Acrylic polymers are the most commonly used self-adhesives, having replaced many compounded natural or synthetic resins. Acrylic offers oxidative stability and resistance to some UV radiations, and are inherently tacky and can be adapted to a variety of uses by copolymerizing two or more monomers, including non-acrylates such as vinyl acetate, N-vinyl pyrrolidone, or N-vinyl caprolactam. The composition of acrylic polymers that are inherently pressure-sensitive is a combination in the polymer chain of soft Tg (low glass transition temperature), hard Tg (high glass transition temperature) and various functional monomers. Normally acrylic PSAs contain typical hard and soft monomers, as well as the types of functionalities that can be incorporated into the polymer chain. Even-though, the heart of their performance lies a delicate balance between seemingly opposing properties, largely dictated by the clever manipulation of their glass transition temperatures (Tg).
What is glass transition temperature (tg)?
Glass Transition Temperature (Tg), is the point at which a material changes from its “glass – like” into a more flexible, rubbery compound.

What happens is that at a specific temperature, the material softens. The plastic quite suddenly goes from being hard and glassy to a soft and rubber-like. If the temperature then decreases again, the plastic/rubber goes back to its previous state. This is true for all the curing adhesives that are commonly found within adhesive technologies, including epoxies, polyurethanes, silicone, and acrylates.
Life above and below tg: the material behavior
The glass transition temperature for an adhesive is the temperature where the modulus of elasticity (or the flexibility) changes drastically. As the temperature reaches this point, the adhesive goes from being glassy to rubbery. With increased temperature, molecules become increasingly mobile and can move around with much more ease. Below Tg the polymers in the adhesive are in a rigid state, effectively “frozen” in place. Only short segments of the polymers can move and within a very limited space. The low mobility of the polymers means that it cannot adapt as easily to distortion – the material is more brittle and will break instead of flexing. Properties that change at Tg include flexibility, creep strength, density, hardness, cohesive strength, modulus, chemical resistance and volume.

THE SIGNIFICANCE OF Tg IN ACRYLIC PSAs;
For acrylic PSAs, achieving the right level of “stickiness” (tack and peel) while maintaining internal strength (cohesion and shear resistance) hinges on carefully selecting and combining monomers with different Tg values – the “soft” and “hard” building blocks

The Role of Soft Monomers: Embracing the Rubbery Side
Soft monomers are the unsung heroes responsible for the immediate grab and conformability that define a PSA. These monomers, when polymerized into a homopolymer, exhibit a low glass transition temperature (typically below -20°C). This low Tg means that at room temperature and even lower, the polymer chains have significant mobility, allowing the adhesive to:
- Wet out the substrate: The soft, flexible nature enables the adhesive to flow and make intimate contact with the surface, even microscopic irregularities.
- Provide tack: The mobility of the polymer chains allows for quick and temporary bonding upon light pressure.
- Offer good peel adhesion: The ability of the adhesive to deform and dissipate energy contributes to its resistance to being peeled away from a surface.
The commonly used monomers that contribute to a soft Tg in acrylic PSAs are 2-Ethylhexyl Acrylate and Butyl acrylate.
The Strength of Hard Monomers: Adding Cohesion and Durability
While soft monomers provide the necessary tack, hard monomers (Table 1) step in to provide the backbone and resilience of the PSA. These monomers form homopolymers with a high glass transition temperature (typically above +20°C). Incorporating them into the acrylic copolymer network imparts:
- High cohesive strength: The stiffer polymer chains resist internal flow and deformation under stress, preventing the adhesive from splitting or leaving residue upon removal.
- Improved shear resistance: The adhesive can withstand static loads applied parallel to the bond line without creeping or failing.
- Increased temperature resistance: The higher Tg contributes to the adhesive maintaining its properties at elevated temperatures.
- Enhanced durability: Hard monomers can improve the overall resistance of the PSA to environmental factors like UV radiation and oxidation.
The Art of Formulation: Finding the Sweet Spot
The magic of acrylic PSAs lies in the copolymerization of these soft and hard monomers, often in conjunction with functional monomers that introduce specific properties like crosslinking sites or enhanced adhesion to particular substrates. By carefully adjusting the ratio of soft to hard monomers, adhesive formulators can tailor the Tg of the resulting copolymer and fine-tune the PSA’s performance characteristics.
- A higher proportion of soft monomers will generally result in a more flexible, tacky adhesive with good peel adhesion but potentially lower cohesive strength and shear resistance.
- Increasing the amount of hard monomers will typically lead to a stiffer, more cohesive adhesive with higher shear resistance and temperature performance, but potentially lower tack and peel adhesion.
In conclusion, the interplay between soft and hard Tg monomers is fundamental to the design and performance of acrylic pressure-sensitive adhesives. By understanding and skillfully manipulating these building blocks, formulators can create a vast array of PSAs tailored to meet the diverse demands of modern applications, from everyday tapes to specialized solutions in automotive, electronics, and healthcare.