In a recent blog, we looked at selecting gaskets for strong acid service. Strong acids and bases (alkalies, or caustics) are commonly found corrosive substances. In this blog, we look at how to choose gaskets for strong caustic service.
Caustics are essentially bases dissolved in water. A base is a substance that can accept a hydrogen ion from another substance. Strong bases give off a hydroxide ion (OH-) when dissolved in water, forming alkaline solutions with a pH greater than 7.0. Strong alkalies, or caustics, include sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (CaOH2), and others. These strong caustics have very high pH values. Because strong caustics are aggressive compounds that are extremely corrosive, achieving effective seals in applications which use these substances can be challenging.
Strong caustics are often used in applications within the following industries:
Pulp and paper
Textiles
Soap and detergents
Bleach manufacturing
Petroleum products
Aluminum production
Chemical processing
Choosing the right gasket for strong caustic service is important since seals that cannot withstand the aggressive chemical attack will likely fail, causing leaks and possible personal injury.
Gasket materials that can be used to produce gaskets for strong caustic service include the following:
EPDM rubber. This elastomer offers good resistance to ozone, steam, strong acids and alkalies, but is not suitable for solvents and aromatic hydrocarbons.
Fluoroelastomer (FKM). This elastomer is a fluorinated hydrocarbon which offers excellent resistance to acids, alkalies, aliphatic hydrocarbons, oils and many aggressive chemicals. It is not suitable for amines, esters, ketones or steam. Viton® is a brand of fluoroelastomer developed by DuPont. Viton® GF-S material demonstrates excellent resistance to strong caustics. Following immersion in a 30 percent potassium hydroxide solution for 1 week at 100°C, this polymer exhibited high retention of properties and low volume swell, even versus a dedicated base resistant TFE / Propylene copolymer.
PTFE**.** PTFE has extremely wide chemical resistance - PTFE gaskets are attacked only by molten alkali metals and fluorine gas. PTFE gaskets also have high compressibility, which allows them to conform well to flange surface irregularities. While these gaskets are easy to handle, have low permeability and an extremely low coefficient of friction, the material can be prone to relaxation and creep. To avoid issues with relaxation and creep, CRG manufactures PTFE gaskets using the following materials:
Restructured manufactured PTFE sheet.Restructured manufactured sheet is now the preferred gasket material for strong caustic service where the operating temperature is less than 204.4°C (400°F). Restructured PTFE sheet will do an excellent job in these applications. CRG uses Tealon™ TF 1570, TF 1580, and TF 1590 restructured PTFE gasket material for moderate and strong caustic service. Tealon™ sheet, manufactured by Teadit North America, is a premium quality restructured sheet that is the exact material equivalent of Gylon®. The unique manufacturing process for Tealon™ results in a sheet that has less creep and cold flow than that normally associated with PTFE gasket material.
Multi-directionally expanded PTFE sheet.CRG supplies gaskets manufactured from Teadit 24SH sheet. 24SH gasket sheet is manufactured by expanding 100% virgin PTFE using a proprietary process that produces a uniform and highly fibrillated microstructure with equal tensile strength in all directions. The resulting product exhibits characteristics significantly different than conventional PTFE sheet. This style is much softer and more flexible than regular PTFE sheet and thus conforms easily to irregular and rough surfaces. In addition, the material is easier to compress and minimizes creep and cold flow. Gaskets made from 24SH sheet will seal all aggressive chemicals over the entire 0-14 pH range, except for molten alkali metals and elemental fluorine.
CRG can supply gaskets produced from EPDM, FKM (including Viton®), PTFE, or other materials especially designed to handle strong caustic service. If you have a strong caustic application, our sales team will be pleased to review your needs and discuss the material options available. Contact our sales team directly at crg@canadarubbergroup.com.