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| Introduction | Peroxide cure | Platinum cure | LSR`s |
| HCR`s | Physical properties | Quality | Processing |
High-performance liquid silicone rubber materials and rapid curing high-consistency rubber formulations have resulted from developments in platinum-catalyzed silicone compounds. They are an aid for the reduction of fabricating costs and increase processing speeds over conventional peroxide-cured silicones.
The surging market for silicone elastomers is beginning to show some trends: for one, a rising number or fabricators are buying finished compounds, rather than doing their own mixing as many have done in the past. In addition, there has been a definite push for advanced process technology to improve output and reduce costs-per-part, especially in light of raw material cost increases that have had an impact on the price of silicone rubber compounds. Platinum cure technology (also known as "addition cure") is playing a significant part in the current drive for more cost-efficient processing.
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For many years, the standard technique for cross-linking silicone polymers was the peroxide cure reaction. There are essentially just two components: peroxide and polymer. Organic peroxides are commonly used with silicone rubber, such as 2,5 dimethyl- 2.5 di(t-butylperoxy) hexane, 2,4 dichloro-benzoyl peroxide, or dicumyl peroxide. The polymer may be methyl- or vinyl-based. Peroxide-initiated cure is classified as a "second order" reaction, and has three steps:
1. When the elastomer is placed in the heated mold, the peroxide thermally decomposes to form free radicals;
2. These free radicals attack either a methyl or vinyl group, depending on the specific polymer and peroxide used, to form active cross-link sites:
3. The active sites on the silicone polymer then combine, forming a carbon-carbon bond.
Despite common use of the term "catalyst", peroxide is really a cure initiator in this instance, not a true catalyst, as it is consumed during the reaction. Cure rate is a function of the individual peroxide used and the temperature of the mold. The cross-link density is dependent on both the vinyl level of the compound and the amount of peroxide in the formulation.
One drawback to peroxide cure is the inability to adjust only the speed of the reaction (cure rate), without significant tradeoffs. For example, molders attempting to accelerate cure or retard scorching by varying the peroxide level will also affect the cross-link density and physical properties of the cured elastomer.
Similarly, reducing mold/cure temperatures to promote better cavity fill in a peroxide-initiated reaction also lengthens molding time and reduces production output. Attempting to accelerate cure by raising the molding temperature increases the risk of scorching.
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Platinum cure technology in molding silicone rubbers has been in use for some time, particularly for applications involving medical components or food processing. Also called "addition cure", the chemical formulation is more complex than peroxide, and it requires that the base polymer contain vinyl groups.
Unlike the peroxide cure mechanism, platinum is a true catalyst here, and is not consumed during the reaction. In fact, an inhibitor is added to the platinum formulation to prevent curing at room temperature. This first order reaction occurs much more quickly than one initiated by peroxide, as it does not depend on a second step which determines the rate.
The rheometer curves represent identical polymers cured by peroxide and platinum, illustrating the difference between the two reaction rates. The platinum curve rises steeply and reaches T90 quickly, flattening out at the material`s maximum torque. In comparison, the peroxide curve rises more gradually, indicating its slower rate of cure
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Some silicone molders are turning to liquid silicone rubber (LSR) compounds to achieve higher output, as the injection molding of LSRs is generally faster than processing high-consistency materials. These two-part, pumpable silicone compounds can be automatically metered, mixed and dispensed and their flow characteristics deliver good fill of intricate mold cavities. Because of the rapid cure time and suitability to automated processing, LSR molding can reduce labor costs and potential for contamination, resulting in lower scrap rates and greater profitability.
New LSR compounds on the market have extended the performance of these elastomers, especially in hot oil service conditions. The new oil-resistant materials exhibit low compression set and excellent heat aging properties directly from the mold, in some cases eliminating the need for expensive post-curing operations.
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New advancements in platinum cure technology are also bringing benefits to molders of high-consistency rubber (HCR). As they are typically supplied as one-part, fully compounded materials. HCRs can be a simpler way to obtain the advantages of platinum-cured processing. With the addition of a stuffer box to feed the higher viscosity material, many injection molding machines can be modified to run HCRs. No transfer pump or meter/mix equipment is necessary.
The faster processing of platinum cured materials brings corresponding savings in labor and utility expenses. These can combine to produce a significant reduction in the fabricator`s unit cost. In all, the lower overheads, direct labor, and reduced scrap with a platinum-cured HCR can slash costs by as much as 50 per cent against a peroxide HCR.
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In addition to processing advantages, fabricators molding platinum-cured HCRs report physical property improvements in the finished parts over peroxide-initiated materials. Increased values for elongation, tensile, tear, and hot tear are common. Compared to peroxide formulations, platinum cured materials also exhibit greater resiliency. Further, because of the chemistry involved, platinum materials are inherently flame retardant.
Table 1 shows a physical profile comparison for two general-purpose, low-strength compounds. The formulations are basically the same, except for the cure system. They were press-cured and post-cured identically, in accordance with ASTM test procedures.
| Table 1 Physical profiles | ||
| Peroxide | ||
| Mold cure | 10 min / 350° F | 1hr/400°F |
| Hardness Shore A | 39 | 41 |
| Tensile PSI | 714 | 855 |
| Elongation, % | 520 | 550 |
| Tear B, PPI | 54 | 61 |
| Specific Gravity | 1.17 | 1.17 |
| Modulus @ 100%, PSI | 141 | 141 |
| Platinum | ||
| Mold cure | 10 min / 350° F | 1hr/400°F |
| Hardness Shore A | 37 | 39 |
| Tensile PSI | 1.136 | 1.050 |
| Elongation, % | 890 | 820 |
| Tear B, PPI | 104 | 105 |
| Specific Gravity | 1.18 | 1.18 |
| Modulus @ 100%, PSI | 106 | 113 |
Under normal molding conditions, the platinum compound would require a much shorter cycle time.
All silicones require some degree of post cure to reach maximum physical properties, especially compression set resistance.
Compression set values of 30-40 per cent directly from the mold are common in platinum compounds; a brief post-cure cycle can usually improve this to 20-25 per cent.
Unlike peroxide cure systems, post curing of a platinum compound serves to complete the cross-link process. Peroxide compounds, on the other hand, require post cure primarily to drive off the residual volatiles from peroxide decomposition, which can potentially revert the polymer to a reduced cure state.
As environmental legislation further restricts manufacturing emissions in all forms, these by-products may be the subject of future regulation. Platinum systems are free of such concerns during molding or post cure, which may become a more important feature over time.
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The platinum system may also have an impact on part quality. By nature, the addition cure process is more forgiving under varying temperatures than a peroxide catalyst. Depending on individual designs and press conditions, temperatures in the mold may vary from the center to the perimeter by as much as 20°F. With a peroxide system, this phenomenon can lead to cure variation, which in turn can cause changes in physical properties (since these are dependent on the cure state).
In a platinum system, the effects of temperature variation are less significant. Cure rate changes only slightly, and there is little difference in the ultimate state of cure.
Overall quality improvements are gained, with lower scrap rates and faster cure cycles giving reduced cost per finished part.
A decrease in part variation after post-cure operations is another benefit of platinum-cured compounds. Because post cure operations continue the cure process in a platinum material, the completed cross-linking produces minor physical changes that may reduce the variation in finished parts. In a peroxide compound, there is no additional cross-linking during post cure, and any variation that exists coming out of the mold will be evident after post cure.
In addition, the induction time of the platinum cure system is controlled by formulation, permitting the material supplier to optimize the compound for the fabricator`s individual molding process. The inhibitor in the compound serves only to delay the beginning of the cure cycle – it does not alter cure speed or final cross-link density.
By manipulating the formulation, the induction time can be changed to suit individual molding requirements. As illustrated in Figure 3, the reaction speed remains constant, therefore the slope of the rheometer curve does not change.
Another quality issue in a peroxide system is incompletely cured flash, due to oxygen inhibition. This partially cured material frequently causes problems by sticking to the mold or the finished part. In contrast, platinum cure is not inhibited by oxygen and the flash cures completely without sticking. This feature alone can reduce mold-open time for between-heat cleanouts, saving time, reducing temperature variation, and conserving energy.
Inherent in the platinum-cured compound is a tendency for molded parts to be less tacky than peroxide materials. This phenomenon further improves handling of the molded part, which is especially beneficial during demolding, deflashing, inspection and packaging.
The faster cure of platinum systems can also benefit part quality in other ways. 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 larger, multicavity molds to attain reasonable production output. Platinum cure technology, however, 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.
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Eliminating the need to mix a two-part material has greatly simplified HCR molding for the fabricator wishing to take advantage of platinum cure technology. The new generation of addition-cure compounds can be supplied in sheet, preforms, or bulk, and overall processing speeds rival those of many LSRs. Fabricators can attain productivity levels previously possible only through liquid injection molding, without the equipment investment required to run low-consistency liquid materials. As with any new material, it is important to optimize the molding process to obtain maximum benefit. The rapid cure may require some process adjustments, as operators learn to accommodate the shorter cure cycles into their routine.
It should be noted that there are a few disadvantages to the platinum system, however. Platinum-based compounds are more susceptible to cure inhibition than peroxide-based compounds are more susceptible to cure inhibition than peroxide-based formulations. Exposure to sulfur, tin, or amine groups can inhibit complete cure. However, these tendencies are well-understood and good communication between supplier and fabricator should prevent any serious problems arising.
It is not often that a new material technology development can have a major positive impact on fabrication costs. More common is the rising price of source materials and labor, coupled with stricter environmental legislation, to make it ever more difficult for the fabricator to profit. But the increasing popularity of platinum cured silicones as an alternative to peroxide cure technology has been reinforced by the development of new one-part HCRs and high-performance LSRs.
The resulting compounds are both application- and process-specific materials that meet or exceed physical property requirements of most elastomer applications and maximize productivity from existing process equipment. They currently represent the height of manufacturing efficiency in the compounding and fabrication of silicone rubber.
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