Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: paul.fr on 31/03/2007 14:39:12
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What makes glue, glue? also, why is "superglue" much stronger than regular glue and the glue on post-it notes so much weaker?
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A question like this would require a handbook by way of reply !
It is obvious that a hot melt glue works along a quite different way compared to a superglue or the glue used in post-it labels.
Roughly, you have the glue and the substrate - or substrates, because you can have to glue together two things that are essentially very different, like metal on glass.
The glue can work like a rivetif it penetrates in low viscous form into the substrates, and becomes solid or high viscous afterwords. This is possible if the substrate is sufficiently porous, or has a really fibrous structure. The transition from low-viscous to solid can be a question of temperature - cooling in the case of hotmelt glues, heating in the case of a glue that gelatinizes like the starch based glue in corrugated board; it can be a question of drying (water or solvent being either evaporated or absorbed by the substrate) or it can be a chemical reaction (two-component glues or cyanoacrylate glues). Glue for wallpaper solidifies by drying : there is water evaporation through the paper, and there is water penetrating into the wall.
There is also the adhesion-cohesion-adhesion type of binding, in which case there is little or no penetration of the glue into the substrate. A typical case of this is when you glue something to glass. Again a low viscosity in the application stage is advisable in order to distribute the glue evenly over the surface(s). Glass glues often solidify by a chemical reaction triggered by UV from the sunlight.
A third possibility is a chemical reaction between the glue and the substrate(s). This is more or less what happens when we use a monomere to glue polymeres, like when gluing pieces of PVC tube together.
In practice you can have a combination of the three.
Other point in gluing : more is rarely better. In fact, you need a good coverage of both faces, and a minimum amount of glue in between. Of course, if you need penetration into the substrate, you will need more glue, same as when the surface of the substrate is very rough.
Apart from the affinity between glue and substrate, the main factor in the binding strength is the molecular weight (or degree of polymerisation) of the glue.
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What about molecular electrostatic attraction (not quite a chemical reaction, because you don't actually get a new molecule formed).
http://www.hero.ac.uk/uk/business/archives/2003/sticking_with_gecko_glue5188.cfm
The researchers, at Manchester’s Centre for Mesoscience and Nanotechnology, built on research by biophysicists into the remarkable ability of geckos to cling to walls and ceilings. The lizards’ secret, it turns out, is millions of microscopic hairs on the pads of their feet. Each hair, or seta, provides a miniscule adhesive force called a Van der Waals force, which operates over very small distances but bonds to just about anything. The combined adhesion of millions of these setae provides a sticking force many times greater than the gecko needs to hang from a ceiling by one foot.
The new adhesive, named ‘gecko tape’ by its creators, contains billions of tiny plastic fibres, each less than a micrometer in diameter, which mimic the hairs on geckos’ feet. The researchers first set out to replicate this gecko feature by fabricating tiny plastic pyramids on a hard backing. This approach was unsuccessful because no surface is perfectly flat, too few of the pyramid tips made contact. So the researchers developed more flexible plastic hairs, each less than two microns high, attached to a soft, pliable backing.
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What about molecular electrostatic attraction (not quite a chemical reaction, because you don't actually get a new molecule formed).
http://www.hero.ac.uk/uk/business/archives/2003/sticking_with_gecko_glue5188.cfm
The researchers, at Manchester’s Centre for Mesoscience and Nanotechnology, built on research by biophysicists into the remarkable ability of geckos to cling to walls and ceilings. The lizards’ secret, it turns out, is millions of microscopic hairs on the pads of their feet. Each hair, or seta, provides a miniscule adhesive force called a Van der Waals force, which operates over very small distances but bonds to just about anything. The combined adhesion of millions of these setae provides a sticking force many times greater than the gecko needs to hang from a ceiling by one foot.
The new adhesive, named ‘gecko tape’ by its creators, contains billions of tiny plastic fibres, each less than a micrometer in diameter, which mimic the hairs on geckos’ feet. The researchers first set out to replicate this gecko feature by fabricating tiny plastic pyramids on a hard backing. This approach was unsuccessful because no surface is perfectly flat, too few of the pyramid tips made contact. So the researchers developed more flexible plastic hairs, each less than two microns high, attached to a soft, pliable backing.
I suppose that in that famous handbook (I'm not ready to write it yet) this would come in the same chapter as "chemical reaction", together with hydrogen bonds and some others. Maybe "chemical bonds" would be a better wording than "chemical reactions".
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ok, to simplify the question. i am making a picture and want to glue some wool to it for hair. i dab some pva glue to the card and put the wool onto the glue, it sticks and adheres. but why, what is so special about this pva that makes the paper and wool "one"
ps, do i get a free copy of your book when it's done?
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I suppose that with "pva" you mean polyvinylalcohol and not polyvinylacetate.
The polyvinylalcohol (sometimes written as PVOH) is a solution (or colloid) in water. Part of the water will evaporate, part of it will be absorbed by the card. This leaves you with a polymer that for some reason adheres well to the wool. This "some reason" is the presence of -OH groops on both the wool and the polyvinylalcohol - so hydrogen bonds can play their role. On the other side some of the polyvinylalcohol will penetrate into the card bith the water, but at a slower rate. In this way, on the side of the card you create the "rivets" as well as having binding at the surface itself between polyvinylalcohol and cellulose fibre, which also shows an abbundance of -OH groups.
Water itself has also -OH groups, but it is not a good glue simply because it does not have the long chain molecules, the water "polymers" are only kept toghether by hydrogen bonds.
Polyvinylacetate (PVAc) would be a latex. It will not perform as well on the wool, but when part of the water is absorbed by the card the latex will coagulate and form a film, which may extend itself around the wool fibres.
And if ever I manage to get any further than a foreword and a table of contents with that book, I will remember to send you a draft version. After all, I will also need a better spelling and grammar control