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Polyurethane Adhesive

One-partsolvent-borne polyurethane adhesive

The one part adhesive is essentially a thermoplastic PU (TPU) dissolved in a

solvent. The solvated TPU is applied to the prepared surface as a contact

cement, or is applied, allowed to dry, and then adhesion generated by

contacting the adherends with heat.

polyether soft segments dominate the adhesive application, many

onecomponent solventborne PU adhesives will employ a polyester soft

segment such as polybutylene adipate for the simple reason that its ability to

crystallize can increase the initial adhesion between adherends.

In one part solvent borne PU adhesives, pretreatment of the surface ie.

etensive roughening of the surface and the use of a primer coating that has

preferential adhesion to the substrate and the TPU.

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Typical primer polymers are substances such as chlorinated polyethylene at

high dilution in an aromatic solvent such as toluene.

!or these systems, the solvent can also be a ma"or participant in the growth

of adhesion by swelling both the substrate and the adhesive polymers to

promote polymer chain entanglement or physical interloc#ing of largerpolymer segments that form as the adhesive laminate swells and then

contracts.

Two part solvent borne polyurethane adhesive

$ two part solvent borne version of PU adhesives is typically an isocyanate

tipped prepolymer with relatively low isocyanate content (%&') on the one

side, and chain etender (diol or diamine), catalyst, and often a crosslin#er

for the second component.

$ two part solvent borne system has the bene*t of allowing the opportunity

to tune adhesive modulus and improve solvent resistance via the cross+

lin#ing capability.

loc#ed isocyanates are standard polyisocyanates prereacted with wea#ly

bonded active hydrogen groups.

-hen heated, the wea#ly reacted components decouple releasing the

original polyisocyanate and the bloc#ing agent.

In addition, PU side reactions, particularly with ambient water, are capable ofproducing / gas. In the limit of high humidity, this can have the very

undesirable e0ect of foaming the adhesive layer.

Hotmelt Adhesives

Hot melt adhesive (HMA) is a form of thermoplastic adhesive that is

commonly supplied in solid cylindrical stic#s of various diameters, designed

to be melted in an electric hot glue gun.

The gun uses a continuous+duty heating elementto melt the plastic glue,

which the user pushes through the gun either with a mechanical triggermechanism on the gun, or with direct *nger pressure.

hotmelt adhesives are thermoplastic materials that

(i) are heated and applied to a substrate in the molten state,

(ii) the bonding substrates are assembled, and

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(iii) the thermoplastic resolidi*es bac# to its thermoplastic state.

non+reactive hot melt adhesive

This is ascribed to the higher viscosity of TPU at standard application gun

temperatures that limit substrate wetting. It is possible to reduce viscosity

and thereby improve hotmelt adhesive properties by addition of a monol(e.g., replacing some amount of butanediol with 'butanol), substitution of

even % of the butanediol chain etender with 'butanol can reduce

molecular weight of the chain limited polymer by a factor of &1 and reduce

the viscosity of the polymer melt by over an order of magnitude based on

the 2 3n rule

formulation of a nonreactivepolyurethane hotmelt adhesive

Reactive Hotmelt Adhesive

4eactive PU hotmelt adhesives are essentially a low percentage isocyanate

prepolymer that is applied to a substrate and allowed to cure by reaction via

adventitious moisture, or by direct reaction with substrates possessing active

hydrogen on their surface.

4eactive hotmelts possess a highly desirable grouping of properties includinglow viscosity at application temperatures, rapid generation of green strength

when using crystalizable soft segments, gradual development of very strong

adhesive bonding, no 5/s, and the ecellent material properties of PU

elastomers.

Formulation of a reactive polyurethanehotmelt adhesive

Polycaprolactone based 36I prepolymer with .1 isocyanate useable as a

reactive PU hotmelt adhesive.

Polyether polyols can also be used to ma#e very good reactive hotmelt

adhesives, especially when the prepolymers are made of blended molecular

weights to provide the mi of low viscosity and rapid green strength build.

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Waterborne PU Adhesives (PUd)

PUds enable delivery of a low viscosity urethane polymer to a substrate, while reducing or

eliminating the use of fugitive organic solvents

The polyol and isocyanate structures in PUd prepolymers can vary widely depending on *nal

application and re7uirements.

-hile PUd prepolymers are typically isocyanate terminated, they can also employ hydroyl+

terminated prepolymers for two component waterborne urethanes.

Prior to dispersing in water, the carboylic acid groups in the prepolymer are neutralized

with a strong base such as triethylamine to convert the acid to the carboylate.

It is this hydrophilic neutralized carboylate that serves to disperse and stabilize the

particles in the a7ueous phase. Immediately following this a7ueous dispersion step, a chain

etender is added to the dispersion to complete conversion of the remaining isocyanate

groups to the *nal polymer within the particles.

6ue to the competing reactivity of water, the selection of chain etender is limited to chain

etenders with higher reactivity toward isocyanates than water. 3ost commonly, these chain

etenders are primary or secondary polyamines that chain etend the polymer via formation

of urea bonds

atch Process for PUd

The PUd ionomer process is often done with low levels of a solvent, such as acetone or n+

methyl pyrrolidinone (n3P) to lower prepolymer viscosity in the process.

In the case of acetone, the solvent is removed in a *nal step, while low levels of high boiling

solvents such as n3P may be left in the *nal product.

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PUd ionomer process