Understanding adhesives: In the context of stickers & labels

This article will give you a good understanding of the different adhesives (glues) used on the back of stickers & labels to help them stick.

Adhesives are used every day in every corner of the world for everything imaginable.

Stickers & labels are one small application.

Most cars are now partly held together using glues, because it's lighter, cheaper & quicker. For example, windshields, brakes, wire harnesses, and exterior trim all tend to be glued.

We're going to focus on adhesives for stickers & labels.

Adhesives summary

There are 4 main categories of adhesive:

1. Solvent
2. Water-based (emulsion)
3. Hot melt

This table provides a quick overview of the main characteristics of each adhesive type, helping you make informed decisions based on your specific application requirements.

Feature	Water-Based Adhesives	Solvent-Based Adhesives	Hot Melt Adhesives
Composition	Polymers (thermoplastics or elastomers) in water	Polymers dissolved in organic solvents	100% solids formulations based on thermoplastic polymers
Application Method	Brush, roller, spray, slot die, beads/ribbons	Brush, roller, spray, beads/ribbons	Roller coating, slot coating, spraying
Setting Time	Quick setting	Variable drying and bonding times	Quick setting
Bond Strength	Strong initial bond strength, may build over time	Good bond strength	Strong initial bond strength
Versatility	Versatile, suitable for various substrates	Versatile, suitable for various substrates	Versatile, suitable for various substrates
Temperature Resistance	Generally good temperature resistance	Good temperature resistance	Good temperature resistance, but limited by thermoplastic nature
Flexibility	Can offer flexibility depending on formulation	Can offer flexibility depending on polymer used	Generally flexible
Cleanup	Easy cleanup with water	Cleanup with specific solvents, may require ventilation	No cleanup required, solid at room temperature
Environmental Impact	Low VOCs, environmentally friendly	May have high VOCs, environmental concerns	Low VOCs, reduced environmental impact compared to solvent-based
Substrate Compatibility	May require permeable substrates in some formulations	Compatibility with various substrates, may be sensitive to contaminants	Compatibility with various substrates

Now, we'll discuss these adhesives in detail below.

Solvent-based adhesives

Solvent-based adhesives typically include polymers dissolved in a solvent. The specific composition can vary based on the desired properties and the types of surfaces the labels will adhere to. Common components include:

• Polymers: Various polymers, such as polychloroprene, polyurethane, acrylics, and natural or synthetic rubbers, are commonly used. The choice of polymer influences the adhesive's characteristics, like flexibility, strength, and tack.
• Solvents: Organic solvents such as toluene, xylene, or acetone serve as carriers for the adhesive components. The solvent evaporates during the bonding process, leaving the adhesive to set.

Characteristics of solvent adhesives

Solvent adhesives have 2 outstanding characteristics.

The first is that they provide great bond strength. Secondly, solvent adhesives can offer flexibility, allowing labels to conform to curved or irregular surfaces.

Another point worth mentioning is that drying and bonding times can be adjusted based on the formulation, offering flexibility in manufacturing processes. But this is nearly always slower than UV-cured adhesives.

Transitioning to water-based alternatives

In response to environmental concerns, some manufacturers are transitioning to water-based adhesives as a more environmentally friendly option.

The bond strength and other characteristics of water-based adhesives are getting better constantly through investment and innovation.

Let's discuss water-based adhesives next.

Water-based adhesives

Water-based adhesives, also referred to as emulsion adhesives, for self-adhesive labels typically consist of polymers dispersed or dissolved in water. The specific composition can vary based on the desired properties and the types of surfaces the labels will adhere to. Common polymers used include:

1. Acrylics: Known for their versatility and ability to adhere to a wide range of surfaces, they offer good adhesion and durability.

1. Vinyl Acetate Polymers and Copolymers (PVAC): Provide flexibility and are often used in combination with other polymers for improved performance.

1. Polyurethane (PUR): This material is known for its excellent bonding strength and flexibility, and it enhances adhesion on challenging surfaces.

Characteristics of water-based adhesives:

• Quick setting: Water-based adhesives often have a quick setting time, facilitating efficient label production.
• Bond strength: These adhesives offer strong initial bond strength, ensuring the labels adhere well to various surfaces upon application.
• Versatility: Suitable for a range of label materials, including paper, film, and specialty substrates. Water-based adhesives can be used for both permanent and removable labels.
• Temperature stability: Water-based adhesives typically exhibit good temperature stability, ensuring labels maintain their adhesion properties in various environments.
• Residue-free removability: Some formulations allow for easy removal of labels without leaving residue on the substrate.

Environmental benefits of water-based

1. Low VOCs: Water-based adhesives contribute to environmental sustainability by having low or no volatile organic compounds (VOCs).
2. Easy cleanup: Water is used for cleanup, reducing the environmental impact compared to solvent-based adhesives.

To summarise, water-based adhesives for coating self-adhesive labels provide a balance of strong bonding performance, versatility, and environmental benefits. Their formulation and application methods are optimised for label manufacturing.

Water-based adhesives have fast been becoming the de facto standard in label applications because of their versatility & lower environmental impact.

Hot melt adhesives

Hot melt adhesives are typically 100% solids formulations based on thermoplastic polymers.

In plain English, this means they melt when they get hot. When thinking of hot melt adhesives, think of sticks you put in a glue gun.

But they actually come in drums, or bags like this:

At room temperature, they're solid. However, heat them up, and they become very runny & tacky.

The composition of hot mets varies, but most include:

1. Thermoplastic polymers: Such as polyethylene, polypropylene, ethylene-vinyl acetate (EVA), or styrene block copolymers. These polymers provide the adhesive with its thermoplastic nature, allowing it to melt and solidify rapidly.
2. Resins: Resins may be added to enhance specific properties like adhesion, tack, and flexibility.

Characteristics of hot melts

1. Quick setting: HMAs offer rapid setting times, promoting fast production processes in label manufacturing.
2. High initial bond strength: Labels coated with hot melt adhesives exhibit strong initial bond strength, ensuring immediate adhesion to various substrates. On the flip side, this makes them difficult to lift & realign if placed incorrectly.
3. Versatility: Suitable for a wide range of label materials, including paper, film, and specialty substrates.
4. Poor resistance to plasticisers: Hot melt adhesives react with the plasticisers in PVC films and change from being tacky to being like un-sticky gunge. The plasticisers 'migrate' (move) from the PVC film to the adhesive to cause this. So avoid using hot melt adhesives with PVC films, especially heavily plasticised films.
5. A lack of temperature resistance: Hot melt adhesives provide good temperature resistance but not at extreme temperatures (100 degrees +). When hot melts get close to their melting point, they lose nearly all their performance.

Considerations of hot melt adhesives

Hot melt adhesives are sensitive to temperature, requiring precise control during application to prevent overheating. You also want to ensure the label won't encounter extreme temperatures when in use. However, for most applications, it's a great solution.

Environmental comments

Hot melts have reduced VOCs compared to solvent-based adhesives as they're 100% solids.

This absence of solvents and the rapid setting nature of hot melt adhesives contribute to energy efficiency in label production.

Summary

I hope this article on the various types of adhesives have helped you to understand the options available and the characteristics of each.

For a quick comparison of the features of each adhesive, see the table shown at the start of this article.