Cellulosic Materials from Cigarette Butts for Additive Manufacturing

doi:10.21203/rs.3.rs-3888177/v1

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Cellulosic Materials from Cigarette Butts for Additive Manufacturing

https://doi.org/10.21203/rs.3.rs-3888177/v1

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In this work we describe the recovery of cellulose acetate (r-CA) polymer from waste cigarette butts (CBs) and subsequent conversion into feedstock for 3D printing technology. The extraction process for CBs includes two stages: initial washes in water, followed by additional washes in ethanol. A final step involves a dissolution and reprecipitation process, resulting in the creation of a fine powder. The recovery polymer has been analysed and compared to commercial cellulose acetate (p-CA) and unsmoked cigarette filter (u-CA) to assess its purity and examine alterations in its physicochemical properties. The CA powder has been also plasticized with different biocompatible plasticizers to improve the CA mechanical properties. We analysed the rheological properties to identify the suitable composition as feedstock for 3D printing.

Cigarette butts

Cellulose acetate

Extraction process

Recovery

3D printing

Cigarette butts (CBs) are one of the most common types of litter all around the world. The main problem with this waste is its disposal method by smokers. As most smokers do not dispose of CBs appropriately in trash bins and discard them anywhere, collecting this litter is a difficult and costly process; this has caused CBs to be observed extensively in various urban areas and public places with consequent effects on environmental resources and living organisms (Jungst et al. 2016; Kalsoom et al. 2016; Kirchmajer et al. 2015; Ligon et al. 2017; Wang et al. 2017; Auriemma et al.2022; Araújo et al. 2019; Torkashvand et al. 2020). The CBs are, in fact, known as toxic litters. In CBs are trapped many pollutants derived from fertilizers used in tobacco cultivation, from processing and cigarette assembly, and formed during smoking. Heavy metals like lead, cadmium, copper, nickel, zinc, and chromium are recognized contaminants found in CBs, seeping into the environment and potentially entering the food chain (Valiente et al. 2020, Kataržytė et. 2020; Yousef et al. 2021; Yousef et al. 2021; Torkashvand et al. 2021; Torkashvand et al. 2022; Dobaradaran et al. 2017; Ghasemi et al. 2022). Alongside heavy metals, CBs contain various identified pollutants, including organic compounds, PAHs, and nicotine (Yousef et al. 2021). Multiple studies indicate that these pollutants escape from discarded CBs, dispersing and contaminating the environment. The extent of this pollutant release hinges on their initial concentration in the CB and environmental factors such as humidity (Dobaradaran et al. 2020). This leakage from CBs leads to water and soil contamination, and the demonstrated toxicity of CBs on diverse organisms underlines the severity of the issue in several cases. The possibility of ingestion of CBs by pet infants, fishes, is another consequence of CBs littering (Dobaradaran et al. 2020; Green et al. 2019; Kurmus et al. 2020; Dieng et al. 2013; Dieng et al. 2014; Lee et al. 2015; Slaughter et al 2019; Parker et al. 2017; Novotny et al. 2011). The CBs are constituted as well by the listed toxic chemicals from the microfibers of cellulose acetate (CA) (Belzagui et al. 2021; Conradi et al. 2022) and so they are also responsible for micro-plastic pollution in the aquatic environment. Extracting value-added materials, such as CA, from such waste for the manufacture of new products not only could allow for a reduction of the overall ecological footprint but also help offset the cost of waste mitigation.

In recent years, three-dimensional (3D) printing has swiftly emerged as a promising technology for crafting intricate structural designs. Among various 3D-printing methods, extrusion-based printing stands out for its advantages, including versatility in material use, user-friendly operation, and cost-effectiveness. This method, also known as Direct Ink Writing (DIW), involves extruding viscoelastic feedstocks referred to as "inks." The primary materials employed as inks encompass polymers like polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), nylon, along with composites, metals, ceramics, sand, and wax (Gibson et al. 2015; Ligon et al. 2017; Wang et al. 2017). Cellulose-based materials, without chemical alteration, are generally deemed impractical for extrusion-based 3D printing due to their thermal decomposition before achieving a meltable, flowable state upon heating (Bao et al. 2015). This is attributed to the robust hydrogen bonding among cellulose molecules. Previous scientific literature on 3D printing using cellulose-derived materials has typically focused on incorporating these materials as low-density fillers within another matrix, commonly another polymer, or using them to produce aerogel-like structures. However, the obtained 3D objects in this way show a low density and dimensional stability. For this reason, alternative strategies must be studied and developed for cellulose-derived materials, such as cellulose acetate.

In this paper, a new method to obtain a novel feedstock for 3D printing with cellulose-based materials by recovery of CBs waste in alternative to fossil-based plastic materials is proposed. The method is based on the combined use of appropriate solvents and plasticizers to address the rheological and printability issues associated with DIW. The function of solvents and plasticizers is to reduce temporarily the strength of hydrogen bonding between molecules of CA and to enable a 3D printing process. A green process, that uses water and ethanol, is proposed to extract CA fibers from CBs waste paired with a dissolution-precipitation technique for recovered CA powder. The morphological analysis of recovered CA fibers and powder has been evaluated by using SEM. The presence of metals, in particular heavy metals, has been also investigated by means of atomic absorption analysis. To evaluate the quality of CA recovered, the results of chemical (FTIR), thermal (TGA, DSC), and mechanical (DMA) properties of films based on recovered CA obtained by means of a solvent cast process have been compared to those obtained for films of CA extracted from commercial and unsmoked filters, and of commercial pure powder CA. Different plasticized, such as Triacetin and Polyethyleneglycol 400 at fixed percentages (Liu et al. 2019; Quintana et al. 2013; De Fenzo et al. 2020) have been also used, in alternative to the currently most widespread industrial plasticizers, such as Diethyl phthalate and Triphenylphosphate, to assess the influence of types of plasticizer on chemical, thermal and mechanical properties of films based on recovered CA. The obtained findings have been then utilized to produce solutions as inks with different solvents, such as ethanol, dimethyl sulfoxide, or a mixture of these and selected plasticizers. The rheological behaviour of these solutions has been analysed at various solvent contents to identify the most suitable solvent or mixture of solvents and content for the preparation of feedstock for 3D printing technology with recovered CA from CBs (Jungst et al.2016; Tekin et al 2023; Cafiero et al. 2023). A comparison with the rheological behaviour of solutions based on pure powder CA has been also conducted.

Materials

Smoked CBs have been collected by Essequadro Eyewear Company S.r.l. (Italy). To evaluate the effect of smoking on the quality of recovered CA, unsmoked cigarette CBs from the brand RIZLA® have been purchased and used in this work. CA powder has been supplied from Merck Life Science (Milano, Italy) and used as a pure reference material (p-CA). Distilled water (DI), Acetone 99% v/v, Ethanol 99% v/v, Dimethyl sulfoxide, Triacetin (TAC) and Polyethyleneglycol 400 (PEG 400) have been purchased from Merck Life Science (Milano, Italy). All the reagents have been used as received.

Green Extraction and precipitation of CA

The CBs were sterilized in an autoclave for 1h at a temperature of 120°C and 1 bar and, then washed in cold water three times so to extend CA fibers and in ethanol 99% w/w twice to remove potential organic compounds. The extraction of cellulose acetate from the cleaned CBs has been performed by using a dissolution-reprecipitation technique. In particular, the cleaned CBs have been dissolved in acetone (5% w/w) for 1 h at room temperature and after the resulting solution has been centrifuged to separate any residues trapped in the filter, mainly soot. The solution has been kept in the ice bath and a non-solvent (ethanol or water) has been added dropwise. The ratio solvent/non-solvent is 1:2. The precipitate has been recovered with filtration and washed twice with ethanol. Finally, the powder has been dried at 80°C for 1h in the oven. The scheme of the recovery process of CA from CBs is reported in Fig. 1. The recovered cellulose acetate powder is hereafter denoted as r-CA. The same procedure has been used to extract cellulose acetate from unsmoked filters RIZLA®. In this case, the extracted cellulose acetate is hereafter denoted as u-CA.

Preparation of CA films

Films based on different CA (r-CA, u-CA and p-CA) have been realized by solvent casting technique. CA was dried in the oven until a constant weight was observed. Then 0,5 g of CA was dissolved in 12 ml of acetone followed by stirring (800 rpm) at room temperature for 1 h. The prepared solution was then filtered to remove dust and other traces of impurities. The solution was left at room temperature for two hours in order to get rid of air bubbles. The prepared clear solution was cast onto polystyrene Petri dishes for film formation and then stored in plastic bags until use. All obtained films were very transparent and free from air bubbles. The same procedure has been used to produce films based on r-CA plasticized with 15 wt% TAC and 10 wt% PEG 400, hereafter denoted as r-CA_15%TAC and r-CA_10%PEG400. The plasticizers, in this case, have been added to the solution after the dissolution of r-CA in acetone and the resulting solution has been stirred for another 10 min to obtain a good dispersion of plasticizer. A digital micrometer (Mitutoyo, Japan) measured the film thickness, which came out to be approximately 500 µm.

Preparation of CA solutions

Solutions of different compositions have been prepared as follows to evaluate the possible use in 3D printing. In a 5 ml vial, 0,5 g of CA was weighed, and 4 ml of solvent or solvent/co-solvent was added dropwise manually mixing. This solution was put in the ultrasonic bath for 20 min until complete homogenization. The composition of polymer solutions prepared for rheological studies is presented in Table 1.

Table 1

Composition of CA Solutions for rheological studies
Sample	CA [% wt]	Acetone [% wt]	DMSO [% wt]
r-CA-15-AC	15	85	-
r-CA-20-AC	20	80	-
r-CA-25-AC	25	75	-
r-CA-20-AC-D	20	40	40
p-CA-20-AC-D	20	40	40
p-CA-20-D	20	-	80

Samples characterization

We bought acetone, 99% v/v ethanol, and other chemical reagents from Merck Life Science, located in Milan, Italy. Every reagent was used exactly as it was given.

To evaluate both structural/morphological properties and thermodynamic/functional properties of CA samples several experimental techniques have been used.

Scanning Electron Microscopy and X-ray microanalysis (SEM-EDX)

the morphology of fibers and powder of r-CA has been investigated by using a scanning electron microscope (Quanta 200 FEG, FEI, The Netherlands). Before the SEM analysis, the samples have been coated with a thin layer (about 10 nm thick) of an Au-Pd alloy by means of a sputter coating system (Emitech K575, Quorum Technologies LTD, UK). X-Ray microanalysis has been performed by EDX Inca Oxford 250 instrument. The cut directions have been the same for all samples.

Atomic Absorption Spectrometer

the analysis of heavy metals present in r-CA has been performed by using an atomic absorption spectrometer (Perkin Elmer Analyst 700, Norwalk, CT, USA) with deuterium background corrector. All measurements have been carried out in an air/acetylene flame. A 10 cm long slot-burner head, a lamp and an air–acetylene flame have been used. The r-CA has been dissolved in 0.5 mL of concentrated HNO₃ and made up to 5 mL with distilled water for the analysis.

Thermogravimetric analysis (TGA)

the thermogravimetric analysis of films has been performed by using a Q500 TGA (TA Instruments, New Castle, Delaware, USA). Samples were heated from 30 to 450°C at a heating rate of 5°C/min under nitrogen and from 450 to 600°C under air.

Differential scanning calorimetry (DSC)

differential scanning calorimetry (modulated—MDSC) of films has been performed with a Q 2000 DSC (TA Instruments, New Castle, Delaware, USA) under nitrogen atmosphere (50 mL/min). The MDSC analysis has been carried out with 5°C/min heating rate, modulation amplitude ± 2°C, and modulation period of 40 s.

Fourier-transform infrared spectroscopy (FT-IR)

FT-IR analyses of films have been performed using a System 2000 FT-IR (Perkin–Elmer, Waltham, MA, USA) at ambient temperature. The samples were analysed in ATR spectra mode from 4000 to 600 cm^− 1 with a wavenumber resolution of 4 cm^− 1 for 64 scans. These analyses have been performed to estimate principal chemical bonds present on the samples and to verify any changes induced by the processes.

Contact Angle measurements

the hydrophilicity of the films has been characterized by water contact angle measurements using a contact angle meter (CA, OCA20, Data physics, Germany). A drop (3 µL) of distilled water has been deposited onto at least ten different sites of the film surface, and the contact angles have been determined at the right and left ends of the drop. The contact angle value has been calculated by averaging the measured values.

Dynamic mechanical thermal analysis (DMTA)

the thermos-mechanical properties of the films have been investigated in shear mode (2 gaps configuration, 5 mm each) using a DMA + 1000 Metravib (ACOEM, Limonest, France SAS). The films have been cut into rectangular samples with a length of 35 mm and width of 15 mm. Thickness were measured by using an outside analog micrometer (0–25 mm model, INECO s.r.l., Osnago, Italy). The linear viscoelastic regime has been preliminary investigated in p-CA samples by performing a strain sweep test at room temperature (25 ± 2°C) and with a frequency of 10 Hz. The upper limit has been found to be 0.08% of strain. DMTA measurements have been performed at a frequency of 10 Hz with a dynamic strain of 0.03%. The temperature ramp has been set at 2°C/min with a temperature ranging from 25°C to 250°C.Transition temperatures were determined from the peak of the damping factor (tan δ) plot, the error being within ± 2°C. Each test was repeated three times.

Rheological Measurements

rheological measurements were performed using a stress controlled rotational rheometer (RheoScope MARS II, HAAKE, Germany) equipped with 20 mm parallel plates. The tests were performed at 25°C under nitrogen atmosphere, using a gap thickness of 0.2 mm. A solvent trap was used to prevent solvent evaporation. Frequency sweep tests from 0.1 to 100 Hz with a fixed strain of ζ = 0.1% were carried out to operate in the linear viscoelastic region.

3.1 Morphological and Elemental Analysis

The morphological evolution of the reprecipitation processes from CA fibers to powder is depicted in Fig. 2. The shape of the r-CA fibers produced during the extraction process is shown in Fig. 2a. Furthermore, it has been feasible to determine by EDX analysis that a titanium coating covers the fibers. There are additional heavy metals including lead (Pb), cadmium (Cd), and zinc (Zn). Cigarette filters get their white color from titanium (Ti), which is found as titanium oxide. Instead, as shown in Fig. 2b), the r-CA powder produced by the dissolution-reprecipitation step exhibits a collapse of the fiber structure as well as a decrease in the concentration of Ti and heavy metals. For fiber and powder composition analysis, three different areas of each sample were analyzed and the average values are shown in the tables in Figs. 2a) and 2b). These data have been confirmed by the research of the heavy metals by flame atomic absorption spectrometer. The results have recorded the concentration of 0.02 ppm of Pb, 0.05 ppm of Zn, and 0.03 ppm of Cd concentrations of heavy metals and the presence of 0.04 ppm of Ti.

Thermal and FTIR analysis

The TGA curves of r-CA, u-CA, and p-CA films are shown in Fig. 3. The curves show that the pyrolysis of p-CA and r-CA films occurs in three steps: the evaporation step of water, the thermal pyrolysis of the cellulose acetate backbone, and the complete combustion after conversion to air at 450°C (Cafiero et al. 2023). The initial thermal degradation temperatures (TI) of p-CA and r-CA samples are 64°C and 277°C, respectively (see Table 3). The TI of r-CA is higher than that of p-CA, but the Tmax is the same at 347°C. For both samples, combustion was complete and there was no residue after 550°C. On the other hand, the TGA curve of u-CA film shows a more significant difference with that of p-CA and r-CA films. This is probably due to the presence of commercial additives that break down during combustion during smoking. At approximately 250°C, an initial degradation with deacetylation of the glucoside moieties is observed, and the most prominent degradation occurs at 350°C due to degradation of the glucoside backbone. (De Freitas et al. 2017).

Table 2

The main thermal parameters of r-CA, u-CA, p-CA, r-CA_10% PEG400 and r-CA_15% TAC films
Sample	T_I [°C]	T_max [°C]
r-CA	277	346
u-CA	169	346
p-CA	64	347
r-CA 10% PEG400	98	346
r-CA 15%TAC	96	343

The TGA curves of r-CA, r-CA_10% PEG400 and r-CA_15%TAC films are reported in Fig. 4. It is evident that the addition of both type of plasticizers lead to a decrease in stability of r-CA over 170°C (see Table 3). However, it should be noted that the degradation profile of r-CA_15%TAC film is more complex that than of r-CA and r-CA_10% PEG400 films; in fact, there are two transitions, one associated with deacetylation (228°C), as previously noted for non-plasticized samples, and another (T = 267°C), probably due to evaporation of TAC. The plasticizer evaporation of TAC in r-CA_15%TAC film occurs at higher temperatures (267°C) with respect to that of the sole plasticizer (T = 258°C) for its interaction with cellulose acetate molecules.

Figures 5a and 5b show the MDSC curves of the r-CA, u-CA and p-CA films. The characterization by MDSC allows the same scan to record the calorimetric curves associated with both reversible (see Fig. 5a) and non-reversible (see Fig. 5b) phenomena in the heating of the sample (Bao et al. 2015). In Fig. 5a it is detected the glass transition region at a jump of Rev Heat Capacity. The corresponding glass transition temperatures (Tg) have been determined as the onset of the MDSC signal from the baseline shift (see Table 3). The u-CA film shows one Tg at 135°C, instead for the r-CA and p-CA two different Tg have been observed, the first is related to the structural reorganization of the material following the loss of water of hydration, and the second corresponds to Tg of CA. Figure 5b displays the non-reversing phenomenon like evaporation of water and solvent used for casting around 70–80°C also decomposition associated with deacetylation around 225°C, as already noted in TGA.

Table 3

The Tg of samples r-CA, u-CA, p-CA, r-CA 10% PEG400, r-CA 15%TAC
Sample	T_g [°C]
r-CA	201
u-CA	138
p-CA	196
r-CA 10% PEG400	160
r-CA 15% TAC	112

The MDSC curves of r-CA, r-CA_10% PEG and r-CA_15% TAC films are reported in Fig. 6a) and b). The addition of 10% wt PEG 400 induces a decrease of about 40°C in Tg, while functionalization with 15% wt of TAC allows to reach a remarkable decrease of about 90°C. This reduction that is function of the plasticizer concentration, is generally attributed to the increase in free volume of the CA molecules resulting from the intercalation of the plasticizer molecules. Previous research has suggested that plasticizer molecules break the polymer-polymer interactions, such as hydrogen bonds and Van der Waals forces. Specifically, the hydroxyl groups of PEG 400 and acetyl groups of TAC are believed to interact with r-CA molecules through dipolar interactions and hydrogen bonding.

A further characterization to evaluate the chemical modification caused by the recovery process and the plasticizer addition is obtained by ATR-FTIR spectroscopy.

Figure 7a) shows ATR spectra of r-CA, u-CA, and p-CA films, while the spectra of r-CA, r-CA_10%PEG400, and r-CA_15%TAC films are reported in Fig. 7b). The p-CA film shows a broad peak at 3490 cm-1, which can be attributed to -OH stretching of non-acetylated cellulose. The absorption peaks at wave numbers 2942 and 1735 cm^− 1 corresponded to CH stretching of the methyl group (-CH3) and carbonyl stretching of the acetate group (C = O). Other peaks at 1642, 1437, 1373, 1211, 1033, and 901 cm^− 1 are H-O-H bending of absorbed water, CH₂ bending, C-H bending vibration of CH₃ of acetyl group, C-O stretching of acetyl group, and C of cellulose backbone and C-O-C stretch, or C-O-C stretch of a glycosidic β-(1→4) bond. (Tekin et al. 2023; Cafiero et. 2023; Fei et al. 2017; De Freitas et al. 2017; Zugenmaier et al. 2004). The ATR spectra of r-CA and u-CA are like those of p-CA, indicating the efficiency of the extraction method proposed in this work. The ATR spectra of r-CA_10% PEG400 and r-CA_15%TAC films don’t show differences with respect to r-CA spectra. This behavior is due because cellulose acetate, TAC, and PEG 400 have the same main functional groups, so the presence of plasticizer in the samples can be detected mainly by the increasing absorbance intensity.

Contact Angle Measurements

The water contact angles of r-CA, p-CA, u-CA, r-CA_10% PEG400, and r-CA_15% TAC films are summarized in Table 4. The sample r-CA shows the hydrophilic nature of cellulose acetate surface, moreover, the r-CA_10% PEG400 displays a significant increase of the surface hydrophilicity compared to the r-CA_15% TAC sample, as a function of the greater number of hydroxyl groups that can interact superficially with water.

Table 4

Contact angle results of r-CA, p-CA, u-CA, r-CA_10% PEG400, r-CA_15%TAC films
Sample	Contact Angle
r-CA	74.7°±1.5°
p-CA	65.0°±4.0°
u-CA	55.8°±1.8°
r-CA_10% PEG400	46.7°±6.8°
r-CA_15% TAC	96.4°±2.4°

DMTA results

To evaluate the storage modulus, G’, and damping factor of CA samples we investigated the variation of mechanical properties. At room temperature, the storage modulus of the p-CA sample is G’ = 1.23 x 10⁹ Pa (see Fig. 8). Being far below Tg, assuming an isotropic linear elastic material with a Poisson’s ratio ν = 0.33, the storage modulus in tensile setting, E’, can be calculated as E’ = 2 G’(1 + ν) = 3.27 x 10⁹ Pa, which is consistent with the storage modulus in tensile setting usually found in the literature for a similar degree of substitution of 2.5 (Zugenmaier et al. 2004; Guo et al. 1993). The p-CA sample shows a glass transition temperature of 225°C, consistent with our DSC measurements. This value is higher than the one found by Bao et al. 2015 (212°C). This discrepancy could be justified by the higher frequency (10 Hz) at which the DMTA test is conducted in the present case concerning the measurements as suggested by, among others, (Zugenmaier et al. 2004). The u-CA sample shows a Tg of 208°C and G’=1.13 x 10⁹ Pa. The lower value of the glass transition temperature compared to the p-CA could be due to additives included in the commercial formulation that plasticize the material. The r-CA sample shows a Tg of 223°C and G’=1.41 x 10⁹ Pa, close to the thermal and mechanical properties of the p-CA sample, which proves a successful recovery process.

The DMTA curves of r-CA 10% PEG and r-CA 15% TAC (and, for proper comparison, the r-CA) films are reported in Fig. 9. The r-CA 10% PEG sample shows a Tg of 133°C and G’=7.39 x 10⁷ Pa. The Tg shows a remarkable decrease of about 90°C with respect to the r-CA sample, while the storage modulus at room temperature decreases of more than one order of magnitude. The effect of the PEG 400 on the thermo-mechanical properties of CA is noticeable; also, the storage modulus starts to drop immediately above room temperature, as supported by compliance values found by Guo et. 1993. For the r-CA 15% TAC film a Tg = 193°C and G’=8.47 x 10⁸ Pa are found. The presence of TAC induces a decrease of the storage modulus, consistently with observations from Quintana et al. 2013.

On comparing the Tg values between the ones obtained from DSC and DMA. Since Tg represents a thermodynamic transition, its value for the same material depends on several factors, such as measurement technique, sample geometry, and anisotropy (Idrees et al. 2018)

Rheological Studies

In the context of 3D printing, it's essential to understand the behaviour of materials, particularly their viscosity, which can impact the ink's flow and the quality of the printed object. Ideally, in 3D printing, materials should exhibit a shear-thinning behaviour, meaning that their viscosity decreases with increasing frequency. This characteristic is beneficial for smooth ink flow through fine deposition needles. To clarify, a 3D printing ink must have reduced viscosity at high frequencies to minimize extrusion pressure through the nozzle. However, after extrusion, it should exhibit increased viscosity at lower frequencies to maintain the printed object's shape stability. In Fig. 10, the complex viscosity of various prepared solutions is displayed as a function of oscillation frequency. Figure 10a) and 10b) illustrate how the viscosity curve is affected by the solvent content and type, while Fig. 10c) and d) compare the rheological behaviour between two samples of cellulose acetate (r-CA and p-CA) under specific solvent conditions (see Table 1). All samples demonstrate a pronounced shear-thinning behaviour related to macromolecular orientation at higher frequencies. It's noteworthy that prior research, such as that by Zepnik et al. 2013, already explored the shear-thinning behaviour of cellulose acetate mixed with a plasticizer, Triethyl citrate (TEC), in the melt state at high temperatures. Their results showed that the viscosity remained relatively high for 3D printing, making it less practical for the application. The approach of using solvent-assisted printing at room temperature may be more advantageous, as indicated by the substantial reduction in viscosity when acetone is used as a solvent. Unfortunately, acetone evaporates rapidly at room temperature, leading to a swift solidification of the solution during rheological testing, resulting in higher viscosity values. To address this issue, we considered using dimethyl sulfoxide (DMSO) as a co-solvent or alternative to acetone. In this case the solutions exhibit a different rheological behaviour, combining a Newtonian region at low frequency and a shear-thinning region at high frequency. To quantitatively assess the degree of shear-thinning in these solutions, we fitted their viscosity curves using the power law equation, which is given as:

η = kγⁿ⁻¹

where η represents viscosity, γ is the shear rate, k is the consistency index, and n is the power law index. These values can be indicative of the materials' printability. In fact, a high value of n suggests improved flowability and better printing performance (see Table 5). In particular, the lower value of n for r-CA-20-AC (see Table 1) and its crossover strain (Fig. 11) suggest that its extrusion will produce a stable and accurate 3D printed object.

Table 5

Fitting parameters of the power law for the different solutions
Sample	K (Pa*sⁿ)	n
r-CA-15-AC	2863	0.23
r-CA-20-AC	7734	0.14
r-CA-25-AC	8018	0.12
r-CA-20-AC-D	20	0.98
p-CA-20-AC-D	23	0.98
p-CA-20-D	32	0.98
p-CA-20-AC	1840	0.24

To check the possibility of using our recovered material as feedstock for SSE 3D Printing, we simulated an extrusion process at room temperature by spinning the recovered cellulose acetate mixed with 10% w/w acetone using a syringe. The wire obtained dries quickly in the air maintaining a good homogeneity. In Fig. 12 we present a scheme modification of the syringe for r-CA ink.

The r-CA is stored in a syringe-like reservoir connected to a dispensing nozzle on the printer head. The displacement of the syringe piston and the flow of ink through the nozzle results in stress inside the nozzle on the printer head, causing the viscosity of the paste to decrease and the ink starts to flow (see Fig. 12). As the r-CA is deposited and the stress disappears, the paste relaxes and forms a solid gel, resulting in the successful buildup of 3D objects. Through a careful control of ink composition, rheological behavior, and printing parameters, 3D structures that consist of continuous solids, high aspect ratio (e.g., parallel walls), or spanning features can be constructed.

Recovery of cellulose acetate from cigarette butts was investigated in this study. We presented a dissolution and reprecipitation process which leads to the formation of a fine powder. The powder can be very useful for sample homogeneity and very easy manipulation. According to the results, the r-CA has a thermal behaviour like p-CA, and different, from u-CA, probably caused by commercial additives. The effect of the PEG 400 on the thermo-mechanical properties of CA is noticeable, and the storage modulus starts to drop immediately above room temperature, probably due to the formation of voids and a non-uniform surface structure. Moreover, we investigated the viscosity of r-CA solution using acetone and DMSO, to check the possibility of using our recovered material as feedstock for Direct Ink Writing.

Author Contributions

Conceptualization, Lucia Sansone and Flavia D’Urso; methodology Lucia Sansone and Flavia D’Urso; validation, Michele Giordano, Ernesto di Maio; investigation, Lucia Sansone, Flavia DiUrso, Paolo Iaccarino, Maria Oliviero; data curation, Lucia Sansone, Flavia DiUrso, Paolo Iaccarino, Maria Oliviero; writing—original draft preparation, Lucia Sansone

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The Authors thank to Essequadro eyewear for providing cigarette butts, funding for the ‘green’ exctraction study and testing support for material development.

Declarations

Not applicable

Ethical Approval

Not applicable

Funding

Any funding received.

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No competing interests reported.

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Cellulosic Materials from Cigarette Butts for Additive Manufacturing

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Abstract

Figures

Introduction

Materials and Methods

Materials

Green Extraction and precipitation of CA

Preparation of CA films

Preparation of CA solutions

Samples characterization

Results and Discussion

3.1 Morphological and Elemental Analysis

Thermal and FTIR analysis

Contact Angle Measurements

DMTA results

Rheological Studies

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1