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

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 cure^1,2, can be determined.

TGA provides information about the thermal stability of the adhesives^3,4, 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 sample^5,6,7, 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 curing^8,9. 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 together^10,11,12. There are also some studies that complement the DSC with rheological measurements^13,14,15. 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

1. Cure the adhesive sample following the manufacturer recommendations, and then evaluate it by a TGA and a DSC test. Record the specific curing conditions.
2. 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 Procedure 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.
3. 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 Procedure, 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

1. Prepare a fresh sample of the adhesive using the ratios and procedures recommended by manufacturer and immediately subject it to the following tests.
2. 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 Procedure, 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.
3. 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 Procedure 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

1. Perform the rheological tests on a rheometer, using a 25 mm parallel plate geometry.
2. 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 Procedure. 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.
3. 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 Procedure. 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.
4. 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 Procedure. 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.
5. Temperature scan test
   1. Perform a temperature scan test following the steps below to verify the cure is complete.
   2. Click the tab Procedure. 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.

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...

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 ...

Materials

Name	Company	Catalog Number	Comments
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