A processing guide to silicone rubber extrusions.

(The first two installments of this series appeared in the August and September, 1998 issues of RW.)

Cooling

When long continuous runs are made at relatively high production speeds, cooling is required. The frictional working of the screw and pressure build-up at the breaker plate and in the extrusion head will all contribute to heat build-up in the equipment. CV vulcanizing systems, because of direct exposure of the die to steam pressure, practically demand cooling of the head. If the compound is allowed to reach a temperature of 120 [degrees] to 150 [degrees] F, scorching or partial crosslinking of the polymer is highly probable. It is advisable to maintain a stock temperature between 80 [degrees] and 100 [degrees] F when possible.

High green strength stocks, long barrel extruders, excessive screw compression ratios, fine mesh screens, (i.e., 100 or higher) when used, and long land and small orifice dies all contribute to frictional heat. The above mentioned conditions, while usually necessary for quality and economy when properly selected, must be considered and proper allowance for cooling must be given.

When considering the modification of present equipment or the purchase of new equipment, plan for cooling in the barrel, screw and head. It may not be necessary to employ all three on all occasions; however, the flexibility is needed.

Vulcanization 

The two methods most commonly used by wire and cable fabricators are the CV, or continuous steam vulcanizing method; and the HAV, or hot air vulcanizing method, primarily used by profile extrusion fabricators. The preference of one over the other depends largely on the size wire and cable manufactured, the space available, the expense outlay permitted and the expected production in terms of footage. The HAV unit demands less floor space in that the HAV tunnel may range from 8 to 30 feet in length, while the CV tube might have a length of 250 feet. The CV system is more expensive to install and requires more floor space. The CV system also generates more start-up scrap which becomes an important consideration for short runs involving larger sized cables.

On the other hand, when long continuous runs of repeated items are involved and when speed enters into the economy, the importance of the CV method is no longer debated. Its economy of operation then quickly covers its initial overhead expense. The CV system is an efficient curing method because it employs the direct heat transfer method. Steam pressures from 150 to 250 psi are desirable. HAV temperatures may range from 300 to 1,000 [degrees] F, depending on the length of the tunnel, size of conductor, thickness of insulation, dimensions of the profile, catalyst concentration, air circulation in the tunnel and effectiveness of preheat on the conductor or carrier. Forced air circulation within the HAV tunnel has been known to double production speed, and it definitely effects a more thorough cure. The air turbulence within the tunnel permits better heat transfer and also acts to remove vulcanization by-products from the immediate curing atmosphere. It is advisable to vent the HAV system whenever possible. To insure optimum efficiency of the HAV unit, it is imperative that silica and curing by-product deposits be removed periodically by brushing out the inside of the tunnel. These deposits, if allowed to accumulate, will affect production speeds and may cause inadequate curing because of the resulting inadequate heat transfer. Brushing out of the tunnel is a rather expedient and efficient preventive maintenance control.

Conductor preheat

Preheating of the conductor to be insulated may be accomplished in several ways, depending on the type of conductor. The decision of whether to use a preheat depends on the type of vulcanization and the demands on production economy and speed of operation.

The three most common methods of preheat are:

* Electrical resistance method;

* gas flame;

* oven preheat.

Probably the most efficient and versatile of the three is resistance heating because the amperage can be adjusted to compensate for variations in production speed. Thus, maintenance of optimum production control is facilitated. Electrical resistance preheaters on the market today are equipped with built-in controls that stabilize temperature automatically with little change in production speed.

The open gas flame method, while effective in some cases, presents the open flame hazard and can more readily form oxidative and flame by-product deposits on the conductor. The gas flame method is also susceptible to moisture pickup, which cannot be tolerated in HAV vulcanizing systems.

There are instances, however, when both the resistance and gas flame preheat methods will not fill the bill. Cases such as large cable diameters and fibrous filler compositions intertwined with spirally wrapped conductor strands often demand the use of an oven preheat. In such cases, the reels of conductor are placed into an oven for periods of approximately 12 hours or more to assure thorough preheat throughout the reel and to eliminate possible entrapped moisture.

Preheating of the conductor is not as essential when the CV vulcanizing method is used; however, it is a prerequisite when using the HAV technique. Entrapped moisture along the insulated conductor, when exposed to vulcanizing temperatures, vaporizes into steam. This condition will cause blisters and possibly effect a poor cure along the conductor in severe cases. Entrapped moisture is not as critical with steam vulcanization since the external pressure of the steam prevents the insulation from blistering. Entrapped moisture, however, is not desirable in any insulation.

The use of vacuum

The utilization of a vacuum pump provides two important advantages. The vacuum should be applied inside the mandrel just before the conductor enters the cavity of the head. A fitting can be adapted to most mandrels with little effort simply by tapping directly into the mandrel or by attaching an extension on the entrance end.

The vacuum, when applied to the conductor? or extrudate, will eliminate blisters in the insulation caused by entrapped air. Small and closely wound conductors usually do not pose a problem; however, large group stranded cables and braided constructions requiring an outer silicone rubber jacket, do in most cases, process more efficiently when a vacuum is used in conjunction with an HAV system. The entrapped air phenomenon does not pose a problem with the CV method.

A second instance in which the vacuum may be used is when it is desirable to obtain an extremely tight insulation around the conductor and to fill the interstices of the conductor more effectively than the pressure type extrusion die alone will permit.

Oven curing

In the early days of silicone rubber, a post-cure was practically essential. Vulcanization alone was not adequate. Today, however, this is no longer the case. Some modern silicone rubber compounds are designed to provide tough, high strength extrusions without the necessity of a post-cure. There are cases, however, where oven curing is required.

High temperature stabilization of properties, oil and compression resistance, outgassing, reversion resistance in high temperature enclosed systems, and various other specification requirements do necessitate oven curing after vulcanization.

Along with improvements in the various other processing aspects of silicone rubber, oven curing has also seen a decided improvement in technique. Post curing in coil form, while still a common method today for large cables and profile extrusions, has been largely replaced by the technique of curing on reels or in stainless steel baskets.

The catalyst most commonly used by the wire and cable industry, bis (2,4 dichlorobenzoyl) peroxide is the system most subject to reversion of the silicone rubber during oven curing. The other catalytic systems used in conjunction with silicone rubber such as dicumyl peroxide (Di Cup 40C); 2,5 dimethyl, 2,4 di (t-butyl peroxy) hexane (Varox), and Addition Cure, are not prone to support reversion. These catalysts, however, require a higher vulcanizing temperature (300 [degrees] F) as compared to 240 [degrees] F for bis (2,4 dichlorobenzoyl) peroxide and benzoyl peroxide. Of the five mentioned catalysts, only bis (2,4 dichlorobenzoyl) peroxide, and Addition Cure can be used with an HAV system. The (2,4 dichlorobenzoyl) peroxide is more prone to support reversion than the other four catalysts.

Silicone rubber is a polymeric material with high average molecular weight that is obtained by linking together fundamental monomer units via the process of polymerization. Under specific conditions, the polymerization reaction for the formation of high polymers becomes reversible. The long chains will cleave into various lower molecular weight fractions.

Acid fragments and [H.sub.2]O by-products given off during the post cure, if not given adequate ventilation, will cause reversion. For this reason, coil form oven curing has been the predominant method. New technology, however, has now given the fabricator either ready made compounds or reinforced gums that can be processed with any catalyst system including bis (2,4 dichlorobenzoyl) peroxide and which can be cured on process reels in six-inch sections of random wound products.

This reversion resistant characteristic has enabled the fabricators to eliminate double handling since the reels or coils can be taken directly from the extruder and placed into an oven for post cure. This process has also enabled the fabricator to obtain longer continuous lengths of finished product.

Braidability

While the application of fiber has its advantages, two problems are sometimes encountered. Low modulus and low durometer silicone rubber can pose a problem during application of the outer braid. Stocks that have a durometer range from 45 to 65 durometer Shore A are the most troublesome. The softness of the insulation allows the tension of the braid fibers to neck down on the insulation, thus causing a backup or gathering effect. The result is lumpy insulation and a distorted braid pattern. Above 65 durometer, silicone will braid equally as well as other insulating mediums.

The second problem which may develop is caused by thermal expansion of the silicone insulation during subsequent lacquer or resin applications. The heat required to cure the resin coatings also causes thermal expansion of the rubber, and if ample room is not allowed between the outer braid and the insulation, the silicone will extrude through the interstices.

Although the low durometer range compounds can be troublesome if not processed properly, there are certain conditions which, when established, enable the fabricator to apply the outer braid effectively. Important considerations when braiding low durometer compounds are:

* Maintain lowest effective yarn tension.

* Use a braid angle between 54 [degrees] and 65 [degrees]. Select the proper size fiber that will allow the proper angle and permit 100% coverage.

* Use a braid pin. This technique entails the use of a steel pin which is inserted under the braid and allowed to protrude beyond the braid gathering point approximately one inch. Experience has shown that wire insulated with a 1/32 inch wall of silicone will require about a 0.015 inch diameter pin. Heavier insulations may require larger pins. By allowing the braid to form around a larger effective diameter, tolerance is [No Continuation On Original Text]

Ted Taylor is Senior Vice President and Managing Partner for Specialty Silicone Products. He has 36 years of experience in silicone which includes several positions at General Electric Co.'s Silicone Products Division.