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Biobased chain extended polyurethane and its composites with silk fiber.

INTRODUCTION

Polyurethane (PU) materials are of commercial interest in many applications because of their excellent properties such as abrasion resistance, chemical resistance, and toughness combined with good low-temperature flexibility (1). The PU materials are constituted by soft (polyol) and hard segments (poly functional isocyanate and chain extender/crosslinker). There are various ways of combining a wide variety of polyols and diisocyanates to produce tailored PU products. Basically, segmented PUs consists of high [T.sub.g] hard segment and low [T.sub.g] soft segment which can be produced by a two-step method that is composed of prepolymerization and chain extension steps. In the prepolymerization step, the isocyanate (--NCO) terminated prepolymers can be formed by the reaction between hydroxyl (--OH) and the excess amount of --NCO groups (3-5). The prepolymers are then chain extended with low-molecular weight materials such as diols, diamines, etc. (2-5). A variety of polyols and diisocyanates have been used in the synthesis of PUs and their effect on the properties has been investigated (5), (6-8).

Recently, the utilization of renewable resources in the synthesis of polymers and fabrication of composites have received considerable attention due to their potential as petrochemical based raw materials substitute. Many researchers have reported the use of natural polymers having more than two hydroxyl groups per molecule either as polyol or as crosslinker in the preparation of PU by allowing them to react efficiently with the diisocyanates (9-12). The results of these investigations have shown that the natural plant components act as hard segments in these PUs. Numerous research papers have been published on the synthesis of chain extended PUs using castor oil as polyol which has two reactive --OH groups and different diols and amines as chain extenders (13-15). A thorough literature survey revealed that the utilization of dicarboxylic acids as chain extender in the synthesis of PU has not been carried out yet. The dicarboxylic acids have the capability to react with pre-PU polymer to form hydrogen bonds (Scheme 1). Hence, the authors have selected the dicarboxylic acid such as gluataric acid as a chain extender for the synthesis of PU. In the previous research, the authors have studied the effect of di and tri carboxylic acid chain extenders such as maleic acid (MA) and citric acid (CA), respectively, on the transport characteristics of n-alkane penetrants into castor oil based PUs (16).

[ILLUSTRATION OMITTED]

The increased green house gas emission and the anticipated depletion of petroleum reserves in the near future directed the researchers to develop environmentally friendly composites based on renewable resources like natural fibers. The advantages of natural fibers over traditional reinforcing fibers such as glass and carbon fibers are low cost, low density, high toughness, acceptable specific strength, enhanced energy recovery, recyclability, biodegradability, etc. (17). Therefore, natural fibers can serve as reinforcement by improving the strength and stiffness and also by reducing the weight of the resulting biocomposite materials, although the properties of natural fibers vary with their sources and treatments (18-20). Natural fibers are largely divided into two categories depending on their origin: plant-based and animal-based. In general, plant-based natural fibers are lignocellulosic in nature and are composed of cellulose, hemicelluloses, and lignin, whereas animal-based fibers are of proteins. Very limited attempts have been made to fabricate the biocomposites using animal based fiber such as silk fiber. Silk fiber is polar in nature due to the presence of various functional groups such as --COOH, --[CH.sub.2]OH, --[NH.sub.2], --NHCO on its surface. Silk fibers are biodegradable and highly crystalline with well-aligned structure. It has been known that they also have higher tensile strength than glass fiber or synthetic organic fibers, good elasticity, and excellent resilience (21).

The present research is aimed to evaluate the effect of silk fiber in biobased chain extended PU. The evaluation is being made on both the neat PUs of TDI and HMDI and their corresponding biocomposites with silk fiber. The initial research has been carried out with the incorporation of small amount of (5% and 10%) silk fiber into biobased chain extended PU. The fabricated neat PUs and their biocomposites have been subjected for physicomechanical, thermal, and morphological studies.

EXPERIMENTAL

Materials

Castor oil was procured from the local market. Its average molecular weight ([M.sub.n]) is 930 and hydroxyl group per molecule is 2.24. Toluene-2, 4-diisocyanate (TDI), hexamethylene diisocyanate (HMDI), and dibutyl tin dilaurate (DBTL) were obtained from Sigma (USA) and were used as received. The organic solvent, methyl ethyl ketone (MEK), is of AR grade distilled before use. Silk fiber was obtained from Department of Sericulture, University of Mysore, Mysore, India. Silk fiber was degummed as per the procedure reported elsewhere (22), (23). The procedure of degumming involves the boiling of silk fiber for about 1 h in an aqueous solution of 0.5% [Na.sub.2][CO.sub.3] followed by washing with distilled water and drying for 24 h at 40[degrees]C. The dried fiber was cut into 5 mm length prior to use.

Preparation of Biobased Chain Extended Polyurethane and Its Biocomposites with Silk Fiber

The preparation of biobased chain extended polyurethane involves a two-step procedure. In the first step, pre-PU was obtained by reacting 2 mol of diisocyanates with 1 mol of castor oil dissolved in methyl ethyl ketone. The reaction was carried out in a three-necked round-bottomed flask equipped with a reflux condenser at 80[degrees]C for 1 h under [N.sub.2] atmosphere. In the second step, 1 mol of glutaric acid was added into the formed pre-PU and the reaction was carried out with continuous stirring at 80[degrees]C for 30 min. The obtained chain extended PU composition was poured into the glass molds coated with silicone realizing agents to cast the neat PU sheets. To study the effect of different diisocyanates, neat PU sheets of both TDI and HMDI have been obtained.

The biocomposites have been fabricated by incorporating the dried silk fiber (5% and 10%) into the obtained chain extended prepolyurethanes. After the incorporation of silk fiber, the composition was refluxed at 80[degrees]C for 1 h. After 1 h, the silk fiber containing biobased chain extended PU composition was poured into a glass mold coated with silicone realizing agent to obtain the bio-composites. The silk fibers have been incorporated into both TDI and HMDI based PU composition.

Techniques

The density, surface hardness, and tensile behavior of neat PUs and their biocomposites was evaluated according to ASTM D 792-86, ASTM D 785, and ASTM D 638 standards, respectively. The tensile strength of the composites was measured at 23[degrees]C and 50% RH with a crosshead speed of 10 mm/min using a 4302 Model Hounsfield Universal Tensile Testing Machine, UK.

The thermal stability of neat PU and their biocomposites was examined up to 600 [degrees]C with [N.sub.2] gas purging using a thermogravimetric analyzer (TGA 2950, TA instruments). A heating rate of 20[degrees]C/min was used. About 20 mg of each specimen was loaded for each measurement. The integral procedural decomposition temperature (IPDT) is used for estimating the inherent thermal stability of the composites. IPDT data were calculated from the thermogram area applying the method reported in the literature (24).

The storage modulus, loss modulus, and tan [delta] of neat PUs and their silk fibre reinforced biocomposite was measured from -60[degrees]C to 120[degrees]C with a purging liquid [N.sub.2] gas using a dynamic mechanical analyser (DMA 2980, TA Instruments). The specimen dimensions were 17.9 X 4.5 X 1.5 mm. A heating rate of 2[degrees]C/min was used to be slow enough to thermally equilibrate each specimen in the furnace. The biocomposite specimen was deformed in a single cantilever bending mode at a fixed frequency of 1 Hz. The oscillation amplitude used was 0.2 mm.

Scanning electron microscope (SEM) studies were conducted (Jeol-JSM, 840A, Japan) to study the morphology of the composites. The tensile fractured samples were gold sputtered before viewing under the microscope. The magnification and the voltage are displayed on the micro-photographs of the samples.

RESULTS AND DISCUSSION

The physicomechanical properties such as density, tensile strength, tensile modulus, percentage elongation at break, and surface hardness of neat PUs of TDI, HMDI, and their corresponding composites with 5 and 10 wt% silk fibers are given in Table 1. The calculated density of neat PUs of TDI and HMDI is 1.108 and 1.055, respectively. The density of all the composites with 5% and 10% silk fiber have exhibited lower density compared to neat PUs. The reduction in density of the silk fiber reinforced PU composites compared to neat PUs may be due to the low bulk density of the silk fiber. The silk fiber reinforced chain extended PU composites have showed higher tensile strength, modulus, and surface hardness compared to neat PUs. The incorporation of 5 and 10% silk fiber found to enhance the tensile strength, modulus and surface hardness of neat PUs. The calculated tensile modulus from the stress-strain curves (see Fig. 1) of the composites found to increase with increasing the silk fiber content compared to neat PUs. The percentage elongation of silk fiber reinforced PU composites found to reduce with increasing the silk fiber content. In general, TDI-based neat PU and its composites with silk fiber has showed higher tensile strength, percentage elongation, and surface hardness compared to that of HMDI-based neat PU and its composites with silk fiber. This can be attributed to the aromatic nature of TDI which imparts stiffness and strength properties. A similar behavior was noticed by Satheesh Kumar et al. in their studies on polyester nonwoven fabric reinforced castor oil based polyurethane composites (25). The increased tensile strength and surface hardness with the incorporation of silk fiber into bio-based chain extended PU suggests that silk fiber acts as a reinforcing fiber.
TABLE 1, Physicomechanical properties of neat PUs and their composites.

Wt (%) of silk       Tensile   Tensile  Elongation  Density   Surface
fibroin in CEPU     strength  modulus    at break   (g/cc)   hardness
                      (MPa)     (MPa)      (%)               (Shore D)

Neat TDI PU            8.6       7.47       170       1.108      30

TDI-PU + 5% silk      12.07     14.64       165       0.986      41
fiber composite

TD1-PU + 10% silk     15.28     16.79       156       0.957      53
fiber composite

Neat HMDI-PU           5.50      3.57       200       1.055      26

HMDI-PU + 5% silk      8.96     10.12       187       0.978      39
fiber composite

HMDI-PU + 10% silk    12.03     14.81       175       0.938      48
fiber composite


[FIGURE 1 OMITTED]

Dynamic mechanical analysis (DMA) is a useful technique to measure the modulus (stiffness) and dampening properties of composite materials. Dynamic storage modulus (E') is an important property to assess the load bearing capability of a composite material. The temperature dependence of storage modulus (G') of neat PUs of TDI, HMDI, and their corresponding composites with different weight percentages of silk fiber is shown in Fig. 2. All the samples have showed a reduction in the G' values with increasing the temperature. The G' value of neat PUs of TDI and HMDI at 30[degrees]C is 1.5 and 0.98 MPa, respectively (Table 2). The incorporation of silk fiber into both TDI- and HMDI-based neat PUs have exhibited an increased G' value. The composites have showed an increased G' with increasing the silk fiber content. The incorporation of 10% silk fiber into TDI- and HMDI-based chain extended PU has enhanced the storage modulus of neat PU by 4.4 and 1.25 folds, respectively. Silk fibers have been found to reinforce the PU matrix by allowing a greater stress transfer at the fiber-matrix interface, therefore increasing the stiffness of the overall material. Glass transition temperatures ([T.sub.g]) are determined from the peak of the tan [delta] (ratio of loss modulus to storage modulus) curves (see Fig. 3). With silk fiber loading, the [T.sub.g] of the chain extended TDI-PU (57 [degrees]C) is shifted to higher values and is measured at 72[degrees]C for a fiber loading of 10%. Latare Dwan'Isa et al. (26) made a similar observation when the silk fiber incorporated into soy oil based biopolyurethane. The authors have noticed that the incorporation of 50% glass fiber into neat soy oil biobased PU has increased the [T.sub.g] from 60 to 90[degrees]C. The tan [delta] of HMDI-based neat PU and its composites have found to be lower than that of TDI- based neat PU and their corresponding silk fiber composites.
TABLE 2. Storage modulus and glass transition temperature ([T.sub.g])
of (A) neat TDI PU, (B) 5% silk fiber TDI PU composite. (C) 10% silk
fiber TDI-PU composite. (D) neat HMDI PU. (E) 5% silk fiber HMDI PU
composite, and (F) 10% silk fiber HMDI PU composite.

Samples    G' (MPa) at 30 C    [T.sub.g] ([degrees]C)

   A             1.50                    57
   B             4.5                     65
   C             6.6                     72
   D             0.98                    48
   E             0.99                    52
   F             1.22                    59


[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The thermogravimetric analysis (TGA) is a useful technique to determine the quantitative degradation based on the weight loss of a composite material as a function of temperature. Figure 4 shows the TGA of silk fiber, neat PUs, and silk fiber reinforced biocomposites. The weight loss as a function of temperature for silk fiber, neat TDI-and HMDI-based chain extended PU and their corresponding composites with 5 and 10% silk fiber is given in

[FIGURE 4 OMITTED]

Table 3. The silk fiber did not show any major weight loss in the temperature range of 100-200[degrees]C as similar to the results reported elsewhere (27). This initial weight loss may be due to the evaporation of moisture. The beginning of the major weight loss of silk fiber found to be above 300[degrees]C. Lee et al. (27) reported that the raw silk fiber found to lose the weight significantly at 250-300[degrees]C. Relatively, the HMDI-based neat PU has showed lower weight loss than TDI-based neat PU. The observed weight loss of all the silk fiber reinforced PU composites likely to be intermediate between neat PU and silk fiber. The silk fiber reinforced TDI and HMDI PU composites exhibited reduced weight loss as compared to their individual components.
TABLE 3. Data obtained from TGA analysis of silk fiber, (A) neat TDI
PU, (B) 5% silk fiber TDI-PU composite. (C) 10% silk fiber TDI-PU
composite, (D) neat HMDI PU, (E) 5% silk fiber HMDI-PU composite, and
(F) 10% silk fiber HMDI PU composite.

              Weight loss (%) at different temperatures
                      temperature ([degrees]C)

  Samples     100    150    200    250    300     350

Silk fiber    3.0    3.5    4.4    8.5    18.5    38.0
    A         0.3    1.5    2.0    5.5    20.0    40.0
    B         0.6    0.6    2.0    4.0    14.0    33.0
    C         0.1    0.9    3.0    5.5    15.0    33.0
    D         0.4    0.4    1.2    3.1    10.0    35.5
    E         0.9    1.5    2.0    4.0    10.0    30.5
    F         1.2    1.5    2.3    5.0    21.3    33.0

  Samples     400     450     500     550     600     IPDT ([degrees]C)

Silk fiber    45.0    49.0    67.0    90.0    95.0           580
    A         58.0    70.0    85.0    97.5    99.0           625
    B         57.0    69.5    84.0    90.0    98.5           636
    C         60.0    70.0    84.0    90.0    98.5           628
    D         49.0    63.0    85.0    96.0    99.0           598
    E         42.5    57.5    84.0    91.5    99.0           643
    F         46.3    61.0    84.0    90.0    98.5           647


Figures 5A-F show the scanning electron microphotographs of tensile fractured neat TDI- and HMDI-based PUs and their corresponding composites with 5 and 10% silk fiber. The neat PUs (Figs. 5A-D) have exhibited a layer like structure with two phases. The two phases may be due to soft and hard segments of PU. Hydrogen bonding between > NH and > C=O group of urethanes result in the phase separation between hard and soft segments of PU. The SEM micropholographs of silk fiber reinforced PU composites (Figs. 5B, C, E, and F) exhibited fiber pull out from the PU matrix during the course of crack propagation. Alhough there is a dispersion of fibers in the matrix, the majority of the fibers appear to be aggregated rather distributed uniformly.

[FIGURE 5 OMITTED]

CONCLUSIONS

The present research concentrated on the characterization of neat PUs derived from castor oil, glutaric acid, and different disisocyanates such as TDI and HMDI. The incorporation of a little amount of silk fiber found to enhance the mechanical properties of neat PU sheets. This may be due to some kind of physical interaction between silk fiber and PU. The mechanical properties of the neat TDI PU sheets found to be higher than that of HMDI-based PU. A significant enhancement in the mechanical properties was noticed after the incorporation of even a little amount of silk fiber into neat PU. DMA studies confirmed that the [T.sub.g] of neat PU has shifted to higher side after the incorporation of silk fiber. The observed thermal stability of the fabricated biocomposites found to be intermediate between that of silk fiber and neat PU. The two phase morphology of neat PU (both TDI- and HMDI-based) was confirmed from the SEM studies. The SEM studies also revealed that the composites have better inter-facial bonding which contributes to the improved mechanical properties.

ACKNOWLEDGMENT

The author would like to acknowledge the financial support of University Grant Commission (UGC) (No. F.145-41/2003 (SR) dtd 27/03/2003), New Delhi.

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K.S. Manjula, (1) M.N. Satheesh Kumar, (2) Bluma G. Soare, (3) Paulo Picciani, (3) Siddaramaiah (1)

(1) Department of Polymer Science and Technology, Sri Jayachamarajendra College of Engineering, Mysore-570 006, India

(2) Faculty of Chemical and Process Engineering, Faculty of Engineering, National University of Malaysia (UKM), Bangi, Selangor, 43600, Malaysia

(3) Institute of Macromolecules, Federal University of Rio de Janeiro, Centro de Tecnologia, BI J, IIha do Fundo, Rio de Janeiro, RJ 21945-970, Brazil

Correspondence to: Siddaramaiah; e-mail: siddaramaiah@yahoo.com

DOI 10.1002/pen.21604

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

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Author:Manjula, K.S.; Kumar, M.N. Satheesh; Soare, Bluma G.; Picciani, Paulo; Siddaramaiah
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:3BRAZ
Date:Apr 1, 2010
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