non-tin catalysts for crosslinkable silane terminated ... · • levels of k-kat 670 were adjusted...

Non-Tin Catalysts for Crosslinkable

Silane Terminated Polymers

K-Kat 670

King Industries, Inc.

Science Road

Norwalk, CT 06852

USA

Silane Chemistry

Terms and Applications

• Organosilane compounds – Monomeric silicon bonded with carbon

• Siloxane compounds (Si-O-Si)

– Polymeric organosilane compounds

• Silanol groups (Si-OH) – Hydrolyzed silane group

• Promote adhesion and reinforcement of:

– Coatings – Adhesives – Sealants – Fillers

Silane terminated Polymer Technologies

• Functionalized backbone with alkoxysilane terminal groups

• Silane terminated polymer backbone chemistries – Polyether – Polysiloxane – Polyurethane

• Cured properties (Tg, flexibility…) depend on the backbone

Silane terminated Polyethers

Si

CH3

Si

CH3

O CHCH2O

CH3

n

MeO OMe

OMeOMe

• Polyether backbone provides low viscosity, low Tg, flexibility over a wide temp. range, low color and odor.

• Can produce versatile sealants and adhesives • Linear MS polymers produce very soft, low modulus sealant with superior workability

and adhesion. • Slightly branched structures provide a higher modulus, with fast and uniform cure. • Superior adhesion to a wide variety of materials without primer. • Excellent elastic behavior and durability. • Great mechanical properties. • Good storage stability, in the absence of catalyst and water. • Fast moisture cure with no bubbling.

Silane terminated Polyurethanes

NX

O

OEt

OEtOEt

Si

CH3

NH

O

NH

XN O CHCH

2O

n

O

CH3

O

Si

CH3

NH

NH

EtO

• Urethane functionality provides compatibility and tear resistance • Polyoxypropylene backbone provides elasticity • Silane end groups serve as crosslinking/chain extension functionalities and an internal primer on

many surfaces. • Combine the benefits of silicones and polyurethanes • Odorless and NCO free and hence not classified • Sealants and adhesives have high mechanical strength and good elastic recovery • Excellent adhesion without primer • Paintability • Bubble-free crosslinking even in humid environment • Useful in

– Bodywork and vehicle construction – Ship building – Airconditioning and ventillation technology – Construction and assembly applications

Market Drivers / Technology Trends

• Organotin compounds such as, for example, dibutyltin dilaurate (DBTDL),and dibutyltin bis(acetylacetonate) are widely used as condensation cure catalysts to accelerate the moisture assisted curing of a number of different polyorganosiloxanes and non-silicone polymers having reactive terminal silyl groups.

– Environmental regulatory agencies and directives, however, have placed increasing restrictions on the use of organotin compounds in formulated products.

– While formulations with greater than 0.5 wt. % dibutyltin presently require labeling as toxic with reproductive 1B classification, dibutyltin-containing formulations are proposed to be completely phased out in consumer applications during next several years.

• Methanol emission from methoxysilane terminated polymers deemed harmful leading to a switch towards ehtoxy silane terminated polymers.

• Non-tin metal catalysts that accelerate the condensation curing of both methoxy and ethoxy terminated moisture curable silanes would be highly desired.

Desired Catalyst Attributes

• Liquid Product

• Stability in 1K system during extended storage periods.

• Ambient moisture cure rate comparable to tin catalysts.

• Imparts good mechanical properties after cure.

• Excellent adhesion to a variety of substrates.

Silanol Formation

Hydrolysis of Alkoxysilanes

Polymer Si

OMe

CH3

OMe Polymer Si

OMe

CH3

OH Polymer Si

OH

CH3

OH

H2O

Catalyst

H2O

Catalyst

Formation of Siloxane Crosslinks by

Condensation of Silanols

Polymer Si

OMe

CH3

OH Polymer Si

OMe

CH3

OH

Polymer Si

OMe

CH3

O Si

OMe

Polymer

CH3

+

Formation of Siloxane Crosslinks by

Condensation of Silanol and Alkoxysilane

Si

OMe

CH3

OH Polymer Si

OMe

CH3

OMe

Polymer Si

OMe

CH3

O Si

OMe

Polymer

CH3

+

+ CH3OH

Acid Catalyzed Hydrolysis of Alkoxysilanes

• At neutral pH, alkoxysilanes hydrolyze at very slow rates, with half-lives > 14hrs

• At pH < 4, the hydrolysis is rapid

• At pH > 10, hydrolysis of the first intermediate, RSi(OR)2OH, is inhibited due to the ionization of the SiOH group.

Acid Catalyzed Hydrolysis of Alkoxysilanes

F. D. OSTERHOLTZ and E. R. POHL* J.Adhesion Sci.Technol.Vol.6, No. 1, pp 127-149(1992)

Acid Catalyzed Condensation of Silanols

Polymer Si

OH

CH3

OH

H

O

H

H Polymer Si

OH

CH3

OH2

Polymer Si

OH

CH3

OH Polymer Si

OH

CH3

O

H

Si

OH

Polymer

CH3

Polymer Si

OH

CH3

OH2

Polymer Si

OH

CH3

O

H

Si

OH

Polymer

CH3

Polymer Si

OH

CH3

O Si

OH

Polymer

CH3

+ + H2O

+

+

+

+

+ +

H2O

+

+ H2O

+ H3O+

Base Catalyzed Hydrolysis of Alkoxysilanes

•General Base Catalysis: Any basic species accelerates the reaction by assisting the removal of a proton from water in the transition state. •Specific Base Catalysis: The hydroxide anion accelerates the reaction rate by directly attacking the substrate.

Base Catalyzed Condensation of Silanols

Polymer Si

OH

CH3

OHR

N

R

H Polymer Si

OH

CH3

NR2

Polymer Si

OH

CH3

NR2

Polymer Si

OH

CH3

OH Polymer Si

OH

CH3

O Si

OH

Polymer

CH3

RN

R

H

+ + H2O

+

+

Optimum pH for hydrolysis and condensation is different

Acid and Base Catalyzed

Hydrolysis and Condensation

Effect of pH on Reaction Rates

-4.5

-1

0 2 4 6 8 10 12 14

pH

log p

K

Hydrolysis Condensation

Hydrolysis

minimum rate ~pH 7

Condensation

minimum rate ~pH 4

pH>10 hydrolysis inhibited due

to ionization of SiOH group

• Hydrolysis

– Rate minimum is at approximately neutral pH ( pH 7)

• Condensation

– Rate minimum is at approximately pH 4

• Each pH change of 1 unit in either direction produces a ten-fold rate acceleration assuming an excess of available water (up to pH 10 for hydrolysis)

• Both hydrolysis and condensation reactions are reversible. Alcohols will reverse the silane hydrolysis

Acid and Base Catalyzed

Hydrolysis and Condensation

Effect of pH on Reaction Rates

Mechanism of Tin catalysis

DBTDL Hydrolysis

DBTDL undergoes hydrolysis and forms an organotin hydroxide, which is the true catalytic species

Sn RR

O

R'

O

O

R'O

O

H

H

Sn RR

O

R'

O

OH

R'O

O

H

+

Frederik Willem, Makromol.Chem. 181, 2541-2548 (1980)

Mechanism of Tin Catalysis

Formation of Organotin Silanolates

Organotin hydroxide reaction with alkoxysilane group to form organotin silanolates

Sn RR

O

R'

O

O

H

Polymer Si

OMe

CH3

OMe Polymer Si

CH3

OMe

O

Sn

R

R

R'O

+ + CH3OH

Mechanism of Tin Catalysis

Formation of Silanol Group

Polymer Si

CH3

OMe

O

Sn

R

R

R'O

Polymer Si

OMe

CH3

OH Sn RR

O

R'

O

O

H

+ H2O +

Tin silanolates react readily with alcohols and water

Mechanism of Tin Catalysis

Formation of Alkoxysilane

Polymer Si

CH3

OMe

O

Sn

R

R

R'O

Polymer Si

OMe

CH3

OH

Sn RR

O

R'

O

O

H

Polymer Si

OMe

CH3

O Si

OMe

Polymer

CH3

+

+

Mechanism of tin catalysis

SnL

SnOH

SnO Si Si

OH

SiOR Si

O Si

H2O

LH

ROH

Evaluation of K-Kat 670 in Silane functional

Polymer Formulations

• Fully formulated single component moisture cure alkoxysilane systems were used

• Uncatalyzed formulations were stored in dispense cartridges. Approximately 30 grams of uncatalyzed material was dispensed into a speed mixer container with a caulk gun before addition of the catalyst. The material was mixed on a speed mixer for 30 seconds at 1500 rpm then 2 minutes at 2200 rpm.

• An adjustable doctor blade was used to apply 3 mm of the blend onto a paper substrate.

• The degree of dryness was determined by using a Model 415 Drying Time Tester* in accordance with DIN 53 150.

• The dryness test involved applying a force onto a paper disk that covered a test site on the casting for 60 seconds. The results are based on the amount of tack and on visual impressions that develop from the applied force. The dryness testing was done at approximately 25°C and 50% relative humidity.

• Degree 1 of the DIN 53 150 method was substituted with a glove test to determine touch dry.

• Hardness of the castings was determined with a Shore A** hardness tester after the castings were allowed to cure under ambient conditions for 2 weeks. Other mechanical properties were measured on an Instron*** tester using dogbone shaped samples cut from the fully cured 3 mm thick castings.

*Model 415 Drying Time Tester, Erichsen GmbH & Co. KG **Instron Corporation, Shore A durometer ***Instron Corporation, Dual column table top model, 30 kN (6700 lbf) load capacity

Dryness Testing Ratings

Degree of dryness (DIN 53 150)

Rating Description

1 Touch dry, no visible residue remaining on rubber glove

2 Paper does not adhere, but visible change with 20g load

3 Paper does not adhere, but visible change with 200g load

4 Paper does not adhere, but visible change to coated surface with 2Kg load

5 Paper does not adhere, no visible change to coated surface with 2Kg load

6 Paper does not adhere, but visible change to coated surface with 20Kg load

7 Paper does not adhere, no visible change to coated surface with 20Kg load

Dimethoxymethylsilane Polymer

Formulation

Component %

Dimethoxymethylsilane polymer 32.8

Plastisizer 16.4

Filler 39.3

Titanium dioxide 6.6

Thixotrope 1.6

HALS 0.3

UVA 0.3

Moisture scavenger 0.7

Adhesion promoter 2.0

100.0

• Levels of K-Kat 670 were adjusted to produce castings that dried at rates similar to the system catalyzed with 0.6% dioctyltin diacetylacetonate (DOTDAA).

• The tin content of DOTDAA is approximately 21%. At 0.6%, the tin content in the formulated control system is approximately 0.12% which would not comply with EU regulations.

• The systems dried similarly with each achieving the highest degree of dryness (Paper does not adhere to 20Kg load, no visible change to coated surface) in 6 hours.

• The DOTDAA and K-Kat 670 catalyzed castings developed similar tensile stress (which can be associated with toughness), modulus (elastic modulus) and strain (elongation).

Dimethoxymethylsilane Formulation

Dryness Development

Dimethoxymethylsilane Formulation

Dryness Development

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

1 2 3 4 5 6 7

Ho

urs

at

25

C/5

0%

RH

Dryness degree(DIN 53 150)

0.6% DOTDAA 2.0% K-Kat 670

Dimethoxymethylsilane Formulation

Properties After 2 Week Cure

0

50

100

150

200

250

300

350

400

Shore A Stress at max, psi Strain at max, % Modulus, psi

0.6% DOTDAA 2.0% K-Kat 670

Trimethoxysilane Formulation

Dryness Development

• DOTDAA was compared to K-Kat 670 in a polyether backbone TMS polymer system.

• The DOTDAA system achieved the maximum dryness rating (passed 20Kg load test) in 5 hours while K-Kat 670 required 6 hours to reach the 7 dryness rating.

• Both castings developed similar mechanical properties after the 2 weeks of ambient cure

Trimethoxysilane Polymer

Formulation

Component %

Trimethoxysilane polymer 32.8

Plastisizer 16.4

Filler 39.3

Titanium dioxide 6.6

Thixotrope 1.6

HALS 0.3

UVA 0.3

Moisture scavenger 0.7

Adhesion promoter 2.0

100.0

Trimethoxysilane Formulation

Dryness Development

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

1 2 3 4 5 6 7

Ho

urs

at

25

C/5

0%

RH

Dryness Degree ( DIN 53 150)

0.6% DOTDAA 2.0% K-Kat 670

Trimethoxysilane Formulation

Properties After 2 Week Cure

0

50

100

150

200

250

300

350

400

Shore A Stress at max, psi Strain at max, % Modulus, psi

0.6% DOTDAA 2.0% K-Kat 670

Diethoxysilane Formulation

Dryness Development

• DOTDAA was compared to K-Kat 670 in a polyurethane backbone DES polymer system.

• DOTDAA was essentially inactive in this system, at even 0.5% and 1.0% on total formulation weight.

• Higher dosages were not evaluated since the tin level incorporated with the 1.0% dosage was more than double the maximum allowed by EU regulations.

• The 3 mm thick castings required more than 120 hours to achieve a dryness rating of 7.

• Dibutyltin dilaurate (DBTDL) was similarly inactive.

• Dry times of the DES system catalyzed with K-Kat 670 were significantly faster than the tin catalyzed systems.

• DOTDAA catalyzed system required a month of ambient cure before it was suitable for testing on the Instron. Even so, the casting exhibited very poor mechanical properties.

• The K-Kat 670 cure profile was unchanged after storage of the polymer containing 2% of the catalyst at 500C for 4 weeks

Diethoxysilane Polymer Formulation

Component %

Diethoxysilane polymer 20.2

Plasticizer 23.1

Calcium carbonate 49.2

Titanium dioxide 3.3

Antioxidant 0.3

HALS 0.3

Moisture scavenger 0.8

Fumed silica 1.4

Adhesion promoter 1.5

100.0

Diethoxysilane Formulation

Dryness Development

0

20

40

60

80

100

120

140

1 2 3 4 5 6 7

Ho

urs

at

25

C/5

0%

RH

Dryness Degree (DIN53 150)

0.5% DOTDAA 2.0% K-Kat 670

Diethoxysilane Formulation

Cured Properties

0

100

200

300

400

500

600

700

Shore A Stress at max, psi Strain at max, % Modulus, psi

0.5% DOTDAA* 2.0% K-Kat 670

Cure Schedule: K-Kat 670: 2 weeks, DOTDAA:1 month

K-Kat 670 for Crosslinkable Silane

Terminated Polymers

Conclusions

• Liquid Product

• Excellent Stability in 1K system during extended storage periods.

• Provides ambient moisture cure comparable to tin catalysts, in methoxy silane terminated polymers.

• Provides superior moisture cure versus tin catalysts in ethoxy silane terminated polymers.

• Imparts good polymer mechanical properties after cure.

• Excellent adhesion to a variety of substrates.

Non-Tin Catalysts for Crosslinkable Silane

Terminated Polymers

Thank You for your interest