hemp fiber composites

Article Study on mechanical properties and thermal stability of polypropylene/hemp fiber composites Journal of Reinforced Plastics and Composites 30...
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Study on mechanical properties and thermal stability of polypropylene/hemp fiber composites

Journal of Reinforced Plastics and Composites 30(1) 36–44 ! The Author(s) 2011 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0731684410383067 jrp.sagepub.com

Pengfei Niu1, Baoying Liu1, Xiaoming Wei1, Xiaojun Wang2 and Jie Yang2,3

Abstract Polypropylene and hemp fiber composites were prepared by melt compounding, followed by injection molding. Maleic anhydride grafted polypropylene (PP–MAH), maleic anhydride grafted styrene–(ethylene-co-butylene)–styrene copolymer (SEBS–MAH) and maleic anhydride grafted Poly(ethylene octane) (POE–MAH) were used as compatilizer to improve the fiber/matrix interactions. The composites were characterized by X-ray diffraction (XRD), dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), mechanical testing and thermogravimetric analysis (TGA). The mechanical properties of polypropylene/hemp fiber composites (fiber loading 30 wt%) at different compatilizer content were analyzed. In comparison to unmodified system, the incorporation of compatilizer (PP–MAH, SEBS–MAH, POE–MAH) can significantly enhance the fiber/matrix adhesion which resulted in higher stiffness and mechanical properties of the composites. The combination of PP–MAH and SEBS–MAH (or POE–MAH) elastomer can be used to optimize the mechanical properties of the composites. The tensile strength, flexural strength, notched and unnotched impact strengths of the composite comprising PP–MAH (5 phr) and POE–MAH (6 phr) were 22%, 8%, 24%, and 82% higher, respectively, than that of unmodified system. Furthermore, the addition of hemp fiber shifted the start of the degradation process and the maximum decomposition temperatures of the components to higher temperatures.

Keywords Hemp fiber, polypropylene/hemp fiber composites, mechanical properties, impact strength, thermal stability

Introduction Natural fibers, such as hemp, flax, jute, and kenef have the advantages of low cost, low weight, reproducibility, ‘eco-friendly’ and bio-degradability compared to glass fiber and carbon fiber.1 Therefore, natural fibers reinforced polymer composites have raised great attention and interests both in academia and industry in recent years. However, the main disadvantages of natural fibers in composites are the poor compatibility between natural fibers and polymer matrix because of the cellulose hydrophilic and polymer hydrophobic nature.2 In order to obtain a useful composite, satisfactory interfacial adhesion between fiber and matrix is necessary. In numerous research studies, the compatibility between composite components was improved either by physical (for example, plasma treatment3) or chemical (alkaline treatment,4 acrylic treatment,5 etc.) modification of the cellulose fibers, or by using coupling agents

(silane coupling agents,6 maleated coupling agents,7 etc.). It should be emphasized that the method of using maleated coupling agents is different from other treatment methods in that maleic anhydride grafted polymer is used to modify not only fiber surface but also the

1

College of Polymer Science and Engineering, Sichuan University, China. Institute of Materials Science and Technology, Sichuan University, China. 3 State Key Laboratory of Polymer Materials Engineering of China, Sichuan University, China. 2

Corresponding author: Jie Yang, Institute of Materials Science and Technology, Sichuan University, Chengdu 610064, China or State Key Laboratory of Polymer Materials Engineering of China, Sichuan University, Chengdu 610065, China Email: [email protected]

Niu et al. polymer matrix to achieve better interfacial bonding and mechanical properties in composites. Accordingly, maleic anhydride grafted polymers are most widely used to strengthen the interfacial adhesion of the composites containing natural fiber reinforcements. Unfortunately, natural fiber reinforced composites exhibit lower impact strengths and smaller elongation at break.8,9 Additionally, investigation related to the fiber structure4 and fiber/matrix adhesion10 did not show any significant improvement in impact strengths of the composites. To improve impact properties of the composites, rubber11 or elastomer12 can be used has been proven. However, it is worth noting that maleic anhydride grafted polypropylene (PP–MAH) compatilized composite has higher yield strength but lower impact strength, while the composites containing rubber or elastomer had higher impact strength but unsatisfactory yield strength compared to unmodified composites. Among natural fibers used in polymer composites, hemp is one of the most productive and useful plants known. It grows quickly in most locations and climates with only moderate water and fertilizer requirements.13 Hemp has many industrial applications. China can be considered as one of the most significant resource country for hemp. However, hemp transformation generates a high proportion of waste. Specifically, textile industry employs less than 70 wt% of hemp, producing significant amounts of waste. Consequently, development of recycling processes of such biomass waste is generating great interest and utilization of it as polymer reinforcement could be an appropriate solution. In addition, hemp fiber (HF) reinforced polypropylene (PP) composite exhibits higher tensile strength, Young’s modulus, and heat distortion temperature. The objective of this work is focused on the mechanical properties of PP/HF composite. PP–MAH was employed to enhance the fiber/matrix interfacial adhesion. In order to overcome the impact strength deficiency, maleic anhydride grafted styrene–(ethyleneco-butylene)–styrene copolymer and maleic anhydride grafted poly(ethylene octane) elastomer (designated as SEBS–MAH and POE–MAH, respectively) were used as impact modifier as well as compatilizer. Furthermore, the combination of PP–MAH and SEBS–MAH (or POE–MAH) was used to improve the yield strength and impact strength of the composite. Finally, thermal stability of the composites was also investigated in this study.

Experimental Materials PP (F401) with a melt flow rate (MFR) of 2.2 g/10 min (230  C/2.16 kg) was supplied by Lanzhou Petrochemical

37 Co. Ltd, China. HFs was obtained from Quartermaster Equipment Institute of the People’s Liberation Army General Logistics Department. PP–MAH (with an MAH content of 1%), SEBS– MAH (Kraton FG1901X, having 1.5% MAH content) and POE–MAH (POE Engage 8999, having 0.8% MAH content) were used in this study.

Preparation of the composites In the preparation of the composites, PP, HFs, and maleated grafted polymer (PP–MAH, SEBS–MAH, and POE–MAH) were first melt-compounding using the twin-screw extruder (CTE-35). The temperatures of barrel and the die were maintained at 190  C during extrusion and the screw speed was 150 rpm. Subsequently, the extrudate was palletized, dried, and injection-molding into standard specimens for mechanical properties testing. The injection-molding temperature and pressure were 200 C and 80 MPa, respectively. Prior to the preparation of the composites, all the components were first dried in an oven at 80 C for at least 24 h.

X-ray diffraction X-ray patterns were obtained with a Philips X’Pert Pro Diffractometer using CuKa radiation ( ¼ 0.1546 nm), voltage of 40 kV and current of 35 mA with 2y increased in steps of 0.03 . The scans were performed in the scattering range of 2y ¼ 5.0–45 .

Dynamic mechanical analysis Dynamic mechanical properties of the composites were studied using a dynamic mechanical analyzer (DMA Q800, TA instrument), analyzing in the three-point bending mode, from 40 C to 50 C under a heating rate of 5 C/min and at a frequency of 1 Hz. The dimensions of each sample were 30 mm  10 mm  4 mm (length  width  thickness).

Mechanical testing Both tensile and flexural tests were performed using a Universal Testing Machine (AG-10TA, Seiko) according to ISO 527 and ISO 178, respectively, at room temperature. The crosshead speed used for type 1A tensile specimens was 5 mm/min. For flexural tests (three point bending), specimen dimensions of 80 mm  10 mm  4 mm (length  width  thickness), a span of 64 mm and a crosshead of 2 mm/min were used. Notched and unnotched Izod impact strength tests for type 1A specimens were carried out using a Chengde Testing Machine (XJU-275) according to ISO 180 at

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Journal of Reinforced Plastics and Composites 30(1)

room temperature. Each value obtained represented the average of five samples.

Morphological study Impact fracture surface morphology of the composites was examined by a JEOL JSM-5900 SEM at an acceleration voltage of 20 kV. All specimens were sputtered with 10 nm layer of gold prior to scanning electron microscopy (SEM) observations.

Thermogravimetric experiments Thermal stability experiments were performed using TA instrument (TGA Q500) at a heading rate of 10 K/min, from 25 C to 600 C under air atmosphere. Samples of about 6 mg were used.

crystals (K) was determined by the ratio between the intensity Ib of the peak for the (300) diffraction plane and the sum of the intensity Ia1, Ia2, and Ia3 of the (110), (040), and (130) diffraction planes as well as Ib of the peak for the (300) diffraction plane. It can be formulated as follows: K¼

I : I1 þ I2 þ I3 þ I

ð1Þ

The calculated K-values were 27% and 19% for PP/ HF (70/30) and PP/HF/PP–MAH (70/30/5), respectively. The lower K-values implied that the addition of PP–MAH significantly decreases the b-form crystal content in the composite. These results are consistent with literature.14

Dynamic mechanical analysis Results and discussion Crystalline structure of PP and PP/HF composites Figure 1 shows the wide angle X-ray diffraction patterns of PP, HFs, and their composites. The peaks at 13.8 , 16.6 , and 18.3 corresponding to the (110), (040), and (130) diffraction planes of the a-form PP crystal were visible both in virgin PP and its composites. The characteristic peak of b-form PP crystal at 15.8 due to (300) crystal plane was also observed in the XRD patterns of PP/HF composites. This phenomenon demonstrated that the incorporation of HFs induced the formation of b-form crystal structure in PP matrix. In addition, with the incorporation of PP–MAH, the two forms of crystals still existed in the composites. The relative amount of the b-form

7000

(2) PP

6000

Relative intensity

(4) PP/HF/PP–MAH (70/30/5) (4)

300

(3) 040 110 130 (2) 002

101 101

10

30

(5) (4)

5000

(3)

4000

(2)

3000 2000 1000 0

(1)

20

Storage modulus (MPa)

(1) HFs

(3) PP/HF (70/30)

300

The temperature-dependent dynamic mechanical characteristics provide an insight into the level of interactions between the polymer matrix and the fiber reinforcement. Figure 2 shows storage modulus of PP and its composites as a function of temperature. The fiber loading was maintained at 30 wt%. The PP–MAH content was 5 phr and the content of SEBS–MAH (or POE–MAH) was 6 phr. The storage modulus of the composites was higher compared to PP throughout the whole temperature range. As expected, the incorporation of POE–MAH, SEBS–MAH or PP–MAH increased the storage modulus of the composites. This may be due to the enhancement of interfacial adhesion between the fiber and polymer matrix. Additionally, the composite containing PP–MAH had the highest storage modulus.

(1) (1) PP (2) PP/HF (70/30) (3) PP/HF/POE–MAH (70/30/6) (4) PP/HF/SEBS–MAH (70/30/6) (4) PP/HF/PP–MAH (70/30/5)

40

–1000 –40 –30 –20 –10 0 10 20 Temperature (°C)

30

40

50

Diffraction angle (2q)

Figure 1. XRD patterns of HF, PP, and PP/HF composites.

Figure 2. Storage modulus vs. temperature of PP and PP/HF composites.

Niu et al.

39

Figure 3(a) and (b) shows the typical SEM micrographs of fracture surfaces of PP/HF (70/30) and PP/HF/PP–MAH (70/30/5) after izod impact testing (notched). Long pulled-out fibers and the corresponding holes were visible in unmodified composites (Figure 3(a)). The surfaces of the pulled fibers were relatively clean. In addition, there were gaps between fibers and matrix. All these observations implied a poor adhesion between fiber and matrix. The incorporation of PP–MAH can significantly enhance the fiber/matrix adhesion as indicated from Figure 3(b).

Mechanical properties of the unmodified and PP-MAH compatilized composites The tensile and flexural strengths of unmodified PP/HF composites at different HFs loadings are shown in Figure 4. The composites exhibited a significant promotion in flexural strength but a slight reduction in tensile strength compared to PP. This may be due to the high mechanical properties of HFs15 but poor

interfacial adhesion between the fibers and matrix (Figure 3(a)). Figure 5 shows the evolution of tensile and flexural strengths with the content of PP–MAH. The HFs loading was maintained at 30 wt% and the PP–MAH content in the composites varied from 0 to 10 phr. It can be seen from Figure 5 that when more than 3 phr PP–MAH was added to the composites, as expected, the tensile and flexural strengths were significantly improved. For example, the tensile and flexural strengths of the composite comprising 5 phr PP–MAH increased by 32% and 24%, respectively, compared to unmodified system. It was associated with the enhanced fiber/matrix interfacial adhesion (Figure 3(b)) which resulted in an improvement of stress transfer from the PP matrix to HFs. However, it is worth noting that the tensile and flexural strengths of the composite did not simply increase with the increase of PP–MAH content but a maximum was observed after adding 5 phr PP–MAH to the composites. Mutje´ et al. has shown the same tendency for PP/HF composites.

Figure 3. SEM micrographs of PP/HF composites: (a) PP/HF (70/30) and (b) PP/HF/PP–MAH (70/30/5).

90 Tensile and flexural strength (MPa)

Tensile and flexural strength (MPa)

70 Tensile strength

60

Flexural strength

50 40 30 20 10 0

Tensile strength

80

Flexural strength

70 60 50 40 30 20 10 0

PP

10 20 Fiber content (wt%)

30

Figure 4. Tensile and flexural strengths vs. fiber content of PP and PP/HF composites.

PP

0

2 4 6 8 PP–MAH content (phr)

10

Figure 5. Tensile and flexural strengths vs. PP–MAH content of PP and PP/HF (70/30) composites.

40

Journal of Reinforced Plastics and Composites 30(1) Figure 6 shows the impact strengths of PP and its composites. In fact, PP did not fracture during the unnotched impact test and in consequence the unnotched impact strength of PP was not present in this article. The HFs loading was maintained at 30 wt% and the PP–MAH content in the composites was 5 phr. Impact strengths of the composites decreased extremely with the incorporation of HFs. Furthermore, PP–MAH induced a further decrease of impact strengths of PP. This is consistent with literature.16,17

20 Notched impact strength

Impact strength (kJ/m2)

18

Unnotched impact strength

16 14 12 10 8 6 4 2 0

PP

PP/HF (70/30)

PP/HF/PP–MAH (70/30/5)

Figure 6. Impact strengths of PP and PP/HF composites.

2500 (1)

(2)

(5)

Load (N)

2000 1500

(3)

(4) 0

5

10

15

1000 (1) PP (2) PP/HF (70/30)

500

(3) PP/HF/POE–MAH (70/30/6) (4) PP/HF/SEBS–MAH (70/30/6)

0

(5) PP/HF/PP–MAH (70/30/5)

0

20

40 60 Displacement (mm)

80

100

Figure 7. Load vs. displacement curves for PP and PP/HF composites.

Impact strength of the elastomer modified composites It has been proved that elastomer (both synthetic and natural) can be used to improve the impact properties of the fiber reinforced composites.12 Figure 7 shows the typical load–displacement curves of PP and its composites. Due to the restrictions of fiber on the polymer matrix motility, the incorporation of HFs resulted in drastic reduction in elongation at break of PP. Additionally, PP–MAH induced a further decrease of elongation at break. In contrary, the presence of SEBS– MAH and POE–MAH elastomer increased the elongation at break by 51% (PP/HF/SEBS–MAH (70/30/6)) and 120% (PP/HF/POE-MAH (70/30/6)), respectively, compared to unmodified composites. The notched and unnotched impact strengths of composites at various contents of POE–MAH or SEBS–MAH are shown in Figure 8(a) and (b), respectively. The HFs loading was maintained at 30 wt% and the elastomer content varied from 0 to 15 phr. In general, the impact strengths of the composites had a significant improvement with the addition of elastomer. In the case of PP/HF/POE–MAH (70/30/6), the

(b) 40

(a) 8 7

POE–MAH

Unnotched impact strength (kJ/m2)

Notched impact strength (kJ/m2)

POE–MAH SEBS–MAH

6 5 4 3 2 1 0

35

SEBS–MAH

30 25 20 15 10 5 0

0

3 6 9 12 Elastomer content (phr)

15

0

3 6 9 12 Elastomer content (phr)

15

Figure 8. Impact strengths vs. elastomer content of PP/HF (70/30) composites: (a) notched impact strength and (b) unnotched impact strength.

Niu et al.

41

addition of POE–MAH induced an increase of both notched and unnotched impact strengths of the composite by 32% and 68%, respectively, compared to unmodified system. Additionally, the impact strengths (both notched and unnotched impact strengths) of the composites comprising POE–MAH was obviously higher than that of SEBS–MAH modified system. The increases in impact strengths and elongation at break of the composites containing elastomer are attributed to the high toughness and high extendability without a permanent deformation of elastomer. SEM examination of fracture surfaces of impact specimens provides valuable information on the toughness of the sample. It is apparent that the surfaces of PP/HF (70/30) and PP/HF/PP–MAH (70/30/5) were relatively flat (Figure 4(a) and (b)) indicating that the samples experienced little plastic deformation and little energy

was dissipated during the impact testing process. In contrast, from Figure 9(a) and (b), rougher surface appearance and numerous micro voids were observed in the micrographs of PP/HF/SEBS–MAH (70/30/6) and PP/HF/POE–MAH (70/30/6). This resulted from the debonding and pulled-out of the elastomer particles from the PP matrix and brought about a large amount of energy dissipation during the impact tests. Consequently, the impact toughness of SEBS–MAH and POE–MAH modified composites was higher than that of composites without elastomer. However, from Figure 7, it is worth noting that PP–MAH compatilized composite had higher yield strengths but had lower impact strengths and elongation at break, while the composites containing SEBS– MAH and POE–MAH elastomer had higher impact strengths and elongation at break but unsatisfactory

Figure 9. SEM micrographs of PP/HF composites: (a) PP/HF/SEBS–MAH (70/30/6) and (b) P/HF/POE–MAH (70/30/6).

(b) 90

(a) 60

POE–MAH

POE–MAH

50

80

SEBS–MAH

SEBS–MAH

Flexural strength (MPa)

Tensile strength (MPa)

70 40 30 20 10

60 50 40 30 20 10

0

0 PP/HF (70/30)

0

3

6

Elastomer content (phr)

9

PP/HF (70/30)

0

3

6

9

Elastomer content (phr)

Figure 10. Tensile and flexural strength vs. elastomer content of PP/HF/PP–MAH (70/30/5) composites: (a) tensile and (b) flexural strengths.

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Journal of Reinforced Plastics and Composites 30(1)

yield strengths compared to unmodified system. Hence, to optimize the mechanical properties of the composites, the combination of PP–MAH and elastomer may be used.

Mechanical properties of the composites modified by the combination of PP-MAH and elastomer The effect of elastomer content on the mechanical properties of PP–MAH compatilized composites is shown in Figures 10 and 11. The fiber loading in the composites was maintained at 30 wt%. The PP–MAH content was 5 phr and the elastomer content varied from 0 to 9 phr. The tensile and flexural strengths of PP–MAH

(a) 8

(b) 40

7

Unnotched impact strength (kJ/m2)

POE–MAH

Notched impact strength (kJ/m2)

compatilized composites reduced slightly with the increasing of elastomer content (Figure 10). Nevertheless, they were still higher than those of unmodified system. The variation in impact strengths of the composites is shown in Figure 11. The addition of elastomer induced a remarkable increase of the impact strength (both notched and unnotched impact strength) of the PP–MAH compatilized composite. As a result, the combination of PP–MAH and elastomer can be used to optimize the mechanical properties of PP/HF composites. Take the composite comprising PP–MAH (5 phr) and POE–MAH (6 phr) for example, the tensile strength, flexural strength, notched and unnotched impact strengths of the

SEBS–MAH

6 5 4 3 2 1 0

POE–MAH SEBS–MAH

30

20

10

0 PP/HF (70/30)

0

3

6

9

PP/HF (70/30)

Elastomer content (phr)

0

3

6

9

Elastomer content (phr)

Figure 11. Impact strengths vs. elastomer content of PP/HF/PP–MAH (70/30/5) composites: (a) notched and (b) unnotched impact strengths.

(a)

(b) (1) HFs

100

(2) PP

(1)

60

(2)

40 (1) HFs

20

(3) PP/HF (70/30)

Deriv. weight (wt%/°C)

Weight (%)

80

(3)

(2)

(3)

(1)

(2) PP (3) PP/HF (70/30)

0 0

100

200 300 400 Temperature (°C)

500

600

0

100

200 300 400 Temperature (°C)

500

600

Figure 12. TGA and DTG curves of HFs, PP, and PP/HF (70/30) composite under air atmosphere: (a)TGA and (b) DTG curves.

Niu et al. composite increased by 22%, 8%, 24%, and 82%, respectively, compared to unmodified system.

Thermal degradation analysis Thermogravimetric curves under air atmosphere of HFs, PP, and PP/HF (70/30) are shown in Figure 12. The fiber filled composite started degradation later than HFs and began to degrade almost simultaneously with PP matrix. In the DTG curves, the maximum degradation rate of the composite was shifted to higher temperature with respect to cellulose (decomposed at about 337 C) and virgin PP (Tdeg ¼ 338 C). The peak degradation rates of PP/HF (70/30) were observed at temperatures 360 and 380 C, respectively. The reason for the higher maximum degradation temperature of the composite compared to PP and HFs is still under research.

Conclusions In this study, the mechanical properties and thermal stability of PP/HF composites were examined. The incorporation of HFs induced the formation of b-form crystal structure in PP matrix. Various compatilizer (PP–MAH, SEBS–MAH, and POE–MAH) were used to improve the fiber/matrix interactions. All composites containing compatilizer showed higher storage modulus and higher interfacial adhesion between fiber and matrix with respect to unmodified composites. The addition of PP–MAH increased the tensile and flexural strengths, whereas the incorporation of SEBS–MAH and POE–MAH elastomer caused a remarkable improvement in impact strength and elongation at break of the composites. The combination of PP–MAH and SEBS–MAH (POE–MAH) elastomer can be used to optimize the mechanical properties of PP/HF composites. The tensile strength, flexural strength, notched and unnotched impact strengths of the composite comprising PP–MAH (5 phr) and POE–MAH (6 phr) increased by 22%, 8%, 24%, and 82%, respectively, compared to unmodified system. In addition, PP/HF composite shows better thermal stability compared with fiber and matrix separately. Acknowledgment The authors acknowledge the Quartermaster Equipment Institute of the People’s Liberation Army General Logistics Department for hemp fiber provision.

Funding This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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