Aromatic Polycarbodiimides: Crosslinkers for Water Based

Adhesives

Chol-Yoo Choi and David R KeelyRohm and Haas Company

Spring House, PA

Crosslinkers in PSAs

• Lower peel strength• Lower peel build• Increase shear• Increase anchorage• Increase water & chemical resistance• Impart clean removability

Applications Using Crosslinkers

• Protective masking films• Mounting tapes• Carpet tapes• Vinyls for graphic arts, lettering• Removable labels

Aromatic pCDI

R N RC N

n

Made by polymerizing di-functional isocyanates

Reaction of pCDI with Carboxylic Acid

N C N + COH

ON

HC

O

NC

O

Reaction occurs in wet emulsion, added just prior to coating

pCDI IR Scan

Main absorption band at 2135 cm-1, well separated from any polymer bands

20

30

40

50

60

70

80

90

%T

20004000 3500 3000 2500 1500 1000 500

NCN Group

Toxicity of pCDI

• Studies– Ames test: negative (test for mutagenic/carcinogenic

potential)– Ingestion LD50: no effects up to highest dosage level

tested (5g/Kg)

• HMIS – 1,2,0 (Toxicity, Flammability, Reactivity)• Supplied as a 50% solution in propylene glycol

methyl ether acetate

Test Conditions

• Samples direct coated to 2 mil treated LDPE film• Coatweight target 0.3 mils dry• Samples covered with silicone release liner,

conditioned at least overnight at 72°F, 50% rH (PSTC conditions) before testing

• Peel measured using PSTC-1 • All failure modes adhesive unless otherwise noted• Crosslinker percentages calculated solid on solid

Potlife of pCDI Crosslinker: Peel Reduction

0.5% pCDI in PSA-1

20

24

28

32

36

0 1 4 8 16 24 96

Hours from mixing to drawdown

Peel

(oz/

in)

24 hr dwellno X-linker

Peel level of un-crosslinked emulsion

Potlife of pCDI Crosslinker: Anchorage

failpasspasspasspasspasspassfail24 hrfailpasspasspasspasspasspassfail1 hr

5 day24 hr16 hr8 hr4 hr1 hrinit.none

Test Method: Immerse test strips in water for specified time, test anchorageby rubbing the exposed adhesive side with the intent to remove it.

0.5% pCDI in PSA-1

Peel Force with Different Levels of pCDI Crosslinker

0

5

10

15

20

25

30

35

none 0.125% 0.25% 0.50%

% pCDI crosslinker

peel

(oz

/in)

1 min1 hour1 day2 weeks

PSA-1.

Typical Crosslinkers Used in Water Based PSAs

• Polyaziridine• Polyisocyanates• Metal Ions – Zinc, Zirconium, etc.

• Crosslinkers available for use in water based polymers are limited compared to solvent based counterparts.

Polyaziridine Reaction with Polymer

Polymer COH

O

+ R N

3

Polymer CO2CH2CH2NHR N

2

Polyaziridine Crosslinker Data

0

5

10

15

20

25

peel

(oz/

in)

0 1% 2% 3%Polyaziridine Level

20 min1 hour24 hours7 days

PSA-1.

Polyisocyanate Reactions with Polymer

Polymer CO2H + R NCO

Polymer CONH R + CO2

H2O + R NCO R N

H

C

O

N

HR

competing reaction – produces urea

Polyisocyanate Crosslinker Data

0

5

10

15

20

25

peel

(oz/

in)

0 2% 4%Polyisocyanate Level

1 min1 hour1 day2 weeks

PSA-1.

Metal Ion Reaction with Polymer

Polymer CO2H + M++

Polymer CO2 M O2C Polymer

Metal Ion Crosslinker Data

010203040506070

peel

(oz/

in)

PSA-4 0%

PSA-40.3%

PSA-20%

PSA-2 0.3%

Percent Zinc Crosslinker

1 hour1 week

Cohesive

Mixed Delamination

Peel Force Comparison of pCDI to Other Crosslinkers

0

5

10

15

20

25

30

peel

(oz/

in)

none Zn (0.3)

NCO(4.0)

AZ (0.25)

pCDI (0.5)

1 hour1 week7d@70°C

PSA-2.

Delamination

Peel Force Comparison of pCDIto Other Crosslinkers

0

5

10

15

20

25

30

peel

(oz/

in)

none Zn (0.3)

NCO(4.0)

AZ (0.25)

pCDI (0.5)

1 hour1 week7d@70°C

PSA-3.

Delamination

Wet Anchorage Test: Comparison of pCDI to Other Crosslinkers

passpasspassfailfail24 hours

passpasspassfailfail1 hour

pCDIAZNCOZnnone

Test Method:Immerse test strips in water for specified time, test anchorage by rubbing the exposed adhesive side with the intent to remove it.

0.5% pCDI in PSA-1

Comparison of Crosslinkers

modmodlowhighlow--Cost in uselowlowhighhighlowlowToxicitylowlowlowlowhighhighWater sensitivitygoodgoodgoodgoodpoorpoorAnchoragelowlowlowlowlowhighPeel buildhighhighhighhighhighnonePeel reduction8+ hr8+ hr8+ hr2 – 8 hrgoodgoodPotlife

pCDIpCDIAZNCOMetal ion

None

Conclusions

• Polycarbodiimides show performance characteristics similar to traditional crosslinkers commonly used in PSA’s

• Benefits:– Peel reduction– Improved anchorage to film– Useful potlife– Low toxicity