SECTION 2.0
Joint Industry Foam Standards and Guidelines
Published: 7/94
TENSILE STRENGTH, TEAR STRENGTH, AND ELONGATION STANDARDS AND GUIDELINES
2.1 Tensile Strength
2.1.1 Tensile strength is measured using the method specified in ASTM D3574. Generally, acceptable tensile strengths are above 8 p.s.i. depending to some extent on the final application of the flexible foam. There are some cases where lower tensile strength foams may be used, but it is generally advisable to use foams that have a tensile strength of at least 8 p.s.i.
2.1.2 Tensile strength alone should not necessarily be used to indicate potential foam problems or to accept or reject shipments of foam. The change in tensile strength from shipment to shipment or lot to lot is a better indicator of potential problems. For example, a particular grade of foam has been running at a tensile strength of 16 p.s.i. plus or minus 2.0 p.s.i. for three or four months, and suddenly, with the next shipment, the tensile strength drops to 8.5 psi. Then, the 7.5 p.s.i. drop in tensile strength should be cause for concern.
2.1.3 When significant changes occur (significant is defined as changes more than the original stated specification tolerances), the vendor should be alerted, and together, the remainder of the properties and test methods on the lot of foam in question should be rechecked.
2.1.4 If the changes are real and not due to testing errors, the testing equipment (yours and the vendors) should be recalibrated. Tensile changes of the order of magnitude mentioned do not occur unless something drastic has changed, and the changes will, in most cases, show up in the other physical properties of the foam.
2.1.5 The key point is that the change in tensile strength from lot to lot is the most important factor. In the more than thirty years that polyurethane foams have been used in furniture, no one has yet defined the absolute values of tensile strength needed for each furniture application. Decisions have been made using much trial and error and the personal experience of the individuals testing the foams in furniture applications. However, it has been clearly observed that significant changes in tensile strength can be prime indicators of significant variations in other more important physical properties, such as density, IFD, compression set, and flex fatigue.
2.1.6 The most serious upholstery problem with low tensile strength is associated with handling of the foams within the upholstery plant and pulling and tugging of the foam during upholstery prepadding. Manual carrying, loading, and unloading foams within a manufacturing operation can lead to tensile breaking, and the pulling and tugging associated with upholstery and stapling foams to the frames when they are used as underpadding can produce tensile breakage with some low tensile foams.
2.1.7 When foams exhibit tensile breaks in actual, in-use circumstances, it is virtually a given that the upholstered piece has been abused or that the original foam tensile and/or tear strength were exceptionally low in the beginning. Both conditions are relatively easily determined by careful testing and records management.
2.1.8 Most furniture manufacturers accept the tensile strength test results of their vendors rather than to routinely test for tensile strength themselves. However, the furniture manufacturer should have the equipment and the ability to perform tensile strength tests for random cross checks and his own R&D efforts.
2.1.9 The major factors which may result in tensile strength testing errors include:
1. --nick or burr in cutting die
2. --poor measurement of cross-sectional area
3. --irregular cutting of sample
4. --excessively high jaw-separation speed*
5. --excessively low jaw-separation speed*
6. --improper reading or recording on force gauge
7. --improperly calibrated force gauge
8. --existing, unseen tears or cuts in sample
9. --slippage of sample in jaws*Since ASTM D-3574 permits a rather wide range on jaw-separation speeds, it is highly recommended that each furniture company should come to an agreement with their vendors on jaw-separation speeds to be used during testing.
2.2 Tear Strength
2.2.1 Tear strength is measured using the method specified in ASTM D-3574. Acceptable tear strengths begin at approximately 1.0 pounds per lineal inch (p.l.i.). As the tear strength progresses below 1.0 pounds per lineal inch, in-plant handling problems are virtually inevitable.
2.2.2 Rubber latex foams, which were in wide use in the furniture industry thirty years ago, had tear strengths which were mostly under 1.0 p.l.i; and these rubber latex foams also presented some very serious in-plant handling tearing problems. Today, some of the more esoteric foams and highly filled foams can present the same in plant handling problems; thus, the furniture manufacturer must carefully evaluate all new foams for potential handling problems.
2.2.3 Tear strength is a greater problem than tensile strength in handling within the plant. Employees tend to get a fist full of foam when handling foam cushions, and this method of handling sometimes tears even the best of foams.
2.2.4 Tear failures on foams in-use are relatively infrequent and are mostly associated with shear forces such as squirming or shearing the cushion across the top of the front rail or shearing the top of an arm with the hands while getting out of a chair or sofa.
2.2.5 Oftentimes, foams are drilled or punched (including cushions) to receive button straps or twines, and tear failure around the drilled or punched holes are inevitable with foams exhibiting tear strengths significantly less than one pound per lineal inch, unless the foam is protected in some manner.
2.2.6 Shear forces during the shipment of upholstered goods can also create tear problems with some low tear strength foams. Many of these problems are simply but inappropriately blamed on the trucker. In many cases, a careful, thorough investigation may show that the foam tears were not the result of poor handling or loading/unloading but were the result of low tear strength.
2.2.7 Low tear strength can also be related to fatigue softening. As is the case with much of the furniture related polyurethane foam data, quantitative proof of the relationship between low tear strength and fatigue softening does not exist. However, there is enough trend type data to be concerned about the possible effects of low tear strength on fatigue softening. The theory of what may happen during flexure lies with the fact that foams are made of billions of tiny gas bubbles (cells) which have very thin cell walls. The total tear strength (as is all of the other properties) is the sum of the strengths of adjacent cell walls. In the case of low tear strengths, each of the cell walls has low resistance to tearing, and in concentrated loading of shear forces or impact loading during sitting there may not be enough cell-wall strength to resist tearing. The cell wall tearing may occur on just a few cell walls, but with time and continued shear loading or impact loading, theoretically, enough cell walls could tear to affect the load bearing properties of the foam, resulting in softening. This type of softening is obviously irreversible.
2.2.8 Relative changes in tear strength can be indicators of potential problems. As was the case in tensile strength, shipment to shipment (or lot to lot) changes in tear strength can be indicators of many serious problems in foam formulation monitoring, changes within a run, or changes in the chemicals used. Tear strength should be monitored carefully.
2.2.9 The major factors which may result in tear strength testing errors include: --nick in sample cutting die --poor sample cutting --undetected tears or voids in sample --incorrect sample dimensional measurement *--excessively fast jaw separation speed *--excessively slow jaw separation speed --incorrect reading of force gauge --slippage of sample in jaws --tear-out too quickly -improper calibration of force gauge. *Since the ASTM allows quite a wide range on jaw separation speed, it is highly recommended that furniture manufacturers should agree precisely with their vendors on an exact speed of jaw separation.
2.2.9.1 Most furniture manufacturers accept the tear strength tests results of their vendors. However, the furniture manufacturer should have both the equipment and the ability to test for tear strength for purposes of cross-checking and their own R&D efforts.
2.3 Elongation
2.3.1 The elongation (elongation at break) is measured using the method specified in ASTM D- 3574. Elongations of under 100% have classically been suspect for potential problems; however, no single production problem or field problem has been statistically, quantitatively associated with low elongation. Certain practitioners use the old adage, "if it doesn't stretch, it'll break", and this is certainly true to some extent; but as the adage relates to upholstered furniture in use and manufacturing foam performance, valid proof has yet to be produced.
2.3.2 It is not possible to judge the performance of any flexible polyurethane foam by using any single physical property alone, and elongation is no exception, but there are some good and acceptable "rules of thumb". Low elongation accompanied by low tensile strength and low tear strength is definite cause for concern--particularly when evaluating new foam types for upholstery applications, and particularly when looking at lot to lot variations of foam physical properties.
2.3.3 As was the case in tensile and tear strength, the change in elongation from shipment to shipment or lot to lot is of more importance than the absolute value of the elongation. For example, if, for the past several weeks, shipments of a specific foam type have ranged from 130% to 150% elongation; and if today's shipment of the same foam is 105% elongation, a change of this order of magnitude is cause for concern; and it is almost a given that significant changes in tensile strength, tear strength, compression set, fatigue, or IFD will also be found.
2.3.4 Elongation by itself is a poor indicator of potential field or in-use problems, but changes in elongation and particularly those which occur along with changes in other physical properties should give rise to much concern and equivalent cross-checks and investigation for the facts.
2.3.5 Most furniture manufacturers accept the elongation test results of their vendors rather than to, routinely, test for elongation themselves; however, the furniture manufacturer should have both the equipment and the ability to test for elongation for purposes of routine cross-checks and their own R&D efforts.
2.3.6 The major factors which may result in elongation testing errors include:
1. --Improper benchmarking .
2. --If the benchmark separation is measured visually, simple errors in reading the moving scale can be made (these are the most common errors).
3. --If the benchmark separation is measured visually and the elongation is relatively high (meaning large distance of separation of the benchmarks), eye angle errors, called parallax errors, can occur.
4. --Parallax errors can occur also in low elongation situations.
5. --If the benchmark separations are measured automatically by the testing equipment, slippage in the jaws can occur.
6. --In the automatic measurement circumstance, improper calibration of the test equipment causes errors.
7. --Excessively fast or slow jaw separation can cause errors.
8. --While running higher elongation samples, the bench-marks may fade or become difficult to read while being stretched.
9. --Undetected tears or voids in elongation sample.
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