Solventless polyurethane adhesive is a kind of green environmental protection food packaging adhesive with great development potential. Compared with traditional solvent-based polyurethane adhesives, solvent-free adhesives do not require the use of organic solvents in the compounding process, thereby avoiding hidden dangers such as solvent residues that endanger food hygiene and safety. In addition, the efficiency of solvent-free compounding is more than 1.3 to 2 times that of dry compounding, and the amount of sizing is small, about 1/2 of that of dry compounding. No drying tunnel is required, which can save costs by more than 20% to 30%.
With the strengthening of environmental protection and safety awareness, solvent-free compounding is receiving more and more attention from flexible packaging companies. As of the end of 2014, the number of solvent-free compounding equipment in the country has reached 700. Because solvent-free compounding technology is easier to get started, after simple training in equipment factories, operators who are accustomed to dry compounding can easily master the compounding technology. Therefore, solvent-free compounding has also achieved rapid progress in the hesitation and worry process of flexible packaging manufacturers. development of.
However, the current level of understanding and mastery of solvent-free composite processes in flexible packaging plants is still not as deep as dry composite processes. Therefore, how to effectively prevent, analyze, and deal with various problems in the process of solvent-free compounding becomes particularly important.
I. Common quality problems of solvent-free compounding
In the process of solvent-free lamination of flexible packaging plants, various phenomena and problems often occur, such as: orange peel, white spots, bubbles, crystal points, pits, wrinkles, tunnels, curls, ink dissolution, darkening of hue, Quality issues such as increased friction coefficient, low disk life, low peel strength, water-cooking and delamination, poor curing, and poor winding.
There are many reasons for these problems: personnel operating habits, technical level, surface flatness and hardness of rubber rollers, surface tension of raw materials, types of inks and printing, types of raw materials additives, glue types, and printing solvents. Components and moisture content, glue ratio, coating amount, machine speed, composite roll temperature, coating pressure, composite pressure, tension in each section, curing temperature, curing time, material structure, workshop temperature, workshop humidity, etc. So many problems, so many reasons, and often there are several problems at the same time, and one problem is often caused by more than one factor, so how do we sort out the ideas and find the problem accurately and quickly in such a complicated situation? Why?
First of all, we can divide the problem of solvent-free compounding into three categories: appearance problems, physical and mechanical performance problems, and glue usability problems; the reasons can be classified according to five important factors affecting product quality: people, machines, materials, Law, ring.
So what is the nature of the problem? The five factors of human, machine, material, method, and environment are mutually influential and cooperative relationships. When these five factors reach a balanced state, a qualified product can be produced; when one of these five factors Or a couple of changes can break this balance and cause quality problems. The process of solving the problem is the process of restoring the balance of the five factors. If this is not properly understood, the problem will often be solved, but the cause of the problem will not be found, and the cause of the problem will not be found, and the problem will not be solved; therefore, it is only possible to accurately grasp the relationship between the various factors. Only by finding a balanced relationship can we find the cause of the problem and the best way to solve it.
Application points of factor analysis
A common problem analysis method is called factor analysis, also known as replacement. That is, keep other factors unchanged, adjust one of them to see if it has an impact on the results and the extent of the impact.
Flexible packaging companies can often use it in the actual analysis of problems, and when replacing a certain factor, if you can get good results, everyone often simply think that the factor is the cause of the problem. This is a typical logical error, and the solution is regarded as the cause of the problem.
For example, a flexible packaging plant uses a solvent-free laminating machine to produce an order with a material structure of BOPP / VMCPP, resulting in compound white spots. When analyzing the problem, the workers believed that the most important factor that caused the composite white point was the coating amount. Therefore, the coating amount was increased to continue production. As a result, the composite white point was significantly reduced, and it could completely disappear after curing. The problem was solved. Therefore, workers will think that the cause of the compound white point problem is the insufficient coating amount.
It was discovered long afterwards that the most fundamental cause of this white spot was the dirty composite roller! Therefore, the problem that could be solved by wiping the composite pressure roller was solved by the workers with a method that greatly increased the cost.
This is a typical logical error that treats the solution as the cause of the problem.
Because most of the time, increasing the amount of coating can effectively improve the problem of compound appearance, but it is not necessarily the real reason. Just improving physical fitness can be beneficial to the treatment of almost all diseases, but not necessarily physical Poor quality will cause the same disease.
If the workers or technicians can comprehensively analyze the five factors of human, machine, material, method, and environment around the problem of compound white points, I believe that the real reason for breaking the balance lies in the "person". In terms of factors, as long as the workers can carefully inspect the cleaning of the equipment every time, it can avoid the problem of significantly increasing the production cost, as in the case of the composite white point problem.
However, if some problems are caused by the superposition of multiple factors, if they are solved by a single factor, the expected effect may not be achieved.
For example, when a company used a solvent-free composite process to compound an NY / PE structure order, the heat-sealed part "wrinkled".
After knowing the situation, I got the following information:
① The "wrinkled" parts are all concentrated in the heat-sealed parts, and the same situation occurs throughout the order;
②Slight "stickiness" after peeling;
③ No similar situation has occurred in the past, and the raw materials, solvent-free glue, ink, process, and equipment have not changed;
④ The coating amount is 1.5g / m2, the recommended ratio is 100: (50 ± 10), and the actual ratio is 100: 58.2;
⑤Temperature and humidity of the workshop is 32 ℃ * 65%;
⑥The ripening condition is 35 ° C * 36h. Prolonging the ripening time has no effect.
If only one factor is analyzed, it is unlikely to produce a "wrinkle" situation. If there is a problem in the ratio, then the 100: 60 and 100: 40 ratios tested in the NY / PE structure process can be determined. Claim;
If it is caused by temperature and humidity, it is usually produced under 40% ~ 70% humidity without any problems;
If it is a printing ink problem, then alcohol-soluble and ester-soluble inks have passed the trial.
Where is the problem?
Later, after careful combing and analysis, the three aspects of change were locked in.
① Proportion, the use ratio of -OH component is close to the highest limit;
② High ambient humidity;
③ The heat-sealed part is printed with white alcohol-soluble ink, and a small amount of ethanol remains.
Although the three factors do not cause "heat-sealing wrinkles" in the single existence, all three factors have a tendency to cause "wrinkling". The combined effect of the three factors has produced a negative synergistic effect. The original wrinkle problem was caused by breaking the original balance.
Fourth, the conclusion
In short, when there is a problem of solvent-free composite quality, it is best to use factor analysis to comprehensively analyze all factors, clarify the relationship between each factor, and find out the point of the factor that breaks the balance. Find the most efficient and least expensive way.