An Aluminum/Polycarbonate (Al/PC) Joint by Homogeneous Low Voltage Electron Beam Irradiation (HLEBI) to PC Prior to Lamination Assembly and Hot-Press

Abstract

A 2-layer aluminum/polycarbonate (Al/PC) joint was fabricated between half specimens of typically difficult to adhere Al and PC without use of welding, fasteners, rivets, chemical treatment or glue by a new double-step adhesion method: applying a low dose of homogeneous low energy electron beam irradiation (HLEBI) to only the PC connecting surface, prior to lamination assembly and hot press at 418 K for 3.0 min under 15 MPa pressure. Experimental results showed 0.30 MGy along with 0.22 MGy had adhesion created in all 11 samples of their data sets [11/11], although data sets of untreated (hot press alone), 0.04, 0.13 and 0.43 MGy had adhesion created in less samples in their data sets at [2/11], [2/11], [6/11] and [10/11], respectively. Moreover, applying the 0.30 MGy HLEBI exhibited the highest mean adhesive force of peeling resistance, ^oF_p over all the data sets, at all peeling probabilities (P_p). Notably, at high-P_p of 0.94 the 0.30 MGy HLEBI raised the ^oF_p significantly, 1517% from 1.48 of the untreated to 23.95 Nm⁻¹. Based on the 3-parameter Weibull equation, the statistically lowest ^oF_p at P_p = 0 (F_s) from 0.30 MGy-HLEBI was the highest value over all other data sets at 3.10 N·m⁻¹. XPS (X-ray photoelectron spectroscopy) of the peeled Al side revealed a C(1s) peak shift in binding energy from 283.8 eV (C-C) to 284.3 eV (C=C), along with increase in O(1s) C=O peak intensity (531.8 eV) indicating the 0.30 MGy HLEBI generates increased reactive double bond (π-bond) sites which can explain stronger ^oF_p of Al/PC joint over the untreated. Since HLEBI cuts the chemical bonds and generates active terminated atoms with dangling bonds in PC polymer, the increased adhesion force in the Al/PC joint can be explained by the chemical bonding at the interface.

1. Introduction

From ancient times, the joining of two different materials has always had wide application in numerous engineering and science fields including building construction, ships, sports equipment and day-to-day articles, and later automotive, airplanes and space vehicles. In recent years, demand for lightweight materials such as polymers and aluminum alloys have been increasing for the benefits of reducing weight and increasing structural strength with high concern for the environment.¹⁾ For example, the amount of plastic in an average family car is reported to have increased from 6 to 15% in the past two decades.²⁾

Aluminum (Al) with its high electrical conductivity, shiny silver color, light specific weight and high corrosion resistance due to passivation is a valuable light structural material utilized for aerospace technology with high specific strength. Polycarbonate (PC) on the other hand, is an engineering plastic of high transparency having mechanical strength more than 150 times that of tempered glass. Because PC is lightweight and exhibits superior properties such as workability, impact resistance, and withstands harsh weather conditions as well, it is widely used for protective articles including cockpit windows, building structures and covers for electronic equipment. Figure 1 shows the structural formula for PC with its two hexagonal hard segments in one monomer.

Fig. 1

Structural formula of polycarbonate polymer.

It is always important to develop a joint with maximum safety enhancement adding minimal weight to the structure for low energy consumption with concern for the environment. However, methods of joining two different materials such as fasteners including bolts and rivets; and adhesive bonding articles such as welding and soldering, has always been some of the serious problems to decay the materials.

While advantages of fasteners are simple processing, high joining strength, and small scatter in the data, disadvantages include increase in weight due to the fasteners and low sealing performance. Moreover, the bolt holes decrease the cross-sectional area and can act as stress concentrators. It is reported drilling holes in FRP laminate composites results in breakage of the reinforcing fibers, peeling of the top plies at hole entry, resin degradation at the hole wall, and delamination of the bottom plies of the laminates.^3,4) The resulting damage can result in generation of fatigue cracks during fatigue.⁵⁾

For adhesive bonding articles, advantages are complete sealing effect, parts are lighter since fasteners are not used, hence no stress concentration due to the bolt hole and no damage due to drilling therefore they typically exhibit higher fatigue strength than bolted joints.⁵⁾ However, disadvantages include adhesion selection is difficult for joints of different materials, additional steps of degreasing and etching the adhering surfaces are needed to obtain high adhesion strength, and chemically treated adhesive joints have the disadvantage of degradation after a few hours by oxidation decreasing bonding strength.⁶⁾ Due to these constraints, high strength adhesive joints are difficult to attain.

Recent advances for Al/PC joints include friction stir welding (FSW) between polycarbonate and AA 7075 aluminum alloy;⁷⁾ friction riveting (FricRiveting), a spot joining process used to fabricate 2024-T351 aluminum/PC joints;⁸⁾ and fabrication of an epoxy-glued Al/PC butt-joint increasing ultimate tensile load 81% and reducing material volume by 15% by flaring outside edges at joining similar in shape to bamboo growth.⁹⁾

In this study, an Al/PC joint was developed without the use of welding, fasteners, rivets, chemical treatment, or glue by a new adhesion method, a double-step treatment consisting of applying low dose (= 0.30 MGy) of homogeneous low energy electron beam irradiation (HLEBI) to the PC prior to lamination assembly and hot-press. While using hot-press without HLEBI the Al and PC are difficult to adhere. However, HLEBI to PC prior to hot-press created a decent mean adhesive force of peeling resistance (^oF_p) of the Al/PC joint.

HLEBI has a successful track record for improving polymer-polymer^10–12) and metal-polymer^13–15) joints. For polymer-polymer joints, polyimide/Teflon (PI/PTFE) layered films have been successfully fabricated by EB irradiation;¹¹⁾ while applying 0.50 MGy EB dose increased adhesive bond strength between PET films and acrylic adhesive.¹²⁾ Homogeneous low voltage electron beam irradiation (HLEBI) has been found to increase adhesive mechanical properties of polymer-polymer joints for biomedical applications of PDMS (polydimethylsilozane)/PTFE,¹⁶⁾ PDMS/PP (polypropylene),¹⁷⁾ and create strong adhesion in the difficult to bond PTFE/PE (polyethylene).¹⁸⁾

For metal-polymer lap-shear joints, applying 0.13 MGy HLEBI to connecting surfaces of the 18-8 stainless steel and epoxy CFRP was found to improve tensile shear strength, τ_B of 18-8/CFRP joints 58% at median accumulative probability (P_B = 0.50).¹⁴⁾ When 0.13 MGy HLEBI was applied to only the 18-8 of the 18-8/CFRP joint, over 200% increase in τ_B at median-P_B = 0.50 was achieved.¹⁵⁾ For Al/epoxy CFRP joints applying 0.22 MGy HLEBI is reported to boost shear strength, τ 45% at low fracture probability, P_s = 0.06.¹³⁾ These effects are mainly caused by surface energy induced by the irradiation with the formation of active terminated atoms with dangling bonds creating chemical bonds between the metal and polymer.¹⁴⁾ When HLEBI improves the joining between different polymers and metals/polymers,^19,20) rapid and safety adhesion by using HLEBI has been successfully developed.

Therefore, we propose making a joint from the widely useful Al and PC without use of welding, fasteners, rivets, chemical treatment or glue by our new double-step adhesion method: applying a low dose of homogeneous low energy electron beam irradiation (HLEBI) to only the PC connecting surface, prior to lamination assembly and hot press aiming to be useful for practical applications.

2. Experimental Procedure

2.1 Preparation of 2-layer Aluminum/Polycarbonate (Al/PC) laminated sheet joints

The 2-layer composite aluminum/polycarbonate (Al/PC) laminated sheet joint samples were constructed with half specimens: Al (10 × 40 × 0.01 mm, Mitsubishi Aluminum Co., Ltd.) and PC (10 × 40 × 0.5 mm, Takiron Co., Ltd.).

Specifically, the preparation steps of the Al/PC joints were as follows. Step 1 is the Al and PC half specimens were cut to size. Step 2 is the novel part of the process, homogeneous low energy electron beam irradiation (HLEBI) was applied to the joining surface of only the PC (see section 2.2) prior to lamination assembly. Step 3 is the Al and irradiated PC were assembled. Step 4 is the Al/PC assembly was inserted into a hot-press at 418 K for 3.0 min under 15 MPa. Note the PC was not desiccated: it was exposed to normal atmospheric conditions. The PC was provided with peel plies on their surfaces that were removed before HLEBI and hot press. “Double-step” refers to the two main steps: HLEBI and hot press. Al sheet samples were washed by acetone, methanol and water before HLEBI.

2.2 Homogeneous low energy electron beam Irradiation (HLEBI)

The connecting side of only the PC was homogeneously irradiated in a jig by an electron-curtain processor (Type CB250/15/10 mA, Energy Science Inc., Woburn, MA, Iwasaki Electric Group Co. Ltd. Tokyo).^21,22) Homogeneous is defined here as constant electron emission as a function of location on sample surface and time by the electron-curtain processor. The homogeneous irradiation was applied to the specimens with the sheet HLEBI with low energy through a titanium thin film window attached to a 550 mm diameter vacuum chamber. A tungsten (W) filament in a vacuum was used to generate the electron beam at a low energy (acceleration potential, V: kV), of 170 keV and irradiating current density (I, A/m²) of 0.0142 A/m². The sheet samples in the aluminum plate holder (0.15 m × 0.15 m) carrying on a conveyor intermittently at a speed of 9.56 m/min were irradiated through the homogeneous electron beam.

One sweep going one way was 0.0432 MGy applied for only a short time (0.23 s) with 30 s interval time between sweeps to avoid excessive heating of the sample.

Figure 2 is a schematic of the electron beam processor. The sheet electron beam generation was in a vacuum above the Ti window. Below the Ti window is the process zone kept under protective N₂(g) at atmospheric pressure with an O₂(g) residual concentration kept below 400 ppm to prevent oxidation of sample. The N₂(g) flow rate was 1.5 L/s at 0.1 MPa N₂(g) pressure. The distance between sample and Ti window was 25 mm.

Fig. 2

Schematic of HLEBI electron curtain processor.

Given the densities (ρ) are 2.70 g·cm⁻³ for Al and 1.20 g·cm⁻³ for PC, the penetration depth (D_th) values of 0.082 mm for Al and 0.184 mm for PC were estimated by assumptions of Christenhusz and Reimer, respectively.²³⁾ In addition, the D_th values of Al (0.121 mm) and PC (0.273 mm) were also calculated by the assumptions of Libby.²⁴⁾

After HLEBI irradiation, the irradiated PC half specimens were assembled with untreated Al half specimens. The Al/PC assemblies were then inserted into a hot-press at 418 K for 3.0 min under 15 MPa.

2.3 90°-peeling test

The 2-layer Al/PC joint composite sample, shown in Fig. 3 was prepared for the 90°-peeling test to evaluate the influence of HLEBI on the mean adhesive force of peeling resistance (^oF_p), as shown in Fig. 4. Peeling adhesive force (F_p) vs. peeling distance (d_p) curves were obtained by using a micro-load tensile tester (F-S Master-1K-2N, IMADA Co. Ltd., Japan) with a strain rate of 10 mm/min.¹⁶⁾ Since the units of the F_p is Nm⁻¹, the ^oF_p was used instead of the adhesive strength, whose units should be Nm⁻². The sample condition of tensile test was as follows:

• (1)   The vertical length from the peeling contact point to the end of the sample was 5 mm.
• (2)   The F_p was determined by using a micro-load tensile tester.

The ^oF_p was estimated by the peeling load and experimental peeling width (10 mm) and length (20 mm, see peeling distance, d_p (mm) from 10 and 30 mm between vertical dotted lines in Fig. 4), respectively. The initial distance before peeling (d_i) was defined at the start point of peeling force, which corresponds to the start point of the first relaxation. The d_i value is ～1 mm.

Fig. 3

Experimental setup of peeling test showing the two half specimens aluminum and PC.

Fig. 4

Peeling load (L_p) - peeling distance (d_p) curves of PC/Al laminated sheets comparing 0.30 MGy HLEBI and untreated at peeling probability, P_p of 0.68. HLEBI is prior to lamination assembly and hot-press.

2.4 Peeling probability

The accumulated probability (P) of Median Rank method²⁵⁾ is one of the convenient ways to analyze mechanical probabilities of adhesive strength,^26,27) adhesive peeling resistance¹⁶⁾ and elasticity,²⁸⁾ as well as strength and impact value on fracture.²⁹⁾ This method is useful for statistical evaluation often used in quality control (QC). Here we evaluate peeling probability (P_p) of the experimental values related to peeling resistance³⁰⁾ expressed by the following equation:   

\[P_{\rm p} = (I - 0.3)/(n + 0.4)\](1)

where n and I are the total number of samples (n = 11) and rank of mean adhesive force of peeling resistance ^oF_p for each sample from weakest (I = 1) to strongest (I = 11), respectively. When I values are 1, 6, and 11, P_p values are 0.06, 0.50 and 0.94, respectively.

2.5 X-ray photoelectron spectrometer (XPS) measurements

X-ray photoelectron spectroscopy (XPS: Quantum 2000, ULVAC Co., JAPAN)¹⁶⁾ was used for surface analysis after fracture. For the 0.30 MGy and untreated samples, both Al and PC peeled connecting surfaces were scanned for elements C, H, O and Al. Narrow scans for the C(1s) and O(1s) signals were performed.

3. Results and Discussion

3.1 Peeling load (L_p)-peeling distance (d_p) curve

Figure 4 shows a comparison of L_p (N) vs. peeling distance, d_p (mm) curves between HLEBI and untreated Al/PC joint showing a joint is created in the difficult to adhere PC and Al by the HLEBI. Although for hot-press without HLEBI only 2 specimens out of 11 could be adhered, by applying HLEBI at 0.30 MGy to the PC prior to hot-press, a peeling load, L_p is generated (0.26 N) compared to the untreated (0 N) where Fig. 4 shows an example at peeling probability, P_p = 0.68. The 0.30 MGy-HLEBI therefore laminates the Al with the PC sheets, creating adhesion of peeling resistance. Note the untreated had no adhesion at P_p = 0.68, hence its L_p values are zero.

3.2 Effect of HLEBI on mean adhesive force of peeling resistance (^oF_p) at each peeling probability (P_p)

Figure 5 plots the relationships between mean adhesive force of peeling resistance (^oF_p) defined as mean of L_p at d_p from 10 to 30 mm (Fig. 4) at each peeling probability (P_p) of the Al/PC laminated sheets for the untreated and HLEBI-treated. Results show applying 0.30 MGy HLEBI gives the highest ^oF_p values at all P_p over all the other data sets. The 0.30 MGy HLEBI dose appears to be at or near the optimum, achieving ^oF_p at medial-P_p = 0.50 of 11.7 Nm⁻¹ compared to no adhesion, 0 Nm⁻¹ for the untreated (without HLEBI). Notably, at high-P_p of 0.94 the 0.30 MGy HLEBI raised the ^oF_p significantly, 1517% from 1.48 of the untreated to 11.91 Nm⁻¹. Figure 5 shows data sets of 0.30 MGy along with 0.22 MGy have adhesion created in all 11 samples of their data sets [11/11]. On the other hand, Fig. 5 shows data sets of untreated, 0.04, 0.13 and 0.43 MGy have adhesion created in less samples in their data sets at [2/11], [2/11], [6/11] and [10/11] respectively.

Fig. 5

Relationships between peeling probability, P_p and mean adhesive force of peeling resistance ^oF_p (Nm⁻¹) of PC/Al laminated sheet joints untreated and HLEBI-treated.

Figure 6 shows the maximum ^oF_p at low-, median-, and high-P_p of 0.06, 0.50 and 0.94 against HLEBI dose occurs in the 0.30 MGy samples at 3.10, 6.20 and 11.91 Nm⁻¹, respectively which are enhanced over that of the untreated at 0, 0 and 1.48 Nm⁻¹. However, in Fig. 6 dotted lines indicate the highest P_p where the ^oFp is zero (low-, median-, or high), for example P_p = 0.50 for the untreated and 0.06 MGy (filled triangles); and P_p = 0.50 for the 0.13 and 0.43 MGy samples (black diamonds) on the abscissa.

Fig. 6

Changes in experimental mean adhesive force ^oF_p at low-, median-, and high-P_p of 0.06, 0.50 and 0.94 of PC/Al joints as a function of HLEBI dose and untreated. Statistically lowest ^oF_p at P_p = 0 (F_s) are shown for 0.22 and 0.30 MGy HLEBI samples. Dotted lines indicate highest P_p where the ^oFp is zero.

On the other hand, the 0.22 and 0.30 MGy data sets had all 11 of their samples successfully adhered hence statistically lowest ^oF_p at P_p = 0 (F_s) (Section 4.1) was calculated (white diamonds) and shows the 0.30 MGy HLEBI enhances reliability and safety of the Al/PC joint with F_s = 3.10 Nm⁻¹. However, the higher dose of 0.43 MGy and lower doses between 0.04 and 0.13 MGy reduced the ^oF_p below that from 0.30 MGy and did not adhere all specimens in their data sets. Therefore, carefulness is recommended when applying HLEBI for practical applications.

3.3 The statistically lowest adhesive force, F_s at P_p = 0

To calculate the statistically lowest ^oF_p value at P_p = 0 for safety design (F_s) often applied to quality control, the F_s is assumed to be attained from the adaptable relationship of the 3-parameter Weibull equation iterating to the highest correlation coefficient (f). The P_p depends on the risk of rupture ([^oF_p − F_s]/F_III).^{16,19,25–32)}   

\[P_{\rm p} = 1 - \exp[-([{}^{\rm o}F_{\rm p} - F_{\rm s}]/F_{\rm III})^m]\](2)

The F_III value is the ^oF_p value, when the term ln[−ln(1 − P_p)] is zero. When P_p = 0, the required ^oF_p value to evaluate new structural materials is defined as the F_s. In predicting the F_s, coefficient (m) and constant (F_III) are the key parameters. The linear logarithmic form of eq. (2) is iterated to obtain the highest f to obtain the F_s values. Figure 7 illustrates the linear relationships between ln(^oF_p − F_s) and ln[−ln(1 − P_p)]. The values of F_III and m are determined by the least-squares best fit method. The m value is estimated by the slope of the relationship when ^eF_s = F_s.

Fig. 7

Changes in correlation coefficient, f against the potential ^eF_s value to obtain the statistically lowest ^oF_s value F_s at P_p = 0 (arrows) for the 0.22 and 0.30 MGy Al/PC joint samples.

Figure 8 shows the linear relationships from the logarithmic form of eq. (2) for the 0.22 and 0.30 MGy data sets in which all 11 samples were adhered. The HLEBI of 0.30 MGy improves the F_s values of the Al/PC laminated sheets over all the other data sets. The 0.30 MGy-HLEBI apparently enhances the F_s from 0 N·m⁻¹ for the untreated to 3.10 N·m⁻¹ as well as at low P_p of 0.06 (the lowest experimental ^oF_p) from 0 for the untreated to 3.44 N·m⁻¹ creating adhesion. Consequently, the 0.30 MGy HLEBI enhances the safety level (reliability) of Al/PC laminated sheets. This indicates with carefulness for optimization, HLEBI induced adhesion can be applied to practical PC/Al joint articles with sterilization without volatilization, when the adhesive force of peeling resistivity is less than 3.10 N·m⁻¹.

Fig. 8

Linear relationships between ln(^oF_p − F_s) and ln[−ln(1 − P_P)] for Al/PC laminated sheets 0.22 and 0.30 MGy HLEBI data sets.

3.4 X-ray photoelectron spectroscopy (XPS) scans of Al and PC peeled surfaces

Fracture analysis by X-ray photoelectron spectroscopy (XPS) narrow C(1s) and O(1s) scans were carried out. The Al and PC peeled surfaces 1) untreated (lamination assembly and hot press alone are compared with; 2) that of the treated with 0.30 MGy HLEBI to PC prior to lamination assembly and hot press since the 0.30 MGy dose gave the best data. Results indicate the 0.30 MGy-HLEBI acts to generate active sites strengthening mean adhesive force of peeling resistance, ^oF_p of the laminated Al/PC joint which generally fractured at the Al-PC interface.

Figure 9 shows the C(1s) scans for the peeled Al (a) and PC (b) surfaces, respectively. Figure 9(a) shows from the C(1s) scan of the peeled Al surface carbon of the PC remains on the peeled Al surface in both the untreated (dotted lines) and 0.30 MGy HLEBI samples (solid lines). However, the C(1s) peak shift in binding energy from 283.8 eV (C-C)³³⁾ to 284.3 eV (C=C)³⁴⁾ indicates the 0.30 MGy HLEBI generates increased reactive double bond (π-bond) sites which can explain stronger ^oF_p of the Al/PC joint over the untreated.

Fig. 9

Carbon (1s) signals in peeled Al side from XPS analysis of Al-PC laminated sheets of untreated and 0.30 MGy HLEBI joint samples. ref) in graph Note x-axis increases from right to left. The untreated sample examined was at high-P_p (>0.80) which adhered (see Fig. 5).

Likewise, Fig. 9(b) shows for the peeled PC surface a shift of the C(1s) peak occurs from 283.7 eV (Al₂C₂O₃/[-CH₂C-H(OH)]_n)³⁵⁾ to 283.8 eV (C-C),³³⁾ This shift can be explained by fracture occuring deeper in the PC side than at the Al-PC interface as evidenced by the 0.30 MGy HLEBI transitioning the fracture from the Al-PC interface with (Al₂C₂O₃/[-CH₂C-H(OH)]_n to deeper in the PC as the C-C peak. Furthermore, additional HLEBI dose of more than 0.04 MGy always decreases the tensile strength, mostly indicating cohesive force, of PC.³⁶⁾ Therefore, the HLEBI appears to make the interface stronger than internal cohesion in the PC itself. Moreover, since carbon-oxygen peaks are not detected,¹⁰⁾ oxygen contamination apparently did not occur by the HLEBI, or exposure to atmospheric oxygen.

Figure 10 shows the O(1s) scans for the peeled Al (a) and PC (b) surfaces, respectively. Figure 10(a) shows the O(1s) peak intensity of the untreated at c/s = 3105 (dotted lines) is reduced to 2857 (solid lines). Therefore, less oxygen remains on the Al side of the 0.30 MGy HLEBI-treated sample. This implies increased carbon of the PC adheres to the Al increasing mean adhesive force of peeling resistance, ^oF_p.

Fig. 10

Oxygen (1s) signals in peeled Al (a) and PC (b) sides from XPS analysis of Al-PC laminated sheets of untreated and 0.30 MGy HLEBI joint samples. ref in graph) Note x-axis increases from right to left. The untreated sample examined was at high-P_p (>0.80) which adhered (see Fig. 5).

In addition, Fig. 10(a) shows although there is a peak shift of O(s1) in Al, the maxima represent the same species: for the untreated, γ-Al₂O₃ at binding energies 531.1 and 531.2 eV^37,38) and α-Al₂O₃ at 351.4 eV³⁷⁾ and for 0.30-MGy HLEBI: α-Al₂O₃ with γ-Al₂O₃ at 531.5 eV.³⁹⁾ Also, the 0.30-MGy HLEBI peak maximum (solid line) spans those for Al(OH)₃ (gibbsite) (531.7 eV)³⁹⁾ and C=O (531.8 eV)⁴⁰⁾ although they are probably superimposed in the shoulder of the untreated (dotted line) peak. Hence, the HLEBI seems to have little effect on changing morphology of the Al₂O₃. This may be due to only the PC being HLEBI-treated before assembly to untreated Al: although electrons probably migrated from the PC to the Al side, the Al side was not directly irradiated.

In Fig. 10(b), the O(1s) scan of the peeled PC surface indicates γ-Al₂O₃ of the Al remains on the peeled PC surface in both the untreated (dotted lines) and 0.30 MGy HLEBI samples (solid lines). However, the increased intensity of γ-Al₂O₃ peaks at 531.1 eV³⁷⁾ and 531.2 eV³⁸⁾ indicate the advantage of more Al adhering to the PC in the 0.30 MGy HLEBI samples. Moreover, the increase in intensity of the C=O peak at 531.8 eV⁴⁰⁾ shows the 0.30 MGy-HLEBI generates increased reactive double bond (π-bond) sites explaining increased ^oF_p over that of the untreated.

Finally, in Fig. 10(b) the broad O(1s) signal peaking from 532.5 to 534.0 eV representing C=O bonds in polymers⁴¹⁾ is virtually unchanged in intensity, however it is extended to higher binding energies by the 0.30 MGy-HLEBI probably generating shorter polymer chain lengths by severing covalent bonds.

It has been previously reported HLEBI increases strength and adhesion of polymers by severing covalent bonds creating dangling bonds.¹³⁾ Dangling bonds in polymers from HLEBI have been detected by electro spin resonance (ESR)¹⁷⁾ and have been reported to enhance adhesion in other polymer/polymer joints.¹⁷⁾ The XPS data indicates a number of possible phenomena: 1) HLEBI to polymers generates double bonds as well as dangling bonds; 2) the dangling bonds are detected as double bonds by XPS; 3) dangling bonds form into double bonds; or 4) two or more of the above.

In summary, the XPS analysis shows the optimum 0.30 MGy HLEBI dose to only the PC connecting surface acts to generate π-bonds in the PC activating strong adhesion to the Al as evidenced by increased ^oF_p. Since lower (0.04 and 0.13 MGy) and higher (0.43 MGy) doses did not adhere all Al/PC joint samples in the data sets, carefulness is highly required when adjusting for optimum HLEBI dose for practical applications.

4. Conclusions

A 2-layer aluminum/polycarbonate (Al/PC) joint was fabricated between half specimens of typically difficult to adhere Al and PC without use of welding, fasteners, rivets, chemical treatment or glue by a new double-step adhesion method: applying a low dose of homogeneous low energy electron beam irradiation (HLEBI) to only the PC connecting surface, prior to lamination assembly and hot press at 418 K for 3.0 min under 15 MPa pressure. Although without HLEBI only two out of 11 samples in the data set could be adhered, decent adhesion of the Al/PC was found when applying 0.30 MGy-HLEBI.

• (1)   Experimental results showed out of 11 samples per data set: applying 0.22 or 0.30 MGy HLEBI resulted in all samples successfully adhered [11/11] and [11/11], more than untreated (hot press alone) [2/11], and those at 0.04 [2/11], 0.13 [6/11] and 0.43 MGy at [10/11]. Moreover, applying the 0.30 MGy HLEBI exhibited the highest mean adhesive force of peeling resistance, ^oF_p values over all the data sets, at all peeling probabilities (P_p).
• (2)   The 0.30 MGy HLEBI dose achieved ^oF_p of 11.7 Nm⁻¹ at medial-peeling probability P_p = 0.50 of the Al/PC joint compared to no adhesion, 0 Nm⁻¹ for the untreated (without HLEBI). Notably, at high-P_p of 0.94 the 0.30 MGy HLEBI raised the ^oF_p significantly, 1517% from 1.48 for the untreated to 23.95 Nm⁻¹.
• (3)   Based on the 3-parameter Weibull equation, the statistically lowest ^oF_p at P_p = 0 (F_s) from 0.30 MGy-HLEBI was the highest value over all other data sets at 3.10 N·m⁻¹.
• (4)   XPS (X-ray photoelectron spectroscopy) of the peeled Al side revealed a C(1s) peak shift in binding energy from 283.8 eV (C-C) to 284.3 eV (C=C), along with increase in O(1s) C=O peak intensity (531.8 eV) indicating the 0.30 MGy HLEBI. HLEBI generates increased reactive double bond (π-bond) sites which can explain stronger ^oF_p of the Al/PC joint over the untreated.
• (5)   Since the 0.30 MGy HLEBI cut chemical bonds and generated active terminated atoms with dangling bonds in PC polymer, the increased adhesion force in the Al/PC joint could be explained by the chemical bonding at the interface. However, the higher dose of 0.43 MGy and lower doses between 0.04 and 0.13 MGy reduced the ^oF_p below that from 0.30 MGy and did not adhere all specimens in their data sets. Therefore, when applying HLEBI to adhere Al/PC for practical applications, carefulness was recommended for maximum safety. Nevertheless, applying the 0.30 MGy HLEBI to only the PC connecting surface prior to lamination assembly and hot press increased the ^oF_p of the Al/PC joint at all P_p.

Acknowledgements

The authors thank Prof. Akira Tonegawa of Tokai University for his useful help. Our sincere gratitude also goes to Eye Electron Beam Co., Ltd. (Gyoda, Saitama, Japan) for their support with this work.

REFERENCES

• 1)  Q. Yang, S. Mironov, Y.S. Sato and K. Okamoto: Mater. Sci. Eng. A 527 (2010) 4389–4398. 10.1016/j.msea.2010.03.082
• 2)  N. Amanat, N.L. James and D.R. McKenzie: Med. Eng. Phys. 32 (2010) 690–699. 10.1016/j.medengphy.2010.04.011
• 3)  R. Zitoune and F. Collombet: Compos. Part A 38 (2007) 858–866. 10.1016/j.compositesa.2006.07.009
• 4)  J. Davim, P. Reis and C. Antonio: Compos. Sci. Technol. 64 (2004) 289–297. 10.1016/S0266-3538(03)00253-7
• 5)  R. Matsuzaki, M. Shibata and A. Todoroki: Compos. Part A 39 (2008) 154–163. 10.1016/j.compositesa.2007.11.009
• 6)  K. Allen: Int. J. Adhes 23 (2003) 87–93. 10.1016/S0143-7496(02)00054-4
• 7)  R. Moshwan, S.M. Rahmat, F. Yusof, M.A. Hassan, M. Hamdi and M. Fadzil: Int. J. Mater. Res. 106 (2015) 258–266. 10.3139/146.111172
• 8)  C.F. Rodrigues, L.A. Blaga, J.F. dos Santos, L.B. Canto, E. Hage, Jr. and S.T. Amancio-Filho: J. Mater. Process. Technol. 214 (2014) 2029–2039. 10.1016/j.jmatprotec.2013.12.018
• 9)  L.R. Xu, H. Kuai and S. Sengupta: Soc. for Exptl. Mech. 44 (2004) 608–615. 10.1007/BF02428250
• 10)  C. Kubo, M. Kanda, K. Miyazaki, T. Okada, M.C. Faudree and Y. Nishi: Mater. Trans. 56 (2015) 1821–1826. 10.2320/matertrans.M2015082
• 11)  A. Oshima, H. Nagai, F. Muto, T. Miura and M. Washio: J. Polym. Sci. Tech. 19 (2006) 123–127.
• 12)  M. Zenkiewicz: Int. J. Adhes. 24 (2004) 259–262. 10.1016/j.ijadhadh.2003.10.008
• 13)  T. Okada, M. Kanda, M.C. Faudree and Y. Nishi: Mater. Trans. 55 (2014) 1587–1590. 10.2320/matertrans.MAW201419
• 14)  A. Minegishi, T. Okada, M. Kanda, M.C. Faudree and Y. Nishi: Mater. Trans. 56 (2015) 1169–1173. 10.2320/matertrans.M2015105
• 15)  A. Minegishi, M. Kanda, M.C. Faudree and Y. Nishi: Mater. Trans. 57 (2016) 513–518. 10.2320/matertrans.MBW201511
• 16)  Y. Nishi, M. Uyama, H. Kawazu, H. Takei, K. Iwata, H. Kudoh and K. Mitsubayashi: Mater. Trans. 53 (2012) 1657–1664. 10.2320/matertrans.M2012124
• 17)  Y. Nishi, H. Kawazu, H. Takei, K. Iwata, H. Kudoh and K. Mistubayashi: Mater. Trans. 52 (2011) 1943–1948. 10.2320/matertrans.M2011129
• 18)  C. Kubo, T. Okada, M. Uyama, M. Kanda and Y. Nishi: Mater. Trans. 55 (2014) 1742–1749. 10.2320/matertrans.MAW201416
• 19)  M. Uyama, N. Fujiyama, T. Okada, M. Kanda and Y. Nishi: Mater. Trans. 55 (2014) 561–565. 10.2320/matertrans.MBW201312
• 20)  T. Shimaru: Technol. Adhes. Seal. 3 (1959) 121–130.
• 21)  M. Faudree and Y. Nishi: Mater. Trans. 53 (2012) 1412–1419. 10.2320/matertrans.M2012145
• 22)  R. Suenaga, M. Kanda, N. Hironaka and Y. Nishi: J. Japan Inst. Metals 72 (2008) 35–38. 10.2320/jinstmet.72.35
• 23)  R. Christenhusz and L. Reimer: Z. Angew. Phys. 23 (1967) 396–404.
• 24)  W.F. Libby: Anal. Chem. 19 (1947) 2–6. 10.1021/ac60001a002
• 25)  T. Nishida and E. Yasuda: Evaluation of Dynamic Properties of Ceramics (Ceramics no Rikigaku Tokusei Hyouka in Japanese), (Nikkan Kogyou Shimbun Ltd., Tokyo, 1986) pp. 50–51.
• 26)  Y. Nishi, H. Sato, H. Takei and K. Iwata: J. Mater. Res. 24 (2009) 3503–3509. 10.1557/jmr.2009.0429
• 27)  H. Sato, K. Iwata, A. Tonegawa and Y. Nishi: J. Japan Inst. Metals 72 (2008) 526–531. 10.2320/jinstmet.72.526
• 28)  H. Takei, M. Salvia, A. Vautrin, A. Tonegawa and Y. Nishi: Mater. Trans. 52 (2011) 734–739. 10.2320/matertrans.MBW201005
• 29)  M. Kanda and Y. Nishi: Mater. Trans. 50 (2009) 177–181. 10.2320/matertrans.MRA2008170
• 30)  K. Oguri, Y. Irisawa, A. Tonegawa and Y. Nishi: J. Intellig. Mater. Struct. 17 (2006) 761–765. 10.1177/1045389X06057532
• 31)  M. Kanda, Y. Miyazawa, M. Uyama and Y. Nishi: Mater. Trans. 54 (2013) 1795–1799. 10.2320/matertrans.MAW201314
• 32)  W. Weibull: Ingeniörs vetenskaps akademien, nr. 153 (Generalstabens litografiska anstalts förlag, Stockholm, 1939) pp. 16–22.
• 33)  S. Takabayashi and T. Takahagi: J. Surf. Anal. 20 (2013) 25–54.
• 34)  Electronic Supplementary Material (ESI) for J. Mater. Chem. Royal Soc. Of Chem. (2011).
• 35)  P. Stoyanov, S. Akhter and J.M. White: Surf. Interface Anal. 15 (1990) 509. 10.1002/sia.740150903
• 36)  T. Takahashi, T. Morishita and Y. Nishi: J. Japan Inst. Metals 69 (2005) 759–762531. 10.2320/jinstmet.69.759
• 37)  A. Lippitz, E. Kemnitz, O. Bose and W.E.S. Unger: Fresenius J. Anal. Chem. 358 (1997) 175–179. 10.1007/s002160050376
• 38)  B.R. Strohmeier, J.A. Rotole and P.M.A. Sherwood: Surf. Sci. Spectra 5 (1998) 18–24. 10.1116/1.1247852
• 39)  P.M.A. Sherwood and S. Thomas: Anal. Chem. 64 (1992) 2488–2495. 10.1021/ac00045a006
• 40)  F. Beguin, M. Inagaki, R. Benoit and R. Erre: Intl. Symposium on Carbon. 2 (1990) 806–809.
• 41)  D.T. Clark and H.R. Thomas: J. Polym. Sci., Polym. Chem. Ed. 16 (1978) 791–820. 10.1002/pol.1978.170160407