Processing Polyester Knitted Fabrics: Best Practices & Techniques

Polyester-based knitted fabric refers to any textile that contains polyester fiber in its composition, ranging anywhere from 5% to 100%. The specific processing method changes depending on the polyester percentage, as well as whether the polyester is in the form of a spun fiber or a filament yarn.

Polyester knitted fabrics are used on a large scale for all types of outerwear. The main reasons for their massive success are their wide patterning potential, excellent durability, high crease resistance, and easy-care properties. Because polyester knitted fabrics are composed almost exclusively of texturized yarn, unfixed fabrics should always be stored or treated at full width to avoid permanent creases and breakages.

Types of Polyester Yarns Used

1. Filament Yarns: Produced in both monofilament and multifilament forms.

2. Textured Yarns: Created from polyester multifilament and texturized either during the drawing process or subsequently during throwing and texturizing.

3. Spun Yarns: Manufactured directly from staple fibers. These can be 100% polyester or blended with other fibers like Viscose, Cotton, Modal, etc.

Key Properties of Polyester Fabrics

• Strength and Stretch Resistance: Fabrics made of regular-tenacity polyester filament yarns are highly durable. They have low elasticity but high stretch resistance, meaning they do not sag or stretch out of shape easily. This makes polyester uniquely suited for knitted garments.

• Dimensional Stability & Wrinkle Resistance: Polyester possesses excellent resilience. It resists wrinkling when both dry and wet, allowing garments to maintain a crisp, pressed appearance even after exposure to rain or humid weather.

• Moisture Management: Polyester is a better conductor of heat than acrylic. It has low absorbency, meaning moisture stays on the surface and evaporates rapidly. While this makes it ideal for water-repellent rainwear, low absorbency can sometimes feel clammy in humid weather. To improve comfort, multifilament polyester yarns are used to spread moisture across a larger surface area for faster evaporation.

• Shrinkage & Care: There is virtually no water shrinkage with polyester fabrics, making shirts, blouses, and slacks safe to launder. When ironing, use low-to-medium heat, as excessive heat will melt the fibers. Note that fabrics made from spun polyester yarns have a higher tendency to pill.

• Outdoor Durability: Polyester fabrics can shrink up to 20% during wet-finishing operations, which is why they are generally heat-set later. They perform exceptionally well outdoors due to their high resistance to sunlight degradation and mildew.

Scouring and Weight Reduction of Polyester Fabrics

Scouring is required for all types of polyester knitted qualities. It is best carried out as a combined single-bath scouring and weight reduction process in an HTHP (High-Temperature High-Pressure) dyeing machine.

1. Pre-Heating and Loading

Load the fabric into water pre-heated to 50°C without any chemicals. A warm wash at this stage helps remove knitting oils easily.

⚠️ Important Note: Avoid adding a wetting agent in the initial bath, as it can lock the oils into the fabric. While knitting oil suppliers frequently claim their products are completely water-soluble, giving the fabric a solo 10-minute warm wash first ensures complete oil removal.

2. The Weight Reduction (WR) Process

The weight reduction of polyester is an alkaline hydrolysis process (saponification of terephthalic ester) caused by caustic soda (NaOH). Under predetermined conditions, this process progressively peels the polyester fiber surface, reducing fabric rigidity and increasing bounce and suppleness without damaging its core properties.

Standard Recipe for Scouring & Weight Reduction

• Caustic Soda (NaOH): 4%

• Wetting Agent: 1%

• Oligomer Remover: 1% (surfactant- or solvent-based oil remover)

• Optional: Add H₂O₂ (Hydrogen Peroxide) and a Stabilizer if processing a cotton/polyester blend to clean both fibers simultaneously.

Process Conditions: The bath requires a minimum temperature of 90°C to activate weight reduction, though it can be safely taken up to 110°C. Hold the temperature for a minimum of 30 minutes.

3. Neutralization

• For Polyester-Rich / 100% Polyester Filament: Neutralize with oxalic acid to prevent potential stains on the fabric.

• For Cotton-Rich Blends: Neutralize with acetic acid.

Process Sequences

The basic finishing processes for 100% polyester filament yarn fabrics are generally arranged in one of three sequences:

1. Scour → Heat-set → Dye (Highly recommended for polyester-rich fabrics to control fabric weight and eliminate crease marks during high-temperature dyeing).

2. Oil Removal ------ Heat-set ------- Scour -------- Dye (This sequence is highly recommended for fabrics with more than 5% stretch fiber/elastane. If de-oiling is neglected prior to heat-setting, the residual oils will permanently bake into the fibers. This leads to a patchy "moiré effect" and significantly reduces both washing and rubbing fastness).

3. Scour → Dye → Heat-set (If cotton content is higher than polyester, weight reduction can be skipped initially and performed after polyester dyeing to act simultaneously as a reduction-clearing step).

Crucial Technical Precautions

• Slitting: Circular knitted fabrics should be slit as soon as possible after knitting (ideally within 24 hours). If they cannot be processed immediately, they must be rolled flat and completely free of creases. Storing goods in tubular form for too long fixes permanent folds into the fabric.

• Pre-stabilizing: If the fabric is prone to creasing during pre-cleaning or if the selvedges tend to curl, run the fabric in a tensionless state through a steaming chamber (saturated steam at 100°C). This allows the fabric to relax and the fibers to crimp.

• Oil Removal: All spin finishes, knitting oils, and impurities must be completely removed prior to dyeing. If spun oil is left on the fabric before heat treatment, the oil will fix unevenly, creating a moiré visual defect after final finishing.

• Tensionless Washing: Scouring should ideally be carried out on continuous full-width washing ranges using soda ash and a high-quality detergent. This allows the fabric to shrink naturally (often over 20%) without longitudinal tension. Do not load Poly-Lycra or 100% polyester fabrics directly into soft-flow machines without this pre-treatment, or you will create hard creases that cannot be removed later.

• Uniform Drying: After washing, rinsing, and hydroextraction, uniform drying is absolutely critical. Any variations in residual moisture content will cause uneven heat fixation, resulting in patchy, uneven dyeing.

• Heat-Setting Parameters: Heat setting is performed in a stenter at 150°C–180°C for 20–30 seconds. Higher temperatures yield better dimensional stability, but setting too high makes the fabric handle flat and harsh.

• Managing Stretch & Elastane: Heat-setting temperatures directly impact fabric stretch. Exceeding 160°C on standard polyester compromises its stretch properties, making it stiff. However, if the fabric contains elastane (Lycra), a minimum temperature of 185°C is required for proper setting.

• Yarn-Dyed & Melange Fabrics: If the fabric uses yarn-dyed or melange fibers containing disperse dyes, the molecular bonds will begin to break down if temperatures cross 145°C. Unfixed dyes will migrate to the surface, causing poor colorfastness. When manufacturing yarn-dyed polyester blended with elastane, it is highly advisable to use easy-setting elastane yarn, which can be safely heat-set at 150°C instead of 185°C.

• Dye Selection: Texturized polyester knitted fabrics are typically dyed at 130°C in an HTHP machine to give a full, bulky handle. If the fabric will undergo high-temperature post-dyeing treatments (such as printing, curing, or high-heat stenter drying), always use disperse dyes with medium to high sublimation fastness to prevent color migration.

• Reprocessing & Redyeing Management

  Reprocessing is an activity no dyehouse manager wants to deal with, but it is an inevitable part of production. If the fabric contains Lycra, attempting to redye it to correct a shade at the standard 130°C will severely deteriorate its stretch properties. This leads to a lower GSM and an uncontrollable increase in fabric width, resulting in complete fabric rejection.

  To prevent this, always conduct a lab study first to determine the threshold where GSM is maintained. When executing a redyeing process, it is highly recommended to either drop the dyeing temperature to 125°C or significantly reduce the holding time at 130°C.

  Woven Fabric Comparison

  If you are processing woven polyester rather than knitted, it is highly advisable to heat-set the fabric prior to jet dyeing. Set the stenter width 2 inches narrower than your final target width, as the fabric width will naturally expand under high temperatures during the dyeing process. Attempting to jet dye woven polyester without this pre-heat setting will cause permanent, unfixable rope marks across the fabric.

  Creating a "Cotton Feel" on Polyester

  Achieving a low-cost "cotton feel" on polyester fabric is currently in high demand. This texture is achieved by sueding (brushing) the fabric. It is critical to perform the sueding process at the grey stage.

  The main challenge this creates for the dyer is managing loose fluff and lint in the jet or soft-flow dyeing machine. Because of this, it is absolutely essential to thoroughly clean the machine filters before starting the dyeing cycle to prevent lint patches and blockages.

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Final Takeaway: There is no single "straight line" formula in textile processing. Successful fabric finishing is always a tailored combination of applications that depends heavily on the specific fiber blend, yarn type, and the expertise of the processor.