Mannich condensates

BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to Mannich condensates prepared from an imidazole, formaldehyde and a polyalkylene polyamine, as hereafter defined. The Mannich condensates are useful, for example, as curing agents for epoxy resins.
More particularly, this invention relates to Mannich condensates prepared from starting materials including an imidazole and a polyalkylene polyamine. The imidazole and the polyalkylene polyamine are reacted with formaldehyde under Mannich condensation reaction conditions to provide the desired reaction product.
In accordance with the preferred embodiment of the present invention the polyamine that is reacted with the imidazole is a diamine and, in this situation, the invention may be illustrated by the following formula: ##STR3## Wherein: R represents hydrogen or a C.sub.1 -C.sub.4 alkyl group;
At least one of R' and R" represents hydrogen and the other represents hydrogen or methyl;
n represents a number having an average value of 0 to about 100;
Q represents --O-- or --NH--; and
Z represents H or ##STR4##
2. Prior Art
The Mannich reaction is a well known reaction which has been extensively reviewed in the literature. See for example, "The Mannich Reaction", Org. Reactions 1, 303 (1942) and "Advances in the Chemistry of Mannich Bases", Methods in Synthetic Organic Chemistry-Synthesis, Academic Press, pgs. 703-775, 1973.
Shimp U.S. Pat. No. 4,417,010 is directed to a liquid curing agent for polyepoxides having terminal epoxide groups which is a liquid blend of a monoimidazole compound and an aliphatic polyol.
Kaufman U.S. Pat. No. 4,420,605 discloses a coating composition comprising a 1,2-epoxy resin, a modified imidazole catalyst and a polyfunctional reactant such as a polyhydric phenol, a polycarboxylic acid or dicyandiamide.
SUMMARY OF INVENTION
It has been surprisingly discovered in accordance with the present invention that a new class of Mannich condensates having utility as epoxy resin curing agents can be provided when an imidazole feedstock is reacted with a polyalkylene polyamine feedstock and formaldehyde under Mannich reaction conditions.
STARTING MATERIALS
The Formaldehyde
Formaldehyde may be employed in any of its conventional forms. Thus it may be, and preferably is, used in the form of an aqueous solution of formaldehyde such as "formalin", and may also be used in "inhibited" methanol solution, as paraformaldehyde, or as trioxane.
The Imidazole Starting Material
The imidazoles to be used as starting materials are imidazoles having the following formula: ##STR5## Wherein R represents H or a C.sub.1 to C.sub.4 alkyl group.
Representative compounds having this structure are imidazole and compounds such as 2-methyl imidazole, 2-ethyl-4-methyl imidazole, 4-methyl imidazole, 2-ethyl imidazole, 4-ethyl imidazole and the corresponding propyl and butyl analogs.
The Polyamine Starting Materials
The polyalkylene polyamines to be used as starting materials in accordance with the present invention are those having the following formula: ##STR6## Wherein: Q represents --O-- or --NH--,
n represents a number having an average value of 0 to about 100, and
at least one of R' and R" represents hydrogen and the other represents hydrogen or methyl.
It will be seen from the above that the foregoing formula includes ethylene diamine, propylene diamine, polyethylene polyamines, polypropylene polyamines, polyoxyethylene polyamines and polyoxypropylene polyamines.
As an example, a polyethylenepolyamine may be used such as one having the following formula:
H.sub.2 NCH.sub.2 CH.sub.2 [NHCH.sub.2 CH.sub.2 ].sub.n NH.sub.2 (III)
Wherein n has a value of about 2 to about 10.
As another example, the amine may be a polyoxypropylene diamine having the formula: ##STR7## Wherein R' independently represents hydrogen or methyl.
Examples of polyoxyalkylenediamines of this formula which are useful in accordance with the present invention are those wherein x has a value of 3 to about 40 such as those compounds known as Jeffamine D (Formula XII) series of compounds available from Texaco Chemical Company.
Another example of a suitable amine to be used in accordance with the present invention is a polyoxyethylenediamine such as one having the formula: ##STR8##
Examples of suitable polyoxyethylenediamines having this formula are given in the following table:
TABLE II______________________________________ Approximate ValueProduct b a + c______________________________________JEFFAMINE ED-600 8.5 2.5JEFFAMINE ED-900 15.5 2.5JEFFAMINE ED-2001 40.5 2.5JEFFAMINE ED-4000 86.0 2.5______________________________________
Still another example of an amine to be used in accordance with the present invention is a polyoxypropylenetriamine having the formula: ##STR9## Wherein the sum of x+y+z is about 4 to 100.
Examples of suitable products having this formula include:
TABLE III______________________________________Product Initiator (A) Approx. MW Moles PO______________________________________T-403 TMP 400 5-6T-5000 Glycerine 5000 85______________________________________
Preparation of the Mannich Condensate
The relative quantities of imidazole, formaldehyde and amine will be determined by the nature of the desired final product. If a monosubstituted Mannich condensate is desired, the imidazole starting material, the amine starting material and the formaldehyde should be used in approximately equimolar amounts. As another example, if a bis Mannich condensate is desired, then about 2 mole equivalents of formaldehyde and about one mole equivalent of polyamine should be used per 2 moles of imidazole starting material.
The reaction is preferably conducted at atmospheric pressure although subatmospheric pressures and/or superatmospheric pressures may be used, if desired.
The reaction is normally conducted at a temperature within the range of about 80.degree. to about 120.degree. when the amine has an average molecular weight of about 500 or less.
However, when higher molecular weight amines are used, it is necessary to use higher temperatures and/or pressures, such as temperatures within the range of about 100.degree. to about 150.degree. C. and pressures within the range of about 1 atmosphere to about 50 atmospheres.
UTILITY OF MANNICH CONDENSATES AS EPOXY CURING AGENTS
A particular utility for which the Mannich condensates of the present invention are well suited is found when the Mannich condensates are used as curing agents for 1,2-epoxy resins.
It is known to use amines such as aliphatic or aromatic amines for curing 1,2-epoxy resins as shown, for example, by Waddill U.S. Pat. No. 4,139,524 and Marquis et al. U.S. Pat. No. 4,162,358. See also, the textbook "Handbook of Epoxy Resins" by H. Lee and K. Neville, McGraw-Hill Book Company, 1967.
Generally the vicinal epoxide compositions that can be cured using the curing agents of this invention are organic materials having an average of more than one reactive 1,2-epoxide group. These polyepoxide materials can be monomeric or polymeric, saturated or unsaturated, aliphatic, cycloaliphatic, aromatic or heterocyclic, and may be substituted if desired with other substituents besides the epoxy groups, e.g., hydroxyl groups, ether radicals, halogenated phenyl groups and the like.
The most widely used epoxy resins are diglycidyl ethers of bisphenol A: ##STR10## where n equals an integer of up to about 10 to 20.
However, these epoxides are representative of the broader class of epoxide compounds that are useful in making epoxy resins.
A widely used class of polyepoxides that can be cured according to the practice of the present invention includes the resinous epoxy polyethers obtained by reacting an epihalohydrin, such as epichlorohydrin, and the like, with either a polyhydric phenol or a polyhydric alcohol. An illustrative, but by no means exhaustive, listing of suitable dihydric phenols includes 4,4'-isopropylidene bisphenol, 2,4'-dihydroxydiphenylethylmethane, 3,3'-dihydroxydiphenyldiethylmethane, 3,4'-dihydroxydiphenylmethylpropylmethane, 2,3'-dihydroxydiphenylethylphenylmethane, 4,4'-dihydroxydiphenylmethane, 4,4'-dihydroxydiphenylbutylphenylmethane, 2,2'-dihydroxydiphenylditolylmethane, 4,4'-dihydroxydiphenyltolylmethyl-methane and the like. Other polyhydric phenols which may also be co-reacted with an epihalohydrin to provide these epoxy polyethers are such compounds as resorcinol, hydroquinone, substituted hydroquinones, e.g., tertbutylhydroquinones, and the like.
Among the polyhydric alcohols that can be co-reacted with an epihalohydrin to provide the resinous epoxy polyethers are such compounds as ethylene glycol, propylene glycol, butylene glycols, pentane diols, bis(4-hydroxycyclohexyl)dimethylmethane, 1,4-dimethylolbenzene, glycerol, 1,2,6-hexanetriol, trimethylolpropane, mannitol, sorbitol, erythritol, pentaerythritol, their dimers, trimers and higher polymers, e.g., polyethylene glycols, polypropylene glycols, triglycerol, dipentaerythritol and the like, polyallyl alcohol, polyhydric thioethers, such as 2,2'- 3,3'-tetrahydroxydipropylsulfide and the like, mercapto alcohols such as .alpha.-monothioglycerol, .alpha.,.alpha.'-dithioglycerol, and the like, polyhydric alcohol partial esters, such as monostearin, pentaerythritol monoacetate, and the like, and halogenated polyhydric alcohols such as the monochlorohydrins of glycerol, sorbitol, pentaerythritol and the like.
Another class of polymeric polyepoxides that can be cured by means of the above-described curing agents includes the epoxy novolac resins obtained by reacting, preferably, in the presence of a basic catalyst, e.g., sodium or potassium hydroxide, an epihalohydrin, such as epichlorohydrin, with the resinous condensate of an aldehyde, e.g., formaldehyde, and either a monohydric phenol, e.g., phenol itself, or a polyhydric phenol. Further details concerning the nature and preparation of these epoxy novolac resins can be obtained in Lee, H. and Neville, K. "Handbook of Epoxy Resins".
It will be appreciated by those skilled in the art that the polyepoxide compositions that can be cured according to the practice of the present invention are not limited to the above described polyepoxides, but that these polyepoxides are to be considered merely as being representative of the class of polyepoxides as a whole.
The amount of curing agent that is employed in curing polyepoxide compositions will depend on the amine equivalent weight of the curing agent employed. The total number of equivalents of amine group is preferably from about 0.8 to about 1.2 times the number of epoxide equivalents present in the curable epoxy resin composition with a stoichiometric amount being most preferred.
Various conventionally employed additives can be admixed with these polyepoxide-containing compositions prior to final cure. For example, in certain instances it may be desired to add minor amounts of other co-catalysts, or hardeners, along with the curing agent system herein described. Conventional pigments, dyes, fillers, flame retarding agents and other compatible natural and synthetic resins can also be added. Furthermore, known solvents for the polyepoxide materials such as acetone, methyl ethyl ketone, toluene, benzene, xylene, dioxane, methyl isobutyl ketone, dimethylformamide, ethylene glycol monoethyl ether acetate, and the like, can be used if desired, or where necessary.

WORKING EXAMPLES
Reaction of Amines, Formaldehyde and Imidazoles
Reaction of Jeffamine D-400 amine with formalin and imidazole is illustrated below. ##STR11##
To a 250 ml 3-necked flask equipped with a stirrer, thermometer and dropping funnel was added 6.8 g of imidazole (0.1 mole) and 40 g of Jeffamine D-400 amine (0.1 mole). The mixture was heated to 90.degree. C. to dissolve the imidazole. The mixture was cooled to 29.degree. C. and 8.1 g of 37% formalin was added portionwise keeping the temperature below 50.degree. C. The contents were held at 45.degree.-50.degree. C. for 30 minutes and then heated at 100.degree. C. and 15 mm. pressure for two hours to remove water. The product was a fluid light colored liquid.
The procedure described above was used to prepare the products described in Table I. In most cases the reactants were heated at 110.degree. C. for two hours prior to going under vacuum. The reactions are believed to take place at the 1-H position of imidazole and the C-alkyl substituted imidazoles. A model compound was prepared from imidazole, formaldehyde and methoxypropyl amine. Analysis by NMR and IR indicated substitution at the 1-position.
Summary of reaction data is shown in Table I, enclosed.
__________________________________________________________________________Example 1Glass Transition Temperatures (Tg) ofAmine.Imidazole.Formaldehyde Condensates Molar Curative Ratio of Notebook Optimum Conc., phr.Condensate Composition Condensate Number Tg, .degree.C. at Opt. Tg__________________________________________________________________________Jeffamine D400.Imidazole.CH.sub.2 O 1:1:1 5728-16 128.6 16Jeffamine D400.2MeImidazole.CH.sub.2 O 1:1:1 5728-25 143.0 16Jeffamine D400.Imidazole.CH.sub.2 O 1:2:2 5728-27 149.8 12Jeffamine D400.2MeImidazole.CH.sub.2 O 1:2:2 5728-26 172.7 11Jeffamine D400.1MeImidazole.CH.sub.2 O 1:1:1 5728-30 126.4 14Diethylenetriamine.Imidazole.CH.sub.2 O 1:1:1 5728-50 154.3 6N--Aminoethylpiperazine.Imidazole.CH.sub.2 O 1:1:1 5728-51 154.3 8Dipropylenetriamine.Imidazole.CH.sub.2 O 1:1:1 5728-59 160.4 6Jeffamine D230.Imidazole.CH.sub.2 O 1:1:1 5728-58 142.0 12Dipropylenetriamine.2MeImidazole.CH.sub.2 O 1:1:1 5906-5 182.0 6Jeffamine D400.2Et4MeImidazole.CH.sub.2 O 1:1:1 5728-29 132.6 19Dipropylenetriamine.2MeImidazole.CH.sub.2 O 1:2:2 5906-7 184.3 6Isophoronediamine.2MeImidazole.CH.sub.2 O* 1:1:1 5906-14 193.1 101,3-Diaminocyclohexane.2MeImidazole.CH.sub.2 O* 1:1:1 5906-15 185.6 8__________________________________________________________________________ *1,3-Diaminocyclohexane and isophoronediamine are cycloaliphatic (e.g., cyclohexyl) diamines.
______________________________________Example 1AGlass Transition Temperatures (Tg):Comparison of Tg of Blends and Condensates Curative Optimum Conc., Phr.Curative Composition Tg, .degree.C. at Opt. Tg.______________________________________Jeffamine D400.Imidazole Blend 124.0 10(8.6-1.4 pbw mixture)Jeffamine D400.Imidazole.CH.sub.2 O Cond. 128.6 16(5728-16)Dipropylenetriamine.2MeImidazole Blend 175.5 5(6.1-3.9 pbw mixture)DPTA.2MeImidazole.CH.sub.2 O Cond. 182.0 6(5906-05)Diethylenetriamine.Imidazole Blend 156.4 4(6.3-3.7 pbw mixture)DETA.Imidazole.CH.sub.2 O Cond. 162.6 6(5906-21)______________________________________
______________________________________Example 2Comparison of Properties: Curing withJeffamine D-400 Imidazole FormaldehydeCondensate and Related Product Blend 5882-44A 5882-77B______________________________________FormulationLiquid epoxy resin (EEW .about.185) 100 100Jeffamine D400.Imidazole.CH.sub.2 O 12 --Condensate (5728-27)Jeffamine D-400 -- 5.2Imidazole -- 1.8Exothermic PropertiesBrookfield Viscosity, cps., .about.25.degree. C. 25000 9000Gel time, minutes (200 g mass) .sup.2 .sup.3Peak exothermic temp., .degree.C. 30.3 29.0Time to peak temp., minutes .about.540 .about.220Physical Properties:.sup.1Izod impact strength, ft-lbs/in. 0.12 0.11Tensile strength, psi 7000 6800Tensile modulus, psi 351000 344000Elongation at break, % 2.9 2.9Flexural strength, psi 12000 11100Flexural modulus, psi 388000 368000HDT, .degree.C., 264 psi/66 psi 136/150 140/152Shore D hardness, 0-10 sec. 85-83 81-79Compression strength,to yield, psi -- --to failure, psi 36600 24650% weight gain,24 hr. water boil 1.7 1.53 hr. acetone boil 0.2 0.3______________________________________ .sup.1 Cured 2 hours, 100.degree. C., 3 hours, 150.degree. C. .sup.2 Time to viscosity of: 50,000 cps 210 minutes 100,000 cps .about.350 minutes Not gelled after 24 hours, .about.25.degree. C. .sup.3 Time to viscosity of 10,000 cps .about.100 minutes 20,000 cps .about.230 minutes 30,000 cps .about.300 minutes 40,000 cps 362 minutes 50,000 cps .about.400 minutes
______________________________________Example 3Comparison of Properties: Curing withDipropylenetriamine (DPTA).Imidazole.Formaldehyde Condensate and Related Blend 5882-46A 5882-77C______________________________________FormulationLiquid epoxy resin (EEW .about.185) 100 100DPTA Imidazole.CH.sub.2 O Condensate 6 --(5728-59)DPTA (5674-16-8) -- 3.2Imidazole -- 1.8Exothermic PropertiesBrookfield Viscosity, cps., .about.25.degree. C. 14000 7200Gel time, minutes (200 g mass) .sup.2 .about.160Peak exothermic temp., .degree.C. 40.0 45.1Time to peak temp., minutes .about.90 140Physical Properties:.sup.1Izod impact strength, ft-lbs/in. 0.15 0.15Tensile strength, psi 7000 7400Tensile modulus, psi 375500 378000Elongation at break, % 2.2 2.8Flexural strength, psi 12350 13300Flexural modulus, psi 388000 394000HDT, .degree.C., 264 psi/66 psi 149/162 149/159Shore D hardness, 0-10 sec. 80-78 82-80Compression strength,to yield, psi --to failure, psi 24300% weight gain,24 hr. water boil 2.2 1.63 hr. acetone boil 0.16 0.22______________________________________ .sup.1 Cured 2 hours, 100.degree. C., 3 hours, 150.degree. C. .sup.2 Viscosity reached 20,000 cps after .about.40 minutes 50,000 cps after .about.130 minutes 90,000 cps after .about.150 minutes
______________________________________Example 4Comparison of Properties: Curing withDiethylenetriamine (DETA).Imidazole.Formaldehyde Condensate and Related Blend 5882-39A 5882-77A______________________________________FormulationLiquid epoxy resin (EEW .about.185) 100 100DETA.Imidazole.CH.sub.2 O Condensate 6 --(5728-80)DETA -- 2.5Imidazole -- 1.5Exothermic PropertiesBrookfield Viscosity, cps., .about.25.degree. C. 13500 6000Gel time, minutes (200 g mass) .about.65 60.8Peak exothermic temp., .degree.C. 163.0 228.0Time to peak temp., minutes .about.155 93.0Physical Properties:.sup.1Izod impact strength, ft-lbs/in. 0.14 0.15Tensile strength, psi 6300 7400Tensile modulus, psi 390000 378000Elongation at break, % 1.9 2.8Flexural strength, psi 11450 13300Flexural modulus, psi 432500 394000HDT, .degree.C., 264 psi/66 psi 153/166 149/159Shore D hardness, 0-10 sec. 86-84 82-80Compression strength,to yield, psi -- --to failure, psi 21800 24300% weight gain,24 hr. water boil 1.9 1.83 hr. acetone boil 0.06 0.12______________________________________ .sup.1 Cured 2 hours, 100.degree. C., 3 hours, 150.degree. C.
TABLE I__________________________________________________________________________REACTIONS OF AMINES, FORMALDEHYDE AND IMIDAZOLES__________________________________________________________________________ Grams Formaldehyde Grams Grams ProductRun No. Amine Moles (37%) Moles Imidazole Moles Wt. (gr)__________________________________________________________________________5728-16 Jeffamine D400 40 gr. Formalin 8.1 gr Imidazole 6.8 gr 47.4 0.1 m 0.1 m 0.1 m5728-21 Jeffamine T403 27.9 Formalin 8.1 Imidazole 6.8 35.8 0.1 0.1 0.15728-22 Jeffamine D2000 200.0 Formalin 8.1 Imidazole 6.8 207.1 0.1 0.15728-25 Jeffamine D400 200.0 Formalin 40.5 2-Methyl 41.0 246.1 0.5 imidazole 0.55728-26 Jeffamine D400 200 Formalin 81.0 2-Methyl 82.0 293.5 1.0 imidazole 1.05728-27 Jeffamine D400 200 Formalin 81.0 Imidazole 68.0 276.2 1.0 0.855728-29 Jeffamine D400 134 Formalin 27.0 2-Et--4-Me 37.0 173.8 0.33 imidazole 0.335728-30 Jeffamine D400 200 Formalin 41.0 N--methyl 41.0 238.2 0.5 imidazole5728-48 Jeffamine ED900 90 Formalin 8.1 Imidazole 6.8 97.4 0.1 0.15728-49 Jeffamine T5000 500.0 Formalin 8.1 Imidazole 6.8 506.7 0.1 0.15728-50 Diethylene- 30.9 Formalin 24.3 Imidazole 18.3 51.1 triamine 0.35728-51 Aminoethyl- 38.7 Formalin 24.3 Imidazole 18.3 59.9 piperazine 0.35728-54 Jeffamine D230 46.0 Formalin 16.2 Imidazole 13.6 59.1 0.2 0.2 0.25728-58 Jeffamine D230 46.0 Formalin 16.2 Imidazole 13.6 61.2 0.2 0.2 0.25728-59 Dipropylene- 25.4 Formalin 16.2 Imidazole 13.8 39.9 triamine 0.2 0.2 0.25728-73 Diethylene- 92.7 Formalin 72.9 Imidazole 54.9 154.6 triamine5728-74 Aminoethyl- 154.8 Formalin 97.2 Imidazole 53.2 217.6 piperazine5728-80 Diethylene- 154.5 Formalin 121.5 Imidazole 92.5 229.0 triamine 0.33 0.335728-81 Aminoethyl- 193.5 Formalin 121.5 Imidazole 91.5 226.8 piperazine5728-82 Ethylenediamine 24.0 Formalin 32.4 Imidazole 27.2 45.2 0.4 0.4 0.45728-83 1,2-propylene 30.8 Formalin 32.4 Imidazole 27.2 40.9 diamine 0.4 0.4 0.45728-84 Aminoethyl- 193.5 Formalin 121.5 Imidazole 91.5 299.0 piperazine5728-86 Jeffamine D400 120.0 Formalin 24.3 Benzimid- 35.5 157.8 0.3 0.3 azole 0.35728-87 Jeffamine D400 120.0 Formalin 48.6 Benzimid- 71.0 196.8 0.3 0.6 azole 0.65728-93 Methoxypropyl- 18.2 Formalin 16.2 Imidazole 13.6 22.9 amine 0.2 0.2 0.25728-96 Methoxypropyl- 27.3 Formalin 24.3 Benzimid- 35.5 60.3 amine 0.3 0.3 azole 0.35906-4 Jeffamine D400 400.0 Formalin 81.0 Imidazole 68.0 477.9(5728-16 1.0 1.0 1.0Scale-up)5906-5 Dipropylene- 126 Formalin 81.0 2-methyl 82.0 215.4 triamine 1.0 1.0 imidazole 1.0__________________________________________________________________________ Product Appearance5728-16 Fluid light colored liquid5728-21 Almost colorless viscous liquid5728-22 Viscous light gold color at room temp.5728-25 Clear yellow liquid5728-26 Golden viscous liquid5728-27 Very viscous gold-orange color at R.T.5728-29 Clear golden liquid5728-30 Clear light yellow liquid5728-48 Clear light yellow liquid5728-49 Viscous light gold color at R.T.5728-50 Orange liquid5728-51 Nonpourable liquid-light colored5728-54 Viscous black liquid5725-58 Burnt orange fluid liquid5728-59 Gold color slight viscous material5728-73 Yellow viscous liquid5728-74 Yellow, nonpourable liquid5728-80 Orange viscous liquid5728-81 Orange-yellow clear solid5728-82 Orange solid at R.T.5728-83 Sticky, gel-like orange liquid5728-84 Nonpourable light yellow5728-86 Slightly viscous dark brown at R.T.5728-87 Very viscous dark brown at R.T.5728-93 Clear golden solid5728-96 Brown liquid-solid5906-4 Yellow-gold liquid5906-5 Yellow fluid liquid
Jeffamine D-230 and D-400 are represented by the following structure:
______________________________________H.sub.2 NCH(CH.sub.3)CH.sub.2 --[OCH.sub.2 CH(CH.sub.3)--] .sub.xNH.sub.2i (XII)JEFFAMINE x (approx.)______________________________________D-400 5.6D-230 2.6______________________________________
JEFFAMINE T-403 has the following structure: ##STR12##
The formula for Jeffamine T-5000: ##STR13##
Amines such as JEFFAMINE-ED 900 may be generically represented by the formula: ##STR14## wherein a plus c equal about 3.5 and b equals about 13 to 46.
Dipropylene triamine refers to the formula: ##STR15##
It will be understood that the foregoing examples are given by way of illustration only and not by way of limitation and that the scope of the present invention is defined solely by the appended claims.

Number	Name	Date	Kind
3677978	Dowbenko et al.	Jul 1972
3787458	Piasek et al.	Jan 1974

Mannich condensates

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

US Referenced Citations (2)

Non-Patent Literature Citations (1)