PMU encapsulation is a well-established technique to protect volatile components in durable microcapsules that are thermally stable and insoluble.
What is PMU Encapsulation?
PMU (PolyoxyMethylene Urea) encapsulation, also known as urea-formaldehyde and urea-melamine-formaldehyde encapsulation, is an emulsion technique used to produce durable core-shell microcapsules with good retention of volatile components. This technique is widely used for the encapsulation of fragrances, cosmetic oils and industrial chemicals. PMU encapsulation is ideal for protecting hydrophobic liquids (i.e., oils) and is called for when capsules must be very small (10-70 µm), thermally stable and have insoluble shells.
PMU encapsulation is done by emulsifying the material to be encapsulated in an aqueous solution and then depositing a tough polymer coating on the droplet. The core material must be mostly insoluble and fairly non-reactive with water, oils and other hydrophobic liquids, as well as some solids and powdered materials. AVEKA has the expertise to design and formulate PMU capsules in our laboratory and then scale to batch production.
The capsule size and shell strength can be varied by controlling process parameters, including:
- Mixing speed
- Reaction time
- Aqueous phase components
- Shell components
- Core materials and properties (density, viscosity, physical chemistry, reactive chemistry)
Typical Core Materials
Hydrophobic materials with melting points below 50°C, such as mineral oil, fragrance oils, waxes and some industrial chemicals, are often encapsulated with PMU. Any liquid with slight miscibility in water, low viscosity and a pH near neutral can in principle be used as a core with this method. Mixtures of hydrophobic liquids are often encapsulated together. In addition, some solid particulate material can be encapsulated with PMU, as long as it is stable in water at low (acidic) pH values.
The capsule shell is a co-polymer of urea and formaldehyde or a co-polymer of urea, melamine and formaldehyde. While formaldehyde is used in the process, the amount of free formaldehyde remaining after the capsules have been made and dried is extremely small.
PMU capsules are true core-shell microcapsules. Depending on the material, core loading can be up to 80 wt% of the capsule. Capsule size can be varied between 10 and 70 µm.
Release Mechanism and Uses
- The core material may or may not be released slowly by diffusion. Depending upon the shell strength used, some fragrances may be slowly released over days when open to the air, while some fragrances can be protected longer than a year.
- The core materials may be released in a burst when the capsules are broken by pressure. Some capsules may be broken by squeezing between thumb and finger or by scratching PMU capsules that have been printed on a page, as in scratch and sniff applications. Other capsules can be designed to release at a higher pressure by adjusting the shell strength.
- The core materials may or may not be released when the capsules are heated. Some phase change core materials such as waxes simply melt upon heating to mild temperatures (≤ 100°C) and then re-solidify upon cooling to room temperature. Some core materials may be released when the capsules are heated above 250°C.
- Batch processing
- Laboratory scale, 100 g to 1 kg/batch
- Production scale, up to 25 kg/batch
- Is the core material(s) insoluble in water?
- Does the core material react with water at pH 2?
- What is the melting point of the core material(s)?
- What are the density and viscosity of the core material(s)?
- What is the target capsule size?
- What is the desired release mechanism?
- How strong does the shell need to be?
- Will the final product be a slurry or a dried powder?