Not all adhesives are designed to be permanent. Some applications require the label to be removed from the surface at some point after application. Other applications may require semi-permanent adhesion - something between permanent and removable – or labels might need to be repositionable or recloseable. Each of these makes different demands on the adhesive technology.
Oil and chemical resistance
Once we know the application and what the adhesive or the label should be capable of doing, we can specify the adhesive chemistry and formulation required.
ADHESIVE TYPES – AN OVERVIEW
Pressure-sensitive adhesives (PSAs) differ from other types of adhesives in that they are able to form a bond at any time, are permanently tacky, are capable of bonding to almost any surface and are adhesive above their glass transition (Tg) temperature. No activation by water, solvent or heat is required to exert a strong adhesive bond on materials as diverse as paper, plastic, glass, wood, cement and metals.
Four well-established pressure-sensitive adhesive technologies are currently used, with a fifth beginning to find commercial applications:
Acrylic solutions. Solvent-based acrylic PSA solution formulations have been widely displaced by water-based and hot-melt systems for economic as well as ecological reasons. Solvent recovery and/or incineration are essential to meet clean air legislation requirements.
Such equipment is expensive and can only be justified for large output operations. Acrylic solution adhesives are therefore not widely used for labels today, except for speciality applications such as durable labels that require chemical resistance and/or compatibility with materials that cause adhesive failure due to plasticizer migration.
Rubber/resin solutions. These are also less frequently used, because they are coated in solvent, except for high performance ‘peelable’ labels and specialities such as oil can labels.
Hotmelts. Hot-melt PSAs are a fast-growing sector, because their conversion performance is now very good and they are competitive. They can be used where an aggressive permanent adhesive is needed with high tack and some recipes have excellent performance in cold and wet conditions, particularly when labeling plastic surfaces. Hot-melts are easy to coat on compact equipment and are also the preferred choice for most in-house converters and printers.
Acrylic dispersions. Water-based acrylic PSA dispersions (emulsions) now represent the dominant technology for the labelstock producer and can also offer a practical option for in-house coaters. The large labelstock suppliers formulate their own adhesives, but ready-to-use formulations are commercially available for small and medium-sized coaters.
Radiation cured. Adhesives can be cross-linked (cured) by electron beam or ultra-violet radiation. This enhances some characteristics such as high temperature performance of hot-melt adhesives. UV Curable adhesives are now used in some speciality tape applications and are also beginning to be used in limited applications by label or forms converters.
Pressure-sensitive label adhesives successfully meet an enormous range of demands. They can provide a permanent bond or can be removable. Some substrates are easy to stick to, for example paper or board, but PSAs can also be formulated to stick to ‘difficult’ surfaces such as plastic containers or moist glass bottles.
Label service temperatures normally range from -20°C (-4°F), or lower for freezer packs up to +200°C (392°F) or more for car engine parts.
The cohesive strength of PSA adhesives can be reduced to allow labels to disintegrate to discourage tampering. The key properties of the main pressure- sensitive adhesives are shown in Figure 4.1.
Figure 4.1 key properties of the main pressure-sensitive adhesives
Before looking in more detail at these different adhesive technologies, we need to ask a more fundamental question. What actually is a pressure-sensitive adhesive and what makes it different from other adhesives?
A pressure-sensitive adhesive is a soft, permanently tacky material which is capable of making an instantaneous bond to almost any surface within a certain temperature range without any additional force being applied.
A pressure-sensitive adhesive has an infinite open time, meaning it is always tacky and always sticky. This is the opposite of adhesives that dry and once dry cannot then be stuck any more. A pressure-sensitive adhesive can be peeled off and still feels sticky.
To achieve this effect, a PSA combines the properties of a solid with the ability to ‘flow’, which makes it possible for the material to form a bond. (Figure 4.2).
Figure 4.2 PSA is a polymer with the characteristics of both a solid and a liquid
The flow is not the same as a liquid, since liquids are not able to form an adhesive bond. Liquid honey, for example, exhibits ‘stickiness’, but will not form a bond, as it does not have the properties of a solid.
Combining the properties of a liquid and a solid in the same material creates a ‘viscoelastic’ property which is the key feature of a pressure-sensitive adhesive.
The study of a material’s response to different modes of flow and deformation is the ‘rheology’ of a material.
Pressure-sensitive materials have an instantaneous tackiness which ensures a material will form some kind of bond within a short time. Raw materials can be selected to be more or less tacky, but for correct bonding the material must also be fluid enough to cover the material surface onto which you want to bond.
The other important property is the shear of an adhesive, otherwise known as the cohesion. This is the resistance required to tear these bonds apart and is a property of a solid.
We know that the ‘solid’ property of a PSA ensures good cohesion, so it is easy to think that to maximize this property requires designing a product with high molecular weights.
That will, for sure, deliver higher shear values, but the downside is that the viscosity of the material becomes so high that processing equipment will no longer properly work. So there is a trade-off involved between cohesion and viscosity.
Certain chemistries offer inherently higher molecular weights. With self-curing solvent acrylics, for example, the curing mechanism used to take out the solvents can itself boost molecular weight.
Similarly, with UV acrylics the chemical cross-linking process delivers a very high molecular weight.
A material’s stiffness properties are known as its ‘modulus’, a property which changes with temperature. At low temperature, materials have a high modulus, which means the material is stiff, and at very low temperatures the material is brittle, with no flexibility.
When the materials are heated, the modulus will start to drop. Starting from a certain temperature, the material has enough energy to show some mobility at a small scale. When polymer chain segments between entanglements have some mobility, the material has reached its glass transition temperature (Tg) range.
By optimizing PSA design to have the Tg at a low temperature, it is possible to have PSA characteristics needed for a deep freeze label, for example.
Pressure-sensitive materials have been found empirically to have a modulus between set values. This is known as the Dahlquist (1966) theorem.
When the modulus is within that range, or window, the material has pressure-sensitive properties. By selecting certain raw materials processed in the right way, the designer ensures that this window corresponds to the temperature at which the adhesive should work. This allows labels to adhere at both extremes of the temperature range.
To take the example of rubber hotmelts, these are made pressure-sensitive by using a blend of elastomers, tackifiers and other additives that result in a mix that has the correct modulus. The individual components – for example the elastomers – have too high a modulus to be pressure-sensitive in their own right. It is the combination of all the materials that makes the final product pressure-sensitive.
An acrylic adhesive is different in that the polymer itself is designed and built to be pressure-sensitive. The raw materials selected –the molecular weight and type of acrylic monomers – ensure that the end product is a pressure-sensitive polymer.
The same applies to water-based acrylics, with the difference that they are made in a process called heterogeneous polymerization. This means that you end up with particles dispersed in water, rather than an organic solution of long polymeric chains that look like entangled spaghetti, which is what a solution acrylic looks like.
UV curable hotmelt is similar in design to a solvent acrylic with the key difference in molecular weight before curing and the curing mechanism itself.
When we examine the modulus of rubber hotmelts, above a certain temperature they fall apart (they ‘melt’), a property which allows them to be processed into an adhesive film. But when they become a liquid, they are no longer pressure-sensitive – they are simply a mix of ingredients that has become liquid.
This process is reversible, so when the hotmelt materials cool down these mixes become pressure-sensitive again.
By contrast solvent and UV acrylics have a curing mechanism such that when they are heated, the modulus does not drop at a certain point. This means the adhesive film does not become a melt, but keeps its pressure-sensitive properties at higher temperatures. This makes this class of adhesives the first choice for really demanding applications where the label must withstand high temperatures in combination with, for example, a high mechanical load.
Designers configuring acrylic polymer chains have a wide choice of functional monomers which form ‘asset groups’ with metals like aluminum to ensure that when the adhesive is cured, a strong chemical bond forms between the polymer chains.
Although UV acrylics work with photo-chemical rather than asset group mechanisms, the end result is the same: a chemical bond between the polymer chains, which means that above a certain temperature the adhesive does not fall apart.
These chemical bonds do not exist in a rubber hotmelt mix – and that is the key difference in high temperature performance.
Of all the available adhesive chemistries, rubber and UV acrylic hotmelt is the easiest to process. It starts as a solid product which is put in a melter, and processing can begin straight away. There are, of course, safety risks associated with hot and sticky surfaces, but equally there are no flammable solvents to handle and no drying process. This makes it a compact setup and explains why rubber hotmelts are very popular from a converting point of view.
Solution acrylics or emulsion acrylics start out as ‘wet’ adhesives that need to be transformed into an adhesive film, and the water or solvents removed. These operations take up a lot of space and require higher levels of knowhow and expertise to ensure the adhesive is properly cured.
Testing protocols exist to ensure these standards are met, but it is a very different setup to hotmelt production.
The adhesive as supplied comes with certain characteristics guaranteed by the manufacturer.
Optical appearance can be transparent, yellow or hazy depending on the end use properties required.
Depending on the technology of the PSA, the manufacturer has to measure and control specific characteristics like a specified solid (non-volatile matter) content, Brookfield viscosity, kinematic viscosity, pH, particle size distribution, etc.
These factors must all be tightly controlled from the manufacturing side to ensure quality of the adhesives batch to batch (Figure 4.3).
Figure 4.3 Checking adhesive polymer properties including appearance, solids (non-volatile matter), Brookfield viscosity
Each PSA technology comes with its own processing challenges (Figure 4.4). Rubber hotmelt has strict viscosity limits and requires protection from oxidation.
For emulsion acrylic key challenges are wetting out of the adhesive and foaming.
For solvent acrylics the challenge lies in handling flammable solvents and ensuring correct cure. For UV acrylics correct usage and maintenance of the UV source to deliver reliable and repeatable cross-linking is essential.
Figure 4.4 Adhesive manufacturing plant
TESTING AND TROUBLESHOOTING
FINAT test methods provide the industry-standard way of measuring factors such as peel, tack and shear (Figures 4.5 and 4.6).
Figure 4.5 Testing for adhesive failure
Figure 4.6 FINAT Peel test
The FINAT test methods offer a reliable standard for measuring key properties of PSAs. They enable suppliers and users of PSA to compare values measured according to these standards. As well as the numerical test values,the observed failure mode of, for example, a shear test, can help understand how an adhesive performs successfully, or fails.
In the case of a removable label, for example, where you do not want the adhesive to stay on the surface permanently, both a shear test and a peel test can distinguish a proper removable PSA from an adhesive that has low adhesion but is not a cleanly removable PSA.
While peel, shear and tack (see information box) are the basic methods for testing adhesives, a range of more general label-specific test methods are also used, for example accelerated aging tests, migration and penetration tests.
Migration can be a particular issue when working with certain paper types and rubber hotmelts, which contain oils and plasticizers that will migrate.
Papers with a closed structure will generally be more resistant to migration. The final application will help decide how much of a problem this is likely to be.
There are also chemical and photochemical ways to test the bond of a label or tape, which all help ensure a better understanding of the integrity of the bond throughout a specified lifetime.
One specific method for labels worth mentioning is mandrel performance, which tests specifically for smaller diameter vials and bottles, ensuring the adhesive will stay stuck and will not start ‘flapping’.
A typical laboratory test is to bond labels onto a small glass or plastic test tube and observe what adhesives are best suited for that application, remembering it is the specific balance of adhesion and cohesion that makes the label work.
Figure 4.7 FINAT Tack test
The role of the PSA in contaminating recycling streams is now widely recognized. If, for example, an adhesive remains on the PET bottle surface after the label is removed, the PET flake recycling stream can be contaminated.
Adhesive manufacturers have responded with adhesives that ‘switch off’ in the presence of the alkaline solution found in recycling wash systems.
The adhesive stays with the label film leaving the polyester clean for recycling.
Adhesive manufacturers are also working on systems which allow cleaner recovery of pulp in the paper recycling process. These include screenable adhesives, which are designed with bigger molecular components.
When paper labels are pulped and soaked in water during the recycling process the adhesive separate from the paper and the larger adhesive particles can then be screened off, allowing smaller paper pulp particles to be separated.
On the sustainability front, compostable PSA adhesives are now available, along with alkali-soluble PSA adhesives and bio-based.
Figure 4.8 FINAT Shear test
Figure 4.9 Specific label tests