General Overview Of Polyesters Biology Essay

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Polyester (aka Terylene) is a category of polymers which contain the ester functional group in their main chain. Although there are many forms of polyesters, the term 'polyester' is most commonly used to refer to polyethylene terephthalate (PET). Other forms of polyester include the naturally-occurring cut in of plant cuticles as well as synthetic polyesters such as polycarbonate and polybutyrate. Polyester may be produced in numerous forms. For example, polyester as a thermoplastic may be heated and processed into different forms and shapes, e.g.: fibers, sheets and three-dimensional shapes. While combustible at high temperatures, polyester tends to shrink away from flames and self-extinguishes. .

Aliphatic polymers are the most used biodegradable polymers in medical applications and have been extensively investigated in the past. Ester linkages are frequently encountered in nature and hence it is expected that synthetic polymers containing such linkages and an appropriate structure would be environmentally degradable.

The superior performance of natural polymers can be attributed to their multi functionality. Separated components have more than one function ,as is evident in wood where a synergistic combination of fiber ( cellulose) ,hemi cellulose ,and lignin ( matrix ) leads to a strong , light weight materials.

On the basis of chemical structure polyesters can be a thermoplastic polyesters polyesters or thermoset polyester 'in which thermoplastics polyesters are the most common.

Polyesters are also used to make 'plastic' bottles, films, canoes, filters, etc.

Polyesters are widely used as a finish on high quality wood products such as guitar, pianos and vehicles cured polyesters can be sanded and polished to a high gloss, durable finish.

GENERAL PROPERTIES

Properties of polyester fibers are strongly affected by fiber structure. The fiber structure, which has a strong influence on the applicability of the fiber, depends heavily on the process parameters of fiber formation such as spinning speed (threadlike stress), hot drawing (stretching), stress relaxation and heat setting (stabilization) speed.

A the spinning threadlike is increased by higher wind-up speed, s the stress inthe PET molecules are extended, resulting in better as-spun uniformity, lower elongation and higher strength, greater orientation and high crystalline. Hot drawing accomplishes the same effect and allows even higher degrees of orientation and crystalline. Relaxation is the releasing of strains and stresses of the extended molecules, which results in reduced shrinkage in drawn fibers. Heat stabilization is the treatment to "set" the molecular structure, enabling the fibers to resist further dimensional changes. Final fiber structure depends considerably on the temperature, rate of stretching; draw ratio (degree of stretch), relaxation ratio and heat setting condition. The crystalline and non crystalline orientation and the percentage of crystalline can be adjusted significantly in response to these process parameters.

As the degree of fiber stretch is increased (yielding higher crystallinity and molecular orientation), so are properties such as tensile strength and initial Young's modulus. At the same time, ultimate extensibility, i.e., elongation is usually reduced. An increase of molecular weight further increases the tensile properties, modulus, and elongation. Typical physical and mechanical properties of PET fibers are given in Table. And stress-strain curves in Fig. 1. It can be seen that the filament represented by curve C has a much higher initial modulus than the regular tenacity staple shown in curve D. On the other hand, the latter exhibits a greater tenacity and elongation. High tenacity filament and staple (curve A and B) have very high breaking strengths and module, but relatively low elongations. Partially oriented yarn (POY) and spun filament yarns, exhibit low strength but very high elongation (curve E). When exposing PET fiber to repeated compression (for example, repeated bending), so-called kink bands start to form, finally resulting in breakage of the kink band into a crack. It has been shown that the compressibility stability of PET is superior to that of nylons.

PHYSICAL PROPERTIES:

The physical properties of polyesters are described here on various conditions as given below in table as follows:

Filament yarn

Staple and tow

Property

Regular tenacity

High tenacity

Regular tenacity

High tenacity

breaking tenacity N/tex

0.35-0.5

0.62-0.85

0.35-0.47

0.48-0.61

breaking elongation

24-50

10-20

35-60

17-40

elastic recovery at 5% elongation, %

88-93

90

75-85

75-85

initial modulus, N/texf

6.6-8.8

10.2-10.6

2.2-3.5

4.0-4.9

specific gravity

1.38

1.39

1.38

1.38

Moisture regian, %

0.4

0.4

0.4

0.4

Melting temperature

258-263

258-263

258-263

258-263

Regular staple for 100% polyester fabrics, carpet yarn, fiberfill, and blends with celluloses blends or wool.

High strength, high modulus staple for industrial applications, sewing thread, and cellulosic blends.

Standard extile -filament yarns for woven and knit fabrics. Tire cord and high strength, high modulus industrial yarns.

measurements are conducted in air at 65% rah and 22oC.fTo convert N/text to ge /den, multiply by 11.33.gThe equilibrium moisture content of the fibers at 21oC and 65% rh.

Fig 1: Typical stress strain curve for PET fibers.

A: high tenacity filament

B: high tenacity staple

C: regular tenacity filament

D: regular tenacity staple

E: poi filament

EFFECTS OF VARIOUS FACTORS ON POLYESTERS

Shrinkage varies with the mode of treatment. If relaxation of stress and strain in the oriented fiber is allowed to occur through shrinkage during fiber manufacture, then shrinkage at the textile processing stage is reduced and initial modulus is lowered. Polyester yarns held to a fixed length under tension during heat treatment are less affected with change in modulus, and reduced shrinkage values can still be obtained. This is very important in fiber stabilization. PET shows nonlinear and time-dependent elastic behavior. It recovers well from stretch, compression, bending, and shears because of its relatively high initial modulus. Extensional creep occurs under load, with subsequent delay in recovery upon removal of the load. But compared with other melt-spun fibers, the creep is small.

HYDROLYSIS OF POLYESTERS

Simple esters are easily hydrolysed by reaction with dilute acids or alkalis.

are your attacked readily by alkalis, but much more slowly by dilute acids. Hydrolysis by water alone is so slow as to be completely unimportant. (You wouldn't expect polyester fleece to fall to pieces if you went out in the rain!)

If you spill dilute alkali on a fabric made from polyester, the ester linkages are broken. Ethane-1, 2-diol is formed together with the salt of the carboxylic acid.

Because Polyesters you produce small molecules rather than the original polymer, the fibres are destroyed, and you end up with a hole!

For example, if you react the polyester with sodium hydroxide solution:

Hydrolysing simple esters:

Technically, hydrolysis is a reaction with water. That is exactly what happens when esters are hydrolysed by water or by dilute acids such as dilute hydrochloric acid.

The alkaline hydrolysis of esters actually involves reaction with hydroxide ions, but the overall result is so similar that it is lumped together with the other two.

Hydrolysis using water or dilute acid:

The reaction with pure water is so slow that it is never used. The reaction is catalysed by dilute acid, and so the ester is heated under reflux with a dilute acid like dilute hydrochloric acid or dilute sulphuric acid.

Here are two simple examples of hydrolysis using an acid catalyst.

First, hydrolysing ethyl ethanoate:

. . . and then hydrolysing methyl propanoate:

Hydrolysis using dilute alkali

This is the usual way of hydrolysing esters. The ester is heated under reflux with a dilute alkali like sodium hydroxide solution.

There are two big advantages of doing this rather than using a dilute acid. The reactions are one-way rather than reversible, and the products are easier to separate.

First, hydrolysing ethyl ethanoate using sodium hydroxide solution:

And then hydrolysing methyl propanoate in the same way:

Hydrolysing complicated esters to make soap

If the large esters present in animal or vegetable fats and oils are heated with concentrated sodium hydroxide solution exactly the same reaction happens as with the simple poly esters.

A salt of a carboxylic acid is formed - in this case, the sodium salt of a big acid such as octadecanoic acid (stearic acid). These salts are the important ingredients of soap - the ones that do the cleaning.

An alcohol is also produced - in this case, the more complicated alcohol, propane-1,2,3-triol (glycerol).

Because of its relationship with soap making, the alkaline hydrolysis of esters is sometimes known as saponification.

Polyester formation and polyester fiber:

Esters are usually prepared by the reaction of alcohols or phenols with acids or acid derivatives. Following are the common methods:

FROM ACID:

1) Acids are frequently converted into their esters via acid chloride

acid acid chloride ester

A carboxylic acid is converted directly into ester when heated with an alcohol in the presence of a little mineral acid, usually concentrated sulphuric acid or dry chloride.

This reaction is reversible, and generally reaches equilibrium when there are appreciable quantities of both reactants and products present.

Acid alcohol ester

2) From acid chloride or acid anhydride:

3) From ester transification:

Esterification using aromatic acid chloride, ArCOCl, is often carried out in presence of base.

A hydroxyl acid is both alcohol and acid. In those cases where a five member ring can be formed, intramolecular esterification occurs.Thus ,a γ and δ-hydroxy acid loses water spontaneously to yield a cyclic ester known as a lactones.

-

γ- hydroxy acid A-γ-lactane

REACTIONS OF POLYESTERS

Esters undergo the nucleophilic substitution that is typical of carboxylic acid derivatives. Attack occurs at the electron-deficient carbonyl carbon, and results in the replacement of the-OR group by -OH,-OR'', or NH2

.

1) RCOOR' + :Z → + :OR'-

These reactions are sometimes carried out in the presence of acid. In these acid-catalyzed reactions, hydrogen ion attaches itself to the oxygen of the carbonyl group, and thus renders carbonyl carbon even more susceptible to nucleophilic attack.

2) RCOOR' + H+ R-

Acid catalysis makes carbon more susceptible to nucleophilic attack.

AMMONOLYSIS OF ESTERS

When ester is treated with ammonia, generally in ethyl alcohol solution, it gives amide. This reaction involves nucliophilic attack by a base, ammonia, on the electron deficient carbon; the alkoxy group,-OR',is replaced by NH2.

Examples:

CH 3COOC 2H5 + NH3 → CH 3CONH2 + C2 H 5OH

Ethyl acetate Acetamide

TRANSESTERIFICATION:

In esterification of an acid, an alcohol acts as a nucleophilic reagent; in hydrolysis of ester, an alcohol is displaced by a nucleophilic reagent. Hence one alcohol is capable of displacing another alcohol from an ester. This alcoholysis (cleavage by an alcohol) of an ester is called transesterification.

RCOOR ' + R"OH RCOOR" + R'OH

Transesterification is catalyzed by acid ( H2SO4 or HCl ) or base (usually alkoxide).

REACTION OF ESTERS WITH GRIGNARD REAGENTS:

Reaction of carboxylic esters with Grignard reagents is an excellent method for preparing tertiary alcohols.

RCOOR' [R-COR"] R-CR"OMgXR" RR"COHR"

+

R'OMgX

Ester 3â-¦ alcohol

REDUCTION OF ESTERS

Esters can be reduced in two ways:

a) By catalytic hydrogenation using molecular hydrogen,

b) By chemical reduction.

In either case, the ester is cleaved to yield a primary alcohol corresponding to the acid portion of the ester.

RCOOR' R-CH2OH + R'OH

Ester 1â-¦ alcohol

Hydrogenolysis of ester:

Hydrogenolysis (cleavage by hydrogen) of an ester requires more severe conditions than simple hydrogenation of a carbon-carbon double bond. High pressure and elevated

temperatures are required. The catalyst used most often is a mixture of oxides known as copper chromites, of approximately composition CuO.CuCr2O4. Example:

CH3(CH2)10COOCH3 CH3(CH)10CH2OH + CH3OH

Methyl laurate Laury alcohol

(Methyl dodecanoate) (1-Dodecanol)

Chemical reduction is carried out by use of sodium metal and alcohol or more usually by use of lithium aluminum hydride.

Example:

CH3 (CH2) 14COOC2H5 CH3 (CH2)14CH2OH

Ethyl palmitate 1-Hexadecanol

(Ethyl hexadecanoate)

HYSTORY OF POLYESTERS:

Polyester was a term given its generation in WH .Caother's laboratory. Carothers discovered that alcohols and carboxylic acids could be successfully combined to form fibers while working for du Pont.

Work of Carother's was used by a group of British scientists -J.R.Whinfild, J.T Dickson, W.K. Britwhistle and C.G. Ritchie. They created the first polyester fiber called Terylene 1939. Du Pont bought all legal rights from British in 1945 and came up with another polyester fiber which then named Dacron.

Polyester was first introduced to the American public in 1951.It was advertised as a miracle fiber that could be worn for 68 days straight without ironing and still look presentable.

In 1958 another polyester fiber celled Kodel was developed by Eastman Chemical Product.

The polyester market kept expanding since it was such an inexpensive durable fiber, amny small textile mills emerged all over the country -many located in old gas stations to produce cheap polyester apprel items.

Polyesters experienced a constant growth until the 1970 when scale drastically declined due to negative public image that emerged in the late 60s as a result of infamous polyesters doubt knit fabric.

Today polyesters are still widely regarded as a cheap, uncomfortable fiber, but even now this image is slowly beginning to change with the emergence of polyester luxury fibers such as polyester microfiber.

pgb4598 - Fotosearch Stock Photography

Fig:2

APPLICATIONS OF POLYESTERS:

Polyesters are basically used is various given purposes….

Abrasive cleaning material

Acoustic barriers

Adhesive

Aerospace and agricultural application

Aircraft application(interior and exterior)

Automobile applications, electronics exterior part.

Bag, bearing, bottles, agricultural chemicals.

Business equipment, calculators, camera applications, carpet banking, coating application, cosmetics, decorative pursues.

Display, electron, eyeglasses, fibers, film, cast, furniture.

INDUSTRIAL IMPORTANCE:

Industries find various uses of polyesters and their applications

Fig:3

.Infactmost of the industry related to fabric work is completely dependent on uses of polyesters and its utility various uses are given as follows:

Fig:4

The automobile industry uses reactive polyester in the form of sheet molding compounds(SMC) to mold exterior panels-lift gates, doors, side panels-for the General Motors mini-vans (Lumina APV, Silhouette and Transport). Exterior panels are also made for Camaro, Firebird and Corvette.

The word fiberglass refers of any substance composed of glass fibers, usually held in a polymers resin matrix (base). However, fiberglass has also come to mean any similar fiber based material, with fibers of various different substances. This material can be made to a variety of flexibilities, and is characterized by its strength and resistance to heat. It can also be made so that it is resistance to certain chemicals and to U.V. light, with these variations due to original monomer mixture chosen.

EFECTS OF POLYESTER ON ENVIRONMENT:

Fig:5

Polyester production or industrial manufacturing, like others stuff cause threat to the environment as there are by-products from the reaction obtained may cause great harm to the surrounding even under the most controlled conditions. So, careful treatment should be done to distillation and all other gases before their disposal in the surrounding. The gases are released into the atmosphere after scrubbing, which removes from the gas stream all the organic vapours that have been produced. The distillates are neutralized, and particulate matter is allowed to settle out before the liquid is disposed of into trades' waste drain.

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