Energy curable pressure sensitive adhesives
In a continuously expanding worldwide market for energy curable pressure sensitive adhesives (PSAs), there are specific trends that appear to be occurring.
Environmental pressures are causing industries to turn their collective backs upon solvent technologies of the past, and accept the solventless systems of the present and future. Converters initially began using hot-melt PSA systems, which offered no volatile components, accelerated process times, and a dramatic reduction in cost and required space needed to operate the tunnel ovens of the past.
However, there are limitations inherent to hot-melt PSAs, such as insufficient high temperature resistance and low chemical resistance, which are due to the thermoplastic nature of the hot-melt adhesives. To combat these issues, the molecules in a PSA need to have crosslinking tendencies, which bring about the necessity of energy curable adhesive.
Ultraviolet (UV) radiation energy or electron beams are used to initiate the chemical process that results in the three-dimensional cross-linked network of monomers and oligomers. These energy curable adhesives can be either hot-melts or room temperature liquids. RAHN has done extensive work studying the tendencies and developing 'Starting Point Formulations' for the latter of these two adhesive types. The following paper delves into the basics of formulating these adhesives with a specific end property in the mind of the formulator.
A liquid radiation curable PSA is comprised of four essential components - elastomer, tackifier, diluent, and photoinitiator. The elastomeric component in this case is a combination of a monofunctional and a difunctional aliphatic urethane acrylate oligomer. The high molecular weights and glass transition temperatures (Tg) of well below ambient temperature allow the oligomer component to offer elastic like properties at room temperature, enabling the adhesive to be extended or compressed upon pressure. Its deformability under light pressure allows it to conform to and wet out a substrate (measured as the internal tack of the system). Upon adhesive removal from that substrate, its elasticity allows it to extend greatly before separating, giving the adhesive good peel and adhesion properties.
The tackifying component of this adhesive is a saturated polyester co-resin. Its function is twofold. Generally speaking, the resin has a much lower molecular weight and higher Tg than the oligomer. This difference allows the elastomer greater mobility in the system, maximising both its deformability under light pressure and its elastic behaviour during adhesive removal.
Besides acting as diluent, the reactive monomer plays a role similar to the tackifying resin, allowing for greater deformability of the adhesive due to its low molecular weight, and depending on the monomer, adding its own flexible behaviour to an adhesive. The Tg of the monomer also assists in defining the overall Tg of the adhesive system.
Finally, the photoinitiator's role is vital as it dictates the manner of cure that the adhesive undergoes. A high surface cure photoinitiator will tend to increase shear properties, but destroy the tack of the system. A great through cure product may leave the surface very tacky but exhibit poor cohesive strength due to the surface not being as well crosslinked, resulting in poor shear properties. A good balance of cure properties is important in maintaining proper pressure sensitive adhesive attributes, which is what this starting point PSA study attempts to consider.
The tack of a pressure sensitive adhesive describes the ability of that system to conform to and wet out upon a given substrate with only minimal external force being applied. An adhesive with high tack will wet out and ‘stick’ efficiently to a respective substrate, while low tack will cause poor immediate adhesion. Each pressure sensitive adhesive is unique, and each takes a specific amount of time to completely wet out upon a substrate.
Adhesion of a pressure sensitive adhesive refers to the measure of amount of force required to remove the respective adhesive from a specific substrate. Three components are involved in this process. The first being the extension of the adhesive itself, the second part involves the deformation of the PSA backing substrate during the removal process, and the third and final step being the actual separation of the adhesive from the substrate surface.
The shear property of a pressure sensitive adhesive relates to the cohesive strength of the respective system. A shear force is applied to a controlled area of a pressure sensitive adhesive tape that has been applied to a standard substrate. Initially, a trapezoidal distortion occurs in the adhesive, until a point is reached at which the adhesive fails either cohesively or adhesively.
The properties of pressure sensitive adhesives are influenced by certain factors. Each component in a formulation plays a very important role in determining the end properties of that respective adhesive. Even slight alterations in a formulation can drastically affect the adhesive's performance.
The tack property is affected mainly by the crosslink density or functionality of the adhesive. A lower crosslink density will generally result in a higher tack value. This will allow the adhesive greater flexibility and assist in its conforming to and wetting out upon substrates.
The adhesion property is a balance between tack and shear performance. Low Tg products that have good cohesive properties exhibit high peel performance. These are typified by large molecular weight urethane acrylates with mono or difunctionality and low Tg's between -10degC and -30degC.
The shear performance of a PSA illustrates the cohesion of the system. The fundamental method of enhancing the shear property is to increase the crosslink density of the adhesive. This is accomplished by increasing the functionality of the system's components. In addition, an increase of the overall Tg of the adhesive will assist in achieving this goal. By utilising higher Tg tackifying resins, diluents or oligomers the shear values of a PSA will rise significantly. These trends also hold true for the heat resistance property of an adhesive, resulting in increased SAFT values.
The concept of this study is to not only offer the formulator functional PSA formulations, but to provide an insight into the methods of altering certain properties that will enable a desired endpoint. It is evident that the delicate relationships between the oligomers, co-resins, different types of monomers and photoinitiators must be perfectly balanced in order to achieve a functional and optimised pressure sensitive adhesive. The pressure sensitive adhesive starting point formulas serve as a basic reference tool for formulators. These formulations can be readily modified to specific applications.
Rahn AG is based in Zurich, Switzerland. www.rahn-group.com
"