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Accelerated Testing of Adhesives

Usage of adhesives as a structural joining method is becoming increasingly popular. Adhesives offer light weight and cost efficient options in design of mechanical members. In many applications, adhesive joints are the main load bearing joint, they also may act as a sealant in a joint. As such, durability of adhesives for long service life need to be accessed based on engineering information that should be available regarding a particular adhesive of choice. One of the ways to obtain the long term performance of adhesive is by use accelerated tests. In these tests possible failure mechanisms of an adhesive system will be simulated in a manner that accelerates time.

Adhesive Failure Prediction

In general, service failures are predicted by accelerated test in which parameters such as temperature, moisture, mechanical loading, etc. are increased in order to reduce the time to failure in the system under investigation. Adhesive systems are no exception to this method for predicting failures in the service environment that they are designed for. The issue with usage of accelerated testing in adhesive stems from their complexity and varied reaction to test condition(s). In other words, an accelerated test regime may induce reaction kinetics that is not applicable to the actual service environment of an adhesive.

Adhesive Stability

Desirable performance of an adhesive system throughout its service time and environment depends on its stability and retaining its properties. However, many reasons can be in play for breakdown of an adhesive bond and like other polymers, prediction of life is not an easy task. One of the main degrading conditions to stability of adhesives is oxidizing environments. Adhesives like other polymers, react with oxygen easily even at low temperatures. Oxidation will cause the long polymer chains to break up and their molecular weight to decrease. This is a non-linear process and tends to approach a critical level below which the mechanical properties are significantly degraded.

Different adhesive systems behave differently in regards to oxidation which means they age differently in an oxidizing environment. So, it is crucial to have a method of predicting the oxidation aging of an adhesive that is applicable to the actual service without producing unwanted stress factors. Use of temperature as an acceleration factor for polymers and adhesives may not be appropriate. This is due to possibilities of reaction kinetics forming at high temperatures that otherwise would not exist at lower service environment.

Traditionally, oxidation induction time (OIT) and/or oxidation onset temperature (OOT) methods are used to predict the behavior of a material for oxidation. These properties are typically found using differential scanning calorimetry (DSC). However, these options use elevated temperatures which limit their use for adhesives and polymers. In these tests, the samples are isothermally or non-isothermally heated and once exothermic heat is detected, OIT or OOT measurements are determined.

Figure 1: Determination of Oxidation Induction Time, OIT (Reference 2)

In order to keep the duration of the tests short (under 2 hours), test temperature is raised above 180°C. Basically, the test article is heated then maintained at constant temperature for some time to stabilize. Oxygen/air is then applied and the time of application to beginning of oxidation is the “oxidation induction time” (OIT). As mentioned above, high temperature exposure may interfere with correlation of the measured data with actual in-service behavior of some adhesive systems. This makes the use these tests limited and at times, unreliable.

This limitation makes alternate tests, which are conducted at lower temperature and collect the same data, more applicable to adhesive oxidation process.

Chemiluminescence (CL)

Production of artificial light can be done in two ways, incandescent light in which light is produced from an object “glowing” hot. Light can also be produced in “cold” manner by photons emitting from energizing electrons which are then grounded and release energy. The process of light emission followed by grounding of the electrons may be very fast. During certain chemical processes such as oxidation, electron excitation and grounding take place which result in production of luminescence.

In Chemiluminescence, light is emitted as oxidation reaction progresses and specific exited species are formed and subsequently grounded. In polymers, initially unstable Alkyl radicals are formed which react further with oxygen to form peroxy. This process is illustrated by “Russel mechanism” depicted in figure 2 right hand side. In this process, as the peroxy radicals (ROO) are relaxed to their grounded state, light is produced by emission of one photon (O*to grounded stated).

The Chemiluminescence light is produced in various wave lengths depending on the material under oxidation reaction. Generally the Chemiluminescence is in the short wave range, however, in some cases it is detected in infra-red (IR) region of light spectrum. Regardless of the emitted light wave length, in Chemiluminescence the intensity of light is measured and is then correlated to the progression of the oxidation process.

Figure 2: Polymer oxidation (left side) and Chemiluminesence by “Russel Mechanism” (Right side) (Reference 1)

Chemiluminescence (CL) light intensity can be measured at various isothermal oxidation cycles; however, the temperature need not be raised to high levels. Correlation of light intensity is made to oxidation process parameters such as Oxidation Induction Temperature (OIT). By obtaining measurements at different temperatures, an accelerated oxidation progression correlation can be established. The prediction of oxidation during service life can then be carried out.

Advantages of Chemiluminescence

Temperature plays a big role in behavior of polymers. Characteristics such as melting, softening, glass transition, etc. are all temperature basic factors in polymers. Some or all of these characteristics happen at low temperatures in some polymers and as mentioned before, tests that exceed these temperatures do not correlate well with actual service life of a part.

One of the most important advantages of Chemiluminescence (CL) in studying the oxidation of polymers/adhesives is that, it does not need to be run at high temperatures. This makes the CL results more applicable to the oxidation rather than other secondary reactions.

Instrumentation set up of  CL tests, can be tailored to the material of use and the amount of sample needed is usually very small (fraction of the actual part). This last point, is mainly due to the reactions that lead to light emission intensity which is not related to mass of the sample.

Predicting oxidation in an polymer/adhesive system using CL measurements has the potential to be very close to the actual service condition of the part. This is due to the higher sensitivity of CL over conventional differential scanning calorimetry (CDS) methods, hence no need to run the CL experiments in high temperatures. Instrumentation for CL measurements is self-contained and easy to set up and use. An example of such instrument is shown in figure 3 showing of the ACL instruments

Figure 3: The basic instrument configuration with the furnace cell (front left hand). (Reference 3)

1 sample

2 oven (silver passivated)

3 atmosphere (gas)

4 optical pathway (L = 55mm)

5 aperture (Dia = 25mm)

6 optical filter (spektrometer

module)

7 detector (photon counting

mode PMT

Figure 4: Basic setup and schematic of CL measuring device (ACL Instruments). (Reference 3)

References:

Ref 1:  ASI Adhesives and Sealants, Simulating the aging of adhesives, Feb 1, 2008, Damian Ferrand, Fabian Kaser, Bertrand  Roduit and Willi Schwotzer, (WWW.adhesivemeg.com/articles/87025-simulating-the-aging-of-adhesives)

Ref 2: NETZSCH, www.netzsch-thermal-analysis.com/us/materials-applications/polymers/determination-of-the-oxidation-induction-time-or-temperature-oit-and-oot/

Ref 3: ACL Instruments, www.aclinstruments.com/fileadmin/aclinstruments.com/documents/PDF/Broschueren/ACL_leaflet_BasicConfiguration1_10_EN.pdf