Alkyd Resins

General

Alkyd resins are a subclass of the polyester class of polymers. Alkyds were invented in 1901 and became commercially available in the 1920s. They are manufactured from synthetic chemicals in combination with (mostly) natural oils and fatty acids by condensation reactions. These oils and fatty acids may either be derived from animal or vegetable origin.

The etymology of the word 'alkyd' comes from a contraction of the names of two of the main components, alcohols and acids ('alc-ids'). The ingredients are reacted together, via a condensation reaction, to produce polymers with chemical links knowns as 'ester groups', hence these resins fall under a class of polymers known as 'polyesters'.

The terminology for these polymers is fairly quaint and depends on the quantity of oil (or fatty acid) in their composition:

• "Long in Oil": those resins containing more than 60% of oil by mass (some consider 55% upwards as long oil alkyds)
• "Medium in Oil": those resins containing 40 to 60% of oil by mass
• "Short in Oil": those resins containing less than 40% of oil by mass

These resins are normally referred to as Long Oil, Medium Oil, and Short Oil alkyds.

Alkyd types

The oil length of an alkyd determines the application areas where these polymers are utilised and this classification will be used below.

Long Oil Alkyds are soft to hard resins with very high viscosity in the raw state. To reduce them to the viscosities suitable for application purposes (for example by brush or roller) they require the addition of a suitable solvent. The solvent used has many generic names. Turpentine, a natural product that is distilled from the resin component of pine trees (seldom used these days due to availability/cost) or 'Mineral Spirits', a synthetic solvent, of similar composition to turpentine, produced by the petrochemical industry.

The oil component can vary quite significantly depending on the final application but is typically based on oxidatively drying oils or fatty acids. For architectural coatings, linseed oil was the original oil of choice, but it fell out of favour, particularly for white and pastel paints, because of its tendency to yellow quite quickly. Natural oils used today in long oil alkyds are typically Soybean and Sunflower Oils which, whilst sometimes called 'non-yellowing' oils, they do, in fact, still yellow, particularly in dark areas (such as cupboards), but less so than linseed oil based alkyds.

Oil based alkyd paints do not yellow in conditions of 'direct sunlight' due to the bleaching effect of the Ultra Violet component of natural sunlight.

Enamel and varnish coatings based on long oil alkyds have high gloss, good flexibility and are suitable for a number of applications. They are the preferred alkyd for wood coatings, especially in exterior conditions, where the natural expansion/contraction of the wood, due to changes in moisture levels which result from changing environment conditions (e.g. dry/wet), can result in the cracking of the coating, thus allowing further moisture ingress, further degradation of the coating and 'flaking' of the paint film.

Medium Oil Alkyds are harder than the long oil resins as they contain less natural oil than the long oil resins, and a greater quantity of synthetic chemicals in the polymer. The lower quantity of oil also means that they have lower solubility in the same solvents as the long oil alkyds. To reduce them to the viscosities suitable for application purposes (for example by brush or roller) they require the addition of suitable solvents, which are normally those used for the long oil alkyds. With Medium oil alkyds, even faster touch dry times can be achieved using solvents from those listed in the 'Medium Solvency' category.

As with the long oil alkyds, the oil component can vary quite significantly depending on the final application. The natural oils used today in medium oil alkyds are typically those that oxidatively dry, such as Soybean and Sunflower Oils, although linseed oil is preferable in metal primer formulations as it improves both the hard dry time and the adhesion to the metal substrate.

Both the touch and hard dry times of coatings based on medium oil alkyds are faster than with comparable long oil alkyds. The harder, less flexible, characteristics of the medium oil alkyds make them less suitable for exterior wood coatings. They are better suited to applications such as metal primers and undercoats. Primers based on medium oil alkyds exhibit faster touch and hard dry times than long oil alkyds, whilst the harder films produced beneficial to the sanding properties of undercoats.

Short Oil Alkyds are harder than the Long and Medium oil resins as they contain less natural oil than either, and a greater quantity of synthetic chemicals. The lower quantity of oil also means that they have lower solubility than the long and medium oil alkyds in the same solvents. To reduce them to the viscosities suitable for application purposes (generally spray application) they require the addition of stronger, mainly faster evaporating solvents. This also results in Short oil alkyds exhibiting faster touch dry times than either the Long or Medium oil Alkyds.

As with the other alkyds, the oil component can vary quite significantly depending on the final application. The natural oils used today in Short oil alkyds are typically Soybean and Sunflower Oils, although Coconut oil, or a synthetic fatty acid, are preferable in wood coatings as these alkyds are lighter in colour, and therefore, more suitable for clear coatings and white/pastel colours. The lower oil length of the Soya and Sunflower Short Oil alkyds also results in less yellowing with time.

The touch dry time of coatings based on Short oil alkyds are faster than with Medium and Long oil alkyds but the ultimate hardness may take longer to achieve due to the lower oil content, if oxidative drying is the secondary curing process. Short Oil Alkyds are better suited to industrial coatings, such as metal primers and enamels, where the end product needs to be handled in a short period of time without marking.

The physical characteristics, and solubility, of these resins is determined, primarily, by the quantity of oil they contain. These, together with other characteristics and their area of application, are summarised in the following table.

General Chemistry

As previously noted, alkyd resins consist of natural oils, or fatty acids, as part of a polymer network containing some synthetic monomers. Before oils can be used, they need to be pared with a suitable glycol using alcoholysis as outlined here: Alcoholysis methodology. Details on oils and fatty acids used in alkyds can be found here: Oils and fatty acids. Additional raw materials commonly used in alkyd esterification reactions are listed under the Raw materials tab.

Long Oil Alkyds (LOA)

A standard long oil alkyd polymer would have the following ingredients

Oils and fatty acids

Soya bean is probably the most widely used oil/fatty acid for the manufacture of LOAs. Sunflower oil is similar to soya bean but has lower linolenic acid content which will theoretically provide somewhat less yellowing. If the LOA is formulated for use in metal primers then Linseed oil, where yellowing doesn't matter, is a better choice. Apart from a faster hard dry time, Linseed might also improve adhesion to the metal substrate, depending on the metal in question. As per the definition of LOAs, the oil content (or fatty acid content determined as the oil) greater than 60% with 62% and 65% oil lengths being common. In some cases, even higher oil length alkyds are manufactured for special purposes.

Glycol

The high oil/fatty acid content of LOAs tends to produce soft polymer films. To counteract this problem the glycol of choice used in LOAs is the 4 functional pentaerythritol. Raw materials like glycerine can be used but the resulting dried films will be very soft. Trimethylol propane is a possible alternative to glycerine, and will give harder films. However, like glycerine, TMP is only 3 functional and achieving a sufficiently high molecular mass becomes problematic due to lack of free hydroxyl groups available and a decreased crosslinking in the polymer lattice.

Diacids

In the majority of cases, phthalic anhydride (PA) is used in the formulation of alkyds as it is inexpensive, reacts rapidly, and only loses one molecule of water due to the presence of the anhydride group when reacted with glycols (hence the final product yield is higher). Isophthalic acid is an alternative to PA but is both higher in cost and produces more water loss because it contains 2 acid groups. IPA does, however, improve the hydrolytic resistance of the polymer which is of benefit in some marine environments

Solvents

Xylene is often used to enable azeotropic distillation of the water of reaction from the esterification process. The general quantity used is roughly 2.5% of the total formulation. In most cases it's acceptable to leave the 'process xylene' as part of the final formulation. But, where environmental legislation dictates, it is normally vacuum distilled from the final polymer prior to the addition of other solvents. In most cases the solvent used to dissolve the polymer is white spirits, also known as mineral spirits or mineral turpentine, the final volatile content of the product being in the vicinity of 30% (i.e. 70% solids) for when packaged in drums, or 45% volatile (i.e. 55% solids) when stored in bulk.

LOAs have very good solubility in many organic solvents dues to the high oil content (even some amount of water may be tolerated as a non-solvent) and low viscosity/high solids LOAs are possible with the use of stronger solvents, like xylene.

Medium Oil Alkyds (MOA)

Medium Oil Alkyds (MOAs) are similar in many ways to the Long oils in that they contain mostly the same raw materials. However, with the lower oil contents of MOAs they tend to be higher viscosity at the same non-volatile contents of LOAs, whilst coating films exhibit slightly lower gloss and less flexible. On the other hand, both touch and hard drying times of coatings based on MOAs are faster, and the final coating films are harder and more scratch resistant.

Oils and fatty acids

The oils/fatty acids used in medium oil alkyds are essentially the same as in LOAs, soya and sunflower, and the same comments apply as they do for the LOAs. In some instances improved drying and/or improvements adhesion of metal primers is required where linseed oil is of advantage, or linseed/tung oil combinations. Care must, however, be taken in the use of tung oil as the high level of unsaturation can cause heat bodying of the oil with a subsequent increase in resin viscosity.

Glycols

Because medium oil alkyds are somewhat more inflexible than LOAs, the usual glycol used with MOAs is glycerine, which provides for somewhat more flexible coatings. Increased hardness can be obtained by using glycerine in combination with trimethylol propane or penta. In some instances, glycerine can be partially, or wholly, substituted by an equimolar amount of penta and MEG, although the resultant resins can show some haze due to solvent compatibility with weak solvents like white spirits.

Diacids

Although it is possible to use other diacids in the formulation of MOAs, phthalic anhydride is the one most likely to be chosen unless for very specialised applications

Solvents

White spirits is the solvent mainly used in MOA resin formulations that are designed for brush application. However, the additional hardness of MOAs over LOAs lends MOAs suitable for spray application also, in combination with a fast evaporating aliphatic solvent, or aromatic or oxygenated solvent.

Short Oil Alkyds (SOA)

Oils and fatty acids

The oils/fatty acids used in medium oil alkyds are essentially the same as in LOAs and MOAs, soya and sunflower, and the same comments apply as they do for the other two. However, where lighter resins and resistance to yellowing are required, use is made of the more saturated coconut oil or coconut fatty acid (best). For hard wearing wooden floor coatings in high traffic areas, use is made of castor oil or castor oil fatty acid. Castor oil has both a low level of unsaturation (hence less yellowing) and also an additional hydroxy group by virtue of the ricinoleic fatty acid content. This hydroxy group, along with the residual hydroxy groups from the esterification reaction, is useful in adding to the 'cross link density' of the film when the resin/coating is reacted with a secondary resin (generally a urea formaldehyde resin) at the time of application. The additional crosslinking improves durability, chemical resistance, and hydrolytic stability.

Glycols

The low oil content, and hardness imparted by the other reactants, restricts the glycol choices in the formulation of SOAs. As such, glycerine is predominantly used to add some flexibility into the polymer backbone. It is, of course, possible to use alternative glycols but there is rarely any need to do so.

Diacids

Although it is possible to use other diacids in the formulation of SOAs, phthalic anhydride is the one predominantly chosen, unless for very specialised applications.

Solvents

The low oil/fatty acid content of the SOAs means that they have low solubility, if any, in weak solvents such as white spirits. Xylene is the most often used solvent, because of its low cost, although other aromatic and oxygenated solvents can be used. hen these alkyds are used for road marking applications, the solvent used is normally toluene.

Resin Examples

The following recipe is for a standard long oil alkyd produced using soya oil using the alcoholysis procedure. The resin is designed to be processed to an acid value of 8 mg KOH/g (on resin solids) and dissolved in white spirits to give a 70% resin solids solution.

The above two recipes, one based on the oil, the other fatty acid, will produce very similar products in many respects. The slight differences in computed values are based on the small differences between the molecular mass of the soya oil and that of the soya fatty acids as used in the computations. Correctly processed, and with the right choice of catalyst and other additives, both will yield alkyds suitable for use in commercial long oil enamel paints.

Although the product based on soya oil has an extra step (alcoholysis) which would increase the manufacturing time by approximately 3 hours, the yield from the soya oil batch will be higher, as indicated, and the raw material cost will be lower (oils are cheaper than the corresponding fatty acids). Thus it is up to the manufacturer to compute which is more economical, a lower cost product or one that takes a shorter time to manufacture. Note the yields are hypothetical computed values and do not take into account any losses incurred during the manufacturing and packaging processes, which will vary from one company to another, and indeed one manufacturing unit to another.