Plastic Material Testing: Resistivity, Optical and Mechanical Properties

Posted on Feb 13, 2026 in Other university degrees and diplomas

Volume and Surface Resistivity Testing

  • Volume Resistivity: Measures the electrical resistance through the bulk of the material. It is the ratio of the potential gradient to the current density. It indicates how well a plastic insulates against leakage through its body.
  • Surface Resistivity: Measures the resistance to leakage current along the surface of the insulating material. It is important for applications where surface contamination or moisture might create a conductive path.
  • Testing: Usually performed using a megohmmeter and a three-electrode system to isolate the volume and surface currents.

2. Haze and Clarity

  • Haze: The percentage of transmitted light that, in passing through a specimen, deviates from the incident beam by more than 2.5° by forward scattering. High haze results in a “milky” or “cloudy” appearance.
  • Clarity: Refers to the “see-through” quality or the ability of the material to transmit fine details of an object viewed through it. While haze relates to wide-angle scattering, clarity relates to narrow-angle scattering (less than 2.5°).

3. Specular Gloss in Plastics

Specular gloss is the relative luminous reflectance factor of a specimen in the specular (mirror-like) direction. It measures the capacity of a surface to reflect more light in some directions than in others. It is measured using a glossmeter at specific angles (usually 20°, 60°, or 85°). High gloss indicates a very smooth, mirror-like surface.

4. Luminous Transmittance

Luminous transmittance is the ratio of the transmitted light flux to the incident light flux. It indicates the “transparency” of a plastic material. It is expressed as a percentage, where 100% would be a perfectly clear material with no absorption or reflection. It is measured using a spectrophotometer or haze meter.

5. Smoke Density

Smoke density testing measures the amount of smoke generated when a plastic material burns or decomposes under radiant heat or flame.

  • Measurement: Usually done in a Smoke Density Chamber (ASTM E662).
  • Significance: It is a critical safety parameter for plastics used in aircraft, subways, and building interiors to ensure visibility for evacuation during a fire. It tracks the “Specific Optical Density” over time.

6. ESCR (Environmental Stress Cracking Resistance)

ESCR (Environmental Stress Cracking Resistance) is the susceptibility of a thermoplastic (mostly polyethylene) to crack formation under the combined influence of mechanical stress and a chemical environment (like detergents, oils, or alcohols).

  • Test: A notched specimen is bent into a U-shape, placed in a test tube with a reagent (like Igepal), and kept at an elevated temperature (50°C). The time taken for failure (cracking) is recorded.

7. Dielectric Breakdown Strength and Test Procedure

Dielectric strength is the maximum voltage a material can withstand per unit thickness before an electrical breakdown (sparking through the material) occurs.

  • Procedure: A specimen is placed between two electrodes in an oil bath (to prevent surface arcing). The voltage is increased at a uniform rate (e.g., 500 V/s) until the material punctures.
  • Formula: Dielectric Strength = Breakdown Voltage / Thickness.

PART – D

(Max 300 words per answer)

1. Weathering of Plastics and Tests

Weathering refers to the degradation of plastics when exposed to outdoor environmental conditions like UV radiation, temperature fluctuations, moisture (rain/humidity), and atmospheric pollutants. This leads to color fading, loss of gloss, chalking, and brittleness.

Tests for Weathering:

  1. Natural Weathering: Specimens are placed on racks facing south at a 45° angle in specific climates (e.g., Florida for humidity, Arizona for UV). It is realistic but very time-consuming (years).
  2. Accelerated Weathering: Uses specialized chambers to simulate years of exposure in weeks.
    • Xenon Arc: Best simulation of full-spectrum sunlight.
    • Fluorescent UV (QUV): Uses UV lamps to simulate the short-wave damaging portion of sunlight, combined with condensation cycles for moisture.
    • Carbon Arc: An older method using glowing carbon rods; less common now.

3. Biodegradable Plastics and Biodegradation Testing

Biodegradable plastics are polymers that can be broken down into water, CO2, and biomass by the action of microorganisms (bacteria, fungi, algae) under specific conditions.

Biodegradation Testing (e.g., ISO 14855 / ASTM D5338):

This method measures aerobic biodegradation under controlled composting conditions.

  1. Setup: The plastic sample is mixed with stabilized organic compost in a vessel.
  2. Conditions: The temperature is maintained at 58°C with constant oxygen supply and moisture.
  3. Measurement: The amount of CO2 evolved is measured over time using an analyzer.
  4. Calculation: The degree of biodegradation is calculated as the percentage of the theoretical maximum CO2 that could be produced from the carbon content of the plastic.
  5. Requirement: Usually, a material is considered biodegradable if it reaches 90% degradation within 180 days.

Would you like me to elaborate on the “Acceptance and Type tests” for pipes and tanks (Part D, Question 2)? α