The research of polyethylene irradiation cross-linking makes progress

The Principle Of Irradiation Dealing With The Change Of Polyethylene Structure, particularly The Change Of Crystalline Morphology Is Introduced.

The Effect Of Crosslinking By Irradiation On Mechanical Properties Of Polyethylene Is Also Discussed.

Introductive polyethylene of Crystallization1 of 　 of Crosslinked of 　 of Irradiation of 　 of Keywords 　 Polyethylene is a kind of wide application produce medium high polymer at daily life and industry and agriculture. Regard half brilliant as material, its function relies on interior strongly collect state structure. Of polyethylene catenary neat the gender gifts its good crystallization ability, crystallization is spent can change inside very big range. On the other hand, close together adhesion is lacked between catenary and catenary, make the generation when whole polyethylene material is experiencing outside force and environmental temperature effect inferior be out of shape or happen destroy, restricted its to apply. Because this basis uses limits and end actually, be necessary to undertake modified to polyethylene, cross-linking is considered as the method of effective. The cross-linking of polyethylene basically uses chemical cross-linking and physical cross-linking. Chemical cross-linking is main make cross-linking lotion in order to cross oxide and silicon alkyl. Physical cross-linking basically is radioactivity of such as nucleus the radiate of the high-energy particle such as source 60Co, 137Cs and neutron, electron or irradiation cross-linking. 1952 Charsby [1] the polyethylene after discovering irradiation produced cross-linking, from now on the cross-linking research of polyethylene spreads out flourishingly. The develops quickly to go up objectively to also be radiation cross-linking research of high-energy radiate device provided solid corporeal basis. Current, the applied rate of irradiation technology and method already was regarded as to measure hi-tech of a country to use one of horizontal symbols. Radiation basically is radiation polymerization and radiation cross-linking in the application in high polymer. The function after radiate of high polymer classics produces bigger change, basically concern with the cross-linking of interior happening and degradation. Chemical cross-linking and radiation cross-linking all differ somewhat to the change of function from executive method. Because chemical cross-linking uses cross-linking agent, origin is rich easy, receive wider application. Undertake as what polymer cross-linking reacts, the fuse-element viscosity of ceaseless heighten makes cross-linking agent is in the dispersive sex in matrix is poorer, appear inhomogenous cross-linking, local happening " anxious burn " phenomenon. Chemical cross-linking agent crosses oxide especially kind the melt temperature that decomposes temperature and polymer is closer, had made oxide suffers lose inevitably, control cross-linking to spend hard; And of agent of silicon alkyl cross-linking decompose need water to do initator, invade as a result of what water divides, deterioration of function of material interpose report, its were restricted to apply on certain level. Action of can even ground is in the high-energy ray that radiation cross-linking uses radiation source to give out on material, the cross-linking of polymer distributings even, and cross-linking is spent control easily, satisfy taller to requirement of polymer cross-linking circumstance circumstance. The another original part of irradiation cross-linking depends on need not adding cross-linking agent, child of cross-linking of attainable tall purity, be in especially medical high polymer material and its domain have tremendous potential application perspective. But, the outcome when sufferring ray effect with phyletic polymer is different. Normally the polymer below irradiation action can produce cross-linking reaction already, also can produce degradation reaction. Accordingly, polymer can be divided for radiation cross-linking model reach radiation degradation. Tell strictly, reaction of the cross-linking when irradiation and degradation reaction are the competitive process that has at the same time. Even if radiation cross-linking polymer, cross-linking of its interior radiate and radiation degradation also undertake at the same time, just cross-linking takes actor just. The happening of cross-linking and degradation and both competition rate divide environment of the radiation dosage that outside be being concerned with structure of high polymer itself, also brings to bear on with place, close rate, radiate (temperature, atmosphere) etc about. Heretofore, the most typical example that uses radiation cross-linking technology at process of cross-linking of high polymer material is the radiation cross-linking of polyethylene. Irradiation cross-linking polyethylene presents a series of exceedingly good chemistry and physical machinery performance (stability of hear resistance, dimension, appropriate model amount, be able to bear or endure the) such as the marked improvement of stress craze. In recent years, study ceaseless development about theory of polyethylene irradiation cross-linking and application, new approach was developed in new material preparation and modified for irradiation technology. The article reacts to its irradiation the influence of fundamental, irradiation to polyethylene structure, function and applied foreground undertake commenting. 2 fundamental 2.

Polyethylene of process of 1 cross-linking reaction classics is high-energy when radiation, except it is on side radical or CH outside producing freedom radical, macromolecule catenary is interrupted to become active freedom base, mutual union builds cross-linking network between freedom radical. The molecular catenary after cross-linking can form H and Y (or T) bodily form structure [2] . The relation between cross-linking reaction process and polymer structure according to radiation dosage by low arrive tall, can divide for the following 4 phase [3] : (The initial phase of 1) cross-linking basically suffers the effect of extreme radical group, expression is orderly cross-linking; (2) cross-linking basically happens in amorphous area, present random cross-linking; (Of 3) cross-linking degree rise further, the molecular catenary of brilliant area surface participates in cross-linking, expression of whole cross-linking process is random cross-linking; (4) waits for brilliant area after complete melt disappears, whole system becomes amorphous form again, the cross-linking of this phase shows the sex that do not have foreword. In addition, irradiation aftereffect is the another feature of irradiation cross-linking method, namely macromolecule catenary is free base athletic ability is poorer, the opportunity of mutual union decreases relatively, the freedom that after the polyethylene after irradiation sets period of time, still exists to did not miss work base. Irradiation hind is proper heat treatment, the mobile capacity that increases freedom radical reacts still but proceed. It is the polyethylene after irradiation in its dissolvent full, the freedom of scour off surface base, can eliminate irradiation aftereffect. 2.

2 cross-linking are right crystal influence 　 Charlesby [1] observe irradiation polyethylene can cause his above all the change that crystallization spends, become gradually after opaque polyethylene classics irradiation namely transparent. The result of X ray diffraction [4] show crystallization peak is reduced, amorphous composition increases, and grain dimension decrescent. DSC is analysed [5 ～ 8] discovery, the melt enthalpy of polyethylene is in small dose (1 ～ 2MGy) the effect suffers scarcely when irradiation. Increase as dosage, hot enthalpy is reduced. But radiation dosage is in 0 ～ 0.

2MGy, put in one lowest to heat up enthalpy value, this also shows crystallization is spent (to it corresponding density) existence minimum is worth [9, 10] . Rijke [11] also deliver small now dose (0.

1MGy) value of lowest of hot enthalpy of the existence when irradiation high density spends polyethylene, think as a result of,this is of cross-linking degree raise the blemish with initiative and crystal prevail on to decrease, a brilliant ply that makes already was generated at the same time attenuate, form not perfect crystal lattice. Mateev [12] with sol - gel law analysed electron beam (EB) the influence of film of polyethylene of irradiation low density to parameter of molecular structure and cross-linking. The unbend chain portion that thinks EB irradiation causes the amorphous division between crystal boundary produces new crystallization. Discover at the same time, the Shu Liujiang that follows irradiation is spent rise, cross-linking is spent increase while the degradation rate in irradiation process also increases, low intensity (Ie=1mA) bundle shed irradiation to spend crystallize rise. Because the unbend catenary of the amorphous part between crystal boundary participates in crystallization to create new crystallization division,this is, isothermal crystallization and drawing action cause the generation of the unbend catenary between crystal boundary. In addition, irradiation causes unbend chain rupture, form macromolecule freedom base, the machine rate that the free catenary of these extreme active links each other is inferior, and be put in local and orderly likelihood, bring about crystallization 2 times. But irradiation dose is higher, the freedom on crystal boundary base excessive arises, brilliant area begins melt, crystallization is spent drop. Of the melt of brilliant area and area of new any crystalline substance the kinetic process that generating is mutual competition. The cross-linking of brilliant area only then of Yu Jing area amorphous change, amorphous changing is area surface begins Cong Jing, increase along with irradiation dosage and expand upcountry. Markovic [13] discovery of new to the polymer after irradiation melt sums up brilliant to spend decrease further. In the meantime, kinetic to crystallization the research that concerns with radiation makes clear as a result, crystallization process has two kinds of different mechanism, respectively correspondence is reached at cross-linking catenary not cross-linking catenary. Kingdom flower [14] with diffraction of ray of extensive role X (WAXD) the high density of 4h of again isothermal crystallization spends melt of the classics after analysing irradiation polyethylene, got parameter of polyethylene brilliant afterbirth follows radiation dosage change to concern. Discovery is microcrystalline in three dimensional space the dimension on each direction follows radiation dosage increase and reduce. Demonstrative irradiation is right the area that be not brilliant, piece brilliant interior is microcrystalline the area is not brilliant between and microcrystalline interior all have destroy effect, put forward thereby " piece brilliant interior destroys mechanism " . Think cross-linking is mixed namely destroy all can happen in brilliant area interior. Via the low density polyethylene of irradiation, its crystallization is spent, melt hot enthalpy did not change almost when small dose, but those who follow dosage is farther rise and reduce [15] . Increase as radiation dosage continuity, crystallization area also can cross-linking. The existence of the recrystallization of the be caused by of macromolecule catenary recomposition that because radiation is caused,the occurrence of peak of absorption of heat of the low temperature on DSC curve proved, its intensity and occurrence temperature all rise along with the addition of radiation dosage. The polyethylene that issues irradiation to different dosage undertakes recrystallization is handled, determine below room temperature its crystallization is spent, when discovery is 175Mrad till dosage, crystallization spends ability to begin to drop somewhat. To high dose (3Grad) the polyethylene of γ irradiation undertakes heating handling, discover brilliant model by orthogonality department change is 6 brilliant is [16] . Xu Xi [17] the effect that studied ultraviolet irradiation spends polyethylene to crystallize to high density. The result makes clear, let time lengthen, crystallization of high density polyethylene is spent increase, melt peak moves to microtherm direction, structure of Dan Jing afterbirth keeps basically changeless. Analyse through heating up weightlessness (the thermal stability that TGA) spends polyethylene to ultraviolet irradiation high density undertakes study, discover along with irradiation time is lengthened, HDPE is initiative decompose temperature, decompose activation energy and reaction progression to reduce. Freeboard element measures polyethylene (UHMWPE) irradiation of classics electron beam, its heat up enthalpy, crystallization to spend, the addition that melt temperature and density all follow irradiation dosage and rise [18] . Vuong [19] with dynamic and mechanical analysis (DMA) the information that obtained a structure to go up, the polyethylene of line form low density that discovers irradiation cross-linking (LLDPE) , 3 on table chart advocate change peak has distinct change along with the addition of radiation dosage: The Chu Nengmo quantity of β change area and bad news can modular quantity increases significantly, α change area has slight addition, but transform in γ the area is invariant. If undertake the sample after irradiation recrystallization, the change of change area is adverse: Radiation dosage increases, modular quantity is reduced. In the meantime, the contribution that the change of the form appearance that causes as a result of recrystallization measures to the model wants the contribution of prep above cross-linking. Undertake recrystallization to gel part, undertake DSC is analysed, through crystallization the peak can prove the crystallization of gel part and sol part have nothing to do. 2.

3 cross-linking are very flabby to the orientaton structure of the influence polyethylene of orientaton calm, be heated or place for long extremely legibility orientaton. Radiation cross-linking is the efficient way that overcomes solution orientaton. PE first after drawing again irradiation processing can get more satisfactory result. Drawing multiple is 5 ～ 6 when, cross-linking rate is top. Research returns discovery, the form appearance of electron beam irradiation and sample and crystallization are spent have immediate concern [20] . The crystallization of HDPE of drawing of irradiation of ray of the γ in vacuum is spent increase along with what absorb dosage and reduce, radiation dosage is achieved when making an appointment with 250kGy, absorb dosage increase to be spent to crystallization do not have an influence basically; In air irradiation comes 1000kGy, the crystallization of drawing HDPE is spent keep basically changeless [21] . 2.

Closely related the degree that the chemical structure of polyethylene of cross-linking of 4 catenary structure changes and cross-linking condition. Of cross-linking structure form existing all the time controversy [22, 23] , namely reticulate to forming Y cross-linking only structure, still form the issue such as T and Y structure at the same time, have not get unanimous verdict up to now. Main reason may depend on what different name and different processing condition issue polyethylene collecting state structure to differ, the polyethylene after bringing about cross-linking thereby reachs difference of the existence on the structure in function. The research of structure of cross-linking polyethylene chemistry is centered at using electronic spin resonance more (ESR) , nuclear magnetic resonance (NMR) , infra-red spectrum (the method such as IR) . Zhu Qingren [24] the network structure after using cross-linking of radiate of 60Co of classics of HDPE of 13C NMR token, no matter be solid state,direct observation arrives or formed after cross-linking of radiate of the PE below melt condition first have the H cross-linking thing with two kinds of Y reticulate structures. Also detect at the same time the change of the other construction unit that accompanies in radiation process. Return discovery at the same time, the Shuang Jian that return form is generated in brilliant area easily, and the Shuang Jian that arrange type is in easily however the area that be not brilliant is generated. Y cross-linking chain dismantles in room temperature preferential form, and H falls in condition of high temperature melt model arise easily. Its reason may be motility of endmost of catenary of solid state PE taller, and the formation that is condensed to be helpful for Y cross-linking catenary at the amorphous area that be not brilliant. Heretofore, the equation that describes cross-linking process applies to the some phase of individual polymer only, return neither one to suit the general comfortable equation of the cross-linking process of most polymer. Even if a kind of polymer, for example polyethylene also fails to find the equation that describes all cross-linking process. Typical equation has the Charlesby-Pinner that is based on statistical theory (C-P) equation: 2 ＝ of ／ of S ＋ S1 the S in type of 1 ／ Q0u1D is ＋ of P0 ／ Q0 sol share number; P0 and Q0 break catenary and cross-linking density for every unit dosage; Before U1 is cross-linking several all degree of polymerization; D is radiation dosage. This equation is pair of radiation cross-linking and degradation is original elaborate, already was accepted extensively. But the diversity as a result of polymer structure, the describes all polymer all cross-linking process that cannot use (or level) . Chen Xinfang [25] be opposite considering molecular catenary extreme the contribution of polyethylene cross-linking, undertook correction to C-P equation, put forward C-L-T equation: D (／ of S ＋ S1 2) the square root that the contribution of this equation of 1/q0u1 of ＋ of Q0 of ／ of 2p0 of ＝ D1 ／ depends on discovering catenary density and radiation dosage becomes direct ratio. To get general comfortable equation, zhang [26] think catenary density should the β with radiation dosage second just become direct ratio, namely Z-S-D equation: D (／ of S ＋ S1 2) the function that the β in 1/q0u1 type of ＋ of Q0 of ／ of =D β P0 is vitrification temperature Tg. 2.

5 cross-linking are mechanical to the polyethylene after the influence cross-linking of mechanical function performance change is bigger, along with the addition of cross-linking degree, the modular quantity of polyethylene, tensile strength has different rate with the denaturation that fight squirm rise. Because cross-linking chain produces between molecular catenary,this is, restricted the slippage of macromolecule catenary, increased capacity of metabolic of be caused by of counteractive and external environment. But after cross-linking reachs certain level, of molecular catenary rupture degradation and brilliant area destroy a phenomenon apparent, stuff becomes fragile, rupture the mechanical function such as percentage elongation drops [27] . Hedvig [28] studied a variety of polymer through the method with isothermal flabby stress (include polyethylene) stress is mixed flabbily interpose report is flabby. The addition that discovers stress and interpose report distributing to follow dosage flabbily fastens transition temperature to be in shift to pure aggregate object, below higher dose, its distributing to broaden as a result of cross-linking flabbily. In its the polyethylene after irradiation crystallization temperature above undertakes isothermal stress is mixed flabbily reply experiment, the sample after discovering irradiation has relative to faster stress flabby process, and be below medium radiate dosage stress is flabby the fastest [19] . Polyethylene of the irradiation in air, its rupture percentage elongation and tensile strength are more apparent than irradiation system falling to drop in vacuum condition [29] . The polyethylene after cross-linking of 3 application foreground produces bigger change on structure and function, can satisfy the circumstance of special requirement. The polyethylene of measurable cross-linking behaves the thermal stability on certain level, the interpose report sex that stabilizes especially. These superior performance make cross-linking polyethylene is used extensively do cable bag to cover. Polyethylene base resistor temperature coefficient (PTC) electric composite material, have resistance - temperature switch property, be paid close attention to extensively consequently. But this kind of material is after the temperature with exceed constant, there is PTC quality no longer between resistor and temperature, lift and drop what contrary resistor follows temperature show negative temperature coefficient (NTC) relation. And classics of this kind of material heats - the reappearance that after cooling loop is used, phenomenon of unwarrantable PTC, NTC reachs room temperature resistor. Reappearance of this kind of blame and cannot control a gender to be reached to production use brought great inconvenience. Eliminate NTC or reduce the electric capacity of NTC phenomenon and stable PTC material, it is to consider to reach use place of this kind of stuff the target of indispensible pursuit. The method that undertakes to PTC material cross-linking is effective [30 ～ 34] . The NTC phenomenon of the PTC material after cross-linking gets effective keep within limits, resistor rises relative superiority or inferiority to fall along with temperature degree alleviates somewhat. Consider to discover, cross-linking is spent raise 70% ～ 80% when, NTC phenomenon just can disappear. The room temperature resistor of the PTC material of certain level cross-linking has increscent appearance. Normally cross-linking can increase the PTC strength of material. Cross-linking eliminates NTC effect to considered as cross-linking to limit the scope that mutual collision combines carbon black particle commonly, and irradiation cross-linking PE arises freedom radical but with the fragrant oxygen of carbon black freedom radical reacts, improve the result of the active force between polymer matrix and electric particle, reduced the detached chance with not reversible carbon black finally, the electric network construction that keeps basically original [35] . We are undertaking irradiation is opposite polyethylene base / in the research that conductivity of composite material of carbon black electric conduction affects, use high-energy electron beam to fall to undertake irradiation to sample at room temperature and melt condition respectively, issue the data after irradiation at constant temperature (120 ℃ of 95 ～ ) undertake isothermal anneal is handled. No matter why is discovery being planted below annealing temperature, the PTC intensity of material of room temperature irradiation all of irradiation of prep above melt. Fall in high dose especially, the PTC intensity of material of melt condition irradiation is extremely faint, what issue irradiation with same dose room temperature is remarkable and different. Condition of proof cross-linking reaction produces major effect to the structure of cross-linking child and function. the data system that studies with respect to us, cross-linking prevents polyethylene crystal to grow is the main factor that affects conductivity of composite material of its electric conduction. The high blame polarity of polyethylene makes it very difficult allow with phase of other polarity data, if sheet relies on mechanical method both lump, the degree mixing a look of more inaccessible ideal. If undertake to its irradiation is handled, introduced the polar group such as carbonyl to enhance the consistence of both greatly. The method that uses irradiation improves the consistence between polyphase much constituent, have do not need to allow agent, normal temperature to often press next advantages that can have, use irradiation law to improve consistence to the material that already shaped especially, it is other method place cannot be likened to [36 ～ 38] . 4 brief summary are odd constituent or system of polyethylene of polyphase much constituent no matter, undertake cross-linking is handled rightly, had obtained tremendous progress in fundamental theory and actual application respect. But in certain and particular facet, still did not reach consensus, if irradiation is revulsive,bigger difference exists between the destruction of crystallization and irradiation to crystallization. Still lack enough understanding to the structure of amorphous area after irradiation and the diversity that make coupling form. Especially the improvement of function of material of functional high polymer or rise, cross-linking serves as one of methods, specific mechanism still remains to study at further development. CNC Milling