Ballistic Tests of Alumina-UHMWPE Composites Submitted to Gamma Radiation

The energy absorption in ballistic tests of alumina-ultra high molecular weight polyethylene (UHMWPE) composites with 60, 80 and 90 wt% alumina submitted to gamma radiation doses of 25, 50 and 75 kGy was investigated. The ballistic tests were carried out at subsonic speed using a compressed air system. The results showed that the composite with 80% alumina irradiated with 50 kGy yields the best ballistic results. Gel content, Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) results showed that this composite is the one with the highest concentration of crosslinks and the lowest volume fraction of amorphous UHMWPE. Scanning electron microscopy (SEM) images of the same composite showed a high pullout, suggesting that gamma irradiation increases the adhesion between alumina and UHMWPE.


Introduction
In the beginning of the 21st century, despite the absence of international conflicts on the scale of the two World Wars, local and regional conflicts involving different tribes, ethnic groups, militias, gangs and heavily armed drug dealers have become a serious threat in several parts of the world.
Standard bulletproof personal protection vests use aramid fabric as a single layer of shielding. This protection is limited to relatively low impacts, up to 9 mm ammunition. Protection against high impact projectiles requires a multiagent system (MAS) 1 . A conventional MAS has, beside the aramid fabric, a ceramic front layer which absorbs most of the impact energy, eroding the projectile tip. However, such protection increases the cost and compromises the soldier mobility due a significant increase of vest weight. In addition, the front layer may be fragmented by the first impact, compromising its resistance to subsequent shots.
The main ceramic materials used for ballistic protection are alumina (Al 2 O 3 ), silicon carbide (SiC) and boron carbide (B 4 C). Alumina has been suggested for ballistic protection due to good physical and chemical properties. However, the low flexural strength and low fracture toughness mean that the use of pure alumina for ballistic protection may lead to catastrophic failure. Moreover, the high density, about 4 g/cm 3 , limits its use in applications where weight is crucial, such as bulletproof vests 2,3 .
According to Figueiredo et al. 4 , alumina-UHMWPE composites may yield a good compromise between high energy absorption and low density, the only problem being low adhesion between the alumina particles and the polymer matrix 5 . The adhesion may be increased by exposure of the composites to gamma radiation 6,7 , that has other favorable effects, such as increased stiffness and stability, as reported by Shafiq. et al. 8 .
On the other hand, according to Hobbs et al. 9 , high radiation doses generate microcracks that weaken the alumina component.
The purpose of this work was to investigate the properties of gamma irradiated alumina-UHMWPE in order to determine the best combination of alumina concentration and radiation dose for ballistic protection applications. The UHMWPE is used to decrease the density and increase the flexural strength, making the shield more suitable for personal protection and avoiding fracture after the first shot 10,11 . There is also an economic factor involved, since the composite is prepared at a relatively low temperature, 230 o C, while pure alumina must be prepared sintering alumina powder at high temperatures, of the order of 1400 o C, a more expensive procedure 12-15 .
Gamma Irradiation of the composites was performed using a Gammacell 220 Excel irradiator with a Co-60 source.

Ballistic tests
For the ballistic tests, an air rifle Gunpower SSS was used with a noise suppressor Padrão Armas. The projectile was a 22 gauge lead shot with an estimated mass of 3.3 g. An Air Chrony ballistic chronograph model MK3, with a precision of 0.15 m/s, was used to measure the impact speed, and a ProChrono ballistic chronograph model Pal, with a precision of 0.31 m/s, was used to measure the residual speed.
The air rifle was positioned 5 m away from the target, consisting of the composite disk attached in an aluminum frame, secured by a bench vise and aligned perpendicularly to the rifle. One ballistic chronograph was positioned 10 cm from the nozzle of the noise suppressor and another was placed 10 cm behind the target.
The energy absorbed by the target was calculated using the equation where m p is the projectile mass, v i is the impact speed and v r is the residual speed 16 .
Taking into account the fact that in the case of personal protection weight may be an important factor, one may define a figure of merit given by the following equation: where m c is the composite mass.

Gel content measurements
The gel content shows the amount of crosslinking in the samples. It was determined according to ASTM D2765 using a Soxhlet device 15 . The samples were extracted using boiling xylene for 6 h and were subsequently washed with acetone and dried at 140 o C. The percent gel content was determined using the following expression: where W o and W 1 are, respectively, the weight of sample before and after extraction.

Differential scanning calorimetry
Differential scanning calorimetry (DSC) analysis was performed with a Netzch DSC 404 F1 Pegasus calorimeter. The measurements were carried out with a heating and cooling rate of 10 o C min -1 under nitrogen (50 mL min -1 ).
The samples were heated from room temperature to 180 o C and cooled to 50 o C and then reheated to 180 o C. The melting temperature and the percent crystallinity was determined only for the first run. The percent crystallinity Xc was calculated using the following expression: where ΔH o and ΔH m are, respectively, the enthalpy of melting of crystalline polyethylene (290 Jg -1 ) and the enthalpy of melting of the sample.

X-ray diffraction
The sintered samples were characterized by X-ray Diffraction (XRD) in an X'PERT PRO PANalytical diffractometer, with monochromatic radiation (Cu Kα, λ = 1.5406 ˚A), step size of 0.05 o s -1 , time per step 150 s and 2θ between 10 o and 90 o . The XRD patterns were refined using the Rietveld method, with the help of the TOPAS Academic version 4.1 software.

SEM images
After the ballistic tests, images of composites with 80% alumina were taken in a FEI Quanta FEG 250 SEM.

Ballistic tests
All shots completely penetrated the disks. Two shots were made in each experiment and eight experiments were performed for each composition. Representative samples are shown in Figure 1 after the first shot.
No significant decrease in energy absorption was observed after the first shot.
Since the test was performed with the samples in the cast aluminum holders, lateral movement was suppressed, reducing the damage to the composite 17,18 . Table 1 shows the average values of the composite mass (m c ). projectile mass (m p ), average impact speed (v i ), average residual speed (v r ), absorption energy (E abs ) and merit factor (FM) for each composition. As expected, m c increases with increasing alumina concentration. Figure 2 shows the FM of composites with different alumina concentrations and radiation doses. One can see that the best compromise between high energy absorption and low weight was displayed by the composite with 80% alumina irradiated with 50 kGy. Table 2 shows that the gel content of UHMWPE, nonirradiated and gamma irradiated with doses of 25 kGy, 50 kGy and 75 kGy. The highest gel content was for the samples irradiated with a dose of 50 kGy, suggesting the concentration of crosslinks is maximum for this radiation dose.
The decrease of gel content for a higher radiation dose is attributed to degradation of UHMWPE, forming free radicals.

DSC
The DSC analyses were performed during the first fusion and crystallization. Table 3 shows that the samples irradiated with 50 kGy had the highest values of crystallinity, and among them the largest value was that of the sample 80/50.

X-ray diffraction
The samples with 80% alumina were investigated further due to the good ballistic performance displayed by the sample with 80% alumina irradiated with 50 kGy. The purpose was to determine why the ballistic performance increased with radiation dose up to a radiation dose of 50 kGy, but decreased when the radiation dose was increased to 75 kGy. Figure 3 shows the XRD patterns of samples A80/00, A80/25, A80/50 and A80/75. Except for a broad line at small angles attributed to amorphous UHMWPE, only the diffraction lines of α-alumina and UHMWPE are seen, showing that there was no phase transformation due to gamma irradiation.     In Figure 4, one can see that the amplitude of the broad line goes through a minimum for a radiation dose of 50 kGy. This suggests that the crystallinity of UHMWPE increases with increasing radiation dose up to a radiation dose of 50 kGy, but decreases for larger radiation doses. several cracks whose interaction creates subsequently a conical damage zone with ductile and fragile failure, similar to that observed in ceramic tiles 19 . Figure 7 show the conical damage zone in the distal face of an A80/50 sample. Figure 8 shows microcracking on the distal faces of A80 and A80/50 samples after the first impact. The main difference between Figures 8a and 8b is that there is evidence for pullout in the irradiated samples as a consequence of increased adhesion between alumina and UHMWPE, as also shown in Figure 9.  Figure 4. In the case of the UHMWPE line, there is a significant displacement to the left for the sample submitted to a radiation dose of 50 kGy. This is attributed to a relaxation due to increasing crystallinity and is consistent with the decrease in the amplitude of the broad line in Figure 4. In the case of the alumina lines, there is a shift to the right in the samples irradiated with 25 and 75 kGy. In the first case, the compression may be due to loss of water and in the second to radiation-induced microckacks.

Images of alumina powder, sample A80/00
and sample A80/50 Figure 6 is an SEM Image of the alumina powder, showing the irregular shape of the grains, which improves the stiffness of the composites.
When impact occurs, compressive shock waves propagate along the thickness of the sample and are responsible for

Conclusions
Ballistic tests were performed on gamma-irradiated alumina-UHMWPE composites. The composite with 80% alumina submitted to a radiation dose of 50 kGy was the one that had the best ballistic properties. Gel content, DSC, XRD and SEM results suggest that this is due to three effects of gamma radiation: an increased extent of crosslinking, a decrease of the volume fraction of amorphous UHMWPE and an increased adhesion between alumina and UHMWPE. The results also showed that increasing the radiation dose above 50 kGy has adverse effects, probably due to the production of a large number of microcracks that leads to a weakening of the alumina particles and to UHMWPE degradation, generating free radicals.