The material selection platform
Coatings Ingredients
The material selection platform
Coatings Ingredients

Haze: An Important Appearance Attribute in Paints and Coatings

Haze in Paints & Coatings
  1. Hazy Appearance of Coatings
  2. What is Haze and How Does it Happen?
  3. What are the Types of Haze & How to Measure it?
  4. Main Causes of Haze in Coatings

Hazy Appearance of Coatings

When we talk about coating finishes, the parameters to be measured are more than just measuring gloss. Apart from gloss, another important parameter includes “haze” which is an important appearance attribute that can be quantified and then used to assess the quality of coated parts.

In several coating applications such as automotive, architectural metallic or non-metallic finishes, surface haze can be problematic as it can impact the harmonious product appearance. Coatings with haze exhibit a slight “milky” finish or bloom observed visually on high gloss surfaces and it is more noticeable when the coated surface is viewed at an angle or under strong light.

Let us now discuss the basic types of haze, why does it happen and the methods to measure it.

What is Haze and How Does it Happen?

Haze is light that has been reflected by small surface structures adjacent to the main specular component* of reflected light, thus obscuring the clarity and brilliance of a surface. Haze can be inherent in the material or a result of surface texture.

With respect to paints and coatings, haze is caused by microstructures or suspended particles which slightly change the direction of a reflected light. The change in direction causes a bloom adjacent to gloss (specular) angle, thus, showing a milky effect. For example, on the high gloss paint surfaces, the incident light coming from an angle of 20° reflects mostly at 20° and a small part of the light scatters around 20°. This defect is perceived by the eye as haze on the paint surface.

Haze on Painted Surface
Haze on Painted Surface
(Source: Konica Minolta Sensing)

* The specular component is the light that is reflected from an object at an angle equal to but opposite the incident light. Specular reflection is a mirror-like reflection of light from the surface where the incident light is reflected into a single outgoing direction.

What are the Types of Haze and How to Measure it?

Controlling haze is a vital part of ensuring a product or application that is of the highest quality and appearance. There are two types of haze that affect a surface or material:

Reflection Haze

Reflection haze is a crucial factor in assessing the appearance of any coated product to achieve highly reflective and smooth finishes.

Reflection haze results from microstructures caused by poor dispersion, polishing, weathering, variations in the coating curing process, or even the coating application method. Reflection haze occurs when the surface diffuses low intensity light adjacent to the main direction of reflection (specular angle) – though the high level of reflection still makes the surface appear glossy and reflective, there is a milky haze on top of it.

Reflection haze is measured using gloss and haze measurements. Most glossmeters measure at the specular angle plus or minus several degrees and therefore cannot report the amount which the specular component spreads.

  • The light that is spread 0.3° from the specular is responsible for distinctness-of-image** gloss.
  • The light that is spread 2° is responsible for a quality known as bloom or narrow-angle reflection haze.
  • The light spread 5° is referred to as wide-angle reflection haze.

Types of Haze
Type of Haze
(Source: Konica Minolta Sensing)

The standards used to measure reflection haze include:

  • ASTM E430 – Standard Test Methods for Measurement of Gloss of High-Gloss Surfaces by Abridged Goniophotometry
    • These test methods cover the measurement of the reflection characteristics responsible for the glossy appearance of high-gloss surfaces. Two test methods, A and B, are provided for evaluating such surface characteristics at specular angles of 20° and 30°, respectively. These test methods are not suitable for diffuse finish surfaces nor do they measure color, another appearance attribute.

  • ASTM D4039 – Standard Test Method for Reflection Haze of High-Gloss Surfaces
    • This test method describes a procedure for using two specular gloss measurements to obtain a haze index for high-gloss non-metallic specimens. It is particularly useful for evaluating the haze in clear finishes on non-glossy substrates, and the haze in reflected images produced by the surfaces of opaque glossy pigmented finishes.

  • ISO 13803:2014 Paints and Varnishes – Determination of Haze on Paint Films at 20° (and ISO 2813:2014 Paints and varnishes – Determination of gloss value at 20°, 60° and 85°)
    • It specifies a test method for determining the haze of coatings. The method is suitable for the haze measurement of non-textures coatings on plane, opaque substrates.

  • ASTM D5767 – Standard Test Method for Instrumental Measurement of Distinctness-of-Image (DOI) Gloss of Coated Surfaces
    • This test method describes the measurement of the distinctness-of-image (DOI) gloss of coating surfaces using electro-optical measuring techniques. The coatings assessed shall be applied to planar rigid surfaces. The light through a small slit is projected on the specimen surface and its reflected image intensity is measured through a sliding combed shutter to provide a value of image clarity.

Reflection haze is of great interest in paints and coatings applications, particularly in powder coatings and other high gloss coatings. 

**Distinctness of Image (DOI) is a measure of how crisp and sharply a reflected image appears.

Transmission Haze

Transmission haze involves appearance of haze in a transparent material. In a transparent material, when light passes through the material, it scatters due to impurities, surface roughness, porosity, etc. resulting in transmission loss. Hazemeters and spectrophotometers are used to measure the level of scattering, light transmitting and light scattering thus, the haze level of the material. The commonly employed standards include:

  • ASTM D1003 – Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics
  • ISO 13468 Plastics – Determination of the Total Luminous Transmittance of Transparent Materials
  • ISO 14782 Plastics – Determination of Haze for Transparent Materials

(Transmission haze mostly linked with transparent materials, i.e., Plastics)

Source: BYK Instruments

Main Causes of Haze in Coatings

In coatings industry, microscopic surface texture can be attributed to several factors as listed below.

Raw Materials Curing Process Post Application
  • Incompatible materials
  • Poor dispersions
  • Pigment properties – particle size and number of particles
  • Binder incompatibility
  • Additives migration to the surface
  • Incorrect solvent blend
  • Curing temperature and speed
  • Drying conditions
  • Stoving/baking conditions
  • Weathering
  • Abrasion
  • Aging and oxidation
  • Poor cleanliness/surface residue

Learn about the factors playing the important role in gloss enhancement and retention in coatings.

Basics of Gloss

Be the first to comment on "Haze: An Important Appearance Attribute in Paints and Coatings"

Leave a comment

Your email address and name will not be published submitting a comment or rating implies your acceptance to SpecialChem Terms & Conditions
Back to Top