The application of magnesium oxide to fluoroelastomer compounds has already been mentioned here on the blog. We talked about how useful this substance is for the curing process in halogenated butyl rubber formulations and explained some benefits of this process. However, you may be asking yourself: how were such interesting results achieved?
In this article, we will review details of the research conducted to understand the benefits of applying magnesium oxide to fluoroelastomer compounds. We will explain the methodology used and the results achieved. All for you to be sure of the effectiveness of this active ingredient in cross-linking the fluoroelastomer rubber. Follow along!
Sample preparation
Magnesium oxide samples were supplied by Bel Mag, extracted from seawater, as is done with all of the company’s products. For the research, three samples from three different batches of magnesium oxide with varied surface areas were used.
In two of these samples, a modification in the surface area was made, resulting in five samples. Thus, to prepare sample 2, calcination was performed at 800 °C for four hours on sample 1. When preparing sample 5, calcination was done at 700 °C for two hours with sample 4. The purpose of this process is to reduce the surface area.
All samples underwent three assessment steps:
1 – Particle distribution
To determine the particle size distribution, the CILAS 1090L particle size analysis equipment was used, which disperses the solid material in a vat containing a liquid where the sample to be analyzed remains insoluble. In this case, isopropyl alcohol was chosen.
2 – Surface area analysis
Also known by the acronym BET (Brunauer, Emmett and Teller), this method consists of using a closed system with constant temperature to determine the amount of gas absorbed by the porous particle. It is a process that generates an increase in the mass of the particle and reduces the pressure of the gas. When these two values are stable on the partial pressure curve, the test is terminated.
3 – Scanning electron microscopy analysis
For this step, a scanning electron microscope with field emission was used, which allows greater approximation and sharpness of the imaging with 50 times the magnification to understand the shape of the particles, their porosity and verify the primary and secondary particles.
After the magnesium oxide samples were characterized, application tests were performed on a fluoroelastomer (FKM) formulation. We show how this step was conducted below.
Application of magnesium oxide to fluoroelastomer compounds
The magnesium oxide samples were subjected to fluoroelastomer application tests in five formulations prepared in the same way: in an open cylinder mixer, carbon black and magnesium oxide were mixed with calcium hydroxide. From this, five formulations were prepared. The only modification in each of them was the magnesium oxide sample.
This material stayed at rest for 24 hours. Part of it was separated for Shore A hardness analysis, which consists of subjecting the material to a pressure applied by a calibrated spring that acts on the conical indenter. The hardness value is a result of the penetration depth into the tested material.
For this, it is necessary to cross-link the test specimen. In this case, this step was carried out in an electric press with a temperature of 175 °C for 6 minutes. The test specimens were 18 mm in diameter and 6 mm in thickness.
But there was also a second part of the material that was used for the rheometric analysis, capable of determining the minimum and maximum torque, Scorch time, T10, T50 and T90. For this, a rheometer at a temperature of 177 °C was used.
Results obtained
Shore A hardness did not show variation between samples, except for sample 2, which was used with magnesium oxide with a surface area of 5m²/g. The maximum and minimum torques had higher values when the surface area of magnesium oxide in fluoroelastomer compounds is also higher. With a smaller surface area, maximum and minimum torque results also reduce.
To determine the Scorch, T10, T50 and T90 times, it was verified that the greater the surface area, the lower the values in these cases. Microscopic images show that the particles are very similar in shape and size, even in samples with high and reduced surface area. In the magnified view of the microscope, it was not possible to observe differences between particles with high and low surface area.
And it was based on this research that it was possible to ensure the efficacy of magnesium oxide for curing fluoroelastomer compounds. Samples 1, 3 and 4 were observed to act as acid receptors in the FKM composition in the bisphenol system, providing efficient curing.
For samples 2 and 5, the initiation cross-link time and the optimum vulcanization time were longer, due to the low reactivity of the magnesium oxide intrinsically linked to its low surface area, which in turn causes the neutralization reaction of hydrogen fluoride to occur more slowly.
We hope that this article has been enlightening and also effective in the mission of proving the effectiveness of magnesium oxide application in fluoroelastomer compounds. If you have any questions, please feel free to contact us. And keep following our blog for more useful information on this topic!
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