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The purpose of this research project was to review the cross-linking techniques used to produce polyethylene pipes for domestic as well as other commercial applications. No matter how remarkable the properties of a plastic may be, it is the effectiveness of its applications that would make the plastic successful or unsuccessful, in the case of polyethylene it would be success. Polyethylene was first used in telephone cable insulation, however after its huge success it was then used in a wide range of other applications. Research showed that cross-linking polyethylene would give it a huge advantage in terms of its use in areas where it could never be used otherwise. This is because the cross-linking would change the properties of the polymer, for example it would be able to withstand very high temperatures. This would be impossible in a non cross-linked polyethylene because at high temperatures it is very easily deformed and has poor mechanical properties. Three different types of cross-linking techniques are reviewed in this project; they include peroxide cross-linking, high energy/electron radiation cross-linking and silane/silicone cross-linking and the latter forms a crosslinked polymer by grafting a specific silane compound that is subsequently used to crosslink the polymer. The report was written & information gathered by reviewing existing journals as well as review books in the field of research, in order to gain a broader insight into not only polyethylene cross-linking techniques, but also their advantages, limitations & challenges, safety aspects, applications & their future prospects.
Aims & Objectives:
The aims & objectives of this research project were to review & discuss the different cross-linking techniques targeted in the chemical industry for the production of polyethylene pipes for use in domestic and commercial applications. Research was collected using books and peer reviewed journals in this field of research.
4.1: An Introduction to Polyethylene & Crosslinked Polyethylene (PEX):
Polyethylene is a long chain carbon-based polymer that in principle is one of the simplest polyolefins and its repeat unit is based on the formula (C2H4)n. It is regarded as one of the most versatile thermoplastics & this is why it is used in a large variety of applications, including wire & cable insulation. It has many advantages such as low cost, easy availability & easy processability. However, the problem with polyethylene is that it has a low melting point which restricts its uses, it has a tendency to crack when stressed and it is soluble in hydrocarbons. As its melting point is not very high, its applications are somewhat limited, however through cross-linking; the thermal stability of polyethylene can be increased. This enables it to be used in more versatile applications with higher temperature requirements, exceeding the temperature limitations of non cross-linked polyethylene. There are many major groups of crosslinked polyethylene (referred to as PEX) materials produced; they include Low Density Polyethylene (LDPE), High Density Polyethylene (HDPE), Linear Low Density Polyethylene (LLDPE) and Ultra-High Molecular Weight Polyethylene (UHMWPE). After the polymerisation process, polyethylene can be modified by chlorination, chemical cross-linking or radiation cross-linking [1-3].
Polyolefins have been classed as the best materials for use in pipes as they had excellent manufacturing and long-term performance. Crosslinked polyethylene (PEX) was found to be more advantageous and superior compared to uncrosslinked polyethylene because crosslinking enhances the polymer in many aspects such as structural aspects & many more. These provide crosslinked polyethylene with many advantages over uncrosslinked polyethylene such as excellent electrical properties, improved heat performance, good chemical resistance & good abrasion resistance .
Crosslinking is one of the most effective methods for modifying the physical, chemical and mechanical properties of a polymer & is a technique whereby either covalent or ionic bonds are created between different molecules in a polymer matrix. As branching is known to promote crosslinking, it is easier to crosslink branched low density polyethylene (LDPE) compared to high density polyethylene (HDPE). Cross-linking restricts the movement of polyethylene chains; this prevents it from deforming when an energy form such as heat is applied to it. Cross-linking polyethylene reduces chain mobility and the advantages is that it forms a large network of high molecular weight which enables it to improve creep, impact strength, electrical properties as well as improved heat performance, abrasion resistance and environmental stress crack resistance. Cross-linked polymers are insoluble due to their three dimensional network structure & most cross-linked polyethylene (PEX) is formed by high density polyethylene (HDPE). Cross-linked polymers are characterised by the network chain length, branch point, crosslink density & the number average molecular weight of a network chain. However, a polymer that is totally cross-linked is insoluble in every solvent and so its characterisation can become difficult. According to research, a small amount of crosslinked polyethylene can enhance the strain hardening behaviour in the elongational viscosity of a linear polymer; this is one of many ways in which crosslinked polyethylene can be of commercial use [3, 5, 6].
Generally, regardless of the crosslinking technique used, PEX has many useful properties [3, 4] shown below:
Room Temperature Properties - Cross-linked polyethylene can be used at elevated temperatures in both pipe and cable applications as it is able to withstand very high temperatures due to the change in its physical, chemical & mechanical properties.
Chemical Resistance - Cross-linking enables the polyethylene to inhibit the attack by harmful chemicals as it prevents their permeation.
U.V. Light Resistance - Before embrittlement can take place, cross-linked polyethylene has more bonds to break than non cross-linked polyethylene, which means the cross-linking makes the polyethylene more resistant to U.V. light.
Environmental Stress Crack Resistance - Cross-linking improves this property at both room temperatures & elevated temperatures & so the cross-linked polyethylene can withstand applied stress as well as known cracking agents.
Temperature Resistance - Non cross-linked polyethylene melts at temperatures between 100oC-130oC, however cross-linked polyethylene can withstand temperatures around 190oC without losing its size or shape [3, 4].
4.2: Methods for Crosslinking Polyethylene:
There are three main methods used in the chemical industry to crosslink polyethylene as follows:
Peroxide Crosslinking: Uses a free radical generating peroxide and this is, also known as chemical-induced cross-linking, to form a crosslinked polymer referred to as 'PEXA'.
High Energy Radiation Crosslinking: Uses ionizing radiation and is also known as radiation-induced cross-linking, to form what is known as 'PEXB'.
Silane Induced Crosslinking (mainly used in cable insulation): Forms a polymer by grafting a specific silane compound, this process leads to a polymer referred to as 'PEXC' products [3, 6, 7, 8].
Each of these methods are described in more details below.
Peroxide Crosslinking (Chemical-Induced Crosslinking):
5.1: Chemistry and Reaction Mechanism of Peroxide Crosslinking of PE and PEXA:
Cross-linking by the use of peroxides is widely studied because it has chemical simplicity and it depends on two factors, firstly the nature of the most easily abstracted hydrogen atom and secondly, the polymer structure surrounding this radical. If the polyethylene is linear then just one type of hydrogen atom is in principle available and the polymer radicals can form cross-links by combination reaction. However, if the polyethylene is non-linear then scission can also occur and can be the predominant reaction of the process. Although the peroxides start to decompose by first order reaction during the process, the decomposition rate can be controlled by raising the temperature .
Peroxide cross-linking involves the formation of polymer radicals through a process known as hydrogen abstraction by peroxy radicals. These peroxy radicals are formed by the decomposition of peroxide. Cross-linking occurs by the coupling of the polymer radicals: 
RO. + CH2CH2 ROH + CH2CH
Figure 1: Coupling of Polymer Radicals 
During peroxide cross-linking, a small amount of peroxide, normally dicumyl peroxide is mixed into the polymer straight after the polymerisation process. The dicumyl peroxide crosslinking reaction of polyethylene is known to be a first-order kinetics reaction regardless of the peroxide concentration. This allows a somewhat stable material to be formed and as long as the extrusion temperature is kept below the peroxides decomposition temperature, it can be handled and processed as thermoplastic polyethylene. As dicumyl peroxide is used, the processing temperature should be kept below 140oC to prevent the polymer from decomposing. After the polymer has been extruded, the temperature can be raised above the peroxide decomposition temperature in order to allow cross-linking to occur. Heat is normally provided for this technique by steam, silicone oil & hot, dry nitrogen. Acetophenone, methyl styrene and methane are formed as by products, but to prevent these affecting the insulation & to prevent these creating voids; the molten polymer during cross-linking & cooling should be kept & maintained under pressure [10, 11].
In order to prevent premature crosslinking, it is vital that the peroxide containing compositions are mixed at temperatures that are above the polymer melting point, but these temperatures should not be really high otherwise the peroxide will start decomposing very slowly. Premature crosslinking can also be prevented by the use of certain additives, for example hindered phenols. Both the degree of crosslinking and the rate of crosslinking can be enhanced further by using 'cure boosters' for example (alpha) methyl styrene dimer .
To enhance the cross-linking, reactive multifunctional monomers are sometimes added, these are more mobile than the polymer chains and therefore, they crosslink different chains by acting as a bridge. This type of cross-linking has a stronger effect compared to radiation cross-linking; this is due to the huge amount of gel fraction formed in the peroxide cross-linked polyethylene. This type of cross-linking is favoured over the other two methods because it has a controlled decomposition rate, a minimum amount of side products are formed and because of its economical process. 
The maximum cross-linking efficiency of peroxide cross-linking is just one cross-link per molecule of peroxide decomposed, however the actual cross-linking is less than that due to side reactions of the initiator and polymer radical. So, if a polymer radical is formed with a large distance between other polymer radicals the cross-linking is affected and becomes deformed as the environment the reaction takes place in is a viscous. Side reactions which include chain scission, hydrogen atom abstraction and combinations with other initiators become possible .
In a study by Zhou and Zhu  using electron spin resonance (ESR) to examine the reaction of HDPE with several peroxy derivatives including dicumyl peroxide, concluded that both alkyl and allylic macroradicals are formed, which by combining together were responsible for crosslinking .
Another study was conducted on the thermolysis of peroxy derivatives & it was concluded that, depending on the structure and type of peroxy derivative used, crosslinking or function grafting of the polymer mainly took place, whilst the use of hydroperoxides induced the formation of carbonyl and hydroxyl on the polymer molecule .
The results from the study of 'Crosslinking of high density polyethylene in the presence of organic peroxides' showed that the extent of crosslinking above a specific dicumyl peroxide concentration increased rapidly. Secondly, during the dicumyl peroxide reaction, alkoxy radicals were more likely to abstract a hydrogen atom rather than undergo coupling with an alkyl radical. Thirdly, by observing the allylic radical by ESR during the thermolysis of dicumyl peroxide in polyethylene showed that ethylene bonds could be produced on the backbone of polyethylene by the disproportionation of an alkyl macroradical with another radical. No other types of ethylene protons were identified which means that hardly any hydrogens in the allylic position were attacked by the cumyloxy radicals. Finally, the decomposition that occured in the molten polymer, and the difference in the dicumyl radical reactivity generated in the homolysis of the O-O bond arising from the fragmentation of centred oxygens is largely responsible for the efficiency of crosslinking polyethylene by peroxide crosslinking. The reaction scheme of this is shown below: 
PE* -MeH + PEH PE*
PE-Me Me* PE* PE-PE (crosslinking)
Way f way e way g
Î” -PhCMe2OH way c
PhCMe2OOC Me2Ph 2PhCMe2O* PE* + PhCMe2O* PE(-H)
Way b (Disproportionation)
Figure 2: Reaction Scheme 
Many techniques can be used to increase the peroxide cross-linking efficiency; this can be done by mixing small amounts of vinyl groups into the structure of the polymer, which is done by copolymerisation. However as crosslinking increases, the density and degree of crystallinity of polyethylene decreases & due to this the PEX gains a more rubber-like behaviour & the modulus of PEX decreases much faster at higher temperatures. 
Studies on the mechanism of peroxide crosslinking of polyethylene resulted in the conclusion that during peroxide crosslinking, the crosslinks between the polymer chain are formed by a process which is similar to the polymerisation of vinyl bonds attached to a copolymer chain. Also from the same study, it was noted that during the peroxide crosslinking process, cumyl peroxy free radicals react by hydrogen abstraction from polyethylene to form cumene hydroperoxide (more thermally stable than dicumyl peroxide) causing the reactivity of peroxy groups to decrease .
Below, is the reaction mechanism of peroxide crosslinking:
Î”H 2 *
+ * + H
* + *
Figure 3: Peroxide Crosslinking Mechanism 
5.2: Properties of PEXA Products, Their Advantages, Limitations and Safety Aspects:
PEX A products are formed by peroxide crosslinking which gives them many properties; these include thermal resistance, mechanical strength, and dielectric loss .
The advantages of PEX A products are no intermediates are required to form the free radicals; they form directly onto the backbone of the polymer, high processing temperature, high temperature resistance, co-vulcanization of rubbers and polyethylene, and no staining on the final product .
The limitations of PEX A products include; imperfections in crystal structures, undesirable by-product formation, agglomeration of cross-linking adjacent to polymer surface, affected by stress during polymerisation, sensitivity to oxygen under curing conditions, releases odours, operated at high vacuum and difficult to control thickness of products .
Regarding safety the peroxides used are unstable compounds which can decompose spontaneously and sometimes explosively, the decomposition can be caused by mechanical effects. If the peroxide is contaminated with other chemicals or metals it can become hazardous, therefore the peroxide cross-linking process has to be controlled in order to prevent contamination & the transportation, storage and handling conditions have to be controlled .
High Energy Radiation Crosslinking (Radiation-Induced Crosslinking):
6.1: Chemistry and Reaction Mechanism of High Energy Radiation Crosslinking of PE and PEXB:
Radiation crosslinking is similar to peroxide cross-linking with the only difference being that the polymer radicals in this case are formed by the interaction of ionising radiation with the polymer. By the cleavage of both carbon-hydrogen bonds and carbon-carbon bonds, the majority of the free radicals are formed in the amorphous and these recombine with each other & this result in cross-links. However, the free radicals formed in the crystalline region become trapped and this can affect the long term oxidative stability of the cross-linked polyethylene. These free radicals may migrate to the amorphous/crystalline interface in the long term and react with diffused oxygen leading to embrittlement [6, 15, 16].
On the polymer chain the primary alkyl radicals react with oxygen this forms peroxy free radicals. These peroxy free radicals are very reactive, however they stabilise themselves by abstracting hydrogens from chains that are close by, this forms hydroperoxides. However, the molecular weight of the polymer becomes degraded over time due to the decay of the hydroperoxides. Hydrogen abstraction leads to a brand new primary alkyl free radical forming which feeds into the oxidation reaction. This causes further degradation & forms more free radicals. As the free radicals undergo an oxidation reaction it causes oxidative embrittlement via the recrystallisation of the degraded short chains that are newly formed [6, 17, 18, 19, 20].
However, oxidative stability can be improved by thermally treating the cross-linked polyethylene after irradiation. One way in which this can be done is by completely melting the crystalline regions; this reduces the amount of residual free radicals so they are literally undetectable, but when crystallisation occurs in the presence of crosslinks it causes lower crystallinity which causes the mechanical properties of the polymer to become reduced [6, 16, 20].
The oxidative stability process mentioned above begins by expanding the polymer to a shape which is easy to apply; the polymer is then cooled in order to maintain the shape that is held due to its crystalline structure. The polymer is then placed into its working position & heated to allow it to shrink to its original shape (elastic memory). Beta rays are normally used in this process although gamma rays can sometimes be used. To enhance the cross-linking, reactive multifunctional monomers are added, these are more mobile than the polymer chains and therefore, they crosslink different chains of the monomer by acting as a bridge [6, 15, 16, 17].
Studies show that irradiation leads to a decrease in molecular weight and melting temperature and an increase in degree of crystallinity and hydrophilicity. A study by Edin  also concluded that irradiation of both low density polyethylene and high density polyethylene induced oxidative degradation and crosslinking reactions resulting in structural changes in the matrix that gave the polymer extra properties. Studies also show that irradiated HDPE gain an increase in oxygen content compared to unirradiated HDPE, this means oxidation of irradiated HDPE was accelerated by an increase in light intensity & also no nitrogen was found during the irradiation process [6, 16, 18].
Studies  show the yield of free radicals were found to be low due to something known as high energy selectivity, so a photosensitiser was used in order to promote selective absorption of the radiant energy and also to accelerate the radical formation which would in turn enhance the crosslinking reaction. Studies  show that the amount of gel content increases rapidly with the increase in irradiation energy, by using a crosslinking agent there was a much more efficient crosslinking in HDPE, temperature increase causes an efficient crosslinking increase & that as laser power increased, the degree of crosslinking increased. 
Research  shows irradiation intensity increased, the melting temperature of irradiated HDPE decreased, but the degree of crystallinity increased. This was probably due to scission of the HDPE chains which was followed by the recrystallisation of the broken chains. The decrease in melting temperature was due to the increase in crystalline defects. The yield strength of irradiated HDPE increased due to polar crystallinity of HDPE growth after irradiation, but a decrease in yield strength could be due to the excessive degradation of the HDPE chains. 
Below, is the reaction mechanism of radiation crosslinking:
(sections of polymer molecules) (radiation removes H atoms leaving free radicals)
(free radicals join together to crosslink molecules)
Figure 4: Radiation Crosslinking Mechanism 
6.2: Properties of PEXB Products, Their Advantages, Limitations & Safety Aspects:
PEX B products are formed by high energy radiation crosslinking which gives them many properties; these include resistance to cold flow/creep, enhanced high temperature stability, resistance to attack by chemicals/solvents, high mechanical strength, excellent electrical properties and it all occurs by a free radical process [6, 16].
The advantages of PEX B products are increased mechanical properties, can induce very quick chemical reactions, high energy concentration on a small surface area, short exposure time, reduces undesirable secondary reactions, low capital costs, environmentally friendly and a very clean. [6, 16].
The limitations of PEX B products include; ionizing radiation sources are bulky, non-user friendly & users need shielding against exposure to the radiation beams, causes extensive degradation in the polymer network causing inefficiency during cross-linking, there is less mobility in crystalline regions so cross-linking occurs mainly in the amorphous region of the polymer, it has limited applications with just polyethylene and ethylene copolymers, only suitable for thin wall parts and it is an expensive process. [4, 6]
Regarding safety, although nearly everyone is exposed to natural radiation, the control of radiation exposure from the use of radiation is a main concern as radiation is very dangerous. It can harm body tissues through over exposure & can cause illnesses like cancer. However, the Health & Safety Executive (H.S.E.) is an institution which regulates radiation exposure by advising, inspecting & then enforcing a procedure by which people who are working with radiation are kept safe. 
7.1: Chemistry and Reaction Mechanism of Silane-Induced Cross-linking of PE and PEXC:
During this crosslinking technique, silane groups are grafted to the polyethylene chain then hydrolysis of the Si-OH groups occurs followed by a condensation reaction that forms Si-O-Si bonds. The siloxane bridges formed are less rigid & so give more flexibility when compared to the C-C bonds formed by peroxide crosslinking & high energy radiation crosslinking. 
As polyethylene has no functional groups whatsoever, cross-linking is not possible and so an initiator is provided, for example dialkyl peroxide. At a high temperature dissociation occurs resulting in free radicals. These are transferred to the polyethylenes free radical sites which then allow silane grafting. The cross-linking process then begins by firstly hydrolysing water with a methoxysilane group, & secondly the formed hydroxyl groups undergo a condensation reaction [8, 10, 22].
As long as the material produced is kept dry, it can be processed as thermoplastic polyethylene. During the extrusion process, a catalyst is blended into the material & then the extruded insulation is cross-linked by exposure to moisture. This moisture is normally provided by hot water tanks or high temperature steam saunas [8, 10, 15, 22].
Yeong-Trang Shieh [8, 15] studied silane grafting reactions of LDPE, LLDPE and HDPE by thermal analysis in differential scanning calorimetry & calculated the apparent activation energy and the order of activation energy was LLDPE>LDPE>HDPE [8, 15].
The effect of molecular structure of polyethylene on silane grafting and moisture crosslinking has been studied. The study showed that at high concentrations of grafting additives, there is no difference between the amount of silane grafting for different polyethylenes. I was also found out that HDPE has more vinyl groups on its polymer chain, but is not very branched, it also has the most allylic hydrogens. Smedberg et. Al. Concluded that vinyl residues on polyethylene chains play a strong role in the generation of polyethylene radicals using peroxide as the initiator/crosslinking agent. So the grafting of HDPE is larger due to its large amount of allylic groups, higher molecular weight and its lower bond dissociation energy of allylic hydrogen. Coil sizes strongly affect grafting & crosslinking behaviour because a large coil interacts easily with other coils & so it increases the possibility of grafting & reduces intermolecular crosslinking. Also at higher concentrations of silane compounds, the consuming of vinyl residues is due to tertiary carbons. The gel content for HDPE increased slowly & then reached a steady level of crosslinking, this could be due to the higher levels of grafted silanes available for HDPE. Finally, the increase in gel content during moisture crosslinking shows the progess of silane crosslinking, which is a result of moisture condensation reactions [8, 15, 22].
To conclude, the silane crosslinking mechanism is different at different crosslinking periods. At short crosslinking periods the ability of polyethylene to form a crosslink network is due to peroxide-induced carbon-carbon entensions formed. At higher periods if crosslinking, silane crosslinks change the chain extended polyethylene into a 3 dimensional network structure in which the diffucsion of water molecules into the polymer structure is an important aspect in the degree of crosslinking [8, 15, 22].
Below, is the silane crosslinking mechanism:
CH3* + * + CH4
+ H20 + ROH
Figure 5: Silane Crosslinking Mechanism 
7.2: Properties of PEXC Products, Their Advantages, Limitations & Safety Aspects:
PEX C products are formed by silane-induced crosslinking which gives them many properties, these include enhanced temperature stability, excellent mechanical properties and thermal resistance. 
The advantages of PEX C products are easy processing, low material cost, low capital investment required, energy saving and higher productivity. 
The limitations of PEX C a product include, it being a slow process & has to be carried out as a batch process. 
There are hardly any safety issues concerning silane-induced crosslinking, however basic laboratory dress code must be obeyed, such as wearing a lab coat and goggles.
Research shows that crosslinked polyethylene has greater advantages when compared to uncrosslinked polyethylene; these include excellent electrical properties, improved heat performance, increased resistance to high temperatures, increased flow resistance, good abrasion resistance, excellent tensile properties & excellent chemical resistance which allow it to be used in a variety of applications all across the world. As crosslinked polyethylene has many advantages, it has gained an increase in its use in applications. For example, it is used to insulate medium and high volt cables and can now be used in piping that is needed for heat distribution processes that are of low temperatures. Polyethylene pipes are also used in mine drainage, because mine water is normally very acidic and can destruct steel pipes, however this problem does not occur with polyethylene pipes which is why there was a large surge in its sellability. Not only this, but as it is light and very easy to move around it can be taken out of a mine and moved into a different place. The flexibility of polyethylene is also advantageous as it can be coiled and then easily distributed to different areas without taking up too much room. Polyethylene pipes can also be used in telephone cable industry especially in the coaxial cable for radio frequency in telephones [4, 23, 24, 25, 26].
Research now needs to be conducted on how to minimise the limitations posed by radiation crosslinking and peroxide crosslinking. The reason being is that the government is now trying to find ways to minimise the environmental impact of industries such as the chemical one. As discussed earlier, the process of silane crosslinking poses no environmental effects therefore it can be viewed as more 'greener' compared to the other two techniques. Not only that, but the more limitations the crosslinking techniques pose, the more time and money is wasted. Therefore, firms will try and use more of the silane crosslinking as it is much cheaper and less problematic. This is why research needs to focus on how to make the other two crosslinking techniques as 'greener' and more environmentally friendly.
The aim of this research thesis was to review the different cross-linking techniques targeted in the chemical industry for the production of polyethylene pipes.
Peroxide crosslinking is carried out using peroxides for example dicumyl peroxide, once the polymer is crosslinked it can then be enhanced by adding monomers for example styrene. The limitations of peroxide crosslinking outweighed the advantages and the major factor was safety, because a peroxide is used in the process extra care needs to be taken to ensure the safety of those carrying out the crosslinking process [section 5].
Radiation crosslinking is similar to peroxide, but instead uses ionising radiation to crosslink the polymer. The overall limitations outweigh the advantages and the safety aspect shows that this crosslinking strategy is non-ser friendy as radiation is used so again extra care needs to be taken for those carryinbg out the process which is timely and more expensive [section 6].
The final crosslinking process studied was silane which uses moisture to crosslink polymers. Compared to the other two it had more advantages and fewer limitations and there were no overall safety issues, it was not only user friendly, but also environmentally friendly .
Overall, regardless of the type of crosslinking process used, crosslinked polyethylene is shown to be more advantageous than uncrosslinked polyethylene. As this is the case polyethylene is now used in many applications worldwide especially in the piping industry and is now being used in the telecommunications industry meaning that there is now an increase in the use of polyethylene and a surge in its production.