DEVELOPMENT OF POLYMERIC FORMULATIONS FOR POTENTIAL APPLICATION IN BLAST MITIGATION

Abstract

Elastomeric coatings are being advocated as excellent candidates for retrofitting applications, particularly for blast mitigation and ballistic protection. Polyurea, an elastomer formed by the reaction of isocyanate and amine, possesses hard domains dispersed randomly within the soft domains, forming a heterogeneous landscape with a nano-segregated microstructure, with each domain exhibiting its own characteristic glass transition temperature. Commercialised in the late eighties, this relatively new entrant in the field of elastomers has received enormous attention in view of its excellent blast mitigation properties and ballistic protection. Since the blast mitigation and ballistic protection ability is decided by the hard and soft domains of polyurea, respectively, the properties needs to be tuned for a particular application through judicious choice of the raw materials as well as the processing technique. In this work, the effect of the various components of the amine resin blend, i.e. soft segment length, chain extender and crosslinker on the material properties of polyurea have been studied. To quantify the effect of soft segment length, a series of polyurea formulations was prepared by reaction of commercially available isocyanate prepolymer with polypropylene oxide based amines of varying molecular weights (230-2000g/mol) by solution casting technique. The effect of increasing the chain length of the soft segment on the mechanical properties of polyurea underboth quasi-static as well as dynamic conditions was determined. The ductility expressed as “elongation at break” was found to increase proportionally with increasing soft segment length, with a concomitant decrease in the tensile strength. All the compositions exhibited sub-ambient glass transition temperatures (Tg), however the same was found to reduce with increasing softsegment length. Time-temperature superposition principle was used to derive the master curves for the compositions. It was of particular interest to establish the effect of increasing soft-segment length on the frequency necessary to initiate the process of dynamic “rubber to glass” transition in polyurea. The frequency essential to initiate the dynamic glass transition process increased was directly proportional to the soft segment length. In view of the fast reaction kinetics, the most important concern associated with the processing of polyurea is the requirement of rapid mixing of reactants; which held back its commercialization till the late 1980s. For all practical applications, polyurea coatings are processed by using reactive spraying where, the isocyanate and amine are stored separately and are forced to react in the mixing-module, just prior to being pushed out of the orifice under pressure. However, the viscosity of isocyanate prepolymer (Side A) is significantly higher than that of the amine resin blend (Side B) even at high temperatures used during processing (~70°C). This results in stoichiometric imbalance which hinders formation of high molecular weight polymer. An experimental study into the consequences of diluting isocyanate with a zero VOC diluent (propylene carbonate) towards processing of polyurea was performed. Viscosity-Blending Index based equations were found to be more reliable towards prediction of isocyanate precursor propylene carbonate blend viscosity. Spraying of undiluted isocyanate precursor with amine formulations led to formation of “noodle-like” fibrous product, irrespective of the type of chain extender being used. Diluting isocyanate with propylene carbonate (10% v/v), led to reduction in the viscosity of the precursor from 85 to 50 mPa.s (at 70°C). This the formulations could be effectively sprayed to form polyurea films with excellent mechanical properties. Dilution with propylene carbonate also increased the “tack free” time appreciably from ~1 to ~4 s, which directly reflects on the improved processability. Rheological studies were performed to quantify the activation energy associated with the isocyanate-amine reaction for polyurea preparation. The properties of polyurea can further be tuned by the prudent choice of chain extenders. An experimental study into the consequences of chain extension of polyurea formulation with different type of chain extenders (both aromatic as well as aliphatic) was undertaken, with an aim to gain an insight into their role in improving properties. Rheological studies were performed to understand the effect of their inclusion on the processing conditions and quantification of the gelation time. Aromatic chain extenders were found to be far more reactive than their aliphatic counterparts, which lead to significantly short „gel-time‟ as quantified by rheometry. The degree of H-bonding was qualitatively established by the red-shift associated with N-H and >C=O bands in the FTIR spectra of the polymer. Introduction of either type of chain extender lead to remarked processability of polyurea formulations through spray coating technique. An optimal aromatic: aliphatic chain extender ratio was found to result in optimal H-bonding, which in turn reflected in terms of mechanical properties. Dynamic studies were performed and all formulations were found to exhibit sub-ambient Tg and is appreciably affected by the type of chain extender used. Higher aromatic: aliphatic chain extender ratio was found to result in higher storage modulus and lower dissipation potential. It was further considered of interest to study the evolution of material properties of polyurea with respect to time. In view of the fast kinetics associated with the isocyanate and amine reaction, the polymerization process occurs in a matter of seconds, which leads to extremely short „tack free‟ times. Polyurea is therefore promoted for all applications where fast curing is beneficial e.g. pipes, vehicle liners and masonry retrofits. Interestingly, although polyurea is considered as an “instantaneous setting” system, the properties of polyurea evolve with time: an issue hadnot been studied systematically. During the processing of polyurea by spray coating process, internal stresses develop within the matrix, which in turn reflected in terms of their inferior mechanical properties. Polyurea was observed to achieve its optimal properties after a finite period time, necessary for relaxation period required to expend the in-built stresses, of the order of ~15 days. In addition to the chain extender and long chain amine, the amine resin blend also contains a crosslinker, which can affects the properties of the coating substantially. To study this, a long chain trifunctional amine was introduced as a co-reactant in the resin blend, the amount of crosslinker being varied from 0-5 mole % (crosslinking density 100-500 mol/m3). The mechanical properties of spray coated polyurea films, both in quasi-static as well as dynamic conditions were determined. Physically crosslinked polyurea coatings (in the absence of chemical cross-linking) exhibited tensile strength ~ 7.4 ± 0.7 MPa and elongation of 121 ± 3.7 %. Introduction of long chain amine led to an improvement in these characteristic properties till a maxima, subsequent to which both strength and elongation decreased. Chemical cross-linking led to restraining of the segmental motions reflecting in terms of increased glass transition temperature, as evidenced by dynamic mechanical analysis and differential scanning calorimetry. The chemical resistance of polyurea also improved substantially due to crosslinking, which reflected in terms of decreased swelling ratio in different organic media. Subsequently, the efficacy of polyurea towards improving the performance of a concrete substrate towards shock and ballistic loading was demonstrated. Concrete tiles (25 cm x 25 cm x 2cm) were coated with polyurea (coating thickness 1-6 mm) and subjected to controlled blast loadings. Unreinforced concrete tiles underwent extensive fragmentation at peak pressures < 50 psi, while polyurea coated tiles could withstand much higher peak pressures. The extent of mitigation increased with the coating thickness and composites with 6 mm polyurea could withstand 87 psi. At higher loadings (Pr = 90 psi), polyurea-concrete debonding was evidenced, however the membrane arrested the fragments formed. Dynamic mechanical studies revealed that polyurea exhibited a dynamic glass transition at ~1015 Hz at room temperatures, which essentially means that polyurea remains as an elastomer under the frequency range associated with the shock tube testing (~102 Hz). Polyurea acts as a catcher system for the fragments formed due to blast loading.