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One aspect of material behaviour that we have not addressed yet except in passing is the influence of lack of isotropy.Recall that isotropy means that the material stiffnesses are identical or constant(iso) in all directions when observed or measured at a point.That is, if we look in different directions from the same point in the material, do we perceive different stress-strain behaviour in some way, shape or form? What's the level of that dissimilarity? That level is reflected in the types of anisotropy that are observed. If the material is simply called anisotropic,
then in every direction we look, we'll perceive a different behaviour.No behavioural characteristic is similar from one direction to another.We'll always see behaviour that's totaly different and without any symmetries in direction.
Transverse isotropy is a simpler lack of isotropy than anisotropy.For this class of materials ,there is one plane in which the beviour is the same in all directions, but perpendicular to that plane or transvewrse to that plane, quite a different behaviour exists.
Those dissimilarities in anisotropic metarial behaviour in different directions can be material nonliearities, they can be simply young's moduli being different in different direcitons, or they can be strength differences or all the foregoing.Thus, the anisotropy desciption does not necessarily mean we're dealing with material nonlinearity, but simply a difference in characteristic material behaviour, what ever that characteristic might be: nonlinearity, modulus, strength, orwhat have you.
(Robert Millard Jones,Deformation Theory of Plasticity,p.68)
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Isotropy. It is the property of the materials by virtue of which the physical properties like elastic modulus, electrical conductivity, index of refraction etc., are independent of direction. In isotropic materials physical properties are same in all directions. In a polycrystalline material, individual grains orient themselves in different directions randomly and so the different physical properties are similar in all the directions resulting in isotropy. Anisotropy. The physical properties of single crystals of certain substances depends upon the crystallographic direction in which the measurements are taken. These physical properties like elastic modulus. electrical conductivity etc., are dependent upon the crystallographic directions and these properties have different values in different directions like [1 0 0] and [1 1 1]. This directionality of properties is termed as anisotropy and is associated with the variance of atomic or ionic spacing with the crystallographic direction. In a polycrystalline material composed of several grains if the grains orient themselves in particular direction (preferred orientation) then the properties would be different in different directions and it would lead to anisotropy and the material is said to have a "texture".
In a polycrystalline material even though each grain may be anisotropic but the poly crystalline aggregate composed of several grain behaves isotropically because of random orientation of different grains in the aggregate.
(A Textbook of Engineering Material and Metallurgy,Amandeep Singh Wadhwa,Er. Harvinder Singh Dhaliwal,2008, p. 87)
4)Hot Adhesives (00.27) [Group: Material]
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--A hot melt adhesive is defined as an adhesive applied from the melt and it gains strength upon solidification and crystallization. Hot melt adhesives are applied without solvents. The increase in solvent emission regulations has increased the demand for hot melt adhesives. Certain types of hot melt adhesives cure over time after application. However, with the general purpose hot melt adhesive, the material is applied as a thermoplastic melt and the resulting adhesive is also a thermoplastic. Attention has to be paid to the type of polimers forming the adhesive, since the resultant material has be low viscosity in the melt (for easy application) but must solidify into a cohesively strong material.
--Hot melt adhesives are divided into two classes; the first class depend upon formulation design. That is, the properties of hot melt come from the combination of components that give the desired balance of properties. This situation should now be familiar, as it was encountered for elastomer based as well as structural adhesives. In the second class of hot melt adhesives, adhesive performance is molecularly designed. That is, the hot melt performance does not come from formulation, but rather from the choice of monomers used to make the base polymer.
(Alphonsus V. Pocius, Adhesion and adhesives technology: an introduction, page 271)
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--A hot melt adhesive is defined as an adhesive applied from the melt and it gains strength upon solidification and crystallization. Hot melt adhesives are applied without solvents. The increase in solvent emission regulations has increased the demand for hot melt adhesives. Certain types of hot melt adhesives cure over time after application. However, with the general purpose hot melt adhesive, the material is applied as a thermoplastic melt and the resulting adhesive is also a thermoplastic. Attention has to be paid to the type of polimers forming the adhesive, since the resultant material has be low viscosity in the melt (for easy application) but must solidify into a cohesively strong material.
--Hot melt adhesives are divided into two classes; the first class depend upon formulation design. That is, the properties of hot melt come from the combination of components that give the desired balance of properties. This situation should now be familiar, as it was encountered for elastomer based as well as structural adhesives. In the second class of hot melt adhesives, adhesive performance is molecularly designed. That is, the hot melt performance does not come from formulation, but rather from the choice of monomers used to make the base polymer.
(Alphonsus V. Pocius, Adhesion and adhesives technology: an introduction, page 271)
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Hot melt adhesives are an important group in the category of physically set-ting adhesives, commonly used for macroscopic assembly processes. Many commercially available thermoplastic hot melt adhesives are polyamides, saturated polyesters, polyolefines, ethylene-vinylacetate-copolymers, block polymers like styrene-butadiene-styrene and polyimides.
The category of reactive hot melts combines the benefits of both hot melts and chemically curing adhesives. Through crosslinking, reactive hot melts become infusible and insoluble. Major constituents for prepolymer reaction are polyurethanes with solid polyesterpolyols. Heating the adhesive induces crosslinking. A new technology are polyurethane hot melt films containing solid isocyanate crosslinkers which are applied prior to the joining process and are activated by heat above melting temperature. Another reactive HMA-systems are indicated by the reaction between thermoplastic polyamide and isocyanates crosslinkes to humidity resistant networks.
The category of reactive hot melts combines the benefits of both hot melts and chemically curing adhesives. Through crosslinking, reactive hot melts become infusible and insoluble. Major constituents for prepolymer reaction are polyurethanes with solid polyesterpolyols. Heating the adhesive induces crosslinking. A new technology are polyurethane hot melt films containing solid isocyanate crosslinkers which are applied prior to the joining process and are activated by heat above melting temperature. Another reactive HMA-systems are indicated by the reaction between thermoplastic polyamide and isocyanates crosslinkes to humidity resistant networks.
The melting temperature of most HMAs ranges between 60°C up to 220°C. That is why substrates sensitive to temperature can be bonded at low tem-peratures. When assembling microsystems, a major benefit of hot melts over viscous adhesive systems is the possibility to apply hot melt systems as pow-der, spheres, balls and films, and as dispersion or solution prior to the joining process, see Fig. 18.1. The actual adhesive bonding process does not neces-sarily have to take place directly after applying the adhesive to the substrate but can be initiated any time during the joining process. Hot melts have no pot life. The adhesive melts through thermal impulse only during the bon-ding process and thus wets the surface of the other substrate. The adhesive bonding process can be started by either directly warming the adhesive or indirectly heating substrate and/or adhesive and substrate. The adhesive sets during cooling. Experiments have proven that hot melts set very quickly, i.e. joints achieve the required handling strength (usually the ultimate strength) in less than three seconds, if suitable heat management is used.
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