6th November 2024
Abstract
Glass flake grade GFE-PF350-Acr manufacture by Glassflake Ltd is an alumina borosilicate glass with average flake diameter 12 µm and flake thickness 350 nm surface treated with 3-(trimethoxysilyl)propyl methacrylate. Glass flake was incorporated into a commercial SLA resin at various loadings and printed using an LCD-SLA printer to manufactured glass flake composites. Glass flakes can be incorporated into the resin by simple means of mixing and do not necessitate an increase in layer curing times. Glass flakes impart an improvement in tensile strength and modulus. The enhancement ranging from 43 – 108 % and is proportional not only to the flake loading, but the print orientation which in turns dictates the flake alignment relative to the applied force. It is also demonstrated that simple modifications such as the addition of a copolymer dispersant have a drastic impact of the uniformity of dispersion of the glass flakes that is crucial in enhancing mechanical properties.
Background
Additive Manufacturing (AM) techniques have the potential to revolutionize industrial manufacturing processes. Early adopters of these techniques such as the dental [i], pharmaceutical [ii], aerospace [iii], military [iv] and automotive industries have demonstrated benefits of productivity by simplifying workflows and removing supply bottlenecks through localisation. As well as reducing waste. Furthermore, AM techniques give the ability to produce structural designs, in a single step, that are in fact impossible using other manufacturing techniques, such as machining or injection molding.
There are two major classes of plastic Additive Manufacturing, often referred to as 3D printing; Fused Filament Fabrication (FFF) and Stereolithography (SLA). FFF uses the extrusion of familiar thermoplastics such as polypropylene, polycarbonate etc. SLA uses specific photo-curable resins and itself encompasses a range of technologies of similar principle where the ultraviolet (UV) light source and method of controlling exposure varies. One such technology, LCD-SLA (Liquid Crystal Display) (Figure 1) discussed herein, uses a UV-LED (Light Emitting Diode) light-source for uniform exposure of the build area and selective exposure is achieved by an LCD masking screen.
Figure 1. LCD-SLA additive manufacturing scheme.
Many AM techniques have historically been used for rapid prototyping purposes but have seen limited application in final product manufacturing on several counts. Namely; equipment limitations such as build volume, reproducibility and precision. All fundamentally cost limitations. But with many new professional 3D printers released each year there are now a host of options for manufacturers at more accessible pricing. Furthermore, many commercial accessories for post processing such as print washing and curing are now available and make the entry to 3D printing more convenient than ever.
One chief limitation more challenging to overcome, is material. Unlike FFF which uses the exact thermoplastics that would be used in traditional manufacturing process, such as injection moulding SLA often uses propriety resins that are simply marketed as being “tough”, “durable” or “flexible” or as analogues of thermoplastic materials such as “ABS-like”. It is often difficult to choose the right material for a particular application, especially for a non-expert, and these resins may not necessarily be of engineering grade quality. They often fall short of the performance of analogous thermoplastics and are sensitive to the conditions of post processing.
There is a demonstrable need not only to diversify and improve the portfolio of commercial resins available but also promote a better understanding of these materials as a whole. An emerging field of materials in SLA AM is composite resins. There are available a range of commercial resins incorporating spherical fillers including alumina or glass microbeads. Providing benefits to properties such as mechanical performance, chemical resistance, UV resistance and temperature performance. The use of anisotropic reinforcing additives in additive manufacturing is less well documented [v].
To this effect we demonstrate a means of adjusting material properties by incorporation of glass flakes. Glass flakes are ubiquitous in coatings systems [vi] and traditional manufacturing of reinforced thermoplastics [vii] and have recently seen interest the manufacture of bio-inspired composites [viii]. Glass flakes are typically superior to spherical fillers in areas such as chemical resistance and tensile properties. Comparable in most other aspects. But notably poorer in impact resistance. Many of the benefits are expected to be universal, regardless of the manufacturing process, but the morphological differences may entail different processing practices.
Glass flakes are characterised as having a platelet structure of a specific thickness and diameter. They are highly planar high aspect ratio particles. They can also be produced with different chemistries and coated with various compounds including organo-silanes to further improve properties such as matrix adhesion and mechanical performance. The process and functionality of using silanes in glass filled systems is well documented [ix].
Aims
The scope of this study is to investigate the potential of glass flakes to improve the properties of SLA resin systems, primarily mechanical. As proof of concept, we report the enhancement of tensile strength and modulus of a commercial resin by addition of glass flake. Demonstrating how easily glass flakes can be added to SLA resins, the primary goal is to promote research in this area.
Methods & Materials
Glass flake materials were manufactured in-house by Glassflake Ltd using propriety means. The material discussed is commercially available grade, GFE-PF-HH-Acr having an average thickness of 350 nm thickness, and average particle size diameter 12 µm. The glass flake was surface functionalized with an organo-functional silane: 3-(trimethoxysilyl)propyl methacrylate), giving acrylic functionality.
Prior to SLA printing glass flakes were added to the commercial resin and dispersed using an overhead stirrer (Radleys Hei-Torque) with A VISCO-JET stirrer head. The glass flakes were added gradually to the required loading to avoid agglomeration. 5 wt. % of BYK057 de-foamer and 0.5 wt. % BYKA55 de-aerator was also added. The resulting mixture is stirred for 1 hr at 600 rpm then 20 mins at 300 rpm. The stirrer head was selected for its moderate shear rate whilst also providing total circulation of the container contents and minimizing foaming. High Shear rates are productive in dispersing glass flakes as they display shear thinning behaviour based on their platelet morphology [x].
Thickness measurements were performed using a Hitachi TM4000 Scanning Electron Microscope (SEM) and a custom automated thickness measurement system. Particle size analysis measurements were made using a Malvern 3000 Mastersizer using dry dispersion. Additional imaging was performed on a Hitachi Flex-SEM II, SEM.
All 3D printed items were produced using a Halot-Sky LCD 3D Printer, with AnyCubic Black commercial resin (405 nm). Layer exposure times were optimized using a variety of open-source calibration models for the base unfilled resin.
Post-processing was performed using a FormWash (2 x 20 min, IDA95 ethanol) and FormCure (405 nm, variable conditions room temp – 70 °C, 10 – 80 mins).
Tensile and flexural test data was acquired using a NETSCH Universal Tester with a 2.5 kN load cell.
Moisture vapor transmission rate (MVTR) was performed according to ISO 2528:2017 using two coating thicknesses; 1 and 2 mm.
Dynamic Mechanical Analysis (DMA) was performed on a Perkin Elmer DMA800. Sample dimensions 15 x 4 x 3 mm flexural, 10 x 4 x 3 tensile (length x width x thickness). Heat Deflection Temperature (HDT) is defined in ASTM D648 as the temperature at which a 0.25 mm deflection is observed under a stress load of 1.8 MPa with a heating rate of 2 °C min-1 and dynamic frequency 1 Hz. In the DMA experiment HDT temperature is approximated as the temperature at which under identical load conditions the equivalent strain in the standard (0.121 %) is achieved for the sample dimensions described.
Differential Scanning Calorimetry (DSC) was performed on a TA Instruments DSC Q20 using a 10 °C / min ramp rate under N2 atmosphere.
Results & Discussion
General Observations
The addition of glass flake did not necessitate any increase in the layer exposure time for successful. As the ECR formulation has high transmittance at 405 nm (> 80 %, ℓ = 1 cm, bulk glass) this was expected. The impact of glass flake addition on the extent of curing from print exposure was also investigated using UV-DSC. But no significant difference was observed. This supports this observation qualitatively but further refinement of the experiment would be required for a quantitative assessment.
Initial cure duration and number of layers were increased and withdraw speeds reduced to promote build adhesion as increasing glass flake content does promote delamination.
Using the commercial resin composite with loadings of up to 20 % wt. of GFE-PF-HH-Acr could be printed without issue. Above 20 % wt. failed prints became more frequent to delamination. Minor incremental increases may have been possible by further optimising the print settings. But this was deemed beyond the initial scope of this stud as it was concluded that the viscosity limit suitable for the printer type was being approached. Many professional systems optimised for higher viscosities exist. Typically, Laser-SLA systems where the exposing light source is of higher intensity or having features such as heated resin vats.
Due to their natural leafing tendency, the glass flake remains parallel to the print platform throughout the print process [xi]. Visual and SEM inspection of the samples suggests the flake is typically evenly distributed throughout the course of the print. There is no obvious gradient of the flake loading across the z-axis of the print because settling of the flake is not an issue within the maximum tested print duration, ca. 6 hours. The acrylic silane surface treatment on the flake likely has some benefit in dispersity and suspension stability but its primary function is to promote adhesion of the flake and resin. Component interfacing is paramount in composite performance. Figure 2 shows the characteristic cross-sections resulting from tensile test failure for an unfilled and glass filled sample. The absence of any voids surrounding the flake suggests good adhesion flake-matrix adhesion.
Figure 2. Flake distribution in final (a) vs. first print layer (b) and tensile break cross-sections for; unfilled resin (c) 20 wt. % GF (d).
Mechanical Testing
Because of the anisotropic reinforcement nature of the glass flake and inherent to the SLA print process, it is expected that depending on print orientation, different degrees of mechanical reinforcement will be observed.
Figure 3 shows a diagram of a representative build plate and the print orientations explored, designated; edge parallel, face parallel, Z, edge 45° and face 45°. The orientation of the flake within the resulting composite and the relative orientation to the applied force in tensile and flexural test modes is also shown.
Figure 3. Orientation of flake alignment vs. applied force as dictated by print orientation.
Figure 4 shows the ultimate tensile strength as a function of print orientation and loading. Relative to the value reported in the material TDS 23.40 MPa it is unsurprising that the ‘z’ aligned sample has the lowest strength as in this case the maximum number of print layers is perpendicular to the applied force [xii]. As are the glass flakes in the filled analogue. In the face parallel and edge parallel orientations the enhancement in ultimate strength was proportional to the loading. The absolute values between these two print orientations are similar as the layer count and orientation of the flakes is similar. In the other print orientations, there was no statistically significant change in ultimate strength based on addition of glass flake. The flexural modulus is directly proportional to the loading for all print orientations (Figure 4b). Concurrently, the extension at break is inversely proportional to the glass flake loading as the glass flake imparts stiffness (Figure 4c).
Figure 5 presents the analogous study for flexural strength (three-point bend). In this case all orientations expect the ‘z’, show a direct correlation of flake loading and strength. However, initially addition of Glassflake reduces the ultimate strength, but at 20 wt. % addition the strength is mostly restored to the initial levels of the blank in the face parallel, edge parallel and face 45 print orientations. This trend suggests that loadings beyond 20 wt. % could produce an improvement in flexural strength but it was not possible to validate given the restraints in viscosity previously mentioned.
Figure 4. a) Ultimate tensile strength, b) tensile modulus and c) strain as a function of glass flake loading and print orientation.
Figure 5. a) Ultimate flexural strength and b) modulus as a function of glass flake loading and print orientation.
Impact of Flake Dispersion
Significant batch to batch variance in the mechanical performance of the commercial base resin is observed. The exact cause of this is unclear. Given the purpose of the study is proof of concept. With aims to report on general means of improvement of performance. Not to achieve a benchmark of performance or overly specific to this resin system. The choice was made to focus on issues relating more directly to the flake as this knowledge would be transferable. Then to narrow the scope to only the edge and face parallel print orientations.
In composite preparations like this, the individual constituent materials may have very dissimilar chemical properties. Regardless of the manufacturing method, achieving a sufficient degree homogeneity is paramount in the ultimate performance.
Initially, large variance between replicate samples was found and reasoned due to occurrences of poorly dispersed glass flakes. Poor performance would therefore not be attributed to a gradient of the flake loading across the z-axis of the print. Instead, there was some evidence of agglomerates that must be reasoned to have persisted from the initial mixing stages of the liquid resin preparation. Such agglomerates reduce the effective aspect ratio of the flake and produce poor interfacing with the matrix as some flakes are not fully wetted by the matrix. Thus, forming nucleation points for failure. Addition of a copolymer dispersant, UNIQPSERSE 650U was found to improve both initial dispersion of the flake and long-term suspension stability.
Figure 6 demonstrates the effect of dispersant addition on the final composite tensile strength. A control experiment confirms the addition of UNIQPSERSE 650U alone has no impact on the tensile properties alone. Addition of 20 wt. % glass flake without a dispersant and a poor mixing regime can be seen to inconsistently impact the tensile strength based on prior reasoning. Each bar represents a minimum of three replicates, and it can be seen in many cases there is no statistically significant improvement. With addition of the dispersant, a consistent and significant enhancement in the ultimate tensile strength is achieved. This ranges from 56 to 99 % for the edge parallel print alignment and from 43 to 108 % enhancement in the face parallel print orientation. The impact of a range of post-cure conditions is also displayed and further discussion comes in the following section.
Figure 6. Effect of dispersant and post-cure conditions on ultimate tensile strength; top) edge parallel, and bottom) face parallel print orientation.
Post-Cure Conditions
Post-processing is an essential part of the SLA print process. Not only does it help produce the final aesthetic finish of a printed piece, it will also terminate any unreacted, potentially hazardous resin monomers. It is also known to drastically impact the final mechanical properties of the printed article. Many commercial resins marketed as being “engineering grade”, or similar, do not reach peak performance without receiving a significant post cure. Formlabs Rigid 10K (a glass microsphere containing resin) for instance advises a 60 min UV-cure at 70 °C, plus an additional thermal cure of 125 minutes at 90 °C [xiii]. As such, the ability to produce composites with superior mechanical properties, obtained with milder post-cure conditions, also presents a significant energy / cost saving opportunity. To this effect we have investigated the impact of post-cure conditions on these glass flake composite prints.
A comparison of both ultimate tensile strength and tensile modulus for the unfilled resin (0 wt. %) and 20 wt. % glass loading as a function of post-cure conditions can be seen in Figure 7. Note there are two components to the post-processing treatment, time, and temperature. The individual contributions of which have not been established. The heat treatments are generally presented as increasing in intensity across the x-axis but is for illustrative purposes only.
It was found that for unfilled samples the ultimate tensile strength was independent of the post cure conditions explored. However, the tensile modulus was found to increase linearly. This can be reasoned due to a degree of increased cross-linking which imparts rigidity. But the commercial resin used is not expected to contain a high portion of crosslinking agents, so the impact is minimal and as demonstrated doesn’t translate to an increase in ultimate strength. Conversely, there is an evident trend of both increasing ultimate strength and modulus for the glass flake filled composites. The impact on the tensile modulus is very similar in the unfilled and glass filled samples because cross-linking is induced in the matrix, which is identical in either case. But the improvement in ultimate strength can be reasoned to due to improved interfacing between the glass and polymer matrix due to additional cross-linking via the silane surface functionalisation of the flake.
Figure 7. Collated mechanical properties of unfilled vs. 20 wt. % glass flake (5 % UNIQ) as a function of post processing conditions.
Other Benefits
Moisture Vapour Transmission
The use of Glass Flake is ubiquitous in anti-corrosive coatings vi. The primary mode of function is to reduce the Moisture Vapour Transmission Rate (MVTR) to the substrate by formation of a “tortuous pathway” of glass flakes distributed throughout the resin matrix. The orientation of the flake is again critical as it dictates the “tortuosity” of the transmission route [xiv].
The same tortuous pathway model can apply to the permeation of other vapours or gases [xv], solvent, oxygen [xvi] etc. which can contribute to resin degradation and mechanical failure and presents benefits in the manufacture of items with sealing requirements.
Table 1 presents the moisture vapour transmission rates of unfilled resin and glass filled composites of two thicknesses. The addition of 20 wt. % glass flake has the approximate effect of doubling the coating thickness.
Table 1. Moisture Vapour Transmission Rates of an unfilled resin and glass flake filled composites.
High Temperature Performance
Being non-combustible glass flake can provide flame retardant benefits to composites. The simplest mechanism being it replaces a fraction of the combustible resin matrix. It also provides mechanical reinforcement to the char. The tortuous pathway effect can thereby function to reduce the volatilisation of resin matrix decomposition products into the gas phase that can propagate the fire event [xvii].
Delayed mechanical failure is also of benefit to reducing fire spread and the ability to perform at elevated temperature opens up different avenues for use for 3D printed items competitive with traditional thermoplastics. Heat Deflection Temperature (HDT) is a measure of a polymer’s resistance to physical deformation under a given load, as a function of temperature. As expected, incorporating glassflake positively impacted HDT, average 3.1 °C (7 % increase) as presented in Table 2. The increased rigidity of the composite imparted by the glass offsetting the matrix softening as temperature rises. Modulus remains independent of print orientation.
Dynamic analysis was also performed under tensile loads. Because HDT is determined by standardised testing procedure specific to the three-point bending mode (see Materials and Methods). Instead the modulus at 44.2 °C, the HDT determined for the blank sample is compared. The flexural modulus at HDT is 43 % of its initial value for 0% GF. Whereas 20% GF retains 51 % of the initial stiffness. The same trend for tensile loads is observed, where 0% GF sample retains 41 % of the original stiffness versus 49 % for 20% GF.
An example DMA plot is given in Figure 8. Glass transition temperature Tg can most easily be defined by peak tan δ. This is also impacted by addition of glass flake. Perhaps counterintuitively, despite an increase in HDT, Tg is slightly lowered (6.3 °C by average). This is reasoned purely as a geometric effect given the broad appearance of the loss modulus (E’’) peak and higher storage modulus (E’) of the composite. Concurrently, no statistically significant difference in the Tg was observed using DSC. Alternately, this could be reasoned due to a slight increase in the matrix free volume imposed by packing restrictions of the matrix macromolecules due the presence of the glass flakes [xviii].
It is also noteworthy that in all cases total sample failure for 0 % GF samples occurred around 140 °C where 20 % GF samples remained intact for the entirety of the measurement range up to 180 °C.
Table 2. Heat Deflection Temperatures of unfilled and glass flake filled composites
Figure 8. Flexural DMA plot, storage modulus (E’), loss modulus (E’’) (inset) and tan δ vs. temperature.
Conclusions
Glass flake composites up to 20 wt. % were manufactured by SLA-LCD 3D printing. We demonstrate that glass flakes can be incorporated into a commercial 3D printing resins by simple mixing methods. Addition of a co-polymer dispersant greatly improves initial dispersion of flake resulting in a uniform composite.
Because of the leafing nature of the glass flake, the flakes maintain alignment parallel to the build plate. Thus, the orientation of the print dictates the relative alignment of the flake providing a means to direct the anisotropic reinforcement properties of the glass flake. Glass flake imparts stiffness and depending on print orientation, an enhancement of tensile strength.
The commercial resin mechanical performance was found to be independent of the post-cure conditions, but both the tensile strength and modulus increased with glass filled composite. This was reasoned due to increased interfacing of the organo-silane functionalisation of the flake and the resin matrix.
Future Work
Low cost of entry resin and 3D printing equipment was selected to demonstrate the simplicity of producing glass flake composites with material benefit. But there is a considerable amount of research yet to be done in this area for performance optimisation. This work with LCD-SLA serves as a proof-of-concept for how the flake morphology behaves within SLA additive manufacturing techniques generally. But it is limited in that it is restricted to low viscosity resins – prohibiting higher glass flake loadings and those of higher aspect ratio. Laser-SLA is significantly less limited in this regard so is a simple route to exploring higher glass flake loadings. Furthermore, resins can be formulated specifically to accommodate large loadings of glass flake or more simply the addition of diluents to commercial resins can be explored.
Further Reading
Many of the observations here are transferrable to other glass flake reinforced plastics. Glassflake Ltd is currently conducting further work in this area not only with glass flakes but also the effect of graphene-coated glass flakes expected to further enhance property having a synergistic effect of both reinforcing materials at a more cost-effective proposition than using graphene alone. The properties affected can be; UV-light resistance, chemical resistance, tensile and other mechanical properties, creep compression and dimensional stability.
Keywords; Additive manufacturing, 3D Printing, SLA Printer, FFF Printer, Glassflake, Glass flake, Flake glass, mechanical properties, polymer reinforcement, aerospace, automotive, dental, medical devices.
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Lee has been with Glassflake since 2018, overseeing all aspects of Research and Development. Lee's work encompasses glass formulation changes, process developments and collaborating with customers to assist in their development projects involving glassflake technology.
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