Specialty chemicals account for 26% of the European chemical industry, however this figure undervalues their total contribution to the EU economy. This is due to the dependence of many other sectors on the intermediate products from these processes, these dependent sectors include construction, cosmetics, household, automotive
Titanium Dioxide (TiO2) is a widely used industrial material, both as a base ingredient and additive to products as diverse as paint, pigments , sun block, toothpaste, paper, photo catalysis and food where it is known as additive E171.
Globally production is approximately 1.5 Million Tons per year of which 60% is used for paints and coatings. Alternatives to Titanium dioxide for white pigments include milled and precipitated calcium carbonate and talc, but since the functional mechanisms are the same the particles size characteristics of the final products are similar. The size of the individual particles of TiO2 is key to its performance in many applications, including paper paint, sun block. For example opaque sun block uses coarse sized particles of 300nm while transparent UV blocking products use finer sizes.
The primary method of production is milling of the raw material which is an energy intensive process which is reflected in the price with the ultrafine grades being up to 10X times more expensive. (Bulk Ti02 for coatings costs $0.90 while Ultrafine TiO2 10-50nm range $2.00-$10.00 ($5.00) (2003))
Use of Titanium Dioxide as white pigment
To provide a white pigment with high opacity necessitates reflecting visible light which as a wave length range of 0.44 (Blue) to 0.70um (Red). Since maximum visible light scattering happens when the particle diameter is equal to the half wavelength the ideal mean particle diameter is in the range0.22 - 0.35um and hence a nominal size approximately 300nm is the target of the milling process.
The exact particle size distribution has an impact on the quality and color of the pigment. With particle sizes below 300nm giving a bluish tint to the product which is used as a selling point in laundry powders but produces poor white paint since it reduces the opacity while slightly larger particle sizes give a brown undertone and very coarse tails in result in a gritty texture and poor performance as a gloss coat.
The need and the challenges for in-situ sensor
Reducing Milling time saves energy but requires a method of determining the end point, when the particle size is 300nm. Since the process is a high concentration and is an extremely opaque medium it is difficult to apply optical methods.
Traditionally samples are removed from the process highly diluted and tested on an optical system such as a UV spectrometer, however the stability of the suspension is very pH dependent and hence dilution can affect particle size and the delay between sampling and result limits the usefulness of the test.
Process Ultrasound Spectroscopy
Ultrasonic Spectroscopy (USS) is the characterization of the ultrasonic response of a material to the propagation
of a low level Ultrasonic wave. It has a number of inherent and useful features:
- Non-destructive, Non-Intrusive and inline
- Can be used on opaque samples
- Can be used on both liquids and soft solids
- Simple sample handling with no requirement for manual intervention
- Able to operate over a wide range of frequencies
The ultrasound wave is a high frequency mechanical wave and its propagation is dependent on materials thermo physical properties.
Rheology is a measure of the time-temperature-dependent flow and deformation properties of liquids and deformable solids under an applied force. Therefore Ultrasonic Spectroscopy can be used to measure the rheological properties of a material. Specifically the attenuation and velocity information can be used to determine the longitudinal bulk modulus of a material.
ITS has developed an Ultrasonic Spectroscopy instrument the U2s which measures the ultrasonic attenuation and velocity of liquids and soft solids, and determines the effects changes in Particle size distribution and
Ultrasound Spectroscopy Characterization Capabilities
The Ultrasonic Spectroscopy data can be used to characterize a range of materials or process properties and behaviors including:
- Measurement of density and or concentration of discreet phases mixed liquids or slurries
- Rheological characterization of liquids and soft solids through phase velocity
- Indicative changes in attenuation spectra due to changes in material structure including particle Size of colloids in both slurries and emulsions
- Relaxation phenomena in biomaterials and soft solids
Concentration Dependence of Titanium Dioxide
The concentration dependence of Titanium dioxide for a single monomodal particle size distribution of 300nm was examined using a concentration ladder. This was produced by diluting a 70%w sample, to provide samples at concentrations of 1, 3, 5, 10, 30, 50 and 70%w.
Ultrasound Spectroscopy provides a method of characterization for changes in the structure of particulate samples at high concentration including concentration and particle size changes.