Messi Biology states that methyl methacrylate (MMA) is a monomer that readily undergoes free-radical polymerization. It is prone to self-polymerization during storage, transportation, and processing, leading to thermal runaway, increased viscosity, and even explosive polymerization accidents. Therefore, in industrial production, a certain amount of inhibitor or stabilizer is often added to delay the onset of the polymerization reaction.
Magnesium oxide (MgO) is a commonly used inorganic alkaline additive. Due to its strong surface alkalinity, good thermal stability, and moderate chemical inertness, it plays an important stabilizing and regulatory role in the MMA system.
1. Absorption and Neutralization of Acidic Impurities
- The MMA monomer may contain trace amounts of methacrylic acid or other acidic substances, which can catalyze the thermal polymerization of MMA.
- Magnesium oxide is alkaline and can effectively absorb these acidic impurities, neutralizing their catalytic activity and inhibiting the self-polymerization reaction.
2. Trapping Free Radicals to Reduce the Rate of Self-Polymerization
- In the MMA system, free radicals are the source of polymerization initiation.
- The surface of magnesium oxide has unsaturated active centers that can adsorb free radicals or the decomposition products of initiators, thereby reducing the concentration of free radicals in the system and delaying the polymerization reaction.
3. Reacting with Initiators (e.g., Peroxides) to Form a Slow-Release System
- In two-component curing systems (such as the benzoyl peroxide-BPO and amine accelerator system), magnesium oxide can adsorb the initiator or its decomposition products, allowing for their slow release, thus achieving a smoother and more controllable curing process.
4. Improving Thermal Stability and Safety
- The addition of magnesium oxide can absorb a certain amount of reaction heat, delaying the peak of the polymerization exotherm and preventing “thermal explosive polymerization.”
- At the same time, as an inorganic material, it does not participate in the reaction nor release gas, enhancing the thermal stability and safety of the system.
5. Regulating Initial Viscosity and Processing Performance
- An appropriate amount of magnesium oxide helps to control the initial viscosity of MMA during storage and use.
- It can provide a better processing window for the impregnation and spreading of composite materials such as MMA-glass fiber and MMA-stone powder.