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to make-even on a laboratory scale-an ‘ E glass com- pletely free from alkali to see what strength would be developed in the fibres in an epoxide laminate.

At the other end of the series, increasing amounts of soda would bring about a situation where an even lower strength would be developed in the fibres, and this does seem an unreasonable assumption.

There is one difficulty in trying to relate the strength to the chemical composition of the glasses in the four cases which have been studied so far. “A” glass differs from the other three glasses in that it has no B,O, in it. This means that the network structure, in this case, must be mainly a silica one, while in the other cases it must be a more complex one containing some boron, giving rise to borosilicate ions in the surface.

So far, in this review of the results which have been obtained it has been assumed that it is the glass surface which is affecting the resin, but it does not follow that this must be so. It could equally well be that the reverse condition is true-namely that the resin system is in some way attacking the glass.

However, no support can be found for either of these hypotheses. Inquiries have failed to discover any infor- mation as to whether a glass surface contaminated with excess sodium ions would affect the cure of an epoxide resin (11). Though it is known that glass surfaces are readily attacked by many chemicals, we know of no evidence to suggest that any of the components of the epoxide resin/curing agent systems is capable of attacking the glass.

The evidence for a relationship between the strength developed and the Na,O or CaO content is sketchy, as

sufficient glasses of similar composition have not yet been studied.

Finally, it seems clear that whatever the cause, the fault must lie entirely at the interface between the resin and the glass. There is no evidence to suggest that the body of the glass is attacked. The mass of the resin is obviously not affected, as there is no evidence of in- hibition or resin degradation.

Literature References 1. Brookfield, Pickthall and Warburton, S.P.I. 16th

Annual Technical and Management Conference, Reinforced Plastics Division, February ( 1961).

2. Thomas, Physics & Chemistry of Glasses, 1, Feb- ruary, 1960.

3. Prebus & Mitchener, Industrial G Engineering Chemistry 46, 147 (1959).

4. Private communication, Pilkington Brothers Limited, England.

5. Weisbart, Kunststoffe-Rundschau, October, 431 (1959).

6. Zachariasen, J.A.C.S. 54, 3841 (1932). 7 . Warren & Biscoe, 3. Am. Cerarn. SOC. 21, 259

(1938). 8. Warren & Biscoe, 3. Am. Ceram. SOC. 21, 257

( 1938). 9. Yates & Trebilcock, S.P.I. 16th Annual Technical

& Management Conference, Reinforced Plastics Di- vision, February (1961).

10. Williams & Weyl, Glass Znd. 26, 275 (1945). 11. Private communication, Shell Chemical Co., Ltd.,

England.

Comments on the Paper Paul C. Yates

E. 1. du Pont de Nemours & Co. Inc.

First I would like to congratulate the authors on their presentation of an interesting and thought-provoking paper. Their work emphasizes the surprisingly large effects of apparently minor variations in the composition of the resins and of the glasses used on laminate strength. It also calls attention to the existence of interactions w-hich show that a particular glass can be satisfactory when used with certain resins, but very much less so with others. The authors have thus re-emphasized to us that a fiberglass-resin laminate is a complex structure containing two solid phases as well as an interface, and that all three are important. The types of interactions between them disclosed in the present paper are a useful alarm signal against attempts to over-simplify the prob- lem.

This work is a continuation and an expansion of a similar study presented by the present authors in con- junction with R. s. Warburton at last year’s meeting. In that paper, the authors gave the experimental and mathematical techniques they have employed to calcu- late the stress applied to the glass as a function of the

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total stress applied to the laminate. It is interesting to discusss the results of the present paper in terms of the final derived equation of their previous paper. For a fixed loading of glass of a given type and a fixed test specimen size, it is possible to express Equation (9) of the previous paper in the following terms:

P/S, = K + CE, where

P = the stress applied to the laminate S, = the resulting stress on the glass K and C are constants and E, is the tensile modulus of the resin.

The equation is expressed in the above form to empha- size the key role pla ed by the elastic modulus of the resin in determining Y aminate strength. If it is assumed that some critical stress for catastrophic failure exists in the glass phase, it can be seen that the corresponding value of applied stress which can be borne by the laminate will be determined by the value of E,.

SPE TRANSACTIONS, OCTOBER, 1962

In the previous paper, the authors used a value of 5 x 1oj as the tensile modulus for Epicote 828 cured in thc manner described. In the present paper, the authors do not clearly state what value they used for the modulus of the various modified epoxy resins. The extent to which the modulus changed as a function of the curing agent was also not mentioned. If the authors have assumed that the modulus did not change as a function of the changes in resin type and curing conditions, but actually the modulus did change substantially, the observed variations in the apparent breaking stress of the glass phase might simply be the result of the varying modulus in the resin phase.

A very interesting paper on the relationship of the mechanical properties of resins to their utility in laminates was presented by H. S. Loveless at last year’s meeting. It was entitled “Study of Toughness of Poly- ester Resins: A Utility Index for Resins used in Glass- Reinforced Plastic Laminates”. Dr. Loveless showed results obtained from a parallel study of the mechanical properties of a series of polyester resins and the strength of laminates formed from them. As a part of this study, he determined the Young’s modulus of cast resins of varying compositions. He found variations in modulus ranging as high as a factor of 10, from values of about 0.6 x lo‘, to 5.8 x 1 0 . If similar differences occurred in the present work as a result of the changed formulations of epoxy resins used, or the variations in curing agents employed, the observed apparent variations in the strength of glass would probably be accounted for. In- cidentally, Dr. Loveless’ work showed that the higher the Young’s modulus of the polyester resins he studied, the higher the strength and toughness of the resulting laminate. This appears to provide experimental confir- mation of the essential correctness of the mathematical treatment of the present authors.

The modulus of resins and its changes with conditions should be studied more fully than it has been previously. For example, the modulus of cellulose acetate drops by a factor of about 10-fold after this resin is exposed in a 90% relative humidity chamber at 85°F. for 48 hours. This was reported by Robert Burns.” Such large changes in modulus as a result of exposure to humid conditions, when considered in the light of the equation given above, might easily account for a considerable portion of the deterioration of the strength of laminates occur- ring under these conditions. A study of the variations of resin modulus as a function of the types of curing agents used, and the curing conditions employed, might therefore provide valuable guidance in selecting those best suited for laminating work. Such a study might also identify whether molecular weight, the number and type of crosslinks, the type of functional groups present, or other fundamental parameters controlled resin modulus. A small amount of such work has been done on thermoplastics, but very little on thermosetting resins.

Another interesting aspect of the authors’ paper was their observation that glass composition strongly in- fluenced the strength of laminates prepared with epoxy resins. Indeed there seemed to be a very consistent increase in strength as the glass composition was varied to substitute calcium oxide for sodium oxide. The

*Modern Plastics, 21, 111-112. (1943).


amount of the strength increase was much greater than would be expected from the differences in strength of the base-glasses tested alone, and greater than the differences in strength between laminates of these same glasses using polyester resins. I t will be recalled that the previous paper by these authors also called attention to this difference. Even greater differences in the strength of the laminates when substituting alkali glass for “E” glass were noted between epoxy and polyester resins if the testing were done after boiling, rather than dry.

I found this of particular interest after I read Dr. Weisbart’s paper on “Investigations of the Strength of Alkali-containing Glass Fibers”, which is also being presented at this symposium. Dr. Weisbart found relatively small differences, in the most favorable cases, between the strength of laminates prepared from alkali- containing fibers and “E” glass fibers. However, Dr. Weisbart’s work was done almost entirely with resins of the polyester type. It therefore seems that his con- clusions are not generally applicable to all resins, but only to the particular type he studied.

These relatively specific interactions between the type of resin and the type of glass used may be caused by the difference in the chemical nature of the glass- resin interface, as suggested by the authors. However, their comments that the surface differences may not reflect the differences in the bulk phase composition, at least to the same degree, are well taken. Some experimental evidence is available for calculating the approxi- mate surface composition of glasses having various oxides present as constituents of typical soda lime-type glasses. Such effects have been measured by Dietzel and Lyon, for example. They found that for a given glass composition, the contribution of a small amount of a particular oxide to the surface tension of the melt could be treated as an additive property. As shown in Figure 1, there is a good correlation between the effect of these oxides on surface tension and the molar volume of that oxide in the glass phase. Figure 2 shows the ratio of the activity of the particular oxide in the surface phase to its activity in the bulk phase. This calculation assumed ideal behavior in the surface region. I t was assumed that the contribution to the surface energy for each oxide was equal to the product of the surface mole fraction and the surface energy of the pure oxide. Al- though most of the oxides obviously do not exhibit ideal behavior, deviations from ideality do not usually appear to exceed a factor of 3- or 4-fold. Such deviations might be explained in terms of the heat of compound forma- tion in the glass melt. In any event, the results indicate that there is at least reasonable relationship between changes in composition in the bulk phase, and those in the surface phase. Thus, the increasingly lower strength obtained as sodium oxide is substituted for calcium oxide may be at least partially due to corresponding changes in the composition of the interface and poorer bonding or more extensive water attack on the sodium- rich surface. This, however, would not explain why the effect is so much more pronounced in epoxy resins than in polyester resins.

An interesting additional contribution factor may be a stress-induced chemical reaction between water vapor and sodium ions. This is suggested by the results ob-

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tained by J. 0. Outwater and Oguzzan Ozaltin in a paper to be presented at this symposium, entitled “The Surface Effects of Various Environments and of Thermo- setting Resins on Glass”. In addition to experiments showing that the strength of glass is sensitively influ- enced by the condition of its surface, this paper shows that, at least with relatively perfect surfaces obtained by HF etching of Pyrex glass rods, there is no indication of serious attack on the glass by a wide variety of liquid environments until the glass is combined with the resin. Even then, if the glass is cured with a resin and a good surface finish, it is found that its strength is usually in- creased by the treatment. It is also found that this strength increase is maintained after boiling. If, however, the surface finish is omitted, a substantial decreuse in strength occurs upon boiling, especiaZEy with the epoxy resin glass composlte. It was further noted that this particular combination, which lost the most strength as the result of boiling, also exhibited the best adhesion.

Studies reported to this symposium last year in papers by I. M. Daniel and A. J. Durelli, and entitled “Photo- elastic Investigation of Residual Stresses in Glass-Plastic Composites”, and by D. C. West and J. 0. Outwater, entitled “The Stress Distribution in the Resin of Rein- forced Plastics”, showed that there are tensile stresses in the resin which result from a differential shrinkage of the resin compared to the glass. These tensile stresses are of the order of several thousand pounds per square inch at room temperature. The existence of such tensile stresses in the resin implies a corresponding radial compressive stress in the outer layers of the glass fibers. The greater the adhesion of the resin to the glass surface, the greater the difference in the thermal coefficient of expansion of the resin and the glass, and the higher the temperature of cure of the resin, the higher will be these compressive stresses in the outer portions of the glass fiber. Such compressive stresses will tend to act as an additional driving force for any chemical reaction which can substitute small atoms such as protons for larger atoms, such as sodium ions. This substitution of protons in the glass structure for sodium ions is a possible ex- planation for the pronounced weakening effects of water vapor on glass fibers.

Studies of the activation energy of the fatigue pro- cesses in the breaking of glass which occur in the presence of humid atmospheres or liquid water, but which do not occur in the absence of water, have shown that they are similar to the activation energy for migration of sodium atoms in the glass structure.

It therefore seems possible that glasses which con- tain large amounts of sodium ion, when combined with highly adhesive resins such as epoxy resins, undergo a reaction between the sodium ions in the glass situated with 5 to 10 atom-layers of the surface and water vapor in their surroundings. The stresses induced in the glass promote the replacement of sodium ions by protons or water molecules. This replacement creates weakened areas in the fiberglass, which may serve as crack nucle- ation sites. Such a reaction would also lower the surface energy of the newly created surface sites, and allow an easier propagation of the cracks.

Resins which did not impose high stresses on curing and glasses poor in sodium ions would not show the effects of such weakening.

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To summarize, this idea of the breaking of glass as a kind of stress-corrosion cracking, promoted by stress- gradients which induce reactions between water vapor and cations in the glass is one which is being intensively studied by people interested in the strength of glass. Extension of these studies to include their importance in fiberglass-resin laminates would appear to be in order.

In this regard, it is of interest that Prof. McGarry of MIT has recently been studying the magnitude of stresses and their distribution in glass fibers of laminates. He found radial compressive stresses of the same order of magnitude as the tensile stresses previously observed by Dr. Outwater in the resin phase, but extremely high longitudinal tensile stresses which approach the ultimate strength of the glass. The interaction of such extremely high stresses with water vapor under conditions of loading to failure of the laminate would seem to offer a very fruitful field for further study. Studies of the internal friction of laminates with and without applied loads, at different temperatures, and in the presence and absence of water vapor might help to indicate the exact nature of the process by which water vapor contributes to their failure. Nuclear magnetic resonance studies of laminates before and after boiling and during testing to look at changes in the environment of bound water and of protons might also be of considerable value.

Author’s Reply Paul Yates is quite correct in re-calculating our Equa-

tion (9) of the 1961 paper which shows the importance of the tensile modulus of the resin.

12.82(D + d ) P W. W, E,.

S, (lbf/in*) = -+-..- G R E,

We have realized the importance of this and have al- ready noted, in other work which we have done, the large variation which can occur under different curing conditions. Unfortunately, it was necessary for us to take the same value throughout all our calculations where standard resins were used, but we employed different values for the modified resins and, in this case, data sup- plied by the resin manufacturers were used in our calculations.

We will stress once again that the degree of extensibility is of great importance, but that this is not an end in itself. It is essential that the high extensibility be combined with correspondingly high tensile strengths for the resin, otherwise it seems to us that unexpected results will be obtained.

We were very interested in two further points which Paul Yates made and we are fully in agreement with him in his remarks about the strains which will set up in the cured structure by the different thermal expan- sions of the resin and the glass. I t is quite clear that there must be great stresses being set up in the cured laminate, and if steps could be taken to prevent this we feel that it would be possible to obtain higher strengths in the laminates.

Finally, the question of requiring good overall adhesion, or only point adhesion, is a very important one, and one which could be the subject of considerable work in the future. THE END


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