The second type of special-purpose plastics: barrier plastics
Barrier plastics refer to a class of polymers with a small pass coefficient for small molecule liquids, gases, water vapors, odors, etc. The barrier capacity of barrier plastic film is expressed by the "transmittance coefficient", and its unit is: ml(cm3,g)•mm/m2•d•MPa, which represents the significance: the volume (ml or cm3) or mass (g) of small molecule substances permeable by small molecule substances per square meter area for 1 day (24 hours) under the temperature of 23 °C, humidity 60%RH and pressure of 1 megapascal at a temperature of 23 °C, humidity 60% RH and 1 megapascal pressure. Usually O2, CO2 and water vapor are three small molecule substances as the standard. The smaller the transmittance coefficient of a material, the better its barrier properties.
Generally, the transmittance coefficient of high barrier plastics is less than 10, and the transmittance coefficient of medium barrier plastics is 10~100. Here are a few commonly used barrier plastics.
(1) Ethylene/vinyl alcohol copolymer
1. Introduction to ethylene/vinyl alcohol copolymer
Polyvinyl alcohol is a copolymer formed by vinyl alcohol monomer and ethylene monomer, abbreviated as EVOH. EVOH combines the excellent barrier properties of polyvinyl alcohol with the ease of processing of polyethylene, as well as the advantages of transparency and gloss, making it the highest barrier resin ever discovered.
Pure EVOH products have high water absorption, which can seriously affect the barrier properties. Therefore, EVOH is generally not used alone, but as an intermediate layer material combined with other plastics to prevent it from absorbing water and ensure its excellent barrier properties. EVOH has good compatibility with other materials, and can be compounded with PE, PP, PA, PET and other materials.
2. Structural performance
(1) Structural characteristics of EVOH The content of ethylene monomer and vinyl alcohol monomer in EVOH is different, and its properties are very different. Ethylene monomer provides water resistance and processability, while vinyl alcohol monomer provides barrier properties. Therefore, the barrier property, tensile strength and impact strength of EVOH decreased, while the water resistance and processability of EVOH were improved while the content of vinyl alcohol monomer was increased, and the barrier property of EVOH was enhanced, while the water resistance and processability of EVOH were reduced. In general, the vinyl alcohol content in EVOH is 66%~82%.
(2) General performance The relative density is 1.2, the transparency of the product is good and shiny, and it is easy to print and heat seal.
(3) Environmental resistance EVOH has good resistance to oil-based organic liquids, but it is not resistant to low-grade alcohols. At 20 °C, after soaking in various liquids and oils for one year, the non-absorption includes: cyclohexane, xylene, petroleum ether, benzene, acetone, etc., ethanol with an absorption rate of 2.3%, and methanol with an absorption rate of 12.3%.
EVOH is resistant to weak acids, alkalis and salts, has good weather resistance and excellent UV resistance.
(4) Mechanical properties EVOH has excellent tensile strength, impact strength and puncture resistance, elasticity, surface hardness and wear resistance. Its tensile strength, impact strength and puncture resistance are comparable to BOPA. Taking EVOH film as an example, the longitudinal tensile strength is 80MPa, the transverse tensile strength is 52MPa, and the elongation at break is 160% longitudinally and 200% transversely.
(5) Barrier performance EVOH has the most outstanding barrier performance. EVOH with 29% ethylene has a transmittance coefficient of 0.1 (cm3) for O2 and 0.4 (cm3) for 38% ethylene. The permeability coefficient to CO2 is 1.5~6 (cm3), and the permeability coefficient to water is 20~25 (g) (38% ethylene).
The barrier property of EVOH decreases rapidly with increasing temperature and is slightly slower than that of PVDC. When the temperature rises from 20°C to 60°C, the transmittance coefficient increases by a factor of 12. The barrier property also decreases rapidly with increasing humidity, even faster than PVDC. When the humidity rises from 20% to 80%, the transmittance coefficient increases by a factor of 10.
3. Molding processing
EVOH has good processability and can be processed by injection molding, extrusion, casting, and compounding. However, extrusion-stretching-blow molding and biaxially stretching film are the most commonly used. If the moisture content is greater than 0.3% before processing, it needs to be dried.
EVOH processing waste can be reused, and the amount added at a time is about 15%.
4. Modified varieties
The purpose of EVOH modification is to improve water resistance and further improve barrier properties. It can be modified by compounding, filling, biaxially stretching, etc.
5. Scope of application
(1) Food packaging due to its good oxygen resistance, its composite film is often used in fresh-keeping packaging materials, such as sausages, milk, roast chicken, condiments, ketchup, juice and other packaging.
PET/EVOH/PET blow molded bottles can be used for the packaging of beer and carbonated beverages.
(2) Non-food packaging such as solvents, chemicals, pharmaceutical packaging, gasoline barrel lining, air conditioning connecting pipe lining, etc.
