Jim et al wrote: > I am getting to the end of my first S & G kit and > have a few questions for the construction gods. > You know who you are so don't be shy. > I'm probably not one of the construction gods you had in mind, but will take a stab at your questions. > What determines the weight of fiberglass cloth? > Say 4 oz verses 6 oz. > The weight designation is in units of weight per square yard. A heavier cloth may have thicker fibers and/or more fibers per inch in the weave. > Is there a good substitute for wood flour? > Depends on the application. Wood flour produces a very strong putty because the wood absorbs the resin and the wood fibers themselves are mechanically strong. Milled fiberglass will also form a hard putty due to the entanglement of the fibers. Microballoons do not absorb the resin and their spherical shape will not form a three dimensional matrix (but can be faired to a smoother surface in a non-structural application). > Why doesn't epoxy bond to older epoxy > (say > 3 days old) unless it has been lightly > sanded first? > If the epoxy is not yet fully cured, application of the new epoxy resin will allow it to *chemically* bond to the prior application. If the epoxy fully cures, there can be no such chemical bonding between the two applications, and the surface must be roughened to provide microscopic grooves for the new epoxy to *physically* adhere to. Also - as the epoxy cures, an amine 'blush' (non-reacted chemicals from the hardener) can rise to the surface of the epoxy, which can inhibit bonding. As the chemistry of the epoxy differs from different manufacturers, it is best to read the technical literature for the exact product that you are using, and follow the manufacturers instructions rather than relying on general epoxy 'wisdom'. > Why does slow cure epoxy create a stronger > bond than 5 minute epoxy? > I don't think the speed of the cure has anything to do with the functional strength of the bond. The epoxy cure is a non-reversible chemical reaction (a thermoset) that creates new chemical bonds, and the epoxy chemist/formulator can design the epoxy resin to work best for different applications, e.g. boatbuilding vs. recoating bathtubs vs. electrical component manufacture. Not only can the epoxy resin - generally based on bisphenol A - properties be modified, as can the amine hardener properties, other additives - reactive or inert - can be added to the system to optimize the epoxy's use in different applications. Properties that can be influenced by design include but are not limited to: speed of cure, viscosity of the resin prior to cure, wetting ability of the resin, flexibility of the resin after cure, ability of the system to tolerate wider ranges of resin-hardener combinations, electrical properties, etc. On the MAS site, there is a statement that their fastest curing epoxy systems forms the strongest bonds. The five-minute epoxy may be formulated with more reactive sites per unit weight of the resin and a more reactive amine, so such a product may be expected to be more brittle/less flexible than a slower-curing epoxy. The challenge in using a five minute epoxy lies in the fact that it's window of usability is so small. There exists a greater likelihood of not fully wetting out the cloth or having the epoxy start to gel before the user is done applying the epoxy to the site of construction/repair. If this happens, the repair/construction of the item with the five-minute epoxy will not be as 'good' as that made with a slower cure product. In other words - the perception of one product having a stronger bond than another involves many factors besides how fast the resin cures. Good online resources for epoxy information are the web sites of the manufacturers for epoxies typically usedin boat buiding (spend some time browsing): http://www.westsystem.com/ http://www.masepoxies.com/ http://www.systemthree.com/ http://www.raka.com/ SAFETY INFO safety info about the epoxy you are using is a MUST READ, as your health can be impacted. Even though I have been mindful of safety considerations, years of epoxy use have resulted in my being sensitized to the point of having to wear a respirator with organic vapor cartridges when I work with epoxy (else I suffer a tightness in the chest that takes more than a day to go away). http://www.masepoxies.com/safety.htm http://www.westsystem.com/ (link to safety info under green 'Go To' bar on left, or do a search for the term 'MSDS' to see not only the MSDS, but a link to General Safety Guidelines) TECHNICAL AND SAFETY http://www.raka.com/epoxyman.htm FAQ/TECHNICAL INFO http://www.masepoxies.com/mas8.htm http://www.systemthree.com/ (download their epoxy manual) http://www.westsystem.com/ (click on gray "Using West System Epoxy" tab on top) Also check out bulletin boards and newsgroups: http://www.kayakforum.com/cgi-bin/Building/index.cgi rec.boats.building Erik Sprenne *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Mon Aug 04 2003 - 06:42:27 PDT
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