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What are adhesives?
Adhesives, commonly known as glues or bonding agents, are substances used to join or bond two or more materials together by surface attachment. Natural bonding agents have been known since antiquity. Egyptian carvings dating back 3,300 years depict the glueing of a thin piece of veneer to what appears to be a plank of sycamore. Papyrus, an early nonwoven fabric, contained fibres of reedlike plants bonded together with flour paste. Bitumen, tree pitches, and beeswax were used as sealants (protective coatings) and adhesives in ancient and mediaeval times. The gold leaf of illuminated manuscripts was bonded to paper by egg white, and wooden objects were bonded with glues from fish, horn, and cheese.
Adhesive: any substance capable of holding materials together in a functional manner by surface attachment that resists separation. “Adhesive” encompasses cement, mucilage, glue, and paste—terms often used interchangeably for any organic material forming an adhesive bond. This article solely discusses organic adhesives, both natural and synthetic, which bind objects such as bricks and beams together through surface attachment, unlike inorganic substances like portland cement, which can also act as adhesives.
During the 18th century, advancements occurred in animal and fish glues, while the 19th century saw the introduction of rubber- and nitrocellulose-based cements. However, decisive advances in bonding agent technology awaited the 20th century, during which natural adhesives were improved and many synthetics emerged from the laboratory to replace natural adhesives in the marketplace. During the second half of the 20th century, the aircraft and aerospace industries profoundly influenced adhesive technology. The demand for adhesives with high structural strength and resistance to fatigue and severe environmental conditions drove the development of high-performance materials. These materials eventually permeated many industrial and domestic applications.
This article begins with a brief explanation of the principles of adhesion and then proceeds to a review of the major classes of natural and synthetic adhesives.
Adhesion
In the formation of an adhesive bond, the interface between the adherend and the adhesive gives rise to a transitional zone known as the interphase. Within this zone, the chemical and physical properties of the adhesive may significantly differ from those in other areas. Experts generally assert that the interphase composition governs the durability and strength of an adhesive joint and primarily transfers stress between adherends. The interphase region often faces environmental attack, which can result in joint failure.
Determining the strength of adhesive bonds typically involves destructive tests that measure the stresses at the point or line of fracture of the test piece. Various test methods, such as peel, tensile lap shear, cleavage, and fatigue tests, are employed across a wide range of temperatures and environmental conditions. An alternative method for characterising an adhesive joint entails determining the energy required to cleave apart a unit area of the interphase. Essentially, the conclusions drawn from such energy calculations are entirely equivalent to those derived from stress analysis.
Key Factors Affecting Adhesive Joint Performance
In the performance of adhesive joints, the physical and chemical properties of the bonding agents are the most important factors. Also important in determining whether the adhesive joint will perform adequately are the types of adherend (that is, the components being joined—e.g., metal alloy, plastic, composite material) and the nature of the surface pretreatment or primer. These three factors—adhesive, adherend, and surface—have an impact on the service life of the bonded structure. The mechanical behaviour of the bonded structure in turn is influenced by the details of the joint design and by the way in which the applied loads are transferred from one adherend to the other.
Implicit in the formation of an acceptable adhesive bond is the ability of the bonding agents to wet and spread on the adherends being joined. Attainment of such interfacial molecular contact is a necessary first step in the formation of strong and stable adhesive joints. Since at least the 1960s, researchers have been studying the precise nature of the mechanisms that generate intrinsic adhesive forces across the interface once wetting is achieved. This research has led to the development of various theories of adhesion. The adsorption theory, which explains the main mechanism of adhesion, posits that substances primarily stick together due to intimate intermolecular contact. In adhesive joints, this contact is attained by intermolecular or valence forces exerted by molecules in the surface layers of the adhesive and adherend.
Mechanisms of Adhesion
In addition to adsorption, researchers have proposed four other mechanisms of adhesion. The first mechanism, mechanical interlocking, occurs when bonding agents flow into pores in the adherend surface or around projections on the surface. The second mechanism, interdiffusion, arises when liquid adhesive dissolves and diffuses into adhered materials. The third mechanism, adsorption and surface reaction, involves adhesive molecules adsorbing onto a solid surface and chemically reacting with it to form bonds. This process varies to some extent from simple adsorption, as described above, although some researchers consider chemical reactions to be part of a total adsorption process rather than a separate adhesion mechanism. Finally, the electronic, or electrostatic, attraction theory suggests that electrostatic forces develop at an interface between materials with differing electronic band structures. In general, achieving the desired level of adhesion for various types of adhesive and adherend involves the interaction of more than one of these mechanisms.
Natural adhesives
Natural adhesives are primarily of animal or vegetable origin. Though the demand for natural products has declined since the mid-20th century, certain of them continue to be used with wood and paper products, particularly corrugated board, envelopes, bottle labels, book bindings, cartons, furniture, and laminated film and foils. In addition, owing to various environmental regulations, natural adhesives derived from renewable resources are receiving renewed attention. The most important natural products are described below.
Adhesive materials
Virtually all synthetic adhesives and certain natural binders are composed of polymers, which are giant molecules, or macromolecules, formed by the linking of thousands of simpler molecules known as monomers. During a “cure” step, the formation of the polymer, also known as polymerisation, occurs simultaneously with adhesive-bond formation (as seen in epoxy resins and cyanoacrylates). Alternatively, the polymer may form before applying the material as an adhesive, as with thermoplastic elastomers like styrene-isoprene-styrene block copolymers. Polymers provide strength, flexibility, and the ability to spread and interact on an adhered surface—properties necessary for achieving acceptable adhesion levels.
Animal glue
Animal glue typically refers to glues derived from mammalian collagen, which constitutes the primary protein in skin, bone, and muscle. When subjected to treatment with acids, alkalies, or hot water, the normally insoluble collagen gradually becomes soluble. If the original protein remains pure and the conversion process is gentle, the resulting high-molecular-weight product is known as gelatin, suitable for various uses including food and photographic products. Conversely, more vigorous processing yields lower-molecular-weight material that is typically less pure and darker in color, referred to as animal glue.
Traditionally, animal glue found applications in wood joining, bookbinding, sandpaper manufacturing, heavy gummed tape production, and similar uses. Despite its initial advantage of high tackiness, much of the animal glue has either been modified or entirely substituted with synthetic adhesives.
Starch and dextrin
Starch and dextrin are extracted from corn, wheat, potatoes, or rice. They constitute the principal types of vegetable adhesives, which are soluble or dispersible in water and are obtained from plant sources throughout the world. Starch and dextrin glues are used in corrugated board and packaging and as wallpaper binding agents.
Natural gums
Natural gums, extracted from their natural sources, also serve as adhesives. Hot water extracts agar, a marine-plant colloid, which is then frozen for purification. Algin results from digesting seaweed in alkali and precipitating either the calcium salt or alginic acid. Harvesting gum arabic involves artificially wounding acacia trees to induce gum exudation. Natural rubber latex, harvested from Hevea trees, is another exudate. Water-remoistenable products primarily utilise most gums.
Casein glue
This product is made by dissolving casein, a protein obtained from milk, in an aqueous alkaline solvent. The degree and type of alkali influence product behaviour. In wood bonding, casein glues are generally superior to true animal glues in moisture resistance and ageing characteristics. Casein is also used to improve the adhering characteristics of paints and coatings.
Blood albumen glue
Glue of this type is made from serum albumen, a blood component obtainable from either fresh animal blood or dried soluble blood powder to which water has been added. The addition of alkali to albumen-water mixtures improves adhesive properties. A considerable quantity of glue products made from blood are used in the plywood industry.
Synthetic adhesives
Although natural adhesives are less expensive to produce, synthetic adhesives are the most important binders. Adhesives based on synthetic resins and rubbers excel in versatility and performance. Manufacturers can produce synthetics in constant supply with consistently uniform properties. Additionally, they can modify synthetics in many ways and often combine them to obtain the best characteristics for a particular application.
The polymers used in synthetic adhesives fall into two general categories: thermoplastics and thermosets. Thermoplastics provide strong, durable adhesion at normal temperatures, and manufacturers can soften them for application by heating without causing degradation. Thermoplastic resins employed in binders include nitrocellulose, polyvinyl acetate, vinyl acetate-ethylene copolymer, polyethylene, polypropylene, polyamides, polyesters, acrylics, and cyanoacrylics.
Thermosets vs. Elastomers
Thermosetting systems, unlike thermoplastics, form permanent, heat-resistant, insoluble bonds that cannot be modified without degradation. The aerospace industry widely utilises adhesives based on thermosetting polymers. Thermosets encompass phenol formaldehyde, urea formaldehyde, unsaturated polyesters, epoxies, and polyurethanes. Elastomer-based adherents may function as either thermoplastic or thermosetting types, depending on whether cross-linking is necessary for the adhesive to perform its function. Elastomeric adhesives possess characteristics such as quick assembly, flexibility, variety of type, economy, high peel strength, ease of modification, and versatility. Major elastomers employed as binders include natural rubber, butyl rubber, butadiene rubber, styrene-butadiene rubber, nitrile rubber, silicone, and neoprene.
Adhesive manufacturers and users face an important challenge: replacing adhesive systems based on organic solvents with water-based systems. This trend has been prompted by restrictions on volatile organic compounds (VOC), including solvents released into the atmosphere contributing to ozone depletion. In response to environmental regulations, adhesive formulations based on aqueous emulsions and dispersions are being developed, while solvent-based adhesives are being phased out.
The polymer types noted above are employed in a number of functional types of adhesives. These functional types are described below.
Structural adhesives
Structural adhesives are adherents that generally exhibit good load-carrying capability, long-term durability, and resistance to heat, solvents, and fatigue. Ninety-five percent of all structural adhesives employed in original equipment manufacture fall into six structural-adhesive families:
- Epoxies: High strength, good temperature/solvent resistance.
- Polyurethanes: Flexible, good peeling, shock/fatigue resistant.
- Acrylics: Versatile, bond to oily parts, quick cure, good properties.
- Anaerobics: Bond metal parts and cylinders (surface-activated acrylics).
- Cyanoacrylates: Fast bonding to plastic/rubber (limited temp/moisture resistance).
- Silicones: Flexible, weather-resistant, good sealing.
Each of these families can be modified to provide adhesives that have a range of physical and mechanical properties, cure systems, and application techniques.
Polyesters, polyvinyls, and phenolic resins are also used in industrial applications but have processing or performance limitations. High-temperature adhesives, such as polyimides, have a limited market.
Contact cements
Contact adhesives or cements are usually based on solvent solutions of neoprene. They are so named because they are usually applied to both surfaces to be bonded. Following the evaporation of the solvent, the two surfaces may be joined to form a strong bond with high resistance to shearing forces. Contact cements are extensively used in the assembly of automotive parts, furniture, leather goods, and decorative laminates. They are effective in the bonding of plastics.
Hot-melt adhesives
Hot-melt adhesives are employed in many nonstructural applications. Based on thermoplastic resins, which melt at elevated temperatures without degrading, these bonding agents are applied as hot liquids to the adherent. Commonly used polymers include polyamides, polyesters, ethylene-vinyl acetate, polyurethanes, and a variety of block copolymers and elastomers such as butyl rubber, ethylene-propylene copolymer, and styrene-butadiene rubber.
Hot-melts find wide application in the automotive and home-appliance fields. Their utility, however, is limited by their lack of high-temperature strength; the upper use temperature for most hot melts is in the range of 40–65 °C (approximately 100–150 °F). In order to improve performance at higher temperatures, so-called structural hot melts—thermoplastics modified with reactive urethanes, moisture-curable urethanes, or silane-modified polyethylene—have been developed. Such modifications can lead to enhanced peel adhesion, higher heat capability (in the range of 70–95 °C [160–200 °F]), and improved resistance to ultraviolet radiation.
Ultraviolet-cured adhesives
During the early 1960s, ultraviolet-cured adhesives became available but experienced rapid development with advances in chemical and equipment technology during the 1980s. These types of bonding agents typically comprise a monomer (which can also serve as the solvent) and a low-molecular-weight prepolymer combined with a photoinitiator. Photoinitiators are compounds that break down into free radicals upon exposure to ultraviolet radiation.
The radicals induce polymerisation of the monomer and prepolymer, thus completing the chain extension and cross-linking required for the adhesive to form. Due to the low process temperatures and very rapid polymerisation (from 2 to 60 seconds), ultraviolet-cured adhesives are making rapid advances in the electronic, automotive, and medical areas. They primarily consist of acrylated formulations of silicones, urethanes, and methacrylates, with combined ultraviolet-heat-curing formulations also existing.
Pressure-sensitive adhesives
Pressure-sensitive adhesives, or PSAs, constitute a significant industrial and commercial market, encompassing adhesive tapes and films utilised in packaging, mounting, fastening, masking, and various applications in electronics and surgery. PSAs effectively bond adherends together when pressure is applied at room temperature. (Unlike contact cements, which do not require pressure for bonding.)
Formulating PSA systems involves using materials such as natural and synthetic rubbers, thermoplastic elastomers, polyacrylates, polyvinylalkyl ethers, and silicones. These polymers, available in solvent-based and hot-melt formulations, are applied as coatings onto substrates like paper, cellophane, plastic film, fabric, or metal foil. With the phasing out of solvent-based adhesive formulations due to environmental regulations, water-based PSAs are expected to see increased utilisation.
Why choose us?
Anglo Adhesives & Services Ltd. is highly recommended for their adhesive solutions and services. With a rich history and extensive knowledge of adhesives, they can provide invaluable expertise to businesses seeking adhesive solutions. Their expertise spans natural and synthetic adhesives, offering a wide range of options for various applications.
Whether you require structural adhesives, contact cements, hot-melt adhesives, or specialised bonding agent formulations like ultraviolet-cured or pressure-sensitive adhesives, Anglo Adhesives & Services Ltd. has the experience and technology to meet your needs. Their commitment to environmental sustainability, including the development of water-based adhesives, demonstrates their dedication to staying at the forefront of bonding agent technology. Partnering with Anglo Adhesives & Services Ltd. means partnering with a company that combines tradition with innovation to deliver top-notch adhesive solutions for today’s demanding industrial and commercial requirements.