2017 Volume 58 Issue 3 Pages 457-464
A 2-layer Titanium/polycarbonate (Ti/PC) laminated sheet treated by homogeneous low energy electron beam irradiation (HLEBI) to only the PC side prior to assembly and hot press at 438 K for 3.0 min under 20 MPa without the use of fasteners, rivets or glue was investigated. Experimental results showed the 0.30 MGy HLEBI dose appeared to be at or near the optimum, achieving mean adhesive force of peeling resistance, oFp at high accumulative probability of peeling Pp = 0.94 of 141.4 Nm−1 compared to 20.0 Nm−1 for the untreated (without HLEBI). Notably, the 0.30 MGy HLEBI raised the oFp at high- Pp of 0.94 significantly, 706%. Moreover, based on the 3-parameter Weibull equation, the 0.30 MGy-HLEBI apparently enhanced the statistically lowest oFp (Fs) from 0 N·m−1 for the untreated to 15.1 N·m−1 for the 0.30 MGy samples, as well as at low-Pp of 0.06 (the lowest experimental oFp) from 0 for the untreated to 25.5 N·m−1 for the 0.30 MGy samples. Based on the results of XPS (X-ray photoelectron spectroscopy) analysis, chemical bonds occurred. When HLEBI cut the chemical bonds and generated active terminated atoms with dangling bonds at PC surface, they probably induced chemical bonding with the Ti. Furthermore, the HLEBI to only the PC side acted to generate the PC activating strong adhesion to the Ti making the interface stronger than the internal cohesion of the PC itself. In addition, 0.30 MGy-HLEBI apparently increased the active bonds sites of C-O and C-C and then decreased the inactive bonds sites of OH on the PC and Ti, resulting in strengthening the peeling resistance. Therefore, increasing adhesion force between the laminated sheets could be explained.
In the past two decades the amount of plastic in an average family car is reported to have increased from 6 to 15%1) since demand for inexpensive lightweight materials have been recently increasing for the benefits of increasing structural strength and reducing weight with high concern for the environment.2) An typical engineering plastic, Polycarbonate (PC) is widely used for protective articles including cockpit windows, building structures and covers for electronic equipment, since the PC exhibits lightweight, high transparency, workability, withstands harsh weather conditions, impact resistance and mechanical strength more than 150 times that of tempered glass. On the other hand, Titanium (Ti) is one of aerospace light structural metals with high specific stiffness, proof stress and tensile strength, as well as high melting point.2) Since Ti is lightweight and exhibits superior properties such as relaxation resistance, impact resistance, and corrosion resistance, it is widely utilized for cleaning free architecture and supersonic airplanes and artificial bone. Both Ti and PC are going to be used in bioadaptable artificial skeletal system and internal organs, as well as dream worthy transport vehicles.
The lamination of two different materials has always had wide application in numerous engineering and science fields including medical artificial organs, airplanes and automobile. Traditional methods of laminating two different materials have always been some of the serious problems to decay the materials. The bolt and fastener are reversible one to join up to fracture, although the rivet and adhesion are irreversible process to join at one time. Developing a joint with maximum safety enhancement adding minimal weight to the structure for low energy consumption is also important. Since both bolt and rivet holes decrease the cross-sectional area, they can act as stress concentrators. Drilling holes in FRP laminate composites reported3–5) results in breakage of the reinforcing fibers, peeling of the top plies at hole entry, resin degradation at the hole wall, delamination of the bottom plies of the laminates3,4) and generation of fatigue cracks.5) In addition, advantages of fasteners are simple processing, high joining strength with small scatter in the data, although disadvantages include increase in weight due to the fasteners and low sealing performance. Although advantages for adhesive bonding articles 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) 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 adhesive strength.6) Due to these constraints, high strength adhesive laminations are difficult to attain.
Homogeneous low energy electron beam irradiation (HLEBI) has a successful treatment for improving polymer-polymer7–12) and metal-polymer13–15) laminations. 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) Since HLEBI improves the joining between different polymers and metals/polymers,16,17) rapid and safety adhesion by using HLEBI has been successfully developed.
Adhesion between Ti and PC has not been improved by HLEBI or hot-press alone. Up to now, this has not been reported: the new HLEBI adhesion method for Ti/PC aims to be an important technology to be developed. Since we propose a new lamination method by applying HLEBI to the connecting surface of the PC and untreated Ti prior to hot-press, we show it is possible to obtain a large adhesive force of peeling resistance (oFp) for Ti/PC laminated sheets to be useful for practical applications.
Figure 1 shows the structure formula of Polycarbonate (PC) (a) and schematic illustration of HLEBI electron curtain processor (b). The 2-layer Titanium/polycarbonate (Ti/PC) laminated sheet was constructed with half specimens: Ti (10 × 40 × 0.02 mm, Niraco Co., Ltd. Tokyo) and PC (10 × 40 × 0.5 mm, Takiron Co., Ltd. Osaka, see Fig. 1(a)). The preparation steps of the Ti/PC sheets were as follows.
Structure formula of Polycarbonate (PC) (a) and schematic illustration of HLEBI electron curtain processor (b).
Step 1 is the Ti and PC half specimens were cut to size. 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.
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 and Fig. 1(b)) prior to lamination assembly. Note “Double-step” refers to the two main steps: HLEBI and hot press. Ti sheet samples were washed by firstly acetone and subsequently methanol to radically and weakly remove oil and fats, respectively. Finally, they were also dipped in distilled water. In the meanwhile each washing prior to assembly and hot-press was performed in the ultrasonic bath after drying.
Step 3 is the half-length of irradiated PC was assembled with the half-length of untreated Ti.
Step 4 is the Ti/PC assemblies were then inserted into a hot-press at 438 K for 3.0 min under 20 MPa.
Step 5 is the characterization of peeling test to evaluate the adhesion and surface analysis of X-ray photoelectron spectroscopy (XPS).
2.2 Condition of HLEBI prior to hot-pressingFigure 1(b) is a schematic of the electron beam processor (Type CB250/15/10mA, Energy Science Inc., Woburn, MA, Iwasaki Electric Group Co. Ltd. Tokyo).18,19) The sheet electron beam generated by a tungsten (W) filament was in a vacuum chamber. The homogeneous irradiation was applied to the specimens with the sheet HLEBI with low energy through a titanium thin film window attached to the chamber with 550 mm diameter. A low energy (acceleration potential, V: kV), irradiating current density (I, A/m2), irradiation current and conveyer speed are summarized in Table 1. 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.
| Irradiation current | 4.7 mA |
| Acceleration voltage | 170 kV |
| irradiating current density | 142 mA/m2 |
| Conveyor speed | 9.56 m/min |
| Dose Correction | RCD Nylon radiometer |
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. The connecting side of only the PC in the aluminum plate holder (0.15 m × 0.15 m) carrying on a conveyor was irradiated through the homogeneous electron beam. Based on the densities (ρ) of Ti and PC, respectively, the penetration depth (Dth) values were estimated by assumptions of Christenhusz &Reimer20), and Libby21), as summarized in Table 2.
| Densities (ρ) | Penetration depth (Dth) | ||
|---|---|---|---|
| Christenhusz & Reime | Libby | ||
| Polycarbonate | 1.20 g･cm−3 | 184 μm | 273 μm |
| Titanium | 4.54 g･cm−3 | 49 μm | 72 μm |
Both Ti and PC peeled connecting surfaces were scanned for elements C, H, O and Ti by using X-ray photoelectron spectroscopy (XPS: Quantum 2000, ULVAC Co., JAPAN)15) for the 0.30 MGy and untreated samples after fracture. Narrow scans were performed for the C (1s) and O (1s) signals.
2.4 Adhesion evaluated by 90°-peeling testFigure 2 shows the experimental setup of peeling test showing the two half specimens Ti and PC (a) and peeling load (Lp) - peeling distance (dp) curves (b) of PC/Ti laminated sheets comparing 0.30 MGy HLEBI and untreated at accumulative probability of peeling resistance, Pp of 0.85. The 2-layer Ti/PC laminated sheet sample, shown in Fig. 2(a) was prepared for the 90°-peeling test to evaluate the influence of Fig. 2(b) shows Lp - dp curves of PC/Ti laminated sheets comparing 0.30 MGy HLEBI and untreated at high Pp of 0.85, when the Lp values are determined by using a micro-load tensile tester (F-S Master-1K-2N, IMADA Co. Ltd., Japan) with a strain rate of 10 mm/min and the vertical length from the peeling contact point to the sample end of 5 mm.16) Based on the curves, the mean adhesive force of peeling resistance (oFp) were estimated by the peeling load and experimental peeling width (10 mm) and length (20 mm, see peeling distance, dp (mm) from 10 and 30 mm between vertical dotted lines in Fig. 2(b)), respectively. The initial distance before peeling (di) was defined at the start point of peeling force, which corresponds to the start point of the first relaxation. The di value is ～1 mm.
Experimental setup of peeling test showing the two half specimens Ti and PC (a) and peeling load (Lp) - peeling distance (dp) curves (b) of PC/Ti laminated sheets comparing 0.30 MGy HLEBI and untreated at accumulative probability of peeling resistance, Pp of 0.85.
The accumulative probability (P) of Median Rank method22) which is useful for statistical evaluation often used in quality control (QC) is one of the convenient ways to analyze mechanical probabilities of adhesive peeling resistance,10) adhesive strength23,24) and elasticity,25) as well as impact value on fracture.26) Here we evaluate accumulative probabilities of peeling resistance (Pp) of the experimental values related to peeling resistance27) expressed by the following equation:
| \[P_{\rm p} = (I - 0.3)/(n + 0.4)\] | (1) |
Here n and I are the total number of samples (n = 11) and rank of mean adhesive force of peeling resistance oFp for each sample from weakest (I = 1) to strongest (I = 11), respectively. Pp values are 0.06, 0.50 and 0.94, respectively when I values are 1, 6, and 11.
Figure 2(b) shows a comparison of Lp (N) vs. peeling distance, dp (mm) curves between HLEBI and untreated Ti/PC laminated sheets showing a joint is created in the difficult to adhere PC and Ti by the HLEBI. Although only little adhesion could be obtained without HLEBI, by applying HLEBI at 0.30 MGy, a peeling load, Lp is generated (135.3 N) compared to the untreated (20.0 N) where Fig. 2(b) shows an example at peeling probability, Pp = 0.85. The adhesion force is mainly caused by chemical bonds and adhesive area. Since residual fine space sites probably exist at adhesive interface, several drops in the curves can be observed, as shown in Fig. 2(b). Therefore, the 0.30 MGy-HLEBI laminates the Ti with the PC sheets, tremendously improving adhesion of peeling resistance.
3.2 Effect of HLEBI on mean adhesive force of peeling resistance (oFp) at each peeling probability (Pp)Figure 3 plots the relationships between mean adhesive force, of peeling resistance (oFp) defined as mean of Lp between dp of 1 and 30 mm (Fig. 2 (b)) at each peeling probability (Pp) of the Ti/PC laminated sheets for the untreated and HLEBI-treated showing applying 0.30 MGy HLEBI gives the highest oFp values at all Pp over all the other data sets. HLEBI dose from 0.13 to 0.43 MGy prior to hot-pressing under 20 MPa for 3 min at 438 K (see in △- 0.13 MGy, □- 0.22 MGy, ◎- 0.30 MGy, ◇-0.43 MGy in Fig. 3) generates the high adhesive force. They are higher than that of hot-pressed PC/Ti sheets untreated (see ● in Fig. 3). The 0.30 MGy HLEBI dose appears to be at or near the optimum, achieving oFp at medial-Pp = 0.50 of 64.9 Nm−1 compared to 13.9 Nm−1 for the untreated (without HLEBI). Notably, at high-Pp of 0.94 the 0.30 MGy HLEBI raised the oFp significantly, 706% from 20.0 of the untreated to 141.4 Nm−1. Figure 3 shows data sets of 0.30 MGy along with 0.22 MGy have adhesion created in all 11 samples of their data sets.
Relationships between peeling probability, Pp and mean adhesive force of peeling resistance oFp (Nm−1) of Ti/PC laminated sheets with and without HLEBI prior to hot-pressing. Here, ●(PC/Ti), ○(0.04 MGy-PC/Ti), △(0.13 MGy-PC/Ti), □(0.22 MGy-PC/Ti), ◎(0.30 MGy-PC/Ti), ◇(0.40 MGy-PC/Ti), ▼(0.30 MGyTi/0.30 MGyPC) and ☆(PC/0.30 MGy-Ti) are for PC untreated/Ti untreated, 0.04 MGy-irradiated PC/Ti untreated, 0.13 MGy-irradiated PC/Ti untreated, 0.22 MGy-irradiated PC/Ti untreated, 0.30 MGy-irradiated PC/Ti untreated, 0.43 MGy-irradiated PC/Ti untreated, 0.30 MGy-irradiated PC/Ti untreated, PC untreated/0.30 MGy-irradiated Ti, and 0.30 MGy-irradiated Ti/0.30 MGy-irradiated PC, PC untreated/0.30 MGy-irradiated Ti prior to assemble and hot pressing, respectively. ☆ is also for 0.30 MGy-irradiated PC/Ti untreated without hot-press.
The weak adhesion force can be also observed for 0.30 MGy-irradiated PC/0.30 MGy-irradiated Ti laminated sheets prior to hot-pressing (see ▼ in Fig. 3). Furthermore, adhesion cannot be detected for the assemble sheets of 0.30 MGy-irradiated PC/untreated Ti without hot-pressing (see ☆ in Fig. 3), as well as untreated PC/0.30 MGy-irradiated Ti prior to hot-pressing (see ☆ in Fig. 3).
Figure 4 shows the maximum oFp at low-, median-, and high-Pp of 0.06, 0.50 and 0.94 against HLEBI dose occurs in the 0.30 MGy samples at 25.5, 64.9 and 141.4 Nm−1, respectively which are enhanced over that of the untreated at 2.10, 13.9 and 20.0 Nm−1. However, in Fig. 4 dotted lines indicate the highest Pp where the oFp is zero (low-, median-, or high), for example Pp = 0.50 for the untreated and 0.06 MGy (filled triangles); and Pp = 0.50 for the 0.13 and 0.43 MGy samples (black diamonds) on the abscissa again showing that carefulness is recommended for optimum dose. On the other hand, the 0.22 and 0.30 MGy data sets had all 11 of their samples successfully adhered hence statistically lowest oFp at Pp = 0 (Fs) (Section 4.1) was calculated (white diamonds) and shows the 0.30 MGy HLEBI enhances reliability and safety of the Ti/PC laminated sheet with Fs = 15.1 Nm−1.
Changes in experimental mean adhesive force oFp at low-, median-, and high-Pp of 0.06, 0.50 and 0.94 of HLEBI-PC/untreated Ti laminated sheets (Solid bold lines) with and without HLEBI at each dose to PC, together with HLEBI-PC/untreated Al38) (Fine lines).
As shown in Fig. 3, the mean adhesive force of peeling resistance oFp (Nm−1) at mid Pp for 0.30 MGy-irradiated PC/Ti untreated (0.30 MGyPC/Ti; ◎) is more than 10 times higher than that of for 0.30 MGy-irradiated Ti/0.30MGy-irradiated PC (0.30 MGyTi/0.30 MGyPC; ▼) and PC untreated/0.30 MGy-irradiated Ti (PC/0.30 MGy-Ti; ☆), respectively. Therefore, the (0.30 MGyPC/Ti; ◎) for 0.30 MGy-irradiated PC/Ti untreated is the best process to utilize for practical articles.
HLEBI dose from 0.13 to 0.43 MGy prior to hot-pressing (see in △- 0.13 MGy, □- 0.22 MGy, ◎- 0.30 MGy, ◇-0.43 MGy in Fig. 3) generates the high adhesive force, higher than that of hot-pressed PC/Ti sheets untreated (see ● in Fig. 3). HLEBI generates active terminated atoms with dangling bonds of polycarbonate polymers. Hot-pressing enhances the contact probability at bonding sites between the active PC-terminated atoms of carbon and oxygen [C and O on PC] and oxygen and Ti terminated atoms on passive film on Ti sheet surface at adhesive interface. The chemical bonds are expressed as a following combinations of ([active C atoms on PC]-[O atoms on Ti], [active C atoms on PC]-[O atoms in atmosphere]-[Ti atoms on Ti], [active O atoms in PC]-[Ti atoms on Ti] and [active C atoms in PC]-[Ti atoms on Ti]) probably occur at adhesive interface.
The contact area with strong chemical bonds, inversely correlated to the residual volume of fine space at interface, is one of the dominant factors of adhesion. The highest oFp (141.4 Nm−1) at high-Pp of 0.94 in the 0.30 MGy-HLEBI samples, which is much higher than that at low-Pp of 0.06 (25.5 Nm−1), can be explained because of broad contact area with strong chemical bonds (see Fig. 3).
On the contrary, the adhesion force values of 0.30 MGy-irradiated PC/0.30 MGy-irradiated Ti laminated sheets prior to hot-pressing (see ▼ in Fig. 3) are lower than that of hot-pressed PC/Ti sheets untreated (see ● in Fig. 3). The weak adhesion can be explained by the weak adhesion, when the repulsive negative Coulomb force between PC and passive film on Ti induced by dangling bonds generated by HLEBI prevents to adhere.
On the other hand, adhesion cannot be detected for the assemble sheets of 0.30 MGy-irradiated PC/untreated Ti without hot-pressing (see ☆ in Fig. 3) because of lack of contact area at interface. Adhesion cannot be also detected for untreated PC/0.30 MGy-irradiated Ti prior to hot-pressing (see ☆ in Fig. 3). It can be explained that dangling bonds in passive film on Ti irradiated cannot catch the terminated surface atoms of inflexible polymers of inactive stiff PC sheets untreated.
4.2 The statistically lowest adhesive force, Fs at Pp = 0The Fs is assumed to be attained from the adaptable relationship of the 3-parameter Weibull equation iterating to the highest correlation coefficient (f) to calculate the statistically lowest oFp value at Pp = 0 for safety design (Fs) often applied to quality control. The Pp depends on the risk of rupture ([oFp − Fs]/FIII).15,18–20)
| \[P_{\rm p} = 1 - \exp [-([{}^\circ F_{\rm p} - F_{\rm s}]/F_{\rm III})^m]\] | (2) |
When the term ln[−ln(1 − Pp)] is zero, the FIII value is the oFp value. The required oFp value at Pp = 0 is defined as the Fs. The Fs, coefficient (m) and constant (FIII) are the parameters. Figure 5 shows changes in f against the potential eFs value to obtain the Fs (a), and linear relationships between ln(oFp − Fs) and ln[−ln(1 − PP)] (b) for the Ti untreated/PC irradiated at each dose-HLEBI.
Changes in correlation coefficient, f against the potential eFs value to obtain the statistically lowest oFs value Fs at Pp = 0 (arrows) (a), and linear relationships between ln(oFp − Fs) and ln[−ln(1 − PP)] (b) for the Ti untreated/PC irradiated at each dose-HLEBI.
The linear logarithmic form of eq. (2) is iterated to obtain the highest f (see Fig 5(a)) to obtain the Fs values. Figure 5(b) illustrates the linear relationships between ln[−ln(1 − Pp)] and ln(oFp − Fs). The values of m and FIII are determined by the least-squares best fit method. When eFs = Fs, the m value is estimated by the slope of the relationship.
Figures 3 and 4 show Fs for the 0.22 and 0.30 MGy samples are lower than the experimental oFp values. The HLEBI of 0.30 MGy improves the Fs values of the Ti/PC laminated sheets. They are higher than all of other data sets. The 0.30 MGy-HLEBI apparently enhances the Fs from 0 N·m−1 for the untreated to 15.1 N·m−1 as well as the lowest experimental oFp at low Pp of 0.06 from 0 for the untreated to 25.5 N·m−1 increasing adhesion. Consequently, the 0.30 MGy HLEBI enhances the reliability (safety level) of Ti/PC laminated sheets. When the adhesive force of peeling resistivity is less than 15.1 N·m−1, HLEBI induced adhesion can be applied to practical Ti/PC laminated articles.
4.3 X-ray photoelectron spectroscopy (XPS) of Ti surfaceWhen HLEBI cuts the chemical bonds and generates dangling bonds with nonbonding electrons in PC, the electrons probably induce chemical bonding and intermolecular coulomb attractive forces. The oFp created between the Ti/PC laminated sheets by the double-step treatment applying HLEBI prior to lamination assembly and hot-press can therefore be explained by creating bonds from the activated electrons of the activated sites. As shown in Fig. 1(a), the structural formula for PC is constructed with its two hexagonal hard segments in one monomer, and is composed of hydrogen (H), carbon (C) and Oxygen (O).
Figure 6 and Fig. 7 show carbon (1s) and oxygen (1s) signals of XPS (X-ray photoelectron spectroscopy) analysis on peeled surface of Ti (a) and PC (b) sides of PC/Ti (Bold lines) laminated sheets with (Solid lines) and without (Broken lines) 0.30 MGy-HLEBI dose to PC. The PC/Ti (Solid lines) lamination is probably caused by the large PC/Ti-adhesion force because of existence of the peak height at the C-C (283.8 eV)28), C=C (284.3 eV)29), C-H (284.6 eV)30) and C-O (285.4 eV)31) on Ti and PC in Fig. 6. The 0.30 MGy-HLEBI enhances the peak height at the C-H on Ti and reduces the height at the C-C on Ti in Fig. 6(a). In addition, the 0.30 MGy-HLEBI enhances the peak height at C-H and C-O on PC and reduces the height at the C-C on PC in Fig. 6(b). As shown in Fig. 7, the 0.30 MGy-HLEBI apparently decreases the O (1s) peaks of inactive bonding sites of OH (530.9～532.0 eV)32), C=O (531.8 eV)33), C=O (532.8 eV)34), C=O (532.5～534.0 eV)35) and TiO2 (530.2 eV)36) groups on the Ti. Since the C=O, C-OH, C-O-OH, C-H and OH are the inactive terminated atoms, they cannot mostly contribute the chemical bonds related to the interfacial adhesion. Thus, the 0.30 MGy-HLEBI induced reduction of the number of OH improves the adhesion force.
Carbon (1s) signals on peeled Ti (a) and PC (b) sides from XPS analysis of PC/Ti (Bold lines) and PC/Al38) (Fine lines) laminated sheets joint samples with (Solid lines) and without (Broken lines) 0.30 MGy HLEBI dose to PC.
Oxygen (1s) signals in peeled Ti (a) and PC (b) sides from XPS analysis of PC/Ti (Bold lines) and PC/Al38) (Fine lines) laminated sheets samples with (Solid lines) and without (Broken lines) 0.30 MGy HLEBI dose to PC.
The 0.30 MGy-HLEBI enhances the peak height at the C-H, and C-O and reduces the peak height at the C-C and C=O on PC in Fig. 6(a). As shown in Fig. 6(b), the 0.30 MGy-HLEBI increases the active bonds of C-O peaks at 285.5 +/− 1.5 eV on PC31). Increasing the oxygen concentration with strong bonding sites of C-O is mainly caused by not only oxygen in PC, but also the oxygen gas contamination, resulting in strengthening the adhesive force of peeling resistance.
Figure 6(b) and Fig. 7(b) and at the C=O in polymers (534.0 +/− 1.0531.0 +/− 1.0 eV)35), C-O-C, C-O-OH, C-OH (533.2 eV)37) and OH (530.9～532.0 eV)32) in Fig. 7. The 0.30 MGy-HLEBI apparently decreases the inactive bonding sites of OH on the PC and Ti. In addition, HLEBI activates the PC surface and enhances the chemical bonds sites of active bonding pairs of C-C and C-O, resulting in strengthening the interface adhesion. It is explained by carbon remaining on the Ti in Fig. 6(a).
When HLEBI activates the terminated electrons at terminated atoms on PC and partly generates active terminated atoms with dangling bonds, they probably induce both chemical bonding and intermolecular coulomb attractive forces to the Ti side. Thus, increasing adhesion force between the laminated sheets can be explained.
Furthermore, the XPS analysis shows the optimum 0.30 MGy-HLEBI dose to only the PC side acts to generate the PC activating strong adhesion to the Ti, and then making the interface stronger than the internal cohesion of the PC itself. However, carefulness is highly required when adjusting for optimal HLEBI dose for practical applications.
4.4 Higher adhesion force of PC/Ti than that of PC/AlAs shown in Fig. 4 (see Bold lines), the PC/Ti laminated sheets for the untreated and HLEBI-treated showing applying 0.30 MGy- HLEBI to PC remarkably gives the highest oFp values at all Pp over all the other data sets. The 0.30 MGy-HLEBI dose also appears to be at or near the optimum, achieving oFp at low and high Pp = 0.06 and 0.94 of 25.5 and 141.4 Nm−1 compared to 10 and 20 Nm−1 for the untreated (without HLEBI), respectively. On the other hand, the 2-layer aluminum/polycarbonate (Al/PC) laminated sheet has been also 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 HLEBI to only the PC connecting surface, prior to lamination assembly and hot press38). As shown in Fig. 4 (see Fine lines), applying the 0.30 MGy HLEBI exhibits the highest mean adhesive force of peeling resistance, oFp over all the data sets, at all Pp.
Both the conversion from Al to Ti and 0.30 MGy HLEBI to PC significantly raises the oFp at all Pp. Based on the XPS results in Figs. 6 and 7, carbon (1s) and oxygen (1s) signals on peeled surface Ti or Al (a) and PC (b) sides of PC/Ti (Bold lines) and PC/Al38) (Fine lines) laminated sheets with (Solid lines) and without (Broken lines) 0.30 MGy HLEBI dose to PC can be detected. Increasing the oFp induced by HLEBI to PC is caused by both decreasing the inactive bonding sites of O-H, C=O and C-OH in PC (Fig. 7(b)) and decreasing the complex alumina of Al2C2O3/[-CH2C-H(OH)]n (283.7 eV)39), Gamma-Al2O3 (531.1 eV)40), Alpha-Al2O3 (351.4 eV)40), Gamma-Al2O3 (531.2 eV)41) and Al(OH)3-Gibbsite (531.7 eV)42) on Al in Fig. 7(a). In addition, as shown in Fig. 6 (see Fine lines), the 0.30 MGy HLEBI increases reactive double bond (π-bond) sites (active bonding sites of C-O in PC) which can explain stronger oFp of Al/PC laminated sheets over the untreated38). On the other hand, effects of conversion from Al to Ti on increasing the adhesion force, oFp induced by HLEBI to PC should be caused by conversion from the complex alumina on Al to simple TiO2 on Ti in Fig. 7(a).
A 2-layer Titanium/polycarbonate (Ti/PC) laminated sheet was created between the difficult to adhere Ti and PC by applying low dose from 0.13 to 0.43 MGy homogeneous low energy electron beam irradiation (HLEBI) to only the PC side prior to lamination assembly and hot-press at 438 K for 3.0 min under 20 MPa atmosphere.
(1) Experimental results showed the strongest adhesion of the Ti/PC was found from the new double-step treatment applying a HLEBI dose from 0.13 to 0.43 MGy, although weak adhesion of the hot-pressed Ti/PC without HLEBI could be obtained. HLEBI up to 0.30 MGy reaching a maximum improved the safety level without radiation damages. The 0.30 MGy HLEBI dose appeared to be at or near the optimum for creating strong adhesion of the Ti/PC laminated sheet, achieving mean adhesive force of peeling resistance, oFp at medial-peeling probability Pp = 0.50 of 64.9 Nm−1 compared to 13.9 Nm−1 for the untreated (without HLEBI). The 0.30 MGy-HLEBI raised the oFp at high-Pp of 0.94 significantly, 706% from 20.0 of the untreated to 141.4 Nm−1.
(2) Based on the 3-parameter Weibull equation, applying HLEBI up to 0.30 MGy prior to hot-press apparently improved the lowest oFp value estimated at the lowest Pp of zero (Fs). The maximum Fs value of the Ti/PC laminated sheets with hot-press after 0.30 MGy-HLEBI dose was 15.1 Nm−1. Consequently, the treatment improved the safety level.
(3) The maximum peeling adhesive force oFp value at high and low Pp (zero, 0.06 and 0.94) of the laminated sheet irradiated at 0.30 MGy were 15.1, 25.5 and 141.4 Nm−1, respectively. However, the higher dose of the treatment applying more than 0.43 MGy HLEBI apparently reduced the oFp at each Pp. Therefore, with careful consideration to dose level, applying HLEBI prior to hot-press proved a useful method for strong and quick lamination of Ti and PC with sterilization without the use of glue.
(4) Based on the results of XPS (X-ray photoelectron spectroscopy) analysis, chemical bonds occurred. When HLEBI cut the chemical bonds and generated active terminated atoms with dangling bonds at PC surface, they probably induced chemical bonding with the Ti. Furthermore, the HLEBI to only the PC side acted to generate the PC activating strong adhesion to the Ti making the interface stronger than the internal cohesion of the PC itself. The 0.30 MGy-HLEBI apparently decreased the inactive bonding sites of OH on the PC and Ti. In addition, HLEBI activated the PC surface and enhanced the chemical bonds sites of active bonding pairs of C-C and C-O, resulting in strengthening the interface adhesion. It was explained by carbon remaining on the Ti. Therefore, increasing adhesion force between the HLEBI-PC/Ti laminated sheets could be explained. Effects of conversion from Al to Ti on increasing the adhesion force, oFp induced by HLEBI to PC should be caused by conversion from the brittle alumina in Al to simple TiO2 in Ti.
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.
