Production process for polyethylene terephthalate (PET)

Production process for polyethylene terephthalate (PET)

Polyethylene terephthalate(PET) is a general-purpose plastic made through polycondensation of PTA with ethylene glycol (EG). This material has many outstanding properties: resistance to both heat and cold, transparency, electrical qualities, chemical proof and abrasion proof; For all these reasons, it is finding a wide variety of applications in the fibers, PET bottle and films.

Production Process for polyethylene terephthalate(PET)

Outline of flow

POLYESTER FIBERS

Polyester fiber is a " manufactured fiber in which the fiber forming substance is any long chain synthetic polymer composed at least 85% by weight of an ester of a dihydric alcohol (HOROH) and terephthalic acid (p-HOOC-C6H4COOH)". The most widely used polyester fiber is made from the linear polymer poly (ethylene terephtalate), and this polyester class is generally referred to simply as PET. High strength, high modulus, low-shrinkage, heat set stability, light fastness and chemical resistance account for the great versatility of PET.

 Applications

Because of their many desirable qualities, polyester fibers and fabrics have many uses.

Polyester is often used in outerwear because of its high tenacity and durability. It is a strong fiber and consequently can withstand strong and repetitive movements. Its hydrophobic property makes it ideal for garments and jackets that are to be used in wet or damp environments - coating the fabric with a water-resistant finish intensifies this effect.

Many jackets and quilted garments are made of polyester. Since polyester can be molded into almost any shape, certain insulating properties can be built-in to the fiber. One method is to create hollow fibers: 

This process traps air inside the fiber which is then heated by the body. The warm air stays inside and helps warm the body in cool weather. A second method is to use crimped polyester in a fiberfill product. The crimp helps to keep in warm air. Polyester is an ideal fiber to use for this since it will retain its shape, unlike its cotton and wool counterparts that tend to flatten out over time, significantly reducing their ability to trap warm air.

Polyester is used in pants, shirts, suits, and bed sheets either by itself or as a blend, because of its wrinkle-resistant property and its ability to retain its shape. Since these garments are frequently worn and washed, its stain-resistance and durability are also desirable.

Polyester also has industrial uses as well, such as carpets, filters, synthetic artery replacements, ropes, and films. 

Making Yarns

After the fiber itself is created, it is made into a yarn. There are two types of polyester yarns: filament and spun. 

Filament yarns are made by taking the single polyester filaments, grouping them together and then twisting or air-entangling them to make them workable. A monofilament yarn has just one, single polyester fiber that is usually not twisted.

Spun yarns are produced in much the same way as a cotton or wool yarn is produced. The long filaments are fist cut into short pieces called staple. These are then combined together and spun to create a yarn made up of thousands of short filaments.

Blends

At this stage, polyester can also be combined with other fibers to produce a variety of effects.
Polyester and cotton is probably the most famous and popular blend. The polyester helps the fabric retain its shape and resist stains and wrinkles. The cotton makes the fabric more absorbent and comfortable.

Polyester is combined with wool to give it wrinkle-resistance and shape retention in all kinds of weather. Since polyester is stronger than wool, it increases the durability and life of the fabric. The wool contributes good draping characteristics and elasticity.

Polyester and rayon is another popular blend fabric. Here again the polyester makes the fabric more resilient and durable, and helps it keep its shape. The rayon adds a different texture, has a good hand, is good for draping, and is more absorbent.

Polyester and nylon produce a strong fabric because of nylon's strength and abrasion resistance and polyester's wrinkle-free properties. This combination produces a yarn that is strong, durable, stable, easy to launder, and resistant to mildew and insects. Problems with this blend, however, are that pilling may occur, and it does not have a very nice hand. Furthermore, since neither nylon nor polyester is very absorbent, the fabric may feel wet and clammy in warm or humid weather.

Weaving

After the yarns are made, they are shipped out to textile mills to be woven into fabric. Polyester can be made into both woven and knitted fabrics (such as the infamous double-knit).

Finishing Processes

Finally, after the fabric is made, one or more of the following finishing processes is often used to improve the quality of the fabric: 

• Heat setting--creates a permanent shape 
• Singeing--improves the hand, reduces pilling, and increases smoothness 
• Anti-static finish--reduces static electricity 
• Water and stain repellency--increases comfort and makes it easier to clean; also used for rainwear 
• Resin finishes--increases ease of care 
• Calendaring--increases smoothness and reduces pilling 
• Embossing--creates a design and/or luster 

The Manufacturing
Process

Polyester is manufactured by one of several methods. The one used depends on the form the finished polyester will take. The four basic forms are filament, staple, tow, and fiberfill. In the filament form, each individual strand of polyester fiber is continuous in length, producing smooth-surfaced fabrics. In staple form, filaments are cut to short, predetermined lengths. In this form polyester is easier to blend with other fibers. Tow is a form in which continuous filaments are drawn loosely together. Fiberfill is the voluminous form used in the manufacture of quilts, pillows, and outerwear. The two forms used most frequently are filament and staple.

Manufacturing Filament Yarn

Polymerization

• 1 To form polyester, dimethyl terephthalate is first reacted with ethylene glycol in the presence of a catalyst at a temperature of 302-410°F (150-210°C).
• 2 The resulting chemical, a monomer (single, non-repeating molecule) alcohol, is combined with terephthalic acid and raised to a temperature of 472°F (280°C). Newly-formed polyester, which is clear and molten, is extruded through a slot to form long ribbons.

Drying

• 3 After the polyester emerges from polymerization, the long molten ribbons are allowed to cool until they become brittle. The material is cut into tiny chips and completely dried to prevent irregularities in consistency.
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Melt spinning

• 4 Polymer chips are melted at 500-518°F (260-270°C) to form a syrup-like solution. The solution is put in a metal container called a spinneret and forced through its tiny holes, which are usually round, but may be pentagonal or any other shape to produce special fibers. The number of holes in the spinneret determines the size of the yarn, as the emerging fibers are brought together to form a single strand.
• 5 At the spinning stage, other chemicals may be added to the solution to make the resulting material flame retardant, antistatic, or easier to dye.

Drawing the fiber

• 6 When polyester emerges from the spinneret, it is soft and easily elongated up to five times its original length. The stretching forces the random polyester molecules to align in a parallel formation. This increases the strength, tenacity, and resilience of the fiber. This time, when the filaments dry, the fibers become solid and strong instead of brittle.
• 7 Drawn fibers may vary greatly in diameter and length, depending on the characteristics desired of the finished material. Also, as the fibers are drawn, they may be textured or twisted to create softer or duller fabrics.

Winding

• 8 After the polyester yarn is drawn, it is wound on large bobbins or flat-wound packages, ready to be woven into material.
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Manufacturing Staple Fiber

In making polyester staple fiber, polymerization, drying, and melt spinning (steps 1-4 above) are much the same as in the manufacture of filament yarn. However, in the melt spinning process, the spinneret has many more holes when the product is staple fiber. The rope-like bundles of polyester that emerge are called tow.

Drawing tow

• 1 Newly-formed tow is quickly cooled in cans that gather the thick fibers. Several lengths of tow are gathered and then drawn on heated rollers to three or four times their original length.

Crimping

• 2 Drawn tow is then fed into compression boxes, which force the fibers to fold like an accordion, at a rate of 9-15 crimps per inch (3-6 per cm). This process helps the fiber hold together during the later manufacturing stages.

Setting

• 3 After the tow is crimped, it is heated at 212-302°F (100-150°C) to completely dry the fibers and set the crimp. Some of the crimp will unavoidably be pulled out of the fibers during the following processes.

Cutting

• 4 Following heat setting, tow is cut into shorter lengths. Polyester that will be blended with cotton is cut in 1.25-1.50 inch (3.2-3.8 cm) pieces; for rayon blends, 2 inch (5 cm) lengths are cut. For heavier fabrics, such as carpet, polyester filaments are cut into 6 inch (15 cm) lengths.