The bakelite is a polymeric phenol and formaldehyde resin, whose exact and chemical definition is that of a polyoxybenzylethylene glycol hydroxide. The emergence and commercialization of this material marked the dawn of the era of plastic; occupied and was part of countless household, cosmetic, electrical, and even military objects.
Its name came from its inventor: the American chemist born in Belgium, Leo Baekeland, who in 1907 achieved the production and improvement of this polymer; and then founding the General Bakelite Company in 1910. At first, while modifying the physical variables involved, Bakelite consisted of a spongy and brittle solid of little value..
After eight years of work in the laboratory, he managed to obtain a Bakelite that was strong enough and thermostable, with a high value as a result of its properties. It was thus that Bakelite replaced other plastic materials of natural origin; the first purely artificial polymer was born.
Nowadays, however, it has been replaced by other plastics, and it is found mainly in accessories or objects from the 20th century. For example, the phone in the image above is made of Bakelite, as are many objects of a similar black color to this, or amber or white colors (resembling ivory in appearance).
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Defined bakelite as a polymeric resin of phenol and formaldehyde, then both molecules must conform their structure, covalently linked in some way; otherwise, this polymer would never have manifested its characteristic properties.
Phenol consists of an OH group linked directly to a benzene ring; while formaldehyde is a molecule of O = CHtwo or CHtwoO (top image). Phenol is rich in electrons, due to the fact that OH, although it attracts electrons towards itself, also collaborates in their delocalization by the aromatic ring.
Being rich in electrons, it can be attacked by an electrophile (a kind of electron hungry); as for example, the CH moleculetwoOR.
Depending on whether the medium is acidic (H+) or basic (OH-), the attack can be electrophilic (formaldehyde attacks phenol) or nucleophilic (phenol attacks formaldehyde). But in the end, the chtwoO substitutes for an H of the phenol to become a methylol group, -CHtwoOH; -CHtwoOhtwo+ in acid medium, or -CHtwoOR- in basic medium.
Assuming acid medium, the -CHtwoOhtwo+ it loses a water molecule at the same time that the electrophilic attack of a second phenolic ring occurs. A methylene bridge is then formed, -CHtwo- (colored blue in the image).
The methylene bridge does not join two phenolic rings at arbitrary positions. If the structure is observed, it will be possible to verify that the bonds are in adjacent and opposite positions to the OH group; these are ortho and para positions, respectively. So, substitutions or attacks to or from the phenolic ring occur at these positions.
Recalling the chemical hybridizations, the carbon of the methylene bridges is sp3; therefore, it is a tetrahedron which places its bonds outside or below the same plane. Consequently, the rings do not lie in the same plane, and their faces have different orientations in space:
On the other hand, when substitutions occur only in -orto positions, a polymer chain is obtained. But, as the polymer grows through the -para positions, a kind of mesh or three-dimensional network of phenolic rings is established..
Depending on the process conditions, the network can adopt a "swollen morphology", undesirable for the properties of the plastic. The more compact it is, the better it will perform as a material.
Taking then the bakelite as a network of phenolic rings linked by methylene bridges, the reason for its properties can be understood. The main ones are mentioned below:
-It is a thermosetting polymer; that is, once solidified it cannot be molded due to the effect of heat, even becoming even more caked.
-Its average molecular mass is usually very high, which makes Bakelite pieces considerably heavier compared to other plastics of the same size..
-When rubbed and its temperature rises, it gives off a characteristic formaldehyde odor (organoleptic recognition).
-Once molded, and being a thermoset plastic, it retains its shape and resists the corrosive effect of certain solvents, increases in temperature and scratches..
-It is a lousy conductor of heat and electricity.
-It emits a characteristic sound when two pieces of Bakelite are struck, which helps to identify it qualitatively.
-Recently synthesized, it has a resinous consistency and is brown in color. When it solidifies, it acquires different shades of brown, until it turns black. Depending on what it is filled with (asbestos, wood, paper, etc.) it can present colors that vary from white to yellow, brown or black..
To obtain bakelite, a reactor is first required where phenol (pure or from coal tar) and a concentrated solution of formaldehyde (37%) are mixed, maintaining a Phenol / Formaldehyde molar ratio equal to 1. The reaction begins of polymerization via condensation (because water, a small molecule) is released.
The mixture is then heated with stirring and in the presence of an acid catalyst (HCl, ZnCltwo, H3PO4, etc.) or basic (NH3). A brown resin is obtained to which more formaldehyde is added and it is heated to around 150 ° C under pressure..
Later, the resin is cooled and solidified in a container or mold, accompanied in addition to the filling material (already mentioned in the previous section), which will favor a certain type of texture and desirable colors..
Bakelite is the quintessential plastic of the first half and mid-20th century. Telephones, command boxes, chess pieces, vehicle door handles, dominoes, billiard balls; any object constantly subjected to slight impacts or movements are made of bakelite.
Because it is a poor conductor of heat and electricity, it was used as an insulating plastic in circuit boxes, as a component of the electrical systems of radios, light bulbs, airplanes, and all kinds of indispensable devices during the world wars..
Its solid consistency was attractive enough for the design of carved boxes and jewelry. In terms of ornamentation, when Bakelite is mixed with wood, the second is given a plastic texture, with which composite planks or boards have been made to cover floors (top image) and domestic spaces..
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