Characterizing photopolymer resins for high‐temperature vat photopolymerization.pdf
Vol.:(0123456789) Progress in Additive Manufacturing  https://doi.org/10.1007/s40964-023-00562-0 FULL RESEARCH ARTICLE Characterizing photopolymer resins for high‑temperature vat  photopolymerization Viswanath Meenakshisundaram1,2 · Keyton Feller2 · Nicholas Chartrain2,3 · Timothy Long4 · Christopher Williams1,2  Received: 11 September 2023 / Accepted: 25 December 2023  © The Author(s) 2024 Abstract The availability of engineering polymers for vat photopolymerization (VP) additive manufacturing is limited. This limitation  primarily stems from the inability of standard VP systems to recoat high-viscosity resins (> 3 Pa s). High-temperature vat  photopolymerization is a new process-based VP platform that enables processing of viscous photopolymer resins (viscosity  > 3 Pa s). Research in this area has been focused on demonstrating expanded access to new polymer families, and studying  the effect of printing temperature on mechanical and esthetic performance of printed parts. However, methods to determine  the printing temperature that prevents the occurrence of thermally induced polymerization (i.e., thermal stability) in the  resin have not been established. In this work, the authors have applied characterization techniques such as thermogravimetric  analysis, Rheology and differential scanning calorimetry to determine the printing temperature for processing viscous  photopolymer resins. As a case study, the developed characterization techniques are applied to: (1) photopolymer that is solid  at room temperature, (2) polymer with viscosity of 21 Pa s at room temperature, and the temperature at which the resins can  be printed without triggering thermally induced polymerization is successfully determined. The results of this work will act  as a materials’ characterization and process parameter development guide for high-temperature VP systems, thus enabling  expansion of VP materials catalogue to engineering materials that were previously unprocessable. Keywords  Hot lithography · Vat photopolymerization · Stereolithography · Viscous photopolymers 1  Introduction Vat photopolymerization (VP) is a polymer additive manu- facturing (AM) process wherein the resin surface is selec- tively exposed to ultra-violet (UV) light to convert irradiated  regions from a liquid to solid state [1, 2]. VP is commonly  used to prototype complex geometries because of its ability  to manufacture parts with attributes such as: high-feature  resolution, superior surface finish, and high-manufacturing  speed [3–5]. While VP systems have been able to meet these  manufacturing metrics for prototypes by using low viscosity  resins (< 3 Pa s) [6], meeting the demand for manufactur- ing end use products with complex geometries and superior  material properties has necessitated the migration towards  the development and use of engineering photopolymers [1,  2]. Among the different strategies available for vat pho- topolymerization of engineering photopolymers, the route  of using high-viscosity resins to access engineering poly- mers has become popular because it offers the following  advantages:   *	 Christopher Williams  	 cbwill@vt.edu 	 Viswanath Meenakshisundaram  	 vmeenak@vt.edu 	 Keyton Feller  	 kfeller@vt.edu 	 Nicholas Chartrain  	 nickchar@vt.edu 	 Timothy Long  	 Timothy.E.Long@asu.edu 1	 Department of Mechanical Engineering, Virginia Tech,  Blacksburg, VA 24061, USA 2	 Macromolecules Innovation Institute, Virginia Tech,  Blacksburg, VA 24061, USA 3	 Department of Material Science and Engineering, Virginia  Tech, Blacksburg, VA 24061, USA 4	 School of Molecular Sciences, Arizona State University,  Tempe, AZ 85281, USA 	 Progress in Additive Manufacturing 1.	 Improvement in thermo-mechanical performance  High viscosity photopolymers often use blends of high  molecular weight backbones with oligomeric moieties.  These inclusions can positively affect the mechanical  performance in areas such as toughness, elongation at  break, and modulus. This strategy offers a path to use  photopolymers for engineering applications [7] 2.	 Predictable control of geometry Reduction in use of  viscosity modifiers such as volatile monomers, solvents  (reactive and unreactive) [8, 9] reduces evaporative  losses during post-processing and lowers post-process  complexity. Reduced shrinkage allows the use of simple  design compensation techniques such as global scaling,  uniform Z-compensation to predictably manufacture  complex parts with features across comparable size  scales. Traditional VP systems find it challenging to handle high- viscosity polymers. One of the sources for this limitation  is the fluid dynamics occurring during the recoating step.  In passive recoating systems where resin is allowed to  settle under self-weight (Top Down or free-surface VP  systems) or allowed to flow due to pressure head (Bottom  up or constrained surface VP system), the rate of flow,  settling time, and distance of resin travel are dependent  on the viscosity of the resin [10]. Increase in resin  viscosity increases settling time, lowers recoat quality  and manufacturing throughput. While addition of active  elements such as recoating blades, recirculation units, or  tilting elements help mitigate the issues, the complex fluid  dynamic interactions introduced by the inclusion of such  elements (i.e., drag force on the printed parts, pressure  differentials created by hollow areas) still make them  inefficient while processing viscous resins [10–14]. Another  challenge with viscous photopolymers is poor/slow reaction  kinetics. Highly viscous photopolymer systems offer low  mobility to reactive species which leads to poor conversion  or insufficient curing that results in green parts that are  incapable of supporting self-weight or shape [15] Hot lithography, or high-temperature VP, is a VP  platform wherein the photopolymer resin is heated to  elevated temperatures to lower the resin viscosity within  the processible limits of the VP apparatus. The use  of high-temperature VP apparatus for manufacturing  complex, high-resolution features with highly viscous  resins has already been demonstrated [16–18]. Majority  of the high-temperature VP research has been focused on  understanding the effect of elevated printing temperature  on the % conversion and thermo-mechanical properties  of the printed parts [19]. While prior research has been  successful in demonstrating the ability of high-temperature  VP systems to handle viscous photopolymers, the tools and  techniques to help VP users study, characterize and develop  printing parameters for high temperature VP systems remain  unexplored. This exploration is even more pertinent given  that it is known that acrylate systems have a tendency  to undergo thermally induced polymerization at high- temperatures [20]. To this end, the primary objective of this work is to  present a set of characterization techniques that can be  used to determine the safe printing temperature for high- temperature VP systems. Specifically, this work details the  application of the proposed characterization techniques  to two case unique high-viscosity photopolymer systems.  The methods to analyze and interpret the characterization  results for process parameter selection are presented. The  proposed methods are then validated using the predicted  printing parameters for actual part fabrication using a  custom high-temperature VP setup. The results of this  work will help provide insight into the thermal stability  of photopolymer resins and enable successful printing of  previously inaccessible polymers. 2  Materials and methods Two material systems exhibiting the following characteristics  were selected as case studies: (1) high viscosity at room  temperature (urethane acrylate), and (2) solid at room  temperature (Bisphenol A dimethacrylate) (Sect. 2.1).  Characterization techniques developed to determine safe  processing temperatures, i.e., process these materials  without initiating thermally induced polymerization are  outlined in Sect. 2.2. The design and construction of the  high-temperature VP apparatus is outlined in Supplementary  Information (SI) Section S1. Methods for specimen  fabrication and post-processing are detailed in Sect. 2.3. 2.1  Materials 2.1.1  Solid photopolymer resin (BPADMA) Bisphenol-A-dimethacrylate (BPADMA), a solid at room  temperatures, was procured from Sigma Aldrich (SKU:  156329). Diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide  (TPO), a photoinitiator, was procured from TCI chemicals  (D3358). BPADMA was heated to 100 ◦ C (melting point:  72–74 ◦ C) and brought to liquid state. TPO (melting point:  88–92 ◦ C) was then added to the liquid BPADMA in the  ratio of 1:100 by weight (TPO:BPADMA). The mixture was  cooled to room temperature before printing. 2.1.2  High‑viscosity photopolymer resin (EBECRYL‑242) A premixed resin containing aliphatic urethane acrylate  oligomer and isobornyl acrylate (IBOA) (30 wt% IBOA)  Progress in Additive Manufacturing	 Parts printed with EBECRYL-242 were immersed in  ethanol (maintained at room temperature) for removal of  cleaning and removal of uncured resin. Immersed parts  did not exhibit signs of warping when they were quenched  in ethanol immediately after printing at 75 ◦ C. Since the  printed parts remain under volatilization temperature, it  is hypothesized that the EBECRYL-242 network remains  plasticized by uncured IBOA and urethane acrylate  oligomers, enabling it to reconfigure and relieve the  thermal stresses, therefore preventing the warping of  parts during quenching. Comparison of EBECRYL-242  parts quenched in hot olive oil and room temperature  ethanol did not reveal visually identifiable difference in  the Honeycomb structure. Finally, self-initiated, cured fragments were not  observed in the resins even after subjecting them to five  thermal and printing cycles. While five cycles of reuse  may not be sufficient to fully explore resin degradation  at elevated temperatures, it provides confidence to the  user that the developed resin characterization techniques  are an appropriate means of identifying the printing  temperature for safely processing photopolymers at  elevated temperatures. It must be noted that the post-process steps indicated  in this work were developed for demonstration purposes  only (i.e., that successful parts can be fabricated with  characterization techniques presented in this work). While  the development of appropriate post-processing methods,  and reformulation strategies are highly formulation and  application specific and is beyond the scope of this work,  the following guidelines are proposed as a starting step for  the users of high-viscosity resins and high-temperature  VP systems:  1.	 Resin removal step: given the high viscosity of the resin,  cleaning and removal of uncured resin from complex  geometries may be more efficient when the part is hot.  Users may consider the use of a hot cleaning medium  (e.g., hot mineral oils) and physical agitation to remove  residual resin from the parts. 2.	 UV post-curing step: UV post-curing may be used to  foster the conversion of unreacted moieties in the green  part and improve the mechanical properties of the  printed parts. 3.	 Secondary thermal treatment step: additional heating  and cooling cycles in an oven could be employed if  warping or distortion is observed in the parts after UV  post-curing. 4.	 Iteration step: Users may want to consider altering  the printing parameters such as printing temperature,  printing intensity, UV dosage or repeating the post- processing steps in different order to achieve desirable  results. 3.4  Broader impacts of high‑temperature vat  photopolymerization and high‑viscosity resins In its current state, vat photopolymerization enables  users to quickly prototype complex geometries with high  resolution and surface finish. The use of simple acrylate,  or methacrylate monomers blended with simple oligomeric  backbones helps meet the requirements of the target  audience. However, these simple photopolymer systems  contain short chain monomers, and photoinitiators that are  prone to extraction in aqueous media. These leachables pose  a serious threat to human health and pose a challenge for safe  disposal. While increased post-curing can help lower these  risks by increasing the crosslinking in the polymer matrix,  the variability in commercial photopolymers and lack of  standardization in the post-process and material composition  continue to elevate the risk of handling and disposing cured  photopolymers [24]. It must be noted that the high-viscosity  resins are not immune to the problems of excess leachables.  While high molecular weight polymers used in viscous  resins may be harder to extract in aqueous media, the viscous  nature of the resin lowers polymer and radical mobility when  processed with traditional VP systems. This often leads to  the incomplete curing, low double bond conversion, and  increased leachables in cured parts [15]. High-temperature  vat photopolymerization systems not only allow for easy  processing of viscous resins, but the method of processing  the resins at elevated temperatures lends the following  advantages:  1.	 Processing the resin at elevated temperatures lowers  the resin viscosity, thus increasing the mobility of  radicals and polymers. This increased mobility increases  the likelihood of photopolymerization events, thus  increasing the crosslink density, double bond conversion  and simultaneously lowers the leachables 2.	 The combination of increased cross link density and low  resin viscosity increases the green strength of the printed  parts. This allows for printing of high resolution, high  aspect ratio features which would have been previously  inaccessible in traditional VP systems. This increase  in design freedom allows for the use of structural  optimization for generating engineered solutions for end  use applications [5, 16, 19] 3.	 Processing photopolymers at elevated temperatures  often alters the curing parameters, namely the critical  energy(Ec ) and the depth of penetration ( Dp ). Parts often  need lower exposure time at elevated temperatures,  thus leading to the formation of more pronounced layer  lines in the part surface. This deterioration in surface  finish may lower increase anisotropic behavior in the  print direction. However, an overall improvement in  geometric accuracy could be expected [19]. 	 Progress in Additive Manufacturing 4.	 The removal of viscous resin from the printed parts  often necessitates the use of heated post-processing  procedures such as hot solvent/mineral oil wash (as  described in Sec. 2.3.4) These post-processing of the  parts at elevated temperatures provides more time for  the polymer matrix to undergo rearrangement, thus  allowing the fabrication of parts with minimal warping  and internal stresses While high-temperature vat photopolymerization offers  several processing advantages, it must be noted that  the selection of the polymer backbone, the final resin  formulation, and the post-processing conditions have  a strong influence on the performance and safety of the  printed device. Given that the advancements in high  temperature VP systems and performance photopolymers  are fairly recent, the number of commercial machines and  materials for high temperature VP are fairly limited and  expensive. However, the growing interest to use polymer  AM for manufacturing of mass customized products is  expected to increase the research and development, and  commercialization interests in the area of high-temperature  vat photopolymerization. 4  Conclusion The primary objective of this work was to develop a set  of characterization techniques that helped determine  the printing temperature that does not induce thermally  initiated polymerization. To this end, two high- temperature VP resins, EBECRYL-242 and BPADMA,  were formulated. First, TGA was used to determine  the degradation and volatilization temperatures of the  resins. Then, variable-temperature rheology was used to  determine the temperature at which thermally induced  polymerization occurred. Then, isothermal DSC was  used to determine the time for which the resin remained  thermally stable at different printing temperatures.  A framework for interpreting and applying the  characterization results for developing printing parameters  was presented. The framework was applied to the  EBECRYL-242 and BPADMA resins and the appropriate  printing temperatures were determined to be 75 and 90  ◦ C, respectively. The findings were validated using the  predicted printing temperatures for successfully fabricating  test specimens with BPADMA and EBECRYL-242 resins.  In conclusion, the successful fabrication of specimens with  previously inaccessible photopolymers provides evidence  to support the use of the presented resin characterization  techniques for determining the printing temperatures for  high-temperature VP systems. 5  Supplementary information Additional information about construction of the  custom high-temperature VP system, working curves,  and additional DSC isotherms can be found in the  supplementary information. Supplementary Information  The online version contains supplemen- tary material available at https://​doi.​org/​10.​1007/​s40964-​023-​00562-0. Acknowledgements  The authors would like to thank Allnex for provid- ing a free sample of EBECRYL-242 polymer. The authors would like to thank the Materials Characterization Lab  and Macromolecules Innovation Institute for providing access to the  characterization instruments used in this work. Funding  This material is based upon work supported by the National  Science Foundation under Grant No.: CMMI - 1762712. Data availability  Data available within the article or its supplementary  materials. Additional data avaialable on request. Declarations  Conflict of interest  The authors have no relevant financial or non-fi- nancial interests to disclose. 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Appuhamillage GA, Chartrain N, Meenakshisundaram V, Feller  KD, Williams CB, Long TE (2019) 110th Anniversary: vat pho- topolymerization-based additive manufacturing: current trends  and future directions in materials design. Ind Eng Chem Res  58(33):15109–15118. https://​doi.​org/​10.​1021/​acs.​iecr.​9b026​79 	 2.	 Tariq A, Arif ZU, Khalid MY, Hossain M, Rasool PI, Umer R,  Ramakrishna S (2023) Recent advances in the additive manu- facturing of stimuli-responsive soft polymers. Adv Eng Mater.  https://​doi.​org/​10.​1002/​adem.​20230​1074 	 3.	 Ippolito R, Iuliano L, Gatto A (1995) Benchmarking of rapid  prototyping techniques in terms of dimensional accuracy and  surface finish. CIRP Ann Manuf Technol 44(1):157–160.  https://​doi.​org/​10.​1016/​S0007-​8506(07)​62296-3 	 4.	 Tumbleston JR, Shirvanyants D, Ermoshkin N, Janusziewicz  R, Johnson AR, Kelly D, Chen K, Pinschmidt R, Rolland  JP, Ermoshkin A, Samulski ET, DeSimone JM (2015) Con- tinuous liquid interface production of 3D objects. Science  347(6228):1349–1352 Progress in Additive Manufacturing	 	 5.	 Arif ZU, Khalid MY, Noroozi R, Sadeghianmaryan A, Jalalvand  M, Hossain M (2022) Recent advances in 3D-printed polylac- tide and polycaprolactone-based biomaterials for tissue engi- neering applications. Int J Biol Macromol 218:930–968. https://​ doi.​org/​10.​1016/j.​ijbio​mac.​2022.​07.​140 	 6.	 Griffith M, Halloran J (1996) Freeform fabrication of ceramics  via stereolithography. J Am Ceram Soc 79(10):2601–2608 	 7.	 Patel DK, Sakhaei AH, Layani M, Zhang B, Ge Q, Magdassi S  (2017) Highly stretchable and UV curable elastomers for digi- tal light processing based 3D printing. Adv Mater 29(15):1–7.  https://​doi.​org/​10.​1002/​adma.​20160​6000 	 8.	 Hegde M, Meenakshisundaram V, Chartrain N, Sekhar S, Tafti  D, Williams CB, Long TE (2017) 3D printing all-aromatic pol- yimides using mask-projection stereolithography: processing  the nonprocessable. Adv Mater. https://​doi.​org/​10.​1002/​adma.​ 20170​1240 	 9.	 Herzberger J, Meenakshisundaram V, Williams CB, Long TE  (2018) 3D printing all-aromatic polyimides using stereolitho- graphic 3D printing of polyamic acid salts. ACS Macro Lett  7(4):493–497. https://​doi.​org/​10.​1021/​acsma​crole​tt.​8b001​26 	10.	 Li X, Mao H, Pan Y, Chen Y (2019) Mask video projection-based  stereolithography with continuous resin flow. J Manuf Sci Eng  Trans ASME 141(8):1–10. https://​doi.​org/​10.​1115/1.​40437​65 	11.	 Renap K, Kruth JP (1995) Recoating issues in stereolithography.  Rapid Prototyp J 1(3):4–16. https://​doi.​org/​10.​1108/​13552​54951​ 00942​23 	12.	 Song X, Chen Y, Lee TW, Wu S, Cheng L (2015) Ceramic fabri- cation using mask-image-projection-based stereolithography inte- grated with tape-casting. J Manuf Process 20:456–464. https://​doi.​ org/​10.​1016/j.​jmapro.​2015.​06.​022 	13.	 Hafkamp T, Baars G, Jager B, Etman P (2017) A trade-off/analysis  of recoating methods for vat photopolymerization of ceramics. In:  Solid freeform fabrication symposium, pp 1–25 	14.	 Pan Y, Zhou C, Chen Y (2012) Rapid manufacturing in minutes:  the development of a mask projection stereolithography process  for high-speed fabrication. In: ASME 2012 international manu- facturing science and engineering conference, p 405 . https://​doi.​ org/​10.​1115/​MSEC2​012-​7232. http://​proce​edings.​asmed​igita​lcoll​ ection.​asme.​org/​proce​eding.​aspx?​doi=​10.​1115/​MSEC2​012-​7232 	15.	 Dickens SH, Stansbury JW, Choi KM, Floyd CJE (2003) Photopo- lymerization kinetics of methacrylate dental resins. Macromol- ecules 36(16):6043–6053. https://​doi.​org/​10.​1021/​ma021​675k 	16.	 Schüller-Ravoo S, Zant E, Feijen J, Grijpma DW (2014) Prepa- ration of a designed poly(trimethylene carbonate) microvascular  network by stereolithography. Adv Healthc Mater 3(12):2004– 2011. https://​doi.​org/​10.​1002/​adhm.​20140​0363 	17.	 Gmeiner R (2018) Method and device for lithography-based gen- erative production of three-dimensional moulds. https://​paten​ts.​ google.​com/​patent/​EP328​4583A1/​en?​oq=​EP+​32845​83+​A1 	18.	 Li X, Xie B, Jin J, Chai Y, Chen Y (2018) 3D Printing temporary  crown and bridge by temperature controlled mask image projec- tion stereolithography. Procedia Manuf 26:1023–1033. https://​doi.​ org/​10.​1016/j.​promfg.​2018.​07.​134 	19.	 Steyrer B, Busetti B, Harakály G, Liska R, Stampfl J (2018) Hot  lithography vs. room temperature DLP 3D-printing of a dimeth- acrylate. Addit Manuf 21(January):209–214. https://​doi.​org/​10.​ 1016/j.​addma.​2018.​03.​013 	20.	 Riazi H, Shamsabadi A, Grady MC, Rappe AM, Soroush M  (2018) Experimental and theoretical study of the self-initiation  reaction of methyl acrylate in free-radical polymerization. Ind  Eng Chem Res 57(2):532–539. https://​doi.​org/​10.​1021/​acs.​iecr.​ 7b046​48 	21.	 Jacobs PF (1992) Rapid prototyping and manufacturing. Funda- mentals of stereolithography, 1st edn. Society of Manufacturing  Engineers (1992) 	22.	 Ors JA, La Perriere DM (1986) Thermogravimetric profile of  decomposition of acrylate systems based on bornyl acrylate  monomers. Polymer 27(12):1999–2002. https://​doi.​org/​10.​1016/​ 0032-​3861(86)​90197-7 	23.	 Pham QT, Hsu JM, Pan JP, Wang TH, Chern CS (2013) Kinetics  of free radical polymerization of bisphenol A diglycidyl ether  diacrylate initiated by barbituric acid. Thermochim Acta 573:121– 129. https://​doi.​org/​10.​1016/j.​tca.​2013.​09.​002 	24.	 Carve M, Wlodkowic D (2018) 3D-printed chips: compatibility  of additive manufacturing photopolymeric substrata with biologi- cal applications. Micromachines 9(2):91. https://​doi.​org/​10.​3390/​ mi902​0091 Publisher's Note  Springer Nature remains neutral with regard to  jurisdictional claims in published maps and institutional affiliations.
Title: Characterizing photopolymer resins for high‑temperature vat photopolymerization ​
Authors: Viswanath Meenakshisundaram, Keyton Feller, Nicholas Chartrain, Timothy Long, Christopher Williams ​
Publisher: Springer Nature ​
Date: 2024
Abstract: The availability of engineering polymers for vat photopolymerization (VP) additive manufacturing is limited. ​ This limitation primarily stems from the inability of standard VP systems to recoat high-viscosity resins (> 3 Pa s). ​ High-temperature vat photopolymerization is a new process-based VP platform that enables processing of viscous photopolymer resins (viscosity > 3 Pa s). ​ Research in this area has been focused on demonstrating expanded access to new polymer families, and studying the effect of printing temperature on mechanical and esthetic performance of printed parts. ​ However, methods to determine the printing temperature that prevents the occurrence of thermally induced polymerization (i.e., thermal stability) in the resin have not been established. ​ In this work, the authors have applied characterization techniques such as thermogravimetric analysis, Rheology and differential scanning calorimetry to determine the printing temperature for processing viscous photopolymer resins. ​ As a case study, the developed characterization techniques are applied to: (1) photopolymer that is solid at room temperature, (2) polymer with viscosity of 21 Pa s at room temperature, and the temperature at which the resins can be printed without triggering thermally induced polymerization is successfully determined. ​ The results of this work will act as a materials’ characterization and process parameter development guide for high-temperature VP systems, thus enabling expansion of VP materials catalogue to engineering materials that were previously unprocessable. ​