High Solids DOTP Plastisols Knife Over Roll Coating onto Release Paper

For release paper coating with high-solids DOTP plastisols, the formulation is differentiated from lower-solids organosol or vinyl solution routes by nonvolatile content above 98 wt% and the absence of a solvent removal stage before gelation. Dioctyl terephthalate, CAS 6422-86-2, is used as the primary plasticiser at loadings between 55 phr and 90 phr, with 70 phr representing a common starting point for fused films requiring Shore A hardness from 55 to 75. The PVC paste resin is an emulsion or microsuspension grade with a K-value between 66 and 78, selected to balance paste viscosity, gelation speed, and final tensile properties. Liquid barium-zinc or calcium-zinc stabilisers are incorporated at 2 phr to 4 phr, and epoxidised soybean oil is added at 3 phr to 5 phr as a secondary stabiliser and co-plasticiser. Fumed silica at 0.5 phr to 1.5 phr may be added to control low-shear viscosity and reduce sag, but it increases high-shear metering resistance and can reduce film gloss. Mixing is conducted in a vacuum dissolver at tip speed between 15 m/s and 25 m/s for 20 min to 40 min, followed by deaeration at 20 mbar to 50 mbar absolute pressure until visible bubble collapse ceases. The compound is filtered through 80 µm to 120 µm screen or bag media to remove agglomerates that would score the knife edge. Viscosity is measured at 23 °C with a Brookfield RV viscometer using spindle 6 at 20 rpm; high-solids DOTP compounds typically fall between 3,000 mPa·s and 12,000 mPa·s. Because DOTP has slightly lower solvation power than DOP or DINP, the mixed plastisol is often matured at 25 °C to 30 °C for 24 h to 48 h to stabilise viscosity and prevent blade gap drift during line start.

What Shear Rate Range Governs Knife-Over-Roll Coating Uniformity?

Knife-over-roll metering is not a simple slot-die process; the paste is flooded behind a stationary blade and dragged through a converging gap by the moving release paper and backing roll. The apparent shear rate in the gap is estimated as line speed divided by gap height. For a line speed of 15 m/min and a wet gap of 250 µm, the apparent shear rate is 1,000 s^-1; at 30 m/min and 150 µm, it reaches 3,300 s^-1. High-solids plastisols are strongly pseudoplastic; a compound with Brookfield viscosity 6,000 mPa·s at 20 rpm may exhibit a cone-and-plate viscosity of 900 mPa·s at 1,000 s^-1 when measured according to ISO 3219 or ASTM D2196. Thixotropic recovery is equally significant: after shearing at 1,000 s^-1 for 60 s, low-shear viscosity may redevelop only 60 % to 80 % of its original value within 5 min. That recovery window controls leveling after the blade. If viscosity recovers too slowly, the wet film retains longitudinal ribs and blade streaks; if recovery is too rapid, the film may hold shear-induced orientation defects. A practical low-shear to high-shear viscosity ratio of 3 to 6 is targeted for release paper coating, and lubricity additives such as fatty acid amides are used at 0.3 phr to 0.8 phr to reduce blade build-up and edge pinning. Blade pressure is monitored with load cells at each end of the blade beam; on a 2.5 m wide line, force variations above ±0.5 kN indicate viscosity drift, edge packing, or non-uniform feed. High-frequency pressure transducers mounted in the backing roll can detect chatter frequencies between 50 Hz and 500 Hz, which are associated with blade resonance and gauge barring.

On a production knife-over-roll line the release paper is unwound under closed-loop tension and the plastisol is deposited from a temperature-controlled trough immediately upstream of the blade. The backing roll diameter is typically 300 mm to 500 mm, with a chrome plating thickness of 50 µm to 100 µm over a ground steel core. Total indicated runout below 5 µm is required to hold wet gauge variation below 5 %. The knife blade is made from through-hardened high-carbon steel or tungsten carbide with a bevelled edge ground to a radius of 0.5 mm; straightness across 2.0 m is held to 0.01 mm. The gap is set by micrometer wedges and verified with feeler gauges at 7 positions across the web. Because the plastisol coating is metered wet and then fused, the wet film thickness must be 1.2 to 1.8 times the desired final thickness, depending on plasticiser content and filler loading. Release paper tension is normally maintained between 60 N/m and 120 N/m; higher tension reduces coating gap variation but can stretch the paper and shift registered patterns. Edge dams contain the paste and define coating width, set 10 mm to 20 mm beyond the desired trimmed width. Trough-level sensors maintain a constant hydrostatic head because level changes of 50 mm alter local pressure and affect wet film thickness. Air entrainment in the trough is suppressed by a low-level return port and a vortex-free feed design. Wet film thickness is measured immediately after the blade with non-contact laser triangulation sampling at 100 Hz; feedback loops adjust blade height to hold short-term variation within ±3 µm. The coating head is enclosed and supplied with filtered air to prevent surface defects from airborne particulates.

Thermal Degradation Pathways in DOTP Plastisols During Forced-Convection Fusion

In the fusion tunnel, the wet plastisol film on release paper is heated rapidly by forced convection and infrared emitters. The film surface temperature must exceed 170 °C for sufficient fusion, but the paper interface temperature may lag by 10 °C to 20 °C because of paper thermal mass and moisture release. Multi-zone ovens are typically configured with a first zone at 160 °C to 180 °C for gelation and moisture removal, a second zone at 190 °C to 210 °C for fusion, and a final zone at 180 °C for equilibration before cooling. At high line speed, residence time in the highest-temperature zone can drop below 20 s, producing a fused skin over a micro-particulate underlayer; tensile elongation then falls below 80 % and delamination may occur during transfer. DOTP is less volatile than DOP and DINP, but extended exposure above 220 °C still causes smoke, surface yellowing, and plasticiser loss. Thermal stabiliser depletion is monitored by static heat aging at 185 °C according to ISO 182-1; a high-solids DOTP film containing barium-zinc stabiliser should remain non-discoloured for at least 30 min. Calcium-zinc stabilised films often require 1.5 to 2.5 times higher loading for equivalent early colour, but may offer lower volatile organic compound emissions. Because release paper can char or embrittle above 210 °C, the maximum useful oven temperature is frequently set by the paper rather than the plastisol. Edge temperature under IR pyrometers should be held within ±5 °C of centreline; wider deviation shifts fusion and creates differential release force across the web. Oven air velocity is typically 15 m/s to 25 m/s, and nozzle height above the web is adjusted to prevent film skinning while maintaining adequate heat transfer. Moisture from the paper is removed in the first oven section; if residual paper moisture exceeds 7 %, steam can disrupt the wet film and produce crater defects.

Release paper selection is defined by surface energy, silicone anchorage, thermal resistance, and resistance to plasticiser absorption. The best performance for high-solids DOTP plastisols is obtained with high-density supercalendered base paper coated with a crosslinked silicone release layer; surface energy measured by contact angle with water and diiodomethane typically falls between 24 mN/m and 34 mN/m. The silicone must be anchored to prevent transfer to the fused PVC film; a rub-off test with adhesive tape after 10 double rubs is used to detect loose silicone. Release force should be low enough for clean separation but not so low that the coating separates prematurely from the paper during winding. Typical release values for silicone-coated papers against a 30 N/m peel angle are 10 cN/cm to 40 cN/cm; values below 5 cN/cm create winding and register instability. Paper moisture content is controlled to 4 % to 6 %; below 3 % the paper becomes brittle, and above 7 % steam can disrupt the wet film during oven entry. The paper must exhibit dimensional stability at 210 °C; cross-direction shrinkage above 0.5 % causes edge weave and coating neck-in. Reuse of release paper is possible only if the silicone layer remains intact and the paper has not been embrittled; production trials often limit reuses to 3 to 5 passes for critical surface appearance grades. Chemical incompatibility between DOTP and some silicone release systems may arise after prolonged contact at 40 °C; migration testing by extraction in accordance with ISO 177 can be used to detect plasticiser interference with silicone cure, but published data for this specific configuration is limited. The unwind and rewind tension profiles are tapered to avoid paper stretching and silicone cracking.

When Coating Thickness Drops Below 80 µm, Pinholing and Pattern Replication Intensify

Thin-gauge high-solids DOTP coatings on release paper exhibit defect modes that are suppressed in thicker films above 150 µm. Below 80 µm fused thickness, the wet film must be less than 100 µm before gelation. At this thickness, a 20 µm particle or entrapped air bubble spans a large fraction of the cross-section and becomes a pinhole after fusion. Vacuum deaeration must reduce air content below 0.2 vol% for thin films, and coating must be performed in a clean environment with air filtration to ISO 14644-1 class 8 or better. Pattern replication from the release paper texture becomes desirable for some synthetic leather finishes, but for smooth films the paper surface roughness must be below 0.5 µm Ra. The knife itself may generate chatter marks if machine speed and blade natural frequency interact; blade holders are damped and the blade angle is set between 20° and 30° from the tangent, balancing metering force against surface smoothness. At high line speeds, air is dragged into the gap by the moving web; a vacuum deaeration chamber immediately before the blade or a reduced puddle height reduces this entrainment. Leveling after the blade is controlled by surface tension and low-shear viscosity; wetting agents at 0.2 wt% to 0.5 wt% may reduce surface tension to 28 mN/m and improve release paper wetting, but excess surfactant can bloom to the fused surface and reduce adhesion of subsequent lacquers or printing inks. Coating weight is verified by in-line beta or X-ray gauges calibrated to ±1 %; thin-film variations above ±5 % produce visible gloss differences after transfer lamination. The gauge profile is usually recorded in 10 mm lanes and trended against blade gap and backing roll temperature.

Plasticizer Migration and Blocking Resistance in Stored Rolls

After fusion and cooling, coated release paper is wound into rolls. DOTP has lower migration tendency than DOP in PVC, but at storage temperatures above 35 °C plasticiser can still migrate to the film surface and accumulate at the release interface over 7 days to 30 days. The migrated layer may act as a weak boundary and reduce adhesion during transfer to fabric or foam. Blocking resistance is tested by pressing two coated surfaces at 50 °C under 5 kPa for 24 h and measuring peel force; acceptable high-solids DOTP films may show peel below 0.5 N/25 mm. Addition of polymeric plasticiser at 5 phr to 15 phr reduces migration but raises viscosity and lowers low-temperature flexibility. Surface gloss and haptics are controlled by the release paper texture and not by post-finishing; therefore any blocking or silicone contamination directly becomes a surface defect. The roll winding tension should be tapered from 80 N/m at the core to 20 N/m at outer layers to avoid blocking and paper creasing. Storage conditions of 20 °C to 25 °C and 45 % to 55 % relative humidity are recommended; higher humidity may re-wet the paper and cause curl, while lower humidity may embrittle the release coating. A pigmented plastisol may be used for opaque films, but filler loadings above 20 phr increase viscosity and reduce tear strength below acceptable thresholds for transfer coating. The release paper is peeled at speeds up to 30 m/min during downstream lamination; static charge generation can exceed 2 kV and requires antistatic bars to prevent dust pick-up and operator hazards.