aromatic polycarbodiimides: crosslinkers for water based ... · aromatic polycarbodiimides:...
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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