英文文献及翻译
J Polym Res (2009)16:165–172DOI 10.1007/s10965-008-9214-2
Mechanical properties of hydroxyl functionalized
chlorinated polyethylene prepared by in situ chlorinating graft copolymerization
Yingying Sun &Gengping Wan &Baoxing Wang &Jiruo Zhao &Ying Feng
Received:21January 2008/Accepted:3June 2008/Publishedonline:4July 2008#Springer Science +Business Media B.V . 2008
Abstract A graft copolymer composed of poly (2-hydroxyethyl acrylate) (PHEA)as branched chains and chlorinated polyethylene (CPE)as backbone, CPE-cg -HEA, was synthesized by in situ chlorinating graft copolymerization (ISCGC).The polymer has special molecular structure with short graft chains and abundant branched points. The mechanical properties of CPE-cg -HEA were studied by tensile testing, differential scanning calorimetry (DSC),dynamic mechanical analysis (DMA).The morphologies of tensile fractured surfaces for CPE and CPE-cg -HEA were investigated by scanning electron microscope (SEM).The testing results indicated the mechanical properties of the in situ chlorinating graft copolymers have greatly improved compared with CPE with about the same chlorine content. Particularly, there was a broad plateau on the stress –strain curve of the graft copolymer, which meant a high elastic-like deformation.
Keywords CPE-cg -HEA . Functionalization . Mechanical properties . Stress –strain curve . In situ chlorinating graft copolymerization Abbreviations HEA CPE ISCGC
CPE-cg -HEA DSC DMA
SEM HDPE GD GPC WAXD Tg E ’tan δscanning electron microscope high-density polyethylene Graft degree
gel permeation chromatography wide-angle x-ray diffraction glass transition temperature storage modulus loss tangent
Introduction
In order to extend the application of chlorinated polyethylene (CPE)or obtain polymers with special use, CPE needs to be modified by physical or chemical methods. There are many reports in recent years about the modification of CPE. Examples of such modification include adding chlorinated paraffin, a hindered phenol compound and organic com-pounds etc. to obtain both good damping property and high stiffness [1–6].By multicomponent mechanical blending technology, a water-swellable elastomer which is compati-bilized with graft copolymers has been prepared using CPE and poly(acrylicacid –acrylic amide) as the chief materials and an amphiphilic graft copolymer as the compatibilizer [7].Carbon black(CB)asreinforcing filler is also filled into CPE for obtaining good properties. For binary systems of CPE and CB, oxidized CB gives a high modulus at low strain amplitude [8].Because both polyurethane and CPE have polar functional groups which may interact to form interchain cross-links on heating, blends of polyurethane and chlorinated polyethylene elastomer have been prepared to obtain better performance properties through interchain cross-linking reaction [9].One of the typical chemical modifications basing on CPE is the preparation of chlor-
2-hydroxy ethyl acrylate chlorinated polyethylene
in situ chlorinating graft copolymerization polyethylene in situ chlorinating graft 2-hydroxy ethyl acrylate
differential scanning calorimetry dynamic mechanical analysis
Y . Sun G. Wan B. Wang J. Zhao (*) Y . Feng
Key Laboratory of Rubber-Plastics, Ministry of Education, Qingdao University of Science and Technology, Qingdao 266042, People ’s Republic of China e-mail:[email protected]
166Y . Sun et al.
osulfonated polyethylene in a tank reactor using chlorinated polyethylene and a gaseous mixture of chlorine, dioxide sulfur and nitrogen initiated by ultraviolet light [10].Covalent cross-linked chlorinated polyethylene thermoplastic elastomer is prepared with reactive blending using dicyclo-pentadiene dicarboxylic kalium [DCPD(COOK)2]as cross linker. This kind of cross-linked polymer can be processed repeatedly [11].Grafting modification is one of the basic ways to modify CPE. The most frequently used grafting method is suspension grafting in water. For example, suspension grafting copolymerization of methylmethacrylate and styrene onto chlorinated polyethylene [12].Few studies about the grafting on CPE have been conducted in recent years, although there were many patents in this respect in early years [13–16].
The traditional method of preparing graft copolymers from chlorinated polymers is as following:for instance, the preparing of CPE graft copolymer, polyethylene is chlori-nated firstly, and then the corresponding monomers are grafted. Our research group has reported in situ chlorinating graft copolymerization (ISCGC)[17, 18]and now the method is adopted in present study for the modification of polyethylene. The way is completely different from the traditional modification way. No any initiator was added into the system and radical reaction was initiated by
Fig. 1Synthetic scheme for the preparation of polyethylene in situ chlorinating graft 2-hydroxy ethyl acrylate (CPE-cg -HEA)
chlorine radical from thermal decomposition of chlorine. The reaction is a gas –solid reaction.
Take polyethylene grafted with HEA by ISCGC as an example, under normal pressure chlorine gas is introduced into the mixture of polyethylene powders and 2-hydroxy ethylacrylate (HEA)under heating. As a result, grafting copolymerization happened while chlorinating reaction of backbone polyethylene was being carried out. Therefore, the grafted chains with reactive functional group were intro-duced. Because of the particularity of the reaction process, graft copolymers have many branched points bearing short side chains [18],which cause remarkable change on physical and mechanical properties to the materials.
Experimental Materials
The starting material in this study was high-density polyethyl-ene (HDPE).HDPE (LG6040)was supplied by LG Chem., Korea. 2-hydroxy ethyl acrylate (HEA)was a commercial grade supplied by Xinglu Chemical Industrial Co. Ltd., China Chlorine was supplied by Haijing Chemical Industrial factory, China. Silicon dioxide was a commercial grade
Cl 2
HCl
2
2CH 2OH
2CH CH 2CHCl
HOCH 2CH 22CH 2OH
Hydroxyl functionalized chlorinated polyethylene
16
167
12S t r e s s (M P a )
CPE
8
100°C.The chlorine gas in the reactor was drawn out by vacuum and then air was let into the reactor. The vacuum and air influx operations were performed alternately some times to ensure the residual chlorine cleaned out completely.
Synthetic scheme for the preparation of CPE-cg -HEA is shown in Fig. 1.
Differential scanning calorimetry (DSC)
800
4
cg -HEA
200
400Strain (%)
600
Fig. 2Stress –strain curves of chlorinated polyethylene (CPE)[Cl%(Wt):36%]and polyethylene in situ chlorinating graft 2-hydroxy ethyl acrylate (CPE-cg -HEA) [Cl%(Wt):36.1%]
DSC measurements were carried out with a PerkinElmer DSC-7calorimeter. The samples were heated from −70to 150°Cat a heating rate of 10°C/minto investigate the glass transition temperature of polymers. The tests were carried out under a nitrogen flow and a sample of 9to 12mg was used. Dynamic mechanical analysis (DMA)
supplied by Qingdao Sodium Silicate Co. Ltd, China. Chlorinated Polyethylene and the graft copolymer (CPE-cg -HEA) were prepared by the author in laboratory. Graft degree (GD)of CPE-cg -HEA is determined by 1H-NMR spectros-copy. The determining details of graft copolymerization of CPE-cg -HEA are going to be published elsewhere. The synthesis of CPE-cg -HEA
ISCGC was carried out [18]in a 500-ml round bottom three-necked flask equipped with a vane stirrer, a thermometer, and a gas delivery tube. 50g HDPE and a certain amount of HEA monomers were added into the flask. It was stirred around 30-min to make HDPE and HEA sufficiently mixed. Then appropriate amount of silicon dioxide was added to avoid the agglomeration of PE. The reaction mixture was deaerated by nitrogen (N2) at about 40°Cfor 15min to eliminate oxygen gas (O2). Then chlorine was introduced into the reactor. The reaction was initiated. The reaction temperature was elevated as the reaction proceeded and was kept in 80±2°Cbefore the chlorine content reached 17%,and then was raised to certain temperature(below140°C)through the reaction process. The reaction process was indicated by the amount of hydrogen chloride (HCl)released from the reaction system. The chlorine gas was stopped and the reaction was terminated when the desired chlorine content was reached. The system was cooled down to below
The dynamic mechanical properties of various samples were determined with a dynamic mechanical analyzer (NETZSCH-DMA242),using a dynamic tensile mode in a rang of −100°Cto 120°Cat a constant frequency of 15.6Hz and a heating rate of 3°C/min.Scanning electron microscope (SEM)
SEM (JEOLJSM-6700F) was used to study the morphologies of fracture surfaces obtained from tensile testing. The fracture surfaces were coated with gold by JFC-1600Auto Fine Coater and then examined by SEM. Mechanical properties
Tensile properties were measured with a GOTECH AI-7000M Universal Testing Machine at room temperature and 55%Relative Humidity (RH).The tensile tests were performed at a crosshead speed of 500mm/min.Reported values were the average values of five individual measurements. Results and discussion
Mechanical properties of CPE-cg -HEA
The structure of CPE-cg -HEA was characterized by FT-IR, 1
H-NMR, gel permeation chromatography (GPC)and
Table 1Mechanical properties of chlorinated polyethylene (CPE)and polyethylene in situ chlorinating graft 2-hydroxy ethyl acrylate (CPE-cg -HEA) Samples
Chlorine content (Wt%)36.036.1
Young ’s modulus (MPa) 1.700.70
Tensile strength (MPa)9.4314.8
Elongation at break (%)647801
Hardness (ShoreA ) 7571
CPE
CPE-cg -HEA
168exo
1
)
v m (w o l F t a e H -50
050100150
Temperature (o C)
Fig. 3DSC thermograms of chlorinated polyethylene (CPE)(1) and polyethylene in situ chlorinating graft 2-hydroxy ethyl acrylate(CPE-cg -HEA ) (2)
wide-angle x-ray diffraction (WAXD)in our past study [17].The mechanical properties of the corresponding polymers were studied in this paper. The stress –strain curves of CPE and CPE-cg -HEA were shown in Fig. 2and the mechanical properties of the polymers were shown in Table 1. By comparing the two curves, it is found that CPE-cg -HEA exhibits typical rubber ’s tensile behavior. A high plateau appears between the elongations of 200%and 600%,which is the feature of rubber, i.e. the strain increasing gradually, yet stress keeping constant. Strain hardening occurred and the stress –strain curve rose sharply after the elongation of CPE-cg -HEA reached 600%.Strain hardening continued until the specimen broke. It differs strikingly from the tensile behavior of CPE. As is shown in Table 1, the tensile strength at break for CPE (chlorinecontent 36.0%)is 9.43MPa, while that of CPE-cg -HEA (chlorinecontent 36.1%)can reach 14.8MPa. Moreover, the elongation of CPE-cg -HEA increased from 647%to 801%.
4000
2
3000
)
a P M 2000
(' E 1000
0-100
-50
50
100
Temperature (°C) Fig. 4Temperature dependences of storage modulus (E ′) of chlori-nated polyethylene (CPE)(1) and polyethylene in situ chlorinating graft 2-hydroxy ethyl acrylate (CPE-cg -HEA ) (2) Y . Sun et al.
The remarkable differences on the properties between the two polymers mentioned above are due to the obvious differences on their molecular structures and chemical compositions. The graft copolymers, CPE-cg -HEA, pre-pared with ISGCG have a special structure with −OH functional groups and abundant branched points and short branched chains [18].That decreases the crystallinity [17]and increases intermolecular distance of the backbone polymer chains, and thus increases flexility of materials. In addition, because of the high chlorine radical concentra-tion in the reactor, the homopolymer (PHEA)in the reaction system have low number average molecular weight of about 1000[17]which contributes to flexility of materials. However, the increases on tensile strength might be attributed to HEA grafted on the backbones which produce intermolecular hydrogen bonds. And these inter-molecular hydrogen bonds act as physical crosslinked points, which make the tensile strength of materials increase.
Large numbers of experimental results all indicated that tensile strength and elongation at break of the graft polymers prepared by ISCGC could increase simultaneously. It is thought that only by hydrogen bonds can we explain these results judging from the structure of the polymers. Further research is still needed in this area. Differential thermal analysis
Figure 3shows DSC thermograms of CPE and CPE-cg -HEA. As are shown on the curves, the glass transition temperature (Tg)of CPE and CPE-cg -HEA are −10.5°Cand −18.2°Crespectively. The decrease of Tg for CPE-cg -HEA is attributed to comprehensive results from abundant branched chains and intermolecular hydrogen bonds in the graft copolymers. On one hand, abundant branched chains make the free volume between backbone chains increase and thus Tg will decrease remarkably. On the other hand,
0.8
0.6
a 0.4
t 1
0.2
-100-50050100
Temperature (°C)
Fig. 5Temperature dependences of the loss tangent (tanδ) of chlorinated polyethylene (CPE)(1) and polyethylene in situ chlori-nating graft 2-hydroxy ethyl acrylate (CPE-cg -HEA ) (2)
Hydroxyl functionalized chlorinated polyethylene 169
Fig. 6SEM micrographs of chlorinated polyethylene (CPE)(A , B ) and polyethylene in situ chlorinating graft 2-hydroxy ethyl acrylate (CPE-cg -HEA) (C , D )
however, intermolecular hydrogen bonds could increase Tg, hence final results is that Tg of CPE-cg -HEA decrease not remarkably, slightly lower than that of CPE. CPE-cg -HEA has a single Tg, which can be interpreted in terms of compatibility. The branched chains of the copolymers prepared by ISCGC [18]are chlorinated at the same time of grafting. Termination reaction occurred frequently since a large number of chlorine radical existed in the system. As
20
130 ˚C ˚C 16
˚C
a result, the branch chains are short; therefore, they are
compatible very well with the backbone polymers. Even though there are a few of intermolecular hydrogen bonds the branched chains on the graft copolymer still make the free volume between main polymer chains increase and thus Tg decreases. At about 50°Ca smaller endothermic peak appears on the either curve of the two systems. There are three endothermal peaks for CPE, however, the attribution of them has been quite a controversial
problem,
Table 2The effect of reaction temperature on Young ’s modulus of polyethylene in situ chlorinating graft 2-hydroxy ethyl acrylate (CPE-cg -HEA) Run
Reaction temperature (°C,°C)a (80,(80,(80,(80,
100) 120) 130) 140)
Graft degree (%)1.621.691.741.52
Young ’s modulus (MPa)171.02.84.74.7
S t r e s s (M P a )
12
120 ˚C
84
[1**********]00
1
234
a
Strain (%)
Fig. 7Stress –strain curves of polyethylene in situ chlorinating graft 2-hydroxy ethyl acrylate (CPE-cg -HEA) [Cl%(Wt):35%]with reaction temperature
HEA:four parts
Reaction temperature:first stage Cl%
170
Table 3The effect of chlorine content of polyethylene in situ chlorinating graft 2-hydroxy ethyl acrylate (CPE-cg -HEA) on Young ’s modulus
Run Chlorine Graft degree Young ’s modulus content(%)(%)(MPa)126.11.3413.3236.41.672.5340.92.463.34
45.0
2.70
4.2
HEA:four parts
Reaction temperature:first stage Cl%17%:140±2°C
and the view which could be widely accepted at present is that the three endothermal peaks are the peaks of crystal phase transition [19].After polyethylene chlorinated or chlorinated and grafted there are flaws of the crystal domains showing a peak appear at a lower temperature. There is no crystal melting peak at around 130°C(themelting temperature of polyethylene) on the DSC curves of both CPE and CPE-cg -HEA. Dynamic mechanical analysis
The temperature dependences of storage modulus (E ′) and the loss tangent (tanδ) for CPE and CPE-cg -HEA are shown in Figs. 4and 5.
It can be seen from Fig. 4that the storage modulus value of CPE-cg -HEA at the lower temperature is larger than that of CPE, which indicates graft copolymer has higher stiffness at lower temperature. The storage modulus value of materials declines sharply as the temperature increases. And the E ’of CPE-cg -HEA graft copolymer declines more sharply. The storage modulus value of CPE is slightly higher than that of CPE-cg -HEA around 0°C.
As seen in Fig. 5, the temperatures corresponding to the maximum value of the tan δpeak were −6.1°Cfor CPE, −8.0°Cfor CPE-cg -HEA. These temperatures can be regarded as Tg of polymers. The Tg obtained from DMA
20
26.1%
15
)
a P M ( 45.0%
36.4%
s 10
s e r t S 5
[**************]00
Strain (%)
Fig. 8Stress –strain curves of polyethylene in situ chlorinating graft 2-hydroxy ethyl acrylate (CPE-cg -HEA) with chlorine content
Y . Sun et al.
16
12
)
P a (M s 8
s 10parts
r e t S 4
[**************]00
Strain (%)
Fig. 9Stress –strain curves of polyethylene in situ chlorinating graft 2-hydroxy ethyl acrylate (CPE-cg -HEA) with HEA parts (Wt%)
curves are higher than that measured by DSC, which is caused by the different way in measuring Tg. The former is dynamic approach and the latter is static one. The tan δpeak of CPE-cg -HEA graft copolymer becomes broader and higher compared with that of CPE. Moreover, the tan δof CPE-cg -HEA from −20°Cto about 20°Cis much larger than that of CPE. In addition, over the glass transition temperature range the energy consumption for inner energy of CPE-cg -HEA is more and the maximum value of tan δpeak is 0.75which is about one fold higher than that of CPE (0.37).It was because the increase of the polarity of the graft polymer and the existence of the hydrogen bonds . Over the range of glass transition temperature of polymers, the material absorbs more vibrating energy and therefore damping action of CPE-cg -HEA is stronger. With increase in the temperature, chain segments can move more freely, thus tan δdeclines sharply. Between 40°Cand 100°C,tanδof CPE-cg -HEA is lower than that of CPE. Fracture surface morphology
SEM images of the tensile fractured surfaces for CPE (A,B) and CPE-cg -HEA (C,D) are displayed in Fig. 6. The fractured surface of CPE is uneven, while that of CPE-cg -HEA is obviously smooth. That is to say there is distinct
Table 4The effect of HEA content on Young ’s modulus of polyethylene in situ chlorinating graft 2-hydroxy ethyl acrylate (CPE-cg -HEA)
Run
HEA content Graft Degree Young ’s modulus (parts)(%)(MPa)110.855.0241.852.5361.752.54
10
1.57
2.0
HEA:one part (Cl%=35.4%);four parts (Cl%=36.4%);six parts (Cl%=34.9%);ten parts (Cl%=35.1%)
Reaction temperature:Cl%17%:136±2°C
Hydroxyl functionalized chlorinated polyethylene difference in morphological structure between CPE and CPE-cg -HEA. By comparing the four SEM micrographs, it can be concluded that graft copolymers have more compact structure; therefore, the rigidity of the materials is greater. By comparing picture B and D, we can draw the conclusion that the fiber drawings on the fractured surface of graft copolymer are wider, which indicates materials have higher toughness.
Effect of synthesis conditions on mechanical properties In this study, the reaction was carried out by two-stage temperature. The reaction temperature was kept in 80±2°Cbefore the chlorine content reached 17%.The reaction temperature was raised to the desired temperature after the chlorine content reached 17%.The effect of the reaction temperature during the second stage on the mechanical properties and Young ’s modulus are shown in Fig. 7and Table 2respectively.
As is seen from the stress –strain curves of the polymers, mechanical properties all have improved greatly as reaction temperature in the second stage increases, while the amounts of HEA and chlorine contents are approximately the same. Chlorination and grafting happened only in amorphous domain of HDPE when the reaction temperature is below the melting temperature. In this case, crystalline regions of HDPE had not been destroyed. And thus the polymer shows typical brittle feature, high modulus and little strain. The samples were in the melting state when the reaction temperature of the second stage approached or surpassed the melting point of HDPE (130°C),for instance, 130°Cand 140°C,chlorination and grafting could happen not only in the amorphous regions but also in the crystalline regions where the sample was melted. Consequently, the mechanical properties of graft polymers obtained have little difference. And thus the curves on 130°Cand 140°Care close. Introduction of chlorine atom and branched chains can destroy the crystalline very effectively. The stress –strain curves of the polymers show high elastic-like plateau, while Young ’s modulus decreases remarkably.
Chlorination and grafting can happen not only in the amorphous regions but in the crystalline regions of HDPE when the reaction temperature of the second stage was kept in 136±2°C.The crystallinity has been investigated in our past study referred in the 18th reference. In the reference, the variation of crystallinity with the chlorine degree was studied by wide-angle x-ray diffraction (WAXD),and the experimental results indicated:with the increase in chlorine content, the crystallinity of polymers decreases. And thus the amorphous regions increase. These changes make the polymer chains move more freely and thus Young ’s modulus decreases (Table3). As a result, the polymer begins to shift from crystalline polymer to elastomer.
171
Polymers with the chlorine content between 35%and 40%exhibit elastic-like behavior and the modulus is very low. As chlorine content continues increasing, the intermo-lecular interaction among the polymers increases so that polymers exhibit plastic behavior, which is indicated on the stress –strain curves (Fig.8).
Figure 9and Table 4display the relation between the adding amount of HEA and stress –strain curves. Graft copolymers prepared by ISCGC give abundant branched points with short branched chains [17].HEA grafted onto the backbone chains increase with the increase in the amount of the monomers added, this results in the increase of the contents of the branched chains in the copolymer, so that the crystallinity decreases. Remarkable high elastic-like platforms appear on the stress –strain curves of the graft copolymers (Fig.9) when the addition of HEA reaches certain amount.
Conclusions
The mechanical properties of hydroxyl functionalized chlorinated polyethylene graft copolymer prepared by ISCGC has greatly improved than that of CPE with approximately the same chlorine content. It exhibits typical rubber ’s behavior. The graft copolymer has only one glass transition temperature which is lower than that of CPE with approximately the same chlorine content. Over the glass transition temperature range the loss tangent (tanδ) peak of the graft copolymer becomes broader and higher, and its value is 0.75which is about one fold higher than that of CPE (0.37).The chlorine content, the addition of monomers, and the second stage ’s reaction temperature of CPE-cg -HEA can greatly influence the mechanical properties of the graft copolymers.
Acknowledgement Contract grant sponsor:National Natural Science Foundation of China
Contract grant number:50373042, 50390090.
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