Chemistry for Everyone edited by
Products of Chemistry
George B. Kauffman California State University Fresno, CA 93740
Artificial Glass—The Versatility of Poly(methyl methacrylate) from Its Early Exploitation to the New Millennium Michael S. Chisholm Ineos Acrylics, PO Box 90, Wilton, Middlesbrough, Cleveland TS90 8JE, England;
[email protected] Group of Imperial Chemical Industries (ICI) at Ardeer in Scotland (3), who established a new method for the synthesis of MMA. This came about during the summer of 1931 when Crawford was involved in the task of finding a laminated safety glass interlayer to replace cellulose nitrate, which had the disadvantage of severely yellowing in sunlight. Crawford recognized the potential of PMMA for this application, but was also aware of the hurdles to commercialization. Coincidentally also in 1931, Rowland Hill of ICI’s Dyestuffs Division at Blackley in Northern England produced PMMA as part of a general investigation on synthetic polymers and resins. However, Crawford’s contribution was the greater, as it was he who discovered an economic commercial process for manufacturing MMA. Crawford’s approach used
There exists a material that is as transparent and crystallike as glass, but is two times lighter and up to eight times less brittle. Unlike glass, this material can be colored in a multitude of shades and tints. It can be molded and worked into an endless number of intricate designs without requiring the furnace-like temperatures needed for glass working. This truly versatile material has been exploited commercially for over 60 years (1). What is it? Its chemical name is poly(methyl methacrylate) or PMMA for short, but it will be known to the general population under trade names such as Lucite, Perspex, or Plexiglas. Today the production of PMMA amounts to over 600,000 tonnes per year (2) and the material is put to a wide variety of uses, such as manufacture of baths, false eyes and teeth, greenhouse glazing, and hospital incubators. Like most synthetic polymers introduced in the 1930s, for example, polyethylene, nylon and neoprene, its history is a fasPh C O O cinating tale of scientific achievement preO dominantly involving the chemical giants of the day. Ph
Early Years
Initiator Decomposition
Ph
CH3
Initiation Ph
H2C C COCH3
+
C O
Ph
CH3 CH2 C
CH3 n
+
CH2 C
CO2CH3
CO2CH3
O
CH2
C O
n
CH2 C CO2CH3
CO2CH3
O
CH3
CH3
CH3 CH2 C
Propagation
CH3
CH3
C CH2 CO2CH3
C CH2 CO2CH3
Termination by Combination
C O
CO2CH3
n H2C C COCH3
O
Ph
CH3 C O CH2 C O
CH3 C O CH2 C CO2CH3 O
Ph
C O O
O
The means to convert water-like methyl methacrylate (MMA) monomer to solid PMMA sheet was first uncovered during the mid-1920s by the German firm, Rohm & Haas AG. The basic chemical reactions involved in the polymerization are depicted in Scheme I. An initiator molecule, such as benzoyl peroxide, thermally decomposes to radical fragments. One of these radicals reacts with an MMA molecule to initiate the growth of a polymer chain. Subsequent MMA molecules add to the chain end by a process called propagation. Finally, termination of chain growth comes about by one of two means, combination or disproportionation. PMMA molecules with molecular weights of up to several million daltons can be achieved by this process. Rohm & Haas were later to scale up and commercialize their PMMA under the trade name Plexiglas, but could not make that advance in those early days because a cost-effective means of manufacturing MMA monomer was not then available. The underpinning work that made PMMA a commercial reality was carried out by John Crawford, a chemist working in the Research Department of the Explosives
Ph
O
+
C O
2
C Ph
n
CH2
CO2CH3
Ph
C O
O
C Ph O
Termination by Disproportionation
CH3
CH3 C
n
CH3 C C CH2 CO2CH3 CO2CH3
CH3 CH2 C
O
CO2CH3
n
CH3 CH2 C CO2CH3
n
O
C Ph O
CH2 CH2 C CO2CH3
+ CH3 H C CH2 CO2CH3
CH3 C CH2 CO2CH3
n
O C Ph O
Scheme I. Free radical polymerization of MMA
JChemEd.chem.wisc.edu • Vol. 77 No. 7 July 2000 • Journal of Chemical Education
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Chemistry for Everyone CH3 HCN
+
CH3COCH3
H3C C CN OH CH3
CH3 H3C C CN
+
2 H2SO4
H3C C CONH2. H2SO4 OSO3H
OH CH3
H3C C CONH2. H2SO4
CH3 H2C C CONH2 . H2SO4
+
H2SO4
OSO3H CH3
CH3
H2C C CONH2 . H2SO4
+
CH3OH
H2C C CO2CH3
+
NH4HSO4
Scheme II. Acetone cyanohydrin route to MMA
the relatively low-cost starting materials acetone and hydrogen cyanide (Scheme II). These were reacted together to form acetone cyanohydrin. Treatment with sulfuric acid followed by addition of methanol resulted in the formation of MMA. ICI soon scaled up Crawford’s process, enabling the largescale manufacture of PMMA during the mid-1930s. Industrialists soon began to appreciate the advantages of PMMA, particularly the high transparency and a waterwhite appearance that was unchanged by exposure to long periods of sunlight or rainfall. These attributes were undoubtedly uppermost in the minds of ICI executives, who decided in 1934 to register their PMMA sheet under the trade mark Perspex from the Latin to ‘see through’, and PMMA powder granules for moulding were called Diakon from the Greek ‘passing through’ and ‘dust’. As well as being lightweight, PMMA sheet was noted to be much less hazardous than glass panes when damaged. It was therefore soon recognized as an ideal glazing material for the aircraft industry and found its first use in 1936 on the new British fighter plane called the Spitfire (3) (Fig. 1). Further impetus for the global commercialization of PMMA was provided by the pre–World War II alliance between ICI and DuPont, aimed at competing more successfully against the German chemical giants of the day such as I. G. Farben (4). The alliance lasted from 1929 until 1944, and led to both companies sharing their respective research on polyethylene, PMMA, nylon, and a synthetic rubber called neoprene (5). By providing details on its manufacturing process for MMA, ICI provided DuPont with the key know-how with which to commercialize Lucite PMMA sheet in 1937.
Figure 1. A typical aircraft canopy (ca.1940).
sheet, albeit in a much developed and automated form. In fact, PMMA sheet manufacture can now be operated as a continuous process, with MMA syrup being introduced at one end of two continuously moving polished steel bands and exiting at the other end as fully polymerized sheet. Shaping of the flat PMMA into curved aircraft cockpit windows was achieved by a process called thermoforming, whereby the PMMA sheet was heated until it softened, whereupon it was draped over a mold and held in place until it had cooled and hardened. Suspension polymerization of MMA followed during the 1940s. This involves dispersing droplets of MMA monomer and water-insoluble initiator in rapidly stirred water containing dissolved protective colloid. This colloid stabilizes the monomer droplets and prevents coalescence. Each insoluble monomer droplet acts as a mini bulk polymerization and results in the formation of small PMMA beads, typically between 50 and 400 µm in diameter. Once filtered, washed, and dried, the beads can be used for a variety of purposes, for example, melted in an extruder to form PMMA sheet or granules, or dissolved in solvent for use in specialized coating applications such as clear lacquers for brass castings. Emulsion polymerization has some similarities to the suspension process, but water-soluble initiators are used and surfactants are employed to suspend the monomer droplets in microscopic micelles. Polymerization occurs principally in these monomer-swollen micelles, which grow as the polymerization continues by diffusion of new monomer and initiator molecules into the micelles from the aqueous phase. The final emulsion usually contains about 40 to 50% by volume of
Production of PMMA Several different approaches have been developed for the commercial manufacture of PMMA, each aimed at producing a unique product form (sheet, beads, pellets or granules, and emulsions). The first commercial plant, built at Billingham in N.E. England in 1936, was aimed solely at producing PMMA sheet for the aircraft industry. This involved pouring flasks of partially polymerized MMA syrup (Fig. 2) between flat sheets of glass separated by a gasket. Each of the resultant glass cells was placed in an oven where the polymerization (termed “bulk” polymerization) was completed. Once cooled, the glass plates were removed to yield the sheet of PMMA. This basic process is still used for the manufacture of PMMA 842
Figure 2. Syrup pouring tables at ICI Billingham (ca.1937).
Journal of Chemical Education • Vol. 77 No. 7 July 2000 • JChemEd.chem.wisc.edu
Chemistry for Everyone
approximately 0.1–0.3-µm diameter polymer particles in water, which is often formulated with fillers and pigments to make products such as paints and inks. Usually, one finds that other methacrylate monomers, such as n-butyl methacrylate, are added in the monomer feed along with MMA to form a copolymer that modifies the properties of the final paint or ink, for example to improve the coating flexibility. Solution polymerization of MMA is a single-phase process that can be operated in either batch or continuous form. Continuous processes usually involve only partial polymerization of the MMA solution. The unreacted monomer and solvent are removed by high-temperature vacuum devolatilization and recycled. This leaves the molten PMMA, which is then extruded into either sheet or granules. Batch processes tend to involve copolymerization of MMA and other monomers to yield a polymer solution, which can then be further formulated for use in applications such as automotive coatings or industrial paints.
Figure 3. Corrugated PMMA roof lights in a commercial garage (ca.1948)
Application Development of PMMA Following the enforced focus on aircraft glazing during World War II, the postwar years finally gave the fledgling PMMA industry the chance to develop new applications for PMMA. Indeed, this task was an urgent one because the large manufacturing capacity built up to satisfy war-time demand for aircraft glazing was now rapidly becoming redundant (6 ). Designers and architects were therefore encouraged to take advantage of the unique properties of PMMA, such as excellent transparency, exterior durability, and easy workability. These properties stem directly from the chemical nature of PMMA. For example, the excellent transparency and workability is attributed to the amorphous, randomly coiled nature of the PMMA chains. Exterior durability stems from the relatively stable chemical bonds in the MMA monomer repeat unit, which are resistant to degradation by UV and visible light. Use in applications where glass was inappropriate was an obvious product development topic to pursue. Thus, one of the first successful new applications in the post-war era was thermoforming of PMMA sheet into corrugated roofing panels. These could be matched to other well-known types of corrugated sheeting and be drilled without shattering to accept standard fixing bolts. Such fabrication is rarely possible with glass, and the use of corrugated PMMA sheet therefore led to new styles of roof lighting that were more efficient and economical. Many new agricultural buildings, warehouses, and factories were fitted with such panels (Fig. 3). The lighting industry also became an important user of PMMA. Light diffusers could be made by either injectionmoulding PMMA granules or thermoforming PMMA sheet. Controlled addition of opal pigments led to PMMA varying widely in optical characteristics from highly diffusing to highly transmitting and made possible a wide range of new soft lighting effects (6 ). Glass technologists were unable to provide similar optical control in glass, and the introduction of the new PMMA materials allowed designers to become much more creative. In the automotive industry, the use of PMMA to make colorless, amber, and red lenses and reflectors became well established. Those early fittings were quite simple and very functional in design. Nowadays, the development of any new
Figure 4. Automotive rear light cluster (1999).
Figure 5. Television implosion guard made from PMMA (ca.1952).
car involves a significant amount of a designer’s time in linking the appearance of the lenses and reflectors with the overall design concept—aesthetics has become as important as function in this case. Key properties such as excellent outdoor weatherability, high light transmission, petrol and antifreeze resistance, high heat resistance, and easy moldability make PMMA an ideal material for this application (Fig. 4). Other applications for PMMA were developed in the 1950s and ’60s. For example, the viewing screens of some of the first mass-produced television sets were fitted with PMMA implosion guards (Fig. 5). Decorative tableware made
JChemEd.chem.wisc.edu • Vol. 77 No. 7 July 2000 • Journal of Chemical Education
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Chemistry for Everyone
Figure 6. PMMA cocktail glasses and tray (ca.1955).
Figure 7. Optical applications for PMMA (1999).
from PMMA became a fashion item (Fig. 6) and hospital incubators were developed for newborn babies. In addition to the high transparency of PMMA, all these uses exploited the lighter weight, improved toughness, easier processing, and safer handling of the acrylic material compared to glass. In addition to glass replacement, MMA also found a niche as an alternative to cast iron in bathtub and spa production. PMMA bathtubs are easier to manufacture than cast iron ones and the installer appreciates the reduced weight. To the user, PMMA has the great advantage of providing a warmer feel and it is also available in a large number of colors, which can coordinate with the decor of the surroundings. More recent applications for PMMA include its use in optics such as light pipes, lenses, and magnifying glasses (Fig. 7). Excellent light transmission and optical clarity are the distinguishing characteristics in these cases. PMMA has attracted the attention of a number of sculptors who have sought to exploit its superb appearance and feel, as well as its relative ease of working. Notable artists who used PMMA during the 1950s and ’60s were Arthur Fleischmann, Einar Andersen, and Edmond Vernassa. More recently, during 1997, Ernest Caballero from California chose PMMA for creating his Cross of New Life sculpture, which is now suspended above the altar in the Holy Family Church at Inverness, Illinois, near Chicago. This sculpture is particularly noteworthy because, at 16 ft tall and 12 ft across and weighing 2,030 lb, it is currently the world’s largest singlemold piece of acrylic artwork. Valued at $750,000, the Cross of New Life represents a magnificent integration of contemporary design, sculpting, casting, and engineering.
to be avoided. Designers were therefore unable to specify PMMA for use in diffusers for high-intensity lights that were left switched on for long periods of time. A means to overcome this shortcoming was identified during the 1960s and 1970s by Rohm & Haas and others. Their approach involved the chemical modification of PMMA through reaction with a primary amine to introduce cyclic imide groups in the PMMA backbone (7):
PMMA Performance Enhancement As the new applications for PMMA became more established, designers and materials engineers began to exploit this material in more demanding situations and within increasingly innovative designs. This led to increasing pressure on the PMMA manufacturers to investigate means to enhance the material performance. For example, commercial PMMA has a glass transition temperature (Tg) (the temperature above which an amorphous polymer changes from a glassy material to one with a leathery and more ductile consistency) of about 105 °C. This clearly sets an upper temperature limit for the use of PMMA in lighting diffusers if distortion or melting is
844
*
CH3
CH3 n
O C
*
C O
RNH 2
* O
OMe OMe
CH3
CH3 n
N
O
*
+
2 CH3OH
R
This causes the polymer backbone to stiffen and increases the Tg to as high as 170 °C, depending on the degree of cyclization. Another aspect of performance that was considered problematic was impact strength. Although PMMA is a relatively robust material and is significantly less brittle than glass, PMMA lighting diffusers are (like glass) not vandal-proof and can be damaged by deliberate (or accidental) knocks. There was therefore a distinct need to develop a tougher grade of PMMA without compromising light transmission and exterior durability. During the 1960s it was well known that the dispersion of microscopic rubber particles into PMMA could increase impact strength. Each rubber particle could act as a microscopic shock absorber and help dissipate the fracture energy of a sharp blow. Unfortunately, these improvements in robustness were accompanied by some loss of optical properties, particularly transparency. The required performance enhancement came about during the 1970s, when DuPont and Rohm & Haas both uncovered a means to synthesize submicron-sized cross-linked rubbery spheres having the same refractive index as PMMA. When these particles are dispersed into PMMA, the rubber-toughened PMMA appears transparent even though the rubber particles are large enough to scatter light (Fig. 8). Lovell et al. (8, 9) have described the technology in some detail. Needless to say, these new impact-modified (IM) grades of PMMA have provided significant improvements in toughness, as demonstrated by the ability of IM PMMA mouldings to have nails driven through them without cracking or for IM PMMA rulers to survive the abuse of boisterous students.
Journal of Chemical Education • Vol. 77 No. 7 July 2000 • JChemEd.chem.wisc.edu
Chemistry for Everyone
Figure 8. Transmission electron micrograph of rubber toughened PMMA (×29,600 magnification).
Future Developments The basic polymerization mechanism employed in the commercial manufacture of PMMA remains essentially the same today as that employed in the very first MMA polymerization plants. In other words, the molecular structure of today’s commercial PMMA is similar to that produced more than 60 years ago. However, the 1990s brought some interesting new developments in MMA polymerization technology. Several university laboratories have been working on new controlled polymerization techniques and there is great interest from industry in evaluating how these could be applied in a commercial setting. For example, achieving control of the stereochemistry of the PMMA molecule is a means to stiffen the polymer chains. This in turn causes an increase in the softening point of PMMA and could lead to new acrylic materials possessing improved high heat characteristics, for example for lighting applications. In some circumstances, a high level of control over stereochemistry of the polymer chain can cause PMMA to be rendered semicrystalline (10). The properties of this new form of PMMA have not yet been examined in detail, but one could imagine it to be significantly stronger and tougher than current commercial PMMA and more similar overall to other semicrystalline polymers such as poly(ethylene terephthalate) or poly(ethylene). New process technologies for manufacture of PMMA have recently been identified. For example, the polymerization of MMA in carbon dioxide as solvent under high pressure has been established in university laboratories (11). The process is unique because the carbon dioxide solvent plus any unreacted MMA monomer can be fully evaporated after the polymerization is complete, leaving dry polymer particles. The process has been touted as a new environmentally friendly way to make polymers such as PMMA. No aqueous effluent is produced, nor is there a need to dry the polymer in a separate process step. Release of pressure from the reactor causes the liquid carbon dioxide solvent to evaporate for recycle back into subsequent polymerization batches. Thus, the process provides benefits to the environment in terms of higher production yield, no aqueous waste, and lower energy usage. Development of the process into a commercial reality is, how-
ever, currently constrained by the need to develop economic high-pressure reactor process technology. New applications for PMMA continue to be found. The use of PMMA to make optical fibers for decorative light pipes and so on is well known, but these applications are set to increase substantially as the field of information technology develops. Similarly, in adhesives, MMA/PMMA syrups have been used for decades for gluing PMMA sheet and moldings. However, a need for new adhesives for construction, automotive, and packaging applications seems to be resulting in the increased development of new methacrylate-based glues. Environmental considerations, particularly recycling, look set to continue to be of high importance to industry. PMMA has the important advantage of being one of the few polymers that can be 100% depolymerized back to the starting monomer simply by heating. New depolymerization recycling processes developed by the PMMA industry capitalize on this feature and provide the potential basis for closed-loop recycling in the future. The continuing story of PMMA since its origins as a safe glazing material looks set to be just as eventful as its past. The versatility of this “artificial glass” will undoubtedly increase as its property envelope becomes broadened and applications that are now just twinkles in the eyes of designers will be established. New and as yet undiscovered controlled polymerization methods for MMA look certain to excite continued interest from academia, and industrialists will, as always, race to be the first to exploit the benefits. Clearly a busy future for PMMA chemists! Acknowledgments I gratefully acknowledge permission from Ineos Acrylics to publish this article and the contributions of my colleagues in its preparation. Literature Cited 1. Chisholm, M. S. Chem. Br. 1998, 34, 33. 2. Fong, W. S. In Poly(methacrylates); Report No. 65A, Process Economics Program; SRI International: Menco, CA, 1991; p 2-1. 3. Kennedy, C. ICI—The Company that Changed our Lives; Paul Chapman Publishing: London, 1993; p 78. 4. Taylor, G. D.; Sudnik, P. E. DuPont and the International Chemical Industry; Twayne: Boston, 1984; p 132. 5. Hounshell, D. A.; Smith, J. K. Jr. Science and Corporate Strategy: DuPont R&D, 1902–1980; Cambridge University Press: Cambridge, UK, 1988; p 476. 6. ‘Perspex’: The First Fifty Years; ICI: Darwen, UK, 1984; p 20. 7. Rohm & Haas. Polymer Treating Process; UK Patent 910,144, Apr 28, 1961. 8. Lovell, P. A.; Sheratt, M. N.; Young, R. J. In Toughened Plastics II: Novel Approaches in Science & Engineering; Riew, C. K.; Kinloch, A. J. Eds.; Advances in Chemistry Series 252; American Chemical Society: Washington DC, 1996; Chapter 15, p 211. 9. Lovell, P. A. Trends Polym. Sci. 1996, 4, 264. 10. Hirano, T.; Yamaguchi, H.; Kitayama, T.; Hatada, K. Polym. J. 1998, 30, 767. 11. Cooper, A. I.; DeSimone, J. M. Curr. Opin. Solid State Mater. Sci. 1996, 1, 761.
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