Nonwoven Materials Produced by Melt Electrospinning of Polypropylene Filled with Calcium Carbonate

Article Nonwoven Materials Produced by Melt Electrospinning of Polypropylene Filled with Calcium Carbonate

Sergey N. Malakhov * , Petr V. Dmitryakov, Evgeny B. Pichkur and Sergey N. Chvalun National Research Centre “Kurchatov Institute”, Akademika Kurchatova pl., Moscow 123182, Russia; [email protected] (P.V.D.); [email protected] (E.B.P.); [email protected] (S.N.C.) * Correspondence: [email protected]

Received: 18 November 2020; Accepted: 6 December 2020; Published: 14 December 2020

Abstract: Nowadays, polypropylene-based nonwovens are used in many areas, from filtration to medicine. One of the methods for obtaining such materials is melt electrospinning. In some cases, it is especially interesting to produce composite fibers with a high degree of filling. In this work, the influence of the filling degree of isotactic polypropylene with calcium carbonate on the structure and properties of nonwoven materials obtained by melt electrospinning was studied. It was shown that electrospinning is possible, even at a filler content of 50%, while the average diameter of the fibers increases with the growth in the content of calcium carbonate. The addition of sodium stearate significantly reduces the diameter of the fibers (from 10–65 to 2–10 microns) due to reducing viscosity and increasing the electrical conductivity of the melt. Wide-angle X-ray diffraction analysis and IR spectroscopy reveal that the initial polymer and composites are characterized by the presence of stable α-form crystals, while nonwovens show a predominance of smectic mesophase. The addition of calcium carbonate leads to an increase in the hydrophobicity of the composite films, the addition of sodium stearate results in a decrease of hydrophobicity, while all nonwovens demonstrate superhydrophobic properties.

Keywords: melt electrospinning; nonwovens; composites; polypropylene; calcium carbonate

1. Introduction Polypropylene is a commodity polymer that combines a number of useful properties, i.e., low density, low moisture absorption, mechanical strength, high chemical resistance, and low cost. Fibers and nonwovens based on polypropylene are of considerable practical interest, e.g., in the fields of filtration [1–3], oil sorption and oil–water separation [4], protective coatings [5] and sensors [6], as well as in medicine and tissue engineering [7,8]. The average diameter of fibers in these materials depends on the production parameters and can vary from a few to dozens of micrometers. A number of methods can be used to obtain such materials, but due to the limited solubility of polypropylene, its processing into a fiber from a solution is difficult and, as a result, it is an object of primarily research interest. In this regard, there are a few works devoted to electrospinning from polypropylene solutions: it is reported that nonwovens were produced from solutions in decalin [9] or from mixtures of solvents based on decalin [10] and cyclohexane [11], while heating the solution to 70–130 ◦C was required to dissolve the polymer. On the other hand, it is possible to produce fibrous polypropylene materials from the melt by electrospinning [2–5,7–9,12–14], melt-blowing [1,15], centrifugal spinning [16] or by combining several of these methods [17]. Moreover, nonwovens can be produced, not only from polypropylene homopolymers, but also from its copolymers [18] and blends with other polymers [19–21]. Currently, polymer composites are the object of close attention from both scientific and practical points of view. The addition of a small amount of filler into a polymer matrix can significantly improve

Polymers 2020, 12, 2981; doi:10.3390/polym12122981 www.mdpi.com/journal/polymers Polymers 2020, 12, 2981 2 of 11 mechanical, electrophysical, thermal, barrier characteristics of the material or give it new properties, e.g., antibacterial or wound healing. Thus, layered silicate fillers (such as montmorillonite) improve the mechanical and thermal properties of the polymer [22]; hydroxyapatite can accelerate the growth of bone tissue [23]; silver nanoparticles provide the material bactericidal properties [24]; titanium dioxide has high photocatalytic activity, which is useful for air and water cleaning [25]; and the addition of carbon nanotubes allows to obtain electrically conductive polymer composites [26]. At the same time, the filler content in these cases is small and varies from fractions to a few percent. A filler can also be used to reduce the cost of manufactured products. In this case, the filler content can reach 50–60%, and it should be inexpensive. One of the most common fillers of this kind is calcium carbonate [27]. Filling polypropylene with calcium carbonate allows to further reduce the price of the material [28–35]. Thus, highly filled composite nonwoven materials based on polypropylene are of practical interest, e.g., for filtration and sorption purposes. However, as of now nonwovens are obtained from pure polymer or with low filler content (up to 5%). One of the factors limiting the usage of the melt electrospinning method is the higher average diameter of the fibers due to the higher viscosity of the polymer melts compared to the polymer solutions. Moreover, the introduction of a mineral filler usually leads to an additional increase in viscosity, and as a result, an increase in the diameter of the fibers [36]. To reduce the melt viscosity, one can increase the temperature [13] or use the polymers of lower molecular weight [8] or introduce commercially available low-molecular-weight additives for polyolefins [37]. In addition, we have previously shown that addition of higher fatty acid salts allows to reduce viscosity and enhance the electrical conductivity of the melts [14]. Thus, the aim of this work is to determine the effect of the high filler content in polypropylene on the possibility of generating fibers by melt electrospinning, as well as to study the morphology, structure, and properties of the obtained nonwoven materials.

2. Materials and Methods

2.1. Materials Polypropylene (PP) Balen 01,270 with a melt flow index of 25.6 purchased from Ufaorsintez (Ufa, Russia) and calcium carbonate (CC) Calmast CM360 (average particle size 0.8 µm) was used. Sodium stearate (SS) was produced by Panreac (Barcelona, Spain) and used (at a concentration of 3 wt.%) to improve viscosity and electrical conductivity of the melts, as described previously [14,38]. All reagents were used as received, without any further purification. Composites for electrospinning were prepared by mixing the components in a twin-screw extruder HAAKE PolyLab (Thermo Fisher Scientific, Waltham, MA, USA) at 180 ◦C, and then re-granulated

2.2. Electrospinning An experimental setup based on a single screw extruder Plasti-Corder PLE-330 (Brabender GmbH & Co. KG, Duisburg, Germany) with four heating zones (Figure1) was used for melt electrospinning. Pellets of the previously prepared composite were loaded into an extruder (2), where it was melted, and then the melt was forced by a screw (3) to the nozzle. Passing through the nozzle (4), the melt formed a primary jet (5), which entered into an electric field created by a high-voltage source (1). Under the action of the electric field, the primary jet was drawn and split many times and drifted to the collecting electrode (6), forming a nonwoven material ready for further use. To prevent premature degradation of the polymer, the temperature of the first zone (T1) was set at 170 ◦C, the temperature of the second and the third zones (T2-T3) was set at 200 ◦C. Electrospinning was performed at a nozzle temperature (T4) of 330 ◦C. The extruder was grounded and the high voltage (135 kV) generated by the Spellman SL130PN30 source (Spellman High Voltage Electronics Corporation, Hauppauge, NY, USA) was applied to the cylindrical drum (collecting electrode) equipped with an electric motor drive. Polymers 2020, 12, 2981 3 of 11 Polymers 2020, 12, x FOR PEER REVIEW 3 of 12

The25 cm, diameter respectively; and width the ofrotation the drum speed were was 15 and1.5–2 25 rpm cm, and respectively; the distance the rotationbetween speed the nozzle was 1.5–2 and rpm the collectorand the distance was 45 cm. between the nozzle and the collector was 45 cm.

Figure 1. Experimental setup for melt electrospinning: 1—high 1—high volt voltageage supply, 2—feeding device (extruder), 3—screw, 4—nozzle,4—nozzle, 5—fiber,5—fiber, 6—collecting6—collecting device.device. 2.3. Characterization 2.3. Characterization Micrographs of the nonwoven materials were taken with a Axio Imager.M2m optical microscope Micrographs of the nonwoven materials were taken with a Axio Imager.M2m optical (ZEISS, Oberkochen, Germany) and an Versa 3D (Thermo Fisher Scientific, Waltham, MA, USA) microscope (ZEISS, Oberkochen, Germany) and an Versa 3D (Thermo Fisher Scientific, Waltham, scanning electron microscope at an accelerating voltage of 5 kV without any conductive coatings. MA, USA) scanning electron microscope at an accelerating voltage of 5 kV without any conductive Image processing and analysis of fiber diameters were carried out using ImageJ 1.49v software. coatings. Image processing and analysis of fiber diameters were carried out using ImageJ 1.49v The melt viscosity was determined using a CEAST Smart Rheo 2000 (Instron, Norwood, MA, USA) software. capillary rheometer (capillary length 40 mm, channel diameter 1 mm) at a temperature of 230 ◦C and The melt viscosity was determined using a CEAST Smart Rheo 2000 (Instron, Norwood, MA, shear rates of 100–3000 s–1. To study the electrical conductivity of the materials, a Novocontrol Concept USA) capillary rheometer (capillary length 40 mm, channel diameter 1 mm) at a temperature of 230 40 broadband dielectric spectrometer (Novocontrol Technologies, Montabaur, Germany) equipped °C and shear rates of 100–3000 s–1. To study the electrical conductivity of the materials, a with an Alpha-A active sample cell was used. The measurements were made at a temperature of Novocontrol Concept 40 broadband dielectric spectrometer (Novocontrol Technologies, Montabaur, 150 ◦C, voltage of 1 V, and frequency of 1 Hz. The samples for the electrical conductivity and contact Germany) equipped with an Alpha-A active sample cell was used. The measurements were made at angles measurements were made with a HAAKE MiniJet II (Thermo Fisher Scientific, Waltham, MA, a temperature of 150 °C, voltage of 1 V, and frequency of 1 Hz. The samples for the electrical USA) laboratory molding machine. The temperature of the polymer melt was 200 ◦C, the temperature conductivity and contact angles measurements were made with a HAAKE MiniJet II (Thermo Fisher of the mold 75 ◦C, injection and post-process pressures 550 and 200 bar, time 30 and 20 s respectively. Scientific, Waltham, MA, USA) laboratory molding machine. The temperature of the polymer melt A disk-shaped mold with a diameter of 25 mm and a thickness of 1.5 mm was used. was 200 °C, the temperature of the mold 75 °C, injection and post-process pressures 550 and 200 bar, Thermal gravimetric analysis was performed with a Pyris 1 TGA (PerkinElmer, Waltham, MA, time 30 and 20 s respectively. A disk-shaped mold with a diameter of 25 mm and a thickness of 1.5 USA) in the temperature interval 50–550 C in a nitrogen flow (100 mL/min). The sample weight was mm was used. ◦ about 10 mg for pellets and 5 mg for nonwoven materials. IR spectra were recorded with a Nicolet iS5 Thermal gravimetric analysis was performed with a Pyris 1 TGA (PerkinElmer, Waltham, MA, (Thermo Fisher Scientific, Waltham, MA, USA) spectrometer with an iD5 ATR accessory in the range of USA) in the temperature interval 50–550 °C in a nitrogen flow (100 mL/min). The sample weight was 550–4000 cm 1 with an averaging of 32 scans. X-ray diffraction (XRD) analysis of the samples was about 10 mg −for pellets and 5 mg for nonwoven materials. IR spectra were recorded with a Nicolet performed on a Rigaku SmartLab (Rigaku Corporation, Tokyo, Japan) diffractometer (CuKα-radiation, iS5 (Thermo Fisher Scientific, Waltham, MA, USA) spectrometer with an iD5 ATR accessory in the λ = 1.5408 Å). Contact angles of the nonwoven materials were measured using a DSA30E drop shape range of 550–4000 cm−1 with an averaging of 32 scans. X-ray diffraction (XRD) analysis of the analyzer (KRÜSS GmbH, Hamburg, Germany). The liquid phase was water and the droplet volume samples was performed on a Rigaku SmartLab (Rigaku Corporation, Tokyo, Japan) diffractometer was 5 µL. (CuKα-radiation, λ=1.5408 Å). Contact angles of the nonwoven materials were measured using a DSA30E3. Results drop shape analyzer (KRÜSS GmbH, Hamburg, Germany). The liquid phase was water and the droplet volume was 5 μL. 3.1. Viscosity and Conductivity 3. Results As has been previously reported in studies on the electrospinning of polymer melts, viscosity and electrical conductivity are key parameters that determine the characteristics of the resulting 3.1. Viscosity and Conductivity fibers [39]. The effect of the filler content on the viscosity is shown in Figure2a. Melts of both pure and filledAs polypropylene has been previously demonstrate reported a non-Newtonian in studies on flowthe electrospinning pattern, while their of polymer viscosity melts, monotonically viscosity andincreases electrical with conductivity the increase inare the key content parameters of calcium that determine carbonate inthe the characteristics composite, which of the is resulting in good fibers [39]. The effect of the filler content on the viscosity is shown in Figure 2a. Melts of both pure and filled polypropylene demonstrate a non-Newtonian flow pattern, while their viscosity

Polymers 2020, 12, x FOR PEER REVIEW 4 of 12 monotonically increases with the increase in the content of calcium carbonate in the composite, which is in good agreement with the published data [31,32]. The addition of sodium stearate reduces the viscosity of the melt by about half, which should simplify the generation of thin composite fibers. Figure 2b shows the results of a study of the electrical conductivity of polypropylene samples with different degrees of filling. As one can see from the obtained data, the specific electrical conductivity of composites increases as the filler content increases. However, the values of specific electrical conductivity are relatively low—in the range of 10−14–10−13 S/cm. The addition of sodium stearate can further increase the conductivity of the material. This effect is caused by the appearance of additional charge carriers, also, their mobility increases with increasing temperature. These factors lead to an increase in electrical conductivity for both polar [39], and non-polar polymers [14]. PolymersThus,2020 , 12the, 2981 addition of calcium carbonate has a dual effect on the characteristics of4 ofthe 11 polypropylene melt in relation to the electrospinning process. On the one hand, an increase in electricalagreement conductivity with the published should promote data [31 ,the32]. formation The addition of thinner of sodium fibers; stearate on the reducesother hand, the viscosityan increase of inthe viscosity melt by aboutleads to half, an whichincrease should in the simplify diameter the of generationthe producing of thin fibers. composite ﬁbers.

(a) (b)

Figure 2. EffectEﬀect of of CaCO 33 contentcontent on on the the viscosities ( (a)) and conductivities ( b) of polypropylenepolypropylene composites.

3.2. FiberFigure Properties2b shows the results of a study of the electrical conductivity of polypropylene samples with different degrees of filling. As one can see from the obtained data, the specific electrical conductivity The process of electrospinning polypropylene starts only at ~280 °C, while polyamide-6, despite of composites increases as the filler content increases. However, the values of specific electrical its higher viscosity, is capable of generating fibers14 at 25513 °C [39]. Thus, it can be concluded that the conductivity are relatively low—in the range of 10− –10− S/cm. The addition of sodium stearate can factor limiting the spinnability of polypropylene at lower temperatures is insufficient electrical further increase the conductivity of the material. This effect is caused by the appearance of additional conductivity, which is significantly lower than that of polyamide. As the temperature increases, the charge carriers, also, their mobility increases with increasing temperature. These factors lead to an average diameter of the obtained fibers decreases, and at 330 °C it is about 10.4 μm. The addition of increase in electrical conductivity for both polar [39], and non-polar polymers [14]. sodium stearate allowed to reduce the average fiber diameter to 2.1 μm (Figure 3a) by decreasing the Thus, the addition of calcium carbonate has a dual effect on the characteristics of the polypropylene viscosity and increasing the electrical conductivity of the melt. The resulting fibers have a round melt in relation to the electrospinning process. On the one hand, an increase in electrical conductivity shape and a smooth surface while a relatively wide fiber diameter distribution is observed. The should promote the formation of thinner fibers; on the other hand, an increase in viscosity leads to an average diameter of the resulting fibers can be controlled within a fairly wide range by regulating increase in the diameter of the producing fibers. the content of sodium stearate [14]. In the case of melt electrospinning of the composites, the average diameter3.2. Fiber Propertiesof the fibers increases as the content of calcium carbonate in the sample grows (Figure 4). In addition, the composite fibers have a non-circular shape and a complex surface morphology (Figure The process of electrospinning polypropylene starts only at ~280 C, while polyamide-6, despite 3b). For the fibers with a high filling degree (40–50%) the distribution of◦ fibers by diameter is close to its higher viscosity, is capable of generating fibers at 255 C[39]. Thus, it can be concluded that bimodal due to an increase in the content of thick fibers. A◦ number of defects are observed such as the factor limiting the spinnability of polypropylene at lower temperatures is insufficient electrical spherical and spindle-shaped thickenings and microdroplets (Figure 3c). It should be noted that in conductivity, which is significantly lower than that of polyamide. As the temperature increases, this case a certain number of unstretched polymer droplets are observed at the edges of the the average diameter of the obtained fibers decreases, and at 330 C it is about 10.4 µm. The addition of collecting drum and around it. This fact can be explained by◦ the size distribution of the calcium sodium stearate allowed to reduce the average fiber diameter to 2.1 µm (Figure3a) by decreasing the carbonate particles. It can be expected that electrospinning produces better results with the use of viscosity and increasing the electrical conductivity of the melt. The resulting fibers have a round shape nano-sized fillers. and a smooth surface while a relatively wide fiber diameter distribution is observed. The average diameter of the resulting fibers can be controlled within a fairly wide range by regulating the content of sodium stearate [14]. In the case of melt electrospinning of the composites, the average diameter of the fibers increases as the content of calcium carbonate in the sample grows (Figure4). In addition, the composite fibers have a non-circular shape and a complex surface morphology (Figure3b). For the fibers with a high filling degree (40–50%) the distribution of fibers by diameter is close to bimodal due to an increase in the content of thick fibers. A number of defects are observed such as spherical and spindle-shaped thickenings and microdroplets (Figure3c). It should be noted that in this case a certain number of unstretched polymer droplets are observed at the edges of the collecting drum and Polymers 2020, 12, x FOR PEER REVIEW 5 of 12

Polymers 2020, 12, 2981 5 of 11 around it. This fact can be explained by the size distribution of the calcium carbonate particles. It can bePolymers expected 2020, 12 that, x FOR electrospinning PEER REVIEW produces better results with the use of nano-sized ﬁllers.5 of 12

(a) (b) (c)

Figure 3. Micrographs of the nonwoven materials: PP+SS (a), (90PP+10CC)+SS (b), (50PP+50CC)+SS (c). Scale bar is 40 μm.

Soaking the composite materials in hydrochloric acid results in the extraction of only a small amount of filler. Even with a calcium carbonate content of 50 wt.%, the total mass loss of the fibrous sample is no more(a) than 13%, i.e., 26% of the filler( bcontent.) This indicates that most(c )of the calcium carbonate is covered by a polymer. SEM images of the samples after soaking in HCl are shown in FigureFigure S1. As3. 3. Micrographs Micrographsone can see, of of the the the nonwoven nonwovensoaking materials: of materials: the nonw PP+SS PPovens+ (aSS), (90PP+10CC)+SS (a), in (90PP hydrochloric+10CC) (b+),SS (50PP+50CC)+SS acid (b), (50PP does+ 50CC)not result+SS in (c). Scale bar is 40 μm. formation(c). Scale of complex bar is 40 µporousm. fibers morphology. Soaking the composite materials in hydrochloric acid results in the extraction of only a small amount of filler. Even with a calcium carbonate content of 50 wt.%, the total mass loss of the fibrous sample is no more than 13%, i.e., 26% of the filler content. This indicates that most of the calcium carbonate is covered by a polymer. SEM images of the samples after soaking in HCl are shown in Figure S1. As one can see, the soaking of the nonwovens in hydrochloric acid does not result in formation of complex porous fibers morphology.

Figure 4. Effect of CaCO content on the average fiber diameters. Figure 4. Effect of CaCO3 content on the average3 fiber diameters. Soaking the composite materials in hydrochloric acid results in the extraction of only a small 3.3. Structural Analysis amount of filler. Even with a calcium carbonate content of 50 wt.%, the total mass loss of the fibrous sampleIt can is be no expected more than that 13%, the i.e.,addition 26% ofof thefiller filler will content. affect the This structure indicates of the that polymer. most of That the calciumis why thecarbonate supramolecular is covered structure by a polymer. of the samples SEM images was characterized of the samples by afterwide-angle soaking X-ray in HCl diffraction. are shown The in Figure 4. Effect of CaCO3 content on the average fiber diameters. X-rayFigure patterns S1. As for one composites can see, the are soakingpresented of in the Figu nonwovensre 5a and for in hydrochloricnonwovens are acid shown does in not Figure result 5b. in Accordingformation3.3. Structural ofto complexX-ray Analysis data, porous the initial fibers polypropylene morphology. and composites based on it are characterized by the presence of the reflections (110) at 14.1°, (040) at 16.9°, (130) at 18.5°, (111) at 21.2°, (131) + (041) at It can be expected that the addition of filler will affect the structure of the polymer. That is why 3.3. Structural Analysis 21.9°,the supramolecular (060) at 25.3°, structure and (220) of theat 28.4°, samples corresponding was characterized to a bystable wide-angle α-form X-ray with diffraction.a monoclinic The crystal latticeX-rayIt [9,40].patterns can be Reflections for expected composites corresponding that are the presented addition to inthe of Figu β filler- reor 5aγ will-form and a forff (e.g.,ect nonwovens the (300) structure and are (117), shown of therespectively) in polymer. Figure 5b. are That not is observed.whyAccording the supramolecular In to addition,X-ray data, calcite the structure initial reflections polypropylene of the samplesappear and in was samplescomposites characterized containing based byon wide-angleitcalcium are characterized carbonate: X-ray di by ff(104) raction. at 29.4°,Thethe presence X-ray (113) patterns at of 39.4°, the reflections for (202) composites at (110)43.1°, areat (018)14.1°, presented (040)at 47.4°, at in 16.9°, Figure and (130) (116)5aand at at18.5°, for 48.4° nonwovens (111) [41]. at 21.2°, An are increase (131) shown + (041) in Figuretheat filler5b. contentAccording21.9°, (060) leads toat to X-ray25.3°, an increase and data, (220) the in at initialthe 28.4°, intensity polypropylene corresponding of these reflections.to and a stable composites α -form based with a on monoclinic it are characterized crystal by latticeDuring [9,40]. theReflections melt electrospinning corresponding toprocess, the β- or the γ-form polymer (e.g., jet(300) has and a very(117), shortrespectively) cooling are time not from a the presence of the reflections (110) at 14.1◦, (040) at 16.9◦, (130) at 18.5◦, (111) at 21.2◦, (131) + (041) temperatureobserved. In aboveaddition, 300°C calcite to room reflections temperature, appear in while samples it is containing simultaneously calcium drawn carbonate: by a strong(104) at electric at 21.9◦, (060) at 25.3◦, and (220) at 28.4◦, corresponding to a stable α-form with a monoclinic crystal 29.4°, (113) at 39.4°, (202) at 43.1°, (018) at 47.4°, and (116) at 48.4° [41]. An increase in the filler fieldlattice (with [9,40 a]. linear Reflections strength corresponding of about 3 tokV/cm). the β- orAsγ -forma result, (e.g., the (300) crystal and structure (117), respectively) of the polymer are not content leads to an increase in the intensity of these reflections. observed. In addition, calcite reflections appear in samples containing calcium carbonate: (104) at During the melt electrospinning process, the polymer jet has a very short cooling time from a 29.4temperature◦, (113) at above 39.4◦ 300, (202) °C to at room 43.1◦ ,temperature, (018) at 47.4 while◦, and it (116) is simultaneously at 48.4◦ [41]. drawn An increase by a strong in the electric filler content leadsfield (with to an increasea linear strength in the intensity of about of 3 thesekV/cm). reflections. As a result, the crystal structure of the polymer

Polymers 2020, 12, x FOR PEER REVIEW 6 of 12

undergoes significant changes (Figure 5b) associated with the appearance of a low-ordered smectic mesophase, which is characterized by the presence of two broad diffuse reflections on the diffraction patterns of materials. At the same time, in thicker fibers (e.g., from pure polypropylene and with the addition of calcium carbonate but without sodium stearate), where the polymer is cooled more slowly, the formation of a certain number of α-form crystallites is observed, and in thinner fibers only mesophase is observed. A similar effect is observed in the case of rapid crystallization (quenching) of polypropylene thin films [40]. A comparative analysis of the obtained results showed that only mesophase is observed at a fiber diameter of up to 8–9 μm. If the fiber diameter is higher, then crystallites of a more equilibrium α-phase are also formed. Its content grows as the fiber diameter increases. In addition, all typical calcium carbonate reflections are observed on X-ray patterns of nonwoven composite materials, the intensity of which increases as the content of filler Polymers 2020, 12, 2981 6 of 11 increases.

(a) (b)

FigureFigure 5. 5.XRD XRD patterns patternsof of ﬁlmsfilms ((aa)) and nonwovens ( (bb)) from from pure pure PP PP (1) (1) and and composites composites of of95PP+5CC 95PP+5CC

(2),(2), (95PP (95PP+5CC)+SS+5CC)+SS (3),(3), (70PP(70PP+30CC)+SS+30CC)+SS (4), (4), (50PP+50CC)+SS (50PP+50CC)+ SS(5). (5). Inset—pattern Inset—pattern of CaCO of CaCO3 powder.3 powder.

DuringThe study the of melt samples electrospinning by IR spectroscopy process, revealed the polymer (Figure jet 6a), has that a very the spectra short coolingof all the time samples from a −1 temperaturecontain antisymmetric above 300 ◦ andC to symmetric room temperature, stretching while vibrations it is simultaneously of C–H in methyl drawn (2953 by and a strong 2872 cm electric, fieldrespectively) (with a linear and methylene strength (2919 of about and 32840 kV /cmcm).−1, respectively) As a result, groups; the crystal bending structure vibrations of theof C–H polymer in undergoesmethyl groups: significant antisymmetric changes (Figure(1450 cm5b)−1) associated and symmetric with the(1378 appearance cm−1); bending of a low-orderedvibrations of smecticC–H −1 −1 mesophase,bonds in methylene which is characterizedgroups: scissoring by the (1457 presence cm ) and of two wagging broad (1304 diffuse cm reflections) [7,8]. Bands on the 1219, di ff1167,raction −1 patterns1101, 998, of materials.973, 941, 899, At 841, the sameand 808 time, cm in in thicker isotactic fibers polypropylene (e.g., from are pure called polypropylene crystallinity andbands with and belong to isotactic chains of various lengths [42]. In composites and fibers containing a filler, the addition of calcium carbonate but without sodium stearate), where the polymer is cooled more bands appear in the region of 712, 874, and 1410 cm−1, corresponding to vibrations of calcium slowly, the formation of a certain number of α-form crystallites is observed, and in thinner fibers only carbonate in the form of calcite [43]. The intensity of these bands increases as the content of CaCO3 mesophase is observed. A similar effect is observed in the case of rapid crystallization (quenching) in the sample increases. In addition, a band at ~1560 cm−1, which is related to antisymmetric of polypropylene thin films [40]. A comparative analysis of the obtained results showed that only stretching vibrations of the carboxylate anion [14] appears in the IR spectra of materials containing µ mesophasesodium stearate. is observed at a fiber diameter of up to 8–9 m. If the fiber diameter is higher, then α crystallitesIt should of a more be equilibriumnoted that depending-phase are on also the formed. conditions Its content of crystallization grows as the fiberof isotactic diameter increases.polypropylene, In addition, the ratio all of typical the intensity calcium of the carbonate absorption reflections bands at are998 observedand 973 cm on−1 changes, X-ray patterns which of nonwovencan be used composite to evaluate materials, the regularity the intensity of the of polymer which increases chain stacking as the content[44]. The of initial filler increases.polymer is characterizedThe study ofby samples a ratio of by A998/A973 IR spectroscopy equal revealedto 0.84, (Figurein film 6samples,a), thatthe as spectrathe content of all of the calcium samples 1 containcarbonate antisymmetric increases, it andincreases symmetric to 0.88, stretching for nonw vibrationsovens it decreases of C–H into methyl0.69-0.71 (2953 (Figure and 6b). 2872 The cm − , 1 respectively)obtained data and indicate methylene a decrease (2919 in and the 2840 proportion cm− , respectively) of polymer chains groups; in bendingthe helical vibrations conformation of C–H 31 in 1 1 methylduring groups: electrospinning. antisymmetric (1450 cm− ) and symmetric (1378 cm− ); bending vibrations of C–H 1 1 bonds in methylene groups: scissoring (1457 cm− ) and wagging (1304 cm− )[7,8]. Bands 1219, 1167, 1 1101, 998, 973, 941, 899, 841, and 808 cm− in isotactic polypropylene are called crystallinity bands and belong to isotactic chains of various lengths [42]. In composites and fibers containing a filler, bands 1 appear in the region of 712, 874, and 1410 cm− , corresponding to vibrations of calcium carbonate in the form of calcite [43]. The intensity of these bands increases as the content of CaCO3 in the sample 1 increases. In addition, a band at ~1560 cm− , which is related to antisymmetric stretching vibrations of the carboxylate anion [14] appears in the IR spectra of materials containing sodium stearate. Polymers 2020, 12, 2981 7 of 11 Polymers 2020, 12, x FOR PEER REVIEW 7 of 12

(a) (b)

FigureFigure 6. 6.FTIR FTIR spectra spectra of of the the produced produced nonwoven nonwoven materials materials (a); (a e);ﬀ ecteffect of CaCOof CaCO3 content3 content on on the the A998A/998A973/A973ratio ratio (b ().b). Inset—spectra Inset—spectra of of pure pure calcium calcium carbonate. carbonate.

3.4.It Thermal should beProperties noted that depending on the conditions of crystallization of isotactic polypropylene, 1 the ratio of the intensity of the absorption bands at 998 and 973 cm− changes, which can be used The produced composites as well as nonwovens were studied by thermogravimetric analysis; to evaluate the regularity of the polymer chain stacking [44]. The initial polymer is characterized the resulting curves are shown in Figure 7. As can be seen from the obtained data, the granule and by a ratio of A998/A973 equal to 0.84, in film samples, as the content of calcium carbonate increases, fiber from a pure polymer decompose almost completely (the residual mass is 0.1–0.2%). The it increases to 0.88, for nonwovens it decreases to 0.69-0.71 (Figure6b). The obtained data indicate a samples obtained with the addition of sodium stearate behave similarly (the residual mass is decrease in the proportion of polymer chains in the helical conformation 31 during electrospinning. 0.5–1.2%). Polypropylene has very low moisture absorption, resulting in a mass loss only of 0.1–0.2% 3.4.in Thermal the range Properties up to 250 °C for both composites and nonwovens based on it. For comparison, the fibers based on polyamide-6, which is a very hygroscopic polymer, can lose up to 4% of its mass up to The produced composites as well as nonwovens were studied by thermogravimetric analysis; 200°C [36]. the resulting curves are shown in Figure7. As can be seen from the obtained data, the granule and For composites, the residual masses correspond to the masses of the calcium carbonate fiber from a pure polymer decompose almost completely (the residual mass is 0.1–0.2%). The samples loadings. For composite nonwovens with a percentage of filler up to 20% inclusive, the residual obtained with the addition of sodium stearate behave similarly (the residual mass is 0.5–1.2%). mass also corresponds to the filler content in the original composites. However, electrospinning Polypropylene has very low moisture absorption, resulting in a mass loss only of 0.1–0.2% in the range from composites with a higher filling degree leads to a small decreasing of the residual mass of up to 250 C for both composites and nonwovens based on it. For comparison, the fibers based on calcium◦ carbonate in the obtained fiber (up to 28.5–29% for a composite with 30% calcium carbonate, Polymerspolyamide-6, 2020, 12, whichx FOR PEER is a veryREVIEW hygroscopic polymer, can lose up to 4% of its mass up to 200 C[836 of]. 12 up to 37–38% for a 40% composite and up to 46–47% for a composite with 50% filling◦ degree). A possible explanation of this fact is that droplets observed at the edges of the collecting drum and around it contain an excess of filler and represent the largest fraction of calcium carbonate particles (or their aggregates) covered with a thin layer of the polymer. The IR spectrum of the dry residue after TGA fully corresponds to the spectrum of pure calcium carbonate (see inset in Figure 6).

Figure 7. TGA curves of the composites (solid lines) and nonwoven materials (dash lines). Figure 7. TGA curves of the composites (solid lines) and nonwoven materials (dash lines).

3.5. Wettability

Polypropylene is well-known as a hydrophobic polymer, so polypropylene nonwovens exhibit hydrophobic and even superhydrophobic properties [9,14]. Films made of pure polypropylene show an average water contact angle of 103 degrees. The addition of 3% sodium stearate (which is an ionic surfactant) leads to a significant increase in the hydrophilicity of the film: the value of the contact angle decreases to 79 degrees. A similar effect was previously observed for samples based on polyamide-6 [39]. For composite films, the contact angle increases as the content of calcium carbonate in the sample increases (Figure 8a). However, nonwovens (both made of pure polypropylene and containing surfactants and fillers) show superhydrophobic properties: the values of the water contact angle are in the range of 143–147 degrees (Figure S2) and do not significantly depend on the presence, concentration, and type of additive, and the exact value of the contact angle is determined by the local surface morphology. If the surfactant is removed from the surface of the film containing sodium stearate by immersing and holding the film in water, the value of the contact angle becomes equal to that of the film from pure polypropylene. In contrast, for nonwoven materials, the value of the contact angle after immersing in water does not change. It should be noted that individual water droplets applied to the surface of nonwoven material do not roll down when the sample is tilted, but are held on the surface even when the fabric is turned upside-down at 180 degrees (Figure 8b). This effect is similar to the rose petal effect and is due to the morphology of the material surface which has a micro-rough structure: part of the applied droplet falls into the space between the fibers and is securely held there, preventing the movement of the droplet, i.e. wetting occurs according to the Wenzel model [45,46].

Polymers 2020, 12, 2981 8 of 11

For composites, the residual masses correspond to the masses of the calcium carbonate loadings. For composite nonwovens with a percentage of filler up to 20% inclusive, the residual mass also corresponds to the filler content in the original composites. However, electrospinning from composites with a higher filling degree leads to a small decreasing of the residual mass of calcium carbonate in the obtained fiber (up to 28.5–29% for a composite with 30% calcium carbonate, up to 37–38% for a 40% composite and up to 46–47% for a composite with 50% filling degree). A possible explanation of this fact is that droplets observed at the edges of the collecting drum and around it contain an excess of filler and represent the largest fraction of calcium carbonate particles (or their aggregates) covered with a thin layer of the polymer. The IR spectrum of the dry residue after TGA fully corresponds to the spectrum of pure calcium carbonate (see inset in Figure6).

3.5. Wettability Polypropylene is well-known as a hydrophobic polymer, so polypropylene nonwovens exhibit hydrophobic and even superhydrophobic properties [9,14]. Films made of pure polypropylene show an average water contact angle of 103 degrees. The addition of 3% sodium stearate (which is an ionic surfactant) leads to a significant increase in the hydrophilicity of the film: the value of the contact angle decreases to 79 degrees. A similar effect was previously observed for samples based on polyamide-6 [39]. For composite films, the contact anglePolymers increases 2020, 12 as, x the FOR content PEER REVIEW of calcium carbonate in the sample increases (Figure8a). 9 of 12

(a) (b)

FigureFigure 8. (8.a)E (a)ﬀ Effectect of of CaCO3 CaCO3 content content on on the the wettability wettability of ofthe the polymericpolymeric ﬁlms;films; ( b) optical optical image image of a of awater water droplet droplet on thethe surfacesurface of of the the (90PP (90PP+10CC)+10CC)+SS+SS nonwoven nonwoven material material after after 60 seconds60 seconds from from

depositiondeposition (top) (top) and and after after rotating rotating the materialthe material by 180 by ◦180°(bottom). (bottom).

4.However, Conclusions nonwovens (both made of pure polypropylene and containing surfactants and fillers) show superhydrophobic properties: the values of the water contact angle are in the range of In this work, melt electrospinning of polypropylene filled with calcium carbonate was 143–147 degrees (Figure S2) and do not significantly depend on the presence, concentration, and type investigated for the first time. It was shown that polypropylene/CaCO3 composites retain the ability of additive, and the exact value of the contact angle is determined by the local surface morphology. to generate fibers from the melt even when the filling degree is up to 50 wt.%. The average diameter If the surfactant is removed from the surface of the film containing sodium stearate by immersing of the fibers increases as the content of calcium carbonate in the composite rises. To reduce the and holding the film in water, the value of the contact angle becomes equal to that of the film from pure average fibers’ diameter, the addition of sodium stearate can be used; it decreases the viscosity and polypropylene. In contrast, for nonwoven materials, the value of the contact angle after immersing in enhances the electrical conductivity of melts. Thus, with the addition of 3 wt.% of the stearate, the water does not change. average diameter of the fibers is reduced by about five times; for a greater reduction, the It should be noted that individual water droplets applied to the surface of nonwoven material do concentration of sodium stearate can be increased. At high degrees of filling, the content of thick not roll down when the sample is tilted, but are held on the surface even when the fabric is turned fibers and the number of their defects grows significantly. Thus, the optimal filling is up to 20–30 upside-down at 180 degrees (Figure8b). This e ffect is similar to the rose petal effect and is due to the wt.%, in this case, such significant changes are not observed. Soaking the material in hydrochloric acid does not result in formation of complex porous fibers morphology, because most of the CaCO3 is covered by the polymer. The study of the supramolecular structure of polypropylene has shown that during electrospinning from the melt, a transition from the stable α-form crystals to the smectic mesophase in nonwoven materials occurs due to rapid cooling of the polymer under high-speed drawing in an electric field. At the same time, the supramolecular structure of the polymer in nonwovens does not depend on the presence of additives but is determined only by the diameter of the obtained fibers. In thick fibers, where the crystallization process is slower, crystallites of a more equilibrium α-phase are formed, while in thinner fibers (up to 8–9 μm in diameter), only a mesophase is formed. Nonwoven materials obtained in this work exhibit superhydrophobic properties and their wetting occurs according to the Wenzel model.

Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: SEM images of nonwoven materials made from PP/CaCO3 composites after soaking in HCl, Figure S2: optical images of water droplet after 60 seconds from deposition on the surface of the nonwoven materials

Author Contributions: investigation and data analysis: S.N.M., P.V.D., and E.B.P.; writing—original draft preparation: S.N.M.; writing—review and editing, supervision: S.N.C.; funding acquisition, S.N.M. All authors have read and agreed to the published version of the manuscript.

Funding: The reported study was supported by the Russian Federation President’s grant for state support of young Russian candidates of sciences (project MK-92.2020.3).

Acknowledgments: The studies were carried out using equipment of the Resource Centers of the National Research Center “Kurchatov Institute”.

Polymers 2020, 12, 2981 9 of 11 morphology of the material surface which has a micro-rough structure: part of the applied droplet falls into the space between the ﬁbers and is securely held there, preventing the movement of the droplet, i.e. wetting occurs according to the Wenzel model [45,46].

4. Conclusions In this work, melt electrospinning of polypropylene filled with calcium carbonate was investigated for the first time. It was shown that polypropylene/CaCO3 composites retain the ability to generate fibers from the melt even when the filling degree is up to 50 wt.%. The average diameter of the fibers increases as the content of calcium carbonate in the composite rises. To reduce the average fibers’ diameter, the addition of sodium stearate can be used; it decreases the viscosity and enhances the electrical conductivity of melts. Thus, with the addition of 3 wt.% of the stearate, the average diameter of the fibers is reduced by about five times; for a greater reduction, the concentration of sodium stearate can be increased. At high degrees of filling, the content of thick fibers and the number of their defects grows significantly. Thus, the optimal filling is up to 20–30 wt.%, in this case, such significant changes are not observed. Soaking the material in hydrochloric acid does not result in formation of complex porous fibers morphology, because most of the CaCO3 is covered by the polymer. The study of the supramolecular structure of polypropylene has shown that during electrospinning from the melt, a transition from the stable α-form crystals to the smectic mesophase in nonwoven materials occurs due to rapid cooling of the polymer under high-speed drawing in an electric field. At the same time, the supramolecular structure of the polymer in nonwovens does not depend on the presence of additives but is determined only by the diameter of the obtained fibers. In thick fibers, where the crystallization process is slower, crystallites of a more equilibrium α-phase are formed, while in thinner fibers (up to 8–9 µm in diameter), only a mesophase is formed. Nonwoven materials obtained in this work exhibit superhydrophobic properties and their wetting occurs according to the Wenzel model.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/12/12/2981/s1, Figure S1: SEM images of nonwoven materials made from PP/CaCO3 composites after soaking in HCl, Figure S2: optical images of water droplet after 60 seconds from deposition on the surface of the nonwoven materials. Author Contributions: Investigation and data analysis: S.N.M., P.V.D. and E.B.P.; writing—original draft preparation: S.N.M.; writing—review and editing, supervision: S.N.C.; funding acquisition, S.N.M. All authors have read and agreed to the published version of the manuscript. Funding: The reported study was supported by the Russian Federation President’s grant for state support of young Russian candidates of sciences (project MK-92.2020.3). Acknowledgments: The studies were carried out using equipment of the Resource Centers of the National Research Center “Kurchatov Institute”. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Hassan, M.A.; Yeom, B.Y.; Wilkie, A.; Pourdeyhimi, B.; Khan, S.A. Fabrication of nanofiber meltblown membranes and their filtration properties. J. Membr. Sci. 2013, 427, 336–344. [CrossRef] 2. Li, X.; Yang, W.; Li, H.; Wang, Y.; Bubakir, M.M.; Ding, Y.; Zhang, Y. Water filtration properties of novel composite membranes combining solution electrospinning and needleless melt electrospinning methods. J. Appl. Polym. Sci. 2015, 132, 41601. [CrossRef] 3. Shen, Y.; Xia, S.; Yao, P.; Gong, R.H.; Liu, Q.; Deng, B. Structure regulation and properties of melt-electrospinning composite filter materials. Fibers Polym. 2017, 18, 1568–1579. [CrossRef] 4. Li, H.; Wu, W.; Bubakir, M.M.; Chen, H.; Zhong, X.; Liu, Z.; Ding, Y.; Yang, W. Polypropylene fibers fabricated via a needleless melt-electrospinning device for marine oil-spill cleanup. J. Appl. Polym. Sci. 2014, 131, 40080. [CrossRef] 5. Lee, S.; Kay Obendorf, S. Developing protective textile materials as barriers to liquid penetration using melt-electrospinning. J. Appl. Polym. Sci. 2006, 102, 3430–3437. [CrossRef] Polymers 2020, 12, 2981 10 of 11

6. Qi, J.; Xu, X.; Liu, X.; Lau, K.T. Fabrication of textile based conductometric polyaniline gas sensor. Sens. Actuat. B Chem. 2014, 202, 732–740. [CrossRef] 7. Li, Q.S.; He, H.W.; Fan, Z.Z.; Zhao, R.H.; Chen, F.X.; Zhou, R.; Ning, X. Preparation and Performance of Ultra-Fine Polypropylene Antibacterial Fibers via Melt Electrospinning. Polymers 2020, 12, 606. [CrossRef] 8. Mazalevska, O.; Struszczyk, M.H.; Krucinska, I. Design of vascular prostheses by melt electrospinning—Structural characterizations. J. Appl. Polym. Sci. 2013, 129, 779–792. [CrossRef] 9. Cho, D.; Zhou, H.; Cho, Y.; Audus, D.; Joo, Y.L. Structural properties and superhydrophobicity of electrospun polypropylene fibers from solution and melt. Polymer 2010, 51, 6005–6012. [CrossRef] 10. Watanabe, K.; Nakamura, T.; Kim, B.S.; Kim, I.S. Preparation and characteristics of electrospun polypropylene fibers: Effect of organic solvents. Adv. Mater. Res. 2011, 175, 337–340. [CrossRef] 11. Lee, K.H.; Ohsawa, O.; Watanabe, K.; Kim, I.S.; Givens, S.R.; Chase, B.; Rabolt, J.F. Electrospinning of syndiotactic polypropylene from a polymer solution at ambient temperatures. Macromolecules 2009, 42, 5215–5218. [CrossRef] 12. Lyons, J.; Li, C.; Ko, F. Melt-electrospinning part I: Processing parameters and geometric properties. Polymer 2004, 45, 7597–7603. [CrossRef] 13. Kong, C.S.; Jo, K.J.; Jo, N.K.; Kim, H.S. Effects of the spin line temperature profile and melt index of poly (propylene) on melt-electrospinning. Polym. Eng. Sci. 2009, 49, 391–396. [CrossRef] 14. Malakhov, S.N.; Belousov, S.I.; Orekhov, A.S.; Chvalun, S.N. Electrospinning of nonwoven fabrics from polypropylene melt with additions of stearates of divalent metals. Fibre Chem. 2018, 50, 27–32. [CrossRef] 15. Oh, H.J.; Bae, J.H.; Park, Y.K.; Song, J.; Kim, D.K.; Lee, W.; Kim, M.; Heo, K.J.; Kim, Y.; Kim, S.H.; et al. A Highly Porous Nonwoven Thermoplastic Polyurethane/Polypropylene-Based Triboelectric Nanogenerator for Energy Harvesting by Human Walking. Polymers 2020, 12, 1044. [CrossRef] 16. Raghavan, B.; Soto, H.; Lozano, K. Fabrication of melt spun polypropylene nanofibers by forcespinning. J. Eng. Fiber. Fabr. 2013, 8, 52–60. [CrossRef] 17. Liu, Y.J.; Tan, J.; Yu, S.Y.; Yousefzadeh, M.; Lyu, T.T.; Jiao, Z.W.; Li, H.Y.; Ramakrishna, S. High-efficiency preparation of polypropylene nanofiber by melt differential centrifugal electrospinning. J. Appl. Polym. Sci. 2020, 137, 48299. [CrossRef] 18. Berber, E.; Horzum, N.; Hazer, B.; Demir, M.M. Solution electrospinning of polypropylene-based fibers and their application in catalysis. Fiber. Polym. 2016, 17, 760–768. [CrossRef] 19. Cao, L.; Dong, M.; Zhang, A.; Liu, Y.; Yang, W.; Su, Z.; Chen, X. Morphologies and crystal structures of styrene–acrylonitrile/isotactic polypropylene ultrafine fibers fabricated by melt electrospinning. Polym. Eng. Sci. 2013, 53, 2674–2682. [CrossRef] 20. Xu, J.; Zhang, F.; Xin, B.; Zheng, Y.; Wang, C.; Jin, S. Melt-Electrospun Polyvinylbutyral Bonded Polypropylene Composite Fibrous Mat: Spinning Process, Structure and Mechanical Property Study. Fiber. Polym. 2020, 21, 1430–1437. [CrossRef] 21. Malakhov, S.N.; Chvalun, S.N. Nonwoven Materials Produced by Melt Electrospinning of Polyamide-6 and Its Blends with Polypropylene, Polystyrene and Polylactide for Oil Spills Removal. AIP Conf. Proc. 2020, 2285, 040012. [CrossRef] 22. Li, Q.; Gao, D.; Wei, Q.; Ge, M.; Liu, W.; Wang, L.; Hu, K. Thermal stability and crystalline of electrospun polyamide 6/organo-montmorillonite nanofibers. J. Appl. Polym. Sci. 2010, 117, 1572–1577. [CrossRef] 23. Rajzer, I.; Kwiatkowski, R.; Piekarczyk, W.; Binia´s,W.; Janicki, J. Carbon nanofibers produced from modified electrospun PAN/hydroxyapatite precursors as scaffolds for bone tissue engineering. Mater. Sci. Eng. C 2012, 32, 2562–2569. [CrossRef] 24. Park, S.W.; Bae, H.S.; Xing, Z.C.; Kwon, O.H.; Huh, M.W.; Kang, I.K. Preparation and properties of silver-containing nylon 6 nanofibers formed by electrospinning. J. Appl. Polym. Sci. 2009, 112, 2320–2326. [CrossRef] 25. Ohama, Y.; Van Gemert, D. (Eds.) Application of Titanium Dioxide Photocatalysis to Construction Materials; Springer Science & Business Media: Cham, Switzerland, 2011. [CrossRef] 26. Spitalsky, Z.; Tasis, D.; Papagelis, K.; Galiotis, C. Carbon nanotube–polymer composites: Chemistry, processing, mechanical and electrical properties. Prog. Polym. Sci. 2010, 35, 357–401. [CrossRef] 27. Xanthos, M. (Ed.) Functional Fillers for Plastics, 2nd ed.; John Wiley & Sons: Hoboken, NJ, USA, 2010. [CrossRef] Polymers 2020, 12, 2981 11 of 11

28. Khunová, V.; Hurst, J.; Janigová, I.; Smatko, V. Plasma treatment of particulate polymer composites for analyses by scanning electron microscopy: II. A study of highly filled polypropylene/calcium carbonate composites. Polym. Test. 1999, 18, 501–509. [CrossRef] 29. Chan, C.M.; Wu, J.; Li, J.X.; Cheung, Y.K. Polypropylene/calcium carbonate nanocomposites. Polymer 2002, 43, 2981–2992. [CrossRef] 30. Zuiderduin, W.C.J.; Westzaan, C.; Huetink, J.; Gaymans, R.J. Toughening of polypropylene with calcium carbonate particles. Polymer 2003, 44, 261–275. [CrossRef] 31. Supaphol, P.; Harnsiri, W.; Junkasem, J. Effects of calcium carbonate and its purity on crystallization and melting behavior, mechanical properties, and processability of syndiotactic polypropylene. J. Appl. Polym. Sci. 2004, 92, 201–212. [CrossRef] 32. Leong, Y.W.; Abu Bakar, M.B.; Ishak, Z.M.; Ariffin, A.; Pukanszky, B. Comparison of the mechanical properties and interfacial interactions between talc, kaolin, and calcium carbonate filled polypropylene composites. J. Appl. Polym. Sci. 2004, 91, 3315–3326. [CrossRef] 33. Leong, Y.W.; Ishak, Z.M.; Ariffin, A. Mechanical and thermal properties of talc and calcium carbonate filled polypropylene hybrid composites. J. Appl. Polym. Sci. 2004, 91, 3327–3336. [CrossRef] 34. Guo, T.; Wang, L.; Zhang, A.; Cai, T. Effects of nano calcium carbonate modified by a lanthanum compound on the properties of polypropylene. J. Appl. Polym. Sci. 2005, 97, 1154–1160. [CrossRef] 35. Rungruang, P.; Grady, B.P.; Supaphol, P. Surface-modified calcium carbonate particles by admicellar polymerization to be used as filler for isotactic polypropylene. Colloid. Surf. A 2006, 275, 114–125. [CrossRef] 36. Malakhov, S.N.; Bakirov, A.V.; Dmitryakov, P.V.; Chvalun, S.N. Nanocomposite nonwoven materials based on polyamide-6 and montmorillonite, prepared by electrospinning of the polymer melt. Russ. J. Appl. Chem. 2016, 89, 165–172. [CrossRef] 37. Dalton, P.D.; Grafahrend, D.; Klinkhammer, K.; Klee, D.; Möller, M. Electrospinning of polymer melts: Phenomenological observations. Polymer 2007, 48, 6823–6833. [CrossRef] 38. Belousov, S.I.; Prazdnichnyj, A.M.; Malakhov, S.N.; Chvalun, S.N.; Shepelev, A.D.; Khomenko, A.Y. Method of Producing Nonwoven Fibrous Material and Nonwoven Material. RU. Patent 2493006; Filed 14.07.2011, 20 September 2013. 39. Malakhov, S.N.; Belousov, S.I.; Shcherbina, M.A.; Meshchankina, M.Y.; Chvalun, S.N.; Shepelev, A.D. Effect of low molecular additives on the electrospinning of nonwoven materials from a polyamide-6 melt. Polym. Sci. Ser. A 2016, 58, 236–245. [CrossRef] 40. Shcherbina, M.A.; Meshchankina, M.Y.; Odarchenko, Y.I.; Machat, M.; Rieger, B.; Chvalun, S.N. From elastomers to thermoplasts—Precise control of isotactic propylene structure and properties and the role of different structural elements in its mechanical behaviour. Polymer 2017, 133, 213–222. [CrossRef] 41. Wang, C.; Piao, C.; Zhai, X.; Hickman, F.N.; Li, J. Synthesis and characterization of hydrophobic calcium carbonate particles via a dodecanoic acid inducing process. Powder Technol. 2010, 198, 131–134. [CrossRef] 42. Zhu, X.; Yan, D.; Fang, Y. In situ FTIR spectroscopic study of the conformational change of isotactic polypropylene during the crystallization process. J. Phys. Chem. B 2001, 105, 12461–12463. [CrossRef] 43. Andersen, F.A.; Brecevic, L. Infrared spectra of amorphous and crystalline calcium carbonate. Acta Chem. Scand. 1991, 45, 1018–1024. [CrossRef] 44. Hendra, P.J.; Vile, J.; Willis, H.A.; Zichy, V.; Cudby, M.E.A. The effect of cooling rate upon the morphology of quenched melts of isotactic polypropylenes. Polymer 1984, 25, 785–790. [CrossRef] 45. Feng, L.; Zhang, Y.; Xi, J.; Zhu, Y.; Wang, N.; Xia, F.; Jiang, L. Petal effect: A superhydrophobic state with high adhesive force. Langmuir 2008, 24, 4114–4119. [CrossRef][PubMed] 46. Bhushan, B.; Nosonovsky, M. The rose petal effect and the modes of superhydrophobicity. Phil. Trans. R. Soc. A 2010, 368, 4713–4728. [CrossRef][PubMed]

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