Joint Industry Foam Standards and Guidelines
DENSITY STANDARDS AND GUIDELINES
1.1. Density continues to be one of the most misunderstood properties of polyurethane foams. Some people mentally relate density to the firmness of foams, and that relationship is totally incorrect. Foams with an extremely wide range of hardness can be made at an extremely wide range of densities.
25% IFD (indentation load deflection, see chapter 4) is a measure of the hardness of foams; and for example, a 20 pound/50 in2 IFD can be made at densities ranging from 1.0 PCF (pounds per cubic foot) to 10 PCF. The key is that density is in no way related to IFD.
1.2. The following chart demonstrates the commercial densities of flexible polyurethane foams used in furniture manufacturing in the United States:
|GENERAL FOAM TYPE||COMMERCIAL DENSITY RANGE|
|CONVENTIONAL FOAMS||0.8 TO 2.5 PCF|
|FILLED CONVENTIONAL FOAMS||1.5 TO 5.0 PCF|
|HIGH RESILIENCY (HR) FOAMS||1.5 TO 6.0 PCF|
|COMBUSTION MODIFIED HR FOAMS||3.5 TO 10.0 PCF|
|MODIFIED HR FOAMS||1.5 TO 3.5 PCF|
|MELAMINE MODIFIED FOAMS(a)||1.5 TO 4.0 PCF|
|FOAMS FOR CALIFORNIA TB-117(b)||1.3 TO 2.0 PCF|
(a) Melamine modified flexible polyurethane foams can be made with a wide range of melamine modification depending on the flame retardancy requirements.
(b)x California TB-117 foams can be made in a very wide variety of densities depending on the user's needs and requirements.
1.3. Density is mass per unit volume. However, in the United States, density is expressed in pounds per cubic foot (PCF), and is calculated as follows:
WEIGHT OF SAMPLE (pounds)
LENGTH(ft) X WIDTH(ft) X HEIGHT(ft)
For example, suppose a typical cushion measures 24 inches x 24 inches x 4 inches, and weighs exactly 2 pounds. The density of the sample is calculated as follows:
First, since density is expressed in pounds per cubic foot, the dimensions must be converted from inches to feet: 24 inches divided by 12 inches per foot equals 2 feet, and 4 inches divided by 12 inches per foot equals 0.33 Feet. The sample was weighed in pounds. The density is calculated by then substituting the converted dimensions into the equation:
___________________________ = DENSITY
2(ft) X 2(ft) X 0.33(ft)
___________________________ = DENSITY
1.32 (cubic feet)
1.52 (pounds per cubic foot) = DENSITY
The density of the sample cushion is then 1.52 Pounds per cubic foot. If the sample is weighed in grams, the grams must be converted to pounds by dividing the weight, in grams, by 454 grams/pound. Then the density is calculated just as it was calculated above.
1.4. There are some do's and don't's in measuring and weighing samples for density computation:
a. Always work with the largest possible sample, because normal weighing and measuring errors become smaller as sample size increases (to a point,) and weighing and measuring errors become larger as sample sizes get smaller.
b. For dimensional measurements, use the most accurate measuring equipment you can buy (a common yard-stick is seldom accurate enough, but a calibrated yard-stick IS ADEQUATE)
c. In measuring dimensions, one must remember that flexible foams bend, contort, and stretch; thus, these factors must not be at all involved in foam dimension measurements.
d. The formula used for measurement of foam densities is based on fact that the foam is either perfectly square or rectangular in all cross-sections --regardless. If any cross-section is something other than perfectly square or rectangular, the density calculation will be incorrect to the degree that the particular cross-section is not rectangular or square. This is the most common error in measuring for calculating density.
e. Usually, the greatest errors in measuring occur in measuring the thickness of the density sample. One excellent way to measure sample thickness is with the indentor foot of an IFD measuring device (Reference ASTM D-3574 b1).
f. When weighing a sample for density calculation, one should always weigh on a scale with adequate pan size to accommodate the sample size used. Don't ever try to balance the foam sample on a small pan or try to keep it from falling by stabilizing it with your hand. Secondly, never try to weigh in the extreme upper or lower end of the weighing range of the scale used because this is where most of the large weighing errors occur. The calibration accuracy of the scale should be checked, routinely, with known, calibrated weights. For the sake of accuracy, many laboratories weigh density samples on gram scales and convert the gram weights to pounds. This procedure is usually very acceptable, because many metric-system laboratory scales are very accurate.
1.5. DENSITY TOLERANCES
There are two basic ways of specifying density tolerances. One is to specify a minimum density, for example, one could specify a foam with a minimum density 1.8 PCF, and all densities for that foam under 1.8 PCF would be considered to be out of specification.
Secondly, one could specify a nominal density with a plus or minus tolerance. For example, one could specify a 1.8 PCF foam with a tolerance of plus or minus 0.1 PCF. This specification would normally be written as 1.8 +or- 0.1 PCF. Thus, all densities for that foam under 1.7 PCF and over 1.9 PCF would be out of specification.
The choice of how one should specify density tolerance is not necessarily dictated by technical considerations as much as economic considerations. One should remember that, in general, smaller tolerances mean higher prices.
Thus, it is generally accepted that a minimum density specification is preferred. If plus or minus tolerances are used, the traditionally acceptable, commercial tolerances are a nominal density plus or minus 0.1 PCF.
1.6. SOME OTHER IMPORTANT FACETS OF DENSITY
One of the best uses of density measurements is quality control screening. Completely testing all foams received every day is not absolutely necessary; however, there are some key properties which should be tested on each shipment received. These properties can be indicative of changes which could cause serious in-use or in-plant problems. Density is one of these properties. For example, suppose one is buying a 1.6 plus or minus 0.1 PCF foam, and for the last two months the density has been checking from 1.50 PCF to 1.61 PCF. However, on today's shipment, the density measures 1.7 PCF.
Today's shipment is still within specification, but there is almost a six percent change over recently received shipments. At the very least, one should ask the vendor for all the physical property tests made on this particular lot of foam, and then, one should review his concerns with the vendor's technical personnel. Because of the complexities of the chemistry of polyurethanes, there are many things that can happen during the manufacturing of foams. A 6% change in density may be meaningless, but it is always best to be cautious. In any good quality control process, the magnitude of relative changes should raise concerns.
1.7. MANAGEMENT OF DENSITY SPECIFICATIONS
Something should be suggested concerning the management of specification ranges and acceptance/rejection criteria. Look again at the 1.6 plus or minus 0.1 PCF foam. As before, suppose that one has been receiving a density range from 1.5 PCF to 1.61 PCF for the last two months. Now, suppose today's shipment measures 1.48 PCF! Do we reject the shipment?
The first thing that should be done is to recheck several other samples, and let's assume that some of them also measure 1.48 PCF; now, do we reject? Rejection isn't recommended unless some of the other physical properties- -notably IFD--are out of specification. Why wasn't rejection recommended based on the 0.02 PCF out of density specification?
First of all, 0.02 PCF if just about the maximum precision to which density can be measured under the best of circumstances; so, those densities could well have been 1.50 rather than 1.48 PCF when one considers the error of measurement potential. It could be stated that we are giving 10 times the 0.02 PCF variance when we use a plus or minus 0.1 PCF tolerance on the nominal. That is true as far as it goes, but the error applies throughout the entire range on the nominal. One could view it this way; the plus or minus 0.1 PCF tolerance is related to how well the foam manufacturing process can be controlled and replicated from day-to-day, and the 0.02 PCF is the smallest error one can expect when weighing and measuring to calculate density. Secondly, perfectly harmless things can and do happen in the foaming process. Some of these happenings can affect density to the degree we have been discussing, and, if one part of a shipment is out of specification by 0.02 PCF, while every thing else in that shipment is in specification, there is a very high probability that it is safe to use that shipment of foam.
If the next several shipments, however, are out of specification by 0.02 PCF or more, on should then contact the vendor and suggest rejection if the density isn't shipped within the agreed upon range.
1.8. FOAM DENSITY VERSUS QUALITY/DURABILITY
The issue of foam density versus foam quality and durability is a very volatile and misunderstood issue. If quality is taken to mean performance to standard (specification), density has absolutely nothing to do with quality. If, for example, a 0.8 PCF foam is within the specifications originally agreed-upon by buyer and seller, the foam is within the expected quality standards and quality is not the issue.
It is very easy to understand the reason for the disparity between quality and durability and the resulting confusion; it is because quality and durability have typically and sometimes traditionally been used synonymously. Durability is described by that property or group of properties which describe or predict how a piece of foam will perform or fail to perform for a satisfactory period of time while in use. In certain well defined circumstances, using the properly applied, statistical theory of probability, it can be stated that density can affect durability directly. The major durability issue is in-use loss of load bearing characteristics or softening. This has classically and traditionally been called flex fatigue or just fatigue. Thus, a more accurate and acceptable statement concerning the effect of density on fatigue properties of polyurethane foams is: as densities of conventional, unfilled foams decrease, the tendency to fatigue soften increases.
Sorting out absolutes on the scale of available densities is very difficult; for example, the magnitude of how much a 1.4 PCF foam softens compared to a 1.6 PCF foam is not yet known, quantitatively.
An acceptable statement says only that the probability of in-use softening is greater with lower density foams. It is a virtual certainty that a 1.3 PCF density foam with a 32 IFD @ 25% will soften excessively in normal use as a seat cushion; conversely, it is virtually an equal certainty that a 1.8 PCF foam will soften less than the 1.3 PCF foam in the same in-use circumstances. The magnitude of the difference in softening between the two foams has not yet been quantitatively and statistically determined or proven.
The statement that 1.8 PCF foams are always better than lower density foams is also an exaggeration of the true facts. Every manufacturer of foams will admit that it is just as easy to err when making a high density foam as it is when making a lower density foam. High density is not the total guarantor of good, in-use performing, durable foams. One must note the use of the words statistical probability and then understand that it is only probable that higher density foams will be more durable than low density foams. The magnitude of the degree of probability is not yet known, and, presently, only a statistical trend exists. The more accurate statement is then: it is highly probable that 1.8 PCF foams will soften less than foams of lower density.
It must be noted that the performance and durability statements made above only apply to conventional, unfilled foams. Separate performance and durability statements must be made on other foam types.
1.9. BALLOTED STATEMENT ON DENSITY VERSUS FATIGUE ON IN USE PERFORMANCE
The following statement was passed by committee ballot:
Years of experience, supported by considerable test data, have given strong indications that polyurethane foams with polymer densities of 1.8 PCF or higher perform better in seating applications than foams with lower polymer densities.
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