Plastic Glazing 101May 31, 2011
Glazing for plastic skylights is usually polycarbonate, copolyester or acrylic. Plastics are a subset of materials known as Polymers. Polymers are formed by chemical reactions in which a large number of molecules called monomers are joined sequentially, forming a chain. In many polymers, only one monomer is present. In others, two or three different monomers may be combined. To make plastic glazing, the most common process is extrusion; sheet can also be formed in a process called cell casting, and injection molding is useful when dimensional precision or varying thickness is desired.
In extrusion, plastic pellets or powders are fed into an extruder, which is basically a metal pipe with a rotating screw inside it, which melts the plastic and forces it into a die which forms the melted plastic into a sheet. In the casting process, partially polymerized plastic is poured into a mold or cast onto rotating stainless steel belts. The polymerization process is then carried to completion.
Acrylic is a tough, transparent polymer of poly(methyl methacrylate) (PMMA) that first appeared on the market as an alternative to glass. It is familiar under the well-known trade names Plexiglass™ and Lucite™. Polycarbonate, known by the trademarked names Lexan™ and Makrolon™, is an amorphous very clear engineering thermoplastic material with exceptional toughness, outstanding impact resistance and ductility, high heat deflection temperatures, inherent flame resistance, good UV resistance and useful engineering properties over a broad temperature range. Copolyester, known by the trade names TiGlaze ST™ and UltraTuf-CX™, is similar in composition to the plastic used in water bottles but is more impact resistant.
Different plastic formulations do offer differing performance characteristics; for example, acrylic materials are typically resistant to solar UV degradation, while polycarbonates and copolyesters are generally more shatter-resistant. Because the final skylight product is engineered to take full account of these performance differences and governed by standards that level the performance playing field, the choice of the type of plastic or plastic vs. glass depends completely on the user’s preference and job-specific requirements.
Plastic-glazed skylights offer a range of advantages:
- An entire skylight structure can be molded from a single piece of plastic, shaped in multiple dimensions to form domes or prismatics that facilitate rainwater runoff. This makes them ideal for flat or low-slope roofs. The plastic configuration can include an integral flange to overlap the mounting frame, thereby minimizing the possibility of water leakage and reducing both framing and maintenance costs. Plastic glazing can also be machined, drilled or sawed like wood and soft metals.
- Plastics can be weather-resistant. Acrylics are inherently resistant to UV light degradation, which can lead to “yellowing” and reduced strength over time. Due to technical advances over recent years, Polycarbonate and Copolyester can be made equally UV-resistant through the use of resin additives, surface coatings, and coextruded cap layers which can significantly extend the service life of the product.
- Due to the inherent permeability of plastics that allows gasses and water vapor to migrate through it, sealed insulating glass (SIG) configurations and protective solar (low-e) coatings are not possible with plastic glazing. However, the U-factor can be reduced through the use of multiple layers of plastic, which has the added advantage of blocking the transmission of unwanted noise. In addition, recent technological advances have led to the use of plastic formulations and coatings that serve to reduce solar heat gain by as much as 20 to 40 percent, while preserving visible light transmission properties. The energy balance trade-offs between heat loss through skylights and the supplanting of electrical energy for artificial lighting is well known.
- Plastic weighs less than glass in the same thickness. So, the addition of plastic skylights to roofs does not significantly increase the load placed on the building’s foundation or frame and are easy to handle during installation.
- Acrylic has a high tensile strength with impact resistance greater than that of annealed glass; it is thus less vulnerable to shattering due to heavy weights or sudden impacts. Under high impact, it typically won't shatter and, if it does break, it fractures into large, dull-edged pieces.
The Role of Standards and Testing
Laboratory performance tests, conducted per ASTM procedures referenced within the section, cover the key performance factors of:
- Weatherability (five years exposure in South Florida or per laboratory test method ASTM G155, Standard Practice for Operating Xenon Arc Light Apparatus for Exposure of Non-Metallic Materials
- Light transmittance, per ASTM D1003, Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics
- Impact strength, per ASTM D6110, Standard Test Method for Determining the Charpy Impact Resistance of Notched Specimens of Plastics
- Smoke requirements (tested per ASTM E84 or ASTM D2843, Standard Test Method for Surface Burning Characteristics of Building Materials)
- Self-ignition temperature requirements (tested per ASTM D1929, Standard Test Method for Determining Ignition Temperature of Plastics)
- Combustibility classification requirements (tested per ASTM D635, Standard Test Method for Rate of Burning and/or Extent and Time of Burning of Plastics in a Horizontal Position)