Viscoelastic foam, also known as memory foam or temper foam, was first commercialised during the mid 60’s as a result of NASA’s AMES Research technology transfer program. This open-cell variety of flexible polyurethane foam (FPF) was distinguished by properties allowing it to redistribute the G-Force suffered by astronauts during take-off and re-entry, and providing commercial pilots a more comfortable seating surface during long flights. Although viscoelastic memory foam production technology has been available for more than 35 years, commercial products have only recently been made widely available to consumers. Viscoelastic pressure distribution performance represents one of the most significant comfort innovations from the FPF industry.
Memory Foam Characteristics
Viscoelastic memory foam is typified by its slow recovery after compression. When a weighted object (for example, the human body) is positioned on a viscoelastic memory foam mattress, the foam progressively conforms to the shape of the object, and after the weight is removed, the foam slowly reassumes its initial shape. Due to this gradual recovery, viscoelastic memory foam also can be described as “slow recovery” foam.
Other characteristics include viscoelastic memory foam’s ability to dampen vibration as well as absorb shock. This latter performance characteristic is confirmed in the laboratory using ball rebound tests. Ball rebound of less than 20% (compared to 50% – 60% with other varieties of FPF) supports the description of viscoelastic polyurethane foam as “dead” or “low-resilience” foam, in that it lacks the surface “springiness” of other FPF products. In fact, certain viscoelastic memory foam products claim to absorb up to 90% of impact. While lack of resilience may appear to be a disadvantage, as discussed later, a “dead” foam can be highly desirable in some applications. In addition to these key advantages, many viscoelastic memory foam products also react to body temperature and ambient temperatures, softening with heat and more easily adjusting to body contours.
Benefits & Applications of Memory Foam
Viscoelastic memory foam’s unique physical characteristics have led to its popularity in the bedding and medical industries. Due to its conforming aspect, viscoelastic memory foam material makes for a comfortable yet supportive mattress or mattress pad, and its low resilience works well in bed pillows.
People with impaired mobility, confined to wheelchairs or hospital beds (especially burn victims) can benefit from the memory foam mattress’ capacity to redistribute weight and surface pressure, potentially reducing decubitus ulcers (bed sores) caused by aggravated pressure between the skin and bony areas of the body such as beneath heels, hips, elbows and the back of the head. If pressure is not relieved through body movement or technology, such as a viscoelastic memory foam mattress surface, blood vessels may compress, inhibiting circulation and possibly irritating the skin tissue. Aside from bedding and medical purposes, viscoelastic memory foam material can be used in standard household furniture (stationary or motion), office furniture, and in some vehicle seating applications. It can cushion sports equipment, power tools, and footwear. Viscoelastic memory foam has also found utility in ergonomic applications such as neck, back and leg pads, as well as in arm and wrist rests for computers. It can act as shock protection within electronics equipment and has been used in specialty packaging, military and commercial aircraft seating, and weaponry (for recoil suppression).
Why isn’t Memory Foam used in more cushioning applications
With such valuable attributes, it would seem that viscoelastic memory foam products should be found in many more comfort cushioning applications. Unfortunately, production economics often restrict end-use applications. Unlike traditional foam processing, viscoelastic memory foam formulation flexibility is more restrictive. Raw materials must be carefully altered to generate foam with varying properties. Cutting, profiling and other fabrication techniques may also require more care due to the slow recovery aspect of the memory foam. These considerations certainly affect product economics.
Memory Foam Production Formulating and Processing
While formulating and processing can be challenging, memory foam manufacturers and chemical suppliers have worked together to develop a range of successful viscoelastic products. For instance, certain formulations can alter the memory foam’s recovery, elasticity, and firmness, giving it the ability to meet a diverse set of end-use needs. Because viscoelastic memory foam is hypersensitive to formulation, the challenge focuses on maintaining product performance consistency. This requires a close working relationship between FPF manufacturers and their raw materials suppliers. With careful attention to quality control procedures, end products can meet or exceed application performance requirements.
Physical Properties of Memory Foam Density
As with all FPF, the density for viscoelastic memory foam is the weight of one cubic foot of material measured in pounds. With conventional FPF formulations, a higher memory foam density generally equates to increased foam durability (retention of performance properties). The same is true of viscoelastic memory foam products. Density enhances durability and the ability of viscoelastic memory foam to maintain its physical performance. Viscoelastic memory foam found in most household and healthcare product applications typically ranges in density from 2 to 6 pounds per cubic foot (pcf).
Firmness
Normally, firmnesses of viscoelastic memory foams can range from supersoft (less than 10 lbs. @ 25% IFD) to semi-rigid (as high as 120 lbs. @ 25% IFD). The potential for surface pressure reduction is closely associated with firmness, and can vary based on the formulation. Viscoelastic memory foam products with a lower IFD tend to exhibit increased conformance and can distribute body weight more efficiently to alleviate pressure. However, if the IFD is very low, and there is not sufficient foam density or thickness to provide support, the product may “bottom out,” negating the benefits of pressure reduction. When performing IFD tests, firmness measurements of viscoelastic memory foam mattresses can be significantly affected by some of the foam’s key characteristics: rate sensitivity (the foam’s rate of recovery after compression), sensitivity to temperature, and sensitivity to humidity. Sample conditioning prior to testing is extremely important.
Rate Sensitivity
Viscoelastic memory foam rate sensitivity, observed as the speed that a foam sample recovers after compression, affects the way firmness (IFD) can be determined in laboratory tests. Because of rate sensitivity, when testing viscoelastic memory foam under load, as in IFD measurement, the speed at which the weight force is applied can alter firmness readings. In other words, if the indentation plate used in the test descends quickly, the memory foam mattress may respond with stiffness, whereas slower speeds may result in different IFD measurements. With viscoelastic memory foam products, IFD tests should show notation of the process speed (rate of deflection) being used so that valid and fair comparisons among foam grades and between laboratories can be consistently made.
Sensitivity to Temperature
The physical properties of viscoelastic memory foam can be greatly influenced by temperature. Even slight changes in room temperature can affect measured firmness and recovery rates. Recovery rate has been positively correlated to heat, so that as the foam increases in temperature, pliability and compression and recovery rates increase. In colder conditions, viscoelastic memory foam products tend to become firmer or even stiff.
Depending on the formulation, some viscoelastic memory foam products can maintain their “memory” feature at as low as 30° F, but the optimum range for best “memory” action is typically between 55° F and 85° F. Research is ongoing to find ways to moderate pliability and allow viscoelastic memory foam products to perform within a broader spectrum of temperatures, especially below freezing. This is especially important in vehicle seating and other applications where ambient temperature cannot be controlled in use. In testing viscoelastic memory foam performance characteristics, it is also important to make notation of the ambient temperature and be certain that all comparison testing be performed under like conditions. Sample must be conditioned prior to testing. Heat (even body heat) can soften viscoelastic memory foam materials and, in extreme situations, affect the foam’s ability to provide support. This process is known as a phase change or relaxation.
Rather than broadening the temperature range at which viscoelastic memory foam retains its slow recovery, foam formulation can be adjusted to narrow the temperature span at which phase change occurs, anticipating the influence of room temperature combined with body temperature. In other words, if the phase change occurs at lower temperatures, its reaction to added body heat could lessen its firmness to the point where support is lost. On the other hand, if the phase relaxation were to happen in warmer conditions, added body heat would not be enough to soften the foam, and the resulting foam rigidity would reduce the potential for pressure relief. Hence, careful control of the phase change characteristics is crucial for the foam to serve its pressure reduction purpose and to provide predictable support.
Sensitivity to Humidity
Not only does viscoelastic foam react to temperature but also to humidity. Viscoelastic products tend to soften in more humid conditions. For example, very pliant foam may feel slick or “buttery” as opposed to coarse, depending on the humidity. Surface feel is generally not a critical factor in end-use applications, because in almost all cases, the foam is enclosed inside a covering material that provides its own surface feel. Firmness that changes with ambient conditions can make accurate calculation of physical properties difficult.
Memory Foam Performance Testing Pressure Reduction and Pressure Relief
Because viscoelastic memory foam can closely conform to the shape of the human body, it can efficiently distribute pressure over the whole surface. Pressure-mapping equipment is often used to calculate the degree of weight distribution. During the mapping procedure, computer programs monitor pressure. The body’s impression is graphed to show which areas endure the most pressure (namely the shoulder blades, posterior region, head, heels, calves, and elbows). Some viscoelastic memory foam producers perform these tests as an indication of how well the foam might act to minimize pressure. In the healthcare area, to be labeled as providing “pressure relief,” readings must be at 32mmHg or lower, whereas “pressure reduction” performance is agreed to occur between 32mmHg and 50mmHg. The adjacent charts help illustrate the process and show some typical pressure readings for viscoelastic memory foam. When readings made on viscoelastic products are compared to those of conventional foam and other common cushioning surfaces, the pressure differences are notable.
Resolving Testing Procedures
With the unique nature of viscoelastic memory foam performance, some traditional FPF test methods do not produce consistent results. For example, measuring durability with the use of a Dynamic Pounding Fatigue Test procedure can be problematic. Due to the foam’s slow recovery characteristic, pounding fatigue data loses validity because the foam has not fully recovered in between each cycle. Compression set testing may provide a better measurement of viscoelastic memory foam durability. The foam industry uses IFD measurements to quantify FPF firmness. Unfortunately, IFD testing may not accurately evaluate viscoelastic memory foam firmness because the foam relaxes during the one-minute hold period before taking the IFD reading. Due to rate sensitivity and other factors, standard IFD testing procedures used throughout the industry are not completely understood and may not accurately measure the firmness characteristics of viscoelastic memory foam products. Recognising both the demand for consistent measurement of viscoelastic memory foam product performance properties and the importance of resolving standardized testing concerns, Polyurethane Foam Association members formed a technical task group to consider these issues. Problematic testing issues have been identified, and, through experimental testing and group testing verification, new test standards may be developed for use with viscoelastic memory foam products.
Environmental Considerations
As with any product, off-gassing naturally occurs. The formulation technologies used to produce viscoelastic memory foam products sometimes create the potential for more noticeable aromatic emissions than typically found in conventionally manufactured foam products. While not harmful, residual odour may be bothersome to some consumers. Therefore, it is advisable to thoroughly ventilate viscoelastic memory foam products prior to final product assembly. In terms of environmental manufacturing objectives and recyclability, the production of viscoelastic memory foam is a friendly addition to a very environmentally conscious industry.
Flammability Considerations
Viscoelastic memory foam can be manufactured to perform with varying degrees of ignition and combustion resistance. Typically, with the addition of flame retardants, viscoelastic memory foam products can be produced to satisfy small open flame tests such as Calif. TB117 and FMVSS 302 and can be combined with barrier materials in mattresses designed to comply with other composite-type flammability specifications. As is the case with all flexible polyurethane foams, great care should be taken to avoid contacting a viscoelastic memory foam product with an ignition source such as a lit match or candle, heating element, sparks, exposed electrical wires, an exposed light bulb or smoking materials. Once ignited, all flexible polyurethane foams have the potential to burn vigorously, emitting great heat and dense smoke.
Memory Foam Summary
Viscoelastic memory foam technology represents new and exciting developments for the FPF industry. It is taking hold in a variety of markets (from medical to bedding to technical applications and more) and offers a unique cushioning alternative to conventional and HR flexible polyurethane foams. Its performance in specialized end-use applications has been well received by consumers. Ongoing industry and individual efforts are focused on improving the control of viscoelastic memory foam’s performance, so that more successful applications can be developed for this unique cushioning, pressure distribution and shock absorbing material.
1. Viscoelastic memory foam is a type of open cell, flexible polyurethane foam.
2. Surface comfort and pressure distribution are closely related to the foam’s ability to conform to body shape.
3. Slow recovery adds comfort characteristics, but also complicates the testing of viscoelastic products.
4. Viscoelastic memory foam firmness, support and height recovery rate are often affected by temperature and humidity.
5. Comparative laboratory testing must match ambient testing conditions for accurate comparisons and replicability of test results.
6. Some standardized FPF performance testing procedures may require special considerations to accommodate viscoelastic’s slow recovery rate.
7. Viscoelastic memory foam products should be thoroughly ventilated after production and during fabrication to reduce the possibility of bothersome aromatic emissions.
8. Viscoelastic performance is a matter of taste. Some like it fast and some like it slow.
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