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    Date:2019/7/29

    Powder is a topic that can not be evaded in the course of drug development. It is also the key intermediate material in the production process of tablets. The success of preparation is closely related to the properties of powder. This paper summarizes the key properties of powder in order to better understand their properties and use their properties in the process of research, development and production.

    Generally speaking, the most direct reaction in the powder mind should be a mixture of granular substances, but the powder is not as simple as it looks from the material composition. What is the powder? The powder is a complex system produced by the three phase interaction of solid, liquid and gas. Of course, it is impossible to use a parameter or equation to characterize its properties in complex systems.Therefore, various parameters, such as particle size distribution (PSD), density, shape, crystal form, moisture, surface morphology, surface area, electrostatic force, capillary force, hydrogen bond, porosity, elasticity, brittleness, moisture absorption and degassing, have been produced.And so on.


                                                                                                            


    Based on the understanding of the three phase interaction between powder, solid,liquid and gas,the apparent properties of the powder can be divided into six main categories:

    One,mobility,characterization of powder flow capacity.

    Two, formability, characterization of the combined forming properties of powders under external forces.

    Three,redispersibility,characterization of the binding strength between powders.

    Four,wettability,indicating thesensitive nature of powder to environmental humidity.

    Five,permeability,characterization of gas through the powder's difficult nature.

    Six,adhesion and adhesion,to characterize the interaction between powders and powders and contact surfaces.

    The properties of each of these powders can also be expressed by a variety of parameters.

    Mobility

    Liquidity is a powder characteristic that developers of solid agents often face. This characteristic also directly affects the feasibility of the process. However, how to better describe the flow characteristics of the powder can be referred to the following parameters.

    The parameter 1: Angles of repose is usually the collapse angle. It is the most commonly used model parameter of powder flow in China. It is calculated by calculating the plane angle of the conical junction formed by the vertical movement of the powder due to gravity action. In general, it is considered that the angle is less than 40 degrees, which can meet the requirement of powder flow in the process of industrial production.  

                                                                                          

    The parameter 2: Carle index (Carr Index) is a more commonly used parameter in foreign countries. It is characterized by its compressibility.

    Carr Index=(DT-DB)/DT*100%


    The parameter 3: Hallson Nabi (Hausner ratio) is a more commonly used parameter in foreign countries, which is used to characterize its fluidity through the compressibility of powder.


    Hausner ratio=DT/DB

    Note: the density of dt: powder and the bulk density of DB powder.


                                                                                         

    The parameter 4: Basic flow energy is a parameter to determine the flow properties of the powder by calculating the powder flow resistance. The smaller the flow energy is, the smaller the flow resistance is, the better the fluidity is. It is the most scientific parameter to characterize the flow property of the powder.

    Friendship hint: compressibility is a parameter that represents the compression ratio of powder under the action of stress, rather than the compression ratio after the stress is removed. Because in a certain compression range, there is elastic deformation in the compression process, and when the stress is removed, elastic recovery will occur. For example, the compression ratio of the different stress applied to the ball is obviously different, and when the stress is removed, the recovery ratio of the ball is at the same compression ratio.Therefore, the degree of compression is a parameter in the state of stress.

    There are many factors that affect the fluidity of powders. We usually explore three phases from the powder, such as physical and chemical properties of solid phase PSD, surface morphology, density, water content in the water phase, and gas degassing direction.

    Compatibility is an important assessment index for process feasibility in the preparation of tablets. It refers to the ability to compress the powder into a certain size physical shape. For tablets, formability is usually expressed by tensile strength, which is the difference of hardness between different powders under different pressure conditions and different hardness under different pressure conditions.


                                                                                  


    The fundamental reason that affects the formability of powder should also be from three angles: solid, liquid and gas. Solid state properties of MICROTEK shape, particle size distribution, shape and density will affect the formability and liquid water content, which will affect the bonding performance of the particle compression interface. The degassing speed and the degree of penetration of the elastic deformation property of gas compression will also significantly affect the formability of the sheet.


    Redispersibility

    This property is more critical in dpi. It refers to the separation process of the mixed powder which was originally combined with the air flow, which is mainly affected by the various forces between the powders. Therefore, the binding form of particles determines the redispersibility of the powders.The strengths and weaknesses of electrostatic force, hydrogen bond, Fan Dehua force and capillary force can be comprehensively analyzed based on the surface morphology, surface area, water content and charge carrying capacity of the powder.


    Moisture absorption

    Wettability is also one of the important characteristics of powder. It is usually ignored in the actual research and development process. However, because the wettability of the mixed powder will be significantly higher than that of a single powder, according to the product principle of critical humidity: rh=rh1*rh2, the product design stage should avoid the severe challenge to the environmental humidity as far as possible.In order to control the feasibility risk of late enlargement, the wetting ability of powders is related to the solid phase properties of powders. Under normal circumstances, amorphous polycrystals are more sensitive to humidity.



                                                                                         

    Gas permeability

    As described above, gas permeability will affect the fluidity and formability of the powder, which is characterized by the resistance of the gas throughout the powder. Generally, the resistance to air flow is small, which means that the aggregation and adhesion behavior between the powders is weak, so the mobility will be better.At the same time, during the compression process, the exhaust performance will be excellent, so the formability can be improved. The permeability of the powder is related to the solid particle surface morphology, PSD, surface area, shape, water content in the liquid phase (high water content, high capillary action, and reduced electrostatic effect).


                                                                                                                           

    Cohesiveness and adhesion

    This property will also affect the technological feasibility of the preparation. The bonding property is the key attribute of whether the powder can be mixed evenly. At the same time, it will also affect the fluidity of the powder. The interparticle adhesion can change the surface properties of the powder and improve the physical properties of the powder. For example, the good adhesion of the flow aid agent can improve the roughness of the particle surface and improve its fluidity.Lubricants improve their flow properties by reducing adhesion between particles and particles and the friction between particles and equipment. The surface properties, shape, particle size distribution and moisture of the powder will affect the adhesion and adhesion of lubricants.



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