Polyethylene Terephthalate (PET): Synthesis, Crystallization & Bottle Processing

Polyethylene Terephthalate (PET) is the premier material for beverage bottling, valued for its high clarity, carbon dioxide barrier, and mechanical toughness. Producing PET bottles requires two stages: injection moulding a preform and then stretch blow moulding (ISBM). Understanding PET polymer synthesis, crystallization kinetics (maintaining high transparency by preventing large spherulite growth), and blow moulding parameters is a core skill for rigid packaging engineers.

• Explain the polymerization chemistry of PET (esterification of TPA and EG).
• Analyze PET crystallization kinetics and the influence of cooling rate on optical clarity.
• Describe the Injection Stretch Blow Moulding (ISBM) process for bottle fabrication.
• Solve blow-up ratio (BUR) and hoop/axial stretch ratio calculations for bottle design.
• Reference international PET testing standards.

3.1 PET Polymerization Chemistry

PET is synthesized via a step-growth esterification reaction between Terephthalic Acid (TPA) and Ethylene Glycol (EG), followed by polycondensation under high vacuum to remove water/EG and build molecular weight ($M_n \approx 20,000 - 30,000$ g/mol). To build higher molecular weights for carbonated bottles, solid-state polymerization (SSP) is performed under inert gas.

3.2 Crystallization Kinetics & Transparency

PET is a semi-crystalline polymer. Slow cooling allows chains to fold into dense spherulites, causing light scattering (opacity). To make crystal-clear bottles, the injection-molded preform must be quenched rapidly in water-cooled moulds (cooling rate $> 100^\circ$C/min) to freeze the polymer in its amorphous state.

3.3 Injection Stretch Blow Moulding (ISBM)

1. Preform Molding: An amorphous preform (test-tube shaped with threaded neck) is injection molded.
2. Reheating: The preform is heated above its glass transition temperature ($T_g \approx 75 - 80^\circ$C) in the ISBM machine.
3. Stretching & Blowing: A stretch rod extends axially to stretch the preform, while high-pressure compressed air (4.0 MPa) expands it radially against the bottle mold. This biaxial orientation induces strain-induced crystallization, increasing bottle strength.

Problem: A packaging engineer designs a 1.0-liter carbonated drink bottle using an injection-molded preform. The design parameters are:

• Preform body outer diameter $D_p = 25.0$ mm and thickness $t_p = 3.5$ mm.
• Preform length below the neck $L_p = 100.0$ mm.
• Final bottle outer diameter $D_b = 82.0$ mm.
• Final bottle length below the neck $L_b = 280.0$ mm. Calculate:

1. The Hoop Stretch Ratio (HSR).
2. The Axial Stretch Ratio (ASR).
3. The Total Blow-Up Ratio (Total Stretch Ratio, TSR). Verify if it falls within the optimum range for PET ($10.0 - 16.0$ for strain-hardening).

Solution:

1. Calculate the Hoop Stretch Ratio (HSR) based on mid-thickness diameters ($D_{p,mid} = D_p - t_p = 25.0 - 3.5 = 21.5$ mm):

1. Calculate the Axial Stretch Ratio (ASR):

1. Calculate the Total Stretch Ratio (TSR):

Interpretation: The design yields a TSR of 10.67, which falls inside the optimum range of 10.0–16.0. This ensures that the PET chains undergo sufficient biaxial orientation and strain-induced crystallization, providing high burst pressure resistance and top-load strength.

Indian bottle converters (e.g., Pearl Polymers) operate high-speed rotary ISBM lines. They purchase bottle-grade PET chips (e.g., from Reliance Industries - Relpet) and monitor the preform heating zone infrared profiles to prevent haze (micro-crystallization) in the bottles.

• ISBM: Injection Stretch Blow Moulding; two-stage process producing oriented hollow plastic containers.
• SSP: Solid-State Polymerization; process raising polymer molecular weight by heating prepolymer below its melting point in nitrogen.
• Hoop Stretch Ratio: Radial expansion factor of the preform.
• Axial Stretch Ratio: Longitudinal stretch factor of the preform.
• TSR: Total Stretch Ratio; product of hoop and axial stretch ratios.

1. IS 12257 — Indian standard specification for PET bottles for packaging drinking water
2. IS 14537 — Code of practice for testing PET containers used for food packaging

1. Explain the thermodynamic difference between thermal crystallization (spherulite growth) and strain-induced crystallization in PET.
2. Why does water absorption in PET chips prior to injection moulding cause degradation? Write the hydrolytic cleavage reaction.
3. A preform with a wall thickness of 4.0 mm is molded. Calculate the minimum cooling rate required to keep the preform fully amorphous.