Questions - Please tell me more detail about interfacial movement between 1) PET & LDPE and 2) BOPP & LDPE.
What is mechanism of swelling and Different thermal expansion of PET, BOPP and LDPE ?
How can I solve or at least minimise the problem of interfacial movement?
I think interfacial movement is the main problem because demetallization is mainly happened in the sealing part of the laminate pouch & at the point where small hole is created to remove air from pouch , hence I need your valuable suggestions for preventing interfacial movement of films in the laminate.
Answers
1. Generally thin polymeric webs are produced by melting and extruding the polymer as a thin strip that is then stretched, both forwards and sideways, that increases the area but reduces the thickness and results in the thin film. The stretching is used to increase some of the film properties such as tensile strength.
Where different polymers are laminated together there may be problems of differential stress because of a number of factors. The two different polymers may have different coefficients of thermal expansion. This means that for the same temperature rise they will expand by different amounts. If they are stuck together this may show up as the laminated film curling.
A second factor may also play a part. When the films are manufactured and stretched the temperature is raised to make the stretching process more easily achieved. Once the film has been stretched the temperature is reduced and the film is locked into the new shape and size. There will be some residual stress within the films. This can mean that if the temperature of the web is raised but without the web having any tensile load applied the web will shrink a little. This shrinkage will be permanent where as the coefficient of thermal expansion is temporary and the expansion reversed as the temperature is lowered. The forwards and sideways stretching may be balanced (equal stretching in both directions) or not. If the web has been stretched a different amount in the forwards direction to the sideways direction the shrinkage will most likely be different in each direction. This ratio of stretching may be different for the different polymers and so may highlight the different shrinkage forces.
In addition to this the stretching affects the web differently at the edges of the web than in the centre of the web. The centre of the web sees the most uniform stress pattern and so sometimes customers of the web manufacturers request rolls slit only from the centre of the web but may expect to pay a premium for these rolls.
Thus laminating rolls of material slit from the edges of the mill roll may have different angles of stress to each other that compound the shrinkage differences rather than cancel out some of the differences. Thus increasing the interfacial stress.
2. Swelling may or may not be a problem. Polymers often contain some moisture & this is removed from the surface during the metallization process. Once out of the metallizer these polymers may, over time, re-absorb some moisture.
Polymers may or may not be stable to solvents. If coatings are deposited onto the polymers, such as adhesives, they may contain a solvent. The solvent may, or may not, migrate through the polymer. There will most likely be some residual solvent in the coating and if the adhesive is used for lamination it may adversely affect the polymer that it is laminated onto.
3. Of the materials mentioned the BOPP and LDPE often contain additives such as slip additives (more so than PET that generally contain fewer additives) that are used to improve the polymer handling characteristics by reducing the coefficient of friction. These additives are detrimental to the adhesion. Often any surface treatment such as flame, corona or plasma is done to remove these slip additives. If the surface is treated it ought to be done on both surfaces as the slip additives will have migrated to all surfaces equally and so if only one surface is treated it may be re-contaminated from the second almost immediately.


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