Identification and characteristics of PSA label materials

As opposed to other forms of labeling such as wet-glue and shrink sleeves, there is no requirement for application of an external adhesive force, and this creates a great advantage for labelstock material – the ability to handle in a clean room environment.

Pressure-sensitive labels are extremely versatile in terms of their ability to function across a vast range of applications and environmental conditions, thanks to a wide choice of adhesives and release liners, and to be dispensed at very high speeds on automated label application lines.

As we have seen, pressure-sensitive label laminate starts with the release base paper or film as a base, which is then coated with silicone, followed by an adhesive coating and then the face material. This face material can be top-coated or primer-coated depending on the print process which follows.

What is the criteria for choosing a face material? It depends, first of all, on the type of printing – thermal printing, digital printing or flexo printing, for example. Based on that, the face material will be different. What finish is required of the label? That, again, dictates the type of face material, whether opaque, clear, metalized or matt finish.

At this point it is worth considering material thickness, particularly as regards film. Different types of container require different material thicknesses to give the required bulk, strength and stiffness characteristics. Thus, for example, PE labels for ampoules are typically in the range 30-50 microns, while PE labels for large containers such as lubricants would be up to 110 microns.

Material thickness, strength and stiffness are also important factors during label conversion. A certain amount of strength in the paper or the film is essential for correct die-cutting matrix removal, as well as dispensing.

There is a complex relationship between material thickness and price. It seems common sense that when the thickness of the material goes down, the price will also go down. But this is not always the case. The price may actually increase. In the case of paper a higher percentage of pulp may be required to give the required stiffness or strength, and similarly the tensile strength and stiffness required in thinner films could also make the cost of the material higher.

Examining materials for adequate strength and stiffness requires firstly checking the GSM and thickness of the material, whether a paper or a film.

In case of paper, checks can also be made for pulp content. Other important factors for paper include roughness, opacity, gloss/matt, brightness, whiteness, shade, tensile strength, whether it contains OBA or not and – very important in today’s climate – whether it is FSC/PEFC certified or not.

In films, characteristics to look for include GSM, thickness, stiffness, gloss/matt finish, brightness, whiteness (for opaque films), shade and tensile strength.

FACE PAPER CHARACTERISTICS

a. Uncoated paper. Paper manufactured in its raw form using pulp. It has a very thin layer on top to allow for printing and to deliver some degree of smoothness. The paper is porous, meaning penetration of ink is very fast. It is mainly used for blank labels and thermal transfer printing. One or two color printing is sometimes used where brand identification is required.

b. Semi-gloss papers. Papers coated with clay, which allows for excellent printability. This is most commonly used for thermal transfer-printed barcodes as well as for prime product labels where thermal transfer over-printability is required.

c. Cast coated paper. Cast coated paper is also known as Mirror Coat paper because of its high gloss. It has a heavy deposition of clay and is highly calendared to give it the required level of gloss. The thickness of the clay coating makes the paper impermeable, meaning the ink stays on the surface, giving a rich printing effect. For this reason, these label papers are widely used in the cosmetics and liquor industries as well as other similar high end FMCG products.

d. Metalized paper. Metalized papers are formed by the deposition of vaporized aluminum onto paper. This can be done either directly, or by a process of transference. Transfer metalizing is carried out by depositing the aluminum vapors onto a polyester film, which is laminated to the paper then delaminated, giving the paper a more glossy look compared to direct metalizing.

Direct metalizing tends to be used for applications like beer labels, while transfer metalizing is used more in the high-end consumer markets such as cosmetics and premium liquors.

e. Direct thermal papers. Direct thermal paper has a heat-sensitive layer impregnated on the paper which turns black on exposure to heat. Thermal printers impart this heat in a particular pattern to give the barcode or variable text. Thermal papers can be both top coated and non-top coated. Non-top coated papers are the most economical grade and they are typically used for various logistics and pricing applications for example in supermarkets. Thermal linerless labels are one of the fastest growing trends in supermarket logistics and pricing labels.
Top coated thermal papers are used in applications where resistance to chemicals, water, heat and abrasion is required. Shelf life can be up to 12 years.

If a longer shelf life is required, thermal transfer ribbons are usually chosen over a direct thermal paper.

FILM CHARACTERISTICS

a. Polyethylene. PE is used in applications where flexibility, durability and resistance to abrasion and impact are required.

On the other hand, PE does not have any resistance to oxidization; it is not 100 percent clear, being somewhat hazy, even in the clear format; it has no resistance to chlorinated hydrocarbons or to harsh outdoor conditions.

Its calliper has to be generally greater than 70 microns for safe dispensing, since PE is mostly used mostly for larger containers.

PE is available in clear and opaque white finishes, as well as metalized, although metalized PE is not so widely used today as there are more cost-effective alternatives.

To identify a PE, separate the laminate and stretch the film, which will deform but will not tear easily. PE, like PP and co-extruded film, floats on water.

b. Oriented Polypropylene. Polypropylene film has several key performance advantages: resistance to tearing, abrasion and chemicals; good outdoor UV stability; and excellent die-cutting and printing characteristics, particularly where ink adhesion is enhanced by corona treatment or a top coating. Its key limitation is less resistance to heat. PP finishes include clear, opaque and metalized.

How do you identify PP film? Take the film, put a notch to it and cut it through. A knife will cut through easily.

c. Polyester. Polyester film has high resistance to heat, high resistance to tearing, high resistance to abrasion, good resistance to chemicals, excellent dimensional stability, resistance to UV – making it excellent for outdoor applications – and resistance to solvents.

Due to its stiffness, it has less conformability than a PE and its cost is higher than PP/PE due to the higher density of the film. Its density is 1.4 as compared to a PP, which about .85 or .9 and PE with a density of 1.

Polyester is available in clear, opaque white and both gloss and matt metalized finishes. How do you identify a Polyester film? Take the film, put a notch to it, try to tear it apart, and it breaks in a zig-zag manner.

d. PVC. PVC may be open to environmental objections, but it still has key applications which cannot easily be replicated by other films.

Its key advantages are a high degree of durability for both indoor and outdoor use and a high degree of flexibility for a semi-rigid material. In addition, it is relatively easy to convert, as corona treatment or top coating is not required for printing to a high standard.

General applications include outdoor advertising and battery labels.

The downside of PVC is undesirable environmental characteristics, as it forms toxins when incinerated. In addition, plasticiser migration has the potential to kill the adhesive in a PS laminate, presenting challenges in adhesive selection.

How do you identify PVC? Separate the laminate and stretch the film and observe how it tears and it will tear differently from the other films.

VISUAL IDENTIFICATION

Another way of identifying face materials is by their gloss and brightness levels and how they change when the labels are printed (Figure 7.1).

Samples of materials printed with the same ink (and on the same press under identical conditions), but with different surface characteristics can look completely different.

CONCLUSION

Given the huge range of choices available to the end user and the label converter, how does one select the right labelstock in totality? Factors that need to be taken into account include:

• The bond required, whether permanent or removable.

• The shape of the container or labeled surface, whether circular or flat, and if circular, what is the diameter and co-diameter of the product?

• The size of the label required will help decide the GSM, or thickness, of the material.

• What is the texture of the surface where the label will be applied? Rough, smooth, porous or a mixture of these characteristics.

• What is the chemical composition of the substrate to be labeled?  Glass, HDP, LDP, corrugated etc. They all have different surface tension and the way they create bonds will be different, requiring different kinds and GSMs of adhesives.

• What are the printing and converting requirements? 

• What finish is required from the label

• What is the application temperature and end use temperature of the label?

• After all these technical considerations have been taken into account, the economics required of the label have to be looked at.

These are among the factors which help decide which label product is appropriate for a particular application. The pace of technology change shows no sign of slackening, giving end users and converters ever greater choice as we move forward.

RECLOSEABLE LABELS CASE STUDY

An excellent case study of materials selection is reclosable labels, an ever-more popular choice for wet wipes, rice and other reclosable packs. Why is PP chosen over PE and Polyester for this application?

The key performance requirement is a label which is flexible enough to adjust to the changing shape of the pack as the number of wipes or the volume of rich, for example, decreases in the flexible pack.

On the surface, one might expect PE to be specified for its flexibilty, but PE would actually stretch too much.

Polyester, meanwhile is too stiff, and PVC is not considered environmentally friendly enough despite having potentially the correct performance characteristics.

For these reasons, PP is chosen. But to ensure it functions correctly, the thickness needs to be increased towards 90 more microns.