High resilient foams

Commonly used isocyanates are toluene dhsocyanate, methylene diphenyl isocyanate, and polymeric isocyanates. Polyols used are macroglycols based on either polyester or polyether. The former [poly(ethylene phthalate) or poly(ethylene 1,6-hexanedioate)] have hydroxyl groups that are free to react with the isocyanate. Most flexible foam is made from 80/20 toluene dhsocyanate (which refers to the ratio of 2,4-toluene dhsocyanate to 2,6-toluene dhsocyanate). High-resilience foam contains about 80% 80/20 toluene dhsocyanate and 20% poly(methylene diphenyl isocyanate), while semi-flexible foam is almost always 100% poly(methylene diphenyl isocyanate). Much of the latter reacts by trimerization to form isocyanurate rings. 

In addition to freedom from bottoming out , most people prefer a seat which effectively provides a soft surface with a firm interior. One measure of the relationship between such surface softness and inner support is the sag factor or support factor. In one commonly used test this is obtained by dividing the force required to compress a foam by 65% of its height by the force needed to obtain 25% sample compression. This generally increases with density but is typically <2.5 for a conventional slabstock foam but >2.5 for a high-resilience foam. 

There is also growing interest in multi-phase systems in which hard phase materials are dispersed in softer polyether diols. Such hard phase materials include polyureas, rigid polyurethanes and urea melamine formaldehyde condensates. Some of these materials yield high-resilience foams with load deflection characteristics claimed to be more satisfactory for cushioning as well as in some cases improving heat resistance and flame retardancy. 

The two primary hydroxyl groups provide fast reaction rates with diisocyanates, which makes this diol attractive for use as a curative in foams. It provides latitude in improving physical properties of the foam, in particular the load-bearing properties. Generally, the ability to carry a load increases with the amount of 1,4-cydohexanedimethanol used in producing the high resilience foam (95). Other polyurethane derivatives of 1,4-cyclohexanedimethanol indude elastomers useful for synthetic rubber products with a wide range of hardness and elasticity (96). 

Molded flexible foam products are becoming more popular. The bulk of the molded flexible urethane foam is employed in the transportation industry, where it is highly suitable for die manufacture of seat cushions, back cushions, and bucket-seat padding. TDl prepolymers were used in flexible foam molding in conjunction with polyether polyols. The need for heat curing has been eliminated by the development of cold-molded or high resiliency foams. 

HR foam (high-resilient foam) is sometimes classified as a cold-molded foam because it can be molded at low temperatures. However, HR foams have slightly different formulations than standard cold-molded foams. There are many kinds of molded-foam formulations in the literature. The following formulations are only a few examples. 

High-Resilient Foam (HR Foam). HR foam is characterized by high ball rebound, low hysteresis loss and high sag factor. HR foam can be produced either as slabstock or as molded foam. 

The transportation industry has many of the worldwide auto producers represented in seven Latin American countries. Brazil is the volume leader (over 1 million autos and trucks in 1979), followed by Mexico, Argentina, and Venezuela. These auto producers are rapidly adopting the concept of molded polyurethane foam seating including the superior cushioning of high resiliency foams. The pressure for increased fuel economy via weight rein Urethane Chemistry and Applications Edwards, K., et al. ... 

In Figure 17, the absorbance due to substituted ureas at 1645 cm" for the three formulation at early times of reaction are given as functions of time. These data are compared with the rise profile, gel profile and cell-opening times previously discussed, Figures 4-10. Concentrations of ureas are measurable from about the time of the peak rate of foam rise and achieve an apparently limiting value after about four minutes, about the time foam rise is complete. "Gel profile" appears to monitor structural developments in the foams which are occurring at the same time as the increase in urea concentration. In the "HR" formulation, onset of "gel" is delayed. This delay in onset of "gel" in high resiliency foams has often been observed (X, ]). 

No such delay, however, is observed in formation of diarylurea. While no direct measurement is yet available, it is taken that "gel profile" measures association of the polyureas into domains. In "HR" foams this domain formation is delayed, permitting larger, more completely aligned polyurea domains and accounting for the principal property characteristics which distinguish high resiliency foam 3). 

The creation of the autocatalytic high MW aminic polyols based on N-methyl substituted poly amines, represents an important development in the area of poly ether polyols for low-fogging flexible PU foams. The VORANOL VORACTIV polyols developed by DOW represent a revolutionary group of autocatalytic polyols with reduced volatile organic compounds (VOC) emissions in PU products, especially in high resilience foams for bedding and automotive seating [149, 150]. 

PIPA polyols, in spite of some disadvantages (tendency to foam shrinkage, and scorching), are used successfully for continuous slabstock flexible PU foams and high resilience foams (cold cure moulding process). 

Cold-cure foaming is used in the production of what is known as high-resilient foams having high sag factor (i.e., ratio of the load needed to compress foam by 65% to the load needed to compress foam by 25%), which is most important to cushioning characteristics. True cold-cure foams will produce a sag factor of 3-3.2, compared to 2-2.6 for hot-cured foams. 

MDI-based systems. Furthermore, changes in polyol technologies designed to improve seat comfort factors can have a negative impact on foam openness and dimensional stability. Specifically, the comfort of high resilience foam may be related to the degree of dimensional stability or foam openness. 

When producing flexible high resilience foam, it is important to provide a wider TPR window to expand processing latitude and at the same time maintain or improve physical properties. This should result in reduced scrap and/or repair rates, providing improved economics for the polyurethane producer. 

Ge J, Zhong W, Guo Z, Li W, Sakai K. Biodegradable polyurethane materials from bark and starch. I. Highly resilient foams. J Appl Polym Sci 2000 77(12) 2575-80. 

Table 4. Typical properties of "high resilience" foams.

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