Pharmaceutical Particle Size Reduction

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Questions and Answers

What process is used to carry out size reduction, also known as comminution?

crack propagation

In crack propagation, what causes bond rupture?

localized stresses

Through what areas of a material do cracks generally propagate?

regions with the most flaws or discontinuities

What effect does rapid crack propagation have on energy?

Excess energy concentrates at other discontinuities. Signup and view all the answers

What is the phenomenon called when rapid crack propagation causes new cracks to form in the material?

cascade effect Signup and view all the answers

What term is used to describe the fracture that occurs when a material breaks instantaneously due to crack propagation?

brittle fracture Signup and view all the answers

What property allows some materials to resist fracture at higher stresses?

plastic flow Signup and view all the answers

How does plastic flow contribute to fracture resistance?

It allows strain energy to be absorbed. Signup and view all the answers

What is another term used for size reduction?

Comminution Signup and view all the answers

Besides aiding in drug delivery, what is one reason for performing size reduction?

To facilitate powder mixing or the production of suspensions. Signup and view all the answers

Name one special function of size reduction mentioned in the text?

Exposing cells in plant tissue prior to extraction of active principles. Signup and view all the answers

What is one property of solid drugs that is affected by particle size?

The rate of dissolution Signup and view all the answers

What is one reason why reducing the bulk volume of a material is advantageous?

To improve transportation efficiency. Signup and view all the answers

Name one method of size separation mentioned in the text.

Sieving Signup and view all the answers

In which chapter can you find a discussion on the significance of particle size in drug delivery?

Chapter 9 Signup and view all the answers

Why might a drug in dry powder inhalation formulation need to have a unique particle size compared to other formulations?

To ensure the drug can be inhaled properly. Signup and view all the answers

What pharmaceutical process does size reduction help to facilitate?

Powder mixing Signup and view all the answers

What is one consideration when selecting a size separation method?

The final required particle size Signup and view all the answers

Besides dissolution, what is another property of solid drugs and excipients that is affected by particle size?

Powder flow characteristics Signup and view all the answers

What does size reduction often involve in the context of plant tissue?

Exposing cells to extract active principles. Signup and view all the answers

What is one key factor that influences the choice of commercial equipment for size reduction?

The size distribution of the material after size reduction. Signup and view all the answers

Name two common methods of size reduction.

Cutting and compression. Signup and view all the answers

Is particle size important for both drugs and excipients? Answer Yes or No.

Yes Signup and view all the answers

What is a practical benefit of size reduction besides ease of mixing, given in the text?

Improved transportation efficiency Signup and view all the answers

Name one specialized method of dry particle size separation, other than sieving.

Elutriation Signup and view all the answers

Besides mean particle size, what other characteristic of powdered material does the method of size reduction affect?

Size distribution. Signup and view all the answers

What is the objective of size separation?

To separate a specific size or a narrow range of sizes from a powder. Signup and view all the answers

True or false: Size reduction is the only factor that pharmaceutical scientists need to understand when manipulating particle size.

False Signup and view all the answers

What do the most efficient commercial size reduction machinery depend on?

The properties and mechanisms of size reduction. Signup and view all the answers

In addition to cutting and compression, name one more size reduction method.

Impact. Signup and view all the answers

Why is size separation often necessary after size reduction?

To obtain the specific sizes or size ranges optimal for the application. Signup and view all the answers

What influences the design of commercial equipment for size reduction?

Influence of size reduction on size distribution. Signup and view all the answers

According to Griffith's theory, crack propagation is related to what energy phenomenon?

relaxation of plastic strain energy Signup and view all the answers

Where is stress concentrated in a material?

at the tip of a crack Signup and view all the answers

In the Inglis equation, what does $\sigma_K$ represent?

the stress multiplier Signup and view all the answers

What does 'L' represent in both the Inglis and fracture stress equations?

the crack length Signup and view all the answers

What does 'r' represent in the Inglis equation?

the radius of curvature of the tip of the crack Signup and view all the answers

What is required for crack propagation in brittle materials and how does that differ from other materials?

Plastic flow is required, and it differs from other materials because brittle materials typically do not exhibit plastic flow. Signup and view all the answers

According to the material provided, how does fracture stress vary with crack length?

Fracture stress varies inversely with the square root of the crack length. Signup and view all the answers

Which scientists modified Griffith's crack theory by considering plastic flow?

Irwin and Orowan Signup and view all the answers

What is the significance of the stress multiplier σK value of 3 for a double surface?

It indicates the ease of comminution, highlighting material behavior under stress. Signup and view all the answers

How does the brittleness or plasticity of a material influence crack propagation?

Brittleness leads to rapid crack initiation and propagation, while plasticity allows for some deformation before failure. Signup and view all the answers

Describe the relevance of the distance between atomic surfaces in relation to crack formation.

The separation of 2 × 10−10 m between atomic surfaces signifies the molecular-level interactions that facilitate crack initiation. Signup and view all the answers

What does a stress concentration factor of approximately 245 indicate for a thin disc-shaped crack?

It suggests a significant amplification of stress at the crack tip, which can lead to crack growth. Signup and view all the answers

Explain how stress concentration diminishes towards the mean stress away from the crack tip.

Stress concentration decreases as distance from the crack tip increases, leading to more uniform stress distribution. Signup and view all the answers

Study Notes

Particle Size Reduction and Size Separation

Particle size greatly affects drug and excipient properties, influencing dissolution and powder flow.
Raw materials often need size reduction before use in pharmaceuticals.
Mechanical properties of solids affect size reduction methods.
The chosen size reduction method impacts mean particle size and the size distribution.

Influence of Material Properties on Size Reduction

Fracture occurs due to localized stress-induced strains in the material.
Cracks propagate through areas with flaws or discontinuities.
Strain energy is related to crack propagation, with stress concentration at the crack tip.
Stress multiplier is calculated using an Inglis equation (equation 10.1).
Brittle materials fracture easily; toughness depends on the ability to resist fracture.
Tough materials can undergo plastic flow, absorbing strain energy without fracturing. Stress and fracture stress is inversely related to the square root of the crack length (equation 10.2).

Energy Requirements of Size Reduction

Only a small percentage of input energy actually reduces size.
Much of the energy is lost in various processes; like: elastic deformation, plastic deformation, crack initiation, friction between particles and the machine, heat, sound, and vibration.
Rittinger's hypothesis (equation 10.3) connects energy input to new surface area created.
Kick's theory (equation 10.4) states that energy is proportional to the log of size change.
Bond's theory (equation 10.5) connects energy to the new crack length.

Influence of Size Reduction on Size Distribution

Milling changes the initial particle size distribution.
Initially, normal distributions shift towards smaller particle sizes.
Continued milling transitions from bimodal to unimodal distributions.
Fine particles may aggregate, and further reduction leads to larger particle sizes and/or agglomeration.

Size Reduction Methods

Cutting methods: Cutter mills (with rotating knives and stationary knives) are used for brittle materials.
Compression methods: Roller mills employ rotating cylinders to compress particles, primarily used for intermediate-sized materials.
Impact methods: Hammer mills use swinging hammers to impact particles causing fracture, typically used for robust materials.
Vibration methods: Vibration mills use vibrations and repeated impacts from balls to reduce particle size.
Attrition methods: Roller mills use attrition to wear away particles, particularly useful for softer materials like gums and pastes.
Combined methods: Processes combining impact and attrition are efficient for processing materials with varying properties.

Size Separation Methods

Sieving: Separates particles based on size using screens with varying mesh sizes.
Sedimentation: Separation based on settling velocities of particles in a liquid.
Elutriation: Uses upward-flowing liquid to separate particles by their different settling velocities.
Cyclone methods: Relies on centrifugal force to separate particles from a fluid stream.

Key Points

Understanding material properties like hardness and toughness helps optimize size reduction and separation.
Selecting the appropriate method for a specific material and application is crucial for efficiency and cost-effectiveness.

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Description

This quiz covers essential concepts in particle size reduction and size separation within the pharmaceutical industry. Learn about how raw materials are processed and the mechanical properties that influence the chosen methods of size reduction. Delve into the fracture mechanics of materials and how these properties affect drug formulation.