Study of application temperature and joint thickness impact on fast fixing effect of one component PUR adhesives

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Study of application temperature and joint thickness impact on fast fixing effect of one

component PUR adhesives

Master thesis

Study programme: N2301 – Mechanical Engineering

Study branch: 2301T048 – Engineering Technology and Materiales

Author: Sudeep Sangamesh Babu

Supervisor: Ing. Martin Seidl, Ph.D.

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Declaration

I hereby certify that Ihave been informed the Act 121/2000, the Copyright Act of the Czech Republic, namely § 60 -Schoolwork, applies to my master thesis infull scope.

Iacknowledge that the Technical University of Liberec (TUL) does not infringe my copyrights by using my master thesis for TUL's internal purposes.

I am aware of my obligation to inform TUL on having used or Iicensed to use my master thesis; in such a case TUL may require compensation of costs spent on creating the work at

up to their actual amount.

I have written my master thesis myself using literature listed therein and consulting it with my thesis supervisor and my tutor.

Concurrently ¹confirm that the printed version of my master thesis iscoincident with an electronic version, inserted into the 15 5TAG.

Date:

Signature:

24/05/2017

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Preface

This publication was written at the Technical University of Liberec as part of the Student Grant Contest “SGS 21122” with the support of the Specific University Research Grant, as provided by the Ministry of Education, Youth and Sports of the Czech Republic in the year 2017.

This work was carried out in the Department of Engineering Technology and Materials at Technical University of Liberec.

I would like to thank all the people who attributed to my project and for the academic support that I have received during my studies at Technical University of Liberec from 2015-2017. I would like to specially thank my supervisor Ing. Martin Seidl, Ph. D. who has been the greatest key behind my development as a researcher and for the guidance in order to finish this research project.

I would also like to thank Ing. Luboš Běhálek, Ph.D. and Ing. Adam Pazourek, Ph.D.for devoting their time and try to help out and do some analysis. I would also like to thank Ing. Svoboda from Magna, for his constant support and advice regarding the research.

Last but not the least, I would like to thank with all my heart to the Faculty of Mechanical Engineering in Technical University of Liberec for giving me this opportunity to do my studies that has groomed me as a researcher. To PhDr. Ivana Pekařová, MA and my parents, who have stood by me for the two years I’ve spent in Liberec, Czech Republic.

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Abstract

Adhesives have existed for centuries, as a material used in the technique of bonding component. In the present days, adhesives with various physical characteristics and chemical composition are used for numerous applications. The strength of an adhesive bond depends on the chemical composition, processing parameters and the field of application. The efficiency of these bonds (joints) strongly depend on the rheological properties of the adhesive.

Temperature, time (waiting time – after the application of adhesive;

curing time – total time for the adhesive to gain complete strength) that impact on the rheological properties has shown interest in the field of research. The core of this thesis was to focus on the thickness of joint and application temperature dependence of one- component polyurethane (PUR) moisture curable adhesive on fast fixing effect of the adhesive.

The impact of rheological properties on the adhesive bond were investigated by Ubbelohde method (Viscosity test), differential scanning calorimetry (DSC), temperature profiles and gangel tests.

These tests were also conducted to observe fast fixing effect of the adhesives. From the results, it was observed that viscosity and application temperature of the adhesive could influence the strength of bonded plastic parts. For higher efficiency of the bond, the processing parameters should be optimized.

Keywords: Polyurethane adhesives, viscosity, DSC analysis, shear test

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Chapter 1 Introduction

Polyurethane is the product of the reaction between diisocyanates or polyisocyanates with polyols. It is a polymer composed of organic elements joined by urethane links. There are two kinds of polyurethanes; most commonly used are the thermo- setting polyurethanes which do not melt when heated and there are also thermo-plastic polyurethanes. Due to the wide spectrum of characteristics of the polymer, it is used also as an adhesive and sealant in major industries [1].

According to Kinloch, “An adhesive may be defined as a material which when applied to surfaces of materials can join them and resist separation [2].” From history, it is known that adhesives were used since the time of Daedalus and his son (~ 1300 BC – 1000 BC), in escaping the Cretan imprisonment using wax (glue) to make wings made out of feathers[2]. In recent days, adhesion technology has improved and used in most of the industries.

Different kinds of adhesives are used for bonding different materials, different types of adhesives are used based on the field of application and so on.

Polyurethane adhesives are employed in major industries such as the automotive, aeronautical and marine industries due to their ease of processing and application. One-component polyurethanes can be classified based on the curing mechanisms as hot-setting and cold-setting adhesives. Application of heat is necessary for the curing mechanism to initiate in the hot-setting one component PUR adhesives whereas, in the cold-setting PUR adhesives, there are many methods used for the initiation of curing mechanism. One of the commonly used method is the curing of one-component adhesives in the presence of moisture at the adherent surfaces [1].

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1.1 Objectives and Approach

The aim of this thesis was to investigate the impact of application temperature and joint thickness in fast fixing effect of one component moisture curable polyurethane (PUR) adhesive.

The substrates selected for were two non-polar polymeric materials; polypropylene with long glass fibres and polypropylene with talcum.

There are series of experiments conducted to understand in detail about the rheological properties of the adhesives. The fast fixing effect of the adhesives in interest were closely observed. The impacts of thickness of joint and overlap of substrates on the strength of bonded plastic parts were detected through modified shear testing (gangel test). There were observations made on the dependence of viscosity and application temperature on the adhesive joint.

1.2 Related work done previously

There are several works by various authors in the field of one- component polyurethane adhesives. Some of the notable work are from the authors A. Pizzi, R.D. Adams, A.V. Pocius, Sina Ebnesajjad and W. Brockmann. From the work of A. Pizzi, a series of experiments were conducted on one-component polyurethane adhesives to check the thermal stability of the structural adhesive.

In the conclusion, he states that the thermal stability of polyurethane adhesives depend on the amount of free isocyanates, degree of polymerization and rate of the reaction [3], [4]. From this paper, it can be understood that the number of free isocyanates, degree of polymerization and rate of reaction also influence other properties of the adhesive and hence adhesion characteristics. There are several US patents for one component polyurethane adhesive applications. Two of the patents are US4511626 and US5922809 [5], [6], in which a detail view on the

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Chapter 2 Adhesive Technology

2.1 Adhesion

Adhesion is a two-dimensional phenomenon which occurs between two adherents (surfaces) and it is considered as an important activity in nature and technology. In general terms, adhesion is the tendency of similar or dissimilar elements or surfaces to cling (join) to one another with or without the help of adhesives (natural or synthetic).

Pocius mentioned that “an assembly made by the use of an adhesive is called an adhesive joint or an adhesive bond” [7].

There are advantages and disadvantages of adhesive bonding.

One of the major differentiation between mechanical fasteners and adhesive bonding systems is that a mechanical fastener such as screw or a rivet should pierce both the adherent surfaces in order to successfully accomplish a joint or an assembly. Whereas while using adhesives, there is no necessity to create a hole or pierce the adherent to achieve the joint. The stresses can be uniformly distributed using the adhesive bonding system, whereas in the case of mechanical fasteners this distribution is difficult to achieve.

There are two classification of adhesive bonding namely structural and non-structural bonds. Structural bonding is bonding for application where the adherents may experience or endure large stresses up to their yield point. These bonds must be able to transmit the forces or stresses without any or minimal loss of integrity within design limits. Non-structural bonds are not required to support high values of forces but should have the strength to hold lightweight materials in place [8].

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2.2 Adhesion Theories

Adhesion is an event in which allows the transference of load from the adherent to the adhesive joint [7]. There are numerous theories of adhesion which have been proposed over a long period of time. Some of the classical adhesion theories are summarized in the figure 1.

Figure 1 Adhesion Theories [1]

The theories of mechanical interlocking, electrostatic, diffusion and adsorption/surface reaction theories propose the mechanisms of adhesion. More recently there are other theories under the surface reaction theory such as the wettability, chemical bonding and weak boundary layer theory which describes the mechanism of adhesion in atomic and molecular levels. Andrew stated “It is often difficult to fully ascribe adhesive bonding to an individual mechanism. A combination of different mechanisms is most probably responsible for bonding within a given adhesive system” [8]. An important

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between the adhesive and adherent. Some interactions scale from the atomic level to the molecular level. For instance, for mechanical interlocking the contact surface of the adhesive and the adherent plays as an important parameter, in the case of electrostatic mechanism, surface charge is the macroscopic factor and so on.

2.2.1 Mechanical Theory

According to this theory, adhesion occurs by penetration of adhesives into pores, cavities and other surface irregularities on the surface of the substrate [8]. Air is displaced from the interface between the contact surface and the adhesive and thus the adhesive penetrating into the surface roughness of two adherents can bond themselves. According to researches, bonding can be enhanced by surface modification of the adherents, however the attainment of good adhesion between smooth surfaces exposes the mechanical interlocking theory. Brewis studied the adhesion between two perfectly smooth mica surfaces and examined the adhesion to optically smooth surfaces, clearly demonstrated that adhesion may be attained with smooth surfaces [9].

The mechanical interlocking model, proposed by McBain and Hopkins as early as 1925 [8], considers that mechanical interlocking, of the adhesive into cavities, pores and surface irregularities is the major factor in determining the adhesion strength. To overcome the difficulty of good adhesion between smooth surfaces, the following approach primarily suggested by Gent and Schultz, Wake proposed that the effect of both mechanical and thermodynamic interfacial interactions could be taken into account as multiplying factors for estimating the joint strength G [8].

G = (constant) x (mechanical interlocking component) x (interfacial interaction component) (Eq.1)

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According to the equation (1), a high level of adhesion should be achieved by improving both the surface morphology and physiochemical surface properties of both the substrate and the adhesive [8]. Apart from the roughness and porosity of the substrate surface, to generate adhesion anchor points, it is necessary that the adhesive has a good filling capacity, the adhesive should be able to penetrate into the valleys, pores and ridges of the substrate surface and this property is directly related to the viscosity of the adhesive.

2.2.2 Polarization Theory

This theory is also known as the “Electronic theory” [1], which describes that the adhesive bond between the substrate and the adhesive strengthens as the intermolecular interaction (forces) between the atoms increases. These intermolecular interactions have recognized the existence of permanent or oscillating dipoles, which interact one with another in chemically saturated systems, but which are also able to induce dipoles in other materials. These interactions are generally lower in strength or have lower binding energies when compared to those of the chemical interactions, they neither change the nature of materials. The interaction can be termed as ‘physical bonds’ and can be divided into three categories;

Permanent dipoles: This kind of dipoles are formed when an atom with a higher atomic number is bound to another atom with a lower atomic number, thus producing a permanent dipole. Theses dipoles are able to build electrostatic attraction forces in the form of a dipole interaction with another permanent dipole. For example, hydrogen has a permanent dipole-dipole bond.

Induced dipoles: When a non-polar atom interacts with a permanent dipole atom, the permanent dipole can induce counter dipoles with which they build up static attraction forces. The binding

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energy is lower when compared to the permanent dipole-dipole bonds.

Dispersion forces: These are the forces which may exist between two non-permanent dipoles. A weak oscillating dipole can be found between the involved atoms, but the statistical probability distribution of the binding electrons is not completely uniform. Their binding energy is generally lower than the permanent dipole-dipole bonds.

These dipoles play a role in the adhesion of adhesive to substrates and vice versa. Generally, an adhesive containing a polar group can be shown to adhere better to a polar substrate or a solid-state material than to a non-polar material [1].

2.2.3 Diffusion Theory

In simple terms, diffusion theory explains the concept of adhesion by the compatibility between polymers and the movement that occur in the polymer chains (see figure 2). When two polymers are compatible, its polymer chains are able to mix up between them, resulting in partial penetration between the two materials, as a result of these penetrations anchorage areas and adhesion points take place. The mobility and degree of

penetration of the polymers is determined directly by their molecular weight. Short polymer chains have higher mobility and penetrate into other material before the long polymer chain diffuse completely.

Figure 2 Rate of diffusion of polymeric chain between components A] Partial penetration of polymer chains B] non- compatible polymers and thus no penetration C] Complete penetration of

polymer chains, as highly compatible polymers [1]

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2.2.4 Chemical Reactions Theory

In the research by McBain and Lee [10] focused on the adhesion of polished steel and aluminium parts with organic adhesives (mainly shellac); the authors referred to ‘cohesive failure’ in the adhesive, with the adhesion forces having proved to be stronger. It was assumed during their research that the binder had a chain structure that was influenced by the surface and reached deep into the structure of the adhesive layer. There was no clear mention of any chemical interactions with the metal as being responsible for this high degree of adherence. These were the earliest result which are used until today for explaining the adhesive strengths of adhesive and the chemical interactions involved which could prove the impact on properties of the bonds.

2.2.5 Adsorption Theory

This theory is also referred to the Thermodynamic theory or Wettability or the Acid-Base theory. The thermodynamic model of adhesion, generally attributed to Sharpe and Schonhorn [11], is certainly the most widely used approach in adhesion science at present. This theory considers that the adhesive will adhere to the substrate because of the interatomic and intermolecular forces established at the interface, provided that an intimate contact is achieved. The most common forces found are the Van der Waals and Lewis acid-base interactions. Fundamental thermodynamic quantities such as surface free energies of both the adherent and the adhesive play an important role in the magnitude of forces in the adhesive bond.

Wettability of the surfaces can be calculated by the simple test of surface tension by dropping a drop of water on the substrate and calculating the contact angle between the drop and the surface using an optical microscope, thus calculating the surface free energy by the law of equation of states. When the adhesive has a

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it is capable of wetting the surface, generating a contact angle less than 90ᵒ, thus achieving adhesion between the adhesive and the substrate. This angle is known as the ‘critical angle for adhesion’.

Against the mechanical theories and the diffusion theory, adsorption theory explains the phenomenon of adhesion without penetration by the adhesive to the substrate; the adhesion is generated by the contact between the adhesive and the substrates [9].

2.3 New Concepts in the Field of Adhesion

There is a need for new concepts to explain the behaviour of adhesive bonding, as the ‘standard theories’ helps us to understand the adhesion phenomena but the information is inadequate. Polymer dynamics plays an important factor in the adhesion capacity of the pressure-sensitive adhesive, which are likely also essential for the adhesion of not only contact adhesives but also hot-melt adhesives. These usually do not form cross-links, even if they did, they are loose cross-links and are not considered to present any chemical reactivity.

From the experiments conducted by Possart W., it is concluded that in addition to the chemical bonds, polymer dynamics also affect the overall adhesive behaviour of reaction adhesives [12]. In the book Adhesive Bonding by Brockmann W., he concludes by stating [1];

“It becomes clear that, when attempting to understand the build-up and behaviour of adhesion, one reaches a dimension which lies between the molecular one and the dimension of matter, the characteristic technical properties of which only takes effect in case of a total quantity of approximately two orders of magnitudes higher than the molecular dimension. Although it has become possible to analyze this dimension only recently, the prevailing – although sometimes unsatisfactory – uncertainty with

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regard to the behaviour of adhesive systems is expected to be overcome within the next few years”.

2.4 Mechanisms of adhesion

For an adhesive to be effective it must have three main properties. It must be able to wet the substrate, it must harden and finally it must be able to transmit load between the two surfaces of substrates being adhered [13]. Adhesion, the attachment between adhesive and substrate, may occur either by mechanical means, in which the adhesive penetrates into the irregularities on the surface of substrate, or by one of several chemical interactions.

The strength of adhesion depends on many factors, including the means by which it occurs. In some cases, an actual bond occurs between adhesive and substrate whereas, in others, electrostatic forces, as in static electricity hold the substrates together. A third mechanism involves the Van der Waals forces that develop between molecules. A fourth means involve moisture aided diffusion of the adhesive into the substrate, followed by hardening [14].

2.5 Surface Preparations

“Surface preparation or treatment is defined as one or a series of operations including cleaning, removal of loose material, and physical and/or chemical modification of a surface to which an adhesive is applied for the purpose of bonding” [15], is the definition according to Ebnesajjad. In bonding of plastics, surface preparations are made in order to increase or induce the surface polarity of substrates (induce surface polarity in case non-polar substrates), improve the surface wettability and creating surface irregularity for better adhesive bonding. The main reasons for surface preparations of substrates are;

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a. To prevent the formation of weak layer for adhesive bonding b. To improve and increase the surface interactions or the degree of

molecular interactions between the adhesive or primer and the substrate surface

c. In the case of polymeric substrates, to induce a polarity on the surfaces of substrates which are non-polar

d. To create microstructures or surface irregularities like valleys and ridges in a microscopic scale on the surface of the substrate e. To optimize the adhesion forces that develop across the interface

and thereby ensure sufficient joint strength

Surface energy plays an important role in the determination of wettability (discussed in the further chapters) of the substrate surface. An important difference between metals and plastics is in their surface energies. Polymers have a lower surface energy compared to those of metallic surfaces and due to the surface energy an intrinsically poor adhesive bond is formed, if the surface of the substrate is not treated [8]. There are various methods of surface preparation such as chemical, physical, bulk and mechanical techniques. Some of them are briefly discussed in this chapter. Chemical treatment techniques include those requiring wet or chemical reactions as the primary means of altering the surface. Physical techniques involve creation of polarity or increasing the surface energy on the surface through various process such as the flame treatment or corona discharge. Bulk treatment techniques involve additives, blending or recrystallization, properties which would affect the bulk properties of plastics [15] (see figure 3 for classification of treatment methods).

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Surface preparation of plastics consist of four stages namely, cleaning, ablation, cross-linking and surface chemical modification [8]. Surface cleaning of plastics is a crucial stage to remove all the impurities (such as dust, grease, oil, rust, scale and miscellaneous dirt) from the surface, with an aqueous solution (alcohol), followed by drying. Solutions are used during the process to remove the mould-release agents or waxes from the surface of the plastic parts. Some of the solutions include methyl-ethyl ketone, acetone and methanol which increases the efficiency of cleaning. Solutions must be chosen, considering the properties of substrates, i.e. the solution should not cause dissolving or degrading of the substrate.

2.5.1 Mechanical Surface Preparation Methods

Surface roughening and sanding of plastics accomplishes the task of removal of loose and unstable polymers from the surface, thus increasing contact surface area. Some of the techniques used in the mechanical surface preparations are abrasion, grit blasting, grinding and brushing.

Figure 3 Surface Preparation techniques [1]

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Surfaces roughened by abrasion usually forms stronger adhesive bonds than the highly-polished surfaces, due to the increase in the contact surface area. A properly abraded surface should not contain any smooth or polished surface areas. Abrasion treatment is usually followed by a degreasing treatment to remove the remaining loose particles [8].

Grit blasting is carried out by treating the surface by a jet of air containing abrasive material (e.g. corundum). The degree of abrasion on the surface of substrate is determined by the hardness of the abrasive material, applied pressure and the distance of the blast nozzle from the surface of the substrate. A localized concentration of energy is created when the blast medium impacts the surface of the substrate. It is assumed that a plasma state is created in the region of impact, and when this plasma state collapses, condensed residues may re-contaminate the adherent surface, hence secondary degreasing is necessary after the process of grit blasting [1].

Grinding is a process of abrasion of surface of substrates with abrasives. The adhesion properties of surfaces are poorer than that of blasted surfaces. Before grinding, it is recommended to clean the surface from all impurities since, if the contaminations are not removed they will be distributed throughout the surface and no improvement in bonding quality is achieved. Belt sanders, angle grinders and random orbital grinders are used for these techniques [1].

During the process of brushing, for a given size of grit, the surface roughness obtained by brushing is less than that achieved through the process of grinding, due to the relatively elastic material used in the brushes when compared to the grinding wheels. During the process of brushing, compressive forces are applied on the adherent surface, which are less than the forces applied through blasting or grinding [1]. This process is implemented for product

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which are painted or laminated in order to preserve the quality of surface.

2.5.2 Chemical Surface preparation Methods

Chemical or electrolytic surface preparation is need to obtain maximum strength and resistance to deterioration. The chemical solution should be carefully prepared for the formation of adequate bond strength. Exposure time of the substrate in the solution decides the activation of surface for bonding, i.e., if the application is short, it does not active the surface, while overexposure leads to chemical reactions and this may interfere in the formation of adhesion bonds [15]. Chemical preparation can have two different effect on the surface of the adherent. In the method ‘acid- degreasing’, non-oxidizing acids are used and the surfaces become not only chemically clean but also creates sub- microscopic structures. This method also creates chemically active sites which would enhance adhesive bonding. Using another method called ‘pickling’ or ‘phosphatizing’, chemical reactions are enhanced by the application of electric current. These methods are usually used for metallic substrates [1].

2.5.3 Physical Surface Treatment Methods

Flame Treatment is a process where the polymer surfaces are gently touched by an open flame with an excess of oxygen. In the case of non-polar substrates, it induces oxidation and removes the weak boundary layer by vaporizing surface contaminations, thus improving the properties of adhesion [1-16].

Corona Treatment is a process in which a high frequency voltage is applied between two electrodes or between an electrode and the substrate material. This high frequency voltage causes the ionization of the atmosphere between the electrode and the substrate and a corona discharge is ignited. The electrons in the

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investigated and concluded that Corona treatment roughens the plastics by degrading and removal of the amorphous region of the polymer surface, causing a much larger adhesive contact area over the surface of the substrate. Also the author says that the crystalline region of the surface is not affected by this treatment [16].

Plasma Treatment is a process in which the surface is oxidized in the presence of oxygen, thus removing organic contaminations from the surface of the substrates [16]. There are different techniques of plasma treatment namely, micro-plasma, atmospheric plasma and low-pressure plasma treatments. In all these techniques, the surface of the substrate is modified for betterment of adhesion properties [17].

2.6 Use of Primers

Primers are adhesive solutions that improve wetting and protects the surface in the dry or cured condition. Primers transform the adherent surface into a polymer surface with a long- term durability and reactivity for good adherence of the adhesive.

Primers contain components which may interfere with the adhesive characteristics if directly added to the adhesive. This opens up a wide spectrum of possibilities in optimizing adhesion properties [1].

2.7 Design of adhesive joints

The designs of joints are optimized in order to avoid peel forces, generate compressive stresses rather than tensile stresses, to obtain maximum strength for a given area of bond and to avoid stress concentration in the bond-line region. The distribution of stress can be improved by the optimization of the design of joint [1]. Strength of an adhesive joint is determined by the mechanical properties of the adherent and the adhesive, the residual internal stresses, degree of contact between the adhesive and the surface and the design of joint. Shrinkage of adhesive during curing may cause volatiles, which may get entrapped at the interface if the

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viscosity of the adhesive is too high. Type of stress acting on the joint also influences the design of joint. Some of the designs based on stresses are shown in figure 4.

Some of the general principles followed to improve the efficiency of the joint are as follows [15];

a. The bonded area should be as large as possible (overlap of substrates), within the allowable geometry and weight constraints b. The bonded area should contribute to the strength of the joint c. The adhesive should be stressed in the direction of its maximum

strength

d. The stresses should be minimized in the direction in which the adhesive is weakest

Figure 4 Design of Joints [1]

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Some of general designs of joints are shown in the figure 5;

Chapter 3 Adhesives

Bonding is the surface-to-surface joining of similar or dissimilar materials using a substance which usually is of a different type, and which adheres to the surfaces of the two adherents to be joined [1]. According to DIN EN 923, an adhesive is a non-metallic substance capable of joining materials by surface bonding (adhesion), and the bond possessing adequate internal strength (cohesion) [7].

The expression adhesive may be used interchangeably with glue, cement, mucilage or paste and is any substance applied to one surface, or both surfaces, of two separate items that binds them together and resists their separation [18]. The basic definition of an adhesive as used by the Adhesive Sealant Council in America is

Figure 5 Types of Joints commonly used [1]

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“A material used for bonding that exhibits flow at the time of application”. Adhesive bonding technology offer great design flexibility as it can be integrated into any complex design and in almost all available industrial sequence of mass production. Over the years as the advancement of knowledge in adhesive science, specific adhesives have been developed that bind very strongly both to organic and inorganic materials. Adhesive bonding can rarely compete with other joining techniques used in the industry.

For example, adhesive bonding applications are limited when it comes to bonding a steel bridge, whereas for lightweight construction of car bodies, joining of aluminium, glass and plastics, adhesive joining offers extremely interesting applications [1].

One of the major advantages of bonding is that little or no heat is necessary to create the joint [7]. When adhesive bonding systems are used for joining, the material (substrate) structure is not macroscopically affected. There are less risks of deflections or internal stresses – which are generally related to the application of heat on the substrates. The disadvantage which is notable when adhesive bonding is used is that the bond has a limited stability to heat and specific production requirements (parameters) have to be met to achieve a successful bond with high strengths.

3.1 Classification of adhesives

There are many methods of classifying adhesives. Some use a system of end use, example adhesives for metals, adhesives for textile, adhesives for plastics etc. However, a system based on the chemical properties, performances and curing mechanism is more appropriate and will be used in this thesis.

3.1.1 Non-curing (pressure-sensitive) adhesives

Pressure-sensitive adhesives belong to the group of highly viscous polymer systems. They partly retain the properties of liquid in their final state, which allows them to completely adapt to the

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adhesive bonds [1]. They are usually designed to form a bond and hold properly at room temperatures, also they typically reduce or lose their bonding strength at low temperatures and reduce their shear holding ability at high temperatures [19].

3.1.2 Physically setting adhesives

If the transition from the liquid state of the adhesive to the solid state takes place by physical processes such as evaporation, solidification of the melt or diffusion processes without chemically changing the polymer components of the adhesive, it is termed as

‘physical setting’ [7].

Contact adhesives are a type of physically setting adhesives which are made from polymer components that are not yet chemically cross-linked. These adhesives should be applied to both surfaces of the adherents and solidification takes place by a drying process before the materials are joined. The adherents are pressed together as soon as the adhesive layers are dry to touch [18]. A diffusion process takes place as pressure is applied on the adherents, firmly joining the adherents together. The strength of bond is increase in hours after the application of pressure. If the surface of the substrate is modified (prepared), and there are irregularities on the surface, partial bond is formed even before the drying process has taken place.

Plastisol adhesives are a type of physically setting adhesives which are extensively used in the lightweight construction in the field of automobiles. These adhesives are formed by the dispersion of a polymer, mostly polyvinyl chloride (PVC), in a plasticizer.

During the curing process, when the dispersion is heated, the polymer dissolves irreversibly in the plasticizer that acts as a solvent, transforming into soft PVC. Additives can be added to improve the heat resistant characteristics of these plastisol adhesives. These adhesives are relatively cheap and high durability of the bond are obtained [7].

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Hot-melt adhesives are a type of physically setting adhesives, which exist in a solid state and do not contain any solvents.

Application of these adhesives are done in the molten state in order to obtain an appropriate wetting of the adherent surface.

Immediately after cooling, hot-melt adhesives have a capability to transmit forces. They can be used at high heating and cooling rates to speed up the production in industries [1]. Hot-melt systems are widely used method to bond windscreens in automotive bodyworks. Hot-melts can be applied at low temperatures, while being able to resist temperatures of 120ᵒC - 150ᵒC in the cured state [7].

3.1.3 Chemically setting adhesives

These are the group of adhesives in which the curing takes place due to the relative reactions between the reactive groups present in the adhesive under specific conditions. The bonds strengthen by means of chemical reactions, which must take place after the application of the adhesive and after joining the adherents. The time needed for curing can be altered by the application of heat.

Two component adhesives are a type of chemically setting adhesives, which consists several components that are mixed in a specific ratio before application. The most important adhesives of this group are the polyesters, cold setting epoxy resins, polyurethanes and acrylic adhesives. For some adhesives, it is possible to apply each of the component to separate substrates being joined together [1]. The term ‘pot-life’ is important with two component adhesive systems, which gives an approximate period of time during which the adhesives can be used after mixing the components, this is in turn dependent on the type of adhesive and the volume prepared.

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Cold setting, one component adhesives are a type of chemically setting adhesives, which employ a wide range of polyols. They are reacted with isocyanates to form an isocyanate - terminated pre- polymer. The resulting isocyanate – terminated pre-polymer is diluted in a non-hydroxyl containing solvent (such as ethyl acetate) and supplied to a converting operation in this form. The adhesive when applied must react with water (either from moisture from the air or from the substances used or by intentional moisture which is induced on the substrates) to form an amine-terminated intermediate that can further react with the isocyanate-terminated polymer. Typical curing rates for the one component adhesives range from 2-7 days [20]. These cold setting one component adhesives can also be initiated by irradiation or by an absence of oxygen. Under the influence of UV light, the system can be accurately controlled. Photo-initiators are dissolved or chemically incorporated with the adhesive systems, which initiate the cross- linking process under irradiation with UV or visible light [1]. The advantage of using cold setting one component adhesive is that it is virtually unaffected over a temperature range from -55ᵒC to +250ᵒC.

Hot setting, one component adhesives are a type of chemically setting adhesive, which consist of low molecular and plasticised substances [1]. This group of adhesives consists mainly of phenolic resins, epoxy resins and polyimides. The cross-linking process takes place in the form of poly-condensation, pressure and heat should be applied in order to remove the water from the bond line during the process. These adhesives are not capable of transmitting forces between the substrates before curing and thus any deformation of the substrate may result in destruction of the bond.

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Chapter 4 Polyurethane Adhesives

Coatings, sealing and bonding is known to man since a long period of time. From the natural resins, oils and fats used in the

‘mummification’ of human bodies to the most recent times, sealants, coating and bonding is an interesting developing field of sciences. As the development of synthetic resins started around the 20^th century, the production of coatings and adhesives vastly improved. A German industrial chemist, Otto Bayer discovered poly-addition process for the synthesis of polyurethanes from polyisocyanate and polyol. Polyurethane is a polymer composed of organic components joined by carbamate (urethane) links. This versatile material can be found in liquid coatings and paints, tough elastomers such as roller blade wheels, rigid insulation, soft flexible foam, elastic fibre or as an integral skin. Polyurethane raw materials are used in the manufacture of foams, industrial rigid foams for insulation, construction industry, energy sector as sealants, energy absorbing components in automobile interiors, packing industry and some application also reach out to the health sector [21]. The major use of polyurethane is in the automobile industry as an adhesive and sealant. The broad range of applications for polyurethanes also include wood finishing, corrosion resistance and construction, textile coatings and many more.

Brockmann defined polyurethanes as the polymers produced by addition reactions between polyisocyanates (di-functional or higher) and hydroxyl-rich-ethers [1]. As of today, polyurethane adhesives are available in solvent based moisture adhesives, thermoplastic hot melts, thermo-setting systems and emulsions.

As there is a wide diversity of mechanical properties of the polyurethane adhesives, they have a broad spectrum of applications.

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4.1 Chemistry and structure

Polyurethanes are formed by reacting a polyol (alcohol with hydroxyl groups) with diisocyanates or polymeric isocyanates in the presence of suitable catalysts and additives. Polyurethane adhesives have a distinctive feature of polymer structure with hard and soft segments. Soft segments are formed by long-chain polyester polyols, which has a low glass transition temperature, whereas the hard segments are created by the cross-linking of

diisocyanates with short-chain diols or diamines. These hard segments of the adhesive have higher glass transition temperature values (see figure 6). Due to the different glass transition temperature ranges, polyurethane adhesives can be generated in two different ranges [1]. Generally, the glass transition temperature of polyurethanes is higher than that of silicones, but lower than that of highly cross-linked, structural epoxy resin adhesives, hence the mechanical properties can be compared to those of the epoxy resin adhesive systems.

Polyurethanes are in the class of compounds called as the

‘reaction polymers’, which include epoxies, unsaturated polyesters and phenolics. The raw materials for polyurethane adhesives are the isocyanates, both aromatic and aliphatic isocyanates are used for the synthesis of polyurethanes. Aromatic isocyanates such as toluene diisocyanates (TDI), methyl-di-phenyl-isocyanates (MDI) are used for the synthesis of polyurethane adhesives. Commonly

Figure 6 Soft and hard segments of polyurethane adhesive [1]

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used aliphatic isocyanates are hexa-methylene diisocyanate, isophorone diisocyanate, tri-methyl hexa-methylene diisocyanate [1] (see figure 7 for types of isocyanates).

Figure 7 Different configurations of isocyanates [1]

The properties of polyurethane adhesives are highly influenced by the type of isocyanate and polyol used for the synthesis.

Isocyanates react with other substances by poly-addition or poly- condensation and this reaction is also based on the electrophilic character of the carbon atom in the double bond system. Another group of polyisocyanates commonly used in the generation of polyurethane adhesives are blocked isocyanates. This is an isocyanate which has been reacted with mono-functional alcohols or amines, to prevent its reaction at room temperature [22]. The commonly used means of generating polyurethanes is the reaction of diisocyanates or polyisocyanates with primary mono-alcohols, di-alcohols and poly-alcohols. Polyols of ether and ester origins are widely employed in the synthesis. Polyether polyols are produced

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they are resistant to alkaline hydrolysis. Polyester polyols have a higher tensile strength and a higher resistance to heat [7].

In the presence of moisture, a two-stage reaction takes place during the synthesis, firstly an unstable carbamic acid is formed, in the second step this carbamic acid splits off carbon dioxide to give an amine. This newly formed amine reacts further with isocyanate groups to give a urea group. If the reaction takes place in room temperature, the urea group formed in turn reacts with free isocyanates, cross-linking the polymer chains to form biurets [7].

4.2 Classification of Polyurethane adhesives

Classification of polyurethane adhesives can be done based on different parameters and characteristics of the adhesives. Each of these systems are mainly based on the type of curing or the components used in the adhesives. But keeping the work of this thesis at view, polyurethane adhesives will be classified based on the components used in the adhesives and the mechanism of curing.

4.2.1 One Component Polyurethane adhesives

A group of polyurethanes called as the ‘Isocyanate-terminated polyurethanes’, which are the pre-polymers when polyol reacts with an excess of polyisocyanates, cure in atmospheric pressure and ambient moisture to form biurets or poly-urea by cross-linking [1], [22], [23]. One component reactive system uses two constituents which will form a polymeric system on the substrate [24]. Hydroxyl polyurethanes are a group of thermoplastic polyurethanes with a hydroxyl group content of 0.5-1.0%, which are produced by the reaction of MDI (diisocyanate) with polyester diols. The crystallinity of the individual hydroxyl polyurethanes determines the curing time of the adhesive [22]. Moisture-curable thermoplastic polyurethane hot-melts are formed by the mixture of hydroxyl polyurethanes with polyisocyanates. At room

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temperature, due to their cross linking, the heat resistivity of the adhesive is increased [1].

4.2.2 Two Component Polyurethane adhesives

This system constitutes of two components which are more reactive at ambient conditions and are mixed just prior to application [22]. They are composed of lower molecular weight polyisocyanates or pre-polymers (compared to those of one component polyurethanes) cured with low-molecular-weight polyols or poly-amines [1]. They are used in the automobile industry in bonding metals to plastics, textile foam coatings and in the wood furniture industry.

4.2.3 Moisture Curable Polyurethane Adhesives

In this thesis, the used adhesive is a moisture curable one component polyurethane adhesive. The curing principle is described by the reaction of isocyanate groups with water.

Polyisocyanate and polyol combinations with an excess of isocyanate groups crosslink at atmospheric moisture to give an insoluble higher molecular weight polyurethane/poly-urea [22]. The properties of moisture-curable one component system are governed by the type of

polyisocyanate used for the synthesis. The drying time depends on the temperature and the amount of atmospheric moisture present during the process of curing (see figure 8 for chemical reaction).

Figure 8 Formation of moisture curable one component polyurethane adhesive

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Comparison of One-component and Two-component polyurethane adhesives [22] (see table 1).

Table 1 Comparison of one component and two component polyurethane adhesive

One-component Two-component

Chemistry is limited to room temperatures

Chemistry can be defined even at higher temperatures

Very long open times Limited open time, variable from seconds to hours No mixing needed Must be meter mixed Simple dispensing equipment

required

Complicated equipment used for dispensing

No flushing required

Flushing and cleaning needed after some predetermined

time Minimum surface preparation

of substrates before application

Best results obtained with primer and prepared surfaces

of substrates

4.3 Processing of One component adhesive systems

Processing any polyurethane adhesive material requires equipment to store, meter, heat, mix and dispense the chemicals such as the polyol or polyether or polyesters and diisocyanates or polyisocyanates [24]. All adhesives undergo a transition from the state of liquid to solid, during the creation of an adhesive joint with the exception of pressure-sensitive adhesives (see chapter 3.1.1 Pressure sensitive adhesives). The chemical components are mixed in the mixing head, which results in the formation of polyurethane. Depending on the application of the adhesive, the mixing head can vary from a high-pressure or low-pressure mix head to apply the adhesive [24] (see figure 9). Viscosity is a

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rheological parameter that has an obligation to bring the adherents in close proximity, and to allow the adhesive forces to originate [1].

Particularly, thermoplastic moisture curable one component polyurethane adhesives are widely used in the automotive and construction industry. For bonding of smaller component in medical and electronic application, UV curing is used for one component adhesives. The specific interaction between the photo initiators and high energy UV radiation allows a very rapid curing process [1]. Use of robots for the application of adhesives and curing using UV radiations are used in industries to increase productivity (see figure 10).

Figure 9 (Left) High pressure mixing head; (Right) Low pressure mixing head for application of adhesive

Figure 10 (Left) Flame treatment using robots; (Right) application of adhesive using robots

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4.4 Field of Application

As the market today is competitive, it is preferred to use complex structures made of different layered materials to achieve the desired combination of properties for respective applications. It is found that one effective way to make strong and durable lamination is through the process of adhesive bonding. Adhesive bonding has its benefits when compared to other processes of joining such as welding or soldering; when these other processes are used for the joining process, the surfaces are permanently changed by thermal stresses whereas, using adhesives the strains are dissipated over the whole surface of the substrate without creating concentrated stress point. Also by using mechanical interlocking systems such as riveting, nailing, sewing or screwing methods, the surfaces are weakened which might affect the application of these materials [22]. Choosing the best adhesive based on the application, one of the important factor to be considered is the type of loads (static, cyclic, impact or dynamic loads) applied later on the joint.

Applications of polyurethane adhesives are very vast and these adhesives are used to bond many different material types including ceramics, metals, glass, plastics and composites. For this thesis, usage of moisture curable one component polyurethane adhesive in the automobile industry is in view and in this chapter, topics related to application of polyurethane adhesive in the automotive industry are described.

Direct Glazing is a process followed in the automotive industry; it constitutes of fitting fixed glass panes with the use of polyurethane adhesives. One component moisture curing products are used as they create an elastic joint between a painted body and the glass;

also, they perform the important function of coping with vibrations and movement. One side of the glass is pre-heated with an adhesion promoting glass primer (discussed in chapter 2.6) to enhance the process of adhesion. These glass primers contain

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silanes and are efficient when applied as a thin coat [1]. The curing time of the moisture cured one component polyurethane adhesive is determined by the diffusion of water vapour, if the humidity is low during the process, it can delay the process of curing. The cured adhesive in this process will have a glass transition temperature lower than -40ᵒC, so the mechanical properties remain unchanged within the whole range of operating temperatures.

Chapter 5 Test Methods and Properties

A wide variety of test methods among different industrial departments, for the evaluation of adhesively bonded joints have been developed and recognized by the ISO (International Organisation of Standardization), EN (European Committee for Standardization) and ASTM (American Society for Testing and Materials). The basic test procedures can be classified into the following;

a. Substrate Properties b. Rheological Properties c. Thermal Analysis

The European guideline CEN/TR 14548 Adhesives – guide to test methods and other standards for the general requirements, characterization and safety of structural adhesives, provide a detailed and comprehensive summary of up-to-date test methods for adhesives in structural applications

5.1 Surface Tension Analysis Surface tension is defined as the work required to increase the area of a surface isothermally and reversibly by unit amount. It is expressed as the

surface energy per unit area. Figure 11 Surface tension

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liquid averages zero. The net force called the ‘cohesion force’ must be counteracted to increase the surface area; the energy consumed by this process is called surface energy (see figure 11 for description of surface tension). Surface energy can be calculated by a series of hypothesis, one of them is by using the Gibb’s free energy and entropy of the liquid surface;

𝐸_𝑠 = 𝐺_𝑠− 𝑇 ^𝑑𝛶_𝑑𝑇 (Eq. 2)

where Es is the enthalpy of the system, Gs is the Gibb’s free energy, T is the absolute temperature and Υ is the surface free energy per unit area. Ebnesajjad defined work of adhesion as “a reversible thermodynamic work required to separate the interface from the equilibrium state of two phases to a separation distance of infinity”

[15]. The equation given by the French scientist A. Dupre explains the relation between the solid and liquid phases at the interface between the adherent and the adhesive [1];

𝑊_𝑎= 𝛶_𝑠+ 𝛶_𝐿− 𝛶_𝑆𝐿 (Eq. 3)

where the surface energy of the solid phase is 𝛶_𝑠, 𝛶_𝐿 is the surface energy of the liquid phase, 𝛶_𝑆𝐿 is the surface energy of the interface between the solid and liquid phase and Wa is the work of adhesion.

An increase in the interfacial energy increases surface tension.

Contact angle is the angle between the surface energy vectors of the solid and liquid phases where all the three phases intersect.

This angle is shown in the figure 12. The degree of wetting of the surface is determined by the contact angle. In the case of an ideal surface, the addition or removal of a small amount of drop will result in the advancement or recession of the drop. The delay in the movement of the drop due to addition of removal is termed as

‘hysteresis. The contact angle formed as a result of addition of liquid is referred to as the ‘advancing angle’ and the angle resulting from the removal of liquid is referred to as the ‘receding angle’.

These angles are used in the calculation of surface free energy.

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One of the theories used for calculations is the ‘Equation of states’, which is defined by the relationship between the contact angles and the critical surface tension. A series of liquid are tested to measure the contact angles against a solid surface. The critical surface tension values are plotted against liquid-vapour surface tension and this plot is referred as the ‘Equation of state plot’. The maximum value of the critical surface tension curve is called the surface tension of the solid surface [15].

Figure 12 Wetting of surfaces and contact angles [1]

5.2 Rheological properties of adhesive

Polymer rheology is the science that deals with the deformation and flow of matter. Most of the plastics exhibit ‘visco-elastic’

behaviour during flow, which means that they exhibit not only viscous characteristics but also elastic behaviour in liquid state [25]. The rheological properties of polymers vary with their chemical composition and structure. One of the most important rheological properties for adhesive bonding is the viscosity of the adhesive. Viscosity can be defined as the resistance offered to the flow of liquid. It can also be defined as the resistance offered by a fluid to gradual deformation by shear or tensile stress. There are different types of viscosities depending on the type of resistance offered. The important classifications are

Dynamic Viscosity also known as shear viscosity of fluids offer resistance to shearing flow, where adjacent layer of liquid move parallel to each other. The speed of different layers varies due to

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the opposition offered to the flow. Dynamic viscosity can be calculated using the equation;

µ =

^𝜏

(^𝜕𝑢_𝜕𝑦) (Eq. 4)

Where, µ is the dynamic viscosity, 𝜏 is the shear stress and (^𝜕𝑢_𝜕𝑦) is the rate of shear deformation.

Kinematic Viscosity is the ratio of dynamic viscosity to the density of the fluid and can be calculated using the equation;

𝜈 =

^µ_𝜌

(

Eq. 5)

Where, ν is the kinematic viscosity and ρ is the density of the fluid.

Relative Viscosity is the ratio of viscosity of polymer solution to the viscosity of pure solvent. This can be calculated using;

𝜂

_𝑟

=

_𝜂^𝜂

0

(

Eq. 6)

Where, ηr is the relative viscosity of the system, η is the viscosity of the polymeric solution and η0 is the viscosity of pure solution.

Apparent Viscosity is defined for polymers when the viscosity of the system depends on the shear rate. For dynamic or shear viscosity, the viscosity of the system is constant independent of shear rate applied to the sample, whereas, the values obtained from the systems which are influenced by the shear rate are known as ‘apparent viscosity’ or ‘apparent shear viscosity’ [26]. Using gangel test, it is possible to calculate the apparent viscosity. The system of adhesive joints is influenced by the temperature and thickness of the bond; and hence the shear rate. The equation for calculating the apparent viscosity are as follows;

𝜂 =

^{2𝑚𝑔ℎ𝑡}_𝐿₂_𝑤

(

Eq. 7)

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where, η is the dynamic viscosity of the adhesive (g/s.mm), m is the mass of load used during the test (g), g is acceleration due to gravity (constant: 9.81 ms^-2), h is the thickness of adhesive (mm), t is the time of failure (s), L is the length of overlap and w is the width of overlap of adhesive

Viscosity can be measured using instruments called as viscosimeters (also known as viscometers) and rheometers.

Rheometers can be called as a special type of viscometer and it is used for liquids which require more parameters to define the values of viscosity. There are two different types of rheometers.

Rheometers which control the applied shear stress on the fluid, are called as rotational shear rheometers. Rheometers which control the applied tensile stress in the fluid, are called as the extensional rheometers. Dynamic rotational shear rheometers are used for characterising and understanding high temperature rheological properties in both molten and solid state of materials, and the components used for the test can vary in design [26].

Capillary design is used for liquid which can be forced through a tube of constant cross-section and known dimensions under conditions of laminar flow. In this method, the pressure drop or flow rate is fixed and the other is measured for variations. These variations are used in computation of shear rate or shear stress.

In rotational cylinder design, the liquid is placed between the cylinders. One of the cylinders is rotated at a set speed and the shear rate of the liquid is measured. Using the resistance (force) offered by the liquid on the cylinder (torque), shear stress is calculated.

In cone and plate design, liquid is placed on a horizontal plate and a shallow cone (known angle) is placed on it. The angle between the plate and cone can be varied depending on the fluid to be

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and the torque on the cone is measured, in turn the shear stress is computed.

Measurement of intrinsic viscosity is another method of measuring viscosity. In this method, the flow time of a polymer solution through a glass capillary at different solution concentrations are measured [26]. A polymer solution passing through a capillary obeys Poiseuille’s law for laminar flow through capillaries, which indicates that the pressure drop is directly proportional to the viscosity of the fluid. This method is known as the Ubbelohde method (see figure 13). The viscosity of fluid can be calculated using the equation;

𝜂 =

^{𝜋𝛥𝑃𝑟}_8𝑙𝑄⁴

(

Eq. 8)

where, η is the viscosity of the fluid, ΔP is the pressure difference of the fluid in the capillary, r is the capillary radius, l is the length of capillary tube and Q is the volumetric flow rate through the capillary [26].

Figure 13 Ubbelohde method [26]

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A polymeric solution of known concentration is put in the reservoir and is pumped to the upper bulb usually by creating vacuum in the chamber. Then the liquid is flows down the capillary by gravity. The time for the liquid to flow between the two marks is recorded. This method is repeated for different concentration of solution [26]. The standard for this test are mentioned in ASTM D445 and ISO 3104.

5.3 Thermal analysis

Menczel states thermal analysis as a family of measuring techniques which measure the material’s response to being heated or cooled (or in some cases, held isothermally). The popular techniques are differential scanning calorimetry (DSC), thermo- gravimetric analysis (TGA), differential thermal analysis (DTA), thermo-mechanical analysis (TMA), dynamic mechanical analysis (DMA), di-electric analysis (DEA) and micro/nano-thermal analysis (µ/n-TA). Some of them are briefly discussed in the following subchapter.

5.3.1 Differential Scanning Calorimetry (DSC)

Using differential scanning calorimetry, (DSC; ISO 11357-1;

ASTM E473) [27], changes in specific heat, heat flow and temperature values can be determined for polymer transition during the glass transition stage. DSC is a technique in which the heat flow rate difference into a substance and a reference is measured as a function of temperature while the substance and reference are subjected to a controlled temperature program. In this test, there is a series of heating and cooling of the material in a controlled environment. The plot of this measurement has heat flux on the vertical axis (y-axis) and temperature or time on the horizontal axis (x-axis). There are certain peaks at certain temperatures, and these peaks are due to the reaction of the material with heat. The peaks can have an endothermic character, as the reaction consumes energy or power or heat. The peaks can

Study of application temperature and joint thickness impact on fast fixing effect of one component PUR adhesives