Fundamental Investigation of Various Factors Affecting the Occurrence of Solvent Cracking for Unplasticized Poly Vinyl Chloride Pipes in Building Equipment

Abstract

We investigated the occurrence of solvent cracking in unplasticized polyvinyl chloride pipes using test materials prepared by conventional methods. We confirmed that voids were formed on the inner surface of the pipe at a holding time of 2 weeks under conditions of room temperature (298 K), 1.0 g of applied adhesive, and 4 N/m U-band tightening torque. The voids formed linearly in the circumferential direction at regions in contact with the U-band and linearly in the axial direction at regions not in contact. Environmental agents of tetrahydrofuran (THF), 2-butane, and cyclohexanone were detected during analysis of void regions on the inner surface of the pipe and residual gas in the pipe. The results suggest that the stress generated by tightening of the U-band and the environmental agents affected void generation. Furthermore, stress analysis simulation of the U-band contact region show the presence of tensile stress at regions in contact with the U-band and compressive stress at regions not in contact. The difference in these stresses and the regularity of the molecular arrangement during manufacturing of the pipe likely influences the direction of void generation, which can progress into cracks.

 

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 70 (2021) 623–627.

Fig. 5 Results of GC/MS analysis at normal section, axial void and circumferential.

1. Introduction

Recently, resin pipes, especially unplasticized polyvinyl chloride pipes, have become increasingly used in buildings.^1–4) This increased demand, however, has led to reports of deterioration phenomena related to usage environment and joining materials. One such phenomenon is environmental cracking, which results in the breakage of resin pipes,^5,6) and includes solvent cracking caused by the effect of solvents.^7–11) The conditions of environmental cracking include: 1) low temperature of 5°C or less; 2) actions leading to excessive stress (thermal stress, flattening, bending, etc.), and; 3) presence of solvents (overcoating of adhesive, preservatives, etc.). However, as yet there are no reports on influential factors and quantitative conditions for occurrence in unplasticized polyvinyl chloride pipes.

In this report, we report the investigation of the influential factors and conditions of solvent cracking using a test tube manufactured by conventional construction methods.

2. Test Method

2.1 Test tube

The test tube used was a transparent tube of impact-resistant unplasticized polyvinyl chloride tube (JIS K 6742). The tube size was 32.0 mm Φ × 1,000 mm L × 3.1 mm T, and used without pretreatment. The adhesive used was an impact-resistant unplasticized polyvinyl chloride pipe adhesive (JWWA S 101).

2.2 Test conditions

Figure 1 shows a schematic diagram of the test tube. The operation was performed as follows, in accordance with the “Work Series Engineering Data of PVC Pipe for Water Supply”¹²⁾ issued by the Japan PVC Pipe and Fittings Association. Both ends of the test tube were wiped with a Kim towel, the amount of adhesive was chosen to avoid dripping (equivalent to 0.56 to 1.08 g), and a closed socket was attached to seal the sample tube. The sealed test tube was fixed to an L-shaped angle with a 4-point U-band and left at room temperature for 2 weeks. Each time, the nut was tightened first by hand and then with a torque wrench (equivalent to a tightening torque of 3.94 to 4.88 N/m). The test was carried out twice, and 3 test tubes were prepared for each test.

Fig. 1

Schematic diagram of test equipment.

2.3 Analytical method

The gas component in each test tube was collected and analyzed using gas chromatography-mass spectrometry (Shimadzu: QP-2010, hereinafter referred to as GC-MS).

Potential solvent crack locations were visually identified and observed with a digital microscope (KEYENCE: VHX-5000) and a scanning electron microscope (Hitachi High-Tech: SU8220, hereinafter referred to as SEM).

In addition, parts were collected, pretreated using a heat desorption device (Nippon Analytical Industry: JTD-505MIII), and analyzed by GC-MS. Figure 2 shows a schematic diagram of the analytical sampling method.

Fig. 2

Schematic diagram of analytical sampling method.

3. Results and Discussion

3.1 Surface observation survey of test tube

Two weeks after the preparation, linear white lines were observed in the test tube at points A and B shown in Fig. 1 along the U-band. This white line showed a difference in direction at each point. It was confirmed that the pipe cross section extends linearly along the pipe axis at 0 and 6 o’clock, and in the circumferential direction at 3 and 9 o’clock.

Figure 3 shows an example of microscope observations at points A and B of the test tubes, and Fig. 4 shows microscope and SEM observations at point A. Voids, which are considered to be signs of solvent cracks, were observed along the linear white line.

Fig. 3

Example of an observation photograph by digital microscope at points A and B.

Fig. 4

Example of an observation photograph by digital microscope and SEM at points A.

The voids formed in extremely fine groups, but were generated linearly while maintaining a certain width. Point A of the U-band and the points of the pipe cross section at 0 and 6 o’clock corresponding to opposite sides were in direct contact with the fixing, bracket, and tightening stress acts.

On the other hand, the points of the pipe cross-section at 3 and 9 o’clock, which correspond to point B and the opposite side, are not in contact with the fixing bracket or directly stressed. A discussion of the effects of stress at points A and B and the directionality of linearly generated voids is given in Section 3.5.

3.2 Void generation on the surface of the test tube and volatile gas

Figure 5 shows the results of GC-MS analysis for surface samples cut from an unaffected normal section, axial void section at point A and circumferential void section at point B. Figure 6 shows the results of GC-MS analysis for the stagnant gas collected from inside the pipe.

Fig. 5

Results of GC/MS analysis at normal section, axial void and circumferential.

Fig. 6

Results of GC/MS analysis at remaining gas in piping.

From Fig. 5, compared to the sample section without voids, the axial and circumferential void sections showed peaks in 2-butane at 96 s, tetrahydrofuran (hereinafter referred to as THF) at 102 s, and cyclohexane at 438 s.

On the other hand, for the stagnant gas results shown in Fig. 6, peaks in 2-butane, THF, and cyclohexanone were observed to be similar to axial and circumferential void sections.

THF is a component involved in solidification that results, for example, from the production of polyvinyl chloride fiber. In polyvinyl + THF concentrated solvent, coagulation is considered to result from the elution and diffusion of THF in water. When the diffusion of THF is rapid, the resulting tension allows the molecular arrangement to become more regular.^13,14)

However, it has been reported that THF, along with cyclohexanone, has the property of promoting the dichlorination proportionally.^15,16) Deterioration may progress gradually in time with chemical deterioration of the resin. Deterioration is considered to be in a solvated state when the affinity between the invading molecule and the polymer is large, and may proceed to further dissolution by induced swelling.¹⁷⁾ This suggests that the larger the amount of THF and cyclohexanone in the adhesive, the more the dichlorination and swelling are promoted.

In this survey, THF and cyclohexanone were detected in the void section samples and stagnant gas, suggesting that it contributes to the generation of voids. In addition, similar results were obtained in both axial and circumferential directions, indicating that in regards to the directionality of void generation, physical influences other than volatile gases promote their penetration into the microstructure.

3.3 Adhesive application amount

The PVC Pipe and Joint Association recommends an adhesive application amount of 2.0 g for PVC pipe 25A. In our experiment, considering construction methods in real-world use, the coating amount was set so that it would not drip, resulting in the range of 0.56 to 1.08 g. Although the amount of adhesive applied was less than half of the recommended amount, but occurrence of voids was confirmed in all test materials. The recommended application amount is considered to be the value intended to ensure joint quality. However, since voids were generated even for less than the recommended amount, it can be inferred they result from the PVC pipe.

3.4 Effect of temperature

All test materials were manufactured and stored in an environment with a room temperature of 298 K. Voids were confirmed in all test materials. A low temperature environment of 278 K (5°C) or less is observed to be one of the conditions for the occurrence of solvent cracks.^7–11) Our results suggest that they can also form in environments from 278–298 K.

3.5 Effect of stress

Figure 7 shows the results of stress generation simulation analysis for the inner surface of the pipe at the tightened section along the U-band. The finite element analysis software ANSYS Workbench was used for the analysis.

Fig. 7

Results of stress analysis simulation for U-band fixed portion.

In Fig. 7(b) it is seen that the distribution of tensile stress shown in red is concentrated at point A and the distribution of compressive stress shown in blue is concentrated at point B on the inner surface of the pipe.

Considering this result and the direction of void generation, voids are generated in the tube axis direction due to tensile stress on the inner surface of point A in contact with the U-band, and in the circumferential direction due to compressive stress on the inner surface of point B not in contact with the U-band. In addition, all test materials were prepared with a tightening torque of less than 5 N/m, with voids observed even at the minimum tightening torque of 3.94 N/m. From this, it was concluded that the tightening stress of 3.94 N/m or more would affect the deterioration.

Polyvinyl chloride and other polymer materials are known to exhibit a phenomenon called craze, in which localized stretching and orientation of molecules occurs over time when stresses above a certain limit are applied while the material remains exposed to environmental agents. It is reported that once craze occurs, the environmental agent permeates the material and the action of the chemical solution directly induces molecular cleavage, i.e. cracks, and the swelling and softening of the polymer by the chemical solution may cause greater plastic deformation.^17–20)

In this test, voids were observed only in the U-band anchorage region, suggesting that the environmental agents THF and cyclohexanone permeated this region under tightening stress, resulting in craze and the formation of voids which can be expected to propagate into cracks over time. The diffusion of THF affects the regularity of the molecular arrangement, suggesting that the direction of stress under tension and compression and the regularity of the molecular arrangement are related to the directionality of the voids. In order to clarify this issue, we plan to investigate this topic in future research.

4. Summary

The following findings were obtained from the test results of this study.

1. (1)    Voids were observed in the U-band tightening area, which is considered to be the starting point of cracking, while voids were not observed in other areas.
2. (2)    The occurrence of voids appears to be influenced by environmental agents THF, 2-butane and cyclohexanone, regardless of the application method of adhesive.
3. (3)    The voids form extremely fine groups, occurring in a straight line with a certain width. The linearity of groups appeared to follow the direction of stress, along the tube axis for tensile stress and the circumferential direction for the compressive stress.
4. (4)    The occurrence of voids, which are considered to be the starting point of solvent cracks, was confirmed for conditions of 1.08 g or less adhesive amount, up to 298 K temperature, and 3.94 N/m or more of tightening torque.

REFERENCES

• 1)  Japanese Association of Building Mechanical and Electrical Engineers: Building Mechanical and Electrical Engineer 2014(12) (2014) 56–118.
• 2)  Japanese Association of Building Mechanical and Electrical Engineers: Building Mechanical and Electrical Engineer 2015(12) (2015) 58–119.
• 3)  Japanese Association of Building Mechanical and Electrical Engineers: Building Mechanical and Electrical Engineer 2016(12) (2016) 55–125.
• 4)  Japanese Association of Building Mechanical and Electrical Engineers: Building Mechanical and Electrical Engineer 2017(12) (2017) 53–112.
• 5)  T. Shiotani, Y. Matsuo, T. Hattori and H. Kawata: Fractography – Analysis and Case Studies, (TECHNO SYSTEM CO., LTD, Toyohashi, 2010) pp. 270–278.
• 6)  Japan Society of Corrosion Engineering: Zairyo-Kankyo-Gaku-Nyumon, (MARUZEN CO., LTD., Tokyo, 2003) pp. 130–134.
• 7)  Y. Otake: Gomu · Purasutikku-Zairyo no Toraburu to Taisaku -Rekka to Zairyo-Sentaku-, (Nikkan Kogyo Shimbun, Ltd., Tokyo, 2005) pp. 104–106.
• 8)   T.  Okuyama: Journal of the Society of Heating, Air-Conditioning and Sanitary Engineers of Japan 83(1) (2009) 25–27.
• 9)   H.  Ishikawa and  Y.  Tomioka: Journal of the Society of Heating, Air-Conditioning and Sanitary Engineers of Japan 79(10) (2005) 961–968.
• 10)   K.  Kabuki: J. Japan Soc. Colour Mater. 54 (1981) 451–459. doi:10.4011/shikizai1937.54.451
• 11)  H. Ishikawa: “Deterioration of Polymer Material”, https://kinki-shasej.org/activities/membership/kankyou269_ishikawa.pdf.
• 12)  Japan PVC Pipe and Fittings Association: Work Series Engineering Data of PVC Pipe for Water Supply, (2012) pp. 23–29.
• 13)   T.  Yoshioka: Kobunshi Kagaku 13 (1956) 77–81. doi:10.1295/koron1944.13.77
• 14)   M.  Taniyama: Kobunshi Kagaku 10 (1953) 126–133. doi:10.1295/koron1944.10.126
• 15)   Y.  Iwami,  A.  Miyauchi and  Y.  Minoura: J. Soc. Chem. Ind., Japan 72 (1969) 2472–2475. doi:10.1246/nikkashi1898.72.11_2472
• 16)   T.  Funazukuri,  T.  Mochizuki,  T.  Arai and  N.  Wakao: Kagaku Kogaku Ronbunshu 20 (1994) 579–582. doi:10.1252/kakoronbunshu.20.579
• 17)   S.  Okuda: Nippon Gomu Kyokaishi 60 (1987) 447–455. doi:10.2324/gomu.60.447
• 18)  K. Mizutani: “Environmental Stress Cracking of Plastics” Technical Sheet of Osaka Research Institute of Industrial Science and Technology, 98037, (1998) http://tri-osaka.jp/Technicalsheet.98037.PDF.
• 19)   H.  Hojo: J. Metal Finish. Soc. Japan 26 (1975) 392–396. doi:10.4139/sfj1950.26.392
• 20)   H.  Hojo: J. Metal Finish. Soc. Japan 26 (1975) 432–436. doi:10.4139/sfj1950.26.432