Characterization of microstructures in sisal fiber composites by Voronoi diagram

Materials

Polypropylene Y1600 was purchased from the plastics of Shanghai Petrochemical Complex, China, with a melt flow index of 16 g/10min. Coupling agent A018 used in the study was a maleated polypropylene (MAPP) supplied by Shanghai Zhongzhen Material Technology Co., Ltd., China. The amount of grafting was 1.1% and the melt flow index was above 70 g/10min. Sisal fibers were obtained from Dongfang Sisal Group Co., Ltd., China. γ-aminopropyltriethoxysilane was supplied by Nanjing Shuguang Chemical Group Co., Ltd., China.

Surface treatment of fibers

The sisal fibers were soaked in 1 and 5 wt% concentration aqueous solutions of γ-aminopropyltriethoxysilane (KH550) at atmospheric pressure. Then after 24 h, they were dried in an oven at 103°C for no weight loss.

Composites preparation

Sisal fibers were dried in an oven at 80°C for 8 h prior to compounding. Then polypropylene, MAPP, and sisal fibers were compounded in a twin-screw extruder (model GE2.8.30-41, Gauder Group, Luxembourg) at 190°C and 180 r/min. The fiber weight fraction was controlled by the extruder speed and the matrix adding. The extrudates exiting from the twin-screw extruder were cooled by water immediately and cut into pellets by a pelletizer. Then these pellets were dried at 80°C for over 10 h and molded into specimens at 190°C using a TTI-80 plastic injection machine (Dong Hua Machinery Co., Ltd., China).The mold temperature was set at 60°C.

Tensile properties

The tensile properties were measured on a CMT 4204 universal testing machine (Shenzhen SANS Testing Machine Co., Ltd., China) at a cross-head speed of 2 mm/min. The tensile samples were prepared and tested according to ASTM D638, type I specification. An extensometer with gage length of 50 mm was used for modulus measurement. At least five samples were tested at ambient conditions.

Morphological characterization

The morphology of fibers and composites were performed using a scanning electron microscope (SEM) (model JSM-6360LV, JEOL, Japan). For morphological characterization, the fracture surface of the tensile specimen was milled and polished by using sandpaper. Then the polished surface was sputter-coated with gold and scanned by the SEM. Matrix crystallizations with sisal fibers were observed by polarizing light microscope (PLM) (model XPR-300C, Shanghai CAIKON optical Instrument Co., Ltd., China.) equipped with a hot-stage device, temperature controller and photocamera.

Voronoi diagram formation

The centroid coordinates of fibers and matrix crystals of the above-mentioned morphological images were captured by Image-Pro Plus 6.0 software. And then Voronoi diagrams were formed according to the x- and y-coordinates using the Matlab ‘Voronoi(x,y)’ command.

Voronoi diagram of fibers

The microstructure of fiber’s cross section is shown in Figure 1. As seen in this figure, the cross sections of sisal fibers show a honeycomb structure. This structure is formed by biological evolution of fibers for millions of years, which has extremely excellent resistance to external forces. As seen in Figure 1(c), the tracheids (elongated cells in some plants that serve in the transport of water and mineral salts) show close and irregular arrangement. This is due to the competition for space in the growth of the plant cell. When its growth is hindered by neighboring cells, the tracheid will be squeezed, resulting in an irregular structure. The formation of Voronoi diagram is similar to the growth of plant cells. The structure of space is divided, each particle occupies a certain space, different from each other. Figure 1(d) was the description of Figure 1(c) by Voronoi diagram method, which showed the similar space structure.

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Figure 1.

Microstructure of fiber’s cross section: (a) microstructure of fibers in composite; (b) microstructure of single fiber; (c) magnified microstructure image of single fiber; (d) Voronoi diagram of single fiber’s microstructure.

Voronoi diagram of matrix crystals

Figure 2 shows the nonisothermal crystallization of matrix in the presence of sisal fiber. The matrix shown in Figure 2(a) to (d) is PP/MAPP blend. Prior to crystallization, the samples were held at 200°C for 8 min to erase the previous thermal effect, then they were allowed to cool. Figure 2(a) to (d) shows the crystallizations at the 132nd, 324th, 336th, 366th second cooled from 200°C, respectively. It can be seen in Figure 2(a) that the matrix is not yet crystallized and is still molten, so it shows a transparent state. After 324 s, more crystals grew, as shown in Figure 2(b), and the structures of crystals were spherulite. There were some small grains in this stage. As seen in Figure 2(c), the matrix further crystallized, the crystals became larger and were subsequently squeezed by each other. After 366 s, the matrix crystallization was more complete, as seen in Figure 2(d), the grains grew larger and were squeezed more obviously. The transparency of the matrix was greatly reduced due to the interactions between the grains. In addition to the spherulite, the transcrystallinity near the fiber is observed in Figure 2(d). The white arrow in this figure pointed out to this structure. Many other researchers had also noticed this phenomenon. For example, Mi et al. ²⁰ studied the effects of PP/MAPP matrix on the crystallization and interfacial morphology of bamboo fiber composites. They concluded that the transcrystallinity appeared in PP/MAPP matrix owing to the better interfacial bonding between fibers and resin. Gassan et al. ²¹ also discovered that the transcrystallinity formation was complete in the presence of MAPP, and MAPP could strengthen the fiber surface nucleation. Wolcott et al. ²² monitored the crystallization of PP/MAPP blend in the presence of wood fibers by using dynamic mechanical spectroscopy. They pointed out that the addition of MAPP promoted the formation of transcrystallinity. The presence of transcrystallinity in composites had important effects on the interface properties. Zafeiropoulos et al. ²³ researched the effects of transcrystallinity on the interface properties by using the single fiber fragmentation test and found that the interfacial adhesion was improved by the presence of a transcrystalline layer.

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Figure 2.

Micrograph showing nonisothermal crystallization of matrix with sisal fiber: (a) 132nd second; (b) 324th second; (c) 336th second; (d) 366th second; (e) Voronoi diagram of matrix’s crystallization at the 336th second.

Figure 2(e) shows the Voronoi diagram of the matrix’s crystallization at the 336th second. As can be seen in this figure, the microstructure of matrix’s crystallization can be characterized well by Voronoi diagram. The crystal growth was the result of grains competition and this was also a type of division for space.

Fiber distribution and tensile properties

Microstructures of fiber distributions and corresponding Voronoi diagrams with and without fiber treatments are shown in Figure 3. For comparisons, the SEM micrographs were taken in the same cross-section of composites. As seen in Figure 3(a), the sisal fibers are not treated. Figure 3(c) and (e) shows the fiber treatments by 1 wt% and 5 wt% KH550, respectively. The MAPP was also added in the composites as seen in Figure 3(c) and (e). For the quantitative analysis of the fiber distribution, the corresponding Voronoi diagrams are described in Figure 3(b), (d) and (f). From the Voronoi diagrams, the area of each polygon can be counted automatically by Image-Pro Plus 6.0 software. The standard deviation of polygon areas is closely related to the fiber distribution. The smaller the standard deviation, the better was the fiber distribution. Table 1 shows the parameters of Voronoi diagrams and tensile properties of composites of Figure 3.

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Figure 3.

Microstructures of fiber distributions and corresponding Voronoi diagrams:(a) and (b) without treatment; (c) and (d) 1 wt% KH550; (e) and (f) 5 wt% KH550.

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Table 1.

Parameters of Voronoi diagrams and tensile properties of composites

Process	Area mean (pixel2)	SD (pixel2)	Tensile strength (MPa)	Tensile modulus (MPa)
Without treatment	1236	743	25.81 ± 0.27	3171 ± 184
1 wt% KH550	1237	524	42.72 ± 0.71	3630 ± 73
5 wt% KH550	1234	519	42.59 ± 0.60	3753 ± 95

SD: standard deviation.

As can be seen in Table 1, the standard deviations of polygon areas with fiber treatments and MAPP compatibilizer were smaller than that without fiber treatment, which illustrated the result of uniform fiber distribution. Therefore, improvement of interface adhesion and reduction of fiber polarity were able to effectively prevent the aggregation between the fibers, thereby enhancing fiber dispersion in the matrix. Raj et al. ²⁴ and Baltazar-y-Jimenez et al. ²⁵ had found the same result in their studies. From this table, with the same extrusion compounding, the tensile properties of composites were increased greatly with fiber treatments and the incorporation of MAPP, which mainly attributed to the improvement of interface adhesion and uniform fiber distribution. It can be seen that with the fiber treatments by 1 wt% and 5 wt% KH550, the tensile moduli of composites were closely related to the degree of fiber distribution.