Surface characterisation and wetting properties of single basalt fibres

Accepted Manuscript Surface characterisation and wetting properties of single basalt fibres Monica Francesca Pucci, Maria Carolina Seghini, Pierre-Jacques Liotier, Fabrizio Sarasini, Jacopo Tirilló, Sylvain Drapier PII:

S1359-8368(16)31486-X

DOI:

10.1016/j.compositesb.2016.09.065

Reference:

JCOMB 4557

To appear in:

Composites Part B

Received Date: 1 August 2016 Revised Date:

28 August 2016

Accepted Date: 20 September 2016

Please cite this article as: Pucci MF, Seghini MC, Liotier P-J, Sarasini F, Tirilló J, Drapier S, Surface characterisation and wetting properties of single basalt fibres, Composites Part B (2016), doi: 10.1016/ j.compositesb.2016.09.065. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Surface characterisation and wetting properties of single basalt fibres Monica Francesca Puccia , Maria Carolina Seghinib,, Pierre-Jacques Liotiera,∗, Fabrizio Sarasinib,, Jacopo Tirill´ob,, Sylvain Drapiera,

Mechanics and Materials Processing dept, Lab. G. Friedel UMR CNRS 5307, Mines ´ Saint-Etienne, 158 Cours Fauriel ´ CS 62362 42023 Saint-Etienne, France b Chemical Engineering Materials Environment dept, Sapienza Universit´ a di Roma, Via Eudossiana 18 00184 Rome, Italy

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Abstract

Basalt fibres are claimed to show better high temperature resistance compared to glass fibres, suggesting better prospects to survive an end-of-life

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composite thermal recycling process while preserving an adequate reinforc-

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ing efficiency. It is well known that sizing agents affect impregnation of fibre reinforcements during composite manufacturing, which influences the overall

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mechanical behaviour of the resulting composites. However, with the exception of some studies about coupling agent effects on the adhesion between

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basalt fibres and matrix, no information are present in literature on the wetting characteristics of basalt fibres. The main contribution of this study is the analysis of the surface properties and wetting behaviour of basalt fibres with sizing optimised for both thermoset and thermoplastic matrices. Measure∗

Corresponding author Email address: [email protected] (Pierre-Jacques Liotier)

Preprint submitted to Composites Part B

October 15, 2016

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ments of static contact angles were performed with a tensiometric procedure

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designed for singles fibres, in order to determine their surface energy and its components. Modifications of surface properties due to chemical treatments and high temperature exposure were also investigated and discussed.

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Keywords: Basalt fibres; Wetting; Surface analysis. 1. Introduction

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Among natural fibres, basalt fibres (BF) have interesting mechanical and thermal properties, which make them suitable to replace glass fibres in composite materials. Their low cost and their ecofriendliness place them as excellent candidates to reinforce composites in both thermoset and thermoplastic matrices, or to produce hybrid composites [1] [2] [3] [4]. Similarly to glass

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fibres, it has been proved that the surface treatment of basalt fibres with a

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suitable sizing enhances the adhesion between fibres and surrounding matrix (in particular) due to the addition of coupling agents like silanes, which signif-

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icantly strengthen the bonding between matrix and fibres. The improvement of adhesion between glass fibres and epoxy or polypropylene matrix is well

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documented [5] [6] [7] [8], but comparatively less information are present in literature about the interfacial adhesion of basalt fibres with thermoset and thermoplastic matrices [9] [10]. Furthermore, the sizing on fibre surface is known to affect the impreg-

nation during composite manufacturing processes [11]. Generally cellulosic and mineral fibres are strongly polar and then difficult to wet by hydropho-

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bic resins [12]. Organosilanes are the main group of coupling agents used

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to enhance the compatibility of any polymer to minerals used in reinforced composites, because they form covalent bonds between both materials. However a study of surface energies and wetting phenomena is mandatory in order to optimise the fibre surface treatments and chemistry of sizing agents

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[13]. Measurements of static contact angles to determine polar and dispersive components of fibre surface energy can be used for surface characterisation of

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raw and sized surfaces. In literature, different studies focused on wettability of glass [14] [15] [16] and carbon fibres [17] [18] [19], also considering the effect of sizing [20] [21], but there is a lack of studies concerning contact angle measurements and surface energies characterisation of basalt fibres [22]. Moreover, independently of the fibre considered, current methods to mea-

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sure the contact angles on single fibres are often ambiguous, not consistent

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with the physical meaning of fibre wetting, and imply some issues that make results of surface energy measurements unreliable [23] [24].

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Since the increase of basalt fibre applications in composite is assessed, for both environmental and economic reasons, another aspect of great interest

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relies in recycling fibres. Among the various methods proposed over the years, the thermal recycling by pyrolysis is the most common and advanced one [25]. The process basically involves incineration of composite wastes in a temperature range, typically from 400 to 700◦ C, able to degrade the

thermoset matrix. Previous works in which glass and basalt fibres were exposed to thermal conditions representative of thermal recycling, showed 3

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that basalt fibres are more resistant to high temperatures than glass fibres

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[26] [27], thus implying a potential reuse of basalt fibres in valuable products. However, upon high temperature exposure, surface energy modifications can occur, partly due to the removal of the original sizing. In this regard, a

study of fibre surface properties before and after recycling has to be carried

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out to assess surface energy modifications. Surface properties should include an attentive study of both morphological and physico-chemical aspects, as

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well as wetting properties, which is the aim of the present investigation. Basalt fibres with two different types of sizing (a first one compatible with epoxy matrix - BF for epoxy - and a second one compatible with polypropylene matrix - BF for PP) were observed with a Scanning Electron Microscope (SEM) and then tested with a tensiometric method to derive surface energy

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and its polar and dispersive components. Static contact angles were measured

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with three liquids, using a recently proposed procedure developed to extrapolate a unique representative contact angle for each fibre/liquid couple, from

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a series of measurements [24]. Knowing contact angles, fibre surface energy and its components, before and after thermal and chemical treatments, were

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determined through the Owens and Wendt (OW) equation [28], combined with the equilibrium Young relation [29] (see paragraphs 2.2.2 and 2.2.3). To investigate the influence of different sizing agents, a wetting analysis has been performed in both fibres in as-received conditions and after a thermal treatment simulating recycling. Moreover, some fibres with coating were also submitted to a soxhlet extraction in acetone, to evaluate the stability of both 4

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sizings. Through this procedure, according to Petersen et al. [30] and Feih

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et al. [31], it is possible to remove the physisorbed and soluble part of the sizing while the covalently bound part can only be removed by thermal treatment. Finally some fibres were submitted to acetone extraction and then to thermal treatments to remove all the sizing. Contact angles, surface energies

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and their components reveal the wetting character of basalt fibres in each

condition, to evaluate potential ability to be impregnated by resins and the

2. Materials and methods 2.1. Materials

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efficiency of recycling treatments for possible reuse.

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2.1.1. Basalt fibres and surface treatments

Two types of basalt fibre rovings were characterised in this study: one

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roving, with a nominal tex of 1200 supplied by Kamenny Vek (BCF-131200-KV12), is characterised by a silane-based sizing compatible with ther-

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moset resins (i.e. epoxy), whilst the second roving, provided by Basaltex (KVT600TEX13E-PP2) with a linear mass density of 600 tex has a sizing

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agent designed to be compatible with thermoplastic matrix (i.e. polypropylene). Both types of basalt fibres have a nominal diameter of 13 µm. Actual fibre diameters were measured by optical microscopy with deviations from the nominal value lower than 1.4 µm over 180 fibres. Surface properties were measured on four samplings for each type of fibres: (1) fibres in as-received conditions; (2) fibres thermally treated in air at 5

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T=400◦ C for 4 h, in order to mimic the minimum temperature of recycling

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conditions; (3) fibres submitted to a chemical treatment (soxhlet extraction in acetone for 24 h). A heating at 80◦ C for 2 h has then been used to remove traces of acetone from fibres. In literature, Gorowara et al. [32] used acetone

extraction for only 2 hours. The duration has been extended in the present

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study to maximize the effect of the chemical treatment. (4) Fibres submitted to chemical extraction and then to the recycling thermal treatment, in order

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to remove completely the sizing [30].

2.1.2. Test liquids for wetting analysis

Four test liquids were used in this study at room temperature. Their properties at room temperature are listed in Table 1: the surface tensions γL ,

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the polar γLp and dispersive γLd components, the density ρ and the viscosity

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η. Water is the most common liquid with a high polar component and surface tension. Diiodomethane is a liquid almost totally dispersive with a

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relatively high surface tension. The ethylene glycol was chosen because it has characteristics between the two others. Finally, n-Hexane is a totally

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wetting liquid because of its very low surface tension, the contact angle of which can be theoretically considered as null (θ = 0◦ ).

2.2. Methods

2.2.1. Fibre surface characterisation In order to evaluate the surface morphology of basalt fibres as a function of applied treatment, a Scanning Electron Microscope (SEM) was used (Jeol 6

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ρ (g/cm3 ) 0.998 3.325 1.113 0.659

γLp (mN/m) 51.0 2.3 19.0 0.0

γLd (mN/m) 21.8 48.5 29.0 18.4

γL (mN/m) 72.8 50.8 48.0 18.4

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Water Diiodomethane Ethylene Glycol n-Hexane

η (mP as) 1.00 2.76 21.81 0.32

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Table 1: Characteristics of test liquids at 20◦ C [33] [34].

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6500F). A gold metallization was realized to improve image quality. 2.2.2. Tensiometric characterisation of contact angles

Generally, a tensiometer measures the meniscus weight m formed by the liquid around fibres. Once the meniscus weight and the wetted length p are known, values of contact angles can usually be derived from the Wilhelmy

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relationship:

(1)

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Fc = m a = pγL cos θ

where Fc is the capillary force, a the acceleration due to gravity and θ the

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contact angle.

In the present study, a DCTA11 tensiometer with a resolution of 10−5 g

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was used to perform measurements of contact angles. The surface detection threshold was set to 5 · 10−5 g and a different number of fibres were clamped

to the tensiometer (2 fibres, 3 fibres or 4 fibres) to compensate for the very small mass change. To make this method accurate, fibres were spaced by, at least, the characteristic capillary length and immersed in the test liquid with a low speed of 0.01 mm/s down to a specific immersion depth (advancing 7

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phase) of 3 mm. Then fibres were maintained in this position for 60 sec-

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onds (static phase), allowing the measurement of a representative value of meniscus weight in static conditions. The time of 60 seconds was considered

sufficient for reaching static conditions due to the low speed of immersion [35].

The immersion depth was set as a compromise between sufficient length to

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ensure a representative value and a short enough length to have an accept-

able duration of experiments. Finally, fibres were withdrawn from the liquid

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at the same speed, up to the initial position (receding phase). One complete cycle in advancing, static and receding phase was performed for each test. The repeatability was also verified. An example of a complete cycle is shown

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on Fig.1 for a test on three BF with epoxy sizing in diiodomethane. .

Figure 1: Example of the three (advancing, static and receding) phases for a test on three BF with epoxy sizing in diiodomethane.

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For measurements of surface energies, the static capillary force should be

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considered, and then the average weight, measured over the static 60 seconds, was considered to calculate the relative angles through Wilhelmy relationship (Eq.1). Five tests were carried out with two, three and four basalt fibres of

each sampling (section 2.1) and for each type of liquid (section 2.2). In these

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tests, the nominal diameter was considered for wetted length calculation,

as no relevant variability was observed in fibre diameters. Following the

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Wilhelmy law (Eq.1), increasing the wetted length of the tested solid (by increasing the number of fibres) in the same liquid, should make the static capillary force vary linearly. Then, according to Wilhemy (Eq.1), the liquid surface tension and the static contact angle should not depend on the wetted length. By plotting the static capillary force Fc (mN ) against the wetted

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length p (m), it is possible to determine a representative value of contact angle

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for each test liquid [24] as the slope of the linear fit which is the product of the liquid surface tension and the cosine of static contact angle (Eq.1). The

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extreme theoretical case ’null length = null force’ has been considered for the fitting. This procedure was validated in previous works for commercial

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cellulose fibres, proving the efficiency for reliably determining representative surface energies [24]. 2.2.3. Characterisation of surface energy To measure dispersive and polar components of basalt fibre surface energy

γS , it is possible to use the Owens and Wendt (OW) equation (Eq.2), coupled

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with the contact angle defined by the Young-Laplace equilibrium (Eq.3) [29]:

γS + γL − γSL = 2(γSd γLd )0.5 + 2(γSp γLp )0.5 .

cos θe =

γS − γSL . γL

(2)

(3)

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With these equations it is possible to determine the dispersive and polar components of surface energy (γSd and γSp ) using at least two liquids, includ-

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ing a purely dispersive one [12] [36]. Using only two liquids results in an overestimation of the dispersive component [37]. Using more liquids should allow more accuracy in the determination of fibre surface energy [38]. Values of contact angles that are inserted in the OW method were measured in static conditions and derived from the method described in paragraph 2.2.2.

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Three liquids were used and the theoretical contact angle of 0◦ for n-Hexane

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was considered as fourth liquid. Owens and Wendt equation can then be

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rewritten as follows:

p p! q γ γSp p Ld + γSd γL | {z } X

q

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γL (1 + cos θe ) p = 2 γLd | {z } Y

(4)

Considering the left hand-side term as the Y ordinate, and the fraction

in brackets in the right hand-side term as the X abscissa, the slope and the

y-intercept of the linear fit are respectively the square roots of polar and dispersive components. 10

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3. Results and discussion

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3.1. Fibre surface analysis

Fig.2 shows SEM micrographs of the two types of basalt fibres. They were observed in the as-received state (1), showing that surface morphology

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is strongly affected by the sizing type, with significant lack of homogeneity for

the sizing optimized for thermoplastic matrices. After the thermal treatment

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(2) it seems that fibres have approximately the same surface morphology, as a consequence of the organic coating removal [25] [30]. From images of basalt fibres after acetone extraction (3) it is possible to note that acetone is not capable of removing the sizing for thermoplastic matrices; the surface state is not modified by chemical extraction. As regards the coating for thermosets,

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it is not possible, at this stage, to conclude about the effect of chemical

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extraction, because the fibre surface already in the as-received state proved to be homogeneous. Finally, acetone extraction in addition to the thermal

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treatment (4) seems to remove completely the sizing like in case (2) when only the thermal treatment was used.

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3.2. Determination of static contact angles In the following, the error bars in all plots of capillary static force against

wetted length represent the standard deviations over five experiments. 1) Contact angles measurements on basalt fibres in the as-received state

(Fig.3):

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Figure 2: SEM micrographs of basalt fibres with sizing agents for epoxy (left hand side column) and PP (right hand side column) after each treatment.

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Linear fits for each couple of basalt fibres and test liquids are well-achieved, with better agreement in the case of fibres with sizing for thermosets. This is likely to be due to the homogeneity of the sizing and thus the better reproducibility of fibre wetted length. Moreover, the slopes with water and with ethylene glycol are very different, while for diiodomethane they are comparable. This may indicate that dispersive functions in both sizings are 12

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quite similar while the difference is relevant in polar groups of sizing agents.

Figure 3: Measured static capillary force versus nominal wetted length as a function of basalt fibre type and test liquid in the as-received state.

2) Contact angles measurements on basalt fibres after thermal treatment

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(Fig.4):

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Slopes, characterising the contact angle, are of the same order of magnitude for each type of sizing. Moreover, slopes obtained with water show a higher

value than in case (1). This may suggest an increase of polar component, as

for glass [39], without sizing. It seems in agreement with what is stated in

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literature [30] about the effects of high temperature exposure able to remove the organic coating.

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3) Contact angles measurements on basalt fibres after chemical extraction with acetone (Fig.5):

It is possible to note quite good linear fits but slope values are significantly different which highlights the different role played by acetone for the two sizings. This behaviour can be better clarified by calculating the surface

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energy.

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4) Contact angles measurements on basalt fibres after both chemical extraction and thermal treatment (Fig.6):

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Values of slopes are quite similar, being very close to the ones obtained after thermal treatment alone (second sampling, Fig.4). These results seem to

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confirm the effectiveness of thermal treatment in removing the coating, thus significantly affecting the surface properties of basalt fibres. With linear regressions and slope values, static contact angles for each

type of basalt fibres were calculated for each sampling. They are summarized in Tables 2 and 3 for fibres compatible with thermoset and thermoplastic polymers, respectively. Values were given by the slope of linear fit. It is worth 14

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Figure 4: Measured static capillary force versus nominal wetted length as a function of basalt fibre type and test liquid after thermal treatment.

noting that standard deviation on angles is acceptable, and that values of angles with water, diiodomethane and ethylene glycol are very similar when

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Figure 5: Measured static capillary force versus nominal wetted length as a function of basalt fibre type and test liquid after extraction in acetone.

the thermal treatment is applied on each type of basalt fibre.

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Figure 6: Measured static capillary force versus nominal wetted length as a function of basalt fibre type and test liquid after extraction in acetone followed by thermal treatment.

3.3. Determination of fibre surface energy Figs. 7 and 8 show plots obtained with values of contact angles following the OW method for four liquids - paragraph 2.2.3 (n-Hexane representing a 17

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θ EG (◦ ) 20.3 ± 1.6 35.1 ± 4.8 45.8 ± 4.8 27.1 ± 5.1

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BF for epoxy θ W AT ER (◦ ) θ DIIODO (◦ ) As received 57.9 ± 1.9 49.9 ± 1.7 ◦ After T=400 C 23.5 ± 4.6 57.7 ± 0.6 After Acetone 69.9 ± 1.3 39.8 ± 2.6 After Acetone + T=400◦ C 27.5 ± 2.3 57.3 ± 2.6

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Table 2: Static contact angles derived from measurements on BF with sizing optimised for epoxy.

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BF for PP θ W AT ER (◦ ) θ DIIODO (◦ ) As received 67.2 ± 1.5 46.2 ± 2.9 After T=400◦ C 21.9 ± 4.9 54.6 ± 0.3 After Acetone 73.8 ± 0.8 52.7 ± 2.3 ◦ After Acetone + T=400 C 17.7 ± 3.7 58.3 ± 2.7

θ EG (◦ ) 59.6 ± 2.2 29.4 ± 1.9 53.2 ± 3.0 40.9 ± 2.8

Table 3: Static contact angles derived from measurements on BF with sizing optimised for PP.

theoretical point with θ = 0). All results show a good agreement with the

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OW equation (Eq. 4) with correlation coefficients higher than 0.9. Fig. 7

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shows that an excellent fit was achieved for basalt fibres with epoxy sizing. The fit with PP sizing remains good but is not as accurate as the previous

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one. This must be due to the inhomogeneity of sizing as observed with SEM (Fig. 1). This results in a larger scatter on surface energy values. It is

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also noticeable that, for each graph, measurements lowering the correlation coefficient are those made with ethylene glycol. This is due to its sensitivity to storage conditions and the theoretical values of surface tension components considered. For further studies, a characterisation of surface tension components of ethylene glycol before each measurement should increase the accuracy of surface energy component measurements [24]. 18

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Figure 7: Linear fits for surface energies of BF in the as-received state and after the thermal treatment.

Surface energies and their components calculated from the OW method

are summarized in Tables 4 and 5. Results show that the chemical treatment has a weak effect on fibre surface energy. Nevertheless, the chemical treatment tends to slightly lower surface energy by lowering the polar component. This could be due to the solvatation of physisorbed coupling agents. 19

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Figure 8: Linear fits for surface energies of BF after extraction in acetone and after extraction in acetone followed by thermal treatment.

It can have an interest since most matrices for composites impregnate fibres in a liquid state that exhibits a mostly dispersive behaviour. The chemical treatment can thus enhance the wettability of fibres by dispersive resins or thermoplastics. However, the chemical treatment can have a negative effect on fibres/matrix bonding. Since the total surface energy is lower, the 20

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bonding energy could be lowered too. Focusing on thermal treatment, it ap-

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pears that the chemical treatment has no further influence, indicating that the physisorbed coupling agents are also pyrolised by the recycling process simulation. The results obtained on both fibres are similar and confirm the

γSd (mN/m) γS (mN/m) 21.29 ± 2.30 44.83 ± 5.07 15.06 ± 6.12 61.42 ± 18.44 23.56 ± 5.10 36.96 ± 9.51 15.86 ± 4.75 60.65 ± 13.92

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BF for epoxy γSp (mN/m) As received 23.54 ± 2.77 After T=400◦ C 46.36 ± 12.32 After Acetone 13.40 ± 4.41 After Acetone + T=400◦ C 44.80 ± 9.17

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polar behaviour of basalt fibres.

Table 4: Dispersive and polar components and the total of BF surface energies for epoxy.

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γS (mN/m) BF for PP γSp (mN/m) γSd (mN/m) As received 14.88 ± 6.06 20.54 ± 6.20 35.42 ± 12.26 After T=400◦ C 46.81 ± 11.27 15.85 ± 5.72 62.66 ± 16.99 After Acetone 12.02 ± 2.71 21.11 ± 3.13 33.13 ± 5.84 ◦ After Acetone + T=400 C 48.45 ± 15.49 14.26 ± 7.33 62.71 ± 22.82

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Table 5: Dispersive and polar components and the total of BF surface energies for PP.

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4. Conclusions

The main focus of this study was to assess the effect of recycling by pyroly-

sis of basalt fibres on surface energy and its components. Two different types of sizing, one specific to epoxy and the other specific to polypropylene, have been studied and subjected to different chemical and thermal treatments.

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The surface state of basalt fibres has been analysed by SEM and non homo-

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geneities on the sizing for thermoplastics have been revealed. Inhomogeneity issues have also been highlighted measuring the surface energies of fibres with both sizings. Indeed, the scatter has been found significantly larger with the

polypropylene sizing. Chemical treatment with acetone has a weak effect on

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surface energies, proving that the physisorbed coupling agents have only an

effect on the polar components of basalt fibres. The thermal treatment has

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been proved to remove the sizing. The surface energy of raw basalt fibres is mostly polar, exhibiting a hydrophilic behaviour. The sizing has thus been designed to lower the polar component of basalt fibres to ensure porosities minimization during manufacturing. Both sizings are pyrolised during the recycling process, leaving raw basalt fibres with comparable surface energies,

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but also comparable ratios of surface energy components.

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