Debonding technologies for adhesive bonded structures

143 Pages·2012·16.6 MB·English

by  Arnaud Concord

Debonding technologies for adhesive bonded structures

Von der Fakultät für Mathematik, Naturwissenschaften und Informatik
der Brandenburgischen Technischen Universität Cottbus

zur Erlangung des akademischen Grades
Doktor der Ingenieurwissenschaften
(Dr. -Ing.)

genehmigte Dissertation
vorgelegt von

M. Sc.
Arnaud Concord

geboren am 15. September 1981 in Saint Martin d'Hères

Gutachter: Prof. Dr. sc. nat. Monika Bauer

Gutachter: Univ.-Prof. Dr.-Ing. Klaus Dilger

Gutachter: PD Dr. rer. nat. habil. Siegfried Vieth

Tag der mündlichen Prüfung: 16.02.2012
II
Acknowledgement

First of all, I would like to thank my supervisor, Professor Monika Bauer, director of the Fraunhofer
PYCO in Teltow and Chair of Polymer Materials of the Brandenburg Technical University Cottbus
for her help and patience during the whole work with this dissertation. I will also give a special
thanks to Georg Wachinger and Doctor Hans Luinge from EADS Innovation Works, who guided me
during my work within the laboratories of EADS, as well as Professor Klaus Dilger and Doctor
Siegfried Vieth, who kindly accepted to be my referees.

I would like to express my gratitude to all those who gave me the possibility to complete this thesis
at the Composite Technology department of EADS Innovation Works: Mrs. Magdalena Kutscha for
her help with the realization and the interpretation of TGA and DSC measurements, Mr. Frank
Strachauer and Mr. Peter Scheid for their help with the manufacture of CFRP panels.

I would also like to thank the employees of the Fraunhofer PYCO who helped me during my working
phases in Teltow, especially Dr. Jürgen Schneider, Mrs. Eike Langkabel and Dr. Norbert Proske.

I have furthermore to thank Dr. Schawe from Mettler-Toledo GmbH who kindly performed the
determination of the kinetic of the decomposition of azodicarbonamide, Mr. Forstner from Astrium
GmbH for the measurements with ion exchange chromatography, Mr. Kolb for acquisition of the
SEM photographs and their exploitation as well as Dr. Raps for his assistance with encapsulation of
graphite with the sol-gel technology, both from the IW-MS department of EADS Innovation Works.

Moreover, I would like to thank Dr. Stehle from the IW-SI department of EADS Innovation Works for
the PVD deposition of CFRP panels, Mr. Jonke from the IW-MS department of EADS Innovation
Works for his help with the electrical arc spraying of CFRP, Mr. Chymyn from ANAC Advanced
Coatings for the coating of CFRP panels with Pyroflex 7D713, Mr. Liepe from AHC
Oberflächentechnik GmbH for the surface treatment of CFRP panels with chemical nickel as well as
Mr. Emrich from Integran Technologies Inc. for the coating of composite panels with Nanovar and
the supply of coated LTM16 samples for trials.

For the trials with powerful mid frequency and stationary high frequency induction units, I would like
to thank Mr. Schuster from IFF GmbH, Dr. Gaille from IW-CT department of EADS Innovation
Works for the trials in an industrial microwave oven and Mr. Emmerich from the Fraunhofer Institute
for Chemical Technology for the investigation with a handheld microwave unit.

Finally, I would like to thank my friends and family who supported me during all this time. Merci pour
tout.
III
Content

Chapter 1: Introduction

1. State of the art ................................................................................................................ 1
2. Aim of the thesis ............................................................................................................. 3
3. Adhesion theories ........................................................................................................... 4
3.1. Wetting theory .................................................................................................................... 4
3.2. Electrostatic interactions .................................................................................................... 5
3.3. Interpenetration by diffusion ............................................................................................... 6
3.4. Mechanical anchorage theory ............................................................................................ 6
4. Main types of adhesives ................................................................................................. 6
4.1. Classification according to the application mode ................................................................ 6
4.2. Classification according to the form they exist .................................................................... 7
4.3. Classification according to the chemical type ..................................................................... 7
4.3.1. Epoxy adhesives ......................................................................................................... 7
4.3.2. Polyurethane adhesives .............................................................................................. 9
5. Conception of a bonding joint ....................................................................................... 10
5.1. Surface treatment ............................................................................................................ 10
5.1.1. Cleaning ................................................................................................................... 11
5.1.2. Mechanical treatment ................................................................................................ 11
5.1.3. Thermal treatment..................................................................................................... 12
5.1.4. Electrical treatment ................................................................................................... 13
5.1.5. Use of primers .......................................................................................................... 14
5.2. Geometry of the bonding joint .......................................................................................... 14
5.3. Composition of adhesives ................................................................................................ 14
5.4. Processing of the adhesive .............................................................................................. 15
5.5. Influence of the bonding gap ............................................................................................ 16
6. Characterization of a joint ............................................................................................. 17
6.1. Mechanical properties ...................................................................................................... 17
6.2. Physico-chemical properties ............................................................................................ 19
7. Debonding .................................................................................................................... 22
8. Foaming agents ............................................................................................................ 23
8.1. Physical foaming agents .................................................................................................. 23
8.2. Chemical foaming agents ................................................................................................. 24
9. Microencapsulation ...................................................................................................... 27
9.1. Aim and use of the encapsulation .................................................................................... 27
9.2. Encapsulation methods .................................................................................................... 29

Chapter 2: Experimental: Materials and analysis methods

1. Substrates .................................................................................................................... 31
1.1. Carbon fibers reinforced polymer: .................................................................................... 31
1.1.1. Carbon fibers reinforced polymer made from preform materials ................................ 31
1.1.2. CFRP made from prepreg materials ......................................................................... 32
IV
1.2. Polyamide 66 ................................................................................................................... 33
1.3. Polyetherimide ................................................................................................................. 34
2. Adhesives used ............................................................................................................ 35
2.1. High strength epoxy adhesive for the use with foaming agents ........................................ 35
2.2. Electrically debonding epoxy adhesive ............................................................................. 36
2.3. Elastic polyurethane adhesive for the use with foaming agents........................................ 37
3. Foaming agents used ................................................................................................... 37
3.1. Expandable graphite in epoxy adhesive ........................................................................... 37
3.2. Azodicarbonamide in polyurethane adhesive ................................................................... 38
4. Shell materials used for the encapsulation ................................................................... 38
4.1. Polyurea .......................................................................................................................... 38
4.2. Parylene .......................................................................................................................... 38
4.3. Melamine formaldehyde ................................................................................................... 38
4.4. Sol-Gel ............................................................................................................................. 38
5. Characterization methods............................................................................................. 39
5.1. Mechanical testing ........................................................................................................... 39
5.1.1. Single lap shear testing ............................................................................................. 39
5.1.2. Fracture toughness energy testing ............................................................................ 40
5.2. Physico-chemical methods .............................................................................................. 40
5.2.1. Thermogravimetric analysis ...................................................................................... 40
5.2.2. Differential scanning calorimetry ............................................................................... 41
5.2.3. Rheometry ................................................................................................................ 42
5.2.4. Chromatography ....................................................................................................... 42
5.3. Scanning electron microscopy ......................................................................................... 43
5.4. Infrared Spectroscopy ...................................................................................................... 43
6. Preparation of the samples ........................................................................................... 43
6.1. Surface treatment of the substrates ................................................................................. 43
6.1.1. Preparation of the CFRP prior to bonding with epoxy & polyurethane adhesives ...... 44
6.1.1.1. Grinding with sandpaper according to DIN EN 13887 ........................................ 44
6.1.1.2. Grinding with Scotch-Brite according to DIN EN 13887 ...................................... 44
6.1.1.3. Plasma treatment ............................................................................................... 44
6.1.2. Surface preparation of the PA 6,6 brackets ............................................................... 44
6.1.3. Surface preparation of the PEI brackets .................................................................... 45
6.1.4. Preparation of CFRP panels for the use with the commercial electrically debonding
adhesive ................................................................................................................... 45
6.2. Formulation of the adhesives ........................................................................................... 46
6.2.1. Formulation of the 2-C model epoxy adhesive .......................................................... 46
6.2.2. Formulation of the commercial polyurethane adhesive Pliogrip 9400/9405 ............... 46
6.3. Realization of bonding joints for the mechanical testing ................................................... 47
6.3.1. Single lap shear samples .......................................................................................... 47
6.3.1.1. CFRP / CFRP .................................................................................................... 47
6.3.1.2. CFRP / brackets ................................................................................................ 48
6.3.2. G1c-samples............................................................................................................. 49
6.3.3. Application of the adhesive ....................................................................................... 49
6.3.3.2. Fracture toughness energy samples .................................................................. 50
V
7. Energy delivery for bulk adhesives and bonding joints ........................................................ 50
7.1. Oven ................................................................................................................................ 50
7.2. Hot air gun ....................................................................................................................... 51
7.3. Microwave........................................................................................................................ 51
7.4. Induction .......................................................................................................................... 51
7.5. Near Infrared .................................................................................................................... 51
8. Ageing of the samples .................................................................................................. 51

Chapter 3: Results & Discussion

1. Foaming agents ............................................................................................................ 52
1.1. Characterization of the commercial foaming agents ......................................................... 54
1.1.1. Characterization of graphite intercalated compounds ................................................ 54
1.1.1.1. Dynamical thermogravimetric measurements .................................................... 54
1.1.1.2. Isothermal thermogravimetric measurements .................................................... 56
1.1.2. Characterization of azodicarbonamide ...................................................................... 57
1.1.2.1. Dynamical thermogravimetric measurements .................................................... 57
1.1.2.2. Decomposition of azodicarbonamide ................................................................. 59
1.1.2.3. Isothermal thermogravimetric measurements .................................................... 61
1.2. Microencapsulation of the commercial foaming agents .................................................... 63
1.2.1. Graphite intercalated compounds ............................................................................. 63
1.2.1.1. Ion-exchange chromatography .......................................................................... 63
1.2.1.1.1. Nord-Min 351 .................................................................................................. 64
1.2.1.1.2. Nord-Min KP 251 ............................................................................................ 65
1.2.1.2. Choice of the encapsulation of the foaming agent .............................................. 65
1.2.1.3. Scanning electron microscopy ........................................................................... 66
1.2.2. Azodicarbonamide .................................................................................................... 68
1.2.2.1. Differential scanning calorimetry ........................................................................ 68
1.2.2.2. Scanning electron microscopy ........................................................................... 70
1.3. Infra-red investigation of the interactions between azodicarbonamide and methylene
diisocyanate ..................................................................................................................... 71
1.4. Characterization of the encapsulated foaming agents ...................................................... 74
1.4.1. Graphite intercalated compounds ............................................................................. 74
1.4.2. Azodicarbonamide .................................................................................................... 76
1.4.2.1. Dynamical thermogravimetric measurements .................................................... 76
1.4.2.2. Isothermal measurements ................................................................................. 77
2. Expandable Adhesives ................................................................................................. 79
2.1. Determination of suitable foaming agent contents in adhesive formulations ..................... 79
2.1.1. Epoxy adhesive ........................................................................................................ 79
2.1.1.1. Mechanical properties of the bonding joints ....................................................... 79
2.1.1.2. Rheological properties ....................................................................................... 81
2.1.1.2.1. Viscosity.......................................................................................................... 81
2.1.1.2.2. Dynamical mechanical analysis ...................................................................... 81
2.1.2. Polyurethane adhesives ............................................................................................ 82
2.1.2.1. Mechanical properties of the bonding joints ....................................................... 82
2.1.2.2. Rheological properties ....................................................................................... 83
VI
2.1.2.2.1. Viscosity.......................................................................................................... 83
2.1.2.2.2. Dynamical mechanical analysis ...................................................................... 84
2.2. Mechanical properties of the bonding joints before debonding ......................................... 85
2.2.1. Fracture toughness energy ....................................................................................... 85
2.2.1.1. Epoxy ................................................................................................................ 86
2.2.1.2. Polyurethanes .................................................................................................... 87
2.2.2. Single lap shear measurements ................................................................................ 88
2.2.2.1. Epoxy adhesive: ................................................................................................ 89
2.2.2.1.1. Single lap shear CFRP/CFRP ......................................................................... 89
2.2.2.1.2. Single lap shear CFRP / PA-bracket ............................................................... 91
2.2.2.1.3. Single lap shear CFRP / PEI-bracket .............................................................. 93
2.2.2.2. Polyurethane adhesive ...................................................................................... 94
2.2.2.2.1. Single lap shear CFRP/CFRP ......................................................................... 94
2.2.2.2.2. Single lap shear CFRP / PA-bracket ............................................................... 96
2.2.2.2.3. Single lap shear CFRP / PEI-bracket .............................................................. 97
3. Debonding process ...................................................................................................... 99
3.1. Investigation of the debonding conditions ........................................................................ 99
3.1.1. Substrates ................................................................................................................ 99
3.1.1.1. Overheating in an oven ...................................................................................... 99
3.1.1.2. Influence of the fracture toughness energy on a second bonding after
overheating ...................................................................................................... 100
3.1.1.3. Temperature stability of CFRP 977-2 substrate ............................................... 100
3.1.1.3.1. Dynamical mechanical analysis .................................................................... 100
3.1.1.3.2. Thermogravimetric analysis .......................................................................... 101
3.1.2. Debondable adhesives ........................................................................................... 102
3.1.2.1. Epoxy adhesive with both graphites ................................................................. 102
3.1.2.2. Polyurethane adhesive with azodicarbonamide ............................................... 104
3.1.2.3. Isothermal measurements ............................................................................... 105
3.2. Cleaning the substrate ................................................................................................... 105
3.2.1. Smooth surfaces ..................................................................................................... 105
3.2.2. Rough surfaces ....................................................................................................... 106
3.3. Energy delivery .............................................................................................................. 106
3.3.1. Hot air ..................................................................................................................... 107
3.3.2. Microwave .............................................................................................................. 107
3.3.3. Induction ................................................................................................................. 109
3.3.4. Near infrared ........................................................................................................... 110
3.4. Mechanical properties after debonding .......................................................................... 111
3.4.1. Epoxy adhesive ...................................................................................................... 112
3.4.1.1. Single lap shear CFRP/CFRP .......................................................................... 112
3.4.1.2. Single lap shear CFRP / PA-bracket ................................................................ 113
3.4.1.3. Single lap shear CFRP / PEI-bracket ............................................................... 114
3.4.2. Polyurethane adhesive ........................................................................................... 115
3.4.2.1. Single lap shear CFRP/CFRP .......................................................................... 115
3.4.2.2. Single lap shear CFRP / PA-bracket ................................................................ 116
3.4.2.3. Single lap shear CFRP / PEI-bracket ............................................................... 117

VII
4. Electrical Debonding .................................................................................................. 119
4.1. Electrical debonding of the adhesive .............................................................................. 119
4.1.1. Investigation of the debonding principle of the adhesive ......................................... 119
4.1.2. Investigation of the electrical debonding capacity of the adhesive on surface
treated CFRP panels. ............................................................................................ 120
4.2. Mechanical tests ............................................................................................................ 121
4.2.1. Single lap shear tests .............................................................................................. 121
4.2.2. Fracture toughness energy ..................................................................................... 123
4.3. Electroless nickel plating ................................................................................................ 124
5. Conclusions ................................................................................................................ 126
5.1. Conclusion regarding thermally debonding adhesives.................................................... 126
5.2. Conclusion regarding electrically debonding adhesives: ................................................ 128
5.3. Perspectives .................................................................................................................. 129

References........................................................................................................................ 131

Glossary ............................................................................................................................ 134

.
VIII
Chapter 1: Introduction

1. State of the art

Composites are increasingly being used by the aerospace industry as a substitute for metals. That
is because composites are much lighter – and weight matters. Every kilo that is lost means lower
fuel costs or higher payload capacity. About a quarter of the structure of the Airbus A380 consists of
lightweight carbon fiber reinforced polymer (CFRP). CFRP materials are being used for the central
wing box, the wing flaps, the entire aft fuselage section including vertical and horizontal stabilizers
and the rear pressure bulkhead. The latter is the “wall” that closes off the rear end of the cabin,
which has to withstand the entire force of internal cabin pressurization during flight [1].

A composite is a material made from at least two non-miscible constituents. In the aerospace
industry, two main different types of composites are used. They are fiber reinforced polymers based
on epoxy or phenoxy resins and glass (GFRP), carbon (CFRP) or aramid fibers as well as a fiber
metal laminate (FML): glass reinforced aluminum (GLARE).
In fiber reinforced polymers, the fibers give the strength and elasticity to the polymer matrix that
sustains the fibers together. It is therefore very important to keep the fibers undamaged in order to
keep the mechanical properties of the composite at their highest level.

Manufacturers of not only commercial airplanes but also military planes and helicopters have
developed various usages of composite materials. Objectives of using composite material have
been in every case to reduce weight of planes and to produce highly performing flying machines.
Composite material also has contributed to those secondary objectives as saving assembling
manpower [2].

Bonding is, in principle, a preferred method for joining FRP parts compared to drilling process steps
for riveting or screwing, since it allows the transfer of forces during load without degrading the
fibers.
It is known that local stress peaks that appear in riveted or screwed parts are homogeneously
spread on the whole bonding surface of adhesively bonded parts [3] as shown in Photograph 1.

Photograph 1: force transfer in screwed parts and in adhesively bonded joints [4]

1
However, a drawback of adhesively bonding joints is that for rework or repair the bonded parts are
difficult to separate without damaging the surface of the FRP parts. Raw force may provoke
delamination of the superficial layer of the FRP on a large area around the bonding joint; solvents or
high temperatures may also chemically or thermally attack the FRP-substrate. The advantages and
disadvantages of adhesive bonding are shown in the following Table 1.

Advantages Disadvantages
Load distribution Temperature limit
Bonding of fragile materials Ageing
Bonding of materials of different kind Reliability
Bonding of thin materials Sensibility to the environment depending of the
High resistance type of adhesive
Retains shape Surface pre-treatment
Flexibility of conception Price?
Reduction of the number of pieces Time of preparation
Water tightness / air tightness Non destructive tests
Separation of the parts for repair, rework or
Aesthetic
recycling
Vibration absorption
Protection against corrosion
Electric and thermal insulation

Table 1: advantages and disadvantages of bonding in assembling technology [5]

Therefore most of the composite parts are still screwed or riveted together in the aerospace
industry.

Several hundreds of thousands of fasteners and brackets are used in airplanes for diverse
applications. Hat racks, tubes for air-conditioning, cables, and insulation materials are fixed with
brackets with different geometries and dimensions on composites (see Photograph 2). Brackets are
fixed every 10 centimeters in cables applications. The brackets are periodically changed for safety
reasons.

Photograph 2: brackets with inserts on a GFRP sandwich plate (source: Diehl Aircabin GmbH, Laupheim)
2