Written by Travis Parkman, Subject Matter Expert
A common use for Dynamic Mechanical Analysis (DMA) is indirect process monitoring and quality control (QC) checks. The type of QC test depends on the nature of the sample, application, and material; however, some of the most common tests I see in ‘industry’ are strain sweeps to understand filler-filler interaction or temperature sweeps to measure the glass transition (Tg) temperature to understand the state of cure. In this blog post, I would like to highlight some great work done by Metravib’s application team on how DMA is used for QC checks via measuring Tg.
Glass Transition Temperature Determination
The glass transition (Tg) temperature defines the region when a material transitions from a hard rigid solid to a softer rubbery material, or vis-versa. There are different approaches for measuring Tg, but DMA is considered the most sensitive approach to determine Tg, especially for cured polymers and filled materials [1].
DMA determines a material’s transition temperature by measuring the material’s viscoelastic properties during a controlled temperature ramp as is shown in Figure 2.
The reduction in the storage modulus (red curve) indicates that the material is undergoing a large change, specifically becoming relatively softer and less stiff. This large drop in stiffness is known as the glass transition or alpha transition. The Tg can be determined by the curve of the storage modulus, which is known as the onset Tg, but typically we see groups use the peak in the loss factor (blue curve) to determine the glass transition. This peak is related to the material relaxing and flowing during the glass transition [1].
Before I jump to the application note, I want to finish off the Tg discussion with a common question I get asked when talking about the glass transition temperature….
“If the Tg can be measured by DMA, Differential Scanning Calorimetry (DSC), and Thermomechanical Analysis (TMA). WHICH APPROACH IS CORRECT?!”
In short, each approach is correct. Each method measures a different event that is happening during the glass transition, which happens at different temperatures. Therefore, my recommendation is to always stick to your industry standard. A common way of describing the glass transition, which I like, is describing this transition over a region, rather than a specific temperature [1].
Using a Metravib DMA to evaluate the state of a cure for an Epoxy
A temperature sweep test was performed on an epoxy material using a Metravib DMA. The epoxy sample was deliberately partially cured for this study to demonstrate the differences in temperature response.
The temperature sweep was carried out from 25C to 250°C at a fixed rate and was repeated twice to show the difference in the state of cure after curing the sample in the DMAs thermal chamber. Figure 3 shows the storage modulus and loss factor plotted against temperature for the two temperature sweeps [2].
Focusing on the loss factor (blue curve), the first measurement shows two peaks, typical for polymer blends of multiple materials. After fully curing the material at an elevated temperature, the second temperature sweep (second measurement) has one distinct peak. When comparing the two runs, it becomes apparent that the epoxy was not fully cured in the first run and the additional peak at 100°C is attributed to the uncured component and the peak near 150°C is attributed to the glass transition temperature.
A key takeaway from this study is that an improperly cured material will not act as intended and will often have poor performance. Let’s shift our focus to the storage modulus (red curve) shown in Figure 3 to demonstrate. The modulus is a measure of how a material can resist a deformation, i.e. high modulus, more force is required to deform. This is critical for epoxy/adhesives to hold in place. The maximum operating temperature for an epoxy is determined by the glass transition temperature, as this indicates the temperature at which a material’s stiffness will decrease and lose its structural integrity. Keeping this in mind, with this example, the results show that the poorly cured epoxy begins to lose its stiffness under 100 °C, even though the glass transition is assumed to be near 150 °C [2].
Closing Remarks
In conclusion, DMA is the most sensitive approach to determine Tg for cured materials and can be used as a quality control check for the state of the cure. In this example, the DMA was able to recognize the difference in Tg of the partially cured state and the fully cured state of the same sample.
Although, this study used an epoxy material, comparing the Tg is used by groups in all different industries, including rubbers and foams [1,3]. If you have an interest in reading more about how DMA can help you, check out our previous blog posts and Metravib’s DMA white paper.
WORK CITED
- Menard, K. P., & Menard, N. R. (2020). Dynamic Mechanical Analysis. CRC Press, Taylor & Francis Group.
- Badard, M. (2023). Curing Quality Control with DMA. Internal MVB Testing Application Note: unpublished.
- Mills, N. (2007). Polymer Foams Handbook. Butterworth-Heinemann, Elsevier Ltd.
About the Author
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Travis obtained his degree in Mechanical Engineering in 2015 from the University of New Brunswick. After graduation, he began his Masters, where he investigated a new method to measure cutting forces produced during machining. This research was later converted to a Ph.D. program, with a focus on identifying and adjusting for inertial effects present in force measurements used to monitor machining processes. Travis is the Metravib Subject Matter Expert. |