EPDM) Surfaces As a Response to Sport Injuries

materials

Article Safety Comes First: Novel Styrene Butadiene Rubber (SBR) and Ethylene Propylene Diene Monomer (EPDM) Surfaces as a Response to Sport Injuries

Cezary Str ˛ak 1, Marcin Małek 2 , Mateusz Jackowski 2,* and Ewa Sudoł 1

1 Construction Materials Engineering Department, Instytut Techniki Budowlanej, ul. Filtrowa 1, 00-611 Warsaw, Poland; [email protected] (C.S.); [email protected] (E.S.) 2 Faculty of Civil Engineering and Geodesy, Military University of Technology, ul. Gen. Sylwestra Kaliskiego 2, 00-908 Warsaw, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-511-06-05-77

Abstract: An athlete’s performance depends not only on the shoes they wear but also on the surface used in sports facilities. In addition, it can signiﬁcantly contribute to reducing injuries, which are easy to get during sports competitions. In the present study, we wanted to investigate whether recycled styrene butadiene rubber (SBR) and ethylene propylene diene monomer (EPDM) could be used in the production of sports surfaces. For this purpose, we designed three different sports surfaces: (1) SBR covered with a thin EPDM spray layer on the top, (2) clean EPDM, and (3) bottom SBR layer with the top layer of EPDM. The test program of these surfaces included in its scope:

shock absorption, vertical deformation, tensile strength, abrasion resistance, and slip resistance tests. Our research also involved the inﬂuence of the substrate under surface, temperature, and surface

Citation: Str ˛ak,C.; Małek, M.; conditions. Presented results show that both materials, in the right proportions, can be used in the Jackowski, M.; Sudoł, E. Safety Comes production of sports surfaces. First: Novel Styrene Butadiene Rubber (SBR) and Ethylene Propylene Keywords: sports injuries prevention; sport surface; safety; rubber; EPDM; SBR Diene Monomer (EPDM) Surfaces as a Response to Sport Injuries. Materials 2021, 14, 3737. https://doi.org/ 10.3390/ma14133737 1. Introduction Over the years, sports have become an indispensable part of human life, no matter if Academic Editor: it is actively practiced or passively watched on television. This trend, however, resulted Edward Bormashenko in a sharp increase in injuries caused by sports accidents taking place during physical activities [1,2]. Particularly popular injuries that athletes suffer are injuries to the limbs, Received: 28 April 2021 including the feet and ankles, with a wide range of mechanisms: light, sharp, and even Accepted: 1 July 2021 Published: 3 July 2021 a long-term or complete disability [3,4]. Many of these injuries can be prevented by appropriate equipment, protective clothing, and safe sports surfaces providing adequate

Publisher’s Note: MDPI stays neutral shock absorption after a fall. with regard to jurisdictional claims in However, specially designed sport surfaces can not only counteract injuries but also published maps and institutional affil- affect the results of athletes [5,6]. For this purpose, it is, of course, necessary to correctly iations. select the component materials that will ensure, for example, adequate adhesion of the footwear to the ground and reduce the possibility of skidding [7]. The final material, which can meet the desired properties depending on the nature of the sport, is most often a combination of several different materials, e.g., polymers and composites. These mixes especially found a place in exposed pitches as they are constantly exposed to changing Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. weather conditions. Polymer materials and composites, when properly designed, can This article is an open access article withstand temperature fluctuations and the accompanying surface contractions, as well as distributed under the terms and the high humidity environment in the fall/spring period [8,9]. That may be a reason why conditions of the Creative Commons recent years showed increased interest in these materials in the sport surface industry, as Attribution (CC BY) license (https:// they are also assessed based on their functionality, strength, and water resistance. Moreover, creativecommons.org/licenses/by/ they are doing remarkably well compared to traditional sport surface materials, such as 4.0/). asphalt or wood, which are way more expensive than polymer or composite materials.

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One of the most suitable polymer materials for sport surfaces is ethylene propylene diene monomer (EPDM), which is an elastomer cross-linked in the sulfur or peroxide vulcanization process [10]. It has great properties for reversible deformation under the influence of mechanical forces while maintaining its structure [11–13]. Furthermore, EPDM exhibits high elasticity at low temperature and high heat resistance, which are crucial when speaking about the application in sports surfaces, especially outdoor ones. This material has a great ability to accumulate energy and high internal friction, as well. Accordingly, in coatings made with EPDM, energy is dissipated as a result of damping during deformation of the rubber, and the mere application of a compressive or tensile load causes elastic deformation of the material [14–17]. The process is completely reversible. These phenomena give EPDM a decisive technological advantage in applications, such as surfaces of sports facilities. In addition, another great rubber material for practical use in these is styrene butadiene rubber (SBR) as it has elasticity, frictional resistance, and mechanical strength similar to EPDM [18–20]. SBR is formed in the polymerization process, which allows for the production of a low reactive viscosity material with all the features of natural rubber, and was firstly developed in the 1930s by Interessen-Gemeinschaft Farbenindustrie AG in Germany [21–23]. Thanks to low costs, it has been used not only in everyday objects but also in protective coatings exposed to impact [24]. As all sport surfaces have direct contact with athletes, they must meet certain operational requirements to specify their safety. To do so, many aspects are taken into account, such as: shock absorption [25], vertical deformation [26], and slip resistance [27,28]. More- over, sport surface should maintain its characteristics so that it can be used for years without visible breakage or abrasion. As presented by Kang and Lee [26], EPDM and SBR surfaces show promising results of reducing the impact after fall. Samples manufactured by them consisted one of four different types of rubber granules (three EPDM and one SBR) and one of six different types of one-component moisture-curing polyurethane resin. The highest force reduction reported by Kang and Lee was about 48% for the surface made from EPDM type C and the resin type D. All sports surfaces showed the vertical deformation in range of 1.1–2.0 mm and the tensile strength in range of 0.21–0.95 MPa. Furthermore, investigation of surface properties and elastomer behavior from EPDM/EOC/PP was also conducted by Uthaipan et al. [29]. They focused on the temperature influence on the tensile strength of surface and on the microscopic studies. The designed surface showed about 1 MPa tensile strength and evenly distributed grains in the composite matrix. Composites containing EPDM were also tested by Basak et al. [30]. They, however, focused on spectroscopic analysis, scanning electron and atomic force microscopy, and adhesion measurements. Today, a growing tendency of waste and by-products usage in material production can be seen, e.g., polymers [31,32], composites [33–37], and even alloys [38]. This article aims to determine the properties of newly designed sports surfaces consisting of EPDM and SBR from recycling. Three different types of sport surfaces were tested, and the scope of the research involved the determination of properties, depending on the substrate used under surface, temperature, and surface conditions (wet or dry). So far, there is no knowledge about sports surfaces obtained from recycling materials’ behavior under specific weather conditions and on top of different substrates. Therefore, this study fills the information gap and presents the impact of underneath substrate used, temperature, and surface conditions on sport surface properties.

2. Materials Sports surface slabs (4 pieces of each type) with dimensions of 1.500 m × 1.500 m × 0.015 m and the following structure, shown in Table1, were prepared. Firstly, rubber granules (recycled EPDM and SBR or only recycled EDPM (Chełm, Poland)) and resin were mixed very carefully according to the designed weight proportions and purred in the mold. Then material was placed inside hot molding press and the 5 MPa pressure in the temperature of 150 ◦C was put on it. Samples then were cured in the laboratory conditions (22 ± 1 ◦C temperature and 60 ± 5% humidity) for 7 days. Then, the sur- Materials 2021, 14, x FOR PEER REVIEW 13 of 15

were mixed very carefully according to the designed weight proportions and purred in the mold. Then material was placed inside hot molding press and the 5 MPa pressure in the temperature of 150 °C was put on it. Samples then were cured in the laboratory conditions (22 ± 1 °C temperature and 60 ± 5% humidity) for 7 days. Then, the surfaces were cut into specimens with dimensions of 1.000 m × 1.000 m × 0.015 m for dynamic tests and of 0.150 m × 0.040 m × 0.015 m with a measurement part in the middle of 0.055 m × 0.025 m × 0.015 m for tensile strength tests.

Materials 2021, 14, 3737 Table 1. Sample specification. 3 of 17

Surface Symbol Description Figure S1 SBR covered with a thin EPDM spray layer on the top Figure 1a faces wereS2 cut into specimens with dimensionsclean EPDM of 1.000 m × 1.000 m × 0.015Figure m for1b × × dynamic testsS3 and of 0.150 mbottom0.040 SBR m layer0.015 with m withthe top a measurement layer of EPDM part in theFigure middle 1c of 0.055 m × 0.025 m × 0.015 m for tensile strength tests.

(a)

(b)

(c)

FigureFigure 1 1.. Digital imagesimages ofof the the cross-section cross-section of of the the sports sports surfaces surfaces slabs slabs prepared prepared for thefor tests:the tests(a) SBR: (a) SBR covered with a thin EPDM spray layer on the top, (b) clean EPDM, (c) bottom SBR layer with covered with a thin EPDM spray layer on the top, (b) clean EPDM, (c) bottom SBR layer with the top the top layer of EPDM. layer of EPDM. 3. Methodology Table 1. Sample speciﬁcation. 3.1. Tests Carried out Depending on the Substrate under Surface Surface Symbol Description Figure S1 SBR covered with a thin EPDM spray layer on the top Figure1a S2 clean EPDM Figure1b S3 bottom SBR layer with the top layer of EPDM Figure1c

3. Methodology 3.1. Tests Carried out Depending on the Substrate under Surface The shock absorption and the vertical deformation tests were carried out in laboratory conditions according to EN 14808:2006 [39] and EN 14809:2005 [40], respectively. Both tests were conducted at three measuring points for each sample, and a device called an artiﬁcial athlete (Elektromechanika Marcin Sienicki, Warsaw, Poland) was used to perform them; see Figure2. The shock absorption was expressed as the percentage reduction in force that a sports surface exerts compared to a hard concrete surface after the fall of device’s foot. Materials 2021, 14, x FOR PEER REVIEW 13 of 15

The shock absorption and the vertical deformation tests were carried out in laboratory conditions according to EN 14808:2006 [39] and EN 14809:2005 [40], respectively. Both

Materials 2021tests, 14 ,were x FOR PEERconducted REVIEW at three measuring points for each sample, and a device called an 13 of 15 artificial athlete (Elektromechanika Marcin Sienicki, Warsaw, Poland) was used to perform them; see Figure 2. The shock absorption was expressed as the percentage reduction in force that a sports surfaceThe shock exerts absorption compared and the to verticala hard deformationconcrete surface tests were after carried the fall out ofin labora- device’s foot. Furthermore,tory conditions the vertical according deformation to EN 14808:2006 was [ 39described] and EN 14809:2005 as the level [40 ],of respectively. vertical Both

Materials 2021displac, 14, 3737ement of thetests sports were conductedsurface measured at three measuring by a sensor points (HBM for each, Poznan sample,, Polandand a device). The called 4 ofan 17 purpose of the testsartificial was to athlete determine (Elektromechanika the level of Marcindeflection Sienicki and, Warsawshock absorption, Poland) was of used the to per- sports surface as a formresult them of running; see Figure and 2. The jumping shock absorptionby the user, was i.e. expressed, to express as the the percentage degree toreduction in force that a sports surface exerts compared to a hard concrete surface after the fall of which the surface is set in motion during an impact. deviceFurthermore,’s foot. Furthermore, the vertical deformation the vertical wasdeformation described was as thedescribed level of as vertical the level displacement of vertical displacof the sportsement surfaceof the sports measured surface by ameasured sensor (HBM, by a Poznan,sensor (HBM Poland)., Poznan The, purpose Poland). of The the purposetests was of to the determine tests was the to leveldetermine of deﬂection the level and of shockdeflection absorption and shock of the absorption sports surface of the as sportsa result surface of running as a result and jumping of running by theand user, jumping i.e., to by express the user, the i.e. degree, to express to which the the degree surface to whichis set inthe motion surface during is set in an motion impact. during an impact.

(a) (b)

Figure 2. The artificial athlete used for test of (a) shock absorption, (b) vertical deformation. (a) (b)

3.2. Tests Carried outFigureFigure Depending 2. 2. TheThe artificial artiﬁcial on the athlete athleteThermal used used Interaction for for test test of of ( (aa )Preceding) shock shock absorption, absorption, Them ( (bb)) vertical vertical deformation deformation.. The tensile strength test with the relative elongation at break and the abrasion re- 3.2.3.2. Tests Tests Carried Carried out out Depending Depending on on the the Thermal Thermal Interaction Interaction Preceding Preceding Them Them sistance test were carried out in accordance with EN 12230: 2003 [41] and ISO 5470-1: 2016 The tensile strength test with the relative elongation at break and the abrasion re- [42], respectively. A ZwickThe tensile machine strength (Zwick, test with Ulm, the Germanyrelative elongation) with a at force break range and the up abrasion to re- sistancesistance test test were were carried carried out out in inaccordance accordance with with EN EN12230: 12230: 2003 2003[41] and [41] ISO and 5470 ISO-1: 5470-1: 2016 10,000 N was used to test the tensile strength; see Figure 3a. The specimen was clamped [201642], respectively. [42], respectively. A Zwick A Zwick machine machine (Zwick, (Zwick, Ulm, Ulm, Germany Germany)) with with a forcea force range range up up to to in the jaws of the machine10,00010,000 N N and wa wass thenused subjected to test thethe tensiletensile to an strength;axialstrength tension; see see Figure Figure until3a. 3abreakage. The. The specimen specimen The was result was clamped clamped in of the destructive forceinthe the jaws wasjaws of recordedof the the machine machine as andthe and thenmean then subjected subjectedvalue of to tosix an an axialmeasurements axial tension tension until until of breakage. breakage. appropr The Thei- result result of ately prepared samples.ofthe the destructive destructive The abrasion force force was resistancewas recorded recorded astest as the wasthe mean mean carried value value of out sixof sixusing measurements measurements the H18 of type appropriatelyof appropr i- abrasive wheel, calledatelyprepared Taber prepared samples. apparatus samples. The abrasion( TABERThe abrasion resistance Industries, resistance test North was test carried was Tonawanda, carried out using out the NY,using H18 USA the type )H18, abrasive type wheel, called Taber apparatus (TABER Industries, North Tonawanda, NY, USA), with a with a load of 1000 abrasiveg rotating wheel at a, called speed Taber of 60 apparatus rpm; see ( FigureTABER 3bIndustries,. The number North Tonawanda, of conducted NY, USA), withload a of load 1000 of g 1 rotating000 g rotating at a speed at a speed of 60 rpm;of 60 seerpm Figure; see Figure3b. The 3b number. The number of conducted of conducted cycles cycles was 1000. Both tests were performed for samples subjected to temperatures of +70 ◦ cycleswas 1000. was 1000. Both Both tests tests were were performed performed for samples for samples subjected subjected to temperatures to temperatures of +70of +70C, ◦ − ◦ °C, +22 °C , and −20°C, +22°C. +22 TheC ,°C and scheme, and20 −20 C. of°C. Thethe The thermal scheme scheme of ofinteractions the the thermalthermal interactionsinteractionspreceding precedingthe tests is the shown tests is is shown shown in Figure4. in Figure 4. in Figure 4.

(a) (b)

Figure(a) 3. The apparatus used for the tests: (a) Zwick machine(b, ()b ) Taber apparatus. Figure 3. The apparatusFigure 3. The used apparatus for the used tests: for ( thea) Zwick tests: (a )machine Zwick machine,, (b) Taber (b) Taber apparatus apparatus..

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FigureFigure 4. The 4. The scheme scheme of thermalof thermal interactions interactions preceding preceding the tensilethe tensile strength strength and and the abrasionthe abrasion resistance resistance tests: testsT1—conditioning: T1—condi- in laboratorytioning in laboratory at 22 ◦C temperature at 22 °C temperature for 24 h, T2—cycle for 24 h, T2 of— highcycle temperature of high temperature interaction, interaction T3—cycle, T3 of— freeze-thawcycle of freeze stresses.-thaw stresses. 3.3. Tests Carried out Depending on the Surface Conditions 3.3. Tests Carried out Depending on the Surface Conditions The slip resistance of the sport surface was determined in accordance with EN 13036-4: The slip resistance of the sport surface was determined in accordance with EN 13036- 2011 [43]. In order to carry out the test, a British pendulum (WESSEX, Aldershot, England) 4: 2011 [43]. In order to carry out the test, a British pendulum (WESSEX, Aldershot, Eng- was used with a CEN type 57 slipper, 76.2 mm wide and 126 mm long, and 55 IRHD land) was used with a CEN type 57 slipper, 76.2 mm wide and 126 mm long, and 55 IRHD rubber hardness. Before testing, the device was calibrated with reference surfaces (glass, rubber hardness. Before testing, the device was calibrated with reference surfaces (glass, reference plate, and polishing paper). The friction force between the shoe and the surface reference plate, and polishing paper). The friction force between the shoe and the surface was determined by measuring the deflection of the pendulum during the movement of the was determined by measuring the deflection of the pendulum during the movement of shoe, using a calibrated scale. The C scale was used [43]. Tests of each sample were carried the shoe, using a calibrated scale. The C scale was used [43]. Tests of each sample◦ were outcarried at 3 measuring out at 3 measuring points for points wet for and wet dry and surfaces dry su atrfaces the temperatureat the temperature of 22 ofC. 22 °C. 4. Results and Discussion 4. Results and Discussion The influence of sample components external structure is not apparent in tests; how- The influence of sample components external structure is not apparent in tests; however,ever it, it may may affectaffect results of of tests tests that that are are carried carried out. out. To get To a getbetter a better understanding understanding of this, of this,preliminary preliminary investigations investigations of the ofmicrostructure the microstructure of the tested of the sports tested surfaces sports were surfaces carried were carriedout on out a stereoscopic on a stereoscopic microscope microscope (DELTA (DELTA OPTICAL, OPTICAL, Nowe NoweOsiny, Osiny,Poland Poland)) enabling enabling ob- observationservation at ata magnification a magnification of 8 ofto 880 to times. 80 times. The surface The surface of each ofcomponent each component grain is rough grain is roughand clearly and clearly visible visible,, as presented as presented in Figure in 4 Figure. In addition4. In addition,, all cross all-sections cross-sections show evenly show evenlydistributed distributed EPDM EPDMand SBR and grains. SBR In grains. case of In surface case of made surface as bottom made asSBR bottom layer with SBR the layer withtop thelayer top of layerEPDM of, there EPDM, is a there visible is alayer visible of polyurethane layer of polyurethane adhesive between adhesive them between (Figure them (Figure5e,f) that5e,f) is that missing is missing when whenit comes it comes to SBR to covered SBR covered with a with thin aEPDM thin EPDM spray spraylayer on layer the on thetop top sample samples.s. However, However, as shown as shown in Figure in Figure 5a, the5a, sprayed the sprayed layer layer is highly is highly porous porous and has and hasgood good penetration penetration into into the the SBR. SBR. In Inaddition addition,, the the morphology morphology of ofthe the cross cross-section-section surface surface waswas examined examined with with aa field-emissionfield-emission scanning electron electron microscope microscope (SEM) (SEM) Sigma Sigma 500500 VP VP (Carl(Carl Zeiss Zeiss Microscopy Microscopy GmbH,GmbH, Köln,Köln, Germany) that that renders renders high high-resolution-resolution images images at at lowlow accelerating accelerating voltage.voltage. The samples were were gold gold-coated-coated before before scanning scanning to topr provideovide an an electricallyelectrically conductive conductive surface. surface. The The accelerating accelerating voltagevoltage waswas 1010 kVkV toto avoidavoid degradation of theof sample.the sample. The The observations observations were were carried carried out out atat fromfrom 0.50 k k to to 10.00 10.00 k k × ×magnification.magnification. TheThe microstructures microstructures were were observedobserved on samples cut cut out out perpendicular perpendicularlyly to to the the sur surface.face. As As presentedpresented in in Figure Figure6 a–I,6a–I, all all rubber rubber samplessamples showshow irregular surface surface and and micro micro-voids-voids in in the internal structure of the recycled rubber that was not filled with the resin binder. This

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the internal structure of the recycled rubber that was not filled with the resin binder. This thephenomenonphenomenon internal structure has has an impact anof the impact recycled on the on roughness the rubber roughness that of the was oftested not the filled surfacestested with surfaces and the their resin and slip binder. their resistance, slipThis re- phenomenonwhichsistance, are discussed which has are an later impactdiscussed in the on laterpaper. the roughnessin the paper. of the tested surfaces and their slip resistance,StudiesStudies which of ofare surfaces surfaces discussed of of EPDMEPDM later in rubber the paper. of of three three different different hardness hardness values values have have also also been beenmadeStudies made by by Mukhopadhyayof Mukhopadhyaysurfaces of EPDM [44 [].44 rubber The]. The author of author three used useddifferent scanning scanning hardness electron electron values microscope microscope have also (SEM) (SEM) been to madetoconduct conduct by Mukhopadhyay his his research research and and [44 tr]. triedied The to to afinduthor ﬁnd patter patterused of scanning offailure failure mode electron mode of ofrubber microscope rubber parts. parts. As(SEM) he As men- heto conductmentioned,tioned his, microscopy research microscopy and technics technics tried areto arefind very very patter important important of failure not not only mode only to toof predict predictrubber the theparts. surface surface As heservice service men- life tionedlifebut but ,also microscopy also get get a abetter better technics inside inside are of of materialvery material important structure structure not and only and rubber rubberto predict grain grain the distribution. distribution. surface service life but also get a better inside of material structure and rubber grain distribution.

(a) (b) (c) (a) (b) (c) FigureFigure 5. Light5. Light microscopy microscopy images images of: ( aof) SBR: (a) coveredSBR covered with awith thin a EPDM thin EPDM spray layer spray on layer the top; on (theb) cleantop; ( EPDM;b) clean (c )EPDM; bottom (c) Figure 5. Light microscopy images of: (a) SBR covered with a thin EPDM spray layer on the top; (b) clean EPDM; (c) SBRbottom layer withSBR layer the top with layer the of top EPDM. layer of EPDM. bottom SBR layer with the top layer of EPDM.

(a) (b) (c) (a) (b) (c)

Figure 6. Cont.

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(d) (e) (f)

(g) (h) (i)

FigureFigure 6. 6. SEMSEM photos photos of: of: SBR SBR ( (aa)) 1.00 1.00 k k magni magnification,fication, ( (bb)) 5.00 5.00 k k magnification, magnification, ( (cc)) 10.00 10.00 k k magnification; magnification; EPDM EPDM ( (dd)) 1.00 1.00 k k magnification, (e) 5.00 k magnification, (f) 10.00 k magnification; connection between SBR and EPDM (g) 0.50 k magnifi- magnification, (e) 5.00 k magnification, (f) 10.00 k magnification; connection between SBR and EPDM (g) 0.50 k magnification; cation; (h) 5.00 k magnification, (i) 10.00 k magnification. (h) 5.00 k magnification, (i) 10.00 k magnification. 4.1. The Shock Absorption Test 4.1. The Shock Absorption Test Table 2 shows the shock absorption test results that are in the range of 33.1–52.4%. Table2 shows the shock absorption test results that are in the range of 33.1–52.4%. Similar range of shock absorption was reported by Khal and Lee [26] as they tested 24 Similar range of shock absorption was reported by Khal and Lee [26] as they tested different types of sport surfaces. Presented by them, test results were between 35% and 24 different types of sport surfaces. Presented by them, test results were between 35% and 48%. The highest shock absorptions in our own study were reported for the SBR covered 48%. The highest shock absorptions in our own study were reported for the SBR covered with a thin EPDM spray layer on the top surface, and the lowest shock absorptions were with a thin EPDM spray layer on the top surface, and the lowest shock absorptions were reported for the clean EPDM surface. In addition, the overall maintained trend for each reported for the clean EPDM surface. In addition, the overall maintained trend for each sports surface was reported, regardless of its component materials and their proportions, sports surface was reported, regardless of its component materials and their proportions, as on a concrete substrate,the sports surfaces obtained the lowest shock absorption values. as on a concrete substrate, the sports surfaces obtained the lowest shock absorption values. Compared to this type of substrate, the shock absorption values increased by about 3–4% Compared to this type of substrate, the shock absorption values increased by about 3–4% and about 14–17% for the asphalt and mineral-rubber substrate, respectively, in every and about 14–17% for the asphalt and mineral-rubber substrate, respectively, in every testedtested type type of of surface. surface. On On the the basis basis of the of theconducted conducted research, research, it was it found was found that the that min- the eralmineral-rubber-rubber substrate substrate allows allows for forthe thegreatest greatest flexibility flexibility of ofthe the surface surface;; thus thus,, it it is is the the most most comfortablecomfortable and and safe safe to to use. use. The The high high flexibility flexibility of of the the combined combined substrate substrate and and surface surface resultsresults in in le lessss stress stress on on the the joints joints and and a a lower lower risk risk of of athletes’ athletes’ injury injury after after the the fall. fall.

TableTable 2. 2. ResultsResults of of tests tests carried carried out out depending depending on on the the substrate substrate under under surface surface (average values) values)..

ShockShock Absorption Absorption (%) (%) Vertical Vertical Deformation (mm) (mm) SurfaceSurface Symbol Symbol Concreteconcrete AsphaltAsphalt Mineral-RubberMineral-Rub- ConcreteConcrete Asphalt MineralMineral-Rubber-Rub- SubstrateSubstrate SubstrateSubstrate berSubstrate Substrate SubstrateSubstrate Substrate berSubstrate Substrate S1S1 38.7 ±38.70.1 ± 0.1 41.841.8± 0.1 ± 0.1 52.452±.40.1 ± 0.1 2.22.±2 0.1± 0.1 2.42.4± ±0.1 0.1 3. 3.33 ±± 0.0.11 S2S2 33.1 ±330.1.1 ± 0.1 36,93±6,90.1 ± 0.1 50.250±.20.1 ± 0.1 1.31.±3 0.1± 0.1 1.51.5± ±0.1 0.1 2. 2.55 ±± 0.0.11 S3 37.0 ± 0.1 40.3 ± 0.1 51.3 ± 0.1 1.6 ± 0.1 1.9 ± 0.1 2.7 ± 0.1 S3 37.0 ± 0.1 40.3 ± 0.1 51.3 ± 0.1 1.6 ± 0.1 1.9 ± 0.1 2.7 ± 0.1

SubstrateSubstrate made made of of a a flexible ﬂexible mineral mineral-rubber-rubber layer layer should should be be used used only only in in multi multi-sport-sport facilities,facilities, i.e. i.e.,, for for practicing practicing many many sports sports (e.g. (e.g.,, school school playgrounds). playgrounds). The The above above statement statement is conﬁrmed by comparing the obtained results with the World Athletics (WA) require-

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Materials 2021, 14, 3737 8 of 17 is confirmed by comparing the obtained results with the World Athletics (WA) requirements [45] for the athletic facilities and the requirements of the EN 14877:2013 [46] standard. By analyzing them, it was found that the results of the shock absorption on the min- ments [45] for the athletic facilities and the requirements of the EN 14877:2013 [46] standard. eral-rubber substrateBy exceed analyzing the them, limits it wasof the found WA that requirements the results of the and shock the absorption requirements on the mineral- of EN 14877:2013 for trackrubber surface, substrate but exceed they the fall limits within of the the WA requirements requirements and of the EN requirements 14877:2013 of EN for multisport facilities.14877:2013 However, for track the surface,results but obtained they fall on within concrete the requirements and asphalt of EN substrates 14877:2013 for multisport facilities. However, the results obtained on concrete and asphalt substrates meet meet the criteria set bythe both criteria the set WA by both and the the WA EN and 14877:2013 the EN 14877:2013 standard standard;; see see Figure Figure 77. .

Figure 7.Figure Shock 7. Shockabsorption absorption test test results results compare comparedd to to EN EN 14877:2013 14877:2013 requirements requirements and WA requirement. and WA requirement. 4.2. The Vertical Deformation Test 4.2. The Vertical DeformationThe Test vertical deformation of tested surfaces is presented in Table2 depending on the substrate used under them. The highest deformation was reported for SBR covered with a The vertical deformationthin EPDM sprayof tested layer surfaces on the top surfaceis presented with the in mineral-rubber Table 2 depending substrate (3.3 on mm) theand substrate used underthe them. lowest The deformation highest wasdeformation reported for was clean reported EPDM surface for withSBR the covered concrete with substrate a thin EPDM spray layer(1.3 mm). on Thisthe showstop surface a correlation with between the mineral vertical- deformationrubber substrate and shock (3.3 absorption mm) as the value distribution of both tests is identical. Furthermore, an increase in deformation and the lowest deformationwas noted was with reported the increase for in clea ﬂexibilityn EPDM of substrate surface used with under the concrete surface. Compared substrate (1.3 mm). This toshows surfaces a correlation on top of the between concrete substrate, vertical surfaces deformation on top of and the shock asphalt absorp- and mineral- tion as the value distributionrubber substrates of both showed tests ais higher identical. vertical Furthermore, deformation of aboutan increase 0.2–0.3 mm in anddefor- 1.1–1.2, mation was noted withrespectively. the increase A similar in flexibility conclusion wasof substrate made by Yukawa used under et al. [47 surface.], who reported Com- that the ﬂexibility of the surface depends not only on the material but also on the substrate pared to surfaces on topunder of it. the In concrete addition, athletes substrate, increase surfaces their leg on stiffness top of (the the stiffness asphalt of and the integratedmin- eral-rubber substratesmusculoskeletal showed a higher system vertical that behaves deformation as a single linear of about spring during0.2–0.3 locomotion) mm and when 1.1– they 1.2, respectively. A similarare running conclusion on a compliant was made ground by compared Yukawa with et running al. [47] on, who a hard reported one, as Katkat that [48] mentions. That is why surface should be tested taking into account the substrate beneath the flexibility of the surfaceit, as well. depends Furthermore, not fromonly the on point the ofmaterial view of thebut results also achievedon the bysubstrate competitive under it. In addition, athletes increase their leg stiffness (the stiffness of the integrated musculoskeletal system that behaves as a single linear spring during locomotion) when they are running on a compliant ground compared with running on a hard one, as Katkat [48] mentions. That is why surface should be tested taking into account the substrate beneath it, as well. Furthermore, from the point of view of the results achieved by competitive athletes, the most advantageous are surfaces installed on rigid substructures, i.e., concrete and asphalt. They allow obtaining better results than on elastic substrates made of mineral rubber materials. Therefore, according to the WA’s recommendations, track and runways should be installed on rigid substrates (Figure 8).

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Materials 2021, 14, x FOR PEER REVIEW 13 of 15 athletes, the most advantageous are surfaces installed on rigid substructures, i.e., concrete and asphalt. They allow obtaining better results than on elastic substrates made of mineral rubber materials. Therefore, according to the WA’s recommendations, track and runways should be installed on rigid substrates (Figure8).

Figure 8.Figure Vert 8.icalVertical deformation deformation testtest results results compared compare to ENd 14877:2013 to EN 14877:2013 requirements andrequirements WA requirement. and WA requirement. 4.3. The Tensile Strength Test Analyzing the test results presented in Table3, the highest values of tensile strength of 4.3. The Tensile Strengthsports Test surfaces were reported after the interaction of high temperature. The noted increases Analyzing the testwere results about 37%, presented 26%, and in 27% Table for S1, 3, S2, the and highest S3 samples, values respectively, of tensile compared strength to the same samples conditioned at 22 ◦C temperature. The tensile strengths of the samples of sports surfaces wereconditioned reported at 22 after◦C temperature the interaction and after of the high freeze-thaw temperature. stresses are The similar noted in the casein- creases were about 37%,of S1 26% and S3, and samples. 27% Thisfor S1, trend, S2 however,, and S3 is samples not relevant, respectively, to samples fully compared made from to the same samples conditionedEPDM because at the 22 values °C temperature. of S2 samples after The freeze-thaw tensile strengths stresses were of about the 13%samples higher than the ones conditioned at 22 ◦C temperature. Furthermore, all tested materials showed conditioned at 22 °C temperaturehigh elasticity. The and relative after elongation the freeze values,-thaw measured stresses during are thesimilar tensile in strength the case test, of S1 and S3 samples.remained This trend at a similar, however level for samples, is not S1, relevant regardless ofto whether samples the samplesfully made were subjected from ◦ EPDM because the valuesto thermal of S2 interaction samples before after or just freeze conditioned-thaw atstresses 22 C temperature; were about see Figures 13% 9higher and 10 . This was not reported, however, for samples S2 and S3 as a signiﬁcant difference in their than the ones conditionvaluesed wasat 22 noted. °C temperature. The highest increase Furthermore, in relative elongation all tested (17.8%) materials was for theshowed bottom high elasticity. The relativeSBR layer elongation with the top layervalues, of EPDM measured surface samples during conditioned the tensile at 22 strength◦C temperature test, remained at a similarand level samples for samples after the interaction S1, regardless of high temperature.of whether In the addition, samples the peakwere value sub- of relative elongation was reached by clean EPDM samples after freeze-thaw stresses. As jected to thermal interactionshown in Figurebefore 11 ,or the just breakpoint conditioned of the sample at 22 was °C difﬁcult temperature to predict; duesee toFigure the highs 9 and 10. This was notporosity reported of the, however material and, for the randomsamples distribution S2 and ofS3 the as rubber a significant grains that difference differ in size. in their values was noted.Only samples The highest of SBR covered increase with in a thin relative EPDM elongation spray layer on (17.8%) the top after was freeze-thaw for the stresses tend to break right in the middle. Overall, the pavement made entirely of EPDM bottom SBR layer withhas the the besttop strength layer of properties EPDM in surface every tested samples condition. conditioned In addition, no at deterioration 22 °C tem- of perature and samples after the interaction of high temperature. In addition, the peak value of relative elongation was reached by clean EPDM samples after freeze-thaw stresses. As shown in Figure 11, the breakpoint of the sample was difficult to predict due to the high porosity of the material and the random distribution of the rubber grains that differ in size. Only samples of SBR covered with a thin EPDM spray layer on the top after freeze- thaw stresses tend to break right in the middle. Overall, the pavement made entirely of EPDM has the best strength properties in every tested condition. In addition, no deterioration of the strength properties as a result of the applied thermal and humidity interactions was reported for each sport surface tested. Similar results of tensile strength were reported by Kang and Lee [26], as they tested EPDM and SBR sport surfaces. They reported the maximum value of this property for clean EPDM type C and resin type B surface (0.95 MPa), which was about equal to S2 samples’ tensile strength. Ethylene propylene diene monomer composite was also tested by Ismail and Mathialagan [49] and Mousa [50]. They reported about 2.2 times higher tensile strength (2.048 MPa and 2.000 MPa) compared to the clean EPDM tested in this study (S2 samples with 0.919 ± 0.05 MPa). This may be due to the additives used by Ismail and Mathialagan, and Mousa in the production

Materials 2021, 14, 3737 10 of 17

the strength properties as a result of the applied thermal and humidity interactions was reported for each sport surface tested. Similar results of tensile strength were reported by Kang and Lee [26], as they tested EPDM and SBR sport surfaces. They reported the maximum value of this property for clean EPDM type C and resin type B surface (0.95 MPa), which was about equal to S2 samples’ tensile strength. Ethylene propylene diene monomer composite was also tested by Ismail and Mathialagan [49] and Mousa [50]. They reported about 2.2 times higher tensile strength (2.048 MPa and 2.000 MPa) compared to the clean EPDM tested in this study (S2 samples with 0.919 ± 0.05 MPa). This may be due to the additives used by Ismail and Mathialagan, and Mousa in the production of EPDM composites, as they both added zinc oxide, stearic acid, tetramethyl thiuram disulﬁde Materials 2021, 14, x FOR PEER REVIEW 13 of 15 (TMTD), and 2-mercapto benzothiazole (MBT) to their composite. Process of the tensile strength testing for all prepared samples proceeded the same way. In addition, all tested samples, regardless of the composition and content of individual of EPDM composites,components, as they showed both added high ductility zinc oxide, during thestearic tests; acid, therefore, tetramethyl their destruction thiuram was not abrupt. First, the sample grew in length (Figure 10b). After reaching the maximum disulfide (TMTD)elongation,, and 2-mercapto the ﬁrst cracksbenzothiazole appeared (Figure (MBT) 10 toc), their which composite. gradually progressed, causing the complete rupture of the specimen; see Figure 10d. Table 3. Results of tests carried out depending on the thermal interaction preceding them (average values). Table 3. Results of tests carried out depending on the thermal interaction preceding them (average values). Tensile Strength (MPa) Abrasion Resistance (g) Surface Symbol Tensile Strength (MPa) Abrasion Resistance (g) SurfaceT1 Symbol * T2 * T3 * T1 * T2 * T3 * S1 0.381 ± 0.03 0.395T1 ± * 0.03 0.520 T2 * ± 0.04 0.728 T3 * ± 0.02 T10.704 * ± 0.02 T2 *0.726 ± 0.02 T3 * S2 0.919S1 ± 0.05 1.042 0.381 ±± 0.030.05 0.3951.154± 0.03± 0.05 0.5201.513± 0.04 ± 0.03 0.728 ±1.4120.02 ± 0.03 0.704 ± 0.021.218 ± 0.7260.03 ± 0.02 S2 0.919 ± 0.05 1.042 ± 0.05 1.154 ± 0.05 1.513 ± 0.03 1.412 ± 0.03 1.218 ± 0.03 S3 0.600S3 ± 0.04 0.570 0.600 ±± 0.04 0.5700.762± 0.04± 0.04 0.7621.423± 0.04 ± 0.03 1.423 ±1.5340.03 ± 0.03 1.534 ± 0.031.431 ± 1.4310.03 ± 0.03 * see Figure 4. * see Figure4.

FigureFigure 9.9.Relative Relative elongation elongation of samples of samples depending depending on the thermal on the interaction thermal preceding interaction tensile preceding strength test.tensile strength test.

Process of the tensile strength testing for all prepared samples proceeded the same way. In addition, all tested samples, regardless of the composition and content of individual components, showed high ductility during the tests; therefore, their destruction was not abrupt. First, the sample grew in length (Figure 10b). After reaching the maximum elongation, the first cracks appeared (Figure 10c), which gradually progressed, causing the complete rupture of the specimen; see Figure 10d.

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(a) (b)

FigureFigure 10. View 10. of View the sample of the during sample the static during tensile the strength static test:tensile (a) thestrength sample mountedtest: (a) the in the sample clamps, mounted (b) the sample in the underclamps, load, (c) ﬁrst(b) cracksthe sample of the sample,under (load,d) complete (c) first fracture cracks of of the the sample. sample, (d) complete fracture of the sample.

(a)

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(a) (b)

Materials 2021, 14, 3737 (c) (d) 12 of 17 Figure 10. View of the sample during the static tensile strength test: (a) the sample mounted in the clamps, (b) the sample under load, (c) first cracks of the sample, (d) complete fracture of the sample.

Materials 2021, 14, x FOR PEER REVIEW 13 of 15

(a)

(b)

(c)

FigureFigure 1 11.1. ViewView of of the the samples samples after after the the static static tensile tensile strength strength test test:: ( (aa)) bottom bottom SBR SBR layer layer with with the the top top layer of EPDM after high temperature interaction, (b) SBR covered with a thin EPDM spray layer layer of EPDM after high temperature interaction, (b) SBR covered with a thin EPDM spray layer on on the top conditioned at 22 °C temperature, (c) SBR covered with a thin EPDM spray layer on the the top conditioned at 22 ◦C temperature, (c) SBR covered with a thin EPDM spray layer on the top top conditioned at 22 °C temperature after the freeze-thaw stresses. conditioned at 22 ◦C temperature after the freeze-thaw stresses.

4.4.4.4. The The Abrasion Abrasion Resistance Resistance Test Test TheThe results results of of weight weight loss loss due due to to abrasion abrasion of of the the surface surface shows shows Table Table 33.. ItIt waswas foundfound thatthat the the impact impact of of the the thermal thermal interactions interactions is is very very insignificant insigniﬁcant,, and the observed slight differencesdifferences result result from from the variability variability of the the surface surface as as all all values values of of the the abr abrasionasion resistance resistance testtest fluctuated ﬂuctuated for for each each surface surface type. type. The The SBR SBR covered covered with with a a thin thin EPDM EPDM spray spray layer layer on on thethe top top (S1 (S1 sample) sample) showed showed the the lowest lowest weight weight loss loss,, which indicates indicates the the highest abrasion resistanceresistance;; see see Figure Figure 1212.. Other Other materials, materials, clean clean EPDM EPDM and and botto bottomm SBR SBR layer layer with with the the top top layerlayer of of EPDM, EPDM, showed showed slightly slightly higher higher weight weight loss loss;; however, however, it it was was still still less less than than four grams,grams, which which is is the the boundary boundary condition condition regarding regarding weight weight loss loss due due to to abrasion abrasion according according to EN 14877:2013 [46]. In addition, for S2 and S3 samples, a slight increase in weight loss was noted after freeze-thaw stresses and the interaction of high temperature. It may be related to the macroscopic degradation of the polymeric matrix which can be observed in terms of change in color and loss of gloss. This finding was reported by Wachtendorf et al. [51], as well, when they tested the influence of weathering on the leaching behavior of zinc and polycyclic aromatic hydrocarbons from synthetic sports surfaces.

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to EN 14877:2013 [46]. In addition, for S2 and S3 samples, a slight increase in weight loss was noted after freeze-thaw stresses and the interaction of high temperature. It may be related to the macroscopic degradation of the polymeric matrix which can be observed in terms of change in color and loss of gloss. This ﬁnding was reported by Wachtendorf Materials 2021, 14, x FOR PEER REVIEWet al. [51], as well, when they tested the inﬂuence of weathering on the leaching behavior13 of 15

of zinc and polycyclic aromatic hydrocarbons from synthetic sports surfaces.

(a) (b)

(e) (f)

Figure 12.12. ViewView ofof thethe samples samples after after the the abrasion abrasion resistance resistance test: test: SBR SBR covered covered with with a thin a thin EPDM EPDM spray layer onon thethe top top ( a(a)) conditioned conditioned at at 22 22◦C °C temperature, temperature, (b) ( afterb) after the the freeze-thaw freeze-thaw stresses; stresses; clean clean EPDM ((cc)) conditioned conditioned at at 22 22◦C °C temperature, temperature, (d) after (d) theafter freeze-thaw the freeze stresses;-thaw stresses; bottom SBRbottom layer SBR with layer withthe top the layer top oflayer EPDM of EPDM (e) conditioned (e) conditioned at 22 ◦C at temperature, 22 °C temperature, (f) after the(f) after freeze-thaw the freeze stresses.-thaw stresses.

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4.5. The Slip Resistance Test The slip resistance values of all surfaces, both wet and dry, are presented in Table4. In wet conditions, the slip resistance values are at a similar level, and any differences result from the material variability and the measurement uncertainty. The best anti-slip properties among the 3 types of tested surfaces were reported for the SBR pavement covered with a thin EPDM spray layer (especially in dry conditions), marked as S1. This surface was characterized by the highest roughness (368 µm). Compared to samples S2 and S3, which had a roughness of about 320 and 231 µm, respectively, its roughness was about 15% and 59% greater. These results showed that the ﬁnish of the top layer is a determining factor in slip resistance. Analyzing the results in relation to the requirements of EN 14877:2013 [46], which are 55–110 PTV units in wet conditions and 80–110 PTV units in dry conditions), it was found that, in dry conditions, all the results were within the requirements, except for the average value of 111 PTV S1 samples. In wet conditions, the obtained values are at the lower limit of the requirements (55 PTV) or are just below this value. As reported bottom SBR layer with the top layer of EPDM surface, marked as S3, did not meet the requirements of the standard specifying outdoor sports surfaces in wet conditions [46].

Table 4. Experimental results of slip resistance (average values).

Slip Resistance Slip Resistance Surface Symbol Wet Samples (PTV) Dry Samples (PTV) S1 56 ± 1 111 ± 1 S2 55 ± 1 106 ± 1 S3 52 ± 1 105 ± 1

5. Conclusions In the present work, we aimed to clarify if recycled styrene butadiene rubber and ethylene propylene diene monomer could be used in sports surface manufacturing. This statement was confirmed by our own experimental studies, and, as obtained results show, all tested materials can be applied in sports facilities, either as track surface or multi-sport surface, depending on the substrate used under them. The obtained results of sport surfaces are as follows: • SBR covered with a thin EPDM spray layer on the top surface showed the highest shock absorption, and clean EPDM surface showed the lowest shock absorption in every substrate tested. • The vertical deformation was in range of 2.2–3.3 mm, 1.3–2.5 mm, and 1.6–2.7 mm for S1, S2, and S3 surface, respectively. • The influence of the substrate on the shock absorption and vertical deformation was proven, as the values differ for the same type of surface depending on the substrate used. • The highest tensile strength was reported for clean EPDM samples (0.919 ± 0.05–1.154 ± 0.05 MPa), and, compared to S1 and S3 samples, they showed about 2.2–2.6 and 1.5–1.8 times higher values, respectively. • The influence of temperature on the tensile strength and abrasion resistance was reported, as obtained results vary for the same sample, depending on the temperature that it was conditioned in. • The slip resistance of all tested surfaces were between 52 ± 1 and 56 ± 1 PTV in wet conditions and between 105 ± 1 and 111 ± 1 PTV in dry conditions, due to which the influence of surface conditions on slip resistance was proven. Materials 2021, 14, 3737 15 of 17

Author Contributions: Conceptualization and Methodology, C.S., M.M., and M.J.; Investigation, C.S. and E.S.; Data Curation, C.S., M.M., and M.J; Formal analysis, M.J.; Funding Acquisition, C.S.; Project Administration, C.S.; Resources, C.S.; Supervision, M.M and C.S.; Validation, M.J.; Visualization, M.J.; Writing—Original Draft Preparation, M.J. and C.S.; Writing—Review and Editing, M.J. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Ministry of Education and Science as part of task No. 1 Influence of operational factors on the safe use of polyurethane surfaces included in the project NZM- 059/2020 Assessment of safety and comfort of use of sports floors and surfaces. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are contained within the article. Conflicts of Interest: The authors declare no conflict of interest.

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