Optimizing practical properties of MMA-based cold plastic road marking paints using mixture experimental design

Highlights

• MMA-based traffic paints were optimized using experimental mixture design.

• Amounts of resin, plasticizer, and silica aggregates were constant in DOE procedure.

• Skid resistance and dirt pick-up as the key factors in crosswalk markings were balanced.

• Among all ingredients, plasticizer showed an effective role on paint performance.

• Experimental measurements coincided with the proposed model by the software.

Abstract

In this study, some practical properties of methyl methacrylate (MMA)-based cold plastic traffic paints were optimized. For this purpose, in the first step, the effect of initiator, benzoyl peroxide (BPO) content on the polymerization of MMA-based traffic paints was evaluated. No-pick-up time, pendulum hardness, tensile strength and color difference (ΔE_w) before and after exposure to the accelerated weathering conditions (AWC) of the samples containing various BPO wt% (0.5, 1.0, 1.5 and 2.0) were determined. In the second step, the MMA-based paint formulation was optimized using mixture experimental design (MED). The amounts of MMA resin, chlorinated paraffin wax (plasticizer) and silica fillers with different average particle sizes were considered as the variables in the paint formulations. Viscosity, reflectance, color difference (ΔE_w) before and after exposure to AWC, skid resistance, and dirt pick-up property were selected as the practical properties and responses in the MED. The region of optimum paint formulations was determined using the quadratic model and selecting the desired end-use properties. The accuracy of the model was evaluated, and the results confirmed an appropriate agreement between the proposed model and experimental measurements.

The results showed that the BPO initiator in low level of usage did not sufficiently affect the curing rate of the paint samples. In addition, high level of BPO led to decrease in mechanical properties of the paint, due to the presence of plasticizer and non-reacted BPO compounds within the polymeric network. The results also confirmed the MED as an operative method to optimize MMA-based pavement marking paints for purpose of balancing between skid resistance and dirt pick-up properties.

Introduction

Road markings are a vital part of safe road network transportation and are considered the principal tools for delineating and directing traffic, warning, regulating and informing drivers [1]. In order to properly delineate the road and maintain road safety, road marking materials must have appropriate quality throughout their service-life [[2], [3], [4]]. Among the different types of traffic paints, two-component methyl methacrylate (MMA)-based systems are usually used for pedestrian crosswalks, traverse-lanes, and others because of these systems are durable. Compared with hot-melt thermoplastic road markings, especially those that are used in warm/hot climate conditions, MMA-based paint materials have promising properties such as fast drying, high durability, appreciate abrasion resistance, sufficient weather resistance, convenient skid resistance, and low dirt pick-up [5]. However, in areas with hot climates and low annual rainfall, the surface of pavement marking quickly adsorbs contaminants and becomes darken. This phenomenon is exacerbated by an improper paint formulation [6].

Usually, for decreasing dirt pick-up, pavement markings are applied with a smooth and low roughness surface. However, during rainfall and when the pavement surface is wet, the marking’s surface becomes slippery and dangerous for pedestrians and vehicles. Therefore, it is desired to optimize pavement marking’s skid resistance and dirt pick-up by adjusting the paint formulation as well as controlling marking surface roughness. However, adjusting multiples properties at once requires prototyping and preparing different coating formulations.

Using the design of experiments (DOE) and systematic approaches are desirable for studying the effects of diverse variables and their relationship to final end-using properties [7]. In recent years, researchers used the DOE method to increase durability and improving the physical properties of pavement marking materials [[8], [9], [10]]. Optimizing throughout conventional experimental design methods (e.g., full and fractional factorial designs) means changing one variable at a time to obtain independent responses. Such methods require performing several experiments and tests to determine the low and high limitations in which all variables meet the acceptable criteria [[11], [12], [13]]. In addition, these methods cannot provide a real optimum, and in some cases can even have different implications with various sets of parameters. Clearly, different types of interactions between the ingredients in paint formulation can affect the properties of the applied paint and are not considered in the traditional methods [[14], [15], [16]].

Mixture experimental design (MED) is a well-known method that is based on statistically analyzing results to minimize the difficulty of obtaining information about complex chemical compositions [[17], [18], [19], [20], [21]]. MED is also identified as an effective method for optimizing the coatings protocols and their ultimate properties [19,[22], [23], [24]]. This approach offers the best answers, especially when it comes to optimizing multiple responses or qualities simultaneously. Considering the interactions between different formulation qualities results in a smaller area in which all responses are optimized; therefore, properties of the composition can be effectively predicted for certain parameters [12,25,26]. MED was used to optimize water-based road marking paint formulations and showed the excellent capabilities of MED compared to the fractional factorial method [11]. Kardar et al. [12] reported MED with quadratic and special cubic models for minimizing the number of experiments for studying the effect of monomers on the structural properties of an epoxy resin. MED was also successfully used for optimizing hot-melt thermoplastic road marking formulations [27].

The main objective of this study was to evaluate the performance of two-component durable traffic paints in simulated practical conditions. Skid resistance and dirt pick-up property as two practical key-properties of MMA-based paints were optimized. To this end, the effect of different levels of an initiator for radical reactions (so-called hardener) on the polymerization quality of a two-component MMA-based resin was studied. Afterward, the formulation of the pigmented part of the paint at a defined wt% of initiator was optimized using mixture experimental design (MED). Based on the software’s suggestion, different paint formulations were prepared and evaluated according to the relevant standard test methods. The DX 7.0.0 program with a quadratic model was exploited to optimize (i.e., set the proportions of each ingredient) the paint formulation to achieve superior end-use properties in MMA-based traffic paints. Fig.1 shows the protocol of used experimental procedures in this study.