Plywood is made by glueing several layers of wood veneer (thin sheets of wood) over each other with the grain pattern alternating at right angles in every subsequent layer.
During the plywood manufacturing process, the individual layers of veneer are first obtained by rotary-cutting a log. For this the timber logs are rotated about their longitudinal axis, while a cutter peels the log at the set thickness (each layer is usually less than 2.5 mm in thickness).
Later on, adhesives are spread on these veneers using a glue-spreader machine. The machine helps in achieving a uniform glue spread. After
this the layers are placed upon each other and strongly pressed together using a hot-press machine. The high temperature and pressure created by the machine, ensures that the adhesive fully cures, thus firmly bonding the layers to each other.
The number of layers in each plywood sheet can vary (could be 3-ply, 5-ply ... 13-ply etc.) depending on the requirement.
There are two major types of adhesives used for making plywood, 'Urea formaldehyde' and 'Phenol formaldehyde'.
1. Urea formaldehyde resins:
These are a type of adhesives known as aminoplastic synthetic resins.
Made by a chemical reaction between Urea and formaldehyde, these adhesives are cheaper than phenol formaldehydes.
Commercial MR grade plywood (Moisture resistant type) is often made using this kind of adhesive.
2. Phenol formaldehyde resins.
This is type of synthetic or artificial polymer that is obtained by a chemical reaction between 'phenol' and 'formaldehyde (methanal)'.
The ratio of formaldehyde to phenol is generally in the ratio 1.5:1. These types of phenolic synthetic resins are called 'resoles'.
Phenolic resins enable a stronger bond compared to urea based resins, and hence these are used in the making of exterior grade plywood (BWR-Boiling water resistant) and marine grade plywood.
Sometimes extenders are added to the glue mixture to reduce the total cost, and to reduce the amount of glue that can penetrate through the individual veneer layers.
The quality specifications for plywood adhesives are detailed in the Indian Standards specification (document number IS:848).
Several types of tests are carried out on such adhered plywood sheets. Some of the important properties checked are glue shear strength, dry strength, resistance to micro-organisms such as fungi, and resistance to water and moisture.
Formaldehyde health risks
In recent years, several concerns have been raised regarding the potential health hazards that can be caused by formaldehyde emissions, and the general consensus seems to be that high level of emissions can pose some health hazards for humans.
In the case of plywood, the synthetic resins used contain formaldehyde, hence it cannot be considered as eco-friendly as natural solid wood. However another important point in this regard is that over time the emissions reduce considerably. Also proper finishing such as by painting or by using laminates also helps in making the product more safer for people to use.
Also, nowadays more eco-friendly types of adhesives (formaldehyde-free) are being researched all over the world to counter this concern.