Test methods for pressure-sensitive labels

After the label has been used, it needs to be recycled or otherwise safely disposed of, and FINAT has developed test methods for this phase in the life of the label, either to control the process or to make sure that materials are suitable for recycling.

RELEASE LINER AND FACE LABEL

The testing requirements for silicone release coating on a paper or film allow manufacturers to control the thickness of silicone, release level and the consistency.

The FINAT test method allows evaluation of silicone coverage on a paper substrate by using a dye stain test. The exact applied amount of silicone on either filmic or paper base liners can be determined according to the silicone coat weight test method. (Figure 8.4).

A sample of the finished laminate is put into an oven with a weight attached and the release force over time is measured. Laminators can also check resistance to UV light and the adhesive coat weight can also be determined.

This may be an effective tool to ensure proper functionality of the label and identify any potential risk such as adhesive bleeding from the label roll.

CONVERTER-LEVEL TESTS

The label converter might firstly want to check the quality of the laminate at goods-in. Another check to make at this stage is the surface tension of a filmic face material, as this relates closely to the adherence of printing inks and whether corona treatment needs to be specified on press (Figure 8.6).

INK ANCHORAGE AND LABEL APPLICATOR TESTS

The release properties of the laminate need to be checked before the label applicator stage. This is particularly critical when high speed automated label applicators are being used, and the label has to lift off correctly each time. This requires a delicate balance.

The label should not release too easily from the backing liner and at the same time should not stick on the liner too long, otherwise the dispensing will not work properly.

So, measuring the release force at this point gives a good indication of how the label will perform at the dispensing stage (Figure 8.8).

A sample of the finished laminate is put into an oven with a weight attached and the release force over time is measured. Laminators can also check resistance to UV light and the adhesive coat weight can also be determined.

This may be an effective tool to ensure proper functionality of the label and identify any potential risk such as adhesive bleeding from the label roll.

CONVERTER-LEVEL TESTS

The label converter might firstly want to check the quality of the laminate at goods-in. Another check to make at this stage is the surface tension of a filmic face material, as this relates closely to the adherence of printing inks and whether corona treatment needs to be specified on press (Figure 8.6).

The converter will also measure the fluorescence and white point of paper substrates for color measurement purposes.

The latest test method is intended for UV curing of transparent lacquers and UV white inks. This method is based on a color-reaction to test for proper curing of UV inks.

After printing, there are tests to check and control die-cutting quality, particularly to ensure the die-cut has not gone too deep (die strike) and that the laminate’s release properties are matched with requirements for high speed matrix stripping (Figure 8.7).

Measurement of adhesion properties are important to ensure the label sticks firmly and instantly to the container surface. Typically this uses the loop tack measurement test which gives an idea of how ‘sticky’ the adhesive is.

Measurement of the adhesive is also a sign of good curing performance of the silicone, since there are no components migrating to the adhesive.

In this sense the loop tack measurement test is a good method for testing correct silicone cure.

Dimensional stability of the label can also be tested, along with a test method for chemical resistance in harsh environments.

Critical to final label quality is to test that the ink has adhered properly. Ink adhesion tests include rub and scratch resistance, and there are further tests for over laminating a protective film.

There are specific rub and scratch resistance tests for UV inks (Figure 8.9).

For testing final adhesion properties, we have already mentioned the loop tack measurement test. But because the label has to stick on a range of different surfaces, the peel adhesion test is critical to specify the label and adhesive on the final application. Peel adhesion tests are carried out at both 90 and 180 degrees peeing angle at speeds of 300mm/min.

Sheer resistance might also be of interest depending on the final application. If a label has to hold something together, or if there is likely to be any force applied against the label, then sheer resistance, or dynamic sheer testing is an important part of the final specification.

Low temperature adhesion tests are required to ensure the proper functionality of a label being dispensed in a cold environment to a container for any liquids or foodstuff.

If labels are going to be applied to smaller diameter curved containers, such as small pharmaceutical vials, then the labels should not be too stiff and must adhere quickly to the container, and the Mandrel Hold test covers this (Figure 8.10).

RECYCLABILITY

Label recyclability can also now be tested under the FINAT Test Methods. We can measure the wash-off properties of a label where labels need to be washed off by water in a separation tank (Figure 8.11). 

Another recently added test protocol identifies any ‘stickies’ such as remaining adhesive or plastic left in the pulp which may contaminate the recycling process (Figure 8.12).