With the ongoing enhancement of living standards, the demand for superior quality in interior building decoration materials and furniture has risen proportionately. Beyond the aesthetic and comfort criteria for buildings, environmental protection is a crucial determinant of product quality, gathering close attention from individuals (Stylianos et al., 2005). Wood materials are indispensable materials in our lives. However, due to the escalation of human activities, the availability of natural wood resources is steadily diminishing. Wood-based panels have progressively gained prominence in the public domain to address the scarcity of wood and optimize its utilization. Owing to their facile processing and dimensional stability, wood-based panels find extensive application in furniture manufacturing and interior decoration. However, the widespread utilization of aldehyde-based adhesives persists in producing and applying wood-based panel products. The formaldehyde release in wood-based panels mainly comes from three sources (Squire et al., 1984, Lu et al., 2003). (1) In preparing aldehyde resin adhesives, formaldehyde reactions often remain incomplete, leading to free formaldehyde. (2) During plywood preparation, formaldehyde is released by thermal decomposition or chemical decomposition of aldehyde resin adhesives. (3) External environmental factors such as humidity, acidity, and temperature can induce aging of the adhesive layer in plywood products during use, resulting in the breakdown of groups within the resin and the generation of free formaldehyde.
Diverse approaches are employed to mitigate the formaldehyde emissions from wood-based panels. Notably, aldehyde-free adhesives, including isocyanate adhesives (Cui et al., 2009), soy protein-based adhesives (Gong et al., 2017), and thermoplastic resin adhesives (Gao et al., 2020), are among the strategies utilized. The issue of formaldehyde release in the adhesive during the plate production process has been addressed; however, the resolution involves a complex procedure with associated high costs (Chen et al. 2021). Furthermore, formaldehyde release can be controlled by adjusting the production process of urea-formaldehyde resin adhesives (Shen et al., 2022, Li et al., 2022). However, the challenge of formaldehyde release persists in wood-based panels. The release of formaldehyde in wood-based panels occurs over an extended period and is characterized by a slow release. Consequently, a technology capable of effectively capturing formaldehyde within aldehyde products for an extended duration holds significant importance.
Microcapsules are composed of shell and core materials, and their diameter is usually defined within 1 ~ 1000 µm (Song 2021). The coated core material can be liquid, solid particles, or gaseous substances. Similarly, the shell material around the core material is usually a synthetic or natural polymer. Microencapsulation can protect the core material and improve its stability (Huanbutta 2008). Moreover, the permeability of the microcapsule shell material can be extended or controlled by changing the external conditions (Zheng et al., 2016; Borodina et al., 2008). Microcapsules have regular or irregular forms, and according to their morphology, they can be divided into three primary forms: single-core microcapsules, multi-core microcapsules, and matrix microcapsules (Fig. 1). Single-core microcapsules are structures formed by single liquid or continuous core material coated with shell material. Pang et al. (Pang et al., 2018) prepared single-core microcapsules by using urea-formaldehyde resin as shell material coated with tung oil, which showed high encapsulation efficiency and certain self-healing functions. Multi-core microcapsules can be broadly defined as microcapsules created by subdividing the core material into several smaller homogenous substances and incorporating them into the shell material. Wu Qiang et al. (Wu et al., 2017) prepared multi-core Janus microcapsules with controllable size and shape structure. For matrix microcapsules, the uniform distribution of core material is crucial (Hu 2016, Li 2019, Liu et al., 2019, Zuo et al., 2019). In addition to the three primary microcapsule forms discussed earlier, microcapsules can be divided into microcapsules and clusters with various composite shells and multi-layer materials.
Figure 1Morphologies of microcapsules (Bah et al., 2020)
The design of microcapsules should have not only the selection of the preparation method but also the careful consideration of both core and shell materials. It is crucial that the shell material is compatible with the core material, devoid of any chemical reactions, and possesses basic properties such as non-toxicity, harmlessness, and cost-effectiveness. In addition, the shell material should also have flexibility, hygroscopic properties, stability and permeability (Bah et al., 2020). Therefore, several kinds of shell materials can be combined to improve the thermal stability, flexibility, or airtightness of the shell material (Wu et al., 2007). Microcapsules with ethyl cellulose as the shell material have the advantages of high stability, good biocompatibility, degradability, excellent film-forming performance, and low production cost. Song et al. (Song et al., 2022) prepared microcapsules using ethyl cellulose as shell material by covering curcumin with solvent evaporation method. The results showed that the microcapsules had uniform particle size distribution, good film formation, and a high coverage rate. The microcapsules with sodium alginate as shell material have the advantages of solid protection, good controllability, and good degradability. Tian (Tian et al., 2019) prepared microcapsules with sodium alginate and gelatin as composite shell material and paraffin as core material using a complex condensation method. The results showed that the microcapsules exhibited a high coverage rate, were non-toxic, environmentally friendly, and displayed commendable thermal stability. The sodium alginate/ethyl cellulose composite shell material was designed to leverage the advantages of both components, effectively enhancing the coverage rate and slow-release performance of the microcapsules.
The ionic gel method is widely used to prepare sodium alginate microspheres. Its working principle is cross-linking cationic and anionic substances by stirring them at high speed to obtain functional sodium alginate microspheres. Popova EV et al. (Popova et al., 2021) prepared polyphenol microcapsules with sodium alginate as the shell material and calcium chloride as the cross-linking agent. The findings demonstrated that sodium alginate polyphenol microcapsules had a high coverage rate and good slow-release performance.
Chlorine dioxide is a strong oxidizing substance that can make formaldehyde to produce carbon dioxide and water by an oxidation-reduction reaction (Zheng et al., 2006; Suo et al., 2006). However, chlorine dioxide has poor stability, is more sensitive to the external environment, and readily decomposes by heat and light. In this study, microcapsules were prepared by utilizing sodium chlorite as the core material, capable of releasing chlorine dioxide under specific conditions, and sodium alginate-ethyl cellulose as the shell material, implementing the ionic gel-solvent evaporation method. Figure 2 shows the successful preparation of microcapsules that can reduce formaldehyde release in aldehyde resin plywood.