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Article

Surface Modification of Commingled Flax/PP and Flax/PLA Fibres by Silane or Atmospheric Argon Plasma Exposure to Improve Fibre–Matrix Adhesion in Composites

by
Wiwat Pornwannachai
1,2,
A. Richard Horrocks
1 and
Baljinder K. Kandola
1,*
1
Institute for Materials Research and Innovation, University of Bolton, Deane Road, Bolton BL3 5AB, UK
2
SCG Chemicals Co., Ltd., 1 Siam Cement Road, Bang Sue, Bangkok 10800, Thailand
*
Author to whom correspondence should be addressed.
Fibers 2022, 10(1), 2; https://doi.org/10.3390/fib10010002
Submission received: 8 December 2021 / Revised: 23 December 2021 / Accepted: 27 December 2021 / Published: 30 December 2021
(This article belongs to the Special Issue Sustainable High Performance Fiber-Reinforced Cementitious Composites (HPFRCCs) and Fiber-Reinforced Polymers (FRPs))
Browse Figures
Versions Notes

Abstract

:
Challenges faced by natural fibre-reinforced composites include poor compatibility between hydrophilic fibres such as flax and hydrophobic polymeric matrices such as polypropylene (PP) or poly(lactic acid) (PLA), and their inherent flammability. The former promotes weak interfacial adhesion between fibre and matrix, which may be further compromised by the addition of a flame retardant. This paper investigates the effect that the added flame retardant (FR), guanylurea methylphosphonate (GUP) and selected surface treatments of commingled flax and either PP or PLA fabrics have on the fibre/matrix interfacial cohesive forces in derived composites. Surface treatments included silanisation and atmospheric plasma flame exposure undertaken both individually and in sequence. 1-, 2- and 8-layered composite laminates were examined for their tensile, peeling and flexural properties, respectively, all of which yield measures of fibre-matrix cohesion. For FR-treated Flax/PP composites, maximum improvement was obtained with the combination of silane (using vinyltriethoxysilane) and plasma (150 W) treatments, with the highest peeling strength and flexural properties. However, for FR-treated Flax/PLA composites, maximum improvement in both properties occurred following 150 W plasma exposure only. The improvements in physical properties were matched by increased fibre-matrix adhesion as shown in SEM images of fractured laminates in which fibre-pullout had been eliminated.

1. Introduction

Natural fibre-reinforced composites are increasingly being used as alternative materials to conventional composites containing glass and carbon fibre reinforcements, for example, due to their renewable and environmentally sustainable properties. However, their uses are mainly limited to semi-structural applications because of their lower strength compared to carbon/glass fibres as well as poor compatibility between hydrophilic and polar natural fibres and hydrophobic non-polar polymer matrices, in particular polyolefins such as polypropylene (PP) and polyethylene (PE) [1,2,3]. This low compatibility can cause weak fibre/matrix interfacial adhesion within composites, which leads to a reduction in the stress transfer efficiency from the polymer matrix to reinforcing fibres, and hence lower mechanical properties than are often expected of natural fibre-reinforced composites [1,3]. In addition, moisture absorption due to the hydrophilicity of fibres can cause dimensional changes, which may further affect the interfacial adhesion [4]. The adhesion can be improved either by using chemical treatments to enhance interfacial, electrostatic or chemical bonding; or by physical means to improve mechanical interlocking between the fibre and matrix [5,6]. There are many reviews and publications in the open literature explaining different types of chemical- [4,6,7,8,9,10] and physical treatments [4,11,12] and characterization of interfacial properties [13]. Amongst chemical treatments, silane treatments have been seen to be most effective in improving compatibility between the two phases, i.e., reinforcing fibres and the polymer matrix [1,2,5]. This is due to the generic chemical structure of silanes with the bi-functional groups, which can form molecular bridges between the two phases. Silane coupling agents have the generic structure R(4-n)-Si-(R’X)n where R and R’ are alkoxy groups, X is an organofunctional group, and n = 1 or 2. The alkoxy group (R) can react with hydroxyl groups present in natural fibres, whereas the organofunctional group (X) may have an affinity to react with the polymer matrix, hence working as a coupling agent to improve the compatibility between the two phases [1,2,3]. The efficiency of silane treatments on natural fibre/polymer matrix depends on the selection of a suitable organofunctional group for each type of polymer matrix.
Physical treatments such as plasma, laser and heat exposure are usually used to etch the fibre surface to improve mechanical locking with the organic polymer matrix [11,14,15]. The efficiency of a plasma treatment generally depends on the type of gaseous environment within the plasma and the activation energy used for generating it [16]. Plasma treatments, however, are known to not only cause etching of the material for better physical adhesion but also can produce free radicals and functional groups on exposed surfaces, the nature of which depends on the material surface chemistry and plasma gases used [17], creating the possibility of chemical bonding between the two components of the composite. There is also possibility of combining plasma and chemical treatments either sequentially (plasma treatment followed by chemical treatment) or simultaneously, as shown by Seghini et al. where plasma/tetravinylsilane treatment has been used to coat flax fibres to improve its adhesion to vinyl ester resin [18].
In this study, different surface modification treatments, using silanes and argon-plasma, and the combination of both, have been used to improve fibre/matrix interfacial adhesion in Flax/PP and Flax/PLA composites. Composites were prepared by melt-pressing flame retarded woven fabrics from commingled Flax/PP) and Flax/PLA fibres, discussed in detail elsewhere [19]. Flame retardant treatments included different commercially available flame retardant formulations commonly used for cellulosic textiles and applied by the pad-dry technique. Of different flame retardant formulations studied initially, the organophosphate guanylurea methyl phosphonate, which showed the best performance was further optimised [20] and chosen for this study. Composites prepared from these fabrics were characterised for their improved mechanical performance by tensile, flexural and peeling tests.

2. Materials and Methods

2.1. Materials

2.1.1. Fabrics

Fabrics with 4 × 4 plain weave structures, produced using proprietary twistless technology incorporating zero-twist yarns, were supplied by Composites Evolution (UK):
  • Commingled Flax/PP woven fabric (50/50 wt-%); 465 g/m2 area density
  • Commingled Flax/PLA woven fabric (50/50 wt-%); 493 g/m2 area density

2.1.2. Flame Retardant (FR)

A commercially available, aqueous flame retardant formulation designed for cellulosic textiles, containing guanylurea methylphosphonate (GUP) as the active FR component, was used. Owing to the commercial sensitivity of the work, the source of FR formulations has not been provided and is identified only by the major active FR component, GUP (Scheme 1), with the representative structure below:

2.1.3. Silane Derivatives

Two types of silane (supplied by Sigma Aldrich, Gillingham, UK) were selected in this study based on the compatibility between their functional groups and polymer matrices. Vinyltriethoxysilane (VTS) with the addition of dicumyl peroxide (10:1 wt/wt) was chosen for Flax/PP, while 3-aminopropyl triethoxysilane (APTES) was for Flax/PLA composite.

2.2. Fabric Treatments

2.2.1. Flame Retardant Application

The Flax/PP and Flax/PLA fabrics were treated with the GUP solution using a typical pad/dry technique, and then dried in an oven at 80 °C. Based on the results in our previous work [20], the GUP contents on the fabrics were adjusted to 0.9%P (phosphorus) and 0.6%P nominal levels for Flax/PP and Flax/PLA, respectively, as these enabled the derived composites to achieve a V-0 rating in the UL-94 test. The GUP content was calculated from the total FR uptake (see Equation (1)).
F R   u p t a k e   o f   f a b r i c   ( % ) = ( W F R   f a b r i c − W U n t r e a t e d   f a b r i c ) W F R   f a b r i c × 100
where WUntreated fabric and WFR fabric are the weights of the fabrics before and after FR treatment, respectively.

2.2.2. Silane Treatment

The VTS and APTES solutions were prepared at different concentrations (1, 2, and 3 wt-%) by adding the required type of silane (VTS or APTES) to mixtures of water and ethanol (20/80 wt-%), and then stirring until completely dissolved. The Flax/PP and Flax/PLA fabrics were treated with these VTS and APTES solutions, respectively, by spraying them onto both sides of the fabrics, and then heating them to 80 °C in an oven for 12 h. The weights of fabrics before and after treatment were recorded, and the percent silane contents calculated (Equation (1)). The details of the treated fabrics are shown in Table 1.

2.2.3. Plasma Treatment

Flax/PP and Flax/PLA fabrics were exposed to argon (Ar) plasma flame at different plasma flame intensities (50, 100, and 150 W) under air atmosphere. The plasma flame unit has been described elsewhere [21] and consists of a plasma containment quartz tube, placed into a Surfatron microwave cavity. This is connected with a SAIREM microwave generator operating at 2.45 GHz with a maximum continuous power output of 300 W. The plasma flame was generated by feeding argon gas (99.99% purity) with a flow rate of 15 L/min into a fused quartz reactor chamber, and ignited by striking using a copper wire. The surfaces of Flax/PP and Flax/PLA fabrics on both sides were exposed to plasma flame at different plasma flame intensities (50, 100, and 150 W) under air atmosphere, and a scanning speed of 3 min/100 cm2, with an average 1.8 s exposure time per unit area. The distance between the nozzle of the plasma quartz tube and the fabrics was kept at 20 mm.

2.2.4. The Combination of Silane and Plasma Treatments for Flame Retarded Fabrics

The optimised condition, discussed in a later section (Section 3.1), using the combined silane and plasma treatment (i.e., 3 wt-% of silane solution and 150 W plasma flame) were used for treating the previously GUP-impregnated fabrics. The fabrics were firstly exposed on both sides to the scanning plasma flame, followed by immediate spraying with the silane solution. These treated samples were then exposed again to the plasma flame.

2.3. Composite Preparation

2.3.1. Single Layered Laminates for Identifying Optimised Conditions of Silane and Plasma Treatments

Single layered laminates of Flax/PP and Flax/PLA were prepared by melt-pressing single layers of non-flame retarded Flax/PP and Flax/PLA fabrics as controls and then respectively flame retarded, analogue precursor fabrics exposed either to the selected silane or plasma treatments at 180 °C and 40 kg/cm2 pressure for 3 min, and then transferred to a cold press to cool down to ambient under the same pressure.

2.3.2. Laminates for Fibre/Matrix Interfacial Adhesion Characterisation

Based on results discussed below, the optimised condition of 3 wt-% of silane solution and 150 W of plasma flame intensity were used for preparing FR-treated Flax/PP and Flax/PLA fabrics using different preparation methods as shown in Table 2. The flame retardant and nominal phosphorus and silicon levels of these treated fabrics are given in Table 3.
The two- and eight-layered laminates from the control and FR-treated Flax/PP and Flax/PLA fabrics were prepared by melt-pressing the required layers of each fabric at 180 °C and 40 kg/cm2 pressure for 3 min, and then transferred to a cold press to cool down under pressure (20 kg/cm2) to ambient. For two-layered laminates, the fabrics were partly wrapped with PTFE tape prior to laminate preparation in order to partly bond two layers together, and leave the un-bonded ends for gripping during the peeling test as shown in Figure 1a.

2.4. Surface Characterisation of Flax/PP and Flax/PLA Fabrics

The chemical characteristics of Flax/PP and Flax/PLA fabric surfaces before and after surface modification treatments were studied using Fourier transform infrared (FTIR) spectroscopy (Nicolet iS10 spectrophotometer, Thermo Scientific) in the attenuated total reflectance mode (ATR). The absorbed IR spectra of the samples were collected in the wavenumber range 600–4000 cm−1.
The physical characteristics of commingled Flax/PP and Flax/PLA fabrics before/after treating with silane and plasma were observed using scanning electron microscopy (SEM, Hitachi S-3400N) with a beam acceleration voltage of 5 kV. The fabrics were gold-coated using a Polaron Range SC7620 Sputter Coater prior to analysis.
The morphologies of the fractured surfaces of Flax/PP and Flax/PLA composite laminates after flexural testing were also studied by scanning electron microscopy as above.

2.5. Mechanical Characterisation of Composite Laminates

Single-layered laminates prepared from control and surface-modified (silane and plasma) Flax/PP and Flax/PLA fabrics, both unretarded and flame retarded, were tested for their tensile properties using an Instron 3369. The five replicate specimens of each sample were tested using a 100 N load cell, a crosshead speed of 1 mm/min and gauge length of 100 mm.
Peeling tests of two layered FR Flax/PP and Flax/PLA laminates were undertaken using the Instron 3369 with a 100 N load cell. The test was conducted by clamping each unbonded end of the specimen firmly in the grips of the Instron, with a grip separation rate of 50 mm/min (see Figure 1a). A typical peeling force (N) versus the distance of grip separation is shown in Figure 1b. From the curve, the average peeling strength was then calculated after disregarding the peeling force of first and last 25 mm of the grip separation distance.
Flexural testing of eight-layered FR Flax/PP and Flax/PLA laminates was undertaken in a three-points bending mode using the Instron 3369, fitted with a 50 kN load cell and at a compression speed of 1 mm/min with a support span length of 100 mm.

3. Results and Discussion

3.1. Optimisation of Silane and Plasma Treatment Conditions

Since tensile properties of composites are fibre reinforcement dependent, the fibre/matrix interfacial adhesion therefore plays an important role on the tensile properties. A greater interfacial adhesion provides better load-transfer between fibre and polymer matrix, and hence results in higher tensile properties of composites [22]. In order to identify the optimised condition of silane and plasma treatments, the tensile properties of single-layered Flax/PP and Flax/PLA laminates prepared from the non-flame retarded fabrics of Flax/PP and Flax/PLA, treated with silane, plasma and plasma–silane treatments using different conditions were evaluated.

3.1.1. Optimisation of Silane Treatment

The tensile properties of silane-treated Flax/PP and Flax/PLA single-layered laminates prepared from the fabrics treated with 1, 2 and 3 wt-% silane solutions are graphically presented as stress–strain curves in Figure 2a,c. In Table 4, the data interpreted from the curves, which are normalised to 50% fibre volume fraction, are reported.

Flax/PP Composites

As seen from Table 4, the normalised tensile properties (50% fibre vol. fraction) show that the tensile modulus of the control Flax/PP single-layered laminate is 3.3 GPa and tensile strength is 42 MPa. On applying 1 wt-% VTS solution (sample Si-Flax/PP (1%)), the modulus increased to 3.7 GPa, increasing further to 4.1 GPa and 4.6 GPa with 2 and 3 wt-% silane concentrations. This effect suggests an improvement in the fibre/matrix interfacial adhesion at each silane level with some effect also on strain-at-break, but only above 2 wt-% silane concentration. The tensile strength, however, remained unaffected within error. In Figure 3a, the IR spectra of flax, PP and Flax/PP fabrics are presented. The IR spectrum of Flax/PP fabrics shows the characteristic peaks of both flax and PP where the main peaks are at 3340 cm−1 (-OH stretching) corresponding to the reactive hydroxyl groups on the surface of flax fibres, and the superimposed IR absorption peaks between 2840–2950 cm−1 (-CH stretching) corresponding to -CH2- and -CH3 of PP [23,24]. With the VTS treatment, the IR spectrum of Flax/PP is slightly changed (Figure 3b) shown by the apparent reduction in the intensity of the 3340 cm−1 (-OH stretching) peak, and the appearance of the peaks at 920 and 970 cm−1 (C-H bending and -CH=CH2 wagging vibrations, respectively) corresponding to the vinyl functional group of VTS [24]. The apparent C–OH peak reduction is a possible consequence VTS hydrolysis, to form reactive silanol groups, Scheme 2a, which can chemically react with cellulosic hydroxyl groups on the surface of flax fibres under heating to form covalent bonds between VTS and flax fibres as shown in Scheme 2b,c [1].
The compatibility between flax and PP of VTS treated Flax/PP is therefore improved, as compared to the untreated one. Moreover, with the addition of dicumyl peroxide (DCP) in the VTS solution, the vinyl groups on VTS treated flax could also chemically react with PP as the DCP could work as an initiator to create free radicals by abstracting hydrogen atoms from the polymer backbone of PP as well as breaking the double bond of the vinyl groups on the surface of VTS-treated flax [25,26,27,28]. These therefore lead to possible chemical bonding between two radicals, and formation of a siloxane bridge between flax and PP as shown in Scheme 3. As a result of the improvement of the compatibility and the formation of siloxane bridge between flax and PP matrix of VTS-treated Flax/PP, the fibre/matrix interfacial adhesion has been enhanced, which has resulted in the increase in tensile properties of the VTS treated Flax/PP laminates, as compared to the control, Table 4.

Flax/PLA Composites

The silane treatment showed the opposite effect on Flax/PLA properties for silane-treated Flax/PP laminates. As can be seen from Table 4, the tensile modulus of Flax/PLA composites decreased from 7.7 GPa to 4.8 GPa when treated with 1 wt-% concentration of APTES solution. This reduction increased at higher concentrations, with values reducing further to 3.9 GPa and 3.1 GPa for Si-Flax/PLA (2%) and Si-Flax/PLA (3%), laminates respectively. Similar to the silane-treated Flax/PP, the effect of APTES on tensile properties of Flax/PLA single-layered laminates can be explained by the changes in surface chemistry of Flax/PLA fabrics after treatment. The IR spectra of flax fabric, PLA and Flax/PLA commingled fabrics are shown in Figure 4a. The IR spectrum of Flax/PLA shows the combination of peaks of flax, at 3340 cm−1 (-OH stretching) corresponding to hydroxyl groups present in both flax and PLA, and at 1749 cm−1 (C=O stretching) for the ester groups (-COO-) present in PLA.
With the APTES silane treatment on Flax/PLA, the intensity of the hydroxyl characteristic peak (3340 cm−1) is reduced, and a new peak at 1559 cm−1 due to N-H bending has appeared (Figure 4b). This may be explained by reaction between the APTES and the reactive hydroxyl groups on surfaces of flax fibres [29]. APTES can undergo hydrolysis and form the silanol groups to react with hydroxyl groups on the surfaces of flax fibres, thereby leading to the reduction in the intensity of the peak at 3340 cm−1 (-OH stretching). The presence of the 1559 cm−1 peak (N-H bending) corresponds to amine groups present in APTES. These changes would be expected to improve the fibre/matrix interfacial adhesion of Flax/PLA composites by hydrogen bonding between amine groups (-NH2) of the APTES-treated flax and the ester groups (-COO-) of PLA. However, this was not the case as the tensile results in Table 4 showed that with the use of APTES these properties of Flax/PLA laminates decreased. The observation that APTES has apparently decreased fibre/matrix interfacial adhesion is similar to that reported in the literature where presence of APTES decreased the interfacial adhesion between glass fibre (hydroxyl-rich surfaces) and a hydrophilic polymer [29,30]. This was explained by the fact that not only could the silanol groups of APTES interact with hydroxyl groups on surfaces of the glass fibres, but also with the amine functionality (-NH2) present, which can lead to a formation of hydrogen-bonding with hydroxyl groups of glass fibres [30]. Since the surface characteristics of glass and flax fibres are similar in terms of being hydroxyl-rich, the mechanism of APTES on flax fibres would be expected to be similar to that of glass fibres. This therefore leads to a proposed modification of APTES-treated flax fibres where the silanol and amine groups of APTES both interact with the reactive hydroxyl groups of flax fibres, thereby leaving the propylene sequence of APTES exposed at the surfaces of APTES-treated flax, Scheme 4. This leads to an increase in the hydrophobic character of the APTES-treated flax fibres in comparison with the control [29]. As a result of this increase in hydrophobicity, the compatibility toward PLA matrix of APTES results in a reduction in the fibre/matrix interfacial adhesion of the APTES-treated Flax/PLA compared to the untreated analogue.

3.1.2. Optimisation of Plasma Treatment

Since the efficiency of a plasma treatment depends on the plasma power [16], in order to find the suitable plasma power, an atmospheric argon-plasma was used at 50, 100, and 150 W to treat Flax/PP and Flax/PLA fabrics prior to single-layered laminate preparation. The tensile properties of the single-layered Flax/PP and Flax/PLA laminates from the plasma-treated fabrics were then evaluated to identify the optimised plasma condition. The stress-strain curves are presented in Figure 2b,d, and the results interpreted from the curves are given in Table 4.

Flax/PP Composites

The tensile moduli and strengths of Flax/PP single layered laminates increased following plasma pretreatment (Table 4) with respective values with a 150 W plasma flame intensity being the highest of all.
To understand the mechanism of action of plasma on the improvement in tensile properties of Flax/PP laminates, the surface characteristic of Flax/PP fabric before and after plasma treatment characterised by IR are given in Figure 5. Relative to the control, spectra of the plasma treated Flax/PP fabrics show negligible changes in surface chemistry after exposure to 150 W plasma power apart from small apparent differences in the relative peak height at 1720 cm−1 with respect to the adjacent 1650 cm−1 band. While in general, plasma exposure may create reactive species and/or functional groups on material surfaces [31], flax fibre surfaces have high concentrations of hydroxyl groups and small changes in IR absorption of these as a consequence of plasma exposure and subsequent air oxidation would be difficult to observe unless simultaneous oxidative conversion to carbonyl groups (with peaks in the region of 1720 cm−1) was occurring. As stated above there is a suggestion of a slight increase in intensity of this peak in the Figure 5b spectrum. Polypropylene, on the other hand, is devoid of any polar groups and so presence of oxidised species would be more easily observable, again as carbonyl groups [32], although any oxidation would most likely be a consequence of air oxidation following argon plasma flame activation by a so-called Bolland and Gee type reaction [33]. Within the error of the FTIR spectra, as stated above the minimal increased carbonyl presence suggests either or both carbonyl group-forming mechanisms occurring albeit a low levels.
Recent work on both cotton and polyester fibres exposed to the same 100% argon plasma torch conditions has shown that free radicals are produced on both fabrics and the wettability of the latter is increased [34]. This suggests that the plasma is indeed powerful enough to activate both cellulosic and PP components, which may then interact and especially in the presence of oxygen to provide the source of the observed increases in tensile properties.
The surface morphologies of untreated and plasma-treated Flax/PP fabrics were also observed by scanning electron microscopy (SEM) and micrographs of the fabrics are given in Figure 6. The SEM image of Flax/PP fabric shows the surfaces of both flax and PP fibres where the latter have smoother surfaces and more uniform diameters in comparison to flax, Figure 6a. After the plasma treatment, the micrographs show obvious changes in the flax fibre surface morphology as can be seen from Figure 6b, which appear rougher than observed in the untreated Flax/PP fabric, while the PP surfaces though ablated, are less affected. This could be explained by the etching mechanism of plasma treatment where the ions produced by the argon plasma flame can bombard and ablate surface regions, especially of the outer layers on the flax fibre bundles [16,35,36]. This suggests that rather than arising from chemical changes on the surface, the improvement in tensile properties of plasma treated Flax/PP in comparison to the untreated one may have a contribution from fibre surface roughening, which increases mechanical interlocking of components within the heat-bonded, flax fibre–polymer matrix [35,36].

Flax/PLA Composites

As can be seen from the tensile results in Table 4, the plasma treatment improved the fibre/matrix interfacial adhesion of Flax/PLA shown by the increase in tensile moduli and strengths of plasma-treated Flax/PLA composites in comparison with the control. While plasma treatment at 50 W plasma flame intensity produced only slight increases in modulus, a significant increase in strength and strain-at-break occurred indicating a sizeable increase in laminate toughness. The improvements in strength and modulus increased further the higher the plasma flame intensity, with 150 W plasma flame giving rise to 18% and 67% respective increases. On comparing the IR spectra of untreated and plasma treated Flax/PLA fabrics, and as with Flax/PP fabrics, similar peaks are observed in both untreated and plasma treated samples, see Figure S1. However, there is evidence of the increase in the functionalities of PLA after plasma treatment published in the literature where X-ray photoelectron spectroscopy (XPS) has been mainly used for characterisation of functional groups [37,38]. Jorda-Vilaplana et al. have reported that after air atmospheric plasma treatment, the functionalities on the surface of PLA were increased due to the formation of peroxide (C-O-O-H), ether (C-O-C), and carbonyl (C=O) functional groups [37]. However, the additional band frequencies of these are not evident in the plasma-exposed spectrum in Figure S1, but given the arguments above for the Flax/PP fabrics, the potential for radical formation on either component and the presence of reactive –OH groups in both fibres present provide a strong likelihood of inter-surface reaction and even cross-linking, thus explaining the significant increase in tensile properties of the derived laminate.
The SEM micrographs of untreated and plasma treated Flax/PLA fabrics are presented in Figure 7, which show that plasma treatment has again caused roughening of the surface of flax as noted previously in Figure 6, whereas PLA fibres seem to be less affected. This indicates that improvement of the fibre/matrix interfacial adhesion in Flax/PLA by increased physical interlocking between fibre and matrix surfaces may be less of a factor than in the Flax/PP laminates where evidence of PP fibre surface ablation is present.

3.2. Fibre/Matrix Interfacial Adhesion in FR-Treated Flax/PP and Flax/PLA Laminates

From the results discussed above, it could be seen that except with the use of silane treatment in Flax/PLA, both treatments improve the fibre/matrix interfacial adhesion in Flax/PP and Flax/PLA composites. The efficiency of these treatments was greater with the use of high concentrations of silane solution and high plasma powers. Therefore, the optimised conditions of 3 wt-% silane solution and 150 W intensity of plasma flame for silane and plasma treatments, respectively, were chosen for preparing FR-treated Flax/PP and flax/PLA laminates in order to study their effects on fibre/matrix interfacial adhesion. Although the silane (APTES) treatment of Flax/PLA did not improve the fibre/matrix interfacial adhesion of the non-flame retarded Flax/PLA, it was decided to include it in the combination in order to study whether it would show the same effect for flame-retarded samples. Fibre/matrix interfacial adhesion was investigated using peeling and flexural bending tests and both two- and eight- layered, FR-treated Flax/PP and Flax/PLA laminates were prepared from the respective precursor fabrics treated with different surface modification treatments as given in Table 2 with details of flame retardant (GUP) contents given in Table 3.

3.2.1. Fibre/Matrix Interfacial Adhesion by Peel Strength Testing

The peeling results evaluated from the two-layered composite laminates of control and FR treated Flax/PP and Flax/PLA are reported in Table 5.

Flax/PP Composites

The results show that while the peeling strength of Flax/PP laminate control was 35 N, in the presence of guanylurea methylphosphonate (GUP) flame retardant alone at 10 wt-% in the Flax/PP laminate, the strength is reduced to 19 N, i.e., 46% reduction in comparison to the control. This shows that GUP interferes with the fibre/matrix adhesion of Flax/PP as it is absorbed on the surfaces of flax fibres. The IR spectra of untreated and FR-treated Flax/PP fabrics presented in Figure 8 show that with the GUP flame retardant presence, the IR peaks relative to the Flax/PP control were significantly changed, with an increase in the intensity of the hydroxyl characteristic peak at 3340 cm−1 (-OH stretching) relative to the overall C-H stretch peak intensity in the 2900–3000 cm−1 region, and the appearance of peaks at 1700 cm−1 (C=O stretching), 1600 cm−1 (-NH2 stretching) and 1312 cm−1 (P=O stretching) [23]. These changes are related to presence of GUP flame retardant on the surfaces of flax fibres in the FR-treated Flax/PP, which has characteristic peaks at 3340 cm−1 (-OH stretching) and 1312 cm−1 (P=O stretching) corresponding to the methylphosphonate moiety (Scheme 1) and 1700 cm−1 (C=O stretching) and a peak at 1600 cm−1 (-NH2 stretching) corresponding to the guanylurea component, see Figure 8b. The presence of these polar groups in GUP will reduce the compatibility between GUP-treated flax and PP as shown by the reduction in peeling strength compared to the untreated one, Table 5.
The use of additional surface modification treatments (silane treatment (Si), plasma treatment (Pm), and the combination of both) to prepare FR (12.2 wt-%) Flax/PP laminates, also resulted in reductions in the peeling strength of FR-treated Flax/PP laminates relative to the control, although to a degree less than that of the non-surface-modified samples, Table 5. Within error it would seem that this reduction is independent of whether the VTS silane or plasma treatments are undertaken alone or in combination. However, that all three sample treatment processes showed similar increases compared to the FR-Flax/PP sample indicates that the immediate loss of interfacial adhesion due to the presence of GUP had been partially offset by each treatment. As can be seen from Table 3, plasma treatment of FR-Flax/PP prior to silane treatment increased silane content (1.3%) in FR-Pm-Si-Flax/PP sample compared to silane-treated only (sample FR-Si_Flax/PP, silane content 0.6%). The IR spectra of these two samples in Figure S2 though do not show any obvious differences in the intensities of peaks representing hydroxyl (3340 cm−1, -OH stretching) and vinyl groups (970 cm−1, CH=CH2 wagging). There is also a possibility that since GUP is not a durable finish, some surface GUP has been removed or redistributed both during silanisation and plasma exposure. This would also explain why the peel strengths had increased slightly.

Flax/PLA Composites

The peeling results in Table 5 show that the Flax/PLA control with a peeling strength of 50 N is much higher than that of Flax/PP (35 N). This is due to the chemical structure of PLA, which contains ester groups (-COO-) in its polymer backbone and which may form hydrogen bonds with hydroxyl groups on the surface of flax fibres, hence yielding higher fibre/matrix interfacial adhesion properties compared to PP matrix which is non-polar. The presence of GUP flame retardant treatment has reduced the peel strength by about 30% to 35 N (FR-Flax/PLA) in a manner similar to that observed for the flax/PP samples. Again, this can be explained by the interaction between flax fibre and GUP flame retardant in the FR-treated fabric thereby reducing flax-PLA hydrogen bond efficiency. As with the flax/PP fabric, addition of GUP gives rise to additional peaks at 3340 cm−1 (-OH stretching), 1700 cm−1 (C=O stretching), 1600 cm−1 (-NH2 stretching) and 1312 cm−1 (P=O stretching) (see Figure S3). The effect of surface modification treatments following flame retardant application on the peeling results in Table 5 shows that APTES silane treatment did not show a clear significant effect on the fibre/matrix interfacial adhesion with respect to the FR-flax/PLA control, although plasma treatment increased it by about 10%. This lack of improvement in peel strength following APTES treatment compares with the absence of any positive effect on tensile properties of Si-Flax/PLA laminate samples (Table 4) However, the increase in peel strength following plasma treatment reflects the improvement in tensile properties of Pm-Flax/PLA laminate samples, suggesting that the fibre/matrix interfacial adhesion within the FR-Flax/PLA matrix was improved, albeit to a smaller extent as a consequence of the presence of GUP. With regard to the combined silane/plasma treatment it would seem that the positive effect of the plasma more than offsets the negligible effect of silane treatment.

3.2.2. Fibre/Matrix Interfacial Adhesion by Flexural Testing

While the flexural properties of composites are mainly matrix-dependent, they are also affected by the interfacial adhesion between fibre and polymeric matrix [19,20,39]. Hence, the fibre/matrix interfacial adhesion of eight-layered laminates from the FR-Flax/PP and FR-Flax/PLA fabrics treated with different surface modifications was also investigated by flexural testing. Owing to different FR/silane contents of the samples (see Table 3), the fibre volume contents were also different, hence flexural modulus and strength of these laminates were normalised with respect to 50% fibre volume fraction) and results are given in Table 6. As can be seen, the flax/PLA laminate control has a higher modulus than the Flax/PP analogue, which is due to the higher modulus of PLA (1.8 GPa) than PP (0.6 GPa) as discussed in detail elsewhere [19,20,40]. The addition of FR increases flexural modulus of the Flax/PP laminate while in Flax/PLA there is no effect. This trend is opposite to that of the peeling test results where FR treatment decreased the peeling strength of both flax/PP and flax/PLA. In the peeling test, the fibre–matrix adhesion is the dominant factor, whereas flexural properties are mainly related to the overall matrix performance. PP is non-reactive towards GUP, hence is not expected to be affected; however, GUP aggregates will be present on PP matrix at the Flax-PP interfaces, which will act as reinforcement and hence increased modulus of the laminate. In addition, the SEM fracture images in Figure 9a for Flax/PP show some degree of fibrillation at the PP fracture zones, whereas those in Figure 9b for FR-Flax/PP show evidence of more brittle fracture as a consequence of GUP presence. However, in FR-Flax/PLA such a stiffening effect of the FR could be compensated by the chemical reactivity of the ester groups of PLA with the FR [19,20].

Flax/PP Composites

The effects of surface modification treatments on the flexural properties of the FR-treated flax/PP laminates are similar to those of the peeling test results in Table 5 in that flexural modulus in particular of FR-treated Flax/PP laminates was improved with the use of surface modification treatments relative to the FR-Flax/PP control. While both VTS silane and plasma alone increased the modulus, the combination of the two showed a further increase to 14.9 GPa, which was the highest compared to other FR-treated flax/PP samples in Table 6.
Fractured regions of the FR-treated flax/PP laminates after flexural testing were observed using scanning electron microscopy (SEM). Figure 9a,b show the fractured regions of control and FR- treated flax/PP laminates without any surface modification. These suggest that flax fibres have been pulled-out from the polymer matrix before breaking, indicating the weak interfacial adhesion between fibre and polymer matrix. However, with the use of surface treatments, the improvement in fibre/matrix interfacial adhesion is clearly observed in that the fibre cross-sections of FR-Si-Flax/PP, FR-Pm-Flax/PP and FR-Pm-Si-Flax/PP laminates in Figure 9c–e respectively show more evident fibre fracture zones coincident with the fractured surface zones of the PP matrix [41,42].

Flax/PLA Composites

The flexural results for FR-treated Flax/PLA composites in Table 6 show a similar trend to those observed in peeling tests in Table 5 in terms of the general incremental changes following each subsequent surface treatment. However, while APTES silane treatment alone did not significantly affect both flexural properties, plasma treatment did significantly improve them, in particular flexural modulus. This 30% flexural modulus increase to 16.0 GPa was sustained following the combined treatment, again reflecting peeling test results for the FR-Pm-Si-Flax/PLA sample. However, in terms of flexural strength, the silane component of the combined treatment slightly offset the advantageous effect of plasma treatment alone in that a reduction to 110 MPa (FR-Pm-Si-Flax/PLA) from the value of 125 MPa was observed for the FR-Pm-Flax/PLA sample. These results are also supported by the SEM micrographs of the fractured regions shown in Figure 10, where the APTES treatment alone did not reduce the fibre-pull out in FR-Si-Flax/PLA (Figure 10c) compared to Flax/PLA and FR-Flax/PLA samples (Figure 10a,b), indicating no effect on fibre/matrix interfacial adhesion. In contrast, with the plasma treatment the fibre/matrix interfacial adhesion of FR-Pm-Flax/PLA and FR-Pm-Si-Flax/PLA samples was significantly improved with clear, coincidental breaking positions of both flax fibres and PLA matrix seen in Figure 10d and to a less extent in Figure 10e. The latter reflects the negative effect of additional silane treatment, as supported by the reduced modulus. Thus it may be concluded that to achieve the maximum improvement of fibre/matrix interfacial adhesion of FR-Flax/PLA in composites, only plasma treatment is required.

4. Conclusions

This work has demonstrated that the weak interfacial adhesion between a natural fibre (flax) and a thermoplastic matrix (PP or PLA) is further reduced by pretreatment of the commingled precursor fabric with a flame retardant, as observed by 30–46% reductions in peeling strength of the derived composites compared to the composites without FR treatment. Different surface modification treatments (silane, argon plasma and the combination of both) on unmodified or FR-treated flax/PP and Flax/PLA fabrics demonstrated different levels of improvement in the fibre/matrix interfacial adhesion in respective FR-Flax/polymer composite laminates. Silanisation with VTS alone improved the tensile modulus of flax/PP laminates by 40% with 3wt% silane treatment, while the converse using APTES was observed in flax/PLA laminates (65% reduction with 3 wt% silane). The former effect was proposed to be associated with radical-initiated cross-linking via the vinyl groups present in the PP while the latter was a consequence of possible reaction between silanol and -NH2 groups in APTES with flax -OH groups thereby reducing flax-PLA hydrogen bonding. For Flax/PP the effect is more than reported for other treatments, such as by using maleic anhydride grafted PP (MaPP), where with 3wt% MaPP, 15% increase in tensile performance has been reported by Keener et al. [43]. Maleic anhydride has also been reported to be effective in Flax/PLA as well, where with 20 wt% MaPLA, 5% increase in tensile strength was observed [44].
Plasma pretreatment of commingled precursor fabrics increased both tensile modulus (115% increase in Flax/PP; 18% in Flax/PLA) and strength (100% increase in Flax/PP; 67% in Flax/PLA) of derived laminates, most likely as a consequence of radically-induced cross-linking at the respective flax–polymer interfaces as well as increased surface physical interlocking as a consequence of surface roughening. However, when the fabric precursor fabrics were pre-impregnated with the flame retardant, GUP, two-layered laminate peel strengths reduced (46% in Flax/PP; 30% in Flax/PLA) while eight-layered composite flexural moduli increased for flax/PP and were little affected for Flax/PLA composites. These effects suggested that the presence of the FR was interfering with the efficiency of interfacial bonding.
Silanisation and plasma pretreatment either alone or in sequence had significant effects in partly restoring peel strengths and flexural moduli of FR-Flax/PP composites. The maximum improvement was obtained with the combination of silane and plasma treatment as shown by the highest peeling strength (31% increase) and flexural properties (38% increased flexural modulus) in FR-Pm-Si-Flax/PP compared to other FR treated flax/PP samples.
For FR-treated flax/PLA, the plasma treatment improved the fibre/matrix interfacial adhesion (30% increased flexural modulus), whereas silane treatment showed no effect. Therefore, to obtain the maximum improvement in fibre/matrix interfacial adhesion of FR-treated Flax/PLA laminates only plasma treatment is required.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/fib10010002/s1, Figure S1: IR spectra of (a) Flax/PLA and (b) Pm-Flax/PLA (150 W) fabrics. Figure S2: IR spectra of (a) FR-Si-Flax/PP and (b) FR-Pm-Si-Flax/PP (150 W) fabrics. Figure S3: IR spectra of (a) Flax/PLA and (b) FR-Flax/PLA fabrics.

Author Contributions

Conceptualization, B.K.K. and A.R.H.; methodology, W.P.; formal analysis, W.P.; investigation, W.P.; resources, B.K.K.; data curation, W.P. and B.K.K.; writing—original draft preparation, W.P.; writing—review and editing, B.K.K. and A.R.H.; visualization, B.K.K.; supervision, B.K.K. and A.R.H.; project administration, B.K.K.; funding acquisition, B.K.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Innovate UK (formerly Technology Strategy Board), grant number TSB Project No 100982.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

Thanks to NetComposites Ltd., IFG Drake Ltd., Tilsatec Ltd., Sam Weller & Sons Ltd. and Exel Composites UK, for providing samples and technical support.

Conflicts of Interest

The authors declare no conflict of interest.

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Fibers 10 00002 sch001 550
Scheme 1. The chemical structure of guanylurea methylphosphonate (GUP).
Scheme 1. The chemical structure of guanylurea methylphosphonate (GUP).
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Figure 1. (a) Peeling test specimen geometry; and (b) the peeling test result of the two layered Flax/PP laminate.
Figure 1. (a) Peeling test specimen geometry; and (b) the peeling test result of the two layered Flax/PP laminate.
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Figure 2. Stress–strain curves of (a,b) Flax/PP and (c,d) Flax/PLA single layered laminates from the fabrics treated with (a,c) silane at different concentrations and (b,d) atmospheric plasma with different plasma flame intensities.
Figure 2. Stress–strain curves of (a,b) Flax/PP and (c,d) Flax/PLA single layered laminates from the fabrics treated with (a,c) silane at different concentrations and (b,d) atmospheric plasma with different plasma flame intensities.
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Figure 3. IR spectra of (a) (i) flax fibres, (ii) PP fibres and (iii) commingled Flax/PP woven fabric; and (b) (i) untreated and (ii) VTS-treated Flax/PP fabrics (3 wt-% VTS Sol.).
Figure 3. IR spectra of (a) (i) flax fibres, (ii) PP fibres and (iii) commingled Flax/PP woven fabric; and (b) (i) untreated and (ii) VTS-treated Flax/PP fabrics (3 wt-% VTS Sol.).
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Scheme 2. The probable interactions between VTS silane and flax fibres in VTS treated Flax/PP fabric.
Scheme 2. The probable interactions between VTS silane and flax fibres in VTS treated Flax/PP fabric.
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Scheme 3. The formation of siloxane bridge between VTS treated flax and PP matrix.
Scheme 3. The formation of siloxane bridge between VTS treated flax and PP matrix.
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Figure 4. IR spectra of (a) (i) flax fibres, (ii) PLA, and (iii) commingled Flax/PLA woven fabric; and (b) (i) untreated and (ii): APTES-treated Flax/PLA fabrics (3 wt-% APTES Sol.).
Figure 4. IR spectra of (a) (i) flax fibres, (ii) PLA, and (iii) commingled Flax/PLA woven fabric; and (b) (i) untreated and (ii): APTES-treated Flax/PLA fabrics (3 wt-% APTES Sol.).
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Scheme 4. The proposed configuration of APTES-treated flax fibres.
Scheme 4. The proposed configuration of APTES-treated flax fibres.
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Figure 5. IR spectra of (a) Flax/PP and (b) Pm-Flax/PP (150 W) fabrics.
Figure 5. IR spectra of (a) Flax/PP and (b) Pm-Flax/PP (150 W) fabrics.
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Figure 6. SEM images of surface morphology of (a) Flax/PP, (b) Pm-Flax/PP (150 W) fabrics.
Figure 6. SEM images of surface morphology of (a) Flax/PP, (b) Pm-Flax/PP (150 W) fabrics.
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Figure 7. SEM images of surface morphology of (a) Flax/PLA, (b) Pm-Flax/PLA (150 W) fabrics.
Figure 7. SEM images of surface morphology of (a) Flax/PLA, (b) Pm-Flax/PLA (150 W) fabrics.
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Figure 8. IR spectra of (a) Flax/PP and (b) FR-Flax/PP fabrics.
Figure 8. IR spectra of (a) Flax/PP and (b) FR-Flax/PP fabrics.
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Figure 9. SEM images of the fractured regions of (a) Flax/PP, (b) FR-Flax-PP, (c) FR-Si-Flax/PP, (d) FR-Pm-Flax/PP, and (e) FR-Pm-Si-Flax/PP 8-layered laminates.
Figure 9. SEM images of the fractured regions of (a) Flax/PP, (b) FR-Flax-PP, (c) FR-Si-Flax/PP, (d) FR-Pm-Flax/PP, and (e) FR-Pm-Si-Flax/PP 8-layered laminates.
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Figure 10. SEM images of the fracture surfaces of (a) Flax/PLA, (b) FR-Flax-PLA, (c) FR-Si-Flax/PLA, (d) FR-Pm-Flax/PLA, and (e) FR-Pm-Si-Flax/PLA 8-layered laminates.
Figure 10. SEM images of the fracture surfaces of (a) Flax/PLA, (b) FR-Flax-PLA, (c) FR-Si-Flax/PLA, (d) FR-Pm-Flax/PLA, and (e) FR-Pm-Si-Flax/PLA 8-layered laminates.
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Table
Table 1. Details of Flax/PP and Flax/PLA fabrics treated with different concentrations of each silane solution.
Table 1. Details of Flax/PP and Flax/PLA fabrics treated with different concentrations of each silane solution.
SampleSilane TreatmentSilane Content on Fabrics (%)
Si-Flax/PP (1%)1 wt-% VTS solution0.6 ± 0.1
Si-Flax/PP (2%)2 wt-% VTS solution2.7 ± 0.1
Si-Flax/PP (3%)3 wt-% VTS solution4.5 ± 0.1
Si-Flax/PLA (1%)1 wt-% APTES solution1.2 ± 0.1
Si-Flax/PLA (2%)2 wt-% APTES solution2.7 ± 0.3
Si-Flax/PLA (3%)3 wt-% APTES solution4.2 ± 0.1
Table
Table 2. Sample preparation methods of FR treated Flax/PP and Flax/PLA fabrics.
Table 2. Sample preparation methods of FR treated Flax/PP and Flax/PLA fabrics.
MethodThe Order of Fabric Treatment
1234
FRFR solution (Pad/Dry)
FR-SiFR solution (Pad/Dry)Silane (Spray)
FR-PmPlasmaFR solution (Pad/Dry)Plasma
FR-Pm-SiPlasmaFR solution (Pad/Dry)PlasmaSilane (Spray)
Table
Table 3. Flame retardant Flax/PP and Flax/PLA laminates treated with different surface modification treatments.
Table 3. Flame retardant Flax/PP and Flax/PLA laminates treated with different surface modification treatments.
Fabric SampleFR Content (%)P Content (%) *Silane Content (%) *
FR-Flax/PP10.91.0-
FR-Si-Flax/PP10.91.00.6
FR-Pm_Flax/PP12.21.1-
FR-Pm-Si_Flax/PP12.21.11.3
FR-Flax/PLA7.20.6-
FR-Si-Flax/PLA7.20.62.1
FR-Pm-Flax/PLA9.40.8-
FR-Pm-Si-Flax/PLA9.40.83.2
Note: * Nominal values calculated from flame retardant add-on or content levels.
Table
Table 4. Tensile properties of single-layered laminates from non-FR fabrics pretreated with either silane or plasma.
Table 4. Tensile properties of single-layered laminates from non-FR fabrics pretreated with either silane or plasma.
SampleTensile Modulus
(GPa)		Tensile Strength
(MPa)		Strain-at-Break
(%)
Flax/PP3.3 ± 0.442 ± 53.2 ± 0.7
Flax/PLA7.7 ± 0.761 ± 62.0 ± 0.9
Silane pretreatment
Si-Flax/PP (1%)3.7 ± 0.140 ± 13.1 ± 0.1
Si-Flax/PP (2%)4.1 ± 0.244 ± 43.1 ± 0.4
Si-Flax/PP (3%)4.6 ± 0.541 ± 22.0 ± 0.4
Si-Flax/PLA (1%)4.8 ± 0.936 ± 71.4 ± 0.4
Si-Flax/PLA (2%)3.9 ± 0.421 ± 40.9 ± 0.3
Si-Flax/PLA (3%)3.1 ± 0.517 ± 51.0 ± 0.4
Plasma pretreatment
Pm-Flax/PP (50 W)4.6 ± 0.360 ± 61.8 ± 0.3
Pm-Flax/PP (100 W)5.0 ± 0.168 ± 62.0 ± 0.3
Pm-Flax/PP (150 W)7.1 ± 0.184 ± 21.7 ± 0.1
Pm-Flax/PLA (50 W)7.8 ± 0.985 ± 143.5 ± 0.3
Pm-Flax/PLA (100 W)8.6 ± 0.691 ± 73.1 ± 0.8
Pm-Flax/PLA (150 W)9.1 ± 0.5102 ± 53.3 ± 0.7
Note: Tensile modulus and strength of the composites were normalised to 50% fibre volume fraction in all samples.
Table
Table 5. Peeling test results of FR treated flax/PP and flax/PLA laminates.
Table 5. Peeling test results of FR treated flax/PP and flax/PLA laminates.
SamplePeeling Strength% Change in Peeling Strength
(N)(w.r.t. Flax/Polym)(w.r.t. FR-Flax/Polym)
Flax/PP35 ± 10--
FR-Flax/PP19 ± 6−46-
FR-Si-Flax/PP24 ± 7−31+26
FR-Pm-Flax/PP23 ± 7−34+21
FR-Pm-Si-Flax/PP25 ± 7−29+31
Flax/PLA50 ± 6--
FR-Flax/PLA35 ± 4−30-
FR-Si-Flax/PLA34 ± 3−32−3
FR-Pm-Flax/PLA39 ± 9−22+11
FR-Pm-Si-Flax/PLA39 ± 5−22+11
Note: [+] indicates an increase in peeling strength. [−] indicates an decrease in peeling strength.
Table
Table 6. Flexural properties of FR (GUP), silane and plasma-treated flax/PP and flax/PLA fabrics as laminates.
Table 6. Flexural properties of FR (GUP), silane and plasma-treated flax/PP and flax/PLA fabrics as laminates.
SampleFlexural Modulus
(GPa)		Flexural Strength
(MPa)
Flax/PP8.8 ± 0.580 ± 1
FR-Flax/PP10.8 ± 0.692 ± 4
FR-Si-Flax/PP12.7 ± 1.780 ± 4
FR-Pm-Flax/PP13.1 ± 0.785 ± 9
FR-Pm-Si-Flax/PP14.9 ± 1.4114 ± 10
Flax/PLA
FR-Flax/PLA		12.4 ± 0.7
12.3 ± 1.0		135 ± 1
109 ± 6
FR-Si-Flax/PLA12.5 ± 0.2105 ± 11
FR-Pm-Flax/PLA16.0 ± 0.2125 ± 1
FR-Pm-Si-Flax/PLA16.0 ± 1.1110 ± 1
Note: All results were normalised to 50% fibre volume fraction.
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MDPI and ACS Style

Pornwannachai, W.; Horrocks, A.R.; Kandola, B.K. Surface Modification of Commingled Flax/PP and Flax/PLA Fibres by Silane or Atmospheric Argon Plasma Exposure to Improve Fibre–Matrix Adhesion in Composites. Fibers 2022, 10, 2. https://doi.org/10.3390/fib10010002

AMA Style

Pornwannachai W, Horrocks AR, Kandola BK. Surface Modification of Commingled Flax/PP and Flax/PLA Fibres by Silane or Atmospheric Argon Plasma Exposure to Improve Fibre–Matrix Adhesion in Composites. Fibers. 2022; 10(1):2. https://doi.org/10.3390/fib10010002

Chicago/Turabian Style

Pornwannachai, Wiwat, A. Richard Horrocks, and Baljinder K. Kandola. 2022. "Surface Modification of Commingled Flax/PP and Flax/PLA Fibres by Silane or Atmospheric Argon Plasma Exposure to Improve Fibre–Matrix Adhesion in Composites" Fibers 10, no. 1: 2. https://doi.org/10.3390/fib10010002

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