HOW PLASTICS ARE MADEINTRODUCTION
When compared to other materials, plastics have unique properties, causing them to contribute greatly to the quality of our everyday life. Plastics, properly applied, will perform functions at a cost 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 has further enhanced our lives. Plastics are used in our clothing, housing, automobiles, aircraft, packaging, electronics, signs, recreation items and medical implants, which are only a few of their many applications.
Most plastics are different from other materials such as glass, steel and aluminum because they can CREEP or change shape when stressed. Ignoring this fact is the greatest cause of failures in designs using plastics. This subject is discussed in greater detail throughout the manual.
This book focuses on three major markets of plastic materials: mechanical and engineering plastics, sign and display and architecture.
The mechanical and engineering plastics market segment, also referred to as engineered plastics, covers plastic materials that are primarily used in wear applications. Typical examples include applications where external loads are applied to the plastic material such as bearings, bushings, wear strips, liners, sheaves and gears. Plastic materials generally used in this market segment are semi-crystalline thermoplastics.
The sign and display market is all around us. These materials are used to sell products, advertise companies and display merchandise. The better they look, the more of an impact they have on consumers’ hearts and minds. Different trends are a constant, always-changing challenge for retailers, designers and advertisers. Examples include store fixtures, tradeshow displays and all kinds of signage. Typical plastics used in this market include acrylic, ABS (acrylonitrilebutadiene-styrene), CAB (cellulose acetate butyrate), foam boards, nylon, polycarbonate, polyesters, polyethylenes, polystyrene and PVC (polyvinyl chloride).
Architecture is the science of designing and erecting buildings (commercial, residential, industrial), which furnishes a practical use along with aesthetic solutions. Plastics continue to make significant inroads into this market due to increasing cost competitiveness with traditional materials (wood and metal) and their properties of interest to architects: durability, color range, transparency, light weight, ability to withstand the elements, high strength-to-weight ratio, ease of cleaning and corrosion resistance, etc. Plastic materials normally seen in the architecture market are acrylic, polycarbonate, PVC, PVC foam, ACM (aluminum composite materials) and CPVC (chlorinated polyvinyl chloride).
Why should plastics be considered as either replacement materials or in a new, ground-up design? Plastics, as a general family of products, share several demonstrable advantages over traditional materials (metals, wood, glass, etc.):
- Weight Savings: For example, nylon is 1/16 the weight of steel, acrylic is 1/2 the weight of glass and PVC is 1/16 the weight of copper. The lower weight per cubic inch of plastics can result in power savings (lower horsepower needs), labor savings (less personnel required to install a product) and other potential benefits
- Corrosion Resistance: Most plastics withstand the effects of common chemicals, water and a wide variety of solvents, acids and other corrosive liquids.
- Impact Resistance: Plastics can take a punch! Their inherent ability to resist breakage from impact makes them a wise choice in tough applications in both commercial and industrial settings.
- Low Friction/Self-Lubricating: There can be considerable cost savings when the need to lubricate is reduced or even eliminated. Plastics can also include additives that improve their inherent good to excellent wear properties. This is especially true in the class of engineering plastics.
- Abrasion Resistance: Closely linked to low-friction characteristics, products that wear longer reduce maintenance needs, promote longer run-times and can result in improved productivity.
- Optical Clarity: Other than glass, what other nonplastic materials are transparent? Plastics can combine light transmittance, light weight, strength and impact resistance to achieve truly unique products.
- Ease of Machining/Fabrication: Plastics are typically easier to machine than most metals, cause less wear on machine tools, are easier to handle during the machining process, reduce finishing costs, are easily decorated and can exhibit integral color.
- Environmentally Friendly: Contrary to some misguided popular opinions, plastics are green! Plastics require far less energy to produce than metals and glass and many types are recyclable. Their light weight helps fuel efficiency on cars, trucks and airplanes. Plastics are good insulators, saving heating and cooling costs, resulting in more comfortable living spaces. In addition, bio-degradable plastics are becoming more readily available.
Remember, it is always important to correctly match the material to the application, keeping in mind several factors, resulting in an optimum value proposition.
HISTORY OF PLASTICS
The synthetic plastic industry started in 1909 with the development of a phenol formaldehyde plastic (Bakelite) by Dr. L. H. Baekeland. 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 1930s. The chemistry for nylons, urethanes and fluorocarbon (Teflon®) plastics were developed; the production of cellulose acetate, melamine and styrene molding compounds began; equipment to perform the molding and vacuum forming processes was made commercially available.
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 postwar era saw the production of vinyl resins, the use of vinyl films, the introduction of molded automotive acrylic taillights and back-lighted signs, and the development of the first etched circuit boards. The injection molding process entered commercial production. Due to the newness of the materials, the properties and behaviors of the plastic materials were not completely 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, transfer molding 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.
Occasionally, plastics are still improperly used and draw negative comments. It is frequently forgotten that materials don’t fail, designs do, and the thousands of successful applications that contribute to the quality of our life are seldom noticed and are taken for granted.
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.
HOW PLASTICS ARE PROCESSED
Plastics encompass a large and varied group of materials consisting of different combinations or formulations of carbon, oxygen, hydrogen, nitrogen and other elements. Most plastics are a solid in finished form; however, at some stage of their existence, they are made to flow 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 50 different, unique families of plastics in commercial use today and each family may have dozens of variations.
How are plastics made? The word “mer” is a Greek word that means “part.” This part of a plastic is a unique combination of atoms combining to form molecules which are called “monomers.” It is like a single link in a chain. “Mono” means “one”; therefore, “monomer” means “one part.” The monomers are then joined together to make long chains that result in a material with a useful blend of properties. Using another Greek word “poly” which means “many,” the long chain of “mers” forms a “polymer.” The monomers are held together in a polymer chain by very strong attractive forces between molecules. Much weaker forces hold the polymer chains together. The polymer chains can be constructed in many ways. Some simplified examples of the way polymers are built are shown in Figures 1.1-1.5:
MONOMERS: A, B, C
Examples of monomers are ethylene, styrene, vinyl chloride and propylene.
Homopolymers are polymers constructed from joining like monomers. Some examples of polymers built this way are polyethylene, polystyrene and PVC.
Copolymers are polymers constructed from two different monomers.
ALTERNATING TYPES: A-A-A-B-A-A-A-B-A-A-A-B
Some examples of alternating copolymers are ethyleneacrylic and ethylene-ethyl acrylate.
GRAFT TYPES: A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A
l l l l l
B B B B B
l I l
B B B
Some examples of grafted copolymers are styrene-butadiene, styrene-acrylonitrile and some acetals.
Terpolymers are polymers constructed from three different materials. An example of a terpolymer is acrylonitrilebutadiene-styrene (ABS).
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.
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 is A- (Figure 1.2), the group of molecules in the copolymer A-B- (Figure 1.3), and in the terpolymer A-B-C- (Figure 1.5). 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 1,000 repeating units, then the polymer has an average “molecular weight” of 60 x 1,000 = 60,000. The molecular weight is a way of measuring the length of the polymer chains 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 (UHMW-PE), which has a molecular weight from 3,000,000 to 6,000,000, can be cast using processes specifically designed to shape it.
CRYSTALLINE AND AMORPHOUS MATERIALS
Polymers are often described as being either crystalline or amorphous when it is actually more accurate to describe plastics by their “degree of crystallinity.” Polymers cannot be 100 percent crystalline, otherwise they would not be able to melt due to the highly organized structure. Therefore, most polymers are considered semi-crystalline materials with a maximum of 80 percent crystallinity.
Amorphous materials have no patterned order between the molecules and can be likened to a bowl of wet spaghetti. Amorphous materials include atactic polymers since the molecular structure does not generally result in crystallization. Examples of these types of amorphous plastics include polystyrene, PVC and atactic polypropylene. The presence of polar groups, such as a carbonyl group CO in vinyl type polymers, also restrict crystallization. Polyvinyl acetate, all polyacrylates and polymethacrylates are examples of carbonyl groups being present and the resulting polymers being amorphous. Polyacrylonitrile is an exception to this since carbonyl groups are present in their structure but they also crystallize. Even amorphous materials can have a degree of crystallinity with the formation of crystallites throughout their structure. The degree of crystallinity is an inherent characteristic of each polymer but may also be affected or controlled by processes such as polymerization and molding.
Crystalline materials exhibit areas of highly organized and tightly packed molecules. These areas of crystallinity are called spherulites and can be varied in shape and size with amorphous areas between the crystallites. The length of polymers contributes to their ability to crystallize as the chains pack closely together, as well as overlapping and aligning the atoms of the molecules in a repeating lattice structure. Polymers with a backbone of carbon and oxygen, such as acetals, readily crystallize. Plastic materials, such as nylon and other polyamides, crystallize due to the parallel chains and strong hydrogen bonds of the carbonyl and amine groups. Polyethylene is crystalline because the chains are highly regular and easily aligned. Polytetrafluoroethylene (PTFE) is also highly symmetric, like polyethylene, with fluorine atoms replacing all the hydrogens along the carbon backbone. It, too, is highly crystalline. Isomer structures also affect the degree of crystallinity. As the atactic stereochemistry resulted in amorphous polymers, those that are isotactic and syndiotactic result in crystalline structures forming as chains align to form crystallites. These stereospecific forms of polypropylene are those which are preferable for structural applications due to their degree of crystallinity.
The degree of crystallinity affects many polymeric properties. In turn, other characteristics and processes affect the degree of crystallinity. Molecular weight will affect the crystallinity of polymers. The higher the molecular weight, the lower the degree of crystallinity and the areas of crystallites are more imperfect. The degree of crystallinity also depends on the time available for crystallization to occur. Processors can use this to their advantage by quenching or annealing to control the time for crystallization to occur. Highly branched polymers tend to have lower degrees of crystallinity, as is easily seen in the difference between branched low-density polyethylene (LDPE) and the more crystalline high-density polyethylene (HDPE). LDPE is very flexible, less dense and more transparent than HDPE. This is an excellent example that the same polymer can have varied degrees of crystallinity. Stress can also result in increased crystallinity as polymer chains align orienting the crystallites. Drawing fibers, the direction of extrusion and gate placement will also affect the orientation of polymers and therefore the crystallites of the material. This allows the processor to maximize the effects and benefits of the inherent crystallinity of the polymer being used in an application. The chart below describes other general effects due to the degree of crystallinity in a polymer.
|COMMON CHARACTERISTICS OF CRYSTALLINE AND AMORPHOUS PLASTICS|
|Higher Percent Crystalline||Higher Percent Amorphous|
|• Higher heat resistance||• Gradual softening/ melting point|
|• Sharper melting point||• More translucent/transparent|
|• More opaque||• Lower shrinkage upon cooling|
|• Greater shrinkage upon cooling||• Greater low-temperature toughness|
|• Reduced low-temperature toughness||• Lower dimensional stability|
|• Higher dimensional stability||• Higher creep|
|• Lower creep|
THERMOPLASTIC AND THERMOSET MATERIALS
The terms thermoset and thermoplastic have been traditionally used to describe the different types of plastic materials. A thermoset allows only one chance to liquefy and shape it. These materials can be cured or polymerized using heat and pressure, or as with epoxies, a chemical reaction started by a chemical initiator.
A thermoplastic can be melted and shaped several times. The thermoplastic materials are either crystalline or amorphous. Advances in chemistry have made the distinction between crystalline and amorphous less clear, since some materials like nylon are formulated both as a crystalline material and as an amorphous material.
Again, the advances in chemistry make it possible for a chemist to construct a material to be either thermoset or thermoplastic. The main difference between the two classes of materials is whether the polymer chains remain linear and separate after molding (like spaghetti) or whether they undergo a chemical change and form a network (like a net) by cross-linking. Generally, a cross-linked material is a thermoset and cannot be reshaped. Due to recent advances in polymer chemistry, the exceptions to this rule are continually growing. These materials are actually cross-linked thermoplastics with the cross-linking occurring either during the processing or during the annealing cycle. The linear materials are thermoplastic and are chemically unchanged during molding (except for possible degradation) and can be reshaped again and again.
As previously discussed, cross-linking can be initiated by heat, chemical agents, irradiation or a combination of these factors. Theoretically, any linear plastic can be made into a cross-linked plastic with some modification to the molecule so that the cross-links form in orderly positions to maximize properties. It is conceivable that, in time, all materials could be available in both linear and cross-linked formulations.
The formulation of a material, cross-linked or linear, will determine the processes that can be used to successfully shape the material. Generally, cross-linked materials (thermosets) demonstrate better properties, such as improved resistance to heat, less creep and better chemical resistance than their linear counterpart; however, they will generally require a more complex process to produce a part, rod, sheet or tube.
Thermoplastics That Can Be
Cross-linked After Processing
HOW PLASTICS ARE MADE
MANUFACTURING THERMOPLASTICS AND THERMOSETS Plastics are changed into useful shapes by using many different processes. The processes that are used to mold or shape thermoplastics basically soften the plastic material so it can be injected into a mold, extruded through a die, or formed in or over a mold. The processes usually allow any scrap parts or material to be ground up and reused. Some of the more common processes are injection molding, extrusion, blow molding, rotational molding, calendering, thermoforming (which includes vacuum forming) and casting.
Thermosets must use a process that allows the material to flow to the desired shape and then become polymerized or cross-linked and rigid. The material cannot be remelted or reused after cross-linking occurs. Some of the processes commonly used to process thermoset materials are injection molding, transfer molding, compression molding, rotational molding, hand (or spray) lay-up, lamination and filament winding.
Injection molding is used to make three-dimensional shapes with great detail. The material is placed in the hopper of an injection molding machine where it is fed into a chamber to be melted. The melting is achieved by conducting heat into the material in a “plunger” machine while the material is primarily heated by shearing or mechanically working the material in a “screw” machine. Several shots of material are being heated and held in the injection unit. The maximum volume of material a machine can inject in a single shot determines its shot capacity. The capacity is given in ounces of a material.
Once melted, the material is forced, under pressure, into the mold where it conforms to the shape of the cavity. The mold is temperature controlled, usually by circulating water through it. Once the part is cooled, the mold is opened and the part removed. The mold is then closed and ready for the next shot. The mold is clamped shut while the material is being injected into the cavity since the cavity pressure may be as much as 5,000 psi (34 MPa). The clamp is sized by the “tonnage” it holds. An injection molding machine will be referred to by its shot size in ounces and its tons of clamping ability. An example would be a 6-ounce (200-g), 80-ton (72.6-metric ton) machine.
The molds are most often made out of hardened steel and carefully finished. They may also be made out of prehard steel, aluminum or epoxy. The type of mold material selected depends on the number of parts to be made and the plastic material to be used. Prototype parts are often machined to test the shape and function of a part before a mold is built.
The injection molding of thermosets is similar to the injection molding of thermoplastics, except the material is kept cool until it is pushed into the heated mold where it is cross-linked. The mold is then opened and the hot, but rigid part is removed.
Extrusion is like squeezing toothpaste out of its tube. The process produces continuous shapes like sheet, pipe, film, tubing or gasketing. The material is fed into the extruder where it is melted and pumped out of the extrusion die. The die and the take-off line shape the material as it cools and controls the final dimensions of the cross section of the shape. The equipment is designed and controlled to produce melted plastic at a very uniform temperature and pressure which controls the size and quality of the extruded product.
The extrusion process is also used with a system of molds and called “blow molding.” This is how bottles such as gallon milk bottles are produced.
In ram extruders, a hydraulic ram forces the polymer through a die. Resin drops from a hopper into the cylinder and a ram slides back and forth to push material into the die where it is forced into a shape and cooled. Ram extrusion is a pressure-sintering process for the continuous production of profiles from high-molecular weight polymers. It is particularly used in the processing of PTFE and UHMW-PE and is generally used for specialty processing.
Melt extrusion is the process where resin drops into the screw. This screw feeds the die with a constant supply of resin in which the product is forced into a shape and comes out the die. Sheet, rod and profiles are made by this process. In the sheet manufacturing process, the hot plastic comes out of a die in the approximate width and thickness needed. It then goes through three rolls that relieve stress and apply a finish to the top and bottom surfaces.
Coextrusion is a process where two different plastic materials or two different grades of the same plastic material are extruded, one above the other material. Coextrusion permits multiple-layer extrusion of film, sheet, pipe, tubing, profiles, wire coating and extrusion coating. It is used often in packaging applications to obtain desired barrier properties. The process eliminates the need for a laminator for plastic.
Paste extrusion is a process for producing thin tubing, thick tubes (liners), wire insulation and unsintered tapes of PTFE. For the PTFE paste technique exclusively, emulsion polymerization powder is used. The paste is a mixture of these spongy PTFE powder types with wetted organic fluids like gasoline, light oil, etc.
The process is done in batch quantities: The PTFE lubricant mixture is pre-pressed to so-called billets (pre-forms, candles) and then put into a paste extruder that works on the principle of the ram extrusion. Then the billets are pressed through a conic nozzle in cold or slightly pre-heated conditions (approximately 40°F to 60°F/4°C to 15°C). The extrudate is separated from the nozzle still containing lubricant within the drying zone and then sintered.
Due to the different demand requirements each paste extrusion is adapted to the customized requirements.
Casting is a process that involves mixing a base resin with a catalyst and pouring the mixture into the mold. These materials can be thermoplastic or thermoset and the process may or may not require adding heat to the mold. This process is used to make sheet, rod, tube and finished or semi-finished parts. The process requires considerable process control to obtain high-quality parts. Tubes, rods, sheets and slabs are often made this way. Several examples of plastics that are cast include acrylic, nylon, epoxy and urethane.
Molds can be made from steel, aluminum, urethane, epoxy, silicone, cardboard and other materials. Much of this depends on how many parts are desired off the tooling and if the part is completed to size and finished out of the mold or if post-finishing is planned.
Compression molding is one of the simplest methods of converting polymers. Material is poured into a cavity and leveled to provide a flat and uniform charge. A top is then put over the cavity and the blank is ready to be put into a heated press. The size and thickness of the cavity dictates the size of the finished product.
The press can be heated in a variety of ways depending on the size of the charge. For sheets and thicker parts, the press is usually steam, hot oil or electrically heated. For smaller parts, it is possible to use electrical cartridges or strip heaters. It is critical to heat the press evenly over its entire surface area to ensure proper curing.
Hydraulic or pneumatic cylinders are usually used to operate the press. Again, it is critical to obtain even pressure over the entire surface area of the charge. The amount of pressure can be adjusted depending on the type of material being processed.
Plastic material in the form of a powder is placed into a hollow mold. The mold is heated and rotated about two axis. The plastic melts and evenly coats the inside of the mold. As the mold cools, the plastic solidifies. This process is good for making large lightweight parts like tanks, children’s toys or canoes.
The charge is generally left in the press through the heating and cooling cycle. This helps to relieve any internal stress. Once the material has been cured and cooled under pressure, it is taken out of the press and the polymer is removed from the cavity. It is often necessary to trim the edges to finish the product.
In transfer molding, only enough material for one shot is placed in a separate chamber or pot. The material is then pushed from the pot into the hot mold and cross-linked. All of the “cured” material is removed from the machine and another charge is loaded for the next shot.
Calendering is a process that usually uses four heated rolls rotating at slightly different speeds. Again the material is fed into the rolls, heated and melted, and then shaped into sheet or film. Polyvinyl chloride (PVC) is the most commonly calendered material.
HAND (OR SPRAY) LAY-UP
Hand lay-up is used to produce products such as fiberglass boats and camper shells. The plastic resin, usually a polyester, is rolled or sprayed with glass reinforcement into a mold. A catalyst is added to the material to cause the material to cross-link or harden at room temperature. This process lends itself to making large and strong parts.
Most laminates are made using thermosets. The materials to be laminated are stacked in a press, clamped and heated. Some examples of laminates using thermosets are plywood (the adhesive), electronic circuit boards, cloth-reinforced phenolic sheet and countertop laminates.
Thermoset plastic industrial laminates are identified in process by three stages — A-, B- and C-stages:
A-stage refers to the key raw materials — reinforcing substrates and resin binders.
B-stage refers to the product produced when reinforcing substrates and resin binders are brought together but not cured. The reinforcing substrate is unwound from a large master roll and dipped into a bath of liquefied resin binder. The reinforcing substrate becomes either saturated, as is the case with absorbent papers and cotton cloths, or coated, as is the case with glass and graphite cloths. Once the wet resin binder is joined with the reinforcer substrate in this method, it is slowly drawn through a long, conveyorized oven where the liquefied resin binder is dried. The result leaves dry semi-cured resin binder in and/or on the reinforcing substrate. Once joined and dried in this fashion, the product is referred to as B-stage or prepreg, and the process described is called B-staging, prepreging or treating.
C-stage refers to sheet, rod, tube, angle or other in their cured stage.
- Sheets — B-stage is sheeted into plys then laid on top of each other into predetermined stacks that will render a given thickness. These stacks are placed into the hydraulic laminating press between two flat surfaces and pressure is applied. While under pressure, heat is introduced to begin the bake cycle. The resin in the B-stage product is reactivated by the heat to a sticky state which moves slowly, filling and bonding the layers together until it eventually hardens and cures. Once plys bond to each other and cure they are referred to as C-stage laminate sheet and the process described is called laminating or pressing.
- Rods — the B-stage is convolutely wrapped under tension onto itself, much like a roll of paper towels is wound. Once the B-stage is rolled to form a rod it is placed into a laminating press which has upper and lower half-round mold cavities. When the two halfround molds close and meet each other a full round is formed. The size of the mold cavity determines the diameter of the finished rod. Once pressure is applied the layers are pressed together filling all voids. Similar pressures and heat cycles employed for making sheet are used. When the layers bond to each other and cure they are referred to as C-stage laminate rod or rolled and molded rod.
- Tubes — rolling tubes are nearly identical to rolling rods with the exception that a steel rod called a mandrel is employed to size and form the inside diameter of the tube. B-stage rolled tubes are usually placed into an oven chamber as opposed to a press. Tube bake cycles compare to those of sheet and rod. Once cured, the center mandrel is extracted. The final cured product is referred to as C-stage laminate tube or rolled tube.
- Angles — this process is nearly identical to that of sheets except the mold cavities are “V” shaped rather than flat surfaces. The final cured product is referred to as C-stage laminate angle or molded angle.
- Other shapes — once cured the end product is referred to as C-stage.
Filament winding is an automated process producing a tube or structure composed of continuous mono-filaments of controlled orientation in a resin matrix, wound on a mandrel. The mandrel is subsequently removed after the resin is cured. The number of layers and filament orientation can be varied to provide the optimum, application-dependent mechanical properties. Storage tanks, streetlight poles and reverse osmosis pressure vessels are three of the many applications of filament winding.
Polymer orientation refers to processes used to align and elongate polymer molecules and crystal structures exclusively in one direction (uniaxial orientation), or in two directions (biaxial orientation). In the case of uniaxial orientation, the principal benefits of the process are improvements in strength and modulus in the orientation direction. Biaxial orientation improves the strength and modulus of the polymer in two directions, but also significantly improves the toughness of the polymer. Polymer orientation processes are used to manufacture polymer fibers, strapping, webbing, film, sheet and profiles.
There are many other processes too numerous to mention in this manual. It is suggested that the reader obtain other literature that can provide more information in greater depth on the various processes.