Mitsubishi Heavy Industries Technical Review Vol. 58 No. 4 (December 2021)
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Development of Destructive Surface Modification of PEEK(Poly Ether Ether Ketone) for High Adhesion StrengthKIYOKA TAKAGI*1

SOTA KAMO*2

KOICHI HASEGAWA*3 TAKUYA MATSUMOTO*4
TAKASHI NISHINO*5

ATSUSHI TAKAHARA*6

Since the early 1980s, Mitsubishi Heavy Industries, Ltd. (MHI) has continued with research
for the application of Carbon-Fiber-Reinforced Plastics (CFRP) in aircraft structures. Poly(etherether ketone) (PEEK) is a thermoplastic resin that has recently attracted attention as a matrix for
CFRP. PEEK is superior mainly in terms of strength, heat resistance and chemical resistance, but
has a drawback of having poor adhesive properties. This report presents the destructive surface
modifications of PEEK enabling stable adhesion that we developed jointly with each of Kobe
University and Kyushu University. The application of these destructive surface modifications canreduce the cost and weight of CFRP structures.

|1. Introduction
CFRP, which is a composite material made up of carbon fiber with thermosetting epoxy
resin, is widely used in aircraft structures, apart from duralumin, a hard alloy of aluminum.
Recently, discussion is going on to shorten the time for forming the parts of aircraft structures byapplying thermoplastic CFRP (CFRTP : Carbon-Fiber-Reinforced Thermoplastics) produced by
combining with carbon fibers instead of thermosetting epoxy resins. Among the thermoplastic
resins, PEEK, which is a super engineering plastic, has often been adopted mainly because of its
strength, heat resistance and chemical resistance. This report focuses on adhesion of CFRTP with aPEEK matrix.
With CFRTP, the use of a hot press machine enables parts to be formed at low cost in a shortperiod of time, but it is difficult to form parts that are larger than the molds (such as aircraft skins)
unless an autoclave is used. This necessitates the use of a technology to assemble CFRP andCFRTP connected together. If bolted, however, the carbon fiber acting as the primary loadtransmitter will be disrupted by bolt holes. The strength will be drastically reduced, leading to theincreased thickness and weight of structures. Therefore, a joint method requiring no bolts, namelyadhesive structure, is preferable.
PEEK exhibits poor adhesive properties, making it impossible to adhere without performing
any surface treatment. For this purpose, there is a method called plasma treatment by whichfunctional groups are introduced on the PEEK surface(1). This method, however, has problems suchas the diminishing of the chemical modification effect with time and the difficulty in defining theintensity of plasma.
This report describes two types of new, unique methods for PEEK surface modification, one
developed jointly with Kobe University and the other with Kyushu University. In these methods,
the formation of covalent bonds – strong chemical bonding – is induced at the interface between
PEEK and the adhesive. The adhesives we used are epoxy based. This type of adhesive is often*1

Deputy Director, Fixed Wing Aircraft Engineering Department, Integrated Defense & Space Systems, Mitsubishi HeavyIndustries, Ltd.

*2

Fixed Wing Aircraft Engineering Department, Integrated Defense & Space Systems, Mitsubishi Heavy Industries, Ltd.

*3

Chief Staff Researcher, Manufacturing Technology Research Department, Research & Innovation Center, Mitsubishi Heavy*4

Associate Professor, Graduate School of Engineering Department, Kobe University*5Professor, Graduate School of Engineering Department, Kobe University*6Professor, Institute for Materials Chemistry and Engineering, Kyusyu UniversityIndustries, Ltd.

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adopted for aircraft structures because of its superior strength and rigidity.

|2. Friedel-Crafts reaction treatment (Kobe University modification)
This chapter summarizes a method to modify the surface of PEEK, which was jointly
developed with Kobe University(2). In the Kobe University modification, epoxy groups are
introduced to the surface of PEEK by the Friedel-Crafts reaction in which epoxy groups areallowed to react with the aryl groups in PEEK through the use of Lewis acids. This enables thePEEK surface to be covalently bonded to amines, which are curing agents for epoxy adhesives,
through epoxy ring-opening polymerization, resulting in strong adhesion.

2.1

Kobe University modification – procedure

In the presence of epoxy groups, Lewis acids used in the Friedel-Crafts reaction promptly
facilitate the opening of the epoxy ring. It is therefore difficult to introduce epoxy groups intoPEEK through a single chemical reaction. Two steps were therefore utilized for this purpose(Figure 1).

Figure 1

Kobe University modification – procedure(1)Step 1
The Lewis acid used was a chloroform solution in which aluminum bromide (AlBr3) wasdissolved. The surface of PEEK was reacted with 10-undecenoic chloride (UC) and theresulting product is referred to as UC-PEEK. A carbon double bond was thus introduced to theterminal of aryl group in PEEK.
(2) Step 2
To replace the terminal carbon double bond with an epoxy ring, Oxone® (potassiumperoxymonosulfate) dissolved in a mixture solvent of acetone and water was dropped on
UC-PEEK before stirring. The resulting PEEK has epoxy groups on the surface and is referred
to as EP-PEEK.

2.2

Kobe University modification – confirmation of the chemical reaction

To examine whether the chemical reaction had progressed as expected, EP-PEEK wasreacted with a fluorinated amine (1H,1H-pentadecafluorooctylamine) to introduce fluorine atoms tothe epoxy groups (the resulting product is referred to as Am-PEEK). The X-ray photoelectron
spectroscopy (XPS) profiles of Am-PEEK are given in Figure 2, which indicates the detection offluorine. It can be therefore inferred that the reaction took place as expected.

Figure 2

Am-PEEK production process and XPS profiles

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Next comes the comparison of the penetration depth of surface modification between theplasma treatment and the Kobe University modification, which was performed by etching thesurface using an argon ion cluster beam. For the plasma treatment, plasma was irradiated for 10
seconds at 100 W under an oxygen atmosphere of approximately 1.2 Pa in a vacuum plasma
apparatus. The results show that the penetration is deeper with the Kobe University modification
than the plasma treatment. The modified layer of Am-PEEK, which is thicker than the plasma
treatment, is considered to more effectively prevent or delay the reorientation/migration offunctional groups at the surface, resulting in the advantage of a long-lasting modification effect.
As PEEK has some crystallinity, the degree of crystallinity was measured by X-ray
diffraction (XRD) before and after the Kobe University modification. The results indicate thatPEEK is 28% crystalline before the treatment and 26% after the treatment, showing almost nodifference (Figure 3). Because of this unaffected degree of crystallinity before and after thetreatment, the impact of the treatment on the matrix strength is expected to be small, which isadvantageous for the surface modification method.

Figure 3 XRD profiles after Kobe University modification

2.3

Kobe University modification – strength properties

As shown in Figure 4, the single lap shear test was conducted using the specimens treated bythe Kobe University modification, to assess the adhesion shear strength.

Figure 4Kobe University modification – lap shear strength test results(Left: lasting effect, right: lap shear strength after thermal annealing)

The specimen material used was a carbon fiber composite with a PEEK matrix. Forcomparison, two types of adhesives were used: 3M Scotch-Weld 2216 B/A Gray (referred to as 3M2216) and an EPONTM Resin 828 curing agent, 1-methylimidazole. With regard to the treatment
before adhesion, the specimens treated with plasma (referred to as PT-PEEK) were also tested inMitsubishi Heavy Industries Technical Review Vol. 58 No. 4 (December 2021)
4order to compare with the Kobe University modification. The adhesive joint area is 13 mm × 13mm.
In this test, shear strength data were collected for (1) different types of adhesives, (2)specimens adhered after being left for 7 days from the treatment to confirm the lasting effect ofmodification and (3) those adhered after the post-treatment annealing at 150°C to evaluate therobustness of surface modification. The results are given in Figure 4. The data shown therein werenormalized by considering the shear strength of PEEK joints adhered using 3M 2216 without
surface treatment as 1.0.
The test results show that (1) there was almost no adhesion strength when specimens werebonded without surface treatment using an EPONTM Resin 828 curing agent (1-methylimidazole) as
an adhesive, while a certain level of adhesion strength was observed in the case of 3M 2216.
Higher adhesion strength was obtained when 3M 2216 was used in combination with the Kobe
University modification.
(2) Regarding the lasting effect of modification, the shear strength was reduced in 7 days
from the plasma treatment, but remained unchanged for the specimens treated by the Kobe
University method (i.e., EP-PEEK).
(3) When it comes to the robustness of surface modification, the decline in the shear strength
after annealing was also smaller with the Kobe University modification than the plasma treatment.
This is presumably because the penetration of surface modification by the Kobe University
modification is deeper than the plasma treatment, preventing the reorientation of functional groups
more effectively and therefore resulting in less decline in the shear strength.

|3. Polymer brush formation modification (Kyushu Universitymodification)
Given in this chapter is a summary of the treatment method developed with Kyushu
University(3)(4). The Kyushu University modification focuses on the chemical structure of PEEK
consisting of units of benzophenone, which is a photoinitiator. PEEK is irradiated with ultraviolet(UV) ray to generate free radicals within it, leading to the formation of a dense polymer brush withepoxy groups (Figure 5).

Figure 53.1

Schematic illustration of Kyushu University modification

Kyushu University modification –procedure

After cleaning, PEEK was immersed in glycidyl methacrylate (GMA) and was irradiatedwith UV ray (wavelength: 240-400 nm) at room temperature. In this way, poly(glycidylmethacrylate) (PGMA), which has many epoxy functional groups, was introduced to the surf ace ofPEEK. The resulting product is referred to as PEEK-g-PGMA. The standard duration of UVirradiation is 120 minutes. The irradiation intensity in the following confirmation test is of twolevels: low (2 mW/cm2) and high (8 mW/cm2), which are respectively referred to as
PEEK-g-PGMA2-120 and PEEK-g-PGMA8-120. The effect of modification was thus evaluated.

3.2Kyushu University modification – confirmation of the chemical reaction
Figure 6 shows the XPS profiles of C1s binding energy for PEEK-g-PGMA. For

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 4 (December 2021)
52comparison, the profiles of the PEEK specimens that were UV-irradiated at 2 and 8 mW/cm for
120 minutes are also given in Figures 6-b’ and 6-c’, respectively. PEEK (Figure 6-a) has threepeaks, each at 284.5 eV for the C-C bond, 286.5 eV for the C-O bond and 291.1 eV, the somewhat
broad peak of which is attributed to aromatic rings (π-π* bond satellite peak). The figures indicatethat UV irradiation or the introduction of PGMA induces the COO bond at 288.7 eV. The molar
ratio of the C-O bond to C-C bond remained almost unchanged in the case of performing only UVirradiation. When PGMA was introduced, however, the proportion of the C-O bond was increased.
This suggests that PGMA has been successfully grafted on the surface of PEEK. In a comparison
between PEEK-g-PGMA2-120 and PEEK-g-PGMA8-120, the latter shows higher intensities of COO
bond and C-O bond, indicating deeper surface modification.
The contact angle of water on the surface was also measured. PEEK (without surfacemodification) has a water contact angle of 79.3° ± 1.8°, while PEEK-g-PGMA2-120 shows a lower
angle of 62.2° ± 2.3°. In the case of PEEK with a surface that was spin coated with PGMA, thevalue is 63.0° ± 2.9°, which is close to that of PEEK-g-PGMA2-120.

Figure 6 XPS profiles of C1s for CFR-PEEK (a), PEEK-g-PGMA2-120 (b),PEEK-g-PGMA8-120 (c), spin coated PGMA (d), PEEK UV-irradiated at 2m/W/cm2 for 120 min (b’) and PEEK UV-irradiated at 8 mW/cm2 for 120min (c’)

3.3

Kyushu University modification – strength properties

The single lap shear test was conducted using the specimens treated by the Kyushu
University modification .
The specimen used was PEEK (thin and thick films: 0.2 mm and 4 mm in thickness) and theadhesive used was EPONTM Resin 828 with ethylenediamine as a curing agent. The size of thespecimen is 5 mm × 20 mm, while the adhesive joint area is 5 mm × 5 mm. The test results aregiven in Figure 7. The data shown therein were normalized by the lap shear strength of
PEEK-g-PGMA2-60 joints. When compared with the untreated specimen, the adhesion strength of
PEEK-g-PGMA2-120 was improved by a factor of approximately 6. While keeping the UV
irradiation intensity constant at 2 mW/cm2, a lap shear strength test was conducted with the varying
duration of irradiation from 20 to 120 minutes. It has been shown that longer exposure results inhigher adhesion strength. Prolonged exposure to UV light is considered to increase the amount ofgrafted PGMA. This therefore indicates that the adhesion strength is directly correlated with theamount of covalent bonds created by the chemical reaction between epoxy groups in PGMA andamines in the adhesive.

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6Figure 7 Kyushu University modification – lap shear
strength test results|4. Test panel production evaluation
As described above in joint research with two universities, the surface modification
technologies for PEEK, the adhesive properties of which are poor, have been established by testingwith coupons. As a preliminary evaluation to apply these technologies for assembling actual
aircraft structures, the skin-stringer panel, which is one of the main components of an aircraftstructure, was test-produced in our company. Figure 8 provides a schematic of the structure. TwoI-shaped stringers on the outer edge and clips are made of CFRTP using PEEK, the skin is made ofa thermosetting composite material (CFRP) and the rib is made of an aluminum alloy. As the Kobe
University modification is a method of immersion in a solution, it is suitable to treat large parts
with complicated shapes. On the other hand, the Kyushu University modification involves only UV
irradiation, not requiring a long time or much labor. Therefore, the stringers were treated using theKobe University modification and the clips were treated with the Kyushu University modification.
The obtained parts were assembled by adhesion. A photograph of the completed test panel ispresented in Figure 9.
Based on this test panel production, we estimated the cost and weight. The results show thatthe cost for assembly is expected to be reduced by about 30%, compared with the conventional
bolting assembly of composite materials. With regard to the weight, the estimated reduction by theassembly with adhesives is approximately 15%.

Figure 8

Schematic of skin-stringer panel

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Figure 9

Photograph of completed skin-stringer panel|5. Conclusion
PEEK is known to have poor adhesive properties. In this development effort, we developedand established surface modification treatments that make it possible to adhere, through joint
research with Kobe University and Kyushu University. In either treatment, epoxy groups are
introduced to the surface of PEEK, thereby inducing a chemical reaction with the curing agent in
the adhesive (an amine) to explicitly realize the formation of the strongest chemical bonding calleda covalent bond. By applying these destructive surface modifications for the test-production of asmall skin-stringer panel in our company, we estimated the weight and cost. The results indicatethat assembly with adhesives has a weight and cost reducing effect. Moving forward, we will
expand their application to actual-scale aircraft structures. PEEK is also known to be highlybiocompatible because of its stability as well as its superior strength, sliding with low friction, etc.
As such, we expect that these technologies will also be useful in fields other than aircraft.
The results presented in this report were obtained from a project commissioned by the New
Energy and Industrial Technology Development Organization (NEDO).

References
(1)
(2)
(3)
(4)

Hiroyuki Yamane, et al., Physical and chemical imaging of adhesive interfaces with soft X-rays,
Communications materials, (2021) 2: 63
Akira Miyagaki, et al., Surface Modification of Poly (ether ether ketone) through Friedel-Crafts Reactionfor High Adhesion Strength, Langmuir, (2019) 35, 9761-9768
Yucheng Zhang, et al., Adhesion enhancement of Poly (etheretherketone) via surface-initiated
Photopolymerization of glycidyl methacrylate, Polymer, 209 (2020) 122971
Yucheng Zhang, et al., Enhanced adhesion effect of epoxy resin on carbon fiber-reinforced Poly(etheretherketone) via surface initiated photopolymerization of glycidyl methacrylate, Polymer, 209(2020) 123036