Sophisticated applications require high performance materials. Acrylic bead polymers are very often chosen due to their unique size and shape. However, the optimum composition of an acrylic bead polymer will vary drastically, depending on the application and the required characteristics.
There are a number of factors that determine the physical and chemical properties of acrylic beads. Three, however, stand out: molecular weight, the residual initiator and the particle size distribution. These factors can be tightly controlled with the right expertise and technology, enabling manufacturers to optimally develop unique polymers which provide superior performance within testing applications.
What controls particle size distribution?
Particle sizes are determined by a combination of chemistry and post-processing. The size of monomer droplets has a significant impact on the final size and distribution of particles within the polymer bead.
The chemistry behind particle sized distribution
The concentration and strength of the granulating agent is an important consideration. This has a direct impact on the Gibbs free energy curve which in turn determines the feasibility of the reaction and thus the droplet configuration.
The formation of monomer droplets is also affected by how the reaction mixture is aggregated during the polymerisation. Simply, greater agitation leads to smaller monomer droplets and a smaller particle size. However, what is equally important is the fluid mechanics exhibited in the reactor vessel. Fluid mechanics are influenced by viscosity (as described by Bernoulli’s principle), geometry of the vessel and agitation amongst many other factors.
Post-processing for optimum particle size distribution
Following downstream post-processing, the high performance polymer beads can be sieved. Sieving is used to influence the particle size distribution by removing fine and coarse particulates. Sieving can also be used to collect a polymer bead fraction and thus narrowing the particle size distribution.
How does particle size affect polymer bead performance?
Powder flow
Powder flow is an important characteristic within a large number of applications. In a large number of industrial processes, powder needs to be sieved prior to use. For this reason, powder flow is a key consideration throughout the production process.
To consider powder flow, it would be prudent to consider how a powder packs in a given area. Finer beads have a smaller amount of space between them causing them to tightly pack together. If acrylic beads are tightly packed, there is less room for them to move and there are more surface-to-surface interactions, resulting in a poor flow. In general, larger beads have a better flow compared to fine beads.
The particle size distribution plays an important role, too. A broad distribution would result in the larger beads packing together, with the smaller beads fitting in the interstices. The Edwards statistical model provides further detail on the statistical mechanics of granular matter (leading to a rare generalised form of the Gibbs-Boltzmann equation for non-equilibrium systems). Essentially, tighter particle size distributions tend to have a better flow.
Powder-liquid systems
Many applications including high performance acrylic bead polymers involve a powder-liquid system. The most common types of liquid used are complementary monomers – which will further polymerise with the monomer – or sometimes a solvent.
When high performance polymer beads are added to a liquid, the beads absorb the liquid and swell. The time it takes to complete the swelling is linked to the depth the liquid needs to penetrate inside the polymer bead. As the particle size increases, the depth the liquid has to penetrate increases, the longer the swelling time.
The fine tuning of the swelling time can be used to optimise a polymers performance. Capillary forces also need to be taken into account when designing the correct performance liquid.
Complementary monomers
Bead polymer-monomer systems are used within a diverse set of industries, especially those requiring higher levels of performance. As a polymer bead is added to a liquid monomer, there are a series of interactions. These complex interactions vary a lot depending on the residual peroxide, molecular weight and the powder-liquid ratio.
As the polymer beads begin to swell, the residual initiator releases and reacts with the liquid monomer. Beads begin to stick together, causing an viscosity increase. Depending on the application and the industry, the optimal viscosity will vary greatly.
For the casting industry, the powder-liquid system produces viscous pourable slurry. This can be poured on a mould prior to casting. Before the slurry is ready to use, it needs to reach the ‘pour-time’. Therefore particles can be used to modify the pour-time with the pour-time increasing as the particle size of the polymer beads particle increases.
In dentistry, the powder liquid system is used in various processes including the production of dentures. The denture base system consists of a pigmented acrylic powder and liquid monomer which are mixed to form dough; it has a similar consistency to play-dough for a designed length of time. Particle size can be used to tailor the denture base characteristics for various geographic climates.
Cosmetic acrylic nail systems use acrylic powders which have a higher residual peroxide content. As a nail technician dips their brush into the monomer and then into the powder, a polymer bead forms due to the swelling of the acrylic bead and the interactions previously described. However, the release of peroxide in this case causes such a large increase in viscosity, the gelling effect occurs and the acrylic system polymerises on the nail once applied. The acrylic polymer bead size is linked to how quickly the polymer bead swells, releasing the peroxide and therefore the setting time of the acrylic nail system.
Solvent
High performance bead polymers can be used as viscosity modifiers within certain systems, such as the production of super glue. For a powder-solvent system, the polymer is essentially an inert substance that increases the viscosity of the system. The beads are added to the solvent and once the beads swell, the polymer chains begin to unwind and dissolve. Under an optimised process, the production time; time it takes the polymer to dissolve; can be shortened by using finer acrylic beads.