A MOLDED-IN-COLOR UV-STABLE SHEET MOLDING COMPOUND:ONE STEP BEYOND

Paul A. Rettinger, Plasticolors, IncorporatedSelby M. Brannon, Plasticolors, Incorporated

Carl E. Gleditsch, Ashland Specialty Chemical Company, Division of Ashland Inc.Michael Sykes, Ashland Specialty Chemical Company, Division of Ashland Inc.

Abstract

An ultraviolet (UV)-stable, pigmentable system,which is designed to be formulated into sheet moldingcompound (SMC) for external structural applications, andwhich eliminates the need for paint, is now available.

Hailed as “the next generation of SMC”, theformula meets original equipment manufacturers’(OEMs) requirements for pickup truck box applications.The material has excellent mechanical properties and after10,000 hours of weathering, the material retains its colorand appearance without a need for painting.

Background

Thermoset composite plastics have been used toprovide aesthetics and durability to the automotiveindustry for more than half a century. In the 1980s,energy costs and average fuel economy requirementswere driving forces for the development of vehicles usingnewer, lighter weight materials. One example of aproduct demonstrating such characteristics was theminivan.

In the early 1990s, the planned development ofplastic cargo boxes for pickup trucks and heavy vehicleswas put on hold. One reason for this delay was that carand truck manufacturers perceived performance issuesassociated with composite materials. An importantsticking point was the inability of classical thermosetmaterials to withstand direct exposure to outdoorelements.

As the decade progressed, driven by a need tocontain costs and improve corporate average fleeteconomy, the transportation industry again slowlycommitted itself to the conversion of pickup boxes,liftgates, tonneau covers and fenders from steel tothermoset composite plastics. Several manufacturershave given the go-ahead to produce commercial pickuptruck boxes utilizing composite materials. Currently, twopickup truck boxes (figure 1) are in production. It isestimated that within two years, four pickup truck models

will be equipped with specialty molded composite boxes,liftgates (figure 2), tonneau covers and fenders.

According to the Automotive CompositesAlliance, the light truck market segment alone consumedan estimated 125 million pounds of composite in the 2001model year. In the transportation industry, demand forthermoset composites is expected to grow from 318million pounds in 2000 to 467 million pounds by year2004. Applications, such as body panels, decklids andhoods will continue, but additional growth remainstargeted at limited production (“niche”) vehicles. Lighttrucks will provide the first significant growth opportunityfor composites since the minivan. Light trucks representthe highest overall sales volume in the automotiveindustry, as they account for more than 50 per cent of allvehicles sold in North America.

A long-standing issue associated with light truckshas been corrosion (i.e. rust) in their metal panels.Corrosion is aesthetically displeasing to consumers, and itlimits the useful life span of the product. Historically, thetransportation industry has adopted strategies forcontrolling or eliminating the problem. Boxes made ofsheet metal go through an expensive four-stage processthat includes galvanizing, surface treating, priming andtopcoat painting. The objectives are to provide a pleasingappearance and to protect the sheet metal.

One of the primary advantages of thermosetcomposites in such an application is that they are notsusceptible to rust. There are limitations, however,including susceptibility to a variety of other processes,such as oxidation, and ultraviolet light degradation.Before light truck manufacturers can use composites, theymust use a primer and topcoat to protect the compositefrom those influences, similar to how metal parts areprimed, painted and top-coated. The costs associated withthe coating materials and application process can beestimated according to the following model:

Total Cost of Coating = Material + Capital +Labor + Scrap+ Rework + (Environmental)

©2001, Ashland Inc. and Plasticolors, Inc.

We have estimated the cost of prime coating atypical six-foot truck box at between $50 and $80 perbox. This is done via typical cost values associated withcoating thermoset composites and does not include anebulous contribution from the environmental costcategory. The costs associated with the primer aresignificant and have become a target for reduction orelimination. The key to accomplishing this is through thedevelopment of composite material that does not requirepaint, primer and otherwise.

Chemical additives relating to the usable lifespan of an exterior composite, and its ability to resist heat,ozone, humidity, chemicals, and ultraviolet light havebeen studied extensively over the past thirty years. Withrespect to materials science, an enormous amount of workhas been done at the system formulation level to optimizepolymer, additive, pigment and filler selections. As aresult, it has been possible to address the many andvariegated causes of organic composite degradation.

However, due to the breadth of issues thatmaterial suppliers must address, it is difficult for onesupplier to address single-handedly all issues associatedwith a new technology or a new application in thetransportation industry. The solution that is ideal for onepurpose is seldom a panacea for all systems and all end-use applications. That fact, coupled with the economicand technical imperatives of automotive vendors andconsumers, has resulted in a multidisciplinary approach toresearch and development, with surprising advantages toquality and value of raw materials and services. Researchprograms that identify specific modes of compositefailure, and develop means of controlling them, offer aneffective means for dealing with these intricacies. Thatleads us to a line of query: Docolorants and other additives play arole in the appearance-relatedlongevity of molded composites? Whatsynergisms may be found betweencolorants, polymers, fillers,catalysts, glass, and other additivesin protracting the useful life span ofan exterior transportation composite?

Scope of Technology

As compared to parts made from manyalternative materials, parts made from SMC are strong,physically durable and cost effective. As previouslymentioned, molded SMC is lighter than steel and will notrust or corrode.

SMC consists of a thermally reactive resin whichcan be chemically reacted or “crosslinked” into an elasticpolymer. Fillers, colorants, additives, catalysts and glass

fibers are added to the resin to produce a reactivecompound that can be molded. Under conditions of heat, achemical reaction takes place in the mold, resulting in athermoset-compounded part of desirable shape,dimensions and mechanical properties.

Traditionally, a drawback to non-coatedreinforced composite is that it does not retain appearancecharacteristics when stored outdoors. When it is exposedto sunlight, oxygen or moisture the color will fade and thesurface will erode and turn dull. When this happens, it ispossible for glass fibers to become visible. While this doesnot effect physical properties of the part, the resultingappearance is not viewed favorably by consumers.

In 1996, several companies formed a jointtechnological research program to investigate causes ofweathering degradation in SMC composite and to devise asolution. The objective of this program has been toeliminate the need for painting composite pickup boxesand other painted automotive components. Designs ofexperiments have identified synergies betweenthermosetting resins, low profile additives and coloranttechnologies. Over the course of several generations ofresearch, it became possible to quantify and optimize suchsynergisms, leading to new levels of acceleratedweathering durability. Consequently, there has been aleapfrog in technology, with the capacity to dramaticallyimpact a number of applications desired by thetransportation industry.

At the present time, most assembly plant truck-boxes, liftgates, tonneau covers and fenders are made ofsteel. With steel, it is necessary for the transportationaftermarket to address cosmetic issues associated withcorrosion. Typically, this is done using either urethane orolefinic bed liners. However, this approach also has somedrawbacks including bed liners that may trap moisture,which could lead to corrosion of the box well. They arealso susceptible to static dissipation and color fading overtime.

One solution to these problems has been to use acomposite and coat the surface with a durable coloredcoating. In this scenario, the composite provides themechanical properties, while the coating supplies theappearance characteristics. This solution contributes anestimated $0.75 per pound to the cost of the part and itreleases volatile organic compounds into the environment.

In order to minimize weight, as well as paintcosts, a durable reduced-weight material would need to befound to replace painted metal. To meet these objectives,the material properties required would includepigmentability, ultraviolet stability and mechanical

durability. A properly formulated molded-in-color UV-stable SMC composite provides for the employment of anon-painted composite, which possessed thesecharacteristics. In this manner, the need for painting,along with some of the associated costs, is eliminated.

As a replacement for steel, traditional SMCtechnology can reduce weight and provide environmentalbenefits to the end user. But, consumers demandvehicular components which are durable, aestheticallypleasing and minimize noise in the driving environment.Only a molded-in-color UV-stable SMC can satisfy theseexpectations.

Experimental Results

As a part of the development process, samples ofblack SMC compounds were subjected to testspecification SAE J-1960-kJ accelerated weathering withthe modification that boro-silicate inner and outer filterswere used in a standard weatherometer. Panels texturedwith Camera Case SZ and Corinthian LJ were exposed to10,000 hours, or 13,200 kJ (kilojoule) of acceleratedweathering. This is the equivalent of nine years ofoutdoor exposure in south Florida.

Under those test conditions, traditional SMCunderwent significant failure within several hundredhours of accelerated weathering exposure. Initially, thetest substrates began to turn from black to gray, and thenthe surface began to erode away.

The surface erosion produced several notable events:

1) Surface erosion produced a significant loss insurface reflectivity, causing substantive dulling.Attempts to restore the original surface finish viawashing and waxing were not effective. Thisfailure was quantified as a loss of gloss (i.e.surface reflectivity) using quantitativemeasurements of light reflected at a 60° angle ofincidence using a standard gloss meter.

2) Black colorant particles normally embeddedbeneath the surface of the substrate becameexposed, resulting in a measurable darkening ofthe substrate. In our work, relationships of light-to-dark were quantified by reflectancespectroscopy of visible light and are expressed interms of “L*” value, as measured using astandard spectrophotometer.

3) The final event of surface erosion is the actualexposure of glass fibers beneath the surface.They appear as tiny silvery strands emergingfrom the substrate.

By contrast, sample specimens of a mold-in-color UV-stable SMC exhibited minimal surface erosion or changein darkness/lightness value and no visible glass exposure,even when subjected to 10,000 hours of xenon arcexposure.

In these findings, two aspects of appearance wereidentified as quantifying visual change: gloss and L*.Gloss, as an aspect of appearance, relates to the scatteringand reflection of light from a substrate. Decreased surfacereflectivity is often the result of microscopic physicaldisruptions in the surface of a material. While a rubber tireand the paint on an automobile may both be black, theblack paint on the automobile is very high gloss while theblack tire has a very low surface reflection. Consequently,a tire and a black automobile appear very different tovisual observers, even though the color itself may besimilar. After accelerated weathering exposure, a molded-in-color UV-stable SMC bears similarities to automotivepaint, whereas traditional SMC resembles an old,unpolished tire. Is it possible for an unpainted SMC tolook like painted metal after weathering? The answer,surprisingly, is yes.

Color change, also an aspect of appearance, isrelative to an individual observer. Color interpretation ismeaningful when quantified in relationship to othercolored materials. Someone who is sitting under afluorescent light source reading a paper may look at thepaper and observe that the print is black; however, theblack print is comparatively gray when seen outdoors nextto the surface of a car tire. One reason this occurs is thatblackness or whiteness will vary according to levels withinany color. Another reason is that color is dependent uponthe light source, which changes when one goes fromindoor fluorescent lighting to outdoor natural sunlight. Inthe case of the black print, the contrast ratio is changedbetween light sources, and the brain adapts to thedifferences in stimulus accordingly.

Naturally, color is concerned with hues of blue,yellow, red and green, from which observers makeadditional subconscious interpretations. However, sincethe SMC of this work is currently black, our work is notfocused simply on hue, but on lightness/darkness,described as the L* aspect of color value.

Note:

For the purposes of this paper, differences incolor as measured via a spectrophotometer, may becomputed according to CIEL*a*b* (1976) colorcoordinates according to the equation:

delta E* =[(L*(standard) - L*(trial))

2 + (a*(standard) - a*(trial))

2 +(b*(standard)

- b*(trial))2]-2,

L* represents a theoretical absolute light/dark value, a*is absolute red/green value, and b* is the value forblue/yellow. Since the compound of this research iscurrently a black-colored compound, L* is the preeminentfactor in determining color constancy.

It was observed in our findings that traditionalSMC composites lose surface integrity within a yearequivalent of south Florida exposure. As previouslyexplained, this failure is demonstrable in terms of L*value change and is marked by a dramatic darkening.

Having identified the problems and developedsome potential solutions, our next step was to initiate theprocess of verifying the performance of molded-in-colorUV-stable SMC, in comparison to a high-performanceautomotive topcoat primer used in production.

L* and gloss data were collected on specimensof the coating through four years of actual south Floridaweathering. Similar data was collected on molded-in-color UV-stable SMC panels tested via SAE J-1960 kJ.Finally, for purposes of correlating artificial to naturalsunlight, samples of UV-stable SMC were submitted toaccelerated weathering via EMMAQUA1, a test protocolwhich uses mirrors to focus sunlight upon specimens at aweather testing laboratory in Phoenix, Arizona. From thistesting, we were able to obtain the following data:

A) Nine-years equivalent south Florida exposure viaa weatherometer (SAE J-1960-kJ)

B) Four-years of actual south Florida data onautomotive topcoat primer

C) Three-years equivalent south Florida exposurevia EMMAQUA (natural sunlight)

Together, tables 1 and 2 suggest a similarity between theweathering performance of a molded-in-color UV-stableSMC and an automotive topcoat primer. But there is anadditional advantage to molded-in-color.

When a painted SMC composite panel isscratched or abraded, the paint layer is removed, revealinga discoloration or color difference between the damagedarea and the area surrounding it. By contrast, a molded-in-color UV-stable SMC, when scratched or abraded,retains very nearly the same color as when it is notscratched or abraded. That represents an additional value-add for appearance-conscious consumers.

Conclusions

In this research, relationships among colorants,additives, fillers and polymers were utilized to prolong theappearance of an exterior SMC compound. Theappearance longevity of a black composite part can bedescribed in terms of changes in L* and gloss retention,that result from degradation of the surface.

A multi-disciplinary approach to developmentwas used to achieve weathering performance in molded-in-color SMC composite that would not have been possible afew years ago – performance that is only possible with thecorrect combination of resin, filler, colorant, and additivepackages.

The beneficiaries of such technology areconsumers, who want better appearance with improveddurability, drivability, and fuel economy – all at a lowercost. By solving technical dilemmas of appearanceretention and at the same time addressing the needs of endusers, a molded-in-color UV-stable compound takes SMCtechnology a step beyond what traditional thinking hasheld possible.

Figure 1: Does your truck have a box made of composite?

If you own a Ford Explorer Sport Trac2, the answer is yes.The composite box inner is molded and painted with aspecial black coating. Notably, the box is not susceptible tocorrosion and is lighter than its metal counter parts, whichhelps to enhance the ride quality and fuel economy of thevehicle. Naturally, a structural part such as this mustwithstand a rigorous regimen of testing, includingdropping a tilted drum filled with water on the floor of thebed. It also needs to be resistant to ultraviolet light.

1Registered trademark of Heraeus DSET Laboratories,Inc.2Registered trademark of Ford Motor CompanyFigure 2: Have you seen the inside of your liftgate?

The tail or liftgate assembly of most pickup trucksconsists of three panels: the inner-gate (facing inside ofthe vehicle), the endgate (what you see from the rear),and a third panel, which is sandwiched between the innerand outer panels for purposes of structural reinforcement.This is the heaviest part of the assembly. However, byreplacing the steel midgate assembly with composite,weight savings of 20–25 per cent can be realized.Reduction in weight of the vehicle is associated withimprovement in gas mileage. This helps provide a moreenvironmentally friendly vehicle that is also more costeffective to use.

Table 1: L* Performance

Table 1 shows the lightness/darkness stability of molded-in-color UV-stable SMC as compared to an automotivetopcoat primer. Data from two accelerated protocols arecompared to south Florida test data on an automotivetopcoat primer. The data suggests that a molded-in-colorUV- stable SMC is likely to have color durability similarto that of a black automotive topcoat primer currentlyutilized in production.

Table 2: Gloss Performance

Table 2 demonstrates how the gloss retention of molded-in-color UV-stable SMC nearly overlaps with that of anautomotive topcoat primer. Again, the topcoat primerdata represents four years of Florida-direct exposure,while the data on molded-in-color UV-stable SMCrepresents equivalent data from the weatherometer (SAEJ-1960-kJ) and EMMAQUA, respectively.

Authors:

Selby M. BrannonPlasticolors, Incorporated

2600 Michigan AvenueAshtabula, OH 44004

Carl E. GleditschAshland Specialty Chemical Company

5200 Blazer ParkwayDublin, OH 43017

Paul A. RettingerPlasticolors, Incorporated

2600 Michigan AvenueAshtabula, OH 44004

Michael SykesAshland Specialty Chemical Company

5200 Blazer ParkwayDublin, OH 43017

L* Stability

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