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Adhesives Tips
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Contents
- Overview
- Paper and Wood Glues
- Matte Medium
- Silicones
- Latex Based Adhesives
- Rubber Cements
- Epoxies
- Conductive Epoxies
7 Jul 01 - Solvent Type Glues
- Super Glues
- Urethane Based Adhesives
- Hot Glue
- Thread Lockers
- Ungluable Materials
- A Long Term Adhesive Test
9 Jun 02
Overview
At some time or another, every modeler will need to use some kind of adhesive to repair a model, scratchbuild a building or attach some scenery. There is no one kind of adhesive that will work in all instances so after a while, you will find yourself acquiring and using several different types of adhesives. Selecting the right adhesive and using is properly is critical to obtain the best results on any given project.
I am not a chemist nor an expert in adhesives, but I have used them on enough different kinds of projects that I feet qualified to write about them.
The term "glue" was originally used to describe organically derived materials that could be used to attach porous objects to one another. The most common starting materials were horse byproducts and wheat. Over the years, the technology of "glues" has advanced so much that a more general term, adhesive, is used to describe the whole family. However, in this page, I will occasionally use the term "glue" when that term is commonly used, such as in "white glue."
Besides the types discussed below, there are many other adhesives that have use in craft projects or industrial processes. I will cover only those adhesives that I have found useful in model railroad work, and in some cases, general household projects.
Besides selecting the right adhesive, a common thread that runs through all adhesives is surface preparation. Almost all of the adhesives described are surface adherents, they work by sticking to a surface, not penetrating the surface (there are exceptions). Therefore for an adhesive to work, the surface to be bonded must generally be dry and free of particulate or organic contamination.
Some adhesives bond instantly or nearly so, others may require many hours to set up and gain strength. Whatever adhesive is used, some form of clamping, fixturing or pressure will be required to hold the work pieces immobile until the adhesive has had a chance to set. Patience is mandatory. The most common cause of failed joints (besides using the wrong adhesive) is that the joint is mechanically disturbed before the adhesive has been given a chance to work.
In all cases, refer to the instructions that comes with the particular adhesive for directions as to how to use that adhesive. These instructions are written by people who know how their adhesive works. Follow them to the letter and you'll generally get good results.
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Paper and Wood Adhesives
Wood is an important material in model railroad construction, both in the structure of a layout itself and in buildings and rolling stock. Paper and cardboard are also common materials.
Probably the safest and most common adhesive used in modeling is ordinary white glue. The most common adhesives in this category are called polyvinyl acetates. This is a water based adhesive that adheres well to paper and wood. It is not particularly strong and it is not tolerant to water, but it is cheap and safe. The material dries milky colored. White glue can be diluted with water and sprayed over texturing materials to hold them in place.
Yellow carpenters glue, sometimes called aliphatic resin, is a better choice to bond wood. This adhesive forms a strong bond that is somewhat tolerant of water. This adhesive dries in a somewhat yellow color. Properly made joints are often stronger than the wood that they attach to.
There are more advanced wood adhesives that are waterproof and in general they are stronger that yellow carpenters glue. Some brands names are Liquid Nails and TiteBond II. These adhesives are suitable for use in outdoor wood structures, even in wet environments.
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Matte Medium
Matte Medium is an acrylic based adhesive that is used by artists to attach paper to backing materials. The material tends to cause less wrinkling than other water based adhesives. I prefer matte medium to attach scenery materials such as dirt, ballast and texture to indoor layouts in preference to diluted white glue. It is more expensive and harder to find, but it works much better. It sprays better from a pump bottle and after it hardens, it leaves no sheen or color and is water resistant. There is also a Gloss Medium that can be used to make water surfaces where you want a shine. Matte medium is useful primarily on porous materials, but since it can surround particulate material, such as ballast, it works fine there too.
This stuff is a little expensive. A 16 oz jar of cost me about $7. I bought it from Dave Frary at Blue Ribbon Models, but he seems to be no longer selling it. The material was manufactured by Ultrecht in Brooklyn NY. I need to start looking around for another source, preferably in larger and hopefully cheaper quantities.
The kind that I get comes as a thick liquid and must be diluted in water before use. The recommended dilution of this material is 1 part matte medium and 3 parts water, however I have found that for attaching ballast, it works a little better when diluted with only 2 parts water.
Matte medium can be used to hold ballast in place in an outdoor environment and it works well. It keeps the ballast from being moved around but is not so strong that the ballast can't be broken up if needed. However, its a little expensive in this application. TiteBond II diluted with water works just as well and costs much less, however TiteBond II does leave a brown cast behind.
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Silicones
Silicone adhesives are one class of highly engineered products. The first and most common of this class is commonly called Silicone Sealer and is manufactured by many companies. The major advantage of silicon sealers is that they adhere well to non porous surfaces and they are absolutely waterproof and somewhat flexible. The downside is that the material is not very strong and virtually nothing will stick to cured sealer. It won't accept other adhesives, paint or even more sealer.
A significantly more advanced silicone adhesive is the GOOP brand. This stuff is magic, it sets fast, sticks to almost anything, and is very strong. It comes in several formulations which vary mostly in viscosity. Crafters GOOP works very well on wood. Marine GOOP is essentially the same stuff but it has a UV inhibitor in it. Use whatever kind you can find, it'll work fine. GOOP can tend to degrade after a few years in outdoors applications.
GOOP will dissolve in solvent based paints and stains which has an upside and a downside. You have to be careful while staining GOOPed structures as they might fall apart on you until the solvent evaporates back out of the material. The upside is that you can slop this stuff all over and it will absorb the stain and not show up as an unstained spot.
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Latex Based Adhesives
Latex is used as a base for some types of adhesives. These are usually fairly strong and reasonably waterproof. Liquid Nails falls in this class. This adhesive is not the strongest of the bunch, but it is usually strong enough and will adhere to wood, metal, glass and masonry and it takes paint very well. Liquid Nails comes in many formulations for specific jobs, but the most applicable is LN-901 exterior grade adhesive. Liquid Nails is available in squeeze tubes or caulking gun tubes. I find that the caulking gun tube is inexpensive (less than $3) and effective as the adhesive is fairly easy to clear from the applicator tip after it has set up. Simply ram the tip out with a metal rod or drill bit and then squirt out the hardened adhesive until fresh adhesive comes out.
A better combination latex and silicone based adhesive is Lexel. Lexel also comes in squeeze tubes and caulking gun tubes but it costs about 3 times as much as Liquid Nails. However, it is stronger, more flexible and has better adhesive strength than Liquid Nails. Lexel comes in white and clear formulations. Lexel also works well in a caulking gun tube. If the tube tip is plugged with a large toothpick or a metal rod, the adhesive will not set up in the tip and the tip is easy to clear. Unlike other silicone adhesives, Lexel will take paint well after it is allowed to set for two days. Lexel adheres to just about anything.
There is also a Lexel formulation called Set N' Stone available that is intended for use with masonry. This is a new product and is not yet widely available.
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Rubber Cements
Rubber cements fall in to three main categories, conventional rubber cement, contact cement and Walther's Goo. Rubber cement is a brown liquid that is usually used to bond paper but it will stick to almost anything that is dry. It is flexible and won't wrinkle paper badly. However, it isn't very strong in tension. Low strength can be an advantage when you want to be able to peel something off. It will rub off non porous surfaces. The work can be moved around somewhat for proper positioning before the adhesive dries.
Contact cement is considerably stronger. This material is usually applied to one surface and then the surfaces are pressed together to spread the adhesive to both surfaces and then pulled apart to allow the adhesive to dry, usually in about 10 minutes. Then the surfaces are pressed together and bonding is instant. The strength of the joint will increase with time although it is usually strong enough to work with immediately. The major downside is that there is NO opportunity to reposition the work. Once the parts are pressed together, adjustment of the position of joint is not possible. Get it right the first time.
Walther's Goo is similar to rubber cement but it is much stronger. The joint is made with the adhesive wet and the adhesive is allowed to set while the joint is clamped. The joints obtained are strong but somewhat flexible. Goo works on both porous and non porous surfaces.
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Epoxies
For all around high strength work, not many adhesives will beat a good epoxy and there are many kinds designed for specific tasks.
Most epoxies are two part materials that are mixed at the time of use. A chemical reaction between the resin and hardener components causes to the mix to set. Generally, epoxies that take longer to set produce stronger bonds. 5 minute epoxies may be handy, but they are generally not strong enough to be practical.
Many epoxies come in two small tubes where you spread out an equal amount of each part and mix the parts. Some come packaged in a double syringe like affair that is handy as the two parts don't get separated once the package is opened.
There are also one component epoxies, but these are usually restricted to industrial processes. These epoxies must be stored in a freezer and cured at high temperature, typically 150°C, hot enough to melt most plastics.
There are also filled epoxies. These adhesives contain some form of structural enhancer that tends to make them behave better in gap filling applications. Devcon Plastic Steel is one of my current favorites. This epoxy hardens black in color and is quite strong. After curing, it can be filed, drilled, sanded and painted. There is another Devcon type that is ceramic filled. It cures off-white and can also be filed, drilled, sanded or painted.
There is a another Devcon epoxy called Plastic Welder. It has a solvent in it as well as the epoxy resin so that it partially dissolves a plastic surface. The epoxy welds itself into the surface. This epoxy will work well with styrene or other plastics that respond to solvents. It works especially well in places where gap filling is desirable.
A very handy form of epoxy is called plumber's epoxy. This comes in a putty roll with the hardener and resin imbedded in a paste and wrapped around each other. The epoxy is mixed by tearing off a piece and kneading it until it is fully mixed and then forming it in place. I've used this stuff underwater, to fix radiators, shower knobs and to add bulk and strength to plastic parts such as coupler mounts. The stuff sets up fully hard in less than an hour and then it can be machined. There are other forms of putty type epoxies that are formulated for specific jobs.
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Conductive Epoxies
There are
conductive epoxies used in many industrial processes, but they are
usually not applicable to home or hobby use. They usually need to be
stored well below freezing and cured at very high temperatures, usually
150°C or so. However,
Chemtronics CW2400 is a two part silver loaded conductive epoxy
that is intended for circuit repairs and cures at room temperature.
Since it is a two part formulation, it can also be stored at room
temperature. It's fairly expensive, I purchased it at Hosfelt for about $15 for a half oz of
the stuff.
I prepared three test
samples with this epoxy to test its current handling capability. Two
code 332 brass rails were joined end to end with about a 10 mil gap
filled with epoxy. Two 18 ga wires were twisted and coated with epoxy.
The stuff doesn't flow well so the joint is pretty ugly. An 18 ga wire
was epoxied to the side of a brass rail section as a power feeder might
be.
The three joints were evaluated with a 4-point probe configuration at 1 amp, 3 amps and 7 amps. On all three samples, the resistance a 1 and 3 amps was the same, but at 7 amps, the resistance started to decrease over a few seconds and it stabilized at about half its original value. As happens with the industrial epoxies, the material appears to "form" under current stress. It is believed that the silver particles in the epoxy start to weld together under current stress and form a permanently better joint. The net resistance of the spliced wire was about 7 milliohms and the rail feeder was 4 milliohms. Both of these values are much higher than a soldered joint but low enough to work in model railroad applications. For example, this might be an acceptable way to attach feeder wires to aluminum or stainless steel track, neither of which are readily solderable. The end to end rail joint had very high resistance, about 50 milliohms. This is probably due to the relatively small cross section and the fairly thick joint.
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Solvent Type Adhesives
Regular model airplane cement is a solvent type adhesive. These cements come in various viscosities, from gel to fully liquid. These are really not adhesives in the normal sense as they don't stick to anything. They are welding agents. They actually dissolve the plastics to be bonded and the dissolved plastics flow together and mechanically weld before the solvent evaporates. They must be used on clean plastic surfaces, paint or other coatings will strongly interfere with the solvent action. Plastic Welder is one of this class of adhesives, but with a real epoxy base as well.
As with many of the more advanced adhesives, the solvents in these compounds are very toxic. Common liquid styrene cement is really just MEK (Methyl Ethyl Ketone) which you can buy by the gallon in home owner's stores. This stuff is especially nasty as it a well know brain rotter. Please use these adhesives in well ventilated area and avoid breathing the vapors.
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Super Glues
A major class of adhesives is called cyanoacrylates, sometimes knows as CA, SuperGlue or KrazyGlue. These are powerful adhesives that work very well in applications where there is no gap between the work pieces. It bonds well to any non porous material. It also bonds VERY well to skin, and quickly at that. CA is very good for attaching detail parts to scratch built projects as very little is needed. One small drop is good for most any application.
CA is usually sold in very small containers. This is not a problem as you rarely will use all the CA in the container before it dries out.
There are gap filling versions of CA as well. Zap CA is one brand that works well and has a good applicator design. Gap fillers are mixed in a gel formulation so that it can bridge small gaps.
Be very careful with CA as you can get your fingers bonded together before you know what has happened. If you do get your fingers stuck, you can work through the adhesive with acetone.
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Urethane Based Adhesives
There is one adhesive family that I know about that is based on urethanes. The one that I have used is called ProBond and is marketed by Elmer's. The claim is that this stuff will glue anything to anything and is waterproof. It does seem to work. The instructions say that the surfaces to be attached should be damp. The adhesive tends to foam a little as it sets so that it tends to squeeze itself out of joints.
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Hot Glue
Hot glue is a special thermal plastic that can be melted from its stick form and then applied to a surface with an electrically heated glue gun. This adhesive works especially well on paper and cardboard. It doesn't adhere very well to wood, probably because of the wood oils. I used hot glue to bond the cardboard strips used to support the Geologically Improbable Railroad, Mountain Division. The major advantage is that the glue sets full hard in about 10 seconds when pressed between the cardboard strips.
I also use hot glue to secure wiring on the undersides of cars and engines as it sticks well to hold the wires in place but it is also completely removable when desired.
A caution with hot glue is that it is HOT and retains it heat well so that if you get it on your fingers, you will get burned.
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Thread Lockers
Thread lockers are adhesives especially designed to hold in shear so that they are useful in preventing the movement of a threaded fastener. Thread lockers come in various formulas for several applications, however the only type that I know about that is useful in model railroad work is the weakest of the family. LocTite brand type 222 Purple Thread Locker can be very useful in particular situations. There are other stronger thread lockers out there, but most are so powerful that if you use them on small screws, you'll NEVER get the screws out if you should need to. Purple thread locker (the color denotes the strength) is sometimes difficult to find. Most stores carry Blue (type 242) thread locker which is too strong. It'll cause bolts much less than 1/4" to break before the locker will let go.
For our small screws, just a very little thread locker is adequate to keep a screw in place. The most common places where a thread locker is needed is on the screws that hold on Aristo steam engine drivers. These seem to come loose all the time. Also the screws under LGB 1600 turnouts have given me fits and I lock them in place before I install the turnouts.
Even Purple thread locker is pretty strong stuff. Apply a very little to a thread from a toothpick dipped in a drop of the locker and then insert the screws. The bond will develop full strength in 24 hours, but it'll be essentially locked almost immediately.
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Ungluable Materials
There are some materials that just resist adhesives. The one that is most common is polyethylene. Nothing sticks to this plastic and solvents don't work. Another is Teflon. LGB and Aristo tie strips fall into this category well. I've had marginal success using GOOP adhesives on tie strips.
Some engineered plastics like Luran-S and Delrin also resist adhesives. In cases like these, it may be necessary to drill holes on the piece to be bonded and force a filled epoxy into the holes to allow the epoxy to mechanically capture the parts to be attached.
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A Long Term Adhesive Test
In mid 2000, I
set out some test samples to age in the weather. I chose to test four
adhesives on both a redwood sample and on JigStones samples cast from
RapidSet concrete. A few months later, I added another JigStones sample
using a premixed generic "thin set" type of tile adhesive. The object
was to set these samples out and inspect it every few months to see if
any of the adhesives had just fallen apart. So far, one has. I have
not yet stressed any of the joints really hard so if one of the samples
has just weakened, I don't know about it.
It would be nice to find a way to accelerate this test because I'd like to know the results as I dive into new projects, but it looks like I'm just going to have to wait. If I had designed a really good experiment, I'd have made more samples that I could test to destruction at intervals.
The four adhesives that I selected were picked because they had all provided good service of some kind in previous projects. These are, ProBond, Liquid Nails LN-901, TiteBond II and clear Lexel. I have not used the ThinSet in any project, but it looks like it might be good as a concrete adhesive. I specifically did not try GOOP brand adhesives as a couple of types had failed miserably when exposed outdoors in a period of a couple of years.
After about 12 months of exposure, only the ProBond seemed to be impacted. Where exposed to the sun, it has turned brown. Where it was not exposed, it still had its light yellowish color.
When checked after two years (in June 02), only the LN-901 wood bond sample had failed in a combination of adhesive and cohesive failure. The part was still attached when first examined, but fell off when touched. All of the others seem to be doing fine for strength.
As for cosmetics, the story is a little different. The ProBond wood joint sample's surface has become rough and crusty and the crusty stuff scrapes off, but the bond is still good. The TiteBond II adhesive has turned dark brown and the edges of the fillet are beginning to flake off, but the part is still stuck. The Lexel sample had turned frosty to dark brown (depending how much sun it saw) from its original clear. The remains of the LN-901 sample had turned crusty white. It's pretty clear that none of these adhesives tolerate sunlight very well.
This page has been accessed times since 30 Oct 1999.
©1998-2002 George Schreyer
Created May 15, 1998
Last Updated Jun 9, 2002
