Porosity is a big issue in the creation of many acrylic-based products that are manufactured using a powder liquid system. Incorrect powder: liquid ratios and gases, such as air or vaporised excess monomer trapped during polymerisation stages, produces voids or bubbles in the finished product. This not only causes aesthetic issues, but also structural weakness and mechanical failure. The polymerisation process is often blamed for porosity, with the curing conditions put under the spotlight. But the real culprit might be hidden further back in the core reactants and even in the properties of the raw materials, including the Poly(methyl methacrylate) (PMMA) bead.
Porosity – not just an aesthetic issue
There’s a reason why acrylic is such a popular choice for casting and modelling. Strong, colourless, inert and mouldable, acrylic is an important base material for dental prostheses, tooth manufacturing, casting resins and artificial nails, to name but a few. But despite its clear benefits, acrylic is not without its problems. Yellowing is the most reported issue, followed closely by porosity. When porosity occurs, it leads to voids or bubbles appearing within the product, which creates unwanted visual characteristics but can also lead to other performance issues.
For nail acrylics, aesthetics is the main concern. In paler shades, bubbles are instantly noticeable and can give an uneven appearance when light hits the nail. When encapsulation is used to embed different coloured glitters, porosity can cause bubbles to gather around the glitter and impair the visual effect.
For dental prostheses and tooth manufacturing, porosity can cause weaknesses that lead to product failure. The bubbles form cavities within the set mould and diminish the mechanical properties of the final product. In a medical device, this can have serious and long-lasting consequences, including device failure.
In casting resins, porosity can cause visual and mechanical issues, depending on the application. Even low porosity levels can cause product failure and high levels of wastage during manufacturing. Porosity are points of mechanical weakness. As a result, the cast article is subject to external pressures, cracks can form at these points of weakness, which in turn can propagate. That can eventually lead to fractures.
First, fix the process…
The introduction of porosity happens during the polymerisation process, caused by multiple factors. These include the chemistry of the raw materials combined with the process used to mix them and the curing conditions. Careful selection of the correct materials that match specific processes, and then making minor changes to the process itself, can mitigate porosity.
Incorrect powder to liquid ratios
In most acrylic applications, including dental prosthesis and casting resin manufacturing, a polymer powder is mixed with a monomer liquid to create a new acrylic polymer in a desired three-dimensional shape.
Commonly, PMMA is used as the polymer powder and methyl methacrylate (MMA) as the liquid monomer. For acrylic nails, the reagents are Poly(ethyl methacrylate) (PEMA) or Poly(ethyl methacrylate-co-methyl methacrylate).
The liquid-to-powder ratio is crucial to ensure the handling characteristics are optimised and for the reaction to proceed at the correct velocity and prevent porosity. The PMMA appears as a fine powder but is actually formed of very small beads. If the powder is not wetted out correctly then small bubbles can appear around the beads. When insufficient monomer is used, the PMMA does not dissolve completely, giving a white frosted appearance. If the monomer ratio is too high, voids can form as the excess monomer is vaporised.
An accelerated polymerisation process can also cause porosity issues. If the curing temperature is too high, or if too much accelerator is present in the monomer liquid, then the polymerisation process can run out of control. Gases form within the acrylic and cannot be released, resulting in bubbles in the final product.
Pressure also has a role to play here. If the pressure in the reaction chamber is too low, then any bubbles formed due to incorrect powder to liquid ratios or auto-acceleration of the exothermic polymerisation reaction will be magnified. The gases will expand and minor porosity will become larger voids and a much bigger problem.
Many of these issues can be avoided by using careful and consistent mixing processing and curing conditions, although this can be easier said than done. The actual on-site conditions can vary widely and manufacturer guidelines can be difficult to follow, particularly when they involve a tight temperature range. Makevale takes time to understand the real-life conditions and challenges that clients face and designs powders and liquids that consider any nuances or restrictions on-site. Careful control of particle size and peroxide levels in the powder and in some case accelerator in the liquid can also help to mitigate environmental challenges, such as high manufacturing temperatures.
…but then look deeper
Porosity is not just a product of the process. It can point to much deeper issues within the underlying chemistry, and even earlier steps such as the pre-production materials specification and procurement. The raw materials should be carefully designed and selected to fit within a specific process. Maintaining a consistent, tightly controlled materials specification alongside strict process controls means the process is both reproducible and so quality issues are minimised.
For example, the base acrylic powder of a particular process can cause many issues if it is not carefully controlled and manufactured to consistent standards. The bead size and its molecular weight can both contribute to the speed of the downstream handling characteristics and therefore the porosity of the end-product. Acrylic beads also have a residual peroxide level. If these levels are too high, then downstream polymerisation will accelerate too quickly, forming bubbles. Too low and the reaction may be incomplete, leading to monomer droplets that could potentially vaporise.
The precise PMMA chemical composition could have far-reaching consequences within a process and is often not realised until the end-product exhibits undesired characteristics. Porosity or yellowing can continue to be a problem if manufacturers do not focus on both the process and the reagents.
The rise of the designer bead
Product innovation and recent advances in polymerisation chemistry mean that more and more uses are being found for acrylics. As we push the boundaries of this process, PMMA bead design becomes more important.
Makevale works with clients across multiple industry sectors to design PMMA beads that meet exact process requirements and product specifications. With 40 years of polymer know-how, Makevale creates PMMA beads with precise molecular weight, surface chemistry, size and shape. This experience extends into the formulation of products for specific applications, including pre-formulated dental liquids and powders, acrylic nail powders for dipped and sculpted nails, and clear casting resin. Carefully controlled levels of accelerators, inhibitors and peroxide minimise side reactions and take into account the actual conditions present when polymerisation occurs. This creates doughs with predictable work times, resins with precise curing windows and high-quality end products.
For further information on our PMMA beads and acrylic powders and liquids, contact our team for free, no-obligation advice.