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Enhancing theperformance of aromaticPU sealant prepolymersusing oxazolidine latent hardeners

Enhancing theperformance ofaromatic PU sealantprepolymersusing oxazolidinelatent hardeners

case study

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Introduction

Whilst there have been a number of technicaladvances in the resins used in construction sealants,polyurethane chemistry continues to offer the bestbalance of cost and performance. The chemistry ishighly versatile, with aromatic and aliphaticprepolymers offering the formulator a range ofproperties to meet differing performancerequirements.

The main advantage PU offers over othertechnologies are:

• better mechanical strength• no toxic by products from the cure mechanism

compared to the production of methanol from silane cured systems

• better substrate adhesion• ease of paintability

The traditional method of curing PU sealants is via thereaction of the terminal isocyanate groups of thepolyurethane binder with moisture. This requires ahigh level of isocyanate content to afford an effectivecure which leads to toxicity issues with residualisocyanate monomer. Another problem is the sideproduct, carbon dioxide, produced with this reactionwhich results in pinholing defects that can thencompromise the integrity and aesthetics of thefinished coating.

An effective way to eliminate gassing and improve thecure performance of the sealant is to introduce anoxazolidine latent hardener such as Incozol LV orIncozol CF. Using such a hardener type allows theformulator to combine the benefits of the oxazolidinehardener with a prepolymer of much reducedisocyanate content. Typically, the isocyanate level canbe reduced by as much as 10-15%.

The benefits of introducing oxazolidine latenthardeners into aromatic PU sealants are:

• Eliminate gassing• Excellent through cure in thick film applications

(compared to moisture cure/aldimine systems)• Retain good workability of the sealant

(skinning control)• Excellent early Shore hardness development

through hardener crosslinking• At recommended levels of addition, there is no

significant impact on mechanical strength or shrinkage through crosslinking

• No detrimental impact on UV resistance

Study Results

All test data was acquired using the prepolymer with oxazolidine only; no fillers or additives were included inthe mixes.

Dealing with moisture ingress

It is vitally important when handling PU prepolymer mixes with oxazolidine latent hardeners that moistureingress is minimised. Good practice recommends nitrogen purging of all mixing reactors and minimumexposure to atmospheric moisture during handling.

A. Oxazolidines eliminate gassing

Traditionally, PU sealants are cured by the reaction of terminal isocyanate groups with moisture. A side productof this reaction is the generation of carbon dioxide which remains trapped in the cured sealant system. Thepictures below illustrate cured PU prepolymer (MDI: Desmoseal M280 & TDI: Desmodur E15) used in sealantsmixed with oxazolidine latent hardener (typically 2% w/w) in comparison to the same prepolymer cured solelywith moisture.

MDI prepolymer Desmoseal M280 cured without oxazolidine (left), cured with 2% Incozol LV (centre) and curedwith 2% Incozol CF (right)

TDI prepolymer Desmodur E15 cured without oxazolidine (left) and cured with 2% Incozol LV (right)

Enhancing the performance of aromatic PU sealantprepolymers using oxazolidine latent hardeners

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B. Through Cure / Hardness Development

The addition of oxazolidine latent hardener not only eliminates sealant defects created by gassing, but alsopromotes faster through cure than by moisture alone. This reflects the ability of oxazolidine latent hardeners tohydrolyse quickly and crosslink the PU prepolymer in preference to the water and isocyanate reaction.

The above figures of hardness development show faster cure when oxazolidine latent hardeners areincorporated into the systems. This results in increased concentration of reactive groups.

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Hardness development of oxazolidine/Desmoseal M280 systems at 4mm thickness

Days of cure

M280

2% CF

2% LV

1% CF

1% LV

5 10 15 20 25 30

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50

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Hardness development of oxazolidine/Desmodur E15 systems at 4mm thickness

Days of cure

M280

2% CF

2% LV

1% CF

1% LV

Enhancing the performance of aromatic PU sealantprepolymers using oxazolidine latent hardeners

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C. Workability: Effect of the addition of oxazolidine latent hardener on mixing with a PU prepolymer

The addition of oxazolidine latent hardener does not have a significant impact on PU prepolymer viscosity andconsequently, does not affect the workability of the sealant. The initial viscosities of the oxazolidine/prepolymerblends shown in the figures below, indicate the influence of the latent hardeners on the systems. Incozol LVeither reduces or retains the viscosity whilst Incozol CF increases it slightly. These differences offer options forformulators to retain the desired workability of the system.

DesmosealM280

1% CF/M280 2% CF/M280 1% LV/M280 2% LV/M2800

10000

5000

20000

15000

30000

25000

40000

45000

35000

Vis

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Initial viscosity of oxazolidine/Desmoseal M280 systems

DesmodurE15

1% CF/E15 2% CF/E15 1% LV/E15 2% LV/E150

2000

6000

4000

10000

12000

8000

14000

Vis

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Initial viscosity of oxazolidine/Desmodur E15 systems

D. Impact on storage stability

The change in viscosity was measured for unopened containers of PU prepolymer (MDI & TDI) mixes withdifferent oxazolidine latent hardeners. Containers were sealed and stored at ambient temperature (23ºC).

As expected, TDI based mixes are more stable than MDI systems due to the increased reactivity of MDIprepolymers. For TDI systems, mixing with a latent hardener shows an initial viscosity rise where moistureingress from the mixing/handling is consumed. Excellent stability follows with little viscosity rise whilst thecontainers are sealed from the atmosphere.

With MDI mixes, the stability is reduced compared to TDI. The data reflects the inclusion of 2% oxazolidine inDesmoseal M280 prepolymer mixes; dilution of the binder and curing agent in fully formulated systems withdry, inert fillers affords improved stability.

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Storage stability of oxazolidine/M280 system at 230C

Month

Desmoseal M280

2% CF/M280

2% LV/M280

1 2 3 4 5 6

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12000

8000

4000

16000

20000

Vis

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Storage stability of oxazolidine/E15 system at 230C

Month

Desmodur E15

2% CF/E15

2% LV/E15

Enhancing the performance of aromatic PU sealantprepolymers using oxazolidine latent hardeners

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E. Impact on tensile strength

The addition of oxazolidine latent hardeners does not significantly alter physical properties such as hardnessand tensile strength in the cured PU system. Crosslinking at low levels does not detrimentally alter sealantperformance.

In general, Incozol CF slightly increases the hardness and tensile strength of the PU prepolymer whilst IncozolLV slightly reduces the hardness and tensile strength. This is largely due to their respective backbone natures.

DesmosealM280

1% CF/M280 2% CF/M280 1% LV/M280 2% LV/M280

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Shore A hardness of oxazolidine/Desmoseal M280 systems

DesmosealM280

1% CF/M280 2% CF/M280 1% LV/M280 2% LV/M280

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0.4

0.2

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100

150

200

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0.6

0.8

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Tensile properties of oxazolidine/Desmoseal M280 systems

UTS (MPa)

Elongation (%)

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DesmodurE15

1% CF/E15 2% CF/E15 1% LV/E15 2% LV/E15

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Shore A hardness of oxazolidine/Desmodur E15 systems

DesmodurE15

1% CF/E15 2% CF/E15 1% LV/E15 2% LV/E15

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600

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1000

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1.5

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Tensile properties of oxazolidine/Desmodur E15 systems

UTS (MPa)

Elongation (%)

Enhancing the performance of aromatic PU sealantprepolymers using oxazolidine latent hardeners

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Conclusion

The data generated from the study demonstrates the advantages of introducing oxazolidine latent hardenersinto aromatic PU sealant formulations.

Firstly, the ability of Incozol latent hardeners to act as moisture scavengers ensures that the reaction betweenany moisture present and the isocyanate is prevented, thereby preventing the generation of carbon dioxide gaswhich would result in sealant defects.

Secondly, the addition of an oxazolidine latent hardener into the formulation will promote faster through curethan that achieved by moisture alone. This is due to the ability of the latent hardener to hydrolyse quickly andcrosslink the PU prepolymer in preference to the water and isocyanate reaction.

Thirdly, introducing an oxazolidine latent hardener allows the formulator to use a prepolymer with reducedisocyanate content. Typically, the isocyanate level can be reduced by as much as 10-15%.

Oxazolidine latent hardeners, such as Incozol LV and Incozol CF, bring added benefits to the PU sealantformulation but they do this without affecting the physical properties of the system. The data indicates thatthere is no significant detrimental impact on the workability of the sealant, the tensile strength and theelongation.

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Incorez Limited

Iotech House • Miller Street

Preston • Lancashire

PR1 1EA • England

t: +44 (0)1772 201964

f: +44 (0)1772 255194

e: info@incorez.com

w: www.incorez.com

Incorez Corporation

79 Bradley Street

Middleton • Connecticut

06457 • USA

t: +1 (860) 613 1613

f: +1 (860) 613 1637

e: info@incorez.com

w: www.incorez.com

Manufacturers of specialist resins & polymers:

• Oxazolidines

• Waterborne epoxy curing agents

• Polyurethane dispersions

• Polyurethane/acrylic hybrid dispersions

• Polyurethane prepolymers

ISSUE 1 - 03/11