Anhydride Based Epoxy

Fracture Toughness and Impact Strength of Anhydride-Cured Biobased Epoxy

Hiroaki Miyagawa, Manjusri Misra, Lawrence T. DrzalComposite Materials and Structures Center, 2100 Engineering Building, Michigan State University,East Lansing, Michigan 48824-1226

Amar K. MohantyThe School of Packaging, 130 Packaging Building, Michigan State University,East Lansing, Michigan 48824-1223

Biobased neat epoxy materials containing functional-ized vegetable oils (FVO), such as epoxidized linseedoil (ELO) and epoxidized soybean oil (ESO), were pro-cessed with an anhydride curing agent. A percentageof diglycidyl ether of bisphenol F (DGEBF) was re-placed by ELO or ESO. The selection of the DGEBF,FVO, and an anhydride-curing agent resulted in anexcellent combination to produce a new biobased ep-oxy material having a high elastic modulus and highglass transition temperature. Izod impact strength andfracture toughness were significantly improved de-pendent on FVO content, which produced a phase-separated morphology. POLYM. ENG. SCI., 45:487– 495,2005. © 2005 Society of Plastics Engineers

INTRODUCTION

The importance of environmentally friendly naturalproducts for industrial applications has become radicallyclear in recent years with increasing emphasis on environ-mental issues, waste disposal, and depletion of non-renew-able resources. There is a growing trend toward developingand commercializing renewable resource–based polymers,which have the advantage of eco-friendliness. Renewableresource–based polymers can replace/substitute for fossil-fuel based polymers on a cost-performance basis.

Petroleum-derived epoxy resins are known for theireasy processability, compositional versatility, superiortensile strength, high stiffness, excellent electricalstrength, and exceptional solvent resistance. Further-

more, cured epoxy resins have good resistance to heatand chemical attack. The chief drawbacks of epoxy resinsare their high cost. Therefore, modifying epoxy resins hasbeen of intense research interest. A tougher, flexiblecrosslinked material can be obtained by incorporating aflexible epoxy resin, curing agent, or reactive additivesinto the network during curing. In other words, the tough-ness of epoxy resin can be improved through a blend withsome functionalized vegetable oils (FVOs). Indeed, thebiobased modifiers have been around for quite some timeand have been utilized to modify other thermoset poly-mers [1–9]. There have been a number of studies regard-ing the improvement of the fracture properties of rubber-toughened and petroleum-based epoxy materials [10 –13]. It was demonstrated that the presence of a secondphase of rubber particles in the epoxy matrix enhancedplastic/shear deformation, which dissipated fracture en-ergy, and consequently, the impact strength and tough-ness were significantly greater with the addition of therubber phase.

United States agriculture produces more than 16 billionpounds of soybean oil annually, only 500 million pounds ofwhich is used in industrial application. Linseed oil is alsoavailable abundantly across the world. FVOs such as ep-oxidized soybean oil (ESO) and epoxidized linseed oil(ELO) are now commercially available in large volume forapplications such as coatings and plasticizer additives. Value-added applications of FVOs will provide benefits to theagricultural industry.

In this paper, biobased neat epoxy materials contain-ing different FVOs were prepared by using an anhydridecuring agent, and the fracture and impact properties ofthese new biobased epoxies are measured and discussed.Fracture and impact failure surface morphologies areobserved by scanning electron microscopy, and the ex-perimentally measured properties are correlated with dif-ferent surface morphologies.

Correspondence to: L. Drzal; e-mail: [email protected] grant sponsors: National Science Foundation Partnership forAdvancing Technologies in Housing (NSF-PATH), Michigan State Uni-versity’s Research Excellence Fund.DOI 10.1002/pen.20290Published online in Wiley InterScience (www.interscience.wiley.com).© 2005 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—2005

EXPERIMENTAL

Materials

Diglycidyl ether of bisphenol F (DGEBF, ResolutionPerformance Products, Houston, TX, epoxide equivalentweight � 172) was used as the main component.30�50 wt% of DGEBF was substituted with the sameweight of 2 different FVOs, ELO (Vikoflex� 7190, Atofina,Booming Prairie, MN; epoxide equivalent weight � 176) orESO (Vikoflex� 7170, Atofina; epoxide equivalent weight� 228). The mixture of DGEBF epoxy and FVO wasprocessed with an anhydride curing agent, methyltetrahy-drophthalic anhydride (MTHPA, Aradur™ HY 917, Hunts-

man Advanced Materials Americas Inc., Brewster, NY;equivalent weight � 159) and 1-methylimidazole accelera-tor (DY 070, Huntsman Advanced Materials AmericasInc.). The amount of curing agent was theoretically calcu-lated to maintain proper stoichiometry.

Dynamic Mechanical Analysis

Dynamic mechanical properties were conducted with aTA Instruments DMA 2980 operating in the three-pointbending mode at an oscillation frequency of 1.0 Hz. Theamplitude and static force of the DMA measurements were75 �m and 1N, respectively. DMA specimens were in theform of rectangular bars of nominal 2.0 mm � 15 mm � 50mm. Data were collected from ambient to 190 °C at ascanning rate of 2 °C/min. The glass transition temperature,Tg, was assigned as the temperature where loss factor was amaximum. A minimum of three specimens of each compo-sition were tested.

Izod Impact Strength

The Izod impact strength was measured for neat epoxyand clay/epoxy nanocomposites at room temperature. Izodimpact specimens with the same dimension as prescribed inASTM D 256 standard were tested with a 453 g pendulum.The dimensions of the notched Izod impact specimens were63.5 mm (length) � 12.7 mm (width) � 10 mm (thickness),with a notch depth of 1.5 mm having a 0.25 mm radius. Allspecimens were cast in silicone molds having the samedimensions. The specimen was held as a vertical cantileverbeam and was impacted on the notched face by a singleswing of the pendulum. Therefore, the crack propagatedfrom the tip of the notch. A minimum of 5 specimens foreach composition were tested.

Fracture Testing

Compact tension (CT) specimens were prepared for frac-ture testing. The crack length a, the width W, and thethickness B of the CT specimens were determined as 10mm, 20 mm, and 5 mm, respectively, based on the ASTMD 5045 standard, as in the previous studies. The notch wasat first made by a band saw, and then the sharp initial cracktip was produced by a guillotine crack initiator and a freshrazor blade. The crack length was measured by opticalmicroscopy after fracture testing. The experiments wereperformed with a crosshead velocity of 15 mm/min to loadthe CT specimens. The fracture toughness was measuredwith at least 3 specimens for each different material. Thecritical energy release rate was calculated from the fracturetoughness in plane-strain state by using the storage modulusmeasured by DMA and assuming the Poisson’s ratio of 0.34for all neat epoxies.

FIG. 1. DMA profiles of DGEBF and biobased neat epoxies containingELO or ESO. (a) Storage modulus. (b) Loss factor.

488 POLYMER ENGINEERING AND SCIENCE—2005

Scanning Electron Microscopy

The fracture surfaces of different biobased epoxy nano-composites were observed with scanning electron micros-copy (SEM). A JEOL 6300 SEM with field emission fila-ment or a JEOL 6400 SEM with LaB6 filament inaccelerating voltage of 10 kV was used to collect SEMimages for all samples. A gold coating, which is a fewnanometers thick, was made on the fracture surfaces to aidin feature resolution.

RESULTS AND DISCUSSION

Dynamic Mechanical Analysis

Figure 1 shows the temperature dependency curve ofstorage modulus and loss factor of anhydride-cured epoxycontaining 30�50 wt% ELO or 30 wt% ESO. In Fig. 1a, thestorage modulus below the glass transition temperature de-creased after the addition of ELO or ESO. In Fig. 1b, onlyone peak appeared in the temperature range between 30 °Cand 150 °C for each epoxy material. The symmetric peak ofthe loss factor, tan �, in Fig. 1b indicates complete cure ofthe anhydride-cured epoxy matrix.

Figure 2 shows the storage modulus at 30 °C, which ismeasured by DMA, of the DGEBF and the biobased neatepoxies cured with the anhydride curing agent, MTHPA. InFig. 2, for example, ELO 50 and ESO 30 indicate that the 50wt% DGEBF was replaced by ELO, or 30 wt% DGEBF wasreplaced by ESO, respectively. After 30�50 wt% DGEBFwas replaced by the same amount of ELO or ESO, thestorage modulus of the processed anhydride-cured biobasedneat epoxies showed an up to 20% decrease. When 50 wt%ELO or 30 wt% ESO was used to replace DGEBF, the

decrease of the storage modulus after processing the anhy-dride-cured biobased neat epoxies was approximately thesame. Therefore, it can be concluded that ELO provideshigher storage modulus when compared with the sameamount of ESO. The storage modulus measured by DMAwas found to be an unbiased estimator of the elastic mod-ulus that was measured by mechanical testing. It had alreadybeen confirmed that the storage modulus was almost thesame as the tensile modulus for anhydride-cured epoxy,because of the lack of plastic behavior in the stress-strain

FIG. 3. Storage modulus at (Tg�30) °C and crosslink density of anhy-dride-cured biobased neat epoxies measured by DMA.

FIG. 4. Izod impact strength of anhydride-cured biobased neat epoxies.

FIG. 2. Storage modulus at 30 °C and glass transition temperature ofanhydride-cured biobased neat epoxies measured by DMA.

POLYMER ENGINEERING AND SCIENCE—2005 489

diagram [14]. Therefore, the storage modulus measured byDMA is used to calculate the critical energy release ratefrom the fracture toughness in the plane-strain state.

Figure 2 also shows the glass transition temperature,which is assigned as the peak position of loss factor curvemeasured by DMA, of the DGEBF and the biobased neatepoxies cured with MTHPA. In Fig. 2, the storage modulusof the processed biobased neat epoxies showed an approx-imately 7�9 °C decrease, after 30�50 wt % DGEBF werereplaced by the same amount of ELO or ESO. In general, a

higher glass transition temperature is obtained when themain component has a lower epoxide equivalent weight andhigher epoxy functionality, because this results in a highercrosslink density [15]. Indeed, ELO has a lower epoxideequivalent weight and higher epoxy functionality (that istheoretically �7) than ESO, whose epoxy functionality istheoretically �6. Therefore, comparing ELO and ESO, itcan be concluded that it is easier to maintain the high glasstransition temperature of the biobased neat epoxy whenELO is used as a partial replacement for DGEBF.

FIG. 5. SEM micrographs of different impactfailure surfaces of anhydride-cured neat epoxies.(a) DGEBF at high magnification. Scale bar � 2�m. (b) Biobased epoxy containing 50 wt%ELO at high magnification. Scale bar � 2 �m.(c) Biobased epoxy containing 30 wt% ESO atlow magnification. Scale bar � 20 �m. (d) Bio-based epoxy containing 30 wt% ESO at highmagnification. Scale bar � 1 �m.


The crosslink density of anhydride-cured biobasedneat epoxies was evaluated by DMA. Figure 3 shows thestorage modulus at (Tg�30) °C and the crosslink density.Interestingly, the storage modulus at (Tg�30) °C did notchange with the substitution with 30�50 wt% ELO, inspite of the slight reduction with 30 wt% ESO. By usingthe glass transition temperature shown in Fig. 3, thecrosslink density can be calculated from the followingequation:

E� � 3�eRT

where, E’, ve, R, and T are the storage modulus at (Tg�30)°C, crosslink density, gas constant (8.314 J/(K � mol)), andtemperature in K, respectively. Although the crosslink den-sity for anhydride-cured ELO was constant with 30�50wt% of ELO, the slight decrease in the crosslink density wasobserved with 30 wt% ESO.

FIG. 5. (Continued)


Izod Impact Strength

Figure 4 shows the Izod impact strength of the anhy-dride-cured DGEBF and biobased neat epoxies. The anhy-dride-cured rigid epoxy sample has a high crosslink density;therefore, the value of the Izod impact strength was rela-tively low. Comparing the DGEBF with the biobased neatepoxies containing 30�50 wt% ELO, the Izod impactstrength was almost the same. As discussed in Fig. 2, aglassy epoxy material, having high glass transition temper-ature, was obtained after replacing 30�50 wt% of DGEBFwith ELO, although the glass transition temperature slightlydecreased. On the other hand, the Izod impact strength wasimproved more than 25% when 30 wt% of DGEBF wasreplaced by ESO.

It was already discussed in Fig. 2 that the storage mod-ulus and the glass transition temperature were almost thesame for both ELO and ESO neat epoxies. To investigatethe difference of the Izod impact strength of the anhydride-cured biobased epoxies, it is necessary to observe the mor-phology of the impact failure surfaces by SEM. Figure 5shows SEM micrographs of the impact failure surfaces ofthe anhydride-cured DGEBF and biobased epoxy materials.In Fig. 5a, the failure surface of the anhydride-curedDGEBF was completely flat and featureless. This suggeststhat the behavior of the anhydride-cured DGEBF was linearelastic, and the crack propagated in a planar manner underthe impact loading, although several small pieces of resinwere found on the failure surface. In Fig. 5b, the failuresurface of the anhydride-cured biobased neat epoxy con-taining 50 wt% ELO was similar to that of the DGEBF,although the failure surface was slightly rougher than that ofFig. 5a. Therefore, the behavior of the anhydride-curedbiobased epoxy containing 30�50 wt% ELO was also lin-ear elastic, and thus the crack propagated in a planar mannerunder the impact loading.

In contrast, the failure surface of the anhydride-curedbiobased neat epoxy containing 30 wt% ESO was muchrougher, and a larger number of the small resin pieces werefound on the failure surface in Fig. 5c. Figure 5d is a highermagnification SEM micrograph of the same failure surfaceof the anhydride-cured biobased neat epoxy containing 30wt% ESO. The regions, indicated with arrows in Fig. 5d, areESO-rich rubber phases. The presence of a second phase isclearly evident in Fig. 5d. It is known that the rubber-toughened epoxy shows much larger improvement on theIzod impact strength, although the solid particle–reinforcedepoxy often showed lower Izod impact strength than theneat epoxy. It is likely that the main epoxy matrix is morerigid than the separated ESO phase, since 100% ESO hasmuch lower storage modulus at room temperature thanDGEBF neat epoxy. Indeed, the loss factor curve for theanhydride-cured biobased neat epoxy containing 30 wt%ESO showed only one peak between 30 and 170 °C in spiteof the phase separation, as discussed in Fig. 1b. This sug-gests that the glass transition temperature of the rubber ESOphase is below 30 °C. The anhydride-cured biobased neat

epoxy containing 30 wt% ESO was not transparent, al-though the anhydride-cured DGEBF and biobased neat ep-oxies containing 30�50 wt% ELO were transparent. Inother words, the lack of the transparency was the result ofthe phase separation. As discussed in Fig. 2, ELO has higherepoxy functionality than ESO. In addition, ELO has lowermolecular weight. Consequently, ELO has higher polaritythan ESO and hence ELO has better solubility and compat-ibility with polar DGEBF [1], while ESO has a largerpossibility to create phase separation than ELO.

The size of the ESO rubber phase was measured to be d� 250�650 nm in Fig. 5d. The void-like feature of the ESOrubber phases was created by distortional pullout of therubbery particles under the impact loading. A much greaterenergy is dissipated to pull out rubber phases because ofexcellent adhesion in the epoxy/rubber interfaces. There-fore, the anhydride-cured ESO neat epoxy having the phaseseparation showed more than 25% higher Izod impactstrength. The change in the crosslink density can also affectthe Izod impact strength. In other words, it was found inFigs. 2 and 3 that the glass transition temperature and thecrosslink density slightly decreased, and this can result inincreased Izod impact strength. However, the contributionof both the decreased glass transition temperature andcrosslink density is small, since the range of these decreaseswas quite small. The DGEBF and ELO neat epoxies that didnot have any phase separation exhibited a lower impactstrength.

Fracture Testing

Figure 6 shows examples of load-crack opening dis-placement (COD) curves of the DGEBF and the biobasedneat epoxies. Non-linearity was not observed in the load-

FIG. 6. Typical examples of load-COD diagram of anhydride-curedbiobased neat epoxies. DGEBF: a � 8.03 mm, W � 21.4 mm, B � 6.55mm; ELO 50: a � 7.73 mm, W � 21.3 mm, B � 5.84 mm; ESO 30: a� 5.24 mm, W � 21.5 mm, B � 6.06 mm.


COD diagrams of both the anhydride-cured DGEBF andELO neat epoxies. Therefore, the maximum load was usedto evaluate fracture toughness. For the anhydride-curedESO neat epoxy, slight non-linearity was observed in theload-COD diagram after COD reached approximately 2.0mm. However, the critical load could not be defined as theintersection of the load-COD diagram and 5% offset linebecause of the linearity in the load-COD diagram. There-fore, the maximum load was used to determine the fracturetoughness of the anhydride-cured ESO neat epoxy. It shouldalso be noted that the size of the plastic zone for all neatepoxies is small enough based on the linear fracture me-chanics, since the load-COD diagram is linear in Fig. 6.

Figure 7 shows the fracture toughness of the anhydride-cured DGEBF and biobased neat epoxies. As seen in Fig. 7,the anhydride-cured ELO neat epoxy showed almost thesame value as the fracture toughness of the anhydride-curedDGEBF epoxy. In contrast, the anhydride-cured ESO neatepoxy showed extremely high fracture toughness, whencompared with the DGEBF and ELO neat epoxies. This wasa result of the presence of a second rubbery phase. This isfurther explained with SEM micrographs.

The fracture properties can also be discussed with thecritical energy release rate shown in Fig. 7. The anhydride-cured neat ELO epoxy has slightly smaller storage modulusthan the DGEBF, as discussed in Fig. 2. Therefore, thecritical energy release rate of the ELO neat epoxy wasslightly higher than that of the DGEBF. In the four differentanhydride-cured epoxies, the ESO neat epoxy has the larg-est critical energy release rate, and was more than 10 timesas large as that of the DGEBF, after 30 wt% of DGEBF wasreplaced by ESO. This large improvement was also seen asa much larger integrated area of the load-COD diagram of

the anhydride-cured ESO neat epoxy, when compared withthe DGEBF and ELO neat epoxies. The improvement ratioof the critical energy release rate with ESO was much largerthan that of the Izod impact strength, because of time-temperature superposition. Under impact conditions, a veryfast loading is applied, resulting in polymer behavior similarto low-temperature fracture.

Figure 8 shows the SEM micrographs of the fracturesurfaces of the anhydride-cured DGEBF and biobased neatepoxies. In Fig. 8a and b, the fracture surface was com-pletely flat. This suggests that the anhydride-cured DGEBFand ELO neat epoxies are brittle and the load-COD diagramwas almost completely linear elastic as discussed in Fig. 6.Hence, the crack propagated in a planar manner, and aminimal fracture surface area was created by the crackpropagation. Minimal fracture surface area means minimalconsumption of the energy for crack propagation. On theother hand, in Fig. 8c, the fracture surface of the anhydride-cured ESO neat epoxy was extremely rough, as the highcritical energy release rate was observed in Fig. 7. Thisfeature in Fig. 8c was clearly distinctive, when comparedwith the completely flat fracture surfaces of the anhydride-cured DGEBF and ELO neat epoxies, which did not havethe second phase as shown in Fig. 8a and b. The roughersurface is identical for dissipating more energy that is due toshear deformation during the crack propagation. It wasreported that the addition of the rubber particles to epoxycould cause (i) localized cavitation in the rubber or therubber/epoxy interface and (ii) plastic shear yielding [10–12]. The change in the crosslink density can also affect thecritical energy release rate. In other words, it was found inFigs. 2 and 3 that the glass transition temperature and thecrosslink density slightly decreased, and this can result inthe increased critical energy release rate. However, thecontribution of both the decreased glass transition temper-ature and crosslink density is too small to obtain 10 timesgreater critical energy release rate, since the decreases ofboth the glass transition temperature and the crosslink den-sity were quite small. For the epoxy, the critical energyrelease rate in Mode II, crack shearing mode, was approx-imately 10 times larger than that of the same epoxy in ModeI, crack opening mode [16]. The ESO rubber phase ob-served by SEM, as shown in Fig. 8c, has the same role aspreviously reported for petroleum-based rubber-toughenedepoxy, and the sight non-linearity for the anhydride-curedESO neat epoxy in Fig. 6 was caused by the plastic shearyielding. As a result, the critical energy release rate wasimproved almost 10 times after 30 wt% DGEBF was re-placed by ESO. Griffith explained that a crack is propagatedwhen the strain energy reaches the certain value, and as aresult new fracture surface is created [17]. A large fracturesurface requires more strain energy to be released as thecrack propagates. Consequently, the fracture surface area islarger when the critical energy release rate is larger. Someenergy was dissipated by the plastic deformation in front ofthe crack tip, since the plastic zone always exists in front ofthe crack tip. However, as discussed in Fig. 6, the size of the

FIG. 7. Fracture toughness and critical energy release rate of anhydride-cured biobased neat epoxies.


plastic zone for the biobased neat epoxy containing 30 wt%ESO is small enough, since the load-COD diagram is almostlinear. As a result, it can be concluded that most of theenergy was dissipated during the crack propagation. Theexperimental results shown in Fig. 7 agreed well with themorphological observations in Fig. 8.

CONCLUSIONSThe DGEBF and biobased epoxies were processed with

an anhydride curing agent. The fracture and impact proper-ties were measured for 4 different anhydride-cured epoxies.The fracture toughness and the critical energy release rate ofthe anhydride-cured epoxy were greatly improved with the

FIG. 8. SEM micrographs of different fracture surfaces of anhydride-cured neat epoxies. (a) DGEBF. Scale bar� 50 �m. (b) Biobased epoxy containing 50 wt% ELO. Scale bar � 20 �m. (c) Biobased epoxy containing 30wt% ESO. Scale bar � 20 �m.


addition of ESO. The improvement of both the impactstrength and the fracture toughness was the result of a phaseseparation of the ESO into rubbery particles. The existenceof the rubber ESO phases added a significant amount ofenergy to the crack propagation process. The fracture andfailure surface morphology correlated well with the exper-imental measurements of the critical energy release rate.

ACKNOWLEDGMENTS

The authors are grateful to National Science FoundationPartnership for Advancing Technologies in Housing (NSF-PATH) 2001 Award No. 0122108 and Michigan State Uni-versity’s Research Excellence Fund for partial financialsupports. The authors are also thankful to Atofina, BoomingPrairie, MN, and Huntsman Advanced Materials AmericasInc., Brewster, NY, for supplying samples.

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FIG. 8. (Continued)


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Anhydride Based Epoxy