Adhesives are substances that are applied to the surface of one element to bond it to another surface, preventing their separation. They are commonly known as glue or adhesive, and can be found naturally or produced synthetically. Adhesive materials are essential in various applications and have seen increasing value in exports and industries in the modern age.
History of Adhesive Use and Manufacturing
Adhesives, widely used in today’s society, have a rich history that can be traced back to ancient times. These materials have evolved from humble beginnings to what they are now. For thousands of years, various forms of adhesives have been used to bond objects together. Archaeologists, in their quest to uncover lost history, have discovered different materials used as adhesives, including substances like beeswax and tree sap. While adhesives had primitive beginnings, they have developed into a billion-dollar industry we have today. (An article titled “The History of Adhesives” from mixerdirect.com)
Use in Ancient Times
- The first human use of adhesives dates back to around 200,000 years ago when humans produced an adhesive material through dry distillation of birch bark. This material was used to attach stone tools to wooden handles and was discovered in central Italy.
- In the Stone Age, humans used composite adhesives. Stone pieces, around 70,000 years old, were found inserted once into covers of wooden shafts coated with an adhesive consisting of plant gum and red ochre (natural iron oxide).
- Archaeologists excavating burial sites from 4,000 years BCE discovered repaired clay vessels sealed with gum made from soap trees. (An article titled “The History of Adhesives and Glues” from arabian-chemistry.com)
- Ancient Egyptians used natural adhesives in the construction of coffins, such as in the case of Tutankhamun’s coffin. They also developed pastes made from starch to bond papyrus to clothing and used a plaster-like substance made from calcined gypsum.
- The Romans used beeswax as a sealant and water barrier between wooden planks in their boats and ships.
These examples demonstrate the long-standing history and varied applications of adhesives throughout ancient civilizations.
- In 1750, Britain issued the first patent for fish glue, creating space for further patent inventions as numerous companies developed different types of adhesives. By 1900, the United States had several glue factories producing adhesive materials from various bases.
- With the ongoing Industrial Revolution, technological breakthroughs led factories to choose new materials for formulating their adhesives. Nitrocellulose became the first wood-derived plastic polymer to be manufactured. It was initially used in the production of ivory balls, known as billiard balls. In 1910, the plastic era began with the production of phenolic thermosetting plastic known as Bakelite.
- Over the past century, advancements in adhesive technology have resulted in the production of various types of polymers. These advancements have not only improved the different properties of adhesives but also altered them. These properties include durability, flexibility, temperature resistance, chemical resistance, and time processing and control.
The adhesive industry, whether natural or synthetic, relies on three main characteristics:
- Surface Cohesion: It refers to the ability of an adhesive material to adhere or bond to a surface. Different types of adhesives vary in their strength of adhesion to different surfaces. Some adhesives are effective on rough surfaces but weak on smooth surfaces.
- Load-Bearing Capacity: It refers to the ability of the adhesive material to withstand various weights and pressures. It is often referred to as the adhesive’s strength. For example, marble cannot be effectively glued using paper adhesive. Therefore, adhesive materials are manufactured based on their load-bearing capacity for different weights.
- Adaptability to Influencing Factors: It refers to the adhesive material’s ability to withstand changing conditions it may be exposed to, such as different temperature ranges, exposure to water, air, or cold temperatures.
Adhesive materials are often made from polymers or resins, which are chemical structures identified as plastic materials in the end.
Some Scientific Concepts Related to Adhesion
- Adhesion: It is a measure of the attractive forces between two different substrates that are bonded together. By assessing the level of adhesion exhibited by an adhesive material, the strength of the bond between the adhesive and the substrate can be determined, along with the cohesive strength, alongside measuring the cohesive forces.
- Wetting: The theory of wetting states that adhesion occurs due to the contact between molecules at the interface of two materials – the adhesive and the substrate. The attractive forces resulting from the contact, such as chemical bonds, van der Waals forces, etc., hold the substrates together.
- Mechanical Interlocking: The theory of mechanical interlocking suggests that adhesion occurs when the adhesive material flows into the cavities and protrusions present on the substrate surfaces and around them. Once the adhesive material mechanically solidifies, it secures the substrates together.
- Electrostatic Attraction: The theory of electrostatic attraction states that adhesion results from the development of electrostatic forces at the contact point (i.e., the interfacial region) between the adhesive material and the substrate due to differences in their electronic structures. These attractive forces resist separation – even if temporarily – forming a bond between the adhesive and the substrates.
- Cohesion: It is a measure of the attractive forces within a material that hold its components together. This value can indicate the strength of the bonds between the components of the adhesive material or the components of the substrate. Some factors that may affect the cohesive properties of the adhesive material include chemical and molecular bonding between the atoms of the adhesive components and the entanglement of molecules (from short chains to long chains). Within the adhesive, cohesion also plays a role between the atoms and molecules comprising the material.
Types of Adhesive Materials
Adhesive materials can be classified into natural adhesives and synthetic adhesives, both of which are involved in the production of various types of adhesives:
Pressure-sensitive adhesives are composed of acrylic, rubber/latex, or silicone. They do not require a solvent, water, or heat to bond, but rely on pressure. If insufficient pressure is applied or the processing temperature is too low, bonding errors such as bubbles or separation may occur.
The ability of these pressure-sensitive adhesives to exhibit quick adhesion is influenced by several factors, including:
- Environmental conditions such as time, temperature, humidity, etc.
- Application conditions, such as application speed, light pressure vs. heavy pressure, cleanliness, etc.
- Chemical composition and structure of the adhesive material itself.
Pressure-sensitive adhesives are manufactured to remain tacky for extended periods. They are coated on fabrics and papers to create adhesive tapes or labels. They can also be formulated to cure into a solid, such as the white glue used in woodworking.
Polymeric adhesives consist of inherently non-adhesive materials, but when mixed with other substances, they undergo a chemical reaction that leads to the entanglement of polymers, forming adhesive compounds or resins. Polymeric adhesives are divided into polyester, polyurethane, acrylic, and epoxy.
Polymer-based adhesives are considered the best for bonding wood but are also used in various other industries, such as glass and automotive.
Electrically Conductive Adhesives
Electrically conductive adhesive products are primarily used for electronic applications where components need to be securely held in place while allowing electrical current to pass through. If you need to repair your electronics and want to avoid the intense heat of soldering and soldering iron that may damage the component, you can use an electrically conductive adhesive instead.
Electrically conductive adhesive materials can rely on various chemical compounds, such as:
- Electrically conductive silicone adhesives, which can be filled with graphite and are often used in anti-static systems. These materials generally have very high viscosity and a thick consistency, making them suitable for larger applications like gasketing or sealing large gaps. Electrical conductivity is somewhat limited (so it is not a good substitute for soldering). Typically, the volume resistivity is around 0.09 ohm∙cm.
- Epoxy adhesive composed of two components that can be filled with silver, providing high conductivity similar to two-part epoxy filled with the same.
- One-part epoxy adhesive material, usually requiring heat curing, so caution must be taken to choose a curing schedule that does not affect sensitive electronic components.
- Single-component epoxy filled with silver can achieve high levels of conductivity, like two-part epoxy filled with metal.
- Silver-filled polyurethane adhesives, which are two-part adhesives, so they either require mixing or are pre-supplied and frozen like epoxies. They can achieve high levels of conductivity (around 0.0001 ohm∙cm to 0.0004 ohm∙cm).
Hot-melt adhesives are solid materials that melt at a specific temperature (typically above 80 degrees Celsius/180 degrees Fahrenheit) during application or shaping, and solidify again upon cooling, ensuring cohesion.
These materials are generally formulated as wax-like sticks, and auxiliary equipment is available for their melting and use. Alternatively, they can be produced as pellets, similar to hot glue used in woodworking.
Some potential base materials for hot-melt adhesive manufacturing include:
- Ethylene vinyl acetate copolymers (EVA)
- Ethylene copolymers
- Polyolefins (PO), usually LDPE but also HDPE
- Amorphous Polyolefin (APO/APAO) compatible with various solvents
- Polyamide and polyester
- Styrene block copolymers (SBC)
- Polypyrrole (PPY)
Non-airborne adhesives are single-component adhesives that remain stable at room temperature for extended periods and only polymerize when oxygen is excluded after assembly with the adhesive.
They are generally formulated with a mixture of monomers and mono-functional and multi-functional acrylate ester resins. This type of material typically has low viscosity and is available in liquid solutions and pastes. They are used for fastening, sealing, and retaining parts with tapered, interlocking, or closely fitted geometries. They are also used for bonding pivot assemblies and sealing flange faces. These materials are primarily used in the automotive industry, optics, fine electronics, and medical industries.
Also known as light-curable adhesives, UV-curable adhesives are available in a range of viscosities and various chemical systems. They are solvent-free materials (100% solid) and are activated by ultraviolet light. These materials include a wide range of chemicals, from standard UV-curable acrylates to UV-curable epoxies and dual-curing adhesives that also cure with an activator, heat, moisture, or anaerobically with a metal.
They are ideal for many applications where glass or UV-transmitting plastics, metals, and other substrates need to be bonded. They are used for glass bonding, architectural bonding, disposable medical devices, lamination of printed surfaces, flat-panel displays, touchscreens, coating, and the manufacturing of respiratory support devices.
Epoxy adhesives are created by polymerizing a mixture of two primary components, resin and hardener. When the resin is mixed with a specific catalyst, the curing process begins, where the molecular chains react chemically and generate heat.
The covalent bonds formed between the epoxy resin’s epoxy groups and the hardener’s amine groups result in cross-linking of the polymer, providing epoxy adhesives with hardness and strength.
Uses of Epoxy Adhesives
Epoxy adhesives are manufactured in various types and formulations to suit a wide range of applications, including:
- Metal bonding
- Plastic bonding
- Cementitious material bonding
Acrylic adhesive is a resin-based adhesive composed of acrylic or methyl acrylic polymers. It is extremely strong and effective in bonding various objects together, with high resistance to environmental conditions. Due to this environmental resistance, acrylic adhesives are commonly preferred in the construction industry.
Acrylic adhesives are among the strongest adhesives available in the market, thanks to two key factors:
- Cohesion: The adhesive’s ability to bond with itself.
- Adhesion: Its ability to bond with other objects.
Acrylic adhesives exhibit excellent cohesion and adhesion properties, which are influenced by factors such as curing time, curing temperature, and viscosity. They are available in liquid or paste form to suit different applications.
Water-based adhesives rely primarily on water for their use and are commonly used in various construction applications. They are made from natural or synthetic sources.
These adhesives are also specifically formulated for specific applications, depending on the type of adhesive and the industry it operates in. They are typically in powder form, like cement and gypsum used in construction, or in liquid form for other uses.
There are four main types of water-based adhesives: vegetable gums, animal/protein glues, resin cements, and latex cements. Each of these different materials is uniquely designed to serve its specific purposes, depending on the adhesive used. The resulting bond can appear as a solid resin or a film-like finish. All of them are solvent-free, making them safe for indoor use and non-ventilated areas. Let’s take a closer look at each of these water-based adhesive types.
Please note that the accuracy and suitability of specific adhesive types and their uses may vary depending on the manufacturer and specific product formulations.