Polyisobutylene Applications

Fuel and lubricant additive. Polyisobutylene (in the form of polyisobutylene succinimide) has interesting properties when used as an additive in lubricating oils and motor fuels. Polyisobutylene added in small amounts to the lubricating oils used in machining results in a significant reduction in the generation of oil mist and thus reduces the operator’s inhalation of oil mist.[2] It is also used to clean up waterborne oil spills as part of the commercial product Elastol. When added to crude oil it increases the oil’s viscoelasticity when pulled, causing the oil to resist breakup when it is vacuumed from the surface of the water.

As a fuel additive, polyisobutylene has detergent properties. When added to diesel fuel, it resists fouling of fuel injectors, leading to reduced hydrocarbon and particulate emissions.[3] It is blended with other detergents and additives to make a “detergent package” that is added to gasoline and diesel fuel to resist buildup of deposits and engine knock.[4]

Polyisobutylene is used in some formulations as a thickening agent.

[edit]Sporting equipment

Butyl rubber is used for the bladders in basketballs, footballs, soccer balls and other inflatable balls to provide a tough, airtight inner compartment.

[edit]Roof Repair

Butyl rubber sealant is used for rubber roof repair and for maintenance of roof membranes (especially around the edges). It is important to have the roof membrane fixed, as a lot of fixtures (i.e., air conditioner vents, plumbing and other pipes, etc.) can considerably loosen it.

Rubber roofing typically refers to a specific type of roofing materials that are made of ethylene propylene diene monomers (EPDM). It is crucial to the integrity of such roofs to avoid using harsh abrasive materials and petroleum-based solvents for their maintenance.

Polyester fabric laminated to butyl rubber binder provides a single-sided waterproof tape that can be used on metal, PVC, and cement joints. It is ideal for repairing and waterproofing metal roofs.

[edit]Gas masks and chemical agent protection

Butyl rubber is one of the most robust elastomers when subjected to chemical warfare agents and decontamination materials. It is a harder and less porous material than other elastomers, such as natural rubber or silicone, but still has enough elasticity to form an airtight seal. While butyl rubber will break down when exposed to agents such as NH3 (ammonia) or certain solvents, it breaks down more slowly than comparable elastomers. It is therefore used to create seals in gas masks and other protective clothing.

[edit]Chewing gum

Molecular structure:

Rubber Chemical Structure

Introduction to rubber chemical structure:

In the organic chemistry section of chemistry ,we learn about the various polymers , monomers,elastomers etc.Monomer is a single unit and when huge number of monomers are combined or say polymerised then polymers are formed.The process of conversion of monomer to polymer is known as p-olymerisation.Elastomer is an another category of polymers having a specific properties of regaining of its structure even if it is stretched.Rubber comes under the category of elastomer.In general life rubber has variety of uses.the important property of rubber is that it regains its structure even if it is stretched.Stretching can be done up to a certain limit.If it is stretched beyond limit then it can break.

Rubber can be found in two forms

1)Natural rubber

2)Synthetic Rubber

Natural rubber is a kind of rubber which which is found directly from the nature.And when the natural rubber is processed under some chemical processes then a new kind of rubber is formed ,this rubber is known as synthetic rubber.We can also say that natural rubber are synthesised from the natural rubber.Both this rubber are of great use because of its specific features.

Structure of rubber:

Main composition of crude rubber is hydrocarbons.It also contains some proteins and materials which are soluble in acetone.The hydrocarbons which possess the properties of rubber are usually high in molecular weight and it ranges from 45000 to 3000000.Isoprene is a monomer of natural rubber.When huge number of isoprene units are polymerised then a polymer is formed .

Cis and Trans Configuration of Rubber

The cis configuration of the natural rubber is the reason for the rubber properties in it. Cis configuration means that extension of the chain is on the same side of the ethylene bond.If the configuration is trans, it means that the extension of chain is on the both sides of ethylene bond,then it is a hard plastic.In case of trans it does not show the properties of rubber.

Synthetic rubber is of great use in the industry.Some of the widely used synthetic rubber are butyl rubber which is formed by the copolymerisation of isobutylene and a little amount isoprene.Another synthetic rubber is Styrene Butadiene Rubber also known as SBR.Buna N and buna S is also a kind of synthetic rubber often use in the industry.

Vulcanisation of Rubber

In the rubber molecules the cross linking between the chains are very less.This leads to the softness in the rubber .To make the rubber hard some chemicals are added to it.The process is known as vulcanisation.In this process the natural rubber is treated with some chemicals ,more often chemical used is sulphur.When sulphur reacts with the natural rubber then it increases the cross linking between the molecules in the rubber.It also forms many sulphide bonds.Due to formation of many new crosslinkings and many sulphide bonds the natural rubber becomes hard.Natural rubber is a kind of thermoplastic,it means that it becomes soft when it is subjected to heat and it becomes hard when it is subjected to cold.

butyl rubber (IIR), also called isobutylene-isoprene rubber, a synthetic rubber produced by copolymerizing isobutylene with small amounts of isoprene. Valued for its chemical inertness, impermeability to gases, and weatherability, butyl rubber is employed in the inner linings of automobile tires and in other specialty applications.

Both isobutylene (C[CH3]2=CH2) and isoprene (CH2=C[CH3]-CH=CH2) are usually obtained by the thermal cracking of natural gas or of the lighter fractions of crude oil. At normal temperature and pressure isobutylene is a gas and isoprene is a volatile liquid. For processing into IIR, isobutylene, refrigerated to very low temperatures (approximately −100 °C [−150 °F]), is diluted with methyl chloride. Low concentrations (1.5 to 4.5 percent) of isoprene are added in the presence of aluminum chloride, which initiates the reaction in which the two compounds copolymerize (i.e., their single-unit molecules link together to form giant, multiple-unit molecules). The polymer repeating units have the following structures:

Because the base polymer, polyisobutylene, is stereoregular (i.e., its pendant groups are arranged in a regular order along the polymer chains) and because the chains crystallize rapidly on stretching, IIR containing only a small amount of isoprene is as strong as natural rubber. In addition, because the copolymer contains few unsaturated groups (represented by the carbon-carbon double bond located in each isoprene repeating unit), IIR is relatively resistant to oxidation-a process by which oxygen in the atmosphere reacts with the double bonds and breaks the polymer chains, thereby degrading the material. Butyl rubber also shows an unusually low rate of molecular motion well above the glass transition temperature (the temperature above which the molecules are no longer frozen in a rigid,glassy state). This lack of motion is reflected in the copolymer’s unusually low permeability to gases as well as in its outstanding resistance to attack by ozone.

The copolymer is recovered from the solvent as a crumb, which can be compounded with fillers and other modifiers and then vulcanized into practical rubber products. Owing to its excellent air retention, butyl rubber is the preferred material for inner tubes in all but the largest sizes. It also plays an important part in the inner liners of tubeless tires. (Because of poor tread durability, all-butyl tires have not proved successful.) IIR is also used for many other automobile components, including window strips, because of its resistance to oxidation. Its resistance to heat has made it indispensable in tire manufacture, where it forms the bladders that retain the steam or hot water used to vulcanize tires.

Bromine or chlorine can be added to the small isoprene fraction of IIR to make BIIR or CIIR (known as halobutyls). The properties of these polymers are similar to those of IIR, but they can be cured more rapidly and with different and smaller amounts of curative agents. As a result, BIIR and CIIR can be cocured more readily in contact with other elastomers making up a rubber product.

Butyl rubber was first produced by American chemists William Sparks and Robert Thomas at the Standard Oil Company of New Jersey (nowExxon Corporation) in 1937. Earlier attempts to produce synthetic rubbers had involved the polymerization of dienes (hydrocarbon molecules containing two carbon-carbon double bonds) such as isoprene and butadiene. Sparks and Thomas defied convention by copolymerizing isobutylene, an olefin (hydrocarbon molecules containing only one carbon-carbon double bond) with small amounts-e.g., less than 2 percent-of isoprene. As a diene, isoprene provided the extra double bond required to cross-link the otherwise inert polymer chains, which were essentially polyisobutylene. Before experimental difficulties were resolved, butyl rubber was called “futile butyl,” but with improvements it enjoyed wide acceptance for its low permeability to gases and its excellent resistance to oxygen and ozone at normal temperatures. During World War IIthe copolymer was called GR-I, for Government Rubber-Isobutylene.

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Polymers

1. Introduction

Prior to the early 1920’s, chemists doubted the existence of molecules having molecular weights greater than a few thousand. This limiting view was challenged by Hermann Staudinger, a German chemist with experience in studying natural compounds such as rubber and cellulose. In contrast to the prevailing rationalization of these substances as aggregates of small molecules, Staudinger proposed they were made up of macromolecules composed of 10,000 or more atoms. He formulated a polymeric structure for rubber, based on a repeating isoprene unit (referred to as a monomer). For his contributions to chemistry, Staudinger received the 1953 Nobel Prize. The terms polymer and monomer were derived from the Greek roots poly (many), mono (one) and meros (part).

Recognition that polymeric macromolecules make up many important natural materials was followed by the creation of synthetic analogs having a variety of properties. Indeed, applications of these materials as fibers, flexible films, adhesives, resistant paints and tough but light solids have transformed modern society. Some important examples of these substances are discussed in the following sections.

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2. Writing Formulas for Polymeric Macromolecules

The repeating structural unit of most simple polymers not only reflects the monomer(s) from which the polymers are constructed, but also provides a concise means for drawing structures to represent these macromolecules. For polyethylene, arguably the simplest polymer, this is demonstrated by the following equation. Here ethylene (ethene) is the monomer, and the corresponding linear polymer is called high-density polyethylene (HDPE). HDPE is composed of macromolecules in which n ranges from 10,000 to 100,000 (molecular weight 2*105 to 3 *106 ).

If Y and Z represent moles of monomer and polymer respectively, Z is approximately 10-5 Y. This polymer is called polyethylene rather than polymethylene, (-CH2-)n, because ethylene is a stable compound (methylene is not), and it also serves as the synthetic precursor of the polymer. The two open bonds remaining at the ends of the long chain of carbons (colored magenta) are normally not specified, because the atoms or groups found there depend on the chemical process used for polymerization. The synthetic methods used to prepare this and other polymers will be described later in this chapter.

Unlike simpler pure compounds, most polymers are not composed of identical molecules. The HDPE molecules, for example, are all long carbon chains, but the lengths may vary by thousands of monomer units. Because of this, polymer molecular weights are usually given as averages. Two experimentally determined values are common: Mn , the number average molecular weight, is calculated from the mole fraction distribution of different sized molecules in a sample, and Mw , the weight average molecular weight, is calculated from the weight fraction distribution of different sized molecules. These are defined below. Since larger molecules in a sample weigh more than smaller molecules, the weight average Mw is necessarily skewed to higher values, and is always greater than Mn. As the weight dispersion of molecules in a sample narrows, Mw approaches Mn, and in the unlikely case that all the polymer molecules have identical weights (a pure mono-disperse sample), the ratio Mw / Mn becomes unity.

The influence of different mass distributions on Mn and Mw may be examined with the aid of a simple mass calculator.

To use this device Click Here.

Many polymeric materials having chain-like structures similar to polyethylene are known. Polymers formed by a straightforward linking together of monomer units, with no loss or gain of material, are called addition polymers or chain-growth polymers. A listing of some important addition polymers and their monomer precursors is presented in the following table.

Some Common Addition Polymers

Name(s)

Formula

Monomer

Properties

Uses

Polyethylene

low density (LDPE)

-(CH2-CH2)n-

ethylene

CH2=CH2

soft, waxy solid

film wrap, plastic bags

Polyethylene

high density (HDPE)

-(CH2-CH2)n-

ethylene

CH2=CH2

rigid, translucent solid

electrical insulation

bottles, toys

Polypropylene

(PP) different grades

-[CH2-CH(CH3)]n-

propylene

CH2=CHCH3

atactic: soft, elastic solid

isotactic: hard, strong solid

similar to LDPE

carpet, upholstery

Poly(vinyl chloride)

(PVC)

-(CH2-CHCl)n-

vinyl chloride

CH2=CHCl

strong rigid solid

pipes, siding, flooring

Poly(vinylidene chloride)

(Saran A)

-(CH2-CCl2)n-

vinylidene chloride

CH2=CCl2

dense, high-melting solid

seat covers, films

Polystyrene

(PS)

-[CH2-CH(C6H5)]n-

styrene

CH2=CHC6H5

hard, rigid, clear solid

soluble in organic solvents

toys, cabinets

packaging (foamed)

Polyacrylonitrile

(PAN, Orlon, Acrilan)

-(CH2-CHCN)n-

acrylonitrile

CH2=CHCN

high-melting solid

soluble in organic solvents

rugs, blankets

clothing

Polytetrafluoroethylene

(PTFE, Teflon)

-(CF2-CF2)n-

tetrafluoroethylene

CF2=CF2

resistant, smooth solid

non-stick surfaces

electrical insulation

Poly(methyl methacrylate)

(PMMA, Lucite, Plexiglas)

-[CH2-C(CH3)CO2CH3]n-

methyl methacrylate

CH2=C(CH3)CO2CH3

hard, transparent solid

lighting covers, signs

skylights

Poly(vinyl acetate)

(PVAc)

-(CH2-CHOCOCH3)n-

vinyl acetate

CH2=CHOCOCH3

soft, sticky solid

latex paints, adhesives

cis-Polyisoprene

natural rubber

-[CH2-CH=C(CH3)-CH2]n-

isoprene

CH2=CH-C(CH3)=CH2

soft, sticky solid

requires vulcanization

for practical use

Polychloroprene (cis + trans)

(Neoprene)

-[CH2-CH=CCl-CH2]n-

chloroprene

CH2=CH-CCl=CH2

tough, rubbery solid

synthetic rubber

oil resistant

3. Properties of Macromolecules

A comparison of the properties of polyethylene (both LDPE & HDPE) with the natural polymers rubber and cellulose is instructive. As noted above, synthetic HDPE macromolecules have masses ranging from 105 to 106 amu (LDPE molecules are more than a hundred times smaller). Rubber and cellulose molecules have similar mass ranges, but fewer monomer units because of the monomer’s larger size. The physical properties of these three polymeric substances differ from each other, and of course from their monomers.

• HDPE is a rigid translucent solid which softens on heating above 100° C, and can be fashioned into various forms including films. It is not as easily stretched and deformed as is LDPE. HDPE is insoluble in water and most organic solvents, although some swelling may occur on immersion in the latter. HDPE is an excellent electrical insulator.

• LDPE is a soft translucent solid which deforms badly above 75° C. Films made from LDPE stretch easily and are commonly used for wrapping. LDPE is insoluble in water, but softens and swells on exposure to hydrocarbon solvents. Both LDPE and HDPE become brittle at very low temperatures (below -80° C). Ethylene, the common monomer for these polymers, is a low boiling (-104° C) gas.

• Natural (latex) rubber is an opaque, soft, easily deformable solid that becomes sticky when heated (above. 60° C), and brittle when cooled below -50° C. It swells to more than double its size in nonpolar organic solvents like toluene, eventually dissolving, but is impermeable to water. The C5H8 monomer isoprene is a volatile liquid (b.p. 34° C).

• Pure cellulose, in the form of cotton, is a soft flexible fiber, essentially unchanged by variations in temperature ranging from -70 to 80° C. Cotton absorbs water readily, but is unaffected by immersion in toluene or most other organic solvents. Cellulose fibers may be bent and twisted, but do not stretch much before breaking. The monomer of cellulose is the C6H12O6aldohexose D-glucose. Glucose is a water soluble solid melting below 150° C.

To account for the differences noted here we need to consider the nature of the aggregate macromolecular structure, or morphology, of each substance. Because polymer molecules are so large, they generally pack together in a non-uniform fashion, with ordered or crystalline-like regions mixed together with disordered or amorphous domains. In some cases the entire solid may be amorphous, composed entirely of coiled and tangled macromolecular chains. Crystallinity occurs when linear polymer chains are structurally oriented in a uniform three-dimensional matrix. In the diagram on the right, crystalline domains are colored blue.

Increased crystallinity is associated with an increase in rigidity, tensile strength and opacity (due to light scattering). Amorphous polymers are usually less rigid, weaker and more easily deformed. They are often transparent.

Three factors that influence the degree of crystallinity are:

i) Chain length

ii) Chain branching

iii) Interchain bonding

The importance of the first two factors is nicely illustrated by the differences between LDPE and HDPE. As noted earlier, HDPE is composed of very long unbranched hydrocarbon chains. These pack together easily in crystalline domains that alternate with amorphous segments, and the resulting material, while relatively strong and stiff, retains a degree of flexibility. In contrast, LDPE is composed of smaller and more highly branched chains which do not easily adopt crystalline structures. This material is therefore softer, weaker, less dense and more easily deformed than HDPE. As a rule, mechanical properties such as ductility, tensile strength, and hardness rise and eventually level off with increasing chain length.

The nature of cellulose supports the above analysis and demonstrates the importance of the third factor (iii). To begin with, cellulose chains easily adopt a stable rod-like conformation. These molecules align themselves side by side into fibers that are stabilized by inter-chain hydrogen bonding between the three hydroxyl groups on each monomer unit. Consequently, crystallinity is high and the cellulose molecules do not move or slip relative to each other. The high concentration of hydroxyl groups also accounts for the facile absorption of water that is characteristic of cotton.

Natural rubber is a completely amorphous polymer. Unfortunately, the potentially useful properties of raw latex rubber are limited by temperature dependence; however, these properties can be modified by chemical change. The cis-double bonds in the hydrocarbon chain provide planar segments that stiffen, but do not straighten the chain. If these rigid segments are completely removed by hydrogenation (H2 & Pt catalyst), the chains lose all constrainment, and the product is a low melting paraffin-like semisolid of little value. If instead, the chains of rubber molecules are slightly cross-linked by sulfur atoms, a process called vulcanization which was discovered by Charles Goodyear in 1839, the desirable elastomeric properties of rubber are substantially improved. At 2 to 3% crosslinking a useful soft rubber, that no longer suffers stickiness and brittleness problems on heating and cooling, is obtained. At 25 to 35% crosslinking a rigid hard rubber product is formed. The following illustration shows a cross-linked section of amorphous rubber. By clicking on the diagram it will change to a display of the corresponding stretched section. The more highly-ordered chains in the stretched conformation are entropically unstable and return to their original coiled state when allowed to relax (click a second time).

On heating or cooling most polymers undergo thermal transitions that provide insight into their morphology. These are defined as the melt transition, Tm , and the glass transition, Tg .

Tm is the temperature at which crystalline domains lose their structure, or melt. As crystallinity increases, so does Tm.

Tg is the temperature below which amorphous domains lose the structural mobility of the polymer chains and become rigid glasses.

Tg often depends on the history of the sample, particularly previous heat treatment, mechanical manipulation and annealing. It is sometimes interpreted as the temperature above which significant portions of polymer chains are able to slide past each other in response to an applied force. The introduction of relatively large and stiff substituents (such as benzene rings) will interfere with this chain movement, thus increasing Tg (note polystyrene below). The introduction of small molecular compounds called plasticizers into the polymer matrix increases the interchain spacing, allowing chain movement at lower temperatures. with a resulting decrease in Tg. The outgassing of plasticizers used to modify interior plastic components of automobiles produces the “new-car smell” to which we are accustomed.

Tm and Tg values for some common addition polymers are listed below. Note that cellulose has neither a Tm nor a Tg.

Polymer

LDPE

HDPE

PP

PVC

PS

PAN

PTFE

PMMA

Rubber

Tm (°C)

110

130

175

180

175

>200

330

180

30

Tg (°C)

_110

_100

_10

80

90

95

_110

105

_70

Rubber is a member of an important group of polymers called elastomers. Elastomers are amorphous polymers that have the ability to stretch and then return to their original shape at temperatures above Tg. This property is important in applications such as gaskets and O-rings, so the development of synthetic elastomers that can function under harsh or demanding conditions remains a practical goal. At temperatures below Tg elastomers become rigid glassy solids and lose all elasticity. A tragic example of this caused the space shuttle Challenger disaster. The heat and chemical resistant O-rings used to seal sections of the solid booster rockets had an unfortunately high Tg near 0 °C. The unexpectedly low temperatures on the morning of the launch were below this Tg, allowing hot rocket gases to escape the seals.

Copolymers

The synthesis of macromolecules composed of more than one monomeric repeating unit has been explored as a means of controlling the properties of the resulting material. In this respect, it is useful to distinguish several ways in which different monomeric units might be incorporated in a polymeric molecule. The following examples refer to a two component system, in which one monomer is designated A and the other B.

Statistical Copolymers

Also called random copolymers. Here the monomeric units are distributed randomly, and sometimes unevenly, in the polymer chain: ~ABBAAABAABBBABAABA~.

Alternating Copolymers

Here the monomeric units are distributed in a regular alternating fashion, with nearly equimolar amounts of each in the chain: ~ABABABABABABABAB~.

Block Copolymers

Instead of a mixed distribution of monomeric units, a long sequence or block of one monomer is joined to a block of the second monomer: ~AAAAA-BBBBBBB~AAAAAAA~BBB~.

Graft Copolymers

As the name suggests, side chains of a given monomer are attached to the main chain of the second monomer: ~AAAAAAA(BBBBBBB~)AAAAAAA(BBBB~)AAA~.

1. Addition Copolymerization

Most direct copolymerizations of equimolar mixtures of different monomers give statistical copolymers, or if one monomer is much more reactive a nearly homopolymer of that monomer. The copolymerization of styrene with methyl methacrylate, for example, proceeds differently depending on the mechanism. Radical polymerization gives a statistical copolymer. However, the product of cationic polymerization is largely polystyrene, and anionic polymerization favors formation of poly(methyl methacrylate). In cases where the relative reactivities are different, the copolymer composition can sometimes be controlled by continuous introduction of a biased mixture of monomers into the reaction.

Formation of alternating copolymers is favored when the monomers have different polar substituents (e.g. one electron withdrawing and the other electron donating), and both have similar reactivities toward radicals. For example, styrene and acrylonitrile copolymerize in a largely alternating fashion.

Some Useful Copolymers

Monomer A

Monomer B

Copolymer

Uses

H2C=CHCl

H2C=CCl2

Saran

films & fibers

H2C=CHC6H5

H2C=C-CH=CH2

SBR

styrene butadiene rubber

tires

H2C=CHCN

H2C=C-CH=CH2

Nitrile Rubber

adhesives

hoses

H2C=C(CH3)2

H2C=C-CH=CH2

Butyl Rubber

inner tubes

F2C=CF(CF3)

H2C=CHF

Viton

gaskets

A terpolymer of acrylonitrile, butadiene and styrene, called ABS rubber, is used for high-impact containers, pipes and gaskets.

For polyisobutylene at a glance, click here!

Polyisobutylene is a synthetic rubber, or elastomer. It’s special because it’s the only rubber that’s gas impermeable, that is, it’s the only rubber that can hold air for long periods of time. You may have noticed that balloons will go flat after a few days. This is because they are made of polyisoprene, which is not gas impermeable. Because polyisobutylene will hold air, it is used to make things like the inner liner of tires, and the inner liners of basketballs.

Polyisobutylene, sometimes called butyl rubber, and other times PIB, is a vinyl polymer. It’s very similar to polyethylene and polypropylene in structure, except that every other carbon is substituted with two methyl groups. It is made from the monomer isobutylene, by cationic vinyl polymerization.

And this is that monomer isobutylene:

Usually, a small amount of isoprene is added to the isobutylene. The polymerization is carried out at a right frosty -100 oC, or -148 oF for you Americans out there. This is because the reaction is so fast we can’t control it unless we freeze it colder than a brass toilet seat in the Yukon.

Polyisobutylene was first developed during the early 1940s. At that time, the most widely used rubber was natural rubber, polyisoprene. Polyisoprene was an excellent elastomer, and easy to isolate from the sap of the hevea tree. Huge plantations thrived in Malaysia and grew hevea trees to supply the world’s rubber needs. There was only one slight problem, and that was that Malaysia had just been conquered by the Imperial Japanese Army, and wouldn’t you know we just so happened to be fighting the Second World War against them right at that moment. Before the war was over more than sixty million people would be dead. Deprived of natural rubber, the Allied nations did some quick thinking and came up with PIB. It obviously worked, because the Allies won the war.

Ok, we didn’t actually invent polyisobutylene during the war. It had been invented long before the war by chemists in Germany. There’s irony! But it wasn’t very useful until American chemists came up with a way to crosslink it. What they did was to copolymerize isobutylene with a little bit, say, around one percent, isoprene. This is isoprene:

When isoprene is polymerized with the isobutylene we get a polymer that looks like this:

About one or two out of every hundred repeat units is an isoprene unit, shown in blue. These have double bonds, which means the polymer can be crosslinked byvulcanization just like natural rubber. What is this vulcanization? To find out, click here.

Stealing Vulcan’s Fire

There was a time long past when the only rubber we had was natural rubber latex, polyisoprene. Straight out of the tree, natural rubber latex isn’t good for much. It gets runny and sticky when it gets warm, and it gets hard and brittle when it’s cold. Tires made out of it wouldn’t be much good unless one lived in some happy land where the temperature was seventy degrees year round.

A long time ago…how long, you ask? It was about a hundred and sixty years ago, 1839 to be exact. This was before there were any cars to need tires, but the idea of a useable rubber was still attractive. One person trying to make rubber more useful was named Charles Goodyear, a tinkerer and inventor, and by no means a successful one at this point. While goofing around in his kitchen with a piece of fabric coated with a mixture of rubber latex, sulfur and a little white lead, he accidentally laid it on a hot stove top. It began sizzling like a mass of really smelly bacon or (strangely enough) burning rubber. Wouldn’t you know, when he took a look at this mass of rubber, he found it wouldn’t melt and get sticky when it was heated, nor would it get brittle when he left it outside overnight in the cold Massachusetts winter. He called his new rubber vulcanized rubber.

Tying it All Together

What had happened here? What did the sulfur do to the rubber? What it did was it formed bridges. Which tied all the polymer chains in the rubber together. These are called crosslinks. You can see this in the picture below. Bridges made by short chains of sulfur atoms tie one chain of polyisoprene to another, until all the chains are joined into one giant supermolecule.

Yes, folks, this means exactly what you think it does. An object made of a crosslinked rubber is in fact one single molecule. A molecule big enough to pick up in your hand.

Wow!

These crosslinks tie all the polymer molecules together. Because they are tied together, when the rubber gets hot, they can’t flow past each other, nor around each other. This is why it doesn’t melt. Also, because all the polymer molecules are tied together, they aren’t easily broken apart from each other. This is why the Charles Goodyear’s vulcanized rubber doesn’t get brittle in when it gets cold.

We can look at what’s going on conceptually, and take a look at the bigger picture. The drawing below shows the difference between a lot of single uncrosslinked polymer chains, and a crosslinked network.

Other kinds of rubber, which chemists call elastomers that are crosslinked include:

Polybutadiene

Polyisobutylene

Polychloroprene

More! More!

But rubber isn’t the only thing that can be crosslinked. Plastics are also made stronger by crosslinking. Formica is a crosslinked material.

Crosslinked polymers are usually molded and shaped before they are crosslinked. Once crosslinking has taken place, usually at high temperature, the object can no longer be shaped. Because heat usually causes the crosslinking which makes the shape permanent, we call these materials thermosets. This name distinguishes them from thermoplastics, which aren’t crosslinked and can be reshaped once molded. Interestingly, the first thermoset was again polyisoprene. The more sulfur crosslinks you put into the polyisoprene, the stiffer it gets. Lightly crosslinked, it’s a flexible rubber. Heavily crosslinked, it’s a hard thermoset. (It was Charles’ brother Noah who made the first polyisoprene thermosets, by the way.) Here are some crosslinked thermosets:

Epoxy resins

Polydicyclopentadiene

Polycarbonates

Crosslinked polymers can also be coatings, adhesives, and electronic parts. Crosslinked materials can’t dissolve in solvents, because all the polymer chains are covalently tied together. But they can absorb solvents. A piece of a crosslinked material that has absorbed a lot of solvent is called a gel. One kind of gel you may be familiar with is crosslinked polyacrylamide. Uncrosslinked polyacrylamide is soluble in water, and crosslinked polyacrylamides can absorb water. Water-logged gels of crosslinked polyacrylamides are used to make soft contact lenses.

The Catch

Crosslinking makes both elastomers and plastics stronger, but there is a problem. Because crosslinked materials don’t melt, it is very hard to recycle them. One answer to this problem is to create crosslinks that can be reversed, or undone, believe it or not. One family of materials using reversible crosslinks are thermoplastic elastomers

What is rubber and why does it stretch?

The terms elastomer and rubber are scientifically identical and interchangeable, although the latter is used in some areas to refer only to natural rubber which comes from the latex contained by some trees and other plants – as opposed to synthetic rubber which is generally an oil by-product. Some Standards attempt to reserve the term elastomer for a crosslinked material (see below for an explanation of crosslinking), but there is no general agreement on this. In our literature we use the terms elastomer and rubber as synonyms.

Elastomers are a class of materials which differ quite obviously from all other solid materials in that they can be stretched, easily and almost completely reversibly, to high extensions. An ordinary postal rubber band illustrates this behaviour. It will generally be made from natural rubber, and can be stretched perhaps 600% (i.e. to seven times its original length), after which – before reaching its ultimate breaking elongation – it can be released and will rapidly recover to almost exactly the original length it had before stretching. The material is said to be elastic.

Most synthetic elastomers are not as elastic as natural rubber, but all can be stretched (or otherwise deformed) in a reversible manner to an extent which easily distinguishes them from all other solid materials. (n.b. a metal spring exhibits high reversible elasticity, but this is a feature of its wound shape. The actual metal itself of which the spring is made only deforms slightly, by twisting locally, at any particular point – nothing like the high deformations of which elastomers are capable.)

Elastomers are a special case of the wider group of materials known as polymers. Polymers are not made up of discrete compact molecules like most materials, but are made of long, flexible, chain-like or string-like, molecules. At this scale the inside of a piece of rubber can be thought of as resembling a pile of cooked spaghetti. In spaghetti, however, the chains, though intertwined, are all separate. But in most practical elastomers each chain will be joined together occasionally along its length to one or more nearby chains with just a very few chemical bridges, known as crosslinks. So the whole structure forms a coherent network which stops the chains from sliding past one another indefinitely – although leaving the long sections of chain between crosslinks free to move. The process by which crosslinks are added is known as vulcanization. To achieve vulcanization the raw rubber is mechanically mixed with a number of compounding ingredients carefully chosen to give the properties required for the particular application. The reason why elastomers behave as they do is associated with the type of molecular structure described above.

Against this background the reason why rubber can stretch so much is that, at normal temperatures, each long chain-like molecule (like any molecule) is in a constant state of agitation (thermal motion). For these flexible long-chain molecules the movement is considerable, and the molecule is agitated so much that it can take up a highly kinked shape. Because of this kinking, the distance between the two ends of the chain is very much less than its fully stretched length. This gives the rubber its flexibility. When a rubber band is stretched some of the highly kinked chains are simply being stretched out. Stretching can then continue until many of the chains are fully extended, or until the rubber breaks.

Low temperature behaviour. What is the ‘glass transition temperature’?

When different elastomers are being described, a fundamental property which is often quoted is the ‘glass transition temperature, Tg’, which differs from one elastomer to another. For example, for natural rubber Tg is -70°C (-95°F). Broadly this means that above -70°C the material behaves as a rubber, but below -70°C the material behaves more like a glass. When glassy, natural rubber is about one thousand times as stiff as it is when rubbery. When glassy a hammer blow on natural rubber will cause it to shatter like a glass; when rubbery the hammer is likely just to bounce off.

Of course in practice the dividing line between the glassy and rubbery behaviour just described is not as sharp as this. In fact the transition is spread over some tens of degrees – but it is centred around -70°C. Thus, although a Tg can be accurately defined (although varying somewhat with the precise test conditions), for practical purposes we have to consider a glass transition region within which the properties are slowly changing from rubbery to glassy or vice versa (the processes are completely reversible). The broadness of this transition region varies from elastomer to elastomer.

Strictly speaking we should only use the term ‘elastomer’ to describe a material when it is above its glass transition temperature, but such a distinction would generally be regarded as pedantic. Definitions of this type (such as ‘elastomer’ or ‘glassy polymer’) are accepted as applying to the state of the polymer at ambient temperature.

The reason for the existence of a glass transition temperature can be understood in terms of the molecular model of rubber which has been described above. It has been seen that, at normal temperatures, the molecular chains are in a constant state of thermal motion, that they are constantly changing their configuration, and that their flexibility makes them reasonably easy to stretch. It is not difficult to appreciate that as the temperature is lowered the chains become less flexible and the amount of thermal motion decreases. Eventually, a low temperature, the glass transition temperature, is reached, where all major motion of the chains essentially ceases. The material no longer has the properties which make it an elastomer, and it behaves as a glass.

For all practical engineering uses of elastomers we require good flexibility – so it is essential that we use them only at temperatures which are comfortably above the glass transition. This is generally no problem for natural rubber with a Tg of -70°C, or cis-butadiene rubber with a Tg of -108°C (-160°F). But many elastomers, especially those which have been designed to be highly heat or oil resistant, have much higher Tgs, and this must be borne in mind when selecting them for service applications. For example some fluoroelastomers, which have excellent oil and heat resistance, have a Tg not far below normal room temperature. This can result in problems if a component required to work at high temperature also has to serve the same function on cooling down, and must be considered in design.

High temperature behaviour.

The limit to the upper temperature at which an elastomer can be used is generally determined by its chemical stability, and will thus vary for different elastomers. Elastomers can be attacked by oxygen or other chemical species, and because the attack results in a chemical reaction, its potency will increase with temperature.

Degradative chemical reactions are generally of two types. The first are those which cause breakage of the molecular chains or crosslinks, softening the rubber because they weaken the network. The second are those which result in additional crosslinking, hardening the rubber – and often characterized by a hard, degraded, skin forming on the rubber component. Selection of a suitable elastomer and the use of chemical antidegradants can reduce the rate of chemical attack.

Why is the stretching reversible?

The section above describes why elastomers can stretch, but does not explain why, when the stretching force is removed, the material returns to its original shape. This can be explained by thermodynamics, and a much simplified description is given here.

When the rubber is ‘at rest’ at normal temperatures the chain-like molecules are in a constant state of agitation and are highly kinked – due to thermal energy. This is a highly disordered state – described thermodynamically as being a state of high entropy. When the chains are stretched (less kinked), a higher state of order is obviously being imposed – in other words the chains are being forced into a state of lower entropy. As it is a fundamental law of thermodynamics that entropy strives for a maximum, the driving force is now back towards the disordered state, and, as soon as the stretching force is removed, the rubber will retract. Other factors, such as the structure of the carbon atom, also play a part, but will not be discussed here.

Fluid resistance

The structure of an elastomer, which we have seen comprises a network of chains, means that there are gaps between adjacent chains. Indeed the elasticity of rubber relies on substantial thermal motion of the chains, which would not be possible if the chains were closely packed. The free volume available in the rubber means that some liquids can enter the rubber and cause swelling – sometimes very large amounts of swelling. For example the ability of oil to swell natural rubber is well known.

Potential for swelling is largely controlled by a thermodynamic property known as solubility parameter which is described elsewhere. All rubbers and all liquids have specific values of solubility parameter, a knowledge of which enables designers to avoid excessive interaction between an elastomer and the fluids which it will contact in service.

Incompressibility

Another property of elastomers which distinguishes them from other solid materials is their incompressibility. For most practical purposes, other than use under very high pressures, elastomers do not change their volume significantly when deformed. A rubber band may stretch 600%, but if its volume were measured in the stretched state it would be found to be almost identical to its unstretched volume.

This has important implications for designing with elastomers as the stiffness of components can be controlled, not just by altering the stiffness of the rubber itself, but by various techniques of geometrical design in relation to the mounting. This phenomenon, known as shape factor effects, will be described in more detail in standard text books, and leads to great versatility in design. In particular it enables rubber components to be designed with different, and controlled, stiffnesses and other properties, in two or even three different directions.

Natural Rubber

Natural rubber is an excellent example of a natural polymer and an elastomer in particular. Elastomers are substances that can be readily stretched. They retract rapidly to their original form when released. It undergoes long range reversible extension under relatively small applied force. This elasticity makes it valuable for variety of uses. Natural rubber is also called plantation rubber.

Sub Topics

Preparation

Structure of Natural Rubber

Preparation

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Latex, the white milky liquid obtained by making a cut in the rubber tree contains 30%-40% of rubber and is a colloidal solution of rubber in water. This is coagulated (changed from fluid to solid state or clotting) with acetic or formic acid and can then be squeezed, rolled, milled and vulcanized.

Structure of Natural Rubber

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Natural rubber is a linear polymer of an unsaturated hydrocarbon called isoprene (2-methyl-1,3-butadiene). There may be as many as 11,000 to 20,000 isoprene units in a polymer chain of natural rubber. In this polymer, the residual double bonds are located between C2 and C3 of isoprene units in the polymer. All these double bonds have cis configuration and thus rubber is cis-1,4-polyisoprene.

Gutta-percha, a naturally occurring isomer of rubber contains double bonds which are trans.

In the structure of rubber, there are no polar substituents. Hence intermolecular attractions are mainly van der Waals interactions. These interactions are again weak because all cis configurations about double bonds do not let the polymer chains come close enough for effective attraction.

Thus the cis polyisoprene molecule is not a straight chain but has a coiled structure. Consequently, it can be stretched like a spring. On stretching the molecules becomes partially aligned with respect to each other and on releasing the force, the chain reverts back to its original coiled state.

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