Small changes in chemical structure show up as shifts in glass transition temperature (Tg). This application note shows how two DMA temperature sweeps on the same epoxy specimen exposed an incomplete cure.
- The first sweep showed two tan δ peaks, near 100 °C and 150 °C, the signature of an under-cured epoxy
- A second sweep on the same specimen showed only the 150 °C transition: the first run had finished the cure
- After curing, the tan δ peak was higher, indicating a distinct viscoelastic behavior
- Tested on the Alpha Metravib DMA+1000 in tension mode, 25 °C to 250 °C at 1 Hz
Tg as a Fingerprint for Cure Quality
Errors in the conception of composite materials can lead to disappointing mechanical properties. DMA is a powerful tool for determining the properties of a wide variety of materials. Among other techniques, DMA is considered one of the most reliable methods for measuring the glass transition temperature (Tg).
The glass transition temperature corresponds to the transition of a material from a hard and relatively brittle “glassy” state to a viscous or rubbery state. This transition is highly dependent on the chemical structure and composition of the material, and even small variations can lead to significant differences in Tg values.
Figure 1. Alpha Metravib DMA
Since different manufacturing or operating conditions can result in variations in the chemical structure of a material, the glass transition temperature can be used to identify manufacturing defects. In other words, DMA can serve as a reliable instrument for chemical process control, such as curing. This application note illustrates how an inadequate curing process can be identified using DMA measurements.
Two Sweeps on One Epoxy Specimen
Specimens
Epoxy materials were studied using the Alpha Metravib DMA+1000 in tension mode. A rectangular epoxy specimen with dimensions of 5 × 5 × 1 mm³ was prepared.
Methods
A sinusoidal dynamic displacement was applied during a temperature sweep from 25 °C to 250 °C at a heating rate of 2 °C/min. A small dynamic strain kept the test within the linear viscoelastic domain, which is recommended for determining Tg.
A static strain was also applied to prevent buckling and to keep the specimen slightly stretched. The test was then repeated on the same specimen to observe changes induced by curing inside the DMA thermal chamber during the first run.
Table 1. Test parameters
| Parameter | Setting |
|---|---|
| Dynamic strain | 0.1% |
| Frequency | 1 Hz |
| Static strain | 0.2% |
| Temperature | 25 °C to 250 °C, 2 °C/min |
Figure 2. Tension mode
Two Transitions Become One
Figure 3 shows the storage modulus E′ and tan δ as a function of temperature at 1 Hz, from 25 °C to 250 °C. The results from two successive measurements performed on the same specimen are shown.
Figure 3. Storage modulus (E′) and tan δ versus temperature at 1 Hz, first and second run on the same specimen
First measurement
A decrease in modulus is observed in two steps: the first above 100 °C and the second slightly below 150 °C. Two tan δ peaks are observed at approximately 100 °C and 150 °C. These results, characterized by two distinct glass transitions, are typical of a material composed of two polymer phases, with each change in E′ or tan δ attributed to one component of the material.
Second measurement
The behavior is markedly different: the decrease in E′ and the tan δ peak observed at 100 °C are no longer present, while the transition around 150 °C remains. The first measurement reveals two tan δ peaks for the epoxy matrix, which is no longer the case during the second measurement.
This behavior indicates that the temperature inside the DMA thermal chamber was sufficient to complete the curing of the specimen. The differences observed between the two runs are therefore attributed to the curing process and the resulting changes in chemical structure. The glass transition observed at 100 °C during the first run is associated with the uncured portion of the material, which becomes fully cured during the first temperature sweep. As a result, this transition is no longer observed during the second measurement.
Additionally, the tan δ peak observed during the second measurement is higher than during the first, indicating a distinct viscoelastic behavior after curing.
What the Data Shows
DMA is a key technique for determining the viscoelastic properties of polymer materials and can therefore be effectively used as a quality control tool. In this example, DMA measurements reveal that the epoxy composite was initially insufficiently cured, as evidenced by the presence of two tan δ peaks instead of a single, expected transition. The first temperature sweep completed the curing process, explaining the disappearance of the lower-temperature tan δ peak during the second measurement.
The bottom line
DMA temperature sweeps can identify incomplete curing by revealing changes in glass transition behavior, making DMA an effective tool for curing quality control.
Curing Quality Control with DMA
Get the complete application note, including all figures and test details, as a PDF.
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