Abstract:This article analyzes the performance,correlation,and role of adhesive leveling at different stages of compounding,which helps us better judge the real cause of compound appearance problems and quickly resolve the problem.
In the process of flexible packaging composite production, the “leveling” of the adhesive has a significant impact on the composite quality. However, the definition of “leveling”, the different stages of “leveling”, and the impact of microscopic states on the final composite quality are not very clear. This article takes solvent adhesive as an example to discuss the meaning, correlation, and role of leveling at different stages.
1.The meaning of leveling
Leveling properties of adhesives：The flow flattening ability of the original adhesive.
Leveling of working fluid: After dilution, heating and other methods of intervention, the ability of adhesive working fluid to flow and flatten during coating operations is achieved.
First leveling ability: The leveling ability of the adhesive after coating and before lamination.
Second leveling ability: The ability of the adhesive to flow and flatten after compounding until it has matured.
2.The interrelationships and effects of leveling at different stages
Due to production factors such as adhesive amount, coating state, environmental state (temperature, humidity), substrate state (surface tension, flatness), etc., the final composite effect can also be affected. Moreover, the multiple variables of these factors can cause significant fluctuations in the composite appearance effect and also result in an unsatisfactory appearance, which cannot be simply attributed to poor leveling of the adhesive.
Therefore, when discussing the impact of leveling on composite quality, we first assume that the indicators of the above production factors are consistent, that is, exclude the influence of the above factors and simply discuss leveling.
First, let’s sort out the relationships among them：
In the working fluid, the solvent content is higher than that of pure adhesive, so the viscosity of adhesive is the lowest among the above indicators. At the same time, due to the high mixing of adhesive and solvent, its surface tension is also the lowest. The flowability of adhesive working fluid is the best among the above indicators.
The first leveling is when the fluidity of the working fluid begins to decrease with the drying process after coating. Generally, the judgment node for the first leveling is after composite winding. With the rapid evaporation of the solvent, the fluidity brought by the solvent is rapidly lost, and the viscosity of the adhesive is close to that of pure adhesive. Raw rubber leveling refers to the fluidity of the adhesive itself when the solvent contained in the finished raw barrel rubber is also removed. But the duration of this stage is very short, and as the production process progresses, it will quickly enter the second stage.
The second leveling refers to entering the maturation stage after the composite process is completed. Under the influence of temperature, the adhesive enters the stage of rapid crosslinking reaction, and its fluidity decreases with the increase of reaction degree, ultimately losing completely.Conclusion: Working fluid leveling ≥ first leveling>original gel leveling>second leveling
Therefore, in general, the liquidity of the above four stages gradually decreases from high to low.
3.The influence and control points of different factors in the production process
3.1Glue application amount
The amount of glue applied is essentially not necessarily related to the fluidity of the glue. In composite work, a higher amount of adhesive provides more adhesive in the composite interface to meet the interface’s demand for adhesive quantity.
For example, on a rough bonding surface, the adhesive supplements the interlayer gaps caused by uneven interfaces, and the size of the gaps determines the amount of coating. The fluidity of the adhesive only determines the time it takes to fill the gaps, not the degree. In other words, even if the adhesive has good fluidity, if the coating amount is too low, there will still be phenomena such as “white spots, bubbles”.
3.2Coating status
The coating state is determined by the distribution of the adhesive transferred by the coating net roller to the substrate. Therefore, under the same coating amount, the narrower the mesh wall of the coating roller, the shorter the travel between the adhesive points after transfer, the faster the formation of the adhesive layer, and the better the appearance. As an external force factor that interferes with the adhesive connection, the use of uniform glue rollers has a more significant positive effect on the composite appearance than those that are not used.
3.3Condition
The different temperatures determine the initial viscosity of the adhesive during production, and the initial viscosity determines the initial flowability. The higher the temperature, the lower the viscosity of the adhesive, and the better the flowability. However, as the solvent volatilizes faster, the concentration of the working solution changes faster. Therefore, under temperature conditions, the solvent evaporation rate is inversely proportional to the viscosity of the working solution. In overproduction, controlling the solvent evaporation rate has become a very important issue. The humidity in the environment will accelerate the reaction rate of the adhesive, exacerbating the increase in viscosity of the adhesive.
4.Conclusion
In the production process, a clear understanding of the performance, correlation, and role of “adhesive leveling” at different stages can help us better determine the true cause of appearance problems in composite materials, and quickly identify the symptoms of the problem and solve them.
Post time: Jan-17-2024