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The effect of the addition of a low profile additive on the curing shrinkage of an unsaturated polyester resin.


Composites made of long, or even continuous glass fibers and unsaturated polyester (UP) resin are widely applied because of the low material costs, low density, ease of manufacturing, and good mechanical properties. However, optimal surface appearance and optimal strength properties may be decreased by large resin shrinkage during curing.

A volumetric shrinkage of about 10% during curing is common for UP resins. Such a shrinkage may cause surface defects with a negative effect on the appearance of the product. Another effect of the shrinkage is the occurrence of residual stresses in the composite, because the glass fibers do not shrink during curing. The residual stresses in the composite may be quite substantial, as presented by Ten Busschen (1) and Zhang (2). Residual tensile stresses exceeding 30 MPa were estimated using finite element calculations for a regular fiber distribution (2). Moreover, irregular fiber distributions may show even higher stress levels (2). Actual composites show quite irregular fiber distributions, and the local tensile stress due to curing shrinkage may well exceed the resin strength as measured in a tensile test. Consequently, initial microscopic resin cracks might be present already after the completion of the curing cycle. Indeed, first microcracks at the fiber resin interface have been observed already prior to loading, Wood and Bradley (3), or in an early loading stage, Bee and Bader (4). Stresses due to thermal shrinkage may enhance this cracking problem. However, the curing shrinkage of the resin is much larger than the thermal shrinkage, therefore curing shrinkage has been chosen as the topic of the investigation described here.

One method of controlling the curing shrinkage is the addition of Low Profile Additive (LPA) in the resin formulation. The LPA is usually a thermoplastic material that can be added to the thermoset resin mixture to compensate for the volumetric curing shrinkage. In the present work, the influence of LPA on the reduction of the curing shrinkage and therefore curing stresses has been investigated.


Four types of experiments was performed, aiming at:

1) The determination of volumetric shrinkage during the curing process.

2) The determination of the development of the shrinkage stress during curing, while constraining shrinkage in one dimension.

3) Tensile tests on cured resin.

4) Curvature measurements on asymmetric composite plates with and without LPA.

Volume Shrinkage

The experiments were performed at three (nominal) temperatures (62 [degrees] C, 72 [degrees] C, and 82 [degrees] C) and various amounts of LPA were added to the resin. The amount of catalyst was also varied occasionally. However, not all experiments were performed at each temperature. The resin chosen for the experiments was Synolite 0423-N-2, which is a "pure maleic acid" unsaturated polyester resin from DSM Resins (The Netherlands), and Synolite ZB 2681-X-1 as an LPA, which is a saturated polyester with a DSM-proprietary chemistry. Synolite 0423-N-2 is a highly unsaturated polyester solution in the reactive styrene monomer (a solid content of about 65%). This resin has a good compatibility with low profile additives such as polystyrene and polyvinyl acetate.

The volume shrinkage experiment was performed in newly designed equipment as shown in Fig. 1. A thin layer of resin (about 3 mm thick) was injected between two steel plates (with thicknesses of 10 and 2 mm). Special care was taken to avoid air inclusions during injection; after each test, the plates were removed to check whether air inclusions had indeed been avoided. The weight of the upper plate, acting on the resin, was neutralized with a balancing system (see Fig. 1). The whole set was assembled in an oven, and resin injection was performed after heating up the assembly to the desired testing temperature. A thermocouple was sealed in the resin between the plates in order to measure the temperature. The tests were performed for various LPA contents and at nominal temperatures of 62 [degrees] C, 72 [degrees] C, and 82 [degrees] C.

The volume change due to curing was identical to the change of the thickness of the resin layer, because the whole area between the plates remained filled with resin and the resin adhered well to the steel plates. Consequently, the volume change is measured as a thickness change, which is determined with an eddy current sensor.

The designed setup [ILLUSTRATION FOR FIGURE 1 OMITTED] in fact provides a volume shrinkage experiment under in-plane (2-dimensional) shrinkage constraints, and considerable in-plane shrinkage stresses may be expected after gelation of the resin. These in-plane curing stresses will have a slight effect on the volume shrinkage. However, the resulting "bulk strain" is low as compared with the "curing strain" and therefore this effect of stresses may be neglected. Moreover, cavitation due to in-plane strain was not observed, and separate density measurements before and after curing, on specimens being cured without constraint, gave identical results as obtained at the beginning and the end of the shrinkage tests between plates. Consequently, the volume shrinkage as measured during the curing process is also regarded as sufficiently accurate.

Stress Under One-Dimensional (l-D) Shrinkage Constraint

The experimental setup is illustrated in Figs. 2a and 2b. A slender cylindrical cavity in a steel mold was filled with resin. The lower side of the cavity was designed with a larger diameter in order to lock the resin during curing. A screw was applied to the upper side of the cavity in such a way that the screw head was below the surface of the resin. The mold cavity was treated with a mold release agent in order to provide decohesion from the mold walls in an early curing stage. In this way a setup is provided yielding a constraint on the resin shrinkage in the longitudinal direction only, namely the vertical direction in Fig. 2, and no constraint on the resin shrinkage in the diameter direction.

Owing to the resin shrinkage, the screw that is connected to a load cell encounters a downward force. The measured force is divided by the (nominal) cross-sectional area of the cylindrical resin specimen in order to obtain the longitudinal tensile stress in the resin as a function of time during curing. Tests were performed for various LPA contents and at curing temperatures of 62 [degrees] C and 72 [degrees] C.

The chosen load cell has an accuracy better than [+ or -]4 N. Considering the diameter of the resin cavity (7 mm), the accuracy of the stress measurement is better than [+ or -]0.1 MPa. This is considerably less than the measured stresses (around 2 MPa), and consequently the stress results are sufficiently accurate.

Tensile Tests

The Young's modulus and fracture stress of cured specimens were obtained from tensile tests on dog-bone specimens. The strain rate was about 0.1% per second. The specimens without LPA were cured at 120 [degrees] C and those with LPA were first cured at 82 [degrees] C and subsequently post cured at 120 [degrees] C.

Curvature Measurements

The amount of LPA, that results in a zero residual stress in the experiment with 1-D constraint was estimated by interpolation of the test results. It appeared to be about 25% (see conclusions). Note that weight percentages are always presented in this paper. Asymmetric composite sheets without LPA and with 25% of LPA were produced at 70 [degrees] C. Curing stresses due to resin shrinkage may cause a curvature of asymmetric laminates. The effect of the addition of 25% LPA on the curvature could be observed.



The results of the tensile tests are summarized in Table 1. The reported values are an average of the results of four tests. It is observed that the Young's modulus decreases almost linearly with increasing LPA content. The LPA appears to have a rubbery character. This was also revealed by DMA tests showing a secondary glass transition temperature at -40 [degrees] C for materials containing LPA. The secondary glass transit_ion temperature coincides about with the glass temperature of the LPA. Such a secondary glass transition temperature of the cured resin is an indication for phase separation. Some consequences of phase separation are discussed in the conclusions. The fracture stress of the specimens shows an initial increase with increasing LPA content and a subsequent decrease. The maximum fracture stress occurs between 7% and 14% LPA, probably at about 10% LPA. A similar trend is observed for the fracture strain. However, the peak value is observed at an amount of about 20% LPA.

The measurements on volume shrinkage and on 1-D constrained shrinkage stress are performed at various LPA-contents, various contents of a Cobalt containing catalyst, and at three temperatures. However, not all combinations of these parameters have been tested. Table 2 presents a survey of performed tests.
Table 2. Survey of the Tests for 1-D Shrinkage Stress Measurements
(1) and Volume Shrinkage Measurements (2) for Different Amounts of
Cobalt Catalyst (in Weight %).

Curing Temperature ([degrees] C)

 62 72 82
Amount of LPA Co % Co % Co %

(%) 0.1 0.2 0.04 0.1 0.2 0.04 0.1

0 (2) (1, 2) (2) (1, 2) (2) (2) (2)
7 (1, 2) (1, 2) (2) (2)
14 (2) (1, 2) (1, 2) (2) (2)
21 (2) (1, 2) (1, 2) (2) (2)
28 (1, 2) (1, 2) (2) (2)
35 (1, 2) (1, 2) (1, 2)

Temperature variations due to exothermic curing reaction have occurred during some of the volume shrinkage experiments. Figures 3 and 4 show some examples for the experiments at 72 [degrees] C and 62 [degrees] C, respectively. A lower curing temperature, a lower amount of catalyst, and a higher amount of LPA tend to reduce the exothermic peak temperature. The peak temperature interferes with the curing reaction and may influence the reaction rate and LPA effect. However, at 60 [degrees] C and larger LPA contents the temperature peak was absent in the present setup. The only effect observed is the effect of resin heating, after the application of resin in the preheated experimental setup. Consequently, the related experimental results are considered to be not influenced by the peak temperature. Moreover, the temperature changes in time are only a few degrees [ILLUSTRATION FOR FIGURE 4 OMITTED]; considering the thermal expansion coefficient of polymers and the large shrinkage due to resin curing, thermal shrinkage may be neglected. Thermal gradients in a 3 mm resin layer will be very small during the experimental observation time. Again, it is concluded that thermal expansion or shrinkage is effectively minimized in the present experimental set-up. Other tests with less LPA and an 80 [degrees] C curing temperature may be inaccurate, but they are still illustrative in a qualitative sense. It can be observed in Fig. 4 that a higher LPA content causes a lower temperature peak. This is expected, because the LPA is a nonreactive thermoplastic component. Thus, its presence dilutes the exothermic heat production and lowers the temperature peak.

The results of the volume shrinkage experiments are presented in Figs. 5, 6, and 7. It is observed in these experiments that a final volume shrinkage of about 10% occurs for resin without LPA. An LPA content of up to 14% does not cause a qualitative change. The effect of low LPA contents may be considered as merely "diluting" the resin shrinkage.

Larger LPA contents show a qualitatively different effect. The shrinkage becomes reversed after some curing time. Mixture expansion starts rather suddenly at a certain time, and it flattens afterwards. The time at which expansion starts becomes shorter with an increasing amount of LPA. The expansion rate also increases with an increasing amount of LPA. Note that the time scale of the process in Figs. 5 to 7 is different. Higher temperatures cause faster curing and earlier LPA effects. Nevertheless, the shapes of the curves remain similar for the different temperatures, suggesting that the observed effects are merely dependent on the resin curing state.

Figure 8 shows the final volume change due to shrinkage as a function of the LPA content. For the three temperatures the curves are almost identical, again indicating that the reaction state is the most important parameter for the effects observed in the Figs. 5 to 7.

Figures 9 and 10 show the stress development in the 1-D constraint test [ILLUSTRATION FOR FIGURE 2 OMITTED] during the curing process. The stress remains zero during the first curing stage where the resin is still a liquid. In this stage, shrinkage is compensated by the flow of excess resin from above the screw-head to the cylinder below it. Stress build-up starts when the gel point is reached. Resins with low LPA contents develop a tensile stress due to shrinkage. The tensile stress decreases after some time for resin with an intermediate LPA content. Resins with large LPA contents (28%) even develop a compressive stress as a result of the resin expansion.

Figure 11 shows a picture of two cured unsymmetric composite laminates made of glass-fiber bundles in two perpendicular directions (cross-ply laminate). The two bundle layers are stitched together with a very thin fiber bundle. The asymmetric laminate without LPA shows a significant curvature due to curing stresses. The asymmetric plate containing 25% LPA in the resin does not show a significant curvature


UP resins may show about 10% volume shrinkage due to curing. A part of this shrinkage occurs after the gel point of the resin. This "solid state" shrinkage causes residual stresses in a fiber reinforced composite due to the shrinkage constraint applied by the stiff fibers. It has been demonstrated by Kiasat et al. (5) that shrinkage stresses develop during an ongoing process of stress buildup, caused by further shrinkage under constraint, and stress relaxation of the curing resin. Consequently, a reduction of the stress buildup should be expected from a reduction of resin shrinkage. The addition of LPA may offer such a shrinkage reduction. Indeed, the addition of LPA is obviously effective for a reduction of the shrinkage stresses in UP resin products. The reduction is caused by two separate effects:

1) Dilution of shrinkage due to the presence of the nonreacting LPA material. Resins with low LPA amount show this effect only.

2) Expansion. starting rather suddenly during curing. This is exclusively observed for LPA contents above about 14%. The resin expansion initially decreases the tensile shrinkage stress that developed previously in an earlier curing stage. Moreover, compressive stresses may develop during further curing.

The relation between the shrinkage and stress buildup is not linear; the changes of the volume before resin gelation have no consequences for the stress buildup (5). About a half the shrinkage has already occurred when gelation starts. Consequently, the very left part in Figs. 5 to 7 has no effect on the shrinkage stress. This explains why compressive stresses may develop, as shown in Figs. 9 and 10, for materials showing an overall shrinkage behavior, as shown in Fig. 8. The expansion during the last curing stage is responsible for the compressive stresses developing in systems with a high LPA content.

The LPA content resulting in zero residual stresses for a completely cured resin may be considered as optimal, from the viewpoint that residual stresses will always decrease the composite quality. Interpolation in the Figs. 9 and 10 reveals that about 25% of LPA may be optimal from that point of view. Indeed, the picture in Fig. 11 shows no curvature for the plate with a matrix containing 25% LPA, indicating the absence of curing stresses in the final curing stage, contrary to the plate without LPA, which shows a significant curvature.

However, other resin properties are also important. The Young's modulus and tensile strength of the resin are seriously decreased if a 25% LPA is applied. An optimal resin formulation remains a compromise, and it requires a decision on the importance of the different resin properties. Such a decision may be influenced by the anticipated resin application as well. Considering the results from the present investigation only, an LPA content between 15% (as a lower limit, regarding the fact that the expansion effect should occur to a significant amount) and 20% (as an upper limit, for retaining a reasonable Young's modulus and tensile strength) would be chosen.

The physical cause of the LPA effect is beyond the scope of the present investigation, which is mainly devoted to the determination of the LPA effect. Nevertheless, some reasoning win be presented here. Phase separation and microvoiding are generally accepted as two explanations. However, microvoiding is a mechanism driven by the presence of tensile stresses. This mechanism cannot explain the present experimental results, because the LPA effect (expansion) proceeds even if the tensile stress is reversed and a compressive stress occurs. Consequently, microvoiding is not a valid explanation for the present LPA effect. Phase separation remains a plausible explanation. Indeed, nonpublished microscopic investigations, showing phase separation, have been performed by our resin supplier (DSM). The LPA is completely soluble in the liquid resin. However, phase separation starts suddenly during curing and proceeds during further curing. Apparently, the LPA becomes insoluble in the resin when its chemistry changes during curing. Phase boundaries may have a lower density than the adjacent phases. Consequently, the developing phase boundaries may cause expansion, It may be expected that the sudden reverse of trends, a change from shrinkage to expansion in Figs. 5 to 7, coincides with the onset of the phase separation. The phase separation explains the expansion. The limited effect of lower LPA contents in Fig. 5 to 10 is consistent with this explanation. Obviously, small contents of LPA do not exceed the solubility limit of the curing resin, and phase separation does not occur. The shrinkage is only diluted for low LPA contents


DSM Resins in Swell (The Netherlands) is thanked for providing the authors with UP resin and LPA. SENTER is acknowledged for funding a significant part of this investigation, under project IOP-PCBP 5.2.


1. A. ten Busschen, PhD thesis, Delft University of Technology, the Netherlands (1996).

2. L. Zhang, PhD thesis, Delft University of Technology, The Netherlands (1995).

3. C. Wood and W. Bradley, ASTM STP 1290, 132-151 (1996).

4. M-Y. L. Bee and M. G. Bader, Proc. ICCM-10, Whistler, B.C., Canada (1995).

5. M. S. Kiasat, A. H. J. Nijhof, H. Blokland, and R. Marissen, Proc. 5th European Conf. on Advanced Materials and Processes and Applications, 2, 95-102, Netherlands Society for Materials Science, Zwijndrecht (1997).
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Author:Liu, C.J.; Kiasat, M.S.; Nijhof, A.H.J.; Blokland, H.; Marissen, R.
Publication:Polymer Engineering and Science
Date:Jan 1, 1999
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