Many PVC processors assume that when they receive resin of a specific K-value, their formulation should remain unchanged without testing. This assumption causes problems. K-value variations between lots can reach 18% in viscosity difference, turning a proven formulation into one that produces fish eye defects. If you see ungelled particles appearing on your film surfaces or embedded in your pipe walls, the root cause often traces back to material or process changes you did not account for.
This guide covers how fish eyes form, what causes them, how to detect them, and how to prevent them through quality control.
What Are Fish Eye Defects?
Fish eyes are ungelled particles in PVC that appear as visible surface defects in finished products. In transparent films, they show as distinct ungelled spots. In pigmented compounds, they remain colorless against the surrounding material, making them more obvious.
The defects form when certain PVC particles resist gelation during processing. Standard S-PVC particles range from 95-250 microns in diameter, with modern narrow distributions at 140-180 microns. These secondary particles are composed of primary particles measuring 1-2 microns each. Fish eyes occur when some particles have different characteristics – lower porosity, different molecular weight, or cross-linked structure – that prevent them from fusing at the same rate as the bulk resin.
PVC has a refractive index of 1.54. When ungelled particles remain in the matrix, light scattering at particle boundaries creates the characteristic “fish eye” appearance. The visual effect is most pronounced in transparent or lightly pigmented products.
Common Causes of Fish Eye Formation
Fish eyes originate from two primary sources: resin quality issues or processing parameter problems. Both require investigation when defects appear.
Resin Quality Issues
Three resin-related factors cause most fish eye formation:
Cross-contamination between grades. When reactors produce multiple K-values, incomplete cleaning leaves residual polymer. A K-57 resin contaminated with K-67 particles creates processing problems. Viscosity varies as K-value raised to the 3.4 power – meaning K-66 versus K-68 resin shows approximately 10.70% viscosity difference. These higher-molecular-weight contaminants require more energy to gel and often remain visible as fish eyes.
Low porosity grains. PVC porosity typically ranges from 10-30%. High-porosity resin (0.41 cc/g) is preferred for plasticized PVC because plasticizer absorbs uniformly. Low-porosity grains (0.25 cc/g) restrict plasticizer penetration, leaving particle cores ungelled during processing.
Batch variability. Even within specification, K-value tolerance of +/- 1 (stated as K-57) means actual range spans 55.6 to 58.4 – an 18% viscosity variation. This explains why the same formulation processes differently between lots.
Processing Parameter Problems
A thick-walled PVC pipe once cracked without ever entering service. Analysis revealed the failure came from severely poor fusion during extrusion – the processing conditions never achieved adequate gelation throughout the wall thickness. Large parts are particularly vulnerable because temperature gradients and shear variations across the cross-section make uniform fusion difficult.
Temperature directly controls gelation. PVC compounds process at 150-200C, with U-PVC typically at 180-190C and high-plasticizer compounds at 130C. Insufficient temperature leaves particles ungelled. Excessive temperature degrades the polymer.
Shear also matters. Low screw speed or worn screw elements reduce the mechanical work available for gelation. Inadequate mixing leaves some particles with insufficient plasticizer contact or heat exposure.
Testing and Detection Methods
Several methods exist for detecting fish eyes and measuring gelation quality. Each has distinct advantages.
| Method | Standard | What It Measures | Best For |
|---|---|---|---|
| DSC | ISO 18373 | Gelation degree (%) | Process optimization |
| Acetone Test | ASTM D2152 | Fusion quality | Quick incoming inspection |
| DCMT | ISO 9852 | Fusion quality | Detailed fusion assessment |
| Optical Microscopy | ASTM D3596 | Fish eye count | Resin qualification |
DSC testing provides quantitative gelation measurement. Samples of 10 +/- 0.5 mg are heated from room temperature to 300C at 10C/min. The optimal gelation range is 85-95%, with 93% typically achievable under controlled processing. If you see this defect, check your DSC results first – values below 85% indicate the process needs adjustment.
The acetone test offers a quick field check but provides only qualitative results. DCMT (dichloromethane test) offers more sensitivity for detailed fusion assessment.
One caution: variability in percent gelation estimates can reach 81.8% between laboratories. Use consistent sampling and testing protocols when comparing results over time. Internal consistency matters more than absolute values.
Prevention and Quality Control
Prevention is more cost-effective than detection and grading. Address the easiest fixes first before investing in equipment or supplier changes.
Process Optimization
Temperature adjustment is the most accessible lever. Before changing formulations or questioning resin quality, verify your temperature profile:
- Check thermocouple accuracy – a 10C error changes gelation outcomes substantially
- Increase barrel and die temperatures incrementally
- Monitor for degradation signs (yellowing, odor)
Shear optimization comes next. Increase screw speed within your process window. Check for screw and barrel wear that reduces mechanical work input.
Review your mixing protocol. Additive addition sequence affects gelation. Verify the temperature of addition and ensure proper torque-temperature-time relationships during compounding.
Formulation adjustments require more effort. Lubricant levels affect fusion – calcium stearate should not exceed 1 phr or it can inhibit gelation. Process aids help if temperature and shear adjustments are insufficient.
Incoming Material Control
The processing window for your formulation is narrower than most processors realize. K-value variations between resin lots translate to substantial viscosity changes requiring lubricant adjustment.
Test K-value on incoming lots. Do not assume supplier consistency eliminates this need. Verify porosity specifications match your application requirements – plasticized products need high porosity, rigid UPVC can use lower porosity grades.
For critical applications like transparent film, specify film-grade resin with documented fish eye counts. Establish incoming acceptance criteria and develop backup suppliers qualified to meet them.
Key Takeaways
Fish eye prevention follows a clear hierarchy. Start with what you can control immediately: temperature settings and shear optimization cost nothing to adjust. Move to mixing protocol and formulation review. Only after exhausting these options should you invest in incoming material testing programs or supplier qualification efforts.
Temperature control remains your primary lever. Most fish eye problems I encounter trace back to process conditions that fail to achieve the 85-95% gelation target. Check your DSC values before blaming the resin.
When resin quality is genuinely the issue, K-value testing on incoming lots catches the 18% viscosity variations that turn a proven formulation into a defect source. The cost of testing is far less than the cost of scrapped product.