With the spate of new, harsh disinfectants being used in healthcare settings — primarily in response to the COVID-19 pandemic — device manufacturers need to consider newer material solutions with improved chemical resistance to withstand the cumulative effects of cleaning. Nithin Raikar, senior business manager, LNP Copolymers focuses on the test for compatibility with healthcare disinfectants among different materials, such as industry‑standard PC blends and advanced PC copolymers.
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According to the World Health Organization, healthcare-associated infections (HAIs) are the most frequent adverse event in care delivery worldwide. As the COVID-19 pandemic has prompted hospitals and clinics to implement enhanced cleaning protocols, there has been a clear shift in the way plastics are viewed to address infection control challenges.
The introduction of new disinfectants has pushed device manufacturers and material suppliers to seek better understanding of the compatibility of new chemical agents on plastic materials. Specifically, device manufacturers need to consider newer material solutions with improved chemical resistance to withstand the cumulative effects of cleaning — the combination of aggressive disinfectants and increased frequency in protocols. Disinfectants such as alcohols, peroxides and quaternary ammonium compounds can cause traditional polymers to become brittle and crack, shortening the lifespan of costly and critical devices.
Medical professionals face a dilemma: How can they trust their devices to keep patients safe and function optimally with increased cleaning? To address this challenge, SABIC has developed a new family of materials. The company’s LNP ELCRES CRX polycarbonate (PC) copolymers can provide superior resistance to some of the harsh disinfectants that are the norm in healthcare today.
Material selection for safer, longer-lasting medical equipment
A primary side effect of repeated cleanings on plastics is polymer embrittlement. When plastics come into contact with chemicals under stress, a phenomenon, known as environmental stress cracking, occurs. In terms of polymer chemistry, exposure to chemicals may result in either physical degradation (stress cracking, crazing, swelling and discoloration) or chemical attack (reaction of chemical with polymer and loss of properties).
Amorphous polymers such as acrylonitrile-butadiene-styrene (ABS) and PC resins were traditionally used for medical device housings and enclosures. When chemical resistance initially became an issue and components made with these materials started to fail from environmental stress cracking, manufacturers began replacing ABS and PC with blends of PC and ABS or semi‑crystalline polybutylene terephthalate (PBT). Even these incumbents, however, can fall short in chemical resistance, especially in view of additional measures to prevent COVID‑19 transmission.
To help maintain the structural integrity of medical devices, SABIC’s Specialties business launched a new product portfolio, LNP ELCRES CRX resins, leveraging a unique new copolymer building block to help meet the needs of this developing market. CRX copolymers are designed to reduce susceptibility to chemical attack and to help minimise crack propagation. In addition to chemical resistance, polymers used in device enclosures and housings need high impact properties to withstand being dropped or resist an external applied force. Additional polymer features include flame retardancy for powered devices and custom colourability to enable styling and aesthetics in part. The retention of impact strength from repeated application of disinfectants is also a key factor for durability over time.
Advanced PC copolymers such as CRX may be a material solution that ticks all the boxes so that hospitals can continue to use aggressive and effective forms of disinfectants to keep patients safe while also keeping medical equipment serviceable for longer periods of time. Device manufacturers may also be less likely to face costly requests to replace devices under warranty.
Environmental stress cracking (ESC) testing
The ultimate laboratory test for a plastic material for use in a finished part would be one that measures the performance over an entire range of temperatures, impact forces, loads, and chemical exposures in its actual end‑use. Unfortunately, such thorough testing has not yet been fully developed and would be extremely costly.
The ESC test has proven to be a useful indicator of expected performance and serves as a screening tool for polymer material candidates. SABIC follows an established ESC testing procedure (ASTM D543) to screen chemicals and environmental conditions that mimic the part application exposure. To test compatibility with various chemicals, SABIC has used a quantitative ESC test that evaluates retention of tensile properties from 3 to 7 days at 1% strain and at room temperature. These properties correlate to plastic failure modes and often provide insights on resistance to fracture. 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 (Figure 1).
Figure 1: SABIC test method.
To demonstrate compatibility with a given chemical agent, a material must achieve >90% tensile stress at yield and 80–139% tensile elongation at break, per ASTM D638: Standard Test Method for Tensile Properties of Plastics.
With no widely adopted industry standard for chemical resistance testing, SABIC pushed the test conditions (the exposure duration and external stresses) to ensure that testing protocols serve as a highly accelerated version of real‑life exposure to disinfectant wipes in a healthcare setting. SABIC continues to collaborate with healthcare OEMs who are conducting their own testing to validate the materials in their applications and environment.
Testing for compatibility with healthcare disinfectants
SABIC evaluated the compatibility between CRX and incumbent materials with 12 leading surface disinfectants widely used to clean devices. Disinfectants used in these products include quaternary ammonium compounds, sodium hypochlorite (bleach), hydrogen peroxide, and ethanol (also referred to as ethyl or isopropyl alcohol). Leading disinfectants such as PDI Sani‑Cloth wipes and Diversey Virex wipes are published on the US Environmental Protection Agency (EPA) list which meet their criteria for combatting SARS-CoV-2, the novel coronavirus that causes COVID-19.1 Several of these disinfectants are listed in Table I. Data shown in the table are based on SABIC’s compatibility criteria scale.
Table I: Test results for compatibility with healthcare disinfectants. (This information should be used as indicative only. This is essentially a ranking test and is not intended to provide data to be used for design or performance prediction. Extensive testing of the finished part is, therefore, strongly recommended. The performance and interpretation of end-use testing are the end producer’s responsibility.)
The materials tested were a PC/ABS blend, a PC/PBT blend and four PC copolymers from SABIC’s LNP ELCRES CRX resin family. Two of the ELCRES CRX grades are semi-crystalline copolymers, the other two are amorphous copolymers.
Results: broad chemical compatibility of PC copolymers
Test results show that all four PC copolymers demonstrated broader compatibility with the selected disinfectant chemical products than the PC/ABS blend. Further, the 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.
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. The purpose of this test is to reveal fracture patterns — either ductile or brittle fractures — as a gauge of impact resistance and toughness. Brittle fracture means that over time and prolonged exposure, the material tends to become more susceptible to stress cracking under application of an external force. Ductile fracture indicates a material with more toughness; that is, more energy is needed to initiate a crack and there is a lower tendency for crack propagation.
For this test, the PC/ABS blend was compared with one of the amorphous PC copolymer materials. Each sample was tested with the high-speed puncture tool before and after exposure to PDI Sani-Cloth AF3 wipes for 3 days. At comparable conditions, data from this testing were used to determine differences in impact force and energy for the materials before and after chemical treatment of the moulded parts.
Results: ductile vs brittle fractures
When the PC/ABS blend sample was exposed to Sani-Cloth AF3 wipes for 3 days and then subjected to the high-speed puncture test, brittle fracture was observed (Figure 2).
Figure 2: Results of high-speed puncture test with PC/ABS blend.
In contrast, the amorphous PC copolymer sample exhibited ductile fracture under the same conditions, with higher force (N) and energy (J) associated to initiate a crack in the puncture test (Figure 3).
Figure 3: Results of high-speed puncture test with amorphous PC copolymer.
Figure 3 does not show any change between the unexposed sample and the chemically treated sample following the high-speed puncture impact test. There is, however, a noticeable difference between the PC/ABS and the CRX1414 copolymer in their 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.
Although ESC data and puncture test results are effective screening tools, the performance and interpretation of end-use testing are important. Extensive testing of the finished part is, therefore, strongly recommended.
Material selection sets the stage
Proper selection of plastics for medical device housings and enclosures will continue to be a focus to help hospitals control HAIs and improve patient safety while at the same time 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 exposed to frequent cleanings.
Once a material has been chosen, other factors affecting ESC come into play. Part design and processing are important to maximise chemical compatibility. Close attention should be paid to good design principles for part geometry and gating to minimise weld lines and avoid sharp corners to reduce areas of stress concentration. Basically, any initial stress points in the part design will become weak links for chemical attack; thus, poor part design can negate good material selection.
As illustrated in Figure 4, a combination of informed material choice, good design principles, and processing and secondary operations that minimise stress can help device makers improve the performance and durability of equipment exposed to today’s healthcare disinfectants.
Figure 4: Synergistic approach to optimising resistance to harsh chemicals.
Note
LNP and ELCRES are both Trademarks of SABIC or its subsidiaries or affiliates.