Practical Application of Intermolecular Forces

1. Practical Application of Intermolecular Forces A Study of Adhesives

2. Introduction • Adhesive – The name for the glue. There are many types of adhesives, which include: • Yellow and White Glue • Instant Glue • Contact Cement • Polyurethane Glue • Epoxy Glue • Spray Adhesive • Construction Glue • Hot Glue • Substrates – The material that is being glued together. Plastic, wood, metal, ceramic, glass, rubber, etc.

3. An adhesive is a substance that sticks to the surface of an object such that two surfaces become bonded.

4. The adhesive will “wet” or cover the substrate, when the Intermolecular forces between the glue and the substrate are stronger than the Intermolecular forces between the glue and glue.

5. Good wetting occurs if the adhesive spreads out over the substrate in a uniform film and in doing so makes a lower degree contact angle between the substrate and the adhesives Poor wetting occurs when the adhesive forms droplets on the surface making a higher degree contact angle

6. For an adhesive to wet a surface, the adhesive should have a lower surface tension, than the solid’s surface energy (or critical surface tension),

7. Adhesive viscosity refers to the resistance for the glue to flow . A low viscous adhesive will flow into the spaces and gaps in the substrates surface producing maximum coverage forming a mechanical bond to the substrate.

9. Adhesive Curing Once the adhesive is applied to the substrate, it will begin to cure, or form a permanent bond Different Adhesives have different mechanisms in which the curing takes place

10. Drying: As the liquid solvent evaporates or is absorbed by the substrate, the solid adhesive is left behind, causing the bond to form These adhesives come ready to use and set to a colorless glue line at room temperature by losing water. Elmer’s glue is one example.

11. Cooling: The adhesive is applied hot, and when cooled it becomes a solid, producing the bond Mostly animal glues, these glues are applied hot and cure by cooling

12. Chemical Reaction: For two part adhesives, there is a reaction that occurs to form a crossed link adhesive Often called Epoxy this glue is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener".

13. Moisture Cure: Adhesive absorbs water from the atmosphere or substrate to form a polymer layer

14. UV Cure: Ultraviolet light is absorbed by the polymer to form crosslinks that cure very quickly. Mostly used in industrial applications, these adhesives are used in manufacturing electronics and create a very strong bond

15. Anaerobic Adhesives: When deprived of oxygen, the adhesive hardens to form the bond Aerobic adhesives remain liquid until isolated from oxygen in the presence of metal ions, such as iron or copper. For example, when an anaerobic adhesive is sealed between a nut and a bolt on a threaded assembly, it rapidly “cures” or hardens to form a tough cross-linked plastic with tenacious adhesion to many metals.

16. So What sticks to what? In the following lab you will investigate wetting and curing with some commonly used household glues