Influence of the Processes Parameters on the Properties of The ...

Chapter 2.
<strong>Processes</strong> to Manufacture Foams and to Functionalize <strong>the</strong> Surface
Ano<strong>the</strong>r approach to create microcellular materials was demonstrated by Shi et al. [1999]. First,
<strong>the</strong>y syn<strong>the</strong>sized a number <strong>of</strong> chemicals soluble in scCO 2 or liquid CO 2 . These chemicals comprise a number
<strong>of</strong> “monomers” containing one or two urea groups and fluorinated “tail” groups that enhance solubility <strong>of</strong>
<strong>the</strong>se compounds in CO 2 . When <strong>the</strong>se compounds were dissolved in CO 2 , <strong>the</strong>ir self-association led to <strong>the</strong>
formation <strong>of</strong> gels. The removal <strong>of</strong> CO 2 via depressurization resulted in <strong>the</strong> formation <strong>of</strong> foams with cells
with an average diameter <strong>of</strong> less than 1 m. Shi et al. [1999] reported that <strong>the</strong> bulk density reduction <strong>of</strong> <strong>the</strong>se
foams was 97% compared to <strong>the</strong> parent materials. Sheridan et al. [2000] studied <strong>the</strong> effects <strong>of</strong> several
processing parameters (such as polymer composition, molecular mass, and gas type) on preparing threedimensional
porous matrices from copolymers <strong>of</strong> lactide and glycolide and demonstrated that crystalline
polymers did not produce foamed materials via this approach, while gas treatment <strong>of</strong> amorphous copolymers
produced matrices with porosity up to 95%. This work also demonstrated that only CO 2 among <strong>the</strong> o<strong>the</strong>r
gases used (e.g., N2 and He) created highly porous polymer structures in <strong>the</strong>se copolymers [Sheridan et al.,
2000] and explained this by <strong>the</strong> possible interactions between CO 2 and carbonyl groups in PLGA [Kazarian
et al., 1996a].
5 Manufacturing <strong>of</strong> <strong>the</strong> Composite Biomaterials
5.1 Fundements <strong>of</strong> Co-grinding Process
5.1.1 Mechanism <strong>of</strong> Size Reduction
The properties <strong>of</strong> a material may be considerably influenced by <strong>the</strong> particle size and, for example,
<strong>the</strong> chemical reactivity <strong>of</strong> fine particles is greater than that <strong>of</strong> coarse particles. In addition, far more intimate
mixing <strong>of</strong> solids can be achieved if <strong>the</strong> particle size is small [Richardson et al., 2002]. Practically every solid
material undergoes size reduction at some point in its processing cycle.
Also reduction in size causes [Richardson et al., 2002]:
Both an increase in area and a reduction in <strong>the</strong> distance.
Separation <strong>of</strong> constituents, especially where one is dispersed in small isolated pockets.
Properties <strong>of</strong> a material may be considerably influenced by <strong>the</strong> particle size.
Chemical reactivity <strong>of</strong> fine particles is greater than that <strong>of</strong> coarse particles.
In addition, far more intimate mixing <strong>of</strong> solids can be achieved if <strong>the</strong> particle size is small.
The mechanism <strong>of</strong> <strong>the</strong> process <strong>of</strong> fragmentation is extremely complex. If a single lump <strong>of</strong> material
is subjected to a sudden impact, it will generally break so as to yield a few relatively large particles and a
number <strong>of</strong> fine particles, with relatively few particles <strong>of</strong> intermediate size. If <strong>the</strong> energy in <strong>the</strong> blow is
increased, <strong>the</strong> larger particles will be <strong>of</strong> a ra<strong>the</strong>r smaller size and more numerous and, whereas <strong>the</strong> number <strong>of</strong>
fine particles will be appreciably increased, <strong>the</strong>ir size will not be much altered. It <strong>the</strong>refore appears that <strong>the</strong>
size <strong>of</strong> <strong>the</strong> fine particles is closely connected with <strong>the</strong> internal structure <strong>of</strong> <strong>the</strong> material.
During <strong>the</strong> course <strong>of</strong> <strong>the</strong> size reduction processes, much energy is expended in causing plastic
deformation and this energy may be regarded as a waste as it does not result in fracture. Only part <strong>of</strong> it is
retained in <strong>the</strong> system as a result <strong>of</strong> elastic recovery. It is not possible, however, to achieve <strong>the</strong> stress levels
necessary for fracture to occur without first passing through <strong>the</strong> condition <strong>of</strong> plastic deformation and, in this
sense, this must be regarded as a necessary state which must be achieved before fracture can possibly occur
[Richardson et al., 2002].
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