Analysis of the properties and structural properties of PA films

2020-06-14 13:12:33
Polyamide (abbreviated as PA), polyamide has the advantages of abrasion resistance, toughness, chemical resistance, heat resistance, cold resistance, easy molding, self-lubrication, non-toxicity and easy dyeing compared with general plastics. It is characterized by low oxygen transmission rate and has attracted people's attention in the field of packaging. Since the amino group on the molecular chain of PA can form strong intermolecular hydrogen bonds, PA has high Tf and thermal stability, high mechanical properties, high oxygen barrier, carbon dioxide inhibition, flavor inhibition, solvent resistance and the like. Of course, the highly polar amino group also causes the water absorption property of the PA. The adsorbed water molecules open the intermolecular hydrogen bond of the PA, so that the rigidity of the PA is lowered and the elasticity is improved, so that the PA has good puncture resistance, viscosity, and thermoforming. performance. These outstanding properties of the PA are retained even after film formation, and these properties are required for the package.
(1) Mechanical properties
PA exhibits high tensile strength, toughness and puncture resistance, so that the composite film prepared on the substrate also has these excellent properties. The mechanical properties of PA are related to the amount of water adsorbed. Water as a plasticizer can reduce the tensile strength and stiffness of PA and improve the toughness, impact properties and elasticity of the material. In the case of PA single mode, PA can absorb more water in a few minutes. Even if PE blocks moisture outside PA, the mechanical ability of PE/PA/PE composite film will be affected by moisture.
(2) Barrier performance
The PA film has moderate barrier properties to oxygen, and its oxygen barrier property is two orders of magnitude higher than that of polyolefin, which is one to two orders of magnitude lower than that of high barrier EVOH film. The barrier properties of EVOH will drop to the level of ordinary PA (such as MXD6) under higher humidity conditions. PA exhibits high barrier properties to low-level small-molecule hydrocarbons, as well as low-polar aromatic hydrocarbons in foods (lemon oil, camphor oil, vanillin, menthol), only for those with high polarity. The molecular resistance, such as water, ethanol, methanol, etc., will reduce the barrier properties of PA.
(3) Thermoforming properties
If the PA film is used as the inner layer (heat seal layer) of the composite film, it is generally required that the outer layer of the composite film should have higher heat resistance than the inner film, so that the PA layer can be heat-sealed, and the outer layer of the composite film is not It will adhere to the heat sealing rod, so as to meet the requirements of the bag making line. If PA is used as the outer layer and PE is used as the inner layer, even if a higher heat-sealing temperature is used, the sealing knife generally does not adhere to the PA film, and the cooling can be cooled.
(4) Other performance
BOPA film has high stiffness, good printing performance, high tensile strength, high puncture resistance and low tensile fracture growth rate. The deformation of the BOPA film under tension is elastic deformation, not viscous deformation, so it is suitable as a substrate for vacuum aluminum plating or vacuum silicon plating film. BOPA also has good printing performance because it has small tensile deformation.
Compared with non-stretched oriented PA (cast PA film, ie CPA), BOPA oxygen barrier is 30% higher than CPA, but this difference is not much different from PET film; for PET, after stretching Biaxially stretched polyester film (BOPET) has a three-fold increase in barrier properties compared to unstretched cast polyester film (CPET). This is due to the fact that the initial crystallinity of CPA is inherently induced, although tensile stress is induced. The shaped PA molecular chain recrystallizes, but the degree of crystallinity is limited, while CPET is in an amorphous state. Therefore, the stretching process induces and accelerates PET crystallization, and the degree of crystallinity increases is particularly obvious, resulting in the barrier property of BOPET. 3 times better than CPET.
Compared with BOPA, CPA has a low modulus, is easy to yield under tension, and then undergoes viscous deformation, and then the molecular chain is stretched and oriented to cause stress hardening. Compared with BOPET, although the elastic modulus of BOPA, its tensile strength and dart impact strength are also higher than BOPET, its barrier to oxygen is higher than BOPET.

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