Causes and Process Rectification Plan for Pinholes and Micro-Needle Eyes in Polyurethane Laminating Adhesive

In the dry lamination production process, pinholes and micro-needle eyes in the adhesive layer are frequent and minute defects, often emerging in the production of transparent composite films and high-gloss films. They manifest as densely distributed or scattered small transparent pores on the film surface, which can be observed with the naked eye. These defects not only damage the smoothness of the appearance but also cause discontinuity in the adhesive layer and weak adhesion. During the steaming process, the products are prone to develop broken points and water seepage, as well as local delamination. These are the key hazards affecting the yield rate of high-end packaging.

 

Poor defoaming of the adhesive liquid is the main cause of pinholes. Excessive stirring of the Polyurethane Adhesive, the inclusion of a large amount of air, and insufficient defoaming time after mixing and standing can result in small bubbles remaining in the adhesive liquid. After coating, the bubbles transfer to the substrate surface along the adhesive layer. After being heated in the drying oven, the bubbles rupture due to heat, and after curing, permanent small pinholes remain, forming large-scale micro-needle eye defects.

Incompatibility between the solvent and drying conditions will aggravate the pinhole defects. Excessive water content and impurities in the solvent, inconsistent evaporation rates, and sudden vaporization and cracking during high-temperature drying can cause the solvent to penetrate the moist adhesive layer and form holes. In addition, uneven temperature in the drying oven, rapid temperature rise, and rapid formation of a protective layer on the surface of the adhesive layer due to the high viscosity of the adhesive can cause dense pinholes. At the same time, excessive viscosity of the adhesive and uneven flowability can prevent the bubbles from merging and dissipating on their own, resulting in the formation of pores after curing.

 

To completely eliminate pinhole defects, it is necessary to standardize the entire coating process. Slow and uniform stirring of the adhesive, sufficient defoaming time, and the prevention of bubble-laden coating are essential. Selecting high-purity ethyl acetate, strictly controlling the water content of the solvent, and matching a reasonable working viscosity to ensure excellent adhesive layer flowability are crucial. Adopting a gradient temperature drying mode allows for the smooth evaporation of gases and solvents. Stabilizing the coating conditions can completely eliminate micro-pinholes and needle-eye defects, ensuring a dense and smooth adhesive layer.