What is the best material for hydraulic seals?

Chemical compound

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Nitrile rubber, also known as nitrile butadiene rubber, NBR, Buna-N, and acrylonitrile butadiene rubber, is a synthetic rubber derived from acrylonitrile (ACN) and butadiene.[1] Trade names include Perbunan, Nipol, Krynac and Europrene. This rubber is unusual in being resistant to oil, fuel, and other chemicals.

NBR is used in the automotive and aeronautical industry to make fuel and oil handling hoses, seals, grommets, and self-sealing fuel tanks. It is also used in the food service, medical, and nuclear industries to make protective gloves. NBR's stability at temperatures from &#;40 to 108 °C (&#;40 to 226 °F) makes it an ideal material for aeronautical applications. Nitrile butadiene is also used to produce moulded goods, footwear, adhesives, sealants, sponges, expanded foams, and floor mats.

Its resilience makes NBR a useful material for disposable lab, cleaning, and examination gloves. Nitrile rubber is more resistant than natural rubber to oils and acids, and has superior strength, but has inferior flexibility.

History

Nitrile rubber was developed in at BASF and Bayer, then part of chemical conglomerate IG Farben. The first commercial production began in Germany in .[2][3]

The Buna-Werke was a slave labor factory located near Auschwitz and financed by IG Farben. The raw materials came from the Polish coalfields.[4] Buna rubber was named by BASF A.G., and through Buna was a remaining trade name of nitrile rubber held by BASF.

Production

Emulsifier (soap), acrylonitrile, butadiene, radical generating activators, and a catalyst are added to polymerization vessels in the production of hot NBR. Water serves as the reaction medium within the vessel. The tanks are heated to 30&#;40 °C to facilitate the polymerization reaction and to promote branch formation in the polymer. Because several monomers capable of propagating the reaction are involved in the production of nitrile rubber the composition of each polymer can vary (depending on the concentrations of each monomer added to the polymerization tank and the conditions within the tank). There may not be a single repeating unit throughout the entire polymer. For this reason there is also no IUPAC name for the general polymer.

Monomers are usually permitted to react for 5 to 12 hours. Polymerization is allowed to proceed to ~70% conversion before a &#;shortstop&#; agent (such as dimethyldithiocarbamate and diethylhydroxylamine) is added to react with (destroy) the remaining free radicals and initiators. Once the resultant latex has &#;shortstopped&#;, the unreacted monomers are removed through a steam in a slurry stripper. Recovery of unreacted monomers is close to 100%. After monomer recovery, latex is sent through a series of filters to remove unwanted solids and then sent to the blending tanks where it is stabilized with an antioxidant. The yielded polymer latex is coagulated using calcium nitrate, aluminium sulfate, and other coagulating agents in an aluminium tank. The coagulated substance is then washed and dried into crumb rubber.[3]

The process for the production of cold NBR is very similar to that of hot NBR. Polymerization tanks are cooled to 5&#;15 °C instead of heating up to 30&#;40 °C close to ambient temperature (ATC). Under lower temperature conditions, less branching will form on polymers (the amount of branching distinguishes cold NBR from hot NBR).

Properties

The raw material is typically yellow, although it can also be orange or red tinted, depending on the manufacturer. Its elongation at break is &#; 300% and possesses a tensile strength of &#; 10 N/mm2 (10 MPa). NBR has good resistance to mineral oils, vegetable oils, benzene/petrol, ordinary diluted acids and alkalines.

An important factor in the properties of NBR is the ratio of acrylonitrile groups to butadiene groups, referred to as the ACN content. The lower the ACN content, the lower the glass transition temperature; however, the higher the ACN content, the better resistance the polymer will have to nonpolar solvents as mentioned above.[5] Most applications requiring both solvent resistance and low temperature flexibility require an ACN content of 33%.

Applications

The uses of nitrile rubber include disposable non-latex gloves, automotive transmission belts, hoses, O-rings, gaskets, oil seals, V belts, synthetic leather, printer's form rollers, and as cable jacketing; NBR latex can also be used in the preparation of adhesives and as a pigment binder.[citation needed]

Unlike polymers meant for ingestion, where small inconsistencies in chemical composition/structure can have a pronounced effect on the body, the general properties of NBR are insensitive to composition. The production process itself is not overly complex; the polymerization, monomer recovery, and coagulation processes require some additives and equipment, but they are typical of the production of most rubbers. The necessary apparatus is simple and easy to obtain.

In January , the European Commission imposed fines totaling &#;34,230,000 on the Bayer and Zeon groups for fixing prices for nitrile butadiene rubber, in violation of the EU ban on cartels and restrictive business practices (Article 81 of the EC Treaty and Article 53 of the EEA Agreement).[6]

Hydrogenated nitrile butadiene rubber (HNBR)

Hydrogenated nitrile butadiene rubber (HNBR) is produced by hydrogenation of NBR. Doing so removes the olefinic groups, which are vulnerable to degradation by various chemicals as well as ozone. Typically, Wilkinson's catalyst is used to promote the hydrogenation. The nitrile groups are unaffected. The degree of hydrogenation determines the kind of vulcanization that can be applied to the polymer.[7]

For more information, please visit hydraulic oil seal manufacturers.

Also known as highly saturated nitrile (HSN), HNBR is widely known for its physical strength and retention of properties after long-term exposure to heat, oil, and chemicals. Trade names include Zhanber (Lianda Corporation), Therban (Arlanxeo [8]), and Zetpol (Zeon Chemical). It is commonly used to manufacture O-rings for automotive air-conditioning systems.[9] Other applications include timing belts, dampers, servo hoses, membranes, and seals.[10]

Depending on filler selection and loading, HNBR compounds typically have tensile strengths of 20&#;31 MPa at 23 °C. Compounding techniques allow for HNBR to be used over a broad temperature range, &#;40 °C to 165 °C, with minimal degradation over long periods of time. For low-temperature performance, low ACN grades should be used; high-temperature performance can be obtained by using highly saturated HNBR grades with white fillers. As a group, HNBR elastomers have excellent resistance to common automotive fluids (e.g., engine oil, coolant, fuel, etc.).

The unique properties and higher temperature rating attributed to HNBR when compared to NBR has resulted in wide adoption of HNBR in automotive, industrial, and assorted, performance-demanding applications. On a volume basis, the automotive market is the largest consumer, using HNBR for a host of dynamic and static seals, hoses, and belts. HNBR has also been widely employed in industrial sealing for oil field exploration and processing, as well as rolls for steel and paper mills.

Carboxylated nitrile butadiene rubber (XNBR)

An alternative version of NBR is carboxylated nitrile butadiene rubber (XNBR). XNBR is a terpolymer of butadiene, acrylonitrile, and acrylic acid.[11] The presence of the acrylic acid introduces carboxylic acid groups (RCO2H). These groups allow crosslinking through the addition of zinc (Zn2+) additives. The carboxyl groups are present at levels of 10% or less. In addition to these ionic crosslinks, traditional sulfur vulcanization is applied.

See also

References

Jan 5,

The Importance of Hydraulic Seal Integrity

Hydraulic seals prevent leakage and loss of fluids from systems. When seals shrink or harden, they can crack and may lose elasticity, leading to seal failure. This can be caused by:

• High operating fluid temperatures
• Fluid degradation

The use of incompatible hydraulic fluids can cause swelling, and/or shrinkage of the seal or chemical attack that can lead to the failure of lip seals as well as O rings.

Types of Hydraulic Seals

The most commonly used material is acrylonitrile or nitrile butadiene rubber (NBR). Relatively inexpensive, NBR exhibits excellent resistance to petroleum-based hydraulic fluids for the temperature range -50°C to 120°C (-60°F to 250°F), but is not resistant to weathering. NBR can be used at temperatures up to 150°C (300°F) but service life will be shortened.

• Compatible fluids: Petroleum-based hydrocarbons including mineral oils, diesel and fuel oils; vegetable oils and greases; HFA, HFB and HFC hydraulic fluids; dilute acids, alkali and salt solutions up to moderate temperatures.
• Incompatible fluids: Highly aromatic hydrocarbons; ketones, acetic acid, and other polar solvents; strong acids, and glycol-based brake fluids.

The second most common material is fluorocarbon rubber (FPM/FKM), commonly known under the DuPontTM trade name VITON® *. It can be used over a wider temperature range of -40°C to 200°C (-40°F to 400°F). There are two main classes of Viton materials, type A and type G. Type A is more common and lower cost than type G, but type G offers improved compatibility with most fluids and resistance to both weathering and ozone.

• Compatible fluids: Petroleum-based hydrocarbons, synthetic hydraulic fluids, fuels, including gasoline/alcohol fuels, aromatics; many organic solvents and chemicals.
• Incompatible fluids: Glycol-based brake fluids; strong alkalis, amines, ammonia, formic and acetic acids, and superheated steam.

Other materials are used for specialised applications such as thermoplastic polyurethanes (TPU) with bio-hydraulic fluids.

Hydraulic Fluid Base Oils

To evaluate the interaction of hydraulic fluids and the sealing materials, it is important to understand the differences between hydraulic fluid base oils.

• API Group I base oils are commonly used for hydraulic fluids. They are refined by a solvent extraction method.
• API Group II and III base oils are refined by hydroprocessing techniques that convert waxes into iso-paraffins without using solvents. These base oils are water white in color and have superior oxidation resistance.
• API Group IV base oils are polyalphaolefins (PAO) whilst API Group V fluids include naphthenic mineral oils, polyalkylene glycol (PAG), natural triglyercide and synthetic esters and other synthetic products.

The majority of base oils used for hydraulic fluids need additives to enhance specific properties. These additives also introduce an additional set of potential interactions with the seal material.

Predicting the Behavior of a Fluid and Seal Material

It is possible to predict the swelling or shrinkage behavior of a seal material with reasonably accurate results under normal operating temperature conditions.

• Comparing the aniline point of the fluid in question to the aniline point of standard ASTM International reference oils produces a useful indication of the oil/ seal compatibility.
• More accuracy can be obtained using the Elastomer Compatibility Index (ECI) method that compares the behavior of a standard NBR compound in a variety of petroleum-based oils to the behaviors of various other nitrile compounds. The results of the compatibility tests are plotted on a graph and used to determine the Swelling Behavior (SB) of the compound in question. Knowing the ECI of the fluid and the SB of the seal material, one can calculate the expected physical interaction of the pair.

How to Pick the Right Seal Material for Your Application

Given the wide range of variables, it is wise to seek guidance from the manufacturers when selecting seal materials for either a new build or maintenance replacement.

It is particularly important to consider the likely impact of any change in hydraulic fluid on the seals already in use before proceeding with a replacement.

For API Group I based fluids operating at temperatures below 100°C (212°F), NBR seals are the most cost-effective choice. Fluoroelastomers (FKM) increase the maximum operating temperature to 200°C (400°F), which is well above the recommended range for ordinary fluids, whilst only slightly reducing low temperature performance. All the other materials exist to fill application niches not covered by NBR or FKM materials.

For applications currently using NBR or FKM seals that are not exhibiting unusual failure modes or unexpectedly short service lives, it is unlikely that changing to a higher performance &#; and usually more expensive &#; material will reduce operating or maintenance costs enough to offset the cost of the premium seals.

For new designs that operate with the same fluids in the same temperature ranges as existing systems, the same seal materials will probably work in the new system too. For new systems that don&#;t duplicate existing designs, either NBR or FKM is a logical elastomer to start with, unless there is something very unusual about the system or known issues with the fluid that will be used.

For more information on hydraulic fluids, elastomer or seal performance, contact your Lubrizol representative.

* Viton is a registered trademark of the E.I. du Pont de Nemours and Company

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