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August 2010
Three Critical "M-Words"
Mechanical Properties
A true understanding of the mechanical properties of raw materials and manufactured products is of obvious relevance to manufacturers. Well established ASTM and ISO tests provide protocols for evaluating raw materials in standard formats. Specific industries and trade associations, all over the world, also promulgate standards.
One can take a portion of compounded plastic pellets and produce well-defined standard shapes for mechanical testing; with tensile (ASTM D638 and ISO 527-1), flexural (ASTM D6395), and compression (ASTM D695 and ISO 604) properties being most common.
For product design and development purposes there are many well established standard test methods to compare and contrast materials. These standard test methods can also be used for lot acceptance testing and for certifying an alternate supplier.
Other standard test methods are product specific. ASTM F2150, for example, is a characterization guide for biomaterial scaffolds. And, packaging is also a critical component that influences the life-cycle of medical devices and products. ASTM F2097, for example, is a standard guide for the design and evaluation of primary flexible packaging for medical products.
Another important variant of mechanical testing is custom testing to predict, verify, evaluate, or explain product performance. Root cause analysis of product failures is often supported by mechanical test data. The sophistication of the tests and the required experience level of the laboratory staff shifts significantly for standard tests compared with product-specific failure analysis investigations.
Morphology
The properties of polymer materials are strongly influenced by the morphology of the plastic material. This is true because virtually every polymer is semi-crystalline. Although the phrase "semi-crystalline " could be inferred to mean "half crystalline " that is not true. Practically speaking, the phrase simply means "partially crystalline " and that's where the technical consequences of morphology begin.
In general, polymers have crystalline regions and amorphous regions. The amorphous region is characterized by the glass transition temperature, is a softening temperature for the polymer, imparts impact resistance, flexibility, optical clarity, and higher gas permeability.
In contrast, the crystalline phase of a polymer is characterized by the melting temperature, sets the upper use limit of the polymer, increases stiffness, can cause opacity, and lowers gas permeability.
A specific polymer does not have a unique pre-defined level of amorphous and crystalline content. The percent crystallinity is influenced by the polymer, additives, process parameters, storage conditions, and end-use environment (exposure to mechanical stress, elevated temperature, or solvents, for example). Mechanical properties and product performance are dependent on the crystallinity which may change over time. Post-processing steps can be taken to produce a more stable polymer system with annealing being a common approach.
Determining the morphology of polymers is typically done using either differential scanning calorimetry (DSC) or x-ray diffraction (XRD) analysis.
The DSC method determines heat capacity as a function of temperature and reveals thermal transitions such as the glass transition temperature and the crystalline melting temperature. Polymers actually have broad melting ranges, very different from the sharp melting peaks for pure organic compounds. As a result, it is important for the polymer engineer to get the full details of the melting transition which includes the onset and termination of melting, not just the peak temperature. ASTM D3418 and ISO 11357-3 describe the application of DSC to plastic materials.
The more sophisticated XRD method is often used to calibrate the DSC method by directly providing the percent crystallinity.
Molecular Weight
The molecular weight of a polymer is second in importance only to the chemical structure of the polymer itself. The molecular weight of a plastic material directly correlates with the "size " of the molecules that form the plastic. Higher molecular weight will generally provide a stronger material but may cause process issues. So, trade-offs are made between processing characteristics and product performance.
Another component of "molecular weight" is the molecular weight distribution. Just as people are not all the same height, the molecules of a polymer also have a distribution of sizes. Polymer distributions can be narrow, broad, or polydisperse. The decision as to whether a distribution is broad or narrow is often process and performance based. An injection molding grade of polymer may be said to have a broad molecular weight distribution whereas the same resin in a blow molding application may be considered narrow. Polydispersity is easier to objectively determine because polydisperse polymers are composed of two or more distinct distributions.
The breadth of molecular weight (MW) distribution curves and the potential for polydispersity cause gel permeation chromatography (GPC) or size exclusion chromatography (SEC) to be the most powerful analysis methods more molecular weight characterization. This is true because the GPC and SEC methods provide the concentration of each molecular size. The analysis requires expert analysts, moderately expensive instrumentation, and frequently very aggressive solvent systems.
Simpler MW analysis techniques are intrinsic viscosity (IV), dilute solution viscosity (DSV), and melt flow index (MFI, or melt flow rate, MFR) tests. Although operator experience and skill, and properly calibrated and validated methods and equipment are still required, the DSV and MFI equipment is low cost compared with the GPC and SEC methods.
The IV and DSV techniques use dilute polymer solutions. Both techniques are excellent for determining average molecular weight. Technologically, these methods are often used to track raw materials and molecular weight shifts in response to processing, sterilization, and shelf-life. The IV and DSV methods provide only "a single data point", not the molecular weight distribution.
Like IV and DSV, the melt flow index value is also a single data point. In this case, however, the polymer is evaluated in the molten state, not as a dilute solution. The most common reasons for performing MFI tests are to verify that raw materials conform to a specification or certificate of analysis (C of A) and for processors to have starting-point values for setting process parameters. Comparison of MFI values for the same polymer purchased from several suppliers, for example, gives polymer processors an idea as to whether process difficulties are likely.
Polymer materials have a wide range of process and performance characteristics that are influenced by many factors. Of those many factors, mechanicals, morphology, and molecular weight are three of the most common. A large percentage of the thousands of industrial-related projects I have worked on involved these three factors. Of course there are other factors that influence process and performance properties of plastics, additives, contamination, and regrind, for example.
We welcome the opportunity to Partner our Experts with your team. You can call us, toll-free (1-877-961-4341) or visit our web-site and connect with us by e-mail or Online Chat.
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In this issue:
Three Critical M-Words
There are many reasons why companies have submitted samples to us for the thousands of projects we have completed.
When I reflect on the many reasons for polymer testing, it occurs to me that there are three principal ones that are of high importance to engineers and researchers who use polymer materials;
- Mechanical Properties
- Molecular Weight
- Morphology
This issue focuses on these three topics.
You can get even more information about these important topics, and others, by visiting PolymerSolutions.com or by talking to one of our Experts.
Call 1-877-961-4341.
Cool Pics
Also in this newsletter is the very popular section, Jay's Cool Microscopy Pics of the Month!
Bonus Content This Month:
- Jay Revealed
- A Few Words From The Founder
- Events PSI is attending
1. Jay is our Microscopy and Physical Testing Expert.
Virtually every day, he is capturing images of interesting subject matter, usually for clients, sometimes just because he can.
A wide range of images are captured by Jay and his team using digital photographs, optical microscopy images, and very high magnification electron microscopy images.
There's more to the man than his images.
In the last issue we offered a $10 Starbucks Gift Card to readers who correctly identified the five true statements from the choices below. No one, not even Jay's coworkers, selected properly.
Curious aren't you?
The questions and the correct answers, with some additional information from Jay himself, is just a click away...
Click to link to Jay's True Answers.
2. A few words from the Founder
Frequently Asked Questions
Q: What is one of the biggest challenges to Polymer Solutions Incorporated?
A: My immediate response to this question is actually two big challenges: "the current economic climate" and "change."
There is no doubt that the economy has caused challenges for businesses in virtually every industrial sector. It is also apparent that for many there will be no return to "the good old days."
We are constantly challenged to do more with less and to provide increased value to our clients while carefully controlling costs.
At Polymer Solutions we are ardent problem-solvers. We also see business "problems" as another opportunity to apply scientific methods: experimentation, analysis, hypothesis-testing, measurements and adjustments to continually improve the business itself.
Change is also an ever-present challenge to Polymer Solutions.
Click here for the rest of Jim's comments.
Where We Are Heading
Polymer Solutions is sending staff teams to:
September 20-23, 2010
The International Tire Exposition and Conference in Cincinnati Ohio
Dr. Jim Rancourt is moderating a session.
September 22-23, 2010
The Orthopedic Design and Technology Conference and Exhibition in Ft. Wayne Indiana
Polymer Solutions is exhibiting.
November 3-4, 2010
The Medical Products Outsourcing Symposium in Waltham Massachusetts
Polymer Solutions is exhibiting.
If you are attending, let's make plans to have coffee together!
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