Polymer compatibility with harsh healthcare disinfectants

According to the World Health Organization, healthcare-associated infections (HAIs) are the most frequent adverse event in care delivery worldwide. COVID-19 has prompted hospitals and clinics to increase protections against infection on top of the stringent sterilization and disinfection protocols already in place. Complicating this ongoing effort are the wide range of surface materials to be cleaned, more-frequent disinfection, and the use of harsh chemicals, which may be incompatible with traditional plastics.

Acrylonitrile-butadiene-styrene (ABS) and polycarbonate (PC) resins were traditionally used for medical device housings and enclosures. Chemical resistance became an issue, and components made with these materials started to fail from environmental stress cracking (ESC). So, manufacturers began replacing ABS and PC with blends of PC and ABS or polybutylene terephthalate (PBT).

PC/ABS and PC/PBT blends are currently the standard amorphous and semi-crystalline materials, respectively, for device housings and enclosures. However, these incumbents can fall short in chemical resistance, especially considering additional measures to prevent COVID-19 transmission.

In addition to chemical resistance, device enclosure and housing polymers need high-impact properties to withstand being dropped or to resist external applied forces. Repeated application of disinfectants can degrade impact strength throughout time, so these properties are tightly connected. Other important material requirements include dimensional stability, custom colorability, and flame retardance for powered devices.

Although high-end, polysulfone (PSU) and polyphenylsulfone (PPSU) offer good chemical and impact resistance, they may be over-engineered for enclosures that do not require sterilization.

Advanced PC copolymers may be the material solution that checks all the boxes. As a potential replacement for conventional resins and blends, PC copolymers can balance mechanical performance (impact, flow) with chemical resistance performance to mitigate crack propagation. Availability of advanced PC copolymers avoids the need to over-engineer these types of applications, providing an alternative, cost- effective solution.

When designing a medical device, OEMs must clearly understand compatibility between candidate polymers and new chemical agents, such as disinfectants, in the context of application requirements. The ESC test can screen polymer material candidates, but there is no current industry standard for chemical compatibility testing. Test methods aimed at simulating end-use conditions can aid in evaluating the effects of commonly used healthcare disinfectants on polymer properties such as impact resistance, toughness, tensile strength, and color stability.

Environmental stress cracking

One of the most common causes of premature failure in plastic parts used in healthcare devices and equipment such as ICU monitors, imaging equipment, infusion pumps, and hospital bed components is ESC.

To prevent HAI spread, hospitals and clinics are thoroughly cleaning and disinfecting medical equipment and high-touch surfaces with disinfectants based on quaternary ammonium, hydrogen peroxide, bleach, and other chemicals.

These measures heighten the risk of ESC, which can lead to poor aesthetics, costly repairs or recalls, downtime of a critical piece of equipment, and a negative patient experience.

Stress cracking depends on more than just the compatibility between a chemical and the resin. The elements that drive ESC are stress (internal in-mold stress and externally applied stress) and chemical exposure (a function of exposure concentration, duration, temperature). When a plastic part under mechanical stress is exposed to aggressive chemicals that penetrate the molecular structure of the resin, polymer chains break down and the material becomes more brittle. Given time, the part can develop a web of thin cracks around the points of stress, called crazing. As these small cracks propagate, the part can develop fractures and fail, particularly if there is an external impact collision or torsional force such as inserting a screw head into a threaded insert.

Other factors contributing to chemical compatibility are stresses introduced by the design, the processing method, and the way the part is used. However, the first step in preventing ESC is choosing the right material for the application.

 

Compatibility testing

To evaluate and compare compatibility with typical healthcare cleaning agents for industry-standard PC blends and advanced PC copolymers, SABIC conducted ESC testing using an aggressive method based on ASTM D543: Standard Practices for Evaluating the Resistance of Plastics to Chemical Reagents. This test continually exposes test bars of the material to strain and a specific chemical agent for three-to-seven days. With no established industry standard, this exposure range was selected to push the material to its test limits.

The test bars are kept saturated with the chemical agent (wrapped in disinfectant wipes) and are bent to a specific strain level (1% in this case) in a test fixture. Constant strain is maintained throughout the test period.

This test method is one of the most stringent protocols and serves as a highly accelerated version of normal exposure to disinfectant wipes in a healthcare setting.

To demonstrate compatibility with a given chemical agent, a material must achieve >90% tensile stress at yield and 80%-to-139% tensile elongation at break, per ASTM D638: Standard Test Method for Tensile Properties of Plastics.

The testing described here was conducted to determine the susceptibility to chemical attack of PC copolymers vs. standard PC blends.

The materials tested were a PC/ABS blend, a PC/PBT blend, and four PC copolymers from SABIC’s new LNP ELCRES CRX family. Two of the ELCRES CRX grades are semi-crystalline copolymers and the other two are amorphous copolymers.

SABIC conducted this ESC study to assess compatibility between incumbent materials against widely used surface disinfectants. Test samples were exposed to 12 different healthcare disinfectant products, such as PDI Sani-Cloth wipes and Diversey Virex wipes.

Disinfectants used in these products include:

• Quaternary ammonium compounds. Cationic surfactants (positively charged surface-active agents) bind readily to the negatively charged surface of most microbes.
• Sodium hypochlorite (bleach). Bleach active ingredients can kill viruses, bacteria and fungi.
• Hydrogen peroxide. Not as strong as bleach, it does have disinfectant properties that effectively kill viruses and bacteria.
• Ethanol. Ethyl or isopropyl alcohol is an effective disinfectant when used at a high enough concentration.

The U.S. Environmental Protection Agency (EPA) has published a list of disinfectants that meet its criteria for use against SARS–CoV-2, the novel coronavirus that causes COVID-191. Several of these disinfectants are listed in Table 1. Data shown in the table is based on SABIC’s compatibility criteria scale.

Results: Broad chemical compatibility of PC copolymers

All four PC copolymers demonstrated broader compatibility with the selected disinfectant chemical products than the PC/ABS blend. Further, semi-crystalline PC copolymers surpassed the PC/PBT blend in compatibility with Sani-Cloth AF3 and Virex TB wipes, considered among the most aggressive products on the market. Specifically, the PC/PBT blend did not meet the compatibility criteria for elongation at break for these two disinfectants.

The new PC copolymers are based on unique building blocks tailored to the backbone of PC to lessen the negative effects of chemical attack and chemical degradation for reduced crack initiation and propagation.

Testing for resistance to external stress

In addition to the ESC test, SABIC conducted a high-speed puncture test on selected material samples before and after exposure to one of the disinfectant chemicals to reveal fracture patterns – either ductile or brittle fractures – as a gage of impact resistance and toughness.

Brittle fracture means that with prolonged exposure, the material tends to be more susceptible to ESC during application of an external force. Ductile fracture indicates a material with more toughness, i.e., more energy is needed to initiate a crack and there is less tendency for crack propagation.

For this test, we compared the PC/ABS blend to one of the amorphous PC copolymer materials. We tested each sample with the high-speed puncture tool before and after exposure to PDI Sani-Cloth AF3 wipes for three days. Face impact testing was completed using the INSTRON CEAST 9350 with a 0.50" hemispherical dart and 10.47kg sled weight. The test data determined differences in impact force and energy for the materials before and after chemical treatment of the molded parts.

Results: Ductile vs. brittle fracture

When exposed to Sani-Cloth AF3 wipes for three days and then subjected to the high-speed puncture test, the PC/ABS blend sample experienced brittle fracture.

In contrast, the amorphous PC copolymer sample exhibited ductile fracture using the same conditions, with higher force (N) and energy (J) associated to initiate a crack in the puncture test.

Figure 3 does not show any change between the unexposed sample and the chemically treated sample following the high-speed puncture impact test. However, there is a noticeable difference between the PC/ABS and the CRX1414 copolymer failure modes. The PC/ABS sample shows significantly more radial cracking and would be classified as a brittle failure, whereas the CRX1414 copolymer has minor radial cracking and would be classified as a ductile failure.

While ESC data and puncture test results are good screening tools, the performance and interpretation of end-use testing are important. Therefore, extensive testing of the finished part is strongly recommended.

 

Material selection sets the stage

Proper selection of plastics for medical device housings and enclosures will continue to be a focus to control HAIs and improve patient safety while reducing operating costs from premature failure of costly equipment. There is a need for advanced materials with improved chemical resistance that can extend the useful life of devices and equipment that get cleaned frequently.

Advanced PC copolymers can surpass traditional PC blends in chemical compatibility with harsh disinfectants and reduce crack propagation that can lead to failure.

Beyond choosing between these two types of materials, device makers and designers must decide whether an amorphous or a semi-crystalline grade is better suited to the application and use environment. In general, amorphous polymers surpass semi-crystalline polymers in ductility, toughness, and dimensional stability for parts with tight tolerances. Semi-crystalline polymers have better chemical resistance due to their inherent chemical structure – but at the expense of dimensional stability due to higher shrinkage and part warpage.

After material selection, other factors affecting ESC come into play. Part design and processing are important to maximize chemical compatibility. Pay close attention to good design principles for part geometry and gating to minimize weld lines and avoid sharp corners to reduce areas of stress concentration. Any initial stress points in the part design will become weak links for chemical attack. Thus, poor part design can negate good material selection.

Poor chemical resistance and weakness in a plastic part can also be attributed to molded-in stresses introduced during processing. Localized in-mold part stresses should be minimized during molding and secondary operations.

As illustrated in Figure 4, a combination of informed material choice, good design principles, and processing and secondary operations that minimize stress can help device makers improve the performance and durability of equipment exposed to today’s healthcare disinfectants.

SABIC
https://www.sabic.com

About the authors: Manish Nandi, Ph.D., is business development leader, healthcare and Nithin Raikar, is senior business manager, LNP copolymers at SABIC. They can be reached at productinquiries@sabic.com.

1 List N: Disinfectants for Coronavirus (COVID-19). EPA website. https://www.epa.gov/pesticide-registration/list-n-disinfectants-use-against-sars-cov-2