64 replies to this topic

[dr thrust]

well to add to the confusion, pyroguide say's raw oil is fine, but kentish says use boiled, so i dont think it matters that much, one point that worried me, that i read was spontaneous combustion of of rags coated in boiled linseed oil! though rapid oxidation of the large surface area the rags heat up!!, not a great idea in a pyro workshop.

also i believe mixture of lampblack/linseed oil should be avoided for the same reason!

[paul]

well to add to the confusion, pyroguide say's raw oil is fine, but kentish says use boiled, so i dont think it matters that much, one point that worried me, that i read was spontaneous combustion of of rags coated in boiled linseed oil! though rapid oxidation of the large surface area the rags heat up!!, not a great idea in a pyro workshop.

also i believe mixture of lampblack/linseed oil should be avoided for the same reason!

Yeah, soak the rags in water afterwards or let them dry in the sun/outside for a few hours or even days.
Friends of mine nearly lost their house because of a fire which was caused by rags soaked in turpentine/oil
which self-ignited after a few hours in the trash-basket...

[digger]

Yeah, soak the rags in water afterwards or let them dry in the sun/outside for a few hours or even days.
Friends of mine nearly lost their house because of a fire which was caused by rags soaked in turpentine/oil
which self-ignited after a few hours in the trash-basket...

We have lots of fires on site due to auto ignition of vegetable oil (planned for and controlled). In the case of soft oils (HO sun, Rape etc) when spent bleaching clay (bentonite) is dropped from bleachers into skip it very often auto ignites several hours later.

[lavenatti]

If I remember correctly, raw linseed oil doesn't dry. It just stays an oily wet coating.

I've only ever seen the boiled linseed oil used as a magnesium coating and that's in a warm pan while stirring to keep it from clumping together as it dries.

Also, check the ingredients. Linseed oil is quite often not what it says. If the ingredients say it's 100% boiled linseed oil it is suitable for your purpose.

[digger]

If I remember correctly, raw linseed oil doesn't dry. It just stays an oily wet coating.

I've only ever seen the boiled linseed oil used as a magnesium coating and that's in a warm pan while stirring to keep it from clumping together as it dries.

Also, check the ingredients. Linseed oil is quite often not what it says. If the ingredients say it's 100% boiled linseed oil it is suitable for your purpose.

I am afraid you do not remember correctly Linseed oil is a drying oil by its nature and content of linolenic acid. The action of heat in air on raw linseed oil will speed up the initiation of the oxidation reactions (hence boiled dries faster than raw), however as previously mentioned pre-boiled can have other additives to speed up/control the rate polymerisation.

So when treating the magnesium with raw oil by heating it in a pan the raw oil takes on the properties of boiled oil. So technically either boiled or raw will do (raw will dry more slowly if not heated), however as mentioned boiled may contain unwanted additives.

For reference below is an excerpt from Bailey's Industrial properties of oils and fats (industry standard reference). Note tung oil may also be an interesting oil to use for coatings.

The structures of oleic, linoleic, linolenic, and eleostearic acids are given in Figure 9. Linseed oil and the tung oil are collectively called “drying oils,” which are defined as liquid oils that dry in air to form a solid film (these oils have iodine values greater than or equal to 150 units). Soybean oil, sunflower oil, and canola oil are semidrying oils, with iodine values between 110 units and 150 units. The drying power of such oils is directly related to the chemical reactivity conferred on the TAG molecules by the carbon-carbon double bonds of the unsaturated acids, which allows them to react with atmospheric oxygen, thus leading to the process of polymerization to form polymeric networks.

Linseed oil, which contains 60% a-linolenic acid, is an example of a nonconjugated oil, which is rich in polyunsaturated fatty acids. These polyunsaturated fatty acids contain double bonds, which are separated by at least two single bonds. The linolenic acid content in nonconjugated oils plays an important role in the drying process that is generally considered to be the result of a process of autoxidation followed by polymerization when the oil absorbs large amounts of oxygen. The process of autoxidation in the case of nonconjugated oil systems begins with the dehydrogenation of unsaturated fatty acids, such as linolenic acid, by means of atmospheric oxygen. As a result, dehydrogenated radicals are formed and chain polymerization starts with the formation of hydroperoxide. Furthermore, cross-linking takes place to form large molecules. A summary of this process is shown in Figure 10.

Tung oil with eleostearic acid as its major component, on the other hand, is an example of conjugated oil systems. The conjugated double bonds of oils, such as those of tung oil, favor polymerization and oxidation more rapidly than nonconjugated oils. The principal drying component of tung oil is eleostearic acid. As a consequence of this polymerization, the resultant product obtained is highly resistant to water and alkali. Drying of films in the case of conjugated oils consists of the following three steps:

1. Induction: This process begins by the autocatalysis of eleostearic acid, and the oxygen uptake starts increasing slowly.

2. Initiation: The film continues to absorb oxygen from the atmosphere and, as a result of this absorption, the mass of the film increases and the double bonds of eleostearic acid undergo a rearrangement process. On rearrangement, hydroxyl and hydroperoxy groups are formed in the film.

3. Cross-linking: As a result of the above two steps, the number of double bonds decreases due to cross-linking and, thus, larger molecules are formed.

For many applications, tung oils often cure so rapidly that a highly wrinkled surface forms. Therefore, it is necessary to modify the reactivity of tung oil, which is possible by reducing the number of double bonds present. The reactivity can be modified by chemical means, such as the Diels-Alder reaction with a reactive dienophile (38) and copolymerization with styrene (39) and diacrylate (40); this reduces the number of double bonds and causes the cure speed of the copolymers to be slower, resulting in a nonwrinkled surface. Also, by controlling comonomer stoichometry, sufficient residual double bonds should remain so that oxidative cure of the copolymers would be still possible.

Edited by digger, 14 April 2008 - 07:57 AM.