(1) Initial Viscosity Issue: Corners Lifting on Automatic Labeling Machines
Situation:
Many manufacturers notice that corners of the labels lift when applied using automatic labeling machines. This is a common issue that may be due to insufficient initial tack or instant adhesion.
Reason:
The problem likely lies in the initial viscosity. When the adhesive is not sticky enough upon first contact with the surface, it causes poor adhesion, leading to corners lifting or peeling off.
Test Method:
To evaluate the initial tack of the adhesive, the slope rolling ball test method (as per GB4852) is used. In this test, a standard steel ball is rolled down an inclined plane and comes into brief contact with the label's sticky surface under slight pressure. The smallest steel ball that the label can adhere to indicates the initial stickiness of the label.
Result:
There are 33 standard steel balls, and the result is determined by the largest steel ball that the label can still stick to. This indicates how strong the initial adhesion is.
Instrument Used:
Initial viscosity tester, which measures the initial tack by analyzing the ball's adherence to the surface.
(2) Stickiness Issue: Label Falling Off After Long Time
Situation:
Some users find that their labels lose adhesion over time and eventually fall off. This issue typically arises from poor retention of stickiness.
Reason:
This is caused by a lack of retained stickiness over time, meaning the adhesive does not maintain its bonding ability after initial application.
Test Method:
To measure the holding stickiness, the GB4851 method is used. A 2 kg standard pressure roller is used to apply the label onto two test boards. The boards are then hung vertically, with a 1 kg weight suspended from the bottom of one test board. The test measures the time it takes for the label to peel off or how much the adhesive moves after a set period.
Result:
The adhesive's resistance to peeling is quantified by the displacement of the specimen over time or the time it takes for the label to completely detach from the surface.
Instrument Used:
Stickiness tester (or holding power tester), which measures the label's ability to stay adhered to the surface under sustained stress.
(3) Peel Strength Issue: Label Peels Easily
Situation:
When labels are applied, they can often be peeled off too easily, suggesting low peel strength.
Reason:
This issue arises when the peel strength of the adhesive is too low, meaning the label is not able to maintain a strong bond once it is applied.
Test Method:
To evaluate the peel strength, the GB2792 method is used. A 2 kg standard pressure roller is used to bond the label to a clean test plate. Then, the free end of the label is folded 180º, and the label is peeled off at a constant speed, and the peel force is measured.
Result:
The peel strength (kN/m) is recorded, and the strength of the adhesive is calculated based on the force required to peel the label from the surface.
Instrument Used:
Electronic peel tester, which measures the force required to peel the label from the surface at a 180º angle.
Summary of Methods and Instruments:
Initial Tack Test: Measured by the slope rolling ball method (initial viscosity tester) to test the first contact adhesion strength.
Holding Stickiness Test: Measured by the holding power test (stickiness tester) to test the ability of the adhesive to resist peeling over time.
Peel Strength Test: Measured by the 180º peel strength method (electronic peel tester) to test how strongly the adhesive bonds to the surface after application.
By using these tests, manufacturers can ensure that their labels perform well under real-world conditions and maintain good adhesion over time.
