Elsevier

Composites Part B: Engineering

Volume 108, 1 January 2017, Pages 143-152
Composites Part B: Engineering

Nanomechanical and anti-stabbing properties of Kolon fabric composites reinforced with hybrid nanoparticles

https://doi.org/10.1016/j.compositesb.2016.09.095Get rights and content

Abstract

In this study carbon nanotubes (CNT) and silica nanoparticles (SiO2) were added to enhance the dynamic mechanical and anti-stabbing properties of the polyurethane/p-aramid multiaxial fabric (Kolon fabric) composites for ballistic protection. The SiO2 were modified (mSiO2) and CNT were oxidised (o-CNT) in order to fabricate o-CNT/mSiO2 hybrid nanoparticles. Various kinds of nanoreinforcement particles were added into poly (vinyl butyral) (PVB)/ethanol solution. The results revealed that the films PVB/o-MWCNT/mSiO2 and PVB/o-SWCNT/mSiO2 yielded 101% and 141% improvement in indentation hardness together with 89% and 117% improvement in reduced elastic modulus, respectively, compared to the neat PVB film. The experiments demonstrated that the Kolon/PVB/o-SWCNT/mSiO2 fabric sample yielded 35% improvement in absorbed energy and 50% improvement in the storage modulus compared to the Kolon/PVB sample.

Introduction

Woven or multiaxial fabrics produced from para-aramid fibres (aromatic polyamide) are efficiently used for soft body armour in ballistic protection and as lightweight vehicle-armour structures. Para-aramid fibres (Kevlar, Twaron, Kolon) are high performance polymeric fibres with high tenacity, high strength to weight ratio, high thermal stability, lightness, flexibility and high impact resistance. They also have their purpose in textiles for fireproof and bulletproof protective clothing, in tyre cords and in high strength ropes. There are other types of polymeric fibres with extraordinary high stiffness beside para-aramid fibres. These kinds of fibres include ultra high molecular weight polyethylene (Dyneema, Spectra, Dacron), PIPD fibres (M5) and PBO fibres (Zylon) [1], [2]. Kevlar® fibre was established by Dupont in 1965 and is the registered trademark for para-aramid (p-aramid) fibres. Kevlar® fibres are made from the aromatic polymer poly-paraphenylene terephthalamide and consist of long molecular chains. The high degree of alignment of the molecular chains provides high tensile strength, high modulus, low elongation at break and high chemical resistance [3]. Cao et al. separated aramid microfibres from commercial Kevlar yarn and observed them under scanning electron microscope. It was determined that the diameter of the individual microfibre was around 14 μm. The surface of an aramid microfibre is almost chemically inert providing a limited number of reactive sites since it consists of closely packed macromolecular chains inside [4]. Among high performance polyaramid fibres are Heracron® fibres from the Korean company Kolon Inc. They possess high strength, thermal and chemical resistance and thus are used in ballistics [5].

In addition to the mechanical properties of the fibres, the energy absorption capacity of the fabric armour also depends on its weaving structure, areal density, surface treatment of yarns and a number of fabric layers. The ballistic resistance of a fabric further depends on the factors which are not connected to the properties of the fabric. These factors, among others, include impact velocity, impact angle and a projectile form [1]. Due to the crimp in woven fabrics, load tension affects in the fabric plane along with the vertical direction to the fabric plane. The load in the vertical direction to the fabric plane affects towards the back of the panel and makes back face deformation. This effect can be diminished by using the unidirectional or multiaxial fabric structures. Interweaving does not exist in these fabrics and the yarns are fastened together with the assistance of a thermoplastic resin at an elevated temperature and pressure. Therefore, the biggest part of impact stress is propagated on the fabric plane, while less part is transferred to the back of the fabric layers. Thus, a smaller back face deformation is made [6].

Poly(vinyl butyral) (PVB) is a flexible and industrially significant polymer, which is produced from condensation of poly(vinyl alcohol) (PVA) with n-butyraldehyde in an acid environment. This polymer possesses high impact strength and impact energy absorption at low temperatures and excellent adhesive properties with a variety of materials (like glass, metals and plastics) [7]. PVB is a polymer with outstanding technical properties. Among these properties, the most remarkable are the following ones: film-forming properties, good water resistance, very good compatibility with organic solvents and crosslink ability with epoxides, phenols and isocyanates. This polymer is applied as a film for laminated safety glass, binder for ceramics and metal powders. It is also used in thermoplastic applications [8]. For example, a specially designed thermoplastic is a Twaron PVB prepreg. When it is exposed to heat and pressure, it can be formed for the body armour: helmets, ballistic vests and vehicle protection [9]. PVB based nanocomposites are generally used in producing thin films which are applied in automobile and aerospace industries.

The addition of nanoparticles to the polyvinyl butyral matrix improves their mechanical properties. Surface modified silica particles are used as reinforcement to enhance the interfacial bonding between the polymer matrix and the nanoparticles, which considerably improve the mechanical properties of the composites. The homogeneous distribution of the silica particles can be obtained by their organic modification which create interfacial interactions between the particles and the polymer matrix [10]. Generally, nanoscale reinforcement in composite laminates develops the fibre-matrix interface and reinforces the matrix toughening properties with their minimal weight. Small sized rigid nanofillers are useful in improving the toughness and the stiffness of composites at the same time [11].

Our previous work reported that the addition of nanoscale ceramic reinforcement to polymer matrices enhances the mechanical properties in the body armour [12]. Nanosilica is among these kinds of reinforcement. Since silica nanoparticles tend to agglomerate, silane coupling agents are used for modifying their surface [13]. The dispersion and deagglomeration of the silica particles and also the formation of the chemical bonds among them and organic constituents are achieved as a result of the modification [12].

Another generally used reinforcement in the body and vehicle armours are carbon nanotubes. They are characterised by their light-weight and excellent energy absorption capacity [14], [15]. The basic problem of CNT is their insolubility in either water or organic solvents. Thus, the dispersion of CNT could be improved by a simple method based on a chemical process. This method consists of grafting carboxylic acid-functionalized carbon nanotubes (CNTsingle bondCOOH) onto an amino-functionalized surface of silica particles (SiO2single bondNH) [16]. In the study, CNTsingle bondCOOH are prepared by using a mixture of concentrated nitric and sulfuric acids, while SiO2single bondNH2 are produced by a silanization with γ-aminopropyltriethoxysilane (AMEO silane).

The purpose of this study was designing a new kind of polyurethane/p-aramid multiaxial fabric composites with improved mechanical properties which are used for ballistic protection. It was achieved by the impregnation of p-aramid microfibres with PVB/ethanol solution which contained different kinds of nanoreinforcement such as silica nanoparticles and carbon nanotubes. Among these nanoreinforcement particles, o-CNT/mSiO2 hybrid nanoparticles presented the innovation in the thermal and mechanical properties of the films and the fabrics that contained them. It turned out that the greatest improvements of these properties were achieved by the samples with the hybrid nanoparticles. The mutual bonding of all the components was achieved by introduction of PVB whereas hydroxyl and ester groups of this polymer reacted with amide groups of p-aramid fibre and, simultaneously, with the functional groups of well dispersed nanoreinforcement in the PVB/ethanol solution.

Section snippets

Materials

A polymer powder poly(vinyl butyral) (Mowital B60H, Kuraray Specialities Europe) and absolute ethanol (Zorka Pharma, Šabac) were used for preparing the PVB solution (10 wt%). The silica nanoparticles were with an average particle diameter of about 7 nm (Evonik-Degussa, Aerosil 380). The multiwalled carbon nanotubes (MWCNT), Cheap Tubes Inc., USA, were added into the PVB solution. The outer diameter of the MWCNT was 13–18 nm, while their length was in the range 3–30 μm. Oxidised single walled

Results and discussion

For the synthesis of o-CNT/mSiO2 hybrid nanoparticles, the separate functionalization of SiO2 nanoparticles and CNT is needed. In the first step, the appropriate modification of silica nanoparticles by AMEO silane was accomplished with the hydrolysis of silane by the loss of alkoxy groups and their chemical reaction with the hydroxyl groups at the silica surface. Then the silane was grafted onto the silica surface, providing free amino groups which could be reactive sites for chemical reaction

Conclusion

In this study a new kind of polyurethane/p-aramid multiaxial fabric composites with improved dynamic mechanical and anti-stabbing properties for ballistic protection were tested. Carbon nanotubes (CNT) and silica nanoparticles (SiO2) were added to enhance the properties of the composite Kolon fabrics. The SiO2 were modified (mSiO2) and CNT were oxidised (o-CNT) in order to fabricate o-CNT/mSiO2 hybrid nanoparticles. The results of this study demonstrated a successful synthesis of o-CNT/mSiO2

Acknowledgements

The authors wish to acknowledge the financial support from the Ministry of Education, Science and Technological Development of the Republic of Serbia through Projects Nos. TR 34011 and III 45019.

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