Castable polyurethane elastomers--serving demanding engineering applications.

The design engineer who needs an elastomer for a demanding application has numerous tools to choose from out of the array of modern materials. The choice for a particular application depends upon a number of factors, including the physical properties needed for the task, the stability of the material in the environmental conditions that will be encountered during its service life and the cost-effectiveness of that choice.

Castable polyurethane elastomer elastomers have been a steadily growing segment of this array for the last half-century. Although some of their applications are well known to the public (roller skate and skateboard wheels and golf ball covers. for example), the major applications are in industrial uses where properties such as extreme abrasion resistance, load bearing ability, cut and tear resistance and environmental aging resistance are required. Another important aspect is their processing. These materials are usually processed as a two-part liquid system, which allows the application of many molding techniques and the use of low cost tooling.

This article is intended to outline the outstanding properties of these castable polyurethane elastomers and to contrast them with other engineering materials. It is also intended to stimulate investigation into new applications by showing how they are used to solve specific engineering problems.

The elastomeric properties of most diene rubbers are due to the crosslinking produced in a vulcanization process, but in castable polyurethane elastomers, a process of chain-extension does the curing and there is relatively little crosslinking. This process produces high molecular weight polymers with so-called "hard blocks" and "soft segments." The elastomeric properties are due to association of the hard blocks and flexibility of the soft segments.

Comparison with other materials

When considering castable polyurethane elastomers for an application, other candidates might be materials such as diene rubbers, metals or plastics. In making comparisons of castable polyurethane elastomers with other candidates for an application, it is useful to highlight some of their attributes. Listed in table 1 are the main advantages of castable polyurethane elastomers compared to the diene rubbers as a group.

Some of the main advantages of castable polyurethane elastomers versus diene rubber are higher abrasion resistance, greater cut and tear resistance and higher load bearing ability.

In addition, most cast castable polyurethane elastomers have natural colors ranging from completely clear and colorless to opaque white or amber. Their ready acceptance of a wide variety of pigments and dyes permits coloring ranging from black to brilliant fluorescent colors of all types. This is especially useful in color-coding of parts. A good example is in business machines where rolls and belts can be color-coded so that correct replacement can be easily made. Even when colored black, there is so little pigment in the castable polyurethane elastomer that the compounds do not leave marks on concrete floors, boat hulls or other surfaces. Since castable polyurethane elastomer compounds usually contain no additives that are not tied into the molecular structure, there can be no migration of materials out of the elastomer to de posit on other surfaces. This makes them attractive for applications where they are in contact with finished surfaces.

Diene rubbers are subject to ozone cracking, particularly around electrical equipment where ozone can be generated. Castable polyurethane elastomers have virtually no ozone-cracking problem since they contain no unsaturation.

The fact that castable polyurethane elastomers are pourable liquids often allows for very inexpensive tooling and makes possible the fabrication of very large or complicated parts.

Most rubbers when compounded up to 90 or 95A durometer have sacrificed many of their physical properties. On the other hand, castable polyurethane elastomers in the 85 to 95A durometer range are approaching the peak of their properties and give extremely good performance at these hardnesses. Castable polyurethane elastomers are available up to about 75D durometer hardness, and although they begin to resemble hard plastics in this range, still have elastomeric properties. There are castable polyurethane materials that go as high as 85D, but their properties are more like nylon, polyacetal, epoxy and other engineering plastics and they can no longer be considered elastomers. On the other hand, there are extremely soft castable polyurethane elastomer materials, giving hardnesses below zero on the A durometer scale (figure 1).

[FIGURE 1 OMITTED]

Polyether-based castable polyurethane elastomers are particularly resistant to molds, fungi and other microorganisms, making them useful in agricultural, sewage treatment and tropical applications. While ultraviolet resistance of castable polyurethane elastomers in general is good, darkening occurs with outdoor exposure. Therefore, for continuous outdoor exposure, dark colors or pigments are often used, sometimes in combination with UV-stabilizer packages. Where light colors must be used and little or no color change can be tolerated, aliphatic diisocyanate-based prepolymers with non-aromatic curatives often are the best choice.

Castable polyurethane elastomers, particularly the polyester types, are resistant to swelling and deterioration by oils, greases and other non-polar solvents.

Rebound, which relates to energy loss in mechanical cycling, is related to the hysteresis properties of the castable polyurethane elastomer. High hysteresis leads to development of heat in dynamic service, and it can range from very low to very high in castable polyurethane elastomers, depending upon the particular structure of the molecule.

Finally, the versatility of cast castable polyurethane elastomers is unmatched by any other type of elastomer. It is often possible to modify one property while holding others more or less constant. For example, it is possible to formulate higher durometer elastomers with low rebound, lower durometer materials with high rebound and vice versa. Similarly, it is possible to maximize flex life or tear resistance without sacrificing much compression set over a wide range of hardnesses. These trade-offs often allow fine-tuning of the material for the best service in the application.

Advantages versus metals

One of the chief advantages of castable polyurethane elastomers versus metal is lighter weight. Parts fabricated from castable polyurethane elastomer weigh far less than metal parts and are much easier to handle and, typically, result in less moving mass when used in machinery. In addition, metal parts tend to generate noise, while castable polyurethane elastomers absorb noise. The reduction of "noise pollution" in the workplace when castable polyurethane elastomers replace metal can often be dramatic.

Castable polyurethane elastomers will also outwear metals in many applications and can be easily cast in inexpensive tooling as mentioned earlier. In contrast, making metal parts requires foundry operations, welding and machining and, as a result, can be very costly, particularly with high hardness alloys.

Castable polyurethane elastomers are also corrosion resistant. In many mining operations, corrosive solutions cause rapid deterioration of steel. Wherever there is a combination of abrasion and corrosion at the same time, such as in copper mining slurry pipelines, the lifetime of metal parts can be remarkably short. Castable polyurethane elastomers, because they have high resistance to both abrasion and corrosion, outlast metal by a large margin in such applications. Some applications, such as cast polyurethane elastomer mining screens, embody all of the advantages described above.

Advantages versus plastics

Plastics represent another group of materials that are sometimes used in the same types of applications as castable polyurethanes. One advantage of castable polyurethane elastomers over plastics is that they are not brittle. Many plastics, particularly in the higher hardnesses, tend to crack and break under impact and shock loading. Although they may appear to be similar to plastics upon casual observation, cast urethanes remain true elastomers, maintaining their high impact resistance even up to very high hardnesses (about 75D durometer).

Urethanes also have elastomeric memory; that is, they can be stretched to substantial elongations and will return to their original dimensions. Most plastics, once they have been stretched beyond a certain yield point, remain permanently stretched. Finally, plastics, as a class, do not have the high abrasion resistance of polyurethanes.

Processing of east polyurethane elastomers

The liquid state of these materials gives the user the ability to manufacture high performance articles with minimal capital investment in plant, equipment or tooling, since most casting operations are carried out at relatively low temperatures (e.g., room temperature to 120[degrees]C (1248[degrees]F) and ambient or low pressures. Molds may be made inexpensively from many types of materials.

Processing of cast polyurethanes consists of mixing two or more liquids, pouring into a mold, curing to a state where the object is self-supporting and can be demolded, then allowing the cure reaction to complete during a post-cure operation. Finishing operations, such as grinding, lathe tooling, etc., if needed, are similar to those performed on other elastomers. Processing operations can be done using hand batching methods, to produce batches ranging from about 300 g (10.6 oz.) to 14 kg (30 lb.) or more, or by using automatic meter-mix machines. Machines are available in sizes that can dispense materials on a continuous basis at rates ranging from less than 100 g (3.5 oz.) per minute to more than 70 kg (150 lb.) per minute.

Before the materials can be mixed and cast, however, the two parts [prepolymer and chain-extender (curative)] must be prepared for use. This consists of melting each of the two materials (if solid), raising them to the appropriate processing temperature, and degassing under vacuum. Vacuum degassing is critical, since prepolymers can contain large amounts of dissolved air and carbon dioxide, which, if not removed, will result in bubbles in the finished casting. Often the curative side does not require degassing because of the viscosity and gas solubility characteristics of many curatives. If the hand batching method is used a second degassing is required to remove air that has been entrained during mixing of prepolymer and curative.

The list of available processing techniques is extensive, allowing the fabrication of the most intricate small articles weighing less than a gram (0.04 ounce) to massive items containing 4,000 kg (8,800 lb.) or more of elastomer. Many of these techniques are listed in table 2. Some of them will be familiar to processors of gum rubber stocks, others are specific to liquids.

After curing to a self-supporting state in the mold, the part is removed from the mold, and the cure reaction is allowed to complete in an oven.

The prepolymer consists of a polyol chain (which can be a polyester, one of two types of a polyether material or several other structures of various molecular weights) combined with a diisocyanate [usually methylene bis-diphenyl diisocyanate (MDI) or toluene diisocyanate (TDI), but can also be paraphenylene diisocyanate (PPDI) or an aliphatic] to give a structure with active isocyanate functionality on each end. Any polyol can be combined with any diisocyanate to give a prepolymer designed to lead to a specific set of physical properties. This is why prepolymer manufacturers have such extensive lists of products. The curative (chain-extender) is usually a material with hydroxyl or amine functionality on each end. When the molder combines the two materials, the resulting reaction converts the materials to high molecular weight linear polymers.

Limitations of east polyurethane elastomers

There are three chief limitations of cast polyurethane elastomers. Most common polyurethane elastomers are not high temperature materials. Owing to certain thermoplasticity in their nature, properties tend to fall off at elevated temperatures. Generally speaking, common urethanes are not useful materials under heavy service loads at temperatures above approximately 220-225[degrees]F (105-107[degrees]C). Newer materials, based on PPDI (para-phenylene diisocyanate) have pushed that range higher, however.

Another limitation is that all polyurethanes are subject to hydrolysis in the presence of moisture and elevated temperatures. The combination of the two factors creates the problem. While at lower temperatures, most cast polyurethane elastomers can withstand continual contact with water for many years; no polyurethane as yet can stand prolonged contact with live steam. In between, there is a wide range of temperature and moisture conditions under which the materials may, or may not, be suitable for use. Newer developments in polyurethane chemistry show promise to push these limits further, however.

Lastly, there are certain chemical environments that are unsuitable for polyurethanes. Very strong acids and bases generally are detrimental, as are certain solvents, specifically the aromatic solvents such as toluene or ketones such as MEK or acetone; and esters such as ethyl acetate. (There are many solvents, on the other hand, which urethanes resist very well and are suited for in-contact service. These include many oils and petroleum-based materials).

Research continues daily in the castable polyurethane elastomer area. Efforts are fairly evenly divided between improvement in properties (higher temperature service, chemical and environmental resistance, etc.) and improvement in processing (safety, user-friendliness, and new molding methods).

Properties

Unlike diene rubbers, where properties are modified by extensive compounding of a base material, when a different set of properties is desired with castable polyurethane elastomers, a different prepolymer and/or curative is selected. This is why suppliers of castable polyurethane elastomer materials have a large selection of prepolymers and curatives. For example, Crompton's list of Adiprene and Vibrathane products consists of over 65 standard and an additional 235 special products.

How does one approach the selection of the best candidate materials from such a vast array? Some guidelines for selecting castable polyurethanes for demanding applications are shown in tables 3 and 4. Listed are various physical and environmental resistance properties and the types of TDI and MDI prepolymers that have the greatest and the least likelihood of success in applications where these properties are important. This is, of course, only a general guideline. Many other materials are not touched upon, such as the benefits of aliphatic isocyanates in UV light resistance or the outstanding dynamic and hydrolytic resistance properties of PPDI-based materials. There are probably many exceptions to these roles, but they can be useful as a starting point in considering materials for a given application. Note that the property is sometimes chiefly controlled by the diisocyanate component and sometimes by the polyol component. Other times the correct combination of the two is critical.

The first property listed, and one that is of primary interest, is hardness. However, since one can obtain all hardnesses with all six basic types of prepolymer systems, this is not a basis for choice among them.

In terms of tensile strength, esters have the edge over ether compounds, although all types have very high tensile strength. However, tensile strength is rarely a key engineering property in polyurethane elastomer applications. Since all types of polyurethanes can have high elongation and high or low modulus, there is really no basis for choice there. Tear strength is an important property in many applications. Here too, the esters have the advantage over the ethers. The lower cost polypropylene glycol (PPG) ethers usually have the lowest tear strength.

In terms of compression set, an important property in applications such as hydraulic seals, the amine-cured TDI compounds, both ethers and esters, tend to have better compression set resistance than diol-cured MDI compounds. Therefore, one might choose a TDI system for the first test compound for an application requiring high compression set resistance.

As for rebound, diol-cured MDI ethers have much higher rebound, or resilience, than amine-cured TDI esters. These examples illustrate the two opposite ends of the spectrum. With many other properties, differences usually are not dramatic, but there is frequently a considerable difference and one should always keep this in mind when making choices.

As for low temperature properties, MDI ethers have the lowest brittle points and best low temperature cracking resistance. Esters are generally not as suitable for low temperature properties. On the other hand, for high temperature properties, the TDI polymers tend to be better than the MDI polymers. (A certain amount of thermoplasticity that the MDI's typically have sometimes shows up in high temperature properties.)

In terms of abrasion resistance, there are two considerations based on the type of abrasion to be encountered. For resistance to sliding abrasion, esters are generally the materials of choice. They perform better than the ethers and much better than the low-cost ethers. On the other hand, for impingement abrasion, where particles are striking a surface at a high angle, the high resilience MDI ethers often perform much better than esters and also much better than low cost ethers. Therefore, to make the optimum choice of material, it is necessary to examine the nature of the abrasion mechanism in the particular application being considered.

Dynamic heat buildup due to hysteresis is important in applications such as wheels and tires. Ethers are better than esters in terms of hysteretic heat buildup. However, some of the recent low-free monomer TDI ester materials have excellent heat build-up characteristics and have narrowed this gap to a large extent.

MDI ethers are more resistant to hydrolysis than TDI ethers. Down at the bottom of the list in terms of this property are TDI esters. In this area, there is a significant difference between the two ends of the spectrum.

Esters have the edge in solvent resistance, ethers generally are not quite as oil- and solvent-resistant as esters. With respect to heat aging, that is, the permanent loss of properties due to prolonged exposure to high temperatures, esters have the edge over ethers, and PPG ethers are the least resistant.

Low hardness formulations are widely used for printing and coating rolls, and TDI esters are the materials of choice because they can tolerate greater loading of plasticizers before significant reduction of physical properties occurs. Ethers generally do not have as high physical properties in low hardness, highly plasticized formulations.

MDI esters and polytetramethylenether glycol (PTMEG)-based ethers cured with 1,4-butanediol are the choice for FDA-approved applications involving dry and wet food contact, particularly for meat or poultry processing. TDIs generally are the least desirable, primarily because of the types of curatives used (aromatic amines).

MDI-based polyurethane elastomers have a greater range of formulation flexibility. To achieve high hardness materials with TDI prepolymers, methylene-bis-orthochloroaniline (MBCA) or another diamine type curative is necessary; whereas with the MDIs there are several diol curatives that can be used, such as 1,4-butanediol and hydroquinone di-(beta-hydroxyethyl) ether (HQEE), and higher molecular weight polyols. Many mixtures and combinations of various diol curatives are possible.

Throughout this list, PPG ethers have appeared in the "weaker" column quite frequently. While their performance characteristics may not be as good as the other materials, very often there are applications where they are desirable. Cost performance is, of course, the reason to use PPG TDI ethers. They are considerably cheaper than premium grade ethers or esters and are useful in parts that are not highly engineered, that is, in applications where the most outstanding properties of polyurethanes are not fully utilized and therefore low-cost materials can perform adequately.

TDI and MDI PTMEG ethers are listed as the weakest in terms of cost, which means that they are the most expensive. However, the true cost in use really depends upon the particular application, because the curative cost must be factored in. Each case must be calculated on an individual basis: How much does the curative cost, what is the ratio used and how does this contribute to total cost? Sometimes the most expensive material to buy may be the best material for the application because of its cost/performance characteristics. This is a consideration with both premium-grade and low-cost materials.

In some applications, one type of castable polyurethane elastomer has advantages that are so outstanding as to make it the major choice to the virtual exclusion of all other types (table 5).

High quality roller skate and skateboard wheels are practically all MDI ethers, mainly because of their high resilience. High resilience gives good speed performance and a smooth ride.

TDI esters are the materials of choice for printing and coating rolls because of their high solvent resistance and good physical properties in low hardness formulations.

It is important that oil pipeline pigs, used for cleaning and servicing lines of all sizes, have high abrasion resistance in addition to high oil resistance to prevent their being worn down by going through miles of pipe. Because of this, esters have been the choice since they combine oil resistance with high sliding abrasion resistance.

For grain handling apparatus (sheet goods, buckets and the like), MDI esters have been the materials of choice. The ester component provides high resistance against abrasive grain and the MDI component permits a variety of compositions that are FDA-approved for contact with dry roods.

As for forklift truck tires, TDI ethers have traditionally served a major segment of this market because of their low heat buildup and high load bearing ability. In applications where the cut and tear requirements are severe due to the presence of scrap metal or other debris on the floor, an ester might be chosen as a trade-off in load and speed performance for greater damage resistance and toughness. As mentioned earlier, the newer low-free TDI esters have much improved heat buildup characteristics and combine the features of heat buildup and cut and tear resistance.

For hammers, TDI esters are the materials of choice because of the combination of high tear resistance with low resilience. Tear resistance is necessary to prevent deterioration of the face of the hammer, and low resilience causes the energy of the hammer to transfer into the object being struck rather than allowing it to bounce back at the user. Formulations suitable for applications ranging from soft mallets to relatively hard metalworking hammers have been developed.

Sandblast curtains and masks require high resilience and abrasion resistance in the impingement mode. MDI ethers, because of their high resilience, allow the particles to bounce back without transferring much kinetic energy in the form of heat to the sandblast curtain or mask.

MDI ethers are also the materials of choice for laundry equipment, such as agitators, pulsators or lifter bars for commercial washing machines because of their outstanding hydrolysis resistance. However, TDI ethers are preferable for paper mill rolls because of their combination of good hydrolysis resistance and excellent hardness stability and dynamic performance. In paper processing applications, it is important that the hardness and dynamic properties of the roll remain constant over a range of operating temperatures so that performance of the roll remains consistent.

MDI esters are preferred for meat-processing equipment because FDA wet food approval is required for this application. These special ester formulations combine the properties of cut and tear resistance and fat and oil resistance with FDA approval. There are now several PTMEG MDI types that meet the wet food requirements as well.

Selecting castable poyurethane elastomers

What approach can a design engineer or manufacturer use to select the best castable polyurethane elastomer for a new application or to improve performance in an existing one? Table 6 lists the suggested steps in the process.

Look at the application carefully and decide which properties are of key importance, both in terms of the physical properties and the environmental resistance needed in the polyurethane elastomer. If you are familiar with the various types available, select a few candidate polymer and curative systems that should offer appropriate performance. Consider engineering design principles regarding the proposed part. Will there be localized flexing at any one spot? If so, can this be distributed over a wider area? Will there be "stress risers" (localized high force concentration) at points of attachment? Can a harder material be used to make a thinner-walled part of the same overall strength? Once you've done this, go to the polyurethane prepolymer and curative suppliers for recommendations and for further information. They will certainly be willing to help you and will welcome discussion of new applications. The suppliers may also help with suggestions of materials or test data that are not available in their published literature.

Next, review your plant capabilities. Make sure the compounds you have chosen are ones that can be run in your plant. either under its present setup or, barring that, consider what sort of investment might have to be made fur processing of the proposed material. Depending on the potential market for the application, new investment may be justified. Although the process is simple in principle, it does require close attention to detail and handling of toxic chemicals. There are many excellent custom molders of high-performance polyurethane elastomers worldwide, and in many cases, the best course of action for a new user of polyurethane elastomers is to approach one of them to have the job done, benefiting from their experience, technique and plant equipment.

Then run whatever preliminary tests are available. If it is a new application where the part must, for example, come in contact with an unusual chemical solution, make sure that the polyurethane elastomer you select can tolerate this. Again, consult the suppliers, who will usually be willing to help you with tests of this type.

At this point, if everything continues to look good, make prototype units of one or more candidate materials. Make sure that the prototypes are well identified so they do not get misplaced in the field and, importantly, so that you can properly track their service history. Get the parts out in actual service; make comparisons of the performance of these test units against whatever is currently being used. If it is a brand new application where nothing like it has ever been done before, test the parts against whatever criteria you and the end-user have in mind. Finally, make sure that you and the end-user agree on the test results and that he or she approves of the part based on the prototype evaluation.

Once you have reached this point you are ready to gear for production and for a successful and profitable business venture.

Summary

Castable polyurethane elastomers offer high performance in demanding applications because of their exceptional abrasion resistance, cut and tear resistance, load-bearing ability, and environmental resistance, and they offer these properties over a wider hardness range than any other elastomer. The liquid state of the casting materials provides for an extensive variety of processing techniques, allowing the manufacture of very small to exceptionally large or complex parts with low capital investment. Castable polyurethane elastomers can replace metals, plastics and other rubbers in many applications to give superior performance and increased cost effectiveness. Manufacturers of materials used in this industry are constantly making improvements in high-temperature performance, environmental resistance and processing ease and safety. Castable polyurethane elastomers are versatile and powerful tools to solve difficult engineering problems, and the list of applications where they are used will continue to grow. This will be driven by design engineers and manufacturers who continue to learn more about their extensive property range and fabrication possibilities for demanding applications, and by polymer chemists who will continue to find ways to improve the physical properties, environmental resistance and processing ease to meet the growing list of industrial uses.

Table 1--advantages
versus rubber

Abrasion resistance

Cut and tear resistance

Higher load bearing

Clarity; translucence

Non-marking, non-staining

Ozone resistance

Pourable; castable

Wider hardness range

Microbial resistance

Oil and petroleum resistance

Low or high rebound

Versatility

Table 2--15 ways to mold castable polyurethane
elastomers

Open casting (most common, easiest, cheapest)
Compression molding (precision parts)
Centrifugal molding (pipelining, sheet goods, multicavity
  molds)
Ribbon Flow moldless casting (for rolls, etc.)
Liquid injection molding (low pressure, bottom fill)
Transfer molding (multiple precision parts)
Rotational molding (makes hollow items)
Vacuum casting (wire or fiber inserts in item)
Pressure casting (pressure chamber)
Reaction injection molding (high pressure impingement
  mixing)
B-staging (when molds can't hold liquids)
Spraying (high pressure solventless and low pressure
  solvented)
Solvent casting (low viscosity for fabric penetration)
Trowelling (repairs and special applications)
Dipping (long pot life heat activated)

Table 3--selection of castable polyurethanes for
demanding applications

Property                      Stronger     Weaker

Hardness                          --          --
Elongation                        --          --
Modulus                           --          --
Tensile strength              Ester        Ether
Tear strength                 Ester        PPG ether
Compression set               TDI          MDI
Rebound                       MIDI ether   TDI ester
Low temperature properties    MIDI ether   TDI ester
High temperature properties   TDI          MDI
Abrasion resistance
  Sliding                     Ester        PPG ether
  Impingement                 Ester        PPG ether
Heat buildup                  Ether        Ester
Hydrolysis resistance         MIDI ether   TDI ester
Oil resistance                Ester        Ether
Heat aging resistance         Ester        PPG ether

Table 4--selection of castable polyurethanes for
demanding applications

Property                   Stronger    Weaker

Low hardness formulation
  (<60A)                   TDI ester   Ether
FDA approval
  (wet and dry food)       MDI ester   TDI
                           and ether
Formulation flexibility    MDI         TDI
Cost                       PPG ether   TDI and
                                         MDI ether

Table 5--specific types of castable polyurethane elastomers for various
applications

Application                      Type

Roller skate/skateboard wheels   MIDI ether
Printing/coating rolls           TDI ester
Oil pipeline pigs                TDI/MDI ester
Grain handling equipment         MIDI ester
Fork lift tires                  TDI ethers, esters
Hammers                          TDI esters
Sandblast curtains and masks     MIDI ethers
Laundry equipment                MIDI ether
Paper mill rolls                 TDI ethers
Metal processing equipment       MIDI esters, ethers *

Application                      Basis

Roller skate/skateboard wheels   High resilience
Printing/coating rolls           Solvent resistance, good physical
                                   properties at low hardness
Oil pipeline pigs                Oil, abrasion resistance
Grain handling equipment         Abrasion resistance, dry food approval
Fork lift tires                  Low heat build-up, cut and tear
                                   resistance
Hammers                          Tear resistance, low resilience
Sandblast curtains and masks     High impingement resistance
Laundry equipment                Hydrolysis resistance
Paper mill rolls                 Dynamics, hardness and hydrolysis
                                   stability
Metal processing equipment       FDA wet food approval

* Special grades

Table--6 selection process

Decide which properties are of key importance--physical
and environmental resistance
Select prepolymer/curative systems that are likely
  candidates
Consider engineering design principles
Consult suppliers for recommendations and further
  information
Review plant capabilities
Run whatever preliminary tests that are available
Make prototype units of one or more candidate
  systems
Field test in actual service, make comparisons, get
approval from future customers
Gear up for production