Results and Discussion

Spacial Orientation of Surface-Grafted Amino-Functional Silane Molecules

Amino-functional silanes such as (gamma)-aminopropyltrimethoxysilane (y-APTMS), (gamma)-aminopropyltriethoxysilane (y-APTES), (2-aminoethyl)-3- aminopropyltrimethoxy silane (2-AE-3-APTMS) are traditionally used as cou­pling agents for improving the strength and long-term durability of adhesively bonded metals or glass fiber-reinforced composites. In these applications, the primary mode of bonding the amino-functional silane to the hydroxyl-rich me­tallic or silicate-based substrate is through the silanol end of the molecule, as illustrated in Fig. 9(a). The secondary mode of bonding is through hydrogen bonding between the silane’s amino groups and either the silanol groups or hydroxyl groups present on the oxidized surface of glass or metal [27-31]. This mechanism is schematically illustrated in Figs. 9(b) and 9(c).

It has been shown [27] that the extent of amino-silane protonation can be correlated to the isoelectric point (IEP) of the respective oxides. The lowest degree of amino-functional silane protonation, e. g., y-APTMS, has been thus observed for magnesium (IEP = 12.0) and the largest on silicon, aluminum, and titanium (IEP = 4.0, 6.0, and 7.0, respectively).

The degree of amine protonation near the substrate surface, in y-APTMS adsorbed onto silicon metal substrate (see Fig. 10(a)) is approximately 28 % [22,27].

Figure 10(b), in turn, illustrates the XPS spectrum of the N1s peak of 2-AE- 3-APTMS silanized low-density polyethylene (LDPE). Two components of the N1s peak were observed near 399.3 and 400 4 eV, which were attributed to free and protonated amino groups, respectively. Based on the relative intensities of the two components, it turned out that about 55 % of the amino-silane was pro – tonated and 45 % contained free amine.

The above observation indicates that both types of amino groups are pres­ent in the interphase:

• Protonated amine groups (-NH3+): hydrogen-bonded to the oxidized polymer surface,

• Free amino groups (-NH2): species available for further reaction with the sealant.

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Based on the above information, it appears that a molecular brush of N – APTMS silane grafted onto the surface of a polyolefin, comprises approximately an equal number of molecules orientated with the “amine group up" (see Fig. 9(a)) and “amine group down" (see Figs. 9(b) and 9(c)). The “amine group up" molecules are available for further reaction with adhesives or sealants.

Adhesion Improvement by Surface-Grafted Connector Molecules Interpene­trating into Silicone Adhesives—The influence of surface-grafted molecular brushes on adhesion of polymeric substrates bonded with silicone adhesives was investigated using the following materials:

• substrate: EVA/PP blend,

• silicone adhesive: RP-4/Rhone Poulenc,

• molecular brush system:

(ii) interpenetrating system: polyethylene imines (PEIs) MW = 800

(N = 19), MW = 2000 (N = 46), MW = 25,000 (N = 581),

MW = 750,000 (N = 17,442).

(iii) chemically bonding system: amino-functional silane: Z-6020.

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401 399

Binding Energy eV)

Binding Energy (eV)

FIG. 10—XPS spectrum of the N1s peak for: (a) y-APTMS silane (0.1 %) grafted onto silicone surface, and (b) 2-AE-3-APTMS (0.1 %) grafted onto oxidized LDPE surface

[19] .

The adhesion quality was assessed by lap-shear tests involving SICOR – treated substrates. These were first surface-activated by corona discharge and subsequently surface grafted using PEIs and Z-6020 silane at the concentration of 0.1 %. The PEIs with MW = 25,000 and MW = 750,000 were also used at con­centrations of 0.5 and 1.0 %.

Lap-shear specimens were prepared as described in the above section on “Shear Strength" with an overlap of 10 mm. They were allowed to cure at room temperature for 3 days prior to testing.

The specimens were tested in a dry condition and after 7 days immersion in water at 40° C.

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The graphs in Fig. 11 [32] demonstrate the difference in effectiveness of interfacial reinforcement using either interpenetrating or chemical bonding mechanisms of interactions. They also illustrate the influence of the type and concentration of interpenetrating graft molecules on the strength of assemblies bonded with silicone adhesive.

The results indicate that the bond strength of specimens modified with inter­penetrating molecular brushes using PEI connector molecules is always greater than that after oxidative treatment only (in this work: corona discharge treatment).

An interesting trend is observed in Fig. 11 regarding the influence of the length of “connector molecules" on the strength of adhesion. The “bare" inter­face of an oxidized polymer produces a bond strength of 180 kPa in dry condi­tion. After 7 days immersion in 40° C water, the bonds between the substrate and silicone adhesive are cleaved resulting in complete loss of strength associ­ated with 100 % delamination of the adhesive at the interface.

For surfaces grafted with interpenetrating only (non-reactive) connector mol­ecules of PEI, it appears that increasing the length of graft chemical molecules results in a corresponding increase in the bond strength, up to approximately

FIG. 11—The influence of PEI molecular weight and chain length (L a N) and amino – silane Z-6020 on the strength of assemblies involving surface-grafted substrates and RP-4 silicone adhesive [32].

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500 kPa for a polymerization index of N = 17,442 and concentration of 0.1 %. The exposure of this type of interface to a 7-day immersion in water at 40° C results in a 35 % loss of strength. An increase in the surface density of the chains, achieved by using higher concentrations of the graft chemicals (0.5 % and 1 %) also appears to increase bond strengths in the case of the PEIs characterized by higher molecular weights (2,000 to 750,000). However, despite the increase in the bond strength, the failure mode is interfacial delamination because of the pull-out of interdigitated molecular chains from the matrix of the silicone elastomer.

When surface-grafted “connector molecules" are chemically reactive with the silicone adhesive, as in the case of the amino-functional silane (Z-6020), the highest degree of interphase reinforcement (690 kPa) is achieved. This particu­lar type of the interface/interphase system (i. e., involving chemically bonded “connector molecules") retains its original bond strength of 690 kPa even after 7 days of immersion in 40°C water.

Another example of the effectiveness of surface-grafted molecular brushes, ca­pable of improving adhesion through interpenetration or through chemical bond­ing with the adhesive, involves substrates that are difficult to bond such as Acetal (polyoxymethylene). The effect of molecular brushes on the adhesion of Acetal to elastomeric silicone adhesives was examined by comparing the peel strength of untreated, corona-treated, and SICOR-treated substrates. The latter involved co­rona oxidation followed by the application of amino – and epoxy-functional silane.

Two types of silicone adhesives were evaluated, i. e., Dow Corning 983 and GE 100. The first of these is chemically reactive with both amino-terminated and epoxy-terminated molecular brushes. Whereas GE 100 does not exhibit chemical reactivity with either type of connector molecule, it is able to interact with the molecular brush structure through the interpenetration of macromo­lecular chains into the matrix of the elastomeric adhesive.

The results listed in Table 3 [32] show that the bond strength of Acetal with­out treatment is very poor, and surface oxidation through corona discharge alone is not sufficient to provide a significant improvement in adhesion. On the other hand, surface-grafted molecular brush provided through the SICOR pro­cess leads to a significant increase in the peel strength.

TABLE 3—Peel force (N) of Acetal /silicone adhesive bonds following various surface treat­ments on the substrates [32].

Acetal surface treatment

Silicone adhesive

Dow Corning 983

GE 100

Strength

Failure mode

Strength

Failure mode

None

0.0

100 % AF

3.65

100 %AF

Corona discharge

3.25

100 % AF

7.5

100 %AF

SICOR (amine grafting)

17.5

80 % CF

19.0

100 %AF

SICOR (epoxy grafting)

24.0

100 %CF

20.0

100 %AF

Note: AF, delamination at the substrate/sealant interface; CF, cohesive failure within sealant

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As would be expected, relatively short and interpenetrating (but non-reac­tive) molecules of graft chemicals (amino – and epoxy-terminated silanes) increase the bond strength of specimens prepared with GE100 elastomer through interdigitation, but are not able to reinforce the interphase to the level required for achieving cohesive failure of the elastomeric adhesive. On the other hand, an appropriate choice of adhesive such as DC 983, which is capable of cross-linking with functional ends of the amino – and epoxy-functional graft molecules, results in high levels of cohesive failure within the adhesive.

Conclusions

(a) The adhesion and fracture performance of interfaces between polymers can be effectively improved and controlled by surface-grafted macro­molecular “connector molecules."

(b) “Connector molecules" grafted onto solid polymer surfaces interact with adjacent materials such as adhesives or sealants through either one or a combination of the following mechanisms:

(i) Interpenetration into the adjacent adhesive, and (ii) chemical reac – tion/cross-linking with the adjacent adhesive.

(c) The effectiveness of the interface reinforcement by surface-grafted con­nector molecules depends on the following factors: (i) The surface den­sity of grafted molecules, (ii) the length of the individual chains of the grafted molecules, and (iii) the optimum surface concentration/surface density in relation to the length of connector molecules.

(d) At the interfaces reinforced with interpenetrating connector chains, a distinct maximum/optimum (rOPT) is recorded for joint fracture energy versus graft density, as expressed by Eq 3. An increase of r above rOPT results in a decrease of fracture energy enhancement because of a decrease in the efficiency of the interdigitated macromolecular chains.

(e) It has been effectively demonstrated that surface-engineered difficult – to-bond polymeric substrates such as polyolefines or polyacetal, “surface tethered" by chemically surface-grafted molecular brushes, the latter provided by a process comprising surface oxidation (e. g., by flame or corona discharge treatment) and application of polyfunctional connector molecules exhibit significantly improved adhesion to elasto­meric silicone sealants and adhesives.

(f) It has been conclusively demonstrated in this paper that adhesion of elasto­meric silicone adhesives to polymers surface engineered through designated types of surface-grafted molecular brushes provided by SICOR process is drastically better than that of the same polymers modified by commodity surface-treatment processes such as corona discharge or flame treatment.