Name: evaluation of the curing of adhesive systems by rheological and thermal testing
Uploaded: 2020-07-03T15:00:00
Description: Watch this Scientific Journal Video about Evaluation of the Curing of Adhesive Systems by Rheological and Thermal Testing at JoVE.com

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 Investigaci&#243;n 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.&#160;We also would like to thank Journal of Thermal Analysis and Calorimetry for its permission for using some data from reference [16],&#160;and the Centro de Investigaciones Cient&#237;ficas Avanzadas (CICA) for using its facilities.

1. Checking the manufacturer curing conditions
<ol>
	<li>Cure the adhesive sample following the manufacturer recommendations, and then evaluate it by a TGA and a DSC test. Record the specific curing conditions.</li>
	<li>TGA test of cured sample
	<ol>
		<li>Perform thermogravimetric tests in a TGA or in a simultaneous DSC+TGA equipment (SDT).</li>
		<li>Carry out a thermogravimetric test of the cured sample following the next steps&#160;to determine the inorganic filler content and the temperature at which the material...

<ol>
	<li>Zhang, Y., Adams, R. D., da Silva, L. F. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effects+of+Curing+Cycle+and+Thermal+History+on+the+Glass+Transition+Temperature+of+Adhesives.">Effects of Curing Cycle and Thermal History on the Glass Transition Temperature of Adhesives.</a> The Journal of Adhesion. 90 (4), 327-345 (2014).</li><li>Wisanrakkit, G., Gillham, J. 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R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multi-nationality+epoxy+adhesives+on+trial+for+future+nanocomposite+developments.">Multi-nationality epoxy adhesives on trial for future nanocomposite developments.</a> Progress in Organic Coatings. 133, 376-386 (2019).</li><li>Kozowyk, P. R. B., Poulis, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+new+experimental+methodology+for+assessing+adhesive+properties+shows+that+Neandertals+used+the+most+suitable+material+available.">A new experimental methodology for assessing adhesive properties shows that Neandertals used the most suitable material available.</a> Journal of Human Evolution. 137, 102664 (2019).</li><li>Tenorio-Alfonso, A., Pizarro, M. L., S&#225;nchez, M. C., Franco, J. 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D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthesis+and+characterization+of+waterborne+polyurethane+adhesives+containing+different+amount+of+ionic+groups+(I).">Synthesis and characterization of waterborne polyurethane adhesives containing different amount of ionic groups (I).</a> Journal of Applied Polymer Science. 102 (6), 5684-5691 (2006).</li><li>Vega-Baudrit, J., Navarro-Ba&#241;&#243;n, V., V&#225;zquez, P., Mart&#237;n-Mart&#237;nez, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Addition+of+nanosilicas+with+different+silanol+content+to+thermoplastic+polyurethane+adhesives.">Addition of nanosilicas with different silanol content to thermoplastic polyurethane adhesives.</a> International Journal of Adhesion and Adhesives. 26 (5), 378-387 (2006).</li><li>Park, Y. J., Joo, H. S., Kim, H. J., Lee, Y. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Adhesion+and+rheological+properties+of+EVA-based+hot-melt+adhesives.">Adhesion and rheological properties of EVA-based hot-melt adhesives.</a> International Journal of Adhesion and Adhesives. 26 (8), 571-576 (2006).</li><li>Kim, H., Kim, J., Kim, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effects+of+novel+carboxylic+acid-based+reductants+on+the+wetting+characteristics+of+anisotropic+conductive+adhesive+with+low+melting+point+alloy+filler.">Effects of novel carboxylic acid-based reductants on the wetting characteristics of anisotropic conductive adhesive with low melting point alloy filler.</a> Microelectronics Reliability. 50 (2), 258-265 (2010).</li><li>S&#225;nchez-Silva, B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thermal+and+rheological+comparison+of+adhesives.">Thermal and rheological comparison of adhesives.</a> Journal of Thermal Analysis and Calorimetry. 138 (5), 3357-3366 (2019).</li><li>Full, A. P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Polymerization+of+tetrahydrofurfuryl+methacrylate+in+three-component+anionic+microemulsions.">Polymerization of tetrahydrofurfuryl methacrylate in three-component anionic microemulsions.</a> Macromolecules. 25, 5157-5164 (1992).</li><li>Pizzi, A., Mittal, K. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Handbook of adhesive technology. , (1992).</li><li>Keenan, M. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Autocatalytic+cure+kinetics+from+DSC+measurements:+Zero+initial+cure+rate.">Autocatalytic cure kinetics from DSC measurements: Zero initial cure rate.</a> Journal of Applied Polymer Science. 33 (5), 1725-1734 (1987).</li><li>Lee, J. Y., Shim, M. J., Kim, S. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Autocatalytic+cure+kinetics+of+natural+zeolite+filled+epoxy+composites.">Autocatalytic cure kinetics of natural zeolite filled epoxy composites.</a> Materials Chemistry and Physics. 48 (1), 36-40 (1997).</li><li>Hayaty, M., Beheshty, M. H., Esfandeh, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Isothermal+differential+scanning+calorimetry+study+of+a+glass/epoxy+prepreg.">Isothermal differential scanning calorimetry study of a glass/epoxy prepreg.</a> Polymers for Advanced Technologies. 22 (6), 1001-1006 (2011).</li><li>Lee, E. J., Park, H. J., Kim, S. M., Lee, K. 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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 ...

Nowadays there is an increasing demand of adhesives. Today&#39;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.

<table>
	<tbody>
		<tr>
			<td>2960 SDT</td>
			<td>TA Instruments</td>
			<td></td>
			<td>Simultaneous DSC/TGA device: Used to perform thermogravimetric tests.</td>
		</tr>
		<tr>
			<td>Discovery HR-2</td>
			<td>TA Instruments</td>
			<td></td>
			<td>Rheometer to perform rheological test.</td>
		</tr>
		<tr>
			<td>MDSC Q2000</td>
			<td>TA Instruments</td>
			<td></td>
			<td>Differential Scanning Calorimeter with optional temperature modulation. Used to peform DSC and MDSC tests.</td>
		</tr>
		<tr>
			<td>Sikafast 5211NT</td>
			<td>Sika</td>
			<td></td>
			<td>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.</td>
		</tr>
		<tr>
			<td>Teroson MS 939 FR</td>
			<td>Henkel</td>
			<td></td>
			<td>T1c: manufactured by Henkel, which is a one component sylil-modified-polymer, whose cure reaction is triggered by moisture.</td>
		</tr>
		<tr>
			<td>Teroson MS 9399</td>
			<td>Henkel</td>
			<td></td>
			<td>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.</td>
		</tr>
		<tr>
			<td>TRIOS</td>
			<td>TA Instruments</td>
			<td></td>
			<td>Control Software for the rheometer. Version 4.4.0.41651</td>
		</tr>
	</tbody>
</table>

evaluation of the curing of adhesive systems by rheological and thermal testing

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.

In order to show the application of the proposed method three adhesive systems are used (Table of Materials):
<ul>
	<li>S2c, a two-component system.</li>
	<li>T1c, a one-component silane-modified-polymer, whose cure reaction is triggered by moisture.</li>
	<li>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.</li>
</ul>
The thermal stability and...

Watch this Scientific Journal Video about Evaluation of the Curing of Adhesive Systems by Rheological and Thermal Testing at JoVE.com

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

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.

The analysis of thermal processes associated to the curing of adhesives and the study of mechanical behavior once cured, provide key information to choose ...

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.

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