On the Para/Ortho Reactivity of Isocyanate Groups during the Carbamation of Cellulose Nanocrystals Using 2,4-Toluene Diisocyanate

Abstract

2,4-toluene diisocyanate (TDI) has been commonly used to bind molecules and polymers onto the surface of cellulose nanocrystals (CNCs). Such a process usually involves two steps: (1) the more reactive para-isocyanates (p-NCOs) of TDI are reacted with the surface hydroxyl groups of CNCs then (2) the ortho-isocyanates (o-NCOs) are reacted with certain desired molecules. During the first reaction, an ideal para/ortho selectivity could be impossible to achieve, as o-NCOs are not fully unreactive. Therefore, there is a need to better understand the reaction between CNCs and TDI towards a maximum para/ortho selectivity. For that goal, CNCs were reacted with TDI under varying temperatures (35–75 °C) and TDI/CNCs molar ratios (1–5). The amount of the reacted TDI was estimated using elemental analysis while the free o-NCO groups were quantified following the hydrolysis method of Abushammala. The results showed that temperature had a negative impact on para/ortho selectivity while TDI/CNCs molar ratio improved it. A maximum selectivity of 93% was achieved using a temperature of 35 °C and a molar ratio of 3. This is a three-fold improvement to that using the traditional reaction conditions (75 °C and molar ratio of 1).

Nano-Brushes of Alcohols Grafted onto Cellulose Nanocrystals for Reinforcing Poly(Butylene Succinate): Impact of Alcohol Chain Length on Interfacial Adhesion

“…7.5% of their total hydroxyl groups available on the surface for a possible chemical modification [22]. The author has also optimized the reaction of these CNCs with TDI towards reacting all surface hydroxyl groups selectively with the para isocyanate of TDI [23]. It was evident that the reaction temperature had a negative impact on the para/ortho selectivity of the isocyanate groups while the molar ratio of TDI/CNCs improved it.…”

“…Overall, a maximum para/ortho selectivity was obtained using 35 • C and a molar ratio of 3. Under these reaction conditions (which are used in this paper), all of the surface hydroxyls of the CNCs are reacted with TDI and 93% of the ortho-isocyanates are available for the following reaction such as the grafting of alcohols in this paper [23]. It is important to mention that the reaction of TDI with CNCs was also optimized to make sure that the reaction does not take place inside the CNCs.…”

“…The CNCs were reacted with TDI following the method of Habibi and Dufresne after minor modifications [20] and using the optimum conditions recommended by Abushammala [23]. Of the 10.4% CNC suspension (equivalent to 1.0 g of dried CNCs (6.2 mmol)), 9.6 g was solvent-exchanged from water to anhydrous acetone using a washing/precipitation procedure (three times), then to anhydrous toluene using the same procedure (twice).…”

“…Since then, the reaction has more often been used to graft functional molecules and polymers onto nanocellulose surfaces [21]. The reaction was only recently optimized to react all the CNC surface hydroxyls with TDI molecules, most of which have their orthoisocyanates available for a further reaction with certain functional molecules [22,23].…”

Self-Assembly of Amphiphilic Linear−Dendritic Carbosilane Block Surfactant for Waterborne Polyurethane Coating

“…(1) (1) Since polyisocyanate is very active, some side reactions take place during the mixing of base and hardener (see Equations (2)-(4)). (2) (3) (4) It is worth noting that the side reaction between polyisocyanate and water, originating from the base and moisture, produces carbon dioxide gas [13]. This will cause defects, because if the applied wet film is too thick, the carbon dioxide gas, which cannot escape, will cause bubbles to form.…”