This summer, a well-known beverage factory began to switch from pull-tab lids to environmentally friendly lids. Its high-carbonated beverage cans (6 packs of shrink film packaging) were stored in the warehouse for about 7 days, and the easy-to-open lids had nicks and cracks. , brought a lot of confusion and trouble to beverage factories and can (lid) suppliers, and enterprises also suffered certain economic losses due to the impact of product quality.

One has to wonder: is this because of the poor quality of the environmental gabble, or is there something wrong with the beverage production process? In order to find out the truth and avoid similar problems from frequently occurring in summer, we will start with the difference in the notch allowance (residual thickness at the notch) between the pull-tab cover and the environmental cover, and further analyze the causes of the easy-open cover. Causes of notch cracking (that is, stress corrosion hereinafter referred to as stress corrosion) prone to occur under high temperature and high humidity conditions in summer, so as to propose relevant prevention and control measures for beverage factories.

Different notch allowances for pull tab caps and eco-friendly caps

For easy-open covers with the same thickness (such as 0.220mm), under normal circumstances, the notch margin of the environmental protection cover is smaller than that of the ring-pull cover, which is specially designed according to the requirements of the opening mechanism. At present, the easy-open lid manufacturers control the notch margin of the two different types of easy-open lids differently, and decide the specific value according to the machine conditions and the thickness of the aluminum material. Table 1 is the control standard for the notch margin of two different easy-open lids produced by a well-known easy-open lid factory. It can be seen that under the same model, the notch margin of the environmental protection lid is usually smaller than that of the pull-ring lid.

Generally speaking, if the notch margin of the environmental protection cover is increased to the control range of the pull ring cover, it may not be able to open normally. Similarly, if the notch margin of the pull ring lid is reduced to the control range of the environmental protection lid, the pop-top beverage product is more prone to the problem of flying tongue.

You may ask: If the notch margin of the environmental protection cover is increased, will there be no notch cracking? The notch margin of the pull ring cover is larger than that of the environmental protection cover, so is there no notch cracking?

the answer is negative. According to the actual feedback from many beverage factories over the years, both the pull ring cover and the environmental protection cover have experienced nick cracking, that is, stress corrosion.

Three causes and three media leading to stress corrosion

Under the high temperature and high humidity environment in summer, beverages with high carbonic acid content tend to crack at the notch of the easy-open lid, which is called stress corrosion (Fig. 1). Stress corrosion is a phenomenon in which the tensile stress and the corrosive environment simultaneously occur in the notch of the aluminum easy-to-pull cover, resulting in notch cracking, and sometimes even discoloration of the pull ring.

Figure 1
Figure 1 Scratch cracking is a distinct feature of stress corrosion.

Figure 2
Figure 2 Stress corrosion can even affect the discoloration of the pull tab.
The three basic factors that produce stress corrosion The sensitivity of materials, tensile stress, and corrosive environment are the three basic factors that produce stress corrosion.

The easy-open cover we usually use belongs to aluminum alloy material, which is a sensitive material.

High-carbonic cans beverages have higher internal pressure, and the notch margin of the easy-open lid is smaller than the thickness of the lid, which is the place with the strongest tensile stress.

During the filling process, the residual beverage liquid on the surface of the cans and the lubricating fluid on the conveyor belt contain chlorides and sulfates, together with the residual moisture on the top of the cans, forming a corrosive environment.

Three kinds of media that produce stress corrosion Residual moisture in the notch, high storage temperature, and humid storage environment are the three media that cause the notch to crack in the easy-open cover.

Small amounts of moisture, chlorides, and sulfates remaining in the score can accelerate corrosion of the score surface.

Higher storage temperatures lead to accelerated formation of corrosion mechanisms and a corresponding increase in stress.

A humid storage environment slows the evaporation of moisture, resulting in a specific “greenhouse” in shrink film packaging.

If the residual thickness increases and the residual moisture still exists, the medium of stress corrosion does not change, and the time for cracking of the notch is only slightly prolonged.

Figure 3
Figure 3 Real photo of stress corrosion in the notch part
Six Prevention and Control Measures for Stress Corrosion

The following six measures can be taken to prevent the occurrence of stress corrosion.

Raise the warmer temperature to prevent condensation. Raise the outlet temperature of the tank warmer above the dew point temperature (2°C) to prevent condensation.

Change the spray water regularly to ensure water quality. Regularly replace the spray water of the tank warmer to ensure that the PH value of the spray water is 6-8, the chloride is less than 10ppm, and the sulfate/nitrate is less than 15ppm.

Removes surface contamination from easy-open lids. Rinse off contaminants on the surface of the easy-open cover, i.e. residual beverages and lubricants on the conveyor belt, with clean water.

Use a blow dryer. It is recommended to install and use a drying device (high-power blower + sufficient number of air knives + preferably single-row drying), and control it in conjunction with the production line to ensure that the conveyor belt can also stop at the same time when the drying device is not running.

Check daily to ensure the cover is air-drying. Check the residual moisture on the easy-open lid surface daily to assess and ensure that the air-drying degree of the lid surface is within the allowable range.

Make sure the warehouse is well ventilated. When stacking in the warehouse, the ventilation of the warehouse should be ensured.

In short, prevent stress corrosion and most importantly make sure the can/lid is air-drying!

It is recommended that can (lid) suppliers remind relevant bottling plants in writing to pay attention to stress corrosion problems before the arrival of high temperature and high humidity in summer each year, and cooperate with the inspection of the water blowing device of the bottling plant to evaluate the air-drying effect of the lid surface.

In the summer of high temperature and high humidity, beverage factories should strictly control the amount of residual water on the surface of the cover, whether it is using a pull ring cover or an environmental protection cover. Both types of lids may suffer from stress corrosion problems if the surface of the easy-open lid is not well-controlled. Since the notch margin of the eco-cap is smaller than that of the ring-pull cap, bottling plants should pay special attention to the air-drying condition of the easy-open cap surface after using the eco-cap, and take stricter control measures to prevent stress corrosion.