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Abstract
We investigated the terahertz optical properties of wood-plastic composites (WPCs) having varying wood powder contents. To evaluate the influence of water uptake, we measured WPC samples under conditions where the water content of each WPC sample was controlled. We found that the refractive indexes and the absorption coefficients of the WPCs increased with accumulating wood powder content. Also, we found that the optical properties of the WPCs were almost constant for wood powder contents ranging from 0 wt% to 40 wt%, even in a high-humidity environment. WPCs are promising materials for terahertz optical components.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Research on terahertz optical components has resulted in important advances in terahertz technology. There are various terahertz optical components, for example, lenses [1–8], diffraction gratings [6,8], wave plates [9–11], polarization splitters [12], prisms [13] and attenuated total reflection (ATR) prisms [14]. Commonly used materials for terahertz optical components include high-resistivity silicon, cyclic olefin polymer (COP), quartz, sapphire, polytetrafluoro ethylene (PTFE) and polyethylene, because these materials have high transparency for terahertz waves. In addition to these, terahertz optical components made of non-conventional optical materials have been reported. In particular, low-cost, sustainable and environmentally friendly materials have attracted attention. For example, a paper terahertz lens [2], a paper wave plate [9] and a paper diffraction grating [15] have been demonstrated. Recently, the terahertz properties of cellulose nanocrystals have also been investigated [16].
In this paper, we propose a wood plastic composite (WPC) consisting of wood and plastic polymer as a terahertz optical material. WPCs are sustainable and environmentally friendly materials because sustainable wood supplies are used. Although WPCs have generally been used for construction applications or ordinary industrial products, they have never been used as the materials for terahertz optical components. The effects of the mixing ratio of WPCs on their physical properties has been reported [17]. We considered that the terahertz optical properties of WPCs can be changed in a similar way, which is achieved by mixing multiple materials to attain the desired optical properties [3].
However, wood is highly hygroscopic, and therefore, it is necessary to consider the influence of the water content of WPCs when employing them as terahertz optical materials. As a first step, we obtained the optical properties of dry WPCs. Second, we obtained the terahertz optical properties of the WPCs when the water content was varied by promoting water uptake. From these results, we investigated the possibility of terahertz optical components based on WPCs.
2. Terahertz optical properties of dry WPCs
In non-destructive testing of WPCs to measure their water content, the terahertz optical properties of WPCs containing 50 wt% and 60 wt% wood powder have been reported [18]. Generally, the upper limit of the wood powder content of WPCs is approximately 60 wt% or 70 wt% because molding becomes difficult if the wood content exceeds this level. Under the above restrictions, a higher wood content is required to give WPC products a wood-like appearance. Therefore, the wood powder content of most WPC products is approximately in the range of 50 to 60 wt% [19]. As possible terahertz optical materials, we considered not only WPCs with wood contents of 50 to 60 wt%, but also WPCs with wood contents of 40 wt% and below.
We prepared WPC samples having different wood powder contents, namely 0, 5, 10, 20, 30, 40, 50 and 60 wt%. The WPC samples had a length l of 60 mm, a width w of 10 mm, and a thickness t of 3 mm. All samples were composed of only wood powder and polypropylene, without any additives. The average diameter of the wood powder particles in the WPC samples was approximately 150 μm. First, the samples were dried in a vacuum drier at 60 °C for one week to a moisture content of 0%. Figure 1 shows a photograph of the samples after the drying process.
Fig. 1 Photograph of WPC samples having wood powder contents between 0 wt% and 60 wt% after a drying process.
Subsequently, we measured the samples with terahertz time-domain spectroscopy. A terahertz beam was focused at the sample position. The terahertz focused beam waist was approximately 1 mm (FWHM). The terahertz refractive indexes and absorption coefficients were determined by fitting experimental transmission data to the single-pass Fresnel Equation [20,21]. Since the diameter of the wood powder particles was close to wavelength of terahertz radiation, scattering occurred in the WPC sample. Therefore, the obtained refractive indexes and absorption coefficients include a scattering component [22]. Figure 2 shows the refractive indexes and power absorption coefficients at frequencies between 0.2 THz and 2 THz. Part of the data for the 50 and 60 wt% samples is not shown because the values could not be measured due to their high absorption. We found that the refractive index increased as the wood powder content was increased. At 1 THz, a change in refractive index, varying between 1.48 and 1.62, was observed by changing the wood powder content from 0 wt% to 60 wt%. The samples showed low dispersion. As shown in Fig. 2(b), the power absorption coefficient increased as the wood powder content increased.
Fig. 2 Optical properties of several WPCs having different wood powder contents and moisture content of 0%: (a) refractive index, and (b) power absorption coefficient.
3. Change in the water content of WPCs by water uptake
We examined the change in water content of the WPCs by water uptake. We submerged WPC samples whose water content was 0% in distilled water. After some time, we removed the samples and measured their weights and their terahertz optical properties. After the measurements, the WPC samples were returned to the water. We repeated this procedure. At every measurement, we wiped water from the surface of the samples. Each sample was placed in the water for a total of 20 days.
The water content was determined from the wet weight, mW, and the completely dry weight, mD, as follows:
The relationship between water content and time is given bywhere K and m represent two fitting parameters over time t. The fitted value of m for the 50 wt% sample obtained in our experiments using Eq. (2) above was approximately 0.71, which agrees well with the value of 0.73 previously reported by another group who obtained the parameters of a 50 wt% WPC [18]. Figure 3 shows the change in water content of the WPC samples as a function of the time that the samples were immersed in the distilled water. The solid line is the fitted result for the 50 wt% WPC obtained by Eq. (2). We observed that the water content tended to be higher for higher wood contents. Also, we found that the water contents of all samples were under the saturation point, assuming that the fiber saturation point of soft wood is approximately 30 wt%.4. Influence of water content on the terahertz optical properties of WPCs
Figure 4 shows the refractive indexes and power absorption coefficients of the WPC samples at 1 THz as a function of the immersion time in the distilled water. The solid lines are linear fits obtained by the method of least squares. Both the refractive index and the power absorption coefficient increased over time for wood powder contents of 60 and 50 wt%. The slope of the refractive index for the 60 wt% sample was 0.0056, and that for the 50 wt% sample was 0.0022. For the samples with wood powder contents between 0 wt% and 40 wt%, all refractive index slopes were under 0.001, and they can be considered to be essentially constant. The slope of the power absorption coefficient for the 60 wt% sample was 1.752, and that for the 50 wt% sample was 0.665. For the samples with wood powder contents of 40 wt% and less, the power absorption coefficient slopes were under 0.2, and they can be considered to be essentially constant. From these results, we consider that we can divide the samples into two groups: those that were affected by water (60 wt% and 50 wt%) and those that were not affected by water (40 wt% and below). Thus, we consider that WPCs having wood powder contents of 40 wt% and below are suitable as terahertz optical materials because of their high tolerance to humidity.
Fig. 4 Optical properties of WPC samples at 1 THz as a function of immersion time in distilled water: (a) refractive index, and (b) power absorption coefficient.
The optical properties of WPCs that contain moisture can be explained by the effective medium theory. In the case where the water content is below the fiber saturation point, the effective permittivity is determined by the permittivity of the dry WPC and the permittivity of bound water [18]. In general, the amount of bound water is increased by water uptake, so that both the refractive index and absorption coefficient increase. The observed properties of the WPC samples containing 50 and 60 wt% wood powder can be explained by this theory. Although the terahertz optical properties of the WPCs having wood powder contents of 40 wt% and below also agreed with this theory, a change in the terahertz optical properties was not shown because the increase in water content was small enough to ignore, and the amount of bound water had almost no effect.
5. Conclusion
We investigated the terahertz optical properties of various WPC samples having wood powder contents in the range from 0 wt% to 60 wt%. We found that the refractive indexes and power absorption coefficients were changed by changing the wood powder contents. For example, the refractive index was shown to vary between 1.48 and 1.62 at 1 THz. In addition, we found that the optical properties of WPC samples containing 40 wt% or less of wood powder were not affected even when immersed in water for a long time. Thus, the WPCs having wood contents of 40 wt% and less are suitable as terahertz optical materials because of their high tolerance to humidity. These WPCs provided a more environmentally-friendly alternative to plastic-based THz optical components.
We found that the optical properties of the 50 wt% and 60 wt% WPC samples were affected by water uptake because of high water absorption at those wood contents. We consider that applying a waterproof coating would be effective in preventing water uptake. If WPCs having wood powder contents of 50 and 60 wt% and protected with waterproof coatings are employed as terahertz materials, WPCs with a wider range of optical properties would become available.
WPC products can be provided stably and easily at low cost since WPC production techniques are well established. WPCs are not only inexpensive and sustainable but are also easy to fabricate into various shapes by molding.
Acknowledgments
The authors thank A. Hiruma, T. Hara and H. Nakagawa of Hamamatsu Photonics, K. K.. The authors would like to thank S. Ogoe and M. Okamoto of Toclas Co., Ltd. for providing the WPC samples.
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