FKM

Key takeaways

FKM belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect FKM to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of FKM from memory before moving on to harder problems.

Reference excerpt

FKM is a family of fluorocarbon-based fluoroelastomer materials defined by ASTM International standard D1418 and ISO standard 1629. It is commonly called fluorine rubber or fluoro-rubber. FKM is an abbreviation of Fluorkautschukmaterial (i.e. fluorine rubber material). All FKMs contain vinylidene fluoride as the common monomer, to which different other monomers are added for specific types and functionalities, fitting the desired application. Originally developed by DuPont (under the brand name Viton, now owned by Chemours), FKMs are today also produced by many other companies, including Daikin (Dai-El), 3M (Dyneon), Solvay S.A. (Tecnoflon), HaloPolymer (Elaftor), Gujarat Fluorochemicals (Fluonox), and several Chinese manufacturers. Fluoroelastomers are more expensive than neoprene or nitrile rubber elastomers, and in comparison they provide additional resistance to heat and chemicals. There are three ways that the FKMs can be separated into classes: by their chemical composition, their fluorine content, or their cross-linking mechanism.

Types On the basis of their chemical composition FKMs can be divided into the following types:

Type-1 FKMs are composed of vinylidene fluoride (VDF) and hexafluoropropylene (HFP). Copolymers are the standard type of FKMs showing a good overall performance. Their fluorine content is approximately 66 weight percent. Type-2 FKMs are composed of VDF, HFP, and tetrafluoroethylene (TFE). Terpolymers have a higher fluorine content compared to copolymers (typically between 68 and 69 weight percent fluorine), which results in better chemical and heat resistance. Compression set and low temperature flexibility may be affected negatively. Type-3 FKMs are composed of VDF, TFE, and perfluoro(methyl vinyl ether) (PMVE). The addition of PMVE provides better low temperature flexibility compared to copolymers and terpolymers. Typically, the fluorine content of type-3 FKMs ranges from 62 to 68 weight percent. Type-4 FKMs are composed of propylene, TFE, and VDF. While base resistance is increased in type-4 FKMs, their swelling properties, especially in hydrocarbons, are worsened. Typically, they have a fluorine content of about 67 weight percent. Type-5 FKMs are composed of VDF, HFP, TFE, PMVE, and ethylene. Known for base resistance and high-temperature resistance to hydrogen sulfide.

Cross-linking mechanisms There are three established cross-linking mechanisms used in the curing process of FKMs.

Diamine cross-linking using a blocked diamine. In the presence of basic (alkaline) media, VDF is vulnerable to dehydrofluorination, which enables the addition of the diamine to the polymer chain. Typically, magnesium oxide is used to neutralize the resulting hydrofluoric acid and rearrange into magnesium fluoride and water. Although rarely used today, diamine curing provides superior rubber-to-metal bonding properties as compared with other cross-linking mechanisms. The diamine's capability to be hydrated makes the diamine cross-link vulnerable in aqueous media. Ionic cross-linking (dihydroxy cross-linking) was the next step in curing FKMs. This is today the most common cross-linking chemistry used for FKMs. It provides superior heat resistance, improved hydrolytic stability and better compression set than diamine curing. In contrast to diamine curing, the ionic mechanism is not an addition mechanism but an aromatic nucleophilic substitution. Dihydroxy aromatic compounds are used as the cross-linking agent, and quaternary phosphonium salts are typically used to accelerate the curing process. Peroxide cross-linking was originally developed for type 3 FKMs containing PMVE as diamine and bisphenolic cross-linking systems can lead to cleavage in a polymer backbone chain containing PMVE. While diamine and bisphenolic cross-linking are ionic reactions, peroxide cross-linking is a free-radical mechanism. Though peroxide cross-links are not as thermally stable as bisphenolic cross-links, they normally are the system of choice in aqueous media and nonaqueous electrolyte media.

Properties Fluoroelastomers provide excellent high temperature (up to 500 °F or 260 °C) and aggressive fluids resistance when compared with other elastomers, while combining the most effective stability to many sorts of chemicals and fluids such as oil, diesel, ethanol mix or body fluid. The performance of fluoroelastomers in aggressive chemicals depends on the nature of the base polymer and the compounding ingredients used for molding the final products (e.g. o-rings). Some formulations are generally compatible with hydrocarbons, but incompatible with ketones such as acetone and methyl ethyl ketone, ester solvents such as ethyl acetate, amines, and organic acids such as acetic acid. They can be easily distinguished from many other elastomers because of their high density of over 1800 kg/m3, significantly higher than most types of rubber.

Applications

Because of their outstanding performance they find use in a number of sectors, including the following:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with FKM

Start with the simplest possible case. Write down what FKM claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to FKM before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about FKM ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of FKM

In research: FKM appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses FKM in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: FKM is common in secondary-school and first-year university syllabi. It links to neighbouring topics Elastomers, Fluoropolymers, Materials science, so understanding it makes those chapters shorter.
In everyday life: Look for FKM outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study FKM in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what FKM means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain FKM out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is FKM in simple terms?

FKM is a family of fluorocarbon-based fluoroelastomer materials defined by ASTM International standard D1418 and ISO standard 1629. It is commonly called fluorine rubber or fluoro-rubber.

Why does FKM matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study FKM?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on FKM.