Residual stresses are the internal stresses that exist inside the molded part in the ab­sence of the external load. The residual stresses in case of injection molding are flow induced and thermal induced stresses. The flow-induced stresses in polymer chains depend on orientation and packing pressure while thermal stresses are induced as a result of non uniform cooling of the molded part.29 In other words, residual stresses in case of injection molding are a result of temperature variations, high pressure generated and polymer chains orientation after cooling.30 The characteristic residual stress distribution in an injection molded part shows tensile stresses at the core and surface, and compressive stresses at intermediate region.2531 The major causes of residual stresses in case of fiber reinforced composites have been listed as high pressure gradient, orientation of the polymer chains, non uniform temperature pro­file and the difference in the thermal expansion coefficient of fibers and the matrix. These stresses are introduced during injection molding process stages of filling, packing, and cooling. Residual stresses in the molded composite cause an early fracture. The stress distribution in injection molded part depends upon the pressure history to which the melt mixture is subjected from start to filling up of the mold cavity.25 The residual stresses in case of injection molded parts may cause defects like stress cracking, warpage and long-term deformation.32 The residual stresses can be reduced by the gradual cooling of the molded part. The gradual cooling of the injection-molded part is achieved by setting higher mold temperature. Heat treat­ment of the injection-molded parts is another way of relieving the residual stresses developed due to non uniform heating. The residual stresses can also be reduced by carefully adjusting the process parameters like screw speed, injection pressure, melt and mold temperature as well as taking the rheological properties of the polymer matrix into consideration. The appropriate mold design helps in the reduction of residual stresses. The mold designing includes the injection gate size and location, cavity shape and vents for air to escape.

8.2 CONCLUSION

The driving forces behind the use of NFCs are environmental as well as economic considerations. The use of NFCs is rapidly increasing in aerospace, automobile, house hold, electric, sports goods and biomedical applications. This has made NFCs as a potential candidate for research efforts. The literature available on NFCs has shown that NFCs have the potential to replace the traditional PMCs in many ap­plications. As the demand is increasing; fast, easy and economical processing tech­niques are required for fabrication of NFCs. Injection molding of NFCs has been reported by many researchers but still much work has not been presented on the selection and optimization of the process parameters. In other words, there is no set of thumb rules available for the processing of NFCs. In this chapter different as­pects regarding fabrication of injection molded NFCs have been discussed. Injection molding process parameters like screw barrel temperature, screw speed, injection speed, injection time, injection pressure, mold temperature and back pressure have been discussed. The study of the effect of these parameters on the injection molding of NFCs is important to ensure a defect free injection molded composite. Distribu­tion and orientation of fibers, fiber attrition and residual stress generation are the main issues that have to be taken care of during injection molding of NFCs. These issues can be minimized by careful selection of process parameters and by optimal mold design. The rheological properties of the polymer matrix should also be taken into consideration while selecting the parameters and designing of the mold.

Finally, it can be concluded that injection-molding process has a huge poten­tial for processing of natural fiber reinforced composites. A judicious selection of process parameters and an optimal mold design can further enhance the application areas of injection molding process in context of the natural fiber reinforced com­posites.

JEOL JSM-6100 SEM at a voltage of 10 kV was used to observe the surface of fibers before and after treatment. This SEM was also used to observe the fracture surface of the composites after tensile test.

11.3.3 GENERAL PROCEDURE FOR BURNING TESTS

A Govmark UL94 chamber was used to conduct burning tests. For each example, results are provided using numbers and the terms “NB” and “G.” The term “NB” means “no burning” and is an indication that there was no flame and no glow after removing the flame. “NB” represents excellent fire resistance as the sample did not continue to burn after the external flame source was removed, thus the sample was self-extinguishing. The term “G” means “glow” and is an indication that the sample continued to glow after removal of the flame. The numbers are the time in seconds that the sample continued to glow after removal of the flame.

Any successful sustainable composite must consider the natural fiber type and con­tent. Within the type, the length, aspect ratio, and chemical composition have a great influence on the processability. Within the content, the stiffness of a composite can be increased significantly by increasing the proportion of natural fibers within the composite assembly.38 The impact strength is also improved by higher natural fiber contents, but can also contribute to composite odor emissions and water uptake.39 A number of other mechanical properties are also attributable to the aspect ratio which influences the quality and quantity of the networking that can be attained in a composite system.40 For cellulose films, the mechanical properties of showed an increase in strength and stiffness with decreasing fiber size, and this stabilized after a certain number of passes in the homogenizer. Finally, in terms of reinforcement, it was found that the critical factor controlling the contribution of NFC to it was its homogeneity rather than its DP.25

In general, by the reinforcement with inorganic particulates, the transparency of films is significantly affected which can limit the potential applications in packag­ing. In case of cellulose-chitosan nanocomposites, the optical transparency of the films is often considered as auxiliary criterion to judge the miscibility of the compo­nents. In an earlier investigation, the transmittance of UV/Vis light (200-1000 nm) of the casted films of cellulose nanocrystals reinforced nanocomposites was shown to decrease slightly from 90 to 85% with 0 to 10 wt.% of CNCs /CNWs. Above 10 wt.% of CNCs loading, the transmittance is significantly decreased.

Fernandes et al.39 have demonstrated an improved transparency with nanocom­posites prepared using longer NFCs nanocomposites in two different types of chito — san with different molecular weight. Both low and high molecular weight chitosan (LCH and HCH) were modified to dissolve in acetic acid and water. The four types of nanocomposites with 0-20 wt.% NFC in HCH and water-soluble HCH (WSHCH) and 0-60 wt.% NFC in LCH and water-soluble LCH (WSLCH) prepared via solu­tion casting exhibited a transparency up to 20-90% depending NFC content. Up to 5 wt.% of NFC, the optical transmittance was unaffected. They have also exhibited a significant increase in thermal stability, for example, the initial-degradation tem­perature (7’di) from 227°C to 271°C.39 In a similar approach, highly transparent (up to 90% optical transmittance) and flexible films were also produced using bacterial cellulose (BC) (Fig. 16.2).44

These materials have the great potential to be exploited in food and antimicro­bial packaging applications where the transparency, mechanical properties, thermal stability are required with inherent antimicrobial activity and biodegradability (after disposal).

Hand pulling spider fibers has been reported in the literature.4252 Applying this methodology, large amounts of variability have been observed in the material prop­erties of the fibers spun from the same spinning dope.42 Furthermore, the feasibility for large-scale production using this approach is unrealistic. Differences between fibers spun from the same spinning dope can be attributed to several parameters that are difficult to precisely control, which include the draw rate and actual draw ratio. Therefore, it seems intuitive that inherent inconsistencies due to hand draw­ing would be introduced into the production of fibers and alternatives, such as the integration of automation into the processing of the fibers, which includes postdraw, is more pragmatic.

1.2 TRANSLATION OF FIBER PRODUCTION TO A LARGE-SCALE FORMAT

4.5.2.1 PREPARATION AND CHARACTERIZATION OF TPG

Vegetable oils such as soybean oil and tung oil (TO) and bio-based phenols such as cardanol (CD) and pyrogallol (PG) are promising raw materials for the preparation of
flexible bio-based phenolic epoxy hardeners. TO is a triglyceride extracted from the seeds of the tung tree (Aleurites fordii), in which approximately 80% of the fatty acid chains is a-eleostearic acid, that is, 9-cis, n,13-trawsoctadecatrienoic acid.6869 Therefore, TO with the conjugated triene moiety shows a characteristic reactivity which is not seen in the convention soybean oil and linseed oil, etc. From the past studies, it was found that the reaction of soybean oil and phenol in the presence of a super acid such as trifluoromethanesulfonic acid or tetrafluoroboric acid pro­duce a complex mixture of phenolated soybean oils oligomerized by Diels-Alder reaction.7071 In contrast, the reaction of TO and phenol smoothly proceed in a mild acidic condition without the formation of oligomerized materials to produce a de­sired TO-phenol resin.7275 PG is obtained by decarboxylation of gallic acid which is a basic component of hydrolysable tannin. In the past studies, PG-formaldehyde resin76 and TO-PG resin77 (TPG) have been successfully synthesized and applied for a thermosetting wood adhesive and a positive photoresist developed by alkaline solutions, respectively. We carried out the preparation and structural analysis by 1H NMR spectroscopy of TPG and used TPG as an epoxy-hardener.25 The reaction of TO and PG in the presence of />-toluenesulfonic acid in dioxane at 80 °C for 3 h gave TPG as a brown viscose liquid in 37% yield (Fig. 4.28). The fact that a considerable amount of TPG is lost during the repeated washing with hot water for the removal of unreacted PG is a reason for the low yield. So, there is a possibility that the yield is improved by the optimization of purification method. As PG has a high reactiv­ity at both the 4- and 6-positions to an electrophile, it is supposed that crosslinking reaction should occur in the reaction with a multifunctional reagent such as TO. Actually, the reaction at a higher temperature than 80 °C or the use of other acid catalysts such as hydrochloric acid and borontrifluoride diethyl etherate resulted in a formation of gelatinous materials. The obtained TPG was soluble to ethanol, acetone, ethyl acetate, tetrahydrofuran, diethyl ether, ^A^-dimethylformamide and dimethylsulfoxide, and insoluble to water, chloroform and hexane.

Figure 4.29 shows FT-IR spectra of TO, TPG and PG. The band at 3375 cm1 due to O-H stretching vibration and that at 1623 cm-1 due to benzene ring frame­work stretching vibration in addition to the band at 1714 cm-1 due to C=O stretch­ing vibration and those at 2950-2840 cm-1 due to sp3C-H stretching vibration were observed for the spectrum of TPG, indicating that pyrogallol moiety and tung oil moiety certainly bonded. Also, the fact that the bands at 993 cm-1 and 732 cm-1 due to =C-H out-of plane bending vibrations of trans and c/s-olefinic moieties, respec­tively observed for TO considerably diminished for TPG, suggesting that the addi­tion reaction of PG to the olefinic moieties of TO certainly proceeded.

TPG

O-H

stretching

1,2,3-trisubstituted

benzene

O-H

stretch і n$

benzene ring Framework ‘ stretching 1623 cm"

Figure 4.30 shows the 1H-NMR spectra of TO and TPG measured in d6-acetone. The 1H signal of methine proton of glyceride unit (Ha and Ha,) in TPG and TO was observed at 5.29 ppm (s) and 5.27 ppm (s), respectively. The integral values of other proton signals were evaluated relative to those of Ha and Ha, signal (1H). Because we could not specify the phenolic hydroxy groups of PG unit in TPG, the H-D exchange reaction was performed by the addition of D2O in a NMR tube. As a result, the 1H signals from 7.92 to 6.71 ppm (6.8H) in d6-acetone disappeared in the spectrum of TPG in d6-acetone/D2O, indicating 2.3 pyrogallol units are added to a TO triglyc­eride moiety. This number is in agreement with the integral values of 1H signals at 6.45 ppm (d, 2.3H, Hf, J = 8.3 Hz) and 6.34 ppm (d, 2.2 H, Hg, J = 8.3 Hz) in the pyrogallol ring of TPG. The fact that coupling constant of two protons (Hf and Hg)

of the pyrogallol ring is 8.3 Hz indicates that electrophilic substitution reaction of the TO-derived carbocation occurred at 4-position of pyrogallol (1,2,3-trihydroxy — benzene), because the product obtained by the reaction at 5-position should have the coupling constant at around 3 Hz. The PG-substituted methine proton (H) is also observed at 3.65 ppm (m, 2.3H). The number of olefins of TO per triglyceride is estimated to be 7.6 from the integral value of the olefinic 1H signals at 6.45-5.41 ppm relative to that of Ha,. Similarly, the number of olefins of TPG is estimated to be 4.3 from the integral value of the olefinic 1H signals at 6.07-5.35 ppm relative to that of H. From their values, the number of diminished olefins of TPG relative to TO is estimated to be 3.3, which is a little higher than the degree of addition of PG (2.3). This discrepancy may be attributed to the possibility that some components of TO with lower olefinic number are eliminated by the purification. Four structural formulae (R) of TPG in Fig. 4.28 are capable as the structures of the pyrogallol — substituted hexadiene moiety of TPG, considering the stability of the carbocation of reaction intermediate, if the horizontally flipped structures of R are omitted. In addition, there is a possibility that the original cis-trans configuration of triene part of TO is transformed to other configurations by the migration of п-bond in the car­bocation intermediates. Among the olefinic proton signals of TPG, the signal at a lower magnetic field (6.07 ppm) is assigned to inner protons (-С^СЯ-СЯ^^) of conjugated diene moiety, and that at a higher magnetic field (5.35 ppm) is related to the protons of isolated olefin moiety. However, we could not assign the olefinic proton signals of TPG more precisely because many structural and configurational isomers are contained.

Biocomposites made up of flax, hemp, coir and jute have potential for application in automobile, building and machineries for noise control purposes. In order to use such materials their physical, mechanical and acoustical properties are needed. Many literature exist where some of these physical properties of biocomposites can be found67.

A biaxial tension device (Fig. 7.2) has also been used to characterize the tensile behavior of the reinforcements.

The biaxial tension device.

Biaxial and uniaxial tension tests as well as tensile test conducted on individual tows can be performed using this device. In-plane shear test can also be performed using the bias test samples. All these specific samples are shown in Fig. 7.3. For synthetic carbon or glass fabrics, the limit to failure is not reached during form­ing, and the tensile test are designed to analyze the possible nonlinearity of the stress-strain curves due to the 2D assembly of the woven textiles generally used. For natural fiber fabrics, the tensile limit of the fabric becomes particularly interesting as the tows used to elaborate the woven fabrics are manufactured from finite length
fibers slightly entangled and held together by a natural binder are not expected to show comparable tensile resistance. The tensile strains for each considered tows are measured using a 2D version of the mark-tracking device described previously. The detailed description of the device as well as the procedure of the test may be found in Reference 67.

Of late, biocomposites are yet to be seen in high magnitude. The depletion of petro­leum resources coupled with awareness of global environmental problem provides the alternatives for a new green material in which the products are compatible with the environment and independent of petroleum based resources. Biocomposite with starch used as a matrix is one of the most popular biodegradable biocomposite and is highly investigated by researchers.5960 Biodegradable matrices are reinforced with natural fibers to improve the composites properties and these composites provide positive environmental advantages, good mechanical properties and light weight.6163

There is a wide variety of studies that have been reported on the performance of incorporation sugar palm fiber in petroleum based matrix. Bachtiar et al.64 carried out a study on properties of sugar palm fiber reinforced High Impact Polystyrene (HIPS) while Ishak et al.65 and Sahari et al.66-67 deals with sugar palm fiber reinforced unsaturated polyester. In the meantime, other research groups have conducted investigation on sugar palm fiber combined with epoxy composites.3,68’69 However no previous research has been done on sugar palm fiber reinforced polymer matrix derived from natural resources. It is important to note that, we can extract the bio­polymer which acts as matrix from sugar palm tree itself. So, the interesting part of these study is we investigate the properties of an environmentally friendly com­posite where the matrix (sugar palm starch) and fiber (sugar palm fiber) are derived from one source, that is, sugar palm tree., that is, sugar palm tree as shown in Fig. 9.12.

12.2.3.1 SCANNING ELECTRON MICROSCOPY (SEM)

The state of dispersion of the wood flour in the polymeric matrix was analyzed with SEM. A FEI Quanta 400 microscope (NE Dawson Creek Drive, Hillsboro, Oregon) working at 30 kV was used to obtain microphotographs of the fractured surfaces of the composites. Samples were cut in liquid nitrogen to avoid any deformation of the surfaces.