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Some Precautions For Using Two-Component Polyurethane Adhesives

Views:34     Author:Site Editor     Publish Time: 2021-04-14      Origin:Site

Some Precautions For Using Two-Component Polyurethane Adhesives



The good composite effect of two-component polyurethane adhesive is related to various conditions. In addition to the use of good quality static mixer nozzles and Epoxy Dispensing Gun, changes in the working environment are also very important factors. In other words, as the seasons and climate change, in order to obtain the ideal composite effect, it is necessary to make some fine adjustments to the glue application process.


Simply put, the seasonal climate change that affects the compound is the change in the two major indicators of environmental humidity and temperature: specifically, in spring and summer, especially during the rainy season, the relative humidity of the air is relatively large, and it can even reach saturation while in autumn and winter. The air is dry and the humidity is low; in terms of temperature, summer is much higher than winter, and the maximum difference between the two can be close to 30-40°C (here is the southern area without indoor heating as an example for comparison). If you do not pay attention to these differences, the following problems are likely to occur during compounding: When the air is humid, the glue is often incompletely cured, that is, it is not dry, and the residual viscosity is large. In serious cases, it can even be peeled off the composite film. It is observed that there is a phenomenon of wire drawing, especially when the film itself is relatively hygroscopic, such as when it is laminated with a nylon film. This phenomenon is more likely to occur; secondly, humid air will condense on the gluing anilox roller, thereby removing moisture. When it is brought into the glue basin, as time goes by, the glue gradually changes from transparent to chaotic, white, and even loses its bonding effect; damp heat and high temperature also make it difficult to preserve the glue. If a good working glue is not used on the same day After leaving it overnight, it will often turn white and agglomerate, forming a gel (jelly-like). In contrast, when the weather is cold in winter, the working glue that has not been used up before still maintains good transparent fluidity overnight, and it can be taken directly without even being mixed into the newly prepared working glue in batches. Come on the machine to use. On the other hand, in winter when the temperature is low, the glue will become sticky and the leveling property will decrease. When the laminating machine is running at high speed, a large number of bubbles are likely to accumulate on the surface of the glue and the edge of the gluing anilox roll in the glue basin. At this time, it may cause the transfer of cavitation and the insufficient glue quantity will affect the composite fastness; at the same time, due to flow The flat dispersion performance is not good, the appearance effect of the composite film will also be worse. For example, the glue has obvious flowing flat lines, sometimes in the shape of orange peel. When used in composite aluminum foil or aluminized film, if the printing surface has a large area of white ink or In light-colored ink, small white spots and spots are more likely to form; in addition, due to the low temperature in winter, the temperature of the curing room is very different from the ambient temperature. If the insulation measures are not adequately done, the heat loss rate will be much faster than that of the high temperature. Summer is fast, which often makes the temperature in the curing room not reach the set temperature (usually 50°C). Therefore, the curing effect is also affected to a certain extent. Under the same curing time, the composite fastness may be lower than in summer.


To analyze the reasons for the above phenomenon, we must understand the influence of moisture and temperature on the adhesive. First of all, for two-component polyurethane adhesives, moisture can react with the NCO groups in the curing agent just like the main agent, namely polyester/polyether polyol. According to calculations, 1g of water can consume 26-32g of curing agent. Of course, this is purely in terms of reaction weight ratio. In practice, the water mixed into the working glue will react with the curing agent when it reacts with the curing agent. Competing. But no matter how much water is involved in the reaction, this is undoubtedly consuming the curing agent, making the amount of its reaction with the main agent not reach the originally set working ratio, thus causing incomplete curing and residual viscosity . The viscosity and reactivity of the adhesive are greatly related to temperature. The viscosity value given by the adhesive manufacturer is measured with a rotational viscometer at 25°C as the standard temperature. This means that in actual use, the temperature of the working environment can be more than 10 degrees higher than that in summer, and it may be higher in winter when it is cold. It is as low as 20°C. The viscosity of the glue is just the opposite of the temperature, that is, the same glue exhibits a low viscosity value at high temperature, good flow and dispersion performance, low temperature, high viscosity and poor leveling. In addition, the cross-linking and curing reaction between the two components of the adhesive is slow at low temperatures and fast at high temperatures. This is precisely why there is a curing room (to speed up the curing reaction speed and increase production effectiveness).


In view of this situation, when using two-component polyurethane adhesives for compounding, the following adjustments can be made according to environmental changes: If the air is humid and the temperature is high, the amount of curing agent should be appropriately increased by 10% to 20% to compensate for the moisture. Consumption; often use dry cotton yarn or cloth scraps to absorb the condensed water droplets on the laminating machine to prevent them from falling into the rubber basin; the unused working glue can be diluted with a small amount of solvent, and then sealed for storage. If conditions permit, it can be placed Keep it in a small refrigerator for better results. When you use it next time, it should be thawed in a closed condition and mixed with the newly prepared working glue. When the temperature in winter is low, the working glue can be prepared by adding more solvents to reduce the viscosity of the system, improve the leveling and dispersion, and also reduce the generation of bubbles in the rubber basin during work. However, the concentration of the working glue will be reduced in this way. If you do not want to change the working concentration, you can replace part of the ethyl acetate with a small amount of acetone, that is, use a mixed solvent of acetone and ethyl acetate as the diluent. The ratio can be 2:8 or 3:7. In addition, when the temperature is low in winter, the temperature of the curing room can be adjusted slightly higher to ensure that the actual temperature can meet the requirements, so as not to affect the curing effect.


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