The ability of the adhesive to wet the surface and then bond to it. The substrate, the labelling storage and service conditions influence adhesion factors which are relevant for both permanent and removable adhesives.
Material properties
Surface tension: Types of surfaces;
- paper,
- glass,
- cardboard,
- metal,
- plastics (HDPE, LDPE, PETE, PS, PP, PVC)
Basic rules: the surface tension of the adhesive must be lower than or equal to the surface tension or object to be labelled. Wetting occurs if the adhesive has a stronger attraction to the surface than to itself.
Acrylic adhesives have a high surface tension; they adhere well to surfaces with a high surface tension like glass, metal and nylon.
Low surface tension – e.g. PP & PE
Very challenging is removable adhesive that exhibit excellent long term removability from a surface with a high surface tension. Overtime removability is reduced.
Roughness: or smoothness affects the contact area of the adhesive. If very rough…contact takes place only on the hills and the valleys will not come in contact with the adhesive. An adhesive for rough surfaces must properly wet the valleys. For removable adhesives, a rough surface is undemanding. It is rare that the adhesion would be too high. It is more likely that the adhesion is too low, which results in the labels coming loose.
Shape: the shape of a surface limits the choice of adhesive e.g. Test tubes where the shape is convex and the radius is tight. On a tight radius, the face material will try to return to its original flat shape. An adhesive with high internal strength (cohesion) is required to keep the label adhered and to avoid edge lift. The flexibility of the face material is also important, very rigid face papers or films are not suitable.
Influencial conditions:
Humidity & Moisture: A real challenge for the adhesive. In the food industry, products are often labelled in a chilled area where condensation forms on the surface of the labelling substrate. Also a challenge for outdoor labelling. Water based adhesives are able to absorb and tolerate a certain amount of moisture. When the moisture levels are too high alternatives are hot melts and solvent based, the 2 do not contain water absorbing substances.
Temperature: At low temperatures the adhesive becomes harder, decreasing the adhesion. At high temperatures the adhesive softens and the softening may increase adhesion because the adhesive flows and the contact area is greater. but if the adhesive becomes too soft, it begins to loose its adhesive properties and bleeds.
Labelling temperature: The best labelling temperature is close to room temperature, if labelling takes place below 32 F on a cold package ( -4F to 32F) choose a deep freeze adhesive. A general purpose adhesive can be applied from chilled areas to room temp (32F to 41F).
Service temperature: Labelling at room temperatures gives the adhesive time to develop a bond to the surface helping the adhesive to resist to higher or lower temp during storage. What are the environmental conditions affecting the labelled product. Where are they used or stored; interior, exterior, humid, condensation, dry, hot.
Resistance to Oxidation and UV light: High temperatures increase the rate of oxidation (aging). Acrylics have a longer shelf life. Rubber based are more sensitive.
Oxidation: the adhesive starts to yellow…looses its properties and hardens so that it is not tacky.
Labelling flexible packages: soft synthetic packages are often multilayer constructions, comprised of different synthetic films, laminated together.
- OPP – oriented polypropylene
- PP – polypropylene
- PET – polyester
- PA – polyamide (nylon)
PP and OPP have a low surface tension and are a greater challenge for the adhesive. PET has a high surface tension – easier for adhesive. Usually low temperature labelling conditions prevail. Packaging films rarely come with a surface treatment, antifogging and slip agents are often added to the film, these create a lower surface tension which is more difficult for adhesive.
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Labelling hard packages: hard synthetic packages include blow and injection moulded and thermoformed. E.g. Ketchup bottles. Typically, paper based face stock are used and also synthetic stocks.
Labelling cardboard: the most difficult is corrugated board. Roughness and recycled fibres, often waxes present a problem and result in lower adhesion.
Direct food labelling: largest market for fruits and veggies. The face material is thin and flexible in paper or synthetic.
Meat labelling: acrylic dispersion is recommended- stock can be applied directly to carcasses.
Home and personal care labelling: the no label look is the most popular. Needs good wetting requirements –resistant to light and UV. Many products come into direct contact with grease, fat and water. The most important properties of the adhesive is the ability to resist water.
Techno chemical labelling: Consider the weather ability and robustness of the adhesive and face stock. Needs very high overall adhesion. The packaging is usually plastic. Generally, opaque face materials are used & the adhesive is tackier or rubber base. Needs good resistance to oil and chemicals.
Pharmaceutical: Wide range of label applications. Small diameters to deal with e.g. syringes, test tubes. Materials to be labelled range from glass to PE & PS. Usually labelled at room temperatures but the service conditions may range from cold storage to a warm water bath.
Laser printing & copying: the adhesive must withstand the heat of the heat fusion process. Must not bleed out from the edge of the sheet before or during sheeting process. Must cut well during sheeting process. For CD labels- removable is not recommended because removal can destroy the disk.
Tire labelling: a real challenge because of the roughness of the surface and the chemical composition of the tire-need very aggressive immediate adhesion. Normally, labelled at room temperatures but storage varies from room temp to warmer or colder