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European Journal of Wood and Wood Products

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Open Access 25.09.2023 | Original Article

Aging effects on the viscoelastic behaviour of products by fused deposition modelling (FDM) made from recycled and wood-filled polymer resins

verfasst von: Antonella Patti, Stefano Acierno, Gianluca Cicala, Domenico Acierno

Erschienen in: European Journal of Wood and Wood Products | Ausgabe 1/2024

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Abstract

In this work, we analyse the thermal aging effects on the thermo-mechanical properties of bio-based specimens realized using fused deposition modelling technology. For the investigations, three commercial filaments made of polylactide acid (PLA) were used. The first filament was a pure virgin PLA (B-PLA); the second one was made from recycled waste production, PLA (R-PLA), and the third one was wood-filled PLA (W-PLA). Such materials were extruded under pre-optimized conditions and thermally aged in an oven at 70 °C. The as-prepared specimens were subjected to dynamic mechanical analysis (DMA) and infrared spectroscopy (IR). The experimental results are presented in terms of storage modulus (E'), loss modulus (E"), tan delta, and absorption spectra at different aging periods (0, 50, 70, 130, 175 days). For B-PLA and R-PLA, the thermal aging results in a decrease in both storage and loss moduli and in an increase in the glass transition temperature (T_g). On the contrary, for the W-PLA the storage modulus increases with the aging time, while the T_g remains constant. The IR spectra support the hypothesis of a degradation mechanism involving hydrolysis and/or hydrogen atom transfer. Based on these observations, we conclude that heat treatments always lead, through polymer degradation and structural changes, to more stable structures. The presence of wood particles slows down the aging process and makes the final products more durable.

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1 Introduction

The concept of biodegradability in polymers is gaining popularity due to the incompatibility of polymeric waste with the environment where it is disposed of after use (Luckachan and Pillai 2011). Different strategies were explored to develop bio-based degradable plastics, i.e. biopolymers, that do not pollute the environment and may be used in place of oil-based traditional polymers (Nampoothiri et al. 2010).

Biopolymers can be made from polysaccharides, proteins, lipids, and bacterial activities (i.e., polyhydroxyalkanoates), conventional synthesis of bio-derived monomers (i.e., polylactides) or synthetic monomers (i.e., polycaprolactone). A wide range of biodegradable polymers has been developed for a variety of applications. Polyhydroxyalkanoates (PHA), produced in the form of granules by various microorganisms, are attractive materials for biomedical applications due to their unique properties such as biodegradability, biocompatibility, and non-toxicity (Sharma et al. 2021). Polycaprolactone (PCL) is approved for use in biomedical devices due to its low degradation rate, high ductility and plasticity (Sachan et al. 2022). Polylactic acid (PLA) is an aliphatic polyester, considered a “green” ecological material (Nampoothiri et al. 2010).

PLA is currently the most promising and commonly used biopolymer in various industries like packaging, textiles, construction, and automotive (Nofar et al. 2019). Being thermoplastic in nature, PLA can easily be processed with traditional technologies in extrusion, compression moulding, injection moulding, and thermoforming (Cisneros-López et al. 2020). PLA is also used in the, more recent, additive manufacturing (AM) technologies; in particular, in the fused deposition modelling (FDM), due to its biodegradability, good mechanical properties, and low crystallization rate (Patti et al. 2022a, b, c, 2023). The latter is a crucial aspect for promoting interlayer adhesion in 3D printed parts. The low melting point of PLA makes this polymer very attractive in order to reduce the energy consumption in 3D printing (Cisneros-López et al. 2020).

At high temperatures during thermal processing or under hydrolytic conditions PLA macromolecules can break down resulting in a decrease in molecular weight (Signori et al. 2009). The effect of extrusion parameters, such as temperature and screw speed, on the structure and properties of various PLA grades was investigated in Mysiukiewicz et al. (2020). The results showed that during extrusion at high temperature (up to 260 °C) and screw speed (up to 250 rpm), the polymer underwent thermo-mechanical and thermo-oxidative degradation (Mysiukiewicz et al. 2020). According to a large amount of published data, PLA thermal degradation was caused by random main-chain scission reactions, depolymerization, oxidative degradation, and transesterification processes (Signori et al. 2009). As a result, the PLA properties were degraded due to permanent physical and chemical changes of the molecular structure or continuous evolutions to achieve a stable configuration of thermodynamic equilibrium (Budin et al. 2019). Such structural changes consisted of permanent chemical changes and breaking of primary atomic bonds, leading to irreversible material changes (Hutchinson 1995).

Often in common polymer processing the materials are rapidly cooled from high temperatures, thus giving rise to non-equilibrium state. Physical aging, or structural relaxation, refers to conformational changes that the polymer chains undergo after processing towards a thermodynamically stable state (Chandra et al. 2018). Physical aging refers only to reversible changes in chemical or physical properties, and results in a decrease in free volume, an increase in glass transition temperature, an increase in stiffness and a decrease in deformability (Cui et al. 2020).

In addition to physical aging other factors, such as the presence of water and crystallization, can also affect the final properties of PLA-based products. Hydrolysis, occurring in the presence of water, reduces the molecular weight and increases the brittleness (Cui et al. 2020). However, hydrolysis is very slow at room temperature and in the absence of catalysts (Cui et al. 2020). It was demonstrated that physical aging of the PLA structure occurred under differed humidity conditions (immersion in water, 50 or 100% relative humidity) at 50 °C (Rocca-Smith et al. 2017). At temperatures below the glass transition, slow molecular rearrangements in amorphous polymers tended to lower free energy and free volume. This resulted in a gradual densification of the polymer structure, which became more glassy and less rubbery. Temperatures close to T_g accelerated the rearrangement of these molecules (Rocca-Smith et al. 2017).

The recyclability of biopolymers is critical for their long-term and cost-effective use. Recycling has proven to be a valuable tool in solving environmental and waste management problems (Shojaeiarani et al. 2019). Mechanical and thermal properties of PLA after five cycles of extrusion and moulding were measured (Shojaeiarani et al. 2019). Compared to virgin PLA, the recycled polymer in the glassy state exhibited a 7.25% smaller decrease in storage modulus and a higher dissipation factor. This indicated the viscous rather than elastic nature of the material (Shojaeiarani et al. 2019). Budin et al. (2019) investigated the mechanical properties of recycled PLA from the injection moulding process. The results showed that the recycled PLA had 11% lower tensile strength, 4% lower hardness and 50% lower impact strength than virgin PLA. The reduced mechanical performance was attributed to thermal degradation of the PLA molecular chains during the injection moulding process.

The presence of various amounts of flax fibers (up to 30%) in the polymer matrix led to reversible (i.e., plasticization) and irreversible (i.e., damage) changes, affecting the mechanical properties of PLA-based biocomposites during aging. Chain mobility increased with increasing the water content and temperature. The ability of molecules to migrate further increased the water absorption within the polymer. The lignocellulosic materials have a high affinity for water, which enhances the moisture storage capacity and diffusion rate of water within the biocomposites. The swelling that takes place during water absorption and desorption damages the flax fibers, causing hoop stress, plastic strain and micro-cracks in the matrix. However, fibers did not always provide a high degree of irreversibility. Indeed, with long-term aging, fibers have shown to delay PLA failure and extend the life of the material (Regazzi et al. 2016). Then, wood particles can slow down the diffusion of degradation products from the polymer bulk to the gas phase and thus protect the polymer from thermal decomposition (Patti et al. 2021).

Polymer degradation is often an uncontrollable process that cannot be stopped even if the material is kept isolated from the aggressive environment in a stress-free state. By amplifying one or more environmental parameters that characterize actual exposure, aging becomes a useful method to predict durability and long-term properties of polymeric materials (Frigione and Rodríguez-Prieto 2021).

In this study, in order to validate the durability of bio-based materials for 3D-printing applications, we investigate the effects of long-term exposure to high temperatures on the thermo-mechanical properties of PLA-based 3D-printed specimens. To this end, a virgin PLA polymer, a recycled PLA, and a wood/PLA composite were used to prepare specimens by additive manufacturing. The thermal aging was conducted in an oven at 70 °C for up to 4200 h (175 days); such conditions could correspond to about 10–15 years of real service time (Hukins et al. 2008; Yarahmadi et al. 2016; Bergaliyeva et al. 2022). Infrared spectroscopy was used to confirm chemical changes in the materials and to validate the potential adverse effects of thermal degradation due to aging.

2 Materials and methods

2.1 Materials

For the experimental campaign we have used three different filaments with a nominal diameter of 1.75 mm. In particular, two types of unfilled PLA, one from virgin granules (B-PLA) and one recycled from filaments waste production (R-PLA); the third one was a composite made from PLA and fir wood pulp (W-PLA). All the filaments were supplied by Eumakers (Barletta, Italy). According to manufacturer information, the basic PLA polymer (cod. IngeoTM Biopolymer 4032D) was supplied by Natureworks (Minnesota, USA). Preliminary thermal analysis confirmed that the composite system contained at least 20% wood by weight (Patti et al. 2022c).

2.2 Sample preparation

These filaments were extruded in a 3D printer (code M200, by Zortax, Olsztyn, Poland) using pre-optimized conditions (Patti et al. 2022b). Z-Suite was printer management software. All filaments were dried in an oven at 70 °C for 5 h prior to the printing to remove any trace of moisture. Rectangular 3D printed specimens, 2 mm thick, 5 mm wide and 25 mm long, were manufactured for thermo-mechanical testing. Table 1 summarizes the processing conditions expressed in terms of temperature, speed, and geometrical parameters used for each material. In general, smaller layer thickness allows to obtain specimens with higher mechanical strength and higher quality (i.e., less visible layer distinction) (Patti et al. 2022a). The B-PLA and R-PLA filaments were extruded at the smallest possible layer thickness (0.09 mm). In this same condition, W-PLA printing encountered frequent nozzle clogging. The layer thickness was increased to 0.19 mm in order to provide a continuous flow of the molten composite through the nozzle, while the retraction speed and distance were reduced compared to the setting used for B-PLA and R-PLA printing.

Table 1

Optimized processing conditions for 3D printing

Technological variables	B-PLA and R-PLA	W-PLA
Layer thickness	0.09 mm	0.19 mm
Nozzle diameter	0.4 mm	0.4 mm
Infill percentage	100%	100%
Platform temperature	70	70
Nozzle temperature	210 °C	210 °C
Printing speed	Default	Default
Retraction speed	27 mm/s	20 mm/s
Retraction distance	2.7 mm	1 mm

Accelerated aging was performed in an oven at 70 °C for up to 4200 h (corresponding to 175 days/6 months). During aging, the samples were supported by wire mesh. According to the so-called “ten degree rule” a 10 K temperature increase doubles the rate of degradation and halves the material’s duration (Yarahmadi et al. 2016). A common method (Hukins et al. 2008) to evaluate the effect of the aging temperature on time considers that the aging rate increased by a factor, f, equal to:

$$f={2}^{\frac{\Delta T}{10}}$$

(1)

where ∆T is the difference between the aging temperature and the reference temperature (i.e., the service temperature). For a material that will endure its entire life at 20 °C, maintaining the material for 1 month at 70 °C would equate to an aging of 2^(70–20)/10 = 2⁵ = 32 months.

3 Characterization techniques

3.1 Dynamic mechanical analysis (DMA)

The machine used for dynamic mechanical analysis was the Tritec 2000 DMA manufactured by Triton Technology Ltd. (Leicestershire, UK). The instrument was operated with a single cantilever holder and 10 mm sample length, a heating rate of 2 °C/min, a maximum displacement of 0.05 mm, and a frequency of 1 for all the samples. The temperature was controlled by a thermocouple placed near the sample. DMA measurements were performed from room temperature to 80 °C in air. Initially, the sample was blocked between the two clamps. During scanning, one clamp stays stationary, whereas the other oscillates sinusoidally. The viscoelasticity of a material is determined by applying an input and measuring the response of the material. Under linear conditions the stress ($\sigma )$ and strain ($\varepsilon$) are both sinusoidal function of time (t) with the same frequency (ω), according to the following relations:

$$\sigma \left(t\right)={\sigma }_{0}\mathrm{sin}\left(\omega t+\delta \right)$$

(2)

$$\varepsilon \left(t\right)={\varepsilon }_{0}\mathrm{sin}(\omega t)$$

(3)

where ${\sigma }_{0}$ is the stress amplitude, ${\varepsilon }_{0}$ is the strain amplitude and δ is the phase shift between force and displacement (δ = 0° correspond to a perfect elastic, Hookenian, material; δ = 90° to a pure viscous, Newtonian, material).

Stress and strain are linked by the complex modulus (E*):

$$\sigma \left(t\right)={E}^{*}\left(\omega \right)\varepsilon (t)$$

(4)

The complex modulus (E*) is determined from measured data and geometry of the sample (Stark 2013). E* can be split in the two components (storage modulus-E′, and loss modulus-E″):

$${E}{\prime}=\frac{{\sigma }_{0}}{{\varepsilon }_{0}} cos\delta$$

(5)

$${E}^{{\prime}{\prime}}=\frac{{\sigma }_{0}}{{\varepsilon }_{0}} sen\delta$$

(6)

The storage modulus represents the elastic response of the material and is related to the energy stored per cycle. The loss modulus represents the viscous response of the material and refers to the loss, or dissipated, energy per cycle. The glass transition temperature is commonly detected as the temperature corresponding to the peak of the dissipation factor curve (tan delta). Tan δ is the ratio between the loss and the storage modulus $\left(tan\delta =\frac{E{\prime}{\prime}(\omega )}{E{\prime}(\omega )}\right)$ (Goertzen and Kessler 2007).

3.2 Attenuated infrared spectroscopy (ATR)

A spectrometer (model Spectrum 65 FT IR) produced by Perkin Elmer (Waltham, MA, USA), was utilized to perform infrared spectroscopic analysis on aged specimens in attenuated total reflectance (ATR) mode. During the investigation, the infrared light passed through a diamond crystal on which the sample was placed. The background spectra are collected using free crystal. A wavenumber range of 650–4000 cm⁻¹, a resolution of 4 cm⁻¹, and a scan number of 16 were used for the test. Three measurements were performed on each specimen, and a representative curve was considered for direct comparison between the ATR spectra. Each curve was normalized considering an internal standard for PLA polymer at 1455 cm⁻¹. This signal is a characteristic peak of the asymmetric bending vibration for the methyl group (CH₃).

4 Experimental results

4.1 Visual changes in samples after heat treatment

As changes in the physical appearance of aged materials can be investigated visually, photographs of the specimens (prior and after heat treatment) are presented in Fig. 1.

From inspection of Fig. 1 we can observe that for B-PLA and R-PLA an exposure to the most severe aging condition (70 °C for 175 days) produced insignificant variations in the color intensity and gloss of the samples without noticeable yellowing. However, for these materials, the heat treatment caused visible deterioration such as sagging and permanent deformation. On the contrary (see Fig. 1c), the wood-based resin W-PLA, subjected to the same thermal treatment, shows no signs of deterioration, with stable color, and undeformed specimens.

Mass variations, measured during the aging process, can be used to monitor decomposition processes as eventual mass loss is due to releases of volatile species into the environment (Andersson et al. 2012).

The mass and thickness of samples are reported as a function of the aging days in Table 2. All the systems show irrelevant (as within the experimental error) variations in both mass and thickness up to 175 days of heat treatment. It should be noticed that the wood-based systems are lighter than that of pure PLA’s. This can be attributed to various reasons including: (i) the wood flour has a lower density (190–200 kg/m³ (Pokhrel et al. 2021)) compared to PLA polymer (1200–1400 kg/m³); (ii) the removal of the pure PLA samples from the support base is more difficult compared to wood composites and some residues of material were left on the underside; (iii) the layer thickness required to print wood-based filaments is greater compared to the layer thickness used to print unfilled polymers (0.19 vs 0.09 mm).

Table 2

Mass and thickness for aged specimens

	B-PLA		R-PLA		W-PLA
	Mass (mg)	Thickness (mm)	Mass (mg)	Thickness (mm)	Mass (mg)	Thickness (mm)
0 days	375 ± 7	2.83 ± 0.04	371 ± 31	2.88 ± 0.06	293 ± 2	2.22 ± 0.01
50 days	374 ± 6	3.08 ± 0.17	369 ± 25	3.00 ± 0.13	290 ± 2	2.28 ± 0.07
70 days	376 ± 8	2.85 ± 0.04	382 ± 23	2.95 ± 0.02	286 ± 12	2.19 ± 0.16
130 days	383 ± 14	3.07 ± 0.25	388 ± 10	2.81 ± 0.07	293 ± 1	2.24 ± 0.15
175 days	377 ± 10	3.01 ± 0.12	379 ± 8	3.17 ± 0.17	295 ± 1	2.28 ± 0.06

4.2 Viscoelastic properties of aged 3D specimens

Figure 2 shows DMA results for virgin, neat, resin (B-PLA), in terms of storage modulus (E′), loss modulus (E″) and tan delta curve as a function of the temperature.

At low temperature (i.e., lower than the glass transition) the polymer is in a glassy state and its storage modulus (see Fig. 2a) is significantly affected by the heat aging and shows a 30% reduction for a 175-day treatment. This behaviour is consistent with the literature where it is often observed a tensile modulus decrease with aging time and temperature. Niaounakis et al. (2011), for instance, found a significant decrease in the tensile properties of PLA samples after 100 days at 50 °C: the Young’s modulus decreased by up to 51.5% and the tensile strength decreased by 65%. Water worked primarily as a degrading (hydrolysis) agent, and the shorter chains formed from PLA breakdown were rearranged in crystallites with low melting point.

The loss modulus of the B-PLA samples decreases with increasing the thermal treatment (see Fig. 2b) thus suggesting a reduction of the macromolecular ability for energy dissipation. This behaviour can be attributed to higher density and packing fraction of the amorphous regions due to the heating treatment. The PLA matrix, characterized by predominantly amorphous structure, undergoes physical rearrangements during the aging process, giving rise to a more stable organization (Pluta et al. 2008).

As aging increases, the tan delta peak (see Fig. 2c), and as a consequence the glass-to-rubber transition takes, shifts to higher temperatures. The peak height shows a slight maximum for intermediate (50, 70 and 130 days) aging times and then, for the longest thermal treatment, its intensity returns to the initial value. The glass transition temperature (T_g) of the pure polymer (B-PLA) is around 66 °C for unaged samples, it shifts to higher values (~ 71 °C for the 70-day aged samples) and lowers (~ 69 °C) for the most severe (175 days) aging conditions. Such a behaviour can be explained by the presence of two counteracting phenomena (Matečić Mušanić and Sućeska 2009): (i) densification, which prevailed during the initial aging period, reduces the flexibility of the polymer chains and produces a T_g increase; and (ii) chain scission, which dominates during the later stages, increases structural homogeneity and produces a T_g decrease.

Figures 3 and 4 show the behaviours of the R- and W-PLA, which are roughly similar to those observed for B-PLA filament.

Figure 3 shows the temperature-dependence of the viscoelastic properties (storage modulus (a), loss modulus (b), and tan delta (c)) as a function of aging duration for R-PLA. The reduction in storage modulus (E′) in the glassy region of R-PLA (see Fig. 3a) is less pronounced compared to the B-PLA and is in the range of 10%. The onset of the glass transition is always delayed in aged samples compared to the unaged material. The peak height of the E″-T curve decreases with aging duration, and the corresponding width also decreases slightly (see Fig. 3b). The entire E"-T curve moves towards higher temperatures as the aging duration is increased. The tan delta peak increases with aging time and shifts to higher temperatures (see Fig. 3c).

The temperature-dependency of the storage modulus (E′), loss modulus (E″) and dissipation factor (tan delta) for wood-based composite before and after heat aging are presented in Fig. 4. In the glassy region the storage modulus shows an increase (up to 13%) due to the aging (see Fig. 4a). On the contrary, the loss modulus peak height decreases and shifts to higher temperatures (see Fig. 4b). The glass transition temperature is only slightly affected by thermal aging with values around 70 °C (see Fig. 4c). This behaviour indicates that no significant molecular events affecting the viscoelasticity of the composites occurs during the thermal treatment (Paunonen et al. 2020). The presence of wood filler contributes to an enhancement of the mechanical properties during the aging by restricting the segmental mobility of the polymeric chains and preventing the enthalpic relaxation of PLA-based composite (Lee et al. 2020). This corresponds to a lower level of tan delta peak in the aged composites (Paunonen et al. 2020).

The increase in storage modulus during the aging can be attributed to stronger interactions at the interfaces between PLA and wood fibers or to the fibers promoting the crystallization (Paunonen et al. 2020). Hydrothermal treatment has been extensively studied to improve the wood properties. This process was found to alter the morphology, the physical and mechanical properties of treated wood. After the hydrothermal treatment, the wood samples have shown reduced presence of hydroxyl (OH^–) and carbonyl groups in the cell walls and improved dimensional stability and resistance to decay (Rowson Ali et al. 2021). In our case, long-time heat treatment above ambient temperature of PLA/wood-based composite could have determined changes in the wood surface chemistry with improved interface interactions between hydrophilic wood particles and hydrophobic biopolymer. Furthermore, particles in filled thermoplastic polymers tend to agglomerate under Brownian motion, thus forming a filler network during aging or further processing (Zhang et al. 2006). This results in increased filler/filler and filler/matrix interactions and increased overall stiffness of the aged wood biocomposites.

The average values of storage modulus (E′) at 30 °C, temperature and intensity of loss modulus peak, and temperature (T_g) and intensity of tan delta peak are summarized in Table 3.

Table 3

E′ at 30 °C, temperature at E″ peak and intensity, T_g and peak intensity of tan delta curve for aged and unaged materials

	E′ (MPa) at 30 °C	T at E″ peak (°C)	E″ peak intensity (MPa)	T_g (°C)	Tan delta peak intensity
B-PLA
0 days	1130 ± 108	58.0 ± 0.4	220 ± 12	65.6 ± 0.2	2.3 ± 0.1
50 days	1030 ± 151	61.8 ± 0.2	187 ± 30	67.7 ± 0.6	2.7 ± 0.1
70 days	497 ± 120	64.4 ± 0.1	74.5 ± 38	70.8 ± 3.3	3.0 ± 0.8
130 days	1190 ± 135	63.0 ± 0.3	207 ± 15	68.1 ± 0.1	2.7 ± 0.1
175 days	844 ± 228	64.5 ± 0.2	140 ± 68	69.1 ± 0.4	2.4 ± 0.1
R-PLA
0 days	1180 ± 119	57.6 ± 0.1	209 ± 22	64.7 ± 0.4	2.2 ± 0.1
50 days	1090 ± 125	62.9 ± 0.2	189 ± 15	68.0 ± 0.5	2.7 ± 0.3
70 days	1080 ± 245	64.6 ± 0.2	181 ± 4	70.4 ± 1.9	2.3 ± 0.2
130 days	1060 ± 883	64.9 ± 0.1	175 ± 18	69.7 ± 0.4	2.1 ± 0.1
175 days	1140 ± 102	64.0 ± 0.6	197 ± 20	68.5 ± 0.6	2.9 ± 0.1
W-PLA
0 days	1030 ± 351	60.0 ± 0.4	170 ± 70	69.5 ± 0.3	2.3 ± 0.1
50 days	1110 ± 103	66.5 ± 2.4	105 ± 62	70.5 ± 0.5	2.1 ± 0.1
70 days	1040 ± 54	65.5 ± 0.2	129 ± 26	70.8 ± 0.4	2.0 ± 0.1
130 days	1150 ± 281	63.9 ± 0.2	155 ± 37	69.2 ± 0.2	2.2 ± 0.1
175 days	1160 ± 59	64.4 ± 0.1	137 ± 4	69.3 ± 0.1	1.9 ± 0.1

4.3 ATR-FTIR spectroscopy on aged parts

To examine microstructural changes due to the aging, ATR-FTIR spectra were collected for each specimen and aging duration from 0 to 175 days. Aging had a qualitatively similar effect on the characteristic absorption bands of PLA polymer in all three materials. The typical bands for PLA polymer are: the stretching vibrations of hydroxyl groups (ν_OH) in the range of 3000–3800 cm⁻¹; C–H alkane stretching vibration (ν_C-H) in the range of 2800–3000 cm⁻¹; carboxyl group stretching (ν_C=O) at 1750 cm⁻¹; C–O vibration of ester group (symmetric stretching) (ν_C-O) at 1183 cm⁻¹. The bands in the range of 1317–1425 cm⁻¹ are generally assigned to various C-H vibrations (Harnnecker et al. 2012; Schramm 2020). The anti-symmetric and symmetric stretching vibrations of C-O bonds (C–O–C, C–OH) are in the region of 890–1160 cm⁻¹ (Schramm 2020).

Figure 5 shows the spectral measurements for aged and unaged samples for the B-PLA resin. Spectra allow to detect changes such as formation/disappearance and increase/decrease of various bands. After 50 days of thermal treatment (see Fig. 5a): (i) absorbance of hydroxyl band (ν_OH) increases significantly while the carboxyl group vibration (ν_C=O) decreases; (ii) the C-O vibration of ester group disappears (ν_C-O), (iii) a new board at 1630–1650 cm⁻¹ peak appears. After 70 days of thermal aging, ν_OH and ν_C-H decreases, while ν_C=O increases.

In general, the hydrolytic degradation of a polymer matrix takes place in two stages: (i) water diffusion into the polymer bulk and (ii) hydrolysis reaction. In contrast to thermal degradation, which requires high temperatures of 200–250 °C and a vacuum at 4–5 mmHg, hydrolysis is a relatively simple depolymerization process that does not necessitate harsh processing conditions or catalysts (Piemonte et al. 2013). Time-evolution of absorption spectra for water molecules diffusion into polycaprolactone (PCL) film were displayed in Peng et al. (2003). Two signals are representative of water absorption in the polymer: the first in the range of 3000–3800 cm⁻¹ was attributed to stretching vibration of OH group (ν_OH) and the second in the range of 1600–1650 cm⁻¹ was attributed to bending vibration of OH group. As water diffused into the PCL film, these signals gradually increased.

In our case, water (coming for moisture penetrating into the non-perfectly insulated chamber) could have diffused into specimens during aging and attacked the double carboxyl bond via the hydrolysis mechanism (Al-Itry et al. 2012). This process results in a random scission of chains.

However, the peak at 1635 cm⁻¹ can be attributed to the stretching vibration of a carbon-double bond (C=C) (ν_C=C) (Alia et al. 2018). This consideration could support the possibility of a second PLA degradation mechanism (Al-Itry et al. 2012) involving hydrogen atom transfer and the creation of terminal vinyl ester and carboxyl groups.

After 130 and 175 days of thermal treatment (see Fig. 5b), all the absorption bands of the aged specimens return to be close to those of starting material. ν_C-H, ν_OH, ν_C=C are slightly higher while ν_C=O, ν_C-O, ν_C-O-C are lower compared to intensities recorded for original polymer.

Findings for B-PLA resin are confirmed for R-PLA (see Fig. 6a) and W-PLA (see Fig. 6b).

Spectroscopic analysis also provided qualitative information on the crystallinity of the samples; indeed FTIR has proved to be a promising tool to characterize crystallinity changes in materials (Abbass et al. 2021). For PLA, the two peaks at 870 cm⁻¹ and 756 cm⁻¹ were considered representative of the amorphous and crystalline phases, respectively (Patti et al. 2022b). Pop et al. (2019) observed a decrease in the ratio of the two band areas A₇₅₆/A₈₆₁ from 3.095 for the PLA-based filament to 1.763 for the printed material. This was attributed to decrease in crystallinity during the PLA printing. Briassoulis et al. (2022) studied the mechanical performance against physical aging of the PLA/PHB blends with olive oil and carvacrol as plasticizing and stabilising additives. The mixture displayed a higher intensity at 756 cm⁻¹ peak compared to basic PLA suggesting a rise in the crystallinity of PLA's phase within the compound.

The crystallinity index (I_c) was calculated through the areas ratio of aforesaid characteristic peaks (Area₈₇₀/ Area₇₅₆) (Stoleru et al. 2017) over the aging time. Data are summarized in Table 4 and suggest that changes in the crystallinity due to the aging are negligible. In all cases, the crystallinity index is close to unity thus indicating that potential densification did not affect the degree of structural order in the solid.

Table 4

Absorbance intensity at 870 cm⁻¹ (A₈₇₀) and at 756 cm⁻¹ (A₇₅₆), and crystallinity index (I_c)

	B-PLA			R-PLA			W-PLA
	A₈₇₀	A₇₅₆	I_c	A₈₇₀	A₇₅₆	I_c	A₈₇₀	A₇₅₆	I_c
0 days	0.848 ± 0.015	0.894 ± 0.026	0.97 ± 0.02	0.749 ± 0.017	0.831 ± 0.031	1.17 ± 0.02	0.811 ± 0.014	0.911 ± 0.009	1.22 ± 0.02
50 days	2.439 ± 0.186	2.621 ± 0.135	1.08 ± 0.00	2.529 ± 0.006	2.672 ± 0.052	1.07 ± 0.02	2.431 ± 0.058	2.538 ± 0.054	0.97 ± 0.08
70 days	1.457 ± 0.041	1.322 ± 0.154	0.99 ± 0.11	1.090 ± 0.406	0.922 ± 0.313	1.13 ± 0.07	1.019 ± 0.086	1.019 ± 0.086	1.18 ± 0.03
130 days	0.899 ± 0.071	0.918 ± 0.101	1.02 ± 0.05	0.904 ± 0.091	0.944 ± 0.090	1.12 ± 0.09	0.899 ± 0.071	0.918 ± 0.101	1.16 ± 0.04
175 days	0.899 ± 0.072	0.982 ± 0.092	1.10 ± 0.02	0.845 ± 0.084	0.903 ± 0.023	1.10 ± 0.01	0.899 ± 0.072	0.982 ± 0.092	1.17 ± 0.02

5 Conclusion

In this study, the thermo-mechanical performance of 3D-printed specimens made from unfilled and wood-filled PLA polymer are investigated as a function of thermal aging.

For unfilled resins subjected to thermal aging, DMA results show a decrease in E' in the glassy region, a shift to higher temperatures and a decreased intensity of E″ peak, an increase in the intensity of the tan delta peak and its shift to higher temperatures. These findings can be explained by the occurrence of two opposing mechanisms: an increment in density and packing fraction during physical rearrangement towards the equilibrium state, and a chain scission mechanism that enhances the structural homogeneity of polymer macromolecules.

For the wood-filled resin, with increasing aging days, E′ in the glassy region increases the E″ peak shifts to higher temperatures and its intensity decreases, the tan delta peak remained unchanged. The different behaviour observed for the wood composite can be attributed to a reduced chains mobility due to macromolecular compaction, particles flocculation, and networks formation of wood particles. Furthermore, the enhanced rigidity of the wood-composite can be attributed to improvements of the filler-matrix interface. ATR measurements reveal possible hydrolysis of the PLA polymer, resulting in a random chain scission, as well as a second degradation mechanism involving a hydrogen atom transfer. The analysis of crystallinity index suggests that the effect of aging on the structural order of the original material was negligible.

In conclusion we can affirm that the thermal aging caused polymer degradation, visible sample deformation, deterioration of mechanical properties and an increase in the glass transition temperature. The presence of wood particles in PLA mitigates such effects and improves the overall product durability.

Acknowledgements

A. Patti wishes to thank the Italian Ministry of Education, Universities and Research (MIUR) in the framework of Action 1.2 “Researcher Mobility” of The Axis I of PON R&I2014- 2020 under the call “AIM—Attrazione e Mobilità Internazionale”. S. Acierno and G. Cicala acknowledge the support of the Italian Ministry of University, project PRIN 2017, 20179SWLKA“Multiple Advanced Materials Manufactured by Additive technologies (MAMMA)”.

Declarations

Conflict of interest

The authors declare no conflict of interest.

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Titel: Aging effects on the viscoelastic behaviour of products by fused deposition modelling (FDM) made from recycled and wood-filled polymer resins
verfasst von: Antonella Patti
Stefano Acierno
Gianluca Cicala
Domenico Acierno
Publikationsdatum: 25.09.2023
Verlag: Springer Berlin Heidelberg
Erschienen in: European Journal of Wood and Wood Products / Ausgabe 1/2024
Print ISSN: 0018-3768
Elektronische ISSN: 1436-736X
DOI: https://doi.org/10.1007/s00107-023-01994-9

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Abstract

Publisher's Note

1 Introduction

2 Materials and methods

2.1 Materials

2.2 Sample preparation

3 Characterization techniques

3.1 Dynamic mechanical analysis (DMA)

3.2 Attenuated infrared spectroscopy (ATR)

4 Experimental results

4.1 Visual changes in samples after heat treatment

4.2 Viscoelastic properties of aged 3D specimens

4.3 ATR-FTIR spectroscopy on aged parts

5 Conclusion

Acknowledgements

Declarations

Conflict of interest

Publisher's Note

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