Pressure-Sensitive Adhesives for Decals and Labels
This article talks about adhesives, looks at various polymer chemistries and curing options available, and provides information.
Decals and labels are everywhere—on products in your house, on your car, and around your neighborhood. Graphic designers create eye-catching designs so the labels can do their job to advertise a product or service, explain how to use it or where to get it, or provide safety warnings.
Design alone, however, does not produce a high quality product. The label needs to function over its intended lifetime, and this requires proper selection of adhesives to attach the product to the desired substrate or surface. Both the label itself and the substrate can be made from a wide variety of materials, including paper, metal, vinyl or various plastics. Some labels are designed for permanent application and need to last for years, while others need to be easily repositionable. The specific application dictates the optimum type of adhesive.
Adhesives for decal and label applications need to adhere readily to the label or decal and the substrate to which it is attached, and the label usually needs to be applied to the substrate without the use of heat or excessive mechanical force. For this reason, pressure-sensitive adhesives (PSAs) are the product of choice. PSAs provide a durable solution that is easy to apply, replacing other materials, such as thermally activated adhesives, liquid glues, or varnishes.
PSAs are often supplied in liquid form, which has several advantages over transfer tape. Since adhesive is only printed on the label where needed, this can eliminate the waste inherent in designs where excess tape material would need to be disposed of, potentially reducing cost. It can be designed not to cover the die-cutting line for labels that will be die cut, avoiding adhesive buildup on the cutting tool. Liquids can be printed in various thicknesses for different applications and can incorporate fine lines or intricate patterns, providing extensive design flexibility.
Liquid adhesives can be applied to label stock using many different methods including screen printing, flexographic printing, gravure printing, pad printing or Mayer rod coating. Adhesive suppliers often specify a preferred application process to achieve optimum results. For example, some adhesives are not suited to screen printing, whereas others are specifically formulated for that method. The elasticity and viscosity of the adhesive affect its ability to effectively transfer to the label material under a given set of coating conditions.
The combination of polymers, tackifiers, thickeners, and other additives in a given formulation determine the properties of the adhesive. PSAs are available in three different polymer families: rubber, silicone, and acrylic. Rubber PSAs have the advantage of excellent adhesion and are resistant to moisture and humidity. One drawback is that rubber degrades with ultraviolet (UV) exposure and is, therefore, not recommended for outdoor applications where it is not possible to protect the rubber from such exposure. Rubber adhesives are best suited to end products that will not be exposed to high temperatures or sunlight and require a strong bond.
Many hot melts are rubber-based. They are applied at elevated temperatures and cure as they cool, forming a high tack material for instantaneous, strong bonds. Some new products with especially aggressive tack are designed for freezer-grade applications.
Silicone can be a good option for labels exposed to extreme temperatures, as it remains flexible and maintains adhesion at temperatures ranging from -50 to +300°C. Silicone PSAs can be customized for various degrees of adhesion, depending on whether the application requires a temporary or permanent bond. They can wet to many different substrates, including many hard-to-bond plastics such as polyethylene, polypropylene, nylons, or even Teflon.
Acrylic PSAs are often made from copolymers that may include vinyl or styrene. They are the most commonly used all-purpose class of materials, as the raw materials can be easily polymerized in atmospheric pressure and the resulting polymers are well-suited to a variety of label applications. Acrylics tend to be priced lower than other options. They are stable over a fairly broad temperature range, from around -40 to +175°C, and are UV resistant. Some formulations have excellent moisture resistance.
Acrylics come in solvent-based or water-based formulations. The polymers in solvent-based products have relatively low molecular weight but are highly crosslinked, producing an easily applied adhesive with very high strength. Water-based formulations are an emulsion with discrete particles of longer polymer chains that coalesce as the water evaporates.
The goal with water-based PSAs is to achieve the performance of solvent-based products without using solvents, and manufacturers are making progress in this direction. One difficulty is in providing good adhesion to a wide variety of substrates. While water-based PSAs bond well to paper and polypropylene substrates, their performance on other plastic surfaces is not as good. Also, the cohesive strength of solvent-based acrylics is much higher.
PSAs are available from many different manufacturers (Table 1). Some boast a wide array of products and pride themselves on being able to supply the right adhesive for any application. Others focus on expertise in a particular technology. The resource list below includes suppliers contacted for this article and specifies what categories of adhesives each manufacturer produces. Within each category there may be many specific products available, and selection depends on the specific application.
Factors affecting adhesive selection
When selecting an adhesive, it is important to consider both the label material and the type of surface to which it will be bonded. Adhesion of the PSA to the label varies with label material. For example, labels printed on vinyl film stock may include plasticizers that reduce adhesion.
Aspects of the label production process, such as ink thickness, may also affect adhesive needs. To minimize costs it is usually best to keep adhesive thickness as low as possible, but rough substrate surfaces require thicker adhesives than do smooth surfaces for adequate performance. Curved surfaces require stronger adhesion to avoid peeling or buckling of the label.
Shear strength, the internal in-plane strength of the adhesive, is an important variable. If it is too low, labels may slide or peel off the surface. If adhesive shear strength is too high, however, the label may not be able to be removed cleanly if it needs to be replaced. Peel strength is related to shear strength and reflects the force needed to debond an adhesive from a surface. This needs to be relatively low for temporary applications and higher for permanent installations, but in any case it must be high enough to withstand forces experienced during the expected lifetime of the label. These forces may range from product handling to wind, depending on the application.
Environmental conditions in the field are a major factor in adhesive selection. What temperature range will the product expect to experience? Will it be installed in a dry or humid environment? Will it be exposed to water? How long does the label need to last? Will it need to be repositioned during use? All these factors affect choice of adhesive, and it may be necessary to balance trade-offs in properties to find the best fit for a given application.
Some applications are especially demanding. One example is advertising on buses (Figure 1), a lucrative, growing market. This application requires large graphics on a flexible substrate that adheres well to the contours of the bus and can be repositioned before final placement. The decal needs to withstand a harsh outdoor environment that may include freezing to very hot temperatures, rain, wind, sunlight and impact from road debris. At the same time, it needs to be removed from the vehicle without damaging the paint once the promotion has ended. These requirements will probably dictate using a solvent-based product.
Under-hood vehicle applications are another example that requires solvent-based PSAs. The combination of heat and chemical exposure would be likely to destroy many other adhesives. Silicone adhesives are a good choice for other high temperature applications, such as electronic components that need to undergo thermal processing after labeling.
Some temporary applications require PSAs that can retain their adhesive properties after being repositioned multiple times, while others, such size labeling on textiles, require that the label adhere until it reaches the end customer. In either case, ease of removal is more important than a strong bond. A specialty water-based acrylic with relatively large, spherical particles in suspension, as opposed to the emulsions common to most water-based PSAs, can work well for these applications. Silicones formulated for low adhesion are another option.
On the opposite side of the spectrum are adhesives that need to adhere securely and stay on. Some of these have tamper-resistant security features, (Figure 2) such as patterned residues that indicate evidence of tampering.
One emerging market in PSAs is UV-curable formulations that can be applied at room temperature. These compounds are liquid at room temperature and usually consist of acrylated monomers and oligomers along with a photoinitiator (PI) that activates when exposed to UV light to polymerize the material. The intensity, wavelength, and duration of UV exposure determines the shear and peel strength of the adhesive. This creates the opportunity to custom-design an adhesive for a given application but also means process control is critical to achieving the desired result. The printing method used to apply UV-curable PSAs can also affect peel. Flexographic printing is often recommended, since screen printing produces a rougher coating with lower peel strength.
One advantage of UV-curable PSAs is the ability to process in-line while printing the label, since the equipment is the same as that used for UV-curable inks. Since the products are 100 % solids, there is no need for the drying ovens that are required for water- or solvent-based acrylics. Also, the humidity controls necessary for proper curing rates of water-based products are not an issue. Cure speeds vary with thickness. A thin coating, 25 µm or less cures relatively quickly, but thicker coatings introduce delay times that impede the efficiency of the overall label manufacturing process.
UV-curable products can be environmentally friendly alternative to solvent-based adhesives in certain applications. Some customers prefer them in cases where the adhesive adheres well to the substrate, the substrate is sufficiently heat resistant to withstand UV-curing, and the product will be used in a benign environment that does not experience high heat or humidity. One caveat is that exposure to sunlight will activate the PI, causing further crosslinking of the polymer and hardening the material. Some formulations, however, can still retain adhesion after exposure to light, although their pressure-sensitive characteristics may be compromised. Substrates with internal UV-curing inhibitors can also extend the use of UV-curable PSAs to applications such as transparent labels exposed to outdoor environments.
Many in the adhesives industry are not convinced that room temperature UV-curable PSAs are the wave of the future, although they may be a solution for manufacturers who regularly use UV-curable inks and want to streamline their production processes. There is concern that limited temperature resistance and relatively low bond strengths make it difficult for today’s UV-curable adhesives to compete with solvent-based acrylics or UV-curable hot melts. Costs are also higher than for competing products, and shelf life is shorter. Some suppliers have removed UV-curable adhesives from the market because of lack of demand. It appears that improvements in both adhesive chemistry and curing equipment may be needed in order to achieve more widespread adoption.
Environmentally friendly products
One thing many customers want is sustainable materials. There are several approaches the adhesives industry is using to move in this direction. In applications where performance requirements do not dictate the use of solvent-based adhesives, other materials allow manufacturers to avoid the emission of volatile organic compounds (VOCs) present in solvents such as toluene and xylene. Water-based acrylics, UV-curable adhesives, and hot melts all fall into the category of solvent-free options.
Further development of water-based and UV-curable PSAs so they can better compete with the performance of solvent-based products is a step toward more environmentally friendly solutions. This includes improvements in properties such as tack and shear as well as developing wider processing windows. Research efforts include working to balance the desire for faster cure times with the need to avoid build up of adhesive on rollers during the coating process.
Another aspect of sustainability is recyclability of paper products. Paper labels (Figure 3) and stamps used by the U.S. Post Office (Figure 4) are coated with recycling-compatible adhesives (RCAs), water-based adhesives that dissolve in water during the recycling process. PSAs that are not recycling-compatible break into small particles that can damage paper-making equipment.
Recycling of polyethylene terephthalate (PET) plastic products such as beverage bottles and clam shell packaging is another important challenge, with demand for recycled PET outpacing supply. Labels or stickers on these products complicate the recycling process since adhesive residue is generally left behind. The Association of Postconsumer Plastic Recyclers (plasticsrecycling.org) states, “It is harmful to recycling for a label and adhesive to be the source of technical interference with the recycling process.” They have created a detailed test protocol for evaluating the recyclability of labels and adhesives. Several water-based acrylic adhesives are listed as meeting this protocol.
Still, either heat or alkaline cleaners are required to remove adhesive residue from PET. A possible solution is adhesives that adhere extremely well to the label material but not as strongly to the substrate, such that the label would remain attached to the bottle or package long enough to serve its purpose to the consumer but could be easily removed in order for the plastic to be recycled. The technology to produce such PSAs is still developing.
A different approach to environmentally friendly adhesives is to move away from polymers that use petrochemicals as raw materials and replace them with plant-based materials. A variety of patents exist in this field. A 2012 patent from the Kansas State University Research Foundation, for example, proposes repositionable PSAs made from plant oil triglycerides. Plant-based materials have also made their way into actual products, such as rosin esters in hot melt PSAs. There is, however, a cost trade-off that makes it difficult for renewable products to compete in the marketplace.
When suppliers are asked what customers are demanding, cost reduction is a common theme. One example is the trend toward thinner label films and liners to reduce cost. These changes in substrates introduce new challenges for manufacturers. The adhesive may not coat the same on a thinner substrate, so it is important for the adhesive supplier to work with the customer to determine an optimum solution that will meet both cost and performance requirements. There is also a drive toward reducing coating thicknesses in cases where adhesion performance will not suffer as a result.
Another cost saving approach is improving throughput, which may favor adhesives with 100% solids that do not require oven drying to cure. Many of these, however, are hot melts. The processing temperature required to apply these adhesives may limit their use on some of the newer label films that have low melting or softening points.
There is also shift from paper to film substrates, giving designers more creative options. Clear labels (Figure 5) that enhance graphics with a no-label look are becoming more popular. These require colorless adhesives that will not yellow over the life of the label, and many manufacturers produce PSAs that meet this requirement.
Even if an adhesive is designed to adhere to a new type of substrate, there can be other concerns for label manufacturers. One example is labels for powder-coated, painted surfaces. The adhesive may bond well, but there can be issues with oozing of adhesive. The adhesive supplier, label manufacturer, and end customer may need to work together to achieve an optimum solution.
What does the future hold for the label printing industry? There is a push toward digital printing of adhesives. Adhesives are not yet available for printing through an inkjet nozzle, but there is a race in the industry to add that capability. The challenge is depositing enough adhesive for a strong bond while avoiding the problem of excess adhesive drying inside the printer.
Labels are being applied to more and more products. One new development is clear labels applied to drinking glasses as an alternative to painted decorations. Here the challenge is developing an adhesive that can withstand the heat and chemical exposure of automatic dishwashing. Some companies report demand for adhesives that can withstand extremely high temperatures that are beyond even the capabilities of silicone materials. Long-term UV resistance is also on the wish list.
The ideal adhesive of the future will be environmentally friendly, low cost, and very strong. It will be able to be applied to any label material attached to any surface under demanding environmental conditions. In reality, of course, many different adhesives will be needed to meet an ever-expanding list of requirements. The industry will continue to innovate and develop a range of products designed to meet customer needs in the years ahead.
Julia Goldstein, Ph.D., is a freelance writer with a background in materials science. She provides commercial writing for companies in the semiconductor and printed-electronics industries.
The author wishes to thank Ingrid Brase (Henkel), Lisa Castillo (KIWO), Shannon Cook (Craig Adhesives), Kyle Rhodes (Dow Corning), Jeffrey Stadelman (MACtac Graphic Products), Randy White (J.N. White Designs) and Mike Witte (Franklin Adhesives) for providing information for this article.