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What is cellular polycarbonate (Twin Wall, Multi Wall) ?.

Updated: Nov 5, 2020

Since cellular polycarbonate (Twin Wall) —or polycarbonate structural sheet (PCSS) as the material is generically known—offers architects a multitude of design solutions for canopies, barrel vaults, skylights, hurricane panels, translucent walls and signage, familiarity with its characteristics, benefits and appropriate use is essential.

Polycarbonates, named because they are polymers containing carbonate groups

(-O-(C=O)-O-), are a widely used plastic made from a particular group of thermoplastic polymers. Unlike monolithic polycarbonate, which has no internal cavities, PCSS is available with multiple honeycomb-like cavities commonly known as “flutes.” All such polycarbonates are versatile due to their high impact resistance, optical properties, temperature and fire resistance.

Polycarbonate is manufactured using extrusion technology. Plastic resin in the form of clear or tinted pellets is melted and pumped through a die. After traveling through the die, the sheets enter a heated oven to relieve tension. They then travel over a series of rollers, are cooled and cut to the final dimensions. This technology allows for the continuous production of any length of product in a wide variety of profiles that is limited only by its ability to be transported. For sheets coming from overseas, the maximum length is 39 ft. That is all that will fit into a 40-ft container. However, manufacturers in the U.S. are beginning to extrude locally and will be bound only by the length of a truck bed.

Key to manufacturing polycarbonate profiles are sophisticated dies that set the width, height, thickness and structure according to the particular specifications of design professionals.

All PCSS sheets are flat except for special profiles such as standing seam panels. They are all flexible and can be cold bent to suit design requirements. The length and width of PCSS flat sheeting can be adjusted to meet specific customer requirements. Standard sheeting sizes in the U.S. are 4 ft or 8 ft wide with a maximum length of 39 ft if they are coming by container and longer if they are produced in the U.S. and can be delivered via truck. These lengths are also true for specially profiled sheets such as standing seam or

Polycarbonate structural sheets (PCSS) are fabricated in many configurations such as three-wall, five-wall and five-wall x-structure, among others. These multiwall sheets can be used as a standard glazing material and, on rare occasion, can be cut to size on site. Applications include greenhouses and signage (usually 6mm and 8mm thick), office dividers and dropped ceilings (usually 16mm). High-impact or heavy snow/wind load applications, such as curtain walls and skylights, will require panels of 16mm, 20mm, 25mm, 32mm, 40mm or 50mm thickness. These thicker panels also offer higher insulating value.

Standing Seam

Standing seam sheets, commonly used for skylights, canopies and wall systems, are typically available in nominal 2-ft widths. More specifically, owing to their European heritage, they are 600mm (23.62 in.) wide. The available lengths are as stated above for PCSS flat sheets. Typically, standing seam systems have polycarbonate battens, which snap over the sheet edges and form the seams. This is the same design that has made standing seam roofing so successful. Aluminum battens can be used if extra spanning capability is desired. The standing seam panels are secured to the substrate (normally purlins or girts) with metal clips. Some manufacturers offer two-piece metal clips which are designed to accept the thermal movement of the panels as well as reducing noise from friction.

Tongue and Groove

A tongue and groove profile is used for vertical walls, office dividers and industrial windows. Joints, which have no battens of any kind, create a visually uninterrupted wall. Tongue and groove panels are typically available in 40mm or 50mm thicknesses. Some panel configurations allow the insertion of steel bars, typically 3/16 in. x 1-1/4 in., and as long as the panel. These bars increase the spanning capability of the panels.

Polycarbonate Properties & Performance

Because of their versatile properties, which position them between commodity plastics and engineering plastics, the second largest use of polycarbonates is for construction material (the first is for electronics components). A testament to the strength of polycarbonate is the fact that airplane windows are made of it. Clarity/Light Transmission/Daylighting

Clear PCSS allows more light transmission than fiberglass reinforced panels (FRP). Soft, even daylighting can be achieved through opal or frosted polycarbonates. Studies have found that access to daylight enhances well being and quality of life. Daylighting is also linked to improved learning, increased retail sales and office worker productivity.

Students with the most daylighting in their classrooms progress faster on math (20 percent) and reading tests (26 percent) than those with the least light in their classrooms. Another study found that diffused daylighting increased retail sales—up to 40 percent for one major retailer.

Further, other research reports that call center workers handled calls 6 to 12 percent faster, while office workers tested 10 to 25 percent better when compared with those working in less natural light. Glare, from any source, potentially decreased office worker performance by 15 percent to 21 percent.

Impact Resistance

PCSS products offer high impact resistance and protection against storm damage and vandalism. Some polycarbonate glazed systems have met the product testing requirements of Florida, Miami-Dade and Texas Building Codes. However, many of those systems require close spacing of framing or other reinforcement, which may raise aesthetic issues.

In some applications PCSS has been demonstrated to be practically unbreakable. Polycarbonate is 250 times more impact resistant than an equivalent thickness of annealed glass and 30 times more than that of non-modified acrylic.

A PCSS surface is softer than that of glass and its structure is more crushable than glass. To address this, some manufacturers incorporate special framing designs to restrict the pressure of the gasketing against the polycarbonate. This is called “controlled gasket pressure.”

Energy Savings

PCSS offers significant benefits in thermal insulation due to the many airspaces between the inner and outer panel faces. Because of the variety of available profiles and resins, it can deliver high levels of light transmission while also maintaining high R-values

(thermal resistance.) One inch of cellular polycarbonate, compared with one inch of insulated glass, has an R-value of 3.84 compared with 2.08 for the glass. Their U-values (rate of heat loss which is the inverse of the R-value) are 0.26 and 0.48 respectively.

Significantly increased thermal performance may be achieved by combining polycarbonates with translucent aerogel, a substance derived from a gel in which the liquid component of the gel has been replaced with a gas.

Aerogel manufacturers claim: * High light transmission * Non-combustible * Totally recyclable * Non-toxic * Silica gel–97% air * Hydrophobic

However, it should be noted that aerogel fillings in cellular polycarbonate are a relatively new item and long-term field performance has yet to be established.

Resistance to Flame/Heat and Smoke

Smoke density, flame spread and self-combustion measurements are all important factors when describing the safety performance of all plastics. When considering these factors, PCSS offers much better results than either FRP or acrylic.

PCSS in various profiles are typically available as Class A-, B- or C-rated materials for flame spread and smoke generation (see sidebar Standards and Definitions.) Properties of PCSS include:

* Self-ignition temperature >842° F (450° C) {Note: IBC 2606.7 requires plastic materials to have a self ignition temperature of 650° F (343° C ) or greater.} * Softening temperature point: 320° F (160° C) (this meets International Code Council acceptance criteria) * Decomposition occurs at approximately 715° F (380° C)

UV Resistance and Yellowing

Sheet degradation is measured using a yellowness index, ∆YI, calculated from spectrophotometric data that describes the change in color of a test sample from clear or white toward yellow. In addition to change in color, ∆YI correlates with a proportionate amount of loss of the sheet's ability to transmit light. A ∆YI over 5 will also have a reduced impact resistance. Different warranties for polycarbonate products reflect the effectiveness of UV protection. When evaluating polycarbonate sheet materials for use as glazing, design professionals should compare warranties to see which offers the best protection against yellowing, impact resistance and light transmission degradation. Most current architectural panels qualify for a ∆YI of 2 or less even after the equivalent of 10 years in a south-facing application.

Thermally Dynamic Material

Polycarbonate expands and contracts thermally more than most materials (about seven times as much as glass or steel and about three times as much as aluminum). It even moves with the temperature differential that occurs from day to night. Thermally induced expansion and contraction of PCSS can amount to 1/8-in. per 3 ft per 100-degree temperature change. Some manufacturers compensate for this by designing retention clips and framing that allows for sheet movement (see Framing below.)

Engineered framing and clip designs can also eliminate sounds caused by the expansion or contraction of PCSS panels.


A 1-in.-thick piece of cellular polycarbonate weighs approximately 0.65 lb/sq ft compared to 6.25 lb/sq ft for 1 in. of insulated glass. This difference is so dramatic that buildings that incorporate PCSS skylights or canopies can be built with lighter, less expensive framing systems. Light weight also offers significant savings in transportation, handling and installation.

Cold Bending

PCSS can be cold bent (curved) lengthwise, usually over a series of supports which are spaced so as to determine the curvature. Once a sheet is removed from its form, it will regain its flat profile. Cold bent polycarbonate is a solution for applications such as canopies and barrel vault skylights. It should be noted that curving also adds significant spanning strength to the panels. Specifiers should be aware that over-bending may constitute a breach of warranty.

Easy to Clean

PCSS products are easily cleaned using detergent and water or power washing. However, one of the virtues of PCSS is that, owing to the light-diffusing nature of the multi-cells, it doesn't show the dirt. This same quality hides scratching or other imperfections. While glass glazing requires periodic cleaning, PCSS glazing is seldom cleaned—thus providing considerable maintenance savings

Cellular or multiwall (PCSS) polycarbonate panels offer considerable versatility for design professionals seeking solutions for canopies, barrel vaults, skylights, translucent walls and signage. Its benefits range from translucency and impact resistance to energy savings and flame and heat resistance. It also can contribute to LEED® points. The key to successful applications is a well-engineered framing system that addresses all of the nuances of this significant and timely building material.

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