Research Background
Nowadays, in order to reduce energy consumption and environmental pollution, it is urgent to develop green energy-saving materials. Transparent wood composite material has the advantages of light weight, high transmittance, low coefficient and good mechanical strength. At the same time, it can also collect sunlight effectively, contributing to energy-saving and comfortable indoor lighting. However, the method of manufacturing transparent wood composite materials generally removes most of the light-absorbing materials (lignin and extracts) or pigment components, and only about 80% of the lignin remains. Intensive chemical treatment is used to severely destroy the original structure of the wood to ensure effective polymer penetration. Therefore, previous researches usually focused on the form and upper part of the optical, mechanical and thermal properties, and rarely discussed the alternate structure, the natural beauty of the original annual growth pattern of wood, and the scalable manufacturing through effective processes.
New Ideas for Research
Recently, the team of Professor Liangbing Hu from the University of Maryland reported on a new type of scalable aesthetically transparent wood. This aesthetic wood is based on the process of selective delignification of space and epoxy resin penetration, and has comprehensive aesthetic characteristics: such as complete wood patterns, excellent optical properties (the average transmittance is about 80%, and the defogging degree is about 93 %), good UV shielding ability and low thermal conductivity. In addition, the rapid manufacturing process and mechanical strength of this aesthetic wood promotes good scalability while saving a lot of time and energy. Therefore, it has great potential in the application of energy-saving building materials, especially glass ceilings, roofs, transparent decorations and interior panels.
Morphological and Chemical Characteristics
First of all, we need to know that due to the obvious microstructure difference between earlywood (EW) and latewood (LW), the growth rings are visible. The researchers designed two kinds of aesthetic woods, one of which uses the difference between EW and LW. Douglas fir has more holes than LW and has thinner cell walls, so it is more convenient to handle. The author used the acidic NaClO2 method to remove lignin after 2 hours, and achieved spatially selective lignin: EW almost turned white, while LW maintained a good pattern. The product thus prepared is called Aesthetic Wood-R.
Scalability and Mechanical Properties
Another product is Aesthetic Wood-L, where researchers use a quarter-slice cutting strategy to construct aesthetic wood with straight grains. They found that using this process gave Aesthetic Wood-L excellent structural integrity and could be mass-produced. Its production principle is: without compromising its aesthetic characteristics and other properties, thicker aesthetic wood can provide better load-bearing performance in construction applications. The obtained aesthetic wood-L (thickness 0.6 mm) has a total transmittance of 87% at 600 nm and a haze removal of 65%. After infiltration, the aesthetic wood showed a large number of aligned dense microchannels along the wood growth direction. Although the inner cavity of LW is much smaller than that of EW, their density is very high, and the same channels and holes are completely filled with polymer due to the effect of glue. Raman spectroscopy imaging revealed that the impregnated polymer was distributed in the cell horn, composite middle layer, cell wall and inner cavity of wood cells. According to Raman spectroscopy, it is found that the polymer penetrates well into the wood cells and forms a firm interface with the cellulose in the wood scaffold.
Thermal Performance
The aesthetic wood can be used as a patterned ceiling in museums or galleries to show its aesthetic effect, and may replace the glass ceiling. At the same time, aesthetic wood also has excellent thermal insulation properties, so it can improve energy efficiency. In addition, aesthetic wood also combines anisotropic heat transfer and low thermal conductivity, which is very beneficial for energy-saving buildings.
