Abstract:The article analyzes in details about the quality influence of leveling property of the adhesive in lamination process.Addition to this,it mentions that instead of judging the leveling performance by judging if there is ‘white spots ’or ‘bubbles’,it is the transparency of the laminated products that could be the evaluation standard of the leveling performance in adhesive.
1.The Bubble Problem and the Leveling of Glue
White spots, bubbles, and poor transparency are common appearance quality issues in the processing of composite materials. In most cases, composite material processors attribute the above issues to poor leveling of the adhesive!
1.1This glue is not that glue
Composite material processors may return unopened and unused barrels of adhesive to suppliers based on the judgment of poor leveling of adhesive, or file complaints or claims with suppliers.
It should be noted that the glue considered to have poor leveling performance is a “glue working solution” that has been prepared/diluted by customers and has a viscosity of a specific value. The returned glue is the unopened original bucket of glue.
These two buckets of “glue” are completely different concepts and things!
1.2Evaluation indicators for glue leveling
The technical indicators for evaluating the leveling performance of adhesive should be viscosity and surface wetting tension. Or rather, “the fluidity of glue” is a combination of “the fluidity of glue” and “the wettability of glue”.
At room temperature, the surface wetting tension of ethyl acetate is about 26mN/m.
The original barrel concentration (solid content) of solvent based polyurethane adhesives used in the field of composite material processing is generally between 50% -80%. Before implementing composite processing, the above-mentioned adhesives need to be diluted to a working concentration of around 20% -45%.
Due to the fact that the main component in the diluted adhesive working solution is ethyl acetate, the surface wetting tension of the diluted adhesive working solution will be closer to the surface wetting tension of ethyl acetate itself.
Therefore, as long as the surface wetting tension of the composite substrate used meets the basic requirements of composite processing, the wettability of the adhesive will be relatively good!
The evaluation of the fluidity of glue is viscosity. In the field of composite processing, the so-called viscosity (i.e. working viscosity) refers to the time in seconds that the glue working fluid experiences when flowing out of the viscosity cup, measured using a specific model of viscosity cup. It can be considered that the working fluid of glue prepared from different grades of original bucket glue has the same “working viscosity”, and its “working fluid” has the same “glue fluidity”!
Under other unchanged conditions, the lower the “working viscosity” of the “working fluid” prepared with the same frame type adhesive, the better its “adhesive fluidity”!
More specifically, for several different grades of adhesives, if the viscosity value of the diluted working solution is 15 seconds, then the working solution prepared by these grades of adhesives has the same “glue leveling”.
1.3The leveling property of glue is a characteristic of glue working fluid
Some alcohols do not form a viscous fluid when the barrel is just opened, but rather a jelly like projectile with no fluidity. They need to be dissolved and diluted with an appropriate amount of organic solvent to obtain the desired concentration and viscosity of glue.
It is obvious that the leveling performance of glue is an evaluation of the working solution formulated into a specific “working concentration”, rather than an evaluation of undiluted original barrel glue.
Therefore, it is incorrect to attribute the poor leveling of glue to the common characteristics of a certain brand of original bucket glue!
2.Factors affecting the leveling of adhesive
However, for the diluted adhesive working solution, there are indeed differences in its adhesive water level!
As mentioned earlier, the main indicators for evaluating the leveling performance of adhesive working fluid are surface wetting tension and working viscosity. The indicator of surface wetting tension does not show significant changes within the conventional working concentration range. Therefore, the essence of poor adhesive leveling is that during the application process, the viscosity of the adhesive abnormally increases due to certain factors, resulting in a decrease in its leveling performance!
What factors can cause changes in the viscosity of glue during its application?
There are two main factors that can cause changes in the viscosity of glue, one is the temperature of the glue, but the concentration of the glue.
Under normal circumstances, the viscosity of a fluid decreases with increasing temperature.
On the user manuals provided by different adhesive companies, the viscosity values of the adhesive solution (before and after dilution) measured using a rotary viscometer or viscosity cup at a liquid temperature of 20 ° C or 25 ° C (i.e. the temperature of the adhesive solution itself) are usually indicated.
On the client side, if the storage temperature of the original bucket of glue and diluent (ethyl acetate) is higher or lower than 20 ° C or 25 ° C, the temperature of the prepared glue will also be higher or lower than 20 ° C or 25 ° C. Naturally, the actual viscosity value of the prepared glue will also be lower than the viscosity value indicated in the manual. In winter, the temperature of the prepared adhesive may be lower than 5 ° C, and in summer, the temperature of the prepared adhesive may be higher than 30 ° C!
It should be noted that ethyl acetate is an extremely volatile organic solvent. During the volatilization process of ethyl acetate, it will absorb a large amount of heat from the adhesive solution and the surrounding air.
At present, most laminating units in composite machines are open and equipped with local exhaust devices, so a large amount of solvent will evaporate from the adhesive disc and barrel. According to observations, after a period of operation, the temperature of the glue working fluid in the glue tray may sometimes be more than 10 ° C lower than the surrounding ambient temperature!
As the temperature of the glue gradually decreases, the viscosity of the glue will gradually increase.
So, the leveling performance of solvent based adhesives actually gradually deteriorates with the prolongation of equipment operation time
In other words, if you want to maintain the stability of solvent based adhesive leveling, you should use a viscosity controller or other similar means to keep the adhesive viscosity stable throughout the application process.
3.Evaluation indicators for correct glue leveling results
The evaluation of the leveling result of glue should be a characteristic of the composite product at a specific stage, and the leveling result of glue refers to the result obtained after the glue is applied.Just like the “designed maximum speed” of a car is a characteristic of the product, the actual driving speed of the vehicle on the road under specific conditions is another result.
Good glue leveling is the fundamental condition for achieving good leveling results. However, good leveling performance of glue may not necessarily result in good glue leveling results, and even if the glue has poor leveling performance (i.e. high viscosity), good glue leveling results can still be achieved in specific situations.
4.The correlation between the results of glue leveling and the phenomena of “white spots” and “bubbles”
Poor “white spots, bubbles, and transparency” are several undesirable results on composite products. There are many reasons for the above problems, and poor leveling of glue is just one of them. However, the reason for poor leveling of glue is not only due to poor leveling of glue!
A poor leveling result of glue may not necessarily lead to “white spots” or “bubbles”, but it can affect the transparency of the composite film. If the micro flatness of the composite substrate is poor, even if the leveling result of the adhesive is good, there is still a possibility of “white spots and bubbles”.
Post time: Jan-17-2024