Materials Science of Adhesives: How to Bond Things Together
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Materials Science of Adhesives: How to Bond Things Together
Costantino Creton and Eric Papon, Guest Editors Abstract This issue of MRS Bulletin provides an overview of the state of the art and emerging trends in the area of adhesives. An adhesive is basically defined by its function, which is to assemble two surfaces together. In order to fulfill this function, the properties of an adhesive must include easy positioning at the interface, rapid and complete bond formation and subsequent hardening, and a bond strength adapted to the specific application (structural, permanent, removable, rigid, or soft). A variety of solutions exist in practice, and their application requires an understanding both of polymer chemistry and materials science. The main families of adhesives, their properties, and their basic design principles will be discussed in the articles in this issue, while this introductory article presents an overview of the functions required of adhesives and future trends. Keywords: adhesion, adhesives, bonding, fracture, interfaces, mechanical properties, polymers.
Historically, the use of adhesives cannot be dissociated from the use of sealants. Two of the earliest documented uses of natural adhesives (from around 1500 BC) are caulking ships with tar and sealing jars of spices with asphalt by the early Phoenicians. The Olmecs in Central America used natural rubber latex to waterproof woven textiles as early as 1000 BC. The next industrial development of adhesives came with the development of the printing press in the 15th century, which required increased production of paper and therefore of natural glues (to hold the cellulose fibers together) and hydrophobic additives (without which the paper would act as a blotter). Although natural adhesives are still used today for certain applications, modern adhesives are closely linked to the development of synthetic polymers in the 20th century. Current world production of adhesives is on the order of 10 million tons a year, and there are more than 10,000 different adhesives available on the market from a vast number of manufacturers.1
MRS BULLETIN/JUNE 2003
From a materials science point of view, what defines an adhesive is not its material type (such as a polymer, a metal, or a semiconductor) but its function. In appearance, this function is rather simple and mundane, namely, to bond two surfaces together. However, the choice of an appropriate adhesive system for an application depends on many factors, such as the required adhesive properties (from permanent to temporary applications), the materials to be bonded, the thermomechanical properties of the bonded joints, and the processing conditions. Making the proper choice requires knowledge of the physics, mechanics, and chemistry of various materials ranging from metals to polymers, composites, and even skin; their surfaces and bulk properties must also be extensively characterized. It is necessary to use specific methodologies to characterize the degree of adhesion (i.e., surface and interface properties) and
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