Steaming bag maturation, bag making process and testing precautions, too detailed!

I. Curing Process

The purpose of curing is to promote the chemical reaction between the main agent and the curing agent of the two-component polyurethane adhesive, making the molecules form a network cross-linking structure, thus having higher composite strength. Therefore, the two important factors of curing - time and temperature - are indispensable.

Curing, whether using external heating or internal heating, aims to achieve stable and uniform indoor temperature. Because different material structures of composite films have different curing requirements, the composite adhesive can only achieve higher bonding strength when it forms a network structure. From the experimental results, the curing time for PET//AL is 36h is more suitable, AL//PA is 24h ，while PA6//CPP needs 60h to achieve the appropriate peel strength. Therefore, to be comprehensive, ordinary two-component adhesives should be at 45-55℃, 24 hours before complete crosslinking. Retorting adhesive composite films require low-temperature long-time curing, while retorting bag adhesives require 72 hours before complete crosslinking and curing. Composite PET When laminating aluminum film, the temperature should be controlled below 50℃, and must not exceed 60 ℃, because the expansion coefficient of aluminum powder is larger than that of PET 20 times. If the temperature is too high, aluminum powder delamination will occur during lamination.

During the curing process, the adhesive still has a certain fluidity , Therefore, hanging curing is the best. However, some adhesives require 15-7 days. This must be done according to the requirements of the product instruction manual of various brands and models, and the curing chamber temperature cannot be too low or the curing time too short.

The temperature in the curing chamber should be maintained uniformly at various points using ventilation and flow. Therefore, while strictly controlling the curing time and temperature, the exhaust fan should be turned on regularly, and the indoor air should be properly exchanged to exhaust the residual solvent released during curing outdoors, supplementing with clean and fresh air to avoid secondary pollution of the composite film and facilitate the circulation of fresh air in the curing chamber. This helps to reduce the amount of residual solvent and odor in the composite, and improves product quality.

Curing can also be carried out by laminating layer by layer and then curing for 24h. That is, according to the characteristics of the adhesive, each layer can be cured for 12~24h After the last layer is completed, the final curing is completed according to the curing time required by the adhesive. Multiple layers can also be laminated at one time before entering the curing chamber, and the final curing is completed according to the curing time required by the adhesive. The advantages of layered curing are stable quality, reduced wrinkling during lamination, and the ability to check the lamination AL Whether there are white spots on the appearance of the product; it can also ensure the stability of product quality. The advantage of one-time completion is high production efficiency and easy management, but there is a quality risk of wrinkling. It is recommended to cure in sections after segmented lamination, and the quality will have relatively high assurance.

The curing temperature is generally controlled within a deviation of 2℃, and the curing time deviation is only +4h, Insufficient curing time or excessively long curing time are both undesirable. , After curing, the material should be allowed to cool at room temperature 24h before slitting and bag making.

For high-temperature retort solvent-free lamination, special attention should be paid to the process design and management of the curing process. The curing temperature and time should be reasonably designed to ensure sufficient curing. It is recommended that the curing temperature should be above 55℃, and the curing time should be extended to 96h above. The door of the curing chamber should not be opened and closed frequently. As far as possible, the door should not be opened after the composite film is put in until the specified time, and then it should be taken out after one-time sufficient curing. According to operational experience, if a product cannot be cured sufficiently at one time, the peel strength of secondary curing is much worse than that of one-time sufficient curing, and it is also easy to delaminate and cause high-temperature retort delamination.

II. Bag Making Process Three Elements and Finished Product Inspection

1. Three elements of bag making: speed ( time ) individual /min 、temperature ( ℃ ) 、pressure (N)

After the composite is cured, the interlayer composite strength and residual solvent content should be tested first. After the data meets the requirements, it can be taken out for bag making. When using composite film to make retort pouches, three-side sealing is generally used to make flat pouches or stand-up pouches, and basically no middle-seam pouches are made. The process parameters of heat-sealing bag making are mainly temperature, pressure, and time, which are complementary and interdependent. Among them, temperature plays a leading role, while pressure and time play a supporting role. If the temperature is too low, no matter how high the pressure is or how long the pressing time is, the heat-sealed layer cannot reach the degree of melting and self-adhesion.

The heat-sealing strength of composite films with different structures, different combinations, and different layers is different, which can be between 45-80N/15mm. For composite films with different structural combinations and different layers, the process parameters of heat-sealing bag making should also be different.

The process parameters of heat-sealing bag making are mainly temperature, pressure, and time, which are complementary and interdependent. Among them, temperature plays a leading role, while pressure and time play a supporting role. Retort pouches use CPP film or HDPE film as the heat-sealable layer, their melting point is higher than LDPE is higher, so the temperature of the hot knife should be much higher than that of LDPE film, mostly in 180-230 ℃ range selection. If the temperature is too low, no matter how high the pressure is or how long the pressing time is, it cannot CPP reach the degree of melting self-adhesion. Only by selecting the appropriate temperature, and then undergoing a certain contact pressure and contact time, can the purpose of heat sealing and bag making be achieved.

When heat sealing bags, the fewer the composite layers and the thinner the composite material, the lower the temperature of the hot knife. The more layers, the thicker, or the composite film containing aluminum foil, the higher the temperature of the hot knife should be. The longitudinal distribution of the hot knife generally has two or three rows, and each row has several knives. The temperature of each knife and each point must be consistent, otherwise, the sealing strength of the bag will be inconsistent, with highs and lows. The pressure of the hot knife pressing down cannot be too small or too large. If it is too small, the strength is insufficient; if it is too large, it may damage the original thickness of the inner membrane of the heat sealing layer, easily squeezing out part of the heat sealing material, making the total thickness of the two inner membranes at the heat sealing welding part smaller than the sum of the original two layers, forming a semi-cut-off state at the weld edge, making the weld brittle and reducing the sealing force.

It is particularly important to note that when the hot knife is pressed down, it must not rely on high pressure on the inner side of the bag and low pressure on the outer side. It is best to adjust the pressure on the inner side of the bag to be slightly smaller, and pay attention to the flatness of the heat sealing silicone pad to avoid uneven surface or bubbles at the heat sealing part. The walking tension of the film during bag making should be appropriate to ensure smooth film running and good heat sealing during each production, and stable bag making quality. Especially the heat sealing layer RCPP is easy to cause poor bag opening, and poor opening should be prevented to prevent the bag from agglomerating. Before production, the coefficient of friction should be controlled. It is recommended that the static coefficient of friction of the film be in the 0.15 － 0.18COF range.

The heat sealing temperature should be controlled between the viscous flow temperature and the decomposition temperature, i.e., Tf---Ta. After the hot sealing, it should be cooled and shaped with a cold knife while maintaining a certain tension to avoid wrinkles and ripples at the heat sealing position. , To achieve good heat sealing effect, the number of heat sealing times should be at least twice. After bag making, the finished bags must be tested for heat sealing strength, elongation at break, impact resistance, oxygen permeability, and sealing performance. After passing the test, they must be tested according to the sterilization conditions of the user before they can be shipped to avoid wasting previous efforts due to quality factors in the last process. If a joint or roll change occurs during bag making, the products 10m within the range must be strictly inspected and controlled. Because the heat sealing temperature and pressure will change and fluctuate to a certain extent during the joint and roll change process, a larger inspection density is beneficial to ensuring the stability of product quality.

In addition, before mass bag making, sample tests should be conducted to determine the processing parameters, including the temperature, pressure, and machine speed of each section of the heat sealing knife, and these parameters can ensure that the heat sealing strength before and after steaming meets the standard. The contents of equal volume are put into a high-temperature steaming tester for a medium resistance test to determine the conformity of the material structure. The width of the heat-sealed edge during bag making should not be less than 5mm. In addition, to ensure that the heat-sealed area is flat and beautiful, the cooling mold should ensure a sufficiently low temperature, generally below 25 ℃. The circulation of cooling water should be fully checked. When using, cold tap water must be allowed to flow in, and the warm water after heat exchange flows out, keeping the knife at a lower temperature. If there is no circulating tap water for heat exchange, the water temperature in the pipe will increase after a long time, and the cooling and shaping effect will not be good. Therefore, don't forget to use cooling water, and don't let the pipe be blocked.

Due to the particularity of high-temperature steaming materials and processes, the sampling ratio of high-temperature steaming products during bag making should be appropriately increased to detect quality abnormalities in the process as early as possible. It is advisable to conduct a steaming test once per production shift. The test method should be correct, simulating the customer's usage conditions, that is, packing the same type of contents with equal volume, according to the customer's sterilization conditions (mainly steaming temperature, time, pressure, such as the condition parameters are 121℃, 40min、0.2-0.24MPa etc.) to detect the high-temperature steaming resistance.

2. Product standards and project testing of high-temperature steaming bags

China's product standard for high-temperature steaming bags, G810004-2008 "Steamable composite film and bags", expands the material structure of the product, including PET/CPP、PA/CPP two-layer transparent type and PET/AL/CPP、PA/AL/CPP three-layer opaque type and PET/PA/CPP three-layer transparent type, and PET/AL/PA/CPP four-layer opaque type, these three categories.

In terms of physical and mechanical properties of the product, three project indicators, namely, medium resistance, peel strength after steaming, and heat sealing strength after steaming, have been added, and the original oil resistance project has been cancelled, highlighting the practicality of the product's high-temperature steaming resistance.

However, the four aspects that are most concerned by producers and users and best reflect the intrinsic quality and usage requirements, namely, "Appearance Quality": 1. Wrinkles; 2. Scratches, burns, punctures, adhesion, foreign objects, delamination; 3. No false sealing at the heat sealing part; 4. Bubbles are not obvious. Nine aspects of physical and mechanical properties: tensile strength, peel strength, sealing performance, puncture resistance, heat sealing strength, water vapor permeability, oxygen permeability, heat resistance and medium resistance, plus hygiene should meet the requirements of GB9683, and the solvent residue should not exceed 10mg/m2 . Among these specific project indicators, peel strength, heat sealing strength, heat resistance and medium resistance performance, and hygiene performance are the key points. Before the product leaves the factory, these four projects must be strictly controlled and tested, and only qualified products can leave the factory.

Among these four projects, peel strength and heat sealing strength have two states: before and after steaming. The retention rate after steaming must reach 75% or 80 % or more to be qualified. The heat resistance and medium resistance must be tested after various media are filled and subjected to high-temperature steaming. Therefore, as a steaming bag material manufacturer, it must have high-temperature steaming equipment and master high-temperature steaming technology. Otherwise, without the test of high-temperature steaming, the manufacturer will not be very confident in the quality of its own products.

3. Testing requirements:

a. Heat resistance and high-temperature medium resistance:

The GB/T 10004-2008 standard for "Plastic Composite Films and Bags for Packaging - Dry Lamination and Extrusion Lamination" highlights the independent testing requirements for the heat resistance and high-temperature medium resistance of retort pouches. Heat resistance and high-temperature medium resistance are 2008 a highlight of the standard. For heat resistance, products used at temperatures of 80 ℃ and above should show no significant deformation, interlayer delamination, or heat-seal delamination after the heat resistance test. For products used at temperatures of 80 ℃ and above, after the high-temperature medium resistance test, there should be no delamination or damage, no significant deformation inside or outside the bag, and the peel strength, tensile strength, breaking nominal strain, and heat seal strength reduction rate should be ≤ 30%。

b. Peel strength, heat seal strength, sealing performance, puncture resistance:

Testing is strictly conducted according to the various product category and grade standard indicators in GB/T 10004-2008 "Plastic Composite Films and Bags for Packaging - Dry Lamination and Extrusion Lamination." Products can only be shipped after passing the tests.

C. Hygiene performance:

The national standard GB/T 10004-2008 "Plastic Composite Films and Bags for Packaging - Dry Lamination and Extrusion Lamination" stipulates that, in addition to the total amount of residual solvent ≤ 5.0mg/m2 and no benzene-based solvents detected, the hygienic performance of food packaging bags should meet GB9683 standard requirements. Each individual material ( substrate, adhesive, ink ) should meet the standards for specific chemical substances in GB/T10004-2008 in 5.7 Table 10 The regulations. GB-T.4.1 The hygiene standards for raw materials and additives also stipulate that raw materials must meet the corresponding hygiene standards for food packaging; additives must meet the hygiene standards for additives used in food containers and packaging materials.

d Sampling requirements:

In a single production run where no changes are made to the raw (auxiliary ) materials and production processes, representative and special samples can be selected for testing.

The number of samples taken continuously should cover the entire length of the sealing knife to effectively avoid missing the detection of local poor sealing.

After the packaging bag machine is running normally, the inspector takes samples after normal debugging. Several sample bags are randomly and continuously drawn from each row in the trial bag, filled with water, sealed, and then boiled. The heat sealing temperature, pressure, and machine speed should be adjusted, and the film roll should be replaced before reconfirmation.

After each shift change, samples should be taken again for water boiling performance testing. Non-conforming products found during the process should be isolated and handled promptly.

When the normal production process conditions change significantly, such as adjustments to machine speed and temperature, or finished bags before and after roll changes, resampling or expanded sampling must be performed for water boiling tests.

III. Matters needing attention in the bag making process

1. Before mass production, sample tests should be conducted to determine the processing parameters, including the temperature, pressure, and speed of each section of the heat sealing knife, and these parameters should ensure that the heat sealing strength before and after boiling meets the standard. Fill with an equal volume of contents and conduct a medium resistance test in a high-temperature boiling test machine to determine the conformity of the material structure.

2. Ensure that the film roll is fully cooled before bag making, and that the composite strength between layers, the residual solvent of the film roll, etc., have met the standards after testing, and the first-piece inspection items have been confirmed.

3. During bag making, the heat-sealed edge width should not be less than 5mm . The heat sealing strength should be 45 ～ 50N or above. In addition, to ensure that the heat-sealed area is flat and beautiful, the cooling mold should maintain a sufficiently low temperature, generally below 25 ℃. The circulation of cooling water should be fully checked.

4. The adaptability assessment of the film roll during bag making should be fully conducted to ensure smooth film feeding, good heat sealing, and stable bag making quality. In particular, RCPP as the heat sealing layer, it is easy to cause poor bag opening, so it is necessary to prevent poor opening from causing bag caking. Before production, the coefficient of friction should be controlled, it is recommended to be 0.15 － 0.18COF range.

5. Due to the particularity of high-temperature boiling materials and processes, the sampling ratio of high-temperature boiling products during bag making should be appropriately increased to enable early detection of quality abnormalities during the process. The boiling test should be performed once per production shift. The test method should be correct, simulating the customer's usage conditions, that is, packaging the same type of contents with equal volume, according to the customer's sterilization conditions (mainly boiling temperature, time, pressure, such as parameter conditions are 121 ℃, 40min、0.2-0.24MPa ) to conduct high-temperature boiling resistance testing.

6. Select the appropriate packaging quantity according to the size and weight of the bag, pay attention to protection, and prevent mechanical damage to the bag.

IV. Backpressure cooling process

Retort pouches generally rupture during retorting. Inspection reveals that no delamination occurs between the layers of the retort packaging bag, and the peel strength is still very high. This indicates that the packaging bag itself may not have quality problems, and it is very likely that the retort operator did not control the backpressure cooling process properly.

What is the backpressure cooling process?

After the insulation and steaming are complete, cold water should be injected from the spray pipe for high-pressure cooling. At this time, the temperature in the steaming pot will drop sharply, while the temperature inside the steaming bag will not drop so quickly, resulting in a difference of tens of degrees in a short time. Generally, water vapor pressure and temperature are proportional; the higher the temperature, the greater the pressure. If high-pressure air is not used for backpressure control at this time, when the pressure difference between the inside of the steaming bag and the space inside the pot reaches a certain value, the steaming bag will expand, like blowing up a balloon, often causing it to burst. It may also happen that during the expansion process, if the composite strength is not in the optimal state, it will burst from the weakest point of the composite strength, causing delamination. Therefore, in the actual operation of cooling, the high-pressure air intake valve must be opened at this moment to maintain the pressure inside the pot at approximately the same value as during insulation to prevent the steaming bag from bursting.

Whether this step can be well mastered is one of the key factors in whether the steaming bag can be used well. If the backpressure cooling process is not strictly controlled, the pressure in the pot will drop too quickly during cooling, causing the steaming bag to burst or leak.

V. Precautions for Retort Pouch Production

1. Substrate balanced thickness, corona treatment effect, and wet heat shrinkage rate

Common substrates for retort pouches include PET, NY 、 AL 、 RCPP etc. The effect of corona treatment on these substrates directly affects the composite strength of the retort pouch. ; During high-temperature steaming, the substrate will shrink to varying degrees. If the corona treatment of the substrate does not achieve the expected effect and the composite strength is low, the layers of the composite bag will not have enough force to resist the shrinkage of the substrate, resulting in a more serious delamination phenomenon after high-temperature steaming. Generally, the PET 、 NY surface tension of the retort pouch used should reach 50mN/m， RCPP should reach 40-42mN/m . In addition, some additives in the substrate, such as opening agents, lubricants, and antistatic agents, accelerate their release to the surface at high temperatures, which may cause a decrease in adhesion. Since high-temperature retort pouches are basically in water during sterilization, the shrinkage rate of the substrate should be measured at 120 ℃ 30min The wet heat shrinkage rate index affects the composite quality. The shrinkage rate should be small and stable, especially BOPA film, RCPP film, SCPP film. The longitudinal and transverse wet heat shrinkage rate is recommended to be no more than 1.5% to be considered a retort-grade film. The balanced thickness of each layer of material should be uniform, with a thickness deviation of no more than 7% and all physical and mechanical performance indicators meet the requirements. Using AL foil products, in addition to meeting its mechanical performance parameters, AL the surface must be dry, and the cleanliness must reach A 、 B grade standard, that is, when checked with distilled water, it must be completely wetted, and the number of pinholes should be as few as possible. The storage time before use should not exceed three months. To enhance the composite strength, the BOPA in the middle should be double-sided corona treated, and the corona value should not be lower than 50 dynes.

2. Retort inks, adhesives, and compatibility of inks, adhesives, and substrates

The production of composite packaging bags is a process of superposition of various materials and various properties, so it requires good compatibility and high-temperature wet heat resistance of substrates, inks, and adhesives. The matching degree between them is very important. If the matching degree is not good, the processed products may have poor appearance or delamination during steaming. According to the adhesion theory, when the surface tension of the adhesive and the adherend is equal and the polarity is the same, the interfacial tension is minimized, and the adhesion strength is the highest. Therefore, the condition of the surface after the ink and adhesive are cured into a film and the surface of the substrate may vary with different manufacturers' products. Whether various surface groups can be well attracted to form intermolecular forces or chemical reactions to form chemical bonds determines whether various surfaces have good adhesion. Therefore, when making retort pouches, the resin type of ink and adhesive should be fully considered to avoid poor overall performance of the retort pouch due to incompatibility of ink, adhesive, and substrate.

The inks or adhesives used in retort pouches are generally two-component polyurethane resin types. The heat resistance of inks or adhesives mainly depends on the heat resistance and moisture resistance of the polyurethane resin. High temperature improves the reaction characteristics of inks and adhesives. If the temperature resistance and moisture resistance of inks and adhesives are insufficient, hydrolysis will occur at high temperatures, causing a decrease in composite strength. In terms of the characteristics of the polyurethane resin system, the amino and ester groups in the resin have strong polarity and are easily attacked by moisture under wet heat conditions, causing hydrolysis. The hydrogen bonds between the polyurethane resin and the substrate surface are easily destroyed, thereby reducing the bonding strength and even causing delamination of the adhesive layer. Currently, the inks and adhesives used in retort pouches are modified resins or additives to improve the adhesion and heat resistance of inks and adhesives. Therefore, the quality of the modified products of the resins used in inks and adhesives determines the quality of the inks and adhesives. The heat-sealing layer of RCPP and SCPP are selected according to different retort temperatures.

Heat-sealing materials resistant to 121℃ or higher commonly use RCPP or SCPP is better. , The thickness must be greater than 60um . It is generally recommended to use imported products. Imported RCPP has a rubber component in the inner layer, which is a block copolymer impact-resistant polypropylene, and the production is more difficult. Generally, domestic RCPP or SCPP uses binary heat-sealing polypropylene, which has poor tensile strength, impact resistance, media resistance, and elongation at break, so the heat-sealing material for high-temperature retort pouches is more resistant to temperature tests.

3. Control of curing time and adhesive amount

Due to the special sterilization method and usage conditions of retort pouches, retort pouches need to have good composite strength to reduce the impact of wet heat conditions on the quality of retort pouches, and the composite strength is closely related to the amount of adhesive and the curing time. Generally, the amount of adhesive for retort pouches needs to reach more than 4g/m2. ; Curing time is another important factor affecting composite strength. The curing time of retort pouches generally needs to reach 72h Although sometimes this time is not reached, the adhesive layer is dry after the bag is peeled open, and it seems that the curing is complete, but in the actual high-temperature processing process, due to the residual stress inside, it will still be far different from the product with sufficient curing time in terms of heat resistance and moisture resistance, and in the subsequent bag-making process, it is easy to produce heat delamination. Therefore, in the production of high-temperature retort pouches, we should not blindly pursue speed, but should prioritize the quality of retort pouches and strictly follow the process requirements.

4. Gold and silver inks are not suitable for retort pouches.

Gold and silver inks and pigments that are prone to penetration, such as carmine, pearlescent blue, and purple inks, will definitely dissolve when they encounter water, plus high temperatures. The copper powder and aluminum powder used in gold and silver inks are metals with relatively high activity, and they are easy to react with water, oil, and oxygen at high temperatures. After oxidation reaction, the appearance shows blackening, greening, or even fading. Therefore, gold and silver inks and inks that are prone to penetration cannot be used for retort pouch products.

5. Uniformity of glue coating and purity of solvent

According to the characteristics of different manufacturers and different types of adhesives, the dry basis weight is generally between 3.5-4.8g/m2. Under the condition of reaching the minimum coating amount standard, the coating deviation at the left, middle, and right positions should not exceed 0.3g/m2 as the benchmark. The curing agent should be added as required. If the humidity in the dry lamination workshop exceeds 75% the amount of curing agent should be appropriately increased, generally increased by 5%-10% If the printed pattern of the product has more yellow, white, and other light-colored areas, the coating amount during production should be increased by 0.3g/m2 to prevent obvious spots after retorting. The use of high-temperature retort adhesive must follow the manufacturer's instructions, such as the ratio, curing temperature and time, etc. The purity of the ethyl acetate solvent used for dilution must reach 99% or more.

6. Appropriately improve the machine processing process parameters

Determine appropriate machine processing process parameters, especially the three-section oven temperature is 5-10℃ higher than that of ordinary products, and the temperature of the composite heating roller is 70-80 ℃, and the composite pressure is increased by 0.1-0.2MPa The initial tension and taper of the winding are determined according to the material type and specifications, so that the winding tension is moderate and the film will not wrinkle. In addition, the preheating roller of the second substrate must be turned on during dry lamination to ensure that both substrates maintain a certain temperature when entering the laminating roller, which is conducive to the activation and flow of the adhesive and enhances the lamination strength. The opening and use of the glue spreading roller, as well as the lamination tension, pressure, temperature, and the wrapping angle between the laminating roller and the substrate, can all affect the product quality.

7, Adhesive Selection

There is a kind of adhesive dye that can dye polyurethane substances to show red, while for films, nylon shows earthy yellow, and other films do not show color. This dye is often used to determine which substrate the adhesive stays on. Therefore, it can distinguish whether the delamination is caused by the quality of the adhesive, ink, or substrate, or by the production process and operation. Under certain conditions, the dye is used for dyeing, and the result is that no color is shown on both sides of the film. This situation indicates that the peeled-off place is not the interface of the adhesive, nor the interface after the transfer of the ink (polyurethane system), because the adhesive and polyurethane ink should both show red after dyeing. This situation is interlayer delamination of the film, which can be verified by further experiments. After rubbing the dyed film with sandpaper on the side containing the heat-sealing layer, dyeing is performed again, and it can be observed that the sanded area shows red. This is because after the sandpaper grinds off the printed film, the polyurethane ink is exposed, thus showing red after dyeing.

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