Precautions for the use of various glues

Glue and sealing glue should be used in certain environment under the conditions of use, and the working conditions have an important influence on the bonding performance.

Among the conditions of use, there are stress conditions, ambient temperature and humidity, chemical media conditions, outdoor conditions and so on.

(1), the stress situation. When the adherend is subjected to peeling force and uneven peeling force, glue with good toughness can be used, such as rubber glue, polyurethane glue, etc.; High glue, such as epoxy glue, acrylate glue.

(2), temperature conditions. Different glues have different heat resistances. According to different temperatures, use different glues.

(3), humidity. Moisture and moisture are very detrimental to the stability of the adhesive interface and can be said to be harmful rather than beneficial. Because the water molecules are small in size and large in polarity, after infiltration and diffusion, they play a hydrolysis effect, which destroys or disengages the adhesive surface, resulting in a decrease in the strength and durability of the adhesive bond. The adhered parts require good water resistance, such as epoxy glue, polyurethane glue, etc.

(4), chemical medium. Chemical medium mainly refers to acid, alkali, salt, solvent, etc. Different types of glue, different curing conditions, have different medium resistance. Therefore, glue and sealant should be selected according to the medium contacted by the adherend.

(5), outdoor conditions. The conditions of the adhesive joints used outdoors are relatively complex. Temperature changes, wind and rain, sun and freezing, etc., will accelerate the aging of the adhesive layer and shorten the service life. Therefore, under outdoor conditions, adhesives with high temperature curing and atmospheric aging resistance should be used for bonding, such as phenolic-acetal adhesive, epoxy-nitrile-butadiene adhesive; silicone sealant should be used for sealing.

Two-component polyurethane adhesive

The good composite effect of the two-component polyurethane adhesive is related to various conditions, among which the change of the working environment is also a very important factor. That is to say, with the change of seasonal climate, in order to obtain the desired compounding effect, it is necessary to make some fine-tuning of the glue 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 spring and summer, especially in the rainy season, the relative humidity of the air is relatively high, and it can even reach saturation. The air is dry and the humidity is low; in terms of temperature, the temperature in summer is much higher than that in winter, and the maximum difference between the two can be nearly 30 to 40 °C (here, the southern area without indoor heating is used as an example for comparison). If you do not pay attention to these differences, the following problems are likely to occur during lamination: when the air is humid, the glue is often not cured completely, that is, it is not dry, and the residual viscosity is large. The phenomenon of wire drawing is observed, especially when the film itself has high moisture absorption, such as when it is compounded with nylon film. It is brought into the glue pot. As time goes by, the glue gradually changes from transparent to chaotic, whitish, and even loses its bonding effect. The hot and humid high temperature also makes the preservation of the glue more difficult. After leaving it overnight, it often turns white and clumps to form a gel (jelly-like). On the contrary, in the cold winter, the working glue that has not been used up before still maintains good transparency and fluidity overnight, and you can directly take it without mixing it into the newly prepared working glue in batches. Come to use it. On the other hand, in winter when the temperature is lower, the glue will become sticky and the leveling will decrease. When the laminating machine is running at high speed, a large number of air bubbles are easily generated in the glue basin and accumulate on the surface of the glue and the edge of the gluing anilox roller. At this time, it may cause the transfer of air bubbles and the insufficient glue amount, which will affect the lamination fastness. The flat dispersion performance is not good, and the appearance of the composite film will also deteriorate. For example, the leveling pattern of the glue is obvious, sometimes in the shape of orange peel. When used for composite aluminum foil or aluminized film, if the printing surface has a large area of ​​white ink or When light-colored ink is used, it is easier to form small white spots and spots; in addition, due to the low temperature in winter, the temperature of the curing room is very different from the external ambient temperature. Summer is fast, which often makes the temperature in the curing room less than the set temperature (usually 50 °C). Therefore, the effect of curing is also affected to a certain extent. Under the same curing time, the composite fastness may be lower than that in summer.

To analyze the reasons for 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 can react with the NCO groups in the curing agent, just like the main agent in it, that is, polyester/polyether polyol. 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 into the working glue is the main agent when it reacts with the curing agent. Competitive. However, no matter how much water is involved in the reaction, this will undoubtedly consume the curing agent, so that the amount of its reaction with the main agent cannot reach the original set working ratio, thus resulting in incomplete curing and residual viscosity. . The viscosity and reactivity of adhesives are strongly related to temperature. The viscosity value given by the adhesive manufacturer is measured with a rotational viscometer at a standard temperature of 25°C, which means that in actual use, the temperature of the working environment can be more than 10 degrees higher than it in summer, and may be higher than that in winter when it is cold. Its lower than 20 ℃. The viscosity of the glue is just the opposite of the temperature, that is, the same glue shows a lower viscosity value when the temperature is high, and the flow and dispersion performance is good. When the temperature is low, the viscosity is high and the leveling is poor. In addition, the cross-linking curing reaction between the two components of the adhesive is slow when the temperature is low, and fast when the temperature is high, which is precisely why there is a curing room (speed up the curing reaction, improve production effectiveness).

In response to such a situation, the following adjustments can be made according to environmental changes when using two-component polyurethane adhesives for compounding: if the air is humid and the temperature is high, the dosage of the curing agent should be appropriately increased by 10% to 20% to make up for the moisture. often use dry cotton yarn or cloth to absorb the condensed water droplets on the compound machine to prevent it from falling into the glue basin; the working glue that cannot be used up can be diluted with a small amount of solvent, and then sealed and stored, if conditions permit, it can be placed Store it in a small freezer for better effect. When using it next time, thaw it in an airtight condition and mix it with the newly prepared working glue. When the temperature is low in winter, some more solvent can be appropriately 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 basin during work. However, the concentration of the working glue will be reduced. If you do not want to change the working concentration, you can use a small amount of acetone to replace part of the ethyl acetate, 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 of the curing room is low in winter, the temperature setting of the curing room can be slightly increased to ensure that the actual temperature can meet the requirements, so as not to affect the curing effect.