Ureolytically Induced Calcium Carbonate Composites as Bioadhesives

Synthetic adhesives, which currently dominate the adhesives market, are generally petroleum-based and commonly release volatile organic compounds (VOCs) during production or use. Indoor accumulation of VOCs can have detrimental human health effects, and outdoor release of VOCs can have adverse environmental effects (<i>e.g.,</i> photochemical smog). Therefore, there is a need for alternative, more sustainable, natural, and natural-synthetic hybrid materials as adhesives. Ureolytically induced calcium carbonate composites (UICCs) are a new design space for sustainable, and low VOC natural adhesives. UICCs are currently used for a broad array of applications, including soil stabilization, concrete remediation, creating subsurface barriers, etc. The aim of this work is to investigate UICC adhesive shear strength and environmental durability.<br/>UICCs are produced by a reaction driven by ureolytic bacteria (<i>e.g</i><i>., Sporosarcina pasteurii</i>) or by crude urease enzyme preparations (<i>e.g.</i>, from Jack bean meal). Urea hydrolysis generates carbonate ions, which can form calcium carbonate in the presence of dissolved calcium. The composite adhesive is formed by aggregation of the urease source (bacteria or extracted enzyme), organics, and calcium carbonate precipitates. To test the adhesivity of the UICC adhesives, a modified ASTM D1002.10 method was used. The adhesive was applied to glass and stainless-steel coupons in single lap joint mode, cured, and then tested in tensile extension to measure its shear strength. The shear strength of the adhesives was optimized for glass and stainless-steel surfaces by varying the type of additive (guar gum and soy protein isolate), the concentration of the urease source, and the concentration of calcium ions. UICC shear strength was also evaluated after exposure to various temperatures (-20°C, 25°, 100°C, and 300°C) and relative humidities (50%, 80%, and immersed in deionized water).<br/>The highest shear strength achieved for the UICCs was 2MPa, as compared to the UICC free formulation at 0.2MPa. Image-based surface coverage analysis indicated that the majority of the samples exhibited a mixed failure (both adhesive failure at the composite-surface interface as well as cohesive failure within the composite). Temperature and humidity testing showed that the shear strength of the UICC composite increased by 41% at -20°C, and by 16% at 50% RH. Up to 75% of the initial shear strength was retained at 100°C. This data demonstrates the potential of UICCs for applications as bioadhesives on a range of surfaces and for common environmental conditions.