1 / 33

WACKER HDK The Fumed Silica

Performance of Wacker HDK?. introduces thixotropic behaviour reduces pigment settling and movement prevents movement of the coating into substrate pores control of picture framing allows flow, levelling and deaeration during stucture build-up provides sag resistance of applied paint a

ferdinand
Download Presentation

WACKER HDK The Fumed Silica

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


    1. Wacker Chemie has been producing fumed silica since 1968 and has become highly proficient in this field. The brand name HDK® is an abbreviation of the German name for sumed silica: „Hochdisperse Kieselsäure“. Wacker HDK® is an inorganic rheology control agent, which introduces thixotropic behaviour. Wacker HDK® converts liquids into solids and solids temporarily into liquids, just as the occasion demands. Wacker HDK® is chemically ultra-pure. There are no metal ions to cause discolouration, nor do any catalytic side-reactions occur. Wacker HDK® is a white non-staining powder, the extreme fineness of which renders it ideal for making even transparent materials. The efficiency of Wacker HDK® is not restricted by narrow temperature tolerances. Since Wacker HDK® has an extremely low water content, it is also suitable for systems sensitive to moisture.Wacker Chemie has been producing fumed silica since 1968 and has become highly proficient in this field. The brand name HDK® is an abbreviation of the German name for sumed silica: „Hochdisperse Kieselsäure“. Wacker HDK® is an inorganic rheology control agent, which introduces thixotropic behaviour. Wacker HDK® converts liquids into solids and solids temporarily into liquids, just as the occasion demands. Wacker HDK® is chemically ultra-pure. There are no metal ions to cause discolouration, nor do any catalytic side-reactions occur. Wacker HDK® is a white non-staining powder, the extreme fineness of which renders it ideal for making even transparent materials. The efficiency of Wacker HDK® is not restricted by narrow temperature tolerances. Since Wacker HDK® has an extremely low water content, it is also suitable for systems sensitive to moisture.

    2. WACKER HDK® The Fumed Silica HDK® is the abbreviation of the German name for fumed silica. HDK® is a registered trademark of Wacker. Wacker HDK® is used to cover three major applications: - rheologicy control of liquids - optimiziation of the free flow properties of powders - anti settling of pigments and fillersHDK® is the abbreviation of the German name for fumed silica. HDK® is a registered trademark of Wacker. Wacker HDK® is used to cover three major applications: - rheologicy control of liquids - optimiziation of the free flow properties of powders - anti settling of pigments and fillers

    3. Performance of Wacker HDK® The most important properties of the Wacker HDK® from the point of view of technical applications are their ability to act as thixotropic and thickening agents. The three dimensional network however brings about some further properties as well. Pigments and fillers are trapped in the network and their settling down is decelerated or prevented. The thickened coating film cannot move so easily into the pores of porous substrates. An effect refered to as hold out. In addition picture framing or fIat edges can be avoided by thickening up a coating. Picture framing is a defect caused by the movement of the coating at the edges as a consequence of an uneven surface tension during drying. The silica structure is able to sufficiently rebuild before material can flow to the edges. As the network buid-up requires a little time after the shear force stops, the viscosity remains low, long enough to allow flow, levelling and deaeration of the coating material. During this time sagging of the coating material is possible. When the viscosity has increased sagging is prevented, however flow and deaeration is stopped. An optimal performance requires a formation of a suitably tight network quick enough to prevent sagging and slow enough to allow optimum film formation. Wacker HDK® can as well be used to improve the free flow properties of powders. The tiny Wacker HDK® particles envelope the powder particles and act as ball bearings between them. Thus powders flow as easily as liquids.The most important properties of the Wacker HDK® from the point of view of technical applications are their ability to act as thixotropic and thickening agents. The three dimensional network however brings about some further properties as well. Pigments and fillers are trapped in the network and their settling down is decelerated or prevented. The thickened coating film cannot move so easily into the pores of porous substrates. An effect refered to as hold out. In addition picture framing or fIat edges can be avoided by thickening up a coating. Picture framing is a defect caused by the movement of the coating at the edges as a consequence of an uneven surface tension during drying. The silica structure is able to sufficiently rebuild before material can flow to the edges. As the network buid-up requires a little time after the shear force stops, the viscosity remains low, long enough to allow flow, levelling and deaeration of the coating material. During this time sagging of the coating material is possible. When the viscosity has increased sagging is prevented, however flow and deaeration is stopped. An optimal performance requires a formation of a suitably tight network quick enough to prevent sagging and slow enough to allow optimum film formation. Wacker HDK® can as well be used to improve the free flow properties of powders. The tiny Wacker HDK® particles envelope the powder particles and act as ball bearings between them. Thus powders flow as easily as liquids.

    4. Wacker HDK® Product Range The Wacker HDK® product range consists of hydrophilic and hydrophobic grades. Hydrophilic Wacker HDK® grades are wettable by water, hydrophobic are not. The differences are reflected by the inscriptions on the bags, in which they are supplied, where an orange inscription denotes the hydrophilic HDK-grades a blue one the hydrophobic grades. The numbers contained in the names in most cases mean the approximate average size of the surface area in square meters per gramm devided by ten. Wacker HDK is supplied in both compressed and uncompressed form. Compressed Wacker HDK grades are denoted by the additional letter P. The Wacker HDK® product range consists of hydrophilic and hydrophobic grades. Hydrophilic Wacker HDK® grades are wettable by water, hydrophobic are not. The differences are reflected by the inscriptions on the bags, in which they are supplied, where an orange inscription denotes the hydrophilic HDK-grades a blue one the hydrophobic grades. The numbers contained in the names in most cases mean the approximate average size of the surface area in square meters per gramm devided by ten. Wacker HDK is supplied in both compressed and uncompressed form. Compressed Wacker HDK grades are denoted by the additional letter P.

    5. Production of Wacker HDK® The manufacturing process is environmentally friendly and takes place in an integrated production that conserves both resources and energy. First volatile chlorosilane is introduced into a hydrogen flame. Pyrolysis is then effected at about 1000-1200°C in the presence of oxygene to yield a mixture of silicondioxide and hydrogen chloride, the latter being used to convert methanol into methylchloride for the Müller-Rochow Production of methyl chloro silanes. Due to the homogenous gas mixture and the short dwell time in the flame uniform spheric primary particles of Wacker HDK® are produced with a narrow particle size distribution their diameters ranging from 5 to 30 nm. Upon cooling down to about 800°C these particles coalesce to much larger units with branched structures, known as aggregates, the particle size of which ranges from 0.1 - 0.25 µm. On cooling down to room temperature the aggregates flocculate to form agglomerates refered to as tertiary structures as well. They have a size of about 10 µm. The short dwell time in the flame prevents crystalline structures to form thus making Wacker HDK® an X-ray amorphous product.X-ray amorphous silicondioxide, according to present knowledge, does not cause silicosis. It is important to note, that the pyrolytic process always yields hydrophilic fumed silica. To obtain hydrophobic products, the surface of the hydrophilic products has to be treated in an own step. This is the reason, why hydrophobic grades are more expensive than the hydrophilic grades, from which they are made. The manufacturing process is environmentally friendly and takes place in an integrated production that conserves both resources and energy. First volatile chlorosilane is introduced into a hydrogen flame. Pyrolysis is then effected at about 1000-1200°C in the presence of oxygene to yield a mixture of silicondioxide and hydrogen chloride, the latter being used to convert methanol into methylchloride for the Müller-Rochow Production of methyl chloro silanes. Due to the homogenous gas mixture and the short dwell time in the flame uniform spheric primary particles of Wacker HDK® are produced with a narrow particle size distribution their diameters ranging from 5 to 30 nm. Upon cooling down to about 800°C these particles coalesce to much larger units with branched structures, known as aggregates, the particle size of which ranges from 0.1 - 0.25 µm. On cooling down to room temperature the aggregates flocculate to form agglomerates refered to as tertiary structures as well. They have a size of about 10 µm. The short dwell time in the flame prevents crystalline structures to form thus making Wacker HDK® an X-ray amorphous product.X-ray amorphous silicondioxide, according to present knowledge, does not cause silicosis. It is important to note, that the pyrolytic process always yields hydrophilic fumed silica. To obtain hydrophobic products, the surface of the hydrophilic products has to be treated in an own step. This is the reason, why hydrophobic grades are more expensive than the hydrophilic grades, from which they are made.

    6. Fumed Silica Agglomerate

    7. Hydrophilic and hydrophobic WACKER HDK® We have learned, that Wacker HDK® exists as hydrophilic and as hydrophobic grades. Now what´s the difference between them ? In general a fumed silica aggregate is composed of the spheric primary particles. Like quartz the primary particles are composed of SiO4 tetrahedra. In contrast to quartz however they are not crystalline but amorphous. The SiO4 tetrahedra are interconnected via siloxane (Si-O-Si) linkages. The Si-atoms on the surface of the Wacker HDK®- primary particles either form siloxane linkages or they bear hydroxy groups. The silanol groups (Si-OH) are of paramount importance to the properties of Wacker HDK® and determine the hydrophilic nature of the silica. They constitute the actual reactive centre, being able to form hydrogen bonds to themselves and other substances. The same type of bonding occurs in hydrophobic Wacker HDK®, which contains less silanol groups than the hydrophilic Wacker HDK®. So the difference between the two types of Wacker HDK® in general is to be seen in the number and availability of the silanol groups. Usually there are around 3 hydroxyfunctions per nm² in hydrophilic Wacker HDK® and approximately 1.5 hydroxyfunctions per nm² in hydrophobic Wacker HDK®. These figures are average values. We have learned, that Wacker HDK® exists as hydrophilic and as hydrophobic grades. Now what´s the difference between them ? In general a fumed silica aggregate is composed of the spheric primary particles. Like quartz the primary particles are composed of SiO4 tetrahedra. In contrast to quartz however they are not crystalline but amorphous. The SiO4 tetrahedra are interconnected via siloxane (Si-O-Si) linkages. The Si-atoms on the surface of the Wacker HDK®- primary particles either form siloxane linkages or they bear hydroxy groups. The silanol groups (Si-OH) are of paramount importance to the properties of Wacker HDK® and determine the hydrophilic nature of the silica. They constitute the actual reactive centre, being able to form hydrogen bonds to themselves and other substances. The same type of bonding occurs in hydrophobic Wacker HDK®, which contains less silanol groups than the hydrophilic Wacker HDK®. So the difference between the two types of Wacker HDK® in general is to be seen in the number and availability of the silanol groups. Usually there are around 3 hydroxyfunctions per nm² in hydrophilic Wacker HDK® and approximately 1.5 hydroxyfunctions per nm² in hydrophobic Wacker HDK®. These figures are average values.

    8. Network Formation by Hydrogen Bridging When Wacker HDK® is added to a liquid, the silanol groups (Si-OH) on adjacent Wacker HDK®-aggregates start to form hydrogen bonds to each other. A three dimensional network of the rigid Wacker HDK®- particles develops and the mobility of the surrounding liquid is restricted. The tighter the network of the Wacker HDK®-particles, i.e. the more Wacker HDK® is added, the greater is the resultant viscosity. Maximum viscosity is attained when the liquid is at rest. When Wacker HDK® is added to a liquid, the silanol groups (Si-OH) on adjacent Wacker HDK®-aggregates start to form hydrogen bonds to each other. A three dimensional network of the rigid Wacker HDK®- particles develops and the mobility of the surrounding liquid is restricted. The tighter the network of the Wacker HDK®-particles, i.e. the more Wacker HDK® is added, the greater is the resultant viscosity. Maximum viscosity is attained when the liquid is at rest.

    9. Reversible Network Formation of Wacker HDK® As long as the coating material is not cured, the network formation is reversible. Fumed silica is supplied to the customer in form of agglomerates. These agglomerates do not have the ideal shape for thickening the respective liquid. They are introduced into the liquid with shearing. The shear force breaks down the agglomerates into aggregates. This dispersion step requires sufficiently high shear forces. The aggregates are able to link reversibly together via some kinds of intercations, where hydrogen bridges form the strongest interparticle links. A three dimensional network of the rigid silica particles is formed and the liquid is trapped in the resulting structure. This results in increased viscosity and yield value. The interaggregate links can be broken by introducing shear forces again. Thus the aggregates are obtained back and the viscosity drops. The interactions between the particles will, however, quickly reform when the shear force is removed. Thus network formation and breakdown is reversible. As long as the coating material is not cured, the network formation is reversible. Fumed silica is supplied to the customer in form of agglomerates. These agglomerates do not have the ideal shape for thickening the respective liquid. They are introduced into the liquid with shearing. The shear force breaks down the agglomerates into aggregates. This dispersion step requires sufficiently high shear forces. The aggregates are able to link reversibly together via some kinds of intercations, where hydrogen bridges form the strongest interparticle links. A three dimensional network of the rigid silica particles is formed and the liquid is trapped in the resulting structure. This results in increased viscosity and yield value. The interaggregate links can be broken by introducing shear forces again. Thus the aggregates are obtained back and the viscosity drops. The interactions between the particles will, however, quickly reform when the shear force is removed. Thus network formation and breakdown is reversible.

    10. The Wacker HDK® decision trees are a recommendation guideline. They contain the most suitable and most general applicable recommendations to achieve the best rheolgical results as to the best of our present knowledge. For any common resin type a selection of Wacker HDK® -grades is given. N20, H20 and H15 are the most versatile products. Wacker HDK® H18 is especially suitable for highly polar systems. Wacker HDK® T 30 and T 40 confer a high degree of transparency in clear coats during manufacture. A high-speed dispersing device such as a high speed dissolver or a mill is advisable in this case. The Wacker HDK® decision trees are a recommendation guideline. They contain the most suitable and most general applicable recommendations to achieve the best rheolgical results as to the best of our present knowledge. For any common resin type a selection of Wacker HDK® -grades is given. N20, H20 and H15 are the most versatile products. Wacker HDK® H18 is especially suitable for highly polar systems. Wacker HDK® T 30 and T 40 confer a high degree of transparency in clear coats during manufacture. A high-speed dispersing device such as a high speed dissolver or a mill is advisable in this case.

    11. As anti foaming aids the hydrophobic grades H15 and H2000 are employed. Hydrophopic Wacker HDK® -grades such as H15 and H20 prevent zinc dust paints from entering into undesirable reactions, e.g. gassing. As anti-settling agent (particularly with heavy pigments) hydrophobic Wacker HDK® -grades are employed. They allow the viscosity to be kept lower than with hydrophilic grades. For improving the free flow of powders the post addition of 0.1 - 0.2% of H15 or H30 is recommended.As anti foaming aids the hydrophobic grades H15 and H2000 are employed. Hydrophopic Wacker HDK® -grades such as H15 and H20 prevent zinc dust paints from entering into undesirable reactions, e.g. gassing. As anti-settling agent (particularly with heavy pigments) hydrophobic Wacker HDK® -grades are employed. They allow the viscosity to be kept lower than with hydrophilic grades. For improving the free flow of powders the post addition of 0.1 - 0.2% of H15 or H30 is recommended.

    12. Dispersion of WACKER HDK® as a function of the equipment Effective product design with the aid of Wacker HDK® presupposes among other things adequately homogenous distribution of the silica. This contributes decisively to optimum efficacy. The transperancy shows the thickening behaviour as a function of dispersing equipment. It becomes clear from the results shown, that with a given concentration higher viscosity values are obtained through the use of better dispersing equipment. Equipment with low to medium dispersing action produces low maximum viscosity values. Even considerable prolonging of the dispersing time does not increase the viscosity of the mixture. High speed equipment however can yield greater viscosity. Effective product design with the aid of Wacker HDK® presupposes among other things adequately homogenous distribution of the silica. This contributes decisively to optimum efficacy. The transperancy shows the thickening behaviour as a function of dispersing equipment. It becomes clear from the results shown, that with a given concentration higher viscosity values are obtained through the use of better dispersing equipment. Equipment with low to medium dispersing action produces low maximum viscosity values. Even considerable prolonging of the dispersing time does not increase the viscosity of the mixture. High speed equipment however can yield greater viscosity.

    13. Dispersion of WACKER HDK® Dissolver Technology Effective product design with the aid of Wacker HDK® presupposes among other things adequately homogenous distribution of the silica. This contributes decisively to optimum efficency. It should be noted, that the dispersing result in the first place is not determined by the dispersing time, but by the shear force, that can be introduced into the liquid. Considerable prolonging of the dispersing time does not increase the viscosity of the mixture. The most commonly used dispersing equipment is the dissolver. With this equipment, best results are obtained by striking a balance between the geometry of the container, the container diameter, the peripheral speed of the disk the height of the stirrer disk above the bottom of the container, the filling height of the material in the container and the flow properties of the material. The given data are intended as guideline: If the diameter of the stirrer disk is D, the diameter of the container should be 2 to 3 times D, the height to which the container is filled with material should be 1 or 2 times D and the dispersing disk is recommended to be in a distance of about 0.5 to 1 times D. With ideal dispersing conditions the doughnut can be seen, when looking into the container from the top. The liquid circulates away from the dissolver disk up the wall and tumbles down on the dissolver disk again. In the latter step the liquid doesn´t strike the outer sphere of the disk but covers it to a certain extend. Effective product design with the aid of Wacker HDK® presupposes among other things adequately homogenous distribution of the silica. This contributes decisively to optimum efficency. It should be noted, that the dispersing result in the first place is not determined by the dispersing time, but by the shear force, that can be introduced into the liquid. Considerable prolonging of the dispersing time does not increase the viscosity of the mixture. The most commonly used dispersing equipment is the dissolver. With this equipment, best results are obtained by striking a balance between the geometry of the container, the container diameter, the peripheral speed of the disk the height of the stirrer disk above the bottom of the container, the filling height of the material in the container and the flow properties of the material. The given data are intended as guideline: If the diameter of the stirrer disk is D, the diameter of the container should be 2 to 3 times D, the height to which the container is filled with material should be 1 or 2 times D and the dispersing disk is recommended to be in a distance of about 0.5 to 1 times D. With ideal dispersing conditions the doughnut can be seen, when looking into the container from the top. The liquid circulates away from the dissolver disk up the wall and tumbles down on the dissolver disk again. In the latter step the liquid doesn´t strike the outer sphere of the disk but covers it to a certain extend.

    14. The figures given here are meant as a guideline for dispersing Wacker HDK® with a dissolver. The dissolver used was a lab instrument. It should be noted, that it is no use to try to disperse Wacker HDK® in a very low viscous liquid, such as water, as it is impossible to introduce shear force into such a liquid, even with a high speed equipment. So a suitable starting viscosity is necessary. If this cannot be achieved, the fumed silica should be prepared as a masterbatch and should be introduced into the respective medium in this predispersed form. The figures given here are meant as a guideline for dispersing Wacker HDK® with a dissolver. The dissolver used was a lab instrument. It should be noted, that it is no use to try to disperse Wacker HDK® in a very low viscous liquid, such as water, as it is impossible to introduce shear force into such a liquid, even with a high speed equipment. So a suitable starting viscosity is necessary. If this cannot be achieved, the fumed silica should be prepared as a masterbatch and should be introduced into the respective medium in this predispersed form.

    15. A processing alternative is provided by the masterbatch method. The advantage of the masterbatch method is that it yields highly concentrated mixtures. The masterbatch method proceeds in two stages. A highly concentrated mixture or masterbatch of Wacker HDK® and the medium for thickening is prepared with highly effective dispersing equipment, such as a three-roll mill. It is then diluted or let down at a low or medium rate of the dispersion to the desired Wacker HDK® concentration. This method results in very good distribution of the Wacker HDK® . A dissolver can also be used to prepare masterbatches.A processing alternative is provided by the masterbatch method. The advantage of the masterbatch method is that it yields highly concentrated mixtures. The masterbatch method proceeds in two stages. A highly concentrated mixture or masterbatch of Wacker HDK® and the medium for thickening is prepared with highly effective dispersing equipment, such as a three-roll mill. It is then diluted or let down at a low or medium rate of the dispersion to the desired Wacker HDK® concentration. This method results in very good distribution of the Wacker HDK® . A dissolver can also be used to prepare masterbatches.

    16. Master Batch vs Standard Method with dissolver in both cases The standard procedure is to introduce the amount of fumed silica needed into the coating material and to disperse it. A processing alternative is provided by the masterbatch method. This method yields highly concentrated mixtures of predispersed fumed silica, which can be diluted to the concentration needed. In the first step a fumed silica concentrate with a concentration as high as possible is produced. Maximum concentrations are around 10% of fumed silica. In our example we used 6 - 9.5% refered to the total resin composition. The fumed silica is accurately dispersed, which results in a considerable increase in viscosity. This concentrate is diluted down to the concentration needed by adding the appropriate amount of resin. As the concentrates are rather high in viscosity, usually slightly better dispersing results are obtained with the masterbatch method. The masterbatch method offers advantages, when no large scale high speed stirring equipment is available, or the fumed silica shall be used in a paint with a very low starting viscosity, in which it couldn´t be dispersed.The standard procedure is to introduce the amount of fumed silica needed into the coating material and to disperse it. A processing alternative is provided by the masterbatch method. This method yields highly concentrated mixtures of predispersed fumed silica, which can be diluted to the concentration needed. In the first step a fumed silica concentrate with a concentration as high as possible is produced. Maximum concentrations are around 10% of fumed silica. In our example we used 6 - 9.5% refered to the total resin composition. The fumed silica is accurately dispersed, which results in a considerable increase in viscosity. This concentrate is diluted down to the concentration needed by adding the appropriate amount of resin. As the concentrates are rather high in viscosity, usually slightly better dispersing results are obtained with the masterbatch method. The masterbatch method offers advantages, when no large scale high speed stirring equipment is available, or the fumed silica shall be used in a paint with a very low starting viscosity, in which it couldn´t be dispersed.

    17. Master Batch vs. Standard Method 0 Hegman = 100 µm 4 Hegman = 50 µm 8 Hegman = 0 µm

    19. Master Batch vs. Standard Method Wacker HDK® N20 has been introdcued into an unsaturated polyester resin. The masterbatch method has been used and compared to the result of the standard method. The masterbatch method has been used as described on the previous slide. With the standard method the 0.8% have been directly incorporated. Two things can be seen from the result. As expected the particle size in the batch prepared utilizing the masterbatch method is slighly smaller than with the standard method. Furthermore after 8 minutes of dispersing no more changes are observed. The grind fineness cannot be further improved just be prolonging the dispersing time.Wacker HDK® N20 has been introdcued into an unsaturated polyester resin. The masterbatch method has been used and compared to the result of the standard method. The masterbatch method has been used as described on the previous slide. With the standard method the 0.8% have been directly incorporated. Two things can be seen from the result. As expected the particle size in the batch prepared utilizing the masterbatch method is slighly smaller than with the standard method. Furthermore after 8 minutes of dispersing no more changes are observed. The grind fineness cannot be further improved just be prolonging the dispersing time.

    20. General Rules for the Performance of Wacker HDK® in liquid systems In general for an optimum performance of fumed silica a good dispersion is necessary. Theoretically overdispersion is possible with shear forces high enough to break down the aggregate structure. In real life however this is not very likely to happen. The fumed silica is expected to introduce thickening by building up a network through interaction just with itself. This makes it strikingly different from the so-called associative thickeners. To allow undisturbed interaction between fumed silica particles, the fumed silica must exhibit a certain degree of incompatibility with the medium, into which it is incorporated. For this reason it is recommended to use hydrophilic fumed silica grades for unpolar systems and to use hydrophobic grades for polar systems. Additives bearing polar groups, capable of hydrogen bridging (such as ethylene glycol and ethylene diamine) can interact with fumed silica hydroxy groups. In case these additives are monofunctional, they mask one hydroxy group of the fumed silica particle and avoid the formation of a hydrogen bridge to another particle. Thus they weaken the network. If they are however multifunctional, they can link fumed silica particles together via interacting with both of them. Thus they gap between neighbouring Wacker HDK® aggregates, create additional bonding sites, enhance the stability of the Wacker HDK® structure and the viscosity rises. Too many of these additives will however cover the surface of the fumed silica entirely and prevent the particles from the necessary direct interaction. In this case the performance breaks down as the particles are solvated.In general for an optimum performance of fumed silica a good dispersion is necessary. Theoretically overdispersion is possible with shear forces high enough to break down the aggregate structure. In real life however this is not very likely to happen. The fumed silica is expected to introduce thickening by building up a network through interaction just with itself. This makes it strikingly different from the so-called associative thickeners. To allow undisturbed interaction between fumed silica particles, the fumed silica must exhibit a certain degree of incompatibility with the medium, into which it is incorporated. For this reason it is recommended to use hydrophilic fumed silica grades for unpolar systems and to use hydrophobic grades for polar systems. Additives bearing polar groups, capable of hydrogen bridging (such as ethylene glycol and ethylene diamine) can interact with fumed silica hydroxy groups. In case these additives are monofunctional, they mask one hydroxy group of the fumed silica particle and avoid the formation of a hydrogen bridge to another particle. Thus they weaken the network. If they are however multifunctional, they can link fumed silica particles together via interacting with both of them. Thus they gap between neighbouring Wacker HDK® aggregates, create additional bonding sites, enhance the stability of the Wacker HDK® structure and the viscosity rises. Too many of these additives will however cover the surface of the fumed silica entirely and prevent the particles from the necessary direct interaction. In this case the performance breaks down as the particles are solvated.

    21. General Guideline for Use Levels of Wacker HDK® In general the dosage levels for appropriate rheology control varies in between 0,1 and 3 wt%. The amount of Wacker HDK® needed to provide rheology control is dependent upon several factors. The most important elements are the polarity of the system, the amount of dispersion energy available in manufacturing the coating and the coating application film thickness. The influence of polarity and its interdependence with dispersion energy in manufacturing has already been discussed. In general, sufficient anti-sag behavior at 2.5 to 10 µm film thickness will require a Wacker HDK® loading of between 0.25 and 0.75 wt.% of the total formula weight. For high build coatings in the 38 to 75 µm film thickness range, loadings of 2 to 3 wt.% may be required. Picture framing is also dependent on the film thickness, requiring the same usage levels as sag resistance. Hold out of a coating applied to a porous substrate will be improved with the addition of 0.20 to 0.75 wt. % Wacker HDK®.In general the dosage levels for appropriate rheology control varies in between 0,1 and 3 wt%. The amount of Wacker HDK® needed to provide rheology control is dependent upon several factors. The most important elements are the polarity of the system, the amount of dispersion energy available in manufacturing the coating and the coating application film thickness. The influence of polarity and its interdependence with dispersion energy in manufacturing has already been discussed. In general, sufficient anti-sag behavior at 2.5 to 10 µm film thickness will require a Wacker HDK® loading of between 0.25 and 0.75 wt.% of the total formula weight. For high build coatings in the 38 to 75 µm film thickness range, loadings of 2 to 3 wt.% may be required. Picture framing is also dependent on the film thickness, requiring the same usage levels as sag resistance. Hold out of a coating applied to a porous substrate will be improved with the addition of 0.20 to 0.75 wt. % Wacker HDK®.

    22. General Rules for the Performance of Wacker HDK® in liquid systems In general for an optimum performance of fumed silica a good dispersion is necessary. Theoretically overdispersion is possible with shear forces high enough to break down the aggregate structure. In real life however this is not very likely to happen. The fumed silica is expected to introduce thickening by building up a network through interaction just with itself. This makes it strikingly different from the so-called associative thickeners. To allow undisturbed interaction between fumed silica particles, the fumed silica must exhibit a certain degree of incompatibility with the medium, into which it is incorporated. For this reason it is recommended to use hydrophilic fumed silica grades for unpolar systems and to use hydrophobic grades for polar systems. Additives bearing polar groups, capable of hydrogen bridging (such as ethylene glycol and ethylene diamine) can interact with fumed silica hydroxy groups. In case these additives are monofunctional, they mask one hydroxy group of the fumed silica particle and avoid the formation of a hydrogen bridge to another particle. Thus they weaken the network. If they are however multifunctional, they can link fumed silica particles together via interacting with both of them. Thus they gap between neighbouring Wacker HDK® aggregates, create additional bonding sites, enhance the stability of the Wacker HDK® structure and the viscosity rises. Too many of these additives will however cover the surface of the fumed silica entirely and prevent the particles from the necessary direct interaction. In this case the performance breaks down as the particles are solvated.In general for an optimum performance of fumed silica a good dispersion is necessary. Theoretically overdispersion is possible with shear forces high enough to break down the aggregate structure. In real life however this is not very likely to happen. The fumed silica is expected to introduce thickening by building up a network through interaction just with itself. This makes it strikingly different from the so-called associative thickeners. To allow undisturbed interaction between fumed silica particles, the fumed silica must exhibit a certain degree of incompatibility with the medium, into which it is incorporated. For this reason it is recommended to use hydrophilic fumed silica grades for unpolar systems and to use hydrophobic grades for polar systems. Additives bearing polar groups, capable of hydrogen bridging (such as ethylene glycol and ethylene diamine) can interact with fumed silica hydroxy groups. In case these additives are monofunctional, they mask one hydroxy group of the fumed silica particle and avoid the formation of a hydrogen bridge to another particle. Thus they weaken the network. If they are however multifunctional, they can link fumed silica particles together via interacting with both of them. Thus they gap between neighbouring Wacker HDK® aggregates, create additional bonding sites, enhance the stability of the Wacker HDK® structure and the viscosity rises. Too many of these additives will however cover the surface of the fumed silica entirely and prevent the particles from the necessary direct interaction. In this case the performance breaks down as the particles are solvated.

    23. General Guideline for Use Levels of Wacker HDK® In general the dosage levels for appropriate rheology control varies in between 0,1 and 3 wt%. The amount of Wacker HDK® needed to provide rheology control is dependent upon several factors. The most important elements are the polarity of the system, the amount of dispersion energy available in manufacturing the coating and the coating application film thickness. The influence of polarity and its interdependence with dispersion energy in manufacturing has already been discussed. In general, sufficient anti-sag behavior at 2.5 to 10 µm film thickness will require a Wacker HDK® loading of between 0.25 and 0.75 wt.% of the total formula weight. For high build coatings in the 38 to 75 µm film thickness range, loadings of 2 to 3 wt.% may be required. Picture framing is also dependent on the film thickness, requiring the same usage levels as sag resistance. Hold out of a coating applied to a porous substrate will be improved with the addition of 0.20 to 0.75 wt. % Wacker HDK®.In general the dosage levels for appropriate rheology control varies in between 0,1 and 3 wt%. The amount of Wacker HDK® needed to provide rheology control is dependent upon several factors. The most important elements are the polarity of the system, the amount of dispersion energy available in manufacturing the coating and the coating application film thickness. The influence of polarity and its interdependence with dispersion energy in manufacturing has already been discussed. In general, sufficient anti-sag behavior at 2.5 to 10 µm film thickness will require a Wacker HDK® loading of between 0.25 and 0.75 wt.% of the total formula weight. For high build coatings in the 38 to 75 µm film thickness range, loadings of 2 to 3 wt.% may be required. Picture framing is also dependent on the film thickness, requiring the same usage levels as sag resistance. Hold out of a coating applied to a porous substrate will be improved with the addition of 0.20 to 0.75 wt. % Wacker HDK®.

    24. Application Lab - Test arrangements We have run a performance test comparing Wacker HDK® N20 with typical and often seen competitive grades. The property determined for the comparison was the Brookfield viscosity, which was obtained in an unsaturated polyester resin. The determination parameters were kept constant. The fumed silica grades were incorporated using a dissolver and stirring at a circumferential speed of 8 m/s for 5 min. Before the determination the preparations were stirred with a propeller stirrer at 750 rounds per minute for 5 min and then kept at 23°C for 60 minutes. The Brookfield viscosity has alwas been determined after 10 rounds of the spindle to obtain comparable values. We have run a performance test comparing Wacker HDK® N20 with typical and often seen competitive grades. The property determined for the comparison was the Brookfield viscosity, which was obtained in an unsaturated polyester resin. The determination parameters were kept constant. The fumed silica grades were incorporated using a dissolver and stirring at a circumferential speed of 8 m/s for 5 min. Before the determination the preparations were stirred with a propeller stirrer at 750 rounds per minute for 5 min and then kept at 23°C for 60 minutes. The Brookfield viscosity has alwas been determined after 10 rounds of the spindle to obtain comparable values.

    25. Wacker HDK®N20 compared to Degussa Aerosil 200 in an UP-resin The first comparison is between Wacker HDK® N20 and Degussa Aerosil 200. The Degussa material was used in different concentrations, where the Wacker HDK® N20 was only used in an amount of 0.7% refered to total formulation. As can be seen from the diagram, the maximum viscosity values obtained with the Degussa materials are lower than that achieved with 0.7% of Wacker HDK® N20 even when 1.0% of the Aerosil 200 are used. That means the use of Wacker HDK® N20 is over 30% more economical, when accurately dispersed, which is quite a big advantage. The first comparison is between Wacker HDK® N20 and Degussa Aerosil 200. The Degussa material was used in different concentrations, where the Wacker HDK® N20 was only used in an amount of 0.7% refered to total formulation. As can be seen from the diagram, the maximum viscosity values obtained with the Degussa materials are lower than that achieved with 0.7% of Wacker HDK® N20 even when 1.0% of the Aerosil 200 are used. That means the use of Wacker HDK® N20 is over 30% more economical, when accurately dispersed, which is quite a big advantage.

    26. Wacker HDK®N20 compared to Cabot´s Cabosil M5 in an UP resin The same picture that was discussed for the comparison of between Wacker HDK® N20 and Degussa Aerosil 200 is obtained, when comparing Wacker HDK® N20 with Cabot´s Cabosil M5. The Wacker HDK® N20 is more economical in use, as with a smaller amount of this fumed silica grade a higher degree of thickening can be achieved, provided the products are both accurately dispersed.The same picture that was discussed for the comparison of between Wacker HDK® N20 and Degussa Aerosil 200 is obtained, when comparing Wacker HDK® N20 with Cabot´s Cabosil M5. The Wacker HDK® N20 is more economical in use, as with a smaller amount of this fumed silica grade a higher degree of thickening can be achieved, provided the products are both accurately dispersed.

    27. Wacker HDK®N20 compared to Tokoyama´s Reolosil QS20 in an UP-resin The same result obtained for the comparison of Cabosil M5 and Degussa Aerosil 200 with Wacker HDK® N20 is obtained when using Tokoyama´s Reolosil QS20. Again the Wacker HDK® N20 is more economical in use or in other words performs better at identical uselevels.The same result obtained for the comparison of Cabosil M5 and Degussa Aerosil 200 with Wacker HDK® N20 is obtained when using Tokoyama´s Reolosil QS20. Again the Wacker HDK® N20 is more economical in use or in other words performs better at identical uselevels.

    28. Wacker HDK®N20 compared to Tokoyama´s Reolosil QS102 in an acrylic based wood coat

    29. This diagram displays the result of a comparison of the Wacker HDK® N20 with Wacker HDK® H18 in a polar vinylester resin based formulation. As can be seen the Wacker HDK® N20 does not increase the viscosity at the dosage levels of 0.6 and 1.8% used. This fumed silica grade here has almost no effect. With the Wacker HDK® H18 at an amount of 0.6% on total formulation, the viscosity at low shear rates is tremendously increased. In general it is found, that for polar media hydrophobic grades perform much better than hydrophilic grades and vice versa, hydrophilic grades perform much better in non-polar systems than hydrophobic grades. This diagram displays the result of a comparison of the Wacker HDK® N20 with Wacker HDK® H18 in a polar vinylester resin based formulation. As can be seen the Wacker HDK® N20 does not increase the viscosity at the dosage levels of 0.6 and 1.8% used. This fumed silica grade here has almost no effect. With the Wacker HDK® H18 at an amount of 0.6% on total formulation, the viscosity at low shear rates is tremendously increased. In general it is found, that for polar media hydrophobic grades perform much better than hydrophilic grades and vice versa, hydrophilic grades perform much better in non-polar systems than hydrophobic grades.

    30. In this test the influence of the concentration of the Wacker HDK® H18 on the performance in a polar vinylester resin based formulation has been examined. As can easily be recognized the higher the concentration, the higher the achievable viscosity. This again is a general rule. The higher the concentration of a fumed silica grade, which is effective in thickening the respective medium, the higher is the increase in viscosity possible. This is true as long as the formulation still stays liquid. In this test the influence of the concentration of the Wacker HDK® H18 on the performance in a polar vinylester resin based formulation has been examined. As can easily be recognized the higher the concentration, the higher the achievable viscosity. This again is a general rule. The higher the concentration of a fumed silica grade, which is effective in thickening the respective medium, the higher is the increase in viscosity possible. This is true as long as the formulation still stays liquid.

    31. In non polar resin systems, hydrophilic, non surface treated Wacker HDK? has an excellent thickening action. But the viscosity of polar systems, such as epoxy, polyurethane and vinyl resins, that have been thickened with hydrophilic HDK? will not remain stable in storage. Even hydrophobic standard products, such as Wacker HDK?H15 fail to completely satisfy customers´ rheological requirements. From the technical point of view the important interplay of shear thinning and yield point formation in liquid systems from a reversible build-up of a stable Wacker HDK? particle network consisting of aggregates and agglomerates and caused by the forces of attraction between Wacker HDK?-surfaces. Unlike the case of non-polar systems, in polare resin systems, classical hydrogen bonding between silanol groups of hydrophilic Wacker HDK? does not work. However if the surface is rendered completely non polar (highly hydrophobic) by silylation, a hydrophobic interaction predominates. The surrounding polar medium forces and allows these hydrohpobic particle interactions. As is seen from the diagram, the hydrophilic silica HDK?N20 (100% Si-OH) and the hydrophobic silica do not stabilize the viscosity or dry film thickness of a 2 pack polyurethane system in the long term. A drop in viscosity and thinner dry films occur, when the systems are applied to vertical surfaces. The highly hydrophobic silica Wacker HDK?H18 (<20% SiOH), however fulfils the requirements imposed on it in terms of rheology and ease of processing. The same is true for a vinyl ester system.In non polar resin systems, hydrophilic, non surface treated Wacker HDK? has an excellent thickening action. But the viscosity of polar systems, such as epoxy, polyurethane and vinyl resins, that have been thickened with hydrophilic HDK? will not remain stable in storage. Even hydrophobic standard products, such as Wacker HDK?H15 fail to completely satisfy customers´ rheological requirements. From the technical point of view the important interplay of shear thinning and yield point formation in liquid systems from a reversible build-up of a stable Wacker HDK? particle network consisting of aggregates and agglomerates and caused by the forces of attraction between Wacker HDK?-surfaces. Unlike the case of non-polar systems, in polare resin systems, classical hydrogen bonding between silanol groups of hydrophilic Wacker HDK? does not work. However if the surface is rendered completely non polar (highly hydrophobic) by silylation, a hydrophobic interaction predominates. The surrounding polar medium forces and allows these hydrohpobic particle interactions. As is seen from the diagram, the hydrophilic silica HDK?N20 (100% Si-OH) and the hydrophobic silica do not stabilize the viscosity or dry film thickness of a 2 pack polyurethane system in the long term. A drop in viscosity and thinner dry films occur, when the systems are applied to vertical surfaces. The highly hydrophobic silica Wacker HDK?H18 (<20% SiOH), however fulfils the requirements imposed on it in terms of rheology and ease of processing. The same is true for a vinyl ester system.

    32. The sample bottle contains 200 g of fumed silica. The most frequently used packaging used for Wacker HDK® grades is the special valved creped paper bags. They maintain product qualitiy and guarantee problem free storage for several months in dry rooms. These bags contain 10 kg for regular grades and 20 kg of the compressed grades. The Wacker HDK® N20 is supplied in big bags with 90 kg contents as well.The sample bottle contains 200 g of fumed silica. The most frequently used packaging used for Wacker HDK® grades is the special valved creped paper bags. They maintain product qualitiy and guarantee problem free storage for several months in dry rooms. These bags contain 10 kg for regular grades and 20 kg of the compressed grades. The Wacker HDK® N20 is supplied in big bags with 90 kg contents as well.

More Related