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Influence of the free isocyanate content in the adhesive properties of reactive trifunctional polyether urethane quasi-prepolymers
ARTICLE IN PRESS
International Journal of Adhesion & Adhesives 26 (2006) 355–362 www.elsevier.com/locate/ijadhadh
Influence of the free isocyanate content in the adhesive properties of reactive trifunctional polyether urethane quasi-prepolymers Ana Luı´ sa Daniel da Silvaa, Jose´ Miguel Martı´ n-Martı´ neza,, Joa˜o Carlos Moura Bordadob a
Adhesion and Adhesives Laboratory, Department of Inorganic Chemistry, University of Alicante 03080 Alicante, Spain b Chemical Engineering Department, Instituto Superior Te´cnico, Av. Rovisco Pais 1049-001 Lisbon, Portugal Accepted 6 June 2005 Available online 25 July 2005
Abstract The effect of the free isocyanate (NCO) content on the adhesive properties of urethane quasi-prepolymers was studied. Polyetherbased isocyanate ended quasi-prepolymers containing between 13 and 21 wt% NCO were prepared by reacting a trifunctional polypropyleneglycol with polymeric MDI. The quasi-prepolymers were characterized by infrared spectroscopy (IR), proton nuclear magnetic resonance ( 1H-NMR) spectroscopy, Brookfield viscosity, and differential scanning calorimetry (DSC). Immediate adhesion of quasi-prepolymers was studied by means of a probe test performed with a texture analyzer. Adhesion properties of the moisture-cured quasi-prepolymers have been obtained from single lap shear tests of vulcanized styrene–butadiene (R1) rubber/cured quasi-prepolymer joints. The increase in the free isocyanate content produced quasi-prepolymers with lower average chain length and less intermolecular interactions between polymer chains. Due to the presence of free isocyanate molecules, lower Brookfield viscosity values and lower glass transition temperatures were obtained by increasing the free isocyanate content in the quasi-prepolymers. The lower the free NCO content, the higher the cohesion and the immediate adhesion of the quasi-prepolymers because of the increase in the average molecular weight. During debonding, fibrillation was observed in the quasi-prepolymers with free NCO content lower than 20 wt%. Single lap shear strength values of R1 rubber/moisture cured quasi-prepolymers joints showed a particular trend as a function of the increase in the NCO content, which can be ascribed to the opposite trends in the cohesion and adhesion properties of the cured quasi-prepolymers. r 2005 Elsevier Ltd. All rights reserved. Keywords: Polyurethane; Lap-shear; Cure/hardening
1. Introduction Urethane prepolymers are typically isocyanate-tipped macromolecular chains containing urethane groups in their backbone, obtained by reaction of a polyol with a molar excess of di- or polyisocyanate. Typically, they have a free isocyanate (NCO) content between 1 and 15 wt% [1]. They are used in the production of paints [2], adhesives [3], sealants [4], and elastomers [5]. Urethane Corresponding author. Tel.: +34 96 5903977; fax: +34 96 5903454.
E-mail address: [email protected] (J.M. Martı´ n-Martı´ nez). 0143-7496/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2005.06.001
prepolymers containing higher free isocyanate content (15–30 wt%) are called isocyanate quasi-prepolymers or semi-prepolymers because only part of the isocyanate molecules contributes to the formation of the backbone oligomer [6]—Scheme 1. The use of prepolymers as intermediate products helps to tailor the properties of the fully reacted polyurethanes; furthermore, the temperature during the synthesis can be more accurately controlled and the formation of chemical groups different than urethane in the molecular structure (e.g. poly(urethane)urea) is favored. Besides, the prepolymers are less
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Scheme 1.
Scheme 2. Scheme 3.
volatile so less hazardous than the monomeric isocyanates. Urethane prepolymers are also very important in certain two component systems, when a homogeneous mixture is required. Isocyanate quasi-prepolymers are extensively used in the production of one-component polyurethane foams [7,8] in building industry and do-it-yourself adhesives [9]. When exposed to air moisture, the quasi-prepolymer cures as the reaction between the free isocyanate groups and the moisture from both the air and substrate occurs, due to the formation of urea groups in a two-step process (Scheme 2). The polyurethane expands to a low-density froth due to carbon dioxide formation and a polyurethane–urea is obtained. Along with other properties, the froth should have an adequate immediate adhesion to a great number of construction materials, and should last for a long period of time. To our knowledge there have been no previous studies dealing with the correlation between the isocyanate content and the adhesion properties of the isocyanate quasi-prepolymers, although a few papers [10,11] have been devoted to prepolymers with low free isocyanate content. Therefore, the aim of this work was to study the effect of the free isocyanate content in the properties of different isocyanate quasi-prepolymers with special attention to the adhesion performance.
2. Experimental 2.1. Materials The raw materials used in this work to produce the isocyanate quasi-prepolymers were polymeric diphenylmethane diisocyanate (pMDI, Dow Chemical, Barcelona, Spain) and a glycerine propoxylated polyether triol (Voranol CP 1055, Dow Chemical, Barcelona, Spain).
Table 1 Formulation of the sulfur vulcanized styrene-butadiene rubber (R1) Ingredient
Percentage (phr)
SBR1502 SBR1904 Carbon black (N-330) Precipitated silica Sulfur Cumarone–indene resin (85 1C) Zinc oxide Stearic acid N-cyclohexyl-2-benzothiazole sulfenamide Phenolic antioxidant Tetramethyl thiuram disulfide Polyethylene glycol (Mw ¼ 4000)
65.0 35.0 23.0 25.0 1.8 3.5 3.8 0.8 1.1 0.8 0.2 1.1
Percentages are given in parts per hundred parts of rubber (phr).
The pMDI (Scheme 3) consists of a mixture of reactive monomers and oligomers of MDI [9,12]. Typically, the monomer fraction accounts for approximately 45–50 wt% of the pMDI and it is composed of a blend of isomers. The oligomer fraction of pMDI is a complex isomeric mixture of oligomers with functionality higher than two. The pMDI used in this study has a free isocyanate content of 31.3 wt% and an average functionality of 2.7, and was used as received. The polyol used has an average molecular weight of 1000 g/mol and an OH value of 156 mg KOH/g, and is mainly composed of secondary hydroxyl terminal groups. The polyol was dried under reduced pressure for at least 3 h at 60 1C prior to use. A sulfur vulcanized styrene–butadiene rubber (R1) containing carbon black and precipitated silica as fillers, has been used to quantify the adhesive properties of the quasi-prepolymers. The formulation of R1 rubber is given in Table 1. It has a tensile strength of 18 MPa and a Shore A hardness of 93 [13].
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Table 2 Amounts of reactants used to synthesize the isocyanate quasiprepolymers
the prepolymers with 20% and 21% NCO content), RV5 (16% NCO), and RV-7 (13–15% NCO).
NCO (wt%)
Polymeric MDI (g)
Polyol (g)
13 14 15 16 20 21
175 194 196 217 224 254
130 130 120 120 80 80
2.2.3. Immediate adhesion measurements Immediate adhesion of the isocyanate quasi-prepolymers were performed at 2571 1C using a Texture Analyzer TAXT2i (Stable Micro Systems, UK) equipped with a cylindrical and flat stainless steel probe with a diameter of 5 mm. Each quasi-prepolymer film 132 mm thick, was placed below the probe, which was moved at 1 mm/s until a 6 N contact force was reached; the force was maintained for 0.01 s; later, the probe was withdrawn from the quasi-prepolymer at 0.5 mm/s and the immediate adhesion was obtained as the maximum force required to withdraw the probe from the quasiprepolymer film. Experiments were replicated six times for each prepolymer and the average was calculated, with an experimental error lower than 10%.
2.2. Experimental techniques 2.2.1. Synthesis of the isocyanate quasi-prepolymers The isocyanate quasi-prepolymers were prepared by reacting the dried polyol with pMDI in a three-necked 1000 mL glass flask, under dry nitrogen atmosphere. Inert atmosphere was used to avoid the ingress of moisture and the consequent formation of urea linkages. The system was kept at 70 1C and mechanically stirred with a glass stirrer at 80 rpm for 3 h. The isocyanate content in the prepolymer was determined by titration with N,N0 -dibutylamine [14]. After preparation, the prepolymers were stored under dry nitrogen in glass containers for further testing and characterization. About 300 g each of six isocyanate quasi-prepolymers with free NCO content between 13 and 21 wt% were prepared. The amounts of polyol and pMDI used to prepare the isocyanate quasi-prepolymers are given in Table 2. 2.2.2. Characterization of the isocyanate quasiprepolymers IR spectroscopy: The quasi-prepolymers were characterized by Fourier transformed infrared spectroscopy (IR) using the transmission mode, in a Bruker Tensor 27 FTIR spectrometer. A drop of each quasi-prepolymer was placed on a KBr crystal and 100 scans were collected at a resolution of 4 cm1. Proton nuclear magnetic resonance ( 1H-NMR) spectroscopy: 1H-NMR spectra of isocyanate quasi-prepolymers were recorded in a Brucker AC300 spectrometer at 300 MHz, using deuterated chloroform (CDCl3) as solvent and tetramethylsilane (TMS) as internal standard. Differential scanning calorimetry (DSC):. DSC measurements were carried out in a TA Q-100 instrument. Aluminum pans containing 8–9 mg isocyanate quasiprepolymer were heated from 80 to 110 1C at 5 1C/min under a nitrogen atmosphere. Brookfield viscosity: The viscosity of the quasiprepolymers was measured at 2571 1C in a digital Brookfield viscometer model DV-I (Brookfield Engineering Laboratories Inc., USA). The measurements were performed at 10 rpm using the spindles RV-4 (for
2.2.4. Single lap-shear tests Adhesion properties of the moisture-cured quasiprepolymers were obtained from single lap-shear tests of R1 rubber/quasi-prepolymer adhesive joints. R1 rubber strip test samples of 150 30 3 mm3 were used. R1 rubber was selected for adhesion measurement because of its low deformation under shear stress conditions during tests. Before the joint formation the R1 rubber was wiped with MEK. After the solvent evaporation for 10 min, 0.1 g quasi-prepolymer was applied in a square area of 9 cm2 (3 3 cm) on one rubber strip and then immediately joined to other R1 rubber strip without applying pressure. The adhesive joints were kept at 35 1C and 95% relative humidity for 72 h to allow the cure of the isocyanate quasi-prepolymer. Tests were performed in an Adamel L’Homargy DY 32 universal test machine by using a crosshead speed of 20 mm/min. Five replicates were tested and averaged with an error lower than 10%. The locus of failure of the joints was assessed by visual observation and by IR spectroscopy, using the attenuated total reflectance technique (ATR-IR) of the failed surfaces after lap-shear tests. The ATR-IR spectra were collected in a horizontal attenuated total reflectance device provided with a ZnSe prism. 200 scans were collected at a resolution of 4 cm1.
3. Results and discussion The IR spectra of all isocyanate quasi-prepolymers show similar bands. Fig. 1 shows as typical example the IR spectrum of the quasi-prepolymer containing 13 wt% NCO. The most important bands are located at 3400 cm1 (free N–H stretching), 3300 cm1 (N–H hydrogen bonded stretching), 2270 cm1 (free NCO stretching), 1725 cm1 (free CQO stretching in
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358 1.6
2270
1.4
Absorbance
1.2
1609
1525
1100
1600
1.0 0.8 1725
0.6 0.4
3400
0.2 -0.0 4000
3000
2000 Wavenumber(cm-1)
1000
Fig. 1. Transmission IR spectrum of the quasi-prepolymer containing 13 wt% free isocyanate.
urethane), 1600 cm1 (N–H bending in urethane), 1609 cm1 (CQC stretching in aromatic ring), 1525 cm1 (C–N stretching and N–H bending from urethane group), and 1100 cm1 (C–O–C stretching in aliphatic polyether) [15]. Urea groups, which have a typical free carbonyl stretching band at 1690 cm1, were not found in the transmission IR spectra of the isocyanate quasi-prepolymers [16]. Fig. 2a shows the CQO stretching region in the transmission IR spectra of the different isocyanate quasi-prepolymers. For comparison, the transmission IR spectra were normalized to intensity and then multiplied by a factor in such a way that the intensity of the band at 1609 cm1 (CQC stretching in aromatic ring) was approximately the same for all quasiprepolymers. According to Fig. 2a, the lower the free isocyanate content, the higher the intensity of the 1725 and 1600 cm1 bands, and the higher number of urethane groups. This is in agreement with a higher degree of chain extension and a raise in the average molecular weight of the quasi-prepolymer by decreasing its NCO content. Furthermore, the decrease in the NCO content produces a decrease in the intensity of the isocyanate band at 2270 cm1, and the CQO band at 1725 cm1 is gradually displaced to lower wave numbers because of the existence of hydrogen-bonded urethane carbonyl [17–19] produced by an increase in the intermolecular interactions between the polymer chains in the quasi-prepolymers containing smaller NCO content. Fig. 2b shows the N–H stretching region in the transmission IR spectra of the quasi-prepolymers. The lower the free isocyanate content, the lower the intensity of the 3400 cm1 band and the higher the intensity of the 3300 cm1 band, indicating an increase in the intermolecular interactions as the free isocyanate content in the quasi-prepolymer decreases. The isocyanate may react with urethane groups to form allophanate linkages (Scheme 4).
Fig. 2. Carbonyl stretching (a) and N–H stretching (b) IR spectral region of the quasi-prepolymers.
O RN
C
OR'
CONHR'' Scheme 4.
This secondary reaction is undesirable, because of branching and cross-linking of the polyurethane structure is produced. Because the carbonyl group of the allophanate linkage appears at 1450–1460 cm1 [20], it cannot be distinguished in the transmission IR spectra of the quasi-prepolymers because of the overlapping of the CH2 and CH3 groups of the polyol at 1454 cm1. To establish the existence of allophanate groups in the isocyanate quasi-prepolymers, the 1H-NMR spectroscopy can be used [21]. Fig. 3 shows the 1H-NMR spectrum of the quasi-prepolymer containing 13 wt% free isocyanate groups as typical example. The characteristic signals of the protons in the polyol are located at d ¼ 1:1 ppm (–CH3) and d ¼ 324 ppm (–CH2–). The protons of the polymeric MDI show signals at d ¼ 3:9 ppm (–CH2–) and d ¼ 7:027:1 ppm (Ar–H) [22]. The signal at d ¼ 5 ppm can be assigned to the propyleneglycol chain’s end methyne proton close to the
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359
Viscosity (Pa.s)
250 200 150 100 50 0 12
14
16 18 free NCO (wt%)
20
22
Fig. 4. Variation of the Brookfield viscosity (2571 1C) as a function of the free isocyanate content in the quasi-prepolymers.
10.0 9.0
8.0
7.0
6.0 5.0 4.0 δ (ppm)
3.0
2.0
1.0
0.0
Fig. 3. 1H-NMR spectrum of the quasi-prepolymer containing 13 wt% free isocyanate (solvent: CDCl3).
carbamate group [21], and at d ¼ 7:3 ppm, the signal due to the N–H proton in the urethane is identified. The N–H proton of the allophanate has a characteristic signal at d ¼ 10:85 ppm [21], and it does not appear in the 1H-NMR spectra of the quasi-prepolymers. Therefore, allophanate linkages were not in the quasiprepolymers prepared in this study. Fig. 4 shows the variation of the Brookfield viscosity as a function of the free isocyanate content in the quasiprepolymers. There is a decrease in the viscosity of the quasi-prepolymer when its free isocyanate content increases. The viscosity of the quasi-prepolymer depends on both the molecular weight and the amount of unreacted pMDI. pMDI has a lower molecular weight than that of the prepolymer, so it may act as a diluent for the prepolymer molecules. Therefore, the higher the free NCO content, the higher the amount of unreacted pMDI, and lower the viscosity of the quasi-prepolymer. Similar results were obtained by Evans and Litt [23]. By using the Flory–Stockmayer approach to gel formation, these authors have demonstrated that the reduction of the molar ratio between isocyanate and hydroxyl (NCO/ OH) groups conducts to polyurethanes with a greater average molecular weight and lower free isocyanate monomer content, leading to higher values of the viscosity. The DSC thermograms of the isocyanate quasiprepolymers show one glass transition. The glass transition temperatures (Tg) as a function of the free isocyanate content in the quasi-prepolymers are given in Fig. 5. Because only one Tg was obtained for each quasiprepolymer, it seems that there is a good miscibility between the prepolymer chains and the unreacted polymeric MDI. As expected, the Tg values decrease when the free isocyanate content in the quasi-prepolymer increases due to the reduction in the average molecular weight.
Fig. 5. DSC thermograms of the quasi-prepolymers with different free isocyanate content.
The evaluation of the immediate adhesion of the urethane quasi-prepolymers is not simple. In this study, a modified probe test was used to obtain reproducible and significant values of immediate adhesion. Fig. 6a shows as an example the variation of the force applied by the probe on the quasi-prepolymer film containing 20 wt% free NCO as a function of the time. Three regions can be identified in the plot: the first region results from the approach of the probe to the quasiprepolymer surface; the second one from the application of a constant force to the quasi-prepolymer (bonding process); the third region results from the separation of the probe from the quasi-prepolymer surface (debonding process). Several parameters can be calculated from the force-time plots that result from the debonding process [24]. Thus, the initial gradient is an indication of the initial strength of the adhesive’s bond. The maximum peak force corresponds to the immediate adhesive strength, and the area under the entire graph is the adhesive work; the area after the peak force is a measure of the quasi-prepolymer’s cohesiveness. Furthermore, the extent of the penetration of the probe during the application of the constant force can also be related to the quasi-prepolymer’s cohesiveness.
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(a)
(b)
(c) Fig. 6. (a) Force–time plot (modified probe test) of the quasiprepolymer containing 20 wt% free isocyanate. (b) Force-time plots of the quasi-prepolymers. (c) Force–time plot of the quasi-prepolymer containing 13 wt% free isocyanate showing fibrillation during debonding.
Fig. 6b shows the force-time curves for the quasiprepolymers and Table 3 shows the values calculated from the debonding region. The higher the free isocyanate content, the lower the adhesive strength of the quasi-prepolymer. Similarly, the adhesive work, the cohesion, and the slope of the initial gradient decrease by increasing the free NCO content in the quasiprepolymer. According to Hammond [25], the bond strength is greater when the ability of the adhesive to distribute the stresses throughout its volume is higher and this stress-distributing property is provided by high
NCO (wt%)
Peak force (N)
Wadhesion (N s)
Cohesion (N s)
Initial gradient (N/s)
13 14 15 16 20 21
2.570.3 2.470.2 2.170.3 1.670.05 0.870.1 0.570.1
1.270.3 0.670.1 0.670.2 0.470.0 0.1470.04 0.0670.01
1.070.3 0.670.1 0.670.2 0.370.0 0.1170.03 0.0570.01
4775 4571 4575 3872 2574 2072
molecular weight and flexible backbone polymers. Because the average molecular weight of the quasiprepolymers decreases by increasing their isocyanate content, the Hammond hypothesis agrees with our results. Furthermore, Fig. 6b shows a reduction in the time necessary to apply the force to the adhesive when the isocyanate contents are increased, as a result of the lower cohesion of the quasi-prepolymer. A pronounced shoulder after the debonding peak of the force–time plots of the quasi-prepolymers, showing higher adhesive strength, can be found (Fig. 6b). This shoulder results from the formation and growing of fibrils during the debonding process [26]. Fibrillation is produced in a three-step process that includes the formation of the cavities, their expansion, and the formation, growing and rupture of fibrils (Fig. 6c). According to Zosel [27] above a critical stress (sc ) nucleation and expansion of the cavities can occur. The nucleation of cavities in the quasi-prepolymers is responsible for the decrease in tensile force, after the maximum strength is reached. Expansion of the cavities is followed by formation and growing of fibrils due to a progressive orientation of the polymer chains in the direction of the tensile stresses. A further increase in the tensile force can cause both instability and the fracture of the fibrils which lead to a macroscopic cohesive failure, or cause the detachment of the fibrils from the surface, originating from an adhesive failure. Several authors have observed these phenomena during the separation process by high-speed photography or video observation [28–30]. If the tensile stress at which the interface between the probe and the adhesive fails, is lower than sc , the debonding occurs before cavitation starts. This situation was observed in the quasiprepolymers with 20 and 21 wt% free isocyanate. However, the quasi-prepolymers with higher molecular weight between entanglements tend to separate by fibrillation [26,27], i.e., the tendency to form fibrils is favored in the quasi-prepolymers with lower free isocyanate (they contain higher amount of polyol). Single lap shear strength values of R1 rubber/cured quasi-prepolymer joints are shown in Fig. 7. As the free isocyanate content increases, the shear strength initially decreases and for isocyanate percentages higher than
ARTICLE IN PRESS Lap shear strength (kPa)
A.L. Daniel da Silva et al. / International Journal of Adhesion & Adhesives 26 (2006) 355–362 13 % NCO
500
16 % NCO
361 21 % NCO
450 400
Fig. 9. Photographs of the failed surfaces after single lap shear tests of different R1 rubber/quasi-prepolymer adhesive joints. Numbers indicate the free isocyanate content in the quasi-prepolymer.
350 300 12
14
16 18 free NCO (wt%)
20
22
Fig. 7. Effect of the isocyanate content in the quasi-prepolymer on the single lap shear strength values of the R1 rubber/quasi-prepolymer adhesive joints (72 h after curing at 35 1C and 95% relative humidity).
higher adhesion Lap shear strength
N
ups gro O C
MW
,int
erm
ole
cul a
r in
ter
act
ion
s
lower cohesion
free NCO (wt%)
Fig. 10. ATR-IR spectra of the failed surfaces after single lap shear tests of the R1 rubber/quasi-prepolymer containing 13 wt% free isocyanate joint and comparison with the ATR-IR spectrum of the moisture cured 13 wt% isocyanate containing quasi-prepolymer.
Fig. 8. Scheme of the variation of the single lap shear strength values as a function of the free isocyanate content in the quasi-prepolymers.
that of 16 wt%, a strength increase results. This trend can be explained considering the properties of the quasiprepolymers. By raising the percentage of free isocyanate, quasi-prepolymers with lower average molecular weight are obtained in which the intermolecular interactions between polymer chains are less important. Consequently, low cohesion quasi-prepolymers are obtained, giving a low lap shear strength value. At the same time, because the isocyanate reacts with hydrogen containing compounds [6], an increase in the isocyanate content, (which occurs in the quasi-prepolymers with a high free NCO content), should enhance the lap shear strength of the adhesive joints. These two opposite trends are shown in Fig. 8, and the combination of both may explain the observed behavior in the lap shear strength values given in Fig. 7. On the other hand, a mixed failure (i.e. adhesion and cohesive failure in the cured quasi-prepolymer), mainly cohesive in the polyurethane (Fig. 9), are assessed by visual inspection and by ATR-IR spectroscopy. Fig. 10 shows a typical example the ATR-IR spectra of the failed surfaces of a joint prepared with 13 wt% isocyanate containing quasi-prepolymer after single lap shear tests, as well as the ATR-IR spectrum of the same quasi-prepolymer after being cured with moisture. Both failed surfaces spectra are similar to the moisture cured
quasi-prepolymer spectrum, what corroborates the cohesive failure in the polyurethane. The most important bands of the cured quasi-prepolymer are located at 1710 cm1 (free CQO stretching in urea), 1650–1700 cm1 (CQO hydrogen bonded stretching in urea), 1530 cm1 (C–N stretching and N–H bending) and 1510 cm1 (N–H bending in urea). Besides, ATR-IR spectra showed that the band at 2270 cm1 due to free NCO stretching, almost disappear due to the reaction of the quasi-prepolymer with moisture, indicating that the curing reaction was nearly completed.
4. Conclusions The increase in the free isocyanate content produced urethane quasi-prepolymers with lower average chain length and less intermolecular interactions between polymer chains. The higher the free isocyanate content in the quasi-prepolymers, lower the Brookfield viscosity values and lower the glass transition temperatures. The immediate and final bond strength were enhanced in the quasi-prepolymers having a lower free isocyanate content as a consequence of the increase in their molecular weight. When the free isocyanate content was lower than 20 wt%, debonding process occurred by cavitation and formation of fibrils, and this became more marked as the free isocyanate content decreased.
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The increase in the free isocyanate content in the quasi-prepolymers showed a particular variation of the single lap shear strength values of the R1 rubber/ moisture cured quasi-prepolymer joints. This behavior was a consequence of the opposite trends in the cohesion and adhesion properties of the cured quasi-prepolymers. A cohesive failure in the cured quasi-prepolymer was mainly observed, suggesting that a relationship between the single lap shear strength values and the properties of the moisture cured quasi-prepolymers may exist.
Acknowledgments Financial support from ‘‘Fundac- a˜o para a Cieˆncia e a Tecnologia’’ by the Portuguese Foundation for Science and Technology (PhD scholarship SFRH/BD/2988/2000) is acknowledged. Authors also thank Dow Chemical Co. for supplying the raw materials used in this study. References [1] Cranley PE. Epoxy and polyurethane adhesives and sealants. In: Gum WF, Riese W, Ulrich H, editors. Reaction polymers. New York: Hanser Publishers; 1992. p. 720. [2] Short WT, Ottaviani RA. Urea flow control agents for urethane paint prepared by reaction of an isocyanate-terminated prepolymer and an ethanolamine. US Patent no. 4,522,986, 1985. [3] Sheikh N, Katbab AA, Mirzadeh H. Int J Adhes Adhes 2000;20:299–304. [4] Hsieh HW, Mahdi SZ. Polyurethane sealant compositions. US Patent no. 6,015,475, 2000. [5] Broos R, Paap F, Maccari B. Process for preparing polyurethane elastomer from a soft-segment isocyanate-terminated prepolymer. US Patent no. 5,418,259, 1995. [6] Szycher M. Structure–property relations in polyurethanes. In: Szycher’s handbook of polyurethanes. London: CRC Press; 1999. p. 3–13. [7] Harrison RP, Scarpati M, Narayan T, Zagata BJ. Water-blown polyurethane integral skin foam. US Patent no. 5,284,880, 1994.
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International Journal of Adhesion & Adhesives 26 (2006) 355–362 www.elsevier.com/locate/ijadhadh
Influence of the free isocyanate content in the adhesive properties of reactive trifunctional polyether urethane quasi-prepolymers Ana Luı´ sa Daniel da Silvaa, Jose´ Miguel Martı´ n-Martı´ neza,, Joa˜o Carlos Moura Bordadob a
Adhesion and Adhesives Laboratory, Department of Inorganic Chemistry, University of Alicante 03080 Alicante, Spain b Chemical Engineering Department, Instituto Superior Te´cnico, Av. Rovisco Pais 1049-001 Lisbon, Portugal Accepted 6 June 2005 Available online 25 July 2005
Abstract The effect of the free isocyanate (NCO) content on the adhesive properties of urethane quasi-prepolymers was studied. Polyetherbased isocyanate ended quasi-prepolymers containing between 13 and 21 wt% NCO were prepared by reacting a trifunctional polypropyleneglycol with polymeric MDI. The quasi-prepolymers were characterized by infrared spectroscopy (IR), proton nuclear magnetic resonance ( 1H-NMR) spectroscopy, Brookfield viscosity, and differential scanning calorimetry (DSC). Immediate adhesion of quasi-prepolymers was studied by means of a probe test performed with a texture analyzer. Adhesion properties of the moisture-cured quasi-prepolymers have been obtained from single lap shear tests of vulcanized styrene–butadiene (R1) rubber/cured quasi-prepolymer joints. The increase in the free isocyanate content produced quasi-prepolymers with lower average chain length and less intermolecular interactions between polymer chains. Due to the presence of free isocyanate molecules, lower Brookfield viscosity values and lower glass transition temperatures were obtained by increasing the free isocyanate content in the quasi-prepolymers. The lower the free NCO content, the higher the cohesion and the immediate adhesion of the quasi-prepolymers because of the increase in the average molecular weight. During debonding, fibrillation was observed in the quasi-prepolymers with free NCO content lower than 20 wt%. Single lap shear strength values of R1 rubber/moisture cured quasi-prepolymers joints showed a particular trend as a function of the increase in the NCO content, which can be ascribed to the opposite trends in the cohesion and adhesion properties of the cured quasi-prepolymers. r 2005 Elsevier Ltd. All rights reserved. Keywords: Polyurethane; Lap-shear; Cure/hardening
1. Introduction Urethane prepolymers are typically isocyanate-tipped macromolecular chains containing urethane groups in their backbone, obtained by reaction of a polyol with a molar excess of di- or polyisocyanate. Typically, they have a free isocyanate (NCO) content between 1 and 15 wt% [1]. They are used in the production of paints [2], adhesives [3], sealants [4], and elastomers [5]. Urethane Corresponding author. Tel.: +34 96 5903977; fax: +34 96 5903454.
E-mail address: [email protected] (J.M. Martı´ n-Martı´ nez). 0143-7496/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2005.06.001
prepolymers containing higher free isocyanate content (15–30 wt%) are called isocyanate quasi-prepolymers or semi-prepolymers because only part of the isocyanate molecules contributes to the formation of the backbone oligomer [6]—Scheme 1. The use of prepolymers as intermediate products helps to tailor the properties of the fully reacted polyurethanes; furthermore, the temperature during the synthesis can be more accurately controlled and the formation of chemical groups different than urethane in the molecular structure (e.g. poly(urethane)urea) is favored. Besides, the prepolymers are less
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Scheme 1.
Scheme 2. Scheme 3.
volatile so less hazardous than the monomeric isocyanates. Urethane prepolymers are also very important in certain two component systems, when a homogeneous mixture is required. Isocyanate quasi-prepolymers are extensively used in the production of one-component polyurethane foams [7,8] in building industry and do-it-yourself adhesives [9]. When exposed to air moisture, the quasi-prepolymer cures as the reaction between the free isocyanate groups and the moisture from both the air and substrate occurs, due to the formation of urea groups in a two-step process (Scheme 2). The polyurethane expands to a low-density froth due to carbon dioxide formation and a polyurethane–urea is obtained. Along with other properties, the froth should have an adequate immediate adhesion to a great number of construction materials, and should last for a long period of time. To our knowledge there have been no previous studies dealing with the correlation between the isocyanate content and the adhesion properties of the isocyanate quasi-prepolymers, although a few papers [10,11] have been devoted to prepolymers with low free isocyanate content. Therefore, the aim of this work was to study the effect of the free isocyanate content in the properties of different isocyanate quasi-prepolymers with special attention to the adhesion performance.
2. Experimental 2.1. Materials The raw materials used in this work to produce the isocyanate quasi-prepolymers were polymeric diphenylmethane diisocyanate (pMDI, Dow Chemical, Barcelona, Spain) and a glycerine propoxylated polyether triol (Voranol CP 1055, Dow Chemical, Barcelona, Spain).
Table 1 Formulation of the sulfur vulcanized styrene-butadiene rubber (R1) Ingredient
Percentage (phr)
SBR1502 SBR1904 Carbon black (N-330) Precipitated silica Sulfur Cumarone–indene resin (85 1C) Zinc oxide Stearic acid N-cyclohexyl-2-benzothiazole sulfenamide Phenolic antioxidant Tetramethyl thiuram disulfide Polyethylene glycol (Mw ¼ 4000)
65.0 35.0 23.0 25.0 1.8 3.5 3.8 0.8 1.1 0.8 0.2 1.1
Percentages are given in parts per hundred parts of rubber (phr).
The pMDI (Scheme 3) consists of a mixture of reactive monomers and oligomers of MDI [9,12]. Typically, the monomer fraction accounts for approximately 45–50 wt% of the pMDI and it is composed of a blend of isomers. The oligomer fraction of pMDI is a complex isomeric mixture of oligomers with functionality higher than two. The pMDI used in this study has a free isocyanate content of 31.3 wt% and an average functionality of 2.7, and was used as received. The polyol used has an average molecular weight of 1000 g/mol and an OH value of 156 mg KOH/g, and is mainly composed of secondary hydroxyl terminal groups. The polyol was dried under reduced pressure for at least 3 h at 60 1C prior to use. A sulfur vulcanized styrene–butadiene rubber (R1) containing carbon black and precipitated silica as fillers, has been used to quantify the adhesive properties of the quasi-prepolymers. The formulation of R1 rubber is given in Table 1. It has a tensile strength of 18 MPa and a Shore A hardness of 93 [13].
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Table 2 Amounts of reactants used to synthesize the isocyanate quasiprepolymers
the prepolymers with 20% and 21% NCO content), RV5 (16% NCO), and RV-7 (13–15% NCO).
NCO (wt%)
Polymeric MDI (g)
Polyol (g)
13 14 15 16 20 21
175 194 196 217 224 254
130 130 120 120 80 80
2.2.3. Immediate adhesion measurements Immediate adhesion of the isocyanate quasi-prepolymers were performed at 2571 1C using a Texture Analyzer TAXT2i (Stable Micro Systems, UK) equipped with a cylindrical and flat stainless steel probe with a diameter of 5 mm. Each quasi-prepolymer film 132 mm thick, was placed below the probe, which was moved at 1 mm/s until a 6 N contact force was reached; the force was maintained for 0.01 s; later, the probe was withdrawn from the quasi-prepolymer at 0.5 mm/s and the immediate adhesion was obtained as the maximum force required to withdraw the probe from the quasiprepolymer film. Experiments were replicated six times for each prepolymer and the average was calculated, with an experimental error lower than 10%.
2.2. Experimental techniques 2.2.1. Synthesis of the isocyanate quasi-prepolymers The isocyanate quasi-prepolymers were prepared by reacting the dried polyol with pMDI in a three-necked 1000 mL glass flask, under dry nitrogen atmosphere. Inert atmosphere was used to avoid the ingress of moisture and the consequent formation of urea linkages. The system was kept at 70 1C and mechanically stirred with a glass stirrer at 80 rpm for 3 h. The isocyanate content in the prepolymer was determined by titration with N,N0 -dibutylamine [14]. After preparation, the prepolymers were stored under dry nitrogen in glass containers for further testing and characterization. About 300 g each of six isocyanate quasi-prepolymers with free NCO content between 13 and 21 wt% were prepared. The amounts of polyol and pMDI used to prepare the isocyanate quasi-prepolymers are given in Table 2. 2.2.2. Characterization of the isocyanate quasiprepolymers IR spectroscopy: The quasi-prepolymers were characterized by Fourier transformed infrared spectroscopy (IR) using the transmission mode, in a Bruker Tensor 27 FTIR spectrometer. A drop of each quasi-prepolymer was placed on a KBr crystal and 100 scans were collected at a resolution of 4 cm1. Proton nuclear magnetic resonance ( 1H-NMR) spectroscopy: 1H-NMR spectra of isocyanate quasi-prepolymers were recorded in a Brucker AC300 spectrometer at 300 MHz, using deuterated chloroform (CDCl3) as solvent and tetramethylsilane (TMS) as internal standard. Differential scanning calorimetry (DSC):. DSC measurements were carried out in a TA Q-100 instrument. Aluminum pans containing 8–9 mg isocyanate quasiprepolymer were heated from 80 to 110 1C at 5 1C/min under a nitrogen atmosphere. Brookfield viscosity: The viscosity of the quasiprepolymers was measured at 2571 1C in a digital Brookfield viscometer model DV-I (Brookfield Engineering Laboratories Inc., USA). The measurements were performed at 10 rpm using the spindles RV-4 (for
2.2.4. Single lap-shear tests Adhesion properties of the moisture-cured quasiprepolymers were obtained from single lap-shear tests of R1 rubber/quasi-prepolymer adhesive joints. R1 rubber strip test samples of 150 30 3 mm3 were used. R1 rubber was selected for adhesion measurement because of its low deformation under shear stress conditions during tests. Before the joint formation the R1 rubber was wiped with MEK. After the solvent evaporation for 10 min, 0.1 g quasi-prepolymer was applied in a square area of 9 cm2 (3 3 cm) on one rubber strip and then immediately joined to other R1 rubber strip without applying pressure. The adhesive joints were kept at 35 1C and 95% relative humidity for 72 h to allow the cure of the isocyanate quasi-prepolymer. Tests were performed in an Adamel L’Homargy DY 32 universal test machine by using a crosshead speed of 20 mm/min. Five replicates were tested and averaged with an error lower than 10%. The locus of failure of the joints was assessed by visual observation and by IR spectroscopy, using the attenuated total reflectance technique (ATR-IR) of the failed surfaces after lap-shear tests. The ATR-IR spectra were collected in a horizontal attenuated total reflectance device provided with a ZnSe prism. 200 scans were collected at a resolution of 4 cm1.
3. Results and discussion The IR spectra of all isocyanate quasi-prepolymers show similar bands. Fig. 1 shows as typical example the IR spectrum of the quasi-prepolymer containing 13 wt% NCO. The most important bands are located at 3400 cm1 (free N–H stretching), 3300 cm1 (N–H hydrogen bonded stretching), 2270 cm1 (free NCO stretching), 1725 cm1 (free CQO stretching in
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358 1.6
2270
1.4
Absorbance
1.2
1609
1525
1100
1600
1.0 0.8 1725
0.6 0.4
3400
0.2 -0.0 4000
3000
2000 Wavenumber(cm-1)
1000
Fig. 1. Transmission IR spectrum of the quasi-prepolymer containing 13 wt% free isocyanate.
urethane), 1600 cm1 (N–H bending in urethane), 1609 cm1 (CQC stretching in aromatic ring), 1525 cm1 (C–N stretching and N–H bending from urethane group), and 1100 cm1 (C–O–C stretching in aliphatic polyether) [15]. Urea groups, which have a typical free carbonyl stretching band at 1690 cm1, were not found in the transmission IR spectra of the isocyanate quasi-prepolymers [16]. Fig. 2a shows the CQO stretching region in the transmission IR spectra of the different isocyanate quasi-prepolymers. For comparison, the transmission IR spectra were normalized to intensity and then multiplied by a factor in such a way that the intensity of the band at 1609 cm1 (CQC stretching in aromatic ring) was approximately the same for all quasiprepolymers. According to Fig. 2a, the lower the free isocyanate content, the higher the intensity of the 1725 and 1600 cm1 bands, and the higher number of urethane groups. This is in agreement with a higher degree of chain extension and a raise in the average molecular weight of the quasi-prepolymer by decreasing its NCO content. Furthermore, the decrease in the NCO content produces a decrease in the intensity of the isocyanate band at 2270 cm1, and the CQO band at 1725 cm1 is gradually displaced to lower wave numbers because of the existence of hydrogen-bonded urethane carbonyl [17–19] produced by an increase in the intermolecular interactions between the polymer chains in the quasi-prepolymers containing smaller NCO content. Fig. 2b shows the N–H stretching region in the transmission IR spectra of the quasi-prepolymers. The lower the free isocyanate content, the lower the intensity of the 3400 cm1 band and the higher the intensity of the 3300 cm1 band, indicating an increase in the intermolecular interactions as the free isocyanate content in the quasi-prepolymer decreases. The isocyanate may react with urethane groups to form allophanate linkages (Scheme 4).
Fig. 2. Carbonyl stretching (a) and N–H stretching (b) IR spectral region of the quasi-prepolymers.
O RN
C
OR'
CONHR'' Scheme 4.
This secondary reaction is undesirable, because of branching and cross-linking of the polyurethane structure is produced. Because the carbonyl group of the allophanate linkage appears at 1450–1460 cm1 [20], it cannot be distinguished in the transmission IR spectra of the quasi-prepolymers because of the overlapping of the CH2 and CH3 groups of the polyol at 1454 cm1. To establish the existence of allophanate groups in the isocyanate quasi-prepolymers, the 1H-NMR spectroscopy can be used [21]. Fig. 3 shows the 1H-NMR spectrum of the quasi-prepolymer containing 13 wt% free isocyanate groups as typical example. The characteristic signals of the protons in the polyol are located at d ¼ 1:1 ppm (–CH3) and d ¼ 324 ppm (–CH2–). The protons of the polymeric MDI show signals at d ¼ 3:9 ppm (–CH2–) and d ¼ 7:027:1 ppm (Ar–H) [22]. The signal at d ¼ 5 ppm can be assigned to the propyleneglycol chain’s end methyne proton close to the
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359
Viscosity (Pa.s)
250 200 150 100 50 0 12
14
16 18 free NCO (wt%)
20
22
Fig. 4. Variation of the Brookfield viscosity (2571 1C) as a function of the free isocyanate content in the quasi-prepolymers.
10.0 9.0
8.0
7.0
6.0 5.0 4.0 δ (ppm)
3.0
2.0
1.0
0.0
Fig. 3. 1H-NMR spectrum of the quasi-prepolymer containing 13 wt% free isocyanate (solvent: CDCl3).
carbamate group [21], and at d ¼ 7:3 ppm, the signal due to the N–H proton in the urethane is identified. The N–H proton of the allophanate has a characteristic signal at d ¼ 10:85 ppm [21], and it does not appear in the 1H-NMR spectra of the quasi-prepolymers. Therefore, allophanate linkages were not in the quasiprepolymers prepared in this study. Fig. 4 shows the variation of the Brookfield viscosity as a function of the free isocyanate content in the quasiprepolymers. There is a decrease in the viscosity of the quasi-prepolymer when its free isocyanate content increases. The viscosity of the quasi-prepolymer depends on both the molecular weight and the amount of unreacted pMDI. pMDI has a lower molecular weight than that of the prepolymer, so it may act as a diluent for the prepolymer molecules. Therefore, the higher the free NCO content, the higher the amount of unreacted pMDI, and lower the viscosity of the quasi-prepolymer. Similar results were obtained by Evans and Litt [23]. By using the Flory–Stockmayer approach to gel formation, these authors have demonstrated that the reduction of the molar ratio between isocyanate and hydroxyl (NCO/ OH) groups conducts to polyurethanes with a greater average molecular weight and lower free isocyanate monomer content, leading to higher values of the viscosity. The DSC thermograms of the isocyanate quasiprepolymers show one glass transition. The glass transition temperatures (Tg) as a function of the free isocyanate content in the quasi-prepolymers are given in Fig. 5. Because only one Tg was obtained for each quasiprepolymer, it seems that there is a good miscibility between the prepolymer chains and the unreacted polymeric MDI. As expected, the Tg values decrease when the free isocyanate content in the quasi-prepolymer increases due to the reduction in the average molecular weight.
Fig. 5. DSC thermograms of the quasi-prepolymers with different free isocyanate content.
The evaluation of the immediate adhesion of the urethane quasi-prepolymers is not simple. In this study, a modified probe test was used to obtain reproducible and significant values of immediate adhesion. Fig. 6a shows as an example the variation of the force applied by the probe on the quasi-prepolymer film containing 20 wt% free NCO as a function of the time. Three regions can be identified in the plot: the first region results from the approach of the probe to the quasiprepolymer surface; the second one from the application of a constant force to the quasi-prepolymer (bonding process); the third region results from the separation of the probe from the quasi-prepolymer surface (debonding process). Several parameters can be calculated from the force-time plots that result from the debonding process [24]. Thus, the initial gradient is an indication of the initial strength of the adhesive’s bond. The maximum peak force corresponds to the immediate adhesive strength, and the area under the entire graph is the adhesive work; the area after the peak force is a measure of the quasi-prepolymer’s cohesiveness. Furthermore, the extent of the penetration of the probe during the application of the constant force can also be related to the quasi-prepolymer’s cohesiveness.
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(a)
(b)
(c) Fig. 6. (a) Force–time plot (modified probe test) of the quasiprepolymer containing 20 wt% free isocyanate. (b) Force-time plots of the quasi-prepolymers. (c) Force–time plot of the quasi-prepolymer containing 13 wt% free isocyanate showing fibrillation during debonding.
Fig. 6b shows the force-time curves for the quasiprepolymers and Table 3 shows the values calculated from the debonding region. The higher the free isocyanate content, the lower the adhesive strength of the quasi-prepolymer. Similarly, the adhesive work, the cohesion, and the slope of the initial gradient decrease by increasing the free NCO content in the quasiprepolymer. According to Hammond [25], the bond strength is greater when the ability of the adhesive to distribute the stresses throughout its volume is higher and this stress-distributing property is provided by high
NCO (wt%)
Peak force (N)
Wadhesion (N s)
Cohesion (N s)
Initial gradient (N/s)
13 14 15 16 20 21
2.570.3 2.470.2 2.170.3 1.670.05 0.870.1 0.570.1
1.270.3 0.670.1 0.670.2 0.470.0 0.1470.04 0.0670.01
1.070.3 0.670.1 0.670.2 0.370.0 0.1170.03 0.0570.01
4775 4571 4575 3872 2574 2072
molecular weight and flexible backbone polymers. Because the average molecular weight of the quasiprepolymers decreases by increasing their isocyanate content, the Hammond hypothesis agrees with our results. Furthermore, Fig. 6b shows a reduction in the time necessary to apply the force to the adhesive when the isocyanate contents are increased, as a result of the lower cohesion of the quasi-prepolymer. A pronounced shoulder after the debonding peak of the force–time plots of the quasi-prepolymers, showing higher adhesive strength, can be found (Fig. 6b). This shoulder results from the formation and growing of fibrils during the debonding process [26]. Fibrillation is produced in a three-step process that includes the formation of the cavities, their expansion, and the formation, growing and rupture of fibrils (Fig. 6c). According to Zosel [27] above a critical stress (sc ) nucleation and expansion of the cavities can occur. The nucleation of cavities in the quasi-prepolymers is responsible for the decrease in tensile force, after the maximum strength is reached. Expansion of the cavities is followed by formation and growing of fibrils due to a progressive orientation of the polymer chains in the direction of the tensile stresses. A further increase in the tensile force can cause both instability and the fracture of the fibrils which lead to a macroscopic cohesive failure, or cause the detachment of the fibrils from the surface, originating from an adhesive failure. Several authors have observed these phenomena during the separation process by high-speed photography or video observation [28–30]. If the tensile stress at which the interface between the probe and the adhesive fails, is lower than sc , the debonding occurs before cavitation starts. This situation was observed in the quasiprepolymers with 20 and 21 wt% free isocyanate. However, the quasi-prepolymers with higher molecular weight between entanglements tend to separate by fibrillation [26,27], i.e., the tendency to form fibrils is favored in the quasi-prepolymers with lower free isocyanate (they contain higher amount of polyol). Single lap shear strength values of R1 rubber/cured quasi-prepolymer joints are shown in Fig. 7. As the free isocyanate content increases, the shear strength initially decreases and for isocyanate percentages higher than
ARTICLE IN PRESS Lap shear strength (kPa)
A.L. Daniel da Silva et al. / International Journal of Adhesion & Adhesives 26 (2006) 355–362 13 % NCO
500
16 % NCO
361 21 % NCO
450 400
Fig. 9. Photographs of the failed surfaces after single lap shear tests of different R1 rubber/quasi-prepolymer adhesive joints. Numbers indicate the free isocyanate content in the quasi-prepolymer.
350 300 12
14
16 18 free NCO (wt%)
20
22
Fig. 7. Effect of the isocyanate content in the quasi-prepolymer on the single lap shear strength values of the R1 rubber/quasi-prepolymer adhesive joints (72 h after curing at 35 1C and 95% relative humidity).
higher adhesion Lap shear strength
N
ups gro O C
MW
,int
erm
ole
cul a
r in
ter
act
ion
s
lower cohesion
free NCO (wt%)
Fig. 10. ATR-IR spectra of the failed surfaces after single lap shear tests of the R1 rubber/quasi-prepolymer containing 13 wt% free isocyanate joint and comparison with the ATR-IR spectrum of the moisture cured 13 wt% isocyanate containing quasi-prepolymer.
Fig. 8. Scheme of the variation of the single lap shear strength values as a function of the free isocyanate content in the quasi-prepolymers.
that of 16 wt%, a strength increase results. This trend can be explained considering the properties of the quasiprepolymers. By raising the percentage of free isocyanate, quasi-prepolymers with lower average molecular weight are obtained in which the intermolecular interactions between polymer chains are less important. Consequently, low cohesion quasi-prepolymers are obtained, giving a low lap shear strength value. At the same time, because the isocyanate reacts with hydrogen containing compounds [6], an increase in the isocyanate content, (which occurs in the quasi-prepolymers with a high free NCO content), should enhance the lap shear strength of the adhesive joints. These two opposite trends are shown in Fig. 8, and the combination of both may explain the observed behavior in the lap shear strength values given in Fig. 7. On the other hand, a mixed failure (i.e. adhesion and cohesive failure in the cured quasi-prepolymer), mainly cohesive in the polyurethane (Fig. 9), are assessed by visual inspection and by ATR-IR spectroscopy. Fig. 10 shows a typical example the ATR-IR spectra of the failed surfaces of a joint prepared with 13 wt% isocyanate containing quasi-prepolymer after single lap shear tests, as well as the ATR-IR spectrum of the same quasi-prepolymer after being cured with moisture. Both failed surfaces spectra are similar to the moisture cured
quasi-prepolymer spectrum, what corroborates the cohesive failure in the polyurethane. The most important bands of the cured quasi-prepolymer are located at 1710 cm1 (free CQO stretching in urea), 1650–1700 cm1 (CQO hydrogen bonded stretching in urea), 1530 cm1 (C–N stretching and N–H bending) and 1510 cm1 (N–H bending in urea). Besides, ATR-IR spectra showed that the band at 2270 cm1 due to free NCO stretching, almost disappear due to the reaction of the quasi-prepolymer with moisture, indicating that the curing reaction was nearly completed.
4. Conclusions The increase in the free isocyanate content produced urethane quasi-prepolymers with lower average chain length and less intermolecular interactions between polymer chains. The higher the free isocyanate content in the quasi-prepolymers, lower the Brookfield viscosity values and lower the glass transition temperatures. The immediate and final bond strength were enhanced in the quasi-prepolymers having a lower free isocyanate content as a consequence of the increase in their molecular weight. When the free isocyanate content was lower than 20 wt%, debonding process occurred by cavitation and formation of fibrils, and this became more marked as the free isocyanate content decreased.
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The increase in the free isocyanate content in the quasi-prepolymers showed a particular variation of the single lap shear strength values of the R1 rubber/ moisture cured quasi-prepolymer joints. This behavior was a consequence of the opposite trends in the cohesion and adhesion properties of the cured quasi-prepolymers. A cohesive failure in the cured quasi-prepolymer was mainly observed, suggesting that a relationship between the single lap shear strength values and the properties of the moisture cured quasi-prepolymers may exist.
Acknowledgments Financial support from ‘‘Fundac- a˜o para a Cieˆncia e a Tecnologia’’ by the Portuguese Foundation for Science and Technology (PhD scholarship SFRH/BD/2988/2000) is acknowledged. Authors also thank Dow Chemical Co. for supplying the raw materials used in this study. References [1] Cranley PE. Epoxy and polyurethane adhesives and sealants. In: Gum WF, Riese W, Ulrich H, editors. Reaction polymers. New York: Hanser Publishers; 1992. p. 720. [2] Short WT, Ottaviani RA. Urea flow control agents for urethane paint prepared by reaction of an isocyanate-terminated prepolymer and an ethanolamine. US Patent no. 4,522,986, 1985. [3] Sheikh N, Katbab AA, Mirzadeh H. Int J Adhes Adhes 2000;20:299–304. [4] Hsieh HW, Mahdi SZ. Polyurethane sealant compositions. US Patent no. 6,015,475, 2000. [5] Broos R, Paap F, Maccari B. Process for preparing polyurethane elastomer from a soft-segment isocyanate-terminated prepolymer. US Patent no. 5,418,259, 1995. [6] Szycher M. Structure–property relations in polyurethanes. In: Szycher’s handbook of polyurethanes. London: CRC Press; 1999. p. 3–13. [7] Harrison RP, Scarpati M, Narayan T, Zagata BJ. Water-blown polyurethane integral skin foam. US Patent no. 5,284,880, 1994.
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