By the ethylene, propylene and non-conjugated diolefin copolymerization of ethylene propylene diene rubber (EPDM) as wire and cable insulation material has more than 60 years of history. EPDM and polyvinyl chloride (PVC) and cross-linked polyethylene (XLPE) and other insulating materials have the same irreplaceable position, and its unique advantages are: longer service life, better resistance to water and oxygen and thermal stability, a wider range of operating temperatures. As the application of EPDM in the field of electrical insulation is more and more extensive, the requirements of EPDM key indicators are also more and more demanding, especially in some high-end applications, the control of key indicators of EPDM has more stringent requirements, only when the corresponding key indicators are in a reasonable range, so that the EPDM to meet the processing, performance and use of the requirements. Take EPDM as an example for the manufacture of medium-voltage wire and cable insulation, if the tensile strength, tearing elongation, breakdown strength, dielectric loss, volume resistivity and other important mechanical and electrical properties of the product can meet the requirements, the raw material EPDM ethylene content, mooney viscosity, diolefin content and purity and other key indicators must have strict limits. There are many factors affecting the breakdown resistance of EPDM wire and cable insulation. In addition to the above mentioned ethylene content, diolefin content and purity, the gel content in EPDM, as well as the adjustment and optimization of the processing formula have an important impact on the product's breakdown resistance. They are not only single and isolated influencing factors, but also work together to have a comprehensive effect on the electrical breakdown resistance of the products. In this paper, some of the factors that affect the breakdown resistance of EPDM are selected for systematic research and analysis, in order to provide important reference for the synthesis of EPDM products for wire and cable insulation.
1 Effect of ENB content on breakdown strength
ENB content (mass fraction) <4.5%, and with the increase of ENB content, the breakdown strength of the product decreases; ENB content (mass fraction) ≥4.5%, and with the increase of ENB content (mass fraction), the breakdown strength of the product increases.
2. Influence of rubber purity on breakdown strength
When EPDM is used in the processing of electrical insulating materials, the purity of the material is required to a certain extent. The purity of EPDM is usually expressed in terms of ash content, which is mainly composed of inorganic salts containing metal ions (vanadium, aluminum, iron, sodium, etc.) introduced during polymerization and post-processing, as well as foreign impurities. The presence of ash, equivalent to a physical "defect", easily become a breakthrough in the rubber products by electric shock. As the ash content in EPDM increases, the metal ion content in the sample also increases, and the breakdown strength of the finished product decreases. Ash (metal ions, foreign impurities) does have a certain impact on the electrical breakdown performance of the product. Considering the effect of metal ions on the breakdown resistance of the product, the metal ions in the test sample will form a localized field inside the product under the action of the external electric field, which will be superimposed with the external field to damage the product. As the content of metal ions in the sample increases, the internal field strength gradually increases, and the possibility of localized aggregation of metal ions also increases, resulting in abnormally high local field strengths, and all these factors will reduce the breakdown resistance of the product. Secondly, consider the impact of foreign impurities on the breakdown resistance of products, rubber products in the foreign impurities mainly include sand and rust, etc., usually, these impurities and rubber products are very poor compatibility, and their presence for the rubber products is equivalent to a "defect", destroying the uniformity and integrity of rubber products. When an external high-voltage electric field acts on EPDM electrical insulation products with "defects", it will be relatively easy to break through the electrical insulation products at the "defects", thus reducing the product's resistance to electric breakdown.
3. Influence of gel content on breakdown strength
Gel is a special structure of "rubber particles" formed during the polymerization of EPDM. Due to its special morphology and structure, its interaction with the normal molecular chain in the rubber product is relatively weak, and its existence can also be regarded as an "impurity" remaining in the rubber product. ". For high-quality EPDM electrical insulation products, the gel content may also have an impact on the electrical breakdown performance of the product. As the gel content in EPDM increases, the breakdown strength of the finished product decreases. Since the gel is a crosslinked mesh structure, its morphology is significantly different from that of normal EPDM molecular chains, and therefore it is less compatible with EPDM molecular chains than normal EPDM molecular chains, and the interaction between the gel surface and normal molecular chains is weaker than that between normal EPDM molecular chains. The interaction force between the gel surface and the normal molecular chains is weaker than that between the normal EPDM molecular chains, and the gel surface is not necessarily "filled" with EPDM molecular chains.
The surface of the gel is not necessarily "filled" with EPDM molecular chains, and there may be some "gaps". Since the existence of the gel will affect the homogeneity of the product, the processed product will form a field gradient around the gel under the action of the external high-voltage electric field. The greater the difference between the morphology and structure of the gel and the surrounding EPDM molecular chains, the greater the formation of the field gradient, the easier it is to break the surface of the gel which has a weaker relative force and there may be a "gap" in the product. The more the difference between the EPDM molecular chains, the greater the field gradient, the more likely to destroy the gel surface where the relative force is weaker and there may be "gaps", forming "defects" in the product, leading to the early breakdown of the sample, and thus reducing the breakdown performance of the sample.
4. Influence of processing formula on breakdown strength
Processing formula is a key link in the processing of EPDM products into electrical insulation products, but also one of the key factors affecting the breakdown performance of electrical insulation products. Adjustment and optimization of the basic processing formula and the addition of electrical insulating additives can significantly improve the breakdown performance of electrical insulation products. After the same EPDM raw material is processed according to different formulas, the mechanical properties do not change much, and the latter is only about 5% higher than the former, but the electric breakdown strength has a significant change, and the latter is about 60% higher than the former. The main reason for the improvement of mechanical and electrical properties is that the marine cable manufacturer has optimized the dosage of diisopropyl benzene peroxide during the processing of the products, so that the double bonds in the side chain of ENB are fully crosslinked, and the residual double bonds are reduced, and kaolin and other electrical insulating additives are added, which increase the adsorbent surface and activity, and can adsorb the conductive ions remaining in the products. The above two points are sufficient to prove the importance of the processing formula to improve the electrical breakdown performance of EPDM electrical insulation products.
5.Conclusion
(1) The double bond in the side chain of the third monomer (ENB) in EPDM is more sufficiently cross-linked.
(1) EPDM in the third monomer (ENB) in the side chain of the double bond, the more fully cross-linked, the lower the number of residues, the more conducive to electrical insulation products (wire and cable insulation) electric breakdown resistance performance.
(2) The higher the purity of EPDM raw material (the lower the ash content), the more helpful for the improvement of electric insulation products (wire and cable insulation layer) electric breakdown resistance.
(3) The lower the gel content of EPDM raw material, the more it helps to improve the electric insulation products (wire and cable insulation) electric breakdown resistance.
(3) The lower the gel content of the EPDM raw material, the better the breakdown resistance of electrical insulation products (wire and cable insulation).
(4) Adjusting the amount of EPDM in the base formulation of electrical insulators (wire and cable insulation) and supplementing it with electrical insulating additives can significantly improve the breakdown resistance of electrical insulators (wire and cable insulation).
