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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