Did you know that soybeans are not only edible, but can also be used to make formaldehyde-free adhesives?

As the saying goes, "no formaldehyde, no board," as the production of artificial boards inevitably involves the use of large amounts of adhesives. Traditional artificial boards are almost exclusively made of "trialdehyde resins," represented by urea-formaldehyde resins, phenolic resins, and melamine-formaldehyde resins. Boards produced with these adhesives will continuously release formaldehyde for decades, polluting indoor air and endangering human health. With increasing emphasis on home improvement safety, consumers are increasingly choosing environmentally friendly formaldehyde-free boards. The production of formaldehyde-free boards is inseparable from the research and development of formaldehyde-free adhesives.

Are formaldehyde-free adhesives mature?

In the article "Types and Current Applications of Formaldehyde-Free Adhesives for Artificial Boards!", Lubanyuan introduced in detail the most common formaldehyde-free adhesives for artificial boards. Among them, the core technology of the more mature MDI adhesive has been mastered, but the relevant patents are in the hands of a few companies. Scientific researchers have devoted more energy to the research of other formaldehyde-free adhesives.

The production of formaldehyde-free adhesives using biomass materials such as modified starch, soy protein, and lignin has great development potential. The development of formaldehyde-free biomass adhesive technology is in line with the national theme requirements for the comprehensive development and utilization of agricultural and forestry biomass, and its core technology is the development of modifiers.

The complex structure and composition of biomass raw materials bring great difficulties to molecular modification. Products prepared by existing methods such as soybean glue and starch glue have problems such as high viscosity, short application period, poor water resistance and long hot pressing time. In particular, the water-resistant bonding strength cannot meet the requirements of industrial application, which greatly limits their application in the wood industry.

As research deepens, more and more modifiers with stable and excellent performance are entering the mature production stage, and the production of formaldehyde-free biomass adhesives can also enter the mass production process. A water-based polymer developed by the Wood Industry Research Institute of the Chinese Academy of Forestry, which is free of harmful ingredients such as aldehydes and benzene compounds, can be used as a special modifier for soy protein. The physical properties and formaldehyde emissions of wood-based panels produced using this polymer meet national standards.

How effective is formaldehyde-free biomass adhesive?

The new formaldehyde-free biomass adhesive for artificial boards (Chem-soy formaldehyde-free biomass adhesive) developed by the Institute of Wood Industry, Chinese Academy of Forestry, has made technical improvements to address the problems of poor water resistance and low bonding strength of products prepared by existing methods, thereby improving the physical and chemical properties of artificial boards.

Technological improvements to solve existing problems

◆ The soybean meal protein directional grafting technology improves the pre-compression and sizing properties of soybean glue.

◆ Synergistic cross-linking enhancement technology solves the technical problems of traditional soybean glue such as high viscosity, low bonding strength and poor water resistance.

◆ The green modification technology of soybean meal protein ensures that no harmful gases are released during the production and use of adhesives.

Evaluation of main properties of adhesives

Formaldehyde-free biomass adhesives are primarily used in the production of veneer products such as plywood, blockboard, and various veneer materials. Key performance indicators for evaluating adhesives include viscosity, solids content, pot life, and bond strength.

◆ Viscosity

Plywood production involves a series of processes, including gluing, cold pressing, trimming, and hot pressing. Therefore, high initial tack is required of the adhesive. A water-soluble polymer, acting as a modifier, reacts with soy protein molecules to form an interpenetrating network adhesive system, imparting excellent initial tack and pre-pressing properties. This allows the cold pressing of five layers of 10 mm thick plywood to be controlled within 40 minutes in summer and no more than 2 hours in winter. The cold pressing performance is comparable to, or even superior to, trialdehyde-based adhesives. The viscosity of the Chem-Soy formaldehyde-free biomass adhesive can reach 26,000 mPa·s, similar to the viscosity of aldehyde-based adhesives after adjustment, making it suitable for direct application in veneer gluing.

◆ Solid content

For veneers with high moisture content, the adhesive's solids content must be kept high, otherwise defects such as blistering will occur. While traditional soy glue has a solids content of 35% to 39%, Chem-Soy adhesives can reach a solids content of 42% depending on process requirements, ensuring product quality and increasing the yield of the finished panels.

◆ Applicable period

Most formaldehyde-free adhesives are prone to drying out due to water loss in the adhesive layer after veneer coating, resulting in poor bonding performance. Chem-soy adhesives contain water-retaining molecular segments within their structure, which increases the shelf life of veneer coating and assembly, resolving the technical bottleneck of requiring cold pressing within one hour after coating to ensure a good molding effect.

◆ Class II water-resistant bonding strength

The bond strength of plywood is related to the amount of glue applied and the hot pressing time. Experimental results show that bond strength increases with increasing glue application, and with increasing hot pressing time, it shows a trend of initially increasing significantly and then gradually increasing. With appropriate glue application and hot pressing time, plywood produced using Chem-Soy adhesive can achieve a Class II water-resistant bond strength of 0.94-1.18 MPa, and will not debond after being boiled in water for more than 4 hours.

Application prospects of formaldehyde-free biomass adhesives

The development of formaldehyde-free biomass adhesives is a key path to transitioning wood-based panels to formaldehyde-free products, aligning with the nation's sustainable development goals. As a relatively new technology, many people lack understanding of and use formaldehyde-free biomass adhesives. Proper use is crucial for fully realizing their performance, and on this basis, formaldehyde-free biomass adhesives are poised for widespread application in the wood industry.

Optimized process parameters

After multiple experimental verifications by the experimenters, the appropriate process parameters for the practical application of formaldehyde-free biomass adhesives were obtained.

◆ Use Chem-soy adhesive to prepare formaldehyde-free plywood

The glue coating amount is 360 g/m², the hot pressing temperature is 120 ℃, the hot pressing time is 66 s/mm, the cold pressing time is 40 min, and the hot pressing pressure is 0.9 MPa.

◆ Use Chem-soy adhesive to prepare formaldehyde-free blockboard

The glue application amount includes poplar core board (single side) 190 g/m² + mahogany surface board (single side) 180 g/m², cold pressing time 40 min, hot pressing temperature 120 ℃, hot pressing time 39 s/mm, and hot pressing pressure 0.8 MPa.

Wide range of application scenarios

Chem-soy adhesives can be adjusted at room temperature and are easy to use. They are suitable for the production of formaldehyde-free plywood, laminated veneer lumber (LVL), blockboard, solid wood composite flooring substrates and other man-made board products, as well as the veneer of particleboard, fiberboard, plywood and blockboard. They have a very broad application prospect.

"Formaldehyde-free" is a hot word in the current home furnishing industry and will be increasingly valued in the future. Formaldehyde-free adhesives are the foundation of formaldehyde-free boards. Formaldehyde-free adhesives are produced using starch, soybeans, etc., which are derived from agriculture and forestry, returning nature to the home!