Dry silane is a modified one-step technology for crosslinking polyethylene; it utilizes the porous PE carrier to absorb the vinyl silane cocktail to get the dry, pellet form vinyl silane crosslinking agent masterbatch. One major concern of the conventional one-step liquid process is the necessity of having 1) a precise injection system for the liquid silane blend. and 2) a long extruder (L/D = 30). This equipment is expensive and has high cost of maintenance. Furthermore, the handling of liquid silane mixtures needs care to assure safety.

Dry silane technology was developed to allow the use of conventional extrusion equipment with a solid silane masterbatch. The heart of this technology is the use of porous polymers as carriers for the silane blends. Producers of wire and cable, pipes, film, sheet and injection-molded parts can thus blend dry silane masterbatch with polyolefin pellets. Dry silane masterbatch must be metered through a suitable gravimetric feeding system in order to ensure an accurate dosing of the silane masterbatch. Ideally the feeder should be equipped with separate feeding sections for color masterbatch and base resin. The blend can then be extruded with a conventional extruder, molded or formed, and allowed to moisture-cure.

Benefits to the user of dry silane masterbatch are:

Existed extruder (L/D =24) can be used,

Masterbatch allows use of local base resins, 

Easy to use,

Faster start-up (shorter down-time),

More consistent dispersion,

Flexible, easily tailored for end application,

Larger diameter XLPE extrusions are feasible,

Lower silane consumption can be achieved,

Elimination of screw-slip (no liquid in barrel),

Less scrap.

Dry silane is a modified one-step PEX-b processing to produce XLPE cable and PEX-b pipe, pellet form dry silane masterbatch can ensure the accurate dosing of the silane crosslinking agent and simplified the equipment requirements of the traditional Monosil one-step processing.

During the extrusion phase the liquid silane cocktail which contains vinyl silane, peroxide, tin catalyst, antioxidant and metal deactivator will be squeezed out and released; their basic chemical mechanism is same to the regular PEX-b processing:

At first Grafting stage, vinylalkoxysilane is grafted via its vinyl groups on polyethylene through a peroxide initiated free radical reaction. After the grafting, polyethylene can still be processed as a thermoplastic resin like regular PE resin as it has not cross-linked. Only this grafted polyethylene treated with moisture like hot water bath (65~100℃), steam bath or even at ambient the grafted vinylalkoxysilane will hydrolyze and condensed, thus the polyethylene will final be cross-linked.

Polyethylene is a volume leader in the global plastic industry with numerous applications. However, its low upper service temperature makes ordinary polyethylene unsuitable for the applications requiring continuously continuous use under stresses at temperatures up to 100℃ (for example in hot water pipes) or occasional increases in temperature above a certain limit during temporary electrical overloads on cables. By cross-linking of polyethylene some important properties would be drastically improved (Table 1). Some applications include heat shrinkable products (in cable installation), heat-resistant foodstuff packaging (up to 200℃), foams for thermal insulation, and chemical-resistant seals. Moreover, cross-linked polyethylene is more resistant to water treeing and tracking which are undesirable in cable insulation & jacketing industry. Besides the capability of cross-linked polyethylene in withstanding higher electric loading, its ability to tolerate thermal shocks, and the small thickness required for insulation, makes it a suitable candidate for cable insulation & jacketing industry. Polyethylene has the melting point of 100~130℃, however, after cross-linking no flow would be noticed even at 150℃ where elastic behavior prevails.

Main applications of silane crosslinking polyolefin are as below:

1.      Wire and Cables (insulation & jacketing layer for LV and MV)

2.      Pipes (hot water, underfloor, gas, chemicals, sewage)

3.      Heat Shrinkable products (tubes, sleeves)

4.      Films (creep resistance and shrinkable applications)

5.      Foams (heat insulation)

6.      ……

Compared to other PEX-b processing our dry silane has the following advantages:

1.   Standard & existing extruders can be used to produce XLPE (i.e. existing PVC extruders might be sufficient and 24 L/D extruders are enough)

2.    Easier startup and Flexibility of use (no liquid, better handling & safety)

3.    Can run cross-linked and thermoplastic materials on same line.

4.    Larger diameter XLPE extrusions are feasible. (through more consistent dispersion),

5.    Dry Silane masterbatch allows use of local base resins (i.e. producer can choose resins from his preferred or current suppliers).

6.    Dry Silane masterbatch can be tailored for end application (i.e. varying cure speed)

A typical Dry-Silane processing unit for the low voltage power cable production consists of the following units:

Raw materials

Base polymer:

The Dry-Silane technology can be used for a wide range of LDPE and LLDPE grades ensuring optimized cost effectiveness.

Dry-silane masterbatch:

Dry-silane masterbatches are delivered in the form of solid free flowing pellets. The dry-silane masterbatches are available with different silane loadings (concentrations). Depending on the different carrier resins and formula, the silane loading can be in the range of 40% up to 70%.

Antioxidant masterbatch:

In case unstabilized base materials are used an antioxidant masterbatch must be used.

Color masterbatch:

It is used for color identification purposes. Usually color masterbatches are fed manually to the dosing system.

UV - stabilizator:

Aerial cables require the application of a UV stabilizator which is handled similar as a color masterbatch.

Raw material preparation

Dry-silane masterbatches are sensitive towards humidity; dry components have to be used.

Predrying of the base polymer to water contents below 300 ppm is required. The polymer transport from the dryer to the dosing station has to be carried out utilizing desiccant air coming from the drying system.

Never dry the dry-silane masterbatch!

In extrusion lines special care is taken that dry-silane masterbatches are in contact with silane resistant materials only.

Handling the dry-silane masterbatch

The shelf life time of the dry-silane masterbatch is apx.6 month. Once when the original packing is opened the dry silane masterbatch should be used up immediately.

Storing of the dry silane has to take place in cool and dry conditions.

Transport respectively storage temperatures above 55°C have to be avoided.

Dosing system

A gravimetric dosing system is absolutely necessary for dry-silane masterbatches with a high silane loading.

For masterbatches with a low silane loading, a volumetric system could be used, but we strongly recommend also for these masterbatches a gravimetric system.

Extruder

For applications, in which Dry-Silane as well as PVC compounds have to be processed, electrically heated/air cooled 24 D extruders are recommended.

Typical output figures of a 120 mm 24D machine are 270 kg/h XLPE, respectively 800 kg/h PVC (with individual screws)

For applications, in which Dry-Silane only has to be processed, 30 D machines are a good choice. The XLPE output of a 120 mm 30 D extruder is apx. 300 kg/h.

In general we use for Dry-Silane applications shallow grooved feeding zones in combinations with barrier type screws.

Separate screws for PVC and dry-silane are provided. For PVC, standard nitrited screws are used; dry-silane XLPE requires screws with a special surface treatment.

Screw heating/cooling

A high performance screw cooling system is essential. Typically cooling capacities of apx. 50 kW is used for a 120 mm main extruder.

We use water based units because of the higher operational dynamic compared to oil based units.

Crosshead

For insulation applications on smaller cross sections usually fixed centered crossheads are used. Compression tooling is used for round conductors.

It is important that the parallel zone on the extrusion tools is kept short.

Best results have been achieved with max. 4mm parallel length.

Manually centered crossheads are commonly used for insulation applications on larger cross sections.

Sector shaped conductors are processed by means of tubing tools.

Die heating system

For improving the surface quality and to reduce the die drewal effect, a powerful die heating system is provided.

The die heating temperature has to be in the range of 300 ~ 320°C, therefore commonly burners (open flames) are used for this application.

Cooling trough

The use of a hot water section after the crosshead is essential. The cooling water temperature in this section is in the range of 60~80°C.

Crosslinking

For an estimation of the crosslinking time the following formula can be used:

Hot set test

The achievable hot set values depend on the absolute silane dosing rate.

The following chart shows the relation between hot set values, absolute silane concentration and line speed.

The data shown have been collected on the basis of a 1,5mm² insulated conductor.

Summary

The Dry-Silane process offers significant advantages compared to the other crosslinking methods for low voltage cables.

The Dry-Silane Technology can be considered as state of the art for this specific cable types in the meantime.

Cable manufacturers can utilize their well-established Monosil or even normal Sioplas extruder to use dry silane.

For any additional information please do not hesitate to contact Capatue.

Notice: Some data above quotes from the document of Rosendahl Nextrom GmbH.

Our dry silane products are mainly used for XLPE wire & cable and PEX-b pipe, those masterbatches contains up to 5 different ingredients:

Surely other ingredients like UV inhibitor could be added according to the customer’s request.

According to different application, different resins & conductors type we develop several different dry silane formula to meet the market demands. They contain different ingredients and have their own features.

VS: Vinyl silane, PO: Peroxide Initiator, CA: Catalyst, AO: Antioxidant, MD: Metal Deactivator.

Briefly we could use the following decision tree to choose the suitable grade for your applications: