Microstructure and Mechanical Properties of Poly (Vinyl Chloride ...

Vol.27 No.5 HUANG Jian et al: Microstructure and Mechanical Properties of Poly(Viny...

886 DOI 10.1007/s11595-012-0568-2

Microstructure and Mechanical Properties of Poly (Vinyl Chloride) Modified by Silica Fume / Acrylic Core-Shell Impact Modifier Blends HUANG Jian1, MA Baoguo2*, LI Xiangguo2, JIAN Shouwei2, TAN Hongbo2 (1.The Green Building Materials and Manufacturing Engineering Research Center of Ministry of Education, Wuhan University of Technology, Wuhan 430070, China; 2.State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology,Wuhan 430070,China) Abstract: This research explored replacing acrylic core-shell impact modifier (AIM) by silica fume to toughen PVC. 100%, 75%, 50% and 25% of AIM (8 phr) were substituted by silica fume in PVC respectively, and then processed by dry blending and twin-screw extrusion. Severe silica fume agglomeration was observed by scanning electron microscope (SEM) in the PVC matrix when 8 phr pure silica fume was used and processed by screw speed of 20 rpm. Its tensile strength was thereby reduced by 38% comparing to unmodified PVC. The silica fume was successfully dispersed while the screw speed was slowed down to 10 rpm to give a stronger screw torque and a longer melt residential time in the extruder. The tensile strength was ‘recovered’ to a level comparable to unmodified PVC. Impact test were performed on all formulations extruded at 10 rpm screw speed and synergetic toughening effect was found with 50% substitution and it had the impact strength that was comparable to 8 phr pure AIM toughened PVC. Key words: silica fume; acrylic impact modifier; synergetic effect; poly (vinyl chloride); tensile strength; impact modification; twin screw extrusion

1 Introduction Acrylic impact modifiers (AIMs) have spherical particle shape with submicron size. The current AIM particle normally has a rigid organic shell that is compatible with the polymer matrix (e g, poly methylmethacrylate (PMMA)), and a rubber core that has a low glass transition temperature and can absorb impact energy (e g, butadiene rubber). Due to the complex manufacturing technique, the price of AIM is generally two to three folds higher than the matrix resin like PVC. Silica fume is a fine spherical amorphous silicon dioxide particle with a submicron particle size. It was known as a by-product during production of silicon or silicon alloys by electric arc furnace [1, 2]. Silica fume had long been an atmosphere pollutant. Until the 1970’s that the value of silica fume was first found to be an additive to make compact Portland cement by the ©Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2012 (Received: Oct. 23, 2011; Accepted: July 19, 2012) HUANG Jian(黄健): P h D; E-mail: [email protected] *Correspondance author: MA Baoguo(马保国): Prof.; Ph D; E-mail: [email protected]

Nordic countries. Then in the 1990’s, silica fume has been found to be applied in rubbers[3, 4]. It is well known that PVC has high compatibility with fillers and additives. Various organic and inorganic additives and fillers can be homogeneously incorporated in PVC to modify its properties, such as changing it from flexible and ductile (Plasticized PVC) to rigid and tough (Unplasticized PVC)[5, 6]. Precipitated calcium carbonate (PCC), as an inorganic IM for PVC, has already been commercialized at present. By acting as stress concentration sites, impact energy can be dissipated through craze/cavitation formation and yielding[7, 8, 13]. Due to its submicron particle size, PCC can effectively toughen PVC through this mechanism. Silica fume, as an inorganic particle with submicron size similar to PCC, has the potential to be used in PVC. However, silica fume tends to agglomerate in plastics because of its high specific area and incompatibility with the organic matrix. Homogeneous compound is therefore difficult to be achieved in a viscous polymer melt. The presence of silica fume agglomerate worsened the mechanical properties of the polymer matrix, limiting its application. Some research has attempted to disperse silica

Journal of Wuhan University of

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fume agglomerates during dry mixing procedure [9]. However, to our knowledge, seldom research has focused directly on dispersing silica fume agglomerates during extrusion and using it to replace AIM. This paper explored substituting AIM by silica fume and analyzed the effect of extrusion parameters on the tensile and impact strength after the substitution. Formulations of silica fume/AIM toughened PVC and corresponding processing parameters were proposed.

2 Experimental 2.1 Materials Suspension polymerized PVC (K value: 65) was used as polymer matrix. Calcium carbonate (Hydrocarb 95T) was used as filler. Titanium dioxide (Kronos 2220) was used as pigment and outdoor stabilizer. Heat stabilizer Naftosafe type P-WX 17146 was from Chemson. An AIM Paraloid KM-366 from Rohm and Haas with aerated density of 0.5 g/mL and median diameter about 0.2 μm was used. Silica fume without any surface modification is from Elkem with density of 2.2 g/mL, specific surface area of 20 m2/g and median diameter of 0.15 μm. The formulation of the sample is listed in Table 1. Sample PVC is a control without IM. Sample SF, A2SF6, A4SF4 and A6SF2 correspond to PVC with substitution of AIM by 100%, 75%, 50% and 25% respectively. Acr is the 100% AIM modified PVC. 2.2 Morphology observation

The particle morphology of silica fume and AIM were observed by type JEM-2100F TEM from JOEL. Tensile fracture surfaces and sample cross-section surface were examined by type JSM-5610LV SEM with EDS attachment from JOEL. The silica fume and AIM powder samples for TEM was dispersed by ultrasound in alcohol solution before observation. The silica fume powder that examined by SEM was sprinkled on a double sided tape first. Loose particles were shaken off then gold plated. Fracture tensile surfaces were characterized just after gold plating. 2.3 Sample preparation

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An 8 L mixer was used for dry blending. Formulations of PVC, Acr: All materials were poured into the 80 ℃ preheated mixing chamber and stirred at 2 000 rpm up to 120 ℃ for 6 minutes. Then the stirrer stopped and the blend was discharged into a cooling chamber and stirred for 5 minutes between 20-30 ℃. Formulations of SF, A2SF6, A4SF4 and A6SF2: All materials except CaCO3 filler and TiO2 pigment were poured into the 80 ℃ preheated chamber to allow silica fume particles to be coated on PVC grains first. Materials were stirred at 2 000 rpm up to 100 ℃ for 6 minutes, when the motor was stopped, and the CaCO3 filler and TiO2 pigment were added. Mixing continued at the same stirrer speed and temperature was raised to 120 ℃ for another 6 minutes. The blend was then discharged and cooled as before. Samples were extruded by a conventional PVC extruder with conical twin screw. A slot die is used to form a square profile. Conventional extruder temperature setting for window profiles was utilized. Two series of samples were produced using screw speed of 20 and 10 rpm respectively. During extrusion, screw torque of the main motor was measured by sensor within the extruder. Melt residential time in the extruder was calculated from the haul-off speed. Melt pressure was measured by force transducers located along the extruder barrel. These extrusion settings were listed in Table 2. Sample names were suffixed with Sr20 and Sr10 corresponding to Setting 1 and Setting 2.

Tensile and impact test samples were machined from the extrudates into required shape. Tensile test was carried out according to ASTM D882[10] at room temperature using extension speed of 10 mm/min. At least 4 samples per formulation were tested. Charpy impact test was performed using notched samples at room temperature according to BS EN ISO 179:1997[11], and 10 samples were tested per formulation.

3 Results and discussion 3.1 Microstructures of silica fume and AIM particles The particle morphology of silica fume and

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AIM was showed in Fig.1. Both silica fume and AIM particles showed similar spherical shape and particle size in the same scale. Comparing to AIM particles, silica fume particles have a broader range of particle size distribution and each silica fume particle has a more clearly defined circular shape. These differences stem from their different preparation conditions. Silica fume was produced in a flue of a furnace while AIM was prepared by carefully controlled emulsion polymerization. Silica fume surface was ‘smoothened’ by the fast air flow within the flue when it densified from melt, resulted in a better defined spherical shape. AIM particles size is tightly manipulated by the control of shear force in an emulsion with assist of surfactants, hence have a more even particle size distribution. 3.2 Tensile and impact properties Representative tensile curves were plotted in Fig.2. Tensile strength results were summarized in Fig.2(c). Each tensile curve from Setting 2 was integrated, then divided by the minimum cross-section area of the tested tensile bar to give a tensile energy intensity (TEI) value. TEI was used to quantify tensile energy consumed on per unit cross-section area of each sample. The averaged TEI value was plotted together with impact strength in Fig.2(d) for comparison. Fig.2(a) and Fig.2(b) showed that when extruder setting changed from Setting 1 to Setting 2, tensile strength of sample PVC, SF and A2SF6 increased, but sample A4SF4, A6SF2 and Acr did not change much. The tensile strength improvement of sample PVC without any IMs is probably due to higher level of fusion between the PVC primary particles. However, as indicated by the long dashed line on Fig.2(b), its tensile curve just went through the yielding point and no obvious plastic deformation was observed. The sample was fractured in brittle mode and toughening is still necessary. Another significant change is the sample SF in Fig.2(a) and Fig.2(b). When processed by Setting 1, the sample SF Sr20 fractured before reaching the yielding point in a brittle mode. Comparing with the unmodified PVC, its tensile strength that showed on

Fig.2(c) was dropped by 38%. In this case, addition of silica fume weakened the tensile strength in PVC. Interestingly, when the screw torque and mixing time increased (Setting 2), the tensile curve of sample SF Sr10 ‘recovered’ and went through yielding point, showing plastic deformation as indicated on Fig.2(b). The higher tensile strength is probably due to improved silica fume dispersion and SEM was performed for



further investigation. The SEM images of the tensile fracture surface from sample SF Sr20 was showed in Fig.3. Severe agglomeration were found on these surfaces. The small square in Fig.3(b) was scanned by the attached EDS equipment and the spectrum was showed on Fig.3(c). The agglomerate in the square was high in elements of silicon and oxygen, confirming it was silica fume. When Setting 1 was applied, these silica fume agglomerates formed defect sites, weakening the tensile strength. Fig.3(d) illustrated the cross-section surface that cut from sample SF Sr10 that extruded by Setting 2. The EDS probe spectrum of point 1 is similar to Fig.3(c), confirming the spherical particle is silica fume. The probe spectrum of point 2 showed strong chlorine peak, confirming the smooth surface was PVC. Most silica fume particles were embedded in the PVC matrix as indicated by square A on Fig.3(d). Other silica fume particles even fell out from the PVC matrix during cutting, leaving holes that just fit the silica fume particle, as indicated by the square B. From these SEM images, it can be confirmed that as stronger screw torque and longer melt residential time were applied, better dispersion of silica fume particles was achieved. The tensile curves of sample A2SF6 (25% AIM) indicated by the small dotted line in Fig.2(a) and Fig.2(b) suggest similar transition from brittle to ductile failure. But the transition and the increase of tensile strength were less obvious than sample SF. As the

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AIM content further increased from 50% to 75% then 100% (long dashed and thin solid lines), large plastic deformation was observed. The patterns of these tensile curves have barely changed regardless of the extrusion setting. Changing screw torque and melt residential time seemed to have little effect on the tensile behavior for these three formulations. Fig.2(c) summarized the effect of replacing AIM by silica fume. AIM has high compatibility with the PVC matrix as it was designed to be and this has been proofed by various studies[8, 9, 12, 13]. Data series of Setting 1 suggested that increasing the loading of AIM from 0% to 100% gradually eliminats the strength weakening effect of silica fume and increases the integrity of the PVC matrix. When changed to Setting 2, tensile strength that modified by silica fume was generally higher than those processed by Setting 1. This can be explained by the distribution and particle characteristic of silica fume. Silica fume particle is rigid and have spherical smooth surface. As silica fume was dispersed in the PVC matrix, individual silica fume particle was embedded in the PVC matrix as can be seen by the SEM image. When silica fume particles got intimate contact with the PVC matrix, Van de Waal’s forces could possibly become effective. In this case, silica fume particles no longer act as defect sites. Hence, the tensile strength was not affected. Since samples extruded by Setting 2 were of stronger tensile strength, they were impact tested and

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the results were showed together with their TEI values in Fig.2(d). Although impact strength and TEI were tested by different methods, these two parameters both quantified the energy used in per unit cross-section area to break the sample apart. Results showed both parameters increased with increasing AIM loading. The TEI gradually increase proportionally with the AIM loading. This can be explained by that AIM has better compatibility with PVC, and the rubbery core of AIM can absorb energy by means of deformation. The material consuming more energy during the test resulted in a higher TEI value and impact strength. An interesting finding here is that the impact strength had a more sudden increase with the AIM loading of 50%, and then the value was stabilized with higher loading of AIM in our study. This phenomenon was explained as follows. The different toughening mechanisms of sample processed by Setting 2 are proposed in Fig.4. Fig.4(a) showed that when only silica fume (8 phr) was used, because silica fume itself was a rigid particle, it cannot absorb energy by deformation and yielding. Hence the impact strength did not increase. Although the impact energy can also be consumed by craze/cavitation formation and yielding of the PVC matrix as found by Refs.7 and 8, it was not found in our study. Since AIM have a rubbery core with low Tg, Fig.4(b) shows that when only AIM was used, its particles were stretched and yielded under an impact force induced stress field. AIM particles consumed part of the impact energy and resulted in a higher impact strength. Fig.4(c) showed that because they have similar particle size, as the silica fume particles dispersed around AIM particles,

it could also concentrate the stress field during impact testing. Because silica fume itself did not deform the test, more intense stress field was formed around AIM particles. This resulted in an even larger extent of deformation and yielding of AIM particles, hence a higher impact strength value. Therefore, silica fume particles can be applyed to replace AIM to act as stress concentration sites. The assisting effect of silica fume with AIM is a synergetic effect. Sample A2SF6 was compounded with both silica fume and AIM, and such synergetic effect was not found. This may because sample A2SF6 has lower tensile strength than any other samples processed by Setting 2 and AIM loading is too low to become effective in toughening PVC. This synergistic toughening mechanism between elastomeric and inorganic rigid particles is still under discussion. However, the result here is in agreement with a recent research which also showed synergistic effect from organic and inorganic IM compound[13]. Above all, our research showed that silica fume can be used as a green and economic co-impact modifier. Due to its smooth spherical shape, similar particles size with AIM and rigidity, dispersing it in a PVC matrix can replace 50% of the AIM by weight to reach similar impact strength in PVC without sacrificing tensile strength.

4 Conclusions a) When PVC modified by 8 phr silica fume (formulation SF) was extruded at 20 rpm, the tensile strength was reduced by 38%, owned to silica fume agglomeration that identified by SEM. The tensile



strength improved with increasing AIM content, probably due to molecular chain entanglement between AIM and PVC. b) The tensile strength was ‘recovered’ when stronger screw torque and longer extruder residential time was applied to extrude formulation SF. The strength recovering effect can be explain by SEM which showed better silica fume dispersion and intimate contact of silica fume particles with PVC. The tensile strength increased with increasing silica fume content, probably stemmed from the Van der Waals forces between silica fume and PVC. c) When 50% by weight of AIM was substituted by silica fume and dispersed, synergetic effect on was found and its impact strength was equivalent to pure AIM. It was probably because silica fume can concentrate stress field around AIM particles and assisted energy dissipation of AIM during impact test.

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