The packaging bag properly controls the ripening process, ensuring product quality!

Curing, also known as solidification, is the process of placing the already laminated film into a drying room (curing room) to promote the reaction and cross-linking of the main agent and curing agent of the polyurethane adhesive and volatilize and remove the solvent remaining in the film. Shunxingyuan Packaging, a professional flexible packaging manufacturer, believes that curing is a crucial process in the production of composite packaging bags and a major factor affecting the quality of printing and lamination. Therefore, today we have specially compiled an analysis of Shunxingyuan Packaging's curing process control techniques to help production enterprises choose higher-quality composite packaging bags.

Composite packaging bag

I. Introduction to the Curing Process

The purpose of curing is to allow the main agent and curing agent of the adhesive to undergo a chemical reaction, fully react and cure within a certain time, and further cross-link. If completely cross-linked, the adhesive molecules are arranged neatly, forming a network cross-linked structure, resulting in higher lamination strength.

Therefore, the two important factors of curing—time and temperature—are indispensable. On the other hand, it can remove the residual solvent in the semi-finished composite film to reduce the amount of residual solvent, thereby reducing poor peel strength, poor opening, and odor.

When using high-barrier materials such as PET and BOPA, it is more difficult to remove the residual solvent during the curing process. It is necessary to remove as much solvent as possible during the printing and lamination process to reduce the adverse consequences of residual solvent.

The curing process mainly controls the curing temperature and time, which are determined by the properties of the adhesive used and the final performance requirements of the product.

Different types of adhesives have different curing temperatures and times. If the curing temperature is below 20℃, the reaction of the adhesive is extremely slow; if the curing temperature is too high, the additives in the substrate film are easily precipitated, affecting the performance of the composite film and increasing odor.

Insufficient curing time means that the adhesive is not yet fully cured, seriously affecting the peel strength of the lamination. However, sometimes increasing the curing temperature and extending the curing time can actually reduce the peel strength of the composite film.

Generally, ordinary two-component adhesives require 45-55℃ and 24 hours to completely cross-link. However, retort pouch adhesives require 72 hours to completely cross-link and cure.

Secondly, it is necessary to remove the residual solvent from the composite film. Therefore, while strictly controlling the curing time and temperature, the exhaust fan should be turned on regularly to remove the residual solvent from the composite film and inject fresh air to facilitate the circulation of fresh air in the curing room and avoid secondary contamination of the composite film.

II Curing Room Requirements

1. The size and location of the Shunxingyuan Packaging curing room are designed for convenient turnover and storage, making it easier for film rolls to enter and exit.

2. The height of the Shunxingyuan Packaging curing room is 2.5m, with vents at the top that directly connect to the outside to exhaust waste gas. There are also small exhaust fans for timed exhaust, and ceiling fans are installed on the roof. The rotation of the ceiling fans helps maintain a uniform temperature in the curing room.

3. The Shunxingyuan Packaging curing room uses electric heating and is equipped with a temperature control device. A mercury thermometer is installed in the room to check whether the temperature is consistent with the set temperature. The four walls, door, and roof of the curing room are made of heat-insulating and heat-preserving materials to ensure uniform temperature around.

4. Shunxingyuan Packaging uses a three-dimensional rack-style overhead horizontal placement for storing films, facilitating rotation. After the composite film is wound, the adhesive will still flow during the curing process.

5. Shunxingyuan Packaging has dedicated personnel to manage and record the curing process, ensuring that it is managed according to the process specifications. If the curing time is too short, it may lead to incomplete curing of the adhesive, affecting peel strength and residual odor. If the curing time is too long, it may lead to poor film opening.

III Control the Curing Process to Ensure Product Quality

Curing process management is very important; it is related to the success or failure of the printing and lamination process of flexible packaging, especially for solvent-free lamination processes. The control and implementation of the curing process are even more critical.

1. Adjust the curing time appropriately according to the final use of the product and the type of adhesive used. To reduce the amount of residual solvent, the curing time can be appropriately extended; the curing time can be appropriately increased for products with high adhesive application; the curing time can be appropriately reduced for products that are wound and shipped.

2. Because films such as PET, BOPA, AL, and CPP have good heat resistance and high shrinkage temperature, the curing temperature can be increased. However, the curing temperature of LDPE, BOPP, and EVA cannot be too high.

Generally, the temperature of products with PE as the inner layer is below 55℃. The number of layers is balanced with time. Products with AL or three or more layers can use a slightly higher temperature. The thicker the material, the longer the curing time, and the better the effect. The more complete the curing time, the more balanced the internal stress, the more thorough the balance, and the more complete the cross-linking reaction of the adhesive.

3. Extend the curing time appropriately for thicker films and larger film roll diameters; the curing time can be extended for products with longitudinal ribs.

However, the curing temperature and time should not be arbitrarily increased. Increasing the curing temperature and extending the time will cause the additives in the material to precipitate (such as a higher content of PE lubricant in films thicker than 60 μm), forming a weak interface layer. The more low-molecular-weight substances accumulated on the surface of the material bonded with the adhesive, the more obvious the decrease in peel strength.

4. PET is a type of material that is difficult to bond. For example, in the composite film with a structure of PET//VMPET//PE, the inner layer VMPET//PE (non-aluminum-plated surface bonded with PE) also shows a significant decrease in peel strength as the curing time increases.

Therefore, in the curing process control of products, it is necessary to ensure the degree of cross-linking reaction of the product adhesive and pay attention to the attenuation changes in peel strength of some special structural products.

To be precise, the longer the curing time, the more complete the cross-linking reaction of the adhesive layer, and the greater the cohesive strength of the adhesive layer. However, the cohesive strength of the adhesive layer is only one of the factors affecting the composite peel strength. For aluminum-plated film composites, it is necessary to increase the curing temperature and reduce the curing time to reduce the probability of aluminum peeling.

When laminating VMPET film, the temperature should be controlled below 50℃, and absolutely do not exceed 60℃, because the expansion coefficient of aluminum powder is 20 times larger than that of PET. If the temperature is too high, delamination of the aluminum powder will occur during lamination.

5. Curing process and solvent-free lamination

Curing temperature and time have a great impact on lamination quality. Curing conditions will have a certain impact on the bonding strength, transferability of the aluminized layer, and coefficient of friction of the heat-sealing layer of the final composite product. It must not be treated carelessly like dry lamination.

Solvent-free composite products cure more slowly and cannot meet the requirements of rapid curing and fast delivery. Because the molecular weight of solvent-free adhesives is much smaller than that of dry lamination adhesives, the curing reaction of composite products requires relatively more time. Generally speaking, curing conditions of 40℃ for 48h or more are necessary (longer for high-temperature retort pouches).

Manual turning can also be used to perform room temperature uniformity operation during the curing process, with a room temperature deviation of less than 2%. To prevent serious wrinkles at the root of the film, it is best to use a 6-inch paper core for winding. Because the complete crosslinking time of solvent-free adhesive is long, in some foreign countries, complete curing takes 7-10 days.

6. Generally, retort pouch composite films require low-temperature, long-time curing.

Because composite films with different material structures have different curing requirements, the composite adhesive can only achieve higher bonding strength when it forms a network structure.

According to the adhesive bonding test results of high-temperature retort pouches, the curing time of PET/AL is 36h, which is more suitable; AL/PA is 24h, while PA6/CPP needs more than 60h to achieve the appropriate peel strength.

Therefore, considering comprehensively, the curing time after the lamination of semi (high)-temperature retort pouches needs to be 72h at 45-55℃ to meet the requirements.

7. Some people in the industry have proposed a secondary curing process or a segmented composite curing process, and after curing, they stay at room temperature for a period of time before making bags to improve the bonding strength of the composite peel and reduce residual solvent. (Please conduct experiments and obtain certain parameters before using the above methods in large quantities).

Another method to accelerate curing, often used in production to control the amount of adhesive applied or as a basis for judging temporary adjustments to the composite process, is: Take about 1 meter of full-width composite film that has just been produced and place it in an 80℃ oven for 30 minutes. Check its appearance and peeling situation to find problems in time and take measures.