CA-glue-induced severe cracking of polystyrene

[robdebie]

I had noted before that small polystyrene strips would crack severely if I bent them first, then glued them on a model's curved surface. Say a 1 x 1 mm strip would break up in 2 to 3 mm segements, and the segments themselves would also show many more tiny cracks. It difficult to photograph, so I can't show it (yet).

Yesterday I made an extension for a 1/25 scale tank trailer (MPC / Lindberg). I first extended the top side of the tank forward with five plastic card 'stringers'. Then I cut a piece of 1 mm plastic card to the required (complex) shape, then pre-curved (rolled) it on the edge of a table. The edges were glued together to form a tube / pipe, and I installed an elliptical bulkhead internally. Lastly added the curved front face of the tank, as an oversize piece, glued on with Zap thin CA, purple label. And then it happened: severe cracking of the bent plastic card - something I had never experienced before.

Is there anyone who understands what's going on here? I have a feeling it's not happening with all CA glues that I use, I should do more experiments.

Rob

Edited September 18, 2024 by robdebie

[peteski]

I have seen this happen too. Not sure if this is caused by certain chemical formulation of that polystyrene sheet, or something else.  Thin polystyrene is usually the most prone to this.  I never really did any analysis.  I do wonder if using BSI Odorless CA glue would prevent this from happening.  Just a speculation.

[Ace-Garageguy]

Only thing I've ever experienced that's similar is cracking of cold-formed styrene strip and sheet when solvent glue hits it (I don't use CA for styrene-to-styrene bonds).

It's a pretty common problem, usually alleviated by heat-forming and tempering thin styrene parts prior to assembly with solvent-type cements.

I'd kinda suspect that whatever solvent is in your "thin CA" to make it thin is the culprit.

HOWEVER...that's just a guess based on similar but not identical experience.

 

Edited September 18, 2024 by Ace-Garageguy
TYPO

[peteski]

1 hour ago, Ace-Garageguy said:

Only thing I've ever experienced that's similar is cracking of cold-formed styrene strip and sheet when solvent glue hits it (I don't use CA for styrene-to-styrene bonds).

Yes, I have also encountered this problem.  In my experience MEK-based cement causes the cracking while Methylene Chloride does not.

As for CA's solvent, I don't believe there is any. CA is simply a liquid resin which polymerizes when it hardens.  It can emit fumes as it sets, but that is not due to any solvent evaporation  it is the resin itself that emits the fumes. If there was solvent., the glue joints would have sink marks.

[Ace-Garageguy]

1 hour ago, peteski said:

As for CA's solvent, I don't believe there is any. CA is simply a liquid resin which polymerizes when it hardens.  It can emit fumes as it sets, but that is not due to any solvent evaporation  it is the resin itself that emits the fumes. If there was solvent., the glue joints would have sink marks.

Then explain the chemistry, please, that accounts for different viscosities of CA, i.e. thin, medium, and thick.

Solvents and inert fillers are used to moderate the viscosity of other "resins", for example styrene monomer as a thinner in the case of polyester, acetone as a thinner in the case of many epoxies, and various rheological and thixotropic agents used as "thickeners" in both.

And as for "sink marks", I routinely bond sheet and strip styrene with solvents, and have no sink marks unless I use way too much.

Edited September 19, 2024 by Ace-Garageguy
CLARITY and ACCURACY

[1972coronet]

Q.C. issue with the production of that particular CA concoction ? 

Bain't that I'm the sharpest butter knife in the tacklebox ; just hazarding a guess...

[peteski]

40 minutes ago, Ace-Garageguy said:

Then explain the chemistry, please, that accounts for different viscosities of CA, i.e. thin, medium, and thick.

Solvents and inert fillers are used to moderate the viscosity of other "resins", for example styrene monomer as a thinner in the case of polyester, acetone as a thinner in the case of many epoxies, and various rheological and thixotropic agents used as "thickeners" in both.

And as for "sink marks", I routinely bond sheet and strip styrene with solvents, and have no sink marks unless I use way too much.

I suspect it is a different chemical (or even molecular) composition.  If you let thin CA glue age, it thickens up (but it is still usable).  If there was a solvent involved, the liquid level would go down, but it stays the same, even though the liquid has thickened up. No solvent to evaporate.

Or it might be something as simple as the thin CA glue is its natural state, and there are thickeners added to make it more viscous.

You also mention no sink marks when using solvents, but that is not a valid comparison.  Those totally evaporate after the plastic is melted. I'm talking about a cement which has some body to it (CA glue is type of acrylic resin).

I don't believe styrene monomer evaporates when polyester resin hardens - it remains in the solidified resin.  Acetone does evaporate from epoxy, so the epoxy remaining in the bond or mold will shrink slightly.

[Ulf]

Over the last thirty years in my work/professional life (which ends today) I have met several professional modelers, house models were built almost exclusively with cardboard for several reasons, cheap, easy to work whit.
House models are unpleasantly expensive and the consequences of a house model failing can be great in terms of being shown to the CEO, board and investors.
Reactions with different adhesives are another problem that makes the professional modelers often use cardboard and white glue.
I have picked up some knowledge over the years and what they really pointed out is the importance of not letting unpainted styrene lie in the sunlight year after year in the workshop, in addition to the importance of buying quality (styrene flakes and profiles) and getting to know the materials and adhesives you use. Any cavities must be ventilated so that solvents do not become trapped, this is especially true of small ones, the construction must be solid and also without too much tension, shape, fit in and glue with as little glue as possible, I always use Revell's pipe glue.

[robdebie]

Thanks all for the replies so far.

A small update: I planned to do some experiments, with bent / folded plastic card strips, but I ran out of Zap thin CA, my prime suspect.

The cracks in the tank trailer are repaired, and no new problems have surfaced.

Rob

 

[Skip]

I remember seeing an Engineering write up about Cyanoacrylate bonding agents (CA Glue) used in industrial and Civil Engineering applications. The takeaway was that CA bonding agents start out as what would be called thin, then bulking agents are added to thicken the respective formulas. (Think bulking epoxy resins with micro-balloons.)

Your lofted framing should be sufficient to hold the exterior skin in place. There may be a couple of methods to Engineer you out of this predicament. First, I think when you bent and formed the skin you created a good deal of stress throughout the skin especially in the tighter radius.  
Next, as Cyanoacrylates cure they off-gas which seems to attack styrene. Case in point - evidenced by all those of us who have ruined clear styrene and acetate windows using Cyanoacrylate where the gasses settled on fingerprints and smudges becoming impossible to polish out. As the Cyanoacrylate gasses harden it greatly reduces any flexibility in the styrene sheet, which may show up in cracking.
 Ulf also makes a good point above, regarding UV light breakdown of the styrene sheet, this is another logical avenue to pursue.

Most of the manufacturers of styrene sheet, Evergreen Plastics, Plastruct, etc. utilize a specific cement formulation chemically engineered for their product, that’s the first place I would start; use their cement. Just make sure it sets and off-gasses completely before closing the skin completely.  

OK, so how do we work around the issue of stress?
If we did some gentle pre-forming around a similar radius and burnished the material slowly with a smooth/polished material around the preform until it is close to the finished overall radius. The way it was bent probably exceeded the minimum bend radius of the material.  Especially  if you attempted to chip-form the skin, that is doing a whole bunch of smaller bends to get the overall shape, then bonded the skin starting at one end working to the opposite. Start in the middle of the skin working from one side of the first bond line then the other until you have to close it from one side. Method two, would be to split the skin into halves, thirds or quarters bonding it to the lofted framework, reducing the overall stress. Likely this a good part of the reason the kit tankers are molded in multiple sections, plus to ease the manufacturing process.

Third method already mentioned is *heat forming the skin using hot water, flame, heat gun, etc. bending it either directly to the lofted frame or onto a similar radius or create a buck from harder material to preform the material, bonding to the lofted frame as above. Working slowly and allowing the cement to off-gas so it isn’t trapped inside the cavity, you may even need some tiny vent holes to allow the gasses to escape. You can either use a drop of CA cement or styrene rod cemented in to close the vents.

*Mr. Obsessive, Bill Geary (forgive me Bill if I misspelled your name), has described a great method of heat forming that he uses to form clear sheet for glass, which looks better than the kit stuff!

I know there are a couple of other Engineers who frequent this board feel free to add from your experience to help solve Rob’s dilemma.

Sorry for writing a book, but I tried to be as thorough as possible to help solve the issue.

[robdebie]

A follow-up. I did a simple test: I bent pieces of 1 mm plastic card into an arch, and fixed the base ends with a piece of tape. Then applied a drop of CA straight from the bottles, on the arch's peak, the area that was most deformed. The pink Zap specimen started cracking within seconds, but nothing happened to the Colle 21 specimens. I bent all specimens identically, but the cracked specimen changed shape due to the cracking.

On the left, the same specimen but upside down. On the right, an earlier test. Different cracking patterns.

An amazing result! But I can't offer an explanation.

It would be great if forum members could repeat this test with their CA glues. If you want to replicate mine: the samples are 80 x 15 x 1 mm, bent to an arch, the ends fixed with tape with a 20 mm distance. CA is applied straight from the bottle, tip against the top of the arch, approximately one drop over an area of 15 x 5 mm.

Rob

Edited November 15, 2024 by robdebie

[peteski]

Obviously some ingredient in the Zap glue reacts with the polystyrene.  What it is causing it? I have no idea.  You live in  Europe. Many of forum members live in USA. The brand of polystyrene might also be in play here.  What brand are you using?

I know that the Zap kicker has acetone and it will affect polystyrene and paint.  I switched to BSI industries accelerator because it does not affect polystyrene or even paint. Actually, I have not used the Zap brand CAs for many years. All I now use is BSI CA glue.

[stitchdup]

is that the archiatectural sheets that have the outer skins shiny? its really bad for this happening and i suspect its due to the inner plastic being a different formula to the outer plastic. if it is these sheets your best option is to use them for mixing filler. the cracking will only spread and get worse. these sheets are meant to be used for flat pieces only. plastruct and evergreen sheets are made to bend but the archiatectural stuff isn't though its cheaper cost makes it tempting

[robdebie]

On 11/16/2024 at 5:51 AM, peteski said:

Obviously some ingredient in the Zap glue reacts with the polystyrene.  What it is causing it? I have no idea.  You live in  Europe. Many of forum members live in USA. The brand of polystyrene might also be in play here.  What brand are you using?

I know that the Zap kicker has acetone and it will affect polystyrene and paint.  I switched to BSI industries accelerator because it does not affect polystyrene or even paint. Actually, I have not used the Zap brand CAs for many years. All I now use is BSI CA glue.

There's no brand on the polystyrene, only a small sticker with a bar code and a brief product description. If I could read the bar code, I would probably know the manufacturer too.

To be clear, I did not use a kicker. Never used that in my life.

Rob

[robdebie]

On 11/16/2024 at 9:26 AM, stitchdup said:

is that the archiatectural sheets that have the outer skins shiny? its really bad for this happening and i suspect its due to the inner plastic being a different formula to the outer plastic. if it is these sheets your best option is to use them for mixing filler. the cracking will only spread and get worse. these sheets are meant to be used for flat pieces only. plastruct and evergreen sheets are made to bend but the archiatectural stuff isn't though its cheaper cost makes it tempting

I would say it's regular polystyrene sheet. The cracking problem is easily avoided by not using pink Zap, so the problem is not too bad. But it is fascinating!

Rob

[peteski]

Yes, it is surprising what sort of chemical interactions can happen with what we perceive as typical plastics and adhesives we use in out hobbies.