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Marsh Tech’s Analytical Services are standard polymer analysis techniques that will characterize the bulk composition, thermal transitions, quantity of fillers or reinforcements, and rheology of a plastic sample. They are the practical starting points for any material identification. The common applications include receiving inspection, process validation, competitive product ID, and failure analysis.
TGA – Thermogravimetric Analysis
A: Description of Equipment
The TGA equipment consists of a very sensitive microbalance, a furnace, a gas switching box, and a temperature controller.
B: Data Collected
The weight change of the sample is collected versus time and temperature. The temperature can be ramped or isothermal from ambient to over 1000 degrees C. Also, the purge gas can be changed as programmed via the gas switching box. A standard TGA is a run from ambient to 900C at 20C/minute with a gas switch from nitrogen to air or oxygen at 700C.
C: Output
A typical TGA curve for a plastic is a plot of sample weight versus temperature. Often the weight is normalized and plotted as a percentage of the initial weight (100%) to make the data easily comparable to other data. The x-axis is usually temperature but can be time if different temperature ramp rates are used or if the run was isothermal. If this is the case, the right axis often becomes temperature and an overlay plot of time versus temperature is given.
D: Analysis
The analysis of a TGA curve involves calculating the change in weight from one point to another. Generally this is calculated from one plateau to another, the loss being polymer, additives, or carbon. The derivative of the weight loss curve can be used to show changes in rate of loss otherwise hard to detect and is especially helpful in isolating the different components of a material.
Any noncombustibles are shown by the remaining weight percent. Often referred to as the residue, the final plateau on a TGA curve represents the portion of the sample that did not burn off. Typically the residue is a glass or mineral reinforcement, but can be a radio-pacifier such as bismuth sub-carbonate or an additive or extender such as talc.
E: When to use
TGA is used to look for the percentage of carbon, noncombustibles, fluoropolymer, or any other components that come off at a given temperature. Carbon and noncombustible levels are obtained by switching to air or oxygen once the TGA curve has plateaued past 600C. Typically the switch is done at 700C and the carbon loss and residue are calculated. (Noncombustibles can be calculated in nitrogen if it is known that only polymer and the noncombustible are present.) Fluoropolymers generally come off at a higher temperature than other polymers (550C versus 450C) allowing the percent PTFE for example to be calculated. Other components, such as the additives and polymers found in highly plasticized PVC, can be quantified for comparative purposes using TGA.
F: Sample description
TGA is performed on a sample up to approximately 100 mg. The sample size is limited by the size of the sample pan and the range of the microbalance. A typical sample size for a plastic is 20 mg to 50 mg.
G: Shortcomings
TGA analysis assumes sample homogeneity. This assumption is often acceptable when analyzing a processed plastic that has been homogenized during its process, but can be misleading in the case of a raw material or other such sample that has gone through little or no homogenizing process.
TGA analysis does not identify any components. The conclusions about what comes off at a given temperature are based on experience and published information.
DSC – Differential Scanning Calorimetry
A: Description of Equipment
The DSC equipment consists of a cell with a reference and sample thermocouple and a temperature controller.
B: Data Collected
The DSC collects heat flow versus time and temperature. The temperature can be ramped or isothermal from -150C to 600C. Typically the data is collected with a purge gas of nitrogen, but air or oxygen are used for oxidative work. First heats or single heats can be used with materials less susceptible to thermal histories. A second heat is collected for materials that look significantly different on a second heat. The second heat is collected after allowing the amorphous material to quench cool or the semicrystalline material to slowly cool.
C: Output
A DSC scan is a plot of heat flow versus time or temperature. When the horizontal axis is time, the right axis is often temperature. Also, normalizing the heat flow with sample weight makes the data easier to compare.
D: Analysis
The analysis of a DSC scan involves identifying any thermal transitions. Amorphous materials will have a glass transition and semicrystalline materials will have a glass transition and a crystalline melt endotherm. (Exothermic reactions are the giving up of energy, and endothermic reactions are the taking up of energy.) Some semicrystalline materials may also exhibit an exothermic reaction while heating indicating crystallization in the DSC cell. All semicrystalline materials exhibit this exothermic reaction when cooled from a molten state. These transitions are thermal characterizations of the polymer. For glass transitions, the onset, mid, and end temperatures are calculated. For melt endotherms, the onset and peak temperature of the melt endotherm along with the heat of fusion (area within the endotherm) are calculated.
E: When to use
DSC is used for thermal characterization of raw materials, processed parts, or competitive products. The unique transitions of each polymer allow one to identify a product or determine if a product is contaminated.
DSC is used to check the crystallinity of some semicrystalline materials after processing. The absence of an exothermic reaction while heating indicates that the sample is fully crystalline as processed.
DSC can be used to look for molded in stresses of a processed part. Often the first heat of a part with molded in stress will have reactions that are do to the thermal history of the part. These reactions are erased by this initial heat, however, and the sample can be analyzed with a second heat for true thermal characterization.
F: Sample description
The DSC sample is limited by the size of the sample pan. Typically DSC samples are between 2 and 20 milligrams.
G: Shortcomings
DSC analysis assumes homogeneity. This assumption is often acceptable when performing competitive product analysis or checking for gross contamination in a processed part because the processing has homogenized the sample. DSC can not be expected, however, to identify an inhomogeneous contaminant in a raw material or processed part unless the suspect area is visible and can be isolated.
There are limited DSC libraries and interpretation is based on material specifications and experience. When a history of DSC on the material is not available, it is recommended that DSC be coupled with FTIR for a higher degree of confidence when performing material IDs.
Some amorphous materials do not exhibit a glass transition by DSC. Other techniques, such as DMA are needed to obtain a glass transition temperature.
FTIR – Fourier Transform Infrared Spectroscopy
A: Description of Equipment
A Fourier Transform Infrared Spectrometer consists of an infrared light source, an interferometer, a detector, and a data station.
B: Data Collected
The FTIR technique is often referred to as a fingerprinting technique. The instrument relies on the physics of electromagnetic radiation in the infrared region and how this radiation is absorbed at certain frequencies by the sample molecules. Using a Fourier transform, the data station transforms the detector signal into a spectrum showing the sample transmittance versus the infrared region wave-numbers.
C: Output
An FTIR spectrum is a plot of % transmittance or absorbance versus wavenumber. (Wavenumber is the reciprocal of wavelength.) The regions of absorbance are indicative of the molecular bonds present in the sample.
D: Analysis
Analysis of an FTIR spectrum often begins with a comparison of the sample spectrum to standard spectra such as those found in the Hummel or Aldrich Polymer Libraries. If a standard match is not found, one can consult with material suppliers or other technical persons knowledgeable in the area of FTIR.
E: When to use
FTIR is used for material characterization. Typical applications include receiving inspection and competitive product identification. It is also used to verify material composition in failure analysis applications.
F: Sample description
The most common FTIR sampling technique is referred to as standard sampling. It requires that the sample be a thin film that is at least translucent in some areas. This technique is well suited to thermoplastic analysis due to the ease with which most thermoplastics melt and press into thin films.
Another sampling technique is Attenuated Total Reflectance (ATR). This technique analyzes the sample as it contacts a crystal sample holder. This technique is used for FTIR analysis of rubbers, thermoplastic elastomers, and any other solid that can be pressed into contact with the sampling surface. This technique does not work well with small or rigid materials that do not provide enough contact area.
There are also gas and liquid sampling attachments.
G: Shortcomings
FTIR analysis does not give information regarding the order or configuration of the molecules in the sample. This means that it is not a good tool for differentiating between members of the nylon family for example. Often another test such as DSC is required to provide a complete material identification.
FTIR patterns overlap. Another test method such as DSC is needed to check for contamination when the suspected contaminant is masked by the main polymer spectrum.