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Talc--the solution to challenges in automotive.

The balance of cost and performance is foremost in consideration for the automotive market. Cost is important due to competition and expected return on investment. Performance can not, however, be compromised due to extended warranties and consumer expectations. The requirements on elastomer components continue to be more demanding, in part due to design. For example, the under-hood temperature continues to increase due to restriction of air flow and the reduction in the size of the engine compartments. Other areas of concern are electrolytic degradation, chemical resistance, permeability, fatigue and crack resistance.

The design engineer/chemist must sort through the various options to meet the challenges required to balance cost and performance. For example, in the case of temperature, one approach is to use expensive elastomers in order to extend the service limits and life. An alternate approach would be to improve the performance of general purpose elastomers to meet these more demanding requirements. The selection of the proper cure systems and antioxidants provides one alternative (ref. 1). In addition, the molecular composition and volume percentage of polymers are important variables (ref. 1). The partial replacement of carbon black with talc has also been shown to improve the thermal performance of elastomers (ref. 2). This approach also reduces compound and finished product cost. This is achieved through lower raw material cost, a reduction in mixing time and improved processability.

In the case of electrolytic degradation, the use of non-conducting industrial minerals such as talc has been shown to eliminate or minimize the problem (ref. 3). This is important to the performance of coolant hose and window seals.

The use of platy fillers to reduce permeability of fluids and gas is well known. The Mistron Vapor talcs discussed in this article have a platy morphology which is preserved by special grinding techniques in order to maintain the aspect ratio. They have been used for over 40 years in the rubber industry to improve the permeability resistance. A further reduction in permeability can be achieved by using a new high aspect ratio product (ref. 4), Mistron HAR talc.

Talc has also been shown to increase the fuel and oil resistance of elastomers (ref. 5). This aspect is even more demanding with the introduction of various additives to fuel and the higher temperature exposure to oil. Even though the polymer selection is the most important parameter in oil resistance, platy fillers such as talc can reduce the effect of exposure.

The environmental aspect is also a consideration. The production of ultra-fine talc requires significantly less energy than that needed to manufacture carbon black. It is estimated that the amount of C[O.sub.2] emitted during the production of talc is only 1/10 of that for carbon black. Therefore, this makes partial substitution of talc for carbon black environmentally attractive.

This article provides supporting evidence for the use of talc in rubber compounds to meet the demanding requirements for the automotive market of today and in the future.

Thermal performance

The degradation of elastomeric compounds is increased by high temperature. Accelerated heat aging is used by the design engineer/chemist to predict the long-term performance of elastomers. Talc improves the thermal aging performance. This is demonstrated in figure 1 by the differences in elongation retention in EPDM using a high-temperature sulfur donor cure system recommended by DSM (ref. 6). In these compounds, 10 to 60 weight percent of the carbon black has been replaced by talc on an equal volumetric basis, i.e., 1.5 parts of talc for each one part of carbon black. The service life of these compounds can be predicted with models generated via linear least-squares fit of the data. For example, the estimated percent retained elongation after 100,000 hours of exposure for the compound with 40% carbon black replaced with talc is two times greater than with carbon black only. This difference is even greater since the talc-reinforced compound had a higher initial elongation than the carbon black control, i.e., 631% vs. 458 %. The estimates of the ultimate elongation of these compounds after 11.4 years of service at 100[degrees]C are 271% with talc vs. 92% with carbon black only.

[FIGURE 1 OMITTED]

Talc also improves the thermal aging performance of EPDM cured with conventional sulfur, non-nitrosamine and EV systems, as shown in figures 2-4. In the compounds labeled MVRE and MVRE-HS (experimental surface-treated MVRE), 40% of N650 carbon black has been replaced with talc on an equal volumetric basis.

Inspection of figures 2-4 shows that partial replacement of carbon black with talc improves elongation retention. These results confirm those obtained by Dunn, et al., which indicated that replacing 25% of the carbon black with silane treated talc improved elongation retention after thermal exposure (ref. 1).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The change in modulus and tear, as shown in figures 5 and 6, is less with the partial substitution of talc for carbon black; however, the changes in tensile strength and hardness are greater.

A similar response is obtained in NBR with both sulfur and peroxide cure systems, as shown in table I. The retained elongation after 42 days of exposure to 100[degrees]C air is significantly higher for the compounds containing talc, i.e., 62 vs. 58% for the sulfur cured and 62 vs. 51% for the peroxide cured. In the compounds containing talc, 40% of the carbon black was replaced with talc on an equal volumetric basis.

Electrolytic degradation

Talc has been used for years in wire and cable because it is electrically non-conductive. From the technical literature, butyl compounds reinforced with Mistron Vapor R exhibited a significantly higher dielectric strength than calcined clay, i.e., 1,150 vs. 650 volts/million (ref. 7). In primary insulation for medium to high voltage cable, surface treated talc was determined to be suitable for 2-35 kV industrial wire applications per ICEA EM60 test (ref. 8)

The non-conductivity of talc is important in applications where electrolytic degradation is possible due to dissimilar metals, as in the case of a steel-reinforced window seal in contact with an aluminum body panel. The insulating value of the compound containing talc is illustrated by the conductive and resistance values in table 2.

The lack of conductivity in tale-reinforced compounds also eliminates the problem of electrochemical degradation in coolant hose.

Permeability resistance

Talc reduces the permeability of rubber compounds to liquids and gases due to platy morphology and high aspect ratio. This is due to an increase in the mean-free path of diffusion caused by the talc platelets as illustrated in figure 7.

[FIGURE 4 OMITTED]

The reduction in air permeability at 60[degrees]C of a bromobutyl compound with equal volumetric replacement of 30% of the carbon black with talc or calcined clay is shown in figure 8.

A similar reduction in permeability of organic vapors from fuel or refrigerant could be expected. This is essential to reduce emissions from polluting the atmosphere.

The addition of talc also reduces the diffusion of liquids. This is demonstrated in the following figure for NBR with two types of fuel. The percentage of carbon black replaced with talc was 0 to 60%. Two parts of talc were added for each part of carbon black removed.

These results are consistent with the observations of Dunn and Vara (ref. 9) in which 25% replacement of N650 black by an equivalent volume of talc reduced the permeation of NBR with 34% ACN to Fuel C and 85/15 blend of C and methanol. Talc substitution also reduced the permeation of both types of fuel in ECO homopolymer and copolymer (ref. 9).

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

The permeability is also related to the aspect ratio of the talc. Specialized grinding techniques can also increase the aspect ratio of ultrafine talc and increase the barrier properties of rubber compounds, as shown in figure 10. This high aspect ratio talc (Mistron HAR) should be considered by design engineers in critical applications.

Chemical resistance

The performance of rubber parts exposed to various environments is primarily governed by the choice of the elastomer. Nevertheless, the chemical resistance of the other components in the formulation can not be overlooked. Talc is chemically resistant to water, acids, bases and organic materials. In addition, its platy morphology reduces the rate of diffusion of chemicals into the rubber. Its use in either mechanical rubber goods and/or hose will not compromise the resistance of the compound to the principle chemical such as refrigerants for AC, or to other automotive fluids such as oil which may be encountered during service. The effect of partial substitution of talc for carbon black is shown in table 3 for EPDM coolant hose. The compound containing talc has superior elongation at break after exposure to various fluids which were required by the OEM. Retention of elongation at break is considered to be the most important factor in the performance of an elastomer.

Another example of chemical resistance involved the exposure of EPDM hose compound to boiling coolant as part of a design experiment (DOE). A fractional-factorial DOE was implemented to evaluate the effect of the following variables on the performance: 1) volumetric percentage of polymer, 2) filler ratio, i.e., percentage of carbon black in filler system, 3) coagent concentration, and 4) zinc oxide level. The 12 compounds in the DOE were evaluated not only for resistance to coolant, but also for thermal aging, compression set, die C tear, cut resistance and tensile fatigue. In all cases, superior performance was achieved by partial substitution of talc for carbon black (ref. 10).

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Weathering resistance

In the case of external exposure such as window seals, talc reduces the effects of weathering on rubber compounds. This is demonstrated by the low value of [DELTA]E in table 4. In addition, talc reduces the change in Ab, which is a measure of the shift on the yellow/blue axis, by a factor of 100 (see figure 11 for an explanation of the lab color scale). Yellowing of the surface is aesthetically unacceptable in external automotive parts.

The Gray scale rating which correlates to uniformity of color meets the specification of OEMs. The lower rating corresponds to a lack of consistent color which suggests the possibility of iridescent bloom. Talc limits the migration of byproducts to the surface, thus reducing iridescent bloom. It also delays the appearance of crystals on the surface from the byproducts of certain peroxides.

The color change with 1,000 hours of exposure in a WOM (Weather-O-Meter) is compared to outdoor weathering in table 5 for an EV cured EPDM. Although the data indicate that the accelerated weathering was more severe, this could be due to a difference in the amount of UV radiation. Nevertheless, the trends are similar. The change in "a" and "b" values with the addition of talc is significantly less than the carbon black control in both outdoor and accelerated weathering. The substantial reduction in Ab translates to a reduction in yellowing of exterior rubber parts.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

Talc also reduces the shift in gloss of exterior rubber parts, as shown in figure 12 for EPDM window seal compounds where 50% of the carbon black has been replaced with talc.

Fatigue

Impulse testing is used to determine the acceptability of hoses for certain automotive applications such as power steering. This test subjects the hose to cyclic fatigue. Failure results from the crack propagation (ref. 11). Cracks are initiated by the combination of environment and stress. The effect of talc on the crack induction and propagation is demonstrated in the following two tables for DeMattia testing.

The number of cycles necessary to induce cracking in the talc-reinforced material is double that of the carbon black control. This indicates that the talc compound has superior fatigue performance. This is confirmed in table 7 by crack propagation data with an initial crack length of 2 mm.

[FIGURE 13 OMITTED]

The addition of talc has been observed to significantly improve the crack/tear resistance of carbon black reinforced elastomers in tension (ref. 2). The improvement in tear resistance is attributed to crack diversion phenomena and a reduction in the stress concentration at the crack tip due to the anisotropic nature of the compound resulting from the alignment of the platy talc reinforcement (ref. 12). This phenomenon was first observed by Eldred at General Motors Research Laboratories in cyclic fatigue testing of Hypalon (ref. 12).

The improvement in performance of compounds containing talc to surface cracks under transient tensile loading is demonstrated in table 8 and figure 13. The tensile specimens were cut with a razor blade across their face in the test region in order to simulate a surface crack.

Inspection of table 8 reveals that the energy to break, which corresponds to toughness, is significantly higher in the compounds with talc, e.g., the energy at break for the sample with 40% replacement is four times greater than the carbon black control.

This resistance to crack propagation is also apparent under dynamic fatigue testing (ref. 2). This is illustrated by the results from Monsanto fatigue-to-failure of EPDM specimens that were precut with a razor blade to a depth of 10% of their thickness. This significant improvement in fatigue equates to greater durability.

Talc also improves the resistance of rubber articles to premature failure due to edge nicks, as shown in table 12. The reason is apparent from the difference in the topography of failure surfaces in figure 14.

The combination of tear resistance and insensitivity to flaws due to talc results in increased toughness and durability of rubber parts thus providing superior performance in service.

Other considerations

The cost of finished rubber goods depends not only on price of raw materials, but also on the cost associated with processing. Talc has been shown to reduce the mixing time by 20% without influencing dispersion of reinforcing fillers (ref. 13). In addition, talc results in the following benefits:

* Lower compound viscosities;

* reduction in heat generation;

* improved mold flow;

* increased extrusion output; and

* reduction in nerve.

The above benefits expand the process window to provide latitude in manufacturing, such as reduction in temperature to avoid scorching.

In the case of molded parts, talc has been shown to improve the hot tear in a peroxide-cured EPDM air duct, as shown in table 10 (ref. 14). This increase in tear properties at elevated temperatures could allow de-molding of parts and consequently higher throughput.

Conclusions

Talc provides the design engineer or chemist with an alternate approach to meet the challenges of today and in the future for the automotive market. Talc has been shown to: * Improve thermal aging;

* eliminate electrolytic degradation;

* reduce permeability;

* provide chemical resistance;

* enhance weathering;

* upgrade fatigue performance; and

* increase toughness and durability.

Talc improves the processability of compounds, thereby reducing production problems and increasing output. In addition, talc can reduce raw material cost by substitution or partial replacement of expensive ingredients.

[FIGURE 14 OMITTED]

References

(1.) John Dunn, Dale Keller and Jeff Peterson, "EPDM compounding for high temperature aging requirements in automotive hose, "technical paper No. 78presented at the 132nd ACS Rubber Division Meeting in Cleveland OH (October, 1987).

(2.) O. Noel and G. Meli, "Synergism of talc with carbon black, " technical paper No. 13 presented at the 174th ACS Rubber Division Meeting in Louisville, KY (October, 2008).

(3.) H. Schneider, H. Tucker and E.T. Seo, "Electrochemical degradation of coolant hoses," technical paper No. 73 presented at the 141st ACS Rubber Division Meeting in Louisville, KY (May, 1992).

(4.) G. Meli, "New high aspect ratio talc improves impermeability of tire inner liners," IRC 2005 Yokohama (Oct. 2005).

(5.) Clark Cable and Charles Smith, "Epichlorohydrin in fuel hose, "technical paper No. 8 presented at the 150th A CS Rubber Division Meeting in Louisville, KY (October, 1996).

(6.) DSM EPDM Product Information, EH-5, "High Quality Heat Resistant Hose," (9/93).

(7.) W.F. Fischer, Technical Bulletin from Enjay Laboratories, "Electrical Grade Fillers in Butyl Rubber and Other Insulation Materials'."

(8.) Cable Technology Laboratories, EM60 Tests, 1/3/95.

(9.) J.R. Dunn and R. G. Vara, "Fuel resistance and fuel permeability of NBR and NBR blends, "presented at the 128th A CS Rubber Division Meeting in Cleveland, OH (October, 1985).

(10.) O.F. Noel, unpublished technical reports from Luzenac, 1998.

(11.) Mark E. Nichols and Robert A. Pett, "Effects of aging during impulse testing on the fracture behavior of power steering hose materials," technical paper No. 33 presented at the 145th ACS Rubber Division Meeting in Chicago, IL (April, 1994).

(12.) R.J. Eldred, "Effect of oriented platy filler on the fracture mechanism of elastomers," Rubber Chem. & Tech., Vol. 61, 619 (1988).

(13.) O. Noel, G. Meli and H. Thakkar, "Talc as a dispersion aid for reinforcing fillers in rubber, "Rubber World Vol. 237 (6), p. 35 (2008).

(14.) O. Noel and G. Meli, "Compounding for injection molding, "Rubber World Vol. 241 s(3), p. 23 (December 2009).

by Oscar F. Noel, III, and Dr. Gilles Meli, Rio Tinto Minerals/Luzenac
Table 1--elongation retention vs. exposure
time for NBR to 100[degrees]C air

Exposure time, days 7 14 21 42

 Elongation
 retention, %

Carbon black/sulfur cure 86 75 70 58
Carbon black + talc/sulfur cure 77 73 72 62
Carbon black/peroxide cure 84 80 63 51
Carbon black + talc/peroxide cure 89 87 76 62

Table 2--electrical properties of talc reinforced EPDM

Carbon black (Spheron 5000) 150 50 100 50

Talc (Mistron Vapor R) -- 100 -- 100

Clay -- 50 50 --

Property (test method)

Resistivity (VW PV 1015), 2.2 x [10.sup.6]
1.1 x [10.sup.15] [ohm].cm

Conductivity (GMI 60459), mV 342 3

Table 3--effect of exposure on EPDM coolant hose

Carbon black, phr 106 80 Limit VW
Talc, phr -- 65 TL 532 61
Ethylene glycol/water 50/50 C 160[degrees]C,
 94 hrs
Hardness change -3 -3 -5 to +2
Tensile strength, MPa 16.6 13.1 >9
Elongation at break, % 255 401 >250
Prestigrade TS 15W40 C 100[degrees]C, 22
 hrs.
Hardness change -16 -22 0 to -22
Tensile strength, MPa 10.9 5.4 >5.5
Elongation at break, % 135 226 >180
Diesel A20INPII @23[degrees]C, 46 hrs.
Hardness change -16 -24 0 to -22
Tensile strength, MPa 4.1 3
Elongation at break, % 95 175 >120

Table 4--color change after 500 hours WOM aging

Carbon black, phr 50 100 50
CaC[O.sub.3], phr 50 50 50
Calcined clay, phr -- -- 100
Mistron talc, phr 100 -- --
[DELTA]b 0.006 0.737 0.652
[DELTA]E 0.610 1.160 0.750
Gray scale rating 4 3 3
(PSA D27 1389)

(1) Compounds are black EPDM automotive window seals;
(2) [DELTA]E = [([DELTA][L.sup.2] + [DELTA][a.sup.2] +
[DELTA][b.sup.2]).sup.0.5] where [DELTA] is the change in value
with exposure.

Table 5--color shift with accelerated
weathering vs. outdoor exposure

FIEF carbon black, phr 120 60 60
Talc, phr 90
Silane treated talc, phr 90
 Accelerated aging 1,000
 hrs. WOM (1)
[DELTA] [L.sup.*] -2.99 -3.17 -2.94
[DELTA] [a.sup.*] 0.87 0.26 0.33
[DELTA] [b.sup.*] 2.91 1.16 1.14
[DELTA] [E.sup.*] 4.3 3.4 3.2
 Outdoor 6 months (2)
Grey contrast(3) 4 4 4
Gloss(4) 8.1 5.7 5.5
[DELTA] [L.sup.*] -0.21 1.45 1.93
[DELTA] [a.sup.*] 0.78 0.07 0.04
[DELTA] [b.sup.*] 2.55 0.54 0.41
[DELTA] [E.sup.*] 2.68 1.55 1.97

(1) PSA D27 1989; (2) Bandol, France per ISO 877 w/45[degrees]
southern exposure; (3) ISO 105-A02; (4) ISO 2813

Table 6--crack induction in DeMattia flex

N660, phr 60 (4) 30
Talc, phr (1) 30

 No. of cycles to failure

Degree 1 (3) 500,000 1,200,000
Degree 2 -- 1,500,000
Degree 3 -- 1,800,000
Degree 4 -- 2,000,000
Break 1,000,000 --

(1) Filler loading adjusted such that 100% modulus equal
to control; (2) samples pre-cut; (3) see appendix for
explanation of degree; (4) samples failed before reaching
criteria

Table 7--crack propagation in DeMattia flex

N660, phr 60 30
Talc, phr (1) 30
Crack length No. of cycles
 to failure
L to L+2 mm 11,000 14,000
L+2 mm to L+6 mm 31,000 78,000
L+6 mm to L+10 mm 46,000 185,000

Table 8--effect of talc on crack sensitivity in EPDM

N650 225 202.5 180 135 90
Talc 0 33.8 67.5 135 202.5
Wt. % N650 replaced 0 10 20 40 60
Tensile strength, MPa 5.70 6.76 9.33 8.53 8.89
Elongation @ break, % 121 147 236 304 478
50% modulus, MPa 1.96 2.02 2.51 1.70 1.65
Energy to break, J 1.11 1.65 3.98 4.58 7.60

Table 9--effect of talc on fatigue resistance
of black EPDM with surface flaw

N650, phr 134 98 74 61
Talc, phr 0 24 46 69
Cycles of failure, # x [10.sup.-6] 0.02 6.2 * 13.8 * 11.0

(1) 40% extension ratio; (2) " no failure; 3) 49 vol. % polymer;
(4) EPDM peroxide hose formula; (5) preconditioned for
100 hours at 100[degrees]C

Table 10--die C and trouser tear properties
as function of temperature

Wt. % N650 replaced 0 10 20 40 60

Die C tear, KN/m

@ 23[degrees]C 16.7 19.1 28.9 26.5 28.6
@ 80[degrees]C 9.4 11.2 11.8 14.0 16.7
@ 121[degrees]C 6.6 8.6 9.5 10.2 10.4

Trouser tear, KN/m

@ 23[degrees]C 3.2 3.9 4.7 8.9 4.9
@ 80[degrees]C 1.4 1.8 1.6 2.6 1.6
@ 121[degrees]C 0.9 0.9 1.2 1.6 1.1

Table 11--appendix--initial properties of EPDM cured with conventional
sulfur, non-nitrosamine and EV systems

Filler system CB MVRE MVRE-HS
Cure system Conv. Conv. Conv.
Mooney scorch, [t.sub.5], min. 3.02 3.93 4.03
Viscosity, [M.sub.L] 1+4 (MU) 105.2 70.8 65.4
Durometer A 76 77 77
Tensile, MPa Mn. 17 16.9 15.9
 Std. dev. 0.39 0.38 1.00
Elongation, % Mn. 354 480 465
 Std. dev. 10.9 20.0 26.9
50% mod., MPa Mn. 2.6 3.0 2.9
 Std. dev. 0.10 0.08 0.06
100% mod., MPa Mn. 5.4 4.1 3.9
 Std. dev. 0.26 0.12 0.09
300% mod., MPa Mn. 15.3 9.6 9.3
 Std. dev. 0.18 0.41 0.17
Die C tear, kN/m Mn. Mn. 33.8 34.0 32.3
 Std. dev. 1.45 1.52 0.80

Filler system CB MVRE
Cure system Non-nitros. Non-nitros.
Mooney scorch, [t.sub.5], min. 16.63 21.01
Viscosity, [M.sub.L] 1+4 (MU) 73.0 56.4
Durometer A 75 76
Tensile, MPa Mn. 16.2 18.1
 Std. dev. 0.30 0.28
Elongation, % Mn. 453 530
 Std. dev. 26.2 3.4
50% mod., MPa Mn. 2.2 2.8
 Std. dev. 0.04 0.07
100% mod., MPa Mn. 4.1 3.7
 Std. dev. 0.08 0.11
300% mod., MPa Mn. 11.8 8.6
 Std. dev. 0.24 0.12
Die C tear, kN/m Mn. Mn. 35.9 35.1
 Std. dev. 3.07 3.78

Filler system MVRE-HS CB
Cure system Non-nitros. EV
Mooney scorch, [t.sub.5], min. 22.51 14.42
Viscosity, [M.sub.L] 1+4 (MU) 54.3 74.8
Durometer A 76 75
Tensile, MPa Mn. 16.3 15.9
 Std. dev. 3.33 0.28
Elongation, % Mn. 533 455
 Std. dev. 56.7 19.5
50% mod., MPa Mn. 2.6 2.1
 Std. dev. 0.11 0.05
100% mod., MPa Mn. 3.3 3.9
 Std. dev. 0.15 0.15
300% mod., MPa Mn. 7.6 11.6
 Std. dev. 0.33 0.18
Die C tear, kN/m Mn. Mn. 35.0 37.9
 Std. dev. 0.45 1.05

Filler system MVRE MVRE-HS
Cure system EV EV
Mooney scorch, [t.sub.5], min. 15.7 16.41
Viscosity, [M.sub.L] 1+4 (MU) 57.5 55.2
Durometer A 73 75
Tensile, MPa Mn. 16.5 15.8
 Std. dev. 0.56 0.69
Elongation, % Mn. 606 556
 Std. dev. 10.0 15.3
50% mod., MPa Mn. 2.3 2.4
 Std. dev. 0.06 0.01
100% mod., MPa Mn. 2.9 3.1
 Std. dev. 0.08 0.03
300% mod., MPa Mn. 6.6 7.4
 Std. dev. 0.15 0.07
Die C tear, kN/m Mn. Mn. 37.8 35.1
 Std. dev. 3.28 1.90

Table 12--effect of talc on EPDM air duct with an edge nick

Wt. % N650 replaced 0 10 20 40
Tensile strength, MPa 3.4 3.4 4.7 4.8
Elongation @ break, % 93 98 129 192
Energy to break, J 0.5 0.6 1.1 1.6

Figure 9--fuel loss vs. % carbob black
replaced with talc for NBR with 34% ACN

Peak loss, gt/[m.sup.2]/day

 Fuel C Fuel C/methanol (85/15)

0% 265 710
20% 220 625
40% 175 600
60% 140 470

Note: Table made from bar graph.

Figure 12--change in gloss of EPDM
window seal compounds after 500
hours exposure in a WOM

 Gloss R85[degrees] Gloss R85[degrees]
 before aging after aging

Carbon black 71 58
Mistron talc 55 55

Note: Table made from bar graph.
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Date:Apr 1, 2011
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