Polyester and nylon are the two dominant synthetic fibers in the sewing thread industry. Together they account for the vast majority of all synthetic sewing thread produced globally. While both are thermoplastics, their different chemical structures produce meaningfully different performance characteristics. This comparison provides thread manufacturers and buyers with a thorough, application-oriented analysis of polyester versus nylon for sewing thread yarn.

Chemical Fundamentals

Polyester (PET)

Polyethylene terephthalate is produced by the condensation polymerization of ethylene glycol and terephthalic acid. The resulting polymer has a regular repeating structure with ester linkages connecting aromatic rings. This structure gives polyester its characteristic combination of properties: good strength, moderate elasticity, excellent UV resistance, and very low moisture absorption.

Nylon 66 (PA66)

Nylon 66 is produced by the condensation polymerization of hexamethylene diamine and adipic acid. The polymer features amide linkages between aliphatic carbon chains. The amide groups can form hydrogen bonds with each other and with water molecules, which contributes to nylon's higher strength, higher moisture absorption, and higher melting point compared to polyester.

These chemical differences at the molecular level propagate to every practical performance characteristic.

Property-by-Property Comparison

Tensile Strength and Tenacity

Nylon 66 wins. Nylon 66 filament yarn achieves the highest tenacity of any common sewing thread fiber, typically in a range that exceeds polyester filament tenacity by a meaningful margin. This is why nylon 66 is specified for the most demanding strength applications such as airbag sewing, parachute construction, and safety harness stitching.

Polyester offers very good tenacity -- more than adequate for the vast majority of sewing applications -- but does not match nylon 66 at the top end.

For spun yarns, the difference narrows because the spinning process introduces additional variables, but nylon staple fiber is not commonly used in spun sewing thread yarn. The nylon advantage is primarily in filament form.

Heat Resistance and Melting Point

Nylon 66 wins. The melting point of nylon 66 (approximately 255 to 265 degrees C) is roughly 40 to 50 degrees C higher than that of polyester (approximately 250 to 260 degrees C). More importantly, nylon 66's softening point is higher, meaning it retains a greater proportion of its room-temperature strength at elevated temperatures.

This thermal advantage is critical in high-speed industrial sewing where needle temperatures can approach the softening point of polyester. Nylon 66 thread provides a larger safety margin before thermal damage occurs.

UV and Weather Resistance

Polyester wins decisively. Polyester has inherently excellent resistance to degradation by ultraviolet light. Extended outdoor exposure has relatively little effect on polyester fiber strength and color.

Nylon, in contrast, is susceptible to UV degradation. Prolonged sunlight exposure causes progressive loss of strength and yellowing. UV-stabilized nylon formulations are available and improve performance, but they do not fully close the gap with polyester's natural UV resistance.

This is the single most important factor favoring polyester for outdoor applications -- awnings, boat covers, outdoor furniture, and any product that will spend significant time in sunlight.

Moisture Absorption

Polyester wins for wet applications. Polyester absorbs very little moisture -- less than 1 percent of its weight at standard conditions. It is hydrophobic, dries quickly, and resists mildew.

Nylon 66 absorbs significantly more moisture -- approximately 4 to 4.5 percent at standard conditions and more at high humidity. Nylon thread swells slightly when wet, and wet nylon loses a small percentage of its dry strength (though it recovers upon drying).

For applications where the thread will be frequently wet or exposed to high humidity, polyester's low moisture absorption is an advantage. For garments, nylon's slight moisture absorption can contribute to comfort because the thread behaves more like the natural fibers it is sewing.

Abrasion Resistance

Nylon 66 wins. Nylon's molecular structure -- particularly the hydrogen bonding between polymer chains -- gives it superior resistance to abrasive wear. In repeated rub testing, nylon 66 consistently outperforms polyester. This is important for products that experience repeated friction on seams: footwear, luggage, belts, and outdoor gear.

Elastic Recovery

Nylon 66 wins for elasticity; polyester wins for dimensional stability. Nylon has higher elastic elongation and better recovery from extension. It stretches more under load and recovers more completely. This is advantageous in applications where the seam must flex repeatedly, such as footwear and athletic equipment.

Polyester has lower elongation, which is advantageous when dimensional stability is required -- the seam should not stretch under load. For seat belts, automotive seating, and applications where seam elongation must be minimized, polyester's lower stretch is beneficial.

Chemical Resistance

Polyester wins overall, with qualifications. Polyester has good resistance to most common chemicals, including many acids, oxidizing agents, and organic solvents. Nylon is susceptible to damage from strong acids and certain oxidizing agents but has good resistance to alkalis and many organic solvents.

For most sewing thread applications, chemical resistance requirements are moderate and both fibers are adequate. For specialized industrial applications where specific chemical exposure is expected, the chemical compatibility should be verified.

Dyeing

Both perform well, with different processes. Polyester is dyed with disperse dyes under high temperature and pressure. The process requires more energy and specialized equipment but produces excellent colorfastness.

Nylon is dyed with acid dyes or metal-complex dyes at atmospheric pressure and moderate temperatures. The process is simpler and less energy-intensive. Nylon achieves vibrant colors with good fastness, though wet-fastness can be a concern if dye selection and after-treatment are not optimized.

Application Recommendations

Footwear and Leather Goods

Recommendation: Nylon 66

The combination of highest strength, best abrasion resistance, excellent elastic recovery, and superior heat resistance makes nylon 66 the standard for footwear and leather goods. Bonded nylon 66 is preferred for multi-directional stitching operations.

Automotive Interiors

Recommendation: Both, application-dependent

For seat covers and interior trim where UV exposure through windows occurs, polyester's UV resistance is valuable. For airbags and seat belts where maximum strength is required, nylon 66 is specified.

Outdoor Equipment

Recommendation: Polyester for UV-exposed; nylon for strength-critical

Tents, awnings, and boat covers benefit from polyester's UV resistance. Climbing gear, parachutes, and load-bearing outdoor equipment use nylon for its superior strength.

Apparel

Recommendation: Both, depending on priorities

Spun polyester and PPC core spun thread are widely used in general apparel for their balanced properties and cost-effectiveness. PCC core spun thread is preferred for cotton garments and denim. Nylon is rarely used in general apparel due to cost and because its advantages are not needed.

Home Textiles

Recommendation: Polyester

Spun polyester provides good performance at the most economical cost for sofas, mattresses, curtains, and bedding. The performance advantages of nylon are typically not required.

Cost Comparison

Polyester is generally less expensive than nylon 66 as a raw material. This cost advantage flows through to polyester sewing thread, which is typically priced below equivalent nylon thread. The magnitude of the difference varies with petrochemical feedstock pricing and market conditions.

For applications where both fibers are technically suitable, polyester offers a cost advantage. Nylon's price premium is justified only when its specific performance advantages -- higher strength, better heat resistance, superior abrasion resistance -- are required by the application.

Making the Decision

Ask these questions when choosing between polyester and nylon:

What is the maximum strength required? If it exceeds polyester's capability, nylon 66 is the answer.
Will the product be exposed to sunlight? If yes and the exposure is significant, polyester is strongly preferred.
What sewing speeds are involved? At very high speeds, nylon's heat resistance provides a safety margin.
Is abrasion a significant factor? For high-abrasion products, nylon's advantage matters.
What is the cost envelope? If performance requirements are moderate, polyester is more economical.

Visit our product pages for detailed specifications: spun polyester yarn and nylon 66 filament yarn. For a broader comparison across all yarn construction types, see our spun vs filament vs core spun vs bonded comparison.

Conclusion

Polyester and nylon 66 are both excellent sewing thread fibers, and the choice between them is genuinely application-dependent. Polyester offers better UV resistance, lower moisture absorption, and lower cost, making it the preferred choice for most general sewing applications. Nylon 66 provides higher strength, better heat resistance, and superior abrasion resistance, making it the standard for the most demanding applications in footwear, leather goods, and safety-critical products. The informed buyer matches the fiber to the specific performance requirements of the end-use application.

Polyester vs Nylon Sewing Thread Yarn: A Comprehensive Comparison