Adhesive Technologies: History and Types

Adhesive can be defined, in its simplest explanation, as any substance capable of holding materials together in a functional manner. As a general term, it includes such terms as cement, glue, paste and putty that create an adhesive bond. Ancient adhesives have been known since ancient times. Papyrus, an early non-woven fabric, contained fibers of reed plants bound together with starch paste. Bitumen, wood resins and beeswax were used in ancient and medieval times as sealants (protective coatings) and adhesives. Gold leaves of illuminated manuscripts were adhered to paper with egg white. Wooden objects were bonded with glues derived from materials such as fish and horn. The technology of obtaining glue from animals and fish advanced in the 18th century. In the 19th century, rubber and nitrocellulose-based cements were introduced. However, steady advances in adhesive technology were not seen until nearly the 20th century. During this period, natural adhesives were developed and subsequently many synthetic varieties emerged from laboratories to replace natural adhesives in the market. In the second half of the 20th century, the rapid growth of aircraft and aerospace industries had a profound impact on adhesive technology. The demand for adhesives with high degrees of structural strength and resistant to both fatigue and harsh environmental conditions ultimately led to the development of high-performance materials used in numerous industrial and domestic applications.

What is Adhesion and How Does it Occur?

In the performance of adhesive bonds, the physical and chemical properties of the adhesive are the most important factors. In determining whether an adhesive joint will perform adequately, the types of adhesive (that is, the components being bonded - for example, metal alloys, plastics, composite materials) and the nature of surface pretreatment or primer are also important. These three factors (adhesive, adherend, and surface) have an effect on the service life of the bonded structure. The mechanical behavior of the bonded structure is influenced by the details of the joint design and the manner in which applied loads are transferred from one adhesive to another. What is effective in forming an acceptable adhesive bond is the ability of the adhesive to spread over and wet the surfaces to be bonded.

Achieving such interfacial molecular contact is a necessary first step in forming strong and stable adhesive bonds. Once wetting is achieved, adhesive forces are generated across the interface through a series of mechanisms. The precise nature of these mechanisms has been the subject of physical and chemical study since at least the 1960s and as a result there are a number of adhesion theories. The main adhesion mechanism is explained by adsorption theory, which states that materials adhere primarily due to intermolecular contact.

In adhesive joints, this contact is provided by intermolecular or valence forces exerted by molecules in the adhesive and bonded surface layers. Physical adsorption arises primarily from van der Waals forces and electrostatic forces between adsorbate molecules and the atoms forming the adsorbent surface. Accordingly, adsorbents are first characterized by surface properties such as surface area and polarity. In addition to adsorption, four other adhesion mechanisms have been proposed. The first is mechanical interlocking, which occurs when an adhesive flows into pores in the adhesive surface or around surface protrusions. The second, interdiffusion, occurs when a liquid adhesive dissolves and diffuses into the bonding materials. The third mechanism is adsorption and surface reaction, where bonding occurs when adhesive molecules are adsorbed onto a solid surface and chemically react with it. Because of the chemical reaction, this process differs to some degree from the simple adsorption described above, but some researchers believe that the chemical reaction is not a separate adhesion mechanism but part of the overall adsorption process. Finally, electronic or electrostatic attraction theory suggests that electrostatic forces develop at an interface between materials with different electronic band structures. Generally, more than one of these mechanisms plays a role in achieving the desired level of adhesion for various adhesive and bonding types.

In the formation of an adhesive bond, a transition region appears at the interface between the adhesive and the adherend. This region, called the interphase, may have chemical and physical properties of the adhesive that differ significantly from the bulk portions. Generally, it is believed that the interphase composition controls the durability and strength of an adhesive joint and is primarily responsible for stress transfer from one adhesive to another. The interphase region frequently undergoes environmental attack, which leads to joint failure.

The strength of adhesive bonds is typically determined by destructive tests that measure stresses that occur at the point or line of fracture in the test specimen. Various test methods are used, including peel, tensile lap shear, cleavage and fatigue tests. These tests are performed over a wide range of temperatures and under various environmental conditions. An alternative method of characterizing an adhesive joint is to determine the energy expended to separate a unit area of the interphase. Results obtained from such energy calculations are, in principle, entirely equivalent to those obtained from stress analysis. Almost all synthetic adhesives and some natural adhesives consist of polymers, which are giant molecules, or macromolecules created by combining thousands of simpler molecules known as monomers. Polymer formation (a chemical reaction known as polymerization) can occur during a "curing" stage in which polymerization occurs simultaneously with adhesive-bond formation (as in epoxy resins and cyanoacrylates) or the polymer material is formed prior to application as an adhesive, as in thermoplastic elastomers such as styrene-isoprene-styrene block copolymers. Polymers impart strength, flexibility and the ability to spread and interact with an adhesive surface – properties necessary for the formation of acceptable adhesion levels.

Types of Adhesives

For various applications, there are numerous types of adhesives. They can be classified in various ways depending on their chemistry (e.g., epoxies, polyurethanes, polyimides), their forms (e.g., pastes, liquids, films, pellets, tapes), their types (e.g., hot melt, reactive hot melt, thermoset, pressure-sensitive, contact, etc.) or their load-bearing capacities (structural, semi-structural or non-structural). Structural adhesives refer to relatively strong adhesives used below the glass transition temperature. This concept is important because above the glass transition temperature polymers are rubbery, while below it they are glassy. Common examples of structural adhesives include epoxies, cyanoacrylates and some urethane and acrylic adhesives. These types of adhesives can carry significant stresses and contribute to structural applications. For many engineering applications, semi-structural (applications where failure would be less critical) and non-structural designs (applications for aesthetic purposes such as facades) are also quite important to the design engineer and provide cost-effective tools for assembly of finished products. These include contact adhesives in which a solution or emulsion containing an elastomeric adhesive is coated onto each of the bonding surfaces, the solvent is allowed to evaporate and the two adhesives are then brought into contact. Examples include rubber cement and adhesives used to glue laminates to substrates. Pressure-sensitive adhesives are very low modulus elastomers that readily deform under small pressures and allow bonding to wet surfaces. When the substrate and adhesive are brought into close contact, van der Waals forces are sufficient to maintain contact and can provide relatively durable bonds for lightly loaded applications. Although pressure-sensitive adhesives are normally purchased as tapes or labels for non-structural applications, they can also come as double-sided foam tapes that can be used in semi-structural applications. As their name suggests, hot melts become liquid when heated, wet surfaces and then solidify to a solid polymer. These materials are used in a wide variety of engineering applications using more sophisticated versions of glue guns commonly used by consumers. Anaerobic adhesives cure in narrow spaces devoid of oxygen; such materials are commonly used in mechanical engineering applications to lock bolts or bearings in place. In other adhesives, cure can be induced by exposure to ultraviolet light or electron beams or can be catalyzed by certain materials such as water, which is ubiquitous on many surfaces.

Adhesives of various chemistries are also available in many different forms. For structural applications, adhesives are available as pastes, liquids, films and supported films. The latter are supported on loose-weave or cloth mesh to improve handling properties and also provide some thickness control. Most of these adhesives produce little or no gas when cured, which significantly reduces the likelihood of voids within the adhesive. Since absorbed moisture can create significant void problems, it is important to keep these adhesives dry. Heat-curing structural adhesives are normally available in two-part forms mixed into a product that cures within a desired time interval through carefully controlled stoichiometry.

One-part forms are also available in which the resin and hardener (cross-linking agent) are already mixed together. These one-part forms must be kept at sufficiently low temperatures where the reaction does not occur early, sometimes using latent cross-linking agents that are inactive at low temperatures. One-part thermoset adhesives generally have limited shelf life and are usually stored at low temperatures, but offer very high performance properties. Pot life refers to the time period after mixing a two-part adhesive during which it remains workable and can still form a satisfactory bond. Materials with very short pot lives cure too quickly and do not give workers sufficient time to assemble the product. Excessively long pot life can delay curing time and slow down the assembly process. Adhesives can be applied in various ways depending on the form they come in. Adhesives can be manually spread onto a surface or distributed using various sophisticated nozzles and robotic equipment already available. Ensuring proper surface cleanliness, providing appropriate fixtures and fastening during cure, and ensuring adequate cure conditions can be important considerations for specific adhesives.

Sources

https://www.britannica.com/technology/adhesive/Synthetic-adhesives https://www.adhesives.org/adhesives-sealants/science-of-adhesion/design-of-adhesives-bonds/types-of-adhesives Pike, Roscoe. "adhesive". Encyclopædia Britannica Online. Encyclopædia Britannica Inc. Retrieved 9 April 2013. Kinloch, A.J. (1987). Adhesion and Adhesives : Science and Technology (Reprinted. ed.). London: Chapman and Hall. p. 1. ISBN 0-412-27440-X. Images: pixabay.com Prepared by: B. Serhat Cengiz