Corte curto de fibra STW tipo 1-PVA

STW Type 1-PVA short-cut fibre is introduced into high-ductility engineered cementitious composites at a volume fraction of 1.5–2.0 vol%. The fibre is characterised by a nominal density of 1.29 g/cm³, tensile strength commonly cited between 1,200–1,600 MPa, elastic modulus of 30–38 GPa, and cut length of 6 mm when specified for ECC matrices. The fibre surface carries an oiling agent, typically 0.8–1.2 wt%, that governs fibre–matrix interfacial bond. An excessively high chemical bond produces fibre rupture before pullout and suppresses multiple-cracking behaviour; an excessively low bond allows slip at small crack openings and reduces ultimate tensile capacity. The matrix design generally uses a water-to-cementitious-material ratio of 0.25–0.30, polycarboxylate-ether high-range water reducer at 0.4–0.8% by mass of cement, and quartz sand with maximum grain size below 0.3 mm to avoid fibre snagging in the mixer.

Production-scale batching on a forced-action planetary pan mixer requires staged addition. The dry cement, sand, and supplementary cementitious materials are blended first until the powder bed is uniform, after which water and dispersant are added and mixed for 60–90 seconds. The short-cut PVA fibre is introduced only after stable paste flow is established; premature addition at dough-like consistencies creates fibre balls that cannot be re-dispersed at fibre contents above 0.8 vol%. A paddle speed of 45–60 rpm is common, and the mixer drive current is monitored because a rise of 15–20% relative to the fibre-free paste usually indicates insufficient slump flow or hydrophobic agglomeration. Moisture absorption beyond 0.5% by fibre mass, frequently observed after storage at relative humidity above 70%, produces cluster formation and requires pre-drying at 40–50°C for at least 12 hours before batching.

Mechanical verification follows ASTM C1609/C1609M for flexural performance and JSCE SF-4 for direct tension. Published data for strained ECC show tensile strain capacity in the range 3–5% and average crack widths below 60 µm when oiling content, matrix toughness, and fibre aspect ratio are optimised. The same formulations commonly fall below 1% tensile strain capacity when fibre addition exceeds 2.5 vol% because inadequate paste volume prevents complete fibre coating and trapped air raises matrix porosity. Hydrothermal curing above 90°C or autoclave conditions can increase interfacial bond and shift the failure mode from multiple cracking to localised fracture; published data for this specific STW Type 1-PVA configuration under autoclave conditions remain limited.

When PVA Short-Cut Fibre Replaces Welded Wire Mesh in Tunnel Shotcrete Linings

For wet-mix shotcrete, the fibre is added at the batching plant or at the nozzle through a pre-wetting hopper. Addition rates for plastic shrinkage crack control are commonly 0.9–1.8 kg/m³; toughness-driven replacement of welded wire mesh in temporary ground support more often uses 4–6 kg/m³ with cut length of 6–8 mm. The fibre must not be added directly into the rotating drum after the cement has begun to hydrate, because localised alkali and shear conditions produce fibre mats that block the pump screen. A screening plate with 12 mm openings and vibratory discharge are used to meter short-cut fibre onto the aggregate belt rather than introducing the fibre through the cement weigh hopper.

Hydraulic performance is controlled by the fresh-mix slump before fibre addition, typically 150–200 mm under EN 12350-2, and by pump hose diameter, which should be at least 2.5 times the cut length. Below this ratio, transient accumulation at a hose coupling creates a fibre mat that reduces effective bore diameter and increases pumping pressure by more than 30% within a few minutes. Nozzle air pressure for dry-mix application is kept at 5–7 bar; higher air pressure increases rebound and preferentially rejects lower-density PVA fibre. In-place density and early strength are verified under EN 14488-3 and EN 14488-7, while residual flexural toughness is assessed with ASTM C1550 or the EFNARC panel test. This substitution is generally limited to linings where crack widths below 0.2 mm are acceptable.

Unlike cementitious applications, the short-cut PVA fibre in wet-laid nonwovens and specialty paper must remain discrete until the drying section, where it becomes a thermally activated binder. Cut lengths are typically 3–6 mm and linear density is specified at 1.0–2.0 dtex to balance dispersion against mechanical entanglement. The fibre is charged to the pulper after the cellulosic furnish has been refined, usually at 5–30% by dry mass depending on whether the sheet is a high-porosity filter medium or a dense overlay. Headbox consistency is held below 0.2 wt% and stock temperature below 40°C to prevent the PVA surface from becoming tacky and forming rope-like aggregates on the forming wire.

Dispersion in white water depends on pH 6–8 and low anionic trash. Polyacrylamide dispersants at 0.05–0.15% by fibre mass are used when PVA loading exceeds 15% because the hydrophilic fibre can form visible flocks that produce low-strength zones in the sheet. Thermal binder activation is performed on a heated calender at 180–220°C with nip pressure of 30–70 kN/m, where the partially hydrolysed PVA surface softens without full melting. If calender temperature drops below 170°C, binder activation is incomplete and the sheet exhibits surface linting; above 230°C, fibre shrinkage and yellowing occur before acceptable bonding develops. Tensile strength is measured under ISO 1924-2, air permeability under ISO 5636-3, and wet-strength retention under ISO 3781. Compliance for food-contact paper is assessed under 21 CFR 176.170 and EU Regulation (EC) No 1935/2004, but end-use confirmation must be obtained from the converter for specific aqueous or fatty food types.

Low-metallic and non-asbestos organic disc pad compounds use PVA short-cut fibre as an alternative or partial replacement for aramid pulp and mineral fibre, primarily to stabilise the preform during cold compaction and reduce density variation before hot pressing. Cut lengths of 3–5 mm are common, and addition levels in published formulations typically range from 2–8 wt% of the finished friction material. The fibre is blended into the dry mix in a ploughshare mixer at 120–150 rpm for 5–10 minutes, after which the mixture is pressed into preforms under 20–40 MPa at room temperature.

The main processing boundary is the decomposition onset of PVA, reported for fully hydrolysed grades in air near 200–220°C. Hot-press cycles for PVA-containing underlayers are therefore held at 140–160°C with press times of 5–8 minutes, followed by post-cure at 170–190°C for 3–4 hours; formulations without PVA may use post-cure temperatures up to 230°C. If the curing oven overshoots above 200°C, the PVA component begins to char and can produce gas pockets at the pad–backplate interface, reducing shear strength under ISO 6312. Brake performance is assessed on a dynamometer using SAE J2568 or ISO 26867, but published data for STW Type 1-PVA in full fade-recovery sequences are limited, and substitution should be validated on the specific pad geometry because the fibre contributes to friction stability but not to high-temperature fade resistance.

Hydrogen Bonding and Short-Cut PVA in Extruded Gypsum Core

In an extruded gypsum core, PVA short-cut fibre improves green strength, edge hardness, and handling resistance before final drying. The fibre is introduced at 0.3–0.8 kg/m³ of board volume or 0.5–1.5 wt% of stucco mass in board lines processing high recycled gypsum content. Because the PVA surface is hydroxyl-rich, the fibre forms hydrogen bonds with free water and gypsum crystal faces during the setting reaction; this interaction reduces edge cracking when the wet board is transported at line speeds above 60 m/min.

The fibre is dispersed in the pin mixer after the stucco and water have formed a uniform slurry. Slurry pH is generally 6.5–7.5 and temperature below 50°C, which avoids premature softening of the PVA surface. Tests on hardened board follow ASTM C473 for flexural strength and nail pull resistance, and the finished board is classified under ASTM C1396/C1396M or EN 13279-1. A practical limitation occurs when high levels of citric acid or other low-pH retarders are used because prolonged wet slurry contact can swell the fibre and reduce pullout resistance before the gypsum sets.

What Are the Mixing Thresholds When PVA Short-Cut Fibre Is Dispersed in NBR and EPDM Compounds?

Short-fibre-reinforced rubber compounds for seals, hoses, and belt reinforcement incorporate PVA short-cut fibre at 10–25 phr, using cut lengths from 3–6 mm. The addition sequence is determined by the polarity of the base polymer. In polar NBR, the hydroxylated PVA surface disperses more readily after carbon black and plasticizer have been incorporated; in nonpolar EPDM, the fibre is preferably pre-treated with resorcinol-formaldehyde-latex or a silane coupling system to reduce fibre-to-fibre self-agglomeration in the internal mixer.

Mixing is performed in an internal mixer with a fill factor of 0.70–0.80 and rotor speed of 40–60 rpm, with the fibre added after the compound temperature reaches 60–80°C. Adding the fibre too early generates excessive shear heating and fibre breakage, reducing aspect ratio and tensile reinforcement efficiency. Mooney viscosity is measured under ISO 289-1; fibre loading above 25 phr can raise Mooney viscosity by 20–35 Mooney units and make calendering width control unstable below 1 mm sheet thickness. After sulfur curing at 150–170°C, the fibre orientation in the extrusion or calender direction produces anisotropic tensile properties. Tensile strength and elongation are tested under ISO 37, tear strength under ISO 34-1, and hardness under ISO 48-4. In nonpolar EPDM, untreated PVA fibre at loads above 12 phr tends to form surface blooms on the uncured sheet after storage; the bloom is caused by absorbed moisture rather than migration, and vacuum drying at 40–50°C for 4–6 hours before mixing is often required when the compound is stored at relative humidity above 60%.