Why Do Self-Adhesive Labels Always Curl? The Truth Behind It May Surprise You!

What Are Self-Adhesive Label Materials?
Self-adhesive label materials are typically composed of three parts: liner, face material, and adhesive. The liner has an oily surface that acts as a release agent for the adhesive, ensuring that the face material can be easily peeled off. The face material, which serves as the primary carrier for the printed content of the label, can be made from various materials such as coated paper, thermal paper, PET, PVC, etc.

Due to both subjective and objective factors, self-adhesive labels often encounter issues such as curling, wrinkling, bubbling, and liner tearing during the labeling process. These problems lead to customer complaints and cause headaches for label printing companies. Among these issues, curling is the most common. In this article, the author, drawing on practical work experience, systematically analyzes the causes of label curling and provides solutions, hoping to offer some assistance to readers.

Factors Affecting Label Curling
01 Adhesive Viscosity and Product Compatibility
If the adhesive of the self-adhesive label material lacks sufficient viscosity, it will definitely lead to label curling. Moreover, due to the wide variety of self-adhesive label materials, there is a compatibility issue between different objects to be labeled and the self-adhesive label materials. For example, AW4200A and AW3209 self-adhesive labels should not be applied to bottles with a diameter of less than 3cm, and hot-melt adhesive self-adhesive labels should not be applied to PVC objects. These precautions are clearly stated in the technical parameters of the self-adhesive label materials. 

Therefore, it is recommended that label printing companies carefully read the product technical parameters when selecting self-adhesive label materials. It is also suggested that when sending samples to end-users for confirmation, the product technical parameters of the self-adhesive label materials should be attached to avoid unnecessary risks due to misuse by end-users.

02 Face Material Flatness
For self-adhesive label materials, the smoothness of the face material is the key factor affecting the overall smoothness of the material, and the overall smoothness is an important indicator to ensure printing quality. Therefore, during the labeling process, it is necessary to check whether the face material of the self-adhesive label is flat. Especially when the label is laminated, it is important to ensure that the lamination tension is not too high. Otherwise, excessive tension will cause the label to curl upwards, resulting in label curling.

03 Label Design
When designing labels, it is best to avoid overlapping of the beginning and end. If this cannot be avoided, cold lamination should be avoided, and the overlapping parts should not be printed or varnished.
When overlapping labels are applied, the following points should be noted:
(1) If the label must be laminated, the label must be treated with corona after lamination to ensure that the surface energy of the film is greater than 38 dynes. Only in this way can normal labeling be achieved.
(2) During printing, the overlapping part should be left blank. The printing surface can be enlarged by 1 to 2 lines towards the overlapping position to ensure that there is no white space after labeling.
(3) For labels that need to be varnished, the original full varnish can be changed to partial varnish, and the overlapping labeling part should not be varnished.
(4) Pay attention to the curing degree of UV ink. Because the surface energy of the over-cured ink layer will be reduced, it will affect the adhesion of the next layer of ink and the adhesion of the adhesive.

04 Object Material
Due to the different materials of the objects to be labeled, their surface energy will vary, and therefore, the same self-adhesive label will show different effects after labeling. Under normal circumstances, the surface energy of different materials of objects to be labeled is as follows: stainless steel > glass > PET > PP > PE.
In addition, for objects of the same material, the surface energy may also be different due to different processing techniques, such as glass bottles treated with silane and plastic bottles with different surface roughness. Therefore, for objects with different surface energy or bottles, self-adhesive labels with different adhesives should be selected for matching.

05 Object Shape
To achieve an ideal labeling effect, the labeling surface of the object should be a plane after unfolding. If the labeling surface of the object is curved in both directions after unfolding (spherical labeling surface), it is difficult to achieve an ideal labeling effect. Therefore, when designing bottles, it is best to avoid such shapes, or when labeling, try to avoid such positions.
In addition, due to production cost pressure, blow molding factories often reduce the weight of the bottle, making the bottle thinner. This leads to a large deformation of the bottle before and after filling, and it is not possible to apply a large labeling pressure. For example, when labeling and then filling with hot liquid at 30℃ to 40℃, part of the bottle will bulge and deform. After cooling, the bottle will shrink, resulting in label curling.

06 Surface Condition of the Object
The roughness of the object's surface has a great influence on the labeling effect. The rougher the surface of the object, the more likely it is to have label curling problems. In addition, if there are oil stains, water droplets, dust and other impurities on the surface of the object, it is equivalent to the adhesive sticking to these impurities instead of the surface of the object, which will also easily lead to label curling.
In addition, whether the surface of the object has been treated with special processes may also affect the final labeling effect. For example, in the process of filling small-mouth glass bottles with medicine, in order to prevent the medicine from adhering to the wall, the bottle surface is washed with silane water in advance. If there is a lot of silane residue after washing, the label will curl after labeling.

07 Labeling Environment
Under normal circumstances, the labeling temperature generally includes four aspects: label temperature, object temperature, labeling environment temperature, and storage temperature within 24 hours after labeling.
(1) The technical parameters of each self-adhesive label material clearly mark the minimum labeling temperature and the range of use temperature. For example, the minimum labeling temperature of NW5539 label products is 10℃, and the range of use temperature is -15℃ to 65℃.
(2) When labeling, the environmental temperature must meet the minimum labeling temperature of the label. Only in this way can the adhesive of the labeled label flow normally and exert normal viscosity.
(3) The surface temperature of the object to be labeled must be above the minimum labeling temperature so that the adhesive, which originally has fluidity, will not have poor or no fluidity after being attached to a low-temperature surface, resulting in reduced adhesion.
(4) The storage environment temperature after labeling is also very important. To maintain the fluidity of the adhesive and allow the adhesive to transition from initial viscosity to final viscosity after labeling, it must be completed in an environment above the minimum labeling temperature.

08 Labeling Process
Different labeling processes can also affect the problem of label curling. For example, an ice cream brand labels ice cream plastic boxes in an environment of 10℃. The ice cream is injected into the plastic box, the box is automatically covered, and then the label is automatically applied to the box cover and around the box. Finally, it is quickly frozen in an environment of -15℃. After 1 to 2 hours, it is transferred to a large cold storage for storage. It was found that most of the labels curled after quick freezing, and most of the labels fell off after being transferred to the large cold storage. To solve this problem, the company labeled the plastic boxes that needed to be filled with ice cream the next day in advance. After 24 hours of labeling, the ice cream was injected, covered, and quickly frozen to form, and then transferred to the large cold storage for storage, which effectively solved the problem of label curling.

09 Labeling Equipment
Different labeling equipment should be selected according to different object surfaces and labeling requirements. Commonly used are sponge wheels, belt wheels, and labeling plates. In addition, the labeling machine should apply appropriate labeling pressure to ensure that the label can be perfectly attached to the object.
When the labeling pressure is insufficient, such as using soft brushes, too soft labeling plates, or the spacing between sponge wheels, belt wheels, and bottles is too large, the label will curl due to insufficient labeling pressure. In addition, if the labeling device ages, it will affect the labeling pressure, and thus may cause the label to curl.

Customer Case of Label Curling
In 2023, a company's PE self-adhesive label curled during labeling and needed to be solved urgently. From the company's site, it was learned that the company took the object to be labeled out of the cold storage, and labeled it when there was still condensation on the surface of the object. After labeling, the product was immediately placed in a cold storage with a very low temperature, and the storage temperature of the label did not reach above 10℃. Therefore, we concluded that the curling phenomenon of the label was caused by environmental problems.

The normal operation is as follows:

The label must be stored in an environment above 10℃ for 24 hours before labeling because the adhesive of this label only has good fluidity in an environment above 10℃, and the viscosity of the adhesive can be exerted normally.

According to the suggestion, the company immediately made improvements and adjusted the place where the labels were stored to ensure that the temperature of the stored labels was above 10℃. After later tracking, the labels produced by the company did not have the problem of label curling.

Summary
Curling of self-adhesive labels during labeling is very common in practical production applications. Both label printing companies and end-users should select the most suitable self-adhesive label materials based on the actual application of the labels and pay attention to the impact of environmental temperature and humidity on the labels. Only by considering various factors can the labeling quality of products be improved and the appearance of products be enhanced.