The Basics - Polymer Definition & Properties

If you're after basic information on plastic materials, this is the place to find it. Here you'll learn the definition and properties of polymers, the building blocks of plastics.

Plastics are polymers. What is a polymer? The most simple definition of a polymer is something made of many units. Think of a polymer as a chain. Each link of the chain is the "-mer" or basic unit that is usually made of carbon, hydrogen, oxygen and/or silicon. To make the chain, many links or "-mers" are hooked or polymerized together. Polymerization can be demonstrated by linking countless strips of construction paper together to make paper garlands or hooking together hundreds of paper clips to form chains, or by a string of beads.

Plastic materials display properties that are unique when compared to other materials and have contributed greatly to quality of our everyday life. Plastics, properly applied, will perform functions at a cost that other materials cannot match. Many natural plastics exist, such as shellac, rubber, asphalt, and cellulose ; however, it is man's ability to synthetically create a broad range of materials demonstrating various useful properties that have so enhanced our lives. Plastics are used in our clothing, housing, automobiles, aircraft, packaging, electronics, signs, recreation items, and medical implants to name but a few of their many applications.

  The synthetic plastic industry started in 1909 with the development of a phenol formaldehyde plastic (Bakelite) by Dr. L. H. Bakeland. The phenolic materials are, even today, important engineering plastics. The development of additional materials continued and the industry really began to blossom in the late 1930's. The chemistry for nylons, urethanes, and fluorocarbon plastics were developed; the production of cellulose acetate, melamine, and styrene molding compounds began; and production of commercial equipment to perform the molding and vacuum forming processes began.

  Acrylic sheet was widely used in aircraft windows and canopies during World War II. A transparent polyester resin (CR-39), vinylidene chloride film (Saran), polyethylene, and silicone resins were also developed. The first polyethylene bottles and cellulose acetate toothpaste tubes were manufactured during this time period.

  The post war era saw the production of vinyl resins started, the use of vinyl films, molded automotive acrylic taillights and back-lighted signs introduced, and the first etched circuit boards developed. The injection molding process entered commercial production. Due to the newness of the materials, the properties and behavior of the plastic materials were not well understood. Many products were introduced that failed, creating a negative impression about plastics in the public's mind.

  Chemists continued the development of materials, such as ABS, acetals, polyvinyl fluoride, ionomers, and polycarbonate. The injection molding, thermoforming, extrusion, transfermolding, and casting processes were all improved. This allowed the industry to provide an even greater number of cost-effective products suitable for many, more demanding engineering applications.

In the early days...
  Bakelite (phenolic resin) is most often actually a product called Catalin. Both, along with Plaskon, are formaldehyde based plastics. Allow me to expand (with liberal borrowing from Dr. Stephen Z. Fadem - a true expert).

  Around the turn of the century, the Belgian born scientist Dr. Leo Baekeland, working as an independent chemist, came upon the compound quite by accident. He sold his rights to Velox to Eastman Kodak for three quarters of a million dollars and started developing a less flammable bowling alley floor shellac; bowling was becoming the latest rage in New York City. Dr. Baekeland soon realized that a resin that was both insoluble and infusible could have a much wider appeal when used as a molding compound. He obtained a patent and started the Bakelite Corporation around 1910.

  Phenolic resin could be produced in a multitude of colors, commonly yellow, brown, butterscotch, green and red. Omitting the pigment could produce a transparent or translucent effect. The resin could be molded or cast, depending on variations in the formula. For molding, the formula was cooked until resinous, spread out in thin sheets to harden, then ground to a fine consistency. At this point, powdered fillers and pigment were added, to enable the resin to be molded and to add color. This mixture was then put through hot rollers which created large sheets of colored, hardened resin. These sheets were then ground into a very fine powder which was molded under high heat and pressure into the final product form. As a molded material the resin's drawback was the limited range of colors which could be created. For casting, the formula was modified slightly, enabling the resin to be poured into lead molds and then cured in ovens until it polymerized into a hard substance. The liquid resin could be tinted to any color or "marbleized" by mixing two colors together.

  For the first ten years or so after its introduction, the resin was used primarily to make electrical and automobile insulators and heavy industrial products. Eventually, uses for the resin spread into the consumer market. Castings were made in the shape of cylinders or blocks, and then sold to novelty and jewelry makers. Industrial designers began experimenting with the new material. Fine craftsmen sculpted the molded products on fast wheels with razor-like tools to carve out designs that the world has not seen since; after World War II, most companies switched to creating designs through the use of patterned molds, instead of hand-carving. Bakelite replaced flammable celluloid, previously the most popular synthetic material for molded items, as a major substance for jewelry production.

  The process to the collector of today may not be significant, as Bakelite is now treasured for its unique, unreproducible beauty. A deeply carved half inch bangle bracelet may sell for $225.00, and a two and one half inch bangle may command $900.00. Bakelite often acquires a patina within a few months to a few years of its date of production, and metamorphisizes into a completely different appearing color. The red, white and blue Bakelite designs of yesterday have mellowed into lovely yellows, reds and blacks, enhancing further the value of those rare pieces which have continued to maintain their original color and luster.

  Bakelite's many uses allowed it to become a standard item in the family home of the 1930s and 1940s. It was frequently found in the kitchen, in the form of flatware handles, rabbit or chicken napkin holders, salt and pepper shakers, or serving trays. During the Depression Bakelite sold more than any other commercial product, and was loved by the public for its brilliant and cheerful colors and its affordability.

  When the Bakelite patent expired in 1927, it was acquired by the Catalin Corporation that same year. They began mass production under the name "Catalin," using the cast resin formula which enabled Catalin to add 15 new colors to the original five produced by the Bakelite Corporation, which used the limited color range molded formula, as well as the now-famous marbleized effect. One of their most notable products was the Fada bullet radio. The Catalin Corporation was responsible for nearly 70% of all phenolic resins that exist today.

  Bakelite-Catalin was sold mostly by Saks Fifth Avenue, B. Altman and Bonwit Teller, but was also on the shelves of F.W. Woolworth and Sears. To the wealthy socialites, whose husbands had fallen on tough times during the Depression, with Tiffany diamonds and Cartier jewelry now well beyond their means, the vibrantly colorful carved jewelry adorned with rhinestones became de riguer for cocktail parties and formal dinners. Yet, Catalin and Bakelite were within everyone's reach with Depression prices ranging from twenty cents to three dollars. Diana Vreeland, editor of Vogue, often spoke of the versatility of Bakelite, as did Elsa Schiaparelli, who was constantly contracting with the Bakelite and Catalin Corporations for exclusive buttons for her dress designs.

  But in 1942 Bakelite and Catalin suspended sales of their colorful cylinders to costume jewelry manufacturers in order to concentrate on the wartime needs of a nation which had totally shifted its focus. Defense phones and aviator goggles, as well as thousands of other Bakelite products, found their way to armed forces around the world. The scheme shifted from the 200 vibrant colors which brightened the dark days of the Depression to basic black, the no-nonsense symbol of a nation at war. By the end of the war, new technology had given birth to injection-molded plastics, and most manufacturers switched to less labor-intensive and more practical means of developing products. The next generation of plastics had been born - Acrylic, fiberglass, and vinyl - and they were molded into products commonplace in our everyday lives today.

  Occasionally plastics are still improperly used and draw negative comments. The thousands of successful applications that contribute to the quality of our life are seldom noticed and are taken for granted. Remember, MATERIALS DON'T FAIL, DESIGNS DO.

  The number of variations or formulations possible by combining the many chemical elements is virtually endless. This variety also makes the job of selecting the best material for a given application a challenge. The plastics industry provides a dynamic and exciting opportunity.

  Plastics encompass a large and varied group of materials consisting of different combinations or formulations of carbon, oxygen, hydrogen, nitrogen and other organic and inorganic elements. Most plastics are a solid in finished form; however, at some stage of their existence, they are a liquid and may be formed into various shapes. The forming is usually done through the application, either singly or together, of heat and pressure. There are over fifty different, unique families of plastics in commercial use today and each family may have dozens of variations.

MONOMERS: A, B, C
Examples of monomers are ethylene, styrene, vinyl chloride and propylene.
 

Figure 1a

HOMOPOLYMERS: A-A-A-A-A-A-A-A-A-
(Polymers constructed from a single material)
Examples of polymers built this way are polyethylene, and some acetals.
 

Figure 1b

COPOLYMERS:
(Polymers constructed from two different materials)
ALTERNATING TYPES: A-B-A-B-A-B-A-B-A-B-
Some examples of alternating copolymers are ethylene-acrylic and ethylene-ethyl acrylate.
 

Some examples of grafted copolymers are styrene-butadiene, styrene-acrylonitrile, and some acetals.
 

Figure 1d

TERPOLYMERS: A-B-C-A-B-C-A-B-C-
(Polymers constructed from three different materials)
An example of a terpolymer is acrylonitrile-butadiene-styrene (ABS).
 

Figure 1e

  The two monomers in a copolymer are combined during the CHEMICAL REACTION of polymerization. Materials called "ALLOYS" are manufactured by the SIMPLE MIXING of two or more POLYMERS with a resulting blending of properties which are often better than either individual material. There is no chemical reaction in this process. Some examples of "alloys" are Polyphenylene Oxide/High Impact Styrene, Polycarbonate/ABS, and ABS/PVC.

MOLECULAR WEIGHT

  It is important for the chemist to know how long the polymer chains are in a material. Changing the length of the chains in a thermoplastic material will change its final properties and how easily it can be shaped when it is melted.

  The "REPEATING UNIT" or molecular group in the homopolymer (Figure 1) is A-, the group of molecules in the copolymer A-B-, and in the terpolymer A-B-C-. The number of repeating units in the polymer chain is called the "DEGREE OF POLYMERIZATION." If the repeating unit has a molecular weight (the combined weight of all of the molecules in the repeating unit) of 60 and the chain or polymer has 1000 repeating units, then the polymer has a "MOLECULAR WEIGHT" of 60 x 1000 = 60,000. The molecular weight is a way of measuring how long the polymer chains are in a given material.

  The molecular weight of plastics is usually between 10,000 and 1,000,000. It becomes increasingly difficult to form or mold the plastic with the application of heat and pressure as the molecular weight increases. A molecular weight of about 200,000 is about the maximum for a polymer to still permit reasonable processability. Some higher molecular weight materials, like Ultra High Molecular Weight Polyethylene (UHMWPE) which has a molecular weight from 3,000,000 to 6,000,000, can be cast using processes specifically designed to shape it.

Molecular Arrangement of Polymers

Think of how spaghetti noodles look on a plate. This is similar to how polymers can be arranged if they lack a specific for or are amorphous. Controlling and quenching the polymerization process can result in amorphous organization. An amorphous arrangement of molecules has no long-range order or form in which the polymer chains arrange themselves. Amorphous polymers are generally transparent. This is an important characteristic for many applications such as food wrap, plastic windows, headlights and contact lenses.

Obviously not all polymers are transparent. The polymer chains in objects that are translucent and opaque are in a crystalline arrangement. By definition a crystalline arrangement has atoms, ions, or in this case, molecules in a distinct pattern. You generally think of crystalline structures in salt and gemstones, but not in plastics. Just as quenching can produce amorphous arrangements, processing can control the degree of crystallinity. The higher the degree of crystallinity, the less light can pass through the polymer. Therefore, the degree of translucence or opaqueness of the polymer is directly affected by its crystallinity.

Scientists and engineers are always producing better materials by manipulating the molecular structure that affects the final polymer produced. Manufacturers and processors introduce various fillers, reinforcements and additives into the base polymers, expanding product possibilities.

The Structure of Polymers

Many common classes of polymers are composed of hydrocarbons. These polymers are specifically made of small units bonded into long chains. Carbon makes up the backbone of the molecule and hydrogen atoms are bonded along the backbone.

There are polymers that contain only carbon and hydrogen. Polypropylene, polybutylene, polystyrene and polymethylpentene are examples of these.

Even though the basic makeup of many polymers is carbon and hydrogen, other elements can also be involved. Oxygen, chorine, fluorine, nitrogen, silicon, phosphorous and sulfur are other elements that are found in the molecular makeup of polymers. Polyvinyl chloride (PVC) contains chlorine. Nylon contains nitrogen. Teflon contains fluorine. Polyester and polycarbonates contain oxygen. There are also some polymers that, instead of having a carbon backbone, have a silicon or phosphorous backbone. These are considered inorganic polymers. One of the most famous silicon-based polymers is Silly Putty.
 

The Structure of Polymers  More 8

Thermal Properties of Polymers More 8