The initial batch of reagents arrived – I was quick to don my lab coat and get mixing :-)
But first a little about the chemistry of the whole process…
Referring to a paper by Joseph Muskin, Matthew Ragusa and Thomas Gelsthorpe, titled “Three-Dimensional Printing Using a Photoinitiated Polymer” they outline a similar reaction very well and also demonstrate a rudimentary projector printing technique. So a directed thanks to them.
The composition of my solution contains:
- Poly (Ethylene Glycol-400) Diacrylate
- Phenylbis (2,4,6 Trimethylbenzoyl) Phosphine Oxide
- Bromothymol Blue Dye
UV light incident upon the photo-initiator (Phenylbis (2,4,6 Trimethylbenzoyl) Phosphine Oxide) is absorbed and free radical species are generated.
The generated species then react with the Diacrylate monomer to initiate polymerisation.
The selected dye was “Bromothymol Blue” chosen for two reasons:
- Acceptable level of light absorbance circa 405nm.
- The colour of Bromothymol Blue dye solutions are highly pH dependant, giving some degree of colour control of the produced parts. In acidic solutions, the dye is bright yellow; neutral solutions (pH 7) the dye is green, and in bases is a deep blue.
The polymer solution is mildly acidic, giving a nice yellow colour to parts. Therefore an aim will be to investigate the necessary amounts of a base (such as NaOH) required to be added to the polymer solution to generate these three different possible colours for part production. It is straightforward to calculate the required amount of a base to be added, however I will revisit this at a later date as it is not a primary goal.
The initial batch produced will be based upon the Muskin et. al formula of 98ml of monomer with 2g of photo-initator.
The solubility of the photo-initiator in the diacrylate is poor, although 2g does eventually dissolve. If I find myself making batches more frequently, I will invest in a magnetic stirrer to speed up the process!
All forms of the polymer solution are stored in amber glass bottles to prevent ambient UV light spoiling the solution.
Initial tests show the prepared solution works well, and early results are very promising. Processing times appear very fast with the current laser prototype. In the coming weeks this will be quantitatively assessed, and a decision made as to wether or not to proceed with the planned upgraded laser design (which would be approximately double the laser power).
Further work: Experiment with dye concentration effect upon slice thickness achieved under standard conditions; and to investigate slight pH changes for creation of yellow, green and blue useable polymer solutions.