1、Natural LatexA. IntroductionB.Control Procedures第1頁(yè)/共21頁(yè)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 modified by t

2、he 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頁(yè)/共21頁(yè)Example of a Control Sheet for LATZ Latex第3頁(yè)/共21頁(yè) An example of a control sheet for the type of natural rubber latex

3、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頁(yè)/共21頁(yè)Control Procedures3456Dry Rubber Content (DRC)Surface TensionpH MeasurementMechanical StabilityKOH NumberTotal Solid Content (

4、TSC)Viscosity271第5頁(yè)/共21頁(yè)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 the sum of the dry rubber content plus the soli

5、d 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頁(yè)/共21頁(yè) The DRC of latex is obtained by coagulating the rubber in the sample of latex to be assayed with acetic acid, then drying, washing and weighing the resul

6、ting 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 proteins, lipids, and mineral salts. In the case

7、 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頁(yè)/共21頁(yè)3. Measurement of Surface Tension Determination of the surface tension of latex is a ver

8、y 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 latex. A low surface tension value indicat

9、es good stability, although for use in dipping systems this could lead to a tendency to form webs of rubber.第8頁(yè)/共21頁(yè) 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 progressive acidification of the late

10、x 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第9頁(yè)/共21頁(yè) In addition, ammonia has an indirect influe

11、nce 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, frequently having to be increased chemically when it is received and siphoned a

12、t its final destination.第10頁(yè)/共21頁(yè) 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 microorganisms, a poorly preserv

13、ed 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 latex and those that enhance sta

14、bility.5. KOH Number第11頁(yè)/共21頁(yè) 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 latex arriving in Eu

15、rope 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 .第12頁(yè)/共21頁(yè) Mechanical stability is defined as the resistance of the latex to flocculatio

16、n 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 Expressed in seconds, this t

17、est 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. 第13頁(yè)/共21頁(yè) Mechanical stability tends to decrease during long periods of transp

18、ortation 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 will be filter

19、able with difficulty. Manufacturers generally avoid this problem by adding stabilizers such as an anionic fatty acid alcohol, for example, lauryl sulfate.第14頁(yè)/共21頁(yè) Among 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