DEVELOPMENT OF POLYMERIC FOAMS FOR MITIGATION OF BLAST EFFECTS

Abstract

Polymeric syntactic foams belong to a class of composite cellular materials, which are prepared by dispersing hollow microbubbles in a suitable polymeric matrix. These cellular materials possess exceptional ability to respond against dynamic loadings. The mechanical properties of the resultant foam are strongly dependent upon the type of the hollow bubbles being incorporated, and lately, hybrid foams have garnered a lot of attention, where another component, usually reinforcing, is included in the composition. This thesis is an attempt to fabricate polymeric syntactic foams containing hollow microbubbles at varying loadings and explore their potential towards blast mitigation. The primary benefit of these foams is their tunability that can produce foams with good specific mechanical properties. Fabrication of syntactic foams requires addressing two issues related to the maximum permissible limit of microbubbles for easy fabrication of syntactic foams and the determination of optimal concentration of hollow microballoons that will favour the formation of strong yet lightweight syntactic foams. In view of this, an in-depth rheological study has been performed on epoxy–glass microbubbles foam formulations by varying the glass microbubbles loading from 10 to 70 % v/v. The studies indicate that although hexagonal close packing of microspheres allows 74 % v/v packing limit, in practice, such high loadings are not feasible, which restricts the upper limit of microbubble packing to 60 % v/v without the aid of solvents. Addition of glass microbubbles (10-60 % v/v) in epoxy resin brings about a sudden increase in the viscosity of the system, ranging from 103 m.Pa.s at 30 °C, for neat epoxy resin, to 105 m.Pa.s,(40 % v/v microbubble loading). At a microballoon loading greater than 60 % v/v, the viscosity becomes prohibitively high for the formulations to be processed by conventional stir casting techniques. Rheological studies also reveal that inclusion of glass microbubbles does not alter the curing profile as evidenced by calorimetric studies; however, there is a slight delay in the curing phenomenon. The same is also evidenced by the appearance of longer gel times during isothermal (60-100 °C) rheological studies. Epoxy-amine reaction is a classical nucleophilic substitution reaction following second order kinetics. The reaction mechanism is not altered upon inclusion of glass icrobubbles. In view of the lower density of the glass microbubbles compared to epoxy, it is highly probable that the hollow fillers drift towards the surface. In fact the digital photographs of epoxy-HGM formulations containing low loadings of glass microbubbles (10-30 % v/v) result in the formation of two separate layers: a microbubble rich phase at the top and epoxy rich phase at the bottom. However, this process require extended time periods: much longer than what is mandated by the epoxy-amine curing reaction. The mechanical properties of syntactic foams (10-70 % v/v glass microbubbles) in the quasi-static regime have also been investigated which suggests a simultaneous decrease in mechanical properties with increasing volume fraction of purposely placed voids. Our studies reveal that formulations with microbubble loading of 40-60 % v/v form stable syntactic foams with no visible evidence of layering and excellent mechanical properties. Specific compressive toughness is maximum for 40 % v/v glass microbubble formulation which indicates its better energy absorption capability compared to other formulations of syntactic foams. The strain rate sensitivity of syntactic foams is confirmed by high strain rate studies performed using Split Hopkinson pressure bar. In line with the quasi-static tests, the flow stress of these foams increases with decreasing microbubble loading. Moreover, the flow stress values increase with increasing strain rates. Shock tube testing of syntactic foams reveals that syntactic foam does not undergo any deformation even at a blast load of ~ 90 psi, unlike control aluminium sheets that underwent extensive deformation (~1700 με) at significantly low blast loads (~ 36 psi). This clearly reveals the role of syntactic foams as core materials in sandwiched configuration for blast mitigation applications. The role of different types of fillers towards improving the mechanical properties, particularly in terms of the energy absorbing ability of syntactic foams has been explored. The incorporation of two-dimensional layered fillers, i.e. molybdenum disulfide (0-0.04 % v/v), reduced graphene oxide (0-1 % v/v) and nanoclay (0-1 % v/v) on the mechanical, rheological and thermal properties of epoxy-hollow glass microbubble syntactic foams have been investigated. The dispersion of these fillers is an extremely important issue which needs to be addressed for obtaining syntactic foams with improved properties. This has been achieved by ultrasonication of these fillers in the epoxy resin for extended periods. However, due to the extremely low loadings of these fillers, their presence could not be discerned in the SEM imaging or PXRD pattern. Isothermal rheological studies were performed at 60 °C, which involved the determination of complex viscosity, storage and loss modulus with time. The studies reveal that the introduction of these layered fillers did not affect the processability of the formulations. The effect of introducing 2D fillers can be clearly seen in the quasi-static mechanical properties of the hybrid syntactic foams. There is a significant improvement in the quasi-static mechanical properties of all 2D filler-reinforced syntactic foams as compared to base foam, however, the extent of reinforcement is much more pronounced for molybdenum disulfide (0.02 % v/v) probably due to the lubricating action of the filler through a crack extension mechanism. High strain rate studies on two-dimensional fillers were also found to follow similar trend as observed during quasi-static tests. The potential of preformed elastomers as a toughening agent has also been explored for epoxy-glass microballoons syntactic foams. Poly(dimethylsiloxane) microspheres have been prepared by suspension polymerization route. The microsphere dimensions range from 58-255 µm by varying the stirring speeds (600-1000 rpm) and feed concentration (30-60 %). The replacement of glass icrobubbles with PDMS microspheres (63 µm, 3-7 % v/v) in syntactic foams (total filler volume fraction of 0.4) did not lead to any significant increase in the viscosity which indicates that the formulations can be processed in the same manner as neat syntactic foams. The extent of improvement upon introduction of elastomeric microspheres (diameter ~ 63 µm, 5 % loading) are 40 % and 185 % respectively in flexural strength and flexural toughness without any undesirable increase in foam density. In another attempt to strengthen epoxy-glass microbubbles syntactic foams, they have been reinforced with electrospun polyamide nanofibers. Nylon 6 nanofibers have been prepared by electrospinning and the operating parameters namely solution concentration and flow rate were varied to obtain fibers of requisite dimensions. These nanofibrous mats were employed for reinforcing glass microbubbles epoxy syntactic foams. The introduction of nanofibers (0.25 % v/v) brought about a marginal improvement (~ 7 %) in the compressive strength, when the direction of loading was perpendicular to the fiber axis. Nonetheless, flexural properties of the reinforced foam are significantly higher, with 75% and 62 % enhancement in flexural strength and elongation, respectively. The mechanical properties of epoxy syntactic foams can also be increased by improving the interfacial adhesion between the matrix and the hollow fillers. We hypothesised that the extent of interfacial adhesion can be strengthened by replacing glass microbubbles with epoxy microbubbles. The improved compatibility between the hollow epoxy microbubbles and epoxy matrix can have a lasting influence on the mechanical properties. In this regard, hollow epoxy microbubbles have been prepared by interfacial engineering wherein the reaction takes place only at the interface of the epoxy microcapsules. This has been made possible by varying the stoichiometric ratio of epoxy and amine hardener from 100:13 to 100:2. Monolithic epoxy microcapsules obtained using a epoxy: hardener ratio of 100:2 were found to possess a core content of 25 %. Effect of incorporation of 40-60 % v/v of epoxy microbubbles on the mechanical properties demonstrated a proportional decrease in the mechanical properties with increasing volume fraction of the microbubbles. At 40 % v/v loading, an improvement of ~ 127 % in the compressive toughness is obtained as compared to neat epoxy. However, due to the relatively higher density of epoxy microbubbles, there is an increase in density of the resultant syntactic foam. This thesis also explores the findings of polybenzoxazines glass microbubble syntactic foams for high-end applications. Thermally stable bisphenol-F based polybenzoxazines syntactic foams have been prepared by the introducing 30-60 % v/v hollow glass microbubbles. The primary objective is to establish the quasi-static mechanical response of such foams. To identify suitable processing conditions, the effect of introducing glass microbubbles on the curing profile of benzoxazine resin has been evaluated by non-isothermal calorimetric studies and rheometry. The curing profiles of syntactic foam formulations remain unperturbed and similar to that of neat polybenzoxazine. The introduction of glass microbubbles leads to an increase in the viscosity of the formulations, and the gel time, as determined by rheology, increases from ~ 3690 to ~ 3800s due to the physical hindrance posed by glass microbubbles. Polybenzoxazines possess a high char yield (~ 48 %) at a temperature of 600 °C and it increases to 83 % (60 % v/v glass microbubble loading). Polybenzoxazines exhibit a quasi-static compressive strength of 171 MPa and the addition of glass microbubbles lead to a decrease in the compressive strength to ~ 60 MPa. Maximum compressive toughness was observed at 40 % v/v of glass microbubble loading which was ~ 200 % higher compared to neat resin. The potential of polybenzoxazines as a matrix material for syntactic foams is thus evaluated.