XPS High-Resolution Spectra – Carbon Chemical States •  Nylon groups on control surfaces, no nylon groups on plasma treated surfaces •  Oxygen containing functional groups after plasma treatment

•  Polar groups promote adhesion [10-12] •  Carboxyl group bond with epoxy adhesive during cure [13]

0 200 400 600 800

282 287 292

Cou

nts

Binding Energy (eV)

C-C/C-H C-N/C-O C=O

COOH/COON

0

10

20

30

40

50

60

0 10 20 30 40 50

Dis

pers

ive

surf

ace

ener

gy (m

J/m

2)

Polar surface energy (mJ/m2)

control high low

Epoxy  Adhesives  [9]  

Reverse the Curse of the Nylon Peel Ply

Ashley C. Tracey & Brian D. Flinn University of Washington Seattle, WA 98195 USA

MOTIVATION

•  Joints are necessary in design of composite structures (Boeing 787, Airbus A350, military aircraft) but unavoidably form localized stresses due to discontinuities à adhesive bonding

•  Materials system specific [1-4] •  Bonds are dependent on the chemistry of the interface and control of surface

chemistry is critical to bond quality [5,6] OBJECTIVE

•  Reverse the curse of the nylon peel ply

CONCLUSIONS

The significant conclusions of this research were: 1.  Atmospheric pressure plasma treatment reversed the curse of the nylon

peel ply à safer and stronger bonds. •  Failure mode changed from adhesion to cohesive/interlaminar

2.  Plasma treatment changed the surface chemistry and surface energy of nylon prepared CFRP.

3.  Increased surface energy and presence of carboxyl groups may be responsible for improved bond quality

FUTURE/ON GOING WORK

•  Time stability/effects of out time after plasma treatment •  Functionalization or removal of peel ply remnants •  Application to other material systems REFERENCES 1.  Hart-Smith, L. J., Redmond, G., and Davis, M. J. The Curse of the Nylon Peel Ply. In 41st SAMPE International Symposium and Exhibition ( 1996). 2.  Phariss, Molly and Flinn, Brian. The Effect of Peel-Ply Surface Preparation Variables on Bond Quality. In DOT/FAA/(AR)-06/28 ( 2006). 3.  Flinn, B. D. and Hickmott, C. W. Effect of Surface Preparation Technique on Bond Quality of AGATE Composite Laminates. In SAMPE 2009 (Baltimore,

MD), Society for the Advancement of Material and Process Engineering. 4.  Flinn, B. D., Cark, B. K., Satterwhite, J., and Van Voast, P. J. Influence of Peel Ply Type on Adhesive Bonding of Composites. In SAMPE 2007 (Baltimore,

MD 2007), Society for the Advancement of Material and Process Engineering. 5.  Hart-Smith, L. J., Brown, D., and Wong, S. Surface Preparations for Ensuring that the Glue will Stick in Bonded Composite Structures. 10th DoD/NASA/

FAA Conference on Fibrous Composites in Structural Design (November 1993). 6.  Kinloch, A. J. Adhesion and Adhesives Science and Technology. Chapman and Hall, New York, 1987. 7.  M. Tuttle, WET v 1.0, 2005, personally distributed executable for generating wetting envelopes. 8.  Belcher, M. A. (Tony), Krieg, K. L., Van Voast, P. J., and Blohowiak, K. Y. Nonchemical Surface Treatments Using Atmospheric Plasma Systems for

Structural Adhesive Bonding. In SAMPE 2013 (Long Beach, CA 2013), Society for the Advancement of Material Process and Engineering. 9.  A. V. Pocius, Adhesion and Adhesives Technology, An Introduction, 2nd ed. Cincinnati, OH: Hanser Gardner Publications, Inc., 2002. 10.  Encinas, N., Oakley, B. R., Belcher, M. A., Blohowiak, K. Y., Dillingham, R. G., Abenojar, J., and Martinez, M. A. Surface modification of aircraft used

composites for adhesive bonding. International Journal of Adhesion & Adhesives. 11.  Encinas, N., Diaz-Benito, B., Abenojar, J., and Martinez, M. A. Surf. Coat. Technol., 205 (2010), 396-402. 12.  Encinas, N., Abenojar, J., and Martinez, M. A. International Journal of Adhesion & Adhesives, 33 (2012), 1-6. 13.  R. J. Zaldivar, J. Nokes, G. L. Steckel, H. I. Kim, and B. A. Morgan, "The Effect of Atmospheric Plasma Treatment on the Chemistry, Morphology and

Resultant Bonding Behavior of a Pan-Based Carbon Fiber-Reinforced Epoxy Composite," Journal of Composite Materials, vol. 44, no. 2, pp. 137-56, 2010.

ACKNOWLEDGEMENTS

Jonathan Morasch, David Pate & Tucker Howie (UW-MSE), Tony Belcher & Kay Blohowiak (Boeing), Dr. Mark Tuttle (UW-ME), Gerry Hammer (UW-NESAC/BIO)

PURPOSE

The purpose of this research was to reverse the curse of the nylon peel ply with atmospheric pressure plasma. Previous research showed that Toray T800/3900-2 CFRP prepared with PFG 52006 nylon peel ply and bonded with MetlBond 1515-3M structural film adhesive failed in adhesion at low fracture energies when tested in the DCB configuration. Other research has suggested that plasma treatment could be able to activate these “un-bondable” surfaces and result in good adhesive bonds. This research aimed to determine if plasma treatment can reverse the negative affects of nylon peel ply preparation on bond quality and determine why or why not using surface characterization measurements.

RESULTS

Atmospheric pressure plasma treatment was shown to reverse the negative affects of nylon peel ply preparation on bond quality. Plasma treatment resulted in safer and stronger bonds. Bond Quality •  Three-fold increase in GIC for

plasma treated samples compared to control

•  Failure modes correlated to fracture energies

Peel ply

Composite

Removal of peel ply from CFRP

Polyester Prepared

Nylon Prepared

SRB Prepared

Failure Mode Cohesive Adhesion Adhesion

GIC 812±35 J/m2 122±15 J/m2 < 94 J/m2 !

11

Table 2 Fracture surfaces of tested laminates (Sample Width = 12.7 mm)

Polyester Prepared Nylon Prepared SRB Prepared

AF555

Failure Mode Cohesive Cohesive & Interlaminar Adhesion

MB1515-3

Failure Mode Cohesive Adhesion Adhesion

The measure of bond quality for our purposes is the critical strain energy release rate of the

bonded laminate, determined by Mode I DCB Testing. As can be seen in

Figure 8, surfaces prepared with SRB displayed consistently poor bonding. Samples prepared

with polyester peel ply had the best consistent bond quality. A dramatic change in fracture

energy was observed when nylon-prepared surfaces were bonded with MB1515-3 rather than

AF555. The fracture mode also changed from cohesive (AF555) to adhesion (MB1515-3) as

shown in Table 2. The mode of failure (cohesive/interlaminar) seen in samples prepared with

polyester peel ply and nylon peel ply bonded with AF555 is more desirable than the adhesion

(interfacial) failure seen in the other samples.

Figure 8 Mode I strain energy release rate of laminates bonded with AF 555 (A) or MB 1515-3

(B)

11

Table 2 Fracture surfaces of tested laminates (Sample Width = 12.7 mm)

Polyester Prepared Nylon Prepared SRB Prepared

AF555

Failure Mode Cohesive Cohesive & Interlaminar Adhesion

MB1515-3

Failure Mode Cohesive Adhesion Adhesion

The measure of bond quality for our purposes is the critical strain energy release rate of the

bonded laminate, determined by Mode I DCB Testing. As can be seen in

Figure 8, surfaces prepared with SRB displayed consistently poor bonding. Samples prepared

with polyester peel ply had the best consistent bond quality. A dramatic change in fracture

energy was observed when nylon-prepared surfaces were bonded with MB1515-3 rather than

AF555. The fracture mode also changed from cohesive (AF555) to adhesion (MB1515-3) as

shown in Table 2. The mode of failure (cohesive/interlaminar) seen in samples prepared with

polyester peel ply and nylon peel ply bonded with AF555 is more desirable than the adhesion

(interfacial) failure seen in the other samples.

Figure 8 Mode I strain energy release rate of laminates bonded with AF 555 (A) or MB 1515-3

(B)

11

Table 2 Fracture surfaces of tested laminates (Sample Width = 12.7 mm)

Polyester Prepared Nylon Prepared SRB Prepared

AF555

Failure Mode Cohesive Cohesive & Interlaminar Adhesion

MB1515-3

Failure Mode Cohesive Adhesion Adhesion

The measure of bond quality for our purposes is the critical strain energy release rate of the

bonded laminate, determined by Mode I DCB Testing. As can be seen in

Figure 8, surfaces prepared with SRB displayed consistently poor bonding. Samples prepared

with polyester peel ply had the best consistent bond quality. A dramatic change in fracture

energy was observed when nylon-prepared surfaces were bonded with MB1515-3 rather than

AF555. The fracture mode also changed from cohesive (AF555) to adhesion (MB1515-3) as

shown in Table 2. The mode of failure (cohesive/interlaminar) seen in samples prepared with

polyester peel ply and nylon peel ply bonded with AF555 is more desirable than the adhesion

(interfacial) failure seen in the other samples.

Figure 8 Mode I strain energy release rate of laminates bonded with AF 555 (A) or MB 1515-3

(B)

DCB fracture surfaces of peel ply surfaces bonded with MB1515-3 adhesive [2]

Adhesion Failure Cohesive/Interlaminar Failure

163

483 498

0

100

200

300

400

500

600

Aver

age

GIC

(J/m

2)

control

low

high

0

20

40

60

80

DI H2O EG GLY DIM

Aver

age

Con

tact

A

ngle

(deg

)

Fluid

control low high

6.6

28.5 32.1 36.4 34.6 32.8 43.0

63.1 64.9

0 10 20 30 40 50 60 70

control low high Surf

ace

Ener

gy (m

J/m

2)

Polar Energy Dispersive Energy Total Surface Energy

To determine why this change in bond quality was observed, witness coupons were characterized with contact angle measurements and XPS.

Wettability Envelopes

Surface Energies

Contact Angle Measurements

DCB Results

appear to be related to the increased concentration of carboxyl groups at the surface of thesubstrate. Covalent bonding between carboxyl groups and epoxides has been observed inthe literature [17]. For many industrial coating applications where chemical resistanceproperties are required, the reaction of epoxy resins with carboxyl groups is used as acrosslinking mechanism. Blank et al. [18] have confirmed that amines can catalyze thereaction of epoxy groups with carboxyl groups as shown in Figure 8. This could lead to thechemical bonding proposed in Figure 9 where a portion of the unreacted epoxide groups inthe adhesive can chemically react with the carboxyl groups on the composite surface, andtherefore enhance bond strength. Reactions between the amine and carboxyl groups arealso possible. Even though this proposed mechanism and data explain observed increasesin bond strength, one cannot draw conclusions without evaluating what contributionsmorphological changes from plasma treatment may have on adhesive strength as a func-tion of plasma treatment.

Scanning Electron and Atomic Force Microscopy

Scanning electron and atomic force microscopy were used to evaluate the plasma treatedsurface of the AS4/E765 composites. Figures 10(a) through 10(e) show the effect of plasmatreatment from 1 pass to 48 passes using SEM. The control (0 pass), 1-pass, 3-pass, and 6-pass specimens appear identical with respect to morphology without any pronouncedchanges in microstructural features. The surface average RMS amplitude is also on thesame relative scale, which is approximately 0.22mm as measured by atomic forcemicroscopy.

The initial surface roughness of the specimen is a function of the bagging/film materialin proximity to the composite sample during gelation; it is not an inherent property of thematerial. It can be controlled and is a function of the composite manufacturer’s processes.Therefore, small variations in surface roughness due to plasma treatment are more difficultto assess on a surface with some initial texture in comparison to a surface that is featureless(e.g. processed on mandrel). Figures 11(a) and 11(b) show the surface topography of both

Composite laminate

CH CH2

CH2

CO

O

O

CH3

CH3

C O

O

CH2 CH CH2

OH

CO

O CO

O CO

O

Figure 9. Possible bonding between activated carboxyl groups on composite surface and epoxide groups inadhesive.

+

R'

O

OHR

OR

OOH

R'

O

Figure 8. The chemical reaction of a carboxyl group with an epoxide group.

The Effect of Atmospheric Plasma Treatment 149

at UNIV WASHINGTON LIBRARIES on February 27, 2011jcm.sagepub.comDownloaded from

appear to be related to the increased concentration of carboxyl groups at the surface of thesubstrate. Covalent bonding between carboxyl groups and epoxides has been observed inthe literature [17]. For many industrial coating applications where chemical resistanceproperties are required, the reaction of epoxy resins with carboxyl groups is used as acrosslinking mechanism. Blank et al. [18] have confirmed that amines can catalyze thereaction of epoxy groups with carboxyl groups as shown in Figure 8. This could lead to thechemical bonding proposed in Figure 9 where a portion of the unreacted epoxide groups inthe adhesive can chemically react with the carboxyl groups on the composite surface, andtherefore enhance bond strength. Reactions between the amine and carboxyl groups arealso possible. Even though this proposed mechanism and data explain observed increasesin bond strength, one cannot draw conclusions without evaluating what contributionsmorphological changes from plasma treatment may have on adhesive strength as a func-tion of plasma treatment.

Scanning Electron and Atomic Force Microscopy

Scanning electron and atomic force microscopy were used to evaluate the plasma treatedsurface of the AS4/E765 composites. Figures 10(a) through 10(e) show the effect of plasmatreatment from 1 pass to 48 passes using SEM. The control (0 pass), 1-pass, 3-pass, and 6-pass specimens appear identical with respect to morphology without any pronouncedchanges in microstructural features. The surface average RMS amplitude is also on thesame relative scale, which is approximately 0.22mm as measured by atomic forcemicroscopy.

The initial surface roughness of the specimen is a function of the bagging/film materialin proximity to the composite sample during gelation; it is not an inherent property of thematerial. It can be controlled and is a function of the composite manufacturer’s processes.Therefore, small variations in surface roughness due to plasma treatment are more difficultto assess on a surface with some initial texture in comparison to a surface that is featureless(e.g. processed on mandrel). Figures 11(a) and 11(b) show the surface topography of both

Composite laminate

CH CH2

CH2

CO

O

O

CH3

CH3

C O

O

CH2 CH CH2

OH

CO

O CO

O CO

O

Figure 9. Possible bonding between activated carboxyl groups on composite surface and epoxide groups inadhesive.

+

R'

O

OHR

OR

OOH

R'

O

Figure 8. The chemical reaction of a carboxyl group with an epoxide group.

The Effect of Atmospheric Plasma Treatment 149

at UNIV WASHINGTON LIBRARIES on February 27, 2011jcm.sagepub.comDownloaded from

Proposed Interaction – Epoxy Adhesive Bonds with Carboxyl Groups on Surface [13]

Poster Entry No. 58378

APPROACH

•  Use atmospheric pressure plasma treatment to remove/functionalize nylon remnants on a carbon fiber reinforced polymer (CFRP) composite surface

•  Toray T800/3900-2 unidirectional surface prepared with Precision Fabrics Group, Inc. (PFG) 52006 nylon peel ply

•  Three scenarios: (1) control peel ply only, (2) low plasma, and (3) high plasma

•  Measure bond quality with the double cantilever beam (DCB) test and relate to surface characteristics: (1) surface energy and wettability envelopes [7] (wetting) from contact angle measurements of multiple fluids, and (2) surface chemistry and potential bonding sites from X-ray photoelectron spectroscopy (XPS)

GIC=

E

A!B

DCB Test

Load

Extension

E

Load vs. Extension Curve

θ

1 µL

EB = hv – EK – w

X-rays photoelectrons

Sample

X-ray (hv) photoelectron

Surface Energy – Contact Angles Surface Chemistry Measurements - XPS

568

PlasmaTreat atmospheric pressure plasma flume [8]

Contact Angle Measurements and Surface Energy •  Significant increase in polar (and total)

surface energy •  Plasma changed the polar character

of surface – polar fluids wet more on plasma treated surfaces

•  Very little change in dispersive surface energy

•  Significant change in surface energy shown by wettability envelopes •  Could help explain difference in

bonding à Characterize chemical nature of surface to determine why surface energy increased

XPS Survey Scans – Composition •  Plasma increased oxygen significantly •  Carbon and nitrogen decreased on

plasma treated surfaces •  Sulfur from proprietary tougheners in

matrix, curing agent?

0

500

1000

1500

282 287 292

Cou

nts

Binding Energy (eV)

C-C/C-H C-N/C-O

N-C=O 0 200 400 600 800

282 287 292

Cou

nts

Binding Energy (eV)

C-N/C-O C-C/C-H

C=O COOH

Control Low High

74.3 64.8 61.8

0

50

100

control low high Ato

mic

Per

cent

C 1s

10.3 8.3

6.4

0

5

10

15

control low high Ato

mic

Per

cent

N 1s

15.0 26.6 30.7

0

20

40

control low high Ato

mic

Per

cent

O 1s

0.4 0.3

1.2

0

1

2

control low high Ato

mic

Per

cent

S 2p