Structure development and interfacial interactions in high‐density polyethylene/hydroxyapatite (HDPE/HA) composites molded with preferred orientation

Abstract

ABSTRACT:Composites of high-density polyethylene (HDPE) filled with sintered and nonsintered hydroxyapatite (HA) powders, designated as HAs and HAns, respectively, were compounded by twin screw extrusion. Compounds with neoalkoxy titanate or zirconate coupling agents were also produced to improve interfacial interaction and filler dispersion in the composites. The composites were molded into tensile test bars using (i) conventional injection molding and (ii) shear-controlled orientation in injection molding (SCORIM). This latter molding technique was used to deliberately induce a strong anisotropic character to the composites. The mechanical characterization included tensile testing and microhardness measurements. The morphology of the moldings was studied by both polarized light microscopy and scanning electron microscopy, and the structure developed was assessed by wide-angle X-ray diffraction. The reinforcing effect of HA particles was found to depend on the molding technique employed. The higher mechanical performance of SCORIM processed composites results from the much higher orientation of the matrix and, to a lesser extent, from the superior degree of filler dispersion compared with conventional moldings. The strong anisotropy of the SCORIM moldings is associated with a clear laminated morphology developed during shear application stage. The titanate and the zirconate coupling agents caused significant variations in the tensile test behavior, but their influence was strongly dependent on the molding technique employed. The application of shear associated with the use of coupling agents promotes the disruption of the HA agglomerates and improves mechanical performance.

Calcium orthophosphate-based biocomposites and hybrid biomaterials

“…Generally, if a poor filler choice is made, the polymeric matrix might be affected by the filler through reduction of molecular weight during composite processing, formation of an immobilized shell of polymer around the particles (transcrystallization, surfaceinduced crystallization, or epitaxial growth) and changes in conformation of the polymer due to particle surfaces and inter-particle spacing [94]. On the other hand, the reinforcing effect of calcium orthophosphate particles might depend on the molding technique employed: a higher orientation of the polymeric matrix was found to result in a higher mechanical performance of the composite [225,226].…”

“…In general, an interaction between the phases in any composite can be either mechanical, when it results from radial compression forces exerted by the matrix on the filler particles (e.g., developed during cooling due to thermal contraction), or chemical, when the reactivity of the filler toward the matrix has an important role. In the latter case, it is important to distinguish a physical interaction from chemical bonding [225]. According to Wypych [906], physical interaction is more or less temporary, implicating hydrogen bonding or van der Waals forces, whereas chemical bonding is stronger and more permanent, involving covalent bond formation.…”

“…Coupling agents establish chemical bridges between the matrix and the fillers, promoting the adhesion between the phases. In many cases, their effect is not unique, influencing also the rheology of composites [225]. Optimization of biocomposite properties with coupling agents is currently an important area of the research.…”

“…Other researchers used glutaraldehyde as a crosslinked reagent in various calcium orthophosphate-based biocomposites [388,392,500,502,503,520,525,585,621,646,649,917]. The interfacial bonding between calcium orthophosphates and other components might be induced by using various coupling agents and surface modifiers, such as silanes [192,234,337,540,[918][919][920][921][922][923], zirconates [225,337,339,914,924], titanates [225,337,924], phosphoric acid [543], alkaline pretreatment [722,725], polyacids [115,116,234], and other chemicals. Besides, some polymers might be grafted onto the surface of calcium orthophosphates [552].…”

“…For example, application of polyacids as a bonding agent for HA/Polyactive TM composites caused the surface-modified HA particles to maintain better contact with the polymer at fracture and improved mechanical properties [115,116,234]. The use of titanate and zirconate coupling agents appeared to be very dependent on the molding technique employed [225]. Silane-coupled HA powders were tested before applying them as fillers in biodegradable composites [921][922][923].…”