Effect of plasticizer on the thermal, mechanical, and anticorrosion properties of an epoxy primer.

The influence of the glass transition temperature and the mechanical and anticorrosion properties of factors such as the amount of plasticizer added to an epoxy primer were investigated by DSC (differential scanning calorimeter), DMA (dynamic mechanical analysis), stress-strain tests, salt fog spray tests, accelerated tests, and electrochemical tests.

The addition of plasticizer results in a decrease in the glass transition temperature and a change in the mechanical properties. Different tests were carried out to the optimum percentage of plasticizer content (1.5-3% weight ratio to epoxy resin) required to obtain the maximum anticorrosion performance of the epoxy primer. These changes are explained by the structural-kinetic effect exerted by the plasticizer on the chemical crosslinking in the course of the epoxy network synthesis and the increase in the excess free volume.

Keywords: Plasticizer, corrosion testing, differential scanning calorimetry, electrochemical impedance spectroscopy, corrosion, corrosion protection, epoxy resins, cure, reaction kinetics, adhesion, iron, cast iron, steel

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Epoxy resins are used in the formulation of primers because of their excellent toughness, adhesion, low permeability, and chemical resistance. Liquid epoxy prepolymers are converted to thermoset materials through their oxirane group by the addition of a suitable hardener. The most widely used epoxy resin is based on bisphenol A and epichlorohydrin. (1) Aliphatic amines are commonly used as hardeners because of their room temperature cure, although the use of a polyamide hardener should enhance the primers flexibility and toughness. (2)

The coating formulation may also be enhanced by plasticizers, fillers, and cure promoters which influence the cure reaction. Moreover, the relative hardener to epoxy ratio or the nature and amount of plasticizer used may also affect the reactivity of the formulation of the final crosslinked polymer that will act as binder in the primer. (3)

The primary role of a plasticizer in a coating, as its name indicates, is to reduce the rigidity of the polymer used as a binder; that is, to render it more flexible. (It will improve the elastic deformability when submitted to mechanical stress.) It may also improve the processibility of the product. The number of commercial plasticizers available is extremely large and relying on the experience of polymer manufacturers and suppliers of plasticizers is still the best way to obtain optimum results at a minimum cost. Esters form the largest class and are of the aliphatic (alkyl) or aromatic (aryl) types. Phthalates are esters of difunctional phthalic acid; examples include di-2-ethyl hexyl phthalate (DOP), dibutyl phthalate (DBP), butyl benzyl phthalate (BBP), etc. (4-7)

Dramatic changes in many physical properties of polymers under the action of different additions of plasticizers have been observed in numerous studies. (8,9) In the case of network polymers (such as epoxy resins), a plasticizer is introduced into the mixture of components before chemical forming and, hence, can affect not only the properties of the resulting polymer but also its formation process. Numerous experimental techniques, such as DSC (differential scanning calorimeter), (10-12) DMA (dynamic mechanical analysis), or mechanical measurements, (13,14) and the results from the study of the cure and properties of epoxy resins have been reported in the literature. All these works have highlighted the close relation between the kinetics of the cure, the network that is obtained, and the final properties of the epoxy.

[FIGURE 1 OMITTED]

In this work, we analyzed the set of properties of an epoxy-amine network polymer obtained in the presence of different additions of the plasticizer butyl benzyl phthalate BBP (Figure 1), taking into account their effect on the formation of the network structure.

EXPERIMENTAL

Paints

The primers were formulated using an epoxy resin based on bisphenol A, an aliphatic polyamine-type curing agent was added in the stoichiometric ratio (26% in weight versus resin quantity), and a tertiary amine was used as a catalyst (it was incorporated in 5% in weight versus resin quantity).

[FIGURE 2 OMITTED]

The values of PVC used in this work were obtained by the procedure developed by Bierwagen. (15) The coatings were formulated at PVC/CPVC = 0.98. Pigments used were a talc and calcium carbonate mixture, titanium dioxide, and included a silica-based ion exchange compound as corrosion inhibitor. Table 1 shows the weight percentages of the components of the formulation (without plasticizer).

Different amounts of BBP were used in the formulation in the following weight to epoxy resin ratios: 0, 1.5, 3, 5, 11, 16, and 25%.

Substrates

Standard test panels made of cold-rolled low-carbon steel were used to ensure uniform substrate surfaces for testing purposes. All test panels were degreased with acetone and paints were deposited by means of a graduated spiral application bar calibrated to give a wet film thickness of 150 [micro]m. The systems were allowed to cure under ambient air conditions for one week prior to testing. Paint dry film thickness was determined by using an Elcometer and the thicknesses were maintained within the 50 [+ or -] 5 [micro]m range.

Testing Methods and Equipment

DIFFERENTIAL SCANNING CALORIMETRY (DSC): A Perkin Elmer DSC7 differential scanning calorimeter was employed for dynamic scans in order to measure the [T.sub.g] of primers with different amounts of plasticizers cured at ambient temperature during one week. The DSC was calibrated with a high purity indium sample. The test samples weighed from 8 to 12 mg. A flow of 20 [cm.sup.3] * [min.sup.-1] of nitrogen was used as purge gas. DSC scans at 10[degrees]C * [min.sup.-1] were performed on cured primers to determine the midpoint [T.sub.g].

MECHANICAL TESTS (STRESS-STRAIN CURVES) ADHESION TEST: Stress-strain curves of cured primers (free coating films without metallic substrate) with different amounts of BBP were obtained with an Instron universal machine. At least five cured samples from the primers were cut according to ASTM D 1708-96 and a 100 N cell was used to perform the tensile tests described in the same normative.

[FIGURE 3 OMITTED]

In order to check primer adhesion to substrate, coatings with different BBP content were formulated and applied between two rectangular and clean metallic substrates, as can be seen in Figure 2. Adherence tests were performed according to ISO 4587 (1979). Almost all the specimens were broken because of a type of adhesion failure.

IMPACT TEST: Impact tests have been performed over coated samples by making the impact from the substrate side (reverse impact) according to ISO 6272. The primers used for this test were formulated with different contents of BBP.

The impact tests consisted of checking the primer delamination resistance to the collision of a ball dropped from different heights directly on to the metal substrate side and evaluating if the coating on the other side had been delaminated or not.

DYNAMIC MECHANICAL THERMAL ANALYSIS (DMA): A dynamic mechanical analyzer (Perkin Elmer DMA) was employed to perform mechanical measurements from -100[degrees]C to 150[degrees]C at 1.5[degrees]C * [min.sup.-1] in primers with different amounts of BBP. The tests were carried out in the dual cantilever mode at a frequency of 1 Hz. All samples tested were cured previously as described.

ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY: Electrochemical impedance spectroscopy (EIS) tests were carried out on coated samples exposed to 3.5% (by weight) NaCl in distilled water for periods up to three months. The three-electrode electrochemical cell was obtained by sticking a glass cylinder on the sample sheet and filling it with the test solution. The exposed surface area was 16.6 [cm.sup.2]. A carbon sheet counter electrode and an Ag/AgCl saturated calomel were used as a reference electrode.

The AC impedance data was obtained at the free corrosion potential using an Autolab PSTAT 30 potentiostat and frequency response analyzer. The impedance tests were carried out over a frequency range of 100 kHz down to 1 mHz using 10 mV amplitude of sinusoidal voltage inside a Faraday cage in order to minimize the external interference on the system. The impedance spectrum was analyzed using ZView software.

The equivalent circuit model, shown in Figure 3, was employed to analyze the EIS spectra. The circuit consisted of a working electrode (metal substrate), a reference electrode, electrolyte resistance [R.sub.s], pore resistance [R.sub.po], coating capacitance [C.sub.c], polarization resistance [R.sub.p], and double layer capacitance [C.sub.dl].

Fitting the EIS data to the circuit determined the values of the equivalent circuit elements. The tolerance of the evaluated data was usually below 0.1 (chi squared).

EQUIVALENT CIRCUIT INTERPRETATION: It is generally assumed that the elements of the equivalent circuits are correlated to the corrosion properties of the system. (16)

Pore resistance [R.sub.po] is a measure of the porosity and degradation of the coating. [R.sub.po] values have usually been related to the number of pores or capillary channels perpendicular to the substrate surface through which the electrolyte reaches the interface. (17) Although the [R.sub.po] can also increase with immersion time, probably as a result of pore or defect blockage by corrosion products, it usually decreases.

[FIGURE 4 OMITTED]

Some authors have found three regions in the time-dependent decreases in [R.sub.po]. It initially decreases rapidly, then slowly (displaying a plateau), and then again rapidly, coinciding with the appearance of the second semicircle. They explain the plateau by making the assumption that the number of pathways formed is approximately constant with time.

[C.sub.c] is the capacitance of the coating and it should be a measure of the water permeation into the coating. It is given by:

[C.sub.c] = [epsilon] * [[epsilon].sub.o] * A/d (1)

where [epsilon] is the dielectric constant of the coating, [[epsilon].sub.o] is the vacuum permittivity, A is the area of the coating exposed to the electrolyte, and d is the thickness. The coating capacitance will usually change during electrolyte absorption because the dielectric constant of water is approximately 20 times greater than that of a typical coating. [C.sub.c] usually increases at the initial stage of exposure, and seems to be a measure of water absorption. When the coating has been exposed for a long time it can be correlated to disbonding and degradation.

[FIGURE 5 OMITTED]

The polarization resistance [R.sub.p] and double layer capacitance [C.sub.dl] are two parameters used to specify the disbonding of the topcoat and the onset of corrosion at the interface. The specific polarization resistance is associated with the charge transfer behavior of the metal substrate. [R.sub.p], like [C.sub.dl], can only be calculated well when at least two time constants are apparent in the spectrum.

The double layer capacitance, [C.sub.dl], is a measure of the area over which the coating has disbonded. It can be measured well only at advanced stages of coating deterioration. The trend of [C.sub.dl] is complex. A change in the [C.sub.dl] value can be associated with the competition between disbonding and corrosion product accumulation at the interface. The [C.sub.dl] value increases as water spreads at the interface and the delaminated area extends. On the other hand, the accumulation of the corrosion product at the interface reduces the area of the double layer capacitor, which will lead to a decrease in the [C.sub.dl] value. Therefore, the change of [C.sub.dl] may depend on which factor, the disbonding or the corrosion product accumulation, dominated during the corrosion process. However, it should be pointed out that both the increase and the decrease in [C.sub.dl] are the results of the occurrence of corrosion at the metal surface and a constant [C.sub.dl] is an indication of a stable interface. (18)

ACCELERATED CORROSION TEST: The accelerated corrosion test applied to the primers consisted only of sample exposition to a salt fog spray chamber in accordance with ASTM B 117.

[FIGURE 6 OMITTED]

Coated specimens were previously scribed through the paint down to the bare metal. The scribe marks were made at right angles to form a 6 cm length cross.

Evaluation of specimens was performed after different periods of exposure time and consisted in the measurement of delamination, which is determined by measuring the distance between the center of the scribed line and the coating delamination line. Evaluation of specimens was performed after different periods of exposure time and consisted of the measurement of delamination, which is determined by measuring the distance between the center of the scribed line and the coating delamination line. In order to have the maximum results precision, several measurements (at least 10) were performed in different places along the scribed line.

RESULTS AND DISCUSSION

Thermal Properties

The fundamental molecular effect of a plasticizer is to interact with the segments of the polymer to speed up the viscoelastic response of the polymer. Thus, a plasticizer increases the molecular mobility of the polymer chains and this increase in mobility produces a lowering of the glass transition temperature ([T.sub.g]). Additionally, in the particular case of a thermoset like an epoxy resin, the possible cause of this reduction in the [T.sub.g] is an increase in the excess free volume and also a decrease in the crosslinking density due to some reactive sites being hindered by the large plasticizer molecules. (1,7)

The [T.sub.g] of an epoxy system increases with crosslinking density as the curing reaction progresses, (19,20) so [T.sub.g] is a good indicator of the curing state of the system. This is especially true in the last reaction stages, when isothermal scans are not able to register the heat flow signal if the generated heat is below the apparatus' sensitivity. (21)

As can be seen in Figure 4, the addition of plasticizer causes a decrease in the thermoset [T.sub.g], although it cannot be considered to be a big change in this parameter (around 10[degrees]C with 25% content of plasticizer by weight versus the weight of epoxy). The effect of plasticizer is more significant for the first 5% of weight added and becomes less pronounced as the amount of plasticizer increases above this percentage, as the literature usually reports. (4,5)

Although the plasticizer can act by hindering reactive species in the epoxy resin and/or the crosslinking agent because of its large molecules, the slight decrease in [T.sub.g] shows that the reduction of the crosslinking density is not very significant. (It is reported in the literature that small changes in the crosslinking density below 2% have a strong effect on [T.sub.g].) (21,22)

Dynamic Mechanical Properties

The change of mechanical relaxation as measured by the peak in the dynamic loss curve is often used to characterize the behavior of polymeric plasticizers and plasticized polymers. (4,5)

Figure 5 shows the loss tangent versus temperature for an epoxy primer containing different amounts of BBP plasticizer. This figure shows the characteristic effect of interaction of a plasticizer with a rigid polymer.

The dynamic loss tangent curve for each primer formulation has a peak at a temperature corresponding roughly to the inflection point in the modulus curve (mechanical relaxation peak or [T.sub.[alpha]]). The plasticizer lowers the temperature at which this inflection point ([T.sub.[alpha]]) occurs due to an increase in the excess free volume and/or a decrease in the crosslinking density (as was observed in the thermal properties). Again, it can be seen that BBP does not exert a large effect on the mechanical relaxation temperature ([T.sub.[alpha]] is only reduced around 12[degrees]C when using 25% of plasticizer).

It is commonly found that plasticizers spread the loss tangent peak over a wide range of temperatures, probably due to an increase in polymer heterogeneity. The large plasticizer molecules are able to decrease the probability of encounters between reactive species of the resin and hardener and so, as plasticizer content increases, there will be more and more epoxy and amine groups unreacted. In the case of the thermoset system being studied, BBP does not seem to produce a large increase in chain heterogeneity (there is no spread of the mechanical relaxation over temperature), so perhaps the reactivity of the species is high enough to prevent it from being too reduced because of the presence of BBP.

[FIGURE 7 OMITTED]

One of the parameters that is obtained in the dynamic mechanical analysis is the storage modulus E'. As the values of E' normally have dispersion in the vitreous state, it is quite common to normalize the curves of this parameter by its values at a temperature like--100[degrees]C (E'(T)/[E'.sub.vitreous(-100[degrees]C)]).

As can be seen in Table 2, the parameter (E'(T)/[E'.sub.vitreous (-100[degrees]C)])[.sub.25[degrees]C] decreases by almost 70% from the value of the coating without plasticizer and 25% with it, which means that the rigidity of the primer at ambient temperature has been strongly reduced while there is almost no change in this parameter at 100[degrees]C. Thus, the practical result of incorporating a plasticizer in the epoxy thermoset system is a product that has a lower degree of rigidity at the normal temperature service than the parent resin.

The change in the mechanical properties is caused by one or both of the following reasons: (1) a significant reduction in crosslinking density which gives rise to a decrease in the rigidity over all the temperature range, and (2) a decrease in mechanical relaxation temperature, so the material can have a liquid-like behavior before and, consequently, a reduction in the modulus at service temperatures. The second effect seems to be the main reason why the mechanical properties drop in this case, because the parameter (E'(T)/[E'.sub.vitreous (-100[degrees]C)])[.sub.100[degrees]C], which is directly related to the crosslinking density of the systems, is quite similar for all formulations. (13,14)

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

In this case, the rigidity does not change uniformly with plasticizer concentration. The first 1.5% BBP lowers the parameter (E'(T)/[E'.sub.vitreous (-100[degrees]C)])[.sub.25[degrees]C] by 34%, 5% BBP does the same by 49%, and 16% does so by 61%. This behavior also shows the importance of the [T.sub.g] of the rigid resin being plasticized. A resin with a [T.sub.g] only slightly above room temperature or near it (as in the case being studied) may be drastically less rigid at room temperature than its initial value by blending it with only 1.5% plasticizer. On the other hand, a resin having a higher [T.sub.g], for example above 100[degrees]C, may show only a small drop in room temperature stiffness with mixtures containing large amounts of plasticizer.

Mechanical Properties

Figure 6 shows stress-strain curves corresponding to tensile tests for primers formulated with different amounts of BBP. Table 3 shows the mechanical parameters obtained from the mechanical tests: elastic modulus (E), elastic limit stress ([[sigma].sub.el]), elastic limit strain ([[sigma].sub.el]), maximum stress ([[sigma].sub.max]), break stress ([[sigma].sub.brk]), and break strain ([[epsilon].sub.brk]).

It can be clearly observed that as the amount of plasticizer increases in the primers, the coatings become less rigid (the Young's modulus E decreases), this effect being especially significant in the first 1.5-3% of plasticizer added. Nevertheless, as the amount of plasticizer increases in the primers there is a drop in the maximum stress ([[sigma].sub.max]) and break stress ([[sigma].sub.brk]). The rigidity decrease is not accompanied by an increase in ductility (the [[epsilon].sub.brk] is reduced), which means that the addition of BBP reduces the mechanical quality of the primer.

Figure 7 shows the shear stress ([[sigma].sub.shear]) needed to pull apart two coupons of metal glued together by the coating in the adhesion test.

It can be clearly seen in Figure 7 that if the plasticizer content increases, there is a reduction of adherence between the primer and the metallic substrate. As the adhesion of the coating to the substrate depends, in one part, on hydrogen bonds between OH groups of the resin and the metallic substrate, the presence of plasticizer in the composition that can be situated between these groups and the metal surface can reduce the adherence of the coating.

Nevertheless, if the epoxy primer does not contain any plasticizer it has a high degree of rigidity and, as the coatings suffer high levels of stress (during the thermoset curing, thermal cycles existence, water absorption, corrosion products development in the metal primer interface ...), the coating may be more likely to delaminate.

Additionally, if the rigidity is high, the epoxy primer will not be able to suffer reverse impacts during service without breaking. Impact tests have been performed on primers formulated with different amounts of BBP, according to ISO 6272. Results show that primers resist the impact of a ball dropped from a height of over 100 cm without breaking when the amount of plasticizer added is more than 3%. If plasticizer content is below 3%, the primer does not resist an impact from a height of even 10 cm.

In regard to the epoxy primer studied in this article, it can be said that the addition of plasticizer improves the impact resistance of the coatings and may possibly reduce the film delamination tendency. However, the BBP presence in the formulations significantly reduced the primer adhesion to the substrate (especially if the BBP percentage is bigger than 3%).

After these results the question may be: Which is the optimum amount of plasticizer to add to the epoxy primer in order to get the maximum anticorrosion performance?

Electrochemical Behavior

Figure 8 shows a Bode plot representing the impedance response (impedance modulus |Z| and phase angle) versus frequency for primers formulated with different amounts of BBP after exposure to electrolyte for 15 days.

As can be clearly seen, primers formulated with 1.5-3% concentration of plasticizer by weight versus epoxy resin have higher impedance modulus than coatings formulated without it. On the other hand, when primers are formulated with more than 3% of BBP (5, 11, 16, or 25%), the impedance modulus are lower than primers with 0-3% of BBP.

Figures 9a-c show the equivalent circuit parameters modelized for EIS results (as was explained in the Experimental Section) corresponding to pore resistance [R.sub.po], polarized resistance [R.sub.p], and double layer capacitance [C.sub.dl]. Results of coating capacitance [C.sub.c] are not shown because they present slight differences between samples and an almost constant value over the exposition time interval of 2 * [10.sup.-9] F. It can also be seen that primers formulated with 1.5-3% of plasticizer had the best anticorrosion properties, with the maximum [R.sub.po] and [R.sub.p], and minimum [C.sub.dl]. If the plasticizer content is over 3%, the primer/metallic substrate interphase is more active ([R.sub.p] is not stable and there is a significant increase in [C.sub.dl], which means a greater tendency towards coating delamination).

As has been studied in this article, the effect of plasticizer on the primer is:

(a) A decrease in the mechanical properties (especially in the rigidity, the Young's Modulus parameter) but an improvement in the impact resistance. Nevertheless, the reduction of the rigidity can prevent the coating delamination because of the application of stress service conditions, change of volume (contraction) during the curing reactions, etc.

(b) An increase in the free volume and a slight decrease in the crosslinking density, which both cause [T.sub.g] to be reduced.

As the primer is formulated with BBP there is a positive effect, which is that the coating film is less rigid and the delamination process is reduced when stress is applied. In addition, the decreased rigidity prevents important coating failure due to the formation of cracking.

On the other hand, the increase of permeability due to the addition of plasticizer can make the active species arrive at the metal surface easier, making the interphase more active and giving rise to coating delamination.

So, from the point of view of the anticorrosion properties, there will be an optimum plasticizer content that will increase all the positive aspects (stability of the interphase and prevention of cracking) without increasing the active species permeability too much. As has been studied for this particular epoxy primer, the optimum amount of BBP is 1.5-3%.

[FIGURE 10 OMITTED]

Accelerated Test

Figure 10 presents the delamination results for primers exposed to salt fog spray test for 215 hr. Results indicate that the minimum delamination is obtained when the primer is formulated with 1.5% of plasticizer, although the primer formulated with 3% of plasticizer has a low delamination value too. There is an increase of the delamination when BBP content is under 1.5% or over 3%.

CONCLUSION

Various epoxy primers were formulated with different amounts of BBP plasticizer and were then studied by means of thermal analysis (DSC), mechanical analysis (DMA), mechanical and electrochemical techniques, and an accelerated corrosion test.

Increased plasticizer content reduces the mechanical relaxation temperature ([T.sub.[alpha]]), reduces the values of the mechanical properties of the primer (especially in the first 5% that is added), and the adherence to the metallic substrate. On the other hand, BBP presence increases the impact resistance. The maximum anticorrosion properties were obtained with 1.5-3% of plasticizer versus epoxy resin by weight.

All these results were interpreted from the structural-kinetic effect of the plasticizer in the course of the epoxy network synthesis, the increases in the excess free volume, and the possible slight decrease in the crosslinking density.

Table 1 -- Components of the Formulation without Plasticizer (in Weight
Percentages)

Formulation Components                                Wt%

Calcium carbonate                                     27.8
Talc carbonate                                         8.3
Titanium dioxide                                      27.8
Silica-based ion exchange compound                     6.9
Binder (epoxy resin + crosslinking agent + catalyst)  29.2

Table 2 -- Main Mechanical Relaxation Temperature and Storage Modulus
(Normalized to Storage Modulus at Vitreous State -100[degrees]C) for
Temperatures of 25 and 100[degrees]C Versus the Amount of BBP in the
Formulation of Epoxy Primers Obtained by Dynamic Mechanical Analysis

% Plasticizer  T[alpha]      (E'(T)/[E'.sub.vitreous -100[degrees]C)]
               ([degrees]C)  [.sub.25[degrees]C]

 0             22.3          11.0 E-3
 1.5           14.7           7.3 E-3
 3             15.2           7.5 E-3
 5              5.6           5.6 E-3
11             20.2           2.8 E-3
16             12.6           4.3 E-3
25              9.6           3.6 E-3

% Plasticizer  (E'(T)/[E'.sub.vitreous -100[degrees]C)]
               [.sub.100[degrees]C]

 0             1.4 E-3
 1.5           1.8 E-3
 3             2.3 E-3
 5             2.8 E-3
11             1.6 E-3
16             1.6 E-3
25             1.7 E-3

Table 3 -- Mechanical Parameters versus Amount of Plasticizer in the
Formulations of Epoxy Primers Obtained from Stress-Strain Curves in
Traction Tests

                  Elastic               Elastic Limit
                Modulus (E)                 Stress
% Plasticizer      (MPa)                    (MPa)

 0              18.478 [+ or -] 0.160   0.022 [+ or -] 0.003
 1.5             7.92 [+ or -] 2.372    0.015 [+ or -] 0.004
 3               9.03 [+ or -] 0.741    0.010 [+ or -] 0.002
 5               6.64 [+ or -] 2.286    0.012 [+ or -] 0.003
11               6.37 [+ or -] 1.660    0.029 [+ or -] 0.009
16               5.15 [+ or -] 1.265    0.021 [+ or -] 0.006
25               3.00 [+ or -] 0.156    0.024 [+ or -] 0.002

                Elastic Limit          Maximum
                Strain                 Stress ([[sigma].sub.max])
% Plasticizer   (mm/mm)                (KN)

 0              0.37 [+ or -] 0.052    0.024 [+ or -] 0.002
 1.5            0.104 [+ or -] 0.030   0.020 [+ or -] 0.001
 3              0.075 [+ or -] 0.019   0.015 [+ or -] 0.003
 5              0.078 [+ or -] 0.052   0.014 [+ or -] 0.001
11              0.183 [+ or -] 0.085   0.015 [+ or -] 0.002
16              0.095 [+ or -] 0.010   0.010 [+ or -] 0.001
25              0.065 [+ or -] 0.009   0.010 [+ or -] 0.001

                Break                       Break
                Stress ([[sigma].sub.brk])  Strain ([[epsilon].sub.brk])
% Plasticizer   (MPa)                       (mm/mm)

 0              2.470 [+ or -] 0.046        0.860 [+ or -] 0.022
 1.5            1.552 [+ or -] 0.165        0.765 [+ or -] 0.083
 3              1.312 [+ or -] 0.060        0.731 [+ or -] 0.028
 5              1.019 [+ or -] 0.105        0.591 [+ or -] 0.065
11              1.470 [+ or -] 0.360        0.747 [+ or -] 0.117
16              1.175 [+ or -] 0.265        0.720 [+ or -] 0.110
25              1.003 [+ or -] 0.056        0.723 [+ or -] 0.027

ACKNOWLEDGMENTS

We would like to thank Ms. Eva Romero Sales and Mr. Jose Ortega for their help and collaboration. The authors are grateful for the economic support of project MAT20000123-P4-03 in this work.

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M.T. Rodriguez, S.J. Garcia, R. Cabello, and J.J. Suay -- University Jaume I*

J.J. Gracenea -- Naber Pinturas y Barnices, S.L. ([dagger])

* Department of Technology, Av. de Vicent Sos Baynat s/n, 12071 Castellon, Spain.

([dagger]) N-332, Km. 279, 46469 Beniparrell, Valencia, Spain.