Solvent-free composite plastic-plastic structure, plastic-aluminum structure and aluminized structure, etc., white spot phenomenon occurs from time to time. After a lot of practical experience accumulation, many discussions and summaries of industry insiders, we have largely controlled this problem in actual operation, but there are differences in product types, materials and processes used by each company, and the reasons for the white spot problem are also different. In order to have a more comprehensive understanding and effective solution to this kind of problem, we should try to control the solvent-free composite white spot problem as much as possible, and then summarize and analyze it to supplement the control and countermeasures when there is a problem in the operation.
I.Cause of Solvent-Free Composite White Spots
With the continuous popularization of solvent-free lamination process and the continuous expansion of application fields, the problem of composite white spots has really troubled many operators, especially for solvent-free composite products with aluminized and aluminum foil structures, and the chance of white spots occurring when laminating with printed films is greater. At present, the basic consensus on the causes of white spots in solvent-free lamination is as follows:
  1. The amount of glue is low
 Low glue loading is one of the advantages of the solvent-free process over dry processing, and in practice, too low glue loading can indeed cause compound white spot problems.
  2. Light-color printing
 In particular, there is a light-colored area of ink layer transition, which is prone to white spots, because the penetration resistance is uneven in the process of stacking the adhesive layer and the ink layer, the resistance is large where the ink layer is thick, and the resistance is small where the ink layer is shallow, resulting in the inconsistent extension strength of the adhesive layer, the relative unevenness between the adhesive layer and the ink layer, and the adhesive layer in the light-colored area is relatively thin after extension, and it is easy to form obvious white spots.
  3. The speed of the compound machine is too fast
 The low amount of glue and the poor leveling after glue coating make the uneven ink layer not fill sufficiently, resulting in white spots.
  4. The glue temperature or ambient temperature is low, and the glue viscosity is high
5. The glue for aluminum foil and aluminization has high viscosity
  When the thickness of the printed pattern ink is uneven, it is difficult for the glue to completely fill the ink pits during the coating and curing process.
  In daily work practice, it is not comprehensive enough to solve the above factors, and the occurrence of white spots cannot be completely controlled.
  Based on the actual experience of dealing with this kind of problem, I believe that the following factors need to be considered:
  1. Consider the matching of ink and glue
  If you consider this problem from the perspective of ink and glue matching, coupled with the influence of machine speed on ink and glue, the white spot problem is indeed related to the matching of ink and glue. If the ink and glue are not well matched, there is no white spot at low speed, and white spot appears at high speed, indicating that the ink and glue can not meet the requirements of solvent-free high-speed production, which affects the production efficiency.
  In view of this situation, it is not objective to say that there is a problem with the glue or the ink has a problem, sometimes the defects of the ink, some properties of the glue can be compensated, sometimes the advantages of the ink can make up for the defects of the glue, and when the two complement each other strongly, there will be no problem;
  2. Solvent-free speed is always a key consideration for compound white spots
  One of the key advantages of solvent-free composite is that it is faster and more efficient than dry compounding machine, but solvent-free high-speed production requires some conditions to guarantee, such as sufficient glue amount, suitable glue viscosity, temperature conditions, printing patterns, etc., which need to be suitable for high-speed production, otherwise it can only be controlled within a certain speed range. In fact, whether solvent-free can give full play to the advantages of high-speed production is also related to the printing film, such as the printing speed is too fast, the influence of the solvent residue of the printing film, the influence of the degree of ink curing, and the influence of variables such as the thickness deviation of the ink layer, which are the reasons for the failure to compose at high speed.
  After the limiting factors of high-speed composite are eliminated, the speed can be appropriately accelerated, and blind high-speed production will bury the hidden danger of white spots.
  3. The influence of printing pattern design
 The influence of printing pattern design is often ignored by everyone, in fact, this is a very common influencing factor, in the ink depth is too frequent, the ground and hollow, whether there is a white ink base, dark and light color demarcation is obvious, etc., improper control, the possibility of white spots is very large. In particular, areas with obvious thickness and thickness drops in the ink layer are more prone to white spots. Of course, in the case of special printing pattern compounding, white spots can actually be prevented by other means of operation.
  4. The thickness of the film is uneven
 In the composite thick PE film, CPP film, etc., due to the uneven thickness of the material, the winding strength is uneven, such as the deviation of the thickness of the raw material causes the material to be loose and tight, and the transverse and longitudinal forces of the two layers of film after the composite are inconsistent, which not only has the possibility of wrinkles, but also has an impact on the generation of composite white spots. White spots in this case are the result of a combination of low solvent-free tack and inconsistent winding tightness. Especially for composite products with two-layer structure, the influence of large deviation in film thickness is more prominent.
  5. The influence of ripening conditions in different seasons
 For example, in the autumn and winter of the north, when the ambient temperature is low, the white spot phenomenon can not be matured with conventional maturation conditions, and a variety of factors are superimposed, and the environmental conditions can magnify the problem, so the maturation conditions should be adjusted according to the actual environmental conditions, and a standardized technical guidance document should be formed.
  6. Changes in ink formulas or replacement of ink manufacturers
 Ink manufacturers will adjust the ink formula with seasonal changes, and if there is a bad situation of white spots in solvent-free lamination, it may also be a problem to adjust the ink formula. For example, if a slow-drying or fast-drying solvent is added, the ink itself changes the dissolution residue of the printing film or the degree of dryness of the ink, or the ink composition changes and the compatibility and compatibility of the original glue is problematic.
  7. The influence of the storage time before the printing film is compounded
 After the printing film is not stored for a certain period of time, the printing film has just been put off the machine, theoretically the ink resin curing is not sufficient, and the adhesion of the printing substrate has not reached the best state, the mutual miscibility of the ink and glue directly under the printing film machine is greater than the strength of the ink after full curing, if it is compounded after several hours of storage, the strength of the fusion reaction after the ink and glue lamination is relatively small. Therefore, there is a hidden danger of white spots in the printing film machine immediately compounded, especially when other factors that appear white spots also exist, the risk is very high. Usually, after the printing film is stored at room temperature for 24 hours, it is better to compound within 48 hours, and the conditions are not allowed, and it should be stored for at least 12 hours.
  8. The influence of temperature and humidity changes in printing workshops and composite workshops
  The processing environmental conditions have a particularly large impact on the glue and the film, the appearance of the product processed by the same production process under high temperature and high humidity conditions and dry and low temperature conditions, the possibility of appearance difference is very large, the environmental conditions of the printing workshop will affect the state of the printing film, and the state of the printing film will definitely affect the effect of the compound, so in the solvent-free composite process, we can not only pay attention to the environmental conditions of the composite workshop, and the printing film when the environmental conditions of the printing workshop change, it is also necessary to pay attention to its correlation with the white spot, and take corresponding preventive measures。
  In addition to the above more conventional influencing factors, the following points are also places to pay attention to: (omitted, please click "Read the original article" for more content)
 II. Key Response Methods and Preventive Measures (omitted, please click "Read the original article" for more content)
 III.Concluding Remarks
Solvent-free compounding produces white spots, and it is very difficult to find a single universally adapted solution. In addition to the most commonly used control means and pay attention to some key details, the actual operation should also be analyzed to find out the real cause of the problem and deal with it accurately. For the occasional white spot problem, we must trace it to the end, find the real variable influencing factors, and accumulate valuable experience for the complete and effective control of the white spot.