A Solution consisting in increasing the blank holder pressure has been already men­tioned in Section 7.3.3.1 to get rid of wrinkles. However, low blank holder pressure

should be preferred to avoid to homogeneity defects caused by too high strained tows. The use of high resistance tows could also be a solution to prevent this defect. Using low blank holder pressure would also delay the appearance of tow sliding de­scribed in Section 7.3.2.2 as this defect only appears when the blank holder pressure is increased. This therefore means that compromises need to be found already at this level to prevent the appearance of these defects.

To prevent the appearance of buckles different solutions may be developed. A first solution consists in designing specific fabric architecture as it was showed in Section 7.3.2.1 that the architecture of the fabric was a critical parameter. Indeed, for the two orientations the buckles appear on the warp tows (on the face C and its opposed edge for 0°orientation and on faces A and B for the 90°orientation).

Reinforcement 1 considered in this study is not balanced since an important space is observed between the weft tows whereas it is almost nonexistent between the warp tows as shown schematically in Fig. 7.26.a. This space controls the appear­ance of buckles. Its presence between the weft tows allows the warp tows to bend out of plane. Between the warp tows the lack of space prevents the movement of the weft tows. As a consequence, a balanced fabric with no space between two consecu­tive warp and weft tows could prevent tow buckling (see Fig. 7.26.b).

This new fabric (reinforcement 3) manufactured with the same un-twisted tows was manufactured by GroupeDepestele and tested under the same process condi­tions as the previous studied fabric. The results presented in Fig. 7.5 confirm the absence of buckles.

A second type of solution was investigated to prevent buckling on the final pre­form; it focuses on the optimization of the forming process parameters so that the local tensions in the preform can be changed in the defect zones. Nevertheless, the change of the local tensions with no modification of stresses in the rest of the fabric is not easy with the geometry of the bank holders used in this study (Fig. 7.1.b).

To reach this goal, new specially designed blank holders have been elaborated to apply minimum pressure to the tows passing by the triple point. New tests are conducted on reinforcement 1 for the 0°orientation. The final preform presented in Fig. 7.28 is obtained for a blank holders’ pressure of 3bar, applied on the warp tows

on which the buckles appeared previously. No buckle is observed on the Face C and its opposed edge, unlike with the previous blank holder system.

7.5 CONCLUSIONS

The possibility of manufacturing complex shape composite parts with a good pro­duction rate is crucial for the automotive industry. The sheet forming of woven reinforcements is particularly interesting as complex shapes with double or triple curvatures with low curvature radiuses can be obtained. To limit the impact of the part on the environment, the use of flax fiber based reinforcements may be consid­ered for structural or semistructural parts. This study examines the possibility to develop composite parts with complex geometries such as a tetrahedron without defect by using flax based fabrics. An experimental approach is used to identify and quantify the defects that may take place during the sheet forming process of woven natural fiber reinforcements. Wrinkling, tow sliding, tow homogeneity defects and tow buckling are discussed. The origins of the defects are discussed, and solutions to prevent their appearance are proposed. Particularly, solutions to avoid tow buckling caused by the bending of tows during forming are developed. Specially designed flax based reinforcement architecture has been developed. However, if this fabric design has been successful for the tetrahedron shape, it may not be sufficient for other types of shapes and that is why the optimization of the process parameters to prevent occurrence of buckles from a wide range of commercial fabrics was also investigated with success.

The effect of polystyrene-block-poly(ethylene-ran-butylene)-block-poly(strene- graft-maleic-anhydride) as compatibilizing agent (2 and 4%) and alkali treatment (4 and 6%) of short SPF on the flexural strength and flexural modulus of SPF/ HIPS composites were studied by Bachtiar et al.116 using 40 wt.% of fiber content. SPF alkali treatment using 6% NaOH solution improved the flexural strength, flex­ural modulus and impact strength of the composites as compared to the untreated composites by 12%, 19% and 34%, respectively. On the contrary, the SPF/HIPS composites treated with compatibilizing agent indicated no improvement in flex­ural strength and flexural modulus. However, significant improvements of impact strength of the alkali and compatibilizing agent treated composites were obtained. The impact strength of the 4% alkali and 3% compatibilizing agent treated compos­ites were about 16% higher than the untreated SPF/HIPS composites. The enhance­ment of the impact strength of alkali treated SPF/HIPS composites were due to: (1) development of rough surface fibers which offers good fiber-matrix adhesion; and (2) removal of hemicellulose and lignin parts of the SPF fibers, whereas the strong cellulose components on the fibers remained. The compatibilizing agent also en­hanced the impact strength of the composites due to chemical reaction of hydroxyl groups of SPF fibers with the anhydride groups of the copolymers which resulted into good interface adhesion between SPF fiber and HIPS matrix.116

9.4 CONCLUSIONS

The rapid advancement in the development of greener materials based on natural fibers and biopolymers is gaining more attention due to increase environmental awareness coupled with depletion of petroleum resources. Utilization of fibers and polymers that are biodegradable and obtained from renewable resources will help to preserve our environment. Hence, sugar palm tree is a potential ‘green resource’ for natural fibers and biocomposites.

Sugar palm tree is one of the multipurpose trees grown in tropical regions. Of recent, sugar palm fiber with its desirable properties has manifested high potential to be used as reinforcement in polymer composites. Sugar palm fibers can be used as reinforcement for bio-based polymers to produce 100% biodegradable compos­ites. The use of sugar palm fiber and bio-based polymers or even petroleum-based polymers to develop greener composites helps in: (1) reducing the negative environ­mental impact of synthetic polymers and fibers; (2) decreasing the pressure for the dependence on petroleum products; and (3) developing sugar palm trees as new crop in the future for tropical countries most especially in Malaysia. When biocomposite materials from sugar palm tree are increasingly used for industrial applications, such would boost Malaysia’s status as global promoter, developer and manufacturer of green composites. This would lead to increasing revenues and create more jobs. The successful development of green composites from sugar palm tree would provide opportunities to improve the standard of living of the sugar palm tree farmers in Ma­laysia. These would generate nonfood source of economic development for farming and rural areas in Malaysia.

9.5 ACKNOWLEDGEMENTS

The authors wish to acknowledge the financial support from Ministry of Education Malaysia with Exploratory Research Grant Scheme (ERGS) project vote number 5527190. Thanks also due to Universiti Putra Malaysia for granting sabbatical leave to S. M. Sapuan in 2013-2014 and to Ministry of Education Malaysia for scholar­ship award (MyPhD) to J. Sahari. The assistance of Dr. Nukman Yusoff of Universiti Malaya is highly appreciated.

14.4.1 MOISTURE

Moisture content of the fibers is due to its hydrophilic nature, the amorphous do­mains of the fiber and also the amount of interfacial area. The hydrophilic character of polysaccharides influences the overall physical properties because water on the fiber surface acts as a separating agent in the fiber-matrix interface in the fibers.35

Therefore, it can be considered a major problem for the use of polysaccharides as reinforcement in biocomposites because it can have a dramatic effect on the bi­ological performance of a composite made from natural fibers, besides affecting drastically mechanical properties of the composites such as compression, flexural and tensile.36

Moisture can be minimized by using coupling agents such as silanes, for in — stance.37 Such water repellency/inactivity can be explained because the fiber-matrix adhesion is improved via chemical silanol bonds as well as hydrogen bonds that reduce any adverse affects from moisture because it cannot penetrate the bonding system. Thus, fiber drying before processing is a key step in any successful biocom­posite processing.

16.4.1 PROCESSING

The nanocomposites of cellulose and chitosan has been motivated by their chemical similarities, the functional properties (antimicrobial, water transpiration) and the improvements in materials (mechanical, physical and barrier) properties achieved in parallel by blending the cellulose and chitosan by dissolution using acidic media and ionic liquids.32 Various organic solvents and mixture of acids have been used to process the composites of chitosan and cellulose. Most reactions consume and pro­duce toxic waste, which is hazardous to the environment. Furthermore, some com­ponents of the processing tend to release the atomic oxygen, which attacks the mac­romolecular chains and degrades them, and severely compromising the mechanical properties of the composites. In order to overcome the above-mentioned issues ionic liquids have been used to dissolve and process cellulose and chitosan composites. A binary system consisting of acidic ionic liquid glycine hydrochloride and neutral ionic liquid 1-butyl-3-methylimidazolium chloride is used as a cosolvent to gener­ate electrospun fibers of chitosan and cellulose. The prepared composite fibers were shown to have excellent thermal and mechanical properties.33 In a similar approach, environmental friendly composites of chitosan and cellulose were prepared using a mixture of sodium hydroxide and thio-urea solvent. The solvent led to chain depoly­merization of both polymers by sustaining the film forming capability. The crystal­line property of the prepared nanocomposites was close to that of cellulose film.34

Interestingly, a blend of chitosan and cellulose also showed an improvement in water vapor even when the blends were not well-miscible. The authors proposed that intermolecular hydrogen bonding of cellulose is supposed to be break down to form cellulose-chitosan hydrogen bonding, while intramolecular and intrastrand hydrogen bonds hold the network flat. These blends membranes demonstrated ef­ficient antimicrobial activity against E. coli and S. aureus3

Chitosan dissolves readily in water or acidic medium which allows one to pre­pare chitosan-based nanocomposites via solution casting3640 or spinning methods.41 Layer-by-layer (LBL) assembly method is also adopted to develop nanocomposites, by alternatively dipping in to chitosan solution and nano-cellulose suspension.4243

MA silk is well known for its outstanding mechanical properties. It has been inten­siveЧly studied by a large number of laboratories across the globe. The majority of these studies have focused on the stress-strain behavior of MA fibers because these values can be normalized to the dimensions of the tested fiber, which allows for comparison across different lengths and thicknesses of materials. Relative to high tensile steel, the breaking stress for dragline silk is comparable but it is considerably more extensible, leading to material that is 30 times tougher than steel.23 Dragline silk also outperforms Kevlar, a synthetic fiber used for ballistic armor, and is 3 times tougher. Analysis of the literature reveals much variability in the published mechan­ical properties for MA silk. In part, some of the differences can be attributed to the fact that different spider species have been used to collect MA fibers for the tests.24 However, it must be emphasized that the methodology for fiber collection can have a significant impact on the mechanical properties.25 In particular, it has been clearly demonstrated that altering the reeling speed or force during the fiber collection can influence the mechanical data.26 Humidity, the age of the spider, ultraviolet light, and the duration of storage of the fibers before analysis have also been reported to have an impact on the mechanical properties of the materials.27

On macroscopic scale, liquid crystalline phase of fluids polymers possesses long — range order. Generally, their molecules have asymmetric shapes to form liquid crys­talline phases and mesogenic groups, linked either on the main chain with flexible segments, or on the side chain attached to the flexible main chain. Some polymers without mesogenic groups are also capable of forming liquid crystalline phases. These polymers have a rigid or semirigid backbone, arising from the steric effects or intramolecular hydrogen bonding, which restricts chain flexibility.

Based on a lattice polymer, Flory was the first to predict that linear rod-like polymer would form ordered phases in concentrated solutions.86 According to this theory, at a given concentration, where the critical volume fraction of the polymer depends on the axial ratio, expressed as a length — diameter ratio, the rigid polymer solution would separate into two phases — isotropic and anisotropic.

Both entropy and enthalpy contribute to the stability of nematic mesophases, although the dominant factor is the entropy term, which is controlled by the size and shape of the molecule.8788 The original theory, which assumed that the polymer chains were completely rigid and rod-like, does not reflect the real situation. Be­sides, polydispersity, distribution of the substituted side groups as well as polymer- solvent interactions affect the flexibility of the polymer, so that even the most rigid polymers will have some degree of flexibility.

Formation of the liquid crystalline phase is highly dependent on specific poly­mer-solvent and polymer-polymer interactions. Transition from the isotropic to the ordered phase of lyotropic systems of semirigid polymers represents a balance of polymer-polymer and polymer-solvent interactions.8990 Not all cellulose derivatives form liquid crystalline phases at high concentrations; it is only those with appropri­ate solubility in a particular solvent that can form liquid crystalline solutions. In some cellulosic/solvent systems, increasing polymer concentration from the semi­dilute state, where already microgels appear, leads directly to the gel state, with no evidence of liquid crystallinity, even if the backbone of these derivatives has simi­lar stiffness with that of the derivatives producing mesophases. Due to the strong polymer-polymer interactions cellulose derivatives are not sufficiently soluble in a particular solvent to achieve the concentrations necessary for mesophase formation. Literature shows that the lyotropic liquids crystalline phase formed by cellulose de­rivatives occurs in highly concentrated solutions, ranging from 20-70% by weight, higher than those of the rigid rods predicted by Flory’s theory. The semiflexible nature of chains is due to substituents nature, substitution degree, substituents dis­tribution for partially substituted cellulose derivatives, temperature and solvents. Thus, the stiffness of cellulose derivatives in solutions is a function of the steric in­teractions occurring between adjacent units, intrachain hydrogen bonding, polymer — polymer and polymer-solvent intermolecular interactions.91

On the other hand, liquid crystal polymer blends have been intensively investi­gated as to their unique electrical, optical, mechanical properties.92 The self-aligning nature of the liquid crystal (LC) is used for obtaining organic polymer thin-film transistors in liquid crystal display devices.93 Recently, a comprehensive study has been devoted to the cellulose triacetate-nitromethane system, to explore its phase separation for different concentration and temperature values.94 The physical state of the polymer is identified within the coexistence phase limits on the phase diagram, which included three types of phase separation: amorphous, crystal, and liquid crys­tal. The limits of the regions determining the coexistence of the liquid crystal and of the partially crystal phase are found to be inside the region of amorphous liquid — liquid phase separation. For cellulose ester-solvent systems, this state diagram is the first experimental evidence for the possible coexistence of several phases with amorphous, liquid crystal, and crystal polymer ordering. The phase state of the sys­tem develops under the influence of temperature and concentration. At temperatures above the binodal and liquidus, all mixtures are homogeneous transparent solutions. If temperature is lowered and the configurative point moves to the metastable range, the system of various concentrations may have various morphologies, depending on polymer concentration and distance from the stability range. For low-concentrated solutions, under conditions of a rather high kinetic mobility of the molecules, amor­phous phase separation is observed with lowering temperature. The solutions are visually characterized by opalescence and occurrence of precipitation. For medium­concentrated solutions with low viscosities, in time the system splits into two phas­es. A dense white precipitate is formed in the polymer-concentrated phase, the other phase appearing as an opalescent solution with suspended particles. Literature9598 shows that transition to LC state is possible under certain thermodynamic condi­tions, when the selective solvation of the different polymeric groups determines the extended helical conformations of macromolecules9499102 and the appearance of liquid crystal ordering. In the same context, the structure, intra and intermolecular interactions, and conformations of macromolecules in cellulose derivative films — under conditions of the liquid crystal state formation during vapor sorption of some solvents — show that the number of intramolecular hydrogen bonds stabilizes the rigid helical conformation of the macromolecules.103 The anisotropic structure is also preserved after desorption of the vapor sorbed solvent. The capability of cel­lulose derivative films to form a liquid crystal state in vapor solvent and to preserve the anisotropic structure after solvent desorption, is especially important for prepar­ing new functional materials.

In crystalline state, the structure of polymers is influenced by solvent and con­centration.6970 Fundamental research on the formation of banded textures in thin — film samples from lyotropic solutions subjected to shear is important, due to the large number of physical interactions here involved.104 105 Surface anisotropy and the mechanical and optical properties of the polymer films,106 together with their potential use as alignment layers for liquid crystal displays, make these systems par­ticularly interesting and promising for new applications. Flow behavior is the most thoroughly studied rheological property. Some studies hypothesized the universal existence of three shear flow regimes to describe the viscosity of polymer liquid crystals namely: a shear thinning regime at low shear rates (Region I), a Newtonian plateau at intermediate shear rates (Region II), and another shear thinning regime at high shear rates (Region III). Region I, observed at low shear rates, shows shear thinning, exhibiting yield stress, as in some plastic materials. This region is charac­terized by distortional elasticity, associated with spatial variation in the director field (average local molecular orientation). Region II is a Newtonian plateau, reflecting a “dispersed polydomain” structure and Region III is a shear-thinning zone, showing viscoelastic behavior. In Regions I and II, the flow is not strong enough to affect molecular orientation while, in Region III, the flow field is very strong, so that the shear induces molecular orientation. Cellulose derivatives do not always cover the entire domain from Region I to Region III, because not every regime lies within the accessible shear rate range.107

In the preceding section, the specific interactions of CAP/HPC blends in 2-me — thoxyethanol are presented, such as hydrogen bonding, ion-ion pairing, and elec­tron-donor and electron-acceptor complexation, which generates miscibility be­tween components, have been discussed. In this context, the ATR-FTIR spectra of CAP, HPC and their blends (100/0, 75/25, 50/50 25/75 and 0/100 wt./wt. CAP/ HPC), plotted in Fig. 3.21, show that equilibrium in polymer blends is assured by the hydrogen bonds.69 A remarkably similar aspect of the spectra is observed for both polymers. Broad transmission bands are distinguished at 3421 cm-1 for HPC and 3431 cm-1, respectively, for CAP, produced by stretching of the — OH groups, at 1723 cm-1 for HPC and 1719 cm-1 for CAP, produced by stretching of the C=O groups from the ester, carboxylic acid, and at 1252 cm-1, respectively, for HPC and 1329 cm-1 for CAP, produced by stretching of the C-O-C ester bond. The pres­ence of hydrogen bond structures in blends are evidenced from peaks shape and intensity of the absorption band of the hydrogen stretching vibration.108 The differ­ences observed among the shape, broadening and shifting of the mentioned peaks for polymer blends suggest the existence of hydrogen bonding generated by — OH, C=O and C-O-C groups. However, the mentioned free and associated groups assure the equilibrium in these polymer blends via hydrogen bonds.

Quantitative measurements of weight loss are shown as TG/DTG plots (Fig. 3.22) for the same blends.69 The highest mass loss was of 100/0 wt./wt. CAP/HPC, followed by 75/25, 50/50 25/75 and 0/100 wt./wt. CAP/HPC. It has been found out

that the components of the blends decompose over different temperature ranges, the films loose water, and the curve for pure HPC shows an one-stage degradation step within the 300-400 °C range, with a maximum at 373 °C. The CAP and CAP/HPC blends showed a two-stage degradation step within the 210-298 °C and 280-560 °C ranges, respectively. The first weight loss is caused by the phthalic anhydride from CAP; with increasing the HPC content, this process is less evident, while the second weight loss is caused by the remaining cellulose. In addition, Table 3.7 shows that a percentage of 10 wt.% from the CAP and HPC mass is lost at 194 °C and 340 °C, respectively. For CAP/HPC blends, the temperatures at which 10 wt.% of their mass is lost represent intermediary values of the above-mentioned ones, increasing with increasing the HPC content.

T, °С

FIGURE 3.22 Experimental TG/DTG curves of CAP, HPC and CAP/HPC wt./wt. blends.69

(a) Degradation stage corresponding to the water loss.

(b) Degradation stage corresponding to the phthalic anhydride and acetic acid losses from CAP.

(c) Degradation stage corresponding to the phthalic anhydride losses from CAP.

(d) Degradation stage corresponding to the thermal decomposition of the remaining cellulose.

Thermogravimetric measurements show that CAP is less thermally stable than the HPC and CAP/HPC blends, as the anhydroglucose units increase the rigidity of the HPC chain.69 These interactions impart specific properties to the polymer blends. In N, N-dimethylacetamide (DMAc), the anisotropic behavior appears under specific conditions of concentration and/or blend composition. Some studies have reported that the polymer structures, their mixing ratio and the used solvent influ­ence the interactions from the systems and, consequently, the ordered domains in rheological behavior.109,110 In this respect, Figure 3.23 presents the modification of dynamic viscosity, П, versus shear rate, Y, for c = 20, 40, 60 wt. % at 25 °C, for
pure CAP and pure HPC in DMAc. In addition, Fig. 3.24 shows the same modi­fication of dynamic viscosity versus shear rate at different temperatures over the 25-45 °C domain for HPC in DMAc.

-10123

log у (S-1)

As one can see, for CAP solutions with lower concentrations, a Newtonian be­havior appears over the entire shear rate domain while, with increasing concentration to 60 wt. %, the thinning effect reduces dynamic viscosity. The shear experiments

performed on lyotropic HPC solutions in DMAc reveal that the viscosity-shear rate dependence at different concentrations appears only in Regions II and III. As concentra­tion increases, the Newtonian plateau becomes smaller, being shifted to lower shear rates. At low shear rate, dynamic viscosity increases with concentration for both cellu­lose derivatives, showing a maximum at intermediary concentration, in the 20-60 wt. % range, which means a transition from the isotropic to the anisotropic phase.

Ordering of macromolecules in both polymers at higher shear rate is associated with a viscosity lower than that of the isotropic solutions, revealing liquid crystal be­havior. Below a certain critical shear rate and at a lower content of HPC, the isotro­pic solutions are Newtonian. Literature data show that the viscosity peak, observed for all lyotropic polymer solutions, is a decreasing function of shear rate, ascribed to several mechanisms.70111 A competition between the ordering induced by shear and that thermodynamically produced was suggested by Hermans,111 while a correlation between maximum viscosity and the anisotropic phase appearance — valid only at lower shear rate — was suggested by Zugenmaier.91

For CAP/HPC blends in DMAc,70 a distinct change in the rheological properties occurs at the critical concentration of 40 wt. %. Therefore, viscosity at higher shear rates takes lower values than at lower concentrations, at equivalent shear rates. Upon formation of the anisotropic phase, viscosity begins to decrease. Considering the de­pendence of viscosity on concentration for concentrations above 40 wt. %, the solu­tion is in a fully liquid-crystalline mesophase. Below this concentration, the solution is biphasic. Similar results were obtained for HPC aqueous solutions89112 or CAP/HPC blends — in which Regions II and III have different extensions, the Newtonian behav­ior decreasing with increasing concentration and HPC content (Fig. 3.25).

Deviation of the CAP solution from the Newtonian behavior is also analyzed by the power law relationship between shear stress and shear rate (equation 11). Figures 3.26 and 3.27 show the values of the flow and consistency indices, obtained at different concentrations and blend compositions, from the slope and intercept of log shear stress versus log shear rate plots, over the domain of lower shear rates. It was observed that, as concentration increases, the resistance of the fluid to the ap­plied rate of shear or force (called shear stress) decreases, causing a decrease of the flow index to values < 1, and an increase in pseudoplasticity. Decrease of the flow index occurs when the HPC content increases and temperature decreases. At the same time, the consistency index increases with increasing concentration and HPC content, and decreases with the increase of temperature.

0.0 0.8 log y, 1/s

FIGURE 3.26 Log-log plots of shear stress versus shear rate for 100/0, 25/75, 0/100 wt./ wt. CAP/HPC blends in DMAc at different concentrations: (■) 20 wt. %; (▲) 40 wt. %; (▼) 60 wt. %.70

Figure 3.28 shows that the flow activation energies for CAP and HPC in DMAc, determined in the region of shear rate with Newtonian behavior, increase with con­centration and are higher for HPC.

Similarly with Fig. 3.13, Fig. 3.29 illustrates the variation of storage and loss moduli versus shear stress at a constant frequency of 1 Hz, for CAP, HPC and CAP/ HPC blend solutions at 60 wt. % concentrations in DMAc. G’ is constant and lower than G», showing viscoelastic fluid properties for 100/0 and 75/25 wt./wt. CAP/ HPC blends over a larger deformation region. Increase of the HPC content deter­mines higher values of G’ than of G», generated by transition from the isotropic to the anisotropic phase.

The ordering tendency in casting solutions of CAP, HPC and their blends in DMAc is illustrated in Fig. 3.30, where moduli G’ and G» are presented as a func­tion of frequency, in the same manner as in Fig. 3.14 (at lower concentrations, in 2-Me).

For all concentrations of CAP and CAP/HPC blends at higher content of CAP, over the low frequencies domain of 0.2-0.9 Hz, the storage and loss moduli are proportional to frequency — when the exponents for G’ and G» take values between 1.6-0.6 and 0.9-0.4, respectively, maintaining the characteristic of a viscoelastic fluid, where G'< G» .76-77-113 These exponents decrease both with increasing polymer concentrations and at higher HPC compositions in the polymer blend. In addition, the frequencies corresponding to the crossover point, which delimits the viscous flow from the elastic one, and for which G’ = G», exhibit lower values for HPC in DMAc, and become higher with increasing the CAP content in polymer blends. A gel characteristic proves the presence of a liquid crystal phase, which appears for HPC, and a higher content of HPC at 40 wt. % and 60 wt. % concentrations. In these cases, the storage modulus, G’, is always higher than the loss modulus, G», over the entire frequency range, being characterized by G’ ~ f0267-0475 and G» ~ f0 249-0399 ; these dependencies are observed when the lyotropic phase becomes predominant.

Literature shows that the liquid crystalline order in cellulose derivative solutions observed by rheological investigations is preserved in solid films by slow evapo­ration of the solvent.114 Consequently, the microstructure of films depends on the drying conditions; also, films prepared under ambient conditions show polydomain

structures with the helical axes of different chiral nematic domains pointing in dif­ferent directions. Moreover, applying a magnetic field during drying increases the size of the chiral nematic domains and affect the orientation of the helical axes with respect to the film plane.115 Studies on the microstructure of such chiral nematic solid films reveal parabolic focal conic defects, a symmetrical form of focal conic defects in which the line defects produce a pair of perpendicular, antiparallel, and confocal parabolas.114 Depending on the substance and manner of sample prepara­tion, the parabolas lie horizontally or vertically in the sample plane.116 In vertical position, the parabolas intersect the top and bottom sample surfaces with their ends. The focal regions are located in midplane. The fact that these structures are captured in a solid film permits to study this highly symmetrical defect structure by other means than optical microscopy, for example, by atomic force microscopy (AFM). In addition, a chiral nematic system with a helical pitch large enough to be resolved by an optical microscope allows visualization of the individual structural layers.

Thus, AFM images from Figs. 3.31-3.34, for cellulose derivative films prepared under ambient conditions, show the polydomain structures with the helical axes of different chiral nematic domains pointing in different directions. Moreover, films obtained from CAP and HPC liquid crystalline solutions in both pure state and in mixture evidence some lights areas whose sizes depend on the HPC content and increase with increasing concentration. Intensity of the polarization colors varies cy­clically, from zero up to a maximum brightness at 45 degrees, for HPC (Fig. 3.31 (b, b,’ b”)). A rotating stage and centration of samples in a polarized light microscope is a critical element for determining the quantitative aspects of the HPC liquid crystal. Centration of the objective make the center of the stage rotation coincides with the center of the field, for maintaining the specimen exactly in the center, when rotated.

In the case of CAP/HPC blends, the number of methylene units (i. e., side chain length) inserted by increasing the HPC ratio in blends has a significant influence on the selective reflection characteristics of cholesteric liquid crystals.117 Some forma­tions of different sizes and intensities, namely droplets, appear. At a c = 20 wt. % concentration for all mixing ratios, the presence of the HPC liquid crystal is visible (white areas), without forming the droplets, such formations tending to appear start­ing from a 40 wt. % concentration (Figs. 3.32-3.34).

Atomic force microscopy studies, according to Figs. 3.33-37, 70 also evidence the occurrence of liquid crystal phases including these formations. Under particu­lar conditions, HPC liquid crystalline films exhibit a characteristic structure, called “band texture,”81,91 consisting of alternating bright and dark areas.

As the percent of the liquid crystalline counterpart decreases, the surface pattern is maintained, while its dimensions become higher and the “small” bands disappear. In the case of CAP/HPC blends in DMAc, hydrogen bonds appear only between HPC and DMAc, therefore band texture is larger and increases with decreasing the amount of HPC. A natural tendency is noticed for the semirigid segments of HPC, namely to self-align into ordered domains, thus lending high performance properties to these materials.118 Also is observed that the LC phases are strongly dependent on the number of methylene units from the side chains of polymer blends.

On the other hand, although in optical microscopy images droplets cannot be seen at c = 20 wt. % concentration, in AFM images they are visible for all films, with the exception of the pure sample. They start to appear from c = 20 wt. %, 50/50 CAP/ HPC wt./wt. blends, their dimensions being around 3.69mm. These values agree with those from literature,119 obtained for hydroxypropylcellulose/polydimethylsi — loxane blends, where the size of droplets varies between 7-15mm. With increasing concentration, domains without droplets appear, as well as areas with droplets in training. They become more visible for 25/75 CAP/HPC wt./wt. blends, increase of the HPC content and, implicitly, of the methylene units leading to a more compact network, caused by HPC/DMAc bonding. Thus, the size of droplets decreases with increasing concentration (Table 3.1). In addition, as shown by Table 3.1 and by the AFM images of films made from pure samples, the surfaces present low roughness, pores appearing only at 75/25 CAP/HPC wt./wt., their dimension increasing with increasing concentration. Both components are present on the surface after solvent evaporation, being stabilized by hydrogen bonds interactions, which leads to the formation of droplets/pores of different sizes and intensities. The periodicity of the

average peak-to-valley height for these images is evident from the profile line plot­ted in Figs. 3.35-3.39.

In the same context, some data on the rheological properties of epiclon — based polyimide/HPC blend solutions have been reported.26 2D topography AFM images (40×40 /um2) of the corresponding films, prepared by mixing a
60 wt. % HPC solution with a 49 wt. % polyimide solution, shows that the hydrogen bonds between HPC and DMAc influence band texture, which increases with decreasing the amount of HPC (Fig. 3.40 (a) and (b)).

As a result of the symmetry properties of the HPC liquid crystal solution, large domains of well-oriented polymer chains are formed during shear flow, while the defects are squeezed into small regions. Under specific shear flow conditions, the cholesteric liquid crystalline cellulose derivatives exhibit unwinding of the cho­lesteric helix and a cholesteric-to-nematic transition.120 When shear is stopped, the system will first relax at a characteristic time to a transient state. In this state, the distortion energy is minimized and the orientational order is maintained, resulting a banded structure. The shear-induced anisotropy is affected by the inevitable relax­ation of the chains, when the external field is removed. By introducing polyimide into the HPC solution, relaxation will take place collectively, due to the fact that the highly concentrated and aligned polymers cannot individually relax and, therefore, the inner stress induces a periodical contraction in the whole liquid crystalline poly­mer and different packing modes are observed. Structural relaxation after cessation of shear depends on the shear history of the mixture and on the dominant mechanism of stress relaxation. The band morphology of the blend is influenced by precursor and polyimide solution composition, solvent evaporation rate, film thickness, rate and duration of shear.121 The effect of the chemical structure and composition on the viscoelastic properties is reflected on the orientation or mobility of segments in the shear field. The inherent long-range ordering tendencies of LC itself — and specifically its pattern-forming properties — open new perspectives to produce or­dered polymer microstructures. The morphology of the ordered domains provides a means of “imaging,”, respectively potentially novel aspects of the pattern-forming LC states. Pure and applied researches related to the shear-induced morphology and structural relaxation after cessation of shear in polymer/LCP blends will become more and more important for developing high performance alignment layers used in display devices.

Consequently, the occurrence of droplets coincides with the thinning behavior evidenced by rheological data, both being caused by the more numerous hydrogen bonding interactions at higher HPC content. In this context, the results are in agree­ment with the literature ones, which show that cellulose derivatives form lyotropic mesophases.

All these results reflect the specific molecular rearrangements produced in the system through modification of the mixing ratio of polymers in certain solvents. As some biological polymers are insoluble in organic solvents or water, ionic liquids have attracted the attention of industry, especially because of the current need for creating a green chemistry environment. Thus, literature recommends 1-butyl-3- methylimidazolium acetate as a solvent for the study of sol/gel and liquid crystal transition of hydroxypropylcellulose (HPC).25 According to the experimental data obtained from the parameters: relaxation time, hysteresis ratio, loss modulus, and also observing the LC textures via a polarization optical microscope, the liquid crys­tal transition concentration of HPC is slightly higher than sol/gel transition concen­tration, and increases with temperature which is also the case of common solvents.

Furthermore, the structural and molecular heterogeneity of cellulose derivatives, their high molecular masses, low diffusion coefficient values, high viscosities of even moderately concentrated solutions, and specific intermolecular and molecule — solvent interactions, as well as their behavior in different conditions raise numer­ous experimental problems. The investigations developed in this chapter describe the character of the structural transformations observed, starting from rheological and morphological data. Knowledge of phase separation kinetics mechanism which modifies the morphology of cellulosic systems allows designing of materials with desired properties for specific applications.

The ratio between the passage ways through the pores and the thickness of the po­rous material is called tortuosity.61 Tortuosity gives the extent of the deviation of the pores from the normal of material thickness.71 The sound absorption performance of porous materials is generally affected by tortuosity which does not allow the sound waves to follow straight paths.72 The air that is forced to follow a tortuous path suffers accelerations which cause momentum transfer from air to the material.35 The value of tortuosity determines the high frequency behavior of sound absorbing porous materials.61

Tortuosity can be measured by an electrical conductivity method.17 In this meth­od, the porous material is saturated with a conducting fluid such as a brine solution, and the electrical resistivity of the saturated porous material and the solution alone is measured and compared.

(28)

where F is the formation factor, and R, and R are the electrical resistance values of

7 ef es

the fluid alone and the porous material saturated with fluid, respectively, which have

the same dimensions, in ohm units. As the electrical resistance is proportional to the length, L, and inversely proportional to the area, S, of the conducting material the ratio between the resistances; formation factor, F, should be tortuosity, Ts, divided by porosity, h, as shown in the following equations:17

Tortuosity contributes to the ‘structure factor,’ Г, along with the effect of in­ner structure of porous materials.40 The structure factor, Г, (x in 18) accounts for an increase in the inertial mass density of the air. In other words, the irregularity of the structure causes an addition of induced mass to the density of air. For fibrous ma­terials, structure factor, Г, is generally between 1.2 and 2.3,13 but it is assumed to be unity in modeling studies by numerous researchers including Allard and Cham — poux,73 Ballagh46 and Cox and D’antonio.14 High structure factor leads to low propa­gation speed, which increases the effective thickness of the absorber; and thus, used in sound absorber design.13 Biocomposites are advantageous in terms of tortuosity due to the fact that the irregular shapes of constituent plant fibers prevent a straight flow of sound waves through the thickness of the absorbent material.

Jute, cotton, coir, flax and their derivatives can be used as acoustical absorber in walls and ceilings of buildings. Such application of the biocomposites helps in con­trolling the reverberation time and the SIL. A typical case study using jute is pre­sented here. Raw jute fiber, chopped jute strands and jute felt as shown in Fig. 6.3 can be used for wall and ceiling treatment. These jute derivatives can be held against a wooden board and faced with a perforated sheet or kept inside a jute cloth packet. The sound absorbing properties of such sample varies with thickness as discussed earlier. These jute derivatives can also be used in office partition and cubical ceil­ings. Since these materials cannot be put by themselves onto a wall they have to be kept inside a jute cloth packet which can be either glued or stapled to the wall. Some of these jute cloth packets can be stapled or screwed against a hard backing surface as shown in Fig. 6.15.

FIGURE 6.15

Natural rubber based jute fiber composite panels have adequate transmission loss and can also be used as partitions in office cubicles. The transmission loss of such panels can also be measured as per ASTM E90 standard38. They also have adequate amount of flexural strength and can be used in such application. A typical used ofjute fiber based panel with a perforated facing of 47% open are ratio used in our office cubical for noise reduction is shown in Fig. 6.16

Typical board size are 600 mm x 600 mm with a thickness of 50 mm. Figure 6.17 shows such boards being used in a room ceiling for controlling the reverbera­tion time. Such jute boards can also be fixed on auditorium walls for improving the acoustics. These boards can be either screwed or nailed into the backing wall at the
corners. These materials have good thermal stability properties and are fire retardant themselves, thus can be safely used56.

These boards have been used in ceilings of rooms for more than three years and have no signs of degradation in their physical condition. Depending upon the requirements, these acoustical boards can be made of larger dimensions for instal­lation in the walls of auditoriums, cinema complexes, shopping malls, hospital, li­brary and airports.

Fiber attrition is a major problem while dealing with natural fiber reinforced com­posites during the melt mixing process.28 The strength of the natural fiber reinforced composites depends on the amount of applied load transmitted to the fibers. The extent of load transmitted depends upon the length of the fiber and the fiber matrix interfacial bonding. The load sharing capacity of the fiber in short fiber reinforced composites (as in case of injection molded composites) depends upon the critical fiber length (CFL). If the fiber length is less than the CFL, debonding of matrix and fiber takes place resulting in failure at low load. If the fiber length is more than the CFL, a failure due to breaking of fibers takes place indicating high composite strength. In case of injection molding process, a significant amount of fiber attrition is found in the molded part due to high shear rates during plasticizing, injection and passage through narrow gates and openings of the mold.26 Fiber attrition also takes place during precompounding process employing a melt mixer, kneader or twin screw extruder. Reduction of the fiber length below the CFL would lead to degra­dation of the composite properties as the short fibers would not be able to bear the load for which the composites are designed. Hence, the determination of the CFL of fibers is important prior to injection molding of fiber-reinforced composites. Al­though, increasing the fiber content would lead to better mechanical properties but the injection molding process limits the amount of fibers that can be injected due to increased viscosity of the mixture and narrow gate and sprue of the mold. To over­come this problem of fiber attrition during processing of NFCs by injection mold­ing, the process parameters and mold dimensions should be adjusted according to the fiber load and viscosity of the melt. The process parameters should be adjusted to cause minimum shear rate during processing of NFCs. Also the gate and sprue dimensions should be increased in order to accommodate the fibers and reduce the shear rate. Better precompounding techniques should be developed to reduce the fiber attrition during precompounding of fiber and polymer. In a study regarding incorporation of a counter rotating extruder for compounding of biopolymer-wood composites, it was reported that a higher aspect ratio of the fibers was achieved due to the use of counter rotating extruder which acts more likely as a refiner and the fiber breakage, compared to other mixing techniques was reduced.6

Prior treated with the chemical the celluloses were cleaned with the detergent at 80°C for 2 h to remove the impurities and contaminants as much as possible then rinsed three times with demineralized water.

Two different fiber treatment processes were used. In one-step treatment pro­cesses (P1), cellulosic fiber was soaked in a prepared solution for a period of time. The fibers were then dried in air for 6 h and then in an oven at 120°C for 2 h prior to testing.

In the two-step treatment processes (P2) cellulosic fiber was soaked in a first so­lution for 5 to 300 seconds. The fibers were then removed from the treating medium and allowed to dry in air for 6 h, and then dried in an oven at 120°C for 2 h. The dried fibers were then soaked in a second solution for 5 to 300 seconds. Finally the fibers were dried in air for 6 h and then in an oven at 120°C for 2 h prior to testing.

11.3.2 COMPOSITE FABRICATION

For phenolic (PF) composites, phenolic resin was then wetted on the fibers and dried in an oven to remove solvent from the resin and to let the resin transfer to stage B before compression. Wabash PC 100-2418-2TM compression was used to fabricate the composites under 100 psi pressure at 150°C. The amounts of resin and fiber in the final product were about 60 wt.% and 40 wt.%, respectively. The thickness of the composite plaque was about 3 mm.

Laminate epoxy composites were prepared by compression molding similar to the fabrication of phenolic composites but at 80°C. The amounts of resin and fiber in the final product were about 60 wt.% and 40 wt.%, respectively.

Laminate unsaturated polyester (UPE) composites were prepared by compres­sion molding similar to the fabrication of phenolic composites but at 50°C. The amounts of resin and fiber used were about 70 wt.% and 30 wt.%, respectively. The UPE resin contains 20 wt.% alumina trihydrate Hubert SB332.