(256r) Investigations on the Mechanical Forces Required for Mechanochemical Synthesis of Hydroxyapatite

Mechanochemistry
is the field of science which studies chemical reactions induced by mechanical
energy1. This paper focuses on the synthesis of hydroxyapatite, a bone
substitute biomaterial, by mechanochemical synthesis, and in particular in
determining the mechanical forces required for the solid state reaction to
occur.

In
recent years, mechanochemistry has become a popular alternative for executing
chemical reactions, as opposed to traditional solvent processes, which make use
of high temperatures and pressures in addition to high volumes of toxic
solvents. Mechanochemical processing exhibits many advantages over traditional
solvent heavy processes. The solid state reactions carried out in a solvent
free environment are a greener alternative which improves the safety of the
chemical reaction and limits the occurrence of undesired side reactions.
Furthermore, the reaction may be carried out at ambient temperatures and
pressures, with no further processing or solvent recovery required.

The
mechanochemical process requires the input of mechanical energy to stimulate a
chemical reaction. The mechanical energy initiates the process in which defects
and dislocations develop in the material, which may cause a solid state change
in the material. When mechanical activation has occurred, the mechanical energy
supplied may potentially cause a change in solid state such as the formation of
a metastable polymorphic form, the amorphous form, or alternatively, chemical
reaction may occur.

For
chemical reactions, the activation energy of the system, also known as
mechanical activation, is the threshold energy that reactants must acquire for
chemical reaction to occur. When the threshold energy level has been obtained,
the reactants will convert to a higher energy transition state, prior to
converting to a lower energy state with the formation of a product, with lower
potential energy and higher stability. In the case of an exothermic reaction,
when the activation energy of the reaction has been attained for a particular
reaction and chemical reaction has initiated, energy is released, to form a
product with a less energy, as described in figure 1. In contrast, an
endothermic reaction involves the absorption of energy for chemical transformation
to occur. Figure 2 demonstrates the absorption of energy for an endothermic
reaction when the activation energy of the system has been achieved. The energy
absorbed is utilised to form the final product with a higher energy state.

Figure 1

An exothermic reaction features a
release in energy when mechanical activation has been achieved.

Figure 2

An endothermic reaction features
absorption of energy when mechanical activation has been achieved.

Hydroxyapatite
was synthesised from calcium hydroxide and diammonium phosphate in a Retsch
Mixer Mill MM 400, varying both the reaction time and milling frequency. The
solid state endothermic reaction was undertaken using molar ratios of calcium
hydroxide and diammonium phosphate according to the following reaction
equation:

The
reaction time for the mechanochemical synthesis of hydroxyapatite was measured
at various milling frequencies between 5Hz and 25 Hz. The synthesis was carried
out in a 25mL stainless steel milling jar, using a powder loading of 1g with
one stainless steel milling ball.

Alternatively,
an exothermic reaction investigating the use of calcium oxide as an alternative
starting reactant was studied. Hydroxyapatite was synthesised using calcium
oxide and diammonium phosphate under identical milling conditions defined
above, according to the following reaction equation:

The
formation of hydroxyapatite and reaction yield was analysed by powder x-ray
diffraction (XRD) and Raman spectroscopy, which monitored the progression of
hydroxyapatite with increasing reaction time and milling speed.

Scanning
electron microscopy (SEM) was carried out to monitor a change in morphology and
size of the milled powder. The change in morphology and size of the powder was
analysed with respect to increasing reaction time and mill speed. Figure 1
below illustrates a change in particle size between the starting reactants
calcium oxide and diammonium phosphate and the resulting milled powder
containing calcium oxide and diammonium phosphate ball milled for 2 hours at a
milling frequency of 20 Hz.

Figure 3

Physical Mixture of
calcium oxide and diammonium phosphate prior to milling.

Figure 4

Ball milled powder
mixture calcium oxide and diammonium phosphate milled for 2 hours at 20 Hz

Mechanochemical
synthesis of hydroxyapatite involves the use of mechanical forces to induce a
chemical change, as opposed to traditional methods which make use of solvents.
The impaction forces present in the milling jar were estimated to determine the
mechanical energy required for the mechanochemical reaction to occur. The
impaction forces in the ball mill were estimated using Newton’s Second Law of
motion, which investigates the change in velocity or acceleration of an object,
when subjected to a change in force. This may be applied to the ball milling
process observed in the Retsch Mixer mill, in which the energy supplied is used
to oscillate the milling jar in a lateral direction. This in turn transfers the
milling ball in the mill jar, which promotes the mixing of powder. The
impaction of the powder and milling ball occurs at the wall of the milling jar.

Newton’s
Second Law of Motion was applied to the milling process used in this study to
estimate the impaction forces at the wall of the mill jar, which were
calculated based on the mass and acceleration of the milling media in the mill
jar, according to the following equation:

)

The
acceleration of the milling ball was calculated using the milling speed, the
dimensions of the milling jar and the pattern of the milling media inside the
mill jars. The mass of the mill ball remained constant for the purpose of this
study. The acceleration of the mill ball varied depending on the mill speed
utilised. The size of the milling jar used also remained constant throughout
the study, and therefore had no impact on the acceleration of the milling ball.

Mechanical
activation, or the activation energy of the reaction, denotes the mechanical
energy essential to initiate a chemical reaction. In the case of the ball
milling process, mechanical energy supplied from the ball mill is absorbed and
mechanical activation will occur. Both the reaction rate constant and
activation energy of the system are reaction specific.

The
activation energy required in the formation of hydroxyapatite was estimated
using the Arrhenius equation defined below:

The
reaction rates for the reactions specified previously were measured at a range
of different temperatures in order to estimate the activation energy for the
reaction.

The
objectives of the study described aims to gain a better understanding for the
mechanochemical process, in particular for mechanochemical reactions and
mechanochemical synthesis of materials. The
mechanical activation and mechanical energy necessary for initiating a mechanochemical
reaction were estimated using the Arrhenius equation and Newton’s Second Law of
Motion. The reaction activation energy, enthalpy of reaction and impaction
forces calculated characterise the mechanochemical process, and aid towards
understanding the mechanochemical process.

  References:

1.           Baláž, P. et al.
Hallmarks of mechanochemistry: from nanoparticles to technology. Chem. Soc.
Rev. 42, 7571–637 (2013).