The elementary knowledge of adhesives5th Installment

The elementary knowledge of adhesives5th Installment

It can certainly be said that we are in the age of the making of things - from building structures and automobiles, to clothing, appliances, and even foods (prepared foods). Fast and cheap are key words among suppliers in their competition to create and bring to market quickly lower cost products which maintain standards of quality. Adhesives technology plays a key role in this process. Broadly speaking, adhesives are substances which consist of various kinds of polymers dissolved in a solvent. As readers of our previous columns may surmise, viscosity is an integral property of adhesives and viscosity measurement performs a key function in development and manufacturing quality control of these products.
Let us take a look at how viscosity is associated with adhesives and products around us, first, by reviewing some related fundamentals.

House building with adhesives?

Almost all of the products around us - whether they be electrical appliances, automobiles, buildings, or articles of clothing - consist of separate parts or components which are assembled together. Obviously, if the assembly times of such products could be reduced, there would be significant advantages gained in terms of costs and delivery time.
House building is a good example. Formerly, houses were hand constructed by skilled carpenters and artisans. Such creations may have significant value but a lot of cost and time are involved. If we think of a house as a commercial product, it would be in the builders interest to consider methods and techniques which would reduce his costs in order to remain competitive.
Here, adhesives can provide such a competitive edge. Separate parts and components can be prefabricated into easer-to-manage units which can be simply assembled on-site with adhesives, without any additional processing or working. The extremely great strength and durability offered by modern adhesives makes this possible.
A good illustration of this is floor construction. Laying of parquet floors by the traditional method involves precise positioning and end nailing of wood slats. It would obviously be more efficient to lay similar flooring as large sheets of wood using adhesives if a similar level of quality can be achieved. Adhesives in fact, are used extensively in many aspects of modern home construction - in wallpapering, for base boards, floor joints, door frames, and bathroom tiling as well as in building framework and exterior wall construction.
The use of adhesives in home building has not been without any negative consequences however. Adhesives have been linked, for example, to the sick house syndrome where formaldehyde and other substances used in adhesives and building materials have been found to emit volatile organic compounds (VOC) which trigger psychosomatic disorders such as headaches and nausea in sensitive people. In response adhesive manufacturers have been working to develop safer products which eliminate or reduce concentrations of harmful ingredients such as formaldehyde.

How do adhesives work?

Before delving into the subject of viscosity measurement, let us look a little at how adhesives work. Adhesive bonding is described as the joining of two surfaces by chemical or physical forces or both. Three bonding principles are involved - mechanical bonding, physical interaction, and chemical interaction.
Of these, physical interaction is most basic and it pertains to intermolecular attraction (Van de Waals attraction force) between the materials being bonded (adherend) and the adhesive. As molecular attraction is stable the materials do not separate.
Mechanical bonding is perhaps intuitively, the easiest to understand. Adhesive flows into microscopic holes of the surfaces of the adherend and hardens to hold the material together - a process referred to as anchoring. Various principles such as these are involved in adhesion, and as such, one cannot tell simply from looking at the object which bonding mechanisms are at work.
What kinds of adhesives are there?
Generally, adhesives are categorized according to the principle ingredient that is dissolved (or mixed) in the solvent base. This includes whether the ingredient is organic or inorganic; if organic, whether it is natural or synthetic; and in the case of synthetic adhesives, the properties of the polymeric material involved. With reference to our basic understanding of adhesives and viscosity, we can categorize these products according to how they harden.

(1) Volatile solvent type adhesives
These adhesives harden as the solvent vaporizes. With the organic type solvents which predominate, VOC is generated during this process.

(2) Water evaporation type adhesives
These adhesives harden as water in the adhesive evaporates or is absorbed. Wood glue is a typical example of this kind of adhesive. There are emulsions and latex types of wood glues. Emulsions are solutions like mayonnaise where the oil and water remain in suspension. Interfacial active agents act to disperse polymeric substances in water and maintain their stability.

(3) Hot melt adhesives
Hot melt adhesives harden when cooled. Glues used in book binding, solders, and asphalt are some examples.

(4) Chemically reacting type adhesives
These adhesives harden due to moisture, addition of catalysts, etc.

Development of adhesives with superior application characteristics

As a major technique employed in the making of things, adhesives must now provide not only the ability to simply bond things together but offer other functions and properties as well. These include, for example, different curing speeds and specific post-cure properties. Sealing materials used in curtain wall construction is one variety of adhesive. Curtain wall construction involves fixed form parts which are assembled on a frame to create exterior walls. All glass-walled buildings, etc., are typical examples of this type of construction. The sealing material, in addition to assuring a waterproof seal, must also have the ability to follow the contours of the structure, possess intrinsic durability, be cost effective and of high quality.
In addition, such sealing materials (as well as adhesives in general) must also allow efficient application - in other words, they must be easy to work with when spread or poured.
Consider the application of adhesive on a vertical surface. We may understand the difficulties encountered if the adhesive drips and runs. On the other hand, although a non-dripping adhesive is good, a problem also arises if too much effort is required to pour or apply it. Thus adhesives must be easy to work with in addition to providing good bonding function. Optimum material consistency is an important characteristic in this regard and in most cases this requires the maintenance of proper viscosity. It is here that viscosity measurement plays an active part.
Our next installment will focus on the key role viscosity measurement has in the manufacture and process control of adhesives.

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