Laser surface modification of polymers to enhance adhesion

Laser surface modification of 

polymers to enhance adhesion 

Andy J Wilson 

EngD Research Student 

TWI Ltd & University of Surrey 

Copyright © TWI Ltd 2012

Overview 

• Introduction to TWI 

• Introduction to University of Surrey EngD 

• Objectives 

• Surface modification by laser treatment 

• Surface characterisation 

• Concluding remarks 

• Future Work 


TWI introduction 

• Independent R&T organisation 

• Specialising in materials joining 

and related technologies 

• Non profit distributing 

• 770+ industrial members in 3500 

locations worldwide – All sectors 

• Five UK technical centres and 

eleven international training 

centres 

Cambridge 

Port Talbot 

• £60M+ turnover, 750+ staff 

Sheffield 

Middlesbrough 


Key industry sectors 

Aerospace Automotive Construction 

Electronics Medical Oil & Gas Power 


TWI expertise in medical products 

• Materials 

• Failure Analysis 

• Surface Modification 

• Joining & Assembly 

• Packaging 

• Inspection and NDT 

• Manufacturing & Product Support 

• Training 


University of Surrey MiNMaT EngD 

• Four-year doctoral programme for researchers who aspire 

to key leadership positions in industry. 

• Tackles an industrially relevant research innovation 

opportunity and incorporates a significant degree of 

advanced characterisation. 

• The majority of the research is undertaken at the sponsor's 

premises. 


1. Polypropylene – Oil & 

Gas Sector 

• Improve adhesion of 

PP insulation to PU 

field joint filler material 

Objectives 

Surface to be optimised 

Polypropylene insulation 

Area for polyurethane 

filler 


Objectives 

2. PEEK – Medical Sector 

PEEK images courtesy of Invibio ® 

• Surface 

modification of 

Victrex ® PEEK 

(unfilled) 

• Nd:YAG laser, 

instead of a gas 

laser (e.g excimer) 

• Characterisation 

of PEEK surface 

and measurement 

of adhesive 

performance 


Medical implants - PEEK 

Specific Tensile Modulus (10 6 Nm kg -1 ) 

30 

25 

20 

15 

10 

5 

0 

Stiffness-to-weight ratios of implantable 

medical materials 

Cortical 

bone 

PEEK 

Medical 

Grade 

Stainless 

Steel 

Medical 

Grade 

Titanium 

Alloy 

Medical 

Grade 

Cobalt 

Chrome 

Alloy 

• PEEK increasingly 

used in long and 

short term 

implantable medical 

devices 

• Alternative to 

traditional metallic 

alloys 

• Closer stiffness-toweight 

ratio to bone 

than metal alloys 

Data provided courtesy of Solvay Specialty Polymers 


Experimental approach 

Nd:YAG laser 

Weight Pulsed laser beam 

Sample 

• Nd:YAG Q-switched 

nanosecond pulse laser 

• Samples manipulated in a raster 

fashion using CNC X-Y table 

• Change power intensity to 

influence surface of PEEK 

• Contact angle measurement 

used to characterise surfaces. 

• Resulting adhesion measured 

using single lap shear 

• FTIR and XPS conducted to 

characterise surface chemistry 


Experimental approach 

Nd:YAG laser 

Weight 

Pulsed laser beam 

Sample 

Untreated 

After laser 

treatment 


Surface characterisation 

• Surface characterisation conducted to 

understand how laser surface treatment affects 

the surface chemistry of PEEK 

Contact angle/surface energy 

FTIR 

XPS 

Kruss Aglient Technologies Thermo Scientific 


Contact angle measurement 

• H 2 O and CH 2 I 2 liquids used to derive surface free energy of 

PEEK surface 

• Water contact angle reduces with increasing laser intensity 

Low water contact 

angle 

= good wettability 

= high surface energy 

Control 

= improved adhesion 

Highest laser intensity 


PEEK surface energy 

Total Surface energy (mJ m -2 ) 

Polar energy + disperse energy = total surface 

energy 

Power Intensity (W mm -2 ) 

80 

70 

60 

50 

40 

30 

20 

10 

0 

• Consistent surface 

energy value up 

until laser intensity 

9.99x10 6 W mm -2 

• Increase in polar 

contribution to 

surface energy 

(consistent with 

introduction of 

polar functional 

groups) 


Lap shear testing 

• PEEK coupons bonded with Henkel Loctite 4061 (medical 

grade cyanoacrylate adhesive) 

• Overlap area 25 mm x 12.5 mm, test speed 1.3 mm min -1 

• Laser treatment increases lap shear strength by up to 13x 

Maximum load (N) 

2500.0 

2000.0 

1500.0 

1000.0 

500.0 

0.0 

Power Intensity (W mm -2 ) 


Fourier transform infrared 

spectroscopy 

• Well documented and used 

method of analysing chemical 

properties of polymer surfaces 

Control 

• Functional groups sought on 

laser treated sample. Resulting 

spectra appear identical, no 

changes observed. 

• Inconclusive - more surface 

sensitive technique required. 

Laser treated 


X-ray photoelectron spectroscopy 

C1s O1s N1s Na1s S2p Si2p Cl2p 

C/O 

ratio 

Control 78.3 16.1 2.1 1.4 1.0 0.8 0.3 4.9 

7.83 x 10⁶ W mm¯² 74.9 21.1 1.2 0.6 0 2.0 0.2 3.6 

2.78 x 10 ⁷ W mm¯² 72.5 21.6 2.4 2.2 0 1.0 0.3 3.4 

• Ratio of carbon/oxygen decreases with increasing laser 

intensity 

• Indicates surface is being oxidised. 

• Functional groups introduced onto surface, most likely 

carbonyls and carboxylic acids. 

• Further XPS analysis to continue. 


Concluding remarks 

• Laser treatment increases lap shear strength of 

adhesively bonded PEEK lap joints 

• Decrease in contact angle shows increase in 

wettability and surface energy. Increase in polar 

surface energy shows functional groups created 

• FTIR is inconclusive, not surface sensitive 

enough – surface specific technique required 

• XPS shows increase in oxygen on PEEK; 

presence of functional groups consistent with 

increase in wettability and reduction in H 2 O 

contact angles. 


Further work 

• Further XPS analysis to be carried out to identify 

functional groups on PEEK surface and further 

characterise trace elements. 

• Further increase of laser intensity to achieve 

ablation of PEEK and other polymers for selective 

removal, to incorporate into next commercial 

project at TWI. 


Thank you 

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Laser surface modification of polymers to enhance adhesion

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