Chemically speaking, it is merely one step from plastics made out of polymethacrylate (PMA) to Perspex, i.e. polymethyl methacrylate (PMMA), production of which began at Röhm & Haas AG in 1928 (Hölscher 1972, 121). In contrast to acrylic acid, methacrylic acid has a methyl group (CH3) and/or is derived from it “by replacing 1 hydrogen atom in it by 1 methyl atom” (Frankland/Duppa 1865, 13; see also Trommsdorff 1976, 224). The small difference is important for the properties of the polymers in question, for example their solubility, flexibility and hardness level:
“In the course of his research into acrylic acid synthesis, Walter Bauer came across methacrylates as the next homologues of acrylates as early as 1919, but did not investigate them any further. It was not until eight years later, when the material properties of acrylates had largely been determined that he pushed for examination of methacrylates by way of comparison. Otto Röhm initially deferred this work in favour of LUGLAS. Two years later, Walter Bauer continued his research work with the aim of examining the suitability of methacrylates for laminated glass. In this context, he made a systematic investigation of the properties of the most important homologues of methacrylates, with the primary focus on methyl methacrylate (MMA) and polymerisation of it. It turned out that polymethyl methacrylate (PMMA) – in contrast to the acrylates investigated beforehand (polyacrylic acid methyl ester and its homologues) – was a hard, clear material.” (Wittig 2007, 35-36)
The advantage was obvious: “Where acrylates were too soft or too tacky and/or swelled badly, it was possible to compensate for such difficulties with the help of methacrylates” (Wittig 2007, 43), because “the polymer methyl ester” represents “the hardest polymer with the highest softening point” (Trommsdorff 1976, 232).
“Polymers of methacrylic acid and its compounds, such as esters, salts, or methacrylic acid amide are possible options for use as plastics” is what it says in the specification of the patent “Plastic” (DRP 656642 of 27. October 1928, inventor: Dr Walter Bauer) granted to Röhm & Haas AG. Moulded products or elastic films, e.g. membranes for linings or covers, as well as lacquers and resins are given as examples of applications for polymerised methacrylic acid ester. The potential to be an organic substitute for glass is only mentioned here on the side:
“It has already been suggested that polymerised itaconic acid ester is used as a plastic and, in particular, as a glass substitute. This invention relates exclusively to use of the functional derivatives of the homologues of acrylic acid. A comparison between the properties of polymers made in accordance with the invention and polyitaconic acid ester shows that there are not only identical properties like crystal-clear transparency but also substantial superiority in various directions. Polyitaconic acid ester is comparatively brittle, whereas polymethacrylic acid esters are, for example, comparatively tough, a property that is of considerable or even – in certain cases – crucial importance for many plastics. Polyitaconic acid esters change to a soft, more or less molten state at temperatures of as low as about 40 to 50°. Polymethacrylic acid esters, on the other hand, can be heated up to temperatures of 100° or more without melting or being changed to their disadvantage.”
The vision: glass substitute
Röhm & Haas started by investigating the potential of the higher methacrylic acid esters for the production of safety glass too. However, the vision behind this was in the final analysis not optimised laminated glass but an independent glass substitute, in other words, the production of acrylic glass (organic glass) as an alternative to silicate glass (inorganic glass). “Polymer methacrylic acid methyl ester made it […] possible to enter the basic applications for glass.” (Trommsdorff 1976, 232) This is confirmed by DRP 724229 “Glass substitute” of 22. March 1932 that is attributed to Röhm and Bauer as inventors:
“It has been determined that copolymers made from acrylic acid esters and methacrylic acid esters are particularly suitable glass substitutes, instead of the polymerisation products of acrylic acid and/or its functional derivatives. […] In contrast to glass substitutes like celluloid panes disclosed in the past, the glass substitute proposed in this invention is exceptionally resistant to light, moisture, petrol, oil and other substances, is very permeable to ultraviolet light and displays excellent mechanical properties, e.g. elasticity and toughness, even at low temperatures. The copolymers are also considerably harder than the polymerisation products of acrylic acid and/or its functional derivatives […] that are the subject of the main patent”.
The main patent mentioned here was DRP 656421 of 7. February 1928, which was entitled “Moulded plastics made from polyacrylic acid, its compounds or blends of the same” and of which Röhm and Bauer were named the joint inventors too. An additional patent (DRP 680679, also dated 7. February 1928) had already been given the title “Glass substitute“, but did not mention polymer methyl methacrylate yet.
Now, in 1932, the key was to focus laboratory work on the development of a polymerisation process that was suitable for the production and marketing of high-quality acrylic glass on what was at the time described as ‘a technical scale’. Following preliminary work of his own in February, Otto Röhm formed “a task force in 1933 consisting of Dr Weisert, Mr Wagner (an engineer) and another staff member […] with the aim of substituting glass […]. The acrylic esters […] had already brought this idea to life.” (Trommsdorff 1976, 232; cf. Ackermann 1967, 114)
“In order to manufacture safety glass (laminated glass), the production of pliable, elastic polymer films with a thickness of approximately 0.2 to 0.5 mm under conditions that bond the panes of glass together to form structures that cannot for all practical purposes be separated has already been disclosed. In contrast to this, polymer structures are produced in accordance with the present invention that are removed from their mould after formation and are used or processed to this end” is the description given in the specification for DRP 659469 of 10. October 1933, which was granted for the “Process for the production of bubble-free moulded structures like panes or panels by the polymerisation of vinyl or acrylic compounds”, particularly of esters of methacrylic acid and acrylic acid or of blends of such compounds. This time, the head of the company, Otto Röhm, and not Bauer is named as the sole inventor. According to the invention, polymerisation itself was “carried out […] at high temperatures in flat chambers that it is advisable to arrange in standing positions and that can be heated and cooled on both sides and can be taken apart and the flat structures produced as a result are removed from the mould once the polymerisation process has been completed”. What are created in this way are, it is claimed, “completely flawless, particularly bubble- and dirt-free structures”, “panes or panels of crystal-clear purity […], that have such smooth surfaces when they come out of the mould that they can be used immediately as a glass substitute and also have special advantages over standard glass, e.g. as regards resistance to breakage, low weight, easy shapability etc.”
Undesirable shrinkage and adhesion
According to Bauer, Röhm & Haas abandoned DRP 659469, however, because: “It has never been possible to produce viable acrylic panels by this process.” (Ackermann 1967, 113) What proved to be problematic in practice were polymer shrinkage and depletion as well as its tendency to stick to the walls of the chamber when the latter consisted of silicate glass – in line with the procedure used in Luglas production (Wittig 2007, 36). “The casting process developed by Dr Bauer” therefore established itself in the end (Ackermann 1967, 114), which was patented as “Process for manufacturing polymerisation products by heating up unsaturated, liquid organic compounds that contract during polymerisation” (DRP 639095 of 8. April 1934). “What was involved here in principle was the use of flat chambers with walls that are sufficiently manoeuvrable in relation to each other, so that it is possible to compensate for the depletion which occurs during polymerisation.” (Ackermann 1967, 46) The specification for DRP 639095 explains:
“In accordance with the invention, polymerisation is carried out by heating the source material in flat chambers with main walls that have the ability to move in relation to each other during the polymerisation process. With the help of such flat chambers, it is possible to carry out polymerisation of the liquid source materials in thin layers that avoid the formation of bubbles. […] If glass panels are used as the main walls of the chambers, moulded structures can be produced with such a smooth surface that either no subsequent treatment of any kind is needed or such treatment can be limited to simple measures like polishing. […] Since the polymer that is produced sticks to the main walls of the chamber, they are moved in relation to each other in accordance with the contraction of the polymer that takes place during the polymerisation process. This reliably avoids deformation or the formation of cavities between the chamber walls and the polymer and counters the formation of bubbles in the polymer. After the polymerisation process has been completed, the polymer structures are cooled in the flat chambers and then removed from the same. Once they have cooled down, the solid panels can generally be prompted to release from the chambers simply by turning the chambers upside down. One way to facilitate removal of the panels from the chamber can if necessary be to line the inside surfaces with substances that stop the polymer from sticking or caking. Thin lining materials or intermediate layers made from viscose, gelatine, paste can, for example, be used for this purpose, in addition to paper impregnated with paste, gelatine and similar substances.”
Wittig 2007, 36-37 summarises the advantages of production of acrylic glass by this process, which is known as casting, as follows: “The silicate panels now followed the shrinkage consistently, so that faults in the material were avoided. In addition to this, the panels of silicate glass could be removed smoothly after the polymer had cooled. As a result, the research scientists obtained the first acrylic glass panes that had ever been produced.”
Industrial production of acrylic glass (trade name: “Plexiglas”) started at the former soap factory Jacobi in Darmstadt, that Röhm & Haas had bought during the First World War (Wittig 2007, 37 and Trommsdorff 1976, 40). The thin panes of organic glass that were produced were initially used for gas masks and watches/clocks, for which “prospective customers were found as early as 1933” (Trommsdorff 1976, 234; see also Wittig 2007, 37).
Rival product: Perspex
Unexpectedly, the monomer MMA and synthesis of it soon became the focus of attention again instead of the polymer PMMA and its potential applications: “Initially, methyl methacrylate was synthesised from acetone cyanohydrin, which was obtained from Deutsche Gold- und Silber-Scheideanstalt in Frankfurt (the name of which was later changed to Degussa), in several stages. This solution was time-consuming, expensive and also problematic, however. In one of the stages of the process, tear gas was released, for example. At another stage, small amounts of the monomer polymerised in the machines, so that they were gradually blocked up with plastic. It took over 30 hours to obtain what was known as the basic MMA alone, which then had to be distilled into pure methyl methacrylate in a subsequent operation.” (Wittig 2007, 37-38) So the news from Great Britain that Imperial Chemical Industries Ltd. (ICI) had succeeded in devising a “considerably more effective synthesis process with a higher yield” (Wittig 2007, 38) attracted a great deal of attention:
“The English scientists mixed the acetone cyanohydrin directly with sulphuric acid and produced basic MMA in a single operation following the addition of methanol and water. All the plastics research scientists involved at the company realised to their alarm that there was no alternative to this synthesis process from the economic point of view. Röhm & Haas therefore attempted to reach agreement with ICI about the obtainment of a licence. At the end of the lengthy negotiations, the British company finally passed its formula for the production of methyl methacrylate on to Röhm & Haas in a contract concluded in the summer of 1936 – albeit without any information about technical implementation. Once Röhm & Haas had developed a process for this, basic MMA could be produced in Darmstadt in only ten hours and without the above-mentioned problems. The company paid an extremely high price for this, however. ICI received the manufacturing process for PLEXIGLAS, which they marketed under the name Perspex (from 1934 onwards, editor’s note). The market for acrylic glass was divided up in a territorial agreement, to avoid mutual competition.” (Wittig 2007, 38; see also Trommsdorff 1976, 230-231 and 254)
Like Röhm & Haas, ICI and particularly the chemist Dr John William Croom Crawford (1901 – 1987) had been carrying out research in the safety glass field and polymerised methyl methacrylate, among other things, in the course of the “investigation of ‘plasticised synthetic resins of the unsaturated type‘ for possible use in the production of safety glass“ (Imperial Chemical Industries 1984, 6). Crawford developed the MMA synthesis process that is outlined above, which went down in history as the “Stevenston Process” (Imperial Chemical Industries 1984, 8), because Crawford’s research department was located in Stevenston/Scotland, and for which ICI secured a patent for Germany as well (DRP 648237 of 2. November 1932). The patent specification was entitled “Process for the production of methyl methacrylate” and ends with the words:
“The methyl methacrylate produced in accordance with the invention can be polymerised by heating, if necessary in the presence of a catalyst, […] or under the influence of ultraviolet light. In this way, a colourless resin polymer is produced that can be used for various purposes, […], e.g. for manufacturing layered, non-shattering glass.”
ICI held a British patent (BP 395,687) about this “new polymerisation product”, which was later named “Perspex” (based on the Latin word “perspicere”, English “to see through”), since 17. November 1931. The inventor named was Rowland Hill (“a British subject”), who worked in the research department of British Dyestuffs Corporation, Hexagon House, Blackley, near Manchester, “but in September 1933 the Dyestuffs Group relinquished their interests in these products to a company called Croydon Mouldrite in which ICI had a majority shareholding“ (Imperial Chemical Industries 1984, 8). Röhm & Haas put it like this, on the other hand:
“Although ICI […] came to hold a patent of its own for polymethyl methacrylate, it still depended on a licence for the casting process developed by Bauer for the production of organic glass panes. Röhm & Haas had secured the relevant foreign rights to this invention. To be fair, credit must be given to the English for Crawford’s accomplishment in the production of monomer methyl methacrylate. The English company filed several patent applications in Germany. In the first one, the cyanohydrin process for the production of acrylic esters was modified in such a way that acetone cyanohydrin was used as the source material instead of ethylene cyanohydrin. In this process, sulphuric acid and methanol were first added at the same time. In a later application, it was explained that it is advantageous to heat the cyanohydrin with sulphuric acid before methanol is added. This modification of the synthesis process has become important for the technical production of methacrylic esters. Röhm & Haas obtained a licence for it. It was not, however, fundamentally crucial for the production of organic glass. Plexiglas was already being produced at Röhm & Haas before the English licence was obtained. In this context, a monomer ester was being used that was obtained from oxyisobutyric acid ester. There are in addition records showing that Bauer already drew attention at Röhm & Haas to the advantage of using oxyisobutyric acid nitrile = acetone cyanohydrin instead of oxyisobutyric acid ester long before the English process was disclosed. In view of the staff available and the greater importance of the other work on hand, it was not, however, possible to process this proposal at Röhm & Haas at this time.” (Ackermann 1967, 103)
Bauer emphasises elsewhere that he personally “tried” at an early date “to identify other dehydration agents”, basing this on minutes of 6. January 1932, which “drew particular attention to nitrile as a source material, which was then used successfully later on by ICI” (Ackermann 1967, 231).
Awarded a gold medal
Since it was considerably less expensive to manufacture monomer MMA after this, Röhm & Haas was now in a position to start upping polymer production (PMMA) too (see Wittig 2007, 38 and Trommsdorff 1976, 230). Acrylic glass had many different properties that made it superior to silicate glass: resistance to breakage, weathering and ageing and, last but not least, its low weight. Customers were found very easily, without any major marketing efforts, at any rate “wherever […] weight was crucial: in aircraft and car manufacturing. For example, the ‘Hindenburg’ Zeppelin was equipped with PLEXIGLAS windows, as were airplanes.” (Wittig 2007, 40) Very soon, “not just merely panels and blocks but also the first pipes were being manufactured from PLEXIGLAS by the centrifugal casting process. At plastics conferences and in discussions with experts, Otto Röhm became increasingly certain that PLEXIGLAS could become his company’s most important product.” (Wittig 2007, 40) In the summer of 1936, Röhm presented his organic glass at the annual meeting of the Association of German Chemists in Munich (Trommsdorff 1976, 237). The innovative new plastic was awarded a Grand Prix and a Gold Medal at the International Exposition in Paris in 1937. Daily production of Plexiglas increased almost ninety times over within three years, between1936 and 1939 – from 250 to 4,700 kilograms (Edschmid 1957, 61). The revenues that the company generated with Plexiglas increased enormously over the same period, from about 2.5 million Reichsmarks to about 11.7 million Reichsmarks (Ackermann 1967, 84), which represented about 80.7 per cent (1936) and about 74.5 per cent (1939) of total sales.
Who should be given the credit for the pioneering invention of acrylic glass and the boom in demand that it triggered? The “Festschrift” published by Röhm GmbH in 2007 “Shaping the future for 100 years” pays unequivocal tribute to Otto Röhm, the founder and head of the company who died in 1939, as the initiator of and driving force behind the development:
“Otto Röhm had […] taken a groundbreaking decision: he had given absolute priority to methacrylates over acrylates. […] When Otto Röhm received the first product sample of PMMA, he decided to hurry research into this substance group along as fast as possible. Röhm was convinced that there were many different potential applications. He recollected later on that his research scientists were sceptical about this plan initially. Their memory of the explosion in one of the first polymerisation trials with pure acrylic acid ester was still too fresh. In the end, he convinced them that this new polymer could become a glass substitute of its own.” (Wittig 2007, 35-36)
Ackermann 1967, 105 quotes Röhm as saying: “Everyone who came into contact with methacrylate – including me – probably realised that it is a valuable material”, only then to disagree fundamentally. Röhm had not encouraged the development: on the contrary, he had obstructed it due to his reservations:
“When Dr Bauer showed him the first polymethyl methacrylate film, Dr Röhm broke the piece of film to demonstrate its uselessness. He did not in the least realise the future significance of the material when he first saw it. This was also the reason why he prohibited work on it rather than encouraging it […] The fact that a decision was taken at the company as late as 30.9.1929 not to file any foreign applications is clear evidence of the negative light in which the company management – and particularly Dr Röhm, who played a major role in this decision – saw methacrylic resins.” (Ackermann 1967, 105)
These diametrically opposite statements give an insight into how the success story of Plexiglas / acrylic did not by any means go smoothly all the time. And that is not all: there was a lengthy dispute between the company management and Bauer about the question of who at Röhm & Haas was entitled to claim to have invented organic glass, which finally led to a complete rift – more about this in the next part of this series.
Katharina Ackermann (Ed.): Dr Walter H. Bauer and his 67 German Imperial Patents. Achievements and experiences of a German research chemist, compiled on the basis of documents. Jugenheim/Bergstrasse: published by the editor in 1967, 264 pages
Kasimir Edschmid: In memoriam Otto Röhm. On the 50th anniversary of the establishment of Chemische Fabrik Röhm & Haas Darmstadt. Darmstadt: Carl Winter 1957, 75 pages
E[dward] Frankland und B[aldwin] F[rancis] Duppa: Analyses of acids from the acrylic acid family. In: Chemistry and Pharmacy Annals 136 (1865), P. 1-31
Friedrich Hölscher: Rubber, plastics, fibres. Six decades of the technical production of synthetic polymers. Munich: Kastner & Callwey 1972 (= series published by the corporate archives of Badische Anilin- und Soda-Fabrik AG, 10), 145 pages
Imperial Chemical Industries (ICI) (publisher): ‘PERSPEX‘. The first fifty years 1934-84. Darwen/Lancashire: author unknown, 1984, 78 pages
Herman F. Mark: Giant molecules. Weert: Time Life 1970 (= Life – Wunder der Wissenschaft), 200 pages
Alfred von Nagel: Ethylene, acetylene. Munich: Kastner & Callwey 1971 (= series published by the corporate archives of Badische Anilin- und Soda-Fabrik AG, 7), 74 pages
Ernst Trommsdorff: Dr Otto Röhm. Chemist and entrepreneur. Düsseldorf and Vienna: Econ 1976, 294 pages
Eva Wittig: Shaping the future for 100 years. Röhm GmbH from 1907 to 2007. Munich: Peschke 2007, 112 pages