Research on the Preparation of Wood Adhesive Active Fillers from Tannin-/Bentonite-Modified Corn Cob

With the swift growth of China’s construction and furniture industries, the artificial plywood industry has emerged as a key foundational sector in the country [1]. This industry is responsible for producing construction building materials and furniture materials. Presently, the primary adhesives employed for artificial plywood in China are formaldehyde-based synthetic resin adhesives. These include urea–formaldehyde resin adhesive, phenol-formaldehyde resin adhesive, and melamine resin adhesive. Together, these three types of adhesives account for over 80% of total adhesive consumption within the wood processing industry [2]. The urea–formaldehyde resin adhesive, in particular, enjoys widespread use in the domestic artificial plywood industry due to its notable advantages, such as corrosion resistance, a colorless cured adhesive layer, ease of application, excellent process performance, low cost, superior bonding performance, and excellent insulation properties. When utilizing urea–formaldehyde resin adhesive for pressing plywood, a curing agent is typically added during the adhesive adjustment process. Additionally, a certain proportion of filler is commonly incorporated [3]. By adding filler to the urea–formaldehyde resin adhesive, adhesive consumption and associated costs can be reduced. Furthermore, this aids in enhancing the initial tackiness of the adhesive [4], preventing excessive adhesive penetration into the wood, and mitigating resulting adhesive deficiency. In addition, the inclusion of filler helps to diminish the internal stress arising from the volume shrinkage of the adhesive during the curing process [5]. This, in turn, improves the durability of the bonding strength and extends the service life of the artificial plywood.

Fillers that are commonly used for urea–formaldehyde resin adhesives possess specific characteristics. They are generally chemically inactive, neutral or close to neutral in nature, possess good water miscibility, and can cure after water evaporation. These fillers exhibit stable viscosity over time and do not significantly prolong the curing time of the adhesive [6]. Importantly, they have minimal impact on bonding strength and durability. Additionally, they are readily available as powder, cost-effective, and have a plentiful supply of raw materials. Various fillers are utilized for plywood production both domestically and internationally. These include wheat flour, coconut shell powder, bark powder, mineral powder, soybean powder, wood powder, and lightweight calcium carbonate, among other mineral components [7]. In particular, domestically, flour is widely employed as a filler for urea–formaldehyde resin adhesive within the plywood industry, with an annual consumption of approximately 300,000 tons. However, this is extensive use of wheat flour in non-food industries. Hence, it is crucial from both economic and societal perspectives to explore cheaper fillers that can effectively reduce formaldehyde emissions [8,9] while being a viable alternative to flour. The utilization of such fillers would help diminish grain consumption in non-food industries.

Corn is a highly important grain, with the largest global plantation and highest yield. Corn cob contains a large amount of hemicellulose, cellulose, and lignin, with rich carbon content. Under dry weight conditions, the crude fiber content is approximately 33% [10]. Among various agricultural byproducts, corn stands out due to its exceptionally high hemicellulose content and significant value in terms of utilization [11]. However, the current efficiency in utilizing corn cobs is remarkably low, resulting in the disposal or incineration of approximately 500,000 tons of corn cobs in China annually. This practice not only contributes to environmental pollution but also wastes valuable renewable resources. In the specific case of utilizing corn cob powder as a filler for urea–formaldehyde adhesive, challenges arise. The powder tends to absorb the adhesive, leading to thickening [12]. Furthermore, it exhibits poor dispersibility within the adhesive solution, causing it to sink or float on the surface.

Tannins are secondary metabolites that have evolved in vascular plants as a means of self-protection. They are found in various plant tissues, including bark, roots, leaves, flowers, and fruits. Among these, bark contains the highest concentrations of tannins, with levels ranging from 20% to 40% in many coniferous tree barks [13]. The presence of tannins in plants helps defend against oxidative stress caused by UV damage and other environmental factors. They also play a crucial role in preventing cell structure damage by neutralizing harmful free radicals and protecting against bacterial and fungal infections [14,15]. In China, bayberry tannin (BT) is a prominent and commercially valuable type of tannin. It is widely available and comparatively inexpensive. The structure diagram is shown in Figure 1.

Bentonite is a naturally occurring clay with montmorillonite as its primary component. The layered two-dimensional grid structure of montmorillonite confers excellent properties to bentonite, including strong adsorption capacity, ion exchange capability, and expansibility [16]. In China, bentonite resources are abundant and widely distributed across more than 20 provinces, resulting in its increasing importance in development and utilization. Sodium-based bentonite, which is obtained through sodium modification of calcium-based bentonite, exhibits superior qualities. These include a higher cation exchange capacity, improved dispersibility, reproducibility, and thermal stability [17]. Furthermore, sodium-based bentonite has a high water-absorption rate and expansion ratio, as well as good thixotropic properties, viscosity, and lubricity of colloidal suspensions. This type of bentonite serves as a fundamental material for the deep processing of bentonite. It has proven effective in treating various pollutants in wastewater, particularly in the enrichment and fixation of heavy metals in large-volume wastewater [18,19]. Sodium-based bentonite also demonstrates efficacy in adsorbing anilines and nicotine in soil, as well as formaldehyde in the air. The structure diagram is shown in Figure 2.