Impact modified thermoplastic molding composition containing polycarbonate and polyester

Abstract

The impact strength of a thermoplastic molding composition containing aromatic polycarbonate resin and thermoplastic polyester is improved upon the incorporation therewith of a particular silicone rubber powder. The silicone rubber powder, added at a level of about 3 to 25 contains a mixture of (a) a polydiorganosiloxane and (b) finely divided silica filler.

Description

The invention concerns a thermoplastic molding composition and more particularly a composition containing polycarbonate and polyester resins.

The invention is based on the surprising and unexpected finding that the impact performance of a thermoplastic composition containing aromatic polycarbonate resin and polyalkylene terephthalate resin is improved upon incorporation therewith of a particular silicone rubber powder. The silicone rubber powder, added at a level of about 3 to 25 percent, relative to the weight of the composition, is characterized in that it contains a mixture of (a) a polydiorganosiloxane and (b) silica.

Thermoplastic aromatic polycarbonate resin is well known and is readily available in commerce. While the impact resistance of polycarbonate makes this resin the material of choice in a variety of demanding applications, attempts at improving this property continue. The literature includes a large number of patents directed to this subject. Thermoplastic polyester resins are also well known and are readily available in commerce. Also known are blends of polycarbonate and thermoplastic polyester.

Of particular relevance in the present context is a paper by R. Buch et al "Silicone-Based Additives for Thermoplastic Resins Providing Improved Impact Strength, Processing and Fire Retardant Synergy". This prior art paper (Dow Corning Corporation) disclosed certain silicone powder resin modifier products termed RM 4-7081 and RM 4-7051 to be useful in reducing the rate of heat release and the evolution rates of smoke and carbon monoxide of burning plastics, including polycarbonate. The relevant properties of compositions containing 99 and 95% polycarbonate, the balance in each composition RM 4-7081, are reported. Also disclosed is the impact strength improvement for engineering resins such as polyphenylene ether (PPE) and PPS. Improved impact strength of polycarbonate compositions is not reported.

Also related is Canadian Patent Application 2,083,014 which disclosed the silicone rubber powder of the present invention as a component in a composition containing poly(phenylene ether) resin.

Polycarbonate molding compositions which contain additive amounts of organosiloxane compounds are known: JP 5,262,960 is said to disclose a low viscosity polycarbonate resin composition which contains organo siloxane and a catalyst. The composition is said to exhibit lower melt viscosity and improved fluidity and moldability without loss of mechanical properties; JP 5,086,278 is considered to disclose an organosiloxane compound and a catalyst as additives to a polycarbonate resin. EP 505,869 disclosed a polycarbonate composition containing a siloxane compound, characterized in its high dimensional stability. Polycarbonate compositions containing a cyclosiloxane compound were disclosed in U.S. Pat. No. 3,751,519 to have good release properties. A thermal oxidative stabilized polycarbonate composition containing a hydrocarbonoxy siloxane compound has been disclosed in U.S. Pat. No. 4,197,384. U.S. Pat. No. 5,322,882 disclosed a composition having improved impact strength containing a polycarbonate/polyorganosiloxane copolymer.

It has now been discovered that certain silicone rubber powders, preferably produced in accordance with the procedure disclosed in U.S. Pat. No. 5,153,238 which is incorporated herein by reference, are useful as impact modifiers in thermoplastic molding compositions containing polycarbonate and polyalkylene terephthalate. The inventive compositions therefore contain about 3 to 25 percent, preferably 4 to 15 percent relative to the weight of the composition, of the silicone rubber powder.

The polycarbonate resins within the scope of the present invention include (co)polycarbonates and mixtures thereof.

The (co)polycarbonates generally have a weight average molecular weight of 10,000 to 200,000, preferably 20,000 to 80,000 and their melt flow rate, per ASTM D-1238 at 300.degree. C., is about 1 to about 65 g/10 min., preferably about 2 to 15 g/10 min. They may be prepared, for example, by the known diphasic interface process from a carbonic acid derivative such as phosgene and dihydroxy compounds by polycondensation (see German Offenlegungsschrifien 2,063,050; 2,063,052; 1,570,703; 2,211,956; 2,211,957 and 2,248,817; French Patent 1,561,518; and the monograph H. Schnell, "Chemistry and Physics of Polycarbonates", Interscience Publishers, New York, N.Y., 1964, all incorporated herein by reference).

In the present context, dihydroxy compounds suitable for the preparation of the polycarbonates of the inventor conform to the structural formulae (1) or (2). ##STR1## wherein

A denotes an alkylene group with 1 to 8 carbon atoms, an alkylidene group with 2 to 8 carbon atoms, a cycloalkylene group with 5 to 15 carbon atoms, a cycloalkylidene group with 5 to 15 carbon atoms, a carbonyl group, an oxygen atom, a sulfur atom, --SO-- or --SO.sub.2 -- or a radical conforming to ##STR2##

e and g both denote the number 0 to 1; Z denotes F, Cl, Br or C.sub.1 -C.sub.4 -alkyl and if several Z radicals are substituents in one aryl radical, they may be identical or different from one another; d denotes an integer of from 0 to 4; and f denotes an integer of from 0 to 3.

Among the dihydroxy compounds useful in the practice of the invention are hydroquinone, resorcinol, bis-(hydroxyphenyl)-alkanes, bis-(hydroxyphenyl)-ethers, bis-(hydroxyphenyl)-ketones, bis-(hydroxyphenyl)-sulfoxides, bis-(hydroxyphenyl)-sulfides, bis-(hydroxyphenyl)-sulfones, and .alpha.,.alpha.-bis-(hydroxyphenyl)-diisopropylbenzenes, as well as their nuclear-alkylated compounds. These and further suitable aromatic dihydroxy compounds are described, for example, in U.S. Pat. Nos. 3,028,356; 2,999,835; 3,148,172; 2,991,273; 3,271,367; and 2,999,846, all incorporated herein by reference.

Further examples of suitable bisphenols are 2,2-bis-(4-hydroxyphenyl)-propane (bisphenol A), 2,4-bis-(4-hydroxyphenyl)-2-methylbutane, 1,1-bis-(4-hydroxyphenyl)-cyclohexane, .alpha.,.alpha.'-bis-(4-hydroxyphenyl)-p-diisopropylbenzene, 2,2-bis-(3-methyl-4-hydroxyphenyl)-propane, 2,2-bis-(3-chloro-4-hydroxyphenyl)-propane, bis-(3,5-dimethyl-4-hydroxyphenyl)-methane, 2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane, bis-(3,5-dimethyl-4-hydroxyphenyl)-sulfide, bis-(3,5-dimethyl-4-hydroxyphenyl)-sulfoxide, bis-(3,5-dimethyl-4-hydroxyphenyl)-sulfone, dihydroxybenzophenone, 2,4-bis-(3,5-dimethyl-4-hydroxyphenyl)-cyclohexane, .alpha.,.alpha.'-bis-(3,5-dimethyl-4-hydroxyphenyl)-p-diisopropylbenzene and 4,4'-sulfonyl diphenol.

Examples of particularly preferred aromatic bisphenols are 2,2,-bis-(4-hydroxyphenyl)-propane, 2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane and 1,1-bis-(4-hydroxyphenyl)-cyclohexane.

The most preferred bisphenol is 2,2-bis-(4-hydroxyphenyl)-propane (bisphenol A).

The polycarbonates of the invention may entail in their structure units derived from one or more of the suitable bisphenols.

Among the resins suitable in the practice of the invention are included phenolphthalein-based polycarbonate, copolycarbonates and terpolycarbonates such as are described in U.S. Pat. Nos. 3,036,036 and 4,210,741, both incorporated by reference herein.

The polycarbonates of the invention may also be branched by condensing therein small quantities, e.g., 0.05 to 2.0 mol % (relative to the bisphenols) of polyhydroxyl compounds.

Polycarbonates of this type have been described, for example, in German Offenlegungsschriften 1,570,533; 2,116,974 and 2,113,374; British Patents 885,442 and 1,079,821 and U.S. Pat. No. 3,544,514. The following are some examples of polyhydroxyl compounds which may be used for this purpose: phloroglucinol; 4,6-dimethyl-2,4,6-tri-(4-hydroxyphenyl)-heptane; 1,3,5-tri-(4-hydroxyphenyl)-benzene; 1,1,1-tri-(4-hydroxyphenyl)-ethane; tri-(4-hydroxyphenyl)-phenylmethane; 2,2-bis-[4,4-(4,4'-dihydroxydiphenyl)]-cyclohexyl-propane; 2,4-bis-(4-hydroxy-1-isopropylidine)-phenol; 2,6-bis-(2'-dihydroxy-5'-methylbenzyl)-4-methylphenol; 2,4-dihydroxybenzoic acid; 2-(4-hydroxyphenyl)-2-(2,4-dihydroxyphenyl)propane and 1,4-bis-(4,4'-dihydroxytriphenylmethyl)-benzene. Some of the other polyfunctional compounds are 2,4-dihydroxybenzoic acid, trimesic acid, cyanuric chloride and 3,3-bis-(4-hydroxyphenyl)2-oxo-2,3-dihydroindole.

In addition to the polycondensation process mentioned above, other processes for the preparation of the polycarbonates of the invention are polycondensation in a homogeneous phase and transesterification. The suitable processes are disclosed in the incorporated herein by reference, U.S. Pat. Nos. 3,028,365; 2,999,846; 3,153,008; and 2,991,273.

The preferred process for the preparation of polycarbonates is the interfacial polycondensation process.

Other methods of synthesis in forming the polycarbonates of the invention such as disclosed in U.S. Pat. No. 3,912,688, incorporated herein by reference, may be used.

Suitable polycarbonate resins are available in commerce, for instance, Makrolon FCR, Makrolon 2600, Makrolon 2800 and Makrolon 3100, all of which are bisphenol based homopolycarbonate resins differing in terms of their respective molecular weights and characterized in that their melt flow indices (MFR) per ASTM D-1238 are about 16.5 to 24, 13 to 16, 7.5 to 13.0 and 3.5 to 6.5 g/10 min., respectively. These are products of Bayer Corporation of Pittsburgh, Pa.

A polycarbonate resin suitable in the practice of the invention is known and its structure and methods of preparation have been disclosed, for example in U.S. Pat. Nos. 3,030,331; 3,169,121; 3,395,119; 3,729,447; 4,255,556; 4,260,731; 4,369,303 and 4,714,746 all of which are incorporated by reference herein.

The thermoplastic (co)polyester suitable in the present invention comprise repeat units from at least one C.sub.6-20 -aromatic, C.sub.3-20 -aliphatic or alicyclic dicarboxylic acid, and repeat units from at least one C.sub.2-20 -aliphatic glycol.

Examples of the dicarboxylic acids include malonic, succinic, glutaric, adipic, pimelic, suberic, azelaic, sebacic, dodecanedioic, 1,4-, 1,5- and 2,6-decahydronaphthalene dicarboxylic acid, and cis- or trans-1,4-cyclohexane dicarboxylic acid. Examples of useful aromatic dicarboxylic acid are terephthalic acid; isophthalic acid; trans 3,3'- and trans 4,4'-stilbenedicarboxylic acid, 4,4'-dibenyldicarboxylic acid; 1,4-, 1,5'-, 2,3'-, 2,6, and 2,7-naphthalenedicarboxylic acid. The preferred dicarboxylic acids are terephthalic and isophthalic acid or mixtures thereof.

The preferred glycol of the (co)polyester includes 2 to 8 carbon atoms. Examples include ethylene glycol, 1,2-propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-, 1,3- and 1,4-cyclohexanedimethanol, neopentyl glycol, and 2,2,4,4-tetramethyl-1,3-cyclobutanediol. The preferred diols are 1,4-cyclohexanedimethanol, ethylene glycol and mixtures thereof.

The preferred (co)polyesters include resins having repeat units from poly(ethylene terephthalate) or poly(1,4-cyclohexylenedimethylene terephthalate). Preferred (co)polyesters comprise repeat units from terephthalic acid, isophthalic acid or mixtures thereof and 1,4-cyclohexanedimethanol. Other preferred (co)polyesters comprise repeat units from terephthalic acid and 1,4-cyclohexanedimethanol, ethylene glycol or mixtures thereof.

The preparation of the (co)polyesters follow conventional procedures well known in the art such as the process described in U.S. Pat. No. 2,901,466 which disclosure is incorporated herein by reference.

The (co)polyesters of the invention have as a rule inherent viscosity of about 0.4 to 1.0 dl/g, preferably about 0.6 to 0.8 dl/g at 25.degree. C. in a solvent containing 60 w/% phenol and 40 w/% tetrachloroethane.

Included among the suitable polyesters are the reaction product of a glycol portion which contains 1,4-cyclohexanedimethanol (CHDM) and ethylene glycol (EG) wherein the molar ratio of CHDM to EG is from about 1:4 to 4:1, with an acid portion comprising at least one of phthalic acid and isophthalic acid. These may be prepared by procedures well known to those skilled in this art, such as by condensation reactions substantially as shown and described in U.S. Pat. No. 2,901,466. More particularly, the acid or mixture of acids or alkyl esters of the aromatic dicarboxylic acid or acids, for example, dimethylterephthalate, together with the dihydric alcohols are charged to a flask and heated to temperatures sufficient to cause condensation of the copolymer to begin, for example to 175.degree. to 225.degree. C. Thereafter, the temperature is raised to about 250.degree. to 300.degree. C., and a vacuum is applied and the condensation reaction is allowed to proceed until substantially complete.

The condensation reaction may be facilitated by the use of a catalyst, with the choice of catalyst being determined by the nature of the reactants. The various catalysts for use herein are very well known in the art and are too numerous to mention individually herein. Generally, however, when an alkyl ester of the dicarboxylic acid compound is employed, an ester interchange type of catalyst is preferred, such as Nail Ti(OC.sub.4 H.sub.9).sub.6 in n-butanol. If a free acid is being reacted with the free glycols, a catalyst is generally not added until after the preliminary condensation has gotten under way.

The reaction is generally begun in the presence of an excess of glycols and initially involves heating to a temperature sufficient to cause a preliminary condensation followed by the evaporation of excess glycol. The entire reaction is conducted with agitation under an inert atmosphere. The temperature can then be advantageously increased with or without the immediate application of a vacuum. As the temperature is further increased, the pressure can be advantageously greatly reduced and the condensation allowed to proceed until the desired degree of polymerization is achieved. The product can be considered finished at this stage or it can be subjected to further polymerization in the solid phase in accordance with well-known techniques. Thus, the highly monomeric condensation product produced can be cooled, pulverized, and the powder heated to a temperature somewhat less than that employed during the last stage of the molten phase polymerization, thereby avoiding coagulation of the solid particles. The solid phase polymerization is conducted until the desired degree of polymerization is achieved. The solid phase polymerization, among other things, results in a higher degree of polymerization without the accompanying degradation which frequently takes place when continuing the last stage of the melt polymerization at a temperature high enough to achieve the desired degree of polymerization. The solid phase process is advantageously conducted with agitation employing an inert atmosphere at either normal atmospheric pressure or under a greatly reduced pressure.

The silicone rubber powder of the invention has an average particle size of about 1 to 1000 microns and contains (i) 100 parts by weight (pbw) of a polydiorganosiloxane and (ii) about 10 to 80 pbw, preferably, about 20 to 50 pbw of a finely divided silica filler.

The polydiorganosiloxane which is characterized in that its viscosity at 25.degree. C. is about 10.sup.6 to 10.sup.9 centipoise is a (co)polymeric resin having siloxane structural units represented by the general formula ##STR3## wherein

R, R' and R" independently denote hydrogen, C.sub.1-10 -alkyl, alkenyl, cycloalkyl radicals or aryl groups, and where p is about 1000 to 8000, preferably about 3000 to 6000 and where the relative weight proportions of n and m are 98.5 to 100:0 to 1.5, preferably 99:1, and where

X denotes a member selected from the group consisting of ##STR4## where R denotes hydrogen, C.sub.1-10 -alkyl, alkenyl, cycloalkyl radicals or aryl groups and where q is 1 to 10.

The organic groups of the polydiorganosiloxane, which may optionally be halogenated, are preferably lower alkyl radicals containing 1 to 4 carbon atoms, phenyl and halogen substituted alkyl radicals. Examples include resins containing dimethylsiloxy units, phenylmethylsiloxy units and dimethylsiloxy units and diphenyl siloxy units. Most preferably, the polydiorganosiloxane contains vinyl group(s) or epoxy group(s) at its chain termination(s) and/or along its main chain. The methods for the preparation of suitable polydiorganosiloxane are well known; a typical method comprises the acid- or base-catalyzed polymerization of cyclic diorganosiloxanes.

The silica filler required in the silicone rubber powder is a finely divided silica selected from among fumed silica and precipitated silica or silica gel. These are well known forms of silica and are readily available in commerce. The suitable silica is characterized in that its surface area is at least 50 m.sup.2 /g, preferably 50 to 900 m.sup.2 /g.

An additional embodiment entails use of treated silica which contains sites bonded to groups X as defined above; the manufacture of treated silica, typically by reacting the silanol groups on the silica surface with about 1 to 2% by weight of an organic alkyl halide compound or an organosilicon halide compound, is known in the art.

Among the suitable compounds, mention may be made of low molecular weight liquid hydroxy- or alkoxy-terminated polydiorganosiloxanes, hexaorganosiloxanes and hexaorganosilazanes.

The procedure for the preparation of the silicone rubber powder has been described in detail in U.S. Pat. No. 5,153,238, the specification of which is incorporated herein by reference. Suitable silicone rubber powder is available in commerce from Dow Corning Corporation under the trademark RM 4-7051 and RM 4-7081.

The preparation of the composition of the invention is carried out following conventional procedures and by use of conventional means such as single, preferably twin screw extruders. Conventional thermoplastic processes are suitable in molding useful articles from the inventive composition.

Compositions in accordance with the invention have been prepared following well-known procedures and their properties determined as described below: In Examples 1-3, the polycarbonate resin was Makrolon 3100 resin, a Bayer product (a bisphenol-A based homopolycarbonate having a MFR of about 6 g/10 min. in accordance with ASTM D-1238 (@300.degree. C. under 1200 g load); the silicone rubber powder was Dow Corning's RM 4-7051. The thermoplastic polyester was Ektar 12822 from Eastman, a polyethylene terephthalate having inherent viscosity of 0.9 dl/g. Composition 2 contained brominated PC oligomer (8.5%), brominated phosphate 3%, bisbenzotriazole 1.5% and PTFE 1%, having no criticality to the improved impact strength of the inventive composition. The notched Izod impact strength was measured at 73.degree. C.

TABLE 1______________________________________ Example 1 2 3______________________________________Thermoplastic polyester, wt. % 30 30 33Polycarbonate, wt. % 62 48 67Silicone rubber powder wt. % 8 8 0Impact Strength, notched 17.6 10.7 2Izod, ft.lb/in 1/8"Impact Strength, notched 14.5 4 2Izod, ft lb/in 1/4"Failure mode ductile ductile brittleFlammability rating UL-94, 1/16" fail V-0 N/ADelamination.sup.1 none none none______________________________________ .sup.1 by visual assessment

In an additional set of examples, the evaluation of which are summarized in Table 2 below, the resinous components of the compositions were the same as in Examples 1-3 above.

TABLE 2______________________________________ Example 4 5 6 7______________________________________Thermoplastic Polyester, wt. % 68 68 62 66Polycarbonate, wt % 32 29 28 25Silicone rubber powder, wt. %. 0 3 10 15Impact strength, notched Izod 1.0 1.3 15.4 14.5ft.lb./in. 1/8"Impact strength, notched Izod 0.8 1.4 3.8 7.7ft.lb./in. 1/4"______________________________________

The data presented above show the beneficial effect on the impact strength, the basis of the inventive finding, in compositions which are polyester-rich. It would be noted that no delamination was observed in the compositions of the invention. Additional data summarizing the results of the evaluation of compositions of the inventions where the resinous components are polycarbonate-rich, are presented in Table 3.

TABLE 3______________________________________ Example 8 9 10 11______________________________________Thermoplastic Polyester, wt. % 33 29 30 25Polycarbonate, wt % 67 68 62 60Silicone rubber powder, wt. %. 0 3 8 15Impact strength, notched Izod 2.3 2.7 17.6 14.2ft.lb./in. 1/8"Impact strength, notched Izod ND.sup.a 2.2 14.5 7.7ft.lb./in. 1/4"______________________________________ .sup.a not determined

Conventional additives may be incorporated in the composition of the invention in the usual quantities. Mention may be made of a thermal stabilizer, a mold release agent, a pigment, a flame retarding agent, a uv stabilizer, a hydrolysis stabilizer, a gamma radiation stabilizer and a plasticizer for polycarbonate compositions, as well as fillers and reinforcing agents such as glass fibers.

Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.

Claims

1. A thermoplastic molding composition comprising (a) a polycarbonate resin, (b) thermoplastic polyester and (c) a silicone rubber powder, wherein said powder is present in the composition in an amount of about 3 to 25 percent relative to the weight of the composition, said silicone rubber powder having an average particle size of about 1 to 1000 microns and containing

(i) 100 pbw of a polydiorganosiloxane having a viscosity at 25.degree. C. is about 10.sup.6 to 10.sup.9 centipoise and siloxane structural units represented by the general formula ##STR5## wherein R', R" and R'" independently denote a member selected from the group consisting of hydrogen atom, C.sub.1-10 -alkyl, alkenyl, cycloalkyl and aryl radicals, and where p is about 1000 to 8000 and where the relative weight proportions of n and m is 98.5 to 100:0 to 1.5, and where X denotes a member selected from the group consisting of ##STR6## where R denotes a member selected from the group consisting of hydrogen, C.sub.1-10 -alkyl, alkenyl, cycloalkyl and aryl radicals and where q is 1 to 10, and

(ii) about 10 to 80 pbw of a finely divided silica selected from the group consisting of fumed silica, precipitated silica and silica gel having a surface area of at least 50 m.sup.2 /g,

said composition characterized in that it has a notched Izod impact strength value which is greater than that of the composition in the absence of said (c).

2. The composition of claim 1 wherein said R', R" and R'" independently are selected from the group consisting of C.sub.1-10 -alkyl radicals; alkenyl radicals; cycloalkyl radicals; and aromatic hydrocarbon radicals.

3. The composition of claim 2 wherein said R', R" and R'" independently are a lower alkyl radical containing 1 to 4 carbon atoms or a phenyl radical.

4. The composition of claim 1 wherein said silica has a surface area of about 50 to 900 m.sup.2 /g.

5. The composition of claim 1 wherein said p is about 5000 to 6000.

6. The composition of claim 1 wherein the relative weight proportions of n and m is 99:1.

7. The composition of claim 1 wherein said silica contains sites bonded to said X.

8. The composition of claim 1 further containing at least one member selected from the group consisting of a thermal stabilizer, a mold release agent, a pigment, a flame retarding agent, a uv stabilizer, a hydrolysis stabilizer, a gamma radiation stabilizer, a plasticizer, a filler and a reinforcing agent.

9. A thermoplastic molding composition comprising (a) a polycarbonate resin, (b) a thermoplastic polyester and (c) a silicone rubber powder, wherein said powder is present in the composition in an amount of about 4 to 15 percent relative to the weight of the composition, said silicone rubber powder having an average particle size of about 1 to 1000 microns and containing

(i) 100 pbw of a polydiorganosiloxane having a viscosity at 25.degree. C. is about 10.sup.6 to 10.sup.9 centipoise and siloxane structural units represented by the general formula ##STR7## wherein R', R" and R'" independently denote a member selected from the group consisting of hydrogen atom, C.sub.1-10 -alkyl, alkenyl, cycloalkyl and aryl radicals, and where p is about 1000 to 8000 and where the relative weight proportions of n and m is 98.5 to 100:0 to 1.5, and where X denotes a member selected from the group consisting of ##STR8## where R denotes a member selected from the group consisting of hydrogen, C.sub.1-10 -alkyl, alkenyl, cycloalkyl and aryl radicals and where q is 1 to 10, and

(ii) about 20 to 50 pbw of a finely divided silica selected from the group consisting of fumed silica, precipitated silica and silica gel having a surface area of at least 50 m.sup.2 /g,

said composition characterized in that it has a 1/4" notched Izod impact strength value which is greater than that of the composition in the absence of said (c).

10. The composition of claim 9 wherein said silica has a surface area of about 50 to 900 m.sup.2 /g.

11. The composition of claim 9 wherein the relative weight proportions of n and m is 99:1.