Three major technical risk control points of solvent-free composite process
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The remarkable advantages of the solvent-free composite process include cost reduction, high efficiency and energy saving, safe and stable production, and improved product quality. It is a production process that is worthy of vigorous promotion and is of great practical value.
However, the current solvent-free composite process has not yet reached the maturity stage, and there are still some technical risks that are easily overlooked in the application process of soft-package enterprises, which leads to the emergence of quality problems. To this end, the author summarized three more common solvent-free composite technology risk control points, and analyzed the reasons, and shared with the industry.
Point one
Technical risk of increasing the amount of A glue in solvent-free compounding
The solvent-free composite two-component (A glue and B glue) adhesive mixing ratio generally refers to the weight ratio of A glue and B glue, rather than the volume ratio of the two. The mixing ratio of two-component adhesives is generally determined by the adhesive manufacturer, because the ratio of adhesives of different brands and different uses is generally different. Although there are many domestic solvent-free adhesive manufacturers, there are not many options for the weight ratio of A glue and B glue. The more common A:B is 100:80, 100:75, 100:50, 100:45, etc. . However, in the actual solvent-free composite production of flexible packaging enterprises, the ratio of the two-component adhesive will be out of adjustment, which will have an impact on the solvent-free composite quality of the flexible packaging products.
Case: The composite structure is a soft packaging bag of OPP/VMPET/PE. After the printing and solvent-free compounding process, there is no quality problem. However, after the slitting bag is formed, the edge banding phenomenon occurs.
The structural composite flexible packaging bag adopts a brand of low-viscosity solvent-free adhesive, and the ratio of A and B glue in actual composite production is 100:67 (Note: according to the above, the normal ratio is relatively close to that. It is 100:75). When analyzing the cause of the failure, the author found that the bag was curled together with the phenomenon that the inner layer of adhesive did not dry. Originally thought to be due to the incomplete degree of maturity of the composite film, and then extended the ripening time and then cut into bags, the problem of bag curling still exists, and the opening is also seriously deteriorated.
In the dry compounding process, there is a saying that “increasing the curing agent (-NCO component) can improve the composite strength”, and it is easy for the industry to “copy” this idea into the solventless compounding process, resulting in more serious Quality accident.
In fact, in the solvent-free composite process, either the -OH excess or the -NCO excess may cause the solvent-free adhesive to dry out, but the -OH excess causes permanent dryness, while the -NCO excess causes It is transient and does not dry. It can also react with water molecules in the air to continue to solidify. A simple way to judge the two is to peel off the dry composite layer and expose it to the air. If the adhesive layer is dry after 24 hours, the -NCO component is excessive; if it is still sticky, it means -OH Excessive components.
Through the above analysis, and then to determine the reasons for the case, combined with the ratio of A, B glue, it is not difficult to find that the bag curl is mainly caused by the excess of A glue (ie -NCO), because the solvent-free composite film roll is larger, Generally, the effect of prolonging the ripening time is not obvious (it must be distinguished from the case where the curing temperature is low), and the water reactive with -NCO must be introduced to participate in the crosslinking curing reaction.
It has been said that placing a few barrels of water in the curing chamber to control the relative humidity of the curing chamber can achieve the effect of introducing moisture reacted with -NCO. However, the author believes that this is only helpful for the curing of the end face and surface of the composite film roll, and has little effect on the middle of the composite film roll, because in the mature environment, the moisture in the air is difficult to penetrate into the middle of the composite film roll, but instead There is a deviation in the overall ripening effect.
The author believes that the more effective method is to roll the composite film on the slitting machine for rewinding and then aging, and increase the humidity of the workshop environment (using a humidifier), so that some moisture is adsorbed between the layers and surfaces of the composite membrane. During the ripening, the water will penetrate into the adhesive layer and cross-link curing reaction with the excess -NCO, so that the self-adhesive layer is further fully cured, thereby effectively releasing the curling of the flexible packaging bag.
Point two
Risk Control of Peel Strength Decay of PE Films in Solvent Free Composites
A certain amount of slip agent is added to the film blow molding to reduce the friction between the films and between the film and the device, so that the surface of the film is smoother, so that the composite film runs smoothly in the production process and on the packaging machine. Under normal circumstances, an excessive amount of slip agent will be added moderately in the PE film, which can solve the phenomenon that the friction coefficient of the solventless composite film is abnormally increased after the ripening, but this will also cause the peeling strength of the composite film to be attenuated. When the aging is completed, the peeling strength of the composite film can be detected to be out of the standard, and in most cases, the peeling strength is attenuated during the process of placing the composite film or during the circulation of the flexible packaging bag. Here I analyze two actual cases as a reference.
Case 1: Composite structure is BOPA/PE film
Main parameters: a brand of solvent-free adhesive, the amount of glue is 1.5g / m2, aged at 40 ° C for 24 hours.
The tracking detection data of the composite film peeling strength is as follows:
On October 13th, the transverse peel strength was 15.37 N/m.
On November 4th, the transverse peel strength was 4.62N/m.
After peeling off the composite layer, the surface of the PE film has a mist-like substance that can be wiped off by hand, as shown in FIG.
On November 18th, the transverse peel strength was 4.69N/m.
Close to the last test data, indicating that the peel strength of the composite film reached a stable value.
In general, flexible packaging companies only detect the peel strength of the composite film shortly after aging, and rarely track the peeling strength of the composite film during the test placement or the flexible packaging bag during the flow process, which should be noticed.
Case 2: Composite structure is matt OPP/PET/antistatic PE
Main parameters: a brand of solvent-free adhesive, the amount of glue is 1.5g / m2, aged at 42 ° C for 24 hours.
The tracking strength of the peel strength was also selected at three time points. The results showed that the longitudinal peel strength was 2.08N/15mm (the fracture of the PET film), 1.82N/15mm, 1.79N/15mm, and the transverse peel strength was 2.97. N/15mm, 2.75N/15mm, 2.69N/15mm.
The longitudinal peel strength was attenuated severely, and the lateral peel strength was not significantly attenuated. Also, on the last test, a white substance which could be wiped off by hand was found on the surface of the PE film, and wiping with a solvent showed that the adhesive layer was on the PE film side.
It can be seen from the above two cases that the migration of the solvent-free adhesive in the PE film will bring out the slip agent, which is the main reason for the deterioration of the peeling strength of the composite film. I suggest that the solvent-free adhesive can be used. The formulated slip agent is added to the formula, which can solve the problem of the change of the friction coefficient of the PE film in the solventless composite, and can also solve the problem of the decay of the peel strength.
Point three
Pay attention to the identification of solvent-free adhesive odor characteristics
I have been asked by the author: "The composite membrane of structure OPP/VMPET/PE has residual odor after solvent-free compounding, but it does not use dry compounding. What is the reason?"
There are many factors causing odors in soft packaging bags, such as residual solvents, ink resins, film raw materials resins, adhesives, etc. Although there is no process risk of solvent residue in solvent-free composites, the resins and formulations in solvent-free adhesives have certain The odor characteristics, some after the composite curing, and some even have some influence after packaging the contents. For flexible packaging companies, it is necessary to pay attention to the identification of the solvent-free composite film after the odor, prevent problems before they occur.
Regarding the detection method of composite film odor, GB/T 10004-2008 "Packaging plastic composite film, bag dry composite, extrusion composite" standard has relevant regulations.
(1) In the GB/T 10004-2008 standard, the detection method of packaging sensory index is specified: the packaging box and the lining packaging film are opened, and the smell is immediately smelled.
(2) The 6.6.16.2 of GB/T 10004-2008 stipulates that a film of 10cm×10cm is taken into strips, placed in a vessel containing 150ml of distilled water, sealed with a lid, and placed in an oven at 60 °C. Or in a water bath, take it out after 30 minutes, open the lid, smell the smell of water vapor, and judge whether there is any smell.
(3) Method 2 is actually less used, usually taking a certain area of the composite film, cutting into a 500ml glass bottle, baking at 60 ~ 80 ° C for 30min, sensory detection of odor. It is also possible to seal the package into air or the actual contents, and then rotate at a temperature of 50 to 60 ° C for several hours to tear the package and sensoryly detect whether there is an odor.
The odor of the composite film is mainly derived from the solvent-free adhesive, so the odor characteristics of the general solvent-free adhesive can be identified by the following methods.
(1) Take a certain amount of different brands of A glue and B glue for sensory detection and comparison.
(2) The composite film of the same structural raw material is compounded with different solvent-free adhesives, and the sampling is performed according to the method 2 of the composite film odor determination.
(3) Add the solvent-free adhesive mixed according to the specification to the glass bottle (the plastic film is placed on the bottom of the bottle, and the adhesive is easy to take out after curing), and the crosslinking reaction is carried out at 40-50 °C. After an hour, the odor characteristics were identified. The method is more suitable for practical applications, including the odor characteristics of the resin itself, the odor characteristics which may be generated by the unreacted low molecular weight component, and the odor characteristics of the side reaction product which may exist during crosslinking curing.