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Interpenetrating polymer network of blocked polyurethane and phenolic resin. I. synthesis, morphology, and mechanical properties.

INTRODUCTION

Interpenetrating polymer networks (IPNs) are a special class of polymer blends in which the polymers are cross-linked (1), (2). These materials are characterized by the presence of two networks strongly entangled ideally only by topological constraints (3), (4). Interpenetrating polymerization is a mode of blending two polymers to produce a mixture in which phase separation is not as extensive as it would be otherwise (5), (6). IPNs synthesized so far exhibit various degrees of phase separation depending mainly on the miscibility of polymers (7-9). It is often important to know the morphology of multicomponent polymer systems, and the factors influencing it, because phase domain size and shape, interfacial bonding, and phase connectivity determine the physical, mechanical, and thermal properties of such materials (10). With highly incompatible polymers, the thermodynamic driving force phase separation is so powerful that gross phase separation occurs before gelation.

The resulting properties are strictly related to both the chemical structure of the networks and the synthesis path of the IPN. Two main synthesis routes are used: sequential IPN (SIPN) (5), (11-15) and simultaneous IPN (SINs) (16-21). SIPNs are formed by swelling a polymer network I with a monomer mixture II, which is polymerized in situ. In SIN, both of monomers or prepolymers with curing agents are simultaneously polymerized and cross-linked by noninterfering reactions. In this case, if one of the two polymers reaches the gelation point before the other, it will tend to be more continuous in space and to dominate the material properties. Among the two modes of synthesis, the SIN is generally the best one to have a high degree of intermixing compared with the SIPN processes because of the compatibility of the monomcric mixture, which is much higher than that of a polymeric mixture (22).

The blocked polyurethane (BPU)/phenolic (PF) system in this study serves as an excellent model SIN system while giving good properties. The morphology of IPN is particularly complicated and has been subjected to many studies (10), (23). During polymerization, two competing processes take place simultaneously. Phase separation of the forming polymer chains proceeds by diffusion through an increasingly viscous medium to form phase domains. The formation of cross-links restricts diffusion, and at gelation, then the present situation is frozen in (10). With highly incompatible polymers, the phase separation is so serious that the gross phase separation occurs before gelation. The SIN process can demonstrate very fine microheterogeneous morphology. The other especially important and fundamental properties of polymer networks are their mechanical behavior. The mechanical properties reveal the strength and modulus of various different IPN components. To obtain the best mechanical properties of IPN materials, the filler added in IPN materials has been investigated in this research.

In this study, the simultaneous polymerization (SIN) method was used to synthesize BPU/PF IPN. The BPU prepolymer with m-xylylenediamine (MXDA) as a chain extender and PF prepolymer using P-toluene sulfonic acid (PTSA) as a catalyst are mixed simultaneously at room temperature, and interpenetrated reaction was carried out at an elevated temperature of 140[degrees]C. The article is focused on the synthesis method, morphology, and mechanical properties of BPU/PF IPN system.

EXPERIMENTAL

Materials

A BPU prepolymer was prepared from toluene di-iso-cyanate (TDI-80, 80/20 blend of 2,4 and 2,6 isomers) and from polyol (branched polyester) at a NCO-to-OH equivalent ratio of 2 to 1, and then was blocked by [epsilon]-caprolac-tam, the blocking reaction proceeds following Eqs. 1 and 2 [24], which was supplied by the Bayer Co., Leverkusen, Germany. It has a viscosity of 31,000 mPa s at 25[degrees]C, a weight-average molecular weight of 4230, three NCO functional groups, and an equivalent weight with respect to 1410, i.e., 4230/3 = 1410. The chain extender (curing agent) of BPU used was MXDA, supplied by the Epocone Chemical Co. Ltd. It has a weight-average molecular weight of 136. The PF prepolymer used was resole type PF 650 (the chemical structure of which is described in Eq. 3) and was supplied by the ChangChun Plastics Co. Ltd., Taiwan, ROC. It has a viscosity of 200 mPa s at 25[degrees]C and a solid content of 60-64%. The catalyst used for chain extension and cross-linking of PF prepolymer was PTSA, which was supplied by the ChangChun Plastics Co. Ltd. It has a specific gravity of 1.2. The filler used was surface-treated kaolin and was supplied by the Yin Chin Co. Ltd., Taiwan, ROC. It has a specific gravity of 2.6 and a particle size of 1.92 [micro]m.

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Preparation of IPN Specimens

Preparation of PF/PTSA Prepolymer. The PF prepolymer was heated to 50[degrees]C and then mixed with 3 phr (parts per hundred parts of resin) of PTSA and was continuously stirred for 10 min to ensure that the mixing was complete. Finally, the PF/PTSA prepolymer was quenched by ice bath for 1 h to prevent further polymerization. The viscosity of PF/PTSA prepolymer measured was 200 ~ 300 mPa s at 25[degrees]C.

Preparation of BPU/MXDA Prepolymer. Because the BPU prepolymer might contain moisture, which can react with NCO functional group at high temperature to generate [CO.sub.2] gas bubbles, the BPU prepolymer was stored under vacuum at 50[degrees]C for 3 h. The blends of the BPU prepolymer with the chain extender (MXDA) were kept at 50[degrees]C and mixed at an equivalent ratio of 1:1 and stirred for 15 min using a high-torque stirrer to ensure that the mixing was complete. The BPU/MXDA prepolymer was stored under vacuum condition because of its susceptibility to moisture.

Preparation of BPU/PF IPN Samples. The BPU/ MXDA prepolymer was blended with PF/PTSA prepolymer at various weight proportions using a high-torque stirrer at 50[degrees]C. The mixtures were molded in an ASTM standard stainless steel mold. The shape and size of the mold were square shaped and 300 X 300 X 3.0 mm (length X width X thickness), respectively. The surfaces of the stainless steel mold have been treated by chrome plating. The mixture of mold was cured in an oven for 4 h, and the temperature was set at 140[degrees]C. Finally, the samples were removed from the mold and kept in a desiccator where the relative humidity was maintained at 50% for at least 2 days before they were tested.

Equipment and Measurement

Fourier transform infrared (FTIR) measurements were carried out with a Model FTIR-5300 (Jasco Co., Japan). The sample was directly dabbed into a KBr pellet or manufactured film for FTIR tests. The viscosity of the prepolymer was measured with a Brookfield RVF model viscometer (Brookfield Co., Middleboro, MA). The tensile strength, modulus, and elongation were measured on an Instron 1123 universal testing machine (Instron Co., Canton, MA) following the specification of ASTM D-638. The sample was dumbbell-shaped, with dimensions of 165 X 19 X 3.0 mm (length X width X thickness) and a crosshead speed of 10 mm/min. The flexural strength and modulus were measured on an Instron 1123 universal testing machine (Instron Co.) following the specification of ASTM D-790. The sample was dumbbell-shaped, with dimensions of 127 X 12.7 X 3.0 mm (length X width X thickness). The span was 90 mm, and the crosshead speed was 2 mm/min. The notched Izod impact strength was measured on a TMI 43-1 (Testing Machine Inc., New Castle, DE) following the specification of ASTM D-256. The sample dimensions were 63.5 X 12.7 X 3.0 mm (length X width X thickness), and the notched depth was 2.5 mm. The surface hardness was measured by means of a Type-A Shore Durometer (Model 473; Teclock Co., Japan) according to ASTM D-2240. The scanning electron micrography (SEM) photographs were obtained on a S-570 Hitachi scanning electron microscope (Hitachi Co., Japan). The microphotographs were taken on the surface obtained by fracturing the specimen in liquid nitrogen and then coating it with gold powder.

RESULTS AND DISCUSSION

Synthesis of BPU/PF IPN

The reaction mechanism can be described by FTIR analysis. The FTIR spectroscopy analysis is based on the peak change of functional groups during the reaction period. Figure 1 shows FTIR spectra of the reaction system of the BPU with chain extender (MXDA) at various reaction times and at a temperature of 140[degrees]C. The spectrum of Fig. la shows no peak at 2270 [cm.sup.-1] corresponding to the isocyanate (NCO) when the reaction started, owing to the NCO-terminated PU prepolymer blocked by the blocking agent ([epsilon]-caprolactam). After heating for 3 min at 140[degrees]C as shown in Fig. lb, the absorption peak intensity of NCO at 2270 [cm.sup.-1] appeared obviously. From the spectrum in Fig. lc, one can observe that the absorption peak intensity at 2270 [cm.sup.-1] (NCO) disappeared obviously after reaction for 4 h at 140[degrees]C. The resulting absorption spectra indicated that the majority of the free isocyanate (NCO) groups were generated at elevated temperature, and then, the free NCO groups could easily be reacted with chain extender (MXDA). Therefore, the reaction process is almost completed in the fourth hour. The deblocking process of BPU was described by Eq. 4 (25), where the blocking agent (EH) is [epsilon]-caprolactam. The free NCO groups can easily be reacted with the chain extender (MXDA). The reaction process is described by Eq. 5 (25).

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Figure 2 illustrates the FTIR spectra of (a) PF prepolymer and (b) PF polymer. In the study, the PF prepolymer polymerized at 140[degrees]C for 2 h. From Fig. 2, one can observe that the absorption peak intensity of OH at 1000 [cm.sup.-1] of PF polymer decrease compared with the corresponding peak of the PF prepolymer during the reaction. Therefore, the reaction process is almost complete in 2 h at 140[degrees]C. The polymerization of PF is represented by Eq. 6.

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Figure 3 shows the FTIR spectra of the reaction system of BPU and PF without MXDA and PTSA at various reaction times at 140[degrees]C. From Fig. 3a, there was no absorption peak intensity of NCO at 2270 [cm.sup.-1] and having absorption peak intensity of OH at 1000 [cm.sup.-1] when the reaction started. After heating at 140[degrees]C for 3 min is shown in Fig. 3b, the absorption peak of NCO of BPU at 2270 [cm.sup.-1] appeared. After heating at 140[cm.sup.-1]C for 16 h is shown in Fig. 3c. the absorption peak of NCO of BPU at 2270 [cm.sup.-1] and OH of PF at 1000 [cm.sup.-1] disappeared during the same time.

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It was found that in the BPU/MXDA system, the absorption peak of NCO disappeared within 4 h. However, in BPU/PF without MXDA and PTSA system, it required 16 h for the peak of NCO to disappear. Results indicated that the NCO group of BPU reacts slowly with the OH group of PF compared with the reaction between NCO group of BPU and [NH.sub.2] group of MXDA. From the above FTIR spectrum analysis, it was also confirmed that in the BPU/PF IPN (BPU/MXDA/PF/PTSA), the major reactions are the polymerization of BPU/MXDA and the self-polymerization of PF.

Morphology and Compatibility

Figure 4 shows the scanning electron micrographs of fracture surface of the pure components PF and BPU and BPU/PF IPN at various BPU contents, respectively. The pure PF expressed smooth and glossy microstructure because it was brittle material (Fig. 4a) (25). The BPU exhibited rough microstructure because it was ductile material (Fig. 4f). Figure 4b-e shows that, as the BPU content of the BPU/PF IPN increased, the microstructure of IPN became rougher. As the BPU content was above 50 wt%, the microstructure is very similar to that of the pure BPU component. It was found that the BPU was dissolved in the PF of the BPU/PF IPN. As the BPU content was above 50 wt% in the BPU/PF IPN, the microstructure was dominated by BPU network. It was found that the BPU network and the PF network penetrated each other, and there is an interpenetrating effect existing in the BPU/PF IPN. The compatibility of the networks was improved. The morphology study showed that the IPN system was heterogeneous and more than one phase existed in the networks.

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Mechanical Properties

Effect of BPU Content. Figures 5-9 illustrated the tensile strength, flexural strength, tensile modulus, flexural modulus, and hardness versus BPU content for unfilled and 10-phr filled (kaolin) BPU/PF IPN. From these figures, it can be found that all of the properties investigated decreased with increase in BPU content. The hard segments of PF seemed to have higher strength and modulus properties, and the soft segments of BPU seemed to have lower strength and modulus properties. The existence of BPU soft segments may partially be dissolved in the PF matrix (as can be seen from the SEM photographs). Therefore, the tensile strength, flexural strength, tensile modulus, flexural modulus, and hardness all decreased with increasing the BPU content.

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The elongation and notched Izod impact strength versus BPU content for unfilled and 10 phr-filled (kaolin) BPU/PF IPN are shown in Figs. 10 and 11. Because BPU may partially be dissolved in the PF matrix (as can be seen from the SEM photographs) and the interpenetration effect will toughen the matrix of the BPU/PF IPN, the elongation and notched Izod impact strength of the BPU/PF IPN increased with increasing the BPU content. From these results it was found that although there is no rubber particle in the matrix, the notched Izod impact strength (i.e., high shear rate in fracturing) still could be improved. The reason is that the soft segment content of the BPU matrix was toughened by the interpenetration effect and showed an increase in notched Izod impact strength. The ductility of the BPU matrix played a very important role in toughening at high shear rate fracturing (i.e., notched Izod impact strength). The result agrees with the toughness mechanism of PU/epoxy graft-IPN proposed by Hsieh and Han (26), (27).

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Effect of Filler (Kaolin) Content. Figure 12 shows the tensile strength and flexural strength versus filler (kaolin) content for BPU (25 wt%)/PF (75 wt%) IPN. It was observed that the tensile strength and flexural strength of IPN increased with filler content to maximum values at 20 phr and then decreased. The tensile modulus and flexural modulus versus filler (kaolin) content for BPU (25 wt%)/PF (75 wt%) IPN is shown in Fig. 13. It was found that the tensile modulus and flexural modulus of IPNs increased with filler content to a maximum and then decreased. The maximum tensile modulus and flexural modulus occurred at 25 and 20 phr filler contents, respectively. The resulting maxima of the mechanical properties (tensile strength, flexural strength, tensile modulus, and flexural modulus) for filled IPN systems was due to the addition of the inorganic filler to the IPN systems, which made the IPN more rigid, thus improving the mechanical properties of IPN with filler content. However, if an excess of filler existed in IPN resin, the IPN systems became more brittle and there was worse dispersion of filler in the IPN resin; thus, the mechanical properties of IPN decreased with filler content. Figure 14 showed the notched Izod impact strength and shore A hardness versus filler (kaolin) content for BPU (25 wt%)/PF (75 wt%) IPN. It can be seen that the notched Izod impact strength of IPN decreased when the filler content increased. The hardness of IPN increased with increasing filler content.

CONCLUSIONS

In this study, BPU/PF IPN was synthesized using SIN method. The IPN was prepared from BPU prepolymer, with MXDA as a chain extender and PF prepolymer using PTSA as a catalyst. From the FTIR spectra analysis, it was found that the major reactions were polymerization of BPU/MXDA and the self-polymerization of PF in the BPU/PF IPN system. It was confirmed from SEM analysis that the BPU/PF IPN compatibility of the both networks was improved, and the system was heterogeneous and more than one phase existed in the IPN.

The tensile strength, flexural strength, tensile modulus, flexural modulus, and hardness of the BPU/PF IPN decreased with increasing the BPU contents. The impact strength and elongation of the BPU/PF IPN increased with increasing the BPU contents. The tensile strength, flexural strength, tensile modulus, and flexural modulus of IPN increased with filler (kaolin) content to a maximum value at 20, 20, 25, and 20 phr, respectively, and then decreased. The higher the filler content, the greater the hardness of BPU/PF IPN, and the lower the notched Izod impact strength of IPN.

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Correspondence to: Chin-Hsing Chen; e-mail: cjx@faculty.pccu.edu.tw

Contract grant sponsor: National Science Council, Taiwan, Republic of China; contract grant number: NSC 98-2221-E-034-001.

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C]2010 Society of Plastics Engineers

Yun-Yun Sun, (1) Chin-Hsing Chen (2)

(1) Department of Cosmetic Applications and Management, Tung Fang Institute of Technology, Hunei, Kaohsiung County, Taiwan, Republic of China

(2) Department of Chemical and Materials Engineering, Chinese Culture University, Yang-Ming-Shan, Taipei City, Taiwan, Republic of China

DOI 10.1002/pen.21826
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Author:Sun, Yun-Yun; Chen, Chin-Hsing
Publication:Polymer Engineering and Science
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Date:Feb 1, 2011
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