Powder Technology in Polymer Industry

 Powder Technology in Polymer Industry

What is Powder Technology?

Particle technology may be a term wont to ask the science and technology associated with the handling and processing of particles. Particle technology is additionally often described as powder technology, particle science and powder science. Particles are commonly mentioned as bulk solids, particulate solids and granular solids. Combining two or more granular or powder ingredients requires an appropriate mixing process, which may be either free or random flow with no attraction forces between the particles or interactive or orderly with the presence of huge active particles that attract others forming stable clumps.

Stages of Powder Technology

1. Sieving

Before starting a powder mixing process, a classification and maybe separation of the particles is necessary. Sieving process is the separation of a mixture of grains of different sizes in two or more plots, through a sieving surface, which acts as a gauge that allows and does not let the grains pass. 

2. Powder mix

Mixture can be defined as the result of combining two or more ingredients. It can be granular or powdery. For such granular or powder mixtures to be formed a suitable mixing process is required.

There are two types of mixtures,

1. Non-interactive or random,

2.  Interactive or ordered.

1. The first are those of free flow, being mixtures of uniform particle size powders or grains, without intraparticle forces of attraction, thus flowing with little interruption. Consequently, each different particle will have the same probability of being found in any portion of the mixture.

2.  Interactive mixing is formed when large active surface particles exist where other particles are attracted. They form stable clusters and the force between the particles belongs to different chemical classes 

It is difficult to define and evaluate the powder mix; but certain quantitative measurements in solids can help estimate mixer performance. The proof of the mixer in practice comes from the properties it provides to the final blend produced by it

where

σ∞ = the standard deviation of a ‘perfectly mixed’ sample,

σo = the stan-dard deviation of a sample at the start of mixing

σm = the standard deviation of a sample.

V = the average fractional volume or mass of a component in the mixture .

Note:- Due to the complexity of the properties of food systems, which may vary during the mixing process, it is extremely difficult to generalize or standardize the mixing operation for various new or traditional applications.

 

Why and which particle properties are important to measure?

 Better control of quality of product (cement, urea, cosmetics etc)

Better understanding of products, ingredients.

Designing of equipment for different operations such as crushing, grinding, conveying, separation, storage etc.

 In addition to chemical composition, the behavior of particulate materials is often dominated by the physical properties of the constituent particles.

 These can influence a wide range of material properties including, for example, reaction and dissolution rates, how easily ingredients flow and mix, or compressibility and abrasivity.

 From a manufacturing and development perspective, some of the most important physical properties to measure are: 

·      Particle size

·       Particle shape

·       Surface properties

·       Mechanical properties

·       Charge properties

·       Microstructure

 

What are Polymers?

Polymers are often used in industry and daily life for purposes such as powder coating, plastic packing, materials for toys, etc. They have been regarded as an important alternative to materials such as steel or wood. New applications of polymers have also been developed, for example, the recycling of single or commingled postconsumer or preconsumer polymeric waste such as used tyres or plastic bottles.

Polymer powders are used in industrial applications in millions of tons today. Several technologies are established to serve the needs and produce appropriate powders for markets like automotive, electronic, public transport, medicine, textile and cosmetic, just to name the most important ones.

 

 Technology used in Polymer Industry

1. Emulsion Drying

Liquid emulsions have distinct advantages over the other oral dosage forms by improving the bioavailability and by reducing the side effects, but the number of emulsion formulations currently in use are few compared with other oral dosage forms thanks to lack of physical-chemical and compliance problems. To overcome these problems dry emulsions are prepared. Dry emulsions are attractive because they're physically and microbiologically stable solid formulations. Dry emulsions are prepared by drying liquid o/emulsions containing a solid carrier in the aqueous phase. The solid carrier provides the dry emulsions with bulk and mass.

Drying may be a difficulty within the preparation of redispersible polymer powder. Not all emulsions are often converted into dispersible polymer powder because these thermoplastic polymer emulsions, film-forming or maybe tacky at temperature, must be converted to free-flowing powders at high temperatures. The diameter of emulsion particle in the emulsion dispersion is about several μm (10-100). The particle size distribution of redispersed liquid emulsion particles is one of the main quality indexes of redispersible polymer powder during dispersion.

                                                                   Fig. Drying Process

In the production process of redispersible polymer powders, polymer powders are polymer “solid” particles transformed from monomer emulsified droplets. Strictly speaking, these particles are not solid indeed, since the polymers considered here are thermoplastic, which will become solids only below a certain critical temperature, referred to as the glass transition temperature (Tg). Only above this temperature will the thermoplastic body lose all of its crystalline properties. However, since polymers are intertwined like nets, this type of fabric is actually still during a quasi-solid state.

The most commonly used drying method of preparing the redispersible polymer powder is spray drying. Reduced pressure drying and freeze drying methods also can be used.

1. Spray Drying: Spray drying is a method of producing a dry powder from a liquid or slurry by rapidly drying with a hot gas. This is the well-liked method of drying of the many thermally-sensitive materials like foods and pharmaceuticals. A consistent particle size distribution may be a reason for spray drying some industrial products like catalysts. Air is the heated drying medium; however, if the liquid is a flammable solvent such as ethanol or the product is oxygen-sensitive then nitrogen is used.

2. Lyophilization or freeze-drying: Lyophilization, or freeze drying, is a process that removes water from a liquid drug creating a solid powder, or cake. The lyophilized product is stable for extended periods of time and could allow storage at higher temperatures. In protein formulations, stabilizers are added to exchange the water and preserve the structure of the molecule.

3. Rotary evaporation: A rotary evaporator (rotovap) may be a device utilized in chemical laboratories for the efficient and delicate removal of solvents from samples by evaporation. When referenced within the chemistry research literature, description of the utilization of this system and equipment may include the phrase "rotary evaporator", though use is usually rather signaled by other language (e.g., "the sample was evaporated under reduced pressure").

Advantages of dry emulsion:-
1. They improve the bio availability of drug substances.
2. Reduced Side effects.
3. Dry emulsions are attractive because they're physically and microbiologically stable formulations.
4. They represent a potential oral drug delivery system for lipophilic and low soluble drug substances.
5. Used for drug substances needing protection against light or oxidation.
6. Dry emulsions provide most constant possible effective blood levels over prolonged durations of therapy.

Disadvantages of dry emulsion:-
1. Preparation needs to be shaken well before use.
2. A measuring device is needed for administration.
3. A degree of technical accuracy is needed to measure a dose.
4. Storage conditions may affect stability.

2. Selective Laser Sintering (SLS) polymer powder technology

Selective Laser Sintering (SLS) is close to be accepted as a production technique ((Additive Manufacturing)
Selective laser sintering (SLS) is an additive manufacturing (AM) technique that uses a laser as the power source to sinter powdered material (typically nylon or polyamide), aiming the laser automatically at points in space defined by a 3D model, binding the fabric together to make a solid structure. It is similar to selective laser melting; the two are instantiations of the same concept but differ in technical details. SLS (as well as the other mentioned AM techniques) is a relatively new technology that so far has mainly been used for rapid prototyping and for low-volume production of component parts. Production roles are expanding because the commercialization of AM technology improves.
In order to understand the most influencing parameter on material characteristics an improved consideration of the most important parameters on SLS powder is necessary. Powder and particle as well as optical, thermal and rheological behavior. It are often understood from figure that it's a posh system of interrelated assets. The different properties are often divided into intrinsic (thermal, optical and rheology) and extrinsic ones (particle and powder). Intrinsic properties are typically given form the molecular structure of the polymer itself and can’t be influenced easily, whereas processing in advance controls extrinsic properties.

Fig. Interconnection of different polymer properties to be organized for providing promising SLS materials;

 

An additive manufacturing layer technology, SLS involves the utilization of a high power laser (for example, a CO2 laser) to fuse small particles of plastic, metal, ceramic, or glass powders into a mass that has a desired three-dimensional shape. The laser selectively fuses powdered material by scanning cross-sections generated from a 3-D digital description of the part (for example from a CAD file or scan data) on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the part is completed.

Because finished part density depends on peak laser power, rather than laser duration, a SLS machine typically uses a pulsed laser. The SLS machine preheats the majority powder material within the powder bed somewhat below its freezing point , to form it easier for the laser to boost the temperature of the selected regions the remainder of the thanks to the freezing point .

 

Application of SLS technology:-

1. SLS is also increasingly being used in limited-run manufacturing to produce end-use parts for aerospace, military, medical, and electronics hardware. On a shop floor, SLS can be used for rapid manufacturing of tooling, jigs, and fixtures. Because the method requires the utilization of a laser and other expensive, bulky equipment, it's not fitted to personal or residential use; however, it's found applications in art.

2. SLS technology is in wide use at many industries around the world due to its ability to easily make complex geometries with little to no added manufacturing effort. Its commonest application is in prototype parts early within the design cycle like for investment casting patterns, automotive hardware, and structure models

Advantages of SLS:-
Any material which will be converted into powder having low melting temperature (350 to 500) are often wont to make parts during this process.
1. No support structure required.
2. Parts obtained are tough.
3. No post curing required.
4. No tooling cost incurred.
5. No wastage of material.
6. Functional metal and ceramic parts can be obtained directly.

Disadvantages of SLS technology:-
1. Surface finish is improper.
2. Parts are porous in nature.
3. Continuous monitoring of the building operation is required.
4. Large amount of time is required to heat up material chamber before building part.
5. Uniform cooling is difficult to maintain in order to avoid warpage.
6. Parts obtained are brittle.

3. Cryogenic grinding system

In order to produce fine polymer powders, a special and unconventional cryogenic grinding system was established using liquid nitrogen, where a jet-vortex mill was used as the grinding mill. The major feature of this grinding process is that heat generation during the grinding period was eliminated. The results suggest that this cryogenic grinding system could also be suitable for studying the grinding properties of polymeric materials.

It may also be helpful in understanding mechanochemistry, e.g., the t-P-T conditions for different mechanochemical processes under cryogenic conditions where Tis the temperature, and P the pressure of the gas mixture in the grinding chamber.

A major problem for polymer application is the fragmentation of particles. Extensive investigation has been performed in the past to understand the evolution of particle size during grinding, particularly in the modeling of size reduction of minerals. However, the performance of a grinding process depends heavily on the materials to be ground, and it's known that pharmaceuticals, food, and polymers behave differently from minerals.

 

                                         Fig. cryogenic grinding system

 

Working:-

Material to be grounded is cleaned manually and fed into the hopper. From the exit of the hopper the material enters into the vibratory feeder, which is positioned with a small inclination towards the entry of the helical screw conveyor, it has ability to control the feed rate. Liquid nitrogen from the storage container is sprayed into the screw conveyor; the time of stay of material in convey or can be maintained by varying the speed of the drive i.e. conveyor drive. When the mill is running, the fabric gets crushed between the studs and cones out through an optional sieve as a ground product.  When the mill is running, the material gets crushed between the studs and cones out through an optional sieve as a ground product. To the bottom of the mills a collecting bin is housed where the ground product gets collected. The vaporized nitrogen from the mill is sucked by a centrifugal blower and thru the filler assembly in fed back mill, and therefore the cyclic process is sustained.

 

Application of Cryogenic Grinding

-Cryogrinding of steel

a. The large amount of heat generated during machining/grinding at high speed and feed rate raises the temperature at cutting zones excessively.

b. To overcome this problem liquid nitrogen is fed to the grinding spot.

 

-Thermoplastics and Thermosets

a.  To which nylon, PVC, polyethylene, synthetic rubber are commonly used in powder form , but not limited to ,a variety of applications such as adhesives, powdered coatings, fillers and plastic sintering and molding.

 

 -Adhesives and Waxes

a. To avoid pliable and sticky of certain materials which is unable in conventional grinding

 -Explosives

a. To grind the explosive materials (TNT) below their ignition temperature

Advantages of Cryogenic Grinding

1.  Increased productivity through optimized particle-size and increased throughput. -Elimination of caking product within the mill.

2.  Decreased wear on grinding equipment.

3. Separation of composite materials within the mill.

4. Higher production rate.

5. Lower energy consumption.

 6. Fine particle size.

 7. More uniform particle distribution

 8. Lower grinding cost.

Conclusion:

·       Particle technology is associated with the handling and processing of particles and it has different stages of development.

·       Particle properties are important to measure as it gives better understanding of products, ingredients and some of the most important physical properties.

·       Polymers are often used in industry and daily life for purposes such as powder coating, plastic packing, materials for toys, etc.

·       Technology used in polymer industry are discussed in like emulsion drying, selective laser sintering and cryogenic grinding.

References

Sekiguchi, M. (1997): “Adhesion  and mechanical strength of a powder bed”, Powder Technology Hand-book, 2nd  ed., eds: Gotoh, Masuda, Higashitani, New York:Marcel Dekker.

Spomenka Simovic. Assembling nanoparticle coatings to improve the drug delivery performance of lipid based colloids. Nanoscale, 2012, 4, 1220-1230.

Deckard, C., "Method and apparatus for producing parts by selective sintering", U.S. Patent 4,863,538, filed October 17, 1986, published September 5, 1989.

Rohit  Saxena, Cryogenic  Grinding: A  Physical  Technique  To Retain Volatile  Content In  Natural  Products, Department  of Humanities and Science.