If anything ends up on the beds of hobbyist-grade laser cutters more often than birch plywood, it’s probably sheets of acrylic. There’s something strangely satisfying about watching a laser beam trace over a sheet of the crystal-clear stuff, vaporizing a hairs-breadth line while it goes, and (hopefully) leaving a flame-polished cut in its wake.
Acrylic, more properly known as poly(methyl methacrylate) or PMMA, is a wonder material that helped win a war before being developed for peacetime use. It has some interesting chemistry and properties that position it well for use in the home shop as everything from simple enclosures to laser-cut parts like gears and sprockets.
Like many of the polymers that the world is built on, PMMA was first commercialized in the early 20th century. The plastic’s root go back much further, though. Acrylic acids, including methacrylic acid, were first synthesized in the mid-19th century. Methyl methacrylate (MMA), the monomer from which PMMA is built, was first synthesized later in that century, and the first successful polymerization was carried out in 1874.
The key to polymerizing methyl methacrylate is the double bond between the two carbons. That bond is part of a functional group called a vinyl group, where the name for other plastics like polyvinyl chloride comes from. In the case of PMMA, the MMA monomers react together in the presence of an initiator compound, like benzoyl peroxide – yes, that benzoyl peroxide. The initiator’s job is to provide lots of free radicals, or unpaired electrons. The free radicals greedily sop up the extra electron in the double bond, reducing it to a single bond and linking an MMA monomer to the initiator. But the resulting molecule is itself a free radical, which can reduce the double bond of nearby MMA monomers, resulting in another free radical. Eventually, the chain reaction runs out of steam, but not before long chains of PMMA are created.
Battle of the Plastics
It would be more than 50 years after the initial polymerization reaction was discovered before PMMA was turned into a commercial product. In the early 1930s, British and German chemists were working independently on PMMA. The British team of Hill and Crawford, working for Imperial Chemical Industries, perfected a method for producing an “acrylic glass” which the company would later market as Perspex. Perspex was lighter, stronger and clearer than regular glass, and as a thermoplastic was able to be pressure or vacuum formed into complex shapes. It was eagerly adopted by civilian aircraft manufacturers to save weight and make their planes more aerodynamic. Military aircraft designers would also take advantage of the properties of Perspex in the run-up to World War II.
On the German side, chemist Otto Rohm, co-founder of industrial giant Rohm & Haas, took a different approach to his process. He saw the value of a composite of regular glass and PMMA, with a layer of the polymer laminated between two sheets of glass. He thought that if he could run the polymerization reaction between two sheets of glass, the PMMA would glue the whole sandwich together into a solid sheet. Sadly, he couldn’t make it work – the glass always peeled away from the PMMA. But it left him with perfect sheets of acrylic glass, with all the same properties of Perspex. He dubbed his product Plexiglas, and it would find just as many civilian and military applications as Perspex.
Having proved itself in the crucible of war, PMMA was poised to take advantage of the post-war boom in consumer products. Acrylics found their way into everything from kitchen utensils to car dashboards, and as aqueous suspensions, PMMA revolutionized the coatings industry by essentially allowing a durable plastic coating to be painted onto a surface. PMMA was also found to be largely biocompatible and became a popular main bone cement for orthopedic prostheses, such as artificial hips and knees.
Cut It, Bend It, Glue It
For the home gamer, PMMA offers a lot of flexibility in designing and building projects. While PMMA filament for 3D-printers is not unknown, it doesn’t have nearly the followings that PLA and ABS have. PMMA mostly shows up as stock for laser cutters and engravers. Given its optical clarity, it may seem odd that a laser can cut PMMA, but the polymer actually absorbs the infrared wavelengths emitted by CO2 lasers quite strongly, enough to burn right through it. There are many online guides to laser cutting acrylic, but the general rule is to get rid of the vaporized PMMA as rapidly as possible, preferably through a downdraft table. Not only is the gas released toxic as hell – think formaldehyde, carbon monoxide, and the original monomer, MMA – it’s also flammable. Leaving it around to burn will only cause problems. Some laser cutters also use a gas assist, gently blowing the vapors away with a gentle stream of nitrogen or compressed air, to help get an optically clear, flame-polished cut.
PMMA’s susceptibility to organic solvents may be a weak point for some applications, but it makes for easy assembly of acrylic parts. Solvent welding with acetone, dichloromethane (DCM), or trichloromethane (chloroform) is a quick and easy way to bond acrylic pieces together. The solvent, which is often mixed with a small amount of MMA, flows into joints by capillary action and dissolves the two pieces together, forming one solid piece of plastic.
[Featured image: Trotec Laser, Inc.]