Acrylic sheet has been on the market for more than 75 years, ever since Rohm & Haas introduced it as Plexiglas® in 1936. Yet, as commonplace as acrylic is, its chemistry has evolved. Sheet manufacturers typically modify the Poly Methyl Methacrylate (PMMA) with additives to change the performance properties of the plastic. Today, a wide range of acrylic products are available to sign makers, most of which are modifications of the original formula.
Acrylic History. The development of acrylic spans nearly one hundred years, before the product was commercial. This plastic product owes its creation to the pioneering work of German scientists. Here is the timeline of the developmental process:
- 1843. Chemists first created acrylic acid.
- 1865. Scientists created methacrylic acid, which is the monomer building block for the acrylic polymer.
- 1877. German chemists, Fittig and Paul, were credited with the development of thepolymerization process, in which methanol is combined with methacrylic acid to form PMMA.
- 1928. The German company, Rohm and Haas, developed acrylic fiber.
- 1936. Rohm and Haas introduces the Plexiglas® brand to the market.
As the world prepared for war, new applications were discovered. As an alternative to glass, acrylic was used for the canopies of fighter aircraft and the bubble turrets of bombers. While pure acrylic can be brittle, it provides much better shatter resistance than glass. In fact, the impact strength of acrylic is at least five times greater than that of glass. It also possesses good optical clarity, nearly as good as glass, and is non-yellowing.
Today, popular brand names, such as Lucite® and Plexiglas®, are household words. Because it is nearly water clear, resists chemicals and provides exceptional outdoor durability, acrylic is ideal for many sign applications, such as internally illuminated signs and P.O.P displays. Compared to polycarbonate and flexible-signface material, it’s also an economical alternative for sign faces.
Manufacturing Acrylic. In the sign industry, leading manufacturers of acrylic sheet include the Arkema Group, CYRO Industries and Plaskolite. The family of acrylic sheet products is divided into three major categories: “cell cast”, extruded, and “continuous cast”. Each manufacturing process is unique. And the finished products produced in each process results in distinctive characteristics.
Cell Cast. Cell cast is manufactured between two sheets of tempered glass. Along the perimeter of the glass a gasket separates the two pieces of glass, acting as a spacer. The thickness of the gasket determines the thickness of the sheet. A syrupy acrylic resin is poured into the mold. To control the heat, which the acrylic syrup generates, the mold is immersed in water.
As the resin cools, it transforms from a liquid state into a solid sheet. After being removed from the mold, the sheet undergoes a post cure process, as the acrylic monomers continue to polymerize. During this post cure phase the cast acrylic sheet can shrink significantly, as much as 20%. The shrinkage, however, occurs in the thickness of the sheet.
The casting process produces acrylic sheet with the best optical clarity, compared to extruded and continuous cast sheet. The plastic itself is also harder and more scratch resistant. The cell cast method is a very traditional way to make acrylic, but it is also a very costly process. Variations in gauge are common. Nevertheless, for custom thickness, custom finishes and custom colors along with the highest quality materials, cell cast sheet has no equal.
Extrusion. In the extrusion process, the manufacturer receives resin in the form of tiny plastic pellets. In the extruder barrel, the pellets are heated, after which they form a molten mass. This molten mass of plastic is forced through a thin slot die. The hot extruded mass is then fed between metal rollers which squeeze the plastic down to its finished thickness. The distance between the rollers and the opening gap of the die determines sheet gauge.
An advantage of this process is that extrusion maintains much tighter tolerances of sheet thickness than in the casting process. More importantly, this is a high speed production process, making it a very cost-effective way to manufacture acrylic sheet. A disadvantage of extruded sheet is that, because it is pressed between the metal rollers, it induces some mechanical stress, which can result in shrinkage in machine direction.
Continuous Cast. Another form of mass producing acrylic sheet is the continuous cast method. In this process, the polymerized acrylic syrup is poured between two polished metal belts. Much of what is used in the sign industry is continuous cast acrylic. To control the gauge of the sheet, gaskets along the edge of the web separate the metal belts. To regulate the curing of the resin the belts pass through a series of heating and cooling chambers.
The continuous cast process produces sheet exhibiting very good optical clarity, consistent gauge, and minimal shrinkage during thermoforming.
Both extrusion and the continuous cast processes are very economical production methods compared to cell cast acrylic. In the continuous cast process the raw materials costs are lower for the resin syrup versus the pellets used in extrusion. The equipment cost of the extrusion equipment, however, is significantly lower than the investment required for continuous cast equipment. As a result, extrusion is the more common production equipment used.
Conclusion. Each acrylic sheet manufacturer makes a variety of products, designed to satisfy particular application requirements. Some additives, such as butyl acrylate, increase impact resistance. Other additives, such as methacrylic acid, increase glass transition temperature for high temperature applications.
The addition of plasticizers, on the other hand, can lower the glass transition temperature, which can make acrylic easier to bend and thermoform, as well as improving the sheet’s impact resistance.
Some modifications have made acrylic products, many of are stronger to resist breakage. Other acrylic products are more heat resistant and can withstand the high temperatures of illuminated sign cabinets. Still other acrylics form at lower temperatures, when thermoforming sign faces.
While these chemical modifications have greatly improved the performance characteristics of acrylic, different chemistries may not be compatible with the other materials that you are using in production. For this reason, as a sign maker or printer, you are ultimately responsible for understanding the raw materials used for a particular application and determining their suitability.
Regardless of how you chose to fabricate or decorate acrylic sheet, the important rules are basically the same:
- Read and heed the manufacturers’ technical bulletins. If you have questions, don’t hesitate to call your distributor for additional information or call the manufacturer directly for advice.
- Test, don’t guess. Test your raw materials for compatibility before going into a production run. When you find a combination that works, stick with it.
- To control your results in production, control your shop environment.
- Proper surface preparation prevents printing, painting and vinyl application problems.
- Document your results. That way, you hopefully won’t repeat any mistakes of the past, when you run a repeat order. What’s more you can reproduce those repeat orders with consistency and control. That way, you will keep your customers happy, so they keep coming back.