When it comes to signage applications, it all comes down to impact strength. Polycarbonate and acrylic plastics are two commonly used substrates for signage applications, and both are strong materials. They look basically identical to the untrained eye, and because of this, people often mistakenly people use the names interchangeably. But for those “in the know,” polycarbonates and acrylics are anything but the same. Knowing these differences will help to ensure the right material is chosen for the specific signage job at hand.
Although the two materials do share some similar properties, the key differences should be explored because signage applications—such as channel letters, cabinet signs, thermo- and vacuum-formed signs—have unique requirements. Variables that come into play for sign applications are strength, ease of fabrication, aesthetic qualities and durability.
When comparing the toughness of polycarbonate and impact-modified acrylic (IMA), several ASTM (American Society for Testing and Materials) test methods are commonly used. In ASTM D-638, also known as the tensile strength test method, the material is held between two clamping jaws, and a device records the force required to stretch the sample to the breaking point. The higher the value of the force needed, the greater the strength of the sheet material. A side-by-side comparison of the ASTM D-638 data for polycarbonate and IMA shows that polycarbonate has a tensile strength of around 9,500 psi, while IMA’s tensile strength is approximately one-third less.
Another industry-standard strength test for plastic is the drop dart test (ASTM D-3763), which measures a material’s impact strength resistance. This test is performed under specific conditions where a 12.7-mm instrumented dart impactor is dropped from a distance of several feet and exerts 200 foot-pounds of force onto a 0.125-inch material sample.
This drop dart test shows that IMA has an impact resistance roughly 50 times that of glass, and polycarbonate’s impact resistance is even more impressive at more than 250 times the strength of glass (see Figure 1). Even though IMA is often substituted for polycarbonate, tests show that polycarbonate material far exceeds the toughness of IMA.
While a material’s strength is definitely important, its ease of fabrication is also a critical component of sign making. Polycarbonate can be bent to a tight radius, 100 times the thickness (R=Tx100), without cracking or crazing. Acrylic, a more rigid thermoplastic, is limited to bending 180-200 times its thickness. Tighter bend radius adds stress to the acrylic sheet and may result in crazing or cracking of the material. Polycarbonate cuts and drills easily with little chance of it chipping or cracking unlike acrylic, due to its brittle nature.
Taking this one step further, we can examine the fabrication processes of the acrylic sheet when compared with polycarbonate. If there is a notch in the edge of a sheet as a result of sawing, routing or drilling, this notched area can be more sensitive to crack propagation. Cracking would make the finished sign no longer viable. In comparing ASTM D256—Notched Izod Comparison Impact Results—one sees that polycarbonate performance is 12 times that of IMA (see Figure 2). Polycarbonate, on the other hand, is easy to handle and virtually unbreakable.
In addition to the material’s strength and durability properties, its aesthetic qualities are important to a sign’s success. Where a clear sheet product’s clarity is critical in glazing applications, it is less important in signage applications since most utilize vinyl graphics, screen prints or paint to achieve the desired look.
IMA and polycarbonate sheet and reel stock are available in standard industry colors—white, blue, red, yellow, orange and green. Some colors offer more than just a desirable hue. They perform an added function, as is the case with light diffusion. IMA and polycarbonate are both available as a white sheet that scatters light, thereby preventing visible hot spots from LEDs in typical flat and formed applications while maintaining high light diffusion. As with other grades, the light-diffusing sheets are easy to fabricate and form, and offer ultraviolet (UV) resistance to enhance long-term, outdoor weathering performance.
UV-Resistance and Weathering
While UV resistance may enhance weathering performance, it does not make the materials immune to the effects of weathering altogether. As with all plastic materials, IMA and polycarbonate will degrade over time when exposed to UV rays and harsh environments. With the advent of newer manufacturing technologies, polycarbonate sheet material with excellent weathering performance properties is becoming available. This UV-enhanced product offers a 10-year protection warranty against UV degradation.
Harsh environments place weathering stress on plastics, and applications in severe weather environments require especially durable materials. Polycarbonate has a severe rating Class Zone 4—based on the ASTM E-1886 and ASTM E-330 standards—making it a good choice for use in areas prone to hurricanes and severe storms. Wind Zone 4 has basic wind speed greater than 160 mph. Numerous grades are listed in Miami-Dade building codes as components of High Velocity Hurricane Zone, HVHZ. Polycarbonate has been tested as a component in a robust framing system and has passed the large missile impact standard per ASTM E-1886. IMA does not carry this rating.
Both acrylic and polycarbonate are generally easy to clean, but a plastic sheet’s chemical resistance varies based on the specific chemicals present. The location and use of a sign often dictates what chemicals it will be exposed to and which material provides better resistance. IMA has a better chemical resistance to certain substances, and polycarbonate is more resistant to others.
The sheet’s chemical resistance comes into play even with routine cleaning. Care must be used when choosing the right cleaner for the job, as solvent cleaners can attack acrylic or polycarbonate substrates. A soft cloth with mild cleaners should be used for both materials with basic, mild soap and water being the best cleansing agents.
Thermoforming sheet material is common in the sign industry. Both IMA and polycarbonate are easy to vacuum form using standard forming equipment. Prior to forming, polycarbonate must be dried, while drying is not necessary for IMA. However, polycarbonate has a shorter forming cycle due to faster cooling and part setup – allowing the formed part to be removed from the tool more quickly. Depending on the formed part design, automated vacuum forming equipment can achieve up to three times the throughput for polycarbonate over IMA.
While the features previously discussed should be the main factors influencing material selection, cost, although not the most important variable, is also in the mix. Besides the upfront cost of the material itself, additional costs may be incurred through shipping and also potential damage of a finished sign while it’s in transit. During shipping, a polycarbonate sign requires less packaging due to the sheet’s durability, thus reducing shipping claims incurred from breakage. These savings can go right to the bottom line.
Visually stunning signs can be created with either IMA or polycarbonate, but the material’s inherent strength, ease of fabrication, aesthetic characteristics and durability influence the success of the end product. This makes material selection a crucial step in sign creation and underscores the need to know the differences between polycarbonate and IMA and how their selection will affect the finished sign.