LAPPEENRANTA UNIVERSITY OF TECHNOLOGY School of Technology LUT Chemistry Laboratory of Fiber and Paper Technology Perttu Muhonen BRITTLENESS OF PAPER Examiners: Professor Eeva Jernström Professor Kaj Backfolk Preface This Master’s thesis has been carried out at Lappeenranta University of Technology. Thesis was done for and financed by UPM-Kymmene Oyj Research Center in Lappeenranta. I would like to express my warmest thanks to the instructor of my thesis D.Sc. (tech.) Markku Ora for all his support, guidance and practical advice during this work and my supervising professors Eeva Jernström and Kaj Backfolk for their good advice. D.Sc. (tech.) Jouko Lehto and Mika Räty, thank you for taking part in the support group and for providing interesting aspects on my work. I would also like to thank Mr. Barry Madden for proof reading the Literature part of the thesis. I am also very grateful to the professional personnel at the laboratories of UPM Research Center and UPM Kaukas paper mill. I would like to show my gratitude to my family for their love, understanding and support through my studies. I would also like to thank my friends for all the unforgettable moments we have seen together. Special thanks I would like to dedicate to Paula: without your support, patience and love this work would never have been completed. Lappeenranta, November 2013 Perttu Muhonen ABSTRACT Lappeenranta University of Technology School of Technology LUT Chemistry Laboratory of Fiber and Paper Technology Perttu Muhonen BRITTLENESS OF PAPER Thesis for the Degree of Master of Science in Technology 2013 110 pages, 58 figures, 15 tables and 11 appendices Examiners: Professor Eeva Jernström Professor Kaj Backfolk Supervisor: D.Sc. (tech.) Markku Ora Keywords: brittleness, viscoelasticity, relative humidity, fracture mechanics Brittleness is a well-known material characteristic but brittleness of paper is vaguely covered. The objective of this thesis was to characterize the phenomenon and causes around brittleness of paper and to clarify if it is a measurable property. Brittleness of paper was approached from the perspectives of paper physics and paper mills. Brittleness is a property of dry paper and it causes problems at the finishing stages of paper machine. According to paper physics, brittle materials fail in the elastic regime, while ductile materials can locally accumulate a plastic deformation prior to the fracture and they are often able to withstand higher stresses. Brittleness of paper is vastly affected by the surrounding conditions: paper as a hygroscopic material tries to get to the equilibrium. It is also affected by the quality of the pulp used. Measurement techniques can be divided into two categories: based on the viscoelastic behavior of paper and on the exposure to the mechanical stress of sort. The experimental part of the thesis was based on the trials with brittle and non- brittle mill-made LWC papers. It is divided into three parts: strength testing of the brittle and non-brittle papers, analysis of the conditions that may contribute the brittleness and the experimental methods to evaluate brittle behavior. The strength measurements confirmed the influence of the moisture content, but only tensile energy absorption and the fracture toughness measurements provided modest differences between the brittle and non-brittle papers. Versatile analysis of the possible contributing factors resulted into speculation, while the brittle papers contained higher amount of starch, triglycerides and steryl esters. The experimental research proved that the formation, the sensory impression and the variation of local strains may contain the crucial information of paper brittleness. TIIVISTELMÄ Lappeenrannan teknillinen yliopisto Teknillinen tiedekunta LUT Kemia Kuitu- ja paperitekniikan laboratorio Perttu Muhonen PAPERIN HAURAUSKÄSITE Diplomityö 2013 110 sivua, 58 kuvaa, 15 taulukkoa ja 11 liitettä Tarkastajat: Professori Eeva Jernström Professori Kaj Backfolk Ohjaaja: TkT Markku Ora Hakusanat: hauraus, viskoelastisuus, suhteellinen kosteus, murtomekaniikka Hauraus on hyvin tunnettu materiaaliominaisuus, mutta paperin haurauteen liittyviä tekijöitä ei tunneta kunnolla. Työn tavoite oli karakterisoida ilmiö ja siihen johtavat syyt. Lisäksi haluttiin selvittää onko paperin hauraus mitattavissa oleva ominaisuus. Paperin haurautta ilmiönä käsiteltiin paperifysiikan ja paperitehtaan näkökulmista. Hauraus on kuivan paperin ominaisuus ja aiheuttaa ongelmia paperitehtaan jälkikäsittelyvaiheissa. Fysiikan mukaan hauras paperi varastoi vedon aikana elastista energiaa, joka vapautuu räjähdysmäisesti paperin hajotessa. Haurauteen vaikuttaa merkittävästi ympäröivä kosteus, sillä paperi hygroskooppisena materiaalina pyrkii tasapainokosteuteen ympäristönsä kanssa. Paperin haurauteen vaikuttaa myös käytetyn massan laatu. Paperin haurauden mittausmenetelmät voidaan jakaa viskoelastiseen käytökseen ja mekaaniseen rasitukseen perustuviin. Työn kokeellinen osa perustui monipuoliseen hauraiden ja ei-hauraiden LWC- tehdaspapereiden testaukseen. Kokeellinen osa on jaettu kolmeen osaan: hauraiden ja normaaleiden papereiden lujuusmittauksiin, paperin haurauteen vaikuttavien olosuhteiden analysointiin sekä paperin haurauden kokeellisiin mittausmenetelmiin. Mittauksissa havaittiin selvästi kosteuden vaikutus paperin lujuuteen. Paperin haurauden arviointiin lujuusmittauksista sopivat vetoenergian absorption sekä murtositkeyden mittaukset tietyin rajoituksin. Haurauteen vaikuttavien ominaisuuksien testauksessa havaittiin hauraiden koepapereiden sisältävän enemmän tärkkelystä, triglyseridiä sekä steryyliestereitä kuin normaalit paperit. Kokeellisista menetelmistä paperin formaatio, aistivaikutelma ja paikallisten venymien hajonta havaittiin käyttökelpoisimmiksi mittareiksi. TABLE OF CONTENTS LIST OF SYMBOLS AND ABBREVIATIONS 1 INTRODUCTION .......................................................................................... 2 2 OBJECTIVE ................................................................................................... 3 3 PAPER DYNAMICS ...................................................................................... 3 3.1 VISCOELASTIC BEHAVIOR OF PAPER ............................................................................ 4 3.2 STRESS-STRAIN CURVE OF PAPER ................................................................................ 4 3.3 EFFECT OF MOISTURE ON PAPER PROPERTIES .............................................................. 9 4 PAPER FRACTURE .................................................................................... 16 4.1 FRACTURE MECHANICS ............................................................................................. 17 4.2 STRENGTH MODELS ................................................................................................... 18 4.3 INITIATION OF THE FRACTURE OF PAPER: MACROSCOPIC FRACTURE.......................... 21 4.4 INITIATION OF THE FRACTURE OF PAPER: MICROSCOPIC FRACTURE ........................... 22 4.5 FRACTURE TOUGHNESS AND FRACTURE PROCESS ZONE ............................................ 23 5 BRITTLENESS AS A PHENOMENON .................................................... 28 5.1 BRITTLE VERSUS DUCTILE FRACTURE ....................................................................... 28 5.2 BRITTLENESS OF METALS AND COMPOSITES .............................................................. 32 6 BRITTLENESS OF PAPER........................................................................ 34 6.1 FACTORS CONTRIBUTING BRITTLENESS OF PAPER ..................................................... 34 6.2 MEASUREMENT TECHNIQUES OF PAPER BRITTLENESS ............................................... 36 6.2.1 Germgård et al. approach ........................................................................................ 36 6.2.2 Brittleness constant based on a stress-strain curve .................................................. 38 6.2.3 Folding endurance.................................................................................................... 41 6.2.4 Wedge test for brittleness ......................................................................................... 42 6.2.5 Standard testing at controlled humidity ................................................................... 44 7 SUMMARY ................................................................................................... 48 EXPERIMENTAL PART ................................................................................... 49 8 INTRODUCTION ........................................................................................ 49 8.1 OBJECTIVE ................................................................................................................ 49 8.2 CHARACTERIZATION OF THE BRITTLENESS OF PAPER ................................................ 50 8.3 TRIAL PAPERS AND TESTING ...................................................................................... 50 9 STRENGTH PROPERTIES OF MILL MADE PAPERS ....................... 51 9.1 TESTING AT 10% AND 50% RELATIVE HUMIDITY ...................................................... 52 9.1.1 Humidity controlling ................................................................................................ 52 9.1.2 Tensile strength and tensile energy absorption ........................................................ 53 9.1.3 Fracture toughness (Fracture tensile energy absorption) ........................................ 54 9.1.4 Tear strength ...........................................................................................................