Adhesive

34Polyurethane Adhesives

Dennis G. Lay and Paul CranleyThe Dow Chemical Company, Freeport, Texas, U.S.A.

I. INTRODUCTION

The development of polyurethane adhesives can be traced back more than 60 years to thepioneering eorts of Otto Bayer and co-workers. Bayer extended the chemistry of poly-urethanes initiated in 1937 [1] into the realm of adhesives about 1940 [2] by combiningpolyester polyols with di- and polyisocyanates. He found that these products made excel-lent adhesives for bonding elastomers to bers and metals. Early commercial applicationsincluded life rafts, vests, airplanes, tires, and tanks [3]. These early developments weresoon eclipsed by a multitude of new applications, new technologies, and patents at anexponential rate.

The uses of polyurethane adhesives have expanded to include bonding of numeroussubstrates, such as glass, wood, plastics, and ceramics. Urethane prepolymers were rstused in the early 1950s [4] to bond leather, wood, fabric, and rubber composites. A fewyears [5] later one of the rst two-component urethane adhesives was disclosed for use asa metal-to-metal adhesive. In 1957 [6] the rst thermoplastic polyurethane used as ahot-melt adhesive (adhesive strips) was patented for the use of bonding sheet metalcontainers. This technology was based on linear, hydroxy-terminated polyesters anddiisocyanates. Additional thermoplastic polyurethane adhesives began appearing in the19581959 period [7,8]. During this period the rst metal-to-plastic urethane adhesiveswere developed [9]. Waterborne polyurethanes were also being developed, with apolyurethane latex claimed to be useful as an adhesive disclosed in 1961 by du Pont[10]. A commercial urethane latex was available by 1963 (Wyandotte ChemicalsCorporation) [11]. The adhesive properties of urethane latexes were explored further byW.R. Grace in 1965 [12]. In the early 1960s, B.F. Goodrich developed thermoplasticpolyester polyurethanes that could be used to bond leather and vinyl [13]. In 1968Goodyear introduced the rst structural adhesive for berglass reinforced plastic (FRP),used for truck hoods [14].

Polyurethane pressure-sensitive adhesives began appearing in the early 1970s [15].By 1978 advanced two-component automotive structural adhesives (Goodyear) were com-mercially available. Waterborne polyurethane adhesives received additional attentionduring this period [16]. In 1984, Bostik developed reactive hot-melt adhesives [17].Polyurethane adhesives are sold into an ever-widening array of markets and products,where they are known for their excellent adhesion, exibility, low-temperature

Copyright 2003 by Taylor & Francis Group, LLC

performance, high cohesive strength, and cure speeds that can readily be tailored to themanufacturers demands [18].

Urethanes make good adhesives for a number of reasons: (1) they eectively wet thesurface of most substrates (the energy level of very low energy surfaces such as polyethy-lene or polypropylene must be raised before good wetting occurs) [19], (2) they readilyform hydrogen bonds to the substrate, (3) their small molecular size allows them topermeate porous substrates, and (4) they form covalent bonds with substrates that haveactive hydrogens. Figure 1 shows the typical mechanism for a urethane adhesive bondingcovalently to a polar surface.

Polyurethane adhesive consumption has been estimated at 217 million pounds (1991)having a value of approximately $301 million (see Fig. 2). Applications contributing to thisvolume are shown in Table 1. It is interesting to note that while the packaging market isthe fourth-largest market in terms of pounds of urethane adhesives sold, it is substantiallylarger than the forest products market and the foundry core binder market in terms of

Figure 2 Polyurethane adhesive consumption, 1991.

Figure 1 Typical mechanism for a urethane adhesive bonding covalently to a polar surface.


dollars. Overall, the polyurethane adhesives market grew at an annual rate of approxi-mately 3% from 1986 to 1991. Specic market segments such as automotive and recrea-tional vehicles easily surpassed the gross national product (GNP) growth rate. In the nextfew years a number of specic market segments are expected to grow at about 5% peryear. These would include vehicle assembly (automotive and recreational vehicles), elec-tronics, furniture, and curtain wall manufacture.

II. APPLICATIONS OVERVIEW

The textile market has traditionally been the largest consumer of polyurethane adhesives.There are a number of high-volume applications, including textile lamination, integralcarpet manufacture, and rebonded foam. Textile lamination occurs through either a solu-tion coating process or ame bonding. Flame bonding textile lamination is accomplishedby melting a polyurethane foam by ame and then nipping the foam between two textilerolls while it is still tacky. Integral carpet manufacture describes carpeting that is manu-factured by attaching either nylon, wool, or polypropylene tufts that are woven through apolypropylene scrim with a urethane adhesive to a polyurethane foam cushion in a con-tinuous process. Rebonded foam is made using scrap polyurethane foam bonded togetherwith a urethane prepolymer and is used primarily as carpet underlay. Durability, exibil-ity, and fast curing speeds are all critical parameters for these applications.

Foundry core binders are isocyanate-cured alkyd or phenolic adhesives used asbinders for sand used to produce foundry sand molds. These sand molds are used tocast iron and steel parts. A fast, economical cure of the sand mold is required underambient conditions.

Packaging adhesives are adhesives used to laminate lm to lm, lm to foil, and lmto paper in a variety of packaging constructions. A broad variety of products are sold tothis market, with solvent-based, high solids, 100% solids, and waterborne adhesives allbeing used. Polyurethane adhesives are considered one of the high-performance productsoered to this industry because of their excellence in adhesive properties, heat resistance,chemical resistance, and fast curing properties. Polyurethane adhesives can also bedesigned to meet U.S. Food and Drug Administration approval, a requirement for foodpackaging applications.

Table 1 Sales Distribution for Polyurethane Adhesives, 1991

Volume Sales

Market segment (lb 106) ( 106)Textiles 82.7 $79.6

Foundry core binders 66.0 62.5

Forest products 30.7 22.6

Packaging 25.0 79.7

Automotivea 6.2 21.4

Footwear 1.6 13.2

Furniture 3.8 15.4

Recreational vehicles 1.6 5.0

Other 0.35 1.8

aDoes not include windshield sealant volumes.


Solvent-borne adhesives represent the majority of the volume in the packagingmarket, with both one- and two-component systems being used. Waterborne polyurethaneadhesives are a much smaller segment that has been driven by environmental considera-tions. Growth has slowed in recent years because of generally inferior performance com-pared to solvent-based adhesives and because most of the major converters have alreadymade capital investments in solvent recovery systems.

Isocyanates are used in the forest products industry to adhesively bond wood chips,which are then pressed to form particleboard and oriented strandboard. Urethanes arealso used to ll knotholes and surface defects in nished plywood boards (plywoodpatch). These lled systems must cure rapidly and be sanded easily.

The transportation market has used polyurethane adhesives for such diverse appli-cations as bonding FRP and sheet molding composite (SMC) panels in truck and carapplications, polycarbonate headlamp assemblies, door panels, and weatherstrip ocking.

The construction market for polyurethane adhesives consists of a variety of applica-tions, such as laminating thermal sandwich panels, bonding gypsum board to wood ceilingjoists is modular and mobile homes, and gluing plywood oors. Early green strength, lowshrinkage, and high bond strength are critical properties.

The furniture industry uses polyurethane adhesives to bond veneers of various com-position to boardstock and metal substrates. Both waterborne and solvent-based adhe-sives are used.

Footwear is a sizable niche for polyurethane adhesives that are used to attach thesoles. Polyurethane adhesives compete primarily with neoprene-based adhesives and havereplaced much of the neoprene volume due to improved performance. However, the over-all market has declined as U.S. manufacturers have moved production overseas.

III. BASIC URETHANE CHEMISTRY

Isocyanates react with active hydrogens as depicted in Fig. 3. This addition reaction occurswith the active hydrogen adding to the nitrogen atom and the electron-rich nucleophile(Lewis base) reacting with the carbonyl group. Generally, the stronger the base, the morereadily it reacts with the isocyanate. Table 2 shows typical reaction rates of some activehydrogen-containing compounds.

As expected, the aliphatic amines and aromatic amines (the strongest bases in thetable) react the fastest. The urethanes industry has taken advantage of this reactivity intwo-component commercial processes, demanding fast cure by using specially designedmetering equipment and spray heads.

Alcohols and water react readily at room temperature. Most urethane adhesivesdepend on the NCO group reacting with either water or alcohols. Primary OH groupsare two to three times as fast as sterically hindered secondary OH groups under equivalentconditions. The reaction rates shown in Table 2 reect uncatalyzed reaction rates andshould be used as an indication of relative reaction rates. Actual rates are dependent on

Figure 3 Reaction of isocyanate with active hydrogen.


solvent, temperature, and the presence of catalysts. Catalysts can signicantly acceleratethese reactions and can in some cases alter the order of reactivity [20].

A. Branching Reactions

There are a number of complex reactions that can occur besides the desired reaction of thepolyol hydroxyl group with the isocyanate group to form a urethane, as shown in Fig. 4.Isocyanates can continue to react with undesirable consequences under conditions of highheat or strong bases. Basic impurities and excess heat catalyze branching reactions, leadingto variations in prepolymer viscosity, gelation, and exotherms. Most basic impurities arisefrom the polyol, since polyols are typically produced under basic condition. As such, thenet acidity of the overall system (contribution of acidic or basic components from thereactants) plays a critical role in determining the nal viscosity achieved [21,22].

The presence of water will lead to the formation of ureas and evolve CO2 as shown inFig. 5. This mechanism is thought to proceed through the formation of an unstableintermediate, carbamic acid, which then decomposes to give CO2 and an aromaticamine. The amine will then react further with another isocyanate to give a urea linkage.All common moisture-cured urethanes give o CO2 upon curing, which can pose problems

Table 2 Typical Reaction Rates for Selected Hydrogen-Containing Compounds

Active hydrogen compound Typical structure

Relativea

reaction rate

Aliphatic amine R NH2 100,000

Secondary aliphatic amine R2 NH 20,00050,000

Primary aromatic amine Ar NH2 200300

Primary hydroxyl R CH2OH 100

Water H O H 100

Carboxylic acid R CO2H 40

Secondary hydroxyl R2CH OH 30

Urea proton R NH CO NH R 15

Tertiary hydroxyl R3C OH 0.5

Urethane proton R NH CO OR 0.3

Amide R CO NH2 0.1

aUncatalyzed reaction rate, 80C [1].

Figure 4 Reaction of polyol hydroxyl group with isocyanate group to form a urethane.


if not properly controlled. Urea groups are known to cause high prepolymer viscositybecause of increased hydrogen bonding and because of their ability to react further withexcess isocyanate groups to form a biuret, as shown in Fig. 6.

At room temperature the biuret reaction proceeds very slowly; however, elevatedtemperatures and the presence of trace amounts of basicity will catalyze the biuret reactionas well as other branching reactions. These would include the formation of allophanategroups, as shown in Fig. 7 (due to the reaction of urethane groups with excess isocyanategroups), or trimerization of the terminal NCO group (to form an isocyanurate), as shownin Fig. 8. Biurets and allophanates are not as stable thermally or hydrolytically as branchpoints achieved through multifunctional polyols and isocyanates. The allophanates shownin Fig. 7 can continue to react with excess isocyanates to form isocyanurates (as shown inFig. 8), a trimerization reaction that will liberate considerable heat. In most cases thedesired reaction product is the simple unbranched urethane or a urea formed by directreaction of an isocyanate with an amine. Ureas are an important class because theytypically have better heat resistance, higher strength, and better adhesion. By controllingthe reaction temperature (typically less than 80C) and stoichiometry, and using a weakly

Figure 6 Reaction of urea with isocyanate.

Figure 5 Reaction of isocyanate with water.


basic catalyst (or none at all), the reaction will stop at the urethane or urea product.Increasing the functionality of the polyol or the isocyanate will achieve branching orcross-linking in a more controlled fashion.

B. Catalysts

As noted previously, strong or weak bases that are sometimes present in the polyols willcatalyze the urethane reaction. The eect of catalysts on the isocyanate reaction is welldocumented. Indeed, the rst reported examples occur in the literature well beforeurethanes became a commercially signicant class of compounds. The rst use of acatalyst with an isocyanate was reported by Leuckart in 1885 [23]. Other early reports

Figure 8 Reaction of allophanate with isocyanate.

Figure 7 Reaction of urethane with isocyanate.


were from French and Wirtel (1926), who used triethylamine to catalyze the reaction ofphenols with 1-naphthylisocyanate [24]. Baker and Holdsworth (1947) detailed themechanism of the urethane reaction [25].

Commercial catalysts consists of two main classes: organometallics and tertiaryamines. Both classes have features in common in that the catalytic activity can bedescribed as a combination of electronic and steric eects. Electronic eects arise as theresult of the molecules ability to donate or accept electrons. For example, in the tertiaryamines, the stronger the Lewis base, generally the stronger the polyurethane catalyst.Empty electronic orbitals in transition metals allow reactants to coordinate to the metalcenter, activating bonds and placing the reactants in close proximity to one another.

Steric eects arise from structural interactions between substituents on the catalystand the reactants that will inuence their interaction. The importance of steric eects canbe seen by comparing the activity for triethylenediamine to that of triethylamine. Thestructure of triethylenediamine (see Fig. 9) forces the nitrogens to direct their lone electronpairs outward in a less shielded position than is true of triethylamine. This results in a rateconstant for triethylenediamine that is four times that of triethylamine at 23C [1].

Organometallic complexes of Sn, Bi, Hg, Zn, Fe, and Co are all potent urethanecatalysts, with Sn carboxylates being the most common. Hg catalysts have long inductionperiods that allow long open times. Hg catalysts also promote the isocyanatehydroxylreaction much more strongly than the isocyanatewater reaction. This allows their use incasting applications where pot life and bubble-free parts are critical. Bismuth catalysts arereplacing mercury salts in numerous applications as the mercury complexes have comeunder environmental pressure.

Catalysts will not only accelerate reaction rates but may also change the order ofreactivity. Table 3 illustrates this behavior. These data indicate that amines do not aectthe relative reactivities of dierent isocyanates and show that Zn, Fe, and Co complexesactually raise the reactivity of aliphatic isocyanates above aromatic isocyanates.

IV. URETHANE POLYMER MORPHOLOGY

One of the advantages that a formulator has using a polyurethane adhesive is the ability totailor the adhesive properties to match the substrate. Flexible substrates such as rubber orplastic are obvious matches for polyurethane adhesives because a tough elastomeric pro-duct can easily be produced. Polyurethanes derive much of their toughness from theirmorphology.

Polyurethanes are made up of long polyol chains that are tied together by shorterhard segments formed by the diisocyanate and chain extenders if present. This is depictedschematically in Fig. 10. The polyol chains (typically referred to as soft segments) impart

Figure 9 Structure of (a) triethylenediamine and (b) triethylamine.


low-temperature exibility and room-temperature elastomeric properties. Typically, thelower-molecular-weight polyols give the best adhesive properties, with most adhesivesbeing based on products of molecular weight less than 2000. Generally, the higher thesoft segment concentration, the lower will be modulus, tensile strength, hardness, and tearstrength, while elongation will increase. Varying degrees of chemical resistance and heatresistance can be designed by proper choice of the polyol.

Short-chain diols or diamines are typically used as chain extenders. These moleculesallow several diisocyanate molecules to link forming longer-segment hard chains withhigher glass transition temperatures. The longer-segment hard chains will aggregatetogether because of similarities in polarity and hydrogen bonding to form a pseudo-cross-linked network structure. These hard domains aect modulus, hardness, and tear

Figure 10 Polyol-chain structure of polyurethane.

Table 3 Gelation Times (min) at 70C

Catalyst TDI

Isocyanate

m-xylene

diisocyanate

Hexamethylene

diisocyanate

None >240 >240 >240

Triethylamine 120 >240 >240

Triethylenediamine 4 80 >240

Stannous octoate 4 3 4

Dibutyltin di(ethylhexoate) 6 3 3

Bismuth nitrate 1 0.5 0.5

Zinc naphthenate 60 6 10

Ferric chloride 6 0.5 0.5

Ferric 2-ethylhexoate 16 5 4

Cobalt 2-ethylhexoate 12 4 4

TDI, toluene diisocyanate.

Source: Ref. 20.


strength and also serve to increase resistance to compression and extension. The hardsegments will yield under high shear forces or temperature and in fact determine theupper use temperature of the product. Once the temperature or shear stress is reduced,the domains will re-form.

The presence of both hard segment and soft segment domains for polyurethanesgives rise to several glass transition temperatures, one below 30C which is usuallyassociated with the soft segment, transitions in the range 80 to 150C, and transitionsabove 150C. Transitions in the range 80 to 150C are associated with the breakup ofurethane hydrogen bonds in either the soft segment or the hard segment. Transitionshigher than 150C are associated with the breakdown of hard segment crystallites oraggregates. Linear polyurethane segmented prepolymers can act as thermoplastic adhe-sives which are heat activated. A typical use for this type of product is in the footwearindustry.

By proper choice of either the isocyanate or the polyol, actual chemical cross-linkscan be introduced in either the hard or soft segments that may be benecial to someproperties. The eectiveness of these cross-links is oset by a disruption of the hydrogenbonding between polymer chains. Highly cross-linked polyurethanes are essentially amor-phous in character exhibiting high modulus, hardness, and few elastomeric properties.Many adhesives fall into this category.

V. PREPOLYMER FORMATION

Most urethane adhesives are based on urethane prepolymers. A prepolymer is madeby reacting an excess of diisocyanate with a polyol to yield an isocyanate-terminatedurethane as shown in Fig. 11. Prepolymers may have excess isocyanate present (quasi-prepolymers) or they may be made in a 2:1 stoichiometric ratio to minimize the amount offree isocyanate monomer present. Most moisture-cured prepolymers are based on 2:1 stoi-chiometric ratios. Two-component adhesives generally are based on quasi-prepolymers,which use the excess isocyanate to react with either chain extenders present in the othercomponent or with the substrate surface.

Prepolymers are isocyanates and react like isocyanates, with several important dif-ferences. Prepolymers typically are much higher in molecular weight, are higher in visc-osity, are lower in isocyanate content by weight percent, and have lower vapor pressures.Prepolymers are important to adhesives for a number of reasons. The desired polymericstructure of the adhesive can be built into the prepolymer, giving a more consistent

Figure 11 Reaction of isocyanate with polyol.


structure with more reproducible physicals. In addition, since part of the reaction has beencompleted, reduced exotherms and reduced shrinkage are normally present. For two-component systems, better mixing of components usually occurs, since the viscosity ofthe two components more closely match. In addition, the ratios of the two componentsmatch more closely. Side reactions such as allophanate, biuret, and trimer are lessened.Finally, prepolymers typically react more slowly than does the original diisocyanate,allowing longer pot lives.

VI. ADHESIVE RAW MATERIALS

Polyols for adhesive applications can be generally broken down into three main categories:(1) polyether polyols, (2) polyester polyols, and (3) and polyols based on polybutadiene.Polyether polyols are the most widely used polyols in urethane adhesives because of theircombination of performance and economics. They are typically made from the ring-opening polymerization of ethylene, propylene, and butylene oxides, with active protoninitiators in the presence of a strong base as shown in Fig. 12.

Polyether polyols are available in a variety of functionalities, molecular weights,and hydrophobicity, depending on the initiator, the amount of oxide fed, and the typeof oxide. Capped products are commercially available as well as mixed-oxidefeed polyols, as shown in Fig. 13. Polyether polyols typically have glass transitionsin the 60C range, reecting the ease of rotation about the backbone and little chaininteraction. As one would expect from such low glass transition temperatures, theyimpart very good low-temperature performance. The polyether backbone is resistant toalkaline hydrolysis, which makes them useful for adhesives used on alkaline substratessuch as concrete. They are typically very low in viscosity and exhibit excellent substratewetting. In addition, their low cost and ready availability from a number of suppliersadd to their attractiveness.

The more commonly used polyether polyols range in molecular weight from 500 to2000 for diols and 250 to 3000 for triols. Lower-molecular-weight, higher-functionalitypolyols are traditionally used in rigid-foam applications but have also been used ascross-linkers for two-component, fast-curing urethane adhesives. Polytetramethylene gly-cols (PTMOs; see Fig. 14) can be considered a subset of polyether polyols. They oer

Figure 12 Ring-opening polymerization to form polyether polyols.


improved physical properties compared to polyethers based on ethylene oxide, propyleneoxide, or butylene oxide, combining high tensile strength (due to stress crystallization)with excellent tear resistance. They are also noted for their excellent resistance to hydro-lysis. They are typically priced at a premium to other polyols.

Polyester polyols are used widely in urethane adhesives because of their excellentadhesive and cohesive properties. Compared to polyether-based polyols, polyester-basedpolyol adhesives have higher tensile strengths and improved heat resistance. These benetscome at the sacrice of hydrolytic resistance, low-temperature performance, and chemicalresistance. One of the more important application areas for these products is in thesolvent-borne thermoplastic adhesives used in shoe sole binding. These products are typi-cally made from adipic acid and various glycols (see Fig. 15).

Some glycerine or trimethylolpropane may be used to introduce branching structureswithin the polyester backbone. Phthalic anhydride may also be used to increase hardnessand water resistance. Inexpensive terephthalic acid-based polyesters from recycled poly-ethyleneterephthalate (PET) resins have more recently become popular.

Figure 13 Various commercially available capped products and mixed-oxide feed polyols.BO, butylene oxide; EO, ethylene oxide; PO, propylene oxide.

Figure 15 Reaction of diol with diacid to form polyester polyol.

Figure 14 Structure of polytetramethylene oxide.


Polycaprolactones (see Fig. 16), another type of polyester polyol, oer improve-ments in hydrolysis resistance and in tensile strength (can stress crystallize) over adipicacid-based polyester polyols. They are typically higher in viscosity and higher in cost thanpolyether polyols of comparable molecular weight. When moisture resistance is critical,urethane adhesives incorporating polybutadiene polyols are used. These products arehydroxy-terminated, liquid polybutadiene resins. The hydrocarbon backbone greatlydecreases water absorption, imparting excellent hydrolytic stability. Polybutadienecompounds also have exceptional low-temperature properties, with glass transition tem-peratures being reported below 70C [26]. These products are priced at a 40 to 50%premium over comparable polyether polyols. The structure of polybutadiene polyols isshown in Fig. 17.

A. Isocyanates for Adhesive Applications

Toluene diisocyanate (TDI) is a colorless, volatile, low-viscosity liquid commonly used inthe adhesives area to manufacture low-viscosity prepolymers for exible substrates. Thestructure of TDI is shown in Fig. 18. TDI is typically supplied as an 80:20 mixture of the2,4 and 2,6 isomers, respectively, with two grades of acidity available. Type I TDI is low inacidity (10 to 40 ppm); type II TDI is higher (80 to 120 ppm). Type II TDI is generally usedfor prepolymer applications because the additional acidity is available to neutralize tracebases found in polyether polyols. These trace bases can cause branching reactions duringprepolymer cooks, causing high viscosities and even gelations if not properly controlled(see Section V). The extra acidity present also serves to stabilize the prepolymer, extendingthe shelf stability. In addition, since TDI is predominately the 2,4 isomer, a reactivitydierence is noted for the isocyanate groups. Since the less hindered site reacts rst, thesterically hindered site is left when prepolymers are formed, leading to prepolymers thatare more shelf stable. TDI prepolymers are used in adhesives for the textile and foodpackaging laminates industry, where a t is found for their low viscosity and low cost.The volatility of TDI and additional handling precautions that must be taken when usingTDI has limited its growth in adhesive applications.

Methylene diphenyl disiocyanate (MDI) is used where high tensile strength,toughness, and heat resistance are required. MDI is less volatile than TDI, making it

Figure 17 Structure of polybutadiene polyol.

Figure 16 Structure of polycaprolactone diol.


less of an inhalation hazard. The acidity levels in MDI are very low, typically on the orderof 0 to 10 ppm, so the trace base levels in the polyols are much more critical in prepolymerproduction than with TDI. The structure of MDI is shown in Fig. 19. There are severalcommercial suppliers of MDI that typically supply grades with 98% or better 4,40 isomer.MDI is a solid at room temperature (melting point 38C, 100F), requiring handlingprocedures dierent from those for TDI. MDI should be stored as a liquid at 115F orfrozen as a solid at (20F) to minimize dimer growth rate. MDI reacts faster than TDI,and because the NCO groups in MDI are equivalent, they have the same reactivity, acontrast to TDI. MDI is used in packaging adhesives, structural adhesives, shoe soleadhesives, and construction adhesives.

Several MDI products have been introduced that address the inconvenienceof handling a solid. They are seeing increased usage in the adhesives industry and areexpected to experience a higher growth rate. Most MDI producers oer a uretonimine-modied form of MDI that is a liquid at room temperature. The uretonimine structureis shown in Fig. 20. In addition, several producers have introduced MDIs containingelevated levels of the 2,40 isomer, as shown in Fig. 21. At approximately the 35%, 2,40

isomer level, the product becomes a liquid at room temperature, greatly increasing thehandling ease. A number of advantages are seen: slower reactivity, longer pot life, lower-viscosity prepolymers, prepolymers with lower residual monomeric MDI, and improvedshelf stability.

Polymeric MDIs are made during the manufacturing of monomeric MDI. Theseproducts result as higher-molecular-weight oligomers of aniline and formaldehydeget phosgenated. A typical structure for these products is shown in Fig. 22. These oligo-mers average 2.3 to 3.1 in functionality and contain 30 to 32% NCO. Much of thehydrolyzable chlorides and color bodies produced in the manufacturing process of MDIis left behind in these products. The acidity levels can be 10 to 50 times the level found inpure MDI, and the products are dark brown in color. The higher acidity level decreasesreactivity; however, this decrease is oset somewhat by the higher functionality.

Polymeric MDIs are typically lower in cost than pure MDI and because of theincreased asymmetry have a lower freeze point (liquids at room temperature). They areless prone to dimerization, and as a consequence are more storage stable than are pureMDI and derivatives. Polymeric MDIs are used whenever the color of the nished

Figure 18 Structure of the 2,4 and 2,6 isomers of toluene diisocyanate.

Figure 19 Structure of methylene diphenyl diisocyanate.


adhesive is not a concern. They are generally not used for prepolymers because high-viscosity branched structures typically result. They are widely used as adhesives in thefoundry core binder area, in oriented strandboard or particleboard, and between rubberproducts and fabric or cord. It is interesting to note that the polymeric isocyanates usedcommercially today are structurally very similar to the Desmodur R (trademark, Bayer)products used over 50 years ago [2].

Aliphatic isocyanates are used whenever resistance to ultraviolet light is a criticalconcern. Examples of aliphatic isocyanates are hexamethylene diisocyanate, hydrogenatedMDI, isophorone diisocyanate, and tetramethylxylene diisocyanate. Structures for thesemolecules are shown in Fig. 23. The aliphatic isocyanates are usually more expensivethan aromatic isocyanates and nd limited use in adhesive applications. Resistanceto ultraviolet light is usually not a critical concern in adhesives because the substrateshields the adhesive from sunlight.

Blocked isocyanates are also used in urethane adhesives. Blocking or maskingof the isocyanates refers to reacting the isocyanate groups with a material that will preventthe isocyanate from reacting with active hydrogen-containing species at room temperaturebut will allow that reaction to occur at elevated temperatures. Blocked isocyanates areeasily prepared and their chemistry has been developed extensively since their inceptionby Bayer and co-workers during the early 1940s [2729]. As an example, the preparation ofa methylethylketoxime blocked isocyanate is shown in Fig. 24.

Blocked isocyanates oer a number of advantages to unblocked isocyanates.The traditional concern for moisture sensitivity can be addressed by blocking theisocyanate. Heat activation is then required, but most commercial adhesive applicationscan meet this requirement. Water-based dispersions and dispersions of the isocyanatein the polyol or other reactive media become possible using blocked isocyanates. Thereare a number of blocked isocyanates commercially available that could be used in adhesive

Figure 22 Structure of polymeric MDI.

Figure 20 Structure of uretonimine.

Figure 21 Structure of the 2,40 isomer of MDI.


applications. Miles (Bayer) produces a series of aromatic and aliphatic blockedisocyanates marketed for primers, epoxy exibilizers, wire coatings, and automotive top-coat applications. Blocked isocyanates are widely patented for fabric laminating adhesives[30], fabric coating adhesives [3134], and tire cord adhesives [3540].

B. Toxicology

Polyether polyols are generally considered to be low in toxicity with respect to eye and skinirritation; however, amine-initiated polyether polyols have been found to be more irritat-ing to the skin and eyes. The manufacturers material safety data sheet (MSDS) shouldalways be consulted before use. Oral toxicity is generally a secondary concern in anindustrial environment. The vapor pressure of polyols is generally negligible; thus vaporinhalation is not usually a concern [41]. Low-molecular-weight glycols (chain extenders)are considered more problematical than polyether polyols. While generally the vaporpressure of these products is low, there are processes that could potentially result invapor concentrations close to the exposure limits [41]. The exposure guidelines for chainextenders may be written to dierentiate between aerosols and vapors. For more specichandling information the manufacturer should be consulted.

Figure 24 Preparation of a methylethylketoxime blocked isocyanate.

Figure 23 Structure of various aliphatic isocyanates.


The toxicology of isocyanates is a primary concern when developing or using poly-urethane adhesives. Respiratory eects are the primary toxicological manifestation ofrepeated overexposure to diisocyanates [4246]. In addition, most of the monomeric iso-cyanates are eye and skin irritants. Precautions should be taken in the workplace toprevent exposure. The risk of overexposure is primarily (but not limited to) allergic sensi-tization with asthma-type symptoms. Manufacturers guidelines (MSDS) should be con-sulted for the most current information and legal requirements.

C. Fillers and Additives

Fillers are used in adhesives to improve physical properties, to control rheology, and tolower cost. The most common polyurethane llers are calcium carbonate, talc, silica, clay,and carbon black. A more rigorous treatment of this subject can be found in Katz andMilewski [47]. Fumed silicas and carbon blacks are used primarily as thixotropes inapplication areas that require a nonsagging bead. Calcium carbonates, clays, and talcsare used to improve the economics of an adhesive formulation. A major concern usingllers with urethane prepolymers is the moisture content associated with the llers. Fillerstypically must be dried prior to use with urethane prepolymers or isocyanates.Hygroscopic llers should be avoided, as moisture introduced by the ller can lead topoor shelf stability of the nished product.

Pigments are sometimes used in polyurethane adhesive systems, but since mostadhesives are generally hidden from view, pigments do not play major roles. Pigmentsmay be used to color the adhesive to match the substrate. Pigments are more typically usedto color one side of a two-component system to help the user distinguish between theisocyanate and the polyol. They are also sometimes used as an aid to judge mix ratios.Carbon black and titanium dioxide are two commonly used pigments.

Plasticizers can also be used in polyurethane adhesives to lower viscosity, improveller loadings, improve low-temperature performance, and plasticize the polyurethaneadhesive. Phthalate esters, benzoate esters, phosphates, and aromatic oils are commonexamples [48]. Plasticizers should be used sparingly, as adhesion will generally decrease aslevels increase.

VII. SURFACE PREPARATION AND PRIMERS

Proper surface preparation is the key to obtaining good adhesive bonds having a predict-able service life. Substrate surfaces may have dirt, grease, mold-release agents, processingadditives, plasticizers, protective oils, oxide scales, and other contaminants that will forma weak boundary layer. When the adhesive fails it is usually through this region, giving alow-strength bond. Some form of surface treatment is necessary to obtain optimum bondstrength. The primary goal of surface treatment is to remove any weak surface boundarylayer on the substrate [49]. A large number of surface treatments have been developed,with many targeted toward specic substrates. These would include mechanical abrasion,etching, solvent cleaning, detergent washing, ame treatments, chemical treatments,and corona discharges [19,5055].

Primers are also used in conjunction with a surface treatment either to improveadhesive performance or to increase production exibility in a bonding operation.Isocyanates have been used for over 50 years as primers on substrates such as rubber,


plastic, bers, and wood [56]. Isocyanates will react with polar groups on the surface andpromote bonding.

Silane coupling agents are commonly used as primers for glass, ber composites,mineral-lled plastics, and cementacious surfaces. The silane coupling agents havebeen found to be especially eective with glass substrates. One end of the couplingagent is an alkoxysilane that condenses with the silanol groups on the glasssurface. The other end of the coupling agent is an amino, mercapto, or epoxyfunctionality that will react with the isocyanate group in the adhesive. Epoxy silaneshave also been used as additives to adhesives to improve water resistance [57]. Otherorganometallic primers are based on organotitanates, organozirconates, and somechromium complexes [49].

VIII. COMMON ADHESIVE TYPES

A. One-Component Adhesives

The oldest types of one-component polyurethane adhesives were based on di- or triiso-cyanates that cured by reacting with active hydrogens on the surface of the substrate ormoisture present in the air or substrate. The moisture reacts with the isocyanate groups toform urea and biuret linkages, building molecular weight, strength, and adhesive proper-ties. Prepolymers are also used either as 100% solids or solvent-borne one-componentadhesives. Moisture-cured adhesives are used today in rebonded foam, tire cord,furniture, and recreational vehicle applications.

A second type of one-component urethane adhesive comprises hydroxypolyurethanepolymers based on the reaction products of MDI with linear polyester polyols andchain extenders. There are several commercial suppliers of these types of thermoplasticpolyurethanes. The polymers are produced by maintaining the NCO/OH ratio at slightlyless than 1:1 to limit molecular weight build to the range 50,000 to 200,000 with a slighthydroxy content (approximately 0.05 to 0.1%). These are typically formulated in solventsfor applications to shoe soles or other substrates. After solvent evaporation heat is usedto melt the polymer (typically 50 to 70C; at these temperatures the polymers reachthe soft, rubbery, amorphous state), so the shoe upper can be press t to the sole. Uponcooling, the adhesive recrystallizes to give a strong, exible bond [58]. More recently,polyisocyanates have been added to these to increase adhesion and other physicalproperties upon moisture curing. In Section IX.B we discuss this in more detail.

The use of waterborne polyurethane adhesives has grown in recent years as they havereplaced solvent-based adhesives in a number of application areas. There are a numberof papers and patents covering the use of waterborne polyurethanes in shoe soles, packa-ging laminates, textile laminates, and as an adhesive binder for the particleboard industry[5962]. Because waterborne polyurethane adhesives have no VOC (volatile organiccontent) emissions and are nonammable, they are more environmentally friendly.Typically they can be blended with other dispersions without problems and exhibitgood mechanical strength. Water-based systems are fully reacted, linear polymers thatare emulsied or dispersed in water. This is accomplished by building hydrophilicityinto the polymer backbone with either cationic or anionic groups or long hydrophilicpolyol segments or, less frequently, through the use of external emulsiers. Figure 25illustrates the more common functional groups that can be built into the urethane mole-cule that will confer hydrophilicity.


A typical example of how these groups are built into the polymer backbone is shownin Fig. 26. A urethane prepolymer is reacted with chain extenders containing either car-boxylates or sulfonates in a water-miscible solvent (e.g., acetone). The reaction product isan isocyanate-terminated polyurethane or polyurea with pendant carboxylate or sulfonategroups. These groups can easily be converted to salts, which as water is added to theprepolymersolvent solution, allows the prepolymer to be dispersed in water. The solventis then stripped, leaving the dispersed product. There are variations on this theme thatallow lower solvent volumes to be used [63]. Long hydrophilic polyol segments can also beintroduced. Chain extenders with hydrophilic ethylene oxide groups pendant to the back-bone are reacted with the prepolymer to form a nonionic self-emulsifying polyurethane.This reaction is also carried out in a water-miscible solvent that can later be stripped fromthe solventwater solution.

Blocked isocyanates can also be considered a one-component adhesive. The use of ablocking agent allows the isocyanate to be used in a reative medium that can be heatactivated. One-component adhesives based on blocked isocyanates are thus not amenableto room-temperature curing applications. The chemistry of these products is covered inmore detail in Section VI.A.

B. Two-Component Adhesives

The second major classication of common polyurethane adhesives is the two-componentsystem. Two-component polyurethane adhesives are widely used where fast cure speedsare critical, as on OEM (original equipment manufacturers) assembly lines that requirequick xture of parts, especially at ambient or low bake temperatures. Two-componenturethanes are required in laminating applications where no substrate moisture is availableor where moisture cannot penetrate through to the adhesive bond. Two-componenturethanes are also useful where CO2 (generated by a one-component moisture cure) ora volatile blocking agent would interfere with the adhesive properties.

Two-component adhesives typically consist of low-equivalent-weight isocyanate orprepolymer that is cured with a low-equivalent-weight polyol or polyamine. They may be100% solids or solvent borne. Since the two components will cure rapidly when mixed,they must be kept separate until just before application. Application is followed quickly bymating of the two substrates to be bonded.

Figure 25 Common functional groups that confer hydrophilicity in the urethane molecule.


Ecient mixing of the two components is essential for complete reaction and fulldevelopment of designed adhesive properties. In-line mixing tubes are adequate for low-volume adhesive systems. For larger-volume demands, sophisticated meter mix machinesare required that will mix both components just prior to application. Commercial systemsfor delivering two-component adhesives are segmented based on the viscosity ranges of thecomponents. The ranges can be broken down into low, middle, and high viscosity, with,for example, Liquid Control Corp., Sealant Equipment and Engineering Inc., and GracoInc., respectively, supplying equipment for the three ranges [64].

In present-day high-speed assembly line operations, adhesives are applied roboti-cally. The adhesive bead is applied quickly and evenly to parts on a conveyor line just priorto being tted. These operations, especially the need to handle the adhered substrates soonafter assembly, demand fast-curing adhesive systems [65]. Two-component adhesives areused to bond metals to plastics in automobiles, to laminate panels in the constructionindustry, to laminate foams to textiles, to laminate plastic lms together, and to bondpoly(vinylidene chloride) lms to wood for furniture. A commercial waterborne two-component adhesive is sold by Ashland under the trademark ISOSET. This system isused for exterior sandwich panels by recreational vehicle manufacturers and is composedof a water-emulsiable isocyanate and a hydroxy-functionalized emulsion latex.

Figure 26 How functional groups are built into the polymer backbone of urethane.


IX. RECENT DEVELOPMENTS

A. Hybrid Adhesives

Over the last four decades there have been a number of attempts to wed the uniquebenets of polyurethane adhesives with the benets of other adhesive systems. Theseattempts have led to the reporting of a variety of urethane hybrids. Early work focusedon simple blends; for example, in 1964 Union Carbide blended organic isocyanates withethylenevinyl acetate copolymers [66]. These blends were used as an adhesive interlayer inglass laminations, particularly safety glass laminates. Similarly, polylurethaneepoxyblends for safety glass laminates were reported in 1970 [67].

More recent eorts have focused on developments that create true hybrids. Forexample, blocked isocyanate prepolymers have been mixed with epoxy resins and curedwith amines [6870]. These blocked prepolymers will react initially with the amines to formamine-terminated prepolymers that cross-link the epoxy resin. Several blocked isocyanatesare commercially available. The DESMOCAP (Bayer) 11A and 12A products are isocya-nates (believed to be blocked with nonylphenol) used as exibilizing agents for epoxyresins. ANCAREZ (trademark, Pacic Anchor, Inc.) 2150 is a blocked isocyanateepoxy blend used as an adhesion promoter for vinyl plastisols. A one-package, heat-cured hybrid adhesive was reported consisting of isophorone diisocyanate, epoxy resin,and a dispersed solid curative based on the salt of ethylenediamine and bisphenol A [71].Urethane amines are oered commercially that can be used with epoxy resins to develophybrid adhesive systems [72].

Urethane acrylic hybrids have been reported based on several approaches. PacicAnchor has developed a urethane acrylate that is commercially available (ANCAREZ300A). Acrylic polyols have been synthesized in the presence of polyether polyols bySaunders for use in two-component structural adhesives with improved tensile andimpact strength [73,74]. Pressure-sensitive acrylic prepolymers with hydroxyl groupshave been formulated with isocyanate prepolymers to give adhesives with improvedpeel strength [75,76]. Aqueous-based vinyl-to-berboard adhesives were reported byChao using water-dispersible MDI with a functionalized acrylic latex and an aqueousdispersion polyurethane to given improved shear and hot peel strength [77]. Acrylo-nitrile dispersion graft polyether polyols have also been used in two-component SMCadhesives [78].

Urethanes have also been used to toughen vinyl-terminated acrylic adhesives forimproved impact resistance. Thus rubber-toughened urethane acrylates [79,80], water-dispersible urethane acrylates [81], and high-temperature-performance urethaneacrylatestructural adhesives have been reported [82]. Polyurethanes terminated with acrylicfunctionality are also used for anaerobic or radiation-cured adhesives with improvedtoughness [83].

B. Reactive Hot Melts

Polyurethane reactive hot melts are 100% solid, hot-melt thermoplastic prepolymers thatmoisture cure slowly after application. Conventional hot melts are known for their quicksetting, excellent green strength, ease of application, and low toxicity. Their primarylimitation is low heat resistance (at elevated temperatures, the adhesive will soften andow) and poor adhesion to some substrates, due to insucient wetting. The use of apolyurethane prepolymer with low levels of free isocyanates as a hot melt oers distinct


advantages: initial green strength is still achieved, and in addition, the isocyanate willmoisture cure slowly, converting the thermoplastic adhesive to a thermoset. There are anumber of recent patents on reactive hot melts [8487]. The tensile strength of the adhesiveincreases, heat resistance is improved, and the nal cured adhesive will not ow at elevatedtemperatures [88]. A limitation of this technology is the need for porous substrates or bonddesigns that will allow the diusion of moisture into the adhesive so that moisture curingwill occur. The adhesive itself must be protected from moisture prior to use. This technol-ogy should be applicable to assembly line operations which require an adhesive that getshigh initial green strength.

C. Pressure-Sensitive Adhesives

The use of polyurethanes in the pressure-sensitive adhesives market has been relativelysmall. Polyurethanes have been somewhat limited to being used as additives to pressure-sensitive adhesives to improve their cohesive strength. Recent developments in the institu-tional carpet backing or automotive carpet oor mat markets suggest that pressure-sensitive urethanes can succeed commercially [89].

X. SUMMARY

Polyurethane adhesives as a class can no longer be perceived as new raw materials. Froma base of 217 million pounds, double-digit growth can no longer be expected. Evenso, signicant growth will continue. Formulators are taking advantage of the tremendousexibility of urethane chemistry in designing new adhesive products. Specialty niches suchas waterbornes and reactive hot melts, for example, will continue to emerge and fuelgrowth. Exciting times lie ahead for innovative formulators of polyurethane adhesives!

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Sept. 2426, 1991.


Handbook of Adhesive Technology, Second Edition, Revised and Expanded Table of ContentsChapter 34: Polyurethane AdhesivesI. INTRODUCTIONII. APPLICATIONS OVERVIEWIII. BASIC URETHANE CHEMISTRYA. Branching ReactionsB. Catalysts

IV. URETHANE POLYMER MORPHOLOGYV. PREPOLYMER FORMATIONVI. ADHESIVE RAW MATERIALSA. Isocyanates for Adhesive ApplicationsB. ToxicologyC. Fillers and Additives

VII. SURFACE PREPARATION AND PRIMERSVIII. COMMON ADHESIVE TYPESA. One-Component AdhesivesB. Two-Component Adhesives

IX. RECENT DEVELOPMENTSA. Hybrid AdhesivesB. Reactive Hot MeltsC. Pressure-Sensitive Adhesives

X. SUMMARYREFERENCES

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