Prevent Corrosion with Kynar® PVDF

Figure 1. Injection molded components and reinforced vessel for mineral processing Graphic courtesy of Arkema

Why PVDF?

PVDF is a high-performance polymer widely used across industries because of its unique properties. Chemical and abrasion resistance, sustained mechanical integrity at elevated temperatures, UV stability and good flame and smoke properties make PVDF a versatile material whose performance is appreciated in many industries (1). In its natural, unfilled form, PVDF is a high-purity fluoropolymer, utilized in applications where the end product’s purity cannot be compromised by even the smallest of contaminants or leachables. Thus, PVDF is one of the world’s most widely used fluoropolymers in industries for chemical, semiconductor, food and beverage, and mineral processing.

With different grades being offered commercially, PVDF is available in many processing components. PVDF resin grades have flexural moduli ranging from 10,000 to 1 million psi. The variations in flexibility of the polymer are attributed to tailoring the chemical structure of the chemical backbone to produce materials termed as PVDF homopolymers or PVDF copolymers. No fillers or plasticizers are added to PVDF to obtain the flexible performance (2). Piping systems, flexible tubing, fittings, pumps, valves, tower packing, wire and cable components and membranes are just a few examples in which PVDF and PVDF copolymers are utilized. Choosing the proper PVDF grade for the desired component is important in maximizing the service life of the system.

Standard PVDF homopolymers and some PVDF copolymers have a maximum use temperature of 150°C. Combined with their robust chemical resistance ranging from pH of <1 up to 13.5 for certain grades, PVDF and PVDF copolymers are a top-choice material of construction for the mining and mineral processing industry. Additionally, new technologies are available that enable PVDF coatings to replace rubbers, fiberglass, urethanes and epoxies as corrosion barriers to protect metal substrates. Case histories are included demonstrating the successes of new PVDF powder coatings application technology in providing solutions for challenging mineral processing.

Chemical resistance

The fluorinated backbone of the PVDF polymer is the driving force behind its good chemical resistance. By weight percentage, standard PVDF homopolymers are 59.4 percent fluorinated and flexible Kynar PVDF copolymers are up to 64 percent fluorinated.

In the mineral processing industry, corrosion in acidic and chlorinated environments is a key challenge for engineers and maintenance professionals to overcome. While handling high concentration acids seems to be fairly common for metals, those materials can be quickly corroded in low acidic environments and acid mixtures that contain sulfur and chlorides. PVDF polymers can handle acids at both high and low concentrations and elevated temperatures, providing a cost-competitive solution for replacing metals.

Typical acids handled by PVDF include sulfuric (up to 97 percent for standard PVDF and 98 percent for Kynar Flex copolymers), methane sulfonic, acetic, nitric, phosphoric, chromic, hydrochloric, hydrofluoric and hydrobromic acids. Additional commonly handled chemicals include metallic chlorides, methane bromine, iodine, hydrogen sulfide, salt water and deionized water. The manufacturers of PVDF and PVDF copolymers as well as PVDF piping manufacturers offer chemical-resistance guides as an important resource to consult for understanding the temperature limitations in each chemical service. Table 1 lists the mechanical properties of various PVDF grades in outdoor chemical-resistance testing.

Abrasion resistance

The abrasion resistance of materials in the mineral and mining industry is also a key property to consider when designing systems for maximum longevity. Mineral processing carries an abundance of solids and liquids mixing together to create slurries. The flow of the slurry can cause an extremely abrasive environment. When the viscosity of the liquid is low, particulates can accumulate at the bottom of the system, thus creating an increased incidence of wear.4 In high viscosity situations, the flow of the viscous materials also can create highly abrasive environments throughout the entire system. PVDF is one of the most abrasion-resistant materials. Table 2 highlights its performance versus other commonly used materials in a Taber abrasion test. In this test, the lower the weight loss, the less material that has abraded off, signifying the best performance. Both PVDF homopolymer and PVDF copolymer versions used in spray-applied powder coatings demonstrate the same relative abrasion values in this test.

PVDF coating technology: Innovation leads to new solutions

Stemming from its use in piping, pumps and other processing equipment, the legacy of PVDF success has now been translated into powder coating technology. PVDF powder coatings offer a variety of benefits for protecting metal substrates from corrosion. The aforementioned properties of abrasion and chemical resistance are crucial reasons for PVDF coating adoption into the mining and mineral processing industry.

Traditional coating/lining equipment includes rubber, fiberglass composites, urethane and epoxy coatings (6). While each of these systems has a niche and proven market, they are not fluorinated polymers and thus lack the combination of enhanced chemical, corrosion, and abrasion resistance. PVDF can offer performance improvement in harsher applications that require a broader range of chemical and thermal resistance. Furthermore, PVDF has no talc, fillers or stabilizers that would leach out over time and could subsequently cause embrittlement of the coating or compromise product purity.