Chlorinated rubber with a chlorine content of 64–65 wt% is charged into a 5,000-L jacketed glass-lined vessel that contains high-density C3 solvent preheated to 40°C under a nitrogen blanket at 0.02 MPa positive pressure. The solvent density is 1.35–1.42 g/cm³ at 20°C when measured according to ISO 2811-1, and the Hansen solubility parameter overlap with chlorinated rubber is sufficient to swell the resin within 10–15 min at 40°C, although published dissolution-rate data for this specific solvent-resin pair is limited. The agitator is a retreat-curve impeller with a diameter-to-tank ratio of 0.55, operated at 40–60 rpm during the swelling phase because higher tip speeds above 2.5 m/s cause local shear heating and generate discolored gel particles rather than a clear solution. Resin sacks are pre-dried at 60°C for 2 h under −0.08 MPa when ambient relative humidity exceeds 60% because free moisture reduces solvent ingress and forms translucent gel specks that exceed 80 μm on a Hegman gauge per ASTM D1210. A low-shear anchor sweep is engaged at 12 rpm for the first 20 min to prevent particle bed compaction, and a solvent-compatible pH electrode in a recirculation loop confirms that the solution remains between 6.8 and 7.2 after addition of 0.25 parts per hundred resin of an acid acceptor such as propylene oxide.

Thermal Degradation Pathways in High-Density C3 Solvent Dissolution

Chlorinated rubber is thermally unstable in halogenated solvents when dissolved metal salts or storage-induced hydrogen chloride are present. The dehydrochlorination reaction is autocatalytic and accelerates sharply above 45°C in the high-density C3 solvent; therefore the batch temperature is maintained at 40°C ± 2°C with a cascade control loop that adjusts jacket cooling-water flow and external heat exchanger bypass. On production-scale equipment, the retention time at 45–50°C must not exceed 90 min because the liberated hydrogen chloride attacks the stabiliser and produces conjugated polyene sequences that discolour the solution from pale yellow to dark amber. A plate-and-frame external heat exchanger with 316L stainless steel plates and cooling water at 10°C is used to maintain the jacket return temperature below 42°C. The rate of heat removal is limited by the viscosity of the swelling resin layer, which reduces heat transfer coefficients in the initial dissolution phase; production records indicate that batch-to-batch variations in resin particle size distribution from 20 μm to 2 mm can change the time to reach 95% optical clarity by 40–60 min. Larger particles require a lower initial temperature of 35°C to avoid thermal runaway at the particle surface. The acid acceptor is metered during the second half of the swelling phase, and the scrubber condensate from the nitrogen purge is monitored for chloride by ion-selective electrode; a chloride concentration above 5 mg/L in the scrubber water triggers an automatic reduction of the batch temperature to 35°C and an immediate addition of 0.1 parts per hundred resin of acid acceptor.

Mechanical agitation scale-up for this system is governed by the dissolution regime rather than the final-solution viscosity. In the early stage, the impeller must provide sufficient bulk motion to keep swollen resin particles suspended while avoiding high shear at the particle surface. The production vessel is fitted with two baffles set at 90° and a retreat-curve impeller running at 45 rpm, equivalent to a tip speed of 1.8 m/s and a power per volume of 0.75 kW/m³. When the solution reaches approximately 80% clarity, a toothed-disc high-shear disperser with a 0.4 mm radial gap and variable-frequency drive is introduced at 1,500 rpm for the remaining gel reduction. The transition point is determined by in-line laser turbidity rather than elapsed time; switching above 1,000 NTU causes the disperser to compact gel bodies on the filter screen and raises the differential pressure across the subsequent filter to 0.6 MPa within 15 min. Pilot-plant records from a 2,000-L batch demonstrate that starting high shear before 70% clarity increases filter changes from 2 to 6 per batch and reduces yield by 3–5% due to hold-up in spent bag filters. The high-shear interval is therefore limited to 10–15 min after the turbidity target is reached, and the disperser is retracted before the solution cools below 30°C to prevent shear-induced chain alignment that raises yield stress.

In rubber-to-metal adhesive compounding, the chlorinated rubber solution at 23–25 wt% solids is transferred through a 50 μm depth filter and then blended with a chlorinated paraffin plasticiser at 5–8 parts per hundred resin in a planetary mixer operating at 35 rpm. The blend is adjusted to a Brookfield viscosity of 1,200–2,000 mPa·s at 25°C using ASTM D1084, with viscosity reversal after 24 h measured to confirm that residual gel structures do not rebuild. Adhesion performance is verified on grit-blasted steel panels using ASTM D429 Method B; peel strengths above 7 N/mm are achieved only when the solution is filtered to remove particles larger than 25 μm, because larger gel bodies behave as bondline flaws under compression moulding at 150°C and 10 MPa. The final adhesive must not contain free amine compounds, because amine-based curatives initiate premature dehydrochlorination of chlorinated rubber in the high-density C3 solvent and produce carbonaceous residue at the rubber-metal interface. The mixer bowl is blanketed with nitrogen at 0.01 MPa positive pressure, and the solvent vapour concentration at the operator station is kept below 1 ppm as measured by a photoionisation detector calibrated in accordance with ISO 16000-6.

What Limits In-Line Filtration Throughput Above 28 wt% Solids Loading?

Filtration throughput is limited by the formation of gel bodies, not by viscosity alone. When solids loading exceeds 28 wt%, the concentration of high molecular weight chlorinated rubber at the particle surface exceeds the solvent penetration front and forms a swollen, impermeable shell around an unswollen core. The resulting bodies are deformable and pass through the first bag filter at 150 μm, then blind the second filter at 50 μm, increasing differential pressure from 0.2 MPa to 0.8 MPa in 20–30 min. A production-scale single-bag housing with stainless steel support basket and polypropylene needle-felt elements is used; the pump is an air-operated double-diaphragm unit with 1:1 ratio and a pulsation dampener set to 0.35 MPa, because centrifugal pumps cause mechanical shear that breaks gel bodies and allows them to pass through the final filter. The batch is diluted with high-density C3 solvent to 26 wt% solids before the second filter pass, and the filter pressure is monitored with a flush-diaphragm pressure gauge readable to 0.01 MPa. Back-pulsing is not permitted because reverse flow disrupts the filter cake and releases previously retained gel bodies into the filtered solution. Filter elements are disposed of after a maximum differential pressure of 0.8 MPa because continued operation compresses the gel layer into a non-cleanable varnish that requires solvent stripping of the housing.

High-Shear Dispersion and Filter Pressure Correlation in Coating Manufacture

Pigmented chlorinated rubber coatings manufactured in high-density C3 solvent require a separate high-shear dispersion step after resin dissolution, and the final filter pressure correlates with the disperser tip speed and residence time. When a toothed disc with tip speed 18–22 m/s is applied to a millbase containing titanium dioxide at 35 wt% pigment volume concentration, the gel fraction is reduced to a Hegman grind below 20 μm within 12 min, but the fluid temperature rises from 25°C to 38°C unless a jacketed dispersion vessel with cooling water at 15°C is used. The millbase is then let down with filtered chlorinated rubber solution at 22 wt% solids, and the finished coating is passed through a 25 μm bag filter. A filter pressure above 0.4 MPa at a flow rate of 8 L/min indicates that disperser residence time was insufficient or that the resin solution contained residual gel shells above 50 μm. On a 1,000-L manufacturing batch, the high-shear step is therefore monitored by both Hegman gauge per ASTM D1210 and in-line differential pressure, with no more than 8% recycle time permitted because extended shear reduces the molecular weight of chlorinated rubber and lowers the tensile strength of the dried film as measured by ISO 527-2 on free films.

For airless spray application of a chlorinated rubber protective coating, the filtered solution is reduced to 18–20 wt% solids and adjusted to 70–90 s Ford #4 cup viscosity at 25°C per ASTM D1200. The fluid is supplied to a 30:1 airless spray unit with a 0.38 mm tungsten carbide tip at an atomisation pressure of 12–14 MPa. Dry film thickness is controlled at 75–125 μm per coat, and sag resistance is measured with ASTM D4400 at 25°C and 60% relative humidity; films above 125 μm develop solvent-pop defects because the high-density C3 solvent has a lower evaporation rate than methylene chloride and becomes trapped under a surface skin. The addition of a thixotrope at 0.2–0.5 wt% based on solution weight is required for vertical substrates, and the thixotropic index calculated from spindle viscosities at 1 rpm and 10 rpm must remain between 3.5 and 5.0 to avoid spray gun spitting. The solvent is not compatible with unlined carbon steel storage because trace moisture extracts chloride and initiates pitting; storage tanks are fabricated from 316L stainless steel or high-density polyethylene with double-walled containment and fitted with desiccant breathers to maintain water content below 0.05 wt% as measured by Karl Fischer titration per ASTM D1364.

When High-Density C3 Solvent Replaces Methylene Chloride in Immersion Stripping

When the high-density C3 solvent is used to replace methylene chloride in immersion stripping of cured chlorinated rubber from steel mandrels, the process window is shifted by the higher boiling point and higher density of the C3 solvent. The immersion tank is operated at 38–42°C rather than 25–30°C used for methylene chloride, and the parts basket is agitated at 0.5–1.0 m/s vertical displacement to prevent a dense solvent boundary layer from saturating at the coating surface. The solvent is recirculated through a carbon bed at 0.5 bed volumes per hour to remove low molecular weight rubber degradation products, and freeboard ventilation is maintained at 0.5 m/s face velocity to keep operator exposure below the applicable occupational exposure limit. The higher density increases pump power demand by approximately 10–15% when transferring the solvent, and the distillation recovery unit is operated under vacuum at 10–15 kPa to keep the reboiler temperature below 90°C, preventing solvent decomposition and acid gas generation. Published stripping-rate data for chlorinated rubber in this specific high-density C3 solvent is limited, so the first production trial is configured with an immersion time of 60 min and an in-line turbidity cutoff of 5 NTU to determine endpoint repeatability before scaling to a continuous agitated tank cascade.

Each production lot is released only after a four-point compliance check for viscosity, gel content, water content, and density. The test methods and acceptance limits used for chlorinated rubber dissolution in high-density C3 solvent are listed in Table 1.

The equipment setpoints listed in Table 2 are used for a 2,000-L batch dissolution unit to control the critical processing window between acceptable dissolution and thermal degradation.

Batch records from a 500-L pilot reactor operating with the same geometry show that the most common deviation is premature filter blinding when the final filtration is conducted above 30°C. Cooling the filtered solution to 22°C before the final 25 μm filter pass increases throughput by 20–30% because the solvated gel fraction contracts and releases occluded solvent. Temperature must not be reduced below 15°C because the high-density C3 solvent viscosity approaches 0.65 mPa·s and the pressure drop across the filter exceeds 0.6 MPa at the same flow rate. The final filtered solution is stored in sealed stainless steel totes with nitrogen headspace at 0.03 MPa and delivered to the coating cellar within 8 h; storage beyond 24 h is not recommended because trace iron from transfer piping promotes acid-catalysed colour drift measured as an increase in Gardner colour from 2 to 6 per ASTM D1544.