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Eve On Electronics

In the making of electronic assemblies - and complete vehicles, for that matter - glues, adhesives and interface materials are widely used. In the subsequent repair of these assemblies, glue often is considered the best method to fix something rather than screwing on yet another expensive assembly.

What glues to use and where, and how to employ them in ways that will provide the best results? It all starts with knowing what you're trying to glue together and then preparing the materials for that bonding process.

There are three basic types of glue adhesion processes. The simplest type is the solvent type of glue. During the semiliquid sem·i·liq·uid  
Intermediate in properties, especially in flow properties, between liquids and solids.

 state of this type of glue, the two parts can be pushed together. As the solvent from the glue dries, the material solidifies and the two pieces are bonded together.

A second type of bonding involves the use of surface pores. All surfaces have some texture, even glass. In automotive applications using plastics, the materials are often filled with glass fiber, talc or mineral fills. The result is that the surface imperfections can be filled with the glue as it's applied. When the glue hardens, these pockets harden, too, but that part of the glue cannot be pulled away without disturbing the material surface. This is a form of mechanical bonding. It works as well as the surface topography allows it to.

The third major type of bonding involves molecular level interaction. In this situation, the electrons of certain types of molecules are not evenly distributed around the nucleus. This means theres a net negative or positive charge at one end of the molecule that can attract an opposite but equal charge on the substrate or glue being used. This is called secondary bonding because it's the chemical bonding of atoms that creates new molecules.

Two kinds of secondary bonding are of interest in the world of glue - dipole-to-dipole bonding and hydrogen bonding hydrogen bonding

Interaction involving a hydrogen atom located between a pair of other atoms having a high affinity for electrons; such a bond is weaker than an ionic bond or covalent bond but stronger than van der Waals forces.
. In dipole-todipole bonding, two oppositely charged ends of two molecules are brought close enough together that the dissimilar charges attract each other, like the poles of a magnet.

A good example of dipole-to-dipole adhesion is water, which is two hydrogen atoms bonded to one oxygen atom. Because of its shape, it's a polar molecule Polar molecule

A molecule possessing a permanent electric dipole moment. Molecules containing atoms of more than one element are polar except where forbidden by symmetry; molecules formed from atoms of a single element are nonpolar (except ozone).
. One of the reasons it wets so well to many if not most surfaces is the dipole-to-dipole adhesion that results from its molecular shape.

The term hydrogen bonding refers to the fact that if hydrogen is bonded to oxygen, nitrogen or fluorine fluorine (fl`ərēn, –rĭn), gaseous chemical element; symbol F; at. no. 9; at. wt. 18.998403; m.p. −219.6°C;; b.p. −188.14°C;; density 1. , there will be a change in electronegativity electronegativity (ĭlĕk'trōnĕgətĭv`ətē), in chemistry, tendency for an atom to attract a pair of electrons that it shares with another atom (see chemical bond).  that will result in the hydrogen atoms then seeking out other oxygen, nitrogen or fluorine atoms. According to according to
1. As stated or indicated by; on the authority of: according to historians.

2. In keeping with: according to instructions.

 a paper written for the SAE by Daryl Doyle of GMI GMI Governance Metrics International (New York, New York)
GMI Giant Magneto-Impedance
GMI Global MSF Interoperability
GMI General Motors Institute
GMI General Mills, Inc.
, it's this combination of dipole-to-dipole and hydrogen bonding that makes water a liquid at room temperature.

So, what all of this means is that before you try to repair a broken plastic part on a vehicle, you need to know what it's made of so you can select an appropriate glue. The good news here is that for the last ten years or so, the International Material Data System and other groups have required that all plastic parts of a vehicle must be labeled with the type of plastic they're made of. If you look at your broken pieces, you'll almost always be able to determine the type of plastic you're dealing with because the name has been molded into the surface.

Correspondingly, glue makers are pretty good about labeling their products as to what types of plastic they'll work with. Plastics widely used in automotive applications include polyesters, polypropylene, ABS, polycarbonate A category of plastic materials used to make a myriad of products, including CDs and CD-ROMs.  and others. Of these, the cheapest to make - polypropylene is the also the hardest to bond to.

GMIs Doyle goes on to say in his paper that "for an adhesive to form a bond with a substrate, some type of interaction must occur. This may include mechanical, physical adsorption adsorption, adhesion of the molecules of liquids, gases, and dissolved substances to the surfaces of solids, as opposed to absorption, in which the molecules actually enter the absorbing medium (see adhesion and cohesion).  or chemisorption chem·i·sorb   also chem·o·sorb
tr.v. chem·i·sorbed, chem·i·sorb·ing, chem·i·sorbs
To take up and chemically bind (a substance) onto the surface of another substance.
." The adhesive has to come in intimate contact with the surfaces being bonded. This is called wetting, and it means the adhesive has spread out to form a thin film over the surface area, with no air pockets formed underneath.

There are a variety of techniques used in the automotive arena to improve adhesion by increasing surface wetting to promote polar bonding. For plastics, the use of plasma etching Plasma etching is a form of plasma processing in which a high-speed stream of plasma is shot (in pulses) at a sample. The atoms of the shot element embed themselves at or just below the surface of the target. The physical properties of the target are modified in the process. , corona etching and/or a flame treatment is common. These methods are used to get the glue to stick to plastic surfaces like polypropylene that would not otherwise have many polar locations on the surface to which the glue can adhere.

The flame treatment should be used only until the surface becomes glossy. You can do this with an oxyacetylene torch oxyacetylene torch (ŏk'sēəsĕt`əlēn), tool that mixes and burns oxygen and acetylene to produce an extremely hot flame. This torch can be used for cutting steel and for welding iron and various other metals. . Just don't overdo it and be careful where else those flames might go. Passing a flame across the surface breaks the chemical bonds that are present at the surface and adds oxygen there. The net result is more polar bonding sites, and the glue sticks a lot better.

Roughening two surfaces to be bonded yields more bonding sites where the hardened glue can achieve a mechanical bond. Cleaning the surfaces of dust and dirt can improve wetting and provides the glue with a solid surface to work on.

One of the gluing problems for plastic parts starts during the molding of these parts. The the has to be designed so the parts can be removed when the injected molten plastic has cooled.

The plastic molder also may use something called processing additives. Sheep dip by any other name, this stuff is poured into the resin pellets to act as a lubricant to facilitate removal of the part from the mold. While it does perform that function, it could mean adhesion problems for glues or epoxies that may be applied later.

According to Doyle's paper, there are many surface preparation techniques that can be used to help give you the best possible bonding surfaces. If you're bonding to metal, the surface must be free of rust or oxidation. Aluminum surfaces may have to be acid-etched to remove its tough oxide layer. Ferrous ferrous (fĕr`əs), iron in the +2 valence state.

Containing or having to do with iron. The difference between ferrous and ferric is the number of valence electrons they contain (ferrous contains two and ferric contains three), which
 metals often have a residual layer of oil to prevent rust. Glue will not stick well to oily metal.

If you're gluing two very dissimilar materials like plastic and metal that have different coefficients of thermal expansion thermal expansion

Increase in volume of a material as its temperature is increased, usually expressed as a fractional change in dimensions per unit temperature change.
, you should use a softer glue that will allow each part some wiggle room wiggle room
Flexibility, as of options or interpretation: ambiguous wording that left some wiggle room for further negotiation.

Noun 1.

It's also possible to sand or abrade a·brade
1. To wear away by mechanical action.

2. To scrape away the surface layer from a part.

abrade (
 the workpieces prior to gluing. This not only cleans them but also can improve the bonding surfaces. Just be sure not to leave sanding residue, dust or dirt on the surfaces. Remember, this is all about clean surfaces. Before you reach for your air gun to blow-dry the part, remember that compressed air compressed air, air whose volume has been decreased by the application of pressure. Air is compressed by various devices, including the simple hand pump and the reciprocating, rotary, centrifugal, and axial-flow compressors.  often contains moisture and metal particles.

In cases where plastic surfaces are not conducive to molecular bonding, more aggressive surface preparation may be necessary. You must add more bonding sites. You can do this with spray cans of adhesion promoter, or the use of flame, corona, plasma or even - if you happen to have a device in your toolbox - laser treatment.

With a broken plastic component, it can be easy to look at the two clean, freshly fractured pieces and think, Oh well, I'll just glue them back together. This is where the residual processing additives can cause trouble. Not only are there residual materials on the outside of the product that helped in mold removal, but there s more of the same stuff locked inside the plastic. The plastic may have warmed while in service, causing the processing additives to "bloom" out on to the surface. It makes a difference only when you try to glue the pieces back together and the glue won't work. The solution is a dab of acetone acetone (ăs`ĭtōn), dimethyl ketone (dīmĕth`əl kē`tōn), or 2-propanone (prō`pənōn), CH3COCH3  to wipe away the contaminating con·tam·i·nate  
tr.v. con·tam·i·nated, con·tam·i·nat·ing, con·tam·i·nates
1. To make impure or unclean by contact or mixture.

2. To expose to or permeate with radioactivity.

 chemical you cannot see.

The nature of the joint itself also a factor in the success of your gluing efforts. You may find that it's possible to reinforce the joint or reduce the forces applied to the joint that caused the failure in the first place.

Probably the most important advice of all is to read the label on the tube or jar of glue. You need to understand what you're working on, what the right choice is for an adhesive and what preparation must be done to make sure it sticks.

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Author:Mike Dale
Date:Jan 1, 2009
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