Plastic's Chemistry

Atoms

Atoms are the smallest particle and the basic building block of all matter. Atoms are very small; over tens of millions of atoms can be placed between the two sides of the paper on this page. Each atom is made up of a nucleus at the center with one of more electrons orbiting around the outside. The structure of atoms are often compared to the planets with the nucleus of the atom at the center, much like the sun at the center of our planets. Then several electrons orbit the center, much like our planets. In Figure 2-1 we see the planetary model for the carbon atom. Our understanding of the carbon atom is very important. The carbon atom plays a critical role in the structure of plastic's atomic structure. Carbon atoms link together to form the string-like molecule found in all plastic materials. Recently, a photograph taken of the carbon, using a powerful microscope, confirmed the atom's planetary shape...in the photograph the carbon resembled a fuzzy ball.

Electrons

As shown in Figure 2-1, the center of the atom is called the nucleus, which contains the sub-atomic particles of protons and neutrons. Electrons orbit the nucleus. The number of electrons equals the number of protons in the nucleus. The physical weight of the atoms is determined by adding the number of electrons to the number of protons. Carbon has a molecular weight of 12. Electrons have a negative charge while the protons have a positive charge. The number of electrons determine the size of the atom. The electrons, depending upon their number, divide themselves in seven levels from the center of the atom. The division is a direct result of their negative charge. The smaller orbits hold fewer electrons than the larger orbits. Thus the farther away from the nucleus, the more electrons in the orbits. Two electrons may orbit in the first level, then 8, 18, 32, 50, 72, and 98 divide into the next six levels. One level must fill completely before beginning a new and larger level. When the last level does not fill completely with electrons, the atom is considered chemically reactive which creates the ability to link easily with other atoms

The electrons in the outermost level are called the valence electrons and they control the chemical bonding of the atom. The valence, which is four in carbon and one in hydrogen, allows the atoms to link together to form large molecules. As the last chapter suggests, there are six primary atoms (carbon, hydrogen, chlorine, oxygen, nitrogen, and fluorine) used to build plastic materials. All six of the atoms lack one, two, or three electrons in their outer most level. Table 2-2 shows these six atoms with their, chemical symbols, atomic numbers, number of electron levels, molecular weight, chemical valence, and the number of bonding sites. The chemical symbol represent the atoms in chemical equations. The atomic number is the number of electrons orbiting the atom center. The molecular weight represents the mass of the atom. The chemical valence represents how many atoms can be linked with it. The number of bonds is the number of chemical bonds that are available with the atom. These atoms are chemically reactive and seek to link with other atoms in order to fill the outside level. This property allows the chemist to bond thousands of atoms into giant molecules that form the micro structure of all plastic materials.

Chemical Bonds

The lack of complete electrons in the outer most orbit determines the atoms ability to bond with another atoms. A covalent bond occurs when two atoms share their electron orbits in order to simulate a completely filled outermost orbit. For example, to form a molecule of methane gas, four hydrogens link up with one carbon. Hydrogen requires two electrons in the first level to be chemically neutral, this means hydrogen is in need of one additional electron. Since carbon has four electrons in its second level, it needs to share with four more electrons to be chemically neutral. The linkage between hydrogen and carbon is formed by sharing its electrons in orbits surroundng the nucleus of each atom. This sharing forms a strong covalent bond. Figure 2-2 shows the hydrocarbon (hydrogen linked with carbons) molecules of methane (gas), pentane (liquid), and polyethylene (solid).

Each of these molecules is made up of covalent bonded carbon and hydrogen atoms. As a molecule increases in size, it weight also increases. The increase in weight changes the molecule from gas to liquid and finally a solid. At room temperature (72 o F) a molecule of methane is in gas form, pentane is a liquid, and polyethylene is a solid. Increasing the temperature to 100 o F would change pentane to a gas and paraffin to a liquid. This liquid form of polyethylene is called a melt. The length (which is analogous to its size) of a molecule may be one carbon, or one million carbon atoms. As the length of the molecule increases, more energy is required to transform it from a solid, to a liquid, or to a gas.

In plastic, short chain molecules are less stable than long chain molecules. For example, polyethylene which is made up of hydrogen and carbons has many different physical properties. Low density polyethylene (trash bags) with molecules that average 20,000 carbons long has a melt temperature that is about 200o F. In contrast, high density polyethylene (milk bottles) with molecules that average 200,000 carbons long has a melt temperature of 425o F. Thus the understanding of atomic bonds and the molecule length are important to our understanding of plastic's physical properties.

Plastics' Atomic Bonds

Covalent Bonds

A covalent bond may be single, double, or triple. Figure 2-3 shows these three types of bonds: triple (acetylene gas), double (ethylene gas) and single (ethane gas).

Single Covalent Bonds

In the single covalent bond shown for ethane, the carbon atoms, with four electrons in its outer orbit, shares space with the adjacent carbon and hydrogen atoms. One electron from each atom combines their orbits to form a figure eight orbit passing around one atom's nuclei and then around other. The sharing of the orbit in a figure eight pattern around both nucleus is a single covalent bond. The single covalent bond is very flexible and allows the molecule to rotate and bend easily. The single bond provides the longer plastic molecules with flexibility.

However, this single covalent bond can also be easily destroyed with heat, pressure, working, or exposure to ultraviolet rays from the sun. Many plastics products lose their color, become brittle, or generally fail because of the destruction of covalent bonds. Ultraviolet rays can break covalent bonds between hydrogen causing plastics molecules to link together. The linkage restricts movement of the molecules and makes the plastic stiffer. Additionally, carbon to carbon bonds can be broken shortening the molecule chain and reducing physical properties. While the plastic part may appear to be unchanged, its physical, chemical, and thermal properties have been significantly altered.

Double Covalent Bonds

Where a single bond acts as a pivot point, the double bond blocks the molecule from bending easily. In a double bond, such as the one seen in Figure 2-3 of ethylene gas, four electrons orbit in a figure eight pattern around the two carbon atoms. The double bond is much weaker than a single bond. The weakness occurs by the four negatively charged electrons repel each other as they are squeezed into close orbit around the two carbons. This double bond is found in the molecule or structure of rigid plastics. Recall the polystyrene coffee cup lid mentioned in from Chapter 1 ? The molecular structure of the lid contains millions of large benzene rings with three double covalent bonds in each ring. The size of the benzene ring and its many double bonds provides the polystyrene used in the lid with its rigid and stiff properties. By contrast, the polyethylene milk container, also illustrated in Chapter 1, has no double bonds. Its physical properties are relatively soft and flexible.

Double bonds in the benzene ring are easily broken by heat, sunlight, and indoor lighting. The breaking of the double bonds forms carbon to carbon rings. These carbon rings darken the plastics and give it a yellow cast. Double bonds in ethylene gas are easily broken to form links with other ethylene molecules. The growth of these formations into long chain structures forms polyethylene plastics. The weak double bond in many plastics is often used as a cross linking site. With the aid of a catalyst, the double bond can be broken without affecting the surrounding single bonds. The broken bond seeks to link up with other broken double bonds cross, linking the material's molecules together. This linkage, called cross links, permanently changes the plastic from a thermoplastic to a thermoset.

Triple Covalent Bonds

The triple bond, shown in Figure 2-3 for acetylene gas, consists of six negatively charged electrons predssed into close orbits around two carbon atoms. The repulsive forces of these electrons is very high; thus, acetylene gas is unstable. It explodes when the pressure rise above 4 pounds per square inch (PSI). Acetylene is often used as a raw material that is converted to ethylene. In the conversion process, the triple bond is broken and two hydrogen atoms are added. The addition of the hydrogen form a double bonded ethylene gas. Triple bonds are not found in any plastic materials because of their unstable properties.

Van der Waal's Force

The secondary forces that act between molecules is called a Van der Waal force; it is named after the German scientist who discovered it. To understand the secondary forces in plastic we must (1) understanding of how the ethylene plastic molecule forms from the smaller ethylene gas molecules and (2) comprehend how these new giant molecules influence each other. It is this influence that is call a Van der Waal's force.

In Figure 2-4a the simple plastic molecule in two dimensional cross section is illustrated. The carbon atoms are show as a straight line. The hydrogen atoms are shown as dots connected to the side of the straight line which are long and chain like. Some are mixed together; some are entangled. However, none are connected to each other, nor do they touch each other. This arrangements is the micro structure of a thermoplastic polyethylene.

If the molecules were connected with cross links, the plastic would be a thermoset. Notice in Figure 2-4a that some molecules are distant from each other, while in other places they are almost touching. The secondary Van der Waal's forces act between the molecules creating a force field, which is analogous to a magnetic field. In Figure 2-4b the force field is illustrated as a tube around the molecule. The closer the molecules are together, the stronger the force. By contrast, the farther apart the molecules are, the weaker the force.

A secondary force field, created by the millions of negatively charged electrons that form covalent bonds around the carbon and hydrogen atoms. The electrons in their orbital paths are repulsed by the other negatively charged electrons: The repulsive force causes the electrons to seek an orbital path of least resistance. The repulsive forces completely surround the molecules. Once the electrons have arranged themselves into orbits of least resistance, the repulsive forces of the adjacent electrons force them to remain in the prescribed orbit. Increasing or decreasing the temperature of the environment accelerates the electrons in their orbits. The change in orbital speed is the only factor that changes the orbital path of the electrons.

The speed of the orbiting electrons is determined by changes in temperature. A lower temperature slows the electron while an increase in temperature accelerate the electron. When an electron increases its speed, it forces the molecule to move farther apart while a decrease in speed forces the molecule to move closer together. The farther apart the molecule the weaker the repulsive force of the adjacent electrons. The close together the stronger the force of the adjacent electron. The stronger force locks the electron into its orbit resulting in a stronger, and stable, plastic. The resistance of the electron to change it orbit and to lock the molecule into the position without a change in temperature is called the Van der Waals force. Van der Waals' force acts throughout a plastic's molecular structure and holds the molecules in position until they are affected by a change in temperature.

Van der Waal's force exists between molecules allowing the molecule to float and move depending upon (1) temperature, (2) pressure, (3) the spaces between molecules and (4) the molecule's length and shape. Since temperature, increases the speed of the electrons also changes their orbital paths. External pressure too, can force the molecule closer together compressing the molecules artificially. This compression creates an internal stress that eventually seeks relief.

The space between the molecules is determined by the form that the molecules take. They can be very open and random, or very closely packed. A longer molecule will be more difficult to move because of its increased mass. If the molecule has a complex shape, it resists compression. All four conditions exist in most plastic materials. The presence of them creates a Van der Waals force that is uneven throughout the material. Tince this reduces the overall strength of the plastic, plastics can adjust when confronted with external pressures.

Van der Waal's force is 20 to 50 times weaker than the covalent bond and varies in strength based on the distance from adjacent electrons. The force is not a physical link (like the covalent bond) and is easily weakened by an increase in temperature. Table 2-3 lists the interatomic distance and disassociation energy for covalent bonds and Van der Waal's force. Van der Waal's force allows many plastic materials to react like a box of rubber bands which are simultaneously resilent.

Increase the temperature of a plastic material and the molecules, will move farther apart. The greater the distance between molecules the lower the strength of the Van der Waal's force. Decrease the temperature of a plastic material and the molecules move closer together. The closer the molecules, the greater the resistance to change, thus increasing the strength of the Van der Waal's force. This expansion and contraction of the molecular structure takes place only in the areas that are affected by changes in temperature. For example, heat one end of a plastic spoon and the handle will remain cool. A metal spoon would rapidly transmit the temperature throughout. In a plastic material held at room temperature, the electrons do not change speed or orbits and the plastic properties remain stable. The resistance of the electrons to change orbits without a change in temperature stabilizes the plastic's microstructures. This stabilization provides plastics materials with their solid qualities.