What are the advantages of using silica sol as the main colloid as glue for straw boards?

The content is excerpted from "Research on the Preparation and Application of Inorganic Environmentally Friendly Adhesives for Straw Boards" in the 6th issue of "Forest Products Industry" magazine in 2018.

Authors: Liu Zhen, Ma Lei, Nie Ning, etc.

my country is a major agricultural country with abundant straw resources. Experts estimate that my country's annual crop straw production reaches 700 million tons, accounting for approximately 30% of the world's total straw. Currently, the effective utilization rate of crop straw is only 30%. Crop straw is rich in cellulose, making it a suitable raw material for wood-based panels. Given the current shortage of timber resources, developing a crop straw-based wood-based panel industry not only transforms agricultural waste into valuable resources and conserves timber resources, but also offers significant economic and social benefits.

Commonly used glue analysis

The production of artificial boards using straw as a raw material typically requires the addition of adhesives and certain production processes. Adhesives are central to straw board production and determine the performance of the resulting boards. Commonly used adhesives include urea-formaldehyde resin adhesives, isocyanates, and plant protein adhesives. Urea-formaldehyde resin adhesives, while relatively inexpensive, are formaldehyde-based adhesives, but their adhesion to straw is poor, resulting in low-strength boards. Furthermore, the free formaldehyde released during straw board production and use can pose significant health risks. Isocyanates are a high-quality adhesive with low toxicity, but their limited raw material availability, complex synthesis process, and high cost lead to high bonding costs. Furthermore, these boards present technical challenges such as flammability, moisture absorption, and high energy consumption during the molding stage. Plant protein adhesives offer advantages such as abundant raw material availability, low cost, and non-toxicity, but their poor water resistance makes them unsuitable for production requirements. To solve these problems, it is urgent to develop an environmentally friendly adhesive for straw boards that is pollution-free, high-strength, water-resistant, and has thermal insulation properties while being low-priced and requiring low energy consumption in board production.

New inorganic environmentally friendly adhesive

We developed an environmentally friendly inorganic adhesive based on silica sol (mSiO2·nH2O) as the primary colloid, with aluminum chloride (AlCl3), aluminum oxide (Al2O3), magnesium oxide (MgO), zinc oxide (ZnO), and aluminum hydroxide [Al(OH)3] as fillers. We applied this adhesive to the production of straw-based boards. We investigated the effects of filler content and adhesive dosage on the properties of the boards by measuring their moisture content, density, compressive strength, flexural strength, 24-hour water absorption thickness expansion, and uniform temperature ignition performance.

Adhesive formula

Extensive preliminary testing revealed that the amounts of AlCl₃, Al₂O₃, MgO, ZnO, and Al(OH)₃ in the adhesive significantly influence the properties of the resulting panels. By varying the amounts of AlCl₃, Al₂O₃, MgO, ZnO, and Al(OH)₃ in the adhesive, the authors tested the panels' moisture content, density, compressive strength, flexural strength, 24-hour water absorption thickness expansion, and uniform temperature ignition performance. Using a single-factor experimental method, they determined the formula for their environmentally friendly inorganic adhesive.

in conclusion

An inorganic, environmentally friendly adhesive for straw-based panels was prepared using silica sol (mSiO2·nH2O) as the primary colloid, with aluminum chloride (AlCl3), aluminum oxide (Al2O3), magnesium oxide (MgO), zinc oxide (ZnO), and aluminum hydroxide [Al(OH)3] as fillers. The optimal formulation for the pressed panels was determined by testing their properties. The filler and silica sol percentages in the formulation were: AlCl36%, Al2O3 2%, MgO 6%, ZnO 2%, and Al(OH)3 2%. The optimal mass ratio of adhesive to straw was 1.0.

The performance indicators of the pressed boards under the optimal adhesive formula and adhesive/straw mass ratio are as follows: moisture content 4.53%, density 888.56 kg/m3, flexural strength 3.56 MPa, compressive strength 3.59 MPa, thermal conductivity 0.13 W/(m·k), 24-h water absorption thickness expansion rate 2.25%, and uniform temperature burning performance 4.53%. These relevant indicators are on par with or better than the corporate standards of similar products.