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Steps to Perform Contamination Analysis in Polymers

How to develop an analytical testing strategy to narrow sources of contamination in plastics, raw materials, and final products

Paula McDaniel, PhD

Paula McDaniel, PhD, spent 23 years in corporate analytical and product development groups at Air Products after receiving her PhD in Physical Chemistry (University of Illinois, 1988). In 2011, she...

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Every industry or manufacturer encounters contamination from raw materials, manufacturing environment, storage, transportation, or use. Ensuring final product performance and safety requires diligence to monitor these many sources. This, however, can be an overwhelming proposition, given the ever-changing landscape of suppliers, regulations, and testing knowledge needed. The guided process outlined here provides tools to rule things in or out, and help you be better prepared to get from contaminant to prevention. 

Contamination issues are common. These scenarios usually start with a visible defect, unexplained odor, discoloration, unexpected mechanical property, or other performance attribute detected internally or from a customer complaint. It is tempting to start testing for everything, but with a few thoughtful steps, options can be narrowed quickly.  

Step 1 – Evaluate the most likely class of contaminant

 This is the chemical analogue of the game “animal, vegetable or, mineral” as the answers lead us down different testing pathways later in the process. Is the contaminant a polymer, and what type or levels might you expect? Is it potentially an inorganic component such as a filler, additive, or machining particle? If an organic chemical, are the suspect compounds in the manufacturing environment (other products or raw materials) or can they come from raw material streams (recycled raw materials) or specific process operations (such as cleaning)?

Step 2 – Manifestation of contamination

Regardless of component class, detailing the conditions under which the problem is manifested helps to narrow possible sources. This bridging step will also direct us to appropriate analytical testing options. The following are examples of clarifying questions:

  •  Is the contaminant a visible particle or localized discoloration and what are the dimensions?
  • Are you seeing changes in overall mechanical or physical properties? This might suggest type of impurities present.
  • Under what conditions does the contaminant manifest itself (temperature, stress, type of light, orientation)? Being able to mimic these conditions will improve likelihood of identification.
  • Do you have a “good” sample for comparison to a “contaminated” material? It can be easier to answer the question, “how is it different?”
  • Is a pure reference of the suspect impurity available? Obtaining a fingerprint for comparison is an common strategy.
  • Contaminant is a known compound that has concentration thresholds established by the relevant regulation (such as California Prop 65). Undesired compounds such as bisphenol A, phthalates, or heavy metals can be present in recycled raw material streams. 

Step 3 – Relevant analytical testing approaches for contaminant classes

 After focusing the potential impurity chemical class and refining conditions for contaminant manifestation, the analytical tools appropriate for each contaminant class can be mined for applicability. For contaminants that are classified as polymeric, inorganic, or organic, specific techniques might apply, which range from routine to advanced. Techniques such as Fourier Transform Infrared (FTIR) or gas chromatography (GC) are tools routinely used in the chemical industry, but depending on steps one and two, impurity type, size, or concentration might fall outside their range of applicability. For example, an FTIR microscope or scanning electron microscope (SEM) might be needed to focus in on a micron-size defect.  Otherwise, the impurity would be obscured by the background matrix when using conventional bulk analysis FTIR techniques.  In addition, morphology, concentration, chemical type, or phase (gas, liquid, solid) will negate some techniques over others.

As a general guideline, for polymeric impurities, DSC, FTIR, FTIR/Raman microscopy, NMR, and GPC can be leveraged depending on levels and matrix interference. XRF, ICP, IC, combustion-IC, SEM-EDS, TGA, and others are suited for identification and quantitation of inorganic impurities. Last, for organic impurities, techniques such as GCMS, NMR, FTIR, GC-FID, LC, and LC-MS might be required depending on levels or volatility of target compound. Deciding which testing pathway to follow can be overwhelming, especially when balancing cost and speed. In step four, guidance on mapping these techniques to the specifics of your problem is described.  As a lab manager, the prospect of understanding the acronyms can be daunting, but planning can be key in surviving these disruptions.  

Triaging a contamination problem requires expertise in your application, chemistry, and analytical testing. A lab manager can get ahead of the game by assessing internal expertise and capability to rapidly remediate these time-critical problems. If internal resources are capacity- or skill-limited, preplanned identification of a consulting partner can result in faster closure of costly time-sensitive problems. Consider identifying a consultant or external contract testing laboratory to leverage as part of your risk assessment plan. Evaluate external partners for their broad analytical technique expertise, ability to perform tailored testing, and delivery of integrated solutions, not just a test result. Using partners to fill gaps in a team’s capabilities is a smart move to speed your resolution process. It can be a smart fiscal decision as well. Maintaining the breadth of equipment needed with expert staff can be an expensive proposition if capabilities aren’t fully utilized, making partnering the right choice on many fronts.

Step 4– Mapping contaminant type to analytical approaches

With a quiver full of analytical approaches, impurity classes defined, and their manifestation clarified, it is time to create a roadmap for your smart testing process. The planning process culmination is summarized below, however, within each category there are multiple analytical technique options with specific strengths or weaknesses. 

  • Impurity type: Polymer 
    • Bulk component: Use thermal properties (melting point), chemical fingerprinting, or molecular weight
    • Trace component: Identify localized specks using microscopic techniques
    • Quantitation needed: Extraction/gravimetric analysis followed by appropriate organic or metals analysis methods
  • Impurity type: Inorganic
    • Bulk/trace component or quantitation: Myriad of metals analysis techniques
  • Impurity type: Organic 
    • Bulk component: Chemical fingerprinting methods
    • Trace component: Mass spectrometry techniques
    • Quantitation needed: Mass spectrometry and other detectors or extraction/gravimetric analysis

The initial findings might point to additional testing. For example, identification followed by reference material comparison can narrow down sources or material grades. Alternatively, a subsequent phase can involve quantitation, which would be important for a final safety assessment.

What’s next after the problem of the moment is solved? In the spirit of continuous improvement, evaluate the potential for this contamination source to reoccur. Consider adding new raw material screening, improve manufacturing material segregation to avoid cross contamination, or other mechanisms to reduce the likelihood of contamination in the future. This is usually an investment well spent to maintain customer confidence.

Contamination and impurities are encountered by most material, chemical, or article manufacturers.  Having a process to facilitate your decision-making as you move from identification to prevention will ensure customer satisfaction and safety. The options for root cause analytical testing can seem overwhelming, but with a process and the right expert partner, resolution and avoidance are within your grasp.