in the present invention , the term a natural rubber latex includes common ones such as anionic natural rubber latexes and cationic natural rubber latexes , unless otherwise noted . on the other hand , a synthetic polymer latex represents a latex comprising a synthetic polymer and which is an anionic synthetic polymer latex or a cationic synthetic polymer latex , unless otherwise noted . the common natural rubber latex such as soctex and asiatex available from socfin company berhad and plantation latex berhad , respectively , to be used in the present invention is a latex comprising an aqueous solution , which flows from the cortex of a rubber plant and contains various organic and inorganic materials , as a disperse medium and the rubber content as a dispersant optionally together with one or more various additives such as a ph adjustor , curing agent , curing accelerator , softening agent , bulking agent , antioxidant and colorant . this latex , which usually contains about 0 . 3 % by weight of ammonia as a ph adjustor , is an anionic latex having a ph value adjusted to be from about 8 to 11 , preferably from about 9 to 11 . the cationic natural rubber latex to be used in the present invention is a so - called acidic latex which has a ph value between about 1 and 5 , preferably between about 1 . 5 and 4 . 7 ( the isoelectric point of the protein serving as a protective colloid ), and more preferably between about 2 and 3 . in this type of latex , rubber particles are positively charged , contrary to those in a common natural rubber latex , and are stabilized by mutual repulsion . such an acidic latex may be prepared by , for example , adding a cationic or nonionic surfactant to the above - mentioned natural rubber latex ( solids content : 20 to 70 % by weight , ph 9 to 11 ) at a ratio of from 1 to 5 % by weight of the former based on the rubber content and then adjusting the ph value to be below 3 by adding an appropriate inorganic or organic acid thereto . conventional application of such an acidic latex had been limited to specific purposes , for example , the treatment of negatively charged fiber or paper or the preparation of derivatives such as rubber hydrochloride , chlorinated rubber or reduced rubber directly from a latex . thus , there had never been known an acid latex blended with a silver compound as the one proposed by the present invention . the synthetic polymer latex used in the present invention comprises polymer ( s ), for example , homopolymers and copolymers of vinyl monomers such as ethylene , styrene , vinyl acetate , vinyl chloride , vinylidiene chloride , acrylonitrile , ( meth ) acrylate , vinylpyridine and methyl vinyl ether ; homopolymers and copolymers of diene monomers such as butadiene , isoprene , 1 , 3 - pentadiene , 1 , 5 - hexadiene , 1 , 6 - heptadiene and chloroprene ; copolymers of the above - mentioned vinyl monomers and diene monomers ; and copolymers of vinyl monomers having functional group ( s ) such as epoxide , amino , carboxymethyl , acid anhydride , hydroxyl , amino , n - methylolamido and isocyanate groups with the above - mentioned monomers ; as the main component optionally together with various additives such as a surfactant , crosslinking agent , bulking agent and softening agent . such a synthetic polymer latex as defined above , for example , acrylonitrile - butadiene copolymer latex ( ph 9 . 5 , nipol 1562 , mfd . by nippon zeon co ., ltd .) and chloroprene latex ( ph 10 . 2 , lm - 50 , mfd . by denki kagaku kogyo k . k . ), is usually used as an anionic system . further , a cationic synthetic polymer latex comprising the above - mentioned polymer ( s ) as the main component together with a cationic surfactant as an emulsifying and dispersing agent may be used in the present invention . this cationic synthetic polymer latex may further contain , for example , a nonionic surfactant , crosslinking agent , bulking agent and softening agent , if required . in the present invention , protein silver is employed as an antimicrobial agent . protein silver is a compound comprising protein and silver as described , for example , in the merk index , 10th ed ., pp . 1223 - 1224 , no . 8363 ( 1983 ). it is preferable that the protein silver contains from 7 . 5 to 8 . 5 % by weight of silver according to the pharmacopoeia of japan . the content of the protein silver varies depending on the purpose . in general , it is preferred that the content of silver based on the solids content of the latex ranges from about 0 . 1 to 10 % by weight , and more preferably from about 0 . 5 to 5 % by weight . a content of silver exceeding 10 % by weight might lower the strength of latex products produced therefrom . when it is less than 0 . 1 % by weight , on the other hand , the antimicrobial effect achieved is unsatisfactory . when the protein silver is blended with the latex , the solid content of the latex varies depending on the purpose , namely , whether the resulting composition is molded to produce , for example , a medical device , or it is used as a coating . it is recommended that the solid content of the latex is from about 20 to 70 % by weight , in general , preferably from about 30 to 65 % by weight , more preferably from about 40 to 60 % by weight . the latex may have any ph value ranging from the alkaline to the acidic range . the antimicrobial latex composition of the present invention may be prepared virtually by any process so long as all components are homogeneously blended with each other . thus , various known processes are available therefor . for example , it is preferable to directly add an aqueous solution of protein silver , more preferably one which has a high protein silver concentration , to a latex . when a cationic natural rubber latex or a cationic synthetic polymer latex is used , a water - soluble silver compound other than protein silver may be blended therewith . examples of appropriate silver compounds include silver salts which are formed by reacting silver with an inorganic or organic acid . preferred examples of the silver salts include silver nitrate , silver lactate , silver chlorate , silver fluoride and silver picrate . the expression &# 34 ; water - soluble &# 34 ; as used herein means having a solubility of 1 . 0 g or above , preferably 5 . 0 g or above , in 100 g of distilled water at 20 ° c . the preferred amount of the water - soluble silver compound blended varies depending on the purpose and the employed silver compound . it is preferred to use from about 0 . 1 to 30 % by weight , more preferred from about 0 . 5 to 10 % by weight , ( in terms of silver ) of the silver compound based on the solid content of the latex . when silver nitrate is used as the silver compound , a high silver content can be obtained by the use of smaller amount thereof as compared with the other compounds . a content of the blended silver exceeding 30 % by weight might lower the strength of molded products or films obtained therefrom . when it is less than 0 . 1 % by weight , on the other hand , the intended antimicrobial effect is not obtained . when the above - mentioned silver compound is used as an antimicrobial agent , an acidic latex having a ph value between about 1 and 5 , preferably between about 1 . 5 and 4 . 7 ( the isoelectric point of the protein serving as a protective colloid ), and more preferably between about 2 and 3 , may be employed as a cationic natural rubber latex . in addition , a cationic synthetic polymer latex having a ph value of from about 1 to 5 may be used . the solid content of the above - mentioned cationic natural rubber latex or cationic synthetic polymer latex varies depending on the purpose , namely , whether the resulting antimicrobial latex composition is molded into a medical device and the like , or it is used as a coating , similar to the case of the protein silver . a solid content of from about 20 to 70 % by weight is general . this antimicrobial latex composition may be prepared by virtually any known method as in the case of the protein silver . for example , an aqueous solution of a water - soluble silver compound , particularly that having a high concentration of the water - soluble silver compound , may be directly added to a latex . the antimicrobial latex composition of the present invention homogeneously contains silver and remains stable for a long period of time . in addition , it can be easily prepared . thus , the antimicrobial latex composition of the present invention can be suitably used , for example , in the production of latex products requiring a sustained antimicrobial activity upon prolonged use , such as medical devices , sanitary goods , and devices for producing food or ice pillows . examples of these latex products include various catheters such as urine catheters , bags such as stercus bags , supplying / drainage tubes , sponges , rubberized fabrics such as bed sheets and diaper covers , bath mats , sizing agents for paper , binders for nonwoven fabric , paintings and adhesives . the present invention is illustrated in greater detail with reference to the following examples and comparative examples , but it is understood that the present invention is not deemed to be limited thereto . in these examples , all the parts are by weight unless otherwise indicated . 0 . 3 part of zinc dimethyldithiocarbamate , 1 . 5 parts of sulfur , 3 parts of zinc oxide and 1 . 2 parts of stearic acid were homogeneously dispersed in 100 parts of a natural rubber latex ( ph 10 . 7 ) having a solids content of approximately 50 % by weight . the latex blend obtained is hereinafter referred to as solution a . separately , 6 . 4 parts of protein silver were dissolved in 20 parts of distilled water and the resulting solution was added to 100 parts of solution a while stirring . thus , a latex composition ( silver content : 1 % by weight ) wherein the protein silver was homogeneously dispersed was obtained . this latex composition was casted onto a glass plate and dried at room temperature . then , it was cured at 70 ° to 80 ° c . for approximately ten hours to form a sheet . the antimicrobial activity of the obtained sheet was determined in the following manner . the sheet was cut and a piece thereof was washed with a 70 % aqueous solution of ethanol under aseptic conditions . after drying and distilling off the ethanol , the sheet piece was placed onto the bottom of a sample vial . then , 200 μl of a culture liquor obtained by incubating escherichia coli at 37 ° c . overnight in a trypticase ® soy broth ( mfd . by bbl microbiology systems vecton dickinson & amp ; co .) medium was placed thereon . after sealing the vial , the bacteria were cultured at 37 ° c . for 18 hours . subsequently , the culture liquor was collected by adding physiological saline solution containing 0 . 1 % of a surfactant ( tween 80 mfd . by yamakei sangyo k . k .) to the vial . then , surviving cells therein were counted according to the colony counting method . the above procedure was repeated except that the culture liquor was directly placed onto the bottom of the vial to thereby give a control case . as a result , the bacterial count of the test case corresponded to 1 . 3 % of that of the control case . the latex of example 1 was then allowed to stand at room temperature for one week . it remained stable without showing any increase in viscosity or aggregation . 6 . 4 parts of protein silver dissolved in 20 parts of distilled water were added while stirring to 100 parts of an anionic styrene / butadiene copolymer latex ( ph 9 . 5 ) having a solids content of approximately 50 % by weight . thus , a latex composition ( silver content : 1 % by weight ) wherein the protein silver was homogeneously dispersed was obtained . with the use of this latex composition , a sheet was formed in the same manner as the sheet described in example 1 . the antimicrobial activity of the sheet obtained was determined in the same manner as the sheet of example 1 . the bacterial count of the test case corresponded to 6 . 5 % of that of the control case . the latex of example 2 was then allowed to stand at room temperature for one month . it remained stable without showing any increase in viscosity or aggregation . five parts of protein silver dissolved in 15 parts of distilled water were added while stirring to 100 parts of an anionic acrylonitrile / butadiene copolymer latex ( ph 9 . 5 , nipol 1562 , mfd . by nippon zeon co ., ltd . having a solids content of approximately 40 % by weight . thus , a latex composition ( silver content : 1 % by weight ) wherein the protein silver was homogeneously dispersed was obtained . with the use of this latex composition , a sheet was formed in the same manner as the sheet described in example 1 . the antimicrobial activity of the sheet obtained was determined in the same manner as the sheet of example 1 . the bacterial count of the test case corresponded to 3 . 5 % of that of the control case . the latex of example 3 was then allowed to stand at room temperature for one week . it remained stable without showing any increase in viscosity or aggregation . six parts of protein silver dissolved in 18 parts of distilled water were added while stirring to 100 parts of an anionic chloroprene latex ( ph 10 . 2 , lm - 50 , mfd . by denki kagaku kogyo kabushiki kaisha ) having a solids content of approximately 50 % by weight . thus , a latex composition ( silver content : 1 % by weight ) wherein the protein silver was homogeneously disposed was obtained . with the use of this latex composition , a sheet was formed in the same manner as the sheet described in example 1 . the antimicrobial activity of the sheet obtained was determined in the same manner as the sheet of example 1 . the bacterial count of the test case corresponded to 4 . 5 % of that of the control case . the latex of example 4 was then allowed to stand at room temperature for one week . it remained stable without showing any increase in viscosity or aggregation . 30 parts of protein silver dissolved in 70 parts of distilled water were added while stirring to 100 parts of solution a prepared in example 1 . thus , a latex composition ( silver content : 5 % by weight ) wherein the protein silver was homogeneously dispersed was obtained . with the use of this latex composition , a sheet was formed in the same manner as the sheet described in example 1 . the antimicrobial activity of the sheet was determined in the same manner as the sheet of example 1 . the bacterial count of the test case corresponded to 8 % of that of the control case . the latex of example 5 was then allowed to stand at room temperature for two months . it remained stable without showing any increase in viscosity or aggregation . 12 parts of a nonionic surfactant aqueous solution ( 10 % by weight ) were added to 100 parts of sectex ( mfd . by socfin company ) while stirring . the resulting latex was adjusted to ph 2 . 5 by adding 8 parts of aqueous hydrochloride solution . 0 . 3 parts of zinc dimethyldithiocarbamate , 1 . 3 parts of sulfur , 2 . 8 parts of zinc oxide and 1 . 2 parts of stearic acid were homogeneously dispersed in 100 parts of the acidic natural rubber latex having a solids content of approximately 50 % by weight . thus , a latex blend containing natural rubber as the main component , which is referred to as solution b hereinafter , was obtained . six parts of protein silver dissolved in 20 parts of distilled water were added while stirring to 100 parts of this latex blend . thus , a latex composition ( silver content : 1 % by weight ) wherein the protein silver was homogeneously dispersed was obtained . with the use of this latex composition , a sheet was formed in the same manner as the sheet described in example 1 . the antimicrobial activity of the sheet obtained was determined in the same manner as the sheet of example 1 . the bacterial count of the test case corresponded to 0 . 8 % of that of the control case . the latex composition of example 6 was then allowed to stand at room temperature for one week . it remained stable without showing any increase in viscosity or aggregation . 23 parts of protein silver dissolved in 60 parts of distilled water were added while stirring to 100 parts of the latex composition prepared in example 6 . thus , a latex composition ( silver content : 4 % by weight ) wherein the protein silver was homogeneously dispersed was obtained . with the use of this latex composition , a sheet was formed in the same manner as the sheet described in example 1 . the antimicrobial activity of the sheet obtained was determined in the same manner as the sheet of example 1 . the bacterial count of the test case corresponded to 5 . 2 % of that of the control case . the latex of example 7 was then allowed to stand at room temperature for one month . it remained stable without showing any increase in viscosity or aggregation . 5 parts of a silver nitrate aqueous solution ( 10 % by weight ) was added under stirring to 100 parts of solution a used in example 1 . the latex blend showed a rapid increase in viscosity and formed a number of small aggregates . a homogeneous sheet could not be formed therefrom . 5 parts of a silver nitrate aqueous solution ( 10 % by weight ) was added under stirring to 100 parts of the anionic acrylonitrile / butadiene copolymer latex composition used in example 3 . aggregates were formed in the latex and a homogeneous sheet could not be formed therefrom . 10 parts of a silver carbonate suspended aqueous solution ( 10 % by weight ) were added to 100 parts of an anionic natural rubber latex ( ph 10 . 0 ) which is defined as solution a in example 1 . after one day , the latex blend formed a number of small aggregates . a homogeneous sheet was not formed therefrom . five parts of a silver nitrate aqueous solution ( 10 % by weight ) were added to 100 parts of the above solution b used in example 6 while stirring . the latex formed showed no aggregation and a latex composition ( silver content : 0 . 6 % by weight ) wherein the silver nitrate was homogeneously dispersed was obtained . this latex composition was casted onto a glass plate and dried at room temperature . then , it was cured at from 70 ° to 80 ° c . for approximately ten hours to form a sheet . the antimicrobial activity of the sheet was determined in the following manner . the sheet was cut and a piece thereof was washed with a 70 % aqueous solution of ethanol under aseptic conditions . after drying and distilling off the ethanol , the sheet piece was placed onto the bottom of a sample vial . then , 200 μl of a culture liquor obtained by incubating escherichia coli at 37 ° c . overnight in a tripticase ® soy broth medium was placed thereon . after sealing the vial , the bacteria were cultured at 37 ° c . for 18 hours . subsequently , the culture liquor was collected by adding physiological saline solution containing 0 . 1 % of a surfactant ( tween 80 mfd . by yamakei sangyo k . k .) to the vial . then , surviving cells therein were counted according to the colony counting method . the above procedure was repeated except that the culture liquor was directly placed onto the bottom of the vial to thereby give a control case . as a result , the bacterial count of the test case corresponded to 0 . 7 % of that of the control case . the latex of example 8 was then allowed to stand at room temperature for one week . it remained stable without showing any increase in viscosity or aggregation . three parts of an aqueous solution of silver nitrate ( 10 % by weight ) were added while stirring to 100 parts of an acidic chloroprene latex ( ph 2 . 5 ) having a solids content of approximately 40 % by weight . thus , a latex composition ( silver content : 0 . 5 % by weight ) wherein the silver nitrate was homogeneously dispersed was obtained . with the use of this latex composition , a sheet was formed in the same manner as the sheet described in example 8 . the antimicrobial activity of the sheet obtained was determined in the same manner as the sheet of example 8 . the bacterial count of the test case corresponded to 10 . 3 % of that of the control case . the latex of example 9 was then allowed to stand at room temperature for one week . it remained stable without showing any increase in viscosity or aggregation . 25 parts of an aqueous solution of silver nitrate ( 30 % by weight ) were added to 100 parts of solution b prepared in example 8 while stirring . the latex showed no aggregation and a latex composition ( silver content : 9 . 5 % by weight ) wherein the silver nitrate was homogeneously dispersed was obtained . this latex composition was then allowed to stand at room temperature for one month . it remained stable without showing any increase in viscosity or aggregation . with the use of this latex composition , a sheet was formed in the same manner as the sheet described in example 8 . the antimicrobial activity of the sheet obtained was determined in the same manner as the sheet described in example 8 . the bacterial count of the test case corresponded to 1 . 3 % of that of the control case . while the invention has been described in detail and with reference to specific embodiments thereof , it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof .
a first antimicrobial latex composition comprising a homogeneous blend of a natural rubber latex or a synthetic polymer latex and protein silver and a second antimicrobial latex composition comprising a homogeneous blend of a cationic natural rubber latex or a cationic synthetic polymer latex and a water - soluble silver compound , wherein each antimicrobial latex composition exhibits excellent long term stability during storage and can be readily prepared .