Preparation of Dispersions PDF

Preparation of solutions, dispersions and emulsions for latex compounding 1  Compounding Ingredients Solid Liquid Water soluble Water insolubl...

Preparation of solutions, dispersions and emulsions for latex compounding 1  Compounding Ingredients Solid Liquid Water soluble Water insoluble Water soluble Water insoluble Added as Aqueous Make dispersion Added as Aqueous Make Emulsion solution solution 2 General Considerations The particle size of dispersions, and the droplet size of emulsions, should, as far as possible, be comparable to the particle size of the latex to which they are to be added. The colloid stability of dispersions and emulsions should be comparable to that of the latex to which they are to be added. 3 General Considerations As far as possible, the pH of solutions, dispersions and emulsions should be similar to that of the latex to which they are to be added. The ionic strength of the aqueous phase of the dispersion should be similar to that of the latex aqueous phase. 4 Preparation of dispersions A coarse slurry of powdered ingredient is made in water along with dispersing agents and dispersion stabilizers and then it is ground in a suitable mill to desired average particle size. Particle size with 5 micron or lower produce a stable dispersion. 5 Equipment for grinding dispersions Types 01 mills Colloid mills Disintegrate aggregates of fine particles But which do not effect any significant reduction in the ultimate particle size Types 02 mills Significantly reduce the ultimate particle size As well as disintegrating any particle aggregates Ex: Ball mills, pebble mills, ultrasonic mills, attrition mills etc 6 Colloid mills Several types of colloidal mills are available Premier Colloid Mill Hurrell Colloid Mill. Toothed colloid mills Trigonal tooth-ring colloid mills. Etc. 7 Colloid mills The main principle is that the slurry of powder is subjected to a high rate of shear thereby causing the agglomerates to break down. 8 Mills which reduce ultimate particle size Ball mills Comprise essentially a cylindrical container in which the slurry is placed together with a charge of balls/pebbles as grinding elements. 9 Grinding occurs because the grinding elements are caused to move relative to each other to the dispersion, and to the walls of the container. When the mill is working properly, the balls get carried round with the container for a short distance and then cascade The process of cascading causes grinding of the particles in the slurry. 10 The container rpm is rather slow to avoid centrifuging of the balls. Critical speed in rpm for a ball mill = 54.2/√R where R = radius of the container in ft. 50% of the volume in a jar is filled with balls and the upper level of the slurry is just above the level of the balls. 11 Preparation of dispersions Attrition mills 12 Attrition mills Advantages compare to the ball mill Fine dispersions at high concentrations can be rapidly produced Grinding times are said to vary from a few minutes to one hour Very little air is entrapped in the dispersion power consumption is relatively low 13  Formulations and preparative procedures for dispersions of solid latex-compounding ingredients The proportion by mass of active ingredient in a dispersion should preferably be a simple sub-multiple, such as 2/3 %(67% m/m), 1/2 (50% m/m), 1/3(33% m/m) or 1/4 (25% m/m), 14 The general procedure for making the initial slurries for the preparation of aqueous dispersions is first to combine the aqueous solutions of dispersion agents and colloid stabilizers with the water, and then to add the dry powder under high-speed stirring. 2 1 1 Colloid stabilizers Dispersion agents 15 16 Sulphur Dispersions Relatively difficult to make Sulphur aqueous dispersion Tends to flocculate and sediment Requires higher proportions of dispersing agents (2.0 – 2.5 phr) and stabilizers (1.0 phr) (casein /ethoxylate stabilizer) Ball-milling for 72 hours is usually considered to be essential for the finest sulphur dispersions. 17 Sulphur Dispersions (Eg: Methylcelluloses) It is often desirable to add a thickener to sulphur dispersions to retard subsequent sedimentation. After the ball milling 18 72 hours 19 Vulcanization Accelerator Dispersions Can be prepared at 50% mlm concentration if desired. However, { Zinc diethyldithiocarbamate} some accelerators are rather prone to flocculate at this concentration. Reduce the concentration to 33% m/m. It may be desirable to add a small amount of a water-soluble hydrocolloid. (e.g. methylcellulose) 20 21 Zinc Oxide Dispersions Activator ZnO is added to rubber compounds to activate sulfur vulcanization and thereby reduce the vulcanization time. Prepared either by ball-milling or by using a colloid mill or an ultrasonic mill (Depending on particle size desired) Ball-milling. Finest dispersions 22 Zinc Oxide Dispersions Ammonium soaps can be used as deflocculant for most solids Should be avoided for zinc oxide Flocculation tends to occur subsequently as a consequence of the formation of zinc-ammine ions and their interaction with the soap anion. 23 Antioxidants The dispersion of some of the solid antioxidants can present difficulty, because of their resinous nature. Various techniques available for overcome this problem Most satisfactory method is use of a fine-particle inert filler as a 'carrier'. kaolinite clays 24 25 Fillers Calcium carbonates and the kaolinite clays are conveniently prepared as 67% mlm dispersions. Some grades can be prepared at 75% mlm concentration possibility is dependent upon the particle size and particle-size distribution of the filler. 26