TPE/TPR thermoplastic elastomer test hardness deviation causes

  • Time of issue:2022-01-08
  • Views:728

TPE/TPR thermoplastic elastomer test hardness deviation causes

  • Categories:Elastomer knowledge
  • Author:SPP
  • Time of issue:2022-01-08
  • Views:728

Hardness is an important performance parameter of thermoplastic elastomer TPE, TPR. For general products, the hardness of TPE and TPR materials is within ±3~5 degrees (A) of the standard hardness. Some products with higher requirements for softness, such as earphone cables, soft TPE products (such as super soft toys with hardness of 0~15A), even require TPE material hardness within ±2 degrees (A) of the standard hardness. Hardness is very important to confirm new product TPE and TPR material samples. Sometimes, inaccurate hardness of material samples may lead to inaccurate sample material hardness, which may result in the sample hardness not meeting the customer's requirements.

  There are several factors for inaccurate hardness testing of TPE/TPR samples, as follows.

  1. Influencing factors of Shore hardness tester

  1.1 The influence of needle length on test results

  The standard provides Shore A hardness tester with a pressurized surface height of 2.5 mm, in a free state when the needle should be zero. When the needle is on a smooth metal plate or glass, the instrument pointer should indicate 100 degrees. If the indication is greater or less than 100 degrees, the exposed needle height is greater or less than 2.5 mm or less than 2.5 mm. In this case, stop. For making corrections. The hardness value measured when the exposed needle height is greater than 2.5 mm is high.

  1.2 Influence of needle end shape on test results

  The Shore A hardness needle end leads to wear under prolonged action, changing its geometry and affecting the test results, with the diameter of the end becoming larger after wear. The results are also different, because the pressure per unit area is different. Caused by. If the diameter is large, the hardness is small and the hardness value is too large, and vice versa.

  2. Ambient temperature coefficient

  Rubber is a polymer material and its hardness value changes with the environment, when the temperature is high the hardness value decreases. Different rubber compounds have different degrees of influence. For example, natural rubber with a slow crystallization rate has a small effect on temperature, while neoprene and SBR have a significant effect. TPE, TPR materials with the same formulation, winter than summer hardness 5 degrees (A) harder.

  3. Effect of reading time

  Shore A hardness tester has a great influence on the test results when measuring. After the pressure pin and sample is pressed, the reading and pointer are immediately stabilized and then read. The results are very different. The former is higher while the latter is lower. The difference between the two can reach 5 to 7 degrees, especially in reconstituted rubber testing. More importantly, this mainly causes creep of the rubber after pressing. The data should be read immediately after pressing the sample (Note).

  4. Influence of sample thickness (specimen)

  The hardness of the measured TPE material is related to the measurement method and the thickness of the part. For the same TPE model material, the thicker the product, the harder it feels, although thickness is practically independent of material hardness. The ASTM method specifies a test plate thickness of 0.250" (the thickness of the test piece is approximately 6.35mm), which is the range where hardness is not affected by thickness.

  Note: Most materials can withstand initial stress, but over time they will yield due to creep and relaxation. Hardness tester readings can be taken immediately or after a certain delay time, usually after 5 to 10 seconds. Immediate readings usually show a higher (or harder) reading than delayed readings. Delayed readings are more representative of the hardness of the material and its elasticity. For weaker, less elastic materials, stronger, more elastic materials are more susceptible to creep.

  Related knowledge.

  1. Penetration depth of the needle

  The most commonly used instrument for measuring hardness is called a Shore hardness tester. This instrument measures penetration depths of 0 to 0.100 inches (about 2.5 mm). A zero reading on the scale means that the indenter penetrates to an extreme depth. Similarly, a reading of 100 means that the indenter did not penetrate.

  2. Selecting a hardness tester for different hardness ranges of materials

  There are three types of Shore hardness testers, namely Shore A hardness tester, Shore D hardness tester, and Shore OO hardness tester. Shore hardness tester A, generally test 30 ~ 90A hardness range is more accurate, hardness less than 30A, with hardness OO test for Shore; hardness higher than 90A, with Shore hardness D mark test, test value more accurate.

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