Ron wrote: >I don't find it in Understanding Wood. Must be looking in the wrong >place. I did find mention of "tends to yield under continued stress" >in the Wood Handbook, with no differentiation made between white and >yellow PVA. There is a considerable and obvious difference between >the two, as anyone familiar with both knows. It also mentions >crosslinkable PVA, which would be the Titebond II equivalents, with >no mention of creep whatsoever. These are all good points. One of the improvements in Titebond II is much reduced creep resistance compared to the white stuff. Mind you even regular titebond II will creep under enough sheer stress. Hide glue will not creep, ever...it's impossible. A hide glue joint can only break, like glass, due to the cross-linked structure. OUTDOOR titebond II is supposed to be a cross-linking PVA derivative of regular titebond II, so presumably that stuff ought not to creep at all either. >I'd still like to find some real information on this for my own >education, if nothing else, should anyone happen across it somewhere. Many of the so-called official glue tests, like the kind you see reported in govt. publications, Fine Woodworking, and especially the industry documentation are simply incorrect, or at the best inaccurate. For instance, most documents list hide glue as having poor resistance to water, AND poor resistance to heat, neither of which is true. Only both of these in combination (e.g. steam) will weaken a hide glue joint. [and incidentally it is possible to remove a hide-glue glued in soundboard without damaging it if you use the right methods]. As for heat, hide glue is used to glue the abrasvie on sandpapers because it is so resistant to heat (friction). And I've seen a piano whose spine had lain in water for decades, enought to rot the wood, yet the hide glue joint was still tenacious. but I'm digressing... Ron asks for evidence - I don't know any quantitative tests, but there is plenty of anecdotal evidence for creep failure of PVA joints subject to excessive sheer stress. Some specific applications that are problematic would include severely bent laminations, cross grain joints with thick components, and especially joints with dissimilar materials. The last two produce very high sheer stress due to differential response to moisture and I have heard of such joints failing (again I don't know if those specific anecdotes were titebond or titbond II, and it is important which one it is). Another problem with titebond is its weak resistance to sheer stress when subject to heat - at least I know the white stuff is, not sure how good the II is on this score. Could be tested pretty easily. I've seen stressed joints fail completely in sheer stress tests at only 200F - hide glue in a side-by-side comparison with increasing temperatures had not failed by the time the wood was charring. Luthiers are very cautious in this respect, because it's not unheard of for naughy violinists to leave their instruments in a car in the sun and have them pop apart if not glued with hide glue. Harpsichord and piano builders who glue cases together with titebond risk problems later, and I know of cases that have literally torn themselves apart at the joints after several years of sheer stress from string tension. This generally results in the board moving north into the gap and jamming the works. Even with titebond II I would not trust a 5 ton string tension on a big Viennese piano. I agree with Ron that most of the bad stories of PVA creep are related to the earlier white stuff. Titebond II is different not just in the colour. But it is still not immune to creep and I would avoid it in the specific applications where excessive sheer stresses can develop. Only a cross-linked glue joint had zero creep. Not that I see any need whatsoever for using any other type of wood glue than hide glue for anything, if only because it's so much easier and pleasant to use it than the muck in the squeeze bottles. ...but that's another rant. Stephen -- Dr Stephen Birkett Associate Professor Department of Systems Design Engineering University of Waterloo Waterloo, Ontario Canada N2L 3G1 Davis Building Room 2617 tel: 519-888-4567 Ext. 3792 PianoTech Lab Ext. 7115 mailto: sbirkett@real.uwaterloo.ca http://real.uwaterloo.ca/~sbirkett
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