International Journal of Adhesion & Adhesives 26 (2006) 555–566 Adhesion of construction sealants to polymer foam backer rod used in building construction A.R. Hutchinson , S. Iglauer Joining Technology Research Centre, Oxford Brookes University, Headington Campus, Gipsy Lane, Oxford OX3 0BP, UK Accepted 6 September 2005 Available online 10 October 2005 Abstract Sealants play a vital role in maintaining the weather tightness of modern high-rise buildings. Typical sealed joints comprise an elastomeric sealant, a backing foam and the sides of the joint corresponding to the cladding panels. Conceptually, the sealant should adhere only to the sides of the joint to enable maximum movement capacity of the material to be utilised; this is known as two-sided adhesion. Previous work has shown that the sealant sometimes adheres to the backing material, resulting in three-sided adhesion, and that this results in a decrease in joint performance. The adhesion between the foam and the sealant was studied using tack and peel tests, and the results were compared with measurements of the surface energies of the foams and the surface tensions of the sealants. Adhesion mechanisms are suggested and peel force–surface energy correlations are presented. Implications for the practical application of the results in building construction are made. r 2005 Elsevier Ltd. All rights reserved. Keywords: A. Sealants; B. Polymer foams; C. Contact angles, Peel; D. Adhesion 1. Introduction Modern high-rise construction techniques employ var- ious forms of curtain walling that consist of large discrete panels attached to a steel or reinforced concrete framework (Fig. 1). The gaps or joints between these panels are generally filled with a gun-applied elastomeric sealant, applied in-situ in order to seal the gaps and to accom- modate movement (Fig. 2). The cladding system of a single building may contain up to 50 km of joints, all of which need to be sealed effectively in order to maintain an acceptable internal environment. The sealant in a sealed joint is required to exhibit good adhesive and cohesive properties in order to guarantee a watertight structure and prevent heat loss. Another key property of a sealant is its ability to undergo extension and compression as the building elements thermally expand and contract. Sealed joints comprise a substrate (usually aluminium or con- crete), a primer (e.g. a silane or an isocyanate dissolved in an organic solvent), the sealant itself (a polymer base with additives such as pigments, fillers and catalysts), and the backer rod, which is generally an expanded polymeric material (polyethylene (PE) or polyurethane (PU) foam). The rod is utilised as a joint filler in order to control the depth of seal, as a support for tooling the sealant, and as a bond breaker to prevent the sealant from adhering to the back of the joint. The relatively high incidence of premature sealant failure in curtain-walled structures can be variously ascribed to inadequate specification, design and detailing, as well as to poor workmanship. Commonly cited technical problems include poor adhesion to substrates, cyclic movement, early movement during cure with one-part materials, and adhesion of sealant to the backing material resulting in three-sided tack or adhesion. [1–4]. This paper presents the results of the investigation of the sealant–foam adhesion that leads to reduced joint perfor- mance [3,4] (Fig. 3). The number of cycles until failure in fatigue were directly correlated with the performance of standard tensile sealed joints. Static joints were not moved ARTICLE IN PRESS www.elsevier.com/locate/ijadhadh 0143-7496/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2005.09.001 Corresponding author. Tel.: +00 44 1865 48 3504; fax: +00 44 1865 48 4179. E-mail address: [email protected] (A.R. Hutchinson).
Transcript
ARTICLE IN PRESS
0143-7496/$ - se
doi:10.1016/j.ija
�Correspondfax: +0044 186
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International Journal of Adhesion & Adhesives 26 (2006) 555–566
www.elsevier.com/locate/ijadhadh
Adhesion of construction sealants to polymer foam backer rodused in building construction
A.R. Hutchinson�, S. Iglauer
Joining Technology Research Centre, Oxford Brookes University, Headington Campus, Gipsy Lane, Oxford OX3 0BP, UK
Accepted 6 September 2005
Available online 10 October 2005
Abstract
Sealants play a vital role in maintaining the weather tightness of modern high-rise buildings. Typical sealed joints comprise an
elastomeric sealant, a backing foam and the sides of the joint corresponding to the cladding panels. Conceptually, the sealant should
adhere only to the sides of the joint to enable maximum movement capacity of the material to be utilised; this is known as two-sided
adhesion. Previous work has shown that the sealant sometimes adheres to the backing material, resulting in three-sided adhesion, and
that this results in a decrease in joint performance. The adhesion between the foam and the sealant was studied using tack and peel tests,
and the results were compared with measurements of the surface energies of the foams and the surface tensions of the sealants. Adhesion
mechanisms are suggested and peel force–surface energy correlations are presented. Implications for the practical application of the
results in building construction are made.
r 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Sealants; B. Polymer foams; C. Contact angles, Peel; D. Adhesion
1. Introduction
Modern high-rise construction techniques employ var-ious forms of curtain walling that consist of large discretepanels attached to a steel or reinforced concrete framework(Fig. 1). The gaps or joints between these panels aregenerally filled with a gun-applied elastomeric sealant,applied in-situ in order to seal the gaps and to accom-modate movement (Fig. 2). The cladding system of a singlebuilding may contain up to 50 km of joints, all of whichneed to be sealed effectively in order to maintain anacceptable internal environment. The sealant in a sealedjoint is required to exhibit good adhesive and cohesiveproperties in order to guarantee a watertight structure andprevent heat loss. Another key property of a sealant is itsability to undergo extension and compression as thebuilding elements thermally expand and contract. Sealedjoints comprise a substrate (usually aluminium or con-
e front matter r 2005 Elsevier Ltd. All rights reserved.
crete), a primer (e.g. a silane or an isocyanate dissolved inan organic solvent), the sealant itself (a polymer base withadditives such as pigments, fillers and catalysts), and thebacker rod, which is generally an expanded polymericmaterial (polyethylene (PE) or polyurethane (PU) foam).The rod is utilised as a joint filler in order to control thedepth of seal, as a support for tooling the sealant, and as abond breaker to prevent the sealant from adhering to theback of the joint.The relatively high incidence of premature sealant failure
in curtain-walled structures can be variously ascribed toinadequate specification, design and detailing, as well as topoor workmanship. Commonly cited technical problemsinclude poor adhesion to substrates, cyclic movement, earlymovement during cure with one-part materials, andadhesion of sealant to the backing material resulting inthree-sided tack or adhesion. [1–4].This paper presents the results of the investigation of the
sealant–foam adhesion that leads to reduced joint perfor-mance [3,4] (Fig. 3). The number of cycles until failure infatigue were directly correlated with the performance ofstandard tensile sealed joints. Static joints were not moved
Fig. 1. Cladding system of a building in London/England.
stat
ic
dyna
mic
hybr
id
hybr
id
0
20000
40000
60000
80000
100000
120000
dyna
mic
stat
ic
Cyc
les
to f
ailu
re
Fig. 3. Fatigue testing results of sealed joints for a silicone sealant.
A.R. Hutchinson, S. Iglauer / International Journal of Adhesion & Adhesives 26 (2006) 555–566556
during their cure, while dynamic joints were moved duringcure in order to simulate real-life panel movement. Hybridjoints were joints which had backer foam attached. It canbe seen that foam-backed joints (hybrid) have a shorterfatigue life, especially if the joint is cycled during curing(dynamic) [1,3–5].
2. Materials
2.1. Foams
Foam is defined as a gas–solid or gas–liquid two-phasesystem. If the gas phase is continuous, the foam is classifiedas open cell foam; if the gas phase is dispersed, then thefoam is classified as a closed cell material. In constructionapplications, both skinned and un-skinned foams are used.The skin is considered to be surface foam cells with ahigher polymer concentration. In the construction indus-try, foams are solid–gas dispersions, which are based upon(PE) or (PU) polymer (more rarely polypropylene). Eightdifferent commercial backing foam materials were used inthe work. Six PE foams and two PU foams wereinvestigated. Both PU foams had no skin and one PEfoam had no skin. The remaining five PE foams had skin.All foams together provided 13 different foam surfaces.These were designated as PE1 to PE6 ‘‘skin’’ or ‘‘no skin’’,and PU1 and PU2 ‘‘no skin’’.
2.2. Sealants
A sealant is a viscoelastic material mixture based on apolymer (e.g. silicone or PU) with additives such aspigments, cross-linking agents, catalysts, fillers and thixo-tropic agents. Such materials can be categorised as eitherone-part sealants or multi-part sealants. One-part sealantscure anisotropically, skin-forming under the influence ofmoisture, while the multi-part sealants show a homoge-neous cure induced by a catalyst or initiator afterthe components are mixed. Some formulations cure bycondensation reactions without catalyst. Typical cure timesare 2–3 weeks for one-part sealants and 1 week for multi-part sealants. Cohesive properties like the modulus and themolar mass of the polymer increase with cure time. Fivesealants were used in this work. These, and the primersassociated with them for the relevant joint substratematerials, are shown in Table 1.
3. Experimental procedures
3.1. Surface free energy analysis
The surface tension of the sealant and the surface freeenergy of the substrate determine the wetting behaviour atthe interface between the two. The degree of wetting is thekey to understanding adhesion; good wetting of thesubstrate is a requirement of good adhesion [6]. Themeasurement of static contact angles, and their insertioninto a combined empirical thermodynamic equation(Eq. (1)), is one way to obtain surface free energies,including their dispersive and polar components.A video contact analyser (VCA) was employed to
measure the static contact angles of 20 mL droplets ofdouble-distilled water, diiodomethane, glycerol and hex-adecane. The comparatively high droplet volume of 20 mL
ARTICLE IN PRESS
Table 1
Materials used in the experimental programme.
Sealant Sealant type Modulus at
25% elongation
(kPa)
Primer
(adhesion
promoter)
Silicone One-part 540 Silanea
Polyurethane One-part 900 Silanea
Modified silicone One-part — Isocyanatea
Polysulphide Multi-part 460 Silanea
Modified silicone
polyether
Multi-part 320 Silanea
aThe silane primers were all manufactured by different producers and
their formulations vary. The exact formulation is confidential to the
company.
Table 2
Material and joint dimensions
Foam 0.125m� 0.025m 0.007m
Steel backer 0.175m� 0.025m� 0.004m
Aluminium peel strips 0.300m� 0.025m� 0.004m
Sealant bead thickness 0.0016m
Bond peel area 0.100m� 0.025m
A.R. Hutchinson, S. Iglauer / International Journal of Adhesion & Adhesives 26 (2006) 555–566 557
was used because it reduced the effect of the very roughsurfaces. The rough foam surfaces were responsible for thehigh standard deviation of the measurements, because theyproduced a wide range of contact angles. In terms ofcontact angle measurements, open cell foams and closedcell foams were treated in the same way. No surface pre-treatments were applied to the polymeric foams, duringwhich surface changes such as dissolution or chemicalreactions with the cleaning solvent could occur. Left andright contact angles were recorded 10 s, 15 s and 20 s afterdispensing the droplets. The time-window was chosen toprevent any liquid–foam interaction. Between five and 12droplets were placed on each surface and all readings wereaveraged. This procedure was repeated for each liquid.Hexadecane is soluble in PE foam, and it penetrates intothe foam phase. Glycerol is hygroscopic and changes itssurface tension properties with exposure to humid air.Within the surface free energy test series, glycerol wasexposed to a laboratory atmosphere for up to 30min.
The surface free energies of each plastic foam werecalculated by inserting the measured contact angles into acomputer program that solves a Girifalco–Good–Fowke-s–Owens–Wendt geometric mean [7]:
glvð1þ cos yÞ ¼ 2
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiðgdlvg
dsvÞ
qþ
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiðgplvg
psvÞ
q� �, (1)
where gsv and glv are the surface energies at thesolid–vapour and liquid–vapour interface, y is the contactangle, and the superscripts d and p represent dispersive andpolar components.
The total surface energy,
gtsv ¼ gdsv þ gpsv. (2)
3.2. 1801-peel tests
Peel testing was carried out in order to obtain semi-quantitative adhesion data regarding the foam–sealantinterface [8], as the level of foam–sealant adhesiondetermines the life-time of a sealed joint [3,4]. The peel
force Fp delivers information about the joint strength andthe work of adhesion at the foam–sealant interface. Themode of failure of the joint has to be taken into account,because it influences peel forces highly and is important inthe data interpretation. Only 100% adhesion failure at thesealant–foam interface provides a reasonable indication ofadhesion. It is difficult to distinguish very thin film cohesivefailures visually, and spectroscopic techniques need to beemployed for a deeper analysis. Peel joint and materialdimensions are shown in Table 2.The foams were cut with a sharp razor blade to the
dimensions given in Table 2, but the skin was not removedfrom the skinned samples. An epoxy adhesive was used toadhere the foams onto steel backing strips; this gave thefoams sufficient stiffness and support for the tests. Asealant layer with a thickness of 1.6mm was applied ontoeach by placing the joints in a rig, which had appropriatedimensions, tooling the uncured sealant onto the foam andwiping off excess sealant with a spatula. An acetone-wipedaluminium peel strip was then immediately placed againstthe uncured sealant. A resultant joint is shown in Fig. 4.Joints were also made in which the surfaces of four of thePE foams had been primed (see Table 1) to simulate theoverspill when priming the sides of a cladding panel priorto sealant application.All specimens were cured for 2 weeks at 23 1C72 1C and
50% relative humidity (r.h.) (75% r.h.). Each specimenwas subjected to a peel load that was exerted at 1801 tothe metal strip incorporated in the sealant. One testsequence with skinned PE foam (PE1) and silicone sealantwas conducted in order to investigate the pressuresensitivity of the sealant–foam system. The pressure wasapplied in the form of 500 g steel weights with a surfacearea of 175mm� 25mm, overlapping the whole bond area( ¼ 2000 Pa). All other peel joints were cured withoutapplying additional pressure.
3.3. Tack tests
Pressure-sensitive tack is defined as the property of amaterial which enables it to form a bond of measurablestrength immediately upon contact with another surface(ASTM D1878-61T [9]). Another definition of tack is theadhesive failure energy determined under conditions of lowcontact pressure and short contact time [10].Tack is a complex property depending on the bond
formation, the substrate preparation, the material separation
ARTICLE IN PRESSA.R. Hutchinson, S. Iglauer / International Journal of Adhesion & Adhesives 26 (2006) 555–566558
and the observed parameters [11]. In order to get a betterunderstanding of the ‘‘instant’’ and short-term adhesion,tack experiments were carried out as depicted in Fig. 5. Atypical tack force–extension graph is presented in Fig. 6.
A procedure was developed in which a sealant layer witha thickness of 1.6mm was pressed onto a PE foam (PE1)surface at a speed of 8.3� 10�6m/s with a constant load of1.5N, and was then immediately separated with a constanttensile speed of 1.67� 10�6m/s. Two tack tests series wereconducted with acetone-wiped anodised aluminium (E6EV1 AlMgSi0,5) instead of a foam substrate. The sealantwas cured for a determined amount of time with thefollowing cure conditions: 23 1C72 1C and 5075% r.h. or23 1C72 1C and 4177% r.h. The force and failure moderequired to detach the foam from the tacky sealant surfacewere recorded. The time to surface free tack was reachedwhen the tensile load reduced to approximately zero andthe failure mode became 100% adhesive.
4. Results and discussion
4.1. Surface free energies of foam materials
Measured contact angles and their standard deviationsare shown in Table 3. Hexadecane droplets spread acrossthe foam samples and their contact angles were not
Peelforce
Aluminiumstrip
Sealant
Foam
Steel
Fig. 4. Schematic of a 1801-peel test.
1.5 N ‘tou
foam
PVC round
bar
sealant
8.3 x 10-6 m/s
Fig. 5. Schematic
therefore considered in the surface energy calculations.Glycerol contact angles for PU2, PE6 ‘‘skin’’ and PE6 ‘‘noskin’’ and hexadecane contact angles of PU1, PU2, PE6‘‘skin’’ and PE6 ‘‘no skin’’ were not measured.Foams have high surface roughnesses, which influence
the contact angles. This effect was analysed using Wenzel’sequation (Eq. (3)) and the Shuttleworth-Bailey equation(Eq. (4)):
cosðyroughÞ ¼ r cosðysmoothÞ, (3)
where yrough is the contact angle on the rough surface,ysmooth the contact angle on an ideal smooth surface, and r
is Wenzel’s factor:
ysmooth ¼ yrough � c, (4)
where c ¼ arc cosðGSA=RSAÞ ¼ arc cosð1=rÞ, GSA is thegeometric surface area and RSA the real surface area.Measurements of RSA were conducted using a laserscanning profilometer, as described elsewhere [12].Adjusted contact angles are listed in Table 4 (Wenzel’sequation) and in Table 5 (the Shuttleworth-Bailey equa-tion).All diiodomethane contact angles and glycerol contact
angles for skinned PE4 and skinned PE5, when adjustedwith the Shuttleworth-Bailey equation, adopt negativevalues. This is physically impossible.
ch’
1.67x10-5 m/s
of ‘tack test’.
Extension [m]
Ten
sile
‘ta
ck’f
orce
[N
]
Fig. 6. Typical tack force–extension graph.
ARTICLE IN PRESS
Table 3
Contact angles with their standard deviations for commercial foams
aPU1 had an inhomogeneous colour distribution: the major parts were green-grey and minor parts were green-yellow.
Table 4
Contact angles and standard deviations, considering surface roughness
with Wenzel’s equation
Foam Contact angle
water (1)
Contact angle
glycerol (1)
Contact angle
diiodomethane
(1)
PE1, skin 82.277.5 77.9711.4 61.9711.8
PE2, no skin 90.375.2 89.8710.8 80.7719.1
PE3, skin 90.371.3 73.676.5 42.471.8
PE4, skin 87.472.8 77.675.1 69.4714.9
PE5, skin 89.875.1 85.277.1 81.8713.2
Table 5
Contact angles and standard deviations, considering surface roughness
with the Shuttleworth-Bailey equation
Foam Contact angle
water (1)
Contact angle
glycerol (1)
Contact angle
diiodomethane
(1)
PE1, skin 15.771.4 7.071.0 –34.8
PE2, no skin 12.770.7 10.171.2 –49.2
PE3, skin 51.870.7 30.272.7 –19.4
PE4, skin 14.870.5 –13.8 –49.7
PE5, skin 10.170.6 –14.1 –35.6
A.R. Hutchinson, S. Iglauer / International Journal of Adhesion & Adhesives 26 (2006) 555–566 559
Foam surface free energies and their components arepresented in Table 6. The second and third dataset showsurface free energies considering surface roughness with theWenzel’s and the Shuttleworth-Bailey equation, respec-tively.
The results for the total surface free energy and itsrespective components calculated with the Shuttleworth-Bailey equation are very high compared with the smoothsurface results, especially the polar component. Because ofthese high surface energies and very low contact angles(some of them reach negative values), the Shuttleworth-Bailey equation is not appropriate for considering theinfluence of foam roughness on contact angles. The totalsurface energy and dispersive surface energy resultscalculated with the Wenzel equation are low comparedwith literature values for pure PE (total surface energy:22.9–36.8mJ/m2, polar surface energy: 0mJ/m2, [11]), butare within reasonable limits, while the polar componentreaches comparatively high values.
It seems that the empirically derived Wenzel and Shuttle-worth-Bailey equations are not valid for highly rough foamsurfaces, as they are clearly not delivering appropriateresults. New equations, which can also be applied to veryrough surfaces, would be useful in this regard.
The solid PE sheet investigated for comparison showed astrong dependency of water contact angles on the locationof the droplet on the sheet. This suggests, in combinationwith the high total surface energy, that it was a hetero-geneous surface and not pure PE. This was confirmed byhorizontally attenuated total reflectance (HATR)-FTIRand XPS spectroscopy data [3,13]. Impurities in the PEcould be release agent, peroxide (which is used as a cross-linking agent) and its residues, antioxidant or otheradditives. Moreover, the polymer chains of the PE werepartly oxidised [3,13]. Oxygen, nitrogen and silicone weredetected on and within the plastic foams by XPS andHATR-FTIR spectroscopy. Groups containing these ele-ments are usually polar and they are responsible for thepolar surface free energy components and the relativelyhigh surface free energies of up to 49.8mJ/m2 for skinnedPE4. The same applies to the solid PE sheet with a totalsurface free energy of 43.9mJ/m2 and a polar surface freeenergy of 6.7mJ/m2.These findings imply an increased total surface free
energy and polar component. Because the Wenzel’sequation fails to deliver this, it is also not appropriate toconsider foam surface roughness, although it delivers betterresults than the Shuttleworth-Bailey equation.
ARTICLE IN PRESSTable
6
Surface
free
energies,withtheirstandard
deviations,ofcommercialfoams
Foam
Surface
free
energy(m
J/m
2)
Standard
procedure
Surface
roughnessconsidered
withWenzel’sequation
Surface
roughnessconsidered
withtheShuttleworth-Bailey
equation
Wenzel’sfactor(r)
Total
Polar
Dis.
Total
Polar
Dis.
Total
Polar
Dis.
Solidpolyethylene
43.971.2
6.776.5
37.277.3
PE1,skin
43.573.7
5.173
38.475.1
29.275
7.675
21.677.5
70.271.1
51.371.4
18.970.6
1.94
PE1,noskin
36.174.7
171.1
35.175.4
PE2,noskin
36.775.3
0.770.7
3675.6
21.474.7
7.975.7
13.578.8
71.470.3
54.371.1
1770.5
5.35
PE3,skin
4772.7
0.370.1
46.772.7
3772.7
1.270.3
35.872.7
55.672.2
18.172
37.574.1
1.28
PE3,noskin
38.674.3
0.370.2
38.374.4
PE4,skin
49.871.8
1.670.7
48.172.3
26.374.7
6.373.6
2078
——
—2.70
PE4,noskin
4875.1
0.770.8
47.274.5
PE5,skin
39.674
1.971.5
37.775
21.473.4
874.6
13.476.3
——
—5.13
PE5,noskin
24.674.7
675.8
18.676.9
PU1,noskin
41.4722.6
33.1724.8
8.377.7
PU2,noskin
38.574.9
2.773.1
35.876.1
PE6,skin
47.572.3
1073.5
37.572.8
PE6,noskin
26.473.4
0.270.3
26.272.7
r¼
Wenzel’sfactor;
r¼
realsurface
area/geometricsurface
area.
A.R. Hutchinson, S. Iglauer / International Journal of Adhesion & Adhesives 26 (2006) 555–566560
The most reliable results using VCA are obtainedconsidering an ideal smooth foam surface, despite the factthat this is obviously not the case.All bulk phases of the foams exhibited smaller polar
surface free energies and total surface free energies,suggesting that the bulk phase is purer PE than the surface.This was confirmed by XPS and HATR-IR studies [3,13].The polar surface free energy of all foams examined was
low (the highest polar surface free energy was for skinnedPE1 ¼ 4.7mJ/m2) and the dispersive surface free energycontributed most to the total surface free energy. Thisresult corresponds to the literature values of the low polarsurface free energy (0mJ/m2) and the high dispersivesurface free energy (22.9–36.8mJ/m2) of pure PE.The total surface free energies of the PU1 and PU2
foams and their polar/dispersive components were foundto be similar to literature results (39mJ/m2 [14]).Problems involved in measuring static contact angles on
polymer foams with the VCA method are mainly thedetermination of Young’s contact angle, i.e. the equili-brium contact angle. Many metastable states are adoptedon rough foam surfaces, which are difficult to distinguishfrom the stable state, and lead to relatively high standarddeviations of the Young’s angles.This was observed for both skinned and non-skinned
foams. For example, PE2 (no skin) and PE5 (skin) showeda similar roughness with Wenzel factors of 5.35 (PE2, noskin) and 5.13 (PE5, skin), and both foams, independent ofskin or no skin, had a high standard deviation in terms oftheir contact angle measurements.
4.2. Peel tests
Load–extension curves were recorded for five replicatesof each peel joint combination. The peel test extension wastaken as the jaw separation of the Tensometer. The first0.06m of extension, which included the initial peak force,and extensions over 0.22m, when the force decreasedstrongly because of separation, were neglected. Therecorded peel force in N (for the 0.06–0.22m interval)was averaged and arithmetic means with standard devia-tions were calculated as shown in Tables 7 and 8, includingtheir loci of failure. The locus of failure was determinedvisually.The reproducibility of the mechanical tests was generally
low (compared to spectroscopic analysis), including peeltests with standard deviations of up to 50% of the nominalvalue for low peel forces. Nevertheless, general qualitativetrends can be derived from these data. Each sealant–foamsystem represents an individual system with different foamand sealant parameters and different adhesion mechan-isms.Peel forces varied strongly for a particular sealant and
foam surface (reading down and across, respectively, inTables 7 and 8), indicating a significant influence of foamsurfaces on peel forces. The influence of the sealant waslower than that of the foam, but still significant with
ARTICLE IN PRESS
Table 7
Average peel forces (N) and loci of failure with standard deviation
Af, adhesion failure. If it was not possible to distinguish between pf(f) and pf(s), then this was considered as af.; pf(f), failure of the primer between primer
and foam; pf(s), failure of the primer between primer and sealant; cfi(s), thin film failure within the sealant; cf(s), cohesive failure within the sealant; cfi(f),
thin film failure within the foam; cf(f), cohesive failure within the foam; Al, failure between aluminium strip and sealant Al***, four samples failed 100%
Al; adv, adhesive failure between foam and epoxy; *1, 4 replicates.
A.R. Hutchinson, S. Iglauer / International Journal of Adhesion & Adhesives 26 (2006) 555–566 561
ARTICLE IN PRESS
Table 8
Average peel forces (N) and loci of failure with standard deviation, for primed foam surfaces
No skin+primer 10.771.5 10.771.4 8.271.0 5.570.6 1473.1
100% af 70% pf(f) 94% af 96% af 87% af
30% pf(s) 5% cfi(s) 4% cfi(s) 8% cfi(s)
1% cf(s) 4% cf(s)
1% cf(f)
Af, adhesion failure. If it was not possible to distinguish between pf(f) and pf(s), then this was considered as af.; pf(f), failure of the primer between primer
and foam; pf(s), failure of the primer between primer and sealant; cfi(s), thin film failure within the sealant; cf(s), cohesive failure within the sealant; cfi(f),
thin film failure within the foam; cf(f), cohesive failure within the foam; Al, failure between aluminium strip and sealant; Al*, one sample failed 100% Al;
Al**, two samples failed 100% Al; Al***, four samples failed 100% Al; adv, adhesive failure between foam and epoxy; *1, 4 replicates.
A.R. Hutchinson, S. Iglauer / International Journal of Adhesion & Adhesives 26 (2006) 555–566562
deviations of up to 600% (e.g. skinned PE5–silicone joint:25.8N; skinned PE5–polysulphide joint: 4.2N).
Peel forces generally lay between 2.5N (unskinned PE1and polysulphide) and 16.6N (skinned and primed PE1and PU), but reached peak values up to 50N (skinned PE4and silicone); this is clearly significant adhesion, similar toaluminium substrate–sealant adhesion! The mode of failurewas influenced mainly by the stiffness of sealant, i.e. itsmodulus at 25% strain (Table 1).
The pattern of data associated with each sealant type issummarised below.
One-part PU: High peel forces with skinned PE1 (primedand unprimed), skinned PE4 (primed and unprimed), andsolid PE. Other PU combinations had low or average peelforces. The mode of failure was mainly adhesion. Theprimer was a silane.
One-part modifiedsilicone: High peel forces with skinnedPE1, skinned PE4, solid PE and PE5 (skinned andunskinned). The remaining combinations had average peelforces. Modes of failures were mixed between adhesion andcohesive failure in foam and sealant, dependent on thefoam surface. The primer was a silane.
One-part silicone: Several high peel force combinations(skinned and primed PE3, skinned PE4, solid PE, skinnedPE5), some average peel forces (skinned and primed PE1,skinned PE1), while the remaining combinations had lowpeel forces. Mixed modes of failures were observed,predominantly adhesion and thin film cohesive in thesealant. The primer was a silane.
Three-part polysulphide: Generally low peel forces.Skinned PE4 (primed) had a high peel force and primedPE1, primed PE2 and skinned PE3 (primed) had average
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peel forces. Mainly adhesion failure, but thin film cohesivesealant failure was observed with some foams. The primerwas a silane.
Three-part modified silicone polyether: High peel forceswith skinned and primed PE1, and skinned and primedPE4. Average peel forces were found with unskinned andprimed PE3, unskinned PE3, skinned PE4, and unskinnedand primed PE4. The remaining combinations had low peelforces. Adhesion failure predominated although thin filmcohesive sealant failure was observed with some foams.The primer was an isocyanate.
It was observed that penetration of sealant into exposedfoam cells was only marginal.
The primer had a clear influence on observed peel forces.It generally increased the peel force, but also decreased it inabout a quarter of all combinations. The primer reactedwith the foam surface, creating a new surface chemistry.Depending on the primer, either a whole new surface (thickprimer) or only a thin molecular layer (thin primer) wascreated. High and average peel forces were often found onprimed foam surfaces.
Higher peel forces were generally found on skinnedfoams, but about a quarter of the non-skinned surfacesshowed an opposite trend, e.g. PE3.
4.3. Tack tests
Tack-free times were obtained for all five sealants asshown in Fig. 7. The tack free times varied with the sealantand they lay between 3 and 6 h for one-part sealants,around 6 h for the three-part polysulphide sealant andaround 24 h for the multi-part modified silicone polyether.Shorter tack-free times for one-part sealants are caused byskin formation [15], while multi-part sealants cure homo-geneously without skin formation.
The multi-part sealants showed an s-shaped curve intheir peak load against time graphs, until they started tofail adhesively or exhibited mixed (adhesion and cohesive)failure. This s-shaped curve is similar to a propertydevelopment versus cure-time curve for multi-part sealants.
The peak load for anodized aluminium was higher thanfor that associated with the skinned PE1 foam (1.2Ncompared with 0.9N for the one-part modified siliconeproduct). Whether this difference is significant was notfurther investigated. It is possible that the difference lieswithin standard deviation limits and is not significant. But,because the used test method is fairly accurate, it isassumed that the difference arose from superior wettingbehaviour on the aluminium, or the presence of a higherconcentration of reactive functional groups on the alumi-nium, inducing a stronger chemical interaction.
One-part sealants showed a decrease in their peak loadafter the first 10–60min, and multi-part sealants after2–8 h. The different chemistry and the different curingprocess for one-part and multi-part sealants lead todifferent tack-free times and maximum tack forces.Inherent in this data are the cure characteristics for the
two multi-part sealants. Tack forces increase until a criticalpoint is reached where they reach a maximum. Furthercuring reduces the tack force. This behaviour might beexplained with forming larger oligomeric molecular struc-tures up to a critical point, where an optimal tackbehaviour is found. At this point, entanglements ofoligomer chains are already strong but the concentrationof reactive functional groups is still high. With ongoingreaction, the concentration of these groups reduces andnon-tacky polymer molecules are generated.
5. Summary and conclusions
5.1. Surface free energies
Contact angle modifications with Wenzel’s and theShuttleworth-Bailey equations, both of which considerthe surface roughness of foams, provide results that are toolow (Wenzel) or far too high (Shuttleworth-Bailey). Bothequations are not suitable in order to consider highly roughfoam surfaces, and the development of a new equation,which is valid for highly rough surfaces, would be useful.The most reliable results were obtained assuming an idealsmooth foam surface, even though that does not corre-spond with reality.Impurities in the PE induced relatively high surface free
energies. The bulk (interior) phases of the foams had lowersurface free energies than their surfaces, because fewerimpurities were present.It is difficult to distinguish between metastable contact
angles and Young’s contact angle on foam surfaces, whichled to a relatively high standard deviation. The foams’composite character must be considered if any comparisonswith solid one-phase materials are undertaken.
5.2. Peel tests
Peel forces generally lay between 2.5 and 16N, withpeaks of up to 50N. Even a small peel force is significant interms of sealed joint performance [3,4] and peak forces of50N are clearly very significant. 50N is an average peelforce between a sealant and a substrate such as aluminium.The cured mechanical properties of sealants, such asmodulus (Table 1), exert a significant influence on jointperformance.The sealant and the foam surface have a strong influence
on the peel force. The locus of failure was influencedmainly by the sealant. One-part PU joints failed predomi-nantly in adhesion, while one-part silicone and one-partmodified silicone joints showed a mixed (adhesive andcohesive in foam and sealant) failure. Three-part poly-sulphide and modified silicone polyether joints showedmainly adhesive failure and partly thin film cohesivesealant failure.The reproducibility of the peel test data was low, but a
general qualitative trend in ‘‘adhesion’’ was obtained.
ARTICLE IN PRESS
Adhesive FailureCohesive Failure
Tack behaviour of polyurethaneon skinned PE1 surfacea
Tack behaviour of 1-part modified siliconeon anodized aluminiuma
Peak
'tac
k' lo
ad [
N]
time [minutes]
Fig. 7. Peak ‘tack’ load–time graphs for five sealants against polyethylene foam and aluminium.(a)Curve condition: 231C721C, 50%rh75%rh.(b)Curve condition: 251C731C, 41%rh77%rh.
A.R. Hutchinson, S. Iglauer / International Journal of Adhesion & Adhesives 26 (2006) 555–566564
5.3. Tack tests
Different tack-free times were found for differentsealants: 3-6 h for one-part sealants, 6 h for polysulphideand 24 h for modified silicone polyether. A maximum tackforce point (critical point) was observed for every sealant.The peak tack force of one-part modified silicone wasabout 40% higher on an aluminium substrate than on a PEfoam (PE1). The significance of this difference was not
verified but, based on the accuracy of the test method, it isassumed that different wetting characteristics or differentsurface chemistries are responsible for this difference.
5.4. Adhesion mechanisms
The major contribution to adhesion arises from chemicalinteractions between the foam surface and the uncuredsealant. Electrostatic interaction and interdiffusion was
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not observed, and mechanical interlocking seemed tocontribute only marginally to the adhesion phenomenoninvestigated. In order to be effective, chemical interactionrequires close contact between the surfaces. This implies aninfluence of the surface free energy, which determines thewetting behaviour of the interfaces. High foam surface freeenergy (with a particular sealant) led to good sealantwetting and increased adhesion through chemical interac-tion. This trend was measured with peel tests and isapparent in the total surface free energy (surface tension)–peel force correlations shown in Fig. 8. The quoted surfacetensions of the uncured sealants were obtained in a manneranalogous to the foam surface free energies [3]. In Fig. 8,white squares correspond to skinned foam surfaces andsolid PE and black squares to unskinned foams.
Skinned foams generally had higher total surface freeenergies than unskinned ones, and higher peel forces wereassociated with skinned foams. One-part sealants gave riseto a higher peel force than multi-part sealants, probablybecause their reactive functional groups are more prone toreact with functional groups present on the foam surfacesand to create chemical bonds at the interface.
Fig. 8. Correlation graphs of total foam surface ten
There is a roughly linear dependency of the peel force(see Tables 7 and 8) on the total foam surface free energy(see Table 6), despite the poor reproducibility of peel forcesand inherent problems with measuring contact angles onfoam surfaces. This trend was observed for both skinnedand unskinned foams. The unskinned foam data are morescattered than the skinned foam data, probably because oflarger surface roughness and associated larger measure-ment errors for peel forces and surface free energies.Nevertheless, it can be assumed that a fairly lineardependency of the peel force on the total surface freeenergy is governing all foams, both skinned and unskinned,and that only the measurement errors distort this picture.The main reason for our assumption is that a foam surfacewith a high surface free energy is better wetted by uncuredsealant than a low surface free energy foam surface. Thebetter wetting of the high surface free energy surface thenleads to higher peel forces simply because of a largercontact area between uncured sealant and foam. Thecontact area should be directly proportional to the peelforce. To verify this assumption, more test data arerequired in order to filter out measurement errors, or more
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accurate tests for surface free energy and peel forcemeasurements must be developed.
6. Implications for construction
6.1. Use of backing foam in cladding and curtain walling
Results reported by the authors elsewhere [4] show thatjoints with open cell PU foam incorporated have generallybetter fatigue resistance and lifetimes. It is recommendedthat sealed joints should therefore use an open cell foambacking. If PE foam is used, then it should be of lowcompressive strength and an unskinned (cut) surfaceshould be presented to the sealant.
The contamination of foam with primer should beavoided otherwise the sealant-to-foam adhesion is in-creased and the joint lifetime will be decreased.
6.2. Variations to test standards
Research has investigated the effects of backing foam onsealed joint performance. The results show that backingfoam can significantly influence the performance of asealed joint, and the associated effects should not beignored by sealant manufacturers or end-users.
Current national and international sealant test standardsused to determine the performance of sealant productsdo not require the inclusion of backing foam in testjoints. The outcome of this research has highlighted theneed to introduce backing foam in current and future teststandards, so that the performance of a sealed joint systemis evaluated.
Acknowledgements
The work reported in this paper formed part of a projectfunded by the UK Engineering and Physical SciencesResearch Council. The project was also supported by sevenindustrial collaborators: CellTex, Dow Corning, Fosroc,Honeywell, Sika, Taywood Engineering and Zotefoams.
References
[1] Jones TGB, Lacasse MA. Effect of joint movement on seals
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