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Laser surface modification of polymers to enhance adhesion part II - PEEK, APC-2, LCP and PA (May 2000)

   
S M Tavakoli, TWI, Cambridge, UK

S T Riches, Micro Circuit Engineering, Newmarket, UK

Paper presented at ANTEC 2000, 7-11 May 2000. Orlando, Florida, USA

Abstract

Excimer lasers have been employed to modify the surfaces of a range of polymers to enhance adhesion. Considerable increases in joint strength were achieved as a result of laser treatment. Many lap shear joints, exposed to hot/wet environments, provided high retention of joint strength and durability. Laser-treated PEEK and APC-2 joints exposed at 50°C and 96%RH for several weeks, showed excellent resistance to ageing.

Introduction

Successful utilisation of adhesives often requires suitable surface treatment of adherends, prior to bonding. Selection and application of an appropriate surface treatment is one of the major factors in achieving good wettability and improving long term durability, whereas, inadequate surface treatment is one of the most common causes of premature degradation and failure. A wide range of surface treatments is available for removing contaminants and weak boundary layers from the surfaces of polymers and metals. These are generally based on mechanical methods, such as abrasion, grit and shot blasting and/or chemical methods, such as solvent degreasing, acid etching, anodising and the use of adhesion promoters or primers. For non-polar polymers or polymers with low surface energy, such as polyethylene, polypropylene or polytetrafluorothylene, it is necessary to introduce chemically reactive surface functional groups in order to improve adhesion. TWI has been working for several years on development of power beam techniques, based on laser and electron beam, for surface treatment of polymeric and metallic adherends and curing of adhesives. [1-6]

Experimental

Adherends

The adherends used in this work were: unreinforced polyetheretherketone (PEEK, Victrex 450G) reinforced PEEK (APC-2), liquid crystalline polymer (LCP 1; 3.9mm thick, cut from injection moulded plate, LCP 2: 3.9mm thick directly injection moulded and LCP 3: 2mm thick, all LCP's were Vectra A130), unreinforced (PA4,6, Stanyl TW300) and reinforced (GPA 4,6 Stanyl TW200-76) polyamide 4,6.

Adhesives

The adhesives used were: PLUS CP792 (a cyanoacrylate by Permabond), Araldite AW106 + HV953V hardener (two part epoxy by Ciba Specialty Chemicals) and AF163-2k (a modified epoxy film by 3M).

Laser treatment

Trials were performed using three excimer lasers: XeCl (wavelength 308nm, Lumonics Model Hyper-X400), ArF (wavelength 193nm) and KrF (wavelength 248nm). Both ArF and KrF lasers were Lambda Physik Model LPX100. The pulse energy of the XeCl laser was 120mJ and, for both ArF and KrF, it was 100mJ, with a pulse duration of 10-15nsec. The beams were focused through a 500mm focal length mirror to produce a rectangular area of 6 x 3mm on the adherend. The samples were mounted on a X-Y linear slide arrangement. The vertical axis was stepper motor driven and a speed of 4mm/sec was used throughout the trials. For each specimen, a treated width of approximately 25mm was produced. This was achieved by traversing the sample through the laser beam in a number of runs. The laser conditions, used to treat various materials, are given in Tables 1 - 5 .

Joint preparation

The laser-treated and untreated (control) specimens were bonded using the three types of adhesives described earlier. The single lap shear joints (nominal bonding area 25 x 12.5mm) were produced using a specially constructed jig. The curing conditions used for the adhesives were according to the manufacturer's recommendations. The methods used for preparing control joints were: PEEK degreased in either 1,1,1-trichloroethane (bonded with CP792) or acetone (bonded with AF163-2K), abraded with 60 grit silicon carbide paper, degreased again then bonded and APC-2, degreased in acetone (bonded with AF163-2k). Various LCP's and PA4,6's adherends were degreased in isopropanol and then bonded with AW106 adhesive. The control PEEK joints, used for durability testing, were degreased in acetone and abraded with 600 silicon carbide paper, followed by degreasing prior to bonding.

Determination of joint strength

The lap shear strengths of the joints, made using PEEK with CP792 adhesive, were determined using a crosshead speed of 1.3mm/minute. The joints, made with PEEK and APC-2 bonded with AF163 adhesive were tested at a crosshead speed of 2mm/min. The joints, prepared using LCP's, PA4,6 and GPA4,6 adherends bonded with AW106 epoxy adhesive, were tested at a crosshead speed of 10mm/min. Normally, three joints were tested for each condition and the mean lap shear strength (MLSS) and mode of failure were determined.

Durability testing

A number of joints made with PEEK and APC-2 bonded with AF163-2k adhesive were durability tested. The aged condition used was 50°C/96%RH over a saturated salt solution. The joints made with PEEK were aged for a period of 1,750 hours. For APC-2 joints, two sets (five joints for each) were aged for periods of 1,030 and 2,060 hours. Each set of joints was removed after a given period of ageing and the lap shear strengths were determined.

The LCP joints were exposed at 70°C and 95%RH for three weeks. These materials were laser-treated at an optimised condition, bonded with the two-part epoxy adhesive (AW106/HY 953V) and then exposed to ageing.

Results and discussion

Effects of laser treatment on joint strength

PEEK

The effects of various laser treatments on the joint strengths of PEEK are shown in Table 1. For PEEK pretreated with the XeCl laser, the MLSS was 4.08MPa with all three joints failing in the parent material (adherend). ArF and KrF treatments also provided joints with high strength. The highest joint strength (5.17MPa) with ArF was achieved using a frequency of 25Hz with two out of three joints failing within the parent material. For KrF lasers, the highest strength was found using a frequency of 2Hz. In this case, two joints failed within the parent material. In contrast, all untreated control joints failed adhesively at the interfacial regions of the joints. The MLSS for these joints was 2.37MPa.

The effects of storage time, for laser-treated PEEK before bonding, are also shown in Table 1. Similar values of MLSS were found for PEEK specimens, bonded five or 15 days after pretreatment, indicating that the effectiveness of laser treatment persisted after 15 days storage.

Table 1 The effects of excimer laser treatments on joint strengths of PEEK bonded with the CP792 cyanoacrylate adhesive.

TreatmentLaser Frequency
(Hz)
Mean Lap Shear Strength
(MPa)
Mode of Failure
XeCl 2 4.08 C (3)*
XeCl 5 3.98 C (2) + A (1)
XeCl 10 1.17 A (3)
XeCl 25 2.99 C (1) + A (2)
ArF 2 2.22 A (3)
ArF 5 2.14 A (3)
ArF 10 4.18 A (3)
ArF 25 5.17 C (2) + A (1)
ArF 25 ‡4.73 C (2) + A (1)
ArF 50 4.65 C (2) + A (1)
KrF 2 4.78 C (2) + A (1)
KrF 2 ‡4.03 C (2) + A (1)
KrF 5 4.57 C (2) + A (1)
KrF 10 2.47 A (3)
KrF 25 2.62 A (3)
D/AB/D - 2.37 A (3)
D - Degreasing    AB - Abrasion
()* - Number of joints failed
C - Cohesive within the adherend
A - Adhesive at the interface
‡ - Stored for 15 days after laser treatment prior to bonding

APC-2

The effects of ArF and KrF laser treatments on lap shear strengths of APC-2, bonded with AF163-2k epoxy adhesive, are shown in Table 2. The mean failure loads, for specimens treated at 5,25 and 50Hz, were 8.8, 9.75 and 9.85kN, respectively. The percentages of location of failure of the specimens lying within the adherend were 22%, 57% and 52%, respectively. KrF laser treatment at 5Hz resulted in a mean failure load of 9.96kN with 48% of failures occurring in the adherend.

Table 2 The effects of ArF and KrF laser treatment on failure load of APC-2 joints.

TreatmentLaser Frequency
(Hz)
Failure† Load( (kN)Mode of Failure
ArF 5 8.8 ± 0.5 22% in CAD
ArF 25 9.75 ± 0.9 57% in CAD
ArF 50 9.85 ± 0.7 52% in CAD
KrF 5 9.96 ± 0.9 48% in CAD
AB - 6.6 CAD
†- Five joints tested for each condition
CAD - Cohesive failure within the adherend

LCP

The effects of laser treatment on joint strengths of various LCP's are shown in Table 3

Table 3 The effects of excimer laser treatments on joint strengths of liquid crystalline polymers bonded with AW106 adhesive

MaterialSample Thickness
(mm)
TreatmentLaser Frequency
(Hz)
Mean Lap Shear Strength
(MPa)
Mode of Failure
Adherend/Bondline
LCP 3.9/0.2 ArF 5 6.23 A (3)
LCP 3.9/0.2 ArF 25 6.48 A (3)
LCP 3.9/0.2 ArF 50 6.48 A (3)
LCP 3.9/0.2 ArF 75 6.63 A (3)
LCP 2/.01 ArF 5 7.35 A (3)
LCP 2/0.1 ArF 25 8.11 A (3)
LCP 2/0.1 ArF 50 5.76 A (3)
LCP 2/0.1 ArF 75 5.99 A (3)
LCP 3.9/0.2 KrF 25 6.11 A (3)
LCP 2/0.1 KrF 25 7.28 A (3)
LCP 2/0.1 KrF 50 6.99 A (3)
LCP 3.9/0.2 Control D 5.54 A (3)
LCP 2/0.1 Control D 7.14 A (3)
C - Cohesive within the adherend
A - Adhesive at the interface
D - Degreasing

Pretreatment of thick LCP adherend with bondline thickness of 0.2 (LCP 1), using ArF at 5Hz, resulted in a MLSS of 6.23MPa. Increasing the frequency to 75Hz, improved the joint strength slightly to 6.63MPa. All the LCP specimens, pretreated with an ArF laser, showed adhesive failure. The MLSSs of all the joints, prepared with the ArF laser, were higher than the control (MLSS = 4.54MPa), prepared after degreasing in acetone. In the case of thin LCP adherends (LCP 3), the MLSS was highest at 25Hz (MLSS = 8.11MPa) reducing with an increase in laser frequency to 75Hz. All the specimens prepared with ArF lasers in this case also failed at the interface. Pretreatment of LCP 1 and LCP 2 with KrF at 25Hz provided lower MLSS at 25Hz (MLSSs = 6.11 and 7.28MPa respectively). The control joints, prepared after degreasing in acetone, gave a MLSS value of 5.54MPa for the thick adherend and 7.14MPa for the thin adherend.

PA4,6

The effects of excimer laser treatment of unreinforced and reinforced polyamide 4,6 are shown in Table 4. The highest MLSS of 7.57MPa was achieved for unreinforced PA4,6 when the lap shear specimens were treated with an ArF laser at 25Hz. All the joints failed within the adherend. Increasing the laser frequency to 50Hz reduced the MLSS to 6.63MPa, with all the joints again failing within the adherend. Treatment with a KrF laser at 5 and 10Hz provided lower lap shear strength (2.09 and 2.20MPa respectively) compared to ArF laser treatment at similar frequencies. However, all of the laser-treated specimens provided higher lap shear strength than the control (1.82MPa). For the glass reinforced polyamide, KrF laser treatment also resulted in higher shear strength than the control. The MLSSs of specimens treated at 5 and 10Hz were 3.74 and 4.90MPa, whereas for the control, the MLSS was 2.57MPa.

Table 4 The effect of excimer laser treatments on joint strengths of unreinforced and glass reinforced polyamide 4,6.

MaterialTreatmentLaser Frequency
(Hz)
Mean Lap Shear Strength
(MPa)
Mode of Failure
PA4,6 ArF 5 5.19 C (1) + A (2)
PA4,6 ArF 10 6.35 C (2) + A (1)
PA4,6 ArF 25 7.59 C (3)
PA4,6 ArF 50 6.63 C (3)
PA4,6 KrF 5 2.09 A (3)
PA4,6 KrF 10 2.20 A (3)
PA4,6 Control D 1.82 A (3)
GPA4,6 KrF 5 3.74 A (3)
GPA4,6 KrF 10 4.90 A (3)
GPA4,6 Control D 2.57 A (3)
C.A.D. As Table 3

Durability testing

PEEK and APC-2 Joints

The effects of ageing of PEEK and APC-2 joints at 50°C/96%RH are shown in Table 5.

Table 5 The effects of ageing on joint strengths of PEEK and APC-2 bonded with AF163 epoxy adhesive.

MaterialTreatment Laser/FrequencyExposure Time/ConditionMean Lap Shear Strength (MPa)Mode of Failure
*PEEK ArF/25Hz 0 12.5 ± 0.35 CAD
PEEK ArF/25Hz 1750hr †11.3 ± 1.8 CAD
PEEK D/A/D 0 2.38 ± 0.32 A
**APC-2 ArF/25Hz 0 29.5 ± 2.7 30% CAH,
70% DF
APC-2 ArF/25Hz 1030hr 31 ± 1.9 16% CAH,
84% CAD
APC-2 ArF/25Hz 2060hr 29.9 ± 2.6 24% CAH,
76% CAD
APC-2 AB 0 21.1 ± 3 -
* = stored for about 25 days prior to bonding
** = stored for about 10 days prior to bonding
D = Degreasing in acetone
† = The average result of seven joints
A = Adhesive failure at the interface
CAD = Cohesive failure within the adherend
CAH = Cohesive failure within the adhesive
DF = Delamination failure within the adherend
AB = Abrasion

The untreated control PEEK failed within the joint. PEEK specimens treated with an ArF laser resulted in a MLSS value of 12.5 ± 0.35MPa, with all the joints failing within the adherend. The same type of joints exposed to 1,750 hours ageing gave a MLSS of 11.3 ± 1.8MPa. The failure positions of these joints were also within the adherend. As seen from this Table, the ArF laser treatment of PEEK at 25Hz provided significant improvement in initial joint strength, i.e. from 2.83 ± 0.32MPa, for the control to 12.5 ± 0.35MPa, for laser-treated specimens. However, the most significant result shown in Table 5 is the high degree of retention (about 90%) of joint strength of PEEK after 1,750 hours ageing, with all the joints still failing within the adherend.

The MLSS of APC-2 joints before ageing was 29.5 ° 2.7MPa. For these joints, the modes of failure were 30% cohesive in the adhesive and 70% failure in the adherend near the surface. The MLSS of joints aged for 1,030 hours was 31 ± 1.9MPa, showing almost no change during exposure. The failure mode was 16% cohesive, 84% in the adherend, with no delamination. Joints exposed for 2,060 hours at 50°C and 96%RH also showed a high retention of strength. The MLSS in this case was 29.9 ± 2.6MPa. The failure mode was 24% cohesive, 76% in the adherend with no delamination.

LCP

The effects of durability testing at 70°C and 95%RH for three weeks, on the lap shear strengths of LCP joints, are shown in Table 6. For LCP 1, after laser treatment at 75Hz, the MLSS before ageing, was 6.63MPa and after ageing was 4.87MPa (retention of 73%). However, the joint strengths of LCP 2 and LCP 3 decreased from 8.44 and 8.11 (unaged) to 2.19 and 1.22MPa (aged), respectively. The highest retention of strength (73%), after ageing was obtained for thick specimens cut from injection moulded plate (LCP 1). The second type of thick LCP adherend (directly injection-moulded) resulted in lower retention of strength (26%). The loss in joint strength was even higher for the thin adherend (15% retention). The cause of this variation is not known.

Table 6 The effects of ageing on joint strengths of liquid crystalline polymers bonded with AW106 epoxy adhesive

MaterialThickness (mm)
Adherend/Bond Line
Treatment Laser/FrequencyExposure Time/ConditionMean Lap Shear Strength (MPa)Mode of Failure
LCP 1 3.9/0.2 ArF/75Hz 0 6.63 A (3)
LCP 1 3.9/0.2 ArF/75Hz 3wks at 70°C/95%RH 4.87 A (5)
LCP 2 3.9/0.2 ArF/25Hz 0 8.44 A (4)
LCP 2 3.9/0.2 ArF/25Hz 3wks at 70°C/95%RH 2.19 A (4)
LCP 3 2/0.1 ArF/25Hz 0 8.11 A (3)
LCP 3 2/0.1 ArF/25Hz 3wks at 70°C/95%RH 1.22 A (4)
A = Adhesive failure at the interface
LCP 1 = Vectra A130, thick adherend, cut from injection moulded plate
LCP 2 = Vectra A130, thick adherend, directly injection moulded
LCP 3 = Vectra A130, thin adherend

Conclusions

As a result of this work the following conclusions can be drawn:
  1. The excimer lasers were all very effective in significantly increasing the joint strength of PEEK bonded with the cyanoacrylate adhesive. XeCl and KrF lasers provided the highest joint strength at 2Hz (MLSS's of 4.08 and 4.78MParespectively), whereas, the ArF laser gave the highest values of MLSS at 25Hz (5.17MPa). Most of these joints failed within the adherend.
  2. For APC-2 joints bonded with the AF163 epoxy adhesive, the ArF laser treatment at 25Hz provided the joints with the highest percentage of failure (57%) within the adherend. The KrF laser was also effective when used at 5Hz.
  3. ArF and KrF laser treatment of different grades of LCP resulted in improvement in adhesion. This was more evident for the thick than the thin adherends, the ArF laser being more effective than the KrF laser.
  4. The ArF laser was very effective in increasing the joint strength of both unreinforced and glass-fibre reinforced polyamide 4,6. For unreinforced polyamide, most joints failed cohesively within the adherend.
  5. Laser-treated PEEK and APC-2 joints, exposed at 50°C and 96%RH for several weeks, showed excellent resistance to ageing. PEEK joints exposed to 1,750 hours and APC-2 joints to 2,060 hours under these conditions showed no lossin strength.
  6. Laser-treated LCP 1 adherends provided high retention (73%) of joint strength after exposure to 70°C and 95%RH for three weeks.

Acknowledgements

The authors wish to thank TWI Member Companies for sponsoring this work.

References

  1. Tavakoli S M and Riches S T: 'Laser surface modification of sheet moulded compound and glass-reinforced nylon'. Unpublished work, 1990.
  2. Tavakoli S M: 'Surface preparation power beams clean up'. Journal of Assembly and Automation, 1994 14 (4) 36-38.
  3. Hunt J A, Tavakoli S M, Williams R L and Riches S T: 'Laser surface modification to improve biocompatibility'. 12 th European Conference on Biomaterials, 12-13 Sept 95, Portugal.
  4. Tavakoli S M: 'Laser surface modification of polymers to enhance adhesion, Part 1 - polyolefins'. ANTEC '96, May 5 - 9 1996, Indianapolis, USA.
  5. Tavakoli S M, Riches S T, Shipman J and Thomas M: 'New coating technologies for wood products, an assessment of rapid curing technologies'. European Coating Journal April 1997 240-246.
  6. Tavakoli S M, Riches S T, Shipman J and Thomas M: 'New coating technologies for wood products curing of pigmented systems using excimer lasers'. European Coating Journal April 1997 390-395.

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