The “Standard for Testing the Quality of Laminating Films†promulgated by the State Press and Publication Administration requires that the paper-plastic composite products have no significant curl after they are divided. In fact, many paper and plastic complex products have obvious curling phenomenon after they are divided. You can unfold it with both hands. You can just let it go, and you quickly retract into a tube. With these curled paper-plastic composite products, it is difficult to restore smoothness with remedies. This causes many inconveniences and wastages in the reprocessing of cuts, folds, and covers, etc. after the film is covered. The curling of paper-plastic composite products has become a serious problem second only to blistering and stripping.
First, the paper-plastic composite product curling its regularity Paper-plastic composite product is composed of paper, BOPP film and adhesive three solid materials, it has a traditional single paper impossible superior performance. The BOPP film enhances the mechanical properties of the substrate-paper and the refraction of light. The adhesive acts as an interface to make the paper and the plastic one.
The different components of both paper and BOPP film determine different changes in the composite process and new natural conditions after compounding. After observing, analyzing, and comparing a longer period of time, a larger range, and more varieties, we found that the curled paper-plastic composite products have one thing in common, namely, "short film length." After the affixed film was completely peeled from the paper, the transverse dimension of the film was always smaller than the same dimension of the paper.
(a) Several factors related to the degree of curling of paper-plastic composite products 1. Paper quantification Low paper curl ratio is higher than that of high-quantity paper.
2. The loose, rough paper texture of paper quality is more serious than that of dense, smooth paper.
3. The tensile deformation of the film is more serious than the stretching deformation of the film.
4. Temperatures Before April and after the end of October each year, the difference between the production workshop and the large air temperature increased, and curling accidents appeared.
5. Humidity Rain, snow, fog and other weather with high humidity are important conditions for inducing curling of paper-plastic composite products.
6. The production process, ie, the glue compounding process, can cause product curling more easily than the precoating film dry compounding process.
(2) Causes of curling of paper-plastic composite products Different heat-pressing working conditions produce different stresses BOPP biaxially oriented films used in paper-plastic composite production are at temperatures higher than the softening point of PP (polypropylene) resins. With mechanical vertical and horizontal biaxial stretching to a certain width (4000-4000mm), a certain thickness (28-15μm), and then cooling set.
The BOPP film is improved in heat resistance and weather resistance by extending the orientation, and the life is prolonged. When the BOPP film is cooled and set, the orientation of the molecular segment is frozen, the expansion coefficient in the stretching direction becomes smaller (including thermal expansion and wet expansion), and the thermal shrinkage rate increases.
When the BOPP film is reheated by the heating roller of the laminating machine, the frozen molecular segments produce a tendency to recover the unstretched size, ie, the film undergoes thermal contraction both in the vertical and horizontal directions. National standards stipulate that the shrinkage of BOPP film in the longitudinal and transverse directions is not more than 4%.
The shrinkage characteristics of BOPP film when it is heated by composites determine that it must produce shrinkage internal stress.
BOPP film in the composite heat shrinkage time, but also have to withstand the hot rolling machine roll rolling and stretching traction pulling mechanism of the two longitudinal forces. These types of forces act on the BOPP film at the same time, increasing its transverse shrinkage and the shrinking tension after it is released from the thermocompression mechanism at room temperature.
The paper in the paper-plastic composite is a net-like fabric made of natural fiber free and filler. The combination of fiber and fiber is always water-based and almost always reversible. Under the bridge of water, loose plant fibers are combined through a series of actions, dehydrated and baked to make paper. When the moisture of the paper increases to a certain value, it will be loosened and become pieces and pulp.
The moisture content of the paper is generally 5-7%, which is equivalent to the equilibrium moisture when the relative temperature is 50-60%. When the relative humidity of the surrounding environment changes, the paper has the function of absorbing moisture or dehumidifying, so as to achieve a new balance between its own moisture and the content of water vapor in the ambient air.
Paper is a hygroscopic material that absorbs moisture from the air. The rate of moisture absorption depends on the amount of water vapor in the air and the temperature of the air. It can also transmit moisture to the air and lose moisture. The rate of water loss depends on the moisture content of the paper and its temperature.
In the paper-plastic composite production process, there are two interrelated movements when the paper and BOPP film pass through the hot lamination mechanism. The first is the thermal movement. The heat of the heating roller is continuously conducted to the pressure rubber roller, the BOPP film, and the adhesive coated on the film and the paper compounded therewith. Followed by fluid movement. The movement of the fluid includes two aspects: one is the leveling of the adhesive and two-way infiltration between the film, the paper and the ink micropores, and the second is the evaporation of moisture contained in the paper under the hot extrusion of the heating roller. Thermal motion and fluid motion are interrelated, the former is the basis of the latter, and the latter is the dynamic response of the former. As a result, most of the adsorbed water and free water present in the pores and cell cavities of the paper are removed, the fibers are dried and shrunk, and the paper size becomes small.
The hot-pressing working conditions of the paper-plastic composites cause the BOPP film to produce shrinking tension. Paper pushes moisture to produce shrinkage stress. The heat shrinkage of the film is mainly manifested in the transverse direction, and the drying shrinkage of the paper is mainly manifested in the opposite lateral direction. Although the reaction of these two different solid-phase substances to heat shows shrinkage, there is a vectorial difference in directionality and size, and the shrinkage tension of the film is greater than the shrinkage stress of the paper.
Second, after the composite environment humidity changes caused by changes in paper stress Paper balance at the same humidity, with the increase of the ambient temperature decreases. For example, when the offset paper is at 18°C, the equilibrium moisture is 6%. At 28°C, the equilibrium moisture is 5.5%. At 42°C, the equilibrium moisture is 5%. The rule is that under a certain humidity, the equilibrium moisture change of the paper is about 0.5% for every temperature change of about 15°C.
The equilibrium moisture of the paper increases with the relative humidity of the air under a certain temperature condition. Under the condition of a relative humidity of more than 75%, a small humidity change will cause the paper to have a large moisture change and deformation.
Moisture absorption or desorption of paper has two characteristics:
1. The directional horizontal stretching is large and the longitudinal stretching is small. The horizontal stretching rate is 2-8 times the vertical stretching rate.
2. Both sides of the fiber due to the paper on the positive and negative sides of the different degrees of alignment, resulting in moisture absorption, the opposite side of the elongation than the front of the larger, the performance of the paper curled to the front; cause dewetting when the negative reduction than the positive The shortening of the surface is large, as the paper curls toward the opposite side. Whichever direction it is curled, it is always oriented along the longitudinal direction of the paper.
After the paper-plastic composite products are separated from the hot-pressing working conditions, most of the external environment is low in temperature and high in humidity. The decrease of humidity makes the BOPP film heat-shrinkable, the humidity increase BOPP film is basically not affected, the water absorption BOPP film shrinks and stereotypes, and the paper contrasts with the shrinking stress that the paper prolongs after absorbing moisture and counteracts the paper pushing water. The shrinking tension of the film is increased so that the film-fed paper is curled toward the front side (film direction) of the paper. This degree of curling increases with the amount of moisture absorbed by the paper).
Third, the prevention of curling of paper-plastic composite products 1. Adopting the new process followed by the old process of applying plastic film to carry out paper-plastic composite production. The operation diagram of BOPP film in the process flow is:
Discharging → Flattening → Gluing → Drying → Flattening → Incipient Sticky → Hot Pressing → Winding → Storage → Slicing Prefabricated film dry composite new process for paper-plastic composite production, BOPP hot melt adhesive precoating The stroke of the film is simplified as:
Discharging → Flattening → Hot-pressing → Winding → Storage → Splitting precoating film Dry-compounding New process than gluing Compounding process is simplified, reducing 3-5 straight rollers and bow-shaped roll before BOPP film coating The flat tension, the squeezing friction of the gluing mechanism, the friction of the flattening rollers in the drying mechanism, and the generation of the integrated stress of the hot gas flow reduce the traction force the host has to overcome the above various outer chimneys, thereby reducing the Heat shrinkage before and after thermocompression bonding at the time of thermocompression bonding.
2. Control the thermal compression bonding temperature The BOPP film's thermal shrinkage increases with increasing temperature. When the temperature is below 95°C, the heat shrinkage is normal, and the effect of shrinking tension on the paper is not obvious. Exceeding this number, heat shrinkage increases dramatically. The upper limit temperature set for the production of paper-plastic composites is 90-95°C.
Also note the paper's response to temperature. Because under certain humidity, the temperature change of 15 °C, paper and plastic composite production and storage temperature difference should be less than 10 °C to 22-25 °C for the best.
3. Control the humidity of the production environment that the coating film coating process uses a lot of solvents. These solvents freely or forcibly generate a large amount of volatile gas during the process of blending, coating, and drying the binder. At the same time, when all kinds of laminating machines work, the electric heating device generates a large amount of thermal energy, and the friction of various components also generates a certain amount of heat during mechanical operation. A microclimate region is formed around the laminator at a high temperature and low temperature (a temperature rise of 10-20°C and a relative humidity decrease of 10-30°C).
In order to continuously reduce the concentration of harmful volatile gases and the temperature around the laminating machine, most production workshops that use air conditioners to regulate the temperature are mostly forced to replace the air flow inside and outside the workshop by means of mechanical ventilation. When the water content in the outside air is greater than the production environment, a large amount of outside air with a relatively high moisture content is introduced, which increases the relative humidity of the production plant, resulting in prolonged paper absorption of the paper-plastic composite product and increased tension in the film surface, causing curling.
According to the moisture content of the paper is equivalent to the relative humidity of 50-60% of the balance of moisture, the humidity control of the workshop is 60%. When the relative humidity is low, water can often be sprayed on the floor to moisturize; when the relative humidity is high, the flow of hot air around the laminator can be increased to dry. The relative humidity difference of the production process should not exceed 10%.
4. Control operation tension The production process of paper-plastic composite production is:
BOPP film → (printed matter) → hot press mechanism → finished product winding In this production process, BOPP film prints are pulled by the power generated by the hot press-fitting mechanism, respectively, and are separated from the material-stripping mechanism and the conveyor belt, and the paper-plastic composite product is collected again. The power traction mechanism of the winding machine is separated from the hot rolling working interface. With the rotation-straight-assured form of movement of the BOPP film throughout the production process, the diameter of each roll of BOPP film continues to decrease, and the diameter of each roll of paper-plastic composite product continues to increase. In order to keep the line speed of the heated roller consistent with the main power, the speed of the BOPP film feeding mechanism must be continuously increased. Its speed is as follows:
Heating roller (ψ270mm): BOPP film (ψ300mm)=1:0.9
Heating roller (ψ270mm): BOPP film (ψ200mm)=1: 1.35
Heating roller (ψ270mm):BOPP film (ψ90mm)=1:2.84
In order to ensure that the film is not forcibly stretched, the tension control of the discharge axis of the BOPP film should be gradually relaxed, and the discharge shaft and the shaft seat, the flattening of each channel, the surface of the adjustment roller should be kept clean and have good lubrication.
The change of finished product winding is contrary to the discharge of BOPP film. It does not gradually decrease, but gradually increases. With each 500 pairs of open sheets as a finished product, the final speed reduction value of the 787×540 split paper is 2.74 times, and the final speed reduction value of the 880×620 split paper is 3.05 times. The tension control of the winding device should be matched.
5. Take the necessary protective measures The necessary protective measures help to avoid or reduce the occurrence of curling of paper-plastic composite products.
(1) There should be a certain storage period before the separation. The firm adhesion of the paper-plastic composite product is not caused by just leaving the hot press-fitting mechanism, but after the adhesive curing and BOPP film and paper stress are adapted to each other. , Not reserved for more than 24 hours after the storage period and prematurely divided, BOPP film shrinkage reaction increased, easily curling.
(2) The moisture-resistance of the paper-plastic composite semi-finished products is higher than that of the paper-plastic composite semi-finished products just peeled off from the laminator. The hotter paper-plastic composite semi-finished products will dissipate heat to the outside while absorbing cold outside air and cooling down. It will cause an increase in humidity. If the semi-finished product is placed directly on the ground, it will be easier for the paper to absorb moisture, resulting in split curling.
(3 points
First, the paper-plastic composite product curling its regularity Paper-plastic composite product is composed of paper, BOPP film and adhesive three solid materials, it has a traditional single paper impossible superior performance. The BOPP film enhances the mechanical properties of the substrate-paper and the refraction of light. The adhesive acts as an interface to make the paper and the plastic one.
The different components of both paper and BOPP film determine different changes in the composite process and new natural conditions after compounding. After observing, analyzing, and comparing a longer period of time, a larger range, and more varieties, we found that the curled paper-plastic composite products have one thing in common, namely, "short film length." After the affixed film was completely peeled from the paper, the transverse dimension of the film was always smaller than the same dimension of the paper.
(a) Several factors related to the degree of curling of paper-plastic composite products 1. Paper quantification Low paper curl ratio is higher than that of high-quantity paper.
2. The loose, rough paper texture of paper quality is more serious than that of dense, smooth paper.
3. The tensile deformation of the film is more serious than the stretching deformation of the film.
4. Temperatures Before April and after the end of October each year, the difference between the production workshop and the large air temperature increased, and curling accidents appeared.
5. Humidity Rain, snow, fog and other weather with high humidity are important conditions for inducing curling of paper-plastic composite products.
6. The production process, ie, the glue compounding process, can cause product curling more easily than the precoating film dry compounding process.
(2) Causes of curling of paper-plastic composite products Different heat-pressing working conditions produce different stresses BOPP biaxially oriented films used in paper-plastic composite production are at temperatures higher than the softening point of PP (polypropylene) resins. With mechanical vertical and horizontal biaxial stretching to a certain width (4000-4000mm), a certain thickness (28-15μm), and then cooling set.
The BOPP film is improved in heat resistance and weather resistance by extending the orientation, and the life is prolonged. When the BOPP film is cooled and set, the orientation of the molecular segment is frozen, the expansion coefficient in the stretching direction becomes smaller (including thermal expansion and wet expansion), and the thermal shrinkage rate increases.
When the BOPP film is reheated by the heating roller of the laminating machine, the frozen molecular segments produce a tendency to recover the unstretched size, ie, the film undergoes thermal contraction both in the vertical and horizontal directions. National standards stipulate that the shrinkage of BOPP film in the longitudinal and transverse directions is not more than 4%.
The shrinkage characteristics of BOPP film when it is heated by composites determine that it must produce shrinkage internal stress.
BOPP film in the composite heat shrinkage time, but also have to withstand the hot rolling machine roll rolling and stretching traction pulling mechanism of the two longitudinal forces. These types of forces act on the BOPP film at the same time, increasing its transverse shrinkage and the shrinking tension after it is released from the thermocompression mechanism at room temperature.
The paper in the paper-plastic composite is a net-like fabric made of natural fiber free and filler. The combination of fiber and fiber is always water-based and almost always reversible. Under the bridge of water, loose plant fibers are combined through a series of actions, dehydrated and baked to make paper. When the moisture of the paper increases to a certain value, it will be loosened and become pieces and pulp.
The moisture content of the paper is generally 5-7%, which is equivalent to the equilibrium moisture when the relative temperature is 50-60%. When the relative humidity of the surrounding environment changes, the paper has the function of absorbing moisture or dehumidifying, so as to achieve a new balance between its own moisture and the content of water vapor in the ambient air.
Paper is a hygroscopic material that absorbs moisture from the air. The rate of moisture absorption depends on the amount of water vapor in the air and the temperature of the air. It can also transmit moisture to the air and lose moisture. The rate of water loss depends on the moisture content of the paper and its temperature.
In the paper-plastic composite production process, there are two interrelated movements when the paper and BOPP film pass through the hot lamination mechanism. The first is the thermal movement. The heat of the heating roller is continuously conducted to the pressure rubber roller, the BOPP film, and the adhesive coated on the film and the paper compounded therewith. Followed by fluid movement. The movement of the fluid includes two aspects: one is the leveling of the adhesive and two-way infiltration between the film, the paper and the ink micropores, and the second is the evaporation of moisture contained in the paper under the hot extrusion of the heating roller. Thermal motion and fluid motion are interrelated, the former is the basis of the latter, and the latter is the dynamic response of the former. As a result, most of the adsorbed water and free water present in the pores and cell cavities of the paper are removed, the fibers are dried and shrunk, and the paper size becomes small.
The hot-pressing working conditions of the paper-plastic composites cause the BOPP film to produce shrinking tension. Paper pushes moisture to produce shrinkage stress. The heat shrinkage of the film is mainly manifested in the transverse direction, and the drying shrinkage of the paper is mainly manifested in the opposite lateral direction. Although the reaction of these two different solid-phase substances to heat shows shrinkage, there is a vectorial difference in directionality and size, and the shrinkage tension of the film is greater than the shrinkage stress of the paper.
Second, after the composite environment humidity changes caused by changes in paper stress Paper balance at the same humidity, with the increase of the ambient temperature decreases. For example, when the offset paper is at 18°C, the equilibrium moisture is 6%. At 28°C, the equilibrium moisture is 5.5%. At 42°C, the equilibrium moisture is 5%. The rule is that under a certain humidity, the equilibrium moisture change of the paper is about 0.5% for every temperature change of about 15°C.
The equilibrium moisture of the paper increases with the relative humidity of the air under a certain temperature condition. Under the condition of a relative humidity of more than 75%, a small humidity change will cause the paper to have a large moisture change and deformation.
Moisture absorption or desorption of paper has two characteristics:
1. The directional horizontal stretching is large and the longitudinal stretching is small. The horizontal stretching rate is 2-8 times the vertical stretching rate.
2. Both sides of the fiber due to the paper on the positive and negative sides of the different degrees of alignment, resulting in moisture absorption, the opposite side of the elongation than the front of the larger, the performance of the paper curled to the front; cause dewetting when the negative reduction than the positive The shortening of the surface is large, as the paper curls toward the opposite side. Whichever direction it is curled, it is always oriented along the longitudinal direction of the paper.
After the paper-plastic composite products are separated from the hot-pressing working conditions, most of the external environment is low in temperature and high in humidity. The decrease of humidity makes the BOPP film heat-shrinkable, the humidity increase BOPP film is basically not affected, the water absorption BOPP film shrinks and stereotypes, and the paper contrasts with the shrinking stress that the paper prolongs after absorbing moisture and counteracts the paper pushing water. The shrinking tension of the film is increased so that the film-fed paper is curled toward the front side (film direction) of the paper. This degree of curling increases with the amount of moisture absorbed by the paper).
Third, the prevention of curling of paper-plastic composite products 1. Adopting the new process followed by the old process of applying plastic film to carry out paper-plastic composite production. The operation diagram of BOPP film in the process flow is:
Discharging → Flattening → Gluing → Drying → Flattening → Incipient Sticky → Hot Pressing → Winding → Storage → Slicing Prefabricated film dry composite new process for paper-plastic composite production, BOPP hot melt adhesive precoating The stroke of the film is simplified as:
Discharging → Flattening → Hot-pressing → Winding → Storage → Splitting precoating film Dry-compounding New process than gluing Compounding process is simplified, reducing 3-5 straight rollers and bow-shaped roll before BOPP film coating The flat tension, the squeezing friction of the gluing mechanism, the friction of the flattening rollers in the drying mechanism, and the generation of the integrated stress of the hot gas flow reduce the traction force the host has to overcome the above various outer chimneys, thereby reducing the Heat shrinkage before and after thermocompression bonding at the time of thermocompression bonding.
2. Control the thermal compression bonding temperature The BOPP film's thermal shrinkage increases with increasing temperature. When the temperature is below 95°C, the heat shrinkage is normal, and the effect of shrinking tension on the paper is not obvious. Exceeding this number, heat shrinkage increases dramatically. The upper limit temperature set for the production of paper-plastic composites is 90-95°C.
Also note the paper's response to temperature. Because under certain humidity, the temperature change of 15 °C, paper and plastic composite production and storage temperature difference should be less than 10 °C to 22-25 °C for the best.
3. Control the humidity of the production environment that the coating film coating process uses a lot of solvents. These solvents freely or forcibly generate a large amount of volatile gas during the process of blending, coating, and drying the binder. At the same time, when all kinds of laminating machines work, the electric heating device generates a large amount of thermal energy, and the friction of various components also generates a certain amount of heat during mechanical operation. A microclimate region is formed around the laminator at a high temperature and low temperature (a temperature rise of 10-20°C and a relative humidity decrease of 10-30°C).
In order to continuously reduce the concentration of harmful volatile gases and the temperature around the laminating machine, most production workshops that use air conditioners to regulate the temperature are mostly forced to replace the air flow inside and outside the workshop by means of mechanical ventilation. When the water content in the outside air is greater than the production environment, a large amount of outside air with a relatively high moisture content is introduced, which increases the relative humidity of the production plant, resulting in prolonged paper absorption of the paper-plastic composite product and increased tension in the film surface, causing curling.
According to the moisture content of the paper is equivalent to the relative humidity of 50-60% of the balance of moisture, the humidity control of the workshop is 60%. When the relative humidity is low, water can often be sprayed on the floor to moisturize; when the relative humidity is high, the flow of hot air around the laminator can be increased to dry. The relative humidity difference of the production process should not exceed 10%.
4. Control operation tension The production process of paper-plastic composite production is:
BOPP film → (printed matter) → hot press mechanism → finished product winding In this production process, BOPP film prints are pulled by the power generated by the hot press-fitting mechanism, respectively, and are separated from the material-stripping mechanism and the conveyor belt, and the paper-plastic composite product is collected again. The power traction mechanism of the winding machine is separated from the hot rolling working interface. With the rotation-straight-assured form of movement of the BOPP film throughout the production process, the diameter of each roll of BOPP film continues to decrease, and the diameter of each roll of paper-plastic composite product continues to increase. In order to keep the line speed of the heated roller consistent with the main power, the speed of the BOPP film feeding mechanism must be continuously increased. Its speed is as follows:
Heating roller (ψ270mm): BOPP film (ψ300mm)=1:0.9
Heating roller (ψ270mm): BOPP film (ψ200mm)=1: 1.35
Heating roller (ψ270mm):BOPP film (ψ90mm)=1:2.84
In order to ensure that the film is not forcibly stretched, the tension control of the discharge axis of the BOPP film should be gradually relaxed, and the discharge shaft and the shaft seat, the flattening of each channel, the surface of the adjustment roller should be kept clean and have good lubrication.
The change of finished product winding is contrary to the discharge of BOPP film. It does not gradually decrease, but gradually increases. With each 500 pairs of open sheets as a finished product, the final speed reduction value of the 787×540 split paper is 2.74 times, and the final speed reduction value of the 880×620 split paper is 3.05 times. The tension control of the winding device should be matched.
5. Take the necessary protective measures The necessary protective measures help to avoid or reduce the occurrence of curling of paper-plastic composite products.
(1) There should be a certain storage period before the separation. The firm adhesion of the paper-plastic composite product is not caused by just leaving the hot press-fitting mechanism, but after the adhesive curing and BOPP film and paper stress are adapted to each other. , Not reserved for more than 24 hours after the storage period and prematurely divided, BOPP film shrinkage reaction increased, easily curling.
(2) The moisture-resistance of the paper-plastic composite semi-finished products is higher than that of the paper-plastic composite semi-finished products just peeled off from the laminator. The hotter paper-plastic composite semi-finished products will dissipate heat to the outside while absorbing cold outside air and cooling down. It will cause an increase in humidity. If the semi-finished product is placed directly on the ground, it will be easier for the paper to absorb moisture, resulting in split curling.
(3 points
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