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Mechanics of Composite Materials

, Volume 49, Issue 3, pp 317–324 | Cite as

Interfacial properties of aluminum/glass-fiberreinforced polypropylene sandwich composites

Article
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Aluminum/glass-fiber-reinforced polypropylene (Al/GFPP) laminates were manufactured by using various surface pretreatment techniques. Adhesion at the composite/metal interface was achieved by a surface pretreatment of Al sheets with amino-based silane coupling agents, incorporation of a polyolefin-based adhesive film and modification with a PP-based film containing 20 wt.% of maleic-anhydride-modified polypropylene (PP-g-MA). In order to increase the effect of bonding between components of the laminates, the combination of silane treatment and the addition of the PP-based film was also investigated. The mechanical properties (shear, peel, and bending strengths) of adhesively bonded Al/GFPP laminates were examined to evaluate the effects of the surface treatments mentioned. It was revealed that the adhesion in the laminated Al/GFPP systems could be improved by the treatment of aluminum surface with an amino-based silane coupling agent. Judging from the results of peel and bending strength, with incorporation of polyolefin-based films, adhesion in the Al/GFPP laminates increased significantly. The modification of Al/GFPP interfaces with a PP-g-MA/PP layer led to the highest improvement in their adhesion properties. The combination of surface modification with silane and addition of PP-based films did not yield the high bending strength desired. This may be due to the insufficient bonding between silane groups and PP-based films.

Keywords

interface adhesion sandwich composite mechanical properties 

Introduction

Metal/polymer laminates have been used in a wide variety of applications in electronics, automotive, defense, and aerospace industries because they tailor the overall mechanical properties of the laminated structures on the basis of properties of their constituents [1]. In recent years, metal-plastic laminates and sandwich sheets have been developed in order to considerably reduce the weight of vehicles and to improve the sound-deadening properties of the material. To date, the focus of research into composite/metal laminates has been on thermoset resins, namely epoxy, and aluminum. Fiber/metal laminates (FML), such as GLARE® and ARALL®, which are made from aluminum and glass or aramid-reinforced epoxy, respectively, have been under development for the aerospace industry since the early 1980s. These materials have been found to exhibit an excellent fatigue resistance, impact resistance, and damage tolerance, and are now finding significant applications in commercial aircrafts. However, such thermoset-based composites are often brittle, and, for optimum consolidation of parts made from prepregs, an elevated processing temperature and pressure are required for a prolonged period [2]. Thermoplastic-based laminates have received comparatively little attention; however, the short production times, high recyclability, and low content of volatiles offered by thermoplastics make them attractive. Other benefits are likely to include: (i) the possibility of reforming and reshaping the components following the manufacture, (ii) the ease of repair, (iii) superb energy-absorbing characteristics, and (iv) a high resistance to localized impact loadings [3]. Polypropylene (PP) has excellently balanced physical and mechanical properties. Glass-fiber-reinforced polypropylene (GFPP) is of particular interest due to its relatively low cost. In recent years, thermoplastic polymer/metal-based components and laminated composites, such as aluminum/thermoplastic/aluminum-laminated sheets and steel/PP or nylon/steel-laminated sheets for the weight reduction of car body panels, have received researchers’ attention. However, PP is generally hydrophobic and shows a low free surface energy, presenting serious difficulties of bonding to other materials, even to polar ones, which has so far limited the widespread use of PP and other polyolefins under mass production conditions where joining is necessary [4, 5]. For this reason, a modification of their surface is necessary to produce well-adhering compounds. As the use of both aluminum and thermoplastics continues to increase, there will be an ever-growing need to efficiently join subcomponents during manufacturing and assembly. While much has been written on the subject of adhesive bonding, the knowledge is still inadequate, and the engineering tools available for the through-life management of adhesively bonded structures are primitive [6].

Many pretreatments are available, ranging from a simple solvent wipe to the use of a series of complex chemical processes. Different groups of materials, i.e., metals, inorganic glasses, plastics, elastomers, etc., tend to require their own specific pretreatments. However, some pretreatments are effective with different groups of materials; for example, silanes can greatly enhance the performance of joints involving either metals or inorganic glasses. With regard to PP composites, the interphase may be tailored with a silane coupling agent, a bonding agent, or an additive agent such as the maleic anhydride into the matrix PP [7].

Demjen et al. [8] focused on the mechanism of interaction between silane coupling agents and the polypropylene matrix. They showed that aminofunctional silanes adhere strongly to the surface of a CaCO3 filler. Chen et al. [9] found that the pretreatment of aluminum surfaces with amino-based silanes led to an increase in the lap shear strength.

Maleic-anhydride-modified PP (PP-g-MA) is a polymer that is widely employed in anticorrosive coatings for metal pipes and containers, metal-plastic laminates for structural use, multilayer sheets of paper for chemical and food packaging, and polymer blends. The authors modified PP by the addition of 5–30 wt.% of PP-g-MA and found that, as a result, the lap shear strength increased. Reyes and Compston et al. [10] applied an amorphous chromate treatment to the aluminum and incorporated a layer of PP-g-MA into the E-glass fiber/polypropylene composite-aluminum interface in order to provide an optimum adhesion between layers.

The objective of this study is to develop fiber/metal laminates based on glass-fiber-reinforced polypropylene composites and aluminum subjected to different pre-treatment techniques. The investigation of various surface treatments on the adhesive properties of fiber/metal laminates is also the aim of the present work

2. Materials

Fiber-metal laminate systems consisting of alternating layers of metal and fiber-reinforced polymer (FRP) composites and bonded by an adhesive layer were prepared. The laminates were manufactured from sheets of 2- and 4-mm-thick aluminum (Al) and GFPP non-crimp commingled glass fiber/PP fabrics (0°/90° biaxial). The noncrimp commingled fabrics used in this study were developed in collaboration with MetyxTM Inc., and their characteristics are shown in Table 1.
Table 1

Characteristics of Glass Fiber/PP Noncrimp Commingled Hybrid Fabrics

Fibers

Tex, g/10,000 m

Composition, wt.%

Nominal weight, g/m2

Weaving angle

Glass

300

60

767

0°/90°

PP

200

40

767

0°/90°

The silane coupling agent, N-(4-vinylbenzyl)-N9-(3-trimethoxysilylpropyl)-ethylenediamine hydrochloride (Z-6032), was provided by Dow CorningTM. Distilled water and glacial acetic acid were used in order to prepare a silanol solution. The polyolefin-based adhesive film (Bemis 6218) was provided by Bemis Associates Inc., USA. DupontTM Fusabond® P613, the maleic-anhydride-modified polypropylene (PP-g-MA) was obtained in a granular form. The density of the Fusabond® P613 was 0.902 g/cm3 and the melting point (T m) 162°C. The polypropylene (MH418), an injection grade homo-polymer with a density of 0.855 g/cm3 and melting point of 160°C, was provided by PETKİM PetrochemicalsTM, Turkey.

3. Preparation Methods of Fiber-Metal Laminates (FMLs)

Al/GFPP laminates were produced by using various surface modification techniques. The bonding between a GFPP composite/metal interfaces was performed by modifying the surfaces of Al sheets with the amino-based silane coupling agent, introduction of the polyolefin-based adhesive film, and modification with the PP-based film containing 20 wt.% of maleicanhydride-modified polypropylene (PP-g-MA).

Silane was used for tailoring the interfaces of GFPP and Al sheet surfaces. For this purpose, Al sheets were first degreased and then modified with silane according to the procedure described by the manufacturer [11]. Alternatively, a PP-g-MA layer was incorporated between the Al sheet and GFP for providing better adhesion. For this purpose, 20 wt.% PP-g-MA/PP films of average thickness 0.5 mm were prepared by extrusion and hot pressing at 185°C under a fixed pressure of 1 MPa. In addition, an as-received polyolefin-based adhesive (Bemis 6218) film was also used for integration of the FML system.

Furthermore, to improve the bonding strength, both the treatment with silane and the addition of a PP-based adhesive film were used for the same FML systems. The preparation steps of the sandwiches were similar to those described above. The surfaces of Al sheets were modified with the silane coupling agent, and then PP-based films were placed between the Al sheet and GFPP layers.

The laminated composites were manufactured from sheets of a 2- or 4-mm-thick Al sheet and a woven cloth consisting of comingled GFPP, as illustrated in Fig. 1. The layers of Al and GFPP were stacked together and pressed at a temperature of 200°C for 10 minutes at a constant pressure of 1.5 MPa. Then the FML specimens were cooled down to room temperature at a constant cooling rate of 15°C/min under a fixed pressure of 1.5 MPa. The laminates containing polyolefin-based adhesive films were prepared in two stages. First, a GFPP composite was prepared, an then, during the final lamination process, the Al sheets, polyolefin-based adhesive films, and GFPP were pressed at 145°C for 5 h at a constant pressure of 1.5 MPa. After lamination, the specimens were cooled down to room temperature at a constant cooling rate of 15°C/min under a fixed pressure of 1.5 MPa.
Fig. 1

Schematic illustration of an Al/GFPP laminate: 1 — GFPP, 2 — Al, and 3 — interface.

4. Mechanical Characterization of Fiber-Metal Laminates (FMLs)

The mechanical properties of FML systems obtained by using various surface treatments were found from the results of lap shear, peel, and three-point bending tests. The lap shear test was performed to evaluate the interfacial properties of the composite/metal laminates. The geometries of the tensile single-lap shear test specimens were selected based on the ASTM D 3164–03 standard and prepared from a 2-mm-thick Al sheet and two plies of GFPP layers. The specimens (20 mm wide) and their preparation steps are shown in Fig. 2. At least four specimens were tested, and shear force–displacement data were collected. The tests was performed in a Schimadzu AGI universal test machine (5 kN) with a crosshead speed of 1.3 mm/min.
Fig. 2

Preparation of lap-shear test specimens (a) and their photo (b).

The peel testing was performed to determine the strength of adhesives in cleavage peel under tensile loading. In order to characterize the adhesion between the Al sheet and the GFPP composite, test specimens were prepared according to the ASTM D-3807 standard. Al sheets were cut into 26-mm-wide and 180-mm-high bars before lamination. The laminated test panels consised of two 4-mm-thick aluminum and two GFPP plies, bonded by using the hot pressing technique. A crack approximately 77 mm long was induced by placing a KaptonTM film between GFPP and the Al sheet before bonding with the PP-g-MA-based film. The peel test specimens and their loading can be seen in Fig. 3. At least four specimens were tested in the SchimadzuTM universal test machine with a crosshead speed of 12.7 mm/min. Load–displacement data were recorded, and the average values of peel strength were calculated.
Fig. 3

Photos of peel test specimens (a) and their loading (b).

The three-point bending tests were carried according to the ASTM D-790M Standard, and load–displacement curves were obtained for all specimens. For this purpose, GFPP was hot pressed between 2-mm-thick Al sheets subjected to the different surface pretreatments described before. The panels obtained were cut into 10-mm-wide and 100-mm-long specimens, which were loaded at a crosshead speed of 2.1 mm/min, as shown in Fig. 4. The support span L was 80 mm, and the bending strength S was calculated by using the formula
$$ S=\frac{{3{F_{\max }}L}}{{2b{d^2}}}, $$
where F max is the maximum load on the load–displacement curve, b is the width of the beam tested, and d is its depth. Load–displacement graphs F–w were obtained during the test.
Fig. 4

Flexural test specimen under a load.

Results and Discussions

Lap shear test results

The interfacial shear stress versus displacement graphs of Al/GFPP FML systems prepared by using various bonding techniques were obtained in lap shear tests. The typical representative data are plotted in Fig. 5. As seen, with increasing displacements, the shear stresses increased in a stable manner up to failure. Sudden drops were observed above the maximum stress level.
Fig. 5

Interfacial shear stress τ vs displacement w for Al/GFPP laminates modified with PP-g-MA (the numbers indicate that five specimens from the same batch were tested).

The shear stresses on the Al/GFPP interfaces modified with a PP-based film reached 5.93 MPa (±0.41). The displacements at the maximum stress (2.9 mm) also increased as compared with those in the cases of other pretreatment techniques. Based on these results, it can be concluded that the introduction of a PP-based film containing 20 wt.% PP-g-MA into Al/GFPP interlayer considerably improves the fracture strength and toughness of the adhesive joint. The increase in the lap shear strength can be ascribed to the contribution of chemical interactions at the interface. Also, due to the good adhesion at the interface, plastic deformation occurs in the interlayer material or matrix, which leads to high toughness values.

As an alternative method, a polyolefin-based adhesive film was incorporated as an interlayer between Al and GFPP. This film gave the lowest interfacial shear strength, equal to 0.57 MPa (±0.05). Also, the displacements at the maximum shear stress were found to be the lowest as compared to those obtained with other pretreatment techniques. This may be connected with the low processing temperature of the polyolefin-based adhesive film (145°C), which is lower than the melting point of PP (165°C). So, no melting on the surface of GFPP during the lamination may cause poor adhesion.

The average interfacial shear strength of silane-treated Al/GFPP samples were calculated to be 2.03 (±0.17) MPa. For the specimens prepared by combining the silane treatment with the use of a PP-based film, the average shear stress was 4.49 (±1.31) MPa. Although a positive effect of the combination was expected, the experimental results showed that it did not yield the high bending strength desired. This may be due to the insufficient bonding of silane groups to the PP-based film.

Peel test results

The load–displacement graphs of Al/GFPP FML systems prepared by using various bonding techniques were obtained from the results of peel tests. The typical representative data are plotted in Fig. 6. The average peel strengths of the Al/GFPP systems are given in Table 2. The load–displacement graphs of silane treated Al/GFPP laminates exhibited the saw-tooth form associated with an unstable propagation of cracks.
Fig. 6

Load–displacement graphs F–w obtained in peel tests of Al/GFPP modified with PP-g-MA (the numbers indicate that five specimens from the same batch were tested).

Table 2

Average Results of Peel Tests for Al/GFPP Prepared by Using Various Bonding Techniques

Bonding technique

Peel strength, N/mm

Silane treatment

0.53 ± 0.06

Introduction of a polyolefin film

2.67 ± 0.79

Introduction of a PP-based film

6.61 ± 0.56

Silane-treated + a PP-based film

3.98 ± 1.22

The load versus displacement graphs of the Al/GFPP laminates modified with a PP-based film containing 20 wt.% PP-g-MA exhibited a much smoother increase in the load in comparison with that in the cases of other treatment techniques. Also, these specimens exhibited the highest values of displacement and stress at break. The combination of surface modification with silane and the introduction of a PP-based film resulted in a lower peel strength that in the case with a PP-based film. This can be explained by the weak interaction of Al sheet and GFPP surfaces due to the thin silane layer.

Flexural test results

The load–displacement graphs of Al/GFPP FML systems prepared by using various bonding techniques were obtained in three-point bending tests. The typical representative data are plotted in Fig. 7. The Al/GFPP laminates modified with a PPbased film exhibited no sudden drop in the load and showed the highest strength (90.79 ± 1.77 MPa) as compared with those given by other bonding methods. This means that the best adhesion obtained with PP-g-MA modification of the interface of Al/GFPP and interlayer materials exhibits a high level of plastic deformations.
Fig. 7

Load–displacement graphs F–w obtained in bending of Al/GFPP modified with PP-g-MA (the numbers indicate that five specimens from the same batch were tested).

For the silane-treated Al/GFPP, the bending loads increased linearly for all specimens and reached a maximum level at about a 2-mm displacement. In the linear region, the response was elastic, and the failure occurred at the maximum load. These specimens exhibited the lowest bending strength (27.22-31.07 MPa) than the other ones. The incorporation of a polyolefinbased adhesive film into Al/GFPP laminates gave a result similar to that reached by using the silane treatment. However, the specimens with a polyolefin-based adhesive film exhibited a higher flexural strength, equal to 69.96 (±4.10) MPa, at higher displacements than the silane-treated Al/GFPP ones.

Conclusions

In the present study, the effects of various surface modification techniques on the adhesive properties of Al/GFPP laminates were investigated. To tailor the interface of Al/GFPP, an amino-based silane coupling agent compatible with polypropylene was employed. Another method was the incorporation of polyolefin-based adhesive and PP-based films containing 20 wt.% PP-g-MA between Al and GFPP. The lap shear, peel, and bending strength tests were performed in order to reveal the effects of the various surface modification techniques on the adhesive properties of Al/GFPP laminates. It was found that the amino-based silane (Z-6032) provided a significant growth in the interfacial shear stresses. The chemical bonding between polypropylene and the amino group of the Z-6032 silane was established successfully, as expected. The polyolefin-based adhesive film gave the lowest interfacial shear strength. The same parameter for the Al/GFPP interfaces modified with the PP-based film was found to be the highest among those given by the other bonding techniques. Although a positive effect of combination of the silane treatment and the use of a PP-based film was expected, the experimental results showed that this did not yield the high bending strength desired. This may be caused by the insufficient bonding of silane groups to the PP-based film.

Judging from the results of peel strength, the best adhesion was achieved by introducing a PP-based film containing 20 wt.% PP-g-MA into the Al/GFPP interlayer. The combination of surface modification with silane and the introduction of a PP-based film resulted in a lower peel strength compared with that of the PP-film modified system. According to the results of lap shear strength obtained for the Al/GFPP, the incorporation of a polyolefin-based adhesive film into the Al/GFPP interlayer gave the lowest values. However, considering the peel strength, the application of silane was less effective then the incorporation of a polyolefin-based adhesive film. It was concluded that, in the shear direction, the silane-treated Al surfaces exhibited a longer resistance than the polyolefin-based adhesive film. Besides, the strength of interfaces with an incorporated polyolefin-based adhesive film were weaker in the peel direction.

The Al/GFPP laminates modified with PP-g-MA exhibited no sudden drop in the load and showed the highest strength compared with that provided by other surface modification techniques. These results indicate that the best adhesion can be obtained by using the PP-g-MA modification of the interface of Al/GFPP, and that the interlayer material exhibits high plastic deformations.

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Copyright information

© Springer Science+Business Media New York 2013

Authors and Affiliations

  1. 1.Izmir Institute of TecnologyDepartment of Mechanical EngineeringUrlaTurkey
  2. 2.Izmir Institute of TecnologyDepartment of Materials Science and EngineeringUrlaTurkey

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