COATINGS & LININGS

Choosing the Correct Material for Tanks

Averie Palovcak, Arkema, Inc., King of When designing tanks for chemical cally, polyvinylidene fluoride (PVDF) and Prussia, Pennsylvania, USA and fluid handling services, engineers PVDF are discussed, showing have a variety of material options. new advancements and fabrication tech- Choosing the correct material from niques that have changed the way engi- metals, , coatings, and combi- neers and plant designers handle their cor- nations thereof can majorly affect the rosion challenges. lifetime of the tank. Plastic tanks can Plastic Materials— be specified over metals due to their A Variety of Choices corrosion resistance, lightweight Changes in a ’s molecular back- properties, and ease of fabrication. bone allow for a profoundly large number This article examines some plastics of possibilities in creating plastic resins. most commonly used for tank con- From varying elements to changing the structions, both as liners inside metal structure and branches, small differences or fiberglass tanks, as well as free- in polymer makeup can account for large standing ones. changes in material performance. Com- monly used families of polymer plastics include polyolefins, polyurethanes, polysty- Corrosion is a challenging problem for renes, polyvinyls, polyesters, and fluo- engineers and maintenance professionals ropolymers. These resins fall into the cate- worldwide, particularly with tanks. Short- gory of —meaning they can term tank corrosion issues include com- be re-melted and re-processed. Thermo- promising product purity and minor leaks. plastics are most often processed via melt More serious problems can cause complete processing techniques, such as extrusion, component failure, releasing potentially injection, or compression molding. The dangerous chemicals into the surrounding temperature profiles and processing condi- areas. tions vary depending on the material of or plastic materials are often choice and it is important for processors to Cemployed as liners or even standalone consult with resin suppliers on optimal parts to combat the issue of corrosion. conditions. Unlike metals, plastics are not susceptible When making a choice on what lining is to corrosion and rusting. However, not all best for the application, examining the plastics are alike and it is important to innate performance properties of the mate- understand the characteristics, advan- rial is crucial. Table 1 compiles some of the tages, and limitations of each material. mechanical properties of commonly used This article explores plastics commonly lining materials at 23 °C. used for tanks and linings to prevent corro- In many instances, plastics are used in sion. For harsh environments with aggres- standalone constructions for tanks, wet sive chemicals and elevated temperatures, benches, and cabinets, so the inherent can be utilized. Specifi- strength of the material is especially impor-

36 JANUARY 2020 WWW.MATERIALSPERFORMANCE.COM tant. Besides the mechanical properties, TABLE 1. PROPERTIES OF PLASTIC LINING MATERIALS1 chemical resistance, permeation, and tem- perature are other areas engineers should Flex Modulus Abrasion consider when designing with plastics. (psi)2 Tensile (psi)3 Hardness (Shore D)4 (mg/1,000 Cycles)5 When commodity materials face chal- PVC 320,000 6,500 80 12-20 lenges in harsh thermal or chemical envi- ronments, fluoropolymers can provide new PVDF 300,000 7,500 78 5-9 solutions. ECTFE 240,000 7,000 75 5-9 For areas where plastics are used as lin- PP 200,000 5,000 60 15-20 ings, there are two principal reinforcement ETFE 170,000 6,500 70 55-65 materials, metal and fiberglass. Common metal substrates include aluminum, car- PFA 85,000 4,000 55 25-35 bon steel (CS), and stainless steel (SS) FEP 80,000 3,000 55 70-80 (Types 304 SS [UNS S30400] and 316 SS PTFE 72,000 3,000 50 500-1,000 [UNS S31600]). One important characteris- tic of a plastic is that a liner in these sys- tems has low permeability. This minimizes the risk of chemicals moving through the liner and becoming trapped between the liner and the outer shell, causing issues with blistering and delamination. Figure 1 shows the permeation rates of oxygen gas through various plastic materials. Oxygen is representative of a small molecule, thereby showing tough testing conditions.

PVDF Copolymers— Flexibility for Linings PVDF copolymers are flexible versions of PVDF. Synthesized by reacting a co- monomer hexafluoropropylene (HFP) with FIGURE 1 Oxygen permeation of tank materials.6 the vinylidene fluoride (VF2) monomer, PVDF copolymers have varying degrees of 7 flexibility, depending on the concentration TABLE 2. MECHANICAL PROPERTIES OF PVDF COPOLYMERS of the HFP. Other common co-monomers Melt Flex Abrasion can include (TFE) and Point Modulus Tensile Hardness (mg/1,000 2 3 4 5 8 its derivatives, ethylene tetrafluoroethyl- (°C) (psi) (psi) (Shore D) Cycles) Impact ene (ETFE), and PVDF 740 170 300,000 7,500 78 5-9 2-4 (PTFE). The HFP is not an additive, plasti- PVDF Flex 157 170,000 5,500 75 6-9 4-10 cizer, or stabilizer to the material. Instead, 2850 it is directly reacted chemically with the PVDF Flex 142 100,000 4,000 65 16-19 10-20 VF2, ensuring that the resin retains its flex- 2800 ibility over time and does not leach out into PVDF Flex 120 35,000 2,200 55 28-33 No Break the chemical media. 2500 While added flexibility is a benefit, especially for tank fabrication, there are other advantages to PVDF copolymers. chemicals from <1 up to 12 pH, the PVDF tion that give the PVDF copolymers First, PVDF copolymers have a higher fluo- copolymers extend the upper range to 13.5 increased resistance to harsher chemicals. rine content. This higher degree of pH. Common chemicals handled by PVDF Higher flexibility also allows tank fabrica- in PVDF copolymers gives a broader range and PVDF copolymers include strong acids, tors to thermoform and manipulate the of chemical resistance. While PVDF homo- chlorides, and deionized or high-purity sheets easier. Table 2 shows the mechanical polymers are rated for continuous use in water. It is the added ductility and elonga- properties of PVDF copolymers.

MATERIALS PERFORMANCE: VOL. 59, NO. 1 JANUARY 2020 37 COATINGS & LININGS

other exotic metals are thought to be the only choice, using a less expensive metal with a lining may provide users with a substantial cost savings and equivalent performance over the lifetime of the tank. Depending on the chemical and tem- perature environment, the thickness of the sheet can vary. While thinner sheets are easier to form around the vessel, the thicker sheets offer better permeation resistance. Consultation with the tank fabricators is recommended. FIGURE 2 Tank-lining steps. Image courtesy of Electro Chemical Engineering & Manufacturing. Application, adhesion, and welding could be slightly modified per the material. Whether the tank is new or used, the lining process can be similar. Figure 2 shows a plan view of the application steps, starting with the primer on the left, and finishing on the right with a cap strip to protect the welded sheets. The following sections describe those steps in further detail, as well as a visual overview in Figure 3.

Cleaning and Blasting Whether the tank is new or has been in

FIGURE 3 Before (left) and after (right) lining a steel tank with PVDF sheets. Images service, the inside of the tank should be courtesy of Electro Chemical Engineering & Manufacturing. thoroughly cleaned before work can start. Typically, an abrasive blast is used to clean the interior of the tank and create an Three main grades of resins are used for 170 °C, it can be processed on standard anchor point. A typical anchor profile is 3 tanks made with PVDF. The first is PVDF equipment used to run polyolefins. Copoly- to 5 mils to allow for good adhesion of the 740 homopolymer used for vessels that are mer grades of PVDF may have an even plastic sheet to the metal. Different types of freestanding. The rigid homopolymer grade lower melting temperature. media are available, ranging from fine, is available in thick sheets of PVDF (>1 in Because fluoropolymers have inher- course, and extra course, and are usually [25.4 mm]) that can be thermoformed and ently non-stick properties, it would be very made of crushed glass or other mineral welded to create tanks. Common grades for difficult to adhere a sheet directly to a abrasive. Harder metals may require harder liners are PVDF copolymers: PVDF Flex metal or fiberglass tank. Because of this, blasting materials. Additionally, fabricators 2850 and PVDF Flex 2800. Copolymers are sheets are available with fabric or glass use standards, such as NACE No. 2/SSPC- 10 recommended in situations where high backing. The fabric backing is co-extruded SP10 to ensure a white metal blast. torque or high stress occurs, such as in onto the sheets as they are being processed, over-the-road trailers that carry aggressive allowing for a strong bond. A common peel Coating and Priming of Tank chemicals, like sulfuric, hydrochloric, test used by some suppliers of the sheets is Once the tank is blasted, a primer can hydrobromic, or other acids. ASTM D17819 in order to ensure a robust be applied to hold the blast and prevent and strong bond without a chance of oxidation prior to the lining process. This is PVDF Sheets delamination. a necessary procedure since there could be In order to fabricate a tank using plastic some time lag before fabrication can be materials, it is first necessary to process Tank-Lining Process completed. plastic into a sheet. Sheets of PVDF or any The process of lining a tank can be simi- other resin are most com- lar for each plastic material. Most often, the Thermoforming Sheets monly manufactured by either compres- liners are applied to CS or SS. Plastic liners, While the tank is being prepped, the sion molding or extrusion. Standard thick- such as the ones described in Table 1, offer sheets and parts for nozzles and heads can nesses of liners are 60, 90, or 120 mils. Since advantages to engineers and designers. PVDF has a relatively low melting point at When Hastelloy†, Inconel†, titanium, or †Trade name.

38 JANUARY 2020 WWW.MATERIALSPERFORMANCE.COM Choosing the Correct Plastic Material for Tanks

be formed. This takes immense detail to understand all the dimensions of the tank while trying to minimize the amount of welding that needs to be done later. Corner angles, nozzle head size, and flanging all need to be considered before thermoform- ing parts. Once the dimensions are very well documented, the plastic parts can be heated appropriately per the material spec- ifications in order to warm up the plastic and physically form the part. Since plastics have different melt temperatures and mechanical properties, the thermoforming temperatures and times must be adjusted to the specific material being employed. FIGURE 4 Over-the-road trailers carrying aggressive chemicals can be lined with PVDF copoly- For PVDF, thermoforming should take mers. (Images courtesy of Electro Chemical Engineering & Manufacturing.) place between two molding plates. PVDF homopolymers thermoforming tempera- ture is recommended at 150 to 160 °C. For ties. This technique takes two sheets of the desired tank geometry. Next, the fiberglass PVDF copolymers, thermoforming is rec- plastic sheet and heats them up under is applied to the exterior, or fabric side of ommended to take place ~15 °C below the pressure to allow the sheet to flow into the sheet using a wet layup process. Dual melting point of the material. each other seamlessly. The material cools laminate tanks can often be a more light- and then has a material between the weight, cost-effective solution for users Applying Plastic to Metal sheets connecting the two. seeking a structurally strong tank. As previously described, a fabric- backed sheet is used to bond the thermo- Testing Conclusions plastic liner to the metal. In addition, an Once the tank is fully lined and all weld- PVDF and other plastics are commonly adhesive is added to provide adhesion to ing is complete, a spark test followed by a used to fabricate tanks. Available in stand- the metal. Some manufacturers have used helium leak test is performed to identify alone and lined configurations, tanks that solvent-based elastomeric adhesive in the pinholes. Pinholes are minute imperfec- have thermoplastics as the contact layer past, but some have developed proprietary, tions where fluoropolymer material is not with the containment media can greatly higher temperature flexible epoxy adhe- present. If a pinhole is identified, it should extend the service life of the system. It is sives. Before the sheets are applied to the be fixed and filled with plastic to cover any important that the proper plastic liner is metal, they should be annealed to alleviate area that is missing material. used, as materials can vary greatly in tem- stresses. A typical annealing time and tem- Overall, the process of lining a tank can perature performance, chemical and per- perature for PVDF can be 135 °C for 2 h. take several weeks depending on the size of meation resistance, and ease of fabrication. Other materials may have different anneal- the tank. Additionally, solutions exist for Engineers and designers who specify plas- ing temperatures. lining over-the-road trailers with PVDF tic liners for corrosion control have come copolymer linings, like in Figure 4. Should a to appreciate their utility in both new tank Welding leak or other defect occur in the tank over designs as well as in retrofitted tanks. Welding can be freehand with a welding time, methods to repair can be relatively rod and heat gun. The operators should be straightforward. These procedures are well Acknowledgments cognizant of hazards with welding and take documented and should be discussed with The author would like to thank Mike precautions to wear proper personal pro- an experienced tank fabricator. Amant, James Doherty, and the rest of the tective equipment. There are standards Electro Chemical Engineering & Manufac- such as AWE G1 committee and/or DVS Dual Laminate Tanks turing team. Their expertise in fluoropoly- 2207-1511 for welding of PVDF or tank fabri- Tanks that have fiberglass as the rein- mer tank linings as well as the images they cators who have developed their own weld- forcement material are also commonly provided were instrumental in the comple- ing procedures for linings. When using butt made with plastic liners. Unlike metal tion of this article. fusion or welding rod, there can be a cap tanks, these dual-laminate tanks start with The author would also like to thank applied to distribute stress of the weld over building the tank from the inside-out. Jason Pomante for his work in contributing a wider joint. However, some organizations Keeping the same fabric-backed sheets, to the article. have developed a flow fusion technique to thermoforming, and annealing recommen- give smooth weld lines and no discontinui- dations, plastic liners are formed into the Continued on page 40

MATERIALS PERFORMANCE: VOL. 59, NO. 1 JANUARY 2020 39 COATINGS & LININGS

Continued from page 39

References 6 G. Carr, “Materials–The Effect of Plastic Tub- 11 DIN-DVS 2207-15, “Welding of Thermoplas- ing Type on Oxygen and Resistivity Measure- tics–Heated Tool Welding of Pipes, Piping 1 UL Prospector Materials Database, https:// ments in High-Purity Water,” Ultrapure Water Part and Panels Made of PVDF” (Berlin, Ger- www.ulprospector.com. J. 17, 10 (2000): pp. 17-21. many: DIN, 2005). 2 ASTM D790-17, “Standard Test Methods for 7 Arkema Inc, Performance Data and Charac- Flexural Properties of Unreinforced and teristics brochure, pp. 4-5. AVERIE PALOVCAK is the Market and Reinforced Plastics and Electrical Insulating Applications Engineering manager for Materials” (West Conshohocken, PA: ASTM 8 ASTM D256-10, “Standard Test Methods for Kynar® PVDF resins in the Technical Poly- International, 2017). Determining the Izod Pendulum Impact Re- mers division of Arkema Inc., King of Prus- sistance of Plastics” (West Conshohocken, sia, Pennsylvania, USA, email: Averie. 3 ASTM D638-14, “Standard Test Method for PA: ASTM, 2018). Tensile Properties of Plastics” (West Con- [email protected]. She is a graduate shohocken, PA: ASTM, 2014). 9 ASTM D1781-98, “Standard Test Method for of Drexel University with B.S. and M.S. Climbing Drum Peel for Adhesives” (West degrees in biomedical engineering. She is 4 ASTM D2240-15e1, “Standard Test Method Conshohocken, PA: ASTM, 2012). an active member of the NACE Specific for Rubber Property—Durometer Hardness” Technology Group (STG) 10 Committee. (West Conshohocken, PA: ASTM, 2015). 10 NACE No. 2/SSPC-SP 10, “Near-White Metal Blast Cleaning” (Houston, TX: NACE Inter- 5 R. Hanselka, R. Williams, “Materials of Con- national, 2006). struction for Water Systems Part 1: Physical and Chemical Properties of Plastics,” Ultra- pure Water J. 4, 5 (1987).

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