Clean Process Manufacturing Detail
Another look at Clean Process Manufacturing
Why keep gloves clean throughout manufacturing?
The obvious reason for keeping critical environment gloves clean throughout manufacturing becomes apparent when particle adhesion forces are taken into consideration. The predominant force of adhesion on small particles less than 10 microns in diameter are Van der Waals forces. Van der Waals forces increase as particle and surface contact area increase in relationship to the overall mass of the particle. The force exertion on particles less than 1 micron in diameter can exceed 100 dynes (forces per unit area of 109 dyn/cm²) or greater. This force far exceeds the gravitational force acting upon the small particle.
Electrostatic forces, although predominant on large particles, must also be considered. These forces are predominant in regards to the attraction force exerted on small particles in the ambient environment. This attractive force causes the initial deposition of particulate to the glove that is then exceeded by adhesion forces.
Capillary forces are considered another form of particle adhesion. These may be caused by high humidity conditions within the manufacturing environment, or immersion and removal from a liquid. All of these conditions are present in glove manufacturing; therefore controlling these conditions becomes a necessity to reduce the adhesion and deposition of small particles. Removal of small particles affected by these forces becomes extremely difficult. Therefore, emphasis must be placed on control measures to reduce or eliminate these forces.
The prevention of particle deposition on the glove surface should be considered rather than relying on virtually non-effective means for removal. The following sections will briefly describe the adhesion forces mentioned above.
Van der Waals forces: The polarization of atoms and molecules cause the Van der Waals forces between molecules possessing dipoles and quadrapoles. For example, solid materials contain local electric fields; these fields originate from the polarization of the constituent atoms and molecules. As the temperature of a material increases so does thermal excitation of the atoms and molecules of the material. Quantum theory explains that electrons of an electrically neutral solid do not occupy fixed states, this results in spontaneous electric and magnetic polarization. Van der Waals forces also include nonpolar attractive forces commonly referred to as London-Van der Waals dispersion forces. These forces can be associated with optical dispersion, explained by the spontaneous polarization of materials. Dispersion forces will create the largest contribution of intermolecular force except where the ability of polarization is minimal compared to that of the dipole movement.
Electrostatic Forces: The most common electrostatic forces are electrostatic image force, and the electrostatic contact potential induced electrical double layer, commonly referred to simply as the electrostatic double layer. The electrostatic image force can be most easily explained as the bulk excess charge on the surface of a material, a particle, or both which produce a coulombic attraction. As for conductors, these excess charges become balanced with contact due to charge flow. Therefore, the adhesion force of electrostatics on conductors is minimal. The opposite is true for nonconductors because charge flow is extremely slow or virtually non-existent to the material in contact, which allows electrostatic attraction to be significant. The electrostatic contact potential induced electrical double layer is the most predominant of the electrostatic forces on small particles. This can be explained as two materials in contact develop a contact potential that is created by the differences of their energy states. Upon contact, electrons are transferred from one solid to another until an equilibrium is met (current flow is equal in both directions). The potential difference which results is called the contact potential difference ranging from near zero to ~ 0.5 volts. This potential difference is referred to as the double layer charge region . With conductors, only the surface layer carries contact charges, although if the materials in contact are semiconductive or insulative, as glove materials are, the charge may permeate into the bulk of the two materials.
Capillary force: Capillary force is a function or interaction of a particle and liquid surface tension. The two most common contributions to this adhesion force are humidity and liquid immersion/removal, both of which create a thin film of liquid by capillary condensation or capillary action between a particle and a surface. This force can not only add to the total adhesion force exerted on a particle, but also in some cases, predominate over many other adhesion forces. Liquid immersion (in most cases) of a particle adhered to a material will reduce the Van der waals forces, virtually eliminate electrostatic image forces, and greatly reduce electrostatic contact potential forces. This is a result of both the enhanced dielectric constant of the liquid, and the sorption phenomena which shield the charge. It is important to understand that this is not always true, liquid molecules and ionic impurities present in the liquid can add to the adhesion force. Although adhesion forces may be reduced with immersion into liquids (particularly with large particles), it must also be understood that when the glove is removed from the liquid re-deposition of particles can occur. If a thin liquid layer is left behind, as a result of the surface tension of the liquid/glove, the total adhesion force of the remaining particles is increased immensely. For example, adhesion of particles can increase if the liquid contains substances that can crystallize which, upon evaporation, will create a solid crystalline bridge. Also, if the particle is water soluble it can become "glued" to the surface or the liquid-particle-surface interface can increase the overall contact area.
This is just a brief description of the adhesion forces and their impact on cleanliness. In addressing glove cleanliness, there are many other concerns that must be considered such as glove surface texture, contact area in relationship to the overall particle mass, and the length of time a particle is in contact with the gloves. Considering the magnitude of adhesion forces it becomes obvious that control measures must be implemented to effectively reduce particulate levels throughout all critical stages of glove manufacturing. Ansell's Clean Process Manufacturing depends upon a series of controls throughout the manufacturing facility to isolate the production lines and to ensure that the gloves are not exposed to contamination throughout the process. A glove that is manufactured clean, is ultimately much cleaner than a glove that is manufactured dirty and cleaned-up later.
Ansell CE Specialist