Diffusion of curing agent through the thickness of a bi-layer EPDM system during the cure

Abstract

Two EPDM rubber layers with different level of curing agent (2 and 10%) pressed and cured together exhibit interesting properties when immersed in toluene. In spite of the large swelling taking place, different for each of the two layers, leading to a strong flexural force, the two components of the bi-layer remain bonded. Simultaneously, a hardness gradient has been measured through the thickness of the final material. This hardness gradient is assumed to result from the diffusion of the curing agent through the thickness of the bi-layer system during the cure process. Deeper insight into the phenomenon is obtained in making calculations by taking into account these facts, either on the process of cure of the bi-layer system or on the process of absorption of liquid leading to large assymmetric swelling. Application of this phenomenon could be made in re-treading old tyres with a roller band made of a new rubber.

Introduction

The process of cure of rubber is of particular interest [1] especially when the associated properties of the final material are considered, whatever they are, but notably the resistance to liquids and the mechanical properties. Both the problems set forth by heat conduction and matter diffusion should be resolved; historically speaking, these two processes are intimately bound together. Conduction heat transfer resulting from random motions was first established mathematically by Fourier [2]. Fick, recognising this fact, put the matter diffusion on a quantitative basis by adopting a similar equation a few decades later (1855). Hence, it follows that Fick's equation of diffusion, in the same way as that of conduction heat transfer, is of value only when the dimension of the material is constant during the process, that is, when the concentration of the liquid in the material is very low. However, the main difference between these two processes appears: on one hand, the conduction heat transfer is associated with a slight change in dimension as the thermal dilatation is rather low; while on the other hand, the matter transfer controlled by diffusion could be the source of an important swelling of the material, this fact being especially true for rubbers. In terms of conclusion, new equations for diffusion have been established either in sheet [3] or in a sphere [4] when the concentration of the liquid is not negligible and provokes swelling. Hence, the simple mono-directional mass transport through a sheet should be revised as not only the thickness is changed but also the other two dimensions [3]. Moreover, when the liquid concentration in the solid is not negligible, we have to understand that the diffusing matter diffuses not only through the solid but also through the liquid located in it, this fact being responsible for an increase in the rate of diffusion with the liquid concentration, which is expressed by a concentration-dependent diffusivity [5]. Finally, as no analytical solution can be found for the equations of diffusion obtained, the problems of diffusion of liquids in rubbers have been resolved numerically by building and using computerised methods [6], [7]. The results are in this way expressed in terms of kinetics of matter transferred and of liquid concentration associated with the change in dimension.

Very often, two matter transfers take place, the solvent entering the solid and enabling an additive of this solid to be extracted into the liquid [8]. In the case of plasticized PVC immersed in a liquid, a maximum in the weight is exhibited, which results from the fact that the rate of diffusion of the liquid into the PVC is higher than that of the plasticizer out. [9]. Oral dosage forms made of a drug dispersed in a polymer can release the drug in the solid state, as the gastric liquid diffuses into the polymer, dissolves the drug and thus enables the drug to diffuse through the liquid located in the polymer [10]. For rubbers, the same phenomenon occurs when the liquid which has entered the rubber, dissolves and removes the additives and the excess of curing agent. It is said that a decrease in weight or in volume of the solid is more serious than the swelling phenomenon, and even a low swelling may be hiding a large deterioration in physical properties [11].

The determination of the parameters necessary to define the process of diffusion is thus a complex operation, because the more simple method, that is measuring the kinetics, is also a tedious time-consuming operation. This is the reason why the method based on using the profiles of concentration, rather shorter in time, has been used: the one, by cutting the material into thin slices [9] and analysing the amount of liquid in each of them; the other, more precise by measuring the I.R. absorption of the diffusing agent in the solid state through a thin sheet of material [12]. In the present case of an EPDM rubber, the diffusion of the curing agent could not be directly observed; but the measure of the hardness through the thickness of two EPDM rubber sheets with different % peroxide as curing agent after their simultaneous cure shows that this hardness did not change abruptly at the interface limiting the two layers [13]. On the other hand, an attempt was made to evaluate the diffusivity from the profiles of concentration of the diffusing substance obtained through the interface separating two polymer layers having initially different concentrations [14]. The kinetics of absorption of styrene and polyester diffusing into cured thermosetting resins was not possible to measure, but advantage was taken of this transfer for repairing broken pieces having the same mechanical properties as the original piece [15], [16].

Moreover, this bi-layer system made of two EPDM layers cured together exhibits a very strong resistance when immersed in toluene, as the layers were not separated even if they were swollen in a quite different way [13]. It should be said that the process of absorption of a liquid by an elastomer is accompanied by a strong force able to provoke not only the swelling but also the destruction of the material [17].

In fact, the process of cure of rubber should be considered through all its complexity, with heat transfer by conduction through the material and the reaction of cure [1]. The kinetics of the overall cure reaction is generally obtained from the measurements made using the moving die rheometer (MDR) run either under isothermal conditions or in scanning mode, but also using calorimetry in scanning mode, which is able to give the cure enthalpy.

The specific problem of curing complex systems made of two layers of rubber is considered where each layer has different percent peroxide as curing agent, 2% for the one and 10% for the other. As no analytical solution exists, calculation is made numerically [18].

The objective of this paper is to show that the profile of hardness measured through the thickness of the cured bi-layer system results from diffusion of the active agent during the heating stage around the onset of vulcanisation, when the temperature is high enough to increase the diffusivity but not so high for the cure reaction to transform the rubber into a three-dimensional network. This matter transfer is responsible for the following two facts:

• A difference in the profiles of temperature, and more important, in the state of cure developed through the thickness of the bi-layer system during the cure.

• A particular behaviour of this bi-layer system when immersed in toluene, which is expressed by the fact that it remains mechanically stable in spite of the large difference in the swelling of each component, which causes in turn strong flexing. The initial plane of the bi-layer system, flat in shape, is recovered after evaporation of the liquid.

Finally, a theoretical study is made for the purpose of explaining that the gradient of hardness determined experimentally through the thickness of the bi-layer system is responsible for a progressive change in the properties through the thickness, such as the state of cure and the diffusion of a liquid with the resulting swelling. An assumption is made for calculation of the kinetics of absorption of the liquid and for determination of the profiles of concentration with the resulting swelling, by relating the extent of swelling to the hardness.