Particle Size Distribution

Detailed Description

Particle size distribution (PSD) is a measurement that indicates the range of particle sizes present in a particulate material, such as a powder, granular material, or suspension. It describes the relative amounts of particles present according to size, typically expressed as a frequency distribution or cumulative distribution. In the context of a Safety Data Sheet (SDS), particle size distribution provides critical information for assessing potential health hazards, handling requirements, and physical behavior of particulate materials.

Particle size can be characterized in several ways:

Diameter: For spherical particles, the diameter is straightforward. For non-spherical particles, various equivalent diameters can be defined:

Equivalent spherical diameter: Diameter of a sphere with the same property (volume, surface area, etc.) as the particle.
Aerodynamic diameter: Diameter of a sphere with standard density (1 g/cm³) that has the same settling velocity as the particle.
Feret's diameter: Distance between two parallel tangents to the particle.
Martin's diameter: Length of the line that bisects the particle image.

Distribution parameters: Various statistical parameters describe the distribution:

D10, D50, D90: Particle size at which 10%, 50%, or 90% of the particles are smaller than this value.
Mean diameter: Average particle size, which can be calculated in various ways (number-weighted, volume-weighted, etc.).
Mode: Most common particle size in the distribution.
Span: Measure of the width of the distribution, often calculated as (D90-D10)/D50.

Importance in Safety Data Sheets

Particle size distribution information in an SDS is important for several reasons:

Inhalation hazard assessment: Smaller particles can penetrate deeper into the respiratory system, with particles <10 μm reaching the lungs and particles <2.5 μm reaching the alveoli.
Dust explosion hazard: Fine particles can form explosive dust clouds, with the explosion risk generally increasing as particle size decreases.
Dermal exposure: Smaller particles may have greater potential for dermal penetration.
Handling considerations: Affects appropriate handling procedures, containment strategies, and dust control measures.
PPE selection: Guides selection of appropriate respiratory protection and other PPE.
Environmental fate: Influences environmental transport, deposition, and bioavailability.
Processing behavior: Affects flow properties, compaction, dispersion, and other processing characteristics.
Nanomaterial identification: Identifies materials with nanoscale dimensions that may have unique hazard profiles.

Measurement Methods

Several techniques are used to determine particle size distribution:

Method	Description	Typical Size Range	Advantages/Limitations
Sieve Analysis	Mechanical separation of particles using sieves of different mesh sizes	38 μm - 125 mm	Simple, inexpensive; limited to coarser particles, dry samples only
Laser Diffraction	Measures angular variation in intensity of scattered light as a laser beam passes through a sample	0.01 μm - 3,500 μm	Wide range, rapid; requires model assumptions, volume-weighted
Dynamic Light Scattering	Measures Brownian motion of particles and relates it to size	0.001 μm - 10 μm	Good for nanoparticles; limited for polydisperse samples
Microscopy (Optical/SEM/TEM)	Direct imaging and measurement of particles	0.001 μm - 1,000 μm	Direct visualization; time-consuming, small sample size
Sedimentation Methods	Based on settling velocity of particles in a fluid	0.1 μm - 100 μm	Provides aerodynamic diameter; time-consuming
Electrical Sensing Zone (Coulter Counter)	Measures electrical impedance changes as particles pass through an aperture	0.4 μm - 1,200 μm	Number-weighted, good resolution; requires conductive liquid
Acoustic Spectroscopy	Measures attenuation of sound waves passing through a particulate system	0.01 μm - 1,000 μm	Works with concentrated suspensions; complex interpretation
Image Analysis	Automated analysis of particle images	0.5 μm - 1,000 μm	Provides shape information; sample preparation critical

Particle Size Categories and Health Implications

Category	Size Range	Classification	Respiratory Deposition	Health Implications
Coarse Particles	10 μm - 100 μm	Coarse	Upper respiratory tract (nose, throat)	Potential irritation of eyes, nose, and throat; generally filtered by nose and throat
Inhalable Particles	2.5 μm - 10 μm	Fine	Tracheobronchial region	Can penetrate into the bronchi and bronchioles; may cause respiratory issues
Respirable Particles	1 μm - 2.5 μm	Very Fine	Alveolar region	Can reach deep into lungs and deposit in alveoli; associated with cardiovascular and respiratory diseases
Ultrafine Particles	0.1 μm - 1 μm	Ultrafine	Alveolar region with high efficiency	High deposition efficiency in lungs; may translocate to other organs
Nanoparticles	<0.1 μm (100 nm)	Nano	Throughout respiratory tract with potential systemic translocation	May cross biological barriers; unique toxicological properties; potential for systemic distribution

Dust Explosion Hazards and Particle Size

Particle size significantly affects dust explosion hazards:

Particle Size Range	Dust Explosion Hazard	Key Considerations
>500 μm	Generally not explosive as dust	May still be combustible; can generate finer particles during handling
100 μm - 500 μm	Low explosion hazard	May contribute to explosion if sufficient fine particles are present
50 μm - 100 μm	Moderate explosion hazard	Can form explosive dust clouds under favorable conditions
10 μm - 50 μm	High explosion hazard	Readily forms explosive dust clouds; high surface area increases reactivity
<10 μm	Very high explosion hazard	Remains suspended longer; lower minimum ignition energy; higher explosion severity

Key dust explosion parameters affected by particle size include:

Minimum Explosible Concentration (MEC): Decreases with decreasing particle size
Minimum Ignition Energy (MIE): Decreases with decreasing particle size
Maximum Explosion Pressure (Pmax): Increases with decreasing particle size
Maximum Rate of Pressure Rise (dP/dt)max: Increases with decreasing particle size
Dust Explosion Class (Kst): Increases with decreasing particle size

Factors Affecting Particle Size Distribution

Production and Processing Factors

Several factors during production and processing affect particle size distribution:

Production method: Different synthesis, crystallization, or comminution methods produce different particle size distributions.
Milling/grinding parameters: Type of mill, energy input, residence time, and media size affect the resulting particle size.
Crystallization conditions: Supersaturation, cooling rate, agitation, and additives influence crystal size and distribution.
Precipitation parameters: Concentration, temperature, pH, and mixing affect precipitate particle size.
Spray drying conditions: Feed concentration, atomization parameters, and drying temperature affect particle formation.
Granulation variables: Binder properties, process parameters, and equipment design influence granule size distribution.

Material Properties

Intrinsic material properties influence particle size distribution:

Hardness and brittleness: Affect breakage behavior during size reduction processes.
Crystal structure: Influences cleavage planes and fracture patterns.
Moisture content: Can affect agglomeration, flow properties, and size reduction efficiency.
Surface properties: Surface energy and charge affect agglomeration tendencies.
Melting point: Materials with low melting points may smear rather than fracture during size reduction.

Storage and Handling Effects

Particle size distribution can change during storage and handling:

Agglomeration: Particles may stick together due to moisture, electrostatic forces, or surface interactions.
Attrition: Particles may break down during handling, transportation, or storage.
Segregation: Different particle sizes may separate during handling or storage.
Moisture absorption: Can lead to caking, agglomeration, or particle growth.
Temperature cycling: May cause recrystallization or changes in crystal structure.

Safety Implications of Particle Size

Particle size has significant implications for safety:

Respiratory hazards: Particles <10 μm can be inhaled, with smaller particles penetrating deeper into the respiratory system. Particles <2.5 μm can reach the alveoli, while nanoparticles may translocate to other organs.
Dust explosion risks: Fine particles can form explosive dust clouds, with the explosion risk generally increasing as particle size decreases. Particles <50 μm are of particular concern.
Dermal exposure: Smaller particles may have greater potential for dermal penetration, especially those <1 μm.
Dustiness: Finer particles remain airborne longer, increasing potential exposure duration.
Agglomeration: Fine particles may agglomerate, affecting their apparent size and behavior.
Surface area: Smaller particles have higher specific surface area, potentially increasing reactivity and biological interactions.
Nanomaterial hazards: Particles with dimensions <100 nm may exhibit unique properties and hazards compared to larger particles of the same material.

Regulatory Considerations

Particle size information has important regulatory implications:

GHS classification: While not specifically mandated for all substances, particle size information is relevant for assessing inhalation hazards and may affect classification.
Nanomaterials: Many jurisdictions have specific reporting or registration requirements for nanomaterials (typically defined as having at least one dimension <100 nm).
Dust explosion hazards: Combustible dust hazards must be communicated in SDSs, with particle size being a key factor.
Workplace exposure limits: Some substances have different exposure limits for different size fractions (e.g., respirable vs. inhalable).
Transportation regulations: Particle size can affect classification for transport, particularly for substances that may be pyrophoric or water-reactive in finely divided form.

In the EU, the REACH regulation requires specific consideration of nanoforms of substances, including detailed characterization of particle size distribution and other physical-chemical properties.

Best Practices

When reporting particle size distribution in an SDS:

Provide quantitative data whenever possible (e.g., D10, D50, D90 values)
Specify the measurement method used (e.g., laser diffraction, sieve analysis)
Include information on the respirable fraction (<10 μm) if relevant for inhalation hazards
For nanomaterials, provide detailed characterization including size distribution, shape, and specific surface area
For materials with dust explosion hazards, include relevant particle size information
Indicate if the material contains a significant proportion of fine particles that may affect handling and exposure control
For materials that may break down during handling, note the potential for generation of finer particles
Ensure consistency between particle size information and hazard classifications in the SDS
Consider including information on dustiness or tendency to become airborne if relevant
For complex distributions, consider providing a graphical representation or more detailed statistical parameters