Raw Materials and Processing of PET Resin for Preform Manufacturing

1. The Base Raw Material

The primary raw material is PET resin (Polyethylene Terephthalate), a thermoplastic polyester.

Chemical building blocks:

Purified Terephthalic Acid (PTA)

Chemical formula: C₆H₄(COOH)₂

A white crystalline powder derived mainly from p-xylene (petroleum-based).

Gives PET its rigidity and barrier properties.

Monoethylene Glycol (MEG)

Chemical formula: C₂H₆O₂

A colorless, syrupy liquid (also from petroleum, via ethylene).

Provides flexibility to the polymer chain.

Polymerization Reaction:

Step 1 – Esterification / Transesterification: PTA reacts with MEG to form Bis(2-hydroxyethyl) terephthalate (BHET).

Step 2 – Polycondensation: BHET molecules join into long chains, releasing water or ethylene glycol as a byproduct.

This produces PET polymer chains.

2. PET Resin Form

After polymerization, PET is solidified and cut into small transparent pellets or granules for easier handling.

Intrinsic Viscosity (IV):
PET resin for preforms typically has IV between 0.76 – 0.86 dl/g, determining molecular weight and mechanical strength.

3. Additives in PET Resin

PET resin for preforms often contains additives to enhance properties:

4. Drying Before Processing

PET resin is hygroscopic (absorbs moisture from air).
If processed with moisture inside, hydrolysis will occur → molecular chain breaks → weak preforms.

Drying conditions:

Temp: 160–180 °C

Time: 4–6 hours

Final moisture content: ≤ 50 ppm

5. From Resin to Preform

Process: Injection molding

PET pellets are fed into a hopper

Melted at ~270–280 °C

Injected into a steel mold shaped like the preform

Cooled rapidly to retain strength

The preform then looks like a test tube with a bottle cap thread — ready for stretch blow molding.

In summary:

The raw material of PET preforms is PET resin pellets made from PTA + MEG via polymerization, sometimes with additives like UV absorbers, colorants, and reheat agents. Before use, the resin must be dried, then injection molded into preforms.