Polychloroprene behaviour in a marine environment: Role of silica fillers

Abstract

Polychloroprene rubbers are widely used in marine structures and often filled with silica in order to increase mechanical properties. The presence of silica fillers leads to a complex degradation of the material. This study aims to understand the deeper degradation mechanisms involved when a silica filled polychloroprene is used in sea water. To do so, 4 polychloroprene rubbers filled with different amounts of silica (from 0 to 45 phr) were aged in natural sea water for 6 months at temperatures ranging from 25 to 60 °C. Moreover, a natural rubber with similar formulation was also considered in order to evaluate the role of the chlorine atom in the degradation. The chemistry and mechanics of the rubber degradation were also studied. In the presence of water and silica fillers, a large decrease in rubber stiffness was observed. This was attributed to the breakage of hydrogen bonds involved in the interaction between the silica and chloroprene matrix and the process is reversible. In the meantime, silica undergoes hydrolysis that leads to silanol formation and so an increase in rubber stiffness when water is removed; this process is irreversible.

Introduction

Polymers are widely used in marine structures in order to reduce weight, to prevent corrosion or improve thermal insulation, or as adhesives [1], [2], [3]. When polymers are used in such harsh environments for a long period (more than 20 years), the question of durability and ageing is raised. The immersion of polymer in sea water leads to a diffusion of water from the external medium into the polymer, the amount and rate of water absorption depends on the nature of the polymer and water temperature. The presence of water within the polymer leads to physical degradation, such as plasticisation where water increases mobility between macromolecular chains and so decreases mechanical properties. This kind of degradation is fully reversible, i.e. when water is removed the mechanical properties revert back to their initial state [4]. The presence of water can also lead to irreversible degradation such as additive leaching or hydrolysis that would also affect mechanical properties of the materials; in this case when the water is removed mechanical properties are still affected [5], [6].

In the specific case of polychloroprene rubbers, they are mainly used as a coating in order to protect structures from corrosion or as sealing materials [7], [8], [9]. Whereas this method is widely used for coating, the durability of such rubber in a severe environment is not fully understood. This can be explained by the fact that the long term behaviour of rubbers depends largely on its formulation [10], [11], [12]. In fact, it has been shown that water absorption in such rubbers is directly related to additives used for the vulcanization process [6], [13]. Moreover, it appears from previous studies that the use of silica in polychloroprene rubber is useful to increase mechanical properties but is also involved in a complex degradation mechanism when used in sea water [14], [15] that will be considered in this study.

It has been observed many times that silica fillers can be used as reinforcing filler in rubbers in the same way as carbon black. In fact, the presence of inorganic fillers leads to an increase in rubber stiffness and stress at break due to the replacement of a soft matrix by hard inorganic filler [16]. Silica fillers are used, instead of carbon black, because it provides a unique combination of tear strength, abrasion resistance and adhesion properties [17]. In the specific case of silica fillers in polychloroprene rubber, it has been shown that reinforcing is not only due to the presence of hard filler in the soft matrix but also due to a strong interaction between the filler and the rubber. In fact, according to Sae-oui et al [18]. (Fig. 1), a hydrogen bond exists between the silanol group present at the silica surface and the chlorine atom that can be represented as follows:

Whereas silica filler allows an increase in the initial properties, it also appears that the presence of such a filler in the polychloroprene (CR) leads to a specific degradation when used in sea water that has been partially studied in the past. We have shown that silica undergoes hydrolysis that leads to the formation of a silanol group and an increase in stiffness in both accelerated and natural ageing [14]. However, in some cases, it has been shown that the CR stiffness is largely decreased when such material is used in sea water [19]. There is a clear lack of knowledge on the degradation mechanism involved in the degradation of silica filled CR when used in a marine environment and thus on the effect of ageing on the mechanical properties.

This study aims to highlight the exact role of silica fillers in the degradation of polychloroprene rubber used in a marine environment. To do so, 5 different rubbers were aged in sea water for 6 months at temperatures ranging from 25 to 60 °C and then characterized in terms of chemistry and mechanical properties. The 5 rubbers that will be presented in the next section were chosen not only in order to study the role of the amount of filler in the CR (0, 15, 30 and 45 phr) but also the role of the nature of the matrix using a natural rubber with the same amount of filler. Experimental results will be presented with the mechanical properties directly after ageing (wet conditions) and after drying in order to distinguish reversible and irreversible processes involved in the complex degradation. Finally, a discussion of the results obtained will be presented in order to conclude on the exact role of silica fillers on the durability of CR in a marine environment.