The good compounding effect of the two-component polyurethane adhesive is related to various conditions, and the change of working environment is also an important influencing factor. In other words, as the seasonal climate changes, in order to obtain the desired composite effect, it is necessary to make some fine-tuning of the adhesive application process.
Simply put, the seasonal climate change that affects the compound is the change of the two major indicators of environmental humidity and temperature: specifically in the spring and summer seasons, especially during the rainy season, the relative humidity of the air is large, and it can even reach saturation and the autumn and winter seasons The air is dry and the humidity is low; in terms of temperature, the summer is much higher than the winter, and the maximum difference between the two can be nearly 30 to 40 degrees Celsius. (Here is an example of the southern area without indoor heating for comparison).
If you do not pay attention to these differences, it is likely that the following problems will occur during compounding: when the air is wet, the adhesive is often not fully cured, that is, it does not dry out, the residual viscosity is large, and even serious. A stringing phenomenon is observed during peeling, especially when the film itself has a high hygroscopicity, such as when it is compounded with a nylon membrane.
Secondly, the moist air will condense on the gluing anilox roller, which will bring the moisture into the glue basin. With the passage of time, the adhesive gradually changes from transparent to chaotic, whitish, and even loses its adhesion ; Humid heat and high temperature also make it difficult to preserve the adhesive. If the prepared working glue is not used up on the day, it will often whitish and agglomerate after being left overnight to form a gel (jelly? ㄠ??. The opposite Yes, when it is cold in winter, the working glue that has not been used up still maintains good transparent fluidity overnight, and it can even be used directly on the machine without having to be mixed into the newly prepared working glue in batches. On the other hand, in the winter when the temperature is low, the adhesive will become sticky and the leveling will decrease. When the compound machine is running at a high speed, a large number of bubbles are likely to accumulate on the surface of the glue and the glue texture. The edge of the roller may cause cavitation transfer, and the amount of upper adhesive is insufficient, which affects the fastness of the composite; at the same time, due to the poor leveling and dispersion performance, the appearance of the composite film will also be poor, such as the leveling of the adhesive More obvious, sometimes orange When it is used in composite aluminum foil or aluminum plating film, if there is a large area of ​​white ink or light color ink on the printing surface, it is easier to form small white spots and spots; in addition, due to the low winter temperature, the temperature of the curing room and the external environment The temperature differs greatly, if the insulation measures are not done in place, the heat loss rate is much faster than the high temperature in summer, which often makes the temperature in the curing room reach the set temperature (generally 50 ℃). Therefore, the effect of curing is also Under certain influences, under the same curing time, the compound fastness may be lower than that in summer.
To analyze the causes of the above phenomena, it is necessary to understand the influence of moisture and temperature on the adhesive. First of all, for two-component polyurethane adhesives, moisture, like the main agent in it, polyester / polyether polyol, can react with the NCO group 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 pure reaction weight ratio. In practice, the water mixed in the working glue is the main agent when reacting with the curing agent. Competing. But no matter how much water is involved in the reaction, it is undoubtedly that the curing agent is consumed, so that the amount of its reaction with the main agent does not reach the originally set working ratio, so it causes incomplete curing and residual viscosity. . The viscosity and reactivity of the adhesive are strongly related to the temperature. The viscosity value given by the adhesive manufacturer is measured with a rotary viscometer at 25 ° C as the standard temperature, which means that in actual use, the temperature of the working environment can be more than 10 degrees higher in summer and cold in winter It may be 20 ° C lower than it. The viscosity of the adhesive is just the opposite of the temperature, that is, the same adhesive exhibits a lower viscosity value when the temperature is high, good flow and dispersion performance, and low viscosity, high viscosity and poor leveling. In addition, the cross-linking curing reaction between the two components of the adhesive, the reaction speed is slow when the temperature is low, and the reaction speed is fast when the temperature is high, which is precisely why there is a curing room (accelerating the curing reaction speed, Increase productivity).
In view of this situation, when using a two-component polyurethane adhesive 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 is appropriately increased by 10% to 20% to compensate for moisture For its consumption; often use dry cotton yarn or cloth shreds to absorb the water droplets condensed on the compound machine to prevent it from falling into the plastic basin; the unused working glue can be diluted with a small amount of solvent and then sealed and stored, if conditions permit, It can be placed in a small freezer for refrigeration and storage, which has a better effect. When it is reused next time, it will be thawed in a closed state and mixed with the newly prepared working glue. When the temperature is low in winter, more solvent can be added to prepare the working glue to reduce the viscosity of the system, improve the leveling and dispersion, and also reduce the generation of air bubbles in the glue bowl 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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