Durability of PBI adhesive bonded joints under various environmental conditions

https://doi.org/10.1016/j.ijadhadh.2019.01.006Get rights and content

Abstract

Structural adhesives are finding increasing use in many applications. However, their utilization at elevated temperature has always been a challenge due to their low thermal and mechanical properties. However, in recent years, the development of high performance polymers have overcome the problem of using adhesive bonding at high temperature to some extent. Polybenizimidazole (PBI) is one such recently emerged high performance polymer with excellent thermal and mechanical properties. It has a tensile strength of 160 MPa and a glass transition temperature (Tg) of 425 °C. Due to its excellent thermal and mechanical properties, it has the potential to be used as an adhesive under various environmental conditions. In the present work, efforts are devoted to explore the potential of using PBI at high temperature and in hot-wet environmental conditions. M21 and DT120 epoxy based carbon fiber composite bonded joints were prepared and tested. Both M21/carbon composite and DT120/carbon composite have exhibited a reduction in joint strength of about 16% and 25% respectively after 1000 h of conditioning in a hot-wet environment. However, a reduction in lap shear strength of 52% and 56% is observed when composite bonded joints were tested at 80 °C.

Introduction

Polymer-based composite materials are getting significant attention for applications in automotive, aviation, spacecraft, civil infrastructure, sports and marine industries [1], [2], [3]. These materials are often joined by adhesive bonding to form structural components. The use of adhesive bonding has significantly increased in recent years as it provides uniform load distribution over larger surface area with very little additional weight and bonded joints show better fatigue resistance than mechanical fastening [4], [5], [6]. Furthermore, the adhesive bonding technique provides more design flexibility compared to the mechanical fastening [7], [8], [9], [10].

Adhesive bonded joints are exposed to various environmental conditions during service life. These environmental conditions can deteriorate the properties of an adhesive which ultimately affect the performance of the bonded joints. High temperature and moisture are the two main factors that need to be considered while determining the long term durability of adhesive bonded joints. Moisture may not only affect the physical and chemical properties of the adhesive itself but it also degrades the interface between substrate and adhesive material [11], [12]. High temperature has also an impact on the bonded joint strength. Studies show that an increase in temperature can result in an increased degradation rate of the adhesive. Physical properties of adhesives are also affected after exposure to high temperature [13], [14]. Tg and thermal stability are the key parameters which can indicate the potential for use of an adhesive for high temperature applications. Polymers with a high Tg exhibit better structural properties at high temperature [15], [16]. On the other hand, the thermal stability of the adhesive at elevated temperature strongly depends on the chemistry of the adhesive [16]. Polymers with aromatic and heterocylic rings in the backbone structure, exhibit high thermal stability and resistance to oxidation [14], [17]. The combined effect of temperature and moisture is even worse for the adhesive properties. Prolonged exposure to high temperature and moisture may induce irreversible changes in the adhesive depending on the type of adhesive. A previous study by Butkus [18] shows that a hot-wet environment is the most detrimental condition which significantly affects the fracture behavior of bonded joints.

Different types of adhesives are now in use in the aerospace, automotive, electronics and marine industries [19]. Epoxy adhesives are the most commonly used structural adhesives because of their low cost and ease of processing [14]. However, at temperatures around 150oC, a noticeable reduction in structural strength of these adhesives is observed [7], [16], [20]. The thermal and mechanical properties of these adhesives are degraded at high temperatures which ultimately degrade the performance of bonded joints. This has posed a challenge for the use of adhesive bonded joints for high temperature applications. However, recent advances in polymer chemistry have allowed the development of high performance adhesives. The application of these adhesives can significantly overcome the problem of using adhesive bonding at high temperatures for an extended period of time [16]. Polybenizimidazole (PBI) is one such high performance polymer which has gained attraction for a wide area of applications. Polybenzimidazole (PBI), a heterocyclic thermoplastic polymer, shows excellent thermochemical properties and outstanding mechanical properties [21]. It has the highest Tg (425 °C) of any commercially available organic polymer [21], has high decomposition temperatures (500–600 °C), good oxidation resistance and maintains excellent strength at cryogenic temperatures [22]. However, the potential of PBI has not been explored as a high temperature adhesive and under hot wet environmental conditions. Therefore, the objective of the present work is to evaluate the performance of PBI as an adhesive under various environmental conditions.

Section snippets

Materials

A 26% concentrated solution of PBI in dimethylacetamide (DMAc) was supplied by CELAZOLE, PBI Performance Products (Charlotte, USA). M21 and DT120 epoxy based unidirectional (UD) carbon fiber prepregs were supplied by MCtechnics Sprl (The Maestrich, Netherlands) and Delta Tech (Sant'Egidio alla Vibrata, Italy) respectively. In the following sections, these composites will be named as M21/carbon composite and DT120/carbon composite. M21/carbon epoxy composite laminate was manufactured using 16

Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) was conducted to determine the thermal stability of PBI. The sample was heated over a temperature range of 25 to 550 °C at a heating rate of 10 °C/min. The TGA curve of PBI exhibits at two-step decomposition process as shown in Fig. 2.

The PBI shows an initial weight loss of about 12% which occurred between 40 °C and 180 °C. The weight loss is more likely due to release of moisture by the polymer and any DMAc that has remained inside the polymer film. After a

Conclusions

The main objective of this study was to evaluate the performance of a polybenzimidazole adhesive under various environmental conditions. Bonded joints of DT120/carbon composite and M21/carbon composite were formed using PBI adhesive. Bonded joints were conditioned in a climatic chamber for 1000 h at 80 °C and 95% relative humidity. Lap shear tests performed on conditioned samples indicated that a hot-wet environment did not significantly deteriorate the properties of the PBI adhesive even after

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