Adhesion is the binding force between two different materials, whereas cohesion is the binding force between two similar materials. When two materials are brought into contact with each other, the surface molecules interact, giving rise to attractive forces that may be physical, chemical or electrostatic corresponding to adsorption, covalent bonding or van der Waals forces, respectively. When the molecules are similar, as in the case of two 'glue molecules,' the cohesive force causes the glue to stick to itself.
When the molecules are dissimilar, as in the case of a glue molecule and a molecule of the substrate the surface the glue is sticking to , the adhesive force holds the glue to the substrate.
Hence, the 'stickiness' of tape is caused by a combination of the molecular forces of the glue material sticking to itself as well as holding onto the substrate. A pressure-sensitive adhesive is a sticky, viscous, liquidlike material that adheres to a surface using only pressure. To function well, it should have good adhesion to a surface and good cohesion, or internal strength. For good adhesion, it is important that the adhesive can readily flow out on the surface. The degree, or freeness, of the flow often determines the intensity of adhesion.
Degree of flow depends on the difference between the surface energy of the material and the surface energy of the adhesive. Pressure-sensitive adhesives tend to flow out on materials having a high surface energy--for instance, metals, glass and plastics such as acrylic, polycarbonate and nylon. Pressure-sensitive adhesives tend not to flow out as easily on low surface-energy materials such as polyethylene, polypropylene and Teflon.
This type of adhesive uses either direct contact or a third surface, such as an adhesive tape, to create the bond. Among the composite applications that are commonly bonded using adhesives are fibreglass, and fibre-epoxies like boron-epoxy and carbon-epoxy. Due to their shear strength and toughness, modern adhesives can also serve to join structural materials, of which plywood is a common and well-known example. One of the main advantages of adhesive bonding is that it allows for the even distribution of stress loads, thereby reducing the stress on the joint.
Since they are applied inside the join, adhesives are also invisible within the assembly. The properties of glues mean that they can form both an adhesive and sealant at the same time, while also resisting vibration and flexing stresses.
The ability to act as a seal allows adhesives to help protect a join from corrosion. While adhesive bonding can be applied to metal-to-metal, plastic, glass, rubber, ceramic and many other material combinations, the process can be used to join irregularly shaped surfaces while offering virtually no change in the dimensions or geometry of the parts. Adhesives also add very little by way of weight to the materials being joined and can be used to quickly bond dissimilar substrates and heat-sensitive materials.
One of the greatest advantages of adhesive bonding is the versatility it affords, allowing for joining of a wide range of items as well as being easy to automate. There are, however, a number of limitations to using adhesives. Firstly, they are prone to failure when exposed to high heat or weathering. They bond to textile fibers, metals, plastics, glass, sand, ceramics, rubber, and wood. Polyurethanes can work well on a wide variety of wood species, particularly on woods with high moisture content or on oily woods, where other glues are not as successful.
Clamping is required until strength is built; a few hours. Full strength is achieved in six to eight hours for a very strong and tough bond. Before completely cured, polyurethane adhesives can be removed using solvents such as mineral spirits or acetone. Dried glue can be sanded. Glue Sticks:. Glue sticks are great for kids! They are a low bonding adhesive, but do provide a permanent bond on various types of paper to include cardboard, foam board, and poster board. Glue dries clear. Application examples: sealing envelopes, applying labels, paper crafting, art projects, scrapbooking.
Not all glues are created equal. There are many variations within each category and from one manufacturer to another. Read the labels for information on toxicity, ventilation, recommended handling and use, as well as durability in a variety of environments.
Apply adhesive evenly and remove excess quickly. Immediately clean and cap the adhesive container to maintain shelf life and performance. For optimum bond strength, it is imperative that the surfaces are residue and dust free. If possible, clean surfaces prior to bonding with a lint-free rag dipped in isopropanol.
Let them dry thoroughly before applying adhesive. If you cannot abrade the surface, try wiping with isopropanol or acetone before applying adhesive. This is particularly helpful for smooth, glossy surfaces that can be harder to bond.
Experiment with the glue on scrap pieces of your project. Check for appearance, adhesion and and resulting bond strength. Yellow Wood Glue: Yellow wood glue is also water based — and is made of the same vinyl acetate polymers as craft glue. Three types of wood adhesives are available: Type-I exhibits some waterproof properties.
Type-II will perform better in exterior conditions. These adhesives generally have a longer open time and can bond at colder temperatures. Both types I and II can be used for exterior applications, such as outdoor furniture and trim. Type-III is not water resistant and is designed for interior use only. Type-III is good for interior woodwork and trim projects. Note: True water resistance for immersion in water requires a marine glue.
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