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One component silicone combines best of HCR, LSR.

Heat cured silicone rubber can be broadly divided into two basic categories, including high consistency rubber (HCR) and liquid silicone rubber (LSR).

The typical HCR is catalyzed with organic peroxide, although specialty grades that utilize addition-cure technology are available for use in a limited number of molding and extrusion applications.

Liquid silicone rubber is normally sold as a two-component, addition cure material that must be mixed in a specific ratio before injection and curing in a mold.

A one-component liquid silicone rubber system has been introduced that offers the best features of both HCR and LSR (ref. 1).

High consistency rubber

High consistency rubber accounts for over 50% of all the silicone rubber processed in North America. This is no accident. HCR enjoys some inherent advantages over LSR. Some of these advantages include:

* Ease of mixing in either an internal mixer or on a roll mill;

* it is readily preformed into manageable shapes for easy introduction into process equipment such as an extruder or molding press;

* long pot life of fully catalyzed compound (in the case of peroxide catalyst);

* peroxide catalyzed HCR is resistant to inhibition by contaminants such as sulfur, amines, metals, etc., in situations where the silicone rubber is processed in close proximity to other types of organic and inorganic rubbers;

* parts can be easily fabricated by injection, compression or transfer molding techniques, or by extrusion into an appropriate cure media;

* high cavitation resulting in high part output per press;

* high versatility that allows HCR to be cured under pressure (such as in a hot mold) or without pressure (such as in a hot air tunnel, salt bath, or by UV or radiation):

* ability to blend high and low durometer rubber to predictable intermediate durometers; and

* uses standard rubber molding equipment.

Some of the disadvantages associated with the processing of HCR include:

* It is labor intensive. Mixing of HCR compounds requires multiple stages in order to prepare the rubber in some shape for introduction into the processing machine. In addition, post fabrication handling may be required to remove cured flash;

* possible contamination at various stages of mixing and preforming;

* waste resulting from flash, gates and large sprues:

* relatively slow cure resulting in long cure times:

* part tolerance is not always very precise and reproducible; and

* may need to post-cure finished parts to remove catalyst by-products or to obtain optimum properties.

Liquid silicone rubber

Up until now, heat cured liquid silicone rubbers have been supplied as two-component, A and B, systems that must be mixed just prior to forming and curing into parts. These systems contain a catalyst, typically a platinum compound, mixed into the A side. The B side normally contains a cross-linking agent, typically a hydrogen-functional polysiloxane. The cure system provides a fast curing, high strength and tear resistant silicone rubber that is used in various applications. including food contact, medical, personal care, automotive and many others.

The advantages of liquid silicone rubber molding over HCR are well documented. Because of its unique advantages, LSR is rapidly replacing HCRs in many molding applications, resulting in substantial saving of resources.

Some of the desirable features of LSR molding include:

* Low viscosity for rapid mold filling even at low injection pressures (ref. 2):

* extremely short cycle times:

* superior physical properties:

* easily automated to achieve maximum productivity while reducing labor utilization and cost:

* low possibility of contamination:

* material is supplied ready to use without additional processing (other than blending of A and B components);

* a post cure is not normally required to remove by-products of curatives;

* precise, consistent tolerances can be achieved; and

* flash-free molding results in as much as 99% reduction in waste compared to HCR. It also allows automated inspection and "pack-at-press" capability.

Some of the inherent disadvantages of the currently commercially available two-component LSR systems include:

* To injection-mold a two-component LSR, components A and B must be mixed in a specifically required ratio (either 1:1 or 10:1) and, shortly thereafter, must be introduced into a hot mold and cured. Off-ratio mixing is a major problem in processing two-component LSR.

* An expensive and sophisticated pumping and metering system is required to maintain optimum operation of a two-part liquid injection system.

* If the mixed components must be stored for any length of time, it is advisable to store under refrigeration to prevent premature cure. It may also be necessary to refrigerate machine components or dismantle the machine for a thorough cleaning to prevent mixed material from curing in the screws of a static mixer or the nozzles of an injector.

* Continuous cooling of the injectors may be required to prevent material from curing inside the injector head and plugging the nozzles.

* During extended shut downs, it may be necessary to purge the machine with one side only, again resulting in imbalance of sides A and B usage.

One-component LSR

A highly innovative one-component liquid silicone rubber that combines the advantages of liquid silicone rubber with that of high consistency rubber has been introduced. As Paul Kehl disclosed (ref. 3), Laur Silicone's new one-component liquid silicone rubber provides viscosity, physical properties, thermal stability, compression set, electrical and other properties comparable to standard heat-cured, two-part liquid silicone rubber. However, one-component LSR goes several steps further. Not only is it supplied as a single component, fully compounded and ready-to-use product, it has a pot life of over one year at room temperature. In addition, it requires a much simpler pumping system to operate. By eliminating the need for precisely mixing equal parts of components A and B, the one-component LSR can be introduced into cure cavities using a pumping system as simple as a Semco cartridge. A specially designed LSR molding machine is not required. Any good HCR press can be easily converted to run one-component LSR.

The physical properties of the one-component LS-7010 series are shown in table 1. Rheological properties of one-component LSR and three commercially available two-part LSRs are shown in figure 1. Using an MDR 2000 rheometer, it can be shown that the two-part LSRs have a slightly faster cure profile than the one-component LSR. The two-part liquid also shows significantly shorter scorch time than the one-component product. However, the longer scorch time of the one-component LSR can be advantageous in filling large and complex molds without significantly sacrificing cure cycle time.

[FIGURE 1 OMITTED]

The one-component liquid system is thermally stable for storage and transportation. Figure 2 shows storage stability for up to one year at room temperature without loss of cure rheology.

[FIGURE 2 OMITTED]

We have demonstrated that one-component LSR could be run on an injection molding press designed to process LSR (ref. 4). Because of the long pot life, one-component fully compounded and pigmented material was pumped using a single LSR pump with no mixer being used.

Additional work has shown that HCR compression molding applications can be converted to LSR compression molding applications using one-component LSR. For example, one application used a pressure pot to deliver fully compounded material to a standard HCR compression mold with minor modifications. Processing time was reduced by as much as 70%, and waste was reduced by over 99% (ref. 5). The properties of LSR have been shown to remain relatively unchanged, whether the material was injection molded or compression molded (ref. 6).

Injection molding of one-component LSR can be speeded up by reducing the scorch time of the material. Table 2 and figure 3 show that scorch times are reduced as the preheat temperature is increased from 100[degrees]C to 130[degrees]C.

[FIGURE 3 OMITTED]

Homogenous and non-homogenous blends

The concept of blending different durometers of HCR to obtain predictable durometers is well known (ref. 7). While this can also be done with two-component LSR, it is rarely done due to the complexities involved. The introduction of one-component LSR makes such blends more practical and economical (ref. 8).

One-component liquid silicone rubber is supplied fully compounded and ready to use. As such, the durometer of the incoming material(s) is well documented. It is feasible, therefore, to blend these materials to a predictable durometer in a manner similar to how HCRs are blended. Current pumping systems with precise metering capability should provide the ratio controls needed to easily blend high and low durometers to intermediate ones. Table 3 shows the results that can be achieved by blending low and high durometer one-component LSR.

In addition to homogeneous blends, non-homogeneous blends of different colors of one-component LSR can be used for dramatic effect. Commercial products of HCR that take advantage of these types of blends have been marketed for many years. However, the use of two-component LSR to make multicolor molded parts is not well known.

The equipment needed to achieve this will vary with the manufacturing method used. Simple compression molding techniques, in which weighed amounts of the various colors are minimally mixed and then introduced into the mold cavity and cured, have been successfully demonstrated at Laur Silicone (ref. 8). It is also entirely feasible to use a standard LSR injection-molding machine. This system could utilize a very short or no static mixer, and the injection of two (or more) colors into the mold cavity with very little mixing. To achieve more color differentiation, a feed system that permits pulsing of the two materials might be considered. Because each of the components is fully catalyzed, homogenous mixing is not required to achieve full cure of each color. Figure 4 demonstrates the effects that can be created using non-homogeneous color blends.

[FIGURE 4 OMITTED]

Non-homogeneous mixing of materials with different durometers can also be used to great effect, especially in the roller industry. Rolls with varying colors and/or hardness can be used to impart unusual texture and color patterns into film, laminates, paper, print and other roller applications. The possibilities are endless and are limited only by one's imagination.

Conclusion

One-component liquid silicone rubber allows the fabricator to overcome many of the disadvantages associated with high consistency rubber by combining the best features of HCR and LSR. Processing time can be reduced by as much as 70%, and waste reduction can be achieved by over 99%, depending on the part and the mold configuration. Rash-free molding, a feature almost exclusive to two-component LSR, is easily achievable with one-component LSR. Flash free molding eliminates the need for post-fabrication operations, such as cryogenic deflashing or die stamping, and allows for pack at press capability.

One-component liquid silicone rubber provides the fabricator with extreme versatility to make both short runs as well as fully automated, high volume production.
Table 1--physical properties of LS-7010 series
press cured 10 minutes at 171[degrees]C

Test description LS- LS- LS- LS-
 7010-30 7010-40 7010-50 7010-60

Durometer A 31 40 50 60
Tensile strength, 5.5 6.1 6.7 7.2
 Mpa
Elongation, % 500 450 360 260
Tear, die B, kN/m 20 32 34 35
Compression set,
 % (Method B, 24 22 22 18
 22 hr./175[degrees]C)
Specific gravity 1.08 1.1 1.11 1.12

Table 2--rheometer data on preheated one-part
LSR run at 171[degrees]C

 Preheat Scorch time Scorch time Scorch time
 temperature TS1 (sec.) TC10 (sec.) TC90 (sec.)

 23[degrees]C 33 31 93
 100[degrees]C 30 28 83
 115[degrees]C 27 25 75
 130[degrees]C 22 20 61

Table 3--properties of blends of low and high
durometer one-part LSR

Material Blend ratio

LS-7010-35 100 80 60
LS-7010-60 0 20 40
Physical properties--cured 10 minutes @ 171[degrees]C
Durometer A 35 40 45
Tensile, MPa 5.7 5.7 6.6
Elongation, % 485 430 435
Tear, die B, kN/m 25.9 29.1 36.6
Specific gravity 1.09 1.1 1.1

Material Blend ratio

LS-7010-35 40 20 0
LS-7010-60 60 80 100
Physical properties--cured 10 minutes @ 171[degrees]C
Durometer A 50 55 60
Tensile, MPa 6.9 6.8 7.2
Elongation, % 385 330 260
Tear, die B, kN/m 38.7 37.3 35.0
Specific gravity 1.11 1.12 1.12


References

(1.) Paul Kehl and Daniel T. Laur, "Next generation of liquid

(2.) Larry Frisch, Dow Coming, "Principles of injection mold design for LSR," Rubber World, December 2003.

(3.) Paul Kehl, "Heat cured One Part liquid silicone rubber," May 2004 International Silicone Conference.

(4.) Rubber & Plastics News, November 1, 2004.

(5.) Paul Kehl, "One Part liquid silicone in conventional high consistency silicone applications," April 2005 International Silicone Conference.

(6.) James S. Curtis, Mathew J. Guoan, William D. Inman, Jr., and Janice D. Worden, Dow Coming, "Liquid injection molding vs. compression molding: Effect on LSR mechanical properties," Rubber World, January 2005.

(7.) "Silastic NPC 40 and 80 silicone rubber products," Dow Coming, 1979.

(8.) Daniel T. Laur, "Homogeneous and non-homogeneous blends of liquid silicone rubber," April 2005 International Silicone Conference.
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Author:Silicone, Laur
Publication:Rubber World
Date:Jun 1, 2005
Words:2129
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