First, let’s tackle the question why water is a problem for a part made of thermoplastics. Textbooks describe that the water uptake for some types of polyamide (PA) is really high in both 50% relative humidity and in water. This alone would not be the problem, but the uptake of water leads to very different properties of materials. For instance, the modulus of polyamide (PA) decreases up to 66% in a humid atmosphere. Thus, knowing the loss of stiffness of a thermoplastic material is essential in constructing polymer parts. How can dynamic mechanical analysis (DMA) help with this issue?
DMA in theory
In this method, a sinusoidal force (StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress) is applied to the sample as input. This results in a sinusoidal deformation (StrainStrain describes a deformation of a material, which is loaded mechanically by an external force or stress. Rubber compounds show creep properties, if a static load is applied.strain) as output. From both parameters, the modulus can be calculated, which refers to the stiffness of the material. But we can find out even more about the material by means of a DMA measurement! Let’s take the example of a man who has a ball in his hand. He drops the ball on the floor, but the ball will not come back to the original height of the man’s hand. This illustrates that the material exerts different behaviors. The stored energy that remains in the ball to come back up from the ground is related to the storage modulus E´. The missing height that the ball does not jump up is related to the dissipated energy associated with the Viscous modulusThe complex modulus (viscous component), loss modulus, or G’’, is the “imaginary” part of the samples the overall complex modulus. This viscous component indicates the liquid like, or out of phase, response of the sample being measurement. loss modulus (E“). In the end, we also get information on the damping behavior of the material.
In the following examples, it is mostly referred to the storage modulus, because this parameter is most closely related to the stiffness of the material and thus most important in the construction of a polymer part.
Example 1: Polyamide 6 under humid atmosphere
The measurement was conducted with an DMA 242 E Artemis combined with a humidity generator. A relative humidity is applied in the furnace, which allows for the measurement of the dynamic mechanical properties of a material under humidity. A PA 6 sample was measured at a frequency of 1 Hz and a temperature of 40°C in tension mode. The relative humidity was stepwise increased from 0% to 75% over time. The stiffness (described by the storage modulus E’) of the material was measured in these relative humidity steps. It is clearly visible that the stiffness of the material decreases with the increase in relative humidity. At 50% relative humidity, the storage modulus decreased by approximately 74%.
Example 2: Polyurethane in an immersion bath
For this example, the DMA 242 E Artemis was equipped with an immersion bath, which is a container made of steel that can be applied on the sample holder. It is applicable for all sample holders and deformation modes of the DMA 242 E Artemis. Polyurethane (PU) was measured at several frequencies, at a temperature of 25°C and in tension mode. In figure 3, it becomes clearly visible at what point the water was added into the container. There is a decrease in storage modulus over the time the water was in contact with the material. The decrease is significant and amounts to about 17%.
The effect of humidity or liquids on materials needs to be kept in mind when constructing polymer parts for different applications. If a part is designed with the original stiffness of the material, the part is likely to fail in its application environment. This can be avoided by testing the dynamic mechanical properties under service conditions by means of dynamic mechanical analysis. Learn more about DMA 242 E Artemis here. Did you know? Water uptake often leads to a change in the dimensions of a part. TMA equipped with a humidity generator may help assess the length change under a humid atmosphere. Follow us on LinkedIn to not miss the article!