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
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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
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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
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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
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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
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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
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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
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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
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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