What makes glues able to stick objects together? The answers are surprisingly complicated and the explanations are different for various types of adhesive, but the following concepts help explain some of the basics. e has two main tasks after it's applied. It must stick to the material to which it's applied, and the glue itself must not break apart internally within the glue layer. The two properties governing this are known as adhesion and cohesion.
Adhesion describes the bond between different types of materials. When considering glue, this is the bond between the glue and the surfaces to which the glue is applied. Strong adhesion is desirable between a glue and a surface. Weak adhesion can be detected by pulling apart a glued joint -- if the joint breaks apart cleanly between the glue and the surface material, that indicates the adhesion between the glue and surface was the weakest part of the joint.
There are several chemical and physical explanations for why a glue adheres to a surface. One or more of these will apply for a given glue and surface combination, but the particular contributions to the adhesion for each of these phenomena will depend on the particular glue and surface.
Chemical adhesion occurs when a glue and a surface form a new chemical compound where they meet. This would happen if the atoms of the glue and surface exchange (ionic bonding) or share (covalent bonding) electrons or if hydrogen atoms are involved and they are attracted to nitrogen, oxygen, or fluorine atoms in the other material (hydrogen bonding).
Diffusive adhesion occurs when molecules from the glue, surface, or both diffuse, or seep, into the other while remaining attached to their own similar molecules. If molecules from one material do diffuse into the other, that interaction may create stronger van der Waals or other bonds.
Dispersive adhesion occurs because of van der Waals forces. This refers to an attraction between molecules that have positive and negative poles (either temporary or permanent) that are attracted to poles in the other materials' molecules.
Electrostatic adhesion occurs when the materials are conductive and pass electrons to/from each other creating an electrical charge difference and attractive electrostatic force.
Mechanical adhesion occurs when a glue can enter the pores in the surface material and harden into some sort of mechanical interlock. Think tiny little glue grappling hooks forming inside the pores of a surface.
Cohesion describes the bond a material has to itself. When considering glue, this is the strength of the glue itself staying together as one unit. Weak cohesion of a glue can be detected by pulling apart a glued joint -- if the joint breaks apart in the glue layer itself, with glue remaining on both surfaces, then the glue cohesion itself was the weakest part of the joint.
Cohesion is an intrinsic chemical property of a molecule. Hydrogen bonding and other van der Waals forces cause cohesion between two like molecules. These forces are due to mutual electrical attraction between molecules in close proximity whose particular properties and structure allow for it.
Have you ever wondered why a glue doesn't harden and stick to the bottle it's in? The answer lies in solvents and hardeners. This is another key aspect of glue -- that it remains in a workable form until applied and cured.
Glues that come in a single bottle and don't need any special additives or treatment to cure (other than time and ventilation) are glue molecules suspended in a volatile solvent that evaporates when it's exposed to air. So as long as a bottle of glue remains sealed airtight, the solvent doesn't evaporate, and so the glue stays liquid. Once it's applied to a joint, the solvent gradually evaporates -- sometimes it takes quite a lot more time to evaporate in a glue joint than just out in open air as a blob of glue. This is because a blob of glue is much more exposed to air than glue inside a tight joint.
Other glues, like epoxies, can come in 2 separate bottles. For 2-part epoxy, the bottles are resin and hardener. Each by itself is just a liquid. When combined, there's a chemical reaction that causes the 2 combined liquids to become solid. So as long as the 2 are kept separate in airtight bottles (a lot of epoxy resins will eventually harden when exposed to air), they'll remain liquid until combined and applied. Note: some epoxies, even when mixed with a hardener, may require additional treatment to fully cure.
Still other glues exist that need some additional treatment to harden. Sometimes this treatment is the application of heat, and sometimes it's exposure to UV (ultraviolet) light or some other form of energy that excites a chemical reaction in the glue or causes the solvent to evaporate.
Cyanoacrylate glues are acrylic resins that cure when exposed to water. When typically applied, the humidity in air contributes the hydroxide ions needed to polymerize (harden) the glue. These need to be applied in a very thin layer to ensure that enough humidity reaches the glue to cure quickly. Various additives can adjust the properties of these glues, resulting in a variety of formulations for specific purposes. Some cyanoacrylates release toxic fumes while drying and should definitely be applied in a very well ventilated space, while other other formulations are much less toxic.
2-part epoxies are the most common hardware store epoxies. These consist of 2 tubes, one containing resin, and one containing hardener. These must be mixed in a precise ratio to ensure proper operation. Epoxy is a very common engineering adhesive. It can be formulated in a lot of different ways and is very versatile. Epoxy is generally a very strong adhesive. Some epoxies are waterproof and can be used in marine applications. Epoxy can be toxic and, again, the mixing ratio matters, so read the label before using.
Hot (melt) glue usually comes in the form of a solid cylinder of glue. It's usually applied with an electric hot glue gun that has a heated tip and a trigger to force the solid cylinder of glue through the heated tip. The advantage of hot glue is that it's usually non-toxic, since it doesn't use any toxic VOC (volatile organic compound) as a solvent like a liquid glue. Hot glue will usually also solidify, or cure, much more quickly than liquid glue, since most hot glues return to their solid form as soon as they cool. Hot glue sticks usually have a very long shelf life. Because nothing evaporates during curing, an applied bead of hot glue will not change size when it cools. After application, hot glue can re-liquify if exposed to high heat, limiting some use cases.
Polyurethane glues are commonly used for wood joints. This glue expands while curing, which changes where and how it's applied as compared to white or yellow PVA wood glues. Generally polyurethane glues are more water-resistant than PVA glues. Cure times and other properties can vary widely between different formulations.
Rubber cement is usually made from latex polymers suspended in a solvent. It's applied, and the solvent evaporates into the air, and the latex polymers are left behind to adhere materials. Rubber cement is commonly used when high adhesion is not needed and not desired -- rubber cement can usually be removed from materials without any damage. Despite it being called "rubber" cement, it's not usually used for patching rubber surfaces (tires, etc.). This is because rubber cement is non-vulcanizing -- it doesn't actually chemically combine and harden against other rubber. Depending on the formulation, rubber cement can release toxic fumes.
Silicone adhesives (also sometimes called silicone sealants) are made from silicon-based polymers. Some formulations will cure just with humidity in the air, and others need UV light or other treatment to fully cure. Silicone adhesives remain flexible after application, are resistant to high heat, and are resistant to water and other liquids. Because of this, they're used for making watertight joints, making flexible couplings for acoustics, etc.
One common use for a solvent cement is "gluing" together plastics, like acrylic. However, an acrylic solvent cement is more of a welding process than a gluing process. Applying a water-thin solvent to acrylic will actually partially dissolve the pieces of acrylic, melting them together, and then evaporate, leaving a single solid piece. Solvent cements are available for a variety of plastics -- PVC, CPVC, ABS, acrylic, etc.
Spray adhesives usually refers to glues that come in an aerosol can. Typically application involves spray a surface with the adhesive and pressing another material down on top to glue it. Spray + pressure = joint. Spray adhesives come in many varieties with many different chemical formulations. In general, the spray adhesive consists of a propellant to make it spray, a solvent to make the glue liquid, and the actual glue. Depending on the formulation, the propellant and solvent portions could be toxic or hazardous to the environment. Usually the glue portion is some type of vinyl polymer or a resin.
Many types of adhesive may come in a variety that can be cured with UV light. Most consumers will never encounter such a glue. However, the advantage of UV-curing glues is that they usually have a long working time (until exposed to UV light) and usually don't contain an evaporating solvent (so less likely to have toxic fumes). However, one must be careful to wear UV-blocking glasses when working with a powerful UV lamp. Normally UV adhesives are used in automated industrial production, but some artists use UV-curing adhesives to take advantage of the long working time.
A vulcanizing cement is used to glue together natural or synthetic rubber. Vulcanizing cements actually cause a chemical reaction in the rubber that forms cross-linking polymer chains between the two materials. The final joint is strong and elastic. Vulcanizing cement is commonly used for patching tires.
Most American school children come to know and love Elmer's white school glue, which is a great example of a white PVA glue. Most white PVA glues are somewhat runny, have around 5 minutes of working time, should be held or clamped for 30 minutes to an hour, and will fully cure in 24 hours. Most white PVA glues will dry clear. These are good for some wood joints and as a decent general light-duty glue for a lot of materials in a pinch. If this glue freezes, it's ruined, so be careful where you store it. Also, it softens with heat, so it's not good for sanding.
Yellow wood glue is also know as aliphatic resin emulsion glue. It's still a PVA glue, just like the "regular" white PVA glue, but it has slightly different formulation and therefore slightly different properties. It's usually thicker to work with, and it's more resistant to heat, so it's better for sanding. And you'll probably want to sand it, since they usually dry yellow. It still can't be frozen. Not all yellow wood glues are formulated exactly the same, so don't assume every yellow wood glue has the exact same properties - always read the label.
Depending on the adhesive used, safety can be very important. Toxic fumes, skin contact, chemical reactions, and flammability are all big concerns. And, let's face it, a lot of kids eat glue -- definitely not healthy.
Although all glues are different, the following basic safety tips are good habits to develop while working with adhesives.