Bio-based products - Bio-based carbon content - Determination of the bio-based carbon content using the radiocarbon method

This European Standard specifies a method for the determination of the bio-based carbon content in products, based on the 14C content measurement.
This European Standard also specifies three test methods to be used for the determination of the 14C content from which the bio-based carbon content is calculated:
-   Method A: Liquid scintillation-counter method (LSC) (normative);
-   Method B: Beta-ionization (BI) (informative);
-   Method C: Accelerator mass spectrometry (AMS) (normative).
The bio-based carbon content is expressed by a fraction of sample mass or as a fraction of the total carbon content. This calculation method is applicable to any product containing carbon, including bio composites.
NOTE   This European standard does not provide the methodology for the calculation of the biomass content of a sample see prEN 16785-1 [5] and prEN 16785-2 [6].

Biobasierte Produkte - Gehalt an biobasiertem Kohlenstoff - Bestimmung des Gehalts an biobasiertem Kohlenstoff mittels Radiokarbonmethode

Diese Europäische Norm legt ein Verfahren zur Bestimmung des Gehalts an biobasiertem Kohlenstoff in Produkten auf der Grundlage der Messung des 14C Gehalts fest.
Es werden auch drei Prüfverfahren festgelegt, die zur Bestimmung des 14C Gehalts anzuwenden sind, aus dem der biobasierte Kohlenstoffgehalt berechnet wird:
-   Verfahren A: Flüssigszintillationszählverfahren (LSC) (normativ);
-   Verfahren B: Beta Ionisation (BI) (informativ);
-   Verfahren C: Beschleuniger Massenspektrometrie (AMS) (normativ).
Der biobasierte Kohlenstoffgehalt wird als Anteil der Masse der Probe angegeben oder als Anteil des gesamten Kohlenstoffgehalts. Dieses Berechnungsverfahren ist auf alle kohlenstoffhaltigen Produkte anwendbar, einschließlich Bioverbundwerkstoffen.
ANMERKUNG   Diese Europäische Norm bietet kein Verfahren für die Berechnung des Biomassegehalts einer Probe; diesbezüglich siehe prEN 16785 1 [5] und prEN 16785 2 [6].

Produits biosourcés - Teneur en carbone biosourcé - Détermination de la teneur en carbone biosourcé par la méthode au radiocarbone

La présente Norme européenne spécifie une méthode permettant de déterminer la teneur en carbone biosourcé dans des produits à partir du mesurage de la teneur en 14C.
Elle spécifie également trois méthodes d'essai à utiliser pour déterminer la teneur en 14C, à partir de laquelle la teneur en carbone biosourcé est calculée :
-   Méthode A : compteur à scintillation liquide (CSL) (normative) ;
-   Méthode B : ionisation bêta (IB) (informative) ;
-   Méthode C : spectrométrie de masse par accélérateur (SMA) (normative).
La teneur en carbone biosourcé est exprimée en fraction de masse d'échantillon ou en fraction de la teneur en carbone total. Cette méthode de calcul est applicable à tout produit contenant du carbone, y compris les biocomposites.
NOTE   La présente Norme européenne ne fournit pas la méthodologie permettant de calculer la teneur en biomasse d'un échantillon ; voir le prEN 16785-1 [5] et le prEN 16785-2 [6].

Bioizdelki - Delež bioogljika - Ugotavljanje deleža bioogljika z radioogljično metodo

Ta evropski standard določa metodo za ugotavljanje deleža bioogljika v izdelkih na podlagi meritve deleža 14C.
Poleg tega ta evropski standard določa tri preskusne metode za ugotavljanje deleža 14C, na podlagi katerih se izračuna delež bioogljika:
– Metoda A: metoda števca s tekočinskim scintilatorjem (LSC) (normativni);
– Metoda B: beta ionizacija (BI) (informativni);
– Metoda C: pospeševalna masna spektrometrija (AMS) (normativni).
Delež bioogljika se izrazi z deležem mase vzorca ali kot delež skupnega deleža ogljika. Ta metoda izračuna se uporablja za vse izdelke, ki vsebujejo ogljik, vključno z biokompoziti.
OPOMBA: ta evropski standard ne zagotavlja metodologije za izračun deleža biomase vzorca. Glej standard prEN 16785-1 [5] in prEN 16785-2 [6].

General Information

Status
Published
Public Enquiry End Date
29-Oct-2015
Publication Date
15-May-2017
Technical Committee
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
03-May-2017
Due Date
08-Jul-2017
Completion Date
16-May-2017

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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.UDGLRRJOMLþQRPHWRGRBiobasierte Produkte - Gehalt an biobasiertem Kohlenstoff - Bestimmung des Gehalts an biobasiertem Kohlenstoff mittels RadiokarbonmethodeProduits biosourcés - Teneur en carbone biosourcé - Détermination de la teneur en carbone biosourcé par la méthode au radiocarboneBio-based products - Bio-based carbon content - Determination of the bio-based carbon content using the radiocarbon method71.040.40Kemijska analizaChemical analysis13.020.55Biološki izdelkiBiobased productsICS:Ta slovenski standard je istoveten z:EN 16640:2017SIST EN 16640:2017en,fr,de01-junij-2017SIST EN 16640:2017SLOVENSKI

STANDARDSIST-TS CEN/TS 16640:20141DGRPHãþD
SIST EN 16640:2017
EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN 16640
February
t r s y
vEnglish Version

Bioæbased products æ Bioæbased carbon content æ Determination of the bioæbased carbon content using the radiocarbon method Produits biosourcés æ Teneur en carbone biosourcé æ Détermination de la teneur en carbone biosourcé par la méthode au radiocarbone

Biobasierte Produkte æ Gehalt an biobasiertem Kohlenstoff æ Bestimmung des Gehalts an biobasiertem Kohlenstoff mittels Radiokarbonmethode This European Standard was approved by CEN on

v December
t r s xä

egulations which stipulate the conditions for giving this European Standard the status of a national standard without any alterationä Upætoædate lists and bibliographical references concerning such national standards may be obtained on application to the CENæCENELEC Management Centre or to any CEN memberä

translation under the responsibility of a CEN member into its own language and notified to the CENæCENELEC Management Centre has the same status as the official versionsä

CEN members are the national standards bodies of Austriaá Belgiumá Bulgariaá Croatiaá Cyprusá Czech Republicá Denmarká Estoniaá Finlandá Former Yugoslav Republic of Macedoniaá Franceá Germanyá Greeceá Hungaryá Icelandá Irelandá Italyá Latviaá Lithuaniaá Luxembourgá Maltaá Netherlandsá Norwayá Polandá Portugalá Romaniaá Serbiaá Slovakiaá Sloveniaá Spainá Swedená Switzerlandá Turkey and United Kingdomä

EUROPEAN COMMITTEE FOR STANDARDIZATION COMITÉ EUROPÉEN DE NORMALISATION EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre:
Avenue Marnix 17,
B-1000 Brussels

t r s y CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Membersä Refä Noä EN

s x x v rã t r s y ESIST EN 16640:2017

EN 16640:2017 (E) 2 Contents Page European foreword ....................................................................................................................................................... 4 Introduction .................................................................................................................................................................... 5 1 Scope .................................................................................................................................................................... 6 2 Normative references .................................................................................................................................... 6 3 Terms and definitions ................................................................................................................................... 6 4 Symbols and abbreviations ......................................................................................................................... 7 5 Principle ............................................................................................................................................................. 8 Table 1 — Advantages and disadvantages of the methods ............................................................................ 8 6 Determination of the 14C content ............................................................................................................... 9 6.1 General ................................................................................................................................................................ 9 6.2 Principle ............................................................................................................................................................. 9 6.3 Sampling ............................................................................................................................................................. 9 6.4 Procedure for the conversion of the carbon present in the sample to a suitable sample for 14C determination ..................................................................................................................... 9 6.5 Measurements .................................................................................................................................................. 9 7 Calculation of the bio-based carbon content ...................................................................................... 10 7.1 General ............................................................................................................................................................. 10 7.2 Reference value for 100 % bio-based carbon .................................................................................... 10 Table 2 — Typical values for biomass fractions .............................................................................................. 11 7.3 Calculation method ...................................................................................................................................... 11 7.3.1 Calculation of the bio-based carbon content by dry mass xB ........................................................ 11 7.3.2 Calculation of the bio-based carbon content xTCB as a fraction of TC ......................................... 12 7.3.3 Examples ......................................................................................................................................................... 12 7.3.4 Examples of calculations xTCB ................................................................................................................... 13 8 Performance characteristics .................................................................................................................... 13 9 Test report ...................................................................................................................................................... 13 Annex A (informative)

Procedures for sampling of products ................................................................... 15 A.1 General ............................................................................................................................................................. 15 Table A.1. — Sampling procedures ...................................................................................................................... 15 Annex B (normative)

Procedure for the conversion of the carbon present in the sample to a suitable sample for 14C determination ................................................................................................. 16 B.1 General ............................................................................................................................................................. 16 B.2 Sample preparation ..................................................................................................................................... 16 Table B.1— Sample preparations ......................................................................................................................... 16 B.3 Preparation for 14C measurement .......................................................................................................... 16 B.3.1 General ............................................................................................................................................................. 16 B.3.2 Reagents and materials .............................................................................................................................. 17 B.4 Combustion of the sample ......................................................................................................................... 17 SIST EN 16640:2017

EN 16640:2017 (E) 3 B.4.1 Combustion of the sample in a calorimetric bomb ........................................................................... 17 B.4.2 Combustion of the sample in a tube furnace or a combustion apparatus ................................ 18 B.4.3 Direct LSC measurement on the product ............................................................................................. 19 B.5 Standardization of LSC and BI measurement results ...................................................................... 19 B.6 Standardization of AMS measurement results ................................................................................... 20 B.7 Symbols and abbreviations ....................................................................................................................... 20 Annex C (normative)

Method A - Liquid scintillation-counter method (LSC) ...................................... 21 C.1 General ............................................................................................................................................................. 21 C.2 Principle ........................................................................................................................................................... 21 C.3 Reagents and materials .............................................................................................................................. 21 C.4 Apparatus ........................................................................................................................................................ 21 C.5 Procedure ........................................................................................................................................................ 22 C.5.1 General ............................................................................................................................................................. 22 C.5.2 Benzene conversion ..................................................................................................................................... 22 C.5.3 Direct absorption of the CO2 in a carbamate solution ..................................................................... 22 C.5.4 Measurement ................................................................................................................................................. 22 C.5.5 Blank correction ........................................................................................................................................... 23 C.6 Calculation of the results ........................................................................................................................... 23 Annex D (informative)

Method C - Beta-ionization (BI) .............................................................................. 24 D.1 General ............................................................................................................................................................. 24 D.2 Principle ........................................................................................................................................................... 24 D.3 Reagents and materials .............................................................................................................................. 24 D.4 Apparatus ........................................................................................................................................................ 25 D.5 Procedure ........................................................................................................................................................ 25 D.6 Calculation of the results ........................................................................................................................... 26 Annex E (normative)

Method B - Accelerator Mass Spectrometry (AMS) ............................................. 27 E.1 General ............................................................................................................................................................. 27 E.2 Principle ........................................................................................................................................................... 25 E.3 Reagents and materials .............................................................................................................................. 27 E.4 Apparatus ........................................................................................................................................................ 27 E.5 Procedure ........................................................................................................................................................ 27 E.6 Calculation of the results ........................................................................................................................... 28 Annex F (informative)

Performance characteristics .................................................................................... 29 Table F.1 — description of sample types ............................................................................................................ 29 Table F.2 — Performance data for 14C methods ............................................................................................. 30 Bibliography ................................................................................................................................................................. 31 SIST EN 16640:2017

EN 16640:2017 (E) 4 European foreword This document (EN 16640:2017) has been prepared by Technical Committee CEN/TC 411 “Bio-based products”, the secretariat of which is held by NEN. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by August 2017, and conflicting national standards shall be withdrawn at the latest by August 2017. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN shall not be held responsible for identifying any or all such patent rights. This document supersedes CEN/TS 16640:2014. This document has been prepared under Mandate M/492 “Mandate addressed to CEN, CENELEC and ETSI for the development of horizontal European Standards and other standardization deliverables for bio-based products”. According to the CEN-CENELEC Internal Regulations, the national standards organisations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom. SIST EN 16640:2017

EN 16640:2017 (E) 5 Introduction Bio-based products from forestry and agriculture have a long history of application, such as paper, board and various chemicals and materials. The last decades have seen the emergence of new bio-based products in the market. Some of the reasons for the increased interest lie in the bio-based products’ benefits in relation to the depletion of fossil resources and climate change. Bio-based products may also provide additional product functionalities. This has triggered a wave of innovation with the development of knowledge and technologies allowing new transformation processes and product development. Acknowledging the need for common standards for bio-based products, the European Commission issued Mandate M/4921), resulting in a series of standards developed by CEN/TC 411, with a focus on bio-based products other than food, feed and biomass for energy applications. The standards of CEN/TC 411 “Bio-based products” provide a common basis on the following aspects: — Common terminology; — Bio-based content determination; — Life Cycle Assessment (LCA); — Sustainability aspects; — Declaration tools. It is important to understand what the term bio-based product covers and how it is being used. The term ‘bio-based’ means 'derived from biomass'. Bio-based products (bottles, insulation materials, wood and wood products, paper solvents, chemical intermediates, composite materials, etc.) are products which are wholly or partly derived from biomass. It is essential to characterize the amount of biomass contained in the product by, for instance, its bio-based content or bio-based carbon content. The bio-based content of a product does not provide information on its environmental impact or sustainability, which may be assessed through LCA and sustainability criteria. In addition, transparent and unambiguous communication within bio-based value chains is facilitated by a harmonized framework for certification and declaration. This European Standard has been developed with the aim to specify the method for the determination of bio-based carbon content in bio-based products using the 14C method. This method using the 14C method is based on the analytical test methods used for the determination of the age of objects containing carbon. This European Standard provides the reference test methods for laboratories, producers, suppliers and purchasers of bio-based product materials and products. It may be also useful for authorities and inspection organizations. Part of the research leading to this document has been performed under the European Union Seventh Framework Programme (see http://www.biobasedeconomy.eu/research/kbbpps/).This document is based on EN 15440 [1] prepared by CEN/TC 343, “Solid recovered fuels”, EN ISO 13833 [2], prepared by ISO/TC 146 “Air quality” and CEN/TC 264 “Air quality”, and CEN/TS 16137 [3], prepared by CEN/TC 249, “Plastics”. The analytical test methods specified in this European Standard are compatible with those described in ASTM D 6866-12 [4].

1) A mandate is a standardization task embedded in European trade laws. Mandate M/492 is addressed to the European Standardization bodies, CEN, CENELEC and ETSI, for the development of horizontal European Standards for bio-based products. SIST EN 16640:2017

EN 16640:2017 (E) 6 1 Scope This European Standard specifies a method for the determination of the bio-based carbon content in products, based on the 14C content measurement. This European Standard also specifies two test methods to be used for the determination of the 14C content from which the bio-based carbon content is calculated: — Method A: Liquid scintillation-counter method (LSC) ; — Method B: Accelerator mass spectrometry (AMS). A third method, Method C: Beta ionization (BI) can also be used for the determination of the 14C content and is described in Annex D (informative) The bio-based carbon content is expressed by a fraction of sample mass or as a fraction of the total carbon content. This calculation method is applicable to any product containing carbon, including bio composites. NOTE This European Standard does not provide the methodology for the calculation of the biomass content of a sample see prEN 16785–1 [5] and prEN 16785–2 [6]. 2 Normative references The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. EN 15400, Solid recovered fuels - Determination of calorific value EN ISO 1716, Reaction to fire tests for products - Determination of the gross heat of combustion (calorific value) (ISO 1716) ISO 1928, Solid mineral fuels — Determination of gross calorific value by the bomb calorimetric method and calculation of net calorific value 3 Terms and definitions For the purposes of this document, the following terms and definitions apply. 3.1 organic material material containing carbon-based compound in which the element carbon is attached to other carbon atoms, hydrogen, oxygen, or other elements in a chain, ring, or three-dimensional structure 3.2 isotope abundance fraction of atoms of a particular isotope of an element 3.3 percentage modern carbon pMC normalized and standardized value for the amount of the 14C isotope in a sample, calculated relative to the standardized and normalized 14C isotope amount of oxalic acid standard reference material, NIST SRM 4990b or NIST SRM 4990c or Sucrose (NIST SRM 8542) Note 1 to entry: In 2016, the value of 100 % bio-based carbon was set at 101,5 ± 0,5 pMC. SIST EN 16640:2017

EN 16640:2017 (E) 7 Note 2 to entry: SRM 4990c, SRM 4990b and SRM 8582 are the trade name of products supplied by the US National Institute of Standards and Technology. This information is given for the convenience of users of this document and does not constitute an endorsement by CEN of this product. Equivalent products may be used if they can be shown to lead to the same results. 3.4 laboratory sample sub-quantity of a sample suitable for laboratory tests 3.5 sample quantity of material, representative of a larger quantity for which the property is to be determined 3.6 sample preparation all the actions taken to obtain representative analysis samples or test portions from the original sample 3.7 beta- particle electron emitted during radioactive decay 4 Symbols and abbreviations 14C carbon isotope with an atomic mass of 14 AMS accelerator mass spectrometry BI beta-ionization Bq Bequerel (disintegrations per second) C symbol for element carbon Cpm counts per minute dpm disintegrations per minute GM Geiger-Müller LLD lower limit of detection LSC Liquid Scintillation Counter or Liquid Scintillation Counting m dry mass of a sample expressed in grams MOP 3-methoxy 1-propyl amine PE Polyethylene PLA poly(lactic acid) pMC percentage of modern carbon REF reference value, expressed in pMC, of 100 % bio-based carbon depending on the origin of organic carbon TC total carbon xB bio-based carbon content by mass, expressed as a percentage of the mass of the sample (dry) xTC total carbon content, expressed as a percentage of the mass of the sample (dry) TCBx bio-based carbon content by total carbon content, expressed as a percentage of the total carbon content SIST EN 16640:2017

EN 16640:2017 (E) 8 5 Principle The 14C present in products is originating from recent atmospheric CO2. Due to its radioactive decay, it is almost absent from fossil products older than 20 000 years to 30 000 years. The 14C content may thus be considered as a tracer of products recently synthesized from atmospheric CO2 and particularly of recently produced bio-products. The determination of the biomass content is based on the measurement of 14C in bio-based products, which allows the calculation of the bio-based carbon fraction. A large experience in 14C determination and reference samples is available from dating of archaeological objects, on which the three methods described in this European Standard are based: — Method A: Proportional scintillation-counter method (LSC), — Method B: Accelerator mass spectrometry (AMS); or — Method C: Beta-ionization (BI). NOTE 1 The advantages and disadvantages of these test methods are given in Table 1. Table 1 — Advantages and disadvantages of the methods Method Additional requests Duration needed for measurement Relative standard deviation Instrumental costs Method A (LSC) Normal laboratory 4 h to 12 h 0,2 % to 5 % Low Method B (AMS) - Large installation - Graphite Conversion device 10 min to 30 min 0,2 % to 2 % High Method C

(BI) - Low background laboratory - Gas purification device 8 h to 24 h 0,2 % to 5 % Low For the 14C LSC measurement a Low Level Counter shall be used. The statistical scattering of the radioactive decay sets a limit, both for method A and B. Thereby both methods need a purified carbon dioxide, otherwise oxides of nitrogen from the combustion in the calorific bomb will result in counting losses by quenching and adulteration of the cocktail in case of LSC measurement. When using method A (LSC), samples with low bio-based carbon content (<10 %) can only be measured with sufficient precision using the benzene conversion procedure or, if applicable, direct LSC measurement, as described in Annex B. NOTE 2 At this moment compact new AMS equipment has become available. In a number of cases, no graphite conversion is required anymore. SIST EN 16640:2017

EN 16640:2017 (E) 9 6 Determination of the 14C content 6.1 General A general sample preparation and three test methods for the determination of the 14C content are described in this European Standard. With this modular approach, it will be possible for normally equipped laboratories to prepare samples for the 14C content, and determine the 14C content with own equipment or to outsource the determination of the 14C content to laboratories that are specialized in this technique. For the collection from the sample of the 14C content, generally accepted methods for the conversion of the carbon present in the sample to CO2 are described. For the measurement of the 14C content, methods are selected that are already generally accepted as methods for the determination of the age of objects. 6.2 Principle The amount of bio-based carbon in the bio-based product is proportional to this 14C content. Complete combustion (see Annex B) is carried out in a way to comply with the requirements of the subsequent measurement of the 14C content and shall provide the quantitative recovery of all carbon present in the sample as CO2 in order to yield valid results. This measurement shall be carried out according to one of the two following methods: — Method A: Liquid scintillation-counter method (LSC): indirect determination of the isotope abundance of 14C, through its emission of beta-particle (interaction with scintillation molecules), specified in Annex C, or — Method B: Accelerator mass spectrometry (AMS): direct determination of the isotope abundance of 14C, specified in Annex E. Possibly, this measurement can also be carried out according to Method C: Beta-ionization (BI): indirect determination of the isotope abundance of 14C, through its emission of beta-particle (Geiger-Müller type detector), described in Annex D. 6.3 Sampling In Annex A sampling methods for products that are mentioned in the scope are given. For any sampling procedure, the samples shall be representative of the material or product and the quantity or mass of sample shall be accurately established. 6.4 Procedure for the conversion of the carbon present in the sample to a suitable sample for 14C determination The conversion of the carbon present in the sample to a suitable sample for the determination of the 14C content shall be carried out according to Annex B. 6.5 Measurements The measurement of the 14C content of the sample shall be performed according to one of the methods as described in Annexes C, D or E. When collected samples are sent to specialized laboratories, the samples shall be stored in a way that no CO2 from air can enter the absorption solution. A check on the in leak of CO2 from air shall be performed by preparing laboratory blanks during the sampling stage. For the determination of the 0 % biomass content the combustion of a coal standard may be used. SIST EN 16640:2017

EN 16640:2017 (E) 10 For validation of the 100 % biomass content, oxalic acid standard reference material, NIST SRM 4990b or NIST SRM 4990c or Sucrose (NIST SRM 8542) may be used. Mixing reference material NIST 4990 with a known amount of fossil combustion aid improves its combustion behaviour, as oxalic acid is difficult to combust due to its low calorific value. For routine checks, a fresh wood sample calibrated against the standard reference material is sufficient. 7 Calculation of the bio-based carbon content 7.1 General The calculation of the bio-based carbon content includes the following steps: a) the determination of the total carbon content of the sample, xTC, expressed as a percentage of the total dry mass; b) the calculation of the bio-based carbon content by mass, xB, using the 14C content value, determined by calculation from one of the test methods specified in 7.3, and applying the correction factors detailed in 7.2, and c) the calculation of the bio-based carbon content as a fraction of the total carbon content,TCBx (see 7.3.2). 7.2 Reference value for 100 % bio-based carbon Before the above-ground hydrogen bomb testing (started around 1955 and terminated in 1962) the atmospheric 14C level had been constant to within a few percent, for the past millennium. Hence, a sample grown during this time has a well-defined “modern” activity, and the fossil contribution could be determined in a straightforward way. However, 14C created during the weapons testing increased the atmospheric 14C level to up to 200 pMC in 1962, with a decline to 102 pMC in 2015. The 14C activity of a sample grown since year 1962 is elevated according to the average 14C level over the growing interval. In addition, the large emission of fossil C during the last decades contributes to the decrease of the atmospheric 14C/12C ratio. In ASTM D 6866-16 [4] the 100 % bio-based C value of 101.5 pMC (for year 2016) is used. This value shall be the base of calculations. Other values are only acceptable if evidence can be given on the pMC value of the b

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