1. Discoloration Mechanisms and Additive FormulatioN
Thi Thu Loan Doan, Dao Thi Thanh Tuyen
University of Science and Technology, The University of Danang - Vietnam
Acknowledgment Dr. Thoi Ho (Flint Hills Resources LP)
Houston, 2019

2. Outline Discoloration during product cycle Discoloration and mechanism
Effects of polyolefin additives
Conclusions

3. Discoloration during Product Cycle

4. Discoloration of during resin production
Main root cause is residual catalyst (Ziegler-Natta, Chromium, Single-site,…)
Appropriate catalyst deactivation process (catalyst kill) will solve the discoloration problem
Chemistry:
Active catalysts normally have color
Deactivated catalyst products are colorless
For example: (R)xTi(Cl)y (color)+ H2O Ti2O (colorless) R= organic group

5. Discoloration During Fabrication Process
Auto-oxidation cycle for polyolefins
Free radicals formed from C-C, C-H bond scission react with other polymer molecules, the oxygen peroxy radicals  react with the polymer hydroperoxides and a new free radical
Under heat, shear, catalyst residues, unstable hydroperoxides are decomposed  formation of alkoxy and hydroxy radicals

6. Discoloration During Fabrication Process
Oxidation of Polyolefins
Initiation:
RH + O2  ROOH (1)
ROOH  RO + HO (2)
 Propagation:
RO + PH  R + ROH (3)
R O2  ROO (4)
ROO + RH  ROOH + R (5)
RO  R1CH=O + R2 (oxidative chain scission) (6)
R  R1CH=CH2 + R2 (thermal chain scission) (7)
 Termination:
2R  R-R (coupling reaction) (8)
2R  RH + Olefin (Disproportionation) (9)
The reactions leading to free radicals being formed on the polymer backbone results in chain linking and/or scission reactionschanging in molecular weight and molecular weight distribution of polyolefins

7. Discoloration During Fabrication Process
Oxidation of Polypropylene
PP: Chain scission is typical, because tertiary C-centered radical of PP is less reactive than the secondary C-centered radical of PE
Probability for PP radicals to combine with each other is lower than that of PE

8. Discoloration During Fabrication Process
Oxidation of Polyethylene
Chain scission
Crosslinking
PE: depending on O2 concentration in the system
No O2: stable
Low O2: Crosslinking (recombination with alkyl radicals )
High O2: Chain scission (oxidation and breaking of polymer 

9. Discoloration During Fabrication Process
Issues During Fabrication Process
Effect on molecular weight (M):
M(chain scission or degradation) and M (branching and crosslinking)
Melt index (MI) change: MI  (PP, PE in high O2 ), MI (PE in low O2 )
Change in mechanical properties
Bulk of polymer melt : soluble O2
Dead spots, and die face: surface O2 (excess)
Formation of Gels (PE)
Discoloration
Formation of black specs
Adversely affect the color, taste and odor properties

10. Discoloration During Fabrication Process
Black specks are formed from localized oxidation of PE:
Extensive time under high temperature
When the additives is completely consumed
Black specks can be controlled by designing the extruder and the die to eliminate dead spots
Hydrogen abstraction
O2 oxidizes PE mainly:
via H- abstraction
after hindered phenol consumed
at dead spots of extruder
Discoloration
Charring
Conjungated polyenes/aromatic (Black specks)

11. Discoloration During Fabrication Process
Inhibited auto-oxidation cycle
Typical antioxidant package: hindered phenol + phosphite

12. Discoloration During Fabrication Process
Polymer: PO + O2  Discolored PO
Hindered phenol: HP + O2  Intense colored quinones
Phosphite: PH + O2  Colorless phosphate
Main mechanism of discoloration is the oxidative discoloration of hindered phenol
Hindered phenol reacts with O2 to protect polymer from degradation but gives colored productsincrease the discoloration
Phosphite scavenges O2 reduce the discoloration of hindered phenols

13. Discoloration Chemistry of Irganox-1076
Klemchuck et al (1991)

14. Discoloration Chemistry of BHT
Klemchuck et al (1991)

15. Discoloration During Fabrication Process
Effect of Hindered phenol on Yellowness Index and Melt Flow Index
Yellowness Index after extrusion
Melt Flow Index #1 #3 #5 PP
-4.88  - - 
3.87
PP+0.1%HP
-1.24
18.70
27.76
3.92
4.87
7.12
PP: Polypropylene BSRTM T3034
HP: Hindered phenol Inganox 1010
Hindered phenols increase the discoloration
Discoloration of PP increases significantly after many times of extrusion
Inganox 1010

16. Discoloration During Fabrication Process
Discoloration of Versify, Engage, PP, PE after heating at 210oC in air
Verify
Engage PP PE
Heating time (min) 30 60 90
120
180
240
Localized oxidation of resin over extensive time under high temperatures→black specks

17. Discoloration During Fabrication Process
Discoloration of polyethylene (PE), polypropylene (PP), Engage and Versify as a function of heating time at 210oC in air
Discoloration of PE is higher than that of PP
Discoloration of Engage is higher than that of Versify
Discoloration of homopolymer higher than that of copolymer

18. Discoloration During Fabrication Process
Yellowness Index of Versify, Engage, PP, PE
Polypropylene
Polyethylene
Engage
(Random Ethylene based POEs)
Versify
(Random Propylene based POEs)

19. Discoloration During Storage
Parts/Films/Fibers turn pink during storage
Discoloration by oxidation of hindered phenol by NOx
Reaction of NOx with hindered phenol about 200 times faster than reaction of Oxygen with hindered phenol
Accelerated by basic (high pH) additives: TiO2, HALS, catalyst kill…
Can be reduced by:
Using appropriate hindered phenols
Reducing NOx in the ware house
Selection of additives to minimize additives interaction
Control radiation of discolored parts with UV light (to destroy polyconjugated double bonds)

20. Discoloration During Storage
Discoloration increase with decreasing hindered phenol, indicating pinking is associated with oxidation of I-1076
LLDPE Resin
density / 1.0 MI
-AO package 500 ppm hindered phenol
NOx exposure via Uni-Charm test method

21. Discoloration During Storage
Kinetics of Discolorations of Hindered Phenols in LDPE
Oven aging at 90oC;
Air oxidation
Uni-charm test
Nitroxide oxidation
Rate of discolorations was determined by following the disappearance of hindered phenol
The reaction of hindered phenol with NOx is much faster than oxidation by air, suggesting that discoloration by NOx is the main contribution of the discoloration during sample storage

22. Discoloration During Storage
Indoor:
Similar to discoloration during storage
Indoor + high temperature:
Air oxidation is the main discoloration mechanism
Outdoor:
For the outdoor applications where the samples are exposed to UV light, the first part of discoloration is due to the oxidation of additives such as hindered phenols and UV absorbers, following by the oxidation of polyolefins
Properly designed hindered phenols and UV absorbers can reduce the first step of the discoloration
Effective HALS can be used to reduce discoloration of polyolefin

23. Discoloration During Storage
Discoloration of Glass Reinforced PP in Weathering Test

24. Conclusions
Gas fading during storage is caused by the oxidation of hindered phenol AO’s via NOx
Oxidation via NOx is much faster than air oxidation
Low gas fading hindered phenols can be used to reduce gas fading
The mechanism of discoloration during out door end use is initially by air oxidation of hindered phenol and UV absorbers and following by oxidation of polyolefin

25. Conclusions
Residual catalyst is the main root cause during resin production. An appropriate catalyst deactivation process can be used to solve the discoloration problem
Discoloration and black spec formation during extrusion is primarily caused by oxidation of hindered phenol AO’s and localized oxidation of resin.
Discoloration of PE is higher than that of PP
Discoloration of Engage is higher than that of Versify
Discoloration of homopolymer higher than that of copolymer

26. Thank you for your attention!