Sustainable solvent and dispersions selection

Afera explored “tapes and sustainability” in 1st Session of TechSem 2021

Afera’s 15 April online Session, part of our 4-part 9th TechSem, was moderated by Afera Sustainability Working Group Chairman Martijn Verhagen, who is also principal scientist at Lohmann GmbH & Co. KG, and included 2 presentations followed by a Q&A section. During Afera’s 90-minute webinar, Dr. Jose Ramon Leiza presented his high bio-content waterborne polymeric dispersions as PSAs, and Dr. Fergal Byrne discussed his new, greener molecule, TMO, which can replace the solvent toluene across the PSA industry. Below, each presenter has contributed an article on his subject:

High bio-based-content acrylate waterborne pressure sensitive adhesives with excellent performance and novel functionalitie

by Dr. Jose Ramon Leiza, professor of chemical engineering at the School of Chemistry of the University of the Basque Country (Spain)

The main objective of this work was to develop high bio-content waterborne polymeric dispersions with competitive performance for their application as pressure sensitive adhesives (PSAs). For this purpose, both commercial and non-commercial biobased monomers having special features were employed.

As a first attempt, 72% bio-content waterborne PSAs containing the commercial biobased monomers 2-Octyl acrylate (2OA, coming from castor oil) and isorbornyl methacrylate (IBOMA, coming from pine tree resin) were synthesised. It was observed that the direct substitution of the oil-based monomers by the renewable counterparts did not provide the same adhesive performance. Consequently, the fine tuning of the microstructure by adjusting the amount of hard bio-based monomer (IBOMA at 15 wt%) and gel content (~60%) yielded a co-polymer with similar peel resistance and loop tack, better SAFT (135˚C vs 70˚C) and 45-times-higher shear strength than the pure, oil-based PSA.
Aiming to solve the conflict between the degree of bio-content and the final performance required for adhesive applications, piperonyl methacrylate (PIPEMA derived from sassafras oil and black pepper) was synthesised and incorporated by emulsion polymerisation in PSA compositions, replacing hard co-monomer IBOMA. It was found that 15 wt% of PIPEMA provided a better viscous behaviour of the adhesive fibrils, maintaining enough stiffness and yielding the maximum dissipation energy value. In addition, benzodioxol structures present in the piperonyl functionality allowed tuning the adhesive performance (increasing cohesion without substantially damaging adhesion) when the adhesive was irradiated with UV-light. Solid state 13C NMR and rheological studies confirmed the formation of a crosslinked network via generation of conjugated structures that increased the solid-like behaviour. It was found that 15 minutes of UV-light curing provided the best balance of adhesives properties, overcoming the performance of an oil-based PSA and keeping a bio-content of 71%.

In an attempt to improve the adhesion and promote the removability of the adhesive tape in water, an isosorbide-based methacrylate mixture (ISOMAraw, coming from glucose) was synthesised and incorporated into the already studied biobased PSA formulations. The monomer mixture consisted of isosorbide dimethacrylate and isosorbide 5-methacrylate (ISOMA) in a 4:1 ratio. The latter one can be isolated by purification of the mixture. It was found that low percentages (1 wt%) of ISOMAraw/ISOMA enhanced both the flexibility and the cohesiveness of the adhesive fibrils because of the presence of crosslinking species and supra-molecular interactions (hydrogen bonding). Furthermore, the low percentage of this sugar derivative monomer promoted removability in water of the adhesive tapes in less than 40 minutes at room temperature, reducing this time by almost 4 times if the temperature is increased to 65˚C.

In view of the results provided by the isosorbide derivative monomers, and aiming to obtain an excellent adhesion and switching-off at the minimum film thickness, bio-based alkali soluble resins (ASRs) were synthesised and used as electrosteric stabilisers for the production of high solids content waterborne PSAs. The use of ISOMAraw in the resin composition not only allowed the grafting to the polymer chains in the polymer particle, but also the enhancement of the mechanical properties as the hydrogen bonding density increased. Regarding this, the existence of a dynamic physical crosslinking provided a good balance of the polymer network viscoelasticity, which was deeply studied by rheological analysis. The nature of the ASR-fortified waterborne PSAs promoted the complete detachment of the adhesive tapes from the glass substrate in basic media in less than 20 minutes at room temperature.

Figure 7.1. Most relevant probe tack curves of biobased waterborne PSA formulations together with the one coming from petroleum-based composition (2Ref). Note that * and ** make reference to 0.025 wbm% and 0.05 wbm% of 2EHTG (chain transfer agent), respectively.

About Dr. Leiza
Dr. Jose Ramon Leiza is a professor of chemical engineering at the School of Chemistry of the University of the Basque Country (UPV/EHU). He graduated in chemistry in 1987 and obtained his PhD in chemical engineering in 1991 at UPV/EHU. He spent sabbatical years at Lehigh University (USA) as a visiting research associate (1994-95) and Queen’s University (Canada) as a visiting professor (2004-05). Dr. Leiza’s current research interests are focused on the following topics: polymer reaction engineering aspects of polymerisation in dispersed media, waterborne polymer/inorganic hybrid nanocomposites for corrosion protection coatings, polymerisation of water-soluble monomers as additives for concrete, renewable resource monomers and their application for the production of green adhesives and coatings, and electrospinning of polymer latexes.

Greener solvents for PSA production

by Dr. Fergal Byrne, postdoctoral research associate at the University of York (U.K.) and co-founder of Addible, a research consultancy specialising in sustainable plastic technologies

Hazardous solvents, such as toluene and hexane, are commonly used in the pressure-sensitive adhesive (PSA) industry. The properties that make these solvents useful also make them difficult to replace with greener alternatives. They are volatile and non-polar hydrocarbons; their volatility means employee exposure can be high if used in an open system; their low polarity means uptake into lipophilic cell membranes is relatively high; and their composition only of carbon and hydrogen means very few changes to functionality can be made without altering their physical properties (boiling point, polarity, etc.). 

Ethers are a class of molecules with a C-O-C functional group in their structure, which have been found to possess many of the desirable properties of hydrocarbons (low polarity, volatility). However, there is a big issue with ethers that prevent their use in some applications: They form explosive peroxides in ambient conditions due to a weakness in their structure. The weakness is the presence of a labile hydrogen atom adjacent to an oxygen atom which can react with oxygen radicals in air. Removing this labile hydrogen atom and replacing it with a more strongly bonded methyl group has allowed us to design out the issue of explosive peroxide formation in a new molecule, 2,2,5,5-tetramethyloxolane (TMO).

This new molecule, TMO, is an ether by definition of possessing C-O-C functionality, but its properties and behaviour have been found to be more like hydrocarbon solvents in many applications. Its boiling point of 112˚C is almost identical to that of toluene (111˚C), it possess similarly low polarity to toluene, and its flammability properties (autoignition temperature and lower explosion limit) are similar to toluene and far superior to traditional ethers. Its in-vitro and in-silico toxicity profiles are very promising, while work is ongoing on its production from biomass to make it truly green. Finally, its atmospheric breakdown is far superior to toluene, meaning potential issues with ozone depletion and smog formations are reduced significantly.

Of particular interest is the use of the new molecule in radically-initiated polymerisations for PSA production, in which the reactivity of the labile hydrogen atom in traditional ether molecules results in low Mw polymers due to chain transfer. TMO does not have this issue but instead yields adhesion, cohesion and tack properties very similar to toluene for the production of a range of polyacrylate polymers produced by Nitto. As such, TMO is likely to be an ideal candidate for replacing toluene across the PSA industry.

About Dr. Byrne
Dr. Fergal Byrne is a postdoctoral research associate at the University of York. Nitto Belgium funded his PhD at the University of York, that aimed to find a greener alternative to toluene for PSA production. Since then, he has worked on the ReSolve project, funded by BBI-JU, which aims to find and scale up the production of alternatives to both toluene and NMP, 2 common industrial solvents. In May 2020, he co-founded Addible, a research consultancy specialising in sustainable plastic technologies.

Q&A section

High bio-based-content acrylate waterborne pressure sensitive adhesives
When asked if Dr. Leiza had managed to locate commercially available bio-based methacrylate sources or still depended on fossil-based feedstocks for the methacrylate part, he indicated that they still depended on the latter, which makes up 70% of the monomers. Perhaps there will be a shift and 100% bio-based methacrylic monomers might become available, if bio-based acrylic acid and methacrylic acid would be commercialised, but at the moment they are not commercially available, although patents exist and some companies have undertaken projects on launching these carboxylic monomers. Dr. Leiza said this is not something they are working on at the moment.

Regarding the mechanism of UV tunability, this lies exclusively with a UV source, which must be irradiated at 254 nm where absorption is maximum for the benzodioxol groups of the PIPEMA monomer. Visible light is not sufficient (in reasonable curing times) to alter the field properties of the adhesive film. 

One of the weaknesses of dispersions lies in initial peel force. There is a certain build-up time because of the particulate structure, so film formation is a bit problematic regarding initial peel. With alkali-soluble resins (ASRs), Mr. Leiza indicated that in comparison with the ASRs they used, the ASRs used in the coating industry usually have a Tg well above room temperature. These high-Tg ASRs will create a shell in the particles that will not facilitate the interpretation of the polymeric chains and good coalescence during film formation. This is overcome with the soft ASRs used in this work.

Mr. Leiza said they have not tested the candidate polymers for ageing properties in specific applications or real situations, including temperature and interaction with face stock (such as high-calcium-carbonate-containing papers). The only property for which Mr. Leiza has checked for is humidity, in order to establish the removability of the PSAs. The adhesive films were put in a chamber with over 90% humidity, and they performed as well as when they are exposed to normal conditions of humidity. 

Is there any work towards the use of such polymers in a 100% solids adhesives formula, for example, in butyl-based products? In principle, they are usable in any polymerisation techniques that go through or via radical polymerisation. So there are no constraints in this sense. 

The adhesive dry film thickness is 100 microns.

Having performed research at an academic level, Mr. Leiza said that he is happy to share their bio-based adhesive technology with people or companies that would be interesting in testing them. 

Greener solvents for PSA production
Dr. Byrne was asked about the combining of toluene with isopropanol and how both are recovered in solvent recovery systems and reused. Could TMO be recovered in the same way? Based on the boiling point of TMO, he doesn’t see why not, but it would require some testing.

During the Nitto test run, the adhesion had higher adhesion but lower cohesion. Dr. Byrne indicated that all the TMO was evaporated from the adhesive, but they do not have an explanation for the lower cohesion. A future project would entail learning why this is happening and perform some optimisation of the adhesive.

If you see stronger legislation regarding the use of toluene, would there be a point in the near future at which price is no longer important for greener toluene adhesive alternatives? It is hard to know. There are restrictions on use in some applications already, and many solvents in general are under pressure now (e.g. NMP and DMF have been banned outright for many applications). Probably the reason toluene has not been as regulated yet is that there are no alternatives on the market for many applications; TMO is the first. Toluene is a workhorse solvent, so banning it would kill a lot of industry. TMO can replace it in many applications, but not all, behaving differently to toluene in strong Bronsted acids. As soon as alternatives are used for toluene in many more applications, the chemical will probably fall under much more regulatory pressure.

In terms of cost, all bio-based molecules are going to experience the bottleneck in biomass pre-treatment in production. It is quite easy to take food resources like sugar cane or cornstarch, which is used in bioethanol made in Brazil and the U.S., because the sugars are there for fermentation or any chemocatalytic treatment. The land used to produce them, however, is better dedicated to food production and food waste or lignocellulosic biomass utilised to produce ethanol. At the moment, the bottleneck lies in doing the pre-treatment on the lignocellulosic biomass, that is adding to the cost, as the technology has not yet been developed enough for this. As companies are now starting to implement new technology in this area, the price will come down. And as regulation of toluene starts to increase, you will see a trade-off between cost and regulation as there will be a squeeze on both ends.

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