Study on effect of injection temperature on shear strength of the direct bond between polyphenylene sulfide and anodized aluminum alloy

A simple and environmentally friendly technique has been proposed to form a composite structure of high interfacial bonding strength between polyphenylene sulfide (PPS) and aluminum alloy (Al). The Al substrate is modified through anodization and enlarging treatments. The influences of injection temperature on the anodic aluminum oxide film surface on the interfacial bond strength of the formed PPS-Al is discussed. The results revealed that when the injection temperature of PPS was set at 310°C, the bonding strength increased to 18.0 MPa. The adhesive strength exhibits a clear dependency on the injection temperature. At higher injection temperatures, PPS exhibits higher flowability and better penetration into nanoscale pores, resulting in a strong interlocking between PPS and anodized aluminum alloy.


Introduction
Polymer-metal hybrid (PMH) structures refer to structures formed by combining polymer materials and metal materials, effectively integrating the advantages of both materials [1].This type of structure combines the lightweight nature and chemical resistance of polymers, with the high strength and stiffness of metals, leading to expanded possibilities and applications for materials in different industries [2].
The implementation of PMH structures can be achieved through various techniques, including adhesive bonding, injection molding, ultrasonic welding, mechanical fastening, and laser methods [3,4].The selection of these techniques depends on specific application requirements, material characteristics, and design considerations.When choosing and applying polymer-metal connection technologies, it is important to consider factors such as material compatibility, connection strength, durability, and cost to ensure the quality and performance of the final connection.Among these techniques, injection molding direct joining (IMDJ) is widely used due to its advantages of high strength, durability, simplicity, speed, and environmental friendliness [5].
Temperature plays a crucial role in the hot bonding process, specifically in the melting and flow behavior during injection molding.In molding, the polymer material needs to be heated to a sufficient temperature to be melted and attain adequate flowability to fill the cavities of the mold effectively.Additionally, the temperature during the injection molding process is closely correlated with the cooling rate.After the injection molding process is completed, the material needs to cool rapidly to solidify and maintain the desired shape.The temperature selection directly affects the cooling rate, which, in turn, influences the crystallization behavior and macroscopic properties of the material.Excessively fast or slow cooling rates can lead to the accumulation of internal stresses or the formation of crystalline defects.These factors can significantly impact the strength of the bonded joint [6].
PPS is a thermoplastic polymer material known for its excellent mechanical properties, chemical resistance, and thermal stability [7].PPS can form strong connections with other materials, providing high connection strength.Compared with some other thermoplastic polymer materials, PPS has a lower melting point and processing temperature, so the requirements for equipment and materials in the thermal bonding process are relatively lower [8].
Oxalic acid anodized aluminum oxide (AAO) film has various characteristics and advantages [9,10].The oxide film has good corrosion resistance, which can protect the aluminum alloy (Al) substrate from external corrosive media and prolong the service life of the Al.In addition, AAO film has good adhesion, can be closely combined with the Al substrate, and is not easy to peel off or fall off [3].In this work, anodization and enlarging treatment were employed to create pores with diameters in several tens of nanometers on the Al surface.A study was conducted to investigate the dependence of bonding strength on injection temperature by varying the injection temperature during the direct injection bonding process.By varying the injection temperature, we can observe the impact on the filling behavior of the nanopores and the subsequent bonding strength of the joints.Higher injection temperatures facilitated better flowability of the molten material, allowing it to penetrate and fill the nanopores more effectively.The mechanical interlocking is formed between the polymer material and AAO film, establishing a strong bond at the interface.

Material
The PPS (GS-40, TOSOH, Japan) was used, and its primary physical properties are presented in Table 1.Rectangular specimens of 5052 Al (UACJ, Japan) were cut into dimensions of 40 mm × 10 mm × 2 mm.

Anodizing
To smoothen a surface and remove grease, the substrate was electro-polished using a 75 vol% HClO 4 + 25 vol% CH 3 CH 2 OH solution for 2 min at 268~278 K and a constant voltage of 25 V.During the anodic oxidation treatment, oxalic acid was used.The one-step anodization process involves anodizing the Al under mild anodization (MA) conditions, followed by anodization voltage under high anodization (HA) conditions.Table 2 lists specific anodization conditions.After the anodization, enlarging treatment of pore was conducted in 5 wt.% phosphoric acid at 25℃ for 45 min.

Injection molding process
After anodizing, the Al sheets were placed into the mold, and PPS pellets were injected onto the Al surface to form the PPS-Al composite structure.Before injection, the PPS particles were dried for 3 h at 120°C to remove any moisture and volatile components from the material.Other processing parameters are presented in Table 3.The PPS-Al composite structure is shown in Figure 1.

Surface characterization and tensile shear test
The interfacial morphologies and fracture surfaces of the as-prepared specimens were examined by scanning electron microscopy (SEM, SU-8000, Hitachi, Japan).X-ray photoelectron spectroscopy (XPS, PHI 5000V VersaProbe Ⅲ, ULVAC-PHI.INC, Japan) was used to analyze the chemical composition of the AAO film surface.The single lap shear experiment was conducted using a universal testing machine (Instron 3300, Japan) at a crosshead speed of 2 mm/min, an average value with standard deviation reported.Figure 2 presents the morphologies of AAO film.With enlarging treatment, the pore diameter and porosity noticeably increase.The pore diameter and porosity of AAO film are still as low as 20 nm and 6.2% (Figure 2a), respectively.After acid treatment, two types of pore structures are observed on the surface of AAO film, with pore diameters measuring less than 40 nm and 70-80 nm, respectively.The total porosity reached 29.3%.The shear strength of PPS material and PPS-Al with different injection temperatures are shown in Figure 3.The shear strength of PPS and bonding strength of PPS-Al increased with the increase in injection temperature.Higher injection temperatures help increase the fluidity and permeability of the polymer, allowing it to better fill cavities in the mold and form tighter connections with surrounding materials, while too high an injection temperature can cause thermal degradation or decomposition of polymers.Therefore, it is observed that the shear strength of PPS increased with the increase in injection temperature.When the temperature reaches 310°C, the bonding strength reaches its maximum value of 18.0 MPa.To verify the composition of the material formed on the AAO film surface, XPS analysis was conducted to examine the surface elements before and after the enlarging treatment.Based on curve fitting using the C-C bond and the results (as shown in Figure 4) revealed that the inner layer of AAO film has a higher content of Al 2 O 3 compared to the outer layer.

Conclusion
In this work, the Al sheets were treated using anodization and enlarging treatment, resulting in the formation of an AAO film with both nanoscale pore structure and boehmite structure.

Figure 1 .
Figure 1.The structure of the specimen (mm).

Figure 3 .
Figure 3.Effect of injection temperature on the PPS and the PPS-Al shear strength.

Figure 4 .
Figure 4. XPS survey spectra (0-600 eV) as a function of AAO film (a) and after acid treatment (b).
By utilizing injection molding technology, the lightweight and high-strength PPS-Al were prepared.Controlling the MATMA-2023 Journal of Physics: Conference Series 2691 (2024) 012064 IOP Publishing doi:10.1088/1742-6596/2691/1/0120645 injection temperature during the direct bonding of PPS-Al significantly enhances the overall bonding strength.These findings will greatly contribute to the advancement of polymer-metal materials in mechanical manufacturing technology.

Table 1 .
Physical properties of PPS.

Table 2 .
Electrolytes and experimental parameters were applied in the anodizing process.

Table 3 .
Experimental conditions of injection molding.