PET Polymer Chemistry: IV, Polycondensation and Why It Matters

What we call polyester is chemically poly(ethylene terephthalate), or PET. Fiber, film and bottle grades share the same backbone; what mainly separates them is chain length, i.e. molecular weight. To understand why a fabric behaves the way it does, you have to look at how the chain is built and how long it is allowed to grow.

A two-step synthesis: esterification, then polycondensation

Modern production starts with direct esterification of purified terephthalic acid (PTA) with monoethylene glycol (MEG); this step yields bis(2-hydroxyethyl) terephthalate (BHET) and short oligomers as intermediates, releasing water as a by-product. In the second step, polycondensation runs under vacuum at high temperature: oligomers couple, excess MEG is continuously distilled off, and the equilibrium shifts toward chain growth. Continuously removing glycol from the system is the real driving force that lets the chain grow.

Side reactions here are not innocent. Diethylene glycol (DEG), formed when MEG reacts with itself, gets built into the chain; as its content rises, the glass-transition temperature (Tg) drops, melting behavior and dyeability shift, and hydrolytic resistance weakens. That is why DEG is a critical parameter typically held to a low percentage (often on the order of ~1%) across the industry.

Catalyst and delustrant: two decisions written into the resin

A catalyst is essential to drive polycondensation. For years the dominant industry choice has been antimony trioxide (Sb2O3), typically on the order of ~200-300 ppm Sb. Environmental concerns and the search for cleaner color have raised interest in titanium-based catalysts, which can be effective at far lower loadings (typically on the order of ~25 ppm); titanium is fast but its tendency toward yellowing demands careful stabilization. Germanium is reserved for niches needing high clarity.

The second decision is luster. Titanium dioxide (TiO2) is added to the melt as a delustrant, scattering light inside the fiber. With a particle size of roughly 0.2-0.3 microns, its dose sets the luster class: below ~0.1% for bright fiber, ~0.3-0.5% for semi-dull, and typically ~1.5-2.0% for full-dull. This choice cannot be undone later; a fabric's sheen begins in the resin.

IV: the one number that reaches the fabric

Molecular weight is not measured directly; in practice it is represented by intrinsic viscosity (IV, dL/g). The higher the IV, the longer the chain and the higher the molecular weight. Textile filament typically runs at roughly 0.60-0.68 dL/g IV; technical yarn and bottle grades are markedly higher. That small number at the spinneret carries the whole story of the chain, from spinnability to final strength.

IV measurement is standardized. ASTM D4603 dissolves PET at 0.50% concentration in a 60/40 phenol/1,1,2,2-tetrachloroethane solvent, measures flow times at 30 C with an Ubbelohde-type glass capillary viscometer, and computes inherent viscosity; ISO 1628-5 is the sibling standard addressing the same subject (with differences such as solvent options and reporting a viscosity number). So when comparing two suppliers' 'IV' figures, the method and solvent used matter.

What IV governs: spinnability, strength, pilling, hydrolysis

Very low IV makes the melt too fluid; the filament becomes unstable, breaks rise and final strength (tenacity) drops. Very high IV raises melt viscosity and spinning pressure, making fine denier harder. The textile range holds that balance. Abrasion and pilling behavior are tied here indirectly too: without adequate molecular weight and proper heat-setting, the surface is more prone to fuzz and pill formation — at fabric level, Martindale pilling assessments (e.g. ISO 12945-2) make this outcome visible.

Hydrolysis is PET's Achilles heel. Water cleaves the ester bond, creating one carboxyl (-COOH) and one hydroxyl (-OH) end with each scission; because carboxyl end groups themselves accelerate hydrolysis, the process is autocatalytic. The result is falling IV, lower molecular weight and loss of strength. That is why drying the chip before spinning (moisture typically below ~50 ppm) and a low starting carboxyl end-group content are the quiet determinants of quality.

Semicrystalline structure, thermal transitions and SSP

PET is a semicrystalline polymer: ordered (crystalline) and disordered (amorphous) regions coexist. The glass-transition temperature (Tg) is typically ~70-80 C and the melting temperature (Tm) ~255-260 C; these transitions are measured by differential scanning calorimetry (DSC) per ISO 11357 (Part 2 for Tg and step height, Part 3 for melting/crystallization enthalpy). The degree of crystallinity governs strength, dimensional stability, dyeability and heat-setting behavior. Tg also defines the fabric's heat-setting and ironing windows.

When higher molecular weight is needed — especially in recycled (rPET) and technical grades — solid-state polymerization (SSP) comes into play. Chips are heated below melting under vacuum or inert nitrogen sweep; as reaction by-products (ethylene glycol, acetaldehyde) diffuse from the pellet interior to the surface and are carried away, the equilibrium shifts to chain growth. SSP raises IV and lowers both carboxyl end groups and acetaldehyde; it is the primary way to recover the IV lost in mechanical recycling for melt processability.

In the end, intrinsic viscosity is the single highest-leverage polymer parameter in a fabric buyer's hands: at the same denier and knit, the IV, DEG, catalyst and TiO2 decisions in the recipe pre-write the fabric's strength, hand, sheen and long-term durability. Asking a supplier 'what is the IV, by which method, and what are the carboxyl end-group and DEG levels' is the fastest way to move past surface-level marketing language.