Figure 4. Comparative plots of (a) normalised hydrogen-bonded urea and (b) normalised intermediate urea as a function of time for the unfilled (UF) and PLSN forms based on CNa+ and C30B.

An FTIR study of Reaction Kinetics and Structure Developmentin Flexible Polyurethane Foam-Layered Silicate Nanocomposites

1.Centre for Materials Science Research, Dalton Research Institute, Manchester Metropolitan University, Chester Street, Manchester, M1 5GD, UK

2.School of Materials, the University of Manchester, Grosvenor Street, Manchester, M1 7HS, UK

KATO, R.1, LIAUW, C. M.1, ALLEN, N. S.1, WILKINSON, A. N.2, STANFORD, J. L.2, FITHRIYAH, N. H.2

ConclusionsAdTR measurements and forced-adiabatic FTIR spectroscopy can be used to investigate the reaction kinetics and structure development in the PLSN foams. The consumption of TDI during the initial stages of the PU copolymerisation was significantly accelerated by the addition of CNa+ and C30B. However, the formation of hydrogen-bonded urea is prevented by the addition of ≥5 wt% CNa+, but is only hindered by the addition of ≥5 wt% C30B probably due to weaker interaction between urea and the C30B.

References[1] Ray, S. S., Okamoto, M., Prog. Polym. Sci., 2003, 28, 1539-1641.

[2] Elwell, M. J., Ryan, A. J, Polymer, 1996, 37, 1353-1361.

Figure 3. Time-resolved FTIR-spectra for the region 1800-1600 cm-1 for the unfilled (UF) and PLSN foams based on CNa+ and C30B.

E-mail: R.Kato@mmu.ac.uk

Figure 1 is a schematic representation of the formation of a flexible PU foam, showing that the simultaneous reactions of toluene diisocyanate (TDI) with the polyether polyol and water produces a segmented multiblock copoly(urethane-urea), which is blown into a foam by the cogeneration of carbon dioxide gas evolved from the water-isocyanate reaction.

1) urea and urethane formation

2) segmented multiblock copoly(urethane-urea) formation

3) phase separation

Figure 1. Flexible PU foam: chemistry and structure.

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IntroductionPolymer layered silicate nanocomposites (PLSN) have been extensively studied in recent years, because PLSN possess many desirable materials properties relative to pristine polymers [1]. Successful formation of a PLSN is dependent on penetration of monomer or polymer into the gallery of layered silicate during polymerisation or melt compounding. There have been a number of studies on PLSN based on solid PU matrices [1], but relatively few on cellular PU nanocomposites. In this study, adiabatic temperature rise (AdTR) measurements and forced-adiabatic FTIR spectroscopy were used to determine the kinetics of both the PU copolymerization and of the microphase separation between poly(ether-urethane) soft segments and polyurea hard segments.

ExperimentalThe sodium montmorillonite (Na-MMT) and organically modified montmorillonite (o-MMT) used were Cloisite® Na+ (CNa+) and Cloisite® 30B (C30B, MMT modified with methyl tallow bis-2-hydroxyethyl ammonium chloride) from Southern Clay Products, respectively. Five foams were produced, namely: an unfilled foam (designated as UF), PLSN foams based on 1 and 5 wt% CNa+ (designated as 1%CNa+ and 5%CNa+), and PLSN foams based on 1 and 5 wt% C30B (designated as 1%C30B and 5%C30B ). Adiabatic temperature rise (AdTR) measurements were carried out using two thermocouples positioned at the midpoint of the mould. Forced-adiabatic Fourier transform-infrared (FTIR) spectroscopy was conducted in attenuated total reflectance (a.t.r.) mode. Both AdTR and FTIR were conducted at the School of Materials, the University of Manchester.

Figure 2. AdTR profiles for unfilled and PLSN foams based on CNa+ and C30B.

Result and DiscussionAdTR measurements

In the initial stage (t < 240 s), it is evident that there is a significant increase in the rate of reaction upon addition of CNa+ and C30B (Figure 2). As the overall heat rise in these systems is expected to be dominated by the reaction between TDI and water (with a molar ratio of reactive groups, water/polyol = 6.4), these significant increases in heat generation by the addition of CNa+ indicate that the water molecules associated with Na+ ions in the Na-MMT are more readily accessible to the TDI. In the case of the PLSN foams based on C30B, it may be that only hydroxyl groups (mainly Al-OH and Si-OH) present at the platelet edges of C30B catalyze the reaction of water molecules with TDI.

Forced-adiabatic FTIR spectroscopy

The evolution of hydrogen-bonded urea (1637 cm-1) and intermediate urea (1655 cm-1) [2] during the foaming reaction can be followed by monitoring the carbonyl region of the FTIR spectrum (Figure 3). In the case of unfilled foam (UF) and PLSN foams containing 1%CNa+ and 1%C30B, the development of hydrogen-bonded (H-bonded) urea at 1637 cm-1 is apparent. For the PLSN foams based on 5%CNa+, an absorption at 1655 cm-1 (intermediate urea) is evident but that at 1637 cm-1 is absent. In contrast, PLSN foam based on 5%C30B shows the development of H-bonded urea. However, the level of H-bonded urea decreased with increasing C30B loading. Figure 4 shows the growth of H-bonded urea and intermediate urea for the unfilled and PLSN foams. It is evident that development of H-bonded urea was observed to occur more quickly in the PLSN foams (Figure 4a). However, H-bonded urea is not formed in the PLSN foams containing 5%CNa+. This may be due to strong interaction of soluble urea with hydrated Na+ ions in CNa+, which would hinder the formation of the H-bonded urea.

The development of H-bonded urea also occurred more quickly in the PLSN foam containing 1%C30B relative to UF. However, the PLSN foams containing 5%C30B exhibited the slower development of H-bonded urea than the UF. Moreover, the formation of the H-bonded urea decreased with increasing C30B content. The interaction of soluble urea with C30B is relatively weak, compared to CNa+, and hence H-bonded urea can form but the rate of formation may be retarded.

The development in the intensity of intermediate urea at 1655 cm-1 can also be used to monitor the structural change during PU foaming reaction (Figure 4b). Interestingly, all the PLSN foams showed more rapid development of intermediate urea than the UF. It may be assumed that the onset of microphase separation occurred at the time of intermediate urea formation. After microphase separation, the intermediate urea and MMT would be concentrated within the hard segment phase. Thus, at MMT loadings above 5 wt%, the formation of H-bonded urea may be prevented by the strong interactions with CNa+ but only hindered by the relatively weak interactions with C30B.

Unfilled foam PLSN foam based on 1%CNa+ PLSN foam based on 1%C30B

PLSN foam based on 5%CNa+ PLSN foam based on 5%C30B

1637 1637 1637

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hydrogen-bonded urea

(1637 cm-1)

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