What is the impact of seasonal environmental changes on the use of adhesives?
Published time:
2021-07-21
The good compounding effect of two-component polyurethane adhesives is related to many factors, and changes in the working environment are also important influencing factors. In other words, with the change of seasons and climate, in order to obtain the ideal compounding effect, it is necessary to make some fine adjustments to the adhesive application process. Simply put, the seasonal and climatic changes affecting the compounding are the changes 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 high, and 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
The excellent compounding effect of two-component polyurethane adhesives is related to many factors, and changes in the working environment are also important influencing factors. In other words, with the change of seasons and climate, it is necessary to make some fine adjustments to the adhesive application process in order to obtain the ideal compounding effect.
Simply put, the seasonal and climatic changes affecting compounding are the changes 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 high, and 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 nearly 30～40℃ (this is a comparison using the indoor, unheated southern region as an example).
If these differences are not paid attention to, the following problems may occur during compounding: When the air is humid, the adhesive often does not cure completely, that is, it does not dry thoroughly, and the residual tackiness is large. In severe cases, pulling threads can be observed when peeling the composite film, especially when the film itself has high moisture absorption, such as when compounding with nylon film, this phenomenon is more likely to occur;
Secondly, humid air will produce condensation on the glue-applying mesh roller, thereby bringing water into the glue basin. Over time, the adhesive gradually changes from transparent to turbid and white, and even loses its bonding effect; wet heat and high temperature also make the preservation of the adhesive more difficult. If the prepared working adhesive solution is not used up on the day, it will often turn white and clump after being left overnight, forming a gel (jelly-like) state.
In contrast, in cold winter, the working adhesive solution that was not used up remains in good transparent fluidity overnight, and can even be used directly on the machine without being added in batches to the newly prepared working adhesive solution. On the other hand, in the colder winter, the adhesive becomes viscous and its leveling property decreases. When the laminator runs at high speed, a large number of bubbles are easily accumulated on the surface of the glue and the edge of the glue-applying mesh roller, which may cause void transfer, insufficient adhesive application, and affect the bonding strength; at the same time, due to poor leveling and dispersion performance, the appearance of the composite film will also deteriorate, such as the leveling lines of the adhesive are more obvious, sometimes appearing orange peel-like. When used to laminate aluminum foil or aluminized film, if the printed surface has a large area of white ink or light-colored ink, it is easier to form small white spots and spots; in addition, due to the low winter temperature, the temperature difference between the curing room and the external environment is large. If the insulation measures are not in place, the rate of heat loss is much faster than in the high-temperature summer, which often causes the temperature in the curing room to not reach the set temperature (generally 50℃). Therefore, the curing effect is also affected to a certain extent, and under the same curing time, the bonding strength may be lower than in summer.
To analyze the causes of the above phenomena, it is necessary to understand the influence of water and temperature on the adhesive. First of all, for two-component polyurethane adhesives, water, like the main agent, polyester/polyether polyol, can react with the NCO groups in the curing agent. It is estimated that 1g of water can consume 26～32g of curing agent. Of course, this is in terms of the pure reaction weight ratio. In practice, the water mixed into the working adhesive solution competes with the main agent when reacting with the curing agent. However, no matter how much water participates in the reaction, this undoubtedly consumes the curing agent, so that the amount of its reaction with the main agent does not reach the originally set working ratio, thus resulting in incomplete curing and residual tackiness. The viscosity and reactivity of the adhesive are closely related to the temperature. The viscosity value given by the adhesive manufacturer is measured by a rotational viscometer at a standard temperature of 25℃, which means that in actual use, the temperature of the working environment in summer can be more than 10 degrees higher than this, while in cold winter, it may be more than 20℃ lower. The viscosity of the adhesive is exactly the opposite of the temperature, that is, the same adhesive shows a lower viscosity value at high temperature, good flow and dispersion performance, and high viscosity at low temperature, poor leveling.
In addition, the crosslinking curing reaction between the two components of the adhesive is slow at low temperatures and fast at high temperatures, which is precisely why there is a curing room (to accelerate the curing reaction speed and improve production efficiency).
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 can be appropriately increased by 10%～20% to compensate for the consumption of water; dry cotton yarn or cloth scraps are often used to absorb the condensed water droplets on the laminator to prevent them from falling into the glue basin; the unused working adhesive solution can be diluted with a small amount of solvent and then sealed for storage. If conditions permit, it can be stored in a small refrigerator for better results. When used next time, thaw it in a sealed state and mix it with the newly prepared working adhesive solution.
When the winter temperature is low, more solvent can be added to the preparation of the working adhesive solution to reduce the system viscosity and improve the leveling and dispersion properties, and also reduce the generation of bubbles in the glue basin during operation. However, this will reduce the concentration of the working adhesive solution. If you do not want to change the working concentration, a small amount of acetone can be used to replace part of the ethyl acetate, that is, a mixed solvent of acetone and ethyl acetate is used as a diluent, and the ratio of the two can be 2:8 or 3:7. In addition, when the temperature is low in winter, the temperature of the curing room can be set slightly higher to ensure that its actual temperature meets the requirements, so as not to affect the curing effect.
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