Formulation 101: Plastisol

Polyvinyl chloride (PVC) plastisols are liquid dispersions of fine PVC resin particles in a liquid plasticizer. When heated, the mixture undergoes gelation and fusion, resulting in a flexible, durable solid film or molded product. Due to their versatility, PVC plastisols are widely used in coatings, flooring, automotive parts, toys, textiles, adhesives, and sealants. Formulating a plastisol requires balancing resin, plasticizer, stabilizers, fillers, and performance additives to meet application-specific needs.

Components of PVC Plastisol Formulation

The art of plastisol formulation lies in balancing viscosity, stability, and performance. Key factors to consider include controlling the viscosity, the gelation temperature which must be matched to the processing conditions (e.g., dip coating, slush molding, screen printing), fusion characteristics which ensure uniform, defect-free films, and the mechanical properties needed for the end-use requirement (e.g., hardness, flexibility, and durability).

Below is an example of a basic starting point formulation, which can be tweaked according to the considerations listed above:

Basic Plastisol Formulation

Notes: phr = parts per hundred by weight.

1. PVC Resin

• Function: The backbone of the plastisol; determines viscosity, fusion characteristics, and physical properties.
• Types: Homopolymer or copolymer grades with varying particle size and porosity.
• Selection Considerations:
  • Fine particle resins (0.2–2 microns): Improve dispersion and lower viscosity.
  • Porous resins: Absorb more plasticizer, lowering viscosity and allowing higher filler loadings.
  • Non-porous resins: Provide strength and hardness but increase viscosity.

2. Plasticizer

• Function: Liquid medium that disperses the PVC resin; imparts flexibility, softness, and processability.
• Common Types:
  • Phthalates (DINP, DOP, DINCH): General-purpose, Widely used, though increasingly regulated in certain applications.
  • Adipates and Sebacates (DOA, DOS): Offer low-temperature flexibility.
  • Trimellitates & Polymerics (TOTM): Provide heat and extraction resistance.
• Levels: Typically 40–100 phr (parts per hundred resin), depending on desired hardness.

3. Stabilizers

• Function: Prevent thermal degradation of PVC during heating and processing.
• Types:
  • Calcium-Zinc stabilizers: Non-toxic, widely used.
  • Organotin stabilizers: Excellent performance but higher cost.
  • Barium-Zinc stabilizers: Good heat stability but declining in use due to environmental concerns.

4. Fillers

• Function: Modify cost, viscosity, hardness, and mechanical properties.
• Examples: Calcium carbonate (CaCO3), clay, silica, or barium sulfate.
• Considerations: Filler particle size and surface treatment affect dispersion and rheology.

Producing a Lab Scale Batch

A typical batch size for testing purposes would be about 3X to 5X, depending on the number of tests being run for the trial. Below is an example of how to scale the formulation to a 3X size:

Basic Plastisol 3X Lab Batch

Equipment needed

• Hobart Mixer – Mixing can be done with a Hobart Mixer or other Mixing Bowl with a Paddle Blade. The advantage of this type of mixer is that you can make small batches and minimize the energy/heat added to the plastisol versus using a lab sized high shear mixer.
• Vacuum Station – A deaeration or vacuum station consists of a vacuum unit under the cabinet with hoses and two way valves that allow either vacuum or external air to be allowed into the bowl.
• Brookfield viscometer (RVT) – for viscosity checks.
• Small convection oven – or IR fusion oven with adjustable temperature (up to 220 °C).

Procedure

1. Mix order (typical): Plasticizer → stabilizers/ESBO → additives (defoamer, rheology) → pigments → fillers → slowly add PVC resin under shear → vacuum de-air (if available).
2. Weight the wet and dry ingredients separately. Weigh liquid components (plasticizer, liquid heat stabilizer, other additives as needed) into beaker and mix to make homogeneous. Stirring by hand usually works.
3. Add the wets to the mixer, and set the mixer to the lowest speed. Then, slowly add PVC resin over several minutes to avoid lump formation. Continue mixing until the dry resin is fully wetted (10–20 minutes total mixing).
4. Increase to higher shear (~1,000–2,000 rpm) for 2–5 minutes to break agglomerates and obtain a smooth plastisol. incorporated). Be careful here as this is where heat can start to build. Avoid excessive heat — keep bulk temp < 40 °C during mixing.
5. The Knit should break down to form a smooth, flowable thick liquid once the resin has been fully wet out by the plasticizer. The material should flow off the paddle mixer once mixing is complete.
6. After mixing, de-aireate using the vacation station for 30 minutes. Start the pump and slowly open valve and the plastisol will rise. This rise is caused by trapped air, from the mixing process, and moisture expanding under the vacuum. Care must be taken so that the foam does not get to the top gasket or into the vacuum hose. If the material rises too high, move the valve to let in some outside air and the bubbles will shrink. Gentle tapping or swirling of the bowl also helps.
7. At the end of the deaeration time the surface of the plastisol should be bubble free or nearly bubble free. Bubbles should not be forming and breaking on their own any longer. If bubbles keep forming then continue vacuuming a little longer. The material is now ready for testing.

Small-scale testing & targets

• Viscosity (Brookfield RVT #4 @ 20 rpm, 25 °C): target 1,000–5,000 cP (1–5 Pa·s) for coating plastisol.
• Visual: uniform, free of lumps and large entrained bubbles.
• Specific gravity: record for QC (depends on filler level).
• Storage stability: note viscosity at T=0 and recheck after 7 days at 25 °C for acceptable thickening (<20–30% rise is reasonable depending on system).

Lab fusion / cure (bench trial)

1. Film application: Apply a thin film (~200–500 µm wet) to cleaned metal or glass test panels (knife coater, drawdown bar, or screen for inks). Record wet thickness.
2. Oven profile (starting point): preheat oven air temp to 170 °C. Start with 4 minutes residence for a 200–300 µm film. Typical fusion window: gel onset ~140–160 °C, full fusion 170–200 °C.
   • For thicker films or molded parts increase time (e.g., 6–8 min) or temperature up to 180 °C as needed.
3. Check results: panels should be glossy/matte per formulation, tack-free, and show no surface pitting or blistering. If fusion incomplete, increase time 1–2 minutes or raise temperature 5–10 °C.
4. Hardness measurement: after cooling to room temp measure Shore A hardness per ASTM D2240 (or target hardness history: ~75–80A for this mix).
5. Adhesion / visual defects: inspect for bubbles, bloom, soft spots, or foaming.

Small-scale molding (optional)

• For dip or slush molding trials, use appropriate mandrels and oven profiles. Thicker molded sections require slower heating ramps to avoid internal voids — start with 150 °C for 2–3 min then 170–180 °C for final fusion.

Troubleshooting (quick guide)

• High viscosity / clumping: likely poor resin wetting — increase mixing time, use higher shear briefly, check resin porosity; consider slightly more plasticizer or small amount of solvent-free coalescent.
• Bubbling / blistering on cure: trapped air or rapid solvent/volatiles release — de-air under vacuum or reduce oven ramp; check defoamer.
• Poor fusion / tacky surface: insufficient temp/time, low ESBO/stabilizer, or incorrect oven profile. Raise time/temp or increase coalescing additives slightly.
• Sagging on vertical parts: viscosity too low—add small fumed silica or thixotrope, or reduce plasticizer level slightly.

Storage and handling after preparation

• Store plastisol in sealed, labeled containers at 15–30 °C (avoid freezing and excessive heat).
• Keep away from direct sunlight, ignition sources, and incompatible materials.
• Before long-term storage, consider light agitation or circulation to prevent settling; if settling occurs, remix under low shear.
• Use within manufacturer-recommended shelf life; run QC checks before use if stored >30 days.

Recommended QC tests for release

• Brookfield viscosity at 25 °C (record spindle & speed).
• Fusion test on standard panel (time/temp).
• Shore A hardness after 24 hours.
• Visual/color match and presence of foreign particles.
• Specific gravity and % non-volatile (if required).