Understanding Polymer Processing

Polymers

Macromolecules

Plastics

1.1	Historical Background

Natural polymeric materials, such as rubber, have been in use for thousands of years. Natural rubber also known as caoutchouc (crying trees) has been used by South American Indians in the manufacture of waterproof containers, shoes, torches, and squeeze bulb pumps. The first Spanish explorers of Haiti and Mexico reported that natives played games on clay courts with rubber balls [1]. Rubber trees were first mentioned in De Orbe Novo, originally published in Latin, by Pietro Martire d'Anghiera in 1516. The French explorer and mathematician Charles Maria de la Condamine, who was sent to Peru by the French Academie des Sciences, brought caoutchouc from South America to Europe in the 1740s. In his report [2] he mentions several rubber items made by native South Americans, including a piston-less pump composed of a rubber pear with a hole in the bottom. He points out that the most remarkable property of natural rubber is its great elasticity.

The first chemical investigations on gummi elasticum were published by the Frenchman Macquer in 1761. However, it was not until the 20th century that the molecular architecture of polymers was well understood. Soon after its introduction to Europe, various uses were found for natural rubber. Gossart manufactured the first polymer tubes in 1768 by wrapping rubber sheets around glass pipes. During the same time period small rubber blocks where introduced to erase lead pencil marks from paper. In fact, the word rubber originates from this specific application — rubbing.

These new materials slowly evolved from their novelty status as a result of new applications and processing equipment. Although the screw press, the predecessor of today's compression molding press, was patented in 1818 by McPherson Smith [3], the first documented polymer processing machinery dates to 1820 when Thomas Hancock invented a rubber masticator. This masticator, consisting of a toothed rotor in a toothed cylindrical cavity [4], was used to reclaim rubber scraps that resulted from the manual manufacturing process of elastic straps, perhaps the first recycling effort. In 1833 the development of the vulcanization process by Charles Goodyear [5] greatly enhanced the properties of natural rubber, and in 1836 Edwin M. Chaffee invented the two-roll steam heated mill, the predecessor of the calendar. It was used to continuously mix additives into rubber for the manufacture of rubber-coated textiles and leathers. As early as 1845, presses and dies were used to mold buttons, jewelry, dominoes, and other novelties out of shellac and gutta-percha. Gutta-percha (rubber clump), a gum found in trees similar to rubber, became the first wire insulation and was used for ocean cable insulation for many years.

The ram-type extruder was invented by Henry Bewley and Richard Brooman in 1845. The first polymer processing screw extruder, the most influential equipment in polymer processing, was patented by an Englishman named Mathew Gray in 1879 for the purpose of wire coating. However, the screw pump is attributed to Archimedes and the actual invention of the screw extruder by A.G. DeWolfe of the U.S. dates back to the early 1860s.

Cellulose nitrate plasticized by camphor, possibly the first thermoplastic, was patented by Isaiah and John Hyatt in 1870. Based on experience from metal injection molding, the Hyatt brothers built and patented the first injection molding machine in 1872 to mold cellulose materials [6].

With the mass production of rubber, gutta-percha, cellulose, and shellac articles during the height of the industrial revolution, the polymer processing industry after 1870 saw the invention and development of internal kneading and mixing machines for the processing and preparation of raw materials [7]. A notable invention was the Banbury mixer, developed by Fernley Banbury in 1916. This mixer, with some modifications, is still used for rubber compounding.

Bakelite, developed by Leo Baekeland in 1907, was the first synthetically developed polymer. Bakelite, also known as phenolic, is a thermoset resin that reacts by condensation polymerization occurring when phenol and formaldehyde are mixed and heated.

In 1924, Hermann Staudinger proposed a model that described polymers as linear molecular chains. Once this model was accepted by other scientists, the concept for the synthesis of new materials was realized. In 1927 cellulose acetate and polyvinyl chloride (PVC) [8] were developed. Because of its higher wear resistance, polyvinyl chloride replaced shellac for phonograph records in the early 1930s. Wallace Carothers pioneered condensation polymers such as polyesters and polyamides. It was not until this point that the scientific world was finally convinced of the validity of Staudinger's work. Polyamides, first called Nylon, were set into production in 1938. Polyvinyl acetate, acrylic polymers, polystyrene (PS), polyurethanes, and melamine were also developed in the 1930s [9].

The first single-screw extruder designed for the processing of thermoplastic polymers was built circa 1935 at the Paul Troester Maschinenfabrik [10]. Around that same time period, Roberto Colombo developed a twin-screw extruder for thermoplastics.

World War II and the post-war years saw accelerated development of new polymeric materials. Polyethylene (PE), polytetrafluoroethylene, epoxies, and acrylonitrile-butadiene-styrene (ABS) were developed in the 1940s, and linear polyethylene, polypropylene (PP), polyacetal, polyethylene terephthalate (PET), polycarbonate (PC), and many more materials came in the 1950s. The 1970s saw the development of new polymers such as polyphenylene sulfide and in the 1980s, liquid crystalline polymers were developed.

Developing and synthesizing new polymeric materials has become increasingly expensive and difficult. Developing new engineering materials by blending or mixing two or more polymers or by modifying existing ones with plasticizers is now widely accepted.

The world's annual production of polymer resins has experienced steady growth since the turn of the century, with growth resins to that of steel and aluminum over the past 70 years. Before 1990, the figure depicts the production of plastics in the Western World and after that, when the iron curtain came down, the worldwide production. In developed countries, the growth in annual polymer production has decreased recently. However, developing countries in South America and Asia are now starting to experience tremendous growth.

Of the over 50 million tons of polymers produced in the U.S. in 2008, 90% were thermoplastics. Figure 1.2 breaks the U.S. polymer production into major polymer categories, including polyethylenes, polypropylene, polystyrene, polyvinyl chloride, and thermosets. Polyethylenes are by far the most widely used polymeric material, accounting for 41% of the U.S. plastic production.

1.2	General Properties

Any plastic resin can be categorized as either a thermoplastic or thermoset. Thermoplastics are those polymers that solidify as they cool, restricting the motion of the long molecules. When heated, these materials regain the ability to “flow,” as the molecules can slide past each other easily. Thermoplastic polymers further are divided into two classes: amorphous and semi-crystalline polymers.

Amorphous thermoplastics have...