Assessment of Long-Term Water Absorption on Thermal, Morphological, and Mechanical Properties of Polypropylene-Based Composites with Agro-Waste Fillers

Abstract

Agro-waste fibres for polymer composite reinforcement have gained increased interest in industry and academia as a more sustainable alternative to synthetic fibres. However, natural fibre composite (NFC) hygroscopicity is still an issue that needs to be solved. This work investigates how prolonged exposure to water affects the properties of the polypropylene (PP)-based injection-moulded composites reinforced with different contents of rice husk (rh) and olive pit (op) fibres. Both rh and op composites became more hydrophilic with increased fibre charge due to the affinity of cellulose and hemicellulose OH groups. Meanwhile, lignin contributes to the protection of the composites from thermo-oxidative degradation caused by water immersion. The PPrh composites had a higher saturation water content of 1.47% (20 wt.% rh) and 2.38% (30 wt.% rh) in comparison to PPop composites with an absorption of 1.13% (20 wt.% op) and 1.59% (30 wt.% op). The tensile elastic modulus has slightly increased, at the cost of the increased saturated composites’ rigidity, in composites with 30% rh and op fibre content (up to 13%) while marginally decreasing (down to 8%) in PP30%op compared to unsaturated counterparts. A similar trend was observed for the flexural modulus, enhanced up to 18%. However, rh and op composites with 30% fibre content ruptured in bending, highlighting their fragility after hydrolytic ageing.

1. Introduction

A need to procure more eco-friendly solutions to guarantee the growing consumption patterns of modern society, striving at the same time to achieve carbon neutrality by 2050, requires reconsidering one’s reliance on petrochemical feedstocks as a source of raw materials [1]. Simultaneously, modern society generates a huge amount of waste related to food production, which, if not properly disposed of, may aggravate the pollution problem more. Reutilization of food industry waste by incorporating it into long-/medium-life consumer goods serves as temporary storage of carbon sequestered from the atmosphere during the growth phase [2]. Annually, the Portuguese agro-industry is required to dispose of more than 30,000 tons of rice husk, a subproduct of paddy rice [3], and an even larger quantity of olive pits, residue from olive oil extraction [4]. These agro-waste hold a huge potential for storing carbon if incorporated into long-life products.

The replacement of synthetic polymers with their bio-based counterparts is gaining increased attention from academia and industry. However, the wholesale solution for the complete replacement of petrochemical polymer by 100% biopolymer is unlikely at present because of associated costs and market availability. For the intermediate solution in the form of natural fibre composites, a considerable body of research exists [5,6,7,8,9,10,11,12,13]. However, natural fillers, despite possessing some useful traits such as lower density and less abrasiveness comparable to synthetic fibres’ specific strength and stiffness properties, are prone to thermal and mechanical degradation during processing. Another disadvantage of incorporating natural filler into the polymer matrix is their difference in polarity, as is the case for apolar polyolefins. It is well known that hydrogen bonding sites in the natural fibres are polar, which is the main cause of their hygroscopicity and the fibres’ agglomeration, which may be a limiting factor for some applications [14,15]. Despite the latter limitations, the choice of polyolefins as a matrix for natural fibre composites may be attributed to their low cost, good mechanical properties, and the possibility of being processed below the degradation temperature of natural fibres by industrial processing techniques such as compression and injection moulding [16]. Moisture absorption mechanisms in natural fibre composites (NFCs) are well documented in the literature. For polyolefins, in most cases, it follows the Fickian diffusion model, in which the kinetics indicate a linear correlation between a fibre charge and the quantity of absorbed moisture [14,16,17,18,19,20,21,22]. Meanwhile, the moisture absorption’s influence on the NFC’s mechanical properties may vary depending on the liquid type, temperature, test duration, and specimen preconditioning after saturation. Most of the studies claim deterioration of the mechanical properties, especially aggravated after immersion at high temperatures [16,18,20,22].

Generally, the effect of moisture absorption is assessed in terms of mechanical performance. However, to the authors’ knowledge, only one study [23] exists regarding the impact of moisture absorption on thermal stability, and no research has been found concerning the thermo-oxidative stability of hydrolytically aged NFCs. Information about the NFCs’ thermal and thermo-oxidative stability is vital for understanding how service conditions may affect their service life expectancy.

It is worth noting that this study was carried out in the framework of the project “OLIpush—Redesign for greater circularity and a smaller environmental foot-print” whose main objective is to identify an eco-friendly biopolymer capable of satisfying the requirements of functional products such as sanitary components. Hence, the present research aimed to evaluate the effect of moisture on the mechanical, thermal, and morphological properties of the rice husk and olive pit polypropylene composites to verify their functional requirements for the production of sanitary components.

2. Materials and Methods

2.1. Materials

Polypropylene-based natural fibre composites with 20% and 30% by weight of rice husk and olive pit fibres were compounded by extrusion. The composites’ preparation method is not reported there, given that it was described in detail in the prior research [24]. The composites’ designations and compositions used along the text are listed in

Table 1. Materials’ compositions [24].

Designation	Composition (%)
	PP 1	Rice Husk	Olive Pits	PPMA 2
PPv	100	-	-	-
PP20%rh	79	20	-	1
PP30%rh	69	30	-	1
PP20%op	79	-	20	1
PP30%op	69	-	30	1

¹ 205CA-40—Polypropylene Random Copolymer by INEOS Olefins & Polymers Europe. ² EXXELOR™ PO 1020 Maleic anhydride functionalized polypropylene by ExxonMobil (Houston, TX, USA).

.

2.2. Methods

2.2.1. Specimens’ Manufacture

The dog-bone (Type I) specimens for tensile [25] and beam-type specimens (127 mm × 12.7 mm× 6.35 mm) for flexural testing [26] were fabricated by injection moulding (injection moulding machine Euroinj D—065, Lien Yu Machinery Co., Ltd. (Tainan City, Taiwan, China). A more detailed description of the specimens’ preparation may be consulted elsewhere [24].

2.2.2. Water Absorption Tests

Before immersion in distilled water at room temperature (23 °C ± 1), the tensile and flexural specimens of the composites and virgin PP were kept in an oven with ventilation for 24 h at 50 °C and afterwards cooled down to 23 °C in a desiccator (HC 200 Humidity Control Cabinet by Guangdong SIRUI Optical Co., Ltd., Guangdong, China) to remove the moisture that might be absorbed from the environment. After conditioning, the initial (dry) weight was measured with a high-precision scale with a resolution of 0.01 mg (an A&D GH-252 scale, A&D Company Limited, Tokyo, Japan).

During immersion, the specimens were removed and weighed at specific intervals, 24 h, 1 week, and afterwards, every 2 weeks until reaching a state of effective moisture equilibrium, which is attained, according to ASTM D5229/D5229M [27], when the average moisture content of the material does not vary more than 0.02% between two consecutive reference time spans. The stainless-steel mesh barriers, shown in Figure 1, were incorporated into the beakers to prevent the specimens from floating.

The weighing was carried out within 2 min to avoid errors due to desorption. Eight tensile and flexural specimens of each material lot were tested to ensure statistical significance. After reaching the moisture equilibrium condition, the tensile and flexural specimens were removed from the immersion beakers, immediately dried, and stored in a vacuum-sealed plastic bag to avoid desorption. The mass of moisture (M) was evaluated by calculating the change in the sample mass to its original mass according to the following Equation (1) [28].

$$M={\displaystyle \frac{\left(M_t-M_0\right)}{M_0}}100\%$$

(1)

Here, M_t is the specimen mass at time t and M₀ is the initial oven-dried mass of the specimen before immersion in water.

The thickness of the tensile specimens was measured using the high-precision digital callipers from Mitutoyo (precision ± 0.01 mm), ensuring accurate results, to verify the swelling (S %) in the thickness direction after reaching the saturation limit (Equation (2)). An average of eight measurements were taken before the moisture absorption test began and after the moisture saturation content was carried out. The thickness swelling was calculated according to Equation (2).

$$S=\left({\displaystyle \frac{h_s-h_i}{h_i}}\right)100$$

(2)

Here, h_i and h_s are the initial and saturated thicknesses.

The diffusion coefficient D (Equation (3)), which represents the water’s ability to penetrate the polymer matrix, was analysed by postulating a Fickian diffusion mechanism after moisture equilibrium was attained [27,28].

$$D=\pi {\left({\displaystyle \frac{h}{4M_m}}\right)}^2{\left({\displaystyle \frac{M_2-M_1}{\sqrt{t_2}-\sqrt{t_1}}}\right)}^2$$

(3)

Here, h is the specimen thickness (mm); M_m is the moisture saturation content (%); and M₂ − M₁ is the linear slope on the moisture absorption curve at a time span, $\sqrt{t_2}-\sqrt{t_1}$ .

In Equation (3), water absorption is considered through the thickness (1D). However, in practice, moisture diffusion occurs from all six surfaces of the specimens under study, requiring a correction factor (Equation (4)) introduced by Shen and Springer [28,29] for moisture ingression through the edges. This enables the determination of the true one-dimensional diffusion coefficient D_x.

$$D_x=D {\left(1+{\displaystyle \frac{h}{L}}+{\displaystyle \frac{h}{W}}\right)}^{-2}$$

(4)

Here, W and L are the length and width of the specimens (mm).

2.2.3. Thermal Analysis

The oxidative resistance of virgin PP and PP composites was determined according to ASTM D3895 [30]. Samples were films, obtained from the specimens and after the long-term water immersion The samples, with a mass of about 8 ± 1.5 mg, uniformly covered the bottom of the aluminium crucible without a lid. It was heated under a nitrogen atmosphere from ambient temperature up to 190 °C at a heating rate of 20 °C/min and maintained isothermally for 5 min. Next, the nitrogen was replaced by oxygen and maintained until reaching the exothermic peak.

The thermal stability of the composites was assessed by thermo-gravimetric analysis (TGA), performed in a Netzsch—Jupiter STA 449 F3 apparatus (NEDGEX GmbH, Selb, Germany) according to E 2550-11 [31]. The samples of 10 ± 1 mg were collected from the tensile tests’ coupons before water immersion. They were heated in an alumina (Al₂O₃) crucible between 30 °C and 600 °C at a constant heating rate of 10°K/min under a nitrogen atmosphere (50 mL min⁻¹) to measure weight change as a function of temperature. In addition, the DTG curve (rate of change in weight) was used to assist in the interpretation of the thermal degradation events.

2.2.4. Morphology Assessment

Morphological analysis of the fractured surface of the tensile specimens was performed using a tabletop scanning electron microscope (SEM) HITACHI TM4000Plus (Hitachi High-Tech Corporation, Tokyo, Japan). The SEM images were acquired at an accelerating voltage of 15 kV.

Assessment of the composites’ surface morphology was carried out under a magnification of 20 by Inspectis HD-009 DIM-U Digital Inverted Microscope (DIM) 8.3 MP Ultra HD (Inspectis AB, Kista, Sweden).

2.2.5. Tensile and Flexural Tests

Before mechanical testing, the water-saturated tensile and flexural specimens were kept in vacuum-sealed plastic bags to avoid moisture desorption. The Young’s modulus and ultimate tensile strength of the samples were assessed using an X-10 kN machine (Shimadzu Scientific Instruments, Columbia, MD, USA) following, respectively, the ISO 527-1 [30] and ISO 178 [31] standards. Eight Type I specimens were subjected to the tensile tests. The experiments were carried out at an ambient temperature (23 ± 1 °C) in two steps. Initially, the specimens were pulled at a rate of 1 mm/min to calculate Young’s modulus. In the second stage, a 50 mm/min tensile rate was applied and maintained until the specimens ruptured. The data from this second test were used to determine the ultimate tensile strength (σ_u) and tensile strain at break (ε_b). Flexural testing (3-point bending) was performed at a 5 mm/min speed. The beam-type test specimen with dimensions of 127 mm × 12.7 mm× 6.35 mm follows the ISO 178 [31] requirement of 20 ± 1 of the length to the thickness ratio.

3. Results and Discussion

3.1. Long-Term Water Absorption Tests

Polymer composites reinforced with natural fibres have poor resistance to moisture due to surface-abundant hydroxyl groups, present in plant fibres, possessing high polarity as well and allowing for interactions between free hydroxyl groups on the fibre’s surface and water molecules via hydrogen bonding [32]. The difference in polarity between polymer matrix and natural fibres causes poor polymer wetting of the latter and hence weak interface adhesion [33]. The water absorption was plotted versus the square root of time for the tensile (Figure 2) and the flexural specimens (Figure 3). At first, it increases linearly and then levels off for a long time until reaching the equilibrium following a Fickian diffusion curve. This moisture absorption behaviour of natural fibre composites has been referred to in many studies [17,18,19]. The moisture saturation equilibrium was reached in 232 days for all rh and op composites independently of the fibre content. As expected, the hydrophobicity of PP results in negligible water absorption (Figure 2) of 0.02% [34].

It is well known that hydrogen bonding sites in the natural fibres, mostly cellulose and hemicelluloses, are the main cause of their hygroscopicity and hence an increase in fibre percentage will promote water absorption [19,20]. Hence, the highest moisture saturation content (M_m) of 2.38% (

Table 2. Water absorption properties.

Property	Specimen Type	PP20%rh	PP30%rh	PP20%op	PP30%op
Moisture saturation content − Mm (%)	TS 1	1.47	2.38	1.13	1.59
	FS 1	1.01	1.63	0.80	1.11
Diffusion coefficient − Dc (× 10−7) (mm2/s)	TS	1.153	1.147	1.216	1.166
	FS	2.764	3.134	2.798	2.572
Time to saturation (days)	TS	232	232	232	232
	FS	232	232	232	232

¹ TS and FS stand for, respectively, tensile and flexural specimens.

), recorded for PP30%rh is to be expected. In general, rice husk fibres contain more hemicellulose and have a more porous structure than olive pit fibres which makes them more hydrophilic [35]. In addition, weak interlocking between the matrix and rice husk fibre’s elongated planar shape [24] allows for a large contact area with moisture when it permeates microcracks in the polymer matrix and reaches the rh fibre. In turn, olive pit fibres are particle-shaped which reduces their exposure area to moisture leading to higher resistance to water uptake. The tensile specimens PP30%op were the worst-case scenario with 1.59% water absorption. An increase in thickness, which is the case for flexural specimens, decreased the water absorption in all the composites tested. The diffusion coefficient represents the ability of water molecules to move between polymer chains. The smallest diffusion coefficient was correlated with higher equilibrium saturation of the PP30%rh tensile specimen. It can be attributed to the continuous filling of more hygroscopic sites of the rice husk fibres, while diffusion speed through the matrix was constrained [36].

The thickness swelling of the composites was assessed only for the tensile specimens. It should be noted that the op composites’ tensile specimens are thicker (Figure 4) than the rh specimens due to the higher density of the former, reported in the previous research [24]. The thickness swelling after water saturation is proportional to the equilibrium water absorption (Table 2), being highest (1.9%) for PP30%rh and lowest (0.8%) for PP20%op after immersion in distilled water for 232 days at room temperature. Higher fibre content was also positively correlated with swelling thickness for all the composites. This is an expected outcome, reflecting how water molecules interact with natural fibre composites. Water absorption starts with the water infiltration and diffusion in the micro gaps between the polymer chains, reaching afterwards to fibres through the flows at its interface with the polymer matrix. So, the major moisture absorption takes place in the fibre via hydrogen bonding and in the cracks at the polymer fibre interface due to the poor wettability of the fibres by the polymer matrix [37]. Therefore, this absorption mechanism implies that increased fibre content will result in more hydrogen bonding sites and microcracks due to imperfect matrix fibre interactions, leading to higher moisture absorption and swelling. A similar trend for an increase in the natural fibre composites’ swelling in thickness direction with an increase in fibre content was reported by other researchers [38,39,40].

3.2. Thermal Properties

3.2.1. Oxidation Induction Time

Oxidation induction time (OIT) measurements were carried out to compare the oxidative resistance at 190 °C of the composites and virgin PP before and after water immersion. The results, listed in

Table 3. Oxidation induction time before and after water immersion.

Material	OIT (s)		OIT↓ (%) 1
	Before Immersion	After Immersion
PPv	17.8 ± 2.5	4.5 ± 0.2	75
PP20%rh	10.2 ± 0.5	5.9 ± 0.6	42
PP30%rh	10.4 ± 0.8	8.0 ± 0.8	23
PP20%op	14.3 ± 0.1	8.6 ± 0.6	40
PP30%op	19.6 ± 1.8	12.2 ± 1.1	38

¹ OIT↓ (%) stands for oxidative induction time reduction before and after water immersion.

, show that before water immersion, PPv and PP30%op were the most resistant to thermo-oxidative degradation, occurring at about 18 s and 20 s, respectively. The latter indicates that at a higher olive pit fibre content (PP30%op), the composite is more resistant to oxidative degradation than PPv. Meanwhile, the rh composites have a thermo-oxidative resistance of 10 s, invariant from the filler amount. These filler-type-related differences can be attributed to higher lignin content in olive pit fibres, which acts as a natural antioxidant [32]. Lignin is also present in rice husk fibres, but in lesser amounts [41]. The high thermal stability of lignin due to its chemical structure was reported to improve the oxidative stability of recycled PP [42] and flax fibre HDPE composites [43].

After water immersion, the oxidative resistance of PPv drops almost four-fold. This drastic reduction may be due to the partial consumption of antioxidants during the processing of specimens and hydrolytic ageing. The latter hypothesis is in line with the findings of Massey et al. [44] They have concluded that hydrolytic ageing was caused by oxidation of the PP surface at 9–10 nm depth. The rh and op composites’ oxidative stability has also decreased after ageing in water but by a lesser degree. Thermo-oxidative degradation of the composites may be promoted by the leaching of soluble and insoluble fibre substances into water, as was observed during the water immersion test (Figure 5) resulting in the specimens’ surface erosion. Water leaching of soluble and insoluble substances from the fibres is a well-known problem for natural fibre composites which weakens the fibre/matrix interface and deteriorates their mechanical properties [45,46]. Surface erosion caused by water leaching may aggravate the previously discussed hydrolysis of the polypropylene matrix. It is worth mentioning that the higher fibre amount and, hence, lignin content significantly dampened the composites’ thermo-oxidative degradation being directly correlated with the OIT reduction.

3.2.2. Thermogravimetric Analysis (TGA) and Derivative Thermogravimetry (DTG)

Thermogravimetric analysis under an inert atmosphere was carried out to investigate the effect of the natural fibre reinforcement on the thermal stability of the composites and the assessment of their filler content. The presence of organic fillers, composed of cellulose, hemicellulose, and lignin, makes these composites especially prone to thermal degradation, limiting the processing temperature range [11].

Hence, it is vital to understand their performance under high temperatures. The results are presented in Figure 6 and Figure 7 and

Table 4. The thermal decomposition data of PPv and the composites.

Material	1st Stage			2nd Stage			Ash Residue 5 (%)
	Ts 1–Te 2 (°C)	TDTG 1st.peak (°C)	Weight Loss 3 (%)	Ts–Te (°C)	TDTG 2nd.peak (°C)	Weight Loss 4 (%)
PPv	―	―	―	313.7–486.3	448.2	99.2	0.8
PP20%rh	233.7–372.1	―	9.8	372.1–494.1	455.1	92.2	6.9
PP30%rh	219.2–373.9	348.9	14.4	373.9–498.0	457.1	89.1	10.2
PP20%op	234.1–376.5	348.5	12.4	376.5–496.4	457.1	94.2	5.3
PP30%op	221.5–376.3	348.6	19.0	376.3–497.7	457.0	91.1	8.0

¹ T_s—onset temperature; ² T_e—end temperature; ³ the weight loss at the end of the 1st. stage; ⁴ the weight loss at the end of the 2nd. stage; ⁵ ash residue at 600 °C.

. Virgin PP co-polymer underwent a single thermal event. It started to degrade at 313.7 °C (T_i), having only one peak (DTG max) in the derivative thermogravimetric curve (Figure 7) at 448.2 °C, designated in Table 4 as T_DTG 2nd._peak. The ash content changes very slightly after 500 °C, volatilizing the PPv almost completely (0.8% ash content) at 600 °C, corroborating the data reported by other researchers [47]. PPv’s nearly complete thermal destruction may be attributed to the polymer chains’ molecular scission and high-temperature volatilization [45,48].

The weight loss occurs in several stages for the rice husk and olive pit composites. At the first barely perceptible stage, between the initial test temperature (30 °C) and the onset temperature of the first stage (T_i), about 1% of the weight is lost due to water evaporation (Figure 6). No moisture evaporation was detected for PPv due to its hydrophobic nature. According to the DTG thermograms depicted in Figure 7, the thermal degradation of the rice husk and olive pit composites occurs in two stages. There are two temperature peaks in the first stage, in which the onset temperatures vary from 219 °C to 234 °C as a function of the filler’s amount and type (Table 4), indicating that the composites with a lower fibre content are more thermally stable. The temperature interval between 219 °C and 310 °C can be attributed to the decomposition of hemicellulose, whose content is proportionally lower in PP20%rh and PP20%op than in their 30% counterparts, leading to a higher onset decomposition temperature (about 234 °C) for 20% composites. Regarding the influence of filler type, op composites showed a marginal advantage. It should be noted that in Table 4, only clearly identifiable temperature peaks were listed. The temperature interval between 220 °C and 310 °C can be attributed to the decomposition of hemicellulose. The first peaks occurring in the interval between 310 °C and 380 °C, as highlighted in Figure 7, are due to the destruction of cellulose [11,49].

In the second stage with the onset temperature varying for rh composites (373 °C) and op composites (376 °C), the maximum weight loss rates happened in a range of 455–457 °C, several degrees higher than in virgin PP, indicating that besides the PP matrix decomposition, the lignin decomposition, which reported to have a more thermally stable polyaromatic structure than hemicellulose and cellulose gradually decomposing over a broad temperature range, is also present at this stage [47,50].

Once the temperature reaches 500 °C, a significant stage is reached as the degradation process is generally complete. At this point, most of the polymer matrix and fibre constituents have been already pyrolyzed and converted to gases and ash. The residual ash content, which provides indirect information about the fibre content in the composites, is a key indicator of this stage. As shown in

Table 5. The relation between the ash residue and the fibre content.

Material	F.C. (%) 1	A.R. (%) 2	A.R./F.C.	A.C.R. 3
PP20%rh	20	6.9	0.35	1.5
PP30%rh	30	10.2	0.34
PP20%op	20	5.3	0.27	1.5
PP30%op	30	8.0	0.27

¹ F.C.—fibre content; ² A.R.—ash residue; ³ A.C.R. is a rate between the ash content at a different content of the same filler (e.g., A.R_{. PP30%rh.}/A.R_{. PP20%rh}).

, the fibre content is proportional to the ash content, independently of the fillers’ type. The higher rate of the ash residue in the rh composite in comparison to the op composites is due to the rice husk’s silica content, which was maintained untransformed in the ash [51] as its decomposition temperature is higher than the temperature range used during the TGA test. Moreover, in all the composites, the ash residue content changes at the same rate as the amount of the filler.

3.3. Morphology Assessment

The fractured cross-sections of the composites’ tensile specimens were analysed with the SEM micrographs (Figure 8 and Figure 9), where the images under the (c) and (d) labels represent the fractured cross-sections of the composite after immersion in water. As expected and reported in detail in the previous research for the composites before water immersion [24], uneven fibre distribution, clamping, and cracks caused by fibre pulling during the fracture are shown in all the images. A non-uniform fibre dispersion in the PP matrix was caused by the extrusion and injection moulding of the composites. So given that SEM imaging of the specimens after water immersion was carried out when the water absorbed during the immersion test had already been desorbed [52], as the specimens were kept under room temperature between the preparation (tensile tests) and SEM observation, it is reasonable to conclude that the absence of the visible alteration of the in-deep thickness specimens’ morphology was due to desorption.

However, considering the erosion of the composites’ surface during immersion, indirectly indicated by the composites’ fibre leaching into the water as demonstrated in Figure 5, the major morphology alterations are to be expected at the specimens’ surface. DIM micrographs were acquired to evaluate the composites’ surface condition before and after water exposure (Figure 10). The left-side images, labelled as (a), (c), (e), and (g), depicted the tensile specimens’ surface before water immersion. As the PP matrix used for compounding the composites is translucent in its original form, the rice husk and olive pit fibres are visible under the specimens’ surface. Moreover, the slight whitish strikes, aligned in the direction of the mould filling, are identifiable with different intensities in all the images mentioned above, indicating specimens’ surface abrasion by fibres during injection moulding. These surface defects were aggravated after the long-term water immersion, as shown in Figure 10’s right-side images (b), (d), (f), and (h), where these whitish strikes appear to gain depth, by the water leaching of soluble and insoluble substances from the fibres as evidenced in Figure 5. The whitening of the rice husk fibres under the specimen surface Figure 10b,d due to water leaching further corroborates this hypothesis. Meanwhile, the op composites are less prone to water leaching, as less insoluble fibre residue deposition was detected at the end of the immersion test (Figure 5b). However, they appear to be more abrasive, especially with higher filler content (Figure 10g,h).

3.4. Influence of Water Absorption on the Mechanical Properties

Before tensile and flexural testing, the water-saturated specimens were preserved in vacuum-sealed bags to prevent desorption. This procedure was adopted to emulate high humidity bathroom conditions, the intended ambient conditions for functional products such as sanitary components for which the composites under investigation are intended as more eco-friendly alternatives. As the evaluation of the dry composites’ fibre type and content on their mechanical properties of the composites was analysed and reported elsewhere [24], this assessment will be solely focused on the impact of water absorption on the mechanical properties by comparing dry and wet counterparts. As it may be seen from

Table 6. Mechanical properties under tension.

Material	S.C. 1	E (MPa)	∆ 2 E (%)	σu (MPa)	σu ↑ 3 (%)	εb (%)	εb ↓ 4 (%)
PP20%rh	b.i.	1377.0 ± 198.2	−0.8	19.7 ± 0.4	15.3	8.6 ± 1.2	15.5
	a.i.	1366.6 ± 59.5		22.7 ± 0.4		7.2 ± 0.9
PP30%rh	b.i.	1322.6 ± 170.63	6.9	19.6 ± 0.6	8.4	8.3 ± 1.4	38.9
	a.i.	1413.73 ± 43.5		21.3 ± 0.2		5.1 ± 0.4
PP20%op	b.i.	1305.8 ± 217.8	−7.8	18.6 ± 0.5	11.4	10.0 ± 1.7	14.9
	a.i.	1204.3 ± 34.1		20.7 ± 0.5		8.4 ± 0.9
PP30%op	b.i.	1249.6 ± 239. 8	13.3	16.6 ± 0.3	8.1	6.8 ± 1.1	19.7
	a.i.	1416.02 ± 106.1		17.9 ± 0.4		5.5 ± 1.0

¹ S.C.—specimen’s condition: (b.i.)—before immersion; (a.i.)—after immersion; ² ∆—difference; ³ ↑—increase; ⁴ ↓—decrease.

, the tensile elastic modulus has decreased slightly in both rh and op 20% wet composites. However, this trend was inverted for higher fibre loading. As shown in Figure 11 and Table 6, the tensile strength was improved between 8 and 15% with a more pronounced improvement for lower fibre content. It may be attributed to the plasticizing effect of water infiltration into the microcracks at the fibre matrix interface and to fibre swelling [18,53]. However, the inverse correlation between the improved tensile strength and the fibre amount may be due to the more pronounced superficial erosion of the wet specimens with a 30% fibre charge. The surface damage may counteract the water and fibre swelling plasticization effect. The vindication of this possibility is a drastic reduction (39%) in the elongation at the break in wet PP30%rh, where the specimen’s surface shows more significant wear induced by long-term water exposure, as seen in Figure 10b. Other wet composites underwent a significant but less drastic reduction in elongation at the break.

After long-term water exposure, the composites’ flexural modulus and strength were improved by 3–18% and 4–11%, respectively, as envisaged in Figure 12 and

Table 7. Mechanical properties under bending.

Material	S.C. 1	Ef (MPa)	Ef ↑ 2 (%)	σf (MPa)	σf ↑ 2 (%)	σuf (MPa)
PP20%rh	b.i.	1263.4 ± 77.1	17.6	30.3 ± 0.8	11.3	―
	a.i.	1485.5 ± 16.0		33.7 ± 0.2		―
PP30%rh	b.i.	1572.4 ± 67.0	3.3	31.4 ± 0.9	5.1	―
	a.i.	1624.1 ± 17.6		33.0 ± 0.3		22.7 ± 3.1
PP20%op	b.i.	1170.5 ± 47.3	17.2	28.6 ± 0.8	7.3	―
	a.i.	1371.7 ± 25.0		30.7 ± 0.3		―
PP30%op	b.i.	1356.5 ± 95.5	13.9	29.0 ± 1.4	3.9	―
	a.i.	1544.8 ± 22.9		30.1 ± 0.3		11.3 ± 1.5

¹ S.C.—specimen’s condition: (b.i.)—before immersion; (a.i.)—after immersion; ² ↑—increase.

. It should be noted that both flexural modulus and strength follow the same trend as the tensile strength, showing a more significant improvement at a lower fibre charge. A more modest increase in the wet specimens’ flexural modulus and strength with a higher fibre charge may be explained by the same conjugation of water-induced filling of the microcrack sand fibre swelling and more significant water-induced specimen surface damage. It is worth mentioning that in the three-point bending mode, the force is applied locally to the surface damaged by water exposure, which at 30% op and rh filler content led to the specimens’ fracture at the ultimate flexural strength (σ_uf) of 23 MPa for PP30%rh and 11 MPa for PP30%rh.

4. Conclusions

The impact of water absorption at room temperature on the mechanical, thermal and morphological properties of the injection-moulded polypropylene composites with endogenous rice husk and olive pit fibre has been studied. The water absorption followed Fickian behaviour for all the composites and took about eight months to reach an equilibrium saturation state. Rice husk composites were more prone to water absorption and thickness swelling, which was less than 2% for the maximum water absorption. Long-term water exposure significantly decreased the thermo-oxidative stability of virgin PP. Meanwhile, lignin in rice husk and olive pit fibres dampened the composites’ thermo-oxidative degradation caused by water immersion. Lower filler content is correlated with the rh and op composites’ higher thermal stability, widening their temperature processing range. Exposure to water results in a slight increase in the tensile and flexural properties, making the composites, however, more brittle, especially critical at a higher filler content. The latter was caused by the degradation of the fibre–matrix interface at the specimens’ surface, observed by microscopy. Attending the project’s objectives, where this study belongs, the most promising candidate for substituting synthetic polymer in technical, sanitary components is a 20% olive pit fibre composite. It aligns with low moisture absorption, dimensional stability, and resistance to thermo-oxidative degradation. In addition, hydrolytic ageing causes reduced deterioration of the mechanical properties of PP20%op in comparison to other NFC composites under investigation.