Bubble Phenomenon
The so-called bubble means that the two layers of film are not completely bonded, and the composite finished film and the substrate have a gap and a gas, and when the aluminized film is combined, a white point like a needle tip is generated. Essentially, the applied glue is not sufficient to fill the voids of the ink layer in the composite film and the pits on the surface of the material, also referred to as bubbles. The bubbles are not necessarily bulged and may exist in other forms. As long as the two films are not completely bonded, the transparency will be different, and we will often say “bubbles”.
1.The Main Reason
Different types of composite substrates have different physical properties (such as gas permeability), and have a great relationship with the formation of composite film bubbles. Generally speaking, a substrate with good gas permeability (such as BOPP, CPP, CPE, LDPE) or a composite substrate with poor gas permeability (such as PVDC, BOPA) and another composite substrate with good gas permeability are compounded. It is not easy to generate bubbles. However, if the two substrates have poor gas permeability, when they are compounded, bubbles are easily generated, such as PVDC and BOPA. This is because the gas permeability of the two composite substrates is relatively poor, and the adhesive inevitably mixes a small amount of air on the compounding system. At the same time, the isocyanate adhesive itself is cross-linked and solidified during the curing stage of the composite film, and also produces gaseous by-products such as carbon dioxide (CO2). In this way, a large number of tiny bubbles are generated, thereby affecting the apparent quality of the composite film. At the same time, in order to make the composite substrate have a certain surface tension, the surface properties of the substrate must be treated, such as corona treatment, to improve the wettability of the surface of the composite substrate and improve the roughness, but the roughness exceeds At certain limits, it is difficult to be filled with viscous glue. Thus, large and small bubbles are formed between these minute engaging faces. The viscosity of the adhesive is also an important cause of air bubbles. Generally speaking, the viscosity of the adhesive is related to the molecular weight of the adhesive, the plasticizer and the type of the diluent, and the adhesive generally has a high viscosity and a poor fluidity at normal temperature. Therefore, in order to lower the viscosity and improve the fluidity, it is required to be higher at the time of compounding. Insulation at temperature. The higher the temperature of the two-component adhesive, the shorter the mixing and storage time, and the larger the ratio of the main agent and the curing agent, the greater the viscosity of the adhesive, the more unfavorable the formation of bubbles, and the more easily the bubbles are formed. Bubbles of the same size, low-viscosity glue can be eliminated, and high-viscosity glue may not work because the high-viscosity glue has a large molecular weight and poor leveling.
2. The Processing Method
(1) The bubble caused by insufficient glue and uneven coating is actually a blank point caused by no glue, which can be eliminated by appropriately increasing the amount of glue applied. Insufficient amount of glue is a relative concept. In addition to the peel strength, the leveling of the coating is an important factor. The leveling of the coating is not only affected by the amount of glue but also The uniformity of the rubber roller, substrate, adhesive, and the viscosity of the adhesive will affect it. The comprehensive conditions are good, and the amount of sizing can be appropriately reduced. This is more pronounced on aluminized and white printed film composites.
(2) Air bubbles caused by too much humidity. The reason is that the carbon dioxide gas generated by the reaction of the NCO component of the adhesive with water vapor is too late to deposit a film and is accumulated between the films, and cannot be volatilized through the film to form bubbles. This phenomenon is solved by properly controlling the ambient humidity during production. Generally, the relative humidity is below 70%.
(3) Since the curing temperature is insufficient and the bubbles are not eliminated, the curing temperature should be appropriately increased. The temperature of the curing chamber is not higher than 50 ° C and not lower than 35 ° C. It is rare to have no bubbles in the composite film, and the curing can eliminate small bubbles. Because the two-component polyurethane adhesive is not cured, the higher the temperature, the greater the fluidity, and the “secondary leveling” can eliminate the bubbles to a certain extent.
(4) Bubbles generated by insufficient drying of the ink solvent are caused by residual solvent in the printing film. Generally, it occurs in a portion where the ink overprinting rate is high, and a part of the bubbles having a low overprinting rate is small. In production, it should be ensured that it is completely dry and then recombined. It can also handle printed film or reduce compound speed resolution.
(5) Air bubbles caused by equipment process reasons, such as damage or foreign matter in the roller of the composite part or the rubberizing part, the steel roller should be replaced and the foreign matter should be removed. (6) Air bubbles caused by the production process. Mainly refers to the control of some process parameters, including composite pressure, winding tension, coating temperature and composite temperature. If the composite pressure is too low, the rubber roller pressure should be increased appropriately. The compounding speed is too fast, and the compounding speed can be appropriately lowered, or the temperature of the rubberizing roller can be increased to increase the fluidity of the adhesive. Air bubbles on the surface of the film caused by dust and impurities should improve the environmental sanitation of the production workshop.
