Adhesion by EPDMs and fluorocarbons to metals by using water-soluble polymers.

The current most widely used adhesive systems, introduced 30-35 years ago for the bonding of elastomers to metals, are based on two-layer systems consisting of a primer layer and a cover cement with a typical thickness of 25-30 [micro]m (refs. 1 and 2).

The primer is usually a combination of halogenated polymers, thermosetting resins and pigments dispersed in organic solvents. The metal-primer interaction is mainly based on physical interactions. A variety of cover cements is commercially available to bond different types of elastomers to metals (ref. 3). These cements generally contain a mixture of halogenated polymers, crosslinking agents like trinitrosobenzene and p-quinonedioxime, oxidizing agents, fillers e.g. carbon black and lead compounds, and organic solvents (refs. 4 and 5).

Because of increasing environmental regulations, particularly to eliminate the emission of organic solvents, and to simplify manufacturing operations, research is focused on developing single layer systems (ref. 6) and waterborne adhesives (ref. 7). These novel systems are based on aqueous systems of polymers, and for this technology emulsifying agents and surfactants are used to form micelles with a diameter of 0.05-0.2 [micro]m (ref. 8). However, storage stability and film formation with these latex systems is very critical.

The silane adhesive systems are a special class of adhesives. These systems are based on silane coupling agents. Silane coupling agents have generally the following formula:

[R.sub.n]Si[X.sub.(4.n)] (ref. 9).

X is an alkoxy moiety, preferably an ethoxy or methoxy, which is hydrolyzable under release of ethanol or methanol and which can provide linkage with inorganic substrates, pigments, fillers or hydroxyl functional polymers. R is a nonhydrolyzable organic moiety that can be an alkyl, aromatic, organo-functional or a combination of these groups. These moieties are either compatible, forming interpenetrating networks, or reactive with organic compounds. Silane adhesive systems are used for the bonding of specialty robbers like fluorocarbon and silicone elastomers (ref. 3).

In the present study, bonding of EPDM and fluorocarbon (FKM) elastomers to metals was investigated by using water-soluble polymeric coupling agents (PCAs), obtained by chemical modification of poly(acrylic acid) (PAA) and polyvinylamine (PVAm). These PCAs should be able to form either physical, covalent or ionic bonds across the polymer-metal interface during evaporation of water. In addition, co-vulcanization should occur with the rubber during molding to form a thermally stable and chemically resistant layer with high cohesive strength. Figure 1 gives the general structure of the polymeric coupling agents (PCAs). The M-groups should show a good metal affinity and the R-groups should co-vulcanize with the rubber during molding.

[Figure 1 ILLUSTRATION OMITTED]

PAA and PVAm were chosen because both polymers are water-soluble and contain both moieties which already have metal affinity. The carboxylic acid groups of PAA are known to interact with metal surfaces (ref. 10). Nakamoto (ref. 10) suggested three different mechanisms:

* Acid-base interactions with hydroxyl groups on a hydrated oxide surface;

* salt bridge formation between free metal ions and carboxylate anions; and

* divalent metallic ion crosslinking reactions.

For the bonding of natural rubber (NR) to steel, Frank et al. (ref. 11) used copolymers of butadiene and methacrylic acid, but these copolymers were applied in a non-aqueous solution.

Hydroxamic acids are well-known for their ability to form complexes with metals, particularly iron (ref. 12). In acid solutions, they form deeply colored complexes with [Fe.sup.3+] which are very stable. Therefore, the carboxylic acid moieties were partially converted to hydroxamic acid moieties by reacting the carboxylic acid groups with hydroxylamine (ref. 13) (M-groups).

Polyamines are known to form chelating complexes with (heavy) metal ions (ref. 14). In this study, polyvinylamine was modified by converting amine groups into iminodiacetic groups which easily form complexes with most multivalent metals (refs. 15 and 16) (M-groups).

For covalent bonding with EPDM and FKM, we also introduced unsaturated groups by reacting the carboxylic acid and the amine groups with allylglycidylether (ref. 17) (R-groups).

Figures 2 and 3 show the structures of the polymeric coupling agents (PCAs) (ref. 18): modified poly(acrylic acid) and modified polyvinylamine, respectively. The effect of the various PCAs on the bonding of EPDM and FKM elastomers to metals was studied. The formed EPDM-metal joints were tested on durability strength in a specific autoclave test. The FKM-metal joints were tested on aging, strength by immersion in a ASTM reference fuel C (a 50/50% mixture of toluene and isooctane) for 70 h at 70 [degrees].

[Figures 2 & 3 ILLUSTRATION OMITTED]

Experimental

Preparation of water-soluble polymeric coupling agents

PCA-1

30 g of a 35 wt. % aqueous solution of poly(acrylic acid) (Aldrich; Mw = 250,000 g/mol) was mixed with 100 ml water. 50 g hydroxylamine-HCl (Aldrich) was added and the pH of the solution was adjusted with aqueous sodium hydroxide (3.9 M) to 3.5. The reaction mixture was stirred at a temperature of 100 [degrees] for 6 h. The resulting yellow solution was dialyzed with a dialysis tube (Mw = 12,000 g/mol: sigma) overnight to remove unreacted hydroxylamine. The remaining solution was freeze-dried to obtain a white soft solid (yield: 9.31 g).

PCA-2

30 g of a 35 wt. % aqueous solution of poly(acrylic acid) (Aldrich; Mw = 250,000 g/mol) was added to 30 ml water and 50 ml ethanol. 12.4 ml allylglycidylether (Aldrich) was mixed with the solution. The reaction mixture was refluxed for 6 h at a temperature of 70 [degrees]. After the reaction, ethanol was evaporated and the solution was washed with toluene to remove allylglycidylether and dialyzed overnight with a dialysis tube (Mw = 12,000 g/mol; sigma). The remaining solution was freeze-dried to obtain a white soft solid (yield: 9.48 g).

PCA-3

First hydroxamic acid moieties were introduced as described above (PCA-1). Subsequently 7.5 g of PCA- 1 was dissolved in 30 ml water and 30 ml ethanol. 9 ml allylglycidylether (Aldrich) was added. The reaction mixture was refluxed for 6 h at a temperature of 70 [degrees]. After the reaction, ethanol was evaporated and the solution was washed with toluene and dialyzed overnight with a dialysis tube (Mw = 12,000 g/mol; sigma). The remaining solution was freeze-dried to obtain a white soft solid (yield: 5.00 g).

PCA-4

2.5 g polyvinylamine-HCl (Aldrich: Mw = 50,000 g/mol) was dissolved in 100 ml water. 8.6 g chloroacetic acid (Aldrich) was added and the: pH was adjusted to 12. The reaction was continued for 24 h at 50 [degrees]. After the reaction, the solution was dialyzed overnight with a dialysis tube (Mw = 1,200 g/mol; sigma). Water was removed by freeze-drying to obtain a white soft solid (yield: 4.5 g).

PCA-5

5 g polyvinylamine-HCl (Aldrich; Mw = 50,000 g/mol) was dissolved in 30 ml ethanol and 70 ml water. 13.8 ml allylglycidylether was added to the solution. The reaction mixture was allowed to stir for 24 h at 60 [degrees]. The solution was washed with chloroform. The yellow solution was dialyzed overnight with a dialysis tube (Mw = 1,200 g/mol; sigma). Water was removed by freeze-drying to obtain a yellow solid (yield: 4.0 g).

PCA-6

First iminodiacetic moieties were introduced as described above (PCA-4). Subsequently 2.3 g of PCA-4 was solved in 30 ml ethanol and 30 ml water. 7 ml allylglycidylether (Aldrich) was added. The reaction mixture was allowed to stir for 24 h at 60 [degrees]. The solution was washed with toluene and the yellow solution was dialyzed overnight for 8 h with a dialysis tube (Mw = 1,200 g/mol; sigma). The remaining solution was freeze dried to obtain a yellow solid (yield: 2.3 g).

Table 1 gives the percentage of converted carboxylic acid groups for the modified poly(acrylic acid) PCAs as analyzed by [sup.1]H-NMR and elemental analysis. From FT-IR and [sup.1]H-NMR it was clear that amine groups of PVAm had reacted with chloroacetic acid and allylglycidylether. However, it was not possible with these techniques to estimate the percentage of reacted amine groups.

Table 1 - Synthesized polymeric coupling agents

Polymeric coupling    Hydroxamic acid   Unsaturated ether
agent                   groups [%]         groups [%]

PCA-1                       15                   -
PCA-2                        -                  11
PCA-3                        5                  12

Adhesion and durability tests

The adhesion test was carried out according to the ISO-5600 standard (ref. 19), although the actual dimensions of the test parts were modified to make the test more economical. [This test is comparable with the BS903: Part A40 (ref. 20) and ASTM D429-81 method C (ref. 21)]. Cylindrically shaped rubber was bonded to two conical metal parts with standard dimensions. The diameter of this cylinder and of the cylindrical portion of the metal parts was 12.5 mm, the half angle of the conical vertex was 45 [degrees] and the distance between the conical tips was 6 mm. The stress is concentrated at the tips of the cones as was indicated by Painter (ref. 22) and shown by finite element analysis (ref. 23). The stress distribution is not even and at the interface the force is not pure tension but has also a peel and shear character.

Preparation of the metal parts and dipping with the polymeric coupling agents

Stainless steel (alloy X12CrMoS17; AISI 430F), aluminum (alloy AlSilMg) and mild steel (alloy 9SMn36) lathe-cut machined parts with a cone roughness of [R.sub.A] [is less than] 0-8 [micro]m were degreased. All the metal parts, except the brass parts, were cleaned for 20 minutes by vapor degreasing with trichloroethylene followed by ultrasonic cleaning in a liquid bath of trichloroethylene for 10 minutes at 80 [degrees]. As a last step, the parts were put in the vapor degreasing bath again for five minutes. The brass parts (alloy CuZn39Pb3) were cleaned ultrasonically in a Deoxidine-solution of 5% in distilled water (pH 1) for ten minutes at room temperature. Deoxidine is an acidic cleaner based on phosphoric acid. The acidic cleaning step was followed by plasma etching with an [O.sub.2]-plasma of 300W for 30 minutes.

The cleaned parts were dipped for 30-60 seconds in a PCA-solution, followed by drying in a vertical position (cone upwards) for 20 minutes at 120 [degrees] in an air circulated oven. Until molding, the pre-treated parts were stored in a drying cabinet; at 40 [degrees].

Preparation of the rubber compounds

An EPDM masterbatch (without the curatives) was mixed using a tangential laboratory mixer with an empty volume of 1.51 at a load factor of 70%. The compound was finished (adding the curatives) using a two roll mill (80 mm diameter 300 mm roll length size). The friction ratio was set at 1:1.2 and the rolls of the mill were cooled at ca. 40 [degrees]. The master-batch was first homogenized on the mill for 45 seconds. Then the coagent and the peroxide were added. After 4.5 minutes the compound had a smooth appearance and was taken off the mill and the EPDM compound was stored for one day at room temperature before usage. Table 2 gives the EPDM compound composition and table 3 the cured EPDM compound properties.

Table 2 - composition of the EPDM compound

Compound                          [phr]

EPDM                               100.0
Silica                             30.00
Carbon black                       60.00
Paraffinic oil                     30.00
Coagent                             2.00
Peroxide (40% on carrier)           8.00
Other ingredients                   7.00
Table 3 - properties of the cured EPDM compound

Vulcanizate properties
Rheometer(a)                     (ASTM-D2084-93)
M1 [-]                                              20.1
Mh [-]                                              56.8
T50 [min.]                                          0:54
Hardness Shore A(b)              (ASTM-D2240-91)      76
Compression set [%](b,c)         (ASTM-D395-89B)    17.5
Tensile strength at              (ASTM-D412-92C)    13.1
 break [N/[mm.sup.2]](b)
Elongation at break [%](b)       (ASTM-D412-92C)     178
Specific gravity [-](b)                             1.16

(a) - measured with a Monsanto oscillating disc rheometer according to ASTM-D2084-93 cured 3 min. @ 180 [degrees] C

(b) - measured on slab (2 mm thickness) according to ASTM-D3182-89 cured 3 min. @ 200 [degrees] C

(c) - 22h @ 200 [degrees] C

Two FKM compounds were tested. Compound FKM-I was based on a terpolymer of tetrafluoroethylene, vinylidenefluoride and hexafluoropropylene containing 69 wt. % fluorine. FKM-1 was peroxide cured. Compound FKM-2 was based on a copolymer of vinylidenefluoride and hexafluoropropylene containing 65 wt. % fluorine. FKM-2 was bisphenol cured. Table 4 gives the properties of the cured FKM-K1 and FKM-2.

Table 4 - properties of cured FKM compounds

Vulcanizate properties                              FKM - 1
Rheometer(a)                  (ASTM-D2084-93)
M1 [-]                                                 2.27
Mh [-]                                                13.58
T50 [min.]                                             1:10
Hardness Shore A(b)           (ASTM-D2240-91)            79
Compression set [%](b,c)      (ASTM-D395-89B)            60
Tensile strength              (ASTM-D412-92C)          11.0
 at break [N/[mm.sup.2]](b)
Elongation at break [%](b)    (ASTM-D412-92C)           362
Specific gravity [-](b)                                2.05

Vulcanizate properties                 FKM-2
Rheometer(a)
M1 [-]                                   1.69
Mh [-]                                  13.79
T50 [min.]                               3:51
Hardness Shore A(b)                        71
Compression set [%](b,c)                   19
Tensile strength                         10.6
 at break [N/[mm.sup.2]](b)
Elongation at break [%](b)                400
Specific gravity [-](b)                  1.91

(a) - measured with an Alpha Technologies MDR2000 according to ASTM-D2084-93 cured 6 min. @ 170 [degrees] C

(b) - measured on slab (2 mm thickness) D3182-89 cured 2 rain, @ 200 [degrees] C and post-cured 7h @ 230 [degrees] C

(c) - 22h @ 200 [degrees] C

Molding of the rubber-metal test parts

Six rubber-metal test parts were prepared in one mold run by hot transfer molding at a vulcanization temperature of 185 [degrees] C for the EPDM test parts and 170 [degrees] for the FKM test parts with a ram pressure of 60 bar for six minutes. The FKM test parts were post-cured for 7 hours at 230 [degrees] to remove volatile reaction products.

Testing of adhesion strength after vulcanization

Prior to testing of the bond strength, at least 16 hours conditioning at room temperature was allowed (ref. 24). Of each mold run, three parts were used for the adhesion tests and three or six parts for the durability tests of the EPDM test parts and the aging tests of the FKM test parts, respectively. Adhesion was tested on a Monsanto Tensometer 10 at a constant crosshead speed of 50 mm/min. Recorded was the maximum force [N] for failure of the test parts and the rubber retention [%R] (100% rubber retention means a complete failure of the test parts in the rubber phase). The reported results are the average of three tests.

Autoclave tests of EPDM test parts

The test parts were immersed in a 50/50% ethylene glycol/distilled water mixture. This system was heated to 135 [degrees] under an over-pressure of 2.5 bar in an autoclave. The test pieces were subjected to this environment for 22 hours. The parts were cooled to room temperature and tested as described above.

Aging tests of FKM test parts

The test parts were immersed in ASTM reference fuel C (ref. 25) (a 50/50% mixture of toluene and isooctane). The fuel tests were carried out with stainless steel chambers which were filled with the fuel mixture and sealed off with a Viton o-ring. The chambers were put in an oven for 70 hours at 70 [degrees]. After 70 hours, the chambers were taken from the oven and some test parts were allowed to cool to room temperature, still soaked in the fuel. After this period, these test pieces were taken out of the fuel, dried with a tissue and immediately tested to avoid evaporation of the fuel. Other pans, after the fuel soak, were put in an oven for 24 hours at 100 [degrees] to evaporate the fuel and tested (dry-out test).

Results and discussion

Effect of PCAs on bonding strength of EPDM

Figures 4 and 5 show the effect of various PCAs on the adhesion of EPDM to stainless steel. As references, the degreased stainless steel parts and unmodified PAA and PVAm were used. For both unmodified PAA and PVAm, 60% rubber retention was obtained. It can be seen that modification of PAA and PVAm improved the adhesion, resulting in 100% rubber retention for PCA-2, PCA-3 and PCA-6. In case of PAA, modification by introducing allylglycidylether groups (PCA-2) had a more pronounced effect on the rubber retention than modification by hydroxamic acid groups (PCA-1). On the other hand, in case of PVAm, conversion of the amine groups into iminodiacetic groups, as well as reaction of the amine groups with allylglycidylether, was necessary to increase both metal affinity and rubber affinity to obtain 100% rubber retention.

[Figures 4 & 5 ILLUSTRATION OMITTED]

From table 5, it can be concluded that PCA-2 and PCA-3 were also successful in bonding EPDM to aluminum and mild steel, resulting in 100% rubber retention. For brass only 70% rubber retention was obtained (ref. 26).

Table 5 - bonding of EPDM to stainless steel, aluminum, mild steel and brass (metal parts dipped in 1 wt.% aqueous solution and dried)

Metal                   PCA-2                PCA-3

Stainless steel         575(a)    100(b)     585(a)    100(b)
Aluminum                536       100        528       100
Mild steel              535       100        542       100
Brass                                        496       70

(a) - maximum force [N]

(b) - rubber retention [%R]

Autoclave test of EPDM-metal bonds

A special demand was that these novel adhesive systems could endure a 22 hour autoclave test in a 50/50% water/ethylene glycol system at 135 [degrees] with 2.5 bar over-pressure. This system represents the environment of an automotive radiator-seal during operation.

In table 6, the adhesion properties are shown and it can be seen that after four hours, PCA-2 and PCA-3 gave the best results. It is also clear that PCA-3 showed excellent endurance even after 22 hours in the aggressive medium. Bonding of EPDM to aluminum and mild steel also gave good durable bonds, showing 100% rubber retention (table 7). In case of brass, no durable bonds were obtained and adhesion decreased to 5% rubber retention.

Table 6 - autoclave test of EPDM-stainless steel bonds after 4, 12 and 22 hours (stainless steel parts dipped in 1 wt. % aqueous solution and dried)

Time [h]   PAA              PCA-1             PCA-2

   4       509(a)   75(b)   505(a)   75(b)    553(a)  90(b)
  12       383      15
  22       386      40

Time [h]   PCA-3

   4       562(a)  100(b)
   12      527     100
   22      541     100

(a) - maximum force IN]

(b) - rubber retention [%R]

Table 7 - autoclave test of EPDM-metal bonds after 22 hours (metal parts dipped in 1 wt.% aqueous solution and dried)

Metal             PCA-2               PCA-3

Aluminum          536(a)    100(b)    528(a)    100(b)
Mild steel        535       100       542       100
Stainless steel                       541       100
Brass                                 242         5

(a) - maximum force [N]

(b) - rubber retention [%R]

Table 8 gives the results for the PVAm-polymers with EPDM-stainless steel. It is clear that PCA-6 showed the best endurance after the 22 hour autoclave test, indicating the positive effect of iminodiacetic as well as allylglycidylether groups in the modified PVAm.

Table 8 - autoclave test of EPDM-stainless steel bonds using PVAm-polymers after 22 hours (stainless steel parts dipped in 1 wt. % [PVAm and PCA-4] or 0.5 wt. % [PCA-5 and PCA-6] aqueous solutions and dried)

Type of PCA    Maximum force [N]   Rubber retention [%R]

PVAm                  133                    0
PCA-4                 143                    0
PCA-5                 220                    0
PCA-6                 476                   100

Bonding of FKM to stainless steel

Table 9 shows the effect of the type of FKM on the bonding to stainless steel by using PCA-3. It can be seen that PCA-3 was successful in bonding to FKM-1 but not to FKM-2. FKM-1 was peroxide cured and FKM-2 was bisphenol cured. Streit et al. (ref. 27) reviewed the available vulcanization systems for fluorocarbon elastomers and the mechanisms of the vulcanization reactions. It is possible that the different adhesion properties of FKM-1 and FKM-2 were related to the difference in curing systems. The FKM. stainless steel bonds were also tested in a fuel system for 70 hours at 70 [degrees] (table 9). For FKM-1, directly after the fuel test, 85%; rubber retention was obtained and after the dry-out step, 100% rubber retention.

Table - 9 effect of type of FKM on bonding of FKM to stainless steel (stainless steel parts dipped in 1 wt. % aqueous solution of PCA-3 and dried

Type of      Before fuel       After fuel     After dry-out
FKM            test              test             test

FKM-1     301(a)   100(b)   281(a)   85(b)   317(a)   100(b)
FKM-2     91       0        33       0       93       0

(a) - maximum force [N]

(b) - rubber retention [%]

Table 10 shows the results for the bonding of FKM-2 to stainless steel using PVAm and PVAm-based PCAs. Application of PCA-4 and PCA-6 was quite successful, the difference possibly indicating an extra effect of unsaturated ether groups in PCA-6. The adhesive properties obtained with PCA-6 differed completely from those observed with PCA-3 for the same rubber-metal combination (table 9). This difference might be attributed to the effect of amine groups, which were not present in PCA-3. It is possible that the 100% rubber retention, found with PVAm at pH 12 (table 10), was also the result of the effect of the amine groups. On the other hand, at pH 5, part of the amine groups in PVAm will be protonated, possibly resulting in a poor adhesive strength. It is well known that fluorocarbon elastomers can also be vulcanized with diamines (ref. 27). Our results might indicate that co-vulcanization of the amine-groups in PVAm and PVAm-based PCAs with the fluorocarbon elastomers had also occurred.

Table 10 - effect of PVAm-polymer on bonding of FKM-2 to stainless steel (stainless steel parts dipped in 1 wt. % [PVAm and PCA-4] or 0.5 wt. % [PCA-6] aqueous solutions solutions and dried)

Type of PCA    Before fuel test

PCA-4           463(a)   70(b)
PCA-6           500      90
PVAm (pH 12)    552      100
PVAm (pH 5)     272      10

(a) - maximum force [N]

(b) - rubber retention [%]

Conclusions

New aqueous bonding systems have been developed for the adhesion of EPDM and fluorocarbon elastomers to a variety of metals, e.g. stainless steel, mild steel, aluminum and brass. These bonding systems consist of water-soluble polymeric coupling agents which are obtained by chemical modification of poly(acrylic acid) and polyvinylamine. In case of poly-(acrylic acid) hydroxamic acid moieties were introduced to increase the metal affinity, whereas unsaturated moieties for co-vulcanization were obtained by reacting carboxylic acid groups with allylglycidylether. The amine groups of polyvinylamine were partially convened to iminodiacetic groups for improved metal affinity and were also reacted with allylglycidylether to obtain the desired unsaturation. By using these water-soluble polymeric coupling agents, the formed bonds of EPDM to stainless steel, aluminum and mild steel could withstand a specific autoclave test (50/50% water/ethylene glycol for 22 hours at 135[degrees]). With brass, further optimization of the bonding system is necessary to improve the adhesion. Fluorocarbon elastomers were successfully bonded to stainless steel. However, polymeric coupling agents based on poly(acrylic acid) were only effective with peroxide cured fluorocarbon elastomers. In that case, the formed bonds of FKM to stainless steel could withstand fuel aging (50/50% toluene/iso-octane for 70 hours at 70 [degrees]). The bisphenol cured fluorocarbon elastomers were successfully bonded to stainless steel with the polyvinylamine based coupling agents.

References

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by C.P.J. van der Aar, L. van der Does and A. Bantjes, University of Twente, The Netherlands; J. Martin, Vernay Laboratories; and F. Roseboom, Vernay Europa B.V.