A new method of cross linking of PE improves properties and widens scope

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A new method of cross linking of PE improves properties & widens scope

PE is one of the most commonly used commodity polymers today. The radiation cross linking of end products made from polyethylene has been industrially carried out for almost half a century. The physical handling limitations of both electron beam accelerators and isotopic gamma irradiators, regrettably restricts the more general radiation modification of end products.

A novel approach is developed in which PE is modified by radiation before conversion to end products.
Three most important modifications taking place in irradiated polyethylenes are:
Oxidation of the polymer when irradiated in air
Oxidation of the polymer when irradiated in air
Low degree of cross linking
Low degree of cross linking
Modification of long chain branching

Modification of long chain branching

The latter two changes have a major impact on the rheological properties of the polymer without significantly increasing the molecular weight thereof, but results in a marked broadening in the molecular weight distribution of these polymers.

The chemical cross linking of PE takes place in the molten state at elevated temperatures and results in a homogeneous cross-link distribution throughout the polymer, without impacting on the molecular weight distribution. The radiation cross linking of PE, however, takes place at ambient temperatures and primarily in the amorphous phase of the polymer resulting in an inhomogeneous cross-link distribution in the polymer. This inhomogeneous cross link distribution emanating from the particular morphological nature of polyethylene, results in a range of novel polyethylene resins with different characteristics. It was observed that the melt flow index of the polymer can be substantially reduced without any negative impact on the spiral flow characteristics resulting in a widening of the molecular weight distribution of the polymer and a markedly enhanced melt strength.

The following changes are the most important when irradiating PE:

Gas liberation

Gas liberation
During the irradiation of polyethylene a gas mixture is formed that consists of 98 % hydrogen, whilst the remaining gasses are the aliphatic compounds methane, ethane, propane and butane. It was shown that these saturated hydrocarbons result from the radiation degradation of the side chain branches of the polymer, as little main chain scission takes place.

Change in nature of unsaturation

Change in nature of unsaturation
It was observed that vinylidene and vinyl unsaturation that is present in the polyethylene, disappears even at very low irradiation doses. This is accompanied by an increase in the trans -vinylene unsaturation and the liberation of hydrogen gas. It is accepted that this trans -vinylene unsaturation is the result of a process of hydrogen abstraction from the hydrocarbon polymer chain.

Cross linking of polymer

Cross linking of polymer
The polymeric free radicals formed during the irradiation of polyethylene can combine to form the well known cross linking in the polymer. This effect is by far the best known because of its significant impact on the properties of the irradiated polymer. This cross linking can be either inter or intra molecular in nature � referred to as H- linking. The type of crosslinking having little effect on the change in polymer properties because of the entanglement of the polymeric molecules.

Introduction of long chain branching

Introduction of long chain branching
An effect that received far less industrial attention than cross linking when irradiating polyethylene, is the enhancement of the long-chain branching (eight C-atoms or more) in the polymer � the so called Y-branching. This results in a broadening of the molecular weight distribution in the irradiated polymer and a lowering of the hydrodynamic volume of the polymer � both effects having significant implications on the properties and processing of such an irradiated polyethylene. It is thus possible to modify the molecular weight distribution, as well as change the ratio between long-chain branching and short-chain branching, by irradiating the polyethylene.

Change in rheological properties

Change in rheological properties
Cross linking in polyethylene during irradiation at ambient temperatures takes place predominantly in the amorphous phase of the polymer, with little cross linking in the crystalline regions of the polymer � the inhomogeneous cross-link distribution mentioned earlier. The melt-flow index of the irradiated polyethylene rapidly decreases with an increase in the irradiation dose, however, the corresponding spiral flow remains virtually independent of the irradiation dose � indicative of the long-chain branching and the associated broadening in the molecular weight distribution. These effects cause the difference in characteristics between radiation-induced and chemical crosslinking of polyethylene.

Oxidation when irradiated in air

Oxidation when irradiated in air
The last radiation-induced effect in polyethylene that received little industrial attention, is the oxidation of the polymer when irradiated in air. Apart from the crosslinking thereof, the radiation treatment of polyethylene in air results in a radiation-induced oxidation of the polymer rendering it with polar functional groups. This results in the modified polymer having adhesive properties to polar materials such as metals � a unique feature for PE.

Physical characteristics:

Physical characteristics:
In addition to the radiation-induced characteristics mentioned earlier, it should be noted that by cross linking the polymer the following properties can be improved:

Thermal:

Thermal:
Dimensional stability
Tensile deformation
Pressure deformation
Environmental stress cracking
Melt strength
Memory effect

Mechanical:

Mechanical:
Cold flow
Abrasion resistance
Modulus
Tensile and bending strength
Impact resistance
Hydrostatic stress strength

PE covers a very wide range of different resins with different characteristics and irradiating these will result in markedly different effects. The following are some of the most important factors impacting on the radiation-induced properties that will be achieved:

		Nature of polymerization process: The high-pressure and low-pressure polymerization manufacturing processes render different resins with different initial characteristics and these will behave differently when irradiated.
It is necessary to introduce certain selection criteria for different PE grades to be modified by radiation before conversion to end products.
		Nature of polymerization process employed :
	Polyethylenes manufactured using catalysts that inherently produce vinyl-unsaturation in the polymer end groups, are highly suitable as starting materials for the radiation modification of the polymer in order to introduce long-chain branching (Y-branching).
		Density of polymer :
	The density of the starting resin should be selected to be in line with the properties required in the end product.
		Melt flow index :
	The selection of the melt flow index of the starting resin should be in line with the conversion process employed in the manufacture of the end product. Experience has shown that the selection of the starting resin should allow for approximately 50% reduction in the required melt flow index.
		Additives in polymer prior to modification :
	As mentioned earlier, the presence of processing aids present in the starting resin can negative impact on the modification process � this is particularly true in the case of antioxidants. It was found that in most cases the best results are achieved by adding the required additives in the form of a masterbatch to the radiation-modified resin, prior to the conversion thereof

The applications of this novel method of cross linking PE are as follows:

		Film blowing
	The radiation modification of linear low-density copolymer polyethylenes (hexenes and octenes) gives resins with high bubble stability with melt flow indices of about 0.3. The resulting films have excellent physical properties combined with the good environmental stress crack resistances associated with the linear low-density polyethylenes. These films are ideally suited for geomembranes and plant protective covers.
		Protective coatings
	The radiation-induced oxidation of polyethylene renders powder coatings that are very tough and with excellent adhesion characteristics to substrates such as steel, concrete and wood. Adhesion strengths as high as 10 MPa to steel pipes can be achieved. Steel pipes have been coated both inside and outside through the fluidized bed method at the same time, using this single layer process, easily passing the required tests for cathodic lack of bonding and environmental stress cracking resistance. By selecting the correct processing and stabilizing additives, these pipe coatings are in agreement with the specifications for potable water and food contact. These PE adhesive coatings are particularly good candidates for marine and chemical plant structures.
		Binders in composite materials
	The adhesive characteristics of the modified polyethylene renders it with excellent binder characteristics and are used as such in the manufacture of wood-polymer composites (WPC).
		Film for lamination
	The adhesive nature of polyethylene films made from the modified resin is suitable for laminating sandwich cardboard structures rendering interesting new composite materials.
		Blow moulding of containers
	Taking advantage of the high melt strength of the modified high-density polyethylene, very large containers can be blow-molded with relative ease.
		High pressure pipes
	The modified HDPE results in a novel resin with greatly enhanced hydrostatic stress characteristics, and with hoop stress values as high as 25 MPa. Such modified polymers are particularly suitable for high-pressure pipes and in hot water applications. This modified resin has been used without any problems over a period of more than two years as a replacement material for polyamides in automotive vacuum brake pipes.
		Recycling of modified polymer
	Apart from the unique characteristics rendered to the polymer resin through the radiation modification thereof, no potentially hazardous cross linking agents are used and the waste materials are fully recyclable � having both very positive health and environmental implications.

This novel technique offers a unique opportunity to develop polymers from different polyethylene grades for different applications. This radiation modification of PE before the conversion to end products renders a unique method for the manufacture of a range of novel polymers with unique characteristics, normally not attainable by the non-modified PE.
(Reference: SPE Annual Technical Conference, May 2004)