What's the difference between MDI glue and urea-formaldehyde glue? A look at sheet metal adhesives

A survey conducted by the Indoor Environment Monitoring Committee of the China Interior Decoration Association reveals that artificial wood panels are the primary source of formaldehyde in indoor environments. Besides the negligible amount of formaldehyde present in the wood itself, the primary culprit is the adhesives added during production. Data shows that my country already leads the world in the production of artificial wood panels and wood-based adhesives. However, little is known about the potential effects of these adhesives on the human body.

Today, I will tell you about the basic knowledge of sheet metal adhesives!

The first is the adhesive for boards, which mainly includes urea-formaldehyde glue, melamine-formaldehyde glue and phenol-formaldehyde glue. As the name suggests, they all have formaldehyde participating in the reaction, specifically:

Urea + formaldehyde + water = urea-formaldehyde glue

Melamine + formaldehyde + water = melamine formaldehyde glue

Phenol + formaldehyde + water = phenolic glue

Although the latter two types of glue, melamine formaldehyde glue and phenol formaldehyde glue, have good environmental performance and the cured glue rarely releases formaldehyde, the price advantage of urea formaldehyde glue is very obvious. Therefore, most of my country's artificial boards are mainly made of urea formaldehyde glue.

Urea-formaldehyde adhesive releases formaldehyde primarily for two reasons: first, residual unreacted formaldehyde in the adhesive; and second, cured urea-formaldehyde adhesive slowly decomposes in a humid environment, converting back into urea and formaldehyde. For the first case, the ratio of urea to formaldehyde can be adjusted and the reaction can be separated into several steps to minimize residual formaldehyde. For the second, a formaldehyde absorber that reacts with formaldehyde can be added to the adhesive. However, achieving this requires numerous experiments to determine the optimal chemical reaction conditions and strict production management. This is difficult for small, unrecognized factories, so choosing board materials requires considering brand awareness for greater safety.

The second is white latex, an environmentally friendly adhesive typically made from polyvinyl acetate emulsion. This glue can be used on its own, requiring no curing agent or heat, and is harmless to the human body. Its concentration can be as high as 60%, with the remainder being water.

The third is neoprene solution glue, often called all-purpose glue. It's often used to bond decorative panels in interior and exterior decoration. Because it uses organic solvents, it's somewhat toxic. Apply the neoprene adhesive to the surface of the decorative panel, let it air dry, and then close it. It crystallizes instantly upon contact, generating a strong initial bond. Therefore, compared to white latex, it doesn't require prolonged clamping, making it more convenient to use.

The fourth is hot melt adhesive. The EVA hot melt adhesive used on the edge banding strip is convenient, fast and environmentally friendly.

Fifth, isocyanate adhesive (MDI). Boards made from it are non-toxic, relatively expensive, and rarely used. Major brands are now gradually promoting this type of board product. For example, formaldehyde-free Australian pine boards and solid wood sliced ​​OSB boards all use imported MDI adhesive and meet the company's formaldehyde-free standards, significantly improving their environmental performance.

Knowledge extension about the bonding principle of glue

There are many theories about how adhesion works, but it can be roughly understood as follows: two surfaces in contact attract each other, but this contact is based on the atomic and molecular scale: the distance between them must be less than 0.0000001 cm. Of course, the smoother the two surfaces, the better the contact.

For example, if you put two pieces of glass together and then pry them apart, you'll feel a faint attraction between them. However, at the atomic scale, the glass's surface isn't perfectly flat, with only about 1% contact. Making them perfectly in contact is easy-fill the gap, perhaps by adding water between the pieces. This makes them difficult to pry apart. But what if you add a curable glue between the pieces? Even two cars wouldn't be able to separate them.

Even the finest wood can never achieve the smoothness of glass, so choosing the right glue for the job is crucial. The worse the smoothness, the more glue is needed. This raises the question of the environmental performance of boards made by small, unknown manufacturers.