Latex Science and Technology MT 3083 PDF

MT 3083 Latex Science and Technology 1 What is a Latex ? Since the 17th century, latex has been used as a term for the fluid substance in plants, deriving from the Latin word for "liquid". Latex is a natural substance which is harvested from plants, but it...

MT 3083 Latex Science and Technology 1 What is a Latex ? Since the 17th century, latex has been used as a term for the fluid substance in plants, deriving from the Latin word for "liquid". Latex is a natural substance which is harvested from plants, but it can also be artificially produced through chemical processes. 2 Polymer Latex – Definition Latex is a stable colloidal dispersion of polymer particles in an aqueous medium (water). 3 What are colloids ? Colloids are mixtures in which microscopically dispersed insoluble particles of one substance are suspended in another substance. 4  What are colloids ? The size of the suspended particles in a colloid can range from 1 to 1000 nm. For a mixture to be classified as a colloid, its suspended particles must not settle. 5  Colloids Lyophobic Lyophilic colloids colloids 6  No mixing tendency between two phases Lyophobic colloids have no affinity for the dispersing medium and are not solvated. Water hydrophobic colloids Eg: Rubber Latices 7  High mixing tendency between two phases Lyophilic colloids highly solvated as well as charged. The charge usually is a result of ionization. hydrophilic colloids If water is the dispersing phase 8  Nonhazardous Economical Aqueous Latices Have relatively high polymer content (continuous phase, dispersion Low viscosities medium – Water) Extremely fine distribution of the Latices polymer droplets High surface area Non- Aqueous Latices (dispersion medium – Organic solvent) 9 Colloidal Destabilization of Latices 10  Colloidal Destabilization of Latices Destabilization of a colloidal suspension results in joining of minute particles by physical and chemical processes. Processes occurs in two stages 11  Gross boundaries between particles disappear Therefore, absence of gross boundaries among the particles can be taken as criteria to distinguish aggregated particles and coalesced particles. 12  Interparticle coalescence of latex At a given temperature, if interparticle coalescence of a latex occurs rapidly That latex is said to be film forming at that temperature 13 If inter-particle coalescence does not occur at a particular temperature , then that latex is said to be non-film forming at that temperature. 14 Minimum Film Forming Temperature (MFT/MFFT) The lowest temperature at which a latex will uniformly coalesce when laid on a substrate as a thin film. Eg: The minimum film forming temperature (MFFT) for natural rubber latex is generally around 15°C to 20°C. 15 Tendency of Film formation depend on Polymer Structure Degree of branching Type of Polymer Degree of crosslinking Increase of temperature Tendency of Film formation Incorporation of Plasticizers 16  Terms used to describe colloidal destabilization Latin term refers to destabilization Denotes a process where particles aggregate in Coacervation large numbers and phases separate from the dispersion medium Dispersion medium is not removed during destabilization 17  The end product of the coacervation is known as coacervate Coacervation Colloidal coacervate dispersion Gelation a gel Coagulation a coagulum Flocculation floccules 18 coagulation Occurs when destabilizing agent is strong and unevenly distributed within the latex Immediate phase separation occurs Separated phase is much harder 19 Flocculation Occurs when particle aggregation is rapid and interparticle coalescence is restricted No phase separation (tiny clusters forms) Occurs when latex is non-film forming at coacervation temperature 20 Gelation Occurs when colloidal stability is gradually and uniformly reduced throughout latex Initially, it is phase change rather than phase separation Take longer time to phase separation. 21