Polyethylene has been crosslinked for many years by a number of methods, yielding benefits such as higher service temperature, reduced deformation under load, improved chemical resistance, increased abrasion resistance, and improved impact properties. A newly developed silane-crosslinking process is said to significantly reduce the cost of making crosslinked PE pipe, an alternative for potable water and radiant heating pipe applications.
A one-step crosslinked polyethylene tubing/pipe system uses a monolayer tubing line to process a resin/silane formulation, producing 1¼2-inch (13 mm) crosslinked PE pipe. The key to the technology is a liquid silane blend, which is absorbed by a porous resin during the material feeding process and continuously introduced with the main resin to the extruder. This makes it possible to combine raw materials and the grafting reaction into a single-step process, allowing for the continuous formation of extruded pipe. It also permits low and consistent silane dosing, achieving a homogeneous blend of raw materials.
This method of silane crosslinking (SXL) offers several processing advantages including high line speed, low energy costs, formulation flexibility, reduced operating costs, and uniform crosslinking.
The system, introduced at a November 2002 tubing/pipe symposium, contains the following extrusion components: a three-component drying system; a four-component dosing station for both polymer and liquid additives; a 75-mm, 32:1 air-cooled extruder with a DSB-V feedscrew; a 1.5 L/D grooved feed section; a 25-ft (7.6m) vacuum tank with contact sizing; a 25-ft (7.6m) spray tank; a puller/cutter; a cutter/blow-off; and OD/wall measurement. The line produced outputs of 80 ft/min (24 m/min) at a processing rate of 225 lb (102 kg) of material/hr.
Crosslinking of Polyolefins
The initial purpose of crosslinking polyethylene was to extend the maximum service temperature. However, other benefits have been associated with crosslinking polymers like polyethylene:
- Higher service temperature (long-term and short-term peak temperatures).
- Reduced deformation under load, providing improved creep and stress rupture performance.
- Improved chemical resistance (e.g., against solvents).
- Increased abrasion resistance (e.g., for cable jackets and pipes).
- Memory effect (e.g., for shrink tubing, shrink film, and stretch wrap).
- Improved impact properties of molded parts and foam.
- Improved flexural modulus.
- Improved low-temperature impact strength.
- Reduced drip phenomena when burning.
Three main technologies have been developed for the crosslinking of polyethylene. These are peroxide, radiation, and silane crosslinking. The use of silanes results in a more flexible and economical process for crosslinking. This moisture-cure technology has been practiced commercially for some time. Contrary to other methods, SXL polyolefins are linked through a silane-oxygen-silane radical rather than a carbon-carbon bond.
Organofunctional silanes can react with a wide variety of organic and inorganic materials. Their unique ability as coupling and crosslinking agents has been proved in a number of applications, ranging from adhesives and coatings to polymer modification as moisture-cure crosslinking agents. Organofunctional silanes are used in many industries, and over the last 30 years, the use of silanes in polyolefins for crosslinking or filler treatment has become common practice.
Silane technology consists of two steps: grafting of vinylsilane onto the polymer backbone; and consecutive crosslinking in the presence of water, generally catalyzed by tin compounds or other suitable catalysts. SXL offers a much greater processing latitude than peroxide or radiation, because fabrication can be performed above the peroxide decomposition temperature. With the silane process, the extrudate can be cooled quickly and prepared for finishing. Limited exposure to moisture and heat after extrusion is sufficient to ensure consistent crosslinking of most polyethylenes.
SXL networks are also more elastic than other crosslinking methods, which explains why they achieve the same mechanical properties. This technology delivers the following advantages compared to peroxide and radiation crosslinking:
- High line speed and low energy costs.
- Numerous process alternatives.
- Reduced initial investment.
- Formulation flexibility.
- Lower overall operating costs.
- Uniform crosslinking.
- Applicability to a wide range of polymers/polymer blends.
Several manufacturers that offer commercial products based on grafted resins (Sioplas) and liquid direct injection materials (Monosil) supply the pipe industry in Europe. These pipes are used in floor heating as well as for potable water and gas supply. The increasing use of crosslinked PE pipes in applications such as the transportation of chemicals and district heating is also due to their improved crack-propagation resistance. They fulfill all the current industrial norms in this area (i.e., ISO/TR 9080, ISO
13479, DIN 16892, DIN 16893, BS EN 1055, and so forth).
The EU Directive 90/128/EEC (?plastic materials and articles to come into contact with foodstuffs?) stipulates that vinyltrimethoxysilanes are authorized for food-contact applications, opening up additional possibilities for silane crosslinkers in potable water pipes. In the United States and Asia, the use of SXL for pipe is growing rapidly.
Under the two-step (Sioplas) method, a silane-grafted resin and a catalyst masterbatch are combined just prior to being fed into the extruder. When combined, the shelf life of the material is very short?especially if exposed to moisture. The extruded pipe is often cooled in a water bath that has the potential of providing the moisture required for the curing. The rate-limiting step for the actual cure of the polymer is the diffusion of moisture. Often a hot water bath, sauna, or low-pressure steam autoclave is used to speed the cure.
In new product evaluations, the Sioplas method is usually the first process used to evaluate the viability of SXL technology. The pipe producer can test a variety of materials quickly with little or no investment.
Another process, called the Spherisil method, is covered by Constab?s (Düsseldorf, Germany) as well as Crompton?s (Geneva, Switzerland) worldwide process patents. These patents relate to a process of crosslinking polymers whereby a silane is fed into an extruder in solid form. The Spherisil process is based on the use of suitable carrier resins (described as particulates in the form of powder, granules, or pellets), absorbing liquid silanes during the metering/dosing process, using suitable equipment. The Spherisil process contains all the necessary ingredients to allow for a one-step, continuous operation. This enables the user to extrude crosslinked PE pipes using single-screw extruders. Water and gas pipes manufactured using the Spherisil dry silane technology show excellent mechanical properties and chemical resistance, and can operate at service temperatures up to 110C.
The Spherisil process provides pipe producers with numerous benefits over other crosslinking methods:
Research at Davis-Standard (Pawcatuck, CT) has shown that 32:1 L/D extruders are sufficient for achieving good grafting and crosslinking performance. A polyethylene-processing screw with barrier elements, a minimum diameter of 45 mm, a compression ratio of 2:1, and screw cooling are all standard recommendations.
Trials with several pipe producers have shown that the use of an extrusion line designed for the Spherisil process provides processors with the flexibility to run crosslinked and thermoplastic pipes on the same line. The compatibility of the Spherisil process with most commercially available HDPE/MDPE resins also enables the pipe producer to ?add? the crosslinker when needed.
The heart of the Spherisil process is the absorption of liquid silane blends by porous carrier resins. This is one major reason for the improvement in dispersion and associated crosslink density of the extruded crosslinked PE pipe, enabling the pipe processor to consistently crosslink much thicker cross sections.
Constab and Crompton continuously screen locally available HDPE base resins from different world regions for their work in crosslinked PE pipe applications.
Crompton?s involvement in organofunctional silanes, organic peroxides, tin catalysts, and antioxidants provides excellent synergies when designing silane blends for crosslinked PE pipes. Silanes are designed to work well with numerous stabilizer/antioxidant packages. Most Spherisil packages contain special stabilizers, used to prevent premature polymerization of the grafting agent and to provide excellent aging properties of crosslinked polyethylene.