ISSN 0012�5008, Doklady Chemistry, 2009, Vol. 428, Part 1, pp. 228–232. © Pleiades Publishing, Ltd., 2009.Original Russian Text © O.V. Startsev, L.I. Anikhovskaya, A.A. Litvinov, A.S. Kroto, 2009, published in Doklady Akademii Nauk, 2009, Vol. 428, No. 1, pp. 56–61.

228

Two main requirements are imposed on polymercomposites produced for manufacturing structuralelements for mechanical engineering. First, thesecomposites must have good mechanical characteris�tics, and second, the reached values of strength, mod�uli of elasticity, and other characteristics must notbecome noticeably worse after long functioning of thematerial in a construction [1]. In operation of machin�ery, polymer composites must be resistant to the actionof moisture, heat, atmospheric oxygen, mechanicalloads, and other factors, which is checked by long andcostly tests [2–5].

The best mechanical properties of polymer com�posites are reached by matching components andselecting optimal molding modes [6]. Usually, aftercompletion of the production cycle, the molded sheetsremain in a nonequilibrium state: they contain inter�nal stresses, low�molecular�weight products, remainsof binder components, open bonds, etc. [2, 4, 6]. Themechanical characteristics of anisotropic polymercomposites are known to depend on the degree of theinitial nonequilibrium [6, 7]. The degree of nonequi�librium is often purposefully increased if it is intendedto reach the best mechanical characteristics of thematerial in the initial state.

Under the action of external factors, the initialnonequilibrium relaxes [7–10]. Transition to an equi�librium state also leads to a change in the set ofmechanical characteristics [9, 10]. This regular relax�ation of the initial nonequilibrium should always betaken into account to avoid wrong conclusions in pre�dicting the lifetime of polymer composites in articles.However, this specific feature of polymer composites isoften ignored. The impairment of the mechanicalcharacteristics during aging tests is interpreted as theresult of accumulation of damages during physico�chemical transformations under the action of externalfactors. But analysis shows that the mechanical char�

acteristics change not because of aging but because ofthe relaxation of the initial nonequilibrium. In thiscase, the material may remain in an excellent state andcan function without further deterioration of the char�acteristics for a long time.

Let us illustrate this regularity by the example ofpredicting the moisture resistance of fiberglass�rein�forced plastics based on adhesive prepregs, namely,plastic composites based on VK�51 epoxy resin andglass fillers for airborne honeycomb structures withcontrolled strength and heat resistance [11, 12].

In the experiments, we determined the moisturesorption and diffusion parameters of these materialsunder conditions imitating the humid tropical cli�mate. Samples were moistened in an air medium in athermostat at a temperature of 60 ± 1°C and a relativehumidity of 98 ± 2% until the kinetic curve reached aplateau. To reduce the estimation error due to theuncontrollable initial moisture content, the samplesbefore the tests were dried until weight stabilization atthe same temperature 60 ± 1°С. The anisotropy offiberglass�reinforced plastics was taken into accountby varying the shapes and sizes of the samples. Themeasurement and data processing procedures weredescribed in detail previously [8–10].

A study of the samples treated in the humid atmo�sphere revealed that there is reversible plasticization ofthe epoxy binder, which is confirmed by a shift of theglass transition temperature toward lower tempera�tures and by a decrease in the dynamic shear modulus(Fig. 1).

In the bulk of the fiberglass�reinforced plastic, thebinder undergoes weak hydrolysis, which furthercauses binder extraction from the bulk of the plastic.This is confirmed by the fact that the weight lossin desorption exceeds the weight gain in sorption by0.8–1.5%.

In production of the fiberglass�reinforced plastic,internal stresses emerge at the interface because ofstrong adhesive bonds. These stresses relax duringmoisture saturation, which is proven by two features.

CHEMICALTECHNOLOGY

Increasing the Reliability of Predicting the Propertiesof Polymer Composites in Hygrothermal AgingO. V. Startsev, L. I. Anikhovskaya, A. A. Litvinov, and A. S. Krotov

Presented by Academician Al.Al. Berlin March 24, 2009

Received March 24, 2009

DOI: 10.1134/S0012500809090079

Altai State University, pr. Lenina 61, Barnaul, 656049 Russia

DOKLADY CHEMISTRY Vol. 428 Part 1 2009

INCREASING THE RELIABILITY OF PREDICTING THE PROPERTIES 229

First, the thickness of the samples that were moistenedand then redried to constant weight irreversiblyincreased by 0.5–0.8%, and second, the thermalshrinking in the direction of the warp of the fiberglasscloth in the initial samples transforms to ordinarythermal expansion after moistening and redrying ofthe fiberglass�reinforced plastic (Fig. 2).

Thus, the moisture plasticizes the binder and bringsthe fiberglass�reinforced plastic structure to a moreequilibrium state. Usually, under the conditions underwhich the structure and properties of a materialchange, moisture diffusion experiments reveal variousnonlinear effects [13]. Approximating such results,many researchers groundlessly complicate moisture

0 50 100 150 200T, °C

0

0.04

0.08

0.12

0.16 ∂G

/∂T

, G

Pa/

°C

46

86

87

5

3

1

0 50 100 150T, °C

G,

GP

a

1

2

0.010

0.008

0.006

0.004

0.002

0

∆L/L0

30 60 90 120 150 180 210T, °C

1

2

3

Fig. 1. Temperature dependence of the dynamic shear modulus of the fiberglass�reinforced plastic based on an adhesive prepreg(cloth T�10�80; binder based on epoxy diane resins ED�10, ED�20, and ED�22, modified by carboxyl resins [11, 12]): (1) initialdried samples and (2) moistened samples (moisture weight content 2.8%). The glass transition temperature was determined fromthe minimum of the derivative of the shear modulus with respect to temperature and is shown by numbers at the top of the figure.

Fig. 2. Thermal expansion in the direction of the warp of the fiberglass�reinforced plastic based on an adhesive prepreg: (1) initialdried samples, (2) maximally saturated samples, and (3) samples after moistening and subsequent drying.

230

DOKLADY CHEMISTRY Vol. 428 Part 1 2009

STARTSEV et al.

transfer models, dealing with only an ordinary transi�tion process. No wonder that this leads to diverse andsometimes contradictory conclusions [13].

Table 1 presents the coefficients of moisture diffu�sion in the fiberglass cloth warp direction for the fiber�glass�reinforced plastics cured at 135°С for 3 h atmolding pressures 0.5 MPa (plastic 1) and 1.0 MPa(plastic 2). In moisture transfer rate, plastic 2 is supe�rior to plastic 1 since the diffusion coefficient of theformer is three times as low as that of the latter. How�ever, during the first moistening, the structural differ�ences in the binder of the plastics obtained under dif�ferent molding pressures are leveled off and the mois�ture diffusion coefficient during the secondremoistening become equal. It is this parameter thatshould be considered basic for any comparative tests.

To confirm the significance of the effect of therelaxation of the initial structural nonequilibrium onthe moisture transfer, we performed several successivemoistening–drying cycles under the above conditions.In each cycle, the maximal moistened and completelydried states were reached. A total of five cycles of total

duration of 1725 days (4.7 years) were carried out. Fig�ure 3 demonstrates the changes in the dry weights ofplastics 1 and 2 for this period.

This example illustrates a typical general regularity.During the relaxation, i.e., during the moistening inthe first cycle, the residues of uncured components ofthe binder are hydrolyzed, and the low�molecular�weight reaction products are desorbed from the sam�ples during subsequent drying. However, with anincrease in the number of cycles, the weight loss isbasically decelerated.

If the state of a fiberglass�reinforced plastic afterthe first moistening–drying cycle is deemed equilib�rium, then, as Fig. 3 shows, the approach to approxi�mating experimental data changes. Conventionalexponential curves of weight loss versus time arereplaced by simple linear graphs. The conclusion ofthe hydrolytic stability of the binder also fundamen�tally changes. First, this stability is determined as veryhigh because the prediction for ten�year operationperiod using a linear dependence indicates only a 1%decrease in the dry weight of the samples both for plas�tic 1 and plastic 2. Second, a change in the moldingpressure of a fiberglass�reinforced plastic does notaffect its hydrolytic stability. The conventionalapproach would lead to wrong conclusions since thetotal change in the weight of the samples of plastics 1and 2, providing the transition process, are different(Fig. 3).

Another good illustration is the results of studyingthe moisture resistance of polymer composites in themost sensitive parameter—shear modulus G in sheetplane. Methodological details of the experiments were

0

−0.5

−1.0

−1.5

−2.0

−2.5

Experimental points for plastic 1 Exponential dependence for plastic 1'

Linear dependence for plastic 1

Linear dependence for plastic 2

Exponential dependence for plastic 2'

Experimental points for plastic 2

7 years 10 years

2 '

1 '

1

2

0 500 1000 1500 2000 2500 3000 3500 4000Time, days

Rel

ativ

e dr

y w

eigh

t, %

Fig. 3. Weight loss for plastics 1 and 2 after completion of moistening–drying cycles.

Table 1. Comparison of the coefficients D (103 cm2/s) ofmoisture diffusion in the fiberglass cloth warp direction forfiberglass�reinforced plastics 1 and 2 during the first andsecond moistening

Number of a moisten�ing–drying cycle

Fiberglass�reinforced plastic

plastic 1 plastic 2

1 25.0 7.7

2 37.6 35.3

DOKLADY CHEMISTRY Vol. 428 Part 1 2009

INCREASING THE RELIABILITY OF PREDICTING THE PROPERTIES 231

described previously [9, 10]. We performed hygrother�mal cycling of 17 variants of fiberglass�reinforced plas�tics obtained while varying molding temperature from120 to 170°С and press curing time from 0.5 to 6 h.The G values were recorded for four moistening–dry�ing cycles, each lasting about 320 days.

As Fig. 1 shows, the moistening is accompanied byordinary plasticization leading to a decrease in theshear modulus of the samples. After drying in each ofthe four cycles, the shear modulus is partially restored.Table 2 summarizes the results. They indicate thatmuch of the decrease in G occurs after the first moist�ening.

Let us take into account that, during the firstmoistening–drying cycle, the initial nonequilibriumrelaxes. Therefore, after the second and next cycles,the degree of irreversible changes in the binderbecomes insignificant and the G values stabilize. Thisregularity is common for all the samples studied.

After four moistening–drying cycles, for all thefiberglass�reinforced plastics tested, the averagedecrease in G is 28% as compared to that in the initialstate. Conventional examination would admit that thisfiberglass�reinforced plastic has insufficient hydrolyticstability and is unfit for long operation in a humidmedium. However, it was established above that, dur�ing the first moistening–drying cycle, the composite

reaches an equilibrium state. It is in this equilibriumstate that the composite occurs throughout the subse�quent tests. Therefore, the properties of plastic com�posites during aging should be compared with those ofthe samples after the first moistening–drying cycle,rather than the initial samples. In this case, as Table 2indicates, the average decrease in G is only 12%. Thisphenomenon characterizes all the samples of fiber�glass�reinforced plastics as a material that is stable to along action of an aggressive humid medium. Table 2shows that one can select such molding modes inwhich the decrease in the shear modulus for almostfour years of hygrothermal action does not exceed5⎯10%.

Thus, in predicting the moisture resistance of poly�mer composites for a long period and selecting optimalmolding modes, the relaxation of the initial structuralnonequilibrium of materials must be taken intoaccount.

REFERENCES

1. Trudy nauchno�prakticheskoi konferentsii “Problemysozdaniya novykh materialov dlya aviakosmicheskoiotrasli v 21 veke” (Proceedings of Scientific and PracticalConference “Problems of Creation of New Materials forAerospace Industry in 21 Century”), Moscow: VIAM,2002.

Table 2. Shear modulus G (GPa) in sheet plane of a fiberglass�reinforced plastic in hygrothermal cycling

Molding conditions

Initial state Cycle 1 Cycle 2 Cycle 3 Cycle 4

∆G between samples, %

Time, h Tempera�ture, °C

between ini�tial sample

and sample af�ter four cycles

between sam�ples after first

and fourthcycles

1 120 7.03 5.75 5.45 4.94 4.7 33.1 18.2

3 120 6.41 5.39 5.44 5.14 4.8 25.1 10.9

6 120 7.49 6.40 5.73 5.72 5.6 25.2 7.3

1 130 6.71 5.85 6.22 6.01 5.5 18.0 6.0

3 130 6.93 5.51 5.52 5.16 5.1 26.4 9.1

6 130 6.86 5.74 5.52 5.45 5.1 25.7 11.1

1 140 7.07 5.89 5.97 5.21 4.8 32.1 17.1

3 140 7.37 6.11 5.65 5.52 5.0 32.2 18.2

6 140 7.13 6.31 5.79 5.45 5.1 28.5 19.0

0.5 150 7.37 6.24 5.98 5.63 5.5 25.4 11.9

1 150 7.28 6.30 5.62 5.51 5.5 24.5 12.7

1.5 150 7.95 7.15 7.11 6.44 6.5 16.4 9.1

3 150 7.77 6.50 5.67 5.45 5.2 33.1 20.0

0.5 170 6.49 5.61 5.16 5.00 4.8 26.0 14.4

1 170 6.80 5.82 4.67 5.07 5.0 26.5 14.1

1.5 170 7.07 4.75 4.47 4.49 4.5 36.4 5.3

2.5 170 6.61 4.74 4.61 4.50 4.4 33.4 7.2

232

DOKLADY CHEMISTRY Vol. 428 Part 1 2009

STARTSEV et al.

2. Startsev, O.V., Extended Abstract of Doctoral (Techn.)Dissertation, Moscow, 1990.

3. Startsev, O.V. and Nikishin, E.F., Mekhan. Kompoz.Mater., 1993, vol. 29, no. 4, pp. 457–467.

4. Startsev, O.V., in Polymer Yearbook 11, Pethrick, R.A.,Ed., Glasgow: Harwood Academic, 1993, pp. 91–109.

5. Startsev, O.V., Krotov, A.S., and Golub, P.D., Polym.Degrad. Stab., 1999, vol. 63, pp. 353–358.

6. Polymer Matrix Composites, Shalin, R.E., Ed., SovietAdvanced Composites Technology Series, Ser. 4, Frid�lyander, J.N. and Marsball, I.H., Ser. Eds., London:Chapman and Hall, 1995.

7. Issoupov, V.V., Startsev, O.V., Krotov, A.S., and Vein�Inguimbert, V., J. Polym. Comp., 2002, vol. 6, no. 2,pp. 123–131.

8. Startsev, O.V., Kuznetsov, A.A., Krotov, A.S., Ani�khovskaya, L.I., and Senatorova, O.G., Fiz. Mezomekh.,2002, vol. 5, no. 2, pp. 109–114.

9. Filistovich, D.V., Startsev, O.V., Kuznetsov, A.A., Kro�tov, A.S., Anikhovskaya, L.I., and Dement’eva, L.A.,Dokl. Phys., 2003, vol. 48, no. 6, pp. 306–308; Dokl.Akad. Nauk, 2003, vol. 390, no. 5, pp. 618–621].

10. Startsev, O.V., Filistovich, D.V., Kuznetsov, A.A., Kro�tov, A.S., Anikhovskaya, L.I., and Dement’eva, L.A.,Perspekt. Mater., 2004, no. 1, pp. 20–26.

11. Fridlyander, I.N., Senatorova, O.G., Anikhov�skaya, L.I., and Sidel’nikov, V.V., in Teoriya i praktikatekhnologii proizvodstva izdelii iz kompozitsionnykhmaterialov i novykh metallicheskikh splavov XXI v. Trudymezhdunarodnoi konferentsii (Theory and Practice ofProduction Technologies of Items Made of CompositeMaterials and New Metal Alloys XXI Century, Pro�ceedings of International Conference), Moscow:Mosk. Gos. Univ., 2001, pp. 122–129.

12. Fridlyander, I.N., Senatorova, O.G., Anikhov�skaya, L.I., Sidel’nikov, V.V., and Dement’eva, L.A.,Entsiklopediya “Mashinostroenie,” vol. II�3: Tsvetnyemetally i splavy. Kompozitsionnye metallicheskie materi�aly (Encyclopedia “Machine Building,” vol. II�3:Nonferrous Metals and Alloys. Composite MetalMaterials), Moscow: Mashinostroenie, 2001.

13. Water Transport in Synthetic Polymers, Iordanskii, A.L.,Stratsev, O.V., and Zaikov, G.E., Eds., New York: NovaScience, 2003.