Properties of fluoroelastomer/semicrystalline perfluoropolymer nano-blends

Abstract

Properties of blends of semicrystalline perfluoropolymers in fluoroelastomers strongly depend on the size of the dispersed phase and are at the best when dispersed phase dimension is well below 0.1 μm, i.e. in the nano-scale region. This fine dispersion is obtained with an innovative mixing technology based on microemulsion polymerization. Further increase of properties can be obtained by generating chemical links between fluoroelastomer and semicrystalline fluoropolymer. Nano-blends combine the performing properties of fluoroelastomers with those of semicrystalline perfluoropolymers. For example, these nano-blends have at the same time the sealing and mechanical properties of fluoroelastomers and the exceptional thermal and chemical resistance, low permeability and low friction coefficient of semicrystalline perfluoropolymers. In addition, as dispersed phase size is below visible light wavelength, finished items made with these nano-blends are optically transparent even when they contain as much as 40 wt.% of semicrystalline perfluoropolymer.

Introduction

Remarkable properties of fluorinated elastomers are their thermal and chemical resistance [1]. For this reason, they are used in hostile environments characterized by high temperature ranges and contact with aggressive fluids. First commercial fluoroelastomers have been made from vinylidene fluoride (VDF), hexafluoropropylene (HFP) and sometimes tetrafluoroethylene (TFE) [2]. During the years, in order to achieve new market needs, fluoroelastomer properties have been continuously enhanced by introducing in polymer composition novel sophisticated monomers able to impart performing characteristics in specific fields of application. A notable example is the use of perfluoromethylvinyl ether (MVE) in fluoroelastomers to obtain compositions dedicated to low temperature applications [2]. Actually, in the last 40 years the right choice of fluoroelastomer composition proved to be an effective way to match new market needs.

In the most recent period however, the “composition” approach is finding more and more troubles to attain new performing properties requested by applications in automotive, oil drilling, electronic, pharmacy and food industry. The sealing capability, high temperature and chemical resistance of fluoroelastomers are no more sufficient to match the material profile requested by these new applications. Actually, besides the properties typical of fluoroelastomers, other properties characteristic of semicrystalline fluoropolymers (such as low friction coefficient, low permeability, high hardness) are needed. For example, automotive is requiring fluoroelastomers with good sealing behavior coupled with low permeability to lubricants and low friction coefficient. Other challenging combinations of properties are high hardness plus good sealing (in oil drilling application) and good mechanical properties plus high purity (necessary in electronic, pharmaceutical and food industries).

These new applications have represented the driving force that has been pushing recent industrial research towards fabrication of completely new compounds based on blends of fluoroelastomers and semicrystalline perfluoropolymers.

Blending of polymers is a well-established technology [3], [4], [5]. Many mixing techniques are today available, the most important being (i) mechanical mixing [6], [7], (ii) core-shell polymerization [8], (iii) latex mixing [9]. When two thermodynamically immiscible polymers are mixed, the final blend is made of a continuous phase of one polymer and a dispersed phase of the other polymer. Standard mixing techniques allow obtaining blends with dispersed particle size, φ ranging from several microns down to 0.1 μm. Effect of φ on thermoplastics as well as thermosets has been investigated by Folkes and Hope [3], while Walker and Collyer [10] studied dispersion of rubber in polymeric materials. They both found that blend properties reach a maximum in correspondence of a critical size of dispersed phase (φ_cr). The φ_cr value varies with polymers but usually lies between 0.1 and 5 μm. In ABS for instance, φ_cr has been estimated between 0.26 and 0.46 μm [3], while in PS φ_cr=2.5 μm, in polymethylmetacrylate (PMMA), φ_cr=0.25 μm and in styrene-acrylonitrile (SAN) φ_cr=0.75 μm [10]. It is worth noting that fluoropolymers do not follow this general rule: their φ_cr appears to be much smaller than usual, being well below 0.1 μm. This feature is shown in Fig. 1, where the effect of the size of perfluoropolymer particles on storage shear modulus, G′ is analyzed. From data in Fig. 1, it comes out that the fluoroelastomer blend with fluoropolymer particles of 0.045 μm performs better than the blend with particles of 0.165 μm (see Table 1 for blends description). So a low φ_cr value has represented for a long time a major problem because, as previously stated, standard mixing techniques can not attain such a fine dispersion.

The size of dispersed phase particles is crucial because blend mechanical properties are strongly affected by interactions between the fluoroelastomer and the semicrystalline perfluoropolymer. These interactions can be seen as the product between specific energy of interaction and interfacial area (i.e. the contact surface extent). Specific interaction energy is determined by chemical–physical nature of the two phases. In case of fluoropolymers, some hints on specific interaction energy can be found out by observing the physical properties of fluorine atom. Fluorine atom is characterized by high electronegativity coupled with low atomic polarizability and low Van der Waals radius. Due to these atomic properties, fluorinated polymers are generally characterized by high intra-molecular chemical bonds and weak inter-molecular interactions. This second aspect is confirmed by the low value of cohesive energy density (CED) defined as:

$$\text{CED}\text{=}\text{Δ}\text{H}{}_{\mathrm{<mtext>v</mtext>}}{}^0\text{−RT}\text{V}{}_{\mathrm{<mtext>m</mtext>}}$$

where ΔH_v⁰ is the enthalpy of evaporation, R the ideal gas constant, T the temperature and V_m is the molar volume. CED is somehow proportional to the energy necessary to disentangle a macromolecule from polymer bulk and move it away. Experimental CED value of PTFE is about 150 J/cm³, while the corresponding value for polyethylene is 250 J/cm³ [11]. This low value of CED confirms the weak specific interaction energy occurring among fluorinated macromolecules and explains why it is so difficult to obtain fluoropolymer blends with good mechanical properties.

From what said above, physical interactions and, thus, mechanical properties can be improved by increasing either (i) specific energy of interaction or (ii) interfacial area. To this purpose, two novel technologies have been recently developed. The first one, named nano-blending technology, is used to increase interfacial area by decreasing the size of dispersed phase (φ) to less than 0.1 μm [12], [13], [14]. The second one (co-curing technology) acts on specific interaction energy: it is increased by introducing chemical links between the fluoroelastomer and the semicrystalline perfluoropolymer during crosslinking step [13].