Downstream polyurethane materials are mostly used in the form of sponge. Amines are often selected as catalysts for these materials. We will briefly introduce several.
First, triethylenediamine (TEDA), referred to as TEDA, is a solid at room temperature, easily hygroscopic, soluble in most organic solvents, catalyzes the reaction of NCO with water in polyurethane, and electron rich tertiary amine compounds can also catalyze the curing of epoxy resin
Because of the saturation of ethyl or the role of electron propulsion, the outer orbit of amino nitrogen is easily occupied by electrons, while ethyl is located in the middle of two nitrogen, making the steric hindrance very small, so it is easier to contact and participate in the reaction; The chemical reaction of polyurethane is mainly the reaction between NCO and active hydrogen. The reaction of NCO with hydroxyl was often called gel reaction, that is, the reaction between isocyanate and polyether. The reaction of isocyanate with water will produce carbon dioxide gas, so the reaction of isocyanate with water is called foaming reaction; In elastomers or solid polyurethane materials, foaming reactions need to be avoided. In sponge and other foam materials, only when the gel and foaming reaction reach an appropriate balance point, can the ideal foam materials be made. Tertiary amine catalysts such as TEDA undoubtedly focus more on the foaming application of polyurethane, while metal catalysts are mostly used for gel reaction. Metals are mostly alkaline. Although the triethylenediamine we mentioned is not a strong base, it also has a very obvious catalytic effect on NCO and active hydrogen. However, why can't metals and tertiary amines be substituted for each other in specific downstream applications? My personal view is still focused on the effective contact or collision in the reaction process. Compared with the reaction between water and NCO, the catalytic rate or reaction conversion of tertiary amine and metal tin or organic bismuth in the early stage should be the same, that is, under the catalysis of these two catalysts, the rate of monohydrogen in water to carbamate is the same. NCO structure is nitrogen linked benzene ring, and nitrogen, oxygen and intermediate carbon atoms form conjugate resonance double bond. Oxygen first gets a hydrogen in water. The resonance of NCO is destroyed. The hydrogen in water is occupied by oxygen first, and then rearranges into nitrogen in the molecule to form carbamate; In the early stage of this process, the catalytic rates of the two are almost the same. With the reduction of the concentration of free hydrogen atoms in the reaction process, the molecular structure of amine catalysts is more likely to be compatible with or in contact with these transition states of NCO. However, NCO that has obtained hydrogen atoms has not been rearranged into stable carbamates, and water molecules that have lost one hydrogen have not yet reacted with NCO to form complexes or rearranged into carbamates. Due to the polymerization of amino compounds, in the subsequent reaction process, more NCOs only choose two hydrogen atoms of water together, so more isocyanate intermediates are not aniline for the time being. The oxygen of water and NCO appears in the form of carbon dioxide, which promotes foaming and also appears in the form of gel; However, due to their different structures and large volumes, metals are difficult to induce aniline, or their probability is relatively lower than that of tertiary amines.
Triethylenediamine itself is a very difficult solid to use. Now we are familiar with A33 catalyst. As long as it is polyurethane, everyone has heard of it. A33 is a solution of triethylenediamine, which overcomes the problem of inconvenient use of solid. TEDA is usually dissolved in small alcohol solvents such as propylene glycol, ethylene glycol, diethylene glycol or dipropylene glycol. Generally speaking, A33 is a triethylenediamine solution with a mass fraction of 33% of dipropylene glycol. Some manufacturers of composite materials will also prepare according to their own requirements. For example, they want to reduce the reaction speed, add macromolecular solvent or polar solvent, and some mix several solvents to improve compatibility and fluidity.
