Handbook of Solvents - George Wypych - ChemTech - Ventech!

956 George Wypych
more to show that, although solvents do contribute to pollution, they also help to produce
materials which are needed for health and well being.
Polyurethanes are materials which have the required properties and biocompatilibity
which makes them good candidate for use in medical devices. Common applications include
pacemaker leads, peripheral and central catheters, feeding tubes, balloons, condoms,
surgical gloves, instrument and appliance covers, wound dressings, and many other. 1-4 Several
methods are used to process polyurethanes. These include injection molding, extrusion,
and solution processing. In solution processing film casting and dip molding are the most
frequent techniques.
Dimethylacetamine, tetrahydrofuran, dichloromethane, methyl ethyl ketone,
N,N-dimethylformamide, N-methylpyrrolidone, cyclopentanone, cyclohexanone, dioxane,
and chloroform are the most commonly used solvents. Most of these are hazardous but used
because they contribute to highly transparent product which is very desirable in medical devices.
Transparent materials can only be made from transparent solutions. 1 These solvents
can dissolve polymers well and form clear solutions. Ease of solvent removal from the material
is very important in formulation design. Obviously, no traces of solvents should remain
in the medical devices since even trace amounts may interfere with the treatment and
the patient’s health. An inappropriate solvent selection may cause the formation of crust as
the solvent escapes. This leads to material discontinuity (e.g., pinholes) which renders the
product inferior. This brings a discussion of solvent evaporation, the rheological properties
of formulation, and formation of multilayer materials.
Good solvents can be used in lower concentration but they result in viscous solutions
which, in dip coating, form thick films which have the potential of blistering on evaporation.
If the solution is diluted, film continuity suffers which increases the number of pinholes.
Rapid evaporation causes a formation of a crust of gelled solidified polymer which
makes solvent removal more difficult and damages the integrity of the layer. Also, material
does not have time to adjust and leveling suffers. On the other hand slow evaporation may
cause dissolution of the layer below the coating in a multilayered products and bubbling between
the layers.
The selection of solvents for dip coating is usually a complex process ultimately
requiring multicomponent solvent mixtures which include a good solvent, a poor solvent,
and a solvent of lower boiling point (sometime called “blush resistor”) to balance viscosity
and rate of evaporation. 2 In wound dressings, the solvents selected affect the material
microstructure which controls the evaporation of exuded body fluids but prevents bacteria
and pathogens from entering the wound. 3 In infection-resistant medical devices, the
antimicrobial agent must be uniformly distributed over all areas of the medical device
which may come into contact with a patient. Otherwise there is a risk of infection. 4 Not all
solvents dissolve antimicrobial agents and swell surface of medical device.
Cleaning of penetrable septa, tubing systems, and infusion and dialysis systems is another
application in which solvents are used. The solvents which are suitable for elastomer
cleaning are dichloromethane, perchloroethylene, halogenated hydrocarbons, and freons. 5
This cleaning method extracts undesirable organic materials from medical devices which
might otherwise be extracted by body fluids. Heat treatment of catheters followed by washing
with a polar solvent increase its surface lubricity. Catheter with poor surface lubricity
often causes frictional pain upon its insertion into the body cavity and damages the mucosal
tissue resulting in cross infection. 6 Film dressings contain two types of solvents: solvents to