Cheryl Weckle, technical service and development, Trinseo, discusses how chemical resistant grades prevent damage to plastics in the fight against Healthcare Associated Infections (HAI).
HAI continue to be a concern in medical environments due to more resistant strains of viral, bacterial, and fungal pathogens emerging all the time. To address this challenge, the World Health Organization, Centers for Disease Control, and other regulatory bodies have prescribed certain steps that can be taken to minimize the risk of infection spread. An important step pertains to cleaning and disinfection of medical equipment surfaces.
Plastics are frequently used for medical equipment housings as a material of choice as a metal replacement for a variety of reasons. These include the potential for cost savings and light weighting, design freedom, parts consolidation, and cosmetic appeal.
The challenge with plastic, which brings so many advantages, is its propensity to fail because of rigorous disinfecting procedures combined with increasing aggressiveness of chemicals. Premature cracking and failure of a plastic part due to the action of stress and contact with a chemical agent is known as Environmental Stress Cracking (ESC). This mechanism can be described as follows:
- Solvent migrates into the polymer surface of a stressed part. The plastic surface is softened or plasticized.
- Small surface defects are formed and stress concentrates around the tip of the crack. Solvent absorption increases in this region.
- Stress is relieved as polymer chains pull apart from one another. The defect widens and eventually leads to cracking and complete failure.
Research and usability studies show that ESC failures are caused by a combination of multiple factors. First is the plastic material itself, which can have more or less resistance to solvents depending on the base polymer structure and the product recipe including additives. The second factor is the stress on the part, which can be due to molding, design, assembly, or even environmental factors. Lastly is the nature of the disinfectant, solvent, or chemical and how frequently it is applied.
From a material design perspective, all these factors need to be balanced against performance property requirements and regulatory compliance concerns. The application itself and environment in which it functions must be evaluated along with different resin types, cleaning options, and protocols. This will result in the most effective material selection that can withstand chemical antagonists necessitated in today’s healthcare environment.
The material selection process should include testing against the disinfectants that would be commonly used in the life of the equipment. For plastic materials it is recommended to perform testing according to ASTM D543-14, where specimens are placed under stress and exposed to chemicals for specified time periods. Following exposure, the samples can be tested for retention of physical properties and data can be analyzed and interpreted with strict attention to detail.
Often the start of failure can be seen when looking under a microscope as tiny stress cracks in the part surface. In other cases, complete brittle failure is immediately evident. While these tests can allow relative comparisons of the performance of materials and disinfectants, it is important that final assessment be done on fully assembled devices and equipment. This is the only way to evaluate and incorporate all aspects of an application’s design which can potentially contribute to its overall durability and life.