An Overview of Polyester and Polyester Dyeing

An Overview of Polyester and Polyester Dyeing (Part-1)
Ardhendu Shekhar Paul
B.Sc. in Textile Engineering.
Specialization in Wet Processing Technology
Primeasia University, Banani, Dhaka.

History of Polyester:
Polyester began as a group of polymers in W.H. Carothers' laboratory. Carothers was working for duPont at the time when he discovered that alcohols and carboxyl acids could be successfully combined to form fibers. Polyester was put on the back burner, however, once Carothers discovered nylon. A group of Brittish scientists--J.R. Whinfield, J.T. Dickson, W.K. Birtwhistle, and C.G. Ritchie--took up Carothers' work in 1939. In 1941 they created the first polester fiber called Terylene. In 1946 duPont bought all legal rights from the Brits and came up with another polyester fiber which they 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 called Kodel was developed by Eastman Chemical Products, Inc. The polyester market kept expanding. Since it was such an inexpensive and durable fiber, amny small textile mills emerged all over the country--many located in old gas stations--to produce cheap polyester apprel items. Polyester experienced a constant growth until the 1970s when sales drastically declined due to the negative public image that emerged in the late 60s as a result of the infamous polyester double-knit fabric!

A polyester is a polymer (a chain of repeating units) where the individual units are held together by ester linkages.

The diagram shows a very small bit of the polymer chain and looks pretty complicated. But it isn't very difficult to work out - and that's the best thing to do: work it out, not try to remember it. You will see how to do that in a moment.

The usual name of this common polyester is poly(ethylene terephthalate). The everyday name depends on whether it is being used as a fibre or as a material for making things like bottles for soft drinks.

When it is being used as a fibre to make clothes, it is often just called polyester. It may sometimes be known by a brand name like Terylene.

When it is being used to make bottles, for example, it is usually called PET.

Polyesters as thermoplastics may change shape after the application of heat. While combustible at high temperatures, polyesters tend to shrink away from flames and self-extinguish upon ignition. Polyester fibers have high tenacity and E-modulus as well as low water absorption and minimal shrinkage in comparison with other industrial fibers. Unsaturated polyesters (UPR) are thermosetting resins. They are used as casting materials, fiberglass laminating resins and non-metallic auto-body fillers. Fiberglass-reinforced unsaturated polyesters find wide application in bodies of yachts and as body parts of cars.

Monomers: Functional Groups
The monomers that are involved in condensation polymerization are not the same as those in addition polymerization. The monomers for condensation polymerization have two main characteristics:.
  • Instead of double bonds, these monomers have functional groups (like alcohol, amine, or carboxylic acid groups).  
  • Each monomer has at least two reactive sites, which usually means two functional groups.
Some monomers have more than two reactive sites, allowing for branching between chains, as well as increasing the molecular mass of the polymer. Four examples of these difunctional monomers were introduced in Part 2 of this tutorial. Here they are again:

Guess the names of each of these monomers. Give the letter that corresponds to the correct name of the structure (use each letter only once). Hints: Glycol means that a molecule has more than one alcohol (-OH) group. Amine means that a molecule has an amino (-NH2) group. Diamine (or diamino) means that a molecule contains two amino groups. Acid means that a molecule contains a carboxylic acid group (-COOH). Click the button when done.

Let's look again at the functional groups on these monomers. We've seen three:
  1. The carboxylic acid group
  2. The amino group (R-NH2)
  3. The alcohol group (R-OH)
The Amide Linkage:
When a carboxylic acid and an amine react, a water molecule is removed, and an amide molecule is formed.

Because of this amide formation, this bond is known as an amide linkage.

The Ester Linkage:
When a carboxylic acid and an alcohol react, a water molecule is removed, and an ester molecule is formed.

Because of this ester formation, this bond is known as an ester linkage.

In Summary:
Monomers involved in condensation polymerization have functional groups. These functional groups combine to form amide and ester linkages. When this occurs, a water molecule in removed. Since water is removed, we call these reactions condensation reactions (water condenses out.). When a condensation reaction involves polymerization, we call it condensation polimarization.

Let's look at a few common examples of condensation polymers.

The Mechanism of Condensation Polymerization:
We know that monomers that are joined by condensation polymerization have two functional groups. We also know that a carboxylic acid and an amine can form an amide linkage, jand a carboxylic acid and an alcohol can form an ester linkage. Since each monomer has two reactive sites, they can form long-chain polymers by making many amide or ester links. Let's look at two examples of common polymers made from the monomers we have studied.

Example 1:
A carboxylic acid monomer and an amine monomer can join in an amide linkage.

As before, a water molecule is removed, and an amide linkage is formed. Notice that an acid group remains on one end of the chain, which can react with another amine monomer. Similarly, an amine group remains on the other end of the chain, which can react with another acid monomer.

Thus, monomers can continue to join by amide linkages to form a long chain. Because of the type of bond that links the monomers, this polymer is called a polyamide. The polymer made from these two six-carbon monomers is known as nylon-6,6. (Nylon products include hosiery, parachutes, and ropes.)

Example 2:
A carboxylic acid monomer and an alcohol monomer can join in an ester linkage.

A water molecule is removed as the ester linkage is formed. Notice the acid and the alcohol groups that are still available for bonding. ( )

Because the monomers above are all joined by ester linkages, the polymer chain is a polyester. This one is called PET, which stands for poly(ethylene terephthalate). (PET is used to make soft-drink bottles, magnetic tape, and many other plastic products.)

Let's summarize:
As difunctional monomers join with amide and ester linkages, polyamides and polyesters are formed, respectively. We have seen the formation of the polyamide nylon-6,6 and the polyester PET. There are numerous other examples.

The above process is called condensation polymerization because a molecule is removed during the joining of the monomers. This molecule is frequently water.

A Simulation of Condensation Polymerization:
During the polymerization process, the monomers tend to form dimers (two linked monomers) and trimers (three linked monomers) first. Then, these very short chains react with each other and with monomers. The overall result is that, at the beginning of polymerization, there are many relatively short chains. It is only near the end of polymerization that very long chains are formed.

Polymerize into dimers, trimers, and so on, the monomers will turn black. Polymerization will continue for a few seconds. Then the display will change into a bar graph entitled "Distribution" and show the progression of the polymerization over time. The x-axis is the number of units in the polymer (the "n" in the formula of a polymer). This is suggested graphically with the series of polymers projected into the screen. As we move to the left, the polymers are longer. The y-axis is the number of polymers. The higher the bar, the more numerous are the polymers. The graph shows dynamically the distribution of polymers in the polymerization as the reaction progresses. Notice that at the beginning of the polymerization, the distribution lies farther to the right, meaning that there are a lot of monomers, dimers, trimers, and other short chains but few long chains. As the polymerization progresses, the distribution shifts to the left, indicating that there are fewer short chains and more of the longer ones.

Some Assumptions:
First, we assume that there is only one type of difunctional monomer, as opposed to two types, as we saw in the two examples in Part 7. If we imagine that the polymers in the simulation are polyamides (like nylon-6,6), then the monomer has one carboxylic acid group and one alcohol group (picture the dimer you saw in Example 1 in the previous section). Second, we assume that there are only 90,000 monomers in the polymerization. In real life, the number of monomers is on the order of 1023. Despite the low number of monomers in the simulation, it does show the correct, real-life distribution of polymer chains over time. 

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About the Editor-in-Chief:

Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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