Potential use of different kinds of carbon in production of decayed wood plastic composite

2.1 Materials

Decayed Pinus massoniana Lamb. was collected from the Liyang Forest region in south of China. The decayed Pinus massoniana Lamb. was dried in air and crushed into 60–80 mesh. Activated carbon (AC), bamboo charcoal (BC), CNT and graphenes (G) were obtained from Chengdu Organic Chemicals Co. Ltd. which sizes were 50–74 μm, 50–74 μm, 10–30 μm and 0.5–3 μm. CS and polyvinyl chloride (PVC) were obtained from Zhengzhou All Stroke Chemical Products Co., Ltd. which deacetylation degree of CS more than 90%.

2.2 Experiment methods

Different portions of the wood flour, AC, BC, CNT, graphenes, and CS granulates were processed in a twinscrew mixer which temperature was 150 °C and crushed them into particles with grinder after natural cooling which volume was about 0.05 cm³. Then piles the pieces in the mold to produce WPC panels with hot-pressing which size was 200 mm × 120 mm × 7 mm and the surface pressure was 5 MPa. The experimental design is presented in Table 1. The WPC panels were prepared for the following tests.

3.1 Physical and mechanical properties of WPC panels

In nature, decayed Pinus massoniana Lamb. widespread. Decayed Pinus massoniana Lamb. powder seldom used to produce WPC. These mechanical properties should have been changed during mold hot-pressing. The measurement results are listed in Table 2.

When the hot pressing temperature at 170 °C, hot pressing time keep at 12 min and chitosan accounting for 3% of total mass, as the mass of carbon increases, the trend of the physical and mechanical properties of WPC was increase first and then decrease. When the proportion of decayed wood and PVC is 40% and 60% in hot pressing at 170 °C with 12 min keeping-time when different kind of carbon and CS accounting for 2% and 3% of total mass, the physical and mechanical properties of WPC achieve the best condition, and the composite index of physical and mechanical properties of WPC achieve the best condition when CNT and CS accounting for 2% and 3% of total mass, the TS, DL, BS and EM was 17.75 MPa, 473.5 N, 31.57 MPa and 2169 MPa. Composite the Different amounts of decayed wood flour and PVC with carbon and chitosan showed that the —OH in the hemicellulose and cellulose molecule and the C—Cl in the PVC molecule have strong polarities, and there is strong intermolecular force between them, chitosan macromolecules and cellulose macromolecules have similar primary structure, and their remote structure is the same, so the two have good compatibility, under certain conditions can occur cross-linked grafting. Due to the better flowability of the molten PVC, it can penetrate and fill the carbon micropores, form a good mechanical connection with carbon, filled with PVC and wrapped carbon can fill the gap between the decayed wood fibers and PVC interface, which play a role in enhancing the interface. Therefore, carbon and CS can form a mechanical force and intermolecular force between WF and PVC. Therefore, WPC with carbon and CS has the best mechanical properties.

3.2 FT-IR analysis of WPC

FT-IR spectra were used to study the structural groups of the WPC. For comparison, the spectra of the five samples listed above are shown in Fig. 1. For WPC, a O—H stretch, —C—H stretch, unconjugated C ⚌ O stretch, C ⚌ C stretch, C—C stretch (in-ring), C—H stretch, C—C stretch, C—O stretch and C—Cl stretchare visible at 3370, 2900, 1736, 1656, 1500, 1434, 1265, 1122 and 608 cm⁻¹, respectively (Xu et al., 2013; Tomak et al., 2013). All spectra showed similar spectral patterns except for different intensities of infrared absorptions. The most typical bands (1656 and 1500 cm⁻¹) represented aromatic regions of lignin (Wen et al., 2014a,b). The transmitted intensities of all peaks in P8 were greater than those of the others which suggest that the proportion of decayed wood and PVC is 40% and 60% in hot pressing could richen the groups at 170 °C with 12 min keeping-time when CNT and CS accounting for 2% and 3% of total mass. As the change of the kind of carbon, the transmitted intensities of all peaks increased gradually, which suggests that the groups were enriched with carbon. Especially, the transmitted intensities of all peaks of P8 sample are better than P11, P5, P2, P14 which showed that CS molecules contain more —OH, so adding CS increases the amount of —OH in the composite, resulting in an increase in the intensity of the corresponding absorption peak, the addition of carbon to the composite material increases the intensity of the absorption, indicating that the addition of carbon introduces a new ester bond in the molecular structure, possibly establishing a new ester bond between the lignin and the carbohydrate, the intensity of C—Cl bond stretching in the WPC with carbon and CS is significantly stronger than that of carbon or CS alone, which may be the combination of carbon and CS weakening the binding strength of C—Cl bond, so that C and Cl atoms are the binding force between other atoms is enhanced, indicating that the combination of carbon and CS can effectively improve the interface between wood fiber and PVC, and CNT is a tubular structure that is more conducive to the combination of molecules (Shah et al., 2005). According to changes in the groups, the proportion of decayed wood and PVC is 40% and 60% in hot pressing at 170 °C with 12 min keeping-time when CNT and CS accounting for 2% and 3% of total mass could provided optimum conditions for performance.

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Fig. 1 FT-IR spectra of WPC.

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3.3 TGA analysis of WPC

WPC are used widely in many applications. Some of these involve brief exposure to high temperature, and in such applications, thermal stability is an issue. TGA is an essential laboratory tool for material characterization and is used to characterize materials in various environments by measuring mass changes in a controlled atmosphere with temperature variations. In controlled hot N₂, WPC lose mass by oxidation, dehydration, hydration, reduction, and decomposition. The P2, P5, P8, P11 and P14 samples were selected to investigate by TGA between 50 °C and 700 °C. The TGA and differential thermogravimetry curves are shown in Fig. 2.

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Fig. 2 TGA/DTG thermal curves of WPC.

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Thermal degradation of the five samples proceeded over a wide temperature range (100–700 °C). At 4%, 52%, 58% and 67% mass loss, the decomposition temperature was 245 °C for the P2, P5, P8, P11 and P14 samples, 355 °C for the P5, P8, P11 and P14 samples, 463 °C for the P5, P11 and P14 samples, 498 °C for the P5 and P11 samples. At 44%, 51% and 57% mass loss, the decomposition temperature was 394 °C, 444 °C and 466 °C for the P2 samples. At the decomposition temperature was 492 °C, the mass loss of the P2 samples were 65% and 70% for P14 and P8 samples. At the maximum temperature of 700 °C, the mass loss was 64.50%, 75.40%, 75.30%, 76.30%, and 73.50% for the P2, P5, P8, P11 and P14 samples, respectively, which suggests that the thermal degradation of WPC was not very different except P2 sample. At 100 °C, the mass loss was 1.30%, 0.80%, 2.60%, 0.70% and 0.90% for the P2, P5, P8, P11 and P14 samples, respectively, which means that water vapor evaporation from the WPC was decreased by the presence of lignin, the initial decomposition temperature of WPC was improved and the heat resistance was better. The composite properties of carbon composites were better than those of CS composites because carbon itself has a high decomposition temperature.

The DTG curves present mass loss rates, with DTG_max being the maximum thermal degradation rate, which can be used to estimate the degree of thermal degradation. The DTG_max values were found to be 297, 311, 309, 309, and 311 °C for the P2, P5, P8, P11 and P14 samples, respectively. A comparative analysis indicated that the thermal stabilities of the P2, P8, P11, P5 and P14 samples increased gradually which means that carbon obviously increase the WPC thermal stability.

3.4 XRD analysis of WPC

I_am was the diffracted intensity of the peak at 2θ = 18° in the amorphous region, and I₀₀₂ was the intensity of the peak at 2θ = 22° in the crystal region. Cr was the relative crystallinity and can be determined from Cr = (I₀₀₂ − I_am)/I₀₀₂ × 100% (Peng et al., 2013, 2012).

In Fig. 3, The P2, P5, P8, P11 and P14 samples were selected to investigate by XRD, I_am of the P2, P5, P8, P11 and P14 samples were 97, 139, 153, 153, and 181 cps, respectively. I₀₀₂ of the P2, P5, P8, P11 and P14 samples were 139, 208, 208, 236 and 222 cps, respectively. The I_am and I₀₀₂ results show that the crystalline cellulose and amorphous cellulose contents increase. Cr for the P2, P5, P8, P11 and P14 samples were 30.22%, 33.17%, 31.70%, 35.17% and 18.47%, respectively. The analytical results show that I_am and I₀₀₂ of the P2, P5, P8, P11 and P14 samples increased first and then decrease, Cr of them also increased first and then decrease which suggests that cellulose was damaged with an high temperature and carbon increased Cr of them. Cr of the P14 sample was lower than other sample. A reasonable amount adding of carbon favored cellulose changes.

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Fig. 3 XRD curve of WPC.

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