The Structure and Performance of Short Glass Fiber/High-Density Polyethylene/Polypropylene Composite Pipes Extruded Using a Shearing–Drawing Compound Stress Field

materials

Article The Structure and Performance of Short Glass Fiber/High-Density Polyethylene/Polypropylene Composite Pipes Extruded Using a Shearing–Drawing Compound Stress Field

Yi Yuan 1,2,3,*, Changdong Liu 4 and Meina Huang 1,2

1 Chongqing Key Laboratory of Manufacturing Equipment Mechanism Design and Control, Chongqing 400067, China; [email protected] 2 Department of Mechanical Design and Manufacturing, College of Mechanical Engineering, Chongqing Technology and Business University, Chongqing 400067, China 3 Engineering Research Center for Waste Oil Recovery Technology and Equipment of Education, Chongqing 400067, China 4 Dongfeng Sokon Motor Co., Ltd., Chongqing 405321, China; [email protected] * Correspondence: [email protected]; Tel.: +86-23-62768400

Received: 20 March 2019; Accepted: 19 April 2019; Published: 23 April 2019

Abstract: Glass fiber reinforced polyolefin composite materials have many advantages regarding their performance and have been widely used in many fields. However, there are few reports on the simultaneously bidirectional self-enhancement of glass fiber reinforced polyethylene/polypropylene composite pipe. To self-reinforce the pipe’s circular and axial properties simultaneously, short glass fiber reinforced high-density polyethylene/polypropylene (SGF/HDPE/PP) pipes were extruded using a shearing–drawing two-dimensional compound stress field pipe-extrusion device. The effects of the rotating speed of the rotating shear sleeve on the orientation, heat behavior, microstructure, and tensile strength of the pipe were investigated in this paper. The microstructure was observed using scanning electron microscopy (SEM), and the crystal diffraction was analyzed using a polycrystalline X-ray diffractometer (WAXD), the heat behavior was measured using a differential scanning calorimeter (DSC), and the tensile strength was tested using a universal electronic tensile testing machine. The results showed that the shear induction effect induced by the shear rotating promoted the formation of the oriented structure of the crystal plate and SGFs along the circular and axial directions of the pipe simultaneously. Furthermore, it increased the crystallinity of the system, and self-improved the pipe’s circular and axial tensile strength at the same time.

Keywords: shearing–drawing compound stress ﬁeld; shear rotating speed; shear inducement; two-dimensional self-reinforcement pipe-extrusion; short glass ﬁber/high-density polyethylene/polypropylene composite pipe

1. Introduction Glass fiber reinforced polyolefin composite materials have many advantages such as a high elastic modulus, good mechanical properties, good dimensional stability, excellent electrical performance, short molding cycle, low cost, recyclability, and so on. Therefore, the composites have a wide range of applications in aerospace, automobile and motorcycle, office furniture, building materials, chemical industry, packaging engineering, and other industries. Therefore, it has been studied by many researchers. The glass fiber/polypropylene (GF/PP) composite properties have been improved using nanoplatelets [1,2]. The researchers examined the properties of GF/PP composite foaming

Materials 2019, 12, 1323; doi:10.3390/ma12081323 www.mdpi.com/journal/materials Materials 2019, 12, 1323 2 of 12 materials [3,4]. The fatigue damage mechanism of GF/PP composites was studied by investigators [5]. Ku-Hyun Jung et al. developed a damage model with the interlaminar fracture toughness of GF/PP composites, and it accurately predicted the impact behavior of the materials [6]. The researchers enhanced the properties of GF/PP composites by using a β-nucleating agent [7–9]. Wu Deshun and Tang Ronghua et al. studied the properties of long glass fiber reinforced polypropylene composites [10,11]. Wang Xuanlun et al. found that the improvement of the interfacial bonding strength could significantly improve the properties of GF/PP composites [12]. The flame-retardant properties of GF/PP composites have been investigated [1,13]. The structure and interfacial shear strength of polypropylene/glass fiber/carbon fiber composites were studied using direct injection molding [14]. Furthermore, the researchers revealed the mechanisms of intrinsic toughening and extrinsic toughening of GF/PP composites [15]. The properties of glass fiber reinforced polyolefin composites are determined by the performances of glass fiber, a polymer matrix, and the interface between them. The interfacial adhesion between the fiber and resin matrix is critical to the properties of the composites to transfer stress through the interface, thus the interface shear strength determines the potential applications of whole composite materials. However, the surface of glass fiber is smooth with a low surface energy, and the wettability to resin is poor. At the same time, the glass fiber is polar, and with a few exceptions, the polymer matrix is generally non-polar, which leads to a vast polarity difference between them. The above reasons lead to the interface adhesion between the glass fiber and resin matrix not being ideal. Furthermore, it restricts the overall performance of composites. To achieve excellent total properties of composites, the adhesion and compatibility between the fiber and resin matrix are require improvement. Therefore, a lot of research has been done in this area. Zhang Daohai et al. enhanced the mechanical properties of long glass fiber reinforced polypropylene composites using grafted polypropylene with glycidyl methacrylate [16]. The performance of GF/PP composites was improved by grafting PP with acrylic acid [17]. Tang Ke et al. used maleic anhydride functionalized polypropylene (PP-G-MAH) to effectively enhance the interface adhesion between the continuous glass fiber and polypropylene resin, which resulted in excellent overall properties for the composites [18]. The performances of GF/PP and PP-banana/GF composites were improved using PP-G-MAH too [9,19,20]. Li Min et al. used polypropylene graft maleic anhydride and ethylene/octene copolymer to significantly improve the tensile, bending, and impact properties of GF/PP composites [21]. Research showed that the interface adhesion between GF and PP affected the GF/PP composite’s properties [22]. It was seen that these studies mainly focused on glass fiber reinforced polypropylene (GF/PP) composites by adding or subtracting or changing the composition of composites. There are few reports studied on the glass fiber reinforced polyethylene/polypropylene blends and their products. In particular, the influence of molding conditions on the structure and properties of glass fiber reinforced polyethylene/polypropylene composite pipes have rarely been reported on. However, for the thin-walled tube under hydrostatic pressure, which was investigated in this paper, its circumferential stress and axial stress were calculated by Bai et al. [23] as follows:

pD σθ = (1) 2t pD σα = (2) 4t where σθ is the circumferential stress, σα is the axial stress, t is the pipe wall thickness, D is the pipe diameter, and p is the inside pressure. In this case, the relationship between σθ and σα is given in Equation (3): σθ = 2σα (3) Therefore, for the thin-walled tube under hydrostatic pressure, its circumferential strength should be two times its axial strength to achieve the balance of pipe properties. However, many studies have Materials 2019, 12, 1323 3 of 12 shown that in the molding process, polymer macromolecules will be arranged and distributed along the flow direction of the melt to form the oriented structure along the route, thus enhancing the strength performance of the product along the path [24–26]. This phenomenon is evident during the formation of the oriented structure of GF too [27]. As a result, the circumferential strength of the pipe made using traditional manufacturing methods is far lower than its axial strength, and it is difficult to satisfy the practical demand for higher circumferential strength of the thin-walled tube under hydrostatic pressure. Although there have been many reports in the research of polymer pipe [28–42], these studies have not been able to effectively realize the self-enhancement of the pipe’s circumferential strength without reducing its axial strength, especially the coincident self-enhancement of the pipe’s axial and circumferential strength. When the melt of GF-reinforced polymer composites is induced by an applied stress, the GF and macromolecule of the polymer under the induction of the stress will be arranged and distributed along the stress direction to form an oriented structure [43]. Furthermore, the oriented structure will self-improve the mechanical properties in the path. Inspired by this, in this study, a shearing–drawing two-dimensional compound stress field pipe-extrusion device was used, which could generate the applied stress along the circumferential and axial direction of the pipe at the same time, and the SGF/HDPE/PP composite pipes with better self-enhanced circumferential strength and self-enhanced axial strength were manufactured using it. The axial and circumferential strength of the tube and its structure and properties were comparatively investigated after the rotating shear sleeve of the device was stationary and rotated. By adjusting the rotating speed of the sleeve in the process of manufacturing, it can change the intensity of the applied shear stress along the circumferential direction of the pipe so its influence on the structure and performances of the tube can be studied. In this way, the circumferential and axial strength of the canal were self-enhanced simultaneously on the premise that no pipe raw materials were added, no raw material composition was changed, and no structure, shape, or size of the tube was changed. In particular, the circumferential strength of the pipe was significantly self-improved. Thus, the results of this experiment better optimized the material properties and better met the actual demand for material properties of a thin-walled tube under hydrostatic pressure.

2. Experimental

2.1. Materials The high-density polyethylene (HDPE) used in this research was type 6100 M and purchased from Beijing Yanshan Petrochemical Co., Ltd. (Beijing, China). Its melting index was 0.42 g/10 min. The 30% short glass ﬁber reinforced polypropylene (SGFRPP) used in this research was granular material and purchased from Chemical Industry Zhonghao Chenguang Research Institute (Chengdu, China). The mass ratio of innovative raw materials in this study was as follows: HDPE/SGFRPP = 60/40.

2.2. The Process of Pipe Extrusion The pipe extrusion device was developed with an inner diameter of 30 mm and an outer diameter of 35 mm. The melt passed through the circumferential shear stress field and axial tensile stress field successively during extrusion. The rotating of the rotating shear sleeve on the mandrel generated the shear stress field on the material along the circumferential direction of the pipe, and the rotating speed could be adjusted. The reduction of the circular runner section size from the shear stress field section to the die section caused an axial drawing stress field along the axial direction on the material. When the composite melt flowed through the two stress fields, it was subjected to circumferential shear and axial drawing action successively, and then formed into the pipe through the extrusion die. The schematic diagram of the extrusion device is shown in Figure1[44]. Materials 2019, 12, 1323 4 of 12 Materials 2019, 12, x FOR PEER REVIEW 4 of 12

Figure 1. Schematic of the structure and principle of shearing–drawingshearing–drawing two-dimensionaltwo‐dimensional compound stress ﬁeldfield pipe-extrusion.pipe‐extrusion.

From the feeding hopper to the die, the extrusion temperatures of the plastic extruder (SJ (SJ-45B,‐45B, Shanghai Extrusion Machinery Factory Co., Ltd., Shanghai, China) successively were were 100, 180, 210, 200, and 180 ◦°C.C. The temperature of the shear stress fieldfield zone was 200 ◦°C.C. The temperature of the extrusion diedie zone zone was was 170 170◦C. °C. The The rotating rotating speed speed of the of extruder the extruder screw screw was 20 was revolutions 20 revolutions per minute per (rpm).minute The(rpm). rotating The rotating speed of speed the rotating of the rotating shear sleeve shear varied sleeve from varied 0 to from 25 rpm. 0 to When25 rpm. the When rotating the speedrotating was speed zero, was the extrudedzero, the pipeextruded was defined pipe was as thedefined conventional as the conventional pipe. When thepipe. shear When rotary the sleeveshear wasrotary rotating, sleeve the was molded rotating, tube the was molded called thetube reinforced was called pipe. the When reinforced the tube pipe. was When extruded the away tube fromwas theextruded die, it wasaway immediately from the die, quenched it was immediately and cooled to quenched finalize the and pipe cooled shape to in finalize room temperature the pipe shape water. in room temperature water. 2.3. Characterization of Pipe 2.3. CharacterizationThin slices of 0.3 of Pipe mm thickness were cut from the core layer at the location of 1.1 mm away from the surface of the pipe. Under nitrogen protection, the second heating melting behavior was Thin slices of 0.3 mm thickness were cut from the core layer at the location of 1.1 mm away measured using a differential scanning calorimeter (DSC, TA2000, TA Instruments, Chicago, IL, USA) from the surface of the pipe. Under nitrogen protection, the second heating melting behavior was at temperatures ranging from room temperature to 200 C at a rate of 10 C/min, and then a delay of measured using a differential scanning calorimeter (DSC,◦ TA2000, TA◦ Instruments, Chicago, IL, more than 3 min at the temperature of 200 C. The following Equation (4) was used to calculate the USA) at temperatures ranging from room temperature◦ to 200 °C at a rate of 10 °C/min, and then a crystallinities of PE and PP in the pipe [45]. delay of more than 3 minutes at the temperature of 200 °C. The following Equation (4) was used to calculate the crystallinities of PE and PP in the pipe [45]. ∆Hf αc = 100% (4) ∆H0 × Hff c 0 100% (4) H α f 0 where c is the crystallinity, ∆Hf is the latent heat of crystallization melting, and ∆Hf is the melt latent 0 0 heatwhere of α thec is same the crystallinity, polymer with ΔH thef is crystallinity the latent heat of 100%. of crystallization In this research, melting, the value and Δ ofH PE’sf is ∆theHf meltwas 0 specifiedlatent heat as of 293 the J/g, same and thepolymer value with of PP’s the∆ Hcrystallinityf was defined of 100%. as 209 In J/g. this research, the value of PE’s ΔHf0 Thewas crystal specified diff asraction 293 J/g, of pipeand the was value analyzed of PP’s using ΔH af0 polycrystallinewas defined as X-ray 209 J/g. Di ffractometer (WAXD, D/MAX-III,The crystal Rigaku diffraction Corporation, of Akishima,pipe was Tokyo,analyzed Japan) using under a polycrystalline the conditions that X‐ray the scanningDiffractometer speed was(WAXD, 0.06 /D/MAXs, the voltage‐Ⅲ, Rigaku was 45 Corporation, kV, and the pipe Akishima, current wasTokyo, 40 mA. Japan) The under 10 mm the10 conditions mm samples that with the ◦ × entirescanning wall speed thickness was 0.06°/s, were directly the voltage cut from was the 45 pipekV, and sample, the pipe and theircurrent axial was directions 40 mA. wereThe 10 marked mm × to10 distinguishmm samples between with entire the axial wall andthickness circular were direction. directly Then, cut from they the were pipe ground sample, flat and on boththeir sidesaxial fordirections testing. were marked to distinguish between the axial and circular direction. Then, they were groundThe flat microstructure on both sides of for pipe testing. was observed using scanning electron microscopy (SEM, X-650, Hitachi Co., Ltd.,The microstructure Tokyo, Japan). Theof pipe rectangular was observed block samples using scanning of 5 mm electron10 mm microscopy were cut from (SEM, the middle X‐650, × ofHitachi the pipe Co., wall. Ltd., Then, Tokyo, the Japan). samples The were rectangular roughly block ground samples to flat of with 5 mm sandpaper × 10 mm along were the cut thickness from the direction.middle of the After pipe being wall. refined Then, and the polished,samples were the polished roughly surfaceground wasto flat soaked with sandpaper in a mixture along etching the solutionthickness of direction. KMnO4,H After3PO being4, and refined H2SO4 forand 24 polished, h. Finally, the the polished samples surface were cleaned was soaked successively in a mixture with etching solution of KMnO4, H3PO4, and H2SO4 for 24 h. Finally, the samples were cleaned successively with 30% of H2O2, deionized water, and acetone, and dried. The temperature was not

MaterialsMaterials2019 2019,,12 12,, 1323x FOR PEER REVIEW 55 of 1212 Materials 2019, 12, x FOR PEER REVIEW 5 of 12 more than 45 °C in the process of all operations, and the samples were sputter-coated with gold more than 45 °C in the process of all operations, and the samples were sputter-coated with gold 30%before of Hthe2O SEM2, deionized observati water,ons. and acetone, and dried. The temperature was not more than 45 ◦C in the processbefore the of allSEM operations, observati andons. the samples were sputter-coated with gold before the SEM observations. 2.4. Tensile Strength Testing of Pipe 2.4. Tensile StrengthStrength TestingTesting ofof PipePipe The tensile strength of the pipe was tested using a universal electronic tensile testing machine The tensile strength of thethe pipepipe waswas testedtested usingusing aa universaluniversal electronicelectronic tensiletensile testingtesting machinemachine (AG-10TA, Shimadzu Co., Ltd., Tokyo, Japan). The stretching velocity during the test was a (AG-10TA,(AG-10TA, Shimadzu Shimadzu Co., Co., Ltd., Ltd. Tokyo,, Tokyo Japan)., Japan). The stretchingThe stretching velocity velocity during during the test the was test a constant was a constant 100 mm/min. The specimen for the circumferential tensile strength test was a circular 100constant mm/ min. 100 Themm/min. specimen The forspecimen the circumferential for the circu tensilemferential strength tensile test strength was a circular test was specimen a circular cut specimen cut from the pipe samples. Its shape and sizes are exhibited in Figure 2a. The sample for fromspecimen the pipe cut samples.from the pipe Its shape samples and.sizes Its shape are exhibited and sizes in are Figure exhibited2a. The in sample Figure for2a. theThe axial sample tensile for the axial tensile strength test was taken as a straight strip specimen. Its form and dimensions are strengththe axial test tensile was takenstrength as a test straight was striptaken specimen. as a straight Its form strip and specimen. dimensions Its areform shown and indi Figuremension2bs [ 44are]. shown in Figure 2b [44]. The two ends of the straight strip specimen were directly clamped on the Theshown two in ends Figure of the2b straight[44]. The strip two specimen ends of the were straight directly strip clamped specimen on thewere upper directly and lowerclamped clamping on the upper and lower clamping heads of the machine respectively for the axial tensile strength test. The headsupper ofand the lower machine clamping respectively heads forof the the machine axial tensile respectively strength test.for the The axial circular tensile sample strength was installedtest. The circular sample was installed on the support blocks, and the support blocks were connected to the oncircular the support sample blocks, was installed and the on support the support blocks wereblocks, connected and the tosupport the connecting blocks were brackets connected through to the connecting brackets through the threaded rods. The connecting brackets were clamped on the threadedconnecting rods. brackets The connecting through thebrackets threaded were clampedrods. The on connecting the upper and brackets lower were clamping clamped heads on of the upper and lower clamping heads of the machine for the circumferential tensile strength test. The machineupper and for lower the circumferential clamping head tensiles of the strength machine test. for The the loading circumferential scheme for tensile circular strength tensile test. strength The loading scheme for circular tensile strength test is shown in Figure 3. testloading is shown scheme in Figurefor circular3. tensile strength test is shown in Figure 3.

(a) (b) (a) (b) Figure 2. Schematic of the specimen for (a) circular tensile strength test and (b) axial tensile strength test. FigureFigure 2. Schematic 2. Schematic of the of thespecimen specimen for for(a) (circulara) circular tensile tensile strength strength test test and and (b (b) )axial axial tensile tensile strength strength test test..

FigureFigure 3.3. The schemescheme ofof loadingloading forfor thethe circularcircular tensiletensile strengthstrength test.test. Figure 3. The scheme of loading for the circular tensile strength test.

3. Results and Discussions 3. Results and Discussions 3.1. Orientation 3.1. Orientation

Materials 2019, 12, 1323 6 of 12

3. Results and Discussions

3.1.Materials Orientation 2019, 12, x FOR PEER REVIEW 6 of 12 WAXD characterization was carried out along the axial and circumferential directions of the WAXD characterization was carried out along the axial and circumferential directions of the conventional pipe and reinforced pipe with a rotating speed of 15 rpm. The results in Figure4 show conventional pipe and reinforced pipe with a rotating speed of 15 rpm. The results in Figure 4 show that the diffraction intensity of the {110} crystal plane at about 14◦ was almost the same as that of that the diffraction intensity of the {110} crystal plane at about 14° was almost the same as that of the conventional and reinforced pipe in both the circumferential and axial directions. However, the the conventional and reinforced pipe in both the circumferential and axial directions. However, the diffraction intensities of the {040} crystal plane at about 16.8◦, {110} at approximately 21.45◦, and {200} at diffraction intensities of the {040} crystal plane at about 16.8°, {110} at approximately 21.45°, and approximately 23.85◦ of the reinforced pipe were significantly stronger than those of the conventional {200} at approximately 23.85° of the reinforced pipe were significantly stronger than those of the tube in both the circumferential and axial directions, indicating that the crystal orientation of the conventional tube in both the circumferential and axial directions, indicating that the crystal reinforced canal was higher than that of the traditional one. Furthermore, the diffraction intensities of orientation of the reinforced canal was higher than that of the traditional one. Furthermore, the all crystal planes of the reinforced pipe in its circumferential direction were higher than those in its diffraction intensities of all crystal planes of the reinforced pipe in its circumferential direction were axial direction. The results indicate that the introduction of shear rotating inducing a rotating shear higher than those in its axial direction. The results indicate that the introduction of shear rotating sleeve induced the molecule structure of SGF/HDPE/PP pipe to obtain a better orientation effect along inducing a rotating shear sleeve induced the molecule structure of SGF/HDPE/PP pipe to obtain a the circumferential direction, and thus the orientation degree along the route was enhanced. The better orientation effect along the circumferential direction, and thus the orientation degree along formation of the more oriented structure along the circumferential direction of the pipe was conducive the route was enhanced. The formation of the more oriented structure along the circumferential to self-improving the circumferential strength, promoting the reasonable distribution of the pipe direction of the pipe was conducive to self-improving the circumferential strength, promoting the properties and the effective utilization of the materials. reasonable distribution of the pipe properties and the effective utilization of the materials.

(a)

(b)

Figure 4. WAXD curves for the (a)) circularcircular directiondirection andand ((bb)) axialaxial direction.direction.

3.2. Heat Behavior The DSC analysis test of the conventional pipe and reinforced pipe with a rotating speed of 15 rpm was carried out, and the results are shown in Figure 5 and Table 1. The results in Figure 5 show that the melting peaks (Tm) of PE and PP in the reinforced pipe were almost the same as those

Materials 2019, 12, x FOR PEER REVIEW 7 of 12 ofMaterials the conventional2019, 12, 1323 one, as indicated by the results of Tm in Table 1, meaning that their crystal plate7 of 12 thickness was virtually equivalent. In Table 1, the results also show that the melting ranges (ΔT) of PE and PP in the reinforced pipe were nearly the same as those of the conventional one too, 3.2. Heat Behavior indicating that their crystal plate thickness uniformity and crystallization rate were virtually the sameThe as each DSC other. analysis However, test of as the shown conventional in Table pipe 1, the and PE reinforced’s crystallinity pipe (α withc) in the a rotating reinforced speed pipe of increased15 rpm was from carried 54.91% out, in and the theconventional results are tub showne to 57.68%, in Figure a 5proportional and Table1 .increase The results of 5.04%. in Figure The5 αc ofshow PP increased that the melting from 6.07% peaks to (T 8.09%m) of, PE a proportional and PP in the increase reinforced of 33.28%. pipe were The almost results the indicate same asthat those the rotatiof theon conventional of the rotating one, as shear indicated sleeve byformed the results an effective of Tm in shearing Table1, meaning-induced that crystallization their crystal effect, plate whichthickness accelerated was virtually the equivalent. formation ofIn Table molecular1, the results crystallization also show of that the the SGF/HDPE/PP melting ranges system ( ∆T) of and PE improvedand PP in thethereinforced crystallization pipe wereeffect nearly of PP the especially. same as thoseThe reason of theconventional for the increases one too, is believed indicating to that be thattheir the crystal rotati plateon of thickness the rotating uniformity shear sleeve and crystallizationinduced the SGF/HDPE/PP rate were virtually system the molecule same ass each to form other. a moreHowever, orderly as shownarrangement in Table structure1, the PE’s, especially crystallinity along ( α cthe) in circumferential the reinforced pipe direction increased of the from pipe. 54.91% This orderedin the conventional structure promoted tube to 57.68%, the cry astallization proportional of increase the system of 5.04%. molecule The sα. cThenof PP, increasedthe improvement from 6.07% of crystallinityto 8.09%, a proportional was helpful increaseto self-improve of 33.28%. the Themechanical results indicate properties that of the the rotation tube. of the rotating shear sleeve formed an effective shearing-induced crystallization effect, which accelerated the formation of molecular crystallizationTable 1. The of second the SGF heating/HDPE DSC/PP test systeming results and improvedof the SGF/HDPE/PP the crystallization pipe. effect of PP especially. The reason for the increases is believed to be that the rotation of the rotating shear sleeve Pipe Type Polymer Type ΔHf (J/g) Tm (°C) ΔT (°C) αc (%) induced the SGF/HDPE/PP system molecules to form a more orderly arrangement structure, especially PE 169 132.02 12.40 57.68 alongReinforced the circumferential direction of the pipe. This ordered structure promoted the crystallization PP 16.9 162.30 10.79 8.09 of the system molecules. Then, the improvement of crystallinity was helpful to self-improve the PE 160.9 132.62 12.49 54.91 mechanicalConventional properties of the tube. PP 12.68 162.08 10.62 6.07

FigureFigure 5. 5. TheThe second second heating heating DSC DSC curves curves for for the the SGF/HDPE/PP SGF/HDPE/PP pipe pipe.. Table 1. The second heating DSC testing results of the SGF/HDPE/PP pipe. 3.3. Microstructure Pipe Type Polymer Type ∆H (J/g) Tm ( C) ∆T ( C) αc (%) The SGFs and crystal chips in the pipe weref observed using◦ SEM. ◦The results can be seen in PE 169 132.02 12.40 57.68 Figure 6a thatReinforced there were few SGFs in the conventional tube arranged and distributed along the PP 16.9 162.30 10.79 8.09 circumferential direction. However,PE the SGFs 160.9in the reinforced 132.62 pipe with 12.49 the rota 54.91ting speed of 15 Conventional rpm was distributed and orientedPP along the circumferential 12.68 162.08 path, as seen 10.62 in Figure 6.07 6b. The results indicate that when the rotating shear sleeve rotated, it induced the SGFs to form an oriented distribution3.3. Microstructure structure along the circumferential route of the pipe. The formation of the oriented structure of SGFs effectively self-enhanced the pipe properties. Furthermore, the results in Figure 6c showThe that SGFs the crystal and crystal plate chips in the in conventional the pipe were pipe observed was less using deformed SEM. The and results oriented can and be seen it was in randomFigure6aly thatdistributed. there were Compared few SGFs with in the the results conventional in the traditional tube arranged tube, andthe crystal distributed plate alongsize in thethe reinforcedcircumferential canal direction. was refined However, and the became SGFs in more the reinforced uniform. pipe Furthermore, with the rotating the crystal speed plate of 15 wasrpm was distributed and oriented along the circumferential path, as seen in Figure6b. The results indicate

Materials 2019, 12, 1323 8 of 12 that when the rotating shear sleeve rotated, it induced the SGFs to form an oriented distribution structure along the circumferential route of the pipe. The formation of the oriented structure of SGFs effectively self-enhanced the pipe properties. Furthermore, the results in Figure6c show that the crystal plate in the conventional pipe was less deformed and oriented and it was randomly distributed. Compared with the results in the traditional tube, the crystal plate size in the reinforced canal was Materials 2019, 12, x FOR PEER REVIEW 8 of 12 refined and became more uniform. Furthermore, the crystal plate was stretched and deformed to stretchedform an oriented and deformed structure to form along an the oriented circumferential structure along direction the circumf of the tube,erential as showndirection in of Figure the tube,6d. Theas shown results in indicate Figure that6d. The the rotationresults indicate of the rotating that the shear rotation sleeve of promotedthe rotating the shear uniform sleeve structure promoted and therefinement uniform of structure the crystal and grain refinement of SGF /ofHDPE the /crystalPP pipe grain and of induced SGF/HDPE/PP the orientation pipe and of theinduced molecule the orientationalong the circumferential of the molecule direction. along the Furthermore, circumferential it self-improved direction. Furthermore, the pipe’s strength it self-improved performance, the pipe'smainly strength the circumferential performance, strength mainly performance. the circumferential strength performance.

(a) (b)

Figure 6. SEMSEM micrographs micrographs of of ( (aa,,cc)) conventional conventional pipe pipe and ( b,,d) reinforced pipe withwith 15 rpm.rpm.

3.4. Tensile Strength 3.4. Tensile Strength To investigate the tensile strength of the conventional and reinforced pipe, the axial and To investigate the tensile strength of the conventional and reinforced pipe, the axial and circumferential tensile strength tests were carried out. As seen in Figure7, the axial and circumferential circumferential tensile strength tests were carried out. As seen in Figure 7, the axial and tensile strength of SGF/HDPE/PP pipe was self-enhanced at the same time when the rotating speed of circumferential tensile strength of SGF/HDPE/PP pipe was self-enhanced at the same time when the the rotating shear sleeve was increased from 0 to 15 rpm. The axial tensile strength increased from rotating speed of the rotating shear sleeve was increased from 0 to 15 rpm. The axial tensile strength 49.68 MPa for the conventional pipe to 50.56 MPa for the most muscular reinforced tube, a proportional increased from 49.68 MPa for the conventional pipe to 50.56 MPa for the most muscular reinforced increase of 1.77%. At the same time, the circumferential tensile strength increased from 18.9 MPa tube, a proportional increase of 1.77%. At the same time, the circumferential tensile strength increased from 18.9 MPa for the conventional pipe to 26 MPa for the most robust reinforced tube, a proportional increase of 37.57%. The results indicate that the rotation of the rotating shear sleeve could self-improve the axial and circumferential tensile strength performance of the pipe simultaneously, and in particular, the circumferential tensile strength performance self-improved significantly. This further optimized the performance of the composite materials and more fully satisfied the practical needs of the use of the thin-walled pipe under hydrostatic pressure. This may be because the shear stress field increased the order of SGFs and molecules of the tube, resulting in the oriented and distributed structure, in particular, the circumferential oriented and distributed

Materials 2019, 12, 1323 9 of 12 for the conventional pipe to 26 MPa for the most robust reinforced tube, a proportional increase of 37.57%. The results indicate that the rotation of the rotating shear sleeve could self-improve the axial and circumferential tensile strength performance of the pipe simultaneously, and in particular, the circumferential tensile strength performance self-improved significantly. This further optimized the performance of the composite materials and more fully satisfied the practical needs of the use of the thin-walledMaterials 2019, 12 pipe, x FOR under PEER hydrostatic REVIEW pressure. This may be because the shear stress field increased9 of 12 the order of SGFs and molecules of the tube, resulting in the oriented and distributed structure, in particular,architecture. the However, circumferential the results oriented in Figure and distributed 7 also show architecture. that the axial However, and circumferential the results in Figure tensile7 alsostrength show of that SGF/HDPE/PP the axial and pipe circumferential decreased w tensilehen the strength rotating of SGFspeed/HDPE exceeded/PP pipe 15 decreasedrpm. This whenindicates the rotatingthat when speed the exceededsleeve rotated 15 rpm. too This violently, indicates the that axial when and thecircumferential sleeve rotated tensile too violently, strength the of axialthe pipe and circumferentialdecreased. The tensile reason strength could be of the that pipe the decreased. adverse effect The reason of the could disorientation be that the and adverse recovery effect of of the disorientationmolecule and and SGF, recovery which was of the caused molecule by andthe shearSGF, which heat generated was caused by by the the sharp shear rotatingheat generated of the bysleeve, the sharp was greaterrotating than of the the sleeve, positive was greatereffect of than the the order positive enhancement effect of the of order the SGFs enhancement and molecules of the SGFsinduced and by molecules the shear induced stress field. by the shear stress field.

Figure 7. TThehe t tensileensile strength ofof the SGFSGF/HDPE/PP/HDPE/PP pipe.pipe. 4. Conclusions 4. Conclusions In this study, SGF/HDPE/PP composite pipes were extruded through a shearing–drawing In this study, SGF/HDPE/PP composite pipes were extruded through a shearing–drawing two-dimensional compound stress field pipe-extrusion device, with the aim to self-improve the circular two-dimensional compound stress field pipe-extrusion device, with the aim to self-improve the and axial properties of the tube at the same time. The change of the structure and performance of the circular and axial properties of the tube at the same time. The change of the structure and pipe after the shear rotary sleeve was stationary and rotated was studied comparatively. The results performance of the pipe after the shear rotary sleeve was stationary and rotated was studied show that the circumferential and axial strength of the tube could be self-enhanced synchronously via comparatively. The results show that the circumferential and axial strength of the tube could be the rotation of the rotating shear sleeve, and in particular, the effect of the circumferential strength self-enhanced synchronously via the rotation of the rotating shear sleeve, and in particular, the self-reinforcement was clear. The axial tensile strength increased from 49.68 MPa for the conventional effect of the circumferential strength self-reinforcement was clear. The axial tensile strength pipe up to 50.56 MPa for the most robust reinforced tube, a proportional increase of 1.77%. The increased from 49.68 MPa for the conventional pipe up to 50.56 MPa for the most robust reinforced circumferential tensile strength increased from 18.9 MPa for the traditional pipe to 26 MPa for the tube, a proportional increase of 1.77%. The circumferential tensile strength increased from 18.9 MPa most robust reinforced one, a proportional increase of 37.57%. The SEM results confirmed that the for the traditional pipe to 26 MPa for the most robust reinforced one, a proportional increase of crystal and SGF in the reinforced tube had been oriented along the circumferential and axial direction 37.57%. The SEM results confirmed that the crystal and SGF in the reinforced tube had been simultaneously. The results of WAXD demonstrated that the orientation degree of the reinforced oriented along the circumferential and axial direction simultaneously. The results of WAXD tube was higher than that of the conventional one, and it had a better orientation effect along the demonstrated that the orientation degree of the reinforced tube was higher than that of the circumferential direction than that along the axial route in the reinforced tube. The results from the conventional one, and it had a better orientation effect along the circumferential direction than that DSC analysis showed that the crystallinity of the reinforced pipe was improved and the crystal grain along the axial route in the reinforced tube. The results from the DSC analysis showed that the was well refined. As a result, the crystallinity of PE increased from 54.91% in the conventional tube crystallinity of the reinforced pipe was improved and the crystal grain was well refined. As a result, to 57.68% in the reinforced one, a proportional increase of 5.04%. The crystallinity of PP rose from the crystallinity of PE increased from 54.91% in the conventional tube to 57.68% in the reinforced 6.07% in the traditional pipe to 8.09% in the reinforced one, a proportional increase of 33.28%. In this one, a proportional increase of 5.04%. The crystallinity of PP rose from 6.07% in the traditional pipe to 8.09% in the reinforced one, a proportional increase of 33.28%. In this way, the properties of the circumferential and axial strength of the SGF/HDPE/PP composite pipe were self-improved concurrently, and it fully meets the functional requirements for the use of the thin-walled tube under hydrostatic pressure.

Materials 2019, 12, 1323 10 of 12 way, the properties of the circumferential and axial strength of the SGF/HDPE/PP composite pipe were self-improved concurrently, and it fully meets the functional requirements for the use of the thin-walled tube under hydrostatic pressure.

Author Contributions: Conceptualization, Y.Y. and C.L.; Funding acquisition, Y.Y.; Methodology, Y.Y., C.L., and M.H.; Project administration, Y.Y.; Writing—original draft, M.H.; Writing—review and editing, Y.Y. and C.L. Funding: This research was funded by the Chinese Chongqing Science and Technology Commission (Project Nos. cstc2015jcyjA50027, and cstc2017zdcy-zdyfX0083), and the Ministry of Science and Technology of the People’s Republic of China (Project No. 2015DFA51330). Acknowledgments: The authors would like to thank Kaizhi Shen for the inspiration on the conception of experimental principles. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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