Dissolution of epoxy thermosets via mild alcoholysis: the mechanism and kinetics study

Thermoset dissolution based on degradable bond or exchange reaction has been recently utilized to achieve thermosetting polymer dissolution and recycling. In this paper, an industrial grade epoxy thermoset was utilized as a model system to demonstrate the thermoset dissolution via solvent assisted transesterification (or alcoholysis) with high efficiency under mild conditions. The anhydride–cured epoxy thermoset was depolymerized by selective ester bond cleavage in 1,5,7-triazabicyclo[4,4,0]dec-5-ene (TBD)–alcohol solution below 180 °C at ordinary pressure in less than two hours. The epoxy dissolution proceeded in a surface erosion mode via transesterification that was coupled with catalyst–alcohol diffusion. Based on this observation, a surface layer model containing three layers, namely the gel layer, solid swollen layer and pure polymer layer was used to analyze the thermoset dissolution kinetics. The epoxy dissolution kinetics was derived from the surface layer model, which could be used to predict the dissolution rate during the diffusion-rate-controlled dissolution process well. The results show that alcohols with larger diffusivity and better solubility lead to a higher alcohol/catalyst concentration in the gel layer and promote faster erosion and dissolution of epoxy. This is the first work to show that it is possible to depolymerize industrial epoxy using the principle of dynamic bonds with fast dissolution rate at mild temperature under ordinary pressure.

The experimental data of swelling within 400 min was fitted by Fick's second law to obtain D.

Hansen solubility parameters (HSP) analysis
The solubility of solvent to epoxy can be evaluated by Hasan solubility parameters (HSP) 1 . The distance parameter between the solvent and the material is calculated according to the follow equation: (S1) where R a is a modified difference between the HSP for a solvent and polymer, δ D , δ P , and δ H are the contributions from nonpolar (dispersion) forces (D), permanent-dipole forces (P) and hydrogen-bonding (H) effects, respectively. The ratio between R a and R o (the sphere of interaction for the materials) is called the relative energy difference (RED).
Good solvents are found within the sphere of affinity which have a RED number less than 1.0. Solvents giving higher RED numbers indicate lower solubility. Based on the Flory-Huggins model, the relationship between the Flory-Huggins interaction parameter (χ) and the HSP solubility parameter is described by the following equation 2 : where β is a constant value (=0.6), V 1 is the molar volume of solvent, R is the gas constant, and T is the absolute temperature.

Dissolution kinetics based on surface layer model
For solid-state kinetic analysis, the mass loss or the conversion fraction (α) can be defined as: where m 0 is the initial weight, m t is the weight at time t.
Scheme S1. The side length of a cubic sample (with an original size of a) changes x. For a cubic sample (side length of a) with the reaction depth of x, α is expressed as： (S5) 3 3 The new surface with 6 square faces has the total area of (S7) where k n is the rate constant of reaction order n. If the dissolution rate is controlled by the resulting reaction interface toward the center of the sample, n is 0. Eq. S8 is simplified as: The integration of Eq. S9 gives the following equation: Scheme S2. The transesterification reaction between ester and alcohol catalyzed by catalyst at elevated temperature.
Based on the reaction kinetics, the transesterification rate is proportional to the hydroxyl concentration, ester concentration and ER rate constant (k ER ). If alcohol is in excess, the reverse reaction can be ignored and the total reaction rate can be expressed: where [C=O] is the concentration of ester group, [OH] is the concentration of hydroxyl group, and k ER is the ER rate constant. For the proposed surface layer model with a thin reactive layer (δ), we can express the solidstate reaction kinetics in a homogeneous transesterification reaction kinetics: (S12) 1 1 where A is the surface area at time t, η is functionality of hydroxyl group in alcohol, φ 1 is the solvent fraction in the swallowing layer after equilibrium, V 2 is molar volume of polymer ((1φ 1 )/V 2 : the ester concentration of swollen polymer, V 1 is molar volume of alcohol (φ 1 / V 1 : the hydroxyl group concentration in the swollen polymer), V surf is the total surface volume within the reactive layer of δ. Based on Fick diffusion law, the diffusion distance is proportional to the square-root of diffusion coefficient D 4 : (S14) We can obtain the dissolving rate by the following equation: Generally, the temperature dependent transesterification kinetics can be described using Arrhenius equation: The mutual-diffusion coefficient (D) of epoxy-solvent can be expressed in the following equation 5-6 : (S17) where D 1 is the solvent self-diffusion coefficient, φ 1 is the solvent volume fraction and χ is Flory-Huggins solubility parameter, ΔE ER is the ER activation energy, ΔE diff is the diffusion activation energy.
Substituting Equation S16 and S18 into Equation S15, and solve the differential equation, we have: (S18) 3