Progress in Adhesion and Adhesives, Volume 6 E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: Adhesion and Adhesives - Fundamental and Applied Aspects
• Sprache: Englisch

Seitenzahl: 1544

Veröffentlichungsjahr: 2021

Beschreibung

With the voluminous research being published, it is difficult, if not impossible, to stay abreast of current developments in a given area. The review articles in this book consolidate information to provide an alternative way to follow the latest research activity and developments in adhesion science and adhesives. With the ever-increasing amount of research being published, it is a Herculean task to be fully conversant with the latest research developments in any field, and the arena of adhesion and adhesives is no exception. Thus, topical review articles provide an alternate and very efficient way to stay abreast of the state-of-the-art in many subjects representing the field of adhesion science and adhesives. The 19 chapters in this Volume 6 follow the same order as the review articles originally published in RAA in the year 2020 and up to June 2021. The subjects of these 19 chapters fall in the following areas: * Adhesives and adhesive joints * Contact angle * Reinforced polymer composites * Bioadhesives * Icephobic coatings * Adhesives based on natural resources * Polymer surface modification * Superhydrophobic surfaces The topics covered include: hot-melt adhesives; adhesively-bonded spar-wingskin joints; contact angle hysteresis; fiber/matrix adhesion in reinforced thermoplastic composites; bioadhesives in biomedical applications; mucoadhesive pellets for drug delivery applications; bio-inspired icephobic coatings; wood adhesives based on natural resources; adhesion in biocomposites; vacuum UV surface photo-oxidation of polymers and other materials; vitrimers and their relevance to adhesives; superhydrophobic surfaces by microtexturing; structural acrylic adhesives; mechanically durable water-repellent surfaces; mussel-inspired underwater adhesives; and cold atmospheric pressure plasma technology for modifying polymers. Audience This book will be valuable and useful to researchers and technologists in materials science, nanotechnology, physics, surface and colloid chemistry in multiple disciplines in academia, industry, various research institutes and other organizations.

Leseprobe

Table of Contents

Cover

Title page

Copyright

Preface

1 Hot-Melt Adhesives: Fundamentals, Formulations, and Applications: A Critical Review

1.1 Introduction to Hot-Melt Adhesives (HMAs)

1.2 Formulation of Hot-Melt Adhesives

1.3 Fundamental Aspects of Adhesive Behavior of HMAs

1.4 Preparation of HMAs Using Various Polymers

1.5 Mechanical Analysis of Hot-Melt Adhesives

1.6 Industrial Applications of Hot-Melt Adhesives

1.7 Current Challenges and Future Scope of HMAs

1.8 Summary

Acknowledgment

References

2 Optimization of Adhesively Bonded Spar-Wingskin Joints of Laminated FRP Composites Subjected to Pull-Off Load: A Critical Review

2.1 Introduction

2.2 Finite Element Analysis of SWJ

2.3 Taguchi Method of Optimization

2.4 Results and Discussion

2.5 Conclusions

References

3 Contact Angle Hysteresis – Advantages and Disadvantages: A Critical Review

3.1 Introduction

3.2 Contact Angle and Hysteresis Measurement

3.3 Advantages of Contact Angle Hysteresis

3.4 Disadvantages of Contact Angle Hysteresis

3.5 Summary

3.6 Acknowledgements

References

4 Test Methods for Fibre/Matrix Adhesion in Cellulose Fibre-Reinforced Thermoplastic Composite Materials: A Critical Review

4.1 Introduction

4.2 Terms and Definitions

4.3 Test Methods for Fibre/Matrix Adhesion

4.4 Comparison of IFSS Data

4.5 Influence of Fibre Treatment on the IFSS

4.6 Summary

Acknowledgements

References

5 Bioadhesives in Biomedical Applications: A Critical Review

5.1 Introduction

5.2 Theories of Bioadhesion

5.3 Different Polymers Used as Bioadhesives

5.4 Summary

References

6 Mucoadhesive Pellets for Drug Delivery Applications: A Critical Review

6.1 Introduction

6.2 Mucoadhesive Polymers

6.3 Pellets

6.4 Summary and Prospects

Conflict of Interest

References

7 Bio-Inspired Icephobic Coatings for Aircraft Icing Mitigation: A Critical Review

7.1 Introduction

7.2 The State-of-the-Art Icephobic Coatings/Surfaces

7.3 Impact Icing Process Pertinent to Aircraft Inflight Icing Phenomena

7.4 Preparation of Typical SHS and SLIPS Coatings/Surfaces

7.5 Measurements of Ice Adhesion Strengths on Different Icephobic Coatings/Surfaces

7.6 Icing Tunnel Testing to Evaluate the Icephobic Coatings/Surfaces for Impact Icing Mitigation

7.7 Characterization of Rain Erosion Effects on the Icephobic Coatings

7.8 Summary and Conclusions

Acknowledgments

References

8 Wood Adhesives Based on Natural Resources: A Critical Review Part I. Protein-Based Adhesives

8.1 Overview and Challenges for Wood Adhesives Based on Natural Resources

8.2 Protein-Based Adhesives

8.3 Summary

General Literature (Overview and Review Articles) for Wood Adhesives Based on Natural Resources

Protein-Based Adhesives

Plant Proteins (including Soy)

Animal Proteins and Other Sources

References

9 Wood Adhesives Based on Natural Resources: A Critical ReviewPart II. Carbohydrate-Based Adhesives

9.1 Types and Sources of Carbohydrates Used as Wood Adhesives

9.2 Modification of Starch for Possible Use as Wood Adhesive

9.3 Citric Acid as Naturally-Based Modifier and Co-Reactant

9.4 Combination and Crosslinking of Carbohydrates with Natural and Synthetic Components

9.5 Degradation and Repolymerization of Carbohydrates

9.6 Summary

General Literature (Overview and Review Articles) for Carbohydrate-Based Adhesives

References

10 Wood Adhesives Based on Natural Resources: A Critical Review Part III. Tannin- and Lignin-Based Adhesives

10.1 Introduction

10.2 Tannin-Based Adhesives

10.3 Lignin-Based Adhesives

10.4 Summary

General Literature (Overview and Review Articles) for Tannin and Lignin

References

11 Adhesion in Biocomposites: A Critical Review

11.1 Introduction

11.2 Biocomposite Processing Methods

11.3 Factors Enhancing Adhesion Property in Biocomposites

11.4 Physical and Chemical Characterization

11.5 Adhesion in Polymer Biocomposites with Specific Applications

11.6 Summary

References

12 Vacuum UV Surface Photo-Oxidation of Polymeric and Other Materials for Improving Adhesion: A Critical Review

12.1 Introduction

12.2 Vacuum UV Photo-Oxidation Process

12.3 Adhesion to VUV Surface Photo-Oxidized Polymers

12.4 Applications of VUV Surface Photo-Oxidation to Other Materials

12.5 Prospects

12.6 Summary

References

13 Bio- and Water-Based Reversible Covalent Bonds Containing Polymers (Vitrimers) and Their Relevance to Adhesives – A Critical Review

13.1 Introduction

13.2 Bio-Based RCBPs

13.3 Water-Based RCBPs

13.4 Summary

13.5 Authors Contributions

13.6 Funding

13.7 Conflict of Interest

References

14 Superhydrophobic Surfaces by Microtexturing: A Critical Review

14.1 Introduction

14.2 Fabrication of Microtextured Surfaces

14.3 Properties of Microtextured Surfaces

14.4 Applications

14.5 Future Outlook

Acknowledgments

References

15 Structural Acrylic Adhesives: A Critical Review

15.1 Introduction

15.2 Compositions and Chemistries

15.3 Physico-Mechanical Properties of SAAs

15.4 Adhesives for Low Surface Energy Materials

15.5 Comparison of the Properties of SAAs and Other Reactive Adhesives

15.6 Summary and Outlook

References

16 Current Progress in Mechanically Durable Water-Repellent Surfaces: A Critical Review

16.1 Introduction

16.2 Fundamentals of Superhydrophobicity and SLIPs

16.3 Techniques to Achieve Water-Repellent Surfaces

16.4 Durability Testing

16.5 Future Trends

16.6 Summary

References

17 Mussel-Inspired Underwater Adhesives-from Adhesion Mechanisms to Engineering Applications: A Critical Review

17.1 Introduction

17.2 Adhesion Mechanisms of Mussel and the Catechol Chemistry

17.3 Catechol-Functionalized Adhesive Materials

17.4 Summary and Outlook

References

18 Wood Adhesives Based on Natural Resources: A Critical ReviewPart IV. Special Topics

18.1 Liquified Wood

18.2 Pyrolysis of Wood

18.3 Replacement of Formaldehyde in Resins

18.4 Unsaturated Oil Adhesives

18.5 Natural Polymers

18.6 Poly(hydroxyalkanoate)s (PHAs)

18.7 Thermoplastic Adhesives Based on Natural Resources

18.8 Cellulose Nanocrystals (CNCs) and Cellulose Nanofibrils (CNFs)

18.9 Cashew Nut Shell Liquid (CNSL)

18.10 Summary

General Literature (Overview and Review Articles) for Wood Adhesives Based on Natural Resources (for further information see [1])

References

19 Cold Atmospheric Pressure Plasma Technology for Modifying Polymers to Enhance Adhesion: A Critical Review

19.1 Introduction

19.2 Atmospheric Pressure Plasma Discharge

19.3 Experimental Setup for the Generation of Cold Atmospheric Pressure Plasma Jet

19.4 Methods and Materials for Surface Modification of Polymers

19.5 Direct Method for the Determination of Temperature of Cold Atmospheric Pressure Plasma Jet (CAPPJ)

19.6 Results and Discussion

19.7 Surface Characterization/Adhesion Property of Polymers

19.8 Summary

Acknowledgements

Data Availability

Conflict of Interest

References

End User License Agreement

List of Tables

Chapter 1

Table 1.1 Primary Constituents for Designing a Typical HMA.

Table 1.2 Some most common polymers used in hot-melts with their characteristics...

Chapter 2

Table 2.1 Dimensions of the SWJ with circular load coupler [5].

Table 2.2 Mechanical properties of HTA/6376 Graphite/epoxy [6].

Table 2.3 Mechanical properties of 6376 epoxy adhesive [5].

Table 2.4 Orthotropic properties of FRP laminates [21, 22].

Table 2.5 Summary of model parameters used in Taguchi orthogonal array.

Table 2.6 The orthogonal array [32] used for the Taguchi study [23].

Table 2.7 Summary of model parameters used in Taguchi orthogonal array.

Table 2.8 The orthogonal array [42] used for the Taguchi study [23].

Table 2.9 Results of ANOVA for weight of SWJ.

Table 2.10 Results of ANOVA for von Mises stress.

Table 2.11 Results of ANOVA for weight of SWJ.

Table 2.12 Results of ANVOA for von Mises stress of SWJ.

Chapter 4

Table 4.1 Influence of the fibre/matrix interfacial shear strength on the mechan...

Table 4.2 An overview of selected test methods which may be used to measure the ...

Table 4.3 Criteria for pull-out specimen preparation from single cellulose-based...

Table 4.4 Parameters for the fabrication of single fibre fragmentation test spec...

Table 4.5 Test parameters for the single element fragmentation test used for cel...

Table 4.6 Influence of fibre surface treatment on the IFSS of different fibre/ma...

Chapter 5

Table 5.1 Types of bioadhesives, their formulations and uses.

Table 5.2 Bioadhesives based on collagen.

Table 5.3 Bioadhesives based on chitosan.

Table 5.4 Bioadhesives based on albumin.

Table 5.5 Bioadhesives based on dextran.

Table 5.6 Bioadhesives based on poly(ethylene glycol).

Table 5.7 Bioadhesives based on poly(lactic-co-glycolic acid).

Table 5.8 Commercially available bioadhesives.

Chapter 6

Table 6.1 Different factors affecting mucoadhesive interactions.

Table 6.2 Classification, categories and examples of different mucoadhesive poly...

Table 6.3 Evaluation of various properties of pellets.

Table 6.4 Mucoadhesive pellets for different categories of drugs prepared by dif...

Chapter 7

Table 7.1 Summary of the measured ice adhesion strengths on various surfaces/coa...

Chapter 8

Table 8.1 Systematic overview on adhesives fully or partly based on natural reso...

Table 8.2 Bonding technologies under special consideration of naturally-based ad...

Table 8.3 Categorization of adhesives (similar to [15]), but extended and with s...

Table 8.4 Overview on most important types of adhesives based on natural resourc...

Table 8.5 Actual and foreseeable requirements for wood adhesives and wood based ...

Table 8.6 Hindrances for expanded use of adhesives based on natural resources.

Table 8.7 Established and possible future wood adhesives based on natural resour...

Table 8.8 Proteinaceous feedstocks for industrial applications.

Table 8.9 Overview on wood adhesives based on vegetable proteins (for soy see se...

Table 8.10 Thermal treatment of soy proteins.

Table 8.11 An overview of wood adhesives based on protein sources.

Table 8.12 Hydrolysis procedures for proteins from slaughterhouse waste and furt...

Table 8.13 Advantages and disadvantages of protein-based adhesives (based on [25...

Table 8.14 Physical/thermal modification of proteins.

Table 8.15 Chemical modification of proteins.

Table 8.16 Enzymatic modification of proteins.

Table 8.17 Proteins in combination with naturally-based components and adhesives...

Table 8.18 Proteins in combination with naturally-based crosslinkers.

Table 8.19 Proteins in combination with synthetic adhesives.

Table 8.20 Proteins in combination with synthetic crosslinkers.

Chapter 9

Table 9.1 Carbohydrates as potential sole wood adhesives.

Table 9.2 Modifications of starch for possible use as improved adhesive.

Table 9.3 Citric acid as naturally-based modifier and co-reactant.

Table 9.4 Combination and crosslinking of carbohydrates with natural components.

Table 9.5 Combination and crosslinking of carbohydrates with synthetic adhesives...

Table 9.6 Resins based on degradation products of carbohydrates.

Chapter 10

Table 10.1 Main chemical composition of condensed flavonoid tannins.

Table 10.2 Types of tannins and their origin (wood or plant species, part of woo...

Table 10.3 Extraction, purification, and modification methods for tannins.

Table 10.4 Hardening of tannins with formaldehyde.

Table 10.5 Hardening of tannins by other aldehydes.

Table 10.6 Hardening of tannins by other crosslinkers.

Table 10.7 Combinations of tannins with natural components.

Table 10.8 Combination of tannins with synthetic components and crosslinkers.

Table 10.9 Advantages and disadvantages of lignin as adhesive.

Table 10.10 Fractionation, molar masses and structural components of lignins.

Table 10.11 Modification of lignins for use as wood adhesives.

Table 10.12 Activation of lignin when used as sole adhesive and curing mechanism...

Table 10.13 Laccase and peroxidase induced activation of lignin.

Table 10.14 Incorporation of lignin into PF resins (LPF resins).

Table 10.15 Lignins and various aldehydes.

Table 10.16 Reactions of lignin with naturally-based components.

Table 10.17 Reactions of lignin with synthetic components and crosslinkers.

Chapter 11

Table 11.1 Polymers, fillers and processing methods for different composites.

Chapter 12

Table 12.1 Wet techniques for surface modification (Courtesy of Dr. Kash Mittal)...

Table 12.2 Dry techniques for surface modification (Courtesy of Dr. Kash Mittal)...

Table 12.3 Resonance emission wavelength lines for gaseous atoms [8].

Table 12.4 Emission maximum for rare gas excimers, Rg2* [27–29].

Table 12.5 Cut-off wavelengths for crystalline materials [7].

Chapter 13

Table 13.1 Bio-based polymers development and comparison with petroleum-derived ...

Chapter 14

Table 14.1 Water contact angles (CAs) on different surfaces with their correspon...

Table 14.2 Contact angles and sliding angles of different liquids on textured PD...

Table 14.3 Water contact angle data for the etched surfaces after fluorination f...

Chapter 15

Table 15.1 Strength of joints of stainless steel and D16AT aluminum alloys using...

Table 15.2 Properties of cured SAA depending on the ratio of MMA and AAEMA comon...

Table 15.3 Influence of methacrylic monomers and polymer thickeners on the stren...

Table 15.4 Influence of the types of monomer CH2=C(CH3)COOR and elastomer on the...

Table 15.5 The adhesion and thermal characteristics of SAAs containing oligobuta...

Table 15.6 Properties of block cured compositions depending on the structure of ...

Table 15.7 Comparison of the strength characteristics of commercial SAA and SAA ...

Table 15.8 The activity of the tertiary amine - benzoyl peroxide systems in the ...

Table 15.9 Properties of triphenylphosphine containing SAAs [47].

Table 15.10 The influence of initiators on the properties of SAAs [48].

Table 15.11 SAA* properties with different initiation systems [9].

Table 15.12 Effect of different nanoparticles (nanofillers) on shear, tensile, a...

Table 15.13 Shear strength for adhesive joints (steel-steel) at different condit...

Table 15.14 Stability of the complex and strength properties of SAA depending on...

Table 15.15 Properties of SAAs containing amine complexes with tributylborane [1...

Table 15.16 The effect of some amines and aminosilanes on adhesion strength prop...

Table 15.17 Properties of SAAs containing various compositions with trialkylbora...

Table 15.18 Strength of adhesive joints bonded with SAAs containing lithium trie...

Table 15.19 The influence of sulfo-containing additives on the shear strength of...

Table 15.20 Strength properties of adhesive joints made with SAA modified with s...

Table 15.21 Comparative properties of industrial SAAs and SAA prepared in accord...

Table 15.22 Shear strength of SAAs containing epoxy compounds for isotactic PP j...

Table 15.23 Shear strength of adhesive joints made using various substrates [158...

Table 15.24 Performance and processing features of reactive acrylic, epoxide, an...

Table 15.25 The influence of the adhesive type on the adhesion strength for bond...

Table 15.26 Comparison of strength properties of cured adhesives depending on th...

Chapter 16

Table 16.1 Typical abrasion test parameters with commonly used abradants. Steelw...

Chapter 18

Table 18.1 Liquified wood.

Table 18.2 Replacement of formaldehyde in aminoplastic and phenolic resins.

Table 18.3 Use of CNFs as components of various naturally-based and synthetic ad...

Chapter 19

Table 19.1 Intensities and wavelengths of four spectral lines at 4 kV.

Table 19.2 Ratio of the intensities of spectral lines at different electron temp...

Table 19.3 Intensities and wavelengths of four spectral lines at 5 kV.

Table 19.4 Variations of contact angles of water and glycerol with APPJ treatmen...

Table 19.5 Variations of surface free energy and its polar and dispersion compon...

Table 19.6 Variation of contact angles of water and glycerol with APPJ treatment...

Table 19.7 Variation of surface free energy and its polar and dispersion compone...

Table 19.8 Variation of contact angles of water and glycerol with APPJ treatment...

Table 19.9 Variation of surface free energy and its polar and dispersion compone...

Table 19.10 Variation of contact angles of water and glycerol with APPJ treatmen...

Table 19.11 Variation of surface free energy and its polar and dispersion compon...

Guide

Cover

Table of Contents

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Copyright

Preface

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End User License Agreement

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Progress in Adhesion and Adhesives

Volume 6

Edited by

K.L. Mittal

This edition first published 2021 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA

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Library of Congress Cataloging-in-Publication Data

ISBN 978-1-119-84665-9

Cover image: The Editor

Cover design by Russell Richardson

Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines

Printed in the USA

10 9 8 7 6 5 4 3 2 1

Preface

The present volume constitutes Volume 6 in the series “Progress in Adhesion and Adhesives” which made its debut in 2015. Volume 5 (published in 2020) was comprised of 13 review articles initially published in the journal Reviews of Adhesion and Adhesives (RAA) in 2019. Volume 4 (published in 2019) consolidated the 9 review articles published in RAA in 2018. Volume 3 (published in 2018) was based on the 12 review articles initially published in RAA in 2017. Volume 2 (published in 2017) documented the 14 review articles which initially appeared in RAA in 2016. The premier volume (published in 2015) in this vein was based on the 13 review articles originally published in RAA in 2014.

It should be recorded for posterity that the inaugural volume in this series was not designated as Volume 1, as at that time we did not have the crystal ball to foretell the future of this series. But now we can gladly report that this series of books have been warmly and enthusiastically received by those with vested interest in Adhesion Science and Adhesive Technology. We feel fully vindicated in developing these books and that they have served their intended mission.

The success of the first five volumes in this series has provided us the impetus to keep the series going. It should be recorded here that the authors of the review articles initially published in RAA have wholeheartedly endorsed this Volume 6.

The sole purpose of RAA has been to publish concise, critical, illuminating, and thought-provoking review articles on any topic within the broad scope of Adhesion Science and Adhesive Technology. The review articles for RAA have been commissioned from internationally-renowned researchers who have been invited to write articles in their specialties. So, the review articles published in RAA have been of the highest caliber.

The rationale for bringing out Volume 6 remains the same as for its predecessors: RAA had limited circulation so these books will provide broad exposure and wide dissemination of very valuable information published in the excellent review articles summarizing the latest status of many topics of contemporary interest, with commentary on future prospects and potential.

Yours truly strongly feels that the present Volume 6 will be very warmly received by the Adhesion & Adhesive community and will serve the intended purpose: to provide a resource for valuable information for both seasoned researchers and the budding scientists who wish to delve into the wonderful world of adhesion and adhesives. This particular volume, as well as its predecessors in this series, should be of interest to a broad range of adhesionists, adhesive technologists, polymer scientists, materials scientists, packaging technologists, and those with keen or peripheral interest in the pervasive wettability phenomena.

The 19 chapters in this Volume 6 follow the same order as the review articles originally published in RAA in the year 2020 and up to June 2021. The subjects of these 19 chapters fall in the following areas:

Adhesives and adhesive joints

Contact angle

Reinforced polymer composites

Bioadhesives

Icephobic coatings

Adhesives based on natural resources

Polymer surface modification

Superhydrophobic surfaces

The topics covered include: hot-melt adhesives; adhesively-bonded spar-wingskin joints; contact angle hysteresis; fiber/matrix adhesion in reinforced thermoplastic composites; bioadhesives in biomedical applications; mucoadhesive pellets for drug delivery applications; bio-inspired icephobic coatings; wood adhesives based on natural resources; adhesion in biocomposites; vacuum UV surface photo-oxidation of polymers and other materials; vitrimers and their relevance to adhesives; superhydrophobic surfaces by microtexturing; structural acrylic adhesives; mechanically- durable water-repellent surfaces; mussel-inspired underwater adhesives; and cold atmospheric pressure plasma technology for modifying polymers.

Now comes the pleasant task of thanking all those who materialized this book. Naturally, first and foremost, I would like to profusely thank the authors of the review articles originally published in RAA in 2020 and in the first half of 2021 for their wholehearted endorsement of the idea of bringing out this volume. The rationale being that this series of books provide an alternative and easily-accessible resource for a wider dissemination of excellent review articles by top-notch researchers in the field published in RAA. I will be remiss if I fail to thank Martin Scrivener (publisher) for conceiving the idea of publishing this series of books, which proved to be opportune and much needed.

Kash Mittal

Hopewell Junction, NY

email: [email protected]

June 30, 2021

1Hot-Melt Adhesives: Fundamentals, Formulations, and Applications: A Critical Review

Swaroop Gharde1, Gaurav Sharma2 and Balasubramanian Kandasubramanian1*

1Nano Surface Texturing Lab, Department of Metallurgical and Materials Engineering, Defence Institute of Advanced Technology (DU), Ministry of Defence, Girinagar, Pune-411025, India

2Nano Fabrication and Characterization Lab, Department of Nanotechnology Centre for Converging Technologies, University of Rajasthan JLN Marg, Jaipur-302004, India

Abstract: Hot-Melt Adhesives (HMAs) are typically used in applications where instant sealing is critically required. HMAs are generally preferred for those applications where processing speed is critical. These materials are widely used in various engineering applications, mainly as sealants in leakages and crack filling of walls and roofs. The industrial use of HMAs is most common in glassware and automobiles for gluing glasses in buildings and bonding heavy motor parts. The formulation of HMAs contains a polymer of suitable nature that makes the base for a strong adhesive, and waxes are added to increase the settling time of adhesive. The tackifiers are used to dilute the polymer to adjust the Glass Transition Temperature (Tg) and to reduce the viscosity for proper flow of hot-melt. This review intends to comprehensively discuss the preparation and formulations of HMAs using various polymer matrices, along with their applications and mechanics. The designing of green HMAs has been discussed in the literature and have been promoted over conventional solvent-based HMAs due to their functionality without Volatile Organic Compounds (VOCs). Various measures, challenges, and resolutions for making hazard-free HMAs have been discussed in the present review.

Keywords: Hot-melt adhesives, formulations, applications, challenges, engineering and industrial sectors

1.1 Introduction to Hot-Melt Adhesives (HMAs)

HMAs are solvent-free thermoplastic materials that are applied in molten state, which upon cooling become solid. An HMA is generally composed of a high molecular weight polymer which is a synthetic elastomer with enhanced viscoelasticity, and a resin for enhancing the properties like gluiness and wetting [1–3]. Here, the polymer provides an appropriate melt viscosity and cohesiveness upon cooling. The precise HMA formulation is dictated by its bonding properties, and application conditions such as time, pressure and temperature. The most common additives for HMAs are rosins, alkyds, and phenol-formaldehyde (PF) resins. Various ranges of suitable polymers are used to make HMAs such as PVAc [poly(vinylacetate)], PBA [poly (butyl acrylate)], polystyrenes, polyesters, EVA (ethylene-vinyl acetate), polyethylene (PE), polyamides, amorphous polypropylene and others. A classic HMA is composed of four core constituents: polymer (~33%), resin (~33%), wax (~32%), and antioxidant (~1%). Here, the resin defines the tackiness of the HMA and also maintains the wetting of an HMA (i.e., for how long the HMA remains in liquid state after its application on the substrate). Relatively, resins have low molecular weights than the polymers but it can be said that a resin is an uncured polymer as most of the available HMAs in the market usually come in two components: resins and hardener. When these parts are mixed, the polymerization starts and the resultant product is a cured polymer. The choice of resin is always determined by its affinity with the main polymer. The resin is insoluble in water, but soluble in alcohol. Resin can also be added directly to the polymer to enhance the mechanical and chemical properties of the particular polymer. Due to the fact that the resins are usually tri- or tetra- functional molecules, and when mixed with difunctional polymers give weak mechanical properties. But, when these materials are allowed to mix with a compatible polymer, they form a cross-linked structure which is mechanically stronger than the polymer itself [4]. With higher quantity of resin, tough and strong hot-melts can be produced, while with less resin, soft and fast settling HMAs can be produced. The variety of primary constituents for a typical HMA are shown in Table 1.1 [3, 5]. HMAs generally exist in granules, powders, films, and blocks forms, where these can be applied to a surface as a solution or emulsion to be heat activated later after evaporation of solvent [6].

For the liquification of Hot-Melt Films (HMFs), thermal activation and solvents are used, and their solidification depends on their open time and set time, where open time is the time interval where the applicable surface remains tacky for pressure-sensitive adhesives (pressure-sensitive adhesives are a type of non-reactive adhesives which make a bond when pressure is exerted to affix the adhesive with the adherend). On the other hand, the set time is defined as the time interval required to obtain bond strength between the surface and adhesive in a temperature range of 20°C to 30°C [1, 5] (Figure 1.1).

Table 1.1 Primary Constituents for Designing a Typical HMA.

Primary Constituent

Physical Properties

Function in HMA

Polymer

High molecular weight (>10000)

Provides adhesive strength

T

g

(Glass Transition Temperature) < Room Temperature

Keeps the tackifiers hot

Cross-linking upon cooling

Reduces the peel force of adhesive

Resin

Low molecular weight (<5000)

Lowers viscosity

T

g

Usually > Room Temperature

Improves HMA wetting

Diluent

Low molecular weight (<1000)

Adjusts T

g

of the system

T

g

< Room Temperature

Dilutes polymer network

Wax

Low molecular weight (<2000)

Increases setting speed of HMA

T

g

< Room Temperature

Provides heat resistance, lowers viscosity

Figure 1.1 Graph showing temperature vs. time curve for a typical HMA.

HMAs have outstanding mechanical and physical properties in comparison to conventional solvent-based adhesives. Generally, HMAs are insensitive to aqueous media which means their bond strengths can rarely be influenced by water or moisture. If HMAs are used on a moist or wet surface then their bonding with the adherend may grow weak with time. Additionally, the technology of HMAs exhibits various unique advantages in comparison to other types of adhesives. The prime benefits of HMAs are their low-cost, elimination of VOCs (Volatile Organic Compounds) discharge in manufacturing, no risk of heat burst (mainly occurs due to the chemical reaction processes in mixing two or more solvents in solvent-based adhesives), no need for adhesive dryers which are conventionally needed to dry the applied adhesive for good settling and maintaining it for long time, and modest application cost, i.e., mainly using a hot glue gun [7].

HMAs are widely used in consumer goods, and engineering sectors for manufacturing pressure- sensitive adhesive tapes, grooved boards, laminated panels of wood, etc. For instance, HMAs with trademarks KR-16-20, Krol, Krok, Krus-2, Stek, and Teplager provided by Jowat Adhesives are being widely used in metalnonmetal bonding along with sealing of polymer composites. For a number of bonding applications, a variety of HMAs perform their function adequately. HMAs are generally preferred for applications requiring critical speeds (critical speed is the theoretical angular velocity of a rotating object such as shaft, leadscrew, gear, etc.). Hot-melts are carrier-free and solidify swiftly upon cooling, and for this reason, they have limited capability to: (i) penetrate the substrates of low-porosity, (ii) dissolve or absorb contaminants on the surface, and (iii) wet-out metals. HMAs are good choices in automated production due to their fast speed of setting. A large variety of HMAs are chemically inert and remain thermoplastic during their use. In this review, we discuss the formulations, design principles, mechanical behavior, HMA tests, and applications, along with the current challenges of HMAs and how to meet them.

1.2 Formulation of Hot-Melt Adhesives

1.2.1 Theories or Mechanisms of Adhesion

Adhesion usually is classified by physical, chemical, and/or mechanical bonding processes, which give the interfacial strength to the joint. The adhesion bonding occurs between the adherend and the adhesive. Over the years, various theories have been established to describe the phenomenon of adhesion. Mechanical bonding is a highly efficient technique for creating joints, but it fails to join smooth surfaces. Chemical bonding is strong but it is difficult to produce in an efficient manner [8–12]. Each theory of adhesion for bonding of materials is discussed in more detail below.

1.2.1.1 Mechanical Interlocking Theory

Mechanical theory is the common and the oldest theory of adhesion. It involves penetration of adhesive in pores, cavities, and irregularities on the surface of adherend and was proposed by McBain and Hopkins in 1925 [13]. It displaces the trapped air at the interface, which determines the adhesive penetration into both adherends. It was also found that adhesives form good bonds with porous and irregular surfaces as compared to smooth surfaces [8–11].

1.2.1.2 Electrostatic Theory

According to this theory, adhesion takes place at the adhesive-adherend interface due to exchange or transfer of electrons across the interface. Such transfer forms an electrical double layer at the adhesive-adherend boundary and results in Coulomb attraction forces between the mating partners [8–11].

1.2.1.3 Diffusion Theory

According to this theory, diffusion is only valid when both the adhesive and the adherend are polymers with relatively long chain-like structures, which deliver mobility, and permit the possibility of the chains to have molecular movements in the sub-molecular range. The diffused interfacial layer has a thickness in the range of 1–100 nm, and it is dependent on various parameters such as pressure, time, temperature, molecular size, and mutual solubility [8–11].

1.2.1.4 Physical Adsorption or Wetting Theory

According to adsorption theory, the forces responsible for adhesion between two materials are secondary valence or van der Waals forces. The process of forming contact between the adhesive and the adherend is known as “wetting”. For an adhesive to wet a solid surface, the adhesive should have a surface tension lower than the critical surface tension of the solid. This is the reason for surface treatment of plastics, which increases their surface free energy and polarity [8–11].

1.2.1.5 Chemical Bonding

This mechanism is quite similar to the physical adsorption but it involves the formation of hydrogen, covalent, and ionic bonds between the adhesive and the adherends. In general, there are four types of interactions during bonding such as covalent, hydrogen, Lifshitz-van der Waals forces, and acid-base interactions, but the exact nature of bond depends on the chemical structure of the adherend (substrate). The fabrication of adhesives based on chemical bonding theory has been adopted for decades by various industries, but the results have been disappointing due to their contamination with external environmental factors like air, moisture, temperature, etc. which made the adhesion unstable and weak [8–11].

1.2.2 Intermolecular Forces between Adhesives and Adherend

The numerous adhesion theories imply the existence of physico-chemical interactions across the interface of adherend and adhesive. The development of an adhesion bond relies on increasing the intermolecular attraction between the two surfaces, both in the polymer bulk and between the adhesive and adherend. Among the different forces accountable for intermolecular attraction, primary forces are dispersion forces acting in all atoms and are responsible for all of the molecular attraction or cohesion in all molecules, excluding the polar molecules. Dispersion forces are short-range interaction forces that are effective at a distance of 4 Å, and rapidly decrease with the inverse 6th power of the intermolecular distance. Hence, the molecules of adhesive polymer must be sufficiently flexible to come within this interaction range with a rigid adherend surface under bond formation conditions. Additional interaction arises between dipoles in molecules, where dipoles arise when the chemically bonded electrons among atoms are shared unequally, thus creating positive and negative charge centres in the molecule. The force of interaction due to permanent dipoles of polar molecules depends on strength of the two dipoles, and is reduced with the inverse 6th power of the distance. Evidently, the dipolar interaction of adhesives will be strong when they possess polar chemical groups [14]. A strong dipolar attraction occurs when the positive centre is connected with the H-atom attached to an electronegative atom, generally N or O, as shown in the following examples

This sharing of proton between electronegative atoms is called the hydrogen bond. It arises in polymers carrying amide (-CONH-), carboxyl (-COOH), or hydroxyl (-OH) groups, and is responsible for adhesion of materials such as proteins, starch, poly(vinyl alcohol), epoxy resins, phenolics to polar substrates, especially to wood which has an abundance of hydroxyl groups [15, 16].

1.2.3 Thermodynamic Model of Adhesion

The thermodynamic work of adhesion, wetting surface tension, solubility parameter or interfacial free energy are responsible parameters for the strength of adhesive bonds. Adhesive bonding is controlled by intermolecular forces, where the attraction forces between adhesive and adherend are defined by an energy interaction that can be characterized by the Lennard-Jones potential [17].

(1.1)

where, F denotes the force of attraction, r denotes the separation distance between an adhesive and adherend with the variables A and C as adhesion constants. The Force of Attraction vs Distance of Separation graph is illustrated in Figure 1.2.

Figure 1.2 Force of attraction vs distance of separation between adhesive and adherend.

The basic equations provided by Dupre and Young-Dupre for the thermodynamics of adhesion are as follows:

(1.2)

(1.3)

where, θLS is the equilibrium contact angle made by L (liquid) phase in contact with S (solid) phase.

The maximum strength and flexibility of an HMA bond are obtained when the interfacial free energy is minimum [18]. The interfacial free energy of the system is obtained by reorganization of Equations (1.2) and (1.3) as:

(1.4)

1.2.4 Bonded Joints