Introduction
Yarn is not inert to its environment. Temperature and humidity conditions during storage, during thread manufacturing, and during the final sewing operation all affect how the yarn behaves. A yarn that performs perfectly in an air-conditioned thread mill may behave very differently in an un-air-conditioned sewing factory in a tropical climate.
This article examines the effects of temperature and humidity on sewing thread yarn and how to manage these environmental factors for consistent performance.
Moisture Regain: The Fundamental Property
What Moisture Regain Means
Moisture regain is the mass of moisture present in a textile material expressed as a percentage of the oven-dry mass. Different fibers have characteristically different moisture regain values: polyester absorbs very little moisture from the atmosphere — typically well under 1 percent regain at standard conditions — while nylon absorbs significantly more.
Moisture regain matters because absorbed water acts as a plasticizer, changing the fiber's mechanical properties. Water molecules penetrate between polymer chains, reducing intermolecular forces and making the fiber more extensible and less stiff.
Equilibrium with Ambient Conditions
Yarn left in any environment will eventually reach equilibrium moisture content with that environment. This equilibration is not instantaneous — it takes hours or days depending on the package size, yarn density, and air circulation. A yarn package brought from a cold warehouse into a warm, humid production area will gain moisture gradually from the outside in.
The practical implication: yarn recently moved between different environments has not yet stabilized and may exhibit changing properties during processing.
Temperature Effects
High-Temperature Behavior
Synthetic yarns soften progressively as temperature increases, well before reaching their melting point. For polyester, significant mechanical property changes begin to occur at temperatures encountered in high-speed sewing. Needle heat — generated by friction between the needle and the fabric — can raise the needle temperature substantially, and the thread passing through the needle eye is heated by contact.
At elevated temperatures, the yarn's modulus decreases — it becomes less stiff — and its elongation increases. The lubricant on the yarn surface may also change viscosity with temperature, altering the yarn's frictional properties. A lubricant that performs well at room temperature may become too thin at elevated temperature, reducing its effectiveness, or may degrade, leaving gummy deposits on thread guides.
Low-Temperature Behavior
At low temperatures, synthetic yarns become stiffer and may lose some elongation. Lubricant viscosity increases, potentially increasing thread tension during sewing if the yarn and machine are cold. For sewing operations in unheated facilities during winter, the initial startup period — before the machine warms up — may see different thread behavior than steady-state operation.
Thermal History and Heat Setting
The yarn's thermal history — the temperatures it experienced during production processes such as drawing, texturing, twisting, and heat setting — affects its response to subsequent temperature exposure. Heat-set yarn has internal stresses relaxed by the heat-setting process. Yarn that was not heat-set retains these internal stresses and may shrink or change properties when subsequently heated.
Humidity Effects on Yarn Properties
Strength Changes with Moisture
Polyester's tensile properties are minimally affected by moisture because it absorbs so little water. For practical purposes, polyester yarn strength can be considered independent of humidity under normal operating conditions.
Nylon's tensile properties are more significantly affected. Nylon loses a measurable percentage of its breaking strength when saturated with moisture compared to its dry strength. This wet-strength reduction is reversible — the strength returns when the nylon dries. For sewing thread applications where the thread will be used on wet fabric or in high-humidity environments, this wet-strength reduction must be accounted for.
Dimensional Changes
Nylon yarn changes dimension with moisture content — it swells slightly and elongates as moisture is absorbed, and shrinks as it dries. This dimensional change, combined with the yarn's tendency to contract when heated, can cause seam puckering if the thread shrinks more than the fabric during washing and drying.
Polyester's dimensional stability with moisture change is excellent. Its minimal moisture absorption means minimal dimensional change, making it the preferred yarn for applications where seam flatness after washing is critical.
Static Electricity
Low humidity environments — common in heated buildings during winter — promote static electricity buildup on synthetic yarns. Static causes yarn filaments to repel each other, making the yarn fluffy and difficult to control. Static discharge can also attract dust and lint to the yarn surface.
Maintaining relative humidity in the range of 50 to 65 percent in sewing and thread manufacturing areas significantly reduces static problems. If humidity control is not practical, anti-static finishes on the yarn or anti-static equipment in the production area may be necessary.
Practical Environmental Management
Storage Environment Recommendations
Yarn should be stored in conditions that minimize property changes before use. Recommended storage conditions include moderate temperature — avoiding extremes — and relative humidity in the range of 50 to 65 percent. The storage area should be protected from direct sunlight — which causes UV degradation and localized heating — and from sources of moisture such as leaks, condensation, or floor washing.
Conditioning Before Use
Yarn brought into the production environment from storage — particularly if storage conditions differ from production conditions — should be allowed to condition before use. Conditioning time depends on the package size and the magnitude of the environmental difference. As a general guideline, palletized yarn should be brought into the production area at least 24 hours before use.
Monitoring and Documentation
Record the temperature and humidity in your yarn storage and production areas. When investigating a quality issue — unexpected thread breakage, tension variation, dimensional problems — environmental conditions should be among the first factors checked. A correlation between performance problems and environmental conditions — for example, breakage increases on dry winter days — points to humidity-related static as a likely cause.
For yarn products with consistent performance across environmental conditions, visit our Spun Polyester Yarn and Nylon 66 Filament Yarn pages.
