Practical Application of Intermolecular Forces

A Study of Adhesives

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.

VOCAB

• adhesion—the force of attraction between unlike molecules; the property of a material that allows it to stick better to something else than to itself

• cohesion—the force of attraction between like molecules; the property of a material that allows it to stick better to itself than to something else 

• dipole moment—the measure of net molecular polarity, where if the difference in electronegativity between atoms is sufficient the resulting molecule will be polar

• van der Waals forces—an alternative name for intermolecular forces, in honor of Dutch scientist Johannes van der Waals

• intermolecular force—an attraction between molecules that holds them together, such as: 

• dipole-dipole force—result of electrical interactions among neutral but polar molecules 

• hydrogen bond—result of electrical interaction between a hydrogen atom bonded to an electronegative O,N, or F atom and an unshared electron pair on another nearby electronegative atom

• ion-dipole force—result of electrical interactions between an ion and the partial charges of a polar molecule

• London dispersion force—result of instantaneous and temporary electrical interaction among molecules that are electrically neutral and that have a zero dipole moment

• What characteristics do glues share with liquids and solids?

• How do you think glues work?• How can we test the intermolecular forces of each

adhesive?• Who can provide a hypothesis for this experiment?• What variables do we need to account for?• What data should we collect?

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

become bonded.

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.

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

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),

Adhesive viscosity refers to the resistance for the glue to flow in relation to its IMFs . 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.

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

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.

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

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".

Moisture Cure: Adhesive absorbs water from the atmosphere or substrate to form

a polymer layer

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

Anaerobic Adhesives: When deprived of oxygen, the adhesive hardens

to form the bondAerobic 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.

How a glue stick works• When an adhesive is pasted on the surface of a material (adherend), the adhesive

penetrates into the small gaps on the adherend surface, and then hardens and does not move any more inside the gap. It is thought that this inability to move is one of the reasons that things attach together. This effect is called the “anchor effect“, because like a ship with a dropped anchor the adhesive does not move. There are chemical bonds between adhesive molecules and adherend molecules, which also make them stick together. These bonds are, for example, hydrogen bond, Van der Waals bond, ionic bond, and covalent bond.   Thus, adhesion occurs not as a single effect but as the sum of several effects above. Polyvinyl pyrrolidone (PVP), the major component of a glue stick, also adheres by several effects such as anchor effect and intermolecular forces. There are also hydrogen bonds (O=C∙∙∙∙∙OH－ ) between carbonyl groups in PVP and hydroxyl groups in paper.  On the other hand, since the attraction force is generated when one molecule comes close to another molecule, it is necessary that the adhesive is brought near to the solid surface by liquid. Because the penetration of the adhesive into the adherend surface is important for adhesion, and the strength of adhesion depends on several effects above, we cannot always say, “to feel sticky = to have adhesive force”.

But if you take two pretty flat items -- two sheets of glass, for instance -- and sandwich them together with just a tiny bit of water, they will stick together with amazing tenacity. The water fills in the gaps, and helps the glass surfaces come into close enough contact that their own electromagnetic forces can make the two panes of glass adhere. The water is acting as a glue. And this is how glue generally behaves whenever two items are pasted together.

Rough and smooth

If you look at most surfaces through a microscope, you will find that they are not smooth.

What an adhesive does is fill in the gaps and build a bridge between two surfaces.

The adhesive must then harden, so that the bond stays together.

So What sticks to what?

In the following lab you will investigate wetting and curing with some commonly used household glues