Moisture Absorption and Its Effects on the Mechanical Properties of Biopolymers Reinforced by Curauá Fiber and Montmorillonite Clay: A Transient Experimental Evaluation

Abstract

Biocomposites are defined as eco-friendly materials from an environmental point of view. Because of the importance of this class of materials, their study is important, especially in moist and heated conditions. In this sense, this work aims to evaluate the transient behavior of moisture absorption and mechanical performance of biocomposites composed of a matrix of high-density biopolyethylene (originated from ethanol produced from sugarcane) filled with curauá vegetable fiber and organophilic montmorillonite clay. For this purpose, dry biocomposites filled with organophilic montmorillonite clay and curauá fiber (1, 3, and 5 wt.%) were prepared using a hand lay-up technique and subjected to moisture absorption and mechanical (flexural and impact tests) characterizations at different times. The experiments were carried out at water bath temperatures of 30 °C and 70 °C. The results have proven the strong influence of chemical composition and temperature on the moisture absorption behavior of biocomposites across time. For a higher percentage of reinforcement on the polymeric matrix, a higher moisture migration rate was verified, reaching a higher hygroscopic equilibrium condition at 16.9% for 5 wt.% of curauá fiber and 10.25% for 5 wt.% of montmorillonite clay particles. In contrast, the mechanical properties of all of the biocomposites were strongly reduced with an increasing moisture content, especially at higher fiber content and water bath temperature conditions. The innovative aspects of this research are related to the study of a new material and its transient mechanical behavior in dry and wet conditions.

1. Introduction

Traditionally, materials have been classified as metals, ceramics, polymers, and composites. For a better understanding, composites can be defined as materials made by a mixture of two or more different materials with a clearly defined interface between them, for example, polymer (the matrix) and fiber (the reinforcement).

It is now well known in the scientific community that the production of petroleum-based materials and fiberglass results in the release of a significant amount of greenhouse gases into the atmosphere [1,2,3,4,5]. Due to this, there is a need to seek alternatives to reduce or minimize this serious environmental and public health problem.

Innovative biopolymers and technological processes related to this type of material have been the subject of various studies in recent years. Known primarily for their environmental benefits, biopolymers (derived from renewable, biodegradable, or compostable sources) have been considered promising alternatives to petroleum-based polymers, since most of this environmentally friendly material can help reduce environmental pollution and greenhouse gas emissions [1,2,3].

Biopolymers are divided into three main categories based on their origin and biodegradable nature: biodegradable bio-based bioplastics (e.g., green polyethylene), bio-based non-biodegradable bioplastics, or bioplastics. Gurunathan et al. [4] point out that all-green biocomposites are materials produced by a combination of biofibers and resins from renewable agricultural and forestry raw materials. This may involve polymers manufactured from a process that reduces the environmental impact of both the process and the product (e.g., green polyethylene). These bioplastics are technically equivalent to their fossil counterparts. In addition, they can be easily recycled mechanically.

Nowadays, there is a bioplastic alternative for almost all conventional plastic materials and their corresponding applications. Bioplastics—bio-based plastics, biodegradable plastics, or both—have the same properties as conventional plastics and, in many cases, even offer additional advantages. These include reduced carbon capture or additional waste management options such as composting. In this sense, green polyethylene (green-PE or bio-PE), produced from sugarcane ethanol, was introduced to the polymer market. In addition to renewability, another advantage of this process is that sugarcane plants absorb CO₂ as they grow, helping to combat the greenhouse effect [2]. Current research indicates that the ratio of polyethylene (PE) production to CO₂ production is 2.1 tons of carbon (CO₂)/tons of polymer (PE). In relation to green-PE, this number has a negative value (−2.5 tons CO₂/tons polymer), indicating that it contributes significantly to reducing CO₂ emissions by carbon sequestration [5].

Today, several bio-based polymers are produced through the polymerization of monomers obtained from natural sources, with bio-PE being the main example. Bio-based polyethylene is an aliphatic thermoplastic synthesized from the polymerization of bioethanol. This input is obtained through the fermentation of sugars from the aforementioned raw materials, such as sugarcane which despite not being biodegradable, maintains the balance of carbon dioxide (CO₂) in nature, since the CO₂ captured from the atmosphere by biomass, when later released into the atmosphere through combustion, is captured again by sugarcane by the process of photosynthesis. The bioethanol is distilled and dehydrated to obtain ethylene, which is then polymerized into bio-PE. The polymer obtained by this process is equivalent to polyethylene of fossil origin, and even its variants (low and high density, linear, and branched) can be obtained. Consequently, bio-PE can be used for any of the many applications of conventional high-density polyethylene (HDPE).

The combination of inorganic and natural materials and biopolymers has been increasingly used to obtain biocomposites. Such an association is also reflected in changes in several of the thermo-physical parameters of the material, such as permeability and thermo-mechanical and water absorption properties. Therefore, in the last decade, the automotive, aerospace, construction, and packaging industries have shown great interest in the development of new composite materials reinforced with raw materials from natural sources, mainly vegetable fibers.

When studying biodegradable polymers such as aliphatic polyesters, Terzopoulou et al. [6] suggested that one of the main challenges faced in the manufacturing of fiber-reinforced polymeric composites is achieving satisfactory interfacial bonding that results in products with better mechanical properties. However, this is difficult due to the hydrophilicity of the fibers and the hydrophobicity of the polyesters used. The final properties of the composites depend mainly on the type of fiber used, its aspect ratio, the orientation and volume fraction of the fibers, and the adhesion strength at the fiber–matrix interface.

Annie Paul et al. [7] suggested that water absorption capacity is one of the important properties that are studied for the formation of polymeric composites with a predominantly hydrophobic matrix, using plant fibers as reinforcement. Hodzic and Shanks [8] studied the water absorption properties of polymeric composites reinforced with plant fibers and concluded that the water permeability factor is limited to the properties of the composites.

Currently, due to their importance, several studies have been devoted to the study of green composites in a dry state, with emphasis on their structure and mechanical properties [9,10,11,12,13,14,15].

On the other hand, recent studies on the problems caused by water absorption in polymeric composites reinforced by plant fibers have been reported in the literature [16,17,18,19,20,21]. According to the cited references [16,17,18,19,20,21], water absorbed by fiber-reinforced polymer composites causes swelling, plasticization, dissolution, leaching, and/or hydrolysis, which cause discoloration, embrittlement, lower resistance to heat and weathering, and lower mechanical properties.

Polymers are highly hydrophobic materials, but they can absorb water when immersed in aqueous media or exposed to humidity. The intensity of this phenomenon depends on several factors, such as polymer polarity, hydrogen bonding capacity, crystallinity of thermoplastic polymers, degree of crosslinking of thermosetting polymers, and manufacturing process. In any case, humidity strongly affects polymer composites, especially when reinforced with plant fibers, due to their highly hydrophilic nature. Moisture can be absorbed by polymer composites reinforced with plant fibers by hydrogen bonds between the polymer and the hydrophilic groups of the filler and through surface microcracks, which are responsible for the transport and deposition of water inside the material. This absorbed water can be found in a free and bound state.

Recently, some researchers have devoted their studies to emphasizing the effect of water absorption on the mechanical properties of polymeric composites reinforced by plant fibers [22,23,24,25,26].

Ladaci et al. [23] using the artificial neural network (ANN) and response surface methodology (RSM) methods examined the water absorption behavior of recycled high-density polyethylene. Biocomposites made from Washingtonia palm waste (PWW) fibers were treated with 3% sodium bicarbonate (NaHCO₃) for 24 h. Various HDPE materials reinforced with different concentrations of PWW fibers (5–30 wt.%) until saturation were developed to investigate the kinetics and diffusion of water absorption in biocomposites. The artificial neural network and response surface methodology methods were applied to model the collection behavior measured in the experiments and improve the setting period and PWW fiber content in the water absorption of RHDPE/PWW biocomposites. Regarding accuracy and reliability, the ANN model outperformed the RSM model, making it suitable for various industrial uses.

Ahmad et al. [24] examined the tensile and flexural mechanical behavior of hybrid biocomposites based on glass/bamboo fibers, nanoclay particles, and epoxy under water immersion. The water-immersed samples showed lower tensile and flexural strengths than the dry ones. Furthermore, all the composites and the epoxy matrix showed a 6–11% increase in tensile and flexural strength, including nanoclay, presenting percentage reductions from 8.6 to 14.8% to 7.4 to 12% in tensile strength.

A study developed by Scida et al. [25] reported the effect of water absorption and hygrothermal aging on the mechanical properties and damage behavior of quasi-unidirectional flax fiber reinforced epoxy (FFRE) composites. The evolution of water absorption for the FFRE composite appears to be Fickian, and the kinetic parameters are influenced by temperature variation. Young’s modulus and tensile strength are clearly affected by hygrothermal aging, with a significant reduction in Young’s modulus, while tensile strength decreases much less for water-saturated composites. The decrease in both properties can be explained by a reorientation of the flax microfibrils and the plasticizing effect of water in the matrix, both stimulated by moisture absorption. Acoustic emission analysis combined with scanning electron microscopy allowed for the effects of hygrothermal aging on the degradation process of the flax fiber composite to be investigated.

Zhu et al. [26] developed a continuous carbon fiber/epoxy composite (CFRE) produced by the pultrusion method and investigated the effects of a hydrothermal environment on the mechanical properties (flexural strength and hardness) and electrical response behavior of the polymer composite. The water absorption rate rapidly increased to 1.02% within 3 days before reaching a relatively stable state. The results indicated that the flexural strength decreased rapidly within 3 days of hydrothermal treatment, followed by a stable trend. Fracture surface analysis indicated that the interfacial properties of carbon fibers in the epoxy matrix decreased after hydrothermal treatment, and more carbon fibers could be extracted from the CFRE in the hygroscopic state. After hydrothermal treatment, the microhardness of the composites was reduced by 25%.

In addition to existing research, this work aims to study water absorption and its effects on composites manufactured by high-density green polyethylene (BPEAD), obtained from ethanol (produced from sugarcane) and reinforced by particles of organophilic montmorillonite clay (OMMT) and by curauá fibers (FC) in different compositions and at different temperatures. The idea is to evaluate the kinetics of water sorption under different experimental conditions and the mechanical behavior of the samples (impact and bending) under saturation conditions after having been subjected to the water absorption process.

2. Materials and Methods

2.1. Materials

In this research, the following materials were used in the experiments: (a) matrix: high-density green polyethylene, trade name SGE7252 (B-HDPE), I’m Green ^® obtained from sugarcane ethanol provided by Braskem company (Maceió, Brazil); (b) reinforcement: natural fibers from Curauá (FC) plant found in the northern region of Brazil, and clay particles (trade mark Dellite 72T, Laviosa Chimica Mineraria S.p.A, Livorno, Italy) derived from a natural montmorillonite. Details about these materials can be found easily in the recently published literature [14,15].

2.2. Reinforcement Preparation

Curauá fibers were received in the form of dried long fibers, without any previous treatment. In sequence, the fibers were combed to remove husks and surface dirt and cut to 2.5 mm in length. After this procedure, an immersion treatment based on a 5% NaOH solution was applied to the fibers for t = 2 h and T = 50 °C. Then, the fibers were washed with distilled water until the remaining washing solution reached pH 7.0 and dried in an oven at T = 60 °C for 12 h.

Natural montmorillonite clay particles were purified and organo-modified by means of an ion exchange reaction in aqueous medium, with quaternary ammonium salt (di-hydrogenated with dimethylammonium).

2.3. Samples Preparation

The biocomposites were obtained mixing green polyethylene (B-HDPE) with treated curauá fibers (FCs) and organophilic montmorillonite clay (OMMT). Further, 10% in weight of polyethylene grafted with maleic anhydride (PE-g-MA) was added to all the studied biocomposites (reinforced by fiber and also reinforced by montmorillonite clay). All the biocomposites were prepared by the melt-state method and processed in a modular, co-rotational extruder, Leistritz LSM 30.34, screw diameter 30 mm and L/D = 29. The following process conditions were used: (a) barrel temperature varying from 160 to 180 °C, (b) screw speed of 150 rpm, and (c) feed rate of 2 kg/h. Following this, the B-HDPE and the biocomposites were molded by injection, in an ARBURG Injector—Goldem equipment, at the following conditions: Tmold = 25 °C, Tzones = 180 to 200 °C, and mold cooling time 5 s. In Figure 1, all of the biocomposites evaluated in this research are illustrated.

2.4. Mechanical Characterization After Water Sorption

The specimens, after the molding process, were immersed in a water bath at temperatures of 30 and 70 °C, and, at certain moments, ranging from 1 to 48 h of saturation, the samples were removed from the water bath and dried quickly with paper towels. Immediately after the manual drying process, the samples were submitted to mechanical impact and bending tests.

The IZOD impact strength test was performed according to the ASTM D 256 standard [27] using CEAST RESIL 5.5 equipment (Charlotte, NC, USA), with 2.75 J impactor. The 2.5 mm notches were produced in a CEAST NOTSCHVIS slot machine. Averages of 5 specimens were used for the impact tests.

For the flexural strength tests, injected specimens measuring 12.9 mm and 3.1 mm wide with a 39.99 mm cross-sectional area were used. The flexural tests were performed on a SHIMADZU machine, model AG-X (Kyoto, Japan), with a 10 kN load cell. For the bending mechanical test, the machine speed was 2.0 mm/min according to ASTM D790 [28]. For each characterization, 5 specimens per sample were used.

The water sorption assays were performed only with the biocomposites that presented better mechanical properties, as reported by Barbalho et al. [14] and Barbalho et al. [15].

2.5. Water Sorption Tests

All specimens were prepared in accordance with ASTM D638 [29]. The geometry of the composites was 30 mm wide, 19 mm long, and 3.1 mm thick with a cross-sectional area of 39.99 mm² (Figure 2).

The water sorption study of the biocomposites was carried out according to ASTM D570 [30]. After the samples were cut with a laser beam to the desired dimensions, the faces of the samples were coated with the liquid biopolymer and left to rest until the curing process was complete. This procedure was necessary to avoid the direct contact of water with the short fibers that were exposed after the cutting process and, consequently, the capillary sorption of water. They were then weighed and placed in an oven at 105 °C for 24 h. Immediately after drying, the samples were weighed again and then immersed in distilled water at room temperature. At predetermined times, until saturation, the samples were taken from the water bath, dried quickly with paper towels, and weighed on a scale, with an accuracy of ± 0.1 mg. At the end of the test, the water absorption curve versus immersion time was obtained, according to ASTM D570 [30]. The absorbed water content (on a dry basis) was calculated by comparing the initial weight and after exposure to the water bath, according to Equation (1). In this equation, m_w is the mass of the wet material and m_s is the mass of the dry material. For each bath, five (5) samples were made available with the objective of measuring the weight gain due to moisture as a function of time and averaging the weight of the five samples.

$$\mathrm{M}\left(\%\right)=\left({\displaystyle \frac{{\mathrm{m}}_{\mathrm{w}}-{\mathrm{m}}_{\mathrm{s}}}{{\mathrm{m}}_{\mathrm{s}}}}\right)\times 100$$

(1)

2.6. The Studied Cases

Table 1. Sample composition and nomination.

Designation	Curauá Fiber (%)	Montmorillonite Clay (%)	PE-g-MA (%)
B-HDPE	0	0	0
B-HDPE/PE-g-MA/FC1	1	0	10
B-HDPE/PE-g-MA/FC3	3	0	10
B-HDPE/PE-g-MA/FC5	5	0	10
B-HDPE/PE-g-MA/OMMT1	0	1	10
B-HDPE/PE-g-MA/OMMT3	0	3	10
B-HDPE/PE-g-MA/OMMT5	0	5	10

summarizes all the conditions studied in this research.

Biocomposites with small fractions of natural fibers and montmorillonite particles as reinforcements were chosen for study for the following reasons: (a) to understand the mechanical behavior of the biopolymer and its interactions with the reinforcement, and (b) to evaluate the effect of moisture and reinforcement content on the mechanical properties of the biocomposite. Future research will be carried out with higher reinforcement contents.

3. Results and Discussion

3.1. Mechanical Properties at the Wet Condition

The results presented here aim to investigate the effect of water absorption, absorption time, and water bath temperature on the mechanical properties of biocomposites. In the polymer matrix, water absorption is usually controlled by diffusion and associated with the movement of water molecules between the microvoids of the polymer chains [31]. In plant fibers, absorption occurs mainly by capillarity due to voids on their surfaces and by the lumen.

Barbalho et al. [14] and Barbalho et al. [15] carried out mechanical experiments using biocomposites reinforced with curauá fiber and montmorillonite clay in the dry state. In view of the better results obtained for the IZOD mechanical impact and flexural properties obtained in these previous studies, and to complement this research, the following cases were selected for the study of water sorption and determination of the mechanical properties of wet biocomposites: (a) biocomposite with 1% mass content of OMMT1 (BPEAD/PE-g-AM/OMMT1) and (b) biocomposite with 3% mass of FC (BPEAD/PE-g-MA/FC3). For this, the biocomposites were saturated by immersion in distilled water at different temperatures (30 and 70 °C) and immersion times (ranging from 1 to 48 h). The results are presented in

Table 2. Impact resistance for biocomposites after different saturation times and temperatures of 30 and 70 °C.

Impact Resistance (MPa)
Dry		Wet
		Saturation Time (h)	Water at 30 °C		Water at 70 °C
OMMT1	FC3		OMMT1	FC3	OMMT1	FC3
293.24 ± 16.86	186.43 ± 9.03	1	291.29 ± 6.67	133.79 ± 4.61	280.45 ± 0.34	137.79 ± 0.35
		3	287.06 ± 5.15	133.16 ± 4.90	286.62 ± 1.36	132.95 ± 0.37
		6	283.50 ± 2.44	133.01 ± 8.64	283.58 ± 0.37	131.99 ± 0.38
		12	282.50 ± 2.38	131.19 ± 4.86	282.63 ± 0.38	131.91 ± 2.90
		24	281.43 ± 8.39	132.70 ± 2.34	281.21 ± 10.39	130.71 ± 2.40
		48	280.67 ± 7.35	129.87 ± 0.34	279.67 ± 1.34	128.45 ± 1.35

and in Figure 3 and Figure 4.

Evaluating the results contained in Table 2 and Figure 3 and Figure 4, it can be observed, in general, that water sorption causes a marked decrease in the impact strength of biocomposites, being more accentuated for biocomposites with a 3% mass of curauá fiber (BPEAD/PE-g-AM/FC3) compared to biocomposites with a 1% mass of montmorillonite clay (BPEAD/PE-g-AM/OMMT1). Further, it was also observed that the water absorption in the montmorillonite clay-reinforced composite was lower compared to the curauá fiber-reinforced biocomposite.

The impact test results after water absorption at 30 °C showed that there was a decrease in impact resistance at the beginning of saturation of 0.66% (t = 1 h), increasing this reduction with immersion time, reaching in 48 h of the process a value of 4.3% for the biocomposite BPEAD/PE-G-AM/OMMT1 in relation to the dry biocomposite. For the temperature of 70 °C, the results showed that there was a higher decrease in the value of the impact resistance, in relation to the temperature of 30 °C, changing from 4.3% to 4.5% in reduction at the beginning of the saturation time (t = 1 h) and becoming higher (4.63%) at the end of the saturation (t = 48 h) for BPEAD/PE-g-AM/OMMT1 in relation to the dry biocomposite. This phenomenon is due to the excellent barrier properties of clay [32]. The presence of organophilic clay affects the porosity, tortuosity, and permeability of the biocomposite, which restricts the flow of water molecules in the polymeric matrix (BPEAD). That is, the impermeable clay platelets generate a more tortuous path for the diffusion of water in the biocomposites. This fluid flow within the biocomposite can be further decreased as tortuosity is optimized by improving clay dispersion in the biocomposites [33]. Similar results were reported by Ramesh et al. [34] when they verified that the resistance to water flow increased to the maximum with the continuous addition of montmorillonite clay to PLA/TKF/MMT biocomposites (PLA-Polylactic Acid; TKF–Treated Kenaf Fiber; MMT–Montmorillonite Clay) when compared to PLA/TKF biocomposite without the addition of MMT. Another determining factor in the evaluation of water sorption in biocomposites is the temperature of the water absorbed by the material. According to Alamri and Low [35], composites subjected to water sorption at higher temperatures have higher diffusion coefficients than those at lower temperatures.

According to Chen et al. [36], when a natural fiber/polymer composite is exposed to moisture, water penetrates and binds to the hydrophilic groups of the fiber, establishing intermolecular hydrogen bonding with fibers and reducing the interfacial adhesion of the matrix with the fibers. The process of the swelling of cellulose fibers causes the development of stresses in the interface region, leading to the appearance of microcracks in the matrix around the fibers. This promotes capillarity and moisture transport via microcracks, causing the deterioration of the fibers and brittleness in the composites.

A similar behavior that verified montmorillonite clay-reinforced biocomposites was also observed for curauá fiber-reinforced biocomposites with 3% by weight of fiber (BPEAD/PE-g-AM/FC3). For this biocomposite, the decrease in impact strength was more pronounced in composites subject to higher temperatures and immersion times. For the immersion water temperature of 30 °C, the results point to a drop of 28.0% at the beginning of the immersion time (t = 1 h), which becomes greater at the end of the immersion time (t = 48 h) with a drop of 32.0% in relation to the dry biocomposite.

For the temperature of 70 °C, the results showed that there was an increase in the reduction percentage of the mechanical property in relation to the temperature of 30 °C, changing from 28.0% at the beginning of the saturation time (t = 1 h) to 31.1% at the end of the saturation (t = 48 h) for BPEAD/PE-g-AM/FC3 in relation to the dry biocomposite. Studies developed by Alsina et al. [37] clearly show that the impact strength of curauá fiber biocomposites in wet conditions is lower than the impact strength of FC composites in dry conditions due to the appearance of microcracks caused by fiber swelling and sorption water temperature variation. Therefore, by fixing the immersion time, biocomposites that are immersed in water at higher temperatures are closer to reaching hygroscopic saturation than those at lower bath temperatures.

Table 3. Flexural strength for biocomposites after different saturation times and temperatures of 30 and 70 °C.

Flexural Strength (MPa)
Dry Biocomposite		Wet Biocomposite
		Saturation Time (h)	Water at 30 °C		Water at 70 °C
OMMT5	FC5		OMMT5	FC5	OMMT5	FC5
14.51 ± 0.80	15.32 ± 0.41	1	13.51 ± 0.30	12.83 ± 0.20	12.38 ± 0.10	12.03 ± 0.40
		3	13.21 ± 0.99	12.42 ± 0.19	12.45 ± 0.71	11.98 ± 0.79
		6	12.28 ± 0.12	12.28 ± 0.22	12.14 ± 0.52	11.75 ± 0.42
		12	12.89 ± 0.55	11.60 ± 0.67	11.98 ± 0.70	10.98 ± 0.67
		24	12.10 ± 0.78	10.64 ± 0.88	11.75 ± 0.65	10.56 ± 0.78
		48	11.05 ± 0.74	10.16 ± 0.79	10.21 ± 0.43	10.05 ± 0.72

and Figure 5 and Figure 6 summarize the flexural strength of the biocomposites with 5% montmorillonite clay (BPEAD/PE-g-AM/OMMT5) and 5% curauá fiber (BPEAD/PE-g-AM/FC5) before and after saturation at different times for temperatures of 30 and 70 °C, respectively. When analyzing these results, it was verified that, in general, water absorption has a negative influence on flexural strength when compared to dry biocomposites.

At 30 °C, the reduction in the bending resistance reached a value of 6.90% at the beginning of the saturation (t = 1 h) and 23.84% at the end of the saturation (t = 48 h) in relation to the dry biocomposite. For sorption at 70 °C, the reduction reached a value of 17.00% at the beginning of the saturation time and 29.63% at the end of saturation both in relation to the dry biocomposite with 5% mass of montmorillonite clay (BPEAD/PE-g-AM/OMMT5). This physical–mechanical behavior is in accordance with the work of Abacha et al. [38], who reported a decrease in flexural strength and flexural modulus of clay/epoxy composites due to water absorption. The decrease in flexural strength can be attributed to the plasticizing effect of water absorption in the polymer matrix, which can lead to decreased interfacial resistance between the matrix and the reinforcement particles. By comparing the BPEAD/PE-g-AM/OMMT5 and BPEAD/PE-g-AM/FC5 biocomposites, we can see a reduction in the flexural strength by nearly 8% after 48h of water absorption at T = 30 °C and 1.6% in the same immersion time and T = 70 °C according to the literature [26].

3.2. Water Sorption Kinetics

Figure 7 and Figure 8 show the water absorption curves (moisture content versus immersion time) according to ASTM D570 [30] for the biocomposites of BPEAD/PE-g-AM/OMMT and BPEAD/PE-g-AM/FC according to Table 1. The general shape of the water absorption curves is similar to that of other fiber-reinforced polymer composites. For all of the studied cases, the average moisture content of the samples increases monotonically along the immersion time in water until reaching a specific limit of moisture.

By analyzing the results, it is verified that after the samples are immersed in water, moisture absorption occurs very quickly in the first 50 h and in a very similar way for each reinforcement content. However, the moisture absorption rate was faster for the biocomposites reinforced by the curauá plant fiber as compared to the montmorillonite clay-reinforced biocomposites. This phenomenon can be explained by the ease with which water molecules move between the microvoids of the polymeric chains and defects in the biocomposites due to adhesion failures between the matrix and the reinforcements. For longer times, the curves change their inclination until they reach a linear plateau that represents the level of moisture saturation (hygroscopic equilibrium condition). This behavior strongly indicates that the reinforcement in both physical situations (fiber and particles) is uniformly well distributed into the polymer matrix. Similar behavior has been reported by other researchers [37,39,40].

Biocomposites with higher concentrations of montmorillonite clay and curauá fiber tend to absorb more moisture, which contributes to the reduction in the mechanical properties of the material. Comparing the values of the moisture content under the hygroscopic saturation condition, it was verified that the biocomposites reinforced by vegetable fibers absorbed more moisture than those reinforced by montmorillonite particles, which can be attributed to the higher hydrophilicity, the sample surface area exposed to water, and higher permeability. On the other hand, even with a very low percentage of reinforcement (1%), the moisture content of the biocomposite in the saturated condition was significantly higher than that presented by the pure matrix. It is believed that higher water bath temperatures lead to thermal expansion of the biocomposites and increased composite porosity, which in turn would increase the moisture migration rate. This phenomenon is well known as thermoactivation. It is important to notice that the degradation of the studied biocomposites was almost insignificant.

4. Conclusions

Currently, due to the increase in environmental pollution, studies focused on new eco-friendly materials, such as renewable, recyclable, and sustainable materials, have made great contributions worldwide. Non-renewable materials are more difficult to degrade and are more aggressive to the environment. Therefore, the development of biodegradable materials is not only a great motivating factor for the development of research in this area but is mainly to strengthen sustainable development worldwide. The present research emphasizes the study of water sorption in biocomposites reinforced by plant fiber and montmorillonite clay particles. Herein, small amounts of the reinforcement together with a compatibilizing agent, PE-g-MA, were used to modify the properties of pure B-HDPE. From the analyses of the results, the following can be concluded:

(a) Water sorption causes a marked decrease in impact and flexural strength values for biocomposites.

(b) The presence of montmorillonite clay in the biopolymeric matrix decreases water absorption compared to curauá fiber-reinforced biocomposites.

(c) The higher the reinforcement contents in the biocomposites, the higher the moisture content at the saturation condition.

(d) Both the economic use of organophilic clay and curauá fiber allowed for biocomposite processing parameters equivalent to those used for pure polymers to be obtained without altering the processability of BPEAD.

Finally, the innovation of the present work lies in demonstrating that small amounts of chemically modified clay and curauá fiber inside the polymer matrix in the presence of water contribute to drastically reducing the mechanical properties of the biocomposites. All these characteristics are of interest for many industries, particularly for the automotive and aeronautical industries. Despite the good characteristics of eco-friendly biocomposites to reduce environmental impacts, it is necessary to devote new related investigations to improve the performance of green polymers and to obtain inexpensive material.