Pressure-Sensitive Adhesives and Applications

Istvan Benedek

• Publisher: CRC Press
• Year: 2004
• Print ISBN: 9780824750596
• eBook ISBN: 9781135537890
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Pressure-Sensitive Adhesives and Applications, Second Edition explains how pressure-sensitive adhesives (PSAs) work, why they are used, and the technology used to manufacture them. This second edition features the latest developments in the field. Dr. Benedek discusses the factors that affect the rheology and special flow characteristics res

1 Introduction

Adhesives are nonmetallic materials [1] used to bond other materials, mainly on their surfaces through adhesion and cohesion. Adhesion and cohesion are phenomena which may be described thermodynamically, but actually they cannot be measured precisely. It was shown [2] that the most important bonding processes are bonding by adhesion and bonding with pressure-sensitive adhesives (PSAs). For adhesives working through adhesion phenomena the adhesive fluid is transformed after bonding (i.e., the build up of the joint) into a solid. In the case of PSAs, the adhesive conserves its fluid state after the bond building too. Thus its resistance to debonding is moderate and the joint may be delaminated without destroying the laminate components in most cases.

Pressure-sensitive adhesives have been in wide use since the late 19th century, starting with medical tapes and dressings. The first U.S. patent describing the use of a PSA—for a soft, adhering bandage—was issued in 1846 [3]. Ninety years later Stanton Avery developed and introduced the self-adhesive label [4]. Two major industries resulted from these innovations: pressure-sensitive tapes and labels. Industrial tapes were introduced in the 1920s and 1930s followed by self-adhesive labels in 1935. About ten years after that, pressure-sensitive protective films were manufactured. The history of PSAs was described by Villa [5]. First, solvent-based PSAs using natural rubber were developed (19th century). In the 1940s hot-melt adhesives were introduced. Pressure-sensitive adhesives are adhesives that form films exhibiting permanent tack, and display an adhesion which does not strongly depend on the substrate [6]. The term PSA has a very precise technical definition and has been dealt with extensively in the chemical literature [7, 8]. However, as discussed in [9], the technical term of PSA in different languages (e.g. pressure-sensitive adhesive, auto-collants, Haftkleber, etc.) is not completely clear. The recent development of pressure-sensitive products without a coated pressure-sensitive adhesive layer, makes the definition of this product group more difficult [9, 10]. The build up and classification of pressure-sensitive products have been discussed in detail in [9].

The function of PSAs is to ensure instantaneous adhesion upon application of a light pressure. Most applications further require that they can be easily removed from the surface to which they were applied through a light pulling force. Thus PSAs are characterized by a built-in capacity to achieve this instantaneous adhesion to a surface without activation, such as a treatment with solvents or heat, and also by having sufficient internal strength so that the adhesive material will not break up before the bond between the adhesive material and the surface ruptures. The bonding and the debonding of PSAs are energy-driven phenomena. Pressure-sensitive adhesives must possess viscous properties in order to flow and to be able to dissipate energy during the adhesive bonding process. However, the adhesive must also be elastic (i.e., it must resist the tendency to flow) and, in addition, store bond rupture energy in order to provide good peel and tack performance. Pressure-sensitive adhesives should possess typical viscoelastic properties that allow them to respond properly to both a bonding and a debonding step. For satisfactory performance in each of these steps the material must respond to a deforming force in a prescribed manner.

Polymers employed as PSAs have to fulfill partially contradictory requirements; they need to adhere to substrates, to display high shear strength and peel adhesion, and not leave any residue on the substrate upon debonding. In order to meet all these requirements, a compromise is needed. When using PSAs there appears another difference from wet adhesives, namely the adhesive does not change its physical state because film forming is inherent to PSAs. Thus, PSAs used in self-adhesive laminates are adhesives which, through their viscoelastic fluid state, can build up the joint without the need to change this flow state during or after application. On the other hand, their fluid state allows controlled debonding giving a temporary character to the bond. Because of the fluid character of the bonded adhesive, the amount of adhesive (i.e., the dimensions of the adhesive layer) is limited; the joint works as a thin-layer laminate or composite. Because of this special, thin-layer structure of the composite, the solid state components of the laminate exert a strong influence on the properties of the adhesive in the composite. Therefore, there exists a difference between the measured properties of the pristine adhesive and of the adhesive enclosed within the laminate.

Adhesives, in general, and PSAs, in particular, have to build up a continuous, soft (fluid), and tacky (rubbery) layer. The latter will adhere to the substrate. On the other hand, the liquid adhesive layer of the PSAs working in the bond has to offer a controlled bond resistance. This special behavior requires materials exhibiting a viscoelastic character. The properties which are essential in characterizing the nature of PSAs comprise: tack, peel adhesion, and shear. The first measures the adhesive’s ability to adhere quickly, the second its ability to resist removal through peeling, and the third its ability to hold in position when shear forces are applied [11].

These properties will be discussed in more detail in Chap. 6, which describes the adhesive properties of PSAs. In order to understand the importance of these properties, it is absolutely necessary to answer the following questions:

What does the viscoelastic character of a PSA comprise?

What is the material basis (main criteria) for the viscoelastic behavior of a PSA?

REFERENCES

1. DIN 16921.

2. R.Köhler, Adhäsion, (3) 90 (1970).

3. J.A.Fries, “New Developments in PSA,” in TECH 72, Advances in PressureSensitive Tape Technology, Technical Seminar Proceedings, Itasca, IL, May, 1989.

4. Der Siebdruck, (3) 69 (1986).

5. G.J.Villa, Adhäsion, (10) 284 (1977).

6. Vinnapas, Eigenschaften und Anwendung, 7.1. Teil, Anwendung, Wacker GmbH, München, 1976.

7. R.Houwink and G.Salomon, Adhesion and Adhesives, Vol. 2, Chapter 17, Elsevier Co., New York, 1982.

8. D.Satas, Handbook of Pressure Sensitive Technology, Van Nostrand Rheinhold Co., New York, 1982.

9. I.Benedek, Development and Manufacture of Pressure-Sensitive Products, Marcel Dekker, New York, 1999.

10. I.Benedek, Pressure-Sensitive Formulation, VSP, Utrecht, 2000.

11. J.P.Keally and R.E.Zenk. (Minnesota Mining and Manuf. Co., USA), Canad. Pat. 1224.678/10.07.92 (US Pat. 399350).

2
Rheology of Pressure-Sensitive Adhesives

Pressure-sensitive adhesives are viscoelastic materials with flow properties playing a key role in the bond forming; their elasticity plays a key role in the storage of energy (i.e., the debonding process). The balance of these properties governs their time-dependent repositionability and bonding strength (i.e., their removability). Their flow properties are useful in coating technology and at the same time detrimental to the converting technology of labels.

Generally PSAs are used as thin layers, therefore their flow is limited by the physico-mechanical interactions with the solid components of the laminate (liner and face) materials. On the other hand the solid components of the laminate are generally thin, soft, viscous, and/or elastic layers, allowing a relatively broad and uniform distribution of the applied stresses. Thus the properties of the bonded adhesive (i.e., its flow characteristics) may differ from those of the pure (unbonded) adhesive. Therefore in this chapter the rheology of pure and coated PSAs will be dealt with separately.

1 RHEOLOGY OF UNCOATED PSAs

It remains difficult to examine the properties of pure (i.e., uncoated or unbonded) PSAs, and to obtain generally valid information. Pressure-sensitive adhesives are seldom used as thick layers between motionless rigid surfaces (i.e., as fluids). On the other hand, as known from industrial experience, the nature of the face stock material or of the substrate used, and their characteristics and dimensions may significantly influence the properties of the PSA laminate. Practically, this disadvantage is eliminated by the use of normalized or standard solid state components. However, a theoretical approach may be used for the investigation of pristine PSAs.

1.1 Properties of PSAs

The adhesive and end-use properties of PSAs require a viscoelastic, non-Newtonian flow behavior which is based on the macromolecular nature of the adhesive. In order to understand the needs and means of viscoelastic behavior one needs to summarize the most important material properties specifically related to PSAs. Generally, adhesives in a bond behave like a fluid or a solid. Fluids are characterized by their viscosity which influences their mobility, whereas solids are characterized by their modulus which determines their deformability. In an ideal case, for Newtonian fluids (or for solids obeying Hooke’s law) the applied force (load) will be balanced by the material’s own mechanical characteristics, that is, the viscosity η or the Young’s modulus E: where τ and σ are the applied stresses, and γ and γ’ are the strain and shear rate, respectively.

(2.1)

(2.2)

As indicated earlier, PSAs originate from a film-forming, elastomeric material, which combines a high degree of tack with an ability to quickly wet the surface to which it is applied, to provide instant bonding at low-to-moderate pressure as a result of its flow characteristics. On the other hand, PSAs possess sufficient cohesion and elasticity, so that despite their aggressive tackiness they can be handled with the...

Table of contents

• COVER PAGE
• TITLE PAGE
• COPYRIGHT PAGE
• PREFACE TO THE SECOND EDITION
• PREFACE TO THE FIRST EDITION
• 1. INTRODUCTION
• 2. RHEOLOGY OF PRESSURE-SENSITIVE ADHESIVES
• 3. PHYSICAL BASIS FOR THE VISCOELASTIC BEHAVIOR OF PRESSURE-SENSITIVE ADHESIVES
• 4. COMPARISON OF PSAs
• 5. CHEMICAL COMPOSITION OF PSAs
• 6. ADHESIVE PERFORMANCE CHARACTERISTICS
• 7. CONVERTING PROPERTIES OF PSAs
• 8. MANUFACTURE OF PRESSURE-SENSITIVE ADHESIVES
• 9. MANUFACTURE OF PRESSURE-SENSITIVE LABELS
• 10. TEST METHODS
• ABBREVIATIONS AND ACRONYMS