(Open Access) Correlation between sub‐Tg relaxation processes and mechanical behavior for different hydrothermal ageing conditions in epoxy assemblies (2010) | Mathieu Chevalier

Q: What future works have the authors mentioned in the paper "Sub-tg relaxation processes and mechanical behavior for different hydrothermal ageing conditions in epoxy assemblies" ?

It invites to be sure that hydroxyether moieties are linked with b1, and to study the relationships between water–hydroxyether interactions and mechanical behavior.

Any correspondence concerning this service should be sent to the repository administrator:

staff-oatao@inp-toulouse.fr

Open Archive Toulouse Archive Ouverte (OATAO)

OATAO is an open access repository that collects the work of

Toulouse researchers

and makes it freely available over the web where possible.

This is an author -deposited version published in: http://oatao.univ-toulouse.fr/

Eprints ID: 3844

To link to this article: DOI: 10.1002/app.31253

URL:

http://dx.doi.org/10.1002/app.31253

To cite this document : Chevalier, M. and Dantras, Eric and Tonon, Claire and Guigue,

Pascale and Lacabanne, Colette and Puig, C. and Durin, Christian ( 2010) Correlation

between Sub-Tg relaxation processes and mechanical behavior for different

hydrothermal ageing conditions in epoxy assemblies. Journal of Applied Polymer

Science, vol. 115 (n° 2). pp. 1208-1214. ISSN 0021-8995

Correlation Between Sub-T

Relaxation Processes and

Mechanical Behavior for Different Hydrothermal Ageing

Conditions in Epoxy Assemblies

M. Chevalier,

E. Dantras,

C. Tonon,

P. Guigue,

C. Lacabanne,

C. Puig,

C. Durin

Laboratoire de Physique des Polyme

res, Institut Carnot CIRIMAT, Universite

Paul Sabatier,

Toulouse Cedex 31062, France

Astrium, 31 avenue des Cosmonautes, Toulouse 31402, France

Centre National d’Etudes Spatiales, 18 avenue Edouard Belin, Toulouse 31401, France

DOI 10.1002/app.31253

ABSTRACT: The

aim of this study is to understand aging

phenomena by monitoring physical parameters after real

and simulated aging experiments. This study focuses on alu-

minum-epoxy assemblies, which are commonly used on

spacecraft structures. Different samples are submitted to

simulated aging tests. Influence of temperature and mois-

ture is analyzed. Evolution with aging is characterized at

two different scales. The macroscopic behavior of the assem-

blies is studied by single lap shear test. A decrease in the

shear rupture stress is observed with increasing temperature

and relative humidity. It is demonstrated that temperature

has more important influence. The molecular behavior in

the adhesive joint is studied by dynamic dielectric spectros-

copy measurements. This experiment gives access to molec-

ular mobility in the adhesive. Dipolar entities are identified

as evolving with aging conditions. The temperature is more

effective than moisture at this scale. An interpretation of the

molecular mobility before and after aging shows that water

is an important parameter of this study. A link between

mechanical and molecular behavior with hydrothermal

aging is found. The decrease of mechanical properties occurs

while failures become interfacial. In the same time, the inter-

actions between hydroxyether and water increase. The

evolution of the macroscopic behavior of the bonded assem-

blies is due to this combination observed at different scales.

Key words: epoxy; adhesive; dielectric spectroscopy;

aging; single lap shear

INTRODUCTION

Aging processes in materials used for space applica-

tions depend on various parameters (on ground and

space environments) and are not fully understood.

The influence of each factor remains obscur. A par-

ticular attention shall be paid on the aging of

bonded assemblies due to storage on ground and

thermal cycling in orbit.

During spacecraft integration, in clean room,

materials are used under humid and thermal con-

trolled conditions: 21  2



C, 50  5%RH. The inte-

gration activities and the storage can last several

years. Consequently, it is necessary to take into

account the time spent in clean room as an aging

factor. An aging process, which begins before the

launch could be the cause of an irreversible damage

during the flight. All the structures of the satellite

are su bmitted to these controlled environmental con-

ditions. The characterization and the simulation of

aging phenomena in bonded assemblies are the chal-

lenge to improve their conception and their durabil-

ity. Checking these assemblies after the launch is

impossible. One alternative is to use accelerated test-

ing on the basis of space industry knowledge and

ESA normative data.

Empirical laws are the only

way to obtain accelerated aging conditions. But, the

link between accelerated testing and real-time aging

is unknown. The aim of this study is to shed some

light on this correlation.

The methodology used for the work presented

later is to carry out an investigation of the mechani-

cal behavior and molecular dynamics evolutions in

thermoset bonded structures exposed to moisture.

The combined use of mechanical and dielectric

measurements has been chosen to evaluate epoxy–

water interactions at macroscopic and molecular

scales.

Epoxy–amine networks are relatively hydrophilic

materials. They are able to absorb 1–6% per

weight of water in many industrial formulations.

Numerous studies have been devoted to the water

Correspondence to: E. Dantras (dantras@cict.fr).

absorption process in these systems but this problem

remains only partly understood.

2–6

Water ingress in

polyepoxy networks leads to a range of effects

: bulk

dissolution by water in the polymer network, mois-

ture absorption on the surface of free volume holes

in the glassy structure, interaction of water with

hydrophilic polar moieties, such as amines and

hydroxyls by strong hydrogen bonds. It is also

reported that water is present in two different forms

in polyepoxy networks

4,7–9

: free water that fills the

microvoids of the netwo rk and bound water that

bonds with polar groups.

A decrease in mechanical properties of bulk and

bonded polyepoxy networks, such as modulus and

rupture strength under humid conditions is

observed. It is rela ted to plasticization.

9–11

But how

absorbed water affects the mobility of polymer

chains is not well know n. According to Mijovic

et al.,

the chemical and physical changes on the

molecular level during the early stage of environ-

mental exposure is the most important point in the

prediction of performance in adhesive joints.

The use of dynamic mechanical and dielectric

measurements allows us to investigate the molecular

dynamics. In poly epoxy systems, two to four relaxa-

tion regions have been reported by different experi-

mental techniques.

12–16

Some authors studied the

influence of water on molecular dynamics of amine

cured epoxy adhesives.

5,8,10,12

They reported that

some relaxational processes were influenced by the

ingress of water into the network. But some results

are contradictory, probably due to the complexity of

the commercial adhesive formulations.

Therefore, the main objective of this study is to

get a simultaneous evaluation of macroscopic and

molecular changes in bonded structures exposed to

hydrothermal environment.

EXPERIMENTAL

Materials

The studied system is an aluminum-epoxy assembly.

The adhesive is a commercial amine-epoxy bicompo-

sant adhesive packaged to be usable immediately.

The two parts are prepared and a nozzle allows us

to make and extrude the mix with an accurate

repeatability. The hardener (part A) is a mix of sev-

eral components, where aliphatic amine is prepon-

derant. The part B is based on diglycidyl ether of

bisphenol-A epoxy resin mixed with other compo-

nents (fillers, catalyst...). Parts A and B are mixed at

room temperature (ratio 2 : 1). The curing process is

7 days at 21  2



C, 50  5%RH. It leads to two

glassy transitions (T

) determined by Diathermal

Scanning Calorimetry measurements. One, equal to



C, is associated with the high-crosslink network.

The second one, equal to 10



C, corresponds to the

low-crosslink regions. Both substrates are wrought

aluminum alloys (2024 or 2017) widely used in aero-

space industry. By this way, differential thermal

expansions (DCTE) between the two substrates are

avoided. This assemb ly is considered as a model.

The joint thickness is ranging from 70 to 200 lm.

Hydrothermal aging conditions

During accelerated hydrothermal aging tests, the

joints are exposed to a warm humid environment

during 7 days. The temperature range is from 20 to



C (step ¼ 20



C) and the relative humidity is

ranging from 20%RH to vapor saturation by steps of

20%RH. Each temperature–relative humidity pair is

tested.

Methods

Single lap shear test

Single lap shear tests are performed at 25



C using

the tensile tension machine Instron 4505 with a 100

kN load cell. The crosshead speed is 1 mm/min.

Three specimens were studied for each test. Data

were recorded with the Serie IX software. The

ruptures were analyzed visually to determine the

percentage of cohesive failure.

Thermo mechanical analysis

Thermo mechanical measurements are performed

using an Advanced Rheometric Expansion System

strain controlled rheometer (TA Instuments) in the

torsion rectangular mode within the linear elasticity

range. Dynamic mechanical storage and loss modu-

lus G

and G

were recorded as function of tempera-

ture from 50 to 100



Cat3



C/min, for an angular

frequency equal to 1 rad/s.

Dynamic dielectric spectroscopy

Broadband dielectric measurements are performed

using a Novocontrol BDS 4000 covering a frequency

range from 10

2

Hz to 10

Hz with 10 points per

decade. Experiments were carried out isothermally

from 150 to 150



C by steps of 5



C. The tempera-

ture is controlled with an accuracy of  0.5



Cbya

nitrogen gas stream heated by a Quatro temperature

controller. The samples are round plate assemblies

(U ¼ 40 mm). We use the aluminum alloy substrates

as electrodes.

The real e

and imaginary e

parts of the rela tive

complex permittivity e* are measured as a function

of frequency f at a given temperature T. Experimen-

tal data are fitted by the Havriliak-Neg ami (HN)

[eq. (1)] function with an additional conductivity

term.

17,18



ðxÞ¼e

 e

ðÞ

1 þ ixs

ðÞ

(1)

where e

is the real permittivity for high frequen-

cies, De is the relaxation strength, a

and b

are

the HN parameters, s

is the relaxation time, x is

the angular frequency, r

is the d.c. conductivity,

and e

the dielectric permittivity of vacuum.

RESULTS

Mechanical behavior

Static mechanical behavior

The shear behavior is characterized at initial state.

The shear rupture stress r

is 27.5  0.3 MPa. The

fracture appearance is cohesive as shown in Figure

1. The cohesive surface is 89  1% of the bonded

surface. These values are considered as reference

values before aging.

Single lap shear tests are performed after different

aging conditions. The shear rupture stres ses are

shown in Figure 2 as function of aging temperature

for different relative humidity percentage. First, we

note that r

is higher than r

at initial state when

the relative moisture percentage of the tests is

40%RH, i.e. lower than clean room conditions. For

this moisture environment, r

value is independent

of the aging temperature. For a given relative

humidity percentage, r

exhibits a linear decrease as

function of temperature. The influence of relative

humidity is also analyzed. For isothermal aging con-

ditions, a nonlinear decrease of r

is observed as

function of moisture amount. The higher the temper-

ature is, the higher the moisture influence.

The evolution of the failures is studied. Figu re 3

shows the failure after aging at 80



C, 95%RH. The

cohesive failure percentage is reported in the

Table I. As function of increasing aging temperature

and humidity, we note that the rupture becomes

more interfacial. For 80



C and 95%RH aging condi-

tions, we observe that the ratio between interfacial

and cohesive failures is about one.

Dynamic mechanical behavior

The storage modulus G

and loss modulus G

are

shown in Figure 4 as function of temperature for the

bulk adhesive at initial state. The conservative mod-

ulus decreases from 1 GPa at 45–0.8 GPa at 40



From 50 to 80



C, the decrease is stronger. G

reaches

10 MPa. This transition is associated with the

mechanical manifestation of glass transition. The

loss modulus exhibits two relaxation phenomena

Figure 1 Fracture appearance for three single lap shear

samples at initial state. [Color figure can be viewed in the

online issue, which is available at www.interscience.

wiley.com.]

Figure 2 Evolution of the shear rupture stress as a func-

tion of temperature and humidity percentage.

Figure 3 Fracture appearance for three single lap shear

samples after 7 days under 80



C, 95%RH. [Color figure

can be viewed in the online issue, which is available at

www.interscience.wiley.com.]

TABLE I

Cohesive Failure Percentage as a Function of

Hydrothermal Ageing Conditions

Aging moisture (%RH)

Aging

temperature (



20 40 60 80

40 N/A 87  478 3 N/A

60 N/A 77  11 65  12 79  9

80 80  275 660 461 7

95 N/A 70  11 61  145 7

called x and a. x peak reaches a maximum at 10



This relaxation is associated with heterogeneities in

the adhesive.

a reaches a maximum at T

tempera-

ture equal to 55



Dielectric behavior

Molecular mobility at initial state

The dielectric loss from 150 to 150



C of an assem-

bly at initial state shown in Figure 5. Six dielectric

phenomena are pointed out. In the low temperature

range a weak and broad relaxation mode is

observed, labeled c. At higher temperature, a more

intense and narrow atypical mode b

is found. Then,

a weak and broad mode is detected: this mode is

called b

. The dielectric loss corresponding to this

relaxation is shown in Figure 6 for temperatures

ranging from 40 to 15



C. In the high temperature

range, three additional relaxations occur: x, a, and

MWS modes.

Secondary relaxation modes have various molecu-

lar origins. The c mode is related to the CH

mobil-

ity in aliphatic chain sequences.

12,13

The molecular

origin of b

and b

is more complex. b modes are a

combination of several molecular entities. According

to Ochi et al.,

b is mainly associated with the

mobility of hydroxyether and diphenyl propane

groups. At higher temperatures, x is associated with

heterogeneities in the adhesive as observed previ-

ously during thermo mechanical measurements.

is attributed to the dielectric manifestation of glass

transition. The MWS relaxation is attributed to

macrodipoles.

Relaxation times s

are determined using the

Havriliak-Negami function [eq. (1)]. The temperature

dependence of c, b

, and b

relaxation times at initial

state is reported in the Arrhenius diagram presented

in Figure 7. The increase of dynamic dielectric spec-

troscopy (DDS) measurement temperature shows

that c, b

, and b

modes follow an Arrhenius law as

expected [eq. (2)].

sðTÞ¼s

exp



(2)

where DH is the activation enthalpy, s

is the pre-

exponential factor, and R is the universal gas constant.

The x, a, and MWS evolution is more difficult to

analyze. At high temperatures, conductivity appe ars.

Consequently, the use of the HN equation is more

complex: we observed a merging between high

temperature relaxation modes and conductivity

phenomenon. The behavior of these relaxation

Figure 4 Storage modulus G

(squares) and loss modulus

(circles) as function of temperature for the bulk adhesive

at initial state. [Color figure can be viewed in the online

issue, which is available at www.interscience.wiley.com.]

Figure 5 Dielectric loss at initial state reported on a loga-

rithmic scale as a function of temperature and frequency.

Figure 6 Isothermal dielectric loss at initial state as a

function of frequency showing b1 relaxation. [Color figure

can be viewed in the online issue, which is available at

Correlation between sub‐Tg relaxation processes and mechanical behavior for different hydrothermal ageing conditions in epoxy assemblies

Figures

Citations

Design and ageing of adhesives for structural adhesive bonding - A review

Influence of hydrogen bonds on glass transition and dielectric relaxations of cellulose

New bonded assembly configuration for dynamic mechanical analysis of adhesives

Dielectric behaviour of an epoxy network cured with a phosphonium-based ionic liquid

Effect of Aging Time at High Temperature on the Shear Strength of Adhesively Bonded Aluminum Composite Foam Joints

References

Nondestructive examination of epoxy adhesive-bonded structures exposed to a humid environment: A comparison of low- and high-frequency dielectric measurements

Mechanical and dielectric relaxations of poly(hydroxy ethers): 1. Low-temperature relaxations

Blistering of Glass-Epoxy Amine Adhesive Joints in Water Vapour at High Pressure. An Indication of Interfacial Crumpling

Related Papers (5)

Long-term behaviour of PEI and PEI-based composites subjected to physical aging

Molecular modeling for calculation of mechanical properties of epoxies with moisture ingress

Epoxy networks based on dicyandiamide: effect of the cure cycle on viscoelastic and mechanical properties

Molecular parameters governing the yield response of epoxy‐based glassy networks

Isothermal physical aging characterization of Polyether-ether-ketone (PEEK) and Polyphenylene sulfide (PPS) films by creep and stress relaxation

Frequently Asked Questions (2)

Q1. What are the contributions in "Sub-tg relaxation processes and mechanical behavior for different hydrothermal ageing conditions in epoxy assemblies" ?

Q2. What future works have the authors mentioned in the paper "Sub-tg relaxation processes and mechanical behavior for different hydrothermal ageing conditions in epoxy assemblies" ?