Bubbles in PR Vacuum & Degassing (a little long)

[bruce119]

OK this is for the guys with a little more experience.

I am adjusting my technique to accommodate the new resin saver molds. Best thing to come along since silicone molds. Thank you Charlie (NewLondon88)

Now my old method was basically mix the cyat. in the resin keeping the bubbles to a minimum. Then pore in a square hard delrin mold. Then pressurize to about 50 lbs. This works great the pressure eliminates bubbles worry free.

Now with the resin saver molds they are sensitive to pressure I found out the hard way. The pressure distorts the silicone as it squeezes to the point with excessive pressure like 50 lbs it will push the tubes into and cutting the mold. Then when curred it gives the appearance the the PR has shrunk and the ends of the tubes are exposed. It took a few conversations with Charlie to figure out what was going on.

Anyway I changed my technique. I am not using pressure any more. But to the point and the question. I heat the PR by putting my mixing tub in a container of hot water to thin it. Helps the bubbles rise I carefully mix the cayt. to not introduce too much bubbles but you always get some.

So I get a vacuum pump so I can pull a good -29. OK I put the warm PR BEFORE adding the cayt. actually there are not many bubbles to begin with. But after about 3-5 minutes under -29 vacuum the resin is loaded with tiny bubbles. I am thinking that the vacuum is expanding dissolved air that is present in the resin. Now the resin that I have has been around for a bout 3 months a 5 gal. pail. Now I am not a chemist but I think that is what is going. It takes me about 3 - 4 cycles of pulling a vacuum to get all the bubbles out before I can add the cayt.

Is anyone else experiencing this. I thought after mixing the cayt. I could put a vacuum on it to get rid of the bubbles make them larger so they rise and pop. But I think mite be going on is there is always dissolved air in resin and the vacuum actually adds air before it can start to remove it.

So I guess you get the picture if anyone else has been down this road would know what I talking about.

Your thoughts.

thanks

 

[ldb2000]

By pulling a high vacuume you are causing the PR to boil , the styrene has a room temp boiling point under 29" of vacuume .

 

[bruce119]

So am I pulling to much pressure at the start. Should I not go over say 15" I'm not quite sure how that works. I know pressure shrinks the air and vacuum expands it. That much I understand I am a SCUBA diver

Am I rite in saying I am reconstituting air from dissolved air in the resin. After 2 -3 pulls of vacuum the bubbles does clear out.

thanks

 

[bruce119]

If I add colorants to the PR I don't use anything, vacuum or pressure, and I don't have any problems. If I do snakeskins or labels (clear PR) then I add the catalyst, pour and vibrate the mold for a couple minutes on the bed of my scroll saw. I finish with 25lbs. of pressure.
I never have used vacuum, never thought I needed to.:wink:

I used to do that worked great with no problems BUT with the new molds you can't go more than 5, 10 at most pressure before the silicone distorts and shrinks in on the tubes.

 

[rherrell]

OK, I didn't realize that about the RS molds.

 

[ldb2000]

I think it was Glass scratcher that gave the info on what the boiling point of PR under vacuum is . Hopefully he will kick in with the exact specs .

 

[TomW]

So am I pulling to much pressure at the start. Should I not go over say 15" I'm not quite sure how that works. I know pressure shrinks the air and vacuum expands it. That much I understand I am a SCUBA diver

Am I rite in saying I am reconstituting air from dissolved air in the resin. After 2 -3 pulls of vacuum the bubbles does clear out.

thanks

Bruce, you are causing the styrene to boil, making bubbles of styrene gas. Think of it like your propane bbq. When you lower the pressure in the bottle, the Propane liquid boils, creating propane gas. Same thing happens to the PR when you reduce the pressure below the boiling point.

I suspect you will be quite happy with pulling a little less vacuum.

Tom

 

[bruce119]

I think it was Glass scratcher that gave the info on what the boiling point of PR under vacuum is . Hopefully he will kick in with the exact specs .

Yea I can understand that kind of but if it was boiling. Wouldn't it boil every time it was was under the vacuum. So is actually bolling or is a point that it releases the dissolved air in the solution. I am just guessing based on some of the stuff I heard here over the past.

 

[BRobbins629]

Your explanation is very close to correct. There will always be some air in solution. Vacuum will reduce the soluability of air creating bubbles (Butch also has a point about boiling the styrene); pressure will increase the soluability - just like a soda bottle which when under pressure you don't see bubbbles. Ultrasonics can also be used to reduce the air in solution. Heating can be a mixed bag. There are valid arguements for increasing or decreasing voids. As for pressure, if you are using PR, there is no need to use 50 pounds of pressure. 15-20 is plenty if all you are trying to do is keep air in solution. Some think that the pressure is shrinking the bubbles, but if you do some quick calculations, you would need considerably more pressure and the effect would be minimal.

 

[its_virgil]

I think Charles, aka glassscratcher, has reported to us that vacuum on PR needs to be less than 27 in. of hg if i remember correctly.
Do a good turn daily!
Don

 

[bruce119]

I can see what is happening. I know what is working for me and it is not all the same procedure for the different things I cast. I am just trying to understand the real reason for what is happening.

The reason I was using vacuum was to completely rid the resin of bubbles before the pore. I had a problem of air sticking and leaching out in spots I resolved that with a coating of CA that is working well for this particular case.

In the past I only had a venturi vacuum (I think that's what they call it) that you hook to the air compressor. Now I could only pull about 10 at the most. i recently went out and bought a vacuum pump now I can just about pin the valve at 29. That is when I discovered the bubbles. So I think I am creating more work with max pressure.

My theory is there is a certain amount of dissolved air in the resin and at a point under vacuum it is pushed or expanded back to air the boiling point that was spoken about. Now I have noticed when I push the vacuum to max 28-29 the first time I get a lot of small bubbles all through the resin. Then I repeat the cycle 2 more times at max vacuum and I get all the air out.

Now my theory and wounder that if there is an advantage to remove the dissolved air (if that is what it is) or more like this is totally unnecessary and just need the vacuum to get rid of the air that is introduced by mixing in the mek. And what is the optimum point to go with the vacuum. I wish I had a clear top for my pot and I could see just what is going on.

So who really knows is my theory correct about dissolved air in the resin and if there is. Is there any benefit to remove the air. I know this is getting technical but I am trying to understand what is happening. Sounds like Bruce thinks I am on the rite track. But I don't I just know what I am seeing and experiencing.

thanks

 

[BRobbins629]

There are only basic reasons why we see bubbles in resin. One is dissolved air and I think most are in agreement on the several ways to minimize. The other cause is that the resin doesn't wet the surface and will repel it in spots. No amount of vacuum or pressure will resolve the later. Applying CA as you found is one cure; cleaning to reduce the surface tension is the other.

 

[ldb2000]

To be perfectly honest , I'm not sure what the problem is to begin with . I pour PR with no vacuum or pressure and get clear cast blanks with no bubbles . Don Ward and several others do as well so why this need to leech all the air out of solution ? I'm not trying to start anything , just wondering .

 

[bruce119]

To be perfectly honest , I'm not sure what the problem is to begin with . I pour PR with no vacuum or pressure and get clear cast blanks with no bubbles . Don Ward and several others do as well so why this need to leech all the air out of solution ? I'm not trying to start anything , just wondering .

No real problem I am just trying to understand some of the science.

My biggest problem in the beginning of this project with overlapping paper was air leaching out of the overlaps and the edges of paper it self. The solution was pressure BUT with resin saver mold that was not an option the molds are too sensitive to pressure. Coating and sealing with CA solved it. I'm just talking in circles now.

I discovered the dissolved air in resin kind of by accident and was curious to find if my theory is correct. It doesn't change anything or really make any difference just wanted to know.

I love the resin saver mold but I got some modifications in mind so they can handle pressure and not squeeze down on the tube.

 

[ldb2000]

You should remember that not all the bubbles you are seeing are air bubbles . The boiling of the styrene I mentioned is a process where the liquid styrene changes to a gaseous state and rises to the surface as gaseous styrene bubbles , so the bubbles you are seeing may not be air but other gases . To know how much air is in suspension would take very complicated and expensive lab equipment .

 

[bruce119]

Got ya I am getting a picture the styrene is that a chemical in PR. I don't recognize that word I think I heard it in plastics.

thanks

 

[ldb2000]

Yes it is a liquid chemical at room temp under normal air pressure . It is one of the chemicals that give PR it's distinctive smell .

 

[bruce119]

Well thanks for the chemistry lessens. I wounder by degassing the styrene if it effects any thing. I didn't notice anything the cast came clear as glass.

thanks again

 

[ldb2000]

It's been a long time since chem101 and I've forgotten so much . You can affect the strength of the resin by boiling off too much styrene . It is the chemical that links the molecules together . Type Styrene into a wiki database to find out how it works .

 

[BRobbins629]

Styrene also acts as a thinner for PR. Without it the PR would be too thick to pour. You can also buy styrene by itself to use as a thinner. Folks that spray fiberglass which is the same chemistry resin, do just that. I'm sure the concentration of styrene along with other things such as how much catalyst, affects the final brittleness of the cast. You can indeed get clear casts without degassing or pressure. For me, its just a little insurance if I've gone to the trouble of casting something I spent a long time creating.

 

[ldb2000]

Here is the wiki entry on polyester resin

"Thermosetting resins are generally copolymers of unsaturated polyesters with styrene. Polyester saturation is governed through the use of maleic acid or fumaric acid. In vinyl esters, saturation (or lack thereof) is found in the alcohol group of the polyester. The double bond of unsaturated polyester reacts with styrene resulting in a 3-D cross-linked structure. This structure acts as a thermoset. The cross-linking is initiated through an exothermic reaction involving an organic peroxide, such as methyl ethyl ketone peroxide or benzoyl peroxide." from www.wikipedia.com

 

[PTownSubbie]

Bruce,

You mentioned air leaking out of seams. How long do you wait after fixing your material to the tube before you pour?

I was having problems with air along the seams of labels until I fixed the label to the tube and waited at least one day before casting. If you don't wait, air will leach out of the seam for some reason. I think the glue needs to fully cure so that it will not create air or something odd like that.

Just an observation of mine.

 

[bruce119]

Bruce,

You mentioned air leaking out of seams. How long do you wait after fixing your material to the tube before you pour?

I was having problems with air along the seams of labels until I fixed the label to the tube and waited at least one day before casting. If you don't wait, air will leach out of the seam for some reason. I think the glue needs to fully cure so that it will not create air or something odd like that.

Just an observation of mine.

Yes i agree it need to cure I had stuff like glue curing after a cast and weird things happening such as CA needs a minimum of 24 hrs it mat seem and feel dry but it is still off gassing and that can give you problems.

My problem with air leaching out in this case was overlapping pieces of paper in my shredded money blanks. I think what was happening there was trapped air pockets here and there and the air seeped out. I only found 2 ways to deal with this pressure the easiest most effective BUT did not work with the resin saver mold. So I now coat the entire blank in CA let it dry 24 hrs and it cured my problems. It just added 2 days to the process. One day to glue the pieces on with Mod Podge (Thank you Don) leave that dry 24 hrs then coat the blank with CA dry 24 hrs. That is working really well for these blanks I am going to try the same process on snake skin. I got the skin on the tube now with the Mod Podge now I am about to coat with CA. Then I will be able to cast tomorrow evening.

I did a cast tonight and didn't use as much vacuum only -15 worked just fine.

Thanks for all the input it helps to try and understand what is going on.

thanks