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.

Q1. Which chemical process is used to increase the molecular weight of PET prepolymer to bottle-grade limits under nitrogen?

• C) Solid-State Polymerization (SSP)

Q2. To maintain high transparency in injection-molded preforms, the PET melt must be:

• B) Quenched rapidly to prevent spherulitic crystallization

Q3. The burst-pressure resistance of PET carbonated beverage bottles is primarily achieved through:

• B) Strain-induced crystallization during biaxial stretching

Q4. A preform with a mid-diameter of 20 mm is blown into an 80 mm bottle. What is the Hoop Stretch Ratio?

• B) 4.0

Q5. Which standard specifies Indian safety requirements for PET packaging in contact with drinking water?

• B) IS 12257