Understand pressure-sensitive adhesives and choose the right one.

Pressure-sensitive adhesive, English name: pressure-sensitive adhesives (abbreviated as PSA). Its bonding process is highly sensitive to pressure, hence the full name. Pressure-sensitive adhesive Pressure-sensitive adhesives are generally not applied directly to the substrate; instead, they are formulated into pressure-sensitive adhesive products—such as tapes and adhesive labels.

Pressure-sensitive adhesive is a viscoelastic material that simultaneously exhibits the viscous properties of a liquid and the elastic properties of a solid; this viscoelastic material possesses both the factors and characteristics necessary to withstand the contact and failure stages of the bonding process.

I. Classification of Pressure-Sensitive Adhesives:

1. Classification by the main constituent of pressure-sensitive adhesives

① Elastic-body pressure-sensitive adhesive

The elastomers used in this type of pressure-sensitive adhesive were initially natural rubber, and over time they have been expanded to include various synthetic rubbers and thermoplastic elastomers. Based on the elastomer employed, these adhesives can be further classified into natural rubber‑based, synthetic rubber‑based, and thermoplastic elastomer‑based pressure‑sensitive adhesives.

a. Natural rubber pressure-sensitive adhesive

This is one of the earliest-developed and still widely produced categories of rubber-based pressure-sensitive adhesives. They are complex formulations based primarily on natural rubber elastomers, blended with tackifying resins, plasticizers, antioxidants, pigments and fillers, and crosslinking (vulcanizing) agents. Because natural rubber exhibits both high cohesive strength and elasticity, and is highly compatible with numerous tackifying resins—resulting in excellent adhesion and superior wetting of substrates—it serves as an exceptionally suitable base material for pressure-sensitive adhesives. Its main drawback is the presence of unsaturated double bonds in its molecular structure, which leads to poor resistance to photo‑oxidative aging. However, through crosslinking and the use of antioxidants, its weatherability and heat resistance can be significantly enhanced. Pressure-sensitive adhesive products of virtually every type can be formulated using natural rubber–based adhesives.

b. Pressure-sensitive adhesives based on synthetic rubber and reclaimed rubber

Pressure-sensitive adhesives formulated primarily from synthetic rubbers such as styrene–butadiene rubber, polyisoprene rubber, polyisobutylene and butyl rubber, as well as chloroprene rubber and nitrile rubber, and supplemented with tackifying resins, plasticizers, antioxidants, and other additives, each exhibit their own distinct characteristics. However, none of these compares in significance to pressure-sensitive adhesives based on natural rubber. Regenerated rubber—particularly those derived from regenerated natural rubber—also delivers commendable performance at a relatively low cost, thereby garnering considerable attention.

c. Thermoplastic elastomer pressure-sensitive adhesive

Thermoplastic elastomers, exemplified by styrene–butadiene–styrene block copolymers (SBS) and styrene–isoprene–styrene block copolymers (SIS), are the primary raw materials for manufacturing hot-melt pressure-sensitive adhesives. Because they do not rely on solvents, hot-melt pressure-sensitive adhesives avoid environmental pollution and offer high production efficiency. In today’s society, where calls for energy conservation and the mitigation of environmental pollution are growing louder, the significance of such adhesives is steadily increasing.

② Resin-based pressure-sensitive adhesive

The resins used in this type of pressure-sensitive adhesive include polyacrylates, polyurethanes, polyvinyl chlorides, and polyvinyl ethers. Among these, polyacrylates are the most widely employed, with their production volume now surpassing that of natural rubber‑based pressure‑sensitive adhesives.

a. Acrylic pressure-sensitive adhesive

Acrylic copolymers, obtained through the copolymerization of various acrylic monomers, constitute the most important class of resin‑based pressure‑sensitive adhesives. Compared with the rubber‑based pressure‑sensitive adhesives mentioned above, they offer numerous advantages: they are colorless and transparent with excellent weather resistance; they typically achieve outstanding pressure‑sensitive adhesion without the need for tackifiers, plasticizers, or antioxidants, resulting in a simplified formulation; and by employing copolymerization and crosslinking, it is possible to tailor these adhesives to meet a wide range of performance requirements. Consequently, over the past two decades, this class of pressure‑sensitive adhesives has advanced rapidly and has now supplanted natural rubber as the dominant type.

b. Silicone and other resin-based pressure-sensitive adhesives

Pressure-sensitive adhesives composed of a blend of silicone resin and silicone rubber exhibit excellent high-temperature resistance and aging resistance, making them an important class of specialty pressure-sensitive adhesives. Their primary application is in the production of various high-end pressure-sensitive adhesive products.

Polyvinyl ethers are one of the earliest-developed classes of resin-based pressure-sensitive adhesives, but their significance has gradually been eclipsed by acrylic pressure-sensitive adhesives.

In addition, resins such as ethylene–vinyl acetate copolymer (EVA), polyurethane, polyester, and polyvinyl chloride can also be formulated into various pressure-sensitive adhesives.

2. Classification by the morphology of pressure-sensitive adhesives

Pressure-sensitive adhesives can be classified into five types: solvent-based, aqueous, emulsion, hot-melt, and calendered.

Emulsion‑type, solvent‑based, and hot‑melt pressure‑sensitive adhesives dominate the market.

3. Whether the pressure-sensitive adhesive’s base polymer is crosslinked

Pressure-sensitive adhesives can be classified into crosslinked and non-crosslinked types. Crosslinked pressure-sensitive adhesives, depending on their crosslinking mechanism, include heat‑cured, room‑temperature‑cured, and photo‑crosslinked varieties. Crosslinked pressure-sensitive adhesives exhibit excellent adhesive strength and are particularly well suited for producing permanent pressure‑sensitive labels.

II. Adhesion Mechanism of Pressure-Sensitive Adhesives

No chemical reaction occurs between the pressure-sensitive adhesive and the substrate; therefore, the adhesive strength between them is primarily governed by intermolecular forces—namely van der Waals forces—between the adhesive’s polymer chains and the substrate.

However, van der Waals forces are short-range interactions that decay rapidly with the sixth power of the distance. To achieve a high‑strength adhesive interface, molecular‑level close contact must be established at the interface, with intermolecular separations typically less than 0.5 nm.

However, solid surfaces typically cannot achieve a level of flatness on the order of 1 nm, which means that pressure-sensitive adhesives must possess excellent wettability to spread uniformly across the substrate, effectively filling in surface irregularities. This ensures that the two adherends make intimate contact through the adhesive, reducing the gap between their surfaces to less than 0.5 nm—within the range where intermolecular forces can develop—and thereby yielding strong adhesive strength.

III. The Relationship Between Application Performance and Structural Physical Properties

During application, pressure-sensitive adhesives primarily exhibit adhesive and release properties, which are typically evaluated using the following parameters: initial tack, peel adhesion, peel strength, and failure mode.

3.1 Initial Adhesion

Initial tack refers to the adhesive force generated by a pressure-sensitive adhesive within an extremely short time upon application of pressure. Good initial tack requires that the adhesive’s polymer chains exhibit high mobility and excellent wettability toward the substrate, enabling rapid wetting of the substrate’s rough surface under relatively low external pressure, thus achieving molecular‑level contact and generating sufficient cohesive strength through numerous intermolecular interactions.

Conversely, polymers with poor segmental mobility exhibit reduced viscoelastic hysteresis and lower initial tack.

From a dynamic perspective, pressure-sensitive adhesives must exhibit liquid-like flow under low-frequency external stress—on the order of 0.01 Hz—in order to increase the contact area with the rough surface of the substrate and thereby enhance adhesive strength. Consequently, initial tack arises only when the polymer’s storage modulus is sufficiently low.

3.2 Adhesive Strength

Adhesive tack refers to the ability of a pressure-sensitive adhesive, once it has bonded to a substrate, to resist sustained shear creep deformation.

In everyday use, you will find that double-sided adhesive tapes gradually lose their adhesion over time and leave residual adhesive marks on the bonded surfaces. This indicates that the primary failure mode of the tape is cohesive failure of the pressure-sensitive adhesive, which in turn suggests that the tape’s peel‑and‑adhere performance is largely governed by the adhesive’s cohesive strength.

Therefore, increasing the storage modulus of the polymer at low frequencies (the frequency of the applied load) can enhance the adhesive’s tack.

3.3 Peel Strength

Peel strength refers to the force required to peel a adhesive layer from a standard substrate at a constant rate and angle, and it primarily reflects the magnitude of the adhesive bond between the pressure-sensitive adhesive and the surface of the adherend.

Researchers have found that the peel strength of all pressure-sensitive adhesives is directly proportional to the ratio of their responses at 435 Hz and 1 Hz. They propose that the quality of a pressure-sensitive adhesive can be characterized by its peel performance at 0.1 Hz and 100 Hz (measured at 25 °C), as well as by its glass transition temperature. Specifically, the 0.1-Hz response correlates with the formation of cohesive bonds, whereas the 100-Hz response reflects debonding behavior. A high‑quality pressure-sensitive adhesive exhibits low peel strength at low frequencies, indicating facile cohesive bond formation, and higher peel strength at high frequencies, reflecting superior resistance to delamination.

3.4 Failure Modes

Failure modes indicate that the failure of pressure-sensitive adhesives primarily occurs in two forms: interfacial failure and adhesive‑layer failure. Interfacial failure arises from poor “stickability” of the substrate and is largely governed by the adhesion strength between the adhesive and the substrate, whereas adhesive‑layer failure is mainly attributable to the adhesive’s low cohesive strength.

In general, during peel testing, the desired failure mode is interfacial failure—leaving no residual adhesive—while cohesive failure of the adhesive layer should be avoided. Therefore, when evaluating peel strength, it is often necessary to account for the transition in the failure mechanism of pressure-sensitive adhesives.

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