Characterisation and degradation of poly(vinyl chloride) in historical and contemporary objects

Rijavec, Tjaša (author), Kralj Cigić, Irena (mentor)

Poly(vinyl chloride) (PVC) objects make up a significant part of contemporary heritage collections. PVC was identified as one of the polymeric materials that are prone to degradation, making it necessary to conduct further studies for an improved understanding of the mechanisms of degradation. PVC objects can become part of collections long after their lifetime, as defined by industrial standards, has ended. The aim of this work was to first develop analytical methods for a more comprehensive characterisation of PVC objects. So far, the chemical composition (identity and content of plasticizers, polymer molar mass and branching) remains largely unknown for objects in heritage collections. To overcome this gap, analytical methods were developed, validated, and then applied to many diverse PVC objects found in a reference collection at the Faculty of Chemistry and Chemical Technology, University of Ljubljana. Gas chromatography with a mass spectrometric detector or a flame ionisation detector (GC-MS and GC-FID) was used for a qualitative and quantitative determination of plasticizersin more than 100 objects. Near-infrared and ATR mid-infrared spectra of the objects were collected non-destructively to create classification and regression models for a quick characterisation of plasticizers in collection objects. Machine learning-supported classification algorithms (Linear Discriminant Analysis (LDA), Naïve Bayes Classification (NBC), Support Vector Machines (SVM), k-nearest neighbours (kNN), decision trees (DT), and Extreme gradient boosted decision trees (XGBDT)), were successfully used to identify the plasticizers in the object. Partial least squares regression was used to determine the content of di(2- ethylhexyl) phthalate and dioctyl terephthalate in PVC objects. Accurate determination of PVC’s molar mass by size-exclusion chromatography with multi-angle light scattering detection required optimising the sample preparation procedure. The optimised method was used to describe the ranges and the distribution of molar mass of historical and contemporary PVC objects and used to investigate changes in molar mass due to accelerated degradation. The main goal of my research was to model the degradation of PVC by developing a damage function, which evaluates the contribution of variables to the yellowing of transparent PVC objects. Elimination of hydrogen chloride from PVC leads to the formation of polyene sequences, resulting in yellowing of the material. Objects from a reference collection were exposed to accelerated degradation experiments, characterised with the developed analytical methods, and the yellowing was expressed as a rate of increasing the value of the b* coordinate in the CIEL*a*b* colour space. Higher temperature and relative humidity increased the degradation rate, while higher plasticizer content and polymer molar mass decreased it. The activation energy of the yellowing was calculated as (86 ± 3) kJ/mol. The lifetime of a PVC object, based on perceptible colour change, can be predicted from its properties and the environmental conditions of storage. Understanding the contribution of variables to degradation can help prioritise conservation efforts and resource allocation in heritage collections of modern materials.

• Language: English
• Keywords: -
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2024
• PID: 20.500.12556/RUL-153517
• COBISS.SI-ID: 181569283
• Publication date in RUL: 11.01.2024

Secondary language

• Language: Slovenian
• Title: Karakterizacija in razgradnja poli(vinil klorida) v zgodovinskih in sodobnih predmetih

Abstract

Predmeti iz poli(vinil klorida) (PVC) predstavljajo pomemben del sodobnih zbirk kulturne dediščine. PVC je bil prepoznan kot eden od polimernih materialov, ki so najbolj podvrženi razgradnji, zato potrebujemo podrobnejše raziskave za razumevanje mehanizmov razgradnje. Predmeti iz PVC lahko postanejo del zbirk tudi po koncu svoje življenjske dobe, ki jo definirajo industrijski standardi. Cilj tega dela je bil najprej razviti analizne metode za podrobnejšo karakterizacijo predmetov iz PVC. Kemijska sestava predmetov (identiteta in vsebnost mehčal, molska masa polimera in razvejanost) v zbirkah kulturne dediščine je do sedaj še v veliki meri neznana. Zato sem razvila, validirala in nato uporabila analizne metode na številnih raznolikih predmetih iz PVC, ki se nahajajo v referenčni zbirki Fakultete za kemijo in kemijsko tehnologijo Univerze v Ljubljani. Za kvalitativno in kvantitativno določanje mehčal v več kot 100 predmetih je bila uporabljena plinska kromatografija z masnim detektorjem ali plamenskoionizacijskim detektorjem (GC-MS in GC-FID). Za izdelavo klasifikacijskih in regresijskih modelov za hitrejšo karakterizacijo mehčal v dediščinskih zbirkah sem neporušno posnela spektre predmetov v bližnjem infrardečem in srednjem infrardečem območju. Za identifikacijo mehčal v predmetih smo uspešno uporabili klasifikacijske algoritme strojnega učenja (linearna diskriminantna analiza (LDA), naivna Bayesova klasifikacija (NBC), metoda podpornih vektorjev (SVM), k-najbližjih sosedov (kNN), odločitveno drevo (DT) in odločitveno drevo z ekstremnim gradientnim vzponom (XGBDT)). Za določitev vsebnosti di(2-etilheksil) ftalata in dioktil tereftalata v predmetih iz PVC smo uporabili parcialno regresijo po metodi najmanjših kvadratov. Za točno določitev molske mase PVC z velikostno-izključitveno kromatografijo z detektorjem na večkotno sipanje svetlobe (SEC-MALS) je bila potrebna optimizacija postopka priprave vzorca. Optimizirano tehniko sem uporabila za opis obsega in porazdelitve molske mase zgodovinskih in sodobnih predmetov iz PVC ter za preučevanje sprememb molske mase zaradi pospešene razgradnje. Glavni cilj moje raziskave je bil modelirati razgradnjo PVC z razvojem škodne funkcije, ki ovrednoti prispevek spremenljivk k porumenelosti predmetov iz PVC. Eliminacija HCl iz PVC vodi do nastanka poliena, ki povzroči porumenelost materiala. Predmete iz referenčne zbirke sem izpostavila poskusom pospešene razgradnje, jih karakterizirala z razvitimi analiznimi metodami, porumenelost pa sem izrazila kot hitrost naraščanja b*-koordinate v barvnem prostoru CIEL*a*b*. Višja temperatura in relativna vlažnost sta povečali hitrost razgradnje, medtem ko sta jo višja vsebnost mehčal in molska masa polimera zmanjšali. Izračunana aktivacijska energija rumenenja je bila (86 ± 3) kJ/mol. Življenjsko dobo predmeta iz PVC, ki temelji na opazni spremembi barve, je mogoče napovedati na podlagi njegovih lastnosti in okoljskih pogojev skladiščenja. Razumevanje prispevka spremenljivk k razgradnji lahko pomaga pri določanju prioritet za konservacijo in pri razporejanju virov v zbirkah modernih materialov kulturne dediščine.

• Keywords: -