When it comes to adhesives, what you don’t know could unglue your printed part

When choosing the right adhesive for your printed part, it’s important to understand that all adhesives are not created equally because all surfaces aren’t the same, nor are all applications. In fact, here at DuraTech, we stock nearly 100 different adhesive configurations to meet the varying needs of our customers.

Many factors affect how an adhesive works (or doesn’t). In this post, we list the top five to help you navigate the crowded, and sometimes confusing, world of what makes things stick together.

Surface energy or surface tension

The majority of adhesives fall into one of two categories: low surface energy and high surface energy. A simple way to understand surface energy (also called surface tension) is to compare a waxed car to an unwaxed car. When water is dropped on the hood of a car without wax, it sheaths off the surface because the surface energy is high, pulling down on the water and causing it to flow. On a waxed car, the surface energy is low, so water beads up and doesn’t flow (wet out) as much. The higher the surface energy, the more “attraction” there is between the materials, which increases contact and facilitates better adhesion.

Surface energy is measured by dynes per centimeter. The higher the dyne level, the higher the surface energy. Adhesives are chemically engineered to work with specific surface energies. An adhesive that will adhere to a high surface energy substrate like metals and certain plastics won’t adhere very well (or not at all) to a low surface energy substrate. On the other hand, a low surface energy adhesive will typically stick to both high and low surface energy substrates.

In addition to the substrate material, paint and other coatings also affect surface energy. Powder coated paint, for example, tends to have low surface energy characteristics.

Surface shape and texture

For a label or overlay to stick to a substrate, the adhesive needs to have adequate surface contact. A flat surface with little or no texture requires only a thin adhesive layer, while a rougher surface requires a thicker layer of adhesive to allow it to make contact with the peaks and flow down into the valleys of the surface texture for more complete coverage (versus just adhering to the peaks).

Here at DuraTech, we typically produce two different types of labels – thin pressure sensitive decals such as .002” polyesters and vinyls up to .004” thick – and thicker overlays, which are made of .003” – .010” polyester and .005” – .030” polycarbonate. Each label type requires a different kind of adhesive.

Standard pressure sensitive label materials come with a pre-applied adhesive that’s thick enough to work well on smooth surfaces. They’re also available with adhesives heavy enough to work on slightly textured surfaces. If a pressure sensitive label needs to adhere to a textured surface, keep in mind that surface anomalies might show through the label due to the thinness of the material.

Overlay materials come in thicker options and offer flexibility for a variety of adhesive types and thicknesses to be applied to them, allowing them to adhere to either smooth or textured, high or low surface energy surfaces. Using overlay materials requires the use of an adhesive transfer tape – a roll of paper backing to which an adhesive is applied. The paper backing with adhesive is then laminated to the part.

Adhesive transfer tapes also come in sheet form, which is a layer of adhesive sandwiched between two paper liners. These sheets come in handy when a blueprint requires holes to be die cut out of the adhesive. If your label, for example, contains a transparent area for certain graphics or an LED window, you don’t want any adhesive in that area because you don’t want it to collect dust or distort your graphics. Double liner adhesives can be die cut to remove the adhesive from areas where it isn’t wanted. The die cut adhesive can then be laminated to the printed sheet leaving transparent areas void of adhesive.

Curved surfaces require a combination of overlay material that’s flexible enough to match the curve and an adhesive that’s strong enough to maintain the hold of the curve and keep it adhered so the label edges don’t lift up. If the surface has a compound curve, not only does the adhesive need to be strong enough, but the substrate must be able to conform in more than one direction.

Finally, surface contact is fundamental to adhesive performance on any surface. To maximize contact, the surface should be dry and clean, firm pressure must be applied to increase the contact and flow of the adhesive to the surface, and time and appropriate temperature must be factored in to increase surface contact and adhesion values.

Initial tack vs ultimate bond

“Initial tack” refers to how aggressively an adhesive adheres to the surface the moment a label or overlay is applied. “Ultimate bond” is the strength of the adhesive after it has fully set. Good initial tack is important for applications that require immediate handling, such as in an assembly line setting. If a label or overlay might need to be repositioned without damaging the part, you’ll want an initial tack that isn’t as strong. When evaluating the ultimate bond strength of an adhesive, wait approximately 48 to 72 hours after it has been applied.

Environmental concerns

Sheer strength is a measure of the internal strength of an adhesive. Firmer adhesives tend to have higher shear strength than softer ones. High shear strength is desirable if forces will be exerted against the adhesive such as applying a heavy part on a vertical surface, like a wall. High shear strength will prevent the weight from sliding down the wall and leaving a gooey mess.

Different adhesives are designed to withstand different environments. Some have better resistance to moisture, temperatures (high and low), UV exposure, solvents and other chemicals. Acrylic adhesives, for example, tend to be more expensive but are much higher performing than rubber based versions which, while less expensive, tend to have poor UV resistance, may fail when exposed to water or humidity and have little chemical resistance.

Specifying the right adhesive for your job

Just like material type, class of ink or embossing tolerances, specifying the right adhesive for a printed part is crucial to the successful outcome of your project. If you’re unsure, consult with our experts and we’ll recommend the adhesive that will work best based on your project requirements.