Haze of plastic materials

Haze in plastics can be defined as the percentage of incident light scattered by more than 2.5° through the plastic specimen. It measures the milkiness of the material (film or sheet). It is expressed in percentage (%). It is an important optical property to measure where true color and visibility are necessary.

While testing a polymeric material or plastics, it is important to understand the complete optical properties. This includes the knowledge of:
 

• transmission,
• haze,
• yellowness,
• refractive index,
• gloss, etc.

Note: The lower the haze value, the higher the clarity. If the haze value is greater than 30%, the material is diffusing.

 

• Impurities: Impurities in plastics, like dirt, dust, and moisture can cause light scattering. This in turn makes the plastics hazy.

• Surface roughness: A rough surface can cause scattering of light leading to haze.

• Inhomogeneities: Inhomogeneities like density variations and filler particles, can also cause light scattering.

• Porosity: Porous plastics are more likely to exhibit haze than non-porous plastics. This is because the pores in the plastic can act as scattering centers.

• Crosslinking: Crosslinked plastics are more likely to exhibit haze than linear plastics. This is because the crosslinking process can create irregularities in the plastic structure.

• Environmental factors: Factors such as weathering or surface abrasion can cause haze in plastics.

• It is used in food and cosmetic packaging. In cosmetic packaging, a soft-focus or frosted finish is achieved using plastics with a high level of haze.
• The plastic panels used in many LED light fixtures are often hazy. This helps to diffuse the light evenly, preventing glare.
• Privacy films are often hazy. They are applied to windows and doors in offices and homes to obscure the view of what is behind them.
• The haze in anti-glare coatings helps to reduce glare and improve visibility.
• Some automotive plastics, such as headlight lenses, are designed to be hazy. This helps to diffuse the light from the headlights and to reduce glare.

• ISO 14782 — It determines the haze of transparent plastic materials.

• ASTM D1003 — It determines the haze and luminous transmittance of transparent plastics.
  
   

Procedure

Hazemeters and spectrophotometers are used to measure the level of haze, light transmitting, and light scattering properties of transparent materials. In hazemeter, the sample is placed between an incandescent light source and photocells. Several parameters are measured, like:
 

• the amount of light transmitted by the sample,
• the light scattered by the sample and the instrument, and
• the total incident light.

From these values, the percentage of transmitted light that is scattered can be calculated. The haze meter measures these variables and interrelates them so that the percentage of scattered light can be read in the meter. 
 

Measurement Principle of Hazemeter (Source: ResearchGate)

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