Waterborne resins

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.

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.

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.

[14] Even though water is present and is a fuel for corrosion, water-based metal coatings based on waterborne epoxy can also be formulated.

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 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.

[20] 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.

With alkyds typical methods include maleinazation of unsaturated fatty acids with maleic anhydride.

To make them waterborne organic acids or anhydrides are added in a two-stage process but there are other methods too.

[31] 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.

PUDs are not usually synthesised with plant based polyols because they don't have other performance enhancing functional groups.

[40] Research has shown that modification of these resin systems with polyaniline improves a number of properties including corrosion resistance.

[42] 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.

Synthetic lattices are usually made by polymerizing a monomer such as vinyl acetate that has been emulsified with surfactants dispersed in water.

[65] Cathodic electrodeposition resins dominate and they have revolutionised corrosion protection in the automotive industry.

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%.

An example would be polyurethane dispersions blended or hybridized with acrylic resins,[68][69] which are commonly used in automotive paint.

Such systems can be made by using acrylic monomers and a polyurethane dispersion which will polymerise simultaneously to give an interpenetrating polymer network, without the need for NMP as a cosolvent.

Using hyperbranched alkyds and modifying them with acrylic monomers and using mini emulsion polymerization, suitable hybrids maybe formed.

As an example, ultraviolet curing coatings that can be electrodeposited and are waterborne hybrids of epoxy and acrylic resins maybe produced.

[83] Hybrid resins used in coatings that are vegetable based, waterborne and UV curable are considered very green and have also been investigated.

[91][92][93][94] Research is also taking place using waterborne alkyd resins hybridized with styrene-acrylic emulsions.

[96] Waterborne resins are already perceived as environmentally friendly but work is ongoing to improve this further by using non-petroleum based raw materials where possible.

[98] Since waterborne resins are usually considered green and environmentally friendly, techniques are being researched that include capturing carbon dioxide from the atmosphere to make the raw materials and then further synthesis.

These factors have to be borne in mind when formulating waterborne resins and other water based systems such as adhesives and coatings.

[116] Over the years they have also been used in polymer modified cements and repair mortars[117] They have also found use in general textile applications including coating nonwovens.

[118] Recent (post 2020) innovations have included producing a waterborne polyurethane that has embedded silver particles to combat COVID.