adhesives

BADI 1

John Errington MSc

Chemistry

• Atoms bond together to form molecules.

• Molecules bond together to form solids.

• There are many different processes through which this bonding occurs.

• The forces that cause bonding between molecules in solids are also responsible for adhesion between different materials when they are glued together.

Ionic bond

The alternate positive and negative ions in an ionic solid are arranged in an orderly way in a giant ionic lattice structure shown on the left. The ionic bond is the strong electrical attraction between the positive and negative ions next to each other in the lattice. The bonding extends throughout the crystal in all directions.

The atom losing electrons forms a positive ion (cation) and is usually a metal.

The atom gaining electrons forms a negative ion (anion) and is usually a non-metallic element.

Metallic bond

• The crystal lattice of metals consists of ions (NOT atoms!) surrounded by a 'sea of electrons' forming another type of giant lattice.

• The outer electrons (-) from the original metal atoms are free to move around between the positive metal ions formed (+).

• These free or 'delocalised' electrons are the 'electronic glue' holding the particles together.

• There is a strong electrical force of attraction between these mobile electrons (-) and the 'immobile' positive metal ions (+) and this is the metallic bond.

• This structure is why metals are good conductors of electricity and heat.

Covalent bond

• Covalent bonds are formed by atoms sharing electrons to form molecules. This type of bond usually formed between two non-metallic elements. The molecules might be that of an element i.e. one type of atom only OR from different elements chemically combined to form a compound.

• The covalent bonding is caused by the mutual electrical attraction between the two positive nuclei of the two atoms of the bond, and the electrons between them.

• One single covalent bond is a sharing of 1 pair of electrons, two pairs of shared electrons between the same two atoms gives a double bond and it is possible for two atoms to share 3 pairs of electrons and give a triple bond.

Here Carbon (2,4) shares outer electrons with Oxygen (2,6)

Each atom now has the “illusion” of having 8 outer electrons, which is a low energy state.

Intermolecular bonding – Van der Waals forcesMolecules overall are electrically neutral, but their charge distribution can vary. If the electrons bunch together at one side that side becomes negative, the other positive. This is called a temporary dipole. Some molecules (e.g. HCl) form permanent dipoles. (see hydrogen bonds)

Dipoles give rise to intermolecular attractions

Imagine a molecule which has a temporary polarity being approached by one which happens to be entirely non-polar just at that moment. ( In reality, one of the molecules is likely to have a greater polarity than the other at that time - and so will be the dominant one. )

As the right hand molecule approaches, its electrons will tend to be attracted by the slightly positive end of the left hand one.

This sets up an induced dipole in the approaching molecule, which is orientated in such a way that the + end of one is attracted to the - end of the other.

Van der Waal forces

An instant later the electrons in the left hand molecule may well have moved up the other end. In doing so, they will repel the electrons in the right hand one.

The polarity of both molecules reverses, but you still have + attracting -. As long as the molecules stay close to each other the polarities will continue to fluctuate in synchronisation so that the attraction is always maintained. This synchronised movement of the electrons can occur over huge numbers of molecules.

This diagram shows how a whole lattice of molecules could be held together in a solid using Van der Waals dispersion forces. An instant later, of course, you would have to draw a quite different arrangement of the distribution of the electrons as they shifted around - but always in synchronisation.

Hydrogen bondWater has a simple molecular structure. It is composed of one oxygen atom and two hydrogen atoms. Each hydrogen atom is covalently bonded to the oxygen via a shared pair of electrons. Oxygen also has two unshared pairs of electrons. Thus there are 4 pairs of electrons surrounding the oxygen atom, two pairs involved in covalent bonds with hydrogen, and two unshared pairs on the opposite side of the oxygen atom. Oxygen is an "electronegative" or electron "loving" atom compared with hydrogen. Water is a "polar" molecule, meaning that there is an uneven distribution of electron density. Water has a partial negative charge ( δ+ ) near the oxygen atom due the unshared pairs of electrons, and partial positive charges (δ- ) near the hydrogen atoms.

An electrostatic attraction between the partial positive charge near the hydrogen atoms and the partial negative charge near the oxygen results in the formation of a hydrogen bond as shown in the illustration. The ability of ions and other molecules to dissolve in water is due to this polarity.

Important mechanisms of adhesion:

Mechanical adhesion (adhesive locks into flaws scratches etc and anchors surfaces together)

Adsorption (usually the most important mechanism) (intermolecular forces are responsible for adhesion)

Chemisorption (chemical bonds tie materials together)Electrostatic attraction

(surfaces become charged oppositely and therefore attract)Diffusion (adhesive blends in to surface)

Mechanical adhesion

The mechanical interlocking theory of adhesion states that good adhesion occurs when an adhesive penetrates into the pores, holes and crevices and other irregularities of the adhered surface of a substrate, and locks mechanically to the substrate. The adhesive must not only wet the substrate, but also have the right rheological properties to penetrate pores and openings in a reasonable time.

This theory explains a few examples of adhesion such as rubber bonding to textiles and paper. Since good adhesion can occur between smooth adherend surfaces as well, it is clear that while interlocking helps promote adhesion, it is not the only mechanism involved in adhesion.

Diffusion theory

The intrinsic adhesion of high polymers to themselves, or to each other, is due to mutual diffusion of polymer molecules across the interface. This requires the macromolecules or chain segments of the polymers (adhesive and substrate) possess sufficient mobility and are mutually soluble (possess similar values of the solubility parameter).

Works in the same way as a weld or brazed or soldered joint.

Electronic theory

The basis of the electrostatic theory of adhesion is the difference in electonegativities of adhesing materials. Adhesive force is attributed to the transfer of electrons across the interface creating positive and negative charges that attract one another. For example, when an organic polymer is brought into contact with metal, electrons are transferred from metal into the polymer, creating an attracting electrical double layer (EDL).

Adsorption theory

Proposes that, provided sufficiently intimate intermolecular contact is achieved at the interface, the material will adhere because of the surface force acting between the atoms in the two surfaces, the most common such forces are Van der Waals forces and are referred to as secondary bonds.

The process of establishing intimate contact between an adhesive and the adherend is known as wetting.

The figure shows good and poor wetting of a liquid (epoxy adhesive) spreading over a surface.

Chemisorption

The chemical bonding mechanism suggests that primary chemical bonds may form across the interface. Chemical bonds are strong and make a significant contribution to the intrinsic adhesion in some cases. Coupling agents and adhesion promoters are often used to help in fixing the adhesive at the surface by chemical reaction.

Table 1    Comparison of the bond energy of different type of tractions [2,3]

TypeBond energy ( kJ / mol )

Ionic 590 ~ 1051

Covalent 63 ~ 710

Metallic 113 ~ 347

Permanent dipole-dipole interactions

Hydrogen bonds involving fluorine < 42

Hydrogen bonds excluding fluorine 10 ~ 26

Other dipole-dipole (excluding hydrogen bonds) 4 ~ 21

Dipole-induced dipole < 2

Dispersion (London*) forces 0.08 ~ 42

       *: London force is one kind of Van der Waals forces.

Traditional glues: ANIMAL AND VEGETABLE GLUE

There are two basic types of natural adhesives which are commonly used: animal and vegetable. A wide variety of vegetable glues are derived from starches, gums, cellulose, bitumen and natural rubber, and have specialized applications. Animal glues are derived from casein (a milk protein used in paint), blood albumen (used in plywood), and collagen (used in woodworking). All of these adhesive products are organic in nature and non toxic to humans.

Animal glues are adhesives which are essentially high polymer proteins derived from hydrolyzed collagen.

• Animal hide and bone glues set in a two part process which first begins by cooling from 145 degrees to room temperature, and then completely drying by evaporation during the next 12 to 24 hours. This allows the traditional woodworker to use this glue to his advantage, since hammer veneering and "rubbed" joints both require a glue with a rapid initial grab as it cools. In addition, the strong initial hold of these glues allows clamps to be removed and reused on another job while the first project dries overnight.

• Protein glues form a chemical (molecular) bond as well as a mechanical bond. This means that fresh animal glue will reactivate and chemically bond to previous animal glue surfaces, as well as forming a strong mechanical bond with wood surfaces and other natural fibers. Hide glue sticks to surfaces by an electrochemical attraction, or "specific adhesion." It is one of the few truly reversible glues, which can be changed from liquid to solid and back again with the addition or subtraction of heat and moisture.

What different types are there? Water based  Solvent Based  Chemical curing

Water based adhesives are made of natural or synthetic polymers, mixed in water, and are used when one of more surface is porous(e.g. wallpaper paste). They are slow to dry and will not stick non-porous materials like plastic. Water based adhesives are typically made of Water - Polymer - Thickener - Additives to improve strength - Biocide which stops them fading in strength with age.

Solvent based adhesives are a synthetic adhesive polymer mixed in solvent. They are used to stick non-porous surfaces and are fast drying.

Chemical curing adhesives involve creating a chemical reaction, and sometimes this involves the use of an additional ingredient as a 'hardener'. They are particularly powerful and are also fast bonding. As well as two-pack adhesives like Araldite the silicone adhesives are also in this group, and the curing process requires the absorption of water as vapour.

Do they all stick one surface directly to another?

No. There are two general types of bonding - direct and contact. In direct bonding the adhesive is applied directly between two surfaces. In contact bonding, you apply a layer of adhesive to each surface, and then the two layers of adhesive are put in contact to stick to each other.

Superglue is new, but surely the process has been around a long time?

Yes, the Ancient Egyptians and the Romans were the first to come up with practical adhesives.

 

They used ground animal bones, flour and water. Modern adhesives based on plastics were only developed this century.

 

Casein adhesiveDid you know you can make glue quite easily yourself, and main

ingredient is milk! Here is the procedure for making your own glue:

• Pour about 50ml of skim milk into a beaker. Add 10ml of vinegar (acetic acid).

• Heat slowly on a hot plate with constant stirring. As soon as the milk begins to curd, remove the beaker from the heat. Continue to stir for about one minute.

• Filter mixture. • After the liquid has been drained, scrape the curds from the filter

paper into the beaker. • Add a little water and .5g of sodium bicarbonate (baking soda). Stir

the mixture until it is creamy smooth. If it still bubbles, add a little more baking soda to completely neutralize the vinegar.

P.V.A. (polyvinyl acetate)

There are two types interior and exterior. Both are used for sticking wood to wood, also for other porous materials such as paper, and for wallpapering – priming walls and putting up borders.

Interior does not hold when it gets wet so anything to be made for outside needs exterior. Exterior is far more expensive than interior so you wouldn’t use it for an inside piece of work. These glues need to be left overnight to dry. PVA is also used for varnish for interior woodwork.

To look at it is a thick white cream.

EPOXY RESIN (e.g. Araldite).

These glues are two separate substances which do not harden unless you mix them together. Once mixed they will stick almost anything, wood to wood, metal to wood, wood to plastic, ceramic pots and so on. They are very expensive so you wouldn’t use them if there was something cheaper you could use. These glues will harden in less than 15 minutes. To look at these substances are thick sticky liquids one which is clear and one which is yellow.

RESIN WOOD GLUECascamite

Only used for sticking wood to wood, excellent for underwater use. Will fill small gaps. Usually comes in powder form and is mixed with water. Dries clear like glass.

IMPACT and CONTACT ADHESIVES

• RUBBER based solutions which are applied to both surfaces. they are left to dry for about ten minutes and the surfaces brought together. The sticking is instant and you do not have time to ‘adjust’ work. Used for sticking tiles to floors, plastic laminates to kitchen surfaces and vinyl backed upholstery. These glues look like ‘runny’ rubber with a dangerous smell which can kill.

• LATEX This type of glue is excellent for sticking most types of materials together. You put glue on both surfaces and allow to become touch dry then put the surfaces together.

POLYSTYRENE CEMENT

Consists of polystyrene dissolved in a solvent to form a nearly saturated solution. It works by dissolving the surfaces of the work pieces, then as the solvent evaporates a real weld is formed.

This glue is not particularly strong, but is ideal for sticking plastic to plastic when the joints are not going to be ‘knocked about’. Used for making models mainly.

A similar glue can be made for joining acrylics such as PERSPEX together, simply by dissolving small pieces of perspex in acetone or amyl acetate.

“Superglue” Cyanoacrylate (C5H5NO2). Cyanoacrylate is an acrylic resin in the form of a thin clear liquid. It forms a strong bond almost instantly. All it requires is the hydroxyl ions in water to trigger the process, and it is just the case that almost any object you might wish to glue will have at least trace amounts of water on its surface. Super glue, undergoes a process called anionic polymerization . The Cyanoacrylate molecules start linking up when they come into contact with water, and they form in chains to make a durable plastic mesh. The glue thickens and hardens until the molecular strands can no longer move.

Even more expensive than epoxy resin, these glues will stick most surfaces which are fairly smooth and which will not allow the glue to ‘soak in’, such as glass, metal, plastic. They can also stick skin so you have to be careful. Once the glue is applied a slight pressure for about 10 seconds will produce a very strong joint.

No More Nails

UniBond No More Nails is a super strong instant grab gap filling adhesive that sticks virtually anything to anything! Multi-Purpose Solvent FreeSafe to use Fills minor gaps in uneven surfaces.Dries white Overpaintable

No More Nails is ideal for fixing

· Skirting boards · Dado rails· Worktops · Architraves· Decorative wall panels· Plasterboard Covings

· Carpet and carpet grippers

UniBond No More Nails Bonds: Wood· Plaster· Brick· Ceramics· Metal· 

Glass· Concrete· Cork· uPVC

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”.

Hot melt glues

• These are gap filling adhesives

Adhesive tapes

Adhesive tapes use an acrylic or rubber based adhesive on a polyester, paper, polypropylene or vinyl film. Can be single or double sided.

High strength VHB (very high bond) tapes are used in applications that require strong, permanent bonds with high shear and peel adhesion.  These products are most suitable for heavy industrial use or external environments. 

VHB tapes distribute stress along the bond-line.  The 'viscoelastic' nature of the solid glue system allows for the adhesive to flow into the microscopic surface of the material to be bonded thus providing 100% 'wet out', resulting in much greater bond performance.

Parameters affecting environmental resistance:

Water is the most commonly encountered environmental that gives the greatest problems in the environmental stability of adhesive joints. The great majority of bonded structures are exposed to moist air, and if the relative humidity is high then over a period of time the strength of joints usually declines. The properties of adhesive are very sensitive to water, and this is come from the fact that adhesives are hydrophilic. The polar groups that confer adhesive properties on a substance also make them hydrophilic.

 

1. Concentration of water: The higher the water concentration, the more rapid and greater the degree of attack.

Furthermore, there often appears to be a minimum concentration below which no environmental failure, or at least no significant attack over a comparatively long time-scale, occurs.

2. Temperature: increasing the temperature of the environmental increases the rate of strength loss.

3. Adhesive type

4. Adherend type

5. Adherend surface pretreatment

6. Applied stress: The rate of loss of strength will be faster if a tensile stress and shear stress is present.

7. Joint design

How does glue work?

If two objects can come very close to one another (and I mean *very* close -- less than an atom's-width apart) then the individual atoms often tend to bond with one another. The bond is an electrical phenomenon, or more broadly put, an electromagnetic phenomenon. In theory, if I could cut a steel bar in half and produce two perfectly smooth faces, when I put the faces back into contact, they would adhere to one another with great force, as if they had been super-glued.

In the real world, though, the steel faces are never perfectly smooth, or even close to it. There are billions of tiny imperfections that prevent the two faces from meeting perfectly, so that there are large (to an atom) gaps between the faces. As a result, they won't stick together.

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.

Links

http://www.gluegunsdirect.com/oldsite/oldsite/gg.latestnews.adhesivevs.html

http://home.pacbell.net/ebeniste/gluearticle.htm

http://cms.3m.com/cms/GB/en/2-37/kcFlrFV/view.jhtml

http://www.chemguide.co.uk/atoms/bondingmenu.html#top

http://campus.umr.edu/piezo/MotorAnalysis/BondingNote/BondingNote.html

http://www.specialchem4adhesives.com/resources/adhesionguide/index.aspx?id=