Read time: 13 mins.
Key highlights:
- An R&D hiccup: Assessing the disruption caused by TPO's new toxicity classifications.
- Gold diggers: How leading photopolymer AM companies are responding.
- High concern: Emphasising the importance of safety protocols in AM.
This article was first published via the Additive Insight newsletter on November 1st, 2024.
On October 19th, 2023, the European Chemical Agency confirmed Diphenyl(2,4,6-trimethylbenzzoyl)phosphine oxide – better known as TPO – had been hit with a reclassification.
The chemical compound, used in photopolymer materials for Digital Light Processing 3D printing technologies, was now officially considered a substance of very high concern, owed to its new reproductive toxicity 1B (repr 1B) and skin sensitisation 1B (skin sens 1B) classifications.
It raised some potential issues in the supply and usage of what is one of the fastest growing 3D printing technology sectors. Are materials with TPO safe to use? Will they be banned entirely? What are DLP materials suppliers doing about it?
Twelve months on, TCT Magazine has spoken to a number of photopolymer 3D printing experts to understand exactly what impact the ECA’s reclassification of TPO is having on the DLP sector.
The gold standard
DLP 3D printing emerged at the turn of the millennium, with a technology traditionally leveraged in the digital cinema and entertainment industry now being used to cure liquid resin into additively manufactured parts.
Since the mid 1980s, those looking to print polymer parts with high resolution and accuracy would do so using a stereolithography system, whereby a UV laser selectively cured the cross-section of layers to produce parts. But, utilising digital light processing technology instead of a laser, EnvisionTEC was blazing a trail of printing parts layer by layer in a smaller form factor than most were used to.
The company filed its first patent in 1999, released its first machine three years later, and paved the way for the likes of Carbon, Photocentric, Azul3D, Axtra3D, Origin and even 3D Systems with its Figure 4 technology to follow.
Though DLP technologies were promising similar benefits to traditional SLA technology, the chemical make-up of the polymers being printed were different. In SLA, machines were using a laser beam at 355-365 nanometres, with cationic photoinitiators packed into the material formulations. But upon the emergence of DLP technologies, light was being emitted at 405nm, which required the materials to use different photoinitiators.
“The obvious ones were from a group termed phosphine oxides and, although 405nm was at the end of their spectrum of activity, they were active and colourless,” Photocentric Managing Director Paul Holt told TCT. “Phosphine oxides such as 2,4,6-trimethylbenzoyldiphenylphosphine oxide were very efficient photoinitiators that absorbed in the long wavelengths of between 380-410nm and were being used industrially, particularly in thick coatings where photobleaching leads to good depth cure.”
Forward AM R&D Manager Marta Ruscello describes the TPO compound as the ‘gold standard,’ with it helping DLP to become a bona fide production tool thanks to its high initiation, no yellowing and moderate price. The technology has enabled some of AM’s flagship applications, such as hearing aids, dental aligner moulds and footwear midsoles, with market research firm Dataintelo suggesting the DLP 3D printing market will become a billion-dollar market by 2032, up from its current value of 202 million USD.
October 2023, however, presented a potential spanner in the works.
