All about Adhesive:
Adhesives, also known as paste, cement, mucilage, or glue. It`s any substance uses to one or both surfaces, of two apart items that binds them together. Adjectives may use in conjunction with “adhesive” to describe properties based on substance’s physical or chemical form, type of materials joined, or conditions under which it`s apply.
Uses of adhesives offers many advantages over binding techniques such as sewing, mechanical fastening, thermal bonding, etc. These include the ability to bind different materials together, to distribute stress more efficiently across the joint, the cost effectiveness of an easily mechanized process. Mainly uses in aesthetic design, and increased design flexibility.
Disadvantages of include reduced stability at high temperatures, relative weakness in bonding large objects with a small bonding surface area. Also difficulty in separating objects during testing. Adhesives are typically organized by method of adhesion. These are organized to reactive and non-reactive adhesives, which refers to chemically reacts in order to harden. Alternatively they can organized by whether raw stock is of natural or synthetic origin, or their starting physical phase.
Adhesives found naturally or produced synthetically. The earliest human use of adhesive-like substances was almost 200,000 years ago. Neanderthals produced tar from dry distillation of birch bark for use in binding stone tools to wooden handles.The first references to adhesives in literature first appeared in near 2000 BC. Greeks and Romans made great contributions to growth of adhesives. In Europe, glue wasn`t widely used until period AD 1500–1700. Until1900s increases in adhesive use and discovery were relatively gradual. Since last century progress of synthetic adhesives accelerated rapidly. Innovation in the field continues to present.
Adhesives are organized by method of adhesion. These are organized to reactive and non-reactive adhesives. It refers to adhesive chemically reacts in order to harden. They can organized by whether raw stock is of natural, or synthetic origin, or their starting physical phase.
There are two types of adhesives harden by drying. Solvent-based adhesives and polymer dispersion adhesives, also known as emulsion adhesives. Solvent-based adhesives are mixture of ingredients (typically polymers) dissolved in solvent. White glue, contact adhesives and rubber cements are members of drying adhesive. As the solvent evaporates, adhesive hardens. Depending on chemical composition of adhesive, they will adhere to different materials to greater or lesser degrees.
Polymer dispersion adhesives:
PDA is milky-white dispersions based on polyvinyl acetate. They used extensively in woodworking and packaging industries. They are also used with fabrics and fabric-based components, and engineered products such as loudspeaker cones.
PSA form a bond by using of light pressure to marry adhesive with adherend. They are designed to have a balance between flow and resistance to flow. The bond forms because adhesive is soft enough to flow (i.e., “wet”) to adherend. Bond has strength because adhesive is hard enough to resist flow when stress is applied to bond. Once adhesive and adherend are in close proximity, molecular interactions, such as van der Waals forces, become involved in bond, contributing significantly to its ultimate strength.
PSA designed for permanent or removable uses. Examples of permanent usage include safety labels for power equipment, foil tape for HVAC duct work, automotive interior trim assembly, and sound/vibration damping films. Some high performance permanent PSAs exhibit high adhesion values and can support kilograms of weight per square centimeter of contact area, even at elevated temperatures. Permanent PSA may initially removable (for example to recover mislabeled goods) and build adhesion to a permanent bond after several hours or days.
They are designed to form a temporary bond, and ideally can be removed after months or years without leaving residue on adherend. Removable adhesives are used in applications such as surface protection films, masking tapes, bookmark and note papers, barcodes labels, price marking labels, promotional graphics materials. Also uses in skin contact (wound care dressings, EKG electrodes, athletic tape, analgesic and transdermal drug patches, etc.). Some removable adhesives are designed to repeatedly stick and unstick. They have low adhesion, and generally cannot support much weight. Pressure-sensitive adhesive is used in Post-it notes.
They are made with either a liquid carrier or in 100% solid form. Articles are made from liquid PSA by coating adhesive and drying off solvent or water carrier. They may be further heated to initiate a cross-linking reaction and increase molecular weight. 100% solid PSA may be low viscosity polymers that are coated and then reacted with radiation to increase molecular weight and form adhesive. They may be high viscosity materials, are heated to reduce viscosity enough to allow coating, and then cooled to their final form. Major raw material for PSA’s are acrylate-based polymers.
They used in strong bonds with high shear-resistance like laminates, such as bonding Formica to a wooden counter, and in footwear, as in attaching outsoles to uppers.
Natural rubber and polychloroprene (Neoprene) are commonly used contact adhesives. Both of these elastomers undergo strain crystallization. In construction industry a major proprietary adhesive known as “liquid nails” is used. This also copes with tasks such as sealing artificial turf.
Contact adhesives must be applied to both surfaces and allowed some time to dry before two surfaces are pushed together. Some of them require as long as 24 hours to dry before the surfaces are to be held together.Once the surfaces are pushed together, the bond forms very quickly. It`s usually not necessary to apply pressure for a long time, so there is less need for clamps.
Also known as hot melt adhesives, are thermoplastics applied in molten form ( 65–180 °C range) which solidify on cooling to form strong bonds between a wide range of materials. Ethylene-vinyl acetate-based hot-melts are particularly popular for crafts because of their ease of use and wide range of common materials they can join. A glue gun is one method of applying hot adhesives. Glue gun melts solid adhesive, then allows liquid to pass through its barrel to material, where it solidifies.
Thermoplastic glue may have been invented around 1940 by Procter & Gamble as a solution to problem that water-based adhesives, commonly used in packaging at that time, failed in humid climates, causing packages to open.
Multi-component adhesives harden by mixing two or more components which chemically react. This reaction causes polymers to cross-link to acrylics, urethanes, and epoxies.
There are several commercial combinations of multi-component adhesives in use in industry. Some of these combinations are:
Polyester resin – polyurethane resin
Polyols – polyurethane resin
Acrylic polymers – polyurethane resins
The individual components of multi-component adhesive are not adhesive by nature. The individual components react with each other after being mixed and show full adhesion only on curing. The multi-component resins can be either solvent-based or solvent-less. The solvents present in the adhesives are a medium for the polyester or the polyurethane resin. The solvent is dried during the curing process.
Pre-mixed and frozen adhesives:
PMFs are adhesives that are mixed, deaerated, packaged, and frozen. As it`s necessary for PMFs to remain frozen before use, once they are frozen at -80 °C they are shipped with dry ice and are required to be stored at or below -40 °C. PMF adhesives eliminate mixing mistakes by the end user and reduce exposure of curing agents that can contain irritants or toxins. PMFs were introduced commercially in the 1960s and are commonly used in aerospace and defense.
They harden via a chemical reaction with an external energy source, such as radiation, heat, and moisture.
Ultraviolet (UV) light curing adhesives:
They also known as light curing materials (LCM), have become popular within the manufacturing sector due to their rapid curing time and strong bond strength. Light curing adhesives can cure in as little as a second and many formulations can bond dissimilar substrates (materials) and withstand harsh temperatures. These qualities make UV curing adhesives essential to the manufacturing of items in many industrial markets such as electronics, telecommunications, medical, aerospace, glass, and optical. Unlike traditional adhesives, UV light curing adhesives not only bond materials together but they can also be used to seal and coat products. They are generally acrylic-based.
Heat curing adhesives:
consist of a pre-made mixture of two or more components. When heat is applied the components react and cross-link. This type of adhesive includes thermoset epoxies, urethanes, and polyimides.
Moisture curing adhesives cure:
when they react with moisture present on the substrate surface or in the air. This type of adhesive includes cyanoacrylates and urethanes.