Chapter 16 Plastics 1. INTRODUCTION AND ECONOMIC FACTORS Having studied some of the basic chemistry and properties of polymers, we now consider in detail the major applications of these fascinating molecules. By far the most important use of polymers is in the plastics industry. Plastics Material and Resin Manufacturing (NAICS 325211) makes up 11% of shipments for Chemical Manufacturing (NAICS 325), the highest percentage of any polymer application. Fig. 16.1 shows the growth of shipments in plastics compared to cellulosic and non-cellulosic fibers and synthetic rubber, other major uses for polymers. Note the very steep incline for plastics, now at $45 billion in shipments. NAICS 325211 includes mainly basic polymer resins and forms, including molded and extruded material. Plastics and Rubber Products Manufacturing (NAICS 326), a NAICS division separate from Chemical Manufacturing, is part of our larger chemical process industries definition of "the chemical industry," as explained in Chapter 1. This class deals with finished consumer products bought retail which contain rubber or plastic material. Table 16.1 shows the breakdown in value of shipments in Plastics and Rubber Products and its subdivisions. It is divided into 78.5% Plastics Products and 21.5% Rubber Products. Plastics products are then subdivided into products such as film, sheet, bags, pipe, laminate, foam, bottles, and miscellaneous. While film, sheet, and bags are the largest subdivision of plastics, the large miscellaneous "other" category demonstrates the breadth and scope of plastics. It cannot be denied that our modern standard of living
Transcript
Chapter 16
Plastics
1. INTRODUCTION AND ECONOMIC FACTORS
Having studied some of the basic chemistry and properties of polymers,we now consider in detail the major applications of these fascinatingmolecules. By far the most important use of polymers is in the plasticsindustry.
Plastics Material and Resin Manufacturing (NAICS 325211) makes up11% of shipments for Chemical Manufacturing (NAICS 325), the highestpercentage of any polymer application. Fig. 16.1 shows the growth ofshipments in plastics compared to cellulosic and non-cellulosic fibers andsynthetic rubber, other major uses for polymers. Note the very steep inclinefor plastics, now at $45 billion in shipments. NAICS 325211 includes mainlybasic polymer resins and forms, including molded and extruded material.Plastics and Rubber Products Manufacturing (NAICS 326), a NAICSdivision separate from Chemical Manufacturing, is part of our largerchemical process industries definition of "the chemical industry," asexplained in Chapter 1. This class deals with finished consumer productsbought retail which contain rubber or plastic material.
Table 16.1 shows the breakdown in value of shipments in Plastics andRubber Products and its subdivisions. It is divided into 78.5% PlasticsProducts and 21.5% Rubber Products. Plastics products are then subdividedinto products such as film, sheet, bags, pipe, laminate, foam, bottles, andmiscellaneous. While film, sheet, and bags are the largest subdivision ofplastics, the large miscellaneous "other" category demonstrates the breadthand scope of plastics. It cannot be denied that our modern standard of living
Plas. M at Is & ResinsOrg. Fib.-Noncell.
Synthetic Rubber
Org. Fib.-Cellulosic
Bill
ions
of
Dol
lars
Figure 16.1 U.S. shipments of plastics, fibers, and synthetic rubber. (Source: AnnualSurvey of Manufactures)
Table 16.1 U.S. Shipments of Plastics and Rubber Products Manufacturing
Figure 16.2 U.S. production of polymers. (Source: Chemical and Engineering News)
would be changed drastically without the plastics industry. Although manycriticisms of "cheap" plastic materials are sometimes justified, no one wouldwillingly return to the preplastic age, and especially have to pay for thedifference. Indeed, many consumer products would not be possible withoutthe availability of plastic materials. It is a high-growth industry.
If we look at pounds instead of dollars, we see the more gradual increasesof the last twenty years in U.S. production (Fig. 16.2) for the five majorpolymers. Be sure to know the important standard abbreviations for themajor plastics: high-density polyethylene (HDPE), low-densitypolyethylene (LDPE), linear low-density polyethylene (LLDPE),polypropylene (PP), poly(vinyl chloride) (PVC), polystyrene (PS), andpolyethylene terephthalate) (PET). LDPE, LLDPE, and PP have been topsfor many years and will probably continue to be the leaders for some time tocome. The LDPE numbers include both LDPE and LLDPE since theirproperties and uses are similar. However, since 1989 separate data forLLDPE is available. Production data for the three separate polyethylenes isgiven in Fig. 16.3. Notice the very rapid increase in LLDPE whileproduction of LDPE remained nearly flat. The average annual % change inLLDPE in the last decade is 7%, while it is only 5% for HOPE and 0.5% forLDPE. PP is also fast growing at 7% per year. Poly(ethylene terephthalate)
LDPE, LLDPE
PVC
PP
HDPE
PS
Bill
ions
of
Pou
nds
Year
Figure 16.3 U.S. production of polyethylenes. (Source: Chemical and Eng-ineering News)
(PET) is not in the figures but is rapidly increasing in plastics use as a clearbottle, especially for soft drinks. It is the major synthetic fiber and will bediscussed more in Chapter 17. As a plastic its production is now up to over4 billion Ib per year.
Price trends in polymers (Fig. 16.4) are more up and down depending onthe economy for a given year. All of these major use polymers in theplastics industry are 30-50 C/lb to be competitive. LLDPE can be mademore economically than LDPE. It is usually about 4 0/lb lower in price.
Table 16.2 shows the amount of plastics produced in the U.S. per personfor selected years. The very large growth rate is apparent until 1980. It isamazing that each of us uses 200 Ib per year.
What was the first synthetic plastic? Although some nineteenth-centuryexperiments should be mentioned, such as the 1869 molding process forcellulose nitrate discovered by John and Isaiah Hyatt, probably the firstmajor breakthrough came in 1910 with Leo Baekeland's discovery of phenolformaldehyde resins (Bakelite®). These are still the leading thermosetplastics made today. The pioneering work of Wallace Carothers at Du Pontin 1929 produced the nylons now used primarily as fibers but known as thebeginning of thermoplastic resin technology.
HOPE
LDPE
LLDPE
Bill
ion
s of
Pou
nds
Year
Figure 16.4 U.S. prices of polymers. (Source: Chemical Marketing Reporter)
2. GENERAL USES OF PLASTICS
Although we will be discussing plastics according to their various typesand what applications each type might fill, it is good to know something
Table 16.2 Per Capita Use of Plastics in the U.S.
Year Lb of Plastics/Person1930 0.251940 1.51950 121960 311970 901980 2091990 210
LDPEPSHOPEPVCPP
Cen
ts/P
ou
nd
Table 16.3 Uses of Thermoplastics
Packaging 32%
Building & construction 14
Consumer products 13
Electrical equipment 6
Furniture 5Transportation equipment 4
Miscellaneous 26
Source: Chemical and Engineering News
Table 16.4 Uses of Thermosets
Building & construction 69%
Transportation 8
Adhesives, coatings 4
Consumer products 4
Electrical equipment 4
Miscellaneous 11
Source: Chemical and Engineering News
general about uses of plastics. Table 16.3 divides the uses of thermoplasticsinto some general areas. Table 16.4 shows some general uses of thermosets.Packaging, the largest use for thermoplastics, includes containers and lids,probably one half of this use, and packaging film, another one third. Liners,gaskets, and adhesives for packaging make up the rest. Building andconstruction, largest use area for the thermosets and second largest forthermoplastics, includes various types of pipes, fittings, and conduit.Plywood adhesives are also big.
3. DEFINITIONS AND CLASSES OF PLASTICS
Your own intuition and experience should give you a good idea of what aplastic is. It is more difficult to define precisely because there are so manytypes, they have such a wide variety of properties, and their methods offabrication are so diverse. Not all polymers are good plastics, although manypolymers serve important plastic applications. Probably the best, simplest,
and all inclusive definition is that plastics are polymers that have beenconverted into shapes by processes involving a flow of the liquid phasebefore solidification. In short, a polymer must be easily shaped if it is toserve in any important plastic application.
The best type of chemical classification of plastics is that same divisionthat we use for all polymers: they are thermoplastic (linear) and thermoset(cross-linked). Unfortunately, there is an overlap in the all-importantmechanical properties when you use this chemical division. In 1944Carswell and Nason categorized plastics by the shape of their stress-straincurves, one of the important properties of any plastic. These curves arepictured in Fig. 16.5. The five major types are: (1) hard-tough, (2) hard-strong, (3) hard-brittle, (4) soft-weak and (5) soft-tough. Generalcharacteristics of these classes and some representative plastics for each typeare given. The words hard, soft, tough, strong, brittle, and weak are notchosen lightly. A hard plastic is one that has a high tensile strength andmodulus; a soft plastic has a relatively low strength and modulus. Toughrefers to a high elongation; brittle refers to a very low elongation. Strongand weak are applied to plastics of moderate elongations and depend on theiroverall tensile strength as well.
Fig. 16.6 is a graph of the range of tensile properties for each specificplastic, plotting tensile strength versus elongation. Note that the hard-toughplastics such as the nylons are in the upper right (high tensile strength, highelongation), the hard-brittle plastics such as the thermosets and polystyreneare in the upper left (high tensile strength, low elongation), and the soft-tough plastics such as low-density polyethylene are in the lower right (lowtensile strength, high elongation). There are no common uses for soft-weakplastics. Specific examples and details for each of these important categoriesof plastics will be given in Section 6.
4. FABRICATION OF PLASTICS
An important step in the manufacture of any plastic product is thefabrication or the shaping of the article. Most polymers used as plasticswhen manufactured are prepared in pellet form as they are expelled from thereactor. These are small pieces of material a couple of millimeters in size.This resin can then be heated and shaped by one of several methods.Thermoset materials are usually compression molded, cast, or laminated.Thermoplastic resins can be injection molded, extruded, or blow moldedmost commonly, with vacuum forming and calendering also used but to alesser extent.
ExamplesElongationat Break
UltimateTensile
StrengthYieldStressModulus
ABSHigh density polyethyleneCellulosicsPolyamidesPolypropyleneFluoroplasticsEngineering plasticsPolyacetal, polycarbonatePolyfmide, polyphenylenesulfide, polyphenyleneoxide
PolysulfonePoly(vinylidene chloride)
HighHighHighHigh
Rigid PVCImpact polystyreneStyrene— acrylonitrile
ModerateHighHighHigh
Class ofPlastic
Hard andtough
Hard andstrong
Figure 16.5 Classes of plastics by shape of stress-strain curve. (Source: Wittcoff and Reuben, IndustrialOrganic Chemicals in Perspective. Part Two: Technology, Formulation, and Use, John Wiley & Sons,1980. Reprinted by permission of John Wiley & Sons, Inc.)
P/F, U/F and M/F resinsPolystyrenePolymethyl methacrylateUnsatu rated polyesterresins
Ep«xy resins
LowHighNo well-defined yield
point
High
Polyethylene waxes
ModerateLowLowLow
Low densitypolyethylene
Plasticized PVClonomer
HighLowLowyield point
Low
Hard andbrittle
Soft andweak
Soft andtough
Figure 16.6 Range of tensile properties for various plastics. (Source: Wittcoff and Reuben, IndustrialOrganic Chemicals in Perspective. Part Two: Technology, Formulation, and Use, John Wiley & Sons,1980. Reprinted by permission of John Wiley & Sons, Inc.)
Hard toughplastics
Hard strongplastics
Hard brittleplastics
Ultim
ate tensile strength (p
si)
Soft tough plasticsElongation at break (%)Soft weak plastics
Nylon 66
PolyacetalPolyimide
Figure 16.7 Compression molding. (Source: Wittcoff and Reuben II and Reuben and
Burstall 1974, Reprinted by permission of Pearson Education Limited.)
Pressure
Flashing
Ejection pin
Hopper
Openable mold
Plastic feed Piston
Heatedsection
Torpedo to ensure bringing ofpolymer into good thermal
contact with walls
Figure 16.8 Injection molding. (Source: Wittcoff and Reuben II and Reuben and Burstall
1974, Reprinted by permission of Pearson Education Limited.)
Compression molding (Fig. 16.7) is practically the oldest method offabricating polymers and is still widely used. The polymer is placed in onehalf of a mold and the second half compresses it to a pressure of about 1ton/in2. The powder is simultaneously heated, which causes the resin tocross-link.
In injection molding (Fig. 16.8) the polymer is softened in a heatedvolume and then forced under high pressure into a cooled mold where it isallowed to harden. Pressure is released, the mold is opened, the molding isexpelled, and the cycle is repeated.
Extrusion (Fig. 16.9) is a method of producing lengths of plastic materialof uniform cross section. The extruder is similar to a domestic mincingmachine with the added facility that it can be heated and cooled. The pelletsenter the screw section via the hopper, are melted, and then pass through the
Feed hopper
Barrel Screw
Head
Figure 16.9 Extrusion. (Source: Wittcoff and Reuben II and Reuben and Burstall 1974,Reprinted by permission of Pearson Education Limited.)
Compressed air
(i) (ii) (iii) (iv) (V)
Figure 16.10 Blow extrusion. (Source: Witteoff and Reuben II and Reuben and Burstall1974, Reprinted by permission of Pearson Education Limited.)
breaker plate into the die. The plastic material is forced out of the die withits cross section determined by the shape of the die, but not identical with itbecause of stresses induced by the extrusion process. Extrusion is also usedto make pellets. A long rod is extruded and then a cutting wheel makessmall pieces from the long rod. This is often used to expel the polymer fromthe reactor and make pellets from it for storage or shipping.
Blow extrusion, in which the initial lump of polymer is formed by anextrusion process, is the most common form of blow molding and is showndiagrammatically in Fig. 16.10. A short length of plastic tubing is extrudedthrough a crossed die and the end is scaled by the closing of the mold.
Figure 16.11 Research injection molding equipment. (Courtesy of Du Pont)
Compressed air is passed into the tube and the "bubble" is blown out to fillthe mold.
Fig. 16.11-16.14 show the types of equipment used in some of theseprocesses.
5. RECYCLING OF PLASTICS
Since about 1990 plastics recycling has been a growing and developingbusiness. It is here to stay in one form or another. Collection and separationare the main problems. PET and HDPE by far are the main plastics that aremost easily recycled. These two polymers are used as containers and easilyreprocessed. Plastic films (especially LDPE) have a potential for recyclingincreases, but this form of plastic material is not as easily collected andstored.
Figure 16.12 Polymer ribbon in a molten state at high temperatures ready to enter an
extruder. (Courtesy of BP Chemicals, Alvin, Texas)
Melt recycling works well only when the processes can acquire largequantities of clear, single polymer material such as PET soda bottles andHDPE milk jugs. For mixed streams separation of different types ofpolymers is a major cost component. Density differences, magneticcharacteristics, color, X-ray, single wavelength infrared, and full-spectruminfrared are some sorting and detection methods.
X-rays can easily determine the presence of PVC via the chlorine atom.Single wavelength infrared is used to separate clear (PET and PVC),translucent (HDPE and PP), and opaque (all pigmented materials andcolored HDPE) streams. Full-spectrum infrared can detect differences in
Figure 16.13 Extruding equipment. The extruder is opened in the middle of this pictureto show the die for making over 1000 simultaneous strands. (Courtesy of BP Chemicals,Alvin, Texas)
Figure 16.14 Research size blow molding equipment. (Courtesy of Du Pont)
each of the plastics because of the unique fingerprint in the infraredspectrum for each plastic. This latter system is 98-100% accurate, except forseparating LDPE from HDPE where it is 90% successful. Markers havebeen considered as a way to code each plastic product with an easilyidentifiable sign. Unique molecular markers on the polymer moleculesduring production have also been given some thought. Many plasticproducts now have the standard visual marker, abbreviation, and numbershown on the surface: PETE (PET), 1; HDPE, 2; V (PVC), 3; LDPE, 4; PP,5; PS, 6; and other, 7.
Total recycled thermoplastic resin production almost tripled between1990 and 1995 and is well over 1 billion Ib per year. The percentagebreakdown of the amount for recycling is the following: HDPE (especiallybottles), 35%; PET (soft drink bottles), 34%; PP (auto battery cases), 14%;LDPE (film), 9%; PS (packaging), 3%; PVC, 0.5%; other, 4%.
6. IMPORTANT PLASTICS
The diversity in properties and uses of plastics is greater than any otherarea of polymer chemistry. It is best simply to select a few of the mostimportant plastics and become acquainted with them individually. In thefollowing sections there is important information on certain polymers havingwide applications as plastics. We will use our general categories of hard-tough, hard-strong, hard-brittle, and soft-tough to determine their order oftreatment and also to emphasize which plastics compete with each other.Although some plastics are similar to others they all have their own set ofadvantages and disadvantages for a given application. Indeed, it is the job ofthe plastics companies to fit the best polymer to a particular use. Thechemistry of manufacture of these polymers is given in Chapters 14 and 15.Production figures are given in Chapter 1 and in this chapter.
6.1 Hard-Tough
6.1.1 High-Density Polyethylene, HDPE
— (CH2-CH2)n—
1. ManufactureIntroduced in the 1950sModerate to low pressuresMetal oxide catalysis (usually)
2. PropertiesNo branches, 90% crystalline, Tm - 1350C, Tg = -70 to -2O0CMore opaque than LDPEStiffer, harder, higher tensile strength than LDPESpecific gravity = 0.96
3. UsesBlow molding (containers and lids, especially food bottles, auto gas
tanks, motor oil bottles), 35%; injection molding (pails,refrigerator food containers, toys, mixing bowls), 22%; film,17%; pipe and conduit, 14%; sheet, 6%; wire and cable, 1%;miscellaneous, 5%
4. Economics2004 demand expected to be 17 billion IbGrowth from 1990-2000 of 5.3%/yr
6.1.2 Polypropylene, PP
Figure 16.15 Polypropylene carpet backing is one important application of thisversatile plastic. (Courtesy of BP Chemicals, Alvin, Texas)
1. ManufactureZiegler-Natta or metal oxide catalysis
2. PropertiesTm = 17O0C, higher than HDPE, can be sterilized at 14O0C for
hospital applicationsTg = -1O0C, more brittle at low temperatures than HDPEStiffer, harder, higher tensile strength than HDPEMore degraded than HDPE by heat, light, and oxygen because of
tertiary hydrogens. Antioxidants and UV stabilizers can beadded.
luggage, steering wheels, battery cases, fan shrouds, air cleanerducts), 31%; fibers and filament (carpet backing, indoor-outdoorcarpeting, rope), 30%; resellers, distributors, and compounders,23%; film and sheet, 11%; blow molding, 2%; miscellaneous,3%
4. EconomicsGrowth from 1990-2000 of 6.5%/yr
6.1.3 Polyethylene terephthalate), PET
1. ManufactureMade from ethylene glycol and either dimethyl terephthalate or
terephthalic acid at 200-30O0C in vacuo2. Properties
Tg = 800C, Tm = 250-265 0C3. Uses
Bottles for carbonated soft drinks, 60%; custom containers forproducts other than carbonated soft drinks, 30%; amorphous(packaging) and crystallized (microwave and oven trays forfrozen foods), 10%
4. EconomicsGrowth from 1989-1998 of 10-15%/yr
Continued strength in the soft drink market and development of PETcontainers for bottled water give an anticipated growth rate of10-15%/yr.
New developments for PET beer containers are overcomingproblems with O2 and CO2 permeability.
Examples of ABS products: radio housings, telephones, pocketcalculators, lawn mower housing, luggage
4. EconomicsU.S. production of ABS resins is 1.4 billion Ib/yr
6.2 Hard-Strong
6.2.1 Poly(vinyl chloride), PVC
1. ManufactureDiscovered in 1915 by Fritz Klatte
Developed in 1926-28 by B. F. Goodrich, Union Carbide, and DuPont
Peroxide free radical initiationSuspension (mostly), emulsion, or bulk procedure
2. PropertiesTm=140°CTg = 70-850C, higher than polyolefms because polar C—Cl bond
gives dipole-dipole intermolecular attractionsLow crystallinityGood impact strengthGood chemical resistanceResistant to insects and fungiNon-flammableEasily degraded by heat and light via weak C—Cl bondBrittle at low temperaturesBecomes a soft-tough polymer and very flexible with 1-2%
plasticizer such as dioctyl phthalate, then competing with LDPE3. Uses
Construction, 76% (including pipe and tubing, roofing, siding,windows and doors, flooring and pipe fittings); consumer goods,6%; electrical fittings and wire and cable coatings, 4%;transportation, 2%; home furnishings, 2%; miscellaneous, 4%
4. EconomicsGrowth has been good at 6.4% per year in the last few years, but
should be slower in the next few years unless the constructionindustry improves.
6.2.2 Polycarbonate
1. ManufactureFrom a bisphenol A and phosgene slurry with a phase transfer
catalyst2. Properties
A very clear, transparent, strong plasticGood mechanical propertiesHigh impact strengthGood thermal and oxidative stability
Low moisture absorption3. Uses
Automotive (instrument panels and lighting systems), 25%; glazingand sheet (windows), 20%; optical media (eyeglasses), 15%;appliances, 8%; computer and business machines (CD and DVDdisks), 7%; medical, 7%; recreation and safety, 7%;miscellaneous, 11%
4. EconomicsOverall U.S. demand over 1 billion Ib/yrGrowth good at 7.5%/yrProjections to continue at 6-8%/yr because of automotive and
computer applications
6.3 Hard-Brittle
6.3.1 Polystyrene, PS
1. ManufacturePeroxide initiationSuspension or bulk
2. PropertiesTm = 2270C, Tg = 940C, wide spread good for processingAmorphous and transparent—bulky phenyls inhibit crystallizationEasily dyedVery flammable—can add flame retardantsNot chemically resistantWeathers poorlyYellows in light—can add UV absorbers
3. UsesPackaging and one-time use, 48%; electrical and electronics, 17%;
construction, building products, and furniture, 13%; consumerproducts, including toys, 9%; medical products, 7%;miscellaneous, 6%
4. EconomicsGrowth from 1990-2000 only 2.5%/yr
Should be about the same in the near futurePolystyrene's chief weakness is its image. There is a continuing
effort to replace it with paper products. The prior use of CFCsas blowing agents in its foam products has contributed to thisnegative image.
6.3.2 Phenol-Formaldehyde, Phenolics, P/F
1. ManufactureOne-stage cured by heatTwo-stage cured by heat and hexamethylenetetramine
2. PropertiesHeat resistanceWater and chemical resistanceDark color
3. UsesPlywood adhesive, 48%; fibrous and granulated wood adhesive,
1. ManufactureMethylolurea formation under alkaline conditions, followed by
heating under acidic conditions2. Properties
White translucent (nearly transparent)Can be pigmented to a wide variety of colorsLight stableLess heat and water resistant than phenolicsGood electrical properties
3. UsesParticleboard, 62%; medium density fiberboard, 19%; hardwood
1. ManufactureFree radical initiatorsHigh pressure and temperature required, 15,000-40,000 psi and 300-
50O0CDiscovered by ICI in the U.K. in 1933, commercialized in 1938
2. PropertiesMuch C4 branching, only 50-60% crystalline, 30 branches per 100
carbonsTm = 115 0C, lower than HDPE and LLDPESpecific gravity = 0.91-0.94, lower than HDPEEasily processedFlexible without plasticizersResists moisture and chemicals—but porous to oxygenEasier processed than LLDPE and has good strength and clarity
Figure 16.17 Polymer storage in pellet form can be done in large silos, each of which
can hold 185,000 Ib (7-10 hr of production). The silos can be mixed to ensure uniformity
before the pellets are added to a tank car holding nearly the same amount as one silo.
Growth from 1990-2000 of 7.4%/yrWill be 5%/yr in the future
Suggested Readings
Chemical Economics Handbook, various articles.Chemical Profiles in Chemical Marketing Reporter, 1-11-99, 6-14-99, 4-10-
00, 4-17-00, 3-12-01, 3-26-01, 4-2-01, and 4-9-01.Kent, Riegel's Handbook of Industrial Chemistry, pp. 623-707.Wittcoff and Reuben, Industrial Organic Chemicals in Perspective. Part
Two: Technology, Formulation, and Use, pp. 39-103.
