Liquid products utilizing microencapsulation can exhibit unique and desirable performance characteristics by modifying the availability of the contents in the microcapsule core. The core, surrounded by a shell wall, can contain fragrances, active ingredients, etc. By managing the onset or duration of release of core ingredients, product ease-of-use or effectiveness can be competitive advantages. The focus here is a water-based final product.
Why polyurea microcapsules?
A microencapsulation process is an investment in know-how and equipment. To justify the investment, this process must be efficient and reproducible in delivering the expected performance and differentiation. The choice of a polyurea-based shell wall can provide certain advantages in efficiency and reproducibility.
What is interfacial polymerization?
The word “interfacial” implies there are multiple phases before the microcapsules are made. Many products are known to contain multiple phases, e.g., emulsions. Here, it is necessary that the core ingredients are each relatively hydrophobic. To synthesize a polyurea shell wall by interfacial polymerization, typically an emulsion is first formed, commonly an oil-in-water emulsion. The oil phase droplets (hydrophobic discontinuous phase) contain a dissolved reactive isocyanate, as well as fragrances, active ingredients, etc. The water phase (continuous phase, surrounds the oil droplets) contains a reactive amine, as well as other inert ingredients to provide stability and convenience in the product. At the interface of the water phase and the oil phase droplets, isocyanate and amine react, making a polyurea shell wall around the droplet core.
Unit Operation Controls
In this case, the fundamental Unit Operations are:
1. Formulating the water phase
2. Formulating the oil phase
3. Making the emulsion by combining the water phase and the oil phase
4. Synthesizing the polyurea shell
5. Adjusting the microencapsulated formulation with additional ingredients
For Operation 1 (formulating the water phase), a key decision is the selection of the reactive amine. Candidates can affect the onset and duration of release for the core contents. Also, there can be no ingredients which interfere with the synthesis of the desired polyurea shell wall structure.
For Operation 2 (formulating the oil phase), a key decision is selection of the reactive isocyanate. Again, candidates can affect the onset and duration of release of the core contents, and, there can be no ingredients which interfere with the synthesis of the desired polyurea shell wall structure. Contained fragrances, active ingredients, etc., cannot interfere with polyurea shell wall synthesis.
For Operation 3 (making the oilin-water emulsion), emulsion droplets should be in the range of 1-10 microns. This is achieved by a combination of surfactants and/or mixing energy.
For Operation 4 (synthesizing the polyurea shell), the initial shell wall will be fragile and needs to quickly achieve a final shell wall of sufficient mechanical strength. Also, there is an expectation that the oil phase droplet of the emulsion is near-quantitatively contained within the shell wall. Control of temperature and mixing energy are key parameters for maximizing the speed and thoroughness in achieving the final shell wall.
For Operation 5 (adjusting), this step is to achieve certain specifications, maximize shelf-life, and promote a positive user-experience by the customer.
Given the implications of Operations 1-5 and the required investment for a microencapsulated product, an early cross-functional review is necessary before proceeding.