Phosphorus-containing epoxy resin for an electronic application
Phosphorus-Containing Epoxy Resin for an Electronic Application
CHUN-SHAN WANG,JENG-YUEH SHIEH
Department of Chemical Engineering,National Cheng Kung University,Tainan,Taiwan,701,Republic of China
Received25July1998;accepted23September1998
ABSTRACT:A phosphorus-containing epoxy resin,6-H-dibenz[c,e][1,2]oxaphosphorin-
6-[2,5-bis(oxiranylmethoxy)phenyl]-6-oxide(DOPO epoxy resin),was synthesized and
cured with phenolic novolac(Ph Nov),4,4?-diaminodiphenylsulfone(DDS),or dicyan-
diamide(DICY).The reactivity of these three curing agents toward DOPO epoxy resin
was found in the order of DICY?DDS?Ph Nov.Thermal stability and the weight loss
behavior of the cured polymers were studied by TGA.The phosphorus-containing epoxy
resin showed lower weight loss temperature and higher char yield than that of bisphe-
nol-A based epoxy resin.The high char yields and limiting oxygen index(LOI)values
as well as excellent UL-94vertical burn test results of DOPO epoxy resin indicated the
?ame-retardant effectiveness of phosphorus-containing epoxy resins.The DOPO epoxy
resin was investigated as a reactive?ame-retardant additive in an electronic encapsu-
lation application.Owing to the rigid structure of DOPO and the pendant P group,the
resulting phosphorus-containing encapsulant exhibited better?ame retardancy,higher
glass transition temperature,and thermal stability than the regular encapsulant
containing a brominated epoxy resin.High LOI value and UL-94V-0rating could be
achieved with a phosphorus content of as low as1.03%(comparable to bromine content
of7.24%)in the cured epoxy,and no fume and toxic gas emission were observed.?1999
John Wiley&Sons,Inc.J Appl Polym Sci73:353–361,1999
Key words:phosphorus-containing epoxy resin;6-H-dibenz[c,e][1,2]oxaphosphorin-
6-[2,5-bis(oxiranylmethoxy)phenyl]-6-oxide;phenolic novalac;4,4?-diaminodiphenyl-
sulfone;dicyandiamide;bisphenol-A;?ame retardant;limiting oxygen index;UL-94V
test
INTRODUCTION
Epoxy resins have excellent moisture,solvent and chemical resistance,low shrinkage on cure,supe-rior electrical and mechanical properties,and good adhesion to many substrates.The versatility in formulation also makes epoxy resins widely applicable industrially for surface coatings,adhe-sives,painting materials,pottings,composites, laminates,encapsulants for semiconductors,and insulating materials for electric devices,etc.1–4 However,the common epoxy systems cannot sat-isfy?eld applications that require high thermal and?ame resistance.Considerable environmen-tal attention and health restrictions have been considered in the last three decades in regard to controlling inherent?ammability of common or-ganic polymer by incorporation of?re-retardant additives.Several approaches for modi?cation of the epoxy backbone to enhance the thermal prop-erties of epoxy resins have been reported.5–8 Flame retardants such as tetrabromobisphenol A,phosphorus–halogen mixtures,ammonium phosphate,and organophosphorus compounds have been used to impart?ame retardancy to epoxy resins.9–12In a?re,halogens(i.e.,of bro
Correspondence to:C.-S.Wang.
Contract grant sponsor:National Science Council of the Re-
public of China;contract grant number:NSC87-2622-E006-001.
Journal of Applied Polymer Science,Vol.73,353–361(1999)
?1999John Wiley&Sons,https://www.sodocs.net/doc/f73243759.html,C0021-8995/99/030353-09
353
mine and chlorine)produce problems of smoke and possibly enhanced toxicity,and corrosion.Or-ganic phosphates have less tendency to cause these problems.Recently,organophosphorus compounds have demonstrated good ability as ?ame retardants for epoxy resins,and also have been found to generate less toxic gas and smoke than halogen-containing compounds.13–21Flame-retardant epoxy resins can be obtained by chem-ically bonding?ame-retardant groups onto epoxy resins,and the permanent attachment of a?ame retardant frequently leads to high ef?ciency in ?ame retardancy.17–18,22–23
In this study,a rigid phosphorus-containing reactive2-(6-oxido-6-H-dibenzo[c,e][1,2]oxaphos-phorin-6-yl)-1,4-benzenediol(ODOPB)is con-verted to a phosphorus-containing epoxy resin, 6-H-dibenz[c,e][1,2]oxaphosphorin-6-[2,5-bis-(oxiranylmethoxy)phenyl]-6-oxide(DOPO epoxy resin),which is expected to exhibit the required ?ame retardancy,less fumes,and higher thermal stability than the conventional?ame-retardant epoxy resin systems containing tetrabromobis-phenol A(TBBA).
EXPERIMENTAL
Reagents
ODOPB was prepared in our laboratory.26Epi-chlorohydrin(EPI)from Janssen Co.and potas-sium hydroxide from Showa Co.were used as received.All solvents were reagent grade or were puri?ed by standard methods before use.The dig-lycidyl ether of bisphenol-A[Bis-A epoxy resin, epoxy equivalent weight(EEW)?189]was ob-tained from Chang Chun Plastic Co.(Taiwan). 4,4?-Diaminodiphenylsulfone(DDS)and dicyan-diamide(DICY)used as curing agents were from Aldrich and used as received.A phenol-formalde-hyde novolac resin(Ph Nov)with an average hy-droxyl functionality of6and a hydroxy equivalent weight of about104(Schenectady Chemical,HRJ-2210)was also used as a curing agent.The A-1 catalyst was(ethyl)triphenyl phosphonium ace-tate acetic complex;Ph3P was triphenylphos-phine,which was used as curing accelerator. Instrumental
Infrared spectra(IR)were obtained using a Nico-let550Fourier Transfer Infrared Spectrophotom-eter.Mass spectra were recorded by a VG70-250s gas chromatography/mass spectroscopy.Differen-tial scanning calorimetry(DSC)data were ob-tained in a nitrogen atmosphere at a20°C min?1 heating rate using a Perkin-Elmer DSC7differ-ential scanning calorimeter.Thermal gravimetric analysis(TGA)was employed with a Perkin-Elmer TGA7at a heating rate of20°C min?1in nitrogen or air atmosphere.Limiting oxygen in-dex(LOI)values were measured on an ATLAS limiting oxygen index chamber.The percentage of O2in the O2–N2mixture just suf?cient to sus-tain the?ame was taken as the LOI.EEW of DOPO epoxy resin was determined by the HClO4/ potentiometric titration method.
Synthesis of DOPO Epoxy Resin
Into a1-L reaction vessel equipped with temper-ature and pressure controls and indicators, means for the continuous or intermittent addition of aqueous sodium hydroxide,means for condens-ing and separating water from a codistillate mix-ture of water,solvent,and epichlorohydrin,and means for returning the solvent and epichlorohy-drin to the reaction vessel was added64.8g(1 equivalent)of ODOPB,185g(5equivalents)of epichlorohydrin,and54g of the methylether of propylene glycol(1-methoxy-2-hydroxypropane) as a solvent.After stirring at room temperature and atmospheric pressure to thoroughly mix the contents,the temperature was raised to65°C and the pressure was reduced to160mm Hg absolute. To the resultant solution was continuously added 8g,2equivalents of a50%aqueous sodium hy-droxide solution at a constant rate over a period of 1h.During the addition of the sodium hydroxide, the water was removed by codistilling with epi-chlorohydrin and solvent.The distillate was con-densed,thereby forming two distinct phases,an aqueous phase(top)and an organic epichlorohy-drin–solvent phase(bottom).The organic phase was continuously returned to the reactor.After completion of the sodium hydroxide addition,the reaction mixture was maintained at a tempera-ture of65°C and a pressure of about160mm Hg absolute for an additional30min.A sample of the reaction mixture was washed to remove salt and subsequently distilled resulting in an epoxidized ODOPB resin(DOPO epoxy resin)having a hy-drolyzable chloride content of0.02%and an epox-ide content of?23.5%,which corresponds to an epoxide equivalent weight of about223?228. Curing Procedure of Epoxy Resins
The thermal and?re retardant properties of the phosphorus-and nonphosphorus-containing epoxy
354WANG AND SHIEH
resin were evaluated by preparing six cured epoxy
resins,i.e.,DOPO epoxy resin cured with Bis-A/Ph
Nov,DOPO/Ph Nov;Bis-A/DDS,DOPO/DDS,
Bis-A/DICY,DOPO/DICY.To obtain a highly
crosslinked polymer with good thermal stability,
one epoxy equivalent weight of the epoxy resins
was cured with one functional equivalent weight
of the curing agents.Curing steps were predeter-
mined by DSC thermograms of respective epoxy/
curing agent compositions(Table I). Preparation of DOPO Epoxy Resin Modified Encapsulants
To determine the amount of phosphorus needed
to achieve?ame retardancy,various amount of
DOPO epoxy resin was added to Bis-A epoxy resin
and cured with Ph Nov.The mixtures of epoxy
resins consisted of Bis-A epoxy/DOPO epoxy(or
Bis-A epoxy/TBBA epoxy)in various weight ra-
tios(87.5/12.5,77.8/22.2,71.4/28.6)were pre-
pared.Ph3P was used as a curing accelerator.
Each epoxy mixture was mixed with Ph Nov and
0.2%Ph3P in a mill at25°C to give thermoset-
table epoxy resin powders.The resin powders
were cured in a mold at150°C and50kg/cm2for
a period of1h and then at185°C for2h,and
further postcured at210°C for3h to obtain cured
specimens.
LOI and UL-94V Flame Retardant Test
The LOI is the minimum fraction of O2in a mix-
ture of O2and N2that will just support?aming
combustion.The LOI test was performed accord-
ing to the testing procedure of ASTM D2836
Oxygen Index Method with test specimen bar of
7–15cm in length,6.5?0.5mm in width,and3.0?0.5mm in thickness.Ten sample bars sus-pended vertically were ignited by a Bunsen
burner.The?ame was removed and the timer
was started.The concentration of oxygen was raised if the specimen extinguished before burn-ing3min or5cm.The oxygen content was ad-justed until the limiting concentration was deter-mined.
The UL-94V test was performed according to the testing procedure of FMVSS302/ZSO3975 with test specimen bars of127mm length,12.7 mm width,and about maximum up to12.7mm thickness.The UL-94V test determines the up-ward-burning characteristics of a solid.Five sam-ple bars suspended vertically over surgical cotton were ignited by a Bunsen burner;two ignitions with10s burning time were applied to each sam-ple bar.The samples of cured epoxy resins with various curing agents of Ph Nov,DDS,and DICY were subjected to the UL-94V test.
RESULTS AND DISCUSSION
Synthesis of DOPO Epoxy Resin
The synthesis of the phosphorus-containing epoxy resin(DOPO epoxy resin)was performed by re-acting ODOPB with EPI(Scheme1).The product was characterized by IR.The IR spectrum of DOPO epoxy resin is shown in Figure1.The peak at917cm?1absorption indicates the oxirane ring and the peak at1180–1120cm?1indicates the P—O—Ph linkage.Other absorption peaks of DOPO epoxy resin are1248cm?1(—P A O)and 1400–1500cm?1(—P—Ph).The EEW of DOPO epoxy resin was determined by the HClO4/poten-tiometric titration method and found to be229 (theoretical is218).
Curing Reactivity
The curing behaviors of the mixtures of DOPO epoxy and various curing agents were studied by DSC.The relative reactivities of various curing agents toward DOPO epoxy resin area
Table I Curing Steps of Epoxy Resins
Epoxy Resin/ Curing Agent
Precure
Temperature
(°C)
Precure
Time
(min)
Curing
Temperature
(°C)
Curing
Time
(min)
Postcure
Temperature
(°C)
Postcure
Time
(min)
Bis-A/Ph Nov15060175120200180 DOPO/Ph Nov15060175120190180 Bis-A/DDS15060175120200180 DOPO/DDS13560155120175180 Bis-A/DICY15060175120200180 DOPO/DICY13060145120165180
PHOSPHORUS-CONTAINING EPOXY RESIN355
shown in (Fig.2).For the epoxy resin used,the exothermic starting temperatures increased in the order of DICY ?DDS ?Ph Nov.A curing agent that exhibits a lower exothermic starting temperature under the same set of curing con-dition is more reactive toward the epoxy resin.Since the reactivity of the hydroxyl group to-ward the oxirane ring is lower than that of the amine group,Ph Nov showed lower reactivity than DDS and DICY.
Dynamic Viscoelastic Analyses of DOPO Epoxy Resins with Various Curing Agents
Dynamic viscoelastic analysis can give informa-tion on the microstructure of cured epoxy res-ins.The tan ?curves for the control network exhibit a major relaxation observed in most ep-oxy polymers.25The transition corresponds to the major T g of the cured epoxy resin,above which signi?cant chain motion takes place.Fig-ure 3showed the storage modulus G ?and tan ?of DOPO epoxy and Bis-A epoxy cured with Ph Nov.The results (shown in Table II)indicated that the DOPO epoxy resin cured with Ph Nov,DDS,and DICY had higher T g (181,254,and 198°C)than that of Bis-A epoxy resin cured with Ph Nov,DDS,and DICY (123,212,and 138°C).The result may be attributed to
the
Scheme 1Synthesis of DOPO epoxy
resin.
Figure 1IR spectrum of DOPO epoxy.
356WANG AND SHIEH
incorporation of a bulky rigid group that in-creases the rotational barrier.
Thermal Properties for Cured Epoxy Resins TGA is the most favored technique for rapid evaluation in comparing and ranking the ther-mal stability of various polymers.Themogravi-metric analyses of both DOPO and Bis-A epoxy resins cured with various curing agents of Ph Nov,DDS,and DICY in nitrogen and air are shown in Table II.The results indicated that DOPO epoxy resin cured with Ph Nov,
DDS,
Figure 2DSC thermograms of DOPO epoxy resin cured with (A)Ph Nov,(B)DDS,and (C)
DICY.
Figure 3Dynamic viscoelastic analysis of cured epoxy resins.(A)and (B)Bis-A/Ph Nov;(B)and (D)DOPO/Ph Nov.
PHOSPHORUS-CONTAINING EPOXY RESIN 357
and DICY have higher char yield than that of cured Bis-A epoxy resin.
TGA thermograms of cured Bis-A epoxy/Ph Nov and DOPO epoxy/Ph Nov in a nitrogen atmo-sphere are illustrated in Figure4.Bis-A/Ph Nov exhibited1%weight loss at257°C and then a rapid weight loss at442°C.On the other hand, DOPO epoxy resin showed its1%weight loss at 207°C,and a10%weight loss at around367°C. DOPO epoxy exhibited a lower?rst step rapid weight loss temperature(at376°C)than that of Bis-A epoxy resin(at442°C).However,unlike the one-stage weight loss behavior of the Bis-A ep-oxy/Ph Nov system,the DOPO epoxy/Ph Nov showed a second-stage weight loss at around 521°C.This phenomenon indicates an important role in improving the?ame retardancy of the resins.While the resin is burning,the phospho-rus-containing groups?rst decompose at around 376°C,and then form a phosphorus-rich residue that prevents further decomposition of the epoxy resin by raising the second decomposition temper-ature to521°C and results in a high char yield. The char yields at700°C for the Bis-A epoxy/Ph Nov system and DOPO epoxy/Ph Nov system were19and48%.TGA thermograms of DOPO and Bis-A epoxy resins cured with various curing agents are shown in Table II.
Table II Thermal Properties of Cured Epoxy Resins Epoxy Resin/
Curing Agent P%T
g (°C)
Temperature of Weight
Loss Rapid Weight Loss Temp(T
r
,°C)
Char
Yield
(%)
1%10%Step I Step II Step III700°C
Air N
2
Air N
2
Air N
2
Air N
2
Air N
2
Air N
2
Bis-A/Ph Nov0.0123267257413367449442621———219 DOPO/Ph Nov 4.8181183207373363377376544521745—3748 Bis-A/DDS0.0212237277371377424408656———011 DOPO/DDS 4.5254187207347327363363557411753—3240 Bis-A/DICY0.0138243243367357398395443429595—08
DOPO/DICY7.2198167159321313212344344491472—28
32
Figure4TGA thermograms of cured epoxy resins in N
2
:(A)Bis-A/Ph Nov,(B) DOPO/Ph Nov and derivative,(C)Bis-A/Ph Nov,and(D)DOPO/Ph Nov.
358WANG AND SHIEH
LOI andUL-94V Test for Cured Epoxy Resins
The?ame-retardant properties of cured epoxy resins were examined by measuring the LOI.To demonstrate the?ame-retardant properties of DOPO epoxy resin,DOPO epoxy,and Bis-A epoxy were cured with(1)Ph Nov,(2)DDS,and(3) DICY for comparison.The LOI values are shown in Table III.Increased char formation can limit the production of combustible carbon-containing gases,decrease the exothermicity due to pyrolysis reactions,as well as decrease the thermal conduc-tivity of the surface of a burning material.24 DOPO epoxy cured with Ph Nov,DDS,and DICY had LOI values of34,32,and36that were higher than those of Bis-A epoxy cured with Ph Nov, DDS,and DICY(LOI?26,22,21).The UL-94Vtest determines the upward-burning charac-teristics of a solid.Five sample bars of each cured epoxy resins suspended vertically over surgical cotton were ignited by a Bunsen burner;two ig-nitions of10s each were applied to the sample. The results of UL-94V and LOI test are listed in Table III.DOPO epoxy resin,besides being supe-rior to phosphorus-free epoxy resin in?ame re-tardancy,also generated much less visible smoke than phosphorus-free epoxy resin.
Dynamic Viscoelastic and Thermal Properties of Flame-Retardant Epoxy Encapsulants
Owing to excellent heat,solvent and chemical-resistance,good adhesion and inexpensiveness, epoxy resin has been widely employed in the en-capsulation of microelectronic devices.In this study,DOPO epoxy and TBBA epoxy(tetrabro-mobisphenol A epoxy resin)were compared for their?ame retardancy in the encapsulation for-mulation.Bis-A epoxy was blended with various amount of DOPO epoxy and then cured with Ph Nov to give the cured products with phosphorus content of0,0.58,1.03,and1.45%(Bis-A/Ph Nov, DOPO-A,DOPO-B,DOPO-C).In the same man-ner,Bis-A epoxy was blended with TBBA epoxy and then cured with Ph Nov to give the cured products with bromine content of4.01,7.24,and 9.91%(TBBA-A,TBBA-B,TBBA-C).Thermal properties of cured epoxy resins are shown in Table IV.It should be noted that T g of cured epoxy resins increased with the increase in phosphorus
Table III UL-94v Test Rating and LOI Values for Cured Epoxy Resins
Epoxy Resin/
Curing Agent P%Average Burning
Time(s)
Visible
Smoke Drip
UL94-V
Classi?cation LOI
Bis-A/Ph Nov0.078Slight No V-226 DOPO/Ph Nov 4.80No No V-034 Bis-A/DDS0.0123Heavy Heavy V-222 DOPO/DDS 4.50No No V-032 Bis-A/DICY0.0158Heavy Slight V-221 DOPO/DICY7.20No No V-036 Table IV Thermal Properties of Cured Epoxy Resin with Various P or Br Contents
Sample P%T
g (°C)
Temperature of Weight
Loss
Rapid Weight Loss Temp
(T
r
,°C)
Char
Yield(%)
1%10%Step I Step II600°C700°C
Air N
2
Air N
2
Air N
2
Air N
2
Air N
2
Air N
2
Bis-A/Ph Nov0.0122267257413367449442——2125019 DOPO-A0.58133267277403403424428—6033136731 DOPO-B 1.03135253273393397409419—59933381134 DOPO-C 1.4513824726938338739941454858739432136 Br%
TBBA-A 4.01130273247387377403411625—2727023 TBBA-B7.24127277261387357406393641—2930226 TBBA-C9.91124297267367383400388647—3134429
PHOSPHORUS-CONTAINING EPOXY RESIN359
content while T g decreased with the increase in bromine content .The result may be attributed to the bulky rigid phosphorus-containing group.TGA were also performed to compare the ther-mal stabilities of the cured epoxy resins;the results are also shown in Table IV.DOPO-A (0.58%P)exhibited higher char yield (31%in N 2at 700°C)and higher thermal stability than any TBBA con-taining resins studied.The result indicates that 0.58%P provided a char yield equivalent to that of 9.91%Br.Figure 5shows the thermogravimetric traces of cured resins of Bis-A ,DOPO-C,and TBBA-C in N 2.The Bis-A resin exhibited 1%weight loss at 257°C and 10%weight loss at 367°C,and then a rapid weight loss at around 442°C in N 2.TBBA-C exhibited 1%weight loss at 247°C and 10%
weight loss at 267°C,and then a rapid weight loss at around 388°C.On the other hand,the phospho-rus-containing DOPO-C showed 1%weight loss at 269°C and 10%weight loss at 387°C.If the thermal stability of cured epoxy resins were compared by 1%and 10%weight loss,the following order may be given:DOPO-C ?TBBA-C ?Bis-A.The rapid weight loss temperature (T r )of Bis-A epoxy resin occurred at 442°C,slightly higher than that of DOPO-C;however,it is unlike the one-stage rapid weight loss of the Bis-A epoxy resin.DOPO-C ex-hibited a higher second-stage rapid weight loss at 587°C.This phenomenon played an important role in providing the ?ame retardancy to the cured ep-oxy resin,and was con?rmed by the higher char yields at 600°C (DOPO-C 43%;TBBA-C 34%;
Table V
UL-94v Test Rating and LOI Value of Cured Epoxy Resins with Various P and Br Content
Sample Flame-Retardant
Element
Average Burning
Time (s)
Visible Smoke
Drip UL 94-V Classi?cation
LOI
P%Bis-A/Ph Nov 0.078Slight
No V-226DOPO-A 0.5832Very slight No V-228DOPO-B 1.036No No V-030DOPO-C 1.450No No V-034Br%TBBA-A 4.0138Heavy No V-227TBBA-B 7.244Slight No V-030TBBA-C
9.91
No
No
V-0
35
Figure 5TGA thermograms of cured epoxy resins in N 2:(A)Bis-A/Ph Nov,(B)TBBA-C,and (C)DOPO-C.
360WANG AND SHIEH
Bis/Ph Nov25%)and700°C(DOPO-C36%;TBBA-C29%;Bis-A19%)for DOPO-C.
LOI and UL-94V Test for Flame-Retardant
Epoxy Resin
Cured epoxy resin with high phosphorus content is expected to have a high char residue on pyrol-ysis.The char residue on pyrolysis is reported to be linearly proportional to the oxygen index for halogen-free polymers.27The?ame-retardant properties of cured epoxy resins with various phosphorus or bromine contents were examined by measuring their oxygen index(LOI).It is clear from the results of Table V that the higher the phosphorus content the higher the LOI value, which agrees well with previous reports.28,29 For the UL-94test,?ve specimens of each cured epoxy resin were prepared and the test results are also shown in Table V.The?ame retardancy of cured epoxy resins increases with phosphorus or bromine content in the cured products.Besides phosphorus being much more effective than bro-mine as a?ame retardant(1.03%P is almost equiv-alent to7.24%bromine by comparing DOPO-B with TBBA-B),it also generates much less visible smoke than the bromine-containing resin. CONCLUSION
The phosphorus-containing DOPO epoxy resin was successfully synthesized.DOPO epoxy resin cured with Ph Nov,DDS,or DICY yields products with higher T g,thermal stability,and better ?ame retardancy than those of Bis-A epoxy resin. DOPO epoxy resin may be suitable as a reactive ?ame retardant by mixing with Bis-A epoxy resin for encapsulation applications.DOPO epoxy resin exhibits better?ame retardancy and thermal sta-bility than that of TBBA,but also generated much less visible smoke during combustion.
Financial support of this work by the National Science Council of the Republic of China is gratefully appreci-ated(NSC87-2622-E006-027).
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PHOSPHORUS-CONTAINING EPOXY RESIN361