How Plastics Are Made

The Basics

The term "plastics" encompasses organic materials, such as the elements carbon (C), hydrogen (H), nitrogen (N), chlorine (Cl) and sulfur (S), which have properties similar to those naturally grown in organic materials such as wood, horn and rosin. Organic materials are based on polymers, which are produced by the conversion of natural products or by syththesis from primary chemicals coming from oil, natural gas or coal.

The plastic production process begins by heating the hydrocarbons in a "cracking process." Here, in the presence of a catalyst, larger molecules are broken down into smaller ones such as ethylene (ethene) C2H4, propylene (propene) C3H6, and butene C4H8 and other hydrocarbons. The yield of ethylene is controlled by the cracking temperature and is more than 30% at 850°C and such products as styrene and vinylchloride can be produced in subsequent reactions. These are then the starting materials for several other types of plastics. Therefore, this process results in the conversion of the natural gas or crude oil components into monomers such as ethylene, propylene, butene and styrene.

These monomers are then chemically bonded into chains called polymers. Different combinations of monomers yield plastic resins with different properties and characteristics. Each monomer yields a plastic resin with different properties and characteristics. Combinations of monomers produce copolymers with further property variations.

The resulting resins may be molded or formed to produce several different kinds of plastic products with application in many major markets. The variability of resin permits a compound to be tailored to a specific design or performance requirement. This is why certain plastics are best suited for some applications while others are best suited for entirely different applications. For instance, impact strength measures the ability of a material to withstand shock loading. Heat resistance protects the resin from exposure to excessive temperatures. Chemical resistance protects the resin from breakdown due to exposure to environmental chemicals.

Some examples of material properties in plastic product applications are:

The Structure of Polymers

Polymers are created by the chemical bonding of many identical or related basic units and those produced from a single monomer type are called homopolymers. 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 carbon backbone.

Diagram of polyethylene, the simplest polymer structure.

Polymers that contain primarily carbon and hydrogen are classified as organic polymers. 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, chlorine, 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 and these are considered inorganic polymers.

The Additives

When plastics emerge from reactors, they do not have the desired properties that make it a material of choice, that is, it is considered a raw material. In order to achieve a commercial product, the plastic is subject to further treatment and the inclusion of additives which are selected to give it specified properties. Most polymers are blended with additives during raw material processing into their finished parts. Additives are incorporated into polymers to alter and improve their basic mechanical, physical or chemical properties. Additives are also used to protect the polymer from the degrading effects of light, heat, or bacteria; to change such polymer properties as flow; to provide product color; and to provide special characteristics such as improved surface appearance or reduced friction.

Two Characterizations Of Plastic

A Thermoset is a polymer that solidifies or "sets" irreversibly when heated. Similar to the relationship between a raw and a cooked egg, once heated, a thermoset polymer can't be softened again and once cooked, the egg cannot revert back to its original form. Thermosets are valued for their durability and strength and are used primarily in automobiles and construction, although applications such as adhesives, inks, and coatings are also significant. Other examples of thermoset plastics and their product applications are:

A Thermoplastic is a polymer in which the molecules are held together by weak secondary bonding forces that soften when exposed to heat and return to its original condition when cooled back down to room temperature. When a thermoplastic is softened by heat, it can then be shaped by extrusion, molding or pressing. Ice cubes are a common household item which exemplify the thermoplastic principle. Ice will melt when heated but readily solidifies when cooled. Like a polymer, this process may be repeated numerous times. Thermoplastics offer versatility and a wide range of applications. They make up the greatest share of plastics used in food packaging because they can be rapidly and economically formed into any shape needed to fulfill the packaging function. Examples include milk jugs and soda bottles. Other examples of thermoplastics are:

source: http://www.holloplastics.com/plasticsinfo/how/index.html 23may01

The Basics of  Blow Molding:

There are three types of blow molding processes --extrusion, injection and stretch blow molding-- that vary widely in applications and thermoplastic resins used.

Continuous Extrusion:

In this method, a separate molding and cooling station on the equipment allows the parison to be continuously formed.  This technique is used mainly for small thin-walled parts ranging up to containers with five gallon capacities.  Parison programming can be used to vary the wall thickness.  Continuous extrusion also allows the use of heat-sensitive materials due to streamlined flow areas and die designs.

Intermittent Parison Extrusion:

This technique is performed in three basic ways --reciprocating, ram accumulator, and accumulator head systems.  All three vary in machine design and the flow of molten resin through the die for parison forming.  However, each system is designed to produce larger, heavier, and thicker parts than continuous extrusion.

Stretch Blow Molding:

Blow moldable grades of material are initially injection molded into preform shapes.  These preforms are then thermally conditioned and then stretched (utilizing pneumatically operated stretch rods) low pressure air, followed by high pressure air up to 40 bar to form axially oriented parts with molded in necks.  The process is used to manufacture PET bottles.

Basic Blow Molding Cycle:

This process utilizes various thermoplastic materials in a solid pelletized state and converts these materials by way of heat, pressure and compressed air into a finished good stat.

The pellitized raw material is conveyed to the feed section of a plasticating extruder by way of a vacuum loader or auger screw.  The raw material is then conveyed forward through the extruder and is plastisized to a molten state of between 350 degrees and 500 degrees F. by way of a feed screw and external heating elements.

The material in a melt state is then reshaped into a round hollow geometry termed a parison.  This parison is then extruded vertically from the head section of the machine through a round die at various outside and inside diameters.

After extrusion of the parison between the two halves of a mold the press section closes encapsulating the parison inside the mold halves.  Upon mold close compressed air is entered into the parison by way of a centrally located air pipe or by piercing air needles.

The molds are chilled with cooled water which transfers the hear form the now formed part inside the mold.  Upon complete part cooling the press section opens and the finished product is removed.  The material which is pinched off outside the mold cavity, or the flash, is then fed into a granulator which cops the flash into a granule size which can be fed back to the feed section of the extruder.

Some of the raw materials used absorb moisture from the atmosphere and therefore require drying by use of a hot air desiccant dryer prior to being conveyed to the extruder.

source: http://www.holloplastics.com/plasticsinfo/definitions/index.html 23may01

Needed to make a useful industrial plastic, is cheap material with very weird properties, specifically, molecules of the material have to react with one another and end up joined, making molecules (monomers) into huge long chains (polymers).

Ethylene, a common component of natural gas, reacts to form very long polyethylene molecules, which plastic bags and milk bottles are made of. The key point is that no chemical reactions "go to completion." That is, 100 molecules of A get together with 100 molecules of B, the result will always be less than 100 molecules of AB, with some left over A and B. The reason is essentially that each time A bumps into B there's always some small chance it won't react, a chance that gets smaller but never vanishes as the reaction gets more favorable.

For decent-sized plastic molecules a *million* or so monomers need to be hooked together. The material has to react with itself so well that only 1 out of every million monomers will be left over when the reaction is complete. (The reaction must go "99.9999% to completion.") There are almost no chemical reactions known that do this, and the few that do are the basis for the entire plastics industry and all life on Earth. (Example: wood and cotton are made of cellulose, which is a chain of 15,000 glucoses -- sugar molecules -- strung together.)

The search for new chemical reactions leading to totally new plastics goes on, because of the huge ($billions) payoff. The odds are better playing the lottery.

source: http://www.newton.dep.anl.gov/askasci/chem99/chem99294.htm

Polymerization Terms

1. BLOWING AGENT OR FOAMING AGENT - Chemicals added to plastics or rubbers that generate inert gases on heating, causing the resins to assume a cellular structure.

2. CATALYST - Chemicals often used to initiate polymerization. They are supposed to act by their presence and not be affected by the chemical reaction which they induce. In plastics, however, the catalyst can be combined and change its structure.

3. COMPRESSION SET - Measures the resistance of material to permanent deformation. In this test a rubber pellet is squashed to 75% of its original height, kept at that amount of deformation for 22 hours at 158°F, then released and allowed to return to original height. The value reported is the percentage not returned to the original height, so the smaller the number, the better.

4. CURE - Polymerization. A change of physical properties by chemical reaction and chain linking of ingredients; usually accomplished by heat and/or catalysts, with or without pressure.

5. ELASTOMER (elastomeric) - An elastic, rubber-like substance, as natural or synthetic rubber.

6. EXOTHERM - A chemical reaction giving off heat. 

7. FILLER - An inert substance added to a plastic to make it less costly, improve physical properties such as hardness, stiffness, and impact strength.

8. FLASH - Extra material attached to a casting along the parting line.

9. GATE - Opening in a mold through which liquid is admitted.

10. HARDNESS - Shore A durometer, Shore D durometer, hardness measures the resistance of a material to indentation. The two hardness measuring devices are basically needles on a spring that measure how far the needle indents the material. The Shore A is a dull needle on a weak spring for measuring elastomers. The Shore D is a sharper needle on a stronger spring for measuring rigid materials. These devices are excellent for measuring and determining if a cast material is curing properly. Most people, however, use the durometer measures as the first criteria for determining the material they need as "I would like to have a Shore A rubber that will . . ." The hardness gives an indication of the type of properties to expect from a material but is not always the indicator of performance.

11. ISOCYANATE RESINS - Based on the combination with polyols (such as polyesters, polyethers, etc.). The reaction joins members through the formation of the urethane linkage. 

12. LAY-UP - Process of placing reinforcing material into position in a mold.

13. MODULUS - A key physical property than any user of a material will definitely experience. The modulus is basically the stiffness of the material, or more specifically, the modulus is the amount of force needed to deform a material a set amount. Modulus is measured in psi (pounds per square inch). Modulus can be measured in any mode of deformation, i.e., tension (stretching), compression, (crushing), flexing, (bending), or torsion (twisting).

14. MOLD RELEASE AGENT (separator or parting agent) - A lubricant used to coat a surface to prevent plastic from sticking to it.

15. MOTHERMOLD - A rigid material used to hold or house a flexable inner mold.

16. POT LIFE AND GEL TIME - These two terms are sometimes used interchangeably but really have two different meanings. Pot life is the amount of time, after two materials have been mixed, that the material remains workable, i.e., pourable for a liquid, trowelable for paste. Gel time is the time after mixing when the materials become a continuous mass.

17. SLURRY - Mixture of a liquid and a fine filler material.

18. SLUSH CASTING - A method for casting thermoplastic in which liquid resin is poured into a mold where a viscous skin forms.

19. SPECIFIC GRAVITY - The specific gravity of water is 1. When a resin has a lower specific gravity than water, less than 1, it is lighter, will float, and has more volume for less weight. Conversely, a resin with a higher specific gravity than water is heavier, will not float, and has less volume.

20. TEAR PROPERTIES - How resistant the material is to tearing measured in pli (pounds per linear inch).

21. TENSILE PROPERTIES - Ultimate tensile strength, elongation at break, 100% modulus. Ultimate tensile strength is the force, measured in psi, needed to stretch a material until it breaks. Elongation at break is how much the material stretches before it breaks, as a percentage of its original dimensions. 100% modulus is the force, measured in psi, needed to stretch the material to twice its original dimensions.

22. THERMOPLASTIC - Material that softens when heated and hardens on cooling.

23. THERMOSETTING - A substance that does not soften when heated.

24. THIXOTROPIC - The ability of a liquid to resist the pull of gravity. Materials that are gel-like at rest and fluid when agitated.

25. UNDERCUT - Protuberances or indentations that lock a solid form into its mold and prevent its removal.

26. VENT - A shallow channel or hole cut into a mold to allow air to escape as it is being deplaced by a liquid.

27. VISCOSITY - Simply the resistance of the material to flow measured in centipoise (cpi). A material with a low viscosity, low cps, and will flow easily. Water's viscosity is 1 cps. Materials with a high viscosity, high cps, will not flow easily. Peanut butter has a viscosity of about 250,000 cps.

source: http://www.synair.com/terms 14nov01

If you have come to this page from an outside location click here to get back to mindfully.org