ctu-n30_eng.pdf

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C o n s t r u c t i o n T e c h n o l o g y U p d a t e N o . 3 0

A typical low-slope roofing systemconsists of three components: a struc-tural deck, a thermal insulation barrierand a waterproofing membrane, whichconsists of reinforcing fibres or fabricsandwiched between two sheets of flexible matrix. The matrix materialis either asphalt- or polymer-based(Figure 1). In “single-ply” membranes,the matrix is made of flexible polymer.

The reinforcement provides dimen-sional stability for the membrane aswell as strength to resist stresses inservice. It is generally made of chopped short glass fibre strandspacked in a mat, or continuous poly-ester fibre arranged in a grid or in anon-woven mat (Figure 2). There are essentially two classes of polymer-

based roofing membranes: thermosets (TS)and thermoplastics (TP) (see Text Box A).

Thermoplastics soften when heated (thisprocess is reversible) but thermosets do not.

Thermosets include the commonly usedethylene propylene diene monomer [alsoknown as ethylene propylene diene terpoly-mer] (EPDM), while thermoplastics encom-pass a wider variety of roofing membranes,including thermoplastic polyolefins (TPOs).

All thermoplastic roofing membranesshare certain characteristics, e.g., seaming

can be done by heat welding. However, forthe most part, they have very differentchemical, physical and mechanical proper-ties. It is impossible to explain these differ-ences here, but there are different ASTMmaterial-based standards for various TP

Thermoplastic PolyolefinRoofing Membranes

Low-slope Roofing Membranes

Asphalt-based

Built-up RoofBUR

Modified Bitumen(Mod. Bit.)

Thermosets(TS)

Thermoplastics(TP)

SBSStyrene-Butadiene-Styrene

(Elastomer)

APPAtactic Polypropylene

(Plastomer)

EPDM(Ethylene-Propylene-Diene

Monomer or Ethylene-Propylene-Diene Terpolymer)

PVCPoly (Vinyl Chloride)

KEE(Ketone Ethylene

Ester)

TPO(Thermoplastic Polyolefin

or Flexible Polyolefin)

CPE(Chlorinated

Polyethylene)

CR(Polychloroprene)

Polymer-based

by Ralph M . Par ol i , Kar en K.Y. Liu an d Terr ance R. Simmons

Thermoplastic polyolefins are a “new generation” of single-ply roofing systems.This Update reviews their characteristics and performance to date and clarifiessome misunderstandings related to terminology.

Figure 1. Categories of low-slope roofing membranes

Figure 2. Reinforcement commonly used in roofingmembranes: (a) random short glass fibre mat; (b) polyester scrim; (c) non-woven polyester mat

A B C



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products. To avoid confusion, one shouldnot just label all of these products simplyas TP products. TPOs differ from other TPsin several respects, and thus must behandled and applied differently.

Definition of TPO Defining “thermoplastic polyolefin” is diffi -cult. “Thermoplastic” is a generic term inpolymer science; it encompasses a class of

polymers that, as mentioned above, softenwhen heated in a reversible process. Theterm “olefin” is even more generic, beingan old chemical name for any moleculecontaining carbon-carbon double bonds (themodern name for this family of moleculesis alkenes). Any polymer formed by chemi-cally linking up many olefin molecules istermed “polyolefin.”

According to the latest draft ASTMstandard for TPOs, the composition is very

non-specific. The standard states that TPOsmust contain more than 95% by mass of

TPO polymer. The polymer itself is notdefined within the standard, which statesonly that the sheet shall contain the

“appropriate” polymers. Because of thisloose definition, there is an endless list of chemicals that would fall under this standard(e.g., polyethylene, polypropylene, andisobutylene, as well as their derivatives).Ideally, manufacturers would specify the exactpolymer in terms of marketing and labelling.

There are a few published papers thatattempt to explain the different types of

TPOs [1-4]. One point is clear, however:unlike plasticized thermoplastic membranes,

TPOs do not contain plasticizers (smallmolecules added during compounding toincrease the flexibility of the product).

Therefore, the problem of plasticizer lossassociated with some plasticized membranesis eliminated.

Confusion in the M arke t Plac e TPO roofing membranes have been in ser-vice in Europe for approximately ten years.

The first appearance of a “TPO-type” roof-ing product in the United States wasaround 1987. As yet, little is known abouttheir durability.

The confusion associated with TPOscomes from both the chemical terminologyand the product marketing. The latter has

promoted mainly the EPDM-like character-istics of TPOs, i.e., it being like rubber withthe added benefits of welded seams (whichEPDMs do not have). Also promoted hasbeen the chemical resistance attributed tothe olefin component of the polymers.Unfortunately, some confusion hasoccurred, especially regarding the use of the term thermoplastic (TP). It is importantto remember that TPOs are thermoplastic(i.e., TPs) but only some TPs are TPOs.

Benefits of TPOs In general, TPO membranes are being

marketed as a product that combines theproperties of EPDM and PVC, without theassociated drawbacks that the latter twomaterials have. In other words, they aresupposed to be as UV-resistant and asheat-resistant as EPDM, and as heat-weldableas PVC.

The following benefits and characteris-tics have been reported for TPOs [1-3]:• environmentally friendly and recyclable• seams can be heat welded

Text Box A

Thermoplastics (TP) vs. Thermosets (TS)

A polymer is a long-chain molecule consisting of many(poly) small repeating units (mer) (~103 – 106) joined end toend. These molecules are tangled in a random manneranalogous to cooked spaghetti.

Polymers can be classified into thermoplastics (TPs) and

thermosets (TSs) depending on their mechanical behaviouron heating and cooling. TPs comprise long-chain mole-cules held together by weak bonds (Figure a). When heat isapplied, the molecules “slide past” one another and thepolymer softens. On cooling, the molecules cannot slidepast each other easily and the polymer hardens. TS long-chain molecules, however, are linked together by smallmolecules via strong chemical bonds, a process sometimesreferred to as vulcanization (Figure b). This three-dimen-sional network is so rigid that the molecules cannot movevery much even when the polymer is heated. Thus, TSs donot soften when heated.

Because of these differences, TP and TS membranes arebonded differently when applied. TPs can be bonded usingheat welding: the hot air melts the polymer at the seam andthe two strips of membrane become fused. TS membranesare usually bonded using adhesives or tapes.

Schematic molecular configurations of (a) a thermoplastic and (b) a thermoset



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• available in many colours

• resistant to heat and UV degradation• resistant to many chemicals• good cold-temperature flexibility• no external plasticizers added.

Performance of TPOs TPO membranes are lighter in weight andeasier to handle than the other thermoplasticmembranes. But, while flexible, they havea rather rigid feel: they tend to hold theirshape, and do not relax quickly. Contractorsappear to be adjusting slowly to them andsome say they are not “contractor-friendly.”

Their comments about TPOs include the

following:• Hot-air-welded seams are easy and clean;• Costs are lower than for other hot-air-

welded membranes;• Mechanically fastened systems (as opposed

to loose laid or fully adhered systems)work well in recover applications withoutadding extra load;

• Non-reinforced flashing membranes areeasy to form for detailing;

• Noticeable changes in colour and textureoccur over time;

• Membranes respond dramatically(expansion and contraction) to tempera-

ture changes;• Cold welds (i.e., welds at temperatures

that are not hot enough) occur fre-quently; the start and stop positions of the robotic welder are especially critical,as are the positions of t-seams;

• Narrow welding window exists betweencold welds and scorch/burn-through (i.e.,welds at temperatures that are too hot);

• Failure of substrate bonding adhesive iscommon (i.e., not sticking to membrane);

• Membranes sometimesrequire solvent wipe (to cleanor prime) before welding;

• Re-welding membranes (inrepair) is problematic afterexposure to sun;

• Black membranes are moredifficult to weld than whiteones.

Current Research Results IRC, in collaboration withBenchmark Inc., a U.S. roofingconsulting firm, initiated afive-year project in 1997 tostudy the long-term perfor-mance of TPO membranes.

Thus far it has been found that the

membranes reinforced with continuouspolyester fibre scrim had significantlyhigher tensile breaking strength (see TextBox B) than those reinforced with randomshort glass fibre mat. With the polyesterscrim (continuous fibre bundles arranged ina grid), the fibres bore the majority of theapplied stress until failure, resulting in highbreaking strength. In the case of the glassfibre mat reinforcement (chopped fibrestrands packed randomly into a mat), thefibre strands flowed and aligned themselveswith the polymer in the direction of theapplied stress, resulting in significantlylower breaking strength — i.e., the strandsmoved with the membrane rather thanstretching and eventually snapping as is thecase with the polyester.

The breaking strength of the polyester scrimmembranes was governed by the number of fibre tows (bundles) across the width of thespecimen. The strength of the fibre towswas the same for all samples, independentof the surface coating. Therefore the tighterthe weave of the reinforcement, the higherthe breaking strength of the membrane.

The difference in weave style had an effecton how a membrane failed as well. Onesample was reinforced with an uncoatedpolyester scrim where the intersecting towswere held together by fine, continuouspolyester fibres. When the membrane waspulled in the machine direction (parallel tothe length of the roll of the membrane), thefine fibres stretched and tightly held ontothe tows in the cross direction (perpendicu-lar to the machine direction), creating highlocalized stresses at the intersections.

Text Box B

Tensile Testing A good roofing mem-brane has to be strongenough to withstandstresses, and flexibleenough to accommodatedeck movement. These

properties are repre-sented by breakingstrength and elongation,which can be measuredby a tensile testingmachine, which pulls arectangular specimen ata constant rate. Themachine plots the reac-tion as a stress-elongation curve. The maximum stress borne by the spec-imen is called the breaking strength while the elongation at which themaximum stress is achieved is called the elongation at break.

Typical tensile stress-elongation curves for a polyester- reinforced TPO membrane at -40°C and at 23°C

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