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Rigel1951
Rigel1951

Dr. Nigel H. Holmes discusses the challenges and opportunities of using nanoparticles in industrial formulations. Some key points include: (1) Nanoparticles provide benefits like improved strength, abrasion resistance, and UV protection, but require careful stabilization to prevent agglomeration. (2) The stabilizing shell surrounding nanoparticles is an important consideration, as it can impact properties like refractive index and cross-linking. (3) Factors like substrate compatibility, manufacturing stability, and particle size distribution must be considered to develop effective commercial formulations.

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The Nanoformulation Toolkit Dr. Nigel H. Holmes Senior Project Chemist MacDermid Autotype Ltd Wantage UK
Introduction The number of papers that have been produced over the last decade on the remarkable properties of nanoparticles is huge. The number of papers providing practical user information is rather less. 1. Is this because the much of the relevant information is buried within the R & D departments of the users and jealously guarded? Very possibly true! 2. Is this because it is simply not a problem? In the view of the speaker, no! 3. Is it because nanoparticles have not made the jump from being materials of scientific interest to materials that are available for industrial use? Yes & no! Points 2 & 3 are closely related.
The very big picture When we talk about the potential industrial use of nanoparticles we are in fact addressing a huge scientific field. A fraction of these are shown below. Physical strengthening of structures using carbon nanotubes Improving the abrasion resistance of materials using silicon and aluminium oxide Improving the gas barrier properties of plastics using nanoclays The production of antimicrobial coatings using nanoparticle silver Imparting UV resistance by the addition of oxides of Cerium, Zinc &Titanium And many, many more.
What you need to consider Development Production Cost Consistency Supply Safety
Do you really need nano? Only choose the nanoparticle route if you believe that the benefits will be measurable and cost effective. Only choose nano if you can identify reliable suppliers. Do not choose the nano-route out of technological vanity.
Size matters 1 The magic of nano is defined by size. Volume = 4r3/3 Surface area = 4r2 Surface area/Volume = 3/r As the radius of a particle decreases then the relative surface area increases. Halving the radius doubles the surface area/volume ratio.
Size matters 2 What is the effect of reducing the particle size? The result is greater attractive forces between the particles. There will be a need to prevent agglomeration. The intensity of light scattering is reduced (Rayleigh Scattering). The physical properties of nanoparticles differ considerably from that of the bulk material. Gold m.pt. 1064属C (Bulk): Gold m.pt. 750-800属C (5nm particle).
Size distribution: a warning. The volume and therefore the mass of a 100nm particle is 1000x greater than a 10nm particle. Only 1 of the large particles in a mix of 1000 of the smaller particles will contribute 50% of the total weight. Beware of particle size distribution by weight.
How are my nanoparticles delivered? Powder Form You choose your formulation (within reason). Very useful for powder coatings Greater variety of materials You will need to disperse and stabilise the particles. Health & safety issues concerning handling. Dispersion form You need to formulate with respect to the dispersion medium. Fewer dispersions than powders. The dispersion is already stabilised. Generally less issues concerning health & safety, but care still required.
What is my particle made of? d(total) is the hydrodynamic diameter. d(particle) d(total) Stabilising shell Core particle
More than just the particle Without stabilisation nanoparticles will rapidly agglomerate not good. The stabilising shell plays a role in determining the bulk properties of the product. We must always consider the particle and shell as a whole entity and not just the particle alone.
Refractive index and stabilising shell 1 The introduction of nanoparticle dispersions provides a means of modifying the refractive index of coatings. The refractive index values of Al2O3, ZnO & CeO2 are significantly greater than those of most organic materials. Altering the refractive index of a coating can alter its optical properties significantly.
The refractive index of mixtures There are several empirical models that can be used to determine the refractive index of a mixture. The simple volume fraction model is shown below. n(coating)=na(Vol %) + nb(Vol%) + Where a,b,c, etc. are the components of the mix.
Refractive index & stabilising shell 2 Note: volume % must be used. Most dispersions quoted as weight % so beware. A ceria nanoparticle dispersion of ~30% by weight sounds impressive, but this translates to a volume % of <5%. If you use only the refractive index of cerium oxide you will get the wrong answer! You need to include the stabilising shell in your calculation.
Effect on refractive index of the stabilising shell Weight of Cerium oxide Volume of Cerium oxide Effective refractive index 100% 100% 1.91 95% 80% 1.80 90% 65% 1.74 80% 45% 1.65 70% 32% 1.59 60% 24% 1.56
Abrasion resistance Silica & Alumina are both significantly harder than organic polymers, therefore significant additions to a formulation should improve the abrasion resistance of the coating. This has been shown to be a fact. If the particles are small enough the clarity of the coating should not be compromised. For ease of formulation it is often more convenient to add dispersions of nanoparticles directly to the formulation. It is possible to combine silica and alumina nanoparticles in UV curable resins for maximum abrasion resistance.
Defining the network Silica nanoparticle dispersions are readily available; Alumina a little less so. In some cases the stabilising shell contains acrylate groups which can cross-link with the resin. This generates a dense cross-linked network. The shell surrounding silica nanoparticles can be particularly thin. An area of 100nm2 might contain between 1 -100 stabilising groups. Both the nanoparticle and the cross-linked acrylate contribute to the overall abrasion resistance.
The idealised cross-linked network Circles represent nanoparticles, rectangles acrylate molecules
Acrylate photopolymerisation Acrylate curing mechanism
Measuring hardness Measurements using different test methods are not directly comparable. The best scientific results for hardness measurement are obtained from nanoindentation Unfortunately most commercial customers do not have access to such equipment. Pencil hardness testing is greatly favoured within industry and by customers. Its quick, its cheap, its woefully inaccurate! A gauge R &R survey of >1000 pencil hardness tests by skilled operatives revealed that the reproducibility of the test is zero.
The variability of 2H pencils Scratch % from 5 pencils: 12x5 individual tests for each pencil 0 10 20 30 40 50 60 Number of scratches (0-5 inclusive) Scratchpercentage Lead 1 Lead 2 Lead 3 Lead 4 Lead 5
The effect of substrate Pencil hardness testing is not independent of the substrate. For the same coating formulation. Metal >2H PET 1H PC 1B The result on flexible substrates is dependent on coating thickness and other factors. Thicker coatings give better results. The substrate bends under the pressure of the test.
Substrates matter The effect of substrate on the hallowed pencil hardness test is a reminder that the substrate can play a role in product performance. For liquid formulations the efficacy of a coating on polyester film might not be repeated on polycarbonate. Polycarbonate is more solvent sensitive than polyester; you might end up severely damaging the substrate if you choose the wrong solvent or acrylate dispersion.
Volume matters - a reminder Most nanoparticle dispersions are designated as weight percentages. A 50% by weight silica nanoparticle dispersion sound impressive (it is). However its only about 22% silica by volume. For denser materials the effect is even greater.
Manufacturing Stability You must consider the whole manufacturing process, not just the formulation in isolation. It is perfectly possible for small levels of impurities in items such as filters to destabilise a formulation with a consequent loss of production. This has been achieved!!
UV stability Nanoparticles offer a means of introducing none fugitive stabilisers to organic coatings. The small particle diameter enables the coating to remain optically clear. Good results have been demonstrated with nanoparticle Cerium Oxide in wood coatings, Titanium Dioxide & Zinc Oxide in sunscreens
Be Aware! Despite its benefits Cerium Oxide can impart a yellow colour to clear coatings. Titanium Dioxide exists in 2 major crystalline forms: Rutile & Anatase. Both provide UV protection. The latter is photoactive, the former is not. If you want a self- cleaning coating choose Anatase, but if you want a stable coating choose Rutile. The above comments are a slight simplification as Anatase can be stabilised.
Zinc Oxide Zinc Oxide is not photo-active and brings no unwanted colour to coatings. It is commercially available as a dispersion from a number of reputable suppliers. It shows a good degree of UV absorption.
Zinc Oxide 2 Remember: the nanoparticle shell must also be considered. Different stabilising materials can show markedly different behaviour with the other ingredients of the formulation. Certain stabilising agents do not have good compatibility with acrylate oligomers. This can lead to agglomeration and loss of performance.
Conclusion Size matters: property enhancement arises out of the increased surface area. Size matters: stability problems arise because of the increased area. Nanoparticles must be stabilised for commercial use. If you dont consider the stabilising shell then you will suffer problems. Possible substrate effects cannot be ignored. Despite all of these hurdles it is possible to formulate with nanoparticles!
Thanks Prof. Steve Abbott for many challenging discussions My colleagues at MacDermid; especially Andy Torrens-Burton, Jason Small, Jeremy Gamble and Lynn Donoghue for their help and patience in the search for commercially viable nanoparticle formulations.

More Related Content

holmes_presentation

  • 1. The Nanoformulation Toolkit Dr. Nigel H. Holmes Senior Project Chemist MacDermid Autotype Ltd Wantage UK
  • 2. Introduction The number of papers that have been produced over the last decade on the remarkable properties of nanoparticles is huge. The number of papers providing practical user information is rather less. 1. Is this because the much of the relevant information is buried within the R & D departments of the users and jealously guarded? Very possibly true! 2. Is this because it is simply not a problem? In the view of the speaker, no! 3. Is it because nanoparticles have not made the jump from being materials of scientific interest to materials that are available for industrial use? Yes & no! Points 2 & 3 are closely related.
  • 3. The very big picture When we talk about the potential industrial use of nanoparticles we are in fact addressing a huge scientific field. A fraction of these are shown below. Physical strengthening of structures using carbon nanotubes Improving the abrasion resistance of materials using silicon and aluminium oxide Improving the gas barrier properties of plastics using nanoclays The production of antimicrobial coatings using nanoparticle silver Imparting UV resistance by the addition of oxides of Cerium, Zinc &Titanium And many, many more.
  • 4. What you need to consider Development Production Cost Consistency Supply Safety
  • 5. Do you really need nano? Only choose the nanoparticle route if you believe that the benefits will be measurable and cost effective. Only choose nano if you can identify reliable suppliers. Do not choose the nano-route out of technological vanity.
  • 6. Size matters 1 The magic of nano is defined by size. Volume = 4r3/3 Surface area = 4r2 Surface area/Volume = 3/r As the radius of a particle decreases then the relative surface area increases. Halving the radius doubles the surface area/volume ratio.
  • 7. Size matters 2 What is the effect of reducing the particle size? The result is greater attractive forces between the particles. There will be a need to prevent agglomeration. The intensity of light scattering is reduced (Rayleigh Scattering). The physical properties of nanoparticles differ considerably from that of the bulk material. Gold m.pt. 1064属C (Bulk): Gold m.pt. 750-800属C (5nm particle).
  • 8. Size distribution: a warning. The volume and therefore the mass of a 100nm particle is 1000x greater than a 10nm particle. Only 1 of the large particles in a mix of 1000 of the smaller particles will contribute 50% of the total weight. Beware of particle size distribution by weight.
  • 9. How are my nanoparticles delivered? Powder Form You choose your formulation (within reason). Very useful for powder coatings Greater variety of materials You will need to disperse and stabilise the particles. Health & safety issues concerning handling. Dispersion form You need to formulate with respect to the dispersion medium. Fewer dispersions than powders. The dispersion is already stabilised. Generally less issues concerning health & safety, but care still required.
  • 10. What is my particle made of? d(total) is the hydrodynamic diameter. d(particle) d(total) Stabilising shell Core particle
  • 11. More than just the particle Without stabilisation nanoparticles will rapidly agglomerate not good. The stabilising shell plays a role in determining the bulk properties of the product. We must always consider the particle and shell as a whole entity and not just the particle alone.
  • 12. Refractive index and stabilising shell 1 The introduction of nanoparticle dispersions provides a means of modifying the refractive index of coatings. The refractive index values of Al2O3, ZnO & CeO2 are significantly greater than those of most organic materials. Altering the refractive index of a coating can alter its optical properties significantly.
  • 13. The refractive index of mixtures There are several empirical models that can be used to determine the refractive index of a mixture. The simple volume fraction model is shown below. n(coating)=na(Vol %) + nb(Vol%) + Where a,b,c, etc. are the components of the mix.
  • 14. Refractive index & stabilising shell 2 Note: volume % must be used. Most dispersions quoted as weight % so beware. A ceria nanoparticle dispersion of ~30% by weight sounds impressive, but this translates to a volume % of <5%. If you use only the refractive index of cerium oxide you will get the wrong answer! You need to include the stabilising shell in your calculation.
  • 15. Effect on refractive index of the stabilising shell Weight of Cerium oxide Volume of Cerium oxide Effective refractive index 100% 100% 1.91 95% 80% 1.80 90% 65% 1.74 80% 45% 1.65 70% 32% 1.59 60% 24% 1.56
  • 16. Abrasion resistance Silica & Alumina are both significantly harder than organic polymers, therefore significant additions to a formulation should improve the abrasion resistance of the coating. This has been shown to be a fact. If the particles are small enough the clarity of the coating should not be compromised. For ease of formulation it is often more convenient to add dispersions of nanoparticles directly to the formulation. It is possible to combine silica and alumina nanoparticles in UV curable resins for maximum abrasion resistance.
  • 17. Defining the network Silica nanoparticle dispersions are readily available; Alumina a little less so. In some cases the stabilising shell contains acrylate groups which can cross-link with the resin. This generates a dense cross-linked network. The shell surrounding silica nanoparticles can be particularly thin. An area of 100nm2 might contain between 1 -100 stabilising groups. Both the nanoparticle and the cross-linked acrylate contribute to the overall abrasion resistance.
  • 18. The idealised cross-linked network Circles represent nanoparticles, rectangles acrylate molecules
  • 20. Measuring hardness Measurements using different test methods are not directly comparable. The best scientific results for hardness measurement are obtained from nanoindentation Unfortunately most commercial customers do not have access to such equipment. Pencil hardness testing is greatly favoured within industry and by customers. Its quick, its cheap, its woefully inaccurate! A gauge R &R survey of >1000 pencil hardness tests by skilled operatives revealed that the reproducibility of the test is zero.
  • 21. The variability of 2H pencils Scratch % from 5 pencils: 12x5 individual tests for each pencil 0 10 20 30 40 50 60 Number of scratches (0-5 inclusive) Scratchpercentage Lead 1 Lead 2 Lead 3 Lead 4 Lead 5
  • 22. The effect of substrate Pencil hardness testing is not independent of the substrate. For the same coating formulation. Metal >2H PET 1H PC 1B The result on flexible substrates is dependent on coating thickness and other factors. Thicker coatings give better results. The substrate bends under the pressure of the test.
  • 23. Substrates matter The effect of substrate on the hallowed pencil hardness test is a reminder that the substrate can play a role in product performance. For liquid formulations the efficacy of a coating on polyester film might not be repeated on polycarbonate. Polycarbonate is more solvent sensitive than polyester; you might end up severely damaging the substrate if you choose the wrong solvent or acrylate dispersion.
  • 24. Volume matters - a reminder Most nanoparticle dispersions are designated as weight percentages. A 50% by weight silica nanoparticle dispersion sound impressive (it is). However its only about 22% silica by volume. For denser materials the effect is even greater.
  • 25. Manufacturing Stability You must consider the whole manufacturing process, not just the formulation in isolation. It is perfectly possible for small levels of impurities in items such as filters to destabilise a formulation with a consequent loss of production. This has been achieved!!
  • 26. UV stability Nanoparticles offer a means of introducing none fugitive stabilisers to organic coatings. The small particle diameter enables the coating to remain optically clear. Good results have been demonstrated with nanoparticle Cerium Oxide in wood coatings, Titanium Dioxide & Zinc Oxide in sunscreens
  • 27. Be Aware! Despite its benefits Cerium Oxide can impart a yellow colour to clear coatings. Titanium Dioxide exists in 2 major crystalline forms: Rutile & Anatase. Both provide UV protection. The latter is photoactive, the former is not. If you want a self- cleaning coating choose Anatase, but if you want a stable coating choose Rutile. The above comments are a slight simplification as Anatase can be stabilised.
  • 28. Zinc Oxide Zinc Oxide is not photo-active and brings no unwanted colour to coatings. It is commercially available as a dispersion from a number of reputable suppliers. It shows a good degree of UV absorption.
  • 29. Zinc Oxide 2 Remember: the nanoparticle shell must also be considered. Different stabilising materials can show markedly different behaviour with the other ingredients of the formulation. Certain stabilising agents do not have good compatibility with acrylate oligomers. This can lead to agglomeration and loss of performance.
  • 30. Conclusion Size matters: property enhancement arises out of the increased surface area. Size matters: stability problems arise because of the increased area. Nanoparticles must be stabilised for commercial use. If you dont consider the stabilising shell then you will suffer problems. Possible substrate effects cannot be ignored. Despite all of these hurdles it is possible to formulate with nanoparticles!
  • 31. Thanks Prof. Steve Abbott for many challenging discussions My colleagues at MacDermid; especially Andy Torrens-Burton, Jason Small, Jeremy Gamble and Lynn Donoghue for their help and patience in the search for commercially viable nanoparticle formulations.