Analysis and Solutions for Steaming Bag Breakage

Common materials for retort pouch production include: PET, aluminum foil, BOPA 、 PE and CPP, etc. These materials have different advantages and disadvantages in terms of mechanical properties (tensile strength, peel strength, impact resistance, etc.) and physical properties (moisture resistance, oxygen resistance, cold and heat resistance, etc.). Therefore, in production, two-layer, three-layer or multilayer composite structures are used according to the type and requirements of the contents to achieve optimal performance. Retort pouches have thin walls and good heat transfer performance, and can reach sterilization standards in a short time, thus saving energy. In addition, retort pouches occupy less space, are easy to carry, and have low transportation and storage costs, so they are deeply loved by enterprises and consumers.

However, in practical applications, retort pouches have certain defects, among which "bag rupture" is the most troublesome problem. To thoroughly analyze the causes of retort pouch rupture, we must first start with the production process. Conventional retort pouches are usually applied in the following production processes ： Raw material acceptance →Processing→Bagging→Heat sealing→Inspection→Heating sterilization→Cooling→Drying and packaging. Bag rupture often occurs during the heating sterilization and cooling stages.

Currently, the industry generally uses the back pressure method to control the rupture rate of retort pouches, that is, compressed air is used to pressurize the pot during the sterilization and cooling stages to balance the pressure inside the bag and reduce the probability of rupture. However, this is only a solution obtained from the process flow. If the retort pouch itself has quality problems, then the above method is counterproductive and loses its practical significance.

In Shunxingyuan Packaging, a professional flexible packaging manufacturer after many experiments, it was found that the rupture of retort pouches mostly occurs in the following parts: rupture at the heat-sealed edge 、 rupture on the inner side of the heat-sealed edge 、 Due to delamination between the heat-sealing layer and the outer layer, the inner layer on the inner side of the heat-sealed edge ruptures.

1、 Rupture at the heat-sealed edge

Currently, retort pouches are generally made into flat mouth bags or stand-up pouches by three-side sealing. After filling by the enterprise, the last side is sealed. Therefore, when analyzing the causes of heat-sealed edge rupture, the source of rupture should be distinguished.

For edge rupture in the bag-making process, it is mainly due to unreasonable heat-sealing technology, resulting in substandard heat-sealing quality of the edge. Heat-sealing technology refers to using a high-temperature heat-sealing knife to apply predetermined pressure to the upper and lower films at the sealing position and maintain it for a period of time, so that it becomes a molten viscous state and adheres. After cooling, it has a certain strength. There are many influencing factors in this process. The heat-sealing temperature, pressure, and time have the greatest influence, with temperature playing a leading role. The heat-sealing temperature is generally between the melting temperature and decomposition temperature of the film. Retort pouches mainly use CPP film or HDPE film as the heat-sealing layer, which has a higher melting point. The heat-sealing temperature is generally set at 180℃ - to 230℃. If the temperature is too low, even if the pressure is high and the time is long, the film cannot reach the molten self-adhesive state. Conversely, if the temperature is too high, the film material at the sealing position is easily extruded under pressure, reducing the thickness and heat-sealing strength of the heat-sealing position.

For edge rupture in the filling and sealing process, in addition to the same heat-sealing process problems, non-standard operation during filling can easily cause contamination of the parts to be sealed, thereby affecting the sealing strength.

2、 rupture on the inner side of the heat-sealed edge

This refers to the phenomenon where the edge does not rupture, but the film at the root of the edge is stretched and broken. The reason is also due to the heat-sealing process of bag making. The combined effect of excessive heat-sealing temperature and pressure, and slow bag-making speed, causes the inner layer film at the edge of the seal to be pressed too thin, reducing the tensile strength and significantly reducing the pressure resistance.

3、 Delamination of the heat-sealing layer from its outer layer, the retort pouch ruptures from the inner layer on the inner side of the heat-sealed edge

Delamination of the composite film layers will greatly destroy the overall tensile strength of the composite film, causing the bag to be easily broken. To solve this problem, it is necessary to understand the main factors causing delamination of the composite film. Taking CPP and other films (heat-sealing layer) - adhesive - aluminum foil - adhesive - PET, etc. as an example of a three-layer composite retort pouch, delamination may occur between any two materials. In mild cases, the composite film shows striped delamination at the stressed parts, and in severe cases, large-area natural delamination occurs after retorting.

The main reasons for this phenomenon are as follows: insufficient wet heat bonding strength of the adhesive; improper ratio of curing agent to main agent in the adhesive; insufficient adhesive coating amount; insufficient curing time, low curing temperature; large thermal shrinkage rate of the inner film itself; incompatibility between the film and the adhesive. 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 adhesion is the strongest. Therefore, it is necessary to reasonably select and match adhesives that are compatible with the film materials.