Introduction to Photocurable Resin

Nancy Zhang first joined Carbon3D as a staff research scientist. While her first years were primarily focused on tailoring resins for a project with Adidas, she has maintained a versatile role throughout her career. After the successful launch of the Adidas midsoles, Nancy moved to a managerial position in R&D where she focuses on elastomer development and formulation. Her sharp enthusiasm and remarkable knowledge base really shows that I was talking to the right person to learn about resins. She is the type of person who knows what makes a resin, how to achieve ultimate elasticity, strength, biocompatibility, “printability” and ensure that the resin is photocurable, or potentially, recyclable. As one can imagine, that’s a lot of demands for one material. As lead of Material Characterization, it’s no wonder she describes her work as “brain gymnastics”.

Prioritizing the mechanical properties and printability of a material resin is the tip of the iceberg in material characterization. Additive manufacturing provides a platform for versatile product manufacturing which implies material versatility along with it.

Comments from the Author: I first envisioned this article as an interview which I held with Nancy Zhang, an R&D Manager of Material Characterization, several months ago. Nancy helped guide a significant portion of my ongoing additive manufacturing research and while I don’t plan to delve deep into the chemistry, I will include enough to discuss how to make photocurable resins for layer by layer additive techniques. Something I’ve taken particular notice to is that there seem to be few papers that make general formulation assessments tied with the mechanical behavior of photopolymer resins in lithography and it could tie means to the need for a thorough review paper.

“Let’s say I’m selecting a material for a car product. I only need to look at the car and the function of the component. I don’t have to think about the car, where it drives, everything outside the car, and being thrown around in the trunk.”

Layer-by-Layer Lithography Printers

The premise of the SLA/DLP process is to solidify photocurable resin using a light source and sequentially lift the solidified part out of a resin reservoir (vat). SLA (stereolithography apparatus) processes can be top-down systems, where a scanning laser is above the resin reservoir tracing the layer of solidified material. Similarly, DLP (digital light processing) processes are typically bottom-up, in which the light source (a projector) is emitting light into a shallow reservoir from below. When a layer is cured, the component is lifted out of the vat, and another layer is cured.

SLA/DLP processes are similar to FDM (fused deposition modeling) methods only by the object of solidifying material layer by layer. Their similarities stop there. Once a part is completed on an SLA/DLP process, there is typically a second step for post processing. Post-processing is used to clean up any uncured material from the surface of the part. When a part is cured, the excess material can be washed away using acetone or other solvent and placed in a UV or thermal oven to cure any remaining material without introducing new resin.

Lithography is another type of additive manufacturing, and differs from SLA/DLP and FDM methods, as it does not solidify material layer by layer; rather, lithography solidifies a part volumetrically. Lithography printers have demonstrated 30-second print times, micro-scale resolution, and heightened material compatibility for printing with softer materials such as polymeric resins, acrylates, and urethanes to generate biocompatible materials. (1)

Many forms of lithographic printers (see image) have a selective viscosity range for rapid printing. The liquid resin should be able to move quickly throughout the vat. That is, after each layer is cured and raised to the subsequent layer, the surrounding resin should fill the void created from the previous layer. This becomes a problem when printing with higher viscosity resins, as it designates slower print speeds to allow the resin to flow. Some estimates in literature designate a resin viscosity below 5 Pa/s for rapid printing. (1,2) This dependence on resin viscosity for layer-by-layer printing suggest the development of printing techniques which are independent of resin flow. (3)*

Within research into additive manufacturing processes, the majority of research has investigated the mechanical properties of AM parts by considering different process parameters such as post-curing time, layer thickness, and orientation. Most of these studies have investigated the material properties of additively manufactured parts by experimentation, but do not provide an accurate prediction of the mechanical properties of the resin before cure. This information is important, since there are a number of parameters that must be taken in to consideration during resin formulation that affect the over material character. The amount of photoinitiator, oxygen presence, apparent viscosity, the curing dose–these are just a few variables that can be adjusted in formulation that significantly impact the final cured part. (4)

*This is the idea behind computed axial lithography (CAL), which I intend to write about in subsequent articles.

“Imagine Printing a Spring”

The viscosity of a photocurable resin has shown to be positively correlated to the elasticity of the post-cured material. High viscosity resins yield parts with higher “green” strength, the strength measured after a stage 1 cure. When a resin is developed, formulators may plan for lower viscosity or increased green strength, though the latter involves an extended print time, which can inhibit the speed of production.

Printing elastomers is more difficult than printing rigid polymers because of the positive correlation between viscosity and green strength. Elastic resins are characteristically less viscous than high strength polymers, which leads to “sticking” at the print interface from surface tension.

“With [additive manufacturing], the idea is that a material can be printed into any object, quickly and easily, and that object can be used in any environment.”

Photoinitiators for Photopolymerization

Photopolymerization is a very general term that relates to any light induced polymerization reaction where an initiating molecule (photoinitiator) induces a chain reaction which combines a large number of monomers or oligomers into a polymer chain. These reactions are referred to as free radical reactions and carried out in three dimensions, such that multiple polymer chains may stack on top of each other (cross-linking) and produce a polymer network. Most resins by themselves are not reactive to light. The photoinitiator has a crucial role in absorbing light energy and reacting with an available element which begins the chain reaction between the resin monomer/oligomer units. (5)

Oxygen Presence

Oxygen is known to be a reactive element, which leads to detrimental effects in free radical polymerization. During a photoinitiation reaction, oxygen will decrease the yield of the initiating species as it bonds with other radicals to produce highly stable compounds that inhibit the growth of a polymer chain. Late studies have indicated that for printing in high viscosity resins, reoxygenation at the layer is much slower, which makes the whole polymerization process easier. On the other hand, low viscosity resins have a rapid reoxygenation time at the layer interface leading to incomplete interfacial layer bonding. (6)

Some reviews have also concluded that the effect of oxygen on layer to layer strength (interfacial strength) is actually promoted by oxygen as it leads to a slower consumption of double bonds at the surface layer**. Because oxygen decelerates the PI reaction, an increase in oxygen will lead to more unconverted bonds, which can react with the subsequent layer subsequently improving layer to layer strength. (7)

At Carbon, the team might use differential scanning calorimetry (DSC) to measures the change of temperature with respect to time to get an estimate for how effective the cure was sine converting double covalent bonds to single bonds generates heat.

Formulation and Characterization

Formulating a new resin is guided by the properties you need. When developing a new material, the Carbon team will make a goal to hit a high elastic modulus, and begin their search by examining the polymer families that can meet that benchmark. As they add properties for their new resin, they narrow down their polymer selection. As with most polymers, the hardest mechanical behavior to make formulation judgements are real-world properties such as UV, polymer aging, and chemical compatibility because they surround the objective of building versatile resins.

This of course, is not everything that goes into the complex science of resin development and there is still a lot to uncover about post-curing processes and curing dose. I’ll save those for another time. For now, there is at least some appreciation for the hard work going on to integrate additive manufacturing in the industry and build access to fully photocurable, bio-compatible, high strength, and maybe one day, fully recyclable resins.

**Zeang Zhao et. al also concluded that, “interfacial strength decreases with curing time and incident light intensity, while the presence of oxygen can significantly improve the strength at the interface. They also found that interfaces with improved strength can be obtained by either decreasing the amount of photoinitiator or by using short chain crosslinkers that can increase the concentration of double bonds.”

References

(1) Yang, Y., Li, L., & Zhao, J. (2019). Mechanical property modeling of photosensitive liquid resin in stereolithography additive manufacturing: Bridging degree of cure with tensile strength and hardness. Materials & Design, 162, 418-428. doi:10.1016/j.matdes.2018.12.009

(2) Quan, H., Zhang, T., Xu, H., Luo, S., Nie, J., & Zhu, X. (2020). Photo-curing 3d printing technique and its challenges. Bioactive Materials, 5(1), 110-115. doi:10.1016/j.bioactmat.2019.12.003

(3) Kelly, B. E., Bhattacharya, I., Heidari, H., Shusteff, M., Spadaccini, C. M., & Taylor, H. K. (2019). Volumetric additive manufacturing via tomographic reconstruction. Science, 363(6431), 1075-1079. doi:10.1126/science.aau7114

(4) Taormina, G., Sciancalepore, C., Messori, M., & Bondioli, F. (2018). 3D printing processes for photocurable polymeric materials: Technologies, materials, and future trends. Journal of Applied Biomaterials & Functional Materials, 16(3), 151-160. doi:10.1177/2280800018764770

(5) Fouassier, J. P., & Lalevée, J. (2012). Photoinitiators for polymer synthesis: Scope, reactivity, and efficiency. Weinheim: Wiley-VCH.

(6) Lalevée, et al., Radical photopolymerization reactions under air upon lamp and diode laser exposure: The input of the organo-silane radical chemistry, Prog. Org. Coat. (2010), doi:10.1016/j.porgcoat.2010.10.008

(7) Zhao, Z., Mu, X., Wu, J., Qi, H., & Fang, D. (2016, June 01). Effects of oxygen on interfacial strength of incremental forming of materials by photopolymerization. Retrieved April 22, 2021, from https://www.sciencedirect.com/science/article/pii/S2352431616301055