Tài liệu Structure Development and Mechanical Performance of Polypropylene docx

Structure Development and Mechanical Performance

of Polypropylene

Structure Development and Mechanical Performance of Polypropylene by Tim B. van Erp.

Technische Universiteit Eindhoven, 2012.

A catalogue record is available from the Eindhoven University of Technology Library

ISBN: 978-90-386-3164-6

Reproduction: University Press Facilities, Eindhoven, The Netherlands.

Cover design: Paul Verspaget (Verspaget & Bruinink) and Tim van Erp

This research is part of the research programme of the Dutch Technology Foundation STW,

”Predicting Catastrophic Failure of Semi-Crystalline Polymer Products”.

Structure Development and Mechanical Performance

of Polypropylene

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de

Technische Universiteit Eindhoven, op gezag van de

rector magnificus, prof.dr.ir. C.J. van Duijn, voor een

commissie aangewezen door het College voor

Promoties in het openbaar te verdedigen

op donderdag 5 juli 2012 om 16.00 uur

door

Tim Bernardus van Erp

geboren te Helmond

Dit proefschrift is goedgekeurd door de promotoren:

prof.dr.ir. G.W.M. Peters

en

prof.dr.ir. H.E.H. Meijer

Copromotor:

dr.ir. L.E. Govaert

Contents

Summary ix

Introduction 1

Background . .................................................................................... 1

Processing-Structure-Properties Relation ..................................................... 3

Scope of the Thesis............................................................................. 5

References ...................................................................................... 6

1 Quantification of Non-Isothermal, Multi-Phase Crystallization 7

1.1 Introduction .............................................................................. 8

1.2 Theory .................................................................................... 9

1.3 Experimental ............................................................................. 12

1.3.1 Materials. ........................................................................ 12

1.3.2 Fast Cooling Experiments ...................................................... 12

1.3.3 Differential Fast Scanning Calorimetry . ....................................... 13

1.3.4 Multipass Rheometer (MPR) ................................................... 13

1.3.5 Dilatometry . ..................................................................... 13

1.3.6 X-Ray ............................................................................ 14

1.4 Results and Discussion .................................................................. 15

1.4.1 Experimental Approach. ........................................................ 15

1.4.2 Fast Cooling Experiments ...................................................... 16

1.4.3 Pressurized Cooling Experiments .............................................. 21

1.4.4 Dilatometry . ..................................................................... 23

1.5 Conclusions .............................................................................. 27

References. ............................................................................... 27

v

vi Contents

2 Rate, Temperature and Structure Dependent Yield Kinetics 31

2.1 Introduction .............................................................................. 32

2.2 Experimental ............................................................................. 33

2.2.1 Materials. ........................................................................ 33

2.2.2 Fast Cooling . .................................................................... 34

2.2.3 X-Ray ............................................................................ 34

2.2.4 Mechanical Testing ............................................................. 35

2.3 Results .................................................................................... 35

2.3.1 Processing - Structure Relation................................................. 35

2.3.2 Yield Kinetics ................................................................... 38

2.3.3 Time-to-Failure. ................................................................. 42

2.3.4 Structure - Properties Relation ................................................. 43

2.3.5 Discussion . ...................................................................... 45

2.4 Conclusions .............................................................................. 48

References. ............................................................................... 49

3 Structure Development during Cooling at Elevated Pressure and Shear Flow 53

3.1 Introduction .............................................................................. 54

3.2 Experimental ............................................................................. 55

3.2.1 Material .......................................................................... 55

3.2.2 Dilatometry . ..................................................................... 55

3.2.3 X-Ray ............................................................................ 57

3.2.4 Transmission Electron Microscopy (TEM) .................................... 58

3.3 Methods .................................................................................. 58

3.3.1 Normalized Specific Volume ................................................... 58

3.3.2 Weissenberg Number ........................................................... 59

3.3.3 Dimensionless Numbers ........................................................ 60

3.4 Results and Discussion .................................................................. 60

3.4.1 Dilatometry . ..................................................................... 60

3.4.2 Morphology ..................................................................... 65

3.5 Conclusions .............................................................................. 73

References. ............................................................................... 73

Contents vii

4 The Oriented Gamma Phase 77

4.1 Introduction .............................................................................. 78

4.2 Experimental ............................................................................. 79

4.3 Results and Discussion .................................................................. 79

4.4 Conclusions .............................................................................. 83

References. ............................................................................... 83

5 Flow-Enhanced Crystallization Kinetics during Cooling at Elevated Pressure 85

5.1 Introduction .............................................................................. 86

5.2 Experimental ............................................................................. 87

5.2.1 Material . ......................................................................... 87

5.2.2 Dilatometry . ..................................................................... 87

5.2.3 X-Ray ............................................................................ 88

5.3 Methods .................................................................................. 88

5.3.1 Normalized Specific Volume ................................................... 88

5.3.2 Weissenberg Number ........................................................... 89

5.3.3 Dimensionless Numbers ........................................................ 90

5.4 Modeling ................................................................................. 90

5.4.1 Quiescent Crystallization ....................................................... 90

5.4.2 Flow Effects on Crystallization. ................................................ 92

5.5 Results and Discussion .................................................................. 94

5.6 Conclusions .............................................................................. 99

References. ............................................................................... 100

5.7 APPENDIX .............................................................................. 102

6 Prediction of Yield and Long-Term Failure of Oriented Polypropylene 103

6.1 Introduction .............................................................................. 104

6.2 Experimental ............................................................................. 105

6.2.1 Material . ......................................................................... 105

6.2.2 Mechanical Testing ............................................................. 106

6.3 Experimental Results .................................................................... 106

6.4 Constitutive Modeling ................................................................... 108

6.4.1 Viscoplastic Model.............................................................. 108

6.4.2 Equivalent Stress ................................................................ 109

6.4.3 Flow Function . .................................................................. 110

6.4.4 Time-to-Failure. ................................................................. 110

viii Contents

6.5 Model Application ....................................................................... 112

6.5.1 Characterization . ................................................................ 112

6.5.2 Validation . ....................................................................... 113

6.6 Conclusions .............................................................................. 114

References. ............................................................................... 116

7 Mechanical Performance of Injection Molded Polypropylene 117

7.1 Introduction .............................................................................. 118

7.2 Experimental ............................................................................. 119

7.2.1 Material .......................................................................... 119

7.2.2 Injection Molding ............................................................... 120

7.2.3 Optical Microscopy ............................................................. 120

7.2.4 Fourier Transform InfraRed (FTIR) Spectrometry ............................ 120

7.2.5 X-Ray ............................................................................ 121

7.2.6 Mechanical Testing ............................................................. 121

7.3 Results and Discussion .................................................................. 121

7.3.1 Microstructure. .................................................................. 121

7.3.2 Mechanical Properties .......................................................... 124

7.3.3 Model Application .............................................................. 126

7.4 Conclusions .............................................................................. 129

References. ............................................................................... 130

Conclusions and Recommendations 133

Conclusions. .................................................................................... 133

Recommendations . ............................................................................. 134

References ...................................................................................... 137

Samenvatting 139

Dankwoord 141

Curriculum Vitae 143

List of Publications 145

Summary

Polymers are known for their ease of processability via automated mass production technologies.

The most important process is injection molding that, due to its freedom in material choice and

product design, allows producing a wide variety of thermoplastic products. Mechanical failure

of these products, either upon impact or after prolonged exposure to load, limits their ultimate

useful lifetime. To predict and control lifetime, understanding of the route from production to

failure, i.e. the processing-structure-property relation, is necessary. This is a complex issue;

especially in the case of semi-crystalline polymers. These are heterogeneous systems comprised

of amorphous and crystalline fractions, of which the latter can be highly anisotropic with size and

orientation that are strongly dependent on the precise processing conditions. As a consequence,

these structural features in the microstructure, and the associated mechanical properties, generally

exhibit distributions containing different orientations throughout a single processed product.

Understanding polymer solidification under realistic processing conditions is a prerequisite to

predict final polymer properties, since only a complete characterization of the morphology distri￾bution within a product can lead to a meaningful and interpretable mechanical characterization.

In this thesis we study the relation between processing conditions, morphology and mechanical

performance of a semi-crystalline polymer, isotactic polypropylene. Key issue is the accurate

control over all relevant processing parameters. Therefore, different experimental techniques are

used to obtain samples at different high cooling rates, at elevated pressures, and high shear rates.

A custom designed dilatometer (PVT-T˙

-γ˙-apparatus) proves to represent the most important and

useful technique.

First, a predictive, quantitative model is presented for the crystallization kinetics of the multiple

crystal structures of polypropylene, under quiescent conditions. The approach is based on the

nucleation rate and the individual growth rate of spherulites of each type of polymorphism (α-, β-,

γ- and mesomorphic phase), during non-isothermal, isobaric solidification. Using Schneider’s rate

equations, the degree of crystallinity and distribution of crystal structures and lamellar thickness

is predicted. Next, the effect of flow is introduced. Flow strongly influences the kinetics of

the crystallization process, especially that of nucleation. Three regimes are observed in the

experiments; quiescent crystallization, flow enhanced point nucleation and flow-induced creation

of oriented structures. To assess the structure development under flow, a molecular-based rheology

model is used. Combining the models derived for quiescent and for flow-induced crystallization,

yields the tool that is capable of predicting the volume distributions of both isotropic and oriented

structures, under realistic processing conditions.

ix

x Summary

The kinetics of mechanical deformations strongly depend on the anisotropy in the crystalline

morphology, thus the local orientation. To study this, uniaxially oriented tapes with a well defined,

and high, degree of anisotropy are used as well as injection molded rectangular plates. Yield and

failure are described using an anisotropic viscoplastic model, applying a viscoplastic flow rule. It

uses the equivalent stress in Hill’s anisotropic yield criterion, and combines the Eyring flow theory

with a critical equivalent strain. Factorization is used and the model is capable to quantitatively

predict the rate, the angle and the draw ratio dependence of the yield stress, as well as the time-to￾failure in various off-axis tensile loading conditions. To use the model, also for other polymers,

characterization of only the isotropic state is sufficient. Therefore, the influence of the cooling

rate on the deformation kinetics is studied in-depth on isotropic systems. Different cooling rates

induce different crystal phases, both the stable α-phase and the mesomorphic phase, while also the

degree of crystallinity and lamellar thickness are influenced. The deformation kinetics prove to be

the same for the different microstructures, which means that the activation volume and energy are

independent of the thermodynamic state. Differences in thermal history are, consequently, solely

captured by two rate constants which are a function of the microstructure.

Introduction

Background

Nowadays, plastics are prominent in our society due to their very wide range of application in

various products and sectors. From a historical perspective this is quite impressive. Compared to

the more traditional bulk materials, the mass production of plastics actually just started, ca. 100

years. In contrast, wood and clay have been used since the existence of mankind, glass for 5500

years, iron for 3500 years, paper for 2000 years and cement for 200 years. Over the last decades,

the worldwide production of plastics has exploded from ∼1 Billion liters in 1950 to ∼240 Billion

liters in 2007. Already in the late 1980s the volume of plastics produced exceeded that of steel

(see Figure 1). The share of plastics has been increasing at the expense of the other bulk materials

and the drivers in this growing demand of plastics are manifold. Economic growth, the increasing

wealth in newly industrialized and developing countries play an important role. This increase is

also partly a result of new needs, which can best be fulfilled by plastics, e.g. safety devices such as

airbags or certain medical devices and implants. Another important driver is material substitution,

e.g. the replacement of glass by polymers in consumer goods such as computer screens and the

replacement of the traditional packaging materials like paper or board. In general, the cost balance

for production and processing of the competing materials is decisive.

Figure 1: Historical world production of plastics and steel [1].

The dynamic development in the demand of plastics is mostly covered by the ”commodity

thermoplastics” PVC, PP, PE and PS [2]. While in the 70s and 80s it was assumed that ”high

performance polymers” would gain an increasing share of the total polymer market, the dominant

1

2 Introduction

market position of commodity thermoplastics has increasingly consolidated itself since then,

see Figure 2. To a great extent this results from a continuous development and modification

of these materials; modern industrial policies demand to achieve this goal without developing

”new” polymers but, instead, making use of ”old” polymers that are based on relatively cheap

and readily available monomers [3]. In addition, another possible explanation is given by the so￾called ”experience or learning curve” which predicts that by doubling cumulative production a

cost reduction of 20-30% is achievable simply by becoming more experienced with the product.

reality

1998

prediction

1975 for 1996

reality

1975

27 Mio t 122 Mio t

HDPE LDPE/LLDPE PP PS PVS

PC PBT PET PA

ABS POM PMMA

LCP

PEEK

PPS PAR PES

88%

12%

<<1%

commodity plastics; 86%

engineering plastics; 14%

high performance plastics; <<1%

Figure 2: Share of commodity-, engineering and high performance thermoplastics in the global

consumption [2].

Among the ”commodity thermoplastics” an important class of polymers are the polyolefins;

mainly PE and PP. The basis of the dynamic development of polyolefins and their still tremendous

potential lies in [4]:

• Their versatility with respect to physical and mechanical properties and applications.

• Their nontoxicity and bioacceptability.

• The energy savings during their production and use, in comparison with other materials.

• Their low cost and the easily available raw materials.

• Their economic, versatile, and nonpolluting production.

The influence of the oil price on the price of petrochemical polymers like polyolefins needs special

attention. From the experience curve theory it is expected that production cost go down over

time due to gained experience. However, this relation can be masked when the costs are mainly

determined by feedstocks which fluctuate in price over time; for bulk polymers like PE and PP

the main feedstock is crude oil. The price of oil is to a large extent correlated (up to 82% for PP)

to the polymer prices showing the importance of the oil feedstock [5]. In view of the high and

ever growing production of plastics, the substantial concomitant environmental impacts and, more

recently, very high oil prices, the replacement of petrochemical plastics by bio-based plastics is

receiving increasing attention [6].

Processing-Structure-Properties Relation 3

The relative size of end-use applications remained fairly stable the last decade with packaging

remaining the largest segment and representing 39% of the overall demand, see Figure 3. However,

this share is lower than the year before (40.1%) due to a higher growth of technical applications in

2010 over 2009. The packaging sector is followed by building & construction (20.6%), automotive

(7.5%) and electrical & electronic equipment (5.6%). ”Others” (27.3%) include various sectors

such as sport, health and safety, leisure, agriculture, machinery engineering, household appliances

and furniture.

PE-LD

PE-LLD

PE-HD PP PS PS-E PVC ABS,

SAN

PMMA PA PET Other PUR

Packaging

Building &

Construction

Automotive

Electrical &

Electronic equipment

Other

Total: 46.4 Mtonne

39.0%

20.6%

7.5%

5.6%

27.3%

Figure 3: Europe plastics demand in 2010 by segments [7].

Society has always quested for new materials that can fulfill new needs or replace existing

materials with ones possessing superior performance and have worked diligently throughout

history to create new materials. Currently, the quest is not just seeking for strong materials, the

desired materials should possess the added value of light weight. Therefore, materials that possess

great specific modulus and strength are nowadays required. This quest comes especially from

fields like transportation, architecture, medical care and social welfare. An illustrative example

is the social and technological requirements and purposes like the reduction of fuel consumption

by automobiles for environmental protection and fuel cost reduction. In Figure 4 it is shown that

bulk polymers have a rather poor position in the specific strength-modulus window compared to

e.g. glass, metals and ceramics. However, from these bulk polymers, and in particular semi￾crystalline polymers, materials can be produced like glass reinforced polymers, high-strength

fibers and composites. As such, their increased specific strength and modulus is competitive with

materials frequently used where high specific strength or modulus is required.

Processing-Structure-Properties Relation

As pointed out, polyolefins constitute an extremely interesting family of materials including large￾volume materials such as polyethylene and polypropylene. For these semi-crystalline polymers

injection molding is one of the most widely employed mass production methods for manufacturing

products. The properties of injection molded products of semi-crystalline polymers strongly

depend on the final morphology, which itself depends on the complete (processing) history of the

4 Introduction

1 10 100 1,000 10,000

1

10

100

1,000

0.1

0.01

SPECIFIC STIFFNESS

(MNm/kg)

Good stiffness￾to-weight ratio Poor stiffness- to-weight ratio

SPECIFIC STRENGTH

(kNm/kg)

Good strength￾to-weight ratio

Poor strength￾to-weight ratio

Rubbers

Foams

Polymers

Nylon

Composites

Ceramics

Metals

and alloys

Glasses

UF

PMMA

Polypropylene

Polyethylene

Figure 4: Specific modulus versus specific strength for different materials.

material, as illustrated in Figure 5. This includes the polymerization, determining the molecular

characteristics, and the thermomechanical history experienced during processing. Understanding

every step from synthesis via processing to the resulting product properties could lead, eventually,

to materials with properties tailored to the application. This thesis studies part of this process,

mainly using polypropylene-based materials, and focuses on the influence of processing on

morphology and morphology on resulting properties.

chemical

composition

material

formulation

additives

processing

thermomechanical

history

crystallization (mechanical)

properties

catalyst

reactor

chain

structure

molecular weigth

(distribution)

this thesis

Figure 5: Flow chart illustrating the processing-structure-properties relationship in semi-crystalline

polymeric materials.

Mechanical performance of polymers is known to be influenced by its molecular properties such

as the molecular weight distribution and its underlying morphology as a result of macromolecular

orientation and thermal history, i.e. factors that are directly connected to processing conditions.

The latter is particularly true for semi-crystalline polymers in which structural features, such as

the degree of crystallinity, crystal size and orientation, may drastically vary depending on the

manner in which the polymer is shaped into the final product. A particularly illustrative example

is given in Figure 6, which shows an injection molded plate of high-density polyethylene (HDPE),

revealing the well-known oriented layers of different thickness at various locations along the

flow path [8]. The observed differences in the microstructure have a dramatic influence on the

macroscopic mechanical properties of samples cut at different loci from the object, which range

Scope of the Thesis 5

from brittle fracture to necking and homogeneous deformation (sections A, B and C resp.). From

this simple example the complexity of the processing-structure-property relation becomes clear,

and it is, therefore, evident that the ability to predict the mechanical properties of polymer products

is uniquely linked to the capability to assess the development of the various structures during

processing within a product.

70 x 70 x 1 mm

injection of

polymer

A B

C

A

B

C

Figure 6: Variation in microstructure over the thickness in a simple product, and the resulting different

mechanical responses of samples cut from different parts of a typical injection molded plaque of

high-density polyethylene.

Scope of the Thesis

Catastrophic failure of polymer artifacts, either upon impact (e.g. of protective products such as

airbags and helmets) or after prolonged exposure to load (for instance supporting structures, high￾pressure pipes), limits their ultimate useful lifetime. Hence, understanding of that process, and,

ideally, being able to accurately predict when and under which circumstances this phenomenon

occurs, is of critical importance, not only for the selection of the materials employed in such

objects, but also for their optimal design for safe use. This issue is especially complex in the

case of semi-crystalline polymers, which are heterogeneous systems comprised of an amorphous

matrix with highly anisotropic crystallites of a size and orientation that are dependent on the

molecular weight distribution and the conditions under which the material is processed. As a

consequence, these structural features, and the associated mechanical properties, generally exhibit

strong variations throughout even a single processed object.