PET Resin for Microfiber Applications

A human hair is approximately 70 micrometers in diameter. At the same time, advanced ultra-fine fibers can be only a few micrometers across—and specialized technologies can push fiber dimensions toward the submicron scale.

In microfiber production, PET resin is more than simply a conventional polyester raw material. As fiber structures become finer and more complex, intrinsic viscosity, melt rheology, thermal stability, moisture control, polymer compatibility, and downstream fiber-opening behavior all become increasingly important.

From core/sheath bicomponent fibers to splittable segmented fibers and islands-in-the-sea technology, modern PET microfiber production is not simply about making a filament thinner. It is about designing the polymer system, fiber cross-section, and downstream process to work together.

Microfiber cleaning cloths, synthetic suede, and high-performance filtration materials may look completely different, but they share one technological foundation: fine fibers with exceptionally high specific surface area.

What Makes a Fiber Ultra-Fine?

Fiber fineness is commonly expressed in denier or decitex (dtex).

Denier represents the weight in grams of 9,000 meters of fiber, while dtex represents the weight in grams of 10,000 meters.

The lower the value, the finer the fiber.

In one commonly cited classification, fibers below approximately 0.7 denier (about 0.78 dtex) are considered ultra-fine, while some industry definitions use 1 dtex as an upper threshold. Terminology therefore varies somewhat between industries and applications.

For PET fibers, a filament around 0.7 denier typically has a diameter of several micrometers, depending on polymer density and cross-sectional geometry.

As fiber diameter decreases, specific surface area increases significantly. Small spaces between neighboring fibers also form fine capillary channels.

These structural changes can provide:

• greater contact area with liquids, oils and particles;
• stronger capillary effects;
• softer and denser textile surfaces;
• finer and more controllable pore structures;
• improved wiping and liquid-management performance.

These properties explain the use of microfibers in synthetic suede, cleaning textiles, filtration, technical nonwovens and specialty fabrics.

From Synthetic Suede to Industrial Microfiber Technology

Commercial ultra-fine fiber technology is closely associated with the development of synthetic suede in Japan.

Companies such as Toray and Kuraray developed technologies combining very fine synthetic fibers with nonwoven structures, polyurethane impregnation, and sophisticated surface treatments. Toray’s ECSAINE™ became one of the best-known early commercial examples.

The important breakthrough was not simply spinning a conventional polyester filament at a smaller diameter.

Instead, manufacturers developed multicomponent filaments that could be spun and processed at a practical size and then split or opened into much finer individual fibers during downstream processing.

That principle remains central to industrial microfiber technology today:

First, create a processable composite filament, then convert it into much finer fibers.

Bicomponent spinning makes this possible.

How Bicomponent Spinning Creates Microfibers

In bicomponent spinning, two polymers are delivered through the same spinning assembly and combined into a defined cross-sectional structure in a single filament.

The function of the final fiber depends on both the polymer pair and the cross-sectional geometry.

For PET-related applications, three structures are particularly relevant:

Fiber Structure	Typical Polymer Roles	Key Material Requirement
Core/Sheath	PET core + modified copolyester sheath	Structural strength + controlled thermal bonding
Splittable Segmented	PET + polyamide segments	Controlled interfacial separation and splitting
Islands-in-the-Sea	PET islands + removable copolyester sea	Stable bicomponent spinning + selective sea removal

Each architecture places different demands on the polyester resin.

Core/Sheath Fibers and Low-Melting Copolyester

In a core/sheath fiber, one polymer forms the inner core, while another surrounds it as the sheath.

A typical polyester system can use conventional PET as the core and a modified copolyester as the sheath.

The PET core provides mechanical strength and dimensional stability. The copolyester sheath can provide thermal bonding or other functional surface properties.

When the sheath has a lower softening or melting temperature than the PET core, heat treatment can soften the outer layer while the core remains structurally intact. Adjacent fibers can then bond without completely melting the entire filament.

This principle is widely used in thermal-bonding fibers, nonwovens, filling materials, and other heat-bonded textile structures.

Low-Melting Copolyester as the Sheath

Low-melting copolyester is therefore an important sheath option for PET-based core/sheath fibers.

Hubei Decon supplies modified copolyester grades that meet different thermal-bonding requirements, with selected grades offering softening or melting behavior from about 90°C to 230°C, depending on polymer design and application.

But bonding temperature alone does not determine whether a resin is suitable for bicomponent spinning.

The sheath polymer must also be evaluated for:

• melt viscosity;
• thermal stability;
• crystallization behavior;
• rheological compatibility with the PET core;
• stability at the actual spinning temperature.

Both polymers pass through the same bicomponent spinning system and must maintain a stable interface.

If their melt viscosities or flow characteristics are poorly matched, the core/sheath geometry can become uneven. This may result in unstable spinning, non-uniform sheath thickness, or filament breakage.

The design objective is therefore:

PET Core → Strength and dimensional stability

Modified Copolyester Sheath → Controlled thermal bonding

The two polymers must be compatible not only in the final fiber, but also during melt spinning.

Splittable Segmented Microfibers

Another important microfiber route is the splittable segmented structure, typically represented by segmented-pie or hollow segmented-pie cross-sections.

In these structures, two polymers—commonly PET and polyamide (PA)—are arranged alternately as multiple segments within one composite filament.

Unlike core/sheath fibers, the objective is not to keep the components permanently together.

The filament is designed to split into multiple finer segments during downstream processing.

Differences in interfacial adhesion and polymer properties allow the segments to separate through suitable mechanical or other processing forces. In nonwoven production, hydroentanglement is one important method for promoting this splitting.

After separation, one composite filament becomes multiple much finer fibers, greatly increasing surface area and creating fine edges and capillary channels.

These characteristics are particularly useful in high-performance wiping materials and synthetic leather substrates.

Side-by-side bicomponent fibers also exist, but differences in shrinkage between the two polymers are more commonly used to create self-crimp or three-dimensional fiber structures than to serve as the principal architecture for microfiber splitting.

Islands-in-the-Sea Microfibers

For producing much finer PET fibers, islands-in-the-sea technology is especially important.

In this structure, many extremely fine polymer filaments—the islands—are embedded within a continuous polymer matrix—the sea.

During spinning, the sea component holds the individual islands together.

Although each island may ultimately become an extremely fine fiber, the overall composite filament remains large enough for conventional spinning, drawing, and textile processing.

After producing the composite fiber, the sea component is removed.

The individual islands are released as independent ultra-fine fibers.

Depending on spinneret design, one composite filament may contain dozens or hundreds of individual islands.

This enables the production of fibers far finer than would normally be practical through conventional single-component melt spinning.

PET Resin as the Island Component

In a PET-based islands-in-the-sea system, spinning-grade PET resin can form the island component.

Once the sea polymer has been removed, these islands become the final PET microfibers.

Important PET resin characteristics include:

• intrinsic viscosity (IV);
• melt viscosity;
• molecular-weight consistency;
• thermal stability;
• crystallization behavior;
• spinning stability.

However, IV alone does not define bicomponent-spinning compatibility.

Intrinsic viscosity is a useful resin specification, but the relationship between the melt viscosities of the island and sea polymers at the actual spinning temperature and shear conditions is more directly relevant to stable flow and uniform island geometry.

If the rheological mismatch becomes excessive, island size and distribution may become less uniform and spinning stability may deteriorate.

Moisture Control Before Spinning

Moisture control is also important when processing PET.

Residual moisture can cause hydrolytic degradation during high-temperature melt processing, reducing molecular weight and melt viscosity.

In a bicomponent system, this is particularly relevant because a change in PET melt viscosity can disturb the intended rheological balance between the island and sea components.

PET resin must therefore be properly dried and protected against moisture pickup before melt spinning according to the requirements of the specific resin and spinning process.

A suitable PET resin for microfiber applications is therefore not defined by a single specification. Consistent polymer quality, appropriate drying, and the correct melt-rheology relationship with the second polymer all matter.

Alkali-Soluble Copolyester as the Sea Component

The sea polymer has a very different function.

During melt spinning, it must provide enough thermal stability and processing compatibility to maintain the composite filament.

After spinning and fabric formation, however, it must be removable under controlled conditions so that the PET islands can be released.

One important option is alkali-soluble copolyester.

In a polyester-based islands-in-the-sea system, conventional PET forms the islands, while a specially modified copolyester forms the surrounding sea.

After spinning, drawing, and fabric formation, controlled alkaline hydrolysis and dissolution remove the sea component, releasing the PET islands and creating the final microfiber structure.

Hubei Decon supplies alkali-soluble copolyester chips that can be evaluated as the sea component for islands-in-the-sea and related fiber-opening applications.

Grade selection should be based on the customer’s actual spinning system, polymer ratio, fiber structure, and alkaline-opening process.

Why Selective Sea-Component Removal Matters

The key question is not simply:

Can alkali remove the sea polymer?

A more important question is:

Can it be removed efficiently without causing unacceptable damage to the PET islands?

Both conventional PET and modified copolyester contain ester bonds. Under sufficiently severe alkaline conditions, PET itself can undergo hydrolysis.

The fiber-opening process therefore aims to selectively remove the sea component.

If removal is too slow, residual sea polymer can keep PET islands connected and prevent complete fiber opening.

If treatment conditions are too aggressive, the PET microfibers themselves may suffer:

• loss of molecular weight or strength;
• excessive weight reduction;
• surface damage;
• reduced process efficiency.

Successful fiber opening depends on the combined effects of:

sea-polymer chemistry, PET/sea ratio, island geometry, alkali concentration, treatment temperature, treatment time and fabric construction.

The sea polymer must therefore provide a sufficiently wide processing window between:

effective sea-component removal and unacceptable degradation of the PET islands.

This makes alkali-soluble copolyester a functional process material, not merely a temporary polymer matrix.

Spinnability and Removability Must Be Balanced

The sea component presents an inherent design challenge.

It needs to be:

stable enough for melt spinning, yet removable enough during downstream fiber opening.

A more highly modified copolyester may respond more readily to alkaline treatment but can also show different melt viscosity, crystallization behavior, or thermal stability.

A polymer with excellent melt-processing stability may require more demanding opening conditions.

For this reason, sea-polymer selection requires a balance between:

melt rheology, thermal stability, spinnability, compatibility, and opening efficiency.

This balance is one of the main reasons specialty copolyester chemistry plays such an important role in islands-in-the-sea microfiber technology.

What About Electrospinning?

Electrospinning provides another route to extremely fine polymer fibers.

A strong electric field stretches a polymer solution or melt into very fine jets, allowing fiber dimensions to reach the submicron or nanometer scale.

This technology is attractive for advanced filtration, biomedical structures, sensors, and other high-value applications.

However, industrial productivity and process economics remain different from conventional melt spinning.

For mainstream PET textile production, bicomponent melt-spinning technologies such as segmented fibers and islands-in-the-sea structures remain particularly important industrial routes to microfibers.

Major PET Microfiber Applications

The high specific surface area, fine capillary channels, and dense structure of microfibers support a wide range of end uses.

Synthetic Suede

Ultra-fine fibers form the structural foundation of many synthetic-suede materials.

Combined with nonwoven processing, polyurethane impregnation, and surface finishing, they can create a soft, dense surface similar to natural suede.

Applications include automotive interiors, footwear, apparel, furniture, and premium surface materials.

High-Performance Wiping Materials

Split and ultra-fine fibers provide large surface areas and fine capillary channels.

Segmented or irregular fiber cross-sections can also provide fine edges that help mechanically capture particles and oils.

These materials are widely used in optical cleaning, electronics, industrial wiping, cleanroom applications, and household products.

Filtration

Fine fibers allow smaller, more controllable pore structures and increase the likelihood of particle interception.

Microfibers are therefore used in various air and liquid filtration applications.

The engineering challenge is to achieve high filtration efficiency without creating excessive pressure drop.

Technical and Medical Textiles

Microfiber structures can provide softness, liquid transport, large surface area, and controlled surface contact.

These characteristics can be useful in selected technical textiles, specialty nonwovens, and medical-material applications.

A Material-System Approach to PET Microfibers

Different bicomponent fiber structures require different polymer functions.

Material	Role in Fiber Structure	Main Function
PET Resin	Core	Mechanical strength and dimensional stability
Low-Melting Copolyester	Sheath	Controlled thermal bonding
PET Resin	Islands	Final ultra-fine PET fibers
Alkali-Soluble Copolyester	Sea	Supports spinning, then enables fiber opening

The important point is that these materials should not be evaluated independently.

Final fiber performance depends on interactions among rheology, thermal behavior, crystallization, moisture control, polymer compatibility, and downstream processing conditions.

PET Resin and Specialty Copolyester Solutions from Hubei Decon

Hubei Decon Polyester supplies specialty polyester materials for differentiated fiber applications, including:

• PET resin for specialty fiber and microfiber applications
• Low-melting and modified copolyester chips for core/sheath bicomponent fibers
• Alkali-soluble copolyester chips for islands-in-the-sea and fiber-opening applications

Selecting an appropriate grade requires more than comparing one number on a technical data sheet.

For technical evaluation, useful information may include:

• fiber structure: core/sheath, segmented or islands-in-the-sea;
• PET/copolyester ratio;
• target filament fineness;
• number of islands;
• spinning temperature;
• PET intrinsic-viscosity requirement;
• target bonding or softening temperature;
• sea-component removal method;
• NaOH concentration;
• opening temperature and treatment time;
• final fiber or fabric application.

With this information, you can evaluate the polymer system against the customer’s actual processing conditions rather than selecting it from a single specification alone.

Frequently Asked Questions

What PET resin properties matter in microfiber spinning?

Important factors include intrinsic viscosity, melt viscosity, molecular-weight consistency, thermal stability, crystallization behavior, moisture control, and spinning stability.

For bicomponent fibers, the melt-rheology relationship between the two polymers under actual spinning conditions can be more important than IV alone.

Can low-melting copolyester be used as the sheath of a PET bicomponent fiber?

Yes. Modified low-melting copolyester can serve as a sheath polymer when its thermal behavior, melt viscosity, stability, and rheological compatibility suit the PET core and spinning process.

The required grade depends on both the target bonding temperature and the bicomponent spinning conditions.

Why is alkali-soluble copolyester used in islands-in-the-sea fibers?

It can serve as a temporary sea matrix that supports the PET islands during spinning and textile processing.

During subsequent alkaline treatment, the modified copolyester can be preferentially hydrolyzed and removed, releasing the individual PET islands as ultra-fine fibers.

What information is needed to select a resin grade?

Useful information includes the fiber structure, polymer ratio, target fineness, spinning temperature, PET IV requirement, and downstream processing conditions.

For islands-in-the-sea fibers, the number of islands and alkaline-opening conditions—including NaOH concentration, temperature, and treatment time—are particularly useful.

References

1. Scheirs, J.; Long, T. E., eds. Modern Polyesters: Chemistry and Technology of Polyesters and Copolyesters. John Wiley & Sons.
2. Toray Industries, Inc. Corporate History. ECSAINE™ ultra-microfiber nonwoven fabric with suede texture entered commercial marketing around 1970.
3. Research on PET/PA6 hollow segmented-pie microfiber nonwovens, including fiber splitting through hydroentanglement and their use in synthetic-leather substrates.
4. Seo M. et al. “A Study on the Alkali Hydrolysis of Sea-island PET Ultra-microfiber.” Textile Coloration and Finishing, Vol. 25, No. 4, 2013, pp. 303–313.

Selecting the Right Polymer System

Microfiber technology continues to push PET processing toward increasingly sophisticated combinations of polymer chemistry, fiber geometry, and downstream processing.

But finer fibers alone do not guarantee better performance.

For PET-based microfiber systems, the real challenge is making PET resin, modified copolyester, bicomponent structure, and downstream processing conditions work together as one complete material system.

For manufacturers developing core/sheath fibers, thermal-bonding fibers, split microfibers, islands-in-the-sea fibers, synthetic suede, or other specialty polyester fibers, Hubei Decon can evaluate suitable polyester materials based on the actual spinning and downstream-processing requirements.