Causes of compound bubble in solvent free

There are many reasons for solvent-free composite bubbles, including humidity, adhesive control (viscosity, standing time, leveling property, glue amount), substrate reasons (surface tension, substrate rigidity, substrate barrier, crystal point impurities on the surface of substrate), composite process (pressure, temperature, winding tension, splicing machine speed), mechanical reasons (such as metering roller, rubber roller Surface roughness, curvature, damage and aging condition of cots and compound rolls). 1. Humidity: the adhesive used for solvent-free composite is polyurethane type, and its main agent is isocyanate. The isocyanate functional group of isocyanate is a very active group, which is easy to react with water vapor in air. When 1tool water (18G) reacts with 1tool diisocyanate (MDI 250g), 1mol carbon dioxide (22.4l at room temperature) can be generated, Thus, a small amount of water can consume a large amount of diisocyanate and produce a large amount of gas. The gas produced is too late to be discharged, and the composite two layers of substrate will swell up during curing, leaving bubbles after the adhesive is cured. When the relative humidity is higher than 85% RH, the probability of bubble generation increases obviously. It is necessary to dehumidify and reduce the exposure time of hygroscopic substrate in the air in the actual production process. 2. Control of adhesive: after the adhesive is prepared, it will react gradually at a certain temperature. With the extension of the service time, the viscosity will increase significantly, which will affect the leveling property when coating. At the same time, it is easy to wrap air to cause blank glue, resulting in bubbles; The prepared adhesive needs to stand, and the main purpose is to reduce the entrainment of bubbles. When the standing time is too short, it is easy to entrain gas into the composite two-layer substrate to produce bubbles; The fluidity and surface tension of the selected adhesive affect its wetting and leveling on other substrates, and bubbles are generated when the leveling performance is poor; When the activation degree of the adhesive is not enough (sometimes the temperature of the composite steel roll is too low), the leveling of the adhesive will be affected, resulting in bubbles; When the amount of glue is too small, there is still blank after leveling under the action of adhesive force, resulting in gas retention and bubbles after curing. 3. Substrate reason: substrate surface tension: when the surface tension of composite substrate is lower than the required value, it will affect the flow and paving of adhesive, resulting in bubbles; Rigidity of base material: when the rigidity of composite base material is too large, it is easy to warp and affect the flattening of base material, resulting in bubbles; Barrier of substrate: as we know, barrier refers to the impermeability of film to gas and water vapor. The higher the barrier, the less the gas spilled under the same conditions, and the bubbles formed between the two layers after curing; Crystal point and impurity on the surface of the substrate: when the crystal point and impurity particle size on the surface of the composite substrate is too large, the roll will be pushed up slightly when passing through the gluing and composite roll, affecting the extrusion and discharge of gluing and gas. After passing through the composite roll, the crystal point will rebound and still prop up the film, leading to bubbles. 4. Composite process: the pressure of composite roller: the pressure of composite roller determines the compactness of composite material, exhausts the gas between the two layers of composite substrate, and makes the adhesive fully level under the pressure. When the pressure is too low, it affects the discharge of gas and the leveling of adhesive, especially when the speed is too fast. Winding tension: the winding tension is too small, which makes part of the composite substrate with higher rigidity warped, and creates a small gap with another composite substrate, forming bubbles; At the same time, due to the influence of moisture and other elements, the gas generated by the reaction is not easy to escape when the winding tension (interlaminar pressure) is relatively low, and bubbles are formed after curing due to the barrier property of the film. Compound roller temperature: the temperature of compound roller is too low, which affects the flow and spreading of adhesive. Air bubbles are formed in the blank. Splicing machine speed: when the splicing time and speed of film coil change and the flatness of splicing substrate change, the pressure of compound roller will change, resulting in the wrapping of air bubbles and the formation of air bubbles. 5. Mechanical reasons: the surface roughness, curvature, damage and aging degree of metering roll, rubber transfer roll, rubber coating roll and compound roll will also cause bubbles. In particular, the rubber transfer roll, because it contacts the metering roll and the coating roll at the same time, realizes the control of the rubber transfer and loading amount through the speed ratio of J three. If the surface hardness and roughness are too large, it will cause the change of the amount of glue and the uniformity of glue to appear blank and form bubbles. If the concentricity of the composite roll is abnormal, it will cause the gas entrainment of the composite film and the formation of bubbles.