PVC: the next 'engineering' thermoplastic?

With improved additive and alloy techniques to boost its heat resistance and impact strength, PVC is being cast in a new role as a low-cost high performer for durable goods.

In just the past few years, processors who once relied on more costly "engineering" thermoplastics like ABS and polycarbonate have switched to a "new," less expensive alternative. That resin, whose utility in durable goods was once thought to belong mainly to pipe and fittings, house siding, and window profiles, is rigid PVC.

PVC users and manufacturers agree that vinyl has a variety of benefits, not the least of which is low cost. Typical injection molding-grade polycarbonate runs about $1.60/lb while a high-quality rigid vinyl for "engineering" applications costs around 85|cents~/lb. Furthermore, because PVC is derived half from petrochemicals and half from common salt, its price tends to remain relatively stable since it is partially insulated from oil-price fluctuations.

Couple this economic advantage with the material's inherent flame retardancy and uv stability, and, users say, vinyl warrants serious consideration by those accustomed to using more traditional engineering materials.

"The PVC and alloys we have today have the properties to be engineering polymers," says Paul Plasschaert, marketing manager for business machines at Monsanto Chemical Group. Recent blending and alloying technologies, as well as advances in property-enhancing additives have given compounders and resin producers the ability to tailor vinyl so it can attain heat-distortion temperatures close to 200 F together with notched Izod impact strengths near 12-14 ft-lb/in., opening up previously impenetrable markets for PVC and its alloys. Industry insiders say they see these inroads continuing to increase in coming years.

TABULAR DATA OMITTED

WHAT'S 'ENGINEERING'?

The consensus obtained from a poll of materials suppliers and users is that "engineering" PVC has a heat-distortion temperature of at least 170 F under 264-psi load, and/or notched Izod impact strength starting at 10 ft-lb/in. As a guide to what's available in that range, we used our PLASPEC On-Line Engineering Databank to select by computer TABULAR DATA OMITTED all grades of rigid PVC and ABS/PVC alloys meeting either of those minimum "engineering" specifications. The results appear in Tables 1 to 4.

TABLE 3--ABS/PVC ALLOYS WITH "ENGINEERING' IMPACT STRENGTH
                             Notched Izod                HDT
@                                                    Trade Name
@1/8"          HDT
@         264                                        Supplier      & Grade
ft-lb/in.      66 psi, F    psi, F
Alpha         Dural 776/X3
  Chemical    Rev. 2         15.3           nr           154
BFGoodrich    Cycovin K25    12.0           170          162
ComAlloy      832-1000       12.0           180          175

Among the earliest and still most successful challenges vinyl has presented to established engineering resins is in injection molded parts for computers and business machines. Once the exclusive domain of polycarbonate, ABS and PPO alloys, parts such as computer and printer housings and copier rollers and covers are now being molded from PVC compounds and alloys with growing frequency. The reasons for this switch, processors and compounders say, transcend vinyl's improved properties.

"The business-machine market segment is in such a cost-reduction mood today that the end users are willing to look at other materials and are forcing their molders also to look at these materials," says Monsanto's Plasschaert.

One molder who has been forced by his customers to explore vinyl is Terry Minnick, president and CEO of Pro Corp., a 31-machine custom injection shop in Florence, Mass. "There's probably not one company that makes business equipment who doesn't want to lower its costs," he says. "I don't have people coming to me these days asking for better performance. I have people coming to me saying they need better economics." Consequently, about 90% of the 2.2 million lb of resin Pro Corp. expects to process this year will be PVC.

To be fair, some say a key factor that helped vinyl make inroads into business machines is the equipment designers' belated recognition that their products were overdesigned, leading to a recent downgrading of material specifications. Whereas two years ago, a 176 F HDT was deemed necessary for plastic computer housings, designers today are satisfied with 160 F.

"Although PVC was not recognized as an engineering material for applications such as business-machine and other housings due to its limited heat resistance, many companies in these markets are understanding that the 155-160 F HDT of PVC, even without even alloying, is in fact adequate for most of these applications," says Christine N. Palombo, marketing manager for Novatec Plastics & Chemicals Co., Inc.

Beyond injection molding of business-machine parts, work is progressing in other durable-goods areas. Recently, PVC and its alloys--such as several Geon compounds from BFGoodrich and Novalloy 9000 from Novatec Plastics and Chemicals--have been successfully applied to thermoforming large, deep-draw industrial parts. For instance, Goodrich is currently experimenting with thermoforming garage-door panels from a PVC alloy.

HOW HIGH IS HIGH?

As Tables 3 and 4 show, PVC compounders can achieve impressive levels of impact strength; but that capability is not particularly new. Rather, there's widespread agreement that today's ability to compound PVC for higher HDTs is the resin's strongest selling point in the "engineering" sphere (see Tables 1 and 2). While 200 F is now generally considered to be the upper limit of vinyl's heat resistance, resin producers would like to extend the material's range. "We want to go to 250 F," says Dr. Clive Copsey, group leader of BFGoodrich's Application Engineering and Design Laboratory in Avon Lake, Ohio. "I think there's probably a need for vinyl with that kind of HDT but there may not really be a need to go to 300 F. The market may not be there."

However, as heat-distortion temperatures become more critical to obtaining new PVC applications, resin suppliers, compounders and processors warn that some of those heat-resistance values should be regarded with appropriate caution. "Those numbers can be deceiving," warns Dr. A. Nelson Wright, v.p. of R&D for Synergistics Industries. "It depends very much if you are looking at heat distortion under load or if you use other ways to measure it."

Many of the vinyl products that give the necessary combination of high HDT and impact strength are actually blends or alloys of PVC and other engineering materials. More costly than straight vinyl, these resins offer the best of both worlds--the inherent flame resistance and weather-ability of vinyl and the strength and heat resistance of the "engineering" resin.

"The quest for higher heat has been going on for quite some time and it's only recently that we've seen some real advances," explains Bill Davis, manager of technical service for Alpha Chemical & Plastics Corp.

Alloys of from 20% to 50% ABS with PVC are one way to beat the heat. Another method that has recently begun gaining wider attention is the addition of special polymeric heat-distortion modifiers. Examples include SMA copolymers, like Monsanto's Cadon modifiers; acrylic-imide copolymers in Rohm & Haas' Paraloid series; Baymod ABS modifier resins from Miles Inc.; and alphamethylstyrene-styrene-acrylonitrile copolymer, like GE Specialty Chemicals' Blendex 586. Taking a different approach, BFGoodrich has been able to boost heat resistance with proprietary glass-coupling technology in its Fiberloc PVC compounds.

Putting everything a vinyl processor needs into one package has so far not proven to be an easy task. "We in the additives industry can handle the heat, we can handle the flow and we can handle the impact," says Kenneth Rozkuszka, market manager for vinyl additives at Rohm & Haas. "But we've yet been able to provide all three at a reasonable cost."

One of the reasons for this, additive suppliers admit, is that high loadings of each modifier are usually required to boost the respective properties. For example, for every 1% of GE's Blendex 586 added to PVC, the HDT rises about 1|degree~F. So that requires 30-40% modifier to raise a typical vinyl resin with an HDT around 145 F to the 175 F level that most agree is a TABULAR DATA OMITTED good level for "engineering-grade" PVC. On the other hand, some additives can be much more efficient than others. Using about 6 phr of GE's Blendex 338 impact modifier results in vinyl with 5 ft-lb/in. notched Izod. Raising the loading just 2 phr to 8 phr raises the impact raises the impact resistance to about 22 ft-lb/in.

NOT ALL GOOD NEWS

Still, users and suppliers say, there is a downside to the engineering vinyl issue. Some problems remain in molding large parts, such as copier housings that can reach 50 in. long. And molders warn that processing PVC requires some special precautions not always needed with more familiar "engineering" resins. For example, molds should be either chrome-plated or made from stainless steel, and must be wiped down between uses to neutralize buildup of acid byproducts of PVC processing. And screws and barrels that have been processing vinyl for prolonged periods are prone to pitting. Such factors impart added cost, concedes Pro Corp's Minnick. "It's not a big cost, but one that has to be factored in when one is considering using vinyl."

PVC's narrow processing window, relatively stiff flow and sensitivity to burning and shearing can cause other problems for inexperienced vinyl molders, long-time users say. But these same molders note that such problems can easily be avoided by careful monitoring of processing parameters.

'ENVIRONMENTAL' FAVORITE?

While PVC in the packaging field has encountered criticism on environmental grounds because it emits hydrochloric acid and possibly other noxious chemicals when incinerated, it is garnering some praise in durable products for one alleged environmental advantage. Health concerns over brominated flame retardants used in ABS and other engineering resins have forced some processors to look for alternatives. And vinyl has an inherent flame retardancy that in most cases eliminates the need for the addition of flame retardants.

"I think this drive to eliminate flame retardants could be the deciding factor in the future success of vinyl," says Rohm & Haas' Rozkuszka. "That has certainly been the case in Europe."