Although there are various indexes, such as deterioration of physical properties due to moisture and heat, regarding the durability of urethane foam, “settling” will be described below. “Settling” means reduction of “thickness” and “hardness” which occurs when urethane foam is used for a long time. The settling is basically suppressed when the density is high, but note that it depends on the characteristics of foam and usages.
The settling, i.e., the reduction of “thickness” and “hardness” is measured by the method specified by the JIS standard.
Reduction of “thickness”
Compression residual strain measurement (JIS K6400-4 4.5.2 A method)
Urethane foam is compressed and released, and the thickness reducing rate is measured. The urethane sample compressed to 75% of the original thickness is stored in the thermostatic oven at 70℃±1℃ for 22 hours, the sample is decompressed, left at room temperature, and thickness is measured to determine how much it is reduced (%). As the value decreases, the durability to compression increases.
Reduction of “hardness”
Repeated compression residual strain measurement (JIS K6400-4 6. A method (Fixed loading method))
The hardness reducing rate is measured when a load is placed on urethane foam repeatedly. First, hardness is measured when urethane foam is compressed to 40% of thickness. Then, it is compressed repeatedly 70 times (a total of 80,000 times) under a load of 750N, and 40% hardness is measured, and the reducing rate is calculated based on the result.
The JIS standard specifies hardness reducing rates in X, V, S, A, and L classes. Refer to our list of physical properties showing classes.
|Repetition compression residual strain test
||Hardness measurement test