The PVDC latex coating process is different from the PVDC resin blown film and extrusion process. The latex coating only has coating spreading and heat drying process. The final film does not contain any additional additives. Therefore, despite the fact that the coating is only 2-3 μm thick, its barrier properties against oxygen and moisture are equivalent to the barrier properties of a 25 μm blown film. This can be seen in Table 1. see.
As a barrier packaging material, PVDC coating film is most suitable for making composite packaging bags, because the mechanical and certain physical properties of the coating film are determined by the substrate to be coated, and the final barrier, fragrance, and oil resistance The performance is provided by the coating. The user only needs to determine the mechanical and physical performance requirements and the degree of protection of the materials to be installed in order to select the barrier capability of the substrate and the coating according to the characteristics of the packaged articles and the environment in which they are to be circulated.
In addition to being directly related to the degree of coating advancement, the barrier capability of PVDC coated anger is related to the film formation process and process control of the coating. For example, the film formation process of PVDC latex coating is: latex state (State 1)→drying (State 2)→deformation thinning state (State 3). State 1 to State 2 must ensure that the coating has a sufficiently high “body temperature”; ensure that there is sufficient energy supply to allow the moisture to evaporate at a uniform rate; ensure that no over-drying occurs, and that the appearance of coated pinholes or surface crusts occurs after the water boils. The water vapor penetrates through the bottom layer and causes coating cracking.
In states 2 to 3, it should be ensured that there is enough air flow velocity to sweep the water film on the coating surface so that there is a reasonable gradient of moisture distribution from the inside to the surface of the coating, so that the moisture can quickly migrate to the surface. . It must be ensured that the state 3 has a reasonable existence time during the drying process to further deform the PVDC coating and fully move the molecules to improve the crystallinity and to squeeze out the low amount of PVDC as much as possible. All of these are conducive to improve the barrier properties of PVDC coating, to ensure that PVDC has a strong solvent resistance and effectively reduce the solvent residue in the printing, composite processing; to ensure that the coating peel strength, improve the coating film gloss and transparency.
It is safe to say that the dry film formation process of PVDC is not so simple. How to fully understand this process, so as to design a more reasonable process control process, this is a long-term problem for PVDC workers, only the process design is reasonable, precise process control, can produce the perfect PVDC coating film, in order to correct Reflect the true face of PVDC barrier packaging material.
To realize this is not easy, so there is no quality assurance for the production of PVDC emulsion coating on the dry complex machine or printing machine, and it can only provide limited barrier performance, and it is very likely to bring other quality problems. Such as oil, poor solvent resistance, composite peel strength and so on. In addition, we can not mark the surface of the coating as flat as the horizontal plane, and microscopically there is a roughness on any surface. It is not difficult to find products with rough coatings on the current domestic market.
The rough coating (even coating flow lines) not only causes the printing to have non-uniform transfer of ink on the screen, but also due to the coating thickness at thick peaks, insufficient drying, insufficient dense coating, insufficient barrier capacity, and solvent resistance. The performance is not strong, and the composite peel strength is not high. This is not strictly a qualified PVDC coated film product.
How to effectively combine equipment structure, latex formulation, tension setting, temperature control, etc. This is the core technical secret of each PVDC coating manufacturer, the so-called KNOW.HOW. In short, PVDC is proved by more than 30 years of use in developed countries. At present, there are no barrier packaging materials that can be replaced by more valuable materials.
The key is how to understand and master the PVDC performance and reasonable processing process in depth. In order to objectively demonstrate its true appearance.