Guide to Working with Plastics: A Primer on Poured Shapes -- Tracking Down Fiberglass Flaws

Common Surface Flaws and How They Are Caused

--Star cracking. Radiating cracks may become visible in the gel coat if it has been applied in too thick a layer or if the underside of the laminate receives a sharp blow.

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Molecules Custom-Built to Serve a Specific Need

Most of the materials used for construction are nature’s gifts and can be manipulated only within fixed limits. Woods can be seasoned and formed, metals tempered and alloyed, stone crushed and shaped, but the basic qualities of each material are inbuilt and unchangeable. Because they are synthetic, however, plastics can be tailor- made: Not only the end product but the very character of the substance can be shaped to its purpose.

Leo Baekeland, a Belgian-born chemist working in New York, was the first scientist to create a truly made-to-order material—earlier versions of plastics, such as celluloid, had consisted of chemically treated natural sub stances. In 1907, searching for a substitute for shellac, Baekelarid mixed and heated two unpromising-looking liquids, phenol and formaldehyde, both smelly derivatives of coal. When his flask cooled, he found a resin altogether unlike its ingredients and far superior to shellac. He had produced the first synthetic plastic, which was later trade- named Bakelite in its creator’s honor.

Today, such metamorphoses of familiar substances or their extracts into synthetics possessing durability and ease of shaping not to be found in nature have become commonplace. Most of them involve a few colorless gases and a volatile liquid or two derived from petroleum or natural gas. Using a combination of heat, pressure, and catalysts—sub stances that hasten chemical change without taking part in it—industry converts these raw materials into the plastics that ease modern life at every turn. The secret of these remarkable transformations, from Baekeland’s successful simulation of shellac to the routine triumphs of today, lies in the invisible realm of molecular structure. A typical plastic molecule strings together hundreds or thousands of carbon atoms, each surrounded by a complement of other atoms. This chain of atoms, al though at best only a few billionths of an inch long, is a molecular giant.

Despite their size and complexity, these giant molecules are relatively easy to assemble. Petroleum and natural gas provide molecules containing only a few atoms, mostly carbon and hydro gen, often arranged like very short sections of the immense carbon chains of plastics. The magic of technology can cause these short sections of chain to line up and fuse together end to end, each of them adding several links to the larger chain. In the language of science, each small molecule is called a monomer; the result of their linking, the unwieldy but invaluable plastic molecule, is a polymer.

In its simplest versions, the plastic polymer consists of a single kind of monomer, repeated hundreds or thou sands of times over. Polyethylene, the waxy, pliant stuff of squeeze bottles, is a scrambled mass of such polymers. The monomers from which they are made are, as the name of the plastic indicates, molecules of ethylene gas.

Scientists can shape the properties of a plastic not only by choosing the monomers, but also by controlling the way these building blocks are arranged at the crucial moment when the monomers unite chemically to form the plastic polymers. In basic, or so-called low-density polyethylene, for instance, numerous side branches trail off the main carbon chains. These branching chains act as spacers, separating the molecules and reducing their tendency to cling to one another. This polyethylene is flexible and lightweight; it softens at moderate heat.

For a polyethylene that withstands higher temperatures, chemists have developed a way to assemble unbranched polymer chains. With no side chains to space them, these molecules pack more closely, cling together more tightly and make up a denser, less flexible plastic with greater heat resistance. Tougher still is a version with links between adjacent polymer chains. Created by drenching the plastic in high-energy radiation, these cross links bind the molecules in a rigid matrix, giving the familiar squeeze-bottle polyethylene enough durability for aerospace uses.

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---The invisible intricacies of plastics. Each of the six-atom monomers in the polyethylene molecule (left) has two atoms of carbon (in color) and four of hydrogen. Before they are strung together, these monomers make up explosive ethylene gas. But when thousands of them fuse, they form a molecule of resilient plastic. Many-branched chains (center) yield a pliant polyethylene that softens at 200° F. Linked in a latticework (below, right), the same chains constitute a polyethylene - that can withstand twice that temperature.