3D printing of ceramics and the influence of additives and binders

Dental 3D printing of ceramics is becoming increasingly important in research. Slip deposition (LSD printing) is promising, as it enables high green density and faster process steps. However, there are many challenges for use in dentistry. Here you can find out more about the importance of additives and binders in optimizing feedstocks for this pioneering technology.

In general, various methods for producing silicate ceramic restorations have been established in dentistry. Conventionally, they can be layered or pressed from pellets. Subtractive manufacturing offers the possibility of producing restorations from industrially manufactured blanks in a time-saving manner in the CAD/CAM workflow. Recently, additive technologies have emerged that promise the production of ceramic restorations using 3D printing. 

Influence of additives and binders on the physical properties of dental silicate ceramic feedstocks 

When it comes to dental 3D printing of ceramics, it is particularly important to emphasize that additive manufacturing has the potential to reduce manufacturing costs and material waste during production. However, long process times, especially for the polymerization of VAT, are currently preventing widespread commercial use. One promising technology is slip deposition (LSD printing), in which a water-based ceramic suspension is used as feedstock. Similar to binder jetting (BJ), this process uses a crosslinker to fix the object cross-section to be printed and prints it into the applied feedstock layer. This makes it possible to achieve a high green density of the printed object (>60%) on the one hand and to reduce the proportion of organic compounds in the printed object on the other. Both of these mean that process steps such as debinding and firing of the printed objects can be accelerated. However, printing dental restorations with this technology is still a challenge due to the precision required and the high demands placed on surface quality as well as mechanical and optical properties. Therefore, the further development of the feedstocks to be used is one of the most important points for optimizing LSD printing technology for use in the dental sector. In addition to the ceramic raw material itself, important adjustments in feedstock development are the additives used. The crosslinker must also be tailored to the feedstock system in order to achieve optimal results.

material and methods

For the investigations, the starting materials were first characterized. The grain size distribution of the ceramic powder (feldspar) and the viscosity of the feedstock formulations were determined. Overall, two different formulations were developed. 

• The control group was a feedstock without additives, consisting only of feldspar powder and distilled water (CG). 
• In formulation AC1, polyvinyl alcohol (PVOH) was used as a binder. 
• In formulation AC2, poly(sodium 4-styrenesulfonate) (PSS) served as binder and polyethylene glycol (PEG) as plasticizer. 

In addition, the influence of the crosslinker in the test recipes was investigated. This is intended to ensure the stability of the green bodies. Disc and beam-shaped test specimens were produced using the slip casting process and characterized in both the green and fired state. In the green state, density and flexural strength were measured, and in the fired state, density, shrinkage, flexural strength, Weibull modulus, fracture toughness, and Martens parameters were measured. The microstructure was also characterized. Test specimens made from commercially available CAD/CAM blocks served as the reference group (TR).

In ceramic 3D printing, the term "feedstock" refers to the raw material used to create ceramic objects. It can come in various shapes. The feedstock is processed layer by layer and then sintered to create a solid and dense structure.

Results

The control group (CG) showed the highest density in the green state but the lowest flexural strength. This indicates that the addition of additives reduces the density but improves the stability and processability of the unfired test specimens. The addition of a crosslinker also increases the stability in the green state and stands out significantly from the values ​​of the test specimens without crosslinker, which is particularly relevant for the uncomplicated washing out process of the objects after printing. After the firing process, the average volume shrinkage for all groups examined is in the range of 34-42% (Figure 2).

In the fired state, there were no significant differences in density between the different feedstock recipes, indicating that almost complete densification was achieved by the firing process. The microstructure analysis confirmed this, as all fired test specimens had a similar microstructure with homogeneously distributed crystalline and amorphous components. However, the reference from commercially available CAD/CAM blocks (TR) showed a higher absolute density and a finer microstructure with a higher crystalline content compared to the slip-cast test specimens. This could be due to a different temperature control in the industrially optimized manufacturing process. The CAD/CAM blocks as reference (TR) had the highest average flexural strength, the highest Weibull modulus and the highest fracture toughness. These results underline the optimized manufacturing process. 

In comparison, however, all slip-cast specimens, especially the experimental formulations with addition of additives (AC1 and AC2), showed promising mechanical properties that were only slightly below the values ​​of the reference.

The results show that the feedstock formulations investigated (AC1 and AC2) are processable for LSD printing technology and can meet the requirements for dental applications. Further investigations to optimize the firing parameters could help to improve the microstructure and mechanical properties of the feedstock formulations and to get closer to the target parameters of the reference.

Conclusion

The results of this in vitro study show that the use of additives such as polyvinyl alcohol (PVA) and poly(sodium 4-styrenesulfonate) (PSS) with polyethylene glycol (PEG) in the formulation of feedstocks improves the mechanical stability in the green state without negatively affecting the material properties in the fired state. The addition of crosslinkers significantly increased the flexural strength in the green state, which is important for handling during the LSD printing process. In the fired state, the slip-cast specimens showed similar properties to commercial CAD/CAM blocks, but with a lower crystal content, which explains the slightly lower fracture toughness. Optimization of the firing parameters could further reduce these differences.

Overall, it can be concluded that the feedstock formulations investigated are promising for LSD printing technology and meet the requirements for dental applications.