Analysis and countermeasures for poor transparency of dry composite products
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Many plastic composite flexible packaging products are required to have high transparency, so it is necessary to use a large blank base film to recombine, but in the dry composite process, the plastic composite flexible packaging often has poor transparency or decline, failing to meet quality requirements, such as finished products. In the form of "fog", there are phenomena such as "small pockmarks", "wave scales", "silk patterns", "mushroom umbrella clouds", "drip splashes", etc., which affect the transparency of composite products, even when complex The finished product is scrapped. After a long period of observation and exploration, we have summed up a set of practical solutions for the industry to refer to practice.
First, the "fog" phenomenon and countermeasures
When the composite finished product is just off the machine, the blank space is “fog”, and it appears randomly. It may be “fog”, the next one is not, and then there are several “fog” after every few sheets. Appear regularly. If the contents are loaded, it is obviously not clean through the observation of the 40-watt fluorescent lamp.
It has been observed that the probability of this phenomenon when compounded with a one-component hot-melt adhesive is higher than that of a two-component reactive adhesive, and is easier in BOPP/OPP, BOPP/CPP structures. appear.
The reasons for this phenomenon are as follows:
1. Poor transparency of composite materials or printed substrates themselves
The film material is generally tested before it is compounded. The detection method is: taking a small section of the film through the light observation, requiring the film material to be smooth, no fine spots, no sticky, no unevenness, and no gray impurities. Therefore, only the high-quality materials produced by regular manufacturers can be combined to produce good finished products.
2. The effect of adhesive
The color of the adhesive is too dark, dark yellow or yellowish red. After drying, this part of the dark color will remain on the film accordingly. Therefore, when the composite product is required to have high transparency, it should be yellowish or even colorless. Adhesive. The correct choice of two-component adhesives can reduce the chance of "fog" in the composite finished product, and even eliminate the "fog" phenomenon.
3. Operation error or drying temperature is not enough
Operational errors or insufficient drying temperatures can also result in a "fog" phenomenon. For example, the amount of sizing is not uniform, one side is thick, and the other side is thin. Therefore, uniform coating is critical in dry compounding processes.
When compounding with a one-component adhesive, it is generally easy to cause uneven coating at the beginning of the winding. If this problem occurs, a composite finished product can be found through light observation.
If it is compounded with a two-component adhesive, it will be difficult to discriminate uneven coating. Generally, a composite product is taken in the winding part, and it is taken out after being solidified in the drying tunnel for 15 minutes. If this "fog" phenomenon still exists, it means that the adhesive is unevenly coated.
In the hot and rainy season, the "haze" phenomenon is most likely to occur, and it is often found in composite membranes of BOPP/OPP, BOPP/CPP, and PET/PE structures. Adjusting the drying tunnel temperature to the optimum state can reduce the probability of occurrence of this phenomenon.
Second, the phenomenon of "small pockmarks" and its countermeasures
The dry composite product often has a bubble-like "small pockmark", which is not a bubble. The small pitting points are unevenly distributed, but the size and thickness are similar, and they are visible in the blank of the substrate. This kind of "small pockmark" plagues many flexible packaging enterprises. By comparison, the two-component two-liquid reactive adhesive has a higher probability of producing small pitting than the one-component hot-melt adhesive, and the BOPP/OPP composite structure It is higher than BOPP/CPP and PET/PE, and there are several reasons for analysis.
(1) The adhesive is mixed with dust and other particles. There is too much dust in the air in the workshop environment. There is dust in the hot air blown out from the drying tunnel after the glue is applied. The dust adheres to the rubber layer and is sandwiched between the two base films. , resulting in opaque composite products.
The solution is to filter the adhesive with a 180 mesh wire mesh to remove insoluble particles from the adhesive. At the same time, we should pay attention to the cleanliness of the workshop, keep the ground clean and free of dust. In the dry compounding, the glue drum and the glue tray must be cleaned before leaving work. Especially in the summer, the indoor temperature is high, the drying is fast, and the residual adhesive should be poured into the barrel before work, and sealed separately.
In addition, the dust in the hot air should be removed with a high mesh filter or other filter material at the air inlet of the drying tunnel.
The two-component reactive adhesive is easily whitened and turbid due to the influence of temperature. The "cloudy white" adhesive can no longer be used, otherwise the uneven distribution of the rubber layer will occur, resulting in an increase in "small pockmarks".
(2) The quality of the base film and composite film is not good. If the OPP heat compound is yellow and old, it should be carefully checked. First, the surface tension value should be measured. The surface tension of BOPP should be 38mN/m, and the surface tension of PET should be 50mN/m. Low surface tension will result in poor wetting of the surface of the material, uneven coating of the glue, voids, and small pitting of the finished product after compounding. Therefore, the film whose surface tension value does not meet the requirements should be used after being processed.
The second is to use a magnifying glass to detect the smoothness of the base film and the composite film, and it is required to have no unevenness. If the surface of the material is uneven and the depth is different, it will not be completely tightly adhered to the film after being pressed by the hot pressing roller, and “small pockmark” will appear. So the selection of materials is very important.
(3) The flatness of the rubberizing roller is poor, the fluidity of the glue is insufficient, and the flatness is poor, which may lead to “small pockmark”. The rubber roller is required to be fine and smooth, and it is preferably a rubber roller made of silicone rubber. The glue has good fluidity, excellent flatness and uniform coating, which can reduce the chance of “small pockmarks” and improve the transparency of the composite products.
Third, the phenomenon of "fish scales" and its countermeasures
The probability of a "fish scale" phenomenon in composite finished products is less than that of "small pockmarks" and "fog", which has two reasons.
(1) The PE film itself has a phenomenon of “fish scales”, which appears in the blank after compounding, resulting in a decrease in composite transparency. The solution is to replace the material.
(2) Occasional shutdown during the production process, the rubberizing roller is not relaxed in time, or the rubberizing roller is not separated from the glue, and the hot pressing roller and the hot drum are not separated. Generally, a wavy irregular lateral indentation occurs in the blank at a pressure of 30 minutes, resulting in poor transparency. Therefore, in the non-working state, the rubberizing roller should be separated from the anilox roller and cleaned, and the hot pressing roller is separated from the hot drum, and the hot drum power source is turned off.
Fourth, other phenomena and countermeasures
There are two main reasons for the phenomenon of “mushroom umbrella cloud”, “drip splash” and “silk pattern” in composite finished products.
(1) The material itself is poor and irregular patterns appear. Just replace it.
(2) The influence of adhesives and drying tunnels. When the temperature enters the drying tunnel, the temperature is too high, and the drying speed is too fast, so that the solvent on the surface of the rubber layer is quickly volatilized, and the surface is crusted. When the heat penetrates into the interior of the rubber layer, the solvent under the coating vaporizes and breaks through the membrane surface to form a volcano. The ring of the mouth, like the "mushroom cloud" distributed on the surface of the film, causes the glue layer to be uneven and opaque. Therefore, the temperature of the drying tunnel should be increased from low to high, so that the solvent is completely volatilized from the inside to the outside in an orderly and uniform manner, leaving no odor. In addition, solvents of different boiling points can be mixed and used, so that when boiling by heat, the high boiling point solvent on the surface of the adhesive layer remains for a long time, and the surface film layer is not crusted, but the amount of residual solvent is required to meet the requirements, and does not exist. Odor and odor problems.