Opus about His Majesty Clay. Part one - introductory

Opus about His Majesty Clay. Part one - introductory

There are areas of knowledge that cannot be measured by a "common yardstick ...". In principle, in my “home” field, colloid chemistry, under this direction you can safely put any fundamental concept, whether it is adsorption (with adsorbents) or adhesion (with adhesives). Honestly, the idea to write about the glue did not arise for me. But when readers in each topic related to polymers start asking to talk about adhesives, you will of course think about it (well, of course you want to steam into “everything must be glued with superglue”). Adhesion and adhesives are a very broad topic, so I decided to take it up, but break the story into several parts. Today, the first part - introductory information. To find out at the expense of what glue glues, what kind of adhesives are and what kind of glue is better for gluing _____ (enter the necessary), we traditionally go under the cut (and bookmark).

Before I begin my narration, I would like to make a small digression-dedication:

Memory of a colloidal Chemist
My head of diploma practice liked to respond to the attacks of colleagues "there are no sensible students now ..." with the phrase "There are no bad students, there is a teacher who takes not the place". Increasingly, I find myself agreeing with this phrase. Students feel the sincerity and mastery of the subject area and “vote” with respect and attendance.

Belarusian science, after the collapse of the USSR, became a thing in itself, strange and even wild in places. It is not surprising that many academicians of Belarus, as a rule, are “widely known in narrow circles,” non-public people, etc. Even though the work was interesting. But more often, dry biographical information on any, self-made html-site of the institute, even approximately can not tell what the person was like. So the doctor of chemical sciences, professor Thomas Fomich Mozheiko was a special guy. Without false modesty it can be said that the entire Soligorsk Klondike is built with the help of his hands and his bright head. I happened to encounter this person for the first time when passing a candidate minimum in colloid chemistry, after which we began to communicate closely and in a friendly way. Considering that the postgraduates in our research institute were defiantly "in no way", it impressed me ... and perhaps because of the meeting with this grandfather, who could briefly explain the essence of the most complicated process and convince us that our common area is the queen of chemistry. , I am writing now a chemical article on Habr, and I don’t wipe my pants for developing or testing ... So, to be honest, all the articles on a colloidal subject should have been in memory “in the memory of F.F. Mozheiko ”, because this man was one of my teachers. Blessed memory to you, ff!

The glue has been used by man since ancient times, we can assume that as soon as the primitive man stuck the silicon tip of his spear to the pole with bitumen or pine resin, and the countdown of the practice of gluing began. In ancient times, everything that came to hand was used as an adhesive. The most commonly used products of animal origin with adhesive properties initially (fish scales, animal veins, etc. substances after heat treatment). It is worth noting that there are areas in which organic glue is actively used so far. Wood glue, casein glue, wallpaper paste. Despite the abundance of synthetic (= chemical adhesives), the options mentioned are still in the ranks and firmly occupy the niche of eco-friendly and cheap adhesive substances that they put.By the way, many modern adhesives are called synthetic resins only in honor of the fact that resin (adhesive substance found in pines and other plants) was one of the first widely used adhesives.

The whole concept of bonding rests on two fundamental phenomena of colloid chemistry - adhesion and cohesion (ok, three, still surface tension).
Adhesion (from Lat. adhaesio - adhesion) in physics - the adhesion of surfaces of dissimilar solid and/or liquid bodies. Adhesion is caused by intermolecular interactions in the surface layer and is characterized by the specific work required to separate the surfaces.

Related to the sound and the meaning of adhesion is the concept of cohesion, which sometimes some people like to confuse.

Not to confuse adhesion with cohesion
... which is the reason for the existence of things in the state in which we used to see them (t in the form of pieces, drops, etc., and not scattered over molecules). This phenomenon is called cohesion :
Cohesion (English cohesion from the Latin. cohaesus - "linked", "linked") - a bond between the same molecules (atoms, ions) within the body within the same phase. Cohesion characterizes the strength of the body and its ability to resist external influence. Cohesion is an action or a property of mutual attraction of identical molecules. This is an intrinsic property of a substance due to the shape or structure of its molecules, causing a change in the distribution of the electrons of the molecules as they approach, creating electrical attraction that can form microscopic structures.

The difference between these fundamental concepts of colloid chemistry is best illustrated by the example of water droplets that form on a window pane during a rain.

The picture shows the confrontation "elements", each of which is engaged in their own business of forming the familiar to us pictures of the world. The shape of the drop gives surface tension . The force of gravity (earthly) - pulls a drop down, drain from the glass. The forces of adhesion and cohesion are struggling with this inexorable force. Earlier, cohesion is manifested, since it takes place already in the drop of water itself. Neighboring molecules stick together with each other and form the very drops, which then painterly slide on the glass. Cohesion binds single molecules into ensembles. But the adhesion attaches the ensembles in the form of drops to the glass, forcing them to hold on to the glass, “pulls up”, forcing them to resist movement under their own weight. Moreover, cohesion is stronger than adhesion, otherwise drops would not be able to form, i.e. rainwater would just spread out evenly over the glass, forming a kind of oil film on the water. By the way, carefully watching the glass during the rain, you can see that the drops roll down the already existing "water paths". This is due to the fact that falling drops of water due to the forces of cohesion try to stick to the water that is already there, and not to the glass. These paths, by the way, are formed due to the fact that when droplets hit the window, water molecules break away from the passing droplets and are captured by the glass.

What does all this have to do with adhesives? And the most direct. Adhesion and cohesion are the main factors in adhesives. Suppose you want to join two pieces of wood, A and B, using glue B.Here you need three different forces: adhesion forces capable of holding together A and B + adhesion forces holding C and B + adhesion forces that hold adhesive B. together.

If everything is clear with the first two forces, I will give a small explanation about the last. The best example is two pieces of loaf glued together with jam or jam. Jam is a classic natural glue (I’ll say a few words about them below) made from sugar and water. While quite effective. When using fairly durable bread (or crackers) and the right mummy jam, it’s quite possible for you to actually hold two pieces together by smearing only one corner. Good jam has strong enough internal cohesion forces (therefore, it is difficult to pull it out of the can, especially pear), but adhesion to other surfaces is excellent. Therefore, it is hard to break a glued sandwich without destroying the loaf (this is most often the case if you move the layers to the side and not apply force perpendicularly). But if the jam “has a weak internal cohesive core,” then it does not matter how well it sticks to the loaf. The two halves will not be able to stick together and will fall apart under the action of gravity.

Another antagonistic example: water and a piece of iron. Both objects are very poorly suitable for gluing under normal conditions, but for various reasons. Water is because its adhesion forces are great and it adheres perfectly to any surfaces, but due to very weak cohesive forces, these surfaces do not adhere firmly to each other and are easy to separate. In a piece of iron, on the contrary, incredibly strong cohesive interactions (responsible for the bond of atoms), moreover, it is so “a thing in itself” that it is almost impossible to achieve adhesion to any other external material. A test for the internal forces of cohesion can be the possibility of dividing the material into pieces. A “piece” of water can easily be separated from the total mass with a finger/spoon, etc., and try to separate a piece of iron with your finger :).

From the above conclusion - in the nature of glue, the main force of co geesi, and in the nature of gluing - the force hell of geesi Since adhesives, as a rule, are rather specific substances, the effectiveness of many of which has been tested by the experience of many generations, I will focus on the adhesion phenomenon (by the way, glue can also be called an adhesive). To date, several different competing/complementary theories have been developed that attempt to explain the appearance of the adhesion phenomenon:

Despite the abundance of materiel, there is still no single answer to the question "what makes glue glue?". But this is not so surprising when you consider how many different types of glue there are and how many different ways to use them. It is believed that for each individual glue and for each individual surface on which it is used, there is an individual combination of different factors that hold these objects together. The process of studying the process of gluing continues today, because even in the 21st century, when “space ships plow through ...” we still do not fully understand what causes substances to stick to each other. Therefore, it is necessary to operate on assumptions and generalizations. Considering which, it turns out that there are four main possible gluing mechanisms: through adsorption, chemisorption, mechanical attachment and diffusion.

Adsorption is the effect of adhesion of surfaces to each other, due to ultra-low attractive forces (the so-called Van der Waals forces, the common name for all intermolecular forces). By the way, these forces are still divided into electrostatic interaction forces ( Kizom forces arising between constant molecular dipoles), polarization forces ( Debye's intermolecular forces between constant and induced dipoles) and the strength of dispersion interaction ( London forces between instantaneous induced dipoles). Dipole = two charges equal largest and opposite in sign, separated from each other at a distance very small compared to the distance to the observation point. and so on. ”This is where all electrostatics holds up (and with it all adhesives). The forces of intermolecular interaction appear, by the way, when participants (atoms and molecules) are very close (less than 1 nm).

When applying the adhesive, the surface of the parts to be glued is wetted and the objects stick to each other. In order for the glue to work, the surfaces must be degreased to the maximum (to spread the adhesive over the surface) and the glue is evenly distributed with a thin layer. In fact, this process is reminiscent of the adhesion of millions of microscopic magnets (which are glue molecules and molecules of bonded materials).

A little remark on perfectly smooth surfaces
gecko . This is a lizard that can easily move on different vertical surfaces (saying marvel like a spider-man). Even Aristotle indulged in idle thoughts about the cause of this phenomenon. Modern scientists have been studying this issue for quite a long time, gradually discarding the vacuum theory (= sticking at the expense of pressure difference), the theory of biological glue (= gives off sticky substance), etc. As a result, they stopped on electrostatic interactions (caused by contact electrification), and not van der Waals or capillary forces. The cause of the phenomenon was bristles covering millions of each paws. The length of each bristle is about 0.1 millimeter (two thicknesses of a human hair). For every millimeter square feet, there are up to 14400 setae (~ 1.5 million per cm 2 ). Each bristle of the end diverges in 400-1000 branches and each branch ends at the end of a triangular plate with a width of 0.2 micrometers. Those. The gecko's foot, about one centimeter square, contacts the surface with approximately two billion ends.

B. Photograph of gecko setae. B. Photograph of a single gecko seta. G. Photograph of the branch at the end of the seta.

Recent studies show that it is this geometry of the legs and the associated electrostatic forces (multiplied by billions of terminations), which together give a result that can hold the weight of the gecko on the ceiling.

It would be surprising if the military did not take advantage of this. In May 2014, DARPA demonstrated its development Geckskin (Project Z -Man), hand tools that allow you to move on vertical surfaces.

True, five years have passed, but for some reason nothing more is heard about Geckskin. Perhaps because it is classified, and perhaps because there is no result.

Geckos and Darpa are all there, in them. And we can have the best ground-in surface for the best illustration of the forces of intermolecular interactions.Each turner-milling machine should know about such a thing as Johansson's tiles, or plane-parallel end measures . These tiles are polished and lapped so that they stick together strongly enough if they are attached with smooth edges to each other. In the picture, thirty-six tiles are held together by atmospheric pressure and by van der Waals forces together:

To those who do not believe that this is possible, I recommend watching the demonstration (clickable):

After adsorption, chemisorption follows relentlessly, but despite the similarity of the names, the essence of the phenomena is radically different. Chemisorption - sticking is due to the formation of chemical bonds between the adhesive and the substances to be glued. In fact, bonding produces a new chemical. During diffusion, bonding occurs due to the mutual penetration of material molecules into each other. Glue molecules are mixed with molecules of bonded surfaces and form a strong bond. Finally, mechanical adhesion occurs when the glue penetrates into the microcracks and cavities of materials and their subsequent physical retention. The picture for clarity shows the numerical values ​​of the energies for various forces occurring during gluing.

Obviously, the best adhesion is formed in the case of chemisorption interaction between the substances being glued, though it is not always possible to achieve this (but it is necessary to strive).

Adhesive Types

From all the above, it follows that any glue will exploit one or another principle described above. Moreover, in the case of adhesives, as in the case of geckos, researchers, too, consensus is usually not observed. But this, in principle, is not so important, because quite a significant practical experience has been gained that allows you to easily select the best adhesives and adhesives for the whole variety of materials. There are many divisions of adhesive substances, I will give the most simple, based on their chemical nature:

Moreover, I would like to note the fact that to this day we mainly actively exploit the development of almost a century ago.Judge for yourself on a brief chronology:

1920 s: proposed adhesives based on cellulose esters, alkyd resins, cyclized rubber, polychloroprene (neoprene), soy adhesives
1930 years: urea formaldehyde invented, pressure sensitive adhesive tapes, phenolic resin based adhesive films, polyvinyl acetate (PVA) wood adhesives
1940 s: nitrilephenol, chlorinated rubber, melamine formaldehyde, vinylphenol and acrylic polyurethanes synthesized
1950 s: epoxides, cyanoacrylates, anaerobic adhesives are presented
1960 s: represented by polyimides, polybenzimidazole, polychinoxaline
1970 -e years: second generation acrylic adhesives, pressure-sensitive acrylics, structural polyurethanes are presented
1980 -e: active development of thickeners for thermosetting resins, water-soluble epoxy resins, contact adhesives, formable and foamed hot melts are presented
1990 -e years: polyurethane-modified modified epoxy resin, curable thermoplastic materials, UV-curable adhesives and visible light offered
2000 -e years: water-based adhesives have been synthesized, one-component and two-component adhesives that do not contain solvents are being actively developed

In most cases, polymers are used as synthetic adhesives; therefore, I recommend reading my two thematic tutorial articles along the way ( A chemist's letter to a 3D printer. Solvents for plastics and protection against them + We return the girl a birdie or RTFM by definition plastics at home ), even though to get used to the "polymer" terminology and see basic information on polymers.

Today, the main development of "custom" adhesive compositions is on the path of increasing environmental performance (often, by the way, to the detriment of the strength of the joint). Structural and industrial compositions are not particularly affected by this, but in general, traditional, time-tested variants are still used there. So, we look for our joined materials in the table below and memorize the necessary type of glue.

Bonus - a comparative review of the strength characteristics of various types of adhesives. Sometimes it is helpful :)

Visual comparison of the strength characteristics of compounds obtained using different types of adhesive compositions

Signatures: CA-cyanoacrylates, MS-based adhesives based on modified silanes, PU-polyurethane adhesives, MMA-methyl methacrylate adhesives, UV adhesives, curable with UV radiation

This concludes the introduction, in the following we will proceed to the consideration of specific types of glue and optimal conditions/materials for its application. Ask questions that concern you in the comments - then in the next part the likelihood of answers will appear.

Used literature
Handbook of Rubber Bonding, Ed., B. Crowther, Rapra Technology Ltd, Shrewsbury, UK, 2000.
D.E. Packham, Handbook of Adhesion, Longman Scientific & amp; Technical, Harlow, UK, 1992.
D.J. Dunn, Engineering and Structural Adhesives, Rapra Review Report No.169, Rapra Technology Ltd, Shrewsbury, UK, 2004.
Skiest, I. The Handbook of Adhesives, 3rd ed., Van Nostrand Reinhold, New York, 1990.
Satas, D. The Handbook of Pressure Sensitive Adhesives, 2nd ed., Van Nostrand Reinhold, New York, 1989.
Petrie, E.M., Handbook of Adhesives and Sealants, McGraw-Hill, New York, 2000.

Source text: Opus about His Majesty Clay. Part one - introductory