Ask Dr. Dave<br>March 2002
QuestionI am doing work on my boat. What should I use for sealing on 1/2-inch-thick acrylic windows, which will be screwed into teak? Also, what would you recommend for bonding fiberglass to steel? When I was in the military, we used a two-part mix, which was great. Will I have to use a two-part sealant and have to etch the surfaces chemically or mechanically?
AnswerThe two-part sealant you used in the military was probably a polysulphide, which is still the type of sealant quite commonly used in boats because of its excellent water resistance. It is not usually recommended for bonding plastics. However, some modern one-part adhesive sealants will almost certainly do the job for you. Marine-grade polyurethanes and silicones will bond well to the three surfaces you mention, although the suppliers for the acrylic may recommend a primer. Both polyurethanes and silicones have good flexibility, which is important in a boat application. The essential difference between them is that the polyurethane will have much higher adhesive strength, which is usually an advantage, but can be a problem if you may want to disassemble the parts later. You should also ask your supplier for UV-resistant-grade sealant for the window application.
QuestionPlease explain to me how anaerobic sealants work and why they don't harden when I get excess on the outside of a joint.
AnswerOne of my former colleagues once gave me a great description of anaerobics. Consider them as materials that behave in a completely opposite manner to a conventional air-drying oil paint. Oil paints are cured by contact with air, while the curing of anaerobics is inhibited by air. The oil paint comes completely sealed in an air-impermeable, tin-plated can and, when brushed on a surface, cures by reaction with atmospheric oxygen, accelerated by so-called metallic "driers" in the paint.
An anaerobic sealant on the other hand is always supplied in a thin-walled plastic bottle or tube that is usually only half filled. This plastic is usually low-density polyethylene, which is highly permeable to oxygen. Anaerobics, as their name implies, only cure when oxygen is excluded, like inside an assembled joint, and are also accelerated by certain metals such as iron or copper, which usually come from the surfaces being assembled.
If you leave an anaerobic exposed to air, like the excess on the outside of a joint, it will never cure because of the stabilizing effect of atmospheric oxygen.
QuestionWe manufacture large numbers of a clear-polystyrene filter assembly for a portable, medical oxygen-supply system. This assembly is attached mechanically to an ABS tube. Because this is a single-use device, we would like to bond the assembly to the tube permanently to prevent re-use of the filter after it has been discarded.
AnswerThese plastics can be bonded with several adhesive types, but it seems that you need something that will be so strong that the plastic will shatter if someone tries to de-bond the parts. Although solvent cements will bond these plastics, I recommend that you contact a manufacturer of 100%-reactive acrylic adhesives, for two reasons: 1. To avoid using toxic or flammable solvents and, 2. To obtain a fast assembly technique that is suitable for your high-volume production.
These adhesives can be formulated with combinations of many acrylic monomers, both acrylates and methacrylates. The acrylic adhesives are available as single-component, heat-cured adhesives; as two-part adhesives; or can be single-component, UV-cured systems.
In your case, I would avoid the heat-cured system because of the low softening points of the plastics and suggest that a UV system would give you tremendous advantages over a two-part system in terms of productivity. Choose the strongest, fastest system available, but also be careful to check that the system you choose does not stress crack the plastic in the liquid state.