METHODS FOR PREPARING AND COAGULATING GRAFT RUBBER LATEX HAVING HIGH THERMAL STABILITY
Technical Field
The present invention relates to a preparation method of a graft rubber latex, more precisely a method of preparing and coagulating a graft mbber latex having excellent thermal stability without using an additional thermo-stabilizer and having increased efSciency in applying to a screw type dewatering machine.
Background Art
A mbber-reinforced resin such as aciylonitrile-butadiene-s^Tene (ABS), methaciylate-butadiene-styrene (MBS) and acrylonitrile-styrene^aciylate (ASA), etc. is obtained by preparing a mbber-reinforced latex by emulsion polymerization, coagulating and drying into a powder. The prepared powder is added in an extruder together witii a resin such as styrene-aciylonitrile copolymer (SAN), polyvinylchloride (PVC) and polycarbonate (PC), etc. leading to the primary process to produce a pellet and the secondary process to inject the pell^ At this time, it is generally preferrwi to add mbber-reinforced powder having moisture content of less than 1% into the extruder for the primary process.
In some cases, flie mbber-reinforced latex is coagulated without drying to prepare a powder containing moisture. This powder is placed in an extruder and moisture of the powder is eliminated
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therein. TTien, the powder is mixed with a resin such as s^rene-acrylonitriie copolymer (SAN), polyvinylchloride (PVC) and polycart)onate (PC), etc. to produce a pellet At this time, the moisture content in the mbt>er-reinforced resin powder is around 30%. If the moisture c(»itent is higher than that, water elimination process in a dehydrator is extended, resulting in em?rs of product quality and decrease of productivity.
Accoridingly, when the powder including moisture is loaded in an extnoder, it is essential to minimize water content to increase productivity. However, the conventional centrifii^ dehydration has its limits in minimizing water content
To overcome the above problem, an alternative method was proposed, that is water content was reduced by using a screw type dewatering machine. But, in that case, an additional higji temperature - high pressure process is required, resulting in the decrease of themial stability of a resin and further deformity of the resin.
TTius, in the rubber-reinforced resin production industiy, it is important to maximize the productivity of the mbber-reinforced resin to increase price competitiveness, in addition to increase the quality of a product
As a method for increasing tiie productivity of a mbber-reinforced resin, a method to increase the extmsion productivity has been proposed Precisely, the amount of each major and minor ingredient to be loaded in an extmder is increased and screw rpm and a temperature in the extmder are up-regulated considering the increased each major and minor ingredient content, so fliat
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the discharging resin is increased. If necessaiy, a specially designed screw can be used to strengthen the mixing process of the ingredients.
This method to increase the extrusion productivity fevors the increase of the productivity and improvement of pn^perties including impact strength owing to the secured diq^ersity of mbber particles, but has disadvantages of reducing themial stability by the increased friction of a resin resulted from the high shear force and accelerating thermal degradation due to the increased temperature. The decrease of thermal stability by themial d^radation causes color change in a final product. Even if the color change might not be significant, color change and gloss reduction will be resulted during the secondary process of injection, making errors in products.
To solve the above problem and to maximize the productivity, thermal stability of a resin prepared by emulsion polymerization has to be fundamentally improved and at the same time a method to improve thermal stability during the process needs to be developed.
The conventional method to improve thermal stability of a resin prepared by emulsion polymerization is focused on gi^-polymerization forming a shell, particularly in the case of using a mbber-reinforced resin having a core-shell stmcture, and secondly on the selection and introduction method of an antioxidant to prevent the resin from being oxidiffid by thermal history during coagulation and drying process. Besides, the conventional method to secure thermal stability during extrusion is generally focused on the minimization of friction between a resin and a banel during the process by the addition of a thermo-stabilizer or a lubricant
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However, the effect of the above conventional methods is vulnerable according to the extrusion productivity requested. That is, to guarantee equal quality of a product before and after increasing tiie productivity, an antioxidant has to be used excessively or various lubricants have to be added or if one lubricant is used, the amount has to be excessive. Therefore, mechanical properties and appearances are to be changed and at the same time price competitiveness might be rather decreased.
Disclosure of the Invention
To overcome the problems of the cCTiventional metiiods, it is an object of the present invention to provide a preparation method of a graft rubber latex having excellent thamal stability without using an additional thetmo-stabilizer and having increased efficiency in applying to a screw type dewatering machine, and a coagulation method of the same.
The above object and other objects of the present invention can be achieved by the following embodiments of the present invention.
To achieve the above object, the jHiesent invention provides a preparation method of a graft mbber latex by polymerization of a monomer mixture comprising one or more compounds selected from the group consisting of a rubber latex, an aromatic vinyl compound, a vinyl c>^n oxnpound and an aciylate compound, characterized by the use of a first hydrophobic (oil soluble) peroxide-based polymerization initiator before the polymerization conversion rate of the moncsner mixture
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reaches 50% and the use of a second hydrophobic peroxide-based polymerizarion initiator after the above time point
TTie present invention also provides a coagulation method of the graft rubber latex, which is characterized by the following steps: early coagulation induced by adding a metal sah to the graft rubber latex; and late coagulation induced by adding an acid to the coagulated latex above, leading to coagulation and aging.
The present invention further provides a preparation method of a graft mbber latex composition, comprising the steps of coagulating the prepared graft mbber latex by the above coagulation method; preparing a rubber resin containing moisture by reducing water content of the coagulated mbber latex to the level of up to 2 -10 % by using a screw type dewatering machine; and mixing and melting the mbber resin containing moisture with one or more resins selected fixjm the group consisting of styrene-acrylonitrile copolymer, polyvinylchloride and polycart»onate, and a lubricant.
A preparation method of a graft mbber latex by polymerization of a monomer mixture comprising one or more compounds selected fi^m the group consisting of a mbber latex, an aromatic vinyl compound, a vinyl cyan compound and an acrylate compound, characterized by treating a first hydrophobic peroxide based polymerization initiator before the polymerization conversion rate of the monomer mixture reaches 50% and then treating a second hydrc^hobic peroxide based polymerization initiator after the polymerization conversion rate reaches 50%.
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Tlie present invention is described in detail hereinafter.
The present inventors have used a rubber latex with high gel content and a hydrophobic polymerization initiator stepwise to prepare the thermoplastic graft rubber latex applicable to a screw type dewatering machine. As a result, a graft nibber latex was prepared by emulsion polymerization with increasing grafting efficiency. Then the present inventors completed this invention by confirming that the mbber resin having excellent thermal stability even at higji temperature and under high shear force can be prepared without additional use of a tfiermo-stabilizer by treating one or more coagulators stepwise.
(A) Preparation of a graft mbber latex
A graft mbber latex of the present invention is prepared by graft-polymerization of 50 - 70 weight part of a mbber latex (mean diameter 2,500 - 5,000 A, gel content: 80 - 99%) and 30 - 50 weight part of a monomer mixture comprising one or more compounds selected fiom the groiq? consisting of an aromatic vinyl compound, a vinyl cyan compound and an acrylate compound.
(a) Preparation of a mbber latex
A polybutadiene mbber latex having small diameter is first prepared and then enlarged to prepare a polybutadiene mbber latex having large diameter, which is used as the mbber latex of the invention.
To prepare the polybutadiene mbber latex having small diameter, a mixture of 100 weight part of 1,3-butadiene, 1 - 4 weight part of an emulsifier, 0.1 - 0.6 weight part of a polymerization initiator, 0.1 -1.0 weight part of an electrolyte, 0.1 - 0.5 wei^t part of a molecular weight regulate
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and 90 - 130 weight part of ion exchange water was reacted at 50 - 65°C for 7 ~ 12 hours, to which 0.05 - 1.2 weight part of a molecular weight regulator was added, followed by fiulher nsaction at 55 -VO'CforS-15 hours.
The preferable mean diameter of the mbber latex having small diameter is 600 - 1500 A and the gel content is 80 - 99%.
To prepare the rubber latex having large diameter, 1.0 - 4.0 weight part of acetic acid solution was added to 100 weight part of the rubber latex having small diameter slowly for one hour to enlarge the particles.
The preferable mean diameter of the mbber latex having large diameter is 2500 - 5000 A and tfie gel content is 80 - 99%. VMm the above gel content range, graft-copolymerization can be effectively finished in the outside of rubber particles, resulting in the excellent impact strength and thermal stability and more over mechanical properties and color can be protected under the high temperature and high shear force s^ for increasing the productivity.
(b) Preparation of a graft mbber latex
The graft mbber latex of the present invention is prepared by graft-copolymerization of 50 -70 weight part of the polybutadiene mbber latex of (a) and 30 - 50 weight part of a monomer mbtture, and at this time a polymerization initiator is added thereto at diflferent time points, before and after the polymerization conversion rate of the monomer mixture reaches 50%.
The monomer mixture is prepared by mbdng one or at least two compounds selected fiom the group consisting of an aromatic vinyl compound, a vinyl cyan compound and an aciylate
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compound.
And 0.1 - 2 weight part of an emulsifier, 0.05 - 2.0 weight part of a polymerization initiator, 0.1 - 0.5 weight part of an electrolyte and 0.2 - 1.0 weight part of a molecular weight regulator can be additionally included in the above.
The aromatic vinyl compound can be one or a mijdure of at least two ccMnpounds selected from the group consisting of styrene, a-methylstyrene, p^nethylstyrene, vinyltoluene, t-butylstyrene, chlorostyrene and their substituents.
The vinyl cyan compound can be one or a mixture of at least two compounds selected from the group consisting of aciylonitrile, methaciylonitrile and their substituents.
The aciylate compound can be one or a mixture of at least two compounds selected from the group consisting of ethylacrylate, methylaciylate, butylaciylate and ethylhexylaciylate.
The emulsifier can be one or a mixture of at least two compounds selected from the group consisting of alkyl aiyl sulfonate, alkalimethyl alkyl sul&te, sulfonated dlQ'lester, and other general adsorptive emulsifiers such as fetty acid soap or rosin alkali salt To guarantee thermal stability of the latex more efiFectively, a reactive emulsifier and a high molecular reactive emulsifier can be added independently or together with the adsorptive emulsifier.
As a polymerization initiator of the invention, hydrophobic pert)xide such as cumenehydro peroxide, diisopropyl benzenehydro peroxide, tertiaiy butylhydro peK)xide, parameihane hydro peroxide and benzoyl peroxide can be used together with an oxidative-reductive polymerization initiator.
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The first hydrophobic peroxide-based polymerization initiator is added within the time point where the polymerization conversion rate of the monomer mixture reaches 50% and the second hydrophobic peroxide-based polymerization initiator is added after the above time point Upcsi completion of the reaction, a hydrophobic peroxide based polymerization initiator and an oxidative-reductive polymerization initiator are added together to eliminate non-reacted monomere.
The first hydrophobic pere)xide based polymerization initiator is exemplified by strong hydrophobic compounds such as cumene hydro peroxide or diisojaopyl benzaiehydro peroxide.
The second hydrophobic peroxide based polymerization initiator is exemplified by comparatively weak hydrophobic compounds such as tertiaiy butyl hydro peroxide.
The oxidative-reductive polymerization initiator can be selected fix)m the group consisting of metal salts including Fe(ll), Fe(III), Co(II) and Ce(IV). The reducing agent can be selected fix>m the group consisting of polysaccharides such as dextrose, glucose and fiuctose, dihydroxy acetone and polyamines. To help a metal catalyst not to loose its activity under various alkali conditions, such compounds as tartaric acid, citric acid, pyrrolephosphoric acid or ethylene diamine tetraacetate can be additionally used.
The electrolyte herein can be one or a mixture of at least two compounds selected fiiom the group consisting of KCl, NaCl, KHCO3, NaHCOs, K2CO3, N^C03, KHSO3, NaHSOj, K4P2O7, K3PO4, Na3P04, K2HPO4 and Na2HP04.
The molecular weight regulator is preferably one or a mixture of at least two mercaptan compounds selected fix)m the group consisting of n-octylmercaptan, n-dodecylmercaptan and t-
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dodecylmercaptan.
In general, the addition of a monomer mixture for the graft polymerization is performed by serial injection, l>atch injection or the combination of these two methods selectively. And, the serial injection is preferred for shell graft reaction, but not always limited thereto. In some cases, 5 - 30% of a monomer mixture is added at a time and the rest portion of the mixture is added serially. At tiiis time, the monomer mixture for the batch injection in the early reaction stage is preferably added alone and the monomer mixture for the serial injection in the late stage is preferably added in an emulsion fonn supplemented with an emulsifier, ion exchange water and a polymerization initiator.
The reaction time for tiie graft polymerization is preferably wi(hin 3 hours and the polymerization conversion rate after the reaction is preferably at least 98.5 % and the preferable molecular weight is 50,000 -150,000.
The graft mbber latex prepared above can additionally include an antioxidant for the prevention of oxidation during the process.
The anti-oxidant herein can be one of the conventional phenol based, phosphorous based or sulftir based antioxidants and is preferably added in the emulsified form (mean diameter 0.5 - 2 p). The preferable content of such anti-oxidant is 0.2 ~ 2 weight part for 100 weight part of the graft mbber latex. The said anti-oxidant content is effective to keep thermal stability during the late reaction stage and does not cause any changes in mechanical properties and color expression.
It is prefen^ to add an anti-oxidant with stirring and slowly to the graft mbber latex at 40 -80°C continuously until coagulation.
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(B) Coagulation of a graft rubber latex
The coagulation method of the graft rubber latex of the invention is characteristically composed of the following steps: early coaguIatitMi induced by adding a metal salt to the graft rubber latex prepared by the method of claim 1; and late coagulation induced by adding an acid to the coagulated latex above, leading to coagulation and aging.
The metal salt herein is exemplified by MgS04, CaCb or Al2(S04)3 and particularly MgS04 or CaCl2 is preferred.
The acid herein is exemplified by suliliric acid, phosphoric acid or hydrochloric acid, and particularly sulfimc acid is preferred
In general, either acid or metal salt is added for coagulation, but it is more preferred in onier to guarantee oxidative stability and improve color quality (whiteness) of a final product to induce early coagulation using a metal salt and then late coagulation using an acid for complete coagulation and aging.
In the late coagulation, pH of the graft mbber latex is preferably 3-7. This pH range has been confirmed to be effective, precisely the latex in this pH range exhibited neither damage on whiteness of a resin usually caused by acid-treatment nor decrease of thermal stability caused by the metal remaining on the resin.
The coagulated graft rubber latex is turned into a solid type having 20 - 40% water content, for which excessive water content is reduced by a dehydrator, followed by hot-air drying to give a powder-type latex. The latex powder is placed in an extruder or the moisture ctMitaining solid latex
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is placed in an extmder where it is through with dehydration and evaporation. Then the latex can be mixed with a resin added as a matrix and a lubricant to produce a pellet
(C) Preparation of a graft rubber resin composition
The preparation method of the graft rubber latex composition of the present invention is composed of the following steps; coagulating the graft rubber latex prepared in the above (A) by the method of (B); preparing a rubber resin containing moisture by reducing water content of the coagulated rubber latex using a screw type dewatering machine to the level of up to 2 -10%; melting and mixing the prepared moisture containing rubber resin with one or more resins selected fiiom the group consisting of styrene-aciylonitrile copolymer, polyvinylchloride and polycarbonate and a lubricant.
hi the step of reducing water content of the coagulated graft mbber latex using a screw type dewatering machine, it is preferred to reduce water content to the level of up to 2 -10% and more preferably up to 5%. With this water content, extruding for dehydration and drying can be omitted and at the same time extrusion productivity can be increased
Best Mode for Carrying Out the Invention
Practical and presently preferred embodiments of the present invention are illustrative as shovm in the following Examples.
However, it will be qjpreciated that those skilled in the ait, on consideration of this disclosure, may make modifications and improvements within the spirit aixi scope of flie
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present invention.
[Examples]
Example 1
Preparation of a graft mbber latex
To a nitrogen substituted polymerization reactor were added 60 wei^t part (solid content) of polybutadiene rubber latex (mbber particle diameter 3,200 A, gel content: 98%), 92 weight part of ion exchange water, and 0.2 weight part of a fetty acid soap. The temperature of the reactor was maintained at SO^Q and 0.1 weight part of cumene hydro peroxide, 0.09 weight part of sodium pyrophosphate, 0.12 weight part of dextrose, and 0.003 weight part of FeS were added to the reactor at once. When the polymmzation conversion rate reached 55%, emulsion containing 28.8 wei^t part of styrene, 11.2 weight part of acrylonitrile, 15 weight part ofion exchange water, 0.3 weight part of tertiary dodecyl mercaptan, 0.6 weight part of a fetty acid soap, and 02 weight part of tertiaiy bulylhydro peroxide was added continually for 2 hours with increasing tiie temperature to 70*0
Upon completion of the continual addition, 0.05 wei^t part of cumenehydro peroxide, 0.043 weight part of sodium pyrophosphate, 0.055 weight part of dextrose and 0.001 weight part of FeS were added at a time at 80°C, and then the reaction mixture was maintained for 60 minutes. The unreacted monomers were removed to complete the polymerization.
Coagulation of a graft mbbo* latex
To the graft rubber latex finished with the above reaction was added 0.5 weight part of an
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antioxidant emulsion (Wingstay-L, mean diameter: 1.0 ^m), followed by primaiy coagulation at VS'C in the presence of 1.5 weight part of MgS04. Then, 0.5 weight part of sulfiaric acid was added additionally, followed by secondary coagulation and aging at 90*0. The coagulation was washed to obtain the graft polymer powder having 25% water content.
Preparation of a graft rubber resin composition
The prepared graft copolymer powda- having 25% water content was loaded in a screw type dewatering machine to reduce the water content to 5%. Styrene-acrylcffiitrile (SAN) copolymer having 28% aciylonitrile content and 100,000 of weight average molecular wei^t and a lubricant were added to the above powder, and the resins mixture was extruded and then molded in an injector to give a sample having the final rubber content of 14%. The physical fHx^jerties of the sample were tested.
Example 2
To the nitrogen substituted polymerization reactor of the Example 1 were added 60 weight part (solid content) of a polybutadiene mbber latex (mbber particle diameter 3,200 A, gel content 98%), 92 weight part of ion exchange water, 7.2 weight part of styrene, 2.8 weight part of aciyloniOile and 0.4 weight part of a fatty acid soap. The temperature of the reactor was maintained at 50°Q and 0.1 weight part of cumene hydro peroxide, 0.09 wei^t part of sodium pyrophosphate, 0.12 weight part of dextrose, and 0.003 wei^t part of FeS were added at a time to the reactor, and the temperature was raised up to 70°C for 20 minutes.
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An experiment was performed in the same manner as described in Example 1 except tha^ with maintaining the raised temperature, emulsion comprising 21.6 weight part of styrene, 8.4 wei^t part of acrylonitrile, 15 weight part of ion exchange water, 0.3 weight part of dodecylmercaptan, 0.6 weight part of a fetty acid soap, and 0.2 weight part of tertiaiy butylhydro peroxide were continually added for 1 hour and 40 minutes.
Example 3
An experiment was perfomied in the same manner as described in the Example 1 except that 60 weight part of a polybutadiene mbber latex (mbber particle diameter: 3200 A, gel content: 70%) was used and only 2 weight part of MgS04 was added to coagulate the prepared graft mbber latex.
Example 4
An experiment was performed in the same manner as described in Example 3 except that 0.1 weight part of tertiaiy butylhydro peroxide was added as a polymerization initiator in the early reaction stage instead of 0.1 weight part of cumene hydro peroxide, and 0.05 weight part of tertiary butylhydro peroxide was added instead of 0.05 weight part of cumene hydro peroxide after the monomer emulsion was added.
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Example 5
An experiment was performed in the same manner as described in Example 4 except that a mixture of 30 weight part of polybutadiene mbber latex having gel content of 98%, and mbber particle diameter of 3200 A and 30 weight part of polybutadiene mbber latex having gel content of 70% with the same mbber particle diameter was used
[Measurements]
The physical properties of the samples prepared in the above examples were tested and the results are shown in Table 1.
*Izod impact strength: measured witii a sample of %" in thickness by ASTM D256.
*Scorch: powders containing 25% moisture content prqDared fiom the graft mbber latex were placed in aluminum foil with exposing on the air to be able to contact oxygen, which was left in a 19^0 hot air oven. Sampling was performed over the time and color change of each sample was investigated. The time of caibonization was measured to evaluate canparative themial stability.
*Maximum oxidation time: samples were left at 190°C in the presence of oxygen and the time point when the wei^t change was most significant was checked and calculated as a value point
*Residence gloss: the pellet obtained fix>m the extmder was loaded in an injection molding machine and stayed there for 15 minutes at 250*0 to give a gloss sample. 45 degree gloss was measured for both the gloss sample and the other sample ejected at 200'*C without staying. The results were compared and bias value was calculated. The lower the bias value, the greater the
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r^idence gloss was.
*Residence discoloration (AE): a gloss sample was prepared by the same manner as described above for measuring residence gloss and L, a, b values of before and after staying were calculated by using Suga color computer and residence discoloration was calculated by the following mathematical formula
[Matiiematical Formula 1]
[Table 1]
Example 1 Example 2 Example 3 Example 4 Examj^eS
Gel content (%) 98 98 70 70 9870
RubbCT latex
Contoit (weight part) 60 60 60 60 30/30
Monomermixture ^^^ ^^ ^^^ ^^ ^^^ ^^
addition
PI . ^ Polymoizatiai
roiymenzanon convasionralfi50% CHP/TBHP CHP/TBHP CHP/TBHP TBHPABHP TBHP/TBHP
initiator , ^ , „
beiOTe'afla-
Antioxidant CQntent(%) 0.5 0.5 0.5 05 0.5
Coagulant MgSOVHaSOj 1.5/0.5 li/0.5 2.0/0 2.0^ 2.0/0
Imp^^^gth ^.^^ 27 28 15 16 20
Fluidity 19 18 19 18 19
Whiteness 56 55 48 45 49
Scoreh(min) 50 45 15 10 13
Residence discolwation 1.9 23 4.0 62 52
Residence gloss A2 A5 AlO Al5 A7
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As shown in Table 1, the graft rubber latex was prepared by treating the first hydrophobic peroxide based polymerization initiator before the polymerization conversion rate reached 50% and then treating the second hydrophobic peroxide based polymerization initiator when the polymerization conversion rate was over 50%. The graft mbber resin compositions of Example I and Example 2 were prepared by double coagulation processes using the first and second hydrophobic peroxide based polymerization initiators, which were confirmed to have excellent physical properties and thermal stability, compared with another sample of Example 3 prepared by a single coagulation process and using the both first and second hydrophobic peroxide based polymerization initiators and also other samples of Examples 4 and 5 prepared by a single coagulation process without the polymerization initiators.
Industrial Applicability
As explained hereinbefore, the present invention provides a method of preparing and coagulating a graft rubber latex having excellent thermal stability without using an additional thermo-stabilizer and excellent applicability to a screw type dewatering machine.
Those skilled in the art will appreciate that the conceptions and specific embodiments disclosed in the foregoing description may be readily utilized as a basis for modifying or designing other embodiments for carrying out the same purposes of the present invention.
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Those skilled b the art will also appreciate that such equivalent embodiments do not depart from the spirit and scope of the invention as set forth in the appended claims.
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1. A coagulation method ofthe graft rubber latex characterized by
early coagulation induced by adding a metal salt to the graft mbber latex and coagulating
the mixture; and
late coagulation induced by adding an acid selected fixm the groi^ consisting
of sulfide acid, phosphoric acid and hydrochloric acid, to the coagulated latex,
leading to coagulation and aging.
2. The coagulation method ofthe graft mbber latex as claimed in claim 1, wherein the pH of
the graft mbber latex in the late coagulation stage is 3 - 7.
3. Methods for preparing and coagulating graft Rubber latex having h i ^ thermal stability as
claimed in any ofthe above claims substantially as described in the specification