Why Common Plastics Are Failing in Hospital Settings

Patient safety and reducing hospital stays is the priority for healthcare institutions and one of the focus areas is preventing patients from getting HAIs (Healthcare-associated infections) during their stay. These HAIs are the responsibility of the institution and have to be treated and paid for by them.

To combat the source of the HAIs, the hospitals have started using very aggressive instant-kill disinfectants and cleaning solutions. These new chemicals are causing plastic components on medical devices to fail. Device makers are scrambling to fix existing products that have worked for years and are now failing. They also need to make sure any new products will meet this new requirement. How can you proactively design and verify your materials to meet these needs?

 

 

 

Types of Chemicals

Let’s start with the chemicals that are affecting these plastics.  Chemicals are broken down into the areas where they are used: medical facilities and home. Most products used in medical facilities will not see the home products and vice versa.

Medical facility chemicals

Chemical used in medical facilities can be broken down into three categories based on their respective level of disinfectant: sterilants and high-level disinfectants, intermediate-level disinfectants, and low-level disinfectants.

Categories	Chemical	Use
Sterilants and high-level disinfectants	Formaldehyde (Formalin)	contact with body fluids
Sterilants and high-level disinfectants	Glutaraldehyde	High level disinfectant for medical equipment
Sterilants and high-level disinfectants	Ortho-phthalaldehyde (OPA)	–
Sterilants and high-level disinfectants	Hydrogen peroxide (H2O2)	surfaces
Sterilants and high-level disinfectants	Peracetic acid	–
Sterilants and high-level disinfectants	Hydrogen peroxide/peracetic acid combination	–
Intermediate-level disinfectants	Sodium Hypochlorite (Bleach)	blood
Intermediate-level disinfectants	Iodophors	antiseptic
Low-level disinfectants	Phenols	surfaces and non-critical medical devices
Low-level disinfectants	Quaternary ammonium compounds (Quats)	non-critical surfaces

Watch out for the most aggressive chemicals or combinations of chemicals including: Bleach, IPA, Glutaraldehyde, OPA, H2O2, Quats, IPA-Quat combination, and Phenols.

Home-use chemicals

Products used at home can be broken down into three categories based on their use: cleaners, topical and food.

 

A great place to start addressing the issues behind some of these household chemicals is to conduct the following screen tests:

Staining:  common offenders are sunscreen, bug spray, catsup and mustard

Chemical attack: common offenders are acetone, IPA, and lotions and soaps that contain high oleic acid, bug spray, safflower oil and sunscreen

TPE, TPU, and acrylic adhesives: common offenders are soaps that contain high oleic acid, safflower oil and other oils.

How environmental stress leads to failure of plastic parts

Environmental stress cracking is considered the leading failure of plastic parts. It is the premature embrittlement and subsequent cracking of plastic due to simultaneous and synergistic action of stress and contact with secondary chemical agents. A primary chemical is one that the part was design for, such as a gasoline fuel line. Failures associated with primary chemical agents are rare because the chemical compatibility is well understood during the design phase. A secondary chemical is one that is not expected to contact the plastic part during its lifecycle.

All plastics undergo stress cracking, and with secondary chemicals this is accelerated. A normal plastic part would undergo stress cracking in air with no chemicals present known as creep rupture. The secondary chemicals accelerate this stress cracking.

Creep

Creep is when a solid material deforms permanently under a constant stress and occurs over time with extended exposure to levels of stress below the yield point. The embrittlement process associated with creep rupture can be accelerated by increase in stress concentration from external forces, internal forces, extreme temperatures, cyclic loading, and chemicals. During environmental stress cracking the chemicals attack the bonding forces of the polymer chains and this reduces the forces to disentangle these chains and reducing the time to failure exponentially.

Cracks

Cracks from environmental stress cracking form at areas of stress concentration such as a notch, defect, surface scratch or a crack. The stress concentration factor due to these conditions can range from 2X to 5X over the normal stress of the part. The first sign that this is happening can be crazes or very fine voids normal to the stress and this starts to form a crack. Cracks can grow and severely weaken the part or create a full failure.

The picture below is of a test fixture that’s straining plastic samples which have been exposed to a specific chemical.  You can see how the plastic samples reacted over a set period of time.

Stress

Stresses may be induced from an applied load or may be trapped during the molding process. The method in which the part is molded must be analyzed to reduce possible residual or induced stress. If the part does have residual stress it can lead to distortion, crazing or creep. The part may functionally be OK but when in the field and used with secondary chemicals it can quickly develop environmental stress cracking at these areas. The environmental stress cracking can quickly develop into cracks or part failures. There are methods to determine if a part has molded in stress such as molding the part in a clear material and then viewing it with a polarized lens or using the test method outlines in IEC 60601-1:2012 15.3.6 Molded stress where the part is baked in an oven at 10⁰C over maximum storage temperature for 7 hours to see if the part is damaged or warped.

Our approach to testing millions of plastic combinations

We have learned that you have to test each plastic and chemical. With millions of plastic combinations,  hundreds of chemicals, and different test methods, there does not seem to be a common database with all the results. We have started our own database with over 70 hospital and household chemicals, and have adapted our own version of testing based on the ASTM D543 Standard practice for evaluating the resistance of Plastics to Chemical Reagents. The test we have been using is very aggressive, but we are trying to simulate 5+ years of use in the field.

We start by bending the sample to induce a 1% strain (0.5-1.5%), add the chemical to a gauze pad, and place it on top of the sample. Put the sample into a sealed bag for 24 hours at room temperature (23⁰C). We then remove the sample and wash and dry it. Once dry, the sample should be inspected for crazing or embrittlement. Finally, an impact test is completed and the results compared to our base sample. Using our control sample (no strain) as 100% strength we classify our chemical results as 100-80% – Pass, 79-60% Concern, 59-0% Fail. *note there is no industry standard and your measurement criteria may vary.

Remember, it is the device manufacturer’s responsibility to perform end-use part testing to determine suitability of materials for the application.

Materials that work

In the past, plastics such as ABS, PC-ABS, and PC have been the standard for medical products. The industry is now seeing these materials that have worked fine for years or decades are now starting to fail when these new aggressive cleaning and disinfection chemicals are being used. Plastic manufacturers are introducing new blends to meet this new challenge such as PC+PBT, PC+ PET, Co-polyester, and Nylon 12. If you ask, the manufactures will send you placards or dog bone samples of their materials for testing.

Summary

Make sure your team understands Environmental Stress Cracking and how this could lead to a field failure of your product. The chemicals that could be used, how these chemicals can cause plastic parts to fail, design considerations in plastic parts that could accelerate these types of failures, how to test chemicals and plastics and finally plastics that work.

*We have found test data from the following manufacturers: Sabic, RTP, Covestro, Bayer, Eastman, Chimei, Trinseo, and Solvay