Bonding

Here's a link for a site which can check various compatibilities for adhesives.  We haven't actuall run this but it looks promising.  Just Click on the image.

or..

3M Industries. Automotive repair and care - Adhesives

Although technical, there is much to be learned here.  We figured this may give some of you a better idea of what you can use for bonding various products and materials.

1.) Bonding Improvement Suggestions

1. Prime the surfaces of the layers to be bonded, i.e., cover the surfaces with a dilute solution of the adhesive mixed with an organic solvent to obtain a dried film thickness between 0.0015 to 0.005 mm (0.00006 to 0.002 in) and cure separated before applying the adhesive to bond the layers together.

2. Consider another adhesive. Epoxies are best, but an alloy such as epoxy-phenolic or epoxy-polysulfone may offer improved peeling resistance.

3. Dilute the adhesive until it has a lower surface tension than either of the bonding surfaces. The surface tensions can be compared by using a wetting test, i.e., by wetting the surface with the adhesive and measuring the "beading" angle. A low beading angle indicates good wetting and an appropriate adhesive. See Figure 2.1.

Figure 2.1 Beading/adhesion relationship

2.) Bonding Adhesives for the Motor System

Theories of Bonding Summary:

Many theories have been developed to explain the process of bonding in adhesive structures. Individually, each of these theories is inadequate to describe the complete process of bonding in most situations. However, each theory contributes an understanding of the overall process of bonding and therefore are important. Figure 3.1 illustrates the five predominant mechanisms of adhesion.

Figure 3.1 Dominant mechanisms of bonding )

Mechanical Bonding

According to the mechanical bonding theory, an adhesive must fill the valleys and crevices (the asperities) of each adherend (body to be bonded) and displace trapped air to work well. Adhesion is the mechanical interlocking of the adhesive and the adherend together, and the overall strength of the bond is dependent upon the quality of this interlocking interface. To this end, chemical or physical abrading is highly recommended for optimum bonding. Abrading the adherend (1) enhances the mechanical interlocking, (2) creates a clean corrosion-free surface, (3) forms a chemically reactive surface, and (4) increases bonding surface area. Diffusion bonding is a form of mechanical interlocking which occurs at the molecular level in polymers.

Adsorption Mechanism Theory

The adsorption mechanism theory suggests that bonding is the process of intermolecular attraction (van der Waals bonding or permanent dipole, for example) between the adhesive and the adherend at the interface. An important factor in the strength of the bond according to this theory is the wetting of the adherend by the adhesive. Wetting is the process in which a liquid spreads onto a solid surface and is controlled by the surface energy of the liquid-solid interface versus the liquid-vapor and the solid-vapor interfaces. In a practical sense, to wet a solid surface, the adhesive should have a lower surface tension than the adherend.

Electrostatic Theory

Electrostatic forces may also be a factor in the bonding of an adhesive to an adherent. These forces arise from the creation of an electrical double layer of separated charges at the interface and are believed to be a factor in the resistance to separation of the adhesive and the adherend. Adhesives and adherends that contain polar molecules or permanent dipoles are most likely to form electrostatic bonding according to this theory.

Weak-Boundary Layer Theory

This theory has been developed to explain the curious behavior of the failure of bonded materials. Upon failure, many adhesive bonds break not at the adhesion interface, but slightly within the adherend or the adhesive, adjacent to the interface. This suggests that a boundary layer of weak material is formed around the interface between the two media. Impurities in the bond and adverse chemical reactions are common causes of weak boundary layers.

Mechanisms of Failure in Bonded Joints

Mechanisms of Failure

The two predominant mechanisms of failure in adhesively bonded joints are adhesive failure or cohesive failure. Adhesive failure is the interfacial failure between the adhesive and one of the adherends. It indicates a weak-boundary layer often from improper surface preparation or adhesive choice. Cohesive failure is the internal failure of either the adhesive or, rarely, one of the adherends. The difference is illustrated in Figure 3.2.

Ideally, the bond will fail within one of the adherends or the adhesive. This indicates that the maximum strength of the bonded materials is less than the strength of the adhesive strength between them. Usually, the failure of joints is neither completely cohesive nor completely adhesive. Measurement of the success of a particular joint is based on the relative percentage of cohesive failure to adhesive failure.

Causes of Premature Failure:The precise cause of premature failures in adhesively bonded joints is difficult to determine. The variety of causes include adverse stresses (peeling, for example), rate of application of stresses, fatigue, temperature, humidity, and solvents. Tests are required to evaluate the cause of adhesive failures for most bonded systems.

Figure 3.2 Adhesive and cohesive failure of bonded joints

Key Requirements for Quality Bonding

All foreign materials, such as dirt, grease, cutting coolants and lubricants, water or moisture, and weak surface scales (e.g., oxides, sulfides), must be removed. Thorough cleaning with various physical or chemical processes removes these contaminants and conditions the surface for bonding. The process of cleaning the surface usually involves several of the following three steps: (1) solvent cleaning; (2) intermediate chemical and/or mechanical cleaning; and (3) chemical treatment. Priming may also be necessary.

Solvent cleaning is the process of removing soil from the surfaces from the adherend using an organic solvent without physically or chemically altering it. The following four processes employ solvent cleaning in an increasing order of severity:

A. Vapor degreasing. Hot solvent in vapor form condenses on the object to be treated and flows away debris.

B. Solvent wiping, immersion, or spraying.

C. Ultrasonic vapor degreasing. Ultrasonic excitation of thin layer of solvent on object. Cavitation of bubbles on surface of object provides scrubbing action. 

D. Ultrasonic cleaning in solvent. Ultrasonic excitation with subsequent solvent rinse of object.

Intermediate treatment of the surface often includes grit blasting, wire brushing, sanding, abrasive scrubbing, scrapping or filing, and alkaline or detergent cleaning. These cleaning methods may be aggressive enough to remove some of the parent material.

Chemical treatment commonly includes acid or alkaline etching of the adherend surface. Etching the adherend removes stubborn oxides and roughens the surface on a microscopic scale. Solvent cleaning should always be performed before chemical treatment.

Occasionally, priming the surface may be necessary. Priming is the process of applying a dilute solution of the adhesive mixed with an organic solvent on the adherend to a dried film thickness between 0.0015 to 0.005 mm (0.00006 to 0.002 in). Priming protects the surface from oxidation, improves wetting, helps prevent adhesive peeling, and serves as a barrier layer to prevent undesirable reactions between the adhesive and the adherend.

Adhesive Selection

Most adhesives in use are actually a combination of components each designed for different functions in the bonded joint. An adhesive can contain any number of the following components: (1) the adhesive base or binder; (2) a hardener (for thermosetting types); (3) accelerators, inhibitors, or retardants; (4) diluents; (5) solvents; (6) fillers; and (7) carriers or reinforcements.

The adhesive base is generally the primary component of any adhesive and is the constituent from which the name of the adhesive is derived. A hardener is a substance added to the adhesive that causes a chemical reaction which cures the adhesive. Accelerators, inhibitors, and retardants control the adhesive's rate of curing. Diluents are liquid components that are added to the adhesive to reduce the concentration of the adhesive base. They usually reduce the viscosity of the adhesive. Solvents are closely related in that they thin the adhesive to make it more spreadable. The difference is that solvents disperse the adhesive base. Fillers are included in the adhesive if the adhesive properties are insufficient alone. Often, fillers are used for reducing cost, improving structural strength, encouraging conductivity, and other similar applications. Carriers or reinforcements are used to support the adhesive during application or storage.

Adhesives can be classified into a number of different categories, but the most convenient system is categorization by chemical composition. Figure 3.3 shows a summary of major structural adhesives categorized by the Society of Mechanical Engineers. Figure 3.4 and 3.5 tabulate the appropriate adhesives to use for particular adherends.

Figure 3.3 Categorization of Structural Adhesives )

Figure 3.4 Metal-adhesive compatibility chart)

Figure 3.5 Non-metal/adhesive compatibility chart)

Notice from Figures 3.4 and 3.5 that the only adhesive judged to be compatible with both stainless steel and ceramics is epoxy, currently used in the piezoelectric motor. Some variations on the basic epoxy adhesive system exist. Epoxy-polysulfone and epoxy-phenolic, both thermosetting alloys, are other possible candidates for the piezoelectric motor system. These two alloys have superior bonding characteristics with ceramics.

Adhesive Types

Aliphatic Resin -- This is the typical high strength wood glue. The yellow is preferred because it indicates a waterproof glue. Examples would be Sig's Sig-ment, or basically any "carpenter's glue". This stuff is dope-proof, fuel-proof and fully sandible. Great for sheeting, however it requires the patience of the old "pin-and-wait-overnight" construction method.

Cyanoacrylate (CA's) -- An instant cure adhesive of various viscosity and gap filling capability. Special formulations are available for plastics and foam. This glue creates strong, virtually instant glue joints, but is not recommended for firewalls as the nitro in glow fuel is a CA solvent. It should also not be used in areas subjected to a lot of flex or excessive vibration as the glue joints created with CA have a tendency to be a bit brittle. Comercial examples include Carl Goldberg's Jet, and Pacer's Zap. Odorless CA's such as the UFO series must be used on foam, and are also recommended for people allergic or hyper-sensitive to CA fumes. You can also purchase cure-accelerators for those of us who find that "instant" just takes too long.

Epoxy -- This is a two part glue which is sold with various cure times and formulations. The two parts must be mixed to cause a cure. As a rule, generally faster curing epoxies tend to become brittle with time. Slow curing epoxy, in contrast, will tend to remain fairly flexible. Use this glue in all high stress areas such as attaching the firewall, landing gear blocks and tail feathers. Commercially available examples include the Hobbypoxy lineup as well as everybody else's house brands. It can be easily cleaned up with alcohol before it cures and is often thinned with alcohol and painted into engine bays and fuel compartments to help fuel-proof the plane. Smart people do this and recommend to all other modellers that they should also do this. Stupid people don't, and eventually their engine flies off with the firewall, leaving the plane behind.

Contact Adhesive -- Can be latex or solvent cement. Use latex when sheeting foam or, if the solvent is not fully dried before joining the sheeting to the foam, any curing fumes may eat away at the foam. This type of glue is applied to both parts being joined and is almost dry on the individual parts before they can be assembled. Once joined, they're joined. Sig sells a latex-based product called Core-bond that's great for sheeting foam wings but it can't be frozen so therefore the garage is not a good place to store this stuff if you live in northern climes.

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