New Solutions for Synthetic Polymer Analysis
Plastics Technology – Web Exclusive
October 27, 2008
Written by Mary Stellmack, Senior Research Chemist, McCrone Associates
The rising prices of petroleum and resin, coupled with recent fluctuations in the United States economy, have the plastics industry looking for new ways to reduce costs. Often, the answer comes from overseas suppliers. Offering reduced labor expenses and advantageous currency exchange, these suppliers help soften the hit from industry pressure. However, overseas suppliers can also offer less sophisticated infrastructure and labor practices, potentially causing quality control losses, financial losses from defects and recalls, and ruined brand reputation.
When contamination and defects become concerns, the causes and sources can be difficult to determine; especially if the amount of contamination is small, or the manufacturing process has not changed since the problem began. Microanalytical testing is an optimal solution because it can not only identify and track contamination problems too small for the naked eye to see, but it can also be used to identify varying chemical compositions and examine crystal structures of the plastics themselves. This process allows scientists to determine if the part was molded at the correct temperature, or if foreign contaminants are present, resulting in visual defects. Inclusions at stress points are a major concern, possibly causing weakness and failure of the part.
Microanalytical testing can also be used to ensure compliance with government regulations and reduce the risk of product liability litigation. With some punitive damage awards reaching millions of dollars, many companies choose to be proactive in their product testing in order to avoid these issues.
COMMON TYPES OF CONTAMINATION/DEFECTS
Some of the more common culprits for contamination and defects include fibers, glass and metal particles, other polymers, insect parts, and dirt. These types of contamination result from primitive technology use and lack of factory infrastructure in some lesser-developed areas. Also, many small plants do not have the necessary resources to isolate and manage their facilities, and may be unfamiliar with their buyer’s industry standards.
Defects in parts can sometimes be traced back to contamination from resin left on the manufacturing equipment from a previous production run. Additionally, products that display visual defects, such as streaks or surface bloom, and reduced performance characteristics, such as poor UV resistance, can have issues with additives not being dispersed well prior to molding.
Before you call for microanalytical testing, gather some information on your process. When this information is shared with the analytical laboratory, you can make the most of your laboratory's experience, and find a solution as quickly as possible.
If contamination occurs, go through your process step-by-step to look for potential sources of the foreign material. Be sure to check the plant as well as the equipment, so as not to miss any environmental factors that could be important. McCrone Associates recently worked with a customer who repeatedly found black specks in their colorless parts. Their machinery was working flawlessly, but upon inspection of the plant, they discovered that there were no screens on the windows. At night, insects invaded the property and got stuck in the resin, and became the mysterious black specks visible in the final product.
It is recommended to check materials at several different stages in the production process to help narrow the search for contamination sources, including checking raw materials before the process even begins. Anything present in the resin, if not removed, will find its way into the final product or part.
If there is a product defect, verify that the grade of resin ordered is the same as what was received—for example, ensure that polyethylene is not being accidentally used to make a polypropylene part, or that a standard grade of resin has not been substituted for an impact-modified grade. When changing material grades, beginning business with a new supplier, or ordering materials with very strict specifications, early research and testing can save a lot of headaches later.,/p>
Even prior to your sample’s arrival at the analytical laboratory, communication is important. The more the laboratory knows about the history of the product and process, the better the scientists can direct their analyses to solve the problem. Many laboratories will work through the production process with customers, and are able to suggest possible sites of contamination—so the more information you can offer, the faster the process becomes, as the laboratory can develop a more direct analytical protocol. A good laboratory will be interested in collaborating on a solution with you.
If the problem is with an older, established product, include a sample of a "good" functional unit molded prior to the problematic run, as a comparison to help scientists diagnose the problem faster and more accurately.
After receiving the sample, the laboratory first completes a visual inspection under a stereomicroscope and a polarizing light microscope. Scientists look for inclusions or defects that can be smaller than 50 µm (micrometers), including spherulites in the plastic which form upon cooling, and other morphological features. By sectioning very thin layers of material from different areas of the part, scientists can compare crystal shapes and sizes to determine whether the part was molded and cooled under proper conditions. The opacity (or lack thereof) in a sample can also be a clue to the additives and fillers used in the process; visual differences might indicate a change in the type of fillers used, or a difference in the formulation or the amount added. Slight color differences can indicate thermal or UV degradation, or the improper loading of a color package. Examination with a fluorescence microscope can show the presence or absence of fluorescent additives.
Upon locating the contaminants, scientists manually isolate them from the sample using microsurgical scalpels, razor blades, and fine needles. Isolated particles can be 10 µm or larger in size. Contaminants smaller than 10 µm can sometimes be analyzed while still embedded in the polymer or isolated for analysis by sectioning around the contaminant. One of the great advantages of microanalytical testing is that an entire part does not need to be sacrificed—only a minute sample is required, and the area from which the sample was removed is often unnoticeable.
A commonly used test in contamination analysis is infrared spectroscopy (FTIR or micro-FTIR), which can be used to identify organic materials and some inorganic materials. In this technique, a 5 μm thick section of the sample or contaminant is isolated by hand and flattened into a thin film on a sample mount. The micro-FTIR, a polarizing microscope interfaced with an infrared spectrometer system, shines a beam of infrared radiation through the sample and records the different frequencies at which the sample absorbs the light. Every material absorbs light at different frequencies and produces a unique infrared spectrum, which is like a fingerprint of the material. McCrone Associates maintains thousands of standards and known materials in its IR library. By comparing the spectra of different sources, it is possible to differentiate a polyester fiber from a polypropylene fiber, or identify different polymer additives. Raman spectroscopy is a complementary technique to infrared analysis, and provides "fingerprints" of many inorganic materials and other opaque samples that are not suitable for IR. At McCrone Associates, the micro-Raman system obtains spectra of samples that are as small as 1μm in size.
For inorganic materials, a scanning electron microscope (SEM) or electron microprobe with energy dispersive x-ray (EDS) or wavelength dispersive x-ray (WDS) technology is used. Samples are bombarded with a focused beam of electrons. The sample absorbs some of the electrons from the beam and emits x-rays at different frequencies, unique to each component of the sample. Two kinds of information are provided: a high quality image of sub-micrometer features of the sample, and a spectrum of elements that are present in the sample. SEM images can be used to examine fracture surfaces, measure the thickness of multi-layered samples, or look for defects in polymer samples. EDS or WDS spectra can identify fillers, indicate differences in filler content, and identify contaminants.
Additive packages can be analyzed by gas chromatography (GC) or liquid chromatography (LC). The sample is injected onto a column, and as it travels through the column, its components are separated and can be seen as individual peaks on a chromatogram. A mass spectrometer (MS) detector analyzes the fragmentation pattern of each peak. The fragmentation pattern or mass spectrum is a fingerprint of the compound, and is compared to reference mass spectra of known compounds in order to identify the unknown sample. The GC/MS method is very useful for the analysis of plasticizers in vinyl materials, and antioxidants in olefinic plastics.
Transmission electron microscopy (TEM) is necessary when looking at materials or contaminants in the nanometer size range. This method is becoming more mainstream as the use of nanomaterials becomes more common in everyday products. In the TEM, electrons travel through an extremely thin sample and form an image, either captured on film or digitally. Excellent for viewing nanofiller dispersion and crystal patterns in materials, this is also one of the most time-consuming methods because it requires intensive sample preparation. Samples must be milled or thin-sectioned to a final size of no more than 1 µm thick—thin enough for electrons to pass through.
For problems involving very thin layers of contamination on a polymer surface, x-ray photoelectron spectroscopy (XPS, also known as ESCA) is the method of choice. In XPS, an x-ray beam is used to generate photoelectrons in the sample, which carry analytical information from only the outermost surface ~5 nm of the sample surface. Thus, this technique is well suited for analysis of thin surface layers and residues on solid samples. In order to analyze greater distances into a sample, XPS is performed while sequentially using an argon ion beam to etch into the sample to expose underlying material for analysis. This technique is known as depth profiling. XPS has been successfully used to measure surface oxidation due to UV exposure, surface chemistry changes after gamma irradiation, and to identify surface bloom and residual mold release agents.
Upon completing the analysis of a contaminant or defect, the laboratory will provide a report to the customer summarizing the analyses performed, usually including photographs and data of the defect to provide the customer with evidence of their findings. The report will summarize the identification of the contaminant, and some possible sources of the problem based on the contaminant composition and the production process as described in the initial consultation with the customer. After making the recommended changes in the production process, the customer may want to re-test their product to ensure that the problem has been eliminated.
To test or not to test; in the wake of several large recalls recently, such as the lead-contaminated toy scare, many companies are wondering if proactive testing makes sense. Defects and recalls can be devastating to a firm’s reputation and future sales, as well as generating fines and possible litigation. Generally, the more expensive the product being sold, the more cost-effective it is to pre-test, due to the high price of replacing and repairing a product once it enters the market. The pharmaceutical and automotive industries have the highest rates of pre-testing due to extensive government regulation of their products and frequent legal actions. Proactive testing may include analysis of incoming raw materials, samples from several steps along the manufacturing process, and a test of the finished product.
When determining how, when, and if one should start proactive testing, a cost-benefit analysis is recommended. If making an expensive or critical part on which lives will depend, proactive testing can reduce or even save a firm from litigation and fines. An important benefit of proactive testing is that problems are caught as quickly as they occur, making their solutions immediate and effective. However, if making a product for a non-critical application such as novelty toys, where some cosmetic imperfections are permissible, proactive testing could be an unnecessary drain on resources. A good-quality laboratory should have no problem quoting a price before testing, and it can counsel customers on whether proactive testing is a wise choice for them.