Improvement of Performance Profile of Acrylic Based Polyester Bio

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Article Improvement of Performance Proﬁle of Acrylic Based Polyester Bio-Composites by Bast/Basalt Fibers Hybridization for Automotive Applications

Anjum Saleem 1,* , Luisa Medina 2 and Mikael Skrifvars 1

1 Swedish Centre for Resource Recovery, Faculty of Textiles, Engineering and Business, University of Borås, 50190 Borås, Sweden; [email protected] 2 Department of Applied Logistics and Polymer Sciences, University of Applied Sciences Kaiserslautern, 67659 Kaiserslautern, Germany; [email protected] * Correspondence: [email protected]

Abstract: New technologies in the automotive industry require lightweight, environment-friendly, and mechanically strong materials. Bast fibers such as kenaf, flax, and hemp reinforced polymers are frequently used composites in semi-structural applications in industry. However, the low mechanical properties of bast fibers limit the applications of these composites in structural applications. The work presented here aims to enhance the mechanical property profile of bast fiber reinforced acrylic-based polyester resin composites by hybridization with basalt fibers. The hybridization was studied in three resin forms, solution, dispersion, and a mixture of solution and dispersion resin forms. The composites were prepared by established processing methods such as carding, resin impregnation, and compression molding. The composites were characterized for their mechanical (tensile, flexural, and Charpy impact strength), thermal, and morphological properties. The mechanical performance Citation: Saleem, A.; Medina, L.; of hybrid bast/basalt fiber composites was significantly improved compared to their respective bast Skrifvars, M. Improvement of fiber composites. For hybrid composites, the specific flexural modulus and strength were on an Performance Profile of Acrylic Based average about 21 and 19% higher, specific tensile modulus and strength about 31 and 16% higher, Polyester Bio-Composites by respectively, and the specific impact energy was 13% higher than bast fiber reinforced composites. Bast/Basalt Fibers Hybridization for Automotive Applications. J. Compos. The statistical significance of the results was analyzed using one-way analysis of variance. Sci. 2021, 5, 100. https://doi.org/ 10.3390/jcs5040100 Keywords: bast fibers; basalt fibers; acrylic resin; mechanical properties; thermal properties

Academic Editor: Jose Castro

Received: 12 February 2021 1. Introduction Accepted: 30 March 2021 New and functional materials with high mechanical performance, dimensional stabil- Published: 4 April 2021 ity, and lightweight are the demands of the modern automotive industry [1–3]. There are strict regulations from legislation forcing the industry to reuse and recycle the materials for Publisher’s Note: MDPI stays neutral environmental protection [4]. So far, the most successful reinforcements in the industry are with regard to jurisdictional claims in glass fibers; yet their use is criticized because of the issues such as high weight (density published maps and institutional affil- 2.5–2.59 g/cm3), non-renewability, and health hazards during processing [5–7]. There is iations. an increasing interest in the use of bast fibers such as flax, kenaf, and hemp fibers [8,9] because of their excellent specific mechanical properties, low density (1.2–1.5 g/cm3), renewability, CO2 neutrality, good life cycle assessment characteristics, and widespread availability [10,11]. Some of them have modulus close to the glass fibers but have much Copyright: © 2021 by the authors. lower strength. However, they have good specific mechanical properties because of their Licensee MDPI, Basel, Switzerland. low density that makes them a preferable alternative to the glass fibers in applications This article is an open access article where the demands are stiffness and lightweight [12]. distributed under the terms and Despite the promising properties, bast fibers have certain drawbacks such as low me- conditions of the Creative Commons chanical properties, hydrophilicity because of the presence of polar hydroxyl groups that Attribution (CC BY) license (https:// hinder the fiber-matrix interaction especially with polyolefins, low thermal stability, and un- creativecommons.org/licenses/by/ even fiber quality because of agricultural factors (soil, growth, climate, harvesting) [13,14]. 4.0/).

J. Compos. Sci. 2021, 5, 100. https://doi.org/10.3390/jcs5040100 https://www.mdpi.com/journal/jcs J. Compos. Sci. 2021, 5, 100 2 of 18

In Europe, bast fiber-reinforced composites are frequently used in the automotive sector predominantly in semi-structural applications [14,15]. Their use in structural applications is restricted because of their limited mechanical profile that is not comparable to glass fibers. Hybridization is an approach that can improve the low mechanical profile of bast fiber composites. It involves the partial replacement of bast fibers in the composites with high-performance fibers such as glass, carbon, or recently basalt fibers [16–19]. Basalt fibers are natural fibers of inorganic origin and because of their environment-friendly processing and recyclability are considered a sustainable alternative to glass fibers [20]. Their mechanical and thermal properties are better than glass fibers and being a sustainable material have a better future in the circular economy [21–23]. The main attraction of bast fiber reinforced composites is their natural, sustainable content. The advantage of hybridizing bast fibers (natural fibers) with basalt fibers (high strength natural fibers) is the improvement of the mechanical performance of bast fiber composites without decreasing the amount of their natural sustainable content [24–26]. The automotive industry is keenly selective in choosing the matrix. It should have strict specifications such as good flexural and impact strength, thermal stability, and short processing time. The most commonly used thermoplastic matrix is polypropylene (PP) because of its good thermomechanical properties, easy processability, and possibility of recycling by reprocessing and no release of volatile organic compounds [10,27]. In our previous work, we have investigated the bast/basalt fibers hybridization in polypropylene matrices [26]. The challenges with polypropylene are its incompatibility with polar bast fibers, limited applications at high temperatures, and structural applications [28,29]. Thermoset polymers are especially suitable for natural bast fibers because of their reactive functional groups that can bond with the hydrophilic bast fibers [12]. Common thermosets used for natural fibers are unsaturated polyester and vinyl ester resins [30]. However, the downside is the release of styrene (a potential carcinogenic) during their processing and use. Other thermoset matrices are phenolic resins with excellent heat resistance, rigidity, and dimensional stability [31]. Their drawback is the emission of formaldehyde (carcinogenic) that causes respiratory problems and eye irritation [32]. A group of eco-friendly formaldehyde-free, single component thermoset acrylic-based polyester resins for bast and wood fibers are the Acrodur resins from BASF SE, that fulfill the specifications required in the automotive sector [33]. They have short crosslinking time, easy handling, and safe processing [34]. Previous work by various authors and the industry with these resins has shown their excellent binding potential with natural plant fibers, reduced processing cost because no surface treatment of the natural fibers is required, improved thermal stability [35–38], and no release of organic substances like formaldehyde or phenol during crosslinking or processing. The only by-product during curing is water, making them an extremely low to non-volatile organic compound releasing environment-friendly substitute of traditional thermosets. Before curing, the resins behave like thermoplastics allowing processing by the methods available for thermoplastics, and after curing, they behave like conventional thermosets being hydrophobic and resistant to wear. The resins are available in solution and dispersion forms. The dispersion form is a latex-modified version of the solution form and offers more ductility for applications where the impact characteristics are required [34,39]. This research work aims to study the enhancement of the mechanical properties of bast fiber reinforced acrylic resin (Acrodur) composites by hybridization with basalt fibers. A mixture of flax and kenaf fibers (1:1) was used as bast fibers. The basalt fibers with a size suitable for thermosets were used as high-performance fibers. The hybridization was studied in pure solution and dispersion form resins, as well as, in a 1:1 mixture of solution and dispersion forms to study if the resin forms or their mixture influence the composite properties. The focus was to develop materials for automotive interior parts which can later be applied to other industrial sectors. The first step in composite fabrication was the production of natural fiber mats by carding and needling. The needled mats were impregnated with resins in a dipping J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 3 of 18

The first step in composite fabrication was the production of natural fiber mats by J. Compos. Sci. 2021, 5, 100 carding and needling. The needled mats were impregnated with resins in a dipping3 pro- of 18 cess using a foulard coating machine. Afterward, the prepregs were compression molded to composites for further characterization. The mechanical performance of the composites processwas studied using by a analyzing foulard coating their mechanical machine. properties Afterward, such the as prepregs tensile, flexural, were compression and Charpy moldedimpact strength. to composites In automotive for further applications, characterization. the density The mechanical of the materials performance is a very of theim- compositesportant factor was because studied it by determines analyzing theirthe speci mechanicalfic strength properties of the such materials as tensile, (strength flexural, to andweight Charpy ratio). impact Lightweight strength. and In automotivehigh-strength applications, materials are the consider density ofed the suitable materials for this is a verysector important [40]. Therefore, factor becausein this work, it determines the mech theanical specific properties strength have of thebeen materials discussed (strength as spe- tocific weight mechanical ratio). properties. Lightweight Several and manual high-strength steps involved materials in arethe production considered of suitable fiber mats for thisand sectorthe resin [40 impregnation]. Therefore, inprocess this work, cause the density mechanical variations properties in the composites. have been discussedThis issue aswas specific resolved mechanical to some extent properties. by density Several normalization manual steps and involved provided in a thesensible production compari- of fiberson of mats the andproperties. the resin The impregnation dispersion in process data is cause common density while variations studying in thethe composites.mechanical Thisproperties issue wasof natural resolved fiber to somereinforced extent composit by densityes [41–43]. normalization Therefore, and providedthe data of a sensiblethe me- comparisonchanical properties of the was properties. analyzed The for its dispersion statistical in significance data is common by one-way while ANOVA studying (anal- the mechanicalysis of variance), properties the statistical of natural tool fiber with reinforced multiple composites comparisons [41– using43]. Therefore, Minitab. theThe dataexper- of theimental mechanical design propertieswas the study was analyzedof the influence for its statistical of two variables, significance basalt by one-wayfiber content ANOVA and (analysisresin forms, of variance),at the specific the statisticalflexural, tensile, tool with and multiple Charpy comparisonsimpact energy using response Minitab. factors. The experimentalThe thermal design behavior was the of studythe composites of the influence was studied of two variables,by differential basalt scanning fiber content calo- andrimetry resin (DSC) forms, and at thethermogravimetry specific flexural, (TGA) tensile, analysis and Charpy to define impact the energy processing response parameters factors. and thermalThe thermal stability behavior of the of composites the composites after was curing. studied Furthermore, by differential the scanningfiber-matrix calorime- inter- tryactions (DSC) were and studied thermogravimetry by scanning (TGA)electron analysis microscopy to define (SEM) the of processing the composites. parameters and thermal stability of the composites after curing. Furthermore, the fiber-matrix interactions were2. Materials studied and by scanningMethods electron microscopy (SEM) of the composites. 2.1. Acrylic Polyester Resins 2. Materials and Methods 2.1. AcrylicTwo forms Polyester of the Resins acrylic resins, solution, and dispersion supplied by BASF SE were used as matrix. The solution form (S) of the resin has two components, polycarboxylic acid Two forms of the acrylic resins, solution, and dispersion supplied by BASF SE were and a polyalcohol (crosslinking agent), which in the curing reaction above 130 °C forms a used as matrix. The solution form (S) of the resin has two components, polycarboxylic acid polyester. In the dispersion form (D), the polycarboxylic acid component is modified with and a polyalcohol (crosslinking agent), which in the curing reaction above 130 ◦C forms a polyester.latex that Inproduces the dispersion a latex-mo formdified (D), thepolyester polycarboxylic with the acidpolyalcohol component after is curing modified (Figure with latex1). that produces a latex-modified polyester with the polyalcohol after curing (Figure1).

◦ FigureFigure 1.1. Polycarboxylic acidacid andand polyalcoholpolyalcohol componentscomponents ofof thethe resinresin reactreact atat aa temperaturetemperature >130>130 °CC inin curing reactionreaction toto formform aa polyesterpolyester [33 [33].]. Both forms can also be mixed in various ratios to tailor the properties from hard Both forms can also be mixed in various ratios to tailor the properties from hard to to brittle, and to adjust their viscosity. In this work, the solution and dispersion forms brittle, and to adjust their viscosity. In this work, the solution and dispersion forms were were mixed (DS) in a 1:1 ratio to study the impact of their combination on the composite mixed (DS) in a 1:1 ratio to study the impact of their combination on the composite prop- properties. The resins have an initial water content of around 50 wt. % and can be further erties. The resins have an initial water content of around 50 wt. % and can be further di- diluted with water to allow improved impregnation of the reinforcement [33]. The technical luted with water to allow improved impregnation of the reinforcement [33]. The technical speciﬁcations of the used acrylic resins are provided in Table1. specifications of the used acrylic resins are provided in Table 1.

Table 1. Technical information about acrylic based polyester resins from manufacturer [33].

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Table 1. Technical information about acrylic based polyester resins from manufacturer [33].

Solid Viscosity pH Resin. Trade Name Content (mPas) Value (%) Solution (S) 950 L (hard and brittle) 900–2500 48–52 3–4 DS 3515 (hard and Dispersion (D) 300–1500 50 3.5 impact modiﬁed)

2.2. Reinforcement Fibers There are some characteristics associated with bast fibers such as naturally occurring irregularities and shortfalls because of weather conditions or the regional cultivation conditions. These issues are almost impossible to control and therefore it is often recommended in the industry to use a mixture of bast fibers to have a broad range of properties and to avoid their shortfall [38,44,45]. Hence, a mixture of 50 wt. % (weight percent) flax (FL) and 50 wt. % kenaf (KE) fibers was used as bast fibers in this work. They were supplied by J. Dittrich & Söhne Vliesstoffwerk GmbH, Ramstein, Germany, with a cut length of approximately 110 mm. Basalt fibers with a thermoset compatible coating (BA) were developed and provided by Deutsche Basalt Faser GmbH, Sangerhausen, Germany, with a cut length of approximately 100 mm. The supplied fibers were tested for their linear density by the vibration method according to DIN EN ISO 1973 [46] and the tensile properties according to DIN EN ISO 5079 [47] using a Favigraph from Textechno Herbert Stein GmbH & Co. KG. The measurements were conducted at a clamping length of 20 mm and with a 20 mm/min test speed. The pre-load weight needed to stretch the fibers according to the linear density was 1000 mg for basalt fibers and 10,000 mg for bast fibers. For each fiber type, at least thirty fibers were analyzed and the results are summarized in Table2. The analysis shows high linear density (81 and 121 dtex) and high breaking force (269 and 437 cN) for bast fibers. This is because the bast fibers are coarser and consist of irregular bundles with variable number of elementary fibers. On the other hand, basalt fibers are fine and brittle, but compared to the bast fibers have almost double the elongation and tenacity. The properties of basalt fibers showed a good correlation with the data provided by the supplier.

Table 2. Properties of applied ﬁbers (as received).

Reinforcement Fibers Properties Basalt Kenaf Flax Elongation (Fmax) (%) 4 ± 1.0 2 ± 0.5 2 ± 0.5 Breaking force (cN) 43 ± 11 437 ± 275 269 ± 112 Tenacity (cN/dtex) 8 ± 1 4 ± 2 4 ± 1 Linear Density (dtex) 6.1 ± 1 121 ± 42 81 ± 38

2.3. Composite Production In material development, the implementation of new materials is convenient when they are processable by established technologies. The common way of producing automotive parts with natural ﬁbers is by compression molding of nonwoven mats because of the simplicity, reproducibility, and low cycle time of the process. [14,48,49]. In this work, composites were prepared by established processing methods such as carding, resin impregnation, and compression molding. The following section provides the systematic details of the composite production process. J. Compos. Sci. 2021, 5, 100 5 of 18

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2.3.1. Fiber Mats Manufacture by Carding densityCarding variations is a in very the importantcomposites. step It was in thequite process challenging of composite to card production.thin and fine Severalbasalt fibers.densitymanual They variations steps stuck such toin as thethe fiber rollercomposites. feeding cards and andIt was the blocked quite collection challenging their of rotation. carded to fiberscardTherefore, thin at this and the stage fine carding basalt cause processfibers.density Theywas variations intenselystuck into thetheoptimized composites.roller cards to find Itand wasthe blocked quitesafe process challenging their rotation.window to card Therefore,for thinthe production and the fine carding basalt of homogeneouslyprocessfibers. Theywas intensely stuck mixed to thenonwovensoptimized roller cards to without find and the blocked fiber safe damage. process their rotation. Aswindow a result, Therefore, for basaltthe production fibers the carding were of alwayshomogeneouslyprocess fed was in the intensely mixedcarding optimizednonwovens machine sandwiched to without find the fiber safe between damage. process the window Ashard a result,and for coarse thebasalt production bast fibers fibers. were of alwayshomogeneouslyAfter fed carding, in the mixedcarding thin fiber nonwovens machine layers sandwiched (fiber without web) fiber betweenwere damage. produced the As hard a with result, and evenlycoarse basalt bastdistributed fibers fibers. were andalways orientedAfter fed carding, in fibers the carding free thin from fiber machine contaminations layers sandwiched (fiber web) such between were as shives, produced the hardsoil, withand and short coarseevenly fibers. bastdistributed fibers.These thinand fiberorientedAfter layers carding, fibers were free thin collected from fiber contaminations layersover a (fiberroller web)rotating such were as in shives, produceda clockwise soil, with and direction evenlyshort fibers.until distributed a These web massthinand fiber orientedof homogeneous layers fibers were free collectedarea from weight contaminations over of a approximately roller rotating such as 1150in shives, a clockwise± 100 soil, g/m and direction2 was short achieved. fibers. until a These Theweb processmassthin fiberof of homogeneous fiber layers web were collection collectedarea weight is over shown of a approximately roller in Figure rotating 2. The in1150 a fibers clockwise ± 100 were g/m directionpredominantly2 was achieved. until a ori- webThe ± 2 entedprocessmass lengthwise of of homogeneous fiber web in the collection web, area weightin the is shown machine of approximately in direction.Figure 2. 1150The cross-layerfibers100 were g/m was predominantlywas not achieved. used in the Theori- process of fiber web collection is shown in Figure2. The fibers were predominantly oriented cardingented lengthwise line because in theisotropically web, in the oriented machine fibers direction. for better The strengthcross-layer properties was not inused the inpro- the lengthwise in the web, in the machine direction. The cross-layer was not used in the carding ductioncarding direction line because were isotropically intended. oriented fibers for better strength properties in the pro- ductionline because direction isotropically were intended. oriented fibers for better strength properties in the production direction were intended.

Figure 2. (a) Supplied basalt, flax and kenaf fibers; (b) fiber web with oriented and evenly distributed fibers after carding. Figure 2. ((aa)) Supplied Supplied basalt, basalt, flax flax and and kenaf kenaf fibers; fibers; ( (b)) fiber fiber web web with with oriented oriented and and evenly evenly distributed distributed fibers fibers after carding. carding. After carding, the fiber webs were needled for compaction (Figure 3). The needled fiber After carding, the fiber webs were needled for compaction (Figure3). The needled matsAfter were carding, stable the enough fiber webs for further were needled processing for compactionby resin impregnation (Figure 3). andThe compressionneedled fiber fiber mats were stable enough for further processing by resin impregnation and compres- molding.mats were stable enough for further processing by resin impregnation and compression sion molding. molding.

FigureFigure 3. 3. (a()a Fiber) Fiber mass mass after after carding; carding; (b (b) )needled needled fiber fiber mass. mass. Figure 3. (a) Fiber mass after carding; (b) needled fiber mass. At this stage, two types of nonwovens were prepared for further resin impregnation. At this stage, two types of nonwovens were prepared for further resin impregnation. One of them with only bast fibers (50 wt. % mixture of flax and kenaf fibers) and the second One ofAt them this stage,with only two basttypes fibers of nonwovens (50 wt. % weremixture prepared of flax forand further kenaf fibers)resin impregnation. and the sec- one with hybrid bast and basalt fibers (70 wt. % bast fibers + 30 wt. % basalt fibers). ondOne one of themwith hybridwith only bast bast and fibers basalt (50 fibers wt. %(70 mixture wt. % bast of flax fibers and + kenaf30 wt. fibers) % basalt and fibers). the sec- ond2.3.2. one Resin with Impregnation hybrid bast and and basalt Compression fibers (70 Molding wt. % bast fibers + 30 wt. % basalt fibers). 2.3.2. Resin Impregnation and Compression Molding The fiber mats of 190 × 250 mm dimensions were impregnated manually with ap- 2.3.2. Resin Impregnation and Compression Molding proximatelyThe fiber 30mats wt. of % 190 of the × 250 acrylic mm resins dimensions so that were in the impregnated final composite, manually there waswith about ap- proximately23 wt.The % fiber of 30 the matswt. resin % of of (100g 190 the ×acrylic fibers 250 mm +resins 30g dimensions resin). so that The in were the impregnated final impregnated composite, fiber manually mats there were was with passedabout ap- 23proximatelythrough wt. % of the the 30 rollers resinwt. % of (100g of the the foulardfibers acrylic + coating 30gresins resin). machineso that The in impregnated rotating the final at composite, a speedfiber mats of there 2 mm/minwere was passed about and through23 wt. % the of rollersthe resin of the(100g foulard fibers coating + 30g remachinesin). The rotating impregnated at a speed fiber of mats 2 mm/min were passedand 4 barthrough pressure. the rollersThe function of the foulard of the foulard coating is machine to squeeze rotating the excess at a speed matrix of from2 mm/min the fabrics and 4 bar pressure. The function of the foulard is to squeeze the excess matrix from the fabrics

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and to achieve 4an bar effective pressure. penetration The function and ofdi thestribution foulard of is the to squeezeresin throughout the excess the matrix fiber from the fabrics mats (Figure 4a).and Since to achieve the matrix an effective was colorless, penetration a small and amount distribution of dye of was the added resin throughout for a the ﬁber visual inspectionmats of (Figurethe resin4a). penetration Since the matrix in the wasfiber colorless, mats. The a smallprepregs amount were of left dye for was added for a about 24 h at roomvisual temperature inspection offor the drying. resin penetration in the ﬁber mats. The prepregs were left for about 24 h at room temperature for drying.

Figure 4. (aFigure) Resin 4. impregnation (a) Resin impregnation using foulard using coating foulard machine; coating (machine;b) compression (b) compression molding of molding prepregs; of (pre-c) test specimen cutting frompregs; cured (c laminates.) test specimen cutting from cured laminates.

The dried prepregsThe driedwere compression prepregs were molded compression to flat plates molded of toabout flat plates2 mm thickness. of about 2 mm thickness. Medina et al. [35]Medina showed et al. in [ 35their] showed study that in their the studymechanical that the properties mechanical of acrylic properties resin of acrylic resin bast composites are dependent on the compression molding parameters such as pressure bast composites are dependent on the compression molding parameters such as pressure and time. A pressure above 60 bar results in fiber damage, which significantly affects and time. A pressure above 60 bar results in fiber damage, which significantly affects the the mechanical properties of the composites. In the automotive sector, press time is a mechanical properties of the composites. In the automotive sector, press time is a very very important economic factor. A short press time is recommended to achieve a short important economic factor. A short press time is recommended to achieve a short cycle. cycle. In this work, the prepregs were compression molded at 150 ◦C and 30 bars for In this work, the prepregs were compression molded at 150 °C and 30 bars for about 3 min about 3 min (under serial conditions the press time can be reduced significantly, even (under serial conditions the press time can be reduced significantly, even possible under possible under 60 s, by increasing the mold temperature). The compression molded 60 s, by increasing the mold temperature). The compression molded samples were char- samples were characterized for their mechanical, thermal and morphological properties. acterized for their mechanical, thermal and morphological properties. The abbreviations The abbreviations and compositions (wt. %) of the composites are given in Table3. and compositions (wt. %) of the composites are given in Table 3. Table 3. Abbreviation and composition (wt. %) of the composites. Table 3. Abbreviation and composition (wt. %) of the composites. Abbreviation Composition (wt. %) Abbreviation Composition (wt. %) S/KEFL S/KEFL 77% KEFL + 23% S 77%(solution KEFL +form 23% resin) S (solution form resin) S/23BA/KEFL S/23BA/KEFL 54% KEFL + 23% BA 54% + KEFL23% S + 23% BA + 23% S D/KEFL D/KEFL 77% KEFL + 23% D 77% (dispersion KEFL + 23% form D (dispersionresin) form resin) D/23BA/KEFL D/23BA/KEFL 54% KEFL + 23% BA 54% + KEFL23% D + 23% BA + 23% D DS/KEFL 77%DS/KEFL KEFL + 23% DS 77%(mixture KEFL +of 23% solution DS (mixture and dispersion of solution form and dispersion resin) form resin) DS/23BA/KEFL 54% KEFL + 23% BA + 23% S DS/23BA/KEFL 54% KEFL + 23% BA + 23% S

The test specimens for mechanical analysis (flexural, tensile and Charpy impact) were cut from the compositeThe test specimens plates using for cu mechanicaltting dies analysisof the specific (ﬂexural, dimensions. tensile and The Charpy de- impact) were tails of testing standards,cut from the sp compositeecimen geometry, plates using and test cutting parameters dies of the are speciﬁc given in dimensions. Table 4. The details of testing standards, specimen geometry, and test parameters are given in Table4. Table 4. Test procedures, standards and parameters for the mechanical characterization of composites.

Specimen Dimen- Test Test Standard Test Parameters sion (mm) DIN EN ISO 178 Span = 32 mm Flexural 80 × 25 × 2 [50] Speed = 2 mm/min

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Table 4. Test procedures, standards and parameters for the mechanical characterization of composites.

Specimen Test Test Standard Test Parameters Dimension (mm) Span = 32 mm Flexural DIN EN ISO 178 [50] 80 × 25 × 2 Speed = 2 mm/min Speed = 2 mm/min Tensile DIN EN ISO 527-4 [51] 250 × 25 × 2 Clamping length at start position = 150 mm Hammer: Weight = 0.951 kg Impact (Charpy) ISO 179 [52] 80 × 15 × 2 Velocity = 2.9 m/s Energy = 3.99 J

3. Results 3.1. Thermal Analysis This section provides an insight into the thermal behavior of the composites.

3.1.1. Thermogravimetric Analysis The uncured resins and their composites were analyzed by TGA and DSC to under- stand the processing parameters and thermal stability of the materials. As an example, the TGA and DSC analysis of the acrylic resins mixture (DS) and its composites are shown below. The uncured resins were dried in an oven at about 50 ◦C for 5 h to reduce moisture before thermal analysis. Figure5a shows the TGA analysis (weight loss and derivative TG) of the uncured resins mixture (DS) at a heating rate of 20 ◦C from 25 to 500 ◦C. The derivative curve shows a first degradation step approximately from 50 to 100 ◦C that may be an indication of the evaporation of adsorbed moisture and ends quite early. The reaction between the acid and alcohol components of the resin, producing polyester and water, starts most probably at about 110 ◦C. The degradation step (starting at about 200 ◦C) could be because of the decomposition of polyester and the formation of some volatile components such as carbon monoxide, carbon dioxide, methane, ethylene, and acetylene. The degradation step at ~300 ◦C most probably represents the oxidation of volatile and charred products [53,54]. A comparison of weight loss (%) of the uncured resin and composites is shown in Figure5b . Curve 1 shows a weight loss of about 60% showing the remaining 40% of the resin at 500 ◦C. Curve 2 is the analysis of the composite with 77 wt. % bast fibers and 23% resin. The bast fibers at 500 ◦C completely degrade producing water and carbon dioxide and approximately 10% of the remaining matrix. Curve 3 presents the analysis of the composite with approximately 23 wt. % basalt fibers, 23 wt. % matrix, and 55 wt. % bast fibers. At 500 ◦C, bast fibers degrade and approximately 33% of the residue remains, consisting mostly of basalt fibers. Figure6 presents the percent weight loss and the derivative thermogravimetric analysis of the composites. The traces of moisture can be seen in a region of 50–100 ◦C. This is because bast fibers have an inherent moisture content that according to literature is favorable for the fiber-matrix interaction [55,56]. Therefore, the bast fibers in this project were processed without preliminary oven drying. The degradation stage from about 200 to 360 ◦C might be because of the degradation of the components such as pectin, lignin, and cellulose, and the production of volatile products. The last stage is due to the oxidation of volatile and charred products as mentioned in the literature about the typical three step degradation process of natural cellulosic fibers [57]. The inherent strength and thermal resistance of basalt fibers in hybrid composites delay the degradation process, as can be seen in the TGA curves of hybrid composite (Figure5b, curve 3) and derivative TG in Figure6b. This indicates that the basalt fibers in hybridization with bast fibers might also support sustaining the degradation of the composites at elevated temperatures. J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 8 of 18

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Figure 5. (a) Thermogravimetric analysis (TGA) of uncured mixture acrylic resin; (b) Uncured mixture resin (curve 1), DS/KEFL (curve 2), and DS/23BA/KEFL (curve 3).

Figure 6 presents the percent weight loss and the derivative thermogravimetric analysis of the composites. The traces of moisture can be seen in a region of 50–100 °C. This is because bast fibers have an inherent moisture content that according to literature is favorable for the fiber-matrix interaction [55,56]. Therefore, the bast fibers in this project were processed without preliminary oven drying. The degradation stage from about 200 to 360 °C might be because of the degradation of the components such as pectin, lignin, and cellulose, and the production of volatile products. The last stage is due to the oxidation of volatile and charred products as mentioned in the literature about the typical three step degradation process of natural cellulosic fibers [57]. The inherent strength and thermal resistance of basalt fibers in hybrid composites delay the degradation process, as can be seen in the TGA curves of hybrid composite (Figure 5b, curve 3) and derivative TG in Figure 6b. This indicates that the basalt fibers in hybridization with bast fibers might also FigureFigure 6. 6.TGA TGAsupport analysis analysis sustaining of of mixture mixture the resin resin degradation composites: composites: of ( ath(a)) DS/KEFL;e DS/KEFL; composites ((bb)) DS/23BA/KEFL.DS/23BA/KEFL.at elevated temperatures. 3.1.2. Differential Scanning Calorimetry 3.1.2. Differential Scanning Calorimetry Figure7a shows the DSC analysis of uncured resin mixture and Figure7b shows the Figure 7a shows the DSC analysis of uncured resin mixture and Figure 7b shows the scans of its composites at a heating rate of 20 ◦C from 25 to 250 ◦C. The ﬁrst heating scan of scans of its composites at a heating rate of 20 °C from 25 to 250 °C. The first heating scan uncured resin mixture (Figure7a, curve 1) shows a peak between 100 and 170 ◦C, which is of uncured resin mixture (Figure 7a, curve 1) shows a peak between 100 and 170 °C, which most probably due to the curing of the resin. This peak disappears in the second heating is most probably due to the curing of the resin. This peak disappears in the second heating scan (Figure7a, curve 2). The heat of reaction ( ∆HDS) calculated for this peak was about scan (Figure 7a, curve 2). The heat of reaction (∆HDS) calculated for this peak was about 145 J/g. 145 J/g. In this work, the composites were cured at 150 ◦C (manufacturer’s recommendation > 130 ◦C) and their scans can be seen in Figure7b, curves 2 and 3. The ﬂat peaks from 120 to 220 ◦C represent most probably the post-curing peaks. From the post-curing peaks (Figure7b ), the residual heat of composites (∆HDS) was calculated as 44 and 41 J/g for DS/KEFL (curve 3) and DS/23BA/KEFL (curve 2). This corresponds to a composite

matrix degree of curing of about 69 and 72% for bast and hybrid composites, respec- Figure 6. TGAtively, analysis when of cured mixture at resin 150 ◦composites:C for 3 min, (a) DS/KEFL; which is ( quiteb) DS/23BA/KEFL. close to the expectations under these conditions [58,59]. 3.1.2. Differential Scanning Calorimetry Figure 7a shows the DSC analysis of uncured resin mixture and Figure 7b shows the scans of its composites at a heating rate of 20 °C from 25 to 250 °C. The first heating scan of uncured resin mixture (Figure 7a, curve 1) shows a peak between 100 and 170 °C, which is most probably due to the curing of the resin. This peak disappears in the second heating scan (Figure 7a, curve 2). The heat of reaction (∆HDS) calculated for this peak was about 145 J/g.

J.J. Compos. Compos. Sci. Sci. 20212021,, 55,, x 100 FOR PEER REVIEW 9 9of of 18 18

Figure 7. DDifferentialifferential scanning calorimetry calorimetry (DSC): (DSC): ( (aa)) First First heating heating scan scan (curve (curve 1) 1) and and se secondcond heating heating scan scan (curve (curve 2) 2) of of uncured acrylic resin mixture (DS); ( b) ﬁrstfirst heatingheating scansscans ofof acrylicacrylic resinsresins mixturemixture (curve(curve 1),1), DS/KEFLDS/KEFL (curve (curve 3) 3) and DS/23BA/KEFL (curve (curve 2). 2).

3.2. MechanicalIn this work, Analysis the composites were cured at 150 °C (manufacturer’s recommendation >130 °C)This and section their presents scans can the be specific seen in mechanicalFigure 7b, curves properties 2 and of 3. the The composites flat peaks andfrom their 120 tostatistical 220 °C represent analysis. most The specific probably values the post-c of theuring properties peaks. were From calculated the post-curing for each peaks specimen (Fig- ureby dividing7b), the residual the measured heat of valuecomposites of the ( property∆HDS) was by calculated the density as 44 of thatand 41 sample. J/g for AfterDS/KEFL that, (curvethe mean 3) and and DS/23BA/KEFL standard deviation (curve were 2). calculated This corresponds for the specific to a composite properties. matrix The densitydegree of of curingthe specimens of about was 69 and calculated 72% for from bast their and weight,hybrid composites, height, width, respectively, and length. when cured at 150 °C for 3 min, which is quite close to the expectations under these conditions [58,59]. 3.2.1. Flexural Analysis 3.2. MechanicalFlexural properties Analysis are preferred for composites in structural applications because theyThis focus section on both presents tensile the and specific compressive mechanic failureal properties modes. They of the are composites determined and in ordertheir statisticalto study theanalysis. ability The of thespecific material values to resistof the deformationproperties were under calculated load. For for each each composite, specimen byat leastdividing five the valid measured tests were value performed. of the property The average by the flexural density properties, of that sample. density, After specific that, theproperties, mean and and standard the standard deviation deviations were calculat (StDev)ed are for given the specific in Table properties.5. The density of the specimens was calculated from their weight, height, width, and length. Table 5. Flexural mechanical properties of composites.

3.2.1. Flexural AnalysisSpecific Specific Flexural ModulusFlexural propertiesFlexural are preferredFlexural for Strengthcomposites in structuralFlexural applicationsDensity because 3 Composite. (MPa)they focus on bothModulus tensile and compressive(MPa) failure modes.Strength They are determined(g/cm in) order (MPa·cm3/g) (MPa·cm3/g) to study the ability of the material to resist deformation under load. For each composite, Meanat least StDev five valid Mean tests were StDev performed. Mean The StDevaverage flexural Mean properti StDeves, Meandensity, specific StDev D/KEFL 4389properties, 593 and 4820the standard 510 deviatio 74ns (StDev) 9 are given 81 in Table 6 5. 0.91 0.05 D/23BA/KEFL 6322 706 6116 490 98 12 96 9 1.03 0.07 Table 5. Flexural mechanical properties of composites. S/KEFL 4568 734 4946 377 76 13 82 8 0.92 0.08 S/23BA/KEFL 5157 453 5606Specific 318 91 9Specific 95 1 0.96 0.11 Flexural Modulus Flexural Flexural Strength Flexural Density DS/KEFL 3500 368 4131 299 63 7 74 3 0.85 0.08 Composite. (MPa) Modulus (MPa) Strength (g/cm3) DS/23BA/KEFL 4330 428 5067 124 80 4 90 5 0.91 0.04 (MPa·cm3/g) (MPa·cm3/g) Mean StDev Mean StDev Mean StDev Mean StDev Mean StDev D/KEFL 4389 593Figure 8 presents4820 a510 comparison 74 of the specific 9 flexural 81 moduli 6 and 0.91 strengths 0.05 of the D/23BA/KEFL 6322 composites.706 It6116 can be observed490 that 98 the properties 12 of 96 hybrid composites 9 1.03 are significantly 0.07 higher than the bast fiber composites. The specific modulus of hybrid composites is on S/KEFL 4568 734 4946 377 76 13 82 8 0.92 0.08 average about 21% and strength about 19% higher compared to the bast composites. S/23BA/KEFL 5157 453 5606 318 91 9 95 1 0.96 0.11 DS/KEFL 3500 368 4131 299 63 7 74 3 0.85 0.08 DS/23BA/KEFL 4330 428 5067 124 80 4 90 5 0.91 0.04

J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 10 of 18

Figure 8 presents a comparison of the specific flexural moduli and strengths of the composites. It can be observed that the properties of hybrid composites are significantly higher than the bast fiber composites. The specific modulus of hybrid composites is on average about 21% and strength about 19% higher compared to the bast composites. The dispersion and solution resin composites show nearly the same average modulus and strength, although it was expected that the dispersion resin (latex modified) composites will have lower flexural properties compared to the solution resin composites [60]. However, the reason that the difference in resins cannot be realized might be because of J. Compos. Sci. 2021, 5, 100 the very low amount of resin in the composites. In the resin mixture composites, however,10 of 18 average specific flexural properties much lower than both the dispersion and solution resin composites indicate the influence of dispersion form in the resin mixture.

Figure 8. Comparison of specificspecific flexuralflexural modulusmodulus and strength of composites.composites. The dispersion and solution resin composites show nearly the same average modulus The equality of the means of the flexural analysis results was compared using one- and strength, although it was expected that the dispersion resin (latex modified) compos- way ANOVA statistical analysis and the key results are presented in Table 6. A p-value ites will have lower flexural properties compared to the solution resin composites [60]. less than or equal to the significance level of 0.05 means that the influence of the factor is However, the reason that the difference in resins cannot be realized might be because of statistically significant and higher means no statistical significance. The influence of the the very low amount of resin in the composites. In the resin mixture composites, however, factors is checked formally using grouping information (GI) from the Tukey method and average specific flexural properties much lower than both the dispersion and solution resin 95% confidence interval (CI) [61]. According to the Tukey method, the means that do not composites indicate the influence of dispersion form in the resin mixture. share a letter are significantly different. The standard error of regression (SE) is an indica- The equality of the means of the flexural analysis results was compared using one-way tor of how far fall the data values from the fitted values and is measured in the units of ANOVA statistical analysis and the key results are presented in Table6.A p-value less the response variable [62,63]. In this case, the experimental design was the study of the than or equal to the significance level of 0.05 means that the influence of the factor is influencestatistically of significantbasalt fiber and content higher (factor) means at no two statistical levels (0 significance. and 23 wt. The %) influenceand resin oftypes the (factor)factors isat checked three levels formally (solution, using dispersion grouping,information and mixture) (GI) at two from response the Tukey variables, method spe- and cific95% flexural confidence modulus interval and (CI) strength [61]. Accordingof the composites. to the Tukey method, the means that do not share a letter are significantly different. The standard error of regression (SE) is an Table 6. Comparison of the means equality using one way analysis of variance—specific flexural analysis. indicator of how far fall the data values from the fitted values and is measured in the units of theSpecific response Flexural variable [62,63]. In this case, the experimentalSpecific design Flexural was the study of Composites the influenceModulus of basalt fiber content (factor) at two levels (0 andStrength 23 wt. %) and resin types (factor)(MPa·cm at three levels3/g) (solution, dispersion, and mixture) at two(MPa·cm response3/g) variables, specific flexural modulus and strength of the composites. Mean StDev 95% CI GI Mean StDev 95% CI GI

Bast Table 6.4632Comparison 527 of the means(4349; equality 4916) usingA one wayp-value: analysis of79 variance—specific 6.2 (76; flexural 83) analysis.A p-value: 0.000 0.000 Basalt/bast 5596 546 (5313; 5880) B 93 6.3 (90; 97) B Specific Flexural SE: 536 Specific Flexural SE: 6 SolutionComposites 5276 479 (4864;Modulus 5688) A B p-value: 88 8.8 Strength(82; 95) A p-value: (MPa·cm3/g) (MPa·cm3/g) Dispersion 5468 830 (5056; 5879) A 0.013 88 10.7 (82; 95) A 0.298 Mean StDev 95% CI GI Mean StDev 95% CI GI Mixture 4599 538 (4188; 5011) B SE: 634 82 9.1 (76; 89) A SE: 9 p p Bast 4632 527 (4349; 4916) A -value: 79 6.2 (76; 83) A -value: 0.000 0.000 Basalt/bast 5596 546 (5313; 5880) BSE: 536 93 6.3 (90; 97) B SE: 6 Solution 5276 479 (4864; 5688) A B p-value: 88 8.8 (82; 95) A p-value: Dispersion 5468 830 (5056; 5879) A0.013 88 10.7 (82; 95) A 0.298 SE: 634 SE: 9 Mixture 4599 538 (4188; 5011) B 82 9.1 (76; 89) A

The data in Table6 shows that the addition of basalt fibers causes a significant increase in the flexural properties of the composites (p-value = 0.000). The Tukey method divides bast composites (0 basalt content) in group “A” and hybrid bast/basalt composites (23 wt. % basalt) in group “B” that indicates that the two groups are significantly different from each other. A comparison of the means of solution, dispersion, and mixture resin composites J. Compos. Sci. 2021, 5, 100 11 of 18

shows that the specific flexural modulus of dispersion resin (A) is significantly different than the mixture resin composites (B). In terms of specific flexural strength, the analysis shows no significant influence of the resin types, they share the same letter “A”. The statistical analysis of the results is quite similar to the analysis of the raw data (Figure8).

3.2.2. Tensile Analysis The tensile properties of the composites are greatly influenced by fiber strength, modulus, and orientation. Table7 presents the tensile analysis of the composites as average values of at least five valid tests on each material.

Table 7. Tensile analysis of composites.

Tensile Speciﬁc Tensile Tensile Speciﬁc Tensile Density Composite Modulus Modulus Strength Strength (g/cm3) (MPa) (MPa·cm3/g) (MPa) (MPa·cm3/g) Mean StDev Mean StDev Mean StDev Mean StDev Mean StDev D/KEFL 6284 629 7486 406 48 5 60 2 0.81 0.02 D/23BA/KEFL 7484 445 9484 421 57 5 72 2 0.79 0.08 S/KEFL 6438 488 7485 306 53 6 62 3 0.86 0.09 S/23BA/KEFL 8780 506 9920 471 65 7 73 3 0.89 0.09 DS/KEFL 5985 544 7305 463 52 3 63 3 0.82 0.03 DS/23BA/KEFL 8156 581 9738 169 60 2 72 4 0.84 0.06

Figure9 presents the comparison of the specific tensile modulus and strength of the composites. The average specific tensile modulus and strength of hybrid composites are significantly higher than for bast-fiber-only composites. The specific tensile modulus of J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 12 of 18 hybrid composites is on an average about 31% and strength about 16% higher compared to only bast fiber composites.

Figure 9. ComparisonComparison of specific specific tensile modulus and strength of composites. The solution, dispersion, and mixture composites have almost the same average The statistical significance of results was analyzed by one-way ANOVA. The influ- specific tensile modulus and strength. The influence of the resin form on the tensile ence of basalt fiber content and resin forms at the response factors, specific tensile modu- properties cannot be established with certainty. It might be because the amount of resin is lus and strength is shown in Table 8. very small in the composites and the influence of fibers at the tensile properties is dominant. Table 8. Comparison of the equality of means using one way analysis of variance—specific tensile analysis.

Specific Tensile Specific Tensile Composite Modulus Strength (MPa.cm3/g) (MPa.cm3/g) Mean StDev 95% CI GI Mean StDev 95% CI GI Bast 7425 378 (7220; 7629) A p-value: 62 2.8 (60; 63) A p-value: 0.000 0.000 Basalt/bast 9714 396 (9509; 9919) B 72 3.1 (71; 74) B SE: 387 SE: 3 Solution 8702 1337 (7881; 9523) A p-value: 67 6.6 (63; 71) A p-value: Dispersion 8485 1123 (7664; 9306) A 0.919 66 6.7 (62; 70) A 0.892 Mixture 8522 1324 (7701; 9342) A SE:1265 67 5.6 (63; 72) A SE: 6

It can be seen that the addition of basalt fibers in the bast fiber composites has a significant influence on the tensile properties of the composites (p-value = 0.000). The grouping information (GI) from the Tukey method and 95% confidence interval (CI) assigns a different letter to the bast and hybrid bast/basalt composites indicating significantly different composites. A comparison of the means in terms of the resin forms shows that they are statistically the same. No significant influence of the resin forms can be established on the specific tensile modulus and strength of the composites.

3.2.3. Charpy Impact Analysis The impact strength of the composites was tested by Charpy impact analysis that determines the amount of energy absorbed by a material during fracture. The results are presented in Table 9 as average values of at least five valid tests for each material.

J. Compos. Sci. 2021, 5, 100 12 of 18

The statistical significance of results was analyzed by one-way ANOVA. The influence of basalt fiber content and resin forms at the response factors, specific tensile modulus and strength is shown in Table8.

Table 8. Comparison of the equality of means using one way analysis of variance—speciﬁc tensile analysis.

Speciﬁc Tensile Speciﬁc Tensile Composite Modulus Strength (MPa.cm3/g) (MPa.cm3/g) Mean StDev 95% CI GI Mean StDev 95% CI GI p p Bast 7425 378 (7220; 7629) A -value: 62 2.8 (60; 63) A -value: 0.000 0.000 Basalt/bast 9714 396 (9509; 9919) BSE: 387 72 3.1 (71; 74) B SE: 3 Solution 8702 1337 (7881; 9523) A p-value: 67 6.6 (63; 71) A p-value: Dispersion 8485 1123 (7664; 9306) A0.919 66 6.7 (62; 70) A 0.892 SE:1265 SE: 6 Mixture 8522 1324 (7701; 9342) A 67 5.6 (63; 72) A

It can be seen that the addition of basalt fibers in the bast fiber composites has a significant influence on the tensile properties of the composites (p-value = 0.000). The grouping information (GI) from the Tukey method and 95% confidence interval (CI) assigns a different letter to the bast and hybrid bast/basalt composites indicating significantly different composites. A comparison of the means in terms of the resin forms shows that they are statistically the same. No significant influence of the resin forms can be established on the specific tensile modulus and strength of the composites.

3.2.3. Charpy Impact Analysis The impact strength of the composites was tested by Charpy impact analysis that determines the amount of energy absorbed by a material during fracture. The results are presented in Table9 as average values of at least ﬁve valid tests for each material.

Table 9. Charpy impact analysis of composites.

Impact Speciﬁc Impact Density Energy Energy Composite (g/cm3) (kJ/m2) (kJ/m2·cm3/g) Mean StDev Mean StDev Mean StDev D/KEFL 11.02 0.50 0.95 0.04 11.67 0.70 D/23BA/KEFL 12.20 1.00 0.95 0.06 12.92 0.90 S/KEFL 8.71 0.61 0.87 0.08 10.01 0.85 S/23BA/KEFL 11.72 0.84 1.04 0.09 11.34 0.65 DS/KEFL 10.37 0.67 0.89 0.04 11.68 0.54 DS/23BA/KEFL 11.90 1.08 0.94 0.09 12.57 0.66

Figure 10 provides a comparison of the Charpy impact energy absorption of the composites. The specific impact energy of hybrid bast/basalt composites is significantly higher than their respective bast fiber composites. The specific impact energy for hybrid composites is on an average approximately 13% higher than bast-only composites. J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 13 of 18

Table 9. Charpy impact analysis of composites.

Impact Specific Impact Density Energy Energy Composite (g/cm3) (kJ/m2) (kJ/m2·cm3/g) Mean StDev Mean StDev Mean StDev D/KEFL 11.02 0.50 0.95 0.04 11.67 0.70 D/23BA/KEFL 12.20 1.00 0.95 0.06 12.92 0.90 S/KEFL 8.71 0.61 0.87 0.08 10.01 0.85 S/23BA/KEFL 11.72 0.84 1.04 0.09 11.34 0.65 DS/KEFL 10.37 0.67 0.89 0.04 11.68 0.54 DS/23BA/KEFL 11.90 1.08 0.94 0.09 12.57 0.66 Figure 10 provides a comparison of the Charpy impact energy absorption of the com- J. Compos. Sci. 2021, 5, 100 posites. The specific impact energy of hybrid bast/basalt composites is significantly higher13 of 18 than their respective bast fiber composites. The specific impact energy for hybrid composites is on an average approximately 13% higher than bast-only composites.

FigureFigure 10. 10. ComparisonComparison of of specific speciﬁc impact impact energy energy of of bast bast fiber ﬁber composites composites with with hybrid hybrid composites. composites.

InIn terms terms of of resin resin forms, forms, the the average average specifi specificc impact impact energy energy of of dispersion dispersion resin resin com- com- positesposites is is better better than than the the solution solution resin resin comp compositesosites and and quite quite close close to to the the resin resin mixture mixture composites.composites. This This observation observation corresponds corresponds with with the the expectation expectation since since latex latex modification modification should improve energy absorption. should improve energy absorption. The statistical significance of the results was examined by one-way ANOVA and the The statistical significance of the results was examined by one-way ANOVA and the results are given in Table 10. results are given in Table 10.

Table 10. Analysis of the equality of means using one way analysis of variance—speciﬁc impact en- Table 10. Analysis of the equality of means using one way analysis of variance—specific impact energy.ergy.

2 3 CompositesComposites SpecificSpeciﬁc Impact Impact Energy (kJ/m (kJ/m·cm2·cm/g)3/g) Mean StDev 95%95% CI CI GIGI pp-value:-value: 0.000 BastBast 10.89 1.081.08 (10.4;(10.4; 11.39) 11.39) A A SE:SE: 1.06 Basalt/bastBasalt/bast 12.26 1.05 (11.78;(11.78; 12.75) 12.75) B B Solution 10.61 1.04 (10.12; 11.09) B Solution 10.61 1.04 (10.12; 11.09) B p-value: Dispersion 12.40 1.01 (11.84; 12.96) A SE:p-value: 0.95 Dispersion 12.40 1.01 (11.84; 12.96) A SE: 0.95 Mixture 12.17 0.75 (11.58; 12.75) A

The analysis shows a significant influence of adding basalt fibers in bast fiber composites on the specific impact energy absorption of the composites (p-value = 0.000). The solution form resin composites are classified in group B, in other words, different than dispersion and mixture resin composites. The dispersion form of the resin is impact modified and the solution form is hard and brittle. Therefore, the average impact properties in dispersion, as well as in resin mixture (1:1 solution and dispersion) composites are better than the solution resin composites.

3.2.4. SEM Analysis The SEM analysis of the composites gave an insight into their morphological characteristics. Figure 11 shows a wide and close view of the solution hybrid composite. Both bast and basalt fibers are homogeneously distributed in the matrix without any overlapping or agglomeration. The basalt-matrix interface shows well-embedded fibers in the matrix without any gaps around the fibers. J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 14 of 18

Mixture 12.17 0.75 (11.58; 12.75) A

The analysis shows a significant influence of adding basalt fibers in bast fiber composites on the specific impact energy absorption of the composites (p-value = 0.000). The solution form resin composites are classified in group B, in other words, different than dispersion and mixture resin composites. The dispersion form of the resin is impact modified and the solution form is hard and brittle. Therefore, the average impact properties in dispersion, as well as in resin mixture (1:1 solution and dispersion) composites are better than the solution resin composites.

3.2.4. SEM Analysis The SEM analysis of the composites gave an insight into their morphological characteristics. Figure 11 shows a wide and close view of the solution hybrid composite. Both J. Compos. Sci. 2021, 5, 100 bast and basalt fibers are homogeneously distributed in the matrix without any overlap-14 of 18 ping or agglomeration. The basalt-matrix interface shows well-embedded fibers in the matrix without any gaps around the fibers.

FigureFigure 11.11.SEM SEM analysisanalysis ofof S/23BA/KEFLS/23BA/KEFL co compositemposite (wide (wide and and close close view). view).

Figure 12 presents the SEM analysis of dispersiondispersion hybrid composite. The analysis of interface shows gaps at the basalt-matrix interface.

Figure 12. SEM analysis of D/23BA/KEFL composite (wide and close view). FigureFigure 12.12.SEM SEM analysisanalysis ofof D/23BA/KEFLD/23BA/KEFL co compositemposite (wide (wide and and close close view). view).

A similar observation can be appreciated in the hybrid resin mixture composite in Figure 13.13. The The fiber-matrix fiber-matrix interface interface shows shows gaps gaps that might be the influenceinfluence of impact modification of the dispersion form resin. The previous literature shows that the composites with effective fiber-matrix interaction absorb less energy than the composites with poor interaction [26,64,65]. It can also be observed in this work that the solution composites (better basalt-matrix interface) have absorbed less energy in Charpy analysis compared to the dispersion and mixture resin composites (gaps at basalt-matrix interface). The bast fibers appear effectively impregnated with the resins indicated by well-coated bast fiber bundles and no fiber pull-out. Matrix and bast fibers break at the same position. This is because the chemical structure of resins (acid-group on polyester) allows chemical bonding with the hydroxyl groups on cellulosic fibers. The basalt fibers however appear not as effectively impregnated by the resins indicated by the pull-out basalt fibers and gaps at the interface. However, the presence of matrix residues at the basalt fibers indicates a certain level of adhesion that can eventually be improved by the optimization of the basalt fibers sizing. J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 15 of 18

modification of the dispersion form resin. The previous literature shows that the composites with effective fiber-matrix interaction absorb less energy than the composites with J. Compos. Sci. 2021, 5, 100 poor interaction [26,64,65]. It can also be observed in this work that the solution compo-15 of 18 sites (better basalt-matrix interface) have absorbed less energy in Charpy analysis compared to the dispersion and mixture resin composites (gaps at basalt-matrix interface).

FigureFigure 13.13. SEMSEM analysisanalysis ofof DS/23BA/KEFLDS/23BA/KEFL co compositemposite (wide (wide and and close close view). view).

4. ConclusionsThe bast fibers appear effectively impregnated with the resins indicated by well- coatedThe bast research fiber bundles work presentsand no fiber the hybridizationpull-out. Matrix of and basalt bast fibers fibers with break bast at the fibers same in position.acrylic-based This is polyester because resinthe chemical composites. structure The of hybridization resins (acid-group was studied on polyester) in three allows resin chemicalforms, namely, bonding solution, with the dispersion, hydroxyl andgroups a mixture on cellulosic of solution fibers. and The dispersion basalt fibers form however resins. appearThe composites not as effectively were prepared impregnated by established by the re processingsins indicated methods by the such pull-out as carding, basalt fibers resin andimpregnation, gaps at the and interface. compression However, molding. the presen The cardingce of matrix process residues was optimized at the basalt to define fibers a indicatessafe processing a certain window level of for adhesion the production that ca ofn eventually nonwovens be with improved homogeneously by the optimization distributed offibers the basalt avoiding fibers brittle sizing. basalt fiber damage. The homogeneous distribution of fibers was confirmed later by SEM analysis of the composites. 4. ConclusionsThe pure resins were studied by DSC to define the processing temperature for the compressionThe research molding. work The presents composites the hybridizatio cured at 150n◦ Cof for basalt three fibers minutes with showed bast fibers a degree in acrylic-basedof curing of about polyester 70%. Thermogravimetricresin composites. The analysis hybridization provided was an insight studied into in thethree thermal resin forms,stability namely, of the solution, composites. dispersion, Basalt and fibers a mixt in basture fiberof solution reinforced and dispersion composites form delay resins. the Thedegradation composites process were thatprepared shows by the established ability of processing basalt fibers methods to sustain such degradation, as carding, toresin an impregnation,extent, at elevated and temperatures.compression molding. The carding process was optimized to define a safe processingThe mechanical window performance for the production of the composites of nonwovens was with discussed homogeneously and compared distrib- in utedterms fibers of their avoiding specific brittle mechanical basalt fiber properties damage. (flexural, The homogeneous tensile, and distribution Charpy impact) of fibers and wasthe statisticalconfirmed significance later by SEM of analysis the results of wasthe composites. analyzed using one-way ANOVA (statistical analysisThe tool).pure Theresins addition were studied of basalt by fibers DSC (23 to wt.define %) inthe bast processing fiber composites temperature significantly for the compressionimproved the molding. mechanical The properties composites of cured the composites at 150 °C for compared three minutes to bast-only showed composites. a degree of curingThe specificof about flexural 70%. Thermogravimetric and tensile properties analysis were provided nearly the an same insight for into solution, the thermal disper- stabilitysion, and of mixture the composites. resin composites. Basalt fibers It was in bast most fiber probably reinforced because composites the amount delay of the resin deg- in radationthe composites process was that less shows and couldthe ability not be of differentiated basalt fibers to by sustain the analysis. degradation, The specific to anCharpy extent, atimpact elevated energy temperatures. absorption was higher for the dispersion and mixture resin composites. It was expectedThe mechanical because performance of the impact of modification the composites of dispersion was discussed form resin. and compared The morpho- in termslogical of analysis their specific of the mechanical composites properties manifested (flexural, very good tensile, wetting and of Charpy the bast impact) fibers withand theall threestatistical resins significance because of of the the chemical results was bonding analyzed between using resin one-way and bast ANOVA fibers. (statistical However, analysismatrix interaction tool). The addition was not veryof basalt effective fibers with (23 wt. the %) basalt in bast fibers. fiber The composites matrix residues significantly at the improvedbasalt fiber’s the surfacemechanical indicate properties a certain of the level composites of interaction compared that canto bast-only be improved composites. by the optimizationThe specific of basalt flexural fibers and sizing. tensile properties were nearly the same for solution, disper- Hybridization of the bast and basalt fibers in acrylic-based polyester resins produced sion, and mixture resin composites. It was most probably because the amount of resin in composites with promising mechanical performance and great potential to replace glass the composites was less and could not be differentiated by the analysis. The specific fibers, at least partially if not fully, in automotive applications. Future research focus is the study of dynamic mechanical behavior, the influence of fiber length, fracture mechanics, water absorption, and numerical modeling, as well as the applications of these materials in other industrial areas such as construction, marine, and household. J. Compos. Sci. 2021, 5, 100 16 of 18

Author Contributions: The individual contributions for the article are as following: Conceptualiza- tion, methodology, administration, L.M. and A.S.; formal analysis, investigation, writing—original draft preparation, editing, A.S.; supervision, review, L.M. and M.S. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors would like to acknowledge the contribution of J. Dittrich & Söhne Vliesstoffwerk GmbH for the bast fibers and BASF SE for providing acrylic based polyester resins. Special thanks to Schuster, at the University of Applied Sciences, Kaiserslautern for providing his lab facilities for mechanical and thermal analysis. We truly appreciate and thank the contribution of Magnus Lundin, from the University of Borås, for the statistical analysis of the results. We are grateful to Hörhammer, from the University of Applied Sciences, Kaiserslautern, for providing thermogravimetric scans of the composites. Thanks to the Leibniz Institute for Composites, Kaiserslautern for providing SEM pictures of the composites. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Osoka, E.C.; Onukwuli, O.D.; Kamalu, C.I.O. Mechanical Properties of Selected Natural Fiber Reinforced Composites for Automobile Application. Am. J. Eng. Res. 2018, 7, 384–388. 2. Beardmore, P.; Johnson, C.F. The Potential for Composites in Structural Automotive Applications. Compos. Sci. Technol. 1986, 26, 251–281. [CrossRef] 3. Faruk, O.; Tjong, J.; Sain, M. (Eds.) Lightweight and Sustainable Materials for Automotive Applications; CRC Press, Taylor & Francis Group: Boca Raton, FL, USA, 2017; ISBN 978-1-4987-5687-7. 4. The European Parliament and the Council of the European Union. Directive 2000/53/EC of the European Parliament and of the Council of 18 September 2000 on End of Life Vehicles. Off. J. Eur. Communities 2000, 269, 34–42. 5. Marsh, G. Next Step for Automotive Materials. Mater. Today 2003, 6, 36–43. [CrossRef] 6. Rajak, D.; Pagar, D.; Menezes, P.; Linul, E. Fiber-Reinforced Polymer Composites: Manufacturing, Properties, and Applications. Polymers 2019, 11, 1667. [CrossRef][PubMed] 7. Yates, M.R.; Barlow, C.Y. Life Cycle Assessments of Biodegradable, Commercial Biopolymers—A Critical Review. Resour. Conserv. Recycl. 2013, 78, 54–66. [CrossRef] 8. Behlouli, K. Natural Fiber Composites in Automotive Applications. In Handbook of Green Materials; World Scientiﬁc: Singapore, 2014; Volume 9. 9. Faruk, O.; Bledzki, A.K.; Fink, H.-P.; Sain, M. Biocomposites Reinforced with Natural Fibers: 2000–2010. Prog. Polym. Sci. 2012, 37, 1552–1596. [CrossRef] 10. Müssig, J.; Graupner, N. Technical applications of natural ﬁbers: An Overview. In Industrial Application of Natural Fibres: Structure, Properties, and Technical Applications; John Wiley & Sons: Hoboken, NJ, USA, 2010; pp. 63–88, ISBN 978-0-470-69508-1. 11. Mohanty, A.K.; Misra, M.; Drzal, L.T. Natural Fibers, Biopolymers, and Biocomposites: An Introduction. In Natural Fibers, Biopolymers, and Biocomposites; Taylor & Francis: Boca Raton, FL, USA, 2005; pp. 2–31, ISBN 978-0-8493-1741-5. 12. Saheb, D.N.; Jog, J.P. Natural Fiber Polymer Composites: A Review. Adv. Polym. Technol. 1999, 18, 13. [CrossRef] 13. Holbery, J.; Houston, D. Natural-Fiber-Reinforced Polymer Composites in Automotive Applications. JOM 2006, 58, 80–86. [CrossRef] 14. Medina, L.A.; Dzalto, J. 1.11 Natural Fibers. In Comprehensive Composite Materials II; Elsevier: Amsterdam, The Netherlands, 2018; pp. 269–294, ISBN 978-0-08-100534-7. 15. Carus, M.; de Beus, N.; Barth, M. Carbon Footprint and Sustainability of Different Natural Fibres for Biocomposites and Insulation Material; Nova-Institute: Hürth, Germany, 2019. 16. Sarasini, F.; Tirillò, J.; Sergi, C.; Seghini, M.C.; Cozzarini, L.; Graupner, N. Effect of Basalt Fibre Hybridisation and Sizing Removal on Mechanical and Thermal Properties of Hemp Fibre Reinforced HDPE Composites. Compos. Struct. 2018, 188, 394–406. [CrossRef] 17. Fiore, V.; Valenza, A.; Di Bella, G. Mechanical Behavior of Carbon/Flax Hybrid Composites for Structural Applications. J. Compos. Mater. 2012, 46, 2089–2096. [CrossRef] 18. Dhakal, H.N.; Zhang, Z.Y.; Guthrie, R.; MacMullen, J.; Bennett, N. Development of Flax/Carbon Fibre Hybrid Composites for Enhanced Properties. Carbohydr. Polym. 2013, 96, 1–8. [CrossRef][PubMed] 19. Öztürk, S. The Effect of Fibre Content on the Mechanical Properties of Hemp and Basalt Fibre Reinforced Phenol Formaldehyde Composites. J. Mater. Sci. 2005, 40, 4585–4592. [CrossRef] 20. Singha, K. A Short Review on Basalt Fiber. Int. J. Text. Sci. 2012, 1, 19–28. [CrossRef] 21. Lopresto, V.; Leone, C.; de Iorio, I. Mechanical Characterization of Basalt Fibre Reinforced Plastic. Compos. Part B Eng. 2011, 42, 717–723. [CrossRef] J. Compos. Sci. 2021, 5, 100 17 of 18

22. Fiore, V.; Scalici, T.; Di Bella, G.; Valenza, A. A Review on Basalt Fibre and Its Composites. Compos. Part B Eng. 2015, 74, 74–94. [CrossRef] 23. Manikandan, V.; Winowlin Jappes, J.T.; Suresh Kumar, S.M.; Amuthakkannan, P. Investigation of the Effect of Surface Modifi- cations on the Mechanical Properties of Basalt Fibre Reinforced Polymer Composites. Compos. Part B Eng. 2012, 43, 812–818. [CrossRef] 24. Umashankaran, M.; Gopalakrishnan, S.; Sathish, S. Preparation and Characterization of Tensile and Bending Properties of Basalt-Kenaf Reinforced Hybrid Polymer Composites. Int. J. Polym. Anal. Charact. 2020, 25, 227–237. [CrossRef] 25. Ricciardi, M.R.; Papa, I.; Lopresto, V.; Langella, A.; Antonucci, V. Effect of Hybridization on the Impact Properties of Flax/Basalt Epoxy Composites: Influence of the Stacking Sequence. Compos. Struct. 2019, 214, 476–485. [CrossRef] 26. Saleem, A.; Medina, L.; Skrifvars, M. Influence of Fiber Coating and Polymer Modification on Mechanical and Thermal Properties of Bast/Basalt Reinforced Polypropylene Hybrid Composites. J. Compos. Sci. 2020, 4, 119. [CrossRef] 27. Czigány, T. Basalt Fiber Reinforced Hybrid Polymer Composites. Mater. Sci. Forum 2005, 473, 59–66. [CrossRef] 28. Erasmus, E.; Anandjiwala, R. Studies on Enhancement of Mechanical Properties and Interfacial Adhesion of Flax Reinforced Polypropylene Composites. J. Thermoplast. Compos. Mater. 2009, 22, 485–502. [CrossRef] 29. Zafeiropoulos, N.E.; Williams, D.R.; Baillie, C.A.; Matthews, F.L. Engineering and Characterisation of the Interface in Flax Fibre/Polypropylene Composite Materials. Part I. Development and Investigation of Surface Treatments. Compos. Part A Appl. Sci. Manuf. 2002, 33, 1083–1093. [CrossRef] 30. Haghdan, S.; Smith, G.D. Natural Fiber Reinforced Polyester Composites: A Literature Review. J. Reinf. Plast. Compos. 2015, 34, 1179–1190. [CrossRef] 31. No, B.Y.; Kim, M.G. Evaluation of Melamine-Modified Urea-Formaldehyde Resins as Particleboard Binders. J. Appl. Polym. Sci. 2007, 106, 4148–4156. [CrossRef] 32. Council on Environmental Health. Chemical-Management Policy: Prioritizing Children’s Health. Pediatrics 2011, 127, 983–990. [CrossRef][PubMed] 33. ACRODUR® Formaldehyde-Free Acrylic Binders. Available online: https://www.basf.com/us/en/products/General-Business- Topics/dispersions/Products/Acrodur-acrylic-resins.html (accessed on 18 October 2020). 34. Islam, M.; Miao, M. Optimising Processing Conditions of Flax Fabric Reinforced Acrodur Biocomposites. J. Compos. Mater. 2014, 48, 3281–3292. [CrossRef] 35. Medina, L.; Schledjewski, R.; Schlarb, A.K. Process Related Mechanical Properties of Press Molded Natural Fiber Reinforced Polymers. Compos. Sci. Technol. 2009, 69, 1404–1411. [CrossRef] 36. Rasyid, M.A.; Salim, M.S.; Akil, H.M.; Ishak, Z.M. Flammability and Thermal Properties Studies of Nonwoven Flax Reinforced Acrylic Based Polyester Composites. In Proceedings of the AIP Conference Proceedings, Langkawi, Malaysia, 20–21 November 2017; p. 030010. 37. Caliendo, H. Natural Fiber Composites Gaining Traction in Automotive. Available online: https://www.compositesworld.com/ articles/natural-fiber-composites-gaining-traction-in-automotive (accessed on 24 August 2020). 38. Saleem, A.; Medina, L.; Skrifvars, M. Mechanical Performance of Hybrid Bast and Basalt Fibers Reinforced Polymer Composites. J. Polym. Res. 2020, 27, 61. [CrossRef] 39. Salim, M.S.; Rasyid, M.F.A.; Taib, R.M.; Ishak, Z.A.M. Processing Parameters Optimisation of Nonwoven Kenaf Reinforced Acrylic Based Polyester Composites. In Proceedings of the AIP Conference Proceedings, Langkawi, Malaysia, 20–21 November 2017; p. 110007. 40. Peirson, B. Comparison of Specific Properties of Engineering Materials; School of Engineering Grand Valley State University: Kochi, India, 2005; p. 14. 41. Rao, K.M.M.; Rao, K.M.; Prasad, A.V.R. Fabrication and Testing of Natural Fibre Composites: Vakka, Sisal, Bamboo and Banana. Mater. Des. 2010, 31, 508–513. [CrossRef] 42. Prasad, A.V.R.; Rao, K.M. Mechanical Properties of Natural Fibre Reinforced Polyester Composites: Jowar, Sisal and Bamboo. Mater. Des. 2011, 32, 4658–4663. [CrossRef] 43. Yousif, B.F. Effect of Oil Palm Fibres Volume Fraction on Mechanical Properties of Polyester Composites. Int. J. Mod. Phys. B 2010, 24, 4459–4470. [CrossRef] 44. Dittenber, D.B.; GangaRao, H.V.S. Critical Review of Recent Publications on Use of Natural Composites in Infrastructure. Compos. Part A Appl. Sci. Manuf. 2012, 43, 1419–1429. [CrossRef] 45. Nimanpure, S.; Hashmi, S.A.R.; Kumar, R.; Bhargaw, H.N.; Kumar, R.; Nair, P.; Naik, A. Mechanical, Electrical, and Thermal Analysis of Sisal Fibril/Kenaf Fiber Hybrid Polyester Composites. Polym. Compos. 2019, 40, 664–676. [CrossRef] 46. ISO. Textiles—Determination of Linear Density—Gravimetric Method and Vibroscope Method; ISO 1973:1995; Beuth Verlag GmbH: Berlin, Germany, 1995. 47. ISO. Textiles—Fibres—Determination of Breaking Force and Elongation at Break of Individual Fibres; ISO 5079:1979; Beuth Verlag GmbH: Berlin, Germany, 1979. 48. Mueller, D.H.; Krobjilowski, A. New Discovery in the Properties of Composites Reinforced with Natural Fibers. J. Ind. Text. 2003, 33, 111–130. [CrossRef] J. Compos. Sci. 2021, 5, 100 18 of 18

49. Ismail, N.F.; Sulong, A.B.; Muhamad, N.; Tholibon, D.; MdRadzi, M.K.; Wanlbrahim, W.A.S. Review of the Compression Moulding of Natural Fiber-Reinforced Thermoset Composites: Material Processing and Characterisations. Pertanika J. Agric. Sci. 2015, 38, 533–547. 50. ISO. Plastics—Determination of Flexural Properties; ISO 178:2019; Beuth Verlag GmbH: Berlin, Germany, 2019. 51. ISO. Plastics—Determination of Tensile Properties—Part 4: Test Conditions for Isotropic and Anisotropic Fibre-Reinforced Plastic Composites; ISO 527-4:1997; Beuth Verlag GmbH: Berlin, Germany, 1997. 52. ISO. Plastics—Determination of Charpy Impact Properties—Part 1: Non-Instrumented Impact Test; ISO 179-1:2010-06; Beuth Verlag GmbH: Berlin, Germany, 2010. 53. Khalfallah, M.; Abbès, B.; Abbès, F.; Guo, Y.Q.; Marcel, V.; Duval, A.; Vanfleteren, F.; Rousseau, F. Innovative Flax Tapes Reinforced Acrodur Biocomposites: A New Alternative for Automotive Applications. Mater. Des. 2014, 64, 116–126. [CrossRef] 54. Braun, E.; Levin, B.C. Polyesters: A Review of the Literature on Products of Combustion and Toxicity. Fire Mater. 1986, 10, 107–123. [CrossRef] 55. Summerscales, J.; Dissanayake, N.P.J.; Virk, A.S.; Hall, W. A Review of Bast Fibres and Their Composites. Part 1—Fibres as Reinforcements. Compos. Part A Appl. Sci. Manuf. 2010, 41, 1329–1335. [CrossRef] 56. Masseteau, B.; Michaud, F.; Irle, M.; Roy, A.; Alise, G. An Evaluation of the Effects of Moisture Content on the Modulus of Elasticity of a Unidirectional Flax Fiber Composite. Compos. Part A Appl. Sci. Manuf. 2014, 60, 32–37. [CrossRef] 57. Ouajai, S.; Shanks, R.A. Composition, Structure and Thermal Degradation of Hemp Cellulose after Chemical Treatments. Polym. Degrad. Stab. 2005, 89, 327–335. [CrossRef] 58. Menczel, J.D.; Prime, R.B. (Eds.) Thermal Analysis of Polymers: Fundamentals and Applications; John Wiley: Hoboken, NJ, USA, 2009; ISBN 978-0-471-76917-0. 59. García-Manrique, J.A.; Marí, B.; Ribes-Greus, A.; Monreal, L.; Teruel, R.; Gascón, L.; Sans, J.A.; Marí-Guaita, J. Study of the Degree of Cure through Thermal Analysis and Raman Spectroscopy in Composite-Forming Processes. Materials 2019, 12, 3991. [CrossRef][PubMed] 60. Mihalic, M.; Sobczak, L.; Pretschuh, C.; Unterweger, C. Increasing the Impact Toughness of Cellulose Fiber Reinforced Polypropy- lene Composites—Influence of Different Impact Modifiers and Production Scales. J. Compos. Sci. 2019, 3, 82. [CrossRef] 61. Lee, S.; Lee, D.K. What Is the Proper Way to Apply the Multiple Comparison Test? Korean J. Anesthesiol. 2018, 71, 353–360. [CrossRef][PubMed] 62. Benkhelladi, A.; Laouici, H.; Bouchoucha, A. Tensile and Flexural Properties of Polymer Composites Reinforced by Flax, Jute and Sisal Fibres. Int. J. Adv. Manuf. Technol. 2020, 108, 895–916. [CrossRef] 63. Minitab, LLC. Example of One-Way ANOVA.Minitab 18 Support; Minitab, LLC: State College, PA, USA, 2019. 64. Ralph, C.; Lemoine, P.; Archer, E.; McIlhagger, A. Mechanical Properties of Short Basalt Fibre Reinforced Polypropylene and the Effect of Fibre Sizing on Adhesion. Compos. Part B Eng. 2019, 176, 107260. [CrossRef] 65. Spearing, S.M.; Evans, A.G. The Role of Fiber Bridging in the Delamination Resistance of Fiber-Reinforced Composites. Acta Metall. Mater. 1992, 40, 2191–2199. [CrossRef]