Story Source: Materials provided by Duke University. Journal Reference : David R. Samson, Charles L. Sleep intensity and the evolution of human cognition. ScienceDaily, 14 December Duke University. Humans evolved to get better sleep in less time: Humans sleep shorter, deeper than our closest animal relatives. Retrieved July 8, from www. Little is known about what genetic or molecular forces drive the need to The new knowledge may be important for estimates of when the common ancestor But now researchers have found a way to capture detailed The observational study in nearly 13, people revealed different patterns of sleep disturbance between Michael is an introvert, preferring to read a book to playing with others.
Mary is the smallest in her class but won't back down from a fight. And David is an amiable kindergartener. They are just normal kids until one afternoon at Judah's soccer game they and everyone in the field suddenly, unexplainably fall asleep. Waking three days later, no one can explain what's happened to them or begin to imagine what's been set in motion. Now they and their friends are something more than just normal kids.
Dubbed "Sleepers" by the press, they find that with the aid of metal bands called bonders they have incredible powers But something else happened when they fell asleep Now their teammates are selling a new designer drug called Ozone at school, Sleepers are disappearing and their friends are going mad. Will these once normal kids be able to stop the darkness descending on them Read more Read less. No customer reviews. Share your thoughts with other customers. Write a customer review. Most helpful customer reviews on Amazon.
February 17, - Published on Amazon. I love genre defying novels like this. Science Fiction. May it contribute greatly to the advancement of adhesive bonding technology. Paul L. According to DIN EN , an adhesive is a nonmetallic substance capable of joining materials by surface bonding adhesion , and the bond possessing adequate internal strength cohesion.
Bonding is a material joining technique that, in the traditional sense, cannot be broken without destruction of the bond. Bonding is by far the most universal joining technique. Virtually all technically useful materials can be joined with each other, and one with another, by means of this surface-to-surface and material-joining technique.
Historically, bonding has long been recognized as a high-performance joining technique. The large majority of original natural binding materials have now been replaced by synthetically prepared adhesives. Meanwhile, high-strength adhesive assemblies have been created with quite short curing periods. In fact, the longstanding problem of extensive curing times necessary to obtain highstrength joints has been almost completely resolved with the introduction of new chemical developments in the creation of adhesives.
For example, one would not consider bonding a steel bridge or a gantry, but for the lightweight construction of car bodies using steel, aluminum, glass and plastics, adhesive joining offers extremely interesting applications. Adhesive joining is particularly well suited to the joining of large-sized surfaces of different materials, such as in the construction of sandwich assemblies. The possibilities, advantages and disadvantages of adhesive bonding compared to other joining techniques are summarized in Table 1.
As a result, the material structure of the adherents to be joined is not macroscopically affected, and deformations or internal stress — which generally are related to the application of heat — rarely occur. From this point of view, there are no limits with regard to the combinations of materials that can be joined. One important disadvantage of adhesive bonding, however, is the relatively poor heat resistance of the bond-line as compared to inorganic materials such as metal or glass.
This applies not only to the manufacturing sequences but also to the ambient conditions in which the joints are produced, because adhesion generally develops only during the production process, and the production parameters can have a decisive effect on the quality of the bond. The same more often than not applies to the cohesion of the adhesive layer.
The technical properties of cohesion only develop during the course of the production process with the exception of pressure-sensitive adhesives after 1 Adhesive Bonding as a Joining Technique different setting processes. As the mechanisms of adhesion and the long-term behavior of adhesives are not yet completely known, it has not been possible to develop strict mathematical models for adhesive joints. The discovery of a wooden, gold-plated Ram statue that contained adhesive made from asphalt indicates that, in Ur, about years later, glue was used for the manufacture of statuettes.
Gluing was also known to have been used in Egypt in BC, since in the tomb of Rekhmara in Thebes the process is depicted on a mural painting dating from that time. The ancient Egyptians presumably used animal glues.
In the tomb of Nebanon and Ipuki, which dates back to the same period, another painting shows the gluing of a shrine. A glue tablet was found in a cave at the upper level of the mortuary temple of Queen Hatshepsut at Deir-el-bahari, i. In the Talmud — a collection of Jewish after-Bible laws and religious traditions — reference is made to casein that had been used many years previously by the Israelites as a binder for pigments, although the large-scale use of casein glue was not introduced until much later.
Glue was also clearly well known in Greece, as the famous legend of Daedalus and Ikarus — which falls into the period between and BC — is based on the failure of adhesive bonds produced using wax Figure 2. He noted that it was remarkable that the wings of the door were left in the glue clamp for four years. The same applies to stone which would never adhere to wood. It can be concluded, however, that the experience gained with adhesives was quite positive. In — AD, Theophilus, in his books De diversis artibus, described different adhesives and mentioned casein glue which, apparently, was known of by the ancient Israelites.
The girders of these airships were made from wood glued with casein see Figures 2. The rough and damp environment, in which naval airships were operated, often resulted in a failure of the glued joints. Their resistance was only slightly improved even when the glued assemblies were exposed to formalin vapors as postcure treatment see Section 5. The bonding industry began to develop rapidly during the seventeenth and eighteenth centuries Figure 2. This was published one year later in Germany, with the title Die Kunst verschiedene Arten von Leim zu machen The art of making different kinds of glue by the Prussian Academy of Sciences.
In his treatise, Duhamel du Monceau provides a variety of recipes for the manufacture of different types of glue, and notes that garlic can be used as adhesion promoter when rubbed on wood before applying the glue. Figure 2. It must be said that the development of adhesives took a long time, despite their widespread use. However, more important progress was made in the nineteenth century, when self-adhesive substances and tapes for use in medicine were developed and employed from the middle of the century onwards . Horace H.
Day was the inventor of pressure-sensitive adhesives based on natural rubber, while in William H. Shecut and Horace H.
Day were granted a US patent for the improvement of adhesive tapes . In Germany, a patent for a tape coated with a pressure-sensitive adhesive based on natural rubber was granted to the druggist Paul C. Beiersdorf in For a long time, early pressure-sensitive adhesives were unable to meet the requirements of industrial applications, although their properties could be improved by these inventions and by various other developmental activities.
The breakthrough was achieved by Richard G. Whereas, until the 2 The Historical Development of Adhesive Bonding s, pressure-sensitive adhesives did not attract much attention, today they have one of the most important growth rates of all groups of adhesives see Section 5. However, the age of synthetically produced polymers truly began when Leo Hendrik Baekeland was awarded a patent for phenolic resins in In , Victor Rollett and Fritz Klatte were granted a patent for the production of polyvinyl acetate PVA , a synthetic raw material that is widely used to the present today, despite not gaining commercial importance until the s.
Although, urea resin had been known since , it was not until — when a curing technique was developed — that it was used in the production of adhesives. According to present knowledge, it was possible to achieve bond lines of high durability. The same effect was achieved later with similar cold-curing systems, many of which are still in use today, having largely displaced the former casein and blood albumin glues see Section 5. These resins contributed greatly to the improvement of aircraft structures, and are still appreciated today for their extraordinary durability, especially in case of aluminum bonding see Section 8.
This was the merit of Eduard Preiswerk who, in , discovered that epoxy resins cured with phthalic anhydride, produced synthetically by Pierre Castan in , offered a wide variety of possibilities to create bonded joints, even between metals . Both, hotcuring and cold-curing epoxy resins have been considered as standard products of structural adhesive bonding technology see Section 5.
The patent for polyurethanes, awarded to Otto Bayer as early as in , marked a milestone in the history of adhesives. It is self-evident that this short historical review cannot be complete. Hence, further detailed information with regard to the development and properties of the different groups of adhesives is provided in the following chapters. It is one of the most important material phenomena in nature and technology. In the realms of technology, a building erected by the Romans will still safely be held together by the adhesion between the mortar and bricks. However, a car tire will only function if adhesion holds together — in an absolutely safe manner — the rubber and fabric made from organic substances or steel cords.
If adhesion is considered as the sine qua non of all forms of adhesive joining techniques, it is reasonable to limit the general view of adhesive phenomena to the study of organic substances which, in most cases, are higher-molecular materials the majority of adhesives , technically useful inorganic materials such as metals, glass, stone and ceramics, and organic materials such as plastics, wood and textiles. Many years of experience gained from adhesion research have shown that it is wise to investigate the different aspects involved in the creation of adhesive joints on the one hand, and their behavior within the assembly on the other hand.
Strictly speaking, adhesive systems virtually can never be destroyed nor fail where adhesion has built up in advance.
Other, more important, distances can only be covered by adhesive interactions in the form of charge transfers that can contribute to the formation of an electrical bilayer. The orders of magnitude of bonding energies that occur in physical bonds, hydrogen bridge bonds and chemical bonds are listed in Table 3. When the adhesion forces are calculated from these bonding energies, the theoretical values given in MPa in relation to a given surface unit are, in most cases, clearly higher than the strength of adhesives made from organic substances. Their theoretical strength, which is calculated from the bonding energy of carbon—carbon bonds, is expected to have an order of magnitude of MPa for a polymer packing with the highest density.
Of course, the values of strength actually measured not only in adhesive assemblies but also for polymer materials, are lower than those calculated from theory because no ideal material combinations or structures are to be expected. If we suppose that the ratio between theoretical and practical values is approximately the same for both the adhesion and the adhesive, it is clearly possible to create a reliable bonded joint with the majority of potential adhesive interactions.
For this, the following additional basic knowledge is required. When creating an adhesive bond, no or only very little energy is added to the system. Wetting is a phenomenon that can be observed when a drop of liquid spreads over the surface of a solid-state material. According to the surface condition, the type of liquid used and the matter to be found in the environment which often is not taken into consideration , the drop of liquid forms a contact angle a between its surface and the surface of the solid-state material.
When the drop of liquid comes to rest, interfacial tensions are acting between the solid-state material and the environment, the liquid and the environment, and the liquid and the solid-state material, respectively. These interfacial tensions or energies thermodynamically determine the angle of contact. When the drop of liquid is at rest, there is equilibrium between the interfacial tensions.
With regards to the wetting criterion, inorganic substances do not represent any problems, provided that they are not covered with contaminants characterized by low surface energies. Table 3. It must be stated, however, that wetting is not the only prerequisite for adhesion. The reasons for this will be provided in the following section. In the following 3. The most important adhesion models are summarized in Figure 3.
The only possible approach was to calculate the energy balance of the process taking place when a solidstate material was wetted by a liquid, assuming that the wetting characteristics — which could be measured macroscopically — were ideal. This thermodynamic approach is based on the fact that, in terms of physics, the surface tension of liquids and solid objects is a type of energy; that is, work is done in order to create the surface of a material.
The aim of these calculations was to deduce the strength of adhesive assemblies from the work of adhesion. This is possible as long as there is no mixing of the substances during the wetting process — that is, there are no penetration or chemical interactions that would resulting in a reversibility of the wetting process. In this case, the work necessary to separate the two materials after wetting is equal to that liberated during the wetting process.
In reality, however, the energy balances involved in the creation and restitution of surfaces or interfaces cannot be applied, even if the interfacial tensions are subdivided into their dispersion force components and their polar force components. However, when the drop is lifted from the polyethylene surface, its surface tension will be similar to that of the polyethylene surface because, despite poor wetting, some slack polyethylene molecules from the solid surface are most likely sticking to the drop material at the moment it is stripped from the surface.
Furthermore, when explaining adhesion by means of the concept of interfacial energies, surface tensions are rather assumed to be material constants. It is assumed that, after separation of the surfaces, the surface tensions of the liquid and the solid material, respectively, are still the same as before wetting. However, present knowledge indicates that, for various groups of materials, this assumption does not correspond to reality, even if there are no components of one material sticking to the other.
Polymers in particular have a distinct ability of restructurization, as has been demonstrated in various recent studies . Restructurization means that polymers are capable of adapting their surface condition — and hence their surface energy — more or less rapidly to the ambient conditions that surround them, even in the solid state. Therefore, it is not logical to proceed on the 3. This may be easily demonstrated with the following model. A third, imaginary surface can be produced by cutting the cone into pieces with a knife. On those surfaces which formerly faced the container the polyethylene chains will be arranged in the form of loops, following the contours of the container.
But, on the surface exposed to the air, they virtually will not present any roughness at all; they may even have slightly begun to oxidize due to the presence of oxygen in the air — a process that takes place more rapidly in the molten state owing to an improved input of energy. Consequently, the polyethylene pieces produced have at least three different surface tensions. If this experiment is performed using containers of the same form, but made from different materials e. But this has been shown not to correspond to reality. By no means can the measurement of interfacial energies be used to explain adhesion.
However, this represented an important problem for adhesive research because the formation of chemical interactions between those different types of material had long been considered impossible. It was known during the late nineteenth century that in matter, physical, intermolecular forces existed as well as chemical interactions. To sum up, these intermolecular interactions have been attributed to the existence of permanent or oscillating dipoles, which interact one with another in chemically saturated systems, but which are also able to induce dipoles in other materials — a process that is particularly obvious in the case of metallic partners.
Those interactions are generally characterized by lower binding energies than chemical interactions; neither do they change the nature of matter to the same extent. Independently of the chemical compatibility, they can, however, take effect between materials of different types. Permanent dipoles: These are found in molecules in which one atom with a higher atomic number e. This homopolar linking is due to the fact that the statistical probability distribution of the electrons in the bonding orbital is shifted towards the larger nucleus, inducing electronegativity and hence a dipole.
The permanent dipole is able to build electrostatic attraction forces in the form of a dipole interaction with another permanent dipole; such forces can be calculated. Hydrogen bridge bonds are dipole—dipole bonds, and have been included in the schematic representation of Table 3.
Induced dipoles: In adjacent molecules without any own permanent polarity, permanent dipoles are able to induce counterdipoles with which they build up static attraction forces characterized by lower bonding energy compared to dipole—dipole bonds. As already mentioned, this approach especially plays a role when trying to explain the adhesion of organic substances with polar groups on bright metal surfaces. Dispersion forces: These may exist between molecules with nonpermanent dipoles, and are attributed to the fact that weak oscillating dipoles can be found between the involved atoms because, at least depending on time, the statistical probability distribution of the binding electrons is not completely uniform.
This in turn may induce weak interactions that take effect between all types of material, including also similar gas molecules. However, their bonding energy is generally lower than that of permanent dipole bonds. In contrast, an adhesive which generally contains polar groups can be shown to adhere better to a polar substrate or solid-state material e. However, in some cases, their bonding energy can only barely explain high-strength adhesion, and it must be appreciated that those bonds which are responsible for physical adsorption can also be destroyed as soon as materials of a higher polarity penetrate the system and break the adhesive bonds by competitive adsorption, taking the place of the polar groups of the adhesive.
Water also migrates to the adhesive zone and considerably impairs or even destroys adhesion based on physical bonds, a process which can often be observed macroscopically. Adhesion with a high resistance to water may occur, however, and is actually often observed between substances of very different types.
Therefore, it must be assumed that there are further types of bond that are exposed to a much lesser extent or not at all to the attack of water. Primarily, these are chemical interactions that will be discussed later. Occasionally, for example, if a polyethylene layer is applied onto a metal, the bilayer can even be observed in the form of an electron enrichment in the polyethylene layer .
It can easily be shown that electrostatic or luminescent phenomena are occurring when adhesive systems are separated. At peeling, there is a crackling noise, but if the peeling is performed more or less rapidly the crackling changes. If the assembly is joined together again, followed by a peeling-off, there is no longer any crackle.
Alternatively, in a photographic dark room, if a modern envelope where the overlaps are pasted together with a pressure-sensitive adhesive see Section 8. However, until now it has not been possible to provide unquestionable proof of the effectiveness of this bilayer in adhesion.
Although there is clearly no doubt that the bilayer exists, it probably does not predominate the adhesive strength in the majority of cases. Owing to their structure, physical low-energy interactions are thought to be effective here, while chemical interactions are hardly assumed to be involved. During the s, Voyutskii explained this phenomenon , his diffusion theory being based on a fact that has often been forgotten, or not been taken into account. Between the points of entanglement, an intense motion of the molecular chains takes place in the form of atomic rotation, at least above the glass-transition temperature, because the free volume allows them to do so, although the polymer as a material itself is in a state of stiffness and rigidity.
When this movement takes place, it cannot be excluded that molecular parts leave the original surface and diffuse into an adjacent polymer. This situation is very easy to demonstrate. You then place the rubber onto the body of a cheap ballpoint pen or if you have one an older fountain-pen made from celluloid. Here, the thermoplastic polymer layers with very smooth surfaces created from the liquid come into intimate contact with each other, and a seamless diffusion process between the molecules from both sides will take place see Section 4. The same applies to pressure-sensitive adhesives characterized by high molecular mobility, as both by means of yield effects taking place on rough surfaces and by diffusion processes taking place between polymers, they build up adhesion over the course of hours, days or weeks.
It appears, therefore, that diffusion theory may be used to explain some interesting adhesion phenomena. Later in this chapter, we note that the molecular mobility which occurs in solid-state polymers, as postulated by Voyutskii, might also be very important for other types of adhesive systems see Section 3. McBain and W. Lee  published the results of the following experiments. Polished steel and aluminum parts were joined together using an organic adhesive made from all components by weight : 50 units of shellac, 5 units of creosote oil obtained from wood tar , 0.
This resulted in remarkable strength values, which were clearly higher when thin adhesive layers j21 j 3 Adhesion 22 layers whilst, under humid ambient conditions, hydroxides could also be created that would be much more chemically reactive in terms of their chromatographic capacity.
The role of chemical interactions in the interpretation of adhesion phenomena changed greatly during the early s, however. Among the emerging highperformance bonding technology, the adhesion behavior of metal surfaces was of particular interest and again subjected to a variety of investigations. Sandstede et al. This indicated that, in terms of the Langmuir adsorption isotherm, acetic acid must bind irreversibly to the metal surface, a process which could only take place provided that a chemical interactions had occurred.
Later, Kautsky and Barusch  stated that aminoamide, when adsorbed onto an oxidized iron surface, also presented low desorbability. By using an infrared spectroscopy technique that had been specially developed for the investigation of interfaces [Fourier transform infrared spectroscopy FTIR had not yet been invented! As reported by Lewis and Forrestal as early as in , the adhesion of plastic materials e.
This provided further evidence for effects caused by the formation of chemical bonds at the interface between metals or their oxides and organic substances. As the irreversible sorption process can, in theory, be attributed to a chemical primary valency bonding to surface atoms , the above-described examples of chemisorptive bonds between organic materials and metal surfaces clearly give rise to the conclusion that chemical interfacial reactions do exist.
This is also valid when the heat of adsorption measured is lower than the energies of chemical bonds, since chemically active adsorbate molecules must dissociate before adsorption can take place, thus consuming a portion of the energy liberated in bond formation. Presumably, the heat of adsorption supplies the activation energy necessary to launch the formation of chemical bonds.
In fact, while adsorption takes place rather rapidly requiring only a few minutes , chemisorption takes longer between 20 min and 1 h . During the late s, Brockmann  extended the results obtained from adsorption and chemisorption experiments most of which were based on the measurement of sorption isotherms to adhesion taking place in bonded joints.
He has already shown that there was no true adhesion failure when metal adherents were separated from a cured phenolic resin adhesive, despite the macroscopic appearance that the separation occurred within the adhesive layer. Rather, by using available measurement techniques, the residues of the adhesive were seen to be spread over almost the entire metal surface. These results were explained by Brockmann on the basis of his investigations into chemical interactions.
In experiments with epoxy resins applied to aluminum surfaces as described in a later publication , Brockmann also detected chemical interactions that occurred to a lesser extent  than in the case of phenolic resins. Although, today this has been established as a foregone conclusion, it has not been shown clearly whether chemical interactions predominate the effect of the adhesion between solid material surfaces and nonreactive adhesives.
However, in order to determine the resistance of an adhesive joint against moisture, knowledge of the nature of the chemical bonds can be of decisive importance. If acid—base bonds, alcoholic bonds e. Chemical bonds only play an important role at the interface if they are resistant to hydrolysis e.
Indeed, this is the case if phenolic resins are used without any auxiliary agent, because hydroxymethyl phenol an important low-molecularweight component of this adhesive is able to form chelates with aluminum oxides or hydroxides by itself. This is considered the main reason for the high durability of aluminum joints bonded with phenolic resins see Sections 5.
In order systematically to optimize the adhesion process, it is essential to utilize currently available analytical methods for determining the chemical nature of substances present on the surfaces to be joined. It is important that such data are known if the type of chemical reaction that may occur with the binder is to be predicted. Similar procedures have been successful with other materials, for example on zinc. Both, reactive silanes and chelate complexing agents stearates lead to considerable improvements in adhesion and durability, and consequently reactive silanes have been used as adhesion promoters and mixed in various adhesives destined for the bonding of inorganic adherents see Section 5.
Even if these promoters do not present any groups that may react with the adhesive, they do improve durability. Organosilanes, for example, promote adhesion on a glass surface without presenting any organofunctional groups — that is, without being able to react chemically with the polymer. The same must be assumed for chelate complexing agents e. Their positive effect with regards to a higher durability may be due to reaction with the thermodynamically instable solid material surface of glass or even metals, thereby reducing instability.
This can easily be demonstrated on oxidized aluminum: if an alizarin chelate is created on pickled or anodized aluminum, scanning electron microscopy SEM can be used to show that, owing to the formation of chelates, the tendency of the oxide to hydrate in humid ambient conditions one of the most important factors which may lead to adhesion failure is drastically reduced.
Whilst adhesion and durability must be considered as a problem of pure adhesive bonding, it is also important — especially in the case of inorganic materials — to realize that the formation of chelates as well as the acidity or basicity of the adhesive system used may reduce or increase the instability of the layers covering the basic material. With weak acids, aluminum oxides resist hydration relatively well. Thus, even in the case of cured phenolic resin adhesives, weak acidity promotes not only chelate formation but also stability within the assembly by improving the resistance of the oxides.
The same applies to iron oxides that are very resistant to hydration under alkaline conditions pH 10— This is the reason that, as long as no acid rain penetrates the system, there is a good resistance of adhesion between concrete and Monier steel. Otherwise, the pH will fall to 10 or even 9. In fact, this is the most important reason why long-term damage has frequently been observed in concrete structures.
This can be 3. For example, if a sheet of gold is rolled up in normal writing paper and left for a certain time, the glue residues originating from the paper will adhere strongly to the gold surface such that it cannot be removed without causing mechanical impairment to the metal surface.
This phenomenon has been recognized for many centuries, one example being the process of goldbeating, where the metal leaves are stored between layers of Japanese paper that do not contain binding glue. The adhesion of gold leaf is very resistant, even in humid ambient conditions, despite the gold leaf itself being soft and having a low resistance. Consequently, the regilding of historical monuments is only necessary when erosion has led to damage of the gold leaf see Section 8.
Both materials can be carefully melted without considerably exceeding the melting point, such that the surface is prevented from oxidizing. A well-cleaned aluminum surface is then coated with the molten polyethylene. As might be expected, if the surface is well wetted the polyethylene can easily be stripped from the aluminum sheet, such that no polyethylene residues can seen macroscopically to remain on the sheet. Hence, it is concluded that although the polyethylene could be stripped off with ease, its residues remained on the aluminum surface, and that cohesive failure had taken place within the polyethylene near the interface between the two materials.
This situation cannot be explained by either of the adhesion theories described in Section 3. However, if the glass surfaces are bonded with acrylic pressuresensitive adhesives, even without using any adhesion promoter, a relatively high water resistance compared to structural adhesives is obtained.
This situation does not apply to adhesion, which common exists as a static, material condition. There is a clear relationship between the degree of molecular mobility, which in turn can be controlled by the degree of crosslinking, and water resistance. It can also be shown that the build-up and regeneration of adhesion — both of which factors are dependent on time and are very pronounced in the case of pressure-sensitive adhesives — support the validity of the hypothesis of dynamic adhesion.
Polymer dynamics, which appear to be a dominant factor in the adhesion capacity of pressure-sensitive adhesives, are most likely also essential for the adhesion of not only contact adhesives but also hot-melt adhesives, which generally do not form crosslinks or only very loose crosslinks and are not considered to present any chemical reactivity. It is remarkable that on surfaces which attribute a good age resistance to an adhesive bonded joint at least as far as we know , such as steel or aluminum surfaces blasted with corundum, the degree of crosslinking near the interface is less than on surfaces which attribute a poorer age resistance to the assembly.
The fact that better, more durable adhesion results from a lower degree of crosslinking is contradictory to standard concepts, as the latter is probably expected to involve a higher rate of water transport. It has long been assumed, and clearly demonstrated in some cases, that the material surface does have an effect on polymers, although the details of this phenomenon are not yet understood .
However, in the case of a lower degree of crosslinking a better polymer mobility can be postulated, which in turn has a stabilizing effect in terms of the dynamic adhesion mentioned above. Previously, adhesion was thought to be irreversibly weakened by humidity, but it is increasingly realized that this effect is in fact not absolutely irreversible. It is known, however, that nanostructured surfaces which can easily be produced on aluminum by pickling, or alternatively by anodizing on zinc or stainless steel, may have a considerable effect on adhesive strength, and particularly on durability when compared to non-nanostructured surfaces of an identical chemical composition.
This phenomenon cannot be explained by simple concepts such as the postulate of micromechanical adhesion because, once again, the failure of adhesion occurs not within the nanostructures but rather within the polymer near the interface. Thus, it is safe to assume that there is an effect of the surface morphology that can be j27 j 3 Adhesion 28 explained not only by the steric hindrance of adsorption or segregation within the nanostructures, but also by orientation effects.
The results of these investigations are presented in Section 7. It becomes clear that, when attempting to understand the build-up and behavior of adhesion, one reaches a dimension which lies between the molecular one and the dimensions of matter, the characteristic technical properties of which only take effect in case of a total quantity of approximately two orders of magnitudes higher than the molecular dimension.
Although it has become possible to analyze this dimension only recently, the prevailing — albeit sometimes unsatisfactory — uncertainty with regard to the behavior of adhesive systems is expected to be overcome within the next few years. However, it is not possible to determine the exact characteristics of adhesion by means of wetting measurements. In addition to physical interactions, it is particularly desirable to have adhesives especially reactive adhesives build up chemical interactions with the adherent surfaces in order to obtain good adhesion.
Chemical interactions modify the nature of the adhesive and the adherent surfaces, respectively, near the interface. Adhesion hardly ever fails where it has developed. Nanostructures or nanomorphologies on the adherents may improve the quality of adhesion, although the reasons for this are not yet known. While various attempts have been made to systemize adhesives, it is virtually impossible due to their nature.
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In contrast to other joining aids, such as welding electrodes, adhesives have not been standardized since, according to current knowledge, standardization would represent an inhibitory factor for further development. At present, there is available a most varied array of adhesives that often differ considerably one from another in terms of their processing properties, their strength and their durability characteristics. Within the adhesive layer, the material must have as low a molecular mobility as possible so as to allow tensile, shear and peel forces to be transmitted, respectively; that is, in technical terms, it needs to behave like a solid object.
A general survey of the types of adhesive available, based on their curing properties and mode of setting, is provided in Figure 4. This allows them to Similar to contact adhesives PVC copolymers Polyamide Adhesive dispersions Heat-sealing adhesives Hot-melt adhesives Physical and chemical curing Neoprene rubber Contact adhesives Physical curing Silane crosslinked polyester Acryle resins Neoprene rubber with isocyanate curing agent Pressure-sensitive adhesives with postcure Contact adhesives with addition of a curing agent EVA, acryle resins Pressure-sensitive adhesives No curing Typical base resin example Type of adhesive Mode of curing Table 4.
Pressure-sensitive adhesives are not only used in the form of adhesive tapes coated with binders on one or both sides, but also as transfer systems which are applied to the solid material, together with a base material which is later stripped off. Another possibility is to apply them to the adherent as aqueous dispersion, the aqueous component evaporating later. Finally, at present, other pressuresensitive adhesives are available which are applied to the surfaces as solvent-free or solvent-containing, low-viscosity systems and later are shortly irradiated with ultraviolet UV light or heated to develop adhesive capacity see Section 5.
Pressure-sensitive adhesives are generally applied to one of the surfaces to be bonded. A good bond is only obtained by uniform short pressing of the adherent surfaces, which must be exactly positioned because the bond has a relatively high strength immediately after the pressing. Pressure-sensitive adhesives have very good peel strength and, in comparison to chemically setting systems, a relatively low shear strength. The reasons for this behavior have been discovered only recently .
Processing aids are rarely needed, and any processing speed is possible. After joining, the bond is immediately ready for mechanical loading. By means of the addition of solvents, a low-viscosity state is obtained which allows the adhesive to wet the surface of the solid material. The adherents are pressed together as soon as the adhesive layers are dry to the touch. The strength of the bond can increase, often within hours, after the application of pressure.
If at least one adherent is porous, the adherents can be joined before the drying process has taken place at all or only partly, because the remaining solvent is capable of evaporating through the porous adherent. In this case, it is also possible to reposition the adherents after having joined them because, in the semi-dry condition, the adhesive still largely presents the properties of a liquid. In the dried or cured condition, the adhesive coat is generally in the thermoplastic state — that is, the molecules are not three-dimensionally crosslinked.