DEVELOPMENT OF VEGETABLE OIL BASED POLYURETHANE ADHESIVES

Abstract

Polyurethane (PU) adhesives are well known for their properties such as excellent adhesion, flexibility, good performance at low-temperature conditions and cure speeds. These are the most adaptable product known in adhesives market due to vast diversity among the choice of the raw materials, which, gives the liberty to user to adjust the properties of the end product. In the recent years, researchers have focused their attention on the synthesis of polyurethane derived from various bio-based resources due to major hitches like depletion of crude oil stock in world, its inflated price and most importantly the environmental concerns. The present research is conducted with an objective to develop polyurethane adhesive by the use of vegetable oils, owing to their low cost, easy availability and nontoxic nature. The experimental work was primarily focused on exploiting the reactivity of unsaturated vegetable oils to prepare a set of polyols, which are one of the important raw material in the production of PU. These polyols were further utilized for the synthesis of polyurethane adhesives by using different synthetic methodologies. Polyether ester polyols were prepared from epoxidation of transesterified canola oil and polyester polyol was prepared by the transesterification of castor oil. The modified polyols were reacted with different types of isocyanate like Methylenediphenyldiisocyanate (MDI), Toulenediisocyanate (TDI), Isophoronediisocyanate (IPDI), Hexamethylenediisocyanate (HMDI) etc. for synthesis of polyurethane adhesives. As synthesizing vegetable oil based polyols covers only one part for sustainable PU synthesis so to eliminate the toxicity of diisocyanates, non-isocyante route was choosen as a worthy alternatives. For this, 5,10,15-tris(pentaflourophenyl)corrole was utilized as novel catalyst for coupling iv reaction between epoxidised canola oil and CO2 (gaseous) to introduce cyclic carbonate moieties in the oil and the carbonated product was further utilized to obtain Non-isocyanate PU by curing with different diamines like EDA(1,2-ethane diamine),HMDA(1,6-hexanediamine),IPDA(Isophoronediamine),PPDA(p-phenylene diamine). The products obtained in these studies were characterized by nuclear magnetic resonance and FTIR spectroscopy for structural analysis and to determine the extent of reaction. TGA and DSC analysis was done to study the thermal stability of prepared adhesive. Influence of various factors such as the NCO/OH molar ratio, hydroxyl value of polyols and the amount of reinforcing fillers on the properties of adhesive were extensively studied. It has been observed that the addition of TiO2 in castor oil based PU adhesive led to a remarkable enhancement in its mechanical and chemical resistance, an increase in Tg value and the adhesion. The adhesive with a mole ratio of NCO/OH equaling to 1.2, filled with 3 wt% TiO2, was found to be much better than the commercial adhesive used for bonding wood, when tested for single lap shear strength in various environmental conditions. Further, the studies conducted to examine the influence of the structurally different diisocyanates on the properties of PU adhesives had shown considerable variations in thermal stability, adhesion strength and environmental resistance. The synthesized adhesive showed much better performance as compared to commercially available adhesive formulation under different conditions of testing. The non-isocyanate PU obtained with the use of 5,10,15-tris(pentaflourophenyl)corrole manganese(III) complex as catalyst had indicated a 1/4th of the reduction in reaction time as compared to the previously reported catalysts in literature studies and the synthesized non-isocyanate PU showed good thermal stability upto 2000C and a tensile strength of upto 8 MPa.