The Science of Plastisol: How Soft Lures Are Engineered for Action

⚠️ Safety Disclaimer

Plastisol formulation and processing involve industrial chemicals, high heat, and specialized equipment. These materials can release hazardous fumes and require strict ventilation, protective gear, and regulatory compliance. Readers should not attempt to make plastisol at home. The information provided here is for technical and educational purposes only, intended for controlled laboratories or manufacturing environments.

Do not use phthalate-based plasticizers in plastisol formulations. Although they were once common in soft plastics, phthalates are now avoided due to documented health and environmental concerns. Safer, non-phthalate alternatives provide the flexibility needed without the risks.

Section 1: Introduction

 The Science of Plastisol

Soft plastic fishing lures are built on plastisol—a suspension of PVC resin in liquid plasticizer. Its unique property of being liquid at ambient conditions and solidifying upon heating makes it ideal for molding complex geometries. The performance envelope of plastisol is controlled by resin particle size, plasticizer chemistry, additive loads, and fusion kinetics.

Plasticizer Landscape and Trade-Offs

Plasticizer choice drives flexibility, migration resistance, fusion behavior, and compliance.

Plasticizer Landscape and Trade-Offs

DEHP (Di(2‑ethylhexyl) phthalate) [Do not use]

•             Pros: High flexibility, strong PVC compatibility, low volatility

•             Cons: Regulatory restrictions, environmental persistence, migration concerns

DINP (Diisononyl phthalate) [Do not use]

•             Pros: Durable, cost-effective, lower volatility than DEHP

•             Cons: Regulatory scrutiny, not eco-preferred

DIDP (Diisodecyl phthalate) [Do not use]

•             Pros: High permanence, heat stability, reduced migration

•             Cons: Higher cost, slower fusion kinetics

DOTP (Di(2‑ethylhexyl) terephthalate)

•             Pros: REACH-friendly, comparable flexibility, lower toxicity profile

•             Cons: Slightly higher viscosity, higher cost

ATBC (Acetyl tributyl citrate)

•             Pros: Biodegradable, food-contact approved, low volatility

•             Cons: Higher cost, reduced tensile strength at high loadings

TEHTM (Tris(2‑ethylhexyl) trimellitate)

•             Pros: Excellent permanence, high-temperature stability

•             Cons: Expensive, slower fusion, requires higher processing temperatures

ESO (Epoxidized soybean oil)

•             Pros: Renewable, acts as stabilizer, improves UV resistance

•             Cons: Limited flexibility, yellowing risk, typically used as co-plasticizer

Example Formulation Ranges for Lure Applications

Formulas are expressed in percent ranges by weight of total plastisol. These are industrial reference ranges, not recipes for home use.

•             Ultra-soft finesse trout worm (neutral buoyancy)

•             PVC resin: 50–60%

•             DOTP: 30–40%

•             ATBC: 3–7%

•             ESO: 1–3%

•             Stabilizer: 0.3–0.7%

•             Pigments + glitter: 1–2%

•             Durable bass craw (high tear resistance)

•             PVC resin: 48–55%

•             DIDP: 35–45%

•             TEHTM: 2–5%

•             ESO: 1–2%

•             Stabilizer: 0.3–0.7%

•             Pigments + salt: 2–5%

•             Eco-conscious soft bait (compliance-forward)

•             PVC resin: 52–58%

•             DOTP: 30–40%

•             ATBC: 5–10%

•             ESO: 1–3%

•             Stabilizer: 0.3–0.7%

•             Pigments: 1–2%

Section 2: Polymer Chemistry of Plastisol

2.1 PVC Resin Morphology

•             Production method: Most lure‑grade PVC resin is produced via suspension polymerization. This yields spherical particles with diameters typically between 0.1–2 microns.

•             Particle size distribution: Smaller particles increase surface area, improving plasticizer absorption and lowering fusion temperature. Larger particles slow fusion but can improve mechanical strength.

•             Molecular weight: Typical lure resins fall in the range of Mw ≈ 80,000–120,000 g/mol. Higher molecular weight resins provide better tensile strength but require more aggressive fusion conditions.

•             Crystallinity: PVC is semi‑crystalline, with crystallinity around 10–15%. This crystalline fraction resists plasticizer penetration, influencing gelation kinetics.

2.2 Plasticizer Compatibility

Plasticizers must be chemically compatible with PVC to prevent phase separation and migration. Compatibility is often assessed using Hildebrand solubility parameters (δ).

•             PVC solubility parameter: δ ≈ 19.2 (MPa)^½.

Explaining Solubility Parameters in Simple Terms

Think of the solubility parameter (δ) as a compatibility score.

               PVC’s score is about 19.2.

               Plasticizers have their own scores.

               If the scores are close (within about 2 points), they “get along” and mix well.

If the scores are far apart, they don’t mix properly—the lure can get sticky, brittle, or leak chemicals.

PVC and Plasticizer “Scores”

Here are some example scores (δ values in (MPa)^½):

•             PVC: 19.2

•             DEHP: ~19.0 → very close, mixes well

•             DINP: ~19.1 → very close, mixes well

•             DOTP: ~19.5 → close, good compatibility

•             ATBC: ~20.1 → slightly higher, but still mixes okay

•             ESO (Epoxidized Soybean Oil): ~17.5 → farther away, so usually used as a helper, not the main player

2.3 Fusion Mechanism

Plastisol remains liquid until heated. Fusion involves three stages:

1.           Gelation onset: At ~160–170°C, resin particles begin absorbing plasticizer. Viscosity rises sharply.

2.           Fusion plateau: At ~180–190°C, particles coalesce into a continuous matrix. Mechanical strength develops.

3.           Post‑fusion stabilization: Holding at ~190–200°C ensures complete absorption and eliminates voids.

Measured by DSC (Differential Scanning Calorimetry):

•             Endothermic peak corresponds to plasticizer absorption.

•             Degree of fusion (DoF) can be quantified as:

•             where  = enthalpy measured during heating.

Simple Explanation

DSC is a tool scientists use to measure how materials change when they get hot or cold.

•             Imagine you’re baking cookies. As the oven heats up, the dough changes—it melts, spreads, and then hardens.

•             DSC is like a super‑precise oven thermometer that doesn’t just measure temperature, but also measures how much energy the dough uses while it changes.

So when scientists heat plastisol, DSC tells them:

•             The exact temperature where it starts to change (gelation).

•             How much energy it takes to fully fuse into a solid.

•             Whether the material is stable or weak when heated.

It’s like watching ice cubes melt: you can see when they start melting, and you can measure how much heat it takes to turn them all into water.

Why It Matters for Lures

For fishing lures made of plastisol:

•             DSC helps scientists know the right temperature to heat the plastisol so it fuses properly.

•             If it’s too low, the lure stays weak and brittle.

•             If it’s too high, the lure can burn or discolor.

So, DSC is basically a lab tool that makes sure lures are cooked “just right.”

2.4 Additives in the Polymer Matrix

•             Thermal stabilizers: Ca/Zn or organotin compounds prevent dehydrochlorination of PVC during heating.

•             Pigments: Dispersed within the plastisol matrix; dispersion quality measured by Hegman gauge (0–8 scale).

A Hegman gauge is a tool scientists use to check how smooth a liquid mixture is.

•             Imagine you’re stirring chocolate milk. If the powder isn’t mixed well, you see little clumps.

•             The Hegman gauge is like a ruler with a groove that gets shallower as you move along it.

•             You spread the liquid across the groove, and the gauge shows you how big the clumps are.

The smoother the mixture, the farther it goes before you see bumps or specks. The rougher the mixture, the sooner you see them.

So the Hegman gauge tells scientists whether their “paint” or “plastisol” is mixed like smooth chocolate milk or still lumpy like chunky oatmeal.

Why It Matters for Lures

When making fishing lures, pigments (colors) need to be smoothly mixed into plastisol.

•             If the mixture is smooth (good Hegman gauge reading), the lure color looks even and professional.

•             If it’s lumpy (bad reading), the lure has streaks or weak spots.

•             Glitter: PET or aluminum flakes embedded in the matrix; particle size affects scattering.

•             Salt: NaCl crystals increase density and sink rate but reduce elongation.

•             Scent oils: Hydrophobic molecules trapped in the plastisol; release governed by diffusion gradients.

2.5 Percent Ranges for Polymer Chemistry

Industrial formulations are expressed in percent ranges by weight of total plastisol.

•             PVC resin: 48–60%

•             Primary plasticizer (phthalate or non‑phthalate): 30–45%

•             Secondary plasticizer/stabilizer (ESO, ATBC): 2–10%

•             Thermal stabilizer: 0.3–1%

•             Pigments + glitter: 0.5–2%

•             Salt (optional): 0–10%

2.6 Engineering Implications

•             Softness vs. durability: Higher plasticizer ratios yield softer lures but reduce tensile strength.

•             Fusion kinetics: Smaller resin particles and compatible plasticizers lower fusion temperature, improving cycle times.

•             Migration control: High molecular weight plasticizers (DIDP, TEHTM) reduce migration but slow fusion.

•             Eco‑compliance: Non‑phthalates (DOTP, ATBC) meet regulatory standards but may require reformulation to maintain mechanical targets.

Section 3: Rheology and Flow Dynamics

3.1 Viscosity Behavior

•             Definition: Viscosity is a measure of how “thick” or “runny” a liquid is. Plastisol viscosity changes depending on how fast it is stirred or injected.

The rheological behavior of plastisol under shear can be described by the power‑law model:

Where each symbol represents a specific property:

• η = Viscosity

•             Technical definition: Viscosity is the measure of a fluid’s resistance to flow. It describes how “thick” or “thin” a liquid is under certain conditions.

•             Units: Pascal‑seconds (Pa·s) or centipoise (cP).

•             In plastisol:

•             High viscosity = plastisol is thick, harder to inject into molds.

•             Low viscosity = plastisol flows easily, fills fine details.

•             Analogy: Honey has high viscosity (thick), water has low viscosity (thin). Plastisol’s viscosity changes depending on how fast it is stirred or injected.

• k = Consistency Index

•             Technical definition: The constant that sets the baseline viscosity of the fluid at a given shear rate. It reflects how “stiff” or “fluid” the plastisol is before shear effects are considered.

•             Units: Pa·sⁿ (depends on flow index n).

•             In plastisol:

•             Higher k → thicker baseline plastisol (more resin, less plasticizer).

•             Lower k → thinner baseline plastisol (more plasticizer, less resin).

•             Analogy: Think of k as the “starting thickness” of pancake batter before you stir it faster.

• γ̇ = Shear Rate

•             Technical definition: The rate at which adjacent layers of fluid move past each other, usually expressed in reciprocal seconds (s⁻¹).

•             In plastisol:

•             Low shear rate = plastisol at rest in a container.

•             High shear rate = plastisol being injected into a mold under pressure.

•             Analogy: Imagine sliding a deck of cards. If you move the top card slowly, that’s a low

shear rate. If you slide it quickly, that’s a high shear rate. Plastisol responds differently depending on how fast it’s “slid” or forced to move.

• n = Flow Index (for plastisol, typically 0.3–0.6)

•             Technical definition: A dimensionless number that describes how viscosity changes with shear rate.

•             If n = 1 → fluid is Newtonian (viscosity stays constant, like water).

•             If n < 1 → fluid is shear‑thinning (viscosity decreases as shear rate increases).

•             If n > 1 → fluid is shear‑thickening (viscosity increases as shear rate increases).

•             In plastisol:

•             n typically ranges from 0.3–0.6, meaning plastisol is strongly shear‑thinning.

•             This allows plastisol to flow easily under injection pressure but remain thick enough at rest to keep pigments and glitter suspended.

•             Analogy: Think of ketchup. At rest, it’s thick. Shake or squeeze the bottle (apply shear), and it flows easily. Plastisol behaves the same way.

Why This Equation Matters for Lure Manufacturing

•             Design implication: By adjusting resin %, plasticizer %, and additives, manufacturers tune k and n to achieve the right viscosity profile.

•             Production implication: Injection molding relies on shear‑thinning behavior (n < 1) to fill fine lure details without defects.

•             Quality implication: Normalization SOPs often specify acceptable viscosity ranges (e.g., 1,000–3,000 cP at 25°C) and flow index values to ensure repeatability across batches.

Engineering implication:

This property allows plastisol to flow easily into fine mold details under pressure but remain thick enough at rest to prevent pigment settling.

3.2 Gelation Curve

•             As plastisol is heated, viscosity first decreases, then sharply increases as particles absorb plasticizer.

•             Gel point: The temperature where plastisol changes from liquid-like to solid-like.

•             Measured by rheometer:

•             Storage modulus (G′): Elastic response (solid-like).

•             Loss modulus (G″): Viscous response (liquid-like).

•             Gelation occurs when G′ surpasses G″.

•             Typical lure plastisols:

•             Gel onset: 160–170°C

•             Full fusion: 180–190°C

3.3 Thixotropy

•             Plastisol exhibits time-dependent recovery of viscosity.

•             When stirred, viscosity drops; when left still, viscosity slowly rises again.

•             Recovery time constant (τ): 30–60 seconds depending on formulation.

•             Practical effect: Prevents pigments and glitter from settling during storage, but requires controlled mixing to avoid air bubbles.

3.4 Flow Dynamics in Molding

•             Injection pressure: Typically 50–150 psi depending on lure geometry.

•             Mold filling: Shear-thinning ensures plastisol fills thin ribs, claws, and appendages without voids.

•             Cooling cycle: 30–90 seconds in aluminum molds; slower in silicone molds due to lower thermal conductivity.

•             Defect risks:

•             Too low viscosity → flashing (material leaks outside mold cavity).

•             Too high viscosity → incomplete fills, trapped air.

3.5 Percent Ranges for Rheology Control

Formulation adjustments directly affect flow behavior:

•             PVC resin: 48–60% → higher resin increases viscosity, slows flow.

•             Plasticizer: 30–45% → higher plasticizer lowers viscosity, improves flow.

•             Secondary plasticizer (ESO, ATBC): 2–10% → modifies thixotropy and recovery.

•             Pigments + glitter: 0.5–2% → excessive loading increases viscosity, risk of streaks.

•             Salt: 0–10% → increases viscosity and density, reduces flowability.

3.6 Engineering Implications

•             Balance is critical: Too thin plastisol causes flashing and weak lures; too thick plastisol causes incomplete fills.

•             Normalization SOPs: Manufacturers often define acceptable viscosity ranges (e.g., 1,000–3,000 cP at 25°C) and shear-thinning profiles to ensure repeatability.

•             Crew training: Operators learn to recognize flow defects visually (short shots, streaks, bubbles) and adjust injection pressure or temperature accordingly.

Section 4: Mechanical Properties of Finished Lures

4.1 Hardness

•             Definition: Hardness measures how resistant a material is to indentation or deformation.

•             Test method: ASTM D2240 (Shore A durometer).

•             Typical ranges for plastisol lures:

•             Ultra‑soft trout worms: Shore A 10–20

•             General‑purpose worms and grubs: Shore A 20–30

•             Durable bass craws: Shore A 30–40

•             Engineering implication:

•             Softer lures (lower Shore A) bend and move more naturally in water, but tear more easily.

•             Harder lures (higher Shore A) resist tearing but may appear less lifelike.

4.2 Tensile Strength

•             Definition: The maximum stress a material can withstand while being stretched before breaking.

•             Test method: ASTM D412 (tensile test of elastomers).

•             Typical ranges for plastisol lures:

•             Soft finesse worms: 2–3 MPa

•             Bass craws and heavy‑duty lures: 4–6 MPa

•             Engineering implication:

•             Higher tensile strength improves durability against aggressive strikes.

•             Lower tensile strength allows more stretch and lifelike action but reduces lifespan.

4.3 Elongation at Break

•             Definition: The percentage increase in length a material can undergo before breaking.

•             Test method: ASTM D412.

•             Typical ranges for plastisol lures:

•             Ultra‑soft worms: 300–400% elongation

•             General‑purpose lures: 250–350%

•             Durable craws: 200–300%

•             Engineering implication:

•             High elongation = stretchy, lifelike movement.

•             Low elongation = tougher, less flexible lure.

4.4 Density and Buoyancy

4.5 Percent Ranges for Mechanical Property Control

Formulation adjustments directly influence mechanical properties:

•             PVC resin: 48–60% → higher resin increases hardness and tensile strength.

•             Plasticizer: 30–45% → higher plasticizer lowers hardness, increases elongation.

•             Salt: 0–10% → increases density, reduces elongation.

•             Pigments + glitter: 0.5–2% → minor effect on mechanics, but excessive loading can reduce elongation.

•             Secondary plasticizers (ESO, ATBC): 2–10% → modify flexibility and toughness.

4.6 Engineering Implications

•             Balance is key:

•             Too soft → lifelike but fragile.

•             Too hard → durable but less natural.

•             Normalization SOPs: Manufacturers often define acceptable ranges (e.g., Shore A ±2, tensile ±0.5 MPa, elongation ±50%) to ensure batch consistency.

Section 5: Pigment and Additive Engineering

5.1 Pigments

•             Purpose: Pigments provide color and opacity to lures.

•             Types:

•             Inorganic pigments (iron oxides, titanium dioxide) → stable, opaque, earthy tones.

•             Organic pigments (phthalocyanine blues, azo reds) → bright, vivid colors.

•             Dispersion quality: Measured using a Hegman gauge, which checks how smooth the pigment is mixed.

•             A high reading (closer to 8) means smooth, no clumps.

•             A low reading (closer to 0) means lumpy, poor dispersion.

•             Engineering implication: Poor dispersion leads to streaks, weak color, and reduced mechanical strength.

5.2 Glitter

•             Composition: PET or aluminum flakes, typically 0.015–0.035 inches in size.

•             Function: Adds flash and visual appeal underwater by scattering light.

•             Optical behavior: Modeled by scattering theory—larger flakes reflect more light, smaller flakes create subtle shimmer.

•             Engineering implication:

•             Too much glitter increases viscosity and causes flow defects.

•             Proper loading (0.1–0.5% by weight) balances aesthetics with flowability.

5.3 Salt

•             Composition: Sodium chloride crystals, ground to <100 microns for uniformity.

•             Function:

•             Increases density → lures sink faster.

•             Adds taste → fish hold onto the lure longer. (maybe: it depends on who you talk to )

•             Engineering trade‑offs:

•             Higher salt (10–30% by weight) increases sink rate but reduces elongation and toughness.

•             Excess salt can cause brittleness and poor mold filling.

5.4 Scent Oils

•             Composition: Hydrophobic oils (anise, garlic, proprietary blends).

•             Function: Diffuse slowly from the lure, creating a scent trail.

•             Diffusion behavior: Governed by Fick’s law of diffusion:

 Diffusion Flux (J)

•             What it is: The “speed” or “rate” at which molecules move through a surface.

•             In lures: Flux tells us how quickly scent molecules leave the lure and spread into the water.

2. Diffusion Coefficient (D)

•             What it is: A number that shows how easily molecules can move through a material.

•             In lures: Small scent molecules have a higher D (move faster), while big heavy molecules have a lower D (move slower).

3. Concentration Gradient ()

•             What it is: The difference in how crowded molecules are between two places.

•             In lures: Right next to the lure, there are lots of scent molecules (high concentration). A few inches away, there are fewer molecules (low concentration). That difference drives diffusion.

4. What Means and How to Find It

•             Technical meaning:  is the rate of change of concentration (C) with distance (x).

•             How to find it:

1.           Measure the concentration of molecules at two points (close to the lure and farther away).

2.           Subtract the two concentrations.

3.           Divide by the distance between those points.

               Engineering implication:

•             Small molecules diffuse faster, releasing scent quickly.

•             Larger molecules diffuse slower, providing long‑lasting scent.

•             Typical loading: <2% by weight to avoid weakening the plastisol matrix.

5.5 Stabilizers and Secondary Additives

•             Thermal stabilizers: Prevent PVC degradation during heating (Ca/Zn, organotin).

•             UV stabilizers: Protect lures from sunlight discoloration.

•             ESO (Epoxidized Soybean Oil): Acts as both a co‑plasticizer and stabilizer, improving UV resistance.

5.6 Percent Ranges for Additive Engineering

•             Pigments: 0.5–2%

•             Glitter: 0.1–0.5%

•             Salt: 10–30% (optional, depending on lure type)

•             Scent oils: 0.5–2%

•             Stabilizers: 0.3–1%

5.7 Engineering Implications

•             Balance is critical: Too much pigment or glitter increases viscosity and causes defects.

•             Salt trade‑off: Improves sink rate and taste but reduces toughness.

•             Scent oils: Enhance fish attraction but must be carefully controlled to avoid weakening the plastisol.