As the diluent level increases, the viscosity passes through a minima and then increases with further dilution. This increase of viscosity is due to flocculation of the plastisol resin. The nature and role of various ingredients are discussed below. They should have the following attributes: 1.
Resistance at room temperature to the plasticizer for stability 2. Good affinity for the plasticizer to rapidly dissolve in it at appropriate temperature for proper gelation and fusion Resins made by mass or suspension polymerization are porous granules and will absorb high levels of plasticizer to form a sticky agglomerate. They are not used for making plastisols.
Paste-grade resins are made by emulsion or microsuspension polymerization and are finished by spray-drying techniques. They have high sphericity and a fairly dense surface, so that penetration of plasticizer at room temperature is low. Particle sizes range from 0.
Particle size and particle size distribution profoundly influence the viscosity of the paste. Because emulsion-grade resins are expensive, incorporation of extender resin lowers the cost. Although paste resins are resistant to swelling or solution by plasticizer at ambient temperature, slow solvation still occurs, otherwise, settling of the resin will take place. The slow solvation results in an increase in viscosity of the paste on storage, known as aging. Higher molecular weight resins give products with superior physical properties, but the fusion temperature is increased.
Alkyl phthalates are the most common primary plasticizers. Care should be taken that the stabilizer is compatible with other liquids of the paste, or else precipitation may occur.
Ba—Zn and Ca—Zn combinations are generally used. Tin-based systems are used where clarity is desired. They affect the flow properties and aging characteristics of the paste. Fillers increase the paste viscosity due to increase of the particulate phase and adsorption of plasticizer by the filler particles. The adsorption can be reduced by using coated fillers, such as with organic titanates. Polyethylene glycol derivatives are generally effective. Various thickening agents like fumed silica, special bentonites, and aluminum stearates are used.
These form a gel structure, and the pastes vary in consistency from butter to putty. They are also known as plastigels.
The decomposition of the blowing agent should occur at or above the fusion temperature for the formation of a closed cell structure. If the blowing agent decomposes completely before gelation, an open cell structure will be formed. The cell size is determined by the rate of decomposition of the blowing agent and the melt viscosity of the fused composition.
Low molecular weight polymers are used in foam formulation, so that the melt viscosity of the fused paste is low. Foam pastes are not commonly deaerated prior to use, because the air present acts as the nucleating agent for cell formation as the azo compound decomposes. Blow ratios are controlled by the quantity of azo compound. A high blow ratio will blow the foam apart during fusion.
The temperature should not rise during mixing. The mixing is generally carried out in vacuum, or entrapped air is removed after mixing by subsequent deaeration. The presence of air may result in bubbles and loss of clarity of the end product. As the temperature of the paste rises, more plasticizer penetrates the polymer particles, causing the paste to swell. On further heating, a solution of polymer and plasticizer is formed, with the formation of a homogeneous plasticized PVC melt fusion temperature. On cooling, solid plasticized PVC is obtained.
The process of gelation and fusion is the conversion of suspended polymer particles in a plasticizer to a solid containing dispersion of plasticizer in a continuous polymer matrix. The products are diverse, including upholstery, luggage fabric, wall coverings, floor coverings, tarpaulins, and shoe uppers. PVC formulations are used in the industry in liquid phase i.
Solid-phase compounding can be broadly categorized into two types: melt compounding and dry blending. Two types of mixers are widely used: the batch-type mixers that offer greater flexibility when frequent product and formulation changes are encountered, and continuous mixers, when large volume and steady throughput are required. The batch-type mixers can be two-roll mills or an internal mixer of the Banbury type, similar to those used for rubber compounding. Continuous mixers are single- or double-screw extruders. The resin is not fused during the compounding operation. The individual particles of the dry blend are much like the initial resin.
Their size is, however, greater due to the absorption of plasticizer and other additives. The resins used for commercially plasticized applications are suspension and mass polymerized homopolymers of PVC.
The resins are porous and have high surface area for rapid absorption of plasticizer. It is desirable to select a resin with a narrow particle size range. Fine particles tend to float in the molten mass, causing surface imperfections in the product.
The dry blends are produced in different mixing equipments. Mainly two types of mixers are used: the steam-jacketed ribbon blender or the high-intensity batch mixer. They are coupled to a cooling blender. In a ribbon blender, the resin and dry ingredients are added first and allowed to mix for a short time to break the agglomerates. The blender may be heated, if required, to increase the absorptivity of the resin for the plasticizer.
The heated plasticizer mix containing other liquid additives is then sprayed onto the resin mix, and the blender is heated. The powder is next discharged to a coupled cooling mixer. In a high-intensity mixer, the procedure of adding the ingredients is similar. In these mixers, heating is mainly due to the mechanical energy of the mixing process i. Sintered dry blends are partly fluxed pellets obtained by heating the dry blend to near its fusion point to sinter the particles into agglomerates.
These can vary in size from coarse powder to regular pellets. These sintered blends are nondusting, easy to transport, more homogeneous than dry blends, and have better processability. The production is usually done in a high-intensity mixer coupled with a cooler mixer. After the dry blend is formed, the temperature of the blender is increased until the particles agglomerate to the desired size.
The blend is then discharged rapidly into the cooling blender, where it is rapidly cooled, and further agglomeration is prevented. The investment on equipment for dry blending is substantially low, enabling processors to carry out their own blending.
Variations in the R and R' segments of the polyaddition reaction shown above permit preparation of polyurethane to meet specific needs. The extent of cross-linking, chain flexibility, and intermolecular forces can be varied almost independently. The range of polyurethane products is thus quite diverse and includes fibers, soft and hard elastomers, and flexible and rigid foams. The two important building blocks are the isocyanates and polyols.
Chain extenders like short-chain diols or diamines, and catalysts are frequently used in the synthesis of the polymer. A brief account of the chemistry of the raw materials and polyurethanes is discussed for a proper perspective. Its most important reaction is the nucleophilic addition reaction of compounds containing an active hydrogen atom.
Primary, secondary, tertiary-OH show decreasing order of reactivity.