Surface Sizing on High-Speed Fine Paper Machines: Rheology, Film Transfer, and Cobb Limits

Oxidized cassava and hydroxyethylated maize starches are applied at the metering size press of woodfree fine paper machines at dry pick-up values between 0.6 g/m² and 1.8 g/m² per side, which corresponds to 0.4 wt% to 1.2 wt% of base sheet mass for grammages from 70 g/m² to 120 g/m². The cooked starch tank is held at 60–65°C and the working solids are maintained between 8 wt% and 12 wt%; Brookfield viscosity measured at 60°C with spindle 2 at 100 rpm is limited to 15–25 mPa·s for metering rod units, because higher viscosities induce rod instability and orange-peel transfer after the film split. On Speedsizer and OptiSizer film-transfer configurations, the pond dwell time is sufficiently short that starch retrogradation at the nip does not dominate, but the capillary number at the pre-metering gap controls the wet film continuity. Machine-side retention of the surface starch is monitored by iodine staining of cross-direction samples, and the resulting surface strength is quantified by low-peel IGT pick tests reported under ISO 3783 and by internal bond measurements under ISO 16260. The water resistance of the sized sheet is measured with the ISO 535 Cobb 60 method; for a 90 g/m² multipurpose office paper, a Cobb value between 22 g/m² and 28 g/m² is generally targeted, with internal sizing from AKD or ASA run separately in the wet end at 0.08 wt% to 0.20 wt% cationic starch as a retention aid. Wet-end cationic starch is not a direct substitute for surface sizing; it contributes to filler retention and zeta potential control at headbox conductivity from 800 µS/cm to 1,500 µS/cm, while surface sizing controls surface strength and curl. Drying capacity after the size press remains the limiting variable on many machines: a starch pick-up of 1.2 g/m² per side adds roughly 5–7 kg water per 100 kg sheet at 10 wt% solids, requiring additional after-dryer evaporation before the reel.

Process conflicts arise when mills attempt to raise size press solids above 12 wt% to reduce drying energy. Oxidized starch with carboxyl content in the range 0.15–0.45% exhibits viscosity instability after prolonged holding at 60°C due to oxidative chain scission and retrogradation of amylose; gel particles larger than 100 µm then accumulate on the metering rod and create streaks. Continuous jet cooking at 125–135°C for 8–15 s provides complete granule disruption, but the cooked starch must be diluted immediately and held under gentle agitation. Batch tanks with steam sparging at 95–100°C followed by cooling to 60°C produce acceptable viscosity if the total holding time from cook to application is less than 4 h; beyond this the viscosity rises and the Cobb 60 response becomes uneven. The interaction with internal sizing chemicals is pH-dependent: at pH below 6.5, residual alum can complex with oxidized starch and increase the wet film viscosity at the size press, while at pH above 8.0 the catalytic activity of residual polyaluminium chloride may accelerate starch hydrolysis. Therefore the surface sizing system is buffered to pH 6.8–7.5 with monosodium phosphate or citric acid, and the starch solution is filtered through 40 µm slotted screens before the supply tank. Broke recirculation containing cationic wet-end starch and surface starch can create pitch-like deposits when the mill closes the whitewater system; this is controlled by measuring cationic demand of the headbox furnish and limiting surface starch loss to the wet end from edge trim re-pulping.

When Borax Gel Point Shifts Under High-Shear Corrugator Transfer

Corrugating adhesive prepared by the Stein-Hall process uses a carrier starch gelatinized with caustic soda and a raw starch suspension that remains ungelatinized until heat and pressure are applied on the hot plates. The carrier portion commonly represents 12–18 wt% of the total starch; caustic soda is added at 0.8–1.5 wt% of total adhesive and borax decahydrate at 0.3–0.8 wt% of total adhesive to crosslink the swollen carrier granules and build tack. Stein-Hall cup viscosity at 38°C is maintained between 20 s and 40 s for single-wall board running on fingerless corrugators at speeds above 250 m/min. The gel point is controlled between 54°C and 62°C by adjusting the borax ratio relative to caustic; below 54°C the adhesive begins to gel before entering the flute tips and starved bond lines appear, while above 62°C the raw starch swells too slowly and green tack is insufficient to hold the combined board before the hot plates. Glue roll application is set with a doctor gap of 0.15–0.35 mm, and the adhesive film transferred to flute tips is evaluated visually for skip and stringing at line speed. Finished board performance is verified by the edge crush test under ISO 3037, flat crush test under ISO 3035, and pin adhesion under TAPPI T 821; for a 5.0 mm flute profile, minimum pin adhesion values in the range 2.5–4.0 N/cm are typically specified in board specifications.

The most common failure mode on high-speed corrugators is shear-induced thinning of the carrier phase before the raw starch gelatinizes. Recirculating pumps and doctor roll metering subject the adhesive to shear rates above 10³ s⁻¹; if the borax level is too low, the swollen carrier granules are insufficiently crosslinked and lose viscosity irreversibly, causing the adhesive to penetrate the medium and produce brown starch lines. Conversely, excess borax raises the gel point beyond the energy available in the hot plate section, so the raw starch remains partially ungelatinized at the dry end. Heat transfer on the hot plates is governed by the steam pressure set between 7 bar and 12 bar; the board surface temperature must exceed 75°C to complete gelatinization of the raw fraction within the available belt time. Water balance is also critical: total adhesive solids range from 20 wt% to 28 wt%, and the wet-bulb depression in the cooling section determines the final moisture of the combined board. Final board moisture measured under ISO 287 is typically controlled between 7.5 wt% and 9.5 wt% to avoid warp. Starch with bimodal granule size distribution and native potato starch in the carrier provide faster gel formation than maize-only systems, but the peak viscosity demand must be matched to the corrugator speed and flute geometry. Operators adjust the borax-to-caustic ratio within narrow limits; a shift of 0.05 wt% borax can alter gel point by approximately 1.5–2.0°C, which is sufficient to cause measurable reductions in shear strength on the single-facer bond.

Gypsum board core formulations incorporating acid-modified starch at addition rates from 0.3 wt% to 0.8 wt% by weight of calcined gypsum are processed through pin mixers and extruder formers at belt speeds of 60–140 m/min with water-to-plaster ratios from 0.65 to 0.85. The starch is pregelatinized by the 170–220°C dryer sections and migrates toward the paper-core interface as free water is removed; this migration develops paper bond measured by wet bond and dry bond procedures described in ASTM C473. At addition levels below 0.3 wt%, the paper-core bond becomes the limiting defect in humidified handling, while levels above 1.0 wt% usually reduce core compressive strength and increase fuel consumption because the starch fraction binds additional water that must be evaporated. Retarder and dispersant chemistry directly alters starch distribution: protein-based retarders at 0.05–0.15 wt% of hemihydrate extend the setting window but can depress pH below 7, while polycarboxylate ether dispersants increase fluidity and thin the starch film. The forming table must maintain a uniform slurry cover because static drainage periods produce starch-enriched zones at the bottom face and starved zones at the top paper. Board physical properties are verified under ASTM C1396/C1396M for flexural strength, edge hardness, and humidified deflection, with typical core compressive strength values after conditioning at 45°C and 90% RH declining by less than 15% from the dry condition when starch addition is within the stated range. Pre-drying of the starch is required in plants where raw starch storage exceeds 60% RH; free moisture above 14 wt% in the starch powder leads to lump formation in the dry feed system and non-uniform distribution in the pin mixer.

Process control of modified starch in gypsum board is less about rheology than about migration kinetics in a non-isothermal setting process. The exothermic recrystallization of hemihydrate generates a temperature rise from 25°C to 55–65°C depending on accelerator dosage; starch gelatinization begins at the paper interface where wet-bulb temperature and local moisture are highest. If the initial starch suspension is too coarse or the acid-thinned viscosity is too high, the starch remains as granules near the core and does not contribute to bond. The use of pregelatinized starch shortens the gelatinization path but raises the demand for mixing water; therefore mills select between acid-modified ungelatinized starch and pregelatinized starch based on the dryer heat balance and paper porosity. Board produced with starch levels above 1.0 wt% often displays calcination marks near the paper edges because the starch film blocks vapor release; this defect is visible as gray bands and corresponds to a drop in wet bond strength measured by the ASTM C473 one-minute water soak method. Published data for the interaction between silicone-treated paper facings and starch migration in this specific configuration is limited; therefore lab-scale pin mixer trials are required before changing starch dosage by more than 0.2 wt%.

On slasher lines running ring-spun cotton and cotton/polyester warp yarns at linear speeds from 80 m/min to 120 m/min, acid-thinned and starch ester size films provide the abrasion resistance required for air-jet insertion at loom speeds above 900 rpm. The size box is maintained at 6–10 wt% solids for fine counts from Ne 40 to Ne 80, with viscosity between 12 s and 22 s measured by a Zahn cup 3 at 85°C. Size pick-up is typically 8–14 wt% on yarn weight, and the dried warp exhibits residual elongation above 4% to withstand drop wires and reed impact. Yarn properties after sizing are verified by ISO 2062 single-end breaking force and elongation; a size film that increases the coefficient of friction by more than 0.1 against a steel reed causes excessive float breakage, while a film that is too brittle generates size dust in the loom shed. The desizing stage is constrained by effluent load: starch esters hydrolyze under alkaline desizing at pH 8.5–9.5 and 85–95°C, but residual starch size contributes COD values from 5,000 mg O₂/L to 15,000 mg O₂/L in desizing wash water before biological treatment. Therefore conversion to low-BOD starch esters or blends with polyvinyl alcohol is evaluated against ISO 14001 environmental compliance targets in integrated mills.

Operational limitations for modified starch in warp sizing are set by the cooking equipment and the drying capacity of the slasher. Steam-injection cookers at 90–95°C with shearing impellers can degrade acid-thinned starch chains if the impeller tip speed exceeds 12 m/s; the resulting viscosity loss reduces size pick-up and increases penetration into the yarn core, which complicates desizing. Trough temperature is held at 80–85°C because cold troughs below 75°C allow amylose retrogradation and skin formation on the squeeze rolls. The squeeze roll hardness is set between 70 Shore A and 85 Shore A; harder rolls increase liquor expression and reduce add-on, while softer rolls carry excess size into the drying cylinders. In high-humidity weaving sheds, starch film softens at equilibrium moisture above 12 wt%, reducing abrasion resistance and increasing loom stops. The combination of modified starch with amine-based antistatic agents is avoided because the amine groups can form hydrogen-bonded networks with starch hydroxyls, raising the glass transition temperature of the dry film and causing it to fracture at the lease rods. Published data for specific high-speed rapier configurations is limited; therefore trial runs on the target loom are required when substitution of native starch by acid-thinned starch exceeds 30 wt% of the size recipe.

What Limits Salt-Tolerant Fluid Loss in Sodium Chloride-Saturated Brines?

Carboxymethyl starch (CMS) and pregelatinized crosslinked starch are used in sodium chloride-saturated drilling fluids at concentrations between 2.0 lb/bbl and 6.0 lb/bbl (5.7 kg/m³ to 17.1 kg/m³) to reduce filtrate invasion under API low-temperature conditions. Filtration performance is measured in a low-pressure filter press according to API RP 13B-1 at 100 psi differential and 25°C; unweighted NaCl saturated systems with 4.0 lb/bbl CMS typically yield API filtrate values below 5.0 cm³/30 min when the starch is prehydrated before addition. Rheological measurements on a Fann 35 viscometer at 600 rpm and 300 rpm are used to derive plastic viscosity and yield point; starch contributes primarily to the low-shear-rate viscosity controlling barite suspension. In the field, the starch is added through a high-shear mixing hopper with hopper screens at 20–40 mesh; premature addition in the slugging stream without prehydration creates fisheyes that plug shale shaker screens. Salt tolerance is achieved through carboxymethyl substitution of the starch hydroxyl groups; substitution levels between 0.3 and 0.6 carboxymethyl groups per anhydroglucose unit provide solubility and salt resistance, but over-substitution above 0.6 increases polyelectrolyte swelling and makes the polymer sensitive to calcium ions in makeup water.

The operational boundary for starch-based fluid-loss additives is thermal hydrolysis. Sodium chloride-saturated systems are generally applied at bottomhole circulating temperatures below 107°C; above 121°C, starch chains degrade rapidly in the alkaline mud environment, and the API filtrate increases after hot rolling aging at 121°C for 16 h. The hot rolling test under API RP 13B-1 therefore discriminates starch from synthetic polymers; a change in filtrate greater than 2.0 cm³/30 min after aging is often used as an acceptance limit for low-temperature wells. Workover and completion fluids using sodium chloride or potassium chloride at 9.0–11.0 lb/gal density have a narrow pH window from 8.5 to 10.0; outside this range the carboxymethyl starch either precipitates under acidic conditions or undergoes rapid oxidative breakdown in the presence of strong oxidizers such as hypochlorite-based biocides. Fermentation control is required in surface pits: starch is a nutrient source, so a non-oxidizing biocide or high-salinity environment is maintained, and pH is monitored every 4 h during hot weather. The combination of starch with polyanionic cellulose can provide complementary filtration performance, but starch should not be used where calcium chloride concentrations exceed 100,000 mg/L or where formate brines dominate, because divalent cations and formate salts alter the hydration of the carboxymethylated starch and published data for this specific configuration is limited.

Tablet Disintegration Forces in Low-Moisture Wet Granulation

Pregelatinized starch and sodium starch glycolate are incorporated into wet-massed granulations for immediate-release tablets at binder levels from 5 wt% to 10 wt% of formulation weight and disintegrant levels from 2 wt% to 4 wt%. Granulation is performed in high-shear mixers with impeller tip speeds of 3–8 m/s and wet mass moisture between 12 wt% and 20 wt%; the binder is usually dispersed in water at 60–80°C before addition to the dry blend. After fluid-bed drying to loss on drying 1.5–2.5 wt%, the granules are milled through 0.8–1.25 mm screens and compressed on rotary tablet presses at compaction pressures from 80 MPa to 180 MPa. Tablet hardness is monitored by diametral crushing and reported in kiloponds or newtons; for a 200 mg immediate-release tablet, a hardness of 60–100 N is generally targeted. Disintegration time is measured under USP <701> and Ph. Eur. 2.9.1 in water at 37°C; the limit is typically 15 min for uncoated tablets and 30 min for film-coated tablets. Sodium starch glycolate functions by rapid wicking and swelling: its settled volume in water reaches 20–30 mL/g, and its swelling capacity is retained even after wet granulation, unlike crospovidone which may lose efficiency when exposed to granulation moisture. Pregelatinized starch contributes plastic deformation during compaction, increasing tablet tensile strength without raising ejection force to the same extent as brittle fillers such as dibasic calcium phosphate.

The main formulation conflict is compression pressure versus disintegration. As compaction pressure exceeds 180 MPa, the pore network of the tablet collapses and the swelling force of sodium starch glycolate becomes insufficient to disrupt the matrix; disintegration time may rise beyond the 15 min limit even though hardness continues to increase. At compression pressures below 80 MPa, capping and lamination occur because the pregelatinized starch binder is not sufficiently deformed. The moisture content of the granules is equally critical: if the water activity of the granulation exceeds 0.4 at compression, sticking to the punch faces is observed, while overdried granules below 1.0 wt% LOD create the opposite problem of insufficient bonding and high friability. Tablet friability is measured according to USP <1216>; a maximum mass loss of 1.0% after 100 rotations is the usual acceptance criterion. Regulatory compliance for the modified starch component is evaluated under FDA 21 CFR 172.892 for food-grade modified starches and the relevant pharmacopeial monograph for pregelatinized starch and sodium starch glycolate. The modified starch must be free of ethylene oxide residues where hydroxyethylation is used; residual ethylene oxide is controlled by the monograph limits and by ICH Q3C residual solvent risk assessment. In formulations with amine-containing active pharmaceutical ingredients, pregelatinized starch can participate in Maillard-like reactions at high granulation drying temperatures above 80°C; therefore fluid-bed inlet air temperature is maintained at 60–70°C when drying amine-containing drugs. The substitution of native starch by pregelatinized starch at levels beyond 10 wt% is not recommended for low-dose tablets because the dilution of the active drug may compromise content uniformity, which is verified under USP <905>.