Tensile property of thin microcellular PC sheets prepared by compression molding

Transcription

1 express Polyme Lettes Vol.1, No.4 (2007) Available online at DOI: /expesspolymlett Tensile popety o thin micocellula PC sheets pepaed by compession molding Z. Xiao, R. Guan *, Y. Jiang, Y. Li College o Chemisty and Chemical Engineeing, Hubei Univesity, Wuhan, , China Received 3 Januay 2007; accepted in evised om 1 Apil 2007 Abstact. Thin micocellula polycabonate (PC) sheets have been pepaed by compession molding. Tensile tests wee peomed on micocellula PC sheets pepaed unde dieent conditions, and esults show that oaming time plays the main ole on the tensile popety. The cellula stuctue pepaed in this pocess has been shown to have a signiicant eect on the coesponding tensile popety o micocellula PC oams. Fo these oams, Single Blend model can be used to pedict the eect o the elative density on the elative stength, but the expeiment data o the tensile modulus is much lowe than that pedicted by Squae-Relationship Model. The eect o cell size on the tensile popety o micocellula PC oams was also investigated, and ound that the cell size can aect the tensile popety, which is dieent om the esult o Kuma. In ode to make the tensile popety pedictable, some mathematical omulas wee also simulated. Keywods: mechanical popeties, micocellula, polycabonate, simulation 1. Intoduction Due to the special stuctue o micocellula oams, they can be used in many aeas, such as packaging, insulation, bone substitute mateial, militay aicat and micoelectonic application. In the micocellula oam application, the mechanical popety is a vey impotant acto to conside. Thee is consideable liteatue on the mechanical popeties o conventional oams [1 8], paticulaly in the book edited by Gibson and Ashby [2], but too limited epots on that o micocellula oams. Since the micocellula oam was successully pepaed by Suh and his co-wokes [9 11] in Massachusetts Institute o Technology, some eseaches have ocused on the tensile popety o micocellula oam. Waldman [12] epoted that the tensile stength o micocellula polystyene (PS) oam ollows a linea elationship with the speciic density, but much highe than that o the stuctual oam when Young s modulus is kept constant. Kuma and Van de Wel [13] ound that the tensile stength o polycabonate (PC) micocellula oam was lowe than that pedicted by the law o mixtue, and Young s modulus was lage than that o stuctual oam. Collias and Baid [14] studied the tensile toughness (the aea unde the stess-stain cuve) o micocellula oams o PS, PC and Styene-Acylonitile (SAN) copolyme. They concluded that micocellula PS oam shows limited impovement in the tensile toughness, but micocellula SAN and PC oams show deteioation in tensile toughness. Late on, Sun et al. [15] caied out mechanical tests both in compession and tension, and they dew a conclusion that Young s modulus and stength ae closely elated to the elative density. Youn and Suh [16] ound that the lexual toughness o polyeste micocellula oam inceases by a acto o 3 o 30% bubble volume action compaed to the neat polyeste, although the stength is about the same. Aoka [17] investigated the eect o the cell size and shape on the * Coesponding autho, ongguan@hubu.edu.cn BME-PT and GTE 217

2 compessive yield stess o micocellula PS oam, and ound that the compessive stength o oam inceases with the incease o cell size, and thei data ae highe than those pedicted by an anisotopic oam model. Recently thee has been some eseach dealing with the eect o elative density on the mechanical popety o micocellula composite oams [18, 19], howeve, they only elated mechanical popety with the elative density, but ewe epots ae available on the eect o pocessing paametes and cell size on the tensile popety. The objective o this pape is twoold. One is to assess the impotance o pocessing paametes which aect the tensile popety. These pocessing paametes include oaming time, oaming pessue and oaming tempeatue. The othe is to investigate the eect o cellula stuctue, that is, the elative density and the cell size, on the tensile popety, and compae the expeimental data with the values pedicted by Gibson and Ashby [2]. 2. Expeimental 2.1. Mateials The polycabonate (PC) sheets used in this wok wee commecial-gade poduct with a thickness o mm, puchased om GE Copoation. Low density polyethylene (LDPE) pellet om Sinopec Beijing Yanshan Chemical Copoation Factoy (Beijing, China), with a melt index 2 g/10 min at 190 C, was used as eceived. Commecial-gade azobisomamide (AC) om Wuhan Hanhong Chemical Reagent Copoation o China (Wuhan, China), was used as blowing agent. The amount o gas yielded, which mainly consists o nitogen, cabon monxide, cabon dioxide and ammonia, is about ml/g. Dicumyl peoxide (DCP), Sinopham Chemical Reagent Copoation, Ltd. (Shanghai, China), was used as eceived. Analytical-gade zinc oxide (ZnO) om Shanghai Shiyi Chemical Reagent Co.Ltd. (Shanghai, China), was used as activate agent o AC decomposition Micocellula PC oam pepaation The pepaation o micocellula PC oam consists o two pocessing steps. Fist, accoding to the omula, 200 g LDPE, 40 g AC, 5 g DCP and 20 g ZnO wee mixed in a two-oll mill as a gas souce o PC oaming. Second, the PC sheets and the mixed LDPE wee placed in a mm mold togethe and loaded in a hydaulic hot pess unde expeimental conditions. Ate the equied oaming time and othe conditions wee eached, the pessue was eleased and the mold was taken out o the hot pess immediately. Then we unloaded the mold, quenched it to oom tempeatue within 30 s; and emoved the micocellula PC oam om the mold. Thus, the micocellula PC samples wee eady o popety chaacteization ate being kept at oom tempeatue o at least 2 days Micocellula PC oam chaacteization The cell size o the oamed samples was calculated accoding to the image obseved unde a Nikon YS2 optical micoscopy (using a 100 ampliy camea lens), ollowing the pocedue descibed peviously by Kuma et al. [20] and Equations (1) and (2). V N 0 3 c πd N = 6 N = 1 V (1) (2) whee V is the volume action o the oamed mateial, D c is the aveage cell size measued om the photogaphs, N is the cell density elative to the oamed mateial, and N 0 is the cell density elative to the neat mateial. The speciic density o oamed samples was measued accoding to the method used by Yuan [19], and concete pocedue was shown in ASTM-792 standad. When the speciic density was less than 1.0, the oamed samples wee immesed into wate by the help o a thin coppe sheet. The speciic density and elative density ae calculated by Equations (3) and (4), espectively. M ρ = M ρ ρ = ρ 0 a w (3) (4) whee ρ 0, ρ is the speciic density o the unoamed and oamed mateial, espectively; M a and M w ae the weight o the samples measued in ai and wate, espectively; ρ is the elative density. 218

3 The micocellula PC sheets wee machined into dumbbell-shape specimens with a gage length o 50 mm (ASTM D638 type II specimen), the specimen thickness vaied with the expansion o oam. Fo each sample, at least ive samples wee tested and the aveage data ae epoted in this aticle. The tensile test was caied out at oom tempeatue using a Shimadzu AG-A all-pupose testing machine (Japan) equipped with a 1000 lb load cell at a constant cosshead ate o 50 mm/min. 3. Results and discussion 3.1. Tensile behavio o unoamed and micocellula PC sheets Figue 1 shows the typical stess-stain cuves o unoamed and micocellula oamed PC sheets, they can be divided into thee egions: in the ist egion stess inceases steeply with the incease o stain; the cental egion in which stess emains constant and stain vaies quickly; and the last egion whee stess vaies with the incease o stain up to actue. The elative density deceases when PC sheet is micocellulaly oamed, the tensile stength at beak, the elongation at beak evidently deceases and the yield point becomes aint. This is because the gas acts as a plasticize. The expeimental data that elect the eect o pocessing paametes on the tensile popety, such as tensile stength, tensile modulus, speciic stength, beaking elongation, ae listed in Table 1 and ae discussed as ollows. Figue1. The stess-stain cuves o unoamed and micocellula PC sheets (oaming time is 10 min, oaming pessue is 20 MPa, oaming tempeatue is 130 C) The eect o pocessing paametes on tensile stength Fom Figue 2, it can be seen that the tensile stength deceases with the incease o each pocessing paamete. Howeve, when oaming time vaies, the tensile stength changes moe than the ones in othe oaming conditions. Hence, combined with Table 1, it can be concluded that oaming time plays the main ole on the tensile stength. Unde the pessue o 18 MPa, 20 MPa and 22 MPa as shown in Figue 2a, the tensile stength is evidently aected by oaming time, and deceases with the incease o oaming time. The oaming pessue can also aect the tensile stength as shown in Figue 2b, when oaming pessue inceases, the Table 1. The expeimental data o the micocellula PC sheets pepaed unde dieent pocessing conditions Foaming time [min] Foaming pessue [MPa] Uppe-plate tempeatue [ C] Cell size [μm] Standad deviation [μm] Cell density N 0 [10 8 cells/cm 3 ] Relative density Tensile modulus [MPa] Tensile stength [MPa] Beaking elongation [%]

4 Figue 2. The eect o pocessing paametes on tensile stength o micocellula PC sheets tensile stength deceases gadually. But the eect o uppe-plate tempeatue and lowe-plate tempeatue on the tensile stength is too limited as shown in Figues 2c and 2d. This is pehaps due to the decease o elative density and the gowth o cell. When oaming time, oaming tempeatue and lowe-plate tempeatue incease, the elative density deceases and the cell size inceases. Thus, the oamed micocellula PC can stand less stess. As o the eect o oaming pessue on the tensile stength, although cell size deceases with the incease o oaming pessue, the deceased elative density plays the main ole in the tensile stength. The esult is the same to that epoted by Gibson [2], the tensile stength is popotional to the elative density. And the oaming time plays moe impotant ole in the deceased elative density than othe oaming conditions in ou consideed expeimental conditions. So the tensile stength deceases with the incease o oaming paametes, especially with the incease o oaming time The eect o pocessing paametes on tensile modulus As shown in Figue 3, tensile modulus deceases with the incease o each pocessing paamete, and oaming time aects tensile modulus to lage extent, the vaiation tend is simila to that o tensile stength. When oaming time inceases, tensile modulus deceases to the lowest point as shown in Figue 3a; and as the othe thee pocessing paametes incease, tensile modulus deceases to dieent points as shown in Figues 3b, 3c and 3d. The eason is simila to that o the tensile stength, the tensile modulus is also a unction o the elative density. The elative density deceases with the incease o pocessing paametes; and deceases much moe with the oaming time than with othe oaming conditions. In addition, the dissolved gas acts as a plasticize when peoming tensile test. So the tensile modulus also deceases with the incease o each pocessing paamete, especially with the incease o the oaming time. 220

5 Figue 3. The eect o pocessing paametes on tensile modulus o micocellula PC sheets The eect o pocessing paametes on speciic stength In this pape, the speciic stength, which is aected by pocessing paametes in a complex manne, is deined as the atio o tensile stength to elative density. The speciic stength o nonoamed PC sheet is 66.1 MPa, in most cases, it becomes lage when PC sheets ae in micocellula oamed state, as shown in Figue 4. When oaming time and lowe-plate tempeatue incease, the speciic stength inceases to a cetain value and then deceases as shown in Figues 4a and 4d. But when the oaming pessue and uppe-plate tempeatue incease, the speciic stength inceases as shown in Figues 4b and 4c. This esult satisies the initial aim o inventing micocellula oam. Ate micocellula oaming, the mechanical popety deceases less compaed to the deceased elative density, so the speciic stength o PC sheet is inceased ate micocellula oaming The eect o pocessing paametes on elongation at beak Elongation at beak is an impotant paamete to assess whethe the micocellula oam is toughened o not, and the elongation at beak o non-oamed PC sheet is % in ou expeiment. Fom Figue 5 we can see that the elongation at beak o micocellula PC sheets is deceased, it agees with the esult o Kuma and Van de Wel [13]. When oaming time inceases, the elongation at beak deceases to lage extent unde the pessue o 18 MPa, 20 MPa and 22 MPa, but the elongation at beak vaies dieently with the incease o each pocessing paamete. This esult may be caused by the deceased elative density and the deteioation o micocells. The elative density deceases with the incease o pocessing paametes, the elongation at beak deceases. When the micocells geneated ae lage than cetain size, they can esult in the embittlement o the oamed sample. Hence, the educed elongation at beak ate micocellula oaming in this eseach may be the esult o lage cell size. 221

6 Figue 4. The eect o pocessing paametes on speciic stength o micocellula PC sheets Figue 5. The eect o pocessing paametes on beaking elongation o micocellula PC sheets 222

7 3.2. The eect o elative density on tensile popety In ode to investigate the eect o elative density on tensile popety clealy, some micocellula PC sheets with the same cell size and the dieent elative density wee tested by using univesal tensile teste. Figue 6 shows the eect o the elative density on the tensile stength and the tensile modulus o micocellula PC sheets, whose cell size is in the ange o 5.0~6.0 µm and the elative density is in the ange o 0.65~0.98. As shown in Figue 6, the elative tensile stength and the elative tensile modulus incease with the incease o elative density. We have compaed ou expeimental data with that pedicted by the Squae-Relationship model and Single Blend model [21]. The two models ae epesented by Equations (5) and (6). E = ρ 2 σ = ρ (5) (6) whee E is the elative tensile modulus o micocellula oam (E /E 0 ), E is the tensile modulus o micocellula oam [MPa], E 0 is the tensile modulus o non-oamed polyme [MPa]; σ is the elative Figue 6. The eect o elative density on tensile popety o micocellula PC sheets tensile stength o micocellula oam (σ /σ 0 ), σ, tensile stength o the oam [MPa]; σ 0, tensile modulus o the non-oamed polyme [MPa]. The compaisons between expeimental data and pedicted value ae shown in Figue 6. The expeimental data o the elative stength at beak appoximately its well with the cuve pedicted by Single Blend model o the elative density is in the ange o 0.65~0.98, but the elative modulus is evidently smalle than that pedicted by Squae-Relationship Model, this may come om the oigin mateial popety and its cell size. In ode to accuately pedict the tensile stength and the tensile modulus elationship with the elative density, the expeimental data between the tensile popety and the elative density wee simulated, and Equations (7) and (8) ae it to ou expeimental data well. E σ 2 = ρ ρ = ρ ρ (7) (8) 3.3. The eect o cell density on tensile popety Thee have been ewe epots about the eect o cell size on the tensile popety. Only Kuma and Welle [22] investigated the eect o cell size on the tensile behavio o micocellula PC, and ound that the cell size had no eect on the tensile behavio. Since the micocellula oams exhibit some special popeties compaed to the conventional oams with the same speciic density, we assume that the cell size should have the eect on the tensile popety. Thus, we selected the micocellula PC oam samples with the same elative density and the dieent cell size, and measued thei tensile popety. Figue 7 shows the eect o cell size on the tensile stength and the tensile modulus o micocellula PC sheets, whose elative density is in the ange o 0.86~0.87 and cell size is in the ange o 3~14 µm. As shown in Figue 7, the elative tensile stength and the elative tensile modulus decease with the incease o cell size, which doesn t agee with Kuma s conclusion [22]. The disageement may be the esult o the dieent way o pepaing the micocellula PC oam. When the cell size becomes lage as shown in Figue 8 and the elative density keeps constant, each bubble can stand less stess 223

8 σ = (10) Dc Fom the esults discussed above, the cell size does aect the tensile popety unde ou expeimental conditions, and the elative tensile stength and elative tensile modulus ae a unction o 1/D c. In ode to pedict the tensile stength and tensile modulus on the basis o elative density and cell size, the elationship between cellula stuctue and tensile popety can be expessed in Equations (11) and (12): E E 0 ρ = A ρ 0 m n 1 D c (11) σ ρ 1 = B σ D 0 ρ0 c i j (12) whee A, B ae itting constants; m, n, i, j ae constants elated to mateial. And these omulas can accuately expess the elationship between the cellula stuctue and the tensile popety. Figue 7. The eect o cell size on tensile popety o micocellula PC sheets Figue 8. The photogaph o optical micoscope o micocellula PC; (a): smalle cell and (b): lage cell when stess was put on micocellula oams; and when the cell size becomes lage and the elative density keeps constant, the deceased cell density esults in ewe bubbles that can shae stess put on micocellula oam, so elative tensile stength and elative tensile modulus decease with the incease o cell size. In ode to pedict the eect o cell size on the tensile stength and the tensile modulus, we also simulated the elationship between cell size and tensile popety, and Equations (9) and (10) ae it to expeimental data well E = D c (9) 4. Conclusions In this pape, tensile tests wee peomed on the thin micocellula PC sheets pepaed unde dieent conditions. The esults show that oaming time plays the main ole on the tensile popety. The cellula stuctue pepaed by compession molding has been shown to have a signiicant eect on the coesponding tensile popety o the micocellula PC oams. We have investigated the eect o the cellula stuctue on the tensile popety, and ound that Single Blend model can be used to pedict the eect o the elative density on the elative stength o micocellula PC oams, but the expeimental data o the tensile modulus is much lowe than that pedicted by Squae-Relationship model. The cell size can aect the tensile popety to some extent in ou expeiment. In ode to accuately expess the elationship between the cellula stuctue and the tensile popety, semi-empiical models wee poposed to it the expeimental data well. Acknowledgements The authos ae gateul to the National Natual Science Foundation o China o inancial suppot ( ). 224

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