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Thermoplastic polyolefin roofing membranes
Paroli, R. M.; Liu, K. K. Y.; Simmons, T. R.
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https://nrc-publications.canada.ca/eng/view/object/?id=9cb06397-b2d5-4311-be46-6c02cb7bbd38 https://publications-cnrc.canada.ca/fra/voir/objet/?id=9cb06397-b2d5-4311-be46-6c02cb7bbd38C 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 typ ical low -slop e roofin g system con sists of th ree com p on en ts: a stru c-tu ral d eck, a th erm al in su lation barrier an d a w aterp roofin g m em bran e, w h ich con sists of rein forcin g fibres or fabric san d w ich ed betw een tw o sh eets of flexible m atrix. Th e m atrix m aterial is eith er asp h alt- or p olym er-based (Figu re 1). In “sin gle-p ly” m em bran es, th e m atrix is m ad e of flexible p olym er. Th e rein forcem en t p rovid es d im en -sion al stability for th e m em bran e as w ell as stren gth to resist stresses in service. It is gen erally m ad e of ch op p ed sh ort glass fibre stran d s p acked in a m at, or con tin u ou s p oly-ester fibre arran ged in a grid or in a n on -w oven m at (Figu re 2).
There are essentially two classes of polymer-based roofin g m em bran es: th erm osets (TS) an d th erm op lastics (TP) (see Text Box A). Th erm op lastics soften w h en h eated (th is p rocess is reversible) bu t th erm osets d o n ot. Th erm osets in clu d e th e com m on ly u sed eth ylen e p rop ylen e d ien e m on om er [also kn ow n as eth ylen e p rop ylen e d ien e terp olym er] (EPDM), w h ile th erolym op lastics en coolym -p ass a w id er variety of roofin g m em bran es, in clu d in g th erm op lastic p olyolefin s (TPOs).
All th erm op lastic roofin g m em bran es sh are certain ch aracteristics, e.g., seam in g can be d on e by h eat w eld in g. How ever, for th e m ost p art, th ey h ave very d ifferen t ch em ical, p h ysical an d m ech an ical p rop er-ties. It is im p ossible to exp lain th ese d iffer-en ces h ere, bu t th ere are d ifferiffer-en t ASTM m aterial-based stan d ard s for variou s TP
Thermoplastic Polyolefin
Roofing Membranes
Low-slope Roofing Membranes
Asphalt-based Built-up Roof BUR Modified Bitumen (Mod. Bit.) Thermosets (TS) Thermoplastics (TP) SBS Styrene-Butadiene-Styrene (Elastomer) APP Atactic Polypropylene (Plastomer) EPDM (Ethylene-Propylene-Diene Monomer or Ethylene-Propylene-Diene Terpolymer) PVC Poly (Vinyl Chloride)
KEE (Ketone Ethylene Ester) TPO (Thermoplastic Polyolefin or Flexible Polyolefin) CPE (Chlorinated Polyethylene) CR (Polychloroprene) Polymer-based
b y Ra lp h M. Pa roli, Ka re n K.Y. Liu a nd Te rra nce R. S im m ons
Thermoplastic polyolefins are a “new generation” of single-ply roofing systems.
This Update reviews their characteristics and performance to date and clarifies
some misunderstandings related to terminology.
Figure 1.Categories of low-slope roofing membranes
Figure 2.Reinforcement commonly used in roofing membranes: (a) random short glass fibre mat; (b) polyester scrim; (c) non-woven polyester mat
2
p rod u cts. To avoid con fu sion , on e sh ou ld n ot ju st label all of th ese p rod u cts sim p ly as TP p rod u cts. TPOs d iffer from oth er TPs in several resp ects, an d th u s m u st be h an d led an d ap p lied d ifferen tly.
De finit ion of T PO
Defin in g “th erm op lastic p olyolefin ” is d iffi-cu lt. “Th erm op lastic” is a gen eric term in p olym er scien ce; it en com p asses a class of p olym ers th at, as m en tion ed above, soften w h en h eated in a reversible p rocess. Th e term “olefin ” is even m ore gen eric, bein g an old ch em ical n am e for an y m olecu le con tain in g carbon -carbon d ou ble bon d s (th e m od ern n am e for th is fam ily of m olecu les is alken es). An y p olym er form ed by ch em i-cally lin kin g u p m an y olefin m olecu les is term ed “p olyolefin .”
Accord in g to th e latest d raft ASTM stan d ard for TPOs, th e com p osition is very
n on -sp ecific. Th e stan d ard states th at TPOs m u st con tain m ore th an 95% by m ass of TPO p olym er. Th e p olym er itself is n ot d efin ed w ith in th e stan d ard , w h ich states on ly th at th e sh eet sh all con tain th e “ap p rop riate” p olym ers. Becau se of th is loose d efin ition , th ere is an en d less list of ch em icals th at w ou ld fall u n d er th is stan d ard (e.g., p olyeth ylen e, p olyp rop ylen e, an d isobu tylen e, as w ell as th eir d erivatives). Ideally, manufacturers would specify the exact p olym er in term s of m arketin g an d labellin g.
Th ere are a few p u blish ed p ap ers th at attem p t to exp lain th e d ifferen t typ es of TPOs [1-4]. On e p oin t is clear, h ow ever: u n like p lasticized th erm op lastic m em bran es, TPOs d o n ot con tain p lasticizers (sm all m olecu les ad d ed d u rin g com p ou n d in g to in crease th e flexibility of th e p rod u ct). Th erefore, th e p roblem of p lasticizer loss associated w ith som e p lasticized m em bran es is elim in ated .
Confusion in t he M a rk e t Pla c e
TPO roofin g m em bran es h ave been in ser-vice in Eu rop e for ap p roxim ately ten years. Th e first ap p earan ce of a “TPO-typ e” roof-in g p rod u ct roof-in th e Un ited States w as arou n d 1987. As yet, little is kn ow n abou t th eir d u rability.
Th e con fu sion associated w ith TPOs com es from both th e ch em ical term in ology an d th e p rod u ct m arketin g. Th e latter h as p rom oted m ain ly th e EPDM-like ch aracter-istics of TPOs, i.e., it bein g like ru bber w ith th e ad d ed ben efits of w eld ed seam s (w h ich EPDMs d o n ot h ave). Also p rom oted h as been th e ch em ical resistan ce attribu ted to th e olefin com p on en t of th e p olym ers. Un fortu n ately, som e con fu sion h as occu rred , esp ecially regard in g th e u se of th e term th erm op lastic (TP). It is im p ortan t to rem em ber th at TPOs are th erm op lastic (i.e., TPs) bu t on ly som e TPs are TPOs.
Be ne fit s of T POs
In gen eral, TPO m em bran es are bein g m arketed as a p rod u ct th at com bin es th e p rop erties of EPDM an d PVC, w ith ou t th e associated d raw backs th at th e latter tw o m aterials h ave. In oth er w ord s, th ey are su p p osed to be as UV-resistan t an d as h eat-resistan t as EPDM, an d as h eat-w eld able as PVC.
Th e follow in g ben efits an d ch aracteris-tics h ave been rep orted for TPOs [1-3]: • en viron m en tally frien d ly an d recyclable • seam s can be h eat w eld ed
Text Box A
T he rm opla st ic s (T P) vs. T he rm ose t s (T S)
A p olym er is a lon g-ch ain m olecu le con sistin g of m an y (p oly) sm all rep eatin g u n its (m er) (~103– 106) join ed en d to en d . Th ese m olecu les are tan gled in a ran d om m an n er an alogou s to cooked sp agh etti.
Polym ers can be classified in to th erm op lastics (TPs) an d th erm osets (TSs) d ep en d in g on th eir m ech an ical beh aviou r on h eatin g an d coolin g. TPs com p rise lon g-ch ain m ole-cu les h eld togeth er by w eak bon d s (Figu re a). Wh en h eat is ap p lied , th e m olecu les “slid e p ast” on e an oth er an d th e p olym er soften s. On coolin g, th e m olecu les can n ot slid e p ast each oth er easily an d th e p olym er h ard en s. TS lon g-ch ain m olecu les, h ow ever, are lin ked togeth er by sm all m olecu les via stron g ch em ical bon d s, a p rocess som etim es referred to as vu lcan ization (Figu re b). Th is th reed im en -sion al n etw ork is so rigid th at th e m olecu les can n ot m ove very m u ch even w h en th e p olym er is h eated . Th u s, TSs d o n ot soften w h en h eated .
Becau se of th ese d ifferen ces, TP an d TS m em bran es are bon d ed d ifferen tly w h en ap p lied . TPs can be bon d ed u sin g h eat w eld in g: th e h ot air m elts th e p olym er at th e seam an d th e tw o strip s of m em bran e becom e fu sed . TS m em bran es are u su ally bon d ed u sin g ad h esives or tap es.
Schematic molecular configurations of (a) a thermoplastic and (b) a thermoset
3 • available in m an y colou rs
• resistan t to h eat an d UV d egrad ation • resistan t to m an y ch em icals
• good cold -tem p eratu re flexibility • n o extern al p lasticizers ad d ed .
Pe rform a nc e of T POs
TPO m em bran es are ligh ter in w eigh t an d easier to h an d le th an th e oth er th erm op lastic m em bran es. Bu t, w h ile flexible, th ey h ave a rath er rigid feel: th ey ten d to h old th eir sh ap e, an d d o n ot relax qu ickly. Con tractors ap p ear to be ad ju stin g slow ly to th em an d som e say th ey are n ot “con tractor-frien d ly.” Th eir com m en ts abou t TPOs in clu d e th e follow in g:
• Hot-air-w eld ed seam s are easy an d clean ; • Costs are low er th an for oth er h
ot-air-w eld ed m em bran es;
• Mechanically fastened systems (as opposed to loose laid or fu lly ad h ered system s) w ork w ell in recover ap p lication s w ith ou t ad d in g extra load ;
• Non -rein forced flash in g m em bran es are easy to form for d etailin g;
• Noticeable ch an ges in colou r an d textu re occu r over tim e;
• Mem bran es resp on d d ram atically (exp an sion an d con traction ) to tem p era-tu re ch an ges;
• Cold w eld s (i.e., w eld s at tem p eratu res th at are n ot h ot en ou gh ) occu r fre-qu en tly; th e start an d stop p osition s of th e robotic w eld er are esp ecially critical, as are th e p osition s of t-seam s;
• Narrow w eld in g w in d ow exists betw een cold w eld s an d scorch / bu rn -th rou gh (i.e., w eld s at tem p eratu res th at are too h ot); • Failu re of su bstrate bon d in g ad h esive is
com m on (i.e., n ot stickin g to m em bran e);
• Mem bran es som etim es require solvent wipe (to clean or p rim e) before w eld in g; • Re-w eld in g m em bran es (in
rep air) is p roblem atic after exp osu re to su n ;
• Black m em bran es are m ore d ifficu lt to w eld th an w h ite on es.
Curre nt Re se a rc h Re sult s
IRC, in collaboration w ith Ben ch m ark In c., a U.S. roofin g con su ltin g firm , in itiated a five-year p roject in 1997 to stu d y th e lon g-term p erfor-m an ce of TPO erfor-m eerfor-m bran es. Th u s far it h as been fou n d th at th e m em bran es rein forced w ith con tin u ou s p olyester fibre scrim h ad sign ifican tly h igh er ten sile breakin g stren gth (see Text Box B) th an th ose rein forced w ith ran d om sh ort glass fibre m at. With th e p olyester scrim (con tin u ou s fibre bu n d les arran ged in a grid ), th e fibres bore th e m ajority of th e ap p lied stress u n til failu re, resu ltin g in h igh breakin g stren gth . In th e case of th e glass fibre m at rein forcem en t (ch op p ed fibre stran d s p acked ran d om ly in to a m at), th e fibre stran d s flow ed an d align ed th em selves w ith th e p olym er in th e d irection of th e ap p lied stress, resu ltin g in sign ifican tly low er breakin g stren gth — i.e., th e stran d s m oved w ith th e m em bran e rath er th an stretch in g an d even tu ally sn ap p in g as is th e case w ith th e p olyester.
The breaking strength of the polyester scrim m em bran es w as govern ed by th e n u m ber of fibre tow s (bu n d les) across th e w id th of th e sp ecim en . Th e stren gth of th e fibre tow s w as th e sam e for all sam p les, in d ep en d en t of th e su rface coatin g. Th erefore th e tigh ter th e w eave of th e rein forcem en t, th e h igh er th e breakin g stren gth of th e m em bran e.
The difference in weave style had an effect on h ow a m em bran e failed as w ell. On e sam p le w as rein forced w ith an u n coated p olyester scrim w h ere th e in tersectin g tow s w ere h eld togeth er by fin e, con tin u ou s p olyester fibres. Wh en th e m em bran e w as p u lled in th e m ach in e d irection (p arallel to th e len gth of th e roll of th e m em bran e), th e fin e fibres stretch ed an d tigh tly h eld on to th e tow s in th e cross d irection (p erp en d icu -lar to th e m ach in e d irection ), creatin g h igh localized stresses at th e in tersection s. Text Box B Te nsile Te st ing A good roofin g m em -bran e h as to be stron g en ou gh to w ith stan d stresses, an d flexible en ou gh to accom m od ate d eck m ovem en t. Th ese p rop erties are rep re-sen ted by breakin g stren gth an d elon gation , w h ich can be m easu red by a ten sile testin g m ach in e, w h ich p u lls a rectan gu lar sp ecim en at a con stan t rate. Th e m ach in e p lots th e
reac-tion as a stress-elon gareac-tion cu rve. Th e m axim u m stress born e by th e sp ec-im en is called th e breakin g stren gth w h ile th e elon gation at w h ich th e m axim u m stress is ach ieved is called th e elon gation at break.
Typical tensile stress-elongation curves for a polyester-reinforced TPO membrane at -40°C and at 23°C
How ever, w h en th e m em bran e w as stressed in th e cross d irection , n o localized stress p oin ts w ere form ed , sin ce th e fin e fibres h eld th e tow s in th e m ach in e d irection by san d w ich in g in stead of p u llin g th em .
The breaking strength of the polyester scrim m em bran es in creased by 20–30% as th e test tem p eratu re d rop p ed to -40°C. How ever, th e m em bran e becam e brittle an d th e u ltim ate elon gation (a m easu re of th e m em bran e’s ability to accom m od ate d eck m ovem en t) red u ced m ore th an 15 tim es. Sin ce th e flexibility of th e p olym er govern s th at of th e m em bran e, it is recom m en d ed th at p olym ers w ith su p erior cold -tem p eratu re m ech an ical p rop erties (e.g., h igh er elon gation ) be u sed in m em bran es in ten d ed for cold tem p eratu re ap p lication s. Th e glass tran sition tem p era-tu re (Tg) is a good criterion for ch oosin g th e
righ t p olym er (see Text Box C).
Cold temperature flexibility was monitored u sin g Tg. It w as fou n d to be in th e ran ge of
-32°C to -37°C, com p arable to th at of oth er sin gle-p ly system s (arou n d -35°C). Th ere w as on e excep tion — a Tgof -54°C. Th at sam p le
also had the highest tensile elongation at -40°C. Th erm ogravim etry (w h ich m on itors th e w eigh t ch an ge of a su bject bein g h eated over a tem p eratu re ran ge) clearly sh ow s th at th ere are at least fou r d ifferen t typ es of TPOs based on w eigh t loss p attern s alon e. Th is sh ow s again th at th e term “TPO” is qu ite gen eric, en com p assin g m an y d ifferen t typ es of p olym ers.
Sum m a ry
Becau se of th e ch em ical term in ology in volved , th e n on -sp ecific ch em ical com p o-sition of th e p olym er an d th e m arketin g focu s of m an u factu rers, th ere is m u ch con -fu sion abou t TPO roofin g m em bran es.
If th e ASTM task grou p d evelop in g th e TPO stan d ard in sists on u sin g th e term “olefin ” in th e stan d ard ’s n am e, th en it sh ou ld con sid er u sin g th e n am e “Flexible or Th erm op lastic Polyolefin ” in th e title, as th is w ou ld be in lin e w ith th e term in ology used in Europe (i.e., flexible polyolefin, FPO). Fu rth erm ore, m an u factu rers sh ou ld con sid er clearly statin g w h eth er th eir p rod u ct is a p olyp rop ylen e- or p olyeth ylen e-based system , to m in im ize p ossible con fu sion .
Ac k now le dgm e nt
Th e au th ors w ish to th an k th e con tractors, bu ild in g ow n ers, an d Ben ch m ark staff w h o collected or provided samples for this project.
Re fe re nc e s
1. Beer, Han s-Ru d olp h . “Flexible Polyolefin Roofin g Mem bran es Prop erties an d Ecological Assessm en t,” Proceed in gs of Waterp roofin g Tech n ology an d th e En viron m en t, 9thIn tern ation al
Waterp roofin g Association Con gress, Am sterd am , 1995, p p . 81-89.
2. d e Palo, Roberto. “Flexible Polyp rop ylen e Alloys: A New Gen eration of Materials for Waterp roofin g Ap p lication s,”
Proceed in gs of Waterp roofin g Tech n ology an d th e En viron m en t, 9th In tern ation al
Waterp roofin g Association Con gress, Am sterd am , 1995, p p . 309-320. 3. Beer, Han s-Ru d olp h . “Lon gevity an d
Ecology of Polyolefin Roof Mem bran es,” Proceed in gs of th e Fou rth In tern ation al Sym p osiu m on Roofin g Tech n ology, Gaith ersbu rg, MD, 1997, p p . 14-21. 4. Foley, Rich ard K. an d William Ru bel.
“Polyolefin s: Th e New Roofin g Tech n ology,” In terface (Jou rn al of th e Roofin g Con su ltan ts In stitu te), October 1997, p p . 30-32.
Dr. R.M. Pa roliis Director of th e Bu ild in g En velop e an d S tru ctu re Program at th e N ation al Research Cou n cil’s In stitu te for Research in Con stru ction .
Dr. Ka re n Liuis a research er in th e sam e p rogram .
Mr. Te rra nce R. S im m onsis with T RS Con su ltin g, Iowa City, Iowa.
“Construction Te chnology Up d a te s” is a se rie s of te chnica l a rticle s conta ining p ra ctica l inform a tion d istille d from re ce nt construction re se a rch.
For more information, contact Institute for Research in Construction, National Research Council of Canada, Ottaw a K1A 0R6
Telephone: (613) 993-2607; Facsimile: (613) 952-7673; Internet: http://irc.nrc-cnrc.gc.ca
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Nation al Research Cou n cil of Can ad a Decem ber 1999
ISSN 1206-1220
Text Box C
Gla ss Tra nsit ion Te m pe ra t ure (Tg)
Wh en a p olym er is w arm , th e m olecu lar segm en ts are con -stan tly ch an gin g p lace by actively slid in g or ju m p in g, an d th e p olym er is flexible. Th is is th e am orp h ou s state. Wh en th e p olym er is cooled , m olecu lar m ovem en t starts to slow. If coolin g is con tin u ed , a tem p eratu re w ill be reach ed at w h ich m olecu lar m ovem en t stop s altogeth er. Th e p olym er loses its flexibility an d becom es brittle. Th is is th e glassy state. Th e tem p eratu re at w h ich th e tran sition from flexi-ble to brittle occu rs is called th e glass tran sition tem p era-tu re (Tg). Tg is a p rop erty of th e p olym er an d is an
im p ortan t con cep t in evalu atin g th e p erform an ce of p oly-m er-based roofin g oly-m eoly-m bran es. Th e ad d ition of p lasticizers (sm all p olym er m olecu les) d u rin g com p ou n d in g h elp s to low er th e Tgby in creasin g th e d istan ce betw een th e p
