Crude petroleum is separated into its various fractions through a distillation process at the oil refinery. After separation, these fractions are further refined into other products which include asphalt, paraffin, gasoline, naphtha, lubricating oil, kerosene, and diesel oil. Since asphalt is the base or heavy constituent of crude petroleum, it does not evaporate or boil off during the distillation process. Asphalt is essentially the heavy residue of the oil refining process.
Distilling the crude

1 The refining process starts by piping the crude petroleum from a storage tank into a heat exchanger or tube heater where its temperature is rapidly raised for initial distillation. It then enters an atmospheric distillation tower where the lighter and more volatile components, or fractions, vaporize and are drawn off through a series of condensers and coolers. It is then separated for further refining into gasoline (considered a "light" distillate), kerosene (considered a "medium" distillate), diesel oil (considered a "heavy" distillate), and many other useful petroleum products.

The heavy residue from this atmospheric distillation process is commonly called topped crude. This topped crude may be used for fuel oil or further processed into other products such as asphalt. Vacuum distillation may remove enough high boiling fractions to yield what is called a "straight run" asphalt. However, if the topped crude contains enough low volatile components which cannot be economically removed through distillation, solvent extraction—also known as solvent deasphalting—may be required to produce asphalt cement of the desired consistency.

Cutting back
2 Asphalt may next be blended or "cut back" with a volatile substance, resulting in a product that is soft and workable at a lower temperature than pure asphalt cement. When the cut-back asphalt is used for paving or construction, the volatile element evaporates when exposed to air or heat, leaving the hard asphalt cement. The relative speed of evaporation or volatility of the cutting agent determines whether a cutback asphalt is classified as slow, medium, or rapid-curing. Heated asphalt cement is mixed with residual asphaltic oil from the earlier distillation process for a slow-curing asphalt, with kerosene for medium-curing, and with gasoline or naphtha for the rapid-curing asphalt.

Emulsifying
3 The asphalt cement may also be emulsified to produce a liquid that can be easily pumped through pipes, mixed with aggregate, or sprayed through nozzles. To emulsify, the asphalt cement is ground into globules 5 to 10 microns and smaller (one micron is equal to one millionth of a meter). This is mixed with water. An emulsifying agent is added, which reduces the tendency of the asphalt and water to separate. The emulsifying agent may be colloidal clay, soluble or insoluble silicates, soap, or sulphonated vegetable oils.

Pulverizing
4 Asphalt may also be pulverized to produce a powdered asphalt. The asphalt is crushed and passed through a series of fine mesh sieves to ensure uniform size of the granules. Powered asphalt can be mixed with road oil and aggregate for pavement construction. The heat and pressure in the road slowly amalgamates the powder with the aggregate and binding oil, and the substance hardens to a consistency similar to regular asphalt cement.

Air Blowing
5 If the asphalt is to be used for a purpose other than paving, such as roofing, pipe coating, or as an undersealant or water-proofing material, the asphalt may be oxidized, or air blown. This process produces a material that softens at a higher temperature than paving asphalts. It may be air blown at the refinery, at an asphalt processing plant, or at a roofing material plant. The asphalt is heated to 500°F (260°C). Then air is bubbled through it for one to 4.5 hours. When cooled, the asphalt remains liquid.

Asphalt Paving Mixtures
Since asphalt cement is a major constituent used in road paving, the following is a brief description of how asphalt paving mixtures are produced. Asphalt paving mixes made with asphalt cement are usually prepared at an asphalt mixing facility. There are two types of asphalt mixes: hot-mix and cold-mix. Hot-mix asphalt (HMA) is more commonly used while cold-mix asphalt (generally mixes made with emulsified or cut-back asphalts) is usually used for light to medium traffic secondary roads, or for remote locations or maintenance use. Hot-mix asphalts are a mixture of suitable aggregate coated with asphalt cement. The term "hot-mix" comes from the process of heating the aggregate and asphalt before mixing to remove moisture from the aggregate and to obtain sufficient fluidity of the asphalt cement for proper mixing and work-ability.

6 Asphalt cement and aggregate are combined in a mixing facility where they are heated, proportioned, and mixed to produce the desired paving mixture. Hot-mix facilities may be permanently located (also called "stationary" facilities), or it may be portable and moved from job to job. Hot-mix facilities may be classified as either a batch facility or a drum-mix facility, both can be either stationary or portable. Batch-type hot-mixing facilities use different size fractions of hot aggregate which are drawn in proportional amounts from storage bins to make up one batch for mixing. The combination of aggregates is dumped into a mixing chamber called a pugmill. The asphalt, which has also been weighed, is then thoroughly mixed with the aggregate in the pugmill. After mixing, the material is then emptied from the pugmill into trucks, storage silos, or surge bins. The drum-mixing process heats and blends the aggregate with asphalt all at the same time in the drum mixer.

7 When the mixing is complete, the hot-mix is then transported to the paving site and spread in a partially compacted layer to a uniform, even surface with a paving machine. While still hot, the paving mixture is further compacted by heavy rolling machines to produce a smooth pavement surface.

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Raw Materials

Methyl methacrylate is the basic molecule, or monomer, from which polymethyl methacrylate and many other acrylic plastic polymers are formed. The chemical notation for this material is CH₂=C(CH₃)

COOCH₃. It is written in this format, rather than the more common chemical notation C₅H₈O₂, to show the double bond (=) between the two carbon atoms in the middle. During polymerization, one leg of this double bond breaks and links up with the middle carbon atom of another methyl methacrylate molecule to start a chain. This process repeats itself until the final polymer is formed.

Methyl methacrylate may be formed in several ways. One common way is to react acetone [CH₃COCH₃] with sodium cyanide [NaCN] to produce acetone cyanhydrin [(CH₃)₂C(OH)CN]. This in turn is reacted with methyl alcohol [CH₃OH] to produce methyl methacrylate.

Other similar monomers such as methyl acrylate [CH₂=CHCOOCH,] and acrylonitrile [CH₂=CHCN] can be joined with methyl methacrylate to form different acrylic plastics. (See Figure 2) When two or more monomers are joined together, the result is known as a copolymer. Just as with methyl methacrylate, both of these monomers have a double bond on the middle carbon atoms that splits during polymerization to link with the carbon atoms of other molecules. Controlling the proportion of these other monomers produces changes in elasticity and other properties in the resulting plastic.

The Manufacturing Process

Acrylic plastic polymers are formed by reacting a monomer, such as methyl methacrylate, with a catalyst. A typical catalyst would be an organic peroxide. The catalyst starts the reaction and enters into it to keep it going, but does not become part of the resulting polymer.

Acrylic plastics are available in three forms: flat sheets, elongated shapes (rods and tubes), and molding powder. Molding powders are sometimes made by a process known as suspension polymerization in which the reaction takes place between tiny droplets of the monomer suspended in a solution of water and catalyst. This results in grains of polymer with tightly controlled molecular weight suitable for molding or extrusion.

Acrylic plastic sheets are formed by a process known as bulk polymerization. In this process, the monomer and catalyst are poured into a mold where the reaction takes place. Two methods of bulk polymerization may be used: batch cell or continuous. Batch cell is the most common because it is simple and is easily adapted for making acrylic sheets in thicknesses from 0.06 to 6.0 inches (0.16-15 cm) and widths from 3 feet (0.9 m) up to several hundred feet. The batch cell method may also be used to form rods and tubes. The continuous method is quicker and involves less labor. It is used to make sheets of thinner thicknesses and smaller widths than those produced by the batch cell method.

We will describe both the batch cell and continuous bulk polymerization processes typically used to produce transparent polymethyl methacrylic (PMMA) sheets.

Batch cell bulk polymerization

1. The mold for producing sheets is assembled from two plates of polished glass separated by a flexible "window-frame" spacer. The spacer sits along the outer perimeter of the surface of the glass plates and forms a sealed cavity between the plates. The fact that the spacer is flexible allows the mold cavity to shrink during the polymerization process to compensate for the volume contraction of the material as the reaction goes from individual molecules to linked polymers. In some production applications, polished metal plates are used instead of glass. Several plates may be stacked on top of each other with the upper surface of one plate becoming the bottom surface of the next higher mold cavity. The plates and spacers are clamped together with spring clamps.
2. An open comer of each mold cavity is filled with a pre-measured liquid syrup of methyl methacrylate monomer and catalyst. In some cases, a methyl methacrylate prepolymer is also added. A prepolymer is a material with partially formed polymer chains used to further help the polymerization process. The liquid syrup flows throughout the mold cavity to fill it.
3. The mold is then sealed and heat may be applied to help the catalyst start the reaction.
4. As the reaction proceeds, it may generate significant heat by itself. This heat is fanned off in air ovens or by placing the molds in a water bath. A programmed temperature cycle is followed to ensure proper cure time without additional vaporization of the monomer solution. This also prevents bubbles from forming. Thinner sheets may cure in 10 to 12 hours, but thicker sheets may require several days.
5. When the plastic is cured, the molds are cooled and opened. The glass or metal plates are cleaned and reassembled for the next batch.
6. The plastic sheets are either used as is or are annealed by heating them to 284-302°F (140-150°C) for several hours to reduce any residual stresses in the material that might cause warping or other dimensional instabilities.
7. Any excess material, or flash, is trimmed off the edges, and masking paper or plastic film is applied to the surface of the finished sheets for protection during handling and shipping. The paper or film is often marked with the material's brand name, size, and handling instructions. Conformance with applicable safety or building code standards is also noted.

Continuous bulk polymerization
The continuous process is similar to the batch cell process, but because the sheets are thinner and smaller, the process times are much shorter. The syrup of monomer and catalyst is introduced at one end of a set of horizontal stainless steel belts running parallel, one above the other. The distance between the belts determines the thickness of the sheet to be formed.

1. The belts hold the reacting monomer and catalyst syrup between them and move it through a series of heating and cooling zones according to a programmed temperature cycle to cure the material.
2. Electric heaters or hot air may then anneal the material as it comes out of the end of the belts.
3. The sheets are cut to size and masking paper or plastic film is applied.