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1、Natural LatexControl and Industrial Procedures,Emily Liu Material Science,Natural Latex,A. Introduction,B.Control Procedures,Introduction,Centrifuged latex is processed into final articles by dipping, extruding, and coating. Different articles have different physical and chemical requirements, which
2、 means that the latex used in their manufacture must be modified by the addition of a great variety of substances that give each formulation its unique characteristics. These characteristics are well-specified and must be carefully controlled.,Example of a Control Sheet for LATZ Latex,An example of
3、a control sheet for the type of natural rubber latex that is commonly used in making surgical gloves is given in the Table 1.,The purpose of this PPT： We explain briefly how some of these characteristics are controlled.,Control Procedures,3,4,5,6,Dry Rubber Content (DRC),Surface Tension,pH Measureme
4、nt,Mechanical Stability,KOH Number,Total Solid Content (TSC),Viscosity,2,7,1,Control Procedures,1.Determination of the Total Solid Content (TSC) of Latex,Add your title in here,The TSC of latex is obtained by drying a sample of the liquid latex to be assayed and weighing the residue. The value obtai
5、ned is the sum of the dry rubber content plus the solid nonrubber contents, which includes the protein. Latex that is used in the production of gloves, catheters, and so on has a TSC of 6062%.,The DRC of latex is obtained by coagulating the rubber in the sample of latex to be assayed with acetic aci
6、d, then drying, washing and weighing the resulting mass of rubber.,The difference between the TRC and the DRC is a measure of the of natural latex.,The difference between the TRC and the DRC is a measure of the total nonrubber contents of the latex. These latter constituents are principally latex pr
7、oteins, lipids, and mineral salts. In the case of HA and LATZ latexes, those are commonly used in the production of gloves, the nonrubber constituents make up between 1% and 2% of natural latex.,2.Determination of the Dry Rubber Content (DRC) of Latex,3. Measurement of Surface Tension,Determination
8、of the surface tension of latex is a very useful measure, as surface tension is a function of: 1. The amount of soap added to stabilize the latex; 2. The natural soaps formed after concentration from fatty acids in the lipid phase of the latex; 3. The mineral salts (mg/Na) that are present in the la
9、tex.,A low surface tension value indicates good stability, although for use in dipping systems this could lead to a tendency to form webs of rubber.,The pH of fresh latex is close to neutrality. It will change more or less rapidly owing to the presence of microorganisms and enzymes, leading to progr
10、essive acidification of the latex and eventually to spontaneous coagulation. Because ammonia is added to preserve the latex, that is, to prevent coagulation, pH determination is important, as the stability of the liquid latex depends to a great degree on its pH.,4. pH Measurement,In addition, ammoni
11、a has an indirect influence on stability, increasing colloidal stability through the formation of fatty acid surfactants. When the pH of a latex is less than 9.5, which may occur, especially in the case of LA latex, then the pH must be watched very closely, frequently having to be increased chemical
12、ly when it is received and siphoned at its final destination.,The KOH number is a measure of the concentration of acid radicals that would have been present as free acids were it not for the presence of ammonia that has converted them to ammonium salts. Since some of these acids are produced by micr
13、oorganisms, a poorly preserved latexone with too many microorganismswill tend to have a high KOH number.,A high KOH number is not, however, necessarily indicative of a poorly preserved latex ,because this test does not distinguish between the ammonium salts of the acids whose anions destabilize late
14、x and those that enhance stability.,5. KOH Number,The KOH number is, nevertheless, an index of the quality of the latex. A high KOH number is usually taken as evidence of degradation of the latex owing to hydrolysis of proteins occurring after too long a period in storage. For example, a very fresh
15、latex arriving in Europe after 6-10 wk in transport from the Far East might have a KOH number of 0.04-0.05, whereas an aged latex would have a KOH number of 0.70-0.8 on arrival. The KOH number of centrifuged or creamed latexes must not exceed 1.0.,Mechanical stability is defined as the resistance of
16、 the latex to flocculation during stirring or agitation. It is measured by the mechanical stability time (MST) test in which latex is stirred rapidly (14,000 rpm) under standardized conditions and the time required for visual flocculation to occur is noted.,6. Measurement of Mechanical Stability,Exp
17、ressed in seconds, this test allows the behavior of the latex to be evaluated during the course of the different operations it undergoes during its transformation into a finished product. The mechanical stability of natural latex should be 650 s.,Mechanical stability tends to decrease during long pe
18、riods of transportation and storage owing to the slow hydrolysis of the stabilizers by the ammonia. It is therefore necessary to be very prudent when a natural latex is received and found to have low mechanical stability, for example, 600 s. Such a latex is susceptible to the formation of grains and
19、 will be filterable with difficulty. Manufacturers generally avoid this problem by adding stabilizers such as an anionic fatty acid alcohol, for example, lauryl sulfate.,Among the physico-chemical properties of latex, viscosity is one of the most important with respect to the production and processi
20、ng of latex products. The viscosity of a fluid is the ratio of the shear stress to the shear rate during its flow.,7. Viscosity,Most latexes exhibit decreasing viscosity with increasing shear rate and for this reason any reference to latex viscosity should include a description of the method of meas
21、urement, the rate of shear and the test temperature.,The viscosity of a particular latex is a function of its origin, including the clone from which it was derived, and of the composition of the latex serum, its total solids concentration, and the means of preservation.,Company Logo,Other Test,Example of a Control Sheet for LATZ Latex,For additional information, refer to: Latex Technology and Processing Natural Rubber Technical Information,Thank you,