PVC insulated and sheathed cables have different levels of oil discharge on the surface during use, causing a big loss to the company as a serious quality accident.
"Oil discharge" means that some of the incompatible components in the PVC migrate slowly from the product to the surface and gradually form a film. It is an oily surface with jelly that is a mixture that are poorly compatible with PVC.
The reasonsof the "oil discharge" phenomenon are brieflyfrom the raw materials and processing technology.
I, Raw materials
1.1 PVC resin
There are many types and grades of PVC. The difference in performance and structure among different PVC has a different influence on the application of the product. The plasticizer's absorption and particle size directly affect the compatibility of the PVC resin.
The synthesis method of PVC resin has different influence on the surface characteristics,the porosity and molecular distribution of the particles, and the compatibility with the plasticizer. When the compatibility is poor,the plasticizer will migrate from the PVC macro-molecular space, and the plasticizer will precipitate on the surface of the cable to form an "oil discharge" phenomenon.
The plasticizer mainly aims to weaken the inter-molecular force of the polymer and increase the mobility of the molecular chain, thereby increasing the plasticity of the polymer, improving the processing property, and improving the elongation and flexibility. In order to avoid the precipitation of plasticizer from PVC sheathed cable, two factors should be considered when using plasticizer:
i ) The migration ability of plasticizers from PVC resins varies with the increment of plasticizer content. The compatibility of the plasticizer with PVC is generally judged by the dissolution index, and the closer it is to PVC, the better the compatibility is.
ii ) Meanwhile, the selection and using of plasticizers should be considered of molecular structure and chemical properties.As important factors, the molecular and concentration of the plasticizer can be adjusted properly, which influence dramatically on the compatibility, processing properties and properties of the PVC product.
The chemical structure of the stabilizer, other additive and the decomposition may cause physical or chemical reaction during processing. The stabilizer may become an oxidation accelerator for the plasticizer, reducing the compatibility of the plasticizer and causing the precipitation. Composite lead is the most widely used as stabilizers, but its lubricity and compatibility are relatively poor,the content percentage must be controlled very well. When the stabilizer is improperly selected or the content is large, it will cause the oil discharge of the cable easily.
1.4 Processing additives
In order to ensure the processing performance of PVC,additives such as lubricants are often added to the mixture. The compatibility of stearic acid lubricants with PVC plasticizers is poor, and the content should not be excessive
2, processing technology
2.1 Kneading temperature and speed
In order to better join together PVC powder and plasticizer, the plasticizer needs to be fully infiltrated into the resin and other powder materials by stirring and kneading. The uneven kneading will result in poor absorption of plasticizer, causing oil discharge finally.
2.2 Granulation process
During the granulation process, the screw extrusion on the material, the temperature and humidity of the processing will affect the uniformity of the material mixing. If the procession is improper, it will also cause oil discharge from the PVC sheath.
2.3 Extrusion process
When extruding PVC onto the cable, the temperature must be controlled well. If the temperature is too high, small molecules will escape to the surface and the resin will be decomposed, which will causes the precipitation of plasticizer.
2.4 Storage and using condition
PVC pellets may cause the precipitation of plasticizers due to factors such as ambient temperature, humidity, build-up pressure and microorganisms. Poorly compatible plasticizers will slowly precipitate as the ambient when temperature increases. As the humidity increases, the plasticizer will slowly hydrolyze after absorbing water, causing precipitation.
II, Tracking &analysis
The oilcollected from the PVC cable surface is analyzed by infrared spectroscopy, it is found that the main component of the precipitate is epoxy ester.During routine performance testing and processing, the pellets showed good performance, it results this auxiliary plasticizer gradually replaces the function of the main plasticizer.But if the quantity is beyond the limitation,the epoxy plasticizers poses a significant risk of "oil discharge" on the cable surface.
III.The detection method
The existing testing standards can not meet the requirements of cable oil performance testing, and it is necessary to find a more reasonable method foe judgment and evaluation the risk of oil discharge.
ISO 177 and ASTM D2199 provide an adsorption method for testing the migration loss of plasticizers. ISO 176 and ASTM D1203 specify the loss of volatilization of plasticizers by activated carbon adsorption. ASTM D2383 and 2391 specify the plasticizer precipitation test method under wet and compression condition.
These standards have great limitations on the detection of wires and cables.
Through continuous experiment, the method of determining the weight of concentrated sulfuric acid can quickly detect the content of epoxy plasticizer in the PVC, and then determine the risk of oil discharge.
IV. Improvement measures
There are two very important indicators in the performance index of epoxy plasticizers: iodine value and epoxy value. The general requirements: epoxy resin >6.0, iodine <3.0.