In the coatings and adhesive industries, elastomers are used primarily because of their remarkable durability and elasticity derived from the partially unsaturated polymer backbone. However, the very source of the desired polymer properties makes these elastomers very susceptible to attack by oxygen, ozone and degradation by way of UV light with subsequent loss of adhesion or other coating properties. Drastic changes can occur in the physical properties of the polymers and coatings by only minor structural modification produced by UV light. As a result, a considerable amount of interest exists in better characterizing the nature of this effect and protecting the exposed surfaces of coatings and adhesives against the attack by UV light.

The absorption of UV light by the polymer provides the energy to break key molecular bonds, such as C=C, C–H, C=O, near the surface of the exposed coating and create free radicals. These free radicals react with oxygen and form peroxy radicals, which attack the polymer molecules in the coating. The absorption of UV light, therefore, induces both oxidative degradation and crosslinking. Antidegradants are used to protect the polymer against this problem.

Extensive work has been done on the effect of UV light on polymers. However, most of the studies have been concerned with the investigation of the changes that occur in the physical properties of the polymers and are not actually directed at obtaining the details of the microstructures. The selection of antidegradants are mostly based on trial and error without consideration of the possible changes that may occur at the structural level of the polymer, which may cause subsequent undesired changes in coating and adhesive properties.

Reflection infrared spectroscopy has been instrumental in showing the changes that occur on the molecular level of the polymer when coated on the substrate.^1-6 This article analyzes the microstructures of natural rubber coated on a metallic surface and attempts to understand and define structural changes that can occur when the polymer is attacked by UV light. This information could be helpful in preventing elastomer-based coatings and adhesives from damage caused by UV light.

This article provides evidence that reinforcing fillers, which play an important role in improving durability and cohesion forces of the system, are also able to protect natural rubber against UV attack. Because of the similarity in structure between natural rubber and many other elastomers, the structural information we deduce from the analysis of this system can be transferred to other elastomer-based coating and adhesive systems.

Composites of synthetic natural rubber, SNR, 99% cis-1,4-polyisoprene, with different reinforcing and non-reinforcing fillers were prepared by mill compounding the filler into the rubber by specific technique developed at Clifton Adhesive. Three different non-black fillers and eight different grades of carbon black with particle sizes ranging from 90 nm (non-reinforcing grade black) to 18 nm (most reinforcing grade black) were used in this study. For each composite, the level of the filler was kept constant at 5 parts per hundred rubber.

Solutions of each composite in toluene were prepared separately. In order to obtain information about the molecular orientation and conformational changes of the polymer on the substrate, each solution was coated on a steel substrate (all coatings having an equal thickness). Infrared reflection spectra of each coated sample was taken (using Buck Scientific Model 500 Spectrometer) before and after exposure to the UV light source. The UV light source was Spectronics Model ENF-260C Spectroline irradiating at 365 nm. The external reflection technique has been discussed elsewhere.¹ The data was stored in the computer for further analysis.

Results and Discussion

UV light initiates free radical oxidation at the exposed surface of the product to generate a layer of oxidized rubber. Moisture and heat can then initiate crazing of the surface, which subsequently can be abraded off. The degradation of unsaturated elastomers is an autocatalytic, free-radical chain reaction, which can be viewed in the corresponding formula.

Discussions will first center on the effect of carbon black on the UV light stability of natural rubber when coated on the substrate and then the effect of non-black fillers will be discussed.

One way to stabilize polymers to photochemical degradation is by the addition to the polymer of compounds, which readily absorb luminous energy and transform it to another type of energy. The new form of energy is expected to be harmless to the polymer. The increase in stability of polymers to photochemical degradation, caused by carbon black, therefore, is thought to be due to the ability of the carbon black to absorb light waves in the ultraviolet and visible ranges and transform luminous energy into thermal energy. The infrared reflection spectra results reported here, however, reveal that in addition to the color of the pigment, its structure and surface area also play an important role in protecting the polymer against UV light and photodegradation.

The reflection spectra of the synthetic natural rubber (SNR) coated on a steel substrate before and after it was irradiated for 12 hours and 24 hours under UV light are compared in Figure 1. There are some obvious spectral changes with irradiation. These changes are mainly the appearance of absorption bands at 1725 cm^-1 attributed to the stretching vibration of C=O carbonyl groups, at 3440 cm^-1 due to stretching vibration of –OH hydroxyl group and at 1160 cm^-1 due to C-O group.^7,8 These chemical changes are indicative of oxidative degradation and the accompanying chain scission.

Figure 2 shows the reflection spectra of SNR filled with two different grades of carbon black, coated on metallic substrates, after exposure to UV light for 24 hours. The spectra represent changes that occur upon irradiation for incorporation of an equal amount of different grades of carbon black in natural rubber. The most striking feature of the infrared spectra in Figure 2 is the absence of the hydroxyl and carbonyl bands for the rubber filled with Vulcan-XC72R (abbreviated as XC72R) carbon black upon irradiation and significant development of the same bands for the rubber filled with the same amount Thermax 907 carbon black upon irradiation. These changes will be discussed shortly.

Nitrogen surface area (specific surface area which is determined by nitrogen absorption capacity) and DBP (the dibutyl phthalate absorption, which is a measure of carbon black aggregate structure) are both a measure of the carbon black’s surface area.

Recently we have shown¹ that for natural rubber-carbon black composites, when laminated between two top coated steel substrates, the lap shear increases with increasing both carbon black nitrogen surface area and DBP absorption values. In that study the adhesion to the topcoat (primer), PC4426, developed at Clifton Adhesive, did not fail nor did the adhesion of the topcoat to the steel. The measured lap shear, therefore, was the measure of the cohesion forces in the system. These cohesion forces are increased by addition of the reinforcing carbon black grades to the rubber.

Our study showed¹ that the lap shear increases are more pronounced and better correlated with increasing DBP absorption values than they are with increasing nitrogen surface areas. The plot of the measured lap shear vs. carbon black DBP absorption values for the synthetic natural rubber-carbon black composites is shown in Figure 3. This figure indicates that the XC72R is a strongly reinforcing grade and Thermax 907 is a weakly reinforcing grade carbon black.

From Figure 2, it is interesting to note that the active carbon black, XC72R, added to the polymer is able to protect the polymer from UV light degradation for 24 hours (under severe irradiation conditions). The rubber filled with the same amount of non-reinforcing grade carbon black, Thermax-907 when exposed to UV light for 24 hours (under the same conditions) had extensive degradation. This is evidenced by significant development of bands at 1725 cm^-1 (C=O) and 3440 cm^-1 (O-H) for the SNR-Thermax 907 system and the absence of the same absorption bands for the SNR-XC72R system upon irradiation.

Previously, we have shown that the conformation and packing structure of natural rubber changes when carbon black is compounded into the polymer.¹ Also, we showed that these conformational changes vary with varying the type of the carbon black in the composite. In that study, we provided some evidence that reinforcing carbon black will not participate in any crosslinks within the elastomer network and most likely the carbon black primary or high structure aggregates reinforce the rubber structure by formation of entanglement network and mechanical interlocking forces. This conclusion is consistent with the recently reported¹ H NMR relaxation results for natural rubber filled with carbon black.⁹ Carbon black is composed of clusters of fused prime particles called primary aggregates. The primary aggregates composed of many prime particles, with considerable branching and chains, are referred to as a high structure black. From the infrared data, therefore, it appears that a high structure carbon black such as XC72R can be present on the elastomer surface and prevent degradation by UV light.

The correlation between infrared spectra and lap shear measurements indicate that the properties of both primary aggregates and spherical particles comprising them are important controlling factors in carbon black performance against UV-light degradation. Higher surface areas, as imparted by finer prime particle carbon blacks, require more energy for wetting and dispersion and they tend to “tie up” more rubber per unit weight of carbon resulting in stiffer compounds than with coarser blacks. The stiffer polymer appears to be more resistant to the UV-light exposure.

To examine the effect of non-black reinforcing fillers on UV stability of the polymer, three different fillers, amorphous fumed silica (SiO₂), titanium dioxide (TiO₂), and calcium carbonate (CaCO₃), were used. Fumed silica with fine particles is the most reinforcing and TiO₂ and CaCO₃ are poorly reinforcing fillers.

Figure 4 shows the comparison of the reflection spectra obtained after 12 hours’ UV light irradiation on SNR and SNR filled with the three non-black fillers. The progression of hydroxyl and carbonyl peak increase for each sample reflects the extent of degradation of the polymer upon the irradiation. Although TiO₂ has a higher hiding power than SiO₂, it is interesting to note that in Figure 4, after 12 hours irradiation, the SNR-fumed silica composite is less susceptible to attack by UV light than SNR-TiO₂. The incorporation of CaCO₃ also is not able to protect the polymer from UV light degradation, as it is evidenced from Figure 4.

The fumed silica, which had very fine particle sizes, has reinforcing properties. The interaction between the filler, which has high surface area particles and the polymer, therefore, apparently results in a loss of preferred conformation of the polymer, conceivably because the polymer forms a strongly absorbed surface layer. This absorbed surface layer makes the polymer less susceptible to attack by UV light.

It should be noted that the decision of antidegradants to further protect the polymer against the attack by UV light is also tied to the type of the interactions that could exist between the antidegradant and polymer or between the antidegradant and the filler used in the system. Evidently, the proper selection of the antidegradant and its interactions with the polymer and reinforcing filler is key in the stabilization mechanism of the system against UV attack. Improper selection of the antidegradant can be partially responsible for the destabilization of the system and coating defects by inducing unstable conformation.

Conclusion

Evidence has been provided that, in addition to the color of the pigment, structure and surface area play an important role in protecting the polymer against UV light. The interaction between the filler and the polymer and the conformation of the filled elastomer is key in stabilizing the system against UV light degradation. The reinforcing fillers, black and non-black, which play an important role in improving durability and cohesive forces of the system, are also able to protect the natural rubber against UV attack.

The interaction between the reinforcing filler, which has high surface area and the polymer results in a loss of preferred conformation of the polymer, conceivably because the polymer forms a strongly absorbed surface layer. This absorbed surface layer makes the polymer less susceptible to attack by UV light.

For more information on elastomer-based coatings and adhesives, contact Clifton Adhesive Inc., Burgess Place, Wayne, NJ 07470; phone 973/694.0845; fax 973/694.5678.