Fully impregnated mainwall insulation

Weight and volume increments during the test are shown in Fig. 5.8. Tensile strength re-tained during the test is illustrated in Fig. 5.9 and E-modulus and toughness in Fig. 5.10, respectively. In Fig. 5.11 the stress versus strain curves are shown. Materials G2E and

Fig. 5.8. Weight and volume increase of different mica tape samples.

Mainwall insulation samples from two different impregnation processes were tested: 1) fully impregnated and transparent samples and 2) samples not completely saturated with the resin.

The purpose of this comparison is to emphasize the importance of the proper manufacturing procedure on the performance of the mainwall insulation. The level of cure was not mea-sured. The determination of full and imperfect impregnation was carried out based on simple observations. It was seen during the cutting process of the test samples that the impregna-tion number 2 had not been good. Furthermore, the samples were not fully transparent, as they were with the impregnation 1. The fully saturated samples are transparent, because the resin, glass, and mica are all transparent and there is no air inside the insulation to diffract the light. The diffraction of light is determined by the speed of light in the medium (Young and Freedman, 2000), which in turn is proportional to the square root of the permittivity of the medium. The permittivities of the insulating materials are similar, but the permittivity of air is significantly lower, which causes the diffraction of light in the voids. The impregnation number 1 refers to a succesful impregnation process, even though it (and any industrial VPI process) is not absolutely perfect.

In the previous chapter it was pointed out that the mechanical parameters except E-modulus are not reliable because of the premature breaking points of brittle resins. With mica tapes, this statement is not valid, because the composite structure is not that brittle.

5.2.1 Fully impregnated mainwall insulation

Weight and volume increments during the test are shown in Fig. 5.8. Tensile strength re-tained during the test is illustrated in Fig. 5.9 and E-modulus and toughness in Fig. 5.10, respectively. In Fig. 5.11 the stress versus strain curves are shown. Materials G2E and

Fig. 5.8. Weight and volume increase of different mica tape samples.

5.2 Screening of mica tapes 121

G2P are not included in Fig. 5.11. They cannot be compared with other tapes, because they are manufactured with a different process and possess a higher quality than the hand-made samples.

Fig. 5.9. Tensile strength remaining after the exposure to wellstream gas mixture.

Fig. 5.10. E-modulus and toughness remaining after the exposure to wellstream gas mixture.

It is clear, based on Fig. 5.8, that RR tapes do not expand as much as VPI tapes and the resin, and furthermore, their weights do not increase as much. It is also evident that the insulation with zinc naphthenate (Z) has expanded over three times as much as the others. Furthermore, it has lost its strength and modulus almost completely, which can be seen in Figs. 5.9, 5.10, and 5.11. The insulation has absorbed a lot of liquids and become soft, and it clearly does not withstand the environment. It is also evident in Fig. 5.8 that the volume increase of material G2P has been notably higher than those of other VPI tapes.

5.2 Screening of mica tapes 121

G2P are not included in Fig. 5.11. They cannot be compared with other tapes, because they are manufactured with a different process and possess a higher quality than the hand-made samples.

Fig. 5.9. Tensile strength remaining after the exposure to wellstream gas mixture.

Fig. 5.10. E-modulus and toughness remaining after the exposure to wellstream gas mixture.

It is clear, based on Fig. 5.8, that RR tapes do not expand as much as VPI tapes and the resin, and furthermore, their weights do not increase as much. It is also evident that the insulation with zinc naphthenate (Z) has expanded over three times as much as the others. Furthermore, it has lost its strength and modulus almost completely, which can be seen in Figs. 5.9, 5.10, and 5.11. The insulation has absorbed a lot of liquids and become soft, and it clearly does not withstand the environment. It is also evident in Fig. 5.8 that the volume increase of material G2P has been notably higher than those of other VPI tapes.

Fig. 5.11. Stress versus strain curves for mica tape samples.

The reference values for tensile strengths of mica tapes can be observed in Fig. 5.11. They differ slightly from the ones reported in (Emery and Smith, 2001) and (Emery and Williams, 2007), but they can be arranged in a similar order: tape with modified glass fabric (G1) is the strongest, Kapton^R backing (K) comes next, and traditional woven glass-fabric-backed tape (G2) has the lowest tensile strength. Furthermore, Emery and Smith (2001) reported that the tape with zinc naphthenate is stronger than the Kapton^R-backed tape. However, Emery and Smith (2001) and Emery and Williams (2007) used somewhat different manufacturing and testing methods and did not reveal their impregnating resins. Furthermore, they did not apply any aging to the samples and discussed only the reference samples.

The mechanical properties, namely tensile strength, of the reference samples depend some-what on the tape structure, but as it can be observed in Fig. 5.11, the tensile strength after the test is virtually the same with materials K, N, G1, G2, and RR1. Excluding the last material, all the tapes have been impregnated with the ”resin” in a VPI process. Material Z contains similar glass fabric as material G2. The only difference of these two materials is the accelera-tor of the resin. As pointed out in the previous section, the resin containing zinc naphthenate does not withstand the environment. Its failure in the wellstream gas has been the reason for the failure of material Z in this experiment. Therefore, it can be stated that the strength after the exposure to wellstream gas environment is mainly provided by the resin and it is almost independent of the tape construction.

Fig. 5.11. Stress versus strain curves for mica tape samples.

The reference values for tensile strengths of mica tapes can be observed in Fig. 5.11. They differ slightly from the ones reported in (Emery and Smith, 2001) and (Emery and Williams, 2007), but they can be arranged in a similar order: tape with modified glass fabric (G1) is the strongest, Kapton^R backing (K) comes next, and traditional woven glass-fabric-backed tape (G2) has the lowest tensile strength. Furthermore, Emery and Smith (2001) reported that the tape with zinc naphthenate is stronger than the Kapton^R-backed tape. However, Emery and Smith (2001) and Emery and Williams (2007) used somewhat different manufacturing and testing methods and did not reveal their impregnating resins. Furthermore, they did not apply any aging to the samples and discussed only the reference samples.

The mechanical properties, namely tensile strength, of the reference samples depend some-what on the tape structure, but as it can be observed in Fig. 5.11, the tensile strength after the test is virtually the same with materials K, N, G1, G2, and RR1. Excluding the last material, all the tapes have been impregnated with the ”resin” in a VPI process. Material Z contains similar glass fabric as material G2. The only difference of these two materials is the accelera-tor of the resin. As pointed out in the previous section, the resin containing zinc naphthenate does not withstand the environment. Its failure in the wellstream gas has been the reason for the failure of material Z in this experiment. Therefore, it can be stated that the strength after the exposure to wellstream gas environment is mainly provided by the resin and it is almost independent of the tape construction.

5.2 Screening of mica tapes 123

Based on the mechanical parameters and increases in weight and volume seen in Figs. 5.8 to 5.10, the RR tapes tolerate the environment better than the VPI tapes. They do not ab-sorb as much gas and, presumably thus, retain their mechanical properties better. However, their mechanical properties have been initially lower. After the test, they have rather similar mechanical behavior as the VPI tapes, as can be seen in Fig. 5.11.

Resin rich tapes contain epoxy novolac resin, the resistance of which to wellstream gas mix-ture was discussed in the previous section. It was found that the epoxy novolac resin gained less weight and expanded less than the epoxy anhydride resin. Nevertheless, the difference in mechanical properties was marginal. The results presented in the previous section (Figs.

5.1 and 5.3) are therefore convergent with Figs. 5.8 to 5.10, although the epoxy novolac resin inside the RR tapes is not the very same as the material A in Section 5.1. Despite seemingly better performance of the RR tapes, it is not fully justified to favor them over the VPI tapes.

All of the VPI tapes, except the one with zinc naphthenate (Z), were still in fair condition after the exposure. The changes in their parameters had not been critical.

The performance of material G2P has been slightly worse than that of the materials impreg-nated with epoxy resin, except material Z. According to Fig. 5.9, the tensile strength has decreased to half during the test, but accordingly, it has diminished also with materials K, G1, and RR2. But the E-modulus of G2P has dropped down to approximately one third, which is roughly 10% less than that of other VPI tapes, except material Z. The hyphothesis based on Sihvo et al. (2008) has obviously been too harsh, because the materials do tolerate the wellstream gas. Nevertheless, similar tape impregnated with epoxy resin using a BCl₃ ac-celerator is able to tolerate the environment better. The observation encourages to use epoxy resin over the common and cost-effective polyesterimide.

Material G2 before and after the test is illustrated in Fig. 5.12. Material P is not included in Figs. 5.8 to 5.10, because it was damaged very badly and no measurements were carried out.

Material P was totally delaminated, which can be seen in Fig. 5.13.

It can be seen in Fig. 5.12 that the insulation has lost its transparency almost completely. It is virtually impossible to discern the logo in the right-hand figure, whereas it is clearly visible on the left. The lighting conditions have not been exactly the same, but the conclusion is still undeniable. No distinctive delamination was observed with a microscope on material G2.

Transparency was weakened as a result of the diffusion of wellstream gas. The transparency of the samples was used as a sort of a quality indicator in the VPI process. A transparent build as seen on the left in Fig. 5.12 represents good impregnation.

The failure of material P is obvious in Fig. 5.13. The backing material has disappeared from the insulation causing total delamination. The insulation was based on resin rich technology and epoxy novolac resin, which means that neither the manufacturing process nor the im-pregnating resin has been the reason for its poor performance. PET film clearly reacts with the wellstream gas.

5.2 Screening of mica tapes 123

Based on the mechanical parameters and increases in weight and volume seen in Figs. 5.8 to 5.10, the RR tapes tolerate the environment better than the VPI tapes. They do not ab-sorb as much gas and, presumably thus, retain their mechanical properties better. However, their mechanical properties have been initially lower. After the test, they have rather similar mechanical behavior as the VPI tapes, as can be seen in Fig. 5.11.

Resin rich tapes contain epoxy novolac resin, the resistance of which to wellstream gas mix-ture was discussed in the previous section. It was found that the epoxy novolac resin gained less weight and expanded less than the epoxy anhydride resin. Nevertheless, the difference in mechanical properties was marginal. The results presented in the previous section (Figs.

5.1 and 5.3) are therefore convergent with Figs. 5.8 to 5.10, although the epoxy novolac resin inside the RR tapes is not the very same as the material A in Section 5.1. Despite seemingly better performance of the RR tapes, it is not fully justified to favor them over the VPI tapes.

All of the VPI tapes, except the one with zinc naphthenate (Z), were still in fair condition after the exposure. The changes in their parameters had not been critical.

The performance of material G2P has been slightly worse than that of the materials impreg-nated with epoxy resin, except material Z. According to Fig. 5.9, the tensile strength has decreased to half during the test, but accordingly, it has diminished also with materials K, G1, and RR2. But the E-modulus of G2P has dropped down to approximately one third, which is roughly 10% less than that of other VPI tapes, except material Z. The hyphothesis based on Sihvo et al. (2008) has obviously been too harsh, because the materials do tolerate the wellstream gas. Nevertheless, similar tape impregnated with epoxy resin using a BCl₃ ac-celerator is able to tolerate the environment better. The observation encourages to use epoxy resin over the common and cost-effective polyesterimide.

Material G2 before and after the test is illustrated in Fig. 5.12. Material P is not included in Figs. 5.8 to 5.10, because it was damaged very badly and no measurements were carried out.

Material P was totally delaminated, which can be seen in Fig. 5.13.

It can be seen in Fig. 5.12 that the insulation has lost its transparency almost completely. It is virtually impossible to discern the logo in the right-hand figure, whereas it is clearly visible on the left. The lighting conditions have not been exactly the same, but the conclusion is still undeniable. No distinctive delamination was observed with a microscope on material G2.

Transparency was weakened as a result of the diffusion of wellstream gas. The transparency of the samples was used as a sort of a quality indicator in the VPI process. A transparent build as seen on the left in Fig. 5.12 represents good impregnation.

The failure of material P is obvious in Fig. 5.13. The backing material has disappeared from the insulation causing total delamination. The insulation was based on resin rich technology and epoxy novolac resin, which means that neither the manufacturing process nor the im-pregnating resin has been the reason for its poor performance. PET film clearly reacts with the wellstream gas.

Fig. 5.12. Material G2 before (left) and after (right) the exposure.

Fig. 5.13. Material P after the exposure.