Labels are big business. A typical label has multiple layers: a topcoat for protection, the face stock, which contains the message in the form of text and/or images, a pressure-sensitive adhesive, and a release liner, which often has a release coating. The release liner and coating are only there to protect the label from sticking to things you do not wish it would stick to. You remove the liner when you wish to apply the label onto your substrate of choice, for example, a bottle containing a drink.

Imagine a label without a release liner and coating, imagine a label that could be activated at the moment you want it to stick to a substrate, a stick-on-demand linerless label.

BonLab has designed and developed a concept and prototype for a sustainable solution: a mesh reinforced pressure-sensitive adhesive for linerless label design.

A mini-emulsion polymerization is a variation on the more conventional emulsion polymerization process in that in the ideal scenario latex particles are formed by monomer droplet nucleation. The monomer droplets are turned into polymer particles. The trick to achieve this is to shrink monomer emulsion droplets to sub-micrometer diameters. For this two ingredients are key, one is a lyophobe, a compound that dissolves in the monomer droplet but does not like to partition into the continuous phase, here water. Typically n-hexadecane is used. This compound suppresses coarsening, also called Ostwald ripening, of the droplets by providing an Osmotic counter pressure. The other essential ingredient is a surfactant which aids to stabilize the large combined surface area of the droplets and keeps then from colliding and fusing (colloidal stability).

The use of molecular surfactants, however, can have negative impacts when the polymer latex is used in formulations and applications as the surfactant can migrate. For example in a clear coating it could lead to uptake of water, causing the transparent coating to become opaque, a phenomenon known as water whitening.

Synthetic polymers in most cases do not have one bespoke molecular weight. A sample typically consists of a large number of individual polymer chains, each having a different molecular weight. The average molecular weights and the shape of the molecular weight distribution are a kinetic fingerprint of how to polymer material was made. The resulting molecular weight distribution dictates physical and mechanical properties.

In free radical polymerizations, four key mechanistic events need to be considered. These are initiation, propagation, termination, and chain transfer. The latter often gets brushed under the carpet in introductory textbooks, but is pivotal.

When one targets polymers of low molecular weight, chain transfer agents are often used. One prominent class of chain transfer agents are thiol compounds, for example n-dodecanethiol. To understand how the molecular weight distribution develops throughout the polymerization process, the ability to determine the reactivity of the chain transfer agent is crucial. This reactivity is often expressed in the form of a chain transfer constant, Ctr, which is the ratio of the rate coefficients of chain transfer and propagation.

Emulsion polymerization is of pivotal importance as a route to the fabrication of water-based synthetic polymer colloids. The product is often referred to as a polymer latex and plays a crucial role in a wide variety of applications spanning coatings (protective/decorative/automotive), adhesives (pressure sensitive/laminating/construction), paper and inks, gloves and condoms, carpets, non-wovens, leather, asphalt paving, redispersible powders, and as plastic material modifiers.

Since its discovery in the 1920s the emulsion polymerization process and its mechanistic understanding has evolved. Our most noticeable past contributions include the first reversible-deactivation nitroxide-mediated radical emulsion polymerization (Macromolecules 1997: DOI 10.1021/ma961003s), and the development and mechanistic understanding of Pickering mini-emulsion (Macromolecules 2005: DOI 10.1021/ma051070z) and emulsion polymerization processes (J. Am. Chem. Soc. 2008: DOI 10.1021/ja807242k). The latest on nano-silica stabilized Pickering Emulsion Polymerization from our lab can be found here.

One quest in emulsion polymerization technology that remains challenging and intriguing is control of the particle morphology. It is of importance as the architecture of the polymer colloid influences its behavioural properties when used in applications. We now report in ACS Nano an elegant innovation in the emulsion polymerization process which makes use of nanogels as stabilizers and allows us to fabricate Janus and patchy polymer colloids.