High-temperature cooking bag breakage occurs frequently. Here are the reasons and solutions.

High-temperature retort pouches, also known as soft cans, are composite pouches used for food packaging that can withstand high-temperature retorting, heating, and sterilization. Because food is filled or packed under vacuum conditions before high-temperature retorting and sterilization, the sterilized food can be stored at room temperature for an extended period.

 

 

They can be eaten cold or hot, heated in under 3 minutes, are convenient to use, easy to dispose of, and take up little space, making them very popular.

However, pouch breakage is common, especially when filling with liquids such as juice or sauce for high-temperature retorting and sterilization. This is a major problem for manufacturers.

Today, Shunxingyuan Packaging will share some solutions based on our investigation, experiments, and analysis of the causes of high-temperature retort pouch breakage.

1. Production Process of High-Temperature Retort Pouches

 

 

Soft plastic packaging often uses gravure printing and dry lamination; the following analysis only covers this process.

The production process is:

Raw material preparation - Printing - Lamination - Curing - Slitting - Pouch making

2. Quality Characteristics of High-Temperature Retort Packaging Pouches

 

 

1. Excellent heat resistance and retort resistance. Dimensions remain stable after retorting, with minimal impact on strength. Based on the sterilization temperature, retort pouches are classified as high-temperature retort pouches (sterilization conditions: 135℃, 30 minutes), low-temperature retort pouches (sterilization conditions: 100℃, 30 minutes), and ultra-high-temperature retort pouches (sterilization conditions: 145℃, a few seconds to a few minutes).

They also exhibit good cold resistance, heat sealability, high heat seal strength, and good heat seal dimensional stability; they do not shrink due to heat sealing and maintain good heat seal strength even when contaminated.

2. Good filling performance, easy opening, good machinability, impact resistance, and pinhole resistance.

3. Good moisture and gas barrier properties, good hygiene, oil resistance, and no odor. Reasonable price and attractive appearance.

3. Quality Characteristics of Commonly Used High-Temperature Retort Packaging Pouch Materials

 

 

1. Inner layer material:

PE maximum use temperature 110℃; special PE maximum use temperature 120℃; RCPP maximum use temperature 125℃, with good stiffness; special CPP maximum use temperature 145℃, all with good heat sealability.

However, each has its drawbacks. For example, PE has a certain odor at 120℃, and cast polyethylene CPE has poor impact strength and a higher heat sealing temperature. Generally, it is used as the inner sealing material.

2. Middle layer material:

AL has good barrier properties, is opaque, and is heat-resistant; NY film has good barrier properties, heat resistance, mechanical properties, and pinhole resistance; PET film has good barrier properties and heat resistance.

3. Outer layer material:

PET film has good mechanical properties, processability, inner layer material protection, and boiling water resistance.

Since the quality characteristics of the materials used in high-temperature retort pouches directly affect the quality of the finished product, raw material inspection is crucial, as evidenced by problems encountered in actual production.

4. Factors Causing Pouch Breakage During High-Temperature Retorting

 

 

1. Influence of Raw Materials

1.1 Surface Tension

Any material surface is subject to a force that compels it to reduce its surface area; this is surface tension. For good ink and adhesive adhesion, the material's surface tension must equal or exceed that of the ink and adhesive. Therefore, material surface tension directly affects the adhesion strength of ink and adhesive to the material.

If the surface tension of the plastic film after molding is insufficient for the ink and adhesive to adhere firmly to the film, especially for non-polar materials such as PE and PP, a certain surface pretreatment is necessary to increase the material's surface tension. Many surface pretreatment methods exist, but corona treatment is the most widely used in industrial production.

1.2 Raw Material Packaging Structure

The heat stability of the surface material of the high-temperature retort pouch after retorting is crucial, especially the dimensional stability after retorting and cooling, otherwise delamination and peeling may occur after retorting and cooling, leading to pouch breakage.

PET is the best surface material for preventing dimensional changes, with a thermal shrinkage rate of only 1% to 1.5%, while NY, as a surface material, cannot exceed 120℃, otherwise it will easily deteriorate and shrink, and its thermal shrinkage rate will reach more than 5%.

Therefore, the material structure selection for high-temperature retort pouches is:

Generally, PET is selected as the surface material; RCPP, a retort-grade CPP, is used as the inner heat-sealing material; the selection and arrangement of NY and AL depend on the contents.

2. Ink

The ink used for printing high-temperature retort pouches must be heat-resistant, otherwise, the packaging bag will peel from the ink layer.

Generally, inks using polyisocyanate resins and an appropriate amount of curing agents containing active hydrogen as binders, such as DIC or Toyo retort inks, are used. These prevent dot spreading, ink layer peeling, and heavy metal migration.

3. Adhesive and Adhesive Concentration

High-temperature retort packaging generally uses dry lamination, typically using polyurethane adhesives composed of a main agent and a curing agent.

For high-temperature retort resistance, high-solid-content aliphatic two-component polyurethane adhesives are usually used. To achieve higher water and heat resistance, about 1% coupling agent can be added to the adhesive.

If the adhesive concentration is too high, it is easy to clog the dots on the adhesive coating screen, resulting in a lower and lower amount of adhesive applied; if the adhesive concentration is too low, the adhesive is not easy to transfer to the material. Usually, the main agent and curing agent are mixed according to the specified ratio, and the solvent is diluted until the working solid content of the mixed adhesive is about 30%.

4. Composite Process Parameters

The setting of composite process parameters directly affects the composite strength of the product and is also a very important link in controlling the high-temperature cooking bag from breaking, such as the setting of adhesive coating amount, tension, and temperature, curing conditions, etc.

4.1 Adhesive Coating Amount

The coating amount is very important for the composite strength. Too little coating amount will not meet the requirements of the composite strength and will produce small bubbles, affecting the appearance of the product; too much coating amount will not only fail to increase the composite strength but also decrease it, and the transparency of the product will also be poor, easy to wrinkle, and increase the cost of the product, so the coating amount must be strictly controlled.

The control of the coating amount is achieved by controlling the depth of the dots on the adhesive coating screen. For the same concentration and the same type of adhesive, the deeper the screen dots, the greater the amount of adhesive applied.

Generally speaking, if the working solid content of the adhesive is 30%, the coating amount (dry basis) of the 140-line screen is 1.5～1.8g/m2, the coating amount of the 120-line screen is 1.8～2.5g/m2, and the coating amount of the 90-line screen is 3.5～4.5/m2.

At the same time, the coating amount is also related to the concentration of the adhesive, the angle of the doctor blade, the hardness and pressure of the pressure roller, and the speed of the machine; under the same other conditions, the greater the concentration, the greater the amount of adhesive applied; the flatter the doctor blade, the longer the soft blade extends, the greater the amount of adhesive applied; the slower the machine speed, the slightly larger the amount of adhesive applied; the smaller the hardness of the pressure roller, the greater the pressure, and the greater the amount of adhesive applied. The hardness of the composite pressure roller is usually 70 degrees.

From the perspective of cost control and quality assurance, a coating amount of 4.5g/m2 is the most suitable.

4.2 Machine Oven Temperature and Tension

Since plastic materials have a certain heat resistance, if the oven temperature and tension are too high, it will cause the stretching of materials such as RCPP, shrinkage after cooling, and then heating and cooling after high-temperature cooking, which will easily cause delamination of the material during expansion and contraction, ultimately leading to bag breakage.

Therefore, the temperature of the oven is usually between 40～90 ℃. Too low a temperature will cause excessive residual solvent. The national standard is not to exceed 10mg/m2, while Europe and Japan require higher, not exceeding 5mg/m2. Moreover, too high residual solvent will erupt during curing or cooking, destroying the adhesive layer and also causing bag breakage.

It is particularly important to note that the oven temperature is usually controlled by three temperature zones, and the temperature is set from low to high. If the first stage is set too high, it will cause local high-temperature skinning on the surface of the adhesive, and the internal solvent cannot be dried, ultimately leading to delamination.

The data in the table are measured after degassing and sealing with 10% saline, cooking at 121℃ for 30min and cooling. The economic daily degree data is the NY/AL layer in PET/AL/NY/RCPP.

4.3 Curing Conditions

Curing, also called curing, is to place the composite material in a constant temperature room at 40～60℃ for a period of time, usually 24～72hr.

The purpose is to allow the main agent and curing agent of the two-component polyurethane adhesive to undergo a chemical reaction, so that the molecular weight increases exponentially, forming a network cross-linked structure, thereby achieving higher composite strength, better heat resistance, and better media resistance.

When the temperature increases, the reaction speed increases, the curing time is shortened, and the composite strength effect is also better. However, the temperature must be increased within the bearing capacity of the material, otherwise, it will cause material deformation and shrinkage. Therefore, the curing temperature is mostly in the range of 40～60℃.

5. Bag Making Process Parameters

In the bag making process, there are three factors that affect the heat sealing strength and sealing performance of the packaging bag: the temperature, pressure, and speed of the heat sealing knife.

The heat sealing strength must reach 65N/15mm. The higher the speed, the higher the heat sealing temperature and pressure should be set; if the temperature is too low and the pressure is too low, the heat sealing strength cannot be reached; if the temperature is too high and the pressure is too high, cracks will appear in the heat sealing area, which will also affect the heat sealing strength and reduce the sealing performance, leading to bag breakage during cooking.

5. Solutions to High-Temperature Cooking Bag Breakage

 

 

There are many factors that affect the breakage of high-temperature cooking bags during cooking, involving raw materials, inks, adhesives, composite and bag making process parameters, etc. Each process must be strictly controlled, and if even one point is not controlled well, it will be a waste of effort.

Through long-term observation and understanding on the production front line, as well as sampling and testing, statistical analysis of experiments and process parameters, the following conclusions and solutions are obtained.

1. Try to use raw materials from fixed suppliers, which is conducive to grasping the processing characteristics of raw materials, and the raw materials should be inspected according to technical conditions.

2. The raw material structure should be determined according to the characteristics of the content, such as acidity, alkalinity, weight, sensitivity to light and oxygen.

Structures without AL can be seen as content, low cost, but the light shielding and oxygen barrier performance is not as good as structures with AL; if it is an acidic or alkaline content: using a structure containing AL, a layer of NY must be added between the AL and the heat-sealing layer RCPP to increase the acid and alkali permeability resistance. At the same time, if the content is liquid cooking sterilization, the sterilization device must have a back pressure device.

3. The ink must use high-temperature cooking ink and must have good affinity with the high-temperature cooking adhesive used.

4. Use a high-temperature cooking two-component polyurethane adhesive.

The ratio of the main agent and curing agent of the adhesive, the working solid content, and the coating amount must be tested experimentally to find the optimal data, and low working solid content and coating amount cannot be used only to reduce costs.

5. The composite and bag making process parameters and curing conditions must be set according to the material structure and the specific situation of the adhesive. Data accumulation and statistical analysis should be carried out, and they cannot be generalized.

6. The composite strength must reach above 7.0N/15mm; the heat seal strength must reach above 65N/15mm.

In addition to the main factors analyzed above, the purity of the solvent, the temperature and humidity of the workshop, the temperature of the compounding machine's second unwinding preheating roller, the temperature and pressure of the heat sealing roller, etc., all affect the performance of the retort pouch.

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