Reprinted from the ACJ - June, 1993
Ever actually see a man hanging from a girder by his hardhat? I always wonder if OSHA has any regulations concerning free hanging from girders on construction sites. Seems a bit unsafe to me. BUT NOT if you are using super duper maxi-bond hardhat epoxy!
Epoxy seems to be the savior of the modern world. Well, maybe not the savior, but it sure comes in handy repairing anything from broken plates and toys to sealing radiator headers and nylon tanks.
The term epoxy has taken on a very broad definition over the years. In very general terms, it is used to describe a two-part adhesive system. Neither part of the system is a glue by itself, but when the two parts are mixed, a chemical reaction occurs that causes the epoxy to harden and secure the piece being repaired.
The two parts are normally called a resin and a catalyst. More appropriate for our discussion is the use of epoxies as coatings to seal and protect surfaces, such as nylon repair compounds, header seal epoxies and gas tank liners.
Epoxy systems have many good qualities that make them convenient and very effective glues and sealants. One of the most appealing virtues is that the hazards associated with epoxy systems are minimal.
With ordinary glues and adhesives (one-part systems), something is lost, usually a solvent, allowing the glue to harden and hold together the parts or form a hard coating. With paste-type glues, the lost solvent is mostly water. With airplane/model glues it's a more volatile solvent such as toluene.
With this solvent loss comes a few problems. One of the drawbacks is shrinkage. As part of the glue disappears/evaporates, gaps or pockets may form—a big problem if you are trying to fill a tube to header leak.
More important from a safety viewpoint is that the solvents lost may be toxic or more often flammable. This solvent loss is called outgassing. A major advantage of epoxy systems is little or no outgassing, thus little shrinkage and no nasty fumes. A side light on the solvent loss is that most glues need air to interact with the glue to allow drying. Not so with epoxies. They cure by chemical reaction and don't need air exchange. They can even cure underwater.
When the two parts of an epoxy system are mixed, a reaction begins which causes links to grow throughout the glue and facilitates the solidification process. Normally this process is exothermic; that is, it gives off some amount of heat. Some systems give off a little heat, some a good deal. One of the hazards associated with epoxies is this heat.
Generally speaking, chemical reactions proceed faster when heated. So as the epoxy hardens, it produces heat, which causes it to harden faster, which produces more heat, which causes it to harden faster … etc. You may be able to see the dog-chasing-his-tail aspect of this situation.
The hazard may be apparent also. If the incorrect amounts of resin or catalyst are mixed, or if a extra large amounts of the parts are mixed, or if a resin and catalyst not designed for each other are mixed, a condition known as a "runaway" can occur. That is when the heat-reaction cycle goes berserk and so much heat is generated that the epoxy itself or some material nearby (rags, paper) ignites. Runaway reactions in chemical production plants have levelled the facilities on occasion.
Now I have yet to hear of a radiator shop destroying itself by an incorrect nylon tank repair, but the concept of the runaway reaction is of concern to us in minor ways. If someone were to mix together the leftover pint of resin with the leftover catalyst and toss the whole thing into the trash, it is possible that the reaction could heat up enough to start the trash on fire.
Often times small amounts of catalyst are enough to cure (harden) larger amounts of resin, so be careful not to use the same stick to mix both parts. You may come back to a rock solid can of resin the next time you want to use it.
Skin contact is a concern when working with epoxy systems. Skin "sensitization" can occur. The curing agents (catalyst) are often amine type compounds and may cause a rash if your skin is exposed to them. Your skin becomes sensitized to the material and all it takes next time you open the can is the fumes to touch your skin and the rash reappears.
Many catalysts are also severe eye irritants. Even a small amount of the material touching your eye can cause scarring. Always wear goggles and wash your hands thoroughly after each use.
The strength and tackiness of curing epoxies can cause another problem: skin bonding. I'm sure most of you have temporarily glued your fingers together with superglue—and the same thing can happen here. So be aware of the cure time the epoxy has. It can be days or as quick as five minutes, and wash up before you have to go through life with hands like Mickey Mouse. I always wear disposable gloves. That way I don't have to worry about skin bonding, sensitization or rubbing my eyes causing irritation later.
The inhalation hazards are minimal for most epoxies, but be careful of the smoke if the epoxy is burned. The smoke is highly toxic. So be sure to work in a well ventilated area when burning off that header seal epoxy. An organic vapor mask would be a smart piece of protective equipment as well.
Ingestion of most epoxy systems causes only minor problems, as few are poisonous. But seeking a medical opinion in case of ingestion is recommended. The same is true for eye contact.
For once, we use a product at work that isn't a cold-blooded killer waiting to strike the first time you make a mistake. Epoxies are very versatile and are not terribly dangerous to handle. Skin irritation and sensitization, and especially eye contact, should be of constant concern when using these compounds. But in general they are safe and will fill a great many more niches in the future.
The only long-term hazard I can think of is illustrated by the new series of commercials for that same glue. That guy is still hanging from that beam!
The above article was written by David M. Brown, Chief Engineer of Johnson Manufacturing Company, Inc. and is published by JOHNSON with the expressed approval of the National Automotive Radiator Service Association and the Automotive Cooling Journal. Other reproduction or distribution of this information is forbidden without the written consent of JOHNSON and NARSA/ACJ. All rights reserved.
JOHNSON MANUFACTURING COMPANY
114 Lost Grove Road / PO Box 96 / Princeton, Iowa 52768-0096
Phone 563-289-5123 or Fax 563-289-3825