silmar 41

[termitedave]

I was wanting to know if you can use the hardner that comes with pr resin with silmar 41 pr resin. Thank you for your time and any help! David

 

[Ed McDonnell]

Hi David - The Silmar 41 I buy always comes with catalyst. Did you get Silmar 41 without catalyst?

In any case, MEKP (methy ethyl ketone peroxide) is the catalyst. Sometimes it's labeled as a hardener, but it's a catalyst. If what you have is MEKP that was intended for another type of PR and it's fresh then it should work with Silmar 41.

Do a small test cast to confirm before mixing up a big amount.


Ed

 

[termitedave]

Thank you for the reply Ed. Mine did come with the catalyst I just had some extra left over from some pr resin I bought out of a place in California not to long ago Thank you again David!

 

[Sylvanite]

In any case, MEKP (methy ethyl ketone peroxide) is the catalyst. Sometimes it's labeled as a hardener, but it's a catalyst.

Actually, MEKP is a hardener, not a catalyst. It initiates, and is consumed by a chemical chain reaction that polymerizes the resin and cross-links the polyester chains. Catalysts initiate a reaction, but are not consumed by it.

Regards,
Eric

 

[Ed McDonnell]

In any case, MEKP (methy ethyl ketone peroxide) is the catalyst. Sometimes it's labeled as a hardener, but it's a catalyst.

Actually, MEKP is a hardener, not a catalyst. It initiates, and is consumed by a chemical chain reaction that polymerizes the resin and cross-links the polyester chains. Catalysts initiate a reaction, but are not consumed by it.

Regards,
Eric

Hi Eric - I would really like to understand the exact chemistry that takes place when PR cures. I've not had any luck searching for something though (I haven't searched extensively:redface. You don't happen to know do you? It would be nice to really understand what I'm doing as I cast PR. More science and less alchemy / black magic would be a good thing.

MEKP isn't like the hardener in epoxy where ratios are critical and the hardener is a substantial part of the cured epoxy. It's a minor part of cured PR. More MEKP makes the PR cure faster, but you can harden PR without it, using heat. So in that respect it's more like a catalyst.

Resin manufacturers / distributors seem to use the words hardener and catalyst interchangeably with respect to PR resin. I wonder why there isn't more clarity on this?

Ed

 

[Sylvanite]

In chemistry terms, a "catalyst" is something that facilitates a reaction, but is not consumed by it. For example, the palladium plating in an automotive catalytic converter is a catalyst because it promotes the oxidation of carbon monoxide to carbon dioxide (2CO + O2 -> 2CO2) but is not altered itself.

As I understand it (not being a chemist myself, but having friends in the fiberglass industry), polyester resin is a mix of unsaturated polyester macromolecules and styrene. It's acutally the peroxide in MEKP that serves as a free radical which causes the styrene molecules to cross-link the polyester chains, forming a strong solid. It's a chain reaction, meaning that the radicals from the MEKP are consumed, but that more radicals are produced. That keeps the reaction going. Because the MEKP does contribute atoms to the molecular reaction, it is not (technically) a catalyst. It is, however, widely referred to as such.

The amount of MEKP used affects the cure rate because more free radicals yield a faster chain reaction. Temperature plays a big role in the reaction rate, as well as accelerators (a.k.a. promoters) or inhibitors added to the resin. Silmar 41 is typically sold pre-promoted with cobalt napthenate.

I hope that helps,
Eric

 

[Ed McDonnell]

Thanks Eric - I had previously looked at the MSDS for Silmar 41 and saw that it was a mix of unsaturated resins and styrene. I know that adding styrene to thin PR will increases the amount of shrinkage as the resin cures. I also know that cured PR will continue to release traces of styrene for quite a while after it is hard (like months).

I use sytrene to thin polyester resin (sometimes Silmar 41, but more often Silmar 249). I've always wondered:

1) how does the ratio of styrene to to resin affect the cured product, or does it make any difference (within reasonable limits)

2) how much of the styrene ends up permanently incorporated into the cured PR and how much is unbound and subject to being ultimately released

3) how much of the sytrene is typically lost during the initial cure (the period until it turns hard.

4) how long does PR continue to emit sytrene and how much (if you read the MSDS on PR / Styrene, it's not something you want to be continually exposed to).

I've also wondered long and hard about how pigments / dyes are bound up in the cured resin. Are they chemically or physically bound? I know that adding copper to PR will super charge the cure so there must be a chemical reaction. You identified cobalt as a promoter. So some metals will speed the curing reaction. That would explain why some coloring agents (that presumably have metal oxides as their coloring component) will accelerate the cure and others won't.

So far I've just been learning by trial and error mixing things together and seeing what happens. Things that work I remember (or try to) and I do them again. Things that don't work (like adding copper to PR) I try not to do again.

Wish I really knew what I was doing. Sadly, I think I've lost way too many brain cells over the years to be able to fully understand even if I could find the information.

Ed

 

[plantman]

I find it best to use the same brands together. They are designed for each other. Mixing any two different products together could make some interesting and maybe some not to safe results, such as toxic or explosive offgassing. There is a reason they instruct you to use in a well ventilated area. I did not know there was a difference myself. Thanks to Eric for pointing this out !! Have a Happy and Safe Holiday Season !! Jim S

 

[termitedave]

Thank You for all the replies!! David