Factors Affecting the Storage and Transportation of Heat-Shrink Labels

Heat-shrink labels, also known as shrink labels, are commonly made from three materials: PVC, PET, and OPS. These are all high-shrinkage labels in the transverse direction and are applied in sleeve form. Other less common types include OPP longitudinally shrink-wrap labels and PLA biodegradable shrink sleeves. Common printing processes include gravure printing, flexographic printing, and digital printing.

During product packaging, distribution, and use, labels must withstand challenges such as resistance to the contents, the impact of transportation conditions on label materials, and the demands of customer usage.

Impact of Contents

When the contents are acidic, alkaline, or corrosive to materials, it is essential to consider the risk of leakage during filling and customer use, which could damage the label. Therefore, materials with better resistance to the contents should be selected as the label substrate. The resistance of common materials to different contents can be seen in the following table:

(Comparison of Weather Resistance of Common Heat-Shrink Labels)

Impact of Transportation and Usage Environment

This can be analyzed from two perspectives: the constraints of the transportation environment on label materials before shrinking, and the performance requirements for labels during the transportation of finished products after shrinking.

Before shrinking, the main challenge is the effect of environmental temperature changes on the material. If the storage and transportation temperature exceeds 25°C without constant temperature control, the labels may naturally shrink and deform, rendering them unusable.

After shrinking, the material is tested by impacts, abrasion during transportation, and preheating of containers and labels during final sales.

In addition to shrinking when heated, heat-shrink labels also exhibit some natural shrinkage under normal temperature conditions. Natural shrinkage refers to the slight contraction of the material when stored under natural conditions. For materials of the same type, natural shrinkage is largely related to the shrinkage rate at low temperatures (60°C–70°C) and the material's inherent stability. The higher the shrinkage rate at low temperatures, the more pronounced the natural shrinkage.

Among the common heat-shrink label materials, OPS has the highest natural shrinkage rate, followed by PVC, while PET has the lowest natural shrinkage rate.

Natural Shrinkage Rate

A recommended method for evaluating natural shrinkage rate is as follows: Similar to measuring shrinkage rate, place the test sample in a constant-temperature water bath at 30°C or 40°C for 7×24 hours continuously, then observe the change in its shrinkage rate.

The comparison of natural shrinkage rates for the three materials is shown in the figure below:

(Comparative Analysis of Natural Shrinkage for OPS, PVC, and PET Materials)

When labels need to be transported over long distances and for extended periods, or when the product production cycle spans a significant duration, the impact of transportation and storage safety must also be considered. If the selected material type cannot be changed, protective measures must be implemented during high-temperature seasons for distribution and storage.

Common Storage Conditions

The recommended storage conditions for common heat-shrink label materials include the following two points:

(1) Store in a constant-temperature environment below 25°C, away from heat sources.

(2) Avoid direct sunlight.

For transportation, arrangements can be slightly more flexible depending on the distance. For example:

For transportation within 6 hours during high-temperature seasons with a maximum temperature below 35°C, direct delivery methods such as overnight shipping can be used.

For distances exceeding 6 hours with maximum temperatures between 30°C and 35°C, or for maximum temperatures above 35°C, it is recommended to use refrigerated trucks for delivery.

During label storage, special attention must be paid to temperature management in constant-temperature rooms and the uniformity of environmental temperature. As evident from the comparison above, OPS is particularly sensitive to environmental temperature. Therefore, greater care must be taken in storing OPS labels. For example, in a light-proof storage room with a constant temperature below 25°C, improper placement or insufficient power of cooling equipment can cause uneven temperature distribution—cooler near the equipment and warmer farther away—leading to noticeable natural shrinkage of the labels.

Similarly, when labels are taken from the constant-temperature warehouse to the production line, the quantity of backup labels on-site should be controlled. It is generally recommended not to exceed a 4-hour supply. Instead, adopt a management approach of frequent retrieval and return of smaller quantities, as the temperature on the production floor is typically not lower than the external natural temperature. Additionally, the storage location for backup labels on-site should be kept away from heat-generating equipment (e.g., high-power motors, heat-shrink tunnels, steam pipes) and must never be exposed to direct sunlight.

During the transportation and distribution of finished products after shrinking and setting, the jostling and impacts during transit can cause abrasion and damage to labels. This is a critical factor that must be considered during label design and material selection. Materials with different densities and surface strengths vary significantly in their resistance to abrasion and impact. Among the three common label materials—PVC, PET, and OPS—PVC and PET have comparable surface strength, while OPS is noticeably weaker. To overcome the weakness of OPS, its surface can be coated with scratch-resistant varnish, or the substrate thickness can be increased. Additionally, PET/OPS/PET co-extruded materials have been developed to combine PET's abrasion and impact resistance with OPS's excellent shrinkage properties.

Special handling methods during end-customer use also impose requirements on label performance. For example, for products like dairy and coffee beverages that require heating in winter, whether the heat-shrink label film can withstand preheating temperatures is a concern designers cannot overlook. For instance, PVC material may turn white (becoming cloudy and losing transparency) when boiled at temperatures above 80°C for about a minute. OPS material is prone to overstretching or even developing holes during secondary preheating, while PET performs well under high temperatures.

Conclusion

The application of a small heat-shrink label involves multiple processes, from materials and contents to containers, equipment, and distribution. In actual industrial production, requirements for efficiency, yield, and automation directly influence the matching selection between labels and other factors, making it difficult to establish a universal evaluation principle. However, this does not mean it is entirely unapproachable. By considering the label application scenario, we can outline the following approach: First, determine the necessary conditions for the label based on container characteristics (e.g., maximum shrinkage rate, material resistance). Then, combine these with the specific needs of product packaging to select matching sufficient conditions (e.g., channel equipment, storage and transportation conditions).