WO2006083506A1 - Light-induced gold iridescence of epdm weatherseals - Google Patents

Abstract

The present invention is directed towards compositions which comprise an ethylene-propylene-diene (EPDM) rubber and a nano-structure carbon black. Furthermore, a method for preventing the development of gold iridescence in carbon black filled EPDM rubbers comprising the step of employing a nano-structure carbon black is disclosed.

Description

LIGHT-INDUCED GOLD IRIDESCENCE OF EPDM WEATHERSEALS BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for reducing surface iridescence from

ethylene-propylene-diene terpolymers exposed to sunlight.

2. Description of Related Art

As is well known to those skilled in the art, extruded and molded rubber compositions

have been widely used in a variety of applications, particularly in the automotive field, for

gaskets, seals, hoses, grommets, tubing, rub strips and bumpers. One type of rubber that has

enjoyed considerable success in those applications owing to its favorable processing characteristics and vulcanizate properties has been the ethylene-propylene-diene terpolymer (EPDM) rubbers. These rubbers are well known to those skilled in the art, and are formed by interpolymerization of ethylene, one or more mono-olefins containing 3-16 carbon atoms, preferably propylene, and one or more polyenes containing a plurality of carbon-to-carbon

double bonds.

In particular, automotive weatherseals are predominantly made from EPDM, which has become the polymer of choice in these applications because of its excellent

weatherability, being impervious to ozone attack and highly resistant to both oxidation and

high temperature, and its ability to be highly extended without losing physical and

performance pi uperties. Unfortunately, when exposed to sunlight, a subtle discoloration of the surfaces of

weatherseals and other articles prepared from EPDM develops. Initially, the discoloration

appears as a blue, red, or rainbow iridescence similar to the sheen of a film of oil on water.

With continued light exposure, this subtle color changes to a bronze or gold color. The

appearance depends upon the angle of view and strength of illumination. When examined

perpendicular to the surface, black predominates. When viewed at an angle, the blue / red /

gold colors become more evident. Moderate light conditions are best for viewing the

iridescent colors. Although the color does not affect the article's properties or function, a

remedy has long been sought.

Various workers have examined this phenomenon, each finding a somewhat different cause and proposed solution. (See, for example, Anon., Iridescence Effects in EPDM, DSM Technical Literature (1987); Eric P. Jourdain, Compounding Parameters to Control the Iridescent Bloom, Paper #42 presented at 149^th meeting of the Rubber Division American

Chemical Society (May 1996); and Lars C. Larsen and Paul A. Danilowicz, Reducing iridescence in EPDM weatherstripping. Rubber and Plastics News, pages 12-14, (May 29, 2000).)

U.S. Patent No. 5,354,793 discloses an accelerator composition and a rubber compounding composition in which a rubber is blended with a combination of three

accelerators and sulfur for curing the rubber while minimizing the formation of iridescent

sheen. The combination of accelerators included a thiolated morpholine, a dithiocarbamate

and a benzothiazole. Despite these efforts, commercial weatherseal products are still susceptible to the

development of gold iridescence due to light exposure or "sunburn". As noted above, this

discoloration is merely cosmetic, and does not affect the part's properties or function.

A common issue with many rubber articles is the appearance over time of a powdery

solid discoloration on the surface. This surface film is known as "bloom". It may be more or

less firmly attached to the surface depending on the specific materials that have migrated to

the surface. The remedy for bloom is straightforward: the material at the surface is removed,

its composition determined, and the dose of the material(s) identified either lowered or their

use eliminated entirely. Similarly, "bleed" is the appearance of a liquid film at the surface, and the procedure

to resolve the undesired film is the same as for bloom.

In. contrast, iridescence cannot be readily removed for identification. Only by using enough force to abrade away the surface physically can be the iridescent colors be removed. Some powerful solvents for EPDM, e.g. methylene chloride, appear to be able to remove the

colors temporarily. However, as soon as the solvent evaporates and the puffed-up surface of the part returns to its normal flat configuration, the iridescent colors reappear.

As noted in U.S. Patent No. 5,354,793, referred to above, for the end uses cited above, fabrication is not complete until the rubber article has been vulcanized or cured to enhance

mechanical strength and stability necessary for prolonged usage. The side chain unsaturation of EPDM permits curing by a variety of mechanisms

including peroxide, sulfur, and resins. The choice of a vulcanizing system is important since

it affects stress-strain properties of the final vulcanizate as well as heat resistance and compression set. Resin cross-linking systems are employed for EPDM when improved heat

resistance is required and when the EPDM is formulated in combination with other rubbers

that cannot be cross-linked with sulfur or peroxide. Peroxide cures are used in specialized

applications instead of sulfur cures to provide articles with improved heat resistance and

compression set. The use of a peroxide cure will have restrictions with regard to the raw

materials used, the curing processes that can be employed, and often vulcanizate

characteristics.

Certain raw materials should be avoided with peroxide cures. Acidic materials, such

as some clays and other fillers, will impair cross-linking owing to heterolytic decomposition

of the peroxide and fewer peroxy radicals will be generated. Many antioxidants will

scavenge the peroxy radicals during cross-linking, resulting in a lower state of cure. Aromatic structures, such as those contained in some process oils, will compete with the polymer for peroxy radicals and also detrimentally lower the state of cure.

During curing when peroxy radicals are generated, contact with oxygen in the air must

be excluded. Therefore, hot air oven cures are unsuitable for peroxide cures and immersion in liquid cure media (LSM) is used instead. Autoclave cures must be thoroughly purged with

nitrogen or steam to exclude residual traces of oxygen. In molded operations, the overflow flash in contact with air will remain tacky and make deflashing of the part difficult.

Vulcanized articles made with peroxide cures can be odiferous or exhibit bloom,

depending on the specific peroxide chosen, Formulations requiring high levels, i.e. greater than twenty percent, of naphthenic or

paraffinic processing oils and carbon black are also slow to cure even with high levels of

peroxide. Sulfur cross-linking systems are used more broadly with EPDM since no special

techniques or processing equipment are required for formulating, extruding or molding, and

vulcanization. By adjusting the level of unsaturation in the base EPDM, sulfur systems can

very economically and effectively be used to control the degree of cure in the fabricated

article, without concern for cross-linking during extrusion or molding. By the proper choice

of an accelerator, very rapid vulcanization cycles can be achieved.

A typical vulcanization recipe for a sulfur cure system would include (1) an activator,

commonly metal oxides, such as zinc oxide, magnesium oxide, manganese oxide; and fatty

acids, such as stearic acid used in conjunction with the metal oxide if an organic accelerator is used, (2) sulfur or a sulfur masterbatch, and (3) an accelerator, needed in order to produce a specific degree of cure in a practical time for commercial use.

In the formulation of EPDM for the applications of interest here, it is generally necessary to incorporate carbon black and plasticizers or processing oils. The carbon black is

used as a reinforcing agent and to provide stability against detrimental radiation and ozone. The processing oil reduces the effective viscosity of the blend so that high Mooney viscosity,

more economical and readily available types of EPDM rubber can be used.

In applications where a carbon black formulation is used, the iridescent sheen referred

to above has been observed on dense and cellular extruded and dense molded parts both prior

to and after vulcanization. It is known that the sheen is a surface phenomenon which exhibits

visual colors of gold, greens and blues, and that the greater the surface area of the extruded or molded part the more intense the condition of iridescent sheen. Broader use of EPDM in

many automotive applications is hindered by the oil-on-water appearance associated with the

iridescent sheen phenomenon. Color coding of the various automotive parts is of particular

concern to today's automotive design engineers, and black parts would be specified more

often if the quality of the black surface could be made compatible with other colors.

The iridescent sheen phenomenon occurs particularly with exposure of the fabricated

article to ultraviolet light (normal fluorescent light has a sufficient UV intensity to activate the sheen) and ozone. It has been determined previously that the appearance of the sheen can

be accelerated by placing a sample of the molded or extruded part in an ozone chamber with

an ozone concentration level of 50 pphm for four hours.

In studying the phenomenon of iridescent sheen, numerous phases of the fabrication process have been examined including formulations, methods of compounding, conditions of extrusion or molding, and methods and conditions of curing. It has been recognized previously that components of the formulation and more typically the sulfur and plasticizers would bloom, i.e., migrate to the surface of the molded or extruded part, Much of the

comppunding literature teaches ways in which sulphur bloom can be minimized by a proper choice of sulfur, for example, reduced use of sulfur by incorporation of organic sulfur

vulcanizing agents. The iridescent sheen observed here is a problem distinguished from that

of sulfur bloom and usually both are not observed with the same formulation.

SUMMARY OF THE INVENTION

The present invention is directed to the prevention of the development of gold

iridescence in EPDM rubbers.

More particularly, the present invention is directed to a method for preventing the

development of gold iridescence in carbon black filled EPDM rubbers comprising the step of

employing as the carbon black a nano-structure carbon black.

In another embodiment, the present invention is directed to a composition comprising

an EPDM rubber and a nano-structure carbon black.

In still another embodiment, the present invention is directed to an article of manufacture comprising a composition comprising an EPDM rubber and a nano-structure

carbon black, whereby said article is free from gold iridescence on its surface after exposure

to ultra-violet light.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) In addition to the EPDM polymer, three main components of a weatherseal formulation are filler, hydrocarbon oil, and curatives - each of which have been implicated as contributing to iridescence. The initial study in the present work was a 2³ designed

experiment to systematically vary the type of filler, oil, and curative system. The goal was to identify both the effects of the individual components and the interactions among the

components as they affect iridescence.

Two additional experiments examined an additional variable - a three-component

additive package for delaying the onset of gold iridescence. Filler

The filler in a typical weatherseal formulation is a semi -reinforcing, furnace-process

carbon black, optionally supplemented with low-cost inorganic fillers, e.g., ground calcium

carbonate. Furnace blacks are manufactured by oxidizing the residual oil fraction from a

refining process. As such, there are trace levels of sulfur, nitrogen, and oxygen in addition to

carbon itself. Furthermore, these blacks contain small amounts of aromatic structures that

cause staining.

As an alternative, a thermal-process carbon black was substituted. Thermal blacks are

made by heating natural gas in an inert atmosphere and are therefore free of sulfur, nitrogen, and oxygen and can be made non-staining. Because thermal blacks provide lower hardness than furnace blacks, the thermal black was supplemented with a high-purity precipitated silica

for an equal hardness comparison.

Oil

Today's paraffinic oils are highly hydro-treated. Like furnace black, they are made from crude oil and contain some aromatic structures, as well as trace amounts of nitrogen and sulfur. In the designed experiment, the paraffinic oil was compared to Synton^™ poly(alpha-

olefin), which is made from decene and contains no sulfur, nitrogen, or residual unsaturation.

Cure System

Weatherseal cure systems are typically based on elemental sulfur together with

organic accelerators. These accelerators generally contain nitrogen in the molecule. A sulfur

cure system was compared to a peroxide cure. Peroxide cures will provide some oxygen in

the vulcanizate, but no sulfur or nitrogen. Accelerator fragments have been identified on the iridescent surface by others , so it was expected that this variable would be highly significant.

Results of the 2³ Designed Experiment

The relative gold ratings for eight combinations of filler, oil, and cure system after 6

days exposure per SAE Jl 960 (exterior test protocol) were determined. It was found that the

filler provided the largest (about 2 points) difference in ratings, with the thermal black

providing consistently lower gold ratings. Nevertheless, it was considered that this difference

might not represent a true reduction in the generation of the gold color because the color is

partially hidden by a change during exposure from a high-gloss (shiny) to a low-gloss (mat)

surface appearance.

The oil provides the next largest change in ratings, with a consistent one point lower rating for the poly(alpha-olefin).

Surprisingly, the cure system does not appear to affect gold ratings by the exterior test protocol. The three-component additive package was effective in only one of two recipes examined. The same formulations were subjected to six days exposure per SAE Jl 885 (interior

test protocol). The filler again provided the largest difference in gold ratings in favor of the thermal black, with this black again changing the surface to low-gloss during exposure.

The cure system did affect ratings in this test, with the sulfur providing 0.5 to 1.5

point better gold ratings. The poly(alpha-olefin) provided 1 to 1.5 point worse results with the thermal black system, but did not affect ratings in the furnace black compounds. The

three-component additive package was effective in both recipes examined. Unfortunately, there appeared to be no highly statistically significant interactions

among the variables by either test protocol. Further, no combination of ingredients provided

a gold-free vulcanizate after light exposure.

Surface Analysis

For samples after light exposure and "as molded", the gold and non-gold surfaces

were examined on a micrometer scale with scanning electron micrograph (SEM) pictures. It

appeared that raised nodules had developed on the gold surface, whereas the black (non-gold)

area remained relatively smooth.

The largest nodule on the gold surface was about the size for which it was thought the

composition could be determined using a narrowly-focused beam from time-of-flight secondary ion mass spectroscopy (TOF-SIMS) to drill down into the nodule. However, this technique could only identify silicone mold release, wherever the surface was sampled. TOF- SIMS of extruded samples (without mold release agent) were also not informative.

A third surface analysis technique employed was X-ray photoelectron spectroscopy (XPS), formerly known as electron scattering for chemical analysis (ESCA). The XPS beam cannot be as narrowly focused as TOF-SIMS, but it can drill down deeper into the surface to

provide the average composition for the surface and the near surface. The XPS analysis

showed the main effect was an increase in oxygen and a decrease in carbon at the surface and

the immediate interior of the gold areas. XPS further showed small, but consistent, increases

in zinc and nitrogen for the gold samples compared to their black counterparts. The large increase in oxygen for the gold samples indicates that light causes an

oxidation reaction, a result in agreement with theory. In fact, simply overheating a

vulcanizate in an air oven (without light exposure) can create the gold color. Neither

antioxidants nor light stabilizer additives can prevent the eventual development of the gold

color.

A recently developed surface analytical technique is atomic force microscopy (AFM).

This method can extend magnification down to the nanometer scale and visualize individual

atoms. AFM was used to examine the black and gold surfaces and it was found that, as with

SEM at larger scale, the black area was relatively smooth. The gold areas exhibited the

development of nodules, with the low-gloss gold area entirely composed of nodules.

Based on the above-described surface analysis results, an experiment was carried out with the gold surface of a light-exposed test pad. A portion of the pad was clamped in a compression set jig for one week at room temperature. When removed from the jig, the compressed portion was no longer gold. A piece of the test pad, with both gold and non-gold (formerly compressed) areas, was exposed to additional light. The gold was found to return to the compressed, non-gold, areas. Similarly, another test pad piece with gold and non-gold areas was put into an air oven for 24

hours at 70⁰C. The gold was also found to return, suggesting thermal relaxation is sufficient

to allow any compressed structures to re-emerge.

Accordingly, it was concluded that removing the gold by compression is not a

permanent solution to the problem. From a theoretical perspective, this experiment suggested that the nodular structures

found on the surface of weatherseal vulcanizates could be associated with the appearance of

gold. For other materials, it is known that small particles can scatter light of particular

wavelengths (See Chad A. Mirkin, George C. Schatz, et. al.. Science, 294, pages 1901-1903

(2001), summarized in Chemical & Engineering News, page 10 (Dec. 2, 2001).) One

example is color-shifting automotive coatings that create "color by physics", a term coined by

Flex Products for their special pigments for paints. (See www.colorshift.com.)

The above work suggested that the formation of nano-scale structure is associated

with the light-induced development of gold iridescence in EPDM vulcanizates. (The term

"nano-scale" refers to dimensions measured in nanometers, i.e., 10^"9 meters.) It was noted

that the size of the structures formed during light exposure was similar to that of ordinary zinc oxide and the semi-reinforcing carbon blacks that are typically used in EPDM compounds.

Accordingly, EPDM compositions comprising fillers and ZnO outside the particle size range of nodules that develop during light exposure were examined by the SAE Jl 885 test procedure. The compositions and results are shown in Tables 1 and 2, respectively.

Compounds were prepared according to ASTM Method D3182, sections 6.2 and 7.2. The ingredients for each recipe - except the curing system (the last six rows in Table 1) - were

mixed in a "B" scale Banbury mixer to make a series of masterbatches. Each masterbatch

was sheeted and cooled, the masterbatches were then returned to the Banbury mixer to add

the curing system in a second, cooler pass to form the fully functional compounds, which

were then sheeted and cooled.

Claims

CLAIMSWhat is claimed is:

1. A method for preventing the development of gold iridescence in carbon black filled

EPDM rubbers comprising the step of employing as the carbon black a nano-structure carbon

black.

2. A composition comprising an EPDM rubber and a nano-structure carbon black.

3. An article of manufacture comprising a composition comprising an EPDM rubber and

a nano-structure carbon black, whereby said article is free from gold iridescence on its surface

after exposure to ultra-violet light.

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