**5. Measurement of interfacial shear stress**

Interfacial adhesion was studied using the microdrop technique, this technique was implemented in 1987 by Piggott 18 and since then it has proven its effectiveness in many studies focusing on the determination of interfacial quality in composite materials 11,19. Basically, the method involves applying a droplet of the matrix to a fiber, this drop must surround the fiber symmetrically so that it may be sustained and separated from the fiber which is normally subjected to tension. The mechanical properties of the fiber-matrix interface were determined in this way (Fig. 13).

Fig. 13. Representation of a fiber pull-out by microdrop

In order to use this technique in the determination of interfacial shear stress in the aramid/PP arrangement, it was necessary to generate small drops of PP on the fiber (at an interval 80 µm to 200 µm diameter), the drops must surround the fiber completely. This is achieved by grinding the matrix and separating it into different particle sizes. The PP powder obtained is deposited on the fibers arranged in aluminum frames (Fig. 14), where it is subsequently heated in a convection stove to melt the thermoplastic matrix.

Fig. 14. Aluminum frame with aramid fibers

The samples obtained were separated according to their diameter; those with diameters between 80 µm and 200 µm were used for the interfacial adhesion tests. This property was determined with a microtensometer equipped with a Newport brand mobile head, which is capable of moving with great precision. The equipment consists of a load cell which registers the force required to separate the drop from the fiber by mechanical extraction. The microvises sustaining the drop move lengthwise along the fiber and drag the drop while the fiber is held by the load cell. The unit comprising the mobile head and microvises moves at velocities defined by the user (in this case 0.5 mm/min).

The value of interfacial shear stress is obtained based on the last load supported by the sample, the diameter of the resin drop deposited on the fiber and the length taken up by the drop. These values are substituted in Equation 3, determining the interfacial shear stress.

$$
\tau\_b = \frac{F\_{\text{Max}}}{\pi dL} \tag{3}
$$

where:

204 Thermoplastic Elastomers

Interfacial adhesion was studied using the microdrop technique, this technique was implemented in 1987 by Piggott 18 and since then it has proven its effectiveness in many studies focusing on the determination of interfacial quality in composite materials 11,19. Basically, the method involves applying a droplet of the matrix to a fiber, this drop must surround the fiber symmetrically so that it may be sustained and separated from the fiber which is normally subjected to tension. The mechanical properties of the fiber-matrix

Load cell Droplet of

Microvices with micrometric advance

matrix

Fiber

In order to use this technique in the determination of interfacial shear stress in the aramid/PP arrangement, it was necessary to generate small drops of PP on the fiber (at an interval 80 µm to 200 µm diameter), the drops must surround the fiber completely. This is achieved by grinding the matrix and separating it into different particle sizes. The PP powder obtained is deposited on the fibers arranged in aluminum frames (Fig. 14), where it

The samples obtained were separated according to their diameter; those with diameters between 80 µm and 200 µm were used for the interfacial adhesion tests. This property was

is subsequently heated in a convection stove to melt the thermoplastic matrix.

Aramid fibers Aluminum strip aluminum

**5. Measurement of interfacial shear stress** 

interface were determined in this way (Fig. 13).

Fig. 13. Representation of a fiber pull-out by microdrop

Mobile head

Aluminum base

Fig. 14. Aluminum frame with aramid fibers

Double-sided strip

Aluminum frame

*<sup>b</sup>* = Maximum interfacial shear stress (Pa).

*FMax* = Maximum force reached in the test (gf).

*d* = Fiber diameter (µm).

*L* = Length of fiber taken up by the drop (µm)
