Two practical case studies on retort pouch issues!

Enterprises often encounter various unexpected situations in the use of retort pouches, such as bag breakage, bag swelling, and wrinkles. If not handled carefully, it is easy to cause product quality problems. Today, Shunxingyuan Packaging, a professional flexible packaging manufacturer, will take you to see the common quality problems and solutions of retort pouches!

 

 

1. Bag breakage caused by poor heat sealing

 

 

A food company in Zhongshan, Guangdong Province, reported that the PA//CPP retort packaging bags processed with 300LM heat-resistant composite ink and PU-1750 heat-resistant adhesive showed a high breakage rate (as high as 45%).

Later, the manufacturer switched to another brand of heat-resistant adhesive, but the same phenomenon occurred.

By inspecting the packaging bags, it was found that the packaging bags had heat sealing quality problems.

When pulling the heat-sealed edge of the packaging bag by hand, the heat-sealing strength 1mm～2mm away from the heat-sealed edge is particularly poor, and the heat-sealed parts of the two layers of CPP film can be easily pulled apart, but the composite strength of PA//CPP is good and cannot be peeled off.

At the same time, put your hand into the bag and use your fingertips to pick the heat-sealed edge. It was found that the heat-sealed parts of the two layers of CPP film in some places can be picked in 1mm～2mm.

Therefore, Shunxingyuan Packaging believes that this is an accident caused by the heat sealing process.

The following are the influencing factors of heat sealing strength:

1. The type and thickness of the heat-sealing layer material have a decisive influence on the heat-sealing strength.

Commonly used heat-sealing materials in composite packaging include CPE, CPP, EVA, and other ion-type resin co-extruded or co-blended modified films, with a thickness generally between 20μm～80μm.

For the same heat-sealing material, its heat-sealing strength increases with the increase of heat-sealing thickness. The heat-sealing strength of retort packaging bags generally requires 40N/15mm～50N/15mm.

Therefore, the thickness of its heat-sealing layer material should be above 70μm.

2. The heat sealing temperature has the most direct impact on the heat sealing strength.

The melting temperature of the heat-sealing material directly determines the minimum heat-sealing temperature of the composite bag.

In the production process, due to the influence of many factors such as heat sealing pressure, bag making speed, and the thickness of the composite substrate, the actual heat sealing temperature used is often higher than the melting temperature of the heat-sealing material.

The smaller the heat sealing pressure, the higher the required heat sealing temperature, the faster the bag making speed; the thicker the surface material of the composite film, the higher the required heat sealing temperature.

If the heat sealing temperature is lower than the softening point of the heat sealing material, no matter how much the pressure is increased or the heat sealing time is prolonged, it is impossible to make the heat sealing layer truly sealed.

However, if the heat sealing temperature is too high, it is easy to cause the heat sealing material at the weld to melt and extrude, resulting in a "root cutting" phenomenon, which greatly reduces the heat sealing strength and impact resistance of the retort packaging bag.

3. To achieve the ideal heat sealing strength, a certain heat sealing pressure is essential. For thin packaging bags, the heat sealing pressure should be at least 2kg/Cm2, and it should be increased accordingly with the increase of the total thickness of the composite film.

If the heat sealing pressure is insufficient, the two layers of film cannot achieve true fusion, resulting in poor local heat sealing, or it is difficult to remove the air bubbles trapped in the weld, resulting in false welding.

Of course, the heat sealing pressure is not the greater the better, it should be appropriate without damaging the weld, because when the heat sealing temperature is high, the heat sealing material at the weld is in a semi-molten state, and too much pressure will easily squeeze the heat sealing material out, making the weld edge semi-cut, the weld brittle, and the heat sealing strength reduced.

4. The heat sealing time is mainly determined by the speed of the bag making machine.

Heat sealing time is also a key factor affecting the sealing strength and appearance of the weld.

Under the same heat sealing temperature and pressure, the longer the heat sealing time, the more fully the heat sealing layer is fused, and the more firmly it is bonded. However, if the heat sealing time is too long, it is easy to cause wrinkles in the weld, affecting the appearance.

5. After heat sealing, if the equal weld is not cooled well, it will not only affect the flatness of the weld appearance, but also have a certain impact on the heat sealing strength.

The cooling process is to shape the weld that has just been melted and heat-sealed under a certain pressure and at a lower temperature to eliminate the process of stress concentration.

Therefore, insufficient pressure, poor cooling water circulation, insufficient circulation volume, too high water temperature, or untimely cooling will all cause poor weld cooling, heat-sealed edge warping, and reduced heat-sealing strength.

6. The more heat sealing times, the higher the heat sealing strength.

The number of longitudinal heat sealing times depends on the ratio of the effective length of the longitudinal welding rod to the bag length.

The number of transverse heat sealing times is determined by the number of sets of the transverse heat sealing device of the machine. At least two heat sealing times are required to ensure good heat sealing strength. General bag making machines have two sets of heat sealing knives, and the higher the degree of overlap of the heat sealing knife positions, the better the heat sealing effect.

7. For composite films with the same structure and thickness, the higher the interlayer peeling strength of the composite layer, the greater the heat sealing strength.

For products with low composite peeling strength, weld failure often causes interlayer peeling of the composite film at the weld first, causing the inner heat sealing layer to independently bear the destructive tensile force, and the surface material loses its reinforcing effect, thus greatly reducing the heat sealing strength of the weld.

If the composite peeling strength is large, interlayer peeling at the weld will not occur, and the actual heat sealing strength measured will be much greater.

By understanding the manufacturer's heat-sealing production process, it is believed that the high rate of bag breakage is caused by the two sets of heat-sealing blades not completely overlapping, resulting in poor welding near the heat-sealed edge, thinning of the CPP film, and damage to some structures. During steaming, the thinned CPP film layer cannot withstand the pressure, resulting in bag breakage. Adjusting the heat-sealing blades to ensure heat-sealing quality solved the problem.

2. Delamination failure of the retort pouch caused by polyurethane ink

 

 

A printing factory in Yibin, Sichuan used a steam-resistant polyurethane ink to print BOPA film and laminate it with CPP. After bag making and retort testing, delamination and wrinkling were found on the BOPA film.

Cause Analysis:

First, let's understand the adhesion mechanism of polyurethane adhesives:

The polyurethane main agent with -OH end groups reacts with the polyurethane curing agent with -NCO end groups to generate high-molecular-weight polyurethane, thus forming adhesion.

Therefore, alcoholic substances with -OH and substances with -NH2 have a greater interference with the lamination process.

Next, we need to analyze the polyurethane ink system. The composition of polyurethane ink includes: pigments, polyurethane resin, auxiliaries, solvents, etc.; the composition of polyurethane ink dilution solvents generally includes: methyl ethyl ketone, ethyl acetate, butyl acetate, isopropyl alcohol, toluene, etc.

Since the polyurethane resin in polyurethane ink itself is also -OH terminated, it can also react with the curing agent of the polyurethane adhesive, reducing the amount of curing agent participating in the reaction of the adhesive main agent and affecting the bonding strength.

Moreover, the alcohol-based dilution solvents in polyurethane ink will also react with the curing agent, affecting the bonding quality.

Solution:

1. First, adjust the composition of the dilution solvent in the polyurethane ink, and try not to use alcohol-based dilution solvents.

If alcohol-based solvents must be used during printing, the residual solvent should be minimized.

2. If white ink is used as a base for the printed film, it is best to add a small amount of hardener to the white ink to improve the lamination strength.

However, the ink with hardener has a shorter shelf life. If there is any remaining ink and a long interval, it should not be used in principle, resulting in significant waste.

3. The purity of ethyl acetate, the dilution solvent used in preparing the adhesive, should be high, and the water and alcohol content should be as low as possible.

In addition, when preparing the adhesive, appropriately increase the amount of curing agent (the increase is 5%~10% of the amount of curing agent, that is, if 1kg of curing agent is added, an additional 0.05kg~0.1kg of curing agent is needed) to ensure that the main agent and curing agent of the adhesive are fully cross-linked.

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