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Influence of temperature and humidity changes in production environment on printing and composite quality
Time: 2015-07-30 Source: Packaging Frontier Reading: 16535 times
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Wenzhou Junchuan Machinery Equipment Co., Ltd.


The so-called seasonal climate change is the change of the two major indicators of environmental humidity and temperature. Generally speaking, during the spring and summer seasons, especially during the rainy season, the relative humidity in the air is relatively large and can even reach saturation. In autumn and winter, the air is dry and humid; in terms of temperature, summer is much higher than winter, and the maximum difference between the two is nearly 30-40 ° C (temperature difference between the south and the north).
If you don't pay attention to these differences, it is likely to cause the following problems during printing and laminating: the adhesive is often not cured completely, does not dry out, and has high residual tackiness, and even serious delamination may occur, especially the absorption of nylon film. It is more tidal, and this phenomenon is more likely to occur.
Although nylon membrane is a polar material, it also undergoes the process of molecular crystallization during the production process, but not all molecules in polyamide can crystallize, and there are also some non-crystalline amide polar groups. These amide groups can interact with water. Coordination of molecules causes the surface of the nylon film to easily inhale highly polar water molecules, which makes the nylon film softer to absorb moisture, weaken the tensile force, and cause tension instability during production. Sometimes a thin layer of water is formed on the surface of the film. Films, barrier inks and adhesives adhere to the film, which affects product quality. Such as wrinkling, warping, curling of the bag mouth, inaccurate overprinting, misalignment of bag making, compound blistering, speckle, crystal point and white point, increased odor, film surface adhesion, and difficulty in coding. In severe cases, it may also cause a decrease in the composite peel strength or an increase in bag breakage during high-temperature cooking, a hardening of the composite film, and an increase in brittleness. These are the quality faults caused by the defects caused by the moisture absorption of nylon membrane.
Once the nylon membrane absorbs moisture, its physical properties will change and become soft and wrinkled. For solvent-free composites running at high speed, moisture absorption and wrinkling is a difficult problem. Secondly, the properties of nylon film thickness balance, film surface smoothness, vertical and horizontal differences in heat shrinkage, surface wetting tension, dosage and floating on heat may affect the quality of solvent-free composite products.
Therefore, special attention must be paid to the production and use of nylon membranes during weather changes or humid and rainy seasons to ensure that the product quality is not affected.
First, strictly control the temperature and humidity changes in the production environment. Due to the moisture absorption of the nylon membrane and a series of adverse consequences caused by moisture absorption, the environment in which the nylon membrane is used and stored is very important.
Nylon film should generally be stored at about 23 ° C and ± 3 ° C. The difference between dry and humidity is preferably within the range of 60% RH ± 5% RH. Nylon film cannot be placed directly on the floor or in the open air to avoid moisture and moisture. The difference between wet and dry conditions in the production environment generally does not exceed 75% RH. If it reaches 80% RH, production (especially composite production) should be stopped to avoid adverse consequences. If it is necessary to start production in a hurry, you can add exhaust fans and dehumidifiers at the production site to strengthen indoor air circulation and reduce the production environment humidity to below 70% RH for reproduction. In short, it must be used with caution when the humidity exceeds 70% RH, and when the temperature is low or the humidity is high, the preheating equipment of the printing and laminating machine must be turned on, the nylon film is preheated first, and dried and attached to the surface of the nylon film Moisture to facilitate the smooth progress of printing and composite production. At the turn of winter and spring, due to the large temperature change, when using a mixed solvent, pay attention to the effective adjustment with the change of air temperature to prevent quality accidents caused by too fast solvent evaporation or too much residual solvent. The workshop environment control can keep the humidity at 50% ~ 60% RH, and the best temperature in summer is 20 ~ 22 ℃. This is because the printing workshop will produce a large amount of heat release during the production process. Controlling the temperature and humidity can ensure the printing quality. primary condition.
In dry and solvent-free composite processes, different types of two-component polyurethane adhesives have different sensitivities to water and alcohols. If it is out of range, it may cause quit phenomenon. The more the sensitivity is out of range, the worse the flexibility, and the compounded product sometimes becomes hard and brittle. Therefore, the compound effect will be seriously affected. This is because the solvent-free polyurethane adhesive contains NCO groups and can react with active hydrogen atoms. These reactions will consume isocyanate, reduce its participation in the glue reaction, and release CO2 at the same time, making the glue The layer contains air bubbles, which is extremely detrimental to the compounding, and also causes the surface to be dry or the adhesive strength to be reduced.
For nylon film, cellophane and other materials with strong moisture absorption, the residual solvents and humidity in the air after printing should be strictly controlled. Among the nylon film family, PA6 film has the largest water absorption rate, and the saturated water absorption rate can reach about 9%. Bidirectionally stretched nylon film will not only wrinkle after moisture absorption, but also will stretch horizontally and shorten vertically, and will increase with humidity. The elongation becomes larger, and the maximum elongation can reach 1.5%. If the humidity is 75% RH, the lateral elongation is about 0.37%, and the longitudinal direction is -0.28%; if the humidity is 85% RH, the lateral elongation is 0.5%, and the longitudinal direction is -0.38%. After the nylon membrane absorbs moisture, its stress-strain curve changes significantly. After the nylon film absorbs moisture, it will not have a large effect on the corona effect, but it will have a large effect on the adhesion of inks and adhesives. Generally, when the moisture content of the nylon film exceeds 3%, it will not only affect the adhesion of the ink and the adhesive, but also make the film feel hard, and the surface will be yellowed to affect the appearance. Therefore, special attention should be paid to the storage and use of nylon membranes to prevent moisture and moisture absorption.
Second, use as little or no alcohol solvents as possible in printing nylon films have their own polyurethane resin inks. When using polyurethane resin inks, pay attention to adding or not adding alcohol-based dilution solvents. Because the polyurethane resin itself is terminated with -OH, -OH can react with the isocyanate group -NCO in the curing agent of the polyurethane adhesive, so that the amount of the curing agent participating in the reaction of the main agent of the adhesive is reduced, which affects the adhesive strength. If you must add alcohol to dilute the solvent, it should be controlled within 5% as much as possible (note the purity of the solvent used). In the printing process, the residual amount of the solvent is reduced to a minimum (generally the total amount is> 3mg / m2).
When using the solvent-free composite process, the residual solvent should be cleaned up in the printing process. Because no solvent is used in the solvent-free composite process, there is no problem of solvent residue. Solvent-free compounding cannot reduce the residual solvents caused by printing, so higher requirements are imposed on the control of residual solvents in printing. Solvent-free composite glue does not contain organic solvents, and does not increase residual solvents, but it does not reduce solvent residues caused by printing. Only by minimizing the printing residual solvent can the odor of the final product be truly eliminated.
The film used for water-based ink printing is best randomized before printing to ensure the adhesion of the ink to the film. When printing white ink underprint, it is best to add a small amount of general-purpose curing agent to the white ink to improve the compound fastness. (Use according to the requirements of the ink manufacturer). Compound production should be carried out as soon as possible after printing.The remaining materials can be packed with packaging films containing aluminum foil to prevent water vapor in the air from invading and being damp.The remaining film should be placed in a dry and ventilated place.It is best to be placed in the curing room 2 to 3 hours before use. In order to dry the water vapor on the surface of the nylon membrane, the material will be protected from moisture and affect the product quality.
3. Select high-quality inks to prevent color migration and color penetration. BOPA film is used in wet seasons or high-temperature and high-humidity weather. Color migration and color penetration in plastic gravure should be prevented. Color migration and color penetration are mainly due to the intense Brownian motion of small molecules of pigments in the ink layer at a certain humidity and temperature, so as to escape from the crystal lattice formed by weak intermolecular forces and follow other organic molecules to perform migration movements. Scatter colors. In high-humidity and high-temperature environments, color pigments such as green lotus color, pink color, etc. can easily cause color migration, especially for printed finished products, which must be piled up carefully.
The higher the temperature and the higher the humidity, the more water molecules, the more severe the decomposition of the colorant crystals is due to the action of water, and the more easily color penetration occurs. For monoazo pigments, due to their small molecular weight, migration is relatively easy; for materials such as biazo pigments and condensed azo pigments such as benzidine yellow and pigment red 44, because of their significantly increased molecular weight, migration is not easy. . Heterocyclic pigments such as Permanent Violet are not easy to migrate due to their large molecular weight and stable chemical properties.
The mechanism of color penetration is more complex than color migration. In addition to the characteristics of the pigment itself, such as molecular weight and the degree of influence of water, it is also related to the characteristics of the film. Color penetration mainly occurs under a certain temperature environment, and the polymer in the plastic film polymer printed with the ink film violently moves to form voids with gaps. When the temperature rises, its pore expansion becomes larger, and at this time, the pigment molecules also make a violent Brownian motion, diffuse to the pores formed by the polymer of the plastic film, and form color penetration. The larger the pores formed between the polymers of the plastic film, the easier it is for pigment molecules to penetrate and the more severe the color migration is.
The causes of color shift and color penetration are generally the result of the combined effects of pigments, temperature, humidity, and film characteristics in the ink layer. For nylon film, non-linear polyethylene and other films, due to the amorphous state, low degree of crystallization, large molecular gaps, and easy to occur color penetration under high temperature and high humidity environment.
Static electricity can also cause color migration. Since the pigment molecules in the ink layer can overcome the internal intermolecular forces and perform Brownian motion, they generally also overcome the forces between the binder resin molecules. Under the action of electrostatic homogeneous repulsion and heterosexual attraction, the binder resin is broken. Package while migrating. For example, some clothing bags printed with gravure plastic surface printing ink, in the process of contact with some clothing with special fiber materials, the printed pattern slowly contacts these clothing materials, causing electrostatic effects. Some pigment molecules will migrate to the clothing material and may penetrate into the interior of the clothing material, staining the clothing and causing certain economic losses.
Generally, the surface resistance of non-treated nylon film is 1013. At this time, the static electricity of the film has no effect, so you can use it with peace of mind. If the surface resistance is 1014, the static electricity will increase during the dry season in autumn and winter. When the surface resistance of the film is 1015, explain this. When the film is too static to use.
Fourth, the climate change during printing causes the ink to stick and block the plate when the spring and summer seasons alternate. The humidity in the air has a large proportion. The exposed inks and adhesives often cause shallow screen blocking and ink knots due to the rapid volatilization of solvents. Phenomenon, whitening and other phenomena, the ink absorbs too much water, which will restructure the molecular structure, resulting in poor usability and affinity, causing more cutting lines, and causing color tone and composite color layering. Inks and adhesives are prone to false-drying after passing through the oven, and deep webs dry more slowly, often with anti-adhesion guide rollers and anti-sticking after winding. At this time, the printing ink should be used less frequently, so as to prevent the ink from deteriorating due to long cycle time.
The humidity of the drying medium (air) has a great effect on the amount of residual solvent. Once the moisture in the air enters the ink, the comprehensive volatility of the solvent will be deteriorated and the volatilization of the solvent will be suppressed. The humidity of the environment doubles, and the drying speed of the ink is generally nearly twice as slow. This is the reason. It is most prone to adhesion failure when printing nylon films in the "rainy season". Therefore, in high temperature and high humidity environments, the printing speed should be slowed down as much as possible. To reduce the contact between the ink and the air to ensure that the ink can be fully dried.
When the spring and summer and winter and spring temperatures alternate, the formula of the printing solvent must be changed accordingly to reduce the occurrence of ink blocking.
The general method is to increase the content of the slow-drying (fast-drying) solvent in the solvent. Correspondingly, the production process, vehicle speed, and exhaust air must be adjusted accordingly to ensure that the solvent is fully volatilized during production. (Quick-drying) Insufficient volatilization of the solvent will produce a series of adverse effects: such as ink adhesion and anti-adhesion, poor compound surface, and insufficient curing of the adhesive after compounding. In actual production, special attention should also be paid to areas with excessive light screens in the printed pattern, because this area is the most prone to "blocking". In special cases, you can make process changes during pre-press plate making to "control in Before, prevention comes first. " The second is the control of ink viscosity. The stability of the ink viscosity is related to the stability of the printing hue. Therefore, when the temperature increases in summer and the solvent volatilization speed becomes faster, the closure of the ink circulation system is particularly important to reduce the solvent volatility and ensure the stability of ink viscosity. (Can be measured once every 30 minutes with 3 # measuring cup)
V. Distinguish the true solvent from the false solvent and adjust it. In the mixed solvent added to the printing ink, some solvents can dissolve the binder in the ink. They are called true solvents. Some solvents cannot dissolve the binder in the ink when used alone. , Called pseudosolvent. Due to the different volatility between solvents, if the true solvent is volatilized first, the proportion of the solvent in the ink will be destroyed, and the phenomenon of precipitation and precipitation of the ink resin will occur. Therefore, real solvents should be added in time. When the solvent balance is poor, some slow solvents should be added to maintain the balance of solvent volatility. Especially after the abolition of benzene solvents such as methylbenzene and xylene, the types of solvents increased, and many special solvents were added in addition to the traditional four major solvent types. The various types of inks are different due to the different ink resins, and the true solvents used are different. It is necessary to find out which type of resin inks are used before they can correspond to which true solvents.
Different resins have different release properties for solvents. Each binder has its own true solvent, sub-solvent and non (pseudo) solvent:
The solubility of the solvent to the chlorinated polypropylene resin is: benzenes> ketones> esters> alcohols.
The solubility of the solvent to the polyurethane resin is: ketones> esters> benzenes.
The solubility of the solvent to the polyamide resin is: benzenes> alcohols> esters> ketones.
6. The quality of the solvent is very important. The water content, alcohol content and impurities of the solvent used must be controlled. The quality of various solvents is very important. For example, the water content and ethyl alcohol content of ethyl acetate can completely affect the quality of dry composite products. The national standard GB3728-1991 stipulates that the content of ethyl acetate in first-class products should reach 99.0% and the moisture content of 0.1%; the content of ethyl acetate in first-class products should reach 98.5% and the water content be within 0.20%. In the dry compound, the moisture content does not exceed 0.20% in order to meet the requirements, and can not contain alcohol, amines, active hydrogens, otherwise the curing agent will be consumed, poor compound peel fastness, dryness, compound delamination or wrinkling will occur. . This is due to the reaction between the -OH-terminated polyurethane main agent and the -NCO-terminated polyurethane curing agent to generate a polyurethane with a larger molecular weight, thereby forming an adhesive force. Therefore, the coordination between them has a fixed ratio, and the alcohols with -OH and the substances with -NH2 have a greater interference with the composite processing, which will not only produce a large amount of dryness, but also remain. There are many solvents, and quality problems such as bubble generation after compounding will continue to emerge.
In order to ensure that the matching of the main adhesive and curing agent of the composite adhesive is reasonable under high humidity weather, considering the high humidity in the air, when formulating the adhesive, the amount of curing agent can be appropriately increased by 5% to 8% to ensure the normal delivery of the adhesive.联 curing. The ambient temperature is high, and the water molecules in the humidity are difficult to volatilize. When the printing is compounded, the increase of air bubbles causes the product to have white spots and the initial viscosity becomes worse, especially the compound fastness quality accident of the cooking bag. In particular, pay attention to the quality problems caused when printing PA nylon and composite aluminum foil in cooking bags. The quality of the solvent determines the residue of the solvent, so strictly controlling the quality of the purchased solvent is a prerequisite to ensure that the printing and compounding residues are qualified. Secondly, when using water-based inks, special attention should be paid to the adhesion rate of inks and the influence of residual solvents. As the water-based ink is a kind of alkaline ink, its pH value is generally between 8.0 and 9.0. The solvent used is soft water. Do not use tap water, pure water and other hard water as thinner to avoid causing adverse quality reactions.
7. Gel and whitening caused by high humidity Because moisture is brought into the adhesive system containing isocyanate group (--NCO), it will cause gel and whitening. This is mainly because: 1 mole of water (H2O) can react with 1 mole of the curing agent of the isocyanate group (-NCO) -containing adhesive to form amines and carbon dioxide R-NCO + H2O → R-NH2 + CO2 ↑, which Although the one-step reaction is not fast, it is much faster than the reaction with the main agent (some people have done experiments more than 10 times faster). Because the curing agent reacts with water first, the ratio of the main agent to the curing agent is changed, so that the adhesive cannot be properly cured, and the CO2 gas generated with water swells the composite product, causing bubbles and pinholes, and reacting with water. The amine produced by the reaction continues to react with 1 mole of isocyanate-containing group (-NCO) to form biuret, R-NCO + R2--NH2--RNHCONHR ↓. This product is a white crystal that does not dissolve in ethyl acetate and shows whitening of the glue solution. The biuret formed gradually accumulates, causing the compound flower roller to block, resulting in insufficient sizing amount and waste products. Some companies often use the compound glue in the morning, and the glue turbidity and whiteness and precipitation in the afternoon are the principle. Therefore, when formulating the adhesive, it should be used with the use, not for too long, and the curing dose should be guaranteed to make it as high as possible (it should be adjusted by 5% to 10% according to the actual situation, generally not more than normal The dosage is 10%.) Generally, the curing amount is controlled below 30%. The larger the proportion of curing agent, the higher the brittleness of the adhesive film and the cohesive force of the adhesive layer. Poor (shrinkage stress offsets part of the peel strength).
8. The maturation process of semi-finished products should not be underestimated. It is very important for the maturation of semi-finished products with good printing compound, which is the key to the quality of the printed compound. The purpose of curing is to allow the adhesive to be further cross-linked. If the cross-linking is completely completed, the molecular alignment can improve the compound fastness. On the other hand, the residual solvent in the semi-finished composite film can be discharged to reduce the amount of residual solvent, thereby reducing problems such as poor peel strength, poor opening, and odor. However, it is more difficult to use high-barrier materials such as PET, BOPA and other materials to discharge the residual solvent during the curing process. It must be eliminated as much as possible in printing and compound drying to reduce the adverse consequences of solvent residue.
Regardless of whether it is used for external heating or internal heating, the purpose is to achieve stable and uniform temperature in the curing room. Generally, curing can be performed according to the required time. If the curing time is too short, the adhesive may not be completely cured, which affects the peel strength and residual odor. If the curing time is too long, the film opening may be poor. From the test results, two or cooked films need to be cured at low temperature for a long time. In addition, when curing the vacuum aluminized film containing PET, the temperature should be controlled below 50 ° C, and must not exceed 60 ° C. Because the expansion coefficient of aluminum powder is 20 times larger than the expansion coefficient of PET, if the temperature is too high, it is easy to cause delamination of the aluminum powder. Therefore, the curing temperature of semi-finished products containing VMPET film should be controlled below 50 ° C.
High-temperature cooking bags generally need to be cured at 45 ~ 55 ℃ for 72h to meet the curing requirements. Some people in the industry have proposed a secondary curing process, that is, after the multilayer substrates are compounded, they are first cured in an oven at 45-50 ° C for 36 hours, and then taken out for slitting and bag making. Then put the rolled semi-finished product and the bagged product at 70 ° C for 12h (PE is the inner film) or at 90 ° C for 3h (CPP is the inner film) for a short time secondary curing (special curing process) ), The curing effect is better than the one-time long-term curing effect.
Among the products with aluminum foil and vacuum aluminum plating, there are also composite aluminum foil and aluminum-plated semi-finished products that are first cured for 24 hours, and then heat-sealed for 24 hours (compositing and curing in stages) to reduce the amount of residual solvents and improve the compounding. Peel fastness.
Before using the above methods in large quantities, please do experiments and obtain certain parameter approval before expanding the use. The curing temperature and curing time are determined by the properties of the adhesive used and the final performance requirements of the product. Different adhesive varieties have different curing temperatures and times. The curing temperature is too low (less than 20 ° C), the reaction of the adhesive is extremely slow; the curing temperature is too high, the substrate film additives precipitate, affecting the performance of the composite film and increasing odor. If the curing time is too long, it will also affect the performance of the composite film and Increase odor, which is mainly caused by the precipitation of processing aids in polyethylene films.
Because the molecular weight of the solvent-free adhesive is much smaller than that of the dry composite adhesive, the curing reaction of the composite product takes a relatively long time. Generally speaking, it is necessary to ensure the curing conditions above 40 ° C for 48 hours (the temperature-resistant cooking bag time is more Longer). After the composite film is rolled up, and the adhesive will still flow during the curing process, it is best to hang the composite film horizontally after the machine is under the machine, or it can be manually turned to perform room temperature uniformity operation on the curing process, and the room temperature deviation is less than 2 %. In order to prevent serious wrinkles at the root of the film, because the solvent-free glue is thoroughly crosslinked for a long time, some foreign countries require complete curing for 7 to 10 days, so it must be cured in strict accordance with the requirements of the adhesive manufacturer. For solvent-free compounding, curing is more important because solvent-free compounding has no initial viscosity, so the curing time and temperature must be strictly controlled to fully cure the composite film, especially for the production of cooking bags. After fully curing, the composite film is naturally Putting it under the condition of more than 24 hours to make bags can not only improve the compound fastness, but also reduce the food pollution caused by first-grade aromatic amines.
Nine, conclusions In summary, the temperature and humidity changes in the production environment will greatly affect our printing and composite quality, each production enterprise should make corresponding effective adjustments and arrangements according to their own production environment and equipment conditions, production processes. Carefully analyze and troubleshoot quality problems in production, find out the cause of the problem, and find the best solution to the problem and improvement methods.

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