Eco-Friendly Adhesives In the World of Sustainable Packaging

Eco-Friendly Adhesives In the World of Sustainable Packaging

Nov 11th 2019

Wait, why are we writing about eco-friendly adhesives as a packaging company? When you think shipping packaging you probably think boxes and mailers. Maybe you think about tissue paper and bubble wrap. You might also think about tape. But only the geekiest among us think about the most detailed nuances of shipping solutions - things like printing ink, release liners on labels, and of course, eco-friendly adhesives.

Since EcoEnclose is proud to be a true ecogeek, we do sit around thinking about and trying to find ways to create the most environmentally friendly adhesives. If you’re with us, read on!

First, what is an adhesive?

Quite simply, adhesives are any substance that sticks two objects together. Basically glue. But how do you make glue eco-friendly? The objects that are being stuck together by our adhesive are considered the “substrates.” Ever made a grilled cheese sandwich? The cheese, once melted and then cooled, 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 Elmers glue. In these types of adhesives, the substrates adhere only 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, there are actually molecular interactions that 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 a pressure sensitive adhesive. Often when a pressure sensitive adhesive is involved, there is also a release liner that protects the adhesive properties until it is ready to be used.
  • Hot melt: A hot melt adhesive is thermoplastic that is melted to become a liquid and then solidifies when cooled. Once solidified, the adhesive forms strong bonds between a wide range of materials. One of the most common examples of this is a glue gun and glue stick.
  • Reactive: There are a wide range of glues in which the adhesive reacts with a substance (sometimes the actual substrate that is being adhered to) and that reaction creates the bond. The most common example of this is a water activated adhesive, such as an envelope that is moistened 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 to make it.

Some are made from renewable resources. Common 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 minerals such as amber, silica, and sulfur.

Synthetic adhesives are those derived from human-made polymers including thermoplastics, thermosets, and elastomers.

While more costly than natural adhesives, these types of adhesives offer greater bond strengths and durability, as well as 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 a number of different places adhesives show up, and a broad set of adhesives we utilize.

Adhesives used to form corrugate for our 100% recycled custom shipping boxes. The liner board and fluting is glued together using a starch-based adhesive.

Adhesives that 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 that are used to form our recycled mailers. This refers to the glues that are used to form the mailers themselves. For most of our paper mailers, a water-based vinyl acetate ethylene adhesive is used. Our poly mailers and poly bags are heat sealed so no adhesive is used.

Self sealing, pressure sensitive adhesives 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. For most of our paper mailers, these are synthetic polymer hot melts that form a pressure sensitive adhesive. For our poly mailers and flap and seals, these are and acrylic based adhesive coated on both sides.

Adhesives on our stickers and shipping labelsThis refers to the adhesive that is revealed when you peel the stickers or shipping labels off of its release liner to expose the adhesive, and then adhere the label onto a substrate. For all of our stickers and labels, these are an acrylic emulsion (pressure sensitive) adhesive.

Adhesives 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 on our water activated tapeOur water activated (i.e. gummed) tapes have a starch-based adhesive that is wetted 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. 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 kids 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 (which is low tack and repositionable) wouldn’t work on the self sealing adhesive for a mailer.

An eco-friendly adhesive’s functionality is driven by not only its formulation, but also how well it bonds with the specific substrates (and the weight of the substrates) at hand. 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, which makes 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 require 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 important 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, result in smog. 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, in certain contexts, they are actually more problematic than renewable counterparts. For example, glutin and casein are two examples of adhesives that are renewable, but are derived from animal proteins and therefore are often considered less sustainable than synthetic counterparts such as acrylic emulsion adhesives. In other scenarios, such as water activated adhesives which are typically derived from plant-based starches, renewable sources are eco-friendly and extremely effective.

Similar to the complexities of bioplastic, it is important to note that a “natural” adhesive is not necessarily compostable and a synthetic adhesive is not necessarily non-compostable. 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 what the adhesive was made from. Which bring 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 material it is being used on.

People often ask for a compostable adhesive, but what they are likely asking for is a package whose components (main material, adhesives, ink, etc) can be composted. Because adhesives are a small component of an overall item, its presence does not typically alter how you can dispose of an item. 

For example, if you have a standard corrugated box with a shipping label on it, 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 it is technically “accepted” by a recycler or composter.

One major certifying body for compostability tests materials by first ensuring they biodegrade (no physical remnants of the material remain) within a set timeframe. They then confirm that a tomato plant can be grown with the output from the composted material. This approach is certainly thorough, but it is forgiving of 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, components of a typical adhesives still do remain in the compost.

In the recycling process, paper is pulped and then screened, 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 that is 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.

What adhesive issues could you run into, especially when it comes to EcoFriendly packaging?

There are three types of “adhesive failures”.

  1. 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.
  2. In some instances, the adhesive itself actually breaks apart, a phenomenon referred to as a cohesive failure. You’ll know this happened if the two substrates that separate each have adhesive residue on them.
  3. Finally, there are instances where the bond between the adhesive and the substrate is actually stronger than the substrate itself. If you pull a piece of tape off of a box and a layer of the box remains on the tape, this would be an example of a substrate failure.

eco-friendly adhesives

In working with 100% recycled (often 90%+ post consumer) corrugate and paper, you may face some adhesive issues. If you do face this, it is largely driven by the characteristics of the recycled paper.

Each time paper is recycled, its fibers become shorter. The shorter fibers of a recycled corrugated shipping box or a recycled mailer makes it a bit harder to form tight bonds with the adhesive it comes into contact with.

We have only come across this issue with pressure sensitive adhesives, not with hot melt or water activated adhesives.

The number one 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 actually 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 largely low or no VOCs but the goal is to eliminate VOCs and any toxins altogether.

Additionally, our adhesives do work with the recycling process, but we know we can continue to better understand the impact of adhesives on the entire recycling process and select 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. Thus far, renewable options are not really available for these purposes, and any option available on a small scale are derived from animals or are not as effective or functional as we need them to be. As the world of adhesives evolves, we will continue to research and test new formulations derived from renewable resources. 

The second, and arguably more critical, opportunity for EcoEnclose in the world of adhesives is to continue to advance the functionality of how our adhesives work with recycled paper. We ship out thousands of packages every month in our own recycled corrugated shipping boxes and mailers, and use our own 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 the recycled paper packaging we work with.

Our 100% Recycled Apparel Mailers are an excellent example of progress we’ve been able to make. In our first iteration, we used a more general adhesive for the self-seal. While this was functional, users did have to apply extensive pressure to create the 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 the more textured surface. We ship about 10% of our own 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 feature an all purpose, all temperature adhesive. While we’ve successfully used them on all of our outbound shipments for the past six months, we do have to apply 5-10 seconds of extra pressure when affixing each of them on our shipments. This issue is driven by a combination of the 100% recycled corrugate and the unique manufacturing process behind the innovative Zero Waste (recycled, recyclable) liner. We are working with our partners to introduce a stronger adhesive to this patented process that can more quickly create a strong bond with recycled corrugate.

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. Here again we have found internally that the added pressure does wonders to address the issues presented by the recycled content and shorter fibers. However, we also believe a stronger, eco-friendly adhesive designed specifically for recycled paper fibers can support this challenge and alleviate the extra pressure needed. 

Head over to our Sustainable Packaging Resource Center to learn more about eco-friendly packaging or peruse our blog for more on recycling, packaging and more!