EVOH can be regarded as the modifier of the EVA. The EVOH, which is a new type barrier material, integrates the good processability of PE polymer and high gas barrier of vinyl alcohol. Its capacity of gas barrier is 100 times higher than PA, and 10000 times higher than PE and PP. The transparency, glossiness, mechanical strength, flexibility, wearing resistance, cold resistance, and surface strength of EVOH are very high. It has excellent performance of obstructing gases, fragrance and resolvent. Therefore, the EVOH is mainly used on hard and soft packing. At the same time, along with the development of the pipe with oxygen barrier, the EVOH is widely used in high-grade pipe more and more.
1. Application of EVOH
In food packing: the plastic film containing EVOH barrier layer is becoming the new standard for food packing film.
In non-food packing: includes packing for resolvent, cosmetic, medicines, toothpaste hose, and electrical products, etc.
For producing plastic oil cell
For producing high-grade pipes: the plastic pipe with EVOH oxygen barrier has excellent weather resistance and gas barrier performance. This kind of pipe can be used as direct drinking water pipe system and high-grade heating pipe.
For producing high-grade cloth with specific function.
For producing high-grade medical instruments.
2. EVOH pipe
2.1 Why use EVOH pipe?
The common plastic pipes have oxygen permeability which will be higher along with increasing of the temperature. The oxygen permeability will induce oxygen content in the hot water enhancing along with the temperature increasing.
The related documents indicate that the oxygen permeability content will be larger than 0.1mg/(L.day) when the water temperature is 40¡æ. This will seriously erode the metal heaters, metal valves, fittings, radiators, and pumps in the heating system. Therefore, plastic pipes with oxygen barrier are used in the hot water system (heating, conditioner, and ground heating, etc.) in Europe. In heating pipe system industry, American, Europe and Australia oblige to use heating plastic pipe with oxygen barrier in heating engineering.
Oxygen permeability comparison between common pipe and pipe with oxygen barrier
|
|
DIN 4726 standard |
EVOH pipe with oxygen barrier |
PE-X pipe |
PP-R pipe |
PE-RT pipe |
PB pipe |
|
Oxygen permeability (mg/1.day) |
Below 0.1 |
0.033 |
3.2 |
6.5 |
4.5 |
6.5 |
2.2 Advantages of EVOH pipe
Long service life: produce according to Germany standary with ensuring 50 years service life;
Strong stress cracking resistance and high temperature resistance at high temperature (up to 110¡æ);
Good memory;
Excellent shock strength at low temperature with the service temperature to -50¡æ;
High wearing resistance and mechanical resistance capacity suiting to be used in bad building site environment.
Excellent chemical resistance.
2.3 EVOH pipe structure
There are three kinds of EVOH pipes according to the different position of EVOH barrier layer positions:
A: internal layer is EVOH oxygen barrier, the outer layer is PE or PEX, and the middle layer is the EVOH pipe with thermosol coating.
B: the internal layer and the outer layer is the PE or PEX, the outer layer is EVOH oxygen barrier layer, and the middle layer is the EVOH pipe with thermosol coating.
C: the internal pipe is PE or PEX, the middle layer is the EVOH oxygen barrier, and the outer layer is PE or PEX; EVOH pipes with thermosoal coating are wrapped between the two layers.
The former two kinds are three layers structure, while the last kind is five layer structure. The five structure EVOH pipe production equipment will be more complex.
The advantages and disadvantages of the three kinds of PE-RT/EVOH pipes:
The EVOH pipe with EVOH oxygen barrier as outer layer: oxygen barrier layer is very easy to be scraped off, and is easy to be damaged causing by impacting of hard object during transportation and construction.
The EVOH pipe with EVOH oxygen barrier as inner layer: it is not convenient to observe the change of the oxygen barrier. When the relative humidity is larger than 60%, the barrier capacity of the oxygen material will drop sharply. Therefore, this kind of pipe is not ideal for heating system.
The EVOH pipe with EVOH oxygen barrier as middle layer: this kind of pipe does not have the above two disadvantages.
Because the oxygen barrier layer is easy to be damaged or abated, the EVOH layer will be little thick in the three-layer structure EVOH pipe than the five-layer structure EVOH pipe. From using performance, cost and quality stability, the EVOH oxygen barrier as the middle layer is better than the other two kinds.
3. EVOH pipe production line
The EVOH pipe production line is similar with common plastic pipe production line, but some key section has special characteristics.
The EVOH resin will decompose when the temperature is too high, so the EVOH will adhere to the metal surface. Therefore, it shall avoid having concave and convex part, sharp corner or dead center on the extrusion components.
3.1 Specially designed EVOH extruder barrel and screw
The material of the EVOH extruder barrel shall be grinding oxidation steel or special alloy, like barrel and screw with surface chromium-plating. It shall avoid usingscrew of low rotary speed and large ratio of length-diameter to shorten the residence of melt inside the barrel, so as to reduce the EVOH decomposing chance because of heating. It needs 4 to 5 zones for controlling the temperature precisely. The material feeding port shall have cooling device to avoid raw material melting prematurely. The EVOH barrel and screw shall be corrosion resistance.
The EVOH extrusion screw shall be suitable to extrude thin layer material. Employ screws of different length-diameter ratios according to EVOH brands. F degree (containing 32M01% ethylene) L/D¡Ý26; E degree (containing 44M01% ethylene) L/D¡Ý24.
3.2 Co-extrusion head
The co-extrusion head of multi-layer is common gradually, especially is widely used in high-grade inflation film manufacturing machine. The path structure of co-extrusion is usually spire distributor. The design, processing and commission of each parameter shall be tested for a long time, so as to fit with used materials.
3.3 High-precise temperature and speed control
The EVOH is sensitive to the processing temperature, so the temperature control of extrusion equipment shall be highly precise. In addition, the thickness of each layer will be thin in multi-layer structure, and the total thickness of small EVOH pipe is thin, so the speed control precise for the pipe production line shall be high. By right of mastering and developing on electrical control technique for many years, Guangzhou Advance-Tech Co., Ltd reaches a high level on high-precise automatic control for the pipe production line. The pipe control system designed and manufactured by the company can compare with the craft brother in the world.
3.4 Special process
The requirement for the EVOH extrusion process is very high with each process such as heating, cooling, diameter calibrating and stretching shall be controlled properly. Guangzhou Advance-Tech Co., Ltd has developed a set of complete process, which ground a stable basis for producing excellent EVOH pipe with oxygen barrier at high speed.
