**3. Biodegradable starch-based composites**

A biocomposite polymer is classified as a material which combines a biodegradable polymer as its matrix and a biodegradable filler as the reinforcement [18]. Such composites are also commonly known as "green composites," and as the focus in this chapter is sustainability, the use of natural fibers as fillers will be the only class investigated [34]. Several researchers have demonstrated a high compatibility between starch- and natural-based fibers such as cellulose derivatives [2, 14, 16, 18, 28, 29, 33]. Considerable improvement in the mechanical properties of starch-based composites coupled with a reduction in water permeability has been reported. Additionally, Reddy et al. has reported the use of nano-fillers specifically cellulosebased in the fabrication of green composites, where significant enhancement in properties is anticipated [35]. The classification of natural fibers is presented in Section 3.1, and an overview of their impact as reinforcements on the TPS matrix is provided in Section 3.2.

#### **3.1 Natural fibers**

Fiber fillers are added as the source of reinforcement and load bearing component within the composite matrix. They are of either natural or synthetic origin such as plants and carbon, respectively. However, natural fibers offer several benefits over their synthetic counterparts, one of them being that they are essentially biodegradable which is considered as a merit for the environment [8]. Additionally, high specific properties such as strength and low density along with being renewable and low in cost have led to their emergence as excellent substitutes for the man-made competitors [4, 36]. The fiber strength comes from the strong inter- and intramolecular bonds that make the fiber stiff and rigid developing intertwined threadlike structures [31]. In addition to the strong bonds, the higher the crystallinity of the filler material, the less exposed areas of the matrix that would absorb water and moisture. Each type differs slightly in their characteristics; there are three major classes upon which this family of fibers are classified: (1) plant-based fibers, usually referred to as bast fibers and are extracted from the outer bark of plant stems, such as flax, jute, and hemp; (2) leaf fibers, which are hard and strong fibers obtained from leaf tissues such as in the case of sisal and pineapple; and finally (3) seed fibers such as cotton and coir [4, 16]. Other types are extracted from wood or grass [4]. **Table 4** illustrates the mechanical properties of the characteristic natural fibers commonly used for each category compared to carbon fibers. It can be observed that the plant-based flax fibers exhibit the highest strength with a maximum of 1500 MPa, while that for the remaining bast and leaf fibers is less than 1000 MPa. Nevertheless, they all show high specific strength and specific stiffness properties compared to carbon fibers, where the specific gravity of carbon is much higher than the natural counterparts.

Furthermore, it is important to note that natural fibers have wax on its surfaces and other elements such as lignin and hemicellulose, which leads to difficulty in the adhesion of the matrix to the fibers. Therefore, to improve the poor linkage and adhesion problem, the fibers undergo a surface chemical treatment before synthesis with the matrix, which also aim to reduce the fiber permeability to water [16]. Most treatments work on removing the hydrogen bonds on the surface so as to make it hydrophobic and to improve the surface roughness.


**Table 4.**

*Mechanical properties of natural fibers compared to carbon fibers [16, 34].*

#### **3.2 Natural fiber-reinforced starch-based composites: performance evaluation**

The main goal driving the fabrication of the natural fiber-reinforced starchbased composites is overcoming the limitations of TPS and attaining better mechanical and physical properties while still retaining the biodegradability attribute of natural materials. This section attempts to evaluate the performance of the different variations of the green composite based on the improvements achieved in the mechanical properties, thermal stability, and biodegradation rates compared to that of the starch matrix. **Table 5** summarizes the outcome of the studies performed on different combinations of natural fiber and starch. Key findings and conclusions are drawn from comparing the data tabulated and will be discussed in the following sections, based on which the DFL approach will be highlighted.

#### *3.2.1 Mechanical properties*

As observed from **Table 5**, there is a general increasing trend in the tensile strength as the fraction of fiber increases in the composite compared to that of pure thermoplastic starch. This is demonstrated in the case of adding flax fibers to TPS, where the tensile strength increased from 50 to 60 MPa as a function of increasing the fiber content from 40 to 50%, respectively. However, increasing the content beyond certain percentages, the opposite occurs where the properties deteriorate instead of improving. This is slightly observed post increasing the flax fiber content to 80%, where the tensile strength reduced to 55 MPa. However, this phenomenon was clearly observed when using date palm fibers, where increasing the fiber composition from 50 to 80% led to a significant 60% decrease in the tensile strength from 32.7 to 12 MPa, respectively. Moreover, in spite of following similar trends, different fibers possess variable properties which eventually lead to major differences in the properties attained. It could be remarked that flax-based starch composites acquire the highest in tensile properties coupled with the highest ductility among other composites. It is clear that the strong flax fibers have imparted their high strength properties (listed in **Table 4**) to the starch matrix and produces a high strength composite with desirable properties. Additionally, the hybrid between date and flax fibers has led to an increase in the tensile strength equivalent to the average increase attained from each type—at the same composition—separately.

method known as thermogravimetric analysis (TGA), where one of the test methods evaluates the temperature it takes to cause a 10% weight loss and records the differences among the different composites, with higher values indicating improved stability. Compared to TPS, the temperature at which 10% weight loss occurs increased from 192 to 229°C and 251**°**C corresponding to a 50 and 80%

Biodegradability is an integral process of biocomposites, which occurs as a result

of microbial bacteria or fungi naturally assimilating the material structure and causing its degradation [16]. The main scheme followed to test the biodegradability rate is by measuring the percentage weight loss during a period of time. Generally, the higher the percentage of fibers, the lower the degradability rate due to the lower degradation rate of the fibers than starch-based polymers, as shown in **Table 5**. During the 1-week test period dictated to measure the rate of biodegradability, only 5% weight loss has occurred in the 80% flax fiber-reinforced TPS compared to the loss of 16% in the 40% flax fiber-reinforced TPS and 30% loss in pure TPS.

The design for a life approach assumes that the lifetime of a product can be estimated based on the rate of biodegradation, which depends on the material

increase in the flax fiber fraction, respectively.

*3.2.3 Biodegradation*

**Fiber Composition**

Hybrid 25(Date) & 25

Short fibercellulose

**Table 5.**

**(%)**

*DOI: http://dx.doi.org/10.5772/intechopen.88425*

*Design for Sustainability with Biodegradable Composites*

50 (unidirectional)

(flax)

**UTS (MPa)** **ε (%) E (GPa) Weight loss**

50 60 5.7 4.3 15.4 229 [2] 80 55 — 2 5 251 [28]

80 12 — 7 10 250 [29]

15 15.43 6.08 364.9 — 350 [39]

1 6.3 42 4.8 23 — [14]

TPS 0 3.8 138 0.5 30 192 [28] Flax 40 50 — 3.5 16 — [28]

Palm 50 28.2 1.82 3.85 18.6 — [2] Banana 50 25.4 2.03 3.71 20.3 — [2] Bagasse 50 29.8 3.27 3.23 20 — [2] Date 50 32.7 — 2.8 18 232 [29]

Sisal 20 2.8 2 151 — — [38] Hemp 20 4 3.4 182 — — [38]

Jute 12.5 5.5 —— — — [40] Lentil flour 0.5 2.1 49 0.86 23.1 — [14]

*Mechanical properties, thermal degradation, and biodegradability of TPS and natural fiber-reinforced TPS.*

**(%)/week**

131 5.8 7.5 15.4 — [2]

43 —— — — [29]

**Temp. at 10% weight loss (°C)** **Reference**

**3.3 Design for a life approach**

**47**

#### *3.2.2 Thermal stability*

Another upward trend is attained in the thermal stability which incrementally increases as a function of increasing the fiber content. Due to the organic nature of the biocomposite constituents, heat application is expected to cause changes in their physical and chemical properties [16]. Thus, thermal stability is tested through a


*Design for Sustainability with Biodegradable Composites DOI: http://dx.doi.org/10.5772/intechopen.88425*

### **Table 5.**

*Mechanical properties, thermal degradation, and biodegradability of TPS and natural fiber-reinforced TPS.*

method known as thermogravimetric analysis (TGA), where one of the test methods evaluates the temperature it takes to cause a 10% weight loss and records the differences among the different composites, with higher values indicating improved stability. Compared to TPS, the temperature at which 10% weight loss occurs increased from 192 to 229°C and 251**°**C corresponding to a 50 and 80% increase in the flax fiber fraction, respectively.

#### *3.2.3 Biodegradation*

Biodegradability is an integral process of biocomposites, which occurs as a result of microbial bacteria or fungi naturally assimilating the material structure and causing its degradation [16]. The main scheme followed to test the biodegradability rate is by measuring the percentage weight loss during a period of time. Generally, the higher the percentage of fibers, the lower the degradability rate due to the lower degradation rate of the fibers than starch-based polymers, as shown in **Table 5**. During the 1-week test period dictated to measure the rate of biodegradability, only 5% weight loss has occurred in the 80% flax fiber-reinforced TPS compared to the loss of 16% in the 40% flax fiber-reinforced TPS and 30% loss in pure TPS.

#### **3.3 Design for a life approach**

The design for a life approach assumes that the lifetime of a product can be estimated based on the rate of biodegradation, which depends on the material

composition and service conditions. Data on biodegradation of various materials, such as that in **Table 5**, can be used to select the appropriate material for a given service environment and expected useful life of the product. The different mechanisms associated with the life cycle assessment of polymers is provided in [9], which provides a comprehensive overview of the needed knowledge for the determination of the useful life of the polymer. Moreover, Elsayed et al. presented biodegradation data related to flax fiber-reinforced starch composites, where the weight loss test was applied for long periods of time. The time needed for a 100% loss in weight was determined to be 6 weeks for TPS coinciding with a less than 40% reduction in the composite [28].

Holbery and Houston have indicated that the use of bast fibers and specifically flax fibers presents a strong competition against E-glass fibers commonly used in composites implemented in automotive applications [43], where the specific strength for flax fiber is 1200 compared to 1275 for E-glass fibers [43]. Other natural fibers suggested for reinforcing plastics are hemp and jute. Their use has been reported in reinforcing PP replacing fiber glass-reinforced plastics in commercial Mercedes-Benz and Ford cars [44]. **Figure 2** demonstrates the use of flax fiber composites in different components of the Mercedes-Benz A-Class vehicle.

However, these composites are argued to be only partially biodegradable due to the synthetic matrices and hence are not an environmentally friendly option. A rather more sustainable option is the use of natural polymers such as starch reinforced by natural fibers. Nevertheless, this option has not been investigated in the literature in applications related to the automotive industry in spite of the benefits these composites offer, which range from the low energy needed for production to being renewable and biodegradable. The case study presented in Section 4.1.2 evaluates the use of natural fiber-reinforced starch as a potential candidate for

For interior panels, the material requirements needed are lightweight and high stiffness. Cost and environmental considerations are other factors considered for the decision-making process. Thus, the material performance index (m) for a stiff light structural member is calculated based on the consideration that a panel is

substitution.

**Figure 1.**

**Figure 2.**

**49**

*Hemp fibers in vehicle doors [45].*

*4.1.2 Performance indices and material requirements*

*Design for Sustainability with Biodegradable Composites*

*DOI: http://dx.doi.org/10.5772/intechopen.88425*

*Mercedes-Benz A-Class vehicle components made of flax fiber composites [46].*

Successful application of the DFL approach entails a proper material selection process to be performed along with sufficient knowledge of the physical and chemical reactions associated with the proposed composites. This will provide the framework and foundation needed for choosing the applicable bio-based alternative and eventually help in controlling the service lifetime of the polymer by either accelerating the degradation process or stabilizing it depending on the application. The case study presented in Section 4.1 elaborates further on the use of material selection and substitution processes to aid in making the proper choice for applications in the automotive industry. The case study is adopted from material substitution cases reported by Farag and published in [16**,** 41].
