The Role of Titanium Dioxide Pigments. In Outdoor Weathering of Rigid PVC

Transcription

1 The Role of Titanium Dioxide Pigments In Outdoor Weathering of Rigid PVC DuPont Titanium Technologies Alex X. Gao, John D. Bolt, Andy A. Feng April 23, 28 Brighton Dome, UK 24 E.I. du Pont de Nemours and Company. All Rights Reserved. The DuPont oval, the miracles of science, DuPont and Product Name are trademarks or registered trademarks of E. I. du Pont de Nemours and Company.

2 2 AGENDA Phenomenon and mechanism of out-door weathering of white rigid PVC PVC degradation scheme with TiO 2 present PVC surface degradation by TiO 2 photo-catalysis Durable TiO 2 vs. Chalking TiO 2 A two-year out-door weathering study Effects of different exposure climates Effects of stabilizer types and TiO 2 loading Effects of TiO 2 chalking

3 3 Out-door Weathering of White Rigid PVC What do we observe? Discoloration (Yellowing): due to formation of conjugated double bonds (polyene) (8 minimum) White->Yellow usually linked to a photolysis phenomenon: hν Photobleaching: due to photo-chemical oxidation of conjugated-double bonds, created during processing Yellow->white Photo-oxidation: hν + O 2 Loss of gloss: due to surface damage (creation of micro-cracks subsequent to PVC chain scissions Photo-oxidation: hν + O2 Also photo-catalysis of TiO2 pigments (Discuss later) Loss of impact strength: due to resin degradation subsequent to PVC chain scissions

4 4 Role of TiO 2 : Interaction with light UV light absorption protection from direct resin degradation Attenuation of 9% UV light -> ~2μm depth Visible light scattering opacity, hiding power, tint strength: hides yellowing Catalytic degradation gloss loss, color fade, chalking

5 5 PVC part PVC Weathering with TiO2 Present Heat UV light Resin Degradation short polyenes hν & heat HCl no reaction TiO2*(hole +,e - ) H2 O O 2 Polyenes n>8 Yellow primary radicals. OH. OOH TiO2 opacity hν + O2? crosslinks scission photo-oxidation (bleaching) Resin Degradation Embrittlement lost impact strength 25 International PVC Conference, J D Bolt, A A Feng, R W Johnson Surface Degradation gloss loss, color fade, chalking

6 6 Photocatalytic Degradation Initiated by TiO 2 TiO 2 + hν hole + + e - light absorption hole + + e - TiO 2 + heat recombination > 99 % hole + + H 2 O (surface) H + + HO primary radicals < 1 % e - + O 2(surface) O 2 - HOO HO + -(CH 2 CHCl) n - degradation products HOO + -(CH 2 CHCl) n - degradation products Recombination vs. radical formation varies with TiO2 surface treatment to produce chalking or non-chalking / durable grades of TiO 2

7 7 Effects of TiO 2 Photo-catalytic Degradation Surface degradation Gloss loss PVC surface become rougher gloss loss is due to TiO 2 catalyzed degradation or direct UV absorption by the resin Color fade resin degradation and void formation lowers refractive index of the matrix, increasing visible scattering efficiency PVC part becomes whiter Chalking physical disintegration of the resin in near surface non-chalking // durable grades of TiO2 are surface-treated to suppress photocatalysis

8 8 Chalking Effects of TiO 2 to Gloss Loss 5 4 Ca-Zn stabilized TiO2 at 5 phr via Xenon-arc Weathering Gloss 3 2 TiO2 #1 TiO2 #3 See 6 o Gloss Loss: Hours in Xenon Weatherometer Gloss Sn stabilized TiO2 at 6 phr TiO2 #1 TiO2 #3 1# R-15 3# Chalking grade Gloss Half-Life: time to reach 5% of initial gloss Hours in Xenon Weatherometer

9 9 Effect of TiO 2 phr on 6 o Gloss Half-Life Pb stabilized Gloss Half-Life (hours) TiO2 # 1 non-chalking TiO2 # 2 non-chalking TiO2 # 3 chalking TiO2 phr

10 1 Acid Solubility Test as an Indicator of Non-Chalking/Durable TiO 2 Most commercially available TiO 2 uses silica surface treatment to achieve non-chalking performance Since silica does not dissolve in acid, acid solubility test can determine how well the silica treatment encapsulates the TiO 2 particles. Lower the acid solubility, higher the durability of TiO 2 General-Purpose/Chalking TiO 2 : >3 wt%, Ti-Pure R-13, R-12 Semi-durable TiO 2 : 2 wt%, Ti-Pure R-92 + Non-Chalking/Durable TiO 2 : <1 wt%, Ti-Pure R-15, R-96

11 11 Comparisons on the Acid Solubility (wt %) for R-15 R and other commercial TiO2 grades for Rigid PVC 4 /% Acid Solubility % R-15 C.1 C.2 C.3 C.4 C.5 C.6 C.7

12 A Two-year Out-door Weathering Study 12 Durable TiO 2 and chalking TiO 2 pigments were included 3 heat stabilizer systems: Pb, Ca/Zn, Organotin (Sn) Stabilizer systems Ca/Zn Pb Sn Stabilizer (phr) TiO2 (phr) CaCO3 (phr) Profiles extruded by a major European PVC resin and dry blend producer on a commercial-scale machine Exposure sites in NA and Europe NA: Phoenix in Arizona and Miami in Florida Europe: Bandol in France and Mechelen in Belgium Study items Effects of different exposure climates Effects of stabilizer types and TiO 2 loading Effects of TiO 2 chalking

13 UV Irradiation in Exposure Sites Resources: Ciba and NASA 13

14 Climate in Exposure Sites Avg Temp (F) Avg Rel. Humidity % Rain (inches) Radiation 1MJ/m2 Netherlands Bandol Miami Phoenix Louisville Lisbon Average Rainfall (mm) Miami Brussels Toulon Fr Lisbon Phoenix Resources: atlasmtt.de/products/natural-weathering-testing-new/sites/ new/sites/ weatherbase.com Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Avg. Maximum Temperature (C) Miami Brussels Toulon Fr Lisbon Phoenix Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

15 15 Delta E* and 6 gloss loss for Ca/Zn Stabilized Rigid PVC Samples Using Ti-Pure R-15 at Four Exposure Sites Ca/Zn Phoenix Bandol Miami Mechelen Phoenix Bandol Miami Mechelen Ca/Zn Gloss Delta E* (a). (b) Time/Month Time/Month Higher temperature, higher UV irradiance and dry climate resulted in the fastest gloss loss rate and higher color change for rigid PVC

16 16 6 Gloss loss for Rigid PVC Stabilized by Ca/Zn, Pb and Sn In n Phoenix and Miami Exposure Tests Phoenix Ca Pb Sn 6 5 Florida Miami Ca Pb Sn 6 Gloss Gloss (a) 1 (b) Time/Monyth Time/Month Time/Month Higher loading levels of durable TiO 2 showed an advantage to retain gloss longer

17 17 Delta b* and 6 Gloss loss for Rigid PVC Comparing Ti-Pure R-15 and Chalking TiO 2 Pigments in a Hot-dry Climate 4 Phoenix, Ca/Zn stabilized 5 Phoenix, Ca/Zn stabilized 3 2 Chalking grade-1 Chalking grade-2 R-15 4 Chalking grade-1 Chalking grade-2 R-15 3 Delta b* 1 6 Gloss (a) (b) Time/Month Time/Month Chalking grades TiO 2 can provide somewhat lower yellowing as the result of stronger surface degradation, but higher gloss loss is clearly found compared with durable TiO2, Ti-Pure R-15

18 18 Outdoor weathering for Ti-Pure R-15 and other commercial TiO 2 grades Delta E* (a) Miami, Ca/Zn Stabillized Time/Month R-15 TiO2-1 TiO2-2 TiO2-3 6 Gloss Miami, Ca/Zn Stabilized Time/Month TiO2-1 TiO2-2 TiO2-3 R-15 Improved gloss and color retention performance by R-15 for exterior PVC application

19 19 CONCLUSIONS The roles of TiO 2 in durability of exterior rigid PVC are: 1) to absorb UV, limiting direct resin degradation, 2) to scatter light, thus masking discoloration from absorption of visible light. Chalking effects of TiO 2 could accelerate the degradation of PVC matrix by the photocatalysis chemistry in the near surface region, where UV is absorbed. Chalking can be suppressed by the silica encapsulation to achieve non-chalking or durable performance. Acid solubility can be used to differentiate the chalking grade and non-chalking or durable grade TiO 2. Chalking by TiO 2 is a disadvantage when gloss retention is important. Chalking is also a disadvantage in color PVC because it promotes color fade. Accelerated weathering gloss loss rates adequately identify chalking versus non-chalking gloss loss for most climates. Higher temperature, higher irradiance and dry climate resulted in the fastest gloss loss rate and higher color change for rigid PVC among four outdoor exposure locations. Dry climate and peak temperatures are key factors in the hot-dry degradation of exterior PVC. Higher loading levels of durable TiO 2 showed an advantage to retain gloss longer and minimize discoloration.

20 2 ACKNOWLEDGEMENT The authors wish to thank the following DuPont colleagues for their support of this study, Ann Delmotte Bert Vanhaecht Antonio Prieto Pablo Aragon Miranda M. L. Lu Robert Johnson Thank You for Your Attention! 2

21 DuPont Titanium Technologies Website: 21