What is it about water based paints?

[sophiecentaur]

I was Nikwaxing my boots the other day. This is a water based wax emulsion and uses no solvent. How does it work - or rather why wasn't the water based approach always used?
There are EU directives about paints that forbid the use of solvent based paints so even car bodies are (will be?) coated with water based paint. In the 1950s you could buy PEP (Plastic Emulsion Paint) and my dad raved about it so it's not a brand new technology. But it's only recently become more or less universal.
What is done differently now that allows all paint to spread, adhere, dry and cure when it's just droplets of plastic suspended in water?
I would have used the Chemistry Forum but the dates of posts indicate that there's not a lot oc traffic there and there must be some Physics involved with the question.

 

[scottdave]

What about latex paint? Here is something - https://www.networx.com/article/paint-types
It's not really scientific answer, but might be useful.

 

[sophiecentaur]

Thanks - but there's no Science there.
Edit: sorry, that was brief and too rude! I read around a lot of Google hits but they were all about what's available and how to use it. The latex paints were the first and then acrylic turned up. But the reasons why oil and other solvents are 'better' is never actually discussed. I imagine there is a lot of commercially sensitive information about the recipes.

 

[Martin0001]

This is related to chemistry involved.
Acrylic or latex paints may be applied as water suspension because ingredients do not react with water and also they will cure (eg polymerize, cross link etc) even if applied in that form. Development was slower due to few obstacles, eg how to make stable, non polymerizing dyspersion and also how to make it adering to painted object well.
There are many superb products on the market which are based on this technology, however these paints are suffering from low resistance to chemical attack.

However there are paints where chemistry is simply not compatible with water, so there is no way to make them in this form.
Examples are:
- old fashion phtalic paints - simply too hydrophobic to use with water.
- 2 component epoxy paints - one of components, the one which contain substituted ethylene oxide (oxirane) functionality is chemically incompatible with water and there would be an issue with very short shelf life..
- modern Zinc ol Aluminium - Etoxysilicate paints would also chemically react with water.

Epoxy paints are difficult to substitute if chemical resistance is important and Zn or Al - etoxysilicate paints are eco friendly methods for good antycorrosion coating (only volatile by-product is ethanol).

I do not expect them banned in EU anytime soon, however for normal household conservation you no longer really need solvent based paints. Alternatives are more than satisfactory.

 

[jim mcnamara]

Moved to Chemistry

 

[sophiecentaur]

@Martin001 Thanks for your response. You have made some comments that ring true to my experience. I guess it's far more knife edged, getting the characteristics right, compared with an oil based paint. It is a massive advantage that you can just wash out the brushes. Dirty white spirit is a pain to get rid of and I often end up burning it in a metal bowl like the Olympic Torch!.

few obstacles, eg how to make stable, non polymerizing dyspersion and also how to make it adering to painted object well.

I bought some paint for garden timber. It is a Ronseal product and I really doubted it would be any good at all. It is as thin as semi skimmed milk but it goes onto rough wood very easily and forms a good tough layer. I couldn't imagine how stuff as thin as that would work but it's really pretty good. It does have a problem as a second coat when the old coat has really cured. It needs scrubbing in so your adhesion comment makes sense to me. I remember my Dad used to use emulsion paint as an undercoat. It gave an easy result but it would chip very easily when covered with oil based gloss.
There are now a number of 'breathable' paints on the market, which seems a bit of an oxymoron but, again, they seem to work.
Cheers