Abstract: Nylon film is a highly hygroscopic film . Once it absorbs moisture, its physical properties change and the film becomes soft and wrinkled. It is a great test for high-speed solvent-free compounding. The thickness and balance of nylon film, the smoothness of the film surface, the vertical and horizontal differences in heat shrinkage, surface wetting tension, and additives will all affect the quality of solvent-free composite products.
In the solvent-free production process, the composite equipment is composed of several main parts, such as a mixed metering and dispensing system, a coating and gluing system, a composite cooling system, and a rewinding and rewinding system. There are many parameters that need to be controlled, and the operation is difficult, especially: tension control (related to the compound pressure and speed ratio), the amount of glue, the coating viscosity and temperature (adjust the temperature of the gluing roller to adjust the viscosity of the glue, control the compound Temperature, compound pressure to adjust the coating amount), curing conditions (according to the different types of glue and post-process requirements curing time) are the most important.
Because the solvent-free compounding speed is very fast, you need to try out or adjust the equipment or process when changing glue. Generally, manufacturers who use well use only one kind of adhesive, and no more than two types of adhesives. Therefore, the glue, equipment, and process are the three. It is necessary to cooperate well to produce qualified products.
The nylon film is a film with high hygroscopicity. Once it absorbs moisture, its physical properties change and the film becomes soft and wrinkled. This is a difficult problem for high-speed solvent-free compounding. Secondly, the uniformity of the thickness of the nylon film, the smoothness of the film surface, the vertical and horizontal differences in heat shrinkage, surface wetting tension, and additives may all affect the quality of the solvent-free composite product.
I. Selection of solvent-free composite raw materials
1. Composite substrate and nylon film
Currently, BOPP, PET, OPA, VMCPP, VMPET, CPP, PE, aluminum foil, etc. are the common substrates for solvent-free composites. When compounding, generally put materials with high rigidity and good coating properties (PET, BOPP, OPA, VMPET, etc.) on the main unwinding station; put easily stretchable materials (PE, CPP, VMCPP, etc.) on the auxiliary unwinding station Station. However, it can also be flexibly selected according to the actual production conditions. For example, when the printing film is compounded with aluminized material, in order to ensure the quality of the composite and production efficiency, the aluminized material can be placed at the main unwinding station. The surface corona parameters of solvent-free composite substrates are very important. Generally, the surface tension of PE and BOPP should be greater than 38 dyne, VMCPP should be greater than 42 dyne, PET should be greater than 45 dyne, and BOPA should be greater than 52 dyne.
In view of the characteristics of high solvent-free compounding speed and low initial viscosity, how can the quality of nylon membranes meet the needs of solvent-free compounding?
Although the nylon membrane is a polar material, although it undergoes the process of molecular crystallization during the production process, not all molecules in the polyamide can crystallize. 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 water film is formed on the film surface, blocking the adhesion of inks and adhesives to the film, and affecting the product quality, such as wrinkling, warping, curling of the bag mouth, inaccurate overprinting, misplaced bags, The composite film is blistering, easy to get spots, crystal points and white spots. Increased odor, film surface adhesion, and difficulty in coding. In severe cases, it can also cause the composite peel strength to decrease, or the ink or adhesive to transfer and degrade, resulting in delamination, decreased coding speed, or unreachable coding, increased bag breaking during high-temperature cooking, and the film feels hard and brittle Increase and so on.
These are all quality failures caused by nylon membranes absorbing moisture.
Therefore, the requirements for nylon membranes are:
1. The thickness and thickness deviation of BOPA film should be controlled within 8%. Because the solvent-free compounding speed is fast, if the thickness and thickness are poor, composite wrinkles and bubbles will occur.
2. The hygroscopicity should be controlled within a certain range, preferably about 1.5% to 2.5%. If it exceeds 3%, it will cause compound drying to slow down, air bubbles to be generated, and dots to be blocked, so that small dots will be lost to print and the nylon film will turn yellow Discoloration.
3. The surface wetting tension should be between 52 and 54 dyne. Corona treatment will affect the barrier properties of the material. Therefore, the corona treatment of the material should be reasonable and should not be too large.
4. The added dose should be controlled within 300 ~ 400PPM, too little affects the opening, too much will destroy the composite strength.
5. The finish of the film is appropriately higher, and it can generally be controlled between Ra0.08 ~ 0.12um. Due to solvent-free compounding, the amount of glue is small. If the film surface is too rough, it is difficult to fill the uneven cells with glue, which will lead to quality accidents such as poor compound strength and white spots.
Due to the special nature of the solvent-free compounding process, it has a strong dependence on the properties of the raw materials. Many raw materials that can be suitable for the dry compounding process cannot be used for solvent-free compounding. All new materials must be tested before they can be used on a large scale. The type of ink, film, content of additives, surface condition, and adhesive will all have an important impact on the final product quality. Therefore, before large-scale production, the original (auxiliary) materials and production processes must be evaluated experimentally to reduce risks.
2.Selection of adhesive
Solvent-free adhesives are mainly one-component moisture-curing adhesives, two-component cold-coating solventless adhesives, two-component reverse hot-coating adhesives, UV-curing adhesives, etc. . There are many factors to consider when selecting an adhesive. The first is the type of packaging content and the type of film material used. Second, if the printing ink is in contact with the adhesive, the compatibility of the two should also be considered. Third, peeling Strength requirements and heat-sealing conditions also greatly influence the choice of adhesive.
In addition, two issues need to be paid attention to: First, the MDI-type isocyanate will gradually migrate to the inner surface through the inner film, and react with water vapor to form a polyurea heat-resistant sealing layer, which affects the heat-seal of the packaging bag . quality. Such problems are prone to occur when the polyethylene film is of poor quality or the adhesive is not selected properly, especially when using two-component adhesives. Second, when using a film with a high content of composite slip agent, due to the migration of the slip agent into the adhesive layer, problems such as poor peel strength, poor heat seal, and increased friction coefficient may occur, affecting its use on packaging production lines. Operability. Therefore, care must be taken in selecting the appropriate adhesive when laminating such films.
At the same time, the amount of glue on different substrates is usually different. Generally speaking, it is generally controlled at 0.8 ~ 3g / ㎡, transparent materials are controlled at 0.8 ~ 1.5 g / ㎡, and aluminized materials can reach 1.5 ~ 1.8 g / ㎡. Paper / plastic composite can be controlled at 1.0 ~ 3.0g / ㎡. Specific parameters should be obtained through field tests. If the amount of glue is too large, it will not only cause uneven rewinding, but also cause the ink to peel and transfer from the printing substrate.If the amount of glue is too small, the peel strength will be reduced, and air bubbles will remain due to the insufficient amount of glue and ink wetting. .
Different two-component polyurethane adhesives are not as sensitive to water and alcohol. For example, the sensitivity of the special adhesive for aluminized film to water or alcohol is in the range of 0.5% ~ 0.8%. If it exceeds this range, it may cause dryness. If the sensitivity is out of range, the higher the flexibility, the worse the compounded product will sometimes become hard and brittle. However, the sensitivity is too small, and the control of the water content or alcohol content of the ethyl acetate solvent must be strict. At present, the total amount of water and alcohol in ethyl acetate sold on the market should be less than 0.5%. If it is more than 0.5%, it can be judged that this solvent does not meet the requirements of the national standard GB12589: the water content of first-grade ethyl acetate does not exceed 0.2% The secondary product does not exceed 0.4%. The moisture content of ethyl acetate in foreign countries is not allowed to exceed 0.05%. This is because if ethyl acetate contains components of OH, -COOH, -NH2 groups, it will seriously affect the compounding effect. This is because the main agent of polyurethane adhesive contains NCO groups, which can react with active hydrogen atoms. These reactions will Consumption of the isocyanate of the curing agent reduces its participation in the glue reaction, and at the same time emits CO2, which causes the glue layer to contain bubbles, which is extremely unfavorable to the compound, causing the surface to be dry or the adhesive strength to be reduced. Nylon membranes are highly hygroscopic materials and are particularly sensitive to water molecules. Care must be taken in this regard.
Second, solvent-free composite process
In solvent-free composite processes, tension control is extremely important and must be very precise. Tension control includes the main unwinding tension, film tension after gluing, auxiliary unwinding tension, rewinding tension, and rewinding taper. The initial viscosity of the solvent-free composite adhesive is zero, and the tension matching must be strictly controlled. This feature of solvent-free compounding places high demands on the equipment. A stable tension control system is the heart of solvent-free equipment. The setting of the tension process parameters is an accumulation of experience for flexible packaging manufacturers. There are certain differences between different equipment, and the difference between different film composites is huge. Only after repeated experiments can a good tension parameter be selected. The material changes after the nylon film absorbs moisture. Soft wrinkles, the tension can be appropriately larger when compounded.
Generally, the tension after film coating is slightly greater than the main unwinding tension, the rewinding tension is slightly greater than the unwinding tension, and the rewinding taper is preferably controlled within 20%. For different material films, the tension of each part in the compounding process is also different. Even the films of the same material produced by different manufacturers need to be adjusted slightly. For example, for a composite film of PA / PE structure, the tension of the PE film is approximately between 1.5 and 2.5N, and the tension of the PA film can be controlled between 7 and 15N according to the actual situation. The method to see if the tension is appropriate is to stop the machine during the lamination process, draw a cross with a blade at the rewinding position, and see the curling direction of the composite film, which direction is curled, indicating that the tension of this layer is too large and needs to be reduced; The tension of the film. The ideal state is to keep the composite membrane flat after marking the cross. It is necessary to adjust the tension according to the different two layers of materials.
The sizing amount is a key factor affecting the quality of the product. To control the sizing amount, you must first adjust the distance between the two rubber holding rollers. Note that the distance between the left and right sides must be the same to ensure the composite film from left to top right. Even glue volume. At the same time, maintain the good smoothness of the coating roller, and no foreign matter can exist. When adjusting the gap between the two ends of the rubber roller, wait until the steel roller is fully warmed up. The adhesive liquid level between the rubber rollers should be kept as low as possible to add fresh glue to the rollers at any time. Since the solvent-free composite product is still in the state of zero initial viscosity when it is taken off the machine, neither the appearance nor the composite strength can be predicted, nor can it be used to test the fast-curing samples like the dry compound, usually only after the composite product is cured for 24 hours. In order to evaluate the final performance. Therefore, due to the small amount of glue and low initial viscosity, if solvent-free compounding is used, the surface finish of the nylon film must have higher requirements. It must be stated at the time of purchase and the supplier is required.
3.Ink and residual solvents
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 does not contain organic solvents, does not increase residual solvents, but it does not reduce solvent residues caused by printing. Only when the printing residual solvent is controlled below 3mg / m2 can the odor of the final product be truly eliminated.
Generally, the type of gravure inner printing ink used for nylon film printing is polyurethane resin ink. The amine group in the ink binder of the polyurethane system ink can digest the curing agent of the adhesive and cause the glue to dry. Secondly, the polyurethane ink system Ternary hydroxy-modified vinyl chloride resin is often added to this, which is also one of the factors that cause the ink and glue to dry. For this reason, when printing with polyurethane-based inks, a hardener should be added, or it is best to use a ketone solvent as the real solvent when mixing the rubber, which is used as the binder resin to dissolve the ink. Our commonly used solvents for polyurethane resin inks are: methyl ethyl ketone (methyl, ethyl ketone), isopropyl alcohol, toluene and other solvents. Of these three solvents, methyl ethyl ketone has the fastest volatilization rate. During the printing process, the amount of methyl ethyl ketone was not replenished in time, which caused the ink in the ink tank to swell due to the poor solubility of the resin and the viscosity increased. Affects the ink's ability to adhere to the film. Therefore, when using polyurethane ink, methyl ethyl ketone must be kept in a sufficient amount in the diluting solvent to meet the needs of dissolving the resin. When printing polyurethane resin inks in summer, the ink viscosity rises rapidly, and the dilution solvent should be replenished in time and continuously during operation to keep the ink in the ink tank always at a relatively low viscosity to ensure the stability of printing quality, so as to avoid image leakage, Quality problems such as small dot loss, chromatic aberration, blocking, sticking back, etc. At the same time, you can also add 8% to 15% curing agent to prevent the phenomenon of glue from drying.
Among the mixed solvents added to printing inks, some solvents can dissolve the binder in the ink, which is called a true solvent, and some solvents can't dissolve the binder in the ink, which is called a pseudo solvent. 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. Various types of inks are different due to the different ink resins, and the true solvents used are different. You must first figure out which type of resin inks are used before you can use the corresponding true solvents.
Moisture can cause gelation and whitening due to the introduction of moisture into adhesive systems containing isocyanate groups (--NCO). This is mainly: 1 mole of water (H2O) can react with 1 mole of the curing agent of the isocyanate group (-NCO) -containing adhesive to generate amines and carbon dioxide R-NCO + H2O → R-NH2 + CO2 ↑. Although this step is not fast, it is much faster than the reaction with the main agent (some people have done experiments, it will be more than 10 times faster). Since 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 cured normally, and the CO2 gas generated with water swells in the composite product, causing air bubbles and pinholes to react with water. The resulting amine 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, resulting in waste products.
The company often appears that the compound glue is still used normally in the morning, and the phenomenon that the glue becomes turbid and white and precipitates in the afternoon is this principle. Therefore, when formulating the adhesive, it should be used with the formulation, not for a long time. Ensure that the curing dose is as high as possible when formulating the adhesive. .
Residual solvents must be removed in the printing process. Because no solvent is used for solvent-free compounding, no solvent remains in the compounding process. Due to the sensitivity of nylon membranes to water molecules, the quality (purity) of the solvents should be strictly controlled when choosing solvents. The quality of the solvent determines the residue of the solvent. Therefore, the addition of solvents such as isopropanol should be particularly careful and controlled during the printing process.
3. Environmental hygiene and temperature and humidity in the production workshop
The solvent-free composite production workshop has relatively strict environmental requirements and certain requirements for humidity in the air. Because the solvent-free compounding speed is fast, if the air has dust, debris, mosquitoes, it will affect the accuracy of the equipment and the transparency of the finished product. In addition, the solvent-free composite adhesive is polyurethane. Its main agent is isocyanate. Its functional group is a very active group. It easily reacts with water in the air. There will be bubbles after the adhesive is cured, so the ambient humidity is normal. Controlled below 75%.
Nylon membrane is hygroscopic, and it will cause a series of bad quality consequences after moisture absorption. Therefore, the use environment and storage environment of nylon membrane have special requirements. Generally, the nylon film should be stored at about 23 ℃ ± 3 ℃, and the wet-dry difference is ideally within the range of 65% ± 5%. Nylon film cannot be placed directly on the floor or in the open air to prevent moisture and moisture. The difference between wet and dry conditions in the production environment generally does not exceed 80%. If it reaches 80%, production (especially composite production) should be stopped to avoid adverse factors. Exhaust fans and dehumidifiers can be added to the production workshop to enhance indoor air circulation. Some companies arrange production of nylon materials at noon. In short, be careful when the humidity exceeds 70%. 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 the moisture attached to the surface of the nylon film is dried to facilitate the smooth progress of printing and composite production.
The condensed water is generally at the exhaust vent of the purifying hood or the exhaust vent of the air conditioner. At the place where the hot and cold air meets, it is easy to slowly condense the water vapor brought by the rise of hot air, and it will fall down to a certain extent, resulting in white Or light yellow trailing marks, which seriously affect the appearance and use of the film. Here, the production workshop is required to regularly wipe off the exhaust vent and all parts that may cause condensation, so as to ensure that the condensation does not fall on the surface of the film. Therefore, in the production process, we must consider the influence of region and temperature difference, and also consider the weatherability of the adhesive, adjust the proportion of the adhesive at any time, otherwise it will cause the glue to dry and the reaction to be incomplete.
Curing is a very important process in the solvent-free composite production process, which is related to the quality of the printing quality. The purpose of curing is to further cross-link the adhesive. If the cross-linking is completely cross-linked, 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 openness, and odor. The use of high barrier materials, such as PET, BOPA and other materials, it is difficult to discharge the residual solvent in the curing process. It must be eliminated as much as possible in printing and compound drying to reduce the adverse consequences of solvent residue.
The curing temperature and time have a great influence on the quality of the composite. The curing conditions will have a certain impact on the composite fastness of the final composite product, the transferability of the aluminized layer, and the friction coefficient of the heat seal layer. It must not be treated as sloppy as dry composite treatment. The solvent-free composite product matures slowly and cannot meet the requirements of fast curing and fast shipment. 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 requires relatively more time. Generally speaking, it is necessary to ensure the curing conditions of 40 ° C and 48h. After the composite film is rolled up, due to the poor initial adhesiveness of the adhesive (usually only 0.2 ~ 0.3N / 15mm), and the adhesive will still flow during the curing process, it is best to hang the composite film horizontally after it is unloaded. It can also be turned manually to perform room temperature uniformity operation on the aging process. To prevent serious wrinkling at the root of the film, it is best to use a 6-inch paper core for rewinding. The deviation at room temperature is less than 2%, and the maturation is 48h.
Due to the long time for complete cross-linking of solvent-free glues, some complete curing in foreign countries takes 7 to 10 days, so it must be cured strictly according to the requirements of the adhesive manufacturer. Regardless of whether the curing is performed by external or internal heating, the purpose is to achieve stable and uniform temperature in the curing chamber. 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-layer or retort films need to be cured at low temperature for a long time, while the aluminized film compound needs to increase the curing temperature and reduce the curing time to reduce the probability of dealumination. High-temperature retort bags generally require 50 ~ 55 ℃ 72h curing can meet the curing requirements. Some people in the industry have proposed a secondary maturation process, that is, after the multilayer substrates are compounded, they are first cured in a drying room at 50 ° C for 36 hours, and then taken out and cut and bagged. 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) ), Its curing effect is better than the one-time long-term curing effect. In products with aluminum foil and vacuum aluminized products, the composite aluminum foil and aluminized semi-finished products are first matured for 24 hours, and then composite heat-sealed layers are then cured for 24 hours to improve the composite peeling firmness. Before mass production of the above methods, it is necessary to do experiments to obtain parameters and consider them feasible.
Analysis of common problems
The biggest feature of solvent-free compound compared with dry compound is that the initial viscosity is low, the coating amount is small, and various quality problems appearing are mostly related to these two factors. Of course, many other factors will also affect the quality of the product. For example, the heat resistance of solvent-free glue is relatively sensitive. A drying temperature below 90 ° C will cause vaporization. And solvent-free glue is most afraid of acidic materials, so the acidity of the content should be considered. It is best to test 3% ~ 5% acidity before mass production, and expand production when feasible.
1) Insufficient sizing, air bubbles caused
The amount of glue is a relative concept. The amount of glue on different substrates is usually different. The specific amount needs to be determined by field tests. If the amount of glue is too small, not only the peel strength will be reduced, but also bubbles will be left because of the amount of glue and insufficient ink wetting. This is even more pronounced on the composite of aluminized and white printed films.
2) Bubbles caused by too much humidity
The adhesive used for solvent-free compounding is polyurethane type. A small amount of water will consume a large amount of isocyanate in the adhesive and generate a large amount of gas. If the generated gas is too late, it will swell the composite two-layer substrate during curing, leaving air bubbles after the adhesive is cured. During the summer rainy season, when the air humidity is high, above 70%, the probability of air bubbles will increase significantly. Therefore, it is very necessary to provide a solvent-free production environment with stable temperature and humidity during the production process.
3) Compound process
Mainly refers to some process parameters of the control equipment, mainly including compound pressure, winding tension, coating temperature, compound temperature, too fast vehicle speed and poor coating effect. Because there is no initial viscosity when the solvent-free compound is just off the machine, if the corresponding parameters cannot be controlled, it will cause bubble generation.
4) Air bubbles caused by poor ink drying
Poor ink drying can also cause bubbles during lamination, which is often overlooked. The air bubbles generated by high ink residue are very characteristic: air bubbles often appear in the part where the overprint rate is high, and there are few air bubbles where the overprint rate is low.
To eliminate such bubbles, you can only deal with printed films, such as re-drying the printed film with a printing machine or a compounding machine, and then compounding. Generally, the bubbles can be eliminated. No longer can only reduce the compound speed. The solvent residue of the poorly dried composite products described above must exceed the standard. The new edition of GB / T10004-2008 stipulates that the amount of residual solvent after printing and compounding cannot be greater than 5mg / ㎡. Therefore, the residual solvent must be controlled in the printing process to prevent the residual solvent from causing a series of quality problems. Generally, the residual amount of solvent must be lower than 3mg / ㎡ after the printing process in order to meet the requirements of GB / T10004--2008 standard.
5) Inadequate maturation temperature control, air bubbles that cannot be eliminated
It is relatively rare for the composite film to have no air bubbles at first, but in general, small bubbles can be eliminated after curing. Because the solvent-free adhesive has a certain fluidity after compounding, we call it "secondary leveling".
At the same temperature, the gel with large molecular weight has poor fluidity. The higher the temperature, the better the fluidity of the glue, the faster the reaction speed of the glue, the faster the viscosity of the glue increases, and the shorter the time it can flow. Therefore, the temperature of the aging room should be well controlled, generally not higher than 50 ° C and not lower than 35 ° C (can be adjusted according to actual conditions). Some maturation rooms are heated by infrared light tubes, and the amount of air circulation in the maturation room is small, resulting in inconsistent temperatures throughout the space, and the actual temperature does not all reach the scale indicated by the thermometer. This can easily cause uneven heating and quality problems.
The so-called white spot refers to the appearance of small white spots in the place where there is ink after compounding.
1) Incompatible ink and glue
In particular, full-print ink printing is easier to produce. This is due to poor compatibility between ink and adhesive, and poor matching. The slightest part is that some of the adhesive penetrates into the ink after compounding, making the ink look grayish, while the ink where there is no adhesive penetrates into white spots and lacks color. The aluminum penetrates through the composite vacuum aluminum plating film. Severe smearing of printing inks can occur in severe cases.
2) The surface finish of the rubber roller or composite roller is too poor or foreign matter is stuck
At this time, it appeared as local intermittent repetitive spots. You can determine which roller has foreign matter by measuring the distance between the two spots that appear repeatedly, and you can clean it.
3) white spots due to poor ink coverage
At this point, white spots appear in white and light-colored inks, while dark parts look good. The solution is to increase the hiding power of the white ink, such as checking the fineness of the white ink, grinding the fine ink, and improving the hiding power. The simplest method is to reprint the white ink once. It can also be tested for fineness and hiding power when purchasing inks, and used after passing.
4) Adhesive coating is uneven, low sizing amount
Insufficient adhesive can penetrate into the whole ink, resulting in white spots, or the compatibility of the adhesive with the ink is too poor. White spots are more likely to appear on composite aluminum-plated films or aluminum foils. Light reflected on the surfaces of aluminum-plated and aluminum foils will pass through the bubbles and encounter cross-sections, which will cause refraction or diffuse reflection to form white spots. Solution: The coating amount can be appropriately increased, and the compound tension can be controlled to make the coating uniform.
5) Uneven tension of substrate film
The tension of the base film is uneven, and it is discounted before entering the composite roll, resulting in poor local composite effect, air bubbles appear, and white spots in the macro.
6) compounding and gluing pressure
The pressure on the compound part or the glue-coated part is not set properly, and the head is big or small. The uneven pressure prevents the air in the glue from being discharged during the compounding process. In addition, the angle between the second substrate and the composite pressure roller cannot be too large during compounding, otherwise air is easily trapped and bubbles are formed.
7) The roughness of the film surface of the two substrates is too large; the additives float up
In the past, the surface roughness of the substrate film was generally 0.08 to 0.16um (the surface roughness of the solvent-free film should be lower). If it is higher than this roughness, then the surface of the film has serious unevenness, unevenness, and it is difficult for ink and adhesive to fill the unevenness, which will cause compound bubbles (white spots or spots in the macro) or compound instability. If the quality of the base film additive is poor or the heat rises, white spots will also be produced.
1) The proportion of glue mixed with glue is incorrect
Customers equipped with glue machines should pay special attention to the maintenance and cleaning of glue machines, especially the sealability of glue machines, otherwise the isocyanate component will easily react with water vapor and deteriorate. If the ratio of the two components produced by the glue machine is wrong, it will cause the glue to not solidify and sticky.
The core of the glue machine is the glue pump. No matter it is a plunger pump or a gear pump, there will generally be an automatic alarm system for out of proportion glue. Generally speaking, the new rubber pump will have no problems in the initial use, but with the extension of the years, if the relevant maintenance and maintenance measures can not keep up. If there is a problem with the glue pump and the adhesive ratio is unbalanced without alarming, a batch quality accident will occur. Generally, as long as the ratio is determined, the refractive index of the glue immediately after mixing is constant at a certain temperature. Although in actual operation, the refractive index will change due to the influence of temperature and test time, but it is regular. . The detection of the refractive index can be performed at any time without affecting the online production, and the operation is very convenient. Enterprises can determine whether the mixed rubber ratio is accurate by detecting the refractive index.
2) The glue is not uniformly stirred
The glue must be stirred thoroughly. Stirring until the glue is clear indicates that it is uniform, especially the manual stirring feels more obvious.
3) Isocyanate component deterioration in glue
The isocyanate component in the glue can easily react with the moisture in the air to make the glue ineffective, so this requires our special attention during storage and use.
Sometimes aluminized materials have stickiness, but transparent materials have no similar problems. This is mainly because the aluminized material has better hydrophilicity than the transparent material, and the water on the aluminized film causes the glue to react. This can be solved by increasing the amount of isocyanate components, or the coating of aluminum plating materials will be improved to a certain extent. At the same time, special attention should be paid to the storage and protection of aluminum plating materials, and never open the packaging before compounding.
1) Composite process
The tension of the two substrates does not match, and the winding tension is inappropriate. Just adjust the process parameters of the equipment. If the thickness of the substrate itself is uneven, and the adjustment by the flattening roller is invalid, the substrate should be replaced.
2) Incomplete curing
Some composite films do not seem to be peeled when peeled at room temperature, but the heat-sealed edges are warped after making the bag, resulting in fine wrinkles, which is a lighter "incomplete curing". Make sure that the curing time is sufficient.
Due to moisture absorption, the film is deformed. Ensure that the purchased membrane can be used up as soon as possible, and unused membrane should be stored in a dry place. Special attention should be paid to the use of moisture-absorbing nylon film materials in high-temperature and high-humidity weather. The packaging film should only be opened before use to avoid moisture deformation.
4) Equipment reasons
A: The roller surface is not clean.
In general, the surface of the guide roller should be clean without any foreign matter adhering to it, and the surface of the guide roller should be mirror-like. If debris is attached, tension may be unstable and wrinkles may occur due to deviation in material operation.
B: The roller level is not enough.
Similarly, the level of the guide rollers can also cause the material walking tension to be unstable and cause wrinkles.
5) longitudinal dead fold
The main reason for this is the uneven tension (poor flatness) of the substrate itself, and the solvent-free compound has strict requirements for the flatness of the substrate, which can be adjusted from the following aspects:
A: Increase the tension of unwinding and rewinding. The two cooperate with each other to prevent shrinkage.
B: Open the main unwinding film preheating system to make the film soft and reduce the stress of the film after lamination;
C: Increase the vehicle speed. There are guide grooves in each guide roller without solvent, and the guide grooves of the two adjacent rollers are exactly opposite, which is to better play a flattening effect during high speed compounding.
5, low peel strength
1) selection of adhesive
PET or NY printed film often has a low peeling degree in the transparent part and a poor or even wrinkled ink part.
This type of film is usually printed with polyurethane inks. Generally, polyurethane binders contain active groups such as hydroxyl groups. When the curing agent in the adhesive is consumed to a critical state by other factors, the consumption of active groups in the ink is increased, resulting in incomplete curing of the adhesive. . The best solution is to add a special ink curing agent to the bottom white ink, otherwise the amount of curing agent in the adhesive can only be increased. The correct ratio of the main agent and the curing agent requires us to adjust according to the actual situation. The degree of "incomplete curing" is different, and the phenomenon is not the same. Some are sticky and some cannot be felt, but the common phenomenon is that the peeling strength is not high.
2) Adhesive coating and sizing amount
As long as this problem can make the amount of glue sufficient, the peeling force can come up immediately. On the other hand, the corresponding glue should be selected according to the actual use. And ensure that the glue is applied evenly and enough.
3) Surface tension (corona value) of composite substrate
The corona treatment of the film is not good, the surface tension of the substrate is low, and the adhesive cannot sufficiently wet the surface of the substrate.
The substrate surface tension value is not enough, and everyone is already familiar with it. Here we want to remind everyone that the surface tension of polar substrates, namely PET and PA film. The surface tension of this kind of substrate itself is very high, it seems that it does not matter if it is not treated or if it is not treated. Especially for VMPET, single-side treated PET for aluminum plating is commonly used in China. The untreated surface is used for lamination with heat-sealed substrates, which sometimes causes poor peel strength. Therefore, the surface tension value must be measured before each roll of film is put into production.
4) Influence of additives in substrate
There are too many additives in the substrate, which will float when exposed to heat, which can cause the adhesive layer to degrade and lose its adhesion.
Fourth, the conclusion
For OPP, CPP and other substrates, many manufacturers have been skilled in solvent-free compounding. For the solvent-free composite process of nylon membranes, companies should carefully consider the water absorption of nylon membranes, pay attention to the choice of membrane flatness, ink, glue, and process parameters, and must first go through a small test to get the correct process After the parameters, mass production.