Effect of colorants on interfacial compatibility in wood �our/Poly ( β -hydroxybutyrate valerate) composites

To investigate the effect of three kinds of colorants on interfacial compatibility between wood and polymer in wood �our/Poly ( β -hydroxybutyrate valerate) (PHBV) composites, three colorants are added to color the poplar wood �our/PHBV composites. The effects of the colorant on the interfacial compatibility of the wood plastic composites (WPCs) are studied. The result shows that the mechanical properties of the WPCs tend to increase �rst and then decrease with increasing colorant. The SEM micrographs of the WPCs show the surface, partially the cracks and grooves of the wood �ber are coated by the dispersed colorant. There are basically no new characteristic peaks in FT-IR spectra, which is proved that three colorants can only enable wood �our to be fully mixed with PHBV. Three colorants can improve the compatibility of wood �ber and PHBV, the mechanical properties of the WPCs, which is signi�cantly increased compared to the WPCs without colorants.


Introduction
With the development of modern technology, people have put forward higher requirements for decoration materials, which are not only relatively low material prices, but also excellent performance.As environmentally friendly materials [1][2], Wood plastic composites (WPCs) have been given considerable interest by researchers and industrialists in recent years because of the increasing price of petroleum, ecological concerns, and the impending depletion of fossil fuels [3].Wood our/polymer composite, as the dominant type in the WPCs, is a product which could be made from small wood particles and thermoplastic polymers.Given that this kind of composite combines the advantages of both wood and polymer, this composite has excellent comprehensive properties and gives full play to the strengths of its components.It can not only overcome the usage limitations of wood due to their low strength and high degree of variability, but also overcome the defects of organic materials of low modulus of elasticity [4].
Wood our/polymer composite can improve the physical and mechanical properties as well as the processing properties of materials, decrease costs, expand the application scale, and increase the added values of materials.Thus, with the above-mentioned properties, this composite has been widely used in construction, auto parts, packaging, tra c, storage, decorating materials, daily appliances, and so on.
However, many researches [5][6][7] have found that the properties of wood our/polymer composites greatly depend on the percentages of their constituents and the compatibility between them.The interfacial compatibility between wood our and apolar polymer caused by their great difference in potential energy is always a crucial problem in further improving the properties of wood our/polymer composites.Some investigators have pointed out that adding auxiliary (interfacial compatibilizer), such as coupling agent, is one of the ways to greatly improve the mechanical properties of the WPCs by improving interfacial compatibility [8][9][10].Colorant is one of the important auxiliaries in the molding process of the WPCs, which has a great in uence on the comprehensive properties of the WPCs.In recent years, many experts and scholars have found that the WPCs can be colored with different colorants to improve decorative performance and mechanical properties.Zhang et al [11] studied the in uence of organic or inorganic pigments on wood our/high density polyethylene (HDPE) composites and found that all colored materials have obtained gorgeous colors, and the mechanical properties of the materials have been improved to a certain extent.Chi et al [12] investigated the effect of species and content of color agents on the mechanical properties and surface color of birch our/HDPE composites by adding white, yellow and red colorants.The results showed that the mechanical properties of the birch our/HDPE composites can be improved effectively and their surface color can be changed by lling color agents.After the color agents added into the birch our/HDPE composites, their mechanical strength and total color difference are gradually enhanced with the increase of color agent content.When the content of color agent is 3.5wt%, the properties of birch our/HDPE composites reach the best.Stark et al [13] examined the effect of incorporating different concentrations of an ultraviolet absorber and/or pigment into wood our/HDPE composites.Lightness and exural properties of the composites were determined periodically during exposure to UV radiation and water spray in a xenon-arc type weathering apparatus.The results showed that both ultraviolet absorbers and pigments provide protection against weathering of the composites.
Turku et al [14] studied the in uence of re retardants, namely aluminum trihydrate, zinc borate, melamine, graphite, titanium dioxide on the durability of polypropylene-based co-extruded WPCs.Schirp et al [15] planed to provide the WPCs with colouration and UV-protection.During the compounding process, pigments were added in powder form or pre-dispersed in a polymer matrix (masterbatch).
Bending strength and modulus of elasticity of the WPCs with coloured bres are slightly reduced compared to the WPCs with uncoloured bres.Water uptake is higher for the WPCs with organic pigments than for the WPCs with inorganic pigments only.Xu et al [16]reported that the hygrothermal ageing properties including water absorption, moisture diffusion coe cient, surface color change, and exural properties of the WPCs were prepared with three different colors (original, red, and chocolate) and found that pigment colored the WPCs have better hygrothermal aging properties including lower water absorption, less total color changes, and higher mechanical property retention rate after 70 days hygrothermal aging test.
The pigments used in this study reduce the colour change of the composites exposed to outdoor weathering as compared with the un-coloured composite [17].The carbon black pigment is more effective than the iron oxide pigment.Moreover, only the carbon black pigment is found to reduce the degradation of the surface layer of the composites.The addition of the carbon black pigment has a positive effect on the dimensional stability of the composites in a water absorption test.Only the combination of the carbon black pigment and wollastonite resulted in a composite which is capable to retain its Charpy impact strength both after one year of outdoor weathering and cyclic treatment.To enrich the colors of wood our/polymer composites, expand their applications, and investigate the in uence and mechanism of colorants on overall properties of the composites, nine groups of wood our/poly (lactic acid) (PLA) composites were prepared in Peng's [18] study by using wood our of poplar and biodegradable polymer PLA composites as raw materials and three types of food-grade colorants ( carminum, tartrazine and fruit green) as additives.The results showed that both the type and loading level of colorants have effects on the color, physical and mechanical properties of WF/PLA composites, and the overall properties of the composites can reach the optimum when adding 2% fruit green.
These previous [19][20][21] studies have con rmed the applicability of colorants, as important coloring additives on ornament and aging properties of the WPCs.However, there is still no report on mechanism of improving interfacial compatibility between wood and polymer in the WPCs by adding all kind of colorants.Therefore, the aim of this study is to investigate the effect of three kinds of colorants on interfacial compatibility between wood and polymer in wood our/Poly (β-hydroxybutyrate valerate) composites.

Experimental Part 2.1 Materials
Poplar (Populus tomentasa Carr.) wood our, with a size passed 80 mesh sieve was provided by Lichang Wood Industry Co. LTD, China, oven-dried at 105℃ for 24 h to a moisture content of less than 3%.Poly (3hydroxybutyrate-co-3-hydroxyvalerate) (PHBV, with a density of 0.9 g•cm -3 and a melt point of 176℃) was purchased from Xinxing Plasticizer Co. LTD, China.Maleic anhydride grafted polypropylene (MAPP, with 1.0 MA% grafting rate and a melt ow index (MFI) of 120.0 g/10 min) supplied by by Chuangjinxin Chemical Technology Co. LTD, China was used as coupling agent.Three different colorants including inorganic titanium dioxide (TiO 2 , white colorant, titanium dioxide with inorganic or organic surface treatment, provided by Dongguan Shanyi Plastics Co. LTD, China), red wood thermochromotropic agent (red colorant, prepared by microencapsulation of an electron transfer type organic compound system with a discoloration temperature of 10℃, provided by Dongfang Discoloration Technology Co. LTD, China) and organic permanent yellow (C 34 H 28 Cl 4 N 6 O 4 , yellow colorant, provided by Yuyao Ranfeng Plastic Dyeing Co. LTD, China) were used.

Preparation of sample
The samples were prepared by compression molding method with a dimension of a size of 250 mm× 250 mm× 2 mm and a target density of 1.0 g•cm -3 .For wood content of 60%, PHBV of 40% and MAPP of 2%, wood our/PHBV composites with three colorants were made at ve different colorants loading levels (1, 2, 3, 4, and 5% of the total weight of wood our and PHBV, respectively ).First, wood our, PHBV, MAPP, and colorant were weighed and then mixed in a high-speed mixer with 1800 r/min for 3min.The mixture was oven-dried, distributed on the mold, and then hot-pressed.The hot press (Carver 3895, manufactured by Mecono Technologies Co., Ltd.) was used to press the panels at 180℃ and 7t for 10min.After hot pressing, the formed panel was pressed at 3 MPa for another 3 min at room temperature in a cold press (XLB, manufactured by China Yadong Machinery Group Co., Ltd.).Then the prepared panels were sawn to speci ed size specimens, which were sequentially dried to oven-dry state.

Mechanical property test
The mechanical properties of the WPCs were characterized by testing the bending properties.The specimens of WPCs with different amounts of coupling agents were installed on the universal testing machine (Instron 3369, made by Instron Corporation, USA) for testing.The load was evenly applied during the test, and the loading speed was 1 mm/min.The maximum load value was recorded.Finally, the modulus of rupture (MOR) and bending modulus of elasticity (MOE) were calculated according to the formula.

Water absorption test
The tests were conducted by immersing specimens in a de-ionised water bath at room temperature for different time durations.The water absorption percentage of specimens was calculated by weight difference between the samples immersed in water and the dry samples according the following formula: where W is water absorption (%), m 1 (g) and m 2 (g) are the mass of the specimen before and after immersing, respectively.

Scanning electron microscopy (SEM)
The morphology of the impact fracture surfaces of the samples was studied by means of a JSM-IT300 LV scanning electron microscopy (Electronics company, Japan).The scanning voltage was 10 KV, and the samples was coated with to eliminate the electron charging effects gold before SEM observation [22].

Fourier transforms infrared (FT-IR) microscopy
FT-IR spectra analysis was used to detect whether a chemical reaction among wood our, PHBV, MAPP, and colorant had occurred by ThermoFisher Scienti c iS5 instrument (Thermo, USA).The KBr pellets were obtained by by special pressing equipment during 13 mm in diameter and 1 mm in thickness a mixture of KBr (100 mg) and solid samples (1 mg) [23].FT-IR spectroscopy was obtained over 16 scans of each spectrum in the transmission mode in the range of 4000 cm -1 to 400 cm -1 with a resolution of 1 cm -1 .[24]

Thermogravimetric analysis (TGA)
The thermal stability of the sample was tested in the TGA-Q50 thermogravimetric analyzer with a condition of 20 ml/min in an N2 atmosphere.The heating process was from room temperature to 600 °C, the scanning rate is 10 °C/min, and the thermogravimetric curve (TGA) and differential thermogravimetric curve (DTG) are obtained.

Mechanical property analysis
The modulus of elasticity and the bending strength, as shown in Fig. 2, tend to increase rst and then decrease with increasing colorant.This trend seems very consistent at various colorants.Fig. 2. shows that when no colorant is added, the modulus of elasticity and the bending strength of the WPCs are 4765.26MPa,33.63MPa, and reach the maximum at white and yellow colorants loading of 3% and at red colorants loading of 2%, respectively.The modulus of elasticity and the bending strength of the WPCs increase by 25.67% and 21.48% respectively when the white colorant is added, and increaseby 37.12% and 32.77% respectively when the yellow colorant is added.When the amount of red colorant is 2%, the modulus of elasticity and the bending strength of the WPCs reach the maximum, which increase by 21.30% and 17.87%, respectively.And it is veri ed that the optimal addition amount of white, red and yellow colorant is 3%, 2% and 3% respectively.
The improvement in the mechanical properties of the WPCs is due to the addition of the appropriate amount of colorant to improve the interfacial adhesion of the composite material [25].White colorants (inorganic titanium dioxide) as inorganic colorants, added into the WPCs can ll the gap between the wood ber.Wood our, PHBV, MAPP and colorant are combined closely to enhance the ability of the WPCs to withstand external forces and to reduce the possibility of deformation [26].Yellow colorants (organic permanent yellow) and red colorants (mahogany thermochromic agents) are organic llers that can be added to the WPCs to improve the compatibility of wood ber and PHBV.However, if too much colorant is added, they will remain at the interface of wood ber and PHBV, causing the molecular chain to slip and crack easily.In this case, the mechanical properties of the WPCs material are weakened [13].

Water absorption
In Fig. 3., as the number of experimental days increases, the water absorption rate of the WPCs initially increases rapidly and then tends to atten out.The water absorption process of the WPCs is a dynamic equilibrium process that reaches saturation within 7-10 days.Because wood ber contains a large number of hydrophilic hydroxyl (-OH) groupsand the composite material has a porous structure, it is extremely easy to absorb water under the action of surface tension.The water absorption strength is proportional to the number of hydroxyl groups and the internal porosity of the material.In general, the water absorption rate of the WPCs is determined by the following two important factors:(1) Hygroscopic characteristics of natural bers, (2) the diffusion rate of water penetrating into the composite material through the gaps and defects between the natural bers and the plastic interface [27].It can be seen from Fig. 3. that the WPCs have the lowest water absorption rate, which resultes in a stronger hindrance to water penetrating into the WPCs.It indicates that the compatibility among wood our and PHBV is good in the WPCs when the colorants of white, red, and yellow are added at 3%, 2%, and 3%, respectively.

Scanning electron microscopy
Fig. 4. (a) shows the microstructure of the cross-section of the composites without colorant.Reaction between PHBV and wood ber is complete under the action of MAPP, due to the roughness is the lowest under this condition.In Fig. 4. (b), (e) and (j), the added colorant begins to disperse on the surface of the wood ber, partially lled into the cracks and grooves of the wood ber, therefore, the mechanical properties of the WPCs material have been improved.Fig. 4. (d) is the microstructure of a WPCs material with the addition of white colorants content of 5%.Excess white colorant is adhered to the surface of wood ber, which can easily cause sliding fracture of molecular chains and serious local accumulation, form a accumulation area with weak mechanical properties.When subjected to external forces, it will rst break from the most weak point, resulting in poorer mechanical properties of the WPCs [4].Similarly, this phenomenon is also seen in cross-section scanning electron microscopy images of the WPCs with red and yellow colorants added.Therefore, the right amount of colorant can improve the morphology of the composites, while too much colorant will reduce the overall performance of the composites.

FT-IR analysis
The most important features of the WPCs are around band at 1536cm -1 attributed to C-C stretching vibration peak of lignin in wood ber, the tensile vibration peak at 1747cm -1 associated to C=O between the wood ber molecules, the absorption peak at 2853cm -1 due to the symmetrical and asymmetrical structure of -CH 2 -in PHBV, at 2934cm -1 and 3429cm -1 , respectively, are the tensile vibration peaks of methyl (-CH 3 ) and OH stretch of OH groups provided by wood bers, as shown in Fig. 5.
As shown in Fig. 5. (a), after the addition of white colorant, the tensile and bending peaks of -CH 2 -in titanium dioxide appear at 2930cm -1 , while the characteristic peak of titanium dioxide appears at 892cm - 1 .It proves that titanium dioxide has been attached to composite materials.As shown in Fig. 5. (b), after adding red colorant, the tensile vibration peak and color characteristic peak of the carboxylic acid appear at 1512cm -1 and 1460cm -1 , respectively.Because in the process of discoloration, the ester carbonyl group in the molecular structure of the red colorant is transformed into a carboxylic acid carbonyl group [28], and its sharp peaks are observed at both 2934cm -1 and 2853cm -1 .It indicates that the red colorant has been attached to the composite.
In Fig. 5. (c), the infrared spectra of the composite material with the addition of yellow colorant shows that the peaks of C-N tensile vibration on the permanent yellow molecular chain appear at 1380 cm -1 and 1260 cm -1 .At the same time, it can be found that the modi ed product has C-Cl from the yellow colorant at 977cm -1 and 826cm -1 .It proves that the yellow colorant has been attached to the composite.In summary, there are basically no new characteristic peaks except for the thermochromic reaction of reddening of temperature.It is proved that three colorants can only enable wood our to be fully mixed with PHBV and create a stronger interface among the WPCs because they are not easy to undergo chemical changes with the substance to be dyed.

Thermogravimetric analysis (TGA)
In the TGA curve of the WPCs with different colorants at heating rate of 10 °C/min (Fig. 6), two thermal degradation processes are observed.It can be divided into three stages in total.The rst stage is the decomposition stage of PHBV and the main initial pyrolysis stage of poplar powder at 250 °C-290 °C, the second stage is the main decomposition stage of poplar powder at 290 °C-400 °C, and the third stage is the deep decomposition of residual bers above 400 °C until the stage of reaction.The decomposition curve of the colorant-added composite material moves to the right compared with the original, indicating that the addition of the colorant can improve the thermal stability of the WPCs.As the amount of colorant added increases, the rst stage of the decomposition curve shifts to the right, suggesting that the colorant can improve the thermal stability of the PHBV, possibly due to the fact that the PHBV surface in the WPCs is covered with a colorant with excellent thermal protection, and adding an appropriate amount of colorant can maximize the grafting rate of PHBV and coupling agent, and too few or too many colorant will affect the grafting ratio [29,30].
In the second stage, as the amount of colorant added increases, the decomposition curve also shows a trend of right shift, which is that the colorant can ll the surface gap at the interface between poplar powder and PHBV.When the PHBV is completely decomposed, the colorant with good heat resistance, lled in the voids on the surface of the wood bers will still improve the thermal stability of the poplar powder.
In the DTG curve of the WPCs (Fig. 7), two main weightless peaks are the peak weightlessness of PHBV closed to 280-300 °C, and the peak weightlessness of wood ber around 370-390 °C separately.With the increase of the colorant content, the weightless peak around 280-300 °C tends to move to the right, which indicates that the addition of colorant reduces the decomposition rate of PHBV.The addition of colorant can improve the thermal stability of WPCs.After the addition of the colorant, the weightless peak near 370-390 °C also tends to move to the right, which indicates that the decomposition rate of wood ber is also reduced.The reason could be that the wood ber is adhered by the decomposed crystals, resulting in higher thermal decomposition temperature of wood our and resulting in lower carbonization rate of wood our.

Conclusion
The of three colorants on interfacial behavior of wood our/PHBV composites are investigated by mechanical properties, scanning electron microscope (SEM), fourier transforms infrared (FT-IR) and thermogravimetric (TGA).
In this study, three colorants can be added to the WPCs to improve the compatibility of wood ber and PHBV, the mechanical properties and thermal properties of the WPCs with colorants were signi cantly increased compared to the WPCs without colorants.The part of colorants can be lled in the gap of the WPCs, which can effectively withstand external forces and reduce the deformation or fracture of the material.The white, yellow and red colorants loading of 3%, 2% and 3% respectively, was a critical point

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Figure 2 Water
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Figure 4 FT
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