1. History of SLA Technology Development

    The rapid prototyping process of UV Laser Curing is now referred to as Stereolithography 3D printing process (abbreviated as SLA). Patented in the United States by Charles Hull in 1984, it is one of the earliest developed 3D printing technologies. Since 3D Systems of America first launched commercialized SLA in 1988, this 3D printing technology has been developing rapidly, which can automatically print out complex 3D objects that are difficult to be made by various other processing methods, representing an epoch-making significance in the field of processing technology. SLA utilizes computer-controlled UV laser to scan point by point on the photosensitive resin liquid level according to the shape of the two-dimensional cross-section and thus to cure the resin. Then the cured resin forms a two-dimensional shape. This process is repeated layer by layer to finally obtain a complete 3D object, with its quality mainly depending on the properties of the photosensitive resin.

    For the power of the UV laser used is very small, usually measured in milliwatts, the photosensitive resin must be sensitive to UV light, so that it can be cured with a relatively small amount of UV exposure. Therefore, developing the photosensitive resin with good photosensitivity and high precision has always been one of the hot spots in the research on SLA 3D printing technology. Meanwhile, due to the presence of small-molecule photosensitive diluents in the photosensitive resin, which is composed of photosensitive prepolymers, photosensitive diluents, and initiators, the parts made from photosensitive resin has poor heat resistance and is prone to thermal deformation. Therefore, developing the photosensitive resin with good heat resistance and high precision has also been one of the hot spots in the research on SLA 3D printing technology.

2. Development of SLA Photosensitive Resin

    The development of photosensitive resin applied in SLA 3D printing can be roughly divided into three stages. In the early stage (1988-1995), the prepolymers of commercialized photosensitive resin for SLA rapid prototyping were acrylate prepolymers with free radical initiators. Under the action of UV light, a radical initiator decomposes into radicals, which initiate the polymerization of acrylate molecules, one by one, to form polymer compounds with high molecular weight. Free radical photosensitive prepolymer acrylate mainly features better photosensitivity compared with that of cationic photosensitive prepolymer epoxy resin; however, it has a greater shrinkage during polymerization, which leads to poor dimensional accuracy of the manufactured parts, and makes it prone to warping and deformation. Therefore, it is difficult to meet the precision requirements and has gradually been replaced.

    In the second stage, pure cationic photosensitive resins composed of prepolymer epoxy resin and cationic initiator were mainly used. There were many types of cationic photosensitive prepolymers, mainly including bisphenol A-type epoxy resin, phenolic epoxy resin and cycloaliphatic epoxy resin. In principle, all of these three types can be used as cationic photosensitive prepolymers with the last type having better photosensitivity. The reason is that the conjugated large bond of benzene ring in bisphenol A-type epoxy resin and phenolic epoxy resin has an inductive effect on the electrons of epoxy group in the molecule, reducing the density of electron cloud of epoxy group and thus decreasing the reactivity with electrophilic reagent protonic acid. In contrast, there is no phenomenon of inducing electrons of epoxy group in the molecule and reducing the density of electron cloud by the conjugated large bond of benzene ring in cycloaliphatic epoxy resin, and they have high reactivity with electrophilic reagent protonic acid.

    In recent years, commercialized photosensitive resin for SLA 3D printing abroad, with its photosensitive prepolymer containing both acrylate and epoxy resin and its initiators including both free radical initiator (to initiate the polymerization of acrylic resin) and cationic initiator (to decompose into protonic acid which is to initiate the ring opening polymerization of epoxy resin under the effect of UV light), features smaller shrinkage rate than double bond rupture of acrylic resin. Therefore, the precision of parts manufactured with such kind of free radical-cationic photosensitive resin is obviously better than that of pure free radical photosensitive resin.

3. Advantages of Epoxy Resin in 3D Printing

    In 3D printing photosensitive resin, the epoxy resin features excellent mechanical properties, stable chemical properties, high/low temperature resistance, low shrinkage rate, low costs, etc.

    From the molecular point of view, the curing process of photosensitive resin is to transform from small molecules to polymers with long-chain macromolecules, with significant changes in molecular structure. Therefore, shrinkage is inevitable during the curing process. The shrinkage of resin is mainly divided into two parts. One is curing shrinkage, and the other is thermal expansion and cold contraction caused by temperature changes when laser scans the liquid level of resin. Besides, the area of temperature increase is small, so the amount of shrinkage caused by temperature change is small and negligible. The impact of the volume shrinkage generated during the photocuring process of photosensitive resin on the precision of parts cannot be ignored. The volume shrinkage generates the shrinkage stress and thus results in the wrap and deformation of parts. Upon acrylic resin curing, the polymerization reaction of carbon-carbon rupture may cause large volume shrinkage, while the ring opening reaction may occur during the process of epoxy resin curing, so the volume shrinkage is relatively small. As can be seen from the results in the figure below, from the shrinkage test with Tetra cycloaliphatic epoxy resin, the volume shrinkage of the cycloaliphatic epoxy resin is significantly lower than that of the acrylate under the photocuring conditions.

Formula: Resin: Photo-initiator=100:0.5 6145-100: Cycloaliphatic Polyurethane Hexaacrylate

    Cycloaliphatic epoxy resin features low viscosity, good weather resistance, low curing shrinkage, high crosslinking density and high reactivity, so it is widely used in SLA 3D printing as the photosensitive resin, and considered as one of the most important matrix oligomer.

    For such applications, the following four cycloaliphatic epoxy resins from Jiangsu Tetra have been fully applied in practice and can meet most requirements of photosensitive resins for SLA 3D printing both at home and abroad.