Effect of hydric aging on the static and vibration behavior of 3D printed bio-based flax fiber reinforced poly-lactic acid composites

Abstract

Moisture accumulation and life span in a hydric setting are major concerns for natural fibers used for the reinforcement of bio-based composites. This article discusses the effect of water aging on the mechanical properties of poly-lactic acid composites reinforced with flax fibers produced by 3D printing. Water absorption of composites is measured and it was observed that the bio-based composite seems to be Fick’s model with a percentage of absorption at saturation equal to 1.2%. To determine the effect of water absorption on static and vibration mechanical properties, various immersion periods are chosen. Water aging obviously influences the stiffness and tensile failure; so, a decline in the Young’s modulus from 2.2 to 1.7 GPa and in strength from 36 to 30.5 MPa is seen. Nevertheless, the damping factor shows an increase by about 16% as a function of the immersion period. These results could be explained by the plasticizing effect of water on bio-based composites, stimulated by moisture absorption.

Keywords

bio-based composite flax/poly-lactic acid water aging absorption static vibration 3D printing

Introduction

The foreseeable depletion of fossil resources (oil, natural gas…), constantly rising oil prices, and a global awareness of the need to limit our impact on the environment, are creating a favorable context for the development of plant-based products. Study in the field of composites has been shifting to bio-sourced materials for many years now, motivated by the environmental and economic context.^1-5 Due to the strong basic mechanical properties of the plant fibers, similar to those of glass fibers¹ but with a much higher damping capacity, the production of polymers strengthened with plant fibers, in particular flax fibers, is in constant expansion.⁶

The challenge of assessing this performance in a hostile setting remains a barrier, and is likely to greatly impede their use, which is a major obstacle that is still limiting the production of these products. Their biochemical composition and structure⁶ may explain the behavior of plant fibers exposed to water. The main structural layer is the secondary cell wall, which is responsible for the majority of the fiber’s mechanical performance and physical properties. It consists of micro-fibrils of crystalline cellulose covered by hemicelluloses and enclosed inside a matrix of pectin. Their hydrophilic nature makes them sensitive to humidity.^7,8

During immersion in an aqueous environment, two forms of degradation occur: physical degradation with plasticizing and swelling effects and chemical degradation caused by matrix hydrolysis and fiber degradation.⁸ Some components on the fiber surface are washed out of the sample during the aging phase in interfacial debonding and transfer to the interfacial region.^9-11 Le Duigo et al.¹² proved that long-term immersion causes enzymatic degradation. All these phenomena cause a loss of mechanical properties and as a result, a decrease in the lifespan of composites filled with plant fiber. Therefore, a more in-depth study and understanding of their behavior during water aging seems indicated. The effect of relative humidity on mechanical properties is dealt with in extensive published research.

Recently, several researchers have performed many experiments to study the impact of water aging on the static and vibration behavior of bio-based composite materials. They showed that exposing a plant fiber composite to a humid environment results in a decrease in mechanical properties when water leaks into the material. For different composites of plant fibers, such as composites with fibers of bamboo,¹² jute,¹³ sisal,¹⁴ hemp,¹⁵ or flax,^16-18 this loss of properties due to water absorption was observed. This decrease in mechanical properties with the rate of moisture absorption may be minimized with the aid of chemical surface treatments or fiber treatments.¹⁶

The characteristics of flax fiber/polypropylene (PP) composites in distilled water at room temperature are investigated by Arbelaiz et al.¹⁹ They show that as the fiber bundle content increases, absorption also increases. The use of maleic anhydride-polypropylene copolymer as the PP matrix changing binding agent reduces marginally the composite’s affinity for water.

The effect of water aging in water bath at room temperature on the mechanical properties is studied by Assarar et al.²⁰ on an epoxy composite reinforced with flax fibers. They show a 63% increase in strain and a decrease in the stiffness modulus and tensile strength of about 39% and 25%, respectively.

Chow et al.²¹ studied composite specimens made from polypropylene bolstered with sisal fibers for treatment by immersion in hot water at 90°C for different times. As the immersion time of the specimens increases, the Young’s modulus and failure strength decrease.

For example, due to plasticization, this tendency toward higher ductility is confirmed by the work of Alix et al.²² on composites containing flax fibers. Also, the increase in damping properties and decrease in bending moduli is proved by the work of K. Cheour et al.²³ on flax fiber reinforced composite materials.

To study the effect of water aging on the load-unload cyclic actions of flax fiber reinforced thermoplastic and thermosetting composites, Chilali et al.²⁴ used monotonic and cyclic load-unload experiments. Results indicate that aging in water induces major deterioration of the mechanical characteristics of these materials by comparing them with non-aged ones. This difference is primarily due to the damage caused by fiber deterioration and the weakness of the interface of the fiber/matrix. Load-unload experiments have demonstrated an improvement in load testing. Due to plasticizing phenomena induced by water absorption, load-unload studies have demonstrated an improvement in stiffness loss with aging.

The combination of natural fibers and bio-based polymers can be an attraction point in view of consumer appeal for these fully bio-based products.

In this context, the 3D printed bio-composite flax/poly-lactic acid (PLA) is studied by Essassi et al.²⁵ static and vibration behaviors were investigated. Results obtained have shown that this composite exhibits important properties. 3D printing is a layer-by-layer automated manufacturing technology that uses digital 3D model data to create items.^26,27 It is used by Antony et al.²⁶ to study mechanical characteristics of hemp/PLA composites with honey comb structure. They concluded that this methodology is crucial in industries to manufacture prototypes. 3D printing process is used to innovate specimens with complicated shapes and to reduce total costs²⁸ and production waste.²⁹ 3D printing improves qualities when compared to the traditional manufacturing process.^30-33

Despite all the advantages presented by bio-composites and 3D printing process, a certain number of barriers in some applications such as humidity must be taken into account. The influence of aging in seawater for different temperatures on the properties of a biopolymer PLA reinforced with flax fibers was examined by Baley et al.³³ They proved that the principal factor of this deterioration is the fiber/matrix interface of biopolymer composites that is weakened by the absorption of water.

This article proposes a study of the effect of water aging on the static and vibration behavior of the bio-based flax/PLA composite. First, composite samples are fabricated by 3D printing technique and the absorption of tap water by the composite to saturation is analyzed. Second, static tensile and vibration tests are carried out on the aged composites in order to highlight the effect of absorption on its behavior.

Materials and experimental procedure

Materials

In this study, the material used is a 1.75 mm diameter roll of PLA reinforced with flax fibers with a density of 1000 kg.m⁻³ and a volume fraction of flax less than 20%. This material supplied by NANOVIA from Louargat, France, is a bio-based, biodegradable, and recyclable material.

The process used to manufacture the specimens is the “Maker Bot Replicator2” 3D printer. The 3D printing process consists of depositing the molten filament to create 0.20 mm thick layers that produce the desired shape with a travel speed of 70 mm/s. It is equipped with three main elements: a spool of filament, an extrusion head from which the material melts and is arranged to create the object and a heating plate on which the specimen is printed.

Following the print settings proposed by the supplier of the material, the temperature of the printer platform is set to 55°C and that of the extrusion nozzle is set to 210°C.

The samples are designed with Solidworks software and then translated into instructions compatible with the Maker Bot Replicator2 in order to be printed. First of all, specimens measuring 25 mm × 25 mm × 5 mm samples are printed. These specimens are used to observe the evolution of the absorption. Second, for tensile tests, specimens with a length (L) of 150 mm, a width (w) of 10 mm, and a thickness (h) of 5 mm (Figure 1(b)) are printed. Finally, specimens with a width (w) of 25 mm, a thickness (h) of 5 mm, and different lengths (L) of 270, 240, and 210 mm are printed to perform vibration tests (Figure 2(b)).

Figure 1.

(a) Experimental tensile test set-up, (b) Tensile test sample.

Figure 2.

(a) Experimental vibration test set-up, (b) Vibration test sample.

Water absorption

The aging process used in this study is the immersion of the samples in tap water at room temperature.

In order to measure the water absorption percentage of flax/PLA, specimens are immersed in water. Then the change in mass of the samples is observed over time. Specimens are weighed with a precise SARTORIUS balance, a scale with an accuracy of 10⁻⁴ g. Samples are periodically removed to be weighed and characterized.

Weight gain is determined as a percentage of initial weight using equation (1)

M_{t} = \frac{W_{t} - W_{0}}{W_{0}} \times 100 (%)

(1)

where

W_{0}

is the mass of the non-aged sample at

t = 0

and

W_{t}

is the mass of the aged specimens.

To characterize the moisture absorption behavior of composite materials, various models have been developed. The diffusion model of Fick³⁴ expressing the mass uptake of the specimens due to absorption at time t to saturation in this study is written as equation (2).^35-37

\frac{M_{t}}{M_{m}} = 1 - \exp [- 7.3 \times {(\frac{D . t}{h^{2}})}^{0.75}]

(2)

where

M_{t}

is the water content at time t,

M_{m}

is the maximum water mass in equilibrium, h is the thickness and D is the diffusion coefficient.

The diffusion coefficient $D$ can be deduced

D = π \times {(\frac{k}{4 M_{m}})}^{2}

(3)

where

k

is the slope of the linear part of the curve

M_{t} = f (\frac{\sqrt{t}}{h})

Because of the small size of the specimens, a correction factor is used

D_{c} = D \times (1 + \frac{h}{L} + \frac{h}{w})

(4)

where

D_{c}

is the corrected transverse diffusion coefficient,

L

and

w

are, respectively, the length and the width of the specimen.

Tensile test

In order to study the influence of the water absorption on the mechanical properties of the material, tensile tests are carried out according to the ASTM D638 standard test method at room temperature. A standard INSTRON hydraulic machine with a 10 kN load cell and a speed of 1 mm/min is used for this test. To measure the displacement of specimens during testing, a longitudinal extensometer is used (Figure 1).

A dry sample is tested to measure the mechanical properties as the reference data. Then, aged samples are tested for several immersion times in order to study the influence of humidity on the mechanical properties of flax/PLA by comparing the results with that of the dry sample. Five specimens are statically tested to failure to take into account the variability of results due to experimental conditions for each immersion time.

Vibration test

The dynamic characteristics of the composite are evaluated using a free vibration test according the ASTM E-756 standard.³⁸ Specimens are supported horizontally and tested in a clamped-free configuration (Figure 2).

The clamping length is set to 40 mm. The specimens are excited using a PCB084A14 impact hammer over three free lengths (230, 200 and 170 mm) in order to obtain various peak frequencies. Then, the recorded excitation and response signals are processed by an acquisition card system and analyzed with NVGate software.

For each beam, the vibration tests are repeated several times and the five best signals are averaged to produce the final data sets. Moreover, to take into account the potentially scattered properties of these natural materials, a minimum of five beams are tested. Then, the data are processed with MATLAB software to generate the beam frequency response function (FRF).^39-41 An automatic routine is then applied to each FRF to detect resonance peaks.

A half power bandwidth method and Matlab software optimization module are used for each mode to measure its resonance frequency fi and its modal loss factor η_i, as shown in Figure 3.

Figure 3.

Half power bandwidth method.

The modal damping factor is calculated using equation (5). The damping factor η_i is the ratio between the bandwidth frequencies where $Δ f_{i}$ the amplitude resonance decreases by 3db, divided by the resonance frequency $f_{i}$

η_{i} = \frac{Δ f_{i}}{f_{i}} = \frac{f_{2} - f_{1}}{f_{i}}

(5)

The Young’s modulus E of the composite for every mode is calculated by equation (6)⁴²

E = \frac{12 ρ l^{4} f_{n}^{2}}{e^{2} C_{n}^{2}}

(6)

where

ρ

is the density of the composite material, l is the free length of the specimen,

f_{n}

is the resonance frequency of the n^th flexural mode, e is the thickness of the specimen in the vibration direction, and

C_{n}

is a coefficient for the n^th mode of a clamped-free specimen, with

C_{1} = 0.55959, C_{2} = 3.5069, C_{3} = 9.8194 and C_{n} = (π / 2) . {(n - 0.5)}^{2} for n 〉 3

.⁴²

These tests are applied to a dry specimen and for other aged specimens for different immersion times in order to identify the effect of humidity on the vibration behavior. Five specimens are tested to take into account the variability of results due to experimental conditions for each immersion time.

Results and discussion

Water absorption of flax/PLA

Figure 4 shows the evolution of water absorption as a function of the immersion time. The water uptake of flax fiber reinforced PLA is shown in Figure 4. The diffusive behavior as shown in Figure 4 is close to Fick model with a water uptake up to saturation after an immersion time of around 1 month and half. Initially, the water absorbed by the composite increases linearly with the square root of time and then slows down before reaching saturation. A Fick diffusion model can describe this diffusive behavior (Figure 4). The weight saturation of the composite is 1.2% with a diffusion coefficient of 4.464.10⁻⁶mm²/s. Table 1 lists the water absorption at saturation, diffusion coefficient, and corrected diffusion coefficient values calculated by using equations mentioned in Water absorption. The second case is used in our study because the calculated experimental ratio $\frac{M_{t}}{M_{m}}$ is higher than 0.6.

Figure 4.

Evolution of water uptake of flax/PLA.

Table 1.

Equilibrium water uptake, diffusion coefficient and corrected diffusion coefficient values.

M_∞%	D×10⁻⁶ (mm²/s)	D_c×10⁻⁶ (mm²/s)
1.2	4.464	2.277

The linear part of this curve provides information on the diffusivity of the water molecules which depends on the rate of penetration of water into the material. The saturation step shows the mass of water absorbed by the material as the aging time tends to infinity. The absorption of water by natural fiber composites has many consequences, including plasticization. Plant fibers are themselves composite materials reinforced with cellulose fibers in an amorphous matrix of highly hygroscopic lignin and hemicelluloses.^43,44 The hydrophilic activity of plant fibers is largely due to water-attracting hydroxyl groups -OH. Hydroxyl groups are picked up by water molecules that are present on the surface of cellulose crystallites in the amorphous cellulose regions or at the level of hemicelluloses.⁴⁵ According to the literature, water seeps into the composite material via the “Vacancy diffusion” process.⁴⁶ The water molecules attach themselves to the hydrophilic groups of the macromolecular chains, which leads to an increase in molecular mobility. In addition, the hollow part in the center of the elemental flax fiber (lumen) strongly contributes to the water absorption process, exhibits high sensitivity to water, resulting in composite materials with higher water absorption capacity.⁴⁷ This can lead to early stress damage, especially at the interface of the fiber matrix, which can potentially affect mechanical properties and significantly reduce durability.

In our study, 3D printing is used to make specimens which are totally submerged in water. Hence, the absorption is directly linked to the bio-based composite components. The two components of our studied bio-based composite absorb water. Le Duigou et al.¹⁷ observed that the manufacturing process does not influence the kinetics of water diffusion of bio-based composites when they are completely immersed. The composite studied in this article is poly-lactic acid reinforced with flax fibers. The prevailing phenomenon of PLA is hydrolysis when it is in the midst of a watery environment.⁴⁸ The syndrome is usually observed in amorphous regions. Although amorphous cellulose and lignin are hydrophilic, hemicellulose is primarily responsible for water absorption which is confirmed by Le Duigou et al.⁴⁹ so the absorption of water of bio-composites is completely controlled by the flax fibers.

Aging effect on the static behavior of flax/PLA

In order to study the effect of humidity on the bio-composite, tensile tests are carried out. Eleven aging periods are chosen from 1 h to 45 days. Five tests are carried out for each immersion time. The tensile tests carried out allow us determine the Young’s modulus, the ultimate tensile stress, and the ultimate strain of the specimens. The Young’s modulus is calculated from the linear part of the stress–strain curves. Three period of immersion (3, 30, and 45 days) are chosen to compare results with the dried composites (0 days).

In order to analyze results, one stress curve for each immersion time is drawn as shown in Figure 5. The curves present the same shape, with an initial linear part.

Figure 5.

Comparison of stress/strain curves for the four immersion times.

The Young’s modulus is calculated from the linear part of the stress–strain curve. By increasing the immersion time, we notice that the slope of the curve decreases. To visualize this loss of linearity, the Young’s modulus is plotted for different elongation ranges for each immersion time (Figure 6(a)).

Figure 6.

Evolution of static properties with absorption: (a) Young’s modulus, (b) Ultimate stress and (c) Ultimate strain.

In the dry case, the bio-based composite has the highest ultimate Young’s modulus value with 2.2 GPa. Likewise, stress decreases from 38 MPa to reach 30 MPa (Figure 6(b)) at saturation. This loss is caused by an increase in the plasticization of composite constituents triggered by the penetration of water molecules into them.^11,17,49 On other hand, the strain increases as a function of immersion time. Also due to plasticization, the strain value increases from 3 to 4% with water uptake as shown in Figure 6(c).

This behavior suggests that these materials’ mechanical properties are proportional to their water content. This outcome was comparable to one found in another research.^50,51 Water immersion may disrupt chemical bonds in composites, impair the bonding between fibers and resins, and generate voids and microcracks on the surface, all of which can contribute to a reduction in the composites' mechanical qualities. They are inevitably affected by this water absorption. This bonding phenomenon makes the material more ductile. This rise was also attributed by Methacanon et al.⁵² to the lubricating effect of water molecules.

Aging effect on the vibration behavior of flax/PLA

Aging effect on flax/PLA Young’s modulus

To study the aging effect on the composite, 11 aging periods from 1 h to 45 days are chosen. Five tests are carried out for each immersion time. The evolution of the dynamic properties for four immersion time (0, 3, 30, and 45 days) is illustrated in Figure 7. Results show that the Young’s modulus decreases as the frequency and immersion time increase.

Figure 7.

Comparison of Young’s modulus for the four immersion times.

The four figures present a linear decrease in the Young’s modulus. Initially, in the dry case, the Young’s modulus of the composite is 2.25 GPa at low frequency. For an immersion time of 45 days at low frequency, the Young’s modulus is 2.02 GPa and at a frequency of 4600 Hz, it falls to 1.80 GPa.

The four immersion time curves are clustered together in the same graph as shown in Figure 7 to compare the multiple immersion time and the effect of aging. It is clear that the Young’s modulus decreases as a function of the time of immersion.

Likewise, absorption and variation in the Young’s modulus for tested specimens are plotted for three chosen frequencies which are, respectively, 250 Hz, 1000 Hz, and 2500 Hz (Figure 8).

Figure 8.

Evolution of flax/PLA Young’s modulus with immersion time for three frequencies: (a) 250 Hz, (b) 1000 Hz, (c) 2500 Hz, (d) comparison.

The Young’s modulus decreases as a function of immersion time and frequency (Figure 8). Degradation of mechanical properties is explained by the intrusion into the structure of the bio-composite of water molecules, breaking down the hydrogen bonds between the polar groups of adjacent macromolecular chains that bind to a water molecule. Breaking the bonds between the chains, which largely ensure the stiffness of the material, apparently increases the mobility of the chains or of the fragments of macromolecular chains ⁵³.

The more flax/PLA absorbs water, the more the Young’s modulus decreases as a function of the immersion time. The stiffness of specimens decreases at high frequency and long immersion duration.

The effect of aging on flax/PLA loss factor

Unlike the Young’s modulus, the damping factor increases as a function of the frequency, as shown in Figure 9. However, it does not give a clear result compared to the immersion time where the four times are plotted together, due to dispersion.

Figure 9.

Comparison of loss factor for the four immersion times.

In order to better illustrate this increase, absorption and variation in loss factor of four tested specimens are plotted for three chosen frequencies which are, respectively, 250, 1000, and 2500 Hz (Figure 10). We observe that the loss factor rises as a function of the frequency and of the time of immersion. Thus, the Young’s modulus and the loss factor have a contradictory tendency as a function of immersion time.

Figure 10.

Evolution of flax/PLA loss factor with immersion time for three frequencies: (a) 250 Hz, (b) 1000 Hz, (c) 2500 Hz, (d) comparison.

The significant damping properties of flax fiber composite may be due to the morphology of flax fibers, which facilitates energy dissipation by friction between cellulose and hemicelluloses as well as by friction between cell walls.⁴⁹ Water absorption’s influence on the mechanical properties was tested in a similar study.⁵⁴ Water absorption causes fiber expansion, which leads to the formation of shear stress at the interface, resulting in fiber debonding from the matrix and a loss in mechanical characteristics. In general, the loss factor of non-aged materials is low because they have few defects and the fiber/matrix interfaces are not yet affected.

Conclusion

The effect of water aging on the static and vibration behavior of the composite produced using 3D printing technology is studied. The experimental study dedicated to the mechanical behavior of non-aged composites is essential to better understand the effects of aging on the damageable behavior of composites. In order to analyze the effect of humidity on static and vibration behavior, absorption is followed and analyzed. The absorption of the bio-composite corresponds to the Fick model. The absorption curve has three parts: the first part where the absorption is accelerated, the second with slow absorption, and the third part with saturation state. In static testing, the tensile test highlights a decrease in the Young’s modulus and an increase in strength due to micro-structural damage and plasticization phenomenon caused by the diffusion of water. Similarly, for dynamic behavior, the vibration test shows a decrease in the Young’s modulus. Nevertheless, the plasticization phenomenon induces an increase in the loss factor. It is found that the loss factor increases as a function of the immersion time and frequency. Subsequently, due to humidity, the mechanical properties will be deteriorated. To sum up, the nature of the composite’s constituents makes it very sensitive to humidity, and for that reason, the mechanical properties are weakened.

Incorporation of acoustic emission, microscopic observation, and numerical simulation, and the application of fatigue tests with the water aging impact would be helpful in future work in order to assess damage mechanisms and determine the lifetime of aged flax/PLA.

Footnotes

Acknowledgment

We gratefully acknowledge M. Peter BURGESS Professor of English (Le Mans University) for providing helpful corrections regarding this study.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Zeineb Kesentini

Jean-Luc Rebiere

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