Epoxy resins. II. The preparation, characterization, and curing of epoxy resins and their copolymers

Epoxy Resins. 11. The Preparation, Characterization, and Curing of Epoxy Resins and

Their Copolymers

S. C. LIN and E. M. PEARCE, Polytechnic Institute of New York, 333 Jay Street, Brooklyn, New York 11201

Synopsis

A polymer with high aromatic ring content in the chain backbone usually has high heat and flame resistance. Three diglycidyl ethers of epoxy resins were prepared from bisphenol A (DGEBA), phenolphthalein (DGEPP), and 9,9-bis(4-hydroxyphenyl)fluorene (DGEBF) in a study of the relation between the cured polymer structure and properties. The epoxy resin prepared from phenolphthalein was separated by liquid chromatography and three fractions were obtained. The fractions had a basic structure of 3,3-disubstituted phthalide and differed only in molecular weight. The DGEPP resin changed color from yellow to red after mixing with trimethoxyboroxine (TMB), the curing agent, and to orange after completing the curing cycle. To prepare a highly crosslinked material with good thermal stability, TMB with three active Lewis sites in a molecule was used as the curing agent. The reactivity of the three different resins toward TMB, measured by differential scanning calorimetry (DSC), was DGEBA > DGEBF > DGEPP. For the same curing conditions the order of crosslink density was DGEBA > DGEPP > DGEBF. To modify the flammability of DGEBA, the conventional epoxy resin, it was copolymerized with DGEPP and DGEBF, the higher-performance epoxy resins. The glass transition temperatures of poly(DGEBA-co-DGEPP) and poly(DGEBA-co-DGEBF) systems deviated from this relationship. The DGEBF copolymers showed an increased char residue (40 wt % a t 700OC) at 20 mole % of DGEBF. This deviation may be due to the lower crosslinking density of this system.

INTRODUCTION

Epoxy resin based on the diglycidyl ether of bisphenol A (DGEBA) has been used widely in many applications such as construction, coating, electrical work, and reinforced plastics. However, the flammability of DGEBA is a major hazard. It has been observed that the oxygen index of DGEBA cured with various curing agents varied between 0.198 and 0.238 and that fillers and flame-retardant additives can increase it to a certain level, depending on the material and amount used.l Because these approaches involve noncompatible additives, large changes in the basic cured resin properties can be anticipated. For the retention of these basic properties a search for inherently high flame-resistant epoxy resins was initiated.

In regard to flame retardancy high char yield may be related to decreased flammability and measured by the oxygen index. Increased char formation can usually limit the production of combustible carbon-containing gases, decrease the exothermicity due to pyrolysis reactions, and decrease the thermal conduc- tivity of the surface of a burning material.2 The char formation of the polymeric material depends on the chemical structure of the polymer segment. It has been reported that the phenolphthalein polycarbonate in a polycarbonate system has a thermogravimetric char yield of about 54%, compared with 20% for bisphenol

Journal of Polymer Science: Polymer Chemistry Edition, Vol. 17,3095-3119 (1979) 0 1979 John Wiley & Sons, Inc. 0360-6376/79/0017-3095$01.00

3096 LIN AND PEARCE

A polycarbonate at 80O0C, and that the oxygen index varies in a similar manner from 47 to 23 (ref. 3).

In thermal analytical studies of the epoxy materials cured by trimellitic anhydride or m-phenylenediamine it was found that polymer with a fluorene or anthrone group between the two phenyl groups showed the best thermal sta- b i l i t ~ . ~

Trimethoxyboroxine (TMB) is a Lewis acid catalyst for epoxy resins. Because TMB has been used in the preparation of hard epoxy coatings5 and has served as primary curing agent or cocuring agent for epoxy resins,6 it has also been used to prepare rigid epoxy f0ams7-10 and transparent epoxy panel^.^ DGEBA cured by TMB had heat deflection temperatures ranging from 80 to 12OOC (ref. ll), was thermally stable up to 4OO0C, exhibited high tensile strength: and could be self-extinguishable with the addition of a mixture of boric acid and sodium borate. Furthermore, three exothermic peaks at approximately 390,430, and 47OoC, with the major exotherm at 43OoC, were found by differential thermal analysis.12

On the basis of these results the material prepared from a diglycidyl ether of phenolphthalein (DGEPP) or 9,9-bis(4-hydroxyphenyl)fluorene (DGEBF) and cured with TMB should show improvement in certain properties, including flammability, over those obtained from the DGEBA-TMB system, provided that good transparency and other desired properties can be obtained.

DGEPP can be prepared from phenolphthalein and epichlorohydrin in the presence of sodium hydroxide.'%16 According to the reaction of phenolphthalein with sodium hydroxide, four structures possibly present in the epoxy resin were proposed. Because of the yellow color and the shift of the 1725-1730-~m-~ band of phenolphthalein to 1750-1760 cm-l in the resin, the quinoid structure was also assumed to be the major product from the reaction.13

By comparing the absorption infrared (IR) spectrum of DGEPP resin obtained directly from the reaction product of phenolphthalein and epichlorohydrin, with some related compounds of phen~lphthalein,'~ and the 13C and proton nuclear magnetic resonance (NMR) spectra of the major component of DGEPP resin from preparative layer chromatography with those of DGEBA and phthalidel8 the phthalide structure rather than the quinoid was confirmed as the major product in the reaction system.

For a better understanding and characterization of the products obtained from the reaction of phenolphthalein and epichlorohydrin liquid chromatography was used to separate each component in the product and to determine their relative amounts. IR, ultraviolet (UV), and proton NMR spectroscopy were used for characterization, and the epoxy equivalent weight and the softening temperatures of the components were measured.

To determine how structural differences affect the reactivity of the epoxy resin DSC was used to investigate the reactions of homopolymer systems cured with an equivalent amount of curing agent.

The degree of crosslinking in a network polymer has a broad effect on the various properties of polymers such as dimensional stability and resistance to solvents. One method of determining the degree of crosslinking is to extract the polymer that is not incorporated into the network structure as a sol fraction. In this study solvent extraction by tetrahydrofuran (THF) was used to determine the amount of gel fraction in different compositions of copolymers and to explain the reactivity of the epoxy resin.

EPOXY RESINS. I1 3097

As expected, DGEPP and DGEBF after curing should give higher thermal stability than the conventionally cured DGEBA. To improve the heat resistance and lower the flammability of the DGEBA system it was copolymerized with DGEPP or DGEBF, and to determine how the properties change with the content of DGEPP or DGEBF, DSC and thermogravimetric analysis (TGA) were used to investigate the various copolymer compositions of DGEBA with a DGEPP or a DGEBF system.

Char formation has been one of the important factors that appear to relate to the flammability of a material. A linear relationship between the oxygen index and the char residue on pyrolysis for halogen-free polymer has been proposed.lg The oxygen index for epoxy resins has also been shown to be nearly independent of cure conditions and tends to be lowered for increasing O/C ratio in the overall composition.l These results, however, have been limited to the relatively few polymers investigated. In the copolymer system it may be expected that each monomer would contribute a char residue or an oxygen index proportional to its composition if two units in the copolymer do not chemically interact in an unusual manner and the copolymer has high molecular weight. The copolymer systems poly(DGEBA-co-DGEPP) and poly(DGEBA-co-DGEBF) were studied to confirm correlations between the char residue or the oxygen index and the composition of copolymers and their general use for protecting the heat resistance and flammability of these copolymer epoxy systems.

EXPERIMENTAL

Preparation of Epoxy Resins

Preparation of Bisphenols

Bisphenols A (4,4’-isopropylidenediphenol: Aldrich analyzed grade) and phenolphthalein [3,3-bis(4-hydroxyphenyl)phthalide: Aldrich analyzed grade] were recrystallized from aqueous and absolute alcohol, respectively. 9,9-Bis(4-hydroxyphenyl)fluorene was prepared from fluorenone (Aldrich

analyzed grade) and phenol (Fisher certified ACS grade) with a catalyst, dry hydrogen chloride, and a cocatalyst, 3-mercaptopropionic acid20 (Aldrich 99+%, nd20 1.4911 and d20 1.218). Dry hydrogen chloride was prepared from sodium chloride (Fisher certified ACS grade) and concentrated H2S04 (Baker analyzed grade). The hydrogen chloride was then passed through concentrated H2SO4 for dehydration. One mole of fluorenone was dissolved in 8 mole of molten phenol, 4 ml of 3-mercaptopropionic acid was added and dry hydrogen chloride was bubbled in for 20 min. The system then became very dark and finally changed to a viscous clear liquid which was purified by steam distillation to remove unreacted phenol, hydrogen chloride, and cocatalyst. After no more phenol was collectible in the distillate the system was treated with 10 liter of cold water and 2.1 mole of sodium hydroxide (Fisher certified ACS grade) to dissolve 9,9- bis(4-hydroxypheny1)fluorene. The mixture was acidified with acetic acid (Allied reagent ACS) to precipitate a white solid which contained hydrate in crystalline form. The dehydration product was a white powder which was recrystallized from toluene (Fisher certified ACS grade) or anhydrous ether (Mallinckrodt analytical reagent). A transparent solvated crystal was collected from toluene or ether solution, washed with a small amount of solvent, and then

3098 LIN AND PEARCE

dried i n uacuo. Last of all, a white powder with a melting temperature of 224OC was obtained.

Synthesis of Diglycidyl Ethers of Bisphenols

The apparatus used was a heated reaction kettle with a thermometer, me- chanical stirrer, and a water-cooled condenser. The reaction kettle was charged with 1 mole of phenolphthalein and specified amount of epichlorohydrin (15, 12, 9.15, 6.08, and 3.2 mole) (Eastman: bp 114-116OC). Because of the low solubility of phenolphthalein in epichlorohydrin, the former was suspended in the latter. The stirrer was started and the reaction mixture was heated to 90°C. Over a period of 3-4 hr 2 mole of sodium hydroxide pellets was added to the reaction mixture and the reaction temperature was maintained between 90 and 100OC. The reaction mixture changed in color from white to deep violet and then yellow. After the color became pale yellow the solution was filtered to separate the solid sodium chloride formed during the reaction. The salt cake was then washed with additional epichlorohydrin and the unreacted epichlorohydrin was distilled off under a vacuum of 30 mm Hg from the filtrate. After no additional epichlorohydrin could be removed under this vacuum the vacuum was decreased to 2 mm Hg for 30 min at 170°C. DGEBA and DGEBF were prepared by the same procedure as DGEPP except that the initial reactant feed molar ratio of the respective bisphenol to epichlorohydrin was 1-10.

DGEBF could be crystallized from its amorphous bulk state or from acetone solution. The crystallization of DGEBF was obtained from the material collected from liquid chromatography if the amorphous resin was given the stress to de- velop the crystalline nuclei. In solution recrystallization 15 ml of acetone was added to dissolve about 10 g of DGEBF resin and absolute alcohol was charged into this solution with vigorous agitation until it turned a little cloudy. The solution then stood for two days to precipitate out a white crystalline powder which was collected by filtration and washed with a mixture of acetone and absolute alcohol (1:l). The crystalline DGEBF had a melting temperature of 132OC obtained from DCS.

Characterization of Epoxy Resins

Liquid Chromatography

Silica gel (Grace) with a mesh number of 60-200 and 5 vol % of acetone (Aldrich spectroscopic grade) in chloroform (Fisher certified ACS grade) were used as absorbent and elution solvent, respectively. The column had an inner diameter of 3.5 cm and a height of 40 cm; 10-15 g of DGEPP was charged. Two different colors, red and yellow, were observed in the column at the beginning of elution. The red material eluted first and became colorless after elution from the column. A higher acetone content eluent (gradient method) was added to remove the slowly moving yellow material. The elution rate was 10 ml/min and the solution was collected in 50-ml portions. The solvent was removed by simple distillation and the residue was weighed.

DGEBF was eluted by the same procedure. The column was colorless at the beginning of and during elution.


Product Analysis

The products collected from the column were identified by UV, NMR, and IR spectroscopy. A Cary-14 recording spectrophotometer was used for UV spectral studies and chloroform (Fisher certified ACS grade) was the solvent. A Varian A-60 NMR spectrometer was used for the NMR measurements and CDC13 (Diaprep; 99-98% minimum isotopic purity) was the solvent.

The IR spectra were obtained on a Fourier transform infrared spectrometer (Diglali, FTS-IR spectrometer, model-20B), and the softening temperature of each component from liquid chromatography was studied by a differential scanning calorimeter (DuPont 900 thermal analyzer).

Epoxy Equivalent Weight Determination

The ASTM D 1652-73 methodz1 was used to determine the epoxy content of the epoxy resins. Two tenths of a gram of the resin was added to a 50-ml flask and then dissolved in 10 ml of 50 vol 9% of chlorobenzene (Aldrich analyzed grade: 99%) in chloroform. The mixture was stirred with a Teflon-coated magnetic stirring bar, and 4-6 drops of 0.1% of crystal violet [4,4',4"-methylidyne- tris(N,N-dimethylaniline), Eastman] solution in glacial acetic acid were added (Eastman: 30.32% in acetic acid by titration). The hydrogen bromide solution was standardized by 0.4 g of potassium hydrogen phthalate each time before use for epoxy equivalent weight determination.

Viscosity Measurement

The viscosity measurement of epoxy resins in this study was carried out with a Brookfield Synchro-Lectic viscometer (Brookfield Engineering Laboratories) that measured the drag produced on a spindle rotating at a definite constant speed while immersed in the material under test. This drag is indicated on a rotating dial by a pointer attached to the spindle shaft and represents the tension produced from a spiral spring, the core of which is fastened to the spindle shaft and the outside coil terminal, to the dial, which is directly connected to the motor shaft. The viscosity was obtained at various temperatures.

Curing Process: Epoxy-Trimethoxyboroxine (TMB) Systems

TMB (Aldrich: 99%, bp 13OoC, n d Z o 1.3996) was used as the curing agent. To compare the results of the cured epoxy resins 9.5 g of TMB was mixed with 1 equiv of epoxy re~in.~JO The curing conditions were investigated by DSC (Du- Pont 900 thermal analyzer) in a nitrogen atmosphere with a flow rate of 0.2 liter/min and a heating rate of 10"C/min.

The DGEPP or DGEBF was powdered at room temperature and mixed with the curing agent. The mixture of the DGEPP-TMB system was red in color. The two-component epoxy system was prepared by mixing DGEPP or DGEBF with DGEBA at 130°C and the curing agent was added to the melt system. The curing was carried out in a nitrogen-filled and sealed glass tube at 135°C for 3 hr, 180°C for 3 hr, and 220°C for another 3 hr in an air-circulated oven.

3100 LIN AND PEARCE

Characterization of Cured Epoxy Resins

Dynamic Thermal Analysis

The thermal properties of the samples in this study were tested on a DuPont 900 thermal analyzer for DSC thermograms and in a DuPont 950 thermogravimetric analyzer for char residue determination. A powder sample with a mesh number of rilore than 170 was used for DSC or TGA. All measurements were carried out in a nitrogen atmosphere at a flow rate of 0.2 liter/min or in vacuo with a heating rate of 10°C/min.

Oxygen Index Measurement

The oxygen index of the various compositions of DGEBA copolymerized with DGEPP were measured with a General Electric model CR280KFllA fluid flammability test kit which extends the oxygen index method to liquids and solid materials that can be melted or tested in powder or pellet form.22

The cured epoxy resin was made into pellet form first and then placed in a sample cup mounted on a cup holder in the flame chamber. The mixture of oxygen and nitrogen passed upward through the chimney at a flow rate of 3-5 cm/sec. The test followed the manual procedures.22 The oxygen index was obtained from the equations as

0 2

0 2 + N2 0 1 =

where the unit of gas quantity is volume.

Solvent Extraction of Cured Epoxy Resins

One gram of cured polymer was charged in a Soxhlet apparatus for 48 hr with THF (Aldrich analyzed grade) as the solvent. The insoluble part was collected and dried until no more weight change could be detected. The gel fraction was calculated as

(1) weight of insoluble material

initial weight of material gel fraction =

RESULTS AND DISCUSSION

Characterization of Epoxy Resin

Structure Study on the Phenolphthalein Epoxy Resin

The original epoxy resin of phenolphthalein produced from the reaction of phenolphthalein, epichlorohydrin, and sodium hydroxide is yellow in color. L0,I3-l6 who prepared this material first, proposed that the product had four possible structures based on the equilibrium reaction between phenolphthalein and sodium hydroxide. Because of the yellow color and the IR absorption band shift, Lo further assumed that the quinoid was the major product in this reaction.


Salazkin, Komarova, and Vinogradova17 compared the absorption IR spectrum of 3,3-(bispheny1)phthalide with those of compounds that have similar or the same structures proposed by Lo and concluded that phthalide rather than quinoid is the basic structure of the major product.

To separate the major product from DGEPP resin Everatt, Haines, and Starkls used preparative layer chromatography to obtain a 70% yield of the major con- stituent. They also studied the proton and 13C-NMR spectra of this major fraction, compared with those of phenolphthalein, phthalide, and benzoquinone, and concluded that the main component of the DGEPP resin is an aromatic diglycidyl ether that contains a lactone ring rather than a glycidyl ether-glycidyl ester compound that contains a quinoid ring.

To confirm the composition and amount of each component in the phenolphthalein epoxy resin liquid chromatography was carried out with 5% by volume of acetone in chloroform as solvent. Figure 1 is the dried weight of the collected material from liquid chromatography as a function of retention volume. Three peaks (defined as A, B, and C) are independent of the initial feed ratio of epichlorohydrin/phenolphthalein in the preparation of DGEPP resin. A, the first fraction, yielded the major product, a colorless, transparent, and viscous material. B, the second fraction, was solid and a very light yellow. The third fraction was a solid yellow resin.

The properties of the components separated from the epoxy mixture are shown in Table I. The softening temperature, measured by DSC, and the epoxy equivalent weight both increased with increased retention volume. If the structure of the epoxy resin is the phthalide derivative, the first peak (the major

..

ELUTION VOLUM ( m I I

Fig. 1. Liquid chromatography of the epoxy resin of phenolphthalein. Original weight, 10.1854 g; recovered weight, 10.3417 g.

3102 LIN AND PEARCE

TABLE I Some Properties of Phenolphthalein Epoxy Resin Fractions Obtained from Liquid

Chromatographya

Epoxy resin fraction Property A B C

~~ ~~

Color White clear Very light yellow Yellow Softening temperature ("C) 15 43 52 Epoxy equivalent weight 233 326 326 625 nb 0.096 0.59 2.19

a Solvent: CHCls/acetone (95/5 by volume); absorbent: silica gel.

peak) will be the monomer of DGEPP resin. No melting point was observed even when the samples were cooled and remeasured; this showed that the epoxy resin was in a glassy state. As in DGEBA p r e p a r a t i ~ n , ~ ~ the epoxy equivalent weight decreased with an increase in the initial feed ratio of epichlorohydrin/phenolphthalein. The amount of monomer formation and epoxy equivalent weight are controlled by the initial feed mole ratio of reactants. The change in weight fraction of fraction A (y) obtained by liquid chromatography with a variation in initial feed mole ratio ( x ) may be generalized and represented as

y = 1.0 - eax (2)

with boundary conditions x = 0, y = 0 and x = 03, y = 1.0, where a is an arbitrary constant equal to -1.5 obtained from Figure 2 in this study.

Liquid chromatography of the IR spectra of the three components produces the same absorption patterns, which show a distinct shift in the 1725-cm-' band of phenolphthalein to 1780 cm-l in the resin (Lo, 1760 cm-'; Salazkin et al., 1765 cm-l). Lo assumed the change of ketonic carbonyl to an ester carbonyl,13 but Salazkin et al.,17 by comparison with model compounds, related it to the normal phthalide carbonyl absorption. Jones et al.24 examined the IR spectra of 23 simple saturated and unsaturated lactones and showed that the unsaturated five-member ring lactones, which included two phthalides, gave the carbonyl absorption between 1772 and 1787 cm-' in CCl4 and 1752 cm-l in CHC13. From these results it can be concluded that the three components have the same structure, which is the phthalide instead of the quinoid or other structures proposed by Lo. The shift in the phthalide carbonyl absorption band in the IR spectrum may be attributed to the formation of hydrogen bonding between a carbonyl group in the lactone ring and a hydroxyl group present in phenolphthalein and absent in its epoxy resin because the proton in the hydroxyl group


I

9 c - 5 -2.0 *

0 5 10 15 20 No. of Motes of Epichiorohydrin b. D t Moles of Phenolphthaiein

Fig, 2. Ln (1.0 - weight fraction of fraction A in DGEPP) as a function of initial feed ratio of epichlorohydrin tophenolphthalein. y = 1.0 - eax; z = 0,y = 0. Whenz = - , y = 1. Slope = -1.5 = a .

has been replaced by a glycidyl group.

The IR spectrum of phenolphthalein shows a strong and broad absorption in the region between 3700 and 2200 cm-l which indicates the formation of hydrogen bonding. Hydrogen bonding formation can decrease the charge density or bond order of the T bond in the carbonyl group and cause a vibration frequency shift to a lower value.

The UV spectra (Fig. 3) of the three components in DGEPP resin also indicate the same structure due to the same absorption patterns. All three components show three absorption maxima at 238,275, and 282 nm, which are the same for pure phenolphthalein in the same solvent (chloroform), which indicates that basically the epoxy resin has the same functional group-the phthalide structure-as phenolphthalein. The only difference between fihenolphthalein and its epoxy resin is the 238-275-nm extinction coefficient ratio. This value, by liquid chromatography, is 3.0 for phenolphthalein and 4.2 for each component of the DGEPP resin. Furthermore, the simple compound phthalide also showed the same absorption^.^^ If the phthalide structure is opened during resin preparation and forms an ester, the absorptions should have some degree of shift, but in this study no shift was observed. It can be concluded that the assumption of the quinoid structure in the epoxy does not occur.

The extinction coefficients for each component in the epoxy resin were calculated on the basis of the titration result of epoxy equivalent weight determi-

LIN AND PEARCE

0.0 7: 20 2LO 260 280 300 3 20

n m

Fig. 3. UV spectrum of phenolphthalein epoxy resin. A23$A275 = 4.2. (A238/A275 = 3.0 for . . . . . . ~ nhennlnhthilein 1

nation and the structure (V) assumed below:

by the equation

mass of epoxy resin in 1 liter of solution epoxy equiv wt X 2 absorbance = E X x (1 + n ) (3)


and 430 + 374n

2 epoxy equiv wt = (4)

The extinction coefficients of the three peaks observed in the spectra are shown in Table 11. In this table it may be observed that the extinction coefficients at X = 275 nm and X = 282 nm are about 1.45 X lo4 liter mole-l cm-l and 0.40 X 104 liter mole-l cm-l, respectively, and are independent of the components of epoxy resin and phenolphthalein. A t X = 238 nm the extinction coefficient increased from 1.42 x lo4 liter mole-l cm-l for phenolophthalein to about 1.90 X lo4 liter mole-1 cm-' for its epoxy resin. From the structure (V), eqs. (3) and (46, and the extinction coefficient the epoxy equivalent weight may be calculated from UV spectral results.

The proton NMR spectroscopic studies of the three components in DGEPP resin show the same pattern in the aromatic region which has two distinct group signals: one signal, between 6 6.64 and 7.40 ppm, had a characteristic AzBz pattern; the other, between 6 7.40 and 7.96 ppm, was complex and was assigned to the ABCI) spin system of the o-substituted phenylene ring. The absorptions occupied the same positions as those obtained by Everatt et al.ls by major component analysis. The integration of the two aromatic groups shows a proton number ratio, 2:1, for all three components in the resin. On the basis of these results it is apparent that the three components have the same basic structure of 3,3-bis(p-substituted pheny1)phthalide.

Figure 4 shows the proton NMR spectra of the glycidyl groups in the three components, from which it can be observed that all have absorption positions at 6 2.60-2.94 ppm, 6 3.16-3.38 ppm, and 6 3.644.30 ppm, which are assigned to the terminal CH2 group of epoxide, -CH group of epoxide, and the other -CHz- group with the ether linkage, respectively. The NMR spectra in this region are also the same as the simple model compound phenyl glycidyl ether. These absorption positions are coincident with those obtained from the pure diglycidyl ether of phenolphthalein.lS

The proton NMR spectra of the three components also show a small difference in signal intensity. For component A the integration shows the ratio of hydrogen atoms as approximately 2:1:2, which is the same as the proposed structure of pure monomer-the lactoid structure. Fractions B and C show a relative increase in the integration of -CHz- with ether linkage, but the ratio of protons of terminal CH2 in the epoxide and -CH groups is still 21. A possible explanation

TABLE I1 UV Spectral Extinction Coefficients of the Liquid Chromatographic Fractions from Diglycidyl

Ether of Phenolphthalein-Based Resin (DGEPP)*

Compound (nm) (nm) (nm) (nm) E238b E275 Ezsz E2381E275

Phenolphthalein 1.42 0.47 0.40 3.0 Fraction A 1.87 0.45 0.39 4.2 Fraction B 1.81 0.43 0.37 4.2 Fraction C 2.01 0.48 0.41 4.2

a Solvent for UV spectral measurement: CHC13. E = ext coeff X liter mole-1 cm-1.

3106 LIN AND PEARCE

I '

I I I

19 3.0 2.0 PPMi 6 1 5.C

Fig. 4. NMR spectra of phenolphthalein epoxy resins collected by liquid chromatography.

is that fractions B and C are not monomer and have a higher n value in the structure (V). The n values of the three components have been calculated on the basis of this structure and the epoxy equivalent weight determined in Table I. This structure shows that the -CH2- group with ether linkage increases in intensity in the NMR spectrum as the value of n increases.

From these results it may be concluded that the phthalide in phenolphthalein is not opened to the ester group during the preparation of the DGEPP resin. The only structure in the product is related to the phthalide even if three fractions were obtained by liquid chromatography. The structure of phenolphthalein in an aqueous alkaline solution is known to depend on the amount of alkali present. The formation of the epoxy resin in a phthalide structure shows that a rearrangement or shift in equilibrium between forms takes place during the reaction and leads to the retention of the lactone ring in phenolphthalein.

The differences among the three components are their colors and the degree of polymerization. The reason for color formation is not clear. The softening temperature increases from A to C and is apparently due to an increase in average molecular weight.


The three components were individually cured with the same amount (9.5 g/equiv of resin) of TMB. After mixing the curing agent with each component the reaction mixture showed red at the beginning and then formed a transparent orange cured epoxide resin. Because the three components with different colors yielded the same final result, it also seemed to confirm the same basic structure in the three fractions.

Properties of Epoxy Resins Derived from Bisphenols

The epoxy resin in this study was prepared by the conventional method by which the epoxy resin should have a general structure:

OH c ~ c x H 2 0 ~ ~ [ --OCH,cH ' cH,o +Ra]:WH2CH-CH2 /9

n = 0,1,2,3 ,...

The properties of the three epoxy resins derived from bisphenols are listed in Table 111.

DGEBF was first prepared by Korshak et al.4 who found that the resin cured with trimellitic anhydride or m-phenylenediamine produced good heat resistance and high thermal stability. The pure crystal of DGEBF monomer had a melting temperature of 132°C. After quenching the monomer melt only a softening temperature at 38OC could be observed. It appeared that DGEBF did not crystallize easily. It was also observed that the recrystallization of DGEBF resin from acetone-alcohol solution was slow. The softening and melting temperatures depend on the crystallinity of the material and the molecular weight of DGEBF. The DGEBF resin obtained directly from the preparation also had a melting temperature that occurred almost at the same degree as the softening temperature. It is reasonable to assume that the cloudiness in the resin is due to the presence of microcrystals and dispersion in the amorphous resin.

TABLE I11 Some Properties of Phenolphthalein Epoxy Resin Fraction Obtained from Liquid

Chromatographya

Epoxy resin fraction Pro D e r t v A B C

Color White clear Very light yellow Yellow Softening temperature ("C) 15 43 52 Epoxy equivalent weight 233 326 625 nb 0.096 0.59 2.19

a Solvent: CHC13/acetone (955 by volume); absorbent silica gel.

3108 LIN AND PEARCE

The viscosities of the three epoxy resins were also measured as a function of temperature (Fig. 5). In this figure the activation energies are shown as 5.7,13.5, and 13.6 kcal/mole for DGEBA, DGEPP, and DGEBF, respectively. The order of viscosity at the same degree above room temperature is DGEBF > DGEPP >> DGEBA. The increase in viscosity from DGEBA to DGEBF corresponded to an increase in side chain stiffness, indicated by the increased aromatic ring content.

Curing Reaction Studies on Epoxy-TMB Systems

The properties and stability of an epoxy resin with a curing agent varies with the degree of curing. Control is achieved by several factors such as the amount of curing agent, the reactivity of the epoxy resin, and the curing conditions. To understand the effect of epoxy reactivity on the curing reaction a DSC thermal analyzer was used to investigate this reactivity with an equivalent amount of the curing agent TMB.

The reaction rate of a crosslinking polymerization system is determined by two factors-the reactivity of the reactant and the mobility of the reactive site-which are dependent on the viscosity of the system. The final stage of a crosslinking polymerization is the formation of a network that limits the mobility

Fig. 5. Arrhenius plots for the viscosities of the three molten epoxy resins. Activation energies (kcal/mole): DGEBF, 13.6 (A); DGEPP, 13.5 (X); DGEBA, 5.7 (0).


A T

0 z 0 (11

of the reagent and thus prevents the process from going to completion. To understand how the structure of the epoxy resin affects the curing process gel fraction determination in the epoxy resin treated under certain curing conditions was considered.

Figure 6 shows the DSC thermograms of DGEBA, DGEPP, and DGEBF resins after mixing with TMB. It can be observed that DGEBA-TMB, DGEBF-TMB, and DGEPP-TMB systems start to release heat at about 90 and 120OC. No exotherms can be detected in the DSC thermograms of the epoxy resins without a curing agent or completely cured epoxy resins in the same temperature range concerned. This indicates that the peaks are attributed to the exothermic curing reactions.

In solvent extraction studies the gel fraction varies with the composition of the copolymers under the curing conditions shown in Table IV. The gel fractions from DGEBF copolymerized with DGEBA generally showed a small increased gel fraction up to 36 mole % of the DGEBF content. They then decreased with increased DGEBF composition and finally reached a 33.3 wt % of crosslinked material. In DGEPP copolymer with DGEBA the gel formation decreased with increasing DGEPP content (the rate of decrease became lower at a DGEPP content higher than 45 mole %) and had a final gel fraction of 54.3 wt %.

On the basis of these results, the lower exotherm starting temperatures for DGEBA and DGEBF curing systems in the DSC thermograms and the com- parative gel fractions obtained from DGEBA homopolymer and its copolymer with DGEBF extended’ to 70 mole % of DGEBA content, it is reasonable to propose that the epoxide groups of both resins have similar or close chemical

DGEBF I, -,

0.2 Qtn. I I 1 I I I I I 1

100 200 300 0 T, OC

Fig. 6. DSC thermograms of DGEBA, DGEPP, and DGEBF after mixing with TMB. Heating rate, 10°C/min in N2 atmosphere.

3110 LIN AND PEARCE

TABLE IV The Relationship between Gel Fraction and Mole Fraction of DGEPP or DGEBF in the

Copolymer with DGEBA*

Copolymer of DGEBA and DGEPP Copolymer of DGEBA agd DGEBF Mole fraction of Gel fraction Mole fraction of Gel fraction

(mole %) (wt %) (mole %) (wt 70)

0.0 84.5 0.0 84.5 13.5 82.8 6.7 86.3 27.6 73.8 9.8 85.2 43.2 58.4 22.9 91.0 58.3 63.7 35.9 94.3

100.0 54.3 36.1 86.4 62.2 63.8

100.0 33.3

a Curing agent: trimethoxyboroxine; 9.5 g/epoxy equiv. Curing condition: 135OC, 3 hr; 18O0C, 3 hr; 218OC, another 3 hr; in sealed tube under Nz atmosphere.

reactivities during curing with TMB. In DGEPP a higher starting exotherm temperature, which is indicative of an epoxy-boroxine system with lower reactivity, was shown. As described earlier, DGEPP changed color from yellow to red after mixing with TMB and to orange at the final curing stage. This may be indicative of a complex formation between DGEPP and TMB. DGEPP, determined previously, is a derivative of 3,3-disubstituted phthalide and has an electron-rich carbonyl group, but the TMB has three affinity boron atoms in a molecule. The phthalide group and the boroxine may form a complex and reduce the possibility of reaction between the epoxide ring and the curing agent. In the polymerization study of phenyl glycidyl ether Lopata and Riccitiello26 proposed a fast initiated mechanism. When the epoxide group forms a carbo- nium ion after rapid reaction with TMB, the initial curing temperature should shift to a higher degree as indicated in the DSC thermogram.

From the Bll quadruple coupling constant in the study of TMB, Ring and K o ~ k i ~ ~ concluded that the resonance structures of this compound are

and that 60% of the structure (VII) is present in this material. The methoxy group also increases the acidity of the boron and thus the double-bond character. This fact indicates that TMB is present with a high electron affinity and 60% of the aromaticity, which is favorable for forming a complex with the electron-rich group, such as carbonyl in phthalide.

The crosslinking density studies by gel fraction determination showed that the gel fraction of the epoxy resin cured under the same curing conditions changed with the resin and composition of the copolymer as described earlier. The order of gel fraction present after the resins had been cured at 135,180, and 220°C for 3,3, and 3 hr sequentially was DGEBA (84.5 wt %) > DGEPP (54.3


wt %) > DGEBF (33.3 wt %). The rate of reaction was dependent not only on the reactivity of the functional group but also on the molecular mobility. It is a well-known trend that a molecular unit with a higher aromatic ring content has a higher rigidity and a higher resistance to segmental or molecular motion if all other factors are equivalent. From the structural consideration of the three epoxy resins the increasing order of aromatic ring content was DGEBF > DGEPP > DGEBA, which also corresponded to the transition temperature and viscosity at the same temperature. As shown in Figure 5 the order of viscosity for the three resins was DGEBF > DGEPP > DGEBA. The energy barrier for the translation of a DGEBA molecule was 5.7 kcal/mole, which is much lower than that of a DGEPP or DGEBF molecule with a value of 13.5 or 13.6 kcal/mole. Because of the much higher viscosities of DGEPP and DGEBF compared with DGEBA resins, both resins increase their viscosities or solidify much faster and their reaction-rate-controlling steps shift from reactivity to diffusion at much earlier stages than the DGEBA resin during the curing process. DGEBF has the highest viscosity or rigidity among the three resins; thus it solidifies and shifts to diffusion control with a slower rate of reaction during curing. This is reflected by the lowest gel formation among the three cured systems. Because of the higher viscosity of DGEBF, compared with DGEPP resin, and the relatively high temperature used in curing both resins, the cured DGEPP may have a higher crosslinking density than DGEBF at the final stage of the curing cycle even though DGEBF is more reactive. Furthermore, DGEBF can be crystallized and the crystals have a melting point of 132°C. Order array is present in the resin during curing and also inhibits the polymerization of this resin. The cloudiness present in the cured DGEBF may be due to these conditions.

The variation in gel fractions with copolymer composition can be similarly explained. The copolymer of DGEBF with DGEBA at a content up to about 30 mole % of DGEBF contains the same or a slightly increased gel fraction. The DSC thermograms showed the reaction of epoxide groups in both resins; thus the curing reaction of the copolymers with a DGEBF content lower than about 30 mole % is probably controlled by the reactivities of the resins up to a high conversion level and then shifts to diffusion control at the latter stages of cure. If the DGEBF content were higher than 30 mole %, the rigidity of DGEBF and its resultant viscosity would cause an earlier shift to diffusion control, hence to lower gel formation. Gel formation decreases as the DGEBF content increases.

Because of the lower reactivity of the DGEPP resin, its copolymer showed decreased gel formation with DGEBA as the DGEPP content increased.

Characterization of Epoxy-Cured Systems with TMB

DSC Studies of Cured Homopolymers and Copolymers

DSC was used to study the glass transition temperatures of the homopolymers, to understand the effect of copolymer composition on the glass transition temperature, and to investigate the heat change during the decomposition of the cured epoxy resins. The thermograms of cured DGEPP, DGEBF, and DGEBA in an inert atmosphere of Nz showed exothermic peaks at approximately 310, 390, and 430OC; 310,380, and 420OC; and 345,395,430, and 470°C, respectively (Fig. 7). This indicated that they may have similar degradation paths even

3112 LIN AND PEARCE

I

0 100 200 300 1,oo 5 00 60 0 7 no 1 , D C

Fig. 7. DSC thermograms of cured epoxy resins. Heating rate, lO"C/min; atmosphere, Na at 0.2 lpm; curing agent, TMB.

though different structural units are present and could be related to degradation; therefore it can be assumed that it initiated from similar structure units such as the aliphatic groups.

The glass transition temperature as a function of composition in copolymers has been studied extensively by many investigators; current theories have been summarized by Wood.28 The glass transition temperature of a copolymer should be equal to some type of weight average of the individual glass transition temperatures of the homopolymers, Tgl and Tg2. For a binary copolymer the Gor- don-Taylor equation which relates the glass transition temperature to the glass transition temperatures of the homopolymers is equivalent to

(5 ) where C1 and C2 are the weight fractions of the constituents and A1 and A2 are constants. This equation can be rearranged to the form

Tg = K(Tg2 - Tg)(C2/C1) + Tgl (6)

(7) where K = A2/A1. Figure 8 shows the glass transition temperature as a function of (T, - Tgl)(l - C2)/C2 and a function of (Tg2 - Tg)C2/(1 - C2) for DGEBA- DGEPP copolymer. From this plot the 1/K and K values can be obtained from each linear relationship, in which case the K parameter for the copolymer of DGEPP and DGEBA cured by TMB is equal to 4.0. Figure 9 shows the glass transition temperatures for various compositions of DGEPP and the theoretical results obtained.

A similar treatment of the copolymers of DGEBF and DGEBA is shown in Figures 10 and 11. The K parameter obtained from Figure 10 is 3.5 for this co-

A1C1(Tg - Tgd + A2CATg - Tg2) = 0

Tg = - ( l / W ( T g - Tgd(CdC2) + Tg2


P b O 80 120 160 '1" - v'1 CLJ

c2

Fig. 8. Glass transition as a function of (Tg - Tgl) C1/C2 and (T,z - Tg) CJCi for the copolymers of DGEBA and DGEPP. K = 4.0,l: DGEBA; 2 DGEPP; C, is weight fraction of component i; curing agent: TMB, 9.5 g/epoxy equiv.

4 2 5 , I

J 0 20 4 0 60 80 loo

w t '/a of O G E P P

Fig. 9. Glass transition against the weight fraction of DGEPP in the copolymers of DGEPP and DGEBA. Curing agent: TMB, 9.5 g/epoxy equiv.

polymer system, based on the glass transition temperatures of the copolymer in which the DGEBF content is below 35 mole % and the hompolymer of DGEBF. From Figure 11 two glass transition temperatures at 350 and 400 K can be observed. The former is obtained directly by the same curing cycle used in the other polymerization systems. The latter was obtained by three additional hours of curing at 220OC. The K value was obtained by using 400 K as the DGEBF homopolymer glass transition temperature, which started to deviate from the Wood equation after the DGEBF content became higher than 35 mole %. The K value was lower than predicted and may have been due to the incomplete cure indicated by the gel fraction studies that showed low gel formation for the high DGEBF content copolymer with DGEBA (Table IV).

3114

425

LIN AND PEARCE

.

( b 2 - h 1 cz_. ( 1 - C 2 )

0 4 8 12 16 f----- - -7-

0 40 80 120 1M) (Ty - T g l ) ( l - C 2 )

c 2

Fig. 10. Glass transition as a function of (Tg - Tgl) C1IC2 and (T,z - Tg) C2/Cl for DGEBA and DGEBF copolymers; K = 3.5,l: DGEBA; 2: DGEBF; Ci is weight fraction of component i; curing agent: TMB, 9.5 g/epoxy equiv.

0 ? O LO GO 80 100 wt. ‘/a 01 DGFBF ( C , )

Fig. 11. Glass transition versus the weight fraction of DGEBF in the copolymer with DGEBA. Curing agent: TMB, 9.5 g/epoxy equiv.

Relationship between Char Residue and Flammability in Copolymer Systems-

Char formation is important to the prediction of the flammability. In this study the char yields of homopolymers and copolymers of various compositions were investigated by thermogravimetry. Figure 12 shows the results of the thermogravimetric measurements of homopolymers in U U C U O .

If the char formation of each monomer unit in a copolymer were independent of one another or there was no chemical interaction between the monomer units,


1 .o

U c 0, 4

g0.5 W

c 4 U

m I*

0 . 0

1.0

P m 0 rt)

0.b n 2 0 0 400 6 0 0

T;C.

Fig. 12. TGA and DTG thermograms of various epoxy resins in uacuo. (--) DGEBA, (- - -) DGEPP, (-.-) DGEBF; heating rate: 10°C/min; W 8.06 mg, DGEBA; 7.39 mg, DGEPP; 7.70 mg, DGEBF.

it may be assumed that the char yield of each monomer unit in a high-molecular-weight copolymer was proportional to its mole fraction:

Yi = XiY," (8)

where Yi and Yio are the char yields of monomer unit i in a copolymer with a mole fraction Xi and its homopolymer, respectively. The total char yield of a copolymer will therefore be equal to the sum of the char yield of each component in the copolymer; for cured DGEPP, TGA in uucuo (Fig. 12) showed additional decomposition at 325 and 27OoC, respectively, when compared with DGEBA. The phenolphthalein polycarbonate also showed initial decomposition at 325°C which was not present in the bisphenol A polycarbonate TGA t h e r m ~ g r a m . ~ ~ This indicated that the decomposition of cured DGEPP at 325°C was related to the decomposition of the phthalide group. The exotherm peak at 310°C in the DSC thermogram of the DGEPP-cured resin could be attributed to this decomposition reaction. The 9,9-bis(4-hydroxyphenyl)fluorene polycarbonate and polyester30 and the copolymer of DGEBA with DGEBF at 36 mole % of the DGEBF content with 94 wt % of the gel fraction did not show decomposition at 270°C. Therefore it may be assumed that the first decomposition stage at 270°C in the cured DGEBF was due to its low degree of cure.

Figure 13 showed the char residue at 700°C as a function of the mole fraction of DGEPP in the copolymer with DGEBA. A linear relationship between char yield and the composition of DGEPP in the copolymer can be observed. A linear relationship between char yield and copolymer composition was also observed in the copolycarbonate of bisphenol A and phen~lphthalein.~~

For DGEBF copolymer with DGEBA there was no linear relationship between char yield and copolymer composition (Fig. 14). The char yield increased rapidly at a low content of DGEBF and reached a maximum value of 40 wt % at about

3116 LIN AND PEARCE

1 ° i 0 0 20 mole 40 of DGEFP 60 80 1 '0

Fig. 13. Char residue at 7OOOC as a function of mole fraction of DGEPP in the copolymers of DGEPP and DGEBA. Curing agent: TMB, 9.5 g/epoxy equiv.

0 0 20 LO 60 80 100

DGEBF (mole %)

Fig. 14. Char residue at 7OOOC as a function of mole fraction of DGEBF in the copolymers of DCEBF and DGEBA. Curing agent: TMB, 9.5 g/epoxy equiv.

20 mole ?6 of DGEBF before decreasing. A possible explanation for this effect is related to the low degree of cure when the DGEBF content was more than 30 mole % in DGEBF and DGEBA copolymers. Another possibility is that an unusual reaction occurred between the two monomer units to form a more thermally stable char structure a t high temperature. Nevertheless, the DGEBF changed the char yield so that it deviated from the predicted linear relationship and the DGEBF increased the char yield to an initial high value of 40 wt % at 20 mole % of DGEBF content a t 700°C. This indicated that DGEBF had some potential as a modifier for the more flammable conventional epoxy material DGEBA.

A similar mathematical treatment could also be applied to the flammability of the copolymer by assuming that the oxygen index of a monomer unit in its copolymer was proportional to the mole fraction in the copolymer if there were no unusual chemical interaction between the monomer units and the copolymers had high molecular weight. The total oxygen index should be equal to the sum


of individual values for each monomer:

01 = zxi01p (9) where 01 and OIp are the oxygen indices of the copolymer and the homopolymer i, and Xi is the mole fraction of monomer i in the copolymer. Figure 15 shows the linear relationship between the oxygen index and the mole fraction of DGEPP in its copolymer with DGEBA.

By combining eqs. (8) and (9) the relationship between oxygen index and char yield can be expressed as

01 = k l Y + k p (11)

where ki = (01; - OI$)/(Yy - Yq) and ka = 01: - Yqk1. Figure 16 shows an oxygen index as a function of the copolymer char yield of DGEPP with DGEBA. The solid line in this figure was calculated by using Yy = 0.24 and Y; = 0.46, obtained from Figure 14, and 01: = 0.205 and 01: = 0.430 taken from Figure 15 and eq. (11). In this manner the char yields and oxygen indices of this copolymer can be predicted from the values obtained in the two homopolymers and copolymer composition.

CONCLUSIONS

DGEPP resin was separated into three fractions by liquid chromatography. From the IR, UV, and proton NMR spectral analysis it was shown that the three fractions in this resin have the same unit structure, the 3,3-disubstituted phthalide, instead of a quinoid structure. The main differences among the three fractions of DGEPP resin were color and degree of polymerization. The average epoxy equivalent decreased with an increased initial feed ratio of epichlorohydrin to phenolphthalein. The weight fraction change in the first fraction (y), which is the monomer of DGEPP obtained from liquid chromatography, with the

Curing Agent . IMB,Y5 g,kpoxyeq

13 20 LO 60 80 100 DGEPP. (mole ‘A)

Fig. 15. Oxygen index as a function of the composition of DGEPP in the copolymer with DGEBA. Curing agent: TMB, 9.5 g/epoxy equiv.

3118

80

60

LIN AND PEARCE

. 7 _ _

‘ O 0 r

.

Char Yield , Y %.

Fig. 16. Oxygen index against char yield of the copolymers of DGEPP and DGEBA at 700OC.

variation in the initial feed mole ratio ( x ) can be generalized and represented by y = 1.0 -eax, where a is an arbitrary constant, -1.5 in this study.

The order of melt viscosity of the three resins was DGEBF > DGEPP > DGEBA. The melt viscosity activation energies for DGEBA, DGEPP, and DGEBF were 5.7, 13.5, and 13.6 kcal/mole. DGEBF can be crystallized from bulk or acetone-alcohol solution, and the crystal has a melting point of 132°C. The order of reactivity toward TMB for the three epoxy resins was DGEBA - DGEBF > DGEPP. The order of crosslinking density after curing at 135,180, and 22OOC for 3,3, and 3 hr, respectively, was DGEBA > DGEPP > DGEBF.

The DSC and TGA thermograms indicated that the three TMB-cured epoxy resins have the same decomposition temperature at about 400OC. The glass transition of a copolymer at a specific composition can be predicted from the Wood equation; it had a K value of 4 for the poly(DGEBA-co-DGEPP) system and 3.5 for the poly(DGEBA-co-DGEBF) system, The latter deviated from the Wood equation when the DGEBF content was higher than 36 wt 70 due to the lower degree of cure and crosslinking. If the char formation of each monomer unit in the copolymer were independent of one another or no unusual chemical interaction occurred between monomer units, the char yield of each monomer unit in the copolymer was proportional to its mole fraction Yi = Xi Yf. A linear relationship between char yield and the composition of DGEPP in the DGEBA and DGEPP copolymers was obtained. A linear relationship between oxygen index and the composition of the copolymers (DGEBA and DGEPP) was also found. The poly(DGEBA-co-DGEBF) showed a maximum char residue (40 wt %) at a DGEBF content of 20 mole %. The decreased char formation at DGEBF contents greater than 20 mole % in the copolymers of DGEBA and DGEBF may be due to a lower degree of cure.

The authors acknowledge support in part by the NASA Ames Research Center, Moffett Field, CA, under grant numbers NSG-2182 and NSG-2147, and the many helpful discussions with J. A. Parker, S. Riccitiello, and G. Fohlen.


References

1. F. J. Martin and K. R. Price, J. Appl. Polym. Sci., 12,143 (1968). 2. E. M. Pearce and R. Liepins, Enuiron. Health Perspectives, 11,69 (1975). 3. J. A. Parker, G. M. Fohlen, and P. M. Sawko, “Development of Transparent Composites and

their Thermal Responses,” paper presented at Conference on Transparent Aircraft Enclosures, Las Vegas, Nevada, February 5-8,1973.

4. V. V. Korshak, L. K. Solv’eva, and I. V. Kamenskh, Vysokomol. Soedin., 13,150 (1971). 5. H. Brunner and M. J. Waghorn, Br. Pat. 910,899 (1962); Chem. Abstr., 58,3564 (1963). 6. H. Lee and K. Neville, SOC. Pet. Eng. J., 16,315 (1960). 7. H. H. Chen and A. C. Nixon, Am. Chem. SOC. Diu. Org. Coat. Plast. Chem. Pap., 23(1), 221

8. H. H. Chen and A. C. Nixon, SOC. Pet. Eng. Trans., 5(2), 90 (1865). 9. J. W. Shepherd, U.S. Pat. 3,310,507 (1967); Chem. Abstr., 67,22,590 (1967).

10. H. L. Lee, US. Pat. 3,378,504 (1968); Chem. Abstr., 69,344 (1968). 11. D. Haworth and G. F. Pollnow, Z&EC Prod. Res. Deu., 1,185 (1962). 12. E. S. Lopata and S. R. Riccitiello, J. Appl. Polym. Sci., 19,1127 (1975). 13. E. S. Lo, Znd. Eng. Chem., 52(4), 319 (1960). 14. E. S. Lo, U.S. Pat. 3,015,647 (1962); Chem. Abstr., 56,8928 (1962). 15. E. S. Lo, U.S. Pat. 3,015,648 (1962); Chem. Abstr., 56,11793 (1962). 16. E. S. Lo, U.S. Pat. 3,008,925 (1962); Chem. Abstr., 56,4974 (1962). 17. S. N. Salazkin, L. I. Kormarova, and S. V. Vinogradova, Zzu. Akad. Nauk SSSR Ser. Khim.,

18. B. Everatt, A. H. Haines, and B. P. Stark, Angew. Macromol. Chem., 56,157 (1976). 19. D. W. Van Krevelen, Polymer, 16,615 (1975). 20. P. W. Morgen, Macromolecules, 3,536 (1970). 21. ASTM D 1652-73, Standard Method of Test for Epoxy Content of Epoxy Resins, ASTM,

22. General Electric Model CR380KFllA Fluid Flammability Test Kit, Test Procedure Manual,

23. H. Lee and K. Neville, Handbook of Epoxy Resins, McGraw-Hill, New York, 1967, pp,

24. R. N. Jones, C. L. Angel], T. Ito, and R. J. D. Smith, Can. J. Chem., 37,2007 (1959). 25. W. A. Schroeder, P. E. Wilcox, K. N. Trueblood, and A. 0. Dekker, Anal. Chem., 23,1740

26. E. S. Lopata and S. R. Riccitiello, J. Appl. Polym. Sci., 21,91 (1977). 27. M. A. Ring and W. S. Koski, J. Chem. Phys., 35,381 (1961). 28. L. A. Wood, J. Polym. Sci., 28,319 (1958). 29. M. S. Lin and E. M. Pearce, unpublished. 30. S. N. Lee and E. M. Pearce, unpublished.

(1963).

144 (1973).

28,341 (1975).

4541K 25-001C.

2-7.

(1951).

Received August 4,1978

Documents

Epoxy resins. II. The preparation, characterization, and curing of epoxy resins and their copolymers