The flow resistivity is defined by the ratio of the static pressure difference (AP )to the product of the velocity V and the thickness l of the porous sample, as given in Eq. (9). Its unit is Ns/m4

For measuring flow resistivity a sample of 25.4 mm thickness was used. With increasing the flow resistivity the sound absorption increases till an optimum value of the resistivity, beyond which the material reflects and the sound absorption de­creases.

INDERDEEP SINGH and SAURABH CHIATANYA

ABSTRACT

With increasing awareness about environmental concerns, Natural fiber composites (NFCs) have emerged as a potential replacement to traditional polymer composites, which are derived from nonrenewable resources and are nonbiodegradable. NFCs can be classified into green and partially green composites based on the polymer matrix used. NFCs have several advantages over traditional polymer composites leading to worldwide increase in their demand. In order to meet this demand, prima­ry processing techniques should be developed exclusively for fabrication of NFCs. Injection molding process is the most extensively used process in the industry for the production of polymer composites. In this chapter, injection molding process parameters and the various issues and challenges during fabrication of NFCs using injection-molding process has been discussed.

8.1 INTRODUCTION

The continuous demand of materials having high strength and stiffness, light weight and low cost has led to the development of fiber reinforced composites to replace metals in structural applications. The polymer matrix composites (PMCs) based on petroleum derived polymers and synthetic fibers have been developed and used as structural material for engineering applications for decades, owing to their high strength and stiffness and light weight.’PMCs have been used for numerous appli­cations ranging from aerospace, automobiles, electrical, sports goods, construction and household items. Due to superior mechanical properties and wide applications, the use of PMCs has increased in the last few decades. Traditionally used PMCs consists of synthetic fibers like glass, aramid and carbon fibers, resulting in com­posites that can be easily used for structural applications. These PMCs are made up of nonrenewable and nonbiodegradable materials and the resulting composites are also nonrecyclable. These drawbacks have led to the disturbance in the ecological balance. Limited petroleum resources, rapidly depleting land fill space and strict en­

vironmental rules and regulations have forced the researchers and plastic industries worldwide to look for alternate matrix and reinforcement materials to overcome the drawbacks as well as to meet the performance of traditional PMC.

The possibility of reinforcing natural fibers into the polymer matrix has been explored by many researchers and industries worldwide. Natural fiber composites (NFCs) have been seen as a potential replacement to the traditional PMCs in many structural and nonstructural applications. Natural fibers are the largest and fastest growing renewable resource of fibers available that can serve as a potential replace­ment to synthetic fibers. NFCs consisting of natural fibers reinforced polymer ma­trix are being developed and studied as a potential material for the replacement of traditional PMCs. Natural fibers have distinct advantages over traditionally used glass fibers as natural fibers are renewable, biodegradable and have a low density (1.2-1.6 g/cm3) as compared to glass fibers (2.5 g/cm3) which result in the fabrica­tion of lighter composites. Natural fiber composites can also be recycled and cause no abrasion to tools used for their processing. Natural fibers can be broadly classi­fied into three types as plant fibers, animal fibers and mineral fibers (see Fig. 8.1).

The incorporation of natural fibers in the petroleum derived polymer matrix re­sults in PMCs called Partially Green Composites (PGCs). PGCs have been studied by many researchers and it has been found that they have a distinct advantage of be­ing recyclable and have comparable properties to traditional PMCs. However, PGCs still remain nonbiodegradable. To overcome the drawbacks of traditional PMCs a material called biocomposite was innovatively developed by reinforcing natural fi­bers into a biopolymer matrix (fully biodegradable polymers) by the DLR Institute of Structural Mechanics, in 1989.2 Biocomposites are still being developed and have attracted the attention of researchers worldwide in the last decade. Biocomposites consists of a renewable and biodegradable matrix (cellulose, starch, lactic acid, etc. derived) like polylactic acid (PLA), poly hydroxyl alkanoates (PHA), polyhydroxy butyrate covalerate (PHBV), etc. and natural fibers (plant, animal and mineral based) like sisal, hemp, flax, etc. The classification of polymers is shown in Fig. 8.2.

Biocomposites derived from renewable natural resources are termed as Green Composites. The cost and availability of biopolymers is restricting their wide spread use. The incorporation of natural fibers into the biopolymer not only reduces its cost (cheaper comparable to glass fibers) and weight but at the same time provides more strength and stiffness compared to base bio polymer.

Due to the increasing awareness among society about environmental concerns, the demand for NFCs has drastically increased during the last decade. According to a market forecasting report by BCC research (leading market forecasting agency), the global market for applications of wood-plastic composites, cellulosic plastics, plastic lumber and NFCs during the 5 year period (2011-2016) is estimated to grow at a compound annual growth rate (CAGR) of 13.8%. The global market for build­ing products and automotive application is expected to experience a growth at a CAGR of 12.4% and 17.1%, respectively.3Also the global use of bioplastics is ex­pected to increase up to 3.7 million metric tons by 2016, at a CAGR of 34.3%.4 With the increase in demand, the need to identify and develop primary processing tech­niques especially for the fabrication of NFCs arises. As other composites, NFCs can also be tailor made according to the specific properties required for specific applica­tions by careful selection of the polymer matrix, natural fiber and a suitable manu­facturing process. NFCs can be manufactured in a similar way as traditional PMCs by compression molding, resin transfer molding, hot pressing, direct extrusion and injection molding. The NFCs are being manufactured using the processes designed for the manufacturing of traditional PMCs for a controllable output. However, the properties of NFCs are highly variable compared to synthetic fibers. The properties of the natural fibers vary in terms of mechanical, thermal and structural properties. Also, there are several problems in fabrication of NFCs such as distribution of fibers in the matrix, fiber attrition during mechanical mixing, interfacial bonding between hydrophobic matrix and hydrophilic fibers and thermal degradation of the fibers during processing. Hence, there is a need for identification and development of pro­cessing technologies for NFCs.

Although compression molded parts exhibit better mechanical properties than other processes, but the compression molding process can only be used for small to medium parts with simple geometries. Short fiber reinforced polymers are used ex­tensively as structural material as they provide superior mechanical properties and can be easily processed by the rapid, low-cost injection molding process.5Injection molding process is used for the fabrication of small to medium parts with complex geometries. The parts which require precision, dimensional accuracy and excellent surface finish can be easily processed by injection molding. Also the polymers and the fibers are exposed to higher temperatures for a very short period preventing their degradation due to exposure to higher temperature for a long time. Apart from these advantages, there are several issues and challenges related to the fabrication of NFCs by injection molding process. This chapter focuses on the identification and selection of the processing parameters to successfully overcome the issues and chal­lenges faced by the industry in fabrication of NFC by injection molding.

ANITA GROZDANOV, IGOR JORDANOV, MARIA E. ERRICO, GENNARO GENTILE, and MAURIZIO AVELLA

ABSTRACT

Over the last decades, due to the increased environmental awareness, numerous stud­ies for production of biocomposites based on natural fibers have been published and many comprehensive reviews have been published. Compared with conventional reinforcements, such as glass and carbon fibers, natural fibers which are renewable resources, offer several other advantages including a wide availability (based on different vegetable species), recyclability, low density and low costs, low abrasion and preserving mechanical properties. Application of cellulose fibers in composites is not only beneficial from an ecological point of view, lowering the environmental impact of the final product within the production, usage and disposal period, but it offers further technical and economical benefits. Especially, natural fiber-reinforced biocomposites have the potential to replace current materials used for automotive industrial applications. In order to obtain composites with the best mechanical prop­erties, most of the research activities in the last decades have been concentrated on the surface physical and chemical modifications of the fibers mainly to optimize their interfacial behavior.

In the first part of this work, besides the overview of the state-of-the-art descrip­tion regarding biocomposites, we will also present characterization results of the lab-scaled flax fiber reinforced biopolymer matrices (PLA) as well as compared with the same-industrially produced composites.

In the second part of the paper, biocomposites based on “self-reinforced cel­lulose” or “all-cellulose” composites prepared from cotton textile fabrics by partial fiber surface dissolution in lithium chloride dissolved in N, N-dimethylacetamide

will be presented. Two different parameters have been studied: (i) surface treatment medium (alkaline/enzyme/bleaching) and (ii) cotton textile preforms (knits, woven).

10.1 INTRODUCTION

Due to the excellent characteristics, such as lower weight/higher strength, fiber-re­inforced composite materials have found wider technical application.12 Composite materials maximize weight reduction (as they are typically 20% lighter than alu­minum) and are known to be more reliable than other traditional metallic materi­als, leading to reduced aircraft maintenance costs, and a lower number of inspec­tions during service. Additional benefits of composite technologies include added strength, noncorrosive materials with superior durability for a longer lifespan. How­ever, because of the remarkable increased environmental consciousness, the substi­tution of traditional synthetic polymer based composites reinforced with glass and carbon fibers with new biocomposites was considered of fundamental importance.

Compared to traditional composites, biocomposites are materials based on natu­ral fibers of different preforms and fabrics and biocompatible polymer matrices. The interest for using biocomposites has increased because they are lightweight, nontoxic, nonabrasive during processing, have low cost and are easy to recycle. Ac­tually, the first natural fiber composites were used more than 100 years ago. In 1896, for example, airplane seats and fuel tanks were made of natural fibers with a small content of polymeric binders. However, these attempts were without recognition of the composite principles and the importance of fibers as the reinforcing part of composites as well as with the importance of having biodegradable matrix. The use of natural fibers was suspended due to low cost and growing performance of techni­cal plastics and, moreover, synthetic fibers. A renaissance in the use of natural fibers as reinforcements in technical applications began during the late twentieth century.

The automotive and space market is growing in terms of quantity, quality and product variety. The key factor for future growth is fuel efficiency. A 25% reduction in vehicle weight is equivalent to a saving of 250 million barrels of crude oil and a reduction in CO2 emissions of 220 billion pounds per year.23 Over the last several years European, American and Japanese recycling regulations have encourage the use of biomass in automotive materials. Additionally, European Union legislation implemented in 2006 has mandated that 85% of a vehicle must be reused or recycled by 2015. Japan requires 95% of a vehicle to be recovered (which includes incinera­tion of some components) by 2015.4

So, in the last 2 to 3 decades, scientists and engineers have worked together to improve and to enhance the performance of natural fiber based biocomposites, as well as to find some other possible application for them. The aim of this review is to give the overview of the state-of-the-art potential of biocomposites and in particular, biocomposites realized with Flax fibers and PLA matrix, and cotton-based “All­cellulose,” composites from lab-scale up to the industrial products.

The tensile properties of tensile strength (oU), Young’s modulus (E), and elongation strain at break (%El) of the HDPE-DDGS composites containing various compos­ites are shown in Table 13.2. The flexural strength (ofm), the flexural modulus or modulus of elasticity in bending (Eb), and the notched iZOD impact strength for the various composites are presented in Table 13.3. The average for the five test speci­mens and their standard error is given for each property. Figures 13.2-13.4 graphi­cally summarize the data in Tables 13.2 and 13.3 by normalizing the outcomes to the HDPE control material. For example, the tensile strength of HDPE-MAPE is 96% of the neat HDPE thus the bar graph of the normalized oU for HDPE-MAPE is 96%. This rendering is employed to clearly illustrate the effect of additives.

TABLE 13.3 Flexural and Impact Properties of HDPE and Composites*

Eb °fm Impact Energy

Composition (MPA) (MPa) (J/m)

HDPE 41.4 ± 0.2a 1169 ± 8a 921.8 ± 1.6a

HDPE-MAPE 40.0 ± 0.1b 1125 ± 5a 924.4 ± 1.3a

HDPE-25DDGS 33.7 ± 0.6c 1272 ± 31b 447.8 ± 2.4b

‘Treatment values with different letters in the same column were significant (p £ 0.05). Means and standard errors derived from five different replicates are presented.

All biocomposites containing DDGS exhibited much lower tensile strength but comparable modulus values compared to the neat HDPE or the HDPE-MAPE for­mulations. Refer to Table 13.2 and Fig. 13.2. The %El values were considerably higher in the unextracted DDGS formulation (HDPE-25DDGS) compared to the STDDGS formulation (HDPE-25STDDGS), which is attributed to the presence of residual oils in this composite (HDPE-25DDGS) which acts as a plasticizing agent interacting with the filler and the resin matrix as shown in Table 13.2 and Fig. 13.2. Similarly, Julson et al.,25 reported the poor mechanical performance of PP — and HDPE-DDGS composites when compared to neat PP or HDPE. To improve the

mechanical properties of the HDPE-DDGS composites the coupling agent MAPE was included in the formulations. Adding 5% MAPE to the DDGS composite formu­lation (HDPE-25DDGS-MAPE and HDPE-25STDDGS-MAPE) resulted in a slight increase in oU but a nominal reduction in E values compared to the corresponding DDGS composites without MAPE (HDPE-25DDGS and HDPE-25STDDGS). Re­fer to Table 13.2 and Fig. 13.2.

Unlike in a previous study30 where a marked increase in the tensile strength was observed when using STDDGS versus DDGS in composites (without MAPE), this study showed only a slight improvement in the tensile strength when using STD — DGS. Although, the modulus significantly increases when using the solvent treated DDGS as shown in Fig. 13.2. However, when MAPE was added to the STDDGS
composite (HDPE-25STDDGS-MAPE) verses original untreated-DDGS formula­tions (HDPE-DDGS or HDPE-DDGS-MAPE) resulted in significantly higher oU values than all other DDGS formulations. In addition, the HDPE-25STDDGS — MAPE formulation compared favorably to the neat HDPE and HDPE-MAPE formu­lations. These results shows the importance not extrapolating and overgeneralizing the results conducted in prior studies to current studies.30 In this study, a high melt­ing HDPE, Paxon BA50-120 with a melting temperature of 204°C was employed; in the previous study a much lower melting HDPE, Petrothene LS 5300-00 with a melting temperature of 129°C was employed. In the previous study, the HDPE — STDDGS-MAPE formulations exhibited a significantly higher tensile strength than neat HDPE30 while in this study the HDPE-25STDDGS-MAPE formulation exhib­ited a oU that was slightly lower than the neat HDPE. See Table 13.2 and Fig. 13.2. Nevertheless, similar trends were found between the two studies employing dis­similar HDPE resins.

Removal of extractables in order to obtain a superior filler has been previously documented.7,30,41’49 It is notable that the HDPE-25DDGS formulation exhibited in­ferior mechanical (oU and E) and flexural (ofc and Eb) values compared to the STD — DGS formulations but had significantly higher %El and impact strength values than other formulations. Refer to Tables 2 and 3 and Fig. 13.2. DDGS contains high levels of crude protein («26%), water (»5.5%), hexane extracted oils («14%), and acetone extractables («3%). The solvent extraction treatment removes oils and polar extractables to obtain the modified DDGS filler (STDDGS). Apparently, the oil and extractables in the DDGS composite formulations interacted with the resin matrix acting as plasticizing agents which allow for greater percentage of elongation at break and impact strength. Conversely, they are responsible for the lower oU, E, ofm and Eb values in the composites. Adding 5% MAPE to the solvent treated DDGS formulations results in lower impact strength but higher o^ and a slight reduction in moduli compared to formulations without MAPE. PW formulations were found to exhibit similar trends in mechanical properties as the DDGS formulations previ­ously discussed. Refer to Table 13.2 and Fig. 13.2.

Chemical modification of STDDGS particles through acetylation (A) or acety- lation/malation (AM) prior to blending with HDPE produced HDPE-25STDDG/A and HDPE-25STDDGS/AM formulations. Surprisingly, these formulations exhib­ited lower tensile and flexural moduli, lower flexural strength, and only a modest increase in tensile strength compared to the untreated formulation (HDPE-25STD- DGS). See Tables 13.2 and 13.3 and Fig. 13.2. Further, the %El and impact strength values declined in the acetylated and acetylated/maleate formulations compared to the untreated formulation; refer to Tables 13.2 and 13.3 and Fig. 13.2. The addition of MAPE to the chemically modified formulations (HDPE-25STDDGS/A-MAPE and HDPE-25STDDGS/AM-MAPE) had little effect on changing the mechanical properties compared to the formulations without MAPE.

Although the improvements due to the chemical modifications are small, they exhibit an expected trend. The tensile strength of the HDPE-25STDDGS composite was 73% of neat HDPE and that of the HDPE-STDDGS/A was 80%. When MAPE is added to these formulations, the oU of HDPE-25STDDGS-MAPE composite was 91% of the neat HDPE and the HDPE-25STDDGS/A-MAPE was 91%, respec­tively. These results indicate the improvement is due to the degree of esterification of the hydroxyl groups by the chemical modification treatments, which were further esterified by the presence of the MAPE coupling agent. The net result is an increase mechanical properties.

Solvent treatment of the PW flour to produce the STPW composites (HDPE — STPW and HDPE-STPW-MAPE) had little effect on their tensile, flexural and im­pact strength properties compared to the nonsolvent treated PW composites (HDPE — PW and HDPE-PW-MAPE). See Tables 13.2 and 13.3, and Fig. 13.3. Apparently the extractables in this wood were not as critical factors affecting the mechanical properties of the composites as in the DDGS formulations.

Chemically modified PW formulations (HDPE-25STPW/A, HDPE-25STPW/A — MAPE, HDPE-25STPW/AM, and HDPE-25STPW/AM-MAPE) exhibited similar trends to those seen for the DDGS formulations. Adding MAPE to the formulation had little effect on changing the mechanical properties of the chemically modified filler composites. Refer to Tables 13.2 and 13.3, and Fig. 13.3.

Despite the numerous publications dealing with the chemical modification (acet­ylation) of wood fiber (WF) and lignocellulosic materials in the literature, few me­chanical evaluations have been conducted on the resultant biocomposites containing chemically modified wood or lignocellulosic fibers.8,48,51’56’61’65 In addition, when the mechanical properties have been analyzed on chemically modified composites the results are rather modest or even negative.65 For example, Ichach and Clemons8 re­ported that HDPE-acetylated pine WF composites exhibited o^ and Eb values were -26 and -16%, respectively, compared to HDPE-untreated pine WF composites. However, the acetylated formulation retained its flexural properties better following weathering and fungal treatments when compared to the untreated HDPE-WF com- posite.8 In another study, HDPE-acetylated-WF composites exhibited ои, E, %El, ofm, and Eb values of +12, -22, +8, +6, and -9%, respectively, compared to HDPE- untreated-WF composites.51 Kaci et al.,48 reported that maleic anhydride acetylated — low density PE (LDPE)-olive husk flour composites exhibited ои, E, %El valves of +9, -27 and +15%, respectively, compared to LDPE untreated — olive husk flour composites. Muller et al.,61 reported that Polyvinyl chloride (PVC)-acetylated-WF composites exhibited ои, %El and impact strength values of+19, +22 and +18%, re­spectively, compared to PVC-untreated-WF composites. Previous reports find that acetylation of WF slightly benefits ov but reduces the tensile and flexural moduli in composites compared to untreated-WF composites. This study confirmed this trend with the chemically modified PW and DDGS composites (Table 13.2). The acetyla­tion and malation of solvent treated DDGS (HDPE-STDDG/A and HDPE-STD — DGS/AM) exhibited slightly higher tensile strength and flexural strength values but significantly lower tensile and flexural moduli values compared to the untreated composites (HDPE-STDDGS) as shown in Table 13.2 and Fig. 13.2. When the malation is provided by the matrix, using the MAPE coupling agent, there is a slight improvement in the mechanical strength properties but a slight reduction in the me­chanical moduli values than when chemical modification is done to the fillers. This is seen by comparing HDPE-STDDGS-MAPE to HDPE-STDDGS as shown in Fig. 13.2. Impact strength of DDGS formulations were negatively affected by the mala — tion (HDPE-STDDGS-MAPE) and chemical modification (HDPE-STDDGS/A and HDPE-STDDGS/AM) treatments compared to the untreated control (HDPE — STDDGS). See Table 13.3 and Fig. 13.2. Similar flexural results are mimicked with the maleate (HDPE-STPW-MAPE) and chemically modified (HDPE-STPW/A and HDPE-STPW/AM) PW composites compared to the untreated control PW compos­ite (HDPE-STPW). Inclusion of the coupling agent (MAPE) with the chemically modified DDGS formulations did improve the modulus of rupture or modulus of

elasticity and could significantly decreased the impact strength values compared to chemically modified formulations without MAPE. Clearly, chemical modification of the two fillers has both positive and negative effects of the mechanical properties to the resulting composites.

Semi-empirical quantum chemistry methods are based on the Hartree-Fock formal­ism, but make many approximations and obtain some parameters from empirical data. They are very important in computational chemistry for treating large mole­cules where the full Hartree-Fock method without the approximations is too expen­sive. The use of empirical parameters appears to allow some inclusion of electron correlation effects into the methods. Within the framework of Hartree-Fock calcu­lations, some pieces of information (such as two-electron integrals) are sometimes approximated or completely omitted. As with empirical methods, we can distinguish if: These methods exist for the calculation of electronically excited states of poly­enes, both cyclic and linear.

AM1 is basically a modification to and a reparameterization of the general theo­retical model found in MNDO. Its major difference is the addition of Gaussian func­tions to the description of core repulsion function to overcome MNDO’s hydrogen bond problem. Additionally, since the computer resources were limited in 1970s, in MNDO parameterization methodology, the overlap terms, Ps and Pp, and Slater orbital exponent’s Zs and Zp for 5- and p — atomic orbitals were fixed. That means they are not parameterized separately just considered as Ps = Pp, and Zs = Zp in MNDO. Due to the greatly increasing computer resources in 1985 comparing to 1970 s, these inflexible conditions were relaxed in AM1 and then likely better parameters were obtained.

Optimization of the original AM1 elements was performed manually by Dewar using chemical knowledge and intuition. He also kept the size of the reference pa­rameterization data at a minimum by very carefully selecting necessary data to be used as reference. Over the following years many of the main-group elements have been parameterized keeping the original AM1 parameters for H, C, N and O un­changed.

Of course, a sequential parameterization scheme caused every new parameter­ization to depend on previous ones, which directly affects the quality of the results. AM1 represented a very considerable improvement over MNDO without any in­crease in the computing time needed.

AM1 has been used very widely because of its performance and robustness com­pared to previous methods. This method has retained its popularity for modeling organic compounds and results from AM1 calculations continue to be reported in the chemical literature for many different applications.

The production of polymer composites comprising of lignocelluloses (LC) fibers will often result in fibers physically dispersed in the polymeric matrix. But in most of the cases, poor adhesion and consequently inadequate mechanical properties re­sult. Hence, surface treatment of the fibers will play a vital role. Generally surface treatment of LC fibers is not required to develop the bonding for the synthesis of biopolymer based composites, in view of the comparable chemical scenery of both the biofiber and biopolymer matrix, which have a hydrophilic nature, unlike the situation with commodity polymers, which have a tendency to be hydrophobic. To improve many specific aspects, such as providing greater adhesion and reduced moisture sensitivity, surface treatment can be useful even in the case of biodegrad­able composites. Although better adhesion between the biopolymers and fibers is contributed by the similar polarities of the two materials yet these results in an increase in water absorption of the composite. Hence, these fibers require suitable surface treatments.

Surface treatment normally involves one of four methods, namely chemical, physical, physical-chemical and physical-mechanical. Chemical methods involve treatment with silanes or other chemicals through chemical functionalization reac­tions and leaching of the surface through alkali or bleaching.109 Physical methods involve treatment by plasma, corona, laser or y-ray and subjected to steam explo­sion.110 Steam explosion process, a high pressure steaming, involves heating of LC materials at high temperatures and pressures followed by mechanical disruption of the pretreated material by violent discharge (explosion) into a collecting tank.111 Mechanical methods involve rolling or swaging and those may damage the fibers. Finally, physical-chemical methods involve solvent extraction of surface gums and other soluble components of the fibers. As an alternative to the methods described above, drying of LC fibers may be an effective process for surface modification, both in terms of cost and improvement in properties.112 It should be noted that all the above mentioned treatments of LC fibers have helped to improve their interac­tion with the matrix materials, increase adhesion of fibers with the matrix through surface roughness of fiber, leading to increased strength or other properties of com­posites through higher fiber incorporation and possibly providing greater durability of the composites.

Natural dragline silk can withstand extreme environmental conditions, such as tem­perature and pressure. MA silks submerged in liquid nitrogen, which at atmospheric pressure corresponds to -196°C, have exhibited increased breaking stress.28 Under these conditions, spider silk has been reported to have 64% higher tensile strength after cryogenic treatment.28 Also, cooling the fibers from room temperature to -60°C has been reported to produce increases in breaking stress and strain, leading to fibers that are tougher.27b Natural dragline silk is also thermally stable to approximately 230°C.29 Other factors that have been shown to influence the mechanical proper­ties of spider silk include humidity, acidity, alcohol treatment (methanol, ethanol and butanol), and special chaotrophic solvents.273-30 In general, treatment of dragline silks with solvents that have increased polarity, the Young’s modulus and breaking strength of dragline silk has been reported to decrease gradually.27a Humidity or incubation of the fibers in water or 8 M urea has been shown to result in shrinking or supercontraction of the fibers. For example, exposure to water can cause some dragline silk threads to shrink by 50% of their original length.31 For the medical in­dustry there are also benefits, for example, spider silk films have been shown to be biocompatible, elicit poor immune responses and have shown benefits in the wound healing process and promoter of coagulation.32 Natural dragline silk has also been demonstrated to provide a suitable matrix for 3-dimensional skin cell culturing.33

Typical research and application areas of polymeric biomaterials include tissue re­placement, tissue augmentation, tissue support, and drug delivery. In many cases, the body needs only the temporary presence of a device/biomaterial, in which in­stantly biodegradable and certain partially biodegradable polymeric materials are used. Recent treatment concepts of scaffold based on tissue engineering principles differ from those based on standard tissue replacement and drug therapies, as the en­gineered tissue aims not only to repair but also to regenerate the target tissue. Cells have been cultured outside the body for many years; however, it has only recently become possible for scientists and engineers to grow complex three-dimensional tissue grafts to meet clinical needs. New generations of scaffolds based on synthetic and natural polymers are being developed and evaluated at rapid pace, aimed at mimicking the structural characteristics of the natural extracellular matrix.122 The natural abundance and biodegradability of cellulose, together with its ability to pro­vide unique properties through diversification of cellulosic structures determine a wide range of biomedical applications. In their native form, cellulosic materials have been widely used in the manufacture of optical products, such as hard contact lenses, due to their excellent clarity, good wettability and high gas permeability, textile fibers, molding powder sheets, optical membranes, etc. In addition, cellulose derivatives evidence excellent properties. Usually, these materials are molded and extruded into various consumer products, such as brush handles, tool handles, toys, steering wheels, or other items.

In view of a direct contact of the biomaterial with blood, a clear understanding of their interactions is a prerequisite. First of all, the material interacts instanta­neously with blood constituents, which is critical in determining their potential side effects on the circulatory system and, eventually, on the whole organism.123 124 Sec­ondly, the interactions with blood can affect the in-vivo pharmacokinetic behaviors of the polymers and their ability to leave the blood compartment and enter other tissues. Blood has important physiological functions and a complex composition, being divided into two compartments, namely plasma — which contains proteins, lipids, salts — and specific cells, including red blood cells, white blood cells, and platelets, as well.

The artificial surfaces interact with blood platelets, initially causing platelet ad­herence and aggregation; when such foreign surfaces are placed in contact with the circulating blood, this interaction is believed to lead to thrombosis and thromboem­bolism, and to the removal of platelets from the circulation.125 The surface charac­teristics of biomaterials, such as the hydrophilicity/hydrophobicity, roughness, and flexibility affect the cell-surface interactions, protein adsorption, behavior of cells adhesion and proliferation, and the host response, too. Therefore, cellular adhesion has a direct bearing on the thrombogenicity and immunogenicity of a specific mate­rial, predicting its blood compatibility and deciding the long-term use for a blood­contacting materials application.

On the other hand, adhesion of red blood cells, platelets or water to the cellulose derivative substrate plays an important role in biomedicine. For analyzing biocom­patibility, the relations between the physicochemical properties of material surface and the adhesion of blood components should be known. Surface wettability, which is associated with surface free energy, has been often related to cell adhesion phe­nomena. In order to study the red blood cells, platelets, or water adhesion as a func­tion of the substitution degree, it is preferable to compare a chemically — homolo­gous series of polymers, for minimizing the contribution of specific interactions between the adherent cells and the chemical groups at the solid surface.

Cellulose derivatives compatibility with blood can be established by equation 12, where Wsw, Ws, rbc, Ws, p, Ws, f, Wsa, and Ws, IgG describe the work of spreading of water, red blood cells, platelets, fibrinogen, albumin, and immunoglobulin G, respectively:78,79,126

where superscripts “d” and “p” indicate the disperse and polar component of the

film surface tension obtained from the Y electron-donor and Y electron-accep­tor interactions, while Ysi indicates the solid-liquid interfacial tension.

When the solid-liquid interfacial tension, Ysl, takes negative values (Fig. 3.41), the interfacial free energy, AGsls, has positive values (Eq. (17), Fig. 3.41) and re­jection between the two surfaces of the same polymer, s, immersed in liquid, along with attraction of the liquid occurs:

The hydrophilic/hydrophobic balance of the polymers can be described by the work of spreading of water, over the considered surface. In addition, when blood

is exposed to a biomaterial surface, the life of the implanted biomaterials is de­cided by adhesion/cohesion of cells. Cellular adhesion to biomaterial surfaces could activate coagulation and the immunological cascades. Therefore, cellular adhesion has a direct bearing on the thrombogenicity and immunogenicity of a biomaterial, thus dictating its blood compatibility. The materials, which exhibit a lower work of adhesion, would show a lower extent of cell adhesion than those with a higher work of adhesion. Polymer interaction with red blood cells is mediated mostly by the hydrophobic interaction with the lipid bilayer (the red blood cell hydrophobic layer containing transmembrane proteins), the electrostatic interaction with the sur­face charges or/and the direct interaction with membrane proteins, depending on polymer characteristics. Figure 3.41 shows generally positive values for the work of spreading of red blood cells, Ws, Ac, and negative values for the work of spreading of platelets, Ws, p, suggesting a higher work of adhesion, comparatively with that of cohesion for the red blood cells, but a lower work of adhesion, comparatively with the work of cohesion for platelets. Blood platelets are essential in maintain­ing hemostasis, being very sensitive to changes in the blood microenvironment. Platelet aggregation is used as a marker for materials’ thrombogenic properties, the polymer-platelet interaction being an important step for understanding their hemato — compatibility.135136 Therefore, considering the exposure to blood platelets, the nega­tive values of spreading work indicate that all compositions of cellulose derivative blends evidence cohesion; this result suggests that polymer blends do not interact with platelets, thus preventing activation of coagulation at the blood/biomaterial interface.

Also, an important problem in the evaluation of biocompatibility refers to the analysis of the competitive or selective adsorption of blood proteins at the biomate­rial surface; predictions about these interactions can be formulated only by knowing exactly the structure of the biomaterial. Initially, the surface of an implanted mate­rial is mainly coated with albumin, immunoglobulins (especially immunoglobulin G (IgG)), and fibrinogen from plasma. These sanguine plasma proteins were selected for the study of the affinity of polymer blends towards physiological fluid media, due to their presence in the biological events from blood. Hence, Fig. 3.41 exhibits negative values of spreading work for all three plasma proteins, revealing that co­hesion prevails, thus favoring a nonadsorbent behavior at the interface, as required by bio-applications. Also, all samples exhibit lower values of spreading work for albumin that, along with the rejection of platelets, emphasizes the important role they play in material-host interactions. On the other hand, CAP/HPC blends may be considered as being compatible with certain elements from the physiological environment (i. e., tissue, cells), since their interaction with the studied biological materials would cause no damage of the blood cells or change in the structure of plasma proteins. All these properties, along with the special microarchitecture of the CAP/HPC blends, recommend them as proper candidates for applications in cellular and tissue engineering.

Another significant observation relates to the interesting combination of CAP/ HPC blends properties, such as suitable cohesion with sanguine plasma proteins and platelets, and small adhesion with red blood cells; these results show them as prom­ising materials for blood-contacting devices (including vascular grafts, stents, pace­makers, extracorporeal circuits, etc.), even if long-term biocompatibility requires, however, administration of anticoagulant drugs (i. e., warfarin, heparin). At the same time, the studies performed have made possible the production of blood-compatible polymeric materials by preparing heparin-containing blends for biomedical fields.

The results reveal that, for tissue engineering, obtaining of some porous and interconnected 3D polymer networks is recommended. Thus, CAP/HPC blends can be used to accomplish the required properties for specific applications making them good tissue-engineered candidates. As a result, considering the traditional processes and the recently developed techniques, the improved ability to control the poros­ity and molecular microarchitecture of the CAP/HPC hydrophobic membranes will drive the research closer to its proposed goals.

This type of membranes — with surface topography and roughness as important factors in determining the response of cells to a foreign material — represents an excellent scaffold for applications in cellular and tissue engineering.137 138 Mention should be made of the fact that fibroblasts and chondrocyte cells were shown to grow well in a 3D porous membrane, evidencing superior properties (specific molecular microarchitecture and controlled porosity) for tissue regeneration applications.

With these techniques, it is possible not only to specifically control individual and group pore architecture, but also to take the next step, namely to create micro­vascular features to improve integration within host tissues. Nevertheless, structural improvement and increased pore interconnectivity of porous scaffolds is claimed for the development of artificial blood vessels or peripheral nerve growth.

3.4 CONCLUSIONS

The chapter reviews especially our recent studies on the modification and applica­tions of some cellulose derivatives from a nanotechnology consideration, involv­ing biomedical applications. Generally, the information for each type of cellulose derivative includes aspects of synthesis, processing and properties in solid state and in solution, thus illustrating a variety of research directions. In this context, stud­ies contribute to a better knowledge of the specific interactions that generate and modify the properties of cellulose derivatives, required by the applications in differ­ent domains. Information regarding the influence hydrogen bonding on properties of cellulose derivative in solvent/nonsolvent mixtures — over a large concentration domain, microstructures and appearance of lyotropic liquid crystal phases and bio­compatibility, reveal important aspects necessary to diversify domain of their ap­plications.

3.5 ACKNOWLEDGEMENT

This work was supported by a grant of the Romanian National Authority for Sci­entific Research, CNCS — UEFISCDI, project number PN-n-RU-TE-2012-3-143 (stage 2013).

KEYWORDS

• Biocompatibility

• Blends

• Cellulose Derivatives

• Hydroxypropylcellulose (HPC)

• Liquid Crystals

• Lyotropic Liquid Crystal

• Nano-Particles

Fibers in fibrous structures have some level of orientation in machine — or cross­direction and parallel to material surface. Hence, fibrous materials are inherently anisotropic and the propagation constant, k, and surface impedance, z1, changes ac­cording to the direction the sound waves propagate. Sound waves must be permitted to enter the material in order for the material to be able to absorb the sound rather than reflect it.17 Fibrous mats with fibers arranged vertically to the surface, such as needle-punched products, allow sound to enter the material.69 Accordingly, fibers ar­ranged perpendicular to the material surface may be placed to face the sound source.

Jute-based composites can be used in the noise control of several home appliances like window air-conditioner, refrigerator, clothes washer and dryer, vacuum cleaner, dishwasher, etc. In many instances noise reduction of around 10 dB has been ob­tained by using the jute derivatives. In few other instances the noise reduction in terms of SPL has been hardly achieved, however there has been a significant im­provement in the sound quality of the sound after the noise treatment. For instance a metallic tonal noise has been suppressed to a pleasant broadband noise. In fact cur­rently noise control engineers are designing products with superior sound quality58. Case studies of noise control from few of them are given in the following sections.