Testing of Thermoplastic Pavement Marking Materials

• Polymeric Resins- maleic modified alkyds derived from trees or hydrocarbon base resins derived from oil
• Retroreflective Elements– Spherical glass that comes in many sizes and clarity or in cluster form
• Pigments– Titanium Dioxide for white markings; lead-free organic pigments for yellow markings and other colors
• Fillers – the most common of which is Calcium Carbonate in the US and Magnesium Carbonate in Europe. Some specifications require friction aggregate such as Quartz
• Plasticizers– liquids such castor oil or solids such as paraffin or low molecular weight oligomers
• Additives– UV stabilizers and antioxidants

• Specific Gravity is a good property to measure since denser materials usually makes the perform better
• Softening Point is traditionally a test performed on hot-melt resins and asphalt; however it is a good predictor for ease of melting, and quality of resin and plasticizers in TPMMs.  
• Glass Bead Content / Bead Quality:  A minimum of 30% spherical glass beads is required in a TPMM to reflect automobile headlights during nighttime driving.  A minimum quality of glass beads for size, sphericalness, refractive index (crystallinity) and impurities can also be required and tested per AASHTO R98 and M247
• Binder Content: A minimum 18% binder content is important so that the material can be extruded at high temperature, adhere to the road surface and not crack after cooling. Some entities believe more binder is better, but the quality of the binder itself plays a large part in the effectiveness of whatever amount of resin is present.
• Titanium Dioxide (TiO₂) / Pigment Content: TiO₂ content levels of 10% (minimum, by weight) are considered important for the color fidelity of white TPMM’s.  Yellow markings (and other colors of pavement markings) should have a minimum amount of pigmentation but most are organic and more difficult to quantify than Ti0₂.
• Flowability of TPMM is simply a crude viscosity test that is used to verify that the product will flow afterbeing heated for 4 hours at 425°F.   If it will not flow, it will not be useable in the field.  High temperature Brookfield viscosity can be performed but is rarely required.  
• Color Stability of TPMM is one of the best predictors of quality of a TPMM formulation.  The material is kept at 425°F for 4 hours, then cooled  and checked for CIE color values (relative to baseline values)using a color meter.  
• Impact Strength can be perfomed with pendulum impact devices or drop-weight impact devices that will help predicate the level of abuse the TPMM can take on the road surface without breaking up.
• Abrasion / Wear Resistance is a recent addition to the testing requirements of TPMM.  Some entities require the TPMM to be tested like a paint using Taber Abraders, and some require testing similar to asphalt using a 3 wheeled device per AASHTO PP103 / PP104.   
• Accelerated Weathering and Color Degradation are required by some entities.  The methodologies and evaluations vary.  As with other tests, no consensus has been reached regarding which methodology predicts real-world results and product quality, since accelerated weathering cannot replicate nature, but rather is used for comparative testing among products.
• Retroreflectivity can be tested in the laboratory after glass beads are applied to a drawn-down of the TPMM. But making these types of samples in the lab won’t be representative of how a contractor applies miles of markings on the roadways.  So most agencies test for retroreflectivity of TPMM using a mobile retroreflectometer unit (MRU) after they are applied to verify their above-federal- minimums.  MRUs are attached to a car or truck and measure reflectivity while being driven 40 mph, taking 60 measurements per second.

Thermoplastic pavement marking materials have evolved over the years. For example, non-AASHTO M249-like thermoplastic products such as “Preformed” thermoplastic pavement marking materials (PreformedTPMM)have emerged. These products are tested using methodologies described in ASTM D8584 “Standard Guide for Testing of Preformed Thermoplastic Pavement Marking Material in the Laboratory and Installed Preformed Thermoplastic Pavement Markings in the Field” (https://store.astm.org/d8584-24.html), which uses methodologies similar to AASHTO T250.  Preformed TPMM’s have carved their niche in the pavement marking industry by being produced in a factory in various symbols and shapes that are difficult to create in the field; e.g., turning arrows, railroad crossing symbols, interstate symbols, stop bars, etc. They can be transported to the field and flamed-down to attach them to the road or runway surface. Therefore, the testing protocols arestill evolving.  

Testing of thermoplastic pavement marking material has changed over the years as the formulations have evolved and unique formulations have been developed. Sensors on driver-operated vehicles as well as  sensors on autonomous automobiles and trucks rely on pavement markings to operate the vehicle safely. A variety of new tests will need to be developed to verify that the sensors and pavement markings are working in unison. Until the methods are developed and validity tests are performed using interlaboratory studies, the existing test methods will simply have to suffice.