The main process parameters of dry adhesive compounding

Dry-glue compound, abbreviated as dry compound, is coated with adhesive on a layer of substrate, dried by a solvent, and then bonded together with a second layer of substrate by heating and pressure.

Dry adhesive compounding is the most important of many compounding processes and is currently widely used in China. It has the advantages of low investment, high yield, strong adaptability, and easy operation. However, the dry complex production process is complicated, the technical difficulty is high, and there are many quality control points. The following describes the main process parameters.

1. Substrate requirements

The bond strength affects many physical properties of the composite film and its products and is the most important quality control index. The necessary condition for a well-bonded composite film is that the adhesive can be firmly adhered to the surfaces of the two films, that is, in close contact with the surfaces of the two substrates, and evenly spreads and fills the voids of the contact surface to wet it.

Films made of non-polar materials such as PE and PP have a surface tension of only 28 dyne, and have poor adhesion to post-processed materials such as printing inks and adhesives. Therefore, before the production of the base film using high frequency and high pressure corona discharge treatment, the surface of the carbonyl and other polar groups and fine physical pits, so that the surface tension of 38 ~ 44 dyne.

The additives in the film and the resin should have good compatibility with the resin. If the interface is polluted due to precipitation, the printing and peeling strength will be reduced.

The composite aluminum foil uses a large amount of lubricating oil during molding, and has a similar effect as a mold release agent. Post-treatment annealing should be complete with cleanliness of Class A.

2. Ink

The composite process strives to select the corresponding composite ink in accordance with the film. Commonly used polyamides should have good affinity with substrates and adhesives, high peel strength, and resistance to the contents. When designing the plate, care should be taken to leave a space around the printed surface, so that it will deviate from the ink layer during heat sealing.

3. Adhesives

The adhesive should have three conditions: (1) easy flow; (2) can wet the surface of the adherend; (3) it can be cured and bonded to the adherend. Currently, two-component type polyurethane adhesives are widely used in China. PU180's standard ratio is: the main agent: curing agent = 100:10 ~ 15.

Adhesive preparation should pay attention to: (1) the ratio should be accurate to prevent incomplete curing; (2) stirring should be sufficient to prevent uneven mixing affect the adhesive force. Due to the small amount of curing agent, it should be evenly dispersed in the solution. The main agent + solvent, curing agent + solvent, and the mixture of the two can be mixed. (3) The prepared solvent should be sealed, and do not mix in water and alcohol. reaction. (4) The viscosity of the adhesive varies greatly and is preferably used within 8 hours. If it exceeds 48 hours, the two-component cross-linking activity is small.

4. Coating technology

The domestic use of gravure roller coating, coating amount is easy to control. The coating thickness is determined by two parameters: (1) the distance between the squeezing roller and the coating roller; (2) the relative speed between the coating roller and the substrate.

The application of adhesive and coating amount are the main factors affecting the peeling strength of the composite film. Inadequate coating results in poor appearance, low peel strength, material delamination, low heat seal strength, and various resistances such as boiling and cooking. decline. Too much binder will make coating difficult, dry slowly and increase cost. After a certain amount of coating, the bond strength no longer increases with the increase of the coating amount. The main reason is that slipping occurs easily, causing internal stress and adversely affecting the strength.

The amount of coating: General dry fruit bag 1.5 ~ 2.5g/cm2 (dry basis); cooking bag 4.0 ~ 5.5g/cm2.

5. Processing temperature control

For uniform coating, the binder is diluted with ethyl acetate and the like to a solid content of 20 to 30%. Therefore, before the compounding, the binder must be heated through a drying tunnel to evaporate. The drying tunnel is divided into three sections and the temperature is gradually increased. The base material is set according to the properties, thickness, and composite speed. The general entrance is 60°C, and the middle to the exit can be 10-20°C.

If the adhesive is dried too much, the adhesive force will decrease, and even some stretched substrates (such as BOPP, etc.) will shrink. If the temperature is too low, the adhesive will not be completely volatilized, and the adhesive force will decrease and there will be odor.

The substrate is preheated before being compounded to reduce the temperature difference with the gluing substrate. The temperature is controlled at about 60°C, and the outlet temperature of the drying tunnel can be referred to.

The temperature of the drying tunnel should also be properly adjusted with the temperature, but the influence of the ambient temperature is very small. The relative humidity of the environment has a great influence on the composite quality. Generally, it is required that RH<80%, and the best RH<70% for aluminum crucibles. If the relative humidity is too high, air bubbles are likely to occur, resulting in a poor appearance and a decrease in peel strength.

The two layers of material are bonded together under the pressure conditions of the composite roll to form a smooth, bubble-free composite material. At this time, the composite roll should be heated to 80 to 100°C (specifically depending on the substrate and speed) to increase the initial tack strength of the cross-linked adhesive so that the two substrates do not move relative to each other.

6. Aging

Two-component polyurethane adhesives maximize bond strength only after adequate cross-link curing. In addition to requiring uniform mixing of the glue, time and temperature are the two main parameters of the curing reaction. Appropriately increase the curing temperature, the reaction rate is accelerated, the longer the aging time, the more complete the curing reaction. According to tests, the strength is improving in 5 to 14 days.

Generally aging temperature control in the 50 ~ 60 °C, the curing is intensified when it is too high, easy to produce bubbles, so that the appearance of bad. Commonly used composite film curing time: BOPP/PE is 24h, BOPP/AL/PE is 48h, and PET/AL/CPP is 72h.

7. other

The entire compounding process is performed under heating, so the material coming out of the composite roll has a high temperature, there is a possibility of blocking, and shrinkage occurs during the natural cooling. In general, a water cooling roll can be added before rewinding so that the temperature is appropriately reduced. (Text: Wang Min)