Kuraray VPB102-PVA Binder Fiber (Special) for Paper Making

Kuraray VPB102-PVA Binder Fiber (Special) for Paper Making is supplied as a highly hydrolysed polyvinyl alcohol staple fibre in cut lengths from 3.0 mm to 6.0 mm. Manufacturer technical data list a nominal linear density of 1.0 dtex, a fibre density of 1.30 g cm⁻³, and a dissolution threshold in neutral water of approximately 80 °C. Moisture content is commonly controlled at or below 5.0 wt% when measured by ISO 638. In papermaking, the fibre is introduced directly into the furnish without prior cooking and remains structurally intact through stock preparation, screening, and sheet formation. Binding action develops in the dryer section when the sheet temperature exceeds 80 °C, causing the polyvinyl alcohol phase to soften, flow into interfibre and filler boundary regions, and form fused bridges after cooling. That thermal activation sequence distinguishes the material from conventional PVOH solution binders, which alter white-water chemistry before sheet formation.

The product is used in wet-laid nonwovens, high-porosity media, filter papers, gasket papers, abrasive backing, and moulded pulp applications where dry strength, dust containment, and filler retention are required without continuous aqueous binder addition. In such grades, the fibre can be added at levels from 2.0 wt% to 8.0 wt% on an oven-dry furnish basis. Above 8.0 wt%, sheet porosity may decline if the dryer section produces continuous films rather than localised fused domains.

What limits fibre-to-fibre bonding during high-temperature wet laying?

Premature dissolution in the approach-flow system is the primary processing risk. The fibre is sufficiently insoluble to survive headbox turbulence at 40 °C to 50 °C, but machine chest temperatures above 60 °C initiate surface swelling and soften the staple. At 90 °C in neutral process water, full dissolution of 3.0 mm cut fibre has been reported within 15 min. Paper machines operating with steam-heated stock chests above 60 °C should introduce the fibre after the approach-flow screen or through a separate cold-slurry line. A cold-water suspension at 20 °C to 40 °C with low-shear mixing for 10 min is commonly used to separate the staple before metering into the white-water stream.

Dryer-section temperature, residence time, and web moisture content control whether the binder fibres form fused bonds or remain inert. On multi-cylinder dryer sections with surface temperatures of 105 °C to 120 °C, the sheet usually must remain above 80 °C for at least 10 s after web moisture falls below 40%. Through-air drying at 120 °C with dwell times of 0.5 s to 2.0 s can activate the fibre in low-density papers because hot air contacts the fibre surface directly. Insufficient dryer energy leaves a dusty non-bonded fibre network; excessive energy creates continuous film domains that lower air permeance. Dry tensile, burst, and air permeance should be tracked using ISO 1924-2, ISO 2758, and ISO 5636-3 to determine the activation window on a specific machine.

On a pilot wet-lay line with a pressure screen and a headbox consistency of 0.5 wt%, 3.0 mm VPB102 fibre is retained at levels comparable to cellulosic fines. An anionic polyacrylamide retention aid at 0.02 wt% to 0.05 wt% on dry furnish mass can improve first-pass retention without interfering with thermal activation. High-shear pulping is not recommended because it generates fine PVA fragments before sheet formation and shifts the fibre length distribution. The finished fibre does not contribute significant mineral ash; raw-fibre ash content is typically below 0.5 wt% when determined by ISO 1762.

Fibre geometry, Ash Retention, and Wet-Web Strength in High-Speed Wet Laying

Fibre length and linear density create a measurable trade-off between formation uniformity and bonding surface area. In laboratory handsheets prepared per ISO 5269-2, 1.0 dtex fibre at 3.0 mm length disperses more uniformly than 4.0 mm fibre, but the longer fibre produces higher wet-web tensile after vacuum dewatering. Addition levels from 2.0 wt% to 8.0 wt% modify dry tensile by progressive fused-network formation; the largest relative gain is normally observed between 3.0 wt% and 5.0 wt%. Below 3.0 wt%, the fused domains may be insufficient to bridge mineral filler and short-fibre regions, particularly on high-speed gap-formers with short dwell times.

Ash retention in filler-loaded sheets is a documented production benefit. In industrial trials with precipitated calcium carbonate at 15 wt% to 25 wt% loading, PVA binder fibre addition reduced white-water filler load and raised sheet ash retention. The mechanism is primarily physical encapsulation of filler agglomerates by fused PVA domains after the dryers, not electrostatic retention in the wet end. Filtrate turbidity measured by ISO 14870 or an equivalent wet-end monitoring method can serve as a proxy for lost filler. Published data for this specific configuration is limited for several paper grades, so machine-specific trials are required before setting release specifications.

Relative to polyvinyl alcohol solution or powder, VPB102 fibre eliminates separate cook-tank equipment and avoids adding water-soluble polymer to the white water. A starch cooker and solution metering pump are not required; the fibre is added through the stock preparation line. Activation occurs after sheet formation, so drainage and formation are affected only by the fibrous phase. Relative to styrene-acrylic latex, the fibre contains no added volatile organic compound and does not form a continuous film at wet-end temperatures, but it does require dryer-section thermal energy. Latex develops strength at lower drying temperatures, whereas VPB102 remains a discrete fibre until the sheet reaches the dissolution threshold. Relative to starch, the polyvinyl alcohol fibre is less biologically accessible, which reduces slime-growth risk in warm, long-retention paper machine systems. Relative to unmodified PVA staple, the VPB102 grade is produced with low hot-water resistance specifically for binder applications; standard PVA staple retains fibre structure at much higher temperatures.

When Borate Ions Are Present in the Process Water

Polyvinyl alcohol fibres are sensitive to borate species because borate ions crosslink vicinal diol units on the polymer chain. Process water containing more than 20 ppm boron as borate can form a gel-like surface skin that slows dissolution and creates stiff spots in the finished sheet. This crosslinking effect is deliberate in some PVA gel systems, but in papermaking it reduces uniform bond development and can increase sheet stiffness unevenly. Process water should be analysed for boron by inductively coupled plasma optical emission spectroscopy per ISO 11885 before VPB102 is used on a line with recycled process water or high boron-containing fillers.

Dissolution is fastest at neutral to mildly alkaline pH, typically from pH 7.0 to pH 9.0. Below pH 6.0, dissolution slows and dryer load may need to increase to reach the same fused-network formation. Persistent operation above pH 10 should be evaluated because high alkalinity can affect residual acetyl groups in the polyvinyl alcohol structure and alter water sensitivity of the bonded domains. The fibre is also incompatible with strong oxidising agents used in some felt-cleaning or deinking operations; carryover of oxidative species into the wet end can degrade the polymer and reduce bond strength.

Compliance position for food-contact paper and paperboard is grade-specific. In the United States, 21 CFR 176.170 lists components of paper and paperboard intended for contact with aqueous and fatty foods, and polyvinyl alcohol may be used subject to extractives limitations. For dry food grades, supplier confirmation against 21 CFR 176.180 is typically required. In the European Union, REACH registration under Regulation (EC) 1907/2006 must be confirmed on the safety data sheet for the specific lot. RoHS assessments under Directive 2011/65/EU usually focus on lead, cadmium, mercury, hexavalent chromium, PBB, and PBDE; PVA fibre is not a typical source of these substances, but the supplier test report should be reviewed for maximum concentration values. The table below summarises the principal compliance references.