Effect of Moisture and Aging on Adhesive Strength Essay

Introduction

Adhesives are substances that hold materials (adherents) together through surface attachments. Adhesives are a compound word that covers cement, glue, paste as well as mucilage among others. Adhesives generate intrinsic adhesion forces on the surface of the adherents. Adhesions can be explained by the adsorption theory, mechanical interlocking theory and the diffusion theory. There are many factors that affect the adhesive strengths and the most notable ones include humidity and ageing. In this experiment, the researcher will investigate how moisture and ageing affect the adhesive strength of the epoxy adhesives. Epoxy adhesives are made from two compounds resins and hardeners.

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The mixtures of two compounds are polymerized. Covalent bonds found between the adhesives epoxies group of the resins and the catalysts amine groups causes cross-linkage of the polymer and in the process dictating the rigidity as well as the strength of the epoxies.

Composite material and fabrication method

The strength of the epoxy adhesives can be determined by the curing conditions that are also determined by the temperature and the types of composite materials used. For example, the type of resins and hardeners used significantly influences the mechanical properties as well as the thermal resistance, electrical resistance and the chemical resistance. Epoxy adhesives are used in a wide range of structural applications and operating conditions. Epoxy adhesives can be either one component or two component adhesives. The one component adhesives must be cured at higher temperatures (250-300F) to maximize their strength and provide excellent adhesion properties. High-temperature curing also enables the material to withstand harsh chemicals and environmental conditions that is why they are also used as alternatives in the application that requires welding and riveting. On the other hand, the one-component systems are usually formulated while pre-catalyzed and in most cases require only moderate heat during the curing processes. It is, however, important to note that air entrapment in the curing process can cause errors in the adhesives. Heat is used in the formulation of the two-component epoxy systems to improve the cross-linking properties.

Resins Transfer Molding Process

BMW and Toyota are the two main car-manufacturing companies that apply the epoxy adhesives in their manufacturing process. The companies use wet compression n moulds usually in combination to or as an alternative to the high-pressure RTM (High-Pressure Resin Transfer Moulding). The HP-RTM is one of the lightweight automotive design processes whereby the fibre performed is made of very low viscosity and the reactive resin is introduced into the cavity at very high-pressure t to ensure that each of the fibres is wetted without leaving air voids (Lee & Park, 2012).

Creating the Resin Perform

The preform or fibreglass reinforcement is created. It is important to note that the preform is hereby the matrix where the resin is injected. The fiber, in this case, is the two dimension woven roving (Neto, Campilho & da Silva, 2012)

Perform layup- after the preform reinforced is done, the perform is packed into the pre-shaped mould cavity

Covering the mould- the mould is then closed and clamped for a smooth finish

Injection- the catalyzed low viscosity resins are injected into the heated mould. The injection should be under high pressures to displace the air bubble through the mould vents.

Curing- curing cycles follow the injection phase to allow the resin to polymerize and become rigid (Araujo, Machado, Marques & da Silva, 2017).

Ageing does not is significantly impacting adhesive strength as compared to humidity. Adhesive durability may only start to deteriorate after 3 years .in retrospect, adhesive strength tends to increase when the material is exposed to a 600C temperature which is consistent with the theory that the adhesive strength can increase at a higher temperature of 66oC and above. An increase IN the water content of all the fatigue joint reduces the fatigue joint strength as well as the lifespan of all the adhesive joints that are bonded with epoxy adhesives. Based on the Paris law curves, humidity increases the water content on joints and reduces the adhesive strengths. The cohesive failure results from poor surface cleaning before adhesives are applied (Saldanha et al., 2013).

Conclusion

The effect of physical and chemical ageing depends on the temperature at which the exposed adhesives are cured. For example, when physically aged materials at 10oC and 40oC followed by higher enthalpy relation, their Tg and relaxation peak values are comparable to those of the materials that were exothermically cured at lower temperatures (20oC of glass transition Temperature- Tg). Ageing at lower temperatures reduces the adhesive strength of epoxy adhesives. Based on the adsorption theory and the fracture mechanics, it is clear that moisture causes a loss of the interfacial fracture toughness. Additionally, the interfacial hydrophobicity and saturation concentration are factors in the analysis. Other factors such as the active nanopore density should also be integrated into investigating the effect of ageing and humidity on epoxy adhesives

References

Araujo, H., Machado, J., Marques, E., & da Silva, L. (2017). Dynamic behaviour of compositeadhesive joints for the automotive industry. Composite Structures, 171, 549-561. doi: 10.1016/j.compstruct.2017.03.071

Lee, Y., & Park, J. (2012). Effect of moisture and drying time on the bond strength of the one-step self-etching adhesive system. Restorative Dentistry & Endodontics, 37(3), 155. doi: 10.5395/rde.2012.37.3.155

Neto, J., Campilho, R., & da Silva, L. (2012). Parametric study of adhesive joints withcomposites. International Journal Of Adhesion And Adhesives, 37, 96-101. doi: 10.1016/j.ijadhadh.2012.01.019

Saldanha, D., Canto, C., da Silva, L., Carbas, R., Chaves, F., Nomura, K., & Ueda, T. (2013). Mechanical characterization of a high elongation and high toughness epoxyadhesive. International Journal Of Adhesion And Adhesives, 47, 91-98. doi: 10.1016/j.ijadhadh.2013.08.001

Ferguson P., Qu J. (2007) The Effect of Moisture on the Adhesion and Fracture of Interfaces in Microelectronic Packaging. In: Suhir E., Lee Y.C., Wong C.P. (eds) Micro- and Opto-Electronic Materials and Structures: Physics, Mechanics, Design, Reliability, Packaging. Springer, Boston, MA