Hi hwuya,
You are asking a some good questions that go to the “heart” of some of the current marketing (and in some cases misinformation) in the industry.
The properties of memory foam are determined by its chemical formulation more than its density. While it’s often true that higher density memory foams are more temperature sensitive and slower responding and firmer than lower density memory foams, there are no “rules” about this. Many people prefer the typically softer “feel” of lower density memory foams even though there would be a durability tradeoff. There is more about the many different properties of different memory foams in post #9 here and post #8 here.
You can see some comments about “so called” soy based or “plant based” foams in post #2 here. While it’s a step in the right direction … it’s mostly about “green marketing” (and in many cases greenwashing) because in most cases the plant oil derived polyols are a chemical derivative of the plant oil and are far from “natural” plant oils (outside of castor based polyols which require much less chemical modification than other plant oils to be used as a polyol because of the hydrogen bonds). I would treat them as close equivalents to regular petrochemical polyfoam and memory foam in terms of durability.
In terms of smell it would depend on the specific formulation of the foam and some plant based foams can have a worse odor than some foams that only use petrochemicals depending on the formulation or the treatment (or in some cases the rancidity) of the plant oil and on how well or completely it reacts in the foam formulation and manufacturing process. One of the limitations of the amount of plant based polyols that can be used in a foam is the odor (although a bigger limitation is the effect on durability and performance with higher plant oil concentrations). Plant based polyols also vary in smell depending on the type of plant oil and the type of chemical modification that it goes through (just like there is a wide range of smell in petrochemical foams) and castor oil is the only one that can be used closer to its more natural form. I would treat odor on a “foam by foam” basis rather than a “category by category” or “generic” basis.
The smell of a polyurethane materials (polyfoam and memory foam) is also a completely separate issue from the safety of the foam because some chemicals that can be smelled are not particularly harmful and some that can’t be smelled are more harmful so the only objective way I know to assess the harmful substances in a foam material that can leach out or the amount of harmful VOC’s that are released is through actual testing and certification. Even with certification … some people will still be sensitive to the smell of some particular formulations of foam or be more sensitive to some of the VOC’s than the limit values … and there can also be differences in the same foam from batch to batch depending on how the formula is adjusted or cured with changing conditions or on how well it is aired out after it is made.
Almost all the memory foam manufactured in the US is made by one of the foam manufacturers that are CertiPur certified*. Individual mattress manufacturers can also choose to become CertiPur certified (even though the foams they use are already certified) but many mattress manufacturers choose not to do this because of the cost involved (although the list is growing*). If the foam is made in the US then the odds are high that it is either CertiPur certified or would pass the CertiPur certification process if it was tested. I would insist on CertiPur certification though if the foam is made offshore because they may be using chemicals or production methods that aren’t used in the US and regulations may be different. Basically the replacement of petrochemical polyols is mainly about using more renewable sources, building a marketing story that attracts “green consumers”, and also is part of a general conservation trend of avoiding petrochemicals which can be more volatile in terms of pricing and less about smell or durability.
ADMIN NOTE: *Removed 404 link|Archived Footprint 1:certipur.us/pages/for-industry/find-a-foam-supplier/|Archived Footprint 2:certipur.us/pages/for-consumers/find-products/ & replaced with latest CertiPUR list
The deeper support layers of a mattress are not generally the weak link of a mattress and once you are past about 2.2 lbs density with conventional or HD foams there would be little difference in durability anyway because higher density than this is usually accomplished with fillers. Foams that are 2.5 lb density or higher are not necessarily HR polyfoams (or cold cured foams) unless they also have a compression modulus of 2.4 or higher and 60% resiliency or higher. True HR polyfoam on the other hand does have higher polymer densities but it’s biggest advantage is that is has better performance and is more “latex like” in its properties. There would be little “practical” difference in terms of durability in a support core compared to say a 2.2 lb conventional foam but depending on the construction it could make a bigger difference in the feel and performance of the mattress for some people. Whether it was important or not would depend on whether it was an improvement in terms of PPP. In most memory foam mattresses (again depending on the specific design) most of what you “feel” would be the memory foam layers.
Gel foams on the other hand may have less smell because the gel materials themselves are non toxic and have little offgassing or smell and anecdotally they appear to have less odor issues “in general” than non gel foams. The gel also has other effects on the foams but there is also a great deal of misinformation in the market about gel materials in general.
Post #2 here and the posts it links and leads to to can help “unravel” some of the information about gel materials (or gel memory foams) on the market because there are many different versions of gel memory foams and many “marketing stories” that may not all be accurate.
As a very broad overview … gel will have some smaller or temporary effect on cooling (depending on the percentage and type of gel), it can have a positive effect on the tendency of memory foam to become softer over the course of the night with increasing temperature or with continuous pressure, and it will generally have a “neutral” effect on foam durability overall although some of the earlier gel formulations with larger particles that can migrate out of the foam more easily can have a more negative effect on memory foam durability and some of the liquid gel additions can have a more positive effect on foam durability because there are no particles and it can form its own polymer matrix when the foam is manufactured. While I haven’t seen any specific testing information about VOC’s in gel foams on a foam by foam basis … it would also make sense that it would have a positive effect on VOC’s and smell because the gel itself is non toxic.
Memory foam heats up throughout the material and having good airflow that has an “escape route” to the outside environment can help with removing any heated air. The movement of the body would provide a “pumping” action to help the air escape from the sides of the vents. Without a method of allowing the air to escape under a memory foam layer (either through the material itself or through vents) there would be nowhere for the warmer air to go. This type of system (punched holes with a ventilation system underneath it) is more effective than simply punching the memory foam full of holes but where the air can’t escape to the same degree (except through the material itself). While the holes may have a small effect on durability “in theory”, I also don’t believe it would have any significant effect on durability in real life because the foam structure is the same (just like the pincores in latex don’t affect durability) and there isn’t enough material removed to make a significant difference.
There are also other methods of reducing the temperature of memory foam that are discussed in post #6 here and there is more about the many factors that can affect the sleeping temperature of a mattress or sleeping system as a whole in post #2 here.
Higher density memory foam will be more durable than lower density until you reach about 6 lbs or so and after that the increasing durability effects of higher density are greatly diminished. As you mentioned … in some higher density memory foam formulations can also result in a foam with reduced durability because of the greater difficulties involved in production or curing of the foam. Basically I would say that “more is better” up to about 6 lbs and beyond that it’s more about the “feel” and performance of the memory foam than it is about durability.
Short term compression of foam materials generally won’t harm them (with a few exceptions of “stiffer” or firmer foam) but if the foam is compressed for more than a few weeks … especially in a humid environment … then it can affect the durability of the foam. Using pressure compression to roll pack a mattress is generally less harmful than vacuum compression to the cell structure of a foam (particularly if the vacuum compression extracts all the air) and in both cases it’s a matter of time and degree. Most foam manufacturers will provide good information to their customers (the mattress manufacturers) about which of their foams is more suitable for compression and which ones may not do as well.
Hope this can help you make a choice or make more meaningful comparisons between some “good vs good” options :).
Phoenix