Polymers: Cornerstones of Construction - Journal of Chemical

Chemical Education Today

Report

Polymers: Cornerstones of Construction by John P. Droske and Charles E. Carraher, Jr.

Table 1. Common Applications of Polymers in Home Construction Application

Typical Polymers Used

Foundation Framing Thermal insulation

Cement Wood Foamed polystyrene, polyurethane, Fiberglas Polyethylene Wood, polyvinyl chloride Acrylics, polyurethanes Polyisoprene (rubber), chlorinated polyethylene Wood, complex silicates (cement/ ceramic tile), carpet (nylon, polyester, and polyolefin fibers) Plywood (sheathing), Fiberglas (shingles), complex silicates (cement, ceramic tile)

Vapor barrier Siding Paints Electrical insulation Flooring

Roofing

Table 2. Distribution of Materials Used in Building and Construction2 Material Concrete* Lumber* Ceramic* Wood panels* Iron and steel Plastics* Other**

Mass (Billions of Pounds)

Amount Used, by Mass %

250 60 50 20 15 10 95

50 12 10 4 3 2 19

*Indicates a polymeric material **Some of these are polymeric, such as paints, sealants, and coatings

1428

Journal of Chemical Education

•

photo: John Droske

Polymers have played a key role in the construction of homes and other important structures for thousands of years. The ancient Sinagua, who lived in Arizona from about 600 to 1425 C.E., used stone and clay along with sycamore beams, reeds, and grasses for their cliff dwellings, an example of which is shown in Figure 1.1 The stone and clay of the walls are inorganic materials, while the trees and other plant matter that made up the roof, floor, and ladders of the cliff dwellings were primarily made of natural polymers, such as cellulose and lignin. Similarly, the log cabins of frontier days in the U.S.—as well as New England Cape Cod style homes—were composed primarily of natural polymers, since they were made of lumber. Other frontier homes, such as Native American homes that were made of animal hides, also were composed of natural polymers, since the hides were primarily proteins, that is, high molecular weight biopolymers. While these represent various kinds of homes that have been

Figure 1. A cliff dwelling in Arizona at Montezuma’s Castle National Monument.

built in the U.S., polymers have been used similarly in home construction throughout the world. The requirements for home construction have changed over the years to include improved energy efficiency, fire retardancy, and convenience, as well as construction cost containment. Modern home construction makes extensive use of polymers, and today the materials are a combination of both natural and synthetic polymers. In addition, the materials include organic polymers, such as wood and plastics, as well as inorganic polymers, such as window glass and cement (Table 1). Polymers offer a very wide range of properties that are desirable in building construction and they account for about 80% by mass of the building materials in a home (Table 2). Natural Polymers

Wood While metal or composite materials have replaced wood framing in some modern construction, the standard wood “2 ⫻ 4” (nominal inches of dimensional lumber) still is used extensively.3 Wood also is used for many other components of a home, including doors, cabinets, flooring, siding, paneling, trim, stairs, shutters, window frames, and furniture. Many of the above components are made from pieces of lumber, while others are laminated, composite materials, such as plywood, that are made of multiple thin layers of wood veneers that are held together tightly with adhesives, primarily phenolic resins. These composite wood products often exhibit higher strength and other improved properties over analogous timber components. Dry wood is almost entirely polymeric, composed of about 40–50% cellulose, 25–35% hemicellulose, and 15–35%

Vol. 83 No. 10 October 2006

•

www.JCE.DivCHED.org


Report

H

H

OH

HO 4

H

O H OH

1 β

in some of the lignin having a somewhat two-dimensional structure similar to a sheet of paper rather than the typical three-dimensional structure for most polysaccharides and other natural macromolecules. The lignin sheets also act as a barrier towards the elements and pests.

H

H

OH

4

H

O HO H

H

O

OH

1 O β

H

n

Figure 2. A small segment of a cellulose chain.

lignin. As shown in Figure 2, cellulose is a polymer composed of thousands of glucose units connected into a polymer chain. The glucose units in cellulose are connected via a ␤-acetal linkage. This linkage differentiates cellulose from starch, which also is a polymer made from glucose units. In starches, the glucose units are connected by an ␣-acetal linkage. The ␤-acetal linkage gives a linear, crystalline arrangement to the chains that allows close contact between hydroxyl groups on adjacent chains. This results in extensive hydrogen bonding between the cellulose chains that imparts great strength and toughness and also causes wood to be insoluble in water. By comparison, the ␣-linkage of starches (e.g., amylose) results in a helical structure where the hydrogen bonding is both interior and exterior to the chain. While this difference in structure affects the physical properties of these materials, it also effects the susceptibility to attack by enzymes. Humans have enzymes that can lyse or break ␣-linkages, while cows, termites, and other species that contain symbiotic bacteria in their digestive systems, furnish the enzymes that are capable of digesting (i.e. breaking) the ␤-glucoside linkages. Thus, humans can digest starches but not cellulose. [The PowerPoint presentation by Harris that is described on p 1435 discusses this bonding as well.] Wood also contains hemicellulose, material that is similar to cellulose, but is lower in molecular weight and is amorphous rather than crystalline in structure. Hemicellulose by itself generally is not associated with the strength of wood, but its association and co-crystallization with cellulose strongly influences the strength of wood. Lignin is the third main component of wood. It is the second most widely produced organic material in nature after the saccharides. It is found in essentially all living plants and is the major non-cellulosic constituent of wood. Lignin is found in plant cell walls of supporting and conducting tissue. It contains a variety of structural units including those pictured in Figure 3. The presence of rigid aromatic groups and hydrogen-bonding involving the alcohol, aldehyde, and ether groups, give a fairly rigid material that strengthens stems and vascular tissue in plants. This allows upward growth and it also allows water and minerals to be conducted through the xylem under negative pressure without collapse of the plant. Lignin’s chemical composition, and strong bonding to cellulose and proteins in the plant, makes it indigestible to plant eaters. During the synthesis of plant cell walls, polysaccharides generally are laid down first. This is followed by the biosynthesis of lignin that fills the spaces between the polysaccharide fibers. This binds the cellulose fibers together and results 1430


•

Synthetic Polymers

Carbon-Based Polymers: Plastics The use of synthetic polymers in building and construction continues to increase at a rapid rate. The use of plastics alone is approaching 10% of the total cost of building and construction in the U.S.2 The major applications of plastic in building and construction include hot and cold water and furnace exhaust pipes, laminated countertops, wood and flooring adhesives, electrical and thermal insulation, siding, and roofing sheathing and panels (Table 3). Commonly used synthetic polymers are those obtained from monosubstituted or 1,1-disubstituted olefins, such as polystyrene and polyvinyl chloride, and those obtained from condensation reactions, such as nylon, polyesters, and polyurethanes. All of the polymers with recycling codes 1–6 find use in construction (Table 3).4 For example, recycling code 1 (PETE or polyethylene terephthalate) is a polyester that is used in carpeting, strapping for lumber bundles, gas and moisture barrier applications, and even in soda bottles for workers! High density polyethylene (HDPE, recycling code 2) is used for plastic lumber, floor tiles, sheeting, garden edging and flower pots, containers for construction goods such as paint and plaster, and fencing. Polyvinyl chloride (PVC, code 3) may be found in water pipes, floor tiles, window frames, paneling, gutters, electrical boxes, and hoses. Polystyrene (recycling code 6) is used extensively in home construction: one common application is the use of large foamed polystyrene sheets as insulation. As energy costs rise, this is becoming more important as it means that much less fuel is needed to heat and cool buildings. The use of foamed polystyrene and polyurethane, and other insulators, such as Fiberglas and blown cellulose, have an important environ-

H3CO O

OH HO

O

HO

OH

HO

OH

HO O

O

H3CO

OCH3 H3CO

O

OCH3

O

HO

O

OCH3 OH

HO

Figure 3. A representative segment of softwood lignin.


•

www.JCE.DivCHED.org

OH


Table 3. Major Applications of Plastic in Building and Construction2 Plastic

Recycling Code

Polyethylene terephthalate Polyethylene

1

Uses Countertops and sinks

2, 4

Plastic lumber; pipes; wire and cable covering; vapor barriers

PVC and chlorinated PVC

3

Hot and cold water pipe; floor tiles; moldings; siding; window frames

Polypropylene

5

Vapor barrier sheeting; brushes; pallets

Polystyrene

6

Insulation and sheathing

Acrylics

Lighting fixtures and windows

Polycarbonates

Windows and skylights

Poly(ethylene oxide)

Roofing panels

Polyurethanes

Insulation and roofing systems

Phenol–formaldehyde resins

Electrical devices and plywood adhesive

Melamine and urea– formaldehyde resins

Laminating for countertops; adhesives for wood, plywood, and particle board

mental benefit in that lower fuel usage also means that less pollution is generated for heating and cooling our homes, schools, and workplaces. Poly(methyl methacrylate) (Plexiglas) and polycarbonate both are used in applications where transparency is important. For example, poly(methylmethacrylate) can be used in place of ordinary window glass (see below) in applications where breakage is a possibility, such as in a storm door. If extra break-resistance is desired, polycarbonate resins may be used. This is the same material that is used to make the outer portion of a CD, as well as motorcycle and sports helmets. As lighter and stronger polymeric materials become available, their impact on the building and construction industry and on other industries will continue to increase. Further, as materials that perform specific tasks become available, they too will become integrated into the building and construction industry. This includes devices for gathering and storing solar energy, and “smart” materials, such as “smart windows” that change color and transparency when a small electric current is applied (1). [Recent research into window coatings is reported in Research Advances on p 1428 of this issue of JCE.] Since windows are a major source of heat losses and gains, these www.JCE.DivCHED.org

windows may play an important role in the energy efficiency, as well as the aesthetic aspects, of homes of the future.

Carbon-Based Polymers: Coatings Coatings are important as insulation for electrical conductors and as protective and decorative finishes on woodwork, metalwork, and plastic components. Construction-grade wiring typically is protected by a polymeric sheath of polyisoprene or chlorinated polyethylene. Although it has not yet found significant use in home construction, in 2000, Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa were awarded the Nobel Prize for the discovery that polyacetylene was an electrically conductive polymer. Maybe someday the electrical circuits in our homes will use polymers as both the electrical conductor as well as the insulation! Home exteriors are either largely or wholly polymeric, being made of inorganic polymers such as stucco or brick that usually are complex silicates (see glass and cement below), or organic polymers such as those found in wood or vinyl siding. Stucco, plaster, and mortar all contain concrete, lime, sand, and water, but in different proportions. These complex silicates often approach the same composition as glass, that is, •


•

they are primarily three-dimensional matrices of silicon dioxide. The composition of vinyl siding can vary but it usually is poly(vinyl chloride). Many exteriors and interiors are painted or coated. This may be for decorative aesthetic purposes or it may be for protection. For household use, most paints are either oil-based paints and varnishes or latex paints. Oil-based varnishes usually consist of a natural or synthetic polymer dissolved in a suitable organic solvent. A polymer film is formed after application by evaporation of the solvent. Other oil-based paints, such as those containing linseed oil, undergo complex crosslinking and polymerization reactions in the presence of oxygen, sometimes promoted by catalysts that have been added to the paint. After application, these paints are no longer soluble, although they may be removed by degradation with an appropriate paint stripper. In recent years, there have been dramatic changes in the coatings industry due to U.S. government regulations concerning emissions of volatile organic compounds (VOC) from coatings. This has led to new coating technologies that are solvent-free or have reduced quantities of solvents. Central to this is the development of water-based coatings, to lower the emission of VOCs and to eliminate the “odor” of the coating during and shortly after application. Most waterborne coatings actually contain about 8–10% non-aqueous solvent and the evaporation of this solvent during drying accounts for the odor associated with latex paints. In addition to polymer and solvent (or the non-solvent, water), paint also contains pigments. Titanium dioxide is the most widely used pigment for white coloring, but it also is used in differently colored paints. Iron oxide pigments are used for brown, yellow, and red colors, and organic compounds, such as phthalocyanine, are used for greens and blues. Inert pigments such as clay, talc, calcium carbonate, and magnesium silicate help the paint to last longer, as do mica chips in some latex paints that actually form a protective coating on drying. Many paints used in moist climates contain other additives designed to fight fungus, rot, and mold. Journal of Chemical Education

1431


Report Latex paint accounts for more than half of all commercial paint sold. It consists of small polymer particles in an emulsion. The polymer, typically an acrylate such as poly(methyl methacrylate), is prepared via a polymerization reaction that occurs in water in small soap bubbles called micelles. A water-soluble initiator is used that diffuses into the micelles and very high molecular weight polymers are obtained. Maintaining a stable emulsion prior to application of the paint is an important part of paint technology, since the polymer is not soluble in water. Emulsion polymerization is a fascinating topic and the effect of microgravity on this method of polymerization was the first scientific experiment that was conducted in the science bay of a space shuttle (2). Interestingly, this experiment resulted in monodisperse polymer particles with a near uniform diameter. The U.S. Department of Commerce estimates that 809 million gallons of architectural coatings were sold in 2004, with total coatings production estimated at 1564 million gallons.

Inorganic Polymers: Silicate Glass Glass is an inorganic polymer composed mainly of SiO2 units. While there are many kinds of glass, the focus here will be on silicate glasses, the common material for window glass. Silicate glass is made by heating well-mixed silica sand and powdered additives, along with a desired amount of recycled glass (known as cullet), in a furnace near 1500 ⬚C to form a viscous, syrupy liquid. The size and nature of the furnace corresponds to the intended use for the glass. Most glass is melted in large tanks that can melt 400–600 metric tons a day. The process is continuous, with the raw materials fed into one end as molten glass is removed from the other end. Once the process (referred to as a “campaign”) is begun, it is continued indefinitely, day and night, often for several years. A key feature of glass is that it has been cooled to a rigid condition without crystallization. The common term “crystal clear” actually is an anomaly, since amorphous glass is clear, while glass that has crystallized is translucent or opaque. Common window glass is soda-lime glass. It contains 60–75% silica, 12–18% soda, and 5–12% lime, and a small fraction of minor ingredients. It has a relatively low softening temperature and low thermal shock resistance and it is not suitable for applications involving rapid temperature changes. It is relatively inexpensive and has high light transmission and optical clarity. Glass is a three-dimensional array that offers short-range order and long-range disorder—it is amorphous with little or no crystallinity. The structure is based on the silicon atoms existing in a tetrahedral geometry with each silicon atom attached to four oxygen atoms, generating a three-dimensional, non-crystalline array. Structural defects occur, due in part to the presence of impurities such as Al, B, and Ca, which may be intentionally or unintentionally introduced. These impurities encourage formation of an amorphous structure upon cooling since the different-sized metal ions disrupt the rigorous spatial organization that is necessary for crystal formation (Figure 4). Most flat glass is shaped by drawing a sheet of molten glass (heated so it can be shaped, but not so it freely flows) www.JCE.DivCHED.org

•

Figure 4. A three-dimensional representation of a multi-component, silicon–dioxide-intense glass (black spheres are silicon, red spheres are oxygen, and green spheres are a cation impurity).

onto a tank of molten tin. Since the glass literally floats on the tin, it is called “float glass”. The glass from the float bath typically has both sides quite smooth with a brilliant finish that requires no polishing. A general purpose fiberglass may contain silica (72%), calcium oxide (9.5%), magnesium oxide (3.5%), aluminum oxide (2%), and sodium oxide (13%). Fibers are produced by melting, with the molten glass being drawn though an orifice. The filaments are passed through a sizing solution and then are wound onto a drum. The take-up rate of the filament is more rapid than the exit rate from the orifice and this aligns the molecules and draws the fibers into thinner filaments with a higher strength-to-diameter ratio and greater flexibility. Fiberglass is used for home insulation and affords important energy conservation and greater comfort.

Inorganic Polymers: Cement Portland cement, the basic ingredient of concrete, is the most widely used, and least expensive, synthetic inorganic polymer. In 2005, more than six billion tons of concrete were made, about one ton for every person on Earth! Concrete is used widely in construction. The name “Portland” is derived from the cement having the same color as the natural stone quarried on the Isle of Portland, a southern peninsula of Great Britain. Common cement consists of lime (CaO, about 65%), silicon dioxide (SiO2, about 20%), and alumina (Al 2O 3, 5–6%) with the remainder small amounts of iron oxide (Fe2O3) and other components. During heating, anhydrous crystalline calcium silicates, made up primarily of tricalcium silicate, Ca3SiO5, and dicalcium silicate, Ca2SiO4, form. Cement is used widely and it has been studied in good detail, yet its structure and formation are not completely known (3). The three-dimensional arrangement of its various atoms has a somewhat ordered array when


•


1433


Report

Figure 5. Proposed steps in the hardening of Portland cement.

a small (molecular level) portion is studied, but as larger portions are viewed, less order is observed. Thus, only an average structure for the cement exists. In addition, the average structure varies with the amount of water and other components added, the age of the cement, and the source of the concrete mix. Characterization of the arrangement of atoms is hampered due to the material being insoluble in all liquids (without decomposition). When anhydrous cement mix is added to water, the silicates react, forming hydrates and calcium hydroxide. Hardened Portland cement contains about 70% cross-linked calcium silicate hydrate and 20% crystalline calcium hydroxide. A typical cement paste contains about 60–75% water by volume and only about 25–40% solids. The hardening occurs through at least two major steps (Figure 5). First, a gelatinous layer is formed on the surface of the calcium silicate particles. The layer consists mainly of water with some calcium hydroxide. After about two hours, the gel layer sprouts fibrillar outgrowths that radiate from each calcium silicate particle. The fibrillar tentacles increase in number and length, becoming enmeshed and integrated. The lengthwise growth slows, and the fibrils now joined up in a sideways fashion, forming striated sheets that contain tunnels and holes. Concurrent with this, calcium ions are washed away from the solid silicate

polymeric structures by water molecules and they react further, forming additional calcium hydroxide. As particular local sites become saturated with calcium hydroxide, calcium hydroxide itself begins to crystallize, occupying once vacant sites and carrying on the process of interconnecting with the silicate bundles. In spite of attempts by the silicate and calcium hydroxide to occupy all of the space, voids are formed, probably from the shrinkage of the calcium hydroxide as it forms a crystalline matrix. These voids have a deleterious effect on the strength of the concrete. Much current research concerns attempts to generate stronger cement by filling these voids. Interestingly, two of the more successful void-fillers are also polymers— dextran, a polysaccharide, and polymeric sulfur.

1434

•


Future Developments It is nearly certain that future developments in materials for home construction will center on reducing labor costs, minimizing environmental concerns, and increasing energy efficiency. As described above, both natural and synthetic organic and inorganic polymers play a leading role in all three of these areas and they are key materials in much of the current research in this field. Because of the cost advantages and versatile properties of polymers, increasing use of polymers in construction applications is expected. Vol. 83 No. 10 October 2006

•

Notes 1. Find more information in A Past Preserved in Stone: A History of Montezuma Castle National Monument by Josh Protas, available online at http://www.nps.gov/moca/ protas/ or in print from the Western National Parks Association (http://www.wnpa.org) (both sites accessed Aug 2006). 2. Frost and Sullivan, a global growth consulting company, reported these data in 1995; it was published in Carraher, C. E., Jr. Seymour/Carraher’s Polymer Chemistry, 6th ed.; CRC Press: New York, 2003. 3. The term “2 ⫻ 4” refers to the thickness and width of lumber pieces before final trimming to a finished size of 1.5 ⫻ 3.5 in. 4. The American Plastic Council has many resources for teachers regarding uses of plastics. See http://www.americanplastics council.org or write to American Plastics Council, 1300 Wilson Blvd., Arlington, VA 22209. The POLYED National Information Center for Polymer Education also has many resources for teachers at http://www.polyed.org (both sites accessed Aug 2006).

Literature Cited 1. Azens, A.; Avendano, E.; Backholm, J.; Berggren, L.; Gustavsson, G.; Karmhag, R.; Niklasson, G. A.; Roos, A.; Granqvist, C. G. Mater. Sci. Eng. B 2005, 119, 214–223. 2. (a) Kornfeld, D. Monodisperse Latex Reactor (MLR), NASA TM-86847; NASA: Washington, DC, 1985. (b)

www.JCE.DivCHED.org


Mulholland, G.; Hembree, G.; Hartman, A. Sizing of Polystyrene Spheres Produced in Microgravity, NBSIR 84–2914; National Bureau Standards: Gaithersburg, MD, 1985. 3. (a) MacLaren, D. C.; White, M. A. J. Chem. Educ. 2003, 80, 623. (b) White, M. A. J. Chem. Educ. 2006, 83, 1425.

John P. Droske is Professor of Chemistry and Director of the POLYED National Information Center for Polymer Education at the University of Wisconsin–Stevens Point; [email protected]. Charles E. Carraher, Jr. is Professor of Chemistry and Biochemistry at Florida Atlantic University and Associate Director of the Florida Center for Environmental Studies; [email protected]. They are co-chairs of POLYED, the education committee of the ACS Divisions of Polymer Chemistry and Polymeric Materials: Science and Engineering.

www.JCE.DivCHED.org

•

Figure 6. “Earthship”, home of the late actor Dennis Weaver, in Ridgway, CO, is 10,000 sq. ft. and made primarily of recycled automobile tires and aluminum cans. The monthly electric bill of about $50 is very low for a home of its size. Photo provided by and used with permission of the family of Dennis Weaver.


•


1435

Polymers: Cornerstones of Construction - Journal of Chemical

Recommend Documents