Dynamic Mechanical Thermal Analysis (DMTA) was performed on the natural fiber composites in order to study and establish the viscoelastic behavior. The vis­coelasticity of materials is determined by applying a sinusoidal strain and measur­ing the stress as response (Essabir et al. 2013b). Measures of viscoelasticity can be expressed by the equation of strain and stress:

e = e0 sin (at)

s = s0 sin (at + d)

where e0 is the strain amplitude, ю is the angular frequency and S is the angular shift between stress and strain.

The dynamic modulus or complex modulus (E*) is given by equation:

E* =>/E’2 + E"2

where E’ is the elastic modulus and E" the viscous modulus. E’ and E" are expressed in the equation:

E’ = 1^—J cosd

( „ A S0

£o

where є0, a0, and m represent respectively the amplitude of the deformation cycle, the amplitude of the stress cycle, and the pulsation in rad/s.

The elastic modulus (storage modulus), is proportional to the energy stored by cycle (elastic behavior). However viscous modulus (loss modulus), represent dissi­pated energy by cycle (viscous behavior). The Loss factor is the relationship between the loss modulus and storage modulus and is expressed by equation:

tanS = E" / E’

In dynamic mode, the mechanical properties of the material depend on the defor­mation of the excitation frequency and temperature.

The Dynamic mechanical thermal tests were performed on a rheometer Solid Analysis (RSA) operating with a dual cantilever configuration. Samples dimension were 45 mm in length, 5.5 mm in width, and 2 mm in thickness. A strain sweep test was carried out and strain from this linear regime was 0.002. After a dynamic fre­quency sweep tests were performed using this strain amplitude (0.002) between

0. 015 and 15 Hz, finally temperature sweep tests were ranged from 30 to 120 °C with a heating rate at 5 °C/min, frequency and strain were fixed at 1 Hz and 0.002 respectively.

Figure 14.8 shows the frequencies sweep tests, complex modulus (E*), and mechanical loss factor (tan S), for PP/Doum composites. It was observed that the complex modulus increases with increases in frequency. This behavior is due to the molecular time response. At higher frequencies have not enough time to relax and attend permanent deformation. It was also observed that the loss factor (tan S) was less sensitive to changes related to the fibers’ content. Generally a change in the tan S curve indicates a relaxation process which is associated to the movement of molecular chains within the polymer structure.

The temperature at which the tanS peak occurs is commonly known as the glass transition temperature (Tg). The glass temperature (Tg) was determined from the deriva­tive curves of tan S vs. temperature (Fig. 14.8). Figure 14.8 shows the Tg variation as function of Doum fibers content in PP matrix. It was observed that the addition of fiber improves the Tg of composites from 76.6 °C for neat PP to 90.25 °C for composite with 30 wt.% fiber content. These results show a gain of gain of 17.8 % in Tg.