Effects of a layered double hydroxide (LDH) on the photostability of wood flour/polypropylene composites during UV weathering

Abstract

In this study, a kind of layered double hydroxide, MgAl–CO₃-LDH, was synthesised and its effects on the photostability of wood flour/polypropylene (WF/PP) composites during weathering were investigated. WF/PP composites with different LDH loading levels were prepared and tested in a QUV accelerated weathering tester for a total 960 h. The surface color, surface gloss, and flexural properties of the composites were tested, accompanied by characterizations using SEM, ATR-FTIR, and TG. The results indicated that (1) the flexural properties of the LDH-loaded composites were improved; (2) the composites containing LDH showed less discoloration, fewer surface cracks, and less loss of flexural strength and modulus than the control group; (3) LDH played a positive effect on alleviating the photo-oxidation process of WF/PP composites, confirming its UV-shielding effect; (4) LDH enhanced the thermal stability of the composites before and after weathering.

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1. Introduction

In recent decades, wood-plastic composites (WPCs), composed of woody materials and thermosetting/thermoplastic resins, have replaced many conventional materials mainly in outdoor applications, such as railings, decking, fencing, windows frames, and playground equipment, etc.^1,2 However, one of the main drawbacks from the consumer's perspective is that WPCs have poor weatherability.

For outdoors applications, the ultraviolet (UV) light in sunlight is reported to be responsible for the photodegradation of woody materials and organic polymers in WPCs. The UV irradiation can principally induce photo-oxidative degradation on the surface layer of WPCs, resulting in discoloration, chalking, cracking, and embrittlement of the composites.^3,4 Wood mainly consists of three primary polymers, namely, cellulose, hemicelluloses, and lignin. Comparatively, lignin is keener to photodegradation and can be degraded into water-soluble products, which eventually leads to the formation of chromophoric functional groups such as carboxylic acids, quinones, and hydroperoxy radicals.⁵ This process is claimed to be responsible for discoloration of WPCs. Similarity, the photodegradation of polyolefins also involves a series of radical-based oxidative process via chain scisson, which then changes the crystallinity of polyolefins.⁶ The process resulted in the embrittlement and rapid deterioration of mechanical properties of WPCs.⁷

In order to improve the UV resistance of WPCs, different kinds of UV absorbers (UVAs) have been introduced into composites during the manufacturing process.⁸ Generally, UVAs in current use can be divided into organic and inorganic materials. Organic UVAs are usually not very stable and easy to migrate and leach from materials surface. In addition, their safety should be taken into consideration when used at high concentration, posing a threat to human health and the environment.^9,10 Moreover, the mechanical properties of polymers would be influenced by organic UVAs inevitably.¹¹ Comparatively, inorganic UVAs including TiO₂, ZnO, SiO₂, and Al₂O₃, have been widely used due to their optical and thermal stability as well as nontoxicity.¹² Literature have shown that the incorporation of TiO₂ to WPCs resulted in less weather-related damage.^13,14 However, as a side-effect, their photocatalytic activity would accelerate the photodegradation process of organic polymers simultaneously.¹⁵ Therefore, it is necessary to find environmental-friendly UV-shielding materials with high stability, excellent light shielding behavior, and of course low cost.

Layered double hydroxides (LDH) are a well-known type of ionic clays made up of positively charged brucite-like layers with an interlayer containing charge compensating anions and solvation molecules.¹⁶ The general formula of LDH can be represented by [M_1−x²⁺M_x³⁺(OH)₂]^x+A_x/mⁿ⁻·nH₂O, in which M²⁺ and M³⁺ are metallic cations such as Mg²⁺, Cu²⁺, or Zn²⁺ and Al³⁺, Fe³⁺, or Cr³⁺. Aⁿ⁻ is an exchangeable inorganic anion such as CO₃²⁻, SO₄²⁻, Cl⁻, NO₃⁻ or various organic anions; and x is normally between 0.2 and 0.4.^17,18 n is the number of water molecules located in the interlayer region together with the anions.^19,20 LDH are available as both naturally occurring minerals and synthetic materials, which have attracted increasing attention due to their potential applications as catalysts, anionic exchangers, UV adsorbents, fire retardant additives, polymer additives, and so on.^21,22 LDH-based UV-shielding materials have shown significantly enhanced performance compared with organic UV absorbents.^23–25 The multi nestification layered structure of LDH imparts the inorganic layer sheets with the physical shield effect against UV light, while the metal elements of layer sheets and negative ions between layer sheets provide LDH with chemical absorbability of UV light.^17,24 This combined two effects result in excellent UV resistance of LDH in the organic materials.²⁶ Previous studies have reported that LDH can inhibit the photo-oxidation of asphalt during weathering, especially at high loading levels.^21,26,27 Moreover, because of the exchangeability of their interlayer anions, LDH can be modified to impart new functions by intercalation of appropriate guest anions into the galleries between the host layers.²⁸ For example, Wang et al.²⁹ have developed several organic UVAs-intercalated LDH systems, which significantly enhance the stability of polypropylene during UV weathering. Yan and co-workers³⁰ incorporated an organic dye molecule into inorganic LDH host matrices and developed a new type of organic-inorganic hybrid photofunctional materials.

In order to extend the service life of WPCs, in this study, wood flour/polypropylene (WF/PP) composites extruded with and without LDH were exposed in an accelerated weathering tester. The objectives of this study were to compare the effects of different LDH loading levels on color fading, flexural properties, and chemical changes in composites during weathering, as well as provide insights into the UV-shielding mechanism of LDH.

2. Experimental

2.1 Materials

Chemicals used to synthesis MgAl–CO₃-LDH were all analytical grade, including Mg(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, NaOH and Na₂CO₃. Wood flour (WF) of poplar (Populus tomentosa Carr.) with size of 60 to 80 mesh was kindly donated by Xingda Wood Flour Company, Gaocheng, China. The average length and diameter of WF was 0.5 mm and 0.15 mm, respectively. Polypropylene (PP) powder with a trade name K8303 was purchased from Beijing Yanshan Petrochemical Co. Ltd., China. It had a melt point around 165 °C and a melt flow index of 1.5 g per 10 min at 230 °C, with the density of 0.9 g cm⁻³.

2.2 Synthesis of MgAl–CO₃-LDH

The MgAl–CO₃-LDH was synthesised by a traditional coprecipitation method in laboratory. Mg(NO₃)₂·6H₂O (25.64 g, 0.1 mol) and Al(NO₃)₃·9H₂O (18.75 g, 0.05 mol) were dissolved in 200 ml H₂O (distilled water) to form a mixed salt solution. Then the obtained solution was added drop-wise to the anion solution containing 10.6 g Na₂CO₃ in 200 ml H₂O. Meanwhile, the pH value of the precipitation solution was kept constantly at 10 using a NaOH solution (4 M). The resulting mixture was aged at 65 °C for 12 h, followed by centrifugation and washed first with deionized water until pH = 7. The slurry was then further washed with acetone intensively before being dried in an oven at 65 °C for 24 h to obtain the LDH powders.

2.3 Preparation of WF/PP composites

The raw materials for manufacturing WF/PP composites contained 40 wt% of WF and the other 60 wt% of PP. The amount of LDH particles with a size smaller than 100 mesh were maintained at 1 and 2 wt% of the total weight of WF and PP, labeled as LDH1 (1 wt%) and LDH2 (2 wt%). Prior to use, the WF was dried in an oven at 105 °C until the weight was constant. Then, WF, PP, and LDH were weighted and mixed in a high-speed blender at about 2900 rpm for 4 min. The obtained mixture was extruded via a co-rotating twin-screw extruder (KESUN KS-20, Kunshan, China) with a screw diameter of 20 mm and a length-to-diameter ratio of 36/1. The corresponding temperatures in the extruder barrel were 165/170/175/180/175 °C from hopper to die zones and the screw speed was set at 180 rpm. After extrusion, the extrudates were cut into small particles of approximately 1 mm and then dried again at 105 °C for 2 h. A hot press (SYSMEN-II, China Academy of Forestry, Beijing, China) was used to produce the composites by compressing the mat at 180 °C with a pressure of 4 MPa for 6 min. After that, the formed mat was pressed at 4 MPa for another 6 min at room temperature in a cold press. The target density of WF/PP composites was 1.00 g cm⁻³ with size of 270 × 270 × 3 mm³.

2.4 Weathering test

Samples of WF/PP composites with and without LDH were photo-aged in a QUV accelerated weathering tester (QUV/Spray, Q-Lab Co.; USA) equipped with Q-Lab fluorescent UVA-340 lamps under the UV irradiation intensity of 0.89 W m⁻² at 340 nm wavelength. Each weathering cycle consisted of 8 h UV radiation at 60 °C and 4 h condensation at 50 °C according to ASTM G 154. The above process was repeated 80 times, giving a total of 960 h of accumulated exposure for each group. Weathered samples were taken out for analysis after 240, 480, 720, and 960 h of exposure.

2.5 Characterization of LDH and WF/PP composites

X-ray diffraction (XRD) were carried out on an X-ray 6000 (Shimadzu, Japan) instrument in reflection mode with Cu Kα (λ = 0.1540 nm) radiation. The accelerating voltage was operated at 40 kV with 30 mA current at the scanning rate of 5° per min, ranging from 5° to 70°.

The average particle size of LDH powders was tested by laser scattering particle distribution analyser (Mastersizer 2000, UK).

The scanning electron microscope (SEM) micrographs were recorded by a Philips (Hitachi S-3400, Japan) SEM analyzer with an acceleration voltage of 15 kV. LDH powders and weathered surfaces of composites were stuck on carbon tape adhered to the stage. Before observation, the samples were sputter coated with a thin layer of gold.

Fourier transform infrared spectroscopy (FTIR) spectra were performed on a FTIR spectrometer (BRUKER, Vertex 70v, Germany) equipped with a ZnSe accessory in attenuated total reflectance (ATR) model in the range of 600–4500 cm⁻¹. The peaks at 3400 and 1650 cm⁻¹ were assigned to hydroxyl (O–H) and conjugated carbonyl group of lignin in WF. For the quantitative analysis, the hydroxyl index and lignin index were defined to show the photo-oxidation degree of the composites, according to the following equations:

Hydroxyl index = 100 × I₃₄₀₀/I₂₉₂₀ (1)

Lignin index = 100 × I₁₆₅₀/I₂₉₂₀ (2)

where, I denotes the peak intensity. The peak at 2920 cm⁻¹, which corresponds to alkane C–H stretching vibration of methylene groups (CH₂), was selected as an internal reference, because it changed the least during weathering.

The thermal properties of composites were determined by thermogravimetric analysis (TGA) (Netzsch DSC-204, Germany). Data were collected upon heating from room temperature to 600 °C at a constant heating rate of 10 °C min⁻¹ under a constant argon flow of 60 mL min⁻¹. Samples with a mass about 6 mg were placed in aluminium pans. For composites, the weathered surfaces (≈50 mm thick) were removed from the samples by a microtome knife before analysis.

2.6 Physical and mechanical properties of WF/PP composites

The surface color of WF/PP composites were measured with a chroma meter (Datacolour DF 110, Germany) according to the CIE L*a*b* color system. The values of L*, a*, and b* can be used to formulate a value for total change as ΔE by the equation:where, ΔL*, Δa*, and Δb* are the differences between values of weathered and unweathered areas of L*, a* and b*, respectively. A low ΔE value corresponds to a low color difference. The surface color for three independent specimens was measured at four different positions on each.

Surface gloss of the samples was measured by a glossmeter (KGZ-1B, Tianjin, China). The test angle was 60° according to ASTM standard D 2457-03 and the measured results were expressed in gloss units (GU). Gloss was measured at five different locations on each sample.

The flexural tests were carried out according to the Chinese standard GB/T 9341-2000. The flexural modulus (MOE) and flexural strength (MOR) of samples were obtained by a three-point bending test at a crosshead speed of 5 mm min⁻¹. The size of the samples was 270 × 270 × 3 mm³ and the weathered surface was placed downward. Four samples of each group were tested. To clearly show the influence of weathering, the MOR and MOE retention ratios were defined as follows.

MOR_{ret ratio} = 100 × (MOR_t/MOR₀) (4)

MOE_{ret ratio} = 100 × (MOE_t/MOE₀) (5)

where, MOR_t and MOE_t are the moduli after weathering time t, MOR₀ and MOE₀ are the moduli before exposure.

3. Results and discussion

3.1 Characterization of MgAl–CO₃-LDH

As shown in Fig. 1(a), the XRD pattern of MgAl–CO₃-LDH exhibited the characteristic pattern of layered materials. The sharp, symmetric peaks indexed as (003), (006), and (009) were observed at low angles, which corresponded to the basal reflection and higher harmonics. In addition, some weaker, asymmetrical peaks appeared at high angles, including the (015), (018), (110), and (113) reflections. The strong intensity of reflections suggested that the synthesized LDH had a well-developed layer structure with good crystallinity.

Fig. 1 Characterization of MgAl–CO₃-LDH by (a) XRD, (b) SEM, (c) particle size distribution, (d) FTIR, and (e) TG.

Fig. 1(b) represents the morphology of MgAl–CO₃-LDH obtained by SEM. It was expected that LDH had formed plate-like particles with polygonal shape, which was typical morphology of LDH as observed by other researchers.^16,31 The surface area mean diameter and the volume mean diameter of MgAl–CO₃-LDH were 14.683 and 22.048 μm, respectively, which indicated the fairly large particle size of the obtained powders. Moreover, it could be clearly seen that LDH platelets overlapped with each other and formed big particles, resulting from the significant aggregation of small LDH particles.²⁰

FTIR spectra of MgAl–CO₃-LDH is shown in Fig. 1(c). The strong broad absorption band around 3425 cm⁻¹ was assigned to the stretching vibration of the hydroxyl groups of LDH layers and interlayer water molecules.²² The band observed near 1627 cm⁻¹ was due to the bending vibration of interlayer water molecules. The three bands at 1364 864, and 670 cm⁻¹ were signals of the stretching mode of CO₃²⁻ groups in the galleries. In the lower wavenumber range, the bands at 936, 790, and 554 cm⁻¹ could be ascribed to O–M–O vibrations in the brucite-like layers of LDH.¹⁴

The synthesized MgAl–CO₃-LDH decomposed at three stages as shown in Fig. 1(d), with the first weight loss up to 130 °C, which was attributed to water loss rising from physically adsorbed water and crystal water.³² With increasing temperature, a weight-loss stage in the range of 130–240 °C was due to the dehydroxylation. The third weight loss between 240 and 460 °C resulted from the decomposition of carbonate ions of the LDH. Comparatively, the residue of LDH (53.06%) was much higher than that of WF (20.79%) and PP (7.03%), indicating the better thermal stability of synthesised MgAl–CO₃-LDH.

3.2 Color change analysis

The color stability of WF/PP composites was evaluated by two parameters, namely, ΔL* and ΔE. Two distinct stages of ΔL* change could be easily observed from Fig. 2(a). The values of all composites increased dramatically during the first 480 h weathering, followed by a stable increase till the end. The lightening of composites was mainly due to the photodegradation of lignin, which degraded into water-soluble products and leached from the weathered surface, leaving cellulose as the main component on the surface.^33,34 Cellulose is whitish and more stable against UV light than lignin. Consequently, the surface of weathered composites acquired a gray appearance, leading to an increase in ΔL* values. It was noteworthy that the ΔL* values of composites containing LDH were much lower than that of control group, especially after 240 h exposure, implying that LDH played a positive role in protecting WPCs from lightening.

Fig. 2 Change in the (a) lightness (ΔL*) and (b) color (ΔE) of WF/PP composites as a function of weathering time.

As expected, the changes in ΔE values exhibited a similar trend to that of ΔL*, which meant that surface whitening was mainly responsible for total color change. Both the values in LDH-loaded composites were always lower than that of the control group throughout the weathering process, demonstrating the UV resistance of synthesized MgAl–CO₃-LDH. Moreover, the color stability of composites was enhanced with the increasing loading level of LDH. Shu et al.³⁵ reported that LDH showed partial blocking in the UV region, resulting from the reflection and scattering of UV radiation by nanoscaled laminates of LDH sheets. In addition, the strong electrostatic interaction and hydrogen bond network at the organic–inorganic interface between LDH and PP molecules also contributed to the good UV-shielding properties of the composites.^26,35

3.3 Surface properties analysis

The surface morphology of WF/PP composites during weathering was observed using SEM (Fig. 3). Before exposure, all composites surfaces were smooth, and the PP flow over WF was evident. After weathering for 240 h, some micro-cracks were observed on the surfaces of samples labeled as C and LDH1, which might be a result of PP chain scission.³⁶ It was interesting that no obvious cracks appeared on surface of LDH2 group, suggesting the anti-aging effect of MgAl–CO₃-LDH. With extended exposure time (480 h), numerous cracks appeared and wood particles protruded from the matrix, which was likely a result of the WF swelling and shrinking after absorbing and desorbing moisture.³ The cyclic process also resulted in pits at the WF/PP interface. As expected, surface deterioration of LDH-loaded composites was less severe than control group. After 960 h weathering, all composites were degraded significantly to the similar degree, accompanied by exfoliation of the upper layer PP.

Fig. 3 SEM images of WF/PP composites after different weathering times.

Surface gloss was reported to be highly related with the surface smoothness. This meant that a high value of surface gloss reflected a smoother surface. As shown in Fig. 4, unweathered composites performed high surface gloss values (around 18 GU), which then experienced a sharp decline only after 240 h weathering. Moreover, composites containing MgAl–CO₃-LDH still displayed higher gloss values compared to control group, especially at a high loading level. However, the efficiency of LDH seemed to be short and temporary for the gloss values of all composites decreased to the same extent after 720 h weathering. This phenomenon could be attributed to the migration of LDH through the cracks and then leached from the exposed surface during condensation process.

Fig. 4 Surface gloss of WF/PP composites after different weathering times.

3.4 Flexural properties analysis

The flexural modulus of rupture (MOR) and modulus of elasticity (MOE) of WF/PP composites before weathering are illustrated in Fig. 5. The loading level of MgAl–CO₃-LDH had an obvious influence on the flexural properties of the composites. Composites containing 1 wt% LDH displayed the highest MOR and MOE values among all groups, which may be attributed to the exfoliation of the rigid LDH nanolayers in PP matrix and strong interfacial interactions between hydroxyl groups of MgAl–CO₃-LDH and polar group of WF through hydrogen bond.³⁷ Compared to control group, the MOR and MOE values of LDH1 were improved by about 23.3% and 17.1%, respectively. This might be resulted from the enhanced interface bonding between WF and PP. It can be seen from Fig. 6(a) that the fracture surface of the control composites displayed holes, fibre pull-out, and matrix cracking, implying the poor WF/PP adhesion. In this situation, stress could not be transferred effectively in the composite matrix and in turn led to lower flexural properties. However, these phenomena were less pronounced in composites containing LDH (Fig. 6(b)). This was because that the majority of the dispersed LDH powders filled the gaps at the interface between WF and PP, which benefited the interfacial bonding properties. The enhanced flexural properties were also probably correlated with the efficient stress transfer of the metal Mg and Al in the LDH layers.³⁸ However, very few improvement was observed when more LDH were incorporated into the matrix (2 wt%), which was probably due to the aggregation of MgAl–CO₃-LDH particles. This was in accordance with the above SEM results (Fig. 1(b)).³⁹

Fig. 5 Flexural MOR and MOE of WF/PP composites before weathering.

Fig. 6 SEM images of composites fracture surfaces. (a) The control group and (b) LDH1 group.

The effects of accelerated UV weathering on flexural MOR and MOE of composites are shown in Fig. 7. After 240 h weathering, composites containing LDH performed a slight decrease in MOR_{ret ratio} compared to the control group, indicating that the addition of MgAl–CO₃-LDH could slow down the photodegradation process. The decreasing strength of composites was mainly attributed to the photo-oxidation of PP through chain scission. With increasing weathering time, the cracked surface layer became fragile and unable to transmit stress into the interior of the composites, leading to the constant decrease from 240 to 720 h weathering. It was obvious that LDH-loaded groups experienced a significant decrease in MOR_{ret ratio} after 720 h exposure, which could be attributed to the migration and leaching of MgAl–CO₃-LDH, leaving pits or holes at the interface between WF and PP, especially at high loading levels. This process posed a negative effect on composites flexural properties. Under UV irradiation, PP chain scissions led to the formation of short chains, which then rearranged into a crystalline phase by recrystallization, benefiting the flexural strength from 720 h to the end of weathering.⁴⁰

Fig. 7 Change in (a) MOR retention ratio and (b) MOE retention ratio of WF/PP composites as a function of weathering time.

As shown in Fig. 7(b), the MOE_{ret ratio} of all composites displayed the similar trend to that of MOR_{ret ratio}, but more pronounced. After 960 h weathering, the MOR_{ret ratio} of the composites were rated as LDH1 (96.26%) > control (90.88%) > LDH2 (86.69%). Obviously, the composites labeled as LDH1 maintained the greatest strength during the whole weathering process. It could be concluded here that the addition of 1 wt% MgAl–CO₃-LDH benefited the flexural properties of WF/PP composites the most before and after weathering.

3.5 Surface chemical change analysis

ATR-FTIR spectra of control WF/PP composites after different weathering times are illustrated in Fig. 8(a). All of the PP matrix and the components of WF including lignin, cellulose, and hemicellulose underwent degradation during UV exposure.^40,41 The intensity of PP associated absorption band at 1452 cm⁻¹ (CH₃ asymmetric deformation) decreased after weathering, indicating the chalking and flake-off of degraded PP. It was obvious that the intensity of the peaks assigned for cellulose at 3400 cm⁻¹ (O–H stretch), 1376 cm⁻¹ (C–H deformation), and 1317 cm⁻¹ (CH₂ wagging in crystallized I cellulose) all increased successively during weathering, suggesting the appearance of cellulose on exposed surfaces due to the exfoliation of PP layer.⁴² In addition, UV irradiation resulted in the increase of the peaks assigned for lignin and its degraded products at 1710 cm⁻¹ (C=O stretching of acetyl or carboxylic acid) and 1650 cm⁻¹ (absorbed O–H and conjugated C–O). The photodegradation of lignin led to the formation of carbonyl based chromophoric groups as well as o- and p-quinonoid structures.⁴³

Fig. 8 ATR-FTIR spectra of (a) the control WF/PP composites after different weathering times, and all composites after (b) 480 h weathering, and (c) 960 h weathering.

ATR-FTIR curves of all composites after 480 h weathering are shown in Fig. 8(b). For composites containing MgAl–CO₃-LDH, the absorption intensities of the peaks mentioned above increased more slightly than those in the control group, especially group labeled as LDH1. However, after 960 h weathering (Fig. 8(c)), LDH-loaded composites exhibited the similar spectra with the control group, suggesting that longer exposure weakened the effects of LDH. By comparison, it could be suggested that MgAl–CO₃-LDH alleviated the photodegradation of WPCs at early stage of weathering. This can be seen more clearly in Fig. 9, where the intensity ratios hydroxyl index (HI) and lignin index (LI) were plotted against weathering time.

Fig. 9 Changes in (a) hydroxyl index and (b) lignin index as a function of weathering time.

Before exposure, the HI of LDH-loaded composites were relatively higher. This phenomenon was expected due to the hydroxyl groups existed in the LDH layers and the interlayer water molecules.²² After 480 h of exposure, the HI calculated for control and LDH1 groups showed the same value, suggesting that the formation of hydroxyl groups was slowed down due to the UV-shielding effects of MgAl–CO₃-LDH. After that, the HI of LDH1 group increased with increasing weathering time, resulting from the migration of LDH to the exposed surface. Likewise, the LI values of the composites were also found to increase with exposure time. The difference was that LDH1 group always showed the lowest LI within the ranges of this experiment. However, LDH2 group showed the highest HI and LI values during weathering, which might be attributed to the extensive migration of MgAl–CO₃-LDH, as well as the higher loading level. The above results clearly indicated that the addition of MgAl–CO₃-LDH (at 1 wt%) markedly led to an enhancement of the UV stability of WF/PP composites, and therefore, the aging of composites was inhibited to a certain extent.

3.6 Thermal stability analysis

Since MgAl–CO₃-LDH had better thermal stability than WF and PP (Fig. 1(d)), the influence of LDH on the thermal stability of WF/PP composites before and after weathering was further investigated by TGA. Before exposure, the 10% weight loss temperature (T_0.1) data and 50% weight loss temperature (T_0.5) data were marked in Fig. 10(a). Surprisingly, with only 1 wt% MgAl–CO₃-LDH, the T_0.1 and T_0.5 of composites were increased by 19.2 and 21.5 °C, respectively. It was interesting that with a further increase in LDH loading (2 wt%), only a slight increase in T_0.1 and T_0.5 can be observed. It was obvious that the thermal stability of LDH-loaded composites was significantly improved comparing to control group. Previous studies reported the similar results.^16,20 Based on TGA data, the LDH loading level should be not be higher than 1 wt% in order to obtain the optimum thermal stability.

Fig. 10 TG curves of all composites (a) before weathering and (b) after 960 h weathering.

TGA curves of all composites after 960 h weathering are plotted in Fig. 10(b). The T_0.1 data were found to shift to lower temperatures, which decreased by 3.6, 19.4, and 14.6 °C for control, LDH1, and LDH2 groups, respectively. As expected, the similar trend was observed for T_0.5. Lignin was reported to contribute to char formation during heating process and such a charred layer could prevent further thermal degradation.⁴⁴ Therefore, the decreased T_0.1 and T_0.5 data could be attributed to the degradation of lignin and the leaching of MgAl–CO₃-LDH. However, the weathered LDH-loaded composites still had higher T_0.1 and T_0.5 values than the control group, indicating the less sever degradation of lignin in composites containing synthesized MgAl–CO₃-LDH. Based on the above results, such kind of composites can also be used as flame-retarding materials due to their better thermal stability.

4. Conclusions

The MgAl–CO₃-LDH was successfully synthesised and introduced into WF/PP composites at two different loading levels. The practical application of UV-shielding performance of MgAl–CO₃-LDH was investigated in composites during accelerated UV weathering. Generally, LDH particles imparted composites with better thermal stability, less color change, and less surface deterioration after weathering. ATR-FTIR results demonstrated that LDH alleviated the photodegradation of WF and PP in composites. In addition, the flexural properties of composites containing LDH were improved before and after weathering. In summary, composites containing 1% wt LDH exhibited better mechanical and thermal properties, while the group with 2% wt LDH performed better color stability and surface morphology after weathering. Therefore, the loading level of MgAl–CO₃-LDH should be considered for practical applications of WPCs. In order to extend the anti-weathering efficacy of LDH, further studies should be focused on improving the bonding properties between LDH particles and WPCs, as well as diminishing their migration and leaching during outdoor applications.

Acknowledgements

This study is financially supported by the Fundamental Research Funds for the Central Universities in China (TD2011-14, BLYJ201510).

Notes and references

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