It was in 1935 when Stanton Avery of Los Angeles first manufactured the self-adhesive label, that labeling began to evolve from traditional wet-glue methods.
He made the first labels that could be applied without moistening or glue. His first product was the Kum Kleen self-adhesive label offered in only one color: white (Figure 4.1). When customers asked to have their names on the labels, he added print, using a hand-cranked printing press. Self-adhesive labeling usage has expanded rapidly since those early days.
In the beginning, the face of the label consisted of plain, white paper. Today the label face substrate may be made of fabric, plastic, or metallic films and it might carry brightly colored designs and pictures.
The self-adhesive system offers a labeling flexibility that wet-glue cannot provide. The lower cost of change parts needed for the applicator is much lower that wet-glue systems, the set-up time for self-adhesive application is much simpler and operates much cleaner.
The modern combination ‘platform press’ used in the self-adhesive industry will produce high quality graphics and embellishments in one pass on the press, giving the facility to produce the printed, embellished and converted labels in a much reduced manufacturing window, compared to wet-glue labels.
The use of plastics for packaging, which has shown a faster growth rate than any other packaging material, has particularly helped boost pressure-sensitive labeling. This especially applies to the shorter-run and luxury end of the market. Typical sectors include toiletries and cosmetics, household products, motor oils and pharmaceutical goods.
Glass containers for foods, drinks and dairy products have historically been labeled with wet-glue labels, or are printed direct. Nevertheless, certain designer products, such as drinks and beers now appear with pressure-sensitive labels. The usage of ‘a no-label look’ filmic label is of particular note here (Figure 4.2).
METHOD OF MANUFACTURE
Pressure-sensitive labels, also known as self-adhesive labels, have been considerably refined over the years.
It is essentially a sandwich of substrates (Figure 4.3). The first part of the sandwich is the face material, which becomes the finished printed label. The reverse of the face material is coated with a pressure sensitive adhesive and the face paper is then laminated onto the liner substrate which is coated with a silicone layer.
A self-adhesive label must have both the ability to ‘hold’ and the ability to ‘release’. It is a contradictory requirement that it must stick to different types of surfaces under varying conditions, yet release easily from its backing paper.
This release coating permits easy removal of both the waste matrix and the label from the release liner, which becomes the carrier for the converted labels. Printing takes place on the face stock and is then die-cut to a shape and supplied in reels.
The face stock can be made from a wide range of substrates, with paper stock or filmic stock being the most widely used. These can be surface coated or uncoated depending on the properties required for the label surface.
Pressure-sensitive adhesives offer an extensive range of adhesive properties. The adhesive/labels are used in a wide range of conditions which demand very high performance levels. These conditions will include high and low temperatures, wet and dry environments and different product surfaces.
MANUFACTURING PRESSURE-SENSITIVE (PS) SUBSTRATES
The manufacture of PS substrates is primarily a coating and laminating process.
The adhesive coating and the laminating of the substrates is done on a coating machine as an in-line operation. A release coating is applied to the liner stock and then dried in-line. The face stock may also require a primer coating and this is also dried in-line. The adhesive coating can be applied directly to the reverse of the face-stock, or by transfer from the liner.
After the coating and drying operations has been completed the two separate substrates (the face-stock and liner) are laminated together to form the pressure sensitive substrate.
The method of PS manufacture is illustrated in Figure 4.4.
Figure 4.4 Typical method of pressure-sensitive manufacture (Picture Source: Label Academy)
There are two methods of applying the adhesive coating to the reverse of the face-stock. The first and most widely used is the direct coating method in which the adhesive is coated directly onto the reverse of the face-stock. The second is transfer coating when the adhesive is coated onto a release liner and then transferred to the reverse of the face-stock after the lamination process.
There are a number of different coating methods used for applying the adhesives.
The thickness of the adhesive is an important factor and must be established before the adhesive is applied.
The coating weight is important for two reasons:-
- It has to be even and consistent to provide the correct adhesion between the liner and the face stock and the label and the product container being labeled
- To apply the optimum/minimum adhesive coat to keep usage to a minimum
Typical coating weights would be in the range of 5-50 gsm for rubber based and aqueous based adhesives
The thickness of the adhesive coating is determined by the following factors:
- The type of face stock to be coated (absorbent – non-absorbent)
- The adhesive system to be used (solvent, emulsion, hot melt)
- The required thickness of the adhesive 0.8 – 5.0+ mils
These factors will determine if the adhesive should be a direct application to the face-stock or transfer coated to the liner.
When this information has been established it is easier to identify the most suitable method of applying the adhesive coating.
APPLYING THE ADHESIVE
There are several different methods of applying the adhesive. It is the type of adhesive that governs the method which is used for the adhesive application.
A typical coating system for aqueous and rubber adhesive coatings would be a Mayer Bar system or a metered system such as a gravure process application.
MAYER BAR COATING METHOD
The Mayer Bar method of application is commonly used to apply low-viscosity pressure-sensitive adhesives (PSAs).
This coating system uses a wired rod to apply the adhesive and is best suited to adhesive types with good flow characteristics.
REVERSE – ROLL COATING
Reverse roll coating (also known as roll-to-roll coating) differs from other coating methods by having two reverse-running nips.
The metering roll and the applicator roll contrarotate and have a fixed gap between the two rollers. The surface of the applicator roll carries an excess of adhesive coating and the thickness of the adhesive coat is established by the nip distance between the two rollers (Figure 4.5).
The meter roller (contra-rotating) wipes the surplus adhesive to leave a precise amount on the applicator roller. The applicator roll running in the opposite direction to the substrate, wipes the adhesive coating onto the substrate.
The gravure method of adhesive coating gives an accurate coat weight without any restrictions on the running speeds of the applicator. A low-viscosity adhesive requiring a low coating weight can therefore be applied at speed.
The coating weight (the amount of adhesive being applied to the substrate) is controlled by the screen cells engraved on the gravure cylinder. The depth and size of the cells can be engraved to a specified size. Smaller shallower cells deliver a lighter coating than a deeper larger cell formation, which delivers a heavier coating weight of adhesive.
Gravure applied coating give a very consistent and evenly coated result, suitable for coating clear filmic substrates used in the pressure sensitive industry.
Figure 4.6 shows the principle of the gravure process with the engraved gravure cylinder running in the tray which holds the viscose adhesive. The doctor blade wipes the cylinder leaving the adhesive in the cells. The adhesive is then transferred onto the substrate which then enters the drying section.
For more detailed info on the gravure process see the Label Academy book on Conventional Printing Processes.
In the knife coating process the coating is applied to the substrate directly via a holding reservoir or an applicator roller (Figure 4.7). The knife which controls the thickness of the adhesive, can be a steel blade or alternatively an air knife. The excess adhesive is removed by the knife which is set to a predetermined height or air pressure. This controls the thickness of the adhesive coating by wiping the surface of the adhesive, giving an evenly applied coating at the required weight.
The hot-melt coating process is a system of applying an adhesive with a 100 percent solids content of wax and polymer resins. The adhesive is heated to a fluid state and then applied to the substrate by an applicator roll, the gravure process or extrusion process. After the adhesive has been applied it is cooled using a chilled roller or chilled air system.
Temperature control is a key factor when using hot-melt adhesives. The temperature controls the adhesive viscosity and the thickness of the adhesive film. This in turn will affect the tack value and the speed of setting i.e. the thicker the adhesive coating the longer it will take to set.
Hot-melt adhesives are 100 percent solids which are supplied in dry form and which melt at temperatures between 266 and 320 degrees F, (130 and 160 degrees C). After the coating process the adhesive coating dries at room temperature, leaving a tacky pressure-sensitive surface.
After the hot-melt coating has been applied, the backing liner is applied to the adhesive coated substrate, thereby laminating the substrate and backing liner together to form the self-adhesive material. The coated material is then passed over a chilled roller to complete the process.
Hot-melt adhesives are used for many adhesive applications, where their immediate speed of setting/ bonding is ideal for fast production. It is often used for more difficult labeling applications where emulsion adhesives can be problematic.
With the hot-melt process higher adhesive coat weights improve adhesion to rough surfaces, but hot-melt self-adhesive formulations are generally unsuitable for labeling plasticized surfaces. The plasticizer will migrate into the adhesive and degrade it, so causing adhesive stringing and strike through of label papers, which may result in bond failure.
Hot-melt adhesives are popular because they are invisible on glass, PET and other plastic containers. They are also suitable for metal applications. Their high tack characteristics make them suitable for difficult applications such as peelable, chill, permanent and deep-freeze label applications.