Poly(methyl methacrylate) beads: transforming monomers into high performance polymers
Poly(methyl methacrylate), commonly abbreviated as PMMA, is one of the most important polymers on the planet. Polymerised using the Methyl methacrylate monomer, PMMA is used in hundreds of applications, each requiring a unique set of characteristics to offer optimum performance.
PMMA is used in everything from biomedicine (including denture bases and bone cements), to glass transportation equipment. It even has properties that create more durable or more resistant to weathering road surfaces. This vast array of applications is testament to the extensive research conducted by chemists into polymerisation techniques and the resulting qualities of PMMA.
The sheer variety of applications is partly due to the mix of characteristics achievable through the multiple polymerisation processes that are available. In this article, we will review four of the most commonly used techniques for synthesising PMMA, including bulk polymerisation, solution polymerisation, emulsion polymerisation and suspension polymerisation.
PMMA made by bulk or solution polymerisation can be ground into smaller particulates, which then makes them suitable for high performance materials. Emulsion and suspension techniques are more conventionally known for their high performance bead polymer production.
Here are the four techniques:
1. Bulk Polymerisation
A mass polymerisation process is where a monomer soluble initiator is added to the monomer. This type of polymerisation would be used for the production of materials such as Perspex and not necessarily high performance polymers. A higher degree of control is necessary to guarantee the required molecular weight distribution and temperatures to closely control the resultant beads.
2. Solution Polymerisation
This polymerisation process involves dissolving the monomer and initiator in a non-reactive solvent. Once the reaction is complete, the solvent is removed to yield the polymer.
This technique offers greater control in terms of thermal control, and the use of solvent reduces the heat generated through polymerisation. However, there are issues created by the solvent. Firstly, they can degrade the final beads and molecular weights tend to be lower, due to the occurrence of chain transfer to solvents.
3. Emulsion Polymerisation
In emulsion polymerisation, the monomer and a surfactant are added to water. This creates an emulsion. The excess surfactant in this emulsion mixture results in the formation of micelles and small volumes of monomer migrate into these micelles. A water soluble initiator is then added to the mixture which in turn reacts with the monomer inside these micelles. The PMMA high performance polymers made from this technique generally have a very small particle size <10µm.
4. Suspension Polymerisation
Suspension polymerisation involves the high-sheer mixing of monomer and initiator in liquid; generally water. Unlike emulsion polymerisation, the initiators used are oil/monomer soluble so the polymerisation occurs in the monomer droplets. For PMMA, the suspension polymerisation method yields perfectly spherical and clear polymer beads that are usually >10µm.
High performance polymers: a question of production processes and control
While each of these techniques will yield different results, high performance polymer beads will, ultimately, only perform as well as the raw materials and accurate manufacturing processes allow them to.
High performance polymers require materials that are high purity and from approved sources, guaranteeing that quality. For manufacturers, preventing contamination is essential, and only specialised equipment can prevent this at high volumes. But what exactly does this equipment offer?
Control is critical for high performance acrylic beads. Raw materials need to be processed and transferred to reaction vessels using specialised equipment. In the reactor, computer controlled processes ensure a highly controlled reaction, depending on the ingredients. This causes the monomer to polymerise, but the temperature needs to be tightly controlled, both ramping and cooling, as does the precise stirring action.
After the reaction is complete, beads should be washed and transferred to a centrifuge where they can dry. Then, advanced sieving equipment can collect, separate and group beads based on the fractions required.
Post-processing is required within some applications. This could include blending beads with flow agents, construct agents, UV stabilisers or coating the bead with peroxide.
High quality beads
Quality controls and checks need to be completed at every stage of the process. In high performance applications, quality is absolutely critical. Take the aerospace or medical industries, where quality materials are vital to the performance of their end applications.
Quality checks are different for every application, though. The checks necessary for the medical and aerospace industries will be very different. Each type of specialised PMMA bead needs to be tested by a unique set of verified measures, depending on the requirements, final application and the manufacturing processes.
Sometimes factors such as the packaging are an afterthought. But in actuality, polymer resins need to be packaged in sterile containers, with descriptive labels and batch numbers. There are packaging certifications issued by entities such as the UN to ensure these packaging procedures are followed.
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