Architecture：Roofing Materials.pdf

611 611 1.0INTRODUCTION The use of polymers as a roofing material is continuously increas- ing. The function of a roof is to protect the building from environmental factors such as light, wind, rain, snow loads, temperature changes, hail, and storms. Therefore, the material used on a roof must withstand those factors for many years. The performance of a material or system depends on the environ- ment and the degrading effects to which it is exposed. Until the early 1990s, methods of measuring the properties and predicting the durability of products were not well known, hence, much of the knowledge about building materials was based on experience from long-time use. An estimate of 90% of all flat roofs in Canada used in industrial, commercial, and public buildings are protected by bituminous roofing, and essentially all sloping residential roofs are covered with shingles having a bituminous-felt base. In recent years, changes in building practices have produced roofs of unusual design for which these materials were not suitable. This led to the development of new rubber and plastic roofing material, which, in turn, led to the development of rubber- and plastic- modified bitumen and bitumen-modified rubbers and plastics.1 15 Roofing Materials 612Chapter 15 - Roofing Materials In 1993 and 2000, the commercial low-slope roofing market in the USA consisted of: The NRCAs 20002001 Annual Market Survey3 reported that in 2000, the low-slope roofing market accounted for 64.1% of the total roofing market, a slight decrease from 68.7% in 1999. The roofing contractors predicted that the low-slope roofing market would be 63.3% in the year 2001. Most types of roofing materials are bituminous and synthetic (polymeric) roofing membranes. The most commonly used roofing and waterproofing membrane is made by combining asphalt or coal tar pitch (bitumen) with felts or mats, or fabrics of organic or inorganic fibers. 2.0BITUMINOUS ROOFING MATERIAL The built-up roof membrane consists of bitumen, reinforced with roofing felts, and aggregates which protect the bitumen from the UV radiation and oxidation. Bituminous materials have been used since 3500 BC. Because of their waterproofing, preservative, and binder characteris- tics, they were utilized by the ancients for the construction of houses and roads.4 Bituminous materials were also used by ancient civilizations such as Egyptians for construction, mummification, waterproofing, preserva- tives, and binders. Type1993220003 NewNew constructionRe-roofing construction Re-roofing Built-up roofing29.0%31.0%22.4%27.8% Single ply roofing38.8%33.3%38.1%33.6% Modified bitumen17.0%21.7%19.1%23.8% Metal3.5%1.6%4.3%2.5% Other types*11.7%12.4%16.1%12.3% * Other types = tiles, PUF, liquid-applied, asphalt shingles, metal 613 The asphalt is a complex mixture of organic and inorganic com- pounds and their complexes. Some of the organic compounds are aliphatics, aromatics, polar aromatics, and asphaltenes. Polar aromatics are respon- sible for the viscoelastic properties of asphalts.5 Over the years, multilayers of tar-based waterproofers replaced the hot asphalt used in roofing. In early 1900, asphalt became available from petroleum refining, and it was followed by oxidized bitumen interlaid with roofing felt and then alternated with a mineral base sheet.4 Asphalt-based materials are used extensively as binders, sealants, and waterproof coatings in diverse applications because of their low cost, inherent cohesive nature, weather-resistant properties, and ease of processing in the molten state.6 Despite its natural viscoelastic properties, asphalt cannot be used as such in roofing applications because of its inherent limitations, such as brittleness at low temperature and flow properties at high temperature. Therefore, studies have been done to improve the properties of bitumen. Combining bitumen with natural or synthetic rubbers or lattices, new materials with higher elasticity, low temperature flexibility, higher strength, and better fatigue resistance can be obtained. Polymer-modified bituminous membranes were developed in Eu- rope in the mid-1960s and have been in use in North America since 1975. The polymeric systems have varied from natural rubber to more complex synthetic systems such as block copolymers and polymer blends. Most common polymers used as modifiers are polyisobutylenes, polybutadienes, polyisoprenes, styrene-butiene-monomer, styrene-butadiene-rubber, butyl rubber, ethylene-vinylacetate (EVA), atactic polypropylene (APP) as well as natural rubber. Polymers, such as atactic polypropylene or styrene- butadiene-styrene (SBS), impart flexibility and elasticity, improve cohe- sive strength, resist flow at high temperatures, and toughness.7 They are the most widely used modifiers of bitumen-based roofing materials. 3.0SYNTHETIC ROOFING MEMBRANES Polymers such as poly(vinyl chloride) (PVC), ethylene-propylene- diene monomer (EPDM), chlorosulfonated polyethylene, ketone ethylene ester (KEE), reinforced polyurethane, butyl rubbers, and polychloroprene (neoprene) have proven to be suitable for roofing membranes.4 In the last ten years, a new synthetic roofing material (thermoplastic polyolefins) Section 3.0 - Synthetic Roofing Membranes 614Chapter 15 - Roofing Materials (TPO) has entered the market. Polymeric membranes have advantages and disadvantages.8 Advantages: Ultra-lightweight (for unballasted, adhered membranes) Adaptable to irregular roof surfaces Isotropic physical behavior Superior architectural quality (color) White membranes have general superior heat reflectivity (for cooling-energy conservation) Better elongation (up to 800% at 21°C, 70°F) Superior performance at sub-zero temperatures flexibil- ity down to at least -40°C, (-40°F) for some materials Easier application Fewer weather restrictions on application Less hazard from moisture entrapment during installation Easier repair of punctures, splits, tears Easier flashing applications at corners and irregular sur- faces where stiffer built-up roof materials are difficult to form Possible greater reliance on factory-manufactured mate- rial quality Disadvantages: Greater requirement for good workmanship More limited range of suitable substrates Less puncture resistance. Hence, greater vulnerability to traffic damage Lack of performance and design criteria comparable to those available for built-up roofs 615 4.0APPLICATIONS Regardless of the chemical composition of the materials, in a number of applications, such as in roofing, the materials are exposed to a wide range of temperature, wind, and load conditions. For example, in northern climates, the temperature of an asphalt or black roof membrane could be as high as 100°C on a hot summer day and as cold as -40°C in winter. Moreover, the material has to sustain the stresses generated by thermal expansion and contraction. Therefore, a combination of high- temperature and low temperature performance is required from end-use consideration. Critical properties such as high softening points, good tracking and flow resistance at elevated temperatures (6070°C for modi- fied bitumens), good impact and crack resistance, good cohesive strength and high elastic modulus are required.6 Exposure of the material to temperature, solar radiation, water, wind, environmental pollution, and stabilizer failure, in addition to roof traffic, poor workmanship, and lack of maintenance will induce either physical or chemical changes or both that affect the properties of the membrane. Therefore, proper evaluation of the membrane is required to determine the type of change. Polymeric roofing membranes are evaluated using various test methods developed for assessment of durability. Mechanical properties of polymeric materials have two facets: one is related to the macroscopic behavior and the other to the molecular behavior.9 Engineers are con- cerned only with the description of the mechanical behavior (physical properties) under the design conditions. Evaluation of the mechanical properties of roofing membranes (tensile properties at different tempera- tures, load at break, elongation at break, and energy to break) provides information about the material structural failures and how it can be improved, but does not offer an explanation. If the failure is related to molecular activity, additional information is necessary to comprehend the problem fully. Current laboratory procedures used to evaluate the durability of roofing membranes utilize artificial weathering devices, which attempt to simulate the primary weathering agents, namely: solar radiation, tempera- ture, ozone, and moisture. After the use of the artificial weathering device, the physical properties are usually compared with an unaged or “original” sample with the results stating “Retains x% of the original physical properties after aging.”10 Section 4.0 - Applications 616Chapter 15 - Roofing Materials Differential scanning calorimetry (DSC), TG, DTA, TMA, and DMA have proven to be useful in the characterization of materials. DSC provides information on the glass transition temperature, vulcanization reaction, and oxidative stability. Thermogravimetry (TG) is applied for the quantitative analysis of the material components. The changes in sample dimensions as a function of time or temperature under a nonoscillatory load are measured by TMA, whereas DMA or DLMTA measures the rheological properties. These methods can also provide information about the thermal stability of polymers, their lifetime or shelf life under particular conditions, phase changes in the polymer, glass transition, the influence of additives, kinetics, oxidation stability, and many others.1120 Thermoanalytical techniques bridge the gap between the traditional engineering evaluation and chemistry. Thermoanalytical methods have long been used to charac- terize construction materials,2128 but they are not widely used to charac- terize roof membrane materials. In 1988, an international roofing committee, working under the auspices of CIB/RILEM (Conseil International du Bâtiment pour la Recher- che, lÉtude et la Documentation/Réunion Internationale des Laboratoires dEssais et de Recherches sur les Matériaux et les Constructions),2930 recommended that thermoanalytical methods be added to the inventory of test methods currently used to characterize roof membrane materials. Since little research had been reported on the application of thermal analysis (TA) methods to roofing, the committee recommended that more research be carried out to provide the technical basis for this application. The recommendation was based on research by Farlling31 and Backenstow and Flueler.32 These authors used TG, DSC, and DMA to characterize EPDM, PVC, and polymer-modified materials. Backenstow and Flueler reported the application of torsion pendulum analysis to characterize the above membrane materials. They concluded that TA techniques were useful for membrane characterization and should be investigated as meth- ods for incorporation into standards. Previous work published by Cash33 on the use of DSC to characterize neoprene, chlorinated polyethylene (CPE) and PVC had shown that DSC could be used to identify not only the components in a single-ply sheet and the manufacturer, but also to differ- entiate between new and exposed materials. In 1990, Gaddy, et al.,34 conducted a study to provide data on the feasibility of using thermoanalytical methods to characterize roofing mem- brane materials. The authors used TG, DSC, and DMA to analyze white and black EPDM before and after laboratory exposure to heat, ozone, UV, and outdoor exposure. The results were compared to changes in load-elongation 617 measured as per ASTM D 412 for rubber sheet materials. The TG, DSC, and DMA results showed only slight changes in the white and black membrane materials after exposure. Such changes (e.g., Tg of ± 10°C) were close to the limits suggested by the CIB/RILEM35 Committee. On the other hand, the percent elongation values displayed relatively large changes compared to changes obtained by TA techniques. In addition, the study showed that the DSC method was less practicable than other methods such as TMA because of analysis time. Based on the results of the study, Gaddy, et al., concluded that the TA methods were more appropriate for determining changes in bulk properties induced during exposure than the elongation measurement because the latter is, to a great extent, influenced by surface characteristics. They also concluded that further research was necessary to integrate TA methods into ASTM standards for EPDM roofing membrane materials. Since previous research3134 had shown that TA methods can detect changes in the properties of roofing membrane materials, Gaddy, et al.,36 investigated the applicability of thermomechanical analysis (TMA) for the characterization of white and black EPDM roofing membrane materials before and after laboratory (heat, ozone, and UV) and outdoor exposure and compared the results with those from load-elongation. The TMA results were similar for the black and white EPDM. The maximum change in glass transition (Tg), as measured by TMA was 12°C. The authors reported that the UV and ozone exposures generally produced greater changes than heat. In some cases, the black material showed slightly greater Tg changes than the white when exposed to UV conditions which was unexpected since it had been previously reported that the titanium dioxide and colored pigments used for white EPDM gave much less UV protection than carbon black. The results of the EPDM study showed that, in general, the changes observed were relatively small and representative of the commercially available white EPDM roofing sheets. Also, it was found that the TMA procedure was readily applicable to the analysis of EPDM roofing materi- als, more practicable, used small specimens, and was reproducible. Accord- ing to the authors, the use of small specimens makes the TMA technique very attractive for the analysis of in-service membrane since small speci- mens may be taken from roofs with minimal disruption to the waterproofing integrity of the membrane. From the results of the study, it was concluded that TMA can be applied to the characterization of EPDM membrane materials and yield results using small specimens: Section 4.0 - Applications 618Chapter 15 - Roofing Materials a) The changes in glass transition (Tg) measured by TMA for the exposed specimens from different manufacturers al- lowed identification of the various products b) Changes in Tg after exposure of 14°C or less, were considered to be minor (compared to ±8°C recommended by CIB/RILEM) c) No differences between the black and white EPDM were observed for the same exposure conditions d) The changes in the Tg of the EPDM obtained from TMA were not as large as those in the percent elongation fo