EVA is the abbreviation for ethylene vinyl acetate copolymer, a polyolefin polymer. Due to its low melting temperature, good fluidity, polarity and non-halogen elements, and can be compatible with a variety of polymers and mineral powders, a number of mechanical and physical properties, electrical properties and processing performance balance, and the price is not high, the market supply is sufficient, so both as cable insulation material, can also be used as a filler, sheathing material; can be made into thermoplastic material, and can be made into thermosetting cross-linking material.
EVA wide range of uses, with flame retardants, can be made into low smoke halogen-free or halogen fuel barrier; choose high VA content of EVA as a base material can also be made into oil-resistant material; choose the melt index of moderate EVA, add 2 to 3 times the filling of EVA flame retardants can be made to extrusion process performance and price of a more balanced oxygen barrier ( filling ) material.
In this paper, from the structural properties of EVA, the introduction of its application in the cable industry and development prospects.
2. Structural properties
When producing synthesis, changing the ratio of polymerisation degree n / m can produce VA content from 5 to 90% of EVA; increasing the total polymerisation degree can produce molecular weight from tens of thousands to hundreds of thousands of EVA; VA content below 40%, due to the presence of partial crystallisation, poor elasticity, commonly known as EVA plastic; when the VA content is greater than 40%, a rubber-like elastomer without crystallisation, is commonly known as EVM rubber.
1. 2 Properties
The molecular chain of EVA is a linear saturated structure, so it has good heat ageing, weather and ozone resistance.
EVA molecule main chain does not contain double bonds, benzene ring, acyl, amine groups and other groups easy to smoke when burning, side chains also do not contain easy to smoke when burning methyl, phenyl, cyano and other groups. In addition, the molecule itself does not contain halogen elements, so it is particularly suitable for low-smoke halogen-free resistive fuel base.
The large size of the vinyl acetate (VA) group in the EVA side chain and its medium polarity means that it both inhibits the tendency of the vinyl backbone to crystallise and couples well with mineral fillers, which creates the conditions for high performance barrier fuels. This is particularly true for low smoke and halogen-free resists, as flame retardants with more than 50% volume content [e.g. Al(OH) 3, Mg(OH) 2, etc.] must be added to meet the requirements of cable standards for flame retardancy. EVA with a medium to high VA content is used as a base to produce low smoke and halogen-free flame retardant fuels with excellent properties.
As the EVA side chain vinyl acetate group (VA) is polar, the higher the VA content, the more polar the polymer is and the better the oil resistance. The oil resistance required by the cable industry mostly refers to the ability to withstand non-polar or weakly polar mineral oils. According to the principle of similar compatibility, EVA with high VA content is used as a base material to produce a low smoke and halogen-free fuel barrier with good oil resistance.
EVA molecules in the alpha-olefin H atom performance is more active, in the peroxide radicals or high-energy electron-radiation effect is easy to take H cross-linking reaction, become cross-linked plastic or rubber, can be made demanding performance requirements of special wire and cable materials.
The addition of the vinyl acetate group makes the melt temperature of EVA drop significantly, and the number of VA short side chains can make the flow of EVA increase. Therefore, its extrusion performance is much better than the molecular structure of similar polyethylene, becoming the preferred base material for semi-conductive shielding materials and halogen and halogen-free fuel barriers. 2 Product advantages
2. 1 Extremely high cost performance
EVA’s physical and mechanical properties, heat resistance, weather resistance, ozone resistance, electrical properties are very good. Select the appropriate grade, can be made heat resistance, flame retardant performance, but also oil, solvent-resistant special cable material.
Thermoplastic EVA material is mostly used with a VA content of 15% to 46%, with a melt index of 0. 5 to 4 grades. EVA has many manufacturers, many brands, a wide range of options, moderate prices, adequate supply, users only need to open the EVA section of the website, the brand, performance, price, delivery location at a glance, you can choose, very convenient.
EVA is a polyolefin polymer, from the softness and use of performance comparisons, and polyethylene (PE) material and soft polyvinyl chloride (PVC) cable material is similar. But further research, you will find EVA and the above two types of material compared with the irreplaceable superiority.
2. 2 excellent processing performance
EVA in the cable application is from the medium and high voltage cable shielding material inside and outside the beginning, and later extended to halogen-free fuel barrier. These two types of material from the processing point of view are regarded as “highly filled material”: shielding material because of the need to add a large number of conductive carbon black and make its viscosity increased, the liquidity dropped sharply; halogen-free flame retardant fuel need to add a large number of halogen-free flame retardants, also halogen-free material viscosity increased sharply, the liquidity dropped sharply. The solution is to find a polymer that can accommodate large doses of filler, but also has a low melt viscosity and good fluidity. For this reason, EVA is the preferred choice.
EVA melt viscosity with extrusion processing temperature and shear rate will increase the rapid decline, the user only need to adjust the extruder temperature and screw speed, you can make excellent performance of wire and cable products. A large number of domestic and foreign applications show that, for the highly filled low smoke halogen-free material, because the viscosity is too large, melt index is too small, so only the use of low compression ratio screw ( compression ratio of less than 1. 3) extrusion, in order to ensure good extrusion quality. Rubber-based EVM materials with vulcanising agents can be extruded on both rubber extruders and general purpose extruders. The subsequent vulcanisation (cross-linking) process can be carried out either by thermochemical (peroxide) cross-linking or by electron accelerator irradiation cross-linking.
2. 3 Easy to modify and adapt
Wires and cables are everywhere, from the sky to the ground, from the mountains to the sea. Users of wire and cable requirements are also varied and strange, while the structure of wire and cable is similar, its performance differences are mainly reflected in the insulation and sheath covering materials.
So far, both at home and abroad, soft PVC still accounts for the vast majority of the polymer materials used in the cable industry. However, with the increasing awareness of environmental protection and sustainable development.
PVC materials have been greatly restricted, scientists are doing everything possible to find alternative materials to PVC, the most promising of which is EVA.
EVA can be blended with a variety of polymers, but also with a variety of mineral powders and processing aids compatible, the blended products can be made into thermoplastic plastic for plastic cables, but also into cross-linked rubber for rubber cables. Formulation designers can be based on user (or standard) requirements, EVA as the base material, to make the performance of the material to meet the requirements.
3 EVA application range
3. 1 Used as a semi-conductive shielding material for high-voltage power cables
As we all know, the main material of the shielding material is conductive carbon black, in the plastic or rubber base material to add a large number of carbon black will seriously deteriorate the fluidity of the shielding material and the smoothness of the extrusion level. To prevent partial discharges in high-voltage cables, the inner and outer shields must be thin, shiny, bright and uniform. Compared to other polymers, EVA can do this more easily. The reason for this is that EVA’s extrusion process is particularly good, good flow, and not prone to melt rupture phenomenon. The shielding material is divided into two categories: wrapped in the conductor outside called the inner shield – with the inner screen material; wrapped in the insulation outside called the outer shield – with the outer screen material; inner screen material is mostly thermoplastic The inner screen material is mostly thermoplastic and is often based on EVA with a VA content of 18% to 28%; the outer screen material is mostly cross-linked and peelable and is often based on EVA with a VA content of 40% to 46%.
3. 2 Thermoplastic and cross-linked flame retardant fuels
Thermoplastic flame retardant polyolefin is widely used in the cable industry, mainly for halogen or halogen-free requirements of marine cables, power cables and high-grade construction lines. Their long-term operating temperatures range from 70 to 90 °C.
For medium and high voltage power cables of 10 kV and above, which have very high electrical performance requirements, the flame retardant properties are mainly borne by the outer sheath. In some environmentally demanding buildings or projects, the cables are required to have low smoke, halogen-free, low toxicity or low smoke and low halogen properties, so thermoplastic flame retardant polyolefins are a viable solution.
For some special purposes, the outer diameter is not large, temperature resistance in 105 ~ 150 ℃ between the special cable, more cross-linked flame retardant polyolefin material, its cross-linking can be selected by the cable manufacturer according to their own production conditions, both the traditional high-pressure steam or high-temperature salt bath, but also available electron accelerator room temperature irradiation cross-linked way. Its long-term working temperature is divided into 105 ℃, 125 ℃, 150 ℃ three files, the production plant can be made according to the different requirements of users or standards, halogen-free or halogen-containing fuel barrier.
It is well known that polyolefins are non-polar or weakly polar polar polymers. As they are similar to mineral oil in polarity, polyolefins are mostly considered to be less resistant to oil according to the principle of similar compatibility. However, many cable standards at home and abroad also stipulate that cross-linked resistances must also have good resistance to oils, solvents and even to oil slurries, acids and alkalis. This is a challenge for material researchers, now, whether in China or abroad, these demanding materials have been developed, and its base material are EVA.
3. 3 Oxygen barrier material
Stranded multi-core cables have many voids between the cores that need to be filled to ensure a rounded cable appearance, if the filling within the outer sheath is made of halogen-free fuel barrier. This filling layer acts as a flame barrier (oxygen) when the cable burns and is therefore known as an “oxygen barrier” in the industry.
The basic requirements for an oxygen barrier material are: good extrusion properties, good halogen-free flame retardancy (oxygen index usually above 40) and low cost.
This oxygen barrier has been used extensively in the cable industry for more than a decade and has led to significant improvements in the flame retardancy of cables. The oxygen barrier can be used for both halogen-free flame-retardant cables and halogen-free flame-retardant cables (e.g. PVC). A large amount of practice has shown that cables with an oxygen barrier are more likely to pass single vertical burning and bundle burning tests.
From the point of view of material formulation, this oxygen barrier material is actually “ultra-high filler”, because to meet the low cost, it is necessary to use a high filler, to achieve a high oxygen index must also add a high proportion (2 to 3 times) of Mg ( OH) 2 or Al ( OH) 3, and to extrude good and must choose EVA as the base material.
3. 4 Modified PE sheathing material
Polyethylene sheathing materials are prone to two problems: firstly, they are prone to melt breakage (i.e. sharkskin) during extrusion; secondly, they are prone to environmental stress cracking. The simplest solution is to add a certain proportion of EVA in the formulation. used as a modified EVA mostly using low VA content of the grade, its melt index to between 1 to 2 is appropriate.
4. Development prospects
(1) EVA has been widely used in the cable industry, the annual amount in the gradual and steady growth. Especially in the last decade, due to the importance of environmental protection, EVA-based fuel resistance has been rapid development, and has partly replaced the PVC-based cable material trend. Its excellent cost performance and excellent performance of the extrusion process is difficult to replace any other materials.
(2) cable industry annual consumption of EVA resin close to 100,000 tons, the choice of EVA resin varieties, VA content from low to high will be used, coupled with the cable material granulation enterprise size is not large, spread in each enterprise each year only in the thousands of tons of EVA resin up and down, and thus will not be the EVA industry’s giant enterprise attention. For example, the largest amount of halogen-free flame retardant base material, the main choice of VA / MI = 28 /2 ~ 3 of EVA resin ( such as the U.S. DuPont’s EVA 265 # ). And this specification grade of EVA so far there is not a domestic manufacturers to produce and supply. Not to mention VA content higher than 28, and melt index less than 3 of other EVA resin production and supply.
(3) foreign companies producing EVA because of no domestic competitors, and the price has long been high, seriously suppressing the domestic cable plant production enthusiasm. more than 50% of the VA content of rubber-type EVM, are a foreign company dominated, and the price is similar to the VA content of the brand 2 to 3 times. Such high prices, in turn, also affect the amount of this rubber type EVM, so the cable industry calls for domestic EVA manufacturers, to improve the rate of domestic production of EVA. More production of the industry has been a lot of use of EVA resin.
(4) Relying on the wave of environmental protection in the era of globalisation, EVA is considered by the cable industry to be the best base material for environmentally friendly fuel resistance. The use of EVA is growing at a rate of 15% per year and the outlook is very promising. The amount and growth rate of shielding materials and medium and high voltage power cable production and growth rate, about 8% to 10% between; polyolefin resistances are growing rapidly, in recent years have remained at 15% to 20% between, and in the foreseeable next 5 to 10 years, may also maintain this growth rate.