Plywood has become one of the most popular building materials, thanks to its useful properties such as ability to withstand moisture and high strength. Despite being robust and versatile, plywood remains affordable and durable for small businesses. A quick glance at the different types of plywood will reveal their importance.
From kitchen cabinets, floorings, and walls to furniture: the entire construction industry revolves around plywood. The use of plywood will only continue to increase as the global market grows because of lack of affordable and reliable alternatives.
Use of Plywood in Construction Engineering
The most prominent use of plywood is in structural applications. Plywood can naturally withstand a lot of stress and full weather exposure. Structural plywood is best used for beams and hoardings, but it is commonly used in crates, bins, internal structures, outdoor furniture, and boxes. Structural plywood is used for wall and roof bracing.
There are several plywood grades in the market, the most common and popular being A-grade which can take some serious beating. It is not uncommon for highly durable boards to withstand continuous boiling without delaminating. However, there is marine plywood which is made with glues that are completely weatherproof and boilproof.
It is also common to use B-bond grade structural plywood. This obviously comes at the cost of durability, but the material is sturdy enough for use in exterior door skins and concrete formwork.
There’s also C-grade plywood, which is un-sanded and likely to have several minor defects that will need to be repaired. C-plywood is used in cases where appearance and durability aren’t that important, such as subfloors and garages. At the very low-end of the spectrum is D-grade plywood, which is un-sanded and guaranteed to have defects that haven’t been repaired. D-grade plywood is identifiable by obvious discoloration and sanding defects.
It is common for manufacturers to pair up different grades of plywood to save on costs. The most popular example is BC-grade, which is a mix of B-grade plywood on one side and C-grade on the other side.
i) Exterior Plywood
As the name suggests, exterior plywood is used outside the house in items that require some serious durability. This type of plywood is bound together using water-resistant glue that doesn’t wear off easily. Some of the most suitable applications for exterior plywood are walls, roof linings, stables, and even outdoor floorings.
ii) Interior Plywood
In this type of construction, aesthetics and appearance take priority. This is because the foundation of the walls and roofs are already laid out, requiring minor aesthetic touch-ups. Interior plywood is not very resistant against outdoor elements and tends to degrade quickly when exposed to moisture and heat.
Some of the popular applications for interior plywood include indoor furniture, interior cladding, and ceilings.
The Different Types of Plywood
i) Marine Plywood
Although its name suggests that marine plywood is naturally water-resistant, it is not. It is not treated with any chemicals, which makes it prone to water damage. That being said, marine plywood is made of high-quality hardwood and secured with waterproof glue.
If you choose higher quality grades, you will end up with strong, lightweight plywood that is free of defects. These qualities make marine plywood the ideal choice for use in building boats and boat parts. It is common to use marine plywood in durable outdoor furniture, making the end product a stronger contender for use in coastal areas after receiving a strong protective finish.
Marine grade plywood is available in the following grades: A, A-B, B, High Density Overlay, and Medium Density Overlay.
ii) Shuttering Plywood
Shuttering plywood is highly dense hardwood covered with a smooth coating of film on either side. It is primarily used in construction applications such as cement formworks. Shuttering plywood is so dense that it has at least nine layers of tightly packed core veneers forced together using high-pressure machines at huge temperatures.
The end result is an intensely durable plywood type with a density of 750kg per cubic meter and thickness of 12mm. This prepares the plywood for use in ceilings, walls, pillars, drains, and highways. Shuttering plywood is reserved for intensive heavy-duty uses where it must withstand massive loads that are typically encountered during the pouring of concrete.
There are several advantages of using shuttering plywood such as:
- High-quality concrete cast surface
- Minimal deflection
- High durability
- Lightweight
- Strong chemical resistance
- Availability in different shapes and sizes
Why Is Waterproof Glue Used in Making Plywood?
The first waterproof adhesive was invented in 1934 when plywood was used in making ships. As a result of its ability to prevent ships from sinking into the sea, the demand for similar adhesives ballooned once the First World War ended. It also started a trend, where manufacturers started looking for innovative solutions for water-based glues.
Waterproof glue is used to increase plywood’s resistance to moisture, humidity, and boiling temperatures. The glue does not fail under such conditions, preventing the wood layers of the plywood from delaminating and falling apart. Many types of glues can be used for this purpose, including melamine, phenolic glue, and urea formaldehyde.
The most widely used glue is urea formaldehyde, for obvious reason. The plastic resin glue is synthesised using urea and formaldehyde. It imbues desirable properties in the plywood such as durability, reliability, and resistance to moisture. The only downside is that it must be used in a well-ventilated area since formaldehyde is toxic to humans.
On the other hand, melamine glue is a hard, thermosetting plastic material that is made frommelamine and formaldehyde. It increases resistance to weathering, which explains why you’ll find it on cabinets and doors.
The biggest disadvantage with the different types of glue is that they are harmful to the skin when exposed.
What Is Plywood Made Out Of?
The plywood, itself, is made from softwood or hardwood, and in some case, a blend of either type of wood. The hardwoods are borrowed from maples, oaks, ash, and mahogany. Softwood is taken from pine, cedar, and redwood trees. Composite plywood is another popular trend that is made with a core of solid timber pieces with a wood veneer for either side. The result is a thick sheet that is durable.
It is common to add additional materials to the front and back veneers to improve reliability and durability. These include resin-impregnated paper, fabric, plastic, and even metal to imbue the plywood with desirable properties such as resistance to moisture, corrosion, and abrasion.
Plywood has several properties, each designed for specific applications.
i) High Panel Shears
It is common to use an odd number of layers to make plywood to increase resistance to bending. The angle at which the veneer grains are arranged might be varied from 90 degrees. Common angles include 45 and 30 degrees to each other. The changes in angle strengthen the plywood. This allows the plywood to be used in bracing panels and fabricated beams.
ii) High Strength
The strength of plywood is only as high as the tree it is extracted from. But it can be strengthened later on during its manufacturing cycle. It is common to add grains of each veneer at 90 degrees to each other to prevent splitting, especially when hammered at the edges. This gives the veneer additional strength and increased stability. This makes it ideal for webbed beams, flooring, and shear walls.
iii) Flexibility
Plywood has the added advantage of being shaped into the manufacturer’s choice. Its thickness can be varied from just a few millimetres to several inches. Manufacturers use additional veneers to increase the plywood’s strength. Each additional layer of veneer increases the strength of the plywood proportionally. It is common to use thin veneers for increased flexibility for use in panelling and ceilings.
iv) Moisture Resistance
There are several types of glues used to bind the pieces of veneer together. The right choice of adhesive gives the plywood special properties such as resistance to moisture and humidity. Adding a layer of varnish and paint also increases resistance to water damage. This allows the veneer to be used in sheds, marine construction, and cladding.
Moisture resistance plays an important role in interior applications as well, including floors and walls. If the plywood is cross-laminated, it ensures the veneers won’t shrink, or expand when exposed to high temperatures and water.
v) Resistance to Chemicals
While plywood is naturally vulnerable to chemical substances, this can be avoided by treating the veneers with preservatives. The result is so effective that it can be used in chemical works such as cooling towers.
vi) Fire Resistance
Although plywood and veneer are well-known fire hazards, manufacturers have been able to increase fire resistance by the use of a chemical coating. This is done by combining non-combustible chemicals such as fibrous cement and plasterboard. It can then be used in structures to make them compliant with local regulations
vii) Insulation
Plywood has naturally high sound and thermal insulation. It is commonly used to insulate floorings, roofs, wall claddings, and ceilings. The high insulation properties of plywood reduce heating and cooling costs over the years.
viii) Resistance to Shock
While plywood is naturally resistant to impact and shock due to high tensile strength, it can be further strengthened via cross lamination of veneers. This is because of the distribution of force over a large surface area, reducing the tensile strength over an isolated area.
The result is plywood that can withstand excessive overloading by up to three times or more before crumbling. This is especially useful during high winds and seismic activity. Plywood with high tensile strength is used in heavy-duty applications such as concrete framework and flooring.
The Manufacturing Process of Plywood – A Brief Overview
The first step is to fell the trees. A high powered chain saw or large hydraulic shears are used to fell large trees. The fallen tree is then further reduced into smaller pieces with a chainsaw. These tree trunks, or logs, are then dragged to a loading area using heavy vehicles. The logs are transported to the plywood mills where they are arranged in long piles known as log decks.
How Is Plywood Prepared in Factories?
Before the veneers can be cut into desired shapes and sizes, the logs are soaked and heated to soften the wood. This process takes anywhere from 12 to 40 hours depending on the type of wood.
The treated logs are transported to a peeler lathe where they are fed to a full-length knife blade one log at a time. This reduces the diameters of the block to about 3 to 4 inches, any remaining wood is removed from the lathe and a new log is entered in its place.
The next process is to cut the veneer into usable widths according to the industry’s requirements for plywood sheets. It is common to arrange the veneer according to their grades. This can be done manually or via optical scanners. The veneer is stored in a low-humid area, known as a dryer to reduce the moisture content of the wood and to shrink the veneer’s size.
These sections of veneer are installed in certain arrangements, crossways or crossbands, depending on the client’s specifications.
A waterproof adhesive is then used to assemble appropriate sections of the veneer. This is usually one manually. Once the glue is properly applied to the sheets, the plywood is sent to a hot press that can handle around 20 to 40 sheets at a time. The hot press creates extreme temperature and pressure settings of around 200 psi and 310 ° F. This establishes good contact between the layers of veneer and allows the glue to achieve its maximum potential.
These sheets are then passed through a set of precisions saws to trim them to accurate widths and lengths. The finished plywood sheets are stamped with appropriate information such as grade, mill number, and other factors.
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