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Budapest University of Technology and Economics

Ildik Kriston

Some aspects of the degradation and sta-

bilization of Phillips typepolyethylene

Ph.D. Thesis

Supervisor:EnikFldes

Laboratory of Plastics and Rubber TechnologyBudapest University of Technology and Economics

Department of Applied Polymer Chemistry and PhysicsInstitute of Materials and Environmental Chemistry

Chemical Research CenterHungarian Academy of Sciences

Budapest, 2010


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1 Ph.D. Thesis

1. Introduction

Polymers and plastics constitute an important part of our life for manydecades now. We use them at home, many parts of cars are made from them,

but often even the active components of electric and electronic devices are

produced from or at least contain plastics. Polyethylene (PE) occupies an im-

portant position even among commodity plastics. It is produced and used in the

largest quantity in Hungary, but also everywhere else in the world. Several

factors led to the success and widespread use of polyethylene. It is cheap andits low density, flexibility, aesthetic appearance and other properties make it

ideal as packaging material. It can be processed easily, PE can be and is

processed practically with all thermoplastic processing technologies including

extrusion, injection molding, blow molding, rotational molding, etc., but the

largest amount of products are prepared by extrusion. The polymer is subjected

to the effect of heat, shear and oxygen in all thermoplastic processing technol-

ogies. As an effect of these factors chemical reactions take place in the

processing machine with a number of consequences. Although PE is a simple

polymer with an apparently well defined structure, its molecular structure often

contains irregularities. Double bonds and other chain irregularities are poten-

tial reaction sites on which chemical reactions take place during processing.

These reactions modify the chain structure of the polymer and also the proper-

ties of the final product. It is obvious that the polymer must be protected

against such changes, which needs stabilizers.

The degradation and stabilization of polymers and especially that of po-

lyethylene is studied for a long time1. Basic processes were revealed and effi-

cient stabilizer packages developed. The combination of a hindered phenolic

1Zweifel, H. Stabilization of Polymeric Materials. Berlin: Springer, 1998.


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Ph.D. Thesis2

primary antioxidant and a secondary, processing stabilizer, very often a phos-

phorous compound is routinely used for the stabilization of polyethylene.

Nevertheless, several questions are completely unclear or have not been dealtwith at all. Among others the hydrolytic stability of antioxidants and its possi-

ble effect on the lifetime of products brought into contact with extractive me-

dia have never been sufficiently investigated and explained. In the framework

of a larger project, we studied processes taking place during the storage of

HDPE in water, the consumption of the additives and the consequent changes

in the structure of the polymer. In the first part of this large project we focused

our attention on changes in the properties of the neat polymer in the hope that

the results may serve as a basis for the study of more complicated systems

containing a stabilizer or even a stabilizer package.

The stabilization mechanism of phenolic antioxidants is widely dis-

cussed in the literature1,2,3. Less attention is paid to the processing stabilizing

mechanism of phosphorous secondary antioxidants. The chemical reactions ofphosphites and phosphonites are considered similar in the literature1. The sta-

bilizing action is attributed to three basic mechanisms: decomposition of hy-

droperoxides, reactions with peroxy and alkoxy radicals4. The reaction me-

chanisms of various phosphorous derivatives (phosphites, phosphonites and

phosphines) determined in solvents at ambient and elevated temperature show

differences only in the reactions with oxy radicals5

. Phosphines yield esters ina higher portion than phosphites due to the predomination of displacement

reactions. The mechanisms of the reactions with hydroperoxide and peroxy

radicals were found similar for the three types of phosphorous compounds. On

2Pospil, J., Polym. Degrad. Stab., 40, 217-32 (1993).3Pospil, J., Polym. Degrad. Stab., 39, 103-15 (1993).4Schwetlick, K., Habicher, W.D.,Angew. Makromol. Chem., 232, 239-46 (1995).5Kochi, J.K., Krusic, P.J.,J. Am. Chem. Soc., 91, 3944-6 (1969).


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3 Ph.D. Thesis

the other hand the studies on the stabilization of polyethylene revealed that not

only the efficiency but also the reaction mechanism of various phosphorous

stabilizers are strongly influenced by their chemical structure6

. Phosphines hadnot been investigated as polyolefin stabilizers previous to the research con-

ducted in our laboratories in cooperation with Clariant and Tisza Chemical

Work. The aim of the larger project run at the Joint Laboratory of the Labora-

tory of Plastics and Rubber Technology at the Budapest University of Tech-

nology and Economics and the Department of Applied Polymer Chemistry and

Physics at the Institute of Materials and Environmental Chemistry, CRC, HASwas the determination of the effect of the chemical structure of phosphorous

antioxidants on the mechanism of processing stabilization in polyethylene. In

the frame of the present work we investigated three phosphorous stabilizers

(phosphites, phosphonites and phosphines) in Phillips type polyethylene. Their

melt stabilizing efficiency was compared, their role in melt stabilization was

determined, and correlation was established between their consumption duringprocessing and the properties of the polymer. In addition, high temperature

reactions of a phosphite stabilizer with reactive species accelerating the degra-

dation of polyethylene during processing (hydroperoxide, carbon-centered,

peroxy and oxy radicals) were investigated by model reactions. Similar model

reactions are run with phosphine and phosphonite stabilizers, but their results

are out of the scope of this thesis and will be reported elsewhere.The work in this thesis formed an important part of this larger project.

One of its five thematic chapters focuses on the effect of water on the degrada-

tion of polyethylene, while the rest on the study of phosphorous antioxidants.

Although the thesis was completed the project continues also in the future.

6

Maloschik, E., Fldes, E., Janecska, ., Nagy, G., Puknszky, B., 42nd Microsymposium ofP.M.M. Degradation, Stabilization and Recycling of Polymers, Prague, 14-17 July 2003.


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Ph.D. Thesis4

2. Materials and methods

The experiments were carried out with ethylene/1-hexene copolymers

of the Tisza Chemical Work (TVK), Hungary, polymerized by Phillips catalyst.

Irganox 1010 (Ciba) was used as phenolic antioxidant. Three commercial

phosphorous secondary antioxidants supplied by Clariant were studied: a hin-

dered aromatic phosphite (Hostanox PAR 24), a hindered aromatic phospho-

nite (Sandostab P-EPQ; consisting of di- and mono-phosphonites) and an aro-

matic-aliphatic phosphine (Pepfine).

The additive-free and the stabilized polymers were pelletized by six

consecutive extrusions at 260 C. Samples were taken after each extrusion

step. Plates of 1 mm and films of 100-200 m thickness were compression

molded at 190 C for characterizing the polymer. Blown films were prepared

at 175, 195, 195, 195, 195 C barrel temperatures, 40 rpm screw speed and 1:4

blow ratio.

The chemical structure of the polymer, the consumption of phosphorous

antioxidants during the processing of polyethylene and the products of the

phosphite stabilizer formed in model reactions were analyzed by Fourier

Transform Infrared (FT-IR) spectroscopy. High performance liquid chromato-

graphy (HPLC) was applied for the analysis of the residual amount of hindered

phenol after the processing of the polymer. The rheological properties of the

polymer were characterized by different methods: MFI was measured at 190

C with 2.16 kg load; dynamic viscosity measurements were carried out at 210

C in the range of 0.1600 s-1; and creep compliance was determined at 190

C, 500 Pa mean stress and 300 s creep/600 s recovery phase times. The mole-

cular mass and mass distribution of the polymer was determined by GPC at

160 C in 1,2,4-trichlorobenzene. The crystalline structure of the polymer was

characterized by thermal analysis. The color of the samples was measured and

the yellowness (YI) and whiteness (WI) indices were calculated as characteris-


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5 Ph.D. Thesis

tic parameters. The residual stability of the stabilized samples was determined

by measuring the oxidation induction time (OIT) in oxygen at 200 C. The

tensile properties of the polymer were measured on dumb-bell specimens,while the mechanical strength of films was characterized by the Elmendorf tear

strength measured in parallel and perpendicular directions to the flow, as well

as by the dart drop strength. The thermal stability of the investigated phosphite

[tris(2,4-di-tert-butylphenyl)phosphite; DTBPP] and its reaction with molecu-

lar oxygen were studied at 200 and 240 C under argon and oxygen, respec-

tively. The reactions of DTBPP with carbon-centered, peroxy and oxy radicals,as well as with hydroperoxide were studied at 200 C. The reaction products

were analyzed by FT-IR spectroscopy, HPLC, and HPLC coupled with a mass

spectrometer (HPLC-MS).

3. Results

In the first part of the research the effect of storage in water was studied

on polyethylene samples extruded 1, 3 and 6 times, compression molded to

plates then stored in closed glass containers in distilled water at 80 C for 1

year. The results proved unambiguously that most chemical reactions are re-

lated to each other and only one or two reactions dominate all changes in struc-

ture and properties, though several reactions are proposed in the literature.

Carbonyl groups play an important role in the processes taking place during

the soaking of the polymer. The chemical changes determine all properties

including molecular mass, crystalline structure, rheological and mechanical

properties.

The efficiency and the reaction mechanism of three phosphorous anti-

oxidants (phosphite, phosphonite and phosphine) were studied under the

processing conditions of polyethylene by multiple extrusions without and in


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Ph.D. Thesis6

the presence of a phenolic antioxidant. We developed FT-IR spectroscopic

procedures to determine the amount of the non-reacted and oxidized phospho-

rus molecules in polyethylene without using extraction.We compared the processing stabilizing efficiency of additive packages

containing 700 ppm of phosphorous stabilizer and 700 ppm phenolic antioxi-

dant. The results revealed that the chemical structure of the phosphorous anti-

oxidant influences significantly the melt stabilizing efficiency of the additive

package during the processing of polyethylene, though the reaction mechanism

of various phosphorous stabiliz-

ers is considered similar in the

literature. The alkyl-aryl phos-

phine investigated has the best

melt stabilizing efficiency by

preventing the recombination of

macroradicals. It is well reflect-

ed in the changes of the rheolog-

ical properties in the course of

multiple extrusions (Fig. 1). On

the other hand, in oxygen at-

mosphere, where the formation

of oxygen-containing groups

dominates, the three different

phosphorous antioxidants have similar efficiency yielding OIT values which

depend only on the concentration of the efficient P(III) molecule and are inde-

pendent from its type. The phosphonite studied protects the most efficiently

the polymer from discoloration, which may be explained by the interaction of

the additives and/or their transformation products.

We investigated the role of phenolic and phosphorous antioxidants in

0 1 2 3 4 5 60.4

0.5

0.6

0.7

0.8

0.9

MFI(g/10min)

Number of extrusions

Figure 1Changes in the melt flow index ofpolyethylene stabilized with 700 ppmphenolic and 700 ppm phosphorous stabi-lisers ( phosphonite, phosphite,

phosphine)as a function of multiple extru-sions.


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7 Ph.D. Thesis

the melt stabilization of polyethylene by studying the characteristics of polye-

thylene processed by multiple extrusions in the absence of any additive, with a

phenolic and one of three different phosphorous antioxidants, respectively,used as a single stabilizer, as

well as with phenol-

ic/phosphorous antioxidant

packages. The results proved

that the chemical reactions

occurring in the first extru-sion play a determining role

in the further reactions of

the polymer during subse-

quent processing steps, even

after the total consumption

of the phosphorous stabiliz-er. Comparing the effect of

single stabilizers we found

that the phenolic antioxidant

does not protect the polymer

from degradative chemical reactions in the first extrusion, while the phosphor-

ous antioxidants have stabilizing efficiency in an extent depending on their

chemical structure (Fig. 2). The phosphite is the least efficient melt stabilizer

among them. The phosphine shows the best performance not only in the first,

but also in the second extrusion. However, the stabilizing effect of single anti-

oxidants disappears fast in subsequent processing operations, as 700 ppm of

phosphorous antioxidants is consumed already in the first extrusion. In two-

component antioxidant packages, the main role of the phenolic antioxidant is

to decrease the oxidation rate of the phosphorous compound. The hindered

0 1 2 3 4 5 6 70

100

200

300

400

500

Elmendorfte

arstrength(g)

Number of extrusion

Figure 2Changes in the Elmendorf tear strengthmeasured perpendicularly to the direction of flowas a function of the number of extrusions. Sam-ples: polymer processed without any antioxidant(), with 700 ppm phenolic antioxidant (), 700ppm phosphite (), 700 ppm phosphonite (),700 ppm phosphine ().


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Ph.D. Thesis8

phenol does not alter the stabilization mechanism of the phosphonite and the

phosphine, but modifies the reactions of the phosphite. The changes in the

characteristics of the polymer during processing are directly related to thedecrease in the number of vinyl groups. In the presence of the phosphine and

the phosphonite less long chain branches form than with the phosphite. The

phenolic antioxidant prevents the long chain branching of the polymer only in

a very small extent. Although on the basis of literature sources we might ex-

pect that the reaction derivatives of the phosphite increase the thermo-

oxidative stability of the polymer, the results of our study do not prove that.

To explore the melt stabilizing mechanism of the antioxidants we com-

pared the additive consumption to the changes in the characteristics of polye-

thylene during processing. The polymer was stabilized with antioxidant pack-

ages consisting of different amounts of phosphorous stabilizers combined with

700 ppm hindered phenol and processed by multiple extrusions. Samples were

taken and films were blown from the pellets after each extrusion step. The

experiments revealed that the synergetic effect of phenol/phosphorous antioxi-

dant combinations compared to single antioxidants can be partly attributed to

the reduced rate of consumption of each type of stabilizer during the

processing of polyethylene. The rheological characteristics of the polymer and

the strength of films processed from that are controlled by the type and the

amount of phosphorous stabilizer. The melt stabilizing efficiency of the phos-

phonite and the phosphine is similar. They differ in their consumption rate: the

phosphonite is consumed fast, while significantly smaller amounts are oxi-

dized from the phosphine. These phosphorous compounds hinder efficiently

the formation of long chain branching of polyethylene above a critical residual

concentration of about 200 ppm (Fig. 3). Below the critical residual concentra-

tion the characteristics of the polymer start to change significantly, as the me-

chanism of inhibition reaction changes; the reactions of the phenolic antioxi-


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9 Ph.D. Thesis

dant (which hardly prevents

the recombination of macro-

radicals) determine stabiliza-tion. The phosphite hinders

the formation of long chain

branches less effectively

than the two other phos-

phorous antioxidants. This

results in a gradual decreaseof film strength even at high

residual concentrations of

the phosphite (Fig. 3). The

discoloration of polyethy-

lene is determined by both

types of antioxidants. Al-though the phosphorous

stabilizers contribute strong-

ly to the color stability of polyethylene by decreasing or increasing it, the reac-

tions of the phenolic antioxidant determine the degree of discoloration at a

given antioxidant pair.

Model reactions were carried out in order to explore the reason of dif-

ferences in the stabilization mechanism of the phosphite in the absence and the

presence of a phenolic antioxidant, and in comparison to that of the phospho-

nite and phosphine. The reactions of the phosphite with molecular oxygen,

hydroperoxide and oxy radicals were studied at 200 C. The experiments

were carried out in melt (without using any solvent) because 1) the question is

the direct reactions of the phosphite with reactive groups and radicals formed

during the thermo-oxidative degradation of polyethylene, 2) the thermo-

0 500 1000 15000

100

200

300

400

Elmendorftearstrength(g)

Residual P(III) compound (ppm)

Figure 3 Correlation between the residualamount of phosphorous antioxidant and theElmendorf tear strength measured in the per-pendicular direction to the flow of polyethylenefilms stabilized with combinations of 700 ppmhindered phenol and various amounts of phos-

phite (), phosphonite () and phosphine ()processed by multiple extrusions at 260 C andfilm blowing at 195 C.


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Ph.D. Thesis10

oxidative degradation of polyolefins is a heterogeneous local process7,8, and 3)

the effect and reactions of solvents can be excluded. The composition of the

solid reaction products was analyzed by several techniques. The results of theexperiments revealed that the investigated phosphite is thermally instable

above its high melting temperature; it starts to decompose at around 200 C.

Besides the known reactions of hindered aryl phosphites, thermal decomposi-

tion and recombination reactions take also place at 200 C. The phosphite

does not react with molecular oxygen, but its decomposition is accelerated by

oxygen and especially by radicals. Accordingly,the heat-stability of phosphor-ous stabilizers has to be taken also into account in their application, as it is one

of the factors which influence the processing stabilization of polyolefins.

4. New scientific results

1. Based on the results of long term soaking experiments we proved that all

reactions taking place in neat polyethylene under these conditions are re-

lated to each other. In spite of the enormous number of reactions proposed

in the literature we proved that only a few dominating ones determine

changes in the chain structure of the polymer. Close correlation exists be-

tween this latter and the properties of the polymer. [1]

2. We developed infrared spectroscopic procedures for the quantitative de-

termination of the consumption of phosphorous secondary stabilizers dur-

ing the processing of polyethylene. [2,3]

7Knight, J.B., Calvert, P.D., Billingham, N.C., Polymer, 26,1713-8 (1985).8Celina, M., George, G.A., Billingham, N.C., Polym. Degrad. Stab., 42, 335-44 (1993).


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11 Ph.D. Thesis

3. We proved that in spite of numerous references claiming the same stabili-

zation mechanism for all phosphorous antioxidants, under the conditions

of polyethylene processing, the effect, efficiency and mechanism of phos-phite, phosphonite and phosphine stabilizers depend on their structure and

differ significantly form each other. [2-4]

4. We showed the first time that phenolic antioxidants do not hinder the

degradation of polyethylene in its first processing operation, but phos-

phorous stabilizers do. The efficiency of these latter depends on their

chemical structure. [3]

5. We found that the reaction mechanism of phenolic antioxidant/phos-

phorous stabilizer packages used extensively in practice depends strongly

on the structure of the phosphorous compound. The phenol does not

change the reaction mechanism of phosphonite and phosphine stabilizers,

but interacts with phosphites. The efficiency of this latter combination isthe smallest of the three. [3,4]

6. We proved that a critical concentration of the phosphorous stabilizer de-

termines the stability of polyethylene and not that of the phenolic antioxi-

dant, as proposed in the literature. Below the critical level the mechanism

of stabilization changes and the reactions of the phenolic antioxidant de-termine property changes. However, this antioxidant cannot protect the

polymer as efficiently as phosphorous stabilizer or phenol/phosphor com-

binations. [4]

7. Through the analysis of the results of well defined model experiments we

showed that the relatively limited efficiency of the phosphite secondary

stabilizer studied results from its instability at the processing temperature


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Ph.D. Thesis12

of polyethylene. Besides the reaction with hydroperoxides, the compound

undergoes fragmentation that is accelerated by oxygen and radicals, and

participates in various recombination reactions. [5]

5. Application of the new scientific results in practice

The research was carried out in cooperation with Clariant, an additive pro-

ducer, and Tisza Chemical Work, producing polyolefin polymers. Therefore

the new scientific results are important not only from the theoretical point of

view, but they can be and are used directly in the development of new polyole-

fin products for specific purposes.


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13 Ph.D. Thesis

6. Publications

6.1. The thesis is based on the following papers

1. Kriston, I., Fldes, E., Staniek, P., Puknszky, B.: Dominating Reactionsin the Degradation of HDPE during Long Term Ageing in Water, Polym.

Degrad. Stab., 93, 1715-1722 (2008).

2. Fldes, E., Maloschik, E., Kriston, I., Staniek, P., Puknszky B.: Efficien-cy and mechanism of phosphorous antioxidants in Phillips type PE,Polym. Degrad. Stab., 91, 479-487 (2006).

3. Kriston, I., Orbn-Mester, ., Nagy, G., Staniek, P., Fldes, E.,Puknszky B.: Melt stabilisation of Phillips type polyethylene, Part I: Therole of phenolic and phosphorous antioxidants, Polym. Degrad. Stab., 94,719-729 (2009).

4. Kriston, I., Orbn-Mester, ., Nagy, G., Staniek, P., Fldes, E.,Puknszky B.: Melt stabilisation of Phillips type polyethylene, Part II:Correlation between additive consumption and polymer properties, Polym.

Degrad. Stab., 94, 1448-1456 (2009).

5. Kriston, I., Pnzes, G., Szijjrt, G., Szab, P., Staniek, P., Fldes, E.,Puknszky, B.: Study of the high temperature reactions of a hindered arylphosphite (Hostanox PAR 24) used as processing stabiliser in polyolefins,Polym. Degrad. Stab., 95, 1883-1893 (2010).

6. Kriston, I., Fldes, E., Puknszky, B.: sszefggs a polietiln feldol-gozsa sorn bekvetkezstabiliztor fogys s a polimer tulajdonsgai-

nak vltozsa kztt,M

anyag s Gumi, 44, 33-37 (2007).7. Kriston, I., Pnzes, G., Fldes, E., Puknszky, B.: A polietiln stabi-

lizlsa,Manyag s Gumiipari vknyv, 6, 25-34 (2008).

6.2. Conference presentations

1. Kriston, I., Fldes, E.: Stabiliztorok reakcimechanizmusnak ta-

nulmnyozsa Phillips tpus polietilnben, Budapesti M

szaki s Gaz-dasgtudomnyi Egyetem, Ipari Nylt Nap, Budapest, February 28, 2006.(lecture)


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Ph.D. Thesis14

2. Kriston, I., Fldes, E.: Stabiliztorok reakcimechanizmusnak ta-nulmnyozsa Phillips tpus polietilnben, XI. Fiatal Mszakiak Tu-domnyos lsszaka, Kolozsvr, March 24-25, 2006. (lecture)

3. Kriston, I., Fldes, E., Puknszky, B.: sszefggs a PE feldolgozsasorn bekvetkez tulajdonsgvltozsok s a stabiliztorok fogysakztt, MTA Anyagtudomnyi s Technolgiai Komplex bizottsga Dok-toranduszok Fruma, Debrecen, April 4, 2006. (lecture)

4. Kriston, I., Simonin, J., Fldes, E., Puknszky, B.: Analysis of Hostanox03 and Hostanox 0310, Clariant Technical Meeting, Rjtkmuzsaj, May25, 2006. (lecture)

5. Kriston, I., Fldes, E., Orbn, ., Nagy, G., Staniek, P., Puknszky, B.:

Two step degradation of polyethylene: the role of antioxidants, MoDeSt2006, San Sebastian, Spain, September 10-14, 2006. (lecture)

6. brnyi, ., Kriston, I., Fldes, E., Orbn, ., Nagy, G., Staniek, P.,Puknszky, B.: Comparison of the rheological characteristics of a Phillipspolyethylene to the properties of films prepared from it, MoDeSt 2006,San Sebastian, Spain, September, 10-14, 2006. (poster)

7. Kriston, I., Fldes, E.: Stabiliztorok hatsmechanizmusnak vizsglataPhillips tpus polietilnben, MTA VII. Tli Iskola, Balatonfred, Febru-

ary 2, 2007. (lecture)8. Kriston, I., Fldes, E.: Polietiln stabilizlsa: primer s szekunder anti-

oxidnsok klcsnhatsa , Olh Gyrgy Doktori Iskola konferencija, Bu-dapest, February 7, 2007. (lecture)

9. Kriston, I., Szijjrt, G., Pnzes, G., Fldes, E., Puknszky, B.: Foszfortar-talm antioxidnsok stabilizlsi mechanizmusnak tanulmnyozsa mod-ellksrletekkel, MTA KK Kutatkzponti Tudomnyos Napok, Budapest,May 23-24, 2007. (lecture)

10. Kriston, I., Szijjrt, G., Pnzes, G., Szab, P., Staniek, P., Fldes, E.,Puknszky B.: Model reactions of an aromatic phosphite antioxidant withoxygen, hydroperoxide and oxygen centred radicals, 27th Meeting of Po-lymer Degradation Discussion Group, Aston University, Birmingham,UK, September 5-7, 2007. (poster)

11. Kriston, I., Szijjrt, G., Pnzes, G., Szab, P., Fldes, E., Puknszky, B.:Foszfit tpus antioxidns stabilizlsi mechanizmusnak tanulmnyozsa,MTA AKI szeminrium, Budapest, October 9, 2007. (lecure)

12. Kriston, I., Szijjrt, G., Pnzes, G., Szab, P., Fldes, E., Puknszky, B.:Foszfit tpus szekunder antioxidnsok stabilizlsi mechanizmusnak ta-


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15 Ph.D. Thesis

nulmnyozsa, Fiatal Analitikusok Eladlse, MKE Analitikai Sza-kosztlya, Budapest, November 20, 2007. (lecture)

13. Kriston, I., Pnzes, G., Szijjrt, G., Szab, P., Fldes, E., Puknszky, B.:Modellksrletek a foszfit szekunder antioxidns reakciinak meg-hatrozsra, Olh Gyrgy Doktori Iskola konferencija, Budapest, Feb-ruary 8, 2008. (poster)

14. Kriston, I., Pnzes, G., Szijjrt, G., Szab, P., Fldes, E., Puknszky, B.:Foszfortartalm szekunder antioxidnsok stabilizlsi mechanizmusnaktanulmnyozsa, MTA Doktori Iskola, Mtrafred, April 21-22, 2008.(lecture)

15. Kriston, I., Pnzes, G., Szijjrt, G., Szab, P., Fldes, E., Puknszky, B.:Comparison of the reaction of phosphite, phosphonite and phosphine po-lymer stabilizers by model experiments, Meeting of the International Ad-visory Board of CRC, Budapest, May 22, 2008. (lecture)

16. Kriston, I., Szab, P. Pnzes, G., Szijjrt, G., Fldes, E., Puknszky, B.:Study of the stabilization mechanism of phosphorous antioxidants bymodel experiments, MoDeSt 2008, Lige, Belgium, September 7-11,2008. (lecture)

17. Kriston, I., Szab, P. Pnzes, G., Szijjrt, G., Fldes, E., Puknszky, B.:

Identification of the reaction products of phosphorous antioxidants byHPLC-MS and FT-IR, MoDeSt 2008, Lige, Belgium, September 7-11,2008. (poster)

18. Kriston, I., Pnzes, G., Szab, P., Fldes, E., Puknszky, B.: Foszforan-tioxidnsok stabilizlsi mechanizmusnak tanulmnyozsa, MTA KutatiFrum, Budapest, October 30, 2008. (lecture)

19. Kriston, I., Szab, P. Pnzes, G., Szijjrt, G., Fldes, E., Puknszky, B.:Identification of the reaction products of phosphorous antioxidants by

HPLC-MS and FT-IR, Olh Gyrgy Doktori Iskola konferencija, Budap-est, February 8, 2009. (poster)

20. Ttraaljai, D., Vmos, M., Kriston, I., Kovcs, J., Staniek, P., Fldes, E.,Puknszky, B.: Effect of additive combinations on the processing and hy-drolytic stability of polyethylene pipes, 28th Meeting of Polymer Degra-dation Discussion Group, Sestri Levante, Italy, September 6-10, 2009.(lecture)

21. Ttraaljai, D., Vmos, M., Kriston, I., Kovcs, J., Staniek, P., Fldes, E.,

Puknszky, B.: Analysis of the efficiency of additive packages in the sta-


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Ph.D. Thesis16

bility of polyethylene pipes, 4th European Weathering Symposium, Bu-dapest, Hungary, September 16-18, 2009. (poster)

22. Kriston, I., Pnzes, G., Szab, P., Fldes, E., Puknszky, B.: Gtolt

aroms foszfit antioxidns stabilizlsi mechanizmusnak feltrsa modellksrletekkel FT-IR s HPLC-MS technikval, Fiatal AnalitikusokEladlse, MKE Analitikai Szakosztlya, Budapest, February 25, 2010.(lecture)

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