Linseed Oil - an overview (2024)

3.7.1 Binders

Linseed oil is probably the best known binder. A level 0.01mm thick coating of linseed oil dries to form a tenaciously adhering, durable film. As a rule it is not easy to mix the powdered colouring matter with the liquid binder. Even after prolonged mixing of the fine powder, unwet powder can still be present and gives rise to unwanted lumps on brushing. It that not every binder can be used for all kinds of colouring matter because sometimes decomposition reactions may occur.

In the past, the main colouring agents were earth colours, charcoal and, later, mineral colours. To some extent, these have now been replaced by synthetic organic coloured lakes. Natural colours were not often used. But as a result of the present “back to Nature” trend the natural colours have been making something of a comeback.

For use in watercolour paints, pigments have to be thoroughly dispersed and ground (particle size 0.25μm), although a slight amount of binder or thickener, such as gum arabic mixed Ca-Ma, K-salt of arabic acid, is often added.

The first earth colours in cave paintings were fixed with sour milk and lye prepared from ashes. At a very early stage slaked lime and potash waterglass (potassium silicate=K₂Si₃O₇ + K₂Si₄O₉) was used, occasionally also in mixtures. Sodium silicate is unsuitable because it effloresces on drying. Since both binders are strongly alkaline, the pigments have to be suitably resistant. On plaster, natural stone and brickwork, as well as on glass and zinc plate, waterglass colours form a very hard dull coating that is lightfast, washable and weather resistant.

Milk contains the albuminous substance casein. It is believed that the Romans stirred milk into the mortar used to build the temple to Minerva in Elis. For mural paintings, fresh milk casein and slaked lime were mixed with aqueous suspensions of the colours. A wide variety of natural oils such as linseed oil, poppyseed oil and nut oil, and resins from different trees, were also used for colour fixation.

The tempera technique reached its peak during the early Renaissance. Its roots reach back to antiquity and it is still in use today. The word tempera comes from the Italian “temperare” = to temper (as of metals) and presumably refers to the fact that the aqueous and oily binders are mixed together, or more precisely, emulsified. Egg white, casein or soap are used as emulsifiers. The basic recipe for genuine tempera consists of about 20% eggs (yolks and whites), about 40% linseed oil and about 40% water. A fine emulsion is prepared from these ingredients and a slight amount of salicylic acid is now added as a preservative.

11.1.4 Flax seed oil

The majority of commercially grown flax seed contains 40–45% of oil and 20–25% protein per weight (Daun and DeClercq, 1994).

Typical linseed oil (LSO) is of great value to nutrition and the chemical industry because of the high content of PUFAs (ALA – up to 65%). However, for the very same reason, this oil is not recommended for use as edible oil for frying and boiling. The polyunsaturated ALA is readily auto-oxidized. Auto-oxidation is the principal cause of the development off-flavors during the storage or cooking process, and finally leads to polymerization of the oil, a feature beneficial in the production of linoleum, but not in the culinary art.

The processing of each type of flax seed differs; flax for industrial use is pressed and then ‘boiled’ – that is, exposed to high temperature (typically 150° or 300°C) in order to initiate the oxidation and polymerization process (van den Berg et al., 2004). This oil is distributed under the name ‘Boiled Flax Oil’ and is not suitable for consumption. Flax oil for human consumption, usually salad oil, or encapsulated oil is ‘cold pressed’ – with temperature within the press lower than 50°C. In both cases, the residue from the press, called ‘cake’, contains 6–12% of oil depending on press type and processing technique (Choo et al., 2007). Solin types of flax are usually processed like other oilseed (i.e., canola) and processing includes a solvent extraction stage, in which the oil remaining in the cake, after pressing of seed, is extracted with hexane, then de-solventized and added to press oil. The residue, now called ‘meal’, makes a valuable source of protein for animal feed. The procedure for solin type of oil was developed and tested for Linola™ flax (Kolodziejczyk and Fedec, 1995).

III.C Air Drying of Oils

Drying oils such as linseed, soybean, and safflower oils have cis-methylene-interrupted unsaturation. Other oils, such as tung oil, contain conjugated double bonds.

The principal film-forming reaction of drying oils is oxidation, which includes isomerization, polymerization, and cleavage. These reactions are catalyzed by dryers such as zirconium, cobalt, and manganese (organometallic salts).

It has been shown that oleic acid forms four different hydroperoxides at C-8, C-11, C-9, and C-10 and that unsaturated double bonds are present on C-9, C-10, and C-8. It is believed that these products undergo typical chain reaction polymerization via the standard steps of initiation, propagation, and termination.

Conjugated oils for use in oleoresinous paints have been produced by the dehydration of castor oil and the isomerization of linoleic and linolenic esters.

Linoleic and linolenic acids or esters can be converted to conjugated unsaturated acids or esters by heating with alkaline hydroxides or catalysts, which shift the methylene-interrupted polyunsaturation to conjugated unsaturations.

In addition to the dehydration of castor oil and isomerization of methylene-interrupted unsaturated oils such as linseed oil, conjugated succinyl adducts can be formed by the reaction of maleic anhydride with non-conjugated unsaturated oils. The succinyl adduct, which is attached at the 9 and 12 positions on the chain, undergoes rearrangement to produce a conjugated unsaturated acid. Additional rearrangement to a trans-trans form occurs at 200°C.

The reaction of 5–8% of maleic anhydride with soybean oil produces an unsaturated oil that is competitive with linseed oil and dehydrated castor oil.

10.03.2.5 Linoleum

‘Bulk polymerization’ of linseed oil in the presence of sufficient oxygen yields a moldable polymer, which serves as bonding material for linoleum. Indeed, the composition of linoleum has remained more or less unchanged since it was invented by Frederick Walton in 1860.⁷¹ Linseed oil is boiled in the presence of oxygen and a dryer, and blended with molten pine rosin to form a thick mixture called ‘linoleum cement’. The cement is combined with cork powder, wood flour, mineral fillers, and color pigments, then poured upon a moving belt that carries the materials to the mixer, and then rolled or calendered upon jute burlap. From the rollers, the linoleum passes directly into huge ovens for the ‘long bake’. At constantly maintained temperatures, the linoleum is cured for a period varying from 3 to 6 weeks to make a tough, rubber-like material of great strength and endurance. Supported by life cycle assessments, linoleum has had a ‘green’ image for decades.⁷² Since the time of its invention, it has been considered to be an excellent material for high-use areas that can be employed anywhere a resilient floor is needed. It is naturally antistatic and antimicrobial, which enables it to be used in high-performance applications such as health-care facilities.

Introduction

Early inks were all pretty similar and consisted primarily of boiled linseed oil (from flax) and carbon black (soot or lampblack). Some milestones are as follows: in the middle 1700s, availability of ink in more colors than just black; 1850s, development of coal tar dyes; 1920s, introduction of synthetic resins; 1978, use of acrylic resins in inks for coated papers.

In 1990, the U.S. printing ink industry had about $3billion in sales. Gravure inks are produced in the highest mass, but lithographic inks are produced with the highest sales (value). About 200 companies produce ink at nearly 500 plants. Inks cost about $5 to $40 per pound.

15.2.4 Finishes and finishing of polyester sailcloth

An early example of an applied finish was the use of linseed oil (from the flax seed) which, when combined with wax, rendered linen canvas sails more water resistant and durable;⁵ they are recorded as lasting up to 40 years with such treatments. Polyester sailcloth has much higher inherent environmental durability, and this, and other performance features, can be enhanced through application of appropriate finishes.

The practice of suction-slot water extraction is becoming more common to reduce the drying-heat energy requirement compared with when mangle squeeze-off of excess water is used. However, the particularly low permeability of sailcloth makes this more difficult. The fabric is best left in a pH condition suitable for resin finishing (normally slightly acidic).

Although quality sailcloth relies largely on the dense construction of the weave for good bias stability and performance, applied resins are often used to gain further improvements by locking the structure (by virtue of adhesion to fibres) and reducing relative yarn movement. A range of hardnesses of finish, controlled by the type and concentration of resin applied (e.g. acrylic, polyurethane, melamine/formaldehyde, alkyd) are offered by sail makers to suit certain types of sailing; a hard finish, for instance, is used in racing applications.

Applied finishes (where applied) consist of a variety of auxiliary products with the aim of improving performance, durability, handle, aesthetics and other properties. For environmental reasons it is more common not to use solvent solutions but to apply the finish from aqueous dispersions, emulsions or solutions – all products often being contained in the same application bath if ionic compatibility allows. A pad mangle is probably the most common machine used for the application stage whereby the chosen concentration of auxiliary in water is contained in the pad trough and the fabric is fed through this to become fully saturated. On subsequent passage through the mangle, excess liquor is expressed out leaving an even and chosen level of auxiliary on the fabric. This continues into the drier where even drying is attempted to reduce migration of applied product from one face of the fabric to the other. Curing of the resin takes place in the later stages of the drying run or is partly delayed until after calendering.

Often, a final finishing stage particularly used for sailcloth is calendering (direct pressure machine; not with a set gap between bowls). This process helps to consolidate the structure and improves the recognised aesthetics for sails of a smooth, slightly lustrous handle; it also improves flatness and lowers the permeability. A friction calender can increase cover factor more on some fabrics, reducing the permeability.

Resin treatment is used where increased stiffness and bias modulus, crease-resistance and springiness are needed in the sail. Resins are invariably of the thermoset types such as melamine/formaldehyde or alkyd (and there is not so much concern over possible skin contact problems as with resins for apparel fabrics), both of which have good adhesion to polyester and impart crisp handle. Such finishes would be expected to achieve the desired durability of effect in service of the sails although it is doubtful whether any textile applied finishes are completely durable and the eventual failure mode of the applied finish could be either separation from the fibre surface or loss of integrity of the resin film. This occurs with repeated sail flexing and high or cyclic bias stress. The resin finish would be expected to aid the sail performance by virtue of holding the fabric structure to resist bias stretching but at high, applied bias loads the hold of the resin will be minimised or lost.

A possible problem arising from sailcloth resin finishing is chalking effect. This occurs when any creasing, sail flexing or scratching of the fabric causes localised powdering of the resin film, leaving opaque, whitish lines and giving an objectionable appearance especially on medium to dark shades. This phenomenon is an indication that there are deficiencies in the resin system or in the application process conditions. Polyester textile is inherently hydrophobic, and more so in continuous filament form and tightly woven as with sailcloth. It thus retains the pad liquor rather poorly and tends to leave deposited resin on the surface of these constructions. For the same reason, migration of applied solids can easily occur, towards the fastest drying face, if the rate of airflow in drying is uneven face to face. Controlled surface deposition of resins and polymers, that form more integrated deposits of solids up to continuous impervious films, is achieved with coating applications. Such systems have performance advantages for higher value sails and are invariably used for spinnakers and some waterproof covers for boats.

It is common practice to incorporate various additives into the resin finishing formulation to impart other useful properties; most of the available ranges of softeners and anti-static agents can aid slick handle and encourage electrostatic charge dissipation. Water-repellent finishes, based on polysiloxanes or fluorochemical, will dramatically reduce absorption of impinged water into the fabric and allow sails to shed water from rain or sea spray extremely readily.

Care in selection is needed for compatibility with other products applied. One also has to be aware of detrimental effects on other properties, e.g. a silicone-based water repellent is much more likely to lubricate the finish and increase the oleophilic effect of the applied resin than a fluorochemical making oily soiling more likely. Conversely, consideration is needed with choice of other components that might contain surface-active agents, which could well dramatically reduce the effect of the water repellents. Silicone elastomers would improve springiness and reduce the tendency for poorly stowed sails to retain a smooth form rather than multi-creased (termed shrinkage in the sailing fraternity). The latter condition from storage reduces the performance of the sail (speed loss) owing to the unwanted surface volume.

As a further measure to protect sails against the elements (i.e. potentially long periods of exposure to sunlight), UV absorbers can be added to the pad liquor, for thermosolling into the fibre or bound on by the resin film; they can also be dyed into the fabric. Such products may assist in reducing the rate of degradation of the polyester in sunlight but will not allow fluorescent colours or optically brightening whites to fluoresce, the incident UV light being absorbed and neutralised rather than being available for re-emission. Sails which fluoresce when under UV light are likely to have had insufficient or no absorber applied. Products that capture light degradation fragments and other types of degradation reaction stabilisers are not available, to the writer's knowledge, for polyester (see nylon, section 15.3.2, in contrast).

Effective anti-fungal and anti-bacterial applied finishes are available if one feels the extra expense is worth it in order to combat musty smells and fungi stains from storage (normally from a damp state) of sails. Loss of strength caused by bacterial and fungal (mould) activity is less of a concern for polyester and nylon fibre than it is for cotton or linen natural fibre degradation.

9.6.1 Epoxidized plant oil-based resins

Boquillon (2006) examined 28 vol.% hemp fibre reinforced/ELO-based resin composites fabricated using hot pressing. The length and diameter of the hemp fibres used were 6mm and 40 μm, respectively. The flexural modulus and strength of the composites were 5.6GPa and 91MPa, respectively, compared with 1.8GPa and 60MPa, respectively, for the neat ELO-based resin. Good fibre matrix adhesion was reported.

Williams and Wool (2000) examined the tensile properties of 20wt.% non-woven hemp fibre mat/AESO composites fabricated using RTM. The tensile modulus and strength of the composites were 4.4GPa and 35MPa, respectively.

The dynamic mechanical properties of hemp fibre mat/AESO composites fabricated using vacuum-assisted RTM were also examined by O’Donnell et al. (2004). The storage modulus and loss modulus of the composites were 2.16 and 0.27GPa, respectively, compared with 1.11 and 0.068GPa, respectively, for the neat resin.

Ramamoorthy et al. (2012) examined the mechanical properties of 40wt.% unidirectional non-woven and biaxial-woven jute/AESO composites fabricated using compression moulding. The jute fibres were treated with 4wt.% NaOH before composite fabrication. The unidirectional composites had better tensile properties than the cross-woven composites. The flexural strength was also slightly higher; however, the flexural modulus was marginally lower.

Åkesson et al. (2009) examined the mechanical properties of 70wt.% flax mat/AESO composites fabricated using spray impregnation followed by compression moulding. The tensile modulus and strength of the composites were 9.7GPa and 78MPa, respectively, while the flexural modulus and strength were 6.9GPa and 98MPa, respectively.

Adekunle et al. (2011) prepared 60wt.% biaxial-woven jute fabric/MMSO composites using impregnation followed by compression moulding. Three different areal weights of jute fabric were used, namely 100, 240 and 300g/m². The jute fibres were treated with 4% NaOH. The mechanical properties of the jute/MMSO composites ranged from 14 to 19GPa for the tensile modulus, from 65 to 84MPa for the tensile strength, from 5 to 8.5GPa for the flexural modulus, from 20 to 137MPa for the flexural strength, and from 11 to 13 kJ/m² for the Charpy impact resistance.

Early Varnishes

The term varnish probably comes from the French vernis which in turn might come from the Latin vitrum which means possibly a glass. The other origin is possibly from mythology in the form of the story of Bernice whose hair was a glossy amber and went into the air to form the milky way. Varnish is called amber in German. The first written records of varnish appear about 350 BC when the writer Pliny told of applying a composition of his own. The Egyptians 1300 - 1400 BC had an oleo-resinous varnish which they applied, probably hot, and which has not cracked today. Even before this period, the Japanese had an Imperial Lacquer Maker. Lucca-resin dissolved in oil was a good varnish used in the early days.

The gums were powdered and boiled with oil and then if the mix was not too thick it was strained. There was little oil and a great amount of resins.

The “fornis” referred to by the monk was most probably sandarac gum. In processing this varnish it was boiled until about a third of the volume was lost. In the manuscript there is mention of cooling but the varnish was probably reheated and applied hot.

Oils with leads were used a bit later. By 1350 the varnishes were still very thick. In 1440, the formula of Theophilus was still used but now 1/4 ounce of alum and 1/2 ounce of incense were added to the oil when boiling and the mix allowed to boil for three or four hours more. The surface was burned for the length of time needed to say three Pater Noster's.

A great amount of excellent research into varnishes of the middle ages was done by Joseph Michelman in his search for the secret of the great violin makers of the period 1550 to 1750 with regard to the finishes on their violins. (see Violin Varnish, Joseph Michelman 1946, Private publication for Joseph Michelman by W. B. Conkey Company, Hammond, Indiana, U.S.A.).

Without going into the many reasons for the nature of the finish, it is sufficient to observe that the pretreatment for the instruments was linseed oil either raw or sun-thickened and sun aged as were all of the subsequent coats also so cured.

The sophistication of the painters, varnishers and gilders of the period was interesting. The wooden substrate was apparently not dyed as we do today, but rather was coated with a transparent but tinted basecoat. In the case of violins, usually a yellow shade. The varnish for this usage were manufactured by the user and for the first time showed multistage production techniques. Using procedures common to the soap and dye industries of this period, apparently a potassium rosinate solution was made using potassium hydroxide and gum rosin. Then a second stage was prepared by precipitating aluminum or iron rosinates from the potassium rosinate solution. These rosinates were filtered, washed and dried. The varnishes were created by dissolving the dried rosinates in turpentine and raw or sun bodied linseed oil. Madder was well known to the dyeing industry of that time as well as the use of alum and zinc sulfate as mordants. Yellows and various shades of brown and red tones were thus available for decoration.

The resultant coatings became turpentine insoluble quite rapidly when exposed to the strong Italian sun but very slowly if not. In Northern Europe, cooked varnishes containing reinforcing resins would have been required in place of the raw linseed oil for satisfactory results.

Alberta was the first to use thinner after boiling. Watin in 1773 wrote an article on manufacture and prices. The first patent for varnish was issued in 1763.

In 1773 Watin in England described Varnish technology reciting formulations utilizing copals and amber, linseed, walnut, hempseed and poppyseed oils with turpentine as the solvent. His book was reprinted (with few and minor changes) fourteen times between 1786 and 1900.

With the advent of the lead pigments, the paint industry broke down into coatings formulated from elaborate blends of oils with resins from every part of the globe combined with numerous pigments on the one hand, and the (to us) traditional lead and oil mixture of white or red lead plus raw and or bodied linseed oil on the other hand. Semi-modern organic coatings (approximately the first 55 years of this century) were interesting combinations of science and art.

Many of the today's coatings systems depended upon research that was done in very early years, a lot of it in the 1800's, but had to wait for application to coatings until such time as other processes created a need for commercialization and thus the availability of the particular resin being considered. Styrene is a classical example of this.

Styrene monomer was first prepared in 1839 from a naturally occurring gum at which time its polymerization to a low molecular weight oligomer was also observed. It took nearly one hundred years to put the polymerization process of styrene on a scientific basis. By 1950 the U.S. production of styrene finally reached 50 million pounds per month.

In the comprehensive Mattiello series published from 1941 to 1946 there is no mention of styrene. Yet, only 25 years later styrene was either the workhorse or the modifier in many polymer coatings. In some respects it replaced rosin as the economic monomer of choice in the building block technique of resinous molecular structure. Inert, generally, coreactive, decently durable when attached to exceedingly durable monomers, its brittleness as a hom*o-polymer was controlled by plasticizing resins or chain modification; for instance, vinyl fluoride and vinylidene fluoride.

During the period surrounding World War II several significant pieces of research were adapted into coatings that have had significant influence on coatings in the later half of this century. These advances include alkyd resins, urethane and epoxy resins and emulsions. Because each of these have their own chapters, we will only treat them briefly in our discussion.

3.3.4 Airic Oxidation

Atmospheric oxygen sometimes causes direct oxidation of the secondary metabolites of herbal medicinal products. Linseed oil, turpentine oil, and lemon oils become resins in this manner. Cannabinol, the principal component of Indian hemp, Cannabis indica, rapidly becomes a resin, similar to oil of turpentine and oil of lemon. The resinous deposits observed on the walls of the storage container indicate such deterioration. In addition to this, the rancidification of fixed oils (such as cod-liver oil or mustard oil) results in the formation of unstable peroxides.