market.[21] As well as patents, doctoral and theses are being done at universities on the subject.[22]

Waterborne polyester resins

see also Polyester resin

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.[23][24]

Waterborne polyurethane resins

see also Polyurethane dispersion

Polyurethanes 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.[25] 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.[26] Waterborne polyurethanes are also available in 2 component versions. As a 2 component polyurethane consists of polyol(s) and an isocyanate and isocyanates react with water this requires special formulating and production techniques.[27][28] The polyisocyanate that is water-dispersible maybe modified with sulfonate[29] for example.

Waterborne lattices

see main article Latex

A latex is a stable dispersion (emulsion) of polymer in water. Synthetic lattices are usually made by polymerizing a monomer such as vinyl acetate that has been emulsified with surfactants dispersed in water.[30]

Waterborne electrophoretic deposition resins

see article Electrophoretic deposition

The 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.[31] Cathodic electrodeposition resins dominate and they have revolutionised corrosion protection in the automotive industry. They are applied as OEM (Original Equipment Manufacture) 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.[32] Waterborne epoxy resins may be modified with acrylate and then further modified with side chains having many fluorine atoms on them.[33] Waterborne resins are also available that use both water and renewable raw materials.[34] Another example is to combine alkyd resins with acrylics to make them waterborne. Using hyperbranched alkyds and modifying them with acrylic monomers and using mini emulsion polymerization, suitable hybrids maybe formed.[35] As well as hybridization of the resins, combination of techniques maybe employed. As an example, ultraviolet curing coatings that can be electrodeposited and are waterborne hybrids of epoxy and acrylic resins maybe produced.[36][37]

Water

see main article Water

Water 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 (ice), liquid (water) and gas(water vapour and steam). At 273.16 K or 0.16 °C (known as the triple point) 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 (mN/metre) which affects its ability to wet certain surfaces. It evaporates (latent heat of evaporation 2260 kJ per kg) 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 (4.184 kJ/kg/K ) 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.

Uses

Waterborne resins find use in Coatings, Adhesives, Sealants and Elastomers. Specifically they find use in industrial coatings,[38] UV coatings,[39] floor coatings,[40] hygiene coatings,[41] wood coatings,[42] adhesives,[43] concrete coatings,[44] automotive coatings,[45][46] clear coatings[47] and anticorrosive applications including waterborne epoxy based anticorrosive primers[48][49][50] They are also used in the design and manufacture of medical devices such as the polyurethane dressing, a liquid bandage based on polyurethane dispersion.[51] Over the years they have also been used in polymer modified cements and repair mortars[52]

See also

References

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External websites

  • Covestro
  • DSM
  • DIC
  • PU General Info
  • Incorez range
  • Allnex website
  • Hexion Waterbone Resins
  • ARKEMA Waterborne Resins
  • Alkyd Emulsions- Van Horn, Metz & Co. Inc.