European Adhesives Handbook

7/27/2019 European Adhesives Handbook

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Author Biography 1/10/2006

David W. Horwat

Lead Engineer

Air Products Polymers, LP.7201 Hamilton Boulevard

Allentown, PA 18195-1501

Phone (610) 481-5646

Fax (610) 481-4839Internet Address: [email protected]

Dave Horwat is currently a Lead Engineer with Air Products Polymers. He has worked in the

adhesives industry for 33 years since receiving his Masters Degree in Chemical Engineering in1973. Employed by Air Products Polymers, he provided formulations development and

technical service to the adhesives industry for emulsion adhesive raw materials for the past 19

years. Markets served are the packaging market, the wood working market, automotiveadhesives, the insulation industry and other industrial bonding markets. Dave has a BS and an

MS in Chemical Engineering from New Jersey Institute of Technology in the USA. Prior to his

experience with Air Products Polymers, Dave worked for 14 years as a solvent borne and waterborne polymer adhesives formulator for a major international adhesives manufacturing company.

Dave began with experience in solvent based adhesives in the construction markets with contact

cement formulation development. Additional work experience included wood and panel bonding,

synthetic and natural elastomer formulations development, pressure sensitive and acrylicadhesives formulations and polymer development. His latest major emphasis is in the

conversion to vinyl acetate/ethylene water based alternatives from solvent, hot melt and other

non aqueous systems. A frequent speaker on technology issues at the Adhesives and Sealantscouncil meetings, he has been published in North American and European adhesives industry

magazines. He is an author of the industry standard training manual Working with Vinyl

Acetate Based Polymers Adhesives Manual published by Air Products Polymers. His focus has

been the mathematical modeling of adhesives systems from a rheological viewpoint.

Dave resides in Emmaus, PA with his wife, three children, three cats and three computers.


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1

Poly (Vinyl Acetate)/Poly (Vinyl Acetate)/Ethylene PolymerEthylene PolymerDispersionsDispersions

for Adhesivesfor Adhesives

Poly (Vinyl Acetate)/ Ethylene Polymer Dispersions

for AdhesivesBy

David W. HorwatLead Engineer

Air Products Polymers, LP.

7201 Hamilton Boulevard

Allentown, PA 18195-1501Phone (610) 481-5646

Fax (610) 481-4839

Internet Address: [email protected]


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2

Introduction

Vinyl Acetate based polymers and copolymers have captured a large share of the adhesives

market since their inception over 40 years ago. They earned this large portion of the market

because they offer the advantages of excellent adhesion, high performance, good productionspeeds, low cost economics, and an ease of handling and processing over many previously usednatural and solvent systems. The relatively polar nature of a vinyl acetate polymer or copolymer

makes it a natural choice for bonding any cellulosic surface like paper, wood or similar

composite materials. If properly flexibilized, adhesion to metallic foils, plastics, films and manymodern printed surfaces is obtained. Vinyl acetate based polymer dispersions are high molecular

weight polymers dispersed in an aqueous media, making a final adhesive film that can be highly

heat, water and humidity resistant. Because they are two phase systems, these polymer

dispersions are higher in solids and faster setting than solution polymers dissolved in water atequivalent machining viscosities. The polymers are saturated chains with no chemical double

bonds , which allows them to remain stable to heat, light, water, oxygen, and micro-organisms.

The polymerization proceeds to nearly 100% percent completion in water, so there is littlevolatile organic content to the dispersion, resulting in an environmentally friendly system.

The Advantages of Polyvinyl Acetate Ethylene Based Adhesives

Adhesion to a wide variety ofsurfaces

High Molecular WeightPolymers

Highly Resistant to HeatHumidity and Water

High Solids at Low ViscosityRapid setting speeds

High levels of Wet Tack

Resistance to Water , Heat and

Solvents can be adjusted easily

Good Machining Properties

Ease of Compounding

Micro-organism resistant

Oxidation Resistant

Non-Flammable

Low Volatile Organic Content

Environmentally Friendly


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3

An additional reason for their growth and popularity is the ease with which the vinyl acetatehomopolymer and copolymer dispersions are transformed into useful, low cost adhesives. One

can develop excellent adhesives by blending common ingredients in relatively simple mixing

vessels. The dispersions can be modified to become slightly water sensitive adhesives allowingeasy cleanup and good machining properties when applied by rollers, nozzles, or spray systems.Conversely, their properties can be modified for improved water resistance, heat resistance or

structural strength also via simple blending.

Adhesive properties that are not capable of being developed through blending are fundamental

polymer dispersion characteristics and are developed by the raw material manufacturer. The

selection of the proper polymer dispersion or blend of dispersions is the key to a successful

adhesives development project. The choice of the proper raw material dispersion can best beaccomplished by examining and understanding four basic aspects of the dispersion i.e., the bulk

polymer properties, any functionality added to the polymer, the colloidal properties and the

particle size distribution. Each aspect of these dispersion properties will be covered in detail forethylene vinyl acetate copolymer dispersions in this chapter.

The Properties of Adhesion Polymer Dispersions

Bulk Polymer Properties

Chemical Functionality

Colloidal Protection System

Particle Size Distribution

1. The Properties of Adhesion Polymer Dispersions: Bulk Polymer PropertiesThe bulk properties are the properties of the polymer within the dispersed polymer spheres. The

chemical composition along with the molecular weight and molecular structure of the polymer

are responsible for the tensile strength, the adhesion characteristics, the toughness, the flexibility,and the heat, oxygen, and water resistance of the bondline. The bulk properties include the

chemical composition determined by the selection of the type of monomers used, the relative

amounts or ratios of the monomers used and the molecular weight of the polymer as determined

by the production process.

A polymer is a chain of molecular links. In the case of vinyl acetate, the chemical schematic of a

vinyl acetate molecule is below. Individually monomers are separate links waiting to be formed

into a continuous chain. Vinyl Acetate monomer has a polar acetate end group and a vinyl ordouble bond in the body of the monomer.


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4

Polymer Bui ld ing:Polymer Bui ld ing:

Monom ers Determin eMonomers Determine

Bulk Prop ert iesBulk Prop ert ies

O

C O

CH3

O

C O

CH3

Vinyl Acetate Monomer Molecules

VAc VAc

Figure 1

It is the vinyl double bond which can chemically open up and cause a monomer molecule to

polymerize with its neighbor. The acetate group does not change in the process. Essentially, a

polymer is a macromolecule consisting of a large number of these chemically combined unitscalled monomers linked together as a chain

By the careful use of a free radical initiator we can join the individual monomer molecules

together in a particular order to form a high molecular weight polymer. Because of the similarityin their structure the vinyl acetate molecules line up to form a regular ordered chain. The order

and consistency of the groups results in a closely aligned polymer chains which reinforce each

other. As a result, poly (vinyl acetate) homopolymer is hard and cohesive.


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5

Polymer Bui ld ing:Polymer Bui ld ing:

Monom ers DeterminesMonom ers Determines

Bulk Propert iesBu lk Propert ies

PVAc PVAc PVAc PVAc

O O O O

C C C CO O O O

CH3 CH3 CH3 CH3

Poly(vinyl acetate)

Polymer: Very Orderly , RegularPolymer: Very Orderly , Regular HardHard

Figure 2

Glass Transition TemperatureA simple way to measure the hardness of a copolymer is to measure the glass transition

temperature of the polymer or Tg. The Tg is the temperature at which the polymer turns from an

amorphous polymer to a polymer glass. Hard polymers have a high Tg; while a flexible polymer

has a low Tg. Because of the closely ordered and aligned chains, pure vinyl acetate

homopolymer has a Tg of about 32 C. The polymer is very glass like at temperatures below

32C. Because we are dealing with a very ordered system which has a high degree of attractionfor each acetate group, poly (vinyl acetate) has a minimum spacing between the chains. This

minimum spacing and an attraction between the groups produces a strong internal cohesive forcewhich limits the rotation and flow of the polymer.

Conversely, the Tg or glass transition temperature is also a measurement of the relative flexibilityof the vinyl acetate polymer, a vinyl acetate/comonomer mixture or a vinyl acetate polymer

/plasticizer mixture. The Tg of vinyl acetate homopolymer is approximately 32C, making it a

quite hard, nearly brittle polymer and a marginal film former at room temperature. Hardness or

brittleness may not be desirable in many instances. Flexibility is usually imparted in some way

to a vinyl acetate homopolymer system. A comonomer or external plasticizer is usually added to

lower the Tg and give flexibility to the adhesive. Most adhesion and film formationimprovements can be made by flexibilizing the polymer and lowering the Tg.

Anything we do to diminish the order or regularity of the chain, lowers the cohesive forces of the

polymer simultaneously increasing the molecular flow, the flexibility and the adhesive ability of

the polymer. We can disrupt this order within vinyl acetate homopolymer by two methods,increasing the spacing between the chains and by disrupting the attraction between the acetate

groups.


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6

External PlasticizationFunctionally, the polymer spacing can be altered by external plasticization by the adhesives

manufacturer. A common way to increase the polymer spacing between the chains and flexibilize

the polymer by reducing inter-chain attraction and allowing the chains to Figure over each otheris the introduction of plasticizer during the compounding step. Plasticizer penetrates the polymersphere and swells the polymer. Because of the chemical compatibility and similarity between

the plasticizer and the polymer, the plasticizer molecule diffuses within the sphere and spreads

the polymer chains apart, reducing the cohesion of the resulting mixture; lowering the Tg andimproving the flexibility and film formation characteristics. This is one reason most vinyl

acetate homopolymer adhesives are commonly formulated with plasticizer.

Plasticizer Molecule

Polymer Spac ing

Determines Polym er

Flexibi l i ty

O O O O

C C C CO O O O

CH3 CH3 CH3 CH3

O O O O

C C C CO O O O

CH3 CH3 CH3 CH3

Figure3

Where rigidity is valued, as in a structural adhesive market - like the wood working adhesives

market, unplasticized homopolymer formulations can be desired to promote high fiber tear andstrong cohesive strengths. Having too hard a film can be self-defeating since optimum film

formation characteristics are needed for best performance. The polymer spheres must flow and

coalesce to form a strong film. Too high a Tg can result in a poorly formed film with poortensile strength.


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For paper and most cellulosic composites, the need for ultimate tensile strength is not great; since

the composite materials tear easily compared to wood. Film formation and the rate of filmformation are now important because the film formation rate is related to how fast a fiber tearing

bond is developed. Industrial processes, with high productivity demands and very fast machines

, require fast film formation rates. Early in the development of vinyl acetate polymers andcopolymers the need to flexibilize and build fast film formation rates was great.

Internal Plasticization

There is a second way of increasing the polymer spacing between the chains to flexibilize thefilm. An alternate monomer with a bulky side group was polymerized into a vinyl acetate

polymer.Polyme r Composi t ion

Determines Poly mer Flexibi l i ty

O C O O

C

CH3

O C CO

O

O O

CH3 CH3CH2

CH2

CH2

CH3

O O C C

C C O CO O O

CH3 CH3 CH2 CH3

CH2

CH2

CH3

Polyvinyl Acetate/Butyl Acrylate copolymerFigure4

This diagram shows butyl acrylate as a comonomer, which effectively hangs a pendant butyl

group from the side of the polymer. The chains spread apart to allow for the spacing necessary

to make room for the butyl group. This extra space between the molecules breaks the attractionof each chain for each other. They are now capable of sliding over each other and flowing

easily. The resulting copolymer is softer, more adhesive, lower in Tg and less cohesive. It can

form films at room temperature easily since the film is forming above the glass transitiontemperature, rather than near or below the Tg. The polymer spheres coalesce easily since they

are softer and flowable rather than hard and glass like.


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These are some other examples of useful vinyl monomers which can be copolymerized with

vinyl acetate. The length or size of the pendant group affects the chain distance and influences

the Tg of the resulting polymer. The short pendant group monomers have high Tgs. The longerpendant lengths have lower Tgs

PolymerPolymer

Compos i t ion:Compos i t ion:

Viny l Monom ersViny l Monom ers

O HC4H

9CHCH

2 2C CO

2CH

2CHC

4 9

C2H5 C2H5

ClOR

O

CH2 CH2O CH3

O

Dioctyl Maleate

Vinyl Chloride Acrylates

EthyleneVinyl Acetate

Polymer: Break up symmetryPolymer: Break up symmetry Add PropertiesAdd Properties

Figure 5

They are all polymerized through their double bonds to build the polymer chain. In addition to

vinyl acetate, the other monomers shown are comonomers which can be polymerized with vinyl

acetate. A large the pendant group in the acrylate family will drop the Tg greatly; a small

pendant group will not drop the Tg of a copolymer appreciably. The Tg of the vinyl chloride

polymer is higher than the Tg of vinyl acetate polymer. With the exception of vinyl chloride, allsoften vinyl acetate polymers and produce improved flexibility and a lower Tg. We can

numerically quantify the amount a comonomer will drop the Tg by comparing the Tg s of the

pure polymer homopolymer


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The Tg of the homopolymer can used as a guide of whether the comonomer is increasing the Tg

of the final copolymer or decreasing it. A Tg higher than vinyl acetate will raise the overall Tgof the copolymer where a lower Tg will decrease the over polymer Tg

Monomer Tg of HomopolymerVinyl Chloride 81 C

Vinyl Acetate 32 C

n-Butyl Acrylate -50 C

2-Ethyl Hexyl Acrylate -66 C

Ethylene -89 C

The relative Tg of each monomer homopolymer can give an idea of the effectiveness of thecomonomer at lowering the Tg of a blend with vinyl acetate. 2-ethylhexyl acrylate introduces a

large, bulky 2-ethylhexyl group when copolymerized on the polymer chain. Its homopolymer has

a Tg of -66C which is very low. A low Tg is an indication of a highly efficient monomer

capable of dropping the Tg significantly per mole percent added. Butyl acrylate introduces aslightly smaller butyl group to the side chain so it is not as flexible per unit mole of monomer as

2-ethylhexyl acrylate. Vinyl Chloride will raise the Tg of the copolymer because thehomopolymer Tg is 81C, which is higher than the 32C Tg of Vinyl Acetate,. Ethylene

introduces a space in the polymer chain and has the lowest Tg of the common comonomers.

The Case for Ethylene:An alternate way to softening the polymer by spreading apart the polymer chains is to disrupt the

order of the acetate groups. When we copolymerize ethylene into the polymer backbone there is

no pendant side group. Ethylene leaves a space, a hole or a swivel point in the polymerbackbone breaking the uniform pattern of the acetate groups.

Polym er Uniformity and

Spacing Determines

Polym er Flexibi l i ty

O O O

C O C O C O

CH3 CH3 CH3

O

C

CH3

O

O

C

CH3

O

O

C

CH3

O

Polyvinyl Acetate/Ethylene CopolymerFigure 6


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The use of ethylene as a Comonomer to suppress the Tg of the vinyl acetate polymer backbonerequire the use of levels of ethylene less than 50 mole percent. Typical VAE emulsions can use

5 to 50 percent ethylene as an additive. Vinyl acetate is the dominant monomer in these systems.

Hot Melt Ethylene Vinyl Acetate polymers are mostly ethylene in composition. Becauseethylene exhibits crystallinity and short intramolecular chain distances when it is the dominantmonomer, low levels of vinyl acetate are used in hot melt based polymers systems to break the

crystallinity of ethylene based polymers. In hot melt, ethylene vinyl acetate systems the vinyl

acetate levels needed to break the ethylene crystallinity are of the levels 18 to 40%

Using ethylene to disrupt the regularity of a vinyl acetate based polymer does not spread the

polymer chains apart since there is no bulky side group. It does break the regularity of the chain

by inserting spaces or holes in the pattern. This reduces the attraction of the acetate groups foreach other by disrupting the distance between them. Because the polymers can now rotate but

the spacing between the chains is the same, the flexibility is improved but the heat resistance is

not substantially impacted. This is the main chemical reason why the ethylene copolymers arewidely commercialized in the marketplace. There is a minimal impact on the cohesive strength

for the improvement in flexibility with ethylene as the flexibilizing comonomer.

Copolymerization with ethylene is a way to internally flexibilize the spheres. The higher theethylene level the greater the internal disruption of the attraction between the groups and the

more flexible is the resulting polymer. The Tg drops as the amount of comonomer increases;

flexibility improves, and adhesion improves.

By improving the flexibility of the polymer, the ability to flow and wet out a surface is greatly

improved, so the adhesion is improved. The film formation and the water resistance is improved

if the spheres flow out and wet out to form a tight continuous film. Conversely, heat resistance,tensile strength and cohesiveness are all measurements of the internal attractive forces within the

polymer. A higher Tg favors these properties


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27

A Typical Tg Curve

Versus Weight

Percent Comonomer

WT. PERCENT ETHYLENE

Tg

in

DegreesCelsius

0

5

10

15

20

25

30

35

0 5 10 15 20 25

Tg

Figure 7

This graph approximately illustrates the drop in Tg and the increased flexibility of a combination

of ethylene with vinyl acetate. Similar curves can be developed for acrylics and even for the post

addition of plasticizers. The slope of the curve would change according to the effectiveness andcompatibility of the plasticizing additive or comonomer. No slope would be as effective as

ethylene.

The Effect of Ethylene as a Comonomer

Decreasing In

creasing

Water Resistance

Ethylene Content

Adhesion to Plastics

Flexibility

Cohesive Strength

Hydrolysis Resistance

Figure 8


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The physical/ performance characteristics change according to the percent ethylene addition. In

addition to the adhesion improvement, the water resistance of the polymer film increases due tothe hydrophobic nature of ethylene. Ethylene doesnt hydrolyze under alkaline conditions so

hydrolysis resistance is improved.

Hydrolysis resistance also improves since the ethylene introduces a space or hole in the vinylacetate segments. Under alkaline conditions, polyvinyl acetate will hydrolyze, exchanging the

acetate group as an acetate salt while replacing it with a hydroxyl group. This is the basic

reaction of the formation of polyvinyl alcohol. An adjacent acetate group will then continue inthe hydrolysis reaction. The resultant process rapidly unzips the polymer; resulting in the

conversion of the polyvinyl acetate to polyvinyl alcohol. This susceptibility to hydrolysis

explains why polyvinyl acetate homopolymer is not recommended for performance under

alkaline conditions. Copolymerization with ethylene, even more than with acrylic14omonomers, effectively stops the hydrolysis reaction. The ethylene position will not promote

the release of an adjacent acetate group easily. The energy to continue the hydrolysis reaction is

higher in this form than in polyvinyl acetate homopolymer. This is why a common use for vinylacetate ethylene copolymers is as a cement modifier or additive. They are resistant to hydrolysis

compared with standard vinyl acetate or vinyl acrylic copolymers polymers. In fact, test films of

vinyl acetate-ethylene copolymers have been exposed to weak alkalis and acids for more than a

year without showing any signs of deterioration.

A wide variety of commercial ethylene contents are available in the industrial dispersions

marketplace. Aside from the chemical benefits of ethylene like hydrolysis resistance or waterresistance, the general physical tradeoffs described below are cohesiveness versus adhesiveness.

Name Tg

C

Peel Adhesion to

PVC foil

N/cm

Cohesive

Strength

N/mm2Poly(VinylAcetate)

Homopolymer

35 Nil 25

Airflex 315 * 17 2.5 13Airflex EP400 5 9 8

Airflex EP401 -15 10 4

Airflex is a Trade Mark of Air Products Polymers, LP

Lowering the Tg by increasing the ethylene content increases the 180 angle peel adhesion value

of a PVC foil wet bonded to plywood. The cohesion values, as measured by tensile strength,decrease in a similar fashion. The beneficial effect of increased ethylene upon adhesion reaches a

point of diminishing returns. At a Tg below 0C, the amount of incremental adhesion

improvement becomes limited. It is the molecular weight of these polymers that limitsadditional adhesion improvement.


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Branching:

In addition to the compositional aspects of the bulk properties, the molecular weight of thepolymer is also a bulk characteristic. In the case of vinyl acetate polymerization, the chain

length or molecular weight is extremely high. In addition to the normal linear chain growth,

branching from the main chain as a side chain or a possible bridge between two chains can occurduring polymerization. Branching or grafting can also link through the protective colloid.Internal branching can increase the molecular weight further by covalently linking the chains of

the polymer within the sphere together. The molecular weight or percent grafting correlates well

to the percent insolubles in a true solvent, like toluene. Highly crosslinked, insoluble vinylacetate polymers and copolymers have very good heat resistance; but have reduced polymer cold

flow properties at room temperature. They take longer to form perfect, continuous films. Lower

molecular weight polymers have more ability to flow at room temperature so their film formation

ability is faster along with their wetting/ flow and adhesion characteristics.

Struct ure of GraftedStruc ture of Grafted

Poly(v iny lPoly(v iny lacetate)acetate)

O O O O

C C C CO O O O

CH3 CH3 CH2 CH3

O O

C CO O

CH3 CH3

Polymer Grafting Determines

Polymer Cohesiveness

Figure 9

Branching increases the cohesive strength of the polymer and can raise the Tg of the polymer

slightly by limiting the polymers flow.


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The effect of increased polymer branching or grafting results in:

The Effect of Increased Polymer Branching

Increased Tensile Strength/ ToughnessIncreased Heat Resistance

Reduced Tackiness of a Dried Film

Increased Solvent Resistance

Increased Acceptance of Polar and Water-Miscible Solvents

This internal grafting or crosslinking increases the tensile strength and the heat resistance of the

polymer. It decreases the solubility of the polymer in polar solvents and reduces the surfacetackiness of the polymer. Grafting can be promoted by the use of poly (vinyl alcohol) as a

colloid and by altering various process variables. In the case of the polyvinyl alcohol stabilized

vinyl acetate ethylene copolymers (VAE), the reduced polymer flexibility limits the polymerwetting and keeps adhesion from continually increasing with increasing ethylene content.

2. The Properties of Adhesion Polymer Dispersions: Chemical Functionality:

Successfully developed adhesives balance the cohesive strength and the adhesive strength of the

formulation to the strength and chemical characteristics of the substrates. Chemical functionality

directly impacts the adhesion aspect of the bond. Since cohesion is related to composition, Tgand other bulk characteristics, it was previously discussed. Added chemical functionality

generally improves the adhesion / cohesion balance, if desired. Prior to optimizing a formulation

for a substrate; a discussion of bonding is necessary.

The adhesives market hosts formulations based upon water-borne polymers, solvent solution

polymers, hot melt polymer systems and 100% solids curable systems. The basic principles of

adhesive bonding apply to all systems. Ideally, the combined forces of adhesion and cohesionshould exceed the tearing strength of the substrate. Of the three modes of failure shown below,

substrate failure is the most desired. Because bond failure could be adhesive or cohesive, proper

bond performance is a formulation balancing act.


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Adhesive BondAdhesive BondFailure TypesFailure Types

Adhesive Glue Line

Substrate Failure

Adhesive Glue Line

Adhesion

Failure

Cohesion Failure

Adhesive Glue Line

Figure10

Anything that interferes with adhesion or substrate wetting reduces the ultimate bond strength.

Because wetting is polymeric flow or wet-out on a substrate, it is highly dependant upon the

liquid or viscous characteristics of thermoplastic polymers. The cohesive forces, often referred

to as elastic forces, are the solid-like properties of the polymer. The cohesive properties areresponsible for physical tensile strength characteristics and the durability under heat or water

exposure. Unnecessary reductions of cohesive strength also reduce the ultimate bond strength.

The optimum bond strength occurs when the proper combination of adhesive strength andcohesive strength exists in a bonded system. The interaction of the adhesive polymer and the

strength of the substrates determine the optimum point.


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0

5

10

15

20

25

30

35

0

Adhesion - CohesionBalance

Increased Crosslinking

Adhesion Cohesion

Bond Strength

Figure 11

There are two phases, which can be addressed by the crosslinking process, the bulk polymer, and

the polymer sphere interface. Intra-particle crosslinking is impacted by the bulk properties of thepolymer used. The polymerization process can be adjusted, along with the basic building block

components to produce a wide variety of dispersed polymers, with many degrees of flexibility

and strengths within the polymer bulk properties. The adjustments, which can be made duringpolymerization, include controlling the molecular weight, adjusting the Tg, adding an internal

crosslinking monomer, and producing internal grafts between polymer chains. This is sometimes

referred to as increasing the gel content or insolubles of a polymer. The polymerization processcontrols all these during manufacture. There is very little that a formulator can do to change the

bulk properties of a polymer dispersion. He can only choose polymers based upon the

information provided.

Chemical Functionality and Crosslinking

Building both cohesive strength and adhesion always results in a higher optimized tensile

strength for a bond. Improvements in the overall balance of polarity vs Tg can be obtained by the

incorporation of a third monomer into a vinyl acetate copolymer dispersion. The most commonfunctional additive monomers would incorporate a carboxyl, amide and Comonomer or hydroxyl

functionality into the polymer backbone.


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Carboxyl FunctionalityA polymer can be chemically modified with a reactive acidic Comonomer to add carboxylgroups into the polymer backbone. The acidic nature of a Comonomer, like acrylic acid, yields

substantial adhesion benefits, especially on metallic foils or other polar surfaces. It also adds a

chemical site on the polymer backbone which can be used for crosslinking and changing thecohesive nature of the polymer through formulation.

Structu re of GraftedStructur e of Grafted

Poly(vinylPoly(vinylacetate)acetate)

O O O O

C C C CO O O O

CH3 CH3 CH2 CH3

O O

C CO O

CH3 CH3

Polymer Grafting Determines

Polymer Cohesiveness

Figure 12The incorporation of carboxyl functionality in a vinyl acetate-ethylene dispersion offers three

advantages over non-carboxylated products. They are: increased adhesion to metals and

polymeric surfaces; reactive sites for cross-linking; and a means for thickening.

Airflex 426 and Airflex 465 dispersions have similar glass transition temperatures with similar

ratios of vinyl acetate to ethylene. They are both stabilized with the same colloidal systems andare manufactured with the same process. The essential difference between the two is that Airflex

426 dispersion contains a carboxyl functional comonomer and as a result adheres much better to

metal substrates than Airflex 465 dispersion.

Annealed and tempered aluminum foil may be bonded to natural kraft paper with Airflex 426

dispersion. The bonds are strong enough to resist 24-hour water immersion. Carboxyl

functionality also increases the adhesion of vinyl acetate-ethylene to several metals.


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Improving the Adhesion to metals via Carboxylation

Airflex 426 Airflex 465

Aluminum 3.0 pli* 2.2 pliBrass 4.2 pli 2.6 pli

Galvanized Steel 2.5 pli 1.4 pli

Cloth to Metal T-Peel (lb/in)

If sufficient carboxyl functionality is present the dispersion will be capable of self-thickeningwith a change in pH. Raising the pH of Airflex 426 dispersion will increase its viscosity. . The

carboxylic acid on the polymer chain is neutralized by the alkali, producing similar ionic

charges, which repel each other, causing the chain to uncoil. Additionally because of the

presence of a neutralized salt on the polymer backbone, the polymer absorbs water and swells,

resulting in a viscosity increase. Uncoiling of the polymer chain can also increase its adhesion tometal substrates because there are more carboxyl groups available for bonding. There is,

therefore, a direct relationship between pH vs. adhesion peel value as well as pH vs. viscosity.

14

Effect of pH on the

Viscosi ty o f Carboxylated

VAE Emulsio n

pH

VISCOSITY(CPS)

0

500

1000

1500

2000

2500

3000

3500

4.0 4.5 5.0 5.5 6.0 6.5

VISCOSITY

Figure 13


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It is necessary to properly buffer a VAE at a high pH to avoid the natural drift towards lower pH

that occurs with the hydrolysis of residual vinyl acetate monomer to acetic acid at a high pH.

The aging characteristics of alkaline VAE formulations need to be monitored for that reason.

If available, functionalities like carboxyl groups can be crosslinked with post added crosslinkers.

Crosslinking will increase the cohesive strength of an adhesive but it may reduce the polymers

flow characteristics and thereby reduce the adhesion. Care must be taken to evaluate the changein both adhesion and cohesion when exploring crosslinkers. Crosslinkers may be used as 1 part

(storage stable) or 2 part systems (mixed systems with a pot life)

Crosslinking Agents

Agent Functional

GroupCrosslinked

Functional

GroupCrosslinked

Hydroxyl Carboxyl

Glyoxal Yes No

Isocyanate Prepolymers Yes No

Zinc Oxide No Yes

Magnesium Oxide No Yes

Zirconium Ammonium

Salts

No Yes

Urea Formaldehyde Yes No

Melamine

Formaldehyde

Yes No

Aziridine No Yes

The phase which can be addressed by the formulator is the interface at the surface of the

particles. When failure is at the interface between the particles, crosslinking can be very

beneficial to developing higher performance systems. Since the polymer interfacial strength isusually weaker than the bulk polymer characteristics, crosslinking between the polymer spheres

can address the weakest link of an adhesive and improve strength. If the polymer is modifiedwith a chemically reactive functional carboxyl comonomer during the polymerization, the

formulator can be left with a very significant spot to perform some crosslinking chemistry. A

common method of crosslinking carboxylic acid functionality is with metal salts. Example metal

salts can include salts of magnesium, calcium, aluminum, zinc and zirconium.


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CrosslinkingCrosslinkingCarboxylic AcidCarboxylic Acid

Groups with MetalGroups with MetalSaltsSalts

COOH

CH2

CH2

+

++

METALor Higher

Anion -

COOH

CH2

CH2

+

Anion -

H +

H +

COO -

CH2

CH2

++

METAL

or Higher

Anion -

-OOC

CH2

CH2

Anion -

Figure 14

The inter-particle bonds are formed as the acid functionality at the sphere surface bonds with

metal ions of opposite charges. There is a strong ionic attraction between a multi-valent metal

ion and an acid group. This can serve to increase the inter-particle strength. The interfacialregion between the spheres is usually the weakest link with water exposure. Since the colloid

and surfactant still present at the spherical interface remain water sensitive, they are plasticized

and solubilized heavily during water exposure. Crosslinking with di-valent and tri-valent metalscan improve the water resistance performance of a dried adhesive polymer film. Tri-valent

Aluminum ions seem to yield the highest performance improvement in water resistance testing

or common metal ions tested. In heat resistance testing, Aluminum performs similarly to othermetal ions like Calcium, Magnesium or Zinc. A test of various anions of aluminum salts show

that all aluminum salts have better water resistance and heat resistance than the controls. The

major performance enhancer seems to be the Aluminum ion itself.


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Amide and Acrylamide Functionality

Poly (vinyl acetate) Homopolymer with

N-methylolacrylamide

Self Cross l inkingSelf Cross l inkingFunct ional i tyFunct ional i ty

VAc

VAc

NMA VAc

O

C O

CH3

C O

CH3

O

C O

CH3

O O

C O

CH3

C O

NH

CH OH2

Figure 15

Amide functionality can promote increased adhesion to polar surfaces. Amides and acrylamidefunctionality can also be crosslinked with external crosslinkers in a manner similar to carboxyl

functionality. Where the carboxyl functionality crosslinked with metal salts is an ionic effect,

impacting primarily heat resistance; crosslinking amide functionality is a covalent bond whichwill generally improve the water resistance of an adhesive. Common two part crosslinking

systems which react to amide groups are melamine formaldehyde or urea formaldehyde

technologies. If the reactive group on the polymer is a methylolated acrylamide, like N-methylol

acrylamide (NMA), the polymer becomes capable of self crosslinking through self condensationwith heat and/ or a mild acidic catalyst. This reaction commonly liberates formaldehyde as a by-

product of the self crosslinking reaction. Most of these crosslinking systems have a limited pot

life and are designated as two part systems

Hydroxyl FunctionalityHydroxyl functionality can be incorporated into a polymer in two ways. A hydroxyl functional

monomer can be copolymerized with the vinyl acetate and ethylene to place a pendant hydroxylgroup on the polymer backbone. Hydroxyethylacrylate is a common comonomer capable of

adding primary hydroxyls to a polymer. If the colloidal system is purely surfactant based, this is

a good method to bring hydroxyl functionality into the polymer.


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The protection system usually has some functionality associated with it because the hydrophilicportion of the surfactant / colloid molecule must be present to be attracted to the water phase.

The second form of hydroxyl functionality can come from the colloidal protection system itself.

It may be possible to perform some chemical reactions on the protective colloid present,depending upon its nature or chemistry. Polyvinyl alcohol is an extremely popular protectivecolloid, as it has many desirable rheological characteristics for the adhesives market and is very

compatible with all vinyl acetate copolymer systems. Polyvinyl alcohol is a prime source of

hydroxyl functionality.

The Emu ls ionStab i l i zer as a Sou rc e

o f Func t iona l it y

PVAc

PVOH

PVAc

PVOH grafted

Increase mol. wt.

Surfactant ProtectedSurfactant Protected

PVOH ProtectedPVOH Protected

-

-

-

-

-

-

-

-

-

Increase mol. wt.

Figure 16

The protective colloid might be capable of being insolubilized or crosslinked, depending on

functionality present on the molecule. During the polymerization process the polyvinyl alcohol isactually grafted to the polymer surface by hydrogen abstraction through the residual acetate

groups. It is an integral part of the polymer, as if it were a hydroxyl containing comonomer.


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Poly(vinylPoly(vinyl acetate)acetate)With GraftedWith Grafted

Poly(vinylPoly(vinyl alcohol)alcohol)

OH OH O OH

C O

CH2

O O

C CO O

CH3 CH3

Figure 17

Surfactants usually exist prior to polymerization as small soap concentrations called micelles.

The monomer can absorb into the micelles, swelling them. There may not be significant graftingof surfactants onto a polymer surface to consider them as sites for crosslinking. Their attachment

is not chemical in nature, merely associative.

A very effective two part crosslinker for crosslinking hydroxyl functionality is the use of

isocyanate prepolymers. These are two part systems where 5 to 15 % of an isocyanate

prepolymer is added to an adhesive prior to bonding. The mixed system can have a useful potlife

of 1-4 hours. When formed within this useful potlife range, the resulting bonds demonstrate highwater resistance. This is the basis for EPI systems (Emulsion Polymer Isocyanate). Wood or

metal to wood bonds made with EPI adhesive systems resist boiling water and have a high

degree of environmental exposure resistance. The use limitations on potlife make one part,

storage stable crosslinking systems desirable. Frequently there is a tradeoff involving a lowerlevel of performance vs two part systems.


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A mild crosslinking system that is a stable one part crosslinker is boric acid. At a pH under 5.0,

boric acid will crosslink adjacent hydroxyl groups on the same molecule reducing the water

sensitivity and creating a tacky rheological flow. Frequently, adhesive compounders will takeadvantage of the boration reaction with poly (vinyl alcohol) stabilized dispersions to createquick-tacking adhesives. This tacky gel can hold substrates together while wet increasing green

strength or wet tack

Crosslinking PVOHCrosslinking PVOHwith Boric Acidwith Boric Acid

CH2

CHOH

CHOH

2

HO

B

OH

HO OH

+

O

B

O

O O

CH

CH2

CH

CH

CH2

CH

+ H2O4

Borate Reaction pH > 5

CH2

CHOH

CHOH

HO

B

HO

OH

+

O

B

O

CH

CH2

CH

OH H2O2

+

Borate Reaction pH < 5

Figure 18

The boration reaction is very pH sensitive. Below a pH 5, PVOH forms a monodiol with theboric acid group which provides a storage stable "tacky" structure. As the pH is raised above 5

the chemistry of the crosslinking changes dramatically. Above a pH of 5, the borated dispersion

will begin to rise in viscosity. The boric acid ion turns into a borate ion and increases its

attraction to two polyvinyl alcohol molecules. The result is a tight insoluble gel. This is areversible reaction with pH.

The table of organic crosslinking agents for the hydroxyl functionality also includes

formaldehyde resins and glyoxal along with a variety of other agents. Most formaldehyde typesof crosslinking agents are very effective in crosslinking hydroxyl functionality. They tend to be

useful as a two-part system where the crosslinker is added directly before the adhesive is used.

These can then crosslink slowly after the bond is made and wetting has occurred to greatly


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enhance performance. As one-part systems, they tend to cause a rise in storage viscosity so their

potential may be limited to two component packages.

PVOHPVOH--

Commercia lCommercia l

Crossl inkersCrossl inkers

Glyoxal Crosslinks TwoGlyoxal Crosslinks Two

Hydroxyl containingHydroxyl containing

polymerspolymers

OH OH

+

H

H

O

O

H+

O

O O

O

n

2

Figure 19

Glyoxal is potentially useful as a one-component system. The crosslinking chemistry proceeds

in accordance with this reaction. It is very powerful reaction and should be used at low levels topreserve storage stability. Again an exploration of the performance benefit of glyoxal as a

crosslinking additive reveals the cohesion/ adhesion balance trade off.


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The Adhesion /The Adhesion /Cohesion BalanceCohesion Balance

Glyoxal with a VAEGlyoxal with a VAEEmulsionEmulsion

0

2

4

6

8

10

12

14

16

18

0 1% 2% 4%

77C HeatResistance x 100(mm/min)

PVC Adhesion

Wt.% Glyoxal

CreepResistanceat77C,

mm/min

Figure 20

The same theme seems to run through these evaluations, i.e. cohesion and adhesion are

antagonistic trade offs. Increasing one usually reflects a decrease in the other. Fortunately,compromise is usually achievable.

3. The Properties of Adhesion Polymer Dispersions: Colloidal Protection SystemThe dispersion polymer is a bit more complex than a solution or 100% solids systems because of

the multiple phases present, namely a polymer solids phase dispersed in an aqueous liquid phase.

To promote stability the particle protection system is present at the interface of the two phases.


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Polymer SpheresDispersed in AnAqueous Media

Protective Colloid

Polymer Phase

A Two Phase SystemA Two Phase System

Liquid Phase

Figure 21

Its presence is needed to keep the spheres apart and separated during manufacture, compounding,

and during storage / handling. If the spheres were to collide and aggregate they would form gritor lumps. They would not be stable towards pumping, application, or materials handling.

The surfactant or protective colloid can use ionic charges or non ionic steric hindrance to keep

the spheres from colliding. There are two general types of protection systems i.e., colloidalpolymers and actual surfactants. Mixtures of both systems are also used to generate combined

propertiesProperties of Various Protection Systems

Surfactants Colloids

Possess a CMC Yes No

Graft to the Particle Possible Yes

Ionic Character Possible No

Molecular Weight Low High

Solution Viscosity Low High

Surfactant Protection/ Stabilization

Surfactants are members of the soap family. They have a hydrophilic end which orients itself

toward the water phase and a hydrophobic end which orients itself toward the polymer phase.Because of their limited solubility in either phase they congregate at the interface of the phases.

They may exhibit a property called micelle formation. This property can create tiny aggregates

of the surfactant in the water phase once a certain concentration is reached. This is the criticalmicelle concentration or CMC of a surfactant. These micelles can act as seeding points for the

formation of polymer particles during the polymerization process. Micelles are small so

polymers created within the micelles tend to be of a small particle size. They may anchor to theparticle by grafting into the polymer as it grows.


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Surfactants may be nonionic or ionic. The characterization depends upon the method of

solubilization that is used in the molecule. A nonionic surfactant will orient itself due to thepolarity of the various functional natures built into the surfactant but it will not actually possessan ionic charge. Nonionic surfactants can be based upon polyethylene oxide or poly propylene

oxide

If an ionic surfactant possesses a negative charge on the hydrophobic group to help stabilize the

polymer particle it is called an anionic surfactant. The ionic charge will always orient toward the

water phase and possess very good solubility in that phase. If the hydrophobic side possesses a

positive charge the surfactant is designated cationic. If a molecule has both charges associatedwith it, it is called amphoteric. Each type of surfactant has different emulsification capabilities,

different attractions toward substrate surfaces, and different stabilities toward chemical additives.

Higher molecular weight polymers that are water soluble but have no micelle formation

capability are usually designated by the term colloids. Because of differences in polarity or

solubility within the polymeric molecule they orient themselves at the particle surface between

the polymer and aqueous phase. Water soluble, colloidal polymers that may be used arepolyvinyl alcohol, cellulosic polymers, starches, dextrins and natural gums. Depending upon the

emulsification ability of each polymer; a costabilized system with a surfactant may be required.

Each is a high molecular weight polymer which adds viscosity to the dispersion. The particlesizes are generally larger than pure surfactant stabilized dispersions. They generally have higher

surface tensions.

.

Colloidal Polymer Protection Systems

Polyvinyl alcoholPartially Hydrolyzed

Polyvinyl alcoholFully Hydrolyzed

HydroxyEthyl Cellulose

Dextrin, Starches

Natural Gums


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Protective Colloids

Hydroxyethyl cellulose, dextrin, starch, gum arabic are used to create specialized dispersionswith unique properties. Each has a unique contribution to the rheological and adhesive

properties of the resulting dispersion. As an example, HEC imparts a thixotropic rheology with

added water resistance and chemical stability. Dextrins and starches impart extra watersensitivity for remoistenable gums, envelopes or tapes. These protection systems usually formthe basis for the family of specialty vinyl acetate homopolymers and copolymers because of the

different properties imparted by the colloidal system to the dispersion.

Polyvinyl alcohol, because of the similar nature to poly(vinyl acetate) is the most common

colloid in the vinyl acetate adhesives market. It comes in a number of degrees of hydrolysis

which affects the emulsification capability of the polymer. The partially hydrolyzed variety has

the best emulsification tendencies and the lowest surface tension of the poly(vinyl alcohol)family. It is the most commonly used colloid. If a high hydrolysis variety is used for water

resistance, a low level of surfactant is sometimes added to enhance the emulsification capabilities

of the overall colloidal protection system.

The chart describes the nature of the emulsifying systems effect on various application

properties. The range of properties affected is broad because there can be radical variations in

average particle size and particle size distribution among these types.

The Effect of Emulsifier on Adhesive Properties

Colloidal Protection Characteristic Surfactant

Protection

Large Particle Size Mean Small

Narrow to Large Particle Size StandardDeviation Narrow

Strong Wet Tack Low

Good Machining Poor

Rapid Speed of Set Slow

Newtonian,High Viscosity

Rheology Newtonian,Low Viscosity

The table above compares the dispersion properties of Colloidal protection systems vs.

Surfactant Protection systems in six different aspects; Mean Particle Size, Particle Size

Distribution, Wet Tack, Machining Properties, Speed of set, and Flow rheology. In general

surfactant protection produces smaller particles with a high surface area. These generally setslower and have lower viscosity than an equivalent colloidally stabilized dispersion.

Polyvinyl acetate can be protected with colloid systems, surfactant systems or a combination ofboth. The type and amount of protection used in the polymerization of polyvinyl acetate have a

great bearing on the physical properties of the resin and on the working characteristics of the

dispersion. Dispersion rheology properties are controlled by the protection system.


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A dispersion which is protected solely with a surfactant (nonionic surfactants are generally used

with polyvinyl acetate) has a fine particle size, a rather narrow particle size distribution and alow aqueous phase viscosity. Because the mean particle size of surfactant stabilized systems is

small and the water phase viscosity is low, the wet tack and setting speed of these products is

low. Their flow rheology may be thixotropic rather than Newtonian so they could shear thinwhen applied by rollers, extrusion, and spray systems. Depending upon the application system aslight amount of shear thinning could be an advantage. It must be balanced against the loss of

setting speed and wet tack development.

The most frequently used protective colloid polymer for vinyl acetate is polyvinyl alcohol. It is

the classic colloid because of a number of advantages. Since partially hydrolyzed polyvinyl

alcohol is a poor emulsifier compared with a surfactant, a polyvinyl acetate dispersion protected

with this colloid would have large particles. The molecular weight of the colloid is largecompared to surfactant systems. This large particle size in combination with a high viscosity

aqueous phase viscosity yields a fast setting, tacky, dispersion that does not change its viscosity

much as it experiences high shear conditions.

Polyvinyl alcohol is a high molecular weight water-soluble molecule. It will therefore contribute

a high degree of wet tack to a dispersion that is not possible with surfactant protection. Polyvinyl

acetate dispersions protected with partially hydrolyzed polyvinyl alcohol have a combination ofproperties usually described as good machining, which include good flow, clean running, easy

cleanup, and a non-slinging or spitting rheology. A high level of wet tack and good machining

properties allow the commercial use of these copolymer adhesives in modern applicationequipment. They work at acceptable line speeds yielding highly productive systems. Dried

films are more water sensitive than surfactant stabilized dispersions; so the clean up of

application equipment is easily accomplished. The environmental impact is minimal since

recycle adhesive bonds can be broken easily by any modern repulping processes.

Part of the highly efficient nature of these systems is due to their rapid speed of set. Colloidally

stabilized polyvinyl acetate copolymer dispersions set by losing water through evaporation intothe air and also by water absorption into the porous substrates to which they are applied. The

large polyvinyl acetate spheres in the dispersion pack together and form capillaries which drive

the water out at a rapid rate. The fine particles in an all-surfactant system are colloidally morestable; they form smaller capillaries and release water out at a slower rate. Dispersions which

combine surfactant and colloid protection will display properties that fall somewhere between

Those properties exhibited by colloid systems or surfactant systems alone


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The Advantages of Polyvinyl Alcohol Protection Systems

Good Machining

Easy Clean Up

Strong Wet Tack

Rapid Setting SpeedHigh Thickening Response

High Heat Resistance

Improved Non Block

Crosslinking Potential

.

Because the partially hydrolyzed poly(vinyl alcohol) molecule has portions which are

poly(vinyl acetate) like, the acetate groups of the colloid can participate in grafting in a similarfashion to the acetate groups within the particle. Poly(vinyl alcohol) becomes chemically grafted

to the particle surface in this process which increases the durability of the protection system to

mechanical shear and compounding ingredients. The grafted colloid can impact the properties of

a dispersion favorably. The lack of shear thinning gives a predictable response on coatingmachinery in terms of consistent glue line application.


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RHEOLOGY OFRHEOLOGY OF

PVOH Stabilized EmulsionPVOH Stabilized Emulsion

vs. an Alternate Colloidvs. an Alternate Colloid

VISCOSITY VERSUS SHEAR RATE

0

500

1000

0 1000 2000 3000 4000 5000

SHEAR RATE (1/SEC)

VISCOSITY

(CPS)

PVOH

Alternate Colloid

Figure 22

This curve shows the viscosity response of a polyvinyl alcohol protected dispersion and a nonpoly(vinyl alcohol) protected dispersion to increasing levels of shear stress. The poly(vinyl

alcohol) protected dispersion resists the shear and maintains a higher viscosity under stress than

the alternate colloid.


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4. The Properties of Adhesion Polymer Dispersions: Particle Size Distribution:

The final element to understanding the characteristics of an adhesive dispersion is the particlesize and particle size distribution. From the previous discussion it is evident the bonding

performance of a dispersion is not just based upon its chemical composition. The colloidal

protection system and the resulting particle size distribution can greatly influence the propertiesof an adhesive dispersion. The size and packing of these spheres influences the wet tack, greenstrength, the film formation, the machining characteristics and the wetting of an adhesive.

Adhesive Properties

Affected by the Particle Size Distribution

The Solids/Viscosity Relationship

Thickening Response

Setting Speed

Wet Tack

Film Formation Integrity

Mechanical Stability

A simple comparison of the solids and viscosity relationship of several dispersions at differentsolids shows how the average size, the breadth of the particle size distribution, can affect the

final properties. The wider the particle size distribution; the higher the solids must be at

equivalent viscosities. Plasticizer thickening response, setting speed, wet tack, and film

properties vary greatly with particle size distribution. Not all dispersions are alike; even thoughtheir bulk properties could be identical. There are a wide variety of performance characteristics

available to the adhesive compounder through the selection of the appropriate raw materials.

This translates to a wide variety of potential adhesives being available to perform their tasks.

The Variety of Properties of TypicalVinyl Acetate Ethylene Copolymers for Adhesives

Solids 50 to 72%

Viscosity 1000 to 10000 mps

pH 4.0 to 6.0

Protective Colloid PolyVinylAlcohol

Tg -20C to 38C


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A Broad Particle SizeA Broad Particle SizeDistributionDistribution

Packing of Non UniformSpheres

Figure 24

Most adhesive dispersions have rather large particle distributions. A mean of 1 micron is typical

for most PVOH stabilized products. The particle size distribution directly impacts a number of

key adhesive rheological and compounding characteristics. The solids/ viscosity curve isdetermined by particle size to a major extent. The thickening response to plasticizer depends

upon polymer crowding. The setting speed and wet tack are influenced by particle size

distribution and the proximity of volume solids to the critical packing factor.


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The Solids/Viscosity Curves ofThe Solids/Viscosity Curves ofConventional and High SolidsConventional and High Solids

Vinyl Acetate EthyleneVinyl Acetate EthyleneCopolymersCopolymers

%Solids

0.5 0.55 0.6 0.65 0.7 0.75 0.8

72%55%

0

20,000

40,000

60,000

80,000

100,000

VISCOSITY

Figure 25

The broader distribution has a lower viscosity at any given solids because of the more effectivepacking of the spheres. The dispersion at 72% solids in Figure has been shifted to the right to

yield equivalent viscosities at higher solids due, in part, to the broader particle size distributionof the particles. Dispersions with a broad particle distribution also have a lower degree of

plasticizer thickening response because they are further from the critical packing factorconcentration at any given viscosity than a more uniform dispersion.

The varied particle size distributions that are possible, along with all the possible combinationsof colloids and their quantities can produce a wide variety of dispersions with a wide variation of

rheological properties. The vast potential of polymer characteristics represents only half of the

potential variety that can be used in the development of an adhesive. Because somecharacteristics are a function of the polymer, polymer selection in adhesive development is

critical. Other performance properties may be based upon speed or wet tack properties which are

particle distribution and aqueous phase viscosity related.


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Additives, such as plasticizer, rheology modifiers, humectants, fillers, and others can also greatly

influence the performance of an adhesive. The individual nature of a bonding process is sospecific due to machine variations, substrate variations and climate variations that frequently

adhesives are formulated specifically to meet the needs of an individual machine running certain

combinations of substrates at variable speeds in a changing environment. This individuality ofthe bonding process is the reason there is no such thing as a universal adhesive.

5. Bond Formation: How do dispersion based adhesives bond?

All adhesives follow a basic three step process to forming an adhesive bond.

The Mechanics of Bond Formation for Adhesives

1. Application as a Liquid

Wetting of Substrates

2. Gain in ViscosityGreen Strength or Wet Tack

3. Setting of Adhesive

Drying

Diffusion of water into substrate

The application as a liquid phase allows for intimate contact on a molecular level. Wetting of the

surface involves attractive forces like Vander Waals or electrostatic forces. The gain in viscositycan involve a rise in viscosity or a phase change. This can involve green strength or early

strength of the bond before it is fully formed. Ultimately a polymeric film is formed and the

glueline is a solid.

The most common bonding method for dispersions is the use of wet combining to form an

adhesive bond. For this method, it is mandatory that a porous surface be involved in the

construction. If a porous surface is not involved then a heat sealing bonding process must be

used. The physical condition of the applied adhesive is a liquid. Most dispersion adhesives canbe applied by rolls, spray, brush, extrusion or other industrial coating processes. When an

adhesive is applied to a porous surface a number of events begin to occur which ultimately result

in a solid adhesive layer bonding two substrates.

Bonds of polymer spheres are formed when the dispersion is placed on an absorbent substrate.

They are applied as a liquid and flow to wet out the bonding surfaces; they gain viscosity and

build green strength; and finally set to their final state. As the water ultimately diffuses away,the polymer solids are left behind to bridge the space between the substrates and form a bond.

The polymers are hydrophobic and permit the water to leave more rapidly. Viscosity and green

strength are built as the spheres of dispersed polymer are left behind and begin to inter react witheach other as the water diffuses away in channels. Ultimately, the spheres touch each other, form

a film and lose their identities and individuality.

Figures 26 to 29 illustrate this process


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The Bond FormationThe Bond FormationStepsSteps

Initial ContactInitial Contact

Adhesive must flow and intimately wet

surfaces contacted

Figure 26

Intimate wetting involves an attractive force built on the similarity in polarity between thepolymer and the substrate as well as a similarity of surface tension between the aqueous phaseand the substrate

As water is absorbed into the porous surface, the concentration of the solids rises in the adhesivelayer. As the solids rises, the spheres become starved of liquid and they come into contact. The

viscosity of the liquid adhesive rises dramatically. This is reflected in the green tack or wet tack

of an adhesive. It is this force which can hold substrates together until the bond is further dried.Many industrial processes require a high level of green strength or wet tack to achieve modern

line speeds. In many cases, the force that fixtures the substrates in place may be required to be

present instantaneously. There is no waiting for drying to occur.


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The Bond Formation Steps

DeWatering via Diffusion and Wetting

Adhesive remains in contact through external compression

or through internal wet tack

Figure 27

With continued loss of water, the spheres interact. They will begin to crowd each other into

formation to begin film formation

.

The BondThe BondFormation StepsFormation Steps

Channel Formation andChannel Formation andFilm FormationFilm Formation

Osmotic pressure forces the water out through

channels

Figure 28

The spheres will begin to deform creating channels through which the aqueous media will

diffuse away. Osmotic pressure will continue to press the spheres together to form a film. This

pressure forces the water out via the channels into the surface nearest the substrate. For diffusion

into the substrate and water absorption to occur; at least one substrate needs to be porous.


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The BondThe BondFormation StepsFormation Steps

Loss of Identity andLoss of Identity andCoalescenceCoalescence

The spheres lose their individual identity and

merge into a single sheet

Figure 29

Ultimately, film formation occurs as a result of the drying step. The spheres lose their individual

identity and merge into a single sheet

We have visual proof of this sequence of events in the bonding process. Through the use of anatomic force electron microscope we can profile the surface condition of the film at any stage in

the film formation process.


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The Bond Formation Steps

Film Formation under the Atomic ForceMicroscope

Initial Film Final Film

Figure 30

A comparison of a dry film that is 24 hours old (initial film) and the same dried film that is 28days old (final film) is revealing. The film at the end of the 24-hour period still shows spheres

with individual identities. The 28-day-old film shows how the polymers have flowed out and

coalesced into a more continuous film. The amount and rate of polymer flow and film formation

depend upon the inherent flow of the polymer itself and the temperature of exposure. A goodbond develops where the adhesive bridges the gaps between the substrate and provides good

wetting or mechanical interlocking between the adhesive and substrate surface. This process isreferred to as the wet bonding method.

Crosslinking, if desired, should ideally occur after the bond has been formed and the polymer has

wet out the substrate. If crosslinking occurs prematurely; it limits the polymer wet out and flow

which could interfere with adhesion. Limiting the adhesion can weaken a bond, although thefinal polymer film is tougher. If there is cohesive failure within the film, two possibilities exist;

failure within the bulk polymer or failure at the surfaces between the spheres. The proper

amount and type of crosslinking may resolve those difficulties.

A dry bonding method also exists and is similar but adds an extra step. Dry bonding can be

accomplished via the use of pressure sensitive adhesives, heat sealing adhesives, contact cementadhesives, or solvent reactivated adhesives. Remoistenable tapes, labels or stickers are includedhere because they actually use water as a reactivation solvent. A dried adhesive film is formed

and then the three bonding steps are applied. Under time, temperature and pressure the adhesive

flows and wets out the opposite substrate. This corresponds to the application as a liquid step.The adhesive is then allowed to cool, crystallize or solidify. Ultimately, the two substrates are

bridged by a polymer that uniformly wets out both sides of the surfaces.


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If the process goes well the bond should appear to look like the schematic drawing at the top of

figure 31(GOOD WETTING). The adhesive has wet and flowed out into the pores of thesubstrate. It has subsequently developed its full set of properties with a large amount of physical

interlock into the surface.

The Wetting ofThe Wetting ofSubstratesSubstrates

Good Wetting

Poor Open Time Poor Wetting

Poor Wetting

Substrate Not WetSubstrate Not Wet



Figure 31

The second schematic (POOR OPEN TIME) shows what happens if the open time of an

adhesive is exceeded. Because the film has dried extensively, the film formation has failed to wet

out the opposite side of the substrates. It fails to interlock and penetrate on that side. As a result

the bond fails because of an improper match of the drying characteristics and the productionprocess. The addition of humectants or hydrophilic additives to slow the water release is the

common way to match the drying rate to a slower process. Lower solids can also help by having

a higher concentration of water present. It is important that the machining, wetting, and otherrheological factors match the production process. Most adhesive failure can be traced to

improper inter relationships between machining and wetting.


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The third schematic (POOR WETTING) shows what happens if the surface tension of the

adhesive is too high. In poor cases, the substrate physically repels the water based adhesive.This can occur due to grease, or other low surface tension, hard to wet materials being at the

surface to be bonded. Because the film has dried but not wet out either surface extensively, the

film fails to interlock and penetrate on any side. Surfactants, which help drop the surface tensionof an adhesive, may help alleviate a wetting problem

The adhesion, cohesion and processing variables have now been covered. Lets try to fit all the

pieces together with some suggestions on how to proceed in developing a number of adhesiveswith a wide variety of application and processing requirements.

6.0 Formulating Vinyl Acetate Ethylene based Adhesives

6.1 Formulating IngredientsHere is a list of typical ingredients which could be used but are not all necessary; unmodified

VAE dispersions can make satisfactory adhesives. Any ingredient can be a possible additive fordispersion adhesives. Sometimes cost, an external requirement or a specification can trim down

the number of allowable ingredients

Typical Adhesive Ingredient Levels

Main Emulsion 100 to 50%

Blending Emulsion 0% to 50%

Plasticizer 0% to 25%

Polyvinyl Alcohol 0% to 10%

Diluent 0% to 30%

Humectant 0% to 30%

Filler 0% to 30%

A useful way to sort through the effects of additives is to sort them according to which phase

they modify i.e., the solid polymer particles or the water soluble phase. An additive like an

alternate dispersion will modify both the liquid and the solid phase. Humectant or watermiscible additives will modify the aqueous phase and its viscosity. An additive like plasticizer

will modify the solid phase. Modifying the solid phase could involve a bulk property change,

like a Tg suppression. It could also involve changing the solids content and the particle sizedistribution. This would alter the relative position of the volume solids of the formulation to the

critical packing factor.

Typical Adhesive Ingredients Affect Differing PhasesMain Emulsion Water Phase Polymer Phase

Blending Emulsion Water Phase Polymer Phase

Plasticizer Polymer Phase

Polyvinyl Alcohol Water Phase

Diluent Water Phase

Humectant Water Phase

Filler Polymer Phase


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Two variables become important in understanding adhesives formulations, the aqueous phase

viscosity and the packing factor. The aqueous phase viscosity is dependant upon the molecularweight and concentration of the aqueous phase colloids and ingredients. It can be affected by the

inter-reaction between these ingredients.

Conceptually, the packing factor of the solids phase is the ratio of the volumetric solids to thecritical packing factor. This is a variable which is affected by concentration and the particle size

distribution of the additives as well as any overall changes in the overall particle size of the blend

which impacts the overall critical packing factor of the system. It is possible to add solids of adifferent particle size distribution and simultaneously raise the solids content and the critical

packing factor. This can explain why blending of dispersions can produce a wide and varied

effect on viscosity.

6.2 Polymer / Solid Phase Modifiers:

Blending with Other DispersionsBlending with other polymer dispersions is a common formulation technique. One can

effectively create your own customized particle distributions by the proper mixing of neat

dispersions.

Care and experience should be used since the solids content, the water phase viscosity, the mean

particle sizes and the standard deviation of the particle sizes are altered as dispersions are

blended. On a bulk properties basis, you can simultaneously alter the adhesion vs cohesionbalance with proper polymer choice.

PlasticizerPlasticizer, the most common additive, penetrates the polymer sphere and swells the polymerspheres increasing their average particle size and volume solids. It has a modest affect changing

their particle size distribution. Plasticizers are added to reduce the intermolecular attraction of

the poly(vinyl acetate) polymers, causing the polymer chains to spread and the poly(vinylacetate) particles to swell. This swelling raises the volume solids, which raises the viscosity, of

the dispersion and destabilizes it for greater wet tack as well as faster breaking and setting

speeds. In addition, the resin in the dispersion gains mobility. Increased mobility helps the resinto wet smooth, nonporous surfaces (e.g., plastics, foils and coated papers) and, consequently, to

increase its adhesion to these surfaces. Moreover, softened polymer particles coalesce more

rapidly and more completely at lower temperatures. Finally, plasticizers will increase the

tackiness of the film, reduce its heat-sealing temperature, and improve its water resistance.


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Fillers

Fillers are added to dispersion adhesives to reduce cost by replacing resin solids withoutdecreasing total solids, to reduce penetration into porous substrates, and to change the rheology

of compounds. Depending on their individual properties, fillers can also add stiffness and

strength or decrease tack and blocking. Uncooked starch fillers, in particular, reduce the coldflow in wood glues. Clays and other fillers impart stiffness to adhesive films. Clays reduce thepenetration of adhesives into porous substrates. Large-particle clays are better able to control

penetration and also impart more rapid setting speed. Calcium carbonates, clays, starch, wood

flours, mica, talc feldspar and other minerals are general-purpose fillers that can be added tomost adhesive dispersions.

6.3 Aqueous Phase Modifiers

Water of DilutionWater performs two functions in diluting dispersions; it dilutes the viscosity of the aqueous

phase dramatically. It separates the particles and lowers the volume solids concentration. Bothfactors contribute to the dramatic drop in viscosity that occurs as water is added to a formulation.

It can act as a temporary plasticizer to the colloid and more modestly to the polymer on a bulk

polymer basis.

Polyvinyl Alcohol

Poly (vinyl alcohol) is used as very important additives or modifiers in polyvinyl acetate /

ethylene dispersion-based adhesives. Poly (vinyl alcohol) adheres particularly well to cellulosicsubstrates such as paper and wood. Adding it to polyvinyl acetate dispersions will increase the

tensile strength of the resulting adhesive. Because it is a hydrophilic polymer, poly (vinyl

alcohol) functions as a humectant to retard the loss of water from the formulation and prolongs

the open time of the adhesive film. Wet tack is increased and can be enhanced further by addinga borated poly (vinyl alcohol).

Poly (vinyl alcohol) is also used as a thickener to increase viscosity and control solids content.When a high-viscosity but low-solids formula is needed, high-viscosity poly (vinyl alcohol)

should be added. When both a high-viscosity and high-solids formulation is needed, a medium-

viscosity grade should be added. All grades demonstrate smooth flow from applicator reservoirsand Newtonian Flow at high speeds. Poly (vinyl alcohol) affords the best means of balancing the

viscosity and solids content of a dispersion adhesive.

Partially hydrolyzed poly (vinyl alcohol) increases the stability of dispersion adhesives byfunctioning as an emulsifier and a protective colloid. These alcohols increase the water

sensitivity of adhesive films, making them very useful in remoistenable adhesives or those

requiring easy cleanup. Low molecular weight poly (vinyl alcohol) is especially useful in

remoistenable adhesives because their ultra-low viscosity permits the use of a large amount ofPVOH, which results in a very tacky, fast grabbing front seal adhesive for envelopes.


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Partially hydrolyzed poly (vinyl alcohol) wets surfaces better than fully or super hydrolyzed

grades and works particularly well in wood adhesives. The addition of partially hydrolyzed poly(vinyl alcohol) to polyvinyl acetate wood glues permits the adhesive to wet the wood and

penetrate the pores, thereby increasing the strength and fiber tear of these adhesives.

Fully hydrolyzed poly (vinyl alcohol) increases the water resistance of adhesive films,particularly the medium and high molecular weight grades. Solutions of super hydrolyzed grades

tend to increase in viscosity as they age and are not recommended as additives to polyvinyl

acetate dispersion adhesives.

Poly (vinyl alcohol) improves the machinability of dispersion adhesives by raising the high shear

viscosity of the adhesive; i.e. spitting and throwing are reduced on high-speed equipment. All

types have surface-active properties that promote thorough wetting of roller applicators andadherends. The lower surface tension of the partially hydrolyzed grades permits better wetting of

hydrophobic surfaces. Since poly (vinyl alcohol) has a higher softening point (180-230C) than

the dispersion to which it is added, it raises the heat sealing and blocking temperature of the filmand increases its overall heat resistance.

Humectants

A humectant is a hydroscopic substance, one that absorbs and retains moisture from theatmosphere. In dispersion adhesives, humectants prevent the surface of the compound from

skinning by keeping it wet. When poly(vinyl alcohol) or starch is present, the humectant holds

water, which plasticizes these materials and keeps them flexible after drying. By retardingdrying, humectants also slow setting speed and extend the open time of the adhesive. Humectants

allow easier cleanup of a dried or semi-dried adhesive. Typical examples of common

humectants are urea, glycerin, propylene glycol, sucrose, sorbitol and many inorganic salts.

Viscosity/Rheology ModifiersAdhesives are most commonly industrially applied by machine. Each type of machine or

applicator has its own viscosity/rheology requirements. Most roll applications use adhesiveswith a viscosity of 1500 to 3000 cPs and operate best with a slightly thixotropic flow. Spray

equipment uses viscosities in the 200 to 800 cPs range. Construction adhesives are applied

through a gun or trowel which needs thixotropic or pseudoplastic rheology. The proper choice ofthickener will control viscosity as well as rheology. Thickeners added to an adhesive will raise

viscosity and permit dilution with water. This reduces the total solids of the adhesive and also

lowers its cost. Thickeners release water slowly, and when combined with lower solids, they

slow the setting (prolong open time) of the adhesive. Thickeners, such as high molecular weightpoly(vinyl alcohol), starch and clay, will improve adhesion to porous substrates by reducing

penetration of an dispersion into substrates and decreasing the likelihood of a starved joint.


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Some thickeners can prevent adhesives from spitting and throwing during high-speed

applications. They permit them to transfer cleanly and break short rather than draw to fibers.Pseudoplastic adhesives can be prepared by adding polyacrylates such as sodium or ammonium

polyacrylate. Starch and cellulosic thickeners will also confer pseudoplasticity. Thickeners that

can be added to adhesive dispersions are: Polyvinyl alcohol, xanthan gum, hydroxyethylcellulose, polyacrylic acid based thickeners, associative thickeners, inverse dispersion thickeners,most water soluble polymers

Other Additives

Wetting Agents and DispersantsWetting agents aid the adhesive to wet the surface of the adherend by lowering the interfacial

surface tension, thereby improving adhesion. When a VAE dispersion is used to bond PVC foils,

the secondary plasticizers in the film exude to the surface, making the film very difficult to wetand bond. The addition of a good wetting agent corrects this situation. Wetting agents help water

penetrate the surface of an adherend. This allows the polymer particles to coalesce and set

rapidly. Dispersants are polyelectrolyte salts that are added to help disperse solids throughout theadhesive. They reduce agglomeration of the added fillers which reduces settling issues.

Certain wetting agents cause foaming if used to excess, and may also increase the water

sensitivity of the adhesive film. Therefore, these should be added in minimal amounts.Typical wetting agents and dispersants that can be used with most VAE dispersions are: Sodium

Dioctylsulfosuccinate, Sodium alkyl benzene sulfonates, Acetylenic glycols, ethoxylated

nonylphenols and many others . The Surfynol surfactant series of wetting agents from AirProducts can also act as defoamers.

Foam-Control Agents/Defoamers

Foam-control agents include antifoam and defoamer compounds. Foam causes problems in boththe manufacturing and the application of adhesives. Air in the formulation increases viscosity

which can be misleading when adding ingredients. An inaccurate viscosity also prevents accurate

metering during application of the adhesive, thereby preventing the deposit of sufficient solids onthe glue line. Defoamers are typically used to destabilize or break down an existing foamy

condition. Antifoams are added to the formulation to prevent foam from occurring. A small

amount of foam-control agent, 0.10 to 0.30 percent of the adhesive, is usually sufficient toprevent or break foam. Caution: Too Much Foam-Control Agent Will Reduce the Wetting Ability

of the Adhesive and Weaken the Bond. A foam-control agent should be retested in each new

adhesive formulation and each time an ingredient is changed. Changing the amount of an

ingredient can change foam-control requirements. The adhesive sample should age beforeanalysis of foam-control efficiency. Some foam controllers may emulsify or separate upon aging

and need to be stirred well before adding.


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BiocidesBiocides are always required when compounding dispersions, especially when animal or

vegetable substances or their derivatives (starches, casein and other proteins, nut shell flours,

sugar and cellulosic resins) are incorporated into VAE adhesives. Growing microorganisms feedon these substances and can generate foul odors, discolor the adhesive, lower its viscosity andweaken the bond. Typically, the inclusion of 0.05 to 0.10 percent of a biocide, based on the total

wet weight of the formulation, prevents microbial growth. Nevertheless, microbes frequently

adapt to a specific biocide and flourish. Biocides should be changed regularly to prevent thisoccurrence. The maximum allowable quantities of all biocides are regulated. The manufacturer

should be contacted for the appropriate use level information.

6.4 Formulation Examples and Logic:There are five major requirements to specify or develop a specific adhesive for your process.

Adhesive RequirementsRequirements Adhesive Property

Bonding Method Polymer flow and Substrate Wetting

Substrates, and their Variability Adhesion

Performance, Specifications Cohesion

Economics Failure AnalysisRisk vs Reward

Plant Conditions Productivity / Statistical Quality Control

The first is to understand the bonding method. The bonding process and the application process

such as roller, brush, spray, extrusion, etc., need to be considered in adhesive development andselection in great detail. The coating weight control is crucial to balancing the performance and

economics of a process. Substrates vary or may need to be switched during a production run sosufficient adhesion is paramount. The final constructions performance characteristics need to be

evaluated in terms of stress, heat, humidity or moisture resistance. Value is cost performancebased. An evaluation of cost must consider the production costs associated with adhesive usage;

this includes mileage and line speeds or prod

European Adhesives Handbook

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