Abstract:
Flame-retardant advanced epoxy resins and cured epoxy resins contain a rigid phosphorus group emanating from a dihydric phenol or naphthol which provides thermal and flame retardant properties. The advanced epoxy resins are suitable for making a fiber-reinforced epoxy resin composite which is useful in the fabrication of printed circuit boards. The cured epoxy resins can be used in semiconductor encapsulation applications.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to flame-retardant advanced epoxy resins prepared from a bisphenol having phosphorus group. The present invention also relates to cured epoxy resins resulting from the advanced epoxy resins, which have excellent flame-retardancy and mechanical properties 
     BACKGROUND OF THE INVENTION 
     Epoxy resins have the excellent characteristics of moisture, solvent and chemical resistance, toughness, low shrinkage on cure, superior electrical and mechanical resistance properties, and good adhesion to many substrates. The versatility in formulation also make epoxy resins widely applicable industrially for surface coatings, adhesive, painting materials, potting, composites, laminates, encapsulants for semiconductors, and insulating materials for electric devices, etc. o-Cresol formaldehyde novolac epoxy (CNE) is the resin typically employed in the encapsulation of microelectronic devices. Several approaches for modification of epoxy backbone for enhancing the thermal properties of epoxy resins have been reported. Aromatic bromine compounds in conjunction with antimony oxide are widely used as a flame retardant for epoxy resins. Tetrabromobisphenol A is a typical example of the aromatic bromine compounds used as a flame retardant for epoxy resins. An excess amount of epoxy resin is reacted with tetrabromobisphenol A to prepare an advanced epoxy resin having two terminal epoxide groups, as shown in the following formula:                           
     wherein EP is a bi-radical group of the backbone of the epoxy resin, and m is an integer of 1-10. The advanced epoxy resin can be used in preparing a flame-retardant printed circuit board (FR-4) by impregnating glass fibers with the advanced epoxy resin and heating the resulting composite to cure the advanced epoxy resin. Furthermore, the advanced epoxy resin can be employed to encapsulate microelectronic devices, in which the advanced epoxy resin is cured at a high temperature with a curing agent, so that an encapsulant having a flame-retardant property is formed. Typical examples can be found in U.S. Pat. No. 3,040,495 (1961); U.S. Pat. No. 3,058,946 (1962); U.S. Pat. No. 3,294,742 (1966); U.S. Pat. No. 3,929,908 (1975); U.S. Pat. No. 3,956,403 (1976); U.S. Pat. No. 3,974,235 (1976); U.S. Pat. No. 3,989,531 (1976); U.S. Pat. No. 4,058,507 (1997); U.S. Pat. No. 4,104,257 (1978); U.S. Pat. No. 4,170,711 (1979); and U.S. Pat. No. 4,647,648(1987)]. 
     Although the tetrabromobisphenol A-containing advanced epoxy resin shows flame retardant property, major problems encountered with this system are concerned with the generation of toxic and corrosive fumes during combustion such as dioxin and benzofuran. 
     The flame retardant having a small molecular weight tends to degrade the mechanical properties of the epoxy resins, and migrate/vaporize from the epoxy resins such that the flame retardancy thereof diminishes. 
     The trend of electronics equipment is being miniaturized and becoming thinner, at the same time the scale of integration of large scale integrated circuits (LSICs) is continuing upward, forcing the design toward larger chips, finer patterns, and higher pin counts that are more susceptible to a high-temperature failure. The prevailing surface mount technology (SMT) also causes the devices being subjected to a high temperature. Therefore, the development of a high-temperature reliable, flame-retardant and environmentally friendly epoxy resin for printed circuit board and encapsulant are essential for semiconductor industry. 
     It is an object of this invention to provide flame retardant advanced epoxy resins and cured epoxy resins with good thermal stability, superior heat resistance, and environment friendly, which are suitable for use in making printed circuit boards and in semiconductor encapsulation applications. 
     It is another object of this invention to provide a method for improving flame retardant properties of epoxy resins. 
     SUMMARY OF THE INVENTION 
     In order to accomplish the aforesaid objects, a flame retardant advanced epoxy resin and a cured epoxy resin disclosed in the prevent invention have the following formula (I):                           
     wherein: 
     m is an integer of 0-10; 
     X=A or B, wherein                           
     when X=A, the formula (I) represents the advanced epoxy resin, X=B, the formula (I) represent the cured epoxy resin;                           
     wherein R 1  is hydrogen or C 1-4  hydrocarbon group;                           
      wherein                           
      or a phenol-aldehyde novolac epoxy resin backbone having the following formula:                           
      wherein 
     R 1  is the same as defined above, g is an integer of 1-6, 
     R 4  and R 5  independently are hydrogen or —CH 3 . 
     The flame retardant advanced epoxy resin and cured epoxy resin of the present invention contain a rigid phosphorus group, which provide not only the better flame-retardant effect and thermal stability than those prepared with a conventional aromatic bromine group, but also generate much less fumes in the combustion test. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides an advanced epoxy resin and a cured epoxy resin containing a flame-retardant phosphorus group, which may be represented by the following formula (I),                           
     wherein m, X, E p  and R are defined as above. 
     A suitable method for preparing the flame-retardant advanced epoxy resin and cured epoxy resin represented by the formula (I) comprises reacting an epoxy resin having the following formula (II) and a phosphorus-containing dihydric phenol or naphthol having the following formula (III) in a molten state or in a common solvent and in the presence of a catalyst:                           
     wherein Ep and R are the same as defined above. 
     Preferably, E p  in the formula (I) is                           
     wherein 
     Q is —C(CH 3 ) 2 —, or 
     E p  is a phenol-aldehyde novolac epoxy resin backbone having the following formula:                           
     wherein R 1  is hydrogen and R 4  is —CH 3 . 
     Preferably, R in the formula (I) is                           
     wherein R 1  is defined as above. 
     In the method for preparing the flame-retardant advanced epoxy resin and cured epoxy resin compound (I), the advanced epoxy resin (I′), i.e. X in the formula (I) is                           
     is prepared when an excess amount of epoxy resin (II) is used to react with phosphorus-containing dihydric phenol or naphthol (III) as shown in the following reaction:                           
     wherein R, Ep, and m are the same as defined above. Preferably, this reaction is carried out at 100° C.-200° C., more preferably at 150° C.-180° C., and in the presence of a catalyst selected from the group consisting of 2-phenylimidazole, 2-methylimidazole, triphenylphosphine, a quarternary phosphoium compound and a quarternary ammonium compound. Examples of the quarternary phosphoium compound include ethyltriphenyl phosphonium acetate and ethyltriphenyl phosphonium halides. Examples of the quarternary ammonium compound are benzyltrimethyl ammonium chloride, benzyltriethyl ammonium chloride and tetrabutyl ammonium chloride. Preferably, the equivalent ratio of the epoxide group in the epoxy resin (II) to the hydroxyl group in the bisphenol compound (III) ranges from 1.5:1 to 5:1; and more preferably 2:1 to 3:1. 
     In the method for preparing the flame retardant cured epoxy resin, i.e X in the formula (I) is B, the phosphorus-containing dihydric phenol or naphthol (III) was used as a curing agent of the epoxy resin (II). The bisphenol (III) can be used alone or together with another curing agent such as phenol-formaldehyde novolac, dicyandiamide, methylenedianiline, diaminodiphenyl sulfone, phthalic anhydride and hexahydrophthalic anhydride. A suitable amount of the curing agent for curing the epoxy resin (II) is the equivalent ratio of the epoxide group in the epoxy resin (II) and the functional groups in the curing agent ranging from 1:1 to 1.2:1. 
     It is apparent that the present invention also provides a method for improving flame-retardant properties of the epoxy resin (II), which comprises reacting said epoxy resin with the phosphorus-containing dihydric phenol or naphthol (III), in which the epoxide group of said epoxy resin is reacted with the hydroxyl group of said phosphorus-containing dihydric phenol or naphthol so that an open ring reaction of said epoxide group occurs. 
     A suitable epoxy resin for use in the present invention can be any known epoxy resin, for examples those having two epoxide groups such as bisphenol A epoxy resin, bisphenol F epoxy resin, bisphenol S epoxy resin and biphenol epoxy resin, and those having more than two epoxide groups such as phenol formaldehyde novolac epoxy and cresol formaldehyde novolac epoxy (CNE). 
     The phosphorous-containing dihydric phenol or naphthol (III) used in the present invention can be synthesized according to the following reaction (IV):                           
     wherein R 1  and R are the same as defined above. The reaction (IV) is preferably carried out at a temperature range of 80° C.˜120° C. in an appropriate solvent such as toluene. 
    
    
     EXAMPLES 
     I. The preparation of a phosphorus-containing dihydric phenol or naphthol 
     Preparation Example 1 
     (DOPO-BQ) 
     500 g of 2-(2-hydroxyphenyl)phenylphosphonic acid (HPPA) was placed in a 300 ml flask which was connected to a vacuum system. The content was heated to its molten state (106° C.) under full vacuum for dehydration. The temperature was slowly increased from 106 to 160° C. until the dehydration was complete. White solids of DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) (yield 93%) with m.p. 119-120° C. after recrystallization from tetrahydrofuran was obtained. I.r.: 1168, 965 (P—Ph), 1196 (P═O), 2384 (P—H), 1587 cm −1  (P—Ph). Anal. Calcd. for C 12 H 9 O 2 P: C, 66.67; H, 4.17; O, 14.81; P, 14.35. Found: C, 66.64; H, 4.31; O, 14.59; P, 14.66. MS, m/z: 216 (100, M + ). 
     To a one liter four-inlet flask equipped with a temperature controller, a reflux condenser, a nitrogen feed and a mechanical stirrer, 1 mole DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 216 g) and 500 ml toluene were added. The mixture was heated to 70° C. and then stirred. The mixture was heated to a temperature of 90° C. and the stirring was continued until DOPO was dissolved completely. To this solution was added slowly 0.9 mole (97 g)1,4-benzoquinone (BQ), and the temperature thereof was increased to 110° C. and maintained at that temperature for two hours after the addition of BQ was completed. The mixture was then cooled to room temperature, filtered, and dried to obtain 2-(6-oxido-6H-dibenz&lt;c,e&gt;&lt;1,2&gt;oxaphosphorin-6-yl)-1,4-benzenediol (DOPO-BQ). Yield, 98%; m.p. 258-259° C. 
     Preparation Example 2 
     (DOPO-MBQ) 
     A bisphenol DOPO-MBQ was prepared from 1 mole DOPO and 0.9 mole methyl-1,4-benzoquinone (MBQ) according to the same procedures as described in Preparation example 1. Yield, 97%; m.p. 263-265° C. 
     Preparation Example 3 
     (DOPO-NQ) 
     A bisphenol DOPO-NQ was prepared from 1 mole DOPO and 0.9 mole 1,4-naphthoquinone (NQ) according to the same procedures as described in Preparation example 1. Yield, 85%; m.p. 282-283° C. 
     Preparation Example 4 
     (DOPO-MNQ) 
     A bisphenol DOPO-MNQ was prepared from 1 mole DOPO and 0.9 mole 2-methyl-1,4-naphthoquinone (MNQ) according to the same procedures as described in Preparation example 1. Yield, 83%; m.p. 289-291° C. 
     II. The Preparation of an Advanced Epoxy Resin Containing Phosphorus Group 
     Example 1 
     An Advanced Epoxy Resin Prepared from Bisphenol A Epoxy Resin and DOPO-BQ 
     To a one liter reactor equipped with a temperature controller, a reflux condenser, a nitrogen feed, a vacuum system and a mechanical stirrer, 400 g diglycidyl ether of bisphenol A (BPA epoxy resin) having an epoxide equivalent weight (EEW) of 185 was added, and heated to 110° C. while stirring and vacuuming (&lt;100 mmHg) for a period of 30 minutes to remove a trace amount of water contained in the epoxy resin. The vacuuming was stopped, and dried nitrogen was introduced into the reactor until the atmospheric pressure was reached. The temperature of the reactor was raised to 130° C., and 210 g DOPO-BQ was then added while stirring. After a molten mixture of DOPO-BQ and BPA epoxy resin was formed, 500 ppm (based on total weight) ethyl triphenyl phosphonium chloride was added, and the temperature of the reaction mixture was increased to 160° C. and maintained at 160° C. for two hours. The equivalent ratio of epoxide group to hydroxyl group was 2.20/1. The resultant product had an EEW of 483. 
     Example 2 
     The procedures described in Example 1 were repeated, except that DOPO-BQ (Preparation example 1) was replaced with DOPO-MBQ (Preparation example 2). The resultant product had an EEW of 483. 
     Example 3 
     The procedures described in Example 1 were repeated, except that DOPO-BQ (Preparation example 1) was replaced with DOPO-NQ (Preparation example 3). The resultant product had an EEW of 483. 
     Example 4 
     The procedures described in Example 1 were repeated, except that DOPO-BQ (Preparation example 1) was replaced with DOPO-MNQ (Preparation example 4). The resultant product had an EEW of 483. 
     Control Example 1 
     The procedures described in Example 1 were repeated, except that DOPO-BQ (Preparation example 1) was replaced with bisphenol A and the equivalent ratio of epoxide group to hydroxyl group was 2.04:1 instead of 2.20:1. The resultant product had an EEW of 483. 
     Control Example 2 
     The procedures described in Example 1 were repeated, except that DOPO-BQ (Preparation example 1) was replaced with tetrabromobisphenol A and the equivalent ratio of epoxide group to hydroxyl group was 2.58:1 instead of 2.20:1. The resultant product had an EEW of 483. 
     III. The preparation of a cured epoxy resin from an advanced epoxy resin 
     Example 5 
     Cured epoxy resins were prepared from the advanced epoxy resins prepared in Examples 1-4 and Control Examples 1-2 with a curing agent (methylene dianiline). 
     Various advanced epoxy resins were mixed with methylene dianiline (1:1 equivalent ratio) at 150° C. and poured into a hot aluminum mould, cured in an oven at 175° C. for one hour, and then postcured at 200° C. for two hours. 
     The flexural properties of the resulting cured epoxy resins were shown in Table 1. The thermogravimetric analysis data of the resulting cured epoxy resins were shown in Table 2. The flame-retardant properties of the resulting cured epoxy resins were shown in Table .3. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 The flexural strength 
               
             
          
           
               
                   
                   
                 Flexural strength at 
               
               
                   
                 Specimens 
                 50° C., dyne/cm 2   
               
               
                   
                   
               
               
                   
                 Control Ex. 1 
                 5.4 
               
               
                   
                 Control Ex. 2 
                 5.6 
               
               
                   
                 Example 1 
                 6.8 
               
               
                   
                 Example 2 
                 6.9 
               
               
                   
                 Example 3 
                 6.9 
               
               
                   
                 Example 4 
                 6.9 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 The flexural strength 
               
             
          
           
               
                   
                   
                 Flexural strength at 
               
               
                   
                 Specimens 
                 50° C., dyne/cm 2   
               
               
                   
                   
               
               
                   
                 Control Ex. 1 
                 5.4 
               
               
                   
                 Control Ex. 2 
                 5.6 
               
               
                   
                 Example 1 
                 6.8 
               
               
                   
                 Example 2 
                 6.9 
               
               
                   
                 Example 3 
                 6.9 
               
               
                   
                 Example 4 
                 6.9 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 The flame retardant properties (UL-94V test) 
               
             
          
           
               
                   
                 Content of P 
                 Burning 
                   
                   
                   
               
               
                 Specimens 
                 or Br 
                 time (sec) 
                 Drip 
                 Fume 
                 Classification 
               
               
                   
               
             
          
           
               
                 Control Ex. 1 
                 0 
                 93 
                 yes 
                 no 
                 V-2 
               
               
                 Control Ex. 2 
                 Br, 17.26% 
                 &lt;1 
                 yes 
                 yes 
                 V-0 
               
               
                 Example 1 
                 P, 2.12% 
                 0 
                 no 
                 no 
                 V-0 
               
               
                 Example 2 
                 P, 2.01% 
                 0 
                 no 
                 no 
                 V-0 
               
               
                 Example 3 
                 P, 1.85% 
                 0 
                 no 
                 no 
                 V-0 
               
               
                 Example 4 
                 P, 1.80% 
                 0 
                 no 
                 no 
                 V-0 
               
               
                   
               
             
          
         
       
     
     Tables 1, 2, and 3 show that the cured epoxy resins of the present invention have good mechanical and thermal properties, and have excellent flame retardant properties, especially no fume and dripping occur in the combustion test, and thus is very suitable for the printed circuit board applications. 
     IV. Using Phosphorus-containing Bisphenol A as a Curing Agent for Epoxy Resin 
     Example 6 
     Various amounts of DOPO-BQ were added to phenol formaldehyde novolac (PN) as a curing agent for cresol formaldehyde novolac epoxy resin (CNE) to determine the flame-retardant effect of phosphorus. The curing agents consisting of DOPO-BQ/PN in various weight ratios (0/100, 25/75, 50/50, 75/25, and 100/0) were prepared. Triphenyl phosphine (Ph 3 P) powder was used as a curing accelerator. The CNE was mixed with the above curing agents and 0.2 wt % Ph 3 P in a mill at 25° C. to give thermosettable epoxy resin powders, wherein the equivalent ratio of epoxide group to hydroxyl group is 1:1. The resin powders were cured in a mould at 150° C. and 50 kg/cm2 for a period of one hour and then at 200° C.; for two hours and further postcured at 260° C. for four hours to obtain cured specimens. 
     For comparison, various weight ratios of tetrabromobisphenol A (TBBA) and PN (25/75, 50/50, 75/25, 100/0) were also used as a curing agent to prepare the cured specimens as above. 
     The cured specimens were subjected to the thermogravimetric analysis and the UL-94V test. The results are shown in Table 4 and Table 5. 
     It can be seen from Table 4 that the Tg values of the phosphorus-containing cured epoxy resin specimens of the present invention were about 40° C. higher than those of the conventional bromine-containing cured epoxy resin specimens. Furthermore, the phosphorus-containing cured epoxy resin specimens of the present invention exhibit higher thermal degradation temperatures and higher char yields in comparison with the conventional bromine-containing cured epoxy resin specimens 
     The data in Table 5 show that 1% phosphorus content of the phosphorus-containing cured epoxy resin of the present invention can produce substantially the same flame-retardant effect as 7˜10% bromine content of the conventional bromine-containing cured epoxy resin. In addition, the phosphorus-containing cured epoxy resin specimens of the present invention generate much less fumes in the combustion test. 
     The results shown in Tables 4 and 5 indicate that the phosphorus-containing cured epoxy resin of the present invention is very suitable for semiconductor encapsulation applications. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 The thermogravimetric analysis data 
               
             
          
           
               
                   
                   
                 Temperature of 
                 Temperature of 
                   
               
               
                   
                 Tg 
                 5 wt % loss, ° C. 
                 10 wt % weight loss, ° C. 
                 Char yield at 700° C., % 
               
             
          
           
               
                 Specimens 
                 (° C.) 
                 In air 
                 In N 2   
                 In air 
                 in N 2   
                 In air 
                 In N 2   
               
               
                   
               
               
                 DOPO-BQ/PN 
                   
                   
                   
                   
                   
                   
                   
               
               
                  0/100 
                 182 
                 397 
                 397 
                 419 
                 419 
                 26 
                 32 
               
               
                 25/75 
                 183 
                 401 
                 405 
                 420 
                 421 
                 32 
                 40 
               
               
                 50/50 
                 184 
                 408 
                 415 
                 417 
                 425 
                 32 
                 41 
               
               
                 75/25 
                 185 
                 411 
                 411 
                 425 
                 427 
                 34 
                 44 
               
               
                 TBBA/PN 
               
               
                 100/0  
                 185 
                 411 
                 413 
                 427 
                 431 
                 38 
                 49 
               
               
                 25/75 
                 146 
                 284 
                 403 
                 339 
                 418 
                 26 
                 36 
               
               
                 50/50 
                 135 
                 387 
                 396 
                 391 
                 399 
                 26 
                 38 
               
               
                 75/25 
                 126 
                 371 
                 390 
                 385 
                 395 
                 28 
                 38 
               
               
                 100/0  
                 113 
                 389 
                 409 
                 389 
                 411 
                 28 
                 39 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 The flame-retardant properties (UL-94V test) 
               
             
          
           
               
                   
                 Burning time 
                   
                   
               
               
                 Specimens 
                 (sec) 
                 Fume a)   
                 Classification 
               
               
                   
               
             
          
           
               
                 DOPO-BQ/PN 
                 P % 
                   
                   
                   
               
               
                   
               
             
          
           
               
                 0/100 
                 0 
                 85.6 
                 − 
                 V-2 
               
               
                 25/75 
                 1.1 
                 8.3 
                 −− 
                 V-0 
               
               
                 50/50 
                 2.0 
                 &lt;1 
                 −− 
                 V-0 
               
               
                 75/25 
                 3.1 
                 0 
                 −− 
                 V-0 
               
               
                 100/0 
                 4.4 
                 0 
                 −− 
                 V-0 
               
               
                   
               
             
          
           
               
                 TBBA/PN 
                 Br % 
                   
                   
                   
               
               
                   
               
             
          
           
               
                 25/75 
                 5.8 
                 16 
                 ++ 
                 V-1 
               
               
                 50/50 
                 12.9 
                 &lt;1 
                 ++ 
                 V-0 
               
               
                 75/25 
                 22.1 
                 0 
                 + 
                 V-0 
               
               
                 100/0 
                 34.4 
                 0 
                 − 
                 V-0 
               
               
                   
               
               
                   a) ++: heavy; +: slightly; −: scarcely; −−: no fume.