Polymer Additives

8.14

Polymer Additives

C Kröhnke, Süd-Chemie AG, München, Germany © 2012 Elsevier B.V. All rights reserved.

8.14.1 8.14.1.1 8.14.2 8.14.3 8.14.4 8.14.4.1 8.14.4.2 8.14.4.3 8.14.4.4 8.14.4.5 8.14.5 8.14.5.1 8.14.5.2 8.14.5.3 8.14.5.3.1 8.14.5.3.2 8.14.5.4 8.14.5.5 8.14.5.6 8.14.6 8.14.7 8.14.8 8.14.9 8.14.9.1 8.14.9.2 8.14.9.3 8.14.9.4 8.14.9.5 8.14.9.5.1 8.14.9.5.2 8.14.9.6 References

Introduction Principles of Polymer Degradation Thermo-Oxidative Degradation Requirements for Polymer Stabilizers Stabilization against Thermo-Oxidative Degradation Stabilization of Polymers during Processing Primary Antioxidants (Sterically Hindered Phenols and Amines) Sterically Hindered Amine Stabilizers Inhibiting Thermal Degradation of Polymers Synergistic Blends of Sterically Hindered Phenols and Organophosph(on)ites Phenol-Free Stabilization Concepts Stabilization of Polymers against Degradation under the Impact of Light Introduction Quenching of Photo-Oxidation with HA(L)S HA(L)S-Based Free Radical Scavengers Inhibition of the photo-oxidation in HA(L)S-stabilized polymers Alkoxyamine hindered stabilizers (NOR HA(L)S) derivatives UV Absorbers Quenchers Practical Considerations for the Use of HA(L)S Multifunctional Additive for Engineering Polymers Metal Ion Deactivators Acid Scavengers Analysis of Stabilizers in the Polymer Matrix Introduction Testing the Polymer Melt Stability during Processing Examination of Long-Term Heat Aging Other Methods Analyzing Long-Term Aging of Polymers Testing of Polymer Stability against Light-Induced Degradation – Natural versus Artificial Weathering Natural weathering Artificial weathering Analytical Methods for Structural Characterization and Quantification of Polymer Additives

8.14.1 Introduction Most of the technically relevant polymers need appropriate stabilization by additives since damaging effects through the impact of heat, shear,1 and ultraviolet (UV) light occur during high temperature processing in the melt and service life of polymers later on. Therefore, specific stabilizers and stabilizer combinations are required to inhibit or at least retard a variety of undesired effects such as oxidation, chain scission, uncon­ trolled recombination, and crosslinking reactions. With appropriate stabilization, changes of chemical as well as phy­ sical properties such as cracking, reduction in gloss, embrittlement, and premature failure are kept under control or are avoided during the service life of stabilized polymer articles. According to a traditional definition, polymer stabilizers are generally classified into the groups of antioxidants and light stabilizers. This partition became questionable since it is known that several groups of stabilizers are suitable to be used for multifunctional purposes, sometimes alone but also in combination with other stabilizers. Polymer Science: A Comprehensive Reference, Volume 8

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Among the total amount of polymer additives – excluding fillers and pigments – less than 10% are dedicated to polymer stabilizers that are commonly used in concentrations from some parts per million to about 1% of the related polymer. Except polyvinylchloride (PVC), roughly 80% of the polymer stabilizers are dedicated to polyolefins.

8.14.1.1

Principles of Polymer Degradation

Historically, theories of polymer degradation are based on studies investigating the autoxidation mechanism of saturated liquid hydrocarbons by Hock and Lang.2,3 Saturated hydrocar­ bons are thermally stable materials, for example, hexadecane is stable up to 390 °C in inert atmosphere. Polymers, particularly polyolefins, start to decompose at significantly lower temperatures under the same conditions because some weak sites, such as unsaturated groups and branching points, decrease the dissociation energy of C–C bonds.4 As an example, the thermal degradation of linear poly­ ethylene in oxygen-free environment starts with random scission of the polymer chains preferentially in allylic positions

doi:10.1016/B978-0-444-53349-4.00212-0

349

350

Polymer Additives

HC2=CH-CH2

R

R

H2=CH-CH2• + •

R1

R polymer segment)

R

CH2 R-CH=CH-CH2

R (

R1 CH2

R1

R-CH=CH-CH2• + •

R1

Scheme 1 Thermal degradation of polyethylene with unsaturated moieties under oxygen-deficient conditions.

RCH2• + R′-CH2• R-CH2• + R′-C•H-R″ R-CH 2CH2• + R′-CH2•

•

R-CH 2• + R′-C H-CH2R″

RCH2CH2R′ R-CH2-CHR′R ′′

R-CH=CH2 + R′-CH3

R-CH3 + R′- CH=CH-R′′

Scheme 2 Termination of C-centered radicals by recombination and disproportionation.

followed by randomly occurring intermolecular hydrogen abstraction and subsequent β-scissions which can lead to long chain branching (see Scheme 1). In addition to depolymerization through β-scission, intraand intermolecular hydrogen transfer occur. So, primary carbon-centered radicals are expected to isomerize by intramo­ lecular hydrogen abstraction (backbiting) and to form secondary radicals, which are more stable. On the basis of activation and bond dissociation energies, Kuroki et al.4 claimed that backbiting reactions and intermolecular radical transfer reactions are much more likely to occur than depoly­ merization reactions.5 Under oxygen-free or oxygen-deficient conditions, recombi­ nation and disproportionation reactions are the most important termination reactions of thermal degradation (Scheme 2). As a result of all these reactions, a significant decrease of the average molecular mass and yield of volatile products take place.6–9

8.14.2 Thermo-Oxidative Degradation The oxidation of hydrocarbons is a free radical-initiated auto­ catalytic chain reaction. The reaction is slow at the start and accelerates with increasing concentration of the resulting hydroperoxides. In addition, ‘free’ carbon-centered radicals react rapidly with so far undamaged polymer segments as well as with oxygen under the formation of peroxide radicals that itself attack further intact polymer chain segments leading to those hydroperoxides and other carbon-centered radicals. The rate constant for the reaction of most alkyl radicals with oxygen is of the order of 107–109mol−1s−1. As formulated for general radical-based degradation path­ ways, this process can be regarded as proceeding in three distinct steps: chain initiation, chain propagation including chain branching, and chain termination (Scheme 3).10 Despite intensive experimental investigations, the origin of the primary alkyl radical R• is still controversial.11 The peroxide radicals form hydroperoxides upon abstrac­ tion of hydrogen from the polymer chain that requires the breaking of a C–H bond, that is, needs activation energy. Therefore, this is the rate-determining step in autoxidation.

The rate of the abstraction reaction decreases in the following order: hydrogen in α-position to a C=C double bond (allyl) > benzyl hydrogen and tertiary hydrogen > secondary hydrogen > primary hydrogen. Primary and secondary peroxide radicals are more reactive in hydrogen abstraction than the analogous tertiary radicals12 and the most reactive are acylperoxide radi­ cals.13 Unstable hydroperoxides can form highly unstable alkoxy and hydroxyl radicals under the impact of heat, UV light, catalyst residues, and even traces of metals and metal ions. Structural changes occurring, for example, in low-density polyethylene at an oxygen content of about 1%, in some other polymers less than 0.0005%. Refined theories on degradation principles of polymers have been published in subsequent decades, for example, by King14 and others studying the polyolefins and their transfor­ mation products under the influence of multiple melt compounding steps under high temperatures and shear rates, starting with polymer out of the reactor toward pelletized products, formulated compounds down to final plastic articles. As the result of those studies, the polymer is subjected to a variety of damaging processes, for example, by the presence of catalyst residues, entrained oxygen, ozone, other types of impu­ rities, high-energy radiation such as γ, X-rays, atmospheric pollutants, microorganisms, and hydrolysis. During repeated heat histories, free radicals are initiated via C–C and C–H bond scission. Once the free radical cycle is initiated, the formed carbon-centered free radicals react not only with other polymer molecules, but also with oxygen that is entrained in the system. As a consequence, formation of peroxide radicals takes place. The peroxide radicals react further with the polymer-generating hydroperoxides. Despite its simplification, the entire course of degradation of polymers in the presence of oxygen is outlined in the so-called ‘autoxidation cycle’ as free radical-initiated chain reac­ tions (Figure 1). In reality, the sequences are much more complex since influence factors such as processing temperature, shear, and catalyst impurities can direct each individual poly­ mer differently. The formation of unstable hydroperoxides, which can also be decomposed by heat, UV light, catalyst residues, or other metallic impurities, ultimately leads to the formation of alkoxy and hydroxyl radicals, as depicted in cycle II. Oxygen-centered radicals can react further with the polymer, leading to the formation of more carbon-centered free radicals, which feed back into cycle I. The reactions leading to free radicals being formed on the polymer backbone result in chain linking and/or chain scission reactions in an effort to quench free radicals. The understanding of polymer degradation mechanism is further complicated by factors such as polymer morphology, diffusion processes of additives in polymers, and interaction of additives and their transformation products.

Polymer Additives

351

R.

R-H, R-R

Chain propagation

.

R + O2

. ROO

. ROO + R-H

. ROOH + R

. RO + R-H

. ROH + R

. HO + R-H

. H 2O + R

. R

O R

1

C=C

+

R C

C

.

.

C

R2

O

ß-Scission

R1

C

R2 + R3

R3 Chain termination

. .

R + ROO

R–O–O–R

. . R + R

R–R

. . R + RO

R–O–R

. 2R

R – H + unsaturated polymer chain

O R

. 2 ROO

+ – R –OH + O2 (disproportionation reactions)

Scheme 3 General course of thermo-oxidative polymer degradation.

R–H (polymer)

Heat, shear, catalyst residues, γ-irradiation Oxygen

RO.+ .OH

R. RO. HO. HOO. ROOH

Cycle II

R. + ROOH

Cycle I

Alky radical Alkoxy radical Hydroxy radical Peroxy radical Hydroparoxide

ROO.

R–H

Figure 1 Simplified autoxidation cycles of polymers.

These crosslinking/chain scission reactions result in fundamental changes to the molecular architecture of the polymer regarding its molecular weight (MW), MW distribution (MWD), as well as the nature of chain branching on the polymer backbone.

In most applications, polymer degradation is an undesir­ able process. However, degradation can be desirable in special applications such as in biomedical, hygienic, and packaging products. Hereby polyesters and appropriate biopolymers which are prone to hydrolysis, for example polylactides, poly­ hydroxyalkanoates such as polyhydroxybutyrate and aliphatic/ aromatic out of butanediol, adipic acid, and terephthalic acid (Ecoflex®), are increasingly used. In addition, naturally occur­ ing polymers like starch are increasingly employed in special applications.15 Those polymers are subjected to ‘biodegrada­ tion’ if their organic components are completely degradable into natural products. Besides hydrolysis, depolymerization can take place by oxi­ dation. In a later stage of degradation, microorganism as well as enzymatic systems can contribute to the process leading to the formation of water, carbon dioxide, methane, and biomass. Among all degradation mechanisms, oxidation is the most important. The auto-initiated oxidation by molecular oxygen is defined as ‘autoxidation’, but this expression is used to describe the reaction of oxygen with organic materials by a free radical process. Degradation pathways of individual polymers during pro­ cessing and service life depend furthermore on their prehistory

352

Polymer Additives

and previous damage. Polymer processing in the melt occurs at temperatures sometimes even above 300 °C under oxygen-deficient conditions within seconds or minutes, which leads preferentially to the formation of carbon-centered radi­ cals ([R●] >> [ROO●]) and a high rate of hydroperoxide decomposition. On the other hand, service life of polymers in the solid state usually takes place from hours to years exposure times under atmospheric conditions. Hereby, with oxygen in the saturation equilibrium, the concentration of peroxide radi­ cals exceeds clearly the concentration of carbon-centered radicals ([ROO●] >> [R●]). The formed hydroperoxides react further at low to moderate hydroperoxide decomposition rates depending on the temperature. Besides the conditions of the environment, structural features of the polymer determine the sensitivity against oxidative degra­ dation as well as the degradation path. Among polyolefins, an ‘increasing tendency’ of thermally induced oxidative degradation from low-density polyethylene (PE-LD) to linear low-density polyethylene (PE-LLD), high-density polyethylene (PE-HD), and polypropylene (PP): PE-LD < PE-LLD < PE-HD < PP. Among polyolefins, PP is particularly prone to degradation that becomes evident especially under long-lasting processing conditions at high temperatures such as rotational molding where kinetic modeling considers two distinct basic approaches: a thermal transfer mechanism with phase changes (e.g., melting and crystallization) simulated by the enthalpy method and a second approach focusing on chemical oxida­ tion and the influence of stabilizing additives to the melt.16 PE-HD shows a degradation behavior depending on the catalyst system used during manufacturing. PE-HD prepared with Al/Ti-based (‘Ziegler’) catalysts decomposes preferentially under C–C bond scission rather than under crosslinking that leads to a decrease of the molecular weight (Figure 2). On the other hand, PE-HD manufactured with chromium-type cata­ lysts degrades in favor under crosslinking leading to a distinct increase of the molecular weight and usually a broadening of the molecular weight distribution. An overview of the degradation tendency and behavior of polyolefins is given in Table 1:

Chain scission (main pathway of Ziegler type) MW, MWD Crosslinking (main pathway of Phillips type) MW, MWD Figure 2 Main degradation paths of PE-HD depending on its prehistory (catalyst system). Table 1

Proportion of degradation pathways of major polyolefins Chain scission

Low-density polyethylene (PE-LD) Minor Linear low-density polyethylene Minor (PE-LLD) High-density polyethylene (PE-HD) Dominant (Al/Ti) Polypropylene (PP) Dominant

Crosslinking Minor Dominant Dominant (Cr) Minor

Degradation pathways of polystyrene derivatives also depend on structural conditions (Scheme 4). Substitution with one single methyl group can substantially influence the direction whether depolymerization or H-transfer is preferred: Figure 3 mediates an impression on the formation of trans­ formation products and particularly on the reconstitution of corresponding monomers during thermally initiated degrada­ tion of important technical polymers. Thermally initiated degradation of aliphatic polyamides can lead to amide pyrolysis at temperatures of about 250 °C and higher as well as inter- and intramolecular transamidation (formation of lactames) reactions at less elevated temperatures (Scheme 5).17,18 Thermal degradation behavior of aromatic polyamides such as poly(1,3-phenylene isophthalamide) and poly(chloro­ 2,4-phenylene isophthalamide) was investigated and the for­ mation of low-molecular-weight compounds was studied by gas chromatography/Fourier transform infrared (GC/FT-IR) spectroscopy and gas chromatography/mass spectrometry (GC/MS). Thermogravimetric analysis (TGA) was applied to study the effect of end-group concentration on the degradation characteristics of the two polyamides. The results of this inves­ tigation suggest that the thermal decomposition of these aromatic polyamides involves homolytic as well as hydrolytic cleavages of the amide units.19

8.14.3 Requirements for Polymer Stabilizers Development and application of polymer stabilizers has to follow certain principles and requirements and must adapt structural features of the corresponding polymer type. Sufficient compatibility of stabilizers in polymer matrices is a highly important factor for their applicability and determines significantly their performance. Tailor-made stabilizers with both an active function and an individual segment influencing the solubility can adapt to a specific environment given by the polymer matrix. Additionally, polymer stabilizers must exhibit sufficient thermal stability and must be nonvolatile at elevated proces­ sing temperatures in the melt and resistant to extraction. Intrinsic color or colored transition products of stabilizers are usually undesired. In practice, especially by use of sterically hindered phenols, discoloration is often observed due to the formation of products with quinoid structures. They show absorption maxima clearly above a wavelength λ = 400 nm in the visible wavelength regime which is shown in Figure 4.20 As far as processing stabilizers are concerned, substances with molecular weights above 300 should be used, preferably in a molecular weight range of 400–800 which enables suffi­ cient mobility in the melt combined with good efficiency but limits undesired volatility. On the other hand, long-term stabi­ lizers such as sterically hindered amine (light) stabilizers (HA(L)S) should have molecular weights of between 1500 and 2500 in order to guarantee a certain stability against migration. Furthermore, odor and taste of stabilizers and their possible transition products must be avoided, as well as any issue of toxicity. Finally, sufficient storage stability should be guaranteed. This is an important aspect especially as far as several organo­ phosphites are concerned which are known to be sensitive against hydrolysis which will be discussed more in detail later.

Polymer Additives

CH3

H CH2 C

353

CH2 CH

CH2

CH3

HC

CH2

CH2

C

H2C

CH3

CH

CH3

Depolymerization

~40%

100%

50%

H-transfer

~60%

0%

50%

Scheme 4 Flow of degradation of different polystyrenes depending on their structural features.

Depolymerization Monomers ‘Intact’ polymer (initial stage)

Intermolecular

Statistical chain scission

Intramolecular

Oligomeric fragments

Chain transfer

Polymer

Monomer yield (%)

Polymethylmethacrylate (PMMA)

100

Poly(α-methylstyrene)

100

Polytetrafluoroethylene (PTFE)

100

Poly(3-methylstyrene)

50

Polystyrene

40

Polyisobutene

30

Polyisoprene

12

Polypropylene

2

Figure 3 Major degradation pathways and products of important technical polymers.

8.14.4 Stabilization against Thermo-Oxidative Degradation 8.14.4.1

Stabilization of Polymers during Processing

Already immediately after manufacture of polyolefins in the reactor, they are extruded and cut into pellets or granules which

are the typical handling form of this material. During this first extrusion step, heat- and stress-induced thermo-oxidative degradation takes place. Such a degradation can be observed by measuring melt properties of the resin. To prevent early degradation of the new material, polyolefins need a base stabi­ lization that consists typically of antioxidants and acid

354

Polymer Additives

Amide pyrolysis

H

O CH2 CH2

OH

CH

NH CH2

CH2

+

CH2 CH2

H2C

CH CH2

NH

−H2C

O C H2 C H2

CH2

CH2

C

N H2 Intermolecular transamidation

O C H2 CH2 C

O NH

H

O

C H2 CH2 N

C

C H2

CH2

C H2 CH2 C

NH

C H2

C H2

C

C H2

C H2

+ CH2

CH2

C H2 CH2

NH O

Intramolecular transamidation

NH CO

NH-CO

CO

CO-NH

NH

Lactame Scheme 5 Main pathways of thermally initiated degradation of aliphatic polyamides.

COOR

COOR ROOC

ROOC

O

A

VIS absorption ε λ max (nm)

O

OH

O

34.800 (l(mole cm−1)) 440

COOR ROOC

OH

O

B

C

106 (l(mole cm−1)) 450

116 (l(mole cm−1)) 420

Substance A strongly absorbs in VIS and therefore leads to discoloration of polymers: For comparison, molar extinction coefficients ε of the commercial UV absorbers are as follows: • Benzophenones (Hostavin ARO 8) − 10.000 (l(mole cm−1)) (λ max 325 − 330 nm) • Benzotriazoles (Tinuvin ‘xy’) − 17.000 (l (mole cm−1)) (λ max 340 − 350nm)

Figure 4 Absorption characteristics of transformation products of sterically hindered phenols.

scavengers.21 Their basic structure and function are described in the following chapters. Additives are added to the polymer melt directly in the reactor or later during the compounding step by feeding into the extruder. Organo(III) phosphites and organo(III) phosphonites are widely used and act during polymer processing in the melt as hydroperoxide-decomposing agents and fall under the defini­ tion ‘secondary antioxidants’. These additives help to maintain

basic physical properties such as molecular weight and color during processing. Such so-called ‘secondary antioxidants’ can usually be added in various product forms (see Figure 5) in a dry state as powders, micropills, granules, and so on often in combination (‘blends’) with other additives like sterically hin­ dered phenols (see below) and acid scavengers. In other cases, hydrolytically sufficiently stable organopho­ sphites are emulsified under aqueous conditions and added to elastomers. Figure 6 exhibits a selection of structures and brand

Polymer Additives

355

Power

O

O P

P

O

O

Compacted (FF)

Extruded granules

Figure 7 Structure of a commercially available organophosphonite (Hostavin® P-EPQ). Pills

Compacted granules

Micropills

substrate play substantial role in initiation, kinetics, and also reaction extent of the substrate oxidative degradation. More complex schemes are continuously under investiga­ tions and development. As a consequence, the ‘trial-and-error’ experimental approach still remains a broadly used method in development of additive packages. As far as polyolefins are concerned, it should be pointed out that organophosphites are effective for processing sta­ bilization of those polyolefins which are prepared by catalyst systems (PP, PE-HD, PE-LLD). Their importance for the stabilization of PE-LD, manufactured by radical polymerization, is less important since the degradation pathway under formation of hydroperoxides is not opera­ tive for PE-LD. Phosphorus-based derivatives (organophosphites and orga­ nophosphonites) are well known to be effective as polymer stabilizers mainly during processing, acting as secondary anti­ oxidants, and are particularly effective in combination with phenols (primary antioxidants). In addition, some phosphites and phosphonites can help in improving the discoloration of polymer articles when they are exposed to heat, UV light, nitrogen oxides (gas fading), and gamma rays. However, phosphorus-based stabilizers are sensitive to heat and moisture so they can cause undesired side problems during processing and in the lifetime of polymer articles. An example presented in Figure 9 demonstrates how phosphorus(V) oxidation products can further hydrolytically decompose.

Figure 5 Different product forms of commercial polymer additives and polymer additive blends. FF, free flowing.

names of such commercially available organophosphites. Figure 7 outlines the chemical structure of the only commer­ cially available organophosphonite. Their efficiency in hydroperoxide decomposition decreases in the order phosphonites > alkylphosphites > arylphosphites > hindered arylphosphites. Five-membered cyclic phosphites are capable of decomposing hydroperoxides catalytically due to the formation of acidic hydrogen phosphates by hydrolysis and peroxidolysis during the reaction. At ambient tempera­ tures, the chain-breaking antioxidant activity of aryl phosphites is lower than that of hindered phenols, because the rate of their reaction with peroxide radicals and their stoi­ chiometric inhibition factors are lower than those of phenols. In oxidizing media at medium temperatures, however, hydro­ lysis of aryl phosph(on)ites takes place giving hydrogen phosph(on)ites and phenols which are effective chain-breaking antioxidants.22 Despite all efforts to clarify in detail the degradation and stabilization pathways of polymers during processing, the pre­ sently proposed schemes for polymer degradation as shown in Figure 8 are simplified and are not able to explain the full complexity of polymer degradation. In reality, various inho­ mogeneities, catalyst residues, and so on in real polymer

R O P

O

R�

O R

O

O

P O

O

O P

O

O

P

R′ O

R R = DTBP NP R� = DTBP DCP R� = TTBP

Hostanox® PAR 24 = Irgafos® 168 TNPP (e.g., Weston® 399) Ultranox® 626 = Hostanox® PAR 62 Doverphos® 9228 Ultranox® 641

DTBP

NP

DCP

C9H19 Figure 6 Selection of structures and related brand names of commercially available organophosphites.

TTBP

O R″

356

Polymer Additives

Heat, shear catalyst residues, γ-irradiation

R−H (polymer)

R· + ROOH

Cycle II RO· + ·OH Reaction of free oxygencentered radicals with phenolic AOs

Alkyl radical traps

Cycle I ROO·

R−H (Polymer)

Hydroperoxides react with organophosphites and organophosphonites yielding inactive products, e.g., R−OH

Degradation path

Reaction of free oxygen− centered radicals with phenolic AO s

Stabilization path

Figure 8 Simplified scheme of pathways of polymer degradation and stabilization.

O O

O P

P O

− 2 equivalents di-tert-butyl-phenol (DTBP)

O

O

HO O P O O

O

O

P

O OH

OH HO

HO

OH +

HO

OH

P

O

HO

Pentaerythritol Figure 9 Hydrolysis of an oxidation product of a bisfunctional organophosphate.

Such undesired processes often lead to a significant loss of performance as well as to possible handling issues since those products can become sticky. Therefore, aim of several investigations in the past two decades was to test new organophosphorus-based compounds more hydrolytically stable than the conventional commercial ones and to present their outstanding performances in improving discoloration during processing as well as when polymer articles are exposed to gamma rays. The comparison has been carried out with standard available products.23 A possible loop­ hole to improve hydrolytic stability is the addition of co-additives of different chemical structures.24 The various chemical structures of such co-additives and the tendency of their

influence are shown in Figure 10. In addition, the rate of hydrolysis of an organophosphite can be drastically reduced by altering the physical product form of the additive package. This selection extends the storage life of a particular organophosphite. Secondary antioxidants are able to prevent the formation of free oxygen radicals through nonradical decomposition of hydroperoxides ROOH (eqn [1]):

ROOH + P(OR1)3

Slower Hydrotalcites

ROH + O=P(OR1)3

(R1 = alkyl, aryl) Faster

Zn-stearate

Zinc oxide

Polyolefin waxes

Glycerinemonostearate (GMS)

Erucamide/oleamide

Ca-stearate

Organic amines, e.g., HA(L)S

Na-benzoate

Silica Fluoropolymers

Metal carbonates Figure 10 Influence of various co-additives on the kinetics of hydrolysis of organophosphates and organophosphonites.

½1

Polymer Additives

O H3C-(CH2)17

O

O

ROOH

C CH2 CH2

2

S

H3C-(CH2)17

−ROH

(I)

H3C-(CH2)17

C CH2 CH2

H3C-(CH2)17

O

O

C CH2 CH2

SOH + H3C-(CH2)17

ROOH

O

C

C

CH2

(v)

(IV)

ROOH

S O

O

O heat

2

O

C CH2 CH2 2 S

O (III)

−ROH

(II)

O (II)

O ROOH

357

ROOH

SO2, SO3 (H2SO4)

(IV) Figure 11 Decomposition of hydroperoxides with thioethers of dipropionates.

In addition, these stabilizers act as synergists and enhance the performance of primary antioxidants. Besides the commonly used organophosphites [P(OR1)3], and organophosphonites [P(OR1)2R2], ‘thioesters’ and ‘thio­ carbamates’ are technically used for several applications.25 All these secondary stabilizers behave highly effective during pro­ cessing of polymers in the melt and protect both the polymer and the primary antioxidant (see below). The key action of ‘dialkyldithiocarbamates’ and ‘thioethers of dipropionates’ can be generally described as outlined in Figure 11. The effi­ ciency of such ‘organosulfur’ compounds is based on the ability of sulfenic acids to decompose hydroperoxides. ‘Sulfenic acids’ are formed intermediately by thermal treatment from sulfox­ ides. In a final stage, sulfuric acid and other sulfur-containing oxidation products are formed. Compounds based on organophospites and organopho­ sphonites with substituents containing sterically hindered piperidine moieties as commonly used in HA(L)S (see below) have been described in the literature but have never received industrial importance.26 ‘Alpha-tocopherol’ as an environmentally friendly com­ pound exhibits excellent thermal stability. Synergistic mixtures with phosph(on)ites or thioesters offer enhanced sta­ bilization properties for polyolefins during processing.27 OH

O

Alpha-tocopherol (vitamin E)

8.14.4.2 Primary Antioxidants (Sterically Hindered Phenols and Amines) A ‘primary antioxidant’ is added to protect the polymer from degradation over long time periods or the plastic product’s

OH

OH R−O−O

−R−O−O−H

R1

R1

O

O R−O−O

R1

R1 O−O−R

Figure 12 Reaction of sterically hindered phenols with peroxy radicals.

lifetime.28 The oldest and technically still highly important class of so-called primary antioxidants consists of ‘sterically hindered phenols’. Their key reaction is the formation of hydroperoxides by hydrogen transfer from the phenolic OH-function to usually heat-generated peroxy radicals, result­ ing in the formation of a phenoxyl radical as outlined in Figure 12. The active group is basically the sterically shielded phenolic OH-group activated by an alkyl substitution (R1) in the 4-position. Several other commercially available alkylated phenols like tert-butyl-methyl-phenols or less hindered phenols are also efficient and. Less hindered phenols with hydrogen atoms in the α- and β-positions to the phenyl ring can undergo dispro­ portionation reactions under formation of quinomethide structures. Also coupling products with irreversibly generated C–C bonds have been observed. The steric hindrance of phe­ nols has mainly several functions by decreasing the accessibility of the hydroxyl group limiting the reactivity toward reactive species like ‘peroxy radicals’ and by stabilizing the intermediate ‘phenoxyl radical’. Furthermore, the steric hindrance by bulky groups neighbored to the phenolic OH-group suppresses the rapid dimerization of two phenoxyl radicals.

358

Polymer Additives

Transformation products of phenols exhibiting quinoid structures give rise to a continuously increasing discoloration of the polymer during its service life. All these reactions includ­ ing formation and scavenging of phenoxyl radicals and generation of transformation products take place already at relatively low temperatures but significantly faster at higher temperatures. Historically, one of the first products applied for stabilizing purposes was BHT (butylated hydroxytoluene). It is still occasionally incorporated in thermoplastic polymers due to the fact that it is a very cheap additive. However, severe problems due to volatilization, blooming, or discoloration (yellowing, pinking, etc.) frequently occur with this product. Therefore, a continuous replacement by less volatile products such as AO-1 took place, where R1 represents a long chain in order to increase the molecular weight of the stabilizer, to decrease the volatility, and to improve the compatibility with unpolar polymer matrices, for example, in case of polyolefins. Product AO-1 is manufactured by various companies and sold under several commercial names such as Irganox® 1076 (BASF), Anox® PP18 (Chemtura), or Hostanox® O 16 (Clariant).

phenolic functions attached to the molecule correlate with their efficiency. Besides their function as efficient H-donors, phenols are able to participate at other reactions which depends on the substitution pattern: phenols with 2-, 4-, and 6-tert-alkyl sub­ stitution where no tautomeric benzyl radical can be formed can react stoichiometrically with peroxy radicals5. Phenols with methylene or methyl substitution at least in 2-, 4-, or 6-position are able to form corresponding quinone methides. Subsequent inter- and intramolecular recombinations often lead to gener­ ally irreversibly formed C–C coupling products (5). A more detailed description of phenolic antioxidants used for industrial applications is given in the literature where the aspect of synergistic blends with organophosphates is also discussed.29 AO-1 and AO-2 are by far the most broadly used phenolic antioxidants. Nevertheless, several further derivatives are avail­ able in the market, such as AO-4 (brand names: Irganox® 3114 (BASF), Hostanox® O 14 (Clariant)), based on a cyclic struc­ tural element. R O R

O

HO

O

N N

N

R = H2C

OH

R

O

O C18H37

AO-4

AO-1

Improved antioxidants have been developed with higher efficiency, reduced migration, and volatility especially suited for the use in PP. Sterically hindered, propionate-type polyphe­ nols like AO-2 still represent the state of the art. AO-2 is sold under commercial brands such as Irganox® 1010 (BASF), Anox® 20 (Chemtura), or Hostanox® O 10 (Clariant).

O

HO

O

For standard processing purposes of polymers, combina­ tions of sterically hindered phenols with organophosphites or organophosphonites are used which stabilize the melt more effectively as these components show a synergistic effect if used together in the formulations. Small aliphatic substituents such as methyl groups in the α-positions to the phenolic OH-group can cause higher efficiency than larger substituents.30 The phenol type AO-4 is used for particular products, such as PP fibers, where it mostly shows a better color stability, for example, in the presence of nitrogen oxide [(NO)x] gases (gas fading, gas yellowing) than the other phenols. The trifunctional phenol AO-5 is claimed to exhibit a very low contribution to ‘water carry over’ (WCO) that makes it very suitable in slit tapes applications.

4 R

AO-2

An obvious advantage of propionate-type polyphenols over a corresponding monophenol is attributed to the fact that the attack of a second peroxy radical is not directed to the first formed phenoxyl radical. The reaction with a second peroxy radical does rather involve a second still intact phenol function in the same molecule. Besides long-term protection of polymers, particularly polyolefins, selected phenolic antioxidants also contribute to the melt stabilization during processing. The performance of a few commercial antioxidants as long-term stabilizers of PP during accelerated heat aging in turbulent airflow ovens is shown in Table 2. It can be demon­ strated that both the structure of the phenol and the number of

R = H2C R

OH

R

AO-5

This product is available under the names Ethanox® 330 (Albemarle), Irganox® 1330 (BASF), or Hostanox® O 13 (Clariant). AO-6, available under the trade name Cyagard® 1790 (Cytec Corp.), shows high resistance against heat-induced migration and is therefore a preferred stabilizer for demanding applica­ tions.

Polymer Additives

359

Performance of phenolic antioxidants (0.1 wt.%) in PP at T = 135 °C in draft air ovens samples

Table 2 Phenol type

Formula

Time to embrittlement/days

AO-1

23

O HO

CH2CH2 C OC18H37

AO-3

79

O OH

O (CH2)6 O

HO

O AO-2

112

O HO

CH2CH2 C O

CH2

C

4 Compression molded plaques, thickness 1 mm; co-stabilizer: 0.1% Ca-stearate; AO-3 represents a difunctional sterically hindered phenol.

R

OH R = H2C

R

R

test criterion ‘time to brittleness’, the tetrafunctional phenolic antioxidant Hostanox® O 10 (AO-2) clearly outperforms the phenol derivative AO-7 (Hostanox® O3), whereas the results are drastically different after such a hot water storage period which can be interpreted that the tendency to being partially hydrolyzed and extracted under the given conditions is clearly higher for the derivative AO-2 as compared to AO-7.

AO-6

OH

Excellent resistance against extraction in aqueous media is provided by AO-7 that is available under the name Hostanox™ O 3 (Clariant). Such an effect can be described based on an experiment using PP plaques before and after 2-month expo­ sure to hot water (T = 85 °C) as depicted in Figure 13. Using the

O

Time to brittleness (weeks) 40 35

O CH2

HO

2 AO-7

Orig.

30 HWT

25 20

8.14.4.3 Sterically Hindered Amine Stabilizers Inhibiting Thermal Degradation of Polymers

15 10 5 0 Hostanox® O 10

Hostanox® O 3

Figure 13 Performance of compression molded PP plaques (thickness 0.5 mm; base stabilization 0.15% phenolic AO, 0.10% organophosphite/ Irgafos 168® = Hostanox® PAR 24, 0.10% Ca-stearate, 0.25% thiocos­ tabilizer/Hostanox® SE 4) before and after 2 months of hot water treatment (HWT) at T = 85 °C; criteria: ‘time to brittleness’.

In the past few decades, piperidine-based sterically hindered amine stabilizers (HA(L)S) gained attention as heat stabilizers for polyolefins at low and moderate temperatures below about 120 °C. It was ascertained that the degradation behavior of polymers, particularly of polyolefins stabilized with HA(L)S, differs significantly from that of phenol-stabilized polyolefins at aging temperatures ranging from T = 100 to 150 °C. The degree of stabilization of a polymer can be tested by measuring the so-called ‘oxidative induction time’ (OIT), a

360

Polymer Additives

R−O R1

N

R1

H

N

H

(O2)

O

O-OH R2

R1

N

H

N

O-O

O

OH R1

+ H

R2

R1

N

OH

Figure 14 Reactions of HA(L)S-based antioxidants with alkoxy radicals and peracids.

standardized test performed by means of differential scanning calorimetry (DSC). The time between the melting and the onset of decomposition under isothermal conditions is gauged. Up to melting, the sample is kept under nitrogen that is replaced at higher temperatures by oxygen. At the end of an induction period phenolic antioxidants exhibit a drastic change in perfor­ mance if a lower critical phenol concentration is reached which depends on temperature and sample geometry. In contrast to this behavior, HA(L)S derivatives show a more gradual deterioration of mechanical properties keeping some mechanical stability particularly at lower aging tempera­ tures. This effect can be explained by a continuous oxidation of HA(L)S-based N–H functions into the N–O• radical by means of peroxy radicals and peracids (Figure 14). The total concentration of N–O• radicals in polyolefins at low and moderate temperatures is higher than at elevated tempera­ tures. In addition, intermediately formed hydroxylamine ethers are able to react quickly with peroxy radicals and peracids, which prevents the acceleration of oxidation pro­ cesses at low temperatures. Scheme 6 gives an overview about various reactions of HA(L)S with hydroperoxides and peroxides.

8.14.4.4 Synergistic Blends of Sterically Hindered Phenols and Organophosph(on)ites It is state of the art for the stabilization of polymers produced on industrial scale to take advantage of synergistically efficient physical mixtures (‘blends’) of stabilizers such as sterically hindered phenols and organophosph(on)ites. Thus, a synergistically efficient base stabilization system can be obtained with a combination of AO-1/PS-1 with an opti­ mum efficiency close to the ratio 1:2. Many other combinations are used under industrial conditions.31 The mutual interaction between such sterically hindered phenols and organophosphites is probably the most known

‘positive’ interaction between two additives. If observed effects of additive blends are less pronounced as the sum of the individual components, the referring system is defined as antagonistic. Occasionally, those antagonistic effects have been observed in some combinations of phenolic antioxidants with sterically hindered amines.

8.14.4.5

Phenol-Free Stabilization Concepts

Within the last decade, there have been new approaches to polymer stabilization. One new stabilizer chemistry is based on the hydroxylamine functionality that can serve as a very powerful hydrogen atom donor and free radical scavenger32 as shown in Figure 15(a) and hydroperoxide decomposer33 as shown in Figure 15(b). For color-critical applications requiring ‘phenol-free’ stabi­ lization synergistic mixtures of sterically hindered amines (HA (L)S) and a hydroxylamine-based stabilizer, used during melt processing, with or without an organophosphate or organo­ phosphonite, can be used to avoid the undesired discoloration typically associated with the oxidation of the phenolic antiox­ idants into products with quinoid structures.34 The use of ‘phenol-free’ stabilization systems has been found to be very effective in color-critical products such as PP and polyethylene fibers, as well as selected TPO applications.

8.14.5 Stabilization of Polymers against Degradation under the Impact of Light 8.14.5.1

Introduction

Polymers including organic coatings – especially if used under outdoor conditions – are exposed to UV radiation in a wave­ length regime below 400 nm. As a consequence, degradation will take place which can be retarded by a proper selection of light stabilizers including UV absorbers.35,36

Polymer Additives

361

1. Decomposition of hydroperoxides NH + POOH

NOH + POH

NR +

N

(1)

R POOH

+ POH

(2)

O 2. Decomposition of activated hydroperoxides (‘sequences’)

P O O H .. N

H

H2O

H O

P O

O P

+ O

O P

N

POOP (3) N OH

H

3. Decomposition of peroxy acids NH + RC OOH O

+ N OH – O C R H O

N OH + RCOH

(4)

O

4. Decomposition of diacylperoxides NH + RCOOCR O

O

N O C R + RCOH O

(5)

O

Scheme 6 Action modes of HA(L)S as preventive antioxidants. P represents a segment of a polymer chain.

UV light-induced degradation of polymers by energy-rich photons proceeds under formation of free radicals within the polymer scaffold, which are converted in the presence of oxy­ gen into the corresponding peroxy and alkoxy radicals as already shown in the ‘autoxidation cycle’. Light-induced degra­ dation can be suppressed by a variety of suitable classes of additives (‘light stabilizers’). UV wavelength from sunlight is an important factor in out­ door degradation of polymers despite the fact that the energy from sunlight is mainly visible, infrared or UV light contributes less than 5% to the total energy of the sunlight. The solar UV radiation spectrum is divided into three different ranges. UV-A represents the energy in the wavelength regime between λ = 400 nm and λ = 315 nm, UV-B covers the λ = 315–290 nm range, and UV-C includes the solar radiation at wavelength of less than λ = 290 nm. Theoretically, polymers with saturated C–C bonds should not absorb UV light since the longest absorption band for these polymers appears in the wavelength regime below λ = 200 nm caused by a σ–σ* transition. Even the absorption of polymers with unsaturated C–C bonds like synthetic rubbers based on butadiene or isoprene copolymers takes place in the wavelength range between λ = 180 and λ = 240 nm associated with a π–π* transition. Longest absorption wavelength of industrial polymers are mea­ sured for polystyrene (λ = 230–280 nm). Therefore, photodegradation has to be attributed to the presence of chromophoric groups in the polymer such as

unsaturated moieties and carbonyl groups, but also to impu­ rities such as catalyst residues and highly absorbing pollutants. The photodegradation pathway of the polymer is quite similar to the one already discussed before. The basic difference is in the initiation step. Scheme 7 represents a very simplified illustration of the initiation step of the photo-oxidation invol­ ving a polymer chain represented by R: A chromophore within the polymer absorbs UV light and moves to an excited state. This reaction toward the formation of the excited state is reversible, but can also lead to cleavage of radical fragments and to the generation of a free radical R*. This free radical can then start the autoxidative degradation process described earlier by quickly reacting with ambient oxygen. A number of options restraining the damage by photo-oxidation are given using the appropriate type, combination, and con­ centration of light stabilizers. Fillers like CaCO3 can influence the light-induced degradation of polymers. Already carbon black is recognized for its ability to stabilize polymers against degradation by UV irradiation.37 However, basically three different major classes of light stabilizers can limit and slow down the photodegradation of polymers: • UV absorbers: by means of their high extinction coefficients below 400 nm, the usually colorless UV absorbers must be able to dissipate the absorbed energy spontaneously as heat rather than as detrimental radiation via excited transition states and final deactivation toward the initial state via fast intramolecular processes.

362

Polymer Additives

(a) H R N

R

O

N

O

–R–H

N

O

–R–H

R

R H R

R N

N

O

O

N

O

–R–H R –R–H

R R

R

R

R R N

R

R

O

N

R

O

H

N

O

–R–H

R

R R

H

R H

(b) H

R N

O

R ROOH

R

N

O

ROH

H2O

R

Figure 15 (a) Carbon-centered radical decomposition via hydroxylamines. (b) Decomposition of hydroperoxides by means of hydroxylamines.

R−H

HO

+

O2

+

2 R−O−O−H

+

H−O−O

H2O2

2 H−O

R−H

H2O

2 R−H

+

R

+ R

2R−O + 2 H2O + 2 R

Scheme 7 Simplified equation scheme of the initial step of photo­ oxidation.

• Quenchers: they deactivate the excited states of the chromo­ phores like carbonyl groups in the polymer, as earlier formed, for example, by thermo-oxidation. Quenchers are usually also efficient sensitizers for the photolysis of hydroperoxides. • Free radicals scavengers: they interrupt the autoxidation pro­ cess by scavenging free radicals.

8.14.5.2

Quenching of Photo-Oxidation with HA(L)S

It has been found that sterically hindered piperidine derivatives (HA(L)S) quench highly efficient the degradation of polyole­ fins yielding trans-vinylidene groups during photo-oxidation which can be reached already at HA(L)S concentrations as low as 0.05%. Through an exciplex polyolefin/oxygen resulting

presumably from the interaction of the polymer triplet state with the ground state of oxygen, a number of quenching path­ ways can be envisaged. Excitation of some impurities can contribute to occupy this triplet state by energy transfer, which represents a special pathway of quenching. In the pre­ sence of HA(L)S, a competition between the polymer and HA(L)S takes place receiving the energy of the impurity, pre­ ferentially at the amorphous-crystalline interphase. Furthermore, the reaction with oxygen can be in competition with the reaction with HA(L)S. Considering the effective con­ centration of HA(L)S and oxygen in polyolefins, this reaction can prefer HA(L)S. A modified course takes into account excitation migration along the polymer chain. Excitation transfer can occur either to a polymer sequence in contact with oxygen or to a polymer sequence in contact with HA(L)S. A further possibility of quenching is the deactivation of the polymer–oxygen exciplex before any formation of a trans-vinylene group and hydrogen peroxide can take place. This alternative requires a direct energy transfer to HA(L)S or an energy transfer to an intermediate HA(L)S–oxygen complex.

8.14.5.3

HA(L)S-Based Free Radical Scavengers

Even with the use of UV absorbers or quenchers, some activated chromophores result inevitably in the generation of free radi­ cals, particularly peroxide radicals. To minimize the damage

Polymer Additives

363

R2 R−O−O

or

R−O−O−H or O2 N

N

N

R1

O

O R2

R1

= H, CH3, acetyl R3−O−O−R2

R3−O−O

Figure 16 Cyclic mode of action of HA(L)S as oxygen-centered radical scavenger.

caused by these entities, highly efficient free radical scavengers (or terminators) have been developed. HA(L)Ss are very efficient stabilizers against the light-induced degradation of most polymers. Their role as free radical scavengers during thermo-oxidative treatment of poly­ mers has been already described in Chapter 3.01. As already indicated, the mechanism of action is based on a complex reaction cycle involving nitroxyl radicals, which are generated by aging or processing the polymer. The action modes of polyolefin degradation in the presence of HA(L)S are still issues of discussion. This refers to the func­ tion of free hydroperoxides and intermediately formed dialkylketones in course of the initiation of photo-oxidation. Historically, nitroxyl radicals deriving from HA(L)S have been interpreted as key intermediates explaining the UV-stabilizing effect. A highly simplified cyclic mode of action of HA(L)S is shown in Figure 16. The stabilizing action is due not to the HA(L)S compound itself (N–R1) but to the nitroxyl radical (N–O.) that is formed by photo-oxidation during weathering. The nitroxyl radical reacts with the polymer radi­ cals formed by photolysis to give ethers (N–O–R), which then reacts with peroxide radicals to generate the nitroxyl radical. In a later stage, attention was paid to other oxidation products. In a different approach, the kinetics of stabilization including stabilizer loss and degradation product formation were consid­ ered toward different mechanisms leading to a deeper insight and a quantitative interpretation of the performance of HA(L)S, particularly in polyolefins.

8.14.5.3.1 Inhibition of the photo-oxidation in HA(L)S-stabilized polymers In the presence of HA(L)S, initiation of photo-oxidation via a polymer–oxygen exciplex is reduced to a minimum. Another most likely intermediate during this process is a HA(L)S– oxygen (charge-transfer) complex since most HA(L)Ss do not absorb UV radiation directly. Excitation of such a complex by direct absorption of UV light will provide sufficient energy for the scission of the relatively strong N–H bond. The amount of HA(L)S disappearing in course of the photo-oxidation is directly proportional to its concentration. In PP during natural exposure the loss of HA(L)S is not independent of its initial concentration. Another difference between polyethylene and PP has been elaborated so far: the UV stability reached with HA(L)S in PP is proportional to the initial HA(L)S concentra­ tion, whereas in polyethylene this UV stability has been ascertained to the square root of this concentration.

In liquids of aliphatic and aromatic amines and even HA(L)S including N-methylated HA(L)S, saturated with oxy­ gen, the appearance of additional absorption bands was already observed which confirms the assumption that HA(L) S–oxygen complexes play an important role as intermediates. Aspects of the outlined mechanisms can be combined into a general scheme as shown in Scheme 8.

8.14.5.3.2 Alkoxyamine hindered stabilizers (NOR HA(L)S) derivatives The latest development in UV stabilization is the use of NOR-type (alkoxyamine; aminoether) HA(L)Ss. NOR HA(L)S derivatives offer superior chemical resistance and stabilization efficiency compared to traditional HA(L)S, when used in the high demanding applications.38 This subclass of advanced steri­ cally hindered aminoethers is characterized by an intrinsically lower basicity, which makes them decisively more resistant to chemical interactions. Equally important, NOR HA(L)S deriva­ tives are very active as stabilizers in severe chemical conditions since they are very efficient in generating active stabilization species. First developed commercially available NOR HA(L)S derivatives are substituted by low-molecular-weight moieties on the nitrogen as shown in Figure 17. For example, in agricultural film applications, pesticides generate acidic volatile compounds that deactivate conven­ tional hindered amines. Similarly, hindered amines are deactivated by the thermal or photogeneration of hydrogen bromide from flame retardants with bromine moieties, which can have a negative impact on light stability of the referring systems. Light stabilization studies conducted with brominated flame retardants, conventional hindered amines, and NOR HA(L)S derivatives confirmed that NOR HA(L)S derivatives perform much better than conventional light stabilizers in the presence of brominated flame retardants. Similar observations were made in agriculture films in the presence of pesticides. A most recent development of such an NOR HA(L)S deri­ vatives follows the demand to have a product with higher compatibility, particularly in polyolefin foils available, mainly for applications in the agricultural sector.39 This product is called Hostavin® NOW whose structure is shown in Figure 18. Artificial weathering under the conditions of a Xe 1200 weatherometer was carried out using ethylene-co-vinylacetate (EVA) film for agricultural purposes in the presence of the pesti­ cide metam sodium (see Figure 19). During development of the product, a significant difference in performance became obvious. Conventional HA(L)S products performed worse compared to

364

Polymer Additives

R4

N

O

R5

O

R6

O

O

R4

O2

R6

H

R5

H

O

R7

H

N R4 O

R6 R5

R4

N

N

+

O

O

R5

OH

R4

R6

R5

R6

H

R6

O

R5 OH

R7

+

N

R6

O

OH

O

O

O R5

R7 O

O O

R7 N H Scheme 8 Simplified modes of action of Hindered Amine (Light) Stabilizer HA(L)S with carbon- and oxygen-centered radicals.

R

R N

(CH2)3

N

(CH2)2

R

R

N

(CH2)3 N

C 4H 9 N

N

C4H9 N R=

N H9C4

N

N N

N

C4H9

C 4H 9

N

(CH2)6

N

N

N N C4H9

N N

N

OC3H7

OC3H7

C4H9

N

N

(CH2)6

N

N

N

OC3H7

OC3H7

N N

N

O

O

OC3H7

H17C8 O N

O C (CH2)8 C O O

Figure 17 Structures of commercially available first-generation NOR HA(L)S derivatives.

O

N O C8H17

C4H9 C4H9

n

N

N

N

N

C4H9

N

N

C4H9

Polymer Additives

O

R

OH

OC8H17

N O

Hydroxybenzophenone: Hostavin R ARO 8

HO N N N

Figure 18 Molecular structure of the commercial NOR HA(L)S derivative Hostavin® NOW.

CH3

OC8H17 Benzotriazoles:

the NOR-containing films. Among them the films containing HA(L)S NOW outperform all other conventional HA(L)S deriva­ tives including the NOR HA(L)S derivatives LS 371.

CH3

N N

UV Absorbers

‘UV absorbers’ have been well known for a long time as light stabilizers of polymers. Materials such as carbon black and titanium dioxide can prolong the service life of polymer arti­ cles. In applications where transparency is necessary, however, these materials are often not an option. As already mentioned, the presence of catalyst residues or other chromophores resulting from the polymerization and processing steps in most industrial polymers can start degrada­ tion processes. UV light absorbers are often used to compete with these contaminant chromophores and preferentially absorb UV light. By means of their high extinction coefficients in a wavelength regime below λ = 400 nm, the usually colorless UV absorbers must be able to dissipate the absorbed energy spontaneously as vibrational energy (heat) rather than as detri­ mental radiation via excited transition states and final deactivation toward the initial state via fast intramolecular processes.40 Suitable UV absorber classes that are readily commercially available comprise, for example, hydroxybenzophenones, ben­ zotriazoles, and hydroxyphenyl-triazines (see Figure 20). Their structural features allow to form intramolecular hydrogen bonds. Their efficiency depends on the concentration of the indivi­ dual UV absorber, its absorption coefficient, and the polymer thickness if the substrate itself is not light absorbing in the given wavelength regime.

abs EB (%)

Tinuvin R P

OH N

8.14.5.4

CH3

H3C

Hydroxyphenyltriazines:

CH3 R

Cyasorb 1164

Figure 20 Important commercially available UV absorbers.

As shown for 2-hydroxybenzophenones, the mechanism of the UV light absorption is based on the formation of a light-induced excited singlet state from which an intramolecu­ lar hydrogen transfer takes place forming a corresponding keto-tautomer that is followed by a fast radiationless deactiva­ tion regenerating the original molecule (Figure 21). Competitive reactions with radicals, for example, peroxide radicals, can occur if the intramolecular H-bond formation is disturbed by nonplanar molecular arrangements. Due to the above-mentioned mechanism, these products are subject to and limited by Lambert–Beer’s law: A ¼ εbc where absorbency A is equal to the product of the molar absorptivity (extinction coefficient), ε, multiplied by the path length b and the concentration c. The Lambert–Beer law implies that thick section films or other articles (with high path length b) at high concentrations c of very effective (high ε) UV absorbers are favored. In practice, however, the surface of the polymer article cannot be

LS 371 1.0 %

LS 944 1.5 % LS 30 1.5 % LS 30 0.7 % / HA(L)S NOW 0.5 %

HA(L)S NOW 0.5 % HA(L)S NOW 1.0 % HA(L)S NOW 2.0 %

900 800 700 600 500 400 300 200 100 0 0

1000

365

2000

3000

4000

5000

6000

7000

8000

X 1200 (h) Figure 19 Exposure of industrial scale EVA films to artificial weathering conditions in the presence of metam sodium.

366

Polymer Additives

O

H

O

O

H

* O

hν

O

H

−ΔT

O

Figure 21 Dissipation of UV light energy in hydroxybenzophenone­ based UV absorber via formation of different tautomeric structures.

completely protected. UV absorbers are generally not used in thin sections. Furthermore, compatibility of these substances limits their use to levels of about 0.5–0.6%. In the course of prolonged service life, UV absorbers are losing their absorption behavior, for example, by reaction of their phenolic function with peroxy radicals. Besides chemical reactions, simple depletion of such compounds can be another reason for the loss of their efficiency. For example, benzophenone-type UV absorbers are described to deplete from polyethylene–vinylacetate copolymer (EVA)-based encapsulants used in solar cells.41 In order to avoid such defi­ cits, attempts have been made to develop UV absorbers incorporated in polymer structures.42 Manifold experimental results indicate that the loss of UV absorbers is initiated by light-induced decomposition reactions rather by migration from polymer substrates. Through combinations with radical scavengers based on sterically hindered piperidine derivatives (HA(L)S) (see above and below), the overall protection of the polymer matrix can significantly be prolonged compared with traditional high­ molecular-weight hindered amines alone measured, for exam­ ple, by improved maintenance of color in pigmented polymers while at the same time maintaining or improving the retention of physical properties.

8.14.5.5

Quenchers

Quenchers are light-stabilizing additives able to take over the energy of photons already absorbed by a suitable chromophore (Chr) of the polymer (see Scheme 9). Carbonyl groups, hydro­ peroxides, and singlet molecular oxygen (or its precursors) are chromophores commonly believed to be involved in the photodegradative mechanisms for numerous hydrocarbon polymers.43

These additives are able to dispose this energy either as heat (‘dark quencher’) or as fluorescent or phosphorescent radiation to prevent degradation.44 Nevertheless, carbonyl quenching additives are rarely effec­ tive stabilizers by themselves because carbonyl group photolysis is only in some cases the single most important degradative process. Hydroperoxide group formation and photolysis are frequently more significant, but the first excited state is dissociative and cannot be quenched. Singlet oxygen can be quenched with certain additives, but there is no evidence that it is a major contribution to the photo-oxidation of any polymer. The mechanism of transfer may occur via an exchange energy transfer or via the ‘Förster mechanism’, which is based on dipole–dipole interactions. Generally, under optimum con­ ditions, quenching can occur within distances of several nanometers if the overlap between the emission spectrum of the chromophore and the absorption spectrum of the quencher is sufficiently good, independently of the polymer thickness. Additionally described in the literature is a collisional energy transfer as long as the distance between the initially excited chromophore and the quencher does not exceed 1.5 nm.45 Among the investigated classes of compounds, Ni chelates are still an important group of quenchers despite severe tox­ icological concerns. The technical interest in these products might reflect their high stability over time and permanence in polymers as well as their chemical resistance versus deactiva­ tion by halogenides or acids. The use of stabilizers based on the heavy metal nickel is under discussion due to industry awareness, initiatives such as Responsible Care, and the availability of non-nickel-containing alternatives such as HA(L)S (see below). Furthermore, these mate­ rials are generally much less efficient than sterically hindered amines (HA(L)S) in terms of light stability and especially of thermal stability. Efficient photostabilization of polymers requires efficient peroxide decomposition and radical scavenging.

8.14.5.6

Practical Considerations for the Use of HA(L)S

Since the efficiency of HA(L)S depends on their concentration and not on layer thickness, this type of stabilizer behaves, there­ fore, equally effectively, no matter whether HA(L)S is used at the surfaces of plastics in films, fibers, coatings, or thick sections. Very often, HA(L)Ss are combined with UV absorbers to improve the efficiency against the degrading effect of sunlight which can be explained by a synergy between UV absorbers and HA(L)S. In fact, the screening effect of the UV absorber diminishes the UV light intensity within the polymer and thus limits somehow the formation of radical intermediates, which finally leads to a reduced degradation of the polymer. HA(L)S-based stabilizers are used for outdoor applications of polymer products where weathering plays an important role.

hv Chr*

Chr Chr

*

+

Q

Chr

Q*

+

*

Q +

heat

Q*

Q +

hv

Q

Scheme 9 Schematic course of excitation and deactivation of chromophores by quenchers.

(‘dark quenching’)

367

Polymer Additives There are some ‘natural’ weathering sites known, for example, in southern France or in Florida, which are often used for testing purposes of mostly industrial formulations. In addition, ‘artifi­ cial’ weathering in closed technical weatherometers is in addition carried out. In both cases, oxygen uptake and other specific parameters can be monitored. Unfortunately, the com­ parison results received by the different weathering techniques lead often to different lifetime ranking of HA(L)S-stabilized polymer formulations.46 Moreover, the impact of the atmo­ spheric constitution, for example, towards the presence and differing local concentration of sulfur dioxide, nitrogen oxides, and ozone, on photo-oxidative processes has been investigated and described.47 A combination of low molecular weight HA(L)S (which can easily migrate to the surface) and high molecular weight compounds (which are more strongly anchored in the bulk polymer) is recommended for practical purposes. Presently, there are a few dozen HA(L)S derivatives commercially offered. Among the most important products, compounds such as Tinuvin® 770, Tinuvin® 622, Chimassorb® 944 (BASF), and Hostavin® N 30 (Clariant) have to be mentioned as outlined in Figure 22. The latter is an oligomeric HA(L)S manufactured by the polymerization of Hostavin® N 20 with epichlorohydrin. It has an excellent resistance to extraction and very low volatility. It is distinguished from other high-

H N

N H

O C (CH2)8 C O O

O

Tinuvin®

770

H3C CH 3

O N CH2 CH2 O H3C CH3

O

CH2 CH2

OCH3 O n

Tinuvin® 622

H N

H

N

N

(CH2)6 N

molecular-weight HA(L)S mainly by its low basicity and high resistance to chemicals that may be present in the environ­ ment (agricultural chemicals, sulfur, acids, etc.). A recent comprehensive review on photochemically degradable polymers with an emphasis on environmental and molecular factors controlling the onset and the rate of degradation is given in the literature.48 A number of principles emerge for the design of viable photochemically degradable plastics. Among the principles discussed are those relating to the effects of chromophores, initiators, antioxidants, tempera­ ture, oxygen diffusion into the plastic, polymer crystallinity, tensile and compressive stress, and the absorbed light inten­ sity on the plastic. In order to obtain a plastic with a controlled lifetime and a specific rate of degradation, many of these parameters can be adjusted in the design stage of the polymer material.

8.14.6 Multifunctional Additive for Engineering Polymers Polar (‘engineering’) polymers being able to activate hydrogen bonds, for example, polyamides, can be efficiently stabilized by a multifunctional stabilizer N,N′-bis-(2,2,6,6-tetramethyl-4­ piperidyl) isophthalamide, commercially available under the trade name Nylostab® S-EED® (Figure 23).49 Nylostab® S-EED® fits well to polymers whose structures allow the molecule to be incorporated via hydrogen bonds, for example, polyamides (Figure 24). Major fields of application of this additive, typically used in a concentration between 0.1 and 1.0 wt.%, are thick sec­ tion injection molded, even glass filled and/or pigmented resins, thin section fibers, monofilaments, films, and blow molded articles. Important benefits of Nylostab® S-EED® are improved thermo-oxidative and UV light stability enhanced output rates due to lowered and constant pressure during processing (see Figure 25), reduced cycle times and lower discoloration, respectively, nonstaining effects. Furthermore, this additive protects colorants and provides better bleach resistance. Other physical properties are at least maintained or even improved. In addition, synergistic effects with oxanilide- and benzylidenemalonate-type UV absorbers are known.

8.14.7 Metal Ion Deactivators N

N

N

HN

n

Besides elevated temperatures, UV light, and high shear forces (e.g., during polymer extrusion), metal ions can give rise to undesired decomposition reaction of polymers. Their ability to

Chimassorb® 944 (CH2)9

O oligomer of H N

C N

and H2C H

O CH CH2

Cl

O

H H

N

H

N

N

O

O

N

H

®

Hostavin N 30 Figure 22 Important commercially available HA(L)S products.

Figure 23 N,N′-Bis-(2,2,6,6-tetramethyl-4-piperidyl) isophthalamide (Nylostab® S-EED®).

368

Polymer Additives

H

O

O

H

N

O

N N O

N

H

O n

A

H O

N A

O

HN

N H A

NH C

B

C

B

(A) Dye-binding sites (B) Dye-and substrate-stabilizing groups (C) Aromatic amide, thermal stability, and substrate-binding sites

Figure 24 Molecular incorporation of Nylostab® S-EED® into polyamide via H-bonds.

60

Extruder engine current (A)

50 40 30 20 10

Blank

S-EED

HALS-1

Additive concentration: 0.3 wt.% 0 0

2

4 6 Extrusion time (min)

8

10

Figure 25 Stability of the polyamide melt processing in the presence of Nylostab® S-EED® during extrusion.

decompose peroxides leading to the formation of reactive inter­ mediate radicals can contribute to accelerated autoxidation. Moreover, such enhanced decomposition can take place if a polymer is placed in direct neighborhood of metals such as copper. This is an important technical issue for wire and cable applications that need a sufficient polymer-coating layer for insulating purposes. Meanwhile, many papers describe those effects in detail.50 Efficient metal (ion) deactivators are based on their ability to form stable complexes with the metal, especially with copper ions. Mainly polyolefins and rubbers need to be stabilized by those metal (ion) deactivators that must resist extraction in aqueous surroundings. An important commercial product is 2′,3-bis[[3-[3,5-di-tert-butyl-4-hydroxyphenyl]propionyl]] propionohydrazide (Irganox MD-1024, BASF).

8.14.8 Acid Scavengers Acid scavengers (other technical expression ‘antacids’) are com­ monly metal salts of weak organic or inorganic acids. Their corresponding free bases are able to efficiently neutralize acidity. The efficiency of an acid scavenger is determined by the reactivity of the salt achieved in the polar polymer matrix as well as by the acidity of impurities that expel a weaker acid from its salt. Besides traditional salts of fatty acids, for example, Ca-stearate and Zn-stearate, inorganic compounds such as hydrotalcites or still zinc oxide are frequently used. Major reasons to use antacids are the presence of catalyst residues in polymer matrices that can generate free acidity during or after catalyst deactivation by steam stripping or solvent treatment.

Polymer Additives

Antacids neutralize this acidity and prevent a couple of unde­ sired side effects such as corrosion of processing equipment.51 Furthermore, particularly the stearate types of acid scavengers can fulfill functioning as slipping agents by reducing shear forces that can be important especially for processing of high­ molecular-weight polymers. In addition, sodium and calcium salts of fatty acids such as Ca-stearate influence the crystalliza­ tion behavior of several technical polymers such as polyolefins, polyethylene terephthalate, and polyamides exhibiting nucle­ ating effects, accelerating crystallization kinetics, and enhancing mechanical properties of the corresponding articles. Besides such main effects, metal stearates also act as lubricants and release agents. Finally, improvement of the performance of piperidine-based HA(L)S as well as their rectified resistance against degradation by external pollutants, for example, pesti­ cides in greenhouses should be mentioned.52 Drawbacks of degradation processes of polymers can be restrained by the use of suitable stabilizers as already discussed above. Those stabilizers and stabilizer combinations are usually employed together with co-additives used for the sta­ bilization of a given polymer grade. Base additive packages required for the stabilization of polymers, particularly polyole­ fins, comprise usually combinations of phenolic antioxidants, phosph(on)ites, and acid scavengers. The cooperative perfor­ mance of such additive combinations is certainly influenced by a proper choice and concentration of all individual components.53 Addition of zinc stearate is known to be technically a super­ ior acid scavenger since it helps to avoid early discoloration of a large series polymer formulations, but it holds for being phy­ sically irritant in humans.54 Exposure to zinc stearate over a long time may develop extensive fibrosis. Although there is no specific information available on the concentration of the exposure leading to such a condition, it is believed that it is very high. Moreover, aspiration of zinc stearate by infants can be associated with respiratory distress and acute pneumonitis.55 Another group of organic antacids are represented by metal lactates, especially calcium lactate (Figure 26) and calcium stearoyl-2-lactylate. Besides their action principle as acid sca­ vengers, these derivatives are able to form chelate complexes even with traces of residual aluminum and titanium in polymer matrices. Their addition can help to avoid discoloration of polymer formulations, particularly in combination with phenols. Synthetic hydrotalcites56 as inorganic acid scavengers recently became important co-stabilizer in polymer formula­ tions. They are primarily utilized in replacing heavy metal-based stabilizing components such as lead stearate, lead phosphite, dibasic lead phthalate, or tribasic lead sulfate and the respective cadmium salts. Mainly two types of hydrotalcites have proven excellent performance in long-term stabilization of polymers, namely,

O

O − Ca2+ −

O OH

O OH

Figure 26 Chemical structure of calcium lactate.

369

a pure Mg/Al hydrotalcite, empirical formula [Mg6Al2(OH) 16CO3·4H2O], (e.g., the commercial products Sorbacid® 911 and Hycite® 713 available from Süd-Chemie AG) and a zinc-containing derivative, [Zn2 Mg4Al2(OH)16CO3·4H2O] (like Sorbacid® 944 of Süd-Chemie AG). Common to all types are their layered structure (see Figure 27) and small particle size (typically 80% < 1 µm) pro­ viding excellent dispersibility in the polymer matrix. Hydrotalcites act in scavenging acidic decomposition pro­ ducts of halogenated polymers such as PVC, polychloroprene (CR), chlorosulfonated polyethylene (CSM), chlorinated poly­ ethylene (CPE), epichlorohydrin (ECO), fluoroelastomers (FKM), and halobutyls (bromobutyl rubber (BIIR), chlorobutyl rubber (CIIR)). In PP and polyethylene, hydrotalcites serve in immobilizing and neutralizing acidic catalyst residues derived from the Ziegler-Natta polymerization process. Hydrotalcites have been added to flame-retardant systems as an inorganic stabilizer and smoke suppressant, especially with halogenated systems. These additives are also used in electrical applications for its antitracking properties.57 Antacids serve several purposes in additive formulations for polymers such as neutralizing the free acidity of catalyst resi­ dues after their deactivation by steam stripping or solvent treatment. But these compounds also act as internal slipping agents in order to reduce shear forces during extrusion which is impor­ tant especially for processing of high-molecular-weight polyolefins and production of polyolefin films in general influ­ encing the crystallization behavior of polymers such as polyolefins as well as some engineering plastics such as poly­ ethylene terephthalate and polyamides. Therefore, they exhibit certain nucleating effects, that is, acceleration of crystalline kinetics and enhancement of mechanical properties of finished polymer articles. Another function is the improvement of dis­ persion of stabilizers in the polymer matrix. In addition, acid scavengers can play an important role for the melt viscosity retention during processing as well as for the long-term stability of the final polymer article.

8.14.9 Analysis of Stabilizers in the Polymer Matrix 8.14.9.1

Introduction

Besides already established technical polymers, the plastics industry is continuously developing new polymeric materials. Most of these products have to fulfill strict requirements during their service life depending on the individual applications. However, indispensable properties of plastic materials can only be achieved by optimal adjustment of the additives pack­ age to the specific requirements. Therefore, the development of tailor-made recipes and also the introduction of novel polymer additives and additive combinations contribute to strength­ ened and continuously improved plastic articles. As a consequence, it becomes mandatory to assess quickly the efficiency of additives with respect to technical and envir­ onmental conditions to which they are likely to be subjected. Since examination of changes of polymer materials at ambient conditions often requires long time periods, several methods have been developed which allow an accelerated degradation. Nevertheless, it has to be ensured that accelerated aging

370

Polymer Additives

Hydroxide layer [MII1−x MIIIx (OH)2]x+ Interlayer: An− anions and water molecules

MII or MIII metal cation OH− anions

Layered crystal structure of hydrotalcite-like compounds Figure 27 Layered structure of hydrotalcites (schematic view).

processes correlate with degradation steps observed under nor­ mal service conditions. Regarding thermal and UV light-induced degradation, various accelerated test methods are established studying the influence of additives on polymer performance.58 TGA coupled to FTIR, TGA/FTIR, has been investigated to study the degradation of several polymeric systems containing additives such as poly(methyl methacrylate), graft copolymers of acrylonitrile–butadiene–styrene and styrene–butadiene with sodium methacrylate and styrene with acrylonitrile, blends of styrene–butadiene block copolymers with poly(vinylphospho­ nic acid) and poly(vinylsulfonic acid), and crosslinked polystyrenes. It must be pointed out that additives may interact with poly(methyl methacrylate) by coordination to the carbo­ nyl oxygen to a Lewis acid and the subsequent transfer of an electron from the polymer chain to the metal atom or by the formation of a radical that can trap the degrading radicals before they can undergo further degradation.

extrusion can be found. Furthermore, advances in molding technology – compression, rotational, and blow molding of polymers – are elucidated. Moreover, alternative processing technologies such as calendaring and coating, foam processing, and radiation processing of polymers are described and even processing of nanocomposites and carbon nanotubes. Finally, the paper addresses postprocessing technologies with chapters on online monitoring and computer modeling as well as join­ ing, machining, finishing, and decorating of polymers.59 Moreover, discoloration before and after processing are characteristic data. Usually, the ‘yellowness index’ (YI) is mea­ sured relative to a given standard (magnesium oxide, barium sulfate).60 A more sophisticated numerical description is based on a three-dimensional color system, considering their shade, brightness, and saturation. Clear, near-colorless polymers can be measured in transmission, whereas the majority of polymers are opaque and can be studied in reflectance.56

8.14.9.3 8.14.9.2 Testing the Polymer Melt Stability during Processing For purposes of polymer processing in the melt, appropriate stabilizers are typically mixed in the substrate to achieve a homogeneous distribution. The influence of particular stabili­ zers is measured taking into account characteristic polymer parameters such as molecular weight and molecular weight dis­ tribution.56 Simulation of severe processing conditions can be reached by means of multiple processing. Changes arise from chain scission and/or crosslinking processes, which immediately influence the melt viscosity of the polymer. Rheological meth­ ods are suitable tools for polymer characterization in connection with degradation taking place during processing. For technical purposes, a relatively simple method is based on the determina­ tion of the so-called ‘melt flow index’ (MFI) and the ‘melt flow ratio’ (MFR) that permit conclusions regarding changes in mole­ cular weight and molecular weight distribution. The MFI is defined as the amount of molten resin passing through a capil­ lary equipment with defined length, diameter, and die geometry under prescribed conditions of temperature, load, and time interval. The quotient of two different MFI values is defined as MFR that is influenced by the actual molecular weight distribu­ tion of the corresponding polymer. Processing techniques are critical to the performance of polymer products that are used in a wide range of industries. Recent papers review comprehensively polymer processing techniques from macro- to nanoscales and describe latest advances in this field. A detailed description of fundamentals of polymer processing on rheology, materials, and polymer

Examination of Long-Term Heat Aging

Conventional techniques determine long-term heat stability of polymer articles at temperatures below melting conditions, for example, during accelerated aging in ‘circulating air ovens’ until degradation effects such as discoloration and embrittlement are visible (Figure 28). Despite the fact that oven aging is widely used, drawbacks are cross-contamination by differently stabilized samples and a

Figure 28 Sample arrangement during accelerated aging in a traditional circulating air oven.

Polymer Additives

relatively inaccurate determination of the moment of embrit­ tlement. A more precise failure criterion is the combined use of spectroscopic methods, for example, monitoring upcoming carbonyl groups by IR spectroscopy. Also mechanical methods measuring elongation, tensile, and impact strength are more reliable to characterize the state of long-term heat-aged poly­ mer samples. ‘Thermoanalytical methods’ are based on DSC and differ­ ential thermal analysis (DTA) are used to determine the time necessary to initiate the oxidation of polymer samples, the so-called ‘oxidation induction time’ (OIT).61 From discussions in the literature, it is known that data received above the poly­ mer melting range do not strictly correlate with data found below its melting range which can be explained with discontin­ uous changes of activation energy and the free volume of the polymer as well as the mobility and solubility of additives in the polymer matrix.

2.5 h

5h

6.25 h

>500 Counts per pixel per 15 min

Figure 30 Visualization of CL of PP samples kept at T = 150 °C after defined times under oxygen using a CCD camera.

peroxy functions. Since new generations of highly sensitive ‘charged couple device’ (CCD) cameras are developed, the CL method became more common and can be used even for routine purposes. Compared with the initially described oven aging method, CL allows a time reduction factor up to 10 if pure oxygen is used and additionally allows to visualize the polymer oxidation in real time (Figure 30). Alternative mechanisms of how CL occurs are discussed in the literature, the so-called chemically initiated electron exchange luminescence (CIEEL), a type of luminescence result­ ing from a thermally initiated electron-transfer reaction, also called ‘catalyzed CL’.64 The CL intensity relates to the concentration of peroxide radicals and therefore makes CL an ideal tool for following the thermal oxidation of polymers in real times.65 CL images can be recorded at different oxidation times. However, the OIT of the polymer samples is defined as the sudden increase of the CL signal (onset) from the baseline level (see Figure 31). The oxidation of unstabilized polymer samples passes off homogeneously, but on the stabilized samples the onset of oxidation and its progression is definitely heterogeneous. Furthermore, it can be observed that the oxidation often begins next to a sample already oxidized due to spreading of decom­ posed, volatile products from the degrading polymer. The retarding effect of different phenolic antioxidants on the OIT has been quantitatively investigated (Figure 32). The difference between the given three commercial antioxidants is

Special long-term methods under thermo-oxidative conditions mainly using thick polymer samples focus on increased oxygen pressure. Attention has to be paid to the possible change from a diffusion-controlled oxidation with preferential oxidation of amorphous surface phases to a homogeneous oxidation of the entire specimen. A versatile but not yet really established method to deter­ mine the lifetime of unstabilized or stabilized polymers is the determination of oxygen. Measurements of the oxygen uptake are described as advantageous over conventional oven aging especially for samples with reduced thickness since evaporation of stabilizers in closed measuring cells can be avoided. As shown for nitrile rubbers, oxygen uptake measurements can be carried out even at ambient temperatures. An upcoming method is based on ‘chemiluminescence’ (CL)62 since some decades ago Ashby described this phenom­ enon during oxidation of polymers.63 An extremely weak photon emission – preferentially at elevated temperatures under a laminar oxygen flow – is probably caused by a bimo­ lecular reaction of two peroxy radicals forming carbonyl groups in excited singlet states and following deactivation steps (Figure 29). The emission intensity correlates with the resulting carbonyl concentration and with the concentration of the earlier formed

R

0

250

8.14.9.4 Other Methods Analyzing Long-Term Aging of Polymers

R + ROH + H2O

3.75 h

371

O2

2 RH ROO

Δ RO OH

Δ

RH

ROH + 1O2 + 3R = O*

ROO

RH

ROOH R

‘Russell mechanism’: bi­ molecular termination of alkylperoxy radicals Quantum yield Φ = 10−9

Figure 29 Simplified autoxidation scheme of polymers with dimerization of two peroxy radicals followed by the formation of alcohol, oxygen, and a carbonyl moiety in an excited singlet state that deactivates under photon emission.

372

Polymer Additives

8.14.9.5.1

O2

Light emission

N2

OIT

Heating time

Figure 31 Time-dependent recording of photon emission evolved from a polymer sample at a constant elevated temperature.

negligible at concentrations about < 300 ppm, whereas at higher concentrations the effectiveness of these antioxidants increases in the order: Hostanox® O 16 < Hostanox® O13 < Hostanox® O10. This ranking corresponds to the ranking determined by conventional oven tests.66

8.14.9.5.2

Artificial weathering

Artificial or indoor exposure samples are subjected to UV radia­ tion from fluorescent lights as well as from UV rays optionally filtered through particular glass filters. More specifically, light sources mostly used are carbon arcs, xenon arcs, mercury arcs, and combinations of fluorescent lamps. An excellent simula­ tion of sunlight is achieved with xenon arc (‘Weather-O-meter’ from Atlas). Other artificial weathering devices combine high UV intensity from fluorescent lamps with water condensation on the samples (QUV, UVCON) or are equipped with medium pressure mercury arcs (SEPAP). Over the last years, testing of polymer samples indicated that the short wavelength as gener­ ated by QUV was inappropriate reaching an acceptable correlation to real outdoor exposure conditions. A series of further parameters must be taken into consideration by asses­ sing the artificial UV radiation exposure on polymer samples such as the energy flux of the light source and its distance from the sample defining the intensity of the radiation on its surface. In addition, the temperature alters the rate of detrimental photochemical oxidation reactions. Usually, temperatures between 55 and 80 °C are required. Finally, the radiation exposure time has an impact.

8.14.9.5 Testing of Polymer Stability against Light-Induced Degradation – Natural versus Artificial Weathering Highly important are methods that help to determine the stability against photo-oxidative polymer degradation because tests under ambient (‘natural’) conditions can take years. Therefore, the development of artificial methods permitting the acceleration is highly important. On the other hand, using appropriate mirrors to multiply the sunlight can accelerate natural light exposure. The best correlation between natural and artificial exposure is achieved with artificial light sources having emission spectra similar to that of sunlight. For example, regarding structural features, the effect of stereoregularity on the photo-oxidative degradation and weath­ ering of polyolefins was examined compared with that of PE­ HD by means of spectrophotometries and molecular weight measurement. It was found that syndiotactic PP is more stable than isotactic PP toward photo-oxidative degradation and to weathering, whereas PE-HD is the most stable among the examined polymers.49

CL OIT (min)

Natural weathering

Real-time (natural) weathering programs represent the stan­ dard to which all other weathering data are compared. Geographically, suitable sites are Southern Florida and Arizona in the United States and Bandol in France providing intensive solar radiation all over the year. Southern Florida is important because its high radiation is combined with high rainfall and humidity. Therefore, this place became an interna­ tional reference climate for gauging the stability of materials particularly of polymers. Standard procedures define the speci­ men exposure on racks, for example, facing due south at an angle of 45° offering a maximum direct sunlight exposure and intensity and washing off during rains. Standard exposure times are generally 6, 12, 24, and 48 months. Modified proce­ dures of outdoor weathering consider additional water spray being a thermal shock by a spontaneous reduction of the sur­ face temperature of the samples. Resulting physical stresses accelerate degradation processes.

2500 O HO

CH2CH2−C−O−CH2−C

2000 4

Hostanox® O 10 1500

RCH2

CH3 CH2R R=

OH

CH3

H 2C

CH2R

1000 HO

500

O CH2CH2−C−OC18H37

Hostanox® O 13

Hostanox® O 16

0 0

200

400

600

800 1000 AO concentration (ppm)

Figure 32 Correlation of the CL induction time with the concentration of different phenolic antioxidants.

Polymer Additives

As in case of thermal degradation processes, the formation of oxidation products such as carbonyl groups in the course of photodegradation can be used as an indicator for proceeding changes in the mechanical properties of the exposed speci­ mens. The influence of oxygen diffusion can also be studied by analyzing the sample’s cross section. It is quite difficult to rank the relative importance of UV light, humidity, and temperature. Proper balancing of all these effects is therefore a most important factor in accelerated weathering. All methods can at least give first indications with regard to the lifetime of plastic materials and the influence of the given stabilizer system. A comprehensive overview on weathering of polymers is provided by Holliwell.67 Despite the fact that the best correlation between natural and artificial exposure is achieved with artificial light sources having emission spectra to that of sunlight, there remains a discussion to which extent comparability between these meth­ ods can be assumed.68

373

spectroscopy.71 Particularly, the concentration of nitroxide radicals of HA(L)S-based stabilizers in polymer substrates can be monitored even over long periods of time by EPR imaging (EPRI), including their spatial distribution within a defined area providing radical concentration profiles along the mag­ netic field direction.72 Indirectly, the impact of polymer additives on stabilization and degradation of polymers can be investigated by character­ izing the referring polymer. For example, the stereoregularity of various PPs (isotactic PP, syndiotactic PP) and PE-HD on the photo-oxidative degradation and weathering was examined and compared with that of PE-HD using established spectro­ scopic and molecular weight measurements. It was found that syndiotactic PP is more stable than isotactic PP against photo-oxidative degradation and weathering, whereas PE-HD is the most stable among the examined polyolefins.73

References 8.14.9.6 Analytical Methods for Structural Characterization and Quantification of Polymer Additives The identification of degradation products provides a better insight into the reactions associated with stabilization. Their qualitative and quantitative determination makes it possible to deduce the original level of stabilization. Furthermore, polymer ingredients degrading stabilizers can be identified. Knowledge on these interactions contributes significantly to improved polymer stabilization.69 In general, analytical characterization of additives in poly­ mer substrates takes place first by extracting the particular product(s) with an appropriate solvent or solvent mixture by means of a Soxhlet extractor. The extent of extraction of an additive out of the polymer matrix depends on parameters such as type of solvent, temperature, time, particle size of the sample, molecular structure and molecular weight of this addi­ tive. Furthermore factors such as migration velocity of the referring additive and migration velocity respectively diffusion of the solvent into the polymer matrix are important features. Such isolation steps may afford protective conditions if the additive can be easily oxidized or hydrolyzed during extraction. In a second step, the isolated products can be subjected to basically all chromatographic and spectroscopic methods being available. Besides such a ‘traditional’ procedure, more sophisticated methods are also known which allow a direct analysis of addi­ tives without applying an extraction step. As an example, HA(L)S in polymers can be characterized using pyrolysis coupled to GC/MS which is applied successfully for fast identi­ fication of those additives. Each of the HA(L)S additives shows different pyrolysis gas chromatograms containing characteristic pyrolysis products. Even HA(L)S additives with very similar chemical structures such as Chimassorb 944 and Chimassorb 2020 can be analytically distinguished. A high-performance liquid chromatography (high-performance liquid chromato­ graphy (HPLC)) method with both UV and evaporative light scattering detection (ELSD) is developed to quantify the var­ ious HA(L)S additives in extracts of polymers.70 Other advanced analytical methods monitoring in situ ther­ mally initiated or light-induced degradation processes of stabilized and unstabilized polymers are based on EPR

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and Lifetime Prediction; Clough, R. L.; Billingham, N. C.; Gillen, K. T., Eds; ACS

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26. 27.

28.

29. 30. 31.

32. 33. 34.

35.

36. 37. 38. 39. 40.

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Biographical Sketch Christoph Kröhnke did his bachelor’s and master’s degrees in chemistry at the Universities of Würzburg and Freiburg, Germany (1971–77). He did PhD thesis on solid-state polymerization of diacetylenes at the Institute of Macromolecular Chemistry under Prof. Dr. Gerhard Wegner, University of Freiburg, Germany (1977–79). He was an assistant at the University of Freiburg, Institute of Macromolecular Chemistry (1980). He held postdoctoral position at the IBM Research Center, San Jose, CA, USA (1981/82) and Research and Development position at the former DuPont Research Center, Neu-Isenburg close to Frankfurt, Germany (1983/84). He was a group leader responsible for synthetic projects at the Max-Planck-Institute for Polymer Research at Mainz, Germany (1984–87). He also held several Research and Development positions at the former CIBA-Geigy, later CIBA Specialities Inc., at Basel, Switzerland (1987–98) which are as follows: • 1998–2007: Clariant Huningue, France, Division Pigments & Additives, various functions in Research, Product Development and Product Application of Polymer Additives • 2007–08: Global Head of Research & Development at BU Performance Packaging, Airsec–Süd-Chemie, Choisy-le-Roi (Paris), France. • 2009–10 Head of Research, BU BPE, Süd-Chemie AG, Moosburg, Germany • since 2011: Head of Departments Functional Materials & Biopolymers and New Catalytic Technology at Corporate Research and Development (CRD) of Süd-Chemie AG in Munich, Germany. Member of the German Chemical Society (GDCh), American Chemical Society (ACS), Bunsen Society, Society of German Natural Scientists and Physicians (GDNÄ), Society of Plastic Engineers (SPE), and Liebig-Society. He is the Head of Research, BU BPE, Süd-Chemie AG, Moosburg, Germany (since January 2009) and a member of the German Chemical Society (GDCh), American Chemical Society (ACS), Bunsen Society, Society of German Natural Scientists and Physicians (GDNÄ), Society of Plastic Engineers (SPE), and Liebig-Society.