Synthetic fibres account for a large share of all the polymer ever manufactured, and most of that fibre is polyester (Geyer et al., 2017). The industry has responded to pressure on virgin feedstock in two ways. One is recycling: post-consumer bottles are washed, flaked and remelted, and the greater part of this material has historically gone into staple fibre and filament and not back into bottles (Shen et al., 2010; Welle, 2011). The other is substitution by polymers made from renewable carbon, of which polylactic acid (PLA), produced from fermented plant sugars, is the most developed (Vink et al., 2003).

A filament spun from a blend of recycled poly(ethylene terephthalate) (rPET) and PLA would combine both responses in one product, and the idea recurs in the literature on sustainable textiles. There is also a less voluntary reason to study the blend. PLA packaging is hard to distinguish from PET by eye, and when it reaches a PET recycling line it becomes a contaminant whose effect on the melt has to be understood (La Mantia et al., 2012). In both situations the practical question is the same: what happens when two polyesters with very different thermal behaviour are forced through one spinneret.

This review assembles the published knowledge that bears on that question. It does not report new spinning trials. Its purpose is to set out, in the order in which a spinning technologist meets them, the phenomena that govern whether an rPET and PLA blend can be spun at all, what the filament is likely to look like, and where the evidence is thin.

The article is a narrative review of the published literature. It draws on standard texts on fibre formation and polymer blends, on established reviews of PET recycling and of PLA processing, and on primary papers that deal with the two polymers together. No formal search protocol was applied, and the selection reflects a judgement about which sources are well established. Much of what can be said about the blend is inferred from the separate behaviour of its components and from the general theory of immiscible melts, and the text indicates where a statement rests on such inference and not on direct observation of rPET and PLA filaments.

Melt spinning asks three things of a polymer: a melt that is stable for the minutes it spends in the extruder and pack, a viscosity that allows steady flow through fine capillaries, and enough melt strength and crystallisability for the thread to survive attenuation and take up an oriented structure (Ziabicki, 1976). PET and PLA meet these requirements under different conditions. Table 1 sets the properties that matter for spinning side by side.

Most pairs of high polymers are immiscible, because the entropy gained on mixing long chains is too small to offset even a weakly unfavourable interaction (Paul and Bucknall, 2000). PET and PLA follow the rule. Studies of melt-mixed blends treat the pair as a two-phase system in which each component keeps its own glass transition and melting behaviour (McLauchlin and Ghita, 2016). The minor component is dispersed as droplets in the major one, and the properties of the product depend on the size of those droplets and on the strength of the interface.

Droplet size in an immiscible melt is set by the balance between the viscous stress that deforms a drop and the interfacial tension that resists it, an analysis that goes back to Taylor (1934). For extruded polymer blends, Wu (1987) showed that the dispersed phase is finest when the viscosities of the two components are close and coarsens as their ratio departs from unity in either direction. This result has an awkward implication for rPET and PLA. At a temperature high enough to melt PET, PLA is far above its own melting point and is losing molecular weight, so its viscosity is low and falling, while the viscosity of the rPET depends on the lot. The viscosity ratio is therefore neither close to one nor constant.

Compatibilisation is the usual answer to coarse morphology and weak interfaces. A block or graft copolymer located at the interface lowers interfacial tension, suppresses coalescence and transmits stress between the phases, and it can be added ready made or formed during mixing by reaction between functional groups (Utracki, 2002). Polyesters offer a further possibility, because ester groups on different chains can exchange in the melt and so generate copolymer at the phase boundary without any additive (Scheirs and Long, 2003). Whether such interchange proceeds far enough between PET and PLA within a spinning residence time, before degradation of the PLA dominates, is not settled by the sources reviewed here. Epoxy-functional chain extenders of the kind used to upgrade rPET react with the end groups of both polymers and are an obvious candidate for a combined role, but their effect on spinnability, as distinct from the properties of moulded specimens, is poorly documented.

A spinning line imposes a sequence of operations, and the blend raises a different difficulty at each one. Figure 1 pairs the stages with the concern that dominates there. The subsections that follow take them in order.

The properties of PLA fibre that differ most from PET are its lower melting point, its lower modulus and its sensitivity to hydrolysis under hot, alkaline conditions, which is why PLA fabrics are dyed at lower temperatures and scoured more gently than polyester (Farrington et al., 2005; Gupta et al., 2007). A blended filament inherits the constraints of its weaker phase: ironing and heat-setting temperatures, dyeing conditions and alkaline finishing all have to be chosen with PLA in mind. Disperse dyes take up differently in the two polymers, so shade and fastness will depend on blend ratio and morphology.

The environmental case needs equal care. Life cycle studies support both the recycling of bottles into fibre and the production of PLA from renewable feedstock when each is taken separately (Shen et al., 2010; Vink et al., 2003). A fibre that mixes the two intimately is harder to place. PLA is unwelcome in PET recycling for exactly the reasons described above, and PET does not biodegrade under the composting conditions for which PLA is designed, so the blend may fit neither recovery route (Ragaert et al., 2017). A claim that the blended yarn is more sustainable than its components would have to be supported by an assessment that includes its own end of life.

Four gaps stand out. The first is the scarcity of spinning studies as opposed to studies of moulded or extruded blends: most of what is known about the pair comes from specimens that never experienced elongational flow. The second is the effect of real feedstock variability, since work with a single well-characterised lot says little about a recycler's month-to-month experience. The third is compatibilisation under spinning conditions, where reactive additives, interchange reactions and degradation compete on a timescale of minutes. The fourth is the fate of the blended fibre in laundering, in recycling and in the environment.

There is also a design question that the literature has not confronted directly. If the aim is a filament that contains both polymers, a bicomponent construction, in which each melt is prepared at its own temperature and the two meet only in the spin pack, avoids most of the thermal conflict that an intimate blend creates (Hufenus et al., 2020). Comparative work on blended and bicomponent routes at equal composition would show whether the blend offers anything that the bicomponent filament does not.

The published record supports a cautious view of rPET and PLA as a spinning blend. The two polymers are immiscible, their viscosities diverge at any common melt temperature, and the temperature required to melt the polyester degrades the polylactic acid. Each stage of the spinning line adds a compromise, from drying through to heat setting. None of these obstacles is unique to this pair, and the tools for dealing with them, including chain extension, reactive compatibilisation, short residence times and bicomponent spinning, are established. What is missing is direct evidence from spinning lines, on realistic recycled feedstock, that the resulting filament is good enough to justify the complication it introduces at the end of its life.

To be declared by the author(s) before publication.

To be declared by the author(s) before publication.