Polyurethane (also called polyfoam or just poly) is the most common type of foam you will see in mattress and in its medium or higher grades can be successfully used as a support layer. Unlike its memory foam cousin, it is made to compress under pressure rather than soften or (melt) under pressure and heat. This gives it a higher resilience and progressive resistance than memory foam and it can hold up the heavier parts of the body much more effectively. This is the reason it can be used as a support layer while memory foam can not. It comes in 3 basic grades and the lower grades have less resilience and less progressive resistance than the higher grades. All its grades can be made in softer or firmer ILD's although the middle HD grade tends to be more commonly available in firmer ILD's. Softness or firmness is not in any way an indicator of its quality and the best indicator of quality and durability in polyfoam is density or weight per cubic foot (in both polyfoam and memory foam). In this article we will deal mainly with polyurethane used as a support core of a mattress however I would also encourage you to read about its use in the comfort layers section as polyfoam in general used in comfort layers is the source of many of the biggest issues in mattress manufacturing and the body impressions and material breakdown that are so common today.
Regular conventional polyfoam: This is the lowest grade of polyfoam and weighs less than 1.5 lbs per cubic foot. It is the least expensive and is not really suitable for use in a mattress at all ... either as a soft comfort layer (unless it is in the range of around an inch or so or less in a quilting layer) or as a support material ... unless the mattress is for occasional use or is meant to last a very short time. Mattresses with regular polyfoam in a support layer should be completely avoided for regular use as they will break down quickly and do not have the desireable qualities that are needed in a support layer.
High Density conventional polyfoam (HD): This is polyfoam that typically weighs 1.5 lbs per cubic foot or more (although I would use 1.8 as a better standard for HD unless you are in lower budget ranges) and can go as high as the mid 2 lb range. It is sometimes difficult to find this in a softer version but the firmer versions are quite suitable for a support layer under memory foam or under a softer top layer of good quality polyfoam or latex. It lasts longer and will keep its firmness longer than lower density conventional polyfoam (depending on the density) but typically not as long as a high quality innerspring or higher grades of polyfoam, or latex foam. It is less resilient and has less progressive resistance (doesn't hold you up as well) as the highest grade of polyfoam, innersprings, or latex. Although it is inexpensive to manufacture, you will often see this grade of foam in the support layers of mid range and even rather expensive mattresses under higher quality materials such as latex or high quality memory foam. The lower end of this range would be a suitable material for a lower or mid range mattress and the higher end of this range would be more appropriate for mattresses that were in a higher budget range. Since the support layers of a mattress are not usually its "weak link" ... I would consider this grade of polyfoam ... especially in the higher densities ... to be perfectly acceptable and a durable choice in a mattress with good quality comfort layers and of course higher density would indicate greater durability and quality. There are some good quality mattresses use this grade of polyfoam (usually in the 2.0 lb range and higher) in their mattresses.
High Resiliency polyfoam (HR): This is the highest grade of polyfoam and weighs 2.5 lbs per cubic foot or more and uses a different formulation than conventional polyfoam. It also must have a support factor (progressive resistance) of 2.4 or higher and resilience of 60% to qualify for this grade. Because it is made with a different combination of chemicals than the other two grades, it is more durable and has better properties than the lower grades and since it can be made from very soft to very firm, it can be used successfully anywhere in a mattress. Because it is also more expensive to manufacture than the lower grades though, and is not as profitable to use, you will rarely see it in mattresses outside of local or regional manufacturers who use higher quality materials in their mattresses. As a support material it would be suitable for use anywhere that latex foam or an innerspring would be used. It is more expensive than many innersprings and less expensive than latex so the price of your mattress should reflect this.
One caution worth knowing about is that HR polyfoam is among the most commonly mislabeled foams and it often seems that any polyfoam that has a density of more that 2.5 lbs/cu. ft. or even lower is called HR yet you will not be told that it does not have the required support factor of 2.4 or higher to be HR. The internet is filled with false claims of HR polyfoam. If you even ask some of these merchants or even many retail outlets about "support factor" or use another term for the same thing (comfort factor, sag factor, or compression modulus) the sales person will often look at you as if you were from another planet and make up an answer that is vaguely connected to how comfortable the mattress is rather than admit that they know very little if anything about one of the most important properties of a mattress core material. These higher density foams that are actually HD grade will not have the same desirable qualities or durability of true HR polyfoam even though the price of the mattress may be as high. The best true HR polyfoams come close to latex and are very good quality, have good resilience, and other qualities and will last for many years.
Unless you specifically know the quality of polyfoam in a mattress core, it is usually wise to assume that it is HD and it should be priced accordingly.
In a lower cost "throwaway" or lower budget mattress, or in a slightly higher priced mattress with high quality materials above it in the comfort layers, HD polyfoam used as a support layer may be part of a good value mattress. Higher densities of HD polyfoam or HR polyfoam can be a good, slightly less expensive alternative to latex in a core layer for those who are on a more restricted budget and are looking for a good quality foam mattress core.