Polymer stability
Assoc. Prof. Dr. Jatuphorn Wootthikanokkhan
Division of Materials Technology, School of Energy, Environment and Materials,
King Mongkut’s University of Technology Thonburi, Thailand
Talk outline
Introduction to polymer degradations
Degradation of PLA
Degradation of polyolefins, EVA
Degradation of PET, PC, Epoxy
Degradation of PVC
Types of polymer degradation
Color changes (darkening PVC)
More brittle (Tire rubber embritlement)
Bio-degradation
Thermal-degradation
Photo-degradation
Mechanical-degradation
Some physical changes of the degraded polymers can be obseved, including
Testing and characterization of polymer stability
Aging tests UV aging and/or thermal aging for a given time
and conditions Retention of strength is determined Changes in chemical structure could be followed
by FTIR
Thermal gravimetric analysis Structural analysis
Molecular weight change (GPC, MFI) Chemical structure changes (FTIR, NMR)
UV aging
Thermal aging
Biopolymers and biodegradable polymers
Biopolymers include PLA, natural rubber
Not all biopolymers are biodegradable
On the other hand, some synthetic polymers can also be biodegradable such as PCL, PBS
Biodegradable polymer-PLA
Degradation Mechanisms of Polymers
การตดัสายโซ่ขาด (Chain scission) การเชือ่มโยงระหวา่งโมเลกุล (Crosslinking) การเกดิปฏกิริยิาพอลเิมอรไ์รเซชนัแบบยอ้นกลบั (De-polymerization) การขจดัหมูแ่ทนทีห่รอือะตอมออกจากโมเลกุล (Elimination) การเกดิปฏกิริยิาแตกตวัโดยมนี ้าเรง่ (Hydrolysis)
ขึน้อยูก่บัโครงสรา้งพอลเิมอร ์และเงือ่นไขสภาวะ บางกรณีอาจจะเกดิขึน้ควบคูก่นัไป
Drying of PLA
• Water content in PLA pellets should be < 500 ppm,
otherwise, degradation via hydrolysis
• Typical conditions
– Air drying, flow rate > 0.5 ft3/min
– Temp ~ 130 ºF (90 ºC) to 190 ºF
– 2 hr. at 190 ºF, 4 hr at 110 ºF.
Water scavengers • By adding some water scavenger, chain scission of the PLA via
hydrolysis can be minimized
• Examples of water scavengers include
– Sodium sulfate
– CaCO3
– CaCl2
– Zeolite
– Silica gel
• Related patents are
• US Patent No. 6,121,410
• US Patent No. 5,338,822
Degradation of polyolefins
Degradation of polyolefins
Induced by either heat or UV radiation (Could be accelerated in the presence of some residual metals such as Fe, Cu)
Proceed via free radical and peroxide intermediates
Free radical chains are finally terminated via either a chain scission (PP, EVA) or a crosslinking (PE), depending on the type of polymers
General degradation mechanism of polymers
Ref: Solar Energy Materials and Solar Cells, 1996, V.43
(Thermal and UV degradation of the polymer lead to EVA browning and bubble formation
This would affect power conversion efficiency of the solar cell modules and service life of the material
Degradation of EVA
EVA browning Bubble formation
Degradation mechanisms of EVA
Ref: Solar Energy Materials and Solar Cells, 1996, V.43
These 2 reactions might be competitive, depending on the oxygen concentration
Stabilization of polyolefins
Types of stabilizers
UV absorbers (benzophenone)
Light stabilizers or free radical scavengers (hindered
amines)
Quencher
Primary antioxidants (phenol compounds)
Secondary antioxidants (peroxide decomposers)
Phosphites
Organic sulfides
http://www.cibasc.com/view.asp?id=6218
Ref
:www.specialchem4polymers.com/tc/Antioxidants/index.aspx?id=
UV spectrum of sunlight
The short wavelength UV radiation below 175 nm is absorbed by oxygen in the layers 100 km above the sea
The radiation between 185 and 290 nm is absorbed by the ozone layer of the stratosphere which begins at about 15 km above the sea level
It is the remaining UV part of sunlight. i.e. radiation between 300 and 400 nm that initiates degradation of plastics on outdoor weathering
Major types of UV absorber
Benzophenone compounds
Benzotriazole compounds
Cinnamate compounds
Energy dissipation in hydroxyphenyl-benzotriazol UV Absorbers www.ciba.com
Assoc. Prof. Dr. Jatuphorn Wootthikanokkhan, KMUTT
Simplified Stabilization Mechanism of Hindered Amine Stabilizers
HALS stabilizing mechanism
Alkyl phenol type (primary antioxidant)
2,6-di-tert-butyl-4-methylphenol
Chain breaking donor mechanism (Plastic Additives Handbook, R. Gachter, H. Muller, Hanser, 1993)
Assoc. Prof. Dr. Jatuphorn Wootthikanokkhan, KMUTT
Ref :www.specialchem4polymers.com/tc/Antioxidants/index.aspx?id=
Assoc. Prof. Dr. Jatuphorn Wootthikanokkhan, KMUTT
http://www.cibasc.com/view.asp?id=6215
Chemical structures of some commercial antioxidants
Assoc. Prof. Dr. Jatuphorn Wootthikanokkhan, KMUTT
Chemical structures of some antioxidants
http://www.cibasc.com/view.asp?id=6215
Stabilization by secondary antioxidants (peroxide decomposers)
Phosphite compounds
Aliphatic phosphite
Aromatic phosphite (more effective)
Sulfur compounds
Phosphite peroxide decomposer (secondary antioxidant)
(J.F. Rabek in Photostabilization of Polymers, Elesvier, 1990)
Phosphite compound also act as an antioxidant (chain breaking mechanism)
(J.F. Rabek in Photostabilization of Polymers, Elesvier, 1990)
Volatility
Some phenolic antioxidants are rather volatile and may be lost from plastics articles, even at low temperatures.
Usually the relative volatilities of antioxidants are determined by thermogravimetry (TGA).
An inverse relationship between volatility (defined e.g. as the temperature corresponding to 50% weight loss) and molecular weight of various commercial antioxidants has been claimed.
Hydrolytic stability
Some antioxidants are sensitive to hydrolysis. (especially phosphites and phosphonites)
Besides, the formation of acidic species (by products) may lead to corrosion of machinery and even to polymer discoloration.
The use of all-aromatic phosphite of high purity (inherently much more resistant to hydrolysis than alkyl or alkaryl phosphites) is recommended.
The use of a combination of the primary and secondary antioxidants
http://www.cibasc.com/view.asp?id=6215
Synergism between antioxidants
By combining primary and secondary antioxidants, synergistic effects can be expected
Example of the synergism is observed when a hindered phenol was used in combination with phosphite for the melt stabilization of polyolefins
http://www.cibasc.com/view.asp?id=6215
Effects of antioxidant on MFI of PP
The above synergistic effect is not always the case
-100
-80
-60
-40
-20
0
20
40
60
80
100
No antioxidants Tinuvin770 Tinuvin770 +
Irgafos168
Tinuvin770 +
Irganox802
Types of Antioxidant
Ch
an
ges i
n T
en
sil
e S
tren
gth
aft
er
UV
Irr
ad
iati
on
(%
)
(0.1 phr of secondary antioxidant)
Discoloration
Depending on the polymer and the environmental conditions of aging, discoloration may be essentially due to the plastics material or to the stabilizer
With polymer little prone to discoloration such as polyolefins and acetals,
yellowing can usually be attributed to the additives, their interaction or their oxidation products
With other polymers e.g. styrenic polymers, polycarbonate, and polyurethane, discoloration originating from the substrate is superimposed by possible discoloration caused by the stabilizers.
Discoloration of EVA film due to peroxide-UV absorber interaction
(P.Klemchuk, Polym Degrad Stab, 55 (1997) 347-365)
Cyasorb UV 531 (uv absorber)
Tinuvin 770 (uv stabilizer)
Naugard P (antioxidant)
Luperox 101 Liquid (curing agent)
Tinuvin 770 (uv stabilizer)
Naugard P (antioxidant)
Luperox 101 Liquid (curing agent)
Color stability
Oxidation products of phenolic antioxidants e.g. 2,6-di-tert-butyl-4-
methylphenol and n-octadecyl 3-(3’,5’-di-tert-butyl-4’-hydroxyphenyl)propionate cause discoloration
Degradations of PET, PC, and epoxy
Thermal decomposition of PET followed by hydrolysis
• Hydrolysis of the vinyl ester yields acetaldehyde (AA)
• Only a few ppm of the aldehyde may impair the taste of the content in soft drink bottle
At presence, there is no direct evidence indicating that AA is carcinogenic
Degradation of polycarbonate s health risk
BPA
Toxicity of BPA (at high dose)
น ้ำหนักอวยัวะสืบพนัธเ์ปล่ียน
อตัรำกำรผลิตสเปร์ิมเปล่ียน
อ่ืนๆ ท่ีเก่ียวกบัฮอรโ์มนเพศและอวยัวะสืบพนัธ์
ผลด้ำนสำรก่อมะเร้งยงัไม่สรปุชดัเจน
[Vom Saal et al., (1997) Proc. Natl. Acad. Sci. USA 94, 2056-2061]
Table 1. Comparison of Allowable Standard of BPA among USA, EU, and Japan.
Organizations
Dietary intake (mg/kg body weight/day)
Migration standard (ppm)
U.S. Environmental Protection Agency (EPA)
0.05 None
The European Commission’s Scientific Committee on Food (SFC)
0.01 (TDI)*
0.6 mg/kg food
Japan 0.05 Not more than 2.5 ppm
* Tolerable Daily Intake (revised in 2002)
• Generally less than 5 ppb (under normal use condition and without damage or
scratch)
•More than 120 times lower than the European’s migration limit (600 ppb).
Migration levels of BPA from various PC products
Migration levels of BPA from various food cans
• The detected BPA level is ~ 10 to 70 ppb
• These levels are ~ 8 to 60 times lower than the FDA limit (0.6 mg/kg or 600
ppb).
(B.M.Thomson and P.R. Grounds, Food Additives and Contaminants, 22(2005)65-72)
Safety limits, and the migration levels of BPA
Concentration
Toxi
city
EU migration limit
High Dose Region
Non Observe Adverse Effect Level
Low Dose Region
(~ 0.02 – 20 g/kg)
0.6 mg/kg
50 mg/kg/day 0.05 mg/kg/day
FDA safety limit
Coconut cream
Low dose hypothesis ?
Tuna
60 times
1,000 times
PC baby bottle
Other canned
food
Measured level of
BPA in human
urine Daily intake level
120 times
Degradation and stabilization of PVC
Thermal degradation of PVC
( )
Cl Cl
Cl
Cl
Cl
Cl
n
) ( n
Polyvinyl chloride, PVC
Degraded PVC
+ x (HCL)
( T > 100 °C)
( )
Cl Cl
Cl
Cl
Cl
Cl
n
) ( n
Polyvinyl chloride, PVC
Degraded PVC
+ x (HCL)
( T > 100 °C)
ต ำแหน่งในโมเลกลุ PVC ท่ีเป็นจุดอ่อน เร่ิมเกิดกำรหลุดออกของ HCl
( CH2 CH CH CH2 )
Cl Cl
Head to Head Structure
CH 2 CH CH
Cl
Unsaturated Bonds at Chain End
TGA thermograms of PVC under different heating rates
Stabilization of PVC
Preventive functions
Absorption of HCl
Prevention of auto-oxidation
Curative functions
Addition to polyene sequence
Effects of phenolic antioxidant on thermal stability of PVC
_Plastics Additives Handbook, edited by R.Gachter and H. Muller, Hanser, 1993
Stabilization of PVC by absorption of HCl using epoxidized fatty acids
Stabilization of PVC via Diels-Alder reaction, using dialkyltin maleate
Stabilization of PVC by absorption of HCl using organotin mercaptides
The reaction proceeds via a substitution mechanism
Stabilization of PVC by addition to polyene sequences
The mercapto compounds (which are released in the course of the
reaction of organotin mercaptides with HCl) are capable of adding to double bonds
Stabilization of PVC by absorption of HCl using cadmium stearate
Thermal dehydrochlorination of PVC at 175 C in the presence of different metal chloride, which may be regarded as reaction by products of the corresponding metal carboxylates (m HCL : liberated
hydrogen chloride, t : time) [ref: Plastics Additive Handbook, R. Gachter and H. Muller, Hanser, 1993, p.290]
Cadmium or zinc stabilizers differ from organotin stabilizer in that the metal chloride formed has a destabilizing effect
Stabilization of PVC by using Ba/Cd carboxylates
RoHS
RoHS ยอ่มำจำก Restriction of Hazardous Substances เป็นขอ้ก ำหนดท่ี 2002/95/EC ของสหภำพยโุรป (EU) วำ่ดว้ยเร่ืองของกำรใชส้ำรท่ีเป็นอนัตรำยในอุปกรณ์เคร่ืองใชไ้ฟฟ้ำและอิเลก็ทรอนิกส์ ซ่ึงหมำยควำมรวมถึงเคร่ืองใชทุ้กชนิด ท่ีตอ้งอำศยัไฟฟ้ำในกำรท ำงำน
1. ตะกัว่ (Pb) ไม่เกิน 0.1% โดยน ้ำหนกั 2.ปรอท (Hg) ไม่เกิน 0.1% โดยน ้ำหนกั 3.แคดเมียม (Cd) ไม่เกิน 0.01% โดยน ้ำหนกั 4.เฮกซะวำเลนท ์(Cr-VI) ไม่เกิน 0.1% โดยน ้ำหนกั 5.โพลีโบรมิเนต ไบเฟนนิลส์ (PBB) ไม่เกิน 0.1% โดยน ้ำหนกั 6.โพลีโบรมิเนต ไดเฟนนิล อีเธอร์ (PBDE) ไม่เกิน 0.1% โดยน ้ำหนกั
Some non-toxic thermal stabilizer for PVC
Ba/Zn carboxylate
Ca/Zn carboxylate
Ba/Ca/Zn carboxylate