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 e....

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 ....

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