# Influence of different long and short ageing protocols on the mechanical behaviour and damage mechanisms of a hemp fibre reinforced polypropylene

Quentin C.P. Bourgogne, Marwa Abida, Olivier Perroud, Vanessa Bouchart

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Quentin C.P. Bourgogne, Marwa Abida, Olivier Perroud, Vanessa Bouchart. Influence of different long and short ageing protocols on the mechanical behaviour and damage mechanisms of a hemp fibre reinforced polypropylene. Next Materials, 2024, 4, pp.100096. 10.1016/j.nxmate.2023.100096. hal-04910209

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Research Article

# Influence of different long and short ageing protocols on the mechanical behaviour and damage mechanisms of a hemp fibre reinforced polypropylene

Quentin C.P. Bourgogne<sup>a, *</sup>, Marwa Abida<sup>b</sup>, Olivier Perroud<sup>c</sup>, Vanessa Bouchart<sup>b</sup>

<figure><img src="image_2.png" /></figure>

ARTICLE INFO

Keywords:

Bio-based composites

Hemp fibres

Ageing

Mechanical behaviour

SEM

IR spectroscopy

ABSTRACT

This paper deals with the study of the evolution of the durability of short hemp fibres reinforced composites subjected to long-term ageing. The compounds were stored for ten years in atmospheric conditions (natural ageing) prior to the study. Therefore, two kinds of specimen are studied. First, the aged compounds were used to moulded tensile specimens. The residual mechanical behaviour of natural aged hemp/PP composites (natural aged specimens) were compared to properties obtained 10 years ago. This storage showed that the hemp is more sensitive to time degradation than PP and was confirmed by the initial degradation of compounds pointed out by ATG. Then, injected natural aged polypropylene reinforced with several hemp fibre weight fractions compounds were subjected to two different protocols in order to characterize their durability (aged specimens). The specimens were subjected to water immersion at ambient temperature and storage in a climatic chamber maintained at 40 °C with a relative humidity of 98%. The fracture surfaces were observed with SEM to identify the physical phenomena leading to the observed loss of mechanical properties. A weight water sorption of 14% with respect to hemp weight fraction was, for the first time, determined as a critical water sorption for which the reinforcement effect becomes negligible. The physical phenomenon responsible for this critical absorption was finally investigated with Infrared Spectroscopy, showing the occurring of a dissolution of the fibres.

1. Introduction

New environmental strategies encourage the use of bio-based ma-
terials in diverse applications. Plant fibre come from renewable re-
sources and require less production energy compared to their
conventional fibres. Then, plant fibre reinforced composites are attrac-
tive due to their relatively low densities and low environmental impact
[1,2]. Vegetal fiber reinforced composites have been successfully used
for lightweight applications in recent year. However, main applications
used thermoset resin and lead to a recyclability issue at end-of-life.
Therefore, the use of thermoplastic polymers as matrix gains interest
in the last decades [1]. However, the degradation temperature of plan
fibres limits the choice of matrix [3]. Low melting temperature ther-
moplastic polymers are usually used in order to encounter this issue.
Among them, polyolefin (polyethylene or polypropylene (PP)) and
polylactic acid (PLA) are the most commonly used materials with

natural fibres [4]. Due to its low density, good processability, dimen-
sional stability, and high resistant to water, alkaline and acidic envi-
ronments [5,7] PP is commonly used.

Among vegetal fibres, those extracted from plant fibre such as flax or
hemp are promising at European scale [6–9]. In comparison to other
commonly, hemp fibres could exhibit better physical and mechanical
properties, especially its specific tensile strength at break and tensile
modulus [10–13]. Despite their promising specific mechanical proper-
ties, their affinity to water molecules remains their major limitation. The
water sorption by hemp fibres is mainly attributed to their chemical
composition and structure [14,15]. Hemp fibres are considered as bio-
composites constituted of cellulose microfibrils held together by a ma-
trix of hemicellulose and lignin [16]. Amorphous cellulose and
hemicellulose are the principal contributors to the moisture sorption,
due to their open structure containing hydroxyl (OH) and acetyl
(C2H3O) groups [17]. Water sorption in plant fibres could also be

<footnote><sup>a</sup> Ecole Nationale d'Ingénieurs de Metz (ENIM), F-57000 Metz, France</footnote>

<footnote><sup>b</sup> Laboratoire d'Etude des Microstructures et de Mécanique des Matériaux, Université de Lorraine, CNRS, ENIM, Arts et Métiers Paris Tech, LEM3, F-57000 Metz, France</footnote>

<footnote><sup>c</sup> Laboratoire d'Etude des Microstructures et de Mécanique des Matériaux, Université de Lorraine, CNRS, Arts et Métiers Paris Tech, LEM3, F-57000 Metz, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: quentin.bourgogne@univ-lorraine.fr (Q.C.P. Bourgogne).</footnote>
caused by capillary effects e.g. via the lumen [16,18]. These phenomena
can be amplified by time and conservation and/or use in atmospheric
conditions for a long time [19]. As the design of structural parts is
conducted to produce reliable properties and long-term lifetime of the
parts, the evolution of the mechanical properties of natural fibres rein-
forced polymers through years needs to be understood.

Several researches focus on the hygrothermal aging of plant fibre
reinforced with thermoset resins and thermoplastics [20–26]. The
studies point out a loss of mechanical properties of natural fibre com-
posites according to the water sorption in material. Plant fibres are
hydrophilic and thermoplastic matrix are mainly hydrophobic. Then,
the water uptake by fibres tends to decrease the interfacial properties
resulting in a composite mechanical properties loss [27]. Moisture
diffusion can also occur through the porosities between the
polymer-fibre interfaces. [28]. Due to different swelling coefficient be-
tween fibre and matrix [29], interfacial bonding could therefore be an
issue Mutjé et al., show that despite the irregular morphology of hemp
fibre that could enhanced fibres matrix adhesion, the tensile strength
and modulus of hemp/PP composites drop compared to those of glass-PP
composites due to poor interfacial resistance [30]. Other authors focus
on the damage mechanisms due to water sorption in natural fibre
reinforced polymer composites [31–33]. The authors pointed out that
the use of IR spectroscopy coupled with other characterization methods
allows a better description of damage mechanisms due to water sorp-
tion. The hydrothermal aging mechanisms of fibre reinforced compos-
ites have also been extensively researched, with processes involved
including swelling, plasticizing, cracking and hydrolysing of the matrix,
and debonding of fibre-matrix interface [34–36]. Several treatments,
such as alkali, acetylation and silane treatment, are used to reduce water
affinity by fibre surface modification [37,38]. The addition of a sec-
ondary reinforcement, such as eggshell nanoparticles was shown to
stabilize the observed behaviour [39,40]. However, as far the authors
knowledge are concerns, few studies focus on long term durability of
vegetable fibre reinforced composites. Cai et al. [41] investigated on
hydrothermal aging evaluation at temperatures from 23 °C to 60 °C over
3 years on unidirectional flax fibre reinforced phenolic (FFRP) resin
composites. Hydromechanical mechanism at microscale was proposed.
The degradation occurs with, interface detachment between the com-
ponents, deformation and rupture of the fibre lumens, followed by dis-
solving of the fibres.

To design reliable mechanical part with natural reinforcement, the
contributions of each component on the sorption process need to be
assessed as well as the long-term ageing of raw materials.

Therefore, the aim of this study is to investigate the long-term
durability and the sensibility of the hemp fibres reinforced PP compos-
ites to thermo-hydro-ageing protocols. The experimental campaign was
conducted on neat PP and PP based composites reinforced with different
weight fraction of short hemp fibres. The paper investigates the damage
phenomena occurring in short fibre composites subjected to water
sorption. Firstly, the paper discusses the effect of years of storage on the
mechanical properties of the composite. Secondly, the paper discusses
the water sorption kinetics of short hemp fibre reinforced PP composites
for several ageing protocols. Then, the effect of water content on me-
chanical behaviour of short hemp fibre reinforced composites is ana-
lysed. Finally, SEM observations of fracture surface coupled with
infrared spectroscopy analyses were used to highlight the damage
mechanisms that occur during ageing in order to propose hypotheses
that explain the weakening of the composite properties and the change
in water sorption kinetics.

## 2. Materials and methods

### 2.1. Materials and processing

The matrix is a commercial block copolymer PP with a density of
0.905 g/cm³ and a melt flow index of 90 g/10 min at 230 °C and 2.16 kg

(according to the ISO standard ISO1133). The compounds were done by
Valagro® by extrusion using a co-rotative twin-screw model Cleextral
BC21 with screw length of 600 mm and length to diameter ratio L/D of
25. The obtained pellets were then maintained at 70 °C for 12 h to
remove residual water.

Industrial extraction and grinding of hemp short fibres were per-
formed using classical process routes [42]. Compounds processing were
done by a company called Valagro® in 2010.

Short hemp fibres reinforced PP composites with four different
weight fractions of fibre were investigated, neat PP and composites
reinforced by 5%wt (PPHF5) and 10%wt (PPHF10) of short hemp fibres.
The samples were obtained with injection process using the following
parameters: nozzle temperature: 200 °C, injection speed: 25 m/min,
injection pressure: 160 bar, holding pressure: 160 bar, holding time: 20 s

These parameters are the same as those used in [43], in order to
avoid any difference in injection process for comparison purposes. They
were also selected in order to limit the shrinkage and the porosity
content. Hence, the obtained porosity content was in the acceptable
standards and equivalent to the one obtained in the reference study.

To limit the influence of melt flow on fibre orientation and spatial
distributions, the injection gate was located in head of sample (tap gate).
The tensile specimen geometry was selected according to ISO 527
standard. The width is 10 mm, the thickness is 4 mm and the gauge
length is 80 mm.

### 2.2. Ageing protocols

The specimens were subjected to two kinds of ageing protocols in
order to compare the effect of long-term ageing and/or use with
hygrothermal ageing. For the first, referred as natural ageing, the
compounds were store in atmospheric atmosphere prior to injection
moulding of tensile specimens. For the second, different hygrothermal
ageing conditions were performed on natural aged injected tensile
specimens to assess the durability.

Different hydrothermal ageing protocols were used in order to
analyse the influence of water content on short hemp fibre reinforced PP
composites behaviour. All specimens were dried at 80 °C for 14 h prior
to ageing. Specimens of neat PP, PPHF5 and PPHF10 were immersed in
water at ambient temperature or stored in a climatic chamber at 40 °C
with a relative humidity of 98% (RH98T40).

The hydrothermal ageing protocols were performed until maximal
water content is reached. The maximum ageing temperature studied was
40 °C in order to avoid any thermal degradation of hemp fibres. The
samples were regularly weighed with a 0.1 mg precision scale, in a
protected environment.

### 2.3. Testing methods

The compounds were stored during 10 years in atmospheric condi-
tions. The natural ageing was therefore analysed using thermogravi-
metric analysis (TGA) with a SDT650 from TA Instrument™. Samples of
approximately 30 mg were analysed with TGA by a continuous heating
to 500 °C at two heating rates: 0.8 and 5 °C/min. Before the test, samples
were dried at 70 °C during 24 h to avoid the influence of water on the
results.

Tensile tests were performed with an INSTRON™ 5585 H equipped
with mechanical grips, and controlled by a 10 kN-capacity load cell. The
local strain was measured with a video extensometer. All tests were
performed in a thermal chamber at a fixed temperature of 30 °C to avoid
any temperature fluctuation. The samples were put in the thermal
chamber 3 min before the tests in order to reach the tested temperature.
The tests were performed at a strain rate of 10⁻³s⁻¹. By weight mea-
surement prior and after the characterization, no significant water loss
was observed.

Observation of the fracture surface were performed with a Scanning
Electron Microscope (SEM) JEOL™ JSM-IT200, in which no
metallization is required, in order to study the damage mechanisms occurring due to water sorption.

The degradation was studied using Infrared Spectroscopy. An IR Alpha 2 spectrometer with diamond ATR from BRUKER™ was used. The scans were performed with 24 measures between 400 and 4000 cm⁻¹.

# 3. Results

## 3.1. Degradation of the compound

The storage in an atmospheric condition for 10 years can lead to a natural degradation of both fibres and matrix, as it was shown by [19]. As the heating rate can have an important influence on the results [44], especially on old samples, the fastest heating rate imposed was 5 °C/min. The results are shown in Fig. 1.

As expected, the Fig. 1 shows a shifting to higher temperature of degradation temperatures with heating rate due to thermal intertie. The thermal degradation of all material shows the same tendency whatever the heating rates. No significative difference in degradation kinetics could be founds between all materials. Indeed, at a heating rate of 0.8 °C/min the degradations start at 200 °C and the evolution in degradation curves (mass loss) remains the same for all materials. A second degradation phase is noticed at the ends of curves around 340 °C. This phenomenon indicates a two-step degradation kinetics. The modification of kinetics is more noticeable on PPH10 degradation curve. Therefore, this phenomenon could be attribute to the presence of hemp fibres. It is worth mentioning that all samples were almost completely degraded at 500 °C, as it was already shown in a previous study [45].

Usually, hemp can still be found at 500 °C, unless a pre-degradation occurred, as it was shown in [19]. The spectroscopy analysis of the residues showed that the ash is mostly composed of approximately 30% of carbonate sodium, 18% of strontium carbonate and 12% of carbonate magnesium hydrate confirming that no hemp remains at 500 °C. This phenomenon was also observed in [19] for aged hemp fibres, confirming that atmospheric storage of the compound for 10 years led to a degradation of the hemp. The influence of this degradation on the mechanical behaviour of the composite is discussed in Section 3.3.1.

## 3.2. Water sorption

The water sorption of each material was studied at different ageing protocols. The injected specimens were taken out of the ageing chamber, wiped off with a soft cloth and weighted. Each specimen was weighted three times at each weight measurement. For each measure, the saved weight value is the stabilized one. The tests were performed on three samples to assess the repeatability (Fig. 2).

As shown in Fig. 2, the water sorption of neat PP aged at a relative humidity of 98% at 40 °C is repeatable with a good accuracy, even for a maximal weight gain of 0.45%. The results of the water sorption kinetics are synthetized in Fig. 3. Only a representative sorption curve for each ageing condition due to the good repeatability of tests (Fig. 2). A plateau can be seen on each curve obtained, meaning that the saturation was reached. The extended measures performed on neat PP and presented in Fig. 3(a), and on a reinforced PP with different fractions of hemp presented in Fig. 3(b) and Fig. 3(c), allowed a good confidence on the identification of saturation as no re-absorption was observed on both neat and on reinforced samples.

No clear difference of maximal water sorption can be observed between composites with studied fibre weight contents. As shown in Fig. 3 (a), Fig. 3(b), and Fig. 3(c), the maximum sorption remains low for fibre weight fraction less than 10%wt. The maximum water content absorbed for high content of fibres can be, however, about 5 times higher than the PPHF10 [21–28]. This high contrast between the composites with high content of reinforcement and the lower fibre content composites can be explained by different mechanisms of water molecules diffusion through the specimens. Indeed, as shown in Fig. 3(a), Fig. 3(b), and Fig. 3(c), the PPHF5 and the PPHF10 reached their maximum water content simultaneously when aged in the climatic chamber (RH98T40). The required time to reach maximal water content of low fibre content composites is similar to neat PP. Thus, the water molecules diffusion in low fibre content composites is driven by the matrix. The water diffusion in the neat PP microstructure remains low.

For a better analysis of the sorption process, the Fick's law [46] was used to model the water diffusion in PPHF (Eq. 1).

$$ \frac{M_t}{M_m} = \frac{4}{\sqrt{\pi} h^2} \text{for } \frac{Dt}{h^2} < 0.05 \qquad (1a) $$

$$ \frac{M_t}{M_m} = 1 - \frac{8}{\pi^2} \exp[-\left[\frac{Dt}{h^2}\right]^2] \text{ for } \frac{Dt}{h^2} > 0.05 \quad (1.b) \text{ where t is time, h is the} $$

<figure><img src="image_3.png" /><figcaption>Fig. 1. Influence of fibre fraction and heating rate on the degradation of a hemp fibre reinforced PP.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 2. Repeatability of the sorption process of polypropylene.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. Comparison of the evolution of water sorption for different ageing protocols of (a) a neat PP (b) a PPHF5 and (c) a PPHF10.</figcaption></figure>
thickness of the sample, D is the apparent diffusion coefficient and Mm is the maximum water sorption. The results are synthesized in Table 1.

As shown in Table 1 and Fig. 3, the water molecules diffusion in PP-based composite is very low. The apparent diffusion coefficients increase with hemp fibre weight fraction. The water diffusion remains slow because PP matrix acts as barrier. The water sorption becomes faster when water molecules reach the hemp fibres. Indeed, the Fick's law constants seems to increase exponentially with the hemp fibre weight fraction when aged in a climatic chamber at 98% relative humidity at 40 °C.

The apparent diffusion coefficients of PPHF5 and PPHF10, immersed in water at ambient temperature are equivalent to the coefficients of neat PP. The ageing temperature has, however, an important role in the diffusion process. The increase of ageing temperature leads to an increase of the PP apparent diffusion coefficient. As a consequence, water molecules reach the hemp fibre and, then, water diffusion is accelerated.

### 3.3. Tensile tests

#### 3.3.1. Influence of 8 years natural ageing

The TGA analysis presented in Section 3.1 showed that a natural ageing of hemp fibre composites occurred due to a storage in an atmospheric environment for 10 years. Tensile tests were performed on injected samples and were compared with results obtained in [43], under the same conditions, following ISO 527 standard. The tests were performed for 3 different strain rates on the different composite reinforced by different fibre volume fractions. The results are presented in Fig. 4. The TGA analysis, presented in Section 3.1, in line with previous results [19], showed that natural ageing of hemp fibres occurred. This degradation is observed on tensile behaviour on all materials in Fig. 4 as the properties were altered by the natural ageing. Even the neat polypropylene is subjected to this natural ageing as presented on Fig. 4(a). The variation of mechanical properties can be seen for all studied strain rates. It is worth noting that the influence of the ten-year natural ageing depends on materials. Indeed, on one hand, at strain rate of $10^{-3} s^{-1}$, the Young modulus of the polypropylene is almost unaffected while its ultimate tensile stress lost 17%. On the other hand, hemp fibre affects the stiffness of the composite more than its ultimate tensile stress. Hence, as hemp is more affected by long-term ageing than polypropylene, as shown in Fig. 1, the Young moduli of reinforced grades are deeply affected while their ultimate tensile stress loss is the same than neat PP, due to the degradation of the polymer matrix.

A synthesis of mechanical parameters in 2013 and 2021 is presented in Table 2. No clear effect of strain rate is observed, only for the polypropylene which is due to the viscoelastic effects appearing at low strain rates, more than the degradation.

It can be concluded that both materials degraded themselves during the storage at ambient temperature and in free air for 10 years. The degradation of the reinforcement is responsible for the decrease of stiffness while the degradation of the PP matrix is responsible for the loss of ultimate tensile stress.

The hemp fibres degraded more than the matrix, leading to a strong decrease of the reinforcement effect. Hence, a PPHF10 aged during 10 years present a stiffness equivalent to a neat and new polypropylene of 2013.

#### 3.3.2. Influence of fibre fraction

This section focuses on the behaviour of naturally aged compounds through the comparison of composite mechanical behaviour after injection. For a better comparison and study of the evolutions, the tests were all performed at strain rate fixed at $10^{-3} s^{-1}$.

First, a comparison of the mechanical behaviour of neat PP and composites reinforced by several hemp fibre contents is performed. at a controlled temperature of 30 °C (Fig. 5).

The fibre orientation repartition is equivalent in the tested specimens, as they were injected in the same mould under similar flow conditions. The difference in mechanical behaviour shown in Fig. 5 is supposed to be dependent on hemp fibre content only. As expected, the composite stiffness increases with the increase of hemp fibre weight fraction. It is worth to note that the ultimate tensile stress is similar for all PPHF. These ultimate tensile stresses are lower than the neat PP. The decrease of ultimate tensile observed could be explained by the stress concentrations at fibre extremities, which are acting as preferential sites of damage initiation and propagation. Moreover, during tensile loading, fibre/matrix decohesion can appear which will also lead to ultimate stress decrease.

#### 3.3.3. Influence of ageing protocols

Tensile tests were performed on aged composites reinforced with different short fibre weight fraction subjected to the different ageing protocols. In order to differentiate the influence of each constituent, the neat PP and the PPHF were subjected to the same ageing protocols. The comparison between the dry-as-moulded (DAM) and aged PP is showed in Fig. 6.

As shown on Fig. 6, the mechanical behaviour of the PP is not deeply affected by water sorption, whatever the ageing protocol. One explanation could be the low water sorption of polypropylene as shown in Fig. 3(a). Thus, at maximal water content of 0.45%wt, the mechanical behaviour variation of the matrix is negligible.

As shown in Fig. 7, the mechanical behaviour of hemp fibre reinforced PP composites is more affected by ageing. The ultimate tensile stress of aged PPHF specimens, whatever the ageing protocol, is 5% less than the one obtained on a DAM material. The evolution of the stiffness, which was negligible in the case of the neat PP, is more pronounced and a decrease of 15% of the Young modulus is assessed. The hemp fibre stiffening is cancelled by the water sorption of hemp. Indeed, the mechanical behaviour of the aged PPHF5 are similar to the mechanical behaviour of the saturated neat PP.

The same trends are also observed for the PPHF10. The stiffness evolution according water content is presented in Fig. 8.

As shown in Fig. 8, the stiffness of the composite decreases with water content to reach the neat PP stiffness. The aged PPHF10 shows no reinforcing effect compared to neat PP. It is also worth noting that no clear difference between ageing protocols is noticeable. The water content is predominant whatever the ageing protocol.

Fig. 8 confirms that, for PPHF, no clear difference between the protocols can be noted. Despite the different protocols' sorption kinetics, the mechanical properties are mainly affected by water content. The ageing time is therefore not sufficient to observe a degradation of polymer chain by chemical reaction. For both PPHF5 and PPHF10, the stabilization of the stiffness occurs for a water content which is inferior to the sorption at saturation.

A critical sorption is identified and proposed for the first time. Above

<table><caption>**Table 1**

Apparent Fick's law constants for studied PPHF for each ageing protocols.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Neat PP</th><th colspan="2">PPHF5</th><th colspan="2">PPHF10</th></tr><tr><th>Mm (%)</th><th>D* 10<sup>-10</sup> (m<sup>2</sup>/s)</th><th>Mm (%)</th><th>D* 10<sup>-10</sup> (m<sup>2</sup>/s)</th><th>Mm (%)</th><th>D* 10<sup>-10</sup> (m<sup>2</sup>/s)</th></tr></thead><tbody><tr><td>Ageing protocol</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>RH98 T = 40 °C</td><td>0.44</td><td>6.0</td><td>1.28</td><td>8.5</td><td>1.40</td><td>9.0</td></tr><tr><td>Immersion ambient temperature</td><td>0.45</td><td>2.5</td><td>1.30</td><td>2.5</td><td>1.32</td><td>2.5</td></tr></tbody></table>
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. Influence of 8 years of natural ageing on the mechanical behaviour of a (a) neat PP (b) PPHF5 and (c) PPHF10.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Table 2</figcaption></figure>

Loss of properties due to long-term natural ageing obtained with tensile tests performed at (a) $10^{-4}$ s$^{-1}$, (b) $10^{-3}$ s$^{-1}$ and (c) $10^{-2}$ s$^{-1}$.

<table><thead><tr><th colspan="4">Strain rate = 10<sup>-4</sup> s<sup>-1</sup></th></tr></thead><tbody><tr><th colspan="4">Grade</th></tr><tr><td>Young modulus loss (%)</td><td>PP</td><td>PPHF5</td><td>PPHF10</td></tr><tr><td>UTS loss (%)</td><td>28.8</td><td>28.9</td><td>32.1</td></tr><tr><td></td><td>5.9</td><td>11.9</td><td>13.2</td></tr><tr><th colspan="4">Strain rate = 10<sup>-3</sup> s<sup>-1</sup></th></tr><tr><th colspan="4">Grade</th></tr><tr><td>Young modulus loss (%)</td><td>PP</td><td>PPHF5</td><td>PPHF10</td></tr><tr><td>UTS loss (%)</td><td>0.9</td><td>31.4</td><td>34.7</td></tr><tr><td></td><td>17.6</td><td>17.1</td><td>16.3</td></tr><tr><th colspan="4">Strain rate = 10<sup>-2</sup> s<sup>-1</sup></th></tr><tr><th colspan="4">Grade</th></tr><tr><td>Young modulus loss (%)</td><td>PP</td><td>PPHF5</td><td>PPHF10</td></tr><tr><td>UTS loss (%)</td><td>0.4</td><td>25.02</td><td>30.5</td></tr><tr><td></td><td>16.7</td><td>16.7</td><td>14.7</td></tr></tbody></table>

this value, the effect of reinforcement of hemp fibres disappears. The obtained critical sorption was obtained following the mixture law:

$$abs_{\text{critical}}(\%) = \frac{abs_{\text{sat}}^{\text{PPHFX}}(\%) - (100 - X(\%))abs_{\text{sat}}^{\text{PP}}(\%)}{X(\%)} \quad (2)$$

$abs_{sat}^{PPHFX}(\%)$ being the sorption at saturation of the studied PPHF, $abs_{sat}^{PP}(\%)$ being the sorption at saturation of the neat PP and $X(\%)$ being the fibre weight fraction. A comparison of the identified critical weight absorbed per mass fraction of each PPHF is given in Table 3.

A sorption of 14% of weight with respect to its mass fraction of reinforcement seems to be enough to induce a major loss of its

<figure><img src="image_9.png" /><figcaption>Fig. 5. Mechanical properties of the PP based composite at 30 °C.</figcaption></figure>

mechanical properties. To the authors knowledge, this aspect of the hemp behaviour was not investigated in the literature. Thus, tensile tests on neat hemp fibres for different water content should be performed in order to refine the estimated critical sorption. Another global comparison of mechanical properties of the different saturated grades is presented in Fig. 9.

The Fig. 9 proposes a comparison of the behaviour of aged PP and aged PPHF. The mechanical behaviour of the neat PP, PPHF5 and PPHF10, when aged, is almost similar. The reinforced composites are however more brittle than neat PP. Their ultimate tensile stress is lower
<figure><img src="image_10.png" /><figcaption>Fig. 6. Comparison of the mechanical behaviour of a DAM and aged neat PP for different ageing protocols.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 7. Comparison of the mechanical behaviour of a DAM and aged PPHF5 for different ageing protocols.</figcaption></figure>

than neat PP. This phenomenon was already noticed for DAM specimen (Fig. 5).

The observations led to highlight the entire loss of reinforcement effect due to ageing, and appearing before the reach of the saturation. To explain this, a new notion was then proposed, called the critical sorption, which seems to be equivalent for both studied reinforced grades. The present study hence showed that an absorption 14% of water with respect to the mass fraction of fibre of the composite leads to a complete loss of reinforcement effect.

The fact that this critical absorption is absolute and equivalent for both grades seems to indicate that this phenomenon is a property of hemp. Tensile tests did not allow, however, to explain the origin of this phenomenon and why it is happening before the saturation. The origin of this phenomenon will thus be discussed in Section 3.4 via SEM observations of the fracture surfaces and IR spectroscopy analyses.

### 3.4. SEM observations and IR spectroscopy

The evolution of mechanical behaviour observed in Section 3.3 could be explained by a fibre/matrix debonding due to the swelling of fibres, phenomenon widely observed in precedent studies [21–28]. In order to

investigate this phenomenon, fracture surfaces of the aged PPHF5 and PPHF10 were observed using SEM. The observation results are presented in Fig. 10.

The fracture surfaces of the PPHF5 and the PPHF10 shown in Fig. 10 show the same damage mechanisms. Even though the specimens were aged and dried before observations no clear evidence of fibre/matrix debonding is visible on fracture surfaces, while this phenomenon was widely observed for high fibre content composites [21–28]. The difference of ultimate tensile stress observed between neat and low fibre content reinforced composites (Fig. 5 and Fig. 9) is thus mainly due to the presence of porosities in the reinforced composites.

The SEM observations of the fibre structure could not confirm the mechanical behaviour of the reinforced composite above a critical sorption, presented in Fig. 7 and Fig. 8 and Table 3. Deeper analyses should be performed. Hence, the DAM and aged composites were analysed with IR spectroscopy. The specimens were dried at ambient temperature for 1 month before the analysis in order to avoid any further degradation of the fibres. Three tests were performed and a comparison with the database was carried out in order to validate the repeatability of the analysis. The results are presented in Fig. 11.

To validate the IR spectroscopy acquisitions, the DAM specimens (PP
<figure><img src="image_12.png" /><figcaption>Fig. 8. Stiffness evolution of PPHF10 for different ageing protocols.</figcaption></figure>

<table><caption>**Table 3**

Critical sorption of short fibre reinforced composites.</caption><thead><tr><th>short fibre weight content (%)</th><th>5</th><th>10</th></tr></thead><tbody><tr><td>Critical sorption (%)</td><td>15.03</td><td>13.79</td></tr><tr><td>Error (%)</td><td>+ 1.2 / - 2.0</td><td>+ 0.1 / - 0.4</td></tr></tbody></table>

and composites) were compared with materials from the IR equipment database. The comparison shows that the DAM specimens and the reference are identical with more than 80% accuracy.

The short fibre reinforced composites (PPHF), aged and tested at maximal water sorption, show a clear difference compared to DAM specimens. Peaks of absorbance are noticeable for a wavenumber of approximately 3300cm⁻¹, around 1600cm⁻¹ and a higher absorbance for wavenumber under 1400cm⁻¹. This difference can be observed for each ageing protocol but is more pronounced on immersed specimens.

Hemp fibres were also analysed and are presented in Fig. 12. The increase of the absorbance for wavenumber of approximately 3300cm⁻¹ and around 1600cm⁻¹, is respectively characteristic of hydrogen bond stretching presented in cellulose and hemicellulose and OH bending vibration of absorbed water. The main phenomenon that occurs in the

ageing protocols used in that study is the water sorption that will modify the interaction between fibre and matrix.

The peak of absorbance for a wavenumber of 3300cm⁻¹ and the continuous spectrum of high sorption for wavenumber under 1400cm⁻¹, shown in Fig. 11 and Fig. 12, are characteristic of cellulose, hemicellulose and lignin present in plan fibres. As infrared is a low penetrant radiation, this analysis shows the presence of hemp on the surface of the aged specimens. The presence of hemp on the surface shows that there is a dissolution of the fibres, allowing them to slip out of the samples.

This observation can thus explain the identification of a critical sorption above which no reinforcement effect remains. The loss of reinforcement effect on the studied grades is thus not due to the fibre/matrix decohesion but due to the fibrils decohesion, leading to a decomposition of the fibre itself.

## 4. Conclusions

An experimental assessment of the long-term durability and the aging sensibility of the hemp fibre reinforced PP composite are proposed. The works focus on the analyses of the influence of natural and accelerated hygrothermal ageing protocols on degradation, water sorption kinetics, tensile thermal mechanical behaviour and damage

<figure><img src="image_13.png" /><figcaption>Fig. 9. Comparison of the mechanical behaviour of saturated short hemp/PP composites.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 10. Fracture surface of an aged (a) PPHF5 and (b) PPHF10.</figcaption></figure>

mechanisms observed by SEM and IR spectroscopy.

The effects of ambient storage for years on the physical and me-
chanical properties of the composite was performed. The TGA analysis
as well as tensile tests showed that a storage in an atmospheric condition
for 10 years leads to a degradation of both matrix and hemp fibre. Both
ultimate tensile stress and stiffness are affected by ambient storage for
years. In spite, water resistance of PP, A loss of 17% of ultimate tensile
stress of PP was measured. This degradation was more pounced for
hemp fibre due to hydrophilic nature. The investigations show a loss of
30% of the hemp stiffness. Such long-term natural ageing thus cannot be
neglected on the mechanical behaviour of the material, especially for
industrial parts designed to be solicited for several years.

The sorption kinetics were studied at several temperatures and
several hygroscopic conditions (water immersion and in atmosphere at
RH98%). Beyond the expected conclusions on the influence of temper-
ature on apparent diffusion coefficients and maximum water content,
the study points out a behaviour which is different from the one ob-
tained for high fibre weight content composite (>20%wt), and studied
in literature. As the studied mass fractions of fibre can be considered as
low, no percolation of fibres facilitating the water diffusion through the
material is observed. In the studied cases, the diffusion and maximum

water absorption is mostly led by the matrix, as PP acts as a water
barrier.

As expected, no loss of mechanical properties of PP is noticeable
when aged for short time aging in hydrothermal conditions. Neverthe-
less, a loss of properties of PPHF is observed due to hydrothermal ageing.
The water content in PPHF is mainly responsible of the evolution of the
mechanical behaviour of PPH. Indeed, at same water content, the aging
condition (immersion, temperature, relative humidity) has no influence
on mechanical properties. When the mechanical properties of aged
composites reach the properties of the matrix no more evolution is
detected. Then, the work introduced a critical water sorption ratio to
explain the effects of water absorption on the mechanical properties.
The critical water sorption ratio defined the threshold where the loss of
properties of composites is maximal. The stabilization of hemp fibre
composites was found for a critical sorption ratio of approximately
water weight fraction of 14% with respect to fibre weight fraction.

The damage mechanisms during loading and ageing were assessed
using SEM observations and IR spectroscopy. SEM observation pointed
out that, whereas pull-out phenomenon can be generally observed for
aged samples with high content of fibre, the fracture surface of studied
aged PPHF shows a good interfacial cohesion, even after ageing. No
<figure><img src="image_15.png" /><figcaption>Fig. 11. Infrared radiation sorption spectrum of (a) a neat PP (b) a PPHF5 and (c) a PPHF10.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Fig. 12. Infrared radiation sorption spectrum of hemp. Red, blue and pink spectra correspond to measures and the green spectrum corresponds to the corresponding spectrum available in the database.</figcaption></figure>

change in the molecular bonds of the PP is observed by IR spectroscopy for the studied ageing time. Indeed, the modification of peak absorbance for aged specimens is located around wavenumber corresponding to OH

hydrogen bonds stretching due to interaction with water. Finally, FTIR analyses pointed that water uptake is enough to cause a dissolution of fibre hemicellulose as traces of hemicellulose were found on the surface
of aged reinforced samples. The major outlook of this work would be to investigate the eventual presence of this critical water absorption on non-naturally aged samples in order to help the design of structural parts made of natural fibre reinforced plastics and predict their real life-time performances. Also, the present study allowed the investigation of the hemp reinforcement through a comparative study between neat and reinforced grades with different fibre contents. In order to enhance the knowledge of the reinforcement, a characterization of single fibres should be conducted.

## Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

## CRediT authorship contribution statement

Conceptualization, Marwa Abida and Vanessa Bouchart; methodology, Quentin C.P Bourgogne, Marwa Abida, Vanessa Bouchart.; validation, Quentin C.P Bourgogne; investigation, Quentin C.P Bourgogne, Marwa Abida, Olivier Perroud, Vanessa Bouchart.; writing—original draft preparation, Quentin C.P Bourgogne, Marwa Abida; writing—review and editing, Quentin C.P Bourgogne, Marwa Abida, Vanessa Bouchart; visualization, Quentin C.P Bourgogne.

## Declaration of Competing Interest

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

## Acknowledgement

We acknowledge the experimental facilities MicroMat from LEM3 (Université de Lorraine - CNRS UMR 7239).

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