Failure of fluorocarbon copolymer pipes subjected to mechanical strain in an alkaline environment
Introduction
Poly (vinylidene fluoride) (PVDF) is a high molecular weight semi-crystalline polymer obtained from the polymerization of 1,1-difluoroethylene (CH2 = CF2). It is widely used in engineering applications such as piping, gasket, filtering membranes thanks to its high mechanical, thermal and chemical stability [1], [2], [3]. Processing parameters can affect the PVDF final properties, including dielectric and mechanical properties. Depending on the crystallization conditions, PVDF may present four different polymorphs that are defined by the conformation of the polymer chains and their crystalline structure: the orthorhombic α, β and δ phases and the monoclinic γ phase [4], [5], [6]. PVDF is rigid, and it is generally poorly suited for objects subject to bending like pipes. It can be co-polymerized to obtain a flexible material such as the poly[vinyl fluoride]-co-[hexafluoropropylene] [PVDF-co-HFP] [7]. PVDF and its copolymers are able to resist several chemicals and harsh conditions; nevertheless, the detrimental effects of strong hydroxide solutions are well known [3], [8], [9]. This copolymer used to manufacture the flexible pipes studied in this work is known under the trade name Arkema Kynarflex® 2800. According to the manufacturer’s datasheet [10], [11], this material is able to withstand several chemicals and harsh conditions. Concerning the reported case, it can resist an aqueous sodium hydroxide [NaOH] solution with a concentration of up to 10% wt. [pH ≤ 13.5] at a maximum temperature of 50 °C; it is not recommended for use at room temperature with NaOH solution with a concentration greater than 50% wt. Several cases of environmental stress cracking affecting PVDF products in service are reported [9], [12], [13], [14] few of them concern also its copolymers and similar fluorocarbon polymers [15], [16]. In order to improve the stability of this specific material in service conditions, it is essential to analyze the failure and understand if the same degradation mechanism applies also on this particular copolymer.
Section snippets
Background and failure description
Pipes [outer diameter 12 mm, inner diameter 10 mm] made in Arkema Kynarflex® 2800 fluorinated copolymer were used to convey NaOH aqueous solution in an industrial disinfection/cleaning plant located in northern Italy. According to the plant management, NaOH solution had a concentration of 30% wt., and the plant operated at room temperature [15–35 °C]. According to plant schematics, pipes were bent with a radius between 90 and 130 mm. It is worth pointing out that, even if regarded as “flexible”
Visual examination and optical microscopy
Evident cuts in the location of the leaking points are visible Fig. 1. These cuts are located on the pipe bends and pass through the entire wall thickness. The surface of the cuts is red-colored; in many cases, reddish stains can also be noticed in the internal surface of the pipe Fig. 2. Details of two different cuts, as seen at OM examination, are shown in Fig. 3. The cut edges appear rough and jagged; a network of material partially interconnecting the opposing walls of the cut can be seen.
Discussion
Although PVDF is regarded as a high chemical-resistant material, its susceptibility to surface degradation in an aggressive environment [in particular alkaline solutions] is well known [3], [29], [30], [31]. Most of the literature concerning practical failure cases deals with filtering membranes [9], [14], [29]; some cases of failed pipes are reported [32]. The most accepted degradation path of PVDF in NaOH encompasses the formation of C
C units in the polymer chain as result of the elimination
Main findings
The results of the background and technical documents show that the pipes in the cleaning/disinfection plant operated very close to the limits suggested by the technical datasheet and resistance chart. The analysis showed that the inner surface of the used tubes was subjected to a chemical degradation attributable to NaOH with visible dehydrofluorination. The mechanical strain of the pipe bendings, combined with this chemical degradation, led to a “solvent crazing”, i.e. destruction of the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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