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Natural versus synthetic fibers

There are a variety of interactions which control the load transfer between fibers and their matrices including chemical bonds, secondary interaction forces (van der Waals, acid/base etc.), and mechanical interlocking. The quality of the fiber/matrix bond significantly affects not only the overall composite properties, but also the water uptake of the composites and authors such as Joseph et al. (2002) have showed that improved adherence between sisal fibers and a polypropylene (PP) matrix, via chemical treatments, can reduce weight gain due to water sorption, by reducing capillary action.

Natural fibers, acting as reinforcement within composites, offer many advantages including good strength properties, low cost, low density, high toughness, good thermal properties, biodegradability, nonabrasive behavior, and widespread availability. However, organic products containing cellulose fibers have several negative characteristics, such as an incompatibility with hydrophobic polymer matrices (Prabakar and Sridhar, 2002) and a tendency to show little resistance to prolonged moisture. Finite natural lengths and large diameters also limit their potential applications. Despite these disadvantages, many previous studies have demonstrated the positive effects of adding natural fibers as composite, mortar, and soil reinforcement, improving the particular compound’s ability to decrease shrinkage and enhance compressive, flexural, and shear strength, if an optimum reinforcement ratio can be found. In many cases today, reinforcing fibers tend to be of a synthetic nature, such as carbon fibers, glass fiber-reinforced plastics, and PP, but in recent years, natural fibers have started to be used as an ecological friendly alternative for soil reinforcement within a variety of material applications. Their shrinkage properties are due to the drying effect, but there are innovative chemical treatments now being developed to counteract their absorption characteristics (Tang et al., 2012). An understanding of the chemical structure of the fiber surface is therefore crucial in order to advance wool processing and finishing technology as well as examine alternative applications.

With respect to PP fiber, it is a thermoplastic polymer used in a wide variety of formats and applications such as plastic food containers, carpets, and insulation. It has a variety of advantageous engineering properties such as resistance to fatigue, physical damage, and freezing, as well as being unusually resistant to many chemical solvents, bases, and acids. PP fibers are generally superior to polyamide fibers, for example, with regards to elasticity and resiliency properties but they have a

Physical structure of the Polypropylene fibers

Figure 4.4 Physical structure of the Polypropylene fibers.

lower wear resistance. Their resistance to various external conditions is largely determined by the effectiveness of added stabilizers. PP filaments and monofilaments are used in the manufacture of floating cables, nets, filter fabrics, and upholstery, whereas PP fibers are used in carpeting, blankets, outerwear fabrics, knitwear, and filter fabrics. PP fibers are cylindrical and usually have a uniform and homogeneous section of around 40 ^m (see Fig. 4.4). They display good heat- insulating properties but are sensitive to heat and ultra-violet radiation.

PP fibers have a low Young’s modulus value, which means that they cannot prevent the formation and propagation of cracks at high stress levels, however they can bridge large cracks in certain circumstances (Qian and Stroeven, 2000; Karahan and Atis, 2011). They have also been used to considerably reduce the amount of cracks in various materials and to enhance residual strength (Xiao and Falkner, 2006; Pliya et al., 2011). There are a few relevant results in natural polymer—soil reinforcement studies incorporating PP fibers. Experimental results from various different studies in academic literature on this subject are contradictory. Several studies carried out (e.g., Uysal and Tanyildizi, 2012; Noumowe, 2005) show a decrease in residual strength in agreement with the hypothesis relating to increased porosity caused by chemical reactions, whilst other studies (e.g., Behnood and Ghandehari, 2009; Chen and Liu, 2004) show improvement in residual strength. Nevertheless, PP’s low moisture absorption rate compared to most natural and nonnatural fibers, makes PP more stable volumetrically (see Table 4.4).

Wool (W) and other natural protein-based fibers are generally obtained from animal hairs and secretions. These protein fibers generally have a greater resistance to moisture and heat than natural cellulosic and vegetal fibers; however protein fibers have little resistance to alkalis, so they are not appropriate for use within mixes that contain cement. A small amount of research has been carried out into the use of animal fibers within composites and Barone and Schmidt (2005), for instance, reported on the use of keratin feather fibers as short fiber reinforcement within

Table 4.4 Fiber absorption assessment

Synthetic fibers

E-glass

Polypropylene

Polyester

Polyamide

Moisture absorption (%)

0.01

0.4

6

Natural fibers

Hemp

Jute

Ramie

Coir

Moisture absorption (%)

8

12

12—17

10

Natural fibers

Sisal

Flax

Cotton

Wool

Moisture absorption (%)

11

7

8—25

10—28

Physical structure of the Wool fibers

Figure 4.5 Physical structure of the Wool fibers.

LDPE composites; this keratin feather fiber they used had been obtained from chicken waste. A very common natural protein fiber containing keratin is wool, which grows outwards from the skin of sheep. Different species of sheep produce different types of wool with varied fiber length, diameter, and other differing physical characteristics. Generally however, fine wool fibers are 40—127 mm in length, 14—45 pm in width, are roughly oval in cross-section and grow in a wavy type of form which gives rise to a degree of twist.

All the same, wool has not been studied in great detail as fiber reinforcement. It is a hygroscopic fiber, which takes up moisture in vapor form, and tiny pores in the cuticle make the fiber semipermeable, allowing vapor to pass through to the heart of the fiber. This means that wool can easily absorb up to 30% of its weight in moisture without feeling damp or clammy, which is obviously a significant advantage to animals trying to keep warm in wet weather.

There is generally a two-phase structure for wool fibers, which consists of a water-absorbing matrix, embedded within nonwater-absorbing cylinders. The macroscopic appearance and physical structure of the wool fibers is shown in Fig. 4.5.

 
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