# Curing - What Actually Occurs?



## RH (Sep 7, 2010)

Doing a complete interior of a customer - big job. As I have been working I gave her the standard spiel about dry time versus cure time and the importance of not messing with the wall surfaces for at least two weeks, a month if possible (two full coats, dark color).

She has her two young adult grand kids living with her and when I got back to the job this past Monday I could see there were areas (when viewed into the dreaded angled light) where something had occurred. Turned out some dirty hands had left marks on the freshly painted walls and the HO had attempted to wash them off. She then expressed concern about the durability of the product (this is one I have been using forever - probably in 85% of our jobs so no worries there on my part). So once again I explained to her that latex paint needs to CURE!!! 

So, what exactly happens during cure time? We all know that it's when paint is working towards it's maximum hardness but what actually occurs when that takes place? Is it simply the water leaving the product? Are the molecules rearranging themselves to a specific pattern? What is happening on a molecular and chemical level during the curing process? If someone has an answer that can be put into laymen's terms I'd appreciate it. Usually just explaining to a HO about the difference between dry to the touch versus full dry/hardness is enough, but with some HOs a little more info would be beneficial (I live and work in a University town - with all that that implies). 

Thanks!


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## stelzerpaintinginc. (May 9, 2012)

You already know how the acrylics you use cure through evaporation of the solvents, coalescence of pigments & binders, etc., but to put into terms which she can relate to, liken the process to having her nails done. They'll be dry to the touch, but she'll still have a period of time where the nails could be easily marred while it's curing.


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## RH (Sep 7, 2010)

stelzerpaintinginc. said:


> You already know how the acrylics you use cure through evaporation of the solvents, coalescence of pigments & binders, etc., but to put into terms which she can relate to, liken the process to having her nails done. They'll be dry to the touch, but she'll still have a period of time where the nails could be easily marred while it's curing.


Yeah, I know, as do all of us, that it's a function of hardness in the final topcoat. But for my own edification, not just to share with a customer, I'd like to know what actually takes place when it occurs. I mean, is it as simple as a certain percentage of water leaving the product over a set period of time based on a variety of conditions - or is it more than that? I suspect it's far more complicated than just evaporation.

I realize that we aren't chemists or physicists, but I was hoping that one of the paint manufacturing guys that lurk here might chime in with some further information. Perhaps NACE? He always seemed to have fairly detailed explanations as to why such and such a product would behave under certain conditions. Maybe he could enlighten me a bit further.


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## Jmayspaint (Mar 26, 2013)

Excellent question. I'm curious about the specifics of this too. 

I'm thinking all the evaporation takes place fairly quickly in the curing process and what we're left waiting on is coalescence.


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## RH (Sep 7, 2010)

Jmayspaint said:


> Excellent question. I'm curious about the specifics of this too.
> 
> I'm thinking all the evaporation takes place fairly quickly in the curing process and what we're left waiting on is coalescence.


I would tend to agree. It would make sense that the bulk of evaporation takes place during what we call the "dry to the touch" phase. Then does the remainder of it, plus coalescence and other processes, occur during the "cure time"? If so, what all needs to coalesce (binders, pigments, and…?) and what might those "other processes" be?


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## Rbriggs82 (Jul 9, 2012)

RH you just gave me a much need blog post topic. :yes: 

I'm waiting for NACE so I can just copy and paste his reply, easy peezy. :jester:


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## The Cutting Edge (May 25, 2014)

I gotta agree that's an excellent question. Pac Man and Drake must be off today.


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## Wildbill7145 (Apr 30, 2014)

The Cutting Edge said:


> I gotta agree that's an excellent question. Pac Man and Drake must be off today.


So, you mean they're not at their respective stores posting on PT when they're supposed to be working?

I'd have to imagine that the majority of the evaporation happens reasonably quickly during the dry to the touch period. As this process continues, I'd wonder if the remainder of the evaporation gets slower and slower as the molecules tighten up. Almost an exponentially slower drying process? Just a guess of course.


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## ExcelPaintingCo (Apr 16, 2011)

Found this on the web:

Did you know that in the early twentieth century the paints used on cars manufacturers need the order of weeks to complete its cure?, Today thanks to the knowledge of the mechanisms involved in the curing process of the paintings we have paints that cure in few seconds.

Always we apply a paint we need to wait a time in which the paint passes from a liquid to a solid state, commonly known as drying time, the knowledge of the mechanisms of curing of the paint as well as the methods used to reduce this time, will allow select the materials and tools needed to do a quality work in a short space of time.

Many times we speak indistinctly of drying, curing and hardening of the paint, but it is important to clarify each of these concepts;

Drying - The paint drying process corresponds to the evaporation of all solvents and diluents added to the paint in order to make it liquid or reduce its viscosity.

Hardening - the hardening of paint corresponds to the process by which the main polymer of the paint it creates and hardens with all other pigments and additives which comprises the paint, creating a solid and adherent coating.

Curing - The paint curing process corresponds both drying and hardening process. 

Curing = Drying + Hardening.

In response to the above definitions now we understand when a painting of two components do not add the hardener, we can observe over time the paint has dried (no smudges when touched) but has not hardened, allowing easy removal of the applied substrate, we say that the paint has dried (the solvent has evaporated), has not hardened (due hardener has not been added) and hence the paint has not cured.

One of the methods most used to classify the families of paint on the market is to identify by their curing mechanism, so we have:

Physical curing paints 

Chemically curing paints

Physical curing paints correspond to the set of paintings where the main resin is already formed, to cure the paint is only required to evaporate the solvent input into the painting itself.

Chemical curing paints correspond to the set of paints where the main resin (polymer) is created through chemical reactions (polyreactions) between various compounds, in this type of cure is necessary both drying and hardening process.

Nitrocellulose lacquers, acrylic lacquers, chlorinated rubber paints and vinyl paints are examples of paint which cured is physical, on the other hand polyurethane, epoxy, phenolic resins and polysiloxanes are examples of paint which cured is chemical.

All physical or chemical cured paints can accelerate or activate the curing process by applying one of the following techniques:

Hot air curing 

Radiation curing

Curing by electrical process 

Hot air curing consist to apply certain temperature to the painted surface, such that the increase in temperature causes a faster evaporation of the solvents and accelerates the chemical reactions take place to create the main polymer or resin, ie the temperature acts as a catalyst which accelerates the drying and hardening process, thereby accelerating the curing process. 

Painting booths with hot air recirculating used this method to accelerate the curing process, being the most used method due to the ease of curing of any workpiece regardless of its geometry, due to the ease of air penetration by any type of hole that has the workpiece. 

Radiation cured used microwave or infrared technology for increasing the substrate temperature so that the heat it propagate along the paint accelerating the curing process, on the other hand ultraviolet radiation technology active the chemical reactions necessary to cause curing of the paint, the latest technologies used laser and plasma to accelerate the curing process.


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## ExcelPaintingCo (Apr 16, 2011)

The remainder of the article:

The main advantage of the radiation curing is the short time need to solidify the paint, achieving curing times in order of seconds, as disadvantage this type of curing only can be applied in areas where radiation emitted can scope, therefore not used in complex workpieces with different shapes and hidden areas. 

Electrical curing method convert electrical power into heat supplying it directly to the painted workpiece so that the heat generated accelerates the curing of the paint, on the other hand the use of electromagnetic fields heated at the molecular level the paints accelerating its curing process.

Now that you know the different methods to accelerate the curing of the paint, you always have to take into account the advice and guide that the paint manufacturer will provide, an increase in excess of temperature on the paint can cause defects such as pinholes and even destroying the paint film.


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## NACE (May 16, 2008)

Rbriggs82 said:


> RH you just gave me a much need blog post topic. :yes:
> 
> I'm waiting for NACE so I can just copy and paste his reply, easy peezy. :jester:


When a coating reaches its full chemical and Abraision resistance. Alkyds. Slow dry. Fast cure. Usually 14 days. Acrylics. Fast dry, slow cure. Up to 30 days to reach full chemical and Abraision resistance and hardness. Color dictates how quickly a coating cures. Darker longer. Lighter, shorter. Temperatures, humidity and air circulation contribute. So many variables effect solvent evaporation, both oil and latex, absorption of oxygen in alkyds, and coalescence in latex. Glycol or UTC colorants soften and weaken a film. The more colorant the weaker, softer, and less Abraision resistant a film is. UTC colorants never fully dry. Waterborne colorants actually dry and improve film integrity and durability. I could go on and on, but basically washing or scrubbing a young paint film that is maybe as thick as rolling paper will not result in a happy customer.


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## ExcelPaintingCo (Apr 16, 2011)

NACE said:


> When a coating reaches its full chemical and Abraision resistance. Alkyds. Slow dry. Fast cure. Usually 14 days. Acrylics. Fast dry, slow cure. Up to 30 days to reach full chemical and Abraision resistance and hardness. Color dictates how quickly a coating cures. Darker longer. Lighter, shorter. Temperatures, humidity and air circulation contribute. So many variables effect solvent evaporation, both oil and latex, absorption of oxygen in alkyds, and coalescence in latex. Glycol or UTC colorants soften and weaken a film. The more colorant the weaker, softer, and less Abraision resistant a film is. UTC colorants never fully dry. Waterborne colorants actually dry and improve film integrity and durability. I could go on and on, but basically washing or scrubbing a young paint film that is maybe as thick as rolling paper will not result in a happy customer.


 What makes you think a bunch of painters would understand a rolling paper analogy? J/k. 
Thanks for your explanation. 

After conducting my own unscientific studies, I've found that low humidity, and air exchange help speed advance cure times.


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## Jmayspaint (Mar 26, 2013)

Thanks guys, interesting stuff. 

Coincidentally, I've been pondering this question lately myself. It's interesting, and good to know the effects of the curing process. 

Perhaps part of what RH is getting at, and what I'm curious about, is getting a basic understanding of the chemical processes. Ya know how there are virtual videos of DNA combining for example. These processes are understood in detail by chemists, just would be nice to have a visual, if approximate, image of this process. 

Is it just that coalescence means the molecules (resins?) bond together over a certain period of time after the solvent is gone? I guess the solvent evaporation is the catalyst for coalescence or oxidation?


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## RH (Sep 7, 2010)

NACE said:


> When a coating reaches its full chemical and Abraision resistance. Alkyds. Slow dry. Fast cure. Usually 14 days. Acrylics. Fast dry, slow cure. Up to 30 days to reach full chemical and Abraision resistance and hardness. Color dictates how quickly a coating cures. Darker longer. Lighter, shorter. Temperatures, humidity and air circulation contribute. So many variables effect solvent evaporation, both oil and latex, absorption of oxygen in alkyds, and coalescence in latex. Glycol or UTC colorants soften and weaken a film. The more colorant the weaker, softer, and less Abraision resistant a film is. UTC colorants never fully dry. Waterborne colorants actually dry and improve film integrity and durability. I could go on and on, but basically washing or scrubbing a young paint film that is maybe as thick as rolling paper will not result in a happy customer.


Thanks for the info NACE. Also, kudos to Excel for finding that info online. I checked for something before posting the thread but all I found was a bunch of hackish crap from some professional painting forum. :whistling2:


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## PNW Painter (Sep 5, 2013)

Rbriggs82 said:


> RH you just gave me a much need blog post topic. :yes:
> 
> 
> 
> I'm waiting for NACE so I can just copy and paste his reply, easy peezy. :jester:



I was thinking the same thing when I started reading this thread. 


Sent from my iPhone using Tapatalk


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## NACE (May 16, 2008)

Jmayspaint said:


> Thanks guys, interesting stuff.
> 
> Coincidentally, I've been pondering this question lately myself. It's interesting, and good to know the effects of the curing process.
> 
> ...


Paint companies have visuals for training purposes on how these processes work. Either training books or countless slide shows with simple diagrams. If you want real technical explanation, chemistry books may show it. Ask your paint rep if there are any training manuals around for new reps. BM has some training modules on their dealer portals. SW always had some excellent training too. Lots of the training is a cut and paste from SSPC or PDCA.


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## NACE (May 16, 2008)

Jmayspaint said:


> Thanks guys, interesting stuff.
> 
> Coincidentally, I've been pondering this question lately myself. It's interesting, and good to know the effects of the curing process.
> 
> ...


 Correct, solvent evaporation starts the process. As solvent is released and evaporated, the curing process and hardness of the film begin. Transfer from a liquid to a solid. Once the film is fully cured, it is effectively inert. I guess you could say that "inertness of the film" is actually fully cured. Lots going on as paint dries and cures. Who said watching paint dry is not exciting!


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## mustangmike3789 (Jun 11, 2011)

jmayspaint said:


> thanks guys, interesting stuff.
> 
> Coincidentally, i've been pondering this question lately myself. It's interesting, and good to know the effects of the curing process.
> 
> ...


i've highlighted the basic points that i will answer to the best of my ability even though i'm not a chemist, but i do work with some and have had the short answer to the question given to me for acrylic resin or coalesence type curing.
As the paint film dries, solvents and co-solvents evaporate, leaving the acrylic resin and pigments. The acrylic resin is like micro-spheres of liquid poymer "plastic" that then begin to touch each other and burst causing a cross linking or layering of the polymer which becomes the dry film of the coating. The more resin in the paint mixture, the glossier the paint will be due to the fact that there will be a thicker film of the "clear" resin holding the pigments in a film. Darker paints that hold more pigments will be softer and dry slower because the pigment to resin ratio changes as more "soft / slow drying" products are being held in the film which can be exposed though the surface of the film and slow down the curing / harding process. 
I hope that this helps and i hope that i got it right. Maybe someone like ric will come along and explain it a little better than i can.


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## PACman (Oct 24, 2014)

The Cutting Edge said:


> I gotta agree that's an excellent question. Pac Man and Drake must be off today.


Football. Sorry.


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## PACman (Oct 24, 2014)

The cure time and hardness of a paint film is an aspect of "quality" that is always overlooked in the "consumer" reporting magazines. Higher quality paint companies that put money into product development instead of marketing will always have products that cure faster and to a harder film then them.For example, Cali Ultraplate will cure so hard in 24 hours that you cannot scratch it with a metal putty knife. It is a urethane modified acrylic. Most products on the market take weeks to get that hard if they ever do.


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## Jmayspaint (Mar 26, 2013)

mustangmike3789 said:


> ....As the paint film dries, solvents and co-solvents evaporate, leaving the acrylic resin and pigments. The acrylic resin is like micro-spheres of liquid poymer "plastic" that then begin to touch each other and burst causing a cross linking or layering of the polymer which becomes the dry film of the coating. The more resin in the paint mixture, the glossier the paint will be......




Thanks guys this is getting good. The quoted paragraph is particularly helpful in forming a mental picture. 

So as the water (solvents) leave the film these microspheres are drawn closer and closer together. When they touch, or get as close as fillers and pigments allow them to, they then "coalesce" to form a continuos, inert film. The closer they can get, the stronger the film.,Sound right?


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## PACman (Oct 24, 2014)

Jmayspaint said:


> Thanks guys this is getting good. The quoted paragraph is particularly helpful in forming a mental picture.
> 
> So as the water (solvents) leave the film these microspheres are drawn closer and closer together. When they touch, or get as close as fillers and pigments allow them to, they then "coalesce" to form a continuos, inert film. The closer they can get, the stronger the film.,Sound right?


Correct. Cross linking acrylic resins create a bond so tight the paint film almost becomes one continuous molecule. I have heard from some people that they actually do and from some people that they don't.

And in the interest of science and science only, I would like to point out that two coats of Behr Marquee semi-gloss on Cover Stain, now having been cured for three months, will scratch off with your thumbnail. California Propaint semi-gloss, a "contractor" grade paint, wouldn't scrape off with my thumbnail after 2 weeks of cure time. Of course it depends on how hard your thumbnail is, so i'm sure there will be some debate.

But the sample is laying right on my front counter if anyone would like to try it.


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## mustangmike3789 (Jun 11, 2011)

Jmayspaint said:


> Thanks guys this is getting good. The quoted paragraph is particularly helpful in forming a mental picture.
> 
> So as the water (solvents) leave the film these microspheres are drawn closer and closer together. When they touch, or get as close as fillers and pigments allow them to, they then "coalesce" to form a continuos, inert film. The closer they can get, the stronger the film.,Sound right?


That's sounds about right or at least that's the way that i understand it.


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## RH (Sep 7, 2010)

Thanks for the great information guys! Very well phrased and relatively easy to understand.


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