Telechelic poly(butadiene) diacrylate for the radical cure of elastomers.

Coagents are rubber chemicals used in the radical cure of elastomers to promote crosslinking reactions and improve physical properties. They are typically multifunctional and can participate in a number of radical reaction mechanisms, the most beneficial including grafting and radical addition. By increasing the crosslink density of the compound, physical properties such as tensile strength, modulus and compression set can be improved (refs. 1 and 2).

The broad group of coagents useful in radical cure is typically subdivided by the influence on the kinetics and extent of vulcanization of the coagent structure (ref. 3). Type I coagents are highly reactive and increase both the rate and state of cure. Typically monomeric, such coagents contain acrylate, methacrylate, or maleimide functionality. They are very polar structures with limited solubility in most elastomer grades. Type II coagents are based on allyl reactive sites and increase the state of cure only. Monomeric forms include allyl-containing cyanurates, isocyanurates and phthalates. Polymeric forms of Type II coagents include poly(butadienes) with pendant vinyl unsaturation.

Ultimate physical properties of the vulcanizate are determined by coagent structural factors, including reactivity, solubility and functionality. The structure-property relationships of common coagents have been previously reviewed (refs. 4 and 5). It has been suggested that the solubility of the coagent in the compound determines if the modification of the network proceeds through grafting of coagents between polymer chains (refs. 6 and 7), the formation of an interpenetrating network of homopolymerized coagents (ref. 8) or the formation of higher modulus filler-like domains of thermoset coagent (refs. 4 and 9). It is important to note that the relative reactivity of the coagent compared to the elastomer also influences the efficiency of coagent reactions. Balancing the relative reactivity of the elastomer and coagent towards available radicals can promote a more homogenous crosslink density, potentially minimizing coagent addition reactions and maximizing grafting of coagent between polymer chains. Solubility of the coagent in the elastomer matrix also promotes network uniformity by achieving a good dispersion of the reactive coagent and minimizing domain formation, which can lead to lower apparent crosslink density.

Structural differences in coagents define the ultimate efficacy of the materials in different elastomer grades and delineate classification as Type I or Type II. The objective of the current study is to evaluate the structure-property relationships of a coagent material with a hybrid structure, containing both Type I and Type II functionality. A telecbelic poly(butadiene) resin containing both in-chain vinyl and terminal acrylate ester groups was synthesized (PBD-DA). The novel structure will be compared to both poly(butadiene) resins with varying vinyl contents and multifunctional acrylate ester monomers in peroxide-cured formulations. The synergistic qualities of the poly(butadiene) diacrylate structure are highlighted by comparison to other polymeric coagent types, and binary blends of high vinyl poly(butadiene) with monomeric acrylate esters. Finally, the relative reactivity of the PBD-DA will be evaluated alongside Type I and Type II coagents in formulations based on elastomers with varying degrees of unsaturation.

Experimental

Coagent materials

Table 1 outlines the commercially available coagent grades used in the study. They are available from Sartomer, and were used without further purification. Type I coagents include 1,4-butanediol diacrylate, trimethylolpropane triacrylate and trimethylolpropane trimethacrylate. Type II representatives include poly(butadiene) resins with 65% and 90% vinyl content. The title poly(butadiene) diacrylate (PBD-DA) is a hybrid structure. Characterization including vinyl content (polymeric forms), molecular weight and number of acrylates/molecule (f), is provided.

Rubber compounding

For direct comparison, the above coagents were evaluated in model compounds at various loadings. The formulations are provided in table 2. A masterbatch containing all ingredients except the coagent and peroxide was prepared in an internal mixer, to which the curatives were later added on a two-roll mill. Curative incorporation time was approximately seven minutes at an average mixing temperature of 85[degrees]C.

Physical testing

Vulcanization behavior was quantified on a Tech Pro MDPT moving die rheometer (MDR) according to ASTM D 5289. Cure temperature for both MDR testing and sample preparation was 160[degrees]C; all samples were cured for 35 minutes. Tensile data were acquired on a Thwing-Albert materials tester following ASTM D 412. Compression set was evaluated after heating at 100[degrees]C for 22 hours (ASTM D 395).

Results and discussion

Structure optimization

Synthesis of a poly(butadiene)-based coagent with elevated vinyl levels and terminal acrylate functionality is possible due to the availability of high vinyl poly(butadiene) diol precursors. Such precursors are produced via proprietary anionic polymerization technology (ref. 10). The poly(butadiene) diol can yield a telechelic diacrylate through a quantitative condensation reaction with acrylic acid. The structure of the poly(butadiene) diacrylate is provided in figure 1. The combined reactivity of both the in-chain vinyl unsaturation and terminal acrylate groups are of interest.

[FIGURE 1 OMITTED]

Methacrylated low vinyl poly(butadiene) materials have been previously reported (refs. 11 and 12). Methacrylate groups are either grafted onto the polymer backbone or reacted at terminal positions. However, methacrylate coagents are less reactive than their acrylate analogues. A comparison of the monomers trimethylolpropane trimethacrylate (TMPTMA) and trimethylolpropane triacrylate (TMPTA) illustrates the differences. Figure 2 compares the 100% modulus and compression set values of vulcanizates prepared from the model EPDM formulation containing increasing amounts of these monomeric coagents. The acrylate group imparts increased modulus and lower compression set. Cure kinetics are also affected, as cure time and relative scorch safety are both reduced versus the methacrylated monomer.

[FIGURE 2 OMITTED]

Low vinyl poly(butadiene) polyols with functionality greater than 2.0 have been available historically. These materials are produced by a radical polymerization process and have inherently low vinyl content and do not provide coagency when applied to radical cure systems. Vinyl unsaturation is more reactive than in-chain double bonds due to reduced steric hindrance (ref. 13), and the reaction mechanism proceeds mainly through addition (versus hydrogen abstraction). To illustrate the influence of vinyl content on the efficiency of the radical cure of elastomers, a series of poly(butadiene) resins (Mw < 5,000 g/mol) was compared in the EPDM model formulation (figure 3). The data suggest that in an EPDM elastomer, low vinyl content of a resinous coagent may actually inhibit crosslinking, while the efficiency of network formation increases linearly at vinyl contents greater than 65%.

[FIGURE 3 OMITTED]

Tetechelic poly(butadiene) containing both high vinyl content and acrylate functionality provides a unique structure incorporating both Type I and Type II features. The remainder of this article is dedicated to comparing the new coagent type to other coagents with partial structural similarity, including multifunctional acrylate ester monomers and high vinyl poly(butadiene) coagents. Structure property relationships can be established when comparing the PBD-DA coagent to binary blends of high vinyl poly(butadiene) and monomeric diacrylates. Cure kinetic parameters of scorch time ([ts.sub.2]) and delta torque will be compared. Key vulcanizate physical properties of 100% strain modulus and compression set will also be provided.

Structure-property comparisons

Monomeric Type I coagents

Monomeric coagents contain a number of structural features, which can be varied for a given application. They are very reactive and produce increased crosslink density in the peroxide-initiated radical cure of elastomers. Typical grades are produced by esterification of linear or branched short chain polyols. For comparison with PBD-DA, 1,4-butanediol diacrylate (BDDA) and TMPTA were selected. The triacrylate product also contains a scorch inhibitor (ref. 14). A disadvantage of using Type I coagents is the reduction in scorch safety associated with the high level of reactivity.

Relative scorch safety of the coagents is provided in figure 4 by comparing [ts.sub.2] values of PBD-DA and the monomeric coagents as a function of loading in the model EPDM formulation. The average scorch times of the controls (no coagent, peroxide only) used in the study was 1.48 minutes. The di- and triacrylate additives produced large reductions in scorch times. TMPTA is one of the most reactive coagent species, and matches the trend of the less active diacrylate only by the inclusion of a scorch inhibitor. In addition, a decrease in delta torque is often seen when using the scorch-inhibited versions of monomers. PBD-DA maintained an extended scorch safety, as the predominant radically active component is vinylic unsaturation. Figure 5 compares the evolution of crosslink density as measured by delta torque (MDR). It can be seen that each coagent exhibited a local maximum in apparent crosslink density with increased loading. PBD-DA increases the apparent crosslink density as loading is increased, and provides a high value at elevated loading.

[FIGURES 4-5 OMITTED]

The physical properties of the vulcanizates are provided in figures 6 and 7, which display modulus and compression set data, respectively. These parameters provide measurements from both tension and compression geometries and highlight two properties most impacted through the inclusion of coagents in formulations. The average modulus of the controls was 1.74 MPa. All three coagents increase modulus versus the control, and the modulus is further increased with coagent loading. The trifunctional coagent (TMPTA) provides the highest modulus values. All coagents reduced the amount of compression set relative to the control average of 24%. However, PBD-DA exhibited similar reductions in permanent set as TMPTA, despite the reduced molar concentration of acrylate functionality.

[FIGURES 6-7 OMITTED]

The PBD-DA polymeric diacrylate was more effective at building crosslink density as a function of loading than the monomeric species. Several factors may contribute. As previously mentioned, the solubility parameter of the polymeric diacrylate is more closely matched to the EPDM polymer than values for the monomeric coagents. Table 3 lists calculated solubility parameters (from group contribution methodology) using published molar attraction constants (refs. 15 and 16). The solubility parameter of PBD-DA is much closer to the hydrophobic elastomers than the monomeric multifunctional acrylates. It is possible that greater solubility may aid in dispersion of the polymeric diacrylate. In addition, the vinyl groups of PBD-DA offer similar reactivity to the unsaturation of the diene monomer of the EPDM polymer. These factors may favor coagent grafting versus radical addition, promoting an increased crosslink density of more uniform distribution.

Polymeric Type II coagents

The activity of PBD-DA is compared in the model EPDM formulation to poly(butadiene) coagents containing 65% vinyl content (PBD-65) and 90% content (PBD-90). A primary advantage to using Type II coagents is maintenance of process safety manifested by longer scorch times.

Figure 8 provides the [t.sub.s2] scorch values as a function of loading for the coagents compared. PBD-65 maintained or increased scorch safety with increased loading, while PBD-90 decreased [t.sub.s2] values with loading. PBD-DA further reduced scorch safety versus PBD-90, while the effect was relatively independent of loading. Delta torque is compared in figure 9. PBD-DA provided the maximum delta torque in this series, followed by PBD-90.

[FIGURES 8-9 OMITTED]

Cured physical properties were also measured. Figures 10 and 11 compare the effect of the coagents on 100% modulus and compression set, respectively. PBD-DA and PBD-90 both increased modulus with loading. PBD-65 provided a minimal increase in modulus at low loading, and reached a plateau at elevated loading. The relative ranking carried over to compression set performance. PBD-65 was not as effective as PBD-90 or PBD-DA at reducing compression set.

[FIGURES 10-11 OMITTED]

The activity of the unfunctional poly(butadiene) coagents correlates strongly with the vinyl content. Such behavior is consistent with previous work that demonstrates a proclivity for radical reaction with vinyl groups versus in-chain unsaturation based on steric hindrance arguments (refs. 17 and 18). PBD-DA contains an identical vinyl content as PBD-65, but provides the highest crosslink density correlation, and matches the modulus and compression set resistance of the higher vinyl PBD-90. These results suggest that the improvement in coagent performance of PBD-DA relative to PBD-65 can be attributed to the activity of terminal acrylate groups. By comparison, the terminal diacrylate functionality provides the rough equivalent of an additional 25% pendant vinyl content in terms of providing cured physical property improvements. The reduction in scorch safety compared to standard polymeric coagents also confirms the added reactivity of the terminal acrylate groups.

Blends of Type I and II coagents

The hybrid structure of the poly(butadiene) diacrylate incorporates Type I and Type II reactive groups in one molecule. By comparing PBD-DA to blends of BDDA and PBD-65, it can be determined if a synergy exists between the attached groups or whether simple blending rules apply. Five blends of the monomeric and polymeric coagents were produced. The molecular weights of PBD-DA and BDDA are approximately 2,000 g/mol and 200 g/mol, respectively. For these materials, 10 grams of PBD-DA has the molar acrylate equivalency of 1 gram of BDDA. BDDA was added to PBD-65 such that the molar equivalency of acrylate functionality would be the same compared to PBD-DA at similar phr loadings. Table 4 outlines the composition of the five blends evaluated.

The blends were mixed into the model EPDM formulation. Figure 12 demonstrates that the apparent crosslink density is greater for PBD-DA versus the blends when compared at similar acrylate/vinyl unsaturation levels. Visual extrapolation of the data would suggest that the local maximum in apparent crosslink density is located at a lower loading for PBD-DA. Scorch safety values (not shown) were similar in each case.

[FIGURE 12 OMITTED]

In addition, cured physical properties indicate differences in the behavior of the resulting networks. As a function of loading, modulus values (100% strain) were greater for PBD-DA than the blends (figure 13). The difference in modulus values increased at elevated loadings, as the modulus/loading slope was greater for PBD-DA than the blend counterparts. Compression set data are compared in figure 14. In this measurement of network integrity, there is little if any difference attributable to the coagents studied. Permanent set was reduced as a function of loading in each case.

[FIGURES 13-14 OMITTED]

The data indicates differences in the apparent crosslink density of the vulcanizates by coagent type when measured under oscillatory shear and in modulus measured under tension. Measured in compression, the series of compounds had similar permanent set values. Drawing definite conclusions regarding the relative reactivity of acrylate ester functionality when attached terminally to a reactive poly(butadiene) or whether present as an individual component in a blend at equimolar concentration may not be possible with this limited data set. However, given that the acrylate radical is more reactive than a vinylic radical, and that acrylate esters attached directly to poly(butadiene) resins are more equitably distributed in an EPDM compound based on solubility parameter calculations, greater tensile modulus and higher crosslink density may be expected versus a binary blend of the monomeric acrylate and poly(butadiene).

Formulation comparisons

The PBD-DA has been compared to other coagent structures in the EPDM model compound. EPDM elastomers contain enough unsaturation such that vulcanizates prepared from peroxides alone possess sufficient, if not optimal, physical properties. By elevating the level of unsaturation in the elastomer, more reactive sites are available for radical cure, and resuiting physical properties generally improve. However, fully saturated elastomers generally require coagents if peroxide vulcanization is to be used (refs. 2 and 19). In addition to the EPDM compounds evaluated above, PBD-DA, PBD-90 and TMPTA were evaluated at 5 phr loading in NBR (unsaturated) and CM (saturated) formulations. The cure behavior and efficiency of the coagent structures can be further compared within the context of elastomer type.

Table 5 lists cure and physical property data for the control (no coagent) compounds of the model NBR, EPDM and CM formulations (provided in table 2). Properties depend primarily on the state of cure trend with elastomer degree of unsaturation. Delta torque increases with increasing unsaturation, while scorch safety and compression set decrease. Modulus is dependent on both crosslink density and filler loading; the latter is disparate in the three formulations. In order to better compare the effect of coagent addition across the formulation series, the experimental data within each formulation were normalized to each respective control (control = 100). In this manner, the contribution to the network density (and resulting physical properties) from the coagent itself can be compared. Differences due to formulation and elastomer are removed.

In figure 15, delta torque is compared across coagent types and formulations. First, the impact of coagent addition was highest in the fully saturated CM formulation. Second, the overall coagent efficiency (regardless of formulation) trends favored structures containing acrylate functionality. In general, cure kinetics trend with acrylate concentration in the formulation. Figure 16 provides scorch safety data for the same series. Between coagent types, a clear trend was evident as scorch safety decreased with increasing acrylate functionality. Across formulations, the saturated system was most affected by coagent addition.

[FIGURES 15-16 OMITTED]

Figures 17 and 18 compare modulus and compression set (respectively) in the same manner. While the trends between coagents were less clear, coagent addition had the least influence on the NBR compounds. Modulus was increased and compression set reduced (relative to control) in the EPDM and CM formulation when acrylated coagents were used.

[FIGURES 17-18 OMITTED]

The addition of reactive coagents influences the cure kinetics and physical properties of the vulcanizate. Regardless of degree of unsaturation, Type I coagents (TMPTA) provide the greatest impact on both the rate of cure (manifested as a reduction in scorch safety) and ultimate properties. The Type II coagent (PBD-90) demonstrated the least impact on properties, but also maintained process safety. The hybrid structure (PBD-DA) contained elements of each structure. Across elastomer types, intermediate scorch safety and improvements in properties result. In the fully saturated compound (CM), PBDDA lengthened scorch safety compared to the scorch-inhibited acrylate monomer, and increased modulus while reducing permanent set relative to the high vinyl poly(butadiene). The influence of coagents on the vulcanization process is decreased as unsaturation increases. Coagent additives compete with the unsaturation in the elastomer for available radicals. At typical loadings (< 10 phr), even highly reactive acrylate functionality may be overwhelmed by the concentration of in-chain and pendant unsaturation on the elastomer. The relative contribution of additional crosslink density through coagent reactions is effectively reduced.

Summary and conclusions

Coagents are utilized in the peroxide initiated cure of elastomers, as they increase the crosslink density through more efficient use of formed radicals. The structure of the reactive coagent influences both the cure kinetics and ultimate physical properties of the resulting vulcanizate. Type I coagents, including multifunctional acrylate esters, can provide large improvements in the state of cure, but decrease the process safety of the compound by accelerating cure rate. High vinyl poly(butadiene) resins are Type II coagents containing less-reactive pendant vinylic structures. Cure rate and scorch safety can be maintained, while effecting moderate increases in crosslink density and physical properties. A coagent possessing both acrylate and vinyl reactive groups can optimize a balance in rate of cure and ultimate physical properties.

A poly(butadiene) diacrylate ester was synthesized by reacting a terminally-functional high vinyl poly(butadiene) diol with acrylic acid. In a model EPDM formulation, this novel structure demonstrated cure kinetics intermediate to the poly(butadiene) resins and the acrylate monomers. Scorch safety was improved versus a scorch-inhibited triacrylate monomer, while modulus and compression set were similar as a function of loading. Compared to poly(butadiene) resins of various vinyl content, the acrylated form matched physical properties of resins containing higher vinyl content, demonstrating the increased reactivity imparted by the addition of telechelic acrylate groups. Compared to binary blends of poly(butadiene) resin and diacrylate monomer with equimolar amounts of reactive groups, the poly(butadiene) diacrylate displayed higher crosslink density and increased modulus as a function of loading. The efficiency of different coagent species was also shown to be inversely proportional to elastomer unsaturation. The greatest impact on cure kinetics and physical properties is found in otherwise unreactive saturated systems.

Several conclusions can be drawn regarding coagent structure-property relationships. By attaching two terminal acrylate groups to poly(butadiene), the resulting activity of the coagent is similar to an unfunctional poly(butadiene) of 25% higher vinyl content. The polymeric form of the diacrylate appears to be more efficient versus monomeric analogues. Activity of the poly(butadiene) diacrylate at equal loading was increased compared to 1,4-butanediol diacrylate, despite carrying 10% of the molar acrylate equivalency. Increased solubility of the polymeric acrylate in the elastomer system was suggested as a possible explanation. Synergy between the highly reactive terminal acrylate functionality and the internal vinylic structures was evident when compared to binary blends of monomeric diacrylate and poly(butadiene), and may also contribute to the efficiency of the polymeric diacrylate coagent compared to monomeric forms.

This article is based on a paper presented at the Fall 170th Technical Meeting of the Rubber Division, ACS, October 10-12, 2006.

References

(1.) A.Y. Coran in Science and Technology of Rubber, F.R. Eirich, ed., Academic Press, New York, 1978.

(2.) P.R. Dluzneski, Rubber Chem. Technol. 74, 451 (2001).

(3.) R.E. Drake, J.J. Holliday and M.S. Costello, Rubber World 213 (3), 22 (December 1995).

(4.) H.G. Dikland, "Coagents in peroxide vulcanizations of EP(D)M rubber," Gegevens Koninklije Bibliotheek, Netherlands, 1965.

(5.) S.K. Henning and R. Costin, Rubber World, 233 (5), 29 (February 2006).

(6.) R.C. Keller, Rubber Chem. Technol. 61, 238 (1988).

(7.) ZH. Murgic, J. Jelencic and L. Murgic, Polym. Eng. Sci. 38, 689 (1998).

(8.) J. Class, Rubber World, 220 (5), 35 (August 1999).

(9.) L. Liu, E Luo, D. Jia and B. Guo, Intern. Polymer Processing XIX (4), 374 (2004).

(10.) "Krasol prepolymers," product bulletin #4885, Sartomer Company, www.sartomer.com (2004).

(11.) J.A. Rosenbaum, et al., U.S. Patent #6,491,598 (2002).

(12.) "Ricacryl methacrylated polybutadiene," product bulletin #4909, Sartomer Company, www.sartomer.com (2004).

(13.) L.D. Loan, Rubber Chem. Technol., 40, 149 (1967).

(14.) L.A. Brooks, et al., U.S. Patent #5,696,190 (1997).

(15.) P.A.J. Small, J. Appl. Chem. 3, 71 (1953).

(16.) J. Brandrup and E.H. Immergut, eds., "Polymer Handbook, Second Edition," John Wiley & Sons, New York, 1975.

(17.) E. Farmer and C. Moore, J. Chem. Soc. 1951, 149.

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(19.) W. Hoffman, "Rubber Technology Handbook," Hanser Publishers, New York, 1989.

by Steven K. Henning and Jeffrey Klang, Sartomer [e-mail]

Table 1 - coagent product identification,
classification and characterization

Identification    Product             Description     Class

BDDA                SR213    1,4-butanediol diac-     Type I
                                           rylate
TMPTA             SR519HP      Trimethylolpropane     Type I
                                      triacrylate
TMPTMA            SR517HP      Trimethylolpropane     Type I
                                  trimethacrylate
PBD-DA              SR307         Poly(butadiene)     Hybrid
                                       diacrylate
PBD-90           Ricon 154        Poly(butadiene)     Type II
PBD-65             Krasol         Poly(butadiene)     Type II
                   LB2000

Identification      Vinyl                  Mw (g/         f
                      (%)                    mol)

BDDA                    -                      198         2

TMPTA                   -                      296         3

TMPTMA                  -                      338         3

PBD-DA                  65                   2,000         2

PBD-90                  90                   5,200         -
PBD-65                  65                   2,000         -

Table 2 - model NBR, EPDM and CM compound formulations

Ingredient (phr)            NBR        EPDM       CM

Nipol DN 3335               100
Nordel I P4640                         100
Tyrin 0136                                         100
Carbon black                100         50          60
Paraffinic oil                          50
Diisodecyl phthalate                                30
PIastHall 7050                5
Calcium carbonate                                   40
Zinc oxide                    5          5
Stearic acid                  1          1
Agerite Resin D               1                      1
Naugard Q                                1
Coagent                 Variable   Variable   Variable
Di-Cup 40KE                 7.5        7.5         7.5

Table 3 - calculated solubility
parameters of common polymers
and selected coagents

Solubility parameter, [delta] (cal/[cm.sup.2]) (0.5)

Polymer                     Coagent

EPDM              7.94      PBD-65     7.97
HVPBd (a)         8.00      PBD-DA     8.05
cis-PI            8.20       TMPTA     9.79
cis-BR            8.26        BDDA     9.86
ESBR (b)          8.33
NBR (c)           9.31
HNBR (c)          9.37

(a)--90% vinyl
(b)--23.5% styrene, 13% vinyl
(c)--28% acrylonitrile

Table 4 - blend compositions of BDDA and PBD-65

            BDDA (phr)   PBD-65 (phr)

Blend A           0.2            2.0
Blend B           0.5            5.0
Blend C           1.0           10.0
Blend D           1.5           15.0
Blend E           2.0           20.0

Table 5 - comparative data for the control formulations (no coagent)

                         NBR      EPDM        CM

Delta torque (dNm)       62.0      18.8      11.6
[t.sub.s2] (min.)         0.58      1.21      2.00
100% modulus (MPa)        6.3       1.8       3.7
Compression set (%)      10.1      14.8      63.0