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Thermal and mechanical properties of polyurethane rigid foam/modified nanosilica composite.

INTRODUCTION

Polyurethanes (PUs) are unique polymer materials with a wide range of physical and chemical properties. With well-designed combination of monomeric materials, PUs can be tailored to meet the diversified demands of various applications, such as coatings, adhe-sives, fibers, thermoplastic elastomers, and foams. A wide range of PU foams from flexible to rigid or from tough to stiff elastomers allows them to use in diverse applications. PU foams have a remarkably broad range of applications, including thermal insulation, cushioning, buoyancy, energy absorption (packaging), etc. Their low density also permits the design of light and stiff compounds, such as aircraft interior panels, structural shapes (transom core, bulkhead core, stringers, motor mounts, etc.). However, PUs also have some disadvantages, such as low thermal stability and low mechanical strength. To overcome these disadvantages, great deals of effort have been devoted to the development of nanocompo-sites in recent years (1-3). Nanocomposites are a new class of materials with improved physical properties, such as thermal, mechanical, and barrier properties, as compared with conventional composites (microcomposites), because of the much stronger interfacial interactions between the dispersed nanometer-sized domains and the matrices (4). Among the inorganic nanoparticles, nanosilica is one of the most interesting materials that many researches have been reported about. To obtain a stronger interaction between the organic and inorganic components, the functionalized silanes are used as coupling agents. These can react with functional groups already present on the polymer molecules, and also can react with silanol groups on the silica surface (3). There were many studies about silica nanocomposites (4-9), also, Lee and his coworkers prepared PU/silica composite via in-situ polymerization in the presence of chemically gamma-glycidoxypropyltrimethoxysilane (GPTS) modified silicas. They reported modification by y-glyci-doxypropyltrimethoxysilane and further modification by diethanol amine and demonstrated the enhanced tensile strength in first modification, when compared with the pure PU. When the doubly modified silica was used, the tensile strength of resulting composites was even more enhanced (4). Chemical modification of nanosilica with 3-aminopropyltriethoxysilane has been reported by Chen et al. (9). They used modified nanosilica silica grafted 3-aminopropyltriethoxysilane (SIAP) in PU elastomer, and reported that the glass transition temperature ([T.sub.g]) of hybrid PU nanocomposites was lower than those of PU nanocomposites without modification of nanosilica. Also, functionalized silica can be dispersed well in the PU matrix via the grafting reaction, when compared with single silica nanoparticles. In this work, the effects of modified nanosilica with n-(2-aminoethyl)-3-aminopropyltrimethoxy-silane (AEAP) on thermal, mechanical, and morphological properties of PU rigid foam were investigated. Characterization was done by Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), thermomechanical analysis (TMA), and scanning electron microscopy (SEM). The tensile properties were also evaluated.

EXPERIMENTAL

Materials

DaltoFoam TA[R] 14,066 polyether polyol (viscous yellow liquid, viscosity: 5260 cps at 25[degrees]C and water content: 2.3%) containing all of required additives and methylene diphenyl diisocyanate (MDI) (Suprasec[R]5005, dark blown liquid, viscosity: 220 cps at 25[degrees]C) for rigid PU foam formulation were purchased from Huntsman. Fumed nanosilica (Aerosil A200, spherical shape and with hydrophilic nature, surface area 200 [m.sup.2][g.sup.-1]) was purchased from Degussa. AEAP (wetting area 358 [m.sup.2] [g.sup.-1]) and toluene were purchased from Merck.

Instruments

Coating of nanosilica surface was confirmed by FT-IR spectra, which were recorded with a Brucker Tensor 27 spectrophotometer. TGA was performed with Perkin-Elmer Pyris Diamond TG/DTA at a heating rate of 20[degrees]C/ min for the analysis of pure and modified nanosilica, and 10[degrees]C/min for foam sample under [N.sub.2] atmosphere.

Tensile test was done using a Hegewald & Peschke-Me[beta] instrument, according to ASTM D412. In this test, Dumbbell specimens were prepared using die. The test specimens cut from cup samples were not <1.3 mm (0.05 inches) and not more than 3.3 mm (0.13 inches thick. The specimen shall be free of surface roughness, fabric layers, etc. They are elongated at a strain rate of 50 mm/min.

Dynamic mechanical analysis was carried out by Dupont DMA 983 instrument according to ASTM E1640. Test specimens are cut (20 x 10.5 x 2.8 mm) from the center of sample and clamped between the movable and stationary fixtures, and then enclosed in thermal chamber. The analyzer applies torsional oscillation to the test sample while slowly moving through the specified temperature range (-150[degrees]C to + 100[degrees]C) at 1 Hz with heating rate 5[degrees]C/min.

Thermomechanical analysis was done on Dupont 2000 instrument. The samples may be of any uniform size or shape, but are ordinarily analyzed in rectangular form. Because of the numerous type of dynamic mechanical analyzers, sample size is not fixed by this method. In many cases, specimens measuring between 1 x 5 x 20 mm and 1 x 10 x 50 mm are suitable. The heating rate was 5[degrees]C/min.

Morphological studies were examined by a Cambridge S360 SEM. The samples were coated with a thin layer of gold by Bio-Rad, E5200 auto sputter coater before observation. To disperse the modified nanosilica in polymer matrix, a ultrasonic homogenizer (IKA[R] T25 Digital) was used.

Preparation of Modified Nanosilica

The amount of AEAP incorporated on to the nanosilica surface could be calculated according to the following formula: (4)

[W.sub.1] (g) = m[W.sub.2] (g) x [[A.sub.nanosilica]/[A.sub.AEAP]], (1)

where [W.sub.1] is weight of AEAP, m is the coefficient (multiple), [W.sub.2] is the weight of silica, [A.sub.nanosilica] is the specific surface area of silica (200 [m.sup.2]/g), and [A.sub.AEAP] is the wetting area of AEAP (358 [m.sup.2]/g). m is usually taken one.

In the present case, 5 g nanosilica in 200 ml toluene was refluxed with mechanical vigorous stirring at 80-90[degrees]C for 2 h. Then, 2.79 g AEAP was added to this solution. The mixture was refluxed for 12 h under the same condition. After isolation and washing with methanol the free AEAP was removed. Finally, modified nanosilica powder (AEAP-silica) was dried in an oven at 70[degrees]C for 24 h and sieved under 125 [micro]m mesh. The schematic structure of modified nanosilica was shown in Fig. 1.

[FIGURE 1 OMITTED]

Preparation of Polyurethane Rigid Foam/Modified Nanosilica Composite

In the first step, modified nanosilica was dispersed in polyol matrix in weight percent of 1, 2, and 3 by vigorous stirring, and sonication with mechanical stirrer (20 min), and ultrasonic homogenizer (20 min), respectively. Polyol is selected for infusion of modified nanosilica as it is less reactive. In the next step, nanoparticulated polyol matrix was hand mixed with MDI in open mold and 300 ml paper cups at 1:1 (polyokMDI) ratio. The samples were allowed to remain at room temperature for 24 h for complete curing and testing.

RESULTS AND DISCUSSION

FT-IR Spectra

Modification of nanosilica was confirmed by FT-IR spectroscopy (see Fig. 2). In the AEAP-silica spectrum, it was seen that the absorption peaks of the C--H stretching vibrations appeared just below 3000 [cm.sup.-1]. This indicates the formation of covalent bond between silanol group on the nanosilica surface, and AEAP was formed (4). The peaks at 1097 and 470 [cm.sup.-1] are attributed to stretching and bending vibrations of Si--O, respectively. Also in Aerosil spectrum, absorption peak at 3442 [cm.sup.-1] is related to silanol group on nanosilica surface.

[FIGURE 2 OMITTED]

Thermogravimetric Analysis

Another method for confirmation of surface coating is TGA. Figure 3 represents the thermograms of pure and modified nanosilica. The weight loss of pure nanosilica was about 4% at 700[degrees]C. It was attributed to adsorbed water on the nanosilica surface. In this temperature (plateau region), the weight loss of modified nanosilica was about 19%. Taking into account of weight loss of pure nanosilica, it could be estimated that the content of organic moiety on the nanosilica surface was about 15%.

[FIGURE 3 OMITTED]

On the other hand, the thermal stability of PU rigid foams including modified nanoparticles was evaluated by this technique. As shown in Fig. 4, with increasing of modified nanosilica, thermal stability was enhanced, because these particles acted as a thermal insulator.

[FIGURE 4 OMITTED]

Mechanical Properties

Table 1 represented the tensile data of PU rigid foams with different percent of modified nanosilica. With increasing of modified nanosilica elastic modulus was enhanced, because of the additional covalent bond and interfacial interaction in boundary layer. Also tensile strength was increased, because of acting of nanosilica as barriers against fracture growth (4). Elongation at break was reduced upto 2%, because additional hydrogen bonding hindered the chain mobility. This parameter in 3% sample had more value. This was attributed to the act of modified nanosilica as a chain extender to some extent.
TABLE 1. Mechanical properties of PU rigid foam with different percent
of modified nanosilica.

Samples (%)  Elastic modulus (MPa)  Tensile strength  Elongation at
                                         (MPa)           break (%)

     0                3.7                 0.5                31
     1                6.3                 0.54               26
     2                6.7                 0.66               24
     3                7.1                 0.68               32

PU, polyurethane.


Dynamic Mechanical Analysis

Dynamic mechanical analysis was used to examine the viscoelastic response of the materials to cyclic deformation and temperature. Figure 5 represented the storage modulus (E') of PU rigid foams with different percent of modified nanosilica. The samples including modified nanosilica, showed reduction in storage modulus in comparison with blank samples. This indicated that the hard phase formation was reduced. Interfacial interaction between functional group (-NH) on the nanosilica surface and free (--NCO) group in bulk polymer disrupted stoichiometry ratio, so that hard phase domain was limited. The loss modulus (E") of these samples was shown in Fig. 6. In 2% and 3% samples, E" was decreased owing to decrease in hard segment. In 1% sample, different result was found. It showed more value in comparison to blank sample. As the loss modulus is viscous response of PU matrix to deformation and by taking loss factor ([E"/E']) into consideration (see Fig. 7), it could be expressed that in the case of 1% sample viscous behavior predominated over elastic behavior, and consequently loss factor (tan [delta]) was enhanced. It means that the increase in damping ability of PU matrix due to reduction of crosslinking density in bulk polymer. It can be concluded that 1% sample has less hard phase domain when compared with other samples. However, the interfacial interaction in boundary layer decreased crosslink density in bulk matrix, and it is believed that the static mechanical behavior was controlled by interfacial properties, and bulk properties influenced dynamic behavior. Similar to these results (enhancement in static mechanical properties and reduction in dynamic mechanical properties), in a type of nanocomposite based on PU was seen, which was reported by Xiong et al. (10) and gave no reason for these observations.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Thermomechanical Analysis

Thermomechanical Analysis results were shown in Fig. 8. The information that can be found from TMA are glass transition temperature ([T.sub.g]) and linear thermal expansion coefficient ([alpha]). In these graphs, [T.sub.g] is where dimension change was strongly decreased. The samples with modified nanosilica, showed lower [T.sub.g] value than pure PU (see Fig. 9). The reduction in hard phase formation and crosslink degree and also increase in soft segment mobility, resulted in decrease in [T.sub.g]. On the other hand, an increase in [alpha]-value in nano-filled samples confirmed dimension instability and reduction in elastic property of cell walls (see Fig. 10).

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

Morphological Studies

The value of cell density of these foams is given in Table 2. Cell density of a cellular structure is estimated using the procedure outlined in reference.
TABLE 2. Cell density of PU rigid foam with different percent on
modified nanosilica.

                Samples (%)                 0     1     2     3

[N.sub.f]([cell/[cm.sup.3]] x [10.sup.5])  1.06  2.23  2.99  6.11

PU, polyurethane.


Cell density ([N.sub.f]) is calculated using the following formula:

[N.sub.f] = [([n[M.sup.2]/A]).sup.[3/2]], (2)

where n is the number of cells, A is the area of the micrograph in [cm.sup.2], and M is the magnification factor that is taken x40 [KAPPA] in this case.

Also, the cell size could be calculated from SEM images. The cell size (d) is measured by dividing the total area of the SEM image by the number of cells, and then the square root is taken to determine the average diameter of a cell (11). On this background, the cell size of foam samples are given in Table 3.
TABLE 3. Cell size of PU rigid foam with different percent on modified
nanosilica.

    Samples (%)      0      1      2      3

Cell size ([mu]m)  695.6  547.7  473.1  396.3

PU, polyurethane.


Scanning electron microscopy image of these samples are shown in Fig. 11. All foams exhibited polygon closed-cell structure with energetically stable pentagonal and hexagonal faces. With increasing in modified nanosilica, cell density was increased and cell size was reduced. These results indicate that the nature of the dispersion plays a vital role in controlling the size of the cell during foaming. Final foam density is controlled by the competitive process in the cell nucleation, its growth, and coalescence. The cell nucleation took place in the boundary between the matrix polymer and the dispersed modified nanosilica (12). These particles act as heterogeneous sites during cell formation (13).

[FIGURE 11 OMITTED]

CONCLUSIONS

The presence of reactive functional group on nanosilica surface influenced the physical properties. Thermal stability of nano-filled foams increased because nanosilica acts as a thermal barrier. In addition, the formation of covalent bonds in the boundary layer improved static mechanical properties in modified nanosilica-contained samples. Obtained data confirmed that the static properties were affected by reinforced interfacial area. On the other hand, dynamic mechanical behavior was controlled by bulk matrix. As the stoichiometry ratio was disrupted by reactive functional groups, the hard phase domain was limited and consequently the storage modulus (E') decreased. Also, the loss modulus (E") diminished (except for 1% sample). The increase of loss factor (tan [delta]) indicated enhanced damping ability of PU owing to low crosslink density of bulk matrix. TMA results showed reduced [T.sub.g] in nano-filled samples. It was because of prevention of soft segment mobility and reduction of hard phase. Moreover, linear thermal expansion coefficient ([alpha]) increased with increasing in modified nanosilica. It was because of decrease of the elastic property of cell walls. The morphological studies showed that the modified nanosilica acted as nucleation sites during cell formation, which led to higher cell density and lower cell size.

REFERENCES

(1.) M. Szycher, Szycher's Handbook of Polyurethane, CRC Press, Boca Raton, FL (1999).

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(3.) H.T. Jeon. M.K. Jang. B.K. Kim. and K.H. Kim. Colloids Surf. A, 302, 559 (2007).

(4.) S. Lee, D.B. Kim, J.H. Sin, Y.S. Lee, and C.H. Nan, J. Ind. Eng. Chem., 13(5), 786 (2007).

(5.) X. Chen. L. Wu, S. Zhou, and B. You. Polym. Int., 52. 993 (2003).

(6.) S.I. Lee, Y.B. Hahn, K.S. Nahm, and Y.S. Lee. Polym, Adv. Technol., 16. 328 (2005).

(7.) R.C.R. Nunes, J.L.C. Fonseca, and M.R. Pereira, Polym. Test., 19, 93 (2000).

(8.) Z.S. Petrovic, I. Javni, A. Walddon, and G. Banhegyi, J. Appl. Polym. Sci., 76, 135 (2000).

(9.) S. Chen. J. Sui, and L. Chen. Colloid Polym. Sci., 283, 66 (2004).

(10.) J. Xiong, Y. Liu, X. Yang, and X. Wang, Polym. Degrad. Stab., 86. 549 (2004).

(11.) X. Han. C. Zeng, L.J. Lee. K.W. Koelling. and D.L. Tomasko, Polym. Eng. Sci.. 43, 1261 (2003).

(12.) S.S. Ray and M. Bousmina, Prog. Mater. Sci. 50, 962 (2005).

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Mir Mohammad Alavi Nikje, Zahra Mazaheri Tehrani

Chemistry Department, Faculty of Science, IKIU, Qazvin, Iran

Correspondence to: Mir Mohammad Alavi Nikje; e-mail: alavim2006@yahoo.com

DOI 10.1002/pen.21559

Published online in Wiley InterScience (www.interscience.wiley.com).

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Author:Nikje, Mir Mohammad Alavi; Tehrani, Zahra Mazaheri
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:7IRAN
Date:Mar 1, 2010
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