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Chlorination of NR: surface and failure analyses in a post-vulcanization bonded composite. (Cover Story).

Normally, elastomers are bonded to rigid substrates during vulcanization. But sometimes, there are advantages to bonding after the rubber article has already been cured. For instance, post vulcanization (PV) bonding makes sense when assemblies contain large, complex inserts that cannot be practically handled in the press. Vulcanization bonding requires lower temperatures and longer times to achieve the highest quality. On the other hand, injection molding processes favor highly efficient (EV), higher temperature cures. Thus, molding techniques generally recommended for good bonding contradict the trends in injection processing (ref. 1).

Frequently, pretreatment of polymers is required to achieve satisfactory levels of adhesion, particularly when the polymers have low polarity. Polyolefin films, for instance, are flame or corona treated prior to printing. Fluorinated polymers such as PTFE are etched with sodium naphthenate prior to bonding. Similarly, non-'polar elastomers often require pretreatment prior to PV bonding in order to create a surface acceptable for bonding. As molded, the elastomer surface may contain adhesive materials that create a weak boundary layer. Waxes, for instance, are often used as antioxidants. Surface treatments for elastomers range from solvent wiping, to mechanical abrasion, to chemical modification. Among the techniques for chemical modification are cyclization with concentrated sulfuric acid and chlorination. Several methods for chlorination are practiced: Exposing the surface to chlorine gas; immersion in acidified sodium hypochlorite; and treatment with solutions of organic chlorinating agents such as trichloroisocyanuric acid (TCI) (figure 1).

[FIGURE 1 OMITTED]

Chlorination and oxidation of various elastomers, including natural rubber (NR), have been described in the literature. Brewis and Mathieson recently reviewed methods for a range of elastomers, concluding that pretreatment with TCI is especially effective on diene-type elastomers (ref. 2). Pastor-Bias, et. al., used IR spectra, SEM and contact angle in their study on SBR (reft 3). Chlorination with TCI led to improved adhesion with a polyurethane adhesive due to a combination of surface modifications including:

* Mechanical (surface roughening);

* thermodynamic (increased surface energy); and

* chemical (removal of adhesive substances, creation of polar groups).

Oldfield and Symes estimated the thickness of the chlorinated layer by energy-dispersive x-ray analysis of cross sections, concluding that chlorine penetration increased with increasing concentration of TCI. In this study, chlorination was accomplished by wiping the surface of the rubber with a tissue soaked in a TCI solution in ethyl acetate. The maximum penetration in a commercial NR formulation was about 5 [micro]m (ref. 4). These authors also reported that, based on TCI concentration, the failure mode varied from fine-grained, shallow tearing in the rubber to a deep, chunky appearance with large amounts of torn rubber. Cutts reported similar failure patterns (ref. 5).

In our study, chlorination with TCI was selected as a pretreatment prior to PV bonding to glass-filled polyamides. Three surface techniques were employed to investigate the chlorination process: Contact angle measurements; reflective FTIR; and electron microscopy with x-ray analysis. While the bonding process itself is considered proprietary, our findings regarding the chlorination process are complementary to those published in the literature. In particular, we found that the pretreatment process plays a key role in the durability of NR/polyamide composite assemblies.

Experimental

Materials

NR plaques (150 mm x 150 mm x 2 mm) were compression molded according to ASTM D3182. The natural rubber compound was based on SMR CV 60 containing paraffin wax, paraffinic oil, carbon black and a typical antioxidant package. It was sulfur cured with a mixed accelerator package consisting of 4-morpholino-2-benzothiazole disulfide (MBS) and N, N'-dibutylthiuram disulfide (Butyl Tuads). Chemlok 7701 (Lord Corp.), a 2% solution of TCI in ethyl acetate, was used as received. Diiodomethane and formamide (Aldrich) and distilled water were used as received for contact angle measurements.

Chlorination process

Strips of rubber approximately 5 mm wide by 75 mm long were cut from the cured slab and then dipped in a beaker containing approximately 250 ml TCI solution for specified times ranging from one to ten minutes. After dipping, the solvent was allowed to flash dry. No further treatment of the surface was performed.

NR/polyamide assembly

Assemblies were prepared from the same natural compound. Following molding, the rubber insert was immersed in the TCI solution for five minutes. A commercially available rubber-to-metal bonding agent was used. The process for forming the bonded rubber/polyamide article is considered proprietary.

Surface energy calculations

Surface energy calculations are relatively straightforward, but provide the least level of detail regarding the physicochemical nature of the surface. Contact angle measurements were taken on a Model 2500 Video Contact Analyzer from AST, Inc. At least eight angles were measured. Standard deviations for each set of contact angle data were below 2 [degrees]. The acid-base model was used to calculate surface energy:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where [THETA] is the contact angle of the test fluid, [[gamma].sup.d] refers to the dispersive portion of the surface energy, [[gamma].sup.+] refers to the acid portion of the surface energy, and [[gamma].sup.--] refers to the basic portion of the surface energy.

FTIR spectroscopy

Fourier transform infrared (FTIR) spectra were recorded on a Perkin Elmer BX II using Spectrum software with a horizontal multiple bounce attenuated total reflectance (HATR) accessory using a zinc selenide crystal at an incident angle of 45 [degrees]. Single bounce ATR spectra of the bulk and failed rubber surfaces from a failed assembly were gathered using a MIRacle accessory with a silicon crystal.

SEM/EDS

Environmental scanning electron microscopy with energy dispersive x-ray spectroscopy (ESEM/EDS) was performed on a FEI Philips XL 30 ESEM-TMP with EDAX Phoenix EDS. Secondary electron images result from interaction of the electron beam with the outermost layer of the sample. No additional sample preparation was required.

Quantitative analysis of the elemental composition with treatment time was taken from K[alpha] x-ray spectra of samples at 200x. Additionally, samples were fractured after freezing in liquid nitrogen; backscattered images were observed, and x-ray maps were performed for several elements, including carbon (C), chlorine (Cl) and sulfur (S). The x-ray maps provide an estimate of the depth of penetration of the chlorinating agent.

Results and discussion

Surface energy

Simply wiping the as-molded rubber surface with toluene led to an increase in the surface energy from 19 to 29 mJ/[m.sup.2]. With one-minute immersion in the TCI solution (figure 2), the surface energy increased significantly as well. Between one minute and five minutes, the surface energy gradually increases and then plateaus. Following the 10-minute sample, an additional data point was taken at four minutes. Surprisingly, the surface energy was only 25.2 mJ/[m.sup.2]. Either the chlorinating agent had been totally consumed or a low energy material such as oil or wax had leached from the rubber and saturated the TCI solution. In order to determine what had occurred, another sample was treated in the same solution for 10 minutes. Its surface energy measured 26.7 mJ/[m.sup.2]. Following a toluene wipe, however, its surface energy increased to 39.19 mJ/[m.sup.2]. This suggests that an ingredient in the rubber is leached from the rubber and deposited on the rubber when relatively little surface area has been treated.

[FIGURE 2 OMITTED]

FTIR spectra

The IR spectra of the NR (figure 3) were recorded as molded and after wiping with toluene. Slight changes in peaks at 2,960, 1,445 and 726 [cm.sup.-1] were observed. A minor peak at 3,026 [cm.sup.-1] was also seen. These peaks are assigned to unsaturated C=C bonds found in a conjugated aliphatic hydrocarbon like NR. Next, IR spectra (figure 4) were collected on samples that had been treated for 1, 3.5 and 5 minutes. Several key peaks were identified and found to increase with immersion time. These peaks were given structural unit assignments. Assignment 1 is for a halogen-substituted unsaturated compound with pendant or terminal vinyl groups. Assignment 2 is for an unsaturated carboxylic acid. Assignment 3 is a primary or secondary alcohol. The IR spectra indicate that the surface of the rubber is both chlorinated and oxidized. Additionally, some of the C=C double bonds along the NR backbone have changed configuration from the cis isomer to 1,2-pendant and vinyl-terminated isomers. Based upon the changes in peak height, the chlorination process does not appear to be complete at one minute, but plateaus by 3.5 minutes, comparable to the surface energy measurements.

[FIGURES 3-4 OMITTED]

SEM/EDS

At 200x magnification, the NR surface prior to chlorination (figure 5) appears to be coated with wax. EDS analysis of this surface yielded peaks expected to be found in NR: Carbon, oxygen, silicon, zinc and sulfur.

[FIGURE 5 OMITTED]

After one-minute immersion in the TCI solution (figure 6), the surface had changed drastically. The waxy layer was no longer present and some fissures were observed. After 3.5 minutes of immersion (figure 7), more severe cracking and pitting was noted. The surface after 5 minutes of immersion (figure 8) was not that different from the 3.5 minute sample.

[FIGURES 6-8 OMITTED]

Elemental analyses for the treated surfaces are summarized in table 1. Chlorine level increases rapidly with the first minute of treatment and then stabilizes after 3.5 minutes. Meanwhile, carbon percentage decreases and sulfur percentage increases slightly. Recall that EDS measures x-rays from within the outer several to hundreds of micrometers, depending upon the energy of the primary electron beam, the atomic mass of the element and the working distance and gas beam path length of the microscope. Thus, changes observed in the weight percent of a given element may not be occurring directly at the sample's surface.

The change in chlorine concentration with time as measured by EDS was plotted (figure 9), along with the peak area for two of the IR peaks (Assignment 1) for chlorine substituted conjugated bonds. The increase in chlorine in the x-ray measurements corresponded to the changes in the IR spectra.

[FIGURE 9 OMITTED]

Next, cross sections were examined by SEM/EDS. With one-minute treatment time, there does not yet appear to be a significant chlorinated layer at the surface. After 3.5 minutes of immersion, the outer 100-120 [micro]m is chlorinated. The thickness of this chlorinated layer is much thicker than that reported by Oldfield and Symes. Note, however, chlorination in our study was accomplished by a timed immersion rather than wiping the rubber surface. With five-minutes immersion time (figure 10), the fissures are evident in the backscattered image (figure 10a). From the carbon (figure 10b) and chlorine (figure 10c) maps, the chlorinated layer is estimated at 200 [micro]m. At the boundary of the chlorinated/non-chlorinated phases (figure 10d), there is a thin layer where the sulfur concentration is greater than in the bulk. The quantitative EDS analysis (table 1) also indicates an increase in sulfur near the surface. During immersion, the carrier solvent apparently extracts accelerator fragments or elemental sulfur for the TCI. Such a sulfur layer could contribute to an increased hardening of the rubber at this region that could potentially lead to failure during fatigue testing.

[FIGURE 10 OMITTED]

Failure analysis

A destructive test of the article after bonding resulted in cohesive failure deep within the rubber, indicating good initial adhesion. In an accelerated, multi-axial fatigue test at ~100 [degrees] C, however, premature failure was observed near the rubber/plastic interface. A thin layer of rubber was found on the polyamide. The failed rubber surface was examined by FTIR (figure 11) and compared with the bulk rubber. Six key peaks (table 2) were found on the failed rubber. A peak analysis based upon a spectral library indicated that the peaks are most likely associated with benzothiazole functionality. MBS, one of the accelerators in the natural rubber compound, is a good match for these key peaks. Given the thin layer of sulfur present in the EDS map of the sample treated for five minutes, we propose that as chlorination occurs, TCI attacks the accelerator in the outer layer of the rubber. Ethyl acetate acts as a carder, depositing the accelerator fragments at the chlorinated/non-chlorinated boundary. With heat aging, the accelerator fragments react with polysulfidic sulfur, leading to hardening of the rubber. Under fatigue, fracture occurs at this boundary, leaving a thin film of rubber on the polyamide surface.

[FIGURE 11 OMITTED]

Conclusions

Chlorination of NR leads to thermodynamic, chemical and mechanical changes of the NR surface. An initial layer of wax appears to be removed within the first minute of treatment. Simultaneously, the surface becomes chlorinated and a rearrangement of the polymer backbone occurs. The thickness of the outer chlorinated layer is on the order of 100 to 200 microns, depending upon immersion time. Chemical and thermodynamic changes appear to be complete between one and 3.5 minutes. However, mechanical changes continue as the outer layer becomes more pitted. With excessive treatment time, oxidation and subsequent migration of one or more sulfur-beating accelerators from the outer layer of the rubber to the chlorinated/non-chlorinated interface appears, leading to increased hardening at this boundary and contributing to bond failure during fatigue testing of a NR/polyamide assembly.
Table 1 -weight % elemental analyses

 Time (minutes)
Element 0 1 3.5 5

 C 60.22 46.16 35.25 38.83
 Cl 0.00 19.16 29.92 25.65
 S 0.89 1.07 1.46 1.21
 0 17.57 12.12 14.13 12.24
 N 17.56 17.84 14.16 18.31
 Zn 2.30 2.09 3.17 2.28
 Si 1.40 1.59 1.90 1.47
Table 2 - spectral assignment for failed rubber
IR spectra

 Peak ([cm.sup.-1]) Assignment

1 3,350 Amino (morpholino)
2 3,025 Aromatic
3 1,590 Aromatic
4 1,525 C-N (benzothiazole)
5 1,480 Aromatic ring
6 1,290 Aromatic or amine function


References

(1.) B.G. Crowther, Rubber to Metal Bonding, Report 87, Rapra Review Reports, 8, No. 3, 21 (1996).

(2.) D.M. Brewis and I. Mathieson, "Pretreatments to enhance the adhesion of elastomers," Rubber Bonding '98, Rapra, Frankfurt, Germany (1998).

(3.) M.M. Pastor-Bias, M.S. Sanchez-Adsuar and J.M. Martin-Martinez, J. Adhes. Sci. and Techol., 8, No. 10, 1093-1114 (1994).

(4.) D. Oldfied and T.E.F. Symes, J. Adhesion, 16, 77-96 (1983).

(5.) E. Cutts, Developments in Adhesives - 2, A.J. Kinloch (edit.), Applied Science, London (1981), Chapter 10.

(6.) K.L. Mittal, edit., Contact Angle, Wettability and
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Title Annotation:natural rubber
Comment:Chlorination of NR: surface and failure analyses in a post-vulcanization bonded composite. (Cover Story).(natural rubber )
Author:Moore, Michael J.
Publication:Rubber World
Geographic Code:1USA
Date:Nov 1, 2001
Words:2399
Previous Article:Comparing curing systems: peroxide-co-agent versus sulfur-accelerator in polyisoprene.
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