Evaluation method of thermal stability of polyolefin materials: oxidation induction period

- Apr 28, 2021-

Polymer materials react with oxygen in the air to cause degradation or cross-linking of polymers, which is called oxidative aging. When the polymer material reacts with oxygen, there is an induction period of oxygen inhalation. Once the induction period has passed, it will switch to the auto-accelerated oxidation stage. Whether a polymer material is prone to oxidation depends firstly on whether it is easy to absorb oxygen. Generally speaking, carbon chain polymers are prone to oxidative aging and become unstable. Polyolefin materials are typical carbon chain polymer materials. Polyolefin materials are usually used as the sheath and insulation layer of optical cables and cables. Therefore, it is necessary to pay attention to its stability in the actual use process. At present, the common indicator for evaluating the stability of polyolefin materials in the industry is the oxidation induction period (OIT).

Oxidation induction period (OIT) is the time when the autocatalytic oxidation reaction of the test sample begins to occur under high temperature (such as 200°C) oxidation conditions. It is a parameter to evaluate the stability level of the material, also called oxidation induction time. Generally, differential scanning calorimetry (DSC) can be used for simple and rapid measurement. The specific measurement and evaluation methods are as follows: the sample and reference are heated at a constant rate in an inert atmosphere (nitrogen), and when the specified temperature (generally 200°C) is reached, they are switched to oxygen or air at the same flow rate. The sample is then kept at this constant temperature until an oxidation reaction is shown on the thermal analysis curve. The oxidation induction time is the time interval from the start of oxygen or air to the start of the oxidation reaction, as shown in the figure.

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There are standard methods for testing the oxidation induction period and the oxidation induction period of polyolefin materials. Such a standard method test usually requires attention to its influencing factors. The key influencing factors that affect the results of the oxidation induction period are as follows, and the influence of these factors should be paid attention to when testing and analyzing the results.

1) Inhomogeneity of samples. Polymer materials are generally blended materials that have been modified and processed. The trace components inside, especially the antioxidants that play an important role in oxidation, are prone to uneven distribution, while the DSC sampling volume is more Therefore, uneven sampling is more likely to occur. It is recommended to take a few more parts for testing in actual testing and analyze the results comprehensively.

2) Oxidation temperature is usually different when other conditions remain unchanged. The oxidation induction time is different. The higher the temperature, the shorter the oxidation induction time. Therefore, the temperature needs to be agreed in advance when evaluating materials. The more commonly used temperature for polyolefin materials is 200℃

3) Gas purity and flow. Nitrogen is used as a protective gas before oxidation. Therefore, if the nitrogen is impure and mixed with oxygen, it will affect the test results. In addition, in the oxidation process, oxygen is the main oxidative active component, so the difference in oxygen flow will also affect the results. The oxidation induction period is usually used to evaluate the stability and anti-oxidation performance of the material. Regarding the oxidation induction period of polyolefins, there are Many standards can be followed. The main indicators for determining thermal stability are oxidation induction time and the oxidation induction time retention rate after a certain condition. Regarding what kind of material needs to meet what kind of judgment requirements, some industry standards or corporate standards have judgment requirements, and materials that do not meet these standards need to be given judgment values according to the use of the product during product design.

In addition to the above determination of the stability of the material, the oxidation induction period can also be linearly functioned by the reduction of the oxidation induction period as the temperature increases, and then the formula is fitted to extrapolate the thermal oxidation life at the operating temperature. Therefore, this is also a method of thermal oxidation lifetime prediction. In the actual evaluation process, it is necessary to formulate a plan according to the characteristics of its own products.

Oxidation induction period test needs Differential calorimetry scanner to perform.welcome to check our product list for details

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