In general, the choice of a packaging material can be driven by economical, marketing, environmental, and processing issues, as well as by the properties of the material itself.

Polymers are nowadays commonly used in every field of human life and activity. Plastics can be employed as either innovative materials or replacement of traditional raw materials with functional or structural performances, due to a very broad range of properties. Conventional polymers show versatility (they can be formed into an enormous variety of complex shapes), lightweight (compared with many other conventional materials, such as glass or steel), durability (they do not corrode or decompose with the passage of time), and good mechanical properties (which can be modulated with relative simplicity).

According to the Association of Plastics Manufacturers (PlasticsEurope), the total amount of plastics consumed in Western Europe in 2003 was estimated in around 39700 ktonnes [1]. The demand as a function of consumption purpose is shown in Figure 1.1. In particular, whereas domestic and building sectors accounted for 20.1% and 18.5%

respectively, packaging was the largest consumer, accounting for 37.2% (around 14800 ktonnes). Over 50% of all Europe’s consumer goods are packaged in plastics [1].

Figure 1.1. Demand as a function of consumption purpose of plastics consumed in Western Europe in 2003

In general, the choice of a packaging material can be driven by economical, marketing, environmental, and processing issues, as well as by the properties of the material itself.

For example, packaging materials for food and hygiene articles have to require a common

set of qualities, like toughness, impact resistance, flexibility, resiliency, and fat-, water- and heat-resistance [2]. The five polymers mostly used for packaging are: polyethylene (PE, commonly used for agricultural films, garbage bags, milk, and chemicals and paint containers), polypropylene (PP, shopping bags and general purpose containers), polystyrene (PS, food and drinks containers), polyethylene terephthalate (PET, bottles and disposable cutlery), and poly(vinyl chloride) (PVC, rigid packaging, hydraulics, and films) [3]. Generally, these polymers offer the main requirements of packaging materials (e.g. good mechanical performances, barrier properties to gases and humidity, transparency, chemical resistance) in combination with large availability, cheapness and easy processability on industrial scale. In Western Europe in 2002 the total consumption of these five polymers for packaging applications was estimated in around 13800 ktonnes [1]. The plastics packaging consumption by polymer type in Western Europe in 2002 is shown in Figure 1.2.

Figure 1.2. Plastics packaging consumption by polymer type in Western Europe in 2002

The fossil origin and the effective biostability can be considered the main drawbacks

related to the production and use of these polymers. Polymer based consumer goods, after

their use, are commonly discarded into the environment, originating thousands of tonnes

of undegradable wastes. Two approaches can be followed in order to preserve the

fast growing of society. Another approach is the active utilization of plastic wastes, by incineration or recycling. Incineration is a relatively expensive process, which is known to generally produce a large amount of toxic gases and carbon dioxide, contributing to the increase of global warming. On the other hand, also recycling is a very expensive process.

Polymeric wastes have to be separated according to the type of plastic, washed, dried, grinded and then reprocessed in order to obtain the desired new product. Further, polymeric materials are typically degraded by a second processing, and the quality of the product made with recycled plastics is commonly low. An effective and promising way aiming to solve these environmental issues can be represented by the use of “green”

polymers, which are produced by renewable resources and easily biodegraded by the

action of natural microorganisms.

1.1. B IODEGRADABLE P OLYMERS

It is commonly accepted that the biodegradation of polymeric materials consists in the disintegration, erosion, dissolution, breakdown and/or chain scission of polymer into metabolizable or excretable fragments in the human body, in animal models, in ex vivo or in vitro test media, which represent, mimic or approximate the body environment [4].

The biodegradation process of a polymeric material is generally composed of two phases, as reported in Scheme 1.1. The first part consists in the physicochemical depolymerization of the macromolecular chains, followed by the conversion of oligomers and monomers in biomass, water, carbon dioxide and methane (mineralization) [5]. This first phase take place outside the micro-organisms (usually a bacterium or a fungus), due to the dimensions and the insolubility of polymeric chains. Extracellular enzymes involved in this phase may act both in a not-selective way (random fragmentation of the polymeric main chain) and selective (sequential fragmentation of the terminal monomeric units in the main chain). In the first case, when a sufficiently low molecular weight is achieved, polymeric fragments can be metabolized by specific microorganisms at the periplasmatic or endocellular level. During mineralization, the cell acquires its metabolic energy [5].

Scheme 1.1. Schematization of the biodegradation process of a polymeric material

The most important biodegradable polymers, divided according to their synthetic and

natural origin with appropriate subclasses, are reported in Figure 1.3 [4]. A very

promising and extensively studied class of biodegradable polymers is represented by polyhydroxyalkanoates (PHAs).

Figure 1.3. General classification of biodegradable polymers

1.2. P OLYHYDROXYALKANOATES (PHA S )

Poly(hydroxyalkanoates) are fully biodegradable polyesters of hydroxyalkanoates (HAs) which are accumulated by several micro-organisms as carbon and energy reserves. HA monomeric units are all in the R configuration due to the stereospecificity of the polymerizing enzyme, PHA synthase [6]. More than 300 different bacteria have been reported to produce various PHAs [7]. General formula of PHAs is shown in Figure 1.4.

n = 1 R = hydrogen Poly(3-hydroxypropionate) P(3HP) R = methyl Poly(3-hydroxybutyrate) P(3HB) R = ethyl Poly(3-hydroxyvalerate) P(3HV) R = propyl Poly(3-hydroxyhexanoate) P(3HHx) R = pentyl Poly(3-hydroxyoctanoate) P(3HO) R = nonyl Poly(3-hydroxydodecanoate) P(3HD) n = 2 R = hydrogen Poly(4-hydroxypropionate) P(4HB) n = 3 R = hydrogen Poly(5-hydroxyvalerate) P(5HV) Figure 1.4. General formula of poly(hydroxyalkanoate)s

Bacteria synthesize and accumulate PHAs as carbon and energy storage materials or as a sink for redundant reducing power under the condition of limiting nutrients in the presence of excess carbon source. When the supply of the limiting nutrient is restored, the PHAs can be degraded by intracellular depolymerases and subsequently metabolized as carbon and energy source [8, 9]. The molecular weight of PHAs usually ranges between 200 and 3000 KDa, depending on the mico-organism, growth conditions (pH, type and concentration of the carbon source) and methodology of extraction and purification [9].

PHAs are accumulated in the cells cytoplasm as discrete granules, the number per cell

sections of PHAs-containing bacteria are observed by transmission electron microscopy (TEM), the PHAs inclusions appear as electron dense bodies, as can be seen in Figure 1.5 for the bacteria Burkholderia Sacchari [10].

Figure 1.5. TEM micrograph of a thin section of Burkholderia Sacchari containing granules of P(3HB)

Since the first finding of poly(3-hydroxybutyrate) P(3HB), in 1926 [11], approximately 125 different monomer units have been detected as constituents of PHAs [6]. In 1974 Wallen et al. reported the identification of hydroxyalkanoates (HA) other than 3HB [12].

One of the factors determining for the type of PHA constituents is the carbon source.

Micro-organisms are capable of producing PHAs (homopolymer and copolymer) from

various carbon sources ranging from inexpensive, complex waste effluents like beet/cane

molasses [13, 14], to plant oils [15] and its fatty acids [16-18], alkanes [19], as well as

simple carbohydrates [6]. The biosynthetic pathway of P(3HB) consists of three

enzymatic reactions catalyzed by three different enzymes: β-ketoacyl-CoA thiolase

(phbA), acetoacetyl-CoA reductase (phbB) and PHB polymerase (phbC). The first

reaction consists of the condensation of two acetyl coenzyme A (acetyl-CoA) molecules

into acetoacetyl-CoA by β-ketoacylCoA thiolase (encoded by phbA). The second reaction

is the reduction of acetoacetyl-CoA to (R)-3-hydroxybutyryl-CoA by an NADPH-

dependent acetoacetyl-CoA dehydrogenase (encoded by phbB). Lastly, the (R)-3-

hydroxybutyryl-CoA monomers are polymerized into PHB by P(3HB) polymerase,

encoded by phbC (Scheme 1.2) [6, 20].

Scheme 1.2. Biosynthetic pathway of P(3HB)

Depending on the number of carbon atoms present in the monomeric units, PHAs can be divided into two main groups, short-chain-length (scl) PHAs (3-5 carbon atoms) and medium-chain-length (mcl) PHAs (6-14 carbon atoms) [21]. The PHA synthase of A.

Eutrophus can polymerize PHAs consisting of 3-5 carbon atoms, while the PHA synthase of Pseudomonas Oleovorans can produce PHAs containing 6-14 carbon atoms [22].

Table 1.1 shows the structures of typical PHA copolymers containing 3(HB) units and

their producers [6].

Table 1.1. Structures of typical PHA copolymers containing 3(HB) units and their producers

Bacterial strain Carbon substrate Random copolymer

Ralstonia eutropha Propionic acid

Ralstonia eutropha Pentanoic acid

3-Hydroxypropionic acid

Alcaligenes latus 1,5-Pentanediol Aeromonas cavie Plant oils

Pseudmonas sp. Sugar

Ralstonia eutropha 4-Hydroxybutyric acid

Alcaligenes latus γ-Butyrolactone

Comamonas 1,4-Butaediol

acidovorans 1,6-Hexanediol

With the aim to produce PHAs at a cheaper price, several researches have been investigating the possibility of producing P(3HB) in transgenic plants. [23-25]. Since β- ketothiolase, the first enzyme of PHAs synthesis, is present in the cytoplasm of higher plants, only the riductase and the PHA synthase are required to synthesize PHA in plant cells. [24]. Transgenic Arabidopsis Thaliana, a small oil seed plant harbouring the A.

eutrophus PHA biosynthesis genes, was constructed. These plants accumulated P(3HB) granules ranging from 0.2 to 0.5 µm of diameter in the nucleus, vacuole and cytoplasm.

However, the amount of P(3HB) accumulated was only 100 µg/g fresh weight.

Furthermore, the plants were impaired in growth, probably due to the severe depletion of the substrate [23, 24, 26]. To avoid this problem and to improve polymer accumulation, further genetic manipulations have been carried out, indicating the plastid as the ideal location for P(3HB) accumulation [24, 25]. More recently, targeting PHA production to the plants plastids, P(3HB) was accumulated with a yield of 10mg/g, representing around 14% of the dry weight [25, 26].

1.2.1. General Properties of PHAs

The most extensively studied PHA is poly(3-hydroxybutyrate) P(3HB). P(3HB) is 100%

stereospecific, with all the asymmetric carbon atoms in the D(-)configuration [8].

Therefore, P(3HB) is very highly crystalline (from 55 to 80%), with a relatively high

stiffness [8, 27, 28]. Glass transition and melting point of P(3HB) are around 5 and 175°C

respectively [29, 31]. P(3HB) shows moisture resistance at ambient conditions,

piezoelectricity, and optical purity [8, 28]. Young modulus and tensile strength of P(3HB)

range usually from 1.5 to 3.5 GPa and from around 30 to 40 MPa respectively, with an

elongation at break of around 5% [22, 30, 31]. However, it has been verified that the

copolymers containing HA units other than 3HB show different thermal and mechanical

properties if compared with P(3HB). As reported in Table 1.2 for various P(3HB-co-

3HV) copolymers with different compositions [22], with the increasing of comonomer

fraction impact strength tends to increase, whereas melting temperature and Young

modulus show the tendency to decrease. Therefore, the copolymers of P(3HB) are in

general more ductile and elastic than pure P(3HB), and they can be melt processed at

Table1.2. Comparison of the mechanical properties of various P(3HB-co-3HV) copolymers with different compositions

Melting Young Tensile Elongation Impact

temperature modulus strength at break strength

PHA (°C) (GPa) (MPa) (%) (J/m)

P(3HB) 179 3.5 40 5 50

P(3HB-co-3HV)

3 mol% 3HV 170 2.9 40 — 60

9 mol% 3HV 162 1.9 37 — 95

14 mol% 3HV 150 1.5 35 — 120

20 mol% 3HV 145 1.2 32 — 200

25 mol% 3HV 137 0.7 30 — 400

Up to now, the commercial use of P(3HB), and of PHAs in general, has been hampered by various drawbacks. Among these, the first one is the noticeable embrittlement that occurs in the polymer after its storage of some days at ambient conditions, due to a very low crystallization rate and a secondary crystallization phenomenon that take place after the crystallization from the melt [32, 33]. Another drawback is represented by the poor melt stability. In fact, it is well known that P(3HB), as well as other PHAs, is rather unstable at temperatures close to the melting temperature. A dramatic drop in the molecular weight has been observed at temperatures even 10°C below the melting point, limiting its melt processability [34, 35].

1.2.2. Processing of PHAs

It is widely believed that P(3HB) thermal degradation reaction occurs almost exclusively

via a random chain scission mechanism (cis-elimination) involving a six-membered ring

transition state (Scheme 1.3) [36-38]. This mechanism, as well as composition and yields

of PHB degradation products, have been determined by several techniques like Gas

Chromatography (GC) coupled with Mass Spectrometry (MS), Fast Atom Bombardment

Mass Spectrometry (FAB-MS), Nuclear Magnetic Resonance (NMR), Fourier Transform

Infrared Spectroscopy (FTIR) and coupled Pyrolysis/GC-FTIR [39-45].

Scheme 1.3. Mechanism of P(3HB) thermal degradation

De Koning found that, keeping P(3HB) at 180°C for 1 hour, the molecular weight can drop of around 50% [46]. Gogolewski et al. [47] processed P(3HB) and P(3HB-co-3HV) (from 5.5 to 22% of 3HV) by injection moulding. It was observed that molecular weight can decrease from 4 to 53 % over a temperature range of 135-160 °C and residence time of 41 s. Less degradation was promoted with increasing content of 3HV in the copolymer.

The lower process temperature adopted for P(3HB-co-3HV) copolymers explained this result. Melik et al. [48] reported the melt behaviour of P(3HB-co-3HV) (12% of 3HV) processed with a torque rheometer, characterizing the effect of temperature and shear on the degradation kinetics. Although the activation energy of chain scission was independent of shear rate, it was observed that the degradation rate increased with increasing shear. It was inferred that this behaviour could be caused by viscous heat dissipation. The influence of temperature and screw speed on the molecular weight and mechanical properties of P(3HB-co-3HV) (ca. 7 % of 3HV) was investigated by Renstad et al. [49]. Processing temperature ranged from 150 to 180 °C and screw speed from 5 to 40 rpm. A decrease of the molecular weight up to ca. 50 % was observed for the higher temperature and lower screw speed. Similar results were found by Chiellini et al. [29], who processed P(3HB) and P(3HB-co-3HV) (13% of 3HV) with a torque rheometer.

Depending on processing temperature (160-180°C) and rotor speed (30-60 rpm), the

molecular weight of the polymers was found to drop of around 40/50%.

1.2.3. Biodegradability of PHAs

The main property of PHAs is undoubtedly their biodegradability. Monitoring the biodegradability of PHAs in various environments such as soil [50], sea water [51, 52], and lake water [52], it was found that the rate of biodegradation of PHAs depends on many factors. These factors are related with both the environmental conditions (temperature, moisture level, pH and nutrient supply) and the properties of the PHAs themselves (composition, crystallinity, additives and surface area). Electron microscopy has revealed that PHAs biodegradation occurs at polymer surface by enzymatic hydrolysis (surface erosion) [6]. Many micro-organisms are able to produce extracellular PHB depolymerises that hydrolyze the solid P(3HB) into water-soluble monomer and oligomers. It has been shown that PHB depolymerises consist of a single polypeptide chain, with a molecular weight ranging from 37 to 60 KDa [53, 57]. In particular, the enzymes contain an N-terminal catalytic domain, a C-terminal substrate-binding domain, and a linker region connecting the two domains. Therefore, the enzymatic degradation of P(3HB) is an heterogeneous reaction composed by two different steps, namely adsorption and hydrolysis. The first phase consists in the adsorption of the enzyme onto the surface of P(3HB) by the binding domain of the enzyme, and the second step is the hydrolysis of the polymer by the active site of the enzyme [6]. A schematic model of enzymatic hydrolysis of P(3HB) single crystal by PHB depolymerase is shown in Figure 1.6 [6].

Biodegradation products of PHAs in aerobic environment are water and carbon dioxide, while in anaerobic conditions methane is also produced [22]. PHAs are compostable over a wide range of temperature, with a maximum biodegradation rate at around 60°C with a relative humidity of 55% [58]. In these conditions, 85% of PHAs were degraded in 7 weeks of exposure [59]. PHAs biodegradation was observed after 254 days of immersion in lake water (Lake Lugano, Switzerland) even at a temperature not exceeding 6°C [59].

The copolymer poly(3-hydroxybutyrate-co-3-hydoxyvalerate) P(3HB-co-3HV) with

various content of 3HV units showed a weight loss of around 60% after an exposure of 6

months in mature compost. Other samples of P(3HB-co-3HV) were totally degraded after

6, 75, and 350 weeks in anaerobic sewage, soil, and sea water, respectively [22]. Koyama

et al. [60] studied the effect of solid-state structure on the enzymatic degradability of

P(3HB) and some copolymers, observing that the hydrolysis rates of PHAs copolymers

films were several times higher than the rates of P(3HB) homopolymer films with the

same degree of crystallinity. The biodegradation of P(3HB) in various environments is reported in Table 1.3 [61].

Figure 1.6. Schematic model of enzymatic hydrolysis of P(3HB) single crystal by PHB depolymerase: structure and morphology of P(3HB) single crystal (a), binding of PHB depolymerase molecules onto the entire surface of P(3HB) single crystal (b), and enzymatic hydrolysis of the P(3HB) crystal edge (c)

Table 1.3. Biodegradation of P(3HB) in various environments

Period of dissolution Average rate of Period of 100%

Environmental of 1 mm thick surface erosion weight loss of

conditions section (weeks) (µm/week) 50 µm film (weeks)

Anaerobic sewage 6 100 0.5

Estuarine sediment 40 10 5

Aerobic sewage 60 7 7

Soil at 25°C 75 5 10

Sea water at 15°C 350 1 50

1.2.4. Applications and Costs of PHAs

Due to their interesting and innovative features, PHAs show a potentially wide range of applications. Other than their biodegradability and renewable origin, which are undoubtedly the main characteristics, PHAs show high crystallinity, optical activity and isotacticity, piezoelectricity, water insolubility, good ultra-violet resistance, and high barrier properties to oxygen [58, 62].

The main application for PHAs is packaging market, mainly in the production of bags, containers and paper coatings. Other similar applications as conventional commodity plastics include disposable items such as razors, utensils, diapers, feminine hygiene products, cosmetic containers, shampoo bottles, and cups. In 1990, a German hair-care company, Wella AG, marketed a shampoo bottle (SANARA) made from BIOPOL, a P(3HB-co-3HV) copolymer produced by ZENECA Bio Products, UK (formerly ICI Ltd) [22, 63]. Other BIOPOL made products like various containers, disposable razors, and food trays have been already sold in Japan [22].

In addition to their potential as plastic material, PHAs are also useful for their stereoregularity. In fact, they can be used as chiral precursors for the chemical synthesis of optically active compounds to be used as biodegradable carriers for long term dosage of drugs, medicines, hormones, insecticides and herbicides [64, 65]. PHAs are also used in bone plates, surgical sutures, blood vessel replacements, and, due to their piezoelectricity, as osteosynthetic materials for the stimulation of bones growth.

However, owing to their relatively slow biodegradation rate and high hydraulic stability

in sterile tissues, the utilization of PHAs for medical and pharmaceutical applications is

still limited [66]. The commercials diffusion and mass use of PHAs are hampered by the

high cost of this class of polymers. The price of the product ultimately depends on the

substrate cost, PHA yield on the substrate, and the efficiency of product formulation in

the downstream processing [67]. This implies high levels of PHA as a percentage of cell

dry weight and high productivity in terms of gram of product per unit volume and time

[58]. The cost of PHAs using the natural producer A. eutrophus is US$16 per Kg which is

18 times more expensive than PP. With recombinant E. coli as producer of PHAs, price

can be reduced to US$4 per Kg, which is close to other biodegradable plastic materials

such as poly(lactic acid) and aliphatic polyesters [58].

1.3. PHA S B ASED B LENDS AND C OMPOSITES

As already pointed out, a large diffusion of P(3HB) as well as other PHAs on industrial scale is hampered by several factors like the inherent brittleness, the poor melt stability and the high production cost. Two approaches have been extensively studied in order to overcome these drawbacks. The first one is the copolymerization of P(3HB) with other PHAs units, that allows the regulation of copolyester thermal and mechanical properties by varying their molecular structure and composition. However, it should be underlined that, if on the one hand PHAs copolymers show an higher biodegradation rate if compared with P(3HB) homopolymer, on the other one the crystallization rate from the melt is undesirably decreased. The second approach, consisting in the preparation of PHAs based blends and composites, shows generally some technical advantages in comparison with the synthesis of new copolymers, such as easy processability, balanced properties, and lower costs.

1.3.1. PHAs Based Blends

Several PHAs properties can be affected as a result of blending with other polymers, such

as crystallization behaviour, physical properties and biodegradation rate. When

immiscible blends are obtained, the degradation behaviour depends on the intrinsic

biodegradability of each component, phase distribution and substrate accessibility to

degrading enzymes. On the other hand, when two components are miscible,

biodegradability is strongly affected by mobility of the amorphous mixed phase, which

forms upon blending. Biodegradation is tipically observed only when the mixed phase is

rubbery. The morphology and biodegradation are also influenced by the blending method

adopted (solvent casting or melt mixing) [68]. Tables 1.4-1.7 show the miscibility of

P(3HB) and P(3HB-co-3HV), the most extensively investigated PHAs, with both

biodegradable and non biodegradable polymers.

Table 1.4. Miscibility of P(3HB) with biodegradable polymers

Polymer Miscibility Reference

Poly(ethylene oxide) Miscible [69]

Poly(ethylene glycol) Miscible [70]

Poly(D, L-lactide) Miscible [71]

Poly(ethylene succinate) Miscible [72]

Cellulose Miscible [73]

Ethyl cellulose Miscible [74]

Cellulose propionate Miscible [75]

Cellulose acetate propionate Miscible [76]

Cellulose acetate butyrate Miscible [76]

Synthetic atactic poly(3-hydroxybutyrate) Partially miscible [77]

Poly(3- hydroxybutyrate-co-3-hydroxyvalerate) Partially miscible [78]

Poly(vinyl alcohol) Partially miscible [79]

Poly(L-lactide) Partially miscible [80]

Poly(butylene succinate-co-butylene adipate) Not miscible [81]

Poli(butylene succinate -co-ε-caprolactone) Not miscible [81]

Synthetic poly(3-hydroxypropionate) Not miscible [82]

Poly(γ-benzyl-L-glutammate) Not miscible [83]

Poly(ε-caprolactone) Not miscible [84]

Starch Not miscible [85]

Table 1.5. Miscibility of P(3HB-co-3HV) with biodegradable polymers

Polymer Miscibility Reference

Cellulose acetate propionate Miscible [86]

Cellulose acetate butyrate Miscible [86]

Synthetic atactic poly(3-hydroxybutyrate) Partially miscible [87]

Table 1.6. Miscibility of P(3HB) with non biodegradable polymers

Polymer Miscibility Reference

Poly(vinyl phenol) Miscible [88]

Poly(vinylidene fluoride) Miscible [89]

Poly(vinyl acetate) Miscible [90]

Poly(epichlorohydrin) Miscible [91]

Poly(vinylidene chloride-co-acrylonitrile) Miscible [92]

Poly(epichlorohydrin-co-ethylene oxide) Miscible [93]

Poly(cyclohexyl methacrylate) Miscible [94]

Poly(vinyl acetate-co-vinyl alcohol) Partially miscible [95]

Poly(methylmethacrylate) Partially miscible [94]

Poly(ethylene-co-vinyl acetate) Partially miscible [96]

Poly(styrene) Not miscible [97]

Poly(ethylene) Not miscible [98]

Poly(propylene) Not miscible [99]

Poly(vinyl chloride) Not miscible [100]

Poly(methylene oxide) Not miscible [101]

Styrene acrylonitrile Not miscible [97]

Ethylene propylene rubber Not miscible [90]

Acrylonitrile-butadiene-styrene Not miscible [102]

Table 1.7. Miscibility of P(3HB-co-3HV) with non biodegradable polymers

Polymer Miscibility Refrence

Acrylonitrile-butadiene-styrene Miscible [102]

Poly(vinyl chloride) Miscible [103]

Poli(ethylene-co-vinyl acetate) Not miscible [96]

1.3.2. PHAs – Bioceramics Composites

PHAs based composites reinforced with bioceramics are a class of emerging materials in the field of biomedical engineering. In particular, the current research is focusing on the development of bioactive and biodegradable composite materials formed by a bioactive inorganic phase incorporated as either filler or coating (or both) into a biodegradable polymer matrix. Bioceramics are inorganic materials specially developed for use as medical and dental implants, such as alumina and zirconia, bioactive glasses, glass- ceramics, hydroxyapatite, and resorbable calcium phosphates [104]. Composites resulting by addition of inorganic bioactive phases in the form of particles or fibers to biodegradable polymers, are increasingly being considered for use as bone tissue engineering scaffolds. This is mainly due to their improved physical, biological, and mechanical properties, and in particular the capacity they offer in tailoring their structure and degradation rate to the specific need of the implant site. In fact, experiments have proven that the mechanical as well as biological performance of bioactive ceramic/polymer composites can be controlled through using different particulate bioceramics and varying the amount of bioceramic particles in the composite. Properties of P(3HB), the application of which in the bone-tissue repairing is restricted by its fragility and poor bioactivity, showed to be greatly improved by the formulation of composites with bioceramics [104]. One of the bioceramic most extensively used as filler for PHAs is hydroxyapatite (HA) [Ca

(PO

)

OH], which is the main crystalline constituent of bones and provides the compression strength [105]. A list of the main PHAs/bioceramics composites literature is reported in Table 1.8.

Ni et al. [106] studied P(3HB) based composites containing 10, 20, and 30 wt-% of HA in vitro, observing the formation of apatite crystals on the surface of the biocomposites after 1-3 days of immersion in simulated body fluid (SBF). The quantity of crystals was proportional to the HA content in the composites. From dynamic-mechanical evaluations, the storage modulus of the composites was found to increase at short immersion times because of the crystals formation. After around two months of immersion, composites moduli decreased, probably due the hydrolytic degradation of P(3HB) by the SBF [106].

Similar results were found using P(3HB-co-3HV) (12% of 3HV) with HA and tricalcium

phosphate (TCP) in a content up to 30 wt-% [107].

Galego et. al. studied biocomposites based on various PHAs (0, 8, 12, 24% of 3HV) containing 60/80 wt-% of HA [108], observing a compression resistance of around 60 MPa, which is very close to the value found in bone tissues of human origin.

Table 1.8. PHAs based composites containing bioceramics

PHA Bioceramic Reference

P(3HB) Hydroxyapatite [106]

P(3HB-co-3HV) Hydroxyapatite and tricalcium phosphate [107]

P(3HB) and P(3HB-co-3HV) Hydroxyapatite [108]

P(3HB) Hydroxyapatite [109]

P(3HB) Hydroxyapatite [110]

P(3HB) Hydroxyapatite [111]

P(3HB) Hydroxyapatite [112]

P(3HB) Bioglass [104]

P(3HB-co-3HV) Bioglass [113]

P(3HB-co-3HHx) Hydroxyapatite [112]

1.3.3. PHAs – Layered Silicates Composites

Another very promising are of composite are the so-called nanocomposites, in which the

filler material has at least one dimension in the nanometric scale. Among the various

nanoreinforcements, layered silicate clay mineral are the most extensively investigated,

due to the easy availability, low cost and environmental compatibility [114]. In particular,

the preparation of polymer based organically modified layered silicate (OMLS)

nanocomposites demonstrated to be effective, if compared with unmodified resin, in the

improvement of a large number of physical properties, including gas permeability,

flammability resistance, thermal and environmental stability, solvent uptake, and rate of

biodegradability of biodegradable polymers [114]. These improvements are generally

obtained at lower silicate content (≤5 wt-%) compared to those of conventional filler

filled systems due to their nanoscale dispersion, which results in high aspect ratio (e.g.

nanocomposites are commonly lighter in weight than conventional composites, and make them competitive with other materials for specific applications.

Depending on the strength of the polymer/clay interaction, two structurally different types of nancomposites are thermodynamically achievable. Intercalated nanocomposites are obtained when the polymer chains can penetrate into the silicates, but the structure of the clay is still crystallographically regular. In exfoliated nanocomposites the silicate layers are separated in the polymer matrix, and the order of the clay is lost (Figure 1.7).

Figure 1.7. Schematic illustration of the two different types of thermodynamically achievable polymer/layered silicate nanocomposites

1.3.3.1. Structure and Properties of Layered Silicates

The commonly used layered silicates for the preparation of polymer/layered silicate nanocomposites (montmorillonite, hectorite, and saponite) belong to the family of 2:1 phyllosilicates [115]. Their crystal structure consists of layers made up of two tetrahedrally coordinated silicon atoms fused to an edge-shared octahedral sheet of either aluminum or magnesium hydroxide. The layer thickness is around 1 nm, and the lateral dimensions of these layers may vary from 30 nm to several microns or larger, depending on the particular layered silicate. Stacking of the layers leads to a regular van der Waals gap between the layers called the interlayer or gallery. Isomorphic substitution within the layers (e.g. Al

replaced by Mg

or Fe

, or Mg

replaced by Li

) generates negative charges that are counterbalanced by alkali and alkaline earth cations situated inside the galleries. This type of layered silicate is characterized by a moderate surface charge known as the cation exchange capacity (CEC), and generally expressed as mequiv/100 g.

Varying from layer to layer, this charge is not locally constant, and it must be considered

as an average value over the whole crystal. Layered silicates have two types of structure,

namely tetrahedral-substituted and octahedral-substituted. In the case of tetrahedrally substituted layered silicates the negative charge is located on the surface of silicate layers, and the polymer matrices can interact more readily with these than with octahedrally- substituted material [114]. Table 1.9 shows the chemical formula and characteristic parameters of commonly used 2:1 phyllosilicates. Their structure is shown in Figure 1.8.

Table 1.9. Chemical formula and characteristic parameter of 2:1 phyllosilicates commonly used in nanocomposites formulation

2:1 phyllosilicates Chemical formula CEC (mequiv/100g) Particle length (nm)

Montmorillonite Mx(Al

Mg

)Si

O

(OH)

110 100-150

Hectorite Mx(Mg

Li

)Si

O

(OH)

120 200-300

Saponite MxMg

(Si

Al

)Si

O

(OH)

87 50-60

Figure 1.8. Structure of 2:1 phyllosilicates

Commonly, two particular characteristics of layered silicates are considered for the

preparation of polymer/layered silicate nanocomposites. The first one is the ability of the

silicate particles to disperse into individual layers. The second characteristic is the ability

to modify their surface chemistry through ion exchange reactions with organic and

inorganic cations. Clearly, these two characteristics are interrelated, since the degree of

consequence, in this pristine state layered silicates are only miscible with hydrophilic polymers, such as poly(ethylene oxide) (PEO) [116] and poly(vinyl alcohol) (PVA) [117]. In order to render layered silicates miscible with non-hydrophilic polymer matrices, the silicate surface has to be converted to an organophilic state. Generally, this situation can be achieved by ion-exchange reactions with cationic surfactants including primary, secondary, tertiary, and quaternary alkylammonium or alkylphosphonium cations. The surfactants in the organosilicates lower the surface energy of the inorganic host and improve the wetting characteristics of the polymer matrix, resulting in a larger interlayer spacing. Additionally, the surfactants can provide functional groups able to react with the polymer matrix, or in some cases initiate the polymerization of monomers to improve the adhesion between the inorganic phase and the polymer matrix [118, 119].

1.3.3.2. Preparation Techniques of Polymer – Layered Silicate Nanocomposites

Polymer/layered silicate nanocomposites are commonly prepared following two approaches: the insertion of suitable monomers in the silicate galleries and subsequent polymerization [120, 121] or direct insertion of polymer chains into the silicate galleries from either solution [116] or the melt [122]. The melt intercalation, during which the molten polymer is able to diffuse between the silicate layers, has recently become the preferred approach, since it is the more compatible with the common industrial processing techniques [123]. This process consists in the annealing of a mixture of the polymer and OMLS above the softening point of the polymer, statically or under shear.

During annealing, the polymer chains diffuse from the bulk polymer melt into the

galleries between the silicate layers. A range of nanocomposites with structures from

intercalated to exfoliate can be obtained, depending on the degree of penetration of the

polymer chains into the silicate galleries. So far, experimental results indicate that the

outcome of polymer intercalation depends critically on silicate functionalization and

constituent interactions. Sinha Ray et al. [124] observed that an optimal interlayer

structure on the OMLS, with respect to the number per unit area and size of surfactant

chains, is most favourable for nanocomposite formation. On the other hand, polymer

intercalation depends on the polar interactions between the OMLS and the polymer

matrix [124].

1.3.3.3. Characterization of Polymer – Layered Silicate Nanocomposites

The most common techniques used in the characterization of polymer/layered silicate nanocomposites are X-ray diffraction (XRD) and transmission electron micrographic (TEM). XRD, by monitoring the position, shape, and intensity of the basal reflections from the silicate layers, can provide useful information about the structure of the nanocomposite (intercalated or exfoliated). On the other hand, TEM allows a qualitative understanding of the internal structure, spatial distribution and dispersion of the nanoparticles within the polymer matrix, and views of the defect structure through direct visualization. However, special care must be exercised to guarantee a representative cross section of the sample. Other techniques that can provide useful informations with respect to clay dispersion are Fourier transform infrared analysis (FT-IR) and gas permeability.

1.3.3.4. Properties of PHAs – Layered Silicates Composites

A list of the main PHAs/OMLS nanocomposites literature is reported in Table 1.10. Maiti

et al. [125] reported the first preparation of P(3HB)/OMLS nanocomposites by melt

intercalation method (twin-screw extruder) using three different kinds of OMLS. XRD

patterns and TEM analysis showed the formation of well-ordered intercalated

nanocomposites. Observing nanocomposites molecular weight by GPC measurements, it

was observed that the nanocomposites based on organically modified montmorillonite

showed severe degradation, but surprisingly no degradation was found with

nanocomposites based on organically modified fluoromica. The authors explained this

phenomenon observing that the presence of Al Lewis acid sites may be one of the

reasons, catalyzing the hydrolysis of ester linkages at high temperature. Biodegradation

tests under compost showed that the degradation started just after one week, and in the

initial stage the weight loss was almost the same for both PHB and its nanocomposites. A

significant deviation occurred after three weeks of exposure, but degradation tendency of

nanocomposites was suppressed. The authors attributed the retardation of biodegradation

of P(3HB) to the improvement of the barrier properties of the matrices after

nanocomposites preparation with OMLS, but they did not report about the permeability

biodegradability and the gas-barrier properties in poly(lactic acid)/OMLS nanocomposites.

Table 1.10. PHAs based nanocomposits containing layered silicates

PHA Layered silicate Preparation Reference

P(3HB) OMLS Melt intercalation [125]

P(3HB) OMLS Solution intercalation [127]

P(3HB) Kaolinite Melt intercalation [128]

P(3HB-co-3HV) OMLS Solution intercalation [129]

P(3HB-co-3HV) OMLS Melt intercalation [130]

P(3HB-co-3HV) OMLS Solution intercalation [131]

P(3HB-co-3HV) OMLS Solution intercalation [132]

a)

OMLS is organically modified layered silicates

Choi et al. [130] prepared P(3HB-co-3HV) based nanocomposites containing 1, 2, and 3 wt-% of organically modified montmorillonite by melting intercalation technique. XRD and TEM indicated a good level of intercalation of the polymer between the clay layers, leading to a general improvement of the copolymer properties. Differential Scanning Calorimetry (DSC) and thermogravimetric analysis (TGA) measurements showed that copolymer crystallization rate from the melt was increased, as well as the thermal stability. Young modulus increased of around 65%, while the stress at break showed only a little increase.

Gardolinski et al. prepared P(3HB)/kaolinite nanocomposites with an intercalation rate

(evaluated by XRD) of around 77% [128]. Thermal stability was increased of around

50°C, while the crystallinity decreased significantly. P(3HB-co-3HV)/OMLS

nanocomposites prepared by solution intercalation showed an increase in tensile strength

of around 30% [131]. An increase in OMLS content resulted in a general decrease of

properties, explained by the authors with a low intercalation of the copolymer between

the layers of the silicate.

1.3.4. PHAs – Natural Fibres Composites

Nowadays the use of natural fibres (NF) as reinforcements in technical applications is taking place mainly in the automobile and packaging industries (e.g. egg boxes). In the automotive industry, textile waste has been used for years to reinforce plastics used in cars, especially in the Trabant [133]. At present many producers, such as BMW, Mercedes-Benz, Fiat, and Renault as well as Indian companies, are known to use NF in blended thermoplastic or resinated thermoset as compression moulded car components [134]. In the case of European producers, local renewable fibres, such as flax and hemp, were used for these cars. As example, the following components were developed for the following applications [135-137]:

• Door panels: Moulded wood, natural fibre mouldings, laminated panels.

• Car roofs: Composites made of natural fibre with epoxy resins or polyurethane composites.

The use of flax fibres in car disk-breaks to replace asbestos fibres is another example of this type of application [137].

1.3.4.1. Properties of Natural Fibres

NF are subdivided on the base of their origins, coming from plants, animals or minerals.

Generally, plant or vegetable fibres are used to reinforce plastics. Plant-fibres may include hairs (cotton, kapok), fibre-sheafs of dicotylic plants or vessel-sheafs of monocotylic plants, i.e. bast (flax, hemp, jute, ramie) and hard-fibres (sisal, henequen, coir) [137]. The availability of large qualities of such fibres with well-defined mechanical properties is a general prerequisite for the successful use of these materials, whereas the lack of this is one of the main drawbacks at the present moment.

Additionally, for several more technical orientated applications, the fibres have to be specially prepared or modified regarding:

• Homogenization of fibre’s properties.

• Degree of polymerization and crystallization.

• Good adhesion between fibres and matrix.

Flax fibres show the highest values of strength, but they are about 30% more expensive than glass fibres (GF) [137]. Further, the price depends on the extent of fibres preparation and pretreatment, e.g. size-finishing including a coupling agent and other surfactants, which are well established for GF. For such applications, NF have to be pretreated in a similar way. In most cases, the substitution of GF by NF is precluded mainly by economic reasons. However, NF offer several advantages over GF, such as:

• Plant fibres are a renewable raw material, and their availability can be considered potentially unlimited.

• When natural reinforced plastics are subjected, at the end of their life cycle, to a combustion process or landfill, the released amount of carbon dioxide of the fibres is neutral with respect to the assimilated amount during their growth.

• The abrasive nature of NF is much lower compared to that of GF, which leads to advantages with regard to technical, material recycling or process of composite materials in general.

• NF reinforced plastics by using biodegradable polymers as matrix are the most environmental friendly materials which can be composted at the end of their life cycle. Unfortunately, the overall physical properties of those composites are far away from GF reinforced thermoplastics. Further, a balance between life performance and biodegradation has to be developed.

NF are in general suitable to reinforce plastics (thermosets as well as thermoplastics) due

to their relative high strength, stiffness, and low density as shown in Table 1.11 [137,

138]. The characteristic values for flax and softwood kraft fibres reach levels close to the

values for GF. Nevertheless and also obvious in Table 1.11, the range of the characteristic

values, which is one of the drawbacks for all natural products, is remarkably higher than

those of GF. This fact can be explained by the differences in fibre structure due to the

overall environmental conditions during growth. NF can be processed in different ways to

yield reinforcing elements having different mechanical properties. The elastic modulus of

bulk NF such as wood is about 10 GPa. Cellulose fibres with moduli up to 40 GPa can be

separated from wood, for instance, by chemical pulping processes. Such fibres can be

further subdivided by hydrolysis followed by mechanical disintegration into microfibrils

with an elastic modulus of 70 GPa. Theoretical calculations of the elastic moduli of

cellulose chains have given values of up to 250 GPa. However, there is no technology

available to separate these from microfibrils [139, 140]. Further as discussed previously,

the fibre properties and fibre structure are influenced by several conditions and varies by area of growth, its climate and the age of the plant [141, 142].

Table 1.11.Mechanical properties of natural fibres as compared to conventional reinforcing fibres

Density Elongation Tensile strength Young modulus

Fibre (g/cm3) (%) (MPa) (GPa)

Cotton 1.5 - 1.6 7.0 - 8.0 287 - 597 5.5 - 12.6

Jute 1.3 1.5 - 1.8 393 - 773 26.5

Flax 1.5 2.7 - 3.2 345 - 1035 27.6

Hemp — 1.6 690 —

Ramie — 3.6 - 3.8 400 - 938 61.4 - 128

Sisal 1.5 2.0 - 2.5 511 - 635 9.4 - 22.0

Coir 1.2 30.0 175 4.0 - 6.0

Viscose (cord) — 11.4 593 11.0

Soft wood kraft 1.5 — 1000 40.0

E-glass 2.5 2.5 2000 - 3500 70.0

S-glass 2.5 2.8 4570 86.0

Aramide (normal) 1.4 3.3 - 3.7 3000 - 3150 63.0 - 67.0

Carbon (standard) 1.4 1.4 - 1.8 4000 230.0 - 240.0

The technical digestion of the fibre is another important factor that determines the

structure as well as the characteristic values of the fibres. As in the case with GF, the

tensile strength of NF also depends on the test length of the specimens, which is of main

importance regarding reinforcing efficiency. Hydrophilic nature is a major problem for all

cellulosic fibres used as plastics reinforcement. The moisture content of the fibres,

dependent on content of non crystalline parts and void content of the fibre, amounts up to

10 wt-% under standard conditions [143]. The hydrophilic nature of NF affects the

overall mechanical properties as well as other physical properties of the fibre itself [144].

1.3.4.2. Composition and Structure of Natural Fibres

Climatic conditions, age and the digestion process influence not only the structure of fibres but also the chemical composition. Component mean values of plant-fibres are shown in Table 1.12 [137, 138]. With the exception of cotton, the components of NF are cellulose, hemicellulose, lignin, pectin, waxes and water soluble substances, with cellulose, hemicellulose and lignin as the basic components with regard to the physical properties of the fibres.

Table 1.12. Composition of different cellulose based natural fibres

Cotton Jute Flax Ramie Sisal

Cellulose 82.7 64.4 64.1 68.6 65.8

Hemicellulose 5.7 12.0 16.7 13.1 12.0

Pektin 5.7 0.2 1.8 1.9 0.8

Lignin — 11.8 2.0 0.6 9.9

Water soluble 1.0 1.1 3.9 5.5 1.2

Wax 0.6 0.5 1.5 0.3 0.3

Water 10.0 10.0 10.0 10.0 10.0

Cellulose is the essential component of all plant-fibres. It is generally accepted that natural cellulose is a linear condensation polymer consisting of d-anhydroglucopyranose units (often abbreviated as anhydroglucose units or even as glucose units for convenience) joined together by β-1,4-glycosidic bonds, belonging to the family of 1,4-β- d-glucans (Fig. 1.9).

Figure 1.9. Molecular structure of cellulose

The pyranose rings are in the

C

conformation, which means that the –CH

OH and – OH

groups, as well as the glycosidic bonds, are equatorial with respect to the mean planes of

the rings [145]. The molecular structure of cellulose is responsible for its supramolecular

structure and this, in turn, determines many of its chemical and physical properties. In the fully extended molecule, the adjacent chain units are orientated by their mean planes at an angle of 180° to each other. Hence, the repeating unit in cellulose is the anhydrocellulobiose unit and the number of repeating units per molecule is half the DP.

This may be as high as 14000 in native cellulose, but purification procedures usually reduce it to the order of 2500 [145]. The mechanical properties of NF depend on the cellulose type, because each type of cellulose has its own cell geometry and the geometrical conditions determine the mechanical properties. Solid cellulose forms a microcrystalline structure with regions of high order, i.e. crystalline regions, and regions of low order, i.e. amorphous regions. As a first approximation, naturally occurring cellulose (cellulose I) crystallizes in a monoclinic unit cell, which contains two cellulose chains in a parallel orientation with a twofold screw axis [146]. Apart from the thermodynamically less stable cellulose I, cellulose may occur in other crystal structures (cellulose II, III, and IV) of which cellulose II is the most stable structure of technical relevance. It can be formed from cellulose I by treatment with aqueous sodium hydroxide (mercerization) or by dissolution of the cellulose and subsequent precipitation/regeneration, as is done in the formation of fibre and film. This monoclinic crystal structure with two antiparallel chains in the unit cell is characterized by the specific unit cell geometry with a modified H-bonding system. The alkalization of cellulose is of considerable importance for cellulose production on commercial scale as a method for increasing the reactivity (activation) of subsequent reactions as well as for the mercerization of cotton. Depending on the concentration of lye, temperature, and mechanical load, it is possible to convert cellulose I into various crystalline alkali forms, each with a different crystal structure and variable NaOH and water content. All forms will then convert into crystalline hydrato cellulose (water cellulose) during washout, and to cellulose II through drying. However, it is not yet understood how the parallel chain arrangement of cellulose I undergoes transition into the antiparallel orientation of cellulose II without an intermediate dispersion of cellulose molecules [146].

Hemicellulose comprises a group of polysaccharides (excluding pectin) that remains

associated with the cellulose after that lignin has been removed. Hemicellulose differs

from cellulose in three important aspects. In the first place, it contains several different

strictly linear polymer. Finally, the degree of polymerization of native cellulose is ten to one hundred times higher than that of hemicellulose. Unlike cellulose, the constituents of hemicellulose differ from plant to plant [145].

Lignin is a complex hydrocarbon polymer with both aliphatic and aromatic constituents (Figure 1.10) [137, 145, 147-149]. Their chief monomer units are various ring-substituted phenyl-propanes linked together in ways, which are still not fully understood. Structural details differ from one source to another [145]. The mechanical properties are lower than those of cellulose.

guaiacyl syringyl p-hydroxyphenyl propane

(a)

(b)

Figure 1.10. Schematic representation of the structural units of lignin (a) and one of the

possible lignin structure (b)

Pectin is a collective name for heteropolysaccharides, which consist essentially of polygalacturon acid. Pectin is soluble in water only after a partial neutralization with alkali or ammonium hydroxide [150]. Waxes make up the part of the fibres, which can be extracted with organic solutions. These waxy materials consist of different types of alcohols, which are insoluble in water as well as in several acids (palmitic acid, oleaginous acid, stearic acid) [150].

A single fibre of all plant based NF consist of several cells. These cells are formed out of crystalline microfibrils based on cellulose, which are connected to a complete layer by amorphous lignin and hemicellulose. Multiple of such cellulose-lignin/hemicellulose layers in one primary and three secondary cell walls stick together to a multiple-layer- composites, the cell, as shown in Figure 1.11. The cell walls differ in their composition (ratio between cellulose and lignin/hemicellulose, Table 1.12) and in the orientation of the cellulose microfibrils.

Figure 1.11. Schematization of a natural fibre cell 1.3.4.3. Surface Modification of Natural Fibres

The quality of the fibre-matrix interface is one of the main parameters to take into account for the application of NF as reinforcement fibres for plastics. Physical and chemical methods can be used in order to optimize this interface. These modification methods exhibit different efficiency for the adhesion between matrix and fibre.

Physical methods Physical methods, such as stretching, calandering, thermotreatment,

and the production of hybrid yarns do not change the chemical composition of the fibres.

thereby the mechanical bonding to polymers. Electric discharge (corona, cold plasma) is another way of physical treatment [137].

Another method of cellulose fibre modification is mercerization [145, 151-154], which has been widely used on cotton textiles. Mercerization is an alkali treatment of cellulose that depends on the type and concentration of the alkaline solution, its temperature, time of treatment, tension of the material as well as on the additives [145, 154]. At present there is a tendency to use mercerization on NF as well. Optimal conditions of mercerization ensure the improvement of the tensile properties and absorption characteristics [145, 152-153]. As discussed above, cellulose forms the main structural component of vegetable NF. However, the non-cellulosic components, e.g. lignin and hemicellulose, also play an important part in the characteristic properties of the fibres.

Hemicellulose, which is thought to consist principally of xylan, polyuronide and hexosan, has shown to be very sensitive to the action of caustic soda, which exerts only a slight effect on lignin or cellulose. Later studies about the alkali treatment of jute-fibres, for instance, report about the removal of lignin and hemicellulose that affects the tensile characteristics of the fibres. When the hemicelluloses are removed, the interfibrillar region is likely to be less dense and less rigid and thereby makes the fibrils more capable of rearranging themselves along the direction of tensile deformation. When NF are stretched, such rearrangements among the fibrils would result in better load sharing by them and hence result in higher stress development in the fibre. In contrast, softening of the interfibrillar matrix negatively affects the stress transfer between the fibril and, thereby, the overall stress development in the fibre under tensile deformation. As lignin is removed gradually, the middle lamella joining the ultimate cells is expected to be more plastic as well as homogeneous due to the gradual elimination of microvoids, while the ultimate cells themselves are effected only slightly. Further, some authors reported about changes in the crystallinity through alkaline treatment on coir [155, 156] and flax [157]

fibres. The increase in the percentage crystallinity index of alkali treated fibres occurs because of the removal on the cementing materials, which leads to a better packing of cellulose chains [155].

Chemical methods Strongly polarized cellulose fibres [158] are inherently incompatible

with hydrophobic polymers [159-161]. When two materials are incompatible, it is often

possible to improve their compatibility by introducing a third material with intermediate

properties between those of the other two. An important chemical modification method is

the chemical coupling method, in which the fibre surface is treated with a compound that forms a bridge of chemical bonds between fibre and matrix, improving the interfacial adhesion. Coupling agents that have been found effective in improving the adhesion between fibres and polymeric matrix are graft copolymers as well as compounds containing methanol groups, isocyanates, triazine, and organosilanes [137].

The unfavourable absorption of moisture by NF is due to the strong interactions between the hydrogen present in water molecules and the hydroxyl groups in cellulose in cell wall polymers. As already shown, reactive organic chemicals are being tied to the cell wall hydroxyl groups of cellulose, hemicellulose and lignin. An essential chemical modification of fibres is thereby acetylation, which consists in the introduction of the acetyl group into fibres surface. Normally the combinations for acetylation with acetic acid or acetylchloride are heated in the presence of a solvent, for example acetic acid or anhydride. In some cases, acetylation is promoted by the use of catalysts. The hydroxyl groups of the cell walls that are to be acetylated are accessible in different ways. Without a good catalyst or co-solvent, only the easily accessible hydroxyl groups are normally reached. By the use of acetic acid only, fibres of southern pine, aspin, bamboo, bagasse, jute, and water-hyacinth, were treated [137]. When some hydroxyl groups are replaced at the cell wall by acetyl groups, the hydroscopity of the cell walls can be lessened. Table 1.13 shows absorbed equilibrium moisture at 65% relative moisture at 27°C of different acetylated NF. It is clear that, in all fibres, the equilibrium moisture decreases with increasing acetyl content, which proves the effectiveness of the process [137].

1.3.4.4. Properties of PHAs – Natural Fibres Composites

A list of the main PHAs/natural fibres composites literature is reported in Table 1.14.

Shanks et al. [162] studied natural fiber/biopolymer composites using flax and PHAs

consisting of P(3HB) and its copolymers with 5% and 12% of P(3HV). The adhesion

between the fibres and the polyesters was found to be better than that of analogous

polypropylene composites. Wetting of the fibres by the polyesters was observed using

SEM. PHAs nucleation was increased by the insertion of flax. Melting temperature was

influenced by the promoted adhesion and copolymerization.

Table 1.13.Equilibrium moisture at 65% relative moisture and 27°C of natural fibres in dependence of the acetyl content after acetylation

Fibre Additional weight (wt.%) Acetyl content(wt.%) EMC (wt.%)

Pine 0.0 1.4 12.0

6.0 7.0 9.2

14.8 15.1 6.0

21.1 20.1 4.3

Aaspen 0.0 3.9 11.1

7.3 10.1 7.8

14.2 16.9 5.9

17.9 19.1 4.8

Bambus 0.0 3.2 8.9

10.8 13.1 5.3

14.1 16.6 4.4

17.0 10.2 3.7

Jute 0.0 3.0 9.9

15.6 16.5 4.8

a)

EMC is the equlibrium moisture content

The bending modulus was increased in the composites and dynamic mechanical analysis provided storage modulus of as much as 4 GPa at 25°C, with a smaller component as the loss modulus. The maximum in the loss modulus curve increased in the composites. The influence of a silane coupling agent, used as adhesion promoter. was found to be beneficial for the material properties of the biopolyester/flax composites. An earlier study showed that flax and PHB had good interfacial adhesion, which was decreased when plasticizers were added [163]. Some plasticizer migrated from the flax to the PHB and caused some complex changes in P(3HB) glass transition, crystallization and crystallinity.

Avella et al. [166] reported on the thermal and mechanical behavior of PHBV copolyester

reinforced with wheat straw fibres. In order to improve the chemico-physical interactions

between the components, the reinforcing agent was previously submitted to treatment

with high temperature steam, leading to fibres richer in cellulose and more reactive. The

addition of straw fibres was found to increase the rate of PHBV crystallization, while the

crystallinity content was not affected. Furthermore, comparison of the mechanical properties has shown that the composites exhibit higher Young’s moduli and lower values of both stress and strain to break than the neat matrix of PHBV. The presence of straw did not affect the biodegradation rate evaluated in liquid environments and in long term soil burial tests. In the composting simulation test, the rate of biodegradation is reduced for composites with more than 10% straw content.

Table 1.14. PHAs based composits containing natural fibres

PHA Fibre Reference

P(3HB) Bambu Colihue [164]

P(3HB-co-3HV) Pine [165]

P(3HB-co-3HV) Wheat straw [166]

P(3HB-co-3HV) Jute [167]

P(3HB-co-3HV) Jute [168]

P(3HB-co-3HV) Pineapple fibres [169]

P(3HO) Cellulose and starch [170]

mcl-PHAs Cellulose whiskers [171]

P(3HB-co-3HV) Flax and jute [172]

P(3HB-co-3HV) Pine pulp and cellulose [173]

P(3HB) Cellulose [174]

P(3HB) Wheat straw [175]

P(3HB) and P(3HB-co-3HV) Cellulose [176]

P(3HB) Flax [163]

P(3HB) Flax [162]

P(3HB-co-3HV) Abaca [177]

P(3HB) Flax [178]

P(3HB) and P(3HB-co-3HV) Flax [179]

The diameter of cellulose fibres decreased when mixed with P(3HB-co-3HV) [174, 176].

Hence, the dispersion of the fibres was found to be higher if compared with analogous

P(3HB-co-3HV) based composites containing pine pulp showed an increase in the

crystallization rate, probably due to the heterogeneous nucleation promoted by the fibres

[173].

1.4. L IFE C YCLE A SSESSMENT (LCA)

The replacement of common petrochemical plastics with biodegradable polymers has become one of the most stimulating fields in scientific research. However, it is now commonly accepted by the scientific community that biodegradability is not a sufficient condition to state the “eco-friendliness” of a polymeric product. There is the need to take into account the environmental impact of all stages of product life, from raw materials extraction, manufacture, use to end-product disposal. As a result, an approach based on Life Cycle Assessment (LCA) methodology seems appropriate in order evaluate the environmental impact related to the production and use of polymeric materials.

1.4.1. LCA Generalities

The environmental Life Cycle Assessment (LCA) is defined as “the compilation and evaluation of the inputs, outputs and potential environmental impacts of a product system throughout its life cycle” [180]. LCA covers all the processes involved in life of the product (product system), from extraction of resources, manufacturing of the product, its use and the waste-management after using the product (disposal, incineration, or recycling). This means that LCA is a cradle to grave approach, beginning with the gathering of raw materials from the earth to create the product and ending at the point when all materials are returned to the earth [181]. However, sometimes it can be justified to limit the analysis to the point where the product leaves the factory (cradle-to-gate analysis). The term “product” has to be considered in a broad sense, and can indicate physical items as well as services. In this view, it is not the product itself that has to be considered but the function that the product fulfils.

LCA is typically used to compare product alternatives, e.g. analysing the origins of a

problem related to a particular product, comparing improvements variants of a given

product, designing new products, and choosing between a number of comparable

products [182, 183]. Since the beginning of its development in the early 90s, LCA has

been applied to several applications in almost all sectors of the economy, such as

packaging, waste management, transport, housing and buildings, and food [182]. LCA

burdens is not possible, a qualitative description of the problem should be anyway provided. LCA is not necessarily specific to location and time. In fact, the processes involved in the product system can be diluted in the time and not concentrated in a single location.

The methodology for LCA has been the subject of standardization. ISO (International Organization for Standardization) has published a series of standards in the 14000 series:

ISO 14040:1997 (Environmental management - Life cycle assessment - Principle and framework), ISO 14041:1998 (Environmental management - Life cycle assessment - Goal and scope definition and Inventory analysis), ISO 14042:2000 (Environmental management - Life cycle assessment - Life cycle impact assessment) and ISO 14043:2000 (Environmental management - Life cycle assessment - Life cycle interpretation). As indicated in the ISO-14040 series [180, 184-186], LCA methodology distinguishes four phases: goal and scope definition, inventory analysis, impact assessment, and interpretation (Figure 1.12).

Figure 1.12. General phases of a Life Cycle Assessment

• Goal and scope definition: LCA application, type, reason and audience are stated, as

well as the geographical and temporal scope. The system boundaries and functional

unit (FU) are also determined.