How does one determine the bond strength of partial cold seal coated flexible packaging substrates? Is there a standard formula to determine the minimum specification of bond strength for different substrates?
Let’s start with basics of the cold seal process. The substrates of a cold seal package are coated with a pressure sensitive adhesive which bonds only to itself. When these substrates are brought together under pressure, they bond to each other. The bonding is usually carried out at room temperature. Application of external heat is not required to achieve a bond.
The final bond strength in a cold seal package, which would generally be measured according to ASTM F88 (Standard Test Method for Seal Strength of Flexible Barrier Materials) is dependent upon several factors. The adhesive formulation is the most important factor affecting bond strength. The formulation can be designed to produce a range of bond strengths. The formulation is usually tailored to the end use application to deliver performance throughout the product life. Your convertor should be able to identify the right formulation to meet the application’s needs. The coating weight also has an impact on the bond strength especially when application weights approach the upper and lower limits. The limits are defined by the application. The convertor of cold seal packaging products determines the proper weight range to achieve the targeted bond levels to meet the application requirements. There are also a large number of factors during the cold seal application process that will influence the bond of the cold seal to itself (including the substrate to which it is adhered). These factors are all controlled in the convertor’s process and include application method, drying process, substrate surface energy, etc.
There is no universal model or formula for calculating the bond strength requirements for cold seal applications. The packaging material supplier should be able to provide guidance on the typical cold seal bond requirements for a given application. Performance testing may be required to verify the cold seal bond strength achieved is adequate for the application. As previously discussed, the bond strength is determined by formulation, coating weight and processing. The convertor of the cold seal material should be able to assist in identifying the seal strength range achieved with a given cold seal formula. The convertor should also have coat weight specifications to insure the proper amount is applied to achieve the desired seal strength.