Structure-property relationship of nano enhanced stereolithography resin for desktop SLA 3D printer

Abstract

Organically modified nanofillers, including nano SiO₂, montmorillonite and attapulgite were loaded to stereolithography resin (SLR). The surface of nanofillers were modified using organic modifier of 3-(trimethoxysilyl)propyl methacrylate (γ-MPS) and (1-hexadecyl)dimethyl allyl ammonium chloride (C₁₆-DMAAC), and were characterized by FTIR and small angle XRD analysis. The morphology of nanocomposites were observed by TEM. Viscosity and curing speed of SLR nanocomposites at increasing nanofillers loading were also studied. The mechanical properties of printed samples fabricated by a home-made stereolithography apparatus (SLA) 3D printer were tested. The influence of nanoparticles on the accuracy was measured and discussed. It was found that addition of 5% w/w of nano SiO₂ increased the tensile strength and modulus by 20.6% and 65.1% respectively, and the printed accuracy was not significantly influenced. This study opens the way to the application of nanocomposites in the desktop level SLA 3D printing.

Introduction

Rapid prototyping, also known as 3D printing, was developed in the late 1970s. In recent years, on account of development in computer aided design (CAD) and material science, the application of 3D printing popularized rapidly. The accessibility of 3D printers for both industrial and general public use has grown dramatically in the past decade [1], [2]. Among various types of 3D printer, stereolithography apparatus (SLA) is used for producing models, prototypes, patterns, and production parts. During the SLA printing process, the stereolithography resin (SLR) is deposited layer by layer based on the prerequisite model, and simultaneously polymerized by an UV laser or other light sources. Typical SLRs used for SLA attached with a 355 nm wavelength laser are the mixture of monomers cured by the combination of cationic mechanisms and free radical mechanisms [3], [4]. After being cured by the UV light, an interpenetrating polymer network is formed. Although 355 nm SLA 3D printing can produce a wide variety of shapes, it is often expensive mainly due to the 355 nm laser device and the cationic photoinitiator.

Recently, desktop level stereolithography apparatus like Formlabs SLA, digital light projection (DLP) and continuous liquid interface production (CLIP) [5] were developed. These desktop level stereolithography apparatuses usually use 405 nm (blue-ray) waveband laser device or DLP projector, and the free-radical polymerization SLR as the printing materials. When using the 405 nm laser source, the beam of light focused onto the bottom surface of a tank filled with SLR. The light beam draws a layer of the object on the surface of SLR forming a cured layer due to photonic polymerization. The cured layer is attached to the base or previous layer and can be peeled off from the silicon attached on the surface of the resin tank. After that, the base is raised to a certain height. Subsequently, more liquid resins refill the gap between the cured part and silicon. By repeating the above steps, the desired part can be “printed” layer by layer. The illustration of 405 nm SLA 3D printer are shown in Fig. 1. These desktop level 3D printers can fabricate models faster than traditional 355 nm 3D printer, and print at a reduced cost, both in terms of 3D printer and SLR.

However, problems like poor mechanical properties and poor accuracy of SLR are common to most desktop level SLA techniques, limiting their application as functional materials [6]. Therefore, the improvement of the mechanical properties of SLRs is of interest for their applications [7], [8]. Recently, the emergence of nanocomposites has attracted great interest amongst researchers. Usually, the nano dispersion phases include nanosized metal [9], inorganic particles [10], [11], [12], [13], fiber [14], graphene-based nanosheets [15], clay [16], [17] and carbon nanotubes (CNTs) [18], [19]. Among various nanomaterials, silicon containing nanomaterials have been extensively investigated due to their interesting mechanical properties and potential technological applications as reinforcing filler in a broad range of polymers. Nanocomposites consisting of a polymer and silicon containing nanomaterials frequently exhibit remarkably enhanced properties including mechanical properties [20], [21], heat resistance [22], low gas permeability [23] and flame retardancy [24]. These enhanced properties in nanocomposites are mainly due to stronger interfacial interaction between the polymer matrix and the surface of the silicon containing nanomaterials. Because of its special surface chemistry, silicon containing nanomaterials can be easily functionalized, and can be readily modified and dispersed in polymer to form nanocomposites.

Although polymer nanocomposites have been well studied, the application of nanomaterials into SLR is less reported. Moreover, the relationship of morphology and structure of nanomaterials on the rheological properties and mechanical properties of SLR is rarely studied. In this paper, the effect of nanomaterials with different morphologies on the properties of SLR was investigated. Three types of silicon containing nanomaterials were used: silica (SiO₂), attapulgite (ATP), and organic montmorillonite (OMMT), corresponding to sphere (0D), rod like (1D) and sheets (2D) respectively. Unlike traditional moulding or casting processes, the primary concern of UV curable resins used in SLA technique are viscosity and refractive index of the resin. Poor flowability will cause difficulties in refilling SLR, and improper refractive index will absorb or scatter UV light, leading to processing defects, incomplete curing, reduction in mechanical properties and accuracy. Therefore, a systematic study is explored on how the introduction of different morphology of nanofillers change the rheology, UV cure kinetic and mechanical properties of nanocomposites.