Promoting aramid-rubber adhesion without epoxies - an alternative approach.

On a weight basis, aramid fibers based on polyparaphenylene terephthalamide) have tensile strengths and moduli greater than that of steel. Couple this with virtually zero shrinkage, outstanding chemical and thermal resistance, and it is not surprising that aramids are making strong inroads into composite applications where high demands are placed on the reinforcement component.

Despite all its attributes, the acceptance of aramid as a reinforcing material in rubber applications is contingent upon its fiber to matrix adhesion. This is especially critical in the tire industry, where passenger and traffic safety are of primary concerns.

For general application reinforcement fibers, e.g. rayon and nylon, sufficient rubber adhesion is achieved by treating and curing the fiber reinforcement with a resorcinol-formaldehyde-latex (RFL) dip prior to its incorporation in the rubber matrix. Here the RF component promotes adhesion to the cord via polar or covalent interactions, while the dried latex rubber (L) covulcanizes with the rubber matrix ensuring adhesion to that material.

For aramids, simple treatment with RFL leads to rubber composites exhibiting low fiber to matrix adhesion with failure often occurring at the fiber-RFL dip interface. This in part is attributed to the physical structure of the aramid, which being 100% crystalline provides an uninviting surface to the components making up the RFL.

Excellent aramid to rubber adhesion can be achieved by first activating the aramid with epoxies prior to the RFL treatment. In practice, two approaches have been adopted. With the first, the epoxy is applied to the aramid during production of the yarn rendering it adhesive activated. This yarn is then twisted into cord which later undergoes the RFL treatment. The second method begins with an already corded aramid fiber. Here the cord is dipped sequentially in an aqueous epoxy solution followed by treatment with the RFL dip. In both approaches the epoxy serves as the glue which binds the aramid with the RFL dip layer. Although little is known about the epoxy-aramid interaction, the bound epoxy activates the aramid surface and reacts with the resorcinol-formaldehyde components in the RFL.

Generally, the epoxies employed in these formulations are low in molecular weight and highly reactive. Given their volatility, and that potential emissions may arise during the dip-operation, alternative aramid activators are being pursued on both toxicological grounds and environmental pressures.

As part of an ongoing program to develop alternative means of promoting aramid to rubber adhesion, aramid yarns and cords were treated with low concentrations of fluorine gas using a continuous reactor set up. Although fluorine is highly aggressive, if handled under controlled conditions, it provides a safe means of altering the surface character of material being exposed to it (refs. 1 and 2). In this article the fluorine-treated aramid cords were later RFL dipped and evaluated for adhesion and fatigue in rubber composites. In addition, surface analyses employing x-ray photoelectron spectroscopy (XPS) were carried out on the fluorinated aramids, and these results were correlated to the ultimate aramid to rubber adhesion mechanism.

Experimental

Aramid yarns

The aramids used throughout this study were based on poly(paraphenylene terephthalamide) and are commercially available from Akzo Nobel and marketed under the trade-name Twaron. Two grades were incorporated in this study, T1000 and T1001; both yarns are 1,680 dtex and consist of 1,000 filaments.

T1000 was employed in all fluorine modifications. T1001, the adhesive activated counterpart of T1000, served as an internal program reference in both adhesion and fatigue assessments. Throughout this article T1000 will simply be referred to as aramid, while T1001 will be described as adhesion activated aramid.

Cord constructions

The following symmetric cord constructions were used throughout this study and are described in turns/meter. For convenience, twist levels/inch are provided in parenthesis.

* two ply - 1 330Z x 2 330S (8.4)

* three ply - 1 245Z x 3 245S (6.2)

* three ply - 1 270Z x 3 270S (6.8)

* three ply - 1 330Z x 3 330S (8.4)

Fluorination studies

Fluorination treatments were carried out in reactors developed by FluoroTec GmbH.

The reactor consisted of a chamber approximately one meter in length (1.5 meters in width). Concentrated fluorine (10 vol%) in nitrogen was fed into the reactor and diluted with air until the desired fluorine concentration was achieved. Within this study, fluorine concentrations were varied between 1 and 3 vol%. Experiments were performed under a slightly negative chamber pressure with effluent gases being quenched in a calcium carbonate column.

Greige aramid yarn or cord was continuously drawn through a fluorination chamber at speeds of 25, 50, 100 or 150 meters/minute. Treated fibers were immediately sealed in a black polyethylene sack and stored until RFL dipping.

RFL dipping

Fluorinated aramid yarns were first converted to cord prior to dipping. Cords undergoing fluorination were RFL dipped as received. All cords (fluorinated aramid and the nonfluorinated adhesion activated reference) were dipped with a standard RFL adhesive followed by a drying and cure cycle which was

Adhesion evaluation

Dipped cords were vulcanized in a natural rubber matrix (standard test compound) and were tested for static adhesion using a peel test method as described in ASTM D 4393-85.

Fatigue test

Goodrich block fatigue (GBF) measurements were carried out using a slight modification of ASTM D 885-62T as described elsewhere (ref. 3). All tests were conducted in a rubber matrix (standard test compound) at 40 Hz under an elongation/compression cycle of +2/-14% for a period of six hours. After cyclic loading, cords were cut from specimen matrices and mechanical properties were determined. These results were referenced against non GBF-tested dipped cords and values are reported as (%) residual strength.

Mechanical properties

Tensile properties were measured on an Instron 4501 tensile testing machine.

X-ray photoelectron spectroscopy (XPS) measurements XPS is an analytical tool capable of assessing a material's surface to a depth of 5 nm. XPS measurements were carried out on a VG Escalab MK II spectrometer using a Mg K-[alpha] radiation excitation source.

Results and discussion

Fluorination and bulk mechanical properties

The fact that aramids have been accepted in reinforcement applications is primarily due to their interesting mechanical and physical properties. Although the aim of this study was to activate an aramid surface to improve its ultimate adhesion to rubber, if doing so were to result in a deterioration in the bulk fiber properties, then such a treatment would be uninteresting from a commercial perspective.

In table 1, the mechanical properties of a starting aramid cord and its fluorinated counterparts are provided. Although the cords were subjected to varying fluorine concentrations and exposure times, tensile tests show that all the fluorinated cords are virtually identical with even a perceptible improvement in strengths in comparison to their untreated counterpart. These results indicate that fluorination does not result in a deterioration of the aramid's bulk properties.

[TABULAR DATA OMITTED]

Fluorination: yarn versus cord

As outlined in the introduction, sufficient. aramid to rubber adhesion often requires priming of the aramid fiber with low molecular weight epoxy reagents. The epoxy can be applied either to the aramid yarn during its production (which is later twisted into cord) or directly onto the cord prior to the application of the RFL dip. Both approaches lead to static peel test adhesion levels of over 300 N/2cm with failure occurring within the test rubber and not at the cord-matrix interface. In order to determine whether fluorination of yarn (which is then twisted into cord prior to RFI. dipping) or a direct cord fluorination has any effect on the ultimate adhesion to some rubber matrix, both materials were fluorinated under identical conditions, and their adhesion results provided in table 2.

[TABULAR DATA OMITTED]

In table 2, cords derived from fluorinated yarns (entries 1C and 3C) exhibited lower adhesion than cords undergoing fluorination under identical reactor conditions (entries 2 and 4, respectively). Interestingly, the monitored fluorine concentration for the treated yarn (and the cords derived from them) is greater than their fluorinated-cord counterparts.

In explaining these observations, it is important to point out that commercial aramid yarn is supplied with a spin finish to protect it during manufacture and any subsequent handling operations. The effect of the surface spin finish can be seen by comparing the XPS data from entries 5 and 6 in table 2. Here the nitrogen contribution increases with the removal of the spin finish, and the naked cord has a C:N:O ratio of 78.7/8.9/12.4 which approaches the theoretical composition of pure poly(paraphenylene terephthalamide) at 78/11/11.

In the fluorination of yarn (in essence a bundle of 1,000 untwisted filaments), a considerable amount of the spin finish is fluorinated in addition to the desired aramid surface. During the subsequent twisting operation into cord, two things probably occur. First, the fluorinated spin finish, rather uniformly distributed throughout the bundle, is squeezed to the outside surface of the cord during twisting, resulting in a considerable amount of measured fluorine at the cord surface. The second event, accounting for the lower ultimate adhesion for cords derived from fluorinated yarns in comparison to their treated-cord counterparts, is that the twisting operation brings non fluorinated filaments (originally in the interior of the yarn bundle) to the cord exterior. This reduces the amount of surface-fluorinated filaments resulting in fewer potential bonding sites available to the RFL dip. In contrast, no such filament reshuffling occurs in cords undergoing fluorination.

To account for the lower XPS-determined fluorine concentration for modified cord as opposed to fluorinated yarn, a possible explanation is that the surface spin finish is fluorinated along with exterior cord filaments. Over time the fluorinated spin finish mixes with interior finish (non fluorinated) via diffusion processes, leaving behind a lower effective amount fluorinate material at the surface. Based on these results, all subsequent modifications were carried out on aramid cord.

Fluorinated cord adhesion and fatigue

In table 2, the aramids undergoing fluorination were passed through a fluorine atmosphere (3 vol%) at 100 and 150 meters per minute. In practice, the handling of tire fabric, often made up of over 1,000 parallel cords, may prove troublesome at such passage speeds. In one set of experiments designed to establish how mild the actual fluorination operation can proceed while still guaranteeing ultimate adhesion to some rubber composite, a series of aramids, representing various cord constructions, were fluorinated using concentrations as low a 1 vol% in combination with speeds as slow as 25 m/min. These cords were later treated with an RFL dip and assessed for their adhesion and fatigue in standard rubber. The results of this are provided in table 3.

Table 3 - adhesion and fatigue results for
aramid cords of various plies and cord
constructions
Entry       Cord        Fluorination     Adhesion     GBF
        Construction   [F.sub.2]/speed     N/2Cm    fatigue
         ply/twist       (%) m/min.                   (%)
7       2/330              2/150           270         --
8       2/330              1/50            294         66
9       2/330              1/25            312         --
10      3/270              2/100           249         64
11      3/270              2/50            250         --
12      3/270              1/100           236         --
13      3/270              1/50            248         --
14      3/270              1/25            253         --
15      3/245              2/100           275         59
17      3/245              2/50            246         --
18      3/300              2/100           254         73
19      3/300              2/50            230         --
20Reference "adhesive activated" aramid(a) 308         62
(a) Reference aramid cord construction: 3 ply; 270 twists per meter.

With regard to adhesion, two-ply constructions (entries 79) exhibited stronger adhesion than their three-ply counterparts (entries 10-19). Within a ply series, ultimate adhesion appears to be insensitive to fluorine concentration, passage speed, or even the twist level in a given cord construction (carried out in the three-ply series). These observations are interesting from a commercial perspective in that acceptable adhesion levels can be achieved under a broad range in processing conditions, and it is worth noting that the adhesion levels attained in entry 9, are on par with those obtained using an epoxy adhesion activated aramid yarn (entry 20). In addition to adhesion assessment, fatigue behavior was also determined on selected cords listed in table 3. In all cases, residual strengths for fluorinated treated cords were in line with their epoxy surface-activated counterparts. Where direct comparisons between constructions were possible (entries 10 and 20), excellent agreement in GBF residual strengths were noted (64 and 62%, respectively). It is also worth noting that within a ply series, residual strength increases with increasing twist levels (refer to entries 15, 10 and 18); this observation is well established reinforcement technology.

Surface analysis

XPS has shown that fluorinated aramids exhibit a slight enrichment of both fluorine and oxygen at the aramid surface (entries 5 and 2], table 4). However, this assessment is qualitative due to the presence of spin finish which not only itself undergoes fluorination, but also being based on polyether formulations interferes with an accurate interpretation regarding the actual modifications at the fiber's surface. As noted earlier in the discussion, removal of spin finish from commercial aramid cord, leads to a surface composition approaching the theoretical value for poly(paraphenylene terephthalamide), C:N:O = 78/11/11 (entry 6, table 4). Therefore in order to gain a greater insight into the fluorination process at the aramid surface, it is first necessary to extract the spin finish from the fluorinated sample. As seen in table 4, entries 21 and 22, XPS surface analysis reveals that extraction results in an increase in the monitored fluorine composition while the higher than normal oxygen contribution is maintained. The increase in fluorine is in line with an earlier observation that fluorinated cords show low fluorine contribution due to a screening effect brought about by a migration of unfluorinated spin finish to the surface. Subsequent removal of this spin finish would then account for the observed increase in fluorine concentrations.

Table 4 - XPS analyses on selected aramid
cords before and after extraction
Sample             Report   Surface analysis (XPS)(a)
description        code      F     C      N     O
Aramid             5        --    81.4   1.7   16.9
commercial
Aramid             6        --    78.7   8.9   12.4
extracted finish
Fluorinated        21       1.8   77.7   0.6   19.9
aramid
Fluorinaed         22       5.5   72.6   3.3   18.6
aramid
extracted finish
(a) F+C+N+O have normalized to 100

Not only can XPS profile the elemental make up at a material's surface, it can also provide insight regarding the environment as to how these elements are bonded there. By viewing the region of the XPS spectra where carbon is monitored (figure 1), spin finish-extracted aramid shows a small peak at 292 eV indicative of carbons participating in the aramid amide linkage (curve 1). As for the fluorinated counterpart, a much broader shoulder is noted in this region. This is believed to represent contributions from carbinols, carbonyls, and carboxyl species in addition to the amide group.

Proposed mechanism

Although fluorine is a diatomic element, because of its reactivity it exists in equilibrium with disassociated fluorine radicals even at ambient temperatures. When in contact with the aramid, these fluorine radicals abstract hydrogens, generating hydrogen fluoride and leaving behind carbon radicals at the fiber surface. In turn, the carbon radicals can be quenched by fluorine radicals, leading to covalently bonded fluorine or react with atmospheric oxygen (present in the reactor chamber), leading to an array of chemically bound oxygen species.

The mechanism how fluorine and oxygen moieties bound at the aramid surface promote adhesion with the RFL dip is still open to speculation. Because of fluorine's high electronegativity, the presence of this element at the aramid surface would be, expected to induce polarities which in turn could interact with the RF via dipole interactions. On the other hand the enrichment of an aramid surface with carbonyl and carboxyl groups would provide an opportunity for condensation reactions or hydrogen bonding with the RF resin. In both cases improved fiber to RFL adhesion would be expected. Once this interface has been established, the ultimate bonding to a rubber matrix occurs via the standard covulcanization of the matrix rubber with the dried latex rubber originating from the RFL.

Conclusion

Fluorination is an alternative method for activating an aramid surface, rendering it more receptive to RFL treatment and ultimately leading to rubber composites possessing good fiber to matrix adhesion. Strong adhesion levels were attained under a wide latitude of fluorination exposures, and fluorination was shown to have no effect on either bulk-cord properties or dipped-cord fatigue. Fluorination circumvents the application of reactive epoxies which in turn has positive environmental consequences.

Figure 1 [ILLUSTRATION OMITTED]

References

[1.] B.D. Bauman, Rubber World, April, 16 (1993). [2.] M.A. Williams, B.D. Bauman and D.A. Thomas, Polym. Eng. and Sci., 31(13) 992-998 (1991). [3.] E.M. Winkler, Rubber Chem., 61(8) 441-448 (1991).

Acknowledegements

"Tape products used in EPDM roofing systems" is based on a paper given at the May, 1995 Rubber Division meeting. "Promoting aramid-rubber adhesion without epoxies - an alternative approach" is based on a paper given at the May, 1995 Rubber Division meeting. "Equipment, sealing for HP/HT completions" is based on a paper given at the October, 1994 Rubber Division meeting.