BAKELITE: STRUCTURE, PROPERTIES, OBTAINING AND APPLICATIONS - CHEMISTRY - 2024

The Bakelite is a polymeric resin of phenol and formaldehyde, the precise chemical definition and is a hydroxide polioxibenciletilenglicol. The emergence and commercialization of this material marked the dawn of the era of plastic; it 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; then founding the General Bakelite Company in 1910. At first, while modifying the physical variables involved, Bakelite consisted of a spongy, brittle solid of little value.

Retro phone made with Bakelite polymer. Source: Pexels.

After eight years of work in the laboratory, he managed to obtain a sufficiently solid and thermostable Bakelite, with a high value due to its properties. Thus, 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Nowadays, however, it has been replaced by other plastics, and it is found mainly in accessories or objects from the 20th century.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 picture above is made of Bakelite, as are many objects of a similar black color to this, or amber or white (resembling ivory in appearance).

Bakelite structure

Training

Formation of a three-dimensional network-type structure of phenol-formaldehyde polymer, bakelite. Source: MaChe.

Defined bakelite as a polymeric resin of phenol and formaldehyde, then both molecules must conform their structure, covalently united in some way; otherwise, this polymer would never have exhibited its characteristic properties.

Phenol consists of an OH group linked directly to a benzene ring; while formaldehyde is a molecule of O = CH ₂ or CH ₂ O (top image). Phenol is rich in electrons, due to the fact that OH, although it attracts electrons towards itself, also contributes to their delocalization by the aromatic ring.

Being rich in electrons, it can be attacked by an electrophile (electron-hungry species); as for example, the CH ₂ O molecule.

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 CH ₂ O replaces an H of the phenol to become a methylol group, -CH ₂ OH; -CH ₂ OH ₂ ⁺ in acid medium, or -CH ₂ O ^- in basic medium.

Assuming an acidic medium, -CH ₂ OH ₂ ⁺ loses a water molecule at the same time that the electrophilic attack of a second phenolic ring occurs. A methylene bridge, -CH ₂ - is then formed (colored blue in the image).

Ortho and para substitutions

The methylene bridge does not bind 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. Then, substitutions or attacks to or from the phenolic ring occur at these positions.

Three-dimensionality of the network

Remembering the chemical hybridizations, the carbon of the methylene bridges is sp ³; 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:

Segment of the three-dimensional structure of Bakelite. Source: Wikimedia Commons.

On the other hand, when the 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.

Properties

Taking then the bakelite as a network of phenolic rings joined 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 by 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 increases, it gives off a characteristic formaldehyde odor (organoleptic recognition).

-Once molded, and as it is a thermoset plastic, it retains its shape and resists the corrosive effect of certain solvents, temperature increases and scratches.

-It is a terrible conductor of heat and electricity.

-Emits a characteristic sound when two pieces of Bakelite are struck, which helps to identify it qualitatively.

-Newly 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.

Obtaining

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 (HCl, ZnCl ₂, H ₃ PO ₄, etc.) or basic (NH ₃) catalyst. 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.

Applications

Plastic wood planks. Source: VarunRajendran at English Wikipedia

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 impact or movement is 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 essential devices during the world wars.

Its solid consistency was attractive enough for the design of carved boxes and jewelry. In terms of ornamentation, when the bakelite is mixed with the wood, the second is given a plastic texture, with which planks or composite boards have been made to cover the floors (top image) and domestic spaces.

References

1. University Federico II of Naples, Italy. (sf). Phenol-formaldehyde resins. Recovered from: whatischemistry.unina.it
2. Isa Mary. (April 5, 2018). Archeology and the age of plastics bakelite in the brody dump. Kale. Recovered from: campusarch.msu.edu
3. College of Science Chemical Education Division Groups. (2004). The Preparation of Bakelite. Purdue University. Recovered from: chemed.chem.purdue.edu
4. Bakelitegroup 62. (sf). Structure. Recovered from: bakelitegroup62.wordpress.com
5. Wikipedia. (2019). Bakelite. Recovered from: en.wikipedia.org
6. Boyd Andy. (September 8, 2016). Leo Baekeland and bakelite. Recovered from: uh.edu
7. NYU Tandon. (December 05, 2017). Lights, Camera, Bakelite! The Office of Student Affairs Hosts a Fun and Informative Movie Night. Recovered from: engineering.nyu.edu