Latex systems

Both functional and decorative coatings can be appHed to paper from latices. The aqueous dispersions can be used on conventional paper converting machinery which usually cannot handle hot melts and solvent coatings. The lack of fire hazard because of absence of solvents is an added advantage of the latex system. 

Lamination. la lamination a film is prepared by caleaderiag or extmsioa (see Laminated materials, plastic). It is thea adhered to a textile at a laminator by either an adhesive or sufficient heat which partially melts the film to obtain a mechanical bond. There are a variety of adhesives available for lamination, including solvent systems, water-base latex systems, and various forms of hot melt adhesives (qv). 

The increase of restrictive regulations on the use of organic solvents favoured the increasing use of latex-based adhesives. Because of the slow evaporation of water, these adhesives show less tack and can suffer bacterial attack. Infrared heaters and ovens can be used to favour the evaporation of water. Often a small amount of solvent (coalescing aid) is added to the latex system to improve wetting and coalescence of the latex particles. 

The term plastisol is used to describe a vinyl dispersion that contains no volatile thin-ners or diluents. Plastisols often contain stabilizers, fillers, and pigments, along with the essential dispersion plastics and the liquid plasticizer. All ingredients exhibit very low volatility under processing and use conditions. Plastisols can be made into thick fused sections with no concern for solvent or water blistering, as with solution or latex systems, so they are described as being 100 percent solids. 

It is generally accepted that the time required for desorption of adsorbed polymer is very long, and this process seems to appear to be irreversible(ljO. Accordingly, it is expected that the high adsorption values which appeared near the LCST may be held for a long time under different temperature conditions. In Table 3, experimental results for irreversibility of adsorption in the HPC-latex systems are shown. After the HPC samples and the latex particles were mixed for 2 hrs at 1+8 °C under the same condition as in the case of the adsorption process, one portion of one of the samples was separated immediately by centrifugation at 1+8 °C. The other half portion of the HPC-coated latex suspension was kept at room temperature for 1+8 hrs and then centrifuged at 6 °C. As... 

Figure 7 Relationship between critical flocculation concentration(CFC) and concentration of HPC-M for HPC-latex systems.

Figure 10. Electron micrograph of composite silica-polystyrene latex system,SPL(-), prepared by using bare silica particles as the seed.

Figure 12. Gel permeation chromatogram of latex polymer separated from composite silica-polystyrene latex system, SPL(HPC).

The two most commonly used latex systems arc acrylic systems (40% of usage), which perform very well, but are relatively expensive, and die vinyl-acrylic copolymer systems (57% of usage and growing), which do not perform as well, but are less expensive. 

The values listed in Table 2 should only be used for qualitative guidance since these can be variations of the c.c.c. with particle size, type and density of surface groupings, time of measurement after adding electrolyte and the presence or absence of stabilising materials such as surfactants. In practice it is advisable to determine the actual value for a particular latex system. 

There are special problems that occur when the particle diameter Is large relative to the wavelength of Incident light. This Is of Interest since many latex systems have particles with diameters of 300 to 1000 nm and a large ratio of particle to fluid refractive Index (1.2 for polystyrene latex). The most common wavelengths for lasers used In light scattering are on the order of 500 nm. 

Dipolar filter techniques (see Section 3) are becoming more common for examining regions of different mobility in polymers.87 In one particular study,88 a dipolar filter is used initially to select 1H spins from mobile regions of coreshell latex systems. Dipolar filters of increasing strength are then applied so as to obtain a characteristic decay curve for each sample. This enabled the mobility of the various components in the system to be compared with the respective pure substances, so that the effect of mixing could be assessed. 

Epoxy dispersions also can easily be blended with other waterborne polymers to make modified latex adhesives. The resulting hybrid adhesive produces performance properties and application characteristics that are superior to those of the originating latex system. 

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