A.Q. Barbosa

Influence of the Size and Amount of Cork Particles on the Impact Toughness of a Structural Adhesive

The inclusion of particles (nano or micro) is a method to improve the mechanical properties, such as toughness, of structural adhesives. Structural adhesives are known for their high strength and stiffness but also for their low ductility and toughness. There are many processes described in the literature to increase the toughness, one of the most common being the use of rubber particles. In the present study, natural micro particles of cork were used with the objective to increase the impact resistance of a brittle epoxy adhesive. The idea is for the cork particles to act like crack stoppers and absorb impact leading to higher absorption of energy. The influence of the cork particle size and amount were studied. Particles of cork ranging from 38 to 250 µm were mixed in the epoxy adhesive Araldite® 2020 from Huntsman. The amount of cork in the adhesive was varied between 1 and 5% by weight. Surface treatment (low pressure plasma) was applied to the cork powder to assess the effect of the interaction adhesive-cork with several degrees of adhesion. This evaluation was made using impact tests and it was evident that impact absorption was related to the size and amount of cork particles in the resin, considering a uniform particle distribution.

Hygrothermal aging of an adhesive reinforced with microparticles of cork

In the present study, natural microparticles of cork are used with the objective to increase the toughness of a brittle epoxy adhesive. The cork particles act as a crack stopper, leading to more energy absorption. This fact occurs because cork presents a remarkable combination of properties (low density, low cost and sustainability of the raw material). Adhesives are susceptible to the presence of moisture in the environment. There are several studies that refer that moisture can degrade the molecular structure of the adhesive, and, therefore, its mechanical properties. The main objective of this research is to investigate the effect of moisture on the degradation of an adhesive reinforced with micro cork particles, knowing that cork presents a great capability to absorb water. The water absorption and desorption characteristics have been studied, for specimens without cork and with 1% cork, 125–250 μm. The moisture uptake behaviour in the adhesive was studied to obtain the coefficient of moisture diffusion. The effect of water exposure on the mechanical properties and glass transition temperature was also investigated. It was observed that the presence of water alters the mechanical properties of the adhesive (with and without cork), but these changes are not permanent.

Effect of the amount of cork particles on the strength and glass transition temperature of a structural adhesive

The inclusion of particles (micro or nano) is a method to improve the mechanical properties, such as toughness, of structural adhesives. Structural adhesives are known for their high strength and stiffness but also for their low ductility and toughness. There are many processes described in the literature to increase the toughness, the use of rubber particles being one of the most common processes. In the present study, natural micro particles of cork were used with the objective to increase the ductility of a brittle epoxy adhesive. The idea is for the cork particles to act like a crack stopper. The influence of the amount of cork particles was studied. Particles of cork ranging in size from 125 to 250 µm were mixed in the epoxy adhesive Araldite 2020 from Huntsman. The amount of cork in the adhesive was varied between 0.5% and 5% in weight. This evaluation was made using tensile tests and it was evident that the failure strain was related to the amount of cork particles in the resin. The results concerning the single lap joints and the glass transition temperature confirm the increased ductility obtained in the tensile tests.

Micro Cork Particles as Adhesive Reinforcement Material for Brittle Resins

Structural adhesives are progressively replacing conventional bonding methods, being constantly adopted for new applications. The most commonly used structural adhesives are epoxies due to their good mechanical, thermal and chemical properties, having a wide range of application. Epoxies are recognized for their high stiffness and strength, induced by their high degree of crosslinking. While the densely cross-linked molecular structure is responsible for the excellent properties of these materials, it also makes them inherently brittle, resulting in low ductility and toughness. Several researchers have, in the past decades, found necessary to mitigate this effect and developed new methods to increase the toughness of structural adhesives. There are many processes depicted in the literature on how to increase the toughness of adhesives. For example, the inclusion of particles (of nano or micro scale) is a successful technique to improve the toughness of structural adhesives. In this chapter, natural micro particles of cork are used with the objective of increasing the toughness of a brittle epoxy adhesive. The fundamental basis of this concept is for the cork particles to act like crack stoppers, leading to more energy absorption. An overview of how the micro cork particles can be used as reinforcement material for brittle resins is described. The main parameters that affect the mechanical properties of composite resin/cork, kinetic and chemical reactions between resin and cork and how this new material behaves in hygrothermal degradation, were analysed. It is concluded that the cork can be used as reinforcing material, promoting increased toughness of the adhesive without any chemical changes in the molecular structure or premature degradation of the adhesive.

Analysis of the effect of size, amount and surface treatment on the tensile strain of a brittle adhesive reinforced with micro cork particles

Nowadays, structural adhesives are increasingly being used for new applications, replacing conventional bonding methods. Epoxy resins are the most common structural adhesives used due to their suitable mechanical, thermal and chemical properties, despite their low ductility. In the past decades, several researchers have found it crucial to reverse these properties and find new ways to increase the ductility of these adhesives. The inclusion of particles (nano or micro) is an effective method to improve ductility of structural adhesives. In the present study, natural micro particles of cork are used with the objective of increasing the ductility of a brittle epoxy adhesive. The concept is for the cork particles to act like as a crack stopper leading to more energy absorption. The influence of the cork particle size, amount and the presence of a surface treatment were studied. The maximum strain of the adhesive was assessed through tensile tests. A Taguchi design experiments was used to understand the influence of each parameter under study (amount, size and presence of surface treatment) and their interaction. Through this research it was possible to settle that cork can improve ductility and cork amount, size and the use of plasma surface treatment have influence on the mechanical properties.