By Melissa Donovan
With more manufacturers considering bringing package decoration in house, digital printing is getting a lot of attention due to its flexibility advantages in terms of versioning and short-run capabilities. For flexible packaging in particular, digital print is of great use. However, flexible packaging has added levels of complication that other types of packaging does not. To build a flexible package, adhesives are used to bond layers of material together. These adhesives must be compatible with digital printing technology.
A Look at the Options
There are three main adhesives used in flexible packaging—water-based, solvent-based, and solventless.
Water-based is a “colloidal system of an acrylic, urethane, or acrylic-urethane hybrid polymer suspended in water, usually run with an isocyanate co-reactant. Solvent-based is a urethane prepolymer in an organic solvent carrier, often run with a co-reactant. Solventless is a 100 percent reactive urethane, in two parts—except for paper applications—that are mixed at point of use,” explains Justine Hanlon, market manager, flexible materials, H.B. Fuller.
Out of three main adhesives, Hanlon says that digital printing tends to favor solventless technology. “The upfront capital investment is lower on solventless machinery and the equipment has a compact footprint, aligning with the market’s advantages of agility, flexibility, and short-run capability.”
Also referred to as “solvent-free,” solventless adhesive is commonly used in the digital space because of its compatibility with HP Indigo ink technology, explains Catherine Heckman, business unit director, Ashland.
While solvent- and water-based technology is used in conjunction with a digitally printed flexible packaging, Hanlon cautions there is a larger capital cost because a large drying oven would need to be incorporated into the process to evaporate the solvent or water.
A Closer Look at Water
Water-based adhesives are relatively new compared to solvent-based adhesives. They are advantageous because of a higher level of eco-friendliness, however there are still challenges that must be overcome and as such the composition of water-based adhesives continues to advance.
“Initially, the polymers available for water-based technology were not capable of achieving high-performance levels in the flexible packaging industry; this has changed completely and is no longer the case,” explains Augusto Cucala, market segment manager, adhesives – dispersions and resins North America, BASF Corporation.
Hanlon notes advantages to water-based adhesives such as lack of volatile organic compounds (VOCs), excellent “green” bonds for quick processing or tandem laminations, and good optical properties.
“They are environmentally friendly due to the ease of disposal and no concerns around regulated emissions, as well as posing a potentially lower health risk for operators. Water-based adhesives also do not generate primary aromatic amines and therefore, cure time for food safety is not an issue,” continues Hanlon.
Cucala explains that when solvent-based adhesives are used, there is a solid content of approximately 40 percent. This means that for every pound of adhesive used, 0.6 pounds of solvent is sent into the atmosphere.
However, water-based adhesives have experienced slow adoption in North America. “This is due to the lack of durability and performance in a range of structures and applications compared to solvent-free and solvent-based,” shares Heckman.
Flexible packaging must undergo demanding technical requirements, and as such, the adhesive must as well. “Retorable bags for example, which after they are manufactured will be put under extreme temperature and pressure conditions, and the adhesive as well as the laminates have to withstand those conditions,” says Cucala.
Water-based adhesives also present challenges like energy consumption to dry the adhesive, co-reactant mixing—which can be slow, and total cost of ownership, adds Hanlon.
Guideline Awareness
Adhesive selection for digitally printed flexible packaging, especially packaging that will be used around food or drink, must involve consideration of regulation and guidelines. There are several that manufacturers need to be aware.
“Manufacturers need to consider national regulations around food safety compliance and employee safety, as well as local regulations for VOCs and global food safety regulations for any markets into which they may ship,” suggests Hanlon.
In the U.S., regulations around food safety are governed by the Food and Drug Administration (FDA), specifically FDA direct food compliance. Other regulations and guidelines include Nestlé Quality Requirements for Vendors of Raw and Packaging Materials; and the Swiss Federal Department of Home Affairs Food Safety and Veterinary Office Ordinance on Materials and Articles, which establishes certain requirements for food contact materials, according to Heckman.
Work with Trust
Flexible packaging adhesives are not one-size-fits-all. More often then not, each flexible package requires a slightly different chemical composition of an adhesive. Adhesive manufacturers offer their own strengths depending on the product type.
There is something to be said for working with an adhesives manufacturer to create the right adhesive for the job at hand. Hanlon says selecting the right adhesive isn’t as easy as buying an out-of-the-box solution. “When engineered for the job, a custom adhesive provides a longer lasting, stronger, lighter, and more reliable bond. It all starts with partnering with a skilled adhesives manufacturer with the expertise necessary to design a solution that’s suited to your needs.”
As such, companies like H.B. Fuller are prepared to work directly with their customers to ensure adhesion success by really getting to know the final product and its intentions. An example, HB Fuller always considers the conditions an adhesive will be exposed to for the application in question, especially if it is experiencing high temperatures such as retort, boil-in bag, or microwave.
BASF’s broad product portfolio of innovative, high-quality, high-performance resins, adhesives, and formulation additives helps formulators and convertors meet a range of flexible packaging needs. The company innovates to make adhesives stick. With dispersions, resins, and additives as well as technical expertise, it supports customers in developing environmentally friendly, high-performance adhesives.
Ashland is committed to developing safe solutions. It listens to the needs of food companies and converters to collaborate on solutions for flexible packaging products that go beyond industry standards to a level that is fit for use. The company’s expertise in design, synthesis, and scale-up of new polymer chemistries helps it develop innovative adhesives, sealants, and coatings that are the basis for next-generation food packaging.
Adhesive of the Future
Adhesive options remain the main three—water-based, solvent-based, and solventless—however their chemical compositions continue to morph and change to best fit the digitally printed packaging product in question. Advancement and evolution takes its cues from macro-type world trends like sustainability.
“As the flexible packaging market continues to become more sustainable, you should expect to see adhesives that allow recycling of structures through mono-material packaging, as well as compostable solutions that allow for certifiable compostable packaging in situations where recyclability is not appropriate, such as liquid or sticky fill goods within the flexible package,” predicts Hanlon.
Heckman’s forecast into the future of adhesives for flexible packaging is similar, sharing that we should be on the lookout for certified laminating adhesives for sustainable flexible packaging.
Similarly, Cucala says BASF is planning to introduce new products in the area of recyclability for the flexible packaging industry. “These products are compostable adhesives, home compostable adhesives, or even adhesives that allow film separation allowing separation of layers and later reprocessability of the single layers for later reutilization as raw material in new plastics.”
Flexible Packaging
Laminating two films using an adhesive is the most common method for assembling flexible packaging. This type of lamination uses two layers of film, foil, or paper. Adhesive options include water-based, solvent-based, and solventless. Digitally printed flexible packaging requires adhesives engineered to work with the technology and as such working with adhesive manufacturers directly is one of the best ways to ensure the right type for the job at hand.
May2021, Industrial Print Magazine