EWS Review complete

5th European Weathering Symposium EWS

WEATHERING AND POLYMER DEGRADATION Luís E. Pimentel Real

Laboratório Nacional de Engenharia Civil Lisboa, Portugal

Weathering Weathering of Polymers and composites is an old story • considerable amount of previous publications • subject is broad and complex Key settings for durability analysis and lifetime prediction of polymers: Ageing methods Degradation and stabilization phenomena Climatic agents and weathering variables affecting performance of polymers. Most suitable instrumental methods of analysis for tracking the evolution of induced degradation in the polymeric materials. 2

Plastic products for outdoor applications Primary design factor: weatherability • Plastic building materials; • Outdoor furniture and surfaces • Composites and polymers used in transports; • Coatings and paints for protection of outdoor surfaces, • Outdoor artwork, dyes, highway pavement markings and road signs; • Textile products; • Biopolymers; • Packaging; • Miscellaneous. 3

Polymer performance Polymer performance is a result of intrinsic properties, additive composition and surface treatments. technical formulations (compounded polymers) development of new materials and stabilizers

complex materials

not completely understood Other factors Thickness Additives History (thermal degradation that is initiated during compounding and fabrication). 4

Degradation of polymers Any change of the polymer properties relative to the initial, desirable properties. Shear stress, heat, light, air, water, oxygen, radiation or mechanical loading

chemical reactions

Change in the physical and optical properties

Changes of the chemical composition and the molecular weight of the polymer Adverse effects on the useful life 5

Thermoxidation, Photoxidation General mechanism

6

Autoxidation use of different types of antioxidants that can intercept radicals and degradation products at different stages of the chain reaction

The key to effective protection against oxidation

7

Photoxidation Most pure polymers are theoretically incapable of absorbing UV light directly. Trace amounts of degradation products or catalyst residues within the polymer, can however absorb UV and initiate photodegradation. The process is autocatalytic. Chain scission, unsaturation, branching, formation of oxidation products (like carbonyl groups) and cromophores, several types of radical products and cross-linking. Cracking, embrittlement, chalking, color changes, loss of gloss and deterioration of physical properties (loss of strength, stiffness of flexibility and scratching. The weather-induced degradation results from competing reaction of these different processes.

8

Photoxidation. DLO Oxygen diffusion limited oxidation (DLO) preferential surface and near-surface changes.

leads

to

Heterogeneous distribution of property changes between the surface and bulk. The oxidation profiles are affected by the particular polymer matrix (nature, transparency and opacity, cristallinity), the severity of the environmental stress and material geometry. Thick samples

Distribution of products strongly unsymmetrical

Thin samples

Even distribution of products through the sample diameter

Differences in concentration gradients between 9 the near-surface and polymer bulk rather small.

Photoxidation. DLO

Heterogeneous distribution of property changes between the surface and bulk. 10

Stabilization To assure good properties and best performance in service, the polymeric resins intended for external applications need to be formulated with the appropriate additives and/or submitted to surface treatments looking for the conditions they will be subjected. Photo-stabilizers and surface coating are commonly used in polymer products to ensure adequate lifetimes.

Plastic products made of thermoplastics that incorporate appropriate weathering protection may be used in various outdoor applications, during many years of service. 11

Stabilizers – Flame retardants Flame retardants are a type of additives that not influence weatherability, but which action is influenced by weathering. In all plastics used outdoors, flame retardants are important and weatherability need to be considered for assuring good outstanding performances. The degradation caused by weathering is usually limited to the surface. In cases that fire retardancy is a bulk property, it changes only by a limited amount. If fire retardancy mechanism is dominated by a surface mechanism, as for the intumescent formulations, an influence of weathering occurred. 12

Stabilizers – Flame retardants Increased public pressure for discontinuing the use of halogenated compounds Ammonium polyphosphate (APP) Metal hydroxides and oxides Melamine based flame retardants Non-halogenated organic phosphorus retardants

based

flame

Layered silicate nanocomposites

13

Flame retardants - APP Ammonium polyphosphate acts as a flame retardant by a chemical effect in the condensed phase called "Intumescence" Mechanism of Action See for instance

http://www.specialchem4polymers.com or http://www.flameretardants-online.com Budenheim is one of the additives manufacturer that has developed finer and coated APP grades 14

Flame retardants - APP APP is an inorganic salt of polyphosphoric acid and ammonia. In contact with water hydrolyses to monoammonium phosphate (orthophosphate). For better weathering performances APP need to be insoluble in water Crystal phase I APP (APP I): Td ≈150°C

Crystal phase II APP (APP II): Td ≈300°C 15

Flame retardants - Metal hydroxides Aluminium trihydroxide (ATH) or magnesium dihydroxide (MDH), coated or not coated. Aluminium trihydroxide (ATH) is selected when processing temperature is under 200°C. When processing temperature exceeds 200°C, Magnesium dihydroxide (MDH) is then required. Research has been focused on surface modification and encapsulation to improve the dispersibility and miscibility of metal hydroxides in polymers matrices and improve the mechanical properties as well as flame retardancy. 16

Flame retardants - Melamine based FR’s Environmental friendliness. Always used together with other flame retardants offering synergistic effects. Melamine can also show considerable contribution to the formation of a char layer in the intumescent process, which acts as a barrier between oxygen and polymeric decomposition gases. When sublimating (>200°C, diluting the fuel gases and oxygen near the combustion source) it absorbs an amount of heat of 29 kcal/mole and when decomposing (>350°C) it absorbs an amount of heat of ~470 kcal/mole, thus acting as a heat sink in fire situations. Melamine exhibits low solubility in water and most other solvents, excellent UV absorption above 250nm.

17

Flame retardants - Melamine based FR’s Pure melamine

Melamine derivatives: melamine borate (MB), melamine phosphate (MP), melamine polyposphate (MpolyP), melamine cyanurate (MC)

MC MP, etc.

18

Flame retardants - Melamine based FR’s Melamine homologues offer an alternative nitrogen source that can be used under such extreme conditions. Melam, melem and melon act in general in the same way as melamine only at higher temperature.

melam (Td≈400ºC)

melem (Td≈500ºC) melom (Td>500ºC) Mechanism of Action

http://www.specialchem4polymers.com/tc/melamine-flameretardants/index.aspx

19

Flame retardants – Other solutions Phosphate and phosphonates based FR's work efficiently and give good UV stability and are used outdoors for roofing and wall sheating Nanocomposites present excellent flame retardant properties. The addition of layered silicate nanocomposites to polymers generally improves its oxygen index and greatly impacts the final flammability, promoting char formation and reducing the total released heat during the burning process. Environmentally safe and economical polymers with efficient FR properties have been developed, as enzymatically and non-enzymatically synthesized polyborosiloxane copolymers. These organic-inorganic hybrid siloxane copolymers and new siloxane-based nanocomposites have great potential as FR materials. 20

Stabilizers – Antioxidants Antioxidants are used to provide polymer protection both against oxidation during melt processing and through the product's life cycle as a 'long term thermal stabilizer”. Processing Stabilizers: to prevent degradation during processing Long term thermal stabilizers: they must function at temperatures considerably below the polymer melting point. Vinyl polymers (as PVC, PVC-C, PVDF, etc.) are a special type of polymers needing addition of thermal processing stabilisers, for avoiding thermal degradation during melt processing. They are not usually classified as antioxidants and they will not included in this class of additives. 21

Antioxidant selection Intrinsic sensitivity to oxidation varies greatly from one thermoplastic to another: PS, PMMA: more stable unsaturated polymers (PP, PB), copolymers: less stable Antioxidant properties: volatility, compatibility, color stability, etc. A single antioxidant can rarely provide complete polymer stability

Synergistic or additive combinations of two or more antioxidants. 22

Antioxidant selection R1: -H, CH3, OR

Phenolic HAS

Lactone & hydroxylamine (Radical scavengers) (RO)3-P

Organophosphorus compounds Antioxidant Hindered Amine (HAS, HALS) Hindered Phenolic Thiosynergist and Phenol Phosphite, posphonite, hydroxylamine, lactone

Melt processing stability

Long term thermal stability

No

Yes (until 110ºC)

Yes (until 300ºC) No Yes (between 150 and 300ºC)

Yes Yes(until 150ºC) No 23

Antioxidant development Recent advance: multifunctional antioxidants, which beneficially combines both primary and secondary antioxidant functions in one compound. Primary

Secondary

24

Heat Stabilizers Heat stabilizers are used primarily to protect the polymer during processing. As they also prevent longer-term heat degradation in the end-use, they are also important during service use of products exposed outdoors. Tin based heat stabilizers (mostly used in U.S.A.) Tin mercaptide: very good heat stability but poor light stability Tin maleate or tin carboxylate-based stabilizers: better light stability but poorer heat stability. Addition of titanium dioxide (restricted to white products) Manufacturers have been developing new stabilizers to improve weatherability. 25

Heat Stabilizers Europe currently uses mostly lead heat stabilizers

Significant development efforts have been made to replace lead stabilizers, with more environmentally friendly mixed metal (mainly Ca/Zn) or organic stabilizers with comparable cost-performance to leadbased stabilizers. Although the organic stabilizers do not give as white a polymer product as lead or tin stabilizers, this is not an issue in colored products.

26

Light stabilizers mechanism To protect the polymer from ultraviolet light degradation

27

Light stabilizers mechanism – before cycle 1 Possibilities to stop photoxidative degradation: 1 - Scavenging of the free radicals as soon as possible after their formation, either as alkyl radicals or as peroxy radicals Degradation: RH + hν R* Stabilization: nickel quenchers act by deactivating the excited states of chromophoric groups by energy transfer: Q*

Q + heat or hν

For energy transfer to occur from an excited chromophore (donor R*) to the quencher (acceptor Q) the latter must have lower energy states than the donor. However, they are being banned because of the environmental impact. 28

Light stabilizers mechanism – before cycle 1 Scavenging of the free radicals can also be done by other free radical scavengers, like sterically hindered amine light stabilizers, lactone and hydroxylamines, which react with carbon centered radicals. Scavenging of alkyl radicals would immediately inhibit the oxidation cycle. Scavenging the extremely reactive alkoxy and hydroxy radicals is practically not possible. HALS do not absorb UV radiation, but act to inhibit degradation of the polymer, by a radical trapping mechanism. Low concentration results in high level of stabilization. High efficiency and longevity (cyclic process wherein the HALS are regenerated rather than consumed). 29

Light stabilizers - HALS Combinations of high molecular weight HALS with low molecular weight HALS provided a excellent balance of UV stability, thermal stability and substrate compatibility in thick section polyolefins (widely used outdoors). NOR HALS (Alkoxyamine hindered amine stabilizer) are the best solution for the stabilization of agricultural films. Unlike conventional HALS, the NOR products have much stronger resistance to pesticide interaction. This class of stabilizers have very low basicity, which makes them less interacting with acidic derivatives from agrochemicals. The NOR products are very efficient because they can directly enter the HALS stabilization cycle. 30

Light stabilizers mechanism – before cycle 1 2 - Prevention of UV light absorption or at least reduction of the amount of light absorbed by the chromophores. Degradation: RH + hν R* Stabilization: UVA’s (UV absorbers) H HO

R

R

N

N hν

N

O

N N

N

R'

R' hν CALOR HEAT

Benzotriazole rearranjement mechanism

31

Light stabilizers - UVA Ultraviolet absorbers function by preferentially absorbing harmful ultraviolet radiation and dissipating it as thermal energy Such stabilizers function according to the Beer Lambert law

In practice, high concentrations of absorbers and sufficient thickness of the polymer are required before enough absorption takes place to effectively retard photodegradation. 32

Light stabilizers - UVA General structure of Benzotriazole

General struture of Benzophenone

The low basicity and non-interacting nature of UVA’s allows it to function well in acidic environments such as vinyl polymers and applications containing halogenated flame retardants. 33

Light stabilizers mechanism – during cycle 1 3 - Reduction of the initiation rate trough deactivation of excited states of the chromophoric groups. Degradation: R* + O2 ROO* Stabilization: HALS (P-NO*) actuating on R* (as nitroxyl radical scavenger) or HALS (P-NOR’), hindered phenol or arylamines actuating on ROO* These hydrogen donors react with oxygen centered radicals (peroxy radicals ROO*) to form hydroperoxides and prevent the abstraction of hydrogen from the polymer backbone. 34

Light stabilizers mechanism – during cycle 2 4 - Arises when the chain branching step is considered, implying the transformation of hydroperoxides into more stabe compounds, without generation of free radicals. Degradation: ROO* + RH

R* + ROOH (1)

Stabilization: organophosphorus compounds (phosphites or thioesters) and hydroperoxide decomposers (as metal complexes of sulfurcontaining compounds)

35

Light stabilizers mechanism– during cycle 2 Transformation of hydroperoxides Degradation: RO* + HO* + RH R* + ROOH (2) R* + ROOH RO* + *OH (3) Stabilization: Phenolic antioxidants or arylamines react with free radicals to yield inactive products ROH and H2O Deactivation or complexing of chromophoric groups other than hydroperoxides may be also important

36

Light stabilizers – Recent developments Non-basic light stabilizer (Tinuvin XT 833) was developed by Ciba in 2008 specifically for highly acidic PVC New UV-absorber was developed by BASF (CGX UVA 006), based on highly stable chromophore of the triazine family which absorption capacity exceedes all other conventional UV absorbers Croda Polymer Additives patented a new developed family of inorganic UV absorbers (Solasorb) that makes use of ultrafine metal oxides (zinc oxide and titanium dioxide), providing longer-term UV protection than traditional organic UV absorbers. Important research has been made in the domain of titanium dioxide technology (mainly by Dupont) 37

Light stabilizers – Recent developments Gabriel-Chemie GmbH, in collaboration with Ciba Specialty Chemicals (now part of BASF), has designed a unique UV stabilizer/flame retardant/color masterbatch (MAXITHEN® PPSYS / PPSYSM) for polypropylene stadium seating. This “all in one pellet” can be used in injection molding, blow molding and extrusion processes.

Other developments can be found in the review

38

Developments in other specific stabilizers Interesting developments were made in the following fields: •

fluorescent whitening agents (FWA, also called optical brighteners), yielding a brighter and fresher appearance to plastic products.

•

nanoclay-based additives for increasing barrier properties (by reducing gas permeation rates through the plastic matrix), having also a synergistic improvement in UV barrier properties.

39

Synergism Addition of synergetic products like pentaerythritol derivatives, carbohydrates and spumific agents (melamine etc.) to APP Other examples of synergism between flame retardants, as well between FR in the presence of nanoparticles have been found Synergistic blends of a nickel quencher and a benzophenone Synergistic mixture of monomeric and oligomeric HALS light stabilizers Combinations of UV benzophenone families stabilizers

absorbers of benzotriazole and with calcium-zinc based thermal

Combinations of primary and secondary antioxidants, such organophosphorous compounds (phosphites and phosphonites) and thiosynergists 40

Antagonism Examples of antagonism between FR and nanoparticles have also been found, depending on the concentration of the nanoparticles “Pinking" phenomenon which frequently occurs in white PVC profiles, containing formulations that include heat stabilizers based on lead and titanium dioxide pigments, upon sunlight exposure, due to interactions between the titanium pigment and the lead stabilizers Photoactivity of titanium dioxide pigments in presence of water, due formation of hydroxyl radicals

Mixtures of hindered amines with thiosynergists Combinations of UV absorbers of benzotriazole benzophenone families with tin based thermal stabilizers: - organotin mercaptide - di-butil-tin-maleate

and

41

Natural weathering Complex phenomenon: • direct effect of heat, light, air, water and mechanical stress • effects of the interaction of these elements Solar radiation, heat and moisture are the major factors that affect polymer durability. Variability from time to time and from place to place Comparisons among outdoor tests obtained at different seasons, years, or locations have been inadequate, even considering the same orientation of the material being tested 42

Natural weathering Principal exposure parameters: • temperature of the air and black panel temperature • relative air humidity and humidity period • Rainfall, rain period and pH value of rain and dew • radiant exposure for global radiation, in the UV range and at 340 nm • concentration of air pollutants (such as O3, SO2 and NOx) Statistical analysis methods, like DOE approaches and principal component analysis, has been used to evaluate consistency and reproducibility of outdoor weathering. The measurement of climatic parameters (minimum, maximum and mean values) can also be used to develop accelerated weathering tests by permitting more realistic simulation of outdoor conditions. 43

Natural weathering - Radiation Ultraviolet radiation promotes the primary photochemical process and plays the major role in degradation. The intensity and spectra power distribution of solar radiation varies widely with: • hour of the day • day and season of the year • altitude, latitude and geographical location • clarity of atmosphere (cloud cover, dust, ozone layer, etc.) Due to seasonal and local variations in the intensity of the sun radiation, it is recognized that “time of exposure” is not a good basis on which to assess outdoor exposure of samples. 44

Natural weathering - Temperature Climate change is expected Exposed plastic surfaces have been reported to reach 40ºC above ambient temperature and cooled as much as 11ºC below ambient temperature, depending of: • total incident radiation • altitude of the sun • ambient temperature and wind • material’s thermal conductivity, absorptivity and emissivity Temperature changes may originate expansion contraction (additional stress-fatigue processes)

and

synergistic effect with solar UV radiation 45

Natural weathering - Water Trend to underestimate some environmental agents, such as water (in form of moisture, snow, rain, fog, dew and frost). However, it plays an important role in the evolution of degradation in certain compounded polymers. H2O may originates dissolution of stabilizers, exudation of plasticizers, hydrolysis, embrittlement, discoloration and cyclic swelling and shrinkage (stressfatigue) due to absorption and desorption. It can influences the mechanism and stoichiometry of photochemical reactions and induce formation of free radicals when interacting with additives showing photoactivity 46

Natural weathering - Other factors oxygen, ozone, pollutants, smoke, dust, fog and clouds atmospheric impurities acid rain (harsh urban and industrial environments) ‘‘dark periods’’ account for differences in the polymer exposed surface arrangement of specimens (with or without a backing substrate or a glass covering) orientation of exposed samples (the total wet time and radiation varies with exposure angle). 47

Climatic accelerated weathering Methods to accelerate exposures under natural sunlight EMMA (Equatorial Mount with Mirrors for Acceleration): • mirrors to intensify the energy of sunlight • forced air cooling to minimize overheating of the samples

Under permission of Atlas Material Testing Technology

EMMAQUA double-sided

48

Climatic accelerated weathering New intensified natural sunlight testing stations, designed as solar concentrators (EMMAQUA, Q-TRAC, TRAC-RAC, SUN-10, HFSF, etc.) All these test stations use an equatorial mount that follows the sun and also concentrates the sun's rays by a battery of mirrors. EMMAQUA field

Results are obtained in approximately one seventh of the time required in natural exposure. Under permission of Atlas Material Testing Technology

49

Climatic accelerated weathering The developments of similar devices have continued. A recently developed Ultra-Accelerated Weathering System (UAWS), installed at ATLAS’ DSET exposure laboratory in Phoenix, Arizona, allows a 63 year (see note) equivalent South Florida UV radiant exposure within a single year of exposure.

Note: This factor of acceleration is indicated by ATLAS, but in my opinion it is not reliable Under permission of Atlas Material Testing Technology 50

Artificial accelerated weathering Major purposes: • determination of relative photochemical stability and weather resistance of different materials (ranking) • prediction of the life expectancy of samples from relative short-term testing (durability) If the natural weathering of a certain polymer is well simulated in laboratory conditions! using longevity functions and establishing a critical value for a specific polymer property, relevant for the service conditions in a certain application Determination of material/product durability when applied outdoors (lifetime prediction) 51

Artificial accelerated aging methods Key features Reliability Degree of correlation

“kinetic approach”, “simulation approach”, “mechanistic approach”

mechanistic approach (Lemaire, Gardette and co-workers) It is possible to accelerate the phenomena of chemical weathering without loss of reliability If chemical mechanism of degradation is the same in natural exposure and artificial weathering conditions, Check the final and the intermediate products of reaction, as well as the conversion paths of these intermediate products. 52

Artificial accelerated aging methods When phenomena of physical transfer are superposed on chemical evolution (as oxygen diffusion of stabilizers or migration) or when a chemical mechanism implies several process of similar importance, all these dynamic processes cannot be accelerated with same factor of acceleration It appears that acceleration effected through increased intensity alone, which is the basis of principle of reciprocity, reciprocity may fail to give good correlation with natural exposures. Secondary processes promoted by other climatic agents play a major rule in polymer degradation and vary with polymer formulation. For this reason, acceleration of only the primary process by increase in the intensity can distort the results even if the spectral distribution is maintained constant. 53

Artificial accelerated aging devices Most currently used apparatus : 1.Equipped with xenon lamp with a glass filter and an UV filter (ATLAS and SOLAR BOX, between other commercial marks); 2. Equipped with medium pressure vapour mercury lamps as used in SEPAP 12-24 weatherometers; 3. Equipped with low pressure vapour mercury fluorescent lamps (UVA-340 and UVB-313), as used in QUV accelerated weatherometer. 54

Artificial accelerated aging devices Devices equipped with mercury radiation sources produce more acceleration of degradation. The short-wave UV radiation sources, as Xenon lamps filtered by quartz glass and QUV UVB-313 lamps, have a lower level of natural exposure simulation and they may produce unrealistically severe results for some materials.

55

Artificial accelerated aging devices New apparatus • NIST Sphere: device designed for weathering of coatings, equipped with eight separate thermally insulated chambers, where eight weathering tests can be carried out simultaneously, permmiting the control of UV, temperature, humidity and pollutants (sulfuric acid aerosol, acid rain, sodium chloride or other salts or gaseous pollutants). 56

Artificial accelerated devices - Reliability The reliability of artificial accelerated weathering devices, for simulating the real world phenomena, depends of several variables, namely the nature of polymer, additives included in formulation and photo-oxidation conditions used Main features: 1. to simulate certain average weather conditions 2. to know the wavelength sensivity of the plastic formulations being tested (for select the optimal test source) 3. to know the chemical evolution of an unstabilized polymer

57

Artificial accelerated devices - Reliability High pressure xenon arc with borosilicate/borosilicate inner/outer filters : radiation source that better simulates terrestrial sunlight Xenon lamps are cooled by forced air or by water circulatings between two glass cylinders (interior and exterior filters) and does not come in contact with the arc. The most reliable programs for simulation of natural exposure include temperature and air humidity control, periodic cycles of water spray, dark periods and condensation

58

Artificial accelerated devices - Reliability The weatherometer should be equipped with: • water filters • aquanizer • cartridge deionizer • water purity meter • spray system • system for auto-adjusting irradiation (solarization!) • calibrated radiometers to check the intensity of the radiation source (340 nm, 300-400 nm and 300-800 nm optional) • calibrated black standard/panel thermometer • relative-humidity control device • Al/inox steel specimen holders (open or backed) • rotation of the cylindrical sample rack or similar means to balance differences of irradiation in the specimens • data-logger to measure the temperature in the exposed 59 surface of specimens (optional)

Artificial weathering – Correlation Simulation of weather natural conditions is a controversial subject Correlation between short-term and long-term tests common failure modes and mechanisms Case studies of correlation are almost limited to physical properties and not focuses on the degradation mechanisms!

60

Artificial weathering – Correlation Accelerated tests are usually deficient to simulate all the effects of the weathering and results of such tests frequently fail to correlate with long term outdoor exposures under natural sunlight. In most instances, the study of the weatherability of plastics follows empirical schemes that show little correlation with the actual effects of outdoor exposure, but there are also some examples of good correlation. Some case studies are available in the review.

61

Artificial weathering – Correlation Correlation between accelerated test methods and geographically different exposure sites has been tried using statistic tests and parameters, namely t test, Z test and using the Spearman rank correlation coefficient Follow the evolution of the measured value of a relevant polymer property (usually optical density or strain at break), for a specific application, after ageing, If the results of accelerated tests yielding the same ranking as that obtained by the natural exposure tests It is possible to determine acceleration factors of accelerated method in comparison with natural exposure and, consequently, to predict the lifetime of a polymer. 62

Artificial weathering – Correlation Best simulation is achieved with devices equipped with filtered xenon arcs, having an automatic control of humidity, temperature, rain cycling, dark periods and specimen rotation Sources having high concentration of UV-B radiation provide fast but unrealistic results due to the promotion of unnatural photo-triggered processes. Even in spite of possible lack of correlation with outdoor exposure, accelerated testing is an extremely useful tool for comparing the relative aging resistance of materials and rapidly screening out materials that have a poor resistance. Thus, ranking performance of different polymers and stabilizers may be done. 63

Artificial weathering – Variability Large amount of variability associated with artificial exposure tests, even when a single device is used Appreciable variation between test laboratories has been found in interlaboratory tests Variability of natural weathering conditions • Differences between the photoxidative and chemical degradation chemistry that occurs during an artificial exposure test and occurring outdoors • Material changes induced by increased intensity of radiation in artificial sources different from those resulting from natural weathering 64

Artificial weathering – Variability Variability of natural weathering conditions • Differences in the spectral energy distribution of indoor ultraviolet sources relatively to sunlight, inclusively in the visible and infrared radiation spectra (which contributes to radiation-induced increase in surface temperature) • Synergistic effect of temperature and radiation (different colors have different surface temperatures) • Differences in the mechanism of oxygen uptake (for example by initiation due to charge transfer complexes and due to normal autoxidation) 65

Instrumental methods of analysis Geburtig, Wachtendorf and Trubiroha, “Springer Handbook of Metrology and Testing, Chapter 15: Material – Environment Interactions”, 2011

Spectroscopic techniques • FTIR • UV-VIS • XPS • Raman, PAS, ESCA, ESRI Microscopy methods SEM Thermal analysis methods • DSC • TGA • DMA

Other methods • Colour and gloss • Traction • Flexion • Impact • Chromatography (GPC, SEC) • VOC analysis • Hyphenated techniques (GPC/FTIR, GC/MS, Micro FT-Raman and Micro-FT-IR) 66

Instrumental methods of analysis - FTIR Problems: • FTIR bands are usually wide and overlapping is possible. • There are different functional groups absorbing in the same region of the spectra (for example C=O groups) • Need of reference peaks to quantification (except when using always the same sample, such as a film) Solutions: • Adjusting of spectra peaks (deconvolution) • Use of derivatization processes. • Combine FTIR with other analytical methods

67

Instrumental methods of analysis – UV-VIS To detect differences of molecular characteristics of polymers, namely to identify chemical modifications. It may also be used as a colourimetric technique, when equipped with an integration sphere

68

Instrumental methods of analysis – XPS X-ray photoelectron spectroscopy (XPS) is very useful for chemical surface characterization • • • • •

to perform elemental analysis to evaluate the effect of weathering to perform elemental analysis to evaluate the effect of weathering to evaluate the presence of pollutants, contaminants and residual content of specific elements, which presence should be avoidable • to evaluate the mechanism of elemental loss during X-ray induced degradation • to measure retention of inorganic elements at surface and its form (bonded or ionic) • to determine the action of stabilizer additives. 69

Instrumental methods of analysis – SEM • The SEM produces images by scanning the surface of a specimen with an electron beam. • Low energy secondary electrons produce a signal that is strongly influenced by topography. • The backscattered electron signal is also sensitive to local topography, but it mainly enables the differentiation between different phases. • With SEM analysis it is possible to evaluate texture modifications of the surface, like surface defects, to detect the presence of discontinuities or flaws and crack networking, as well as to detect different phases formed as the result of accumulation of segregated additives 70

Derivatization techniques Derivatization techniques are realized in three steps: 1. The weathered samples are submmited to solvent extraction. 2. The components of the extracts are separated using suitable methods, including separation cromatography. 3. The extracts are sequentially submitted to organic reactions (with NO, SF4 or NH3) forming new products which characteristic functional groups that permit a clear identification of precursors, by titration or using FTIR. Hyphenated techniques, as GC-MS, may also be used for separating and identify products from the extracts

71

Lifetime prediction Useful lifetime corresponds to the end of the service life, which in general terms it is “the point in time, when the foreseen function is no longer fulfilled”. Service life is the time that a specimen extracted from a sample behaves according its specifications with a previously defined probability Exact prediction of the useful lifetime of a given polymer is a specific geographic location is still the dream of both consumers and manufacturers.

72

Lifetime prediction Is a currently adopted procedure to estimate service life of polymeric materials by extrapolation of accelerated aging data, in cases that a valid correlation between a short-term and a long-term test is obtained. As the majority of methods presently employed for the determination of weatherability of plastics tend to be predictive and extrapolatory, the accuracy and reliability of lifetime prediction methods are very important.

73

Lifetime prediction methodology 1. The problem should be explicitly defined before attempting to solve it. 2. Service life should be defined such that a) it can be measured (quantitatively) and b) it can be related to inservice performance. 3. We should be open to new approaches and methods rather than blindly accepting those of tradition. 4. We should use simple and systematic procedures having a basis in logic, common sense, and material science. 5. We should be aware that unsystematic, qualitative accelerated ageing test data can be used to make anything look good, bad, or indifferent.

74

Lifetime prediction methodology 6. We should recognize that a) it is impossible to simulate all possible weathering stresses in the laboratory, and b) it is not necessary to do it anyway. 7. We should ensure that degradation processes induced by accelerated tests are the same as those encountered in-service. 8. The degradation factors should be measured. 9. We should be wary of the correlation trap. 10. We should recognize that, by using systematic, quantitative procedure, valid acceleration tests can be developed.

75

Lifetime prediction models Service life prediction can be based on two different principal approaches: deterministic approach and probabilistic (or stochastic) approach. The durability models can also be divided into deterministic or stochastic models and can be based on empirical or analytical grounds Empirical models: based on experience and test results, applying correlative and other statistical methods (viewpoint of engineers). Analytical models: based on laws of nature and fundamental reasoning, thorough analysis of degradation mechanisms and kinetics (viewpoint of material scientists). 76

Probability models Stochastic or probability models take into account the scatter of data. Degradation is generally regarded as a stochastic process The probabilistic methods are used to estimate service life or maximum lifetime, trough the application of a statistical cumulative distribution (usually Weibull) to the results of lifetime determined for several specimens, quantifying uncertainties in the form of density distributions. Submitting a new specimen to different levels of weathering stress it is possible to describe the behavior of the specimen as function of the stress variable and build a diagram of probability of failure-stress-time to failure, which permits to describe the service life as a function of the stress parameter at a predefined level of reliability. 77

Probability models – Weibull

t    α   F (t ) = 1 − e

β

−

1. Cumulative distribution of Weibull (2 parameters) Example (color difference), with failure criteria of ∆E=0,9:

∆E = a1 − e

a2t

2. Confirm the agreement between model and experimental values obtained (∆E versus weathering time)

3. Plot the cumulative probability plot in function of failure time for each experimental condition (for example Temperature) and for failure criteria (∆E=0,9) 4. Check the influence of environment parameters (for ex. temperature) in the distributions, considering α and β as functions of applied stress (for ex. Temperature)

SLT = F 1(T )[−ln(1−Φ )]F ( ) 1 2

T

maximum service lifetime (SLT) equation 78

Probability models – Weibull case study 5. Considering that a shift of curves (for lower values of lifetime) occurs with increased temperature, evaluate the influence of temperature in the Weibull parameters α and β 6.Considering, as example, that this influence occurs only for α (scale parameter), apply an Arrhenius model for α and plot log α versus 1/T (K-1) 7.If it is a linear fit replace α by the model and insert it in the maximum service lifetime (SLT) equation:

SLT =α [−ln(1−Φ)]β

1

1

n m+ SLT =10 T

[−ln(1−Φ)]

β

79

Probability models – Weibull case study 8 – Construct a plot of probability of failure-stress-time to failure (PST plot), for a previously fixed level of reliability (1-Φ from 90% to 99%), using an average value for β (shape parameter). 9 - From this plot determine time to failure for each temperature for the selected probability level. For probability theory and different durability designs, involving probabilistic and mixed models, see for instance the Reports compiled by CIB W080 / RILEM 175-SLM Service Life Methodologies and CIB W080 / RILEM 140 Prediction of Service Life of Building Materials and Components: P. Jernberg, C. Sjöström, M.A. Lacasse, E. Brandt, T. Siemes, “Guide and Bibliography to Service Life and Durability Research for Buildings and Components, PART I – Service Life and Durability Research”, Publication 295, (2004), ISBN: 90-6363-041-7,

Hovde, J.; Moser, K. “Performance Based Methods for Service Life Prediction”, CIB Report: Publication 294, (2004), ISBN: 90-6363-040-9 80

Deterministic models In the design of deterministic durability models the scatter of data (degradation, performance or service life) is not taken into account. With known values of parameters the model yields only one value (of degradation or performance or service life) which is often the mean value. In some cases, deterministic models are formulated to give an upper or lower fractile value instead of the mean Analytical models of service life prediction using accelerated testing data (property decay) are based on: 1. Kinetics 2. Arrhenius law 3. Principle of reciprocity 4. Mathematics and statistics 81

Deterministic models - Kinetics The kinetic approach describes the value of a plastic property as a function of composition variables, which are kinetically related to the initial concentrations (before exposure) and to the time and variables of exposure. These chemical models emphasize the relation between chemical composition and weatherability and represent an attempt to obtain a correlation between the durability of a plastic material and its composition before exposure. The Kinetic approach is difficult to be employed in the analysis and correlation of weatherability data, due to difficulty in determining change in composition upon exposure and the absence of explicit relationships between plastic properties and compositional variables 82

Deterministic models – Kinetics: Kamal An example of such approach is the exposure parameters model of Kamal, which differs from other kinetic models, because it uses constants that reflect the combined effects of both the kinetic parameters and the exposure variables.

c dt w P = P0 ⋅ e Where P is the evaluated property with an initial value P0, cd is a degradation constant and tw the weathering time

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Deterministic models – Kinetics: Kamal                − ET   − EUV   − E Hr  α   P ln = ∑ C T exp RT  ∆t + C UV I UV exp RT  ∆t + C H R exp RT  ∆t  HR       P0 k =1    

T , HR: temperature (K) and wetting time (h-1) measured periodically in the surface at each ∆t; IUV: measured irradiance at [300-400 nm]; α: degradation constant of the material (≈0,3); Ci: deterioration constants (h-1), with value < 0; Ei: Activation energy (kJ/mol)

The most common application of this technique is the study and evaluation of the effects of additives and modifiers on the weatherability of plastics and it has been most successful in the areas of product development. 84

Deterministic models - Arrhenius law An example of such approach, based on chemical reaction theory, is the life prediction system for polymeric products used outdoors proposed by Tomiita and Kishima. In this model, a material property at any time tn is estimated by integrating the small property difference in every time interval These property differences can be calculated by inputting the degradation material constants obtained from different artificial accelerated degradation tests (thermal, humidity and weather tests) and the values of climatic agents at each time interval (temperature, ultraviolet energy and wetting time). The service life corresponds to the time when the property fall below a critical level. 85

Deterministic models - Arrhenius law: Tomiita

where P is the current property of the material, P0 the initial property of the material, CH the thermal deterioration constant of the material, EH the activation energy of thermal deterioration (kJ/mol), R the gas constant (8,314x10-5 kJ/mol K), T the absolute temperature of the material (K), and t the elapsed time.

By various assumptions and developing this equation Tomiita has been able to calculate the degradation curve as a function of solar radiation energy, SD, daily average wind speed, WD, average daily temperature in the daytime, TD, and night-time, TN, and the daily equivalent black panel temperature, BPT. 86

Deterministic models - Arrhenius law: Bruijn Bruijn (1996) proposed a version of the Arrhenius prediction of reaction rate that accounts for UV intensity: K is the reaction rate as a function of the activation energy, temperature, and UV intensity (I). I is the cumulative radiant exposure at the time to failure divided by the time to failure, thus normalizing the irradiance by time to predict a constant intensity of radiation. The standard reaction rate, k0, and the degree of stabilization, α, are also constants. The activation energy of all the reaction kinetics involved decreases with time, as the propagation of free radicals continues to accelerate the oxidation. So a simple inverse relationship was proposed: 87

Deterministic models - Arrhenius law: Bruijn • UV intensity, I, is a constant over the exposure time (irradiance normalized by time) • α changes over time of exposure, as the stability of the polymer decreases with time due to rapid chromophore generation in oxidation and the consumption/migration of additives. The value of α has been reported to vary between 0.5 for non-UV stabilized plastics and 1 for stabilized plastics. If the stability decreases with time, then the value of α is also inversely proportional to time. For integration simplicity, the UV intensity and α were combined as a function of t squared, to show greater dependence on time than the activation energy: C represents a constant based on the UV intensity and the initial value of α. 88

Deterministic models - Arrhenius law: Bruijn Substituting these functions of time provides the proposed relationship of reaction rate to time of exposure: If the fraction of the original value either gained or lost is defined as the property change, y, induced by chemical reactions: Then the profile of y as a function of time [y(t)] is the function represented in the typical degradation profile of any property for property change with time 89

Deterministic models - Arrhenius law: Bruijn for property change with time

Thus

to calculate the time to 50% change in a property:

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Deterministic models - principle of reciprocity The reciprocity rule requires the relative rate of the degradation process to be a linear function of the intensity of light incident on the sample. Several studies indicate that the principle of reciprocity is not valid for a great variety of polymers, like PUR coatings, PE, PP, PS, PC, SAN, ABS and PVC. Trubiroha, Geburtig and Wachtendorf, V. “The Principle of Reciprocity and its Limits”, 3rd European Weathering Symposium: Natural and Artificial Ageing of Polymers”, (2007), Proceedings pp. 243-258 Martin et al. “Reciprocity Law Experiments in Polymeric Photodegradation: a Critical review”, progress in Organic Coatings 47, (2003), pp. 292-311 91

Deterministic mathematical models Longevity functions: equations for the rate of weathering based on multivariable regression, allowing the determination of the value of the constants of the equation by measurements of exposure variables and the resulting changes in properties at short intervals (Langshaw, Kamal, Preisendorfer, etc.) Karpukhin (rate of oxidation of PE)

where c is the carbonyl concentration and I the irradiation intensity

Minsker (degradation in PVC)

where U is proportional to the activation energy, W is the relative humidity and H is the u.v. dose for wavelengths below 400 nm. 92

Deterministic Longevity functions The mathematical models based on empirical measurements, and consequent longevity functions developed, are theoretically unjustifiable. They do not necessarily reflect the true mathematical relationship between exposure conditions and the resulting changes in plastic properties. Kamal/Saxon

(Q) + b (I ) + c (O2) + d (OX ) + e (H 2 O) + f (τ ) + + g (Q ) (I ) + ... + z (Q ) (I ) (O 2 ) (OX ) (H 2 O ) (τ ) z

W =a

s

y

t

x

α

m

β

δ

p

σ

π

q

υ

W: weathering rate; Q: heat; I: irradiance a, b, c, ….β,σ: constant; etc. 93

Recent polymer developments Recent developments in polymeric materials and composites designed for outdoor applications include: • oxidation resistant crosslinked UHMWPE • PLA, PHBV and PCL with enhanced barrier properties to UV light, oxygen, and water • Protein-based plastics • Natural fiber reinforced plastic composites • Improvements in energy efficiency materials (developments in surface technology and in new high thermal insulation polymers) • Development of pultruded panels for ventilated facade of buildings and for incorporation into window frames providing both better thermal insulation 94

Recent polymer developments Recent developments in polymeric materials and composites designed for outdoor applications include: • Nanocomposite thermoelectric materials (NcTMs) • Stimuli responsive materials and smart polymers (thermotropic and organic thermochromic materials for adaptive solar control) • Smart polymer composites containing aggregachromic dyes of different nature and structure changing insulation properties according temperature of the environment. • Composites based on oxide nanofillers in polymers, namely TiO2, ZnO, and CeO2, show improvements in exposure to UV radiation

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THE END

Many thanks for your kindly attention 96