Pressure-sensitive label being applied to a product.

Pressure Sensitive Labels vs. Self-Adhesive Labels: Is There a Difference?

What’s the difference between pressure-sensitive labels (PSL) and self-adhesive labels? Nothing. They use the same construction and application method, although these names are often confused with other types of labels. Why does the same label go under two names, and what makes them so popular?

The Development of Pressure-Sensitive Adhesive Labels

Pre-applied adhesive labels have been around since the middle of the 19th century. However, these early labels required something to activate the glue. For example, when you lick an envelope to seal it. The only pressure-sensitive adhesives on the market were in bandages and dressings. These used a rubber-based glue applied just before application and had a limited working life.

In 1935, Stanton Avery started a business to sell the first commercial pressure-sensitive labels. He created a new process based on wet glue technology that didn’t require activation. Instead, the glue was kept tacky by a removable backing layer, like a modern-day sticker. He marketed them as self-adhesive labels since they did not require any activation steps before use. The user could just remove the release liner to apply. 

Although originally sold blank, companies immediately saw the benefits of this new type of label. Soon, Avery started making pre-printed labels to apply to products. While these early labels were successful, it took a while for them to take the labeling market by storm. Acrylic acid was the only pressure-sensitive adhesive available, and it had limited strength. These labels were particularly prone to flagging, forcing Avery to use round labels for his early product runs. It wouldn’t be until after WWII that the first high-quality acrylic polymer glue reached the market. Breakthroughs in the late 1950s led to hydrogen bonding monomers, polymer crosslinking, and copolymerization. These processes added structure to these glues, increasing their strength.

Pressure-sensitive labeling really came into its own with the introduction of acrylates. While there are other pressure-sensitive glues on the market, there are few tacky options that do not need additives. Cutting out additives doesn’t just make these glues cost-effective to manufacture; it also limits or eliminates problems with migration, changes in strength, and allergic reactions. Acrylates are also highly resistant to oxidation and exposure to chemicals and light. With some modifications, they can resist extreme temperatures and exposure to petroleum products. This versatility has helped self-adhesive labels become the most popular technology used for commercial product labeling.

What Makes These Labels Stick — and What Does Pressure Have To Do With It?

Adhesives are broadly classified as structural or pressure-sensitive. Structural adhesives change chemically to stick to substances. The trigger could be the evaporation of solvents, UV radiation, direct chemical reactions, or temperature changes.

Pressure-sensitive adhesives can best be described as flexible solids. When they’re applied to a container, they flow across the surface. However, once applied, they won’t flow enough to peel off. In a way, they act like Lego bricks. While they flex enough to easily snap together, it takes more effort to separate them.

Why Use Two Terms for the Same Label?

It comes down to how you look at the application. Despite the separation between consumer and commercial use, all of them use similar construction within label materials, from the label stock to the face stock. The top coats may differ due to variations in the label printing process between home and commercial uses. Home label making usually doesn’t involve elaborate digital printing, embossing, or laminating

If you’re looking at these labels from a consumer standpoint, you’re focused on a one-off application method. If you grab a roll of tape or an address label (like the die-cut labels from your inkjet printer), you can slap it on without thinking about glues or drying time. Avery marketed his first labels as “self-adhesive” to emphasize this ease of use. Look at labels marketed to office and home users; you’ll see this term used on them.

If you’re looking at these labels from a manufacturing standpoint, you’re focused on the application method and how it fits your production system. With these labels, you don’t need a heat source, nor do you need to worry about application and drying times. When you choose to use these labels, the biggest concern is pressure. There are several application methods to consider, as well as ways to prevent flagging, bubbling, and skewing. This is why commercial labels are called “pressure sensitive.” 

How Does Pressure Sensitive Label Application Work?

Pressure-sensitive adhesive is thin and can’t flow into large gaps. It only works with smooth product surfaces. They’re commonly on smooth-sided aluminum cans, but not ribbed tin cans. Likewise, pressure-sensitive labels work well on other smooth surfaces (including cardboard, plastic, and glass packaging) and don’t risk graphic distortion like in shrink sleeve labels

Each layer of the label (from the liner to the top coat) can be changed to meet packaging requirements. This lets you build the perfect custom label, considering everything from moisture exposure to label design and graphics choices. The labeling machine needs to peel the labels from the liner and apply them evenly to the containers. These machines use one of three methods, with variations in speed and position for accuracy.

Merge

The label is wiped onto the label as it’s peeled off of the backing. While this is the simplest solution, the conveyor and applicator have to work at the same speed. However, there are ways around this. For example, our trunnion roller labeling system traps the container and moves in time with the applicator. This increases application accuracy, reducing skew on long labels.

Air Blow

A burst of air pushes the label away from the applicator and onto the container. This system works in any position and doesn’t have to be in time with the conveyor. As soon as a container is in position, the machine applies the label. This works well for applying labels on uneven surfaces, like wrapped produce. Our 360a Series applicator can be configured for air blow, making it easy to set up a system for odd-shaped containers.

Tamp/Blow

The label is dispensed onto a tamp pad which uses negative pressure generated by a muffin fan or by a vacuum valve bank to keep the label in place. A jet of air overcomes this pressure, pushing the label onto the container. The tamp pad can move, which lets you get the label close to recessed surfaces before application. Our 3600a-PA dual-action tamp printer applicator uses the flexibility of this system to apply labels from any angle and at high speed.

Take Full Advantage of a Pressure Sensitive Labeling System

If you think either pressure-sensitive labels or self-adhesive labels may be the answer to your labeling problems, contact CTM Labeling Systems. Our local distributors will work with you to set up a labeling solution that fits your production needs and the demands of your customers.