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    Home > Coatings News > Paints and Coatings Market > Molecular thermal motion and powder bonding process

    Molecular thermal motion and powder bonding process

    • Last Update: 2022-09-20
    • Source: Internet
    • Author: User
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    Abstract: Molecular thermal motion is closely related
    to the macroscopic properties of polymers.
    The theory of molecular thermal motion has positive significance for bonding process, and by controlling the factors affecting molecular thermal motion, bonding bonding process can be improved to ensure the continuous stability
    of bonding powder quality.


    1.
    Introduction


    Advocating green environmental protection and reducing VOCs emissions are the online coatingol.
    com of the country's current environmental protection policy
    paint.
    Powder coatings have no VOCs emissions at room temperature, and compared with paints, they have the advantages
    of efficient Eficiency, energy-saving Energy Saving, environmentally friendly Environmental Friendly, and excellent performance Excellent finishing.


    Therefore, "paint to powder" has become a hot topic
    in the discussion of environmental protection control in the paint industry.
    On July 3, 2018, the State Council issued the Three-Year Action Plan for Winning the Blue Sky Defense War, which clarified the overall ideas, basic objectives, main tasks and safeguard measures for the prevention and control of air pollution, and promoted the process
    of changing metal paint to metal powder.


    Paint is a liquid paint, adding metallic effect pigments is still easy to disperse and construct; Powder coating is solid, adding metal effect pigment is difficult to disperse uniformly, and the charging properties of the base powder and metal pigment are different, and it is easy to appear "zebra crossing", "yin and yang side", "flower or cloud spots", color difference, gun head powder and other drawbacks
    during construction.


    To avoid these defects, the addition of metallic effect pigments can be solved
    using the bonding process.
    Under different external conditions, the thermal movement of molecules of the same substance is different, and it will show different macroscopic properties
    .
    Through the control of the influencing factors of molecular thermal motion, it has guiding significance
    for the quality assurance of Bonding powder.


    2.
    Molecular thermal motion


    A molecule is the smallest particle
    that keeps the original chemical properties of a substance unchanged.
    The non-stop, irregular motion of a large number of molecules in an object is called molecular thermal motion
    .
    Temperature determines how intense the thermal movement of molecules is
    .


    The typical phenomenon of molecular thermal motion performance is diffusion, and the test for observing molecular thermal motion is Brownian motion
    .
    The phenomenon that different substances can enter each other when they come into contact with each other is called diffusion; The phenomenon in which suspended particles are constantly doing irregular motion is called Brownian motion
    .
    But molecular thermal motion is not Brownian motion
    .


    We can see in our daily lives the phenomenon of diffusion due to molecular motor performance, such as diffusion in ink water Figure 1; In fact, the molecules are very small, and the molecular movement cannot be seen by the naked eye (it is impossible to observe how the screws that cannot be removed by the extruder screw are stuck) can only be observed under the electron microscope, as shown in Figure 2;


    In 1827, the British botanist Robert Brown observed the non-stop irregular movement of pollen with an ordinary microscope in an aqueous solution of pollen particles, indicating that water molecules are moving irregularly, as shown in Figure 3
    .


    2.
    1 Characteristics of thermal motion of polymer molecules


    Thermodynamic polymers have three major characteristics: 1) the multiplicity of molecular motion units; 2) dependence on molecular motion time (relaxation characteristics); 3) Dependence on molecular motion temperature
    .


    Polymer molecular thermal motion units have multiplexes (diversity), which can be side groups, branched chains, links, segments and entire molecular chains
    .


    Under the action of the outer field (thermal, force or electric field), the polymer gradually changes from a equilibrium conformation through molecular thermal motion to a new equilibrium conformation adapted to external conditions.

    This time-dependent process is called the relaxation process, and the time to complete the process is called relaxation time
    .


    The dependence of molecular motion temperature, one is that the temperature rise activates the internal energy of the moving molecules, and the other is that the temperature rise expands the volume of the polymer, which is conducive to the movement
    of the molecules.


    2.
    2 Mechanical state of thermal motion of amorphous polymer molecules


    The thermal motion of amorphous polymer molecules has two transitions and three mechanical states
    .
    Two transformations: 1) glass transition; 2) Viscosity flow transition
    .
    Three mechanical states: 1) vitreous; 2) High elasticity; 3) Viscous flow state
    .


    The same polymer, the structure is unchanged, due to the different thermal motion of molecules, can exhibit different macroscopic properties
    .
    Amorphous polymers do not have a fixed melting point Tm, and when the temperature rises and the thermal motion of the molecule accelerates, there will be a temperature that begins to soften, that is, the softening temperature Ts (softening point: the starting temperature of the viscous flow state).


    When the temperature transition will also show different mechanical states, according to the temperature region can be divided into glass state, high elasticity and viscous flow state three mechanical states
    .


    The transition between high elasticity and glass (glass and high elasticity) is called glass transition, and the corresponding transition temperature is the glass transition temperature Tg; The transition temperature between the high elasticity state and the viscous flow state (viscous flow state and high elastic state) is called the sticky flow temperature Tf
    .
    Figure 4 below:

    Below the glassing temperature (below the A point), the polymer is in a glassy state, showing the nature of plastic, deformation is caused by the slight telescopic movement of the bond angle and the bond length, and the molecular chains and segments do not move, only those smaller moving units such as side groups, branched chains and small chain links can move
    .


    Above the glass transition temperature (above the A point), although the molecular chain cannot be moved, the chain segment begins to move, the deformation is large and reversible, and the high elasticity of
    the rubber is exhibited.


    The temperature rises again, showing the mechanical characteristics of large deformation value and irreversible, the coordinated movement of the chain segments causes the centroid movement of the molecular chain, and the macromolecule is displaced when it is subjected to external forces, and cannot be restored
    .


    Causes the entire molecular chain to move into a viscous flow state
    .
    The glass transition and the viscous flow transition are not phase transitions (the non-crystalline is a liquid phase structure, and the glass state is called supercooled liquid), but only a change
    in the mechanical state.


    3.
    Powder bonding process


    Bonding is a transliteration of bonding in the foreign language, Chinese meaning bonding
    .
    The purpose of the powder bonding process is to bond the powder foundation with the metallic effect pigment or additive added later
    .


    So that the pigment or additive with different density and chargability from the foundation can be evenly diffused in the bottom powder, and will no longer be layered or precipitated to separate, becoming a uniform powder to facilitate the application
    of construction.


    The powder bonding process is a paradoxical and comical process
    .
    In order to bond firmly, the powder must be soft to be adhesion, but the agglomerated powder cannot be used
    after too soft glass transformation.


    Therefore, the bonding process can only be completed
    within the relaxation process of glass transition.
    In this way, the powder not only achieves the bonding effect, but also reversely restores the original flowability
    of the powder.
    Figure 5 below:

    The control bonding process requires a deep understanding
    of the glassing temperature.
    Glassing temperature is defined as the temperature
    at which the polymer molecular chain segment begins to move or freeze.
    Its significance is as follows:

    When bonding heats up, the molecular thermal motion accelerates and the conformation rearranges into a state
    of higher energy.
    However, the relaxation time of conformational rearrangement has not yet adapted to the heating rate
    .


    The bottom powder chain segment does not fully move into a high elastic state, that is, the mechanical state has not changed, but it is adhesion, and the bonding cold cylinder cools down sharply to return the movement of the chain segment back to its original state, only slight clumping, and the vibrating screen powder is loose
    .
    Like when we have a meeting, everyone sits neatly in their seats, and when the order to adjourn the meeting is announced, everyone will enter the state of leaving the venue, but it will take time
    to walk from the seat to the door of the conference room.


    Suddenly, the change of circumstances announced that the meeting would not be adjourned for the time being, and everyone had not yet walked out of the door, and after listening to the order, they returned to their original seats (the time for the dispersal to leave the meeting had not yet adapted to the speed of the dispersal, and the suspension of the meeting returned to their seats).


    Bonding bonding is similar to ultrasonic welding, the welding body maintains the original mechanical state, which is an example
    of the body without glass transition.
    Figure 6:

    The mechanical state of the bottom powder production process has changed, which is an example
    of the glass transformation of the main body.
    Figure 7 below:

    Powder coating (plastic powder) as an amorphous polymer, its mixed → extrusion tableting → ACM grinding production process is the molecular thermokinetic mechanical state from glassy → high elasticity → glass state change process
    .


    This production process includes extrusion heating, intensification of molecular thermal motion, vitrification transition of chain segment movement, and pressure cooling of roller rolls, weakening of molecular thermal motion, and "freezing" glass transition
    of chain segments.
    Extruded tablets undergo two vitrification transitions
    .


    Ice crystals have a fixed melting point and can coexist with an ice-water mixture at a critical melting point temperature
    .
    Amorphous polymers do not have a fixed melting point, so there is no melting point critical temperature
    .


    There are only two mechanical state transitions, this article only discusses the bonding of glassy transition critical state, the vitreous state of high elasticity coexist, for the vitrification after the transition of bonding (such as: roller roll pressure plus metal pigment bonding, bonding most of the agglomeration and then grinding) This article does not discuss
    .


    For the phenomenon of glass transition, there is still no perfect theory that can make a correct explanation
    that is completely in line with the experimental facts.
    There are many theories that have been proposed, and there are three main ones: free volume theory, thermodynamic theory and dynamic theory
    .
    Each theory can only explain some of the experimental phenomena
    in the vitrification transition.


    The influencing factors of glass transition temperature are structural factors (internal factors) and external conditions (external factors
    ).
    Factors such as main chain flexibility, intermolecular forces, relative molecular mass, copolymerization, crosslinking, plasticization, external force size action time, and temperature ramp rate will all affect Tg
    .
    Figure 8 below:



    4.
    Quality control of Bonding powder


    4.
    1 Foundation formulation design


    The bottom glass transition temperature can be controlled
    by selecting a resin (intrinsic factor).
    Figure 8 shows that the flexibility of the main chain and the average relative molecular weight have an effect on
    Tg.


    Resin molecular chain flexibility ↓, segment length ↓, Tg↓, the stronger the rigidity of the resin molecular chain, the longer the chain segment, the higher the Tg; When the average relative molecular weight of the resin increases, Tg increases, but when the average relative molecular weight increases to a certain extent, the glass transition temperature tends to a fixed value
    .


    The variety or amount of metallic effect pigments is different, and the bottom powder formulations
    of different Tg can be designed accordingly.
    Secondly, the primer of the flash silver bond powder can be designed to gel fast - high Tg - cover the base powder, and the gelatinization slow - low Tg - transparent bottom powder mixed
    .


    Bond timing, temperature dependence of molecular thermal motion, low Tg transparent powder molecular chain segments move first, bonding flash silver more, high Tg cover the bottom powder molecular chain segment after the movement bond less
    .


    When curing, the fast gelatinizing masking powder gels first, and the slow gelatinizing transparent primer also melts and flows and floats on the gel covering base, and there will be a special interspersed flash effect
    after curing.


    In addition, spherical axis effect additives such as alumina C/white carbon black/wax powder can also be added to the bonding time, which instantly reduces the friction external force of the applied base powder, coordinates the constant temperature, and helps bonding
    .


    4.
    2 Bonding Equipment


    The most primitive bonding machine was a cylinder block, which followed the characteristics of molecular thermal motion temperature/time dependence for rapid cooling, and later bonding machines were designed with two cylinder blocks
    .


    Different effects of heating methods: steam, water or oil medium cylinder wall conduction heating, to a transfer process, often the cylinder wall overheating sticky powder, but the paddle wing slow turn and not easy to destroy the effect pigment; High-speed friction heating is more uniform, but the high-speed rotation of the paddle wing is easy to destroy the effect pigment
    .


    The combination of various heating methods such as medium, friction and radiation waves can make each molecule do thermal motion at the same time, and the temperature uniformity is better
    .


    Zepplin is the representative of Bond equipment, and the glass transition can be controlled
    online.
    The thermal motion of the base powder is monitored not only by temperature, but also by changes in current caused by torque
    .
    Fully intelligent PLC interface
    .
    Figure 9 below:

    The multi-layer and multi-angle design of the paddle wing of Jishenglong Company, the powder friction trajectory rolling motion, to ensure that the bottom powder is heated evenly
    .
    Figure 10 below:

    The post-pigment of Dongshi Company adopts the auxiliary heating method design
    .
    When the amount of pigment is added in large amounts, it avoids a sharp drop in the temperature of the bottom powder caused by a large amount of heat absorption of the pigment itself, which has a helping effect on bonding
    .
    Figure 11 below:


    4.
    3 Bonding process settings


    The settings of the bonding process include the control of the setting of the bonding temperature, the bonding time and the heating rate
    .


    Temperature has two effects on the thermal motion of
    molecules.
    One effect is to activate the motor unit
    .
    The increase in temperature increases the energy of the thermal motion of the molecules, and when the energy increases enough to overcome the barrier required for the motion unit to move in a certain way, the motion unit is in an activated state, thus starting a certain way of thermal motion
    .


    Another effect is that the temperature increase causes the polymer to expand in volume, which increases the free space of the molecule, and when the free space reaches the necessary size for the movement of a certain unit of motion, the unit of motion can move
    rapidly.


    As the temperature rises, the result of both actions speeds up the relaxation process, or shortens the relaxation time
    .
    If the temperature of the polymer system increases, the relaxation time of the motion unit is shortened
    .


    Bonding temperature setting: In the early stage of bond powder development, the control accuracy of the equipment was poor, and the bottom powder was not specially designed
    .
    Bonding often knots
    .


    Therefore, the bonding temperature is generally set below Tg (below the "A point" in Figure 4), so that the problem of the cylinder is solved, but from the perspective of the thermal motion of the molecule, the macromolecular chain segment does not move, and the bonding effect is definitely poor;


    Now, the accuracy and automation of bonding equipment have been improved, the bottom powder is also specially designed, the bonding temperature is generally set above Tg (between the "A point and B point of Figure 4"), the recommended setting of indoor powder of Shenglong equipment is 60 ° C ~ 70 ° C, the outdoor powder is recommended to be set 65 ° C ~ 75 ° C, and the bottom powder and the effect of the pigment are smoothly bonded under the link segment movement of the pigment molecules, and the effect of bonding is better
    .


    Bonded time setting: Is based on
    the time it takes to meet the bonding of the base powder and metallic pigments.
    At the same Tg, the temperature of the bottom powder ↑, relaxation time ↓
    .
    The bonding time corresponding to the substrate and equipment characteristics can be set to prevent agglomeration and ensure the bonding quality
    .


    Figure 8 indicates that the ramp rate has an effect on
    Tg.
    When measuring Tg in DSC, it can be seen that the faster the temperature rise, the higher the
    Tg.
    The general temperature rate is changed by 10 times, and the difference between Tg and Tg is 3 to 5 °C
    .
    Therefore, after the highly intelligent bonding machine accurately detects the temperature change of the bottom powder, the heating rate is automatically controlled through PLC programming to improve the bonding effect
    .


    5.
    Conclusion


    If the critical temperature range for glass conversion in Figure 4 of the foundation design is very narrow, it is as difficult
    for the subsequent bonding work as it is to train a colorblind person to be a colorist.


    If the bonding equipment cannot control the temperature uniformity of the bottom powder in the cylinder, no matter what kind of foundation the bonding is a pain point
    .
    In addition, the equipment relies on manual operation, and it is difficult to adapt to the pace
    of molecular thermal movement without PLC intelligent control.
    Highly intelligent equipment can make good products
    .


    The setting of the bonding process is determined by referring to the characteristics of the powder and equipment (data acquisition and control accuracy, degree of automation, etc.
    ), which is the logical link between the powder and the equipment, and builds a bridge
    for the bonding powder.


    The theory of molecular thermal motion runs through the entire bonding process, supports the continuous improvement of the bonding process, and plays a pivotal role
    in the continuous stability of the quality of the bonding process.
    Don't forget that the molecular thermal movement is intense enough that the cylinder bottom powder and metal pigment dust cloud are in danger of explosion and must be protected
    by an inert atmosphere such as nitrogen.


    Conclusion:


    The design of the bottom powder formula is the premise of doing a good job of bonding powder, bonding equipment is the basis for doing a good job of bonding powder, bonding process is the bridge to do a good job of bonding powder, molecular thermal motion theory is the key to doing a good job of bonding powder, and plays a huge role
    in the continuous stability of quality.


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