Introduction

Thermochromic wood, as a thermochromic material, can undergo a reversible change in colour with respect to temperature stimulus1,2,3,4,5, that has the advantages of renewability, low cost, easy preparation, easy modification and so on. Thermochromic wood can be used in many fields, such as: temperature sensors6,7,8, smart window frame9,10,11, temperature control coatings12,13, thermal energy storage14, camouflage15, etc. Infiltrating thermochromic dyes into wood materials will help to improve the seasonal visual effect of wood, that can enrich the decorative effect of wood materials, can be used as temperature indicators, and can provide a new solution to energy consumption for interior buildings16,17. Professor Fu Feng and coworkers have investigated the development of thermochromic wood18, the preparation and properties of thermochromic thin wood-based veneer (thickness: 0.7 mm)19,20,21, and multi-functional thermochromic energy-storage wood materials22. Xiaodong Zhu and coworkers have prepared a thin thermochromic wood-based veneer (thickness: 0.17 mm) that can be obtained after impregnation with a thermochromic compound suspension for 2.0 min at 65 °C16.

Poplar is mainly distributed from low altitudes to 4800 m above sea level, and between latitudes 22° to 70° N23,24,25. Farmed poplar is an important agro-forestry tree species in many nations due to its rapid growth, short rotation on stand, multiple uses, and high economic value26,27,28,29. Reasonable use of farmed poplar can meet the human demand for thermochromic materials and avoid the consumption of natural forest resources.

In the present research, a thicker, poplar-based, thermochromic composite (R-PTC, thickness: 5.0 mm) was prepared by infiltration of red thermochromic dyes (R-TD) into the pre-treated poplar veneer (thickness: 5.0 mm) and compacting the resulting poplar-based composite (Fig. 1). This type of R-PTC can undergo a colour change from light-red to dark-red at about 38 °C to 46 °C, can revert to light-red at below 38 °C after about 14 h, and the peak of colour change is at about 42 °C.

Figure 1
figure 1

Preparation of R-PTC.

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Results and discussion

Fourier transform infrared spectroscopy (FTIR) was used to investigate changes in the composition of the cell wall of poplar specimens before and after pre-treatment. FTIR spectra were measured using an FTIR-850 (Gangdong, Tianjin, China). In the FTIR spectrum, the band at 1505 cm−1 represents the aromatic skeleton vibration of the lignin28,29,30,31. The band at 1235 cm−1 represents the characteristic vibration of hemicelluloses, and that at 1735 cm−1 signifies the presence of a C=O functional group32,33,34,35. The 1505 cm−1, 1235 cm−1, and 1735 cm−1 peaks represent lignin, hemicelluloses, and C=O functional group, respectively. After pre-treatment, the 1505 cm−1, 1235 cm−1, and 1735 cm−1 peaks of pre-treated poplar specimens are lower than the peaks of the original poplar specimens in the FTIR spectra (Fig. 2), proving that lignin, hemicellulose, and the C=O functional group may had been changed therefrom. As Table 1 shows, the absolute-dry mass of original poplar specimens (60 mm × 60 mm × 5 mm) was about 4.244 ~ 4.391 g, and the absolute-dry mass of pre-treated poplar specimens (60 mm × 60 mm × 5 mm) was about 3.113–3.382 g, and the absolute-dry mass of pre-treated poplar specimens was less than that of the original poplar specimens, with the mass being reduced by about one-third of its original value after pre-treatment. Therefore, to compare with the original poplar specimens, the lignin, hemicellulose, and the C=O functional group may had been largely removed in the pre-treated poplar specimens.

Figure 2
figure 2

FTIR spectra: original, and pre-treated poplar specimens.

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Table 1 The absolute-drying masses: original, and pre-treated poplar specimens.
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Original, and pre-treated poplar specimens were cut from the longitudinal direction, these sections were examined by using Sigma 300 scanning electron microscopy (Zeiss, Germany). Figure 3a,b are SEM images of longitudinal direction from original, and pre-treated poplar specimens, respectively. As the red arrows show (Fig. 3), the cell cavity space was obviously enlarged after pre-treatment, that improved the channel of R-TD infiltration.

Figure 3
figure 3

SEM images: original, and pre-treated poplar specimens.

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DSC test of R-TD and R-PTC. The phase-change temperature of thermochromic dyes can affect the temperature during dye-infiltration and the colour-change temperature of a thermochromic material19,20. A differential scanning calorimeter (DSC) can be used to measure phase-change temperature (Fig. 4): the peak value of phase-change temperature was 46.97 °C in R-TD, and the peak value was 41.78 °C in R-PTC. As Fig. 4 shows, the phase-change temperature range of R-TD is about 30 °C to 62 °C, and the phase-change temperature range of R-PTC is about 38 °C to 46 °C. In R-PTC, the covalent bond between wood fibers and R-TD that potentially improved the starting temperature of phase-change temperature range, and the water molecule of wood that possibly reduced the peak value and range of phase-change temperature. Therefore, comparing with R-TD, the R-PTC had lower peak value of phase-change temperature, and narrower phase-change temperature range.

Figure 4
figure 4

The DSC curve of R-TD and R-PTC.

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Thermochromic properties of R-PTC. According to CIELab measurements (International Lighting Committee, 1976) and the environmental temperature range of R-PTC, the R-PTC specimens were placed into a constant-temperature box for about 10 min at a specific temperature from 20 to 60 °C (Table 2), and the colour parameters (L, a, and b) of R-PTC surface were measured using a CR-400 colorimeter (Konica Minolta, Japan) at each specific temperature (Table 2). Each colour parameter is the average value of three measurements at different positions (●, ■, and ▲) on the R-PTC surface (Fig. 5a). In Table 2, ∆Eab is the value of the colour difference, and the magnitude of ∆Eab affects the strength of human vision (Table 3)36,37; ∆Eab is given by:

$$ \Delta E_{ab} = \sqrt {(L_{n} - L_{0} )^{2} + (a_{n} - a_{0} )^{2} + (b_{n} - b_{0} )^{2} } $$

L0, a0, and b0 are the colour parameters of the R-PTC surface at 20 °C; Ln, an, and bn are the colour parameters of the R-PTC surface at 22 °C, 24 °C, 26 °C, …, 58 °C, and 60 °C, respectively.

Table 2 The colour parameters of the R-PTC surface at temperatures from 20 to 60 °C.
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Figure 5
figure 5

(a) The colours of the R-PTC surface changed from light-red (below 38 °C) to dark-red (above 42 °C). (b) The curve of ∆Eab. (c) The colours of the R-PTC surface changed before and after ultraviolet light treatment. (d) The thermochromic mechanism of R-TD.

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Table 3 The relation between ∆Eab and identification ability of human vision.
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As shown in Table 2 and Fig. 5b, at 38 °C to 42 °C, ∆Eab exceeded 3.0 and gradually reached 12 (Table 2). Data in Table 3 show that the colour differences in R-PTC changed at 38 °C and did so to a much greater extent at 42 °C to the human eye. In Fig. 5a, the colour of the R-PTC surface is seen to have changed from light-red (at < 38 °C) to dark-red (at ≥ 42 °C). The thermochromic properties of R-PTC basically accords with the DSC test of R-PTC, the peak value all appeared at about 42 °C, and the range all appeared at about 38 °C to 46 °C.

After heating the R-PTC specimens in a constant-temperature box (Symor, Heifei, China) for about 150 h at 80 to 100 °C, the weight of heat-treated R-PTC was reduced about 21% of original R-PTC. As Fig. 5c shown, at ●, ■, and ▲ positions on the R-PTC surface, the colour parameter of three positions were measured before and after the R-PTC specimens were exposed to ultraviolet light (120 mw/cm2) for about 150 h, and its ∆Eab was 5.7, recognition change.

The colour of methyl red (C15H15N3O2) is red and acid form (HMR) when its pH value was ≤ 4.4 and its wavelength was at 520 nm, and the colour of methyl red (C15H14N3O2) is yellow and alkaline form (MR) when its pH value was ≥ 6.2 and its wavelength was at 430 nm38,39,40,41. R-TD is the tetradecanoic acid tetradecyl ester (C28H56O2) and methyl red (C15H15N3O2) mixture. In R-TD, C28H56O2 is the hide-colour solvent, and C15H15N3O2 is the developer. As Fig. 5a,d shown, the R-TD was light-red and its developer was acid form (HMR) at about 20 °C, and the R-TD was dark-red and its developer was alkaline form (MR) at about 42 °C.

Mechanical characteristics of R-PTC. The longitudinal compressive strength and tensile strength, and radial hardness of R-PTC were measured. Compared to the original poplar specimens, the R-PTC had a slightly lower longitudinal compressive strength, tensile strength, and radial hardness (Fig. 6). The mechanical characteristics of pre-treated poplar were reduced after the loss of the lignin during pre-treatment: this also influenced the mechanical characteristics of the R-PTC specimens, therefore, improving the mechanical characteristics of R-PTC without affecting its thermochromic properties will form a focus of future research.

Figure 6
figure 6

(a) Longitudinal compressive strength. (b) Longitudinal tensile strength. (c) Radial hardness.

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Microscopic features of R-PTC. The microscopic features of R-PTC were examined by SEM. Figure 7a,c show SEM images of radial-cut and longitudinal-cut pre-treated poplar specimens, respectively. Figure 7b,d show SEM images of radial-cut and longitudinal-cut R-PTC, respectively. Compared with Fig. 7a,c,b,d show that the R-PTC was able to be infiltrated by R-TD, which as the red arrows show, R-TD had been attached to the cell wall of wood. In R-PTC, the R-TD gathered around the fibers and on the surface of cell wall, and the R-TD liked irregular ball-like object that combined with the fibers of wood (Fig. 8).

Figure 7
figure 7

SEM images: pre-treated poplar specimens (a, c) and R-PTC specimens (b, d).

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Figure 8
figure 8

SEM image: the R-TD in R-PTC specimen.

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Conclusions

The colour of the newly-developed R-PTC could change from light-red to dark-red at 38 °C to 46 °C, can gradually revert to light-red at below 38 °C after about 14 h, and the peak of colour change is at about 42 °C. When the temperature of object at ≥ 42 °C, the user will feel scalded pain after touching the object. Therefore, R-PTC could be used in material used to make furniture capable of indicating the surface temperature to potential users, allowing them to assess likely scalded pain. Furthermore, future work will focus on improvement of its mechanical characteristics.

Experimental

Materials and chemicals. The original poplar (60 mm × 60 mm × 5 mm) was purchased from Mudan Wood Co., Ltd. (Suqian, China). Sodium hypochlorite (NaOCl, > 98%), deionised water, and absolute ethyl alcohol (C2H6O, > 99.5%) were purchased from Aladdin Biochemical Technology (Shanghai, China). Tetradecanoic acid tetradecyl ester (C28H56O2, > 99%) and polypropylene wax were purchased from Shandong Usolf Chemical Technology Co., Ltd. (Linyi, China). Methyl red (C15H15N3O2, > 99%) was purchased from Fangzheng Reagent Factory (Tianjin, China).

The preparation process of R-PTC included pre-treatment of the specimens, preparation of R-TD, R-TD infiltration, and compaction (Table 4).

Table 4 The chemical formula and method of preparation of R-PTC.
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Sample pre-treatment. After drying the original poplar specimens in a constant-temperature box (Symor, Heifei, China) for about 3 h at 80 to 90 °C, the sample was dipped in NaOCl solution (0.81 mol L-1 in deionised water) for about 48 h at 15 to 25 °C. Then, the specimens were steamed over deionised water for about 6 h at 100 to 120 °C. After repeating these pre-treatment, all added chemicals were removed from the specimens by rinsing in hot distilled water, and subsequent preservation in C2H6O.

Preparation of R-TD and R-TD infiltration. R-TD was prepared by mixing C28H56O2 (10 g) and C15H15N3O2 (0.1 g) for about 0.5 h at about 75 to 80 °C. The pre-treated poplar specimens were then removed from the C2H6O and dried in a constant-temperature box for about 1 h at 80 to 90 °C. The resulting R-TD was infiltrated into the lumen of pre-treated poplar sample by R-TD impregnation treatment in a beaker under ultrasonication in a DR-LQ20D ultrasonic cleaner (Derui, Shenzheng, China) for about 2 h at 75 to 80 °C, and an applied ultrasonic power of 80 W.

Compaction and painting treatment. After R-TD infiltrating the sample, the poplar-based thermochromic composite (R-PTC) specimens were compacted in a 150 T universal test press (Suzhou, Shanghai, China) under an applied stress of 4.5 MPa for about 0.5 h at 15 to 25 °C, and the R-PTC samples were covered by polypropylene wax for preventing R-TD from overflowing from R-PTC under the action of phase-changed temperature.

Statement. All methods were performed in accordance with the relevant guidelines and regulations.