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

Ana D&#237;az-D&#237;az; Barbara S&#225;nchez-Silva; Javier Tarr&#237;o-Saavedra; Jorge L&#243;pez-Beceiro; Silvia G&#243;mez-Barreiro; Ramon Artiaga

doi:10.3791/61468

JoVE Journal > Engineering

Engenharia

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

Published: July 03, 2020

doi:

10.3791/61468

Ana Díaz-Díaz*¹, Barbara Sánchez-Silva*¹, Javier Tarrío-Saavedra*¹, Jorge López-Beceiro*¹, Silvia Gómez-Barreiro*¹, Ramon Artiaga*¹

¹Escola Politécnica Superior,Universidade da Coruña

Summary

An experimental methodology based on thermal and rheological measurements is proposed to characterize the curing process of adhesives with to obtain useful information for industrial adhesive selection.

Abstract

The analysis of thermal processes associated to the curing of adhesives and the study of mechanical behavior once cured, provide key information to choose the best option for any specific application. The proposed methodology for the curing characterization, based on thermal analysis and rheology, is described through the comparison of three commercial adhesives. The experimental techniques used here are Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and Rheology. TGA provides information about the thermal stability and filler content, DSC allows the evaluation of some thermal events associated to the cure reaction and to thermal changes of the cured material when subjected to temperature changes. Rheology complements the information of the thermal transformations from a mechanical point of view. Thus, the curing reaction can be tracked through the elastic modulus (mainly the storage modulus), the phase angle and the gap. In addition, it is also shown that although DSC is of no use to study the curing of moisture curable adhesives, it is a very convenient method to evaluate the low temperature glass transition of amorphous systems.

Introduction

Nowadays there is an increasing demand of adhesives. Today's industry demands that adhesives have increasingly varied properties, adapted to the growing diversity of possible new applications. It makes the selection of the most suitable option for each specific case a difficult task. Therefore, creating a standard methodology to characterize the adhesives according to their properties would facilitate the selection process. The analysis of the adhesive during the curing process and the final properties of the cured system are crucial to decide whether an adhesive is valid or not for a certain application.

Two of the most commonly used experimental techniques to study the behavior of adhesives are Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA). Rheological measurements and thermogravimetric tests are also widely used. Through them, the glass transition temperature (Tg) and the residual heat of curing, which are related to the degree of cure¹^,², can be determined.

TGA provides information about the thermal stability of the adhesives³^,⁴, which is very useful to establish further process conditions, on the other hand rheological measurements allows the determination of the gel time of the adhesive, analysis of the curing shrinkage, and the definition of the viscoelastic properties of a cured sample⁵^,⁶^,⁷, while the DSC technique allows measurement of the residual heat of curing, and discernment between one or more thermal processes that can take place simultaneously during the curing⁸^,⁹. Therefore, the combination of DSC, TGA and rheological methodologies provide detailed and reliable information to develop a complete characterization of adhesives.

There is a number of studies of adhesives where DSC and TGA are applied together¹⁰^,¹¹^,¹². There are also some studies that complement the DSC with rheological measurements¹³^,¹⁴^,¹⁵. However, there is not a standardized protocol to address the comparison of adhesives in a systematic way. That comparison would all to better choose the right adhesives in different contexts. In this work, an experimental methodology is proposed for doing a characterization of the curing process through the combined use of the thermal analysis and rheology. Applying these techniques as an ensemble allows to gather information about the adhesive behavior during and after the curing process, also the thermal stability and the Tg of the material¹⁶.

The proposed methodology involving the three techniques, DSC, TGA and rheology is described in this work using three commercial adhesives as an example. One of the adhesives, hereinafter referred to as S2c, is a two-component adhesive: component A contains tetrahydrofurfuryl methacrylate and component B contains benzoyl peroxide. The component B acts as an initiator of the curing reaction by causing the tetrahydrofurfuryl methacrylate rings to open. Through a free radical polymerization mechanism, the C=C bond of the monomer reacts with the growing radical to form a chain with tetrahydrofurfuryl side groups¹⁷. The other adhesives, T1c and T2c, are the one- and two-component versions from the same commercial house of a modified silane polymer adhesive. The curing process begins by the hydrolysis of the silane group¹⁸, which can be initiated by ambient humidity (as in the case of T1c) or by the addition of a second component (as in the case of T2c).

Concerning the application areas of these three different systems: the adhesive S2c was designed to substitute, in some cases, welding, riveting, clinching and other mechanical fastening techniques and it is suitable for high strength fastening of concealed joints on different types of substrates including top coats, plastics, glass, etc. The T1c and T2c adhesives are used for elastic bonding of metals and plastics: in caravan manufacturing, in the railroad vehicle industry or in shipbuilding.

Protocol

1. Checking the manufacturer curing conditions

Cure the adhesive sample following the manufacturer recommendations, and then evaluate it by a TGA and a DSC test. Record the specific curing conditions.
TGA test of cured sample
1. Perform thermogravimetric tests in a TGA or in a simultaneous DSC+TGA equipment (SDT).
2. Carry out a thermogravimetric test of the cured sample following the next steps to determine the inorganic filler content and the temperature at which the material starts to degrade. Do not exceed that temperature in further tests.
3. Open the air stopcock. Switch on the SDT (or TGA) apparatus. Open the SDT control software.
4. Open the furnace of the SDT and place two empty capsules: one will be the reference capsule and the other will contain the sample.
5. Close the furnace and press the bottom Tare.
6. Open the furnace and place a sample size of 10-20 mg in the sample capsule.
7. Fill the information about the sample in the tab Summary.
8. Open the tab Procedimento and click Editor. Drag the segment type Ramp to the Editor screen. Establish the ramp as 10 or 20 °C/min to 900 °C. Click OK.
9. Open the tab Notes. Choose Air as the purge gas and establish a flow rate of 100 mL/min. Click Apply.
  NOTE: TGA test has two objectives: 1) to determine the inorganic filler content and 2) to determine the temperature at which the material starts to degrade. For the first objective the test has to be performed in an air atmosphere. For the second one, an air atmosphere represents the most common situation in normal use.
10. Close the furnace.
11. Start the experiment.
DSC test of cured sample
1. Carry out the DSC tests on a standard DSC or on a modulated temperature DSC (MTDSC) instrument working in standard mode, use aluminum crucibles. Carry out a DSC test of the cured sample following the next steps to study the following parameters: the Tg of the material, a possible residual curing and the Tg_∞ of the sample.
2. Open the nitrogen stopcock. Switch on the DSC apparatus. Open the control software of the DSC instrument.
3. Click Control | Event | On. Then click the tab Tool | Instrument Preferences, choose DSC and establish a standby temperature of 30 °C.
4. Click Apply. Click the tab Control | Go to Standby Temperature and wait for at least 45 min before starting any experiment.
5. Open the tab Summary. Click Mode and choose Standard.
6. Open the tab Procedimento, click Test and choose Custom. Click Editor.
7. Drag an Equilibrate segment indicating the temperature at which to start the experiment (that temperature should be relatively low, for example -80 or -60 °C).
8. Drag the segment type Ramp to the Editor screen. Introduce a heating rate of 10 or 20 °C/min and the final temperature into the command editor window. The final temperature is tentatively chosen to allow for a complete cure and must be lower than the degradation temperature obtained from the previous TGA test.
  NOTE: These recommended heating rates are proposed as a starting point that will probably work fine in most cases. However, these heating rates can be modified to improve sensitivity or resolution.
9. Drag the segment type Ramp to the Editor screen. Similarly, to the previous step, introduce a 10 or 20 °C/min cooling rate to a temperature tentatively below the glass transition.
10. Drag the segment type Ramp to the Editor screen. Introduce a 10 or 20 °C/min heating rate to a temperature slightly below the degradation temperature.
11. Open the tab Notes. Choose Nitrogen as the flow gas and establish a flow rate of 50 mL/min. Click Apply.
12. Fill the information about the sample in the tab Summary.
13. Click Control | Lid | Open. Place a reference pan and a pan with a sample of 10-20 mg weight inside the DSC cell.
14. Launch the experiment by clicking Start.

2. DSC analysis of a fresh sample

Prepare a fresh sample of the adhesive using the ratios and procedures recommended by manufacturer and immediately subject it to the following tests.
Ramp curing test
1. Perform a heating-cooling-heating test as indicated below to obtain the curing enthalpy of the adhesive, the final glass transition on heating and to establish the range of temperatures where the curing process starts.
2. Open the tab Summary. Click Mode and choose Standard.
3. Click the tab Tool | Instrument Preferences, choose DSC and establish a standby temperature of 10 °C. Click Apply. Click the tab Control | Go to Standby Temperature,
4. Open the tab Procedimento, click Test and choose Custom. Click Editor. Drag the segment type Equilibrate at -80 °C to the Editor screen. Drag the segment Ramp and establish 10 or 20 °C/min to (a temperature slightly below the degradation temperature obtained from the TGA test).
5. Insert the segment Equilibrate at -80 °C. Then drag the segment Ramp, establish 10 or 20 °C/min to (the same temperature as before). Click Ok.
6. Fill the information about the sample in the tab Summary.
7. Click Control | Lid | Open. Place a reference pan and a pan with the freshly prepared sample of 10-20 mg weight inside the furnace.
8. Start the experiment.
Isothermal curing test
1. Taking into account the DSC plot of the curing in ramp, choose several temperatures at the beginning of the exotherm to execute the isothermal experiments.
  NOTE: The isothermal experiments will allow to evaluate the maximum degree of curing that can be obtained at each temperature.
2. Open the Summary tab. Click Mode and choose Standard.
3. Open the Procedimento tab, click Test and choose Custom. Click Editor. Drag the segment type Ramp to the Editor screen. Introduce a 20 °C/min to the chosen isothermal temperature.
4. Introduce an Isothermal segment for time enough to complete the cure at this temperature. It is possible, for example, to establish 300 min, but the test can be stopped when the heat flow curve is flat.
5. Introduce a command segment Equilibrate at 0 °C. Add a Ramp segment, establish a heating rate between 2 and 20 °C/min (in the example 2.5 °C/min was chosen) to the maximum temperature, which was chosen from the TGA test in order not to compromise the thermal stability of the adhesive.
6. Drag the Mark end of cycle segment to the editor window. Insert another Equilibrate segment, this time with a temperature of -80 °C. Add another Ramp segment with a heating rate between 2 and 20 °C/min (in the example 2.5 °C/min was chosen) to the same temperature indicated before. Click Ok.
  NOTE: A set of heating rates are suggested. Probably, most of them work correctly and depending of the nature of the curing process, mainly its kinetics, and the sensitivity and resolution required, some of these heating rates could be better. If the evaluation is done with comparative purposes the same conditions should be used for each studied adhesive system. In order to minimize the time elapsed from mixing the components to the beginning of the isothermal experiments, the temperature of the DSC cell should be adjusted to a temperature lower than the isothermal temperature before mixing both components.
7. Click the tab Tool | Instrument Preferences, choose DSC and establish a temperature lower than the isotherm temperature of the experiment. Click Apply. Click the tab Control | Go to Standby Temperature.
8. Fill the information about the sample in the tab Summary.
9. Click Control | Lid | Open. Place a reference pan and a pan with the sample of 10-20 mg weight inside the furnace.
10. Start the experiment.

3. Rheological analysis

Perform the rheological tests on a rheometer, using a 25 mm parallel plate geometry.
Logarithmic strain sweep test
1. Do an exploratory logarithmic strain sweep test following the steps below to set up the strain amplitude to be used in the curing study of the adhesive in the rheometer. Perform the test with a fresh sample (before curing).
2. Open the air stopcock. Switch on the rheometer apparatus. Open the rheometer control software.
3. Place the specific geometry on the rheometer.
4. Click Zero Gap.
5. Click the tab Geometry. Choose the specific geometry.
6. Open the tab Experiment.
7. Fill the information about the sample in the tab Sample.
8. Click the tab Procedimento. Choose Oscillation Amplitude. This experiment can be performed at room temperature (the actual temperature is annotated), and with a frequency of 1 Hz and a logarithmic sweep from 10^-3 to 100% of strain.
  NOTE: To prepare a sample of the two-component system, weigh components at room temperature, about 20 °C to the exact proportions recommended by the manufacturer. Then mix both components.
9. Place the sample on the bottom plate with the upper plate separated about 40 mm from the lower plate. Lower the upper plate until a gap of about 2 mm is observed between both plates. Trim off the excess adhesive.
10. Start the experiment.
Isothermal multifrequency curing test
NOTE: This test shows if there is or not gelation and, in case of gelation, it provides the gelation time. In addition, the contraction and the evolution of G’ and G’’ can be observed along the curing process.
1. Follow the subsequent procedure to monitor the curing of the adhesive.
2. Click the tab Procedimento. Choose Conditioning Options. Establish the Mode Compression, Axial Force 0 N and Sensitivity of 0.1 N. Click Advance and establish a Gap change limit of 2000 µm in the up and down directions.
3. Insert a new step of an oscillatory time sweep. This experiment can be performed at room temperature (the actual temperature is annotated), the duration of the test as a function of the estimated curing time based on the Data Sheet of the adhesive, and the percentage of Strain which is chosen from the result of the previous logarithmic strain sweep test. Choose Discrete and then set the frequencies 1, 3 and 10 Hz for all samples.
4. Remove the previous sample, do the Zero Gap and place a new sample. Then proceed as in step 3.2.9.
5. Start the experiment.
  NOTE: Do not remove the sample at the end of experiment. It will be used in the next experiment.
Torque sweep test
1. Once the curing test ends, proceed to the torque sweep test following the steps below to find out the linear viscoelastic range for the previously cured material.
  NOTE: The extension of LVR can be determined either by applying strain sweep test, mostly in controlled-strain rheometers, or torque or stress sweep test, mostly in controlled-stress rheometer. However, in some rheometers both methods can be used.
2. Click the tab Procedimento. Choose the Oscillation Amplitude. This experiment can be performed at room temperature (the actual temperature is annotated), with a frequency of 1 Hz and a logarithmic sweep from 10 to 10000 µNm of torque.
  NOTE: Use the same sample that was left in the instrument from the previous experiment.
3. Start the experiment.
  NOTE: Do not remove the sample at the end of experiment. It will be used in the next experiment.
Temperature scan test
1. Perform a temperature scan test following the steps below to verify the cure is complete.
2. Click the tab Procedimento. Choose Temperature Ramp. Initiate the experiment from room temperature, establish a ramp rate of 1 °C/min, which ensure a uniform distribution of temperature into the sample without consuming too excessive time, a frequency of 1 Hz and a given Torque amplitude, which is chosen from the previous Torque sweep test.
  NOTE: Use the same sample that was left in the instrument from the previous experiment.
3. Close the furnace of the rheometer. Open the air stopcock of the furnace.
4. Start the experiment.
  NOTE: If the next experiment is needed, do not remove the sample at the end of experiment. In that case it would be used for the next experiment.

Representative Results

In order to show the application of the proposed method three adhesive systems are used (Table of Materials):

S2c, a two-component system.
T1c, a one-component silane-modified-polymer, whose cure reaction is triggered by moisture.
T2c, a two-component system. It is a silane-modified-polymer too, but the second component is aimed to make the curing rate a little more independent from the moisture content of air.

The thermal stability and the amount of filler of the cured adhesives are analyzed by TGA. Figure 1 shows the thermogravimetric plots obtained in air from the three adhesives. In the case of S2c a slight mass loss is observed from about 50 °C, which is probably related to moisture volatilization. The onset of the main degradation process appears at 196 °C. For T1c and T2c, the degradation onsets appear at slightly higher temperatures: 236 °C and 210 °C, respectively. These degradation temperatures should be not reached in further DSC or rheology experiments. The residue at 600 °C probably corresponds to inorganic fillers. It amounts 37.5% for T1c, 36.9% for T2c, and 24.6% for S2c. In the case of S2c an important mass loss is observed in the 600-800 °C range which suggests that CaCO₃ is the main filler component since it is a typical filler which decomposes in that range of temperature in air. A mass loss of 10.32% was observed what corresponds to a 23.5% of CaCO₃ in the cured sample.

Figure 1: TGA curves of the three adhesives. The curves were obtained from cured samples using air as the purge gas. Please click here to view a larger version of this figure.

Following the procedure, the next step consists of performing DSC tests of cured samples. Figure 2 shows the heat flow curves obtained. The S2c was previously cured at room temperature (approx. 20 °C) during 95 min. The T1c (moisture curing system) and T2c were previously cured at room temperature for 48 h.

Figure 2: DSC heat flow curves obtained from cured samples of the three adhesives: S2c (A), T1c (B), T2c (C). Please click here to view a larger version of this figure.

Figure 2A shows no evidence of residual cure. A small deviation from the baseline is observed at about 60 °C during the first heating ramp. It could be considered a manifestation of a glass transition, but it is practically negligible, and it would be better to wait for the rheological test to confirm. A glass transition temperature at 60 °C was specified by the manufacturer but it is not observed in this DSC plot. At -67 °C, there is a tiny drop in the heat flow signal that suggest a possible glass transition of a component of the adhesive. Figure 2B shows a clear glass transition at -66 °C. There is also an endothermic peak between 65 °C and 85 °C on heating and the corresponding exotherm on cooling at 53 °C. The shape and size of these peaks suggest possible melting and crystallization processes of a polymeric compound. The only important event in Figure 2C is a glass transition at -64 °C.

The next results are also related to DSC tests. Figure 3 shows the curing plot of a S2c sample at 20 °C/min in a heating ramp. That ramp will be followed by a cooling and heating ramps not displayed in this Figure. The curing enthalpy of the adhesive, 171.5 J/g, is obtained by integration of the peak. The shape of the exotherm suggests an autocatalytic curing reaction¹⁹^,²⁰^,²¹, which would correspond to the methyl methacrylate free radical polymerization of the S2c adhesive²².

Figure 3: DSC heat flow curves obtained from a fresh sample of the S2c adhesive system Please click here to view a larger version of this figure.

In the case of T1c and T2c no curing exotherm was observed by DSC, as expected for moisture curing adhesives. Rheology studies of the curing will be of highest interest for these systems.

In order to evaluate the degree of curing that can be achieved at different temperatures isothermal DSC experiments were performed only for S2c, since the moisture curable systems cannot be tracked by DSC. For T1c and T2c samples, rheological measurements such as G’ or the gap can be used to track the advancement of the curing reaction at any temperature at which the experiment is performed. Table 1 shows the curing enthalpy values obtained at three temperatures. The degree of curing is calculated by comparing the curing enthalpy obtained at each temperature to that obtained in a heating ramp. The one used to calculate the values displayed on Table 1 was obtained at 20 °C/min.

Temperature (°C)	Curing enthalpy (J/g)	Degree of curing (%)
10	162.1	94.5
15	166.0	96.8
20	169.5	98.8

Table 1: Curing enthalpy and the degree of curing values resulting from the isothermal cure of S2c samples at different temperatures.

Figure 4 shows how the residual cure is much smaller in the case of the sample cured at the higher temperature. That is so because the degree of curing achieved at 20 °C is higher than that obtained at 10 °C, as it can be observed in Table 1.

Figure 4: Specific heat flow plots obtained in the first and second heating scans from S2c samples isothermally cured at the indicated temperatures. Please click here to view a larger version of this figure.

Important features of a curing process that were not observed by DSC are the gelation, the shrinkage produced by the curing and the change of the moduli along the cure process. The latter is especially important in the case of moisture triggered curing, since in these systems the conversion of the curing process cannot be tracked by DSC. These missing features can be evaluated by rheology.

The first rheological test performed with each sample consists of a strain sweep that allows to see the linear viscoelastic range from which a strain value will be chosen for the next experiment, an isothermal multifrequency test with the following frequencies: 1, 3 and 10 Hz. (6.28, 18.85 and 62.83 rad/s). Figure 5 corresponds to the cure of a fresh S2c sample that is placed between the parallel plates of the rheometer. The gelation time of the material can be observed as the point where the phase angle, δ, becomes frequency independent, according to the Winter and Chambon criterion²³^,²⁴. The gelation time is the time from mixing the two components to the instant when the phase angle curves obtained at different frequencies cross. After the gelation, the Tg continues to increase until a value somewhat above the cure temperature. The high filler content of this sample, about 23%, is the reason why a higher value of G’ than of G” is obtained throughout the test. Figure 5 also gives information about the shrinkage of the adhesive along the curing, that is about 6.5% in 10 minutes. A value of 20.5 MPa the modulus is obtained after about 11 minutes from mixing the components. After that instant, the moduli and the gap change only very little.

Figure 5: Plots resulting from the isothermal curing of a S2c sample in the rheometer at room temperature. Please click here to view a larger version of this figure.

Performing the isothermal multifrequency test at different temperatures, it would be possible to evaluate how the gel time varies with the curing temperature. In the case of the T1c and T2c systems, Figure 6 and Figure 7, there is no sign of gelation of the adhesives. A comparison of the slopes of the moduli of both adhesives reveals that T2c cures faster than T1c, which is normal since T2c has an additional compound to accelerate the curing reaction. An important increase of the storage modulus is observed in both cases, reaching an almost constant value after 24 h. A value of 0.94 MPa is observed for T1c and 1.2 MPa for T2c, which are much smaller than that observed for S2c.

Again, a high filler explains that G’ is consistently higher than G” along the test. The behavior of tan δ in both cases, seems to be related to the shear that those thixotropic materials undergo between the plates of the rheometer and also because of the curing process.

On the other hand, the contraction observed for both T1c and T2c systems in 24 h, 0.65% and 0.89%, respectively, are very little in comparison to that observed for S2c in 15 minutes, 5.7%.

Figure 6: Plots resulting from the isothermal curing of a T1c sample in the rheometer at room temperature. Please click here to view a larger version of this figure.

Figure 7: Plots resulting from the isothermal curing of a T2c sample in the rheometer at room temperature. Please click here to view a larger version of this figure.

The temperature scan tests of the cured samples are needed to evaluate the linear viscoelastic range (LVR) of the cured samples. The LVR is usually determined either by applying strain sweep test, in controlled-strain rheometers, or stress or torque sweep test, in controlled-stress rheometer. However, in some rheometers both methods can be used. On this occasion torque sweeps were done.

Figure 8 shows the results of a temperature scan of the S2c sample that was cured for one hour in the rheometer. The glass transition can be easily identified as a drop in G’, and as broad peaks in G’’ and in the phase angle, δ. The value of Tg, measured as the δ peak, is 60.2 °C.

Figure 8: Temperature scan test performed in the rheometer with a cured S2c sample. Please click here to view a larger version of this figure.

Temperature scans of fully cured T1c and T2c adhesives are reflected in Figure 9. The scan of T2c does not shows any relaxation in all temperature range. That can be of interest in case a consistent behavior is sought in that range of temperature. On the other hand, the moduli of the scan of T1c show a slow decrease until up to 60 °C, and then a more intense decline between 60 °C and 80 °C to then persist constant until the end of test.

Figure 9: Temperature scan tests of the cured adhesives T1c and T2c. Values of G’, G’’ and δ were obtained from a 1°C /min temperature scan. Please click here to view a larger version of this figure.

Discussion

A preliminary TGA test of each adhesive is always a fundamental step as it gives information about the temperature range at which the material is stable. That information is crucial to correctly setting up further experiments. In addition, TGA may also inform about the filler content, which can be very insightful to understand that storage and loss modulus may not to cross along the cure.

On the other hand, DSC allows to study the cure of most thermosetting systems but not of those whose cure reaction is moisture triggered. Rheology allows to track the cure of any system, moisture triggered or not and is the right technique to compare them. However, it must be taken into account that a typical limitation of rheometers is the minimum temperature at which a curing test can be performed. Fortunately, most adhesives are intended to be used at room temperature or higher.

Most flexible adhesives have a glass transition temperature at sub ambient temperatures. Some components of semi-rigid systems may have a low Tg too but it is frequent that common rheometers cannot reach that low temperature. Many commercial DSC can easily reach -80 °C and thus can be used to determine that low Tg.

An interesting feature of some rheometers is the possibility of applying an almost null axial force, which allows to track the gap changes due to the adhesive contraction along the cure. That feature was not common in the past but nowadays many rheometers incorporate that feature. Another interesting advantage of rheology respect to DSC is the possibility of identifying the gel point through the phase angle at different frequencies. That is useful to see it the adhesive is a thermoset or not and, if so, to measure the gel time, a critical factor that is directly related to the working time at a given temperature.

A critical step within the protocol is the use of appropriate ratios and procedures recommended by manufacturer with two component systems, as well as adjusting both DSC temperature and time expended to launch experiment for freshly prepared samples. In relation to the rheological test, it is important to keep the heating rate at low values to ensure a uniform distribution of temperature, also for DSC test the chosen heating rate should take into account aspects such as sensitivity and resolution.

The experimental results that can be obtained by the proposed methodology allow to better understand how time and temperature parameters involved in the preparation of any adhesive joint may affect the technological properties of the adhesives. For example, in the case of thermosets, it is important to complete the application of the different elements of a joint before gelation occurs, and it is also important to keep the elements in their place until about a 90% of the maximum modulus is reached. This methodology can help to choose between adhesives with different reactivity, modulus, or contraction in the curing.

From all the above, it can be deduced that the convenience of the elaboration of a methodology for the systematic study of the cure of adhesive systems through two techniques, thermal analysis and rheology, which complement each other efficiently to achieve a complete characterization of the cure for very different systems.

Declarações

The authors have nothing to disclose.

Acknowledgements

This research has been partially supported by the Spanish Ministry of Science and Innovation [Grant MTM2014-52876-R], [MTM2017-82724-R] and by Xunta de Galicia (Unidad Mixta de Investigación UDC-Navantia [IN853B-2018/02]). We would like to thank TA Instruments for the image showing the scheme of the rheometer used. This image is included in the Table of Materials of the article. We also would like to thank Journal of Thermal Analysis and Calorimetry for its permission for using some data from reference [16], and the Centro de Investigaciones Científicas Avanzadas (CICA) for using its facilities.

Materials

2960 SDT	TA Instruments	Simultaneous DSC/TGA device: Used to perform thermogravimetric tests.
Discovery HR-2	TA Instruments	Rheometer to perform rheological test.
MDSC Q2000	TA Instruments	Differential Scanning Calorimeter with optional temperature modulation. Used to peform DSC and MDSC tests.
Sikafast 5211NT	Sika	S2c: a two component system manufactured by Sika. It is based on tetrahydrofurfuryl methacrylate and contains an ethoxylated aromatic amine. The second component contains benzoyl peroxide as the initiator for the crosslinking reaction.
Teroson MS 939 FR	Henkel	T1c: manufactured by Henkel, which is a one component sylil-modified-polymer, whose cure reaction is triggered by moisture.
Teroson MS 9399	Henkel	T2c: a two component system manufactured by Henkel. It is a sylil-modified-polymer too but the second component is aimed to make the curing rate a little more independent from the moisture content of air.
TRIOS	TA Instruments	Control Software for the rheometer. Version 4.4.0.41651

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Díaz-Díaz, A., Sánchez-Silva, B., Tarrío-Saavedra, J., López-Beceiro, J., Gómez-Barreiro, S., Artiaga, R. Evaluation of the Curing of Adhesive Systems by Rheological and Thermal Testing. J. Vis. Exp. (161), e61468, doi:10.3791/61468 (2020).