Foam

Subscriber access provided by Northern Illinois University

Article

Structure-Properties Relationship of a Novel Multilayer Film/ Foam Material Produced through Co-extrusion and Orientation Hanguang Wu, Jingwei Zhang, Cong Zhang, Jingxing Feng, Md. Arifur Rahman, and Eric Baer Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03418 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Structure-Properties Relationship of a Novel Multilayer Film/Foam Material Produced through Co-extrusion and Orientation Hanguang Wu1, Jingwei Zhang2, Cong Zhang2, Jingxing Feng2, Md. Arifur Rahman2*, Eric Baer2 1

Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing, 100029, China 2

Center for Layered Polymeric Systems (CLiPS)

Department of Macromolecular Science and Engineering Case Western Reserve University, Cleveland, OH 44106-7202, USA

*Corresponding authors: Tel.: +1 216 317 0457 E-mail addresses: [email protected] (Md. Arifur Rahman)

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 23

ABSTRACT: The PP multilayer film/foams prepared by multilayer co-extrusion technique possesses a unique solid/porous alternating horizontal architecture, in which the film layers can effectively suppress the premature rupture of cells in foam layers. This paper introduces a strong and lightweight layered cellular structure developed through optimized uniaxial drawing of the co-extruded PP multilayer film/foams. Moreover, systematic study reveals the effects of orientation on the structure and properties of the oriented PP multilayer film/foams. Formation of subcellular interconnected porous structure decreases the bulk density with increasing draw ratio (DR) up to DR = 3.5. In addition, strain induced increase in the orientation of α crystals and crystallinity of PP during uniaxial drawing improve the tensile strength and modulus of oriented multilayer film/foam significantly. The oriented PP multilayer film/foam with the lowest density and improved tensile properties can be obtained at DR = 4 at the optimized drawing condition. Key Words: PP Multilayer Film/foam Material, Orientation, Structure-Properties Relationship

2


Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Polymeric foams are widely used as the lightweight alternatives to their solid counterparts in order to provide some unique properties such as low density, high heat insulation, noise insulation and high shock absorption. Among polymeric foams, polyolefin foams are mostly commercially used. Furthermore, the polypropylene (PP) foams have recently been widely used as a kind of polyolefin foam products because of their high chemical resistance, high thermal stability, and cost-effective synthesis. However, the foaming of PP is sometimes uncontrollable compared with polyethylene (PE) foaming because of its low melt strength, resulting in a poor cell morphology and the relatively low tensile properties of PP foam products at a low density 1, 2 In order to obtain the materials with improved mechanical and thermal properties while maintaining the similar low density, one common strategy is to modify the cell morphology and cell density of PP foams either by crosslinking or by adding micro/nano fillers 2-4. Another widely used strategy is to prepare multilayered materials through laminating PP foam polymers with polymeric films, which will significantly suppress the crack of the cells when the materials are subjected to a stress5-7. Baer et al. proposed a novel multilayer co-extrusion technique, which is capable of economically producing microlayered film/foam materials with anisotropic cells8-11. They developed PP based film/foam structures through building a unique solid/porous alternating, continuous horizontal architecture, in which the film layers can effectively control the growth of the cells and suppress the break of the cells during co-extrusion.8 Rahman et al. showed that layer uniformity of multilayer film/foams can be improved by controlling the viscosity of film and foam layers10, 11. Such multilayer film/foams can have wide range of applications e.g. as excellent sound absorber, light weight flame retardant composite for aerospace interior, and also as filtration membrane when layers are vertically aligned and oriented 12-14. However, in most cases the obtained multilayer film/foams do not possess adequate physical strength at low bulk density, which limits their structural applications.8 Thus, an effective approach is necessary to maintain the low density as well as high mechanical strength of PP foam materials.

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 23

The uniaxial or biaxial drawing of polymer films is a well-known strategy to modify the microstructural features of polymer chains, as well as the bulk properties of the polymer films, including the increased stiffness and strength, the improved barrier properties, and the enhanced transparency15-19. Moreover, according to the previous studies about the oriented polymeric foams, strong foam materials with much higher mechanical properties can be achieved if the orientation of the cells can be maintained after the processing or post-drawing19. Therefore, Orientation can be considered as a useful strategy to improve the mechanical properties of the multilayered film/foam materials. In addition, the presence of continuous solid film layers in multilayered film/foam materials effectively confined the cells in the foam layers, and highly improved the fracture strain during orientation10. Thus, multilayer film/foam materials would be an ideal system to develop a highly oriented structure with low density and enhanced mechanical properties. However, the work about the orientation of multilayered film/foam materials has not been reported before. In the present work, taking advantage of the unique solid/porous alternating architecture of the multilayer film/foam material, we developed a novel foaming material with low density, high tensile strength, and improved modulus through drawing the co-extruded PP multilayer film/foam samples. Then, we carefully studied the effects of orientation on the structure and properties of multilayer PP/PP film/foam, and a structure-property relationship of the oriented PP multilayer film/foam material was proposed. In this study, we explored a now technique to produce the polymeric cellular material with highly improved mechanical properties while maintaining the low density. Furthermore, we aimed to deep understand the mechanism about the effect of orientation on the density and mechanical properties of the multilayer film/foams based on PP. 2. EXPERIMENTAL SECTION 2.1. Materials PP (H700-12, supplied by the Dow Chemical Company) was used for the fabrication of multilayer PP/PP film/foam. The physical properties of neat PP are 4


Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reported in Table 1. Actafoam (AZ -130 Galata Chemicals) was used as the chemical blowing agent (CBA) (1.5 wt%) for foaming PP.

Table 1. Characteristics of Materials Used to Produce Multilayer PP/PP Film/Foams. Polymer

MFIa (g/10 min)

Density (g/cm3)

Tm (oC)

Crystallization Point (oC)

PP

12

0.896

165

119

MFI= Melt flow index; Tm= Melting point. a

at 230 oC and 2.16kg.

2.2. Preparation of the Samples 2.2.1. Processing and preparation of multilayer PP/PP film/foams The PP multilayer film/foam structure was prepared by using the same two-component microlayer coextrusion setup described in our previous works (as shown in Figure 1)

8, 10

. One extruder contained the foam layer polymer (PP) and

chemical blowing agent, the other extruder contained the film layer polymer (PP). After merging in the two-component feed block, the foam and the film layers were formed into multilayers using the layer multipliers. In this study, 3 multipliers were used to prepare PP multilayer film/foam structure with 16 layers, and the volumetric composition of film and foam layers is controlled at 50/50 by equalizing the pumps rate. A processing temperature of 195 oC was used for optimum foaming based on the decomposition kinetics of Actafoam AZ 130. A 3” exit die was used at the end of the multipliers. A chill roll setup was used as a sheet take-off. Uniaxial drawing of PP multilayer film/foam samples with dimension of 30×30×1.5 mm3 was carried out in a mechanical testing machine (MTS Alliance RT/30) at a speed of 100%/min 140 oC was selected as the drawing temperature, which is 25 oC below the melting point of PP, and the reason is discussed in the following context (Section 3.1). It should be noted that the as-extruded PP/PP film/foams are annealed at 140 ºC for 30 min before initiating the uniaxial drawing in 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 23

the mechanical testing machine, because the crystallinity of PP changes with annealing time and temperature

20, 21

. The samples with different deformation are

obtained by stopping the orientation of multilayer film/foams at different strains and cooling at the room temperature.

Figure 1. Two-component setup for the co-extrusion of multilayer film/foam system

2.2.2. Post-processing of PP multilayer film/foam: Uniaxial Drawing Figure 2 shows the uniaxially drawn samples having different orientation strains. It can be seen that different zones on each sample occurred different deformation during the drawing. In order to clearly present the relationship between the properties and the deformation of the oriented PP/PP film/foam sample, we selected the zone with the maximum deformation on each sample for the following morphology, density, crystal structure, and tensile properties measurement. The deformation of this zone is defined as the draw ratio (DR) in the following context, which was calculated by using the following equation: DR =

ld lo

(1)

where ld and lo are the length of the mostly deformed zone before and after drawing, respectively (see Figure 2(b)). From Figure 2(a), it can be seen that a maximum strain 6


Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of 1250% was achieved by orienting the film/foam samples at the selected orientation temperature. The structures of the oriented samples were studied to understand the effect of orientation on their properties.

Figure 2. Uniaxial drawing of PP multilayer film/foam samples. (a) stress-strain curve obtained during the uniaxial drawing of PP/PP film/foam samples at 140 oC and at a speed of 100%/min; (b) the visual aspects of uniaxially drawn samples. 2.3. Characterization and testing of as-extruded and uniaxially oriented multilayer PP/PP film/foams 2.3.1. Morphology Studies Scanning electron microscope (SEM) (JEOL) was used to observe the as-extruded and oriented multilayer film/foam layered structures in each sample. Film/foam samples were cut both in extrusion and transverse directions with sharp blades at room temperature and then sputter coated with gold (8 nm) for observation in SEM with an emission voltage of 30 kV. Same method was followed to observe the surface topology of oriented film/foam samples. 2.3.2. Bulk Density Measurement A liquid displacement method (ASTM D 3575-93, W-B) was used to measure the bulk density of the foam/film samples. Each sample was tested at least 5 times and the average value was taken. 2.3.2. DSC Analysis 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 23

The melting point and crystallinity of each PP/PP film/foams samples were determined by using a differential scanning calorimeter (DSC) (Pyris1, PerkinElmer) in nitrogen atmosphere at a constant heating rate of 10 ⁰C/min in the temperature range 0 ~ 200 ⁰C. Melting point was determined from the peak value of endothermic peak in the second heating scan. The percent crystallinity of each as-extruded and oriented film/foam sample was determined by using the melt enthalpy of fusion from DSC analysis. The percentage crystallinity for each sample was calculated using the equation

χ c (%) =

∆H i × 100% ∆H io

(2)

where ∆Hi is the heat of fusion measured for an endotherm in a given sample and ∆Hi0 is the heat of fusion for a completely crystalline PP 22, 23. 2.3.3. WAXD Analysis The polymer chain orientations of the as-extruded and uniaxially drawn PP multilayer film/foam samples were characterized by using Rigaku MicroMax 200 + S 2D X-ray spectroscopy which produced monochromatic Cu Ka radiation source (k = 0.1542 nm). The X-ray generator operated at 45 kV and 0.88 mA. The beam time was 3 h for each sample. The x-ray shot on the sample from the transverse direction, and the beam size is 700 µm 24. We used Herman’s orientation factor (fc) to characterize the extent of crystal alignment in the drawing direction of a film/foam composite material. fc for the PP crystal in oriented PP/PP film/foam samples can be determined by using the following equation 25, 26: f c = (3〈 cos 2φ j , z 〉 − 1) / 2

where 〈 cos 2φ j , z 〉 = ( ∫

π

0

2

(3)

I (φ j , z ) cos 2φ j , z sin φ j , z d φ j , z ) ( ∫

π

0

2

I (φ j , z ) sin φ j , z d φ j , z ) . The value

of fc varies between －0.5 for perfect perpendicular orientation and 1.0 for perfect parallel orientation. 2.3.4. Uniaxial Tensile Testing 8


Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Tensile deformation behavior of uniaxially oriented PP multilayer film/foam was investigated on microtensile bars (1.5 × 4.7 × 25) mm3. Tensile tests were performed in a mechanical testing machine (MTS Alliance RT/30) with a strain rate of 10%/min and the stress-strain curves were generated from the load-displacement curve obtained from the machine. All tests were performed at room temperature. 3. RESULTS AND DISCUSSION 3.1. Preparation of Multilayer PP Film/Foam Samples with Oriented Film Layers and Elongated Cells This innovative approach to fabricate robust multilayer film/foam is achieved in two main steps: (a) Co-extrusion of multilayer film/foam; (b) Uniaxial orientation of multilayer film/foam. The co-extrusion process for the production of film/foam layered system is different from the production of multilayer polymer films.10 This is because one extruder is used to compound the chemical blowing agent with the polymer melt. As described in our previous studies, film polymer melt and foam polymer melt streams (polymer melt with decomposed chemical blowing agent) merge in the feed block, then flow through the multipliers and get doubled through a process of cutting, spreading and stacking the viscoelastic polymer melt. When the multilayered melt stream exits the exit die, it experiences a pressure drop and the cells start to form and grow due to the nucleation and converging of the dissolved gas. The cell growth is stabilized as the sheet cools down, and the multilayer PP/PP film/foam with enclosed cells is obtained. Table 2 shows the properties of the as-extruded multilayer PP/PP film/foam sample, and its microstructure is indicated in Figure 3. Totally 16 alternating solid/porous horizontal layers can be observed in the sample, which are multiplied by using three multipliers.

It can be noticed that cells in foam layers are

more elongated to the flow direction whereas cells are ellipsoidal in the extrusion direction, indicating an anisotropic cellular morphology in film/foam composite structure caused by the flow of the polymer melt through the multipliers. Such anisotropic orientation of film/foam improves toughness, tear resistance and puncture 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 23

resistance in comparison with film/foams having isotropic structures. Therefore, we tried to uniaxially draw the co-extruded multilayer PP/PP film/foams to achieve higher anisotropy and improved mechanical properties.

10


Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Properties of co-extruded PP multilayer Film/foam.

a

Film/Foam Systems

Foam Content (v%)

CBA Content (wt%)

Number of Layers

Average Cell Sizea (µm)

PP/PP

50

1.5

16

120 ± 40

Average cell size was calculated from SEM images.

Figure 3. SEM images showing the layered morphology of PP multilayer film/foams with 16 layers produced by coextrusion and multiplication technology: (a, b) The layered morphology in the extrusion direction; (c, d) The layered morphology in the transverse direction. At high post-drawing temperatures, the PP crystals are large and more perfect, and the crystals network can more effectively reduces the relaxation of the amorphous phases at elevated temperatures. Therefore, the PP films and PP fibers drawn at the temperatures slightly below their melting point possess the highest stiffness and tensile properties

27, 28

. However, for the PP multilayer film/foam, an excessive high

temperature leads to the rupture of the PP cells, and hence significantly deteriorates the microstructure and performances of the oriented PP multilayer film/foams. Therefore, we optimized the temperature of 140 ºC (25 oC below the melting point of PP) as the temperature for the orientation of PP multilayer film/foam. In addition, the 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 23

speed of the test and geometry of samples were also optimized to obtain the maximum draw ratios without rupturing the whole structure of samples. The stress-strain plot of the orientation and the uniaxially drawn multilayer PP film/foam samples are shown in Figure 2. It can be seen from Figure 2(a) that the sample exhibits a large fracture strain (1250 %), which is due to the linear chain structure of PP. The PP film/foam samples were collected at different orientation strains, as shown in Figure 2(b). As mentioned above, the mostly deformed zone on each sample was selected for the microstructure and properties measurement, and the deformation of this zone is reflected by using draw ratio (DR). Interestingly, the surface of oriented film/foam samples become very reflective with increasing deformation, which is mainly due to the diffuse reflection of light from the rough surface of foam samples. 3.2. Structure-Properties Relationship of Oriented Multilayer PP Film/Foams 3.2.1

Effect of orientation on microstructure and crystallization of PP multilayer

film/foam Figure 4 shows the microstructures of multilayer film/foams with different DR. It can be observed that the cells in foam layers get elongated to the drawing direction with increasing DR, and the aspect ratio of cells increases as the draw ratio increases. The similar phenomena were also observed during the uniaxial drawing of LDPE/LDPE film/foams10. In addition, cells are densified and elongated at higher strain. After the densification strain, fractures take place in cell walls and produces cracks in film layers, which eventually causes rupture of the whole film/foam samples. It is important to mention here that DR of 9.0 was chosen as the maximum draw ratio as it is evident that the large fractures are created on the surface of samples which deteriorate the structural integrity for further characterization.

12


Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. SEM images of PP multilayer film/foams with different DR and the change in aspect ratio of cells as the function of DR. It is evident from Figure 4 that both cell walls and film layers in PP multilayer film/foam are oriented as the DR increases during the uniaxial drawing. In addition, this drawing can also lead to the chain orientation of PP, and eventually influences both its crystallization kinetics and the final polymorphic morphology 29, 30. Therefore, we also investigated the crystallinity and the crystal orientation of PP in the multilayer film/foam samples with different DR by using DSC and XRD, respectively. Figure 5 shows the results obtained from DSC studies. It can be noticed that the melting peak for PP is evident in each thermogram and a small hump close to the main melting peak can be observed which is attributed to β crystallites of PP 31. The area under the melting peak changes with the draw ratios, indicating the variation of the crystallinity of the samples 29, 30. Figure 5(b) shows that crystallinity changes from 34% to a maximum of 46% for the samples with DR= 9.0. There is a sharp increase from 34% to 42% when DR increases from 1 to 2.5, indicating that the crystallinity of PP in multilayer film/foams enhances most significantly at low orientation. Such increase in crystallinity will affect the strength and stiffness of oriented film/foam samples, which are discussed later.

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 14 of 23

(b)

Figure 5. (a) DSC thermograms of as-extruded and oriented multilayer PP/PP film/foams obtained from the first heating scan. (b) The effect of draw ratios on the crystallinity of oriented film/foam samples. The effects of uniaxial orientation on the orientation of crystal domains of PP were evaluated by comparing the WAXD patterns of as-extruded film/foam sample with the oriented samples at different DR, which is shown in Figure 6. For the as-extruded sample, isotropic ring X-ray diffraction can be observed for the (110), (130), and (111/041) reflection plains, representing random crystal orientation of PP crystal in as-extruded sample. In comparison, the oriented samples show sharp peaks on the equator at 2θ =14.1⁰, 17.0⁰ and 18.5⁰, referring to PP (110), (040), and (130) planes of PP α crystals, respectively 32, 33. The reflections get sharper with the increase in DR. Compared to as-extruded PP/PP film/foam samples, oriented samples at DR= 9.0 exhibited significant preferential orientation in the stretching direction. Such shaper reflections of crystal planes indicate their polymer chain orientation. As mentioned in Section 2.2.3, we use Herman’s orientation factor (fc) to characterize the extent of crystal alignment in the drawing direction of a drawn PP multilayer film/foam, and the variation of fc with DR of the oriented PP multilayer film/foam samples is shown in Figure 6(e). It can be seen that all the uniaxially drawn samples have highly oriented α crystals of PP along the longitudinal direction. Considering fc = 1 represents a fully oriented sample, the crystal orientation of the sample with DR of 9.0 is almost fully aligned along the longitudinal direction during the orientation process. 14


Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Effect of uniaxial drawing (at 140 oC, 100%/min) on the crystal orientation of PP multilayer film/foam samples: (a) as-extruded sample; (b) DR = 2.5; (c) DR = 4.0; (d) DR = 9.0; (e) The Herman’s orientation factor as the function of DR. 3.2.2 Structure-Bulk Density Relationship of Oriented Samples The change in bulk density of oriented samples as a function of their DR has been studied. Figure 7 shows the changes in bulk density as a function of local strain variation, which is directly related to the DR of samples. The bulk density of PP film/foams decreases with increasing stain of up to DR = 3.5 and above DR = 3.5 the 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

bulk density slowly increases with increasing draw ratio (DR = 9, ρ = 0.48 g/cm3). To further investigate the mechanism of density variation in oriented samples. SEM analyses were carried out at higher magnification at the cross-section of oriented samples in order to correlate the structural changes to the density change of the samples.

Figure 7. Variation in bulk density of oriented multilayer PP/PP film/foams as the function of DR. Figure 8 shows the SEM images of the cross-sections of oriented PP multilayer film/foams with different DR, and it reveals some interesting features of the oriented samples. It can be observed that at a DR of 2.5, cells are more elongated and small fractures are evident in cell walls which produce interconnected cellular morphology (Figure 8(a)). The fractures in cell walls are in the size range of 0.5 to 1 µm. As the draw ratio increases to DR = 3.5 such fractures are transformed into large pores (5 to 10 µm) and interconnected cellular morphology becomes more evident (Figure 8(b)). Such interconnected subcellular structures are evident in samples having DR > 4.0. On the contrary, through pores can be observed in samples having DR = 4 (Figure 8(c)). This is due to further increase in strain where the samples strain hardens (as shown in Figure 3(a)). Thus, the mechanism for initial reduction in bulk density can be attributed to the formation subcellular interconnected porous structure without rupturing the upper 16


Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


layers thus increasing the volume and reducing the density from 0.55 to 0.35 g/cm3. On the other hand, with further increase in DR (i.e. DR > 4.0) through pores are created and thus losing the air inside cells. Thus, the bulk density increases slowly to 5.0 g/cm3 at a DR of 9.0 due to the presence of more solid components and increased crystallinity of PP.

Figure 8. Mechanism of the density variation in uniaxially oriented PP/PP film/foams. Schematic shows the change in cellular morphology at different DR i.e. (a) DR=2.0, (b) DR=3.5, (c) DR>4.0 and corresponding changes in bulk density of oriented PP/PP film/foam samples. 3.2.3 Structure-mechanical properties relationship of oriented samples Figure 9 shows the tensile properties of as-extruded PP multilayer film/foam sample and the oriented multilayer film/foam sample with different DR. It can be seen that the stress-strain behavior of as-extruded PP multilayer film/foams show a gradual yielding leading to a linear plastic deformation which is typical to multilayer PP/PP film/foams. The Young’s modulus and tensile strength of the oriented samples both increase with the increase in DR (see Figure 9(b) and Figure 9(c)), and the sharpest increase appears between DR=1.0 to DR=6.0. The DSC results and WAXD studies on 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

oriented samples reveal that the crystallinity and the oriented α crystal lamella of PP increase significantly at this range, indicating that the increase in Young’s modulus and strength of oriented PP/PP film/foam samples are correlated to the increased crystallinity as well as the orientation of α crystal lamella in PP. This conclusion is similar with the previous studies about the effect of drawing on the crystal structure of oriented PP films and fibers

33-35

. The mechanical properties of the oriented PP

multilayer film/foams do not change significantly when the DR of is high than 6.0, attributing to the small increase in the crystallinity as well as the crystal orientation of PP. On the other hand, the cavities on the film layers and the through pores formed on the oriented PP/PP film/foam samples at the high DR have a deteriorate effect on the tensile strength of the sample, which offsets the strengthening effect of the PP crystal.

Figure 9. Effect of orientation on the mechanical properties of uniaxially drawn multilayer PP/PP film/foam samples. (a) stress-strain behavior of uniaxially drawn 18


Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


samples with different DR, (b) Young’s modulus and (c) Stress at break as the function of draw ratio. From the above results, we can see that for the oriented PP multilayer film/foams drawn at temperature of 140 oC and speed of 100%/min, the DR of 4.0 is the best orientation degree to obtain the samples with lowest density (≈0.35 g/cm3) and the relatively ideal mechanical properties (modulus = 400 MPa, tensile strength = 27 MPa). 4. CONCLUSIONS By orienting both microstructural layered morphology of PP multilayer film/foam and the crystal lamella in PP, while maintaining the layered cellular structure, a novel strong and lightweight material was created. Moreover, we carefully studied the effects of orientation on the structure and properties of PP/PP film/foam, and a structure-property relationship of the oriented PP multilayer film/foam was proposed. With the increase in DR, the density of the PP multilayer film/foam material decreases first till DR = 3.5, attributing to the formation of the subcellular interconnected porous structure at the low DR. At the higher DR (DR > 4.0), the density of the sample started to rise with the increase in DR due to the formation of the through pores and the escape of the air inside cells. Because of the enhanced crystallinity as well as the crystal orientation of PP chains during drawing, the tensile strength and modulus of oriented multilayer film/foam improve significantly with the increase in DR, but the mechanical properties are deteriorated because of the break of the film layers at a relative high strain (DR > 6.0). Therefore, for the oriented PP multilayer film/foams drawn at temperature of 140 oC and speed of 100 %/min, the DR of 4.0 is the best orientation degree to obtain the samples with lowest density (0.35 g/cm3) and the relatively ideal mechanical properties (Modulus = 400 MPa, Tensile strength = 27 MPa). Such oriented multilayer PP/PP film/foams can also have interesting sound absorbing and heat insulation properties, which require further investigations.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected] (Md. Arifur Rahman)

ACKNOWLEDGEMENT: Financial support from the NSF Science and Technology Center for Layered Polymeric Systems (Grant 0423914) is gratefully acknowledged. We also would like to acknowledge China Scholarship Council for financial support.

REFERENCES (1) Saiz-Arroyo, C.; Rodríguez-Pérez, M. A.; Tirado, J.; López-Gil, A.; Saja, J. A., Structure– property relationships of medium-density polypropylene foams. Polym. Inte. 2013, 62, 1324-1333. (2) Zhai, W.; Wang, H.; Yu, J.; Dong, J.; He, J., Cell coalescence suppressed by crosslinking structure in polypropylene microcellular foaming. Polym. Eng. Sci. 2008, 48, 1312-1321. (3) Zheng, W.; Lee, Y.; Park, C., Use of nanoparticles for improving the foaming behaviors of linear PP. J. Appl. Polym. Sci. 2010, 117, 2972-2979. (4) Lee, Y. D.; Wang, L. F., Properties of polypropylene structural foam crosslinked by vinyltrimethoxy silane. J. Appl. Polym. Sci. 1986, 32, 4639-4647. (5) Wolinski, L. E.; Harris, R. G., Oriented foam laminar structures. U.S. Patent 3,619,344, Nov 9, 1971. (6) Dukess, J., Method of co-extrusion with foam core. U.S. Patent 4,206,165, Jun 9, 1980. (7) Passaro, A.; Corvaglia, P.; Manni, O.; Barone, L.; Maffezzoli, A., Processing-properties relationship of sandwich panels with polypropylene-core and polypropylene-matrix composite skins. Polym. Composite. 2004, 25, 307-318. (8) Ranade, A. P.; Hiltner, A.; Baer, E.; Bland, D. G., Structure-property relationships in coextruded foam/film microlayers. J. Cell. Plast. 2004, 40, 497-507. (9) Barger, M. A.; Bland, D.; Mazor, M. H.; Baer, E.; Dooley, J.; Garcia, J. A., Anisotropic foam-film composite structures. U.S. Patents 7,993,739, Aug 9, 2011. (10) Rahman, M. A.; Andrade, R.; Maia, J.; Baer, E., Viscosity contrast effects on the structure– Property relationship of multilayer soft film/foams. Polymer 2015, 69, 110-122. (11) Rahman, M.A. and Baer, E. Layer Integrity in Polyethylene Based Multilayer Film/Foams and Their Applications. SPE ANTEC Orlando, 2015, 2789-2794. (12) Zhao, T., Yang, M., Wu, H., Guo, S., Sun, X. and Liang, W., Preparation of a new foam/film structure

poly

(ethylene-co-octene)

foam

materials

and

its

sound

absorption

properties. Materials Letters, 2015, 139, 275-278. 20


Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13) Lee, S., Maia, J., Dayton, O.H. Improving the mechanical properties and flame retardancy of multilayered PP foam/films via the introduction of flame retardants. SPE ANTEC Indianapolis, 2016, 1837-1841. (14) Rahman, M. A., Wang, J., Zhang, C., Olah, A., & Baer, E. Novel micro-/nano-porous cellular membranes by forced assembly co-extrusion technology. European Polymer Journal, 2016, 83, 99-113. (15) Lin, Y.; Hiltner, A.; Baer, E., Nanolayer enhancement of biaxially oriented polypropylene film for increased gas barrier. Polymer 2010, 51, 5807-5814. (16) Dias, P.; Lin, Y.; Hiltner, A.; Baer, E.; Chen, H.; Chum, S., Effect of chain architecture on biaxial orientation and oxygen permeability of polypropylene film. J. Appl. Polym. Sci. 2008, 107, 1730-1736. (17) Chu, F.; Yamaoka, T.; Kimura, Y., Crystal transformation and micropore formation during uniaxial drawing of β-form polypropylene film. Polymer 1995, 36, 2523-2530. (18) Uejo, H.; Hoshino, S., Structure of biaxially oriented polypropylene film. J. Appl. Polym. Sci. 1970, 14, 317-328. (19) Benning, C. J., Polyethylene Foam III-" Orientation in Thermoplastic Foams". J. Cell. Plast. 1967, 3, 174-184. (20) O'kane, W. J.; Young, R. J.; Ryan, A. J., The effect of annealing on the structure and properties of isotactic polypropylene films. J. Macromol. Sci. B 1995, 34, 427-458. (21) Lorenzo, A. T.; Arnal, M. L.; Sánchez, J. J.; Müller, A. J., Effect of annealing time on the self ‐ nucleation behavior of semicrystalline polymers. J. Macromol. Sci. B 2006, 44, 1738-1750. (22) Ran, S.; Zong, X.; Fang, D.; Hsiao, B. S.; Chu, B.; Phillips, R. A., Structural and morphological studies of isotactic polypropylene fibers during heat/draw deformation by in-situ synchrotron SAXS/WAXD. Macromolecules 2001, 34, 2569-2578. (23) Li, J.; Cheung, W.; Jia, D., A study on the heat of fusion of β-polypropylene. Polymer 1999, 40(5), 1219-1222. (24) Hu, Y.; Topolkaraev, V.; Hiltner, A.; Baer, E., Measurement of water vapor transmission rate in highly permeable films. J. Appl. Polym. Sci. 2001, 81, 1624-1633. (25) Li, J.; Cheung, W., A correction function to determine the β-fusion heat in a mixture of α-and β-PP. J. Therm. Anal. Calorim. 2000, 61, 757-762. (26) Stepaniak, R.; Garton, A.; Carlsson, D.; Wiles, D., The characterization of nylon 6 filaments by x‐ray diffraction. J. Appl. Polym. Sci. 1979, 23, 1747-1757. (27) Loos, J.; Schimanski, T., Effect of postdrawing temperature on structure, morphology, and mechanical properties of melt-spun isotactic polypropylene tapes. Macromolecules 2005, 38, 10678-10685. (28) Turetskii, A.; Chvalun, S.; Baranoy, A.; Zubov, Y. A.; Prut, E., Structure and physico-mechanical properties of isotactic polypropylene oriented at various temperatures. Polym. Sci. U.S.S.R 1988, 30, 1993-1999. (29) Somani, R.; Yang, L.; Sics, I.; Hsiao, B.; Pogodina, N.; Winter, H.; Agarwal, P.; Fruitwala, H.; Tsou, A. In Orientation-induced crystallization in isotactic polypropylene melt by step-shear deformation, Macromol Symp, 2002, 105-117.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

(30) Nozue, Y.; Shinohara, Y.; Ogawa, Y.; Sakurai, T.; Hori, H.; Kasahara, T.; Yamaguchi, N.; Yagi, N.; Amemiya, Y., Deformation behavior of isotactic polypropylene spherulite during hot drawing investigated by simultaneous microbeam SAXS-WAXS and POM measurement. Macromolecules 2007, 40, 2036-2045. (31) Offord, G. T.; Armstrong, S. R.; Freeman, B. D.; Baer, E.; Hiltner, A.; Swinnea, J. S.; Paul, D. R., Porosity enhancement in β nucleated isotactic polypropylene stretched films by thermal annealing. Polymer 2013, 54, 2577-2589. (32) Huang, M. R.; Li, X. G.; Fang, B. R., β‐nucleators and β‐crystalline form of isotactic polypropylene. J. Appl. Polym. Sci. 1995, 56, 1323-1337. (33) Sadeghi, F.; Carreau, P. J., Properties of uniaxially stretched polypropylene films. Can. J.Chem. Eng. 2008, 86, 1103-1110. (34) Alcock, B.; Cabrera, N.; Barkoula, N.-M.; Reynolds, C.; Govaert, L.; Peijs, T., The effect of temperature and strain rate on the mechanical properties of highly oriented polypropylene tapes and all-polypropylene composites. Compos. Sci. Technol. 2007, 67, 2061-2070. (35) Ward, I., Optical and mechanical anisotropy in crystalline polymers. Proceedings of the Physical Society 1962, 80, 1176.

22


Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


“For Table of Contents Only”

23


Foam

Recommend Documents