Sustainable Three-dimensional Printing of Waste Paper-based Functional Materials and Constructs

doi:10.21203/rs.3.rs-4535114/v1

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Sustainable Three-dimensional Printing of Waste Paper-based Functional Materials and Constructs

https://doi.org/10.21203/rs.3.rs-4535114/v1

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published 28 Sep, 2024

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3D printing is a prominent technology across various industrial sectors, and its increasing popularity urgently calls for sustainable 3D printing materials. However, the availability of such materials remains limited. Here, we present a low-cost strategy that harnesses waste papers as a feedstock for developing sustainable 3D printing inks. Our strategy offers remarkable printability and utilizes widely available biodegradable paper wastes to produce 3D printed constructs with satisfactory mechanical properties for common applications and shape stability for servicing at high temperature. Our constructs can be efficiently recycled into inks for reprinting, and our method can be applied to various types of waste papers. By employing multi-material printing, our approach can be extended to produce multi-colored constructs, security information printings and mechanically appealing designs. Our strategy offers an innovative and sustainable solution that addresses the need for repurposing paper wastes, which would otherwise end up in landfills, while concurrently reducing the reliance on virgin plastics for 3D printing.

3D printing

sustainable

waste paper

recyclability

3D printing has been offering remarkable flexibility and diversity in material and structure designs, revolutionizing a wide variety of industrial sectors, including manufacturing, automotive, medical and laboratory industries[1–5]. As the technology continues to evolve, the increasing demand for 3D printing materials exacerbates the environmental concerns about the materials used[6]. Among 3D printing materials, thermoplastics (e.g. polylactic acid and acrylonitrile butadiene styrene) and light-curable resins are widely used[2]. However, these materials, typically based on non-renewable resources, often exhibit poor degradation rates (Supplementary Table 1)[7–9]. Their production requires high energy demand (> 43 MJ/kg), and results in a high global warming potential (> 2 kg CO₂ eq/kg) (Fig. 1a and Supplementary Table 2)[10]. Additionally, light-curable resins can lead to harmful health effects to users due to the emission of volatile organic compounds (VOCs), and the uncured resin wastes are known to present hazards to the environment and aquatic ecosystems[11, 12]. Although recent advances have established various sustainable and recyclable 3D printing materials, including recyclable thermosetting photopolymers[6, 13–15] and cellulose-based materials[16–19], these materials still encounter certain limitations restricting their widespread adoption. One drawback lies in the production and recycling processes of recyclable thermosetting photopolymers, which involve inefficient, complex and costly chemical treatments. Additionally, while numerous studies have depicted the production of 3D printing materials and hydrogels using cellulose microcrystalline, nanocrystal (CNCs) and nanofibers (CNFs) materials with improved printability and stimuli-responsive functionalities, these studies often rely on virgin cellulose feedstocks for production[20–22]. The reliance on virgin resources contradicts the principles of sustainability. It is also worth noting that the current processes to produce nanocellulose, such as CNCs, are highly energy-intensive, time-consuming and require the use of toxic chemicals[23]. Detailed discussion about the limitations of cellulose-based 3D printing can be found in Supplementary Note 1. Therefore, it is highly desirable to explore the possibility of 3D printing using waste materials that are recyclable, degradable, easy to process and can possess adequate mechanical properties for daily applications. The exploitation of such materials would align with the principles of the 3Rs-reduce, reuse and recycle.

One promising sustainable, biodegradable and recyclable choice for 3D printing is waste papers. Derived from renewable lignocellulosic biomass, papers have been integral to our daily lives for over 2,000 years[24]. Today, a multitude of paper products has been created for various everyday practices, from documentation to packaging, and the global production rate of papers reaches nearly 400 million tons per year[25]. However, the current recycling rate of papers is unsatisfactory, contributing around 10–25% of the global landfill waste[26, 27]. The accumulation of paper waste in landfills poses a significant environmental burden. Therefore, with the growing demands for 3D printing materials, repurposing waste papers into printing materials at their end-of-life presents a favourable solution to address both the environmental impact of 3D printing and the paper waste issue.

Here, we present a simple yet economical strategy for repurposing diverse types of waste papers, such as office papers, newspapers and cardboards, into sustainable 3D printing inks (Fig. 1b). By utilising waste paper fibres to formulate the inks, our inks possess excellent printability, and do not require elevated temperature for extrusion. Complex geometrical objects with overhanging designs can be created with ease, while possessing numerous remarkable properties, such as exceptional thermal resistance of up to 200 ^oC, sufficient mechanical integrity for general use, and the capability to be recycled into 3D printable inks when it reaches the end of its lifespan. Through multi-material printing of different types of paper inks, we demonstrated that multi-coloured constructs, encodable prints for information security and mechanically appealing biocomposite designs can be realised. Our strategy offers a range of environmental and economic benefits, not only providing waste papers a second life but also paving the way for the sustainable future of 3D printing.

2.1. Waste papers as sustainable inks for 3D printing

Our strategy is capable of creating printable inks from waste papers that possess the essential rheological properties required for direct ink writing (DIW) (Fig. 1b), a popular and economical extrusion-based 3D printing technique[3, 13]. By formulating paper-based inks that are suitable for DIW, the widespread utilization of the inks can be enhanced. (Supplementary Fig. 1 depicts the ink preparation procedure. We first used waste office papers that are prevalent at home and workplace to exemplify our method. Waste papers are broken down into small fibres using low-cost shredding and grinding methods (Fig. 1b and (Supplementary Fig. 1). Subsequently, the paper fibres were mixed with water and passed through a sieve to filter out large particles (> 100 µm). The above mechanical treatments transformed the paper fibres from ribbon-shaped (655 µm in fibre length) to fibrillar and sponge-like morphologies (10.6 µm in fibre length) that are suitable for extrusion (Fig. 1c and (Supplementary Fig. 2). Centrifugation was then used to attain a soggy paper pulp composed of lignocellulosic fibres. Finally, a paper ink was obtained by mixing the paper pulp with a small amount (10 w/v%) of polyacrylamide (PAM).

Our paper inks exhibit the requisite rheological characteristics for direct ink writing. Compared with inks composed of only PAM, our paper ink possesses a significantly higher viscosity that favours printability (Fig. 2a). In addition, the ink exhibits a pronounced shear-thinning response. The apparent viscosity decreases sharply by four orders of magnitude (10⁴ to 10⁰ Pa·s) when the shear rate is increased from 10^− 2 to 10² s^− 1, enabling a smooth extrusion[28]. The low yield stress behaviour of our ink further facilitates the extrusion at modest pressure. Above the yield stress of ~ 3 × 10² Pa, the storage modulus of the ink drops dramatically (Fig. 2b). Our paper ink also exhibits a fast elastic recovery behaviour that promotes shape retention after extrusion. The ink can rapidly recover its storage modulus (> 10⁴ Pa) after shearing above its yield stress, indicating its ability to preserve the printed shape for supporting successive layers (Fig. 2c). All of these properties make our ink highly suitable for DIW. PAM serves as a dispersant in our ink formulation to prevent sedimentation of the paper fibres, and its concentration is optimised. The use of 10 w/v% of PAM can significantly prevent ink sedimentation and ensure smooth printing (Supplementary Fig. 3–4). Without PAM, sedimentation of paper fibres occurs, leading to nozzle clogging. To enhance the eco-friendliness of our inks, we have also tested the use of natural dispersants, such as sodium hyaluronate and sodium alginate (Supplementary Fig. 4–5). Similar dispersion effects and printability were observed. Of note, for the rest of the experiments, we mostly used PAM as the dispersant unless otherwise specified.

By leveraging the rheological properties of our paper inks, our ink can be printed layer-by-layer under their own weight with high precision (Supplementary Video 1). Sharp filamentary shape was enabled with no significant spreading marked at the intersection for various infill patterns (i.e., honeycomb, rectilinear and triangular infills, (Supplementary Fig. 6f and Supplementary Videos 2–3). In addition, through control of the printing setting (e.g., printing rate and extrusion flow rate), only a slight geometry deviation was observed between the computer-aided design (CAD) model and the 3D printed (3DP) model ((Supplementary Fig. 6e. To gain further insight into the printability of our paper ink, we fabricated a series of 2D and 3D objects with complex geometric features, such as a spiral ring, a staircase and an ancient Chinese temple (Fig. 2d). Noteworthily, as explicitly demonstrated in the 3D printed temple, suspended and overhang features with an inclination angle of 45^o can be neatly printed, similar to the maximum overhang angle in thermoplastic extrusion-based 3D printing[29]. Moreover, unlike conventional thermoplastics used in extrusion 3D printing (i.e., fused deposition modelling) that require high temperature for extrusion[30], our paper ink can be easily extruded at ambient conditions. The printed features remain stable during a 2-hour printing time, indicating the potential for achieving large-scale printing with considerable height using our inks. After printing, the 3D printed constructs were dried by either freeze-drying or ethanol exchange drying methods, resulting in solidified 3D objects.

2.2. Longevity, recyclability and applicability to a wide range of paper types

The paper constructs have a long service lifetime. We found that the structure of the constructs remains stable, and no sign of micro-organism growth was found over storage of 18 months under ambient temperature and humidity (Supplementary Fig. 7). In addition to the service lifetime, the paper inks exhibit a long shelf-life. The ink maintains its printability after storage for at least 8 months in a sealed bottle (Supplementary Fig. 8). No significant change in the rheological properties was observed, indicating the stable dispersion of paper fibres within our formulated ink (Supplementary Fig. 8a-d).

Circular life cycle is an important aspect towards a sustainable 3D printing practice, given the high failure rate during the 3D printing resulting from incorrect geometry or printer malfunctions[31, 32]. When the constructs reach the end of their useful life or possess printing flaws, our paper-based 3DP constructs can be recycled and processed into printable inks using an approach similar to that used for preparing the virgin inks, where mechanical treatments were used to cleave the printed constructs and lignocellulosic fibres of papers were collected for recycling ((Supplementary Fig. 9a). The recycled inks exhibit no observable change in the rheological properties. Similar ink viscosity and storage modulus were attained, despite being recycled for five times (Fig. 2e and (Supplementary Fig. 9b). Thus, excellent printability was preserved in the recycled prints (Fig. 2f).

Our protocol can be readily extended to repurpose diverse types of waste papers. In addition to the office papers, we could readily create inks from newspapers and cardboards that inherit their colour characteristics using the same ink preparation method (Fig. 2g). When formulated with the same pulp-to-PAM ratio, these paper inks could be engineered with rheological properties analogous to those of office papers, including storage moduli, shear-thinning properties and elastic recovery behaviour (Supplementary Fig. 10–11). Therefore, good printability was similarly realised without the need for reoptimizing the printing parameters (Fig. 2g).

2.3. Multi-material printing of coloured and information-encrypted paper constructs

Multi-material printing offers the capability to produce constructs with disparate properties, such as colour and structures. Our office papers inks can be easily stained with dyes owing to the whiteness of office papers. Through multi-material printing of office paper inks dyed with different colours, we successfully created sophisticated colourful constructs, such as a paper vase (Supplementary Fig. 12 and Supplementary Video 4). This is in contrast to the dark lignin inks developed in previous studies that have colour-tuneability limitations[33, 34]. A wide range of colourful paper inks can be readily crafted for producing attention-grabbing objects.

Further, by harnessing the fluorescent whitening agents present in high-brightness office papers ((Supplementary Fig. 13), we created inks with fluorescent and encodable identities as information carriers, analogous to the “invisible ink” in steganography. Through co-printing this ink with a non-fluorescent paper ink made from tissue papers, we created an encoded motif of the inventor of the papermaking process, ‘Cai Lun’ (Fig. 3a). The encoded motif was scarcely visible under ambient lighting but decrypted readily under UV light (Fig. 3b). Despite multiple paper inks being involved, no delamination was observed, and the embedded fluorescent pattern remained intact under severe structural deformation, including bending, folding and rolling (Fig. 3c and Supplementary Video 5). Additionally, the information system can be encrypted, decrypted and recycled. As an example, we created a QR code with concealed information that is decodable by scanning the code on a smartphone under UV light (Fig. 3d – f). Moreover, the 3DP samples can be permanently destroyed and reformulated into another batch of paper inks (Fig. 3g), readily applicable for another cycle of 3D printing of a different information-embedded print (Fig. 3g) or a print for other general purposes. This highlights the potential of our waste paper ink for information encryption and security printing technology.

2.4. Functional paper constructs with stiff and compliant mechanical designs

Next, we explore the mechanical properties of our paper constructs dried by different methods, including freeze-drying, ethanol exchange drying and air drying, as these properties are crucial in determining their practical applications (Fig. 4a and (Supplementary Fig. 14). Our office paper constructs possess specific tensile moduli ranging from 10^− 3 to 10^− 5 GPa/(kg m^− 3), which is approximately an order of magnitude higher than that of typical elastomeric materials (Fig. 4a). Additionally, their specific strengths range from 10^− 2 to 5 × 10^− 3 MPa/(kg m^− 3), approaching the values reported for elastomers (Fig. 4a). The satisfactory structural integrity of the construct is further demonstrated by its load-bearing capability. The 3DP construct that is solidified by freeze-drying can bear a mechanical load of 5,000 times heavier than its own weight without structural collapsing (Fig. 4b). Constructs made from the recycled inks, the newspaper inks and the cardboard inks exhibit mechanical properties (i.e., elongation at break, fracture strength and Young’s modulus) of the same order of magnitude ((Supplementary Fig. 15–16). Noteworthily, our paper-based constructs possess excellent shape stability at high temperature and can excel in applications that require high-temperature resistance, which overcomes the thermal softening limitation encountered in many 3D printing thermoplastics. After being subjected to a high temperature of 200 ^oC on a hot plate for 2 hours, the structural integrity of the paper constructs experienced no observable change, yet traditional thermoplastics (e.g. polylactic acid and thermoplastic polyurethane) became softened and deformed (Fig. 4c and (Supplementary Fig. 17).

Natural structures combining stiff and compliant constituents always demonstrate extraordinary flexibility and strength[35, 36]. While prevalent in natural materials, this integration remains far less explored in engineering structural materials[37, 38]. Thus, exploiting heterogeneous biocomposite designs, such as biomimetic and metamaterial designs, in 3D printing holds great promise for attaining intriguing mechanical behaviours. Our waste paper ink is a desirable eco-friendly candidate for tailoring the mechanical properties of biocomposite constructs[10, 39]. By incorporating waterborne polyurethane, an eco-friendly polymer matrix[40], with our paper ink, a range of stiff-to-compliant mechanical behaviours can be readily attained ((Supplementary Fig. 18). Such dissimilar characteristics can be integrated into a single construct by multi-material printing. We created heterogeneous Miura origami and tessellated structures by co-printing a stiff ink composed of the office paper ink and a soft ink composed of the paper ink and PU (Fig. 4d, Fig. 4g and (Supplementary Fig. 19). Both heterogeneous constructs, consisting of stiff domains (1150 MPa in Young’s modulus) and soft domains (223 MPa in Young’s modulus), exhibit a significant reinforcement in mechanical properties. Compared with the homogeneous constructs entirely made with the soft ink, higher fracture strength and higher toughness were observed under compression (Fig. 4e and 4h). The hard constituents retain the high stiffness, while the soft constituents serve to concentrate the applied stress and provide a means of dissipating strain, as illustrated in our finite element modelling results (Fig. 4f and 4i, (Supplementary Fig. 20 and Supplementary Videos 6–7). This highlights the potential of our waste paper ink for expanding the 3D printing design space towards programmable stiffness designs, creating eco-friendly constructs with reinforced mechanical properties.

In summary, we developed a low-cost strategy that utilise waste papers as a feedstock for creating sustainable 3D printing materials, opening a new avenue for sustainable and economical 3D printing. Differing from the existing works on 3D printing cellulose-based materials, our work does not rely on virgin wood or agricultural feedstocks, which might require costly and time-consuming ink preparation. Instead, we make use of waste papers that are cheap and abundant, making our approach highly scalable. Moreover, our approach can easily process waste papers into 3D printing inks using simple and non-toxic solvents. Hence, our inks well align with the sustainability principles of 3Rs.

Our waste paper inks can be printed at ambient temperature, and the resulting paper constructs exhibit remarkable thermal stability of up to 200 ^oC, which sets them apart from thermoplastics that require high temperature for extrusion and can deform at high temperature (Fig. 5a). Our approach is applicable to a range of waste papers, including those typically considered non-recyclable for paper production, such as tissue papers. Through combining different waste paper inks, potential applications in multi-colour printing, security information printing and mechanically intriguing designs are enabled. Furthermore, the natural lignocellulosic fibres of our paper constructs offer recyclability and biodegradability potential, providing a sustainable end-of-life solution for 3D prints. This stands in stark contrast to thermoplastics, which can take 10–1000 years to degrade or are even non-degradable (Fig. 5b). Our method is cheaper than other natural materials or materials commonly used in 3D printing as we utilise wastes. Overall, our strategy is not only economical and eco-friendly, but also increases the circularity of waste papers, and reduce the reliance on virgin plastics.

Materials

Waste office papers (Deli A4), tissue papers (Vinda gentle facial tissue), newspapers and cardboards (Express box) were collected in work places and were used directly. Polyacrylamide (PAM, MW = 3,000,000) was purchased from Sinopharm Chemical Reagent Co., Ltd (China). Waterborne polyurethane (PU) was purchased from King Chemical Co., Ltd (China). All chemicals were used without further purification.

Preparation of waste paper pulps

To prepare printable paper inks, we first created a paper pulp composed of lignocellulosic fibres from waste papers ((Supplementary Fig. 1). First, waste papers were cut into small sheets by a paper shredder with a shred width of 4 ~ 5 mm (Deli GA311), and was ground in a mortar grinder (RM 200, Retsch, Germany) with excess water for 3 hrs. This prolonged grinding time ensures the fiber particles reached the minimal achievable particle size of the grinder. The resulting mixture was resuspended and filtered through a 150-mesh sieve to remove any large paper particles. The suspension was loaded into 50-ml conical tubes and centrifuged at 1000 rpm for 3 min to separate the paper fibers from the solution. The supernatant was then removed, leaving a paper pulp. For every batch of the solution, we measured the paper content of the paper pulp. This involved drying 0.1 g of the paper pulp at 100°C for 60 minutes, and then weighting the resulting dried pulp to calculate the solid content of the pulp for the following ink preparation. The same protocol was used to create all types of waste paper pulps, including office papers, newspapers, cardboard and tissue papers used in this study.

Preparation of 3D printable paper inks: (Supplementary Fig. 1 denotes the ink preparation procedure. The waste paper inks were composed of PAM (MW = 3,000,000), deionized water and paper fibre in a weight ratio of 0.1:8.9:1, unless otherwise specified. We added paper pulp, which contains water, to the inks. Therefore, the amounts of deionized water and paper pulp added into the inks were determined based on the measured paper content and water content of the paper pulp. All types of paper inks, including office paper, newspaper, cardboard and tissue paper inks, were prepared using the same formulation ratio.

For the paper filler-reinforced PU inks, two inks were prepared in this study. The ink formulation contains paper fibres, PAM, PU and water. The water content in both inks is 80% by mass. The remaining 20% is composed of paper fibres, PAM and PU at mass ratios of 1:0.15:2 and 1:0.15:2, respectively. The amounts of deionized water and paper pulp added were determined based on the paper content and water content of the paper pulps. All inks were homogenized and degassed using a planetary mixer (AR-100, Thinky) at 1000 rpm for 2 mins prior to printing. The prepared inks were stored at room temperature in a sealed container when not in use.

3D Printing of the pulp inks.

All 3D printing experiments were conducted using a multi-material direct ink writing printer (3D Bio-Architect WS, Regenovo). The printed structures were designed using a 3D modelling software (Cinema 4D, MAXON), and converted into G-code using a slicing software (Regenovo). Prior to printing, the ink was drawn into a syringe barrel and degassed. The barrel was then loaded into the syringe holder of the printer. A printability test was performed to optimize the printing parameters, including printing speed, extrusion pressure and nozzle-to-stage distance. The optimized parameters were found to be an extrusion pressure of 100 kPa, a printing speed of 20 mm/s and a nozzle-to-stage distance of 0.3 mm. All experiments were carried out using nozzles with an outlet diameter of 0.34 mm.

Solidification of the 3DP constructs.

After printing, the printed structures were dehydrated and solidified. Three drying methods were tested in this study, including freeze-drying, air-drying and ethanol exchange drying. Unless otherwise noted, all 3D structures were freeze-dried, and all 2D prints were air-dried. For freeze-drying, the 3D constructs were placed in a refrigerator at -20°C for 12 hr to ensure they were frozen prior to loading them to a freeze-dryer (Scientz, China). The constructs were then freeze-dried at -80 ^oC under a pressure of 1 Pa for 12 hr. For air-drying, the 2D prints were left at ambient conditions until fully dried, which took 4–12 hr in this study, depending on the size of the print. Samples dried via ethanol exchange drying were also prepared for mechanical tests. For ethanol exchange drying, the constructs were transferred to an anhydrous ethanol solution and soaked for 12 hrs. The constructs were then dried at ambient temperature on a sieve that enables ease of removal after drying. We noted that shrinkage of the 3D printed constructs happened with the ethanol exchange drying method. To precisely preserve the designated shape, a scale factor of 2.8 was applied to the print model for shrinkage compensation.

Construct recycling

To recycle the printed constructs, a process similar to the paper ink preparation was adopted. This process involved mechanical shredding, grinding, filtration, and centrifugation to obtain a pulp precipitate. The yield of the lignocellulosic paper fibre was around 98% for each recycling cycle. The resulting paper pulp was then mixed with PAM in a mass ratio of 10:1 to obtain a recycled paper ink. The printed samples created using the recycled paper ink were then air-dried at room temperature and subsequently subjected to mechanical testing.

Finite element modelling

All numerical simulations in this work were carried out using a commercial finite-element software, ABAQUS (Version 2022, SIMULIA, France). Two finite element models, the Miura-origami model and tessellated model, were developed to simulate the stress distribution or strain distribution within the models under z-axis compression. The models, consisting of stiff domains and soft domains, were built according to the geometrical dimensions stated in Fig. 4d and Fig. 4g, with the thickness of the Miura-origami model as 2 mm. The experimentally-measured stress-strain curves and Poisson’s ratio of the soft domains (paper ink mixed with PU) and the hard domains (paper ink) were imported as the material parameters of the respective domains for further simulation.

For simulation of the Miura-origami model, an implicit dynamic analysis was adopted where a slow loading (1 mm/min) was employed to emulate the quasi-static compression used in the experiments. For simulation of the tessellated model, a non-linear static finite element simulation was adopted which activating nonlinear configurations. A compression speed along the z direction was set as 1 mm/min which was applied to the top surface of the model. A mesh convergence test was conducted to obtain an optimal mesh size for accurate simulation results at an acceptable simulation time.

Optical microscope observation

Optical microscope (DM 2700 M, Leica, Germany) was used to visualise the paper fibre length before and after grinding. The fibres in the pulp were dispersed in water and a dried droplet of the dispersion was observed using the microscope.

Scanning electron microscopy

Before carrying out imaging using a field-emission SEM (Merlin, Zeiss), the samples were fractured under liquid nitrogen, and sputtered with platinum using a sputter coater (Q150TES, Quorum). SEM was performed at an acceleration voltage of 5 kV and an electric current of 100 pA.

Fiber length statistics

To evaluate the distribution of length of the paper fibres before and after the mechanical treatments, 0.01 g of paper pulp was dispersed in 5 mL of water. The solution was placed in an ultrasonic bath for 20 minutes to ensure proper dispersion, and then a droplet of the solution was placed on a dust-free glass slide and allowed to dry under ambient air. Micrographs of the samples were taken using an optical microscope. The length of the fibres in the micrographs were measured using ImageJ software (Version 1.54h). The number of fibres in each group of samples is n > 500.

Confocal microscopy: To evaluate the geometry deviation between the computer-aided design (CAD) model and the 3D printed model, a staircase sample (dimensions: 5.5 mm in height, 10 mm in length, and 10 mm in width) was fabricated using a paper ink, and its dimensions were acquired using a laser scanning confocal microscope (VK-X1000, Keyence).

Rheometry

All rheological measurements were carried out using a rheometer (DHR rheometer, TA instruments) equipped with a parallel-plate geometry (20 mm in diameter) at room temperature, and a gap size of 1000 µm was used. Shear viscosity measurements were performed in a steady-state flow mode over a shear rate range of 0.01–500 s^− 1. Dynamic oscillatory strain amplitude sweep measurements were conducted at a frequency of 10 rad/s in an amplitude range of 0.01% − 200%. Oscillatory thixotropy and rotational thixotropy tests were also performed to determine the time-dependent moduli and viscosity recovery behaviours of the inks. In oscillatory thixotropy tests, an altered shear rate was applied stepwise between 0.1% and 200%, while in rotational thixotropy tests, an altered shear stress was applied between 0.1% and 200%. The results were analysed using TA Instruments TRIOS software.

Mechanical testing

All tensile tests were conducted using a universal testing machine (CMT2000K, SANSI, China) equipped with a 500 N load cell. Samples (10 mm X 10 mm X 0.2 mm) were clamped between two grippers, and were tested at deformation rates of 1 mm min^− 1 (waste paper samples) and 20 mm min^− 1 (pure PU) at room temperature. Compression tests were conducted using an electronic universal testing machine (MTS, 100 kN load cell, USA) at a deformation rate of 1 mm min^− 1.

Photograph and video recording

Unless otherwise specified, all photographs in this work were taken with a SONY camera (A7RIV, Japan), and all videos were recorded using a Nikon camera (D7500, Japan).

Statistical Analysis

All the results in this study were presented as mean ± S.D., and all the mechanical properties presented in this study were measured from at least three parallel samples. Data distribution was assumed to be normal for all the parametric tests, but not formally tested, and no significant difference analysis was performed. The statistical analyses were carried out with the OriginPro 2021 software.

Supporting Information

Supporting Information is available from the author. The online version contains supplementary material available at

Author contribution

Chengcheng Cai, Pei Zhang, Iek Man Lei, Ben Bin Xu and Ji Liu wrote the main manuscript text. Chengcheng Cai and Pei Zhang conducted all experiments. Yafei Wang, Ben Bin Xu and Yun Tan performed numerical simulation and analysis. Chengcheng Cai, Pei Zhang and Ji Liu conceived the idea and designed the research. All authors reviewed the manuscript.

Funding

This study was supported by the National Natural Science Foundation of China (52373139), Natural Science Foundation of Guangdong Province (2022A1515010152), Basic Research Program of Shenzhen (20231116101626002), and Scientific Research Platforms and Projects of University of Guangdong Provincial Education Office (2022ZDZX3019). This work was also supported in part by the Science, Technology and Innovation Commission of Shenzhen Municipality (ZDSYS20220527171403009). I.M.L. acknowledges the funding support from the Science and Technology Development Fund, Macau SAR (0119/2022/A3 and 0009/2023/ITP1), and the Research Grant from the University of Macau and the University of Macau Development Foundation (SRG2022-00038-FST and MYRG-GRG2023-00225-FST-UMDF). B. B. X. is grateful for the support from the Engineering and Physical Sci- ences Research Council (EPSRC, UK) grant-EP/N007921.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Competing interests

Ben Bin Xu is one of the deputy editors of ACHM, so he should be excluded from the assessment process of this manuscript.

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No competing interests reported.

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Sustainable Three-dimensional Printing of Waste Paper-based Functional Materials and Constructs

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Abstract

Figures

1. Introduction

2. Results

2.1. Waste papers as sustainable inks for 3D printing

2.2. Longevity, recyclability and applicability to a wide range of paper types

2.3. Multi-material printing of coloured and information-encrypted paper constructs

2.4. Functional paper constructs with stiff and compliant mechanical designs

3. Conclusion

4. Materials and Methods

3D Printing of the pulp inks.

Declarations

References

Additional Declarations

Supplementary Files

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Journal Publication

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