Study of Grain Formation in Linseed Oil‐Based
Paints
Ria Afifah Almas
Master Thesis
Supervisor: Anders Larsson, Ph.D RISE Bioeconomy and Health Ann‐Charlotte Hellgren, Ph.D Research Consultant at Engwall o Claesson AB Examiner: Prof. Mats Johansson KTH Royal Institute of Technology Department of Fibre and Polymer Technology
Stockholm, Sweden November xx, 2020
Abstract
A study of grain formation in linseed oil‐based paint have been conducted. The grains were formed in linseed oil paints with either Yellow 1, Yellow 2, Black, or Red as the pigments. There are two types of grain formation which has been observed from the paint samples. (a) The grains which immediately appear in the newly made paint with Yellow 1, Yellow 2, and Red paints (b) while the other type is the grain that developed as the function of storage time in Black linseed oil paint. The aims of the study were to identify the content of the grains, determine the parameter which induced the grain formation, and to propose a method that can minimize the grain formation for both types of grains. The project has been performed by analyzing the linseed oil paint's chemical composition, producing the linseed oil paint by varying the parameters such as oil content, metal driers, dispersing agents, and the grinding process of the paint. The grains were analyzed regarding chemical composition, morphology, surface analysis, and molecular interaction using analytical methods such as FTIR, pyrolysis GC, SEM‐EDS, XPS, and profilometry. The result showed that the immediately formed grains were caused by the accumulation of the pigment particles. High content of iron was found in the grains of Yellow 1 paint. Therefore, the immediate grains in the Yellow 1, Yellow 2, and Red linseed oil paints were due to the partial pigment deagglomeration during the grinding process. The grain formation could be reduced by optimizing the grinding condition in the paint formulation. The grains developed over time in the Black paint have been identified as most probably caused by pigment re‐agglomeration because of insufficient particle stabilization by the dispersing agents. In this case, the grains could be reduced through the different polymeric chains in the dispersing agent structure. The chemical content of the grain itself could not be identified in our experiments and needs to be studied further with advanced analytical methods. The contribution of metal soaps to the grain formation in the linseed oil paint is still not clarified.
Table of Contents
Chapter 1 ... 4 Introduction ... 4 1.1 Background of the study ... 4 1.2 Purpose of the Study ... 5 Chapter 2 ... 7 Theoretical Background ... 7 2.1 Linseed oil ... 7 2.2 The Drying Process of Linseed oil ... 8 2.3 Linseed Oil as Paint ... 9 2.4 Processing of the Paint ... 10 2.5 Stabilization of the particles ... 11 2.6 Metal soap formation in linseed oil ... 12 2.7 Methods of Analysis ... 14 Chapter 3 ... 16 Materials and Methods ... 16 3.1 Materials ... 16 3.2 Experimental Methods ... 16 3.3 Methods of Analysis ... 17 Chapter 4 ... 20 Results ... 20 4.1 Analysis of the Component in the Linseed oil Paint ... 20 4.2 Analysis of linseed oil paint with Yellow 1 pigment ... 24 4.3 Analysis of Immediate Grains on the Other Iron Pigments ... 31 4.4 Analysis the grain formation in the Black Paint ... 34 Chapter 5 ... 44 Discussion ... 44 5.1 The Presence of the Raw Linseed Oil in Paint Formulation ... 44 5.2 Chemical Dispersing Agent ... 44 5.3 Mechanical Dispersion Process ... 46 5.4 Methods to Reduce the Grain Formation in Linseed Oil Paint ... 47Chapter 6 ... 49 Conclusion ... 49 Chapter 7 ... 50 Future Work Research ... 50 Reference ... 51
Chapter 1
Introduction
1.1 Background of the study
Linseed oil has been widely used as a varnish, wood protection, and oil‐based paint since the 15th century.[1] In the paint industry, linseed oil is used as a binder as well as the solvent and constitutes a renewable resource. The linseed oil paint is one of the sustainable resources with few, mostly non‐toxic additives that generate an environmentally friendly product. Moreover, the paint only contain small amounts of volatile organic compounds (VOC) with no hazardous waste when the paint is decomposed.[2]
The linseed oil paint produces a complex multi‐layered structure due to the heterogeneous mixture between fatty acid hydrocarbon chains from the linseed oil and inorganic compounds such as pigments, fillers, and dispersing agents.[3] The binder of the paint, linseed oil, has the capability to form a stable, continuous drying film with good mechanical properties. However, it also has the tendency to change in terms of optical appearance due to the long‐term degradation of the drying oil.[4] The Engwall o Claesson Company has been producing and selling linseed oil paints for over 100 years. In some cases, there is a problem in the manufacturing process of linseed oil paints with some specific pigments. The problem initiates lumps or grains in the finished product that affect the properties and aesthetical function of the paint. The grains appear on the dry film surface of the paint that sometimes resembles sandpaper. The grains are most often formed when the paint is produced with the pigments which refer to Yellow 1, Yellow 2, Black, and Red. This is a complicated situation in the manufacturing process that causes problems for customers as well as for the company. This complicated phenomenon is most certainly due to the chemical nature in the paint composition. Engwall o Claesson has a well‐equipped laboratory but incapable to investigate in detail the cause of this phenomenon and how to avoid it.
The problem with grain formation is mostly evident in the freshly made paint. Sometimes, especially in the case of a Black paint, the grains develop after the paint had been stored for some weeks in the can. Therefore, the grains which appear on the black paint surface is labeled as developed grains. Based on the observation, it produces a smaller size of grains on the film surface compare to for instance Yellow 1 paint. Figure 1 below shows the grains on film surface of linseed oil paint when it was freshly made and aged 2 months in the can.
Figure 1. The grains on the film surface of (a) Yellow 1 freshly made, (b) Black freshly made, (c) Black aged 2
months.
In this study, the grain formation in linseed oil paint can be categorized into two types of formed grains, i.e., grains formed immediately during paint preparation and grains developed over several weeks in the can. The particles might flocculate in the storage can due to van der Waals attraction and deficiency in steric stabilization of the particles.[5] Therefore, the first hypothesis is that the grain formation might be caused by incomplete deagglomeration and re‐ agglomeration of the inorganic particles in the paint formulation. The initially agglomerated particles might not be deagglomerated completely due to not enough mechanical energy and time in the paint preparation process. The second hypothesis is that the developed grains are formed due to re‐agglomeration of pigments upon storage. It might be caused by incomplete stabilization of pigment particles causing them to agglomerate upon storage.
Another hypothesis is that the grain formation might be induced by metal soap interaction. It has been reported that zinc soaps can form in old oil paintings and create protrusion in layered surfaces. [6] The metal soap can be produced by interaction of the fatty acids carboxylic groups with zinc or other metal ions present in the paint formulation.[7]
1.2 Purpose of the Study
There are three purposes of this research study. The first purpose is to identify the composition of the grains themselves. The next one is to analyze which condition or parameter in the linseed oil paint promotes the grain formation. The final purpose is to find a method to reduce or minimize the grains formation in the linseed oil‐based paint, i.e, improve the recipes for these paints.
Figure 2. Schematic approach in this research. The approach to determine the reason how and why the grain phenomenon appear in linseed oil paints has been performed through several steps. The study of the grain formation was started by analyzing the composition of the paint. Second, the influence of additives containing metals towards the drying paints appearance was investigated. It was followed by studying the effect of the dispersing agents towards the drying paints appearance and varied the parameters which affects the dispersion during paint process such as grinding speed and time. The grains were also analyzed further both in atomic and chemical content by using analytical methods such as Scanning Electron Microscope (SEM), Energy Dispersive X‐Ray Spectroscopy (EDS), Fourier Transform Infrared Spectroscopy (FTIR), Pyrolysis Gas Chromatography (GC), and X‐Ray Photoelectron Spectroscopy (XPS).
Chapter 2
Theoretical Background
2.1 Linseed oil
In general, linseed oil is derived from flax . It is one of the essential crops with dark brown seeds that are around 4‐6 cm long, oval, and flat in shape. [1] The seeds are grown into fiber or oil crops derived from fiber‐type or oil‐type varieties. The linseed oil can be processed by different methods either by cold pressing, hot pressing, solvent extraction, and refining with pre‐pressing beforehand. [8] In the pressing method, there are cold and hot‐pressed methods to produce linseed oil. Both of them are performed by grinding the seeds and pressing with high power. With the high pressure, the powder seeds will be extracted into oil. The grinding in the hot‐pressed method is proceeds at high temperature while the cold‐pressed method should not be above 40°C. The cold pressed has a lower amount of extracted oil but with a better quality of oil than the hot‐pressed one.[9] The crude linseed oil which is produced by cold‐pressing mainly consists of triglycerides with both unsaturated and saturated fatty acids, less content of proteins, antioxidants and other impurities.[8][9]
Composition of Linseed oil
In general, the linseed oil consists of triacylglycerids with monocarboxylic acids such as palmitic acids (6‐7%), stearic acids (3‐6%), oleic acid (14‐24%), linoleic acid (14‐19%), and linolenic acid (48‐60%). [10] The percentage of fatty acids content depends on the climate where the seeds have been planted. The region with a colder climate tends to produce linseed oil with higher iodine value as it correlates with the amount of unsaturated fatty acids. Linseed oil which is produced in Sweden typically contains 60% linolenic acid and 15% of mixture between linoleic and oleic acids. [11] The fatty acids in linseed oil are two saturated fatty acids and three unsaturated fatty acids. The saturated acids are palmitic with sixteen carbon (C16) and stearic acid (C18) while oleic, linoleic, and linolenic acid belong to the unsaturated type. [10] These three unsaturated fatty acids have a big contribution to the drying process of linseed oil. The structures of unsaturated fatty acids in the linseed oil is presented in Figure 3 below. Figure 3. Structure of unsaturated fatty acids in linseed oil [1]
The oleic acid has one carbon double bond while linoleic and linolenic have two and three double bonds, respectively. In linseed oil, the carbon double bond has a cis configuration (unconjugated) which is formed at the ninth carbon atom in the fatty acid chains. [1] Classification of Linseed Oil The unsaturated fatty acids, primarily linoleic acids, are easily reacted with oxygen and form a crosslink network. The crude oil can be processed in different ways to induce different properties in the oil. The first one is raw oil which typically consist of crude linseed oil produced by cold pressing without any further treatment. [9] Treatment by heating the oil near the boiling point is called boiled oil, although the temperature is controlled below the boiling point. This starts a process of oxidation polymerization of the triglycerides forming a polymeric network that increases the molecular weight of the binder. Additives such as metallic driers are used to accelerate the pre‐polymerization. The colour of the oil become darker than raw oil due to some production of unsaturated ketones. [9][12]
The so‐called stand oil is processed by heat treatment of raw oil above 270 °C under inert gas for a few days to induce polymerization of the double bonds without oxidation. The process increases carbon‐carbon bonds instead of carbon‐oxygen bonds which makes the oil less susceptible to chemical breakdown. It produces a network which has the highest viscosity of all processed types of linseed oils. [1][13] The linseed stand oil also thickens the oil, improve the gloss, and reduce the yellowing in the paint.
2.2 The Drying Process of Linseed oil
In industrial application, the oil dries, i.e. polymerize, due to the auto‐oxidation reaction when the oil is exposed to air. The high content of polyunsaturated fatty acids in linseed oil is susceptible to oxidation reaction. Linseed oils also have faster oxidation rate than other vegetable oils, mainly due to high amount of linolenic acid. All the polyunsaturated fatty acids are able to oxidize slowly at room temperature while the monosaturated fatty acids need a higher temperature to be oxidized or with the presence of additives.[9] It has been reported that metal, light, heat, and enzymes could also increase the oxidation rate. The drying oil will solidify during film formation when crosslinking occurs in the double bonds of unsaturated fatty acids.[1] The dry film is formed after the paint is successfully drawn onto the surface of a substrate. To properly achieve good surface protection, there are some requirements of the dry film that need to be fulfilled. The dry film should have good mechanical strength in terms of hardness, toughness, flexibility, and durability.[14] It should be able to withstand high force without being cracked or ruptured, endure the abrasion, and bear deformation when the temperature is changing drastically.[15] These properties can be obtained through intermolecular crosslink in the film or use polymer with high molecular weight as the binder.[14] Aside from hardness, the dry film also needs to provide good flexibility, especially in wood coating. [15] At last, the dry film must have high durability in terms of resisting the weathering effect. The function of the dry film can be lost due to changes in modulus, loss of strength and adhesion, discoloration, loss of gloss,
and embrittlement.[15] The coating is vulnerable to the presence of oxygen, UV light and moisture since it breaks the chemical bond of the polymer.[16]
2.3 Linseed Oil as Paint
The linseed oil paint is solvent‐free and consists of oil, pigments, filler, and small amounts of additives. Pigments and extender fillers are solid materials in the paint media which provide colour, hide the surface of substrates, and modify the films properties.[5] [17] In general, the composition of the paint consists of binder, pigment, extenders, and additives. Binder The linseed oil binder provides bonding between the pigment particles and holds the particles to the surface.[14] [16] It is also called a film former since it produces a thin film when the polymer dries after application in the coating process. [14] Pigment and Extender The solid insoluble particles in the paint mixture are called pigments and extenders. The main purpose of the pigment is to provide the colour and hiding power in the paint while the extender usually an inert mineral that can improve the solid content of the paint without giving any colour. [14] [16]The particle size and surface area contributes to the properties of the paint. The surface area increases as the particle size decrease. Hence, the fine particles will obtain better hiding power and good colouring effect. [16]
In general, there are two types of pigments: inorganic and organic pigments. The inorganic pigment usually consists of an oxidized metal forming compounds such as titanium oxide which provides white colour and hiding power. Others important type of inorganic pigments are iron oxides since they give multiple colorants, such as yellow, red, brown, and black. There are synthetic and natural iron oxides pigments. The natural iron oxides are found in the earth‐ground such as ochre, umber, and sienna. Nowadays, the synthetic one is extensively used in paint formulation since they provide consistent results while natural iron oxides vary in composition and are difficult to process.[16] The most important property of inorganic pigments is their colour stability towards breakdown by the sunlight. In addition, iron oxide pigments can improve the mechanical strength of the dried film and decrease the moisture permeation into the films.[18] Another type of insoluble inorganic particles, mostly minerals, in the paint medium are extenders or fillers. They are added since they increase the paint volume and are considerably cheaper than pigment particles. The chemical groups which are common in extenders are carbonates, silicon dioxides, silicates, and sulphates. [16]
Liquids
The liquid components in the paint are solvents, thinners and/or oils.[14] The solvent must be able to disperse the binder without any chemical reaction. In linseed oil, the oil itself act as both binder and solvent since its viscosity is low enough to give the paint a viscosity suitable for application of the paint with a brush or a roller. In some situations, linseed oil is diluted with hydrocarbon solvent to provide better penetration of the paint into porous substrates, but most often the paint is made 100% with oil.
Additives
Additives are small quantities of chemical substances which put into the paint mixture to obtain certain properties during the paint production or to the finished product. [5] Additives which needed in a linseed oil paint are mostly dispersing agents, driers, rheological additives, and fungicides (for outdoor paint).[16] In general, additives have the functions to improve important characteristics in paints including surface activity, chemical stability, solubility, and application properties. However, each additive may have multiple functions, both with a positive and negative effect. For example, a thickener not only work as anti‐settling agent and increase the viscosity but also affect the gloss of the coating layer and counteract surface film defects. Dispersing agents can also interact as polymerization catalyst and stabilizing the particles.[5] Other important additives are driers which contain metal ions. They will affect the drying behaviour of oils and decrease drying time to form better‐polymerized films.[19]
2.4 Processing of the Paint
All the paint ingredients are mixed in a mill or pigment dissolver to produce the paint. The main principles are dispersion and breakdown of the pigment particles in the liquid medium. The pigment dispersion process can be considered to occur in three steps: (1) Wetting of pigment powders; (2) Separation of pigment particle agglomerates; (3) Stabilization of separated pigment particles. Wetting of Pigment Powders The particles will be dispersed as a colloidal suspension into the paint’s mixture. The maximum stability of the particles requires that the surface area of each particles is wetted with the wetting agents or in this case the oil to replace moisture and air in the particle agglomerates.[14] This process is called pigment wetting, as the pigment/air interface is replaced by pigment/paint medium interface. The wetting of pigment particles depends on the pigment surface, the cavities between agglomerate particles, and the property of the liquid medium.[5] The pigment wetting involves the surface tension of the liquid phase, the free surface energies of the particles, and interfacial tension between the particle surface and liquid phase. It is also influenced by the viscosity of the liquid phase with the wetting becoming poor as the viscosity is increased.[20]
Separation of pigment particle agglomerates
After the wetting, the pigment particle agglomerates need to be broken up to smaller agglomerates or at best primary particles. It is achieved by addition of mechanical energy and sufficient time to break up the agglomerates and physicochemical strategies to dissolve particle necks which otherwise can hinder the break‐up of the agglomerates. The deagglomeration of the pigment particles is carried out by grinding the pigments into the liquid medium.[5] The grinding step is performed by using a mill or dissolver that produces high shearing motion to squeeze the aggregates of particles in the opposite direction.[14] The mechanical actions will reduce the particle size from the agglomerated state to non‐agglomerated state or primary particles. Stabilization of separated pigment particles
The freshly separated particles have a high tendency to re‐agglomerate and therefore need to be stabilized by polymer chains in the liquid medium. Each particle of the pigment will be anchored to polymer chains by intermolecular attraction. Moreover, the chain should also connect to the liquid medium. It separates two particles by putting polymer chains in between.[14] In the dispersion process, the dispersant usually contains a polar group to attract the pigment surface particles. The dispersant should also be able to interact with the solvent mixture.[5]
2.5 Stabilization of the particles
The stability of the particles in the paints is an important aspect, hence the dispersion stage of the pigment into the paint is essential. The poor quality of dispersion generates low colour strength, sedimentation, and separation of phases.[5] However, in the practical paint system, the pigments particles are never dispersed completely and always contained coarse particles. The coarseness of the particles is closely related to the energy which has been applied to the pigment particles during dispersion and to the dispersion tactics in general such as when and how much dispersants and wetting agents are added. [4] The newly separated and dispersed particles will tend to re‐agglomerate due to attractive forces between the particles. The re‐agglomeration of dispersed pigments particles is called flocculation. The Brownian movement occurs in the suspension system and particles move around in the liquid phase.[20] Eventually, collision will occur between pigment particles in the dispersed system.[5] The attractive and repulsive forces between the particles will determine whether the particles remain stable or if they will re‐agglomerate.[20] Therefore, a dispersing agent is needed to maintain the dispersed particles in the deagglomerated state.[5] To separate each particle in colloidal systems using water or polar solvents as the medium, it can rely on the electrostatic charge of the particles.[20] Steric Stabilization The presence of polymer molecules will stop flocculation of particles as they adsorb onto the pigment surface. Polymer molecules must be adsorbed onto one pigment particle only. As two pigment particles covered with polymers approach each other the particles will separate when
there become too much polymers in a limited space and the high concentration of polymer will cause the particles to separate due to a high osmotic pressure. This type of stabilization is called steric stabilization.[20][21] Higher concentration of polymer in the region between two particles will be restored to equilibrium by diffusion of molecules at the osmotic pressure.[14] Figure 4. Schematic illustration of polymer adsorption [20] Homopolymer will adsorb first if they have low solubility in the liquid phase. Copolymers with surface‐affinic groups may adsorb through chemisorption instead of physisorption on the pigment surface. The simple mechanism of polymer adsorption is explained through a dissolved linear homopolymer. Several monomers in the linear polymer will have contact with the pigment surface (trains) and adsorption occurs. The rest of the monomers in the polymer molecules form tails and loops sticking out from the surface into the solution (Figure 4). This is due to the nature of polymer which constantly changes their conformations. The crosslink polymer has more restrained structure and less ability to have different conformations.[20]
To fully stabilize the particles, the polymer molecules need to completely cover the particle’s surface area and bind strongly during the adsorption. Therefore, the homopolymer has lower steric ability than copolymer. In the homopolymer, the chains tend to have one interaction with the pigment surface or molecules in the liquid phase, while copolymer can interact with both sides.[5] The larger the particle surface area, the higher volume of adsorbed polymer and this increase the repulsive energy between the particles.[20] 2.6 Metal soap formation in linseed oil Metallic soap with the formula (RCO2)M is described as material that consist of metal with long‐ chain carboxylates forming a salt with the positively charged metal ion and the negatively charged carboxylic group on the fatty acid. The M in the formula is the metal ion, such as Zn, Cd, Pb, Ba, Ca, Co, Cu, Al, and Fe while R is a branched or linear alkyl group.[22] It is a preferable condition since the presence of metal‐carboxylate on the pigment surface decreases the interfacial energy between the pigment surface and the binding medium.[7]
Metal soaps have been found in the old oil paintings and cause deterioration of the paint surface. In the case of oil paintings, the common heavy metal such as Zn, Pb, and Cu are typically used as a pigment. Those metal ions will react with free fatty acids in the oil medium. It has been observed that lead (Pb) soap is frequently formed and create protrusion up to 200 microns that may break the layered paint surface.[23] The zinc oxide is also typically used as a white pigment. One study case showed that zinc oxide with linseed oil tend to have hard and brittle film after several years. The pigments form metal soaps which induce hydrolysis of triglyceride in the fatty acids content. This reaction is usually called saponification and it causes the paint surface to be darker and brittle.[19] The oil painting also has insufficient inter or intralayer adhesion as well as protrusion due to the presence of zinc soap.[6] The zinc metal ion has been reported to adsorb polymer which contains the carboxylic acid group, especially on the oxidized zinc surface. [7] Figure 5. Proposed mechanism of metal carboxylate in the pigment with binding medium[7] It has been proposed that the mechanism of metal soap formation is through the migration of metal ions to the binding medium (Figure 5). The mechanism explains the ability of metal ions to diffuse into the binding medium through single carboxylate molecules to the next one. This mechanism occurs under the assumption that there is a presence of free saturated fatty acid in the binding medium.[7] Other factors such as relative humidity which above 50% also promote cleavage of fatty acid chains and generate high amounts of diacids.[6] The metal ions act as Lewis acids or bases that locally encourage metal soap formation and accelerate hydrolysis reactions in the fatty acid chains.[4]
2.7 Methods of Analysis
In this study, advanced chemical instrumentations are used to identify and analyze the grain formation on the linseed oil paint surface. The instruments are also widely used to detect the presence of metal soap in oil paintings. To characterize the metal soap and deterioration in oil paintings, a wide range of methods such as FTIR, SEM‐EDS, Raman spectroscopy, and GC‐MS are commonly used.[23] FTIR Information about sample composition can be obtained from FTIR. The method of FTIR combine with SEM manages to detect the degradation degree from the surface of the sample.[24] The method could characterize oxidation products in the drying oils and help to understand the decomposition of hydroxy peroxide in the dried linseed oil film.[25] The principle of infrared spectroscopy is an analysis of sample’s chemical interaction through infrared light. The beam of infrared light will be directed at the sample and the wavelength that adsorbed is equal to the molecular vibrations of the molecules in the sample. Thus, FTIR will provide chemical and structural information based on those specific molecular vibrations.[26] It has been reported that FTIR could detect localized carboxylate groups in the lead soap through a specific IR band in the 1510–50 cm−1 region while zinc soap is around 1585‐1530 cm−1.[27][7] SEM‐EDS The elemental analysis of the material could be performed by electrons microscope. The method provides observation of the surface and cross‐section of the material such as iron powder.[28] It produces magnified images in a microscopic scale and gives information related to the size, shape, composition, and crystallography of the material. This method involves emission from electron sources which create the focused beam of energetic electrons. The sample will interact with the electrons beam and produces two electrons: backscattered electron and secondary electrons. The backscattered electron is large fraction energy of beam electrons that emerge from the specimen after it is scattered and deflected by the atoms in the sample. On the other hand, EDS or semiconductor energy dispersive X‐ray spectrometer is contained X‐Ray spectrum with a specific energy that correspond to a specific element. The spectrum could identify and quantify the elements in the sample except H and He since it does not emit X‐Ray.[29] The combination of SEM and EDS methods has been used to provide mineral composition from archeology objects.[24]
XPS
Another surface analysis method is X‐Ray Photoelectron Spectroscopy (XPS). It is a highly surface sensitives method to analyze the chemical composition from 2‐ 10 nm of the sample surface. The depth of analysis is usually lower than 10 nm for metal oxides.[30] This method provides quantification of atomic composition which is derived from the intensity of the peak. It usually also contains hydrocarbon as a contaminant; thus, relative sensitivity factor (RSF) is usually included to give suitable results of atomic % composition.[31]
Pyrolysis GC The zinc soap in the paint sample can be detected by using pyrolysis GC‐MS. It has been reported to identify a high level of oleic acid in a pigmented paint.[6] The pyrolysis GC‐MS analysis also claimed as a powerful method to analyze polymer and organic matrix such as protein, polysaccharides, polymerized natural resin, and drying oil.[24]
Profilometer
Profilometry is widely used to observe and evaluate the metallic surface. It produces an image with surface height. The basic principle of profilometry is measures the surface roughness by using a probe as mechanical contact that applies across the surface. The probe observes the contour of the surface with the axis of x and y resolution is around 5 μm and the z axis resolution is 0.01 µm. This method is more suitable to analyze large area with a substantial slope on the sample surface. It also provides reproducibility of the data and less expensive compare to microscopic techniques.[32]
Chapter 3
Materials and Methods
3.1 Materials
In this study, all ingredients in the linseed oil paint composition such as the oil, iron oxide pigments, dispersing agents, fillers, anti‐fungicide, and the drier which consist of cobalt(II)‐2‐ ethylhexanoat were obtained from Engwall o Claesson, AB. Several different dispersing agents have been used in this project. They are referred as dispersing agent A, B, C, D, E, and F. The contents of the dispersing agents are presented in Table 1 below.
Table 1. The chemical substances of dispersing agents
No Dispersing Agent Chemical Substance
1 A Solution of a polycarboxylic acid salt of polyamine amides 2 B Block copolymer with basic, pigment‐affinic groups 3 C Hydrotreated heavy naphtha 4 D Polyether phospate 5 E Neutralized amine 6 F Polymeric dispersant 3.2 Experimental Methods Paint Formulation and Processing A 500 g batch of linseed oil paint was made by mixing all ingredients to a high‐speed dispenser, Dispermat CV. The ingredients were put sequentially according to the standard paint formulation. The linseed oil was the first one to be added into the can, followed by the dispersing agent. During this process, the mixture was stirred at mixing rate of 300 rpm. After that, the pigment and filler were added, and all the substances were mixed in the grinding process at 2000 rpm for 20 min. A saw disc impeller was used to provide high shear force with diameter 5 cm and the height of the blade was 0.5 cm. The paint mixture was cooled down after the grinding process by stirring the mixture at 600 rpm for 20 mins. The other paint ingredients were added during this stage, such as the rest of the linseed oil, the cobalt drier, and the anti‐fungicide. All the ingredients were weighed g on a laboratory balance, Mettler Toledo ML3002E.
Table 2. Chemical composition of the linseed oil paints [33] Chemical Compounds Amount ( wt %) Raw linseed oil 20‐30 Stand linseed oil 10‐20 Iron(III) oxide 0‐25 Filler 0‐40 Zinc oxide 4‐7 Dispersing Agent * <1 Anti‐fungicide* < 0.3 Cobalt(II)‐2‐ethylhexanoat <0.2 Additives* <1.8 *Dispersing agent: Fatty acids, C18, unsatd., dimers, reaction products with N,N‐Dimethyl‐1,3‐propanediamine and 1,3 propanediamine * Antifungicide: 4,5‐Dichloro‐2‐octyl‐isothiazolone, Iodopropynyl butylcarbamate * Additives: Propylene glycol methyl ether, Zirconium salt (IV) 2‐ethylhexanoate The Paint Film Process
The finished paint was drawn to 15 cm x 10 cm leneta paper surface. The film was made by standard film applicator Sheen 1107A with the thickness was set to 100 µm. The aging process of the paint was observed by using 1‐month old aging of the paint samples and the film was made with the same procedure. Before the old paint was drawn to the substrate, the paint had been shaken by shaker, Corob TM at 600 rpm for 1 min. Study the parameter in the Linseed Oil Paint Process The influence of each parameter in the grain formation of linseed oil paints were determined by varying the type of oil (raw, boiled, and stand oil), amount of drier (full and half recipe), and type of dispersing agent. The mixing rate during the grinding process was also varied between 2000, 2600, and 3200 rpm as well as the grinding time which was varied from 10 up to 50 min. Each of the linseed oil paints were prepared with the same procedure and followed the paint standard formulation from the company. 3.3 Methods of Analysis Linseed Oil Content The linseed oil samples were analyzed by AAK Sweden AB. The samples differed by the variances of raw, boiled, and stand linseed oil. The saturated and unsaturated fatty acid contents in the three types of linseed oil were analyzed by standard method IUPAC 2.304. The peroxide value was measured by standard method AOCS Cd 8b‐90(m) while the iodine value was analyzed according to IUPAC 2.205. The amount of free fatty acid in the linseed oil sample was measured by following IUPAC 2.201(m) standard method.
SEM‐EDS The analysis of powdered pigments, grains area, and the cross‐section on the paint’s film surface was performed by SEM, SU3500 Hitachi Japan. The electron micrographs were taken by using electron beam at 20 kV and backscattered electron detector (BSE). In the Yellow 1 paint sample, the grains area was analyzed by removing the coating on the grains surface and compare with the grains area without coating removal and non‐grains area. The elemental composition of the grains was measured through energy dispersive spectroscopy (EDS), Bruker Corporation USA. The variable pressure mode was used as the operating method. ATR‐FTIR The Yellow 1 paint with boiled oil, Black paint with raw oil, Black paint with boiled oil, and pure boiled oil film were investigated by FTIR spectroscopy, Varian 680‐IT FTIR spectrometer, equipped with a DTGS detector. The system was operating in ATR mode. An ATR crystal of Diamond with ZnSe, having a contact area of 2 mm and a penetration depth of 2 µm, was used. Background and sample spectra were scanned using a spectral resolution of 4000 – 650 cm‐1; 32 scans were collected. Spectra were ATR and baseline corrected using a Varian Resolution Pro software. All the samples contains cobalt(II)‐2‐etylhexanoat and varied between with and without DA‐A content. The reference of the spectra was using Epoxy bisphenol A‐ ESBO with gold ion and all samples were plotted in absorbance unit (Abs) unit.
XPS
The grains area in the Black‐raw+stand oil 1‐month old was analyzed by XPS, Kratos AXIS Ultra DLD, United Kingdom. The monochromatic Al were used as the X‐ray source. The sample measurement was performed below 1 mm2 with the signal was collected in the area around 700 x 300 μm. The atomic percentages of the samples were analyzed in a wide spectrum and quantified from the curve peak of each element. The analysis of carbon in the sample were performed through operating at high resolution. The carbon signals were shifted because of the different functional group in the sample. The carbon peaks which shifted were adjusted with the carbon at C‐C curve peak and used 285 eV as the reference value. Profilometer The grains surface area in the Black‐raw+stand oil 1‐month was analyzed with a stylus instrument, DektakXT profiler, Bruker. The probe with a diamond tip was used with a radius of 2 mm while the force on the tip was adjusted to 1 mg. The scan of the grains area was measured vertically with the range was set to 524 mm and vertical resolution was controlled at 8 nm. In the line scan, the length of scan was carried at 2 mm with time of measurement was addressed for 30s. The same setting was used in the map measurement with the distance between the line scan in the x axis was set to 2mm.
Pyrolysis GC
The analysis of organic composition on the film surface of the Yellow 1 paint were conducted by using pyrolysis‐gas chromatography/mass spectrometry (Py‐GC/MS). The chromatogram was plotted against the retention time and the compound was injected into the capillary column. In the sample, the polar compounds such as acids and alcohol were derivatized by methylation using tetramethyl ammonium hydroxide (TMAH). The peaks in the mass spectra were compared with the National Institute of Standards and Technology (NIST) mass spectral library in order to identify the compounds in the sample. Viscometry The viscosity measurement of the paint samples was performed by using Viscometer, Brookfield DVE. The liquid paint sample was poured to a cup with the volume was around 200 mL. The liquid paint was measured at 50 rpm with shear rate was set to 0.5 s‐1. The viscosity was obtained in centipoise unit (cP).
Chapter 4
Results
The biggest problem with grains development is in the representative recipe where the Yellow 1, Black, Yellow 2, and Red pigments are used. All these pigments are iron oxide powder with different structures and atomic composition. The Yellow 1 pigment consists of FeO(OH) with gamma crystal modification, while the Yellow 2 and Black pigments have a composition of FeO(OH) and Fe3O4, respectively. The Red pigment also consists of Fe3O4 and other minerals such as CaCO3. The grains were formed when these pigments were dispersed with the linseed oil. Figure 6 shows the grain formation in all linseed oil paints samples.
Figure 6. The grain formation on paint’s film surface of (a) Yellow 1; (b) Red; (c) Yellow 2; and (d) Black
4.1 Analysis of the Component in the Linseed oil Paint
To evaluate the cause of the grains, it was crucial to know the chemical contents of the linseed oil and the pigments itself. In this study, the linseed oil and the pigments were further investigated using advanced analytical instruments.
4.1.1 Composition of fatty acid in the linseed oil
The three linseed oil samples contain either raw, boiled, or stand oil that were used in the paint formulation. In general, linseed oil has two saturated and three unsaturated fatty acids in their structure. The two saturated fatty acids are palmitic acid (16:0) and stearic acid (18:0). On the other hand, the unsaturated fatty acids consist of 18 carbon chains with different numbers of the carbon double bonds which belong to oleic acid (18:1), linoleic acid (18:2), and linolenic acid (18:3).[9]
Table 3. Chemical composition of linseed oil in the paint formulation
Compounds Raw Oil wt % Boiled oil wt % Stand oil wt %
C18:1 20.5 20.7 25.2
C18:2 15.5 15.7 8.8
C18:3 55.2 49.8 18.2
Free fatty acid 0.76 1.2 3.0
Peroxide value 5.8 meq/kg 5.6 meq/kg 13 meq/kg
Iodine value 190 171 37
Unknown 0.2 2.0 31.2
Stand oil has the lowest amount of unsaturated fatty acid. This is because the stand oil is produced by heating raw linseed oil up to 270°C which increases the polymeric network. On the other hand, the boiled linseed oil was only heated under the boiling point, hence, the oil was much less polymerized compare to the stand oil. With this follows that stand oil has the lowest iodine value which is correlated to the degree of unsaturation.[34] Conversely, the peroxide value of stand oil was the highest due to the highly polymerized network in the stand oil itself. The high value of unknown compounds in the stand oil could be the effect of the pre‐treatment process. The stand oil has been heated above its boiling point under inert and oxygen‐free atmosphere.[9] Thus, the unsaturated fatty acid in the stand oil have been polymerized already which made the large fraction molecules could not be detected by the analytical method. 4.2.1 Penetration level of the linseed oil in wood The polymerization of fatty acid in linseed oil will impact the viscosity of the oil. The viscosity becomes an important aspect since it affects how far into the wood porous system the oil can penetrate. An increased contact area between the paint and the wood substrate will increase the adhesion between the substrate and the coating. Hence, the adhesion would be varied depending on the type of linseed oil.
Linseed oil paint has commonly been used to preserve wooden surfaces. As a result, the oil’s ability to penetrate the wood surface of the substrate has been checked in order to know how viscosity affects the coating penetration into the wood. Figure 7 below presented the penetration of each linseed oil type into the wood surface of the substrate.
Figure 7. Graph of oil penetration in wood substrates according to standard formulation
The viscosity of the oils has also been measured. The raw oil has the lowest viscosity at 64 cP while boiled and stand oil are around 152 and 2096 cP, respectively. According to the oil penetration result in Figure 7, it clearly shows that raw oil has the highest ability to penetrate a wooden substrate. On the other hand, the stand oil itself has the lowest ability to penetrate the wood substrate. This was due to the difference in viscosity, molecular weight and cross‐link density. The stand oil has the highest molecular weight and viscosity due to post‐processing treatment of the oil. Therefore, the raw oil has faster diffusion through porous wood species since it has the lowest viscosity and molecular weight. The combination of raw and stand oil would give better paint properties that accommodate the adequate viscosity of the paint. Therefore, in this study, the paint sample standard formulation also used the combination of raw and stand oil as the binder.
4.1.3 Analysis of pigments powders
The pigments that cause the biggest problem with grain formation which are Yellow 1, Yellow 2, Black, or Red were analyzed for morphology and atomic composition by SEM‐EDS. The morphology of each pigment was represented in the Figure 8 below.
The pigment samples were analyzed with 65 times magnification, except for Red pigment which had 55 times magnification. It could be observed from the SEM image that all types of pigment particles are polydisperse and agglomerated in the dry state. The SEM images were obtained from dry pigment particles without being crushed in a mortar.
Figure 8. SEM analysis of pigments (a) Yellow 1; (b) Yellow 2; (c) Black; and (d) Red It has been stated from the manufacturer that the Black pigment was micronized beforehand. However, the SEM image could not observe the difference in morphology of micronized Black pigment because of all the pigments particles were agglomerated in dry powdered state. On the other hand, the EDS analysis was able to detect the atomic percentage of the pigment’s composition and shown on Table 4 below. The analysis was performed with the samples were crushed by using a mortar and the magnification was up to 300 times.
Table 4. EDS result of powdered pigments sample
No Pigments Atomic content (%)
C O S Si Fe Al Mg Ca Mn Ti P
1 Yellow 1 4.30 63.48 ‐ ‐ 32.03 0.19 ‐ ‐ ‐ ‐ 0.06
2 Yellow 2 6.96 61.71 0.36 ‐ 30.55 0.06 ‐ ‐ ‐ 0.38 ‐
3 Black 8.68 53.19 ‐ 1.01 34.93 0.27 0.29 ‐ 0.84 0.79 ‐
4 Red 18.33 58.14 ‐ 0.43 5.87 0.16 ‐ 17.07 ‐ ‐ ‐
Iron (Fe) and oxygen (O) were the most abundant elements for all the pigments except the Red one. The amount of oxygen also around double the amount of iron. This result supports the theory that all of the pigments mostly consist of iron oxide, except Red pigment which mostly contains calcium carbonate (CaCO3). In the Red pigment, the amount of carbon and calcium is equal while the ratio of oxygen was three times higher. This is the typical atomic composition of CaCO3. Meanwhile, carbon (C) and silicon (Si) were found in the EDS results due to contamination
c
d
b
a
from the method’s preparation and laboratory environment.[24] In the case of Red pigment, higher carbon content was caused by calcium carbonate from the pigment composition. 4.2 Analysis of linseed oil paint with Yellow 1 pigment It is important to study the dispersion process of the paint in order to know how the grains are developed on the film surface. As a result, the experiments were started by producing the paint according to the standard formula. The standard formula of the paint is made by grinding all the composition such as linseed oil, dispersing agent, pigment, and filler. Further, other additives were added after the grinding.
It could be observed by using a microscope that indeed there was grains which protrude from the film surface of Yellow 1 sample. The grains that appears on Yellow 1 sample was formed immediately after the paint was applied to the Leneta paper substrate. Figure 9. Micrograph of grain formation on Yellow 1 paint’s surface 4.2.1 The effect of linseed oil type in the grain formation The first step to identify the factor which promoted the grain formation was to vary the linseed oil type as the binder. The Yellow 1 paint samples were varied by using only raw, boiled, or stand oil of each paint formulation. The other parameters were kept the same in the formula. The effect of difference linseed oil type towards the grain’s development could be observed in Table 5 below. The content of the Table included the observation of film surface appearance after the paint has been newly made.
Table 5. Effect of oil type to the grains in Yellow 1 paint samples
No Sample Type of oil Film’s Surface (fresh)
1. Yellow 1 standard Raw + stand oil Lot of grains
2. Yellow 1 Raw oil Lot of grains, wrinkles
3. Yellow 1 Boiled oil Lot of grains
4. Yellow 1 Stand oil A very few amounts of grains
Overall, all the different types of linseed oil in the Yellow 1 paint samples gave substantial grain formation except the stand oil paint sample. The Yellow 1 paint sample with only stand oil has low mobility since the paint was too viscous. Hence, the stand oil was not suitable as binder alone
even though only a few amounts of grains appeared. Meanwhile, it can be seen that the raw and boiled oil contributes more to the grain formation compared to the stand oil due to the significant difference in the viscosity.
4.2.2 The effect of metal driers in the grain formation
The metal soap formation is known to exist in old oil paintings.[35] Thus, another logical parameter to vary was the driers concentration. The driers which consist of organometallic compounds could be the source of metal ions in the metal soap formation. In this paint formulation, a complex of cobalt was used as drier. The cobalt drier amount was varied by using full amount and half amount of the standard recipe. With this approach, it could be detected if metal soaps were generated the grain formation by varying the amounts of cobalt drier in the paint formulation. Table 6. Effect of cobalt drier’s amount to grain formation in Yellow 1 paint samples.
No Sample Amounts of
driers
Film’s Surface (fresh) 1 Yellow 1– Raw oil Full recipe Lot of grains 2 Yellow 1 – Raw oil Half recipe Lot of grains 3 Yellow 1 – Boiled oil Full recipe Lot of grains 4 Yellow 1– Boiled oil Half recipe Lot of grains
The drier was added to paint formulation since it accelerates the air‐drying coating system. It helps the transformation of the liquid paint coating into a solid film of paint in the adequate length of time.[20] The effect of too much drier in the formulation can be observed if wrinkles appear on the film surface. If the amount of cobalt drier were too much, it would only dry the outermost part of the paint and the resultant film hinders oxygen to penetrate into the wet paint underneath. However, if the amounts of driers were too little, it would slower the drying rate and the film surface become tacky since it does not dry in adequate time. Freshly made Yellow 1 paint samples have developed lots of grains despite the different amount of cobalt drier in the formulation. The result did not indicate that the grains is related to metal soap formation from cobalt ions. Therefore, it was speculated that another parameter has greater influence regarding grain formation in the Yellow1 paint rather than metal soap formation.
4.2.3 The role of dispersing agent to the grain formation of Yellow 1 paint
The dispersing agent is one of the additives in the paint formulation. The dispersing agent consists of polymer with high affinity for the pigment surface which is able to stabilize the inorganic particles in the medium and reduce agglomeration. It also helps the wetting process of the pigment into the liquid medium. There were two dispersing agents in the Yellow 1 paint formulation, one was dispersing agent B (DA‐B) which was added during the grinding process and
thickening agent which was added during the final stage (let down) of the paint formulation process. The effect of DA‐A presence in the Yellow 1 paint was observed through the film surface condition.
Table 7. The effect of using DA‐A in the Yellow 1 paint samples
No Sample Dispersing Agent Viscosity (cP) Film’s Surface
(fresh)
1 Yellow 1 – Raw oil DA‐A + DA‐B 4600 Lot of grains
2 Yellow 1 –Raw oil DA‐B 2700 Lot of grains
3 Yellow 1 – Boiled oil DA‐A + DA‐B 3600 Lot of grains
4 Yellow 1 – Boiled oil DA‐B 1600 Lot of grains
It could be observed that the sole effect of DA‐A in the Yellow 1 paint was thickening the paint mixtures and increase the viscosity. Aside of that, there was no difference in the grain formation by removing the DA‐A compounds in the Yellow 1 formulation. This was since DA‐A was only acted as thickener and the role of dispersing agent was provided by the presence of DA‐B. The dispersing agent was added to stabilize the particle and avoid re‐agglomeration. The initial hypothesis was the presence of grains might be due to problem in the particle’s dispersion. Therefore, the next experiment was made to observe the effect dispersing agents had on the grain formation. This was performed by changing the chemical composition of the dispersing agent. Hence, the other dispersing agents were labeled as dispersing agent C (DA‐C), dispersing agent D (DA‐D), dispersing agent E (DA‐E), dispersing agent F (DA‐F). The chemical composition of these dispersing agents was already discussed (Chapter 3). The grains on the film surface and the viscosity of Yellow 1 paint were presented in Table 8 below. The Yellow 1 paint sample was made according to the standard recipe which use the combination of raw and stand linseed oil as the binder while other ingredients were kept the same except the dispersing agent. Table 8. The effect of using dispersing agents with various chemical composition of Yellow 1 paint
No Sample Dispersing
Agent
Viscosity (cP)
Film’s Surface (fresh)
1 Yellow 1 ‐ Raw + Stand oil DA‐B 4000 Lot of grains
2 Yellow 1 ‐ Raw + Stand oil DA‐C 2100 Lot of grains
3 Yellow 1 ‐ Raw + Stand oil DA‐D 1900 Medium grains
4 Yellow 1 ‐ Raw + Stand oil DA‐E 1216 Few amount grains,
craters
5 Yellow 1 ‐ Raw + Stand oil DA‐F 2160 Lot of grains
The difference in chemical composition of the dispersing agents influence the dispersion and stability of the particles. The most significant result is that few grains are formed in the Yellow 1
paint with DA‐E. The result indicates that DA‐E improves the dispersion of particles. However, the viscosity of the paint decreases significantly leading to formation of craters.
It was found in the standard paint formula that paints containing the dispersing agent DA‐D formed less grains compared to paints containing DA‐B. Further experiments were conducted by producing Yellow 1 paints with DA‐D and DA‐B increasing both grinding speed and time. At optimal conditions less grains were formed in paint formulations containing DA‐B compared to DA‐D. Therefore, it can be claimed that DA‐B has better performance as dispersing agent than DA‐D in the Yellow 1 paint formulation.
4.2.4 The effect of grinding process to grain formation in Yellow 1 paint
It has been stated that the dispersion process is extremely important since it determines the particle size and distribution in the liquid medium and stabilization state of the particle.[5] Without a good dispersion process, a poor particle deagglomeration process and/or poorly stabilized deagglomerated particles can lead to subsequent agglomeration of the particles in the paint samples. Aside from dispersing agents, mechanical action such as shear forces is necessary to mix paint’s composition and disperse the particles evenly. This force was obtained through grinding the paint’s composition by using a pigment dissolver in a given length of time. In this study, grinding condition of the Yellow 1 standard formula was stated at 2000 rpm for 20 minutes. Thus, further experiments were performed by changing the grinding speed and time to obtain the optimum conditions. Grinding Speed In this experiment, the grinding speed of Yellow 1 was varied between 2000, 2600, and 3200 rpm in order to see the effect of grain formation on the film surface. The paint samples were made with boiled linseed oil as well as a combination of raw and stand linseed oil as the binder. Other paint compositions were kept the same as standard formula, except the amounts of cobalt driers was cut into half of the standard recipe. Table 9. The variation of grinding speed in Yellow 1 paint samples
No Sample Grinding speed (rpm) T (°C) Film’s Surface (fresh)
1 Yellow 1– Boiled Oil 2000 40 Lot of grains 2 Yellow 1 – Boiled Oil 2600 45 A few amounts of grains 2 Yellow 1– Boiled Oil 3200 50 A few amounts of grains 4 Yellow 1– Raw + stand oil 2000 62 Lots of grains 5 Yellow 1 – Raw + stand oil 2600 59 A few amounts of grains Based on the observation data, the grain formation is less common with higher grinding speed in the case of Yellow 1 samples. Thus, the dispersion process was improved with higher grinder speed. This indicated that the grain formation was due to the poor dispersion process in Yellow 1 paint. There was also no significant difference between the grinding speed of 2600 rpm and
3200 rpm regarding the partial pigment deagglomeration of Yellow 1 samples. It was concluded that the grinding process has already reached optimum condition at 2600 rpm.
Grinding time
The other parameter during the grinding process is the length of time. The Yellow 1 sample showed good film’s surface with only a few amounts of grains by grinding the sample for 10 to 40 mins. In this experiment, the period of grinding has been conducted with raw and stand linseed oil mixture as the binder. On the other hand, the optimum grinding speed has been determined to be 2600 rpm; thus, these experiments were carried at this optimum grinding speed.
Table 10. The variation of grinding time in Yellow 1 paint samples
No Sample Grinding time (min) T (°C) Film’s Surface
1 Yellow 1– Raw + stand oil 10 42 Medium grains 2 Yellow 1 – Raw + stand oil 20 62 Few amounts of grains 2 Yellow 1– Raw + stand oil 30 66 Very few amounts of grains 4 Yellow 1– Raw + stand oil 40 70 Very few amounts of grains Based on Table 10 above, the presence of grains of Yellow 1 paint samples have been decreased with increasing of time during the grinding process. This trend reached optimum condition at 30 mins grinding time, since there was no significant difference in terms of grain formation for Yellow 1 paint in 30 and 40 mins, respectively. Hence, the optimum grinding process was achieved at 2600 rpm for 30 min length of time. Therefore, it can be stated the major cause of the immediate grains in Yellow 1 was related to incomplete deagglomeration and stabilization of the pigment particles. Figure 10. Film surface of Yellow 1 paint (a) before grinding optimization (2000 rpm, 20 min) and (b) after grinding optimization (2600 rpm, 30 min) 4.2.5 Analytical Results of Yellow 1 Paint
The immediate grains which appears on the Yellow 1 paint samples has been reduced by optimizing the grinding process. However, the grains content itself has remained unknown. Thus,
analysis of the grains with advanced physico‐chemical instrumentation has been conducted in the Yellow 1 paint. Pyrolysis GC The Yellow 1 paint with boiled linseed oil as the binder had been used as the pyrolysis GC sample. Analyses was made of the dried paint film with samples taken from an area with grain and from an area without grain. The chromatogram of the sample was presented as the retention time of each pyrolysis compounds for both the grains area and outside the grains area of Yellow 1 paint. Figure 11. Chromatogram of Yellow 1‐boiled oil paint The grains and non‐grains area were mainly consisted of fatty acids (16:0, 18:1 and 18:0), oxidized fatty acids, and degraded fatty acids (C8, C9 and C10 decanedioic acid). The smaller fragments of fatty acids were generated from the long chains of fatty acids (C18) which had been degraded by heating in the pyrolysis method. It can be observed that there is no significant difference in the fatty acid composition between the film surface with and without grains. Another possibility is that free fatty acid composition in the sample was comparatively small compared to the total amount of polymerized fatty acid chains. Hence, it would be hard to detect if there were free fatty acid which correlated to the metal soap formation. FTIR Another analytical approach to evaluate the grains content is through FTIR which could indicate the metal carboxylate peak of an eventual metal soap formation. The Yellow 1 paint with boiled
