NaturalLatex天然胶乳胶乳英文橡胶实用教案

1、Natural LatexA. IntroductionB.Control Procedures第1页/共21页第一页，共22页。Introduction Centrifuged latex is processed into final articles by dipping, extruding, and coating. Different articles have different physical and chemical requirements, which means that the latex used in their manufacture must be modi

2、fied 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. 第2页/共21页第二页，共22页。Example of a Control Sheet for LATZ Latex第3页/共21页第三页，共22页。 An example of a control sheet for the ty

3、pe 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.第4页/共21页第四页，共22页。Control Procedures3456Dry Rubber Content (DRC)Surface TensionpH MeasurementMechanical Stab

4、ilityKOH NumberTotal Solid Content (TSC)Viscosity271第5页/共21页第五页，共22页。Control Procedures1.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 obtained is th

5、e 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%.第6页/共21页第六页，共22页。 The DRC of latex is obtained by coagulating the rubber in the sample of latex to be assayed with ac

6、etic acid, 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

7、latex proteins, 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第7页/共21页第七页，共22页。3. Measurement of Surface

8、 Tension Determination 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) tha

9、t are present in the latex. 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.第8页/共21页第八页，共22页。 The pH of fresh latex is close to neutrality. It will change more or less rapidly owing to the presence of micr

10、oorganisms and enzymes, leading to progressive 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 p

11、H. 4. pH Measurement第9页/共21页第九页，共22页。 In addition, ammonia has an indirect influence on stability, increasing colloidal stability through the formation of fatty acid surfactants. When the pH of a latex is , which may occur, especially in the case of LA latex, then the pH must be watched very closely

12、, frequently having to be increased chemically when it is received and siphoned at its final destination.第10页/共21页第十页，共22页。 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

13、ammonium salts. Since some of these acids are produced by microorganisms, 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

14、 the ammonium salts of the acids whose anions destabilize latex and those that enhance stability.5. KOH Number第11页/共21页第十一页，共22页。 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 p

15、roteins occurring after too long a period in storage. For example, a very fresh 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 l

16、atexes must .第12页/共21页第十二页，共22页。 Mechanical stability is defined as the resistance of 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

17、for visual flocculation to occur is noted. 6. Measurement of Mechanical Stability Expressed 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

18、 natural latex should be 650 s. 第13页/共21页第十三页，共22页。 Mechanical stability tends to decrease during long periods 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

19、 low mechanical stability, for example, 600 s. Such a latex is susceptible to the formation of grains and will be filterable with difficulty. Manufacturers generally avoid this problem by adding stabilizers such as an anionic fatty acid alcohol, for example, lauryl sulfate.第14页/共21页第十四页，共22页。 Among

20、the physico-chemical properties of latex, viscosity is one of the most important with respect to the production and processing 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 measurement, the rate of shear and the test temperature. 第15页/共21页第十五页，共22页。 The viscosity of a particular latex