How to Choose Materials for Heat-Applied Labels to Achieve Stronger Adhesion?

I. Adhesive Selection: The Core Determinant of High Adhesion

Adhesives are the direct source of adhesion in heat-applied labels. Different adhesive types vary significantly in molecular structure, degree of crosslinking, and applicable temperature ranges. Selection should be based on the substrate material (such as metal, glass, plastic, or paper) and the usage environment (ambient / low temperature / high temperature; humid / dry).
Modified and crosslinked adhesives are preferred to improve long-term adhesion strength and durability.

1. Crosslinked Acrylic Adhesives: The All-Round Choice for High Tack and High Holding Power

Acrylic pressure-sensitive adhesives (PSAs) are the mainstream choice for heat-applied labels. Crosslinked acrylic adhesives form a three-dimensional network through chemical crosslinking, delivering 30–50% higher adhesion compared to standard acrylic adhesives, making them the first choice for high-adhesion requirements.

Material characteristics:
High crosslink density in the molecular chains provides both strong initial tack (instant adhesion upon application) and excellent holding power (long-term adhesion without lifting or peeling). They offer outstanding weather resistance, operating reliably in temperatures from –20°C to 80°C, with strong resistance to UV exposure and moisture, minimizing edge lifting or adhesive failure caused by environmental changes.

Modification and enhancement options:
Adding tackifier resins (such as rosin resins or terpene resins) increases the glass transition temperature (Tg) of the adhesive and enhances molecular affinity with polar surfaces (e.g., glass and metal).
Incorporating nano-silica improves mechanical interlocking strength, making the adhesive particularly effective on rough surfaces.

Typical applications:
High-end consumer product labels (jewelry, cosmetic containers), identification labels for metal profiles in doors and windows, and outdoor equipment nameplates where strong adhesion is critical.

2. Modified Hot-Melt Adhesives: A High-Adhesion Solution for Low-Temperature Environments

Hot-melt adhesives generate adhesion by melting during application and solidifying upon cooling. Modified hot-melt adhesives (such as EVA-modified hot melts and polyamide hot melts), enhanced with tackifiers and anti-aging additives, offer significantly higher bonding strength than conventional hot melts and perform especially well in low-temperature environments.

Material characteristics:
Extremely strong initial tack with rapid setting, forming a firm physical bond with the substrate surface.
EVA-modified hot melts with paraffin-based tackifiers improve low-temperature flowability and maintain high adhesion even at –10°C.
Polyamide hot-melt adhesives provide superior heat resistance and are suitable for labels requiring high-temperature sterilization, such as healthcare and wellness products.

Key parameters:
Select hot-melt adhesives with a moderate melt index (MI) of 15–25 g/10 min.
A low MI results in poor flow and insufficient wetting of the substrate, while an excessively high MI may cause brittleness, reducing holding power.

Typical applications:
Food and healthcare product labels for cold storage, outdoor building material labels used in winter, and high-speed production lines requiring fast bonding.

3. Rubber-Based Adhesives: High-Adhesion Solutions for Rough and Oily Surfaces

Rubber-based adhesives (natural rubber or synthetic rubber systems) offer excellent flexibility and wetting performance, making them ideal for rough or oily surfaces. Their adhesion can be further enhanced through modification.

Material characteristics:
Flexible molecular chains allow the adhesive to fill micro-gaps on the substrate surface, creating strong mechanical interlocking.
Butyl rubber modification significantly improves oil and solvent resistance, enabling adhesion to oily metal surfaces (such as uncleaned door and window hardware).
Chloroprene rubber modification enhances aging resistance, preventing adhesive tackiness degradation or failure over time.

Limitations and optimization:
Standard rubber adhesives have relatively poor weather resistance and should be used with protective overlamination to prevent UV-induced aging.
Vulcanization and crosslinking treatments improve heat resistance and holding power, extending service life.

Typical applications:
Identification labels for industrial equipment with rough metal surfaces, automotive component labels, and building pipeline markings in oily environments.

II. Substrate Optimization: Upgrading the “Adhesive Carrier”

The substrate acts as the carrier for the adhesive layer. Its surface tension, smoothness, and thickness directly affect coating quality and adhesion transfer.
To enhance overall adhesion, substrates with high surface tension, low shrinkage, and strong compatibility with the adhesive should be selected, with surface pretreatment used to strengthen adhesive bonding.

1. Substrate Material Selection: Prioritize High-Density, Low-Porosity Materials

Higher substrate density reduces adhesive penetration, allowing a more uniform adhesive layer to form, thereby improving adhesion.

Recommended substrates:

PET (polyester) film:
Smooth, dense surface with high tensile strength and low shrinkage (≤0.5%). After surface treatment, surface tension can exceed 52 dyn/cm, offering excellent compatibility with acrylic and hot-melt adhesives. Ideal for high-adhesion heat-applied labels.

PVC film:
Good flexibility and moderate cost. Matte or gloss surface finishes can be applied. When paired with rubber-based adhesives, it provides excellent adhesion, especially for curved surfaces (such as cosmetic bottles or curved door and window profiles).

Coated paper (laminated type):
Standard coated paper is highly porous and allows adhesive penetration, reducing adhesion. PE lamination creates a dense surface layer, improving adhesive uniformity and making it suitable for high-adhesion paper-based labels.

Avoid common pitfalls:
Avoid untreated kraft paper or standard writing paper. These porous substrates absorb adhesive, resulting in thinner surface adhesive layers and significantly reduced adhesion.

III. Surface Processing Enhancement: Precise Control of the Adhesive Layer for Stable Adhesion

Adhesive layer thickness, coating uniformity, and protective lamination directly affect real-world adhesion performance.
To achieve stronger and longer-lasting adhesion, refined process control is required to ensure full contact between the adhesive layer and the substrate surface.

1. Precise Control of Adhesive Thickness: Thin, Uniform Coatings Perform Better Than Excessive Thickness

Greater adhesive thickness does not necessarily result in stronger adhesion. Overly thick adhesive layers are prone to cohesive failure (internal adhesive rupture), which reduces holding power.
A thin, evenly applied adhesive layer allows better surface wetting and stronger molecular interactions with the substrate.