Alkyd resin

1.1 Introductory
Paints have been used for decorative propose for many centuries. The
cavemen were probably the first to use the paint to record their legacy. The used
the paints as the means of communication and decoration. Paint making and
paint application were mint to be and art rather than science. But after industrial
revaluation the whole scenario has been changed which lead sciences to
emphasis more on better understanding of composition, constitution, application
and testing of paints to meet the new requirement arose due to evaluation. This in
turn slowly transferred the art obtain paint making into science which lead to
discoveries of new materials and new ways of application of paints, to meet the
stringent requirements of protection coupled with decoration in certain instance
under divers services condition.

Major constituents of paint generally are pigments, binders and solvents with
smaller quantity of additives. A dispersion of pigment in the binder constituent
the paint film, the property of which depend on the nature of binder to large
extend, but nature and quantity of pigments also effect the property of paint film.
Pigments generally gives aesthetic appearance it indicate color and opacity to
paint film and some pigments also provides protection to much or less extent, to
cured film from UV radiation and penetration of liquid and this can be attributed
to their chemical composition and structure of pigments. Solvents are volatile
compounds which leave the film by evaporation and so do not affect the
performance properties of dry paint film. They are in corporate in paints to
provide ease of processing during paint manufacturing and ease application.

Additives are included in a paint system for many different regions. Thought
they are present in relatively small quantities, they can significantly influence
properties of the liquid paint and/or the drive paint film. They are used either to
overcome some defects or to provide certain properties and to improve overall
performance for coating system. To reach the high quality requirements up today,
additive play a major role in formulation and functioning paint system. The
produced thesis work is in contents of modification of alkyd resin. Hence, more
trace about the review on resin has been given.

1.2 Resins (i.e. binders) for coating industries
A wide range of binders are commercial available for coating industries now a
days but earlier chemist had to depend on natural product like, natural resin oil
until the attempts were to modified the natural resin to produce synthetic natural
products and lead to availability of synthetic polymers used in recent age. Very
first synthetic plastic suitable for coating material and which is still in use today
was Phenolic resin, developed by Leo Backeland in earlier 1900s. After this, there
came the new resin called alkyl in 1923 which is used till date. After the industrial
revolution, extensive research in the field of polymer chemistry resulted in the
availability of numbers of different resins for suitable for surface coating.

Resin is the continuous phase in a paint film and is largely responsible for the
protective and the general mechanical properties of the film. A large numbers of
resins with different chemical composition, solubility, nature of film formation,
application characteristics and performance properties of their films are available
for surface coating industries. It is generally found that the performance
properties of a coating systems are best at higher molecular weight of resin, but it
will be difficult to process high molecular weight during paint manufacturing or
even at the time of paint application whereas at low molecular weight polymer
will facilitate of processing during manufacturing of a coating system but at the
loss of performance properties. To achieve optimum balance between ease of
processing and performance properties to distinct methods are used to arrive at
final films.

In the first approach film formation takes place solely due to evaporation of
solvents from the film and no chemical reaction any sort is involve in the film
formation. The resin used is of sufficiently high molecular weight to provide
better performance properties but the dry paint film remain sensitive to parent
solvent. Such type of resin is known as non-convertible resins.
In the second method initially a low molecular weight per polymer is used to
provide ease of processing and application which is then converted to a high
molecular weight polymer to provide better performance property through
chemical reaction known as curing reaction involves the use of either simple
chemicals called curing agent or another resin with some special functional group.
Such resins are referred as convertible resins.

1.2.1 Alkyd Resins
Alkyd resins form the largest group of the synthetic resins available to the
paint industries and consumption is greater than that of the any other resin
for wide variety of application due to their low cost and versatility. Alkyds
are formed by polycondensation of a dibasic acid and a fatty acid and in a
strict manner alkyds can be defined as oil modified polyester resin. Most
commonly used dibasic acid in alkyd synthesis is Phthalic anhydride while
glycerols, Pentaerythritol, trimethylol propane, glycols etc. are used as fatty
acid determines the dyeing characteristics of the alkyd resin. Saturated fatty
acid yields non-dyeing or plasticizing resin while drying properties are
conferred by unsaturated acids. Alkyds are compatible with most of the
resins used in paint industries like rosin, epoxy, Phenolic resin, amino resin,
polyurethanes etc. so it can be easily modified to achieve specific
properties.

Idealized alkyd structure based on glycerol

The product obtained from the condensation polymerization of a polyhydric
alcohol and polyfunctional acid is known as polyester resin and widely used in
surface coating industries. The most commonly used polyfunctional acids are
maleic anhydride, Phthalic anhydride, Adipic acid, sebasic acid etc. whereas
ethylene glycol, propylene glycol, diethylene glycols are used as polyhydric
alcohols. Depending upon the raw material used, Polyester can be either
saturated polyester or unsaturated polyester. Both find extensive use in surface
coatings. Unsaturated polyester is cured by many different ways but the basic
reaction of curing is free radical initiated addition reaction. Free radical
initiation can be achieved either at elevated temperature by use of an electron
beam or UV beam or certain metal ions as a catalyst by decomposition of
peroxides.

Unsaturated polyesters are generally dissolved in reactive solvent such as
styrene to produce final resin and then curing is carried out. Saturated
polyesters are such formulated so that they produce high hydroxyl contents as
they are mainly used in production of polyurethanes. They also react with some
amino resins.
1.3 Alkyd Resins
Alkyds are essentially short-branched polyesters formed by poly
condensation of a dibasic acid and a polyhydric alcohol in the presence of the
glycerides oil or oil derived fatty acids. The presence of the oil facilitates good
pigment wetting properties and, when unsaturated, allows coherent films to be
formed on cure but properties like hardness, durability color and gloss retention
are adversely affected.

Alkyds are relatively inexpensive in terms of raw materials and
manufacturing costs. Alkyd resins are easy to pigment and are compatible with
most substance used in surface coating industries and have they can be easily
modified for special application.
Other polymers may offer better properties in some specific area of
application but alkyds have widest range of acceptable properties in terms of
surface coating along with cost and versatility.

1.3.1 Raw Materials for Alkyd Resins
1.3.1.1 Dibasic Acids

The main dibasic acids most commonly used is Phthalic anhydride when
improved film hardness and chemical resistance are required, isothalic acid and
terphthalic acid are also employed in alkyd resin manufacture. Phthalic
anhydride is widely used as it does not suffer from major solubility problem and
reacts at low temperature as compare to other anhydrides. Trimellitic anhydride
has been increasable used in water soluble or water dispersible alkyds.

The most widely used polyhydric alcohols are glycerol and Pentaerythritol
but in some specific areas of application trimethylol propane and sorbitol are
also employed.
Glycerol contain two primary and one secondary hydroxyl group and it can
be used in alkyds of all fatty acids contain or oil lengths were as Pentaerythritol
contains four primary hydroxyl groups and thus from relatively complex resin
with better performance properties like fast drying greater hardness , better gloss
etc. The higher functionality of Pentaerythritol limits its used in short and
medium oil types of alkyds as it gel rapidly but widely used for long and
medium long oil alkyds.

1.3.1.3 Oils/ Oils derived fatty acids.

The oils used in paint industries are derived mainly form vegetable and to a
much Sesser extent animals sources. They are testers of glycerol and fatty acid,
non-volatile and unstable at higher temperature. These oils very in properties
according to nature of fatty acids combined with glycerol it indicated they may
be saturated or unsaturated. In the case of unsaturated oil, oil possess important
properties of setting slowly to a solid and adherent film when spread on surface
and expose to the air. This process is known as drying and oils can be classified
into following groups based on their drying properties.
 Dryings oils E.g. Linseed oil, Tung oil etc.
 Semi drying oil E.g. Soyabean oil, tobacco seeks oil etc.
 Nondrying oils E.g. Caster oils etc

Drying oils of linseed type consist of glycerol of fatty acids containing two
or three isolated double bond whereas in case tung oil it consist of conjugated
unsaturated fatty acid which result in greater reactivity and drying properties
than linseed oil. Semi drying oils consist of acids with only one or two double
bonds. Nondrying oils contain glycosides from saturated fatty acids which have
no drying properties or may contain small amount of acid with one double
bond.
Fatty acids are large group of compounds consisting of long hydrocarbon
chain of above 16 carbon atoms and upwards, attached to a carboxyl group and
they are present in oil in varying amounts as glycosides.

These fatty acids are either saturated or unsaturated and in latter case they
can be conjugated or non-conjugated. Thus fatty acids are one which governs
the drying properties of oils and in terms of alkyls. This is discussing in detail
letter in this chapters. Fatty acids can be separated from oils by hydrolysis of
glycerides/oils.

1.3.2 Manufacture of Alkyds Resins
Alkyds are essentially short-branched polyester chains formed by the
polycondensation of dibasic acid and a polyhydric alcohol in the presence of
glycerides oil or oil derived acids. Glycerides oils are not readily reactable with
other raw materials used in alkyds resin manufacture as they are tri-glycerides
and due to stearic hindrance do not react easily. To overcome this problem of
reactivity following techniques are employed.
Alcoholysis of oil
This is a technique in which oil is pre-reacted with a polyol to convert tri-
glycerides oil into reactive monoglyceride which can easily take part in
polycondensation reaction. During alcoholysis of oil ester interchange takes
place between oil and polyol.

Alcoholysis is generally carried out at temperatures of 240 oC -260 oC in
presence of basic catalysts like litharge, lead acetate, lithium hydroxide etc. The
presence of basis catalysis greatly speeds up the ester interchange between oil
and polyol. Alcoholysis is normally carried out under inert atmosphere to
prevent ingress of air which can lead to discoloration of product.

Mechanism of the acid-catalyzed transesterification of vegetable oils.

Mechanism of the base-catalyzed transesterification of vegetable oils.

The progress of alcoholysis reaction is monitored by measuring the tolerance
of reaction mixture to alcohol. Initially the tolerance of oil-polyol mixture to
alcohol is low but as the reaction proceeds and ester interchange between oil and
polyol takes place resulting in monoglyceride formation, tolerance of reaction
mixture increases. This technique is also known as monoglyceride (MG)
process.
Acidolysis of oil
In this technique oil is first reacted with acid. This technique is normally
used only where there are problems of reactivity or solubility of dibasic acids
encountered. Acidolysis requires temperature above 260 oC and even presence of
catalyst does not affect the reaction rate much and process take much longer
time. Thus

There is more risk of discoloration and polymerization of oil associated with
acidolysis of oil.

Fatty Acids Process
As the name itself indicates, this process uses the fatty acids derived from
oils. Fatty acids separated from oil are used and not as such oil is used. Since
fatty acids contain carboxylic group, they easily react with polyol and so there
is no need to carry out alcoholysis or acidolysis of oil. Thus fatty acid, polyol
and dibasic acid are together charged directly and polycondensation is carried
out to produce alkyd resin. Fatty acids are expensive as compare to oils but they
facilitate shorter process time and better products. To make it cost effective
generally a mixture of fatty acids and oil is frequently used.

Polycondensation
During polycondensation reaction in alkyd resin manufacturing a series of
simple esterification reactions takes place in which a hydroxyl group reacts with
a carboxyl group to form an ester link with removal of a molecular of water as a
byproduct.
Commercially two techniques are employed in alkyd resin manufacture.

Fusion Process
In this process, the reactants (after alcoholysis, if oil is used) are charged into
the reactor together at temperature of between 180 oC and 260 oC. An inert gas
is purged in the reactor to prevent ingress of air and to facilitate removal of
water of the reaction which is essential for progress of polycondensation
reaction. Since in this process higher temperature is encountered, there is a
considerable loss of volatile reactants for which allowance has to be made to
have necessary degree of process control. This process is generally used for
longer oil length formulation where loss of volatile reactions will not have much
serious effect on process control parameters.

Solvent Process
This process is widely used in alkyd resins manufacture as it has many
advantages over fusion process. In this process reactants are heated together at
temperature between 200 oC - 240 oC with a solvent (most commonly used
solvent is Xylene) which helps in many ways during the process. The solvent
and water of reaction form an azeotropic mixture which is distillated off from
the reactor; condensed and passed into a separator where water is removed as
waste and solvent is recycled to reactor. This will also result in short process
time without encountering higher temperature.

The blanket of solvent vapor prevents ingress of air thus enabling low color
products with minimum use of an inert gas. Since solvent is being recycled to
the reactor most of the volatile reactants lost from the reactor are returned with
solvent to reactor, thus providing much better control over process control
parameters. Solvent process offers a uniform resin composition with a narrow
molecular weight distribution and higher molecular weight resins with a
corresponding improvement in drying and film performance properties.
Polycondensation is monitored by measuring residual acid and hydroxyl
value at regular interval of time till the required molecular weight at specified
residual acid and hydroxyl value has been achieved. When this point is reached,
the alkyd is cooled down to 180 o C to prevent further reaction and diluted with
solvent to the required solids or nonvolatile content.

1.3.3 Classification of Alkyd Resins
The nature and the amount of oils/fatty acids determine the characteristics of
an alkyd resin and thus alkyds are classified in terms of oil length and oil type.
1.3.3.1 Oil Length
This is the amount of oil/fatty acid present as percentage of the nonvolatile
content. Although it does little to describe the complex polymer system, it is
universally used throughout surface coating industry and is a convenient method
of classification for alkyds. On the basis of oil length, Alkyds can be classified
as:

1) Long oil alkyd -Oil content greater than 55% W/W.
2) Medium oil alkyd- Oil content 45-55% W/W.
3) Short oil alkyd-Oil content less than 45% W/W.
1.3.3.2 Oil Type
Depending upon nature of oil i.e. either saturated or unsaturated, alkyds can
be further classified as:
a) Oxidizing Alkyds
Oxidizing alkyds contain drying or semi-drying (unsaturated) oils or fatty
acids and film formation takes place by oxidation. The oil length in such alkyds
is usually in excess of 45% W/W.

b) Non-oxidizing Alkyds
A non-oxidizing alkyd contains non-drying (saturated) oils or fatty acids and
is not able to form film by oxidation. They form film by reacting with other
polymers and this is known as curing. Non-oxidizing alkyds generally have oil
lengths below 45%.
From above classification, it is not necessary that all oxidizing alkyds are
used in air- drying systems and all non-oxidizing alkyds are cured. The
characteristics of an alkyd resin and mainly the film properties are related to the
oil type and oil length. The longer oil length, the more the alkyd reflects
properties of oil and shorter the oil length, the more properties of polyester
chains predominates. When selecting an alkyd for a particular application, it is
necessary to specify both oil length and oil type to achieve balance between
properties derived from polyester chain and those derived from oils.

1.3.3.3 Area of Application
On the basis of area of application, alkyds can be further classified as:
I) Air Drying Alkyds
The unsaturation in the fatty acid chains is unaffected during alkyd resin
manufacture and so the mechanism of film formation in such alkyds is same as
that of oils. The molecular weight of alkyds is higher than that of oil so less
cross-link are required to form a film resulting in rapid drying as compare to
corresponding oil.

Semi-drying oils discolor less on drying than the highly conjugated drying
oils. Keeping the above point in mind, resin formulator can employ semi-drying
oils in alkyds to have rapid drying capability with better color. Oil lengths of 50-
60% are generally used for such alkyds but it must be remembered that though
higher oil length provide improved drying rates, they also adversely affect color
and gloss retention as well as durability of film.
II) Force Drying Alkyds
Alkyds based on unsaturated fatty acids and oils can be cured rapidly at
temperatures in the range of 10-160 oC because increased temperature greatly
accelerates the oxidation of fatty acids. Moreover, in this case C-C bond type
cross links are formed and the resulting films are more chemically resistant and
durable than in case of air-drying systems where C- O-O-C bond type cross
links are formed.

Rapid curing means that short oil lengths can be used and providing more
polyester components to have improved durability, gloss retention and color.
Oil lengths of 40-50% are normally employed for this type of resins.
III) Plasticizing alkyds
Since alkyds have better compatibility with resins like nitrocellulose, amino
resins etc. they are used as a plasticizer with such resins to provide increased
flexibility, adhesion and gloss. Plasticizing alkyds are not intended to make
films on their own so they are generally formulated on non-drying oils or fatty
acids like castor oil and fatty acids derived from castor oil or coconut oil and
coconut oil derived fatty acids. The later being less used as it is more expensive.

IV) Curing Alkyds
A co-cured surface coating system can be formulated on amino resin/alkyd
resin combination. For such a system alkyd with shorter oil lengths having
higher proportions of free hydroxyl groups is used. These free hydroxyl groups
of alkyd react with reactive groups of amino or Phenolic resin during curing and
film formation takes place. The amino resin /alkyd resin combination produces
films with improved hardness, exterior durability, alkali resistance and cure
time. The amino resin/alkyds resin combination provides low color products
whereas the Phenolic resin/alkyds resin combination results in darker color
products but they offer improved chemical resistance.

1.3.4 Modification of Alkyd Resins
No single polymer exhibits all desired properties of a surface coating
system. The polymer has at least one property that is less than ideal.
Commercial coatings are formulated in such a way to achieve balance between
advantageous and disadvantages properties of a base polymer system.
Combination of different types of polymers is often used to obtain a
coating with more desirable properties of individual polymers. It is very
well known that alkyds have better compatibility with a wide range of
reactive chemicals and polymeric materials such as amino resin, Phenolic
resin, acrylics, vinyl etc. and they can be easily modified with such
materials to achieve desirable properties. The chemical modification of
alkyds through fatty acid unsaturation or through functional has been
extensively studied.

1.3.4.1 Modification with Amino Resins
The amino resin/alkyd resin combination results in films with improved
hardness, exterior durability, alkali resistance, mechanical properties and solvent
resistance.The success of this modification lies in the compatibility of two
resins. It has been reported that generally short oil alkyds with high proportions
of hydroxyl group provide good compatibility with alkylated urea-formaldehyde
of melamine formaldehyde. To achieve rapid curing an acid catalyst is
commonly employed in alkyd-amino system. Such modified system is widely
used in industrial baking finishes, generally for top coats as they provide low
color products.

1.3.4.2 Modification with Nitrocellulose
In this modification, alkyd mainly acts as a resinous plasticizer which imparts
adhesion, flexibility and gloss to nitrocellulose. Alkyds generality based on
coconut oil and castor oil with oil length of 55% are used for such system.
However best compatibility is attained with short oil alkyds due to high degree
of polarity obtained via ester and hydroxyl group. Castor oil based short oil
alkyds are most commonly used as the hydroxyl groups in castor oil provide
excellent compatibility, flexibility and adhesion at lower cost but where the cost
is not prohibitive, coconut oil based alkyds are also used to achieve better
desired properties. Such modified system is employed in automotive lacquers
extensively and has also been recommended for traffic paints.

1.3.4.3 Modification with Chlorinated Rubber
Chlorinated rubber is compatible with alkyds of similar linearity and low
polarity. Such modified system shows improved toughness, adhesion, durability,
drying rate and excellent resistance to acid, alkali and water [37]. Such system
is mainly used in concrete floor paints, swimming pool paints and road marking
paints.
1.3.4.4 Modification with Phenolic Resins
Phenolic resins and in particular resole type, when combined with drying oil
alkyds shows excellent gloss retention durability and better resistance to water
and alkali [38]. Such systems are cured at elevated temperatures and products
are darker in color so mainly used in undercoat applications and primers.

1.3.4.5 Modification with Silicone Resins
Silicone modified alkyds are produced by copolymerization alkyd with
certain silicone intermediates i.e. low molecular weight siloxane to give
polymers with highly improved durability and gloss retention and increased
heat resistance. They are also characterized by good drying properties and out-
standing weather resistance.
1.3.4.6 Water-soluble Alkyds
One of the important and interesting modification of alkyd resin in recent
years is water soluble alkyd in which water-soluble or water-reducible alkyd
resin is produced which upon curing results in a water-resistant film. The resin
for its modification is formulated in such a way that it contains a number of free
hydroxyl and carboxyl groups in a well distributed manner throughout resin
molecule which are neutralized by organic bases, generally tertiary bases are
preferred. Curing takes place by stoving in which organic base is volatilized and
cross linking occurs.

1.3.4.7 Modification through Unsaturation of Alkyds
Alkyds based on drying oil fatty acids are used for such modification in
which alkyds are co-polymerized with vinyl or acrylic monomers like styrene,
methyl methacrylate, n-butyl acrylate etc. The resulting polymers possess the
desires application and wetting properties of alkyds along with strength,
chemical and weather resistance of acrylic or vinyl resin.
Another way to prepare such systems is condensation of hydroxyl groups of
alkyds with carboxylic acid groups of acrylic or vinyl copolymer. This way of
modification is technically very attractive as it does not limit the use of
unsaturation of alkyds. Saturated fatty acids containing alkyds with free
hydroxyl groups can also be used for such modification and moreover the
formation of acrylic or vinyl homopolymer is eliminated. Acrylic or vinyl
modified alkyds are widely used in fast drying finishes.

1.3.4.8 Modification through Functional Groups of Alkyds with
Isocyanates
Alkyds resins are frequently modified by reaction with species containing
isocynates group to give resin with improved film hardness and durability. The
hydroxyl groups of alkyds react with isocynates compound (mainly
diisocyanates) to give a high molecular weight three dimensional network
obtained by joining the alkyd chains together with urethane linkages. The cured
films exhibits faster drying rates and improved chemical and abrasion resistance.
Such systems find wide applications in wood finish and quick drying paints for
high performance applications .

Alkyd resin

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