Article

Polypropylene/Short Glass Fibers Composites: Effects of Coupling Agents on Mechanical Properties, Thermal Behaviors, and Morphology Jia-Horng Lin 1,2,3 , Chien-Lin Huang 4 , Chi-Fan Liu 5 , Chih-Kuang Chen 6 , Zheng-Ian Lin 1 and Ching-Wen Lou 7, * Received: 13 October 2015; Accepted: 20 November 2015; Published: 2 December 2015 Academic Editor: Wen-Hsiang Hsieh 1

2 3 4 5 6 7

*

Laboratory of Fiber Application and Manufacturing, Department of Fiber and Composite Materials, Feng Chia University, Taichung City 40724, Taiwan; [email protected] (J.-H.L.); [email protected] (Z.-I.L.) School of Chinese Medicine, China Medical University, Taichung City 40402, Taiwan Department of Fashion Design, Asia University, Taichung City 41354, Taiwan Department of Fiber and Composite Materials, Feng Chia University, Taichung City 40724, Taiwan; [email protected] Office of Physical Education and Sports Affairs, Feng Chia University, Taichung City 40724, Taiwan; [email protected] The Polymeric Biomaterials Laboratory, Department of Fiber and Composite Materials, Feng Chia University, Taichung City 40724, Taiwan; [email protected] Institute of Biomedical Engineering and Materials Science, Central Taiwan University of Science and Technology, Taichung City 40601, Taiwan Correspondence: [email protected]; Tel.: +886-4-2451-7250 (ext. 3405); Fax: +886-4-24510871

Abstract: This study uses the melt compounding method to produce polypropylene (PP)/short glass fibers (SGF) composites. PP serves as matrix while SGF serves as reinforcement. Two coupling agents, maleic anhydride grafted polypropylene, (PP-g-MA) and maleic anhydride grafted styrene-ethylene-butylene-styrene block copolymer (SEBS-g-MA) are incorporated in the PP/SGF composites during the compounding process, in order to improve the interfacial adhesion and create diverse desired properties of the composites. According to the mechanical property evaluations, increasing PP-g-MA as a coupling agent provides the composites with higher tensile, flexural, and impact properties. In contrast, increasing SEBS-g-MA as a coupling agent provides the composites with decreasing tensile and flexural strengths, but also increasing impact strength. The DSC results indicate that using either PP-g-MA or SEBS-g-MA as the coupling agent increases the crystallization temperature. However, the melting temperature of PP barely changes. The spherulitic morphology results show that PP has a smaller spherulite size when it is processed with PP-g-MA or SEBS-g-MA as the coupling agent. The SEM results indicate that SGF is evenly distributed in PP matrices, but there are distinct voids between these two materials, indicating a poor interfacial adhesion. After PP-g-MA or SEBS-g-MA is incorporated, SGF can be encapsulated by PP, and the voids between them are fewer and indistinctive. This indicates that the coupling agents can effectively improve the interfacial compatibility between PP and SGF, and as a result improves the diverse properties of PP/SGF composites. Keywords: polypropylene (PP); Short glass fibers (SGF); coupling agent; mechanical properties; thermal behaviors

Materials 2015, 8, 8279–8291; doi:10.3390/ma8125451

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Materials 2015, 8, 8279–8291

1. Introduction Thermoplastic polymer has formed a new trend in material development, and it has the advantages of low production cost, great diversity, sufficient sources, light weight, good physical properties, and chemical resistance, as well as various efficient manufacturing process. Therefore, there are a great number of plastic products commercially available. Polypropylene (PP) is the most consumed polymer globally, and it has a comparably light weight. Because of its good processing features, high chemical stability, and electrical insulation, PP has been commonly used in the spare parts of vehicles, bicycles, electronic products, civil necessities, medical instruments, and chemical products. However, it has low mechanical properties and does not have any functions, which restrict its application ranges [1–5]. Current studies reinforce PP in order to have greater mechanical properties; reinforcing materials include rigid particles fillers, such as nanoclay [6–8], calcium carbonate [9], and silicon dioxide [10,11], as well as short fibers, such as glass fiber, carbon fiber [12–17], and basalt fiber [18]. In particular, short glass fiber causes PP to be the strongest mechanically. PP is a nonpolar polymer and has a poor interfacial adhesion with short glass fiber (SGF). This disadvantage is improved by using a coupling agent [19–26] or increasing the surface roughness of fibers during the compounding process [27–29]. Wong et al. examined three coupling agents for the mechanical properties and interfacial compatibilities of composites made of PP and recycled carbon fibers (CF). The test results showed that the tensile and flexural strengths of the composites were significantly improved. The interfacial compatibility of PP and CF was dependent on the acid anhydride group and molecular weight [30]. Broughton et al. used aminosilane and titanate as the coupling agent for glass flake and PP and found that the improved interfacial adhesion of the composites resulted in greater tensile, flexural, and impact strengths [31]. In contrast, Eslami-Farsani et al. added nanoclay to basalt fiber/polypropylene composite in order to increase the roughness of the fibers. The test results indicated that after the adhesion of nanoclay to the basalt fibers, the layered structure improved the interfacial adhesion of two materials, and thereby improved the mechanical properties of the composites [27]; however, the reinforcement was not permanent. In contrast, using a coupling agent can improve the interfacial adhesion between constituent materials, and this method forms a chemical reaction between the matrix and fibers, thereby effectively and permanently improving the mechanical properties of the composites. This study aims to produce composites that meet the requirements of different products and to expand the applications of the composites, and compares different characterizations of the PP/SGF composites produced with two different coupling agents. In this study, short glass fibers that possess high strength, high modulus, and good thermal stability are synthesized with PP in order to reinforce the mechanical properties of PP. Two coupling agents (i.e., maleic anhydride grafted polypropylene, PP-g-MA, and maleic anhydride grafted styrene-ethylene-butylene-styrene block copolymer, SEBS-g-MA) are used for a greater combination between PP and SGF. PP-g-MA has the same molecular structure that PP has, and thus enhances the bonding between two materials. Similarly, SEBS-g-MA, an elastomer, has a structure that includes an ethylene-butylene segment that is compatible with PP. Therefore, SEBS-g-MA can improve the compatibility between PP and SGF, and provides the composites with impact resistance. Finally, the influences of these coupling agents on the mechanical properties, thermal behaviors, spherulite structure, and interfacial adhesion are examined. This study can thus serve as a reference for the selection of a coupling agent according to the diverse applications of the composites. 2. Experimental Section 2.1. Materials Polypropylene (PP; YUNGSOX 1080; Formosa Plastics Corporation, Taipei, Taiwan) was a homopolymer with a melt flow rate of 10 g/10 min (ISO1133). Short glass fiber (SGF; 202P; Taiwan

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Glass Ind. Corp., Taipei, Taiwan) had a length of 3.2 mm and a diameter of 13 µm, and was treated with a silane coupling agent. Maleic anhydride grafted polypropylene (PP-g-MA; DuPont Fusabond P613) was purchased from DuPont, Wilmington, DE, USA. Maleic anhydride grafted styrene-ethylene-butylene-styrene block copolymer (SEBS-g-MA; Kraton FG1901X) was purchased from Kraton, Houston, TX, US. Physical properties of materials are summarized in Table 1. Table 1. Physical properties of materials. Polypropylene, PP; Short Glass Fiber, SGF; Maleic Anhydride grafted Polypropylene, PP-g-MA; Maleic Anhydride grafted Styrene-EthyleneButylene-Styrene block copolymer, SEBS-g-MA. Material

Density (g/cm3 )

Melt Index (g/10 min)

0.900 0.903 0.910

10 (230 ˝ C / 2.16 Kg measured) 120 (190 ˝ C / 2.16 Kg calculated) 22 (230 ˝ C / 5 Kg measured)

PP SGF PP-g-MA SEBS-g-MA

Diameter Length (µm) (mm) 13 -

3.2 -

Graft Weight (%) 0.5 1.5

2.2. Methods Various amounts of PP, a specified amount of 25 wt % of SGF, and 2, 4, 6, or 8 wt % of a coupling agent (PP-g-MA or SEBS-g-MA) were mixed to form different PP/SGF/PP-g-MA blends and PP/SGF/SEBS-g-MA blends. Different blends were dried in an oven at 80 ˝ C for 8 h in order to remove moisture, after which they were made into PP/SGF/PP-g-MA pellets and PP/SGF/SEBS-g-MA pellets via a single screw extruder (SEVC-45, Re-Plast Extruder Corp., Miaoli, Taiwan), in which the temperatures of the three barrels and the die were 210 ˝ C, 220 ˝ C, 230 ˝ C, and 210 ˝ C, respectively, and the screw speed was 36 rpm. The pellets were then dried in an oven at 80 ˝ C for 8 h, followed by being made into composites by using an injection machine (Ve-80, VICTOR Taichung Machinery Works Co., Ltd., Taichung, Taiwan), in which the temperatures of three barrels and the nozzle were 210 ˝ C, 220 ˝ C, 230 ˝ C, and 210 ˝ C, respectively. The control group was the pure PP/SGF composites made of 25 wt % SGF and 75 wt % PP. 2.3. Measurements 2.3.1. Tensile Tests An Instron 5566 Universal Tester (Instron, Canton, MA, USA) was used to measure the tensile strength and tensile modulus, as specified in ASTM D638-10. Samples were made into dumbbell-shapes according to ASTM D638 Type IV. The crosshead speed was 5 mm/min. There were a total of 5 samples for each specification. 2.3.2. Flexural Tests The flexural test was performed by using an Instron 5566 Universal Tester (Instron, Canton, MA, USA), as specified in ASTM D790-10. The load and flexural modulus of the samples was measured. There were a total of 5 samples for each specification, and each sample had a size of 127 mm ˆ 12.7 mm ˆ 3.2 mm. The test speed was 2 mm/min, and the support span was 50 mm. The test results were then used to calculate the flexural strength with the following Equation (1): σ f max “

3PL 2bd2

(1)

where P is the load (N); L is the support span (mm); b is the sample width (mm); and d is the sample thickness (mm).

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2.3.3. Izod Impact Tests This test followed ASTM D 256-10. An Izod impact strength tester (CPI, ATLAS, Mount Prospect, IL, USA) was used to measure the impact strength of the samples. The samples had a 45˝ V-shaped cut with a depth of 0.25 mm, and were sized as 63.5 mm ˆ 12.7 mm ˆ 3.2 mm. There were a total of 5 samples for each specification. 2.3.4. Differential Scanning Calorimetry (DSC) Composite samples of 8–10 mg were then placed in the DSC (Q200, TA Instruments, New Castle, DE, USA). Samples were heated from 40 ˝ C to 200 ˝ C at 10 ˝ C/min increments, and were isothermally kept at 200 ˝ C for 10 min, in order to delete the thermal history. Next, samples were cooled to 40 ˝ C with the same increments. During the second cycle, the samples were heated and then cooled between these two temperatures with the same increments. The crystallinity of composites was calculated from Equation (2). The enthalpy corresponding to the melding of 100% crystalline PP is 209 J/g [32,33]. ∆Hm XC “ (2) ˝ ˆ p1 ´ f q ∆Hm where XC is crystallinity; ∆Hm is the apparent enthalpy of crystallization; ∆H˝ m is the enthalpy corresponding to the melting of 100% crystalline PP; and f is the weight fraction of SGF. 2.3.5. Polarized Light Microscopy (PLM) The spherulite morphology of the samples was observed by using a PLM (BX51, Olympus, Tokyo, Japan). A small amount of sample was placed on a glass slide and was then melted to form a film at 200 ˝ C. Samples were then cooled to 130 ˝ C at 10 ˝ C/min increments, and were kept at 130 ˝ C for the observation of spherulite morphology. 2.3.6. Scanning Electron Microscope (SEM) Samples were affixed to the sample holder by using carbon paste and were then coated with a thin gold layer. The fractured surface of the samples was then observed by using an SEM (S3000N, Hitachi, Tokyo, Japan) at a voltage of 15 kV. 3. Results and Discussion 3.1. Effects of Two Coupling Agents (PP-g-MA or SEBS-g-MA) on Mechanical Properties of PP/SGF Composites The tensile properties of various PP/SGF composites are compared to those of pure PP/SGF composites (i.e., the control group), as indicated in Figure 1 and Table 2. The composites that are incorporated with 8 wt % PP-g-MA have a tensile strength of 79.0 MPa, in comparison to that of the control group (67.6 MPa). The tensile strength of the composites is proportional to the amount of PP-g-MA. In contrast, the composites that are incorporated with 8 wt % of SEBS-g-MA have a lower tensile strength (50.6 MPa) than that of the control group. The tensile strength is inversely proportional to the amount of SEBS-g-MA. The tensile modulus of the PP/SGF composites incorporated with 8 wt % PP-g-MA is 2004 MPa, which is greater than that of the control group (1998 MPa). However, increasing PP-g-MA hardly influences the modulus of the composites. Conversely, the tensile modulus of the composites incorporated with 8 wt % SEB-g-MA is 1683.6 MPa, which is lower than that of the control group. Furthermore, the tensile modulus has a decreasing trend with increasing SEBS-g-MA. Figure 2 and Table 2 indicate the flexural properties of various PP/SGF composites that are in conjunction with different coupling agents. The incorporation of 8 wt % of PP-g-MA results in a greater flexural strength of the composites (123.4 MPa) in comparison to that of the control

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group (99.3 MPa). The flexural strength increases as a result of the increasing PP-g-MA. However, incorporation of 8 wt of SEBS-g-MA decreases the flexural strengthstrength of the composites to 81.3 MPa. the incorporation of 8% wt % of SEBS-g-MA decreases the flexural of the composites to Moreover, the flexural strength of the composites shows a declining trend with increasing 81.3 MPa. Moreover, the flexural strength of the composites shows a declining trend with increasing SEBS-g-MA content. content. SEBS-g-MA

Figure 1. (a) Tensile strengthstrength and (b) tensile modulus PP (Polypropylene)/SGF (short glass fiber) Figure 1. (a) Tensile and (b) tensile of modulus of PP (Polypropylene)/SGF (shortcomposites. glass fiber) composites. Table 2. Mechanical properties of pure PP matrices and PP/SGF composites treated with various amounts coupling agents. Table 2. of Mechanical properties of pure PP matrices and PP/SGF composites treated with various amounts of coupling agents.σT (MPa) Composite PP 21.0 ± 0.8 Composite σT 67.6 (MPa) PP/SGF25 ± 0.9 PP 21.0 ˘ 0.8 PP-g-MA (2 wt %) 76.8 ± 0.2 PP/SGF25 (4 wt %) 67.677.7 ˘ 0.9 PP-g-MA ± 0.4 PP-g-MA (2 wt %) 76.8 ˘ 0.2 PP-g-MA (6 wt %) 80.0 ± 0.6 PP-g-MA (4 wt %) 77.7 ˘ 0.4 PP-g-MA wt %) ± 0.5 PP-g-MA (6 wt(8%) 80.079.0 ˘ 0.6 PP-g-MA (8 wt %) ˘ 0.5 SEBS-g-MA (2 wt %) 79.061.3 ± 1.5 SEBS-g-MA (2 wt %) ˘ 1.5 SEBS-g-MA (4 wt %) 61.357.8 ± 0.9 SEBS-g-MA (4 wt %) 57.8 ˘ 0.9 SEBS-g-MA (6 wt %) 53.4 ± SEBS-g-MA (6 wt %) 53.4 ˘ 0.8 0.8 SEBS-g-MA wt %) 50.650.6 ± 0.9 SEBS-g-MA (8 wt(8%) ˘ 0.9

ET (GPa) 0.8 ± 0.032 E1.9 T (GPa) ± 0.026 0.81.9 ˘ ±0.032 0.008 1.91.9 ˘ ±0.026 0.013 1.9 ˘ 0.008 2.0 ± 0.022 1.9 ˘ 0.013 0.016 2.02.0 ˘ ±0.022 2.01.8 ˘ ±0.016 0.035 1.8 ˘ 0.035 1.8 ± 0.014 1.8 ˘ 0.014 0.016 1.71.7 ˘ ±0.016 0.012 1.71.7 ˘ ±0.012

σF (MPa) 47.9 ± 1.3 σ99.3 F (MPa) ± 2.7 47.9 ˘ ±1.3 120.0 4.2 99.3 ˘ ±2.7 123.3 1.0 120.0 ˘ 4.2 127.2 ± 0.9 123.3 ˘ 1.0 123.4˘±0.9 2.8 127.2 123.4 97.8˘± 2.8 2.9 97.8 ˘ 2.9 90.6 ± 1.0 90.6 ˘ 1.0 84.8 1.0 84.8 ˘±1.0 81.4˘±1.2 1.2 81.4

EF (GPa) 1.3 ± 0.051 3.5EF±(GPa) 0.045 1.3±˘0.268 0.051 3.2 3.5 ˘ 0.045 3.4 ± 0.031 3.2 ˘ 0.268 3.5 3.4±˘0.032 0.031 3.6 3.5±˘0.035 0.032 3.6±˘0.320 0.035 3.2 3.2 ˘ 0.320 3.2 ± 0.031 3.2 ˘ 0.031 3.0 3.0±˘0.052 0.052 2.9 2.9±˘0.076 0.076

IS (J/m) 44.8 ± 0.0 (J/m) 73.7IS ± 3.3 44.8 ˘ 0.0 101.9 ± 7.5 73.7 ˘ 3.3 103.5 ± 10.1 101.9 ˘ 7.5 103.5 ± 3.6 103.5 ˘ 10.1 97.5 ± 5.6 103.5 ˘ 3.6 ˘ 5.6 77.797.5 ± 0.0 ˘ 0.0 82.077.7 ± 3.5 82.0 ˘ 3.5 86.286.2 ± 2.7 ˘ 2.7 90.390.3 ± 2.7 ˘ 2.7

Note.σσTTisisthe the tensile ET EisT the tensile modulus; σF is the strength;strength; EF is the flexural modulus; Note. tensilestrength; strength; is the tensile modulus; σFflexural is the flexural EF is the flexural and IS is the impact strength. modulus; and IS is the impact strength.

The The flexural flexural modulus modulus of of the the PP/SGF PP/SGF composites increases to 3565.9 MPa as a result of the the incorporation incorporation of of 88 wt wt % % PP-g-MA, PP-g-MA, in in comparison comparison to to that that of of the the control control group group(3503.4 (3503.4MPa). MPa). The The flexural does not notfluctuate fluctuatewith withincreasing increasingPP-g-MA. PP-g-MA.InIn contrast, flexural modulus of flexural strength does contrast, thethe flexural modulus of the the PP/SGF composites decreases to 2937.6 MPa, comparisontotothe the control control group. The flexural PP/SGF composites decreases to 2937.6 MPa, inincomparison modulus modulus is is inversely inversely proportional proportional to to the the content content of of SEBS-g-MA. SEBS-g-MA. The The impact impact strength strength of of the the PP/SGF PP/SGF composites that are incorporated with different coupling agents agents is is indicated indicated in in Figure Figure 33and andTable Table2.2.The Theimpact impactstrength strengthincreases increasesfrom from73.6 73.6J/m J/mto to97.5 97.5J/m J/m when the PP/SGF composites are incorporated with 8 wt % of PP-g-MA. Meanwhile, the impact when the PP/SGF composites strength composites are incorporated with strength increases increases from from 73.6 73.6J/m J/m to to 90.2 90.2 J/m J/m when the PP/SGF PP/SGF composites 8 wt wt%%ofof SEBS-g-MA. SEBS-g-MA. In In summary, summary, increasing increasing coupling coupling agent, agent, either either PP-g-MA PP-g-MA or or SEBS-g-MA, SEBS-g-MA, improves the impact strength of PP/SGF composites. Table 3 summarizes the variation in tensile, improves the impact strength of PP/SGF Table tensile, flexural, flexural, and and impact impact properties properties as as percentages. percentages. The The tensile, tensile, flexural, flexural, and and impact impact strengths strengthsof ofPP/SGF PP/SGF composites are proportional to the content content of PP-g-MA that is incorporated with the composites. The mechanical properties are reinforced of that is incorporated with the composites. The mechanical properties are reinforced due due to improved the improved interfacial compatibility between PP and There is esterification between to the interfacial compatibility between PP and SGF.SGF. There is esterification between the the hydroxyl groups on the surface of SGF and the acid anhydride groups of PP-g-MA, and hydroxyl groups on the surface of SGF and the acid anhydride groups of PP-g-MA, and the covalent the covalent is then formed, asinindicated Figure 4. As a result, increases PP-g-MAthe increases the bond is thenbond formed, as indicated Figure 4.inAs a result, PP-g-MA interfacial interfacial compatibility between PP and SGF, and thereby improves the mechanical properties of compatibility between PP and SGF, and thereby improves the mechanical properties of the

composites [30,34,35]. However, when PP-g-MA reaches 8 wt %, the aforementioned properties of the composites first exhibit an initial steep increase, 8283 followed by a gradual increase or even a steady 5

Materials 2015, 8, 8279–8291

the composites [30,34,35]. However, when PP-g-MA reaches 8 wt %, the aforementioned properties Materials 2015, 8, page–page Materials 2015, 8, page–page of the composites first exhibit an initial steep increase, followed by a gradual increase or even a decrease, which iswhich ascribed to a saturated content ofcontent acid anhydride groups of PP-g-MA. addition, steady decrease, is ascribed to a saturated of acid anhydride groups ofInPP-g-MA. decrease, which is ascribed to a saturated content of acid anhydride groups of PP-g-MA. In addition, theaddition, tensile modulus andmodulus flexural and modulus of the composites changebarely after the conjunction of In the tensile flexural modulus of thebarely composites change after the the tensile modulus and flexural modulus of the composites barely change after the conjunction of PP-g-MA. There are twoThere possible factors. SGF in PP/SGF composites is composites the major material that conjunction of PP-g-MA. are two possible factors. SGF in PP/SGF is the major PP-g-MA. There are two possible factors. SGF in PP/SGF composites is the major material that withstands by an externally force. PP-g-MA only improve interfacial material thatdeformation withstands deformation by anasserted externally asserted force.can PP-g-MA can onlythe improve the withstands deformation by an externally asserted force. PP-g-MA can only improve the interfacial compatibility between PP and SGF, butSGF, cannot resist deformation. Another factor factor is that interfacial compatibility between PP and buthelp cannot helpthe resist the deformation. Another compatibility between PP and SGF, but cannot help resist the deformation. Another factor is that and PP similar molecular weights. Therefore, flexural isPP-g-MA that PP-g-MA andhave PP have similar molecular weights. Therefore,the thetensile tensilemodulus modulus and flexural PP-g-MA and PP have similar molecular weights. Therefore, the tensile modulus and flexural modulus of of the the composites composites are are not not correlated correlatedwith withthe the conjunction conjunctionof ofPP-g-MA. PP-g-MA.Such Suchaa result resultisis in in modulus modulus of the composites are not correlated with the conjunction of PP-g-MA. Such a result is in linewith withthe thefinding findingof ofthe thestudy studyby byWong Wongetetal.al.[30]. [30]. line line with the finding of the study by Wong et al. [30]. Thetensile tensileand andflexural flexuralstrengths strengthsofofPP/SGF PP/SGF composites composites decrease decrease as as aa result resultof ofthe thecombination combination The The tensile and flexural strengths of PP/SGF composites decrease as a result of the combination of SEBS-g-MA. SEBS-g-MA. This This coupling coupling agent agent isis an an elastomer, elastomer, which which possesses possesses low low tensile tensile and and flexural flexural of of SEBS-g-MA. This coupling agent is an elastomer, which possesses low tensile and flexural strengths. A greater content of SEBS-g-MA causes the the tensile tensile and and flexural flexural strengths strengths to to decrease. decrease. strengths. strengths. A greater content of SEBS-g-MA causes the tensile and flexural strengths to decrease. However, the the improved improved compatibility between to However, between SEBS-g-MA SEBS-g-MAand andSGF SGFlends lendslimited limitedreinforcement reinforcement However, the improved compatibility between SEBS-g-MA and SGF lends limited reinforcement to thethe tensile and flexural strengths indicated in in Figure 5, 5, thethe combination of to tensile and flexural strengthsofofthe thecomposites. composites.AsAs indicated Figure combination the tensile and flexural strengths of the composites. As indicated in Figure 5, the combination of SEBS-g-MA results turn improves improves their their of SEBS-g-MA resultsininthe thechemical chemicalreaction reactionbetween betweenPP PP and and SGF, SGF, which in turn SEBS-g-MA results in the chemical reaction between PP and SGF, which in turn improves their interfacial compatibility. compatibility. This forfor elastic deformation of the interfacial Thisimproved improvedinterfacial interfacialcompatibility compatibilityallows allows elastic deformation of interfacial compatibility. This improved interfacial compatibility allows for elastic deformation of the composites under an an impact, and toughen the the the composites under impact, andthe thedeformation deformationcan canthen then absorb absorb energy energy and toughen composites under an impact, and the deformation can then absorb energy and toughen the composites, thereby thereby increasing increasing the the impact impactstrength. strength. Although the toughness toughness of of the the composite composite composites, composites, thereby increasing the impact strength. Although the toughness of the composite increases as as aa result result of of the the combination combination of of SEBS-g-MA, SEBS-g-MA, the the tensile tensile and and flexural flexural strengths strengths of of the the increases increases as a result of the combination of SEBS-g-MA, the tensile and flexural strengths of the composites decrease. Elastomers improve the toughness of the composites at the cost of sacrificing composites decrease. Elastomers improve the toughness of the composites at the cost of sacrificing composites decrease. Elastomers improve the toughness of the composites at the cost of sacrificing theirrigidity rigidityand anddimensional dimensionalstability stability[36]. [36]. their their rigidity and dimensional stability [36].

Figure 2. (a) Flexural strength and (b) flexural modulus of PP/SGF composites. Figure Figure2.2.(a) (a)Flexural Flexuralstrength strengthand and(b) (b)flexural flexuralmodulus modulusofofPP/SGF PP/SGF composites. composites.

Figure 3. Impact strength of PP/SGF composites, as related to various coupling agents. Figure 3. Impact strength of PP/SGF composites, as related to various coupling agents. Figure 3. Impact strength of PP/SGF composites, as related to various coupling agents. Table 3. Variations in tensile, flexural, and impact properties in percentage (%). Table 3. Variations in tensile, flexural, and impact properties in percentage (%). Coupling Coupling Agent (wt %) Agent (wt %) PP/SGF25 PP/SGF25 2 wt % 2 wt %

σT (%) σT (%) 0 0 14 14

ET (%) ET (%) 0 0 −3 −3

PP-g-MA PP-g-MA σF (%) EF (%) IS (%) σF (%) EF (%)8284 IS (%) 0 0 0 0 0 0 21 −9 38 21 −9 38

6

σT (%) σT (%) 0 0 −8 −8

SEBS-g-MA SEBS-g-MA ET (%) σF (%) EF (%) ET (%) σF (%) EF (%) 0 0 0 0 0 0 −8 −2 −8 −8 −2 −8

IS (%) IS (%) 0 05 5

Materials 2015, 8, page–page Materials 2015, 8, 8279–8291

4 wt % 6 wt % 8 wt %

15 −2 24 −4 41 −14 −11 −9 −8 18 1 28 1 41 −21 −14 −15 −13 Table 3. Variations in tensile, flexural, and impact properties in percentage (%). 18 0 24 2 33 −25 −15 −18 −16

Coupling Agent (wt %) σT (%) Materials Materials 2015, 2015, 8, 8, page–page page–page PP/SGF25 0 wt % % 14 442 wt 15 wt % 15 4 wt % 15 66 wt 18 wt % % 18 6 wt % 18 88 wt 18 wt % % 18 wt % 18

ET (%)

PP-g-MA σF (%)

0

0

´3 −2 −2 ´2 111 000

21 24 24 24 28 28 28 24 24 24

EF (%)

IS (%)

σT (%)

ET (%)

0 ´9 −4 −4 ´4 111 22

0

0

38 41 41 41 41 41 41 33 33

´8 −14 −14 ´14 −21 −21 ´21 −25 −25 ´25

0 ´8 −11 −11 ´11 −14 −14 ´14 −15 −15 ´15

SEBS-g-MA σF (%) EF (%)

Figure 4. Chemical reaction between PP-g-MA and SGF.

0 ´2 −9 −9 ´9 −15 −15 ´15 −18 −18 ´18

0 ´8 −8 −8 ´8 −13 −13 ´13 −16 −16 ´16

12 17 23 IS (%) 0 5 12 12 12 17 17 17 23 23 23

Figure 4. Chemical reaction between PP-g-MA SGF. Figure4. 4. Chemical Chemical reaction reaction between betweenPP-g-MA PP-g-MA and andSGF. SGF. Figure Figure 5. Chemical reaction between SEBS-g-MAand and SGF.

3.2. Effects of Two Coupling Agents (PP-g-MA or SEBS-g-MA) on Thermal Behaviors of PP/SGF Composites In comparison to the crystallization temperature (Tc) and melting temperature (Tm) of pure PP matrices and PP/SGF composites, PP/SGF composites that are incorporated with PP-g-MA have a Figure 5. Chemical reaction between SEBS-g-MA and and SGF. Figure5. Chemical reaction reactionbetween betweenSEBS-g-MA andSGF. SGF. slightly greater Tc, but aFigure similar5.TChemical m, as indicated in Figure SEBS-g-MA 6a,b, respectively. The crystallization temperature of PP/SGF composites increases from 116.4 °C to 118.8 °C when 3.2. Effects of Two Coupling Agents (PP-g-MA or SEBS-g-MA) on Thermal 3.2. Effects Effects of of Two TwoCoupling CouplingAgents Agents(PP-g-MA (PP-g-MAor orSEBS-g-MA) SEBS-g-MA)on onThermal Thermal Behaviors Behaviorsofof of 3.2. the composites are incorporated with 8 wt % of PP-g-MA. For PP/SGFBehaviors composites, SGF exists in PP, PP/SGF Composites PP/SGF Composites PP/SGF Composites and is thus able to increase the crystallization temperature of PP. The subsequent combination of In comparison to the crystallization temperature (T ))and and melting temperature (T m) of pure PP In comparison comparison tothe the crystallization temperature (Tcc)cbe and meltingtemperature temperature (T of pure purehas PP PP-g-MA enables SGF to becrystallization effectively distributed in and compatible with PP. As a(T result, SGF In to temperature (T melting PP mm)) of matrices and PP/SGF composites, PP/SGF composites that are incorporated with PP-g-MA have andPP/SGF PP/SGF composites, PP/SGF are incorporated incorporated with PP-g-MA have aaa amatrices greater and surface area in PP matrices, whichcomposites induced thethat nucleation of PP, andwith eventually augments matrices composites, PP/SGF composites that are PP-g-MA have slightly greater T c , but a similar T m , as indicated in Figure 6a,b, respectively. slightly greaterTTcc,,of but similar TTmm,, as as indicated indicated in in Figure Figure 6a,b, 6a,b, respectively. respectively. the crystallization PPaa[37]. slightly greater but similar The The crystallization crystallization temperature temperature of of PP/SGF PP/SGF composites composites increases increases from from 116.4 116.4 °C °C to to 118.8 118.8 °C °C when when the composites are incorporated with 8 wt % of PP-g-MA. For PP/SGF composites, SGF exists in the composites are incorporated with 8 wt % of PP-g-MA. For PP/SGF composites, SGF exists in PP, PP, and and is is thus thus able able to to increase increase the the crystallization crystallization temperature temperature of of PP. PP. The The subsequent subsequent combination combination of of PP-g-MA PP-g-MA enables enables SGF SGF to to be be effectively effectively distributed distributed in in and and be be compatible compatible with with PP. PP. As As aa result, result, SGF SGF has has aa greater greater surface surface area area in in PP PP matrices, matrices, which which induced induced the the nucleation nucleation of of PP, PP, and and eventually eventually augments augments the the crystallization crystallization of of PP PP [37]. [37].

Figure 6. 6. DSC DSC curves curves for for (a) (a) melting melting behavior behavior and and (b) (b) crystallization crystallizationbehavior behaviorof ofPP/SGF PP/SGF composites. composites. Figure

The Tm of PP/SGF composites that are incorporated with PP-g-MA remains at 165.4 °C. The crystallization temperature of PP/SGF composites increases from 116.4 ˝ C to 118.8 ˝ C when Furthermore, the melting temperature does not pertain to the amount of PP-g-MA. Namely, the the composites are incorporated with 8 wt % of PP-g-MA. For PP/SGF composites, SGF exists in conjunction of PP-g-MA is not correlated with the crystal structure and thermal stability of PP. 7(b) Figure Figure 6. 6. DSC DSC curves curves for for (a) (a) melting melting behavior behavior and and (b) crystallization crystallization behavior behavior of of PP/SGF PP/SGF composites. composites. 8285 The The TTmm of of PP/SGF PP/SGF composites composites that that are are incorporated incorporated with with PP-g-MA PP-g-MA remains remains at at 165.4 165.4 °C. °C. Furthermore, the melting temperature does not pertain to the amount of PP-g-MA. Namely, the Furthermore, the melting temperature does not pertain to the amount of PP-g-MA. Namely, the

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PP, and is thus able to increase the crystallization temperature of PP. The subsequent combination of PP-g-MA enables SGF to be effectively distributed in and be compatible with PP. As a result, SGF has a greater surface area in PP matrices, which induced the nucleation of PP, and eventually augments the crystallization of PP [37]. The Tm of PP/SGF composites that are incorporated with PP-g-MA remains at 165.4 ˝ C. Furthermore, the melting temperature does not pertain to the amount of PP-g-MA. Namely, the conjunction of PP-g-MA is not correlated with the crystal structure and thermal stability of PP. Materials 2015, 8, page–page Similarly, in comparison to the Tc and Tm of pure PP matrices and PP/SGF composites, the PP/SGF composites that are incorporated with SEBS-g-MA have a slightly greater Tc and a Similarly, in comparison to the Tc and Tm of pure PP matrices and PP/SGF composites, the similar Tm . ˝ C when PP/SGF composites that are incorporated with from SEBS-g-MA a slightly greater Tc and similar Tm. The Tc of PP/SGF composites increases 116.4 ˝have C to 118.8 they are aincorporated The T c of PP/SGF composites increases from 116.4 °C to 118.8 °C when they are incorporated with with 8 wt % of SEBS-g-MA. For PP/SGF composites, the Tc of PP increases as a result of the 8conjunction wt % of SEBS-g-MA. For PP/SGFthat composites, the Tc of combined PP increases as athe result of the conjunction of SGF. SEBS-g-MA is subsequently with composites serves asofa SGF. SEBS-g-MA that is subsequently combined with the composites serves as a nucleating agent, nucleating agent, which causes nucleation of PP and thus augments the Tc of PP/SGF composites. ˝ ˝ which causes nucleation of PP and thus augments the T c of PP/SGF composites. In addition, the of In addition, the Tm of the PP/SGF composites increases from 166.21 C to 167.51 C whenTmthe the PP/SGF composites increases 166.21 to 167.51 °CAccording when the to composites are incorporated composites are incorporated withfrom 8 wt % of°C SEBS-g-MA. the Tm results, the Tm of ˝ with 8 wt % of SEBS-g-MA. According to the T m results, the Tm of PP/SGF composites remains at 166.4 °C, PP/SGF composites remains at 166.4 C, which indicates that SEBS-g-MA does not correlate with which indicates that SEBS-g-MA not correlate the crystal structure and thermaldoes stability of PP. with the crystal structure and thermal stability of PP.

3.3. Effects Spherulite Morphology Morphology of of Effects of of Two Two Coupling Agents (PP-g-MA or SEBS-g-MA) on Spherulite PP/SGF Composites PP/SGF composites composites as as related related to to various various coupling coupling agents agents are are indicated in The PLM images of PP/SGF Figure 7. Figure Figure 7a 7a shows shows that that when when SGF SGF is is combined combined with with PP, PP, SGF serves as its nucleating agent. The distribution of SGF in PP matrices results in an increasing amount of spherulites, which in turn As aa result, result, the the spherulite spherulite morphology is prevents the spherulites from being formed completely. completely. As incomplete, and spherulites have a smaller size. composites, SGF SGF is evenly When 8 wt % of PP-g-MA or SEBS-g-MA is incorporated incorporated with with SGF/PP SGF/PP composites, distributed in PP matrices, as indicated in Figure 7b,c. A phenomenon that is similar to that observed namely, the spherulites have an incomplete structure and a smaller size with in Figure 7a is found; namely, the identical reason addressed for Figure 7a [36,38]. The optical microscopic images of PP/SGF composites that that have have been been treated treated with with different different PP/SGF composites C, after coupling agents are shown in Figure 8. PP is at a melting state when at a temperature of 200 ˝°C, C, which which the majority of spherulites start to crystallize along the fibers at a temperature of 130 ˝°C, exemplifies that SGF is the nucleating agent for PP. PP. Nevertheless, SGF is evenly distributed in PP as a result of the combination of PP, and thus there are more nucleating points created. The majority of PP’s spherulites are formed surrounding the fibers, and at the same time increases the crystallization PP, as temperature of PP, as indicated indicated in in Figure Figure 6b 6b and and Table Table4.4.

Figure PP/SGF composites that areare composed of Figure 7. 7. Polarized PolarizedLight LightMicroscopy Microscopyimages images(100×) (100ˆ)ofofthe the PP/SGF composites that composed (a) 0 wt % of a coupling agent; (b) 8 wt % of PP-g-MA as a coupling agent, and (c) 8 wt % of SEBS-gof (a) 0 wt % of a coupling agent; (b) 8 wt % of PP-g-MA as a coupling agent, and (c) 8 wt % of MA as a coupling agent. agent. SEBS-g-MA as a coupling

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Materials 2015, 8, page–page

Figure microscopy images (100×) of theofPP/SGF composites that arethat composed of (a) 0 wt Figure8.8.Optical Optical microscopy images (100ˆ) the PP/SGF composites are composed of % of a coupling agent; (b) 8 wt % of PP-g-MA as a coupling agent; and (c) 8 wt % of SEBS-g-MA as of a (a) 0 wt % of a coupling agent; (b) 8 wt % of PP-g-MA as a coupling agent; and (c) 8 wt % coupling agent. SEBS-g-MA as a coupling agent. Table Table 4. 4. Thermal Thermal behaviors behaviors of of PP/SGF PP/SGFcomposites. composites. PP-g-MA ΔHm (J/g) ˝TPP-g-MA m (°C) Tc (°C) Xc (%) ∆H m (J/g) T m ( C) T c (˝ C) X c (%) PP 93.1 166.0 111.4 44.5 PP 93.1 166.0 111.4 44.5 PP/SGF25 61.5 165.2 116.4 39.2 PP/SGF25 61.5 165.2 116.4 39.2 116.8 44.8 2 2 68.4 68.4 165.7165.7 116.8 44.8 4 4 67.1 67.1 164.8164.8 116.9 45.1 116.9 45.1 6 6 75.5 75.5 165.4165.4 118.1 52.1 118.1 52.1 8 65.1 165.1 118.8 46.0 8 65.1 165.1 118.8 46.0

Coupling Agent Coupling Agent (wt %) (wt %)

ΔHm (J/g) ∆H m (J/g) 93.1 93.1 61.5 61.5 66.0 66.0 59.2 59.2 62.6 62.6 59.6 59.6

SEBS-g-MA SEBS-g-MA Tm (°C) Tc (°C) Xc (%) T m (˝ C) T c (˝ C) X c (%) 166.0 111.4 44.5 166.0 111.4 44.5 165.2 116.4 39.2 165.2 116.4 39.2 166.6 117.2 43.243.2 166.6 117.2 166.2 118.2 166.2 118.2 39.839.8 167.1 118.5 167.1 118.5 43.243.2 167.5 118.8 42.2 167.5 118.8 42.2

Note. ∆Hm is the melting enthalpy, Tm is the melting temperature, Tc is the crystallization temperature, and Xc

Note. ΔH m is the melting enthalpy, Tm is the melting temperature, Tc is the crystallization temperature, is the crystallinity. 2015, 8, page–page crystallinity. andMaterials Xc is the

3.4. Effects of Two Coupling Agents (PP-g-MA or SEBS-g-MA) on Spherulite Morphology of PP/SGF Composites Figure 9 illustrates the SEM images of PP/SGF composites that are made with different amounts of coupling agent. The increasing amount of coupling agent, regardless of it being PP-g-MA or SEBS-g-MA, decreases the pull-out of SGF and the interstices between SGF and PP. In addition, the incorporation of a coupling agent also results in a rugged surface of PP/SGF composites. Therefore, PP-g-MA and SEBS-g-MA can effectively improve the interfacial compatibility between PP and SGF. Figure 10 shows the same images as those in Figure 9, but has a greater magnification. Figure 10 indicates that increasing PP-g-MA or SEBS-g-MA can distinctively decrease the interstices between SGF and PP. Meanwhile, the PP remarkably adheres to SGF, and SGF does not exhibit a pull-out phenomenon [30,34,35,37]. These results signify that the interfacial compatibility between PP and SGF has been improved, and the composites thus have greater mechanical properties, as exemplified in Figures 1–3 and Table 2. These results confirm the findings of the study by Tjong et al. [34,35,37].

Figure 9. SEM images (100×) of PP/SGF composites, which are incorporated with (a) 0 wt % of a

Figure 9. SEM images (100ˆ) of PP/SGF composites, which are incorporated with (a) 0 wt % of a coupling agent; (b) 2 wt %; (c) 4 wt %, (d) 6 wt %; and (e) 8 wt % of PP-g-MA; as well as (f) 2 wt %; coupling wt%; %;and (c)(i)4 8wt (d) 6 wt %; and (e) 8 wt % of PP-g-MA; as well as (f) 2 wt %; (g)agent; 4 wt %;(b) (h) 62 wt wt%, % of SEBS-g-MA. (g) 4 wt %; (h) 6 wt %; and (i) 8 wt % of SEBS-g-MA.

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3.4. Effects of Two Coupling Agents (PP-g-MA or SEBS-g-MA) on Spherulite Morphology of PP/SGF Composites Figure 9 illustrates the SEM images of PP/SGF composites that are made with different amounts of coupling agent. The increasing amount of coupling agent, regardless of it being PP-g-MA or SEBS-g-MA, decreases the pull-out of SGF and the interstices between SGF and PP. In addition, the incorporation of a coupling agent also results in a rugged surface of PP/SGF composites. Therefore, PP-g-MA and SEBS-g-MA can effectively improve the interfacial compatibility between PP and SGF. Figure 10 shows the same images as those in Figure 9, but has a greater magnification. Figure 10 indicates that increasing PP-g-MA or SEBS-g-MA can distinctively decrease the interstices between SGF and PP. Meanwhile, the PP remarkably adheres to SGF, and SGF does not exhibit a pull-out phenomenon [30,34,35,37]. These results signify thatwhich the interfacial compatibility Figure 9. SEM images (100×) of PP/SGF composites, are incorporated with (a) between 0 wt % ofPP a and SGF coupling has beenagent; improved, and the composites thus have greater mechanical properties, as exemplified (b) 2 wt %; (c) 4 wt %, (d) 6 wt %; and (e) 8 wt % of PP-g-MA; as well as (f) 2 wt %; in Figures 1–3 These results confirm the findings of the study by Tjong et al. [34,35,37]. (g) 4 wt %; and (h) 6Table wt %; 2. and (i) 8 wt % of SEBS-g-MA.

Figure Figure 10. 10. SEM SEM images images (800×) (800ˆ)of ofPP/SGF PP/SGFcomposites, composites,which whichare areincorporated incorporatedwith with (a) (a) 00 wt wt % % of of aa coupling agent; (b) 2 wt %; (c) 4 wt %; (d) 6 wt %; and (e) 8 wt % of PP-g-MA; as well as (f) 2 wt coupling agent; (b) 2 wt %; (c) 4 wt %; (d) 6 wt %; and (e) 8 wt % of PP-g-MA; as well as (f) 2 wt %; %; (g) 4 wt %; (h) 6 wt %; and (i) 8 wt % of SEBS-g-MA. (g) 4 wt %; (h) 6 wt %; and (i) 8 wt % of SEBS-g-MA.

4. Conclusions

10

This study successfully improves the compatibility between PP and SGF for their composites by using PP-g-MA and SEBS-g-MA, and thereby augments the tensile strength, flexural strength, impact strength, thermal behavior, and compatibility. The test results have proven that SGF is a good reinforcing fiber, and the conjunction of 25 wt % of SGF improves the tensile, flexural, and impact strengths of PP. In addition, the incorporation of 8 wt % of PP-g-MA as a coupling agent provides the composites with 18% greater tensile strength, 24% greater flexural strength, and 33% higher impact strength; however, it does not benefit their tensile modulus or flexural modulus. Moreover, the incorporation of 8 wt % of SEBS-g-MA as a coupling agent provides the composites

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with 23% greater impact strength, but 25% lower tensile strength, 16% lower tensile modulus, 18% lower flexural strength, and 16% lower flexural modulus. The test results also indicate that for PP/SGF composites, SGF serves as the nucleating agent, which increases the crystallization temperature of PP, but decreases the size of the spherulites of PP. Using PP-g-MA or SEBS-g-MA as the coupling agent allows for an even distribution of SGF, which at the same time provides more nucleating points for PP. As a result, the crystallization temperature of PP is increased and the spherulite size of PP is decreased. Hence, using these two coupling agents positively improves the interfacial compatibility, which is exemplified by the facts that the PP matrices enwrap the surface of SGF, and the mechanical properties of the PP/SGF composites are enhanced. The PP/SGF composites proposed by this study can also provide feasibilities for different practical applications by adjusting the amounts of PP-g-MA and SEBS-g-MA in order to render the composites with desired synthetic properties and diverse uses. Acknowledgments: The authors would especially like to thank Ministry of Science and Technology of Taiwan, for financially supporting this research under Contract MOST 103-2221-E-035-027. Author Contributions: In this study, the concepts and designs for the experiment, all required materials, as well as processing and assessment instrument are provided by Jia-Horng Lin and Ching-Wen Lou. Data are analyzed, and experimental results are examined by Chien-Lin Huang, Chi-Fan Liu, and Chih-Kuang Chen. The experiment is conducted and the text is composed by Zheng-Ian Lin. Conflicts of Interest: The authors declare no conflict of interest.

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