PS Adhesives

Without an adhesive, a label is just a piece of paper or film. Of a pressure sensitive label’s three core components – substrate, adhesive and liner – it’s fair to say that the adhesive is the part that puts “pressure sensitive” in a pressure sensitive label. Perhaps it doesn’t get the recognition that papers, films and inks get in a label’s success, as adhesives are not visual, but their importance should not be understated. Adhesives are all about performance.

The sticky business is big business. According to Transparency Market Research (TMR), the global market for adhesives was valued at $22.7 billion in 2012 and is expected to reach $31.64 billion by 2018, growing at a CAGR of 5.7% from 2012 to 2018. Along with the construction and automotive industries, packaging is a growth end use market and is expected to be a key driver in the adhesive industry’s growth.  Geographically, the Asia Pacific region is dominating the global adhesives market, accounting for over 40% of the total market in 2012. Asia Pacific is also expected to be the fastest growing market, at an estimated CAGR of 5.2% from 2012 to 2018. Asia Pacific was followed by Europe and North America, in terms of adhesive consumption in 2012. According to TMR, North America is a saturated market and is expected to show moderate growth over the forecast period.

Chemistry
The adhesive manufacturing process is steeped in science, and manufacturer’s knowledge and chemistry expertise play a crucial role in developing specific adhesives tailored to end use requirements.

Polymerization is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. That’s an awfully scientific definition, and it’s one that’s important to PSA manufacturing.

Todd Hunt, director of sales and market development – Polymer, for Franklin Adhesives & Polymers, a division of Franklin International, says that several types of polymerization techniques or processes can be used in the manufacture of pressure sensitive adhesives. “The two types we use are free radical emulsion polymerization and solution polymerization. The advantage to emulsion polymerization is that we can use water as the reaction vehicle, which can help absorb the heat of the exothermic reaction (a chemical reaction that releases energy in the form of heat). In addition, we can generate higher molecular weights very quickly and create small, uniform particles, which enables us to design and customize the product specifically to its end use,” Hunt says.

Franklin Adhesives & Polymers employs solution polymerization, using water as the solvent. “We cannot produce higher molecular weight material through solution polymerization as readily as we can through emulsion polymerization; therefore, solution polymers are characteristically very low in shear resistance but typically higher in peel and tack performance. We focus on water-based polymerizations since they have the inherent benefits of being non-hazardous, non-flammable, non-toxic, have lower VOCs, and are lower in cost than solvent-based polymerizations,” explains Hunt.

Regardless of the process or system used to manufacture PSAs, efficacy and consistency is paramount. John Buday, business account manager, Tape and Label, for H.B. Fuller Company, says, “The processes used to manufacture pressure sensitive adhesives are heavily dependent on the type of adhesive being manufactured. Since many PSAs are designed for specific applications, it is critical to have an effective and reproducible process. This is especially true because, compared to other adhesives, the tolerances of PSAs are quite narrow. And in addition to needing adequate mixing so products are homogenous, you must have a balanced manufacturing scale,” Buday says, “Ultimately, you want to be in position to make cost-effective batches while at the same time maintaining flexibility for shorter specialty runs.”

Rubber and acrylic
There are distinct “systems” of pressure sensitive adhesive applications. Rubber-based and acrylic-based adhesive systems continue to dominate the pressure sensitive labeling industry. According to Al Kuhl, technical service and support representative for MACtac, silicone-based adhesives have made some inroads into the label market, especially for applications requiring high temperature resistance, but remain too costly for the typical general-purpose label application.

“The decision for choosing a rubber-based adhesive versus an acrylic-based adhesive must be based on the requirements of the application because each adhesive system offers distinct properties and benefits,” Kuhl says. “Rubber-based adhesives offer excellent initial tack, often referred to as ‘thumb appeal,’ very good adhesion to low-energy substrates, like plastics, excellent water-resistance, and can be suitable for direct food contact applications. Acrylic-based adhesive systems offer a broad service temperature range, a long shelf life suitable for archival applications, UV energy resistance, good flexibility for wrapping small diameter cylinders and good chemical resistance.”

Kuhl points out that the differences between rubber-based and acrylic-based adhesive systems have narrowed in recent years. He explains: “The clarity and converting benefits of acrylic adhesives have been matched with the new and improved rubber-based adhesive technology. In similar fashion, the water resistance benefits of rubber-based adhesives are nearly matched by improvements in acrylic adhesive formulations.”

Todd Hunt notes a third type of adhesive prevalent in the label market – modified acrylic. “These adhesives are formulated acrylic polymers that address areas where all-acrylic polymers might fall short. Like rubber-based products, modified acrylics offer improved quick tack and adhesion to LSE (low surface energy) materials. However, in some cases, this modification can decrease the resistance to some solvents, plasticizers and UV exposure. Depending upon the end use, modified acrylics can be dialed in to meet the needs of the application. For example, some can be tackified with rosin ester or hydrocarbon resins to improve tack, yet not lose shear adhesion. Some can be cross-linked through functional monomers to build shear adhesion without sacrificing tack.”

Adhesive selection
Labels serve a wide range of functions. Some, like certain durable labels, are meant to remain adhered to a structure indefinitely, while others are designed to be moved around from place to place, with the end user being able to peel the label off easily from one thing and affix it to something else – without compromising adhesion and other properties. So it stands to reason that choosing an adhesive has everything to do with the label’s purpose.

Buday, of HB Fuller, says that in order to select the best adhesive for a PS label, you should start by considering the lifecycle of the label. He says, “You’re considering the simple stuff, such as what packaging type works best for the label or which one creates the greatest value for the dollar. Then, once the label is made, what is going to happen to it? Where will it go on the product and for how long? Is it permanent, removable or repositionable? These are all important questions and factors before we even get to what the label is being applied to,” Buday says.

As the entire lifecycle of the label, from converting to environmental resistance and durability must be considered during adhesive selection, several factors can affect the success of every label application. According to MACtac’s Al Kuhl, most of these factors can be grouped into three major categories – surface conditions, environmental conditions and application conditions.

Surface conditions, Kuhl explains, refers to the substrate for the label application. “These factors include the substrate’s surface texture (rough or smooth), the surface contour (flat or curved), the surface energy of that particular substrate (high, medium or low), and the presence of any surface contaminates, including coatings, mold release, anti-static agents or slip agents like silicone and water,” he adds. “Water should also be considered as a contaminant that can affect good bonding of a pressure sensitive label.” 

Environmental conditions refer to the conditions to which the label and the substrate will be exposed, including exposure to UV light, humidity, the presence of solvents and chemicals in the environment, and temperature extremes that the label will experience during its lifecycle. Thirdly, Application conditions refer to the factors on how the label is applied. “Questions that will affect the success on the adhesion of the label include: Is the label applied by machine or by hand? Is sufficient bonding pressure used during label application? Is pressure applied evenly over the entire surface of the label? How long is the pressure applied to the label? What is the temperature and humidity during the application? It is important to remember that most PS adhesives take longer to bond in cold temperature conditions,” says Kuhl.

Franklin Adhesive & Polymer’s Todd Hunt concludes that while several factors need to be considered when choosing the adhesive for a PS label, the choice is first and foremost dependent upon the performance needed for the surface and conditions in which it will be applied and used. “The performance of an adhesive offering is well documented by nearly all suppliers. Characteristics such as peel, tack, and shear (cohesive strength) are the main drivers for PSAs.”

Hunt also emphasizes that the contour of the substrate to which the label will be affixed is a primary consideration. “Corners, tight angles and tight radii of curvature all place additional stress on the adhesive label. Some adhesives are designed to withstand these forces and prevent problems with label ‘wing-up’ or curling when removed from the release liner and applied to the surface.”

Surface energy is also of the utmost importance, Hunt adds. “Materials with low surface energy are difficult for adhesives to wet out for good anchorage. Some substrates require surface treatment, such as corona treating, primers or top-coating to obtain improved adhesion.”

Adhesives doing more
The term “adding value” is a prevalent within the label industry, and it applies to PS adhesives. Security and anti-counterfeiting, convenience and sustainability are all areas area where adhesives can add value.

”Valuable brands need to have labels that stay in place and can indicate tampering in the event that someone tries to counterfeit the products,” says Al Kuhl. “And market growth in smaller size products, prepackaged foods and convenience items lead to new and sometimes demanding labeling requirements, such as different bottle and packaging types, flame treated or not, etc.”

Sustainability is increasing in prevalence in the global adhesives industry, says Franklin Adhesives & Polymers’ Hunt. “We see more companies seeking benign water-based PSAs rather than solvent-based PSAs. Although solvent-based products are generally higher performing, novel formulation additives and innovative polymerization processes are yielding higher performance, water-based PSAs that approach solvent-based performance. 

Buday, of HB Fuller, also sees adhesives being asked to do more than simply “stick.” He says, “We’re seeing more interest in minimizing post-consumer waste. For containers that will be recycled, it’s important that we offer adhesives that comply with the recycling process. On the whole, I’m seeing a client base that is once again opening up to new ideas and fresh ways of looking at things. We’re trying to solve problems collaboratively and the dialogue is about as good as I’ve ever seen it.”

New Products
MACtac’s PUREtac CL216 is its latest adhesive technology. Touted as being the only optically clear adhesive specifically designed to go on and stay on, it’s ideal for packaging and labeling applications for health & beauty, food, beverage and household cleaner products. “MACtac PUREtac CL216 exhibits superior clarity, high tack and peel, a quick, strong bond, and excellent adhesion to low-energy surfaces. It is unique to the market, giving you an edge to grow and protect the customer base you already have,” Kuhl says.

H.B. Fuller features a robust line of pressure sensitive hot melt adhesives for the label market. “We have everything from freezer grades to ultra-removable grades,” Buday says. “We’re continually innovating and pushing the envelope of what adhesives can do. We’re looking at everything from PSAs that adhere at -60 degrees Fahrenheit service temperature to ones that can withstand shears with 10 pound weight. And that’s just the baseline of what we are doing. These products are not commercially available yet, but we’re getting close and I can see ‘families’ of products to follow.”

Franklin Adhesives & Polymers offers a broad family of water-based emulsion PSA products under the brands Covinax and Micronax. Micronax is an ‘ultra-removable’ based on proprietary microsphere technology. “Microspheres allow the label to be removed and repositioned without damage to either the label or the substrate. End users can apply and remove labels coated with Micronax without damaging even delicate archival grade paper or newspaper. These products are very low in peel performance by design and can be removed and repositioned over and over again,” explains Hunt.