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Pharmaceuticals: Plastics

vs. Stainless Steel... Which

is Better?

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Reprinted from Chemical Engineering Progress

By Rich Greene.

 

Pharmaceutical Plants: Choose Your Material

 

If you've ever opened a faucet at home to find that the water coming out

had a brown color, then you know part of the problem — rust from the

iron in your pipes. While this is bothersome at home, it is unacceptable

at a pharmaceutical plant, particularly in a purified-water line.

 

Pharmaceutical plants have been traditionally using Type 316L stainless

steel (SS) as the preferred material-of-construction. Since Type 316L

SS contains about 70% iron, it can rust under aggressive conditions.

(The "L" is for low carbon (< 0.03%), needed to help prevent

intergranular attack.) In high-purity water systems, deposits on stainless

surfaces are thin and are referred to as "rouge" due to their

reddish-brown hue. However, rouge is only part of the problem. One

way to eliminate rouge is by using plastics instead of stainless steel.

 

Selecting the most suitable material of construction for pharmaceutical

pipes, equipment and other systems is a complex issue that has

aroused debate among those in the pharmaceutical community. To

help clarify this issue, the New Jersey section of the International

Society of Pharmaceutical Engineers (ISPE; Tampa, FL; www.ispe.org)

held a forum on plastics vs. stainless steel at its meeting in Somerset,

NJ (June 11, 2002). "Choosing the optimum material is far from easy,"

says George Black, a communications consultant who organized the

forum. "A similar problem was faced by the semiconductor industry

more than 10 years ago. The decision that pharmaceutical companies

must make is more complex," he says.

 

For instance, the water-quality standards required for the process fluids

used in the manufacture of sophisticated electronic equipment could

readily be judged by performance-testing the units. "With performance

as the judgment criterion, the increased benefits offered by the

thermoplastic materials were easy to evaluate and justify in terms of

quality and cost," says Black. "Plastics won this battle because they

proved cost-effective and met or exceeded equipment test standards."

The effect of impurities in pharmaceutical water is another story. Every

change in the manufacturing procedure or equipment that might affect

the purity of water for ingestion or injection in a human being has to be

validated before use. This validation requires extensive testing beyond

chemical analysis.

 

Designers of pharmaceutical and biotechnology manufacturing and

processing systems must overcome more than just a natural resistance

to change. They need to address problems such as getting

internal-management approval, justifying capital costs involved in

making the change, securing validation, and dealing with the potential

liability, if these changes affect the health of their customers. As

Shakespeare so aptly put it, "There's the rub."

 

Will the pharmaceutical industry do what the semiconductor

manufacturers did — and switch to plastics? The choice will depend on

the material's ability to resist leaching, withstand heat, and show good

structural integrity. But by far, the key issue is avoiding contamination

(see the sidebar).

 

FORESTALLING ROUGE

 

The culprits behind rouge are dissolved iron and oxygen. Elemental iron

(Fe+…) is insoluble in water, but oxygen converts it into Fe+†, which

precipitates as either Fe•O– or Fe(OH)–. One gram of iron will cover an area

of 65 x 65 ft… (6.04 m… x 6.04 m…), three atoms deep. This is a huge surface.

Oxygen can enter a system through seals, such as on a pump impeller or by

diffusion through plastics, which are permeable to gases.

 

Rouging in water-for-injection (WFI) systems can occur due to active

corrosion, such as on non-stainless steel components. But far more

frequently, rouging results from simple oxidation of trace quantities of

dissolved iron. Engineers who are familiar with water treatment will recognize

this as aeration. "It may be far easier to prevent aeration and the resulting

precipitation than to eliminate trace iron from process streams and systems,"

says ISPE forum speaker David O'Donnell, manager, technical services, for

Rath Manufacturing Co. (Janesville, WI; www.rathmfg.com). "While plastics

certainly are iron-free, if there are other components somewhere in a system

that are made of steel, there still can be a problem," he says. "Even some

'high-end' plastics are actually permeable to gases (e.g. O•, N•, He, H•, CO•)

and many solvents. It is this permeability that leads over time to blistering

behind plastic-lined items," says O'Donnell.

 

In general, there are three types of rouge. Type 1 is due to the

dissolution of steel, such as is found on a pump impeller. Type 2 results

from active corrosion and Type 3 is due to high temperatures.

Passivation is a technique commonly used by pharmaceutical

companies to rid a system of rouge. "Passivation cleans out rouge, but

it doesn't prevent Types 1 and 3 from happening again. If oxygen

permeates the system, rouging can recur," says O'Donnell.

 

Type 2 is often the result of improper welding techniques that leave a

heat-affected zone (HAZ) near the weld. Type 2 rouge strikes the HAZ,

not the weld itself. "During fabrication, manual field welding should not

be allowed," says O'Donnell. "Use automatic orbital welding whenever

possible to prevent heat tints from forming. Follow this with

post-passivation acid treatment and then neutralize with an alkaline

rinse," he recommended.

 

The 300 series stainless steels are sensitive to chlorides at a pH of 6.5-8

and at temperatures less than 140°F (60°C). Although Type 316L

tolerates about 1,000 mg/L of chlorides, care must be taken in wet-dry

zones, where concentrations can reach 26,000 ppm in the worst case

(for magnesium chloride).

 

Still, stainless steel hasn't become the historic material-of-choice

because it presents problems. It does have numerous positive

attributes. It is impermeable to oxygen, other gases and solvents. It

has 10 times the thermal conductivity of plastics, which makes it well

suited for heat-transfer surfaces. Stainless steel is strong, too. Its yield

and tensile stresses are at least 10 times those of thermoplastics, and it

requires about 800°F (427°C) to initiate creep. Many plastics creep at

room temperature. Therefore, plastic piping must be supported

properly to ensure its integrity. Steel pressure ratings are generally 20

times higher than those for plastic. "In practice, a 1 in. O.D. 300-series

stainless-steel pipe can withstand 1,600 psi (11,032 kPa), while a similar

plastic pipe can tolerate a maximum pressure of 75 psig (5.17 bar), but

only at room temperature. At 200°F (93°C), plastic loses half of its

strength," says O'Donnell.

 

PROPERTIES OF PLASTICS VARY

 

While Type 316L has basically one set of properties, those of plastics

vary from type to type. Improved properties generally come at a price,

so the less-costly plastics, such as polyvinyl chloride (PVC) and

polypropylene (PP), generally do not perform as well as the

fluoropolymers, namely polyvinylidene fluoride (PVDF), Type 316L's

main competitor in pure-water systems. PVDF melts at 352°F (178°C),

allowing it to be steam-sterilized, but rigid PVC starts to decompose

near the boiling point of water, making sterilization possible only by

chemical means. "PVC and PP must be sterilized with hydrogen

peroxide or chlorine, which both require a rinse afterward. PVDF not

only tolerates steam, but also withstands sterilization by ozone. Ozone

has a half-life that is measured in minutes, so no post-sterilization

cleaning is needed," says Gary Dennis, worldwide market manager,

technical polymers for Atofina Chemicals (Philadelphia, PA; www.

atofina.com). Still, PVC and PP are inexpensive and are common in

chemical process industries (CPI) plants.

 

COST VS. PERFORMANCE

 

Fluoropolymers are available that can outperform PVDF in temperature

resistance, such as perfluoroalkoxy (PFA) resin, which can be used at

operating temperatures to 500°F (260°C). But these formulations are

expensive and usually exceed the needs of a pharmaceutical plant.

"PVDF is among the hardest fluoropolymers and has among the highest

tensile strength in this family of plastics. It even has better abrasion

resistance, often referred to as particulation, than stainless steel," says

Dennis. Particulation is measured by the amount of polymer abraded

from a surface by a rotating wheel. PVDF's particulation is 5-10

mg/1,000 cycles of rotation, while that for SS is about 50 mg/1,000

cycles.

 

Fluoropolymers have excellent chemical resistance including to

deionized water, high thermal stability and they resist degradation by

sunlight. Also, since they have low coefficients of slip, microorganisms

(notably, fungi and bacteria) generally do not grow on them. On the

contrary, metal surfaces cannot easily be smoothed out to a degree that

can compete with plastics. Microbial-induced corrosion (MIC) is not

uncommon in chemical plants, (i.e., in heat exchangers), and can take

place on micropolished stainless-steel surfaces in pharmaceutical

facilities. In chemical plants, biocides can be added to process water to

prevent MIC in cooling towers and heat exchangers. Not so in an

ultrapure water system.

 

Although quite smooth, polymer surfaces can contain molecules that

can leach out into water streams. In its virgin form, PVDF is highly pure

and contains no additives. Thus, nothing will leach out. But polymers

such as PVC and PP can have additives. These include plasticizers, heat

stabilizers and flame retardants.

 

PVDF, however, is not an inexpensive plastic, but piping and vessels

fabricated out of it, or lined with it, are said to be about 10% cheaper

than comparable all-stainless systems. Although neither material (PVDF

or stainless steel) is perfect, each has substantial advantages and

proven track records in pharmaceutical plants, as well as in other CPI

installations. "Since pharmaceutical plants traditionally use stainless

steel, switching to plastics isn't going to be easy," says Black. But the

switch may be on.

 

"Eventually, the semiconductor industry embraced the high purity and

corrosion-resistance advantages of plastics. It is only a matter of time

before the pharmaceutical industry follows suit," says Rick Bolger,

marketing manager for Plast-O-Matic Valves (Cedar Grove, NJ; www.

plastomatic.com). Obviously, some applications will forever be

stainless steel, due to high-pressure and temperature considerations.

But for many applications, change is inevitable. "We find that interest is

growing for thermoplastics in the pharmaceutical community,

particularly for homopolymer PP and a number of fluoropolymers,

including PVDF," says Bolger.

 

The debate will result in an education process on both sides: The plastic

piping industry has historically been geared toward the needs of the

core CPI segments and toward semiconductor manufacturers. "These

needs don't necessarily apply to pharmaceutical industries," says

Bolger. "It's likely that some new products will arise as a result." By the

same token, the pharmaceutical engineer will need to learn how to

design a plastic system, including understanding the materials and the

numerous joining techniques available. The plastic piping industry is

positioned to solve contamination problems, and the pharmaceutical

engineer is geared toward improving quality and efficiency, so plastics

may make further inroads into this area. Some companies have had

plastic pure-water systems in-place, and have already found success

 

PLASTICS VS. STAINLESS: THE DEBATE GOES ON

 

PRO-PLASTICS

 

"The question has been raised as to the suitability of sealless,

magnetically-coupled pumps for use in pharmaceutical and other

high-purity water systems. The reason for concern is the potential for

bacteria formation in the gap between the inner magnet and the

containment can. This gap is frequently less than 0.03 in. Current

thermoplastic mag-drive designs, however, offer clearances of 0.09-0.10

in. Further, they provide wide, open fluid passages for the continuous

flow of fresh liquid, as well as drain plugs to assure complete clearing of

the passages within the pump and in the suction piping back to the

shutoff valve." — Dan Besic, chief engineer, Vanton Pump & Equipment

Corp. (Hillside, NJ), 2001 technical seminar.

 

PVDF and PP plastic systems are inherently superior to stainless steel in

that they are manufactured from 100 % pure resin. Unlike stainless

steel alloys, there is virtually no difference in the chemical composition

of manufactured lots of material. The welding process or chemical

cleaning procedures do not degrade the corrosion resistance of

thermoplastic systems." — Roger Govaert, Asahi/America, and Albert

Leughamer AGRU Kunstsofftechnik, in Ultrapure Water, Dec. 2001.

 

PRO-STAINLESS

 

"Both plastics and metal have logical industrial applications. Metals are

favored for larger, industrial systems operating at higher temperatures.

Under these conditions stainless steels and the higher alloys make a lot

of sense.

 

"Plastics have very low strength and temperature tolerance relative to

metals. Many WFI systems operate at 180°F (82°C) and use steam

cleaning protocols. Metals tolerate steam cleaning with no difficulty.

Plastics typically soften considerably at such temperatures.

"WFI systems should be designed to minimize oxygen to avoid this.

Avoiding the use of plastics would be one key step, as plastics are

permeable to oxygen. The use of nitrogen blanketing in accumulation

tanks would be another step, as would the use of magnetically-coupled

pumps (no rotating shaft seals exposed to process steam).

 

"Metals have no significant flow limitations. Plastics suppliers often use

10-12 ft/s (3.048 - 4.587 m/s) as a maximum flow speed, particularly on

plastic-lined items to avoid damage." — Dave O'Donnell, manager,

technical services, Rath Manufacturing Co. (Janesville, WI)

 

AN EXISTING PVDF INSTALLATION

 

As evidence of the possible wave of the future, Christ, Ltd. built a

purified water-treatment plant and distribution system for Sulzer

Orthopedics in Winterthur, Switzerland. The plant has been in

operation since 1966. Christ was awarded the contract because it had

successfully built similar pharmaceutical facilities requiring purified

water that met European Pharmacopoeia and other guidelines on sterile

products. High requirements were set in regard to the distribution

system. Design factors included:

 

• No deadspaces within the system

• Low surface roughness of piping material and welding seams

• Dynamic operation of the system with a high velocity

• Periodic disinfection with ozone.

 

The complete distribution loop was installed using bead- and

crevice-free, high-purity PVDF piping. The pure-water plant easily and

continuously meets the requirements set by Sulzer Orthopedics in

regard to conductivity, capacity and yield, and the water-purity quality

meets and surpasses the requirements set by United States

Pharmacopeia (USP). For example, the target conductivity for the water

was set at <10 μS/cm and the plant's diluate is below detection limits.

The chloride target was 5.3 ppm, and, it too is below detection limits.

 

Copyright 2016 - Vanton Pumps (Europe) Ltd - All rights reserved

About Us

In the 1950, Vanton developed a revolutionary all-plastic pump for use in conjunction with the first heart-lung device. The design limited fluid contact to only two non-metallic parts: a plastic body block and a flexible liner. This was the birth of our Flex-I-Liner rotary pump. Its self-priming sealless design made it an industry standard for the handling of corrosive, abrasive and viscous fluids as well as those that must be transferred without contaminating the product. Vanton now offers the most comprehensive line of thermoplastic pumps in the industry.

 

 

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