Epoxy resins composites for X-ray shielding materials additivated by coated barium sulfate with improved dispersibility

Highlights

• High performance barite/epoxy resin composites for X-ray shielding were produced.

• Dispersibility of particles is improved through surface functionalization.

• Liquid assisted grinding (LAG) was used to functionalize the additive surface.

• Additive sedimentation was eliminated, and mechanical performances improved.

• Liquid assisted grinding was used for the first time in the polymer field.

Abstract

Epoxy resins additivated by barium sulfate proved to be promising low cost, easy workable and environmentally friendly alternative to lead and steel as X-ray shielding materials, but the composites tends to be stratified, with the additive accumulating in the bottom side of the sample. This sedimentation process has been, at first, studied by in situ X-ray powder diffraction, thermogravimetric techniques and X-ray tomography and then inhibited by exploiting finer barite sources, implementing a grinding procedure, combined to a surface modification of the inorganic additives. Stearic acid and sodium dodecyl sulfate were used to coat barite surface, using a liquid assisted grinding (LAG) approach. The functionalized additives resulted more compatible with the resin and their dispersion within the polymer resulted much improved. The produced composite samples were then studied by optical and electron microscopy, X-ray radiography, X-ray diffraction, thermogravimetric analysis and tensile strength test. The use of a finer additive and the grinding procedure allowed to limit the sedimentation and induced a marked hardening of the samples, with the drawback of a reduction of their plasticity. Stearic acid coating was able to eliminate sedimentation maintaining good mechanical properties.

Introduction

Environmentally friendly X-ray shielding materials exploit minerals with high Z number elements such as Ba [1], Bi [2], Gd [3]. To obtain objects and artifacts with good mechanical performances such compounds are used as additive for polymers to obtain high performing composites. However, composite materials development must face the issues due to an unsuitable interaction between the different components used in their preparation. The low compatibility among the components might induce a reduced uniformity of the composite materials, with a general decrease of their performances and increase of defects. These effects are particularly common when dealing with polymers additivated with inorganic materials. Common behaviors are segregation, sedimentation and formation of voids and, in general, of interfaces between non or weakly bound components. These phenomena have been observed for instance in rubbers [4], biopolymers [5], bio-resorbable materials [6] and thermoplastic polymers [7]. The problem is well documented for epoxy composites, especially for fiber-reinforced materials [8], [9], [10]. These problems were faced by some of us [1] when dealing with epoxy resins additivated with commercial barium sulfate, to obtain X-ray shielding materials with low cost, easy workability and environmental impact smaller than lead and steel. In fact, the produced samples showed segregation and sedimentation of barite with a dramatic reduction of mechanical properties. This issue can be solved by disaggregating the particles, reducing their size and functionalizing their surface to improve the compatibility with the organic matrix [11], [12]. In the case of inorganic/epoxy resin composites, the main issue is the striking contrast between the polarity of the inorganic surface (additives are typically oxides, carbonates and sulfates) and the hydrophobicity of the polymer [13]. The typical approach is using low cost surfactants or fatty acids, able to bond the inorganic surface with their carboxylate or sulfonate heads and compatible with the polymers exploiting their apolar tails. The more common compounds used to obtain hydrophobic coated (nano) particles are stearic acid (SA) [14], [15], [16] and sodium dodecyl sulfate (SDS) [17]. Nihal Tüzün et al. demonstrated that finely dispersed barite gave good results in adhesion test compared to coated calcite [18]. Sun et al. proposed the modification of barite with sodium stearate in a water slurry to improve hydrophobicity [19]. The addition of bismite fillers modified by multiwalled carbon nanotubes allowed to improve dispersion and thus increase the X-ray attenuation coefficient of the composite [20]. The addition of silica nanoparticles to bismite allowed to improve thermal stability [21]. Nanometric Gd₂O₃ appeared to be the unique filler giving good dispersion and attenuation coefficients without surface modification [3]. The aim of the present work is obtaining X-ray shielding materials using the methodology by Lopresti et al. [1], but by extensively studying and optimizing the interactions at the organic-inorganic interface to improve additive dispersion and increase attenuation coefficients, when using a low cost additive as barite. At first, the sedimentation process during composite gelation was studied by in situ X-ray powder diffraction (XRPD), X-ray tomography and thermogravimetric (TGA) analysis to understand the features and critical issues of additive sedimentation. Then, the inorganic additive surface are functionalized, to obtain a better dispersion, limited or absent sedimentation and good mechanical performances. Low cost inorganic fillers (barium sulfate) and functionalizing agents (SA and SDS) were used. To have a facile and easily scalable functionalizing strategy, liquid assisted grinding [22], [23] and quasi solid-state reactions is used [24], for the first time in polymer additive field. On the sample preparation view point, the goal is understanding the better preparation conditions to obtain well dispersed samples and to reduce sedimentation. TGA, SEM and XRPD were used to investigate additive dispersion and sedimentation. The effect of the functionalization on mechanical properties was investigated by a tensile strength test to infer possible applications in the real world. On a more general viewpoint, we intend to demonstrate the potentialities of LAG approach in the field of polymer additive coating.