Effect of Matrix Orientation and ted crystalline structure ......Pineapple leaf fiber Polypropylene...

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Pineapple leaf fiber Polypropylene Composite Matrix orientation Orien-ted crystalline structure Fiber alignment

Composites reinforced with aligned short pineapple leaf fiber (PALF) and polypropylene (PP) were successfully prepared and studied. First, PALF -PP composites were prepared by melt mi-xing on a two-roll mill and taken out to form uniaxial ‘prepreg’. The ‘prepreg’ was stacked and compressed at 175 and 195 °C to form uniaxial sheets. Small and wide angle x-ray scattering patterns reveal that the crystalline ori-entation and stacked structure are still preserved in sheets compressed at 175 °C. However those compressed at 195 °C lose all. Composites prepared at 175 °C display greater modulus and strength. This is due to the combined effect of matrix and PALF orientation.

Effekt der Matrixorientierung und des Fasergehalts auf die Eigenschaften von uniaxialen Ananasblattfaser– Polypropy-lenkomposite Ananasblattfaser Polypropylen-Ver-bundwerkstoff Matrixorientierung Orientierte Kristallstruktur Faseraus-richtung

Mit ausgerichteten kurzen Ananasblatt-fasern (PALF) und Polypropylen (PP) ver-stärkte Verbundwerkstoffe wurden er-folgreich hergestellt und untersucht. Zunächst wurden PALF-PP-Verbund-werkstoffe durch Schmelzmischen auf einer Zweiwalzenmühle mit unter-schiedlichen PALF-Gehalten hergestellt. Dann wurde die homogene geschmol-zene Mischung unter leichtem Strecken herausgenommen, um einachsiges „Prepreg“ zu bilden. Das „Prepreg“ wur-de unter Beibehaltung der Faserorien-tierung gestapelt und dann bei 175 und 195 °C zu einachsigen Verbundfolien gepresst. Klein- und Weitwinkel-Rönt-genstreuungsmuster (SAXS und WAXS) zeigen, dass die kristalline Orientierung und die Stapelstruktur bei den 175 °C komprimierten Verbundfolien erhalten bleiben. Verbundwerkstoffe weisen ein höheres Elastizitätsmodul und eine hö-here Festigkeit auf.

Figures and Tables: By a kind approval of the authors.

1. IntroductionFiber reinforced polymer composites are important. They are widely used in vari-ous applications such as aerospace, au-tomotive parts and sport equipment [1-2]. With global warming issues and the demand for environment-friendly prod-ucts [3], as well as reduction in CO2 emis-sion, the current trend of composite de-velopment is focused on using bio-based materials. Many natural fibers such as sisal, flax, jute, coir and pineapple leaf fiber (PALF) have been studied for use as a reinforcing agents [1, 4-5] in polymer matrix composites. Because of the tem-perature sensitivity of a lignocellulosic component, normally semi-crystalline, lower processing temperature matrix polymers like polyethylene and polypro-pylene are used as the matrix for such natural fiber composites [6-7]. The pro-cessing of natural fiber polymer compos-ites can be done using various tech-niques such as compression molding, in-jection molding and extrusion molding [8]. Compression molding has received more attention because of lower energy consumption, preserving the fiber length, also the capability to shape com-plex products. As one important exam-ple, semi-finished sheet products have been used for manufacturing parts in automotive applications [8]. However, for natural fiber reinforced semi-crystal-line polymers, the mechanical properties of the composites not only depend on the strength of natural fiber but also on the conditions of processing. So, to achieve the best properties of such ma-terials, optimization of the manufactur-ing process is necessary [9].

The properties of a semi-crystalline polymer like PP are particularly sensitive to crystallization conditions. Then crys-tallite orientation can have a strong ef-fect on the resulting properties of the fi-nal applications [2]. As with other filled systems, the presence of natural fiber can cause shearing at the interface be-tween the fiber and the molten matrix during processing. This can lead to an

oriented crystalline structure. This phe-nomenon is observed in natural fiber filled PP processed with intense shearing flows [10], injection molding [11], and extrusion [12].

Recent work in this laboratory on pol-ymer matrix reinforcement using PALF prepared by a novel method has shown promising results. This PALF is finer than that prepared by the conventional meth-od [13] and has more surface area. The reinforcement effectiveness of this fiber has been convincingly demonstrated [13-15]. Uniaxial PALF-PP composites have mechanical properties much im-proved over those of the starting materi-al [13]. However, it is not clear whether the mechanical properties of the com-posite are determined solely by the fiber reinforcement or whether there are oth-er factors [16]. Since cellulose fiber is known to induce crystallization in poly-propylene [17-18], the increase in fiber surface area and also their alignment could influence the crystalline structure. In addition, crystallization of PP can easi-ly be induced in the sheared melt [19].

Thus, the objective of this work is to explore the effect of matrix orientation

Effect of Matrix Orientation and Fiber Content on the Properties of uniaxial Pineapple Leaf Fiber – Polypropylene Composites

AuthorsNanthaya Kengkhetkit, Phitsanulok, Taweechai Amornsakchai, Nakhon Pathom Thailand Corresponding Author:Nanthaya KengkhetkitFaculty of Science and Agricultural TechnologyRajamangala University of Technology Lanna,Phitsanulok CampusMuang, Phitsanulok 65000, Thailand E-Mail: [email protected]


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on the mechanical properties of uniaxial PALF-PP composites. Composite ‘pre-pregs’ with aligned PALF were first pre-pared on a two-roll mill by slight melt stretching. Then, the ‘prepregs’ were compressed to form a sheet at two differ-ent compression temperatures. One temperature was just high enough to melt the matrix for sheet consolidation, while the other was high enough to de-stroy any structure previously formed in the ‘prepreg’. Mechanical properties, thermal properties and x-ray diffraction patterns of the materials prepared in this way were investigated.

2. Experimental2.1 MaterialsPolymer Matrix: Isotactic polypropylene (extrusion grade, Moplen400H) was ob-tained from Basell Advanced Polyolefins (Thailand) Co. Ltd. Density and melt flow index of this polymer matrix are 0.9 g/cm3 and 2.4 g/10min, respectively.

PALF Preparation: Fresh pineapple leaves were collected from cultivation areas in Kok Kwai District, Amphor Ban Rai, Uthai Thani Province, Thailand. PALF was separated from pineapple leaves using a disc-milling method. The range of diameter and average diameter of PALF so obtained are 3-68 µm and 18.70 µm, respectively [13].

2.2 Composite preparationMixing: PP matrix and various predeter-mined amounts of PALF were mixed on a Bench-Type two-roll mill (W100T, Dr. Col-lin GmbH, Germany). The temperature of the front and back roll were 185°C and 175°C, respectively. At first, PP was pre-heated and melted on the mill until a uniform band formed for 5 minutes and then PALF was added. The total mixing time was approximately 20 minutes.

‘Prepreg’ preparation: The homoge-nous molten composite mixture was cooled. It was then taken out with a very narrow roll gap using 0.5 mm nip gap and slight melt stretching to induce the fiber orientation as shown in Fig. 1 with drawing speed of about 1 m/min. Prior to final compression molding this sheet was called ‘prepreg’ [14, 20]. Finally it was allowed to solidify under ambient conditions.

Compression molded sheet prepara-tion: Composite sheets were prepared by stacking the ‘prepreg’ retaining the fiber direction and then compression molding to form uniaxial composite sheets of 1 mm thickness. This was done at tem-

perature of 175 °C and 195 °C under a pressure of 500 psi for 5 min, followed by cooling under the pressure of 1,000 psi for another 5 min. Composites are coded as XPALF-PP, where X is the weight per-centage of PALF in the composites.

2.3 Composite CharacterizationPALF orientation: A piece of ‘prepreg’ was extracted with toluene in a Soxhlet ap-paratus overnight to remove the PP ma-trix. Specimens, after the extraction, were observed under an optical micro-scope. The fiber alignment in a cryogenic fracture surface of 30PALF-PP, in the di-rection perpendicular to machine direc-tion, was observed with scanning elec-tron microscope (SEM) (Hitachi; model S-2500, Japan).

Crystalline structure and orientation: Small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) were performed on the beamline BL1.3W:SAXS at the Synchrotron Light Research Insti-tute (Public Organization) (SLRI), Thai-land. The X-ray energy was 9.5 keV. The patterns were recorded with a Mar de-tector and data analyzed with the insti-tute’s self-developed program.

Surface morphologies in the longitu-dinal and transverse directions of the composites were observed with a scan-ning electron microscope (SEM) (Hitachi; model S-2500, Japan) and an atomic force microscope (AFM) (Nanoscope IIIa, model Ns3, Digital Instruments, USA). Composite surfaces in both directions were etched at room temperature for 2 h, with 1% potassium permanganate in a

Fig. 1: Photograph of PALF alignment step on a two-roll mill during the preparation of uni-axial PALF-PP ‘prepreg’. Schematic on the right illustrates the alignment of PALF which is represented with black lines.

Fig. 2: Optical micrograph of 30PALF-PP ‘prepreg’ after washing with hot toluene (a). SEM micrograph of the cryogenic fracture surface of 30PALF-PP composite perpendicular to the machine direction (b).

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mixture of concentrated sulfuric acid, 85% orthophosphoric acid and water with the volume ratio of 10:4:1, respec-tively [21-22].

Thermal properties: Melting and crys-tallization temperatures (Tm and Tc) and heat of fusion (∆H) of the composites were determined with a differential scanning calorimeter (DSC, Q200, TA In-struments). The heating rate was 10°C/min. The degree of crystallinity (Xc) was estimated using the following equation.

𝑋𝑋𝑐𝑐(%) =𝛥𝛥𝐻𝐻𝑓𝑓

𝛥𝛥𝐻𝐻100%×

100(1 −𝑊𝑊𝑓𝑓) (1)

Where ∆Hf is the experimental heat of fusion, ∆H100% is the heat of fusion of 100% crystalline PP taken as 209 J/g [23] and Wf is the weight fraction of PALF in the composite.

Mechanical properties: Tensile testing was carried out on a universal testing machine (Instron 5569, High Wycombe, UK) with a 1 kN load cell. Specimens were cut with a scroll saw from the ‘pre-preg’ and compression molded sheets. Cuts were along the fiber direction gi-ving a strip 10 mm wide. The gauge length was 50 mm and the extension rate was 5 mm/min. Secant moduli at 1% stain and tensile strength at yield were determined. The values averaged from five specimens are reported.

3. Results and discussion

3.1 Proof of PALF alignment in the com-positeFig. 2(a) displays the optical micrograph of the surface of 30PALF-PP ‘prepreg’ af-ter being washed with hot toluene. PALF fibers in the ‘prepreg’ are mostly aligned in the machine direction. Both fine and bundle fibers are seen. Fig. 2(b) is the SEM micrograph of the final composite surface, cryogenic fractured perpendicu-lar to the machine direction, showing uniaxially oriented fibers.

The alignment of PALF was confirmed by determining the mechanical properties of the composites in two perpendicular directions. Fig. 3 displays the tensile mod-uli and strengths, in the perpendicular and parallel directions, of PP and 30PALF-PP composite pressed at 195°C. The compos-ite is stronger and stiffer in the parallel di-rection than in the perpendicular direc-tion. This indicates that the sheet prepara-tion technique used in this work can main-tain the alignment of PALF in the composites. The effect of fiber alignment

Fig. 3: Tensile modulus at 1% strain and tensile strength of the30PALF-PP composites pressed at 195°C, in parallel and perpendicular directions.

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Fig. 4: WAXS and SAXS patterns of composite ‘prepregs’ and final sheets compressed at 175°C and 195°C (the fiber direction is vertical)

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giving anisotropic properties to the com-posites seen here is similar to that ob-served in other systems [14-15, 24].

3.2 Crystalline structure and orientation of the matrixFig. 4 shows the 2D WAXS and SAXS pat-terns of ‘prepreg’ and compression mold-ed sheets of PP and 30PALF-PP compos-ites. PP in all samples displays the α crys-talline structure [19]. PP and PALF-PP prepregs clearly show crystalline orienta-tion and also show two-spot SAXS pat-terns, indicating stacked-lamellae mor-phology [25]. Some crystalline orientation and some stacked structure are still ob-served in sheets compressed at 175°C. However, those compressed at 195°C lose all of the matrix orientation and stacked structure. The orientation of PP crystal-line in this composite is similar to that observed in other fiber-reinforced PP sys-tems [16, 24, 26]. However, its effect on mechanical properties is much stronger (see below).

Azimuthal scans of the (110) reflec-tion of the WAXS pattern are displayed in Fig. 5. For each set of data, i.e. ‘pre-preg’, sheet compressed at 175°C and 195°C, the scans of PP (110) reflection in neat PP and 30PALF-PP are compared as two superimposed lines. The observed characteristic of orientation in the sam-ples is similar to that of PP observed in other work. It has been attributed to what is called a parent and daughter structure [26-27]. The azimuthal inten-sity profile clearly indicates that the ‘prepregs’ have a high degree of PP ori-entation. In addition, the degree of ori-entation of PP in PP ‘prepreg’ is much higher than that in 30PALF-PP ‘prepreg’. However, after compression molding at 175°C, the some relaxation occurred and now the degree of matrix orienta-tion in 30PALF-PP sheet is higher than that in PP. Composite sheets com-pressed at 195°C completely lose this orientation. These observations can be explained as follows. At a high com-pression temperature (195°C), PP chains have high energy and can easily relax during the compression process. This loss of matrix crystal orientation during high temperature processing has been reported earlier [14, 28]. When a lower compression temperature is used, the relaxation of PP chains is possible, but the chain movement is too slow to fin-ish during the compression process.

Additional evidence from the SEM and AFM micrographs of etched 30PALF-

Fig. 5: Azimuthal scans of the (110) reflections of PP in neat PP (O) and in 30PALF-PP (x) for ‘prepregs’ (top black) and sheets compressed at 175°C (middle grey) and 195°C (bottom light grey).

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Fig.6: SEM micrographs of etched surface of 30PALF-PP ‘prepreg’ and compressed sheet at 195°C in parallel and perpendicular directions.

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PP samples are shown in Fig. 6 and Fig. 7, respectively. These reveal the difference between the PP structure in ‘prepreg’ and that in sheet pressed at 195°C. The ‘prepreg’ sample shows different PP ori-

entation in both the parallel and perpen-dicular directions, as has been observed previously in drawn tape [29]. The struc-ture of the other sample displays a spherulitic structure in both directions.

3.3 Thermal propertiesDSC curves (1st melting and cooling) of PP, 10PALF-PP, 30PALF-PP composite sheets pressed at 195°C are displayed in Fig. 8. The melting temperature of the crystal-line phase, crystallization temperature and degree of crystallinity of all samples obtained from DSC are shown in Table 1. The presence of PALF fiber causes no sig-nificant change in the melting tempera-ture of PP in the composites. In all types of composite samples (prepregs, sheets pressed at 175°C and sheets pressed at 195°C) these are around 162-164°C.

When crystallization behavior is con-sidered, the onset temperature of crys-tallization of all PP types is around 115-116°C. With PALF, the onset of crystalli-zation occurs at a higher temperature of around 120-122°C. It does not seem to depend very much on PALF content. However, with a greater amount of PALF, the crystallization is completed faster.

In contrast to the melting and crystal-lization temperatures, the degree of crys-tallinity of the PP is affected by the pres-ence and amount of PALF. The degree of crystallinity of all composites is higher than that of neat PP and increases with increasing PALF content. As with other natural fibers, PALF acts as a nucleating agent [30] and so the PP chains can be oriented during alignment of the PALF [31]. The degree of crystallinity of com-posites is affected also by the compres-sion molding temperature. ‘Prepreg’ showed the highest degree of crystallini-ty, followed by sheets pressed at 175°C and then by sheets pressed at 195°C. In those composites with a high PALF con-tent, the PP chains cannot easily relax within the short compression time. As a result, 30PALF-PP composites, both in the forms of ‘prepreg’ and as compression molded sheets, show only a slightly high-er degree of crystallinity (Table 1).

3.4 Mechanical propertiesTensile moduli and strengths of all types of PALF-PP composites containing differ-ent amounts of PALF are displayed in Fig. 9a and Fig. 9b, respectively. These me-chanical properties are affected by both the PALF content and the preparation method. The latter is known to influence matrix orientation.

With regard to strength, ‘prepreg’ dis-plays the highest value followed by sheets pressed at 175°C and 195°C, re-spectively.

In the case of neat PP, the tensile mod-uli of all types are almost the same, indi-

Fig. 7: AFM micrographs of etched surface of 30PALF-PP ‘prepreg’ and compressed sheet at 195°C in parallel and perpendicular directions.

Fig. 8: DSC melting (upper) and cooling (lower) curves of PP sheet(□), 10PALF-PP sheet (), 30PALF-PP sheet (O) composite sheets pressed at 195°C.

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cating a slight effect of chain orientation. In contrast, the mechanical properties of all the composites are strongly affected by both matrix orientation and fiber con-tent. The ‘prepreg’ composites display the highest tensile modulus and tensile strength due to the combined effect of both. When these ‘prepregs’ are trans-formed into sheets by compression molding, the PP matrix can melt and re-laxation can occur, although the PALF alignment remains the same. At the compression temperature of 175°C, this effect is small and generally matrix ori-entation is maintained. The composite sheets display only a slight increase in modulus and strength over those of their respective ‘prepregs’. However, if the compression temperature is too high, complete relaxation results and the me-chanical properties are determined sole-ly by the PALF orientation and content. For samples with high PALF content, however, crystalline orientation has little effect on either the modulus or the strength.

It is interesting to compare, from two different viewpoints, our results and those reported in the literature. First we shall consider the reinforcing effect of our PALF with that of other natural fibers. For sheets prepared at 195°C, the rein-forcement effect of PALF is much greater than that of banana [32] and bamboo [33] when prepared under similar condi-tions. This is due mainly to the excellent mechanical properties of PALF. Next is the effect of crystal orientation. There are reports of attempts to introduce crys-tal orientation in fiber reinforced PP com-posites through shear induced crystalli-zation. One example is by using normal injection molding of vetiver grass rein-forcement [11]. Another is by using a more sophisticated oscillatory shear in-jection molding technology with glass fiber reinforcement [24]. Only the latter shows that crystalline orientation makes a significant contribution in improving the composite mechanical properties. For vetiver grass reinforcement, crystal-line orientation occurred only at the very thin skin of an injection molded part and was not enough to play any part in im-proving the mechanical properties.

The simple technique to introduce crystalline orientation reported here has also been used to improve the mechani-cal properties of PALF reinforced PP based thermoplastic elastomer [14]. So this technique is very effective in introducing crystalline orientation, as well as fiber

1 The melting temperature of the crystalline phase, crystallization temperature and degree of crystallinity of all samples obtained from DSC curvePP composites 1st heating (10°C/min) Cooling (10°C/min)

Xc (%) Tm onset (°C) Tm (°C) Tc onset (°C) Tc (°C)PP prepreg 47 155.8 164.0 116.6 111.710PALF-PP prepreg 51 155.8 161.8 120.1 115.930PALF-PP prepreg 58 154.9 162.3 121.2 117.3PP sheet175 46 151.4 164.4 116.4 112.0PP sheet195 44 150.7 164.0 115.9 112.010PALF-PP sheet175 52 151.9 162.5 119.6 115.410PALF-PP sheet195 47 150.9 162.4 120.1 116.530PALF-PP sheet175 59 151.6 161.2 122.1 116.330PALF-PP sheet195 57 150.5 163.3 120.1 117.0

Fig. 9: (a) Tensile moduli and (b) strengths of ‘prepregs’ and sheets containing different PALF content compressed at 175 °C and 195 °C.

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orientation, in making thick, fiber rein-forced, composite parts.

4. ConclusionsThe mechanical properties of uniaxial PALF-PP composites are significantly en-hanced by the combined effects of ma-trix orientation and fiber alignment and content. Slight melt stretching during extraction from a two-roll mill provides fiber alignment and also introduces a certain degree of matrix orientation. The presence of PALF slows down the relaxa-tion of PP matrix and allows crystalliza-tion with preferred crystalline orienta-tion to occur and become frozen within the ‘prepreg’. This crystalline orientation can be preserved in a subsequent com-pression molding step by choosing an appropriate melting temperature. How-ever, if the compression molding temper-ature is too high, complete relaxation of the PP matrix occurs and the composite properties are determined only by the PALF reinforcement.

AcknowledgementsThe authors gratefully acknowledge the Synchrotron Light Research Institute (SL-RI) and also would like to thank staffs at Beamline BL2.2 at SLRI for technical help and Center of Excellence for Innovation in Chemistry (PERCH-CIC), Office of the Higher Education Commission, Ministry of Education (OHEC) for partial funding.

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Effect of Matrix Orientation and ted crystalline structure ......Pineapple leaf fiber Polypropylene...

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