Platinum-Cure Technology Eases Processing 01 Molded ...Platinum-Cure Technology Eases Processing 01 Molded Silicone Rubber . By Russ Tanton, George Sullivan and Marty Matevia . Suppliers

Platinum-Cure TechnologyEases Processing 01 Molded Silicone Rubber By Russ Tanton, George Sullivan and Marty Matevia

Suppliers working in tandem with users can yield the most efficient use of materials and technology in producing a custom mixed compound.

Maximizing process efficiency is for handling a n d finishing tech­ used, one based on organic peroxide, a critical element of success in niques, etc. But fabricators are work­ the other on platinum. molding silicone rubber. ing with their suppliers in increasing

Although fabricators have tradition­ numbers to develop new compounds, Peroxide-Cure Technology ally equated raw material prices with customized not only to meet physical Crosslinking of silicone polymers manufacturing costs, in many cases property specifications, but also to by peroxide chemistry has been a material costs account for 25% or less make the most of their individual standard technique for many years.

of the finished molding processes. The technique of Chemically, the pro cess is quite sim­About the product's selling marketing a new and innovative sili­ ple. There are es sentially just twoAuthors price. Therefore , cone material as a global design solu­ components: the peroxide and the Russ Tanton is far greater poten­ tion is gradually giving way to the polymer . The polymer may be development tial for impact on custom compound, as codevelopment methyl- or vinyl-based. Organic per­programs man­ overall cost re­ concepts increasingly drive material oxides such as 2,5 dimethyl-2,5 ditt ­ager for Dow sides in the most advances. butylperoxy) hexane or 2,4 di­Corning STI's efficient use of Fabricators search continuously for chlorobenzoyl peroxide are commonly West region. labor resources , ways to increase production speeds, used with silicone rubber, as is George Sullivan reductions in ov­ especially in compression- and trans­ dicumyl peroxide. is a technical ser­ erhead and mini­ fer-molding operations . Some mold­ Despite the use of the term "cata­vice specialist mized scrap. ers are turning to liquid silicone rub­ lyst," peroxide is , in a strict sense, a and reliability From an eco­ bers (LSRs) to achieve higher output, cure initiator, not a true catalyst; it engineer for nomic stand­ as the injection molding of LSRs is consumed during the reaction. A Dow Corning point, material often is 50-100% faster than process­ peroxide-initia ted reaction is classi ­STI's Midwest re­ suppliers have ing high-consistency material. The fied as a second order reaction; it has gion, with a pri­ had limited op­ capital equipment investment can be three steps . When the ela stomer is mary focus in portuni ty to af­ prohibitive, however, and molding placed in the heated mold, the perox­platinum-cure fect processing , techniques are substantially differ­ ide thermally decomposes to form materials and except as it di­ ent. fr ee radicals. These free radicals LSRs. rectly reI a tes to Cure technology is one area that is attack e ither a methyl or a vinyl Marty Metevia is the molding oper­ under constant scrutiny by silicone group, depending on the polymer and a technical ser­ ation: for exam­ suppliers as faster cure reduces cycle peroxide used, to form active vice specialist for ple, reduced ma­ times and helps fabricators to raise crosslink sites . The active sites on STI's West re­ terial variation , their production levels. Two primary the silicone polymer then combine, gion. recommenda tions cure systems are currently being forming a carbon-carbon bond.

Reprinted from ELASTOMERICS, July. 1992 © 1992 by Communication Channels Inc., Atlanta, U.S.A.

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Peroxide cure rate

Time

Figure 1. Peroxide cure rate (left) compared to platinum cure rate.

The cure rate depends on the indi­vidual peroxide and the temperature of the mold. The crosslink density is a function of both the vinyl level of the compound and the amount of per­oxide in the formulation. One draw­back to peroxide cu re is the inability to adjust only the speed of the reac­tion (cure rate) without substantial tradeoffs. For example, molders often attempt to accelerate cure or retard scorching by varying the peroxide level. However, changes to the perox­ide level also affect the crosslink den­sity and physical properties of the cured ela stomer.

It is possible to improve induction time to a ll ow the material greater opportunity to completely fill the mold ca vity in a peroxide-initiated reaction by lowering the mold/cure temperature. Unfortunately, temper­ature reduction also lengthens mold­ing time and reduces production out­put. Attempts to accelerate cure by raising the molding temperature increase the risk of scorching.

Platinum-Cure Technology Platinum-cure technology has been

available for some time, particularly for application s involving medical materials and food/drug production. Also called "addition cure," the for­mulation is more complex than per-

QI :J

E' o I-

Figure 2. Platinum cure rate ad ­justment .

Platinum cure rate

oxide compounds and requires that the base polymer contain vinyl groups.

Unlike the peroxide-cure mecha­nism, platinum is a true catalyst and is not consumed during the reaction. In fact, an inhibi tor is necessary in the platinum formulation , as the

part quality . Many fabricators take advantage of the cure rate by using smaller molds with fewer cavities , which allow them to hold much tighter tolerances on finished parts. The slower-curing peroxide system may require the use of much larger multi-cavity molds to attain reason­able production output. But plat­inum-cured silicone makes it possible to reach the necessary productivity levels with fewer cavities, which facilitates better temperature control and flow characteristics . The ulti­mate result is higher part quality and fewer rejects.

In addition , the induction time of the platinum-cure system is con ­trolled by formulation , permitting the material compounder to optimize the formulation to the fabricator's individual molding operation. Th e inhibitor in the compound serves only to delay the beginning of th e cu r e cycle; it does not alter cure speed or crosslink density.

By manipulating the formulation , the induction time can be changed to

pia tinum-catalyzed reaction occurs suit individual molding require­even at room temperature. This first ments. The speed with which the order reaction occurs much more quickly than one initiated by peroxide , as it does not depend on a second order, rate-deter­mining step.

The rheometer curves shown in Figure 1 repre­sent id en tical polymers cured by the peroxide and platinum mechani sms and illustrate the differ­ence between the two reaction rates. The plat­inum curve rises steeply and reaches complete cure quickly, where it flattens out at the materi­al's maximum torque, near the ideal cure rate. (Ideal cure rate is defined

The platinum

system's faster

processing

brings savings

in labor and

utility expenses

per part.

actual r eaction itself occurs remains constant. The slope of the rheome­ter curve does not change, as seen in Figure 2.

The platinum system also is much more forgiv­ing under varying tem­peratures. Depending on individual designs and press conditions, temper­a t u r es in the mold may vary from the center to the perimeter by as much as 20°F . With a peroxide system, this can lead to a variation in the state of cure. Temperature differ­ences within the mold can cause a wide varia­

as the point at which tc(90 )-ts2=0. ) The peroxide cure, by comparison, rises more gradually, indicating its slower rate of cure.

Processing Advantages Faster processing is an important

advantage to the platinum system, which brings a corresponding savings in labor and utility expenses per part. These can combine to produce a significant reduction in the fabrica­tor's cost per fini shed unit. Molders report an average output improve­ment over peroxide-cured materials of around 50%, and many have real­ized gains of 100% or more, depend­ing on the size and complexity of the parts being produced.

The faster cure of the platinum materials can have a direct impact on

tion of physical properties in a perox­ide compound, as these are depen­dent on the cure state.

In a platinum system, the effects of temperature variation are les s signif­icant. Cure rate changes only slight­ly, and there is little difference in the ultimate state of cure . Overall quali­ty improvements are the result, with lower scrap rate s a n d faster cure combining to reduce the cost per fin­ished part.

The mechanism of the peroxide system often produces incomplete curing of the flash due to oxygen inhibition . This flash frequently causes problems by sticking to the molded part. In contrast, platinum cures are not inhibited by oxygen. As a result, the flash cures completely and does not stick to the part or the

mold . This feature alone ca n r educe m old-open time for be t wee n -h ea t clean outs, saving time, re ducing tem­per a ture variation and conse r ving ene rgy .

I nhere nt in th e pl a t inum -cured com pound is a tenden cy for mold ed par ts to be less tacky than peroxid e materials . This phen omen on fu r the r im pr ov es handli n g of the molded part, which is esp eciall y beneficial during demolding, deflashing, inspec­tion and packaging.

Physical Properties I n a d d it ion t o t he p r ocessing

advantages cited, fabricators employ­ing the pla ti num-cure system rep ort phys ical prop er ty improveme nts in the fin ished parts over peroxide-initi­ated materials . In cr eased values for elongat ion, tensile, tear and hot tear are com m on , and in so me cases a 40% gain can be realized (Ta ble I).

All s ili con es r equir e pos t cu r e to some degree in orde r to obtain maxi­mum ph ysical pr operties , es pecia lly compression se t resistance. Compres ­s ion set values of 30-4 0% a re com­mon in platinum-cu red com poun ds

of suc h concer ns during mold ing or po st cu re, a fa ct that is lik el y t o increase in importance over time .

A decrease in part variation after post cur e operations is an other ben e­fit of pl a tinum-cured compounds . Because pos t cure operations con ti n­u e t he cu re proces s in a pl a tinum mat erial , the completed crosslinki ng p r odu ce s m inor ph ysi cal cha n ges that re duce the variation in the fin­ished parts . In a peroxide compound, there is no addi ti on a l crosslinking during post cure, and any variation from the mold als o will be appa ren t after post cure .

Com pared to the ir peroxid e coun­terpa r ts , pla tinum-cured material s a lso exhi bit greate r r esiliency. Fur­t her, b ecau s e of the che m is t ry in vol ved , p la tin u m ma t eri al s a re inherently flam e retardant .

New Material Developments In the past , on e of the drawbacks

to platinum-cured s ili cone has been th e necessity of supplying th e mate­ria l in tw o pa rts . Adv ancements in co m pou n di ng t echn ol ogy a t Dow Corning ST I have produced custom-

t a ilor ed , one -pa r t materials that are sup­

Table I-Physical profiles plied to the fabr icator

Peroxide result

Mold cure. hr/400°F Hardness. Shore A Tensile. psi Elongation. % Tear B. ppi Specific gravity Modulus at 100%. psi

Platinum result

Mold cure. hr/400°F Hardness. Shore A Tensile. psi Elongation. % Tear B. ppi Specific gravity Modulus at 100%. psi

Immediate 10 min/350°F

39 714 520 54

1.17 141

Immediate 10 min/350°F

37 1136 890 104 1.18 106

di r ectly from the m old ; bri ef post­cure opera tions can decrease these to 20-25%.

P ost cu r ing of a pl at inum com­pound se rves t o com p lete th e crosslinking process, as the sys te m is not oxygen inhibit ed . P eroxid e com­pounds, on the other han d, re quir e post cure primarily t o dr ive off the residual products of per oxid e decom­position, which can potentially revert t he pol ymer to a r edu ced s ta te of cure . As env ironme nta l r egula ti on s conti nu e to tighten on manufacturing processes of all kinds , th ese by-pro d­u cts m a y come u nd e r increase d scruti ny. Platinum systems are fr ee

bulk, and overall pro­ce s sin g s pee ds rival thos e of th e LSRs. Fa bricators can attain pr odu c­t ivity levels previous ly possibl e only throug h injection mold in g, witho u t th e equi pme nt in vestment r equired to run the low-consistency liquid sili ­cone s.

As with any n ew materi al , opti ­miz ati on of the molding proces s is nec essary to obt ain maximum bene­fit . Th e ra pid cu re may require some process a dj us t me n t s , as op erators learn t o a cco mm oda t e th e shor ter cure cycles in to thei r routine. Cooper­a ti on between fab ri cator a nd com­pound s u pplie r is hi gh throu gh ou t the development pr ocess . Enginee rs

from both parties are pr esent during initial trial s an d pil ot runs to fine­tune the pr ocess and determine what adjustments may be necessary in th e material to optimize the production.

To obtain maximum ben efit from custom com pounding, it is essentia l that the m ater ial s u pp lie r des ign com po u n ds b as ed on t he physi cal properti es required of the fini sh ed part and th e s p ecific method s employed by the fabricator during molding. Failure to do so is likely to cause inferio r performa nce of the end product or r educed efficie ncy of the fabrication pr ocess .

Unlike peroxid e m a t er ial s , e ve n those th at are custo m-m ixed, th e cu re cycle of pla t inum-based silicones can be manipula t ed t o suit an in di ­vidual fabricator 's opera tio n , raising output to maximum levels and main­taining much lower scrap rates than are common with peroxide. Once th e optimum m ater ial formulation is r eached , STI en ters t he com pou n d formulation a nd specifications in a so phistica te d info r mation manage­ment system th at tracks raw materi ­a l data, t oler an ces , manufac turing detail s and te st r es u lts . Statistical process cont ro l as sures con s is t en t quality of th e cus tom-mixe d material according to th e needs of the individ­ual fabricator for wh om the product has been dev eloped . This data can be t ra ns fer re d t o cus to mers for their own use in qu ali ty control programs.

Th ere are few disadvantages to th e pl atinum sys te m . Pl a t inum-bas ed comp ounds te nd to be more sus cepti ­ble to cure inhibition th an peroxide­based materials. Exp osure to sulfur, tin or amine gro up s can inhibit com­plete cure. But these tendencies are well known of pl atinum-cure sys­te ms, and the cooperation between supplie r and fab ricato r prevents any serious probl ems. As molder s become awa re of th e nu an ces of the new com­pound , they can in tegrate th em into a new, optimized process flow.

It is rar e for a n ew elas t om er development to have a major positive impact on fabrication cos t s. More common is the r ising pri ce of source materials and lab or , cou ple d with stric te r environm ental legisla ti on , to make it e ve r mor e di fficul t for the fabricator to profi t . The in cr easing popularity of pla tinum as an alterna­tive to per oxide-cu re techn ology has been augmen ted by th e development of the new, one-part materials .

The resulting series of custom com­pounds meet or exceed physical prop­er ty requirements of the applica ti on a n d maximi ze p r od u cti vity from exis ti ng process eq u ipment. Th ey represent th e pea k of the partnering conce pt in th e com pounding and fab ­rication of silicone ru bber.

Post cure 1

41 855 550 61

1.17 141

Post cure 1

39 1050 820 105 1.18 113

fully compounded and r eady to run. Unli ke th e two-comp on ent products and previou s one -par t ma t e r i al s , t hese compounds h av e a she lf life that is mea­sured in months rather th an days or even hours .

Eliminating th e need to mix a two-part mate­r ia l has greatly simp li­fie d molding for t he fa br ica to r wishing to take advan tage of plat­inum-cure technology . The new generati on of platinum- cu r e co m ­pounds can be supplied in s h eet , preforms or