John Crawford

/John Crawford
John Crawford 2016-12-06T05:35:20+00:00

John
Crawford

synthesised
methyl methacrylate

(methyl
2-methyl-
propenoate)

 

Two separate lines of research in two different parts of ICI came together in the birth of "Perspex".
Rowland Hill in the laboratories of British Dyestuffs Corporation at Blackley near Manchester was studying artificial resins and polymers, including poly(methyl 2-methylpropenoate), otherwise known as polymethyl methacrylate or PMMA. A patent was applied for in 1931 and this covered not only the polymer and its use as a thermoplastic material but also the preparation of the monomer. The patent noted:- "It is a matter for surprise that the polymer of methyl methacrylate differs so sharply in physical properties from the polymers of known closely related compounds… " This was a reference to other acrylc polymers which were much softer.

At the same time, Dr. John Crawford in the laboratories of Explosives Group of ICI at Stevenston in Scotland was investigating the suitability of various resins and polymers as interlayers for safety glass. The cellulose based materials then being used had the disadvantage of turning yellow in sunlight which poly(methyl 2-methylpropenoate) did not. John Crawford recognised, as had Rowland Hill, the unique combination of physical and optical properties shown by poly(methyl 2-methylpropenoate).

Both researchers recognised also that, although the patent had specified a way to prepare the monomer, methyl 2-methylpropenoate, the process described in the patent was not capable of producing it in large quantities at an economic price. Before the polymer could be properly developed and made commercially available a way would have to be found to make the monomer on a commercial scale.

John Crawford’s great contribution to the story of  was the discovery of a commercially viable manufacturing process for methyl 2-methylpropenoate.

The process used cheap and readily available starting materials, namely propanone, hydrogen cyanide, methanol and sulphuric acid. A patent covering this process was granted on 12th August 1932.

The process is known as the Stevenston Process and, with some modifications, it is still used today, throughout the world, for the manufacture of methyl 2-methylpropenoate and related materials, the first stage in the manufacture of the family of plastics materials commonly known as ‘acrylic plastics’ or simply ‘acrylics’.

It was in 1932 that John Crawford discovered the above process and was able to make methyl 2-methylpropenoate in 45 Kg batches. The way now lay open for the development of the polymer as a commercial material. The first sheet of clear poly(methyl 2-methylpropenoate) was made at ICI’s Ardeer Works in 1933. The technique devised then still forms the basis for the method used today for large scale production of cast acrylic sheet. The technique is possible because methyl 2-methylpropenoate is a chemical whose polymer is soluble in the monomer. As the chemical polymerises it changes from a low viscosity liquid, through a progressively thickening syrup, to a hard tough solid.

When the first sheet was cast in 1933 the monomer was heated and partially polymerised to form a thick syrup. This syrup was then poured into a cell made from two sheets of glass separated at the edges by a compressible gasket. The cell was then placed in an oven at about 313 K to complete the polymerisation. Afterwards the cell was cooled and the glass plates removed leaving a sheet of clear, hard, tough poly(methyl 2-methylpropenoate). The gasket used must be compressible to allow for the contraction in volume, with its consequent increase in density, that occurs when methyl 2-methylpropenoate is polymerised. The polymerisation reaction is exothermic, that is to say heat is given off as the reaction proceeds, and it is essential to control the temperature of the polymerisation very carefully. If the temperature is allowed to get too high the reaction proceeds rapidly with the generation of so much heat that the monomer boils, forming large bubbles in the rapidly hardening material. A new material had arrived – "Perspex" was born.

The task of making methyl 2-methylpropenoate monomer was now given to ICI’s General Chemicals Division at Billingham on Teesside. Research into the manufacture of sheet materials from this monomer was also transferred to Billingham and by 1934, the year in which the Trade Name "Perspex" was registered, a semi-technical plant to produce sample quantities of the new "Perspex" sheet had been established there.

Only some five years had elapsed between the original investigation into poly(methyl 2-methylpropenoate) as a possible interlayer material for safety glass and its introduction as a transparent, thermoformable sheet material.

It has been said that, when the first sheet of polymer was cast, researchers were still thinking of it as an interlayer for safety glass and were disappointed when it did not stick to the glass sheets in the cell! Nowadays, manufacturers of cast poly (methyl 2-methylpropenoate) sheet would be disappointed if the polymer did stick to the glass.