Article pubs.acs.org/IECR
Preparation of Lignin−Clay Complexes and Its Effects on Properties and Weatherability of Wood Flour/Polypropylene Composites Yao Peng, Wen Wang, and Jinzhen Cao* MOE Key Laboratory of Wooden Material Science and Application, Beijing Forestry University, Qinghua East Road 35, Haidian 100083, Beijing, China ABSTRACT: In this study, two lignin−clay complexes, namely lignin−diatomite (L-DMT) and lignin-montmorillonite (L-MMT), were prepared. Then, 1 wt % of DMT, MMT, L-DMT, and L-MMT were incorporated into wood flour/polypropylene (WF/PP) composites. The changes in their surface color and flexural and impact properties during accelerated ultraviolet (UV) weathering were tested, accompanied by characterizations through UV−visible spectra, scanning electron microscopy, and attenuated total reflection Fourier transfrom infrared spectroscopy. The results showed that (1) lignin deposited on the surface of MMT after treatment, and part of them even penetrated into the micropores of DMT; (2) all additives exhibited good UV absorption ability and protected the composites from photodegradation to different extents; (3) composites containing lignin−clay complexes showed less severe surface deterioration and less reduction in mechanical properties during weathering; and (4) L-MMT exhibited photocatalytic effect at the later stage of weathering, while L-DMT was the most effective additive combination on retarding the photodegradation of composites as used in this study because of its excellent UV-shielding effect. their strength and thermal stability.10 However, its effect on polymers during UV weathering is still controversial. Pagacz et al.11 showed that organically modified MMT (OMMT) deteriorates the weathering resistance of polyvinyl chloride (PVC). However, Oliveira et al.12 reported the antiweathering effect of OMMT in polystyrene (PS) due to its UV-screen effect and barrier effect against diffusion of oxygen during weathering. It was also proposed that MMT could provide some stabilization effect for polymers during UV exposure.11 Diatomite (DMT) is a type of natural amorphous siliceous mineral, which has many unique properties including high permeability, high porosity, and large specific surface area.13 DMT is abundant in North China, especially in Jilin province.14 Its highly porous microstructure makes it an excellent adsorbent for lignin, dyes, and metal ions.15 However, its application as photostabilizer has rarely been studied. Lignin is produced as a byproduct of the paper and pulp industry, and its potential to be used in polymer materials has been studied for about 30 years.16 Lignin has a complex polymer structure containing methoxyl groups, phenolic hydroxyl groups, and some terminal aldehyde groups.17 It can act as a stabilizer against UV degradation or thermo-oxidation because of its phenol groups, which are able to scavenge free radicals.18 However, lignin can be easily degraded into water-soluble
1. INTRODUCTION The consumption of wood-plastic composites (WPCs), made from thermoplastics and woody fibers, has registered a significant increase over the past decades due to the use of recycled materials.1,2 They have been widely used in a variety of applications, such as windows, decking, landscaping timbers, playground equipment, automotive parts, etc.3 However, when used outdoors, both the woody fibers and the polymer matrix experience photodegradation upon exposure to ultraviolet (UV) radiation and moisture.4 The weathering of WPCs finally results in severe degradation of aesthetic appeal as well as their physical and mechanical properties.5 The incorporation of additives into WPCs has been proven to be a cost-effective means to improve their photostability, such as adding UV absorbers (UVAs) and antioxidants.6 Commercial UVAs can be divided into organic and inorganic materials. Organic UVAs are usually toxic and not very stable, so their safety should be taken into consideration when used at high concentrations.7 Although inorganic UVAs, such as TiO2, ZnO, SiO2, and Al2O3, are promoted as nontoxic and thermally stable, their photocatalytic activity as a side-effect should not be neglected.8 Therefore, it is necessary to find environmentfriendly UV-blocking materials with high stability, excellent light-shielding behavior, and of course low cost. Recently, some types of clays, such as montmorillonite (MMT) and diatomite (DMT), as fillers incorporated in polymer material have attracted a great deal of interest. MMT is a typical and widely used natural clay, which belongs to the general family of 2:1 layered silicates.9 It has been proven to be an efficient and promising enhancer for polymers, increasing © XXXX American Chemical Society
Received: July 12, 2016 Revised: August 13, 2016 Accepted: August 26, 2016
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DOI: 10.1021/acs.iecr.6b02660 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
2. MATERIALS AND METHODS 2.1. Materials. Wood flour with size of 60−80 mesh was prepared from poplar (Populus tomentasa Carr.), which has an average length of 0.5 mm and average diameter of 0.15 mm. Polypropylene (PP) powder (trade name K8303) with density of 0.9 g/cm3 was purchased from Beijing Yanshan Petrochemical Co. Ltd., China. Its melting point was around 165 °C, and the melt mass-flow rate was 1.5 g/10 min at 230 °C. Alkali lignin (L) powder, which is extracted from pulping black liquor, was supplied by Shan Feng Chemical Co., Ltd., Changzhou, China. Na-montmorillonite (MMT) was purchased from Nanocor Inc., U.S., with a specific gravity of 2.6 and pH of 9−10 at 5% w/w in water. The mean interlayer distance of Na-MMT is 1.417 nm. Diatomite (DMT) powder was obtained from Xilong Chemical Co., Ltd.; Guangzhou, China. 2.2. Preparation of Lignin−Clay Complexes. According to a previous study, the weight ratio of lignin to clay was maintained at 1:1.20 For the preparation of lignin−montmorillonite complex, 10 g of MMT and 10 g of lignin were suspended in 300 mL of distilled water and then stirred at room temperature for 12 h. After that the mixture was ball-mixed at a speed of 150 rpm for 3 h. After centrifugation, the L-MMT complex was dried in an oven at 65 °C for 24 h to obtain powders. The lignin−diatomite complex was prepared in the same way. 2.3. Composite Manufacturing. The WF/PP composites contained 40 wt % WF and another 60 wt % PP. The addition level of lignin, DMT, MMT, L-DMT, and L-MMT (
