Kuraray K-II WQ941-Fibra PVA solúvel em água (Dissolve a 95 ° C)

Kuraray K-II WQ941 is a water-soluble polyvinyl alcohol staple fiber specified for high-temperature dissolution at 95°C in water. The product belongs to the K-II family of polyvinyl alcohol fibers, CAS 9002-89-5, manufactured by wet spinning and drawing into a controlled crystalline structure that restricts solubility below the thermal threshold. It is normally supplied as cut staple in lengths of 4 mm or 6 mm and linear densities from 1.4 dtex to 1.7 dtex, with exact values reported on the lot certificate of analysis. The fiber remains intact in aqueous processing below approximately 80°C and dissolves only after sufficient residence at or above 95°C. This property creates a processing window for wet-laid nonwovens, papermaking, and other converters that require a temporary fiber phase.

Compared with warm-water PVA grades that dissolve at 70–80°C and cold-water grades that dissolve below 60°C, WQ941 tolerates higher wet-end temperatures, heated dryer cans, and hot-water showers without premature fiber loss. Standard non-water-soluble Kuralon polyvinyl alcohol fiber is used as a permanent reinforcing phase and does not dissolve. The K-II WQ941 grade is therefore selected when the process stream runs above 50°C but extraction can be performed with boiling or near-boiling water. Published grade-specific data for all water-soluble K-II subgrades is limited; grade selection should be confirmed by controlled bath testing under the target temperature-time profile.

How does the 95°C solubility threshold alter wet-laid nonwoven retention and dryer-section survival?

On a pilot Fourdrinier wet-laid line with headbox stock temperature controlled at 45–55°C, WQ941 can be blended with cellulose, glass, or synthetic staple at addition levels from 2 wt% to 10 wt% without measurable dissolution. The slurry can be held in pulpers and machine chests for 20–40 min without headbox consistency drift or increased filtrate chemical oxygen demand. Anionic or nonionic retention aids are preferred; strongly cationic polyelectrolytes may bridge the partially hydrolyzed fiber surface and reduce dispersion quality. Vacuum dewatering and wet pressing at linear loads up to 80 kN/m do not harm the fiber geometry because the web temperature remains below the thermal threshold. On multicylinder dryer cans, surface temperatures may reach 120°C, but the fiber remains intact because web moisture content drops quickly and dry heat does not solvate the polyvinyl alcohol phase. Extraction is then conducted in a separate bath or shower at 95–98°C with residence time from 30 s to 5 min, depending on basis weight, water turnover, and mechanical turbulence.

High-shear dispersion should be limited to the minimum energy required to separate fiber bundles. Excessive refining at high temperature fibrillates the fiber surface and increases the surface area, which can make the apparent dissolution onset appear below the nominal 95°C threshold. The stock should not be recirculated through centrifugal cleaners for more than 15 min at temperatures above 60°C unless the system is designed to handle dissolved polymer accumulation.

After the sheet has been resin-bonded, thermally bonded, or hydroentangled, the sacrificial PVA phase is removed to create a controlled void network. Extraction should be arranged countercurrently to prevent redeposition of dissolved PVA as the bath cools. Bath PVA concentration can be monitored by total organic carbon analysis or by a colorimetric boric acid-iodine complex method, with the method selected according to the site discharge permit. The extraction step is often the rate-limiting operation on converting lines because low bath agitation or insufficient overflow can leave residual PVA that reduces final porosity and air permeability.

As a dissolved species, PVA raises chemical oxygen demand in the extraction bath. For a hypothetical line producing 10,000 m²/day of 50 g/m² web with 5 wt% WQ941, the input fiber mass is approximately 25 kg/day. The extraction bath overflow and any downstream biological treatment must be sized for this organic load; otherwise bath viscosity increases and washing efficiency falls.

When Hydroentanglement Uses 80–90°C Water Jets Before the Final 95°C Extraction Step

In hydroentanglement with water-jet manifolds operating at 80–90°C, WQ941 provides temporary support to the fiber web only if the cumulative time-temperature exposure remains below the dissolution induction period. The control variable is the integral of jet temperature and residence time across all manifolds, recirculation loops, and wash boxes. At 85°C, partial surface dissolution may begin after several minutes, forming a PVA-rich boundary layer that raises recirculated water viscosity and can deposit on perforated drums. Lower-temperature water-soluble PVA grades would not survive this operation because their dissolution onset lies within the jet temperature band. To avoid defects, the water system should be operated with overflow bleed to limit dissolved solids, and the fiber should be validated in pilot trials with the specific jet pressure, drum speed, and manifold configuration. After entanglement, the web transfers to an extraction bath at 95–98°C where the fiber dissolves and is removed through overflow.

Removable PVA fiber forms have also been used in composite preforms to create microchannels or void networks after matrix cure. WQ941 can be laid into a carbon or glass preform, subjected to resin infusion, and extracted after cure with heated water. The process requires the cured matrix to be sufficiently open to water transport and dimensionally stable at the extraction temperature. Anhydride-cured epoxy systems with glass transition temperatures above 110°C are generally more suitable than flexible or low-Tg matrices. Published data for this specific configuration is limited, and cure exotherms above 180°C may alter PVA crystallinity and reduce the subsequent dissolution rate.

Aqueous Stability Boundaries in Borate-Containing Dispersions

Borate ions form reversible covalent complexes with vicinal diol units on the PVA chain and can gel the fiber surface or the extraction bath. Borax-containing retention aids, starch additives, or process water with elevated boron are incompatible unless the ratio and pH have been validated. Gelation can occur at low borate concentrations, particularly at alkaline pH and higher molecular weight. Processing pH is typically maintained between 4 and 9; outside this band the acetate functionality and surface charge shift, altering dispersion stability and dissolution behavior. Polyvalent cations such as Fe³⁺ and Al³⁺ should be controlled because they can bridge carboxylate/acetate sites and induce agglomeration. Storage is recommended below 65% relative humidity at 20–25°C. The fiber is hygroscopic, with moisture regain at 65% RH of approximately 4–5%. Material that has absorbed excess moisture should be dried at 105°C to constant mass before blending to avoid bridging in screw feeders and nonuniform dispersion.

Bales are typically supplied as compressed sheet-wrapped units in the range of 100–200 kg, sealed in moisture-barrier film. The bales should be conditioned in the production hall for 24 h before opening to prevent condensation on the fiber surface. Once opened, the fiber should be consumed within the shift schedule specified by the facility because moisture uptake can alter electrostatic behavior and make web formation less uniform.

Tensile and dimensional stability limits impose narrower dry-heat boundaries than the dissolution temperature alone

Water-soluble PVA fiber grades generally exhibit lower dry tenacity than non-water-soluble Kuralon reinforcement grades because the reduced crystallinity required for solubility also reduces load-bearing capacity. Supplier literature for water-soluble PVA staple commonly reports dry tenacity in the range of 3 cN/dtex to 5 cN/dtex and elongation at break of 15–25%, but these values are grade-dependent and should be taken from the certificate of analysis. The fiber should not be exposed to prolonged dry heat above 160°C, because discoloration and embrittlement may occur before any dissolution-related failure. For operations involving through-air dryers or infrared panels, the web surface temperature should be checked with an infrared pyrometer rather than inferred from the air setpoint.

Fiber diameter can be estimated from linear density and PVA density. At 1.4 dtex and a polyvinyl alcohol density near 1.28 g/cm³, the nominal filament diameter is approximately 12 μm. This geometry is useful in filtration media because the extracted pore width is initially defined by the fiber diameter. However, wet pressing and drying collapse and consolidate the web, so final pore size must be measured by capillary flow porometry such as ASTM F316 rather than inferred from fiber diameter alone.

Can WQ941 be used in alkaline or cementitious process streams?

No. Alkaline conditions can accelerate surface swelling and dissolution of water-soluble PVA, particularly at temperatures above 60°C. Cementitious boards, mortar reinforcement, and structural fiber-cement applications require non-water-soluble Kuralon or another high-tenacity PVA fiber. WQ941 is intended for neutral to mildly acidic process waters; if the process stream contains sodium hydroxide or other alkalinity above pH 9, a laboratory immersion test is required before production. The high-pH aqueous phase in cement pore water can also react with acetate and hydroxyl functionality, making the fiber unsuitable for permanent alkaline service.

Which standards govern specification of water-soluble PVA fiber and extraction verification?

Retention of water-soluble PVA fiber is influenced by cut length and the presence of surface finish. Long 6 mm fibers provide greater wet-web tensile strength but increase the risk of wrapping around rolls or forming fabric contamination. Short 4 mm fibers disperse more easily in low-consistency headbox stock. The choice should be matched to the forming section configuration and the target sheet basis weight. At basis weights below 30 g/m², the shorter cut length is preferred to reduce pinholing. Sheet air permeance after extraction can be characterized by ISO 5636-3 for paper and board or ISO 9073-15 for nonwovens.

For food-contact or medical packaging applications, the converter must not assume that the dissolution temperature establishes suitability. The extracted PVA is normally removed from the finished web; residual PVA content should be quantified by extraction followed by total organic carbon analysis or a validated colorimetric procedure. If residual PVA remains in an article intended for skin contact, the finished article must meet the relevant migration limits under the applicable jurisdiction. Textile embroidery backing and support yarns represent an additional use class, but WQ941 is appropriate only when the downstream washing process is conducted at or above 95°C. Many textile finishing operations run at 60–80°C; in those conditions a lower-temperature water-soluble grade is more appropriate because WQ941 would leave undissolved residue. The high dissolution temperature becomes a process defect if the substrate cannot tolerate boiling-water baths without dimensional change.