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The ties that more than bind

Specialty, multi-functional adhesives are a concept for the times.

Sterling Anthony, CPP
Sterling Anthony, CPP
The term "commodity," as applied to lamination adhesives, continuously is losing allure, as attention is directed to their "specialty" counterparts. It used to be that if an adhesive adequately held together the substrates of a lamination, that adhesive—barring major disqualifiers—was assured a market; after all, that pretty much was all adhesives were expected to do. That time is gone, shown the exit by the ever-increasing demands placed upon packaging as a strategic tool for competitive advantage. No component is exempt from expectations of delivering more, nor should it be.  
What defines a specialty adhesive is relative (much like "smart" packaging); nonetheless, the designation should be reserved for formulations that provide a meaningful extra (which, too, is relative). What is less conditional is that whatever is promoted as a specialty adhesive should be the product of a collaboration among brand-owner (end-user), converter, and raw-material supplier. The benefits from a specialty adhesive need not distribute evenly among those three, as long as each obtains something that improves its competitiveness.

The specific trends and drivers behind specialty adhesives are diverse and some contradict others. Managers have many research & development options, including the following:

Low-energy substrates

The bond between an adhesive and a plastic substrate is one of polarity: charges (negative and positive) in the adhesive link with their opposites on the substrate's surface. The surfaces of plastics, however, are mostly non-polar—low-energy, in the industry vernacular—making adhesion difficult, unless surface energy is increased through corona treatment (better suited to film than is flame treatment).
An adhesive that bonds with low-energy substrates, without the need for surface treatment, carries the benefit of eliminating a process, but there is more to consider. Plastic surface polarity also can be a requirement for inks, lacquers, and overcoats; therefore, the ideal would be for the technology behind the adhesive to be applicable to those other chemical substances. That way, surface treatment is eliminated, period. Brand-owners, by strongly communicating their requirements upstream, can incentivize raw-material suppliers and converters to work in tandem to develop mutually beneficial technology.

Besides elimination of surface treatment, another approach is adhesives that bond at polarities that are lower than traditional requirements. The effects from corona treatment are not permanent, but degrade over a course of days. An adhesive that still can bond at a lower dyne level (the unit of measure for surface energy) provides its user with greater flexibility as to scheduling and contingencies.
Thinner laminations

Reduce the thickness of the substrates being bonded and the result is a thinner lamination. No revelation there, but below the benefits associated with source reduction lurk challenges for adhesives. First, thinner substrates still have low-energy surfaces and are subject to the challenges previously discussed. Secondly, thinner substrates present other challenges that are a function of their thinness.

A substrate should be chosen on the basis of what it contributes to the lamination; hence, different substrates, there to perform different functions, might not lend themselves to uniform reduction. Thinner substrates are more likely to stretch under shear, but, substrates of varying thinness within the same lamination might stretch varyingly. Such uneven reactions can cause ruptures within the adhesive, unless it has sufficient cohesive strength. Increasing the amount of adhesive is not the automatic (nor necessarily desirable) answer because laying it on thick is the antithesis of source reduction. Adhesive strength, cohesive strength, and thin layers make for a difficult balancing act, but it's one that offers financial rewards to companies that can achieve it.

From a related perspective comes this question: Why not an adhesive formulated to add to the stiffness of a thinner lamination without a proportional increase in the amount of adhesive applied? Stiffness is a property useful in a lamination's machinability but also comes into play, let's say, in a stand-up pouch's ability to stand-up. Despite being thinner, a lamination might have to satisfy tactile requirements, if, for example, consumers prefer a certain feel-to-the-touch. Proceeding down the list of faculties, stiffness even might have to manifest itself to the sense of sound. An example is snack food packaging, that when handled, makes a crunching sound, remindful of the contents. Stiffness is a multifaceted property, and it makes financial sense to explore whether some facet(s) can be achieved with an adhesive, since that component is usually the least expensive within a lamination.

Braking barriers

When a lamination has to protect against moisture, gas, or light, the barriers are provided by one or more substrates. There is no official category of barrier adhesives, but that need not be an indictment against its feasibility. The concept can be thought of as auxiliary to the preceding discussion on thinner laminates. Reworded, if source reduction reduces barrier, to what extent might the adhesive compensate? The aforementioned elements that necessitate barrier pose different levels of challenge, but that's no reason to permit conventional thinking and convenient assumptions to thwart the imaginative investigation of possibilities.

The notion of barrier can be widened to include protection of the other components of a lamination. Electron-beam (EB) radiation has its benefits as a method of curing adhesives (see Searching for a cure, in the 3/9/10 Packaging Insights Newsletter) but can penetrate down to a heat-seal coating, negatively affecting it. It establishes the need for EB-curable adhesives formulated to absorb the necessary amount of energy, while serving as a barrier that protects other coatings.
On the flipside, adhesives are capable of attacking another coating, which can happen when a solventless adhesive is applied to a substrate, which in turn, is laminated to a reverse-printed substrate. Adhesive and ink not only are in direct contact with one another, but remain so for the days required for the adhesive to fully cure. During that time, the adhesive can penetrate the ink, resulting in poor bonding that can lead to delamination, or at the very least, to poor print aesthetics. Such results occur repeatedly when the ink in question is water-based (a choice that's growing in popularity for environmental reasons) but solvent-based inks also are vulnerable. An adhesive that needs no barrier between itself and the ink addresses a demonstrated need.

Bio-materials and sustainability

Even some brand-owners who remain unconvinced that there's a bright future for bio-materials in packaging are not willing to risk being wrong and lose market share. That guarantees continued investigation and experimentation, some of it directed at laminations, and thereby driving the need for compatible adhesives.

Polylactic acid (PLA), the biggest member of the category, unsurprisingly has the interest of raw material (adhesive) suppliers, and there are adhesives on the market that purport to be formulated for PLA. What should not be assumed, however, is that a PLA substrate is all bio-material. It likely contains a variety of additives to improve such properties as impact resistance, melt strength, tensile strength, shear strength, and machinability. What that means is that the adhesive should be compatible with the PLA as modified by additives. That's an easier task when additives and adhesive come from the same raw material supplier and have been co-formulated for compatibility.

If the adhesive, itself, is a bio-material, the sustainability of the lamination is potentially raised. Consequently, adhesives derived from plants have been commercialized and development efforts continue.
On the other hand, laminations have been criticized for a lack of recyclability, owing to their disparate components. Whether laminations ever will be as recyclable as some of the more homogenous types of packaging is debatable; regardless, adhesives can play a role, either by being removable (at reasonable cost), or if not, being recyclable along with the substrate(s).

Layers of opportunities

Adhesives can be of the special variety in many more ways than mentioned herein. Opportunities with adhesives are layered, like the laminations themselves. In the end, a lamination is a good analogy for the needed partnership among brand-owner, raw-material supplier, and converter—each performing a function that contributes to the whole, bonded by mutual interests.

Sterling Anthony is a consultant, specializing in the strategic use of marketing, logistics, and packaging.  His contact information is: 100 Renaissance Center-176, Detroit, MI 48243; 313-531-1875 office; 313-531-1972 fax;
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