Nanocomposite materials based on polyurethane intercalated into montmorillonite clay

Nanocomposite materials based on polyurethane intercalated into montmorillonite clay

Materials Science and Engineering A 399 (2005) 368–376 Nanocomposite materials based on polyurethane intercalated into montmorillonite clay Ahmed Reh...

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Materials Science and Engineering A 399 (2005) 368–376

Nanocomposite materials based on polyurethane intercalated into montmorillonite clay Ahmed Rehab ∗ , Nehal Salahuddin Chemistry Department, Faculty of Science, University of Tanta, 31527 Tanta, Egypt Received in revised form 31 March 2005; accepted 7 April 2005

Abstract Polyurethane organoclay nanocomposites have been synthesized via in situ polymerization method. The organoclay has been prepared by intercalation of diethanolamine or triethanolamine into montmorillonite clay (MMT) through ion exchange process. The syntheses of polyurethane–organoclay hybrids were carried out by swelling the organoclay into different kinds of diols followed by addition of diisocyanate. The nanocomposites with dispersed structure of MMT was obtained as evidence by scanning electron microscope and X-ray diffraction (XRD). The results shows broaden with low intense and shift of the peak characteristic to d0 0 1 spacing to smaller 2θ and the MMT is dispersed homogeneously in the polymer matrix. Also, the TGA showed that the nanocomposites have higher decomposition temperature in comparison with the pristine polyurethane. © 2005 Elsevier B.V. All rights reserved. Keywords: Polyurethane nanocomposites; Nanocomposites; Polyurethane organoclay; Intercalated polymers; Layered silicate; Polymer–clay nanocomposite

1. Introduction Polymer composites were widely used in electronic and information products, consumer commodities and the construction industry. In these polymer composites, inorganic materials were used to reinforce polymers with the idea of taking advantage of the high heat durability and the high mechanical strength of inorganic and the ease of processing polymers. Clays have been extensively used in the polymers industry either as reinforcing agent to improve the physicomechanical properties of the final polymer or as a filler to reduce the amount of polymer used in the shaped structures, i.e., to act as a diluent for the polymer, thereby lowering the economic high cost of the polymer systems. The efficiency of the clay to modify the properties of the polymer is primarily determined by the degree of its dispersion in the polymer matrix, which in turn depends on the clay’s particle size. However, the hydrophilic nature of the clay surfaces impedes their homogeneous dispersion in the organic poly∗

Corresponding author. Tel.: +20 40 350804; fax: +20 40 350804. E-mail addresses: [email protected], [email protected] (A. Rehab). 0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.04.019

mer phase. The interfacial incompatibility between inorganic and organic polymers existed owing to the difference in the nature of their individual intermolecular interaction forces and often caused failures in these inorganic–organic composites. One approach to alleviate the interfacial and the tenacity problem in these polymer composites is to chemically bond the inorganic and polymers through the sol–gel method [1]. The composite materials prepared by sol–gel method suffered the drawback of large shrinkage during the removal of the solvent. The other approach is to uniformly disperse the inorganic in the polymer matrix in the nanometer scale to form inorganic–polymer nanocomposites [2]. Nanocomposites are a class of composites in which the reinforcing phase dimensions are in the order of nanometers [3]. Layered materials are potentially well suited for the design of hybrid nanocomposites, because their lamellar elements have high in-plane strength, stiffness and a high aspect ratio [4]. The smectic clays (e.g., montmorillonite) and related layered silicates are the materials of choice for polymer nanocomposite design for two principal reasons: first, they exhibit a very rich intercalation chemistry, which allows them to be chemically modified and made compatible with organic polymers for dispersal on a nanometer length

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scale. Second, they occur ubiquitously in nature and can be obtained in mineralogically pure form at low cost. The layered clay–polymer nanocomposites can be prepared by replacing the hydrophilic Na+ and Ca+ exchange cations of the native clay with more hydrophobic onium ion to form a polymer-clay hybrid through two ways. The first is the intercalation of a monomer into the clay interlayer and subsequent heat treatment for polymerization [5]. The second is the direct intercalation of a preformed polymer into the layered clay [6]. Since the development of Nylon-6–clay nanocomposite by Toyota researchers [7], extensive studies on polymer–clay nanocomposites have been investigated in order to obtain new organic–inorganic nanocomposites with enhanced properties. The use of clay or organically modified clay as precursors for preparation of nanocomposites has been studied into various types of polymer systems including polyamide 6 [8], polyoligo (oxyethylene) methacrylate [9], epoxy [10], polyimide [11], polyester [12], polypropylene [13], polyacrylamide [14], polypyrrol [15], polystyrene [16], poly(p-phenylene vinylene) [17], polyethylene oxide [18], polycaprolactone [19] and polymethyl methacrylate [20]. Polyurethane (PU) elastomers are a family of segmented polymers with soft segments derived from polyols and hard segments from isocyanates and chain extenders [21]. PU elastomers represent one of the most attractive elastomers because they have the advantages, such as the best abrasion resistance, outstanding oil resistance and excellent lowtemperature flexibility. They also exhibit the widest variety of hardness and elastic moduli that just fill in the gap between plastics and rubbers. The first example of elastomeric polyurethane–clay nanocomposites with greatly improved performance properties compared to the pristine polymer was reported by Wang and Pinnavaia [22]. Preparation, characterization and properties of polyurethane–clay nanocomposites have been reported by a various researchers [23–33]. However, there also exist some disadvantages concerning with thermal stability and barrier properties. To overcome the disadvantages, the present work will discuss our initial efforts to synthesis different structure of PU–MMT nanocomposites. Since the physical properties of the resulting materials are derived from their structures and study the effect of organoclay percentage on the resulting nanocomposites (Scheme 1).

2. Experimental

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Co. Germany; 1,3-butylene glycol, diethylene glycol and triethylene glycol from Aldrich; tolylene-l,4-diisocyanate (TDI) from Fluka were used as supplied. Dimethylformamide (DMF) from Adwic (Egypt) was used after distillation and drying over molekularSieb. 2.2. Preparation of materials 2.2.1. Preparation of modified clay Ia,b The MMT (10 g) was swelled in 600 ml of distilled water followed by addition of 20 g diethanolamine dropwise with stirring. The suspension was stirred for 24 h at room temperature followed by addition of dilute HCl (1:1) to obtain slightly acidic medium (pH ∼5.5–6) then the stirring was continued for 24 h at room temperature. The suspension was allowed to stand for a few hours, filtered off using sintered glass (G4), washed many times with distilled water, then dried at ∼35 ◦ C under vacuum to yield 10.85 g of MMTdiethanolamine intercalate product. The product was retreatment with diethanolamine by swelling in mixture of 300 ml DMF and 300 ml water followed by addition of 20 g of diethanolamine and the procedure was repeated as previously to give 11.05 g of MMT-diethanolamine intercalate Ia . The intercalation of MMT (10 g) with triethanolamine (20 g) was carried out by the same procedure described in synthesis of Ia to give 11.5 g of Ib . The structural properties were measured directly by infrared (IR), Fig. 1; thermogravimetric analysis (TGA), Fig. 2; calcinations, elemental microanalysis, swelling data and X-ray diffraction (XRD), Fig. 3. 2.2.2. Preparation polyurethane–MMT composites The materials were prepared by swelling the modified clay in the diol followed by addition of the diisocyanate as in the following procedure. 0.52 g of Ia swelled in 4.66 g (50 mmol) of 1,3-butylene glycol for ∼5 h with stirring followed by addition 8.87 g (50 mmol) of tolylene-2,4-diisocyanate with stirring at room temperature (∼20 ◦ C). The polymerization was started after a few minutes (the viscosity of the mixture was increased) and completed very fast. After about 2 h, the formed solid product was suspended in DMF then precipitated in distilled water. The white powder product was filtered off and washed several times with water then dried under reduced pressure at −35 ◦ C to give 13.2 g (94% yield) of (IIa ). The other samples (IIb–h ) were prepared by the same procedure using different amount of modified clay (Ia ) with different diols and another modified clay (Ib ) with the same diisocyanate as illustrated in Table 1.

2.1. Materials Montmorillonite (Na-MMT) minerals were supplied by ECC America Inc., under the trade name Mineral ColloidBP as fine particles with an average particle size of 75 ␮m and cation exchange capacity (CEC) of 90 m equiv./100 g ˚ Diethanolamine from Riedeland interlayer spacing of 9.6 A. De Haen AG Seelze-Hannover; triethanolamine from GDR

2.2.3. Preparation of linear polymers The linear polyurethanes were prepared by polycondensation technique using a mixture of diol and diisocyanate as in the following procedure: 2.83 g (30 mmol) of 1,3-butylene glycol was cooled in ice bath then added 3.6 ml (4.35 g, 30 mmol) of 2,4-tolylene diisocyanate with stirring. The mixture was stirred for a few minutes then the temperature

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2.2.4. Analytical procedures Infrared spectra were carried out on a Perkin-Elmer 1430 Ratio-recording infrared spectrophotometer using the potassium bromide disc technique in the wavenumber range of 4000–400 cm−1 . Thermogravimetric analysis was obtained by using a TGA 50 Shimadzu (thermal gravimetric analyzer). The heating rate was 10 ◦ C/min in all cases in the temperature range ∼30–800 ◦ C in nitrogen atmosphere. Calcination measurements: A definite weight of the sample was introduced into a porcelain crucible and dried in an electric oven at 120 ◦ C overnight, then introduced into an ignition oven and the temperature was increased to 1000 ◦ C and adjusted at this temperature for 15 h. The loading of each sample expressed as the weight loss by ignition per 100 g of the dry sample. The data of all prepared samples are listed in Table 4. X-ray diffraction measurements were carried out using a Phillips powder diffractometer equipped with a Ni-filtered ˚ at scanning rate 0.005◦ s−1 , Cu K␣ radiation (λ = 1.5418 A) ◦ diverget slit 0.3 . Measurements were made for the dried product to examine the interlayer activity in the composite as prepared. Morphology of the composite was examined by a Joel JXA-840 scanning electron microscopy (SEM) equipped with an energy dispersive X-ray detector to examine the morphology and particle size of MMT in the polymer–MMT composites. Specimen was deposited on double-sided scotch tape and examined at their fracture surface.

3. Results and discussion To disperse MMT nanolayers in a polyurethane matrix, it was necessary to first replace the hydrophilic inorganic exchange cations of the native mineral with more organophilic diethanolamine or triethanolamine. The ion exchange was carried out between sodium cation in MMT and ammonium groups in diethanolamine or triethanolamine. The presence of these group in the galleries of MMT renders the

Fig. 1. Infrared spectra of the organoclay Ia,b and PU–organoclay nanocomposites IIa–h and linear polyurethanes IIIa–d .

increased gradually to the room temperature (∼20 ◦ C) for about 2 h. The formed solid product was dissolved in DMF then precipitated in distilled water. The white powder was filtered off, washed several times with water then dried under reduced pressure at ∼35 ◦ C to give 5.3 g (74% yield) of product IIIa . The other samples (IIIb–d ) were prepared by the same procedure using different diols and the same diisocyanate as illustrated in Table 2.

Scheme 1. Synthesis of intercalated polyurethanes.

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MMT organophilic and promote the absorption of diol into the interlayer of MMT and improve the particle–matrix interactions, since diethanolamine and triethanolamine contains functional group which react with diisocyanate. Polyurethane nanocomposites were prepared by solvation of the organoclay with the diol. It was found that the modified clay was swelled

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easily in the diols at room temperature. This solvation was followed by adding the diisocyanate. The synthesis of new organic–inorganic nanocomposite materials was achieved by the intercalation of polyurethane onto functionalized montmorillonite clay through in situ polycondensation polymerization technique. The nanocom-

Fig. 2. TGA thermogram of PU–organoclay nanocomposites (a) IIa–d with different ratios of organoclay Ia and linear polyurethanes IIIa , (b) IIe–h with different diols, (c) TGA thermogram of linear polyurethanes IIIa–d .

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Table 1 Polymerization data of intercalated samples IIa–h Modified clayb

Run

IIa IIb IIc IId IIe IIf IIg IIh a b c d

Diisocyanatea

Diol

Product

g

wt%c

Typed

g

mmol

g

mmol

g

Yield (%)

0.52 1.17 1.65 2.80 0.594 0.591 0.595 0.256

3.83 7.98 10.91 17.21 3.749 3.703 3.717 3.641

1,3-Bu-G 1,3-Bu-G 1,3-Bu-G 1,3-Bu-G TEG DEAm DEG 1,3-Bu-G

4.66 4.62 4.63 4.60 7.12 5.80 5.84 2.43

50 50 50 50 47 55 55 25

8.87 8.87 8.87 8.87 8.18 9.57 9.57 4.35

50 50 50 50 47 55 55 25

13.2 11.3 14.0 14.0 10.65 10.79 11.85 5.51

93.9 77.1 92.4 92.5 67.21 67.61 74.03 78.37

Diisocyanate is 2,4-tolylene diisocyanate. Modified clay in all cases is Ia except in the sample IIh the modified clay is Ib . wt% = (weight of dry modified clay/total weight of all components introduce the polymerization process). 1,3-Bu-G, butane diol; DEAm, diethanolamine; DEG, diethylene glycol; TEG, triethylene glycol.

Table 2 Polymerization data of linear polymer samples IIIa–d Run

IIIa IIIb IIIc IIId

Diol

Diisocyanateb

Product

Typea

g

mmol

g

mmol

g

Yield (%)

1,3-Bu-G DEG TEG DEAm

2.83 3.25 4.556 4.4

30 30 30 40

4.35 4.35 4.35 4.35

30 30 30 30

5.311 1.233 5.083 5.366

73.97 16.22 57.07 61.33

aprotic solvents than the non-polar solvent (the swelling followed the order DMF > 1,4-dioxane > water > acetone > benzene. The IR spectra of all the prepared samples were illustrated in Fig. 1 and Table 3. The spectra of modified clay Ia,b shows that the reported NH stretch band near 3425 cm−1 and NH bend band near 1630 cm−1 are shifted quite substantially to regions associated with + NH3 vibration which facilitate the ion exchange with MMT. A characteristic band at 464 cm−1 for Si–O and at 3626 cm−1 for OH group are shown. This free OH band at 3626 cm−1 in organoclay was disappeared in nanocomposite indicating the strong interaction are occurring between OH group in organoclay and the isocyanate forming the isocyanate linkage. Comparing the + NH3 band near 1630 cm−1 in organoclay with nanocomposite, it is clear that this band is shifted to higher wavelength near 1710 cm−1 indicating that an interaction occur between organoclay and the polymer. The spectra of polyurethane IIIa shows the absorbance appeared at 1724 cm−1 that was assigned to hydrogen-bonded urethane carbonyl (C O), 1413 cm−1 to a secondary urethane amide (C–NH). The spectra of the synthesized PU–modified clay shows IIa , peaks at 1712 was caused by the stretching of urethane carbonyl group (C O) and the 2927 and 2864 cm−1 were due to the asymmetric and symmetric C–H stretching vibration. The 3317 cm−1 peak resulted from the N–H group in hydrogen bonding; the main features of various bond vibration and hydrogen bonding of these PU–modified clay nanocomposites remained the same as that of neat PU. These results deduce that there were no major chemical structural changes in PU, owing to the presence of organoclay.

a 1,3-Bu-G, butane diol; DEAm, diethanolamine; DEG, diethylene glycol; TEG, triethylene glycol. b Diisocyanate is 2,4-tolylene diisocyanate.

posites were synthesized through the intercalation of diols into organoclay Ia,b interlayers followed by addition of TDI to produce the intercalated polyurethanes IIa–h . The yields of the products were ranging from 67% to 94%, as shown in Table 1. It was found that the 1,3-butylene glycol gives high yields than the other diols in the polymerization into organoclay. The different ratios of organoclay used during the polymerization do not appear as an important factor to affect the yield percent of the product. It was found that the percentages of yield in the nanocomposites is higher than the yield percentage of linear polyurethane (Table 2), which may be attributed to catalytic effect for the clay. The structural composition and properties of the product materials was determined by several analytical techniques. The data in Table 3 illustrate that a high intercalation yields for Ia,b occurred. Also, the swelling data indicated that the organoclay Ia,b account for higher swelling in the organic solvents and lower swelling in water. While, the affinity to water still present due to the presence of OH group (Ib > Ia ). Moreover, the swelling behavior increased in the polar and Table 3 Characterization of modified montmorillonite clay Run no.

Ia Ib a

IR (ν, cm−1 )

Microanalysis

Loading

Swelling (%)

C%

H%

N%

wt%

mmol/100 ga

Acetone

H2 O

Bz

DMF

Dioxane

Free OH

CH aliphatic

3.2 6.3

1.9 2.9

1.4 1.35

13.6 17

111.4 104.6

37 133

142 203

30 19

168 328

95 181

3434 3358

2930, 2852 2934, 2899

Number of mmol of nitrogen per 100 g of clay, from calcination; (loss of weight/molecular weight) × 1000.

N+ 1520 1489

Si 0 1046, 523, 464 1045, 523, 465

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Fig. 3. XRD pattern of the organoclay Ia and PU–organoclay nanocomposites (a) IIa–d , (b) IIa and IIe–g and (c) IIh .

Table 4 X-ray diffraction and thermal analysis data for the intercalated samples Sample

X-ray dataa 2θ

Ia Ib IIa IIb IIc IId IIe IIf IIg IIh a b

6.2 6.35 5.9 6.0 6.0 5.7 Shoulder 4.5 4.5 4.0

TGA datab

Calcination d-Spacing 14.26 13.94 14.98 14.73 14.73 15.50 >49 19.64 19.64 22.09

Polymer (%)

Clay (%)

Weight loss (%) in first stage

Weight loss (%) in second stage

Residue

11.7 15.6 96.96 97.2 89.54 90.68 – 96.4 98.1 –

88.3 84.4 3.04 2.8 10.46 9.32 – 3.6 1.9 –

16.5 20.3 55.1 59.5 58.4 57.4 50.8 54.4 62.2 51.5

– – 41.5 33.8 32.5 28.1 40.9 42.8 34.4 41.7

83.5 79.7 3.4 6.7 10.1 14.5 8.3 2.8 3.4 1.8

˚ 2θ (◦ ); d-spacing (A). First stage ≈30–350 ◦ C, second stage 350–800 ◦ C and residue at 800 ◦ C.

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Thermal analysis of polyurethane and intercalated materials were determined by both calcination and TGA data listed in Table 4 and Fig. 2a–c. The data and the figure shows the weight loss encountered during heating the PU–modified clay materials were ranging from 85% to 98% as determined by both TGA and calcination. The associated weight loss evident in TGA curves is nearly compatible with the calcination measurements. The TGA curves for all samples indicate that there are two stages of decomposition. The first stage is the major and sharp, which involve the thermal decomposition of the intercalated polymers, specially the polymers present on the surfaces of the layers of the clay. The decomposition temperature in this stage was started at ≈200 ◦ C and take place to ≈350 ◦ C, which corresponds the weight loss ranging from 54% to 62%. In this stage, there is no clear difference between

the samples. Also, it was found that the composites degrade slightly faster than the pure polymer. This may be attributed to the degradation of the small molecules between the interlayers. The second stage is broad, in which the weight loss ranging from 32% to 42% in the temperature range ≈300–700 ◦ C. In this stage, the composites displayed higher thermal resistance than pure polymers. This stage was attributed to further decomposition of the rest intercalated polymers, specially the polymers present in the interlayers of the clay or some salts in the interlayer of the clay or interval the clay mineral loses OH groups and the crystallographic structure collapsed [9]. The crystal structure of MMT consists of two-dimensional layers formed by fusing two silica tetrahedral sheets to an edge-shared octahedral sheet of aluminum hydroxide. Stack-

Fig. 4. (a) Scanning electron micrograph of PU–organoclay nanocomposites IId , (b) Elemental mapping for Si of PU–organoclay nanocomposites IId .

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ing of layers of clay particles are held by weak dipolar or van der Waals forces [34]. XRD is powerful technique to observe the extent of silicate dispersion, ordered or disordered structure in the polyurethane nanocomposites. Fig. 3a and c show typical XRD for the organoclay Ia,b . The 0 0 1 reflection has sharp intense peak at 2Θ = 6.2, 6.35 for Ia and Ib , respectively. The d0 0 1 spacing was calculated and listed in Table 4 from peak positions using Bragg’s law d = λ/2 sin θ. It is clear ˚ increased to (14.26, that the d-spacing for Na-MMT (9.6 A) ˚ since the small inorganic Na+ cation is exchanged 13.94 A) by onium group in ethanolamine and diethanol amine through an ion exchange process. Fig. 3a–c presents three series of XRD corresponding to polyurethane clay nanocomposites: (a) with different ratios of organoclay, (b) with different types of diols and (c) with different types of organoclay. In polyurethane clay nanocomposites IIa–d , the position of the peak corresponding to intercalated organoclay show some change to smaller 2Θ = 6.0–5.7 as in Table 4 and Fig. 3a. It is note worthy that the sharp peak obtained in organoclay Ia due to a more narrow distribution of the interlamellar spacing become broad and have small intensity. The intensity decrease (i.e., broadness increase) with decreasing the percentage of organoclay. This suggests that the stacking of the silicate layers become disordered. In one earlier work [35], the same results obtained for polymethyl methacrylate–MMT composites. On the contrary, it is interesting to find that at constant ratio of organoclay, the peak characteristic to 0 0 1 ˚ This plane in IIf,g is shifted to higher d-spacing = 19.64 A. confirmed that the polyurethane is intercalated between the layers. However in IIe , XRD is featureless of ordered structure and there is no apparent peak of the clay that can be detected as in Table 4 and Fig. 3b. It is clear that the type of diol affect on the structure of the resulting nanocomposites. This diol is used to swell the organoclay before addition toluene diisocyanate Fig. 3c. In IIh , the peak characteristic to 0 0 1 is shifted to smaller 2Θ = 4 and the intensity of the peak is small. The broadness of the peak may suggest that clay show some mixture of intercalated and exfoliated structure. However, the exfoliated structure of the silicate layers is not judged from only this diffractograms. These results confirm that modified MMT with different chemical structure, different percentage of clay lead to various degree of the dispersion in the polymer matrix. These results similar to the one described using another structures in PU nanocomposites [36]. SEM examination of the fracture surface of the compression-molded samples did not reveal the inorganic domains at the maximum possible magnification. Fig. 4a shows a micrograph of the fracture surface at 9000 magnifications. It is observed that there is no mineral domains could be seen. The search for any aggregation was aided by an energy dispersion X-ray probe. An image for element mapping for Si was shown in Fig. 4b. The uniformity of the white dots representative of Si, indicates that the mineral domain are submicron and are homogenously dispersed in the polymer matrix.

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4. Conclusion A series of polyurethane organoclay nanocomposites were synthesized by in situ polymerization using different kinds of diols and toluene diisocyanate in the presence of montmorillonite clay modified with diethanolamine or triethanolamine. The infrared spectroscopy confirms the interaction between the polymer and silicate layers. X-ray analysis showed that ˚ or more with some disthe d-spacing increased to about 22 A order for low MMT content, whereas for higher content, the intercalated clay rearranged to a minor extent. SEM results confirm the dispersion of nanometer silicate layers in the polyurethane matrix.

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