Laser Welding of Plastics E-Book

Rolf Klein

• Herausgeber: Wiley-VCH
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 292

Veröffentlichungsjahr: 2012

Beschreibung

This is the first detailed description in English of radiation and polymeric material interaction and the influences of thermal and optical material properties. As such, it provides comprehensive information on material and process characteristics as well as applications regarding plastic laser welding. The first part of this practical book introduces the structure and physical properties of plastics, before discussing the interaction of material and radiation in the NIR and IR spectral range. This is followed by an overview of the physical foundations of laser radiation and laser sources used for plastic welding. The third part describes the main processes of laser welding thermoplastics, as well as possibilities of process control, design of joint geometry, material compatibilities and adaptation of absorption of plastics to NIR radiation. Finally, the author explains applications of laser welding plastics using several industrial case studies from the automotive industry, household goods, and medical devices. Tailored to the needs of everyone dealing with laser welding of plastics, especially engineers in packaging, component manufacturing, and the medical industry.

Leseprobe

Contents

Cover

Related Titles

Title Page

Copyright

Introduction

Chapter 1: Material Properties of Plastics

1.1 Formation and Structure

1.2 Types of Plastics

1.3 Thermal Properties

1.4 Optical Properties

References

Chapter 2: Laser Sources for Plastic Welding

2.1 Properties of Laser Radiation

2.2 Types of Lasers

2.3 Beam Guiding and Focusing

2.4 Principle Setup of Laser Welding Systems

References

Chapter 3: Basics of Laser Plastic Welding

3.1 Heat Generation and Dissipation

3.2 Theory of Fusion Process

3.3 Material Compatibility

References

Chapter 4: Process of Laser Plastic Welding

4.1 Basic Process Principles

4.2 Process Types

4.3 Adaption of Absorption

4.4 Design of Joint Geometry

4.5 Methods of Quality Monitoring and Control

References

Chapter 5: Case Studies

5.1 Automotive Components

5.2 Consumer Goods

5.3 Electronic Devices

5.4 Medical Devices

5.5 Others

Index

Related Titles

Elias, H.-G.

Macromolecules

Volume 4: Applications of Polymers

727 pages with 295 figures and 193 tables

2008

Hardcover

ISBN: 978-3-527-31175-0

Kannatey-Asibu, E.

Principles of Laser Materials Processing

approx. 820 pages

Hardcover

ISBN: 978-0-470-17798-3

Harper, C. A.

Handbook of Plastic Processes

approx. 744 pages

2006

Hardcover

ISBN: 978-0-471-66255-6

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Library of Congress Card No.: applied for

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

Bibliographic information published by the Deutsche Nationalbibliothek

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.d-nb.de.

© 2012 Wiley-VCH Verlag & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany

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Print ISBN: 978-3-527-40972-3

ePDF ISBN: 978-3-527-63698-3

oBook ISBN: 978-3-527-63696-9

ePub ISBN: 978-3-527-63697-6

Introduction

Laser is a short form for “light amplification by stimulated emission of radiation.” The first theoretical description of stimulated emission of radiation was given by Einstein in 1917. It took many years for a first technical realization of a laser source based on Einstein's theory by Mainman in 1960, developing a solid-state ruby laser emitting red laser radiation. In 1972 lasers entered industrial application for metal sheet processing. From this time laser processing of metals, especially laser cutting and welding steel sheets or stainless steel, changed from an exotic processing tool to well-established industrial applications from small-size serial to large-scale production.

Up to the early 1990s, laser welding of thermoplastics was a potential but exotic way for joining plastic components. Available laser sources for plastic welding at this time were CO2 or Nd:YAG lasers having high investment costs not capable of economical industrial application. Also, the technique of through transmission laser welding (TTLW) was not developed yet.

Then, two fundamental developments were made almost simultaneously, giving a basis for introduction of laser plastic welding into industrial application: development of TTLW as a new processing technique for laser welding plastics and development of high-power diode lasers previously known as low-power laser sources produced in mass production for example, for communication technology, computer data storage or consumer goods like CD players.

The opportunity for mass production of high-power diode laser sources generating decreasing investment cost for such laser sources as well as high plug efficiency compared to other laser sources like Nd:YAG lasers enabled development of laser welding plastics in conjunction with the new TTLW process ready for introduction into the market. As a result, laser welding thermoplastic components entered the market rapidly. One of the first industrial applications for laser welding plastic components entering mass production was an electronic car key, starting production in 1997 for the new Mercedes Benz type 190.

Since that time laser welding plastics has grown rapidly as an alternative joining technology in competition with conventional joining technologies like heat contact, ultrasonic, vibration and other welding methods.

Advantages of laser welding plastic components compared to conventional joining technologies are localized heat input to the joint interface without damaging of sensitive inner components like electronics or mechanics by heat or internal mechanical forces, extremely reduced welding flash while maintaining part geometry and visual appearance as well as generating weld seams of high mechanical strength and outstanding quality.

Laser welding of thermoplastic components enables flexible production with economical benefits from small-scale production with varying geometries of the work pieces up to industrial mass production with high output rates.

Even laser welding of thermoplastics seems to be an investment-intensive production technology, considering the entire production chain using laser welding in comparison with conventional joining technologies may result in reduced efforts for component preparation and logistics as well as high joint quality and increased production output. Highly developed quality monitoring and online control during laser welding enables industrial production of thermoplastic parts by laser welding with reduced scrap rate compared to conventional joining processes. However, laser welding in industrial applications has to meet the economic conditions compared to competitive joining technologies.

This book gives a basic introduction to the principles, processes and applications of lasers for welding thermoplastic materials. The first part of the book gives an introduction into the structure and physical properties of plastics, especially to thermoplastics and thermoplastic elastomers, considering the interaction of material and radiation in the NIR and IR spectral ranges. Secondly, a brief introduction into the basics of laser radiation and laser sources used for plastic welding is given. The third part describes the main processes of laser welding thermoplastics as well as the possibilities of process control, design of joint geometry, material compatibilities and adaption of absorption of plastics to NIR radiation. The fourth part of the book will explain applications of laser welding plastics by several industrial case studies.

The book is targeted at students in physics, material science, mechanical engineering, chemistry and other technical subject areas in universities and universities of applied sciences as well as engineers in product and/or process development and production engineers in the field of automotive, consumer goods, electronics, medical devices, textiles and others who will use or already use laser welding of plastics.

I want to give special thanks to all who supported me by realization of the book. Special thanks go to Mr. Brunnecker from LPKF, Mr. Hinz from Leister and Mr. Rau from bielomatik for their support by case studies from industrial applications of laser welding plastics, pointing out the outstanding technical and economical opportunities of this process today.

Dr.-Ing. Rolf Klein, Groß-Umstadt, Germany, May 2011

Chapter 1

Material Properties of Plastics

1.1 Formation and Structure

The basic structure of plastics (or polymers) is given by macromolecule chains, formulated from monomer units by chemical reactions. Typical reactions for chain assembling are polyaddition (continuous or step wise) and condensation polymerization (polycondensation) [1] (Figure 1.1).

Polyaddition as chain reaction: Process by chemical combination of a large number of monomer molecules, in which the monomers will be combined to a chain either by orientation of the double bond or by ring splitting. No byproducts will be separated and no hydrogen atoms will be moved within the chain during the reaction. The process will be started by energy consumption (by light, heat or radiation) or by use of catalysts.

Figure 1.1 Processes for generating plastics and examples [1].

Polyaddition as step reaction: Process by combination of monomer units without a reaction of double bonds or separation of low molecular compounds. Hydrogen atoms can change position during the process.Polycondensation: Generation of plastics by build up of polyfunctional compounds. Typical small molecules like water or ammonia can be set free during the reaction. The reaction can occur as a step reaction.

The monomer units are organic carbon-based molecules. Beside carbon and hydrogen atoms as main components elements like oxygen, nitrogen, sulfur, fluorine or chlorine can be contained in the monomer unit. The type of elements, their proportion and placing in the monomer molecule gives the basis for generating different plastics, as shown in Table 1.1.

Table 1.1 Examples of some common plastics and their monomers.

The coupling between the atoms of a macromolecular chain happens by primary valence bonding [2]. The backbone of the chain is built by carbon atoms linked together by single or double bonding. Given by the electron configuration of carbon atoms, the link between the carbon atoms occurs at a certain angle, for example, for single bonding at an angle of 109.5°. Atoms like hydrogen, which are linked to the carbon atoms, hinder the free rotation of the carbon atoms around the linking axis. The “cis”-link of carbon atoms has the highest bonding energy while the "trans"-link has the lowest (Figure 1.2) [3].

Figure 1.2 Potential energy for rotation of ethylene molecules around the carbon-linking axis [3].

Depending on the type of bonding partners several chain conformations are possible. Examples of such conformations are zig-zag conformation (e.g., PE or PVC) or helix conformation (e.g., PP, POM or PTFE) (Figure 1.3) [2].

Figure 1.3 Conformation types of macromolecules.

The chain length and by this also the molecular weight of macromolecules have a statistical distribution [4] (Figure 1.4). By influencing the conditions of the polymerization process, the average molecular weight and the width of the distribution function can be controlled within certain limits.

Figure 1.4 Statistical distribution of macromolecule chain length using polyvinylchloride (PVC) as an example [4].

During the polymerization process, depending on the type of polymer, side chains can be built to the main chain in a statistical way [5]. As for the length of the main chain, frequency and length of the side chains depend on the macromolecular structure and the physical/chemical conditions of the polymerization process [6].

An example for the order of size of macromolecules is the length and width of polystyrene molecules with an average molecular weight of 105. Corresponding to the molecular weight the macromolecular chain consists of a number of approximately 2 × 105 carbon atoms. The average distance between each carbon atom is 1.26 × 10−10 m. Using this distance and the number of atoms in the chain takes to a length of 25 × 10−6 m and 4–6 × 10−10 m width for a stretched chain.

The statistical forming of the macromolecular structure of plastics results in the fact that physical properties of plastics, like temperatures of phase changes, can only be given as average values. Unlike materials like metals, phase changes of plastics occur in certain temperature ranges. The width of such temperature ranges is dependent on the homogeneity of the materials structure [6].

The physical and chemical structure of the macromolecule is given by the primary valence bonding forces between the atoms (Figure 1.5) [1]. The secondary valence bonding forces, like dispersion bonding, dipole bonding or hydrogen bridge bonds, have a direct influence to the macroscopic properties of the plastic like mechanical, thermal, optical, electrical or chemical properties.

Figure 1.5 Context of molecular and macroscopic material properties [1].

The secondary valence forces are responsible for the orientation of the macromolecules among themselves [6–8]. During processing of plastics the orientation of molecule segments can result in an orientation of segments of the macromolecular chain. Under suitable conditions, like specific placements of atoms in the monomer structure and by this within the macromolecular chain, a partial crystallization of the plastic is possible. The strength of the secondary valences is directly correlated with the formation of the macromolecular chains. The strength increases with increasing crystallization, with higher polarity between the monomer units, decreased mobility of molecule segments and increased strapping of chains with others. Because of the small range and low energy of secondary valences in comparison with the main valences, effects caused by them are strongly temperature dependent.

In the case of possible atom bonds between macromolecular chains, a crosslinking of the molecule structure can happen. While secondary valences can be dissolved with increasing temperatures and rebuilt during cooling, atom bonds cannot dissolve reversibly. By dissolving these bonds the plastic will be chemically destroyed.

Taking the chemical structure and the degree of crosslinking between the macromolecules, plastics can be classified as thermoplastics, elastomers and thermosets (Figure 1.6) [1]. Compounds like polymer blends, copolymers and composite materials are composed of several base materials. This composition can be done on a physical basis (e.g., polymer blends or composite materials) or on a chemical basis (copolymers).

Figure 1.6 Principle structure of linear (A), with side chains (B) and crosslinked macromolecules (C + D). Chain structure (A) and (B) are thermoplastic types, structures with low crosslinking (C) elastomers and with strong crosslinking thermosets (D).

1.2 Types of Plastics

Caused by the macromolecular structure and the temperature-dependent physical properties plastic materials are distinguished into different classes. Figure 1.7 gives an overview of the classification of plastics with some typical examples.

Figure 1.7 Classification of plastics.

Thermoplastics are in the application range of hard or tough elasticity and can be melted by energy input (mechanical, thermal or radiation energy). Elastomers are of soft elasticity and usually cannot be melted. Thermosets are in the application range of hard elasticity and also cannot be melted.

Plastics as polymer mixtures are composed of two or more polymers with homogeneous or heterogeneous structure. Homogeneous structures are for example copolymers or thermoplastic elastomers, built by chemical composition of two or more different monomer units in macromolecules. When using thermoplastic monomers such plastic material can be melted by thermal processes. Heterogeneous structures are for example polymer blends or thermoplastic elastomers, built by physical composition of separate phases from different polymers. Polymer blends with thermoplastic components also can be melted by thermal processes.

Plastic composites consist of a polymeric matrix with integrated particles or fibers. When using thermoplastics as matrix, such composites can be melted. If thermosets are used as matrix the composite cannot be melted.

Characteristic of the different classes of plastics are the phase transitions that occur in contrast to metallic materials in temperature intervals. Data given in tables (e.g., [9]), are usually mean values of such temperature intervals.

Phase-transition temperatures are dependent on the molecular structure of the plastic. Limited mobility of the molecule chains, for example, by loop forming, long side chains or high molecular weight cause an increased phase-transition temperature [6]. A large variance of the molecule chain length or number and length of side chains also have an effect on the spreading of the phase-transition ranges.

1.2.1 Thermoplastic Resins

Thermoplastic resins consist of macromolecular chains with no crosslinks between the chains. The macromolecular chains themselves can have statistical oriented side chains or can build statistical distributed crystalline phases. The chemistry and structure of thermoplastic resins have an influence on the chemical resistance and resistance against environmental effects like UV radiation. Naturally, thermoplastic resins can vary from optical transparency to opaque, depending on the type and structure of the material. In opaque material, the light is internally scattered by the molecular structure and direct transmission of light is very poor with increasing material thickness.

Thermoplastic resins can be reversibly melted by heating and resolidified by cooling without significant changing of mechanical and optical properties. Thus, typical industrial processes for part manufacturing are extrusion of films, sheets and profiles or molding of components.

The viscosity of the melt is dependent on the inner structure, like average molecular weight and spreading of the molecular weight around the average value. According to DIN EN ISO 1133:2005–2009 [10], the melt-flow index (MFI) is a measure for the melt viscosity. The MFI gives the amount of material that will be extruded in 10 min through a standardized nozzle diameter by using a determined force.

Low MFI values signify high viscosity with glutinous flow behavior of the melt (materials for extrusion). Increasing MFI values result in decreasing viscosity and lighter melt flow behavior (materials for molding). It has to be noted that MFI values are only a rough estimation for the melt flow behavior because the structure viscosity of thermoplastics strongly depend on the loading [11].

The macromolecular structure of thermoplastics is given by the chemical structure of the monomer units, the order of the monomer units in the molecule chain and the existing side chains. A pure statistical distribution of the macromolecules results in an amorphous material structure, but also semicrystalline structures can occur depending on the material. Therefore, thermoplastic resins are differentiated into amorphous and semicrystalline types [1, 6].

1.2.1.1 Amorphous Thermoplastics

Amorphous thermoplastic resins consist of statistical oriented macromolecules without any near order. Such resins are in general optically transparent and mostly brittle. Typical amorphous thermoplastic resins are polycarbonate (PC), polymethylmethacrylate (PMMA), polystyrene (PS) or polyvinylchloride (PVC).

Table 1.2 shows examples of amorphous thermoplastic resins with typical material properties.

Table 1.2 Examples for amorphous thermoplastic resins with typical material properties according to [1]

Temperature state for application of amorphous thermoplastic resins is the so called glass condition below the glass temperature Tg. The molecular structure is frozen in a definite shape and the mechanical properties are barely flexible and brittle (Figure 1.8).

Figure 1.8 Temperature behavior of amorphous thermoplastic resins (schematically) [1].

On exceeding the glass temperature, the mechanical strength will decrease by increased molecular mobility and the resin will become soft elastic. On reaching the flow temperature Tf the resin will come into the molten phase. Within the molten phase the decomposition of the molecular structure begins by reaching the decomposition temperature Td.

1.2.1.2 Semicrystalline Thermoplastics

Semicrystalline thermoplastic resins consist of statistical oriented macromolecule chains as amorphous phase with embedded crystalline phases, built by near-order forces. Such resins are usually opaque and tough elastic. Typical semicrystalline thermoplastic resins are polyamide (PA), polypropylene (PP) or B (POM) (Table 1.3).

Table 1.3 Examples for semicrystalline thermoplastic resins with typical material properties according to [1]

The crystallization grade of semicrystalline thermoplastic depends on the regularity of the chain structure, the molecular weight and the mobility of the molecule chains, which can be hindered by loop formation [6]. Due to the statistical chain structure of plastics complete crystallization is not feasible on a technical scale. Maximum technical crystallization grades are of the order of approximately 80% (see Table 1.3).

The process of crystallization can be controlled by the processing conditions. Quick cooling of the melt hinders crystallization. Slowly cooling or tempering at the crystallization temperature will generate an increased crystallization grade. Semicrystalline thermoplastics with low crystallization grade and small crystallite phases will be more optically transparent than materials of high crystallization grade and large crystallite phases.

Below the glass temperature Tg the amorphous phase of semicrystalline thermoplastics is frozen and the material is brittle (Figure 1.9). Above the glass temperature, usually the state of application [1], the amorphous phase thaws and the macromolecules of the amorphous phase gain more mobility. The crystalline phase still exists and the mechanical behavior of the material is tough elastic to hard. Above the crystal melt temperature Tm the crystalline phase also starts to melt and the material becomes malleable. As for amorphous thermoplastics, the flow ability of semicrystalline thermoplastics in the molten phase is characterized by the melt-flow index MFI.

Figure 1.9 Temperature behavior of semicrystalline thermoplastic resins (schematically) [1].

The melt temperature of semicrystalline thermoplastics depends among other things on the size of the crystallites and the ratio between the amorphous and crystalline phases. Larger size and a higher proportion of crystallites will increase the melt temperature (Figure 1.10) [12]. As with amorphous thermoplastics, degradation of semicrystalline thermoplastics will start in the molten phase by exceeding the decomposition temperature Td.

Figure 1.10 Influence of the crystallite size to the melt temperature for PA6 fiber material [12].

1.2.2 Elastomers

Elastomers are plastics with wide netlike crosslinking between the molecules. Usually, they cannot be melted without degradation of the molecule structure. Above the glass temperature Tg, as the state of application (Figure 1.11), elastomers are soft elastic. Below Tg they are hard elastic to brittle. The value of the glass temperature increases with increasing number of crosslinks. Examples of elastomers are butadiene resin (BR), styrene butadiene resin (SBR) or polyurethane resin (PUR) [13].

Figure 1.11 Temperature behavior of mechanical properties of elastomers (schematically) [1].

Raising temperature affects an increase of elasticity, caused by reducing the stiffening effects of the crosslinks and increasing the mobility of the molecule chains. On exceeding the decomposition temperature Td, the atom bonding within and between the molecule chains will be broken and the material will be chemical decomposed.

1.2.3 Thermosets

Thermosets are plastic resins with narrow crosslinked molecule chains [1]. Examples of thermosets are epoxy resin (EP), phenolic resin (PF) or polyester resin (UP).

In the state of application (Figure 1.12) thermosets are hard and brittle. Because of the strong resistance of molecular movement caused by the crosslinking, mechanical strength and elasticity are not temperature dependent, as with thermoplastics or elastomers.

Figure 1.12 Temperature behavior of mechanical properties of thermosets (schematically) [1].

Thermosets cannot be melted and joining by thermal processes like ultrasonic welding or laser welding is not possible. On exceeding the decomposition temperature Td, the material will be chemical decomposed.

1.2.4 Polymer Compounds

The term polymer compound summarizes materials like polymer blends, copolymers and thermoplastic elastomers (TPEs). Polymer compounds are physical or chemical composed from different polymers to achieve special material properties like elasticity or fatigue strength.

1.2.4.1 Polymer Blends

Polymer blends are combinations of different polymers [14], usually mixed in the molten state. After solidification the different polymeric proportions are combined by physical but not chemical reaction (Figure 1.13).

Figure 1.13 Schematic molecule structure of polymer blends.

The extent to which a mixture can be achieved depends on the miscibility of the polymers among each other. Chemical, thermal or mechanical properties of polymer blends are defined by the type of different polymers used and their proportions within the polymer blend.

Polymer blends, designed from thermoplastic materials, can be joined together by thermal processes like ultrasonic or laser welding. Examples of thermoplastic polymer blends are PC/ABS, PC/ASA or PPE/SB (see Table 1.4).

Table 1.4 Examples of thermoplastic polymer blends. Condition of application, specific weight and typical mechanical strength [15]

1.2.4.2 Copolymers

Copolymers are built by chemical composition at least from two different monomer units. Processes to built up copolymers are block polymerization, group transfer polymerization or graft copolymerization [1, 6, 16]. Examples of copolymers are ABS or SAN (see Table 1.5).

Table 1.5 Examples of thermoplastic copolymers. Conditions of application, specific weight and typical mechanical strength [1]

Beside grade of polymerization, chain-length distribution, type of end groups and chain side branches, composition and distribution of monomer units inside the molecule chain have to be known to achieve specific chemical, thermal, optical or mechanical properties of the copolymer. Especially influential on the properties is the regularity of the chain composition, which means a statistical or more regular distribution of the different monomers within the molecule chain (Figure 1.14) [11].

Figure 1.14 Schematic build up of copolymers.

1.2.4.3 Thermoplastic Elastomers

Thermoplastic elastomers (TPEs) are elastic, flexible polymers with similar qualities as elastomers or rubber but of a thermoplastic nature [17, 18]. TPEs close the gap between stiff thermoplastics and vulcanized elastomers. Due to the thermoplastic nature, TPEs can be processed to parts by extrusion and molding and can also be joined together or to other thermoplastic material by adhesive bonding, solvent bonding and welding processes or by coextrusion and multicomponent injection molding.

In principal, the material group of TPEs consists of two different base structures as a physical or chemical mixture, polymeric blends and block copolymers. Depending on the molecular structure given by the thermoplastic component, both of them could be amorphous or semicrystalline.

TPE blends consist of a thermoplastic matrix, for example, PP or PE, and softer particles, for example, EPDM, which are well dispersed in the matrix (see Figure 1.15). Two types of TPE blends are available:

Thermoplastic vulcanization elastomers (TPE-V): are TPE blends with a chemically crosslinked elastomer proportion produced by dynamic vulcanization that is a process of intimate melt mixing of a thermoplastic polymer like PP and a suitable reactive elastomer like EPDM.Thermoplastic polyolefin elastomers (TPE-O): two-component elastomer systems consisting of elastomers like EPR and EPDM finely dispersed in a thermoplastic polyolefin (e.g., PP).

In block copolymers, the hard and soft segments are linked within the macromolecules (Figure 1.16). Materials used as hard segments are for example, styrene and for soft segments butylenes. Common block copolymers are:

Styrene block copolymers (SBC, TPE-S): consist of block segments of styrene monomer units and elastomer monomer units. Their most common structure are linear A–B–A block type: styrene-butadiene-styrene (SBS), styrene-isoprene-styrene (SIS), styrene-ethylene/butylenes-styrene (SEBS) or styrene-ethylene/propylene-styrene (SEPS) type.

Figure 1.15 Schematic structure of TPE blends [18].

Thermoplastic polyurethane elastomers (TPE-U): were first commercialized in the 1950s and are one of the oldest TPE types in existence.Copolyester elastomers (COPE): are a family of engineering thermoplastic elastomers based on copolyester chemistry. They have both hard and soft parts. The hard segment is a semicrystalline polybutylene terephthalate (PBT), while the soft segment is made of amorphous glycol.Copolyamides (COPA, TPE-A): also called polyether block amides (PEBA), are extremely versatile, high-performance engineering thermoplastic elastomers that combine the properties of nylon and elastomers. The polymer structure consists of a regular linear chain of rigid polyamide segments, usually based on polyamide PA 6 or high-performance PA12 infiltrated with flexible polyether segments.

Depending on the type of TPE, a wide variation from very soft to more rigid materials is given. The hardness values can vary in a wide range of shore A values. Table 1.6 gives an overview about typical thermal and mechanical properties of TPEs.

Table 1.6 Examples of thermoplastic elastomers. Condition of application, specific weight and typical hardness values [18]

Table 1.7 Examples for thermoplastic composites with glass fibers. Condition of application, proportion of glass fibers, specific weight and typical mechanical strength [15]

Figure 1.16 Schematic structure of TPE block copolymers [18].

Because of low melting temperatures TPEs can easily be processed by molding or extrusion within a temperature range of 190–240 °C (depending on the TPE type). But to achieve a good homogenization during the processing high shear forces have to be used.

Uncolored TPEs can vary from optical transparency to opaque, depending on type and structure of the material. In opaque material, the light is internally scattered by the molecular structure and direct transmission of light is very poor with increasing material thickness.

Most of the TPEs show good weather and chemical resistance. Natural TPEs are usually colorless transparent or opaque and can be easily colorized.

1.2.5 Polymer Composites

Polymer composites are composed of a polymer matrix material (thermoplastic or thermosets) with organic or inorganic fillers (Figure 1.17) [1] like mineral pigments, short fibers, long fibers, continuous fibers, paper or fabrics to enhance the mechanical properties for special applications.

Figure 1.17 Classification of polymer composites [1].

Particles like mineral powder, wood flour or carbon black are used to increase the stiffness of the matrix material. The fatigue strength of the matrix material usually will be not increased, but is sometimes decreased. Short fibers, long fibers and continuous fibers from glass, carbon or aramid cause an increase of the fatigue strength (Figure 1.18), although the effect depends on the orientation of the fibers [19].

Figure 1.18 Dependence of the reinforcement on the type and structure of the filler material [11].

Continuous fibers from glass, carbon or aramid will influence the mechanical properties of the polymer compound by the adjustable orientation of the fibers. Besides increasing fatigue strength and stiffness the temperature-dependent expansion of the compound can also be decreased [20].

Polymer compounds with thermoplastic matrix usually can be melted by thermal processes like welding, but not polymer compounds with thermoset matrix.

1.3 Thermal Properties

Successful processing of thermoplastic resins by laser radiation needs a basic knowledge of the temperature dependence of the thermal material properties. The temperature ranges of the different phase transitions (e.g., glass transition and softening temperatures of amorphous thermoplastics, melt temperatures of crystalline phases of semicrystalline thermoplastics) but also material properties linked with heat conduction (e.g., heat capacity, heat conduction or specific volume) influence of laser power and processing speed of a laser application.

Thermal properties of thermoplastics strongly depend on the molecular structure. Orientation and length of macromolecular chains, number and distribution of side chains, crystalline structure or level of molecular links influence such thermal properties.

Typical phase transitions of thermoplastic resins are glass transition, melting of crystallites and thermal degradation of macromolecular chains. Physical properties like specific volume, heat capacity, heat conduction or thermal conduction, which characterize the material behavior regarding thermal energy absorption and transport, partly show a distinctive dependence of the material temperature and vary particularly in the ranges of phase transitions.

1.3.1 Phase Transitions

Depending on the physical and chemical structure of thermoplastic resins, the following phase transitions will occur on increasing material temperature [1, 6]:

1.3.1.1 Glass Transition (Tg)

Below the glass temperature (Tg) the mobility of the molecules (Brown's macromobility) is strongly curbed by intermolecular interaction. There are no position-change processes and only restricted thermal induced movements of chain segments or side chains. At the glass temperature Brown's micromobility of chain segments and side chains starts to occur and the plastic becomes softer but is still mechanical stable. Before reaching the glass temperature second-order relaxation processes are possible, single-molecule segments obtain a restricted mobility.

1.3.1.2 Flow Temperature (Tf)

On increasing temperature the hindering influence of intermolecular interaction decreases. On reaching the flow temperature (Tf) complete macromolecular chains can slip against each other (Brown's macromobility). The amorphous structures of the plastic become softer and start to melt. No chemical degradation of the macromolecules of the plastic will occur in this state.

1.3.1.3 Crystallite Melting Temperature (Tm)

By reaching the crystallite melting temperature (Tm) of semicrystallite thermoplastics the near forces, responsible for crystallite forming, will vanish and the crystallites start to melt. Because the temperature range of crystallite melting exceeds the flow temperature of the amorphous state, the entire thermoplastic will be plasticized. As long as no thermal degradation will occur in the molten phase, the resin can reversibly get back into the solidified state by cooling. Depending on the cooling conditions (speed and duration of cooling) crystallite phases will again be generated. The size and distribution of these crystallites can be differing from the original status.

1.3.1.4 Thermal Decomposition (Td)

Exceeding the decomposition temperature (Td) in the molten phase of thermoplastics and thermoplastic elastomers, the macromolecules start to decompose caused by intensive thermal oscillations. Separation of monomer units (e.g., PMMA) [6], oxidation or chemical conversion into reaction products like HCl during decomposition of PVC are possible reactions [4]. The resin will be irreversibly chemically modified.

The decomposition products will be separated as gaseous phase or will remain as components in the residual material. The start of decomposition, which means the value of the decomposition temperature, is greatly dependent on the intensity and duration of the thermal input. The decomposition temperature is lower by long duration and low intensity than by short duration and high intensity of the thermal input.

For laser welding of thermoplastic resins and thermoplastic elastomers phase transitions in the thermal range from room temperature up to the start of degradation are of interest. Table 1.8 summarizes phase-transition temperatures of typical thermoplastic resins. The indicated temperatures refer to average values or temperature ranges of phase transitions.

Table 1.8 Examples for phase-transition temperatures for thermoplastic resins [1, 6]

Plasticization of amorphous thermoplastics starts with exceeding the flow temperature (Tf) and for semicrystalline thermoplastics with exceeding the crystallite melting temperature (Tm). Figure 1.19 shows for a number of thermoplastics a compilation of the temperature ranges of the molten phase and the start of decomposition (from [1]). The decomposition temperatures in Table 1.8 or Figure 1.19 are dependent on the reference values of thermal degradation under vacuum [21] or estimated values (from [1]) and can possibly differ somewhat under atmospheric influence.

Figure 1.19 Melt-temperature ranges and decomposition in the molten state of some thermoplastic resins [1].

Increasing the temperature of a solid material will be done by energy input for example, using friction energy, ultrasonic energy or absorption of radiation. In the range of phase transitions an additional energy input is necessary to start the phase transition. For amorphous thermoplastics phase transition will occur at the flow temperature and for semicrystalline thermoplastics at the crystallite melting temperature. To start a phase transition an additional energy input as melting energy (or melting enthalpy) is necessary. The height of the melting energy for semicrystalline thermoplastics is dependent on the grade of crystallinity of the material.

Table 1.9 gives examples of the melting energy relative to the mass proportion of the crystalline phase of semicrystalline thermoplastics.

Table 1.9 Melt energy for some typical semicrystalline thermoplastics [1]

ResinMelting Energy Δ [J/g]HDPE310PA 6213PETP145POM333PP238

1.3.2 Specific Volume

The volume of thermoplastics expands with increasing temperature. Caused by more intense thermal oscillation of atoms and molecule elements the balance positions of the oscillation segments are moving apart [22]. As for example, heat capacity or heat conductivity, the type of bonding and caused by this the material structure is an important influence on the quantity of the volume change. For the temperature dependence of the specific volume a distinction is given between amorphous and semicrystalline plastics (Figures 1.20a and b) [23].

Figure 1.20 Schematic behavior of specific volume in dependence from the temperature for amorphous (a) and semicrystalline thermoplastics (b) [23].

Within the glass state of amorphous thermoplastics the specific volume shows a linear increase with increasing temperature (see Figure 1.20a) [23], described by Equation (1.1):

(1.1)

V0: volume at start of heating

β: cubic expansion coefficient

ΔT: increase of temperature.

On exceeding the glass-transition temperature the linear increase rapidly rises, depending on the distribution of the molecular weight. This means with increasing temperature the increase of the specific volume will be intensified, caused by decreasing strength of molecular bonds in the molten state. In Figure 1.21a a graphic portrayal of the temperature-dependent course of the specific volume for some amorphous thermoplastics is given.

Figure 1.21 Specific volume of amorphous (left) and semicrystalline thermoplastics (right) in correlation with the temperature [23].

Below the glass temperature the volume change of semicrystalline thermoplastics rises almost linearly with increasing temperature. On exceeding the crystallite melting temperature a difference to the linear behavior occurs. The volume change on increasing temperature will be almost linear again if the plastic is completely plasticized. Figure 1.21b shows the temperature-dependent course of the specific volume from some semicrystalline thermoplastics.

Depending on the cooling rate out of the molten state a different crystallization of the thermoplastic material can occur [24]. The faster the cooling the smaller will be the crystalline phases in dimension and they will be less numerous. The specific volume of a fast-cooled semicrystalline thermoplastic with low crystallization will be higher than of a slowly cooled material with high crystallization.

1.3.3 Heat Capacity