**2. Biocomposites**

Polymer composites constitute a broad group of materials, composed of the macromolecular matrix and various fillers. Currently the filler market for plastic composites is dominated by calcium carbonate (40%) and glass fiber (31%) and some other inorganic fillers such as talc, mica and clay. Although the conventional fillers offer property changes in the composites, their high density is not beneficial to fuel savings in automotive applications. Polymer composites with cellulose fillers are growing rapidly, mainly in the construction and automotive industry. The main advantage of such composites is lower density in comparison to that of glass fiber reinforced plastics. In Fig. 1 the density of polypropylene and PP filled with wood flour (WF), without or with of a compatibilizer (PP/WF/comp) has

Lightweight Plastic Materials 293

Natural fillers are biodegradable and derive from renewable resources, which is higly advantageous for sustainable development (Ellison et al., 2004; Bledzki et al., 2008; Oksman Niska & Sain, 2008; Klesov, 2007; Kozlowski & Kozlowska, 2005). Apart from the ecological reasons, natural fillers offer possibility of reinforcing thermoplastic matrices (Figs. 3 and 4)

> without talc with talc

without talc with talc

PE

PE

0 20 30 40 50 Fiber content [%]

Increase in the stiffness of thermoplastic composites filled with cellulose fibers depends on a matrix polymer nature – it is particularly significant to composites based on the low density polyethylene (LDPE). In case of LDPE filled with 30wt.% of hemp fibers Young modulus

> 0 20 30 40 50 Fiber content [%]

Fig. 4. Tensile strength of LDPE and LDPE composites with hemp fibers

Fig. 3. Young modulus of LDPE and LDPE composites with hemp fibers

increased for 300%, whereas at 50% loading it is for 5-times higher (Fig. 3).

and provide weight savings in comparison to glass fibers (Table 1).

0

5

10

Tensile strength [MPa]

15

20

Young modulus [MPa]

been compared to that of PP composites with glass fibers (GF). One can observe an increase in density with the filler content for all composites, however markedly higher density is that of PP/GF materials.

Fig. 1. Density of polypropylene and PP composites with wood flour

Low density of polymer composites filled with natural fibers was received due to a specific hollow structure of the fibers (Fig. 2), which is totally different from a bulky structure of glass fibers. Cellulose fibers do not exist in nature as separate items, but they form bundles. Each bundle contains 10-60 fibers diameter of 10-17 microns, linked together with pectines.

Fig. 2. Flax fibers bundle cross-section

been compared to that of PP composites with glass fibers (GF). One can observe an increase in density with the filler content for all composites, however markedly higher density is that

> PP 20 30 40 50 Filler content, % PP/WF PP/WF/comp PP/GF

Low density of polymer composites filled with natural fibers was received due to a specific hollow structure of the fibers (Fig. 2), which is totally different from a bulky structure of glass fibers. Cellulose fibers do not exist in nature as separate items, but they form bundles. Each bundle contains 10-60 fibers diameter of 10-17 microns, linked together with pectines.

of PP/GF materials.

Fig. 1. Density of polypropylene and PP composites with wood flour

Density, kg/m3

Fig. 2. Flax fibers bundle cross-section

Natural fillers are biodegradable and derive from renewable resources, which is higly advantageous for sustainable development (Ellison et al., 2004; Bledzki et al., 2008; Oksman Niska & Sain, 2008; Klesov, 2007; Kozlowski & Kozlowska, 2005). Apart from the ecological reasons, natural fillers offer possibility of reinforcing thermoplastic matrices (Figs. 3 and 4) and provide weight savings in comparison to glass fibers (Table 1).

Fig. 3. Young modulus of LDPE and LDPE composites with hemp fibers

Increase in the stiffness of thermoplastic composites filled with cellulose fibers depends on a matrix polymer nature – it is particularly significant to composites based on the low density polyethylene (LDPE). In case of LDPE filled with 30wt.% of hemp fibers Young modulus increased for 300%, whereas at 50% loading it is for 5-times higher (Fig. 3).

Fig. 4. Tensile strength of LDPE and LDPE composites with hemp fibers

Lightweight Plastic Materials 295

natural fillers (Fig. 1). Another approach might be hydrophobisation of cellulose fibers (esterification). Although polymer composites with natural fillers have been commercialized, their industrial applications are in some sectors limited because of their

The original processing technology of biocomposites was nonwoven technology, which is a normal production precursor to compression moulding. Further developments extended the processing technology to extrusion or injection molding of composites reinforced with short cellulose fibers. What should be considered at processing of thermoplastic composites is the melt viscosity, which increases after addition of fillers. Melt flow rate (MFR) of LDPE and composites with hemp fibers has been presented in Fig. 5. Flowability of composite filled with 30wt.% of fibers is for 2.5 times lower than that of neat LDPE, droping down at 50wt.% of hemp for 6 times in comparison to MFR of polyethylene. For that reason either higher pressure or higher processing temperature has to be used in order to shape high quality products.

low impact strength and high density compared to natural wood and polyolefines.

0 20 30 40 50 Fiber content [%]

Particular type of biocomposite is the particle board, which is a composite material made from small pieces of wood or other lignocellulosic material (branches, stem, saw dust, straw or bagasses) that are mechanically pressed into sheets being bonded with a resin. Similar sandwich structure have composites which external skin layers are made of a bulky plastic, whereas a core is lightweight (density of 40-70 kg/m3), made either as a foam or honeycomb structure. Such sandwich materials (Panelplus panels) while used in the manufacture of truck bodies weigh 60% less than the equivalent plywood panels (Institute

Hollow, lightweight plastic components may be manufactured also by the gas or water assisted injection molding. In this technology the mould is only partially filled with a polymer melt, afterwards a gas or water is injected to pack the mould completely. After mold cooling and solidification of a polymer the gas or water is evacuated from the cavity

Fig. 5. Melt flow rate of LDPE and LDPE composites with hemp fibers

PE

without talc with talc

of Materials, Minerals and Mining, 2004).

and the hollow part is ejected (Fig. 6).

MFR [g/10min]

Tensile strength of LDPE composites filled with natural fibers also enhances as a function of the filler content – at 30wt.% an increase for 60% was observed in comparison to neat LDPE, while at 50wt.% of hemp fibers the improvement reached 100% (Fig. 4).

Looking at the data presented in Table 1 and considering that the glass-fibre reinforced polypropylene composites are widely used in automotive components (front/end carriers, door panel supports, dashboards, consoles, seat backs, headliners, package trays, underbody shields etc.) one should expect significant environmental profits from replacing glass fibers with natural fibers (NF).


Table 1. Comparison of data related to materials used for manufacturing modern composites

The fibers used most frequently for reinforcement of polymer composites are the bast fibers grown in the climate of Europe (flax and hemp) and sub-tropical fibers (kenaf, jute and sisal) imported mainly from Asia. Composites filled with wood/natural fibers are called biocomposites or wood polymer composites (WPC), sometimes also a term "artificial wood" is applied. Their properties combine higher stiffness, hardness and better dimensional stability in comparison to plastics and lower density in comparison to mineral fillers. Currently biocomposites are being used by Fiat, Ford, Opel, Daimler Chrysler, Saturn, BMW, Audi, Peugeot, Renault, Mercedes Benz and Volvo.

Different polymers can be used as a matrix for biocomposites, and the loading of cellulose fillers usually vary within a range of 10 to 70%. Although thermosets were used first as a matrix for wood composites, currently thermoplastics use to be applied for manufacturing of biocomposites. Most frequently used matrices are polypropylene, polyethylene, polyvinylchloride and polystyrene. Depending on a chemical structure of the matrix the interaction at the polymer-filler interface is of different strength. This is crucial for the mechanical properties and melt viscosity of composites. High adhesion is expected if both components have polar groups. Unfortunately, this is not a case of the most popular biocomposites, which are composed of hydrophobic polyolefines and hydrofilic cellulose fibers (-OH groups). Therefore, extensive research has been performed to enhance interfacial adhesion and improve dispersion of cellulosic fibers in polyolefines. Frequently used solution is addition of compatibilizers composed of blocks interacting on a physical or chemical way with each component of the composite. Good results are reported on using maleated polypropylene (PP-g-MAH) for compatibilization of PP-based composites with

Tensile strength of LDPE composites filled with natural fibers also enhances as a function of the filler content – at 30wt.% an increase for 60% was observed in comparison to neat LDPE,

Looking at the data presented in Table 1 and considering that the glass-fibre reinforced polypropylene composites are widely used in automotive components (front/end carriers, door panel supports, dashboards, consoles, seat backs, headliners, package trays, underbody shields etc.) one should expect significant environmental profits from replacing glass

Price [€ / kg] 1.4 2.2-3.0 1.5-2.2 Density [g/cm3] 0.92 1.2-1.5 2.5-2.8 Energy [MJ/kg] 101.1 3.4 48.3 CO2 [kg/kg] 3.11 0.64 2.04 SOx [g/kg] 22.2 1.2 8.8 NOx [g/kg] 2.9 0.95 2.9 BOD [mg/kg] 38.37 0.265 1.75 PM [g/kg] 4.37 0.2 1.03 COD [g/kg] 1.14 3.23 0.02 Table 1. Comparison of data related to materials used for manufacturing modern

The fibers used most frequently for reinforcement of polymer composites are the bast fibers grown in the climate of Europe (flax and hemp) and sub-tropical fibers (kenaf, jute and sisal) imported mainly from Asia. Composites filled with wood/natural fibers are called biocomposites or wood polymer composites (WPC), sometimes also a term "artificial wood" is applied. Their properties combine higher stiffness, hardness and better dimensional stability in comparison to plastics and lower density in comparison to mineral fillers. Currently biocomposites are being used by Fiat, Ford, Opel, Daimler Chrysler, Saturn,

Different polymers can be used as a matrix for biocomposites, and the loading of cellulose fillers usually vary within a range of 10 to 70%. Although thermosets were used first as a matrix for wood composites, currently thermoplastics use to be applied for manufacturing of biocomposites. Most frequently used matrices are polypropylene, polyethylene, polyvinylchloride and polystyrene. Depending on a chemical structure of the matrix the interaction at the polymer-filler interface is of different strength. This is crucial for the mechanical properties and melt viscosity of composites. High adhesion is expected if both components have polar groups. Unfortunately, this is not a case of the most popular biocomposites, which are composed of hydrophobic polyolefines and hydrofilic cellulose fibers (-OH groups). Therefore, extensive research has been performed to enhance interfacial adhesion and improve dispersion of cellulosic fibers in polyolefines. Frequently used solution is addition of compatibilizers composed of blocks interacting on a physical or chemical way with each component of the composite. Good results are reported on using maleated polypropylene (PP-g-MAH) for compatibilization of PP-based composites with

BMW, Audi, Peugeot, Renault, Mercedes Benz and Volvo.

**PP Natural fibers Glass fibers** 

while at 50wt.% of hemp fibers the improvement reached 100% (Fig. 4).

fibers with natural fibers (NF).

composites

natural fillers (Fig. 1). Another approach might be hydrophobisation of cellulose fibers (esterification). Although polymer composites with natural fillers have been commercialized, their industrial applications are in some sectors limited because of their low impact strength and high density compared to natural wood and polyolefines.

The original processing technology of biocomposites was nonwoven technology, which is a normal production precursor to compression moulding. Further developments extended the processing technology to extrusion or injection molding of composites reinforced with short cellulose fibers. What should be considered at processing of thermoplastic composites is the melt viscosity, which increases after addition of fillers. Melt flow rate (MFR) of LDPE and composites with hemp fibers has been presented in Fig. 5. Flowability of composite filled with 30wt.% of fibers is for 2.5 times lower than that of neat LDPE, droping down at 50wt.% of hemp for 6 times in comparison to MFR of polyethylene. For that reason either higher pressure or higher processing temperature has to be used in order to shape high quality products.

Fig. 5. Melt flow rate of LDPE and LDPE composites with hemp fibers

Particular type of biocomposite is the particle board, which is a composite material made from small pieces of wood or other lignocellulosic material (branches, stem, saw dust, straw or bagasses) that are mechanically pressed into sheets being bonded with a resin. Similar sandwich structure have composites which external skin layers are made of a bulky plastic, whereas a core is lightweight (density of 40-70 kg/m3), made either as a foam or honeycomb structure. Such sandwich materials (Panelplus panels) while used in the manufacture of truck bodies weigh 60% less than the equivalent plywood panels (Institute of Materials, Minerals and Mining, 2004).

Hollow, lightweight plastic components may be manufactured also by the gas or water assisted injection molding. In this technology the mould is only partially filled with a polymer melt, afterwards a gas or water is injected to pack the mould completely. After mold cooling and solidification of a polymer the gas or water is evacuated from the cavity and the hollow part is ejected (Fig. 6).

Lightweight Plastic Materials 297

Cellular structure of foams allows also sound and vibration damping, which has been used

Cellular plastics may be manufactured either by the periodic or continuous technology, using chemical or physical foaming agents. The conventional foams have cells of large size (0.1-1 mm) and broad size distribution (Fig. 8), therefore their mechanical properties are inferior to that of bulky polymers. The cell density of conventional foams is in a range of 104

The microfoams contain much higher number of cells (>109/cm3), which size is markedly smaller (ca. 10 μm). Such materials exhibit better performance than the conventional foams,

in sound insulating panels, upholstery in furniture, car seats, protective pads etc.

with higher mechanical properties and better thermal insulating characteristics.

Fig. 8. Polystyrene foams – conventional (left) and microfoam (www.trexel.com)

excluding manufacturing of expanded polystyrene (EPS), they are rarely applied.

solvent, the empty holes form cells of the resulted foam.

Foaming techniques can be divided into several groups. Similar to the plastics processing technology, they can be divided on continuous (extrusion foaming) and periodic (injection molding or press foaming) processes. Periodic technology requires a long time and

Other method of rather seldom use is manufacturing of polymer composites filled with easily soluble compounds, like salt or sugar. After the filler is eluted with an appropriate

Cellular structure can be also formed by sintering of polymer powders at high temperature. Soft surfaces of neighboring spheres stick each other, whereas the free volumes between

Akzo Nobel offers a foaming method based on mixing of a matrix polymer with thermoplastic spheres filled with volatile hydrocarbons (Expancel). At heating the polymer becomes soft, while the hydrocarbon evaporates, expanding the material. Initial sphere diameter is 12 μm, which after expansion increases to 40 μm. The material of spheres should be compatible with the matrix polymer, whereas a hydrocarbon is selected depending on a required decomposition temperature. The spheres of Expancel are added to a polymer in an


**3.1 Methods of foaming** 

them create foam cells.

Fig. 6. Principle of gas-assisted injection molding (acc. to www.cinpres.net)
