interlab test on polymers determination of antioxidants in polyolefins

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Interlaboratory Tests

Interlaboratory test on polymers: determinationof antioxidants in polyolefins

Axel Rittera,*, Elisabeth Michela, Manfred Schmida, Samuel Affolterb

aEMPA (Swiss Federal Laboratories for Materials Testing and Research), Lerchenfeldstrasse 5, CH-9014 St Gallen, Switzerland b Interstaatliche Hochschule fu r Technik Buchs NTB, Werdenbergstr. 4 CH-9470 Buchs, Switzerland

Received 6 October 2004; accepted 23 November 2004

Abstract

This report presents results obtained from two interlaboratory tests performed in 2000 and 2002. Up to 13 participants,

mainly from industry and research institutes analysed two different antioxidants in four polymer matrices. The measured data

were collected by EMPA and evaluated using a robust statistical method. Repeatability (r ) and reproducibility data ( R) were of

special interest, as well as the combined uncertainty of measurements (uc).

Samples of non-stabilised polyolefins were homogeneously doped with accurate well-known quantities of Irgaphos 168 and

Irganox 1010. Since no generally valid and universal standard methods for the determination of antioxidants in plastics exist,

the interlaboratory test participants were permitted to select an analysis technique of their choice. Almost all laboratories

determined the antioxidant contents with HPLC after different pre-treatments. As a result, the most precise HPLC methods

could be identified and will be described in this article.

The relative repeatability of the determinations was between 1.3 and 5.5%, and the relative reproducibility was in-between 12

and 28% for both antioxidants. No matrix or analyte dependence was observed. The combined calculated uncertainty of

measurement (uc) in all laboratories was between 36 and 86%. Therefore analysts in different laboratories have to expect largevariations in results when comparing the results of antioxidant analysis on the same polymer.

q 2005 Elsevier Ltd. All rights reserved.

Keywords : Interlaboratory test; Antioxidant analysis; Robust statistical method; Repeatability; Reproducibility; Combined uncertainty;

Irgaphos 168; Irganox 1010

1. Interlaboratory tests

The Swiss Federal Laboratories for Materials Testing and

Research (EMPA) organises interlaboratory tests on poly-

meric materials on a biennial basis. The participants are

usually industrial or private laboratories and laboratories atinstitute that test, research and develop polymeric materials.

The present interlaboratory tests tookplace in 2000and 2002.

ISO 5725 (1994) describes the procedures for inter-

laboratory tests. The present interlaboratory tests were

statistically analysed using the robust statistical method

described by Lischer [1].

The exact procedure used for the evaluation of our

Interlaboratory Tests and a detailed description of the

statistical parameters is described elsewhere [2]. Some

important statistical expressions for a common comprehen-sion of the applied statistical parameters are listed in Table 1.

2. Antioxidants in polyolefins

2.1. Introduction

Non-stabilised polymers usually undergo reactions with

oxygen causing degradation and aging readily. Such

oxidation reactions negatively influence the properties of

Polymer Testing 24 (2005) 498–506

www.elsevier.com/locate/polytest

0142-9418/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymertesting.2004.11.012

* Corresponding author. Tel.: C41 71 274 7782; fax: C41 71 274

7788.

E-mail address: [email protected] (A. Ritter).

http://www.elsevier.com/locate/polytest

http://www.elsevier.com/locate/polytest



polymers even at very low conversion rates. The oxidative

sensitivity of polyolefins, e.g. polyethylene, is apparent at

room temperature. The use of suitable stabilizers (anti-

oxidants) is one possible solution to overcome this problem.

Antioxidants are used in almost all commercial polymers in

small amounts up to 2%, w/w (20,000 mg/kg or ppm). They

should provide efficient protection of the polymer during

processing and end use for the given environmentalexposure conditions.

Fig.1 shows thegeneral pathway (circle) of the process of

polymer autoxidation and the mode of action of antioxidants

[3]. The polymer RH decays under the influence of shear

strain (t), temperature (D), light (hn) or traces ofredox active

metals (MnC) into free radicals. These radicals can be

disposed of by antioxidants. Antioxidants can be divided into

two different groups: radical scavengers and hydro peroxide

decomposers. The type and quantity of an antioxidant or a

combination of different antioxidants used in a polymer

compound depend on the intended application and is a

determining factor for the long-term behaviour of a polymer.Therefore, one important question of all polymer

producers is: ‘how much antioxidant is in my polymer’?

This is a question, which should be answered by analysts

with accuracy. An overview of tools and techniques

available for analyzing additives is provided and summar-

ized by Zweifel [3].

The intention of this laboratory test was, beside an exact

statistic evaluation, to compare the different analytical

methods used for quantification of antioxidants, because

many different determination and sample preparation

methods are commonly used.

2.2. Preparation of the samples

Due to the fact, that no polymeric reference materials

with known antioxidant content are commercially available,

EMPA decided to prepare some special samples for this

interlaboratory test in-house, in order to be sure about the

exact additive content.

Thus, two samples (samples 1 and 2) based on

polypropylene (PP) were produced in laboratory scale by

compounding non-stabilised PP with known amounts of

additives by extrusion. The resulting polymer was cut into

pieces, powdered in a mill under cooling with liquid

nitrogen and sieved with a 1 mm wide meshed sieve, inorder to produce homogeneous samples.

The second two samples (samples 3 and 4) based on of

PP and PE-LLD were produced by Ciba Specialty

Chemicals (Basel). The amount of the added additives was

disclosed to EMPA.

Some details of sample compositions are described in

Table 2.

The homogeneity of the doped polymer samples was

controlled by EMPA with HPLC before mailing and

delivering the samples to the participants of the Inter-

laboratory Test.

2.3. Used antioxidants

The antioxidants intended for identification in the

Interlaboratory Test was a radical scavenger (Irganox

1010 as proton donor) and a hydro peroxide decomposer

(Irgaphos 168 as a oxidizable phosphite). These two

Table 1

Statistical expressions

sr Standard deviation of repeatability absolute

sr,relative Standard deviation of repeatability relative

sR Standard deviation of reproducibility absolute

sR,relative Standard deviation of reproducibility relative

uc Combined uncertainty of measurementResult with

99.7% confidence

interval

ucZ3sR

Fig. 1. General scheme of action of antioxidants.

A. Ritter et al. / Polymer Testing 24 (2005) 498–506 499



antioxidants were chosen due to their widespread usage in

many recipes for the production of commercial polymers.

Chemical details of both antioxidants are described in

Table 3.

2.4. Determination

2.4.1. Available methods

There are only a few normative standards governing the

chemical analysis of antioxidants in polymers (see Table 4).

There are many possible analytical methods currently in use,

briefly described in Table 5, which can be separated into

direct and indirect methods. For the indirect methods

suitable sample preparation to dissolve the antioxidants

homogeneously precedes the analytical determination.

Extraction according to Soxhlet, dissolving/reprecipitation

and swelling of a polymer are widespread methods for

sample pretreatment. The most important determinations

are based on chromatography, mass spectrometry andspectroscopy methods, whereby GC and HPLC are the

most commonly used analytical methods.

2.4.2. Used methods

In most cases the participants applied HPLC for the

analysis in principle, but with great variations in the

analysis parameters. Gas chromatography, photometry and

X-ray fluorescence were only rarely used as described in

Table 6.

Table 3

Description of the antioxidants

Irganox 1010 Irganox 1076 Irgaphos 168

Synonyms Pentaerythritol

tetrakis(3-(30,50-di-tert-

butyl-40-hydroxyphenyl))

Octadecyl-3-(3,5-di-tert.butyl-4-hydroxy-

phenyl)-propionate

Phenol, 2,4-bis(1,1-

dimethyl-ethyl)-, phosphite

(3:1)

Benzenepropanoic acid, 3,5-bis

(1,1-dimethylethyl)-4-hydroxy-,

2,2-bis[[3-[3,5-bis(1,1-dimethy-

lethyl)-4-hy-droxyphenyl]-1-oxo-

Propoxy] methyl]-1,3-propane-

diylester

Tris(2,4-di-tert-butyl-

phenyl) Phosphite

CAS-number [6683-19-8] [2082-79-3] [31570-04-4]

Molecularweight (g/mol) 1177.6 531.0 646.9

Formula C73H108O12 C35H62O3 C42H63O3P

Structure

Table 2

Description of the samples, number of participants per sample

Sample Polymer State Added additives Participants

1 PP, nature Powder 500 mg/kg Irganox 1010 6

2000 mg/kg Irgaphos 168 9

2 PP, nature Powder 1500 mg/kg Irganox 1010 6

500 mg/kg Irgaphos 168 9

3 PP, nature Granulate 800 mg/kg Irgaphos 168 13

400 mg/kg Irganox 1010 13

110 mg/kg Irganox 1076

(not to be determined)

4 PE-LLD, nature Granulate 520 mg/kg Irgaphos 168 13

170 mg/kg Irganox 1010 13




HPLC is one of the indirect methods. As sample

preparation is needed for HPLC analysis; most participants

performed a process starting with dissolving the sample

followed by reprecipitation of the polymer. Extraction by

Soxhlet was also used in some cases. Microwave and

accelerated solvent extraction and stirring in a solvent were

used once.

2.5. Results

2.5.1. Determination of Irganox 1010

Fig. 2 shows a representation for all polyolefins

investigated, including the results of the Irganox 1010

determination in sample 3. This type of comparison sheet is

one of the possible results of the interlaboratory tests and is

revealed to the participants. From such plots, the

participants can estimate their position in the whole field

of results and assess their reliability (e.g. finding a

systematic error during the measurement). This figure

shows the median values of all participants, the theoretical

gravimetric value of Irganox 1010 in sample 3 and the

repeatability and reproducibility standard deviation for this

determination.

The results of the determination of Irganox 1010 in all

samples are shown in Table 7 in detail. Absolute and relative

statistical terms of repeatability and reproducibility (sr, sR,sr,relative, sR,relative,) are listed. All samples show a lower

median value than theoretically predicted (‘true value’).

This difference, ranging between 2.2 and 28.6%, could most

likely be explained by a partial loss of antioxidants resulting

from the compounding process or the analysis procedure.

The data of sr,relative were between ca. 2.2 and 5.5%, and

therefore the repeatability and precision were considered

acceptable and in the expected range. The standard

deviations of the reproducibility sR,relative were quite large

between ca. 12.3 and 26.6%. This demonstrates the

problems that arise in comparing the results between

different laboratories.

The ratio sR / sr is a measure of the quality of a

determination and should be in a range about 2–3 for good

interlaboratory tests. This is not fulfilled by this determi-

nation with a ratio sR / srZ4.9–9.0 and therefore the results

seem to be a little bit uncertain.

Table 4

Actual standards for determination of antioxidants in polyolefins

Country Standard Title

GB BS 2782-4: Method 434B: 1977 Methods of testing plastics. Chemical properties. Determination of

antioxidants in polyolefin compounds by ultra-violet absorption of the

chloroform extract

GB BS 2782-4: Method 434D: 1975 Methods of testing plastics. Chemical properties. Determination of

antioxidants in polyolefin compounds by a spectrophotometric method

GB BS 6630: 1985 ISO 4645-1984 Method for identification of antidegradants in rubber and rubber products by

thin layer chromatography

GB BS 7164-31.1:1997 ISO 11089:1997 Chemical tests for raw and vulcanized rubber. Determination of anti-

degradants. High-performance liquid chromatography

US ASTM D 5524 Test method for determination of phenolic antioxidants in high density

polyethylene using liquid chromatography

US ASTM D 5815 Test method for determination of phenolic antioxidants and erucamide slip

additives in linear low density polyethylene using liquid chromatography

Table 5

Possible methods for determination of antioxidants in polyolefins

Direct methods

Determination with pyrolysis/gas chromatography/mass spec-

trometry (e.g. using the commercial system of Shimadzu)

Time of flight secondary ion mass spectrometry (TOF-SIMS)

Matrix assisted mass spectrometry (MALDI)

Direct probe insert mass spectrometry (DIP-MS) Indirect methods

Determination with gas chromatography (also high temperature

GC is possible)

Determination with liquid chromatography (HPLC)

Determination with thin layer chromatography (DC)

Determination with spectrometric methods

In case of Irgaphos 168, determination of phosphorus content by

photometry or emission spectrometry

Antecedent one has to carry out a suitable sample preparation in

order to separate the antioxidants from the polymer matrix and to

dissolve them homogeneously. The following methods are

theoretically possible:

Sample preparation for indirect methods

Extraction according to Soxhlet with a suitable solvent (typically

for several hours)

Accelerated solvent extraction (ASE)

Dissolving the polymer and reprecipitation

Hot or cold swelling of polymer and diffusion of additives into

the solvent

Disaggregation of the system with microwaves (determination of

phosphorus)

Disaggregation according to Parr with oxygen (determination of

phosphorus)

The suitability of the indirect methods depends strongly on the

optimization of the extraction methods. Only a high retrieval rate

indicates that the extraction method is trustworthy.




The combined uncertainty of measurement (uc) i n a

laboratory is strongly affected by standard deviation of

reproducibility (sR). Within a 99.7% confidence interval the

combined uncertainty of measurement (uc) is the threefold

reproducibility (sR). At this interpretation the combined

uncertainty was calculated between 36.9 and 79.8%, which

is considered as a rather high value.

2.5.2. Determination of Irgaphos 168

As a representative for all polyolefins investigated, the

results of the Irgaphos 168 determination in sample 4 are

shown in Fig. 3. The median values of all participants, the

theoretical gravimetric value of Irgaphos 168 and the

repeatability and reproducibility standard deviation for this

determination are visualized in such plots. All results of

Table 6

Applied methods by the participants

Analysis method Analyte Sample preparation Used solvent for extraction

GC/MS Irgaphos 168 Soxhlet extraction, 8 h Dichloromethane

GC Irgaphos 168 Soxhlet extraction Dichloromethane/cyclohexane (3:1)

Pyrolysis/GC/MS Irganox 1010, Irgaphos 168 None None

XRF Irgaphos 168 None None

Photometry of phosphate Irgaphos 168 Schoniger digestion None

HPLC Irganox 1010, Irgaphos 168 Solution and precipitation Xylene (S), acetonitrile (P)

HPLC Irganox 1010, Irgaphos 168 Solution and precipitation Cyclohexane/THF/xylene (1:1:1) (S),

ethanol (P)

HPLC Irganox 1010, Irgaphos 168 Solution and precipitation Xylene (S), methanol (P)

HPLC Irganox 1010, Irgaphos 168 Solution and precipitation Toluene (S), methanol (P)

HPLC Irganox 1010 Solution and precipitation p-Xylene (S), acetonitrile (P)

HPLC Irganox 1010, Irgaphos 168 Solution and precipitation Toluene (S), chloroform (P)

HPLC Irganox 1010, Irgaphos 168 Microwave extraction Heptane/acetone

HPLC Irganox 1010 Soxhlet extraction Dichloromethane/cyclohexane (3:1)

HPLC Irganox 1010, Irgaphos 168 Soxhlet extraction, 8, 16 h Dichloromethane

HPLC Irganox 1010, Irgaphos 168 Soxhlet extraction, 1.5 h Ethylacetate

HPLC Irganox 1010, Irgaphos 168 Soxhlet extraction, 2,18, 48 h ChloroformHPLC Irganox 1010 Soxhlet extraction, 6 h Acetone

HPLC Irganox 1010, Irgaphos 168 24 h, 40 8C by stirring Isooctane

HPLC Irganox 1010, Irgaphos 168 ASE Unknown

S, solution; P, precipitation; ASE, accelerated solvent extraction; XRF, X-ray fluorescence analysis.

Fig. 2. Content of Irganox 1010 in mg/kg in sample 3.




the statistical evaluation of the measurements of Irgaphos

168 are shown in detail in Table 8, where all absolute andrelative statistical terms of repeatability and reproducibility

(sr, sR) are listed. All Irgaphos 168 samples show similar

results as the aforementioned value for Irganox 1010,

having a similar difference between median values

measured and the theoretically predicted, ‘true’ value. The

discrepancies were between 7.0 and 25.9%. The data of

sr,relative were about 1.3–4.05%, so the repeatability and

precision were again considered acceptable as they were in

the expected range. The standard deviations of the

reproducibility sR,relative were quite large, between 12.1

and 28.6%. Also the comparison of results amongst different

laboratories seems to be critical again, as it was with Irganox

1010.The ratio sR / sr is about 5.3–21 and therefore very high,

which leads to the assumption that the results are not very

reliable.

Furthermore, the combined uncertainty of measurement

(uc) was calculated between 36.3 and 85.8%, which are also

rather high values.

2.5.3. Comparison of the results

The most important results are listed in Table 9. For both

Irganox 1010 and Irgaphos 168, considerable differences

Table 7

Data of interlaboratory test, determination of Irganox 1010

Parameter Sample 1 Sample 2 Sample 3 Sample 4

Type of polymer PP PP PP LLDPE

True value (mg/kg) 500 1500 400 170

Content av. (median) (mg/kg) 357 1253 371.3 166.2

Relative underestimate (%) K28.6 K16.5 K7.2 K2.2

sr (repeatability) (mg/kg) 19.5 29.8 8.3 5.19

sr,relative (%) 5.46 2.37 2.2 3.1

sR (reproducibility) (mg/kg) 95 154.6 74.7 38.2

sR,relative (%) 26.6 12.3 20.1 23.0

sR / sr 4.9 5.2 9.0 7.5

uc (99.7% confidence interval)Z3sR (mg/kg) 285 464 224 115

uc (99.7% c.i., relative) (%) 79.8 36.9 60.3 69

No. of labs 6 6 12 12

Fig. 3. Content of Irgaphos 168 in mg/kg in sample 4.




between measured and ‘true’ contents were detected. The

reasons are as follows:

1. During the compounding process parts of the anti-

oxidants can degrade by thermal stress or will be

vaporized. The chemical compounds are sensitive to air

at elevated temperature and react, e.g. by cleavage, to

undefined oxidised end products.

2. During the analysis procedure, especially the extraction

and clean-up steps, parts of antioxidants can be lost or

remain in the matrix. These parts of the analyte cannot be

detected by the analysis procedure.

The standard deviation of the reproducibility sR between

the analytes and samples are displayed in Figs. 4 and 5. .

There is no significant trend observable with these data; thereproducibility seems to be neither depending on the kind of

analyte nor on the kind of polymer matrix. All values were

found to be in a range between 12 and 28%. Therefore, the

accuracy of determination of antioxidants in polymers lies

in this range.

The ratios sR / sr of different kinds of interlaboratory tests

are compared in Table 10. Ratios sR / sr!3 are a performance

figure of a reliable interlaboratory test, but often

the numbers are greater. In this test the range of ratios was

between 5 and 20, so the comparability between different

laboratories is not very good.The combined uncertainty of measurement of this

interlaboratory test is very large as compared to other

analytical methods. The reason is most likely the complexity

of the determination, the extensive sample preparation and

the analysis of potentially unstable compounds.

2.6. Conclusions

The present report indicates what repeatability and

reproducibility standard deviations must be taken into

consideration when the amount of antioxidants are deter-

mined in plastic matrices. An overview of all the differentanalytical methods applied by the participants is summar-

ized in Table 11, illustrating that only two methods gave

reliable results.

Methods of pyrolysis, photometry, XRF and GC/MS

were proven to be uncertain methods. The quantitative

results were either too high or to low and so these methods

cannot be recommended. HPLC was the method of choice

when the pre-treatment was optimised, which is very

important to get reliable results.

Table 8

Data of interlaboratory test, determination of Irgaphos 168

Parameter Sample 1 Sample 2 Sample 3 Sample 4

Type of polymer PP PP PP PE-LLD

True value (mg/kg) 2000 500 800 520

Content av. (median) (mg/kg) 1482 401 743.9 441.3

Relative underestimate (%) K25.9 K19.8 K7.0 K15.1

sr (repeatability) (mg/kg) 33.7 16.2 10.7 5.9

sr,relative (%) 2.27 4.05 1.4 1.3

sR (reproducibility) (mg/kg) 179.1 108.8 106.1 126.3

sR,relative (%) 12.1 27.1 14.3 28.6

sR / sr 5.3 6.7 10 21

uc (99.7% confidence interval)Z3sR (mg/kg) 537 326 318 379

uc (99.7% c.i., relative) (%) 36.3 81.3 42.9 85.8

No. of labs 9 9 13 13

Table 9

Comparison of the results

Irganox1010

Irgaphos168

Comment

Relative under-

estimate of

determinations

2–29% 7–26% Probably lost during

the take-up to the

polyolefin or the pro-

cedure of analysis

Relative com-

bined uncer-

tainty of

measurement

37–80% 36–86% Large uncertainty for

an analytical method

sR / sr 5–9 5–20 High value, low

comparison between

laboratories Fig. 4. Comparison of sR,relative between the analytes.




HPLC analysis after microwave extraction was the most

successful method when a mixture of heptanes and acetone

was used for extraction. The second method, also a HPLC

method, is preceded by another sample preparation. The

polymer was dissolved in an aromatic solvent like xylene

and then precipitated by methanol under intensive stirring.

This method is recommended and is easy to execute.

2.7. The Horwitz graph

In 1980, Horwitz first described the relation between theconcentration of an analyte and the standard deviation of

the reproducibility of interlaboratory tests [4]. By plotting

the relative standard deviation of reproducibility versus the

concentration of the analyte in a x, y-plot (graph see Fig. 6),

Horwitz assessed, that the value of reproducibility is almost

doubled when the concentration is decreased a hundred

times. Today an interlaboratory test is regarded as valid

when the relative standard deviation of the reproducibility is

in-between 0.5 and 2 times the predicted value [5].

Fig. 6 shows the typical coherence between concen-

tration of the analyte and the reproducibility sr (thick line),

as well as the results of this interlaboratory test with the

regression curve (dotted line). Important analytical methods,

like doping or food analysis, usually obey the common

Horwitz curve. For the antioxidants analysis the law does

not apply because the values of sR are three times higher

than the Horwitz graph. The reason is that polymer analysis

is more imprecise due to the complex polymer matrix,

Table 10

Ratio sR / sr in interlaboratory tests

Literature Analytes Matrix Number of inde-

pendent tests

Range sR / sr Average sR / sr

Interlaboratory test of determination

of metals in stabilised water, internal

report EMPA, Nr. 840’203, 1999

8 heavy metals Water 24 0.9–6.5 2.7

Interlaboratory test: analysis of wood

and wood ash, internal report EMPA,

Nr. 46’011, 1998

24 heavy metals Ash of

wood

24 1.7–8.9 5.4

Interlaboratory test: eluate test of

concrete, internal report EMPA, Nr.

46’003, 1996

8 heavy metals and

2 anions

Water 214 2.5–77 18.7

Analytical quality assurance, interla-

boratory test Nr. 8, heavy metals,

Bayerisches Landesamt fur Wasser-

wirtschaft, 1997

24 heavy metals Water 24 1.9–4.3 3.0

Antioxidants in polyolefins 2 antioxidants in 4

samples

Polyolefins 8 5–20 8.7

Fig. 5. Comparison of sR,relative between the samples.

Table 11

Assessment of the analytical methods

Method Irganox 1010 Irgaphos 168

Pyrolysis/GC/MS Too low and too

high values change;

the method is

uncertain

Too low and too

high values change;

the method is

uncertain

Photometry – Too low values

XRF – Too high values

GC/MS after Soxh-

let extraction

– Too low and too

high values change;

the method is

uncertain

HPLC after micro-

wave extraction

Reliable values

when heptane/

acetone used for

extraction

Reliable values

when heptane/

acetone used for

extraction

HPLC after accel-

erated solvent

extraction

Too low values Too low values

HPLC after sol-

ution and precipi-

tation

Reliable values by

using aromatic sol-

vents and methanol

Reliable values by

using aromatic sol-

vents and methanol

HPLC after Soxhlet

extraction

Too low values Too low values




which is inhomogeneous and difficult to prepare. Therefore,

when comparing different laboratories one must take into

consideration that the combined uncertainty of measure-

ment is greater than that of many common analytical

techniques.

Acknowledgements

The authors wish to thank Dr M. Weber who handled the

statistical software and evaluated the statistical data and

Ciba Specialities, Basel, for the preparation of some

samples. We would also like to thank all our colleagues at

the Institute who made the interlaboratory tests possible andthe companies and institutes that supported the project

financially or supplying test materials free of charge.

References

[1] P. Lischer, Robust statistical methods in interlaboratory

analytical studies in: H. Rieder (Ed.), Robust Statistics, Data

Analysis, and Computer Intensive Methods, Lecture Notes in

Statistics (109), Springer, New York, 1996, pp. 251–265.

[2] A. Ritter, E. Michel, M. Schmid, S. Affolter, Polym. Test. 23/4

(2004) 467–474.[3] H. Zweifel (Ed.), Plastics Additives Handbook, Hanser,

Munich, 2000, ISBN: 1-56990-295-X.

[4] W. Horwitz, L.R. Kamps, K.W. Boyer, J. AOAC 63 (1980)

1344–1354.

[5] M. Thompson, P.J. Lowthian, J. AOAC Int. 80 (1997) 676–680.

Fig. 6. Horwitz curve.


interlab test on polymers determination of antioxidants in polyolefins

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