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1 Training Courses on Polyurethane Adhesives Part I: Formulation How to Formulate Polyurethane Based Adhesives: A Complete Overview of Key Ingredients and Latest Formulation Techniques Edward M. Petrie (SpecialChem Expert Team)

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Page 1: Polyurethane Adhesives

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Training Courses on PolyurethaneAdhesives

Part I: Formulation

“How to Formulate Polyurethane Based Adhesives:A Complete Overview of Key Ingredients and Latest

Formulation Techniques ”

Edward M. Petrie(SpecialChem Expert Team)

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Seminar Objectives

Learn capability of polyurethane adhesivesLearn about their versatility and breadth Learn the formulation possibilities

Chapter 1 - Introduction

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Seminar Contents - Part I 1) Slide Show PresentationChapter

1. Introduction 2. Types of PU adhesives3. PU chemistry

• Morphology and physical properties4. PU adhesive composition5. Isocyanates and Polyols6. Additives and other raw materials7. Common formulations8. Current and future development

Chapter 1 - Introduction

60 minutes

30 minutes

2) Questions and AnswersOn-line interaction

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Chapter 1

Introduction

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DefinitionPU adhesives contain a number of urethane groups in the molecular backbone or are formed during use

Urethane group

Many complex reactions can occur simultaneously or sequentially

O

N – C – O

H

• Diversity

• Versatility

• Diversity

• Versatility

Chapter 1 - Introduction

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Why the Attraction to PU Adhesives?Good adhesion achieved through:− Chemical bonding with water, hydroxyls on substrate surface− Physical bonding through van der Waals forces and low surface energy− Mechanical through low viscosity and diffusion into porous substrates

Specific mechanical properties achieved through molecular composition− Toughness and other viscoelastic properties− Elasticity and modulus− Crosslinking density

Many types, methods of application, and cure speeds are possible

Chapter 1 - Introduction

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Advantages and Disadvantages of PU Adhesives

Advantages− Highly versatile chemistry− Flexible, tough, or rigid− Bonds a variety of plastics− Can be formulated as 1 or 2

component system− Slow to fast curing time− Good low temperature

properties− Good environmental

resistance− Good chemical, oil

resistance below 50C

Disadvantages− Moisture sensitivity during

storage and application− Only average bond strength

to metal without a primer− Precise mix ratio required

for certain products− Requires good mixing − Maximum temperature of

100-150C for specially formulated PUs

− Hydrolytic stability is a problem for certain types

Chapter 1 - Introduction

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Chapter 2

Types of Polyurethane Adhesives

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Classification of PU Adhesives and Sealants

Polyurethane adhesives vary widely in composition and are used in many different applications and in

various market segments

Chapter 2 – Types of PU Adhesives

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Chapter 3

Polyurethane Chemistry

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Polyurethane Chemistry Urethanes are generally produced from isocyanates, represented as:

R – N = C = O

Properties revolve around the universal and rapid reaction between the NCO group and any organic compound containing an extractable hydrogen atomReaction with a hydroxyl terminated polymer (e.g., alcohol, polyester or polyether polyol, derivatives of castor oil or hydrocarbons)

H O| ||

R – N = C = O + HO - R’ R – N – C – O – R’(Isocyanate) (Alcohol) (Urethane)

Chapter 3 – PU Chemistry

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Isocyanate – Polyol Reaction

Polyether or Polyester DiisocyanatePolyol

HO~~~~~~~ OH + OCN – R – NCO

O O

~~~~O – C – NH – R – NH – C – O~~~~~

Urethane Group

Polyurethane

Chapter 3 – PU Chemistry

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Chapter 4

Polyurethane Adhesive Composition

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Polyurethane Adhesive CompositionIsocyanateActive Hydrogen CompoundCatalyst Chain ExtenderAdditives− Drying agents− Antioxidant− UV Inhibitor− Antimicrobial− Leveling agent− Colorant− Adhesion Promoters− Fillers and Extenders− Other Polymers

Chapter 4 – PU Adhesive Composition

Ed
Note
In many applications, e.g., bonding of rigid metal, polyisocyanate concentrations of up to 50% above the stoichiometric amount are used. For this discussion polyols and prepolymers are considered under “active hydrogen compounds”. In bonding flexible materials, a higher amount of polyol (e.g., about 10% above the stoichiometric amount) can be an advantage. Here the unreacted polyol acts as a plasticizer. All auxiliary materials are generally added to the polyol for optimum storage stability. The polyisocyanate is diluted only in special cases and is normally added as a second component immediately prior to use.
Liyan
Note
Unmarked définie par Liyan
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Manufacture of Polyurethane AdhesivesUrethane adhesive and sealants are generally made in a batch process

Prepolymer is made in a standard mix tank

Adhesive or sealant is made in a high torque mixer− Mixer control is important for consistent viscosity

It is necessary to avoid moisture contamination− Reactions run under a nitrogen blanket or in a vacuum

Reactors are always jacketed for temperature control

Chapter 4 – PU Adhesive Composition

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Chapter 5

Isocyanates and Polyols

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Isocyanates

MDI (methylene diphenyl diisocyanate)

TDI (toluene diisocyanate)

Polymeric isocyanates used for crosslinking (e.g., polymethylene polyphenylene isocyanate)

Prepolymers – Isocyanate prereacted with some or all of the polyol (not all of the polyol is used) – very low viscosity

Chapter 5 – Isocyanates and Polyols

Ed
Note
The number of available isocyanates and their derivatives is very large. Only a fraction of the materials are described here. Many isocyanates can be used which results in the diversity of PU adhesives. The isocyanate groups are highly energetic and readily react with a wide variety of other chemical groups. Aromatic isocyanates are typically less expensive than aliphatic and are faster reacting. Aromatics are not as light stable as aliphatics, nor are they as resistant to oxidation. The most commonly used isocyanates in PU adhesives are MDI and TDI, both aromatic isocyanates. Aromatic isocyanates are normally preferred for PU adhesives because they are more reactive and more economical. Aliphatic isocyanates are also used but in smaller volumes. TDI is used to manufacture low viscosity prepolymers. It is not used commonly in adhesive formulations because of its high vapor pressure and concerns relative to safety and toxicity. MDI is somewhat more expensive than TDI. However, it has much lower vapor pressure. MDI is less reactive than TDI and provides a higher tensile strength, modulus, and greater toughness and heat resistance. A disadvantage of MDI is that it is a solid at room temperature (m.p = 38C). It also tends to react with itself and form unwanted byproducts. These byproducts tend to crystallize out and must be minimized. To avoid this problem MDI is stored at -20 to 10F as a solid or at 105-115F as a liquid. MDI is usually shipped and handled at the latter temperatures. As a result, MDI is more difficult to handle than TDI with the exception of its lower vapor pressure. However, several forms of liquid MDI are available. Uretonimine modified MDI is a liquid at room temperature. Liquid MDIs can also be made by increasing the 2,4’ isomer. Polymeric MDI, a liquid at room temperature, is a low cost material with a low freeze point. It is more storage stable than pure MDI. Polymeric MDI is not generally used for prepolymers because the viscosity is too high and it is not used for applications where color is a concern.
Liyan
Note
Unmarked définie par Liyan
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IsocyanatesTriphenylmethane triisocyanate (Desmodur R)

Polymethylene polyphenyl isocyanate (PAPI)

Aliphatic isocyanates - Less reactive, non-yellowing, more resistant to UV, slightly higher thermal resistance, lower resistance to oxidation, more expensive− HDI (hexamethylene diisocyanate)

− IPDI (isophorone diisocyanate)

O = C = N - - CH2 - - N = C = O

MDI (methylene diphenyl diisocyanate)

Chapter 5 – Isocyanates and Polyols

Ed
Note
HDI is an aliphatic isocyanate. It has a relatively high vapor pressure. HDI is usually reacted further with either itself or with other isocyanates to form a higher molecular weight isocyanate with usually increased functionality. IPDI is less reactive. This gives more control in prepolymer and adduct formation. Blocked isocyanates are also used in adhesive formulations. The isocyanate groups are reacted with a material that prevents reaction at room temperature, but will allow it to occur at elevated temperatures. This will be discussed under “single component PU adhesives”.
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Polyols

Polyether Polyol− Low temperature properties− Hydrolysis resistance− Lower cost− Easier processing− Low viscosity− Not as reactive as polyester

Polyester Polyol− Good adhesion− High strength, modulus, and

hardness− Better resistance to

oxidation, elev. temp.− Linear to highly branched

R R

HO – (CH2-CH-O)n – R’ – (O-CH-CH2)m – OH

Polyether polyol

R R

HO – (CH2-CH-O)n – R’ – (O-CH-CH2)m – OH

Polyether polyol

OH – R – O –(CO-R’-CO-O-R-O)n – H

Polyester polyol

OH – R – O –(CO-R’-CO-O-R-O)n – H

Polyester polyol

Chapter 5 – Isocyanates and Polyols

Ed
Note
Polyols are simply materials that contain two or more hydroxyl groups. Typically low molecular weight polyols (<2000) provide the best adhesive properties. The most commonly used polyols are polyether (made from the reaction of ethylene oxide or propylene oxide with a starting compound)) and polyester (adipate based) polyols. These are generally low viscosity materials with good wetting properties. Polyether polyols are the most widely used in PU adhesives because of performance and economics. Tg is about -60C and, therefore, the resulting adhesive has good low temperature performance. They are also useful on alkaline substrates such as concrete because of their resistance to alkaline hydrolysis. Polyester polyols are available in structures ranging from linear to highly branched. With more branching, more hydroxyl functionality is available for crosslinking. Polyester polyols have higher tensile strength and greater heat resistance than polyether polyols, but they have poorer hydrolytic resistance, low temperature performance and chemical resistance. They are often used in solvent based thermoplastic adhesive for the shoe sole binding. Polyether polyols are made by the reaction of ethylene oxide or propylene oxide with a starting compound. Other polyether polyols are polytetramethylene ether glycol (PTMG) polyols. They are somewhat more stable to UV light and oxidation, but are somewhat more expensive the PO and EO types of polyols. Polyester polyols are usually formed by the condensation reaction of a glycol and a dicarboxylic acid such as adipic acid. There are several types of polyester polyols. Propylene oxide ones are the more common because pure polyethylene oxide polyols tend to be too hydrophilic.
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Polyols

Polytetramethylene ether glycol (PTMG)

Polycaprolactone glycol

Polybutadiene (PolyBD) glycol− Improved hydrolytic stability− Lower moisture vapor transmission− Lower tensile strength and modulus

Others− Polycarbonates− Fatty alcohols− Castor oil− Polyester amide

Chapter 5 – Isocyanates and Polyols

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Polyols

Important factors in selection of a polyol:

Flexible RigidMolecular Weight 1000-6500 150-1600Functionality 2 to 3 3 to 8Hydroxyl value (mg KOH/g) 28-160 250-1000Glass Transition Temperature -40 to –60C +60 to –40CNCO / OH Ratio Lower Higher

Chapter 5 – Isocyanates and Polyols

−Chemical type

−MW

−Functionality

−Chain structure

−Chemical type

−MW

−Functionality

−Chain structure

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Comparison of PU Polyols

Chapter 5 – Isocyanates and Polyols

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Prepolymers

A prepolymer is a low molecular weight urethane polymer that retains the ability to react further

Prepolymers are formed by partially reacting the available isocyanate groups with a curative− Partial reaction is possible by controlling the ratios of the reactants− Called “xmers”

Important parameters in prepolymer design:− Mc: average MW between crosslinks− % NCO: if value is too high in one-part systems gassing can occur− NCO/OH ratio control mole % of xmer formed

Chapter 5 – Isocyanates and Polyols

Ed
Note
The desired polymer structure of the adhesive can be built into a prepolymer. The prepolymer will have a more consistent structure with more reproducible physical properties (less side reactions). Most PU adhesive are based on urethane prepolymers. A prepolymer is made by reacting excess of diisocyanate with a polyol to yield an isocyanate terminated urethane. Prepolymers for moisture cured polyurethanes are generally formed from a 2:1 stoichiometric ratio. Prepolymers are isocyanates and act like isocyanates, but they have high molecular weight, high viscosity, lower isocyanate content, and lower vapor pressure. Theoretically there is no need to make a prepolymer in a two component adhesive - an unreacted isocyanate in one component can react with a curative on the other. However, there are difficulties in handling unreacted isocyanates (solids at room temperature or relatively high vapor pressures). Since part of the reaction has been completed with a prepolymer, there is reduced exotherm and shrinkage. Also better mixing ratios and viscosities assist application. The prepolymer reacts more slowly than other systems yielding longer pot life. Since there is an excess of isocyanate groups the molecule formed has a limited molecular weight.
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Chapter 6

Additives and Other Raw Materials

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Catalysts, Chain Extenders, Crosslinkers

Catalysts− Tertiary amines (e.g., triethanol amine)− Transition metals (e.g., dibutyl tin dilaurate, DBTDL)

Chain Extenders (diamines or diols)− Reacts with isocyanate then phase separates into domains (“hard

segments”)− Each extender molecule more than doubles the length of the rigid

segment− Used to produce stiffer “hard segments”− Used to crosslink

Crosslinkers− Low molecular weight compounds containing active hydrogen

with a minimum functionality of 3

Chapter 6 – Additives and Other Raw Materials

Ed
Note
Commercial catalysts consist of two main classes: organometallics and tertiary amines (triethylene diamine, triethyl amine, etc.). Catalysts not only accelerate but they may also change the order of reactivity. It is important to note that there are no catalysts for urethanes that accelerate only one type of reaction. Therefore, a lot of effort goes into finding a catalyst that is as specific as possible. Adhesives based on polyether polyols usually need a stronger catalyst than polyester based adhesives. DBTDL is one of the most common organometallic catalysts. If used in a system in which both polyol and water, transition metals are capable of reacting with isocyanates, it will strongly favor the reaction with polyol. Transition metals affect the aging properties of the bonds to a higher degree than tertiary amines. The chain extenders consist usually of short chain diols such as 1, 4-butanediol, 1,6-hexanediol, 1,4-bis(hydroxyethyl) derivative of hydroquinone, oxypropylene and oxyethylene derivatives of bisphenol A. Also, alkanolamine or diamines serve as chain extenders. Chain extenders are low molecular weight difunctional compounds containing active hydrogen. They are often used to cure prepolymers. They are used to link diisocyanate to create “hard” segments with higher Tg. Their role is to increase the molecular weight of urethane polymers without affecting elongation at break. The choice of an extender can have a significant influence on final physical properties. The purpose of crosslinkers are to crosslink the polymer network and so increase the hardness and modulus of the polyurethane.
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Other Curatives

Polythioethers− React with isocyanate− Provides some of the properties of polysulfide sealants

Amines− Aromatic amines provide rigid, hard adhesives− Aliphatic amines (e.g., polyetheramine) are extremely reactive with

isocyanatesNecessary to slow down the rate of reaction for practical use; therefore, blocking agents are used

Blocking Agents− Reversibly react with isocyanates (phenols, methylethylketoxime,

caprolactam, malonates, etc.)

Chapter 6 – Additives and Other Raw Materials

Ed
Note
Polythioethers cured isocyanates - properties include fuel and chemical resistance, improved thermal properties, and improved adhesion. Pure polythioethers and hydroxyl terminate polythioethers have been used. Polyetheramine are simply polyether polyols where the terminal hydroxyl groups have been laminated. Thereby urea groups are formed instead of urethane groups (with polyols). Urea compounds have higher thermal resistance and better hydrolytic stability. Blocking agents are materials that reversibly react with isocyanates. Examples include phenols, malonates, methylethylketoxime, and caprolactam. These react readily with isocyanates, but the products that form are not stable. In the presence of high temperature or strong nucleophiles these products revert to their original starting compounds. Aliphatic amines displace these blocking agents at room temperature. (Commonly used in two component PU adhesives.) For one-component systems using amine curatives, a blocked prepolymer is also made. Before the amines are added they are converted to ketamines. The ketamine can be mixed with the urethane prepolymer and other additives to form an adhesive. When the adhesive is applied, the ketamine reacts with water to regenerate the amine which then reacts with the blocked polymer.
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AdditivesUsually added to the polyol componentGenerally does not take part in the reactionAll additives must be free of water (or foaming and poor adhesion); solvents must have a H20 and alcohol content of less than 0.02%− Stabilizers

Antioxidant (e.g., phenolic) at a 2% levelUV stabilizerAntimicrobial at a 0.3% levelHydrolysis stabilizer

− Adhesion promoters (e.g., organosilane)− Fillers− Drying agents at a 2-5% level

Triethyl ortho formateCalcium sulfateMolecular sieves – Zeolite

− Others

Chapter 6 – Additives and Other Raw Materials

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Plasticizers, Extenders, and SolventsNot usually required – urethane is typically flexible enoughPlasticizers are used to improve elongation, reduce prepolymer viscosity, and lower hardness− Phthalate type plasticizer commonly used

Extenders are used to primarily lower cost− Aromatic oils− Asphalts− Coal tars

Solvent containing adhesives are formulated with dry organic solvents− Ketones, lower alkyl esters (e.g., acetates), methylene chloride and

trichloroethylene− Certain flow agents (cellulose acetate butyrate, vinyl acetate copolymers,

etc.) can be added

Chapter 6 – Additives and Other Raw Materials

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Chapter 7

Common Formulations

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PU Adhesive Formulation

Reactive Polyurethane Adhesives− One-component adhesives (solvent, water, and 100% solids) − Two-component adhesives (solvent, water, and 100% solids) − Reactive hot melts− Wood binders

Non-Reactive Polyurethane Adhesives− Solvent borne adhesives− Hot melt adhesives− Water based adhesives

Chapter 7 – Common Formulations

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Two Component Reactive AdhesivesGenerally composed of a diisocyanate terminated prepolymer as one component, and a polyol and polyamine crosslinking agent and catalyst as a second componentSolventless and low solvent content adhesivesFunction as an adhesive or sealant

Chapter 7 – Common Formulations

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Typical Two Component PU Formulation

Chapter 7 – Common Formulations

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Moisture Cured One Component AdhesivesMoisture cure− Ambient conditions, shelf life, and

foaming can be a problem− Can be applied either with or

without solvent

Chapter 7 – Common Formulations

Ed
Note
Moisture cured polyurethanes react with active hydrogen atoms on the surface or the moisture that is present in the air or in the substrate. The presence of water will lead to the formation of ureas and evolution of CO2. The reaction proceeds through the formation of an unstable intermediate, carbamic acid, which then decomposes to give CO2 and a aromatic amine. The amine then reacts further with another isocyanate to give a urea linkage. The urea group reacts further with excess isocyanate groups to form a biuret. The evolution of CO2 can pose problems in certain applications if not properly controlled. TDI or MDI based prepolymers are most frequently employed either alone or with the addition of a small amount of PMDI. Cure can be accelerated with the use of catalysts, although the storage stability may be adversely affected. Factors such as %NCO and type and functionality of components will affect strength, adhesion, and environmental properties. The curing conditions are very important for the curing rate of PU adhesives.
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Blocked One Component AdhesivesReaction is reversible, and generally temperatures of 120-160C are required for deblocking

O O|| ||

OCN – R – NCO + 2BH B – C – HN – R – NH – C – B

Blocking agents include phenols, branched alcohols, methylethyl ketoxime and e-caprolactam

Chapter 7 – Common Formulations

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Other One Component PU Adhesive TypesAnaerobic− Prepared by reacting hydroxyethyl methacrylate with TDI or an NCO

terminated prepolymer; a hydroperoxide is added, and the mixturestored in an oxygen permeable container with air

− Polymerization takes place when access to oxygen is eliminated in the joint

Solid Polyol− A solid polyol, such as pentaerythritol, is simply mixed into the

adhesive/sealant. When heated, the polyol melts and reacts with the isocyanate

Chapter 7 – Common Formulations

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Reactive Hot Melt (HMPUR) AdhesivesSolid polyether or polyester polyol mixtures are reacted with anexcess of diisocyanate to produce a prepolymer

The prepolymer is cooled into a hot melt product that can flow at relatively low temperatures 85-140C

Molten prepolymer has a high degree of green strength

Once applied the residual non-reacted isocyanate groups react with moisture to form a thermosetting structure

Prepolymer contributions:− Higher NCO/OH ratio: lower melt viscosity, lower elongation, higher

tensile modulus and extended open times

− Lower NCO/OH ratio: thermoplastic characteristics with improved flexibility and toughness

Chapter 7 – Common Formulations

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Isocyanate Wood Binders, Primers, SolutionsAromatic isocyanates (predominantly MDI) used for manufacture of: − Oriented strand board (OSB), − Medium density fiberboard (MDF), and − Particleboard

Isocyanates solutions can be used as primers for coatings and adhesives

Isocyanates solutions can be blended with various types of elastomers (solvent blends) as rubber cements

SBRNatural RubberNeopreneNitrile

Chapter 7 – Common Formulations

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Solvent Based and Hot Melt Non-Reactive Adhesives

Thermoplastic polyurethanes such as Estane (Goodrich) and Desmocoll (Bayer)

Solvent systems can either be applied directly (permeable substrate), as a contact adhesive, or as a coating and then hot pressed

Hot melt adhesives are most commonly applied as an extruded film for laminating applications

Properties depend on formulation and selection of diisocyanates, polyols, chain extenders, and plasticizers

Chapter 7 – Common Formulations

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Water Based AdhesivesPU lattices can be prepared in various ways:− Special monomers are polymerized in an aqueous medium to produce a

thermoplastic PU− Thermoplastic PU is dissolved in solvent and emulsified in water, then the

solvent is removed− Isocyanate terminated PU prepolymer is blocked and emulsified in water

together with a crosslinking agent.

Chapter 7 – Common Formulations

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Water Based AdhesivesAqueous polyurethane dispersion usually contain:− 40% thermoplastic resin (for aqueous inertness)− Aliphatic diisocyanate polymer (for light stability), and − An anionic surfactant

Can be cured at room temperature; however strength, adhesion, and water resistance are greater when dried for several minutes at 120-175C

Chapter 7 – Common Formulations

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Chapter 8

Current and Future Development

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Recent Development Activities

Examples of development:Hybrid PU adhesives

Ethylene / vinyl acetate copolymersEpoxyAcrylics

Blocked isocyanatesPressure sensitive acrylic prepolymersUrethane toughened acrylate adhesives Reactive hot meltsPressure sensitiveAnaerobic and radiation curing mechanismsNon-sagging without fillers

Chapter 8 – Current and Future Development

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Reactive Epoxy Urethanes“Mixture” of epoxy and polyurethane oligomers with pendant epoxy groups

Curing of epoxy groups unites the urethane and non-urethane components

Tough, durable adhesive films

Good adhesion to oily surfaces and plastics

Chapter 8 – Current and Future Development

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End of Presentation for Session I

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Seminar Contents - Part II

Selecting and Applying Polyurethane Adhesives:Overview of major applications and key performances to match your

end-use requirements1. Introduction

Markets and applicationsComparison to other adhesive systemsRange of types and properties

2. Application variablesHealth and Safety

3. Reaction rates4. Types of polyurethane adhesives and their use5. Stability in various environments

Part II: Wed May 26, 2004

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Backup Slides and

Additional Material

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Common PU Adhesive Reactions

Chapter 3 – PU Chemistry

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Common PU Adhesive Reactions

Chapter 3 – PU Chemistry

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Reaction Versatility

Isocyanates react with active hydrogen compounds in the order:R2NH > R-NH2 > Ar-NH2 > R-OH > R2OH ~ H20 > Ar-OH > R-SH

Isocyanate coreactants of commercial significance are:− Polyester Polyols− Polyether Polyols− Amines

Polymerization and/or crosslinking will occur depending on the amount and nature of the diisocyanates, stoichiometric amount and type of comonomer, and conditions of curing

As with other polymers, additives are used to further modify thesystems for specific end-use properties

−Polycaprolactones−Polyglycols−Natural and Hydroxyl Containing Oils

Chapter 3 – PU Chemistry

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Crosslinking CapabilityLinear, thermoplastic PUs are obtained by using compounds with two reactive groups such as diisocyanates and diols

Crosslinking occurs when:− Polyols with three or more hydroxyl groups (i.e., a functionality of 3 or

more) are reacted with an isocyanate, or when

− Isocyanates with three or more Isocyanate groups are reacted with a polyol.

− There is an excess of isocyanates (linkages of allophanate and biuret).

The amount of crosslinking determines the stiffness of the polymer, strength, and resistance to environmental factors

Chapter 3 – PU Chemistry

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Crosslinking

Polyurethanes exhibit both chemical crosslinking and “physical”crosslinking

Physical and chemical crosslinking sometimes overlap

Crosslinking does not always produce the expected effect because of disruption of the domain structure (especially at lowlevels of crosslinking)

Chapter 3 – PU Chemistry

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Polyurethane MorphologyPU molecule consists of two separate phases or microdomains (pseudocrosslinking)− Hard segment

polyisocyanate and low molecular weight hydroxy compound (a short chain polyol or a diamine chain extender)Crosslinks the soft segment

− Soft segmentsHydroxy terminated diolsGreater flexibility and elongation, resistance to low temperatureLower hardness, modulus, abrasion resistance

At elevated temperatures two phases become one amorphous, soft phase

Chapter 3 – PU Chemistry

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Polyurethane Morphology

1 – Flexible PUR

2- 40% rigid segments

3- 60% rigid segments

4- Rigid PUR

Chapter 3 – PU Chemistry

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Polyisocyanates Provide Varied Adhesion Properties

Isocyanates react readily with a variety of other functional groupsDi- and polyisocyanates can undergo self-polymerization to form three dimension resins in situIsocyanates are quite soluble in many organic substances, and due to their small molecular size readily permeate insoluble porous structuresThe reaction of di- and polyisocyanates with hydroxyl bearing polyesters and polyethers produce the strong, polar, hydrogen bonded flexible PUs which wet substrates very wellIsocyanates provide elastomer- metal bonds with flexible to rigid gradation in physical properties between the elastomer and metal which provides superior fatigue life

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Polyols and Hydrolytic Stability

Hydrolysis Resistance as a Function of Change in Stress at BreakAll polyurethane elastomers: 80 shore A, immersed in water at 80C

Chapter 5 – Isocyanates and Polyols

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Polyols

Properties of Adhesive Films Based on Mixtures of PolyolsPolyol PO1 = 5% hydroxyl content

Polyol PO2 = 1.3% hydroxyl content

A = modulus at 100% elongation

B = tensile strength

C = elongation at break

Chapter 5 – Isocyanates and Polyols

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Fillers

Fillers that reduce shrinkage, improve strength, reduce costs, adjust viscosity

BarytesHydrated AluminaClays

Particulate fillers increase density and hardnessCaCO3 has a catalytic effect on isocyanate reactionAluminum hydroxide reduces flammabilityCarbon black reinforces and provides UV protectionColloidal and microexpanded silica are thixotropic agents

Flaky fillers are used for surface properties, appearance, etc.Fibrous fillers (organic and inorganic) reinforce PU; generallyincrease rigidity and elastic modulus

Quartz FlourSlate FlourFumed Silica

Chapter 6 – Additives and Other Raw Materials

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ThixotropyThixotropic agents are used to provide non-sag properties or to reduce penetration in absorbent substrates

Usually fumed silica or bentonite is used as an additive to provide thixotropy

However, carbon black, fibers, and treated clays can also increase thixotropy

Bayer has developed amine terminated prepolymers (ATEP) that provide fast “B-stage” reaction for thixotropy− Generally need automated meter, mix, and dispense equipment for fast B-

stage development

Chapter 6 – Additives and Other Raw Materials

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Silane Primer / Additive Improves Moisture Resistance

Adhesion promoters can be applied as a primer or as a component in the adhesive formulation

Generally silanes are used to provide:

−A chemical bridge between the surface and organic polymer or between organic polymers−A barrier to prevent moisture penetration into the interface−Effective dispersion of fillers and reduction in the apparent viscosity of the system Isocyanate Terminated PU, Adhesion to Al

(1. Urethane is Adiprene L-100, 2. Substrates degreased and etched, 3. Most commonly

recommended for pot life)

Chapter 6 – Additives and Other Raw Materials

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UV Stabilizers and AntioxidantsRequired with aromatic isocyanates and polyether polyolsUV absorbers and stabilizers are commonly employed when PU is exposed to light − Carbon black− Titanium dioxide− Tinuvin P (Ciba-Giegy)

Antioxidants neutralize free radicals that are formed by the reaction of various chemical bonds with oxygen− Iganox 1010 (Ciba-Giegy)− Hindered phenols and amines

UV stabilizers and antioxidants work synergistically, so they are often used together

− Zinc oxide− Certain molybdates

Chapter 6 – Additives and Other Raw Materials

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Stabilizers – Hydrolytic StabilityImproved hydrolytic stability (most often required for polyester urethanes)− Carbodiimide (e.g., Staboxal P

from Bayer) − Satrastab developed by SATRA

(Shoe and Allied Trades Assoc., Kettering, England)

− Elimination or reduction of ester groups in the polyol

− Hydrolysis resistance increases in the order of ether > polycaprolactone > polyester

Chapter 6 – Additives and Other Raw Materials

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Other StabilizersFungicides (most often required for polyester urethane)− Copper-8-quinolinolate− N-(trichloromethylthio) phthalimide (e.g., Fungitrol 11 from Nuodex)

Improved heat resistance− Non-stabilized PU are not recommended for service temperatures greater

than 100C− Heat resistance can be improved through exclusion of catalyst residues

from the polymer and by tailoring the polyurethane structure− Antioxidants and hybrid with epoxy and other resins

Chapter 6 – Additives and Other Raw Materials

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Polymers Sometimes Used with PU Adhesives

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Polyurethane Metal Adhesive

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Solvent-Free General Purpose Reactive Adhesive

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Solvent Based Reactive Adhesive

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Thermoplastic PU Contact Adhesive