What is the difference in chemical structure between recycled polyester staple fiber and virgin polyester staple fiber? ​

From a chemical perspective,Recycled polyester staple fiberThe core molecular main chain structure of native polyester staple fibers is consistent, both are linear polymers connected by ester bonds (- O-CO -) to repeat units of ethylene terephthalate (PET), with a molecular formula of [- OCH ₂ CH ₂ OCOC ₆ H ₄ CO -] ₙ. This is also the fundamental reason why both belong to the category of "polyester (PET)". However, due to differences in raw material sources and recycling processes, there are significant differences in the details of their chemical structures, which can be explored from the following five dimensions:

1、 Differences in monomer purity and trace impurities

The raw materials for the production of native polyester staple fibers are high-purity petrochemical grade monomers: terephthalic acid (PTA) and ethylene glycol (EG), with a purity of usually ≥ 99.9% and no other organic/inorganic impurities (such as dye molecules, polyolefin fragments, metal ions, etc.). Therefore, the PET molecular chains generated by polymerization only contain PET repeating units, and the chemical structure is single and pure.

Recycled polyester staple fiberThe raw materials used are recycled PET products (such as waste beverage bottles, textile scraps, waste clothing, etc.), and there may be residues in the raw materials:

Additives added during the processing of native products (such as antioxidants, plasticizers, UV absorbers);

Other polymers mixed in during the recycling process (such as polyethylene, polypropylene, nylon, etc.);

Dye molecules and printing pastes carried by waste textiles;

Dust and metal ions adsorbed in the environment.

These impurities will exist in the molecular chains of recycled PET in the form of "block", "graft" or "physical inclusion", resulting in significantly higher "heterogeneity" of its chemical structure compared to native polyester.

2、 Differences in molecular weight (degree of polymerization) and distribution

Native polyester staple fibers are produced through controllable polymerization processes such as continuous ester exchange polycondensation, which can accurately control the polymerization degree (n-value) of PET. The polymerization degree is usually concentrated in the range of 100-150, and the molecular weight distribution index (PDI, weight average molecular weight/number average molecular weight) is relatively narrow, generally 1.8-2.2. The molecular chain length uniformity is high, and the chemical structure stability is consistent.

The raw material of recycled polyester staple fiber (recycled PET) already has "molecular chain breakage":

Waste PET is subjected to light, heat, and oxygen during use, which can cause hydrolysis or thermal degradation of ester bonds, leading to molecular chain breakage and a decrease in polymerization degree (such as PET polymerization degree of waste beverage bottles, which is usually 10% -30% lower than that of native PET).

During the recycling and regeneration process (such as melt extrusion, depolymerization and re polymerization), the high temperature environment will further exacerbate ester bond breakage, and the degree of breakage will be uneven.

This makes the polymerization degree of recycled PET usually lower than that of native PET (mostly 80-120), and the molecular weight distribution is wider (PDI is mostly 2.5-3.5), with large differences in molecular chain length and more pronounced "dispersibility" of the chemical structure.

3、 Differences in Terminal Structure and Reaction Activity

The end groups of PET molecular chains are mainly hydroxyl (- OH) and carboxyl (- COOH), and their quantity and type directly affect the subsequent processing properties of fibers, such as dyeing and hydrolysis resistance.

The end group structure of native polyester staple fibers is controllable and stable: by adjusting the amount of "end capping agent" (such as benzoic acid) added in the later stage of polymerization, the ratio of - OH to - COOH in the end group can be accurately controlled. The carboxyl content is usually controlled at 20-40 mmol/kg, the hydroxyl content is lower, the end group reaction activity is uniform, and the chemical behavior during subsequent spinning and dyeing is stable.

The end group structure of recycled polyester staple fibers is significantly affected by the "degradation regeneration" process:

The hydrolysis/thermal degradation of recycled PET can lead to ester bond breakage, generating more end groups containing - COOH and - OH, and the carboxyl content is usually higher than that of native PET (up to 50-80 mmol/kg);

If "end group repair" is not carried out in the recycling process (such as adding chain extenders), degradation products such as aldehyde groups (- CHO) and ketone groups (- CO -) may remain in the end groups. These unsaturated end groups are prone to further oxidation, leading to a decrease in the chemical stability of recycled PET (such as yellowing and hydrolysis).

4、 Differences in branching/cross-linking structures

The PET molecular chain of native polyester staple fibers is mainly linear in structure, with only a small amount of branching monomers (such as polyols) introduced during special functional modifications (such as anti-static and flame retardant). The branching degree is low (branching point density<0.1/100 repeating units), and there is almost no cross-linking structure. The chemical structure is mainly linear polymers.

If recycled polyester staple fibers are used as raw materials from "waste textiles" (especially waste clothing that has been dyed and sorted), there may be:

Residual "crosslinking agents" (such as resin finishing agents) added during textile processing lead to the formation of a small amount of covalent crosslinking bonds (- C-O-C - or - C-N-C -) between the molecular chains of regenerated PET;

The high temperature and high pressure environment during the recycling process may cause "self crosslinking" between molecular chains, forming trace cross-linking structures.

These branched/cross-linked structures will break the linear characteristics of native PET, resulting in differences in the solubility and melt flowability of recycled polyester compared to native polyester.

5、 Differences in crystallinity and crystal form

Although crystallinity belongs to the "aggregated structure", there are differences in their crystallization characteristics due to the influence of chemical structures such as molecular weight and impurities

The molecular chain length of native polyester staple fibers is uniform, with few impurities. During the crystallization process, the molecular chains are easily arranged in a regular manner, and the crystallinity is usually high (30% -40%). The crystal structure is mainly stable in the "triclinic system", and the crystal structure is regular.

Recycled polyester staple fiberDue to the wide distribution of molecular weight and impurities, the regular arrangement of molecular chains is hindered, and the crystallinity is usually 5% -10% lower than that of primary polyester (mostly 25% -35%); At the same time, impurity molecules may act as "crystal nuclei" to interfere with crystal growth, resulting in the inclusion of small amounts of unstable "amorphous regions" or "metastable crystal forms" in the crystal structure, and the uniformity of the crystal structure is lower than that of primary polyester.

Summary: The Essence of Core Differences

The difference in chemical structure between the two is not due to the difference in main chain structure, but rather a chain reaction of "raw material purity → polymerization process → molecular details": primary polyester is a "forward synthesis" from pure monomers to regular molecular chains, with controllable and uniform chemical structure; Recycled polyester is a "reverse regeneration" process that involves the degradation of raw materials and the repair of molecular chains. Its chemical structure is influenced by residual raw materials and the degree of degradation, exhibiting characteristics of "high impurities, uneven molecules, and high end group activity". These differences also directly lead to differences in physical properties (such as strength, heat resistance) and processing properties (such as spinning breakage rate, dyeing uniformity) between the two.