Viscoelastic Stress Relaxation Mechanisms in Sized Bast Fibre Warp Sheets

Decay

On a stalled rapier loom, static tension drops thirty percent within the first two hundred seconds of an unscheduled stoppage. Backrest load cells record this drop as the composite structure of flax bundles and starch-based size yields under continuous load. Bast yarns behave nothing like synthetic filaments or carded cotton.

Their elementary fibres, bound by pectin and hemicellulose middle lamellae, display pronounced time-dependent deformation dictated by an internal spiral fibrillar angle of eight to eleven degrees. Held at an open shed angle of twenty-eight degrees, the sustained stretch on the upper warp sheet forces molecular rearrangement through the non-crystalline polysaccharide matrix.

Stress relaxation follows a logarithmic path while the machine stands idle. Primary creep sets in as soon as the shed forms, driven by instantaneous elastic strain and molecular uncoiling, while secondary creep develops over minutes of stoppage as viscous flow takes over. The initial tension of 0.45 centinewtons per dtex decays along a predictable curve, yet the sized shell encasing the technical fibres complicates this dissipation.

If the loom restarts without compensation, the beat-up force strikes an overly compliant warp, pushing the fell line toward the reed and leaving a dense band of picks.

Static tension across a 260-ends-per-decimetre flax sheet falls by 0.14 centinewtons per dtex after four minutes of shed dwell at 65 percent relative humidity.

A four-element Burgers model describes the viscoelastic behavior here: a Maxwell spring and dashpot account for immediate elasticity and steady viscous flow, while the parallel Kelvin-Voigt element captures the delayed, reversible deformation typical of plant cell walls under load. Sizing polymers shift the yarn’s retardation spectrum. Technical flax yarn sized with etherified potato starch shows an average retardation time between twelve and forty seconds.

Without size, the bast bundles slip faster and more locally, as the inter-fibre pectin gives way with no outer shell to resist shear.

Temperature and moisture dictate the dashpot viscosities. When ambient humidity shifts, water molecules plasticize both the native pectin and the external sizing film, accelerating stress loss along the free yarn span between whip roll and drop wires. In sheds running rapiers at 480 picks per minute, routine pauses for weft package changes or broken ends cause sharp tension dips.

As the warp slackens, individual ends sink below the trip threshold of electronic drop wires.

Whether high-speed shedding can ever decouple shear in the natural middle lamella from plastic flow in synthetic size films under static tension remains an unresolved problem on the mill floor.

Film

Dried size coats the outer filaments and penetrates twenty to thirty-five percent into the bundle core. Formulations combining polyvinyl alcohol and carboxymethyl cellulose form a sheath with its own distinct glass transition and relaxation profile. At standard conditions ~ twenty degrees Celsius and sixty-five percent relative humidity ~ this size matrix stays glassy, showing an elastic modulus above 2.2 gigapascals against roughly twenty-five gigapascals in the flax bundle core.

That ten-to-one stiffness mismatch concentrates heavy interfacial shear along the boundary during cyclic shed cycling.

Interfacial Shear Transfer Mechanics

Opening the shed transfers tension from the sizing coat into the core bast fibrils through adhesive shear. If the recipe runs short on plasticizer, that interface breaks down under repeated cycling. Micro-cracks spread through the starch-polyvinyl film, lowering the effective cross-sectional stiffness of the warp end.

Much comes down to penetration depth in the size box: a shallow crust flakes off in the harness eyes, while liquor that penetrates too deeply leaves the yarn stiff and brittle, inviting snap-offs at beat-up.

Size liquor concentration governs wet pick-up and yarn stability on the loom. Mixes prepared at eleven percent solids deposit eight to ten percent dry add-on on wet-spun flax. Higher add-on restricts instantaneous relaxation by locking the elementary fibres inside a stiff polymer sleeve, but too much deposition binds adjacent ends together, causing splits and breaks at the lease rods behind the loom.

Moisture exchange with the weaving room air is continuous. Sized bast warps dry out rapidly on the beam if protective polyethylene wrapping is torn or stored improperly. Losing just two percent moisture raises the size film’s glass transition temperature by eight degrees Celsius, embrittling the sheath and gutting its flex fatigue resistance; sheds running dry beams see warp stops climb past 4.2 per hundred thousand picks.

Cohesive strength between bast fibrils depends entirely on moisture stability within the amorphous cell wall polymers.

Chemical sizing formulations display characteristic failure sequences under tension relaxation:

• Interfacial debonding starts when differential Poisson contraction between the cellulosic core and the starch shell overcomes the adhesive bond.
• Transverse matrix microfissuring forms across the outer jacket at peak shed opening, opening routes for moisture loss.
• Fibrillar slippage spreads through the elementary bundles once plastic flow relieves the confining pressure of the outer polymer sheath.
• Progressive shed sag produces mispicks across the cloth width as slack lower sheets drop into the path of the rapier head.

Where a flexible polymer yields under stress, a dry bast yarn simply breaks.

Stop

Every machine stop breaks the dynamic equilibrium among let-off, shedding tension, and cloth take-up. While electronic let-offs hold running tension steady via dancer rolls or backrest load cells, an end break or exhausted weft package triggers an immediate stop with the warp locked at full shed displacement. Stress relaxation then bleeds tension from the yarn sitting between the weaver beam and the breast beam.

Shed geometry largely determines how severe that loss will be. In an asymmetrical shed, the upper warp sheet endures sixty percent greater extension than the lower sheet. Over a five-minute stop, ends in the upper shed relax substantially faster than those below.

Because the let-off drive restarts against the average load across the backrest, it cannot balance the uneven tension between the sheets, setting up distorted sheds, caught ends, and heavy starting marks.

Standard ISO 13934 test strips cut from cloth sections containing restart marks demonstrate a sixteen percent reduction in initial tensile modulus.

Mechanisms behind cloth starting marks divide into distinct structural faults:

1. Fell line creep occurs when the relaxed warp permits the cloth fell to pull back toward the reed under residual take-up tension, crowding picks into a dense band upon restart.
2. Tension drop thinning happens when the automated let-off feeds too much yarn at restart to hit running setpoints quickly, spreading picks too thinly over several insertions.
3. Sizing fracture occurs at beat-up when re-tensioned, cold-drawn starch shatters under reed impact, packing size powder into the reed dents.

Mark severity tracks stop duration closely. A pause under thirty seconds produces little visible fell movement, whereas stops past two minutes allow substantial secondary creep throughout the bast bundles. To prevent defects, weavers must dial in the controller’s anti-mark parameters, either advancing the fell or easing the take-up clutch before the first pick goes in.

Uncorrected fell movement leaves persistent optical banding across the fabric, leading finishing houses to reject entire broadcloth runs.

Audit

Evaluating sized bast warps demands dynamic testing rather than static checks. Nominal warp tension figures supplied by mills often mask localized stress decay across the sheet. Reliable incoming inspection tests the yarn under cyclic loading on constant-rate-of-extension instruments, cycling between 0.2 and 0.8 centinewtons per dtex at speeds that match running shed frequencies.

Inspection Parameters and Standards

Greige grading relies on the ASTM D5430 four-point system, which places start marks and stop bands in the heaviest penalty class. Any start mark running longer than two hundred millimetres across the width draws the maximum four points. In standard European apparel contracts, accumulating more than twenty-four points per one hundred square metres causes outright rejection.

Meanwhile, size add-on is confirmed by ISO 7211 desize washing to verify uniformity across the beam.

Sizing uniformity is checked through core samples taken across the weaver beam; size add-on between the flanges and the center must not vary by more than 1.2 percent by weight. Moisture variation through the package causes similar headaches. If inner yarn layers stay damper than the exposed outer wraps, stress relaxation rates drift as the beam weaves down, changing warp tension over the course of the run.

The weaver’s sizing formulation matches traditional specifications and machine settings remain identical to previous lots.

Ledger

Loom time governs the economics of bast yarn weaving. Running dense linen on a positive rapier carries an hourly overhead between twenty-two and thirty-four euros before yarn costs. When sizing fails to control stress relaxation, efficiency collapses: stops climbing from 1.5 to 4.0 per hundred thousand picks drag machine efficiency from eighty-eight percent down to seventy-one percent, driving weaving cost per linear metre up by twenty-six percent.

Worked Cost Breakdown under Creep Failures

A standard production run illustrates the financial fallout from warp relaxation defects, based on parameters typical for commercial linen apparel fabric:

• Cloth specification comprises a plain interlacing at 185 centimetres reed width, 22 ends per centimetre, 20 picks per centimetre, utilizing wet-spun flax 36 metric count.
• Weaving machinery utilizes positive rapier looms operating at 450 picks per minute at an assumed baseline efficiency of 85 percent.
• Standard warp lot involves a 4,500-metre weaver beam requiring approximately 176 operational loom hours under nominal conditions.
• Overhead cost base represents an all-inclusive shed rate of 28.00 euros per machine hour, encompassing power, labor, plant depreciation, and space allocation.

Uncontrolled relaxation quickly turns expected operating margins into dead shed hours. Consider a warp lot hobbled by poorly plasticized size and high static relaxation:

Between slack shed sag and drop-wire trips, the stop rate climbs from 1.2 to 3.8 stops per hundred thousand picks. At an average repair time of 2.5 minutes per break, total downtime across the 4,500-metre beam rises from 2.8 hours to 9.2 hours. Operating efficiency drops from 85 percent to 74 percent, pushing the time needed to weave out the warp from 176 hours to 202 hours ~ an extra 26 machine hours per beam.

Those extra 26 loom hours at 28.00 euros per hour add 728.00 euros in direct weaving overhead to the beam. Across a net yield of 4,350 finished metres, that surcharge adds 0.167 euros to every linear metre. When set marks force menders to cut out flawed sections, yield drops another 2.5 percent ~ sacrificing 108 metres worth 6.40 euros per greige metre.

Total losses reach 1,419.20 euros per beam.

Commercial purchase contracts routinely require a clause establishing that greige goods showing fell line shifts or set marks beyond ASTM D5430 Grade B allow the buyer an immediate fifteen percent invoice debit against the weaving mill to offset mending and downgrading costs.