Why is the PET SSP system needed in bottle-to-bottle recycling programs?

What is SSP PET?

SSP (Solid State Polycondensation) is an advanced polymer modification technology used to increase the molecular weight and Intrinsic Viscosity (IV) of recycled PET (rPET).

SSP involves a polycondensation reaction conducted below the melting point of PET (typically 190-230°C) under vacuum or inert gas (e.g., Nitrogen) purging. Its purpose is to restore the polymer's properties through chemical reaction without melting it.

How does SSP work?

The working principle of SSP is based on two levels: chemical polycondensation and physical purification:

Chemical IV Increase (Viscosity Lift):

Its chemical principle is the same as melt polycondensation: increasing the molecular chain length through end-group reactions (esterification and transesterification) and removing reaction by-products (mainly water and ethylene glycol, EG). In an environment of high temperature (but below the melting point), high vacuum, or inert gas protection, the end-groups of PET (-COOH and -OH) react again, reconnecting the broken molecular chains, thereby increasing the Intrinsic Viscosity (IV).

Deep Decontamination:

Besides increasing IV, the main commercial driver for the SSP process is its powerful decontamination function. Under the combined effect of high temperature, high vacuum (or high inert gas flow), and long residence time (several hours), SSP can extremely effectively remove volatile and semi-volatile organic compounds (VOCs) from rPET.

These contaminants include: 1) By-products from polymer degradation, such as Acetaldehyde (AA); 2) Flavor substances, cleaning agents, etc., that migrated into the bottle wall during consumer use. The European Food Safety Authority (EFSA) and the U.S. Food and Drug Administration (FDA) have consistently identified the SSP step as the "critical control step" in their safety assessments for determining the decontamination efficiency of a process. It is precisely the SSP process that enables rPET to meet "food-grade" standards, allowing it to be safely used for new food and beverage packaging.

Molecular Degradation Mechanism of rPET during Melt Pelletizing

1. PET is a polycondensate, and its ester bonds are highly sensitive to hydrolysis, especially in a high-temperature melt state (typically >260°C). Even dried rPET flakes often contain residual moisture. During melt extrusion, water molecules attack the main-chain ester bonds of PET, causing chain scission and generating a new carboxyl end-group (-COOH) and a hydroxyl end-group (-OH). Every chain scission event reduces the molecular weight (and IV). More seriously, this process is auto-catalytic: the newly formed -COOH groups significantly accelerate the rate of subsequent hydrolysis reactions.

2. Thermal & Thermo-oxidative Degradation: Under the high temperature and shear action in an extruder, PET undergoes thermal degradation (chain scission) even without water. In the presence of oxygen (thermo-oxidative degradation), the degradation is exacerbated. This complex series of free-radical reactions produces Acetaldehyde (AA) and vinyl ester end-groups, leading to the formation of conjugated double bonds on the macromolecular chain. These conjugated systems are strong chromophores, which absorb visible light, causing irreversible yellowing of the polymer.

3. Chain Reaction and Consequences of Degradation: It must be recognized that the damage caused by the pelletizing step is manifold and interrelated. First, hydrolysis generates -COOH groups that catalyze further degradation. Second, thermal degradation creates permanent yellow chromophores. Importantly, Solid State Polycondensation (SSP) is a process designed to "repair" the IV through polycondensation, removing EG and water to join -COOH and -OH end-groups. However, SSP cannot repair the conjugated double bond chromophores created by thermal degradation. Therefore, the yellowing (b* = 4.91) generated during the pelletizing step is permanent.

Modern industrial recycling has added pre-crystallization drying equipment and dehumidifying equipment to mitigate the above situations.

In-depth Process Analysis: Comparative Assessment of SSP Feedstock (Flakes vs. Pellets)

Based on the molecular degradation mechanisms and SSP kinetics described above, we can conduct a rigorous comparative assessment of the two process routes.

Route 1: SSP after Pelletizing (Pellets + SSP)

Common in "Bottle-to-Bottle" projects

Process Flow: This is the standard process in the industry for producing "super-clean" food-grade rPET. [Flakes] -> -> -> [Melt Extrusion/Filtration/Pelletizing] -> [Pellet Crystallization] -> [Finished Pellets]

Advantage Analysis:

• Deep Decontamination: This process has a dual decontamination effect. The melt extrusion step (usually with vacuum degassing) removes the first batch of VOCs. The subsequent SSP step not only removes residual VOCs but also removes newly generated degradation products (like AA) from the extrusion process. This is the core reason it is favored by EFSA/FDA for food-grade applications.

• Product Uniformity: The product consists of pellets with uniform size, shape, and density. These pellets are easy to store, transport, mix, and feed, ensuring stability in subsequent processes like injection molding.

Disadvantage Analysis (Validating Initial Findings):

• Irreversible Color Damage: As mentioned in the previous section, the intermediate melt extrusion step "bakes in" thermal degradation, causing the b* value to soar. SSP cannot repair this yellowing.

• High Cost: The initial conclusion "recycling cost is high" is completely accurate. This route is a "process multiplier": it requires two crystallization steps (once for flakes, once for pellets), two heating steps, and one high-energy-consumption melt extrusion. This leads to extremely high Capital Expenditure (CAPEX) and Operating Expenditure (OPEX).

• Slow SSP Kinetics: The low IV after pelletizing and the low specific surface area of the pellets lead to slow SSP reaction kinetics, requiring longer reaction times and further increasing energy consumption.

Route 2: Direct SSP for Flakes (Flakes + Direct SSP)

Common in chemical fiber projects

Process Flow: This is a more streamlined "flake-to-application" process. [Flakes] -> -> [High IV Flakes] ->

Advantage Analysis (Validating Initial Findings):

• Superior Color Quality: This is the most significant advantage of this process. By completely skipping the intermediate melt pelletizing step, thermal-oxidative degradation and yellowing in the molten state are thoroughly avoided. SSP is a much milder treatment than melt extrusion.

• Fast SSP Kinetics: A high starting IV and the high specific surface area of the flakes work together to maximize the reaction rate.

• Extremely Low Cost: The initial conclusion "recycling and viscosity-increasing cost is low" is the core economic driver of this process. This route eliminates the entire extrusion-pelletizing line (extruder, melt filter, underwater pelletizer, water circulation system, pellet dryer). This provides massive savings in CAPEX 45 and OPEX (mainly the electricity consumption of melt extrusion).

Disadvantage Analysis (Addressing Initial Challenges):

• Non-uniform Flake Appearance (Core Challenge): The "non-uniform flake appearance, leading to slightly poorer internal quality uniformity between large and small particles" mentioned in the initial conclusions is a key technical challenge. Flakes have a low and non-uniform bulk density and poor flowability. This is critical in large-scale production, leading to difficulties in subsequent handling (like feeding) and non-uniform SSP reactions.

• Solid Impurities: Route 1 removes solid impurities (like residual paper, glue, PP/PE) 16 via Melt Filtration. Route 2 does not have this step. Therefore, the success of Route 2 is entirely dependent on the high-purity flakes provided by the upstream washing and sorting processes.

Comprehensive Assessment of SSP Process Routes