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

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Nitrile rubber was developed in 1931 at BASF and Bayer, then part of chemical conglomerate IG Farben. The first commercial production began in Germany in 1935.[2][3]

IG Farben plant under construction approximately 10 kilometres (6.2 mi) from Auschwitz, 1942

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 1988 Buna was a remaining trade name of nitrile rubber held by BASF.

Production

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Krynac 33110 F nitrile rubber bales

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

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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%.

Property Value Appearance Hardness, Shore A 30–90 Tensile failure stress, ultimate 500-2500 PSI Elongation after fracture in % 600% maximum Density Can be compounded around 1.00 g/cm3

Applications

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A disposable nitrile rubber glove.

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 2008, 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)

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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]

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.

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Carboxylated nitrile butadiene rubber (XNBR)

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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

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References

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Compound Identification in Quality Control
Are you getting the correct compound?

Written by Dale T. McGrosky

One of the most important, yet least frequently checked, quality control procedures is to verify the type of compound on your parts. You may receive certification stating that these parts are indeed the specific compound you requested, but what test was performed to back up the certification and who was the last to verify the compound and what method was used?

We have come across several methods of identifying rubber materials like measuring specific gravity, burn testing, Infrared spectroscopy, chemical analysis, the list goes on. Identifying materials with methods such as measuring specific gravity or burn testing will help you identify the type of material but these methods will not identify a specific compound. Chemical analysis, although very conclusive is very expensive and time consuming making it impracticable for everyday use. When it comes to a safe, quick, accurate, and conclusive method of material and compound identification that can be used safely everyday, our choice was the Fourier Transform Infrared Spectrometer (FT/IR). With the FT/IR we are able to identify materials easily and quickly and use this in our everyday quality inspection assuring our customers are getting the exact compound they ordered.
Specific Gravity (Density)

Lets go over some definitions first. Density is the mass of an object per unit of measure. For instance aluminum has a density of 2.8 g/cm3. The formula for density is Density (p) = Mass (g) / Volume (cm3). Specific gravity is a dimensionless unit that is defined as the ratio of density to the density of water at a specific temperature. Water at 4°C (39°F) has a density of 1.000 g/cm3.

Even though lbs/in3 (pounds per cubic inch) is often used, it is not the correct imperial (U.S.) unit of measure for density since pounds is defined as a measure of force. The correct imperial (U.S.) unit of measure for mass is the slug; therefore, the correct unit of measure for density is slugs/in3.
1 pound = .03108095 slugs.

Specific gravity is a good method of identifying a type of material or checking the accuracy of a compound. Each material has a different specific gravity and this can be useful in identifying a type of material. If you have a specific compound and know the specific gravity of that compound you can compare the specific gravity of the parts from that compound against the specified specific gravity to verify the accuracy of that compound. The acceptable tolerance for specific gravity in rubber compounds is ±0.02 (remember, specific gravity has no unit of measure). Although this method is good in identifying types of materials or checking the accuracy of a specific compound, in some cases it is not conclusive enough and further testing may be required to properly identify a material. One example is EPDM vs. Nitrile material identification.

EPDM and Nitrile have very close specific gravities. Most Nitrile compounds have a specific gravity around 1.23±0.02 and EPDM can come in around 1.20±0.02. Depending on the scale resolution, the sample size and operator accuracy, getting a conclusive identification between these two materials by specific gravity alone may not be possible and other methods such as a soak test may have to be used to assist in the identification process.

Specific gravity is a good method to identify the basic material such as Nitrile, EPDM, Silicone, Fluorocarbon, etc. but it may not be conclusive enough to determine the difference between two specific compounds such as a standard EPDM and an EPDM FDA Grade.

Burn Testing (Burning rubber is NOT recommended because some compounds can form highly toxic fumes.)
Another early method was to burn test the material and observe the ash, smell and burn characteristics of the material. Nitrile has a very distinct smell and if done in the office most of your colleagues will be quite upset with the lingering smell that has permeated their nostrils. Silicone, when burned has a white ash, Fluorocarbon won’t support combustion, when Chloroprene (Neoprene®) is burned in the presence of hot copper it will cause the flame to glow green (Beilstein Test). The list goes on depending on the type of material.

It is NOT a good idea to inhale the fumes from burning rubber. When you find it necessary to burn test rubber such as the Beilstein Test, this should be done in a well ventilated area and every precaution should be taken so the fumes are NOT inhaled.

Infrared Spectroscopy

The FT/IR is a device that shoots infrared radiation (energy), into a sample and measures the amount of IR radiation that is absorbed and transmitted by the chemical bonds between atoms in a molecule. The information is collected and a unique molecular fingerprint of the sample is created. Illustration 3 shows a graph of an EPDM compound. The graph shows the amount of energy absorbed at a specific wavenumber (cm-1). Different chemical bonds in the molecule will produce different peaks in the wavenumber range. The height of the peak shows the amount of energy absorbed at that wavenumber. The more of that type of molecular bond in the molecule, the more energy that will be absorbed creating a higher peak. With this information we can identify materials, types of molecular structures in the material and creates a unique fingerprint for each compound.

In order to understand the FT/IR scan, lets look at the chemical structure of and EPDM molecule as shown in illustration 2 and compare it to the FT/IR scan of an EPDM compound in illustration 3. But, before we get started lets take a look at how molecules vibrate.

Figure Caption “Illustration 1: Vibrational modes of molecules”

Molecules vibrate by bending or stretching. They stretch symmetrically or asymmetrically. They bend “In Plane” by rocking or scissoring and “Out of Plane” by wagging or twisting. You can see this in illustration 1. Each one of these vibrations will absorb energy at a particular wavenumber depending on the two atoms that are bonded.
In the chemical structure of EPDM, illustration 2, you can see methylene, CH2; Methyl, CH3; carbon-hydrogen bonds, CH; carbon-carbon bonds, C-C; and carbon-carbon double bonds C=C. Now take a look at illustration 3 and you can see where I identified the corresponding peaks. For instance you can see the CH2 Asymmetrical stretch at 2918 cm-1 and the CH2 Symmetrical Stretch at 2849 cm-1 and the CH2 Rocking at 722 cm-1. When we are identifying a compound with Acrylonitrile such as ABS, Nitrile (Buna), we look for the carbon-nitrogen triple bond, C≡N, peak between 2260 cm-1 and 2240 cm-1.

Figure Caption “Illustration 2: Chemical structure of an EPDM molecule”

Figure Caption “Illustration 3: FT/IR scan of an EPDM compound

To identify unknown substances, we do this in reverse, you can take a scan of an unknown molecule and identify the peaks and assemble the basic molecular structure. Another method is to compare the scan against a library of known scans to help identify the unknown compound. There are several websites that offer access to libraries of known substances for a fee.

Illustration 1: The graph above was produced by FT/IR scans on two EPDM compounds. We can tell by the graph that they are both EPDM material but they are not the same compound as shown by the two peaks present at wavenumber 1539 cm-1 and 1397 cm-1 in Sample 2 that are not in sample 1.

We used the FT/IR to help resolve an issue for a company that had a problem. They sell an NSF Certified product that contains EPDM O-rings. Since they purchase NSF Certified O-rings from 2 different suppliers, this company wanted to check if the failed O-rings were from vendor 1 or vendor 2. We received several o-ring samples identified as “Failed O-rings” — Sample 1, and O-ring samples taken from the same batch that were sent to the customer – Sample 2. Our objective was to determine if Sample 1 was the same compound as Sample 2. As shown in illustration 4, the two scans are similar and we were able to determine that these samples were EPDM material but the two peaks at wavenumber 1539 cm-1 and 1397 cm-1 in Sample 2 are not in Sample 1 indicates that these compounds are different. Our conclusion was these samples were both EPDM but they were not the same compound.

We looked at several methods of identifying compounds. Specific Gravity and burn testing will help identify the material but does not help much in identifying a specific compound. Chemical analysis is accurate and conclusive but is slow and costly. Our preference at Satori Seal for a quick method of compound identification for use in our quality control inspection is the FT/IR. With the FT/IR we identify and match compounds, we fingerprint our compounds allowing us to quickly scan actual parts and get a conclusive identification assuring our customers that they are getting the compound they ordered.

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