Methyl methacrylate-based acrylics are best known for their rapid cure and extreme versatility

Methyl methacrylate-based acrylics cure fast and are long lasting.

By Wilfried Riesterer

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Acrylic reactive resins stand apart from other floor resurfacing and repair materials. Methyl methacrylate (MMA), the monomer used in acrylic reactive resins, provides them with their fast cure and physical performance characteristics. MMA systems consist of a primer (to seal the substrate), body coat (thin coatings as high as 20 mils or trowel-applied, self-leveling toppings of 1/8-inch to 1/4-inch thick) and topcoat (to seal the finished floor).

In industrial environments, acrylic reactive resins are used for new construction and floor renovations. They are best known for their rapid cure and versatility. MMA-based acrylics cure within one to two hours in temperatures ranging from -20 degrees Fahrenheit to more than 100 degrees Fahrenheit.

Figure 1. Warehouse floor application.

Substrate preparation

As with any floor resurfacing, substrate condition and preparation ensures a successful installation and a long-lasting bond.

Acrylic reactive resins are used on concrete, interior asphalt, wood, metal and some synthetic substrates. The substrate must be dry, sound, clean and contaminant-free. Allow new concrete to cure for at 28 days. Spalls, hollow surrounding areas, cracks, etc., must be repaired properly.

Shot blasting is the recommended substrate preparation method. It uses an enclosed dust-free process that can be adjusted to produce a range of surface profiles. Shot blasting removes laitance and weak concrete. It's often used in conjunction with power scrubbing to leach oils, fats and other contaminants from the substrate.

Once the substrate is cleaned, prepared and dried properly, bond tests determine the adequacy of substrate preparation. Bond tests mix quartz sand and a small batch of the selected primer system and apply it in palm-size patties 1/8-inch to 1/4-inch thick at various locations throughout the job site. After one hour, the patties must cure tack-free. They are then removed with a chisel and hammer.

The entire bond test area should show only concrete with fractured aggregate attached to its underside. If only laitance, fines or a small amount of concrete is attached, or if interface between patty and substrate is tacky, further substrate preparation is required. Proper tests help ensure that the bond will be deep and long-lasting bond (see Figure 2).

Figure 2. This is an example of a complete, successful bond test. The entire bond test area shows concrete with fractured aggregate attached to the entire underside.

Physical performance characteristics

Chemical components and cure.

MMA-based resins cure via a radical polymerization reaction (see Figure 3). They have a low viscosity that is unaffected by temperature. MMA crystallizes at - 55 degrees Fahrenheit; full cure may be achieved within a couple of hours even at -20 degrees Fahrenheit. Adding "radical donors" in the form of 50% granular benzoyl peroxide powder initiates the curing process. The acrylic molecules, which are responsible for the performance properties of reactive resins, are already present and need not be produced by a chemical reaction. The sole function of the initiator is to override the inhibitor in the resin (which is added for storage and shipping) and to initiate the polymerization reaction. Thus, the initiator has no impact on the physical properties of the cured material. The chemical chain formation of the polymerized resin shows clearly that the main molecular chain is surrounded and protected by branched groups. This is why MMA- based acrylics have such excellent weathering and chemical resistance.

Figure 3. The radical polymerization reaction of MMA-based acrylic reactive resins.

The initiator almost dissolves instantaneously, giving the cured material its consistent and predictable physical properties. Once begun, the curing process is unstoppable, and is 99% complete in one to two hours. At that time, no unreacted monomer remains. This results in a system having stable physical properties. The fast-curing process allows quick repair and surfacing at a fraction of the time required for other flooring materials.

Strength

For toppings less than 1/4-inch thick, it's important that the material be capable of withstanding impact loads and temperature fluctuations without disbonding. The two variables having the greatest effect on substrate compatibility are not the compressive or tensile strength, but the modulus of elasticity and the coefficient of thermal expansion. For applications, more than 1/4 inch thick, these properties increase in importance.

Acrylic reactive resins used for industrial flooring have a relatively lower modulus of elasticity, which means they are capable of passing on impact and other forces into the substrate without disbonding or sustaining damage.

Reparability

MMA-based acrylics offer a welded chemical bond between coats. This eliminates the need for expensive, time-consuming preparation of existing acrylic floors. No mechanical preparation or removal of existing flooring is necessary. Reactive acrylic resins bond to themselves, regardless of how long the installation has been in place. Fresh resin slightly dissolves the old surface and then cures with the new material (in one hour), forming a perfect, welded, monolithic bond and coating.

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