Correlation of Test Methods to evaluate the Thermal Stability of neat PVC Resin

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Erik Richardson
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1 Correlation of Test Methods to evaluate the Thermal Stability of neat PVC Resin PVC The 10 th International Conference on PVC Innovation Technology Sustainability Networking Brighton Dome, UK April 22 nd, 2008 Alex Wegmann; Shandy Li Ciba R&D Center, Plastic Additives, Shanghai, China 1

2 Contents Introduction Chain stoppers / antioxidants used in Suspension-PVC (S-PVC) polymerization Efficiency of chain stopping Sterically hindered phenols as chain stoppers Thermal stability of PVC (degradation and discoloration) Evaluating thermal stability of PVC by different methods Evaluation of neat PVC resins Evaluation of PVC compounds Correlation between different methods

3 PVC industry 3

4 S-PVC Polymerization Process Deionized water Suspension agent VCM Initiators PVC Polymerization PVC slurry Chain stoppers Antioxidants Stripping Drying Chain stoppers terminate the polymerization and quench catalyst residues, thereby avoiding further polymerization reactions that can lead to polymer deposits in strippers and dryers Some sterically hindered phenols also demonstrate a quite good efficiency as chain stoppers; in addition, they are also very effective antioxidants, protecting PVC under thermooxidative conditions 4

5 Structure of chain stoppers / antioxidants Acetone-thio-semicarbazone (ATSC) Bisphenol A (BPA) Ciba IRGANOX 1076 CGX AO 145 (80%) IRGANOX 1076 (20%) Ciba IRGANOX 245 =Ciba IRGASTAB PVC 11 5

6 Evaluating chain stopping performance Pressure drop during PVC polymerization: Chain terminator is added at 70% conversion rate to check its efficiency in the lab process 12 Reaction Pressure (bar) Delta P 0 Ideal chain stopper (100% effective) Delta P < Delta P < Delta P partial chain stopping effect Delta P : No chain stopper (Maximum pressure drop) Reaction Time (hour) No chain-stopper: Ideal chain-stopper: Non-ideal chain-stopper: VCM consumption causes pressure drop Terminates polymerization completely (no pressure drop) Pressure drop only reduced (not 100% effective) 6

7 Efficiency of different chain stoppers 50ppm ATSC 150ppm IRGASTAB PVC ppm IRGANOX ppm IRGANOX 1076 without chain stopper Pressure drop (bar) Hindered phenols can be almost as good as ATSC in chain terminating. The degree of sterical hindrance of the phenol group seems to determine the efficiency in chain stopping: Full sterical hindrance (2 tert-butyl groups), like in Ciba IRGANOX 1076, shows a very low chain stopping efficiency. Less hindered molecules, like IRGANOX 245 (1 tert-butyl, 1 methyl group) and CGX AO 145 (1 methyl, 1 secondary alkyl group) are better in chain terminating 7

8 Thermal degradation of PVC Dehydrochlorination (DHC): Autoxidation: DHC and autoxidation proceed simultaneously and influence each other HCl (autocatalytic) and O 2 (defects formed by autoxidation) catalyze DHC DHC is reduced during processing by adding heat stabilizers (metal soap) Autoxidation is reduced by antioxidants during production and processing 8

9 Discoloration occurs during degradation R H H H R + 3 HCl oxygen R Cl Cl Cl PVC start of DHC at structural defects ("irregular structures"): R R Cl Cl O Cl Cl R Cl polyene (light colored) HCl halochromism H R + C H H carbenium ion (polymethine) deeply colored (mesomeric) oxygen oxygen O O cyclic peroxides not conjugated (colorless) R O keto-polyene (deeply colored) via peroxy radicals chain scission: terminal carbonyl and carboxyl groups A direct consequence of the dehydrochlorination ( unzipping ), also involving oxidation and chain-scission reactions, is discoloration of PVC 9

10 Evaluation of thermal degradation of PVC Evaluation of neat PVC resin (without any heat stabilizer or other components added): Dehydrochlorination (DHC) Discoloration after compression molding Static oven aging of PVC powder (discoloration) Evaluation of PVC compounds (formulated with heat stabilizers and possibly other components) Discoloration after processing on a 2-roll-mill (dynamic heat stability) Discoloration of PVC sheets in a gradient oven (static heat stability) Measuring the molecular weight decrease due to chain-scission of PVC with a torque rheometer 10

11 Evaluation of neat PVC resin Dehydrochlorination Compression molding carrier gas carrier gas + HCl conductivity cell HCl heating block sample holder

12 Evaluation of dehydrochlorination (DHC) Conductivity (micro Siemens / cm) Control IRGANOX 1076 IRGASTAB PVC 11 On heating, PVC releases HCl, that increases the conductivity or ph of an aqeous solution in which HCl is trapped Time (min) 12

13 Dehydrochlorination (DHC) performance 150ppm IRGANOX ppm IRGASTAB PVC 11 50ppm ATSC+100ppm BPA 50ppm ATSC+100ppm IRGANOX ppm IRGANOX ppm ATSC Time (minutes) to reach a conductivity of 200 micro Siemens/cm Sterically hindered phenols can increase the stability of PVC resin. Especially, partially hindered phenols are very effective, e.g. IRGASTAB PVC 11, IRGANOX 245. Fully hindered phenols, like IRGANOX 1076, or completely unhindered phenols, like Bisphenol A (BPA), show lower efficiency. IRGANOX 1076 shows lower DHC performance than IRGANOX ATSC, because part of the IRGANOX 1076 is used up for chain stopping Pure Chain stoppers, like ATSC, show low performance in DHC test 13

Discoloration during compression molding 150ppm IRGANOX 245 150ppm IRGASTAB PVC 11 50ppm ATSC+100ppm IRGANOX 1076 Yellowness index Lightness 50ppm ATSC+100ppm BPA 150ppm IRGANOX 1076 50ppm ATSC 30 45

14 Discoloration during compression molding 150ppm IRGANOX ppm IRGASTAB PVC 11 50ppm ATSC+100ppm IRGANOX 1076 Yellowness index Lightness 50ppm ATSC+100ppm BPA 150ppm IRGANOX ppm ATSC The lower the yellowness index (YI), the higher the lightness (L*) of PVC plaques made from neat PVC resin, and thus the better the thermal stability. Sterically hindered phenols can effectively reduce the discoloration of PVC resin at high temperature 14

15 Evaluation of PVC compounds t D torque A B X C time

16 Discoloration after 2-roll-mill processing Yellowness index ppm ATSC 250ppm IRGASTAB PVC PPM IRGANOX mins 10mins 15mins 20mins 25mins 30mins PVC resin 100 phr Processing times (minutes) Tin stabilizer 1.5 phr Process aid 1.9 phr PVC sheets containing sterically hindered phenols have better color retention compared to PVC sheets with only chain stoppers 16

17 Correlation between DHC and molding Yellowness index Yellowness index / lightness*1.5 Time to reach a conductivity.. of 200 micro Siemens/cm Lightness*1.5 DHC stability ppm ATSC 150ppm IRGANOX ppm ATSC+100ppm BPA 50ppm ATSC+100ppm IRGANOX ppm IRGASTAB PVC ppm IRGANOX 245

18 Correlation between DHC and molding DHC measurements reflect the first phase in the degradation process of PVC; it is based on the release of HCl from the PVC resin. Measuring the discoloration of PVC resins (e.g. by compression molding) reflects the final stage of the degradation process, involving different degradation mechanisms (DHC and autoxidation) A correlation exists between DHC and the discoloration of PVC plaques: the higher the DHC stability, the better the color in the compression test 18

19 Conclusion and summary Partially hindered phenols show the best performance in thermal stabilization of neat PVC resin (during polymerization, stripping, and drying) Partially hindered phenols show, at concentrations needed for good thermal stability of the resin, chain stopping almost as good as ATSC Good correlation between DHC (HCl release) and compression molding (discoloration)

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20 Thank you for your attention! Please visit our internet site: For further information, please contact: Alex Wegmann Application Technology Shandy Li Application Technology 20

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