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FENESTRATION MAY/JUNE 1993
PVC Withstands test of Time and Temperature
Industry standards and testing provide assurance that properly designed vinyl windows can be used in warmer climates.
By Douglas L. Cole
As polyvinyl chloride windows and doors have gained more acceptance in the marketplace, the questions asked have gone beyond whether or not vinyl is clear enough to see through. Now that people realize PVC is the framing material, the questions concern what will happen to the material in the regions of the United States with warmer climates.
Will the PVC disintegrate and disappear in a week, a year, or a decade, and leave the glass suspended in the opening by itself? Will heat turn the framing into a soft, melted material unrecognizable as a window? Will the window's color change from white to yellow or brown to beige? If the PVC framing is hit with some object, will a chunk break Fig. I off, leaving a jagged gap where there once was a smooth surface?
Most of these concerns are material based and can be addressed in the formulation of the compound. Rigid PVC without additives is brittle and translucent, discolors under ultraviolet light, and sticks to metal when processing in an extruder at 360 to 400·F. Other concerns are in product design and function: how much surface is exposed, what loads wind, gravity, handling are expected, and how reinforcing can help ensure long term durability. Homeowners want to know how they can be assured of quality windows and especially PVC windows. They might ask what industry standards exist and what they mean. Starting with the basic material, the American Society for Testing and Materials gives the standard specification for rigid PVC and related plastic building product compounds. The following requirements fall under ASTM D4216:
O Base resin.
O Impact strength.
O Tensile strength.
O Tensile modulus.
O Deflection temperature under load.
O Coefficient of linear expansion.
O Weatherability color change.
O Weatherability impact retention.
Moving to the extruding process, the standard specification for white PVC exterior profile extrusion used for assembled windows and doors is ASTM D4726. Included in this specification are:
O Material class from ASTM D4216.
O Impact resistance.
O Weathering maxi mum color change; mini mum impact retention.
O Dimensional stability maximum shrink age.
With these two specifications met, customers can be assured the window or door has a good start. Then, how the product will perform as a finished unit is covered under one of two specifications: AAMA 101 V86 voluntary specifications for PVC prime windows and sliding glass doors, established by the American Architectural Manufacturers Association; or ASTM D4099 the standard specification for PVC prime windows and sliding glass doors. Areas of testing include:
O Air infiltration.
O Water resistance.
O Wind loads.
O Operating forces.
O Deglazing.
Test methods for forced entry are covered under ASTM F588 Window Assemblies and ASTM F842 Sliding Door Assemblies. If a window is made and classified under all these requirements, it will start out as an excellent product. But will it last? That depends on how well it was designed for the area in which it will be used.
Design for durability
The window design must take into account the product function and the environment in which it will be stored, transported, and eventually installed. Although storage and transportation environments can be controlled to some extent, if required, the installation environment cannot. So a number of considerations are involved in the compounding and extrusion design.
O Hot direct sun. The UV or IR (infrared) wavelength of sunlight is about the only condition that affects PVC. Other elements, such as moisture or salt air, have no effect on most PVC windows. UV affects the impact strength and color retention of the window back to ASTM D42 16 and D4726 but most of the degradation is just on the extrusion surface, affecting appearance only. UV caused degradation can be controlled with the right compounding and extrusion process. Coextruded exterior surfaces and exterior laminated films or other resin blends also can improve the weathering characteristics, whi le still keeping within the definition of a PVC extrusion. The more critical factor is heat build up from IR that can cause distortion of the framing materials. But design considerations, reinforcing, or using materials with higher heat distortion resistance numbers can help prevent this problem. White and pastel colored extrusions generally will perform in all U.S. climates, while darker colors may absorb more heat. ASTM _ 4803, the test method for predicting heat build up in PVC building products, can predict heat build up on different colors and materials. Compounding to keep this build up as low as possible yields very good performance in hot climates.
O Coefficient of expansion. How can you make a window from a material with expan sion two or three times that of aluminum? I tried to measure this expansion by using a glazed window covered with dry ice mini mal change. Then I heated up the window to 160·F still minimal change. These tem perature changes were applied to the exterior surface and maintained for four to si x hours, so what happened to the expansion? I found that although the outside surface temperature was changing, the inside surface seemed to stay at room temperature, 70·F (Fig. I ). Remember, PVC is a good insulator. This part was in equilibrium. Thus, expansion is no more of a problem on PVC hollow framing sections than on aluminum.
O Gravity. The ability of the window framing material to carry the glass loads must be developed in the design stage. The loads must be transferred from the glass to the framing material and eventually to the building (Fig. 2). If all these loads are accounted for and handled by section design and/or reinforcing, the finished product will remain serviceable for many years.
O Gas pressure. This is not about gas between the lites of the insulating glass unit, but the air in the hollow chambers of the extrusions. Windows that have fusion welded corners present a possible problem in hot climates. As the exterior surface temperature increases, the trapped air in the hollow chambers increases in pressure. If the material around this chamber cannot sustain this pressure, then the extrusion will distort. I call this ballooning (Fig. 3). This distortion usually is visible on the sill section, meeting rails, or west facing jambs, where there is too much direct sunlight. Gas pressure problems can be prevented by having thick enough walls around the chambers or by venting the hollow chambers.
O Hardware. The attachment of locks, keepers, pulls, pivot bars, balances, rollers, roto operators, and other hardware must be considered in the design of PVC windows. What is the service life required for each? If they are screwed into the PVC and the loads on the screw are minimal only holding the part in place then one wall is sufficient. If the require ments are greater, then two walls are used; and if heavy, long term use is expected, then two walls and metal reinforcing, or metal reinforcing only, are required (Fig. 4).
O Hollow extrusions. One of the fundamental principles used in PVC window extrusion design is that hollow extru sions are advantageous. In fact, hollow sections are required for high performing PVC windows. Hollow extrusions solve long time window design problems of torque, water drainage, and appearance no more unsightly pockets and voids visible to the public (Fig. 5). And the smooth covered surfaces can be cleaned easily. The extrusion process is much more difficult for hollow sections than for solid ones but that's another department's problem.
Long term life
Using the right material and the right design doesn't prove the window will last even a year. But tests and studies do. ASTM and the Society of Plastic Industry are developing a new procedure for environmental testing of finished products: ASTM Draft E06.51, Revision No. 15 Practice to determine the effects of temperature cycling plus IR radiation on fenestration systems. The procedure exposes the window to room temperature on the inside surface and varies the exterior air temperature and IR surface temperature. This cycle test can be run from hot to cold for any specified time. To test the effect of warm climates, I have run this kind of test at the warmer end of the cycle and allowed the high temperature (ambient and IR) to stabilize for four to six hours and a number of cycles. This is an expensive test, but it does correlate closely to outside conditions. The IR light source is perpendicular to the window and heats areas the sun may never see, but the test is still informative. Another way to find out about long term results is to install a PVC window in the hottest area and come back a year or more later to see if it has changed. An interesting study along these lines is being conducted by three organizations: SPI, AAMA, and the Dade County, FL, Building Department. The groups have been inspecting PVC windows installed in Florida in 1975 to see how they currently perform if they are still there. The windows were located and inspected in March of this year. The results showed the glass was still surrounded by PVC framing, the color white was still white, and the windows still operated, though the hardware and replaceable items needed maintenance. Can PVC windows be made for war'm climates? Yes. The following independent test results confirm the right design works in warm climates:
O Material ASTM D4216.
O Extruding ASTM D4726.
O Structural AAMA 101 V 86 or ASTM D4099.
O Forced Entry ASTM F588 and ASTM F842.
O Heat Build up ASTM D4803.
O Environmental Testing ASTM Draft E06.51.
The real question about PVC windows is not will they last, but how will we get rid of them 50 years from now? Just kidding the answer is, of course, recycling.
Douglas L. Cole is manager/research & developmentf orMikron Industries, Inc., the extruder of vinyl window and door profiles based in Kent, WA. He has heen in the fenestration industry for 30 years and currently serves as chairman of the code & regulatory committee of the Vinyl Window & Door Institute, a division of the Society of the Plastics Industry.
FENESTRATION MAY/JUNE 1993 Vol 6, No. 5
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