as is described above , the essential ingredients in the silicone - based antifoam composition of the invention include the above defined components ( a ) to ( d ), of which the most characteristic is the component ( a ). this component ( a ) is a polyoxyalkylene - modified diorganopolysiloxane which is , different from conventional polyoxyalkylene - modified diorganopolysiloxanes in the prior art , modified characteristically with two different kinds of polyoxyalkylene groups denoted by the symbols g 2 and g 3 in the general formula ( i ). when such a polyoxyalkylene - modified diorganopolysiloxane is compounded with the other components each in a specified proportion to prepare an antifoam composition , a very strong foaming - suppressing effect with excellent durability can be obtained by the addition of a small amount of the composition to an aqueous foaming liquid . furthermore , the antifoam activity is very stable even at high temperatures or under high shearing force and not decreased by dilution so that the antifoam composition can be used advantageously in the dyeing process without causing the troubles of uneven dyeing or appearance of oil spots . the component ( a ) is a polyoxyalkylene - modified diorganopolysiloxane represented by the general formula me . sub . 3 - a g . sup . 1 . sub . a si -- o --(-- simeg . sup . 2 -- o --). sub . m --(-- simeg . sup . 3 -- o --). sub . n --(-- sime . sub . 2 -- o --). sub . p --(-- simer . sup . 1 -- o --). sub . q -- sig . sup . 1 . sub . a me . sub . 3 - a . ( i ) in the formula , me is a methyl group , g 1 is a group selected from the group consisting of a methyl , r 1 , g 2 and g 3 groups , g 2 and g 3 are each a polyoxyalkylene group having a molecular weight of 500 to 3000 or , preferably , 1000 to 2000 and represented by the general formula in which r 2 is a combination of ethylene units and propylene units with the proviso that the molar fraction of ethylene units in the overall r 2 units is from 10 to 30 % in g 2 and from 65 to 85 % in g 3 , the balance being propylene units , q is a hydrogen atom or a monovalent organic group selected from the group consisting of alkyl groups of 1 to 8 carbon atoms , acetyl group and isocyanato group , r is a positive integer of 2 to 6 or , preferably , 2 or 3 and s is a positive integer of 5 to 50 or , preferably , 15 to 45 , r 1 is a monovalent hydrocarbon group having 3 to 20 carbon atoms , the subscript a is 0 or 1 , the subscripts m and n are each a positive integer of 1 to 30 , the subscript p is a positive integer of 10 to 100 and the subscript q is a positive integer of 1 to 10 . when the molecular weight of the polyoxyalkylene groups g 2 or g 3 is too small , the diorganopolysiloxane as the component ( a ) cannot be imparted with high hydrophilicity while , when the molecular weight thereof is too large , the viscosity of the diorganopolysiloxane as the component ( a ) would be too high to be rapidly dispersible in the foaming liquid causing inconvenience in working . it is preferable that the molar ratio of the polyoxyalkylene groups g 2 : g 3 is in the range from 0 . 3 : 1 to 3 . 0 : 1 . when g 1 is g 2 or g 3 , the calculation of this molar ratio should of course be made by taking g 1 into calculation . when the molar ratio of g 2 : g 3 is too small , the dispersibility of the diorganopolysiloxane as the component ( a ) is adversely influenced while , when the molar ratio is too large , the antifoam activity of the antifoam composition would be decreased . the group denoted by q in the above given general formula ( ii ) representing the polyoxyalkylene group is a hydrogen atom or a monovalent organic group selected from the group consisting of alkyl groups having 1 to 8 carbon atoms such as methyl , ethyl , propyl , butyl and hexyl groups , acetyl group and isocyanato group of the formula -- nco . r 1 in the general formula ( i ) is a monovalent hydrocarbon group having 3 to 20 carbon atoms . when the number of carbon atoms in r 1 is larger than 20 , the dispersibility of the diorganopolysiloxane as the component ( a ) is unduly decreased and the antifoam activity of the composition is decreased . the subscripts m and n in the general formula ( i ) are each a positive integer of 1 to 30 with the proviso that m + n is in the range from 3 to 40 or , preferably , from 3 to 20 . the subscript p is a positive integer in the range from 10 to 100 or , preferably , from 20 to 60 . when the value of p is too small , the viscosity of the component ( a ) is unduly decreased so that the antifoam composition would be somewhat unstable in storage to cause separation of the component ( a ) while , when the value of p is too large , the viscosity of the component ( a ) would be so high that the dispersibility thereof is greatly decreased resulting in poor workability . the subscript q in the general formula ( i ) is a positive integer not exceeding 20 . these subscripts should preferably have such values that the weight proportion of the siloxane moiety in the component ( a ) is in the range from 5 to 20 % or , in other words , the component ( a ) consists of from 5 to 20 % by weight of the siloxane moiety and from 95 to 80 % by weight of the polyoxyalkylene moiety corresponding to g 2 , g 3 and g 1 , when g 1 is g 2 or g 3 . the polyoxyalkylene - modified diorganopolysiloxane as the component ( a ) can be prepared by a known method involving the addition reaction of so - called hydrosilation . for example , a methyl hydrogen polysiloxane having hydrogen atoms directly bonded to the silicon atoms is subjected to the hydrosilation reaction with two kinds of different polyoxyalkylene compounds having a vinyl or allyl group at a molecular chain end corresponding to the groups g 2 and g 3 and an α - olefin compound having 3 to 20 carbon atoms in a molecule corresponding to the group denoted by r 1 in the presence of a catalytic amount of a platinum compound . alternatively , an α - olefin compound having 3 to 20 carbon atoms in a molecule is first reacted with a methyl hydrogen polysiloxane to introduce the groups r 1 thereinto and then the r 1 - containing methyl hydrogen polysiloxane is reacted with two kinds of polyoxyalkylene compounds to introduce the polyoxyalkylene groups g 2 and g 3 . the amount of the polyoxyalkylene - modified diorganopolysiloxane as the component ( a ) in the inventive antifoam composition is in the range from 60 to 70 parts by weight assuming that the total amount of the components ( a ) to ( d ) is 100 parts by weight . when the amount of the component ( a ) is too small , the composition is poorly dispersible in aqueous foaming liquids . when the amount thereof is too large , on the other hand , certain difficulties are encountered in the preparation of the composition which should be dispersed in the form of an aqueous emulsion in use due to a decrease in the self - emulsifiability . the component ( b ) in the inventive antifoam composition is an oil compound consisting of ( b - 1 ) a polydimethylsiloxane fluid or so - called dimethyl silicone oil and ( b - 2 ) a finely divided silica powder . the dimethyl silicone oil is a polydimethylsiloxane having a linear molecular structure although a small number of branches in the molecule may have no particularly adverse influence . the terminal group at the molecular chain end is preferably a trimethylsilyl group . the dimethyl silicone oil should have a viscosity in the range from 10 to 100 , 000 centistokes or , preferably , from 100 to 5000 centistokes at 25 ° c . in view of the balance between the dispersibility and workability , for which the viscosity should be as low as possible , and durability in the antifoam activity , for which the viscosity should be as high as possible . when the viscosity of the dimethyl silicone oil is too low , the stability of the composition in the form of an aqueous emulsion is decreased . when the viscosity thereof is too high , the workability with the antifoam composition would be decreased . the finely divided silica powder compounded with the above described dimethyl silicone oil is a well known material and any of wet - process and dry - process silica fillers can be used without particular limitations although it is preferable that the silica powder has a specific surface area of at least 100 m 2 / g as determined by the bet method . various commercial grades of precipitated silica fillers , silica xerogels and fumed silica fillers as well as those after a hydrophobic treatment with an organosilicon compound are available on the market including aerosils ( products by nippon aerosil co . ), nipsils ( products by nippon silica co . ), cab - o - sils ( products by cabot corp . ), santocels ( products by monsanto chemical co .) and the like . the oil compound as the component ( b ) is prepared from 85 to 95 % by weight of the dimethyl silicone oil and from 15 to 5 % by weight of the finely divided silica powder . when the weight proportion of the dimethyl silicone oil is too large , the composition comprising the oil compound cannot exhibit full antifoam activity . when the weight proportion of the silica powder is too large , dispersibility of the composition would be unduly decreased due to increase in the consistency . the amount of the oil compound as the component ( b ) in the inventive antifoam composition is in the range from 10 to 20 parts by weight per 100 parts by weight of the total amount of the components ( a ) to ( d ). the antifoam activity of the composition would be decreased when the amount of the component ( b ) is too small while the consistency of the composition would be too high to cause a decrease in the workability when the amount of the component ( b ) is too large . the component ( c ) in the inventive antifoam composition is a polyoxyalkylene compound which is a copolymer consisting of oxyethylene units and oxypropylene units and serves as an emulsification aid to promote emulsification of the above described components ( a ) and ( b ) and the component ( d ) described below in water to give a stable emulsion . the polyoxyalkylene compound should have an average molecular weight in the range from 500 to 3000 or , preferably , from 1500 to 2500 . when the molecular weight thereof is too small , no full effect can be exhibited on the stabilization of the aqueous emulsion . when the molecular weight thereof is too large , on the other hand , the polyoxyalkylene compound has a viscosity too high to have good dispersibility in water . it is important that the polyoxyalkylene compound as the component ( c ) is a copolymer of from 10 to 85 % by moles or , preferably , from 30 to 70 % by moles of the oxyethylene units and from 15 to 90 % by moles or , preferably , from 70 to 30 % by moles of the oxypropylene units . when the molar fraction of the oxyethylene units is too small , the polyoxyalkylene compound would be too hydrophobic to satisafatorily act as an emulsification aid . when the molar fraction of the oxyethylene units is too large , on the other hand , the polyoxyalkylene compound would have a too large value of hlb to serve as an emulsification aid . the amount of the polyoxyalkylene compound as the component ( c ) in the inventive antifoam composition is in the range from 10 to 20 parts by weight per 100 parts by weight of the total amount of the components ( a ) to ( d ). when the amount thereof is too small , the aqueous emulsion of the composition would be poor in the storage stability while , when the amount thereof is too large , the aqueous emulsion of the composition would be somewhat unstable in dilution . the component ( d ) in the inventive antifoam composition is a polydimethylsiloxane terminated at each molecular chain end with a silanolic hydroxy group or dimethyl hydroxy silyl group and serves as a &# 34 ; binder &# 34 ; between the components ( a ) and ( b ) by increasing the miscibility of these components . namely , the interfacial tension between these components , inherently poorly miscible with each other , is decreased by the silanolic hydroxy groups . the hydroxy - terminated polydimethylsiloxane as the component ( d ) should have a viscosity in the range from 10 to 500 centistokes or , preferably , from 30 to 200 centistokes at 25 ° c . when the viscosity thereof is too low , the aqueous emulsion of the composition would be somewhat unstabe sometimes to cause a trouble due to oil spots in dyeing . when the viscosity thereof is too high , on the other hand , the above mentioned effect on the components ( a ) and ( b ) cannot be fully exhibited unless the amount of the component ( d ) is unduly increased . the amount of the hydroxy - terminated polydimethylsiloxane as the component ( d ) in the inventive antifoam composition is in the range from 5 to 15 parts by weight per 100 parts by weight of the total amount of the components ( a ) to ( d ). when the amount thereof is too small , the desired effect cannot be fully exhibited as a matter of course . when the amount thereof is too large , on the other hand , the composition would have an unduly low viscosity eventually causing settling of the finely divided silica powder . the antifoam composition of the invention can be readily prepared by uniformly blending each a specified amount of the above described components ( a ) to ( d ) using a suitable blending machine such as homomixers and the like . the order of blending of the respective components is not particularly limitative . it is sometimes advantageous that the blending work is performed under heating at a temperature in the range from 40 ° to 150 ° c . in the following , the antifoam composition of the invention is illustrated in more detail by way of examples and comparative examples . the antifoam compositions prepared in the examples and comparative examples were evaluated for several items by the testing methods shown below . a 100 g portion of a 10 % by weight aqueous dispersion of the antifoam composition is taken in a 200 ml glass beaker and kept standing for 24 hours at 25 ° c . to be visually examined and rated according to the following criteria for the ratings of a , b , c and d . b : very slight but noticeable oil spots or settling or floating of silica powder a 100 g portion of a 0 . 2 % aqueous solution of sodium oleate is taken in a graduated measuring cylinder of 1000 ml capacity and the solution is admixed with 2 . 0 g of a 10 % by weight aqueous dispersion of the antifoam composition . air is blown at 25 ° c . into the solution through a fritted glass ball at the bottom at a rate of 1000 ml per minute for 20 minutes to cause rising of foams and the total volume of the liquid portion and the foam layer is recorded . a polyester cloth weighing 10 g is dyed at 135 ° c . taking 10 minutes in an aqueous dyeing bath of 2 liters volume containing 0 . 1 g of a dye ( s - bgu , a product by hoechst co . ), 4 g of a surface active agent ( emal nc - 35 , a produce by kao co . ), 4 g of acetic acid and 10 g of the antifoam composition under testing by using a high - temperature dyeing machine ( model jf , manufactured by werner mathis co .) and evenness of dyeing is visually examined and rated according the following criteria for the ratings of a , b , c and d . b : slight unevenness of dyeing but not greater than in blank test without addition of antifoam composition an antifoam composition was prepared in each of these examples 1 to 4 and comparative examples 1 to 3 by uniformly blending , in a homomixer , 650 g of ( a ) one of the polyoxyalkylene - modified diorganopolysiloxane oils a - 1 to a - 4 and b - 1 to b - 3 , respectively , shown by the average structural formulas below , in which me is a methyl group and pr is a propyl group , with 15 g of ( b ) a silicone oil compound prepared by compounding 45 % by weight of a first polydimethylsiloxane having a viscosity of 1150 centi - stokes at 25 ° c . and terminated at each molecular chain end with a trimethylsilyl group , 45 % by weight of a second polydimethylsiloxane having a viscosity of 100 centistokes at 25 ° c . and terminated at each molecular chain end with a trimethylsilyl group and 10 % by weight of a finely divided silica filler ( nipsil 9019 , a product by nippon silica co . ), 150 g of ( c ) a poly ( oxyethylene - oxypropylene ) copolymer having a viscosity of 1250 centistokes at 25 ° c . and expressed by the structural formula and 80 g of ( d ) a polydimethylsiloxane having a viscosity of 56 centistokes at 25 ° c . and terminated at each molecular chain end with a dimethyl hydroxy silyl group . a - 1 : me 3 si -- o --(-- simeg 2 -- o --) 2 . 9 --(-- simeg 3 -- o --) 7 . 1 --(-- sime 2 -- o --) 37 --(-- simepr -- o --) 3 -- sime 3 , g 2 is a group of the formula --(-- ch 2 --) 3 -- o --( c 2 h 4 o ) 27 --( c 3 h 6 o ) 9 -- c 4 h 9 and g 3 is a group of the formula --(-- ch 2 --) 3 -- o --( c 2 h 4 o ) 6 --( c 3 h 6 o ) 24 -- h . a - 2 : me 3 si -- o --(-- simeg 2 -- o --) 5 --(-- simeg 3 -- o --) 5 --(-- sime 2 -- o --) 37 --(-- simepr -- o --) 3 -- sime 3 , in which g 2 and g 3 each have the same meaning as above . a - 3 : me 3 si -- o --(-- simeg 2 -- o --) 5 . 8 --(-- simeg 3 -- o --) 4 . 2 --(-- sime 2 -- o --) 37 --(-- simepr -- o --) 3 -- sime 3 , in which g 2 and g 3 each have the same meaning as above . a - 4 : me 3 si -- o --(-- simeg 2 -- o --) 7 . 5 --(-- simeg 3 -- o --) 2 . 5 --(-- sime 2 -- o --) 37 --(-- simepr -- o --) 3 -- sime 3 , in which g 2 and g 3 each have the same meaning as above . b - 1 : me 3 si -- o --(-- simeg 4 -- o --) 55 --(-- sime 2 -- o --) 37 --(-- simepr -- o --) 3 -- sime 3 , in which g 4 is a group of the formula --(-- ch 2 --) 3 -- o --( c 2 h 4 o ) 20 --( c 3 h 6 o ) 20 -- co -- me . b - 2 : me 3 si -- o --(-- simeg 4 -- o --) 10 --(-- sime 2 -- o --) 37 --(-- simepr -- o --) 3 -- sime 3 , in which g 4 has the same meaning as above . b - 3 : me 3 si -- o --(-- simeg 4 -- o --) 20 --(-- sime 2 -- o --) 37 --(-- simepr -- o --) 3 -- sime 3 , in which g 4 has the same meaning as above . table 1 below shows the results of the evaluation tests of these seven antifoam compositions undertaken according to the procedures described above . table 1______________________________________polyoxyalkylene - stability defoamingmodified silicone against activity dyeingoil dilution test , ml test______________________________________a - 1 b 155 ba - 2 a 190 ba - 3 a 165 ba - 4 a 145 bb - 1 c 165 cb - 2 c 200 bb - 3 d 190 c______________________________________