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What is the leveling property of laminating adhesive? How to improve the leveling results of lamination?

“White spots”, “bubbles” and “poor transparency” are common product appearance quality problems in dry lamination processing. In most cases, the processors of lamination materials will attribute the causes of the above problems to the “poor leveling property of the glue”. Is that really the case? This article will explore the issue of adhesive leveling property in dry lamination.


Table of contents:

Concept: The relevant concepts of glue leveling property

Evaluation indicators of “glue leveling”

Factors affecting the “leveling property of glue”

How to improve the leveling results of adhesive

Q&A

Product recommendation


Concept:

First we need to understand what is adhesive leveling property, and what are the evaluation indicators of leveling property.

Glue leveling property: The flow and leveling ability of the original adhesive.

Working solution: The original barrel concentration adhesive can be mixed with a suitable amount of ethyl acetate diluent to form a specific viscosity “working solution”.

Glue leveling property: The flow and leveling ability of the adhesive working solution that can be applied after being diluted, heated, and other methods.

Evaluation indicators of “glue leveling”

The technical indicators for evaluating the characteristic of “glue leveling” should be the viscosity and surface wetting tension of the working solution, or “glue leveling” is a combination of “glue fluidity” and “glue wetting”.

The viscosity of working solution: In the field of lamination processing, for solvent-based dry lamination glue, the so-called viscosity (i.e., working viscosity) refers to the time in seconds measured by using a specific type of viscosity cup when the working solution flows out of the viscosity cup.

Surface wetting tension: The surface tension of the diluted working solution will be closer to the surface tension of ethyl acetate itself. Therefore, as long as the surface wetting tension of the lamination substrate used meets the basic requirements of lamination processing, “glue (working fluid) wetting” will be relatively good!

Factors affecting the “leveling property of glue” In lamination packaging

“Glue leveling” actually refers to “working solution leveling”, and there are indeed differences in leveling for working solutions that have been diluted. As mentioned earlier, the indicators for evaluating the leveling of the working solution are mainly surface tension and working viscosity. Among them, the surface tension indicator will not change much within the normal working concentration range, so the so-called “poor glue leveling” actually is: in the application process, due to some factors, the viscosity of the working solution abnormally increases, resulting in poor fluidity.

There are two main factors that cause changes in glue viscosity, namely:

Glue temperature: Under normal circumstances, the viscosity of fluids will decrease with increasing temperature. Ethyl acetate is a highly volatile organic solvent, which absorbs a large amount of heat from both the glue solution and the surrounding air during its volatilization process. At present, most solvent-based dry lamination machines have open-type gluing units, and some are equipped with local exhaust devices, so a large amount of solvent will volatilize from the glue pan and barrel during equipment operation. According to observation, after running for a period of time, sometimes the temperature of the working solution in the glue pan will be more than 10°C lower than that of the surrounding environment.

Glue concentration As the temperature of glue gradually decreases, its viscosity will gradually increase! At the same time, with the continuous volatilization of solvent, the concentration of glue is also constantly increasing! As the working solution concentration increases, its viscosity will also gradually increase!

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Other factors that cause poor “glue leveling results”: The processing status and blockage status of mesh holes on the gluing roller, the application status of the smoothing roller, the positive and negative pressure status in the drying oven, the running speed of the lamination machine, winding status of the laminated film and “microscopic surface smoothness” are some other factors that affect “glue leveling results”.

How to improve glue leveling results

1. The fluidity of solvent-based dry lamination adhesive (working solution) actually gradually deteriorates as equipment running time (solvent volatilization) increases. To maintain stability in solvent-based adhesive (working solution) fluidity, we should use a viscosity controller or other similar means to keep adhesive (working solution) viscosity and temperature stable throughout the application.

2. The viscosity of solvent-free dry lamination adhesive is affected by body temperature (after heating and mixing) and the degree of completion chemical reaction between the main agent and curing agent during residence on the bearing roller/metering roller. Generally speaking, higher body temperature means lower viscosity; longer residence time means higher viscosity.  Therefore, to maintain stability in solvent-free adhesive (working solution) fluidity, we should try to increase adhesive temperature and minimize adhesive residence time in bearing roller/metering roller (try to lower glue level height).

3. Correct use of smoothing roller:

① Maintain a larger “wrap angle” between the smoothing roller and the carrier film;

② Make the smoothing roller rotate in the opposite direction to the running direction of the glue-bearing film.

③ Adjust the speed of the smoothing roller to make the glue-bearing film have good transparency.

4. Adjusting the appropriate winding state

The concept of “secondary leveling” is often mentioned in the industry. It is believed that the main agent and curing agent in the glue layer that has just been coated on the carrier film and has been dried have not completed their cross-linking and curing reaction, so the glue layer still maintains a certain degree of “creep”. Under the action of appropriate compound film winding pressure, the “raised” part of the glue layer will be “flattened”, while the “recessed” part of the glue layer will be “filled” (secondary leveling), thus making the fully matured compound film have better appearance and transparency, and play a significant role in eliminating “white spots”. Under the above premise, appropriate paper core size, winding tension and contact roller pressure will make laminated film roll pressure (hardness) maintain at a reasonable level, and make glue layer get good “secondary leveling” results.

Q&A: Is “glue leveling result” the main cause of the “white spot” and “bubble” phenomenon

“White spots”, “bubbles” and “poor transparency” are several results that people do not want to see on lamination products. There are many reasons for these problems, and “poor glue leveling result” is only one of them, but the reason for “poor glue leveling result” is not simply “poor glue leveling property” “Poor glue leveling result” does not necessarily lead to “white spot” or “bubble” problem, but it will affect laminated film’s “transparency”. If the lamination substrate’s microscopic smoothness is poor, even if the “glue leveling result” is good, there may still be a “white spot” or “bubble” problem.

The evaluation indicator of “glue leveling result” should be the lamination product’s transparency.  Not whether there are “white spots” or “bubbles”.

Bangda 598A/B solvent-free series receives a good reputation on the leveling property of adhesive. It is polyester-based, has low odor, good performance on PE, good leveling properties, and fast cure. It is boiling resistant, and in some cases, it can do 121°C.

There are many brands of dry lamination adhesives for flexible packaging in the world. They have different advantages and competitive edges in terms of product quality, performance, innovation, and customer service. Welcome to visit Bangda pu-based adhesive for dry lamination to explore more details and differences as well as make your profit.

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