Eco-Friendly Adhesives In the World of Sustainable Packaging
You probably think of boxes and mailers when you think of shipping packaging. Maybe you think about tissue paper and bubble wrap. You might also think about tape.
But only the most packaging-obsessed among us think about the most detailed nuances of sustainable shipping solutions - things like printing ink, release liners on labels, and eco-friendly adhesives.
Since EcoEnclose is proud to be a true EcoGeek, we sit around thinking about and trying to find ways to create the most environmentally friendly adhesives. So if you're with us, read on!
What is an adhesive?
Quite simply, adhesives are any substance that sticks two objects together—basically, glue. The objects stuck together by our adhesive are considered the "substrates." Ever made a grilled cheese sandwich? Once melted and cooled, the cheese could be considered an adhesive that holds the two pieces of bread (substrates) together.
What kind of adhesives exist?
There are so many different ways to think about adhesive categories.
For example, they can be characterized by how they form the bond between substrates.
- Drying. Some adhesives work by drying. Think rubber cement and Elmer's glue. In these types of adhesives, the substrates adhere after the solvent or carrier dries, and the adhesive hardens.
- Pressure-sensitive: This type of adhesive is solid enough to retain its shape but soft enough to flow and press into the substrate when pressure is applied. When that adhesive is pressed into the substrate, molecular interactions firm and strengthen the bond. The most common example of a pressure-sensitive adhesive is Scotch tape or any other brand of clear household tape. Post-it notes and Band-aids are also an example of pressure-sensitive adhesives. Often, when a pressure-sensitive adhesive is involved, a release liner protects the adhesive properties until it is ready to be used.
- Hot melt: A hot melt adhesive is a thermoplastic that is melted to become a liquid and then solidifies when cooled. Once hardened, the adhesive forms strong bonds between various materials. One of the most common examples is a glue gun and glue stick.
- Reactive: There is a wide range of glues in which the adhesive reacts with a substance (sometimes the actual substrate being adhered to), creating the bond. The most common example is a water-activated adhesive, such as a moistened envelope and then sealed. The water activates the adhesive - typically made with potatoes or corn starch - allowing it to stick to the paper.
Alternatively, adhesives can also be characterized by the raw materials used.
Some are made from renewable resources. Familiar natural sources of adhesives include starches (such as corn, potatoes, sugarcane, and wheat), natural resins (such as gum arabic), casein (a milk protein), and other animal sources (such as beeswax, shellac, and gelatin rendered from animal hides, hooves, or bones). Some are made from natural but non-renewable resources. These adhesives are made from amber, silica, and sulfur minerals.
Synthetic adhesives are derived from human-made polymers, including thermoplastics, thermosets, and elastomers. While more costly than natural adhesives, these types of adhesives offer more incredible bond strengths and durability and provide more options for customization.
Other ways to characterize adhesives include their form (liquid, paste, powder, solid) or how they are typically applied.
Where do adhesives show up in EcoEnclose packaging?
Across our packaging solutions, there are several different places where adhesives show up, and we utilize a broad set of adhesives.
Adhesives are used to form corrugate for our 100% recycled custom shipping boxes. The liner board and fluting are glued together using a starch-based adhesive.
Adhesives are used to glue our RSC and FPF boxes. Note that our other box styles do not require any glue). These are hot melt thermoplastic adhesives.
Adhesives are used to form our recycled mailers. This refers to the glues that are used to create the mailers themselves. A water-based vinyl acetate-ethylene adhesive is used for most of our paper mailers. Our poly mailers and poly bags are heat-sealed, so no adhesive is used.
Self-sealing, pressure-sensitive adhesives are used on our mailers. This refers to the adhesive that you - the end-user - would use to seal the mailing envelope once you've put your products inside. These adhesives are covered with a release liner that you peel off to expose the adhesive for closing. These are synthetic polymer hot melts that form a pressure-sensitive adhesive for most of our paper mailers. These are acrylic-based adhesive coated on both sides for our poly mailers and flap and seals.
Adhesives are used on our stickers and shipping labels. This refers to the adhesive revealed when you peel the stickers or ship labels off its release liner to expose the adhesive and then adhere the tag onto a substrate. Our stickers and labels are acrylic emulsions (pressure sensitive) adhesive.
Adhesives are used on our pressure-sensitive tape. EcoEnclose offers two different types of pressure-sensitive tape - a cello and kraft flatback tape. Both of these tapes have a pressure-sensitive rubber adhesive.
Adhesives are used on our water-activated tape. Our water-activated (i.e., gummed) tapes have a wetted starch-based adhesive to activate the adhesive.
What to consider when choosing an adhesive?
Functionality and effectiveness
Choosing an adhesive must be specific to the application.
This is true even in your home. For example, you might use crazy glue (a contact adhesive) if you are trying to repair a mug but you'd use PVA (e.g. Elmer's) glue for a kid's art project and perhaps double-sided (pressure sensitive) tape for a scrapbook.
For packaging, the adhesive must, first and foremost, function effectively for the situation at hand.
For example, the adhesive used on a post-it note (low tack and repositionable) wouldn't work on the self-sealing adhesive for a mailer.
An eco-friendly adhesive's functionality is driven by its formulation and how well it binds with the specific substrates (and the weight of the substrates) at hand. For example, some adhesives work beautifully on stainless steel but don't work as well on paper.
It also must align with how it is being applied. For example, we cut RSC boxes and then use a hot melt machine to glue them together. We need the adhesive to bond the corrugate together as quickly as possible, making a hot melt adhesive that solidifies in 5-10 seconds ideal. It wouldn't make sense for us to use a drying adhesive, which often requires minutes or even hours to dry (we'd have a warehouse full of boxes waiting to dry before we can fold them or send them out)!
Sustainability and health
In addition to matching the functionality and application needs of the situation, there are essential sustainability considerations.
Release of VOCs (or Volatile Organic Compounds): Volatile organic compounds (VOCs) are emitted as gases from certain solids or liquids. VOCs include a variety of chemicals, some of which have adverse health effects and, when combined with nitrogen oxide, resulting in smog. For example, drying adhesives can be volatile, emitting VOCs when the carrier dries. In comparison, hot melt adhesives are non-volatile.
How the adhesive is derived: As described above, adhesives can be made from renewable resources, from natural but non-renewable resources, or can be synthetic/human-made (often derived from polymers).
Ecologically, there can be benefits to adhesives derived from renewable materials. However, they are more problematic than their renewable counterparts in specific contexts. For example, glutin and casein are two examples of renewable adhesives derived from animal proteins and, therefore, are often considered less sustainable than synthetic counterparts such as acrylic emulsion adhesives. However, in other scenarios, such as water-activated adhesives, typically derived from plant-based starches, renewable sources are eco-friendly and highly effective.
Similar to the complexities of bioplastic, it is essential to note that a "natural" adhesive is not necessarily compostable, and a synthetic adhesive is not necessarily non-compostable. Furthermore, each formulation (and there are countless adhesive formulations out there!) is unique. More importantly, the end-of-life scenarios related to an adhesive are not necessarily driven by the adhesive's materials. This brings us to the following.
End of life: Adhesive end of life is similar to ink.
In most instances, an adhesive's individual, isolated end-of-life characteristics aren't as critical as how well the adhesive's end-of-life path aligns with and supports the ideal end-of-life of the substrate's material.
People often ask for a compostable adhesive, but they are likely asking for a package whose components (primary material, adhesives, ink, etc.) can be composted. Because adhesives are a small component of an overall item, their presence does not typically alter how you can dispose of an item.
For example, suppose you have a standard corrugated box with a shipping label. In that case, you could compost the entire package (if your local composter accepts corrugate, which many now do) or recycle it.
This doesn't mean that vast quantities of the adhesive itself would necessarily be compostable or recyclable or that adhesives pose no concerns or contaminations even when technically "accepted" by a recycler or composter.
One primary certifying body for compostability tests materials by ensuring they biodegrade within a set timeframe. Here, biodegrade means no physical remnants of the material remain. They then confirm that a tomato plant can be grown with the output from the composted material. This approach is certainly thorough, but it forgives contaminants such as ink and adhesives because of how small a percentage these substances make up. Even though the certification process is forgiving of these contaminants, typical adhesive components remain in the compost.
Paper is pulped and then screened in the recycling process, and ideally, the adhesives are screened out during this process. However, historically, pressure-sensitive adhesives (such as a stamp on an envelope) are shredded into small particles during the pulping process. Because they are shredded so finely, they would then go through the screen and enter the pulp, ready to be remade into paper. This can result in 'stickies' that can build up on process equipment and cause defects in the final paper product.
This concern led to the development of the Recycle Compatible Adhesive (RCAs), developed for paper to paper adhesives.
Recyclable adhesives resist the forces that would pulp them and remain large enough to be removed by screens.
The vast majority of EcoEnclose's packaging, labels, and stickers are made with adhesives that respond well in repulping environments, making them Recycled Compatible. This - more than "compostability" is the most critical eco characteristic we look for when it comes to adhesives.
What adhesive issues could you run into, especially when it comes to eco-friendly packaging?
There are three types of "adhesive failures."
- The most common is that the adhesive itself does not establish a strong enough bond with the substrate. For example, if you have two substrates attached by an adhesive and then pulled apart if the adhesive remains fully attached to one substrate and not the other, this would be considered an adhesive failure.
- In some instances, the adhesive breaks apart, a phenomenon referred to as a cohesive failure. You'll know this happens if the two substrates that separate each have adhesive residue.
- Finally, there are instances where the bond between the adhesive and the substrate is stronger than the substrate itself. For example, you remove the tape from a box, and a layer of the box remains on the tape.
You may face some adhesive issues when working with 100% recycled (often 90%+ post-consumer) corrugate and paper.
If you do face this, it is largely driven by the characteristics of recycled paper.
Each time paper is recycled, its fibers become shorter. The shorter fibers of a recycled corrugated shipping box or a recycled mailer make it harder to form tight bonds with adhesives.
We have only come across this issue with pressure-sensitive adhesives, not hot melt or water-activated adhesives.
The primary way to address an adhesive failure on recycled paper or corrugated is to apply more pressure for 5-15 seconds when applying the label, self-seal, or sticker.
This added pressure creates a stronger and more interlocked bond (remember that there are molecular interactions that firm up the bond!).
What's next for EcoEnclose's eco-friendly adhesives?
As with everything about our business, we know there is always room for improvement.
The first opportunity to improve is around sustainability - VOCs, compatibility with recycling, and, over time, utilizing readily renewable resources.
Our adhesives are mainly low or no VOCs but the goal is to eliminate VOCs and any toxins.
Additionally, our adhesives do work with the recycling process. Still, we know we can continue to understand better the impact of adhesives on the entire recycling process and unique formulations that will best lead to clean, high-value recycled paper.
Many people ask us if we are exploring renewable, non-synthetic adhesives.
We do use renewable resources for our carton sealing tapes and the production of our corrugated sheets. Thus far, we have not considered these options in our mailers, shipping labels, or stickers because they are not readily available for these purposes. Options that DO exist are derived from animals, are not as effective or functional as we need them to be, OR are derived from "renewable" resources that we need to protect and do not want to utilize for our packaging.
As the world of adhesives evolves, we will continue to research and test new formulations derived from renewable resources.
With this in mind, the essential eco improvement opportunity for EcoEnclose in the world of adhesives is to continue to advance the functionality of how our adhesives work with recycled paper.
Every month, we ship out thousands of packages in our own recycled corrugated shipping boxes and mailers and use our shipping labels and stickers. Between our own experience and the feedback you've so generously provided us, we are always looking for ways to improve our adhesives and how well they interact with our recycled paper packaging.
Some EcoEnclose adhesive wins.
In our quest to continuously understand and advance the world of sustainable adhesives, we've already had a handful of exciting wins! Here are three examples.
100% Recycled Apparel Mailers: We used a more general adhesive for the self-seal in our first iteration. While this was functional, users did have to apply extensive pressure to create a strong bond between the self-seal and the mailer. As we've learned and improved the mailer, our current version features two main changes: a stronger adhesive, and we've reversed the paperboard, so the seal is formed with a more textured surface. We ship about 10% of our orders using our Paper Apparel Mailers, and our team is thrilled with these improvements! The seal is now quick and strong!
Our Zero Waste Shipping Labels: Originally, our Zero Waste Labels featured an all-purpose, all-temperature adhesive. We successfully used the first iteration of these labels but found that we had to apply 5-10 seconds of extra pressure when affixing each of them to our shipments. This issue was driven by the 100% recycled corrugate and the unique manufacturing process behind the innovative Zero Waste (recycled, recyclable) liner. About a year after launching the first version of these labels, we worked with our partners to change the adhesive formulation to a High Strength Corrugate version. We have never looked back! These new labels work beautifully with 100% recycled corrugate and paper mailers.
Hi-Tac Shipping Labels: Our 100% recycled padded mailers present two challenges as a substrate. First, they are fully recycled with those shorter fibers. Second, they feature an uneven surface. Customers sometimes found that shipping labels - such as the free UPS labels - did not adhere well to these mailers. So we launched our HiTac line of shipping labels for padded mailers. These labels feature a hot melt adhesive with a significantly higher bonding strength. Once these labels touch your padded mailers, they will not rip off!