High-Performance Adhesives Based on Maleic Anhydride-g-EPDM Rubbers and Polybutene for Laminating Cast Polypropylene Film and Aluminum Foil

The adhesion between aluminum (Al) foil and cast polypropylene (CPP) film laminated with mixtures of amorphousand crystalline-maleic anhydride-grafted ethylene-propylene-diene monomer (MAH-g-EPDM) rubbers and highly reactive polybutene (HRPB) as the adhesives was investigated. Specifically, the HRPB was used as an adhesion promoter of the MAH-g-EPDM rubbers and CPP as well as a compatibilizer of two kinds of MAH-g-EPDM rubbers having limited miscibility. To introduce strong chemical bonds between the MAH-g-EPDM rubbers and Al foil, the surface of Al foil was treated with 3-aminopropyl triethoxysilane (APTES). The weak adhesion between Al foil and MAH-g-EPDM rubbers was improved by imidization between the amine groups (–NH2) of APTES and the maleic anhydride groups (MAH) of MAH-g-EPDM rubbers. The effects of the composition of adhesives, tempering time and adhesive thickness were also studied to optimize the adhesion of the CPP/Al foil laminates. We concluded that MAH-g-EPDM rubber based adhesives containing HRPB can be applied for the lamination of Al foil and CPP films to satisfy the requirements of high-performance packaging materials for various purposes.


Introduction
Aluminum (Al) foils are widely used as protective materials in coating and packaging in electronic devices because of their resistance against air and moisture [1][2][3][4][5].However, poor mechanical properties of Al foil make it prone to tearing, greatly limiting its application.Therefore, the lamination of Al foils with polymer films such as polypropylene (PP) and polyethylene (PE) have been investigated to enhance their performance.The laminated polymers efficiently protect the Al surface from damages that decrease the protective ability of the Al foil [6][7][8][9][10][11][12][13][14][15].In particular, such polymer-based Al-foil laminates have been studied as a soft packaging material for lithium-ion batteries.These polymer-based Al-foil laminates are more suitable in terms of price, weight and heat dissipation compared to the conventional metal containers used for the packaging of lithium-ion batteries [1,14,15].In this study, cast polypropylene (CPP) film was investigated as a protective material for the Al foil.To obtain a CPP-laminated Al foil with high durability, it is critical to achieve strong interfacial adhesion between the polymer layers and Al foil.Resins derived from polyurethane (PU) and epoxy resin (EP) have been widely used as adhesives as well as coating materials [8][9][10][11][12][13][14][15], which can be suitable candidates Coatings 2019, 9, 61 3 of 14

Materials
CPP films (thickness: 40 µm) and Al foil (thickness: 80 µm) foils were supplied from LG Chem in Dae-jeon, Korea.Two kinds of MAH-g-EPDM rubbers, EPDM-A (KEPA-1150) and EPDM-C (KEPA-1130), were purchased from Kumho Polychem in Dae-jeon, Korea.EPDM-A is amorphous while EPDM-C is semi-crystalline due to the presence of 30 wt.% PP.According to the supplier, both EPDM-A and EPDM-C are based on MAH-g-terpolymers of ethylene, propylene and ethylidene norbornene (ENB) in which MAH contents are 0.7 wt.%.HRPB (HRPB 750, M W : 770 g/mol) was obtained from Daelim Industry in Yeo-su, Korea.APTES and acetic acid, which were used to modify the surface of Al foil, were purchased from Sigma-Aldrich Chemical in Yong-in, Korea.Xylene was used as solvent for the mixtures of EPDM-A, EPDM-C and HRPB.AIBN was used as a radical initiator for cross-linking the adhesives and CPP.Both xylene and AIBN were also purchased from Sigma-Aldrich Chemical in Korea.All chemicals were used as received.The chemical structures of the raw materials are presented in Figure 1.

Materials
CPP films (thickness: 40 µm) and Al foil (thickness: 80 µm) foils were supplied from LG Chem in Dae-jeon, Korea.Two kinds of MAH-g-EPDM rubbers, EPDM-A (KEPA-1150) and EPDM-C (KEPA-1130), were purchased from Kumho Polychem in Dae-jeon, Korea.EPDM-A is amorphous while EPDM-C is semi-crystalline due to the presence of 30 wt.% PP.According to the supplier, both EPDM-A and EPDM-C are based on MAH-g-terpolymers of ethylene, propylene and ethylidene norbornene (ENB) in which MAH contents are 0.7 wt.%.HRPB (HRPB 750, MW: 770 g/mol) was obtained from Daelim Industry in Yeo-su, Korea.APTES and acetic acid, which were used to modify the surface of Al foil, were purchased from Sigma-Aldrich Chemical in Yong-in, Korea.Xylene was used as solvent for the mixtures of EPDM-A, EPDM-C and HRPB.AIBN was used as a radical initiator for cross-linking the adhesives and CPP.Both xylene and AIBN were also purchased from Sigma-Aldrich Chemical in Korea.All chemicals were used as received.The chemical structures of the raw materials are presented in Figure 1.

Adhesive Preparation
All adhesive samples were prepared by solvent mixing.Xylene was used as solvent to dissolve EPDM-A/EPDM-C to produce a homogeneous mixture.The EPDM-A/EPDM-C mixture (70/30 by wt.%) was placed in a four-necked flask under nitrogen purging.HRPB (0-25 wt.% of MAH-g-EPDM rubbers) was then added to the flask followed by adding xylene to prepare a solution of fixed solid content (10% by wt).Subsequently, the solution was mechanically stirred at 110 °C for 1 h to obtain a homogeneous mixture of EPDM-A/EPDM-C and HRPB.

Modification of Al Surface
To introduce strong chemical bonds between Al foil and the adhesive, the surface of Al foil was chemically treated with APTES.A 1 wt.% solution of APTES in water was prepared at room temperature.Subsequently, the pH of the solution was adjusted to 5 by adding acetic acid dropwise under stirring.The prepared APTES solution was homogeneously spray-coated on the surface of Al foil.The APTES-coated Al foil samples were placed in a convection oven at 110 °C for 1 h to modify surface of Al foil, facilitating the formation of strong chemical bonds between APTES and Al foil via silanization.The treatments were based on the sol-gel reactions of silane coupling agent on the Al as reported in the literature [47].

Adhesive Preparation
All adhesive samples were prepared by solvent mixing.Xylene was used as solvent to dissolve EPDM-A/EPDM-C to produce a homogeneous mixture.The EPDM-A/EPDM-C mixture (70/30 by wt.%) was placed in a four-necked flask under nitrogen purging.HRPB (0-25 wt.% of MAH-g-EPDM rubbers) was then added to the flask followed by adding xylene to prepare a solution of fixed solid content (10% by wt).Subsequently, the solution was mechanically stirred at 110 • C for 1 h to obtain a homogeneous mixture of EPDM-A/EPDM-C and HRPB.

Modification of Al Surface
To introduce strong chemical bonds between Al foil and the adhesive, the surface of Al foil was chemically treated with APTES.A 1 wt.% solution of APTES in water was prepared at room temperature.Subsequently, the pH of the solution was adjusted to 5 by adding acetic acid dropwise under stirring.The prepared APTES solution was homogeneously spray-coated on the surface of Al foil.The APTES-coated Al foil samples were placed in a convection oven at 110 • C for 1 h to modify surface of Al foil, facilitating the formation of strong chemical bonds between APTES and Al foil via silanization.The treatments were based on the sol-gel reactions of silane coupling agent on the Al as reported in the literature [47].

Adhesive Coating on Al Foil and Lamination with CPP
The prepared EPDM-rubber-based adhesive dissolved in xylene were mechanically mixed with 1, 3 or 5 wt.% of AIBN at room temperature to obtain homogeneous mixtures, which were then coated on APTES-coated Al foil.Uniform adhesive thickness was obtained using a bar coater and the thickness of adhesive layer was measured with a micrometer.To remove solvent, samples were placed in a vacuum oven at 80 • C for 120 s.Finally, CPP was placed on the dried adhesive-coated Al foil and laminated at 110 • C with a speed of 10 mm/s for the efficient penetration of the adhesive from the Al foil up to the CPP film.To evaluate the effect of tempering, the samples were aged in a convection oven at 80 • C for 5 days.
In this study, adhesion between Al foil and CPP film was facilitated by using mixture of two types of MAH-g-EPDM rubbers and HRPB as an adhesive.To improve the weak adhesion between the adhesive and the Al foil, the surface of the Al foil was treated with APTES, resulting in the formation of strong imide bonds between APTES and MAH-g-EPDM rubbers during the lamination step.The imide bonds caused strong adhesion between CPP and APTES-treated-Al foil.Tempering at 80 • C further enhanced the interfacial interaction between the adhesive and the CPP via the radical polymerization of MAH-g-EPDM rubbers and HRPB, which was used to not only increase the compatibility between the different kinds of MAH-g-EPDMs but also facilitate the easy penetration of the adhesive based on its good compatibility with CPP.The overall process is illustrated in Figure 2.

Adhesive Coating on Al Foil and Lamination with CPP
The prepared EPDM-rubber-based adhesive dissolved in xylene were mechanically mixed with 1, 3 or 5 wt.% of AIBN at room temperature to obtain homogeneous mixtures, which were then coated on APTES-coated Al foil.Uniform adhesive thickness was obtained using a bar coater and the thickness of adhesive layer was measured with a micrometer.To remove solvent, samples were placed in a vacuum oven at 80 °C for 120 s.Finally, CPP was placed on the dried adhesive-coated Al foil and laminated at 110 °C with a speed of 10 mm/s for the efficient penetration of the adhesive from the Al foil up to the CPP film.To evaluate the effect of tempering, the samples were aged in a convection oven at 80 °C for 5 days.
In this study, adhesion between Al foil and CPP film was facilitated by using mixture of two types of MAH-g-EPDM rubbers and HRPB as an adhesive.To improve the weak adhesion between the adhesive and the Al foil, the surface of the Al foil was treated with APTES, resulting in the formation of strong imide bonds between APTES and MAH-g-EPDM rubbers during the lamination step.The imide bonds caused strong adhesion between CPP and APTES-treated-Al foil.Tempering at 80 °C further enhanced the interfacial interaction between the adhesive and the CPP via the radical polymerization of MAH-g-EPDM rubbers and HRPB, which was used to not only increase the compatibility between the different kinds of MAH-g-EPDMs but also facilitate the easy penetration of the adhesive based on its good compatibility with CPP.The overall process is illustrated in Figure 2.

Characterization
The 180° peel test was conducted for all prepared laminated samples on a universal testing machine (UTM) (LR5K Plus, LLOYD, West Sussex, UK).The width and length of samples were fixed at 15 and 60 mm, respectively and the tests were conducted at room temperature with a crosshead speed of 250 mm/min.The peel strength was determined by using following equation, where F is the average peel force (N) and w is the width of specimen (mm).The peel tests of CPP/adhesive/Al foil laminates were conducted with 5 specimens per sample and the average values were taken for the analyses.The thermal properties of the prepared EPDM based adhesive and raw material were evaluated by differential scanning calorimetry (DSC) (TA, Q20, New Castle, DE, USA).DSC was calibrated using around 5 mg of indium as a standard for the quantitative measurements.
The DSC measurement was performed from −80 to 200 °C in N2 gas environment at a heating rate of 10 °C/min twice for the samples.The APTES coating and imidization between APTES and the MAH of the adhesive were analyzed by energy-dispersive X-ray analysis (EDX) (SUPRA 40VP, Carl Zeiss, Oberkochen, Germany) and Fourier transform infrared spectroscopy (FT-IR) (FT/IR 300E, JASCO,

Characterization
The 180 • peel test was conducted for all prepared laminated samples on a universal testing machine (UTM) (LR5K Plus, LLOYD, West Sussex, UK).The width and length of samples were fixed at 15 and 60 mm, respectively and the tests were conducted at room temperature with a crosshead speed of 250 mm/min.The peel strength was determined by using following equation, where F is the average peel force (N) and w is the width of specimen (mm).The peel tests of CPP/adhesive/Al foil laminates were conducted with 5 specimens per sample and the average values were taken for the analyses.The thermal properties of the prepared EPDM based adhesive and raw material were evaluated by differential scanning calorimetry (DSC) (TA, Q20, New Castle, DE, USA).DSC was calibrated using around 5 mg of indium as a standard for the quantitative measurements.
The DSC measurement was performed from −80 to 200 • C in N 2 gas environment at a heating rate of 10 • C/min twice for the samples.The APTES coating and imidization between APTES and the MAH of the adhesive were analyzed by energy-dispersive X-ray analysis (EDX) (SUPRA 40VP, Carl Zeiss, Oberkochen, Germany) and Fourier transform infrared spectroscopy (FT-IR) (FT/IR 300E, JASCO, Easton, MD, USA).The FT-IR spectra were collected in ATR mode using a diamond crystal at 4 cm −1 resolution from 4000 to 500 cm −1 by scanning 100 times for the samples.The field-emission scanning electron microscope (FE-SEM) (SUPRA 40VP, Carl Zeiss, Germany) was used to analyze the morphologies of the adhesion surfaces after peel testing.For the measurement via EDX and FE-SEM, the surfaces of peel tested samples were coated with conductive gold (Au) layer.

Adhesion Based on EPDM-A, EPDM-C and HRPB
Many parameters can affect the adhesion strengths of adhesives.In this study, we evaluated the effects of adhesive composition, concentration of radical initiator, adhesive thickness and thermal tempering.Two kinds of MAH-g-EPDM rubbers, EPDM-A and EPDM-C, were tested as the adhesives between CPP and Al foil.EPDM-A is amorphous and shows good affinity for CPP during coating.In contrast, EPDM-C contains 30 wt.% PP.Although the crystallinity of EPDM-C may result in better mechanical properties, it cannot exclusively be used as the adhesive because its crystallinity results in limited film formation on Al foil on drying.Thus, mixture of EPDM-C and EPDM-A in xylene was prepared and HRPB was subsequently added to improve the compatibility between EPDM-C and EPDM-A.The mixtures were initially tested with various weight compositions.For example, when the content of EPMD-C exceeded 30 wt.%, the mixture lost flowability.Thus, the mixing ratio of EPMD-A:EPDM-C was fixed at 7:3 by weight to obtain MAH-g-EPDM rubber blends (EMPD-B).
Figure 3 shows the peel strengths of CPP/Al foil laminates without tempering, which were laminated by EPDM-B or EPDM-A with different HRPB contents.The thickness of adhesives was fixed at 15 µm and it was well-controlled by employing a bar coater during adhesive application on Al foil.Compared with EPDM-A-based CPP/Al laminates, EPDM-B-based CPP/Al laminates showed much higher peel strengths regardless of the HRPB content because 30 wt.% of crystalline PP in EPDM-C might introduce large enhancement in the modulus and yield strength of adhesive itself [25,26,48,49].The peel strength of CPP/Al foil laminate increased with increasing HRPB content until 15 wt.%, after which the peel strength decreased.These results indicate that the optimal adhesive for CPP/Al laminate is the EPDM-B, containing 15 wt.% of HRPB (EPDM-B-15).Easton, MD, USA).The FT-IR spectra were collected in ATR mode using a diamond crystal at 4 cm −1 resolution from 4000 to 500 cm −1 by scanning 100 times for the samples.The field-emission scanning electron microscope (FE-SEM) (SUPRA 40VP, Carl Zeiss, Germany) was used to analyze the morphologies of the adhesion surfaces after peel testing.For the measurement via EDX and FE-SEM, the surfaces of peel tested samples were coated with conductive gold (Au) layer.

Adhesion Based on EPDM-A, EPDM-C and HRPB
Many parameters can affect the adhesion strengths of adhesives.In this study, we evaluated the effects of adhesive composition, concentration of radical initiator, adhesive thickness and thermal tempering.Two kinds of MAH-g-EPDM rubbers, EPDM-A and EPDM-C, were tested as the adhesives between CPP and Al foil.EPDM-A is amorphous and shows good affinity for CPP during coating.In contrast, EPDM-C contains 30 wt.% PP.Although the crystallinity of EPDM-C may result in better mechanical properties, it cannot exclusively be used as the adhesive because its crystallinity results in limited film formation on Al foil on drying.Thus, mixture of EPDM-C and EPDM-A in xylene was prepared and HRPB was subsequently added to improve the compatibility between EPDM-C and EPDM-A.The mixtures were initially tested with various weight compositions.For example, when the content of EPMD-C exceeded 30 wt.%, the mixture lost flowability.Thus, the mixing ratio of EPMD-A:EPDM-C was fixed at 7:3 by weight to obtain MAH-g-EPDM rubber blends (EMPD-B).
Figure 3 shows the peel strengths of CPP/Al foil laminates without tempering, which were laminated by EPDM-B or EPDM-A with different HRPB contents.The thickness of adhesives was fixed at 15 µm and it was well-controlled by employing a bar coater during adhesive application on Al foil.Compared with EPDM-A-based CPP/Al laminates, EPDM-B-based CPP/Al laminates showed much higher peel strengths regardless of the HRPB content because 30 wt.% of crystalline PP in EPDM-C might introduce large enhancement in the modulus and yield strength of adhesive itself [25,26,48,49].The peel strength of CPP/Al foil laminate increased with increasing HRPB content until 15 wt.%, after which the peel strength decreased.These results indicate that the optimal adhesive for CPP/Al laminate is the EPDM-B, containing 15 wt.% of HRPB (EPDM-B-15).The DSC thermograms also confirmed the excellent affinity between CPP and EPDM-B-15.Figure 4b S1 of Supplementary material.These results indicate that EPDM-B-15 showed good compatibility with CPP and the crystallinity of CPP was suppressed by the penetration of HRPB in EPDM-B-15.
We also evaluated the effect of the concentration of radical initiator (AIBN) on peel strength of CPP/Al foil laminate (Figure 5).The addition of AIBN was expected to introduce cross-linking among the EPDM-B-15 and CPP via free-radical polymerization.EPDM-B-15 adhesives containing different amounts of AIBN (0, 1, 3 and 5 wt.%) were prepared and evaluated for the CPP/Al laminates.The EPDM-B-15/AIBN mixture was coated on the Al substrate with a thickness of 10 µm and laminated at 110 • C with CPP.After 1 day of tempering at 80 • C, laminates of CPP/Al foil were evaluated by peel testing.The addition of AIBN generally increased the peel strength of CPP/EPDM-B-15/Al foil laminates, although the peel strength was maximized at the AIBN content of 1 wt.% (Figure 5).The decrease in peel strength upon the addition of larger amounts of AIBN may have resulted from the chain scission of EPDM-B-15 main chains; it was previously reported that excess active free radicals might have caused the main-chain scission of the MAH-g-EPDM or PP in MAH-g-EPDM, resulting in a decrease in the MW of the main backbones or disproportionation during cross-linking process [50][51][52].On the basis of these results, an in EPDM-B-15 containing 1 wt.% of AIBN content was used in subsequent experiments.Although the incorporation of AIBN resulted in a significant increase in the peel strength of the laminates, adhesive failure was observed at the interface between the adhesive and Al foil as shown in Figure S1 in Supplementary material.In order to improve the adhesion between the adhesive and Al foil, Al foil was treated with silane coupling agent, 3-aminopropyl triethoxysilane (APTES), to induce cohesive failure via chemical bonding of Al foil with MAH of EPDM.

Surface Treatment of Al Foil with APTES
To introduce strong chemical bonds and obtain strong adhesion between EPDM-B-15 and Al foil, the Al surface was coated with APTES and thermally treated at 100 °C following the process reported by Denoyelle et al and Plueddemann [47,52].The processes are described in Section 2.3 and schematically presented in Figure 6.7c).No visible peaks are seen in the spectrum of Al foil.In contrast, the spectrum of APTES-treated Al foil shows peaks corresponding to -CH stretching (symmetric and asymmetric) at 2900 and 2860 cm −1 along with -NH peaks at 3300 and 3370 cm −1 .These peaks are the characteristic of the hydrocarbon chains and primary amine groups of APTES.In addition, Si-O stretching peaks were detected at 1200 cm −1 .The -Si-O peak at 1000 cm −1 is

Surface Treatment of Al Foil with APTES
To introduce strong chemical bonds and obtain strong adhesion between EPDM-B-15 and Al foil, the Al surface was coated with APTES and thermally treated at 100 • C following the process reported by Denoyelle et al. and Plueddemann [47,52].The processes are described in Section 2.3 and schematically presented in Figure 6.

Surface Treatment of Al Foil with APTES
To introduce strong chemical bonds and obtain strong adhesion between EPDM-B-15 and Al foil, the Al surface was coated with APTES and thermally treated at 100 °C following the process reported by Denoyelle et al and Plueddemann [47,52].The processes are described in Section 2.3 and schematically presented in Figure 6.treatment was also confirmed by FT-IR (Figure 7c).No visible peaks are seen in the spectrum of Al foil.In contrast, the spectrum of APTES-treated Al foil shows peaks corresponding to -CH stretching (symmetric and asymmetric) at 2900 and 2860 cm −1 along with -NH peaks at 3300 and 3370 cm −1 .These peaks are the characteristic of the hydrocarbon chains and primary amine groups of APTES.In addition, Si-O stretching peaks were detected at 1200 cm −1 .The -Si-O peak at 1000 cm −1 is attributed to the formation of an Si-O-Si network resulting from the silanization of APTES.The peaks at 3500 cm −1 (-OH of silanol) and 920 cm −1 (Si-O-Et of APTES) provide some evidence for the incomplete hydrolysis and silanization of APTES; however, the formation of Si-O-Si networks on the Al surface, as demonstrated by the FT-IR spectra, suggest that most of APTES participated in the treatment [53].The highly reactive -NH 2 group of APTES, which persists even after the treatment process on Al foil, provide chemically active sites for reaction with the MAH groups of MAH-g-EPDM rubbers.Many researchers have reported that strong imide bonds can be formed via ring-opening and cyclization reactions between amine and MAH groups [40][41][42][43][44][45][46]54,55].Nuclear magnetic resonance and FT-IR analysis could be the most powerful tools for confirming the formation of imide bonds between APTES-treated Al foil and EPDM-B-15; however, these methods could not be employed in this study because of the low proportion of imide bonds within the CPP/EPDM-B-15/APTES-treated Al foil laminates.Thus, we directly reacted EPDM-A with APTES in the presence of solvent to verify the feasibility of imidization under lamination temperatures of 110 • C.
Figure 8 shows the scheme of imidization between the MAH groups of EPDM-A and -NH 2 groups of APTES.Imidization of APTES and EPDM-A was conducted and 110 • C for 15 min with magnetic stirring in xylene and monitored by FT-IR.To confirm the formation of imide linkage, we monitored the peaks at 1860 cm −1 , which correspond to the asymmetric stretching of anhydride carbonyl groups (C=O) of MAH in EPDM-A; after the imidization with APTES, these peaks were disappeared, providing strong evidence of reaction of MAH groups of EPDM-A for imidization at 110 • C in the laminator.The characteristic anhydride symmetric carbonyl stretching at 1786 cm −1 and carboxylic groups stretching at 1711 cm −1 were also monitored.The peak at 1786 cm −1 was shifted to low wavenumbers (1779 cm −1 ) with strong decrease in intensity.On the other hand, the peak at 1711 cm −1 shows the significant increase in its intensity with shifting to 1707 cm −1 due to the formation of imide groups [55,56].These results imply that imide bonds were formed between the EPDM-A and APTES-treated Al foil after lamination at 110 • C.These imide bonds are expected to greatly improve the peel strength of CPP/Al foil laminates by preventing adhesion failure between adhesives and Al foil.
carboxylic groups stretching at 1711 cm −1 were also monitored.The peak at 1786 cm −1 was shifted to low wavenumbers (1779 cm −1 ) with strong decrease in intensity.On the other hand, the peak at 1711 cm −1 shows the significant increase in its intensity with shifting to 1707 cm −1 due to the formation of imide groups [55,56].These results imply that imide bonds were formed between the EPDM-A and APTEStreated Al foil after lamination at 110 °C.These imide bonds are expected to greatly improve the peel strength of CPP/Al foil laminates by preventing adhesion failure between adhesives and Al foil.

Adhesive Strength of CPP/APTES-Treated Al Foil Laminates
Silane treatment on the Al surface with APTES was expected to largely improve the weak adhesion between Al foil and EPDM-B-15 through imidization (see Figure 8).Improved adhesion between APTES-treated Al foil and EPDM-B-15 was confirmed via the 180° peel strength.The results of peel testing are shown in Figure 9a,b.Figure 9a 10b).However, when compared with CPP/EPDM-B-15/APTES-treated Al foil, the CPP/EPDM-B-15/Al foil was found to show relatively little improvement in peel strength with time for tempering.Furthermore, the significant improvement in peel strength was not observed above 45 µm.This trend was mainly attributed to the weak interfacial adhesion between EPDM-B-15 and Al foil substrate that resulted in an adhesive failure on the Al surface during the peel test (Figure 10a).These findings strongly suggest that a significant increase in peel strength depending on the thickness is possible with strong adhesion of EPDM-B-15, with both APTES-treated Al foil and CPP exhibiting cohesive failure.Thermal tempering time also affected the peel strength.Radical polymerization largely depends on the half-life time of the radical initiator: the half-life of the initiator used in this study (AIBN) was 80 min at 80 °C [57].Although the theoretical time for the consumption of AIBN in radical polymerization is approximately 14 h, tempering times of up to 5 days were studied to provide sufficient free-radical-induced crosslinking time.Adhesive strength gradually increased with increasing tempering time up to 3 days in both CPP/EPDM-B-15/APTES-treated Al foil and CPP/EPDM-B-15/Al foil laminates.This indicates that tempering times longer than the half-life of the radical initiator is effective for implementing high cross-linking between CPP and EPDM-B-15, which results in a significant improvement in peel strength.10b).However, when compared with CPP/EPDM-B-15/APTES-treated Al foil, the CPP/EPDM-B-15/Al foil was found to show relatively little improvement in peel strength with time for tempering.Furthermore, the significant improvement in peel strength was not observed above 45 µm.This trend was mainly attributed to the weak interfacial adhesion between EPDM-B-15 and Al foil substrate that resulted in an adhesive failure on the Al surface during the peel test (Figure 10a).These findings strongly suggest that a significant increase in peel strength depending on the thickness is possible with strong adhesion of EPDM-B-15, with both APTES-treated Al foil and CPP exhibiting cohesive failure.Thermal tempering time also affected the peel strength.Radical polymerization largely depends on the half-life time of the radical initiator: the half-life of the initiator used in this study (AIBN) was 80 min at 80 • C [57].Although the theoretical time for the consumption of AIBN in radical polymerization is approximately 14 h, tempering times of up to 5 days were studied to provide sufficient free-radical-induced crosslinking

Figure 1 .
Figure 1.Chemical structures of the raw materials.

Figure 1 .
Figure 1.Chemical structures of the raw materials.

Figure 2 .
Figure 2. Overall scheme for CPP and Al foil adhesion.(a) The prepared adhesive is uniformly coated on the APTES treated Al foil using a bar coater; (b) The EPDM-A/EPDM-C form maleimide groups via imidization with the -NH2 groups of APTES and wet onto CPP during lamination at 110 °C; (c) The mixture of EPDM-A/EPDM-C and HRPB form a cross-linked structure with CPP via radical polymerization during tempering.

Figure 2 .
Figure 2. Overall scheme for CPP and Al foil adhesion.(a) The prepared adhesive is uniformly coated on the APTES treated Al foil using a bar coater; (b) The EPDM-A/EPDM-C form maleimide groups via imidization with the -NH 2 groups of APTES and wet onto CPP during lamination at 110 • C; (c) The mixture of EPDM-A/EPDM-C and HRPB form a cross-linked structure with CPP via radical polymerization during tempering.

Figure 3 .
Figure 3. Peel strengths of CPP/Al foil laminates vs HRPB (wt.%) content of the adhesives, which were laminated with EPDM-B (■) or EPDM-A (•).The failures occurred at interfaces of adhesives and Al foils regardless of concentration of HRPB.

Figure
Figure4ashows the DSC traces of the raw materials, EPDM-B and EPDM-B-15; the results are summarized in Table1.The melting point (Tm) of EPDM-C was approximately 168 °C, which is attributed to the crystallinity of PP.In contrast, the Tm of EPDM-A was not observed because of its amorphous nature.EPDM-B, the mixture of EPDM-C and EPDM-A, showed a slight decrease of Tm compared with EPDM-C.It was expected the enthalpy change (∆Hm) at Tm of EPDM-B would be about 7 J/g based on the EPDM-C content (30 wt.%) of the EPDM-B.It is of interest to note the ∆Hm of EPDM-B was 16 J/g possibly due to the nucleation effect of crystalline PP in the system.On the

Figure 3 .
Figure 3. Peel strengths of CPP/Al foil laminates vs HRPB (wt.%) content of the adhesives, which were laminated with EPDM-B (I) or EPDM-A (•).The failures occurred at interfaces of adhesives and Al foils regardless of concentration of HRPB.

Figure 1 Figure 4 .
Figure4ashows the DSC traces of the raw materials, EPDM-B and EPDM-B-15; the results are summarized in Table1.The melting point (T m ) of EPDM-C was approximately 168 • C, which is attributed to the crystallinity of PP.In contrast, the T m of EPDM-A was not observed because of its amorphous nature.EPDM-B, the mixture of EPDM-C and EPDM-A, showed a slight decrease of T m
shows the DSC curves of EPDM-B-15, CPP and CPP coated with 5-µm-thick EPDM-B-15 at 110 • C. The solvent in EPDM-B-15 and CPP coated with EPDM-B-15 was fully removed in convection oven at 80 • C for 1 day.Although only 5 µm of EPDM-B-15 was coated on CPP (The weight ratio of coated EPDM-B-15 and CPP was 1:8.5),EPDM-B-15 caused the T m of CPP to decrease from 155 to 152 • C and the ∆H m of CPP to decrease from 50 and 38 J/g.Considering that EPDM-B-15 has a higher T m than CPP (T m of EPDM-B-15 and CPP: 162 and 155 • C respectively), the significant decrease in T m of CPP after coating with EPDM-B-15 is a result of the high crystallinity of CPP, lowered by HRPB in EPDM-B-15.The possible penetration of HRPB to CPP was confirmed by the swelling of CPP in HRPB as described in Table

Coatings 2018, 8 ,
x FOR PEER REVIEW 7 of 14 the peel strength of the laminates, adhesive failure was observed at the interface between the adhesive and Al foil as shown in FigureS1in Supplementary material.In order to improve the adhesion between the adhesive and Al foil, Al foil was treated with silane coupling agent, 3-aminopropyl triethoxysilane (APTES), to induce cohesive failure via chemical bonding of Al foil with MAH of EPDM.

Figure 5 .
Figure 5.Effect of radical initiator content on the peel strength of EPDM-B-15-based CPP/Al foil laminates.The failure occurred at interfaces between EPDM-B-15 and Al foil regardless of concentration of AIBN.

Figure 6 .
Figure 6.Mechanism for the surface treatment of Al foil with APTES: (a) hydrolysis of APTES in acidic condition; (b) silanization between oxidized Al foil and silanol.

Figure
Figure 7a,b show the SEM and EDX results of the Al foil and APTES treated Al foil after peel test.As shown in the SEM images, no visible differences in surface morphology were observed between Al foil and APTES-treated Al foil because of the thin layer of APTES coated on the Al foil.However, the EDX data show obvious differences in surface composition between Al foil and APTEStreated Al.Only two elements, Al and oxygen (O), were detected in Al foil.The small amount of O detected in Al foil implies that Al was easily oxidized without thermal or chemical treatment in the air.In contrast, carbon (C), O and silicon (Si) were detected on the APTES-treated Al foil surface and oxygen content also largely increased, indicating the successful treatment with APTES.The APTES treatment was also confirmed by FT-IR (Figure7c).No visible peaks are seen in the spectrum of Al foil.In contrast, the spectrum of APTES-treated Al foil shows peaks corresponding to -CH stretching (symmetric and asymmetric) at 2900 and 2860 cm −1 along with -NH peaks at 3300 and 3370 cm −1 .These peaks are the characteristic of the hydrocarbon chains and primary amine groups of APTES.In addition, Si-O stretching peaks were detected at 1200 cm −1 .The -Si-O peak at 1000 cm −1 is

Figure 5 .
Figure 5.Effect of radical initiator content on the peel strength of EPDM-B-15-based CPP/Al foil laminates.The failure occurred at interfaces between EPDM-B-15 and Al foil regardless of concentration of AIBN.

Coatings 2018, 8 ,
x FOR PEER REVIEW 7 of 14 the peel strength of the laminates, adhesive failure was observed at the interface between the adhesive and Al foil as shown in FigureS1in Supplementary material.In order to improve the adhesion between the adhesive and Al foil, Al foil was treated with silane coupling agent, 3-aminopropyl triethoxysilane (APTES), to induce cohesive failure via chemical bonding of Al foil with MAH of EPDM.

Figure 5 .
Figure 5.Effect of radical initiator content on the peel strength of EPDM-B-15-based CPP/Al foil laminates.The failure occurred at interfaces between EPDM-B-15 and Al foil regardless of concentration of AIBN.

Figure 6 .
Figure 6.Mechanism for the surface treatment of Al foil with APTES: (a) hydrolysis of APTES in acidic condition; (b) silanization between oxidized Al foil and silanol.

Figure
Figure 7a,b show the SEM and EDX results of the Al foil and APTES treated Al foil after peel test.As shown in the SEM images, no visible differences in surface morphology were observed between Al foil and APTES-treated Al foil because of the thin layer of APTES coated on the Al foil.However, the EDX data show obvious differences in surface composition between Al foil and APTEStreated Al.Only two elements, Al and oxygen (O), were detected in Al foil.The small amount of O detected in Al foil implies that Al was easily oxidized without thermal or chemical treatment in the air.In contrast, carbon (C), O and silicon (Si) were detected on the APTES-treated Al foil surface and oxygen content also largely increased, indicating the successful treatment with APTES.The APTES treatment was also confirmed by FT-IR (Figure7c).No visible peaks are seen in the spectrum of Al foil.In contrast, the spectrum of APTES-treated Al foil shows peaks corresponding to -CH stretching (symmetric and asymmetric) at 2900 and 2860 cm −1 along with -NH peaks at 3300 and 3370 cm −1 .These peaks are the characteristic of the hydrocarbon chains and primary amine groups of APTES.In addition, Si-O stretching peaks were detected at 1200 cm −1 .The -Si-O peak at 1000 cm −1 is

Figure 6 .
Figure 6.Mechanism for the surface treatment of Al foil with APTES: (a) hydrolysis of APTES in acidic condition; (b) silanization between oxidized Al foil and silanol.

Figure
Figure 7a,b show the SEM and EDX results of the Al foil and APTES treated Al foil after peel test.As shown in the SEM images, no visible differences in surface morphology were observed between Al foil and APTES-treated Al foil because of the thin layer of APTES coated on the Al foil.However, the EDX data show obvious differences in surface composition between Al foil and APTES-treated Al.Only two elements, Al and oxygen (O), were detected in Al foil.The small amount of O detected in Al foil implies that Al was easily oxidized without thermal or chemical treatment in the air.In contrast, carbon (C), O and silicon (Si) were detected on the APTES-treated Al foil surface and oxygen content also largely increased, indicating the successful treatment with APTES.The APTES

Figure 7 .
Figure 7. SEM images and EDX data of Al foil (a), APTES-treated Al foil (b) and FT-IR spectra of Al foil and APTES-treated Al foil (c).

Figure 8 .
Figure 8. Schematic for the imidization of the MAH groups of EPDM-A and amine groups of APTES (a) and FT-IR spectra of EPDM-A (black) and maleimide synthesized from reaction between EPDM-A and APTES (red dot line) (b).

3. 3 . 14 Figure 8 .
Figure 8. Schematic for the imidization of the MAH groups of EPDM-A and amine groups of APTES (a) and FT-IR spectra of EPDM-A (black) and maleimide synthesized from reaction between EPDM-A and APTES (red dot line) (b).
depicts the peel force versus extension curve and Figure 9b depicts peel strength of CPP/EPDM-B-15/Al foil and CPP/EPDM-B-15/APTES-treated Al foil after tempering at 80 °C for 1day.As shown in Figure 9, CPP/EPDM-B-15/APTES-treated Al foil shows a much higher average peel force (7.85 N) and peel strength (0.54 kN/m) than those of CPP/EPDM-B-15/Al foil (average peel force: 4.68 N, peel strength: 0.31 kN/m).The significant improvement in peel force and peel strength of CPP/EPDM-B-15/APTES-treated Al foil, around 42%, can be attributed to the enhanced adhesion between APTES-treated Al foil and EPDM-B-15 by the formation of maleimide bond.

Figure 9 . 14 Figure 10 .
Figure 9. Results of the 180 • peel test after tempering at 80 • C for 1 day: (a) typical peel force (N) vs. propagation extension (mm) of CPP/EPDM-B-15/APTES-treated Al foil and CPP/EPDM-B-15/Al foil, (b) peel strength (kN/m) of CPP/EPDM-B-15/APTES-treated Al foil and CPP/EPDM-B-15/Al foil.When Al foil and CPP were laminated with EPDM-B-15 and subjected to peel testing after 1 day of tempering, adhesive failure on the Al surface occurred.As shown if Figure 10a, after peel testing, the smooth and bare surface of Al foil was observed due to the adhesive failure between Al foil and EPDM-B-15.This indicates that the adhesion between Al foil and EPDM-B-15 without chemical bonding is much weaker compared with that between CPP and EPDM-B-15 crosslinked by radical polymerization.In contrast, cohesive failure of the adhesives occurred in CPP/EPDM-B-15/APTES-treated Al foil, which was observed on the surface of APTES-treated Al foil, as shown in Figure 10b.Moreover, the EDX results on the assigned area in SEM images

Figure 11
Figure 11 shows the adhesion of CPP/EPDM-B-15/Al foil and CPP/EPDM-B-15/APTES-treated Al foil laminated with different thickness of EPDM-B-15 and aged for various times at 80°C.The peel strength increased with increasing adhesive thickness in the tempering time range that was investigated, regardless of the presence of APTES on Al.The peel strength of CPP/EPDM-B-15/APTEStreated Al foil significantly increased with an increasing thickness of EPDM-B-15 with cohesive failure of EPDM-B-15 (Figure10b).However, when compared with CPP/EPDM-B-15/APTES-treated Al foil, the CPP/EPDM-B-15/Al foil was found to show relatively little improvement in peel strength with time for tempering.Furthermore, the significant improvement in peel strength was not observed above 45 µm.This trend was mainly attributed to the weak interfacial adhesion between EPDM-B-15 and Al foil substrate that resulted in an adhesive failure on the Al surface during the peel test (Figure10a).These findings strongly suggest that a significant increase in peel strength depending on the thickness is possible with strong adhesion of EPDM-B-15, with both APTES-treated Al foil and CPP exhibiting cohesive failure.Thermal tempering time also affected the peel strength.Radical polymerization largely depends on the half-life time of the radical initiator: the half-life of the initiator used in this study (AIBN) was 80 min at 80 °C[57].Although the theoretical time for the consumption of AIBN in radical polymerization is approximately 14 h, tempering times of up to 5 days were studied to provide sufficient free-radical-induced crosslinking time.Adhesive strength gradually increased with increasing tempering time up to 3 days in both CPP/EPDM-B-15/APTES-treated Al foil and CPP/EPDM-B-15/Al foil laminates.This indicates that tempering times longer than the half-life of the radical initiator is effective for implementing high cross-linking between CPP and EPDM-B-15, which results in a significant improvement in peel strength.

Figure 10 .
Figure 10.SEM images and EDX results of the Al surfaces of CPP/EPDM-B-15/Al foil and CPP/EPDM-B-15/APTES-treated Al foil laminates after peel test.The peel test was conducted after tempering of the laminates at 80 • C for 1 day: (a) Al surface of CPP/EPDM-B-15/Al foil; (b) APTES-treated Al foil surface of CPP/EPDM-B-15/APTES-treated Al foil; (c,d) EDX results in the area assigned with white dotted line in (a) and (b) respectively.

Figure 11
Figure 11 shows the adhesion of CPP/EPDM-B-15/Al foil and CPP/EPDM-B-15/APTES-treated Al foil laminated with different thickness of EPDM-B-15 and aged for various times at 80 • C. The peel strength increased with increasing adhesive thickness in the tempering time range that was investigated, regardless of the presence of APTES on Al.The peel strength of CPP/EPDM-B-15/APTES-treated Al foil significantly increased with an increasing thickness of EPDM-B-15 with cohesive failure of EPDM-B-15 (Figure10b).However, when compared with CPP/EPDM-B-15/APTES-treated Al foil, the CPP/EPDM-B-15/Al foil was found to show relatively little improvement in peel strength with time for tempering.Furthermore, the significant improvement in peel strength was not observed above 45 µm.This trend was mainly attributed to the weak interfacial adhesion between EPDM-B-15 and Al foil substrate that resulted in an adhesive failure on the Al surface during the peel test (Figure10a).These findings strongly suggest that a significant increase in peel strength depending on the thickness is possible with strong adhesion of EPDM-B-15, with both APTES-treated Al foil and CPP exhibiting cohesive failure.Thermal tempering time also affected the peel strength.Radical polymerization largely depends on the half-life time of the radical initiator: the half-life of the initiator used in this study (AIBN) was 80 min at 80 • C[57].Although the theoretical time for the consumption of AIBN in radical polymerization is approximately 14 h, tempering times of up to 5 days were studied to provide sufficient free-radical-induced crosslinking