I have been doing a lot of plastic kit structure models over the past two yrs and have always preferred the weld-type bond created by styrene adhesives. Of course, this works best BEFORE one paints!
I turn to CA adhesives- the instant and the medium types- when painted parts are to be put together, so this now has me thinking of the “quality” of that bonding action. If I use CA on two painted parts, what I really seem to be doing is joining two paints together, which happen to be attached to the underlying material (I also join wood to plastic as I model).
Can anyone with expert knowledge of CA enlighten me as to what is really going on? I know the basic concept of CA, but am no materials engineer by a long shot.
CA, or super glue, is a contact cement. It bonds with whatever solid object it touches, but it doesn’t penetrate it as far as I know. Unless the paint is especially thin and porous, the glue is most likely bonding to the paint alone.
You have already found that styrene cement (MEK the active ingredient) will bond the plastic by actually melting/ fusing those surfaces. I will still prefer to scrape the paint from mating surfaces and use Faller Xpert or Testor’s Model master to bond similar styrene plastics. Some like Tenax, which is neede for acylics. I will use CA for joining disimilar materials especially wire details, cast brass/ white metal etc. The bond may be strong but tends to be brittle and won’t withstand any force or flex. I will, however use CA to cement some small detail parts in plastic that won’t see any abuse.
You are right in that bonding painted parts w/ CA will only bond paint to paint. The joint is really only as good as the bond of the paint to the plastic surface.
Of coarse CA isn’t the recommended product to bond any porous materials even metal or plastic to wood. It will work but the CA will suck into the porous wood fiber and leave little for the bond. I will add CA to the joint or wood, wipe clean let dry a minute or two, then bond the parts. This will hold true for many other glues and cements when one surface is very porous. Spray adhesives, contact cement, Pliobond and even wood glues. An example of this is when I will glue up ballustrde (railing) and the fittings. One application of glue to the end grains will not bond those 2 rails/ fittings as the carpenters glue will wick into the porous end grains ( this will happen to even some of the hardest and tightest graining of wood) Epoxies will eliminate most of this, but tend to be too difficult to work with under most conditions. It will mostly depend on how secure a true bond you are looking to have.
This doesn’t apply to many of the bonds for our needs in structure building or layout construction, as the joints need not be that critical a bond.
I have used Allene’s tacky for small quick bonds on basswood or balsa for structures but many still like to use yellow wood glue ( better bond will just need more time in hold
Basically, this stuff does not air dry, it needs a slight amount of water for the chemical/bonding reaction to take place. The above link will give you a detailed description of the chemistry involved in this interesting adhesive.
That’s why it seems to work better in the summer when the humidity is higher. I’ve also found that freshly cleaned surfaces don’t seem to bond as readily as those that you’ve handled with your hands - the moisture from your fingers helps aid the reaction.
That’s probably why it is so good at gluing you fingers together.
It was originally developed as an adhesive for strain gages, small devices that are used to determine how much load or stress/strain that has been applied to a structure. It requires high strength and a thin adhesive film.
While ca may bond on contact, a true contact cement is applied to both surfaces to be joined, then allowed to dry to the touch. When the two surfaces are then brought together (contact), the bond is instantaneous and permanent. Most ca allows for at least minor “working” time.
A very good and inexpensive cement for styrene is good old lacquer thinner. I buy it by the gallon, then decant it into smaller containers depending on its end-use, as I also use it as a paint thinner (Floquil, Dullcote, Scalecoat, Testors, Humbrol, Pactra, and SMP Accu-Paint) and for stripping paint from brass and cast metal models.
Ca is useful for attaching dissimilar materials, but performs best when used in conjunction with a mechanical joint, such as a mounting pin within a properly-sized hole or for the joining of interlocking surfaces.