PREPARATION AND CHARACTERIZATION OF POLYURETHANE - Fe POWDER COMPOSITES

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1 Powder Metallurgy Progress, Vol.11 (2011), No PREPARATION AND CHARACTERIZATION OF POLYURETHANE - Fe POWDER COMPOSITES M. Špírková, R. Bureš, M. Fáberová, M. Trchová, A. Strachota, L. Kaprálková Abstract A series of organic-inorganic composites in the form of films was prepared. The organic polymeric matrix was formed from the commercial product KRASOL LBH P (linear liquid polybutadiene terminated with primary hydroxyl groups), aliphatic diisocyanate (1,6- hexamethylenediisocyanate), and 1,4-butanediol. Powder iron (ASC ) of micrometre size formed the inorganic part of the composite (5 to 80 wt. %). It was found that Fe content considerably influences mechanical, thermomechanical and surface properties, as detected by tensile tests, dynamic mechanical thermal analysis and hardness measurements. While pure organic matrix is typical elastomeric, qualitatively different mechanical properties were detected for composites containing more than 30 % of Fe. Bimodal character of Fe area fraction, hardness and resistivity vs. Fe content dependences were found. Keywords: polyurethanes, polybutadiene diol, Fe powder, organicinorganic composite INTRODUCTION Polyurethane (PU) systems are characterised by extensive range of end-use properties (e.g., densities, hardness, stiffness). They are used in building, transportation, furniture (bedding) and footwear in the form of foams (flexible, semi-rigid, rigid), soft solid elastomers or hard solid plastics [1]. The most common polyurethanes (PUs) are based on polyether (PE) or polyester (PES) polyols and aromatic di- or poly- isocyanates. Polybutadiene- (PB) or polycarbonate- (PC) based polyols belong to the group of specialty polyols due to enhanced end-use properties of PB-PUs and PC-PUs compared with PE-PUs and PES-PUs [1,2,3]. For example, PB-PUs exhibit superior water-resistant properties, high elasticity, very good low-temperature characteristics, excellent insulation characteristics and superior resistance. They are useful mainly in special applications like in encapsulation of electronic components [1]. PB-PU elastomers exhibit also outstanding resistance to aggressive aqueous media (acidic and alkaline solutions) [4]. This paper presents our preliminary results of study of novel PU composites made from PB-based macrodiol, aliphatic diisocyanate, 1,4-butanediol and micrometer-size Fe powder. Dibutyltin dilaurate was used as a catalyst and one-stage procedure was used in all cases. As all films were prepared by an identical procedure, the influence of Fe concentration on structure and surface properties could be tested. Milena Špírková, Miroslava Trchová, Adam Strachota, Ludmila Kaprálková, Institute of Macromolecular Chemistry ASCR, Nanostructured Polymers and Composites Department, Prague, Czech Republic Radovan Bureš, Mária Fáberová, Institute of Materials Research, Slovak Academy of Sciences, Košice, Slovak Republic

2 Powder Metallurgy Progress, Vol.11 (2011), No EXPERIMENTAL Chemicals Polybutadiene diol, α,ω-di(2-hydroxyethyl)-polybutadiene [Krasol LBH-P 2000 (LBH-P)], kindly provided by Kaučuk, now Synthos Kralupy), 1,-6 hexamethylene diisocyanate (HDI; Fluka), butane-1,4-diol (BD, Fluka), dibutyltin dilaurate (DBTDL; Fluka), and Fe powder (ASC , Hoganas AB Sweden) were used as received. Characteristics of LBH-P: M n 2194; water content: 0.01 wt. %, OH value: (0.873 meqiv OH /g), viscosity at 25 o C: mpa.s. Polyurethane composite film preparation As this study is of a preliminary character, all films were prepared without any solvent by one-step procedure in order to gain basic information about the character of the organic-inorganic (O-I) composites compared with pure PU matrix. Macrodiol, BD (chain extender) and Fe powder were mixed and degassed. Finally, HDI and DBTDL (1 % DBTDL in oil Marcol) were put, the mixture was degassed and spread on modified polypropylene sheet at constant layer thickness (500 μm) using a ruler. Samples were kept in the inert milieu at 90 C for 24 h. Ratios ([OH] macrodiol /[OH] extender = 1.0 and [NCO]/([OH] macrodiol + [OH] extender ) = 1.05) were used in all cases. Catalyst concentration, c DBTDL, was equal to wt. %. Fe powder concentration was up 80 wt. %. Methods of characterization ATR FTIR ATR FTIR spectra were measured using a Thermo Nicolet Nexus 870 FT-IR spectrometer (Madison, WI, USA) in an H 2 O-purged environment with MCA detector. The Golden Gate TM single reflection ATR system (Specac Ltd., Orpington, Great Britain) was used to measure the ATR spectra of samples over a wavelength range of cm 1. Typical parameters were: 256 sample scans, resolution 4 cm -1, Happ-Genzel apodization, KBr beamsplitter. Tensile characterization Static mechanical properties were measured on an Instron model 5800 (Instron Limited, UK). Specimens with gauge length 25 mm were tested at 23 C at a test speed of 0.17 mm.s -1. The reported values are averages obtained from at least five specimens. Dynamic Mechanical Thermal Analysis (DMTA) DMTA was carried out on ARES-LS2 from Rheometrics Scientific (now TA instruments) using an oscillation frequency of 1 Hz, deformation ranging automatically from 0.01% (glassy state) to 3.5% (maximum deformation allowed), over a temperature range of 100 to 120 C, at a heating rate of 3 C/min. The standard specimens dimensions were 25 mm x 8mm x 0.5 mm. Storage modulus (G ), loss modulus (G ) and loss factor (tan δ, tan δ = G /G ) were measured. In this work, the glass transition temperature, T g, was defined as (tan δ) maximum. Metallography and image analysis Inverted metallographic Microscope Olympus GX 71 equipped with the polarizer was used for qualitative and quantitative metallograhic analysis and 12 Mpix camera Olympus DP12 for image acquisition. Software ImageJ [5] and Statistica 7 were used for Image analysis, quantitative metallography and statistics evaluation.

3 Powder Metallurgy Progress, Vol.11 (2011), No Hardness Hardness tester HPO 250 with Vickers indentor was used to measure relative hardness of the composites. Two values of relative hardness HV5 and HV10 were measured according to STN EN ISO 6507/1 standard. Electric properties Resistivity of composites was measured using Teraohmmeter Sefelec M1501P. Area of electric contacts was circular with diameter 10 mm. Measurement range from 4 kω to Ω or from 0.01 pa to 20 ma in regime of picoammeter. Measurement voltage was adjustable volt by volt from 1V to 1500VDC. Measurement speed was selectable from 1 to 10 readings/s. RESULTS AND DISCUSSION ATR FTIR ATR FTIR spectra were measured on the pure PU matrix, and on PU O-I composites containing 5 to 80 wt. % of Fe powder. ATR FTIR spectra are shown in the Fig.1, where two distinct dependences (of very similar shape within this type) are very well visible: one type is detected for the pure PU matrix and for composites with Fe contents up to 30 wt. %, and the other was distinguished for composites containing 40 to 80 wt. % of iron. FTIR results strongly support the results of other analytical methods, especially mechanical analysis; see later. Fe [%] = PU_Fe ATR FTIR Absorbance Wavenumbers, cm -1 Fig.1. ATR FTIR spectra of PU-Fe composites (Fe contents is given directly in the Fig.; left).

4 Powder Metallurgy Progress, Vol.11 (2011), No Tensile properties Tensile properties of the neat PU and PU-Fe films were measured. Stress-at break, σ b, elongation-at break, ε b, Young modulus, E, and toughness (expressed as the energy necessary to break the sample) were measured, and they are given in Table1. Tab.1. Tensile properties of PU-Fe composites Code Stress-atbreak [MPa] Elongation-atbreak [%] Young modulus E [MPa] Toughness [mj/mm 3 ] PU-0Fe PU-5 Fe PU-10 Fe PU -20 Fe PU-30 Fe PU- 40 Fe PU- 50 Fe PU- 60 Fe PU- 70 Fe PU- 80 Fe In accordance with FTIR results, two regions of tensile characteristics are detectable: (i) PU matrix and samples with Fe content 30% exhibiting elastomeric character of the neat PU and PU composites: ε b is of 10 2 % order, σ b or E are of 10 0 MPa orders, which leads to toughnes of 10 0 to 10 1 mj/mm 3 ; and (ii) PU composites containing 40 wt.% of Fe are characterised by a dramatic ε b decrease, substantial E increase (three orders of magnitude), ca 5 times σ b increase, but at the same time radical ε b decrease (two orders of magnitude) compared with composites containing 30% wt.% of Fe. Dynamic mechanical thermal analysis (DMTA) Thermomechanical properties were tested by DMTA. Figure 2 shows temperature dependences of storage shear modulus G (Fig.2 a) and tan δ (Fig.2b). Similar to results given by other methods used for PU-Fe composite characterization, two regions are well detectable: samples with lower Fe content ( 30 wt. %) featuring by T g between -20 and -30 o C, while composites with higher Fe contents (up to 50 %) are characteristic by T g at temperature over 50 o C, whereas samples contaning 60 and more % of Fe are without any characteristic (tan δ) peak in the temperature scale from -100 to 120 o C.

5 Powder Metallurgy Progress, Vol.11 (2011), No E+11 1.E shear modulus G' [Pa] 1.E+09 1.E+08 1.E+07 20, E Temperature [ C] tan(delta) [1] , , , Temperature [ C] 0, 10 Fig.2. Storage modulus (a) and tanδ (b) vs. temperature dependences for PU-FE composites. (Fe content in % is given directly in the Figure).

6 Powder Metallurgy Progress, Vol.11 (2011), No Microstructure analysis PU-Fe composites were cut to a cylindrical shape of diameter 10 mm and fixed in epoxy resin, then ground on SiC and polished using spray with polycrystalline diamond 0.25 μm size. Samples were observed in normal and polarised light to evaluate planar distribution of Fe particles and pores. Acquised images in Fig.3 were processed using image analyzer to remove the noise, image optimisation and measurement. Area fraction, size and shape characteristics of iron particles were evaluated (Tab.2). Statistic analysis of measured characteristics in Fig.4 showed that PU-Fe composite exhibits two types of microstructure- Fe content dependences. Transition between microstructure of PU matrix and Fe matrix in composites is not smooth. There are two peaks in statistical distribution of size and shape characteristics. Planar distribution of Fe indicates the same tendency, as shown by changes in area fraction of Fe on metallographic sample surface. The distribution of Fe particles in the composites is homogenous up to 30 wt. % Fe. Iron particles are distributed as aglomerates in the case of high content of Fe wt. %. Distribution of Fe phase is heterogeneous, but distribution of the iron aglomerates is homogeneous. The most heterogeneous microstructure from the view point of Fe distribution was observed in composites with content wt.% of Fe. 20 % Fe 20 % Fe polarised 30 % Fe 30 % Fe polarised 40 % Fe 40 % Fe polarised 50 % Fe 50 % Fe polarised 60 % Fe 60 % Fe polarised 70 % Fe 70 % Fe polarised 80 % Fe 80 % Fe polarised PU-matrix Fig.3. LOM Images of PU-Fe composites observed in normal and polarised light.

7 Powder Metallurgy Progress, Vol.11 (2011), No Tab.2. Size, shape and planar distribution characteristics of Fe particles in composite Sample Area Fraction [%] Feret diameter [μm] Circularity PU-20Fe PU-30Fe PU-40Fe PU-50Fe PU-60Fe

8 Powder Metallurgy Progress, Vol.11 (2011), No Sample Area Fraction [%] Feret diameter [μm] Circularity PU-70Fe PU-80Fe Hardness Hardness was measured for fast information about properties of prepared PU-Fe composites. It is not possible to compare values of hardness with other metallic materials due to a very high elastic deformation and relaxation of (polymeric) PU matrix. However, the measurement of the relative hardness is a useful when comparing changes of surface hardness properties in in relation to Fe content, as shown Table 3. It is evident that values of relative Vickers hardness increase with increasing Fe content, with two step changes: between 20 and 30 wt. % of Fe, and between 30 and 40 wt. % of Fe. The values of standard deviation in Table 3. validate sufficient accuracy of the hardness measurements. Tab.3. Relative Vickers hardness of PU-Fe composites. Sample HV5 HV10 Standard deviation PU-0 Fe PU-5Fe PU-10Fe PU-20Fe PU-30Fe PU-40Fe PU-50Fe PU-60Fe PU-70Fe PU-80Fe HV5 and HV10 mean values calculated from 11 measurements Resistivity Values of resistivity measured at voltage 10 V, dwell time 600 sec. are shown in Table 4. Sample thickness was measured after 600 s after applying pressure to the holder, because the pressure of the clamp holder on the composite caused thickness changes during

9 Powder Metallurgy Progress, Vol.11 (2011), No measurement. Dimensional changes influenced by density and resistivity of composite were tested. In general, expected could be monotonic decreasing function of resistance on Fe content. It was found that measured dependence resulted in two maxima, similary as in the case of metallografic analysis (Fig.4). Tab.4. Resistivity of PU-Fe composites Sample Thickness [mm] Resistance [TΩ] * Normalised Resistance [TΩ] Specific Resistivity [Ωm] PU E11 PU-5Fe E11 PU-10Fe E11 PU-20Fe E10 PU-30Fe E10 PU-40Fe E11 PU-50Fe E11 PU-60Fe E11 PU-70Fe E11 PU-80Fe E9 * Resistance normalised to thickness 100 μm Fig.4. Trend curves of dependence of evaluated properties on Fe content: area fraction of Fe(a), hardness (b), resistivity(c). The tendency to multimodality of relation between selected characteristic of composite and Fe content is documented in Fig.4. Curves of area fraction of Fe (a), hardness (b) and resistivity (c) vs. Fe content are bimodal. First peak is related to PUmatrix composite and second one corresponds with composition of Fe-matrix composite.

10 Powder Metallurgy Progress, Vol.11 (2011), No Transition between the composite structures is not smooth and it is accompanied by step change of the properties, as evident from tensile, DMTA and microstructure analysis. CONCLUSIONS The series of novel PU-Fe composites with content of Fe from 5 to 80 wt. % was prepared in the form of free-standing films. Analyses of mechanical and electrical properties indicate two typical combinations of properties of the investigated composite. Low contents of Fe up to 30 wt.% lead to elastomeric composite with homogeneous distribution of iron particles and lower content of small bubles ( PU-matrix composite ). These composites exhibit high specific resistivity, high ductility, high toughness and relative low hardness and low tensile strength. Contents about 70 to 80 wt. % of iron lead to agglomeration of Fe in composites ( Fe-matrix composite ). Hardness and strength of these composites are higher, specific resistivity is also high, while ductility and toughness rapidly decrease. Mixed microstructure with Fe particle and Fe aglomerates was observed in the range between 30 and 60 wt.% of Fe content, as proved by statistic image analysis of these composites. Transition microstructure causes anomalous changes in properties of PU-Fe composites. Acknowledgement Presented results were obtained within the work on the projects P108/10/0195 (Grant Agency of the Czech Republic; IMC) and VEGA 2/0149/09 (IMR). REFERENCES [1] Hepburn, C.: Polyurethane Elastomers. London : Elsevier Science Publishers, 1992 [2] Špírková, M., Matějka, L., Hlavatá, D., Meissner, B., Pytela, J.: J. Appl. Polym. Sci., vol. 77, 2000, no. 2, p. 381 [3] Špírková, M., Pavličević, J., Strachota, A., Poreba, R., Bera, O., Kaprálková, L., Baldrian, J., Šlouf, M., Lazić, N., Budinski-Simendić, J.: J. Eur. Polym. J., vol. 47, 2011, p. 959 [4] Pytela, J., Sufčák, M. In: Proceedings of the International Polyurethane Industry Conference, UTECH The Hague, 2000, Paper 9 [5] Rasband, WS.: ImageJ, U. S. National Institutes of Health, Bethesda, Maryland, USA,