Bad transparency analysis and countermeasures in dry compounding process

After a long period of observation and exploration, our factory has concluded a set of practical and feasible solutions and achieved initial results.

"Mist" phenomenon and countermeasures

When the composite finished product was just removed from the machine, the blank space was “hazy” and randomly appeared. It may appear as a “mist” on this sheet, and the next one did not appear, and then the “haze” appeared again after a few sheets. Appears regularly. If the contents are put in, the observation through a 40-watt fluorescent lamp is clearly not clear.

We note that the probability of this phenomenon occurring when recombining with one-component hot melt adhesives is higher than that of two-component reactive adhesives, and it is also compared in BOPP/OPP, BOPP/CPP structures. comes out often.

There are several reasons for this phenomenon.

1. Poor transparency of the composite or printed substrate itself

Prior to compounding, film materials are generally tested. The detection method is: taking a small part of the film through the light observation, the film material is required to be smooth, no fine spots, no sticky, no bump inequality, and no gray impurities. Therefore, only the selection of high-quality materials produced by regular manufacturers can produce good products.

2. The effect of adhesive

The color of the adhesive is too dark, dark yellow or yellowish red. After drying, this darker color will remain on the film accordingly. Therefore, when the finished product is required to have high transparency, yellowish or even colorless Adhesives. The correct choice of two-component adhesives can reduce the chance of “hazy” appearances in composite finished products and even eliminate “haze” phenomena.

3. Operation error or drying temperature is not enough

Misoperation or drying temperature will not produce the same "haze" phenomenon. For example, the amount of sizing is uneven, with one side being thicker and the other side being thinner. Therefore, uniform coating in the dry composite process is critical.
When compounding with single-component adhesives, coating unevenness often occurs at the just-wrapped position. If this problem occurs, a composite product can be found through light observation.

If two-component adhesives are used for compounding, it will be difficult to identify uneven coating. Our factory generally takes a piece of compound finished product in the winding section and puts it into the drying tunnel for curing for 15 minutes. If this “haze” phenomenon still exists, it means that the adhesive is unevenly applied.

In the high temperature and rainy season, the phenomenon of “haze” is most likely to occur, and it often appears in the composite film of BOPP/OPP, BOPP/CPP, and PET/PE structures. Adjusting the temperature of the drying tunnel to the optimal state can reduce the occurrence of this phenomenon.

"Small pits" phenomenon and countermeasures

Dry composite products often appear like "small pits" like bubbles, careful analysis is not a bubble. Small spots are unevenly distributed, but their size and thickness are similar and can be seen in the blank space of the substrate. This “small pit” plagues many flexible packaging companies.
We have compared, the two-component two-liquid reactive adhesive has a higher probability of producing small pits than the one-component hot-melt adhesive, and the BOPP/OPP composite structure is higher than the BOPP/CPP, PET/PE. There are several reasons for analysis.

(1) Adhesives are mixed with particles such as dust. There is too much dust in the air in the workshop environment. After the gluing, the hot air blown in the drying tunnel also has dust, and the dust sticks to the adhesive layer. When compounded, it is sandwiched between two base films. , resulting in opaque composite finished product.

The solution is to use a 180-mesh metal wire mesh for adhesive filtration to remove insoluble particles from the adhesive. At the same time, we should pay attention to the cleanliness of the workshop, keep the floor clean and free of dust. In dry compounding, plastic buckets and glue trays must be cleaned before off duty. Especially in summer, the indoor temperature is high and the drying is fast. Before each shift, the residual adhesive should be poured into the barrel and closed separately.

In addition, dust from the hot air should be removed with a high mesh filter or other filter material at the air inlet of the drying tunnel.
Two-component reactive adhesives are susceptible to whitening and turbidity due to the influence of temperature. The “cloudy white” adhesive cannot be reused. Otherwise, uneven distribution of the adhesive layer will occur, resulting in an increase in “small pits”.

(2) The quality of base film and composite film is not critical. OPP hot material should be examined carefully if it is yellow and old. First, the surface tension should be measured. The surface tension of BOPP must reach 38mN/m, and the surface tension of PET must reach 50mN/m. Low surface tension leads to poor wetting of the surface of the material, uneven coating of the glue, voids, and appearance of small pits after compounding. Therefore, the surface tension value of the film can not meet the requirements, to be processed before use.

Followed by the use of a magnifying glass to detect the smoothness of the base film and composite film, requiring no bump. If the surface of the material is uneven and varies in depth, it cannot be completely and tightly adhered to the film after being pressed by a hot-pressing roll, and "small pits" will appear. So the selection is critical.

(3) Poor smoothness of the rubber roll, insufficient fluidity of the glue, and poor leveling result in “small pits”. It is required that the rubber roller be fine and smooth, and it is preferably a rubber roller made of silicon rubber. The glue liquid has good fluidity, excellent flatness, and even coating, which can reduce the chance of “small pits” and at the same time improve the transparency of the composite finished product.

"Fish Scales" Phenomenon and Countermeasures

The probability of "fish scales" appearing in composite products is less than that of "small spots" and "haze". There are two reasons for this.

(1) The phenomenon of “fish scales” appeared on the PE membrane itself, which appeared in the blank after compounding, resulting in decreased composite transparency. The solution is to change the material.

(2) During the production process, it was accidentally stopped, the squeezing roller did not loosen in time, or the squeegee roller was not separated from the glue liquid, and the hot-press roller and the hot drum were not separated. Normally, wavy irregular transverse indentations appear in the blank space during the 30-minute press, resulting in poor transparency. Therefore, in the non-working state, the rubber roller and the anilox roller should be isolated and cleaned. The hot roller is separated from the hot drum and the hot drum power is turned off at the same time.

Other phenomena and countermeasures

There are two main reasons for the appearance of "mushroom umbrella cloud", "splashing water", and "silk pattern" in composite products.

(1) The material itself is bad and irregular patterns appear. Just replace it.

(2) The effect of adhesives and drying ducts. At the beginning of entering the drying tunnel, the temperature is too high, the drying speed is too fast, the solvent on the surface of the adhesive layer evaporates quickly, and the surface crusts. After the heat penetrates into the adhesive layer, the solvent below the film vaporizes and breaks through the film surface to form a volcano. The mouth of the mouth, like "mushroom cloud", is distributed on the surface of the film, causing the adhesive layer to be uneven and opaque. Therefore, the temperature of the drying path should be increased from low to high in order, allowing the solvent to volatilize from the inside to the outside in an orderly and uniform manner, leaving no odor. In addition, the solvents with different boiling points can be mixed and used so that the high-boiling solvent on the surface of the adhesive layer will remain for a long time without being affected by the heat, and the surface film layer will not be crusted, but it is necessary to ensure that the amount of residual solvent meets the requirements and does not exist. Odor, odor problem.