Kuraray K-II WN4 41-Water Soluble PVA Fiber (Dissolves at 40°C)

Wet-laid filtration media lines operating at wire speeds above 120 m/min employ Kuraray K-II WN4 as a fugitive pore-forming binder rather than as a permanent structural fiber. The fiber is introduced at 2 wt% to 7 wt% of total dry furnish; below 2 wt% the pore-opening effect is masked by calendering, while at loadings above 7 wt% dissolved PVA can transfer to the surface of the first dryer can and generate a tacky deposit that increases sheet breaks. Stock pH is maintained between 6.0 and 7.5, and furnish temperature is held below 30°C because premature dissolution at or near the 40°C threshold raises dissolved-polymer content in the white water and consumes cationic retention aid. Machine chest residence is limited to 20 min or less in closed-loop systems. The sheet is formed on an inclined wire former, dewatered by vacuum boxes, and dried on a multi-cylinder section with first-can surface temperature of 95°C to 110°C; at this stage the PVA fiber enters solution and redistributes around cellulose or polyester staple intersections. After rewetting, the PVA phase leaches out and leaves a controlled void network. Sheet tensile is reported under ISO 9073-3, thickness under ISO 9073-2, and air permeability under ISO 9237; the air-permeability measurement is useful because the pore volume created by PVA removal is not reliably captured by basis weight alone. Published data for this specific K-II WN4 wet-lay configuration is limited, so mill validation should include a white-water solids mass balance and a PVA concentration check before full-scale roll production.

What limits continuous running time on circular seaming lines when K-II WN4 replaces solvent-soluble yarn?

Temporary separation courses in garment panels are knit from K-II WN4 filament or staple yarn at counts between Nm 50/1 and Nm 80/1, with the selected count determined by machine gauge and stitch length rather than by fiber strength alone. On a single-jersey circular machine running at 0.8 m/s to 1.4 m/s, feed tension is set between 2 cN and 4 cN; tensions above 5 cN produce filament fraying at the needle latch, and the resulting PVA dust accumulates in the trick groove. The yarn absorbs moisture rapidly above 55% relative humidity, which softens the fiber surface and increases yarn-to-metal friction; this is the primary continuous-running limitation observed on production-scale machines in humid knitting and weaving sheds. Storage before knitting is therefore kept at 20°C to 28°C and below 50% RH, with cones sealed in moisture-barrier film. After garment assembly, the separation yarn is removed in a hot-water bath set at 40°C for 10 min to 15 min at a liquor ratio not below 20:1; the bath must be agitated or the PVA dissolution front stalls at the stitch interstices. Wet strength of PVA water-soluble yarn is lower than dry strength, so the bath is not used for load-bearing seams. Tensile properties are reported under ISO 2062 or ASTM D2256/D2256M-21; converting mills should use the supplier certificate of analysis because water-soluble PVA tenacity and elongation are sensitive to storage humidity. Residual PVA on the garment after washdown is checked by a cool-water rinse and hand-feel test, because finite PVA residue can re-deposit on polyester knitting oils and create a stiff edge at the former separation line.

Infection-control laundry bags fabricated from the 40°C grade demand hydroentanglement rather than needlepunch bonding.

A production-scale hydroentanglement line can process K-II WN4 staple into a 30 g/m² to 60 g/m² fabric that retains sufficient wet strength for handling contaminated linen at room temperature but disperses at 40°C in the washer. Needlepunch bonding is generally unsuitable because the high barb density and needling density required for PVA staple can fracture the water-sensitive fiber and generate short-fiber debris that does not dissolve uniformly. The preferred forming route is carding followed by low-pressure hydroentanglement using water at 10°C to 25°C; if the recycled water temperature in the injector manifold exceeds 35°C, fiber dissolution begins on the belt and creates a translucent PVA film that blocks the perforated sleeve. Fabric tensile is measured following ISO 9073-3, and bursting strength is tested to ISO 13938-1:2019; a burst strength of 80 kPa to 150 kPa at fabric weights below 50 g/m² is regarded as workable for bag conversion, but the final specification depends on bag size and seam construction. Seams are closed with water-soluble adhesive tape rather than stitching thread because thread holes remain visible after dissolution and can trap insoluble lint. Bags are used in healthcare and containment wash processes where contaminated linen is sealed at the point of use and loaded directly into a washer; the 40°C dissolution point is compatible with cold pre-wash cycles and avoids manual reopening of soiled textiles. The fabric must be stored in sealed polyethylene outer bags at 10% to 40% RH because high humidity reduces dry burst strength and can cause blocking between adjacent layers. REACH registration status for the PVA polymer and local waste-water permits for dissolved PVA should be confirmed before implementing closed-loop laundry water recovery, since dissolved PVA contributes to chemical oxygen demand and can interfere with membrane filtration if not degraded.

For high-stitch-count emblems and standalone lace constructions processed on multi-head embroidery lines, K-II WN4-based soluble backing is used to avoid tearing fragile stitch patterns during manual release. The backing sheet is calendered to a thickness of 0.20 mm to 0.40 mm and is hooped together with the face fabric, then embroidered at speeds of 700 to 1,000 stitches per minute using a 75/11 or 80/12 needle. The backing must resist needle puncture deflection at high stitch density; if the sheet thickness is below 0.15 mm, densely packed satin stitches cut through the backing and create loose fibers that wrap around the needle, causing thread breaks. After embroidery, the assembled piece is passed through a 25°C rinse to remove water-soluble patterning oils and non-ionic lubricants, then through a 40°C spray or immersion stage for 5 min to 10 min; the second stage dissolves the PVA backing without disturbing the embroidery structure. The wash tank is renewed at intervals based on dissolved solids; dissolved PVA concentrations above 2 wt% in the tank make the water slippery and reduce the dissolution rate of the remaining backing. Tensile strength of the backing is measured to ISO 9073-3, and bending stiffness is evaluated by the converter because needle penetration force depends on stiffness rather than tensile strength alone. Residual backing is monitored by the absence of a white film on dark embroidery after the final cold rinse; any residual PVA film becomes stiff after textile finishing and is detectable as a mirror-like patch under oblique light. Where the finished product is exported to apparel brands, the converter should verify dissolved PVA in wastewater against the receiving region’s chemical oxygen demand limits.

When a water-soluble core fiber must disappear without collapsing the cotton sheath in hollow-core yarn manufacture

In core-spun yarn manufacture, K-II WN4 is used as a sacrificial core component that is removed after ring-frame spinning to produce a low-density hollow-core cotton or cotton-blend yarn. The process uses a PVA filament or staple core at 10% to 25% of final yarn mass, fed through a core-yarn attachment with controlled pretension of 5 cN to 12 cN; if pretension is too low, the PVA core migrates to the yarn surface during twisting, and if it is too high, the cotton sheath opens under the core tension and produces a slub-like fault. The sheath is spun from combed cotton or cotton/polyester staple at a twist factor of 3.8 to 4.2 (αe), because a compact sheath with adequate radial pressure is required to withstand the core removal stage and maintain the hollow channel. After the yarn is wound onto dye packages, the PVA is dissolved in a pressure dyeing vessel or package wash line at 40°C for 20 min to 40 min, with liquor flow reversed at intervals to prevent the collapsed PVA layer from channeling at the package flanges. The resulting hollow-core yarn exhibits lower bulk density and higher thermal insulation than equivalent solid yarn, but its tensile strength is reduced; yarn breaking force is evaluated under ISO 2062 and the strength loss is normally balanced by increasing the sheath cotton count by one or two yarn counts. The dissolution bath must be kept free of anionic acrylic size, because PVA can co-precipitate with acrylic size under acidic conditions and form a water-insoluble complex that cannot be rinsed from the cotton sheath. Published data for this exact K-II WN4 core-spun configuration is limited; package density, flow reversal frequency, and dissolved-PVA concentration should be validated on a pilot package machine before bulk production.