Effect of inorganic pigments on the hygrothermal properties of wood-plastic composites

10 July 2015
Changyan Xu
The addition of iron oxide pigments to wood-plastic composites results in an increase in the flexural strength and moduli they exhibit after hygrothermal aging tests.

Due to the introduction of wood fibers, wood-plastic composites (WPCs) are hygroscopic in nature (i.e., they attract and absorb water). This absorption of moisture, which occurs in wet environments and/or under high humidity, leads to dimensional instability, microcracks, degradation, mass loss, susceptibility to bio-deterioration, and significant reduction in the mechanical stiffness and strength of WPCs.1

To enhance the market appeal of plastic products, the addition of pigments is a common practice.2 In general, inorganic iron oxide pigments are characterized by their excellent durability, high inherent opacity, good UV-screening properties, low toxicity, and low cost. However, few studies have been conducted on the incorporation of inorganic iron oxide pigments into WPCs.3–5

The hygrothermal aging test is the most robust and reliable method for quickly evaluating the long-term service performance of moisture-affected fiber-reinforced polymer composites. The information obtained from this test can be used to predict the outdoor performance of WPCs that are exposed to rain and environmental moisture.

To test the effect of pigments on the hygrothermal aging properties of WPCs, we fabricated a control sample and two colored samples made of recycled high-density polyethylene (HDPE) highly filled with wood waste, calcium carbonate, and inorganic fillers at a ratio of 27/73.6 Characteristics of these fillers and the WPC formulations that we employed can be seen in Tables 1 and 2. The WPC lumbers were fabricated in a commercial production line via compounding, followed by an extrusion process. These specimens were then immersed in distilled water at 45°C for 70 days. We subsequently investigated the hygrothermal aging properties of the samples.

Characteristics of the inorganic fillers.

Inorganic materialsMolecular formulaMolar mass g/molDensity g/cm3Particle size nmParticle shapeSuppliers
Iron Red 190Fe2O3159.695.24700SpheroidalYiPin Pigments
Iron Black 722Fe3O4232.65.18350AcicularYiPin Pigments
Calcium carbonateCaCO3100.092.833000IrregularHainan Kunlun

Formulation of the WPCs (wt%). HDPE: High-density polyethylene. MAPE CMG9804: Maleic-anhydride-grafted polyethylene CMG9804. PE-Wax BN-208: Polyethylene wax BN-208.

WPCsWoodHDPECaCO3MAPE CMG9804PE-Wax BN-208Iron Red 190Iron Yellow 313Iron Black 722

We found the pigment-colored WPCs to better withstand the hygrothermal aging test. Compared to the control sample, the properties of these WPCs include lower water absorption, less total color change, and a higher mechanical-property-retention rate. Among our samples, the combination of iron oxide red, yellow, and black pigment exhibited the best aging performance in terms of dimensional stability, color stability, and the retention rate of mechanical properties.

The water-absorption curves of different samples are illustrated in Figure 1. Generally, the water absorption of the specimen increases with immersion time and reaches a maximum (equilibrium) value at a saturation point of approximately 20 days, beyond which no more water is absorbed and the water content of the composites remains constant. For the sample without pigments (original color, the control), the equilibrium for water absorption is 11.49%. For the pigment colored-1 (red) and colored-2 (brown) samples, the equilibria are 10.34 and 10.52%, respectively: see Table 3. In addition, the diffusion coefficients of pigment-colored WPCs are slightly lower compared to the control specimens.

Water absorption of wood-plastic composites (WPCs) as a function of aging time. Control: No pigment. Colored-1: Red iron oxide pigment. Colored-2: Red, yellow, and black iron oxide pigments (brown).

Water absorption (Mm) and diffusion coefficient (D) of WPCs.

D (×10−14m2/s)3.83 (0.18)3.76 (0.32)3.46 (0.21)
Mm (%)11.49 (0.46)10.34 (0.31)10.52 (0.15)

The hygrothermal aging treatment changed the color of all samples, but the patterns of lightness change (ΔL*) were very different between the control and the colored samples. Figure 2 shows ΔL* to increase for colored WPCs, indicating fading or lightening with aging time. The control, however, darkened with aging time. The lightening rate of the colored WPCs is slightly increased with time, whereas the darkening rate of the control sample was relatively fast over the first 42 days and became slower thereafter. Figure 3 shows the change in total color (ΔE*) of both control and colored WPCs to increase with aging time. The brown sample exhibited the lowest rate of increase for ΔE*, demonstrating that darker WPCs have better resistance to color change during hygrothermal aging.

Brightness change (ΔL*) of the WPCs as a function of aging time.

The total color change (ΔE*) of WPCs as a function of aging time.

We found the addition of pigments to result in comparable initial flexural properties compared to the control specimen: see Tables 4 and 5. All WPCs exhibit gradually decreasing flexural properties during the hygrothermal aging test, indicating that the degradation effect of interfacial bonding due to water and heat plays an important role in this phenomenon. It is also interesting to note that the deterioration of the flexural modulus (modulus of elasticity, MOE) caused by hygrothermal aging is more severe than that of flexural strength (modulus of rupture, MOR). The retention rate of MOR was in the range of 68–76.5%, whereas the MOE was only 50–56% for all WPC specimens after 70 days of aging.

Effect of aging time on the flexural modulus of rupture of the WPCs.

Retention rate (%)68.0173.2176.47

Effect of aging time on the flexural modulus of elasticity of the WPCs.

Retention rate (%)50.8454.9256.00

In summary, we found WPCs colored with iron-oxide pigments to exhibit better performance after hygrothermal aging than an uncolored sample. In addition, the water-absorption equilibrium and total color change of the colored WPCs were lower. In the next stage of our research, we intend to fabricate WPCs from wood-flour residues and recycled HDPE with nanocellulose derived from banana- or coconut-palm-petiole fibers as reinforcement. We will then investigate the dose and surface morphology of the nanocellulose; the content and species of inorganic pigments; and the effect of the resin/filler ratio on the mechanical and hygrothermal aging properties of the obtained WPCs.


Changyan Xu
Nanjing Forestry University


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  6. C. Xu, C. Xing, H. Pan, L. M. Matuana and H. Zhou, Hygrothermal aging properties of wood plastic composites made of recycled high density polypropylene as affected by inorganic pigments, Polym. Eng. Sci., 2015. First published online: 7 January 2015

DOI:  10.2417/spepro.005944

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