Cracking in curing elastomers.

A problem which may occur with sealants used in construction is cracking related to joint movement while curing. Construction sealants are neutral elastomers curing with air moisture. The hardening can last several hours, especially when humidity is low.

Thermal expansion of construction materials leads to joint expansion and contraction with temperature changes. J.M. Klosowski (ref. 1) has shown that the temperature differences can reach 80 [degrees] C on building walls between day and night. Expansion joints can move up to 25%, even 50% in extreme conditions.

Slow curing and joint movement lead to cracks in the seal. These cracks can lead to premature failure of the seal. Some sealants fail during movement, while some others do not crack. No clear relationship has been established between sealant properties and elongation at crack. M. Koike (refs. 2 and 3) studied seal properties after cracking for polysulfides, polyurethanes and silicones. E.A. Collins, C.H. Chen and J.P. Padolowski (ref. 4) characterized viscoelastic properties of sealants in relation to their performance. D.R. Flackett (ref. 5) described the effect of strain on silicone sealants and related crack resistance to rate of surface curing.

Silicone sealants are condensation elastomers based on polydimethylsiloxanes, silane crosslinkers and mineral fillers. The curing proceeds at room temperature by hydrolysis of organosilicon bonds followed by condensation of silanol with organosilicon bonds under the action of tin or titanium organometallic compounds. Silica fillers reinforce the elastomers through hydrogen bonding, between silanol groups on the silica surface and the polysiloxane chains.

Experimental

The cracking behavior is usually characterized by elongation where the first cracks appear. The elongation test was performed at 23 [degrees] C and 50% relative humidity, with an elongation rate of 50%/min. The test samples, 12 mm width x 8 mm depth at center x 100 mm length, were prepared between two steel substrates, on a closed cell polyethylene backer rod.

Several silicone sealants were used for this study, including Rhodia Silicones' acetoxy sealant 3B, oximino sealant 7B and alcoxy sealant 5C.

Results and discussion

Few previous studies focused on influence of curing time before elongation. It is, however, a key parameter. To follow the cracking versus time, the sealant sample was partially cured and then strained. The curve in figure 1 shows that the elongation at crack quickly decreases to a low value.

[Figure 1 ILLUSTRATION OMITTED]

The silicone sealants used contain polydimethylsiloxanes endcapped with silane crosslinkers. The polymer reactive end-groups are hydrolyzed with air moisture, which slowly diffuses in the sealant from the exposed side. The silanols groups condense with reactive end groups and build a three-dimensional network (figure 2).

[Figure 2 ILLUSTRATION OMITTED]

Three different crosslinkers were used, including an acetoxysilane releasing acetic acid, an oximinosilane releasing an oxime and an alcoxysilane releasing an alcohol. Acetoxysilanes hydrolyze and condense faster that oximinosilanes which are faster than alcoxysilanes (figure 3).

[Figure 3 ILLUSTRATION OMITTED]

The three sealants present different cracking behaviors. All can show cracks but the curves - elongation at crack versus curing time - are different (figures 4-6).

[Figures 4-6 ILLUSTRATION OMITTED]

The acetoxy sealant cures very quickly. It can only crack for a short period between one to thirty minutes; the elastic properties quickly develop in 30 minutes. The oximino sealant is slower; its brittleness period is longer, between one and 240 minutes before showing elastic properties. The alcoxy sealant is even slower. Its sensibility to strain is high from the beginning to several hours.

The oxime system is an intermediate case. Similar general behavior is observed for acetoxy or alcoxy systems if the time scale is changed. It is compressed for the acid product, which leaves quickly the brittleness period. The scale is expanded for the alcohol system, which can crack during a long time.

We can also observe differences between the minimum elongation values. For the acetoxy sealant, the transition between a viscous liquid with a yield point to an elastomer is fast. The product quickly becomes elastic. This prevents a low minimum elongation value. The oxime minimum is lower than the alcohol minimum. The cure rate due to the crosslinker does not justify this difference which is more related to formulation differences.

Just after application, the sealants do not crack. They are still viscous liquids. A simple flow is observed. Upon surface exposure to moisture, they quickly enter into a state where the material cracks if strained. The elongation at crack is low. Then the sealants become elastic; they are able to resist to strain and the cracks appear for higher elongation. The curves show two changes: First, a viscous liquid which fluidity decreases, this is the brittle period; then, a solid which cannot flow but can stand strain with an elastic stretching. This is the end of the brittle period.

Hypotheses can be stated. For the uncured sealant, resistance to cracking is related to elastic limit and yield point. The extension is not elastic anymore. If the stress exceeds the elastic limit, the product cracks. If the material cohesion is high enough, the stress reaches the yield point and the product flows. During curing, the yield point increases and exceeds the elastic limit. The network is brittle, close to the gel point. When chains percolate, the liquid with a yield point becomes an elastic solid.

A. Zosel (ref. 6) observed similar behavior when studying adhesive energy failure. He reported that fracture energy has a maximum related to crosslinking. Before the gel point, the behavior is a liquid with cohesive failure. During crosslinking, the adhesion goes through a maximum and decreases because cohesion increases and rupture becomes adhesive. The energy goes through an optimum between good extensibility and high modulus.

We observed that the brittleness period depends on the rate of crosslinking. Fast reaction reduces the period length. A mathematical model can explain this influence.

To evaluate crosslinking density in a sealant, J. Falender (refs. 7 and 8) used a gas diffusion equation and considered a first order reactivity.

He obtained the following equations where:

C = water concentration;

[C.sub.8] = water concentration in atmosphere;

D = water diffusion constant in sealant = 1.8 x [10.sup.-3] [cm.sup.2] .[min.sup.-1];

K = reaction rate constant;

t = time;

L = sealant thickness (figure 7); and

[Figure 7 ILLUSTRATION OMITTED]

Q = crosslinking density.

The rate of water concentration increase in the sealant is due to the water diffusion reduced by water consumption.

dC/dt = dCdiff/dt + dCcons/dt = [Dd.sup.2]C/[dx.sup.2] - KC

The crosslinking density can be calculated:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

(The formulation is shown in figure 8).

[Figure 8 ILLUSTRATION OMITTED]

Crosslinking densities were calculated for two model systems. A reaction rate constant of one [min..sup.-1] was used to model a fast sealant and a constant of 0.01 [min..sup.-1] was used to model a slow sealant. The crosslinking profiles calculated for 100 h. curing time show clear differences in figure 9. When the reaction is fast compared to diffusion, there is a diffusion control. As soon as a water molecule diffuses in the uncured sealant, it reacts. A front of fully crosslinked elastomer progresses in the sealant like a piston. On the other hand, when the reaction is slow compared to diffusion, there is a reaction control. The water molecules diffuse before reacting. This is a progressive curing leading to a broad crosslinking gradient.

[Figure 9 ILLUSTRATION OMITTED]

The rate of curing of acetoxy sealant is faster than oximino, which is faster than alcoxy. By using K = 1, K = 0.2 and K = 0.05 [min..sup.-1], it is possible to model their curing. It is estimated (ref. 7) that a crosslinking density of 1 mole.[m.sup.-3] corresponds to the network at skin over time or tack free time. A full crosslinking corresponds to 10 to 50 mole.[m.sup.-3], depending on the molecular weight of the precursor polymer.

It is possible to correlate the model to the cracking of sealants. The curing of an acetoxy sealant and of an alcoxy sealant is illustrated in figures 10 and 11. The hypothesis that we use is that the network is brittle when the crosslinking density is between 0.1 and 1 mole.[m.sup.-3]. Below 0.1 mole.[m.sup.-3], the sealant flows; above 1 mole.[m.sup.-3] it can be extended.

[Figures 10-11 ILLUSTRATION OMITTED]

We previously saw that the brittle period was very short for an acetoxy sealant and that it was longer for the alcoxy sealant. The crosslinking profile models can explain the difference. For a short air exposure time of t = 0.025 h., the fast curing sealant has crosslinked on a small thickness, there is no noticeable crosslinking for the slow curing sealant in figure 12. The crosslinking density is between 0.1 and 1 mole.[m.sup.-3]. It is brittle. It cracks under strain. The crack is localized at the surface since the deeper layer can still flow. We have a shallow crack for a short curing time. On the other hand, for a longer exposure time of t = 0.5 h., the slow sealant reaches a crosslinking density corresponding to brittleness on a much thicker section in figure 12. A deep crack is obtained under strain. For the same exposure time, the fast sealant does not crack anymore since the surface layer has reached a crosslinking density above 10 mole.[m.sup.-3], giving elastic properties.

[Figure 12 ILLUSTRATION OMITTED]

The experimental observations correspond to the models as shown on figure 13. The acetoxy sealant presents surface cracks after a few minutes and then resists to strain. The alcoxy sealant leads to deeper cracks later and for a longer period.

[Figure 13 ILLUSTRATION OMITTED]

Conclusions

The cracking of silicone sealants is explained by the brittleness of the partially cured elastomers. The elongation at crack is related to the liquid-solid transition. Crack elongation reaches a minimum at the cure beginning and gradually increases while crosslinking. A crosslinking mathematical model based on water diffusion and hydrolysis reaction confirms the influence of reaction rate and explains cracking differences between sealants.

References

(1.) J.M. Klosowski in "Sealants in Construction," M. Dekker Inc. Ed., N.Y. & Basel, 183 (1989).

(2.) M. Koike, K. Tanaka and Y. Munakata, Report of the Research Laboratory of Engineering Materials, Tokyo Institute of Technology , 4, 173 (1979).

(3.) M. Koike, Report of the Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 8, 167 (1983).

(4.) E.A. Collins, C.H. Chen and J.P. Padolowski, Journal of Rheology 32 (2), 163, (1988).

(5.) D.R. Flackett, "Science and Technology of Building Seals, Sealants, Glazing and Waterproofing," Second Volume ASTM 1142, J.M. Klosowski Ed., ASTM, Philadelphia (1992).

(6.) A. Zosel, J. Adhesion 30, 135 (1989).

(7.) J. Falender, ACS Polymer Preprints 20 (2), 467, (1979).

(8.) W. Schoenherr and J. Falender, ACS Polymer Preprints 22 (1), 190, (1981).