Ethylene-vinyl alcohol copolymer (EVOH)
EVOH can be regarded as a copolymer of ethylene and vinyl alcohol according to its molecular structure, which is +CH2-CH2fCH-CH2)m. In EVOH, the content of the ethylene component, OH, is usually in the range of 30% to 50% (molar fraction). Since EVOH is a copolymer structure, its properties change accordingly with the different contents of ethylene and vinyl alcohol. As the ethylene content increases, the barrier to non-polar gases (oxygen, carbon dioxide, ammonia, etc.) decreases while the moulding and processing properties and moisture resistance improve accordingly.
The characteristics of EVOH are as follows.
(1) Main Advantages
Barrier oxygen and other non-polar gases through the performance is particularly good; has outstanding fragrance retention; health performance is excellent, can be used for food, pharmaceutical packaging; oil resistance, resistance to organic solvents performance is superb; has good transparency and gloss; printing performance is excellent; environmental protection and adaptability, the waste can be melted regeneration processing, combustion processing and recovery of heat energy does not produce environmentally hazardous toxic and hazardous substances; moulding processing Excellent performance, can be applied to general-purpose plastic moulding equipment for processing.
(2)Main disadvantages
Expensive, EVOH price is not only much higher than ordinary polyolefin plastics, and more expensive than the price of nylon 6; heat resistance is limited, even if the cooking level of EVOH, made of co-extruded composite film, the highest can only be subjected to high-temperature cooking at 121 ℃.
(3) EVOH Forming Processing Points
EVOH has certain hygroscopicity, the moisture content of the granule should be controlled below 0.3%, if the material is damp, it must be pre-dried before use, otherwise the product will have defects such as bubbles.EVOH has enough thermal stability, but compared with general-purpose plastics such as polyethylene, EVOH's thermal stability is poor, when the temperature exceeds 230℃, the part of the structure of the ethylene alcohol in EVOH will be rapidly reduced, and it can be used as a composite film. When the temperature exceeds 230℃, the structural part of vinyl alcohol in EVOH will be decomposed rapidly, so the temperature of extrusion moulding must be controlled below 220℃. EVOH stays in the extruder for a long time, it will be gelatinous because of the heat of the resin, which will lead to trouble in operation, so when short-term shutdown is needed in the process of production, it is necessary to add a small amount of lubricant into the EVOH and supply high-viscosity polyolefin resin in order to exclude the EVOH resin in the extruder. When the production process requires a short-term shutdown. When the production process needs to shut down for a long period of time, the equipment should be cleaned up immediately after stopping the operation.
2. Polyvinylidene chloride (PVDC)
Polyvinylidene chloride resin is also a co-extruded barrier multilayer composite film barrier materials, polyvinylidene chloride is a homopolymer of vinylidene dioxygen, it is non-toxic, tasteless, good transparency, high mechanical strength, good toughness, resistance to oils and organic solvents; heat-shrinkage and self-adhesion is good; but also has self-extinguishing, taste preservation, as well as excellent moisture, mildew, etc., and at the same time, has excellent printing and heat sealing properties, but also on the In terms of barrier property, it is a very ideal barrier material, but the application quantity is not much yet. Polyvinylidene chloride not only has high performance in blocking oxygen and other non-polar gases (oxygen barrier and EVOH resin is comparable), but also has high water and moisture resistance, its water and moisture resistance even exceeds that of HDPE resin, which ranks first among the commonly used plastics. But PVDC resin has two major defects: one is poor thermal stability, film-forming conditions are very harsh, even after plasticising, stabilising the formulation of PVDC plastics, but also must be applied with special alloy screws, barrels and die head of the special co-extrusion equipment, under strict control of the process conditions in order to successfully produce multi-layer co-extruded film containing PVDV barrier layer, so the development of co-extruded film PVDC class The second is containing PVDC barrier film environmental protection adaptability is poor, due to the lack of thermal stability of PVDC, containing PVDC barrier film edges and recycled materials, can not be regenerated through hot melt pelleting recycling, while the PVDC incineration will produce vinyl chloride and other hazardous substances, and even produce a strong carcinogenic substance dipyrimidine, and therefore subjected to environmental protection people to blame, and this is also the most important reason why the PVDC is still in use. At the same time, the incineration of PVDC produces harmful substances such as vinyl chloride and even the strong carcinogen dipyrimidine, so it has been criticised by environmentalists.
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