Two Powerful Thermal Methods
Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry is an analytical method in which a sample is subject to programmed heating and cooling protocols between prescribed temperature limits. The heat capacity of the sample is measured as a function of time and temperature in a controlled ambient environment. For example, a plastic sample may be cooled to -80 °C, heated from -80 °C to +250 °C at 10 °C per minute, cooled back to -80 °C at 10 °C per minute, and then reheated.
The first heating cycle provides information about the thermal transitions within the sample: side group rotations, segmental motions of the polymer chain, and melting of crystalline domains, for example. Some subtle thermal transitions and the specific details of the melting endotherm are caused by the process history imparted to the plastic. Therefore, it is often very important to compare a "second heating cycle" DSC curve with the "first heating cycle" DSC curve to distinguish properties characteristic of the polymer with those properties imparted by the process.
In failure analysis investigations it is a common approach to compare the first and second heating cycles of "good" and "bad" samples or from "failed" and "non-failed" regions of a manufactured product. A challenge to the interpretation of the data sets is that a "non-failed" region may actually be a "not failed yet" region, meaning the properties of the failed and non-failed regions are not substantially different. In those cases, additional samples or analytical methods must be applied.
The DSC method is also often used to determine residual cure in two-part polymer systems, epoxies for example. The physical properties of epoxy polymers depends on the molecular structure of Part-A and Part-B, proper mixing and mix ratio, and the cure time and temperature. Using the DSC method of analysis one can readily determine the extent of cure of an epoxy system. Most often, clients simply want us to answer the question "is the epoxy fully cured?" DSC is a great tool for that job.
What We Use DSC For...
Biosorbable and biodegradable polymers have degradation rates and physical properties that are strongly influenced by their level of crystallinity. In the case of semi-crystalline polymers, the extent of crystallization is influenced by molecular weight, impurities, and process conditions, and the crystallinity can change with time. DSC is a powerful analytical method for determining the level of crystallinity that exists in manufactured products during the lifetime of the product and as a function of intentional variations.
Encapsulating resins, epoxies in particular, achieve their desired properties when the mix ratio and cure profile is consistent with the "recipe." Insufficient cure will cause off-gassing, poor solvent resistance, reduced thermal stability, low modulus, and reduced abrasion resistance. The DSC method can readily detect a residual cure exotherm, a direct indicator of incomplete cure. In addition, the glass transition temperature can be documented in the epoxy as produced by the manufacturing process (DSC first heating cycle) and the maximum glass transition temperature achievable with the epoxy system (DSC second heating cycle). For failure analysis the comparison of the first and second heating cycle DSC curves typically proves to be very diagnostic.
The DSC method of analysis can also be used to distinguish specific members of certain classes of polymers: various polyethylenes and polyamides, for example. In the case of polyamides, a materials identification by Fourier transform infrared spectroscopy may reveal with high reliability that a certain product is comprised of either polyamide 6 (Nylon-6) or polyamide 6,6 (Nylon-6/6). The spectral features make a definitive interpretation solely by FTIR unreliable. Coupled with DSC however, because of the substantial different melting peak endotherm temperatures (220 °C for PA-6 compared with 260 °C for PA-6/6), these two amides are readily distinguished. It is actually often the case that multiple analytical methods are required to provide the level of conclusive interpretations in failure analysis projects.
Polymer Solutions Incorporated can develop and validate custom DSC methods. We can also use established ASTM or ISO Methods. Examples of ASTM Methods of DSC include:
ASTM D3895 Standard Test Method for Oxidative-Induction Time of Polyolefins by Differential Scanning Calorimetry
ASTM D4419 Standard Test Method for Measurement of Transition Temperatures of Petroleum Waxes by Differential Scanning Calorimetry
ASTM D7426 Standard Test Method for Assignment of the DSC Procedure for Determining Tg of a Polymer or an Elastomeric Compound
ASTM E1356 Standard Test Method for Assignment of the Glass Transition Temperatures by Differential Scanning Calorimetry
ASTM E1858 Standard Test Method for Determining Oxidation Induction Time of Hydrocarbons by Differential Scanning Calorimetry
ASTM E1952 Standard Test Method for Thermal Conductivity and Thermal Diffusivity by Modulated Temperature Differential Scanning Calorimetry
ASTM E2009 Standard Test Method for Oxidation Onset Temperature of Hydrocarbons by Differential Scanning Calorimetry
The International Organization for Standardization (ISO) has over a dozen DSC Standards with the most recent three being:
ISO 28343 Determination of glass transition temperatures by DSC (DSC)
ISO 11357-1 Differential scanning calorimetry (DSC) Ė Part 1: General Principles
ISO 11357-6 Differential scanning calorimetry (DSC) Ė Part 6: Determination of oxidation induction time (isothermal OIT) and oxidation induction temperature (dynamic OIT)
Thermogravimetric Analysis (TGA)
Thermogravimetric analysis is an analytical method in which a sample is subjected to a programmed heating protocol to a prescribed upper temperature. The mass of the sample is measured as a function of time and temperature within a controlled ambient environment. Most samples, especially plastic and rubber materials will lose mass as the sample is heated. At moderate temperatures, or even at room temperature, the sample may lose mass due to the release of adsorbed moisture or process solvents. At higher temperatures, the sample will release additional volatile and semi-volatile ingredients, including process aids, additives, and plasticizers. And, at even higher temperatures, the polymer will decompose and produce volatile constituents leaving principally carbon black and inorganic fillers behind. By intentionally switching the ambient environment from nitrogen to air, the carbonaceous residues can be converted to carbon dioxide gas leaving the inorganic components behind, principally as metal oxides. Thus, the mass loss versus temperature profile provides a basic compositional profile for the sample. This TGA method is very useful for comparing rubber compounds to each other and for evaluating masterbatch compounds.
In failure analysis investigations a good and bad region from a manufactured product or, alternatively, the exact same location from multiple types of manufactured parts are compared with each other. Some polymer compositions, those used in some medical implant applications for example, are not supposed to contain any inorganic fillers. The TGA method is a very simple approach for determining if that is in fact the case. The residues that are released from the sample during the TGA experiment can next be characterized using inductively coupled plasma (ICP) optical emission spectroscopy (OES) of the dissolved residue. If the TGA provides an insufficient quantity of residue, a larger sample can be generated using a digestion protocol. The particle size, shape, and morphology, together with the specific chemical elements that comprise the residue can be determined using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS).
What We Use TGA For...
TGA is a method that involves monitoring the mass of a sample as the temperature is increased. The temperature at which the organic portion begins to volatilize is related to the thermal stability. In the case of a failure analysis, "good" and "bad" samples can be compared to see if this temperature varies between the two samples. A lower onset of degradation may indicate that insufficient stabilizers are present, for example.
TGA is also used to determine compositional information about samples. Organics will volatilize away by about 400 °C to 500 °C. If the testing is performed in nitrogen, this will leave behind any carbon black and inorganic fillers (things like talc or titanium dioxide). Switching the gas to air will cause the carbon black to volatilize, so exploiting the gas switch allows one to determine compositional information. This is covered in ASTM D6370, which applies to rubbers. In addition, the remaining material, which is the inorganic filler, can then be examined using other methods, such as scanning electron microscopy with energy dispersive analysis (SEM-EDS) to determine form and composition.
In some cases, TGA can be useful to detect the presence of volatile components at a temperature of interest. A sample can be held isothermally and the mass monitored for some length of time. This may be of interest if, for example, residues are being noted during a processing step. Comparison of samples can indicate if one contains either more of a volatile component or other low molecular weight material.
Polymer Solutions Incorporated can develop and validate custom TGA methods. We can also use established ASTM or ISO Methods. Examples of ASTM Methods of TGA analysis include the following:
ASTM E1131 Standard Test Method for Compositional Analysis by Thermogravimetry
ASTM E1877 Standard Practice for Calculating Thermal Endurance of Materials from Thermogravimetric Decomposition Data
ASTM E2008 Standard Test Method for Volatility Rate by Thermogravimetry
ASTM E2105 Standard Practice for General Techniques of Thermogravimetric Analysis (TGA) Coupled With Infrared Analysis (TGA/IR)
ASTM E2550 Standard Test Method for Thermal Stability by Thermogravimetry
ASTM D6370 Standard Test Method for Rubber-Compositional Analysis by Thermogravimetry
ASTM D6382 Standard Practice for Dynamic Mechanical Analysis and Thermogravimetry of Roofing and Waterproofing Membrane Material
The International Organization for Standardization (ISO) also publishes TGA Standards, including:
ISO 11358 Thermogravimetry (TG) of polymers -- General principles
ISO 13358-2 Thermogravimetry (TG) of polymers -- Part 2: Determination of activation energy
ISO 9924-3 Determination of the composition of vulcanizates and uncured compounds by thermogravimetry -- Part 3: Hydrocarbon rubbers, halogenated rubbers and polysiloxane rubbers after extraction
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.
In this issue:
Two Powerful Thermal Methods
There are many thermal methods of analysis that are used to determine the response of plastic and rubber materials to a change in temperature. These analytical methods are very important for at least two reasons.
First, many plastic and rubber materials are not used only at "room temperature" but instead are exposed to low temperatures or to high temperatures. For example, the external components of commercial aircraft may start a flight at temperatures near 100 įF and climb to a flight altitude that reduces the temperature to -50 įF.
Second, the change in a physical property as a function of temperature can be used to understand fundamental characteristics of polymeric materials and can also be used to provide highly detailed comparisons among materials.
Common physical properties that are measured include thermal stability, heat capacity, electrical conductivity, and dimensional changes.
Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) are the most requested thermal analysis methods. This issue focuses on these two methods.
You can get even more information about these important topics, and others, by visiting our web-site or by talking to one of our Experts.
Bonus Content This Month:
- An Amazing Time-lapse Video
- A Few Words From The Founder
- Events PSI is attending
1. Environmental Stress Cracking
(ESC) of plastics is a major cause of failure. Each class of polymer material has its own Achillesí Heel, an extreme sensitivity to families of chemicals that cause severe damage to an otherwise robust plastic.
Acrylonitrile-butadiene-styrene terpolymer (ABS) is highly sensitive to DPM exposure (dipropylene glycol monomethyl ether), polyethylene (PE) is sensitive to silicone oil, and chlorinated polyvinylchloride (CPVC) is highly sensitive to phthalate esters.
CPVC is used in piping systems to transport fluids. In the field, there are numerous routes of unintentional exposure of CPVC to phthalate ester plasticizers, such as dioctylphthlate (DOP). Exposure of the CPVC to DOP in the presence of sufficient stress can induce an ESC event.
View the time-lapse video that demonstrates the power of a drop of DOP on an otherwise robust CPVC pipe segment. We have compressed 900 minutes into 100 seconds.
Click here to view the video.
2. A few words from the Founder
Frequently Asked Questions
Q: What is it like to serve as an expert in law suits?
A: That is a short question that has a long answer with many facets. First of all, careful consideration needs to be given to the subject content and the expectations of the client. If I am confident that my background, training, and experience can provide real value to the client, then we move to the next step. The next step is a conflict-of-interest check to be sure that Polymer Solutions has not had previous interactions with parties of the law suit that would be viewed as a conflict.
Then, the details of the clientís expectations and the scope of the engagement are discussed and specific boundaries for the scope of the project are set. Agreeing to serve as an expert carries with it the implicit assumption that I will be available 24/7 and that the requirements of the case will trump virtually all other professional and personal obligations. This availability can at times be very inconvenient but it is necessary because the courtís calendar and schedule are very inflexible.
Q: Is a legal project different from an industrial project?
A: Legal projects do have a very high level of scrutiny and review, by the client, the opposing law firms, and especially the opposing experts. Therefore, clarity and completeness are critical. It has to be expected that every result, data point, opinion, assumption, and interpretation will be questioned. This is true from the simplest to the more complex subject matter. It is vital that the work be thorough, logical, and understandable. Legal projects are most similar to medical client GMP projects, another sector of analysis that requires very thorough documentation. Although our quality system is in force for every project that is undertaken, those projects for which a client has asked for a specific method to be applied to their sample is most different from the litigation oriented projects. The reporting in these cases often involves a Certificate of Analysis rather than a technical report.
Q: What is the most unusual case you have worked on?
A: I have had the opportunity to work on industrial espionage, patent infringement, personal injury, and product liability cases. Each one has very interesting aspects or features. Two come to mind, one being my first case and the other being an international case.
Q: What was your first case?
A: The first legal case I worked on involved involuntary manslaughter charges against a heating system repairman who was accused of causing a home resident's death. The coronerís report indicated the presence of carbon monoxide in the decedentís lungs, taken as proof he had died as a result of inhalation of carbon monoxide from a faulty heater installation.
The coroner used gas chromatography (GC) with a thermal conductivity detector (TCD) and concluded that carbon monoxide was in the decedentís lungs, thereby proving he had died as a result of carbon monoxide exposure.
I explained to the court that the TCD is not able to identify a chemicalís identity. I further explained that the use of a single chromatography column and a peak having a retention time consistent with carbon monoxide does not verify that the peak is in fact carbon monoxide. The coroner should have used a second different chromatography column or should have used a spectrometric detector.
Further, the coroner had indicated an approximate concentration of carbon monoxide but did not account for the fact that carbon monoxide is present in the earthís atmosphere. The formation of carbon monoxide within the lungs of the decedent, post mortem, was also not addressed by the coroner.
The heating system repairman was acquitted.
Where We Are Heading
Polymer Solutions is sending staff teams to:
The Medical Products Outsourcing Symposium in Costa Rica May 11 through 13, 2011.
Polymer Solutions is a Silver Sponsor of this event.
The Medical Products Outsourcing Symposium in Waltham Massachusetts October 19 through 20, 2011. Polymer Solutions is exhibiting.
If you are attending, let's make plans to have coffee together!