Evaluation of the Curing of Adhesive Systems by Rheological and Thermal Testing

Our methodology combines thermal analysis and rheology to characterize the curing process of an adhesive and to obtain useful information for industrial adhesive selection. This technique allows the creation of a standard guide for the curing process study of adhesive systems, making it easier to compare different adhesives. This methodology can also be used as an accepting criterion in the quality control of adhesive systems.

To perform a thermogravimetric test to determine the inorganic filler content and the temperature at which the material starts to degrade, open the procedure tab and click editor. Drag the ramp segment to the editor screen and establish the ramp as 10 or 20 degrees per minute to 900 degrees Celsius. Click OK and open the notes tab.

Select air as the purge gas and set the flow rate to 100 milliliters per minute. Then close the furnace and start the experiment. To perform a differential scanning calorimetry test of a cured sample, open the procedure tab, click test, and select custom.

Click editor and drag an equilibrate segment indicating the temperature at which to start the experiment. Drag a ramp segment to the editor screen and introduce a heating rate of 10 or 20 degrees per minute and the final temperature into the command editor window. Drag a ramp segment to the editor screen and introduce a 10 or 20 degrees per minute cooling rate to a temperature tentatively below the glass transition.

Drag another ramp segment to the editor screen and introduce a 10 or 20 degrees Celsius per minute heating rate to a temperature slightly below the degradation temperature. Open the notes tab and select nitrogen as the flow gas. Set the flow rate to 50 milliliters per minute and click apply.

Then place a reference pan and a pan with a sample inside the DSC cell and launch the experiment. To analyze the fresh sample through a heating-cooling-heating test, open the procedure tab and click test and custom. Click editor and drag an equilibrate at minus 80 degrees Celsius segment to the editor screen.

Drag a ramp segment and set the heating rate to 10 or 20 degrees Celsius per minute to a temperature slightly below the degradation temperature and insert another equilibrate at minus 80 degrees Celsius segment. Then drag a ramp segment and set the heating rate to 10 or 20 degrees Celsius per minute to the same temperature as before. Click OK.Then place a reference pan and a pan with the sample inside the DSC cell and click start to launch the experiment.

To perform an isothermal curing test, open the procedure tab, click test, and select custom. Click editor and drag a ramp segment to the editor screen. Introduce a 20 degrees Celsius per minute to the chosen isothermal temperature.

Then introduce an isothermal segment with enough time to complete the cure. To check the degree of curing reached, introduce at equilibrium at zero degrees Celsius segment, add a ramp segment, and set the heating rate between 2 and 20 degrees Celsius per minute to the maximum temperature. Drag the mark end-of-cycle segment to the editor window and insert another equilibrate segment with a temperature of minus 80 degrees Celsius.

To obtain the final glass transition, add a ramp segment with a heating rate between 2 and 20 degrees Celsius per minute to the same temperature as indicated before and click OK.In the tool tab, select instrument preferences and DSC, and set a temperature lower than the isotherm temperature of the experiment. Click apply and open the control tab to select go to standby temperature. Then place a reference pan and a pan with a sample inside the DSC cell and click start.

To perform a logarithmic strain sweep test, open the procedure tab and select oscillation amplitude. Set the experimental temperature to room temperature, the frequency to one Hertz, and the logarithmic sweep from one times 10 to the negative three to 100%of strain. Place the sample on the bottom plate with the upper plate separated about 40 millimeters from the lower plate and lower the upper plate until a gap of about two millimeters is observed between both plates.

Then trim the excess adhesive and start the experiment. To monitor the curing of the adhesive, click the procedure tab and select conditioning options. Set the mode to compression, the axial force to zero Newtons, and the sensitivity to 0.1 Newtons.

Click advance and set the gap change limit to 2, 000 microns in both the up and down directions. Insert a new oscillatory time sweep step and set the experimental temperature to room temperature, the test duration as a function of the estimated curing time based on the data sheet of the adhesive and the strain percentage acquired from the previous logarithmic strain sweep test. Select discreet and set the frequencies one, three, and 10 Hertz for all of the samples.

Then load a new sample and start the experiment. To perform a torque sweep test, open the procedure tab and select oscillation amplitude. Then set the temperature to room temperature, the frequency to one Hertz, and the logarithmic sweep from 10 to 10, 000 micronewton meters of torque and start the experiment.

From the torque sweep test, choose a torque amplitude within the linear viscoelastic region to use in the temperature ramp test, then select temperature ramp and set up the experiment at room temperature with a ramp rate of one degree Celsius per minute to ensure a uniform distribution of the temperature into the sample, a frequency of one Hertz, and the torque amplitude determined from the torque sweep test. Close the furnace of the rheometer and open the air stopcock of the furnace. Then start the experiment.

These thermogravimetric results showed different degradation temperatures and different inorganic fillers for each studied adhesive. The mass loss observed between 600 and 800 degrees Celsius suggest the presence of calcium carbonate as filler. For this two-component adhesive, in the heat flow curve, there was no evidence of residual cure and the small deviation cannot be assigned with certainty to the glass transition reported by the manufacturer.

In this table, the degree of curing of a two-component system at different temperatures was calculated by comparing the curing enthalpy acquired at each temperature to that obtained in a heating ramp. Through a rheological multi-frequency test of a fresh two-component adhesive sample, the gelation time of the material can be observed as the point at which the phase angle becomes frequency independent. In these isothermal multi-frequency tests using the one and two-component silane polymer adhesive systems, no sign of gelation is observed and a comparison of the slopes of the moduli of both adhesives reveals that the two-component silane polymer adhesives cures faster.

In this rheological temperature scan test of a two-component adhesive sample cured for one hour, the glass transition can be clearly observed. Do not delay starting the test when using a fresh sample and be sure to prepare a thoroughly mixed solution when using a two-component system.