Coatings Technology Handbook Episode 2 Part 3 pps

The horsepower of the disperser is related to the blade diameter and the anticipated load the blade will create at a given speed and resistance.. However, as the blade diameter increases

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45-4 Coatings Technology Handbook, Third Edition

45.6 What Different Types of Dispersers Are Available, and What

Type Is Best for Me?

Dispersers are available with single-speed, two-speed, and variable-speed mixing shafts Some are directly mounted atop a tank and are fixed to operate with the blade in only the original mounting position (Figure 45.3) Other tank-mounted dispersers can raise and lower the blade by several feet (to better control the vortex) without exiting the tank Another design, perhaps the most popular, places the disperser atop a hydraulic lift (similar to the ones used at gas stations to lift automobiles) that is mounted

to the floor (Figure 45.4) The lift enables the operator to raise the blade completely out of the mixing vessel and change to another vessel This technique uses small portable tanks (up to 500 gal) that can be rolled away on wheels or picked up with a fork truck Larger stationary tanks are often centered within the arc of rotation from the center of the hoist to the center of the mixing shaft The bridge containing

DK4036_book.fm Page 4 Monday, April 25, 2005 12:18 PM

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A Practical Guide to High-Speed Dispersion 45-5

the mixing shaft at one end and the motor at the other is then rotated from one tank to the next Choosing the best configuration of available designs is a combination of functional need and economic justification

An experienced process engineer or consultant familiar with dispersers is a good investment

45.7 How Do I Select the Proper Size Disperser?

The size of a disperser is generally thought of in terms of horsepower However, there are dispersers that are dimensionally very large but use relatively small amounts of horsepower These are exceptions

to the rule

The horsepower of the disperser is related to the blade diameter and the anticipated load the blade will create at a given speed and resistance The resistance is a function of the rheology of the dispersion

as well as the viscosity and density However, as the blade diameter increases, the horsepower increases disproportionately For example, if a 12 diameter blade were to draw 20 hp in a non-Newtonian system (viscosity changes with shear), doubling the blade diameter could increase the horsepower demand by

a factor of 5 That means a 24 diameter blade of the same design, working in the same product, would require 100 hp The larger blade would also pump considerably more so it would lend itself to working

in a much larger (perhaps five times the volume) tank and producing a much greater amount of finished product in the same time period

Horsepower requirements are interrelated with blade diameter, tank diameter, batch size, rheology, viscosity, and density Variations outside recommended operating parameters usually result in compro-mises in performance, such as poor particle separation, extended dispersion times, and a decrease in quality of the finished product

45.8 How Do I Select the Proper Size Tank for My Disperser?

The ideal tank for most dispersers is slightly taller than wide Dished- or bowl-shaped bottoms aid in preventing solids from accumulating in sharp corners associated with flat bottoms Equally as important, dished bottoms drain to the center, where a discharge valve can be installed Flush bottom ball valves welded into the center of the dished bottoms further enhance the ease of discharge and cleaning Optimum tank geometry is an integral part of several aspects that need be considered and are listed later in this chapter

45.9 How Do I Select the Proper Size Blade for My Disperser?

The blade is sized based upon the flow characteristics of the product and the desired degree of dispersion The thicker the product, the larger the blade diameter must be in comparison to the tank diameter Conversely, the thinner the product, the smaller the blade diameter must be in comparison to the tank diameter This comparison is called the blade-to-tank ratio Thick products like heavy, flowable pastes may require a ratio of 1/2:1 Moderate products like paint require 1/3:1 ratio, and thin products like stains can work with up to a 1/8:1 ratio

For example, if the blade-to-tank ratio is 1/3:1, and the tank diameter is 6 ft, the blade diameter would

be 2 ft

45.9.1 How Do I Know When It Is Time to Replace My Blade?

Once the batch formula has been process optimized, the typical time required to reach maximum dispersion should range from 20 to 30 min after the last ingredients have been added Longer times do not usually result in better dispersions and, in some cases, can be detrimental because of the higher batch temperatures generated by the high shear disperser blade As the blade begins to wear, longer batch times are required to get to the optimized dispersion standard Sawtooth-type disperser blades should be replaced once the blade tips are worn to one-half their original height

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45.9.2 What Type of Blade Works Best on My Disperser?

High shear disperser blades are available in a range of styles and sizes (Figure 45.5) They can be generally categorized into two groups: open sawtooth and ring type Both categories work well when used under the proper operating conditions

The open sawtooth blade is the most popular because of its low cost, ease of cleaning, and general utility It is available in a wide range of tooth designs As the teeth increase in size and become more aggressive in shape, the pumping ability of the blade increases However, as pumping (turbulent flow) increases, shear decreases A high pumping saw blade still generates significant shear compared to a low shear paddle blade agitator This aspect is an important consideration when determining exactly what is

to be achieved in the finished product

The ring-type blade is a powerful tool for optimizing disperser performance It is more expensive to purchase and consumes more horsepower than the saw blade It typically runs at higher tip speeds (5700+ fpm) and performs more like a rotor stator Instead of solely depending upon the face of the disc and the configuration of the sawtooth for shear and flow, much of the ring blade’s work is done hydraulically,

as centrifugal force forces the product between the contoured rings, creating velocity differentials and a high-pressure zone within, and then instantaneously discharging into the low-pressure area outside the rings, creating a film splitting venturi effect Additional heat is created as a by-product of the higher shear However, in some cases, this higher shear level eliminates or greatly reduces any subsequent milling that may have previously been required

45.10 What Other Factors Affect the Performance of My

Disperser?

Formulating for a disperser is an important part of reaching optimum dispersion Optimizing a formula can sometimes have more to do with how and when ingredients are added, because basic recipe changes may not be acceptable In most instances, rapid addition of about one-half the total amount of powders into the liquid vehicle is acceptable, although careful observation of each initial formula is prudent to ensure that powders are not floating on top of the batch for more than a few seconds Adding dry powders too rapidly can “choke” the blade and may result in an incomplete, unstable dispersion The last half of the powders should be added progressively more slowly until the final percentage completes the formula The blade speed should be adjustable from a minimum of one-half the final tip speed at the beginning

of the powder addition to the maximum of the final tip speed as the batch thickness and flow slow This procedure helps prevent splashing and overvortexing, which are inefficient for dispersion and can cause

FIGURE 45.5 Disperser blades.

"F" style: the "Saw Tooth" blade

"E" style: the "Cutter" blade

"G" style: the

"High Vaned" blade DK4036_book.fm Page 6 Monday, April 25, 2005 12:18 PM

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A Practical Guide to High-Speed Dispersion 45-7

excessive air entrapment in the dispersion Assuming the rules of tank size, horsepower, blade size, etc., have been followed, most dispersions are completed within 20 to 30 min after the last of the powders have been properly added Continuing the dispersion process beyond that time is usually unproductive and can actually cause harm to some ingredients if the temperature continues to rise

Typically, dispersers perform best when the flow pattern is doughnut shaped and the blade tips are traveling at about 5000 ft/min in a medium viscosity (1500 to 5000 centipoises) Lower tip speed may

be acceptable at higher viscosities, and higher tip speeds may be acceptable at lower viscosities to get to the same shear rate and stress In other words, shear is a function of blade tip speed and product rheology

45.11 How Do I Operate My Disperser for Optimum

Performance?

The following is a summary of the detailed aspects outlined in the above paragraphs Optimum perfor-mance requires the following criteria:

1 Start with a clean tank

2 Correct blade-to-tank ratio

3 Proper formulation

4 Suitable blade in good condition

5 Highest appropriate blade tip speed

6 Correct tank geometry (length compared to width)

7 Sufficient horsepower

8 Proper technique of adding raw materials

9 Proper rheology

45.12 What Safety Measures Must I Follow and Why?

1 Read the operation and safety instructions supplied by the manufacturer If they are not available, call the manufacturer and request additional copies

2 Be certain that all operators are properly trained on the use and drilled on the potential dangers involved with the disperser

3 Do not operate the machine unless all the appropriate safety features are in place and are func-tioning properly On hoist mounted units that raise and lower the blade, these features include but are not limited to mixing shaft guard, tank holder with limit switch, and limit switch on lift

to prevent machine from operating with blade or shaft within reach of the operator

The disperser is a very fast and powerful machine Serious and fatal accidents can occur in a split second of carelessness Human reflex is no match for the instantaneous danger of operating a disperser unsafely Never sacrifice safety for convenience

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III

Materials

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46 Acrylic Polymers

46.1 Introduction 46-1 46.2 Chemistry and Manufacture 46-1 46.3 Versatility of Acrylics 46-2 46.4

46.5 Coating Techniques 46-8

46.1 Introduction

Since their introduction decades ago, acrylic polymers have gained a strong foothold in the coatings and allied industries as a result of their improved flexibility and adhesion compared to polyvinyl acetate emulsions, phenolics, and styrene-butadiene latex combined with their moderate cost In addition, their significantly improved outdoor durability, including resistance to ultraviolet degradation, has mandated their use in several applications In many respects, the name “acrylic” has become synonymous with a high performance level in a polymer system

Presently, acrylics are available in three physical forms: solid beads, solution polymers, and emulsions The emulsion form is by far the dominant form in use today This is due generally to the ease of tailoring properties, and the lower hazards and manufacturing costs compared to the solid and solution polymers

46.2 Chemistry and Manufacture

46.2.1 Monomers

Acrylic monomers are esters of acrylic and methacrylic acid Some common esters are methyl, ethyl, isobutyl, n-butyl, 2-ethylhexyl, octyl, lauryl, and stearyl The esters can contain functional groups such

as hydroxyl groups (e.g., hydroxyethyl methacrylate), amino groups (e.g., dimethylaminoethyl methacry-late), amide groups (acrylamide), and so on, in addition to the carboxylic acid functionality of the unesterified monomer Acrylic monomers can be multifunctional (e.g., trimethylolpropane triacrylate,

or butylenes glycol diacrylate, to mention two) The polymer chemist has a wide range of monomers to select from when designing a specialty polymer system

Typically, mixtures of comonomers are chosen for the properties they impart to the polymer Adhesive strength, for example, is increased by using monomers with low glass transition temperatures such as butyl acrylate or 2-ethyl hexylacrylate Members of the carboxylic acid group of acrylic and methacrylic acids also tend to increase the adhesive properties of polymers Cohesive strength is usually imparted by the harder acrylic monomers such as methyl methacrylate and methyl acrylate Molecular weight is also a significant contributing factor, and these two parameters must be carefully balanced by the polymer chemist

Ronald A Lombardi

ICI Resins US

James D Gasper

ICI Resins US

Coatings • Adhesives • Inks

Application Areas 46-4

Coaters • Knife over Roll Coaters • Reverse Roll Coaters

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46.4.2.1 Heat-Sealable Adhesives

Heat sealing is used for bonding two substrates where one or both are impervious to water Typically, the more heat- and solvent-resistant substrate is coated first, and the solvent or water is driven off in an oven At this point, the web can be wound up on itself (if the dried adhesive is tack free or nonblocking) and heat sealed to the secondary substrate at a later date Alternatively, the adhesive-coated substrate can

be laminated simultaneously to the second substrate to form the finished product The latter case is the laminating adhesive technique, discussed later When heat is applied to activate or soften the adhesive, two conditions must be met for adequate bonding:

1 The adhesive must have sufficient flow at the activation temperature to properly wet out the secondary substrate

2 The adhesive must have a chemical affinity or specific adhesion to the particular substrate This comprises the secondary chemical bonding forces, which give rise to what we call adhesive bonding Many applications involving food packaging fall into this category Examples include lidding-type adhesives for coffee creamers and jams and the blister packaging of pharmaceuticals

46.4.2.2 Laminating Adhesives

Laminating adhesives function much the same as heat-seal adhesives except that the temperature neces-sary to activate the adhesive is much lower Where heat-seal adhesives may require an activation tem-perature of 120°C (250°F), laminating adhesives can be designed to function anywhere between room temperature and 90°C (200°F) Recently, new low temperature curing types of adhesive have, to some degree, replaced two-component solvent-polyurethane adhesives in the flexible packaging area.13 Design-ing a room temperature curDesign-ing mechanism into an acrylate system further enhances opportunities to replace solvent systems.14 Considerable progress has also been made using low temperature curing acrylics

in the industrial area.15

Typical applications include vinyl-to-wood laminating and bonding vinyl to ABS for automotive interiors

46.4.2.3 Pressure-Sensitive Adhesives

The pressure-sensitive method of bonding is often called the “one-way” bonding method The adhesive

is coated onto the substrate either directly or by transfer coating The adhesive is protected by a release liner until it is ready to be used When application is desired, the liner is removed and the adhesive-coated substrate is bonded to the other substrate using pressure alone Pressure actually activates the adhesive, hence the name of this method Upon firm pressure, the tacky adhesive mass actually flows and bonds itself both mechanically and chemically to the other surface

However, to be functional, a pressure-sensitive adhesive must be more than just very tacky Flypaper has a very high degree of tack but lacks internal strength or cohesiveness A functional pressure-sensitive material will have high tack or adhesive strength combined with high cohesive strength, the basis require-ment for any adhesive

Pressure-sensitive adhesives are used in many tape areas such as packaging, masking, electrical mend-ing, medical, and mounting The graphic arts area includes vinyl decals and special decorative films such

as clear and metallized polyester films These areas generally require an outstanding balance of properties that must be retained under severe outdoor exposure and temperature extremes Such properties as outstanding resistance to ultraviolet light degradation and plasticizer migration are required Adhesives for this area must produce clear and colorless films, dictating the use of acrylics exclusively.16

Solvent-based polystyrene acrylics (PSAs) have been traditionally used in these areas More recently, they have been replaced in varying degrees with their emulsion counterparts.17 However, there still exist technical areas in which the solvent systems prevail.18 These include applications requiring high levels of heat, water, and solvent resistance

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increases coating weight, whereas slowing the applicator roll decreases coating weight The primary reason

for using a reverse roll coater is that a precise, uniform coating weight across the web can be attained

Dried film tolerances of ~0.0001 in are possible The other desirable feature is that coating weight

thickness is independent of the backing weight variation This is exactly opposite to the relationship on

a knife over roll coater Coating thickness in a reverse roll coater is generally a function of the following:

• Gap between applicator and metering roll

• Applicator roll speed

• Adhesive solids, viscosity, and rheology

Two primary coating supply systems are used when applying adhesives or coatings by reverse roll For

higher viscosities (10,000 to 20,000 cp), a nip-fed arrangement is utilized For lower viscosities (3000 to

10,000 cp), a pan-fed system is preferred

The principal drawback to the reverse roll coater is the expense, as high precision rolls and bearings

are required However, the initial investment is usually outweighed by the improvement in quality of the

finished product

References

1 J A Brand and L W Morgan, U.S Patent 4,546,160; S C Johnson & Son, Inc

2 K W Free, U.S Patent 3,821,330; E I Dupont de Nemours and Co

3 K Shimada et al., U.S Patent 3,968,059; Mitsubishi Raon Co Ltd

4 H F Mark, Encyclopedia of Polymer Science and Technology, Vol 5 New York: Wiley, 2003, pp

801–859

5 B.F Goodrich Chemical Company, Latex Product Date, Bulletin L–12, p 14

6 Rohm and Haas Company, Bulletin SP197, Special Products Department

7 B.F Goodrich Chemical Company, Latex Bulletin L–10, p 4

8 B.F Goodrich Chemical Company, Latex Bulletin L–19, p 12

9 R A Heckman, J Prot Coat Linings, 3 10 (1986)

10 J E Fitzwater, Jr., J Water-Borne Coat., 8, 3 (1985)

11 J E Fitzwater, Jr., J Water-Borne Coat., 7, 3 (1984)

12 G Pollano and A Lurier, J Water-Borne Coat., 9, 1 (1986)

13 P Foreman, Paper Film and Foil Converter, November 1982

14 R A Lombardi, Adhes Age, February 1987

15 A H Bealieu and F P Hoenisch, J Water-Borne Coat., 10, 1 (1987)

16 D Satas, Handbook of Pressure Sensitive Adhesive Technology, 2nd ed New York: Van Nostrand

Reinhold, 1989

17 R A Lombardi, Higher Performance Water-Borne Pressure Sensitive Adhesives. Pressure Sensitive

Tape Council Seminar, Itasca, IL, 1987

18 F T Koehler and J A Fries, Acrylic Emulsion PSAs: Performance vs Acrylic Solutions. Spring Seminar,

Adhesive and Sealant Council, Atlanta, 1985

19 G Sen, Am Ink Maker, 65, 12 (1987).

20 M T Nowak, Am Ink Maker, 62, 10 (1984).

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47 Vinyl Ether Polymers

47.1 General 47-1 47.2 Monomers 47-1 47.3 Production of Polymers 47-1 47.4 Products on the Market 47-2 47.5 Properties 47-2 47.6 Applications 47-3 Bibliography 47-3

47.1 General

The general formula for vinyl ether polymers used in the production of adhesives and coatings is as follows

The consistency of these polymers depends on their molar mass and ranges from viscous oils to rubbery solids

Vinyl ether was first converted into a resinous polymer more than a century ago Between 1920 and

1930, it became readily accessible by the techniques of Reppe chemistry and thus attracted industrial interest Means were then investigated for polymerizing it In 1938 the large-scale production of vinyl ether polymers commenced in the Ludwigshafen works of the former IG-Farbenproduktion (now the Carbide Corporation, which relinquished the field in 1976

47.2 Monomers

The Reppe reaction between acetylene and an alcohol gives rise to vinyl ether:

HC≡CH + H–OR → CH2=CH–OR

It is still the only method of producing vinyl ethers that has acquired industrial significance The

47.3 Production of Polymers

Vinyl ethers can be easily polymerized in bulk or in solution, by batch or continuous techniques Owing

to the considerable heat of reaction, careful control and elaborate equipment are essential The monomers and the initiator are metered continuously into the reactor, and the polymerization reaction sets in within

a few minutes After the reactor has been completely charged, it is closed, and polymerization proceeds further under pressure The boiling point of the monomer or the solvent governs the rate at which the

Helmut W J Müller

BASF AG, Ludwigshafen/Rhein

main production site of BASF AG) GAF Corporation started production after 1940, followed by Union

properties of the constituent monomers are listed in Table 47.1

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