Simple Non-Equilibrium Atmospheric Plasma Post-Treatment Strategy for Surface Coating of Digital Light Processed 3D-Printed Vanillin-Based Schiff-Base Thermosets

A simple non-equilibrium atmospheric plasma post-treatment strategy was developed for the surface coating of three-dimensional (3D) structures produced by digital light processing 3D printing. The influence of non-equilibrium atmospheric plasma on the chemical and physical properties of vanillin-derived Schiff-base thermosets and the dip-coating process was investigated and compared to the influence of traditional post-treatment with UV-light. As a comparison, thermosets without post-treatment were also subjected to the coating procedure. The results document that UV post-treatment can induce the completion of the curing of the printed thermosets if complete curing is not reached during printing. Conversely, the plasma post-treatment does not contribute to the curing of the thermoset but causes some opening of the imine bonds and the regeneration of aldehyde functions. As a consequence, no great differences are observed between the not post-treated and plasma post-treated samples in terms of mechanical, thermal, and solvent-resistant properties. In contrast to the UV post-treatment, the plasma post-treatment of the thermosets induces a noticeable increase of the thermoset hydrophilicity ascribed to the reformation of amines on the thermoset surface. The successful coating process and the greatest uniformity of the lignosulfonate coating on the surface of plasma post-treated samples are considered to be due to the presence of these amines and aldehydes. The investigation of the UV shielding properties and antioxidant activities documents the increase of both properties with the increasing amount and uniformity of the formed coating. Interestingly, evident antioxidant properties are also shown by the noncoated thermosets, which are deduced to their chemical structures.

Determination of the resins' working curves.The cure parameters of the resins were determined by implementing the Jacobs working curve model according with equation 1.The method correlates the resins' penetration depth (Dp) indicating how far light can penetrate into the resin, with the critical dose (Ecrit), intended as the amount of light required for polymerization to proceed until a solid material is formed.Disks (3 mm in diameter) of each resin formulation were cured over the screen of the DLP 3D printer by exposing a small circle of light (385 nm, 28 mW/cm 2 ) at varying durations (from 80 s to 100 s).After a washing in DCM and a UV lamp post-curing (3 min/side), the thicknesses of the cured samples (Cd, μm) were measured using a micrometer thickness gauge.Thickness measurements were performed in triplicate for each sample.Each resin's working curve is obtained as a semi-logarithmic plot of Cd vs light irradiation dosage (Emax, mJ/cm 2 ), which is the product of the UV light intensity (mW/cm 2 ) and exposure time (s).Table S1.Mechanical properties of all the thermosets before and after post-treatments.

Sample Strain at break [%] Stress at break [MPa] Elastic Modulus [MPa]
Table S2.Thermal properties of all the thermosets before and after post-treatments.Table S3.Weight increase of the thermosets after the lignosulfonate coating procedure.The test was performed in triplicate.Samples were weighted before and after the coating procedure.The weight increase was determined according with the following equation:

Sample Tg
Where wf is the weight of the thermoset after the coating procedure and wi is the initial weight of the thermoset.

Figure S4 .
Figure S4.Working curves for the printing of R1 and R2 resins.

Figure S8 .
Figure S8.ATR-FTIR spectra of T1_UV, T1_UV_C and lignosulfonate.After the lignosulfonate coating procedure, an increase of the intensity in the region of the -OH absorption band can be observed on the surface of T1_UV_C.

Figure S9 .
Figure S9.ATR-FTIR spectra of T1_Plasma, T1_Plasma_C and lignosulfonate.The presence of the lignosulfonate coating on the surface of T1_Plasma_C after the coating procedure is clearly confirmed.

Figure S11 .
Figure S11.ATR-FTIR spectra of T2_UV, T2_UV_C and lignosulfonate.After the coating procedure, a slight increase of the intensity in the region of the -OH absorption band can be observed on the surface of T2_UV_C.

Figure S12 .
Figure S12.ATR-FTIR spectra of T2_Plasma, T2_Plasma_C and lignosulfonate.The presence of the lignosulfonate coating on the surface of T2_Plasma_C after the coating procedure is clearly confirmed.

Figure S13 .
Figure S13.ATR-FTIR spectra of T1_Plasma_coated immersed in water for 72 h, T1_Plasma_coated and T1_Plasma.The presence of the coating on the surface of T1_Plasma_C can be detected also after 72 h immersion in water.

Table S4 .
Residual DPPH° expressed as an average value with standard deviation for all the T1 samples without and with coating.

Table S5 .
Residual DPPH° expressed as an average value with standard deviations for all the T2 samples without and with coating.