Water-Based Acrylic Latexes are excellent Pressure Sensitive Adhesives when Branched

Water-based pressure sensitive adhesives (PSAs) are typically made by emulsion polymerization using a low glass transition temperature base monomer, such as n-butyl acrylate or 2-ethyl hexylacrylate, together with a range of functional comonomers. Typically these include a high glass transition temperature comonomer, such as styrene or methyl methacrylate and monomers that can promote wetting and undergo secondary interactions such as (meth)acrylic acid.

A golden rule for good adhesive performance is that the polymer latex particles must contain a certain fraction of gel, that is, cross-linked material. This typically is 50-70%. This gel content optimizes the balance between tack and cohesive forces within the adhesive.

Schematic of the branched polymer chain architecture approach to deliver good performance in pressure sensitive adhesives (PSAs).

In our paper, led by PhD researcher Emily Brogden and published in the RSC journal Polymer Chemistry, we challenged this view. Prof. dr. ir. Stefan Bon says: “We designed a range of polymer latexes with low gel content. To balance the adhesive forces, we introduced a branched polymer chain architecture instead. We show that excellent water-based acrylic PSAs can be made using this approach.”

Moreover, the chemical composition of our PSA was designed with sustainability in mind. The base monomer selected was the bio-based 2-octyl acrylate, the high Tg component bio-based 2-isobornyl acrylate. The other monomers and chain transfer agent show promise to be or become fully bio-based.

The paper entitled “Water-Based Polymer Colloids with a Branched Chain Architecture as Low-Gel Pressure-Sensitive Adhesives” has gold open access and can be read here:

https://doi.org/10.1039/D4PY00522H

Grafting polymers to graphene oxide goes better through branching

Single-layer graphene is interesting as a flexible 2D material, with xy-dimensions variable up to a centimetre in length and a z-thickness of a single carbon atom. It conducts heat and electricity, has excellent mechanical strength, and is impermeable to gases except hydrogen gas. Its drawback: how to disperse it in a liquid. When you try to do this flexible sheets of graphene tend to stack as a result of attractive van der Waals interactions, making it virtually an impossible material to disperse as single sheets.

As a compromise, people came up with graphene oxide (GO), it’s oxidised analogue. The presence of oxygen atoms containing functional groups, such as hydroxy, epoxy, carboxylic acid, ketone, or aldehyde, provides it with a polarity, which makes it possible to disperse GO as single sheets in polar solvents, such as water or DMSO at low concentrations in the absence of electrolyte or other colloidal particles. The drawbacks: graphene oxide sheets have holes. Whereas graphene can be seen as a pristine, flexible bedsheet, graphene oxide is a bedsheet attacked by moths. This makes the material less suitable for barrier property reinforcement. Graphene oxide is also less electrically conducting as oxidation has partially destroyed its conjugation. Nevertheless, it still shows great potential for a range of applications, for example, those that need mechanical reinforcement, such as polymer nano-composites, but only if we can enhance its dispersibility in liquids or polymer melts.

Grafting polymer chains from the surface of GO allows it to be dispersed more easily. This can provide additional electrostatic and steric stabilization against sheet stacking at higher concentrations in less polar solvents or contain soluble polymer chains or non-interacting particles that can trigger depletion flocculation.

There are several approaches to accomplishing this, each with advantages and disadvantages. A particular challenge is linking a sufficient mass of polymer chains to the GO’s surface. In our gold open access paper recently published in the RSC journal Polymer Chemistry, we report a way to address this, employing radical polymerization reactions that lead to a branched polymer architecture. Methacrylate-based macromonomers made by catalytic chain transfer polymerization were used together as molecular weight regulators with dimethacrylates as crosslinkers to introduce branching points.

You can read the paper here:

https://doi.org/10.1039/D4PY00300D

BonLab designs a stick-on-demand adhesive for linerless labels

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. The idea was worked out by Emily Brogden and prof. dr. ir. Stefan Bon, in collaboration with UPM Raflatac Oy, a global supplier of label materials for branding and promotion, information and functional labelling (patent application: WO2023105120A1). The complete study, which was done at the University of Warwick, is now published in the new journal RSC Applied Polymers.

A water-based pressure sensitive adhesive (PSA) is contained in a hard 3D mesh structure. This replaces the standard PSA layer of a label. It provides structural support upon storage and prevents the adhesive layer from sticking. In adhesive jargon: it shows excellent blocking resistance.

Upon short heat treatment, for example, upon contact with a hot sterilized glass bottle, the mesh will soften, allowing for adhesion to occur.

The 3D mesh structure was generated using a binary mixture of water-based polymer dispersions. One component is a standard PSA latex, and the other is a polystyrene polymer colloid. Film formation of the blend followed by a short annealing time led to the desired phase-separated mesh structure, as displayed in the figure below.

Visualization of the mesh structure in an acetone etched PVAc-PS model film (Film 1) a-c. a) SEM image of the top surface, b) a tilt adjusted image of the cross section after cutting away a section using FIB SEM with a film height of approximately 20 𝜇𝑚, and c) micro-CT 3D reconstruction of the etched film. Visualization of a similar mesh structure in a PSA-PS film (Film 2) d-f. d) SEM image of the top surface of Film 2, annealed for 60 𝑚𝑖𝑛 e) SEM image of the top surface (prepared similiarly to Film 2 but with only a 19 𝑚𝑖𝑛 annealing time) with a clean crack after freezing in liquid nitrogen and f) a cross section of Film 2, where the PS is tagged with hostasol methacrylate (green), taken using confocal microscopy (the original top surface of the cast film is at the top of the image).

Prof. dr. ir. Stefan Bon says: “I am immensely proud of the hard work done by all people involved to get the idea from a sketch on a piece of paper to a working prototype. A big thank you to PhD researcher Emily Brodgen, the paper's first author, who has been the driving force behind turning a concept into reality and has worked tremendously hard on the project for the last couple of years. The work has received excellent feedback from the academic and industrial scientific communities, with Emily winning multiple prizes for best talk and scientific poster.”

The paper can be accessed from the RSC Applied Polymers website:

https://doi.org/10.1039/D3LP00224A

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