**4. Processing of aramid/PP samples tested at high impact**

For the processing of samples, an atactic PP in film form was used; the mechanical properties of this polymer are shown in Table 1. The aramid fabric used to reinforce the PP is a balanced woven used for personal body armor which was donated by the fabric Company Carolina Protect Ballistic, the properties of this fiber are included here.


Table 1. Properties of the materials used in the aramid/PP composite material

During processing, the samples were divided into two main groups; the first comprising plates in which the aramid and the PP matrix are molded in a single stage, forming a multilayer fiber-reinforced composite (Fig. 10a), and the second comprising arrangements of composite materials where each layer of aramid was independently molded keeping the same volume fraction ratio (Fig. 10b).

Fig. 10. Configuration of laminates, a) consolidated, b) independent

All these laminates, both consolidated and independent were formed by thermo-molding in a 25 ton press with automatic control of pressure and temperature, which guarantees less variation between the properties of samples within a particular batch. The molding conditions are shown in Table 2. Stacking of material in the molding process is another important factor as it generates a good distribution of the matrix in the fabric, Fig.11 shows this configuration, which grows in accordance with the number of layers required in the material.

Fig. 11. Stacking of material in the molding process

For the processing of samples, an atactic PP in film form was used; the mechanical properties of this polymer are shown in Table 1. The aramid fabric used to reinforce the PP is a balanced woven used for personal body armor which was donated by the fabric

**Kevlar® Fabric724 15 Polypropylene film form 16** Yarn type Kevlar® 129 Elastic modulus 680.6 MPa Yarn count 1000 denier Maximum stress 35.83 MPa Weave Plain Maximum deformation 18.89% Weight 207 g/m2 Glass transition -10 °C a -18 °C Count 24 yarns/inch Melting point 175 °C

During processing, the samples were divided into two main groups; the first comprising plates in which the aramid and the PP matrix are molded in a single stage, forming a multilayer fiber-reinforced composite (Fig. 10a), and the second comprising arrangements of composite materials where each layer of aramid was independently molded keeping the

All these laminates, both consolidated and independent were formed by thermo-molding in a 25 ton press with automatic control of pressure and temperature, which guarantees less variation between the properties of samples within a particular batch. The molding conditions are shown in Table 2. Stacking of material in the molding process is another important factor as it generates a good distribution of the matrix in the fabric, Fig.11 shows this configuration,

Laminates

Company Carolina Protect Ballistic, the properties of this fiber are included here.

Table 1. Properties of the materials used in the aramid/PP composite material

Fig. 10. Configuration of laminates, a) consolidated, b) independent

Fig. 11. Stacking of material in the molding process

Woven

which grows in accordance with the number of layers required in the material.

same volume fraction ratio (Fig. 10b).

a) b)

**4. Processing of aramid/PP samples tested at high impact** 


Table 2. Laminate molding conditions

Both molding conditions and stacking configurations generate 64% fiber volume fraction in the composite material. This value was obtained thanks to previous studies in which the percentage contained in the composite was varied, it also coincides with values reported in the literature, where the fiber volume fraction recommended is 60% to 70% 17.

The aramid/PP composite material was subjected to the following high impact tests; first, consolidated and independent laminates with two to six layers were characterized at high impact. Each batch per layer consists of six samples in order to determine the ballistic limit in each point and thus create a comparative curve between the ballistic limit and the number of layers between both laminate configurations, independent and consolidated. Subsequently, an intermediate point of four layers was used to carry out the following tests; trauma in four-layer consolidated laminates, trauma in four-layer independent laminates, trauma in aramid fabric arrangements without polymeric matrix, and the velocity curve of residual impact-energy in four layer laminates using ballistic gelatine. Fig. 12 shows a general representation of how a high impact test is carried out with the use of witness material and ballistic gelatine. One important difference in these two tests is that the witness material must be in direct contact with the sample being tested, while the ballistic gelatine may or may not be in contact, in this case it is not in contact with the sample.

Fig. 12. Representation of an impact test on a sample with material placed behind it.

Advantages of Low Energy Adhesion PP for Ballistics 205

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

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

*Max*

(3)

*b F dL*

**6. Results of the high impact tests on aramid/PP composite material** 

A comparison of ballistic behavior was carried out between aramid/PP consolidated and independent laminates with 2 to 6 layers, both with the same fiber volume fraction (64%). Energy absorption is calculated with the projectile mass and at the ballistic limit of the laminate, which determines the kinetic energy that the composite material is capable of absorbing before failure. An increase in the number of consolidated laminates led to a nonlinear increase in absorbed energy (Fig. 15). This non-linearity is a consequence of the rigidization undergone by the material as it becomes thicker, thereby causing a restriction in

The independent laminates showed an approximate increase of 13% in energy absorption in comparison with their counterpart, consolidated laminates (Fig. 15). The amount of energy absorbed by each laminate is dominated by the rigidity of the material, which increases with the number of layers, resulting in a reduction in the slope of the curve. Composite materials based on epoxy resin show an abrupt change in the slope at four layers, in contrast with the thermoplastic composites in this study which did not register this behavior, even at six

Fig. 16 shows energy absorption normalized as a function of the number of layers, from this it can seen clearly that an increase in the number of layers in an arrangement reduces its capacity to absorb energy; however, the reduction was lower for independent aramid/PP

velocities defined by the user (in this case 0.5 mm/min).

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

*d* = Fiber diameter (µm).

its transversal deformation.

layers 20.

arrangements.

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

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

stress.

where:
