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Rubber hardness estimation and adjustment method

Jun 21, 2018

The relationship between formula design and rubber hardness



Raw rubber varieties

Vulcanization system

Reinforcing Filler

Softening plasticizer


Factors Affecting Shore A Hardness Measurement


1. Effect of sample thickness


Shore A hardness value is measured by the depth of the pressure needle into the sample, so the thickness of the sample directly affects the test results. The specimen is deformed by the pressure and is thick, and the portion where the pressure is applied becomes thinner, and the hardness value increases. Therefore, the hardness of the sample is small and the hardness value of the sample is large and the hardness is small.


2. The effect of the length of the pressure pin on the test result


The standard stipulates that the height of the pressurized surface exposed by the pressure needle of the Shore A durometer is 2.5mm, and the pointer should indicate zero in the free state. When the pressure needle is on a smooth metal plate or glass, the instrument pointer should indicate 100 degrees. If the indication is greater than or less than 100 degrees, the pressure needle exposure height is greater than or less than 2.5 mm or less than 2.5 mm. In this case, it should be stopped. Use it for correction. The measured hardness value is higher when the pressure needle exposed height is greater than 2.5mm.


3. The effect of the shape of the end of the needle on the test result


The pressure needle end of the Shore A durometer cause wears under long-term action, which changes its geometry and affects the test results. The larger diameter of the worn end is also measured because of its unit area. The pressure is different. If the diameter is larger, the hardness value measured by the smaller pressure is larger, and vice versa.


4. Influence of temperature on test results


Rubber is a polymer material, its hardness value changes with the environment, and the hardness value decreases when the temperature is high. Different rubbers have different influences. For example, natural rubber with slower crystallization has less influence on temperature, while neoprene rubber, styrene-butadiene rubber, etc. have a significant effect.


5. Effect of reading time


The Shore A hardness tester has a great influence on the test result when the reading time is measured. Immediately after the pressure needle and the sample are pressed, the reading and pointer are stable and then read. The results are quite different. The former is high and the latter is low. The difference between the two is about 5 to 7 degrees, especially in the synthetic rubber test. Significantly, this mainly causes the rubber to creep after being pressed. Therefore, the data should be read immediately after the sample is pressurized.


Rubber hardness adjustment method


1. Adjust the amount of filler


The hardness is adjusted by increasing or decreasing the filler in the rubber material. The hardness of the rubber material with high hardness is relatively low, and the unfilled rubber with high filler not only has poor self-viscosity of the rubber material but also has difficulty in handling and large hysteresis of the vulcanizate and cannot meet the user requirements.


2. Adjust with the sulfur amount


Adjust the hardness according to the amount of sulfur in the rubber. The hardness of the rubber compound increases with the increase of the sulfur content. This method is easy to grasp in production, the glue content is relatively stable, the viscosity of the rubber material is good, and it is easy to operate. The finished product can also have appropriate elasticity. The disadvantage is poor heat resistance.


3. Adjust with plasticizer


In the rubber material below 60 degrees (Shore A) hardness, since the sulfur content is already very low, and the lower hardness is required, the above two methods have no effect, and the plasticization in the increasing and decreasing rubber can be adopted. The amount of agent to obtain the required low hardness.

The basic hardness of various rubber


NR, low-temperature SBR, CIIR 40

Oil filling (25phr)

SBR 31

High-temperature SBR 37

Oil filling (37.5phr)

SBR 26

IIR 35

NBR, CR, CSM 44

NBR 46 with more than 40% acrylonitrile content


In each of the above base compounds, the change in hardness value for each 1 part of filler or softener is as follows:


FEF, HAF, EPC +0.5

ISAF +2.5

SAF fumed white carbon black +2.5

SRF +0.33

Hydrous Silica +0.4

Thermal cracking carbon black or hard clay +0.25

Calcium carbonate +0.167

Surface treated calcium carbonate +0.142

Mineral rubber -0.2

Lipid Plasticizer -0.67

Aliphatic or naphthenic -0.5

Aromatic oil -0.588


Rubber hardness estimation method:


Estimated hardness = rubber base hardness + filler (or softener) dosage × hardness change.