Compounding plastics and rubber for the medtech sector

Nathan Doyle and Nick Norton, Compounding Solutions offer an overview of plastics and rubber compounding for the medtech sector

Specialty plastic compounding is more than mixing two or more components together. A superior quality compound starts with the selection of the right inputs; the rest is science devoted to maintaining the high quality that the medtech industry requires and deserves. After input selection, the next steps include drying the materials to ensure the integrity of the materials is maintained; process development to prevent degradation during compounding; selecting the appropriate pelletisation method and selecting pellet size that device manufacturers are able to process efficiently.

Materials selection

Perhaps the most important part for materials selection is that device manufacturers should always partner with their compounders. Making the compounder aware of information about the final application of a material greatly helps custom compounders develop a material formulation and process that results in excellent filler and pigment dispersion, meets specifications and will work in the desired application for the desired amount of time. When it comes to catheter tubing or micro-moulding applications, the wall thickness of the tubing extrusion or micro-moulded part is critical information, as this will help to ensure a well-dispersed compound free of surface imperfections. This is especially important for very thin walled applications that can be as thin as 0.001”.

Material selection is about more than choosing materials simply because the properties look good on paper. There are right and wrong decisions and the material combinations selected can make or break any new project. A large portion of materials used in medical devices are combinations of polymers and radiopaque fillers, such as barium sulfate, bismuth subcarbonate, and tungsten. Even selecting a radiopaque filler is not cut and dry; there are polymer and filler combinations that do not react well when combined. One of those combinations is thermoplastic polyurethane (TPU) with bismuth subcarbonate. Combining TPU and bismuth subcarbonate results in the rapid degradation of the polymer and all desired physical and mechanical properties are greatly compromised. There are also certain polymer combinations that are incompatible, such as polyethylene with nylon and TPU with nylon. Consult with your compounder about your formulation for any potential compatibility problems.

Colourants

In recent years, the United States Food and Drug Administration (FDA) has put limitations on the colourants that can be used in medical devices. The creation of Title 21 CFR Part 73 Subpart D has a limited number of colourants that have been deemed acceptable for use in medical devices without batch certification. Using the appropriate colourants can help speed up the FDA review process to save time and money. These requirements greatly limit the end colours that are achievable for devices and tubes. If there are any doubts about whether a colour should be used for the development of a new product, compounders should be able to provide the advice needed to avoid any setbacks due to colourants.

The processes used involves more than the temperatures used during compounding. There are a number of technologies that can set a compounder apart from the rest, whether it is how they dry materials, the feeding systems they use, or designing an optimal screw design to impose the correct amount of shear and residence time, the pelletisation methods or the frequency and type of quality checks being performed during compounding.

Drying process

There are numerous materials that require drying prior to processing, whether for compounding or device manufacturing through extrusion or injection moulding. These requirements are often critical, as certain materials do degrade if processed without being dried properly. Drying not only requires the equipment for the actual drying but you should also have the ability to check moisture levels with an appropriate moisture analyzer. Whether you are a compounder or a device manufacturer, moisture analysis is a key part to ensuring product quality and functionality. It is also vital to keep materials dry throughout processing, as some can quickly absorb moisture from the air and cause material degradation. Compounders and processors should have effective methods for maintaining appropriate dryness for all components.

Feeding systems play a large role in achieving high-quality product that is consistent pellet to pellet and lot to lot. Volumetric feeding systems are less accurate and feed by discharging a constant volume of material; this is an efficient feeding method for less technical applications, where accuracy is not overly critical. Compounding for medical materials requires a very high degree of feed accuracy to ensure consistency and quality; that is where loss in weight feeding systems come in. These systems feed based on a set discharge rate. Feeding by weight ensures that every pellet is a consistent formula.

Screw designs are often customised by compounders for certain applications and materials. There are two types of mixing that can be targeted via screw designs: distributive and dispersive. With dispersive mixing, particles (ie. second polymers, liquids, etc.) are reduced in size but it does not typically spread the particles evenly in the polymer. Distributive mixing, on the other hand, homogenises a material; it distributes fillers and additives more evenly in space but does not change the size of particles. Each material and filler likes its own type of mixing to ensure maximum dispersion. Maintaining plastic properties in screw design is a balance between shear and residence time in the extruder. The goal of compounding should be to impose the right amount of shear to disperse fillers while pushing the material out of the extruder as fast as possible so the polymer is not subjected to an unnecessarily long heat history. Exposing polymers to high shear rates or longer heat histories than what is needed leads to polymer degradation.

Pelletisation of the materials can effect a device manufacturer’s process and it is very important to work with your compounder to ensure a proper pellet size is produced. Compounders may have their own internal pellet size specifications, but those may not feed right into small, single screw tubing extruders. An incorrect pellet size or shape can lead to feed and pressure fluctuations during processing, which leads to issues including lower yields and higher scrap rates for device manufacturers.

Importance of partners

Making sure you partner with a compounder that has an audited quality system is a must. The proper quality checks must be done to ensure consistency and quality. Properties such as dispersion and melt flow rate are always vital in medical compounds, no matter the final application. Filler and colourant dispersion is especially critical in thin walled catheter applications and it is very important that compounder has the proper technology to properly evaluate dispersion. Ultra fine microscopy is a great tool for evaluating dispersion and particle size. Melt flow index or capillary rheometry should always be performed to ensure there is no thermal degradation of the polymer after compounding.

The many processes and technologies involved in compounding for the medical device industry are constantly evolving. It is important to find a compounder with a high commitment to quality, technology and a willingness to partner with device manufacturers to ensure open communication of specifications. Compounding Solutions has long had an open door policy; meaning customers are encouraged to take an active roll in the development of their materials. Compounded plastic materials save lives; compounding is about being partners and having more than just a supplier/customer relationship.