Support Factor
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http://www.pfa.org/intouch/new_pdf/hr_IntouchV3.1.pdf The ILD (aka IFD) numbers we toss around are at 25% compression. The Polyurethane Foam Association refers to this as "surface feel" or "firmness". Foams are also measured at 65%, but this number is almost never mentioned. The importance of this number is that two foams can have the same ILD at 25%, but quite different ILD's at 65%. The one with the higher 65% number will be more supportive. This is sometimes referred to as "deep down support." The ratio of the 65% ILD to 25% ILD is "support factor," or "compression modulus." Support is a key function of flexible polyurethane foam. In many ways, it is the most important function foam can provide. In many ways, foam support and firmness are interrelated. In fact, compression modulus is measured by taking the ratio of two foams firmness measurements. And foams with high levels of support can actually feel firmer than foams with the same density and surface firmness measurement but which have lower support levels. Generally speaking, the higher the density, the more "supportive" a foam will be. Typical poly foams will have support factors under 2.0, "High performance" foams will have support factors above 2.0. The higher the density, typically the better the ability of the foam to provide support. This thread in the old forum contains some good discussion on the difference between "firmness" and "support".Support is also important in that foam with higher support can be specified with softer surface IFD, without sacrificing the ability of the foam to bear a load. This message was modified Sep 9, 2007 by haysdb
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Re: Support Factor
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The problem with talking about support factor is that it's of limited practical use since we almost never know either the support factor or the 65% ILD so that we can calculate it ourselves. What I personally come away with is that a person seeking firmness needs to look for not only high ILD's but high density as well. Note that Latex can be (mostly) excluded from this discussion because latex density and ILD are directly related - as latex ILD increases, so does density. This is not true for other types of foam. With memory foam, density has always been a part of the discussion, so with memory foams it's well understood that density and "support" are directly related. Support factor becomes most meaningful when talking about polyurethane foam. There isn't much discussion of PU foam here, but this is definitely where PU foam gets a bad, albeit richly deserved, reputation. The problem (I think) isn't that PU foam is bad, per se, but that low density PU foam is bad. High density (HD and HR) poly foams can be of a quality approaching that of latex [1]. The problem is, such high quality PU foam is rarely used. Support factors of PU foams can range from 1.6 or even less, to as much as 2.9. [2] This is more theory than something I can prove, but I believe a conventional mattress (either innerspring or all foam) can be of high quality if made with high quality PU foam. The density of PU foam matters because it's directly related to support factor and longevity, and more attention needs to be focused on this. [1] High Resilience Polyurethane Foam with the Performance of Latex Foam Latex-like flexible polyurethane foam and process for making same (a link to BASF's patent) [2] I don't actually know what the low-end is, but I do know that poly foam exists with a support factor as high as 2.9. I have already linked to that source earlier in this thread so I will not do so again here. This message was modified Sep 15, 2007 by haysdb
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Re: Support Factor
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Post deleted by author. I incorporated this post into the previous one. This message was modified Sep 15, 2007 by haysdb
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