Graded latexes

There are several visually graded latex NRs, including ribbed smoked sheets (RSS) and crepes such as white and pale, thin and thick brown latex, etc. Two types of raw NR are field latex and raw coagu-lum, and these two types comprise all NR ( downstream ) grades. Depolymerized NR is used as a base for asphalt modifiers, potting compound, and cold-molding compounds for arts and crafts. 

Several preservative systems have been developed for NR latex to suppress the proliferation of bacteria. Ammonia is the most common preservative used for NR field latex. It is a known mild bactericide and also functions as an alkali to increase the pH of the latex. The normal practice is to add some ammonia to the field latex upon arrival at the factory to prevent auto-coagulation before further treatment. Since the field latex contains only about 30% solids, it is unsuitable for most technical appKcations. Thus field latex needs to be further preserved, purified and concentrated to about 60% sohds in many commercial technical grade latex concentrates. Long-term preservation requires strong preservatives and/or higher dosages of these. Latex concentrate is preserved usually with about 0.7% of ammonia at a pH of about 10.5. This is known as HA latex concentrate. Alternatively a low ammonia concentration of 0.2% is used in conjunction with tetramethylthiuram disulfide (TDTM) as copreservative. This latex concentrate, known as LA-TZ, contains 0.025% TMTD/ZnO and 0.05% ammonium laurate. HA and LA-TZ latex concentrates are the two most common latex concentrates marketed. 

Poly etrafluoroethylene is manufactured and sold in three forms granular, fine powder, and aqueous dispersion each requires a different fabrication technique. Granular resins are manufactured in a wide variety of grades to obtain a different balance between powder flows and end use properties (Pig. 1). Pine powders that are made by coagulating aqueous dispersions also are available in various grades. Differences in fine powder grades correspond to their usefulness in specific appHcations and to the ease of fabrication. Aqueous dispersions are sold in latex form and are available in different grades. A variety of formulation techniques are used to tailor these dispersions for specific appHcations. 

AH-acryHc (100%) latex emulsions are commonly recognized as the most durable paints for exterior use. Exterior grades are usuaHy copolymers of methyl methacrylate with butyl acrylate or 2-ethyIhexyl acrylate (see Acrylic ester polymers). Interior grades are based on methyl methacrylate copolymerized with butyl acrylate or ethyl acrylate. AcryHc latex emulsions are not commonly used in interior flat paints because these paints typicaHy do not require the kind of performance characteristics that acryHcs offer. However, for interior semigloss or gloss paints, aH-acryHc polymers and acryHc copolymers are used almost exclusively due to their exceUent gloss potential, adhesion characteristics, as weU as block and print resistance. 

In recent years, synthetic polymeric pigments have been promoted as fillers for paper. Pigments that ate based on polystyrene [9003-53-6] latexes and on highly cross-linked urea—formaldehyde resins have been evaluated for this appHcation. These synthetic pigments are less dense than mineral fillers and could be used to produce lightweight grades of paper, but their use has been limited in the United States. 

Eield latex and field coagulum are the source materials for all varieties and grades of dry natural mbber that include the conventional International grades as weH as the Technically Specified Rubbers (TSR). 

Latex Sheet material Blend Field-grade material ... 

Fig. 3. Typical factory processing lines for (a) latex-grade mbber, and (b) field-grade mbber.

Partially hydrolyzed poly(vinyl alcohol) grades are preferred because they have a hydrophobic /hydrophilic balance that make them uniquely suited for emulsion polymerization. The compatibUity of the residual acetate units with the poly(vinyl acetate) latex particles partly explains the observed stabilization effect. The amount of PVA employed is normally 4—10% on the weight of vinyl acetate monomer. The viscosity of the resulting latex increases with increasing molecular weight and decreasing hydrolysis of the PVA (318). 

Masterbatches with Carbon Black. Two producers (Zeon Chemicals and Copolymer) offer nitrile mbber—carbon black masterbatches. These grades are prepared by mixing the carbon black with the mbber latex prior to the coagulation and drying process. 

NR latex grades are described by the method of concentration used. Evaporation, creaming and centrifuging are the most common methods used in the industry. 

In solid form, the natural rubber is graded according to the content of dirt remaining from the precipitation of latex at the plantation. Eight basic NR types have been traditionally recognized internationally. Only the so-called ribbed smoked sheets and the pale crepes are normally used for adhesives. The predominant grade system, the Standard Malaysian Rubber system, has been used since 1965. 

It has to be kept in mind that the polymer properties desired in polychloroprene latex formulations may be totally different from those needed in dry grades. Polychloroprene latices generally exhibit lower contactability than dry polychloro-prenes because of the presence of residual soaps and salts, appreciable gel content and segregation of tackifier [79]. For this reason, the polychloroprene latices have usually medium to low crystallinity. 

History. This includes the severity grading of the clinical reaction, the time of administration and onset of symptoms, the concomitant use of other drugs, foods or compounds (latex), previous history of drug allergy, atopy in the personal or family history, other underlying conditions such as mastocytosis or Cl esterase inhibitor deficiency. The actual preparation in its galenic identity should be stored or at least listed. 

The lowest grade of plantation crepe rubber. It is made from earth scrap (rubber from latex which has fallen on the ground) and any other plantation salvage scrap. It was formerly called Rolled Brown Crepe. 

The remaining latex is processed into dry rubber as sheets, crepes and bales. There is an International Standard for the Quality and Packing for Natural Rubber grades, the so-called Green Book , published by the Rubber Manufacturers Association. The following grades of NR listed in the Green Book are sold to visual inspection standards only ... 

Uses. Plastics and synthetic rubber are the major uses for styrene. They account for the exponential growth from a few million pounds per year in 1938 to more than 8 billion pounds today. The numerous plastics include polystyrene, styrenated polyesters, acrylonitrile-butadiene-styrene (ABS), styrene-acrylonitrile (SAN), and styrene-butadiene (SB). Styrene-butadiene rubber (SBR) was a landmark chemical achievement when it was comrner-cialized during World War II. The styrene derivatives are found everywhere—in food-grade film, coys, construction pipe, foam, boats, latex paints, tires, luggage, and furniture. 

The nonionic surfactant, nonylphenol deca(oxyethylene glycol) monoether, NP-EO10, supplied by Berol Kemi AB, Stenungsund, Sweden, was of technical grade and used without further purification. The main impurity is free polyethylene oxide. Analysis of the sample gave a polyethylene oxide content of = 3% (4). Note, that polyethylene oxide adsorbs on polystyrene latexes ( ), but a monolayer is reached at solution concentrations that are 10 times the concentration required to obtain a monolayer coverage with NP-EO q. The free polyethylene oxide, therefore, is expected to have negligible influence on the adsorption measurements. 

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