Density vs strength is the gauging criteria for choosing physical properties and PolyU is stronger when considering this. Temperature effects is another point of consideration and PolyU is better in this respect also.
KRron ----- Original Message ----- From: "Stephen Jacobs" <ask...@microlink.zm> To: "'KRnet'" <kr...@mylist.net> Sent: Friday, July 23, 2004 7:33 AM Subject: RE: KR> Foam vs. Foam I understand the physical (chemical) differences between Styrofoam (polystyrene?) and Polyurethane. I have used Styrofoam in various densities from 2 to 6 lb/cub ft - the higher density stuff appears to be mechanically as strong (or stronger) than PolyU when used as a composite core material - is this correct? This may be a naïve question as I have not seen (touched) the stuff you call Lastofoam (Divinicel?). Maybe this is a mechanically stronger polyU, but the stuff locally available is very fragile - I can poke my finger all the way into it without much effort and the surface readily crumbles almost to a dust when rubbed - not an ideal surface to glue anything to. My wife uses it for flower arrangements. Styrofoam has its own draw backs in that Vinyl Ester resins (and most solvents /avgas) will attack it. I need to appreciate the structural qualities /suitability for use in the KR type construction. In a perfect world (and the US) a large variety of core materials can be purchased without any trouble - in fact, someone will deliver whatever you desire to your front door. This is not the case for me in Africa. I can however source good quality (not re-cycled) white Styrofoam in many densities from South Africa. The stuff is easy to "slice" into sheets of any thickness with a hot wire. It does not sand as well as polyU, but the higher density stuff is not too bad - it can be done. I am looking to do Mark L style sandwich wing skins - glass on both sides of the skin core. The logistics of shaping the material aside - can Styrofoam be used to the same good effect? The theory says that the strength of a composite sandwich increases almost exponentially with thickness. I cannot see that polyU will outperform Styrofoam in compressive or tensile strength - maybe the shear capability is the issue? I will appreciate any informed advice Regards Steve J _______________________________________ to UNsubscribe from KRnet, send a message to krnet-le...@mylist.net please see other KRnet info at http://www.krnet.org/info.html
