Question re. lamination problems.

We are doing water-based lamination with metallized BOPP film we also do it with metallized CPP film. However when we do adhesive lamination with BOPP we find problem of corrosion i.e. water attacks metallization layer. We have a doubt that the composition of metallized layer being coated on CPP is different than that of BOPP as they are from two different suppliers. Could you please guide us that what could be there in the metallized layer that is so hydrophilic that it does not allow moisture to evaporate?

Answer.

Cast PP and Biaxially Oriented PP differ where the PP is a mixture of amorphous and crystalline material. The orientation process can result in some alignment within the polymer of the crystalline material. This alignment changes the performance of the polymer is all sorts of ways including tensile performance and also permeation performance.  It is likely that the moisture barrier of the BOPP is better than that of CPP.  This might be a contributing factor.

What you do not mention is what the thickness is of each of the materials and this can also be a factor.  The barrier performance of any material is thickness dependent. Thus, if the CPP and BOPP are of different thickness then this too could affect the performance.

Similarly if the metallization thickness is different in the two samples then this too could result in a differing barrier performance.  Keep in mind that if the materials are metallized by different suppliers there may also be other differences in the metallized layer.  The faster the rate of aluminium deposition the smaller will be the crystal size of the aluminium coating. If the metallization is done at different pressures this too will result in differences within the coating. Higher pressure will result in a less dense coating and lower permeability for the same thickness.  Measuring the coating thickness and also the Optical Density (OD) of each coating will give an indication of how similar the two metallization processes are.  If the thickness were the same but the OD different, or the OD the same but the thickness different it would indicate there are differences in the metallization process. If both thickness and OD are the same, or very similar, it would indicate the metallization processes are also similar.  Another difference could also be in the quality of the coating as described by the number of pinholes per unit area.  The higher the level of pinholes the greater the permeability of the coating as well as the greater the number of starting points for any corrosion.  The surface roughness of the substrate can also affect the nucleation and growth of the aluminium and hence the barrier performance. The higher the surface roughness the worse the barrier performance is likely to be.  Other factors that could affect the corrosion of the metallized film are the temperature and humidity conditions seen by the roll of material.  If the metallized film is re-wound hot, as in greater than 30 deg C, in the metallizer it is likely that the aluminium coating will grow a thicker oxide more rapidly than if the temperature at the re-wind is closer to ambient, generally less than 30 deg C.  After metallization if the rolls are stored in a high humidity atmosphere they will hydrate the aluminised film and lead to more rapid oxidation than if stored in a low humidity atmosphere. Adhesion is another possible factor that could have an effect. It would be expected that the system with the lower adhesion to suffer from higher permeation and more likely to have a greater propensity to corrosion.   I would normally expect the metal to CPP adhesion to be lower than that of metal to BOPP.   

I would suggest that measuring the barrier performance, OD, metal thickness and adhesion of each of the metallized materials before lamination would be a useful comparison. 

I would expect that the barrier performance of the BOPP would be higher than that of the CPP thus making the removal of the water from the adhesive slower than for the CPP.

I hope that this gives you some points that you find useful in helping solve this problem.