Plastics Engineered Product Design Part 11 docx

380 Plastics Engineered Product Design Regarding articles, educational information, and Networking in addition to sourcing vendors and selecting materials, the Internet makes it easy to

380 Plastics Engineered Product Design

Regarding articles, educational information, and Networking in addition to sourcing vendors and selecting materials, the Internet makes it easy to locate article archives, register for educational programs, and network with other professionals

Many industry trade associations have Web sites that provide a number

of resources for designers For example, the Web site of the PD3 (Product Design and Development Division) of the Society of Plastics Engineers (www-pd3.org) contains a Design Forum or chat area where users can discuss design challenges and exchange advice They also provide a schedule of educational programs and links to helpful design articles

The IDSA (industrial Designers Society of America) (wwwidsa.org) provides similar links, as well as opportunities to locate reference materials, job openings, and suppliers

PLASTIC PERFORMANCE

OVERVIEW - .-

I I I I I I

Throughout this book many different properties are reviewed What follows provides additional information on the properties for different plastics As a construction material, plastics provide practically unlimited

benefits to the design of products, but unfortunately, as with other materials, no one specific plastic exhibits all these positive character-

istics The successful application of their strengths and an understanding

of their weaknesses (limitations) will allow designers to produce useful and cost cfficient products With any material (plastic, steel, etc.) products fail not because of the material’s disadvantage(s) They fail because someone did not perform their design approach in the proper manner

to meet product performance requirements The design approach includes meeting required performance of material and its fabricating process that operate within material and process controllable variables (Chapter 1, Variables)

There is a wide variation in properties among the over 35,0000

commercially worldwide available materials classified as plastics They now represent an important, highly versatile group of commodity and engineering plastics Like steel, wood, and other materials, specific groups of plastics can be characterized as having certain properties Many plastics (that are extensively used worldwide) are typically not as strong or as stiff as metals and they are prone to dimensional changes especially under load or heat They are used instead of metals, glass, etc

(in millions of products) because their performances meet require- ments However there are plastics that have very high properties (Fig

6 l), meet dimensional tight requirements, dimensional stability, and are stronger or stiffer, based on product shape, than other materials

382 Plastics Enqineered Product Design

Figure 6.1 Mechanical and physical properties of materials (Courtesy of Plastics FALLO)

Specific Gravity Modules of Elasticity

For room-temperature applications most metals can be considered to

be truly elastic When stresses beyond the yield point are permitted in the design permanent deformation is considered to be a fknction only

of applied load and can be determined directly from the usual tensile stress-strain diagram The bchavior of most plastics is much more dependent on the time of application of the load, the past history of loading, the current and past temperature cycles, and the environmental conditions Ignorance of these conditions has resulted in the appearance

on the market of plastic products that were improperly designed

6 - Plastic performance 383

The plastics material properties information and data presented are provided as comparative guides; readers can obtain the latest and more detailed information from suppliers and/or sof'tware programs (Chapter

5) Since new developments in plastic materials are always on the horizon it is important to keep up to date It is important to ensure that the fabricating process to be used to produce a product provides the properties desired Much of the market success or failure of a plastic product can be attributed to the initial choices of material, process, and

For many materials (plastics, metals, etc.) it can be a highly complex process if not properly approached particularly when using recycled plastics As an example, its methodology ranges from a high degree of subjective intuition in some areas to a high degree of sophistication in other areas It runs the gamut fiom highly systematic value engineering

or failure analysis such as in aerospace to a telephone call for advice fiom a material supplier in the decorative houseware business As reviewed at the end of this chapter there are available different publications, seminars, and software programs that can be helpful

Plastics are families of materials each with their own special advantages and drastically different properties An example is polyethylene (PE) with its many types that include low density PE (LDPE), high density

PE (HDPE), High molecular weight PE (HMWPE), etc The major consideration for a designer and/or fabricator is to analyze what is required as regards to performances and develop a logical selection procedure from what is available

Recognize that most of the plastic products produced only have to

meet the usual requirements we humans have to endure such as the environment (temperature, etc.) Thus there is no need for someone to identify that most plastics cannot take heat like steels Also recognize that most plastics in use also do not have a high modulus of elasticity or long creep and fatigue behaviors because they are not required in their many respective designs However there are plastics with extremely high modulus and very long creep and exceptional high performance fatigue behaviors These type products have performed in service for long periods of time with some performing well over a half-century For certain plastic products there are definite properties (modulus of elasticity, temperature, chemical resistance, load, etc ) that have far better performance than steels and other materials

The designer can use plastics that are available in sheet form, in I-

beams, or other forms as is common with many other materials Although this approach with plastics has its place, the real advantage cost

384 Plastics Engineered Product Design

with plastic lies in the ability to process them to fit the design shape, particularly when it comes to complex shapes Examples include two or more products with mechanical and electrical connections, living hinges, colors, snap fits that can be combined into one product, and so

on

Designing is the process of devising a product that llfills as completely

as possible the total requirements of the user, and at the same time satisfies thc needs of the fabricator in terms of cost-effectiveness (return

on investment) The efficient use of the best available material and production process should be the goal of every design effort Product design is as much an art as a science Guidclines exist regarding meeting and complying with art and science

Influencing Factor

Design guidelines for plastics have existed for over a century producing many thousands of parts meeting service requirements, including those subjected to static and dynamic loads requiring long life Basically design is the mechanism whereby a requirement is converted to a

meaningful plan The basic information involved in designing with

plastics concerns the load, temperature, time, and environment As reviewed throughout this book there are other important performance requirements that may exist such as aesthetics, non-permeability, and cost

In evaluating and comparing specific plastics to meet these require- ments, past experience and/or the material suppliers are sources of information It is important to ensure that when making comparisons the data is available where the tests were performed using similar procedures Where information or data may not be available some type

of testing can be performed by the designer’s organization, outside laboratory (many around), and/or possible the material supplier if it warrants their participation (technicalwise and/or potential costwise) If little is known about the product or cannot be related to similar products prototype testing is usually required

When required, plastics permit a greater amount of structural design freedom than any other material (Chapter 4) Products can be small or large, simple or complex, rigid or flexible, solid or hollow, tough or brittle, transparent or opaque, black or virtually any color, chemical resist or biodegradable, etc Materials can be blended to achieve different desired properties The final product performance is affected

by interrelating the plastic with its design and processing method The designer’s knowledge of all these variables can profoundly affect the ultimate success or failure of a consumer or industrial product

6 - Plastic performance 385

For these reasons design is spoken of as having to be appropriate to the materials of its construction, its methods of manufacture, and the loads (stresses/strains) involved in the product's environment Where all these aspects can be closely interwoven, plastics are able to solve design problems efficiently in ways that are economically advantageous It is important to recognize that these characteristics of plastics exist This book starting with Chapter 1 provides their characteristics and behavior

Select i ng plastic

_ I _ - ~

It is unfortunate that plastics do not have all the advantages and none

of the disadvantages of other materials but often overlooked is the fact that there are no materials that do not suffer from some disadvantages

or limitations The faults of materials known and utilized for hundreds

of years are often overlooked; the faults of the new materials (plastics) are often over-emphasized

As examples, steel is attacked by the elements of fire [1500 to 2500°F (815 to 1370°C)] They lose all their strength, modulus of elasticity,

etc Common protectivc practice includes the use of protective coatings (plastic, cement, etc.) and then forgetting their susceptibility to attack is all too prevalent Wood and concrete are useful materials yet who has not seen a rotted board (wood on fire, etc.) and cracked concrete Regardless this lack of perfection does not mean that no steel, wood, or concrete should be used The same reasoning should apply to plastics

In many respects, the gains made with plastics in a short span of time far outdistance the advances made in these other technologies

To significantly extend the life of structural beams, hardwood (thicker than steel, etc.) can be used; thus people can escape even though the wood slowly burns The more usehl and reliable structural beams would be using reinforced plastics (RTs) that meet structural

performance requirements with even a more extcnded supporting life than wood To date these RPs are not used in this type of fire

environment primarily because of their high cost

Even though the range of plastics continues to be large and the levels of their properties so varied that in any proposed application only a few of the many plastics will be suitable A compromise among properties, cost, and manufacturing process generally determines the material of construction Selecting a plastic is very similar to selecting a metal Even within one class, plastics differ because of varying formulations (Chapter

1), just as steel compositions vary (tool steel, stainless steel, etc.)

386 Plastics Engineered Product Design

For many applications plastics have superseded metal, wood, glass, natural fibers, etc Many developments in the electronics and trans- portation industries and in packaging and domestic goods have been made possible by the availability of suitable plastics Thus comes the question of whether to use a plastic and if so, which one

As an initial step, the product designer must know and/or anticipate the conditions of use and the performance requirements of the product, considering such factors as life expectancy, size, condition of use, shapc, color, strength, and stiffness These end use requirements can be ascertained through market analysis, surveys, examinations of similar products, testing, and/or experience A clear definition of product requirements will often lead directly to choice of the material of construction At times incomplete or improper product requirement analysis is the cause for a product to fail

As a general rule, until experience is developed, it is considered desirable to examine the properties of three or more materials before making a final choice Material suppliers should be asked to participate

in type and grade selection so that their experience is part of the input The technology of manufacturing plastic materials, as with other materials (steel, wood, etc.) results in that the samc plastic compounds supplied fiom various sources will generally not deliver the same results

in a product As a matter of record, even each individual supplier

furnishes their product under a batch number, so that any variation can

be tied down to the exact condition of the raw-material production Taking into account manufacturing tolerances of the plastics, plus variables of equipment and procedure, it becomes apparent that checking several types of materials fiom the same and/or from different sources is an important part of material selection

Experience has proven that the so-called interchangeable grades of materials have to be evaluated carefully as to their affect on the quality

of a product Another important consideration as far as equivalent grade of material is concerned is its processing characteristic There can

be large differences in properties of a product and test data if the processability features vary from grade to grade It must always be remembered that test data has been obtained from simple and easy to

process shapes and does not necessarily reflect results in complex product configurations This situation is similar to those encountered with other materials (steel, wood, glass, etc.)

Most plastics are used to produce products because they have desirable mechanical properties at an economical cost For this reason their mechanical properties may be considercd the most important of all the

6 - Plastic performance 387

physical, chemical, electrical, and other considerations for most applications Thus, everyone designing with such materials needs at least some elementary knowledge of their mechanical behavior and how they can be modified by the numerous structural geometric shape factors that can be in plastics

Comparison

The following information provides examples of guidelines on performance comparisons of different plastics As an example, if the product requires flexibility, examples of the choices include poly- ethylene, vinyl, polypropylene, EVA, ionomer, urethane-polyester, fluorocarbon, silicone, polyurethane, plastisols, acetal, nylon, or some

of the rigid plastics that have limited flexibility in thin sections

The subject of strength can be complex since so many different types exist: short or long term, static or dynamic, etc Some strength aspects are interrelated with those of toughness The crystallinity of TPs is important for their short-term yield strength Unless the crystallinity is impeded, increased molecular weight generally also increases the yield strength However, the crosslinking of TSs increases their yield strength substantially but has an adverse effect upon toughness (Chapter 1)

Increasing the secondary bonds’ strength and crystallinity than by increasing the primary bond strength increases long-term rupture strengths in TPs much more readily Fatigue strength is similarly influenced, and all factors that influence thermal dimensional stability also affect fatigue strength This is a result of the substantial heating that is often encountered with fatigue, particularly in TPs

Polystyrene, styrene-acrylonitrile, polyethylene, acrylic, ABS, polysulfone, EVA, polyphenylene oxide, and many other TPs are

satisfactorily odor-free FDA approvals are available for many of these plastics There are food packaging and refrigerating conditions that will eliminate certain plastics Melamine and urea compounds are examples

of suitable plastics for this service

Thermal considerations will eliminate many materials Examples for products operating above 450°F (232°C) include the silicones, fluoro- plastics, polyirnides, hydrocarbon resins, methylpentene cold mold, or glass-bonded mica plastics may be required A few of the organic

plastic- bonded inorganic fibers such as bonded ceramic wool, perform well in this field Epoxy, diallyl phthalate, and phenolic-bonded glass fibers may be satisfactory in the 450 to 550°F (232 to 288°C) ranges A

limited group of ablation material is made for outer space reentry

388 Plastics Engineered Product Design

Between 250 and 450°F (121 and 232°C) glass or mineral-filled phenolics, melamine, alkyd, silicone, nylon, polyphenylene oxide, poly- sulfone, polycarbonate, methylpentene, fluorocarbon, polypropylene, and diallyl phthalate can be considered The addition of glass fillers to

the thermoplastics can raise the useful temperature range as much as 100°F (212°C) and at the same time shorten the fabricating cycle

In the 0 to 212°F range, a broad selection of materials is available Low temperature considerations may eliminate many of the thermoplastics Polyphenylene oxide can be used at temperatures as low as -275°F Thermosetting materials exhibit minimum embrittlement at low temperature

Underwriters’ Laboratory (UL) ruling on the use of self-extinguishing plastics for contact-carrying members and many other components introduces critical material selection problems All thermosets are self- extinguishing Nylon, polyphenylene oxide, polysulfone, polycarbonate, vinyl, chlorinated polyether, chlorotrifluoroethylene, vinylidene fluoride, and fluorocarbon are thermoplastics that may be suitable for applications requiring self-extinguishing properties Cellulose acetate and ABS are also available with these properties Glass reinforcement improves these rnatcrials considerably

Many TPs will craze or crack under certain environmental conditions, and products that are highly stressed mechanically must be checked very carefully Polypropylene, ionomer, chlorinated polyether, phenoxy, EVA, and linear polyethylene offer greater freedom from stress crazing than some other TPs Solvents may crack products held under stress Toughness behaviors and evaluation can be rather complex A definition

of toughness is simply the energy required to break the plastic This energy is equal to the area under the tensile stress-strain curve The

toughest plastics should be those with very great elongations to break, accompanied by high tensile strengths; these materials nearly always have yield points One major exception to this rule is RPs that use

reinforcing fibers such as glass and graphite that have low elongation For high toughness a plastic needs both the ability to withstand load and the ability to elongate substantially without failing except in the case of Rps (Fig 6.2)

It may appear that factors contributing to high stiffness are required This is not true because there is an inverse relationship between flaw sensitivity and toughness; the higher the stiffness and the yield strength

of a TI?, the more flaw sensitive it becomes However, because some load-bearing capacity is required for toughness, high toughness can be achieved by a high trade-off of certain factors

6 * Plastic performance 389

I : ~ ~ i x p 6.2 Toughness behaviors (courtesy of Plastics FALLO)

// -

Elastic Limit Percent

Crystallinity increases both stiffness and yield strength, resulting usually

in decreased toughness This is true below its glass transition

temperature (T,) in most noncrystalline (amorphous) plastics, and

below or above the Tg in a substantially crystalline plastic (Chapter 1)

However, above the Tg in a plastic having only moderate crystallinity,

increased crystallinity improves its toughness Furthermore, an increase

in molecular weight from low values increases toughness, but with

continued increases, the toughness begins to drop

Deformation is an important attribute in most plastics, so much so that

it is the very factor that has led them to be called plastic For designs

requiring such traits as toughness or elasticity this characteristic has its

advantages, but for other designs it is a disadvantage However, there

are plastics, in particular the RPs, that have relatively no deformation or

elasticity and yet are extremely tough where (a) toughness is related to

heat deflection or rigidity and (b) toughness or impact is related to

temperature for polystyrene (PS) and high impact polystyrene (HIPS)

This type of behavior characterizes the many different plastics available

Some are tough at room temperature and brittle at low temperatures

Others are tough and flexible at temperatures far below freezing but

become soft and limp at moderately high temperatures Still others are

hard and rigid at normal temperatures but may be made flexible by

copolymerization or adding plasticizers

By toughness is meant resistance to fracture However, there are those

materials that are nominally tough but may become ernbrittled due to

390 Plastics Engineered Product Design

processing conditions, chemical attack, prolonged exposure to constant stress, and so on A high modulus and high strength with ductility is the desired combination of attributes However, the inherent nature of plastics is such that their having a high modulus tends to associate them with low ductility, and the steps taken to improve the one will cause the other to deteriorate

Soft, weak materials have a low modulus, low tensile strength, and only moderate elongation to break According to ASTM standards, the elastic modulus or the modulus or elasticity is the slope of the initial straight-line portion of the curve Hard, brittle materials have high moduli and quite high tensile strengths, but they break at small

elongations and have no yield point Hard, strong plastics have high

moduli, high tensile strengths, and elongations of about 5% before breaking Their curves often look as though the material broke about where a yield point might have been expected

Soft, tough plastics are characterized by low moduli, yield values or plateaus, high elongations of 20 to 1,000%, and moderately high breaking strengths The hard, tough plastics have high moduli, yield points, high tensile strengths, and large elongations Most plastics in this category show cold drawing or necking during the stretching operation The

RPs will have at least one modulus but some materials can have two or three

Although impact strength of plastics is widely reported, the properties have no particular design values and can be used only to compare relative response of materials (toughness, etc.) Even this comparison is not completely valid because it does not solely reflect the capacity of the material to withstand shock loading, but can pick up discriminatory response to notch sensitivity

A better value is impact tensile, but unfortunately this property is not generally reported The impact value can broadly separate those that can withstand shock loading vs those that are poor in this response Therefore, only broad generalizations can be obtained on these values Comparative tests on sections of similar size which are fabricated in accordance with the proposed product must be tested to determine the impact performance of a plastics material The laminated plastics, glass- filled epoxy, melamine, and phenolic are outstanding in impact strength Polycarbonate and ultrahigh molecular weight PE are also outstanding in impact strength

In general, rigid plastics are superior to elastomers in radiation resistance but are inferior to metals and ceramics The materials that

will respond satisfactorily in the range of 1O1O and l o l l erg per gram are

glass and asbestos-filled phenolics, certain epoxies, polyurethane, poly- styrene, mineral-filled polyesters, silicone, and hrane The next group

of plastics in order of radiation resistance includes polyethylene, melamine, urea formaldehyde, unfilled phenolic, and silicone plastics Those materials that have poor radiation resistance include methyl methacrylate, unfilled polyesters, cellulosics, polyamides, and fluoro- carbons

Maximum transparency is available in acrylic, polycarbonate, polyethylene, ionomer, and styrene compounds Many other thermoplastics may have adequate transparency

Urea, melamine, polycarbonate, polyphenylene oxide, polysulfone, polypropylene, diallyl phthalate, and the phenolics are needed in the temperature range above 200°F (93°C) for good color stability Most

TPs will be suitable below this range

Deteriorating effects of moisture are well known For high moisture applications, polyphenylene oxide, polysulfone, acrylic, butyrate, diallyl phthalate, glass-bonded mica, mineral-filled phenolic, chlorotrifluoro- ethylene, vinylidene, chlorinated polyether chloride, vinylidene fluoride, and the fluorocarbons should be satisfactory Diallyl phthalate, polysulfone, and polyphenylene oxide have performed well with moisture/steam on one side and air on the other (a troublesome combination), and they also will withstand repeated steam autoclaving Long-term studies of the effect of water have disclosed that chlorinated polyether gives outstanding performance Impact styrene plus 2 5 w %

graphite and high density polyethylene with 15% graphite give long- term performance in water

Depending on what is required, the different plastics can provide different rates of permeability properties As an example certain polyethylenes will pass wintergreen, hydrocarbons, and many other chemicals It is used in certain cases for the separation of gases since it will pass one and block another Chlorotrifluoroethylene and vinylidene fluoride, vinylidene chloride, polypropylene, EVA, and phenoxy merit evaluation

There are materials with low or no permeability to different environ- ments or products Barrier plastics are used with their technology not becoming more complex but more precise Different factors influence performance such as being pinhole-free; chemical composition, crosslinking, modification, molecular orientation; density, and thickness The coextrusion and coinjection processes are used to reduce permeability while retaining other desirable properties Total protection against vapor transmission by a single barrier material increases linearly

392 Plastics Engineered Product Design -

with increasing thickness, but it usually is not economical Thus extensive use is made of multiple layer constructions This composite would include low cost as well as recycled plastics that provide mechanical support, etc while an expensive barrier material thickness is significantly reduced

With crystalline plastics, the crystallites can be considered impermeable Thus, the higher the degree of crystallinity, the lower the permeability

to gases and vapors The permeability in an amorphous plastic below or not too far above its glass transition temperature (Tg) is dependent on the degree of molecular orientation It is normally reduced when compared to higher temperatures, although small strains sometimes increases the permeability of certain plastics The orientation of elastomers well above their Tg has relatively less effect on the overall transport property Crosslinking thermoplastics will decrease perme- ability due to the deaease in their diffusion coefficient The effect of crosslinking is more pronounced for large molecule size vapors The addition of a plasticizer usually increases the rates of vapor diffusion and permeation (Chapter 1)

The permeation of vapors includes two basic processes: the sorption

and difhsion of vapors in the plastic As an examplc in the packaging industry, the resistance of moisture is essential for the preservation of many products The loss of moisture, flavor, etc through packaging materials may damage foodstuff The prevention of the ingress of moisture by a barrier is essential for the storage of dry foods and other products In other applications, the degree of resistance to water and oxygen is important for the development of corrosion resistance coatings, electrical and electronic parts, etc

Fluorination is the process of chemically reacting a material with a

fluorinc-containing compound to produce a desired product As an

example it can improve the gasoline barrier of PE to nonpolar solvents

A barrier is created by the chemical reaction of the fluorine and the PE, which form a thin (20 to 40 mm) fluorocarbon layer on the surface Two systems can be used to apply the treatment depending on the results desired With the “in-process” system, such as that used during blow molding PE gasoline tanks, fluorine is used as a part of the parison expanding gas in the blowing operation (the result is no gasoline leakage) The barrier layer is created only on the inside In a post- treatment system, bottles and other products are placed in an enclosed chamber filled with fluorine gas This method forms barrier layers on both the inside and outside surfaces

Worksheet

The first step in selecting a plastic for a product to be fabricated is to

determine its complete requirements Since there could be a tendency

to overlook certain properties because they may appear to be insignifi- cant or overlooked, it is vital to ensure that the product will perform during packaging, shipment, and/or in service Selecting an optimal material for a given product must obviously be based on analysis of the requirements to be met A simplified approach involves comparing the

specific service requirements to the potential properties of a plastic What follows is a simplified but practical material-selection approach This “longhand” system has been uscd for almost a half century during which time it became a basis in many fast computerized software material selection databases

A simplified approach is where one starts by selecting the design criteria

as well as potential plastics of interest and incorporating them into a

table format checking off only the major criteria across the worksheet Follow by setting up a comparison of the performance requirements for the potential plastics being considered and transfer the bold-faced numerical rating in each selected criteria column to the worktable Add these numbers across the worktable to determine the plastic group with the lowest-point subtotal that will be the best plastic for a given application on a performance basis Next add in the cost factor and

total it to find the plastic group with the lowest number that results the best choice based on a cost-performance evaluation

Follow by determining the specific plastic within the plastic group selected The plastic with the lowest final total will be the best for the application on a cost-performance basis

Plastics behave differently when exposed to temperatures; most plastic can take greater heat than humans There are some plastics that cannot

394 Plastics Engineered Product Design

take boiling water and others operate at 150°C (300°F) with a few up

to 540°C (1000°F) Most are not effected by low temperature (below

fieezing) The flexible (elastomer) plastics at room temperature become less flexible as they are cooled, finally becoming brittle at a certain low temperature Then there are plastics that reach 1370°C (2500°F) with

exposures in fiactions of a second Performance is influenced by short

to long time static and dynamic mechanical requirements An excellent test if a plastic can take heat is put in your automobile trunk or a railroad boxcar where temperatures can reach 55°C (1 30°F)

Important to understand that there is a temperature transition in plastics; also called ductile-to-brittle transition temperature It is temperature at which the properties of a material change Depending

on the material, the transition change may or may not be reversible A few other characteristics are presented, The plastic softening range temperature is the temperature at which a plastic is sufficiently soft to

be distorted easily A number of tests exist and the temperatures arrived

at may vary according to the particular test method Softening range is sometimes erroneously referred to as the softening point Temperature stability identifies the percent change usually in tensile strength or in percent elongation as measured at a specified temperature and compared

to values obtained at the standard conditions of testing

Data obtained by testing different impact properties at various temperatures produces information that is similar to an elongation vs temperature curve As temperatures drop significantly below the ambient temperatures, most TPs lose much of their room-temperature impact strength A few, however, are on the lower, almost horizontal portion of the curve at room temperature and thus show only a gradual decrease in impact properties with decreases in temperature One major exception is provided by the glass fiber RPs, which have relatively high

Izod impact values, down to at least 4 0 ° C ( 4 0 ° F ) The S-N (fatigue)

curves for TPs at various temperatures show a decrease in strength values with increases in temperature However the TSs, specifically the

TS RPs, in comparison can have very low losses in strength

Plastics can be affected in different ways by temperature It can influence

short- and long-time static and dynamic mechanical properties, aesthetics, dimensions, electronic properties, and other characteristics Fig 6.3 provides a guide relating time at temperature vs 50% retention mechanical and physical properties Testing temperature was at the exposure temperature of test specimens

As the temperature rises thermoplastics (TPs) are effected In com- parison thermosets (TSs) are not affected The maximum temperatures

6 - Plastic performance 395

ure 6.3 Guide t o temperature versus plastic properties (Courtesy of Plastics FALLO)

under which plastics can be employed are generally higher than the temperatures found in buildings, including walls and roofs, but there are those such as LDPE that are marginal and cannot carry appreciable stresses at these moderately elevated temperatures without undergoing noticeable creep Many plastics can take shipping conditions that are more severe than their service conditions With a closed automobile trunk or railroad boxcar temperatures reach at least 52°C (126°F); a temperature endurance test could be run in these closed containers or other containers

Plastic strength and modulus will decrease and its elongation increase with increasing temperature at constant strain Curves for creep isochronous stress and isometric stress are usually produced from measurements at a fixed temperature (Chapter 3) Complete sets of these curves are sometimes available at temperatures other than the ambient It

is common to obtain creep rupture or apparent modulus curves plotted against log time, with temperature as a parameter (Fig 6.4)

A set of creep-rupture curves at various temperatures (Fig 6.5) can be

extended to provide data to obtain longer time data With these data projecting the lowest-temperature curves to longer times as a straight line would produce a dangerously high prediction of rupture strength

396 Plastics Engineered Product Design

FFg~ire 6.4 Effect o f temperature on creep modulus

A

Temperature increasing

LOG TIME (HOURS)

An advantage of conducting complete creep-rupture testing at elevated temperatures is that although such testing for endurance requires long times, the strength levels of the plastic at different temperatures can be developed in a relatively short time of usually just 1,000 to 2,000 h The Underwriters Laboratories and other such organizations have employed such a system for many decades

Thermal Property

Different plastics provide a wide range of temperature capabilities with

a wide difference between TPs and TSs The more common TI? follows different phases as it is subjected to heat Fig 6.6 shows a plot for a TP

~!~~~~ 6.5 Temperature effect on creep-rupture

up to the temperature where it disintegrates

TPs’ properties (and processes) are influenced by their thermal characteristics such as melt tempcraturc ( T,), glass-transition temperature ( Tg), dimensional stability, thermal conductivity, thermal diffusivity, heat capacity, coefficient of thermal expansion, and decomposition (Td) Table 6.1 provides some of this data on different plastics (also applicable data for aluminum and steel) All these thermal properties relate to how to determine the best useful processing conditions to meet product performance requirements There is a

maximum temperature or, to be more precise, a maximum time-to- temperature relationship for all materials preceding loss of performance

or decomposition

Heat history or residence time develops when TPs repeatedly exposed

to heating and cooling cycles such as when recycled Certain TPs can be indefinitely granulating (scrap, defective products, and so on) During the heating and cooling cycles the performance and properties of certain plastics will not change or be insignificantly affected However thcrc arc TPs that have minor to completely destructive results TPs that are heat sensitive or those with certain additives and/or fillers are subject to destruction on their first recycling If incorrect methods were used in granulating recycled material, more degradation will occur

TPs are subjected to various degrees of dimensional stability

Dimensional stability is the temperature above which plastics lose their

dimensional stability For most plastics the main determinant of

dimensional stability is their T,, With highly crystalline plastics is T, not

a major problem (Chapter 1) The crystalline plastics in the range between T, and T, are referred to as leathery, because they arc made

up of a combination of rubbery noncrystalline regions and stiff crystalline regions The result is that such plastics as PE and PP are still

usefd at lower temperatures and nylon is useful to moderately elevated temperatures even though those temperatures may be above their respective T,

Plastic memory is another behavior of TPs When subjected to heat they can be bent, twisted, stretched, compressed, or squeezed into various useful shapes, but eventually, especially if you add heat, they return to their original form This behavior, can be annoying When

Table 6.1 Thermal properties of materials (Courtesy o f Plastics FALLO)

~~ ~~~

Glass Thermal Heat Thermal Thermal Density Melt transition conductivity capacity diffusivity expansion Plastics skm3 temperature temperature (10-4 cal/scmT) cal/g“C 1P4 cmZ/s W c m / c m “C (morphology) lb./ft3) T,,, “C C9 T, “CC9 @TU/lb “ 9 (BTU/lb ‘fl 10-3ft!/hc) (1@6in./in 4

3

3

6 4.7

5

3000

(0.068) (0.290) (0.145) (0.140) (0.087) (0.073) (0.073) (0.114) (0.1 45) (0.121) (72.50

0.9 0.9 0.3 0.075 0.45 0.5 0.5 0.56 0.5 0.6 0.23

(0.004) (0.004) (0.003) (0.001 1

(0.002) (0.002) (0.002) (0.002) (0.002) (0.002)

3.5 13.9 9.1 6.8 5.9 3.8 5.7 8.9 7.8 6.2

4900

(1.36) (3.53) (2.64) (2.29) (1.47) (5.41

(2.2) (3.45) (3.0) (2.4) (1 900)

19 (10.6)

= Crystalline resin A = Amorphous resin

6 - Plastic performance 399

Figure 5.6 Example of TP modulus of elasticity versus temperature

STAGES GLASSY TRANSITION RUBBERY MELT FLOW

During this shaping they do not alter their molecular structure or grain orientation to accommodate the deformation permanently Plastics temporarily assume the deformed shape but always maintain internal stresses that want to force the material back to its original shape

From a design approach plastic memory can be built into the product during fabrication The tendency for the product to move into a new shape is included as an integral part of the design After the product is assembled in place, a small amount of heat can coax that part to change shape TP products can be deformed during assembly then allowed returning to their original shape In this example products can be stretched around obstacles or made to conform to unavoidable irregularities without permanent damage

Most TPs naturally have this memory capability Polyolefins, neoprene, silicone, and other crosslinkable polymers can be given a memory either

by radiation or by chemically curing Fluorocarbons, however, need no curing With fluorocarbons such as TFE, FEP, EWE, ECTFE, CTFE, and PVF useful high tcmperamre or wear resistant applications are possible