An experimental study of the self-healing behavior of ionomeric systems under ballistic impact tests

An experimental study of the self-healing behavior of ionomeric systems under ballistic impact tests

A. M. Grande*a, S. Coppia, L. Di Landroa, G. Salaa, C. Giacomuzzob, A. Francesconib,

M. A. Rahmanc aDept. of Aerospace Engineering, Politecnico di Milano, via La Masa 34, 20156, Milano (Italy)

bCISAS "G. Colombo", University of Padova, via Venezia 15, 35131, Padova (Italy) cDept. of Mechanical and Industrial Engineering, University of Brescia, via Valotti 9, 25123,

Brescia (Italy)

ABSTRACT

This research deals with the investigation of the self-healing behavior after ballistic damage of ethylene-methacrylic acid ionomers and theirs blends with epoxidized natural rubber (ENR). The self-healing capability was studied by ballistic puncture tests under different experimental conditions as sample thickness, bullet speed, diameter and shape. Bullet speed ranging from few hundreds meters per second to few km/s were employed. The healing efficiency was evaluated by applying a pressure gradient trough the plates and by checking for possible flow at the damage zone. A morphology analysis of the impact area was made observing all samples by scanning electron microscope.

Keywords: self-healing, ionomer, epoxidized natural rubber, ballistic impact, blend

1. INTRODUCTION Recently, the study of self-healing materials has attracted the interest of many researchers who have developed different approaches to achieve this behavior. The repair strategies designed and implemented today can be grouped mainly into three types: repair by liquid agent incorporated in the matrix, thermally induced repair carried out by a solid-phase material, and, finally, intrinsic repair by the use of ionomers1.

The first technique is based on the presence in the material of a storage system, microcapsules or microvascular system, which hosts healing, reactive substances2. When the material is damaged, cracks propagate and break the storage medium thus releasing the healing agent; usually this consists of a monomer which reacts with a catalyst dispersed in the polymer matrix, allowing the mending of the damaged component.

The second approach involves the use of special thermoset matrices. These resins, when fractured and subsequently heated, have the ability to restore the broken chemical bonds3. This technique can potentially increase the number of repair cycles using reversible covalent bonds and does not need to incorporate the healing agent in the polymeric matrix, although external intervention with a heating system is needed.

Ionomers are polymeric materials designed and developed in the early sixties, which exhibit self-mending behavior; they are currently taken into account also for new applications in the aerospace field. Among the advantages of these materials is the instant repair of the damage without the use of external intervention. However, the use is presently limited to the case of damage caused by events that pierce the component, for example, the penetration of a projectile4,5; the repair occurs only under strict conditions of temperature, speed and shape of the impacting object6-8. The ionomers are a class of polymers that contain a certain percentage (often up to 20%) of ions along the polymer molecule. These ions form aggregates that play a crucial role in defining the physical and mechanical properties of these materials. The phenomenon of self-repair occurs spontaneously without any external intervention as a result of impacts in which the energies involved are high enough to allow the impacting object to pass through the material in a very short time. The potential applications of these materials are numerous and it is therefore understandable the strong interest in the study of

*[email protected]; phone +39 02 2399-8642; fax +39 02 2399-8028;

Behavior and Mechanics of Multifunctional Materials and Composites 2012, edited by Nakhiah C. Goulbourne, Zoubeida Ounaies, Proc. of SPIE Vol. 8342, 83420U · © 2012 SPIE

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the mechanisms of repair in order to exploit and possibly produce new materials with the same capacity over extended conditions.

Self-healing behavior of ionomers has been reported in several references, in which the self-healing phenomena following high energy impact or puncture test, such as bullet penetration, was investigated4-8. The self-healing process of ionomers arises from the inherent chemical structure and morphology of these polymers, i.e. there are no added “healing agents”. The process can in principle be repeated many times and in that way it is distinctive from other systems, which usually cannot repair themselves frequently. These unique features of ionomeric systems generate remarkable opportunities for these polymers in environments prone to impact related damage, since the actual impact can itself initiate healing. Several researches have been dedicated to complete understanding of the self-healing mechanism of these materials.

The self-healing behavior is however usually presented within a limited range of ambient conditions, where the mechanical characteristics are subjected to little variations. To overcome what are the limits of the thermo-mechanical characteristics of pure ionomers highlighted in the literature6-8, it was decided to develop mixtures in order to investigate the possibility of obtaining new materials with the same self-repairing behavior, but with different mechanical properties. This technique is a common route to obtain polymer mechanical performances spanning over wider limits. In this regard, it was decided to produce blends combining pure ionomer with epoxidized natural rubber (ENR). In previous studies, the authors have investigated the self-healing response of blends of ionomers with elastomers and thermoplastic polymers as well as the effect of blending on overall thermo-physical properties of such materials9-11.

In this paper, the self-healing behavior following projectile impact of poly(ethylene-co-methacrylic acid) with acid groups partially neutralized with sodium ions (EMNa) and its blends with ENR is investigated. Different ballistic tests at low and hyper-velocity were carried out. A first phase of activity, consisting of sample preparation, definition of the puncture-testing instrumentation and projectile shape, was followed by a second phase of analysis and discussion of the self-healing results and individuation of significant parameters as thickness/bullet diameter.

The self-healing capability has been studied by ballistic puncture tests; different conditions were assessed by varying sample thickness (0.5 - 5 mm), bullet impact velocity (180 - 4000 m/s) and bullet diameter (1 - 14 mm).

At tested impact speed, pure sodium based ionomer presents self-repair ability up to a specific thickness/projectile diameter ratio. Considering blends, at low and medium impact speed (less than 1000 m/s), EMNa/ENR mixtures exhibit complete repairing just after the ballistic tests at any compositions. Hypervelocity impact tests, instead, showed that complete healing did not occur at these speeds for all blends.

After all impact tests, the healing efficiency was evaluated by applying a pressure gradient. Hole closure was tested both by following vacuum decay and by checking for possible flow of a fluid droplet placed at the damage zone with the applied pressure difference. A morphology analysis of the impact zones was made observing all samples by scanning electron microscope both in the bullet entrance and exit sides.

2. EXPERIMENTAL PART 2.1 Materials

The material used for the development of the following work is a thermoplastic ionomer known by the trade name Surlyn® (DuPont). On the market there are several grade of Surlyn®, which differ in percentage and kind of cations inside them. In this research the polymer used has was the Surlyn® 8940, characterized by a content of 5.4 mol% acid groups, of which 30% neutralized with sodium. The material was supplied courtesy of DuPont Italy in pelletized form. Density for this ionomer is 0.95 g/cm3.

The epoxidized natural rubber (ENR-50), density 1.02 g/cm3, was provided by SAN-THAP International Company Ltd, Thailand. This polymer is based on natural rubber (cis 1,4-polyisoprene) with epoxidised groups randomly distributed at 50%. Different compositions of EMNa/ENR were melt in a discontinuous mixer, (Brabender Plastograph, Germany) at 120 °C with a specific screw speed of 80 rpm for 15 minutes. Prepared blends had a composition varying from 15% up to 50% of ENR weight content. All polymers were dried in a vacuum chamber for 12 hours before samples production.



Plate specimens of variable thickness were prepared using the compression molding technique and setting the temperature of the mold at 180 °C; during cooling stage the pressure was maintained until sample removal from the mold. PTFE films were placed between the prepared materials and the mold to avoid sticking of the two parts during samples production.

2.2 Ballistic tests

A first set of ballistic puncture tests on pure ionomer and its blends with ENR was performed at Fiocchi Munizioni Ballistic Lab. by shooting 4.65x19.2 mm bullets through 2 mm thickness square plates at 23 °C in the speed range of 700-730 m/s.

The low velocity impact tests were carried out with a compressed air gun using steel balls as bullets. The diameter and the mass of the spheres varied from 2.5 to 14.5 mm and from 0.6 to 11.9 g, respectively. The bullet velocity was set up at 180 m/s. Samples thickness was explored in the range between 0.6 to 2.6 mm.

Hypervelocity impact were performed at CISAS Hypervelocity Impact Facility using a two-stage Light-Gas Gun (LGG)12. In this activity 1.5 mm aluminum spheres where used and the two bullet speed were studied, 2 and 4 km/s. Sample thickness tested were 2, 3 and 5 mm.

All specimens were observed by scanning electron microscope both in the bullet entrance and exit sides to have evidence of hole closures. SEM analyses were also performed to evaluate the morphology of the damaged surfaces of the specimens. To check for the ability of healing leakage tests were carried out; a pressure difference of 0.9 bars was initially applied by a vacuum pump in a closed chamber, sealed with the tested polymer plate. Air tightness through the hole was tested following vacuum decay and by checking for possible flow of a fluid droplet placed at the damage zone with the applied pressure difference. When the hole was healed no appreciable vacuum decay was detected within the specified time range but for non-healed samples vacuum decay was observed within a few seconds. A sketch of the experiment system and a picture of the instrumentation are showed in Figure 1.a and 1.b, respectively.

(a)

(b)

Figure 1. Sketch of the experiment system (a) and a picture of the instrumentation during a leakage test.

3. RESULTS AND DISCUSSION 3.1 Self-healing evaluation

Previous experimental activities have shown how, after ballistic impacts with 4.65x19.2 mm bullets, EMNa/ENR blends showed complete self-repairing phenomenon (Figure 5.a); as expected, also pure EMNa showed extended self-healing capability after the bullet penetration (Figure 3.a). The healing of each material was checked with air-flow test. Interestingly, all the EMNa blends containing ENR showed good maintenance of self-repairing effect even with the highest ENR content9-11.



The low velocity impact tests (180 m/s) on pure ionomer have instead identified a critical ratio between sample thickness and bullet diameter below which full repair was not observed (Table 1). Also in this cases, healing was tested applying pressure difference in the damage zone and following vacuum decay. Preliminary tests on EMNa/ENR blends have demonstrated that, at these low velocities, self-repairing capacity is well maintained for all compositions; further tests will be carried out in order to identify the critical functioning parameters of the these materials.

Regarding hypervelocity impact tests results are summarized in Table 2. At the lowest projectile velocity, all ionomer samples exhibited complete hole closure, but leakage test showed a loss of pressure of the 3 mm thickness specimen. At the higher bullet speed instead only the 3 and 5 mm thickness samples exhibited complete hole closure. Preliminary tests on EMNa/ENR blends showed that full healing did not occur at micrometeorites impact speed (Figure 3.b).

Table 1. Results of ballistic impact tests at low speed, d=diameter [mm], t=thickness [mm]. Shaded cells indicate unrepaired tested specimens.

t/d 2.35 3 5 6.34 8 10 12 14.27 0.6 0.26 0.2 0.12 0.09 0.08 0.06 0.05 0.04

1.02 0.43 0.34 0.2 0.16 0.13 0.1 0.09 0.07 1.86 0.79 0.62 0.37 0.29 0.23 0.19 0.16 0.13 2.62 1.11 0.87 0.52 0.41 0.33 0.26 0.22 0.18

Table 2. Results of ballistic impact tests at hypervelocity impact speed.

Diameter (mm)

Velocity (km/s)

Thickness (mm)

Hole closure

Repaired (leakage test)

1.5 1.93 2 Yes Yes 1.5 1.80 3 Yes No 1.5 1.64 5 Yes Yes 1.5 3.90 2 No No 1.5 4.00 3 Yes Yes 1.5 4.10 5 Yes Yes

Figure 2. Example of specimen tested at low impact speed and steel spheres used as projectiles.

3.2 Morphological analysis

Microscope analysis after ballistic tests highlighted different morphologies of the impacted areas; on the other hand, some similarities suggest that similar repair mechanisms occur. For instance, both SEM microscope observations of pure ionomer after ballistic tests with 4.65 mm bullet (Figure 3.a) and low velocity impacts (Figure 3.b) evidenced complete



Iono_Hyper0003 FL xlOO 1mm

In002

hole closure although in the first one a more extended viscous response of the material appears due to the greater energy exchanged during the impact. Either damaged areas show the characteristic striations radially distributed around the crater caused by an intense plastic deformation in ductile polymers. The ballistic tests on blends have highlighted a different behavior; in particular, the elastic response seems much more extensive as evidenced in Figure 5.a.

Morphological analysis after hypervelocity tests showed, instead, complete different conformation of the impacted sites. On the inlet side, debris of the fired aluminum sphere can be observed (Figure 4.a), nevertheless on the other side there are no residues of the projectile (Figure 4.b). The morphology of the impact area on the back part of the samples exhibits a clearly defined molten zone of approximately the same diameter of the impacted sphere. In addition, the presence of voids, caused probably by the generation of volatile substances during the impact, can be clearly observed. In the case of EMNa/ENR blends a hole of diameter approximately equal to that of the fired ball is present (Figure 5.b); no appreciable self-repairing phenomenon was observed for these materials.

(a)

(b)

Figure 3. Pure EMNa ionomer tested with 4.65 mm bullet at 700 m/s (a) and 5 mm steel ball at 180 m/s (b), sample thickness 2 mm.

(a)

(b)

Figure 4. SEM micrographs of bullet enter (a) and exit (b) zone of pure EMNa sample, 2 mm thickness, impacted at 1.9 km/s by an aluminum sphere of 1.5 mm in diameter.



(a)

(b)

Figure 5. 50% EMNa 50% ENR blend tested with 4.65 mm bullet at 700 m/s (a) and 1.5 mm aluminum sphere at 4 km/s (b), sample thickness 2 mm.

4. CONCLUSIONS The self-healing behavior of EMNa ionomer and EMNa/ENR blends after ballistic puncture tests was investigated in different experimental conditions. SEM micrographs and de-pressurized air flow tests confirmed the self-repairing behavior for tested materials. The results presented show that the self-repairing behavior is well maintained in EMNa and EMNa/ENR blends at low impact speed The EMNa ionomer shows self-repairing behavior also at hypervelocity speed, while no healing was observed for EMNa/ENR blends in the same experimental conditions.

The availability of blends with physical and mechanical properties modulated over a wide range, yet maintaining efficient self-repairing behavior, remarkably extends the potential application areas of such materials. For example, ionomer based materials may find employment in multilayer composites to obtain multifunctional structures for space application able to withstand ballistic impacts..

ACKNOWLEDGEMENTS

The sponsorship of project SMAT (ID 12775772) is gratefully acknowledged. The authors would like to thank Mrs. M.R. Pagano and Mr. M. Pardi for their contribution. The authors wish also to thank the company DuPont (Italy) for providing the material.

REFERENCES

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An experimental study of the self-healing behavior of ionomeric systems under ballistic impact tests

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