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Article Hybrid Self-Reinforced Composite Materials Based on Ultra-High Molecular Weight

Dmitry Zherebtsov 1,* , Dilyus Chukov 1 , Eugene Statnik 2 and Valerii Torokhov 1 1 Center of Composite Materials, National University of Science and Technology “MISiS”, 119049 Moscow, Russia; [email protected] (D.C.); [email protected] (V.T.) 2 Skolkovo Institute of Science and Technology, 143026 Moscow, Russia; [email protected] * Correspondence: [email protected]

 Received: 2 March 2020; Accepted: 3 April 2020; Published: 8 April 2020 

Abstract: The properties of hybrid self-reinforced composite (SRC) materials based on ultra-high molecular weight polyethylene (UHMWPE) were studied. The hybrid materials consist of two parts: an isotropic UHMWPE layer and unidirectional SRC based on UHMWPE fibers. Hot compaction as an approach to obtaining composites allowed melting only the surface of each UHMWPE fiber. Thus, after cooling, the molten UHMWPE formed an SRC matrix and bound an isotropic UHMWPE layer and the SRC. The single-lap shear test, flexural test, and differential scanning calorimetry (DSC) analysis were carried out to determine the influence of hot compaction parameters on the properties of the SRC and the adhesion between the layers. The shear strength increased with increasing hot compaction temperature while the preserved fibers’ volume decreased, which was proved by the DSC analysis and a reduction in the flexural modulus of the SRC. The increase in hot compaction pressure resulted in a decrease in shear strength caused by lower remelting of the fibers’ surface. It was shown that the hot compaction approach allows combining UHMWPE products with different molecular, supramolecular, and structural features. Moreover, the adhesion and mechanical properties of the composites can be varied by the parameters of hot compaction.

Keywords: UHMWPE fibers; self-reinforced composites; single composites; hybrid materials

1. Introduction The ultra-high molecular weight polyethylene (UHMWPE) is the main used for the acetabular cup of artificial hip joints due to its outstanding physical and mechanical properties over other . At the same time, different reports have a common conclusion that the UHMWPE is one of the leading contributors to failure in total joint replacements [1]. In other words, the generated debris due to particles and creep of the UHMWPE component results in joint loosening, osteolysis, and further required medical revision or even surgery [2,3]. Instead of total joint replacements, the UHMWPE is a widely used material for skis, neutron shielding, trabecular bone tissue replacement, and other industrial and consumer applications [4–8]. There are many approaches to the improvement in mechanical stability (hardness, ), tribological behavior (wear resistance), or adhesive (roughness of surface) properties of the polyethylene. The most popular ones are the reinforcement with nanotubes [9], [10], kaolin or zeolite fibers [11,12]; filling of silver [13], zirconium/titanium/hafnium [14], alumina [15], or surface modification by argon plasma [16], chemical etching, electrostatic spraying techniques [17,18], and ultraviolet grafting [19]. On the other hand, many studies are focused on altering physical and chemical properties of the UHMWPE to increase wear resistance, using gamma-irradiated cross-linking [20,21], which correlates with decreases in wear. At the same time, this technique can result in a reduction of chemical stability, ultimate tensile strength, and strength of the

Materials 2020, 13, 1739; doi:10.3390/ma13071739 www.mdpi.com/journal/materials Materials 2020, 13, 1739 2 of 11

UHMWPE [22]. reinforcement is also one of the promising methods for obtaining UHMWPE based composites with enhanced properties [23,24]. Many studies have proved that crystallinity plays an essential role in the tribological behavior of the UHMWPE. For instance, Karuppiah showed that an increase in crystallinity decreases the friction force and the scratch depth [25]. A similar result was reported by Wang, who demonstrated that the degree of crystallinity could be increased by controlling pressure and temperature during the material formation process [26]. Besides, there is a well-known fact that maximum crystallinity can be achieved only in fibers that have a highly oriented fibrillar . This structure can be achieved during gel-spinning followed by orientational drawing [27]. However, it is complicated to produce a bulk monolithic material from fibers due to the inertness of the UHMWPE. In 1975, Capiati and Porter presented a unique concept of reinforced polymeric materials. The idea was to produce a composite material where a reinforcement and a matrix should be made from the same polymer but with different morphologies [28]. Such composite materials are commonly called self-reinforced composite materials. As compared with traditional composites, in self-reinforced composite materials a strong and uniform fiber-matrix interface can be achieved because the problem of material difference is solved. Moreover, usually SRC has a low compared to traditional composites based on carbon or fibers. This difference is achieved because polymeric fibers, as a reinforcement, have a lower density compared to traditional reinforcements (approximately 1.0 g/cm3 versus more than 2.0 g/cm3). In addition, the SRC can be easily recycled because there is no need to separate the composite components. Different crystalline forms of amorphous-crystalline polymers (polymorphism), the existence of different supramolecular , or different grades of one polymer can be used to obtain the SRC. For example, a single-polymer composite can contain a tougher phase as a reinforcement and a less tough phase as a matrix. For example, β- can be used as a matrix for α-polypropylene reinforcing elements [29]. However, there is a problem related to quite close melting points of the used materials, which can be solved using various types of the same polymer, for example, high- and low-density polyethylene (HDPE and LDPE), as a reinforcement and a matrix, respectively [30]. However, these materials do not align with the true concept of self-reinforcement and cannot be applied to biomedical applications [31]. In our previous , a method for the production of SRC materials based on UHMWPE fibers was proposed [32]. The developed technique is focused on hot compaction of UHMWPE fibers, which allows avoiding the relaxation processes resulting in a decrease in mechanical properties. Moreover, the proposed approach allows a more precise control of the melting process by shifting the melting point that occurs due to high hot-compaction temperatures [33]. Also, it is complicated to obtain a composite with complex shapes using hot compaction of UHMWPE fibers. Moreover, it is difficult to mold products using subtractive methods. Thus, it seems an economically and technically sound solution to make only a functional layer from the SRC. However, it is problematic to combine UHMWPE-based SRC with other materials due to the inertness of UHMWPE fibers. This problem can be solved using an isotropic UHMWPE as a part of the product. The isotropic part will not have a functional role as a self-reinforced composite material. This approach allows saving expensive UHMWPE fibers and makes it easier to process the product. An important property of such a composite is an adhesion at the interface between an isotropic UHMWPE and a self-reinforced composite material based on UHMWPE fibers. The mechanical, tribological, and other properties of the SRC were studied in our previous [34,35]. It was shown that in terms of the mechanical properties, the coefficient of friction, wear rate, and creep resistance, the designed SRC is far superior to the isotropic UHMWPE. The main goal of this study is to combine an isotropic and self-reinforced UHMWPE to create a multilayered single-polymer-based composite structure and investigate its mechanical and structural properties. Materials 2020, 13, 1739 3 of 11

2.Materials Materials 2020, and13, x FO MethodsR PEER REVIEW 3 of 11

TheThe UHMWPE UHMWPE powder powder grade grade GUR GUR 4120 4120 (molecular (molecular weight weight of of approximately approximately 5.0 5.0 ×10 106 6g g/mol.)/mol.) × waswas used used to to produce produce an isotropican isotropic part part of the of multilayeredthe multilayered sample. sample. The powder The powder was heated was heated up to 180 up◦ Cto at180 a pressure°C at a pressure of 25 MPa ofin 25 a MPa stainless- in a stainless mold-steel to produce mold to 80 produce10 280 mm × 103 rectangular× 2 mm3 rectangular samples. × × Thesamples. UHMWPE The fibers UHMWPE SGX (DSM Dyneema, SGX (DSM Heerlen, Dyneema, The Netherlands)Heerlen, Netherlands) with an average with diameter an average of 15diameterµm and of a linear15 μm density and a oflinear 220 Dtexdensity were of used220 Dtex for the were preparation used for ofthe the pr SRC.eparation Hot compaction of the SRC. was Hot usedcompaction for producing was used rectangular for producing samples rectangular with 80 samples10 3 mm with3 in 80 size × 10 for × single-lap3 mm3 in size shear for tests single (SRC-lap × × wasshear 1mm tests in (SRC thickness was 1 and mm isotropic in thickness UHMWPE and isotropic was 2 mmUHMWPE in thickness). was 2mm The in fibers thickness) were wound. The fibers on awere special wound winding on a formspecial to winding achieve theform unidirectional to achieve the state unidirectional (Figure1a). state After (Figure that, the1a). wound After that, fibers the werewound placed fibers into were a mold placed with into isotropic a mold UHMWPEwith isotropic plates UHMWPE and polytetrafluoroethylene plates and polytetraflu (PTFE)oroethylene plates. The(PTFE PTFE) plates. plates The allow PTFE separating plates allow the initial separating fibers andthe initialthe isotropic fibers andUHMWPE the isotrop in certainic UHMWPE places as in showncertain in places Figure as1b. shown This separationin Figure 1b allows. This controlling separation the allows contact controlling area between the contact the self-reinforced area between compositethe self-reinforced and the isotropic composite UHMWPE. and the isotropic The PTFE UHMWPE. plates were The removed PTFE plates after hot were compaction removed andafter the hot samplescompaction for single-lap and the samples shear tests for weresingle obtained-lap shear (Figure tests were1c). obtained (Figure 1c).

Figure 1. Scheme of obtaining the hybrid composite material based on ultra-high molecular weight polyethyleneFigure 1. Scheme (UHMWPE) of obtaining for the the single-lap hybrid shearcomposite tests. material The winding based of on initial ultra UHMWPE-high molecular fibers ( aweight) and partspolyethylene of hybrid ( compositeUHMWPE material) for the beforesingle- (lapb)and shear after tests (c). hotThe compaction. winding of initial UHMWPE fibers (a) and parts of hybrid composite material before (b) and after (c) hot compaction. The size of the SRC and the isotropic UHMWPE contact (lap shear overlap) was approximately 2 10 10The mm size. Aof temperature the SRC and ranges the isotropic from 155 UHMWPE◦C to 170 contact◦C and (lap a pressure shear overlap) of 25 MPa was or approximately 50 MPa were × used10 × for10 mm hot compaction.2. A temperature The heatingranges from time and155 °С the to 170 °С timeand a were pressure 50 min of and25 MPa 10 min, or 50 respectively. MPa were usedThe for SRC’s hot compa rectangularction. samples The heating of a size time of and 80 the10 molding2 mm3 timewere wereobtained 50 minto determine and 10 min, the × × influencerespectively. of the hot compaction parameters on the SRC’s properties using the same hot compaction parameters.The SRC’ Thes rectangular flexural test samples and the of DSC a size analysis of 80 were × 10 carried× 2 mm out3 were to determine obtained tothe determine effect of hot the compactioninfluence of on the the hot SRC’s compaction structure parameters and mechanical on the properties. SRC’s properties using the same hot compaction parametersHot compaction. The flexural allows test melting and the a surfaceDSC analysis of each were initial carried fiber (Figureout to determine2a), the melted the effect part afterof hot coolingcompaction forms on a the matrix SRC’s of structure the SRC withand mechanical an isotropic properties structure. (Figure 2b). Moreover, the volume of theH remeltedot compaction UHMWPE allows fibers melting can bea surface changed of varying each initial the temperaturefiber (Figure and2a), the pressuremelted part during after hotcooling compaction. forms a matrix of the SRC with an isotropic structure (Figure 2b). Moreover, the volume of the remeltedThe single-lap UHMWPE shear fibers tests were can be carried changed out varying according the to temperature the standard and test the method pressure for during lap shear hot adhesioncompaction for. fiber reinforced plastic bonding ASTM D5868-01 on a Zwick Z020 universal testing machine (Zwick Roell Group, Ulm, Germany) at a speed of 10 mm/min. The flexural tests were carried out on the same machine at a speed of 10 mm/min. For these tests, rectangular samples 80 10 2 mm × × in size were used. A NETZSCH DSC 204 F1 differential scanning calorimeter (NETZSCH Group, Selb, Germany) was used for a study of the SRC samples melting enthalpy. The measurements were carried out in argon atmosphere with the heating rate of 10 K/min according to ASTM D3417-83. A scanning electron microscope Hitachi TM-1000 (Hitachi, Marunouchi, Chiyoda-ku, Tokyo, Japan) was used to study the structure at a backscattered electron image mode. To conduct SEM observations, the samples were etched using the following : one volume of orthophosphoric acid, two volumes of sulfuric

Materials 2020, 13, x FOR PEER REVIEW 3 of 11

The UHMWPE powder grade GUR 4120 (molecular weight of approximately 5.0 × 106 g/mol.) was used to produce an isotropic part of the multilayered sample. The powder was heated up to 180 °C at a pressure of 25 MPa in a stainless-steel mold to produce 80 × 10 × 2 mm3 rectangular samples. The UHMWPE fibers SGX (DSM Dyneema, Heerlen, Netherlands) with an average diameter of 15 μm and a linear density of 220 Dtex were used for the preparation of the SRC. Hot compaction was used for producing rectangular samples with 80 × 10 × 3 mm3 in size for single-lap shear tests (SRC was 1 mm in thickness and isotropic UHMWPE was 2mm in thickness). The fibers were wound on a special winding form to achieve the unidirectional state (Figure1a). After that, the wound fibers were placed into a mold with isotropic UHMWPE plates and polytetrafluoroethylene (PTFE) plates. The PTFE plates allow separating the initial fibers and the isotropic UHMWPE in certain places as shown in Figure 1b. This separation allows controlling the contact area between the self-reinforced composite and the isotropic UHMWPE. The PTFE plates were removed after hot compaction and the samples for single-lap shear tests were obtained (Figure 1c).

Figure 1. Scheme of obtaining the hybrid composite material based on ultra-high molecular weight polyethylene (UHMWPE) for the single-lap shear tests. The winding of initial UHMWPE fibers (a) and parts of hybrid composite material before (b) and after (c) hot compaction.

The size of the SRC and the isotropic UHMWPE contact (lap shear overlap) was approximately 10 × 10 mm2. A temperature ranges from 155 °С to 170 °С and a pressure of 25 MPa or 50 MPa were used for hot compaction. The heating time and the molding time were 50 min and 10 min, respectively. The SRC’s rectangular samples of a size of 80 × 10 × 2 mm3 were obtained to determine the influence of the hot compaction parameters on the SRC’s properties using the same hot compaction parameters. The flexural test and the DSC analysis were carried out to determine the effect of hot compactionMaterials 2020, 13on, 1739the SRC’s structure and mechanical properties. 4 of 11 Hot compaction allows melting a surface of each initial fiber (Figure2a), the melted part after cooling forms a matrix of the SRC with an isotropic structure (Figure 2b). Moreover, the volume of acid, and 2% wt/vol of potassium permanganate. The samples’ surface was prepared by cutting in the remelted UHMWPE fibers can be changed varying the temperature and the pressure during hot liquid nitrogen and etched for 4 h according to a method proposed in [36]. compaction.

Figure 2. Scheme of the self-reinforced composite (SRC) preparation: initial UHMWPE fiber (a) and self-reinforced composites after hot compaction (b).

3. Results and Discussion A highly oriented fibrillar structure of the initial UHMWPE fibers is a reason for their high mechanical properties in addition to a high degree of crystallinity (96.8% for the used fibers). The oriented structure is stable at an ambient temperature, while relaxation processes occur at temperatures close to the fibers’ melting temperature. These relaxation processes cause the disorientation of molecular chains, which results in the transformation from a fibrillar to lamellar structure with folded chain crystals. The lamellar structure of UHMWPE is mainly isotropic and shows poor mechanical properties as compared with the fibrillar structure. Moreover, this transformation causes a decrease in the crystallinity of UHMWPE. Crystallinity of the samples (DC) can be calculated as the relation of the area under the melting peak (sample’s melting enthalpy, ∆H) to enthalpy of melting of 100% crystalline sample, (∆H100 = 291 J/g)[37]: ∆H DC = (1) ∆H100 Thus, the fraction of the preserved fibers and the isotropic UHMWPE (which was formed by melting of the fibers’ surface) can be calculated from the SRC melting enthalpy using the crystallinity of the fibers (Dc(fiber)) and the isotropic UHMWPE (Dc(isotropic)):

X( f iber) D ( f iber) ∆H100 + X(isotropic) D (isotropic) ∆H100 = ∆H, (2) × C × × C × where X(fiber) and X(isotropic) are fractions of the preserved fibers and the isotropic UHMWPE, respectively, and ∆H100 is the enthalpy of melting of 100% crystalline sample. It was determined for the used UHMWPE fibers that crystallinity of the initial fibers and the remelted UHMWPE fibers was 96.8% and 54.4%, respectively. Thus Equation (2) can be rewritten for the used fibers:

X( f iber) 0.968 291[J/g] + X(isotropic) 0.544 291[J/g] = ∆H. (3) × × × × The DSC analysis was carried out for the SRC’s samples that were obtained at 25 MPa and at a temperature range from 155 ◦C to 170 ◦C with a step of 5 ◦C. The DSC analysis showed that the melting enthalpy was 267 J/g, 264 J/g, 252 J/g, and 200 J/g for the samples obtained at 155 ◦C, 160 ◦C, 165 ◦C, and 170 C, respectively (Figure3a). Using Equation (1), crystallinity was 92.0 1.1%, 91.0 1.3%, ◦ ± ± 87.0 0.9%, and 69.0 1.2% for the samples obtained at 25 MPa and at 155 C, 160 C, 165 C, and ± ± ◦ ◦ ◦ Materials 2020, 13, x FOR PEER REVIEW 5 of 11

fraction of the preserved fibers was calculated as 88%, 85%, 76%, and 34% for the samples obtained Materialsat a temperature2020, 13, 1739 range from 155 °C to 170 °C, respectively. It is well known that for these materials5 of 11 , the DSC curves are characterized by two main peaks, the first of which (“high-temperature” peak) corresponds to the fibers and the other peak (“low-temperature” peak) corresponds to the matrix. It 170 C, respectively (Figure3b). Thus, an increase in hot compaction temperature results in a decrease can◦ be noted that the endoderm peaks at approximately 155 °C (“high-temperature” peak) decrease in SRC’s crystallinity caused by the fibers fraction reduction. Using Equation (3), the fraction of the with an increase in hot compaction temperature. It also confirms that the preserved fibers’ content preserved fibers was calculated as 88%, 85%, 76%, and 34% for the samples obtained at a temperature reduces with an increase in temperature of hot compaction [38]. Moreover, the small peak at a range from 155 C to 170 C, respectively. It is well known that for these materials, the DSC curves lower temperature◦ can◦ be observed for the samples obtained at 170 °C. It is related to the are characterized by two main peaks, the first of which (“high-temperature” peak) corresponds to the appearance of a high amount of matrix phase that was caused by intense fiber melting at high fibers and the other peak (“low-temperature” peak) corresponds to the matrix. It can be noted that the processing temperatures. endoderm peaks at approximately 155 C (“high-temperature” peak) decrease with an increase in hot The single-lap shear test was ◦ carried out to determine the influence of hot compaction compaction temperature. It also confirms that the preserved fibers’ content reduces with an increase in temperature on shear strength between the SRC and the isotropic UHMWPE. An increase in hot temperature of hot compaction [38]. Moreover, the small peak at a lower temperature can be observed compaction temperature results in an increase in shear strength, which were 2.10 ± 0.10 MPa, 2.20 ± for the samples obtained at 170 C. It is related to the appearance of a high amount of matrix phase 0.11 MPa, 2,90 ± 0.12 MPa, and◦ 3,80 ± 0.14 MPa for the samples obtained at a pressure of 25 MPa that was caused by intense fiber melting at high processing temperatures. and at 155 °C, 160 °C, 165 °C, and 170 °C, respectively (Figure 3b).

Figure 3. Results of differential scanning calorimetry (DSC) curves (a) and shear strength of hybrid compositesFigure 3. andResults crystallinity of differential (b) of scann the self-reinforceding calorimetry composites (DSC) curves based (a on) and UHMWPE shear strength produced of hybrid at 25 MPacomposites and various and temperatures. crystallinity (b) of the self-reinforced composites based on UHMWPE produced at 25 MPa and various temperatures. The single-lap shear test was carried out to determine the influence of hot compaction temperature on shearAn strengthincrease between in shear the strength SRC and is the caused isotropic by UHMWPE. better consolidation An increase between in hot compactionthe isotropic temperatureUHMWPE results and the in anremelted increase fiber in shear fraction strength, that whichincrease weres with 2.10 an 0.10increase MPa, in 2.20 hot-compaction0.11 MPa, ± ± 2.90temperature.0.12 MPa, However, and 3.80 crystallinity0.14 MPa for of thethesamples SRC decreases obtained from at a 92.0 pressure ± 1.1 of % 25 to MPa 69.0 and± 1.2 at % 155 for C,the ± ± ◦ 160samples◦C, 165 obtained◦C, and 170 at a◦ C,temperature respectively range (Figure from3b). 155 °C to 170 °C, respectively. Consequently, the higherAnincrease temperature in shear causes strength an isincrease caused by in better shear consolidation strength while between the fiberthe isotropic fraction UHMWPE decreases and and themechanical remelted fiber properties fraction of that the increases SRC reduc withe, which an increase was proved in hot-compaction by the flexural temperature. tests. Moreov However,er, the crystallinityobtained shear of the stre SRCngth decreases is slightly from higher 92.0 as 1.1%compared to 69.0 with1.2% the forlaminated the samples sheets obtained of UHMWPE at a ± ± temperatureobtained at range 132 °C from and 155 13.8◦C MPa to 170 by◦ hotC, respectively. compaction Consequently,with an the higher matrix temperature (range of causes 2.1–3.8 anMPa increase in this in paper, shear strengthagainst approximately while the fiber 2 fraction MPa in decreases [39]). and mechanical properties of the SRC reduce,The which flexural was strength proved byand the the flexural flexural tests. modulus Moreover, were determined the obtained depending shear strength on hot iscompaction slightly higherparameters as compared for the withSRC’s the samples laminated obtained sheets at of 25 UHMWPE MPa and obtaineddifferent attemperatures 132 ◦C and 13.8(Figure MPa 4) by. It hot was compactionshown that with the anflexural polyurethane strength matrixincreases (range from of 110.2 2.1–3.8 ± 1.5 MPa MPa in thisto 117.3 paper, ± 1.7 against MPa approximatelywith increasing 2 MPahot compaction in [39]). temperature, which causes an increase in the matrix content. An increase in the matrixThe flexuralcontent strengthresults i andn a thebetter flexural load modulus transfer were to reinforcing determined elements. depending However, on hot compaction the flexural parametersmodulus decreases for the SRC’s from samples 37.7 ± 1.8 obtained GPa to at18.0 25 ± MPa 1.4 GPa and diforff erentthe samples temperatures obtained (Figure at a temperature4). It was shownrange thatfrom the 155 flexural °C to strength170 °C, respectively. increases from This 110.2 behavior1.5 MPais caused to 117.3 by a 1.7decrease MPa within th increasinge preserved ± ± hotfibers compaction content temperature, and the relaxation which processes causes an that increase accompanied in the matrix a decrease content. in Anthe increase stiffness in of the the matrixcomposites content [40] results. in a better load transfer to reinforcing elements. However, the flexural modulus decreases from 37.7 1.8 GPa to 18.0 1.4 GPa for the samples obtained at a temperature range from ± ± 155 ◦C to 170 ◦C, respectively. This behavior is caused by a decrease in the preserved fibers content and the relaxation processes that accompanied a decrease in the stiffness of the composites [40].

Materials 2020, 13, x FOR PEER REVIEW 6 of 11 Materials 2020, 13, 1739 6 of 11

Figure 4. Flexural strength and flexural modulus of the self-reinforced composite based on UHMWPE Figure 4. Flexural strength and flexural modulus of the self-reinforced composite based on produced at 25 MPa and different temperatures. UHMWPE produced at 25 MPa and different temperatures Also, the shear test was carried out for the samples obtained at 50 MPa and at a temperature range fromAlso, 155 ◦theC toshear 170 ◦testC with was 5 carried◦C step. out The for photos the samples of the samples obtained are at shown50 MPa in and Figure at 5aa,b. temperature The shear rangestrength from for 155 these °C to samples 170 °C with has a 5 similar °C step increasing. The photo behaviors of the samples as for the are samples shown in obtained Figure at5a,b 25. The MPa. shearThe shearstrength strength for these was samples 1.10 0.12 has MPa,a similar 1.40 increasing0.16 MPa, behavior 2.30 0.18as for MPa, the samples and 2.90 obtained0.09 MPa at 25 for ± ± ± ± MPa.the samplesThe shear obtained strength at was 155 ◦1.10C, 160 ± 0.12◦C, MPa, 165 ◦ C,1.40 and ± 0.16 170 ◦MPa,C, respectively 2.30 ± 0.18 (FigureMPa, and5c,d). 2.90 It ± can 0.09 be MPa seen forthat the the samples shear strengthobtained for at the155 samples°C, 160 °C, obtained 165 °C at, and a higher 170 °C, pressure respectively (50 MPa) (Figure is lower 5c,d) as. comparedIt can be seenwith that the samplesthe shear obtained strength at for a pressure the samples of 25 MPa obtained andat at the a higher same hot pressure compaction (50 MPa) temperature is lower range. as comparedThis difference with isthe caused samples by lower obtained fibers’ at remeltinga pressure in of the 25 samples MPa and obtained at the at same a higher hot compactionpressure and temperatureat the same range hot compaction. This difference temperature. is caused The by lower lower fibers’ fibers’ remelting remelting in was the also samp provedles obtained by the at DSC a higheranalysis, pressure which and showed at the an same increase hot incompaction crystallinity temperature. for the samples The obtainedlower fibers’ at a pressure remelting of was 50 MPa also as provedcompared by the with DSC crystallinity analysis, ofwhich the samples showed obtained an increase at 25 in MPa. crystallinity Crystallinity for the decreases samples from obtain 94.0ed at0.9% a ± pressureto 84.0 of1.7% 50 MPa with as an compared increase inwith hot cryst compactionallinity of temperature the samples for obtained the samples at 25 obtainedMPa. Crystallinity at 50 MPa ± decrease(Figure5s d);from whereas 94.0 ± crystallinity0.9 % to 84.0 for ± 1.7 the % samples with an obtained increase atin 25 hot MPa compaction decreases temperature from 92.0 for1.1% the to ± samples69.0 1.2 obtained (Figure at3b). 50 TableMPa 1(Figure presents 5d) all; whereas the values crystallinity of the shear for strengththe samples and obtained crystallinity at 25 for MPa the ± desamplescreases manufacturedfrom 92.0 ± 1.1 at % a pressure to 69.0 ± of 1.2 25 ( orFigure 50 MPa 3b). and Table temperatures 1 presents from all the 155 values to 170 ◦ ofC. the shear strength and crystallinity for the samples manufactured at a pressure of 25 or 50 MPa and temperaturesTable 1. Shearfrom 1 strength55 to 170 and °C. crystallinity of the SRC obtained at different temperatures and pressures.

Hot Compaction 25 MPa 50 MPa Temperature, ◦C Shear Strength, MPa Crystallinity, % Shear Strength, MPa Crystallinity, % 155 2.10 0.10 92.0 1.1 1.10 0.12 94.0 0.9 ± ± ± ± 160 2.20 0.11 91.0 1.3 1.40 0.16 93.0 1.2 ± ± ± ± 165 2.90 0.12 87.0 0.9 2.30 0.18 89.0 1.3 ± ± ± ± 170 3.80 0.14 69.0 1.2 2.90 0.09 84.0 1.7 ± ± ± ±

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Figure 5. Photo of the samples (a,b), -strain curves (c) and shear strength (d) of the hybrid Figure 5. Photo of the samples (a,b), stress-strain curves (c) and shear strength (d) of the hybrid composites based on UHMWPE produced at 50 MPa and various temperatures. composites based on UHMWPE produced at 50 MPa and various temperatures. To study the composites’ cross-section, the samples were cut in the direction perpendicular to the Table 1. Shear strength and crystallinity of the SRC obtained at different temperatures and fibers’ axis. The cutting was made in liquid nitrogen and the followed etching was made to show the pressures. structural features of the composites. The surfaces were studied using scanning electron microscopy (Figure6). Two top pictures show the structure25 MPa of the SRC’ samples obtained at50 155MPa◦C and at a pressure Hot Compaction of 25 MPa (Figure6a) or 50 MPaShear (Figure strength,6b). TheCrystallinity, potassium permanganate Shear strength, etching isCrystallinity, more e ffective for Temperature, °C the isotropic UHMWPE. Thus, theMPa isotropic UHMWPE% matrix, formedMPa as a result of the% fibers’ surface melting, was etched more effectively than the preserved part of the fibers. Thus, the etching grooves characterize the155 thickness of the2.10 remelted ± 0.10 fibers92.0 layer, ± and1.1 depending1.10 ± on 0.12 hot compaction94.0 ± parameter, 0.9 it changes in a range160 of 1–3 µm.2.20 It can ± 0.11 be seen that91.0 due ± 1.3 tothe high1.40 processing ± 0.16 pressure,93.0 the ± 1.2 transverse section of the fibers165 becomes nearly2.90 ± ellipse.0.12 The melted87.0 ± 0.9 UHMWPE 2.30 fills ± the 0.18 gaps between89.0 the± 1.3 fibers and forms a uniform,170 monolithic composite3.80 ± 0.14 material. 69.0 The ± amount1.2 of the2.90 melted ± 0.09 UHMWPE84.0 is ± su1.7ffi cient to connect the fibers with each other. The fibers in the SRC obtained at a lower pressure have a rounder shapeTo than study that the of atcomposites’ higher pressure. cross-section, The etching the samples grooves were are widercut in forthe the direction samples perpendicular produced at ato lowerthe fibers’ pressure, axis. which The cutting means higherwas made fibers’ in remelting liquid nitrogen at pressure. and the It was followed also confirmed etching was by the made DSC to analysisshow the (Figure structural7). Crystallinity features of was the 87composites.0.9% and The 89 surfaces1.3% for were the samplesstudied using obtained scanning at 25 MPa electron and ± ± 50microscopy MPa, respectively. (Figure 6). Two top pictures show the structure of the SRC’ samples obtained at 155 °C and at a pressure of 25 MPa (Figure 6a) or 50 MPa (Figure 6b). The potassium permanganate etching is more effective for the isotropic UHMWPE. Thus, the isotropic UHMWPE matrix, formed as a result of the fibers’ surface melting, was etched more effectively than the preserved part of the fibers. Thus, the etching grooves characterize the thickness of the remelted fibers layer, and depending on hot compaction parameter, it changes in a range of 1–3 μm. It can be seen that due to the high processing pressure, the transverse section of the fibers becomes nearly ellipse. The melted

Materials 2020, 13, x FOR PEER REVIEW 8 of 11

MaterialsUHMWPE 2020, 13 , fillsx FO Rthe PEER gaps REVIEW between the fibers and forms a uniform, monolithic composite material.8 of 11 The amount of the melted UHMWPE is sufficient to connect the fibers with each other. The fibers in UHMWPEthe SRC fills obtained the gaps at a between lower pressure the fibers have and a formsrounder a uniform shape than, monolithic that of at composite higher pressure. material. The Theetching amount grooves of the meltedare wider UHM forWPE the samples is sufficient produced to connect at a lower the fibers pressure with, whicheach other. means The higher fibers fibers’ in theremelting SRC obtained at pressure. at a lower It was pressure also confirmed have a rounder by the DSC shape analysis than that(Fig ure of at 7 a).higher Crystallinity pressure. was The 87 ± etching0.9 % grooves and 89 ±are 1.3 wider % for forthe the samples samples obtained produced at 25 at MPa a lower and pressure50 MPa, ,respectively. which means higher fibers’ remeltingAlso, at pressure. the interfaces It was also between confirmed the self by- reinforcedthe DSC analysis composites (Figure bas 7a).ed C onrystallinity UHMWPE was and 87 ±the 0.9 isotropic% and 89 UHMWPE± 1.3 % for theare samplesshown inobtained Figure at6c ,25d. MPaThese and hybrid 50 MPa, comp respectively.osites were obtained at 165 °C andAlso, at a the pressure interfaces of 25 between MPa (Figure the self 6c)- reinforced or at 50 MPa composites (Figure 6d bas).ed It can on UHMWPE be seen that and the the fibers isotropicpenetrate UHMWPE into the areisotropic shown UHMWPE in Figure 6 andc,d. formThese a hybriduniform comp interfaceosites withoutwere obtained gaps and at structural 165 °C anddefects. at a pressure Such an of interface 25 MPa (providesFigure 6c an) or effective at 50 MPa load ( Figure transfer 6d between). It can be the seen layer thats and the results fibers in penetrate into the isotropic UHMWPE and form a uniform interface without gaps and structural Materialsrelatively2020 high, 13, 1739 interlayer shear strength. 8 of 11 defects. Such an interface provides an effective load transfer between the layers and results in relatively high interlayer shear strength.

Figure 6. SEM images of the SRC’s and hybrid composites’ surface of samples obtained at 155 °C , 25 MPa (a), 155 °C , 50 MPa (b), 165 °C , 25 MPa (c) or 155 °C , 25 MPa (d).. FigureFigure 6. SEM 6. SEM images images of the of the SRC SRC’s’s and and hybrid hybrid composites composites’’ surface surface of samples of samples obtained obtained at 155 at 155 °C ,◦ C,25 25 MPaMPa (a), ( a155), 155 °C ,◦ 5C,0 50MPa MPa (b), ( b1),65 165 °C ,◦ 25C, MPa 25 MPa (c) (orc) 155 or 155 °C ,◦ 25C, MPa 25 MPa (d) (..d ).

Figure 7. DSC analysis of the samples obtained at 165 ◦C and pressure of 25 MPa (red curve) and 50 MPaFigure (black 7. DSC curve). analysis of the samples obtained at 165 °C and pressure of 25 MPa (red curve) and 50 MPa (black curve). FigureAlso, 7. theDSC interfaces analysis of between the samples the self-reinforcedobtained at 165 composites°C and pressure based of 25 on MPa UHMWPE (red curve) and and the 50 isotropic MPaThe (bl ackDSC curve). curves were obtained for the samples obtained at 165°C and a pressure of 25 MPa or UHMWPE are shown in Figure6c,d. These hybrid composites were obtained at 165 ◦C and at a pressure50 MPa ( ofFigure 25 MPa 7) to (Figure compare6c) orthe at dependence 50 MPa (Figure of crystallinity6d). It can beon seenthe content that the of fibers the preserved penetrate fibers into . The DSC curves were obtained for the samples obtained at 165°C and a pressure of 25 MPa or theAccording isotropic to UHMWPE the Clausius and–Clapeyron form a uniform relation, interface the me withoutlting temperature gaps and structural increases defects.with an Suchincrease an 50 MPa (Figure 7) to compare the dependence of crystallinity on the content of the preserved fibers. interfacein pressure provides [33]. Thus an eff, ectivehigher load pressure transfer allows between preserving the layers higher and results volume in of relatively each UHMWPE high interlayer fiber. According to the Clausius–Clapeyron relation, the melting temperature increases with an increase shear strength. in pressure [33]. Thus, higher pressure allows preserving higher volume of each UHMWPE fiber. The DSC curves were obtained for the samples obtained at 165 ◦C and a pressure of 25 MPa or 50 MPa (Figure7) to compare the dependence of crystallinity on the content of the preserved fibers. According to the Clausius–Clapeyron relation, the melting temperature increases with an increase in pressure [33]. Thus, higher pressure allows preserving higher volume of each UHMWPE fiber. For example, crystallinity was 87% and 89% for the samples obtained at 25 MPa and 50 MPa, respectively. Thus, the proposed approach to obtaining the SRC provides various ratios of a matrix to the oriented phase. This allows obtaining composites for various fields of application, the mechanical and other functional properties of which will be determined by the technological parameters of their production, which allows varying the properties of materials depending on the requirements for their application. Materials 2020, 13, 1739 9 of 11

4. Conclusions An approach to obtaining UHMWPE-based two-layered composite materials was proposed, and the structure and mechanical properties of the composites were studied. The first layer of the composites was an isotropic UHMWPE, and the other one was a self-reinforced unidirectional composite material based on UHMWPE fibers. Hot compaction allowed creating a hybrid material formed by the surface melting of the UHMWPE fibers and the remelted UHMWPE formed a matrix of the SRC and bound the SRC and the isotropic layer. The shear strength between two layers was 2.10 0.10 MPa, 2.20 0.11 MPa, 2.90 0.12 MPa, ± ± ± and 3.80 0.14 MPa for the samples obtained at a pressure of 25 MPa and at temperatures of 155 C, ± ◦ 160 ◦C, 165 ◦C, and 170 ◦C, respectively. A similar trend was observed for the samples obtained at a pressure of 50 MPa and the same conditions. The shear strength increased from 1.10 0.12 MPa to ± 2.90 0.09 MPa for the samples obtained at a temperature range from 155 C to 170 C, respectively. ± ◦ ◦ This behavior was caused by an increase in the matrix content in the SRC, which bonds the isotropic and SRC layers. Crystallinity, as a direct indicator of the fiber content, was 92.0 1.1%, 91.0 1.3%, ± ± 87.0 0.9%, and 69.0 1.2% for the samples obtained at a pressure of 25 MPa and temperatures from ± ± 155 ◦C to 170 ◦C, respectively. Thus, the adhesion between the layers increased with an increase in matrix content, which is caused by an increase in hot compaction temperature. However, higher temperatures resulted in lower fiber content that was accompanied with poorer SRC’s properties. It was shown that the SRC’s flexural modulus decreases from 37.7 1.8 GPa to 18.0 1.4 GPa with ± ± increasing hot-compaction temperature. However, an increase in the matrix content allowed a slight increase in the flexural strength of the SRC (from 110.2 1.5 MPa to 117.3 1.7 MPa), which was ± ± related to a better load transfer to the preserved fibers. The SEM examination was used to study the structure of the hybrid and self-reinforced composites based on UHMWPE. Using a special etching approach to visualization, it was shown that the hot compaction allows melting only the surface of each fiber. The amount of the remelted UHMWPE fibers is sufficient to fill the gaps between the fibers. Also, it can be noted that in the hybrid composites, the fibers penetrate into the isotropic UHMWPE and form a large surface contact between the SRC and the isotropic UHMWPE layers. Such an interface provides an effective load transfer between the layers and results in relatively high interlayer shear strength.

Author Contributions: Conceptualization: D.Z. and D.C.; methodology: D.Z. and D.C.; validation: D.Z., D.C. and E.S.; investigation: D.Z. and V.T.; writing—original draft preparation: D.Z. and E.S.; writing—review and editing: D.Z. and D.C.; visualization: D.Z. and V.T.; supervision: D.C.; funding acquisition: D.C. All authors have read and agreed to the published version of the manuscript. Funding: The reported study was funded by Russian Science Foundation grant No. 18-73-00211. Acknowledgments: The authors thank E. Bazanova, director of NUST MISiS Academic Writing Office, for her critical reading of the manuscript and many helpful suggestions. Conflicts of Interest: The authors declare no conflict of interest.

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