There are many advantages to ultraviolet (UV)-curable systems, like energy saving, rapid curing, environmentally friendly, and high cost performance. These systems are used often with UV-assisted 3D printing, which deposits layers of photocurable liquid polymer crosslinked by an external UV light source to make an object. Silicone materials are promising for 3D printing use due to their unique properties, like thermal resistance, good transparency, and UV resistance.
A team of researchers from Hangzhou, China recently published a paper, titled “Preparation and performance of ultraviolet curable silicone resins used for ultraviolet cured coating and ultraviolet-assisted 3D printing materials,” that detailed how they synthesized a type of UV-curable silicone resin “by an easy controlled method of sol-gel with 3-methacryloxypropyltrimethoxysilane, methyltrimethoxysilane and dimethyldiethoxylsilane.”
The abstract reads, “Types of curable methacryloxypropyl silicone resins were prepared by a cohydrolysis condensation reaction with 3-methacryloxypropyltrimethoxysilane, methyl trimethoxy silane, and dimethyl diethoxy silane. The properties of the transparent resins cured by the ultraviolet were investigated in detail. The ultraviolet cured silicone material transparency was higher than 95% in the light wavenumber range of 400 nm and 800 nm, pencil hardness from 6 B to 6 H, and thermal decomposition temperature higher than 150°C. The silicone materials prepared by ultraviolet-assisted 3D printing had a water absorption coefficient of 0.21 wt% and a line thermal expansion coefficient of 5.27 × 10−4 m/k. It showed that the silicone resins can be candidates for ultraviolet cured transparent coating and ultraviolet-assisted 3D printing.”
A sol-gel method was used to synthesize the transparent silicone resins, which had various amounts of R/Si for UV-assisted 3D printing. These pre-polymer materials were 3D printed on an SLA 3D printer, and the resulting resins were deposited on slides and UV-cured.
The researchers measured the materials’ dynamic viscosity, along with the hardness of the cured samples and their apparent curing contents. They also recorded the transmittance spectra of the silicone resin pre-polymers cured on a slide, carried out thermogravimetric (TG) analysis of the 3D printed silicone resins, and performed differential scanning calorimetry (DSC) measurements of the final products.
By decreasing the n(R)/n(Si), the researchers found that they could increase the viscosity of the pre-polymers, as well as the pencil hardness of the UV-cured products.
“The TGA analysis (Fig. 4) showed that all the thermal decomposition temperature for the ultraviolet cured silicone materials higher than 150°C, which means that the cured silicone resins were all with fairly good thermal stability,” the researchers wrote.
The cured material is still sticky, with low hardness, if the amount of 2-hydroxy-2-methyl-phenyl-propane-1-one is less than 10 wt% of silicone resin pre-polymers – possibly because it wasn’t totally cured “for the insufficient amount of ultraviolet initiator.”
But, if the amount of UV initiator reached 10 wt%, absorption at 2333 cm−1 and 1625 cm−1 for methacryloxypropyl disappeared. Excessive amounts of UV photoinitiator will make the material’s properties worse, which is why the correct amount is 10 wt% of the silicone resin pre-polymer.
The researchers also investigated the influence of UV curing time, and found that when it was less than 30 seconds, the material would not completely cure. But, the cure time did not have much of an effect on the cured material’s gelation rate and hardness.
“This phenomenon can be explained by the FT-IR analysis given by Fig. 7 that the characterization peaks at 1630 cm−1assigned to the unsaturated double bond of the methacryloxypropyl almost vanished after the silicone resin cured by ultraviolet for 30 s, which demonstrated that the silicone resins were cured completely,” the researchers explained.
By observing the performance of the UV-cured silicone resins, it’s obvious that they can be used as transparent coating.
“Ultraviolet curable transparent acrylic modified resins were prepared by cohydrolysis condensation reaction with 3-methacryloxypropyltrimethoxysilane, methyl trimethoxy silane and dimethyl diethoxy silane. It showed that the prepared transparent acrylic modified resins can be cured completely cured by 10 wt% 2-hydroxy-2-methyl-phenyl-propane-1-one for 30 s with transmittance up to 95% in the light wavenumber range of 400 nm and 800 nm, pencil hardness from 6 B to 6 H, thermal decomposition temperature higher than 150°C,” the researchers concluded. “It also exhibited that the silicone resin prepolymers can be used for ultraviolet-assisted 3D printing. The silicone materials prepared by ultraviolet-assisted 3D printing are with coefficient 0.21 wt% and the line thermal expansion coefficient 5.27 × 10−4 m/k. It showed that the silicone resins can be a candidate material for ultraviolet cured transparent coating and ultraviolet-assisted 3D printing.”
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