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MMS 7, MMS 8 and MMS 11 IAG Meeting

A Review of Current Trends in Surface Pretreatment prior to Structural Adhesive

BondingJohn Bishopp

Star Adhesion Limited, Waterbeach Cambridge

November 20th 2003 at NPL, Teddington, Middlesex

From

To

Adequate Surface Pretreatent prior to bonding is required to ensure structural integrity

Surface Pretreatment: Raison d’Être

• To form a strong, integrally-bonded, load-bearing structure, the surface of the adherend must be pretreated before application of the adhesive

• Vital for good environmental or thermal durability

1


• The surface must be as clean a as possible, removing any weak boundary layers such as:

• oxide shale • weak oxide layers• release agents• low molecular weight species• contamination in general

2


• Surface roughness can be adjusted

• For metallic adherends, this also increases the stability of the surface oxides and can maximise potential interactions across the interface.

• Surface free energy of the adherend is raised to as high a level as possible - to maximise its wetting characteristics

3


• This general principle is as true for wooden substrates as it is for the “high tech” carbon fibre laminates

• It was with the bonding of plywood, in aircraft structures, that the requirement for surface pretreatment first came to light

4

Origins of Surface Pretreatment

• Solution: The case-hardened plies were treated with glass-paper, sanding almost down to Tego glue film but leaving an unbroken surface of wood

• November 1937: Tego-bonded plywood, when new, could not be bonded using Aerolite urea-formaldehyde adhesives. This was attributed to a “case hardening” effect in the outer plies

• September 1938: Similar problem found by Norman be Bruyne when repairing a Desoutter monoplane; weak bonds between the plywood and the spruce longerons

• Final Solution: Findings were reported to the Air Ministry in 1938 but the technique of sanding along the grain prior to bonding was not made mandatory until 1942

Substrates for Structural Bonding

• Aluminium• Stainless Steel• Titanium• Fibre Reinforced Composites

• Thermoplastic/Thermoset Matrices• Aramid, Carbon and Glass Reinforcement

Surface Pretreatment: General Methodology

• General Procedure is:

• Surface Modification

• Clean

• Deoxidise [metallic substrates]

Surface Pretreatment: Methodologies

• “Light” Abrasion: wire wool or Scotchbrite• “Heavy” Abrasion: grit-blast with wet or dry

alumina particles or chilled iron shot• Chemical: Acid pickling• Chemical: Anodising in acidic or alkaline

electrolytes• Chemical: Phosphating• Chemical: Sol-Gel techniques• Activated Plasma

1

Surface Pretreatment: Methodologies

For pretreatment techniques for other metallic substrates and for full details of the chemical pretreatment processes covered here, refer to, for example, The “Redux Bonding Manual” at:

http://www.hexcelcomposites.com/Markets/Products/Adhesives/default.htm

2

Pretreatment of Aluminium

The oxides associated with aluminium have high surface energy but are relatively unstable; they vary in thickness from a few nanometres to up to 10 to 20 microns and all have a certain degree of micro-roughness

Aluminium Substrates: Methods of Pretreatment

Activated plasmaSurface Bombardment

Sol-gel proceduresChemical

Boric acid-Sulphuric acid anodizingElectrochemical

Sulphuric acid anodizingElectrochemical

Phosphoric acid anodizingElectrochemical

Chromic acid anodizingElectrochemical

P2 EtchChemical

Chromic-sulphuric acid pickleChemical

Grit-blast with alumina particles plus silane treatment‘Heavy’ Abrasion

Grit-blast with alumina particles‘Heavy’ Abrasion

Wire wool or Scotchbrite‘Light’ Abrasion

Aluminium: Mechanical Abrasion

Aluminium: Schematic of Chemical Baths

Aluminium: Acid Etchants

8 to 1560 to 7048.037.015.0P2

Time[mins]

Temperature[°C]

H2O[%]

H2SO4[%]

FeSO4[%]

2402270.223.46.4FPL - RT106568.326.75.0Optimised FPL58280.316.53.2Alcoa 3306065.027.57.5DIN 53 2813060 to 6570.323.36.4DEF STAN 03-2

15 to 306873.224.32.5FPL

Time[mins]

Temperature[°C]

H2O[%]

H2SO4[%]

Na2Cr2O7. 2 H2O [%]

Process

Aluminium: Chromic-Sulphuric Acid Pickle

UK Defence Standardization, DEF STAN 03-2, Issue 3, 1995

Aluminium: P2 EtchRodgers, N.L., 13th Natl. SAMPE Tech. Conference, 1981

P2 EtchFPL Etch

Davis, GD and Venables, JD, ‘Surface Treatment of Metal Adherends’, Adhesion Science and Engineering – Volume 2: Surfaces, Chemistry and Applications, Edited by Chaudhury, M and Pocius, AV, Elsevier, 2002, pp 947 – 1008

Aluminium: Chromic Acid AnodisingUK Defence Standardization, DEF STAN 03-24, Issue 3, 1997

Aluminium: Phosphoric Acid AnodisingBoeing Specifications, BAC 5555, Issue N, 2001

Durability: “Conventional” Pretreatments

Bishopp, JA, Sim, EK, Thompson, GE and Wood, GC, ‘The Adhesively Bonded Aluminium Joint: The Effect of Pretreatment on Durability’, J Adhesion, 26, 1988, pp 237 – 263

Exposure: 85% R.H. at 70°C

Durability: Grit-Blast/Silane Pretreatment

Mazza, JJ and Kuhbander, RJ, ‘Grit-Blast/Silane [GBS] Aluminium Surface Preparation for Structural Adhesive Bonding”, WL-TR-94-4111, Materials Laboratory, Air Force Materiel Command, 1999

26

28

30

32

34

36

1 10 100 1000

Exposure Time [Hours]

Cra

ck L

engt

h [m

m]

PAA Grit Blast/SilaneExposure: 95 to 100% R.H. at 60°C

Durability: Chromic-Sulphuric Acid Pretreatments

0102030405060708090

1 10 100 1000


Cra

ck L

engt

h [m

m]

PAA FPL FPL/NTMP

Ahearn, JS, Davis, GD, Sun, TS and Venables, JD, In: Mittal, KL (Ed) ‘Adhesive Aspects of Polymer Coatings’, Plenum Press, 1983, p 281

Exposure: 95 to 100% R.H. at 60°C

N CH2

CH2

H2C P OH

OH

O

PHO

OH

O

P

OH

OHO

Aluminium: Chromium-Free Anodising

Sulphuric Acid Anodise + Phosphoric Acid Dip

Boric Acid-Sulphuric Acid Anodise

UK Defence Standardization, DEF STAN 03-15, Issue 3, 1997

Boeing Specifications, BAC 5632, Issue C, 1999

05

1015202530354045

0 20 40 60 80 100


Cra

ck E

xten

sion

[mm

]

0

1

2

3

Cra

ck E

xten

sion

[mm

]

Anodising Temperature: 25°C Anodising Temperature: 35°C

BSAA: Effect of Anodising Temperature

Yendall, KA, Critchlow, GW, Andrews FR and Bahrani, D, ‘An Evaluation of Chromate-Free Anodising Processes for Aerospace Applications’, Extended Abstracts for Euradh 2002, IOM Communications, London, 2002


3537394143454749515355

1 10 100 1000


Cra

ck L

engt

h [m

m]

PAA CAA BSAA/25°C

Durability: BSAA Pretreatment



0

2

4

6

8

10

12

14

Max

imum

Loa

d to

Fai

lure

[kN

]

CAA 40 /50V BSAA at 35°C CAA 4 0 /50V BSAA at 35°C

0 Days 30 Days 60 Days 120 Days

Durability: BSAA Pretreatment

Yendall, KA, Critchlow, GW, Andrews FR and Bahrani, D, ‘An evaluation of chromate-free anodising processes for aerospace applications’, Extended Abstracts for Euradh 2002, IOM Communications, London, 2002

Exposure: Deionised Water at 60°C

Unclad 2024-T3

Unclad 7075-T6

Aluminium: AC Anodising

• Previous examples: All under DC conditions• AC anodising of aluminium: uses sulphuric and

phosphoric acids as electrolytes• Significantly thinner oxide films: 0.2 µm for sulphuric acid

[10 µm for DC] and 0.1 µm for phosphoric acid [0.5 to 1.0 µm for DC]

• Process times much faster: seconds rather than 20 to 30 minutes

• Temperatures usually higher at about 65°C• The durability of the bonded joints: Nearly as good as with

DC anodising

1

Aluminium: AC Anodising

100nm

Oxide

Al

100nm

Oxide

Al

Phosphoric Acid Sulphuric Acid

Bjørgum, A, Lapique, F, Walmsley, J, and Redford, K, ‘Anodising as Pre-Treatment for Structural Bonding’, Int. J. of Adhesion and Adhesives, 23, No. 5, 2003, pp 401 - 412 2

Aluminium: Sol-Gel Techniques

• A technique still in its relative infancy

• Different from all other chemical pretreatment processes as it relies on chemical rather than physical bonding

• A combination of hydrolysis and condensation reactions which lead to the formation of purely inorganic or hybrid inorganic/organic polymer networks

• The hydrolysis reactions, which lead to the formation of the hydroxide groups on the metallic substrates, are key; the hydrolysed oxide surface promotes the condensation reactions

1


• Alkoxides based, for example, on aluminium, titanium silicon or zirconium will all hydrolyse in aqueous media

• Silicon alkoxides will readily hydrolyse. However condensation reactions, leading to structure growth, are relatively latent, compared with the above.

Significant degree of user control

2


Schematic of a Sol-Gel StructureDavis, GD and Venables, JD, ‘Surface Treatment of Metal Adherends’, Adhesion Science and Engineering – Volume 2: Surfaces, Chemistry and Applications, Edited by Chaudhury, M and Pocius, AV, Elsevier, 2002, pp 947 – 1008 3


• This sol-gel system is a dilute aqueous solution of tetra-n-propoxy zirconium with a silane coupling agent: glycidoxy trimethoxy silane

• Acetic acid added to control stability and rate of reaction

• AC®-130 of Advanced Chemistry & Technology [originally Boegel]

• Formulation is such as to give optimum compatibility as well as having the ability to form strong bonds and to enhance the final surface durability

4

Durability: Sol-Gel Pretreatments

29

30

31

32

33

34

1 10 100 1000


Cra

ck L

engt

h [m

m]

PAA Sol-Gel

Blohowiak, KY, Krienke, KA, Osborne, JH and Greegor, RB, Proc. Workshop on Advanced Metal Finishing Techniques for Aerospace Applications, Keystone, CO, 1998


Ti

SS

FRP

Pretreatment of Stainless Steel

The oxides associated with low carbon steels are very stable butare thin and very smooth; they have none of the micro-roughness which is typical for other substrates

Stainless Steel Substrates: Methods of Pretreatment

Plasma Spray CoatingSurface BombardmentSol-gel proceduresChemicalCLP: Ciba Laser PretreatmentLaser TreatmentAcid EtchingChemicalGrit-blast with chill iron shot, glass or alumina‘Heavy’ Abrasion

Stainless Steel: Acid Etching

• Sulphuric Acid/Oxalic Acid

• Hydrochloric acid/Formalin/Hydrogen Peroxide

Both techniques can lead to excessive “smut” formation which can be removed by immersion in chromic-sulphuric acid

Stainless Steel: Ciba Laser Pretreatment

• Apply Primer to Substrate

• Laser Treatment

Broad, R, French J and Sauer, J, ‘New Effective, Ecological Surface Pretreatment for Highly Durable Adhesively Bonded Metal Joints’, Int. J. of Adhesion and Adhesives, 19, Nos. 2 – 3, 1999

Ciba Laser Pretreatment: Performance

0

5

10

15

20

25

Deg reased Deg reased /Primed Deg reased /Laser Treated Deg reased /Primed /LaserTreated : CLP

Lap

Shea

r St

reng

th a

t 22°

C [M

Pa]

Before Ageing 14 Days Cataplasma

Broad, R, French J and Sauer, J, ‘New Effective, Ecological Surface Pretreatment for Highly Durable Adhesively Bonded Metal Joints’, Int. J. of Adhesion and Adhesives, 19, Nos. 2 – 3, 1999

Durability: Ciba Laser Pretreatment

0

5

10

15

20

25

30

35

Cleaned Uncleaned Aged * Cleaned Uncleaned Ag ed*

Lap

Shea

r St

reng

th a

t 22°

C [M

Pa]

Before Ageing 14 Days Cataplasma 1000Hours Salt Spray

Stainless Steel

CLPWithout CLP

0

5

10

15

20

25

30

35

40

45

Cleaned Uncleaned Ag ed * Cleaned Uncleaned Ag ed *

Lap

Shea

r St

reng

th a

t 22°

C [M

Pa]


CLPWithout CLP

Aluminium

0

5

10

15

20

25

30

35

40

Cleaned Uncleaned Ag ed * Cleaned Uncleaned Ag ed *

Lap

Shea

r St

reng

th a

t 22°

C [M

Pa]


CLPWithout CLP

Titanium

*Aged: 7Days at 40°C and 100% R.H.

Stainless Steel: Plasma Spray Coating

Davis, GD, Groff, GB, Biegert, LL and Heaton, H, J. Adhesion, 54, 47, 1995

Expensive and need to control temperature but initial results are good

Pretreatment of TitaniumThe oxides associated with titanium are significantly thinner than those grown on aluminium; all have a certain degree of micro-roughness.

If high temperature adhesives are to be used [350° to 400°C cure] very thin oxide layers are required otherwise the oxygen from the oxide layer will dissolve in the base metal giving rise to voids and microcracks

Titanium Substrates: Methods of Pretreatment

Pasa Jell® treatmentChemical

Acid etch plus phosphate-fluoride treatment Chemical

Chromate free: alkaline perborate treatmentChemical

Sol-gel proceduresChemical

Sodium hydroxide anodizingElectrochemical

Sulphuric acid anodizingElectrochemical

Chromic acid anodizingElectrochemical

Alkaline peroxide etchingChemical

Grit-blasting‘Heavy’ Abrasion

Titanium: Overview

• Abrasive Blasting: Dry blasting with 40 to 50 µm alumina grit, is an ideal pretreatment for titanium; either on its own or as a forerunner to chemical pretreatment

• Acid Etching: Not generally used as the sole chemical pretreatment prior to structural bonding; an invaluable intermediate stage prior to a phosphate-fluoride treatment or to anodising. Etchants include: sulphuric acid and nitric acid/hydrofluoric acid and hydrochloric acid/orthophosphoric acid mixtures

1

Titanium: Phosphate-Fluoride

• The anatase oxide coating produced consists of titanium, oxygen, phosphorus, fluorine and aluminium

• The thickness of the oxide coating is in the range of 150 nm to 295 nm

• Adherend surface is etched in a 3% hydrofluoric acid and 15% nitric acid solution

• Etched adherend is immersed in an aqueous solution containing 5% anhydrous trisodium phosphate, 2% potassium fluoride and 2.6% hydrofluoric acid for 2 minutes

The phosphate-fluoride treatment involves two steps:

US Patent 2 864 732

Titanium: Overview

• Acid Etching: Pasa-Jell and alkaline peroxide etching are “stand alone” techniques

• Pasa-Jell: The etchant comprises a mixture of nitric acid, fluorides, chromic acid, coupling agents and water. Temperature of 38°C and a final oxide thickness of 20 nm. Surface is macro-rough and durability is not as good as with other pretreatments

• Alkaline peroxide etching : The etchant comprises a mixture of sodium hydroxide and hydrogen peroxide. Temperature of 55° to 65°C and a final oxide thickness of 60 to 200nm. Surface is micro-rough but bath stability is poor

2

Titanium: Pasa-Jell


SAE Standard, ARP 1843A, Revision A, 1991

Titanium: Alkaline Peroxide Etching


Durability: Various Pretreatments

Brown, SR, In: “Proc. 27th Natl. SAMPE Symposium, SAMPE, 1982, p 363

MFP: Modified phosphate fluoride

PF: Phosphate fluoride

DA: Dapcotreat

DP: Dry Pasa-Jell

LP: Liquid Pasa-Jell

TU: Turco 5578


Titanium: Overview

• Acidic Electrolytes: The etchant stage uses a mixture of nitric and hydrofluoric acids. Anodising is accomplished at about 20 V. Bath temperature is 40°C for chromic and 21°C for sulphuric

• Alkaline Electrolytes: The cleaned substrates are anodised in sodium hydroxide at 20°C and under 10 V. Procedure is seen as a replacement for alkaline peroxide etching

3

• Anodising: Both acidic [chromic acid and sulphuric acid] and alkaline [sodium hydroxide] are effective electrolytes in which to anodise titanium

Titanium: Sodium Hydroxide Anodising

Kennedy, AC, Kohler, R and Poole, P, Int. J. of Adhesion and Adhesives, 3, No. 3, 1983, pp 133 - 139

Titanium: Overview

• Chromate-Free Pretreatment: Developed by NASA/LaRCthe sulphuric acid etched substrates are immersed in an aqueous alkaline perborate solution. Oxide films are about 2µm thick and exhibit good thermal stability

• Sol-Gel Procedures: Similar approach as for aluminium pretreatments but wider selection of silanes are used. Novel system uses polyimide-silica hybrids: pendent phenylethynyl imide oligomeric bis-silanes with tetraethoxy silane

4

Titanium: Sol-Gel Silanes

C2H5 O Si

O

C2H5

O

C2H5

NH2

C2H5 O Si

O

C2H5

O

C2H5

NH2

C2H5 O Si

O

C2H5

O

C2H5

CH2 CH2 CH2 NH2

p- and m-aminophenyl triethoxy silane

γ-aminopropyl triethoxy silane

Durability: Pasa-Jell and Sol-Gel Pretreatments

0

500

1000

1500

2000

2500

3000

3500

4000

Sol-Gel Pasa-Jell 107 Turco 5578

Frac

ture

Tou

hnes

s [J

/m²]

Before Ageing 5000 Hours "hot/wet" 5000 Hours "hot/dry"

Cobb, TQ, Johnson, WS, Lowther, SE and St Clair, TL, J. Adhesion, 71, 115, 1999

Cracked-Lap Shear

Pretreatment of Fibre Reinforced Plastics

Two types of fibre-reinforced laminates: •thermoplastic Matrices: e.g. polyetheretherketone, polyethersulphone •thermosetting matrices: e.g. epoxy, phenolic, BMI, PI, cyanate ester

Fibres – carbon, glass, quartz, ceramic and aramid have little effect on surface pretreatment methodologies

Fibre Reinforced Plastics: Ideal Surface Pretreatment

The optimum pretreatment for the preparation of composite substrates for adhesive bonding can be defined as one which removes all the surface matrix system, to expose the reinforcing fibres, without doing any damage to the fibres themselves

Fibre Reinforced Plastics: Methodologies for Thermoplastic Matrices

• As moulded – i.e. no pretreatment• Abraded and solvent cleaned• Corona treated• Oxygen plasma treated• Excimer-Laser Ablated

Fibre Reinforced Plastics: Abrasion versus Surface Bombardment

• Kinloch, Blackman et al found that fracture toughness after abrasion pretreatment was an order of magnitude lower than following corona or plasma treatment

• Locus of failure moved from the interface to cohesive within the adhesive

• Level of pretreatment required was somewhat dependant on adhesive cure temperature and was different dependant on whether corona discharge or plasma treatment was used

Fibre Reinforced Plastics: Thermoplastic Matrices – Various Pretreatments

Blackman, BRK, Kinloch, AJ and Watts, JF, ‘The Plasma Treatment of Thermoplastic Fibre Composites for Adhesive Bonding’, Composites, 25, No. 5, 1994, pp 332 - 341

As moulded

Abraded and solvent cleaned

Corona treated [20 J.mm-2]

Oxygen plasma treated

[10 minutes]

Fibre Reinforced Plastics: Thermoplastic Matrices – Laser Ablation

Buchman, A, Dodiuk, H, Rotel, M and Zahavi, J, ‘Laser-induced Adhesion Enhancement of Polymer Composites and Metal Alloys’ in ‘Polymer Surface Modification: Relevance to Adhesion’, Ed. Mittal, KL, VSP, The Netherlands, 1996 p 119 - 212

High Energy [1 J/P cm², 10 pulses] Low Energy [0.18 J/P cm², 100 pulses]

Fibre Reinforced Plastics: Thermoplastic Matrices – Surface Bombardment

It could be argued that, in spite of the excellent results seen when using corona discharge, plasma treatment and laser ablation surface pretreatment techniques, it is unlikely that they will be cost effective and will be difficult to introduce for the treatment of large components. However, in view of the poor performances now seen by using conventional abrasion techniques, it is probable that ways round these difficulties will have to be found

Fibre Reinforced Plastics: Methodologies for Thermoset Matrices

• No surface pretreatment• Solvent clean• Peel ply/Tear ply

• Peel ply: coated with a release agent• Tear ply: uncoated

• Hand abrasion techniques• Grit-blasting techniques

• Dry abrasion• Wet abrasion

• Surface Bombardment• Laser Ablation, Plasma, Corona, Flame

Fibre Reinforced Plastics: Thermoset Matrices

• Peel ply/Tear ply: Suitable perforated fabric incorporated in the lay up of RTM, RIM or prepreg-based components. Stripping off the peel ply just prior to bonding removes some of the excess cured matrix material; exposes a fresh, clean surface for bonding. However, the degree of resin removal is not uniform across the laminate surface and a relatively resin-rich surface layer still remains

• Solvent clean: Removes some, but not all, surface contamination; little better than no pretreatment

• No pretreatment: Surface remains contaminated with mould release agents, airborne dirt, oils, et cetera. Relatively thicklayer of cured matrix resin at the proposed adhesive interface between composite and adhesive

1

Fibre Reinforced Plastics: Peel Ply


• Hand abrasion techniques: Use of wet-and-dry paper, often augmenting the peel ply approach. Techniques are highly operator-dependent as well as being arduous and slow. Fibre damage is a distinct possibility. Automating the process makes the operation more controllable and more conducive to routine production

2


• Wet grit-blasting: The abrasive grit, usually alumina or silica, is mixed into a slurry with water and this mixture is used to abrade the composite surface. The water not only acts as a cushion, which means that a more gentle abrasion results, but it is also constantly washing down the surface which means that there is less chance for residual contamination to remain on the bonding surface

3

• Grit-blasting techniques: More controllable as grit chemistry and hardness, grit size, air-line pressure, distance from the blasting head to the job, rate and frequency of pass are all readily programmable.

• Dry grit-blasting: Cleanliness of the grit is a concern also possibility that surface contamination can be embedded in the component

Fibre Reinforced Plastics: Wet Abrasion


• Surface modification techniques: Similar techniques to those for thermoplastic matrices: laser ablation of the surface or plasma, corona or flame treatments to modify the chemistry of the surface. The latter can introduce highly polar groups onto the surface to improve bonding across the interface

4

Fibre Reinforced Plastics: Thermoset Matrices – Surface Bombardment

Even if beneficial it is unlikely that they will be cost effective and will be difficult to introduce for the treatment of large components. In the light of the good performances which are possible using peel plies and abrasion techniques, unlike the situation which exists for thermoplastic composite substrates, these techniques are only likely to find applications where small, delicate components are to be bonded

mms 7, mms 8 and mms 11 iag meeting€¦ · · 2014-10-30... mms 8 and mms 11 iag meeting ......

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