Substrate Secrets

I was recently asked the following question.

1. What is the reason for bubble formation in laminates? We are using a solvent-based adhesive system.

2. Also, the bubble problem is not observed for the same thickness of polyethylene without nylon layer (polyamide) and printed PET (Mylar ) laminate.

Bubbles are sometimes caused by air trapped in the nip or from shrinkage of the laminating adhesive as it cures or perhaps from incomplete drying of the solvent.  However in this case it appears that what ever is causing the bubbles might b trapped between layers of Nylon and PET.

Because you are using a solvent based system and the use of Nylon and PET give rise to the bubbles, I think there are two possible causes.

First is incomplete drying of the solvent from the lamination.  In this case the LDPE would not be a permeation barrier to the solvent and drying would proceed rapidly and would not reach saturation pressure in the adhesive causing a bubble.  Once the bubble forms and the adhesive cures it cannot collapse as the solvent permeates out.  You might determine this by taking fresh laminate of each product design and putting in a sealed jar.  Let sit several hours and then smell the head space to see if there is solvent smell.  You could compare the LDPE and nylon laminations to see if there is solvent and a rate difference but checking over a period of time, or using a gas chromatograph to check head space composition specifically looking for solvent.

Second, I assume the adhesive is a urethane and cures by moisture absorption.  During curing gasses are  given off and could be the source of the bubble formation.  For the LDPE case the water absorbed during drying could be sufficient for curing at a moderate rate and the LDPE prevents pick up of “extra” water for faster curing.  In the case of the nylon layer or PET, they will contain absorbed water and could be a source of water to increase the curing rate of the adhesive creating faster and greater off gassing from the cure of the adhesive, faster than it can diffuse out of the adhesive forming a bubble in the soft curing adhesive.

Then, as the adhesive cures the bubble becomes rigid and cannot collapse as the gasses diffuse thru the surrounding films.  You might need to condition the nylon and PET films in a dry room prior to use to lower the moisture level in the film or note seasonal differences (low humidly and high humidity seasons) from past converting history.  Also could measure moisture content of films or dry before lamination (run through dryer once or twice before lamination to “dry” nylon layer and PET films and then see what happens in the lamination).