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Evaluating the performance of polymeric roofing materials with thermal analysis.



The choice of roofing materials is quite varied, ranging from asphalt-based or modified-asphalt (APP and SBS See Small Business Server. ) to polymer-based materials such as thermoplastic olefins (TPO (Twisted Pair Only) Refers to the use of twisted pair wire when other options are available. For example, a TPO suffix at the end of 3com Ethernet adapter model numbers indicates the card has only an RJ45 connector. ), polyvinyl chloride polyvinyl chloride (PVC), thermoplastic that is a polymer of vinyl chloride. Resins of polyvinyl chloride are hard, but with the addition of plasticizers a flexible, elastic plastic can be made.  (PVC PVC: see polyvinyl chloride.
PVC
 in full polyvinyl chloride

Synthetic resin, an organic polymer made by treating vinyl chloride monomers with a peroxide.
) and ethylene-propylenethene monomer (EPDM EPDM Ethylene-Propylene-Diene-Monomer
EPDM Enterprise Product Data Management
EPDM Ethylene Propylene Dimonomer (industrial/commercial piping/plumbing components)
EPDM Engineering Product Data Management
). This variety motivated the international roofing industry to shift towards using both engineering and chemical principles in solving roofing problems. In 1988, a joint CIB/RILEM international roofing committee was established to investigate the applications of thermal analysis in the characterization of roofing membranes (ref. 1).

Thermal analysis is not widely used in the roofing industry but is gaining popularity (refs. 2-21). Thermoanalytical techniques can be used to monitor a wide array of material characteristics. Some of the applications include enthalpy enthalpy (ĕn`thălpē), measure of the heat content of a chemical or physical system; it is a quantity derived from the heat and work relations studied in thermodynamics. , weight-loss, thermal stability, coefficient of thermal expansion coefficient of thermal expansion,
n See expansion, thermal coefficient.
 (CTE (Coefficient of Thermal Expansion) The difference between the way two materials expand when heat is applied. This is very critical when chips are mounted to printed circuit boards, because the silicon chip expands at a different rate than the plastic board. ) and the glass transition temperature The glass transition temperature is the temperature below which the physical properties of amorphous materials vary in a manner similar to those of a solid phase (glassy state), and above which amorphous materials behave like liquids (rubbery state).  (T[.sub.g]). Thus, these techniques can assist in selecting roofing materials for a particular application. For example, the T[.sub.g] is an important characteristic that should be considered for the cold temperature performance of roofing membranes. Below T[.sub.g] the material will be rigid and hard. Yet, above T[.sub.g] the material will be flexible. Generally, the strength of polymeric materials above the glass transition temperature is lower than the strength below T[.sub.g]. Other properties that vary with T[.sub.g] are thermal expansion coefficient, heat capacity and thelectric constant (refs. 22-26)

There are four main thermoanalytical techniques that are commonly used to determine and monitor the changes in a roofing membrane. They are thermogravimetry (TG), differential scanning calorimetry Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature.  (DSC (1) (Digital Signal Controller) A microcontroller and DSP combined on the same chip. It adds the interrupt-driven capabilities normally associated with a microcontroller to a DSP, which typically functions as a continuous process. See microcontroller and DSP. ), thermomechanical analysis (TMA TMA Turnaround Management Association
TMA Texas Medical Association
TMA Transportation Management Association
TMA Training and Management Assistance (a component of OHRD, which is a component of OWR)
TMA Tooling & Manufacturing Association
) and dynamic mechanical analysis (DMA (1) (Digital Media Adapter) See digital media hub.

(2) (Document Management Alliance) A specification that provides a common interface for accessing and searching document databases.
). In this article, a brief overview of these thermoanalytical techniques is presented. In addition, the data obtained by the various techniques on EPDM and PVC roofing membranes are presented.

TG

Thermogravimetry measures the change in mass of a material as a function of time at a determined temperature (i.e., isothermal i·so·ther·mal
adj.
Of, relating to, or indicating equal or constant temperatures.



isothermal, isothermic

having the same temperature.
 mode) or over a temperature range using a predetermined pre·de·ter·mine  
v. pre·de·ter·mined, pre·de·ter·min·ing, pre·de·ter·mines

v.tr.
1. To determine, decide, or establish in advance:
 heating rate. Essentially, a TG consists of a microbalance mi·cro·bal·ance  
n.
A balance designed to weigh very small loads, up to 0.1 gram.

Noun 1. microbalance - balance for weighing very small objects
balance - a scale for weighing; depends on pull of gravity
 surrounded by a furnace. A computer records any mass gains or losses. Thus, this technique is very useful in monitoring heat stability, and loss of components (e.g., oils, plasticizers plasticizers

mostly triaryl phosphates, such as tricresyl, triphenyl phosphates, which are poisonous. See also triorthocresyl phosphate.
 or polymers).

DSC

DSC is widely used in providing valuable information on chemical and physical properties of materials. The DSC technique measures the amount of energy (or heat) absorbed or released as the material is heated, cooled or held at an isothermal temperature. A DSC thermal curve shows the amount of heat evolved or absorbed as a function of temperature or time. This technique yields thermodynamic ther·mo·dy·nam·ic
adj.
1. Characteristic of or resulting from the conversion of heat into other forms of energy.

2. Of or relating to thermodynamics.
 data such as enthalpy, and specific heat, as well as kinetic data.

The shape and appearance of DSC curves can give a clue as to the type of transition taking place. Generally, first-order transitions such as melting give distinct peaks. These peaks can be integrated and a value for enthalpy (AH) can be determined. In the case of second order transitions such as the glass transition, a step-wise increase in heat capacity is observed. This is also detected in the DSC by a step change in baseline slope. In the case of heavily plasticized materials (e.g., roofing swnples) a broad transition is obtained and the step change is difficult to detect.

Calibration of a DSC instrument is important to obtain useful data. Normally, the melting points of pure standards are used. It is best to use a standard that will have a transition in the same temperature range as the samples being studied.

The glass-transition temperature may be determined by taking the middle of the change in baseline (half-height method). This method requires establishing a tangent line. This step is facilitated by using the first derivative of the heat-flow signal. T[.sub.g] values obtained by DSC are generally different from those obtained by dynamic techniques such as DMA since DSC is a static technique.

DMA (refs. 27-31)

The DMA technique measures the stressstrain relationship for a viscoelastic Adj. 1. viscoelastic - having viscous as well as elastic properties
natural philosophy, physics - the science of matter and energy and their interactions; "his favorite subject was physics"
 material. A real and imaginary component of modulus can be obtained by resolving the stress-strain components: [sigma] = [epsilon] E'sin ([omega]t+ [epsilon]o E"cos ([omega]t)

The storage modulus, E', is defined as E' = ([sigma]o/[epsilon]o) cos [delta] and is a measure of recoverable strain energy in a deformed body. The loss modulus, E", is associated with the dissipation of energy Same as Degradation of energy, under Degradation.

See also: Dissipation
 as heat due to the deformation of the material and is defined as E" = ([sigma]o/[epsilon]o) sin[delta]. The ratio E"/E' yields the loss tangent or damping factor (tan [delta]) which is the ratio of energy lost per cycle to the maximum energy stored and therefore recovered, per cycle.

A typical dynamic mechanical analysis curve shows either E', E" or tan[delta] plotted as a function of time or temperature. In general, the most intense peak observed for either E" or tan [delta] in conjunction with a relatively pronounced drop in E' corresponds to the glass transition. However, to prove that this relaxation event is a T[.sub.]g, a DMA multiplexing experiment to establish the activation energy would be required. The Tg may be affected by the crosslink density or degree of crystallinity and is directly related to the amorphous region within a polymer.

Care should be taken when reporting the glass-transition temperature obtained by DMA. The transition temperature determined by DMA (or other dynamic techniques) is not only heating-rate dependent but also frequency dependent. In addition to heating rate and frequency, the mechanical/rheological property E', E" or tan [delta]) used to determine the T[.sub.g] must also be specified. It has been found that the E" peak maximum at I Hz corresponded closely with the T[.sub.g] obtained from volume-temperature measurements (ref. 20).

TMA (refs. 28 and 32)

TMA, as defined by ASTM ASTM
abbr.
American Society for Testing and Materials
 E473-85, is a method for measuring the deformation of a material under a constant load as a function of temperature while the material is under a controlled temperature program. The measuring system consists of a linear voltage differential transformer (LVDT LVDT Linear Variable Differential Transformer
LVDT Linear Variable Displacement Transducer
LVDT Linear Variable Differential Transducer
LVDT Linear Voltage Differential Transformer
LVDT Low Voltage Differential Transceiver
LVDT Low Voltage Differential Transducer
) connected to the appropriate probe. Various probes are available and the measurements can be done in either compression, expansion, penetration, flexure flexure /flex·ure/ (flek´sher) a bend or fold; a curvation.

caudal flexure  the bend at the aboral end of the embryo.

cephalic flexure  the curve in the midbrain of the embryo.
 or in tension mode. It is this variety of probes which allows for the measurement on samples of different configurations. Any displacement of the probe generates a voltage which is then recorded. The dimensional change of a simple with an applied force is measured as a function of time or temperature. The plot of expansion (or contraction) vs. temperature (or time) can then be used to obtain T[.sub.g], the coefficient of thermal expansion (CTE), softening temperature and Young's modulus.

The change in linear dimension as a function of temperature can be described by the following:

[Mathematical Expression Omitted]

where [alpha][.sub.1] is the coefficient of linear expansion, and L[.sub.1] and L[.sub.2] are the lengths of the specimen at temperatures (or time) T[.sub.1] and T[.sub.2], respectively. If the difference between T[.sub.2] and T, is relatively small, then the equation can be represented by: L[.sub.2] - L[.sub.1] = L[.sub.1][alpha][.sub.1](T[.sub.2-T.sub.1]) or it can be rewritten as: [alpha][.sub.1] = (1/L[.sub.1])/([delta]L//[delta]T)

Therefore, the slope of the curve of length vs. temperature yields [alpha][.sub.1]L[.sub.1] and the coefficient of linear thermal expansion is obtained by dividing by L[.sub.1].

There are some drawbacks with thermomechanical analysis. Proper calibration is required to obtain reliable and reproducible data. Other sources of errors include slippage of the probe on the specimen and specimens undergoing creep in addition to length changes.

Experimental

The various experimental conditions are outlined in references 5-7, and 10, 11 and 13.

Results and discussion

Figures I and 2 contain the derivative weight loss (DTG DTG Date-Time Group
DTG Digital Television Group (UK trade association)
DTG Distance To Go
DTG Days To Go
DTG Digital Transmission Group
DTG Direct Trunk Group
DTG Digital Trunk Group
DTG Dance Theatre of the Gospel
) as a function of temperature of two EPDM roofing membranes. These EPDM membranes had been heat-aged at 100[degree]C for up to 28 days. It is quite apparent from these curves that one sample figure 1) is more stable than the other figure 2). The observed weight loss figure 2) between 200[degree]C and 400'C is due to loss of oils. This loss significantly affected the in-service performance of this membrane, e.g., severe shrinkage was observed.

A similar experiment was carried out on some PVC membranes. The results are shown in figures 3 and 4. As can be seen, the differences between the two are more subtle than in the case of the EPDM membranes. The sample shown in figure 4 has actually lost 4% of its original weight while the one in figure 3 has remained stable. This loss could be attributed to loss of plasticizer.

The results obtained by oscillating os·cil·late  
intr.v. os·cil·lat·ed, os·cil·lat·ing, os·cil·lates
1. To swing back and forth with a steady, uninterrupted rhythm.

2.
 DSC on some PVC and EPDM roofing membranes are shown in figures 5-8. It has been found that it is generally difficult to measure the T[.sub.g] using conventional DSC. However, this was not the case using oscillating DSC. The deconvoluted curves are very similar to what would be obtained by conventional DSC. The Cp curve allows for an easy separation of the T[.sub. g] component from the rest of the curve. Thus, one can more easily determine if the changes in the baseline can be attributed to the T[.sub.g].

DMA curves showing E" results are shown in figures 9 to 12. Once again, one can clearly discern between a thermally stable membrane and an unstable one. In the case of the EPDM, it is obvious that the peak maximum shown in figure 9 is not significantly changing, even after being exposed for 28 days at 100[degree]C. This is definitely not the case for the EPDM sample shown in figure 10. This sample has a T[.sub.g] which shifts from approximately -70[degree]C in the as received mode to -40[degree]C after 28 days at 100[degree]C. Similar observations are recorded for PVC samples. Figure 11 contains the E" curve for a PVC sample exposed to 100[degree]C for up to 28 days. As can be seen, the T[.sub.g] (approximately 35[degree]C) does not appear to change. The data for another PVC sample is shown in figure 12. There it is quite obvious that the T[.sub.g] has shifted significantly. Changes in T[.sub.g] can be attributed to various factors (loss of oils or plasticizers, crosslinking, etc.). Since below the Tg the membrane is stiff, DMA allows the membrane manufacturer/user to determine some low temperature behavior of these materials.

A typical TMA curve for an EPDM membrane is shown in figure 13. From such a curve, one can obtain the coefficient of thermal expansion (CTE) and determine the glass-transition temperature. A plot of CTE vs. heat-aging days figure 14) clearly demonstrates how stable one membrane is vs. the other.

Summary

Thermal analysis shows much promise in providing quick and reliable data regarding the stability of polymeric roofing membranes. It has been shown how the relative stability of PVC and EPDM membranes can be determined by TG, oscillating-dsc, DMA and TMA. Each technique provides information which is complimentary to the other. Furthermore, this data can be of assistance when trying to understand why a membrane is exhibiting peculiar behavior.

In the future, one would hope that these techniques be incorporated into the relevant membrane standards. In Canada, the next version of the PVC standard will be addressing this issue. Another aspect to be considered is the use of these techniques in service-life-prediction. Using a kinetic approach it may be possible to determine the approximate service-life of some of these materials. Work in this area, has recently been initiated at NRC NRC
abbr.
1. National Research Council

2. Nuclear Regulatory Commission

Noun 1. NRC - an independent federal agency created in 1974 to license and regulate nuclear power plants
 and data will hopefully be forthcoming in the near future.

References

[1.] "Performance testing of roofing membrane materials, " Recommendations of the Conseil International du Batiment pour la Recherche I'Etude et la Documentation (CIB CIB
abbr.
Latin cibus (food)
) W.83 and Reunion International des Laboratories d'Essai et de Recherche sur les Materiaux et les Constructions RILEM RILEM Reunion Internationale des Laboratoires et Experts des Materiaux, Systemes de Construction et Ouvrages (French: International Union of Laboratories and Experts in Construction Materials, Systems, and Structures) ) 75-SLR Joint Committee on Elastomeric, Thermoplastic and Modifted Bitumen bitumen (bĭty`mən) a generic term referring to flammable, brown or black mixtures of tarlike hydrocarbons, derived naturally or by distillation from petroleum.  Roofing, RILEM, Paris, France, November 1988. [2.] C G. Cash, Single ply roofing technology: ASTM STP STP or standard temperature and pressure, standard conditions for measurement of the properties of matter. The standard temperature is the freezing point of pure water, 0°C; or 273.15°K;.  790, pp. 55-64, (1982). [3.] D. Backenstow and P. Flueler, "Thermal analysis for characterization, " Proceedings, 9th Conference on Roofing Technology, National Roofing Contractors Association, Rosemont, IL, pp. 54-68, April 1987. [4.] G.D. Gaddy, WJ. Rossiter, Jr, R.K. Eby, Roofing research and standards development: ASTM STP 1088, pp. 3 7-52, 1990). [5.] O. Dutt, R.M. Paroli, N.R Mailvaganam and R.G. Turenne, "Glass-transitions in polymeric roofing membranes, " Proceedings, 1991 International Symposium on Roofing Technology, 495-501, 1991), F Kocich, editor [6.] R.M. Paroli, O. Dutt, Polymeric materials science and engineering Materials science and engineering

A multidisciplinary field concerned with the generation and application of knowledge relating to the composition, structure, and processing of materials to their properties and uses.
, 65, 362-363 1991). [7.] R.M. Paroli, O. Dutt, A.H. Delgado, M. Mech, Thermochimica Acta, 182, 303-317, 1991). [8.] G.D. Gaddy, W.J. Rossiter, Jr, R.K. Eby, "The application of thermal analysis to the characterization of EPDM roofing membrane materials after exposure in service, " Proceedings, 1991 International Symposium on Roofing Technology, 502-507, 1991, F Kocich, editor [9.] G.D. Gaddy, WJ. Rossiter, Jr and R.K. Eby, ASTM STP 1136, 168-175, 1991). [10.] R.M. Paroli, O. Dutt, A.H. Delgado and H.K. Stenman, Journal of Materials in Civil Engineering 5(l), 83-95,(1993). [11.] J.J. Penn and R.M. Paroli, "Evaluating the effects of aging on the thermal properties of EPDM roofing materials," Proceedings of the 21st North American North American

named after North America.


North American blastomycosis
see North American blastomycosis.

North American cattle tick
see boophilusannulatus.
 Thermal Analysis Society (NATAS NATAS National Academy of Television Arts & Sciences
NATAS North American Thermal Analysis Society
NATAS Nation Ahead of Time and Space (band) 
) Conference, Atlanta, Georgia, Sept. 13-18, 1992, pp. 612-617. [12.] Oba, and F. Bjork, Polymer Testing, 12, 35-56 (1993). [13.] M. Paroli, TL. Smith and B. Whelan, "Shattering of unreinforced PVC roof membranes: Problem phenomenon, causes and prevention," NRCA/NIST Tenth Conference on Roofing Technology, April 22-23, 1993, pp. 93-107. [14.] James J. Penn, Ralph M. Paroli, Thermochimica Acta, 226, 77-84 1993). [15.] Rodriguez, O. Dutt, R.M. Paroli and N.P. Mailvaganam, Materials and Structures, 26 (160), 355-361 1993). [16.] Oba, Flat roofs: Investigation of heat welding techniques for polymer-modifted bituminous bi·tu·mi·nous  
adj.
1. Like or containing bitumen.

2. Of or relating to bituminous coal.

Adj. 1. bituminous - resembling or containing bitumen; "bituminous coal"
 roofing membranes, " dissertation, Royal Institute of Technology, Sweden 1994). [17.] Oba, and M.N. Partl, Empa-forschungs- und Arbeitsbericht No. 13616: FE 147'135. Swiss Federal Laboratories for Materials Testing and Research, Switzerland (1994). [18.] Oba, and. A. Roller, Characterization of polymer-modified bituminous roofing membranes using thermal analysis, Materials and Structures, in press (1994). [19.] Ralph M. Paroli and James Penn, Assignment of the Glass Transition, ASTM STP 1249, R.J. Seyler, Ed., American Society for Testing and Materials, Philadelphia, 1994, pp. 269-276. [20.] Paroli, R.M., Dutt, O., Smith, T.L. and Whelan, B.J., Roofing research and standards development: Volume 3, ASTM STP 1224, Thomas J. Wallace and Walter J. Rossiter, Jr., Eds., American Society for Testing and Materials, Philadelphia, 1994, pp. 139-147. [21.] Ralph M. Paroli, O. Dutt, Ana H. Delgado and William Lei, Polymeric materials science and engineering, 72, 378-379 (1995). [22.] Feldman, D., Polymeric building materials, Elsevier. Science Publishers Ltd., New York, 1989. [23.] Billmeyer, F. W., Textbook of polymer science, 3rd Ed., John Wiley and Sons, New York, 1984. [24.] Young, R.J., Introduction to polymers, Chapman and Hall Chapman and Hall was a British publishing house, founded in the first half of the 19th century by Edward Chapman and William Hall. Upon Hall's death in 1847, Chapman's cousin Frederic Chapman became partner in the company, of which he became sole manager upon the retirement of , New York, 1981. [25.] Bikales, N.M., Mechanical properties of polymers, John Wiley and Sons, New York, 1971. [26.] Flory, P.J., Principles of polymer chemist , Cornell University Press, 1967. [27.] Murayama, T., Dynamic mechanical analysis of polymeric material, Elsevier Scientific Publishing Company, New York, 1978. [28.] Wendlandt, W. Wm., Chemical analysis 19: Thermal analysis, 3rd Edition, John Wiley and Sons, 1986. [29.] Campbell, D. and White, J.R., Porymer characterization: Physical techniques, Chapman and Hall, New York, 1989. [30.] Crompton, T.R., Analysis of polymers: An introduction, Pergamon Press, 1989. [31.] Skrovanek, D.J. and Schoff, CK, Progress in organic coatings, 16, 135-163 1988). [32.] Riga, A.T, Neag, M.N. (editors), Materials characterization by thermomechanical analysis, ASTM STPI STPI Software Technology Parks of India
STPI State-Trait Personality Inventory
STPI Stentor Telecom Policy, Inc.
STPI Software Test Process Improvement
STPI Services Transaction Program Interface (IBM) 
 136, 1991.
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Title Annotation:includes bibliography
Author:Delgado, Ana H.
Publication:Rubber World
Date:Jul 1, 1996
Words:2651
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