Composite Panels: Particleboard and Medium-Density Fiberboard  

Composite Panels Help Save a Piece of History

The Cathedral Sainte-Cecile, originally built in 1935 in Salaberry-de-Valleyfield, Quebec, survived a serious fire three years ago. The ornate ceiling was made from colorful hand-painted details cut from pressed wood fiber panels and nailed to larger sheets that spanned the arches.

Baupré & Michaud Architects, specialists in historic and religious buildings throughout eastern Canada, wanted to recreate the ceiling as faithfully as possible. However, the original material literally burned like paper in the fire, preventing them from using most of it. They also wanted to improve the acoustical properties of the space, and chose a composite wood panel, medium-density fiberboard (MDF), that met flame-spread requirements without special treatment.

First, they replaced the wooden ribs between the concrete arches, and then installed 5/8-inch-thick MDF panels to span the ribs. A second layer of MDF was machined into detailed parts, hand-painted and glued to the panels to recreate the original ornate design. Wood furring strips used between the ribs and the panels varied the offset of the panels from nine to 12 inches to further control reverberation.

An experienced high-end architectural millwork firm sized and machined the MDF panels based on designs recreated by the architect and draftsmen in the shop. They used the original 1934 shop drawings and the parts of the original pieces recovered from the fire. Precise drawings of the structure allowed CNC equipment to cut parts that fit perfectly when installed.

Cathedral Sainte-Cecile, Salaberry-de-Valleyfield, Quebec, reconstruction by Baupré & Michaud Architects. Source: Second Wave, Composite Panel Association

Found in myriad projects from the cathedral ceiling in Quebec to renovated rowhouses in Philadelphia, composite wood panel products are an essential building material. One of the great advantages to building and fabricating with composite panels is the materials' ability to be custom engineered for a great range of furniture, architectural, and construction applications.

The most commonly used composite wood panels for interior applications are Particleboard and Medium-Density Fiberboard (MDF). Although raw composite panels are rarely seen anywhere on a project, they're a critical component of most interior architecture. (Hardboard composite panels, which are manufactured differently from particleboard and MDF, are used for exterior siding, interior wall paneling, household and commercial furniture, and industrial and commercial products.)

Europe and the U.S. started producing particleboard commercially during the 1940s, when there was an insufficient supply of wood panels available due to World War II. Today, the North American market's annual production capacity is 7.645 billion square feet (3/4 inch basis).

MDF was developed to help fill the growing need for an economical panel that could easily perform in the same areas as particleboard. The first plant was built in Deposit, NY in 1965. By 1970, production had grown to 215 million square feet (3/4 inch basis). Today, with a U.S. annual capacity of 2.208 billion square feet (3/4 inch basis), it is one of the most rapidly growing composite board products to enter the world market in recent years.

What is Particleboard?

Particleboard consists primarily of cellulosic particles (usually wood) of various sizes that are bonded together with a synthetic resin or binder under heat and pressure. It is used as a substrate for many applications and is available in many different thicknesses and panel sizes. It is also used for case goods, store fixtures, cabinet doors, drawer fronts, wall panels, countertop core, miter folding, ready-to-assemble furniture, shelving, door-core, agricultural box ends, and mobile home decking.

Most particleboard manufacturers have created niches in the market that fit their particular abilities. Using different species of raw material and press sizes, some specialize in thin 1/4-inch panels for drawer bottoms and cabinet backs, while others produce 11/16-inch panels for the hardwood plywood industry, 11/8-inch stock for office furniture or 1 1/2-inch panels for door core, or even low-density panels (density is measured in lbs. per cu. ft.).

Typically, particleboard has several layers of different materials-a core and two outside layers, or faces. Face material is fine and uniform-characteristics that lend themselves to laminating and cutting with minimum chip-out. The core is made from coarser and larger particles to reduce the overall density and maintain desired physical and mechanical properties.

A non-structural product, particleboard is generally made from recovered and recycled wood waste. Sources range from post-industrial waste from saw mills, planer mills, plywood plants and so forth, to waste from pallets, construction and demolition sites. Raw materials vary depending upon what is available in the region-Douglas fir and Western Pine in the west, Southern Pine in the south and southeast, and hardwoods in the east and northeast.

Variations in the manufacturing process, such as modifying particle geometry, resin levels, and board density, allow production of panels suitable for a wide range of specific end uses. Incorporating additives during manufacturing provides greater dimensional stability, better fire retardancy, and moisture resistance, as well as additional characteristics.

Particleboard also accepts a variety of overlays, including high-pressure laminate, low-pressure laminate, low basis-weight paper, veneer, vinyl, coatings and decorative metals, making it an even more versatile product.

Because of freight costs, shipping particleboard across the country is generally not desirable, although there are exceptions when certain specifications are not available regionally. Each species has different properties and requires slightly different processing.

Benefits of Particleboard

Particleboard offers many benefits:

• Has a smooth surface
• Is available in a wide variety of panel sizes and thicknesses
• Accepts many types of laminates, veneers, and coatings
• Thin laminate can be applied without telegraphing (evidence of bumps or color from the substrate transferred through the laminate)
• Readily accepts most edge treatments
• Has no surface patches or core voids
• Cuts cleanly without splintering
• Specialized particleboard is available for specific applications.
• Can have resins or chemicals added to enhance fire retardancy or moisture resistance

Manufacturing Particleboard

To achieve the desired particle geometry, raw material must first be cleaned and sized, then mechanically resized by means of a refiner and a series of screens. Once sized, the material passes through a dryer and then to a storage silo. Urea formaldehyde (UF) resin is applied prior to forming the mat. (Additives to enhance characteristics like fire retardancy or moisture resistance can be applied at this stage.)

Source: Weyerhaeuser Company

Mats are then formed on a caul plate or belt, loaded into the press and subjected to heat, pressure and time until the resin has cured. After the press cycle is complete, the panel is removed from the press, transported through a board cooler, and then hot-stacked to await sawing into finished panel sizes and sanding. Alternatively, the master panels may be sanded before being cut into finished sizes.

What is MDF?

Like particleboard, MDF is a composite panel product that typically consists of cellulose fibers (usually wood) combined with a synthetic resin or other suitable bonding system and joined together under heat and pressure. Additives may be introduced during manufacturing to impart additional characteristics.

Used in the manufacture of furniture, cabinets, door parts, moulding, millwork, and laminate flooring, MDF panels are manufactured in a variety of dimensions and densities. Like particleboard producers, MDF manufacturers create niches for themselves with products such as 3mm panels for the door skin industry, 11/16-inch panels for the hardwood plywood industry, 1-1/4-inch stock for crown moulding, or 2-1/2-inch thick material for the furniture industry.

Benefits of MDF:

MDF is a higher-end, non-structural interior product compared to particleboard. It is used in the most detailed of applications because it offers more design flexibility than particleboard.

• Its surface is flat, smooth, uniform, dense, and free of knots and grain patterns.
• The surface features make finishing operations easier and more consistent, especially for demanding uses such as direct printing and thin laminates.
• MDF can be overlayed with items that include veneer, high- or low-pressure laminate, low basis weight papers, vinyl and hot stamp foils or treated with a powder coating. Some MDF can be embossed.
• It can be easily routed and drilled. The homogeneous density profile of MDF allows intricate and precise machining and finishing techniques for finished products.
• MDF holds precise tolerances in accurately cut parts.
• It has acoustical dampening properties.
• There are specialized MDF panels for specific applications.
• MDF has the ability to take dyes and pigments, producing colored panels.

Manufacturing MDF

MDF is made from much of the same types of recovered and recycled wood waste as particleboard, but is "cooked" using a different manufacturing process.

Source: Weyerhaeuser Company

Introduced into the high-energy refining system, the recycled waste is literally cooked in a steam pressurized "digester." This allows the raw material to become softened and to actually change physically and chemically so that the fibrous material becomes less susceptible to the influences of moisture and less brittle as the lignin (the substance that, with the cellulose, forms the chief part of the woody tissue) softens.

The material remains under pressure as it is processed through rotating refiner plates, getting rubbed apart into uniform fiber size. Resin is usually applied in the "blow line" as the fiber exits the refiner, i.e., before drying, but it is sometimes applied after drying (see Figure 3). The most common binder for MDF is UF.

Other types of resins and additives can be used to provide special properties such as moisture resistance or fire retardancy. The wood-fiber-resin combination is dried to a uniform moisture content, formed into a homogeneous mat or in separate face and core layers of random fiber orientation and hot pressed to complete the rough manufacture.

Standards, Certifications, and Codes

The American National Standard for Particleboard, ANSI A208.1 (see table 3 online), classifies particleboard by density and class and is the voluntary particleboard standard for the North American industry. The standard, which covers physical and mechanical properties and dimensional tolerances as well as formaldehyde emission limits, was developed through the efforts of the Composite Panel Association (CPA, see sidebar), producers, users, and general interest groups.

The ANSI standard for Medium Density Fiberboard, ANSI A208.2 (see Table 4 online), is the North American industry voluntary standard for MDF and classifies MDF by physical and mechanical properties and identifies product grades. Specifications identified in the standard include physical and mechanical properties, dimensional tolerances, and formaldehyde emission limits.

A summary of the ANSI Property Requirements are included in CPA's Buyer's and Specifiers Guide to North American Particleboard, Medium Density Fiberboard, and Hardboard Products and Manufacturers, 2005, which is available online (www.pbmdf.com). Copies of the complete ANSI Standards for Particleboard and MDF are also available from CPA.

Third-party certification to ANSI Standards is required for many applications of composite panels. For example, the U.S. Department of Housing and Urban Development (HUD) requires the physical and mechanical properties of manufactured home decking to be third-party certified. Many building code jurisdictions require the physical and mechanical properties of particleboard underlayment and stair tread to be third-party certified. HUD requires third-party certification of formaldehyde emissions for particleboard used in mobile homes. The state of Minnesota requires MDF used as a "building material" to comply with HUD formaldehyde emissions for particleboard.

Formaldehyde in Particleboard and MDF

Approximately 80-85 percent of the typical composite panel is wood. The rest consists of resin binders, additives, and water. The adhesive (resin) most often used by the North American composite panel industry is UF. This resin is strong, colorless, economical, and provides performance criteria specified for most interior uses. The "formaldehyde" used in UF resins is a colorless chemical that is part of a large family of "volatile organic compounds" (VOCs)-those that become a gas at normal room temperature.

Over the past 15 years, technology has significantly reduced the free formaldehyde, which contributed to emissions. Panels produced today, on average, emit only about one-sixth as much formaldehyde as those produced in the early 1980s.

ANSI A208.1-1999 established voluntary formaldehyde emission limits for particleboard. These include emission limits of 0.30 parts per million (ppm) for standard industrial grades and 0.20 ppm for flooring grades. Emission limits are based on values determined under specified conditions in a standard large chamber with product loading ratios of 0.425 square meter/cubic meter (0.13 square feet/cubic feet). ANSI A208.2-2002 sets the voluntary formaldehyde emission limit for MDF at 0.30 ppm at a loading ratio of 0.26m 2/m3 (0.08 ft2/ft3).

Formaldehyde emission limits established by the U.S. composites industry have played a major role in bringing emission levels down. In fact, the industry average is well below the voluntary ANSI standards for both particleboard and MDF. Some U.S. particleboard and MDF producers are also certified by third parties as meeting quality and environmental standards such as the International Organization for Standardization (ISO) 9000, ISO 14000, Scientific Certification Services (SCS), or the CPA's Environmentally Preferable Product (EPP) Certification Program.

Various overlays and surface treatments are also known to significantly reduce product emissions even further. Effective barriers can reduce emission levels by 95 percent or more. These barriers are most effective when all surfaces are treated, i.e., for maximum emission reduction, edges, notches, and holes also need to be edge banded, laminated, finished or covered with hardware. For additional information about emissions, see the CPA Technical Bulletin VOC Emission Barrier Effects.

U.S. and Canadian composite producers who are members of the CPA voluntarily meet the EPA and ANSI standards for formaldehyde emissions. They also use significant amounts (up to 100 percent) of recycled and/or recovered fiber as their raw material. Be aware, however, that safety and production standards vary from country to country, as do raw materials sourcing practices and requirements.

Oriented strandboard (OSB), plywood, and some specialty MDF products use phenol-formaldehyde. However, phenol-formaldehyde requires a different manufacturing process involving higher employee safety and manufacturing costs. Agri-board uses a third type of adhesive known as methyl diisocyanate adhesive, which also involves extremely complex and expensive employee safety precautions.

LEED Certification

As more owners become "green conscious" and seek certification from the U.S. Green Building Council's Leadership in Energy and Environment (LEEDâ„¢), there is a growing interest on the part of architects to seek out products with lower VOC emissions. Some manufacturers are offering composite panels with no added urea formaldehyde in order to reduce formaldehyde emissions.

At present, the majority of potential points available from Particleboard and MDF products are in the category of materials and resources (see Table 5 online). For example, the use of typical Particleboard and MDF could help reach the goal of specifying 5 percent or 10 percent recycled content for the whole project (worth one point each). For a building in San Francisco, specifying composite panels manufactured locally (within 500 miles) in California or Oregon could help achieve the project's goal of using 20 percent locally manufactured materials (worth one point).

Specifying composite panels manufactured locally from raw materials harvested locally could potentially help earn another point. Other potential LEED points could be gained by specifying certain specialty composite panels, such as agri-board or formaldehyde-free MDF. However, designers need to be aware there may be difficulty sourcing these specialty products or meeting other specifications of the project.

Green Globes Rating

Green Globesâ„¢ is another "green" rating for commercial buildings, adopted in the U.S. in 2004 from a Canadian protocol of the same name. It is one of only two green building rating systems recommended by the Canadian government.

Weyerhaeuser Company (NYSE: WY), one of the world's largest integrated forest products companies, was incorporated in 1900. In 2004, sales were $22.7 billion. It has offices or operations in 19 countries, with customers worldwide. Weyerhaeuser is principally engaged in the growing and harvesting of timber; the manufacture, distribution, and sale of forest products; and real estate construction, development and related activities. Weyerhaeuser Composite Panels, a business unit within Weyerhaeuser Company, manufactures Duraflake® and Ultrapine® particleboard and Premier® and Colorburstâ„¢ medium density fiberboard in the United States. Additional information about Weyerhaeuser's businesses, products and practices is available athttp://www.weyerhaeuser.com

Green Globes uses performance benchmark criteria, just as LEED does. However, one major benefit of Green Globes is that it compares building designs to data that reflects real building performance vs. the performance of hypothetical structures. It also encourages builders and designers to consider sustainability and green elements early in the project rather than adding expensive technologies later in the design.

Green Globes awards points for acoustical comfort, use of an integrated design process, emissions and effluents reductions and "minimal consumption of resources (reused, recycled, local, low-maintenance materials, certified wood) as well as reduction, reuse, and recycling of demolition waste." Using composite panels contributes to potential Green Globes points. For more information on the Green Globes rating system, visithttp://www.thegbi.com/commercial/greenglobes/index.htm.

Flame Spread Requirements for Composite Panels

Most code requirements for wood product interior finish materials are expressed in terms of flame spread index numbers. These values are determined in the standard U.S. flame spread test, ASTM E-84, conducted by such organizations as Underwriters' Laboratory and the Hardwood Plywood and Veneer Association.

Different maximum indices are permitted depending upon building occupancy, location of the material in the building, and the presence of sprinklers. The index is calibrated based on a scale where a noncombustible material is 0 and red oak flooring is 100. Class I or A designates flame spread range 0-25; Class II or B, flame spread range 26-75; and Class III or C, flame spread range 76-200.

Several composite panel manufacturers supply flame spread ratings for fire retardant products (Class I or A), which achieve that rating by using special treatments. Most wood products, however, including particleboard, hardboard, and MDF, are presumed to have a flame index of less than 200-class III or C-making them acceptable under current building codes for a wide range of interior finishes. Depending upon thickness, particleboard indices range from 145 to 156, and MDF indices range from 90 to 140. Indices for factory finished composite panels with vinyl or paper overlaid composite panels range from 100 to 180.

Some national and local codes require lower flame spread ratings. To meet those requirements, designers can specify specialty fire-retardant products from the handful of manufacturers who offer composite panels with a Class I or A classification.

Moisture Resistance

Because composite panels are mostly organic wood material, they are affected by the same environmental factors that affect timber and solid lumber and are therefore susceptible to water and moisture.

Moisture resistance is achieved through careful engineering to address two essential properties: resistance to dimensional change and retention of strength when exposed to water or elevated humidity. As a general rule, wood shrinks or swells in proportion to the volume of water lost or gained. When wood is green, it is saturated with water in both cell cavities and the cell walls. The water in the cell cavities is called "free water" and the water in the cell walls is called "bound water." Normally, free water is removed completely during the drying process. Some bound water remains and is in equilibrium with the relative humidity of the air. As the relative humidity of the air changes, the moisture content will correspondingly change. It can affect panel gluing, finishing, and dimensional stability.

All wood species generally follow the moisture content curve as a function of relative humidity (see chart). Composite panels follow a somewhat modified curve and will equilibrate at a lower moisture content for a given relative humidity (due to additional heat treatments in the processes for these products). Since they are made from wood fibers or particles reconstituted in a more random orientation, the dimensional changes due to moisture or temperature variations are more uniform across the width and length than for solid wood.

Source: Weyerhaeuser Company

The ANSI MDF Standard defines three levels of moisture resistance. The first level, designated as MR10, requires a product to have a thickness swell performance that is equal to or less than half (50 percent) of the grade's thickness swell requirement. The second level, MR30, requires testing according to the six-cycle ASTM D 1037 accelerated aging test. The highest level, MR50, requires a product conform to the requirements of both MR10 and MR30.

In addition to these standards, the CPA sponsors an industry-voluntary specification for intermediate moisture resistance that includes a test method. The CPA Voluntary Specification For Intermediate Moisture Resistance, CPA-IMR-VS-01-2004 provides performance requirements that differentiate standard particleboard and MDF.

There are many applications for composite panels which may call for enhanced moisture resistance treatment. These include cabinetry, moulding, and countertops, especially for kitchens, bathrooms, and labs, where incidental moisture may occur.

As in the case of reducing VOCs emission levels, the main technology used to increase a panel's moisture resistance revolves around the type of adhesive used to bind the wood particles and fibers together. Melamine-fortified UF resins and phenol-formaldehyde are frequently used to improve a panel's moisture resistance, but can increase manufacturing costs. Phenol-formaldehyde resins, while providing durability to a panel, may darken its the color. The use of methyl diisocyanate adhesives in composite panels is limited to particleboard/MDF specialty products and agri-fiber-based panels. This adhesive typically provides a strong, moisture-resistant bond.

It should be noted that the addition of a decorative surface to a composite panel, particularly when the panel is "encapsulated"-covered completely on both broad surfaces and all edges-further enhances the moisture resistance properties of the panel.

The typical properties manufacturers improve are thickness swell, linear expansion, residual internal bond strength and bending strength. The goal is to provide a substrate whose behavior in the presence of incidental moisture is known and acceptable for the intended end use.

Handling and Storage

All types of composite panels will perform better if properly handled and stored. Since a smooth panel surface is essential for both particleboard and MDF, units should be protected against exposure to water and high humidity.

• Always wear proper eye, hearing, and respiratory protection when fabricating particleboard and MDF.
• Effective storage and handling should begin with a critical observation of inbound shipments. Photograph any problems before unloading and contact the panel manufacturer if necessary, to resolve any issues.
• Unload units under cover where possible. Avoid unloading during inclement weather. Tight straps, which may indent the upper corners of a bundle, are an indication of built-up stresses induced by exposure to high humidity.
• Most panels tend to absorb moisture into their edges more rapidly than through the panel surfaces. This unequal rate of moisture absorption can cause unequal stresses to build in the panels that will be relieved when the panels are cut. Strips from the outer edges may bend, creating what is commonly called a "banana" cut. Allowing the cut part to come to equilibrium will normally straighten these strips.
• Do not store materials outside or in locations where they may be exposed to water or high humidity.
• Stack units on a hard, level surface that is clean, dry, and away from open doorways and running machinery that could create airborne contaminants.
• Support bolsters that are misaligned, uneven or missing may cause high bending stresses, which could result in permanent warping or damage.
• Materials slated for gluing, laminating or other finishing may need a week or more before coming to temperature and moisture content equilibrium. Such materials should be "conditioned" by separating the panels with clean, dry spacer sticks or by placing panels in a spacing rack to provide good air circulation across all surfaces. Adequate conditioning time should be scheduled into the fabrication process, especially when temperature extremes exist during shipment.
• Temperatures should be kept as close to 70 degrees F (21 degrees C) as possible. Avoid storage in extremely cold or hot temperatures. Warming panels in winter takes just as long as cooling them in summer. While the top few panels may feel comfortable to the touch, the middle ones could be either too hot or too cold. Cold panel and/or cold ambient temperatures will slow the cure rate of laminate adhesives.

Minimizing Warp in Laminated Particleboard and MDF

Warp is defined as the deviation of the geometry of a panel from an initial state of flatness.

• Always select flat panels for substrates. Consider the substrate properties, including stiffness, thickness, linear expansion, and uniformity. These can be evaluated from the manufacturer's specifications or standards. The greater the thickness of the material, the better it will resist moisture-related expansion stresses.
• A cause of warp in laminated panel products is unbalanced panel construction.
• As different materials are rigidly bonded together, moisture content changes may occur. In response to the changes in moisture content, the materials attempt to change dimensions. When that happens, stresses can accumulate. Warp results when these stresses become excessive and are no longer balanced on the two surfaces.
• Selection of laminates and balanced construction go hand-in-hand. The laminates and/or coatings applied to each face of a particleboard or MDF substrate should be similar in properties. Generally, this is best achieved by using the same material to cover both sides of a substrate.
• Unusually moist or dry conditions should be avoided in the laminating and storage environments. The moisture content of wood-based materials and laminates is dependent on the amount of moisture in the air. When these materials are moved from one environment to another, the moisture content changes. Resulting dimensional changes can be substantial.
• It is unlikely that the moisture content of the laminate and the substrate will be in equilibrium with the laminating environment when they are delivered to the laminating shop. Allow sufficient time for the laminate and substrate to adapt to the laminating environment. It can take two or more weeks to reach a satisfactory equilibrium. Seasonal changes and air circulation around the materials will influence the time it takes.
• Once assembled, differences in the expansion or shrinkage characteristics of the laminate and substrate can produce stresses, which cause warped panels.
  Ideally, laminates and substrates should be stored and assembled in conditions similar to the finished product application environment.
• Laminates at one equilibrium moisture content condition should not be applied to particleboard/MDF of a different condition. If they are, as the moisture content equalizes, the particleboard/MDF substrate may expand or contract while the laminate seeks the opposite. When bonded with rigid adhesives, the components cannot move in relation to each other. This creates stresses at the substrate/laminate interface, which can result in a warped panel.
• A well-balanced laminated panel can exhibit temporary warp due to unequal rates of moisture gain or loss by the face and back laminates. However, as soon as the laminates equalize, these stresses diminish, and the panel returns to its flat condition. This ability to equalize and return to the flat condition at any humidity is an important attribute of the balanced panel.

Installation

Even with a perfectly balanced panel, installation conditions can cause moisture unbalance that results in warp. The laminated panel should not be exposed to extreme variations in humidity during final installation.

• Changes in humidity at the installation site can result in stresses that cause warp as the entire panel equalizes to the new service environment. Panel conditioning prior to final installation improves product reliability.
• The design of laminated panel applications must also consider the service environment. Applications that expose one surface of a panel to a warm humid atmosphere and the other to a dry atmosphere will result in moisture unbalance that can cause warp. Long expanses of panels, such as display cases or walls, may warp if they do not include expansion joints, reveals or other design considerations allowing for dimensional changes resulting from seasonal variations in the environment. Panels that are butted edge-to-edge and rigidly fastened may buckle (a form of restrained warping) due to expansion stresses as moisture content increases.

Source: Weyerhaeuser Company

Important Note for Designers:

Depending upon their application, composite board ratings and compatibilities may change. Manufacturers draw designers' attention to the following:

• Particleboard and MDF are for nonstructural, interior use only
• In colored particleboard or MDF, color variation from one manufacturing run to the next can occur, as well as from panel to panel.
• Some laminates and coatings applied to fire-rated composite board may change the flame spread rating.
• Standard available woodworking glues have been successfully used in lamination. However, some adhesives may have compatibility problems with the chemical system used in the manufacturing process. Any adhesive should be tested for compatibility with the chemical system in the composite product prior to full- scale gluing. Questions should be directed to the glue supplier.
• When using composite panels in wall systems, an integral vapor barrier must be a properly installed part of the wall in either of the following conditions:
  • The wall has an exterior side
  • The wall separates spaces conditioned unequally (for example, between a heated and unheated space)

Table 4