At 04:21 PM 7/31/2002 -0400, Mark Holm wrote: >Hi Bill, > >Great discussion of gluing from someome who obviously was paying attention >in Surface Chemistry class.
I teach the Surface Chemistry class. ;-) >I have one little quibble / bit of advice from someone who used to take >apart PVC for a living. It has to do with your statement: > >>After that NEVER TOUCH THE SURFACE TO >>BE PAINTED. I always where vinyl gloves. > >Vinyl often means PVC or a PVC copolymer, though technically it applies to >a wider group of polymers. PVC is a rigid material. In order to make it >flexible, large quantities of an oily liquid known as plasticizer is often >blended in. Plasticizer is exactly the last thing I would want on a >surface to be bonded: lousy cohesion. There is always some present at the >surface of a plasticized PVC. Not exactly. The use of mobile or volatile plasticizers is pretty old technology. Yes, PVC is a brittle, rigid polymer. Modern flexible PVC polymers utilize very bulky side groups along the polymer backbone. These bulky grafted compounds provide the same flexibility as the older oils, but are not volatile and do not move around. Any modern book on polymer chemistry will go into more detail than I am doing here. Yeah, the older cars had that lovely new car smell and would crud up the windows with time. Not so with new cars. Naugahide is nothing more than a canvas fabric impregnated with plastiziced PVC. That new car smell use to be the unreacted polymer and the pasticizer. Carcinogenic and mutanogenic but didn't they smell great! New cars (and some used cars) are often treated with a perfume to allow them to smell like new cars without the problems. The older naugahide would crack with time because the volatiles would leave making the PVC more brittle and allowing it to shrink. I wear PVC gloves without hesitation on bonding surface. At 11:46 AM 7/31/2002 -0400, Don Stackhouse wrote: >First, I'd like to thank Bill Johns for his outstanding discussion of the >details of adhesive bonding. > >I have just a couple of other comments to add. > >There is a tradeoff involved in the cohesive properties of the adhesive. A >joint may fail in tension or shear (where the whole surface of the bond is >stressed), or it can fail in peel (where just the edge of the bond is >stressed, such as when you grab the end of a piece of tape and peel it >off) or in impact. In general, harder adhesives such as many epoxies are >optimized for tension and shear, and because of this they are not as good >in peel or impact. Softer, more rubbery adhesives such as Goop or RTV >silicone sacrifice tensile and shear strength to get more peel and impact >strength. Depending on the nature of the joint and the loads on it, you >may need more of one or more of the other. Excellent thoughts. When I hit the send button on that long post I thought of all the things I didn't write about and joint design is one of them. Your observations are exactly on the mark. For brittle materials cleavage/peel failure loads are often less than 10% of shear. Pure tensile loads are not good either unless you match the Poisson's ratio of the adhesive and substrate closely, often an impossible task. For optimal joint performance of hard/brittle adhesives shear is the best mode of loading. Actually the rubbery adhesives are also much better in shear, but aren't sensitive to crack tip radius as discuss below. Thus, they are used for cleavage/peel mode load applications. >Some epoxies, such as the original epoxy Burt Rutan used for the VariEze, >are "elastomer modified"; i.e.: they have rubbery stuff added to them that >trades off a little of the tensile and shear strength in order to get a >fairly big improvement in peel and impact strength. Among epoxies in >general, a laminating epoxy will tend to be more optimized for the >strength and stiffness of the laminate, and therefore tends to be more to >the tensile and shear direction, while an epoxy designed to be used as an >adhesive will tend to be softer, more towards the peel and impact side of >things. There are appropriate applications for both. For example, the >propeller company I used to work for used one type of epoxy in the >composite laminates themselves, and a different type in the bond between >the metal fittings in the blade roots and the laminates in the composite >blade shell. Good info. The rubber-toughened epoxies are well-developed technology, but hard for hobbyist to pick and choose when selecting a glue at the local shop. This technology is best understood in the area of fracture mechanics. Consider glass as in window glass. You start a crack growing and it takes stunningly little load to get the crack to grow. The stresses at the crack tip scale inversely with the radius of the crack tip and in brittle materials that is of an atomic dimension. With a dispersion of micron-sized particles of various rubber, the crack motors along until it hits one of these rubber blobs and the crack tip radius grows about 4 orders of magnitude. This is a classic toughening mechanism. >As far as the practice of scrubbing or sanding the aluminum, be careful >what you scrub with. --snip-- > DO NOT EVER use steel wool against aluminum. Many shops that work with > aluminum alloy aircraft parts won't even allow steel wool anywhere near > the building! Rubbing steel wool on aluminum leaves thousands of > microscopic steel splinters imbedded in the aluminum surface, which are > nearly impossible to eliminate once imbedded. These proceed to react with > any moisture or other chemicals in the environment to create little > batteries that severely corrode the aluminum surface. Besides destroying > any adhesive bonds that may be involved, these corrosion pits also make > excellent initiation sites for fatigue cracks. Absolutely, good point. If you feel you simply must use a metal wool for a cleaning process, get the matching metal. A little surfing on the web will get you to purveyors of aluminum, stainless steel and bronze wools of a variety of coarseness. I like Scotchbrite pads, they are cheap and available. And I did emphasize doing really good rinse after a scouring with Scotchbrite pad. This is because some scouring powders have various bleaching agents in them and they can be alkaline in nature. Deadly on aluminum if not thoroughly rinsed off. >Carbon (such as the fibers in our composites, or the graphite in a pencil >lead) will also attack aluminum. In fact, there aren't too many things >that carbon won't attack, with the possible exception of titanium, certain >stainless steels, and gold, among a few others. Absolutely. Never bond CF directly to any metal if you intend to have it around for very long. These unforeseen batteries are deadly. >In bonding carbon composites to aluminum, it's generally considered a good >practice to put a couple layers of glass between the carbon laminate and >the aluminum surface. That's what I've done in the past. No, it won't weaken the structure. > Aircraft bolts are typically plated with cadmium to keep the steel in > the bolt from reacting with the aluminum aircraft structures. Zinc > plating would be even better for preventing galvanic corrosion, although > the cadmium is more common in fasteners because of some of its other > properties. Yeah, one neat quality is that it turns colors when it corrodes and a quick inspection with a flashlight will flag areas for attention/repair. The zinc is better but doesn't turn colors with galvanic activity. Cheers, Bill PS. Tack the wood structure together with the CA and then hit it with "Yellow" glue. It will be stronger and tougher than using either adhesive alone. If you've seen me land, you'd know why tough and strong are important to me. ;-) -- There is no such thing as a pretty good alligator wrestler. Bill Johns Pullman, WA RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
