By Abhijit Phadke
In this article we will take a detailed look at what Phenolic plywood is, how its made, and its use in the building construction industry.
Phenolic plywood also known as phenolic film-faced concrete shuttering plywood, is widely used in the construction industry to create the wooden moulds that are used for casting concrete such as while making the concrete slabs and pillars of buildings. Amongst the major advantages of using film-faced plywood for concrete casting, is that these boards are completely water-proof, the concrete does not stick to their surface, and moreover these plywood sheets can also be reused again after the concrete casting process is completed.
How phenolic plywood is made:
Basically, phenolic plywood is waterproof grade plywood, having a phenolic film on its surface. During the final stage of the manufacturing process a phenolic film is applied over the surfaces of the plywood. This is the reason it is also known as film-faced plywood.
The basic steps in the manufacture of phenolic plywood are listed below.
1. Good quality timber logs obtained from hardwood trees are used as the raw material.
2. Slices of wood veneers are obtained from these logs. For doing this, the timber logs are mounted on a machine, that rotates the logs about their longitudinal axis, while a cutter peels off layers from the log. These layers are the veneers.
3. The veneers are graded according to quality, and stacked upon each other.
4. Synthetic phenolic resin, made from phenol formaldehyde, is evenly spread over the veneers to glue them together.
5. The multiple layers of veneers (plies), arranged in alternating grain pattern, are then hot-pressed to form a strong and permanent bond.
6. Preservative treatment is carried out on the plywood by adding suitable chemicals.
7. In the final stage, a very thin film of phenolic resin is applied over the surface of the plywood. This film is made by soaking and saturating a paper in phenolic resin. Once the resin cures, the paper becomes hard and the film is ready. The film is then hot-pressed and melted over the surface of the plywood, to get a smooth hard finish.
8. The shiny plywood is now ready for quality testing and dispatch.
The outer glossy surface of most phenolic plywoods is usually of a deep red colour, however dark brown or black coloured ones can also be found.
It's interesting to note, that decorative laminates (such as Sunmica, Formica and other brands) are also made using a similar process. In the case of laminates, a brown craft paper soaked in phenolic resin and a decorative paper soaked in transparent melamine resin are hot-pressed together under high pressure, to make the decorative laminate sheets that are used as an overlay over plywood.
Quality tests for Phenolic plywood:
Similar to Marine grade plywood, the quality tests for phenolic film faced shuttering plywood also include a water-resistance tests during which sample pieces of the plywood are submerged into boiling water for a period of 72 hours, and then removing and cooling them by plunging into cold water. The still wet pieces are then tested for strength and also checked so as to confirm that the outer phenolic layer has not deteriorated in any manner.
The Indian Standard quality specification for Concrete Shuttering plywood is detailed in document IS:4990. Manufacturing companies who meet the prescribed quality standards can obtain a certificate from BIS (Bureau of Indian Standards), and then become eligible to put the trusted ISI mark on their plywood sheets.
Important properties of Phenolic plywood:
1. Durable: The concrete casting process involves alternate wet and dry conditions. The plywood has to be durable to withstand these changes.
2. Very strong: Concrete is made by mixing cement, sand and crushed stone together with water. While the concrete cures and becomes hard it exerts force on the shuttering plywood mould. So the plywood has to be strong to withstand these forces.
3. Reusable: Using Phenolic film faced concrete shuttering plywood is advantgeous because after the concrete casting process is completed, the plywood shutters can be dismantled and then these sheets can be reused again elsewhere. However for this to happen, the plywood sheets need to be carefully handled both while erecting the form work, and while dismantling, so that they remains good enough for reuse.
4. Shiny, glossy surface: The phenolic film lends the plywood with a very smooth glossy surface. This is a very useful characteristic in the case of concrete shuttering work, because the concrete does not stick to the plywood because of this smooth laminated film surface. The smooth film surface also gives the resulting concrete a smoother finish compared to other methods, and very often additional plastering of such concrete is not required since it is already quite smooth.
Here it needs to be noted that not all concrete shuttering plywoods are film-faced. They also come in other varieties such as plain or oil-coated. However the phenolic film-faced ones are the best of the lot.
5. Dimensionally stable: Since it is used in making the shutters used for concrete casting, it's dimensions have to be stable even under the excess heat conditions that get generated while the concrete cures.
6. Costly: Phenolic plywood is a costly material just like Marine grade plywood, since it is made to meet higher quality standards, compared to furniture-grade plywood.
Phenolic plywood lasts longer and serves its purpose better, when the sheets are well-cared for. Avoiding scratches, wear and tear, cleaning the surface, and applying mould-oil (parting oil) on the surface time and again, all help to increase its quality for repeated reuse and help to make it more durable.
After the ply has been used for several times for casting concrete, it can later still be used for making good quality utility furniture such as work-benches, table-tops, and for outdoor woodwork. For such other uses, buying used phenolic plywood can be very cost-effective, since it can be purchased at significantly lower prices, compared to buying brand new plywood of the same kind.
And finally a few last words to try and clear some of the term-related confusion that is the hallmark of the woodworking industry.
What is 'Phenolic bonded plywood'?
Phenolic bonded plywood would simply mean a plywood that has been made using synthetic phenolic resins. To understand this point we need to know the kinds of glues used for making plywood.
Basically, there are two main kinds of resins that are used for bonding the plywood veneers to each other.
1. UF (Urea formaldehyde) which is used for making the lesser quality commercial MR (Moisture Resistant) plywood that is used for making indoor furniture, and
2. Phenolic resins (Phenol formaldehyde) that are used for making the better quality BWR (Boiling Water Resistant) grade of waterproof plywood. Phenolic resins are synthetic plastic resins, and hence enhance the water-proofing quality. Extenders are added to the resin in this case.
In addition to the above, there is also a third variety in plywood veneer bonding, and it is the use of unextended phenolic resins (meaning undiluted resins) for glueing the veneers together. This is the method used for making high-quality Marine grade plywood, as also for making Phenolic film-faced Concrete Shuttering plywood.
So, the term 'phenol bonded plywood' can be used to refer to any type of plywood in which the individual veneers (plies) have been bonded together using phenolic resins.