Adhesives and Fillers used in Model Making

ADHESIVES AND FILLERS USED IN MODEL MAKING
.
1.
2.
3.
4.
5.
6.
7.
Cyanoacrylate (CA or Superglue)
Solvent weld
Epoxy
Polyester Filler
Glaze
RTV or Silicone
Soldering.
Cyanoacrylate CA or Superglue is an adhesive
which is ideal for joining most materials together
where there is a close fit between the parts. The
material cures rapidly when AIR is excluded from it.
A good example would be where the handle on a cup
is broken. The handle can often be place carefully
back into position and there is almost no joint visible
because the two parts mate back together so well. When this happens, the air is excluded and the
Superglue cures within a few seconds.
Superglue comes in two types, a thin watery material, and a gel. The gel contains some filler to thicken
it; this helps to fill small voids and keep the material in position in cases where a thinner material may
run off.
One very useful quirk Superglue has is that it can be rapidly set off by Baking Soda. The Baking Soda
also thickens the liquid, filling gaps. A typical example would be where a rod goes through a larger hole,
and the rod needs to be glued in place and the hole filled. The rod would be placed into the hole and
Superglue allowed to run into the hole and around the rod. Once that had set up, Baking Soda would be
lightly brushed into the hole until it was full. More Superglue would be run over the Baking Soda, which
would absorb the liquid, then immediately set up.
Superglue was invented by Eastman Kodak and used frequently in the Vietnam war as a human tissue
adhesive to stop bleeding. The original Eastman formula was not FDA approved for medical use,
however, because of a tendency to cause skin irritation and to generate heat. In 1998 the FDA
approved 2-octyl cyanoacrylate for use in closing wounds and surgical incisions. Closure
Medical has developed medical cyanoacrylates such as Dermabond, Soothe-N-Seal and BandAid Liquid Adhesive Bandage.
Solvent Weld Liquid ( A cohesive)
This material is usually used on several THERMO-plastics, like Styrene, ABS
and PVC. This watery liquid is usually Methylene Chloride, an unpleasant
solvent, which is dangerous if inhaled in quantity and is a known carcinogen.
Methylene Chloride is the main ingredient in paint-stripper and it literally
dissolves the plastic paint. A real weld is created during this process, because
the plastics are literally turned to liquid at the joint, as the solvent dissolves
them. The same thing happens to a metal being welded, but heat is used to
dissolve the metal. The two surfaces flow together, and slowly the solvent
evaporates (like the heat leaving the weld) and the two parts solidify together, now as one. It is
extremely difficult to break this join, unlike an adhesive bond.
Because the material is extremely watery, it will run in between closely mated surfaces, making it ideal
for assembly work. The two objects can be clamped together in a dry state, and copious amounts of
solvent weld applied to the join, where it will, by capillary action, soak into the joint thereby effecting
the weld.
Solvent Weld Gel.
This material works in exactly the same way as the
Solvent Weld Solution, however it has a gelling agent
added to thicken the material. This allows it to fill slight
voids, but more importantly, it does not run all over the
model melting important detail.
Epoxy Resins
Epoxies come in many forms. As a fast cure adhesive (5 minute), slow cure (30 minutes - 1 hour set-up),
a coating, or a filler adhesive. They all have precise mixing ratios which must be adhered to, because
part A or part B (A= Base, B= solidifier) both have a precise number of long chains of molecules which
must match each other when mixed together. To better understand this, imagine a ladder with one side
removed. The left side with the rungs still attached is Part A, the right side is Part B. When you mix
Parts A and B together, you must have the same amount or length of ladder sides so that there are
enough holes for all the rungs to fit in. If one side of the ladder is shorter than the other, a mis-match
occurs and the resin will not set properly, seriously weakening it. Insufficient hardener will cause the
material to be rubbery, too much will make it set too fast and then become extremely brittle.
Epoxies can have all sorts of filler added to them, ceramics; micro-balloons, sawdust, sand, metal flakes,
etc. etc. These fillers do not effect the mix ratios of the raw resin and hardener, however the cure time of
a resin can be slowed up slightly by preventing exothermic heat building up within the resin.
Epoxy resin has almost no shrinkage on cure, unlike polyesters. It also has incredible bond strength. It
can be used as a filler (with suitable additives) or as a thin set adhesive.
Epoxy resins are the ideal material for lay-up of laminated hulls. 5 Minute Epoxies are fast curing, but
lack the adhesive strength of the slower curing systems.
Polyester Filler
Car body fillers are the polyester materials we are referring to here. They are set
up using small amounts of peroxide hardener, supplied in a tube. Mix ratios are
not at all critical, and the useable life can be shortened or lengthened by
judicious amounts of hardener. In ambient temperatures, ‘useable life’ would
normally be about 5-10 minutes if a squeeze of approx 1” of hardener is added to
about two golf ball sizes of the resin. A chemical reaction takes place that will
continue to slowly cure all the resin if the slightest amount of hardener is
introduced. It is therefore critical not to introduce contaminated tools into the
resin tub.
There are several types of polyester resins.
Hair Filled – this has short lengths of fiberglass strands already mixed into the
resin, adding great strength to and otherwise brittle material. This is ideal for
filling gaps, like rust holes in car body panels.
Regular Filler – an economy paste material, for general dent filling, similar to
‘hair-filled’.
Lightweight Filled -usually filled with Cabosil or Micro-balloons, (A flour like
material) it is a paste which is creamy, and easily fills areas like dents in body
panels. It is an ideal product for filling large areas on model work.
Glaze– usually filled with a talc-like filler, the material is made slightly runny, so
it will flow into pin-holes and small crevices. A very useful filler for model
making.
Polyester resins have a high shrinkage factor after cure, and their bond strength is
nowhere as good as Superglue or epoxy, which is why they are generally relegated
to use as a filler, not subject to great stresses. However, most model applications
are not subject to the stresses that a vehicle body panel is subjected to, so they will
generally do a good job.
RTV or Moisture Cured Silicone Rubber
Truly a remarkable material, RTV (Room Temperature Vulcanization) Silicone
sealants are a great asset in model making where a flexible joint or repair is
needed.
RTV Silicones need nothing other than a little moisture in the air to cure, a process
that usually takes several hours to full cure. Simply apply the material to the part
and set aside overnight. Adhesion is tenacious to almost all materials, however,
this it be used to glue parts together that may need to be parted later for
maintenance. The material will shear at the joint when subjected to excessive
stresses.
BONDING TECHNIQUES
In all bonding situations, a solvent weld is going be the
ultimate method of joining two styrene parts together.
However, this method of bonding cannot be employed to
join different types of materials, such as styrene and
fiberglass, or fiberglass and epoxy. The solvent will only
dissolve thermoplastics, that is to say, those resins that are
injection molded by heat. Epoxy and GRP or polyester
harden chemically, so the solvent will not provide an
adequate ‘melt’.
In this diagram, solvent welding will work perfectly to join
the parts together. In joint #2, a fillet of styrene can be
added to fill the void, much like a welding rod is
employed. Similarly, styrene rod can be added to the
internal corners of # 3 & # 4. These additions will greatly
strengthen the joint, acting as buttresses to the flexing.
Liquid epoxies will work well on joints like #1, #3, and #4. These joints can be further strengthened by
using paste epoxies or polyesters as buttresses in the corners. Liquid epoxies take much longer to set
up,but their adhesion properties are greater. The joints can also withstand greater flexural stresses.
CA may also be used to bond #1, #3 and #4 together. This adhesive is ideal where an instant bond is
required. In the case of #2, Baking soda an be brushed into the V joint with a soft brush, the CA allowed
to flow into the powder, the baking soda will immediately set up the CA, forming an instant solid filler.
CA joints tend to be more brittle than an epoxy, and may shear if subjected to sudden impact.
#3 joint technique is probably the strongest, because the wall thickness has been doubled, as has the
bond surface area.
#1 joint technique, the butt joint, is probably the weakest because it has the least bonded surface area.
However, it will fair well if put into a straight pull, the parts being pulled in opposite directions. If
flexed, it is likely to fair due to ‘peel adhesion’ failure.
#3 joint technique, the overlap joint, will fare badly in a straight pull apart, due to ‘shear failure’,
whereas it will fare excellently in a flexed situation.
If you are not familiar with these adhesives and materials, it is worth the time to do a few experiments
just to be able to get a feel for what will fail or succeed in certain situations.
In all circumstances, SURFACE PREPARATION and BAD MIXING are the main causes of failure.