Handbook of Plastics, Elastomers and Composites Part 5 pps

The selection of production equipment and processes for mosetting molding compounds commences with the compound designer’s formulation,which designates the type and quantity of the vario

Thermosets, Reinforced Plastics, and Composites 161

catalyst pigment, and lubricants The compounds are free-flowing granular in form and areavailable in opaque colors (mostly red) They are readily moldable in compression, trans-fer, and injection molding processes

2.7.7.3 Reinforcements. The reinforcements available are quartz, “E”-type glass bers, and fused silica

fi-2.7.7.4 Specifications

Military (MII-M-14)

MSG: Mineral filler, heat resistant

MSI-30: Glass filler, impact value of 3.0 ft-lb, heat resistant

UL. The UL ratings are (1) thermal index rating of 464°F (240°C) and(2) flammabilityrating of 94 VO in 1/16-in sections

2.7.7.5 Data sheet values. Table 2.30 provides a property value list for the siliconefamily of molding compounds

2.7.7.6 Applications. The silicones are nonconductors of either heat or electricity;have good resistance to oxidation, ozone, and ultra-violet radiation (weatherability); andare generally inert They have a constant property profile of tensile, modulus, and viscosityvalues over a broad temperature range 60 to 390°F (13 to 166°C) They also have a low

glass transition temperature ( T g ) of –185°F Encapsulation of semiconductor devices such

as microcircuits, capacitors, and resistors, electrical connectors seals, gaskets, O-rings,and terminal and plug covers all take advantage of these excellent properties

2.7.7.7 Suppliers

Dow Corning Corporation

Cytec Fiberite

General Electric Company Silicone Products Division

(See App C for supplier addresses.)

2.7.8 Composites

2.7.8.1 History 15 The introduction of fiberglass-reinforced structural applications inl949 brought a new plastics application field which began with the consumption of 10 lband burgeoned into the annual usage of over 1 to 2 billion lb over the next several de-cades This usage has been, and still is, taking place in application areas that take advan-tage of the extraordinarily low-weight/high-strength ratio inherent in these compositematerials

A thermosetting matrix is defined as a composite matrix capable of curing at some

tem-perature from ambient to several hundred degrees of elevated temtem-perature and cannot bereshaped by subsequent reheating In general, thermosetting polymers contain two ormore ingredients—a resinous matrix with a curing agent that causes the matrix to poly-merize (cure) at room temperature, or a resinous matrix and curing agent that, when sub-jected to elevated temperatures, will commence to polymerize and cure

2.7.8.2 Resins (matrices). The available resins are polyester and vinyl esters, ureas, epoxy, bismaleimides, polyimides, cyanate ester, and phenyl triazine

poly-Polyester and vinyl esters. Polyester matrices have had the longest period of use,with wide application in many large structural applications (see Table 2.31) They will

cure at room temperature with a catalyst (peroxide), which produces an exothermic tion The resultant polymer is nonpolar and very water resistant, making it an excellentchoice in the marine construction field The isopolyester resins, regarded as the most wa-ter-resistant polymers in the polymer group, have been chosen as the prime matrix materi-als for use on a fleet of U.S Navy mine hunters.

reac-Epoxy. The most widely used matrices for advanced composites are the epoxy resins,even though they are more costly and do not have the high-temperature capability of thebismaleimides or polyimide The advantages listed in Table 2.32 show why they arewidely used

Bismaleimides (BMIs). The bismaleimide resins have found their niche in

high-tem-perature aircraft design applications where temhigh-tem-perature requirements are in the 177°C(350°F) range BMI is the primary product and is based on the reaction product from me-thylene dianiline (MDA) and maleic anhydride: bis (4 maleimidophynyl) methane (MDABMI) Variations of this polymer with compounded additives to improve impregnation arenow on the market and can be used to impregnate suitable reinforcements to result in high-temperature mechanical properties (Table 2.33)

Polyimides. Polyimides are the highest-temperature polymers in the general advanced

composite, with a long-term upper temperature limit of 232 to 316°C (450 to 600°F) ble 2.34 is a list of commercial polyimides being used in structural composites

Ta-Polyureas. Polyureas involve the combination of novel MDI polymers and eitheramine or imino-functional polyether polyols The resin systems can be reinforced withmilled glass fibers, flaked glass, Wollastanite, or treated mica, depending on the compoundrequirements as too processability or final product

Thermosets, Reinforced Plastics, and Composites 165

Cyanate ester and phenolic triazine (PT). The cyanate ester resins have shown rior dielectric properties and much lower moisture absorption than any other structuralresin for composites The physical properties of cyanate ester resins are compared to those

supe-of a representative BMI resin in Table 2.33 The PT resins also possess superior temperature properties, along with excellent properties at cryogenic temperatures Theyare available in several viscosities, ranging from a viscous liquid to powder, which facili-tates their use in applications that use liquid resins such as filament winding and transfermolding

elevated-2.7.8.3 Reinforcements

Fiberglass. Fiberglass possesses high tensile strength and strain to failure, but the realbenefits of its use relate to its heat and fire resistance, chemical resistance, moisture resis-tance, and very good thermal and electrical values Some important properties of glass fi-bers are shown in Table 2.35

Graphite. Graphite fibers have the widest variety of strength and moduli and also thegreatest number of suppliers These fibers start out as organic fiber, rayon, polyacryloni-

trile, or pitch called the precursor The precursor is stretched, oxidized, carbonized, and

graphitized The relative amount of exposure to temperatures from 2500 to 3000°C willthen determine the graphitization level of the fiber A higher degree of graphitization willusually result in a stiffer (higher modulus) fiber with greater electrical and thermal con-ductivities Some important properties of carbon and graphite fibers are shown in Table2.36

Aramid. The organic fiber kevlar 49, an aramid, essentially revolutionized pressurevessel technology because of its great tensile strength and consistency coupled with lowdensity, resulting in much more weight-effective designs for rocket motors

Boron. Boron fibers, the first fibers to be used in production aircraft, are produced asindividual monofilaments upon a tungsten or carbon substrate by pyrolytic reduction ofboron trichloride (BCL) in a sealed glass chamber Some important properties of boron fi-bers are shown in Table 2.37

2.7.9 Molding Compound Production 16

2.7.9.1 Introduction. The selection of production equipment and processes for mosetting molding compounds commences with the compound designer’s formulation,which designates the type and quantity of the various ingredients that make up the com-pound These molding compounds are a physical mixture of resin, reinforcement or filler,

ther-catalyst, lubricant, and color The resin, by far, is the key component in any thermosetting

molding compound, since it is the only component that actually goes through the chemical

reaction known as polymerization or cure during the molding process Also, because of

this curing quality, the resin, production process, and equipment are governed by the need

to understand this chemical reaction with its effects on the production process and/orequipment Also, the resin is the primary flow promoter and chief provider of the desiredelectrical insulating properties of the final molded product

The next most important component is the reinforcement or filler, because the type andquantity of either will determine the manufacturing process and equipment This will beseen in the following sections, which describe the various processes and their equipment

Thermosets, Reinforced Plastics, and Composites 169

The compounds utilizing “fillers” as opposed to “reinforcements” can be processed witheither the “dry” or the “wet” (solvent) process, since the compound formulations includefree-flowing granular fillers that are not as susceptible to degradation when exposed to the

hot roll mill phase of the operation The catalyst in each compound serves as the reaction controller, with the type and quantity of catalyst acting to either accelerate or inhibit the

curing rate in both the production and the molding phases

Lubricants, which provide a measure of flow promotion, mold release, and barrel lifeduring molding, are generally internally supplied but are occasionally provided as an ex-ternal addition All thermosetting molding compound colors are opaque, with the pig-ments or dyes heat stable within the molding process temperature range of 200 to 400°.Coloring does have a large effect on the manufacturing process when the product line in-cludes a wide variety of colors such as are common to the DAP, melamine, urea, and ther-moset polyester compounds Choice of production equipment has to be designed to meetthe need for quick and easy color changes

2.7.9.2 Production processes

Dry process (batch and blend). The dry or nonsolvent process, illustrated in Fig.

2.15, employs low-strength, low-cost, free-flowing granular fillers and involves the use ofribbon or conical mixers to homogenize the dry ingredients prior to feeding the mix ontothe heated roll mills where the mix is compounded (worked) for a specific time and tem-perature Once the mix or hatch has been worked to the proper consistency and tempera-ture, it is then fed onto a three-roll calendering mill where it is shaped to a specific widthand thickness to allow the sheet to pass into a grinder, and then onto screens to obtain thedesired granulation and for dust removal The thickness and temperature of the calenderedsheet is controlled for ease of granulation If the sheet is too warm, it will not cut cleanly;

if the sheet is too cool, it will be too fragile to produce clean, even size particles

An individual batch is generally 200 lb, which eventually is blended with other hatchesinto 2500- to 5000-lb blends that are ready for shipment to the customer The batch and

Figure 2.15 Batch and blend dry production method.

blend process is employed for short runs, especially where the production schedules callfor a variety of colors The equipment must he such that it allows for relatively quick colorchangeovers.

Wet process (pelletized). The wet process, shown in Fig 2.16, when low-strength,low-cost, free-flowing granular fillers are used, can produce free-flowing pelletized mate-rial, with water as the solvent and the ingredients thoroughly mixed in a kneader and thenauger fed to a heated extruder The extruder screw densifies the wet mass, forcing it out ofthe extruder head, which contains many small through orifices that determine the diameter

of the pellets, and the fly-cutters working across the face of the extruder head determinethe pellet length The extruded pellets then require a drying operation prior to final blend-ing for shipment

Wet process (high strength). As can he seen in Fig 2.17, this process involves theuse of mixers, mobile carts, air-drying rooms, prebreakers, hammer mills, lenders, and ex-truders The basic purpose of the entire process is to provide minimum reinforcement deg-radation so as to maintain sufficient fiber integrity to meet the various mechanical strengthrequirements which are generally set by military or commercial specifications

The process begins with the mixing of all the ingredients, except the reinforcement, into

a suitable solvent Once the mix has been properly dispersed, the reinforcement is added,keeping the mixing time to a minimum to preserve the fiber integrity Solvent recovery ispossible during this phase of the operation as well as later on in the drying phase The wetmass is removed from the mixer and spread onto wire trays capable of holding about 25 lb.These trays are loaded into a mobile cart and placed in a drying room, generally overnight.After drying, the now-hardened slabs of compound are fed into a “prebreaker,” which tearsthe slab into pieces that are then sent to a hammer mill for particle size reduction Theeventual particle size is determined by the size of the openings at the bottom of the mill.There is no need for blending, since the ultimate flow properties of the compound aregoverned by the resin, reinforcement, and catalyst mix There has been little, if any, tem-perature imposed upon the materials that might affect the flow properties

High volume (general purpose). The method for producing large volume runs ofgranular compounds, generally of a single color, flow, and granulation, shown in Fig

2.18a, involves the use of extruders or kneaders into which the compound mix (wet or dry)

is fed for “working” or homogenizing prior to being extruded out the exit end of the unit

The open buss kneader (Fig 2.18b) works with an external screw bearing in the

produc-tion of pastel colors in the urea and melamine compounds When processing epoxy, esters (with or without glass fibers), and phenolics, the external bearing is not required.Both the open buss kneader and the Werner Pfliederer compounding unit will process thecompounds and also control the granulation and flow properties

poly-Sheet process (SMC). Almost regardless of the specific resin used in the SMC cess, the compound manufacturing technique is the same as that shown in Fig 2.19, withthe doughy material and its reinforcement being covered on both lower and upper surfaceswith a thin film of polyethylene The finished product is then conveyed onto a rotatingmandrel and wound up until it reaches a preset weight, and then it is cut off The sheetsready for use or for shipment generally weigh about 50 lb, are 4 ft wide, and are approxi-mately 0.075 to 0.250 in thick Formulations generally consist of an unsaturated polyesterresin (20 to 30 percent), chopped glass rovings (40 to 50 percent), fine particle size cal-cium carbonate, filler, catalyst, pigment, and modifiers The resin system can be epoxy,polyester, or vinyl ester to meet the need of the marketplace

pro-Sheet process (TMC). The production of TMC, shown in Fig 2.20, differs from that

of SMC in that the glass fibers are wetted between the impregnating rollers before being

Figur

deposited onto the moving film TMC sheets can be produced in thicknesses of up to 2 inwith glass lengths of 1 in/min at loading levels of 20 to 30 percent These sheets are gener-ally compression molded using matched metal-hardened steel molds Packaging and ship-ment of the unmolded product is similar to that of the SMC products.

High strength (BMC). Bulky high-strength compounds are produced with the batchmethod during which the resin, lubricants, catalyst, and chopped glass fibers (1/8 to 1/2 inlong) are all compounded in relatively low-intensity mixers The mixing procedure is care-fully monitored to achieve the highest possible mechanical properties with the leastamount of fiber degradation The finished product is shipped in bulk form using vapor bar-rier cartons with the compound in a sealed polyethylene bag These compounds are alsoavailable in a “rope” form in any length or diameter specified by the customer The shape

is attained by feeding the doughy material through an extruder, with the extruder nozzleproviding the desired shape and length

Quality assurance. Thermosetting compound production uses comprehensive qualityassurance programs to ensure that customers receive products that meet their specificspecifications, regardless of the process or equipment employed Formulation and process-ing specifications require that the incoming raw materials meet specific quality standardsand that the manufacturing processes be carefully monitored to make certain that the finalcompound meets a designated property profile

The formulation requirements are based on meeting certain standards relating to thefollowing property standards:

Thermosets, Reinforced Plastics, and Composites 173

(a)

(b)

Figure 2.18 (a) Continuous dry high-volume method and (b) open

bus kneader.

Figure 2.19 Sheet molding compound (SMC) process.

Figure 2.20 Thick molding compound (TMC) process.

Thermosets, Reinforced Plastics, and Composites 175

■ Surface finish

■ Color

These specifications are established by either the military or commercial users or by

ASTM and UL standards The compound designer’s primary task is one of selecting the

individual ingredients that, within a cost range, will not only create the appropriate pound but will also function properly in the manufacturing process Process proceduresare governed by a set of standards that spell out check points along the line to aid in theprocess control The formulation will furnish the necessary information relating to the ac-quisition of raw materials along with the basic standards by which each ingredient is ac-cepted for incorporation into the compound mixture

com-The specific ingredients are resin reinforcement, pigments or dyes, lubricants, and vents Each of these ingredients has specific characteristics that are measurable and con-trollable, and the compound producer can, and often will, supply the molder with test data

sol-on each productisol-on blend the customer has received The special characteristics of each ofthese ingredients are as follows:

Resin Viscosity, gel time, cure rate, and solubility

Reinforcements and fillers Aspect ratio, moisture content, fiber size and length, purity,

and color

Pigments and dyes Solubility, coatability, and thermal stability

Catalysts Solubility, reaction temperature, and purity

Lubricants Solubility, melting point, and purity

Solvents Purity and toxicity

Compounds manufactured to meet Mil specifications will be subjected to a certificationprocess involving the documentation of actual property values derived from the testing ofthe compound both during, and upon completion of the manufacturing procedure The val-ues called for are as follows:

■ Volume and surface resistivity

Certification will cover a blend of 2500 lb or more, with the actual values recorded andfurnished to the customer on request The alternate certification process involves the com-pounder furnishing a letter of compliance that confirms that the actual blend involved doesmeet all the requirements of the specification

2.7.9.3 Testing equipment and procedures. Most compounders employ a variety

of testing tools to control and monitor their manufacturing processes, as well as their R&Dprograms, and for troubleshooting problems encountered in the field A list of such equip-ment follows

Unitron metallograph. This is a sophisticated metal detection device used to checkproduction as well as troubleshoot areas.

Scanning electron microscope (SEM). An SEM is employed to examine surfacesfrom low magnification up to a 100,000× enlargement It is an excellent research andproblem-solving tool

Dynamic mechanical spectrophotometer (DMS). The DMS measures the tic properties of polymers, thus determining a compound’s viscosity and elastic modulus,following the change of these properties over time and changes in temperature

viscoelas-Infrared spectrophotometer. The infrared spectrophotometer can provide

informa-tion concerning the composiinforma-tion of a compound such as degradainforma-tion, replacement for ers and reinforcing agents, and it can be used to evaluate the purity of resins, fillers,catalysts, reinforcements, solvents, and lubricants

fill-Capillary rheometer. The Monsanto capillary rheometer measures the viscosity

prop-erties of polymers and provides a direct measure of viscosity and the change in viscositywith time and flow rate at plastication temperatures The capillary orifice simulates thegate and runner system of actual molding conditions, thus providing valuable flow infor-mation for molding compounds

Brabender plasticorder. The plasticorder is a small mixer capable of measuring theviscosity and the gel time of thermosetting molding compounds with results that can becorrelated to the performance of a compound during molding conditions

High-pressure liquid chromatograph. This device is available in two forms:

1 Gel permeation Measures the molecular weight distribution and average molecular

weight of the molecules in a sample of a compound

2 Liquid Detects and measures the amount of chemical constituents present in a

com-pound

Thermal analysis (TA). DuPont’s TA equipment is available in four modules that vide information regarding the effect of temperature on a compound’s physical properties

pro-1 Differential scanning calorimeter (DSC) This measures heat uptake or heat release of

a compound as the temperature is raised and also the heat effects associated with terial transitions such as melting

ma-2 Thermal analyzer (TMA) A thermal analyzer measures the variation in the length of

a sample as temperature is increased It is good for comparing this property with asample of another compound TMA also measures thermal transition points by pre-dicting the point and rate at which a compound will melt as well as determining thetemperature at which blistering will occur if a molded part has not been properlypostbaked

3 Dynamic mechanical analyzer (DMA) Measures a compound’s modulus (stiffness) as

its temperature is raised This instrument has provided interesting insights into theproperties of phenolics as well as those of DAPs, thermoset polyesters, silicones, andepoxies, by indicating the ability of thermosets to retain their modulus at elevatedtemperatures

4 Thermogravimetric analyzer (TGA) TGA measures weight changes in a sample as the

temperature is varied, providing a useful means to determine degradative processesand heat resistance in polymeric compounds

Thermosets, Reinforced Plastics, and Composites 177

Particle size analyzer (PSA). A particle size analyzer is an accurate and automaticdevelopment tool that allows for a very rapid measurement of particle size distribution inpowder or slurry compounds

Humidity chamber. A humidity chamber is employed to measure the effects of perature and humidity cycles on molded parts

tem-Instron testers. Instron testers will measure flexural, tensile, and compressive

strength as well as stress-strain curves at ambient temperatures, and, when fitted with anenvironmental chamber, the flexural and tensile tests can run at elevated temperatures

2.7.9.4 Rheology (flow testing). Easily the foremost characteristic of a ting molding compound is its ability to flow under pressure within the confines of theheated mold This property value is of utmost importance in the eyes of the molder andwill vary according to the molding method, mold design, molding equipment, and cer-tainly the configuration of the molded part

thermoset-Since the molding compound is subjected to elevated temperature and pressure in themolding process, its ability to flow is greatly affected by the chemical reaction takingplace as a result of these conditions As a general rule, the speed of this chemical reactionwill double with every 10° increase in temperature In every thermosetting molding cycle,regardless of the type of compound, mold, or molding press used, the molding compoundwill go through the typical thermosetting reaction curve shown in Fig 2.21 At the top leftside (A), the compound is at room temperature and 0 pressure With pressure applied andthe compound increasing in temperature, its viscosity decreases as shown on the slope at

B This decrease in viscosity continues along the B slope as the compound temperature creases until the compound reaches its peak of flow at C, just prior to a rapid acceleration

in-of the reaction as the curve turns upward to D, thus completing the cure

Every individual molding compound possesses its own flow characteristics, which areaffected by the resin, catalyst, and reinforcement ratio as well as the type and content of all

the ingredients that make up the complete formulation The desired rheology or flow

prop-erty requirements of a specific compound will be determined by

Molding method Compression, transfer, injection, mold designs, part configuration,

number of cavities and their location in the mold, and size and location of the runnerand gate system

Figure 2.21 Typical thermosetting curve.

Flow specifications Generally identified as stiff, medium, or soft or by a designated

cure rate or flow time All thermosetting molding compounds possess flow tics that are both measurable and controllable, with the important characteristics beingthe rate of curing, speed of the flow, distance of the flow, and finally the amount of com-pound used during the flow time

characteris-2.7.9.5 Flow testing procedures. The five most widely used flow testing dures have one main purpose—to provide specific and detailed information based on thecompound’s intended use Compounds that are designated for use in compression moldswill have decidedly different flow requirements than if the intended use is in either a trans-fer or an injection molding process

proce-In all of the following flow tests, with the exception of the Brabender, three elementsare always kept constant during the testing procedure:

■ Amount of compound Charge weight

■ Mold temperature Usually 300°F

■ Molding pressure Usually 1000 lb/in2

Cup closing (test and mold) (Fig 2.22). With the mold set at 300°F (148°C) and amolding pressure of 1000 lb/in,2a room-temperature charge of compound is placed in thelower half of the mold, and the mold is closed The time required for the mold to com-pletely close is recorded in seconds The longer the time, the stiffer the compound; theshorter the time, the shorter the flow Generally speaking, stiff flow compounds will be

15 s or more, whereas the medium flow compounds will be in the 8- to 14-s range, and thesoft flow compounds closing in less than 8 s

Figure 2.22 Cup closing test and mold.

Thermosets, Reinforced Plastics, and Composites 179

Disk flow I (test and mold) (Fig 2.23). With the mold in the open position, a sured amount of room-temperature compound is placed on the lower half of the mold, andthe mold is closed and then reopened as soon as the compound is cured The molded disk

mea-is then measured for diameter and thickness The thinner the dmea-isk, the softer the flow; thethicker the disk, the stiffer the flow

Disk flow II (test and mold) (Fig 2.24). With the diameter of the disk being used asthe gauge, the molded disk is placed on a target of concentric circles numbered 1 to 5 Adisk matching the #1 circle on the target will he designated as 1S flow, whereas a diskmatching the #5 circle will carry a 5S How The higher the number, the softer the flow

Orifice flow I (test and mold) (Fig 2.25). This flow test involves the use of a moldwith a lower plate containing a cavity into which a measured quantity of room-tempera-ture compound is placed The upper plate has a plunger with two small orifices cut into theouter circumference, as shown in Fig 2.25 The test generally uses a charge of 12 to 15 gand a mold temperature of 300°F (148°C) The molding pressure employed can be 600,

900, 1800, or 2700 lb/in2, depending on the molding process to be used With the chargedmold on, the heated platens of the press close, and the compound is forced out of the twoorifices The mold is kept closed until the compound has stopped flowing and is cured Oncompletion of the molding cycle, the cured compound remaining in the mold is extractedand weighed to determine the percentage of the flow For example, if 30 percent of the

Figure 2.23 Disk flow test and mold (I).