UV inkjet printing is a rapidly growing technology in which the UK currently holds an industry leading position. The process involves the deposition of ink droplets followed by UV curing that solidifies the ink on the surface via photo-polymerisation. It can be used on almost any surface, and for a wide variety of labelling and packaging applications across a range of sectors, including industrial, automotive, pharmaceutical, food, home and personal care products, etc.
Ink formulation styles are evolving because of a requirement for low viscosity and rapid curing, which is driven by a desire to print at high speeds which compete effectively with flexographic plate printing, and also because of the statutory requirement to minimise the presence of migratable species in food packaging applications.
The photochemical pathways of a few photoinitiators have been studied in the past, but most of those compounds are no longer considered suitable for the latest applications and current food packaging, and these earlier studies were generally in a solvent or high viscosity acrylate that does not represent the composition of a modern UV curable inkjet ink.
To establish the photochemical pathways and reaction rates of contemporary photoinitiators in current and future UV inkjet formulations. In particular, to study the consequences of changing from the monomeric photoinitators that were used in the past to the new generation of photoinitiators, which are usually also multifunctional, the use of low vs high molecular weight acrylates in the curing process, and the effect of non-reactive solvent.
The overall plan is to develop a detailed mechanistic understanding that can lead to the design of new formulations that achieve a faster and higher degree of polymerisation, and a lower yield of unwanted photochemical by-products.
A range of photoinitiators will be studied by time-resolved techniques, in which the molecules are pumped by pulsed laser excitation and the excited states and other intermediates are probed using methods such as UV-vis absorption or emission, infrared absorption or NMR spectroscopy. The photoinitiator excited state lifetimes and photophysical and photochemical decay pathways will studied; the excited states cleave to form radicals that initiate the curing process, and the rates of reaction of those radicals with acrylates or other substrates will be deduced. The effects of oxygen or other components in formulations that may quench the excited states to lower the yield of radicals, or terminate the radical polymerisation reactions, will also be studied. This general approach is well established in the literature, and these studies will extend across a range of timescales; they will be complemented by steady-state studies, product studies, and computational chemistry.
In addition to studies at York, practical outcomes for formulation and curing performance will be studied using facilities available at Domino, near Cambridge. For example, rates of cure can be measured on a sub-second timescale at Domino by infrared, rheological or dielectric methods, double bond conversion may be estimated by infrared absorption, and the content of untethered molecules can be quantified by extraction tests. Facilities to print under test conditions are also available at Domino.
Studies of the photochemical pathways of the new generation of polymeric photoinitiators will be novel because there are very few published studies to date, and there are currently no systematic mechanistic studies of the consequences of changing the rest of the formulation.
The student will receive training on photochemistry and a variety of time-resolved, steady-state and computational techniques at York. Training on market relevance, UV ink formulation, and cure evaluation techniques will be provided by Domino.
All Chemistry research students have access to our innovative Doctoral Training in Chemistry (iDTC): cohort-based training to support the development of scientific, transferable and employability skills: https://www.york.ac.uk/chemistry/postgraduate/idtc/
The Department of Chemistry holds an Athena SWAN Gold Award and is committed to supporting equality and diversity for all staff and students. The Department strives to provide a working environment which allows all staff and students to contribute fully, to flourish, and to excel: https://www.york.ac.uk/chemistry/ed/
. This PhD project is available to study full-time or part-time (50%).
This PhD will formally start on 1 October 2020. Induction activities will start on 28 September.