1 Aging and Hail Research of PVC Membranes Frank J. Foley, Jim ...

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From the 87 membrane samples, three analyses were made:1. Solvent extraction of plasticizer content - hidden flap and exposed portionof the same membrane.2. Micrometer measurement of membrane thickness - hidden flap andexposed portion of the same membrane.3. Hail simulation testing of membrane panels.3.0 TEST PROCEDURESSolvent Extraction of Plasticizer ContentTo explore the relative rates of plasticizer loss among these four major manufacturers,plasticizer was solvent-extracted from both the hidden flap and from the exposedportion of the membrane. Solvent extraction follows ASTM D 3421 protocol. Theunderlying concept of solvent extraction is that a bath of aggressive solvent easilyseparates the liquid plasticizer from the polymer resin. The ASTM protocol defines thatthe average extraction is plus or minus 3.3% of a complete extraction of liquidplasticizer from the PVC membrane.Prior to extraction, the sample is weighed and then placed into a bath of solvent for adefined period of time where the plasticizer is drawn out of the sample. The bathedsample is allowed to air dry and is weighed a second time. At this point, the plasticizeritself is captured as a dilution into the liquid solvent. The reduction in weight of the postbathsample represents the amount of removed liquid plasticizer. (For additionalchemical analysis of the type of plasticizer, the solvent may be evaporated, leaving theplasticizer as a residue).By extracting plasticizer from both the hidden flap and the exposed portion of the samemembrane panel, measurements of plasticizer content in both the exposed and hiddenportions of the membrane can be made. The differences between these two contentsreflect the amount of plasticizer that migrated from the membrane as a result ofexposed weathering. Two qualifications in this analysis are highlighted:1. Solvent extraction is only made from the top layer of the membrane.PVC roofing membranes are usually a composite of films above andbelow the fabric reinforcement. Mechanical separation was made of thefilm above the reinforcement and only the top film were analyzed forsolvent extraction. Since the top film withstands direct sunlight andweathering, analysis only of the top film dramatically reveals theweathering effect of exposure.10
Underlying logic would support that the content and quality of plasticizerbelow the reinforcement serves as a reservoir of replacement forplasticizer, which degrades and departs the membrane from the topsurface (a sort of “balanced equilibrium”). Within this logic, however, theweave structure of the fabric reinforcement and the presence of a coatingaround the reinforcement potentially promote or limit the amount ofmigration from bottom film to top film. Regardless of logic, if the layerabove the fabric surfacing loses significant plasticizer, then the top surfacebecomes embrittled and loses impact resistance.2. The hidden flap undergoes some small degree of aging.Unlike the weathered portion of a cross-seam sample, the portion ofmembrane hidden under the flap looks new and feels supple. However,because the hidden flap is exposed to ambient temperature and humidity,the content of the plasticizer measured in the hidden flap is not exactly thesame content of plasticizer existing in the membrane when the productwas first shipped. This analysis does not incorporate an estimate of thissmall rate of hidden flap aging.Micrometer Measurement of Membrane ThicknessAs conventional PVC membranes are exposed and aged, a change in thickness occurs.As an additional analysis, micrometer measurements of the cross-seam samples weremade for both the hidden flap and the exposed portion. Measurements were made infive locations on each side (hidden and exposed) of each cross-seam sample and theaverage of the five measurements are reported. Metric data is offered simply as aconversion.Hail Simulation Testing of Membrane PanelsJim D. Koontz & Associates, Inc. used the larger 3-foot by 3-foot (.9 m by .9 m) sectionof each exposed membrane for hail simulation testing. The following conditions weredefined for this simulated hail testing.Simulation of Hailstone ProjectileSpheres of frozen water were used as projectiles for impact testing against the largemembrane samples. The ice sphere test method was selected since laboratory cast icespheres closely correlate with hail. Prior studies have shown that approximately 75-percent of large-sized hail is spherical or nearly spherical in shape. 6 Hail density rangeshave been reported between 0.02 lb/inch 3 and 0.03 lb/inch 3 (0.7 and 0.91 gm/cm 3 ) thelatter value being the density of pure ice. The ice spheres used in this research werecast pure ice (Photo 2).11
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