Effect of calcium carbonate particle size and surface pretreatment on polyurethane composite Part 2 – phase behaviour D. Vrsaljko*1, I. Sˇmit2 and V. Kovacˇevic´1 Effects of untreated calcium carbonate (CaCO3) of different sizes, and silane pretreated CaCO3 fillers on properties of polyurethane (PU) composites (PU/CaCO3) were investigated. Small angle X-ray scattering research of phase structure and morphology has shown significantly stronger influence of the CaCO3 nanofillers than the CaCO3 microfiller on a crystallisation process (domain/ grain restructurisation) of the PU matrix. This influence also increased with higher filler loading. Wide angle X-ray diffractograms investigations proved that the presence of nanofiller in the PU composites affects the nucleation and crystal growth of polyester segment domains. The conclusion is that the nanoparticles of CaCO3 filler are situated preferably in the polyurethane soft segment domains and decrease the crystallite size of polyester domains. Keywords: Polyurethane composites, Small angle X-ray scattering, Wide angle X-ray diffractograms, Silane pretreatment
Introduction In the first part of this paper1 the correlation between the calculated adhesion parameters, i.e. interfacial free energy, thermodynamic reversible work of adhesion and coefficient of wetting, based on measured surface free energies of the pure components by using the contact angle method and mechanical properties of the polyurethane (PU) composites was made. Lower surface free energy of the micro-CaCO3 filler than surface energy of the unmodified and silane modified nanofillers resulted in lower work of adhesion for the PU/micro-CaCO3 than for the PU/nano-CaCO3 composites. These results implied weaker interactions at the PU/micro-CaCO3 than at the PU/nano-CaCO3 composites interface followed by the lower tensile strength. Nanofiller pretreatment by 3-aminopropyltriethoxysilane resulted in the specific silane structures at the filler surface that remained after extraction with solvent and in some extent improved interactions at the interface with the PU matrix.
Experimental Materials Polyurethane used in the present paper was a commercial Bayer linear hydroxyl polyurethane in the form of granules (Desmocoll 130). It is a flexible polyurethane matrix with a medium crystallisation rate which gives
1
Faculty of Chemical Engineering and Technology, University of Zagreb, Marulic´ev trg 19, 10000 Zagreb, Croatia 2 Rud– er Bosˇkovic´ Institute, Bijenicˇka 54, PO box 180, 10002 Zagreb, Croatia *Corresponding author, email
[email protected]
ß 2008 W. S. Maney & Son Ltd. Received 20 September 2007; accepted 14 December 2007 DOI 10.1179/143307508X304264
solutions of low viscosity (200¡100 mPa s measured according to ISO 3219). Fillers used were three types of calcium carbonate (CaCO3), microfiller Kredafil RM-5 (RM) with a primary particle size of 5 mm supplied by Industrochem (Croatia) and two types of nanofillers (fillers with primary particle size ,100 nm) of commercial precipitated calcium carbonates: Socal U1 (U1) and Socal U3 (U3) with primary particle sizes of 80 and 20 nm respectively, both supplied by Solvay (Germany). Silane pretreatment of Socal U3 was carried out with 3-aminopropyltriethoxysilane (ABCR Gmbh & Co. KG, Germany) with concentrations as follows: 2%, 10 g silane on 500 g CaCO3 (10AM); 10%, 50 g silane on 500 g CaCO3 (50AM); 30%, 150 g silane on 500 g CaCO3 (150AM). Details of the Socal U3 silane pretreatment procedure used in this paper are described in Ref. 2.
Composite preparation Into a mixture of the PU matrix and the CaCO3 filler a solvent acetone was added. The weight ratio of the PU matrix to acetone in the mixture was 14–15%. The mixtures were then mixed with a magnetic stirrer 3–4 h until homogenisation. After that the mixtures were poured into a teflon foil covered dish and dried at room temperature for seven days allowing the solvent to gradually evaporate until constant weight. Composites with 3, 6, 12 and 18 vol.-% of the filler in the form of films with thickness y0?4 mm were prepared.
Methods of characterisation Optical microscope (Leica DMLS) equipped with a digital camera (Olympus) was used for the morphology observations of thin crossed microtome sections and prepared thin films under parallel and crossed polars.
Materials Research Innovations
2008
VOL
12
NO
2
72
Vrsaljko et al.
Effect of calcium carbonate particle size and surface pretreatment on PU composite: Part 2
1 Polarisation micrographs of A pure PU, B PU with 18% of U1 filler and C PU with 18% of U3 filler
The small angle X-ray scattering (SAXS) experiments were carried out at the synchrotron facilities of Elettra (Trieste, Italy), on the SAXS beamline,3 using synchrotron radiation with wavelength l50?154 nm (photon energy of 8 keV). The measurements were taken using a Gabriel type, gas filled 1D detector. The spectra were corrected for background intensity and detector response. The wide angle X-ray diffractograms (WAXD) of the films were taken by a Philips diffractometer with monochromatised Cu Ka radiation in the diffraction range of 2h55 to 40u. The degree of crystallinity of PU matrix wc,x, was evaluated by the Ruland method4 wc,x ~
Ic Ic zKIa
(1)
where Ic and Ia are integrated diffraction intensity from the crystalline and amorphous PU phases respectively. The value of correction factor was K51?00 in accordance to proposal of Pompe et al.5 The crystallite size L, was calculated by the Scherrer formula6 from half-maximum width bs of Bragg reflections observed at diffraction angle 2h L~
Kl bs cos h
(2)
where K is a constant that is commonly assigned a value of unity (K51?00) and l is wavelength of the Cu Ka ˚ ). radiation (l51?541 A
Results and discussion Morphology of PU/CaCO3 composites Comparison of polarisation micrographs of the examined samples shows the most extensive morphological changes between the neat PU (Fig. 1a) and the PU composites with nanofillers U1 and U3 (Fig. 1b and c). The SEM micrograph of the neat PU in Part 1 of this paper (Ref. 1) reveals the morphology with bright grains or granules (y1 mm in size) and elongated morphological forms that look like granular aggregates (long up to 10 mm). This observation is in good accordance with the conclusion of Foks et al.7,8 that segmented polyurethanes may reveal, beside the usual spherulites, also granular aggregates of sizes exceeding domains. The granules y1 mm in the present case may correspond to equally sized granular elements. The bundles of parallel stacked lamellae with length of 0?7 to 0?9 mm observed in the TEM microscope were arranged radially around the centre of spherulites.8 These granules are also comparable with small PU spherulites with diameter of the
order of microns.9 The bright grains in polarisation micrographs presented in Fig. 1a are without feature of a Maltese cross even at higher magnification. The introduction of the CaCO3 nanofiller into the PU matrix changes somewhat the morphology of the PU matrix (Fig. 1b). The elongated granular aggregates in neat PU in Fig. 1a (long up to 10 mm) seem to disappear and the polarisation micrographs of PU composite with nanofiller in Fig. 1b and c still exhibit mainly spherical grains or granules. Obviously, the filler nanoparticles increase heterogeneous nuclei density in the PU matrix by decreasing its granular size. The comparison of Fig. 1b with c indicates that smaller particles of the U3 (20 nm) filler affect nucleation of the PU matrix stronger than the U1 (80 nm) particles. This difference in morphology between the PU/U1 and PU/U3 composites contribute to the difference in mechanical properties. Higher increase in elongation at break at optimal filler loading y6 vol.-% was followed by increased strength at the same time in the composite with U1 nanoparticles of lower aggregates than U3, as was shown in Part 1 of this paper.1 The literature10 shows that nanoscale microstructure leads to strong interfacial interactions that might result in the significant improvement of usually opposite composite properties such as strength and elongation at break.
Structural characteristics of PU/CaCO3 composites The SAXS experiments were performed to investigate the supermolecular structure of the PU/CaCO3 composites. It is a study of structural change of PU microphase separated structure consisting of immiscible hard and soft chain segment domains in the PU matrix upon introduction of the different CaCO3 grades. Broad scattering peak in SAXS curve of the neat PU in Fig. 2a indicates a wide domain distribution. The introduction of microfiller into the PU matrix reduces steadily the intensity of this maximum (Fig. 2b and c), whereas this maximum in the SAXS curves of PU/ nanofillers is not recognisable (Fig. 2d–f). The parameters directly determined from the measured intensity in SAXS curves with broad and/or slight maxima, like in the case of some semicrystalline polymers, could be inaccurate. In these cases Lorentz correction is applied to the SAXS curves for determination of the correct positions of peaks.11 Therefore, Lorentz correction was applied for determination of the domain or long periodicity in the PU and PU/CaCO3 composites. The authors use squared form IS2 (or IQ2) instead of a multiplication factor of sin2h according to equation11–13
Materials Research Innovations
2008
VOL
12
NO
2
73
Vrsaljko et al.
Effect of calcium carbonate particle size and surface pretreatment on PU composite: Part 2
2 Small angle X-ray scattering curves of a pure PU and of composites filled with b 3% RM microfiller, c 18% RM microfiller, d 18% U1 nanofiller, e 18% U3 nanofiller and f 18% 150AM nanofiller
I1 (S)~I(S)S2
(3)
where I(S) is the scattering intensity and S is the value of scattering vector (S~ 4p l sin h). This literature11 shows that the absence of a sharp interference peak is an indication of the absence of the regular superstructural organisation. The scattering curve of a real system is always the sum of the scattering intensities of the different phases and interactions between phases. Lorentz corrected SAXS curves of the samples can be seen in Fig. 3. The SAXS curve of the neat PU exhibits two slight broad peaks in addition to the emphasised first maximum (Fig. 3a). Because of partial overlapping, deconvolution of three peaks is shown in the figure inserted in Fig. 3. The S values relationship of these peaks (reciprocal interplanar spacing relationship) S1/S2/ S350?057 : 0?1135 : 0?21
