ALKID RESINS
IV CHEMISTRY OF ALKYD RESINS l. Polymerisation Mechanisms
3. Important Parameters for Formulating Alkyd Resins
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specific paint application, there are several resin-related parameters which must be considered. They may be quantitative or qualitative, and the experienced resin chemist uses them. knowing their effect in the final paint formulation.
(i) Hydroxyl Value.
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excluding solvent content of the resin). It enables the determination of the amount of free and available hydroxyl functionality in the alkyd resin.
This parameter will influence the following properties of the resin:
Polarity of the resin
Water resistance - a high OH view.
• Crosslinking - a certain OH value is essential if the OH groups on the alkyd are to be used in cure or crosslinking reactions with isocyanate or amino-resins
• Adhesion to substrate.
These are extremely important parameters since the oil length and properties of the oil used will fundamentally determine the nature of the alkyd resin. The nature of the oil and oil length
f an alkyd controls the following parameters of any paint formulated from the alkyd:
il length is a highly practical concept which facilitates the classification of alkyd resins into ifferent groups. It corresponds to the percentage of oil present in the resin expressed as o
• exterior durability
• chemical resistance
• dilution properties and compatibility O
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triglyceride. Consider an example of an alkyd resin consisting of 878 g (1 mole) of Soya bean oil, 184 g (2 moles) of glycerol and 444 g (3 moles) of phthalic anhydride. The alkyd resin obtained has an oil length of:
%
As a function of this parameter or criterion, the following classification can be established:
SHORT OIL ALKYDS <45%
KYDS >45%; <55%
LONG OIL ALKYDS >55%
s explained earlier, the definition of short medium and long oil alkyds may vary from hen the oil length is greater than 75 %, the resin is no longer considered as an alkyd resin
NGTH ON PROPERTIES OF ALKYD
R MEDIUM OIL AL
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formulator to formulator. The differences are normally only a few percent.
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but a modified oil instead. The properties obtained will be closer to those of the base oil.
Table 2-6 and Figure 2-15 indicate the influence of oil length on the resin properties:
TABLE 2-6: EFFECT OF OIL LE
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Stoving Whit
Oil length Air drying finish Self colour Flexibility
e
TABLE 2-7: EFFECT OF OIL LENGTH AND DEGREE OF UNSATURATION (IODINE VALUE) ON THE PROPERTIES OF ALKYD RESIN.
Air drying obviously requires the presence of fatty acids, so it is obvious that air drying increases as oil length increases. Stoving (baking) systems require compatibility of the rosslinking resin, frequently melamine, with the alkyd, and as oil length increases, this ecreases. The yellowing tendencies of an alkyd are related lo unsaturation, so ellowing increases with increasing oil length.
hence flexibility increases with oil length.
ompatibility with white spirit increases with increasing oil length as does ease of
ii) Functionality.
knowledge of the functionality of the resin system enables the formulator to predefine the
following properties of the paint and resin:
cular weight 2. hardness
4. compatibility c
compatibility d y
Fatty acid chains are inherently flexible, C
application, as the fatty acid chains confer low surface tension and hence ease of surface wetting.
(i A
level of crosslinking and condensation (the amount of reaction or polymerisation).
A functionality equal to 2 is obtained uniquely with difunctional monomers. The degree of condensation will influence, amongst others, the
1 . mole
3. drying characteristics
5. the relationship between viscosity and non-volatile content (solids) levels 6. gloss
7. flexibility
The value of the functionality can be easily calculated using the theory of condensation, after scribing to each basic component of the resin a real (practical) and non theoretical
knowledge of the functionality of the resin system also enables one to estimate whether the sin can be manufactured. The performance of many alkyd resins improves the nearer they re processed to the point of gelation. Thus it is necessary, particularly for air drying alkyds, process as close to the point of gelation as possible without loosing the resin through the rmation of a gel. The functionality of a system is a good indicator as to how far the resin an be processed without gelling.
he Carothers and Patton equations enable the measurement, for an alkyd resin, of the Patton
1. functionality.
3. final acid value of the resin.
is possible to calculate the gel acid value of the reaction. The higher die selected
t to make - gelation could occur during manufacture. On the other hand, the same resin is prepared with slightly less functionality with, for example, a gel acid value
here are other constants which can be used such as the Patton constant or the hydroxyl
s.
Another approach, which has been traditionally or estimating functionality, is to utilise
the functionality and mol ted. As a generalisation,
e closer this value is to 2, the greater the possibility of gelation at some extent of reaction.
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functionality. As examples, the functionality of trimethylolpropane is three, because it has three primary and non sterically hindered hydroxyl groups. Contrast this to the triol glycerol whose practical functionality is two, despite the presence of three OH groups. This is because only two of the OH groups are primary and a single secondary OH group has little actual reactivity in the polymerisation reaction. Hence a polymer of phthalic anhydride and glycerol will yield an essentially linear polyester with little branching or crosslinking, assuming equal moles of the two reactants. If the glycerol is replaced with pentaerythritol, of functionality four, with its four primary OH groups, extensive branching and crosslinking would occur with equal moles of anhydride and polyol.
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gel value. During the condensation polymerisation, the acid value of the resin gradually reduces with a simultaneous elevation in the viscosity as a function of the following parameters:
2. hydroxyl value.
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functionality, the higher the gel acid value. If an alkyd resin with a final acid value of 5 to 10 has to be prepared, and the gel acid value is 0, there is a strong possibility that the product would be very difficul
if
of -15, there should be no production problems.
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excess, but those cited above remain the ones most used. Today, all of the cited parameters are calculated using computers and often using programmes developed by the formulators.
Therefore, nowadays, all that is needed is to enter the different constituents of the resin as well as the quantities by weight in order to immediately have the desired parameter
used f
ecular weights of the components to be reac th
Resins with functionalities above 2 can be prepared without gelation.
The alkyd resin either has an acid or hydroxyl excess. In most cases (>99.9%) this is a of reactable equivalents can be calculated. For a hydroxyl excess this is 2 x COOH equivalents, because every OH group can only react with one COOH. The total number of moles of reactants present at the start of the reaction is also required. The average functionality ( F(average) ) can then be calculated as follows:
( F(average) ) = total number of reactable equivalents / total number of moles of reactants Consider some examples.
Example 2-1.
nHO-R-OH + nHOOC-R'-CO (2n -2)-CO-R'-COOH + nH2O
F ) = 4n/2n = 2
eactable equivalents = 2 COOH and only 2 of the 4 OH groups = 4 Total number of moles F ) = 4n/3n = 1.33
lolpropane and phthalic anhydride reacting.
t some point during the preparation of this alkyd gelation will occur.
hydroxyl excess. From a knowledge of the functionality and molecular weight of each reactant the total number
OH → HO-R-O-{CO-R'-C0-0R-O}
Reactable equivalents = 2 COOH and 2 OH = 4n. Note that there is no hydroxyl or acid excess. This does not happen in practice for alkyd resins and this is only being used as an illustration.
Total number of moles present at start = 2n ( (average)
Note that this case is unique, in that although the functionality is 2, gelation should not occur because the system is linear. No reactant has a functionality greater than 2.
Example 2-2.
2 HO-R-OH + HOOC-R'-COOH → HO-R-OOC-R'-COO-R-OM R
present at start = 3 ( (average)
Consider now a system with a trifunctional polyol which would more closely resemble an alkyd formulation. Here because at least one reactant present has a functionality greater than two, gelation is a real possibility unless the amounts of reactants are carefully controlled.
Example 2-3.
Consider stoichiometric amounts of trimethy 2 TMP + 3 PA → polyester + water
Reactable equivalents = 2 x 3 OH (from TMP) + 3 x 2 COOH (from PA) = 12. In this example there is neither acid nor hydroxyl excess.
Total number of moles present at start = 2 + 3 = 5 ( F(average) ) = 12n / 5n = 2.4
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There are two ways to avoid gelation. The first, which is unsatisfactory, is to stop the reaction short at an acid value higher than the predicted gelation acid value. The other, which is used in practice is to incorporate a proportion of monofunctional material. This is normally a fatty cid or benzoic acid or derivatives. In this role the acid is often referred to as a 'chain stopper'.
In practice it limits the growth of the chains, thereby stopping. the build up of a network.
Consider the previous reaction with benzoic acid (BA) present. For this example, there is no excess of hydroxyl to enable comparison with the above calculation.
Example 2-4.
2 TMP + 2 PA + 2 BA → Polyester + water
eactable equivalents = 2 x 3 (TW) + 2 x 2 (PA) + 2 x 1 (BA) = 12. This is the same value of
(average)
can has been significantly reduced. There is still the
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l has a a
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reactable equivalents as in the previous example.
Total number of moles present at start = 2 (TMP) + 2 (PA) + 2 (BA) = 6 F ) = 12n/6n = 2.
(
s be seen the functionality of the system A
possibility of gelation at low acid values. In practice a hydroxyl excess would be used.
Consider the above reaction with excess TMP.
Example 2-5.
2.1 TMP + 2 PA + 2 BA → polyester + water
The number of reactable equivalents remains at 12, because there are only 6 COOH groups available for reaction.
The number of moles present at the start is increased to 6. 1.
( F(average) ) = 12 / 6.1 = 1.9
It should be possible to prepare this alkyd at low acid value without gelation, provided no losses occur. Some of the low molecular weight polyols are sufficiently volatile at reaction temperatures to be distilled along with the water of reaction. This tends to be a greater problem. with polyesters than alkyds, due to the nature of the polyols commonly used.
Phthalic anhydride can also be lost. Obviously any change in the number of moles or reactable equivalents through losses or purity of the starting materials will affect the
nctionality. A difunctional material containing some trifunctional materia fu
functionality greater than two. In practice, this is not a problem. for alkyds used for surface coatings, unless low grade low cost raw materials are being used.