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Article The Effect of Poly(Vinyl Chloride) Powder Addition on the Thermomechanical Properties of Epoxy Composites Reinforced with Basalt Fiber

Danuta Matykiewicz 1,* , Kamila Sałasi ´nska 2 and Mateusz Barczewski 1

1 Institute of Materials Technology, Faculty of Mechanical Engineering, Poznan University of Technology, Piotrowo 3, 61-138 Pozna´n,Poland; [email protected] 2 Central Institute for Labour Protection—National Research Institute, Department of Chemical, Biological and Aerosol Hazards, Czerniakowska 16, 00-701 Warsaw, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-616-475-858

 Received: 4 July 2020; Accepted: 12 August 2020; Published: 15 August 2020 

Abstract: The aim of the article was to determine the effect of the poly(vinyl chloride) additive (PVC) on the thermomechanical and fire properties of epoxy composites reinforced with basalt fabric. Ten-layered composites with 2.5, 5 and 10 wt.% of PVC powder were fabricated using hand lay-up. The following features were evaluated for composites: structure (by scanning electron microscopy, SEM), thermomechanical properties (by dynamical thermomechanical analysis, DMTA), mechanical properties (in bending, tensile and interlaminar shear strength tests), hardness (using the Barcol method), thermal stability (by thermogravimetry, TGA) and fire behavior (using a cone calorimeter). It was found that the introduction of micron PVC powder into the epoxy matrix improved the thermomechanical properties of composites, such as storage module, and mechanical properties, such as flexural strength and modulus, as well as hardness.

Keywords: epoxy; poly(vinyl chloride); basalt fiber; thermomechanical properties

1. Introduction Technological development in various branches of industry creates the need for new construction materials [1–3]. Therefore, in recent years composites have become a widely used construction material in many industries and the requirements for these materials are continually growing [4–7]. Due to the low strength and fire resistance of epoxy composites, various types of fillers and reinforcing fibers are introduced in order to improve their properties [8–10]. Due to the variety of interactions that may occur between individual components in a composite, the assessment of its properties is critical in order to determine its application potential. Epoxy reinforced with different types of particles can be classified as hybrid materials, which are designed using substances with different properties and form in order to achieve a significant improvement in the properties of the polymer matrix [11,12]. The following techniques are used to produce layered composites, depending on the shape, type of , permeability of the fibers and the time needed to make the element; hand lay-up process, resin , vacuum-assisted resin transfer molding, and hot or cold pressing. The influence of selected forming methods on the properties of finished products has been widely described in the literature [13–16]. Basalt fiber (BF) is characterized by high strength, as well as chemical and thermal stability, which is why it has found wide application as a reinforcing material in polymer composites. Sarasini et al. proved that hybrid composites based on aramid and basalt woven fabrics have a better impact energy absorption capability than composites reinforced only with aramid fabric and enhanced damage tolerance [17]. Wang et al. described the degradation of the tensile

Materials 2020, 13, 3611; doi:10.3390/ma13163611 www.mdpi.com/journal/materials Materials 2020, 13, 3611 2 of 13 properties of prestressed basalt fiber-reinforced polymer (BFRP) and hybrid FRP tendons in a marine environment [18]. Tirillò et al. described the effects of basalt fiber hybridization on high-velocity impact response of fabric-reinforced epoxy composites [19]. Double modification of epoxy resin—through simultaneous reinforcement with a fiber filler and introducing dispersed powder filler—creates an opportunity to improve the mechanical, thermal, electrical or tribological of the composites. Li et al. used nanoparticles, such as montmorillonite, nano SiO2 and montmorillonite/nanoSiO2 double nanoparticles, for the modification of the epoxy/basalt fiber composites [20]. The combined nano-modifier composed of montmorillonite and nano SiO2 improved the interlaminar shear strength and impact strength of the epoxy/basalt fiber composites. Subagia et al. investigated the effect of different tourmaline micro/nano particle loading on the tensile and flexural properties of a basalt fiber-reinforced epoxy [21]. They found that the addition of tourmaline significantly improved the tensile and flexural strength and modulus of the basalt/epoxy composite. Bulut proved that the addition of nanopellets at low concentration into the epoxy matrix improves the mechanical properties of epoxy composites containing basalt fiber [22]. Natural graphite flakes used as a filler in the production of an epoxy composite reinforced with basalt fibers improved their mechanical properties, as described in the paper [23]. Khosravi et al. described the influence of the addition of multi-walled carbon nanotubes (MWCNT) to the epoxy matrix on the mechanical properties of unidirectional basalt fiber/epoxy composites [24]. While Kim et al. used MWCNT-coated basalt fibers as a reinforcement for epoxy resin [25], these composites showed highly improved flexural behaviors compared to the reference material. Abdi et al. reported that, in the case of basalt–epoxy composites modified with nano CaCO3, the tensile and flexural strength values increase with the amount of a introduced nanofiller as a result of improved load transfer between the nanocomposite matrix and BF [26]. In turn, Surana et al. used sawdust from the industry as a filler for basalt–epoxy composites, which improved the mechanical properties [27]. Obtaining a specific modification efficiency depends on the amount of polymer matrix filler used; for nanofillers it is usually up to 1% and it can reach up to 40% by weight in the case of micro fillers. Products made of PVC are characterized by favorable properties and a relatively long life cycle. Still, finding an area to use recyclate from post-consumer PVC products seems to be an important ecological and technological issue. To the best of the authors’ knowledge, the available literature does not mention the use of powdered PVC as an effective modifier of the properties of basalt–epoxy composites. It should also be emphasized that the application of poly(vinyl chloride) powder to modify the properties of other fiber reinforced composites has not been described. In our previous work, we presented poly(vinyl chloride) powder as an inexpensive flame-retardant epoxy resin [28]. Therefore, the purpose of this work was to determine the possibility of using powdered PVC as a modifier in an epoxy composite reinforced with basalt fiber. The following features were evaluated for composites: structure (by scanning electron microscopy, SEM), thermomechanical properties (by dynamical thermomechanical analysis, DMTA), mechanical properties (in bending and interlaminar shear strength tests), hardness (using the Barcol method), thermal stability (by thermogravimetry, TGA) and fire behavior (using a cone calorimeter).

2. Materials and Methods

2.1. Materials

Epoxy resin Epidian 6 (EP6) based on A (BPA) ( at 25 ◦C; 10,000–15,000 mPas) with agent—triethylenetetramine (Z1)—both produced by CIECH Sarzyna S.A., Poland—were used as the matrix of the composites. Suspension poly(vinyl chloride) Polanvil S-67 (PVC) in the form of powder with particle size below 250 µm and basalt fiber woven fabric (BASALTEX)-type BAS 210.1270.P with 210 g/m2 were used as the filler. The epoxy resin was mixed with Z1 at the ratio: 13 parts Z1 per 100 parts EP6 by weight. Materials 2020, 13, 3611 3 of 13

2.2. Sample Preparation The epoxy resin was mixed using a mechanical stirrer Disperlux (proLAB, Gda´nsk,Poland), with 2.5, 5 and 10 wt.% of PVC powder to the total weight of composition, respectively, under subatmospheric pressure conditions (0.2 Bar). The resulting compositions were mixed with a curing agent Z1 [28]. The epoxy composites were fabricated in a mold using hand lay-up, cured at ambient temperature (23 ◦C) for 24 h and post-cured at 80 ◦C for 3 h. They contained 10 layers of woven fabric. The samples were designated as 0PVC; 2.5PVC; 5PVC; 10PVC adequately to the PVC powder content.

2.3. Methods The morphologies of the composites were monitored by Scanning Electron Microscopy (SEM). The fracture surfaces of the samples after impact tests were photographed at the magnification of 1000 with a scanning Carl Zeiss Evo 40 (Oberkochen, Germany) and with an electron accelerating voltage of 12 kV. All materials were covered with a layer of gold. Dynamic thermomechanical analysis (DMTA) was used to assess the thermomechanical properties of the composites using the Anton Paar MCR 301 apparatus (Graz, Austria) in the torsion mode, at a frequency of 1 Hz, in the temperature range of 25 ◦C–180◦ C and a heating rate of 2 ◦C/min. The transition temperature (Tg) was determined for the maximum tan δ value. The material properties for bending were determined during a three-point bending test in accordance with DIN EN ISO 14125 using a Zwick Roell Z010 (Ulm, Germany) machine at a test speed of 1 mm/min and with a 10 kN load sensor. The width of the sample was 10 mm, the length of the span was calculated as 16-times the thickness of the sample. Tensile mechanical behaviors were analyzed in accordance with ISO 527-4 by an INSTRON 4481 universal testing machine at 25 ◦C with a testing speed of 5 mm/min and a load cell of 50 kN. Interlaminar Shear Strength Test (ILSS) was carried out in accordance with the EN2563.Zwick Roell Z010 machine at a test speed of 1 mm/min and with a 10 kN load sensor. Specimen dimensions for the short-beam shear test were 20 mm 10 mm 2 mm and the length of the span was 10 mm. × × The internal laminar shear strength of the composites was calculated using Equation (1).

3P τ = (1) 4bh where: τ—the maximum ILSS at failure; P—the maximum load on the specimen; b—the width of the specimen; h—thickness of the specimen. The hardness of the materials was determined with an ASTM D2583 standard Barcol hardness tester GYZJ-934-1 (Barber Colman Co., Loves Park, IL, USA). The assessment of thermal stability of composites by the thermogravimetric method (TGA) was conducted in an air atmosphere in the temperature range from 30 ◦C to 900 ◦C and at the heating rate of 10 ◦C/min using a Netzsch TG 209 F1 apparatus (Selb, Germany) in ceramic pans. The initial decomposition temperature T10% was defined as the temperature at which the weight loss was 10% and the residual mass was determined at 900 ◦C. The maximum temperature of the thermal degradation and rate was also determined based on derivative thermogravimetric curves (DTG). The fire behavior was determined by a cone calorimeter (Fire Testing Technology Ltd., East Grinstead, UK), in accordance with ISO 5660-1. The samples (100 100 4 mm3) were × × wrapped in an aluminum foil and placed on the holder in a steel frame, providing a surface area of 88.4 cm2. All specimens were irradiated horizontally at a heat flux of 35 kW/m2. After tests, the residues were photographed using a digital camera (EOS 400 D, Canon Inc. Tokyo, Japan).

3. Results

3.1. Composites Structure The fracture morphology of composite samples was assessed using the SEM method. For all the tested samples, no significant differences were observed at the epoxy fiber-matrix interface (Figure1a–d). Materials 2020, 13, 3611 4 of 13

MaterialsMaterials 2020 2020, ,13 13, ,x x FOR FOR PEER PEER REVIEW REVIEW 44 of of 13 13 This may indicate a good combination of all components in this layered composite. The fibers in the in the composites 5PVC and 10PVC were found to be more closely connected than in the reference incomposites the composites 5PVC and5PVC 10PVC and 10PVC were found were tofound be more to be closely more closely connected connected than in than the reference in the reference sample sample (Figure 1c,d). Obtaining a favorable composite structure by properly connecting the epoxy (Figuresample1 (Figurec,d). Obtaining 1c,d). Obtaining a favorable a favorable composite comp structureosite by structure properly by connecting properly theconnecting epoxy matrix the epoxy with matrix with the fiber and led to the improvement of the mechanical properties of the materials matrixthe fiber with and the filler fiber led and to the filler improvement led to the improvem of the mechanicalent of the properties mechanical of the properties materials of [29 the]. Moreover,materials [29]. Moreover, SEM images of samples after ILSS tests are presented in Figure 2. The resulting [29].SEM Moreover, images of samplesSEM images after ILSSof samples tests are after presented ILSS tests in Figure are presented2. The resulting in Figure pattern 2. The of resulting damage pattern of damage in all cases was similar. The single pulled out fibers were not observed, which patternin all cases of damage was similar. in all Thecases single was pulledsimilar. out The fibers single were pulled not observed,out fibers whichwere not confirms observed, their which good confirms their good connection with the matrix. Arrows indicate the distribution of PVC particles. confirmsconnection their with good the matrix.connection Arrows with indicatethe matrix. the Ar distributionrows indicate of PVC the particles.distribution of PVC particles.

FigureFigure 1.1. StructureStructureStructure ofof compositescomposites (( aa)) 0PVC;0PVC; (( bb)) 2.5PVC;2.5PVC; (( cc)) 5PVC;5PVC; (( dd)) 10PVC. 10PVC.

FigureFigure 2.2. StructureStructureStructure ofof compositescomposites afterafter ILSSILSS testtest (( aa)) 0PVC;0PVC; (( bb)) 2.5PVC;2.5PVC; (( cc)) 5PVC;5PVC; ( (dd)) 10PVC. 10PVC.

Materials 2020, 13, x FOR PEER REVIEW 5 of 13

3.2. Thermomechanical Properties The thermomechanical properties of the composites are one of the critical criteria determining the area of their application as construction materials. Therefore, the dynamic thermomechanical analysis provides important information on material properties, such as temperature, storage modulus, and damping factor. The storage modulus describes the amount of energy stored by the sample during one cycle of measurement [30]. Figure 3 shows the dependence of storage Materialsmodulus2020 and, 13 ,damping 3611 factor on temperature. Two peaks were observed on the tan δ curves5 offor 13 composites with PVC; the first one in the temperature range of 90–100 °C, which can be attributed to 3.2.the glass Thermomechanical transition of Propertiespoly(vinyl chloride), and the second one in the range of 120–130 °C, which can be attributed to the glass transition of epoxy resin. It should be emphasized that, for all the tested PVC-modifiedThe thermomechanical composites, the properties value of of storage the composites modulus are was one high of theer criticalthan for criteria the reference determining sample. the areaG’ values of their determined application at as30 construction°C for 2.5PVC, materials. 5PVC and Therefore, 10PVC samples the dynamic were thermomechanical similar and averaged analysis 3200 providesMPa, whereas important for the information sample 0PVC on it material was 2870 properties, MPa (Table such 1). asIt can glass be transition concluded temperature, that well-dispersed storage modulus,PVC particles and dampingcan improve factor. stiffness The storage and act modulus as a reinforcement describes the amountin epoxy of composites, energy stored which by the is sampleconsistent during with one our cycle previous of measurement work [28]. [Thermomechanical30]. Figure3 shows theproperties dependence of fiber-reinforced of storage modulus materials and dampingare strictly factor dependent on temperature. on the type Two of peaks fiber, wereits weave, observed the ontype the of tan matrixδ curves used, for its composites continuity with and PVC; the thepresence first one of other in the reinforcing temperature molecules. range of 90–100Mineral◦C, fill whichers, if cannot befunctionalized, attributed to most the glass often transition physically of poly(vinylinteract with chloride), the polymer and the matrix, second while one inPVC, the rangedue to of the 120–130 Cl atom◦C, present, which can may be be attributed reactive to with the glassnon- transitioncrosslinked of epoxy epoxy resin. It should [31]. Therefore, be emphasized when that,analyzing for all the the G’ tested value PVC-modified above the glass composites, transition thetemperature value of storageat 130 °C, modulus a favorable was effect higher is than observed for the with reference the presence sample. of G’modifier values particles determined in the at 30epoxy◦C formatrix, 2.5PVC, because 5PVC these and 10PVCvalues are samples much were higher similar than andfor PVC averaged for samples 3200 MPa, 2.5PVC whereas and for5PVC. the sampleThis may 0PVC indicate it was the 2870 favorable MPa (Tablethermomechanical1). It can be concludedproperties of that these well-dispersed materials and PVC higher particles resistance can improveto temperature stiffness changes and act [9]. asa reinforcement in epoxy composites, which is consistent with our previous work [28]. Thermomechanical properties of fiber-reinforced materials are strictly dependent on the type of fiber, its weave, the type of matrix used,Table its1. DMTA continuity analysis and results. the presence of other reinforcing molecules. fillers, if not functionalized, most often physically interact with the polymer matrix, while PVC, due to the ClName atom present, G’(MPa) may at be 30 reactive °C G’(MPa) with non-crosslinked at 130 °C TgI peak epoxy (°C) monomers TgII peak (°C) [31 ]. Therefore, 0PVC 2870 128 116 − when analyzing the G’ value above the glass transition temperature at 130 ◦C, a favorable effect is observed with2.5PVC the presence of3250 modifier particles in 225 the epoxy matrix, 97 because these 130 values are much higher than for5PVC PVC for samples3180 2.5PVC and 5PVC. 221 This may indicate 100 the favorable 132 thermomechanical properties of10PVC these materials 3260 and higher resistance 100 to temperature changes 96 [9]. 127

Figure 3. DMTA curves of investigated materials. Table 1. DMTA analysis results.

Name G’(MPa) at 30 ◦C G’(MPa) at 130 ◦C TgI peak (◦C) TgII peak (◦C) 0PVC 2870 128 116 − 2.5PVC 3250 225 97 130 5PVC 3180 221 100 132 10PVC 3260 100 96 127 Materials 2020, 13, x FOR PEER REVIEW 6 of 13 Materials 2020, 13, 3611 6 of 13 3.3. Flexural Behaviors 3.3. FlexuralThe bending Behaviors resistance of fiber-reinforced composite materials is important for their functional properties.The bending Flexural resistance strength of values fiber-reinforced for the modifi compositeed 2.5PVC materials and 5PVC is important composites for theirwere functional similar to properties.the unmodified Flexural sample strength (Table values 2). for Figure the modified 4 shows 2.5PVC the dependence and 5PVC composites of the obtained were similar results to the of unmodifiedmechanical sampletests on (Table the content2). Figure of4 theshows PVC the additive. dependence However, of the obtained a significant results improvement of mechanical of tests this onvalue the contentis observed of the from PVC 250 additive. MPa However,to 275 MPa a significant for the composite improvement with of the this highest value iscontent observed of fromPVC 250powder MPa (10 to 275 wt.%). MPa The for values the composite of the flexural with the modu highestlus were content similar of PVC for powderall the modified (10 wt.%). materials The values and ofslightly the flexural highermodulus than for the were reference similar sample. for all the The modified addition materials of PVC powder and slightly may improve higher than the rigidity for the referenceof the tested sample. materials The additionand make of it PVC possible powder to ob maytain improve a composite the rigiditywith specific of the functionality tested materials [21]. and make it possible to obtain a composite with specific functionality [21]. Table 2. Mechanical properties of composites.

Flexural TableFlexural 2. Mechanical propertiesTensile of composites.Tensile Elongation at Name FlexuralStrength ModulusFlexural TensileStrength Strength ModulusTensile Modulus Elongation at Name Break (%) Strength(MPa) (MPa) Modulus(GPa) (GPa) (MPa)(MPa) (GPa)(GPa) Break (%) 0PVC0PVC 250 250 14± 14 10.1 10.1 ± 0.50.5 307 307 ± 9 9 7.7 ± 1.2 7.7 1.2 4.7 ± 0.2 4.7 0.2 ± ± ± ± ± 2.5PVC2.5PVC 252 252 12± 12 11.6 11.6 ± 0.50.5 262 262 ± 8 8 7.8 ± 0.8 7.8 0.8 4.4 ± 0.2 4.4 0.2 ± ± ± ± ± 5PVC 257 11 11.8 0.7 255 7 7.5 0.2 4.1 0.2 5PVC 257± ± 11 11.8 ± 0.7 255 ± 7± 7.5 ± 0.2 ± 4.1 ± 0.2 ± 10PVC 275 13 11.8 0.6 263 3 8.7 0.6 4.4 0.1 10PVC 275± ± 13 11.8 ± 0.6 263 ± 3± 8.7 ± 0.6 ± 4.4 ± 0.1 ±

FigureFigure 4.4. MechanicalMechanical propertiesproperties ofof thethe composites.composites.

3.4.3.4. TensileTensile BehaviorsBehaviors TensileTensile strengthstrength valuesvalues usuallyusually didiffffererfrom fromflexural flexuralstrength. strength. ForFor thethe testedtested materials,materials, aa decreasedecrease inin tensiletensile strengthstrength waswas notednoted inin thethe groupgroup ofof modifiedmodified PVCPVC compositescomposites comparedcompared toto thethe unmodifiedunmodified compositescomposites (Table(Table2 ).2). Nevertheless,Nevertheless, Young’sYoung’s modulusmodulus values values for for 2.5PVC 2.5PVC and and 5PVC 5PVC samples samples were were similarsimilar toto thethe 0PVC0PVC sample,sample, andand aa significantsignificant improvementimprovement inin thisthis parameterparameter waswas notednoted forfor thethe 10PVC10PVC sample.sample. This is is consistent consistent with with our our previous previous result results,s, where where the the addition addition of ofPVC PVC powder powder improves improves the thestiffness stiffness of materials of materials [28]. [28 In]. Inorder order to to compare compare th thee obtained obtained mechanical mechanical properties properties of thethe testedtested composites,composites, aa tabletable withwith literatureliterature datadata onon similarsimilar systemssystems basedbased onon epoxyepoxy resinresin reinforcedreinforced withwith basaltbasalt fiberfiber andand modifiedmodified withwith powderpowder fillersfillers isis presentedpresented (Table(Table3 ).3). In In the the case case of of 10-layer 10-layer basalt–epoxy basalt–epoxy composites modified with graphene nanoplatelets (GNPs) (0.1–0.3 wt.%), similar level values of

Materials 2020, 13, 3611 7 of 13 composites modified with graphene nanoplatelets (GNPs) (0.1–0.3 wt.%), similar level values of flexural strength (275 MPa) and tensile strength (263 MPa) were obtained for composites with PVC [22]. Langovan et al. described that the addition of silica nanoparticles (SiO2) at 1.5 wt.% to epoxy/basalt composite pipes improved their flexural properties [32]. In the case of nanofillers, their addition at the level of 0.1–1 wt.% is enough to obtain improved properties. For fillers with a larger particle size, a much larger amount of additive is required. The beneficial effect of introducing sawdust (at 2 wt.%) into the epoxy matrix for six-layer basalt composites was described by Surana et al. [27].

Table 3. Mechanical properties of basalt/epoxy composites with different filler.

wt.% of Fillers Flexural Flexural Tensile Strength Tensile Modulus Ref. and Name Strength (MPa) Modulus (GPa) (MPa) (GPa) 0 GNPs 210 4.8 212 13.1 0.1 GNPs 273 8.1 240 15.9 [18] 0.2 GNPs 232 7.8 224 14.2 0.3 GNPs 220 5.7 223 13.3 0 SiO 108 9 2 − − 0.5 SiO 109 9.1 [26] 2 − − 1 SiO 132 9.6 2 − − 1.5 SiO 137 10.9 2 − − 0 194 7.2 − − 2 sawdust 252 7.5 [22] − − 4 sawdust 205 7.0 − − 6 sawdust 177 6.5 − −

3.5. Interlaminar Shear Strength Interlaminar Shear Strength is the possibility to resist the inter-laminar deformation under load [33]. This is especially important in the case of composites reinforced with fibers and modified with other particles. The composites ILSS as well as the maximum load values determined, and the maximum load are summarized in the Table4. The ILSS values for the modified composites were higher than those of the reference sample, which may indicate the beneficial effect of the PVC addition, and is in good agreement with the results obtained in the flexural test. This confirms the good strength of the interfacial bond between all composite components. In the work of Scalici et al. [13] the ILSS value for eight-layer basalt–epoxy composites was 18.9 MPa for composites produced by resin infusion and 21.1 MPa for composites produced by vacuum bagging. Therefore, it can be concluded that the results obtained for the tested PVC composites are satisfactory. Moreover, according to the literature, the Cl atom of PVC can attack the oxirane ring and chemically bond to the epoxy matrix [31]. Additionally, the recorded maximum force for all PVC composites was higher than for the neat sample. The highest values ILSS and p were recorded for the 5PVC sample. In the case of 10PVC these values were lower, which may be due to the tendency of a large amount of PVC particles to agglomerate. Figure5 presents the dependence of load on displacement for investigated materials. The curves of all samples are characterized by the first linear region, followed by a sudden decrease and increase in load.

Table 4. ILSS of composites.

Name ILSS (MPa) Max. Load, P (N) 0PVC 22.6 0.6 756 45 ± ± 2.5PVC 23.9 0.3 960 25 ± ± 5PVC 26.7 0.4 1100 22 ± ± 10PVC 23.8 0.8 806 60 ± ± Materials 2020, 13, 3611 8 of 13 Materials 2020, 13, x FOR PEER REVIEW 8 of 13

Figure 5. TheThe curves curves of load on displacement for investigated materials.

3.6. Hardness 3.6. Hardness The Barcol hardness methods determine the hardness of composite materials by the depth of The Barcol hardness methods determine the hardness of composite materials by the depth of penetration of the indenter into the material [34]. Barcol hardness values were, respectively, 45 for the penetration of the indenter into the material [34]. Barcol hardness values were, respectively, 45 for 0PVC sample, and an average of 55 for samples modified with powdered PVC, which confirmed the the 0PVC sample, and an average of 55 for samples modified with powdered PVC, which confirmed thebeneficial beneficial effect effect of introducing of introducing this this additive additive into into the epoxythe epoxy matrix. matrix. 3.7. Thermal Stability 3.7. Thermal Stability The thermogravimetric method was used to evaluate the thermal stability of the composites. The thermogravimetric method was used to evaluate the thermal stability of the composites. The The results are summarized in Table5. The introduction of PVC powder reduced the T10% value of results are summarized in Table 5. The introduction of PVC powder reduced the T10% value of composites, caused by a partial distribution of PVC grains at a lower temperature than the pure epoxy composites, caused by a partial distribution of PVC grains at a lower temperature than the pure epoxy resin. The residue mass after the test was similar in all cases, and on average reached 50 wt.%. It should resin. The residue mass after the test was similar in all cases, and on average reached 50 wt.%. It be emphasized that the addition of PVC to the epoxy matrix delayed its degradation. This confirms should be emphasized that the addition of PVC to the epoxy matrix delayed its degradation. This the shift in the maximum decomposition temperature DTG peak towards higher temperatures for confirms the shift in the maximum decomposition temperature DTG peak towards higher composite samples compared to the reference sample. The characteristic thermogravimetric curves are temperatures for composite samples compared to the reference sample. The characteristic shown in Figure6. A two-stage distribution was observed for all the tested composites. thermogravimetric curves are shown in Figure 6. A two-stage distribution was observed for all the tested composites.Table 5. TG and DTG values of the materials examined in air atmosphere.

Table 5. TG and DTG values of the materials examinedDTG Peak in air atmosphere.Max Degradation Name T10% (◦C) Residual Mass (%) Temperature (◦C) Rate (%/min) T10% Residual DTG Peak Max Degradation NamePVC 271.0 1.05 287.8 17.74 0PVC(°C) 350.4 Mass (%) 51.5Temperature (°C) 331.6 Rate (%/min) 5.78 2.5PVCPVC 271.0 337.6 1.05 50.9 287.8 352.5 17.74 5.24 0PVC5PVC 350.4 330.7 51.5 47.9 331.6 347.2 5.78 5.64 2.5PVC10PVC 337.6 317.5 50.9 45.4 352.5 351.2 5.24 5.70 5PVC 330.7 47.9 347.2 5.64 10PVC 317.5 45.4 351.2 5.70

Materials 2020, 13, 3611 9 of 13 Materials 2020, 13, x FOR PEER REVIEW 9 of 13

FigureFigure 6.6. The characteristic thermogravimetricthermogravimetric curves ofof thethe composites.composites.

3.8. Fire Behavior 3.8. Fire Behavior ConeCone calorimeter teststests were conducted to investigate the fire fire behavior of epoxy resin composites, andand thethe relevantrelevant data,data, suchsuch asas time to ignition (TTI), time to flameoutflameout (TTF),(TTF), averageaverage heatheat release raterate (av-HRR),(av-HRR), totaltotal heatheat releaserelease (THR),(THR), maximummaximum averageaverage raterate ofof heatheat emissionemission (MAHRE),(MAHRE), firefire residueresidue andand specificspecific extinctionextinction areaarea (SEA)(SEA) areare listedlisted inin TableTable6 .6.

Table 6. Cone calorimeter results of investigated composites (irradiance 35 kW/m2). Table 6. Cone calorimeter results of investigated composites (irradiance 35 kW/m2). TTI TTF av-HRR MARHE THR Fire Residue SEA Sample Fire (s) TTI (s)TTF (kWav-HRR/m2) MARHE(kW/m2) THR(MJ/m 2) (%) SEA (m2/kg) Sample Residue 0PVC 63 (11 a(s)) 321 (32)(s) 239.0(kW/m (8)2) (kW/m 292.2 (10)2) (MJ/m 63.02 (2)) 51.1 (3)(m2/kg) 865.2 (32) (%) 2.5PVC 61 (10) 333 (13) 242.0 (13) 318.1 (2) 66.2 (6) 50.5 (2) 848.3 (42) a 5PVC0PVC 54 (12)63 (11 303) 321 (25) (32) 245.4 239.0 (34) (8) 292.2 316.3 (10) (18) 63.0 52.9 (2) (20) 51.1 (3) 50.5 (1) 865.2 (32) 898.2 (28) 10PVC2.5PVC 63 (5) 61 (10) 375 333 (27) (13) 215.1 242.0 (23) (13) 318.1315.2 (16)(2) 66.2 68.1 (6) (8) 50.5 (2) 47.5 (2) 848.3 (42) 923.7 (54) 5PVC 54 (12) a 303The (25) values 245.4 in parentheses (34) 316.3 are the (18) standard 52.9 deviations. (20) 50.5 (1) 898.2 (28) 10PVC 63 (5) 375 (27) 215.1 (23) 315.2 (16) 68.1 (8) 47.5 (2) 923.7 (54) As presented in Tablea The6, the values addition in parentheses of PVC powderare the standard resulted deviations in TTI reduction,. with the single exception of the 10PVC composite; moreover, in most cases, a higher TTF was observed. TheAs presented HRR curves in Table of composites 6, the addition are shown of PVC in Figurepowder7. resulted All the investigated in TTI reduction, materials with exhibitedthe single curvesexception consisting of the 10PVC of two maxima,composite; the moreover, first peaks in were most a resultcases, of a thehigher rapid TTF initial wasincrease observed. in HRR, while the secondThe HRR appeared curves at of the composites end of burning. are shown Contrary in Figu to compositesre 7. All the without investigated PVC, materials higher values exhibited were obtainedcurves consisting for the first of peak,two maxima, and the the second first peak peaks flattened were a asresult the proportionof the rapid of initial the additive increase increased. in HRR, Thewhile average the second values appeared of HRR at were the end similar of burning. to those Co obtainedntrary to forcomposites the reference without material, PVC, higher and a values slight decreasewere obtained was noted for the for first composites peak, and with the the second highest pe amountak flattened of PVC as the only proportion (Table6). Theof the congruous additive reductionincreased. inThe av-HRR average values values (by of HRR approx. were 10%) similar for to epoxy those composites obtained for containing the reference 10 wt.%material, of PVC and powdera slight wasdecrease observed was innoted our previousfor composites work [28 with]. PVC thehas highest self-extinguishing amount of PVC properties only (Table and does 6). The not self-ignitecongruous [reduction35]. Howeever, in av-HRR the introduction values (by approx. of only 10%) high for content epoxy of composites powdered containing PVC into 10 the wt.% epoxy of resinPVC haspowder a positive was observed effect on thein our fire previous resistance work of composites, [28]. PVC suchhas self-extinguishing as a reduction in heat properties release rate.and Indoes our not previous self-ignite article, [35]. a Howeever, reduction inthe HRR introduction and increase of only in TTIhigh was content achieved of powdered with 40 wt.%PVC into of PVC the additiveepoxy resin [28 ].has a positive effect on the fire resistance of composites, such as a reduction in heat release rate. In our previous article, a reduction in HRR and increase in TTI was achieved with 40 wt.% of PVC additive [28].

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Figure 7. Representative heat release rate curves of the composites. FigureFigure 7. 7. Representative Representative heat release raterate curves curves of of the the composites. composites. Maximum average rate of heat emission (MAHRE), corresponding to the peak of the cumulative MaximumMaximum average average rate rate of of heat heat emission emission (MAHRE), corresponding corresponding to to the the peak peak of of the the cumulative cumulative heat emission divided by time, is another important parameter, used to assess the influence of materials heatheat emission emission divided divided by by time,time, isis anotheranother important parameter,parameter, used used to to assess assess the the influence influence of of on fire development. Compared to the reference material, the MARHE of composites with PVC was materialsmaterials on on fire fire development. development. Compared Compared to the reference material, material, the the MARHE MARHE of of composites composites with with higherPVC andwas reachedhigher and similar reached values similar regardless values regardle of the poly(vinylss of the poly(vinyl chloride) chloride) contribution. contribution. As shown As in PVC was higher and reached similar values regardless of the poly(vinyl2 chloride) contribution. As Table6, the total heat release (THR) of the 0PVC sample was 63 MJ /m , while that2 of the composites, shownshown in in Table Table 6, 6, the the total total heat heat releaserelease (THR)(THR) of thethe 0PVC0PVC sample sample was was 63 63 MJ/m MJ/m, 2while, while that that of ofthe the in most cases, increased due to the addition of PVC. In some cases THR may be dependent on a length composites,composites, in in most most cases, cases, increasedincreased duedue to the additionaddition ofof PVC.PVC. In In some some cases cases THR THR may may be be of flame burning time (TTF), however, the 2.5PVC and 5 PVC samples showed similar values of TTF to dependentdependent on on a alength length of of flame flame burning burning timetime (TTF), however,however, the the 2.5PVC 2.5PVC and and 5 PVC5 PVC samples samples showed showed thesimilar reference values sample. of TTF to the reference sample. similar values of TTF to the reference sample. FireFire residues residues of of the the reference reference material material andand compositescomposites with with 2.5–5 2.5–5 wt.% wt.% of of PVC PVC were were similar similar and and Fire residues of the reference material and composites with 2.5–5 wt.% of PVC were similar and reachedreached approx. approx. 51%. 51%. The The addition addition ofof aa higherhigher amou amountnt of of thermoplastic powder powder caused caused a decrease a decrease in in reached approx. 51%. The addition of a higher amount of thermoplastic powder caused a decrease in thethe investigated investigated features. features. However, However, the the results results did did not not reflect reflect the the initial initial shareshare ofof PVC.PVC. TheThe reduction in the investigated features. However, the results did not reflect the initial share of PVC. The reduction thein yield the yield of residue of residue with with increasing increasing PVC PVC amount amount is is confirmed confirmed byby the photographs photographs of of samples samples after after in the yield of residue with increasing PVC amount is confirmed by the photographs of samples after conecone calorimetry calorimetry tests tests (Figure (Figure8 ).8). The The addition addition ofof PVCPVC powder did did not not have have a apositive positive effect e ffect on on the the cone calorimetry tests (Figure 8). The addition of PVC powder did not have a positive effect on the smokesmoke emission emission either. either. The The specific specific extinctionextinction area (SEA) (SEA) increased increased along along with with the the amount amount of ofPVC, PVC, smoke emission either. The specific extinction area (SEA) increased along with the amount of PVC, andand the the highest highest values values (increase (increase of of approx. approx. 7%) 7%) was was recorded recorded for for composites composites modifiedmodified withwith 10 wt.% of and the highest values (increase of approx. 7%) was recorded for composites modified with 10 wt.% poly(vinylof poly(vinyl chloride). chloride). The The increase increase in in this this value value was was significant significant for the 10PVC 10PVC sample, sample, while while for for the the of 2.5PVCpoly(vinyl and chloride). 5PVC samples, The increase these values in this were value simi waslar significant to the reference for the sample.10PVC sample,The growth while in for the the 2.5PVC and 5PVC samples, these values were similar to the reference sample. The growth in the fumes 2.5PVCfumes andemission 5PVC as samples, a result of these PVC valuespowder were addition simi islar consistent to the reference with the previoussample. resultsThe growth [28]. in the emission as a result of PVC powder addition is consistent with the previous results [28]. fumes emission as a result of PVC powder addition is consistent with the previous results [28].

Figure 8. Photographs of samples after cone calorimetry tests: (a) 0PVC, (b) 2.5PVC, (c) 5PVC, (d)

10PVC. Figure 8. PhotographsPhotographs of of samples samples after after cone cone calorimetry calorimetry tests: tests: (a) (0PVC,a) 0PVC, (b) (2.5PVC,b) 2.5PVC, (c) 5PVC, (c) 5PVC, (d) (10PVC.d) 10PVC.

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4. Conclusions The beneficial effect of introducing a powder filler in the form of poly(vinyl chloride) as a modifier in epoxy composites has been shown. The introduction of micron PVC powder into the epoxy matrix improved the thermomechanical properties of composites, such as storage modulus and mechanical properties, such as flexural strength and modulus and ILSS, as well as hardness. The TGA analysis has proven that the addition of PVC to the epoxy matrix retarded its rate of degradation. A slight decrease in the heat release rate was recorded for composites with the highest amount of PVC. Composites with 2.5 and 5 wt.% PVC showed similar fire properties to the reference sample. As for utility considerations, it is important to design materials that are safe for the environment. The introduction of powder fillers into layered composites is an easy method of obtaining materials with specific properties already at the stage of their components without the need for subsequent processing.

Author Contributions: Conceptualization, D.M.; methodology, D.M. and K.S.; validation, M.B.; formal analysis, M.B.; investigation, D.M., M.B. and K.S.; data curation, D.M. and K.S.; writing—original draft preparation, D.M.; writing—review and editing, D.M. and K.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Ministry of Science and Higher Education in Poland under Project No 0613/SBAD/4630. The cone calorimetry investigations were realized using equipment allocated in the Central Institute for Labor Protection—National Research Institute. Conflicts of Interest: The authors declare no conflict of interest.

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