weathering resistance development of polymer.pdf

7/28/2019 Weathering Resistance Development of Polymer.pdf

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WEATHERING RESISTANT DEVELOPMENT OF POLYMER

BENNI RAMADHONI

2012021680


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INTRODUCTION

Polymer and polymer composites are increasingly being used in a wide range of applications where long-

term service in different environments is required. As a consequence, there is growing demand for

manufacturers to guarantee the life expectancy of their products, particularly where inspection can be

difficult or the possibility of failure catastrophic. Examples of such applications include gas pipelines,

chemical storage tanks, underground cabling, aerospace components, personnel safety equipment and

medical implants. Moreover, stringent product guarantees are also increasingly being demanded for

engineering components in products such as cars and domestic appliances, where consumers often view

extended lifetime warranties as a sign of product quality.

Among the parameters influencing the degradation of organic materials, especially environmental

related parameters, it is daylight, combined with the effect of heat, oxygen, water, and humidity whichacts as the main parameter of stress in outdoor conditions. When a reasonable amount of daylight

passes through windows, indoor conditions can also play a degrading role, primarily affecting the

appearance of the material, and are given a lot of attention in the conservation field. Even fluorescent

light, usually used to reproduce daylight, was revealed to be potentially damaging. Strictly speaking, only

complete darkness would prevent the occurrence of photochemical reactions.

The failure of polymeric materials can be increased due to the presence of impurities, improper

formulation, design flaws and processing factors. Deterioration of mechanical properties such asembrittlement or cracking, discoloration and loss of transparency of polymer products can be observed

[1].

DEGRADATION MECHANISMS

Effective stabilizers can be developed if the process of photo-oxidative degradation of polymeric

materials is understood well. The polymer photo-oxidation is started with the absorption of photon by

polymer to produce radicals followed by the photo-physical and chemical conversions of these activated

species. The photon energy is sufficient to rupture the weaker covalent bonds in polymers at

wavelengths of less than 400 nm and the strongest bonds at 300 nm. Only ~ 6% of solar energy is

associated with radiation of wavelength less than 400 nm.


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It has also been proposed that under the influence of ultraviolet radiation, polymers can form charge-

transfer complexes which capable to produce radicals. Singlet oxygen, resulting from the interaction of

ultraviolet radiation and molecular oxygen and its subsequent attack on polymers has been suggested as

significance, because it is on the surface that the intensity of damaging radiation is at a maximum during

subsequent exposure [2].

The photo-oxidation of most polymers proceeds by a radical chain mechanism which involves the

various steps: initiation, propagation, possibly branching and termination. The propagation, branching

and termination steps are believed to closely similar between thermal and photo-oxidation of polymeric

materials whereas the initiation processes are different. Strength of some common polymer bonds is

shown in Table 1 and the spectral sensitivity of some polymers is shown in Table 2. Table 3 shows the

absorption of UV-visible radiation by common synthetic and natural polymers. As the initiation started

and the radical formed, the process is continued with propagation, branching and termination. C-H and

C-C bonds on the carbon adjacent to the radical site are undergo cleavage as they have much lower

bond dissociation energies than those associated with the primary polymer molecule chain. Chain

scission is thus an expected degradation reaction in polymers whenever alkyl or alkoxy radicals are

formed [3].

Table 1. Strength of some common polymer bonds


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Table 2. Spectral sensitivity of some common polymers

Table 3. Absorption of UV-visible radiation by common synthetic and natural polymers

THE DEVELOPMENT OF WEATHER RESISTANCE OF POLYMERS

It is very important to increase the stability of polymers and plastics that exposed to heat, light,

atmospheric oxygen and other environmental agents and weathering conditions. Some polymer such as


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PP and PVC is inherently very photo-chemically labile therefore limits them to outdoor application. They

are photo-degraded fast over periods ranging from months to a few years.

The photo-stability of a polymer can be lowered due to the presence of impurities such as catalyst

residues, trace metals and structural irregularities. This problem can be addressed by eliminating orreducing these entities by modifying the polymerization or fabrication process. It is possible to alter the

polymer structure but this approach is seldom adopted because the cost is generally high and also such

modification can change the physical and mechanical properties of resulted polymer. Applying some

additives into polymer matrix is more common method. These additives include a wide range of

materials which are effective in a variety of ways. The most effective way to stabilize polymer against

photo-oxidation can be achieved by adding an absorber or reflector into polymer matrix. If the radiation

is absorbed by the polymer and activated species formed, it is still possible to deactivate these groups

before damaging chemical process by employing additives which quench the excited chromophores and

dissipate the absorbed energy harmlessly [2].

APPLICATIONS OF NANOPARTICLES AS PHOTOSTABILIZING AGENTS

The integration of inorganic nanoparticles into polymers allows the possibility to incorporate the

properties of inorganic nanoparticles to polymers matrices. Decreasing the size of inorganic particles

allows for the functionalization of transparent polymers without significant loss of transparency by

avoiding them to agglomerate from each other inside the polymer matrix [4].

There has been extensive interest in using inorganic nano-particles since inorganic UV absorbents, such

as nano-ZnO, TiO 2 and CeO 2, virtually do not migrate in a polymeric matrix, and have excellent photo-

and thermal stability. Also zinc oxide and titanium dioxide are non-toxic and chemically stable under

exposure to both high temperatures and UV. Furthermore, nanoparticles have a large surface area to-

volume ratio that results in a signicant increasing of the effectiveness in blocking UV radiation when

compared to bulk materials.


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Figure 1. Decreasing the size of inorganic particles allows for the functionalization of transparent

polymers without significant loss of transparency if the particles are isolated from each other inside the

TITANIUM DIOXIDE NANOPARTICLES

Currently, TiO 2 nano-particles have become attractive multi-functional materials. TiO 2 nano-particles

possess properties such as higher stability, long service life, safe and non-toxic. TiO 2 can be used as UV

blocker because of its good reflective properties and UV absorption ability.

The TiO 2 nano-particles possess high surface area per particle size ratio, which causes a significant

increasing of the effectiveness in protecting UV irradiation when compared to micron-size TiO 2 [5].

TiO2 nano-particles exist in three forms: anatase, rutile and brookite. Only Anatase and rutile are readily

to be used in various applications. Anatase can degrade polymers under the action of UV radiation due

to its photo-catalyst property which can generate higher population of hydroxyl free radicals. Rutile

absorbs UV light up to the proximity of visible light wavelengths and also transparent at visible light

wavelength and very high refractive index. Hence rutile TiO 2 can be used in the fabrication of visually

transparent UV filters. The anatase-rich mixtures with rutile are more effective photo-catalyst than pure

anatase.

TiO2 nano-particles can be used directly in applications such as PET textile / TiO 2 nano-particles

composites. To get good results, TiO 2 nano-particles can be added to the polymer matrix during the

polymerization process, fiber manufacture or in the process of finalization. The key issue of using

nanoparticles is that the nanoparticles should be dispersed homogeneously or by avoiding the

nanoparticles aggregation. Many methods available are the sol-gel blending, the melt blending, the in

situ polymerization, in situ forming nanoparticles and in situ polycondensation process. In in situ

polycondensation process, the TiO 2 nano-particles was treated with a coupling agent to introduce some

organic functional groups onto the surface of TiO 2 nano-particles. The functionalized TiO 2 nano-particles


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were then dispersed in a solvent such as ethylene Glycol (EG) and the solution then reacted with

terephtalic acid (PTA) to further going into polycondensation. Finally PET/ TiO 2 nano-particles were spun

into fiber. The UV-blocking mechanism of PET/ TiO 2 nano-particles composite can be deducted due to

the energy of UV-ray is absorbed by TiO 2 nano-particles of which its band gap lies in the UV-ray solar

spectrum. Figure 2. shows the spectrograms of UV-ray transmitting through different fabrics [6].

Another application of titanium dioxide nanoparticles is the using of titanium dioxide nanoparticles as

the photo-stabilizer for wool to prevent the photo-yellowing problem. Wool is easier become yellow

compared to other fibers such as nylon, acrylic, cotton and polyester. UV-ray was demonstrated to make

wool become yellow where visible light tend to bleach the wool. The strength loss of wool due to

polypeptide chain scission can be considered as a photo-degradation of wool. Figure 3. shows the

photo-yellowing trends of pure wool and wool doped with photo-catalyst agent P-25 TiO 2 nano-particles

and wool doped with fluorescent whitener agent (FWA). The result suggested that P-25 TiO 2 doped wool

has a better yellowness index compared to untreated one. All samples that treated with P-25 TiO 2

experienced a significantly slower rate of photo-yellowing index. Increasing the P-25 TiO 2 concentration

also effectively reduced the photo-yellowing of wool. Unlike some synthetic polymers in which photo-

degradation are accelerated in the presence of photo-catalyst P-25 TiO 2, but in the case of wool, P-25

TiO2 acts primarily as UV absorber resulting in slower photo-yellowing rate [7].

Figure 2. The spectrograms of UV-ray transmitting through different fabrics:

(a) Pure PET, (b) 1 wt.% nano-TiO 2, (c) 2 wt.% nano-TiO2


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Figure 3. Photo-yellowing trends of pure wool and wool doped with TiO 2 nano-particles and wool doped

with fluorescent whitener agent (FWA)

Figure 4. Doping rates of P-25 TiO 2 and yellowing trends of doped wool

ZINC OXIDE NANOPARTICLES

Zinc oxide (ZnO) is a mineral and can be prepared in particles that have an optimal size of 20-30 nm. ZnO

is also usually coated with silicon oils, SiO 2, or Al2O3 in sunscreen formulations. Additionally, ZnO is


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considered a better sunscreen ingredient than TiO 2 because it is more transparent for a given

concentration and is more protective against UV light.

ZnO can be incorporated into epoxy to obtain ZnO/epoxy nanocomposites with high UV shielding

efficiency. Transparent epoxy is usually used in standard white LED technology as packaging materialsbecause it possess excellent transparency, high Tg, low water absorption and easy to be processed.

Packaging materials for UV light based white LED required to possess high visible light (> 400 nm)

transparency and high UV light (< 400 nm) resistance. Optically transparent composites have been

obtained by incorporation of a low content of fillers with dimensions far below the wavelength of light.

ZnO possess high UV shielding material with low refractive index. However, as ZnO nanoparticles

content increases, the visible light transparency decreases dramatically. Also, if ZnO nanoparticles are

too small they will causing a blue-shift phenomena and hence reducing the UV shielding efficiency.

Therefore, the fine size as well as concentration of ZnO nanoparticles is critical to obtain high UV

shielding efficiency. There are so many synthesis methods to obtain fine size ZnO nanoparticles namely:

precipitation, hydrothermal, sol-gel and microemulsion. Among all, the precipitation method makes

possible for production of large quantities as well as low cost. The transparent ZnO/epoxy

nanocomposites is obtained by dispersing ZnO nanoparticles in curing agent using ultrasonic technique

for 10 min following by mixing with epoxy and stirred. The mixture then cured by heating it.It can be

deduced from figure 5 that the increasing concentration of ZnO nanoparticles also increase the

absorbance of UV-light region (< 400 nm) which means that UV radiation is consumed by the zinc oxide

alone and prevent UV radiation to reach the polymer matrix from in this case the epoxy hence providing

protection from being damage by the UV radiation. But further increasing the ZnO nanoparticles will

also decrease the transparency of the sample where in this white LED application transparency is one of

the important parameter [8].

OTHER METHODS

Weather resistances can also be increased by formulating new material namely reactive light stabilizer

(UCHA). UCHA can be synthesized by combining a UV absorbance group, a radical scavenger and acryl

groups in the same molecule via processing method called UV-curing. This UCHA was synthesized by

reacting some material namely methyl ethyl ketone (MEK) with toluene HDT, 4-metoxyphenol, DBTDL,


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4-hydroxy-2,2,6,6-tetramethylpiperidine, Tinubin 400 and pentaerythritol triacrylate. Figure 6 shows the

QUV weathering of unstabilized and stabilized coating [9].

Figure 5. Transmittance of ZnO/epoxy nanocomposites with different ZnO contents. Z0, Z5a, Z5b, Z5c,

Z5d and Z5e denote 0, 0.03, 0.05, 0.07, 0.1, 0.15 wt. % ZnO

Figure 6. QUV weathering of unstabilized and stabilized coating with 2 wt. % of UCHA.

Another method to improve the weatherability of polymer is by the using of fluoropolymers. Over the

last decades, fluoropolymers have gained in importance in the coating market, mostly because of

their good resistance to UVA, UVB and to corrosive chemical agents [10]. Their bond strength (C F)


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stabilized the structure decreasing the chemical degradation such as scission of the polymer chains,

scission of end chains, etc [11].

When exposed to UV light, acrylics coatings undergo significant discoloration and chalking. Acrylics

contain esters and possibly other functional groups sensitive to both photochemical degradation andother types of degradation such as hydrolysis [12]. Therefore, coating acrylic with fluoropolymer such as

PVDF can increase its resistant to UV radiation. Figure 7 shows the change in delta E* of the acrylic and

PVDF-acrylic coatings [13] .

Figure 7 Change in delta E* of the acrylic and PVDF-acrylic coatings

In Figure 7, the change in delta E* decrease significantly in the system with PVDF. It is very clear that

PVDF increase the stability of acrylic under the expose of UV radiation.


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CONCLUSIONS

1. Some polymer such as PP and PVC is inherently very photo-chemically labile therefore limitsthem to outdoor application.

2. This problem can be addressed by modified the structure of polymer or adding some material tocombat the severe effect of weathering especially due to UV radiation.

3. Modifying the polymer structure is relatively high cost compared to applying additives intopolymer matrices.

4. Nanosized UV adsorber such as TiO 2 and ZnO nanoparticles can effectively enhance theweatherability of polymer.

5. Synthesizing new material i.e. UV absorber and filter also possible to increase the performanceof polymer regarding to outdoor application.

6. Polymer can be coated with fluoropolymer materials which inherently UV resistant to increasethe weather resistant.


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REFERENCE :

1. J. Scheirs, Compositional and Failure Analysis of Polymers - A Practical Approach; John Wiley &Sons, Ltd, 2000

2. J.F. Rabek, Photostabilization of Polymers - Principles and Applications; Elsevier Applied Science,1990

3. A. Davis, D. Sims, Weathering of Polymers; Applied Science Publishers, 1983

4. H. Althues, J. Henle, S. Kaskel, Chem. Soc. Rev., 2007, 36, 1454-1465

5. Yadav A, Prasad V, Kathe AA, Raj S, Yadav D, Sundaramoorthy C, et al. Bull. Mater. Sci., Vol. 29,No. 6, November 2006, pp. 641 645

6. K Han, M Yu - J Appl Polym Sci, 2006, 100, 1588-1593

7. H. Zhang, K.R. Millington, X. Wang, Polym Degrad Stab, 2009, 94, 278-283

8. Y.Q. Li, S.Y. Fu, Y.W. Mai, Polymer, 2006, 47, 2127-2132

9. S.C. Jang, S.C. Y, J.W. Hong, J. Ind. Eng. Chem, 2005, 11, 964-970

10. L. Sung, S. Vicini, D. Ho, L. Hedhli, C. Olmstead, K. Wood, Polymer, 2004, 45, 6639 6646

11. Anonymous, Introduction to Fluoropolymers, Zeus Technical Whitepaper, 2006, 1 9

12. K. Wood, Effect of fluoropolymer architecture on the exterior weathering of coatings, in: XXVIFATIPEC Congress, Dresden, 2002

13. V. Landry, P. Blanchet, Prog. Org. Coat, 2012, 75, 494 501

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