A water repellent PP film or foam panel is used inside every single auto door. The PP film or foam panel can prevent water from entering into an auto interior during raining or car washing. Conventionally, butyl rubber-based sealant is used to bond these PP films or foam panels onto the door body. Butyl rubber-based sealant does offer excellent moisture impermeability. However, its low cohesion characteristic is not favorable for repositioning purpose in the after service market. The PP film or foam panel should be removable and re-mountable easily without scarifying moisture impermeability after repairing.
Hot melt pressure sensitive adhesives (HMPSAs) have been introduced to this application area in the past few years. Since SBC-based HMPSA is much more elastic than butyl rubber-based sealant, it can be removable cleanly during repairing service. However, the high elasticity and cohesion of SBC-based HMPSA sometimes cause adhesion failure between adhesive and PP surface. This deficiency may result in moisture permeation into the interior of the auto body. To reduce the elasticity of HMPSA, a foam-able HMPSA containing Nitrogen is introduced for this purpose. After foaming, the 50-70% solid content HMPSA becomes softer, less rebounded, and more deformable.
In the real world application, HMPSA is foamed in a hot melter and applied onto the PP film or foam panel according to a fixed contour by a nozzle accompanied with a XY-robotic arm. The sticky adhesive foam strip is covered by release paper. At the auto assembly conveyor, the release paper is peeled off and the PP film or foam panel is attached onto the auto body by a hand roller. The selective HMPSA for this application should impart good adhesion performance to PP film or foam panel and auto body at low, room, and high temperatures. Upon separation, the foamed HMPSA should remain on the PP film or foam panel without transferring any residue to the auto body.
Most importantly, HMPSA should possess very high cohesive strength during foaming process at elevated temperatures. Consequently, the skin of HMPSA foam will not break and create some uneven porosity between the bonding interface. The undesired porosity may cause eventual moisture permeation and bond failure. In addition to performance advantages of the newly introduced foam-able HMPSA technology versus butyl rubber sealant, by means of this foaming technology, the total amount of adhesive consumed per door is greatly reduced.