Pressure-Sensitives Adhesives

Considerations for choosing, types of adhesives and material selection

This article is part of a new series about the basics of label printing, in cooperation with Label Academy.

When selecting an adhesive, the type of application and end use of the product to which the label is to be attached, will often determine the type of adhesive to be used.

Key considerations are:

• Is the label to be permanent or removable?
• Storage environment including the range of temperatures, moisture and their potential effect on the label
• The surface to which the label will be applied i.e. smooth/rough, hot, cold, frosted, wet, dry, paper, plastic or metal
• Will label be used for direct contact with food?
• Is there a medical application involving a sterilization process or skin contact?
• Is the label to be used for children’s toys?

PERMANENT	REMOVABLE
GENERAL PURPOSE	LONG TERM REMOVABLE
DEEP FREEZE	SHORT TERM RESPOSITIONABLE
WET PACK
LOW SURFACE ENERGY
WATER SOLUABLE
UNDERWATER
HIGH TEMP
HIGH SHEAR INDUSTRIAL

Types Of Adhesive

There are a number of variables involved in the manufacture of pressure-sensitive adhesive products (PSA).

Although the process of coating and laminating is uncomplicated, not all PSAs can be coated using every type of coating system.

PSAs use differing adhesive technologies and each of these technologies have varying ranges of viscosity, solids content and coating weights, which may be required for specific applications.

Each of these items can affect the adhesive-coating. These variations will affect the adhesive properties and the choice of the right adhesive and method of application is very important. Figure 4.8 summarizes the main types of PSAs.

MAIN TYPES OF PRESSURE SENSITIVE ADHESIVES

The main adhesive formulations used for pressure sensitive adhesives are as follows;

• Acrylic
• Emulsion acrylics (water-based)
• Solution acrylics (organic solvent-based)
• Radiation curable (UV and EB) – 100 percent solids
• Rubber
• Solvent rubber
• Hot melt rubber (100 percent solids)
• Radiation curable (UV and EB-electron beam) – 100 percent solids
• Silicone adhesives

BASE COATING AND TOP COATING OF SUBSTRATES

There are other processes that are used to optimize the performance of materials. Base coating is used to create a base layer to assist the printing and embellishment processes by improving the ink key and ink lay, and by producing a smoother substrate surface, particularly when the foiling process is used.

The method of application for this type of coating is predominantly via the flexo process, although the screen and the gravure processes are also used. Roller coating too is also used as a base coating process.

The printing units are generally positioned immediately after the unwind unit of the press and can be used to apply an overall coat or alternatively a spot coating.

Top coating or over varnishing of the face substrate is widely used in the self-adhesive label market. The coating is applied as a protective layer after the printing process has taken place giving protection from abrasion and increasing the product resistance. It is also used to produce an alternative finish to the label and enhance the shelf display qualities of the product.

Top coating is also used to aid ink adhesion when secondary overprinting is required, for example date or batch coding data.

Top coated UV over varnishes are typically used to assist product detection via the label applicator or to improve filling line performance.

CORONA DISCHARGE

Corona treatment is another process often used to improve the basis for adhesion of applied printing inks, adhesives, lacquers, etc.

To obtain good adhesion it is necessary to increase the surface energy of the substrate. The surface of film or other material to be treated is bombarded with electrons to increase the surface wettability of the substrate (generally filmic).

Corona discharge is often used on polymer-based substrates that have low surface energy leading to poor adhesion of inks, glues and coatings.

A full explanation of corona treatment can be found in the Label Academy book on Conventional Printing Processes.

MATERIAL SELECTION

Every label application must be carefully considered to ensure that the substrates selected can meet the expected performance criteria.

• The following issues will impact on substrate selection;
• The ability of the substrate to withstand the printing process to be used
• The level of absorbency required i.e. the ability of a label substrate to retain liquids, moisture and inks
• The required rub resistance of the label surface
• The required material surface characteristics i.e. smoothness/roughness
• The chemically compatibility of the material to cater for inks, varnishes, solvents and the contents of the products the label may come into contact with
• Stability in different end-use environments and during application and handling

SUBSTRATES USED IN THE MANUFACTURE OF PRESSURE SENSITIVE LABELS

A wide range of paper and filmic materials are used in the manufacture of pressure-sensitive labels.

PAPER SELF-ADHESIVE SUBSTRATES

The main types of paper substrate used in the label market give excellent anti-fungal and wet strength features and fall into the following categories;

Coated Paper – Gloss and Semi-Gloss

• Matt coated
• Semi-gloss
• Gloss
• Cast coated

Uncoated Papers – matt finish

• Litho
• Vellum
• Textured paper – gives a ‘laid’ traditional appearance
• Metallics – highly reflective surface – supplied in a range of colors and tints and holographic effects

FILMIC AND PLASTIC SELF-ADHESIVE SUBSTRATES

Whilst paper face materials continue to be the most common face-stock used in the label industry there has been an increasing rise in the use of filmics (Figure 4.9).

These substrates are available in a wide variety of constructions, such as co-extruded, blown and engineered films, all with a wide range of functional properties.

Some of the advantages and disadvantages of filmic substrates are highlighted below.

Advantages

• Can offer transparent features including a ‘no-label’ look when used on clear containers
• Durable and can offer conformable, ‘squeezable’ properties.
• Resistance to chemicals, grease, water, moisture etc
• Plastic labels can be compatible with the packaging material being used and this can aid recyclability
• High gloss finish without over laminating or varnishing

Disadvantages

• Can be more difficult to print and convert
• They are often more expensive than paper substrates

Filmic substrates fall into four main groups which form the majority of the film face-stock used today. These are PE, PP, PET, and PVC.

POLYETHYLENE – PE

Low density polyethylene has the highest share of the filmic facestock market and is widely used in the primary self-adhesive labeling market, particularly in health and beauty and the household chemical markets.

There are two types of PE film which are available; Blown PE and Cast PE films.

The key characteristic of polyethylene labels are;

• Excellent tear resistance
• Minimal shrinkage
• Good printability
• Low stiffness
• Good squeeze-ability
• Good resistance to moisture and chemicals
• Environmentally friendly
• Low transparency
• Can be prone to stretching

POLYPROPYLENE – PP

Polypropylene films offer cost-effective and high-yield face-stock and are steadily challenging other films for primary product labeling and variable information printing.

An inherent advantage of polyolefin films over other film types is their lower density range, facilitating removal and recovery in recycling operations with non-polyolefin containers and in their obvious compatibility with polyolefin containers.

Other key characteristic of polypropylene films labels are as follows;

• PP films may be stretched in one direction (OPP) or two directions (BOPP), or co-extruded usually in 3 layers
• High resistance to tearing and excellent dispensability
• Good die-cutting and printability
• Film flatness and resistant to moisture, abrasion, chemicals
• Good dimensional stability (OPP/BOPP) and excellent conformability
• High clarity and low cost

POLYESTER (PET)

PET films are preferred where durability, higher temperatures and enhanced chemical resistance are required. The major applications are in consumer durables and in the automotive sector.

The key characteristic of PET labels are;

• High durability and clarity and therefore Ideal for clear-on-clear/no-label look applications
• Good resistance to heat, stretch, tearing, chemicals, moisture
• Good dimensional stability and dispensability
• High cost

MANUFACTURING BLOWN AND CAST FILMS

Filmic substrates are manufactured by extruding liquid polymers. This process involves feeding polymer materials from a hopper into an extruder unit in pellet, powder or granule form. The material is heated and the liquefied polymer is injected into a die and by using air directed through the die a bubble, is formed at the desired thickness of film.

The manufacture of cast film differs from blown film manufacturing as the liquid polymer is passed through a flat die which produces flat film.

After leaving the flat die, the continuous sheet of liquid film is cooled which freezes the film, the edges of the film are trimmed and the film is wound into reels.

Cast films can be co-extruded in multi-layers (usually three) with each layer optimized for different characteristics (Figure 4.10).

RELEASE LINERS

The release liner (also known as the backing paper or carrier) forms the base of the self-adhesive substrate sandwich. The surface of the liner material is coated with silicone which allows the label face substrate to be easily and accurately dispensed on the labeling line.

Release liners are produced using both paper and filmic materials.

Super-calendared papers offer a robust and consistent surface which is an important pre-requisite for accurate and consistent die-cutting. They also offer good transparency which allows easy label edge detection during application. There are three main types of paper liners;

• Glassine
• Kraft – super calendared
• Paper with thin laminated PE film

FILMIC LINERS

There are two main types of filmic liners;

• Polyester
• Polypropylene

Filmic liners offer a number of advantages over paper liners. They are light and thin and because of their very smooth surface give an excellent wet out for the adhesive layer. This property enhances the adhesive transparency necessary for missing label identification.

The high tensile strength of filmic liners makes them ideal for high speed label application by removing the possibility of web breaks on the filling and label applicator lines.