阅读说明：本技术 一种乙烯基酯树脂及其制备方法与应用 (Vinyl ester resin and preparation method and application thereof ) 是由 李会峰 于 2019-10-09 设计创作，主要内容包括：本发明涉及高分子材料技术领域,具体公开了一种乙烯基酯树脂及其制备方法与应用。本发明的乙烯基酯树脂,含有一种或多种如下结构的树脂H,其中,n为1～3；m为1～3；R<Sub>1</Sub>为-H或-CH<Sub>3</Sub>,R<Sub>2</Sub>为—(CH<Sub>2</Sub>)<Sub>2</Sub>—、—(CH<Sub>2</Sub>)<Sub>4</Sub>—或<Image he="173" wi="203" file="DDA0002226962410000011.GIF" imgContent="drawing" imgFormat="GIF" orientation="portrait" inline="no"></Image>本发明的制备方法包括以下步骤：(1)将溴化双酚A型环氧树脂A与一元羧酸反应制得树脂C；(2)将树脂C与二元羧酸反应制得树脂E；(3)将树脂E与酚醛型环氧树脂F反应制得树脂G。此方法制备简单,制得的改性乙烯基酯树脂外观为无色透明、强度高、耐腐蚀,兼顾阻燃、耐热等性能,可通过光固化方式进行固化,适于应用于环保工程。<Image he="172" wi="700" file="DDA0002226962410000012.GIF" imgContent="drawing" imgFormat="GIF" orientation="portrait" inline="no"></Image>(The invention relates to the technical field of high polymer materials, and particularly discloses a vinyl ester resin and a preparation method and application thereof. The vinyl ester resin contains one or more resins H with the following structures, wherein n is 1-3; m is 1-3; r 1 is-H or-CH 3 ，R 2 Is (CH) — (CH) 2 ) 2 —、—(CH 2 ) 4 -or The preparation method comprises the following steps: (1) reacting brominated bisphenol A epoxy resin A with monocarboxylic acid to prepare resin C; (2) reacting the resin C with dicarboxylic acid to obtain a resin E; (3) and reacting the resin E with the phenolic epoxy resin F to obtain a resin G. The method is simple to prepare, and the prepared modificationThe vinyl ester resin is colorless and transparent in appearance, high in strength, corrosion-resistant, flame-retardant, heat-resistant and the like, can be cured in a photocuring mode, and is suitable for being applied to environmental engineering.)

1. A vinyl ester resin comprising one or more resins H of the structure:

wherein n is 1-3; m is 1-3; r₁is-H or-CH₃，R₂Is (CH) — (CH)₂)₂—、—(CH₂)₄-or

2. The vinyl ester resin according to claim 1, wherein R is R₁is-CH₃，R₂Is (CH) — (CH)₂)₄—。

3. A process for the preparation of a vinyl ester resin, characterized in that the reaction scheme is as follows:

wherein n is 1-3; m is 1-3;

the method comprises the following steps:

step (1): reacting brominated bisphenol A epoxy resin A with monocarboxylic acid shown as a formula B to prepare resin C;

step (2): reacting the resin C with dicarboxylic acid shown as a formula D to obtain a resin E;

and (3): reacting the resin E with phenolic epoxy resin F to obtain resin G;

the monocarboxylic acid is one or more of acrylic acid and methacrylic acid; the dicarboxylic acid is one or more of itaconic acid, adipic acid and succinic acid.

4. The method of claim 3, wherein the steps (1) to (3) are carried out in the presence of a phosphorus catalyst, a phosphite antioxidant and a phenolic inhibitor.

5. The method of claim 4, wherein the phosphorous-based catalyst has the structure:

6. The method of claim 5, wherein R is C₁₄Alkyl carbon chain of (2).

7. The method of claim 4, wherein the phosphite antioxidant has the structure:wherein R is₃、R₄Identical or different, each independently is C₁-C₄Alkyl carbon chain of (2).

8. The method of claim 4, wherein the phenolic polymerization inhibitor is one or more of hydroquinone, p-methoxyphenol, 2, 6-di-tert-butyl-p-cresol.

9. The method of claim 4, wherein when the phosphorus-based catalyst is tetradecylbenzylphosphonium chloride, the phosphite antioxidant is ethylpropylphosphite, and the phenolic polymerization inhibitor is hydroquinone.

10. The method according to claim 4 or 9, wherein the mass ratio of the phosphorus-based catalyst to the brominated bisphenol a-type epoxy resin a is (0.5 to 0.7): 100, respectively; the mass ratio of the phosphite antioxidant to the brominated bisphenol A epoxy resin A is (0.3-0.5): 100, respectively; the mass ratio of the phenolic polymerization inhibitor to the brominated bisphenol A type epoxy resin A is (0.5-0.7): 100.

11. the process according to claim 3 or 4, wherein the molar ratio of brominated bisphenol A epoxy resin A to the monocarboxylic acid in step (1) is 1: (0.95-1); the molar ratio of the brominated bisphenol A epoxy resin A to the dicarboxylic acid is 1: (0.95-1).

12. The method according to claim 3 or 4, wherein the molar ratio of the brominated bisphenol A epoxy resin A to the novolac-type epoxy resin F is 1: (0.95-1).

13. The method as claimed in claim 3, wherein the reaction temperature of steps (1) - (3) is 105-115 ℃.

14. The method according to claim 3, wherein the reaction of step (2) is terminated when the acid value is less than 190 KOHmg/g.

15. The method according to claim 4, wherein the method comprises the steps of mixing the brominated bisphenol A type epoxy resin A with the phosphorus catalyst, the phosphite antioxidant and the phenolic polymerization inhibitor, and adding the monocarboxylic acid obtained in the step (1) to obtain a first reaction system; after the reaction of the first reaction system is finished, adding the dicarboxylic acid into the first reaction system to obtain a second reaction system; and when the acid value of the second reaction system is less than 190KOHmg/g, adding the novolac epoxy resin F into the second reaction system to obtain a third reaction system.

16. The method of any one of claims 3-15, wherein the reaction profile further comprises:

the steps further include:

and (4): and reacting the resin G with the monocarboxylic acid to obtain a resin H.

17. The process of claim 16, wherein the step (4) is carried out in the presence of the phosphorus-based catalyst, the phosphite antioxidant and the phenolic inhibitor in the process of any one of claims 4 to 10.

18. The method as claimed in claim 16, wherein the reaction temperature of step (4) is 105-115 ℃.

19. The method according to claim 16, wherein when the acid value of the third reaction system in the method of claim 15 is less than 5KOHmg/g, the monocarboxylic acid of the step (4) is added to the third reaction system to obtain a fourth reaction system; and finishing the reaction when the acid value of the fourth reaction system is 10-15 KOHmg/g.

20. The method of claim 16 or 17, wherein the steps further comprise:

and (5): dissolving the resin H in a solvent containing unsaturated double bonds.

21. The method of claim 20, wherein the solvent is one or more of vinyl toluene, styrene, 2-phenyl-1-propene, methyl acrylate, methacrylate, glycidyl methacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate.

22. The method according to claim 20 or 21, wherein the mass ratio of the solvent to the resin H is (0.6-1.4): 1.

23. the method as claimed in claim 20, wherein the temperature of the dissolution in the step (5) is 50-60 ℃.

24. Use of a vinyl ester resin according to claim 1 or 2 or a method according to any one of claims 3 to 23 in the field of environmental engineering, in particular in environmental engineering for desulfurization and denitrification by photocuring molding.

Technical Field

The invention relates to the technical field of high polymer materials. In particular to a vinyl ester resin and a preparation method and application thereof.

Background

Vinyl ester resins are modified epoxy resins obtained by reacting an epoxy resin as a matrix with an acid containing an unsaturated bond, and are generally called Vinyl Ester Resins (VER), also known as epoxy vinyl ester resins, which are thermosetting resins. The vinyl ester resin has the excellent characteristics of epoxy resin, has higher strength and excellent corrosion resistance, and has wide application prospect. According to different use requirements, various modified vinyl ester resins have been researched and developed, so that the application fields of the vinyl ester resins are more and more extensive. Among them, research aiming at comprehensively improving the flame retardancy, heat resistance and appearance color performance thereof to meet the requirements of various engineering (for example, engineering in the field of flame retardation and environmental protection and the field of photocuring) has become a hot spot.

Chinese patent application CN102898775 discloses a furan resin material for corrosion and high temperature resistance of a chimney, which is prepared by reacting furfuryl ketone and furfuryl alcohol monomer with anhydride at 80-110 ℃, prepolymerizing, and then adding inorganic filler to finally synthesize a product with high temperature resistance, corrosion resistance and flame retardant property. However, the resin has disadvantages in that: 1. the synthesized product belongs to a prepolymer product, needs to be heated in site construction, and has high working condition requirement; 2. the resin material has large brittleness after being cured, poor tensile bending property and easy aging.

Chinese patent application CN102924680 discloses a carbamate modified vinyl ester resin which takes a brominated bisphenol A epoxy structure as a main body and takes

The resin has a plurality of excellent chemical and physical properties such as high temperature, toughness, flame retardance, good corrosion resistance, room temperature curing and the like as a terminal group, and simultaneously has excellent toughness and impact resistance. However, the resin has the disadvantage that although the resin can meet the general corrosion resistance requirement, the structure of the resin is introduced with the oxazolidinone five-membered ring

It is very easily decomposed in a strong acid environment, limiting its use. And in the course of its reaction, a toxic benzyldimethylamine is used as a catalyst, and its residue may have an influence on the environment.

Chinese patent application CN104031213 discloses a scheme of replacing unsaturated acid in the traditional vinyl ester resin formula with a phosphorus reactive flame retardant to react with novolac epoxy resin and only adding an additive flame retardant, so as to finally obtain the low-smoke halogen-free high-temperature resistant flame retardant vinyl ester resin. The invention has the problems that the used reactive phosphorus flame retardant has low phosphorus content and cannot obviously improve the flame retardant effect, and the additive flame retardant is still added to cause the delamination of the resin product, the reduction of the corrosion resistance and the reduction of the mechanical property.

Chinese patent application CN109467651 discloses a photo-cured epoxy vinyl ester resin obtained by the reaction of epoxy resin, amino acrylate and organic unsaturated acid. The light curing speed is improved, but the product is light yellow transparent liquid, and deep curing is influenced by wavelength absorption during light curing.

In summary, in the conventional resin technology, in order to pursue flame retardancy, temperature resistance and other properties, corrosion resistance and mechanical properties of a resin part are affected, or non-environment-friendly raw materials are introduced to achieve the purpose. Moreover, the prior art cannot obtain the colorless and transparent product appearance on the basis of considering flame retardance, temperature resistance, corrosion resistance and mechanical properties, and limits the application of the resin.

Therefore, it is necessary to provide a new (flame retardant, high heat resistant, colorless) vinyl ester resin, a preparation method and applications thereof to solve the drawbacks of the prior art.

Disclosure of Invention

In order to solve the problems of the prior art, it is an object of the present invention to provide a vinyl ester resin having excellent overall properties.

In order to achieve the purpose of the invention, the technical scheme of the invention is as follows:

a vinyl ester resin comprising one or more resins H of the structure:

wherein the content of the first and second substances,n is 1 to 3; m is 1-3; r₁is-H or-CH₃，R₂Is (CH) — (CH)₂)₂—、—(CH₂)₄-or

The vinyl ester resin provided by the invention has high strength and corrosion resistance, and has flame retardance, heat resistance and colorless performance.

Particularly, the Hazen color number of the vinyl ester resin provided by the invention is less than or equal to 5.

The vinyl ester resin can be cured and molded by normal temperature curing, thermocuring, photocuring and other modes.

Preferably, in the vinyl ester resins of the present invention, R₁is-CH₃，R₂Is (CH) — (CH)₂)₄And the influence of the modified double bond on the final curing performance of the resin due to steric effect is avoided.

It is another object of the present invention to provide a process for the preparation of vinyl ester resins.

In order to achieve the purpose of the invention, the technical scheme of the invention is as follows:

a process for preparing a vinyl ester resin having the following reaction scheme:

wherein n is 1-3; m is 1-3;

the method comprises the following steps:

step (1): reacting brominated bisphenol A epoxy resin A with monocarboxylic acid shown as a formula B to prepare resin C;

step (2): reacting the resin C with dicarboxylic acid shown as a formula D to obtain a resin E;

and (3): reacting the resin E with phenolic epoxy resin F to obtain resin G;

the monocarboxylic acid is one or more of acrylic acid and methacrylic acid; the dicarboxylic acid is one or more of itaconic acid, adipic acid and succinic acid.

The method comprises the steps of carrying out ring-opening addition reaction on a brominated bisphenol A epoxy resin A and monocarboxylic acid shown as a formula B to obtain a resin C, carrying out ring-opening addition reaction on the resin C and dicarboxylic acid shown as a formula D to obtain a resin E, and carrying out addition reaction on the resin E and phenolic epoxy resin F to obtain a resin G.

The invention changes the addition mode of the conventional two types of resin combination, firstly the brominated bisphenol A type epoxy resin is respectively subjected to ring-opening addition reaction with the monounsaturated carboxylic acid and the dicarboxylic acid in two steps, and then the brominated bisphenol A type epoxy resin is added with the phenolic aldehyde type epoxy resin, so that the molecular chain segments in the finally obtained combination of the brominated bisphenol A type epoxy resin and the phenolic aldehyde type epoxy resin are uniformly distributed, and the final product performance of the resin is not unbalanced and the quality is not influenced due to the problem of chemical selectivity.

Preferably, in the method of the invention, the epoxy equivalent of the brominated bisphenol A type epoxy resin A is 390-410 g/mol.

Preferably, in the method of the present invention, the epoxy equivalent of the novolac epoxy resin F is 175-185 g/mol, and the relative molecular weight is 525-555.

Wherein, the monocarboxylic acid is selected from one or more of acrylic acid and methacrylic acid, and the molecular weight thereof is small, so that the double bond of the final product can not influence the curing effect due to incomplete reaction of steric effect in the curing process. The dicarboxylic acid is selected from one or more of itaconic acid, adipic acid and succinic acid, and the carbon chain of the dicarboxylic acid is short, so that the reaction in the reaction step (3) is realized.

In the method, the steps (1) to (3) are all reacted in the presence of a phosphorus catalyst, a phosphite antioxidant and a phenolic polymerization inhibitor.

Aiming at the reaction system, the inventor particularly selects a phosphorus catalyst, a phosphite antioxidant and a phenolic polymerization inhibitor to jointly form a catalytic system, and the phosphorus catalyst, the phosphite antioxidant and the phenolic polymerization inhibitor do not react with each other in the reaction system to cause catalyst inactivation, so that the corresponding functions of the phosphorus catalyst, the phosphite antioxidant and the phenolic polymerization inhibitor in the catalytic system are ensured to be played.

In the method of the present invention, the structure of the phosphorus-based catalyst is:

wherein R is C₁₀-C₁₅Alkyl carbon chain of (2).

The phosphorus catalyst has a long-chain alkyl structure, so that the inactivation in the catalysis process of the reaction system is avoided, and the high purity and colorless appearance of the final product are ensured; preferably, R is 14 carbons (C)₁₄) The alkyl carbon chain of (2) to more favorably realize the catalytic effect.

In the method of the invention, the structure of the phosphite antioxidant is as follows:

wherein R is₃、R₄Identical or different, each independently is C₁-C₄Alkyl carbon chain of (2).

The phosphite antioxidant has a trisubstituted phosphorus structure, so that the antioxidant effect is realized in the reaction system, the problem of yellowing of the product after antioxidation is avoided, and the colorless of the final product is ensured.

In the method, the phenolic polymerization inhibitor is one or more of hydroquinone, p-methoxyphenol and 2, 6-di-tert-butyl-p-cresol.

The invention further researches a specific catalyst system, when the phosphorus catalyst, the phosphite antioxidant and the phenol polymerization inhibitor are selected to form the specific catalyst system for use, the colorless appearance of the product is not influenced, and the bromine element in the brominated epoxy compound is ensured to be stabilized on a benzene ring in the whole reaction process, so that the problem that the final product cannot form colorless transparent appearance due to cracking or dissociation is avoided.

Preferably, when R is C₁₄When the alkyl carbon chain (i.e., the phosphorus-based catalyst is tetradecylbenzyl phosphorus chloride), R is₃Is ethyl, said R₄Is propyl (i.e., phosphites are antioxidant)The agent is ethylpropyl phosphite), and the phenolic polymerization inhibitor is hydroquinone. This combination can achieve a lighter color end product.

In the method, the mass ratio of the phosphorus catalyst to the brominated bisphenol A type epoxy resin A is (0.5-0.7): 100, respectively; preferably 0.6: 100, respectively; the mass ratio of the phosphite antioxidant to the brominated bisphenol A epoxy resin A is (0.3-0.5): 100, respectively; preferably 0.4: 100, respectively; the mass ratio of the phenolic polymerization inhibitor to the brominated bisphenol A type epoxy resin A is (0.5-0.7): 100, respectively; preferably 0.6: 100.

in the method of the invention, each reaction step is carried out under the protection of a mixed gas of 10% oxygen and 90% nitrogen (volume percentage).

In the method of the invention, the reaction in the step (1) is finished (the reaction is complete) when the acid value of the system is less than or equal to 0KOH mg/g.

Preferably, in the method of the present invention, the molar ratio of the brominated bisphenol a-type epoxy resin a to the monocarboxylic acid in step (1) is 1: (0.95-1).

In the method of the present invention, the reaction of the step (2) is terminated when the acid value is less than 190 KOHmg/g.

Preferably, in the method of the present invention, the molar ratio of the brominated bisphenol a-type epoxy resin a to the dicarboxylic acid is 1: (0.95-1).

In the method of the invention, the reaction in the step (3) is finished when the acid value of the system is less than or equal to 5KOH mg/g.

Preferably, in the method of the present invention, the molar ratio of the brominated bisphenol a-type epoxy resin a to the novolac-type epoxy resin F is 1: (0.95-1).

In the method of the present invention, the reaction temperatures of the steps (1) to (3) are all 105-115 ℃.

Mixing the brominated bisphenol A type epoxy resin A with the phosphorus catalyst, the phosphite antioxidant and the phenol polymerization inhibitor, and adding the monocarboxylic acid in the step (1) to obtain a first reaction system; after the reaction of the first reaction system is finished, adding the dicarboxylic acid into the first reaction system to obtain a second reaction system; and when the acid value of the second reaction system is less than 190KOHmg/g, adding the novolac epoxy resin F into the second reaction system to obtain a third reaction system.

In the method of the present invention, the reaction sequence further comprises:

the steps further include:

and (4): and (3) reacting the resin G with the monocarboxylic acid (shown as a formula B) to obtain a resin H.

In the step (4), the resin G and monocarboxylic acid are subjected to ring-opening addition reaction to obtain a resin H. In the step (4), the monocarboxylic acid is one or more of acrylic acid and methacrylic acid.

In the method of the present invention, the step (4) is performed in the presence of the phosphorus catalyst, the phosphite antioxidant and the phenolic inhibitor (specifically, the selection and the amount are the same as above).

In the reaction in the step (4), the resin G prepared in the step (3) is used as a raw material, and reacts with the monocarboxylic acid in the presence of the phosphorus catalyst, the phosphite antioxidant and the phenol polymerization inhibitor. Preferably, the reaction of step (4) may also be carried out by directly adding the monocarboxylic acid to the third reaction system.

Preferably, in the method of the present invention, the amount of the monocarboxylic acid added in the step (4) is preferably added to ensure that all epoxy groups in the resin G are ring-opening-added with the monocarboxylic acid.

Preferably, the molar ratio of the resin G to the monocarboxylic acid in step (4) is 1: (1.95-3.9).

In the method of the present invention, the reaction temperature in the step (4) is 105-115 ℃.

In the method of the present invention, when the acid value of the third reaction system is less than 5KOHmg/g, the monocarboxylic acid in step (4) is added to the third reaction system to obtain a fourth reaction system; and (3) finishing the reaction in the step (4) when the acid value of the fourth reaction system is 10-15KOHmg/g to obtain the resin H.

The steps in the method of the present invention can be performed separately or sequentially in the same reaction system, and when performed separately, the amounts of the phosphorus-based catalyst, the phosphite antioxidant and the phenolic polymerization inhibitor used in the steps can be the same or different within the range defined in the present invention, and it is particularly preferable that the steps in the method of the present invention are performed sequentially in the same reaction system to further reduce the synthesis cost and simplify the operation.

In the method of the present invention, the steps further comprise:

and (5): dissolving the resin H in a solvent containing unsaturated double bonds.

The present invention also includes a step of further mixing the resin H with a solvent (crosslinking agent).

In the method of the present invention, the solvent is one or more of vinyltoluene, styrene, 2-phenyl-1-propene, Methyl acrylate, methacrylate (Methyl propionate), glycidyl methacrylate (glycidyl methacrylate), 2-Hydroxyethyl methacrylate (2-hydroxymethacrylate), and hydroxypropyl methacrylate (Hydroxy propyl methacrylate).

The mass ratio of the solvent to the resin H is (0.6-1.4): 1, preferably 1: 1, so as to be more beneficial to obtain viscosity more suitable for operation and facilitate the application of the product.

In the method of the present invention, the temperature of the dissolution in the step (5) is 50 to 60 ℃ to facilitate the mixed dilution of the resin H and the solvent.

In the method, after the step (4) is finished, the temperature of the reaction is reduced to 50-60 ℃, a solvent containing unsaturated double bonds is added, and the mixture is mixed and stirred uniformly.

As an embodiment of the invention, the reaction process of the method provided by the invention is as follows:

wherein n, m, R₁、R₂The selection of (2) is as above.

The invention further aims to provide an application of the vinyl ester resin or the method for preparing the vinyl ester resin in the field of environmental engineering, in particular an application of desulfurization and denitrification environmental engineering formed by photocuring.

The vinyl ester resin can be used as matrix resin for preparing photo-curing sheet molding compound (photo-curing SMC), can be prepared into a prepreg sheet molding compound (photo-curing sheet molding compound) in advance by using the resin, matching with a reinforced fiber material and an auxiliary agent, is constructed in a patch mode on a construction site, and is cured by ultraviolet lamp light irradiation.

The construction scheme for preparing the SMC sheet by utilizing the resin has the characteristics of fast construction process, low VOC, fast curing speed, deep curing thickness, flame retardance and the like, greatly reduces the whole construction period, improves the construction efficiency and the construction safety, can be suitable for the field of environmental protection engineering, and particularly can be applied to the application field with the requirements of fast construction, low VOC, flame retardance, high heat resistance and high corrosion resistance. The method can be used as a new material solution in the desulfurization and denitrification environmental protection engineering of the thermal power plant, meets the market demand of the environmental protection engineering construction of the related anticorrosion field represented by the application of the desulfurization and denitrification environmental protection engineering of the thermal power plant, and contributes to the strength of the environmental protection cause.

The invention has the beneficial effects that:

the vinyl ester resin of the invention has the following characteristics:

(1) the flame-retardant resin has excellent flame retardance (the oxygen index of pure resin is greater than 34), so that the fire risk of a construction site can be reduced;

(2) the heat resistance is excellent, the thermal deformation temperature of the resin is higher than 150 ℃, and the thermal deformation temperature of a composite material forming product can be ensured to be more than 200 ℃;

(3) the ultraviolet pure resin has colorless and transparent resin appearance (hazen color number is less than or equal to 5), and can be cured with the thickness of 5-10 mm after being irradiated by ultraviolet light or visible light for 20-30 minutes, and the ultraviolet pure resin has deep curing depth (the curing depth can reach 10 mm);

(4) excellent mechanical properties (bending, stretching, etc.) in physical properties;

(5) has excellent corrosion resistance and can resist corrosion of most chemical substances such as sulfuric acid, hydrochloric acid, nitric acid and the like.

The method for preparing the vinyl ester resin can ensure that the molecular chain segments in the combination of the brominated epoxy and the novolac epoxy resin are uniformly distributed, the resin performance is uniform, and the colorless appearance of the product is ensured by a specific catalytic system. In addition, the method is simple and convenient, and is beneficial to industrial popularization.

The vinyl ester resin provided by the invention has the advantages that the flame retardance, the heat resistance and the deep photocuring performance are improved on the basis that the performances of the vinyl ester resin comprehensively meet the high strength and the corrosion resistance of common vinyl ester resins. And the subsequent preparation of the photo-curing SMC can be further realized by matching with various thickening aids and fibers.

The vinyl ester resin is suitable for being applied to environmental protection engineering, and is particularly suitable for meeting the application requirements of quick construction, low VOC, flame retardance, high heat resistance and high corrosion resistance.

Detailed Description

Preferred embodiments of the present invention will be described in detail with reference to the following examples. It is to be understood that the following examples are given for illustrative purposes only and are not intended to limit the scope of the present invention. Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the spirit and scope of this invention.

The experimental procedures used in the following examples are all conventional procedures unless otherwise specified. Materials, reagents and the like used in the following examples are commercially available unless otherwise specified.