Adhesive Bonding by the Numbers

About this book

An adhesive joins parts by doing two things at once. As a liquid, it has to wet the surfaces, flowing into contact with them closely enough for intermolecular forces to act. As a solid, it has to carry load, spreading stress over an area instead of concentrating it at a few holes or weld spots. Most bonding failures come from one of those two jobs going wrong: a surface that was never wetted properly, or a joint whose shape put the adhesive into peel, cleavage or a strain it could not take.

This book teaches the arithmetic behind both jobs. It starts with surface tension, contact angle and the surface energy of solids, which decide whether an adhesive wets. It moves to surface preparation, cleaning solvents and shop conditions, then to cure and its dependence on temperature. It then treats the bonded joint as a structure: shear in lap joints, peel and cleavage, thermal movement between unlike materials, and durability under heat and moisture. It ends with testing and with proving a bonding process in production.

It is for design, manufacturing and quality engineers, workshop supervisors and technicians who choose, specify, apply or test structural and elastic adhesives on metal and plastic parts. It assumes algebra, a little trigonometry and basic strength of materials. Adhesive chemistry is described only as far as it changes the numbers. Each chapter works the running case with real numbers, includes one plausible mistake and how it was found, and ends with an exercise and its worked answer.

The running case

Every chapter follows one fictional workshop. This is an illustration. Plant K builds outdoor electrical kiosks: powder-coated aluminium cabinets that stand beside roads and in substations. Each cabinet door is a 2.0 mm 5052-H32 aluminium skin, 800 by 1,800 mm, and the plant makes about 40 doors a week. Two changes to the door are planned.

• Bonded stiffeners. Until now, three hat-section stiffeners of 1.6 mm aluminium were fixed inside each door with welded studs and nuts. The studs mark the painted outer face and the marks have to be filled and sanded. The plant wants to bond the stiffeners with a two-part toughened epoxy along their two 20 mm flanges.
• A bonded viewing window. A new door type needs a 6 mm polycarbonate window, 300 by 200 mm, so that meters can be read without opening the cabinet. The window is to be bonded to the inside of the door over a 260 by 160 mm cut-out, with a 20 mm bond flange all round.

The story begins with a trial batch: some stiffeners peel away from the skin at their ends during a drop test, and the first windows show fine cracks in the polycarbonate next to the bond after a hot week. The plant’s manufacturing engineer is asked to find out why and to prove a process that works.

The plant, its products, its suppliers, its adhesives and every measured value are invented. None describes KEVOS® work, a real company or a real product. Adhesive properties quoted for the case are illustrative data-sheet values, not the properties of any named product.

How to use the book

Read Chapters 1 to 3 in order: they set out why bonding works and how to measure a surface’s readiness for it. Chapters 4 and 5 cover preparation and cure, the process steps where most production faults arise. Chapters 6 to 8 treat the joint as a structure. Chapter 9 covers durability, Chapter 10 testing and Chapter 11 production control and proof.

KEVOS® tools are linked where they perform a step of the method, with a note of exactly what each one calculates and where its model stops. Where a tool solves a related problem rather than the chapter’s own, the text says so.

Source limits and professional responsibility

The relations used here are the established ones of surface science and joint mechanics: Young’s equation, the work of adhesion, the Owens and Wendt two-liquid method, the shear-lag analysis of lap joints and Arrhenius temperature dependence. Each chapter states the assumptions they need. Test standards are named by number and scope only; their procedures, specimen drawings and acceptance values govern real testing and are not reproduced. Adhesive manufacturers’ data sheets and application instructions govern the use of any real product.

Bonded joints can carry safety-critical loads, and adhesives and cleaning solvents carry health and flammability hazards covered by their safety data sheets and by work health and safety law. Real structural bonding, qualification and approval are matters for qualified engineers working with the adhesive supplier and to the applicable standards. This book is educational. It explains how the numbers are made and how far to trust them; it is not a bonding procedure, a qualification or a substitute for testing the real joint.