Plastics Instructional Module VI


 

Applications

 

  1. Polymer Applications
    1. Teacher Information
    2. Polymer Coatings Lab
    3. Polyesters – Teacher Information
    4. Preparation of Polyesters Lab

 

  1. Biomedical Applications of Plastics
    1. Teacher Information
    2. Student Hand-out

Plastics:  Meeting the Medical Needs of a Growing Population

by Sarah Battjes

c.      Interactive Teaching Session – Orthopedic Implants

d.     Activity – Quiz

e.      Bioabsorable Polymers

1.     Teacher Information

2.     Student Lab

f.       Microencapsulation

1.     Teacher Information

2.     Student Lab

g.      References

 

3.     Environmental

a.      Factual Information on plastics and Recycling

b.     References

c.      Activity – Grocery Store Dilemma

1.     Teacher Information

d.     Activity – Plastic Cycle Representation

1.     Teacher Information

e.      Demonstration – Soda Bottle Preforms

1.     Teacher Information

f.       Activity – Group Debate

1.     Teacher Information

g.      Lab – Cellulose Degradation by Microorganisms Found in Soil

 

 

POLYMER APPLICATIONS

 

            Today, polymers are big business.  Over 4000 major synthetic polymers and plastic resins are already on the market and hundreds more appear each year.  Business analysts estimated that production of plastics alone exceeded 46 billion pounds in the United States in 1985.  The value of U. S. plastics shipments is approaching the $20 billion mark, placing them third among chemical products behind organic chemicals and drugs.

 

 

Fibers

 

            The fiber and textile industry has traditionally used many of the natural polymers—silk and wool of animal origin and cotton, kelp, sisal, flax, and others of plant origin.  Synthetic polymers have been developed that mimic and extend the properties of these natural materials.  Comparisons of these natural synthetic materials are given in a table on the next page.

            These properties determine the applications of the fibers.  There are synthetic polymers that closely approximate natural fibers in strength and percent elongation.  These polymers are spun, and during processing and fabrication can be made to be similar in feel and bulk to the natural fibers, and hence are useful in clothing.  There are also synthetic polymers that have high breaking strength and are used in ropes and fabrics that are subjected to extreme wear; and come that are resistant to heat along with high tensile strength, and are used in protective clothing, filter cloths, and as an asbestos substitute where heat and flame resistance are needed.  The special applications of these synthetic polymers are based in part on resistance to biological attack by fungi and insects, and on the fact that they can be spun as continuous filaments of high strength, crimped to mimic natural fibers, or even spun as hollow filaments or filaments with a variety of cross sections.

            Synthetic fibers are spun from a liquid melt or solution and consist of randomly oriented molecules that must be drawn or stretched to produce crystallinity.  This results in an array of molecules oriented parallel to each other having great strength.  The polymers exhibiting very high strengths actually exist as liquid crystals in the solutions being spun.  It is interesting to note that the difference in structure that leads to the high strength and temperature resistance of Kevlar® aramid fiber over that of Nomex® fiber, is the shift from meta isomers to para isomers.  This aligns the axis of the resulting polymer in a symmetrical linear arrangement (Kevlar polymer), instead of the asymmetrical arrangement found in Nomex polymer.

            This leads to a rod-like configuration in Kevlar fiber that aligns the liquid crystals as it is spun, giving a high degree of crystallinity instead of the random coiled arrangement in Nomex fiber.  This results in greater strength and stiffness.

 

Polymer Fiber Properties

 

 

Generic Name

 

 

Chemical Name

Breaking Strength

g/denier

 

Elongation at Break %

 

 

Thermal Stability

Wool

Protein

1-2

20-40

Decomposes 150°C

Cotton

Cellulose

3-6

3-10

Decomposes 130°C

Silk

Protein

18

20-40

Decomposes 150°C

Rayon (Viscose)

Regular

High Tenacity

Regenerated Cellulose

 

1-2

3-6

 

15-30

9-26

 

Softens 150°C

Decomposes 150°C

Acetate

Cellulose acetate

1-1.5

25-45

Sticks 175°C; softens 205°C

Spandex

Lycra®

(Du Pont)

 

Block poly

(urea-urethane)

 

.3-.5

 

400-800

 

Sticky at 215°C

Flurocarbon

Teflon®

(Du Pont)

 

Poly (tetrafluroethylene)

 

2-10

 

10-20

 

Melts at about 290°C

Polyester

Dacron®

(Du Pont)

 

Poly (acrylonitrile)

 

2-10

 

10-20

 

Sticky 230°C

Acrylic

Orlon®

(Du Pont)

 

Poly (acrylonitrile)

 

2-3

 

34-50

 

Sticky 250°C

Nylon

Polyaklylamides

3-10

15-60

Melts 215-260°C

Olefins

Polyethylene (linear)

Polyethylene (branched)

Polypropylene

 

 

1-3

 

 

3-7

 

 

3-8

 

20-80

 

 

10-45

 

 

14-80

 

Shrinks 5% 75°C, softens 100°C & melts

Shrinks 5% 75°C, softens 115°C & melts

Shrinks 5% 100°C, softens 140°C

Aramide

Nomex®

(Du Pont)

Kevlar®

(Du Pont)

Polyaromaticamides

 

4-5.5

 

20-30

 

22-32

 

2.5-4

 

Decomposes 370°C

 

Decomposes 500°C

Glass

Silica-silicates

9.5-20

3-5.5

Softens at 730-800°C

Asbestos

Silicate

10-20

2-4

Melts at 1520-3300°C

 

Based in Part on information in TEXTILE WORLD, 1978, 126(8), 57

 

 

            At the other end of the spectrum of properties, we find the spandex elastomer fiber.  It is a block polymer of urea and urethane, which is melt spun with a finishing agent which prevents the filaments of the elastomer from sticking together.  It has excellent elastic properties and is used along with other fibers in the fabrication of elastic fabrics.

            The use of polymer fibers has been the basis of the paper industry for centuries.  Synthetic fibers are also being used in non-woven fabric-paper.  Very strong, temperature resistant non-woven fabrics have been made using paper-making machines.  Nomex fibers are blended with polymer binders in making fabrics of unique properties.  Nomex aramid paper (Du Pont) has excellent thermal-electrical carrier properties plus great strength that make it ideal as an insulator in electrical motors.  It is also utilized in making unique honeycomb laminates that have high strength, low mass, and excellent insulation capabilities.  These laminates are used extensively in construction of aircraft, skis, boats,  and the space shuttle and space vehicles.

            Hollow fibers are an interesting extension of polymeric materials into a new and expanding application area.  Their insulating properties are due to dead air space, and they maintain this insulating ability even when wet. Because they don’t absorb water.  Their uses are well known in cold weather gear and sleeping bags.  Their use in artificial kidneys as an exchange membrane has made a relatively small (7 x 20cm), highly efficient, easily used device in the treatment of kidney diseases.  They act as a semipermeable exchange membrane in the purification of the blood by hemodialysis.  The device utilizes a bundle of thousands of hollow fibers as the membrane.  A not-so-well advertised but important use of hollow fibers is found in water desalinization units.  Utilizing the process of reverse osmosis, sea water can be forced through the walls of these tubes, leaving dissolved impurities concentrated in the waste water and demineralized water in the effluent.

 

 

 

 

 

 

 

 

 

 

Figure 1
 

 

 

 

 

 

 

 

 

 


A unit such as described, uses tens of thousands of hollow tube fibers to give a very large surface area where this exchange can take place.  In the separation of hydrogen from natural gasses, a typical unit contains over 10,000 fibers (.8 mm outer diameter, .4 mm inner diameter) packed in an exchanger 20 cm in diameter and 3 m long.

 

Films

            The use of polymeric materials in making films involves the extrusion of the polymeric melt.  Where strength is desired in the film, two methods are possible:  biaxial drawing of the film is used to develop crystallinity, or a process of blow molding of the polymer with the subsequent formation of a blown tube of polymer.  The list of polymers that have been used successfully in film application is lengthy and includes polyethylene, polypropylene, cellulose acetate, polyvinyl chloride, and polyethylene terephthalate.  Properties of these films include exceptionally strong films as well as the flexible, transparent, or translucent materials we are familiar with.  The applications of polymeric films are many, and we will examine only a few:  coatings, adhesives, exchange membranes, and photosensitive films.

            Coatings         Materials are coated with films of polymers for a number of reasons:  to improve their finish for aesthetic reasons, to increase their abrasion or water resistance, to protect them from corrosion, or to protect materials from contamination by corrosion products.  Almost all types of polymers are involved in these polymeric finishes.  A typical product such as an automobile body might have a variety of different polymers used in the coating process.  The methods of applications are different for various types of polymers.

            Thermoplastic polymers (lacquers) can be applied using a solvent system.  In addition to the polymer binder and the solvent, the coating composition may contain a plasticizer, pigments, surfactants, antioxidants, modifiers for flow characteristics, and more.  The best properties are developed when high molecular weight polymers are used.  These polymers have high viscosities and a high ratio of solvent to polymer must be used.  Recovery and/or disposal of the solvent is therefore a problem because of the amount of solvent.  Coatings can also be removed by reapplication of the solvent.

            Thermosetting polymers can be applied in the monomer form.  They are usually low viscosity, eliminating the solvent difficulty, but curing techniques and time for the reaction must then be considered in their use.

            One of the newer types of coating involves the use of cathodic electro-deposition of an ionic polymer.  This method has an advantage because no solvents are introduced into the atmosphere and there is little waste of the polymer.  If the polymer is not deposited on the object, it is still available for the next item to be coated.

 

 

            A typical coating on a car body might include these layers:

 

 

Text Box: Figure 2