Will epoxy glue that material?

[bainbridgeisland]

At work I am often asked if epoxy can be used to glue ‘X’ material. Over the years, I have found that many Shipwrights, Naval Architects and Engineers are not knowledgeable about key aspects of how glue works. Here is one aspect:

Surface wetting is an essential element of a glue joint. Without wetting, strong bonds are impossible.

There are two primary aspects of wetting, lateral spreading of the adhesive film and penetration of the fluid adhesive into the profile of the surface.

Lateral spreading can be intuitively understood by comparing the surface of a just-waxed car and the old TV commercial about ‘Cascade’ dishwasher soap. Water on a just-waxed car beads up. In other words, it has poor surface wetting. Water on dishware (mixed with Cascade soap) sheets off, it doesn’t bead up. In other words, it has good surface wetting.

The reason lateral spreading is important is that a glue that wets well has more bond area per square inch of joint area. This reduces the load per square inch of adhesive actually loaded. Think of it this way. The beads of water on the waxed car actually only contact the car in a small percentage of the area. If the beads of water represented glue contact area, there just wouldn’t be much contact between the glue and the surface. Thus the smaller contact area must carry the entire load.

Penetration of the fluid into the profile of the surface improves bonding by increasing surface area and by mechanical interlocking of the glue to the surface. Capillary action is increased when glues that ‘wet’ well are used. The ability to penetrate is affected by viscosity.

O.K. How does one use this information?

1. When choosing an adhesive, be sure the ‘critical surface tension’ of the glue is equal or lower than the ‘critical surface tension’ of the surface you want to bond. When this is the case, you will see good lateral spreading. You may also see ‘critical surface tension’ called ‘surface tension’, ‘apparent surface free energy’ or ‘surface energy’. The ‘critical surface tension’ of epoxy is usually between 43 and 47 dynes/cm (or 0.043 to 0.047 N/m). There are additives that can lower this a little. Verify the ‘critical surface tension’ of your glue with your manufacturer if you have a critical glue joint with a material that is close.

Here are the ‘critical surface tensions’ (dynes/cm) of some common materials:
43 to 47 Amine cured epoxy
57 sanded Maple
47 planed Maple
46 Polycarbonate
31 Polyethylene
39 Methylmethacrylate
33 Polystyrene
39 Polyvinyl chloride
43 Polyethylene terephalate (PET)
46 Nylon-6,6
19 P.T.F.E
Greater than 500 = metals
73 Water
48 Ethylene glycol
29 Lube oil
21 Silicone oil

2. Be sure the surface is not contaminated. You can see from above that a spritz of silicone oil, oil from your hand or some other ‘low surface energy’ material would harm your glue joint.

3. Don’t increase the viscosity of your glue too much. Doing so will inhibit flow of the fluid into the profile and weaken the bond. You can get around this somewhat if low viscosity epoxy is rolled onto the surfaces before applying the high viscosity material.

4. Newly prepared surfaces bond better than old surfaces because they wet better (fewer contaminates).

5. Sanded woods generally wet better than planed woods (better profile for interlocking).

One caution: This is not the whole picture. There are other elements of good adhesive joints besides surface wetting.

David Mancebo

[Richard Smith]

Thanks. Good info.

I've been using Bob S's electric blanket technique to warm the wood prior to wetting out. It really helps with viscosity and absorption of the epoxy into the surface of the wood as it cools.

I think I read somewhere that in certain circumstances, or on certain woods, sanding the surface might adversely affect an epoxy bond. Do you know anything about that?

[erster]

Thanks. Good info.

I've been using Bob S's electric blanket technique to warm the wood prior to wetting out. It really helps with viscosity and absorption into the the surface of the wood as it cools.

I think I read somewhere that in certain circumstances, or on certain woods, sanding the surface might adversely affect an epoxy bond. Do you know anything about that?
Some types of sandpaper releases the gluing agent that can cause a film to be created on the surface. Proper wiping is also key when sanding, depending on your type of sandpaper.

[Pierre LaRochelle]

David,
Do you have any experience with bonding UHMW P.E. plastic? I often wondered how good a grounding shoe the material would make if not for some of its pesky personallities like a high coeff. of thermal expansion and a near impossible surface to bond most materials to.

PL

[Scott Rosen]

Thanks. This is good information.

[paladin]

so....it's not a good Idea to wax the wood before gluing it.......?......:D


....and I assume tack cloths are still used in wiping the surface clear.....

[bainbridgeisland]

David,
Do you have any experience with bonding UHMW P.E. plastic? I often wondered how good a grounding shoe the material would make if not for some of its pesky personallities like a high coeff. of thermal expansion and a near impossible surface to bond most materials to.

PL

Gluing ultra high molecular weight polyethylene (UHMWPE) with epoxy is not practical.

However it is not impossible. Polyethylene has no electrons available for sharing (all P-orbitals are filled for those familiar with chemistry). So electron-sharing bonding (covalent) is not possible. (This is not exactly true since the exterior electrons can be stripped from polyethylene using electrically charged gas, i.e. plasma.) Thus, only mechanical bonding is really available.

So, if one could create an aggressive enough surface, somewhat of an initial bond is possible.

However, polyethylene has a high coefficient of thermal expansion. This means it changes shape more than most materials as temperature changes. Too much strain due to thermal changes between different materials can cause a glue bond to fail. So this limits the surfaces you would want to bond with to those with similar coefficients of thermal expansion.

Also, all polyethylene materials are subject to 'creep'. This is a very slow permanent deflection due to sustained loads. What I am saying is that if you are gripping polyethylene by any kind of mechanical pressure, over time the polyethylene will slowly give in to the pressure. Thus the grip will be diminished.

I would plan to grip UHMWPE with lightly loaded fasteners instead of adhesive. This has proven to be successful.

David Mancebo

[Pierre LaRochelle]

Thanks David!
The use of fasteners and recessed slotted holes in the UHMWPE to allow for movement seemed to be one good alternative...I'll have to experiment a bit before commiting it to the keel. The stuff does appear to have potential for wooden boat applications where bearing and wear surfaces exist, not to mention for sheaves and other running rigging applications.

PL

[G. Schollmeier]

Many dog sleds have their runners treaded with UHMW. The strips are held in place with a dove tail slot; these runners get a tremendous amount of abuse before the strip needs to be replaced.
Gary