OIT Test for Polyolefin Insulations and Jackets of Cables
   
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OIT Test for Polyolefin Insulations and Jackets of Cables
Polyolefin materials are generally used for insulations and jackets of electric cables and optical cables. The thermal oxidative stability of polyolefin materials can be used to evaluate the service life of electric cables and optical cables. Oxidative Induction Time (OIT) is a relative measurement of the ability for stabilized material inhibits oxidation and decomposition. It is used for evaluating the level of stabilization of the material. The OIT is the time interval between the initiation of oxygen flow and the onset of the oxidative reaction .The time interval for which the antioxidant stabilizer system present in a test specimen inhibits oxidation is measured while the specimen is held isothermally at a specified temperature in a stream of high-purity oxygen, so that the OIT of material sample can be determined. Generally it may be simply and fast measured by using Differential Scanning Calorimeter (DSC). 
 
Air and sunshine is essential to survival of human being and plant growth on the earth, but they play  bad and damaging role in the application process of polyolefin materials. Thermal oxidation and light oxidation of polyolefin materials used in insulations and jackets of cables happened due to the oxygen from air and the ultraviolet light from sunshine will result in the deterioration of physical and mechanical properties of these materials thereby loosing their original functions and application value. The speed of thermal oxidation and light oxidation and the capability of anti-thermal oxidation, anti-light oxidation reaction of different plastics materials exhibit large difference because of the difference of their molecular structure, polymerization process, processing technology, application environment and conditions. Antioxidant and light stabilizer added in plastics materials can effectively inhibit or reduce the rate of thermal oxidation and light oxidation reaction, and significantly improve the performance of heat-resistance, light-resistance, so that postponing the degradation time and aging process of the of plastics materials, hence extending the service life of the plastics products. However, some suppliers used recycled plastics or added insufficient and poor antioxidant and light stabilizer in order to reduce the cost, resulting in the poor oxidative resistance of the polyolefin materials in the natural environment, and the mechanical and physical properties, and service life of cables deteriorated. It leads to bigger harm to all walks of life. Therefore to test the thermal oxidative stability is very important, and the OIT of materials is an important indicator reflecting thermal oxidative stability. The service life of products and the performance of raw materials can be identified by measuring OIT. 
 
The OIT measurement instrument, DSC is capable of measuring the antioxidant properties of materials quickly and accurately. The measurement principle is as follows: specimen and a reference material (e.g. α-Al2O3, melting point 1500 ) are heated at a constant rate in an inert gaseous environment (e.g. a flow of nitrogen). When the specified temperature is reached, the atmosphere is changed to oxygen maintained at the same flow rate. The specimen is then held at constant temperature until the oxidative reaction is displayed on the thermal curve. To measure the time interval that the specimen and the reference material spend from starting the oxygen flow to the onset of the exothermic effect (autocatalytic oxidation) resulting in heat flux difference (Since the chosen reference materials all have very high melting point, and are inert materials exothermic reaction and endothermic reaction will not occur at the chosen testing temperature and conditions), this time interval can then be used to judge the thermal stability performance of the specimen, the longer the OIT value the better the thermal oxidative stability of the material is. The onset of oxidation is signaled by an abrupt increase in the specimen’s evolved heat or temperature and may be observed by DSC. The OIT is determined from the data recorded during the isothermal test. The OIT is an assessment of the stabilization level of the material tested. OIT method is a method that under pure oxygen at high temperature accelerates the aging of materials so as to extrapolate the thermal oxidation life of a material. The OIT under normal atmospheric conditions will be longer. Generally, it is believed that if OIT is 1 min, the service life of the product will be one year.

OIT measurement of Polyolefin materials is based on the following standards:
[1] ISO 11357-6 Plastics-differential scanning calorimetry (DSC)-part 6:determination of oxidation induction time
[2] The China General Bureau of Quality and Technical Supervision. GB / T 2951.9-1997 Appendix B polyolefin insulated wires of copper oxidative degradation test method
[3] ASTM D 4565.17-1999 standard test methods for physical and environmental performance properties of insulations and jackets for telecommunication wire and cable oxidative induction time
[4] JB/T 10696.8-2007 test methods for wire and cable mechanical, physical and chemical properties, Part 8: oxidative induction time
 
Different product standards have different judging requirements, for example, YD/T1174-2001 communication cables – central office coaxial cable specifies that the OIT ( before aging) should not be less than the 30min, and the OIT( after aging) should not be less than 21min; while ANSI / SCTE 74 2003 Specification for braided 75Ω flexible RF coaxial drop cable specifies that the OIT (before aging) should not be less than 20min, and the OIT(after aging) should be 70% of the initial value.
 

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