The Culture made by Adhesives7th Installment

The Culture made by Adhesives7th Installment

Adhesives' ancient links with our lifestyle and culture

In our past two columns on adhesives, we covered some basic information on how adhesives work with examples which illustrate how extensively these products are used in modern society. We have also looked at the major role that viscosity plays in product development and manufacturing control of adhesives. On a slightly lighter note, we close out this series on adhesives.
Adhesives have been in existence since ancient times. Discoveries of artifacts (dating to approx. 2700BC) from the world’s oldest civilization - the Sumerian culture in Mesopotamia - included items of shell and blue lapis lazuli (lazurite) which had been bonded together with pitch. The pitch used is assumed to be naturally occurring asphalt and as such asphalt is viewed widely as the world’s oldest adhesive. Asphalt is a product of crude oil and is similar to the coal tar byproduct of the coking process of coal.
Asphalt is also referenced in the Old Testament in ‘Noah’s Ark’ and the ‘Tower of Babel’. It supposedly was used as a sealer for the Ark and as a glue for the bricks used in the building of the Tower - applications which still have relevance in the present day.
In Japan, asphalt as an adhesive appeared among the historic remains of the Jomon Period. Earthenware containing asphalt was uncovered at the Osaka Uemichi Ruin in Niigata Prefecture (approx. 4,500?, 2,500 years ago). At other sites, arrowheads and lance points unearthed yielded evidence of asphalt which was used to attach them to the shafts. Adhesives were also used to repair earthenware.
Asphalt flows naturally from the ground and it is apparent that the area in Japan where this substance could be obtained is the region of crude oil deposits in present day Akita and Niigata Prefectures. This is supported by compositional analysis of asphalt found at historic sites in Hokkaido which confirmed the asphalt’s Akita origin. This also evidence that asphalt was transported widely from Honshu to Hokkaido in earthenware jars during the Jomon Period, an example which may shed further light on the extent and influence of this early culture. Thus it seems that along with function, ancient asphalt has also transcended time to provide us with something of cultural value.
As we descend in time, Europe following the Stone Age, saw extensive use of asphalt not only as an adhesive, but also as a paving material for roads. A culture based on woodcraft developed in Asia and, instead of mineral-based asphalt, there was widespread use of adhesives derived from plants such as lacquer and starch and glues of animal origin.
In Japan, lacquer evolved not simply as an adhesive, but as an essential material for crafts. Lacquer, like asphalt, was used as an adhesive for arrowheads and other items but its unique coloring has also been exploited since ancient times as a coating for handcrafted works of art. In fact the term “japan” is now synonymous with the best in Japanese lacquer ware in a similar manner that “china” has come to symbolize porcelain.

The hidden but key role of adhesives in the growth of industrial output

Metal, during the growth of the machinery industry during the Industrial Revolution, was used not simply as a material (for containers and the like) but for parts of components which were assembled together to provide function in mechanical devices, devices which successively led to the creation of other machines. During this evolution in the use of metals, adhesives which were in use up until that time could not provide sufficient bonding strength required by such metal tool parts and this stimulated the development of new joining techniques for metal parts - techniques such as welding processes which involved the melting of base material; swaging where the metals were bent and folded together; and the use of rivets and nuts & bolts. These techniques remain as standard practices in the metal industry even today.
The aftermath of the Second World War saw dramatic progress in research on materials (e.g. molecular structure). Techniques developed enabled creation of man-made synthetic materials which, together with traditional plant and animal-based and mineral-based materials, resulted in a profusion of materials available for potential use in a wide range of applications. As new materials based on synthetic polymers replaced metal and found use in an increasing variety of products, adhesives reemerged as an important building component - a return which involved further advancements in the state of the art of adhesives. Using screws to join hard pieces of plastic may not provide an optimum solution because of cracks or damage to the material that may result. Traditional adhesives such as starch-based glues however lack sufficient strength for such types of application and the situation spawned further development of new adhesives and bonding techniques compatible with these new materials.
As expressed in the phrase, “necessity is the mother of invention”, development in new materials has thus coincidentally given rise to adhesive products which are capable of meeting the bonding requirements of such materials. In addition to bonding strengths greater than what had been previously achievable, these adhesive products must now provide other functions as well. Indeed it may be apt to describe this trend as a “revolution in bonding” as it is having a profound industry wide impact on products as well as production processes.
The airplane is a good example which illustrates this point. Adhesives are extensively employed in the assembly of airframes. These adhesives are of course, superior in terms of strength and resistance to deterioration. Until such adhesives became available, methods such as welding, riveting, and bolt & nut joints, etc., were employed to join metal parts of the airframe. There were drawbacks with these methods however. Welds deteriorate and spot joints created by rivets, bolts & nuts tend to create points where stresses concentrate (the no. 1 cause of metal fatigue) and they also tend to loosen over time. These techniques also add weight. Joints created with adhesives however allow dissipation of stress over a wider area. Adhesives also make possible the bonding of metals with non-metallic materials and they offer other advantages which enhance safety as well as provide other benefits. Less use of welds, rivets, and nuts & bolts in addition to use of non-metallic materials and thin sheet metals mean lighter aircraft. Of course lighter weight of aircraft and mobile objects allows higher speeds and energy-savings and provides other basic performance advantages and this ultimately translates into an improvement in service.
Another important advantage adhesives provide is freedom in design. The development and production of the automobile you love with its sleek lines might just have been made possible because of adhesives.

Evolving functions and issues pertaining to adhesives

As we have seen, there has been a transformation in traditional product specifications and manufacturing processes attributable to the progress in adhesives. As utilization of adhesives increased however, there have been additional demands on these products to provide functions in addition to bonding performance in a manner similar to other component parts. Adhesives which can respond to a variety of needs are referred to as ‘functional adhesives’. Structural adhesives used in construction of aircraft and automobile airframes and bodies must not only be able to withstand great loads but must also resist the effects of heat and chemicals, requirements which place these adhesives into one category of functional adhesives. Improved adhesive resistance to heat allows separate machining of parts of a complex mold which can then be joined together with such adhesives at a great reduction in cost.
With increasingly severe demands being placed on adhesives - such as in the bonding of parts which can withstand extreme loads, the bonding of micron-level parts, and joining of concrete parts underwater, etc. - accurate quantitative control of adhesive functions becomes imperative. Bonding speed is one such control factor. Fast curing instant glues are a typical example. Another factor is the manner in which adhesives harden. Timing of the curing process might be based on factors other than the time elapsed from when the adhesive is spread. The curing process could instead be ‘triggered’ by a condition more appropriate to the application such as contact between the parts joined, or exposure to ultraviolet or electronic beams, for example.
A third factor is the post-cure characteristics of the adhesive. For example, in lens manufacture, the degree of transparency or refractive index would understandably be important for the adhesive to meet its intended purpose. In aerospace applications, resistance to heat may be critical. Compatibility with very low temperatures would be important for liquid gas storage tanks and depending on the application, other requirements such as electrical conductance and elasticity may also be demanded. To insure that adhesives can meet the required specifications, control of their performance is critical.
As capabilities of adhesives expand to meet increasingly demanding specifications, these products are finding use in development and manufacture of a broader range of products. In recent years, applications have moved beyond industrial products to include medical uses (e.g. for tissues and in dentistry), for example. In such manner adhesive products play a strong, though perhaps less than visible, role in support of the functioning of our society.
Finally, let us return to the issue of the impact progress in adhesives has had on the environment. In the first column (Column 5) in this series on adhesives, the issue of the ‘sick house syndrome’ which is caused by volatile organic compounds (VOC) found in adhesives and building materials was raised. In addition to VOC and relative to adhesives in general, other problems which need to be addressed include proper disposal of adhesive solutions and effluents and disposal of lead which is a component of solder used in semiconductor packaging. Activity underway in effort to solve these problems include changes to more ecologically compatible water-based, water soluble, and biodegradable polymeric type adhesives and development of metallic nano-particle electrically conductive pastes which are used in thermoplastic electrically conductive adhesives as a replacement for solder.
An emerging trend with environmental implications is the increasing emphasis being placed on the ‘strippability’ of adhesives. Domestic manufacturers have begun to actively embrace Life Cycle Assessment (LCA) which takes into advance consideration the dismantling and tearing-down of products to the smallest possible parts level in order to facilitate their reuse and disposal. Parts assembled with adhesives that can be easily removed is thus a focus of recent research. This requires that adhesives provide secure and strong bonding function under certain conditions but also allow easy removal when necessary - a seeming contradiction in needs at first glance. For an industrial society increasingly dependant on adhesives however, this is a crucial issue and one which is spurring development of new adhesives - products such as heat strippable epoxy resins, heat expanding microcapsules, and adhesives with resin mixtures which harden when water is absorbed.
Adhesives will continue to evolve as new materials are developed and applications grow in sophistication. With regard to adhesive product development and quality control, viscosity measurement is expected to play an increasingly larger role in general adhesive property testing (e.g. in accordance with the standard, General Testing Methods For Adhesives, JIS K6833, currently under revision).

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